Photophysical Characterization of a Ruthenium(II) Tris(2,2′-bipyridine)-Doped Zirconium UiO-67 Metal–Organic Framework

Photophysical Characterization of a Ruthenium(II) Tris(2,2′-bipyridine)-Doped Zirconium UiO-67 Metal−Organic FrameworkWilliam A. Maza and Amanda J. Morris*

Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States

*S Supporting Information

ABSTRACT: The photophysical properties of ruthenium(II) tris(5,5′-dicarboxy-2,2′-bipyridine), RuDCBPY, doped into the metal organicframework of Zr6O4(OH)4(BPDC)6, RuDCBPY-UiO67 (where BPDC is4,4′-biphenyldicarboxylic acid), are presented as a function of the degreeof RuDCBPY doping. Steady-state diffuse reflectance of RuDCBPY-UiO67 powder shows an absorption maxima at 455 nm, which iseffectively insensitive to doping concentration. The energy of theemission maxima and excited state lifetimes are, however, quite sensitiveto concentration of RuDCBPY in UiO-67. At low doping concentrations,the emission maxima lies at ∼630 nm. The emission decay can beadequately modeled using a single discrete exponential decay functionwith an observed lifetime of 1.4 μs. The emission lifetime and the energyof the emission maxima are found to decrease with increased RuDCBPYconcentration. However, at higher doping the emission decay becomes nonexponential. Equally adequate fits to the data wereobtained using stretched exponential and biexponential functions. A two-state model is presented in which the nonexponentialbehavior observed at higher RuDCBPY doping concentrations is due to two separate solvation environments within UiO-67. It isproposed that a single RuDCBPY preferentially occupies the larger octahedral cages of UiO-67 by incorporation into thebackbone of the cage and experiences a dimethylformamide (DMF)-like solvation environment. At higher doping concentrations,in addition to incorporation of RuDCBPY into the backbone of the octahedral cavities, populations of encapsulated RuDCBPYcan also be found in separate octahedral UiO-67 cavities. Encapsulation is assumed to restrict the solvent occupancy within thepore; thereby limiting the solvation of RuDCBPY by DMF. This leads to the disparity in lifetimes between the slow (>100 ns)and fast (∼20−30 ns) components of the lifetime decay, as well as the lack of doping concentration dependence on the fast decaycomponent. The decreased lifetime of the slow phase with increased doping concentration is attributed to intermolecular energytransfer between neighboring RuDCBPY molecules incorporated into the backbone of the octahedral cages.

■ INTRODUCTION

Metal−organic frameworks (MOFs) are a diverse class ofhighly tunable, functional materials. These present attractiveplatforms for a variety of applications including chemicalstorage/transport, separation, sensing, and catalysis.1−14 Inparticular, a large body of work has been dedicated to reactionscatalyzed by MOFs either thermally or electrochemi-cally.1,2,15−31 The variety and specificity of reactions carriedout by these materials can be tuned by selective engineering oftheir components. Reactivity can be designed into the materialsthrough the choice of the metals comprising the nodes as wellas the geometry, size, and functionalization of the ligandsforming the backbone and struts of the framework. Theseligands afford dimensionality and pore volume to theframework. Functionality can also be obtained via postsyntheticmodification of the metal nodes or ligand struts. In addition,materials can be permanently doped with a chemical guest viaencapsulation or substitution of one or more of the ligandstruts.32−37

The excited state properties of the ligand struts or dopantscan be exploited to enhance the catalytic or electrocatalytic

function of the material. Generation of the ligand/dopantexcited state effectively alters the redox potential and, thereby,the driving force for a given reaction involving electron transferaccording to the Rhem−Weller equation (eq 1).38,39

Δ = ° − ° −+ −G E E EET /0 0/ 0,0 (1)

E°+/0 and E°0/− refer to the oxidation and reduction potentialsof the donor and acceptor, respectively. E0,0 is the energydifference between the ground and excited state of thephotoactive reactant. The high surface area and porous natureof the MOF reduce the limitation imposed by diffusion ontothe probability of reaction, thereby reducing the need forextremely long-lived excited states.It is well-known that environmental effects, such as solvation,

polarity, and viscosity, can have dramatic effects on thephotophysics of chromophores in solution.40−44 For chromo-phores incorporated into materials, the steric, geometric, and

Received: January 31, 2014Revised: March 11, 2014Published: April 4, 2014

Article

pubs.acs.org/JPCC

© 2014 American Chemical Society 8803 dx.doi.org/10.1021/jp501140r | J. Phys. Chem. C 2014, 118, 8803−8817

pubs.acs.org/JPCC

torsional constraints imposed on the photoactive species mayalso affect the photophysics and photochemistry of thechromophores.45−53 Understanding the functional role ofMOF−host materials in determining the photophysical proper-ties of their photoactive guests is, therefore, paramount indesigning appropriate host−guest systems to carry outphotochemical reactions.Previously, a fairly water stable Zr-based MOF containing

4,4′-biphenyldicarboxylate struts with a molecular formula ofZr6(μ3-O)4(μ3-OH)4(BPDC)6 (BPDC = biphenyldicarboxylicacid), also known as UiO-67, was developed at the Universityof Oslo (“UiO”).33,54 UiO-67 has been shown to be quiteresistant to a variety of different solvents of varying polarity, yetshows instability in aqueous 0.1 M HCl and 0.1 M NaOHsolutions.55 The Zr6O4(OH)4(CO2)12 clusters come togetherto form two types of cavities: a tetrahedral pore of 11.5 Ådiameter and an octahedral pore of 23 Å diameter with 6.5 Åwindows.56,57 Lin and co-workers have successfully doped UiO-67 with a variety of transition metal coordination compoundsincluding a modified ruthenium(II) tris(2,2′-bipyridine),RuBPY, in which one of the bipyridyl ligands was functionalizedat the 5,5′-positions with carboxylic acids, DCBPY.33,58

DCBPY, which is isostructural with BPDC, imparts the abilityof RuDCBPY to replace BPDC within the UiO-67 framework.The rich excited state properties of RuBPY make it an ideal

photoactive compound for doping within a MOF matrix. Insolution, the relatively long-lived (∼600 ns) emissive tripletmetal-to-ligand charge transfer (3MLCT) state forms with aquantum yield of unity and affords the complex an additional 2eV of free energy for redox reactions. The excited stateproperties of the RuDCBPY-UiO-67 complex are hereindescribed as a function of RuDCBPY doping concentration.It was found that the excited state properties of RuDCBPYincorporated into UiO-67 are concentration dependent, whereat low doping concentrations the photophysical propertiesresemble that of RuDCBPY in dimethylformamide (DMF) andare significantly perturbed with increased doping. Probablequenching mechanisms are discussed within the context of thenature of UiO-67 pore occupancy by RuDCBPY.

■ EXPERIMENTAL SECTIONAll chemicals and solvents including RuCl3·xH2O (38−42%Ru), 2,2′-bipyridine (BPY, 99%), 2,2′-bipyridyl-5,5′-dicarbox-ylic acid (DCBPY, 95%), 4,4′-biphenyldicarboxylic acid(BPDC, 98%), ZrCl4 (98%), DMF (>99%), methanol(MeOH, >99%), and benzoic acid (BZA, 99%) were used asobtained without further purification from either FisherScientific or Sigma-Aldrich.Preparation of Ru(II) Bis(2,2′-bipyridine) (2,2′-Bipyr-

idyl-5,5′-dicarboxylic acid) Dichloride, RuDCBPY.RuDCBPY was synthesized by first synthesizing Ru(II)bis(2,2′-bipyridine) dichloride, Ru(BPY)2Cl2, by the methodof Sullivan et al.,59 which was used as precursor for thepreparation of RuDCBPY by an established procedure.60

Preparation of UiO-67. UiO-67 was synthesized by amodification of the procedure given by Schaate et al.61 Briefly,0.13 g of ZrCl4 (0.56 mmol) and 3.4 g of BZA (28 mmol, 50mol equiv) were dissolved by sonication (5 min) in 20 mL ofDMF. This was then added to a 6 dram scintillation vialcontaining 0.14 g of BPDC (0.58 mmol), and the solution wassonicated for an additional 5 min. The mixture was then heatedat 120 °C for 2 days. After cooling to room temperature, theresulting crystalline white powder was filtered and washed with

DMF (∼10−20 mL × 3) and acetone (∼10−20 mL × 3), thenallowed to air-dry.

Preparation of RuDCBPY-UiO-67 and [email protected] and RuBPY@UiO-67 were prepared by aprocedure identical to that described for UiO-67 with theaddition of 2.2 mg of RuDCBPY. For those higher loadedpreparations, the amount of RuDCBPY added to the reactionmixture varied from 2.2 mg (at low doping, ∼3 mm) to ∼50 mg(highest doped material, ∼45 mm). In the case of RuBPY@UiO-67, 45 mg of RuBPY was used.

Characterization of Materials. X-ray powder diffraction(PXRD) patterns were collected using a Rigaku Miniflexequipped with a CuKα radiation source (λ = 1.5418 Å) at ascanning speed of 0.8 min/deg with a step size of 0.02°. FTIRspectra were obtained with a Cary 670 FTIR spectrometerequipped with a Golden Gate single reflection diamond ATRattachment. The spectra represent an average of 64 scansbetween 400 and 4000 cm−1 with a spectral resolution of 4cm−1.MOF powders used for steady-state fluorescence, polar-

ization, and fluorescence lifetime measurements were mountedon a glass slide (cut to fit the diagonal of a standard 1 cm2

cuvette) with a thin layer of vacuum grease. The slide was thenplaced in a quartz 1 cm2

fluorometer cuvette, sealed with arubber septum and parafilm, and purged for 30−45 min withN2. Steady-state absorption and diffuse reflectance spectra wereobtained using a Cary 5000 UV−vis−NIR spectrometer.Steady-state fluorescence were obtained using a front-facegeometry with a Cary Eclipse Fluorescence Spectrometer. Thefluorescence spectra presented were reconstructed fromnormalized emission spectra of solid samples excited at 440,450, and 460 nm in order to identify and correct for bandsarising due to Raman scattering as is common for solid samples(Figure S1 in Supporting Information).Fluorescence lifetime decays of the powders were acquired

also using a front-face geometry with an Applied Photophysicsmodel LKS.60 laser photolysis system. Samples were pumpedusing the third harmonic of a Continuum Surelite SLI-10Nd:YAG laser (6−8 ns pulsewidth, λexc = 355 nm). Zero ordersignals were passed through a Spectrakinetic monochromator(model 05-109, bandpass 4.65 nm/mm), amplified using aSpectraphysics photomultiplier and digitized with a HPInfinium 500 MHz oscilloscope (2 GS/sec sampling). Sampletemperatures were regulated using a VWR model 1150Srefrigerated circulator attached to the sample cuvette temper-ature block. A clean glass slide mounted in a quartz cuvette withthe same front-face geometry was used to collect laser scatterfrom the fluorescence decays, which was used as the instrumentresponse function.

Determination of RuDCBPY Loading of RuDCBPY-UiO-67. DeCoste et al. have recently reported on the stabilityof UiO-67 under various solvent conditions and have found thismaterial to be the least stable in aqueous solutions of NaOH.55

Therefore, in order to determine the degree of loading ofRuDCBPY in RuDCBPY-UiO-67, known amounts of materialwere suspended in known volumes of either 0.1 M NaOH or0.1 M KOH and sonicated for 5−10 min. The solutions werethen filtered using 28 mm syringe filters with 0.45 μm pores.Optical densities at 450 nm were recorded and the solutionconcentrations of RuDCBPY determined assuming theextinction coefficients are similar to RuBPY at 450 nm (14.6mM−1 cm−1).62 The material loading was then calculated fromthe solution concentrations obtained and reported in terms of

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp501140r | J. Phys. Chem. C 2014, 118, 8803−88178804

micromoles of RuDCBPY per gram of RuDCBPY-UiO-67material (e.g., millimolal, mmolal).Calculation of van der Waals Radii. The structure of

Ru(BPY)2Cl2 was generated and geometry optimized using aMM+ force field with the HyperChem program suite, thechlorine atoms were then removed, and the QSAR(quantitative structure−activity relationship) calculation wasperformed to obtain the van der Waals (VDW) volume.Molecular radii were then estimated from the VDW volumeassuming a hard sphere approximation.

■ RESULTSRuDCBPY in Solution. In DMF, the absorption spectrum

of RuDCBPY resembles that of RuBPY with a maxima in thevisible at 462 nm and a higher energy shoulder at 434 nm. Inwater, while there is little change in the position of the 434 nmshoulder, the absorption maxima displays a hypsochromic shiftto 449 nm and a lower energy shoulder appears at ∼487 nm(Figure 1). This broad absorption band has been attributed to aRu(dπ

6) → Ru(dπ5)BPY(π*) metal-to-ligand charge transfer

(MLCT) transition of singlet character (see Juris et al. andreferences therein).60,63 Xie et al. have reported similarabsorption values for RuDCBPY in water (pH = 7), with amaxima at ∼450 nm and a concomitant shift to higher energywith decreased pH.60 Bathochromic shifts in the absorptionmaxima were also observed for RuDCBPY in acetonitrile (456nm) and ethanol (456 nm) relative to water at pH 7.60

An emission maxima at 625 nm is observed in DMF forRuDCBPY (Figure 1). In neat water (pH ≈ 7), the emissiondisplays a large bathochromic shift with a maxima centered at∼690 nm. Xie et al. reported that the weak emission observedat pH 7 (Φfl ≪ 0.01) centered ∼650 nm decreased significantlyand underwent a large bathochromic shift as the solution pHwas decreased (∼680 nm at pH 1.7).60 The emission maxima inacetonitrile and ethanol were, however, shifted to higher energyrelative to water (606 and 600 nm, respectively).The emission decay of RuDCBPY in DMF at room

temperature displayed an exponential dependence on time(Figure 1). Fits to a discrete single exponential function yield alifetime of 880 ns, similar to that of RuBPY in DMF (∼900 ns,see Table 1). The emission lifetime was also found to betemperature dependent. This temperature dependence of theemission lifetimes of RuBPY and other ruthenium(II)-polypyridyl complexes has been shown to arise from thermalpopulation of two additional states lying slightly above theemissive 3MLCT: a metal-centered triplet (3dd) state lying∼2500−4000 cm−1 above the emissive state and a fourth3MLCT lying ∼100−1000 cm−1 above the emissive state.64−66

These are summarized schematically in Figure 2, and theobserved emission decay is expressed mathematically by eq 2:

τ = + +− − Δ − Δk k ke eE k T E k Tobs

1o 1

( / )2

( / )1 B 2 B (2)

where the first term, ko, is the sum of the radiative andnonradiative decay rate constants for the emissive triplydegenerate 3MLCT manifold, k1 and k2 are the nonradiativedecay rate constants of the thermally populated 3dd and fourth3MLCT, respectively, and ΔE1 and ΔE2 are the energydifference between the emissive 3MLCT and either the 3ddor fourth 3MLCT, respectively. Although the observed lifetimeat a given temperature may have some contribution from thelast term corresponding to population of the fourth 3MLCT, atroom temperature determination of k2 and ΔE2 can be difficult

if thermal population of the 3dd dominates or if the rate ofdecay from the fourth 3MLCT is approximately equal to ko, i.e.,ko ≈ k2 exp(−ΔE2/kBT), k2/ko → 0, or k2 exp(−ΔE2/kBT) ≪k1 exp(−ΔE1/kBT). In such cases, eq 2 reduces to eq 3.67

τ = +− − Δk k e E k Tobs

1o 1

( / )1 B (3)

In systems where steric confinement or a mixed ligandcoordination sphere about the metal center stabilizes the3MLCT thereby making the 3dd inaccessible, the second termin eq 2 can be negligible and the parameters k1 and ΔE1 in eq 3are replaced by k2 and ΔE2. Examples of this will be discussedlater.Fits of the temperature dependence on the emission lifetime

of RuDCBPY in DMF to eq 3 reveal corresponding ko, k1, andΔE1 (∼9 × 105 s−1, ∼2 × 1011 s−1, and ∼2500 cm−1,respectively) to be very similar to those found by Casper andMeyer for RuBPY in DMF (Table 2, Figure S2 in SupportingInformation).65 In water, however, the emission lifetime ofRuDCBPY is considerably faster than in DMF (∼38 ns at room

Figure 1. Comparison of ( a) steady-state absorption spectra, (b)steady-state emission spectra (λexc = 440 nm), and (c) emission decaysof RuDCBPY in water (black) and DMF (red).



temperature). This large discrepancy in lifetime is not at allsurprising in light of the known sensitivity to solvent(particularly to solution pH) displayed by mono-, bis-, andtris-substituted RuBPY derivatives in which the carboxylic acidfunctional groups are placed at varying positions on the BPYligand.60,68−71 Indeed, Nazeeruddin and Kalyanasundaram havereported a lifetime of 37 ns for the tris-DCBPY substitutedRuDCBPY.68 The observed lifetime displayed very littletemperature dependence, and therefore, adequate fits couldnot be obtained with eq 3.Lumpkin et al. have suggested that for mixed ligand

polypyridyl complexes of Ru(BPY)2 and 2,2′-bipyridyl-4,4′-dicarboxylic acid (4,4′-DCBPY), the 3MLCT localizes onto the4,4′-DCBPY because of the lower energy of 4,4′-DCBPY(π*)relative to the BPY(π*).67 The lowering of the 4,4′-DCBPY-(π*) was rationalized as a consequence of the electron-withdrawing nature of the carboxylic acids. Localization of the3MLCT onto the 4,4′-DCBPY increases the ΔE1 correspondingto the 3MLCT and 3dd energy gap, rendering the 3ddinaccessible. The temperature dependence observed in this

system was attributed to thermal population of the fourth3MLCT.67

It is possible that in aqueous solutions of RuDCBPY theDCBPY(π*) is similarly stabilized making the 3dd inaccessible.Furthermore, substitution at the 5,5′-positions may alsodecrease the magnitudes of either the k2 or ΔE2 parameterscorresponding to population of the fourth MLCT. This wouldmake the exponential term in eq 3 approximately equal to ko inthe temperature range monitored (5° to 35 °C), resulting in anapparent temperature insensitive emission lifetime.

RuDCBPY-UiO-67 Powder. The solvothermal reactionbetween ZrCl4 and BPDC in DMF results in a fine whitepowder (UiO-67). The addition of 50 mol equiv (relative toZrCl4) of BZA increases the crystallinity of the powder, as hasbeen systematically demonstrated by Schaate et al.61 Theexperimental PXRD was in excellent agreement with PXRDpatterns simulated from the previously reported structural data(Figure 3).72

The same solvothermal procedure for UiO-67 performed inthe presence of less than 1 mol % RuDCBPY (relative toBPDC) yielded similarly fine powders of pinkish hue, whichwere observed to show reddish emission under UV irradiation(Figure S3 in Supporting Information). PXRD patterns of theRuDCBPY-doped material synthesized in the presence of BZAwere almost identical to that of the undoped UiO-67 material.This agrees with those results reported by Wang et al. forsimilarly RuDCBPY-doped UiO-67.34

FTIR spectra of as-synthesized RuDCBPY-UiO-67 werecompared to that of BPDC and undoped UiO-67 (Figure 3).The assignment of the vibrational transitions for UiO-67 havebeen made and discussed by others.55,73−75 Considerableagreement was observed between RuDCBPY-UiO-67 andUiO-67 with minor differences in band positions and shapes.A negligible (∼6 cm−1) shift is observed in the symmetricstretching band of the carboxylate (OCO) band between UiO-67 (∼1403 cm−1) and RuDCBPY-UiO-67 (∼1409 cm−1). The

Table 1. Summary of Fluorescence Data

stretched exponential fit discrete exponential fit

sampledoping(mmolal) τ (ns) β

⟨τ⟩(ns) ⟨k⟩ (s−1) f1 τ1 (ns) f 2 τ2 (ns)

⟨τ⟩(ns) ⟨k⟩ (s−1)

RuDCBPY-UiO-67 2.6 1.0 1370 ± 30 1370 7.3 × 105

6.8 1.0 720 ± 8 720 1.4 × 106

16.4 1.0 645 ± 6 645 1.6 × 106

20.6 107 ± 3 0.57 ± 0.01 390 5.8 × 106 0.42 ± 0.01 23 ± 2 0.58 ± 0.01 204 ± 4 121 8.3 × 106

44.7 48 ± 2 0.53 ± 0.01 228 1.2 × 107 0.52 ± 0.01 24 ± 1 0.48 ± 0.01 168 ± 2 93 1.1 × 107

RuBPY@UiO-67 1.3 862 ± 0.70 ± 0.01 1712 9.2 × 105 0.32 ± 0.01 242 ± 55 0.68 ± 0.03 1340 ± 90 987 1.0 × 106

RuBPY in DMF 1.0 920 ± 10 916 1.1 × 106

RuDCBPY in water 1.0 38 ± 1 38 2.6 × 07

RuDCBPY in DMF 1.0 880 ± 10 880 1.1 × 105

Figure 2. Energy level diagram of the ground and excited states ofRuBPY and their corresponding deactivation pathways.

Table 2. Results of the Fits of the Temperature Dependence of Emission Lifetimes of RuDCBPY-UiO-67 Powders

doping (mm) ko (105 s−1) k1 (s

−1) ΔE1 (cm−1)

RuDCBPY-UiO-67 2.6 8.2 ± 0.6 (2.3 ± 1.9) × 1011 2700 ± 3006.8 12 ± 1 (4.5 ± 4.3) × 1010 2300 ± 30016.4 15 ± 6 (1.4 ± 1.1) × 1011 2600 ± 20020.6 43 ± 1 (3.0 ± 6.0) × 1013 2700 ± 95044.7 50 ± 2 (9.8 ± 8.8) × 1010 2500 ± 300

RuBPY@UiO-67 1.3 13 ± 1 (8.0 ± 1.7) × 1010 2490 ± 60RuBPY in DMF 8.1 ± 0.1 (2.2 ± 2.8) × 1012 3100 ± 300RuDCBPY in DMF 9.6 ± 0.3 (2.5 ± 4.7) × 1011 2500 ± 500



CC ring-skeleton vibrations at ∼1545 and ∼1360 cm−1 in UiO-67 were observed to shift to slightly lower energy (∼1541cm−1) with some broadening, in the former case, and higherenergy (∼1373 cm−1) in the latter case. The 1373 cm−1 band isnearly occluded by the symmetric OCO band at 1409 cm−1 inRuDCBPY-UiO-67. Such small changes indicate little pertur-bation of the overall UiO-67 structure upon incorporation ofdoping concentrations of RuDCBPY.Diffuse reflectance spectra obtained for the material displays

an absorption maxima at ∼455 nm, which is in good agreementwith the observed maxima previously reported at 454 nm forthe same material (Figure 4). This transition is attributed to theformation of a singlet metal-to-ligand charge transfer state(1MLCT) discussed above which is characteristic of ruthenium-(II)-polypyridyl coordination compounds.33,63,76,77 The energyof the transition observed at 455 nm for the ground state to1MLCT was observed to be slightly bathochromically shiftedrelative to RuBPY in DMF (∼453 nm) and hypsochromicallyshifted relative to RuDCBPY in DMF (460 nm).78

Remarkably, the excited state properties of RuDCBPYincorporated into UiO-67 at a doping concentration of 3 mmdeviate markedly from those in aqueous solution, but resembleRuDCBPY in DMF. The steady-state emission spectrum of thematerial displays a maxima centered around 630 nm (Figure 4).This is bathochromically shifted relative to the previouslyreported emission maxima of 614 nm.58 This is also shifted withrespect to RuBPY and RuDCBPY in DMF (620 and 625 nm,respectively). However, the emission maxima is significantlyhypsochromically shifted in relation to RuDCBPY in water.33

The temperature-dependent emission lifetime of RuDCBPY-UiO-67, at a doping concentration of 3 mm, under anaerobicconditions, was found to decay exponentially with an observed

lifetime of ∼1.4 μs at room temperature. Results of the fit to thetemperature-dependent data to eq 3 for RuDCBPY-UiO-67yield a ko of (8.2 ± 0.6) × 105 s−1, a k1 of (2.3 ± 2.0) × 1010 s−1,and a ΔE1 of 2711 ± 342 cm−1, which were very similar tothose found for RuBPY in DMF (ko = 8.1 × 105 s−1, k1 = 2.2 ×1012 s−1, and ΔE1 = 3110 cm−1).The color of the synthesized powders (and subsequent

emission as a result of UV-lamp irradiation) was observed tointensify with increasing amounts of RuDCBPY in the startingreactant mixture. As a result, the photophysical properties ofthe powders were also observed to be affected by the degree of“loading” of RuDCBPY in RuDCBPY-UiO-67. Specifically,although the absorption maxima seem relatively unaffected byincreased concentration of RuDCBPY in the material, theemission spectra demonstrated a general bathochromic shift ofthe 3MLCT band from ∼630 to ∼650 nm (Figure 4). Inaddition, the band shape of the spectra displays increasedbroadening with increased loading and an appearance of ashoulder at ∼500 nm similar to that observed for RuDCBPY inwater at ∼495 nm.Similarly, the emission lifetimes were dramatically affected by

increasing the concentration of incorporated RuDCBPY inRuDCBPY-UiO-67 (Figure 5). The decay rates obtained byexcitation at 355 nm were adequately fit to a single exponentialdecay function up to concentrations of approximately 20 mm.At higher loadings, equally adequate fits were obtained by useof either the sum of two exponential functions or stretchedexponential functions (also known as the Kohlrausch or

Figure 3. (Top panel) PXRD patterns of UiO-67 (red) andRuDCBPY-UiO-67 (blue) powders along with the predicted patternof UiO-67 (black) based on crystallographic data (CCDC no.889530). (Bottom panel) FTIR spectra of the BPDC ligand (black),UiO-67 powder (red), and RuDCBPY-UiO-67 powder (blue).

Figure 4. (Top panel) Normalized diffuse reflectance spectra ofRuDCBPY-UiO-67 powders as a function of RuDCBPY loading; 3mm (black), 7 mm (red), 16 mm (blue), 21 mm (pink), and 45 mm(green). The reflectance measurements were converted using theKubelka−Munk function. (Bottom panel) Normalized emissionspectra of RuDCBPY-UiO-67 powders at various RuDCBPY loading;3 mm (black), 7 mm (red), 16 mm (blue), 21 mm (pink), and 45 mm(green).



Kohlrausch−Williams−Watts decay function). The latterfunction is expressed as

= τ− βI t A( ) e t( / )

(4)

where τ is the observed lifetime at the maxima of thedistribution and β is a parameter related to the width and shapeof the distribution and ranges between 0 and 1. Emissiondecays with a β of 1/2, 3/8, and 3/10 have been attributed todonor luminescence quenching via the dipole−dipole, dipole−quadrupole, and quadrupole−quadrupole resonance energytransfer mechanisms, respectively.79,80 At β =1 the distributionbecomes a δ-function, and therefore, eq 4 reduces to a discreteexponential.40,81 The average lifetime, ⟨τ⟩, and decay rate, ⟨k⟩,calculated from the τ obtained from the Kohlrausch decay fit(and describe the area under the distribution of lifetimes), aregiven by eqs 5 and 6,81

τ τ ββ

⟨ ⟩ = ΓΓ

⎛⎝⎜

⎞⎠⎟

(2/ )(1/ ) (5)

τβ

⟨ ⟩ = Γ +− ⎛⎝⎜

⎞⎠⎟k 1

11

(6)

For exponential decays, the average lifetime is given by eq 7,40

∑τ τ⟨ ⟩ ==

fi

n

i i0 (7)

where f i is the fractional contribution of the ith exponentialcomponent of lifetime τi to the total amplitude of theexperimental lifetime decay. Therefore, for single exponentialdecays ⟨τ⟩ = τobs. It was found that the residuals obtained(Figure S4 in Supporting Information) for the Kohlrauschdecay fits were equally good relative to the double exponentialfits. The results for both sets of fits are presented in Table 1.The long lifetime component of the emission decays wasobserved to be somewhat temperature dependent, and the fitsto eq 3 are summarized in Table 2.UiO-67 containing encapsulated RuBPY, RuBPY@UiO-67,

at a doping concentration of 1.3 mm have also beensynthesized. The observed PXRD pattern and FTIR spectraare nearly indistinguishable from that of undoped UiO-67(Figure S5 in Supporting Information). The diffuse reflectancespectrum of RuBPY@UiO-67 (Figure S6 in SupportingInformation) was found to be very similar to that of RuBPY

in water and DMF displaying a maxima at ∼455 nm.63 Thesteady-state emission spectra (with a maxima at 593 nm) wasobserved to be hypsochromically shifted relative to RuBPY inwater (λmax ≈ 610 nm),62 DMF (λmax ≈ 630 nm),78 andRuDCBPY-UiO-67 at 3 mm doping.The emission lifetime at room temperature could not be

adequately fit to a single discrete exponential decay. A summaryof the fits to biexponential and Kohlrausch functions are listedin Table 1; the resulting fits and residuals were equally good(Figure S7 in Supporting Information). Analysis of thetemperature dependence of the emission lifetime indicated atemperature sensitive long lifetime component of ∼1.3 μs andtemperature insensitive short lifetime component of ∼240 ns(Figure 6). Fits of the temperature dependence of the longlifetime component are summarized in Table 2 and resemblethose observed for RuDCBPY-UiO-67.

■ DISCUSSIONRuDCBPY-UiO-67 and RuBPY in Confinement. To date,

RuBPY has been incorporated into a wide array of materialssuch as clays,82−84 zirconium phosphate,85−97 glasses,98−101 andcoordination polymers (e.g., zeolites51,101−106 and metal−organic frameworks35,48,58,107−112). For the most part, incor-poration within the material has been via intercalation orencapsulation between layers or pores of the materi-als.35,51,102,103,106,112,113 However, there are few examples, ofwhich RuDCBPY-UiO-67 is one, in which modified-RuBPY hasbeen incorporated into the material as a linker or strut, therebycomprising a portion or the whole of the material back-bone.33,34,108,109,114,115

Confinement into porous materials having restrictedgeometry has been shown to have pronounced effects on thephotophysical properties of RuBPY and its deriva-tives.48−51,102,103,108,111,113−115 For example, in zeolite-Y,encapsulation of RuBPY led to red shifts in the absorptionspectra of the 1MLCT and emission spectra of the 3MLCTtransitions relative to water.49 Fits to the temperaturedependence of the emission lifetimes to eq 2 indicated a koof ∼4 × 105 s−1, a k1 of 11 × 107 s−1, and an ΔE1 of 890cm−1.49,65 Comparison of the k1 and ΔE1 obtained from the fitto those of RuBPY in water were significantly different (with k1≈ 5 to 7 orders of magnitude less than the expected, 1012−1014s−1, and ΔE1 ∼3 times smaller than the expected, 2500−4000

Figure 5. Emission lifetime decays obtained at room temperature ofRuDCBPY-UiO-67 at various RuDCBPY loading: 3 mm (black), 7mm (red), 16 mm (blue), 21 mm (pink), and 45 mm (green).

Figure 6. Temperature dependence of the observed emission decayrates of the long phase at various RuDCBPY loading: 3 mm (black), 7mm (red), 16 mm (blue), 21 mm (green), and 45 mm (pink). Solidlines represent best fits of the data to eq 3



cm−1 gap).65,66 In contrast to RuBPY in water, the valuesobtained in zeolite-Y were consistent with population of thefourth 3MLCT, similar to results observed for RuBPY incellulose.67,116 It was argued that the observed trend in zeolitewas due to an increase in the energy of the 3dd (which isantibonding with respect to the Ru−N bond) above theemissive 3MLCT manifold as a result of confinement within thezeolite-Y pore making it thermally inaccessible.49

Encapsulation of RuBPY within a MOF composed of Zn(II)and trimesic acid (RuBPY@USF2) resulted in considerablydifferent energetics compared to zeolite-Y.35 It was found thatwithin this MOF environment of slightly larger dimensionsthan zeolite-Y (14 Å in diameter for the larger pore with 9 Åwindows), ko was on the same order of magnitude as that ofRuBPY in zeolite-Y (∼6 × 105 s−1). However, the valuesobtained for the exponential term were indicative of thermalpopulation of the metal centered 3dd state with k1 ≈ 2 × 1015

s−1 and a ΔE1 of ∼4600 cm−1.35 Similar to RuBPY in zeolite-Y,the increased spacing between the emissive 3MLCT and 3ddwas attributed to restricted elongation of the Ru−N bond uponpopulation of the 3dd due to confinement. However, unlikeRuBPY in zeolite-Y, the perturbation caused by confinement inUSF2 was not enough to make the 3dd entirely inaccessible.The increase in the nonradiative term k1 was rationalized asarising from increased coupling between the 3dd and theground state, due to perturbation of the equilibrium geometriesbetween the ground and 3dd state, affecting the Huang−Rhysfactor in the energy gap law.117,118

In the preceding examples, the framework topology,specifically the steric confinement of the pores, have quitedifferent effects on the excited states of RuBPY. Therefore,given the topology of UiO-67 is considerably different than thatof RuBPY@zeolite-Y and RuBPY@USF2, the photophysics ofRuDCBPY can also differ relative to the aforementionedmaterials. The geometry of the Zr6O4(OH)4(BPDC)12 clustersof UiO-67 result in the formation of tetrahedral and octahedralcavities of ∼12 and ∼23 Å interior diameter with ∼7 Å sizedadjoining windows.56,57 A cursory QSAR calculation indicates avan der Waals diameter of ∼9.9 Å for Ru(bpy)2 within a hardsphere approximation. Therefore, within this approximation, itis possible for the BPY ligands of RuDCBPY to beaccommodated within both UiO-67 pores, though lessconstricted within the octahedral cages than the tetrahedral.Consequently, the resulting photophysical properties ofRuDCBPY are distinct relative to each of the aforementionedmaterials. Despite the two different environments within UiO-67 that may be populated, it is argued later that only one, theoctahedral cavity, is occupied.The normalized emission lifetimes observed for RuDCBPY-

UiO-67 powder at ∼3 mm loading were found to be adequatelymodeled using a discrete single exponential function withlifetime ∼1.4 μs. The absence of multiexponential kinetics areindicative of the dilute nature of the doped material, minimizingany distance-dependent RuDCBPY···RuDCBPY interactions.Fits of the temperature dependence of the observed lifetime toeq 3 indicate a k1 on the order of 1011 s−1 lying ∼2700 cm−1

above the emissive 3MLCT manifold resembling those valuesobserved in DMF. This is not surprising considering thermalgravimetric analysis suggests at least 26% of the weightcomposition of UiO-67 is due to DMF solvent.119

Clearly, upon incorporation into UiO-67, the photophysicalproperties of RuDCBPY resembles those observed forRuDCBPY and RuBPY in DMF solution and within a variety

of materials such as RuBPY@USF2. It is likely thatdeprotonation in DMF and/or coordination of the DCBPYligand of RuDCBPY into the framework of UiO-67 significantlydestabilizes the energetics of DCBPY, raising the redoxpotential above that of BPY. By doing so, the 3MLCT ofRuDCBPY in RuDCBPY-UiO-67 localizes onto one of the twoBPYs, which protrude into one of the two UiO-67 cavity types,quite likely the larger octahedral cavity. Close inspection of theUiO-67 structure reveals distortion of the planarity of theBPDC linkers (Figure 7). It is reasonable to assume, given that

the PXRD patterns indicate that RuDCBPY-UiO-67 andundoped UiO-67 are isostructural, that a similar distortionoccurs in the DCBPY ligand upon coordination to the Zr-clusters. A symmetry reducing distortion in the DCBPY ligandcan very well disrupt the π-network of the ligand, affecting itsground and excited state properties.120−122 Therefore, it isplausible that the magnitude of the emission energy and

Figure 7. (A) Observed emission decay rates of RuDCBPY-UiO-67,(B) ratio of observed lifetime over reciprocal k′, obtained from eq 14,and (C) energy transfer efficiencies, ΦRET, at various dopingconcentrations of RuDCBPY. The solid line represents a best fit ofthe data to (A) eq 12, (B) eq 14b, and (C) eq 16b.



lifetime are due to formation of a BPY-localized 3MLCT withinthe interior of a solvent (largely DMF) occupied cavity. TheDMF environment within the cavities is confirmed bythermogravimetric analysis of UiO-67.54,73,119

The observed RuDCBPY-UiO-67 powder emission lifetimewas also observed to be sensitive to RuDCBPY loadingconcentration. Specifically, the emission lifetime of the 3MLCTwas found to decrease with increased RuDCBPY loading up toa concentration of ∼20 mm at which point the observed decaydeviated from discrete monoexponential behavior. At thesehigher loading concentrations, adequate fits to the data wereobtained using both the Kohlrausch stretched exponentialfunction mentioned above, and a sum of two discreteexponential functions. In the former case, stretched exponentialfits indicate a doping concentration dependence on the lifetimebut not the β values. In the latter case, biexponential fits suggesta doping concentration dependence on the lifetime of the slow(>100 ns) phase in the latter case but not on the fast phase(∼20 ns).Emission lifetime decays have been observed to be

nonexponential in MOF systems in which incorporation ofDCBPY-modified RuBPY acts as pillars or struts within thebackbone of the framework.108,109,114,115 The multicomponentdecays have generally been fit to a sum of two discreteexponential processes comprising a slower, >100 ns,component and a faster, <100 ns, component. Thebiexponential nature of the decays have been explained asarising from at least two distinct emissive populations ofRuBPY, where the excited state of the faster populationemerges due to quenching of the slower population via energytransfer in regions of high RuBPY density. This argument isbased on the observed RuBPY concentration dependence onthe emission lifetimes in zeolites, zirconium phosphates, andelectropolymerized thin films.50,82,90,123

RuDCBPY-UiO-67 Self-Quenching. The clear RuDCBPYloading concentration dependence on the long emissionlifetime is indicative of quenching by RuDCBPY···RuDCBPYinteractions. Self-quenching of the RuDCBPY 3MLCT canoccur by either energy or electron transfer, both of which aredistance dependent. However, oxidative or reductive quenchingaccording to reactions 8 and 9

* +

→ +

+ +

+ +

RuDCBPY RuDCBPY

RuDCBPY RuDCBPYk

2 2

3ox (8)

* +

⎯→⎯ +

+ +

+ +

RuDCBPY RuDCBPY

RuDCBPY RuDCBPYk

2 2

3red (9)

is thermodynamically unfavorable (ΔE° ≈ −0.4 V for reactions8 and 9), assuming that the redox potentials of RuDCBPY aresimilar to RuBPY.124−126 Therefore, the deactivation of theexcited state occurs via energy transfer. Diffusional or collisionalquenching is improbable due to incorporation of RuDCBPYinto the framework of UiO-67, which restricts translationalmotion. Even in the case of RuDCBPY encapsulation withinthe cavities, the small (diameter ≈ 7 Å) windows connectingthe cavities would restrict intercavity migration of RuDCBPYso that RuDCBPY···RuDCBPY collisions are negligible.A general scheme involving nondescriptive, nondiffusional

quenching of the excited state is described by eqs 10 and 11

* → + + Δ′

hvRuDCBPY RuDCBPYk

(10)

* +

→ + + Δ

RuDCBPY RuDCBPY

RuDCBPY RuDCBPYkq

(11)

where k′ is the emission decay rate constant at infinite dilutionof RuDCBPY within the material and contains thoseparameters defined in eq 2. The parameter kq is the quenchingrate constant so that the rate equation for deactivation of theRuDCBPY excited state is given by eq 13.The observed lifetime at a given doping concentration of

RuDCBPY is then represented by eq 12.

τ= ′ + = ′ + ″k k k k

1[RuDCBPY]

obsq

(12)

− * = ′ + *t

k kd[RuDCBPY ]

d( [RuDCBPY])[RuDCBPY ]q

(13)

Fitting the data given in Table 1 for the long lifetimecomponent of RuDCBPY-UiO-67 to eq 12 (Figure 7) yields ak′ of 5.7 × 105 s−1 and a kq of 1.3 × 108 m−1 s−1. Thisbimolecular quenching rate constant, kq, is found to be fasterthan with RuBPY self-quenching observed in cellulose (∼8 ×106 M−1 s−1),127 zirconium phosphates (7 × 105 M−1 s−1),85

and an order of magnitude slower (1.1 × 109 M−1 s−1) than indoped Zn(bpy)3(PF6)2 crystals.128 However, there is somedisagreement among these examples regarding the dominantmechanism of self-quenching. In zirconium phosphates, Colonet al. have argued that quenching occurs via electron transfer(e.g., not by Dexter’s exchange mechanism) between a groundand excited state RuBPY (vide infra).85 The self-quenchingobserved for RuBPY in zeolite and cellulose with increasedloading was, however, attributed to a triplet−triplet annihilationmechanism via exchange.127−129

The concentration dependence on the long component ofthe emission lifetime is indicative of a distance dependence onthe self-quenching mechanism resulting from RuDCBPY···RuDCBPY interactions in RuDCBPY-UiO-67. It is well-knownthat quenching by electron transfer and energy transferprocesses are both inherently dependent on the intermoleculardistance between donor and acceptor species. However, as theelectron transfer self-quenching mechanism is thermodynami-cally unlikely (vide supra), the discussion below will focus onquenching by energy transfer.Homo- and hetero-energy transfer can occur by Coulombic

interactions via an exchange or dipole−dipole reaction. Threedifferent regimes have been identified for dipole−dipoleresonance energy transfer reactions (RET). These regimesare characterized by strong coupling, weak coupling, and veryweak coupling (Forster regime) between donor and acceptortransition dipoles. Consequently, the dependence of the RETrate on distance varies depending on the coupling strengthregime. A number of examples where homo- and/or hetero-RET behavior has been observed between organic linkers in anumber of MOFs have been reported.130−134 Quenching ofRuBPY 3MLCT in a number of different RuBPY-based MOFshas been ascribed to an energy migration via exchangemechanism.109,111,114,115 Yet, it is not uncommon for RETinvolving RuBPY in solution to occur via a dipole−dipolemechanism.132−142

In the exchange mechanism of energy transfer kRET falls offexponentially with respect to r as143

= − −k k e r R LRET o

2( )/c (14a)



where ko is the limiting energy transfer rate constant at theintermolecular contact distance, Rc, and L is the average Bohrradius of the interacting species.40 Inokuti and Hirayama80 haverelated this energy transfer mechanism to Perrin’s “sphere ofquenching” model144 and the concentration of acceptor, C,

ττ

= −e C C

o

( / )o

(14b)

where80

π=C

N R3000

4oA o

3(14c)

and Ro are analogous to the characteristic critical concentrationand distance, respectively, which emerge from Forster’s dipole−dipole mechanism (vide infra). They represent the concen-tration and distance at which the transfer rate is half themaximum rate (e.g., equal to the intrinsic decay rate of thedonor in the absence of the acceptor). A fit of the data to eq14b yields a Co of 0.011 ± 0.002 m, which approximatelycorresponds to a Ro of 33 Å via eq 14c (Figure 7). This distanceis, however, uncharacteristically long for an exchangemechanism (Ro < 15 Å).40,145−148 It is possible, then, thathomotransfer proceeds according to a dipole−dipole mecha-nism.In the dipole−dipole approximation described by Forster for

very weak donor−acceptor interactions, the rate of resonanceenergy transfer (eq 16a), kRET, is inversely proportional to thesixth power of the intermolecular distance, r.149,150

* +

⎯ →⎯⎯ + *

+ +

+ +

RuDCBPY (1) RuDCBPY (2)

RuDCBPY (1) RuDCBPY (2)k

2 2

2 2RET(15)

In this mechanism, depopulation of a 3MLCT on RuDCB-PY(1) is coupled to population of a 3MLCT on RuDCBPY(2)of the same lifetime (assuming both reactants are found insimilar environments so that the photophysical properties ofthe accepting RuDCBPY are identical to the donatingRuDCBPY). Therefore, quenching via this dipole−dipolemechanism of energy transfer should not affect the observedemission lifetime. However, impurities, crystal lattice defects, orresonant vibrations inherent to the MOF may act as energytraps leading to the decrease in the emission life-time.108,109,111,114,115,123,127 Forster defined kRET by eq16a.150,151

τ= ⎜ ⎟

⎛⎝⎜

⎞⎠⎟⎛⎝

⎞⎠k

Rr

1RET

o

o6

(16a)

Here, τo is the fluorescence lifetime of RuBPY in the absence ofquencher and Ro is a characteristic distance at which theprobability of energy transfer is equal to the probability of thedecay of the 3MLCT. The relationship between intermoleculardistance and acceptor concentration is

∫

π

ξπ

Φ = −

= −ξ

⎛⎝⎜

⎞⎠⎟

⎡⎣⎢⎢⎛⎝⎜

⎞⎠⎟

⎤⎦⎥⎥⎡⎣⎢⎢

⎛⎝⎜

⎞⎠⎟⎤⎦⎥⎥

CC

CC

CC

x x

exp 1 erf

erf( )2

exp( )d

RET1/2

o o

2

o

1/2 0

2

(16b)

where ΦRET is the efficiency of energy transfer defined as

ττ

Φ = −1RETo

C is the concentration of the energy acceptor and Co is acharacteristic critical concentration, which is inversely propor-tional to the third power of Ro (defined above) according to

π=C

N R3000

2 oo 3/2

A3

(16c)

and therefore related to the probability of energy transfer(RET) via eq 16aa. A fit of the data to eq 16b yields a Co of0.016 ± 0.003 m, which results in a Ro of approximately 54 Å(Figure 7). This result is, however, strange considering the pooroverlap between the emission and absorption spectra ofRuDCBPY.Forster has shown that the Ro can be approximated from the

overlap, J, of the donor and acceptor emission and absorptionspectra, respectively, if the quantum yield of emission, Φem, ofthe donor in the absence of acceptor and the index of refractionof the medium, n, are known.149,150

∫κπ

λ ε λ λ λ=Φ

RN n

I9000(ln 10)

128( ) ( ) do

62

em5

A4 D A

4

(17)

The integral in eq 17 is known as the overlap integral, J, whereID is the donor emission normalized so that the area under theemission is unity, εA is the extinction spectra of the acceptor,and λ is the wavelength. The κ2 term is a parameter dependenton the relative orientation of the transition dipole moments ofthe donor and acceptors taken to be 2/3 for a completelyrandomized distribution of orientations.Ro values have been calculated (Figure S8 in Supporting

Information) based on the overlap of the diffuse reflectance andemission spectra of RuDCBPY-UiO-67 at ∼3 mm for the fullrange of potential Φem (from 0 to 1) and using two values for J.J values were obtained by (1) dividing diffuse reflectancespectra by the doping concentration (1.4 × 1012 molal−1 cm−1

nm4) and (2) assuming the extinction of RuDCBPY at lowdoping is similar to RuBPY in solution (1.4 × 1015 molal−1

cm−1 nm4). From these a Ro of ∼33 Å is estimated from theformer J value and ∼11 Å for the latter J value (assuming theemission quantum yield and the index of refraction are similarto that of RuBPY in DMF: 0.0068 and 1.43, respectively).Incongruence between the Ro values calculated from thespectral overlap and data fits in Figure 7C may be indicative ofa non-Forster type mechanism.Inokuti and Hirayami generalized that the time-dependent

emission decay was proportional to the inverse nth power ofthe donor−acceptor distance according to eq 18.80

τ τ= − − Γ −⎜ ⎟ ⎜ ⎟

⎡⎣⎢⎢

⎛⎝

⎞⎠⎛⎝⎜

⎞⎠⎟⎛⎝

⎞⎠

⎤⎦⎥⎥I t I

tn

CC

t( ) (0) exp 1

3 n

o

3/

(18)

Here, I(t) and I(0) are the emission intensity at time t ≠ 0 andt = 0, respectively, C is the doping concentration, and Co isgiven by eq 14c. It should be noted that for n = 6 eq 18 can berewritten as eq 16b and that the Forster dipole−dipolemechanism (for very weak coupling between donor andacceptor) is the dominant mode of energy transfer. For n = 8or 10, higher order interactions (i.e., dipole−quadrupole andquadrupole−quadrupole, respectively) dominate.80 Fitting thedecay data for RuDCBPY-UiO-67 at doping concentrationbetween 3 mm and 21 mm indicate n = 3.5 ± 0.3 and Co =



0.038 ± 0.008 mm, which correspond to a Ro of 22 ± 5 Å basedon eq 14c. A 1/r3 dependence on the energy transfer rate isconsistent with a “weak coupling” mechanism of energy transferin which the transfer is excitonic in nature.152

In this weak coupling limit of dipole−dipole energy transfer,the interaction energy between donor and acceptor is largecompared to the very weak coupling limit. The interactionenergy is also larger than an intramolecular vibronic transition,yet smaller than the absorption bandwidth. The strength of thecoupling results in a number of implications. First, the vibronicexcitation is delocalized over the interacting pair, although notto the extent as in the strong coupling case.40,153 Second, theinteraction leads to slight splitting of the vibrational transitionpractically observed as broadening of the absorption band withconcomitant hypo- or hyperchromism.152,154 Third, the RETprocess occurs before equilibration/relaxation of the Franck−Condon vibrational state.40,153,155 As a consequence, kRET istypically on the order of a molecular vibration (∼1012 s−1).Despite the 1/r3 dependence on the emission decay observed

here, some inconsistencies must be pointed out. First, althoughthe diffuse reflectance spectra of RuDCBPY-UiO-67 (Figure 4)do show some evidence of broadening, contributions to theabsorption by encapsulated RuDCBPY (see below) complicatesunequivocal assignment of spectral changes as excitonic innature. Second, according to eq 12, k″ (where k″ = kRET = kobs− k′) ranges between 1.6 × 105 to 5.4 × 106 s−1 with increaseddoping. It should be noted that similar kRET have been noted forForster-type RET reactions involving RuBPY or RuBPYderivatives.135−142 Kenkre and Knox have proposed that theregion between the Forster (very weak coupling) and Perrin(weak coupling) regime in which the distance dependence onkRET is either 1/r6 or 1/r3, respectively, is not necessarilydiscontinuous.155 Therefore, it is possible that a complex

system, like RuDCBPY-UiO-67, may lie somewhere along thatcontinuum.

Occupational Conformations of RuDCBPY within UiO-67. It has already been pointed out that upon incorporation ofRuDCBPY into the UiO-67 backbone, it can occupy one of twocavities (Figure 8), either a large octahedral cavity or smallertetrahedral cavity. These cavities have been reported to havediameters of ∼23 and ∼12 Å, respectively.56,57 However, it ispossible that population of the octahedral cavity is preferredover the smaller cavity due to size and steric restrictions. Inaddition, given the size of the octahedral cavity and reducedeffective diameter of RuDCBPY upon incorporation into theUiO-67 backbone, i.e., d(VDW) ≈ 10 Å for Ru(BPY)2 vs ∼12Å156 for Ru(BPY)2(DCBPY), incorporation of a secondRuDCBPY into the UiO-67 backbone comprising theoctahedral cavity seems to be possible with little distortion ofthe cage. Encapsulation of RuDCBPY by the octahedral cage isalso at least somewhat probable due to its size, evidenced bysuccessful encapsulation of RuBPY in UiO-67, [email protected] increasing doping concentrations of RuDCBPY, the

emission lifetime decay of RuDCBPY has been found todecrease significantly. This is attributed to the presence of atleast one additional nonradiative deactivation pathway as aresult of RuDCBPY···RuDCBPY energy transfer via long-rangedipole−dipole interactions near the Forster regime. Interest-ingly, at higher concentrations of RuDCBPY, the emissiondecay was heterogeneous, indicating the presence of more thanone emissive RuDCBPY population, each with differinglifetimes. This is further evidenced by the nonlinearity of theaverage decay rates, ⟨k⟩, and ⟨τ⟩/⟨τo⟩ as a function of thedoping concentrations.The origin of the heterogeneity can be explained by invoking

a mechanism involving a distribution of quenching processes

Figure 8. (Top panel) UiO-67 structure (left) with isolated octahedral cavity (middle left), tetrahedral cavity (middle right), and close-up ofdistorted BPDC ligand (right). (Bottom panel) Potential occupational configurations of RuDCBPY incorporated into UiO-67 including a singlyoccupied octahedral (left) and tetrahedral (middle left) cavity, a doubly occupied octahedral cavity (middle right), and a RuDCBPY encapsulatedwithin an octahedral cavity (right); RuDCBPY molecules are highlighted in blue, and the hydrogens were removed for clarity.



(especially given the decay data fit equally well to a Kohlrauschdecay model). However, this mechanism is rejected here due to(1) the lack of heterogeneity at low doping concentrations, (2)geometric and steric constraints imposed by the symmetriesand sizes of the two cages, and (3) diffusional restrictionsimposed by incorporation into the backbone of the material.Therefore, a two state model is more attractive.It is proposed that, at low concentrations, RuDCBPY is

incorporated into the backbone of UiO-67 preferentially singlyoccupying the octahedral pores. At higher concentrations,RuDCBPY is both incorporated into the backbone andencapsulated within separate octahedral pores (singly occupyingeach pore). The long and short emission lifetime componentsare ascribed to these two proposed populations of RuDCBPY;the long lifetime component is attributed to the incorporatedpopulation and the short lifetime component is assigned to theencapsulated population, based on the following rationalization.It is possible that encapsulation of RuDCBPY within the poreincreases the volume occupied by RuDCBPY. This, in turn,decreases the number of DMF solvent molecules that canconcomitantly occupy the same pore. Consequently, thesolvation of RuDCBPY by DMF within the pore is reduced.The resulting implication is that the pore, devoid of DMF,imposes a water-like dielectric on RuDCBPY, explaining theresemblance of the short lifetime component of RuDCBPY-UiO-67 to RuDCBPY in water. The little degree of spectraloverlap between the encapsulated RuDCBPY ground →1MLCT and 3MLCT → ground transitions in RuDCBPY-UiO-67, coupled with the smaller Φem, result in a Ro ≪ 10 Å(according to eq 17) for these encapsulated populations. As aconsequence, quenching of the encapsulated population ofRuDCBPY would be expected to be negligible and thereforerelatively insensitive to doping concentration, which is, indeed,observed. It is acknowledged that other scenarios are plausible,though much more complex.

■ CONCLUSIONS

The excited state properties of RuDCBPY-doped UiO-67,RuDCBPY-UiO-67, have been examined as a function ofdoping concentration. It was found that at low dopingconcentrations of RuDCBPY-UiO-67 the photophysical prop-erties, including formation of the 3MLCT, energy of the3MLCT, energy spacing between the 3MLCT and the thermallyaccessible metal centered 3dd state, and the 3MLCT lifetime,resemble that of RuDCBPY in DMF solution. Upon increasingthe doping concentration the diffuse reflectance and emissionspectra trend toward spectra resembling that of RuDCBPY inaqueous solution. The observed emission decays at higherRuDCBPY doping were biphasic with a concentrationdependent long lifetime component and concentrationindependent short lifetime component resembling the lifetimeof RuDCBPY in water. The concentration dependence of thelong lifetime component was attributed to quenching of the3MLCT by an energy transfer mechanism given electrontransfer is thermodynamically unfavorable. Previous reportsinvolving incorporation of RuBPY into the backbone of metal−organic frameworks or by encapsulation attribute homotransferof energy to be due to a Dexter-type mechanism.109,111,114,115

However, in the present system, (1) the observed kRET (k″ fromeq 12), (2) the dependence of the observed decay rate ondistance, and (3) the results from a modified sphere ofquenching model (eq 14b) are indicative of a dipole−dipole

RET process lying on a continuum between the Perrin weakcoupling and Forster very weak coupling regimes.155

To explain the biphasic nature of the decay, a two-statemodel is adopted. At low doping concentrations, RuDCBPY isincorporated within the backbone of the material, and its twofree bipyridyl ligands occupy the DMF-filled larger octahedralcavities. The solvation environment within the cavity at theselow concentrations resembles DMF in bulk solution. At higherdoping concentrations, RuDCBPY is proposed to also beencapsulated within the octahedral cavities. In the pores inwhich RuDCBPY is encapsulated, the solvation environmentresembles that of bulk water. This is presumably due toexclusion of DMF from the cavity as a result of larger porevolume occupied by the encapsulated RuDCBPY.Functional metal−organic frameworks engineered to contain

photoactive groups may undergo energy and/or electrontransfer in order to facilitate certain reactions. Incorporationof photoactive compounds whose photophysical properties arewell understood, such as RuBPY and its derivatives, can offerinsight into the structure−function relationship betweenchromophoric guest molecules and their material hosts. It isclear from this and other reports that the structure, topology,and the environment within the pores of the host material playa large role in determining the photophysical properties andthereby the photoreactivity of the chromophore/materialhybrid.

■ ASSOCIATED CONTENT*S Supporting InformationTemperature dependence of RuDCBPY in water and DMF,characterization data pertaining to RuBPY@UiO67, andcomparison of goodness of fit and residuals of the biexponentialand distribution fitting functions, as well as the method ofreconstruction of the emission spectra. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*(A.J.M.) E-mail: [email protected].

Author ContributionsAll authors have given approval to the final version of themanuscript.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank and acknowledge financialcontributions from the Virginia Tech Department of Chemistryand the Virginia Tech Institute for Critical Technology andApplied Science (ICTAS).

■ ABBREVIATIONSRuBPY, ruthenium(II) tris(2,2′-bipyridine) dichloride;RuDCBPY, ruthenium(II) bis(2,2′-bipyridine)-mono-(2,2′-bi-pyridyl-5,5′-dicarboxylic acid) dichloride; BPY, 2,2′-bipyridine;DCBPY, 2,2′-bipyridine-5,5′-dicarboxylic acid; BPDC, biphe-nyldicarboxylic acid; UiO67, University of Oslo framework-67;ZrCl4, zirconyl chloride hydrate; MeOH, methanol; DMF,dimethylformamide; PXRD, X-ray powder diffraction; FTIR,Fourier transform infrared; MLCT, metal-to-ligand chargetransfer



http://pubs.acs.org

mailto:[email protected]

■ REFERENCES(1) Kumar, R. S.; Kumar, S. S.; Kulandainathan, M. A. Highlyselective electrochemical reduction of carbon dioxide using Cu basedmetal organic framework as an electrocatalyst. Electrochem. Commun.2012, 25, 70−73.(2) Hinogami, R.; Yotsuhashi, S.; Deguchi, M.; Zenitani, Y.; Hashiba,H.; Yamada, Y. Electrochemical reduction of carbon dioxide using acopper rubeanate metal organic framework. ECS Electrochem. Lett.2012, 1, H17−H19.(3) Lee, Y. R.; Kim, J.; Ahn, W. S. Synthesis of metal−organicframeworks: A mini review. Korean J. Chem. Eng. 2013, 30, 1667−1680.(4) Nakagaki, S.; Ferreira, G. K. B.; Ucoski, G. M.; Castro, K. A. D. D.Chemical reactions catalyzed by metalloporphyrin-based metal−organic frameworks. Molecules 2013, 18, 7279−7308.(5) Wei, Z. W.; Yuan, D. Q.; Zhao, X. L.; Sun, D. F.; Zhou, H. C.Linker extension through hard-soft selective metal coordination for theconstruction of a non-rigid metal−organic framework. Sci. ChinaChem. 2013, 56, 418−422.(6) Fu, Y. Y.; Yan, X. P. Metal−organic framework composites. Prog.Chem. 2013, 25, 221−232.(7) First, E. L.; Floudas, C. A. MOFomics: Computational porecharacterization of metal−organic frameworks. Microporous Mesopo-rous Mater. 2013, 165, 32−39.(8) Ricco, R.; Malfatti, L.; Takahashi, M.; Hill, A. J.; Falcaro, P.Applications of magnetic metal−organic framework composites. J.Mater. Chem. A 2013, 1, 13033−13045.(9) Regati, S.; He, Y. B.; Thimmaiah, M.; Li, P.; Xiang, S. C.; Chen,B. L.; Zhao, J. C. G. Enantioselective ring-opening of meso-epoxidesby aromatic amines catalyzed by a homochiral metal−organicframework. Chem. Commun. 2013, 49, 9836−9838.(10) Borjigin, T.; Sun, F. X.; Zhang, J. L.; Cai, K.; Ren, H.; Zhu, G. S.A microporous metal−organic framework with high stability for GCseparation of alcohols from water. Chem. Commun. 2012, 48, 7613−7615.(11) El Osta, R.; Carlin-Sinclair, A.; Guillou, N.; Walton, R. I.;Vermoortele, F.; Maes, M.; de Vos, D.; Millange, F. Liquid-phaseadsorption and separation of xylene isomers by the flexible porousmetal−organic framework MIL-53(Fe). Chem. Mater. 2012, 24, 2781−2791.(12) Liu, B. Metal−organic framework-based devices: separation andsensors. J. Mater. Chem. 2012, 22, 10094−10101.(13) Van Heest, T.; Teich-McGoldrick, S. L.; Greathouse, J. A.;Allendorf, M. D.; Sholl, D. S. Identification of metal−organicframework materials for adsorption separation of rare gases:Applicability of ideal adsorbed solution theory (IAST) and effects ofinaccessible framework regions. J. Phys. Chem. C 2012, 116, 13183−13195.(14) Wang, W. J.; Dong, X. L.; Nan, J. P.; Jin, W. Q.; Hu, Z. Q.;Chen, Y. F.; Jiang, J. W. A homochiral metal−organic frameworkmembrane for enantioselective separation. Chem. Commun. 2012, 48,7022−7024.(15) Hosseini, H.; Ahmar, H.; Dehghani, A.; Bagheri, A.; Tadjarodi,A.; Fakhari, A. R. A novel electrochemical sensor based on metal−organic framework for electro-catalytic oxidation of L-cysteine. Biosens.Bioelectron. 2013, 42, 426−429.(16) de Sousa, P. M. P.; Grazina, R.; Barbosa, A. D. S.; de Castro, B.;Moura, J. J. G.; Cunha-Silva, L.; Balula, S. S. Insights into theelectrochemical behaviour of composite materials: Monovacantpolyoxometalates @ porous metal−organic framework. Electrochim.Acta 2013, 87, 853−859.(17) Guo, Z. G.; Reddy, M. V.; Goh, B. M.; San, A. K. P.; Bao, Q. L.;Loh, K. P. Electrochemical performance of graphene and copper oxidecomposites synthesized from a metal−organic framework (Cu-MOF).RSC Adv. 2013, 3, 19051−19056.(18) Phan, N. T. S.; Nguyen, T. T.; Vu, P. H. L. A copper metal−organic framework as an efficient and recyclable catalyst for theoxidative cross-dehydrogenative coupling of phenols and formamides.ChemCatChem 2013, 5, 3068−3077.

(19) Phan, N. T. S.; Nguyen, T. T.; Nguyen, K. D.; Vo, A. X. T. Anopen metal site metal−organic framework Cu(BDC) as a promisingheterogeneous catalyst for the modified Friedlander reaction. Appl.Catal., A 2013, 464, 128−135.(20) Phan, N. T. S.; Nguyen, T. T.; Nguyen, V. T.; Nguyen, K. D.Ligand-free copper-catalyzed coupling of phenols with nitroarenes byusing a metal−organic framework as a robust and recoverable catalyst.ChemCatChem 2013, 5, 2374−2381.(21) Nepal, B.; Das, S. Sustained water oxidation by a catalyst cage-isolated in a metal−organic framework. Angew. Chem., Int. Ed. 2013,52, 7224−7227.(22) Phan, N. T. S.; Nguyen, T. T.; Ho, P.; Nguyen, K. D. Copper-catalyzed synthesis of -aryl ketones by metal−organic frameworkMOF-199 as an efficient heterogeneous catalyst. ChemCatChem 2013,5, 1822−1831.(23) Wu, F.; Qiu, L. G.; Ke, F.; Jiang, X. Copper nanoparticlesembedded in metal−organic framework MIL-101(Cr) as a highperformance catalyst for reduction of aromatic nitro compounds. Inorg.Chem. Commun. 2013, 32, 5−8.(24) Hamidipour, L.; Farzaneh, F. Cobalt metal organic framework asan efficient heterogeneous catalyst for the oxidation of alkanes andalkenes. React. Kinet. Mech. Catal. 2013, 109, 67−75.(25) Wan, Y.; Chen, C.; Xiao, W. M.; Jian, L. J.; Zhang, N. Ni/MIL-120: An efficient metal−organic framework catalyst for hydrogenationof benzene to cyclohexane. Microporous Mesoporous Mater. 2013, 171,9−13.(26) Phan, N. T. S.; Nguyen, T. T.; Nguyen, C. V.; Nguyen, T. T.Ullmann-type coupling reaction using metal−organic frameworkMOF-199 as an efficient recyclable solid catalyst. Appl. Catal., A2013, 457, 69−77.(27) Shi, D. B.; Ren, Y. W.; Jiang, H. F.; Lu, J. X.; Cheng, X. F. A newthree-dimensional metal−organic framework constructed from 9,10-anthracene dibenzoate and Cd(II) as a highly active heterogeneouscatalyst for oxidation of alkylbenzenes. Dalton Trans. 2013, 42, 484−491.(28) Li, Z. H.; Xue, L. P.; Wang, L.; Zhang, S. T.; Zhao, B. T. Two-dimensional copper-based metal−organic framework as a robustheterogeneous catalyst for the N-arylation of imidazole witharylboronic acids. Inorg. Chem. Commun. 2013, 27, 119−121.(29) Dong, X. W.; Liu, T.; Hu, Y. Z.; Liu, X. Y.; Che, C. M. Ureapostmodified in a metal−organic framework as a catalytically activehydrogen-bond-donating heterogeneous catalyst. Chem. Commun.2013, 49, 7681−7683.(30) Zhao, Y.; Deng, D. S.; Ma, L. F.; Ji, B. M.; Wang, L. Y. A newcopper-based metal−organic framework as a promising heterogeneouscatalyst for chemo- and regio-selective enamination of beta-ketoesters.Chem. Commun. 2013, 49, 10299−10301.(31) Chen, G. Z.; Wu, S. J.; Liu, H. L.; Jiang, H. F.; Li, Y. W.Palladium supported on an acidic metal−organic framework as anefficient catalyst in selective aerobic oxidation of alcohols. Green Chem.2013, 15, 230−235.(32) Gross, A. F.; Sherman, E.; Mahoney, S. L.; Vajo, J. J. Reversibleligand exchange in a metal−organic framework (mof): Toward mof-based dynamic combinatorial chemical systems. J. Phys. Chem. A 2013,117, 3771−3776.(33) Barrett, S. M.; Wang, C.; Lin, W. B. Oxygen sensing viaphosphorescence quenching of doped metal−organic frameworks. J.Mater. Chem. 2012, 22, 10329−10334.(34) Wang, C.; Xie, Z. G.; deKrafft, K. E.; Lin, W. L. Doping metal−organic frameworks for water oxidation, carbon dioxide reduction, andorganic photocatalysis. J. Am. Chem. Soc. 2011, 133, 13445−13454.(35) Larsen, R. W.; Wojtas, L. Photophysical studies of Ru(II)tris(2,2′-bipyridine) confined within a Zn(II)-trimesic acid polyhedral metal−organic framework. J. Phys. Chem. A 2012, 116, 7830−7835.(36) Li, T.; Kozlowski, M. T.; Doud, E. A.; Blakely, M. N.; Rosi, N. L.Stepwise ligand exchange for the preparation of a family ofmesoporous mofs. J. Am. Chem. Soc. 2013, 135, 11688−11691.(37) Han, Q.; He, C.; Zhao, M.; Qi, B.; Niu, J.; Duan, C. Engineeringchiral polyoxometalate hybrid metal−organic frameworks for asym-



metric dihydroxylation of olefins. J. Am. Chem. Soc. 2013, 135, 10186−10189.(38) Rehm, D.; Weller, A. Kinetics and mechanism of electrontransfer in fluorescence quenching in acetonitrile. Ber. Bunsenges.-Ges.Phys. Chem. 1969, 73, 834−9.(39) Rehm, D.; Weller, A. Kinetics of fluorescence quenching byelectron and hydrogen-atom transfer. Isr. J. Chem. 1970, 8, 259−71.(40) Valeur, B.; Berberan-Santos, M. N. Molecular FluorescencePrinciples and Applications, 2nd ed.; Wiley VCH: Weinheim, Germany,2012.(41) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.;Springer: New York, 2006; p xxvi.(42) Becker, R. S. Theory and Interpretation of Fluorescence andPhosphorescence; Wiley Interscience: New York, 1969; p xiv.(43) Vining, W. J.; Caspar, J. V.; Meyer, T. J. The influence ofenvironmental-effects on excited-state lifetimes: the effect of ion-pairing on metal-to-ligand charge-transfer excited-states. J. Phys. Chem.1985, 89, 1095−1099.(44) Durr, H.; Bouas-Laurent, H. Photochromism: Molecules andSystems, rev. ed.; Elsevier: Amsterdam, The Netherlands, 2003; p liii.(45) Hashimoto, S. Zeolite photochemistry: impact of zeolites onphotochemistry and feedback from photochemistry to zeolite science.J. Photochem. Photobiol., C 2003, 4, 19−49.(46) Mohanambe, L.; Vasudevan, S. Aromatic molecules in restrictedgeometries: Photophysics of naphthalene included in a cyclodextrinfunctionalized layered solid. J. Phys. Chem. B 2005, 109, 22523−22529.(47) Choi, J. R.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Evaluatinghost−guest interactions in a metal−organic framework using apolarity-sensitive probe. J. Phys. Chem. Lett. 2010, 1, 1101−1106.(48) Sen, R.; Koner, S.; Bhattacharjee, A.; Kusz, J.; Miyashita, Y.;Okamoto, K.-I. Entrapment of [Ru(bpy)3]

2+ in the anionic metal−organic framework: Novel photoluminescence behavior exhibiting dualemission at room temperature. Dalton Trans. 2011, 40, 6952−6960.(49) Maruszewski, K.; Strommen, D. P.; Kincaid, J. R. Zeolite-entrapped ruthenium(II) complexes with bipyridine and relatedligands. Elimination of ligand-field-state deactivation and increase in3MLCT state lifetimes. J. Am. Chem. Soc. 1993, 115, 8345−8350.(50) Turbeville, W.; Robins, D. S.; Dutta, P. K. Zeolite-entrappedRu(bpy)3

2+: intermolecular structural and dynamic effects. J. Phys.Chem. 1992, 96, 5024−5029.(51) Vitale, M.; Castagnola, N. B.; Ortins, N. J.; Brooke, J. A.;Vaidyalingam, A.; Dutta, P. K. Intrazeolitic photochemical chargeseparation for Ru(bpy)3

2+-bipyridinium system: Role of the zeolitestructure. J. Phys. Chem. B 1999, 103, 2408−2416.(52) Ito, A.; Stewart, D. J.; Knight, T. E.; Fang, Z.; Brennaman, M.K.; Meyer, T. J. Excited-state dynamics in rigid media: Evidence forlong-range energy transfer. J. Phys. Chem. B 2013, 117, 3428−3438.(53) Casadonte, D. J.; Mcmillin, D. R. Hindered internal-conversionin rigid media: thermally nonequilibrated 3IL and 3CT emissions from[Cu(5-x-phen)(pph3)2]

+ and [Cu(4,7-x2-phen)(pph3)2]+ systems in a

glass at 77 K. J. Am. Chem. Soc. 1987, 109, 331−337.(54) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;Bordiga, S.; Lillerud, K. P. A new zirconium inorganic building brickforming metal organic frameworks with exceptional stability. J. Am.Chem. Soc. 2008, 130, 13850−13851.(55) DeCoste, J. B.; Peterson, G. W.; Jasuja, H.; Glover, T. G.;Huang, Y.-G.; Walton, K. S. Stability and degradation mechanisms ofmetal−organic frameworks containing the Zr6O4(OH)4 secondarybuilding unit. J. Mater. Chem. A 2013, 1, 5642−5650.(56) Katz, M. J.; Brown, Z. J.; Colon, Y. J.; Siu, P. W.; Scheidt, K. A.;Snurr, R. Q.; Hupp, J. T.; Farha, O. K. A facile synthesis of UiO-66,UiO-67 and their derivatives. Chem. Commun. 2013, 49, 9449−9451.(57) Bon, V.; Senkovska, I.; Weiss, M. S.; Kaskel, S. Tailoring ofnetwork dimensionality and porosity adjustment in Zr- and Hf-basedMOFs. CrystEngComm 2013, 15, 9572−9577.(58) Wang, C.; Xie, Z.; de, K. K. E.; Lin, W. Doping metal−organicframeworks for water oxidation, carbon dioxide reduction, and organicphotocatalysis. J. Am. Chem. Soc. 2011, 133, 13445−13454.

(59) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Mixed phosphine 2,2′-bipyridine complexes of ruthenium. Inorg. Chem. 1978, 17, 3334−3341.(60) Xie, P. H.; Hou, Y. J.; Zhang, B. W.; Cao, Y.; Wu, F.; Tian, W. J.;Shen, J. C. Spectroscopic and electrochemical properties of ruthenium-(II) polypyridyl complexes. J. Chem. Soc., Dalton Trans. 1999, 4217−4221.(61) Schaate, A.; Roy, P.; Godt, A.; Lippke, J.; Waltz, F.; Wiebcke,M.; Behrens, P. Modulated synthesis of Zr-based metal−organicframeworks: From nano to single crystals. Chem.Eur. J. 2011, 17,6643−6651.(62) Lin, C. T.; Boettcher, W.; Chou, M.; Creutz, C.; Sutin, N.Mechanism of the quenching of the emission of substitutedpolypyridineruthenium(II) complexes by iron(III), chromium(III),and europium(III) ions. J. Am. Chem. Soc. 1976, 98, 6536−6544.(63) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;Vonzelewsky, A. Ru(II) polypyridine complexes: photophysics,photochemistry, electrochemistry, and chemi-luminescence. Coord.Chem. Rev. 1988, 84, 85−277.(64) Van Houten, J.; Watts, R. J. Temperature dependence of thephotophysical and photochemical properties of the tris(2,2′-bipyridyl)-ruthenium(II) ion in aqueous solution. J. Am. Chem. Soc. 1976, 98,4853−4858.(65) Caspar, J. V.; Meyer, T. J. Photochemistry of Ru(bpy)3

2+:solvent effects. J. Am. Chem. Soc. 1983, 105, 5583−5590.(66) Barigelletti, F.; Juris, A.; Balzani, V.; Belser, P.; Vonzelewsky, A.Excited-state properties of complexes of the Ru(diimine)3

2+ family.Inorg. Chem. 1983, 22, 3335−3339.(67) Lumpkin, R. S.; Kober, E. M.; Worl, L. A.; Murtaza, Z.; Meyer,T. J. Metal-to-ligand charge-transfer (MLCT) photochemistry:experimental-evidence for the participation of a higher lying MLCTstate in polypyridyl complexes of ruthenium(II) and osmium(II). J.Phys. Chem. 1990, 94, 239−243.(68) Nazeeruddin, M. K.; Kalyanasundaram, K. Acid−base behaviorin the ground and excited-states of ruthenium(II) complexescontaining tetraimines or dicarboxybipyridines as protonatable ligands.Inorg. Chem. 1989, 28, 4251−4259.(69) Cherry, W. R.; Henderson, L. J. Relaxation processes ofelectronically excited-states in polypyridine ruthenium complexes.Inorg. Chem. 1984, 23, 983−986.(70) Miksovska, J.; Larsen, R. W. Time-resolved photoacoustics studyof the ruthenium(II) bis(2,2′-bipyridine)(4,4′-dicarboxy-2,2′-bipyri-dine) complex. Inorg. Chem. 2004, 43, 4051−4055.(71) Fernando, S. R. L.; Maharoof, U. S. M.; Deshayes, K. D.; Kinstle,T. H.; Ogawa, M. Y. A negative activation energy for luminescencedecay: Specific solvation effects on the emission properties of bis(2,2′-bipyridine)(3,5-dicarboxy-2,2′-bipyridine)ruthenium(II) chloride. J.Am. Chem. Soc. 1996, 118, 5783−5790.(72) Yang, Q. Y.; Guillerm, V.; Ragon, F.; Wiersum, A. D.; Llewellyn,P. L.; Zhong, C. L.; Devic, T.; Serre, C.; Maurin, G. CH4 storage andCO2 capture in highly porous zirconium oxide based metal−organicframeworks. Chem. Commun. 2012, 48, 9831−9833.(73) Chavan, S.; Vitillo, J. G.; Gianolio, D.; Zavorotynska, O.;Civalleri, B.; Jakobsen, S.; Nilsen, M. H.; Valenzano, L.; Lamberti, C.;Lillerud, K. P.; Bordiga, S. H2 storage in isostructural UiO-67 and UiO-66 MOFs. Phys. Chem. Chem. Phys. 2012, 14, 1614−1626.(74) Marquez, F.; Tocon, I. L.; Otero, J. C.; Marcos, J. I. A prioriSQM vibrational spectrum of 2,2′-bipyridine. J. Mol. Struct. 1997, 410,447−450.(75) Valenzano, L.; Civalleri, B.; Chavan, S.; Bordiga, S.; Nilsen, M.H.; Jakobsen, S.; Lillerud, K. P.; Lamberti, C. Disclosing the complexstructure of UiO-66 metal organic framework: A synergic combinationof experiment and theory. Chem. Mater. 2011, 23, 1700−1718.(76) Juris, A.; Campagna, S.; Balzani, V.; Gremaud, G.; Vonzelewsky,A. Absorption-spectra, luminescence properties, and electrochemical-behavior of tris-heteroleptic ruthenium(II) polypyridine complexes.Inorg. Chem. 1988, 27, 3652−3655.



(77) Kalyanasundaram, K. Photophysics, photochemistry and solar-energy conversion with tris(bipyridyl)ruthenium(II) and its analogs.Coord. Chem. Rev. 1982, 46, 159−244.(78) Nakamaru, K. Solvent effect on the nonradiative deactivation ofthe excited-state of tris(2,2′-bipyridyl)ruthenium(II) ion. Bull. Chem.Soc. Jpn. 1982, 55, 1639−1640.(79) Berberan-Santos, M. N.; Bodunov, E. N.; Valeur, B.Luminescence decays with underlying distributions of rate constants:General properties and selected cases. Springer Ser. Fluores. 2008, 04,67−103.(80) Inokuti, M.; Hirayama, F. Influence of energy transfer by theexchange mechanism on donor luminescence. J. Chem. Phys. 1965, 43,1978−1989.(81) Berberan-Santos, M. N.; Bodunov, E. N.; Valeur, B.Mathematical functions for the analysis of luminescence decays withunderlying distributions 1. Kohlrausch decay function (stretchedexponential). Chem. Phys. 2005, 315, 171−182.(82) Ghosh, P. K.; Bard, A. J. Photochemistry of tris(2,2′-bipyridyl)ruthenium(II) in colloidal clay suspensions. J. Phys. Chem.1984, 88, 5519−5526.(83) Dellaguardia, R. A.; Thomas, J. K. Photoprocesses on colloidalclay systems: tris(2,2′-bipyridine)ruthenium(II) bound to colloidalkaolin and montmorillonite. J. Phys. Chem. 1983, 87, 990−998.(84) Krenske, D.; Abdo, S.; Vandamme, H.; Cruz, M.; Fripiat, J. J.Photochemical and photocatalytic properties of adsorbed organo-metallic compounds. 1. Luminescence quenching of tris(2,2′-bipyridine)ruthenium(II) and tris(2,2′-bipyridinechromium(III) inclay membranes. J. Phys. Chem. 1980, 84, 2447−2457.(85) Colon, J. L.; Yang, C. Y.; Clearfield, A.; Martin, C. R.Photophysics and photochemistry of tris(2,2′-bipyridyl)ruthenium(II)within the layered inorganic solid zirconium-phosphonate sulfophe-nylphosphonate. J. Phys. Chem. 1990, 94, 874−882.(86) Ferragina, C.; Di Rocco, R.; Giannoccaro, P.; Patrono, P.;Petrilli, L. Intercalation of tris-(2,2′-bipyridyl) ruthenium(II) in alpha-and gamma-zirconium phosphate: synthesis, thermal behaviour and X-ray characterization. J. Inclusion Phenom. Mol. Recognit. Chem. 2009, 63,1−9.(87) Kumar, C. V.; Williams, Z. J. Supramolecular assemblies oftris(2,2′bipyridine) ruthenium(II) bound to hydrophobically-modifiedalpha-zirconium phosphate: photophysical studies. J. Phys. Chem.1995, 99, 17632−17639.(88) Kumar, C. V.; Williams, Z. J.; Turner, R. S. Supramolecularaassemblies of metal complexes: Light-induced electron transfer in thegalleries of alpha-zirconium phosphate. J. Phys. Chem. A 1998, 102,5562−5568.(89) Marti, A. A.; Colon, J. L. Direct ion exchange of tris(2,2′-bipyridine)ruthenium(II) into an alpha-zirconium phosphate frame-work. Inorg. Chem. 2003, 42, 2830−2832.(90) Marti, A. A.; Colon, J. L. Photophysical characterization of theinteractions among tris(2,2 ′-bipyridyl)ruthenium(II) complexes ion-exchanged within zirconium phosphate. Inorg. Chem. 2010, 49, 7298−7303.(91) Peng, Y.-L.; Shi, S.-K.; Ye, J.-P. Assemblies of tris(2,2′-bipyridyl)ruthenium(II) into zirconium phosphate layered materials and theirphotophysical properties. Acta Chim. Sinica 2008, 66, 937−942.(92) Shi, S.; Peng, Y.; Zhou, J. Enhanced stable luminescence andphotochemical properties of ruthenium complex/zirconium phosphatehybrid assemblies. J. Nanosci. Nanotechnol. 2009, 9, 2746−2752.(93) Shi, S.; Zhou, J.; Zong, R.; Ye, J. Spectroscopic studies of tris(2,2′-bipyridine)ruthenium(II) intercalated to modified alpha-zirconiumphosphate. J. Lumin. 2007, 122, 218−220.(94) Vliers, D. P.; Collin, D.; Schoonheydt, R. A.; Deschryver, F. C.Synthesis and characterization of aqueous tris(2,2′-bipyridine)-ruthenium(II)-zirconium phosphate suspensions. Langmuir 1986, 2,165−169.(95) Vliers, D. P.; Schoonheydt, R. A.; Deschrijver, F. C. Synthesisand luminescence of ruthenium tris(2,2′-bipyridine) zirconium-phosphates. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2009−2019.

(96) Xiang, M. H.; Shi, X. B.; Li, N.; Li, K. A. Fluorescence propertyof tris(2, 2 ′-bipyridine)ruthenium(II) dichloride immobilized in thegallery of gamma-zirconium phosphates. Chin. Chem. Lett. 2007, 18,89−90.(97) Yeates, R. C.; Kuznicki, S. M.; Lloyd, L. B.; Eyring, E. M.Tris(2,2′-bipyridine) ruthenium(II) electron-transfer in a layeredzirconium-phosphate lattice. J. Inorg. Nucl. Chem. 1981, 43, 2355−2358.(98) Matsui, K.; Momose, F. Luminescence properties of tris(2,2′-bipyridine)ruthenium(II) in sol-gel systems of SiO2. Chem. Mater.1997, 9, 2588−2591.(99) Matsui, K.; Sasaki, K.; Takahashi, N. Luminescence of tris(2,2′-bipyridine) ruthenium(II) in sol-gel glasses. Langmuir 1991, 7, 2866−2868.(100) Reisfeld, R. Spectroscopy and applications of molecules inglasses. J. Non-Cryst. Solids 1990, 121, 254−266.(101) Avnir, D.; Kaufman, V. R.; Reisfeld, R. Interfaces and confinedenvironments 0.23. Organic fluorescent dyes trapped in silica and silicatitania thin-films by the sol-gel method: photophysical, film and cageproperties. J. Non-Cryst. Solids 1985, 74, 395−406.(102) Coutant, M. A.; Le, T.; Castagnola, N.; Dutta, P. K. Zeolite-induced solvation effects on excited-state properties of Ru(bpy)3

2+:Implications for intrazeolitic photochemical quenching reactions. J.Phys. Chem. B 2000, 104, 10783−10788.(103) Briot, E.; Bedioui, F.; Balkus, K. J. Electrochemistry of zeolite-encapsulated complexes: new observations. J. Electroanal. Chem. 1998,454, 83−89.(104) Ledney, M.; Dutta, P. K. Oxidation of water to dioxygen byintrazeolitic Ru(Bpy)3

3+. J. Am. Chem. Soc. 1995, 117, 7687−7695.(105) Yue, Y. F.; Qiao, Z. A.; Li, X. F.; Binder, A. J.; Formo, E.; Pan,Z. W.; Tian, C. C.; Bi, Z. H.; Dai, S. Nanostructured zeoliticimidazolate frameworks derived from nanosized zinc oxide precursors.Cryst. Growth Des. 2013, 13, 1002−1005.(106) Dutta, P. K.; Incavo, J. A. Photoelectron transfer from tris(2,2′-bipyridine)ruthenium(II) to methylviologen in zeolite cages: aresonance raman-spectroscopic study. J. Phys. Chem. 1987, 91,4443−4446.(107) Carson, F.; Agrawal, S.; Gustafsson, M.; Bartoszewicz, A.;Moraga, F.; Zou, X.; Martin-Matute, B. Ruthenium complexation in analuminium metal−organic framework and its application in alcoholoxidation catalysis. Chem.Eur. J. 2012, 18, 15337−44.(108) Kent, C. A.; Liu, D.; Meyer, T. J.; Lin, W. Amplifiedluminescence quenching of phosphorescent metal−organic frame-works. J. Am. Chem. Soc. 2012, 134, 3991−3994.(109) Lin, J.; Hu, X.; Zhang, P.; Van, R. A.; Beratan, D. N.; Kent, C.A.; Mehl, B. P.; Papanikolas, J. M.; Meyer, T. J.; Lin, W.; Skourtis, S.S.; Constantinou, M. Triplet excitation energy dynamics in metal−organic frameworks. J. Phys. Chem. C 2013, 117, 22250−22259.(110) Qi, X.-L.; Liu, S.-Y.; Lin, R.-B.; Liao, P.-Q.; Ye, J.-W.; Lai, Z.;Guan, Y.; Cheng, X.-N.; Zhang, J.-P.; Chen, X.-M. Phosphorescencedoping in a flexible ultramicroporous framework for high and tunableoxygen sensing efficiency. Chem. Commun. 2013, 49, 6864−6866.(111) Kent, C. A.; Liu, D.; Ito, A.; Zhang, T.; Brennaman, M. K.;Meyer, T. J.; Lin, W. Rapid energy transfer in non-porous metal−organic frameworks with caged Ru(bpy)3

2+ chromophores: oxygentrapping and luminescence quenching. J. Mater. Chem. A 2013, 1,14982−14989.(112) Larsen, R. W.; Wojtas, L. Photoinduced inter-cavity electrontransfer between Ru(II)tris(2,2′-bipyridne) and Co(II)tris(2,2′-bipyr-idine) Co-encapsulated within a Zn(II)-trimesic acid metal organicframework. J. Mater. Chem. A 2013, 1, 14133−14139.(113) Incavo, J. A.; Dutta, P. K. Zeolite host-guest interactions:optical spectroscopic properties of tris(bipyridine)ruthenium(II) inzeolite-y cages. J. Phys. Chem. 1990, 94, 3075−3081.(114) Kent, C. A.; Liu, D. M.; Ma, L. Q.; Papanikolas, J. M.; Meyer,T. J.; Lin, W. B. Light harvesting in microscale metal−organicframeworks by energy migration and interfacial electron transferquenching. J. Am. Chem. Soc. 2011, 133, 12940−12943.



(115) Kent, C. A.; Mehl, B. P.; Ma, L. Q.; Papanikolas, J. M.; Meyer,T. J.; Lin, W. B. Energy transfer dynamics in metal−organicframeworks. J. Am. Chem. Soc. 2010, 132, 12767−12769.(116) Allsopp, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J.; Carassiti, V.;Traverso, O. Inorganic photophysics in solution 0.3. Temperatureactivation of decay processes in the luminescence of tris(2,2′-bipyridine)osmium(II) and tris(1,10-phenanthroline)osmium(II)ions. J. Chem. Soc., Faraday Trans. 1 1979, 75, 353−362.(117) Caspar, J. V.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J.Application of the energy-gap law to the decay of charge-transferexcited-states: solvent effects. Chem. Phys. Lett. 1982, 91, 91−95.(118) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J.Application of the energy-gap law to the decay of charge-transferexcited-states. J. Am. Chem. Soc. 1982, 104, 630−632.(119) Shearer, G. C.; Forselv, S.; Chavan, S.; Bordiga, S.; Mathisen,K.; Bjorgen, M.; Svelle, S.; Lillerud, K. P. In situ infrared spectroscopicand gravimetric characterisation of the solvent removal anddehydroxylation of the metal organic frameworks UiO-66 and UiO-67. Top. Catal. 2013, 56, 770−782.(120) Arulmozhiraja, S.; Fujii, T. Torsional barrier, ionizationpotential, and electron affinity of biphenyl: a theoretical study. J.Chem. Phys. 2001, 115, 10589−10594.(121) Fukuda, R.; Ehara, M. Electronic excited states and electronicspectra of biphenyl: a study using many-body wavefunction methodsand density functional theories. Phys. Chem. Chem. Phys. 2013, 15,17426−17434.(122) Imamura, A.; Hoffmann, R. Electronic structure and torsionalpotentials in ground and excited states of biphenyl, fulvalene, andrelated compounds. J. Am. Chem. Soc. 1968, 90, 5379−5385.(123) Devenney, M.; Worl, L. A.; Gould, S.; Guadalupe, A.; Sullivan,B. P.; Caspar, J. V.; Leasure, R. L.; Gardner, J. R.; Meyer, T. J. Excitedstate interactions in electropolymerized thin films of Ru(II), Os(II),and Zn(II) polypyridyl complexes. J. Phys. Chem. A 1997, 101, 4535−4540.(124) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.; Sutin, N.Lifetimes, spectra, and quenching of the excited-states of polypyridinecomplexes of iron(II), ruthenium(II), and osmium(II). J. Am. Chem.Soc. 1980, 102, 1309−1319.(125) Sutin, N.; Creutz, C. Light-induced electron-transfer reactionsof metal-complexes. Pure Appl. Chem. 1980, 52, 2717−2738.(126) Sutin, N. Light-induced electron-transfer reactions. J. Photo-chem. 1979, 10, 19−40.(127) Milosavljevic, B. H.; Thomas, J. K. Photochemistry ofcompounds adsorbed into cellulose. 1. Decay of excited tris(2,2′-bipyridine)ruthenium(II). J. Phys. Chem. 1983, 87, 616−621.(128) Ikeda, N.; Yoshimura, A.; Tsushima, M.; Ohno, T. Hoppingand annihilation of (MLCT)-M-3 in the crystalline solid of[Ru(bpy)3]X2 (X = Cl−, ClO4

− and PF6−). J. Phys. Chem. A 2000,

104, 6158−6164.(129) Turbeville, W.; Robins, D. S.; Dutta, P. K. Zeolite-entrappedRu(Bpy)3

2+: intermolecular structural and dynamic effects. J. Phys.Chem. 1992, 96, 5024−5029.(130) McCarthy, B. D.; Hontz, E. R.; Yost, S. R.; Van Voorhis, T.;Dinca, M. Charge transfer or J-coupling? Assignment of an unexpectedred-shifted absorption band in a naphthalenediimide-based metal−organic framework. J. Phys. Chem. Lett. 2013, 4, 453−458.(131) Shustova, N. B.; McCarthy, B. D.; Dinca, M. Turn-onfluorescence in tetraphenylethylene-based metal−organic frameworks:an alternative to aggregation-induced emission. J. Am. Chem. Soc. 2011,133, 20126−20129.(132) Jin, S. Y.; Son, H. J.; Farha, O. K.; Wiederrecht, G. P.; Hupp, J.T. Energy transfer from quantum dots to metal−organic frameworksfor enhanced light harvesting. J. Am. Chem. Soc. 2013, 135, 955−958.(133) Lee, C. Y.; Farha, O. K.; Hong, B. J.; Sarjeant, A. A.; Nguyen, S.T.; Hupp, J. T. Light-harvesting metal−organic frameworks (MOFs):efficient strut-to-strut energy transfer in bodipy and porphyrin-basedMOFs. J. Am. Chem. Soc. 2011, 133, 15858−15861.(134) Son, H. J.; Jin, S. Y.; Patwardhan, S.; Wezenberg, S. J.; Jeong,N. C.; So, M.; Wilmer, C. E.; Sarjeant, A. A.; Schatz, G. C.; Snurr, R.

Q.; Farha, O. K.; Wiederrecht, G. P.; Hupp, J. T. Light-harvesting andultrafast energy migration in porphyrin-based metal−organic frame-works. J. Am. Chem. Soc. 2013, 135, 862−869.(135) Demas, J. N.; Mandal, K.; Pearson, T. D. L. Singlet energy-transfer from the charge-transfer excited-state of tris(2,2′-bipyridine)-Ru(II). J. Photochem. 1981, 17, 168−169.(136) Mandal, K.; Pearson, T. D. L.; Demas, J. N. Singlet energy-transfer from the charge-transfer excited-state of tris(2,2′-bipyridine)ruthenium(II). J. Chem. Phys. 1980, 73, 2507−2509.(137) Mandal, K.; Pearson, T. D. L.; Krug, W. P.; Demas, J. N.Singlet energy-transfer from the charge-transfer excited-state oftris(2,2′-bipyridine)ruthenium(II) to laser-dyes. J. Am. Chem. Soc.1983, 105, 701−707.(138) Gryczynski, Z.; Maliwal, B. P.; Gryczynski, I.; Lakowicz, J. R.Long-wavelength long-lifetime luminophores for cellular and tissueimaging. Proc. Soc. Photo-Opt. Instrum. Eng. 2004, 5323, 88−98.(139) Kang, J. S.; Piszczek, G.; Lakowicz, J. R. Enhanced emissioninduced by FRET from a long-lifetime, low quantum yield donor to along-wavelength, high quantum yield acceptor. J. Fluoresc. 2002, 12,97−103.(140) Lakowicz, J. R.; Piszczek, G.; Kang, J. S. Long-wavelength long-lifetime high quantum yield luminophores. Biophys. J. 2001, 80, 367a−368a.(141) Lakowicz, J. R.; Piszczek, G.; Kang, J. S. On the possibility oflong-wavelength long-lifetime high-quantum-yield luminophores. Anal.Biochem. 2001, 288, 62−75.(142) Maliwal, B. P.; Gryczynski, Z.; Lakowicz, J. R. Long-wavelengthlong-lifetime luminophores. Anal. Chem. 2001, 73, 4277−4285.(143) Dexter, D. L. A theory of sensitized luminescence in solids. J.Chem. Phys. 1953, 21, 836−850.(144) Perrin, F. La fluorescence des solutions, induction moleculaire,polarisation et duree d′emission, photochimie; Masson: Paris, France,1929.(145) Speiser, S.; Hassoon, S.; Rubin, M. B. The mechanism of short-range intramolecular electronic-energy transfer in bichromophoricmolecules. 2. Triplet triplet transfer. J. Phys. Chem. 1986, 90, 5085−5089.(146) Hassoon, S.; Lustig, H.; Rubin, M. B.; Speiser, S. Themechanism of short-range intramolecular electronic-energy transfer inbichromophoric molecules. J. Phys. Chem. 1984, 88, 6367−6374.(147) Ermolaev, V. L. Energy transfer in organic systems with theparticipation of the triplet state. III. Solid solutions and crystals. Usp.Fiz. Nauk 1963, 80, 3−40.(148) Strambini, G. B.; Galley, W. C. Laser-pulsed phosphorescencestudies of the distance dependence for triplet−triplet energy transfer. J.Chem. Phys. 1975, 63, 3467−3472.(149) Forster, T. Zwischenmolekulare Energiewanderung UndFluoreszenz. Ann. Phys. Berlin 1948, 2, 55−75.(150) Forster, T. 10th Spiers Memorial Lecture: transfer mechanismsof electronic excitation. Discuss. Faraday Soc. 1959, 7−17.(151) Forster, T. Zwischenmolekulare energiewanderung undfluoreszenz. Ann. Phys. Berlin 1948, 437, 55−75.(152) Kasha, M. Energy transfer mechanisms and the molecularexciton model for molecular aggregates. Radiat. Res. 1963, 20, 55−70.(153) Clegg, R. M.; Sener, M.; Govindjee. From Forster resonanceenergy transfer to coherent resonance energy transfer and back. Proc.SPIE 2010, 7561, 75610C.(154) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Exciton model inmolecular spectroscopy. Pure Appl. Chem. 1965, 11, 371−92.(155) Kenkre, V. M.; Knox, R. S. Theory of fast and slow excitationtransfer rates. Phys. Rev. Lett. 1974, 33, 803−806.(156) Rillema, D. P.; Jones, D. S.; Levy, H. A. Structure of tris(2,2′-bipyridyl)ruthenium(II) hexafluorophosphate, [Ru(bipy)3][P6]2: X-raycrystallographic determination. J. Chem. Soc., Chem. Comm. 1979,849−851.



Documents

Photophysical Characterization of a Ruthenium(II) Tris(2,2′-bipyridine)-Doped Zirconium UiO-67 Metal–Organic Framework