PAPER www.rsc.org/pps | Photochemical & Photobiological Sciences

Photoinduced electron transfer reaction of tris(4,4¢-dicarboxyl-2,2¢-bipyridine)ruthenium(II) ion with organic sulfides†

Eswaran Rajkumar and Seenivasan Rajagopal*

Received 25th April 2008, Accepted 25th September 2008First published as an Advance Article on the web 15th October 2008DOI: 10.1039/b806974c

[Ru(dcbpy)3]2+ (dcbpy = 4,4¢-dicarboxyl-2,2¢-bipyridine) ion, in the excited state, undergoes facileelectron transfer (ET) reaction with aryl methyl and dialkyl sulfides and the quenching rate constant, kq

value is sensitive to the structure of the sulfide. The detection of the sulfur radical cation in this systemusing time-resolved transient absorption spectroscopy confirms the ET nature of the reaction. Thesemiclassical theory of ET has been successfully applied to the photoluminescence quenching of[Ru(dcbpy)3]2+ with sulfides. This is the first report for the generation and detection of sulfide radicalcations from the excited state reaction of Ru(II) complex with organic sulfides.

1. Introduction

Organic sulfur compounds are known to undergo fast one electronoxidation reactions, owing to their relatively low ionizationpotentials.1 Thus, suitable oxidants can remove an electron froma lone pair on the sulfur atom, the likely site of the oxidativeattack, to produce molecular radical cations, which are importantintermediates in a variety of chemical and biological processes.1–4

The oxidation of organic sulfides has also been found to beof potential use in the decontamination of toxic chemicals.5

Most investigations have been carried out to provide informationon reaction pathways of dialkyl sulfide radical cations, whilemuch less attention has been given to radical cations fromaromatic sulfides.5–7 The chemistry of sulfide radical cations hascontinued to attract considerable interest for theoretical as wellas for experimental chemists.4,8–12 Recently Baciocchi, Steenken,Bobrowski and our group13–16 recorded the absorption spectra ofsulfide radical cations formed from the photochemical oxidationof dialkyl as well as alkyl aryl sulfides. The thermal oxidation oforganic sulfides16–19 by various oxidants [Fe(NN)3]3+, [Ru(NN)3]3+

(NN = 2,2¢-bipyridine, 1-10-phenanthroline and their derivatives),Cr(VI) and Cr(V) and photochemical oxidation with [Cr(NN)3]3+20

has been proposed to proceed through an electron transfermechanism from the dynamic study carried out in this laboratory.Recently Zhou et al.21 reported that the sulfoxidation of organicsulfides by molecular oxygen was efficiently catalyzed by usingruthenium(III)-mesotetraphenyl porphyrin chloride.

Luminescent d6 transition metal complexes, in particularRu(II) complexes, are useful photosensitizers for energy andelectron transfer processes.22,23 The excited state properties ofthe tris(2,2¢-bipyridine)ruthenium(II) complex [Ru(bpy)3]2+ (bpy =2,2¢-bipyridine) is dramatically affected by the introduction ofelectron -donating and -withdrawing groups in the 4,4¢-position of2,2¢-bipyridine.23–28 By modifying the structure of the ligand, it is

School of chemistry, Madurai Kamaraj University, Madurai, 625 021.E-mail: seenirajan@yahoo.com† Electronic supplementary information (ESI) available: Fig. S1 and S2,Table S1. See DOI: 10.1039/b806974c

possible to tune the excited state properties like emission lifetime,quantum yield and wavelength of the emission maximum as wellas redox potential. In order to analyze the effect of introducingelectron withdrawing groups in the 2,2¢-bipyridine on the excitedstate redox properties of Ru(II) complexes, carboxylic, sulfonic,phosphonic and trifluoromethyl groups have been introducedin the 4,4¢-position of 2,2¢-bipyridine and the photophysicalproperties of these complexes have been extensively studied.23–28

Interestingly, a significant increase is observed in the excitedstate reduction potential of [Ru(NN)3]2+ by introducing the –CO2H groups at the 4,4¢-position of 2,2¢-bipyridine rings. Infact the reduction potential of tris(4,4¢-dicarboxyl-2,2¢-bipyridine)ruthenium(II) (1.55 V vs. SCE) is 0.8 V higher than that oftris(2,2¢-bipyridine)ruthenium(II) (0.76 V vs. SCE). Among theligands carrying electron withdrawing groups indicated above, theligand carrying carboxylic acid received special attention, becauseof its simple method of synthesis and its extensive applicationin the construction of devices to achieve efficient solar energyconversion29,30 and for use as sensors.31

In our recent report,26 we have shown that *[Ru(dcbpy)3]4- isa better oxidant than *[Ru(bpy)3]2+ in aqueous alkaline medium(pH 12.5) and undergo photoinduced electron transfer reactionswith phenolate ions and the reaction is more exergonic by 0.2 eV.As far as the dcbpy ligand is concerned the important point is thatdcbpy exists as an anion (carboxylate ion) in alkaline mediumbut it is neutral (carboxylic acid) in acetonitrile medium. Thisaspect is significant here because –CO2H (Hammet s value =0.40) is a better electron-withdrawing group than –CO2

- (s =0.11) which makes *[Ru(dcbpy)3]2+ (E0 = 1.55 V) a better oxidantthan *[Ru(dcbpy)3]4- (E0 = 1.00 V). Because of its favorablereduction potential we thought it would be interesting to study theluminescence quenching of *[Ru(dcbpy)3]2+ by various alkyl andaryl sulfides in CH3CN to check the efficiency of *[Ru(dcbpy)3]2+

as a photosensitizer. It is important to mention that the reactionbetween the excited state [Ru(bpy)3]2+ and organic sulfides is highlyendergonic (0.5–1.09 V).

Interestingly *[Ru(dcbpy)3]2+ undergoes facile electron transferreaction with organic sulfides and the reaction is followed by steadystate as well as by time resolved techniques. In the present study

This journal is © The Royal Society of Chemistry and Owner Societies 2008 Photochem. Photobiol. Sci., 2008, 7, 1407–1414 | 1407

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we have investigated the excited state electron transfer reactionof [Ru(dcbpy)3]2+ (dcbpy = 4,4¢-dicarboxyl-2,2¢-bipyridine) withseveral alkyl and aryl methyl sulfides. As far as we know thisappears to be the first report for the generation and detectionof sulfide radical cations from the excited state reaction of[Ru(NN)3]2+ complexes with organic sulfides.

2. Experimental

2.1 Materials

4-Methylpyridine, RuCl3·3H2O and organic sulfides were ob-tained from Aldrich and used as obtained. The ligand 4,4¢-dimethyl-2,2¢-bipyridine has been obtained from 4-methylpyridineusing Pd/charcoal as catalyst.32 The ligand 2,2¢-bipyridine-4,4¢-dicarboxylic acid has been obtained by the oxidation of 4,4¢-dimethyl-2,2¢-bipyridine using a literature procedure.33 The tris-chelated complex [Ru(dcbpy)3]2+ was prepared by a publishedprocedure.32,33

2.2 Absorption and emission spectral measurements

Sample solutions of the metal complexes and the quenchers havebeen freshly prepared for each measurement. The absorptionspectral measurements were carried out using a SPECORD S100diode-array spectrophotometer. Emission intensity measurementswere carried out using FP-6300 spectrofluorometer. All the samplesolutions used for emission measurements were deareated forabout 30 min by dry nitrogen gas purging by keeping solutionsin cold water to ensure that there is no change in the volume ofthe solution.

2.3 Luminescence quenching measurements

The photochemical reduction of [Ru(dcbpy)3]2+ complex with alkyland aryl methyl sulfides has been studied by the luminescencequenching technique. The sample solutions were purged carefullywith dry nitrogen for 30 min. The luminescence measurementswere performed at different quencher concentrations and thequenching rate constant, kq, values were determined from theStern–Volmer plot using the equations given below.

I 0/I = 1 + K sv[Q]

K sv = kqt 0

Here I 0 and I are the luminescence intensities of Ru(II) complexin the absence and presence of quencher, K sv, the Stern–Volmerconstant, kq, the quenching rate constant and t 0, the luminescencelifetime of [Ru(dcbpy)3]2+ in the absence of quencher. Selectedemission quenching experiments were also followed by lifetimemeasurements. The emission lifetime was monitored in the absenceand in the presence of quencher.

2.4 Luminescence lifetime measurements

Time resolved fluorescence measurements were carried out using adiode laser-based time correlated single photon counting (TCSPC)spectrometer from IBH, UK. In the present study, a 452 nm diodelaser (50 kHz) was used as the excitation source and Hamamatsuphotomultiplier tube was used for the fluorescence detection. The

instrument response function for this system is ª1.2 ns and thefluorescence decay was analyzed by using the software providedby IBH (DAS-6) and PTI global analysis software.

2.5 Transient absorption measurements

Transient absorption measurements were made with a laser flashphotolysis technique using an Applied Photophysics SP-QuantaRay GCR-2(10) Nd:YAG laser as the excitation source.34 Thetime dependence of the luminescence decay is observed usinga Czerny-Turner monochromator with a stepper motor controland a Hamamatsu R-928 photomultiplier tube. The productionof the excited state on exposure to light of wavelength 355 nmwas measured by monitoring (pulsed xenon lamp of 250 W)absorbance change. The change in the absorbance of the sampleon laser irradiation was used to calculate the rate constant as wellas to record the time-resolved absorption transient spectrum. Thechange in the absorbance on flash photolysis was calculated usingthe expression

DA = log I 0/(I 0 - DI)

DI = (I - It)

where DA is the change in the absorbance at time t, I 0, I and It

are the voltage after flash, the pretrigger voltage and the voltageat particular time respectively. A plot of ln(DAt - DA•) vs. timegives a straight line. The slope of the straight line gave the rateconstant for the decay and the reciprocal of rate constant gavethe lifetime of the triplet. The time-resolved transient absorptionspectrum was recorded by plotting the change in absorbance at aparticular time vs. wavelength.

3. Results and discussion

The structure of the complex and the quenchers used in the presentstudy are shown in Chart 1. The absorption and emission spectraldata, the excited state lifetime (t) and redox potential value of[Ru(dcbpy)3]2+ in acetonitrile solution were measured and the datacollected in Table 1. The ground state absorption spectrum of[Ru(dcbpy)3]2+ is shown in Fig. 1. The strong absorption at 300 nmcorresponds to the p–p* (LC) transition and the low energymaximum absorption at 467 nm is assigned to the dp–p* (MLCT)transition. The MLCT transition involves electronic excitationfrom the metal orbital [dp(Ru)] to the ligand centered acceptorp* orbital (ligand). The emission wavelength of Ru(II) complexoccurs at 636 nm which originates from the 3MLCT of the Ru(II)complex. From the photophysical data collected in Table 1 itis seen that the absorption maximum of MLCT transition isred shifted to the tune of 17 nm from 450 to 467 nm and theemission maximum by 24 nm from 612 to 636 nm when the ligandbpy is replaced by dcbpy. The introduction of –CO2H group inbpy lowers p* level of the ligand and thus shifts the LC and

Table 1 Absorption and emission spectral data, excited state lifetime andreduction potential of [Ru(NN)3]2+ complexes in acetonitrile at 298 K

Complex labsmax/nm lem

max/nm t/ns E0 Ru2+*/+ vs. SCE/V

[Ru(bpy)3]2+ 450 612 850 0.76[Ru(dcbpy)3]2+ 467 636 1080 1.55

1408 | Photochem. Photobiol. Sci., 2008, 7, 1407–1414 This journal is © The Royal Society of Chemistry and Owner Societies 2008

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Chart 1 Structure of [Ru(dcbpy)3]2+ and organic sulfides.

Fig. 1 Absorption and emission spectrum of [Ru(dcbpy)3]2+ in acetoni-trile at RT.

MLCT transition to the red. The stabilization of 3MLCT statein [Ru(dcbpy)3]2+ compared to the parent complex [Ru(bpy)3]2+

leads to the increased lifetime which is confirmed from the datacollected in Table 1. In order to check the ground-state complexformation between [Ru(dcbpy)3]2+ and the quencher, increasingconcentrations of three aryl methyl sulfides(methyl phenyl,methyl p-methoxyphenyl and p-bromophenyl methyl sulfides) areadded to the [Ru(dcbpy)3]2+ complex and the spectra recorded atdifferent [sulfide]. There is no significant change in the absorptionspectrum of [Ru(dcbpy)3]2+ in the presence of sulfides under thepresent experimental conditions, which helps us to conclude thatthe contribution from the static quenching is negligible here.

3.1 Luminescence quenching rate constants

The emission intensity as well as excited state lifetime of[Ru(dcbpy)3]2+ are efficiently reduced in the presence of sulfides inacetonitrile and are analyzed in terms of Stern–Volmer equationand the observed quenching rate constant, kq, data are collectedin Table 2. The kq data given in Table 2 show that the value of kq

is sensitive to the oxidation potential of sulfides.18,19,35 The changein the luminescence intensity of *[Ru(dcbpy)3]2+ in the presence ofdifferent concentrations of methyl phenyl sulfide is shown in Fig. 2.

Fig. 2 The change in luminescence intensity for the excited state of theRu(II) complex, [Ru(dcbpy)3]2+ with different concentrations of methylphenyl sulfide (from top : 0.5 ¥ 10-4, 1 ¥ 10-3, 4 ¥ 10-3, 8 ¥ 10-3, 10 ¥ 10-3,2 ¥ 10-2, 4 ¥ 10-2, 10 ¥ 10-2 M respectively).

Table 2 Bimolecular quenching rate constants, kq, for the *[Ru(dcbpy)3]2+ complex by organic sulfides in CH3CN at 298 K

Quenchera (E0ox/V)b DG0/eV l0/eV kq/M-1 s-1 k23 (exp.)/M-1 s-1 k23 (calc.)/M-1 s-1

Diethyl sulfide (1.65) 0.10 — 5.0 ¥ 105 — —Dipropyl sulfide (1.63) 0.07 — 5.6 ¥ 105 — —Di-sec-butyl sulfide (1.65) 0.1 — 8.3 ¥ 105 — —sec-Butyl sulfide (1.65) 0.1 — 2.2 ¥ 105 — —Methyl phenyl sulfide (1.53) -0.02 0.82 8.7 ¥ 106 1.6 ¥ 106 2.7 ¥ 106

Methyl p-methoxyphenyl sulfide (1.26) -0.29 0.65 3.2 ¥ 108 5.0 ¥ 107 4.2 ¥ 107

Methyl p-tolyl sulfide (1.41) -0.14 0.71 2.4 ¥ 107 4.0 ¥ 106 3.4 ¥ 106

Methyl p-fluorophenyl sulfide (1.54) -0.01 0.78 2.5 ¥ 106 4.6 ¥ 105 5.4 ¥ 106

Methyl p-chlorophenyl sulfide (1.55) 0 0.76 2.0 ¥ 106 3.5 ¥ 105 4.1 ¥ 106

Methyl p-bromophenyl sulfide (1.56) 0.01 0.74 1.3 ¥ 106 2.2 ¥ 105 2.7 ¥ 106

Methyl p-nitrophenyl sulfide (1.85) 0.30 0.72 5.9 ¥ 105 9.7 ¥ 104 2.1 ¥ 105

a Alkyl sulfides 0.005–1 M and alkyl aryl sulfides 0.0005–0.1 M. b Ref. 17,18,35.

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A typical Stern–Volmer plot for the excited state of the Ru(II)complex with methyl phenyl sulfide obtained from steady-stateand time-resolved measurements are shown in Fig. 3. The Stern–Volmer plots from the emission intensity data (Fig. 3) are linear forall photoredox systems, indicating that dynamic quenching is thepredominant process and the contribution from static quenchingis negligible.

Fig. 3 Stern–Volmer plot for the excited state of the Ru(II) complex,[Ru(dcbpy)3]2+ with methyl p-methoxyphenyl sulfide.

The quenching reactions carried out in the present study occurby ET mechanism (vide infra) and can be discussed using Scheme 1.The primary photochemical step in the reactions involves anelectron transfer from the sulfur atom of the quencher to the tripletstate of [Ru(dcbpy)3]2+. This is confirmed by direct observation oftransient species Ru+ and sulfide radical cation. For the excitedstate of transition metal complexes, only energy and electrontransfer quenching processes have been considered.23 Since thetriplet energy level of the sulfides (>3.0 eV)17,18,35 is above theavailable excitation energy of [Ru(dcbpy)3]2+ (~2.1 eV), electronicenergy transfer is less probable. The results of both steady-stateand time-resolved experiments indicate that the effect of addingsulfides on the luminescence of [Ru(dcbpy)3]2+ must be associatedwith only a dynamic quenching process through electron transfer(Scheme 1). Sulfides with lower oxidation potentials exhibit higherquenching rate constants, a trend indicative of electron transferquenching. The quenching rate constants have been correlatedwith the oxidation potentials of organic sulfides. The plot of ln kq

vs. oxidation potentials of organic sulfides is linear, which providesadditional support for the electron transfer mechanism (Fig. 4).The operation of an electron transfer quenching mechanism isfurther supported by the transient analysis using flash photolysistechnique (vide infra).

Scheme 1 Mechanism for the reaction of the excited state of the Ru(II)complex [Ru(dcbpy)3]2+ with sulfides.

Fig. 4 Plot of ln kq vs. oxidation potential of sulfides.

3.2 Application of Hammett equation to the quenching rateconstant data

Fig. 5 shows the Hammett plot36 i.e., log kq versus s (Hammettsubstituent constants) plot, for the luminescence quenching of*[Ru(dcbpy)3]2+ with aryl methyl sulfides. The introduction ofelectron-donating groups like methyl and methoxy in the aromaticring of PhSMe increases the rate of the electron transfer andelectron-withdrawing groups like F, Cl, Br, NO2 decrease the rateof the electron transfer. Application of the Hammett equationfor the analysis of these electron transfer rate data gives a largenegative r value (r = -4.0) which indicates a substantial develop-ment of positive charge on the sulfur atom of the quencher in thetransition state. The log kq values have also been correlated withBrown–Okamoto’s s+ values also. It is found that the correlationwas better with Hammett’s s values than Brown–Okamoto’s s+

values. The better correlation observed with the Hammett’s svalues can be rationalized on the basis of a small extent of chargedelocalization in the aromatic ring in the transition state leadingto the formation of the sulfide radical cation.

Fig. 5 Hammett plot for the log kq vs. s .

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3.3 Transient absorption spectral studies

One convenient detection method for monitoring the formationof sulfur radical cation during the course of a redox reaction istransient absorption spectroscopy. Unfortunately, sulfur radicalcations from alkyl sulfides have only very weak absorptions inthe near UV region though they form strongly absorbing dimerswith two centered three electron bonds with unreacted thioethermolecules.12 On the other hand, monomeric sulfur radical cationsfrom aromatic sulfides do absorb in the visible region. This featureof aromatic thioethers makes them ideal precursors of sulfurradical cations that can be used for probing the mechanism ofET quenching of excited states.

In order to gain more insight into the nature of the quenchingprocess, the progress of the reaction of the long-lived excited stateof [Ru(dcbpy)3]2+ complex with methyl p-methoxyphenyl sulfidewas followed using time-resolved absorption studies by meansof <8 ns laser width at 355 nm excitation. Fig. 6 and Fig. 7show the transient absorption spectra of [Ru(dcbpy)3]2+ recorded

Fig. 6 Transient absorption spectra of *[Ru(dcbpy)3]2+ in acetonitrile at298 K recorded 500 ns after laser flash.

Fig. 7 Transient absorption spectra of *[Ru(dcbpy)3]2+ in the presence ofmethyl p-methoxyphenyl sulfide in acetonitrile at 298 K recorded 3 ms afterlaser flash.

in the absence and presence of 0.5 M methyl p-methoxyphenylsulfide obtained after 500 ns and 3 ms following laser flash as afunction of wavelength. In Fig. 6, the band at 380 nm correspondsto the dcbpy anion radical on the basis of its similarity (inlocation) with the spectrum of 2,2¢-bipyridine radical anion andthe bleaching around 450 nm is due to the loss of ground stateabsorption, dp–p* (MLCT) transition. The bleaching around 600–700 nm corresponds to the light emission from the relaxed excitedstate to the ground state. In the presence of sulfide (Fig. 7),a new transient species is formed around 580 nm. The broadband around 550–620 nm was assigned to the radical cations ofalkyl aryl sulfides.6,9 Del Giacco et al.6 reported the reactivityand photokinetic behavior of alkyl aryl sulfides in the oxidationsensitized by triplet chloranil in organic solvents. The broad bandcentered at 550–620 nm was assigned to the radical cations ofalkyl aryl sulfides. The peak formation around 580 nm in thepresence of methyl p-methoxyphenyl sulfide is attributed to themethyl p-methoxyphenyl sulfide radical cation and another bandat 510 nm to the formation of [Ru(dcbpy)3]+ species.26 In addition,the decay traces of sulfide radical cation (580 nm) could be fittedto a second-order rate function to give a rate constant of 6.7 ¥105 M-1 s-1. The decay kinetics of *[Ru(dcbpy)3]2+ measured at580 nm in the absence and presence of methyl p-methoxyphenylsulfide are shown in Fig. 8. It is important to point out that forthe first time the formation of sulfur radical cation is observed inthe 3MLCT excited state electron transfer reaction of [Ru(NN)3]2+

with organic sulfides.

Fig. 8 The transient kinetics of *[Ru(dcbpy)3]2+ were measured at 580 nmin the absence and presence of methyl p-methoxyphenyl sulfide (complex,lower traces; complex and sulfide, upper traces).

3.4 Dynamics of electron transfer reaction of [Ru(dcbpy)3]2+

complex with organic sulfides

After establishing the electron transfer nature of the quenchingprocess, we applied the semi-classical theory of electron transfer(eqn (1))37,38 to the above redox reaction. The rate of ET from adonor to an acceptor molecule in a solvent is controlled by freeenergy change of the reaction (DG0), the reorganization energy(l) and the electron transfer distance between the donor and theacceptor.

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k h H kT S m

G mh

S m

m

et DA (e / )=

- + ∞ +

- -

=

•

Â4 42 2 1 2

0

2

p p

¥

/ ( ) !

exp[ ( ) /

/l

l nD 44lkT ] (1)

In eqn (1), HDA is the electronic coupling coefficient between theredox centers and its value here is 2 ¥ 10-3 eV,25,26 the reorganizationenergy l is composed of solvational lo and vibrational li

contributions with s = li/hn, n is the high-energy vibrationalfrequency associated with the acceptor and m is the density ofproduct vibrational levels. The terms h and k are Planck’s andBoltzmann’s constants, respectively.

According to Rehm and Weller, the free-energy change ofelectron transfer (DG0) can be calculated from eqn (2).39

DG0 = E(D/D+

) - E(A/A-

) - E0–0 - e2/ae (2)

where E(D/D+

) is the oxidation potential of electron donors, E(A/A-

),the reduction potential of acceptor, E0–0 the lowest excited stateenergy of [Ru(dcbpy)3]2+, and e2/ae a Coulombic term. TheDG0 values thus estimated for the reaction of different donorswith [Ru(dcbpy)3]2+ in CH3CN are given in the Table 2. Thereorganization energy (l) is the sum of two contributions, l =lo + li, where li represents the activation of the vibrationalmodes of the reactants and lo represents the changes in the solventstructure around the reactants, which strongly dependent on thesolution medium. The value of lo is evaluated by using dielectriccontinuum model, eqn (3).

lo = e2/4peo (1/2rD + 1/2rA - 1/d) (1/Dop - 1/Ds) (3)

where e is the transferred electronic charge, eo the permittivity offree space, Dop and Ds the optical and static dielectric constants,respectively. The terms rD and rA are the radii of the electron donorand acceptor, respectively and d is the sum of radii, rD + rA. Thevalues of rD and rA are estimated by MM2 molecular model andthe values are 3. 85–4.73 A and 9 A. The value of lo calculatedusing eqn (3) is 0.65 eV for methoxyphenyl methyl sulfide and0.82 V for methyl phenyl sulfide. The value of li is found to be0.2 eV and is employed in the calculation of the rate constant forET reaction.40 Thus the total reorganization energy, l, value forthis redox system is in the range of 0.85 -1.02 eV. Since DG0 andl values are known the value of rate constant for electron transferfrom sulfide to *[Ru(dcbpy)3]2+ can be calculated using eqn (1). Ineqn (1), HDA = 2 ¥ 10-3 eV, l = 0.85–1.02 eV, n = 800–1200 cm-1

and T = 298 K. These values are the optimum values for thereaction, chosen by a trial and error method.41

According to Scheme 1, the excited state acceptor (*RuII) andthe ground state donor (sulfide) molecules diffuse together to forman encounter complex, (*RuII ◊ ◊ ◊ S). This encounter complex thenundergoes a reorganization to reach the transition state whereET takes place from the donor to the acceptor to give an ion-pair species, (RuI ◊ ◊ ◊ S+∑), the successor complex. The parametersk12 and k21 are the diffusion-controlled rate constants for theformation and dissociation of the encounter complex (*RuII ◊ ◊ ◊ S),respectively. The k23 and k32 are the rate constants for the forwardand reverse ET reaction. The successor complex can undergoeither a back electron transfer to form the encounter complex(k32), or can form the separated species [Ru(NN)3]+ and S+∑ (ksep),which in turn can undergo back electron transfer to form thereactants in the ground state (k34). As the present study aims

at the dynamics of ET reaction from the organic sulfides to theexcited state [Ru(dcbpy)3]2+ as a function of exergonicity, we didnot make any attempt to measure the of quantum yields of theinitial transients [Ru(NN)3]+ and S+∑ and identification of finalproducts of the reaction.

By applying steady-state treatments to the short lived speciesin Scheme 1, the following expression (eqn (4)) for the observedbimolecular quenching rate constant, kobs(kq) can be derived.

kk

k k Kqeq

=+

12

12 231 ( / )(4)

K eq is the equilibrium constant for the formation of theencounter complex. The value of k12 is calculated from eqn (5).39,41

k12 = 2RT/3000h [2 + rD/rA + rA/rD] f (5)

where f -1 = d∫

eu/kT dr/r2 with u = ZDZAe2/DS[eKd/1 + Kd]e-Kr/rwhere K = (8pe2Nh/1000DSkT)1/2 and rD and rA are the radii ofthe reactants and ZA and ZB are the charge of the reactants andDS is the static dielectric constant and h is the viscosity of themedium.

The diffusion rate constant, k12, calculated according toSmoluchowski42 for non-charged molecules, has a value of 1.5 ¥1010 dm3 mol-1 s-1. K eq was estimated using the Fuoss and Eigenequation (eqn (6)).42

K eq = (4pNd3/3000) exp(-wr/RT) (6)

where wr is the work required to bring the reactants to theseparation distance d. Since we use neutral quenchers throughoutthis study, wr is zero. The value of K eq is found to be in the range5.88 to 6.69 M-1 for the excited state of Ru(II) complex with sulfides.Since the values of k12 and K eq are known the value for k23, therate constant for the process of ET in the encounter complexcan be calculated from the observed kq values using eqn (4) andthe values are plotted against DG0 in Fig. 9. The rate of electrontransfer increases by decreasing the free energy of the reaction.The values of k23 (ket) have also been calculated using semiclassicaltheory from eqn (1). The calculated ket values were also plottedagainst -DG0 values (Fig. 9). The data given in Fig. 9 show aclose agreement between the experimental and calculated values.

Fig. 9 Plot of log k23, M-1 s-1 vs. DG0, eV for the excited state of theRu(II) complex, *[Ru(dcbpy)3]2+ with sulfides (★ experimental value, �calculated value).

1412 | Photochem. Photobiol. Sci., 2008, 7, 1407–1414 This journal is © The Royal Society of Chemistry and Owner Societies 2008

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Thus, the semiclassical theory of ET reproduces the experimentalresults favorably confirming the success of the theory of ET andthe operation of ET mechanism of the reaction.

4. Conclusion

The excited state of [Ru(dcbpy)3]2+ complex undergoes facile oneelectron transfer reaction with organic sulfides. The detection ofsulfur radical cations as transient species confirms the electrontransfer nature of the reaction. This seems to be the first reportfor the generation and detection of sulfur radical cation using anexcited state Ru(II) complex as the oxidant. The successful ap-plication of semiclassical theory of ET to the photoluminescencequenching of [Ru(dcbpy)3]2+ with organic sulfides also supportsthe ET nature of the reaction.

Acknowledgements

S. R thanks the Department of Science and Technology (DST),New Delhi, India for sanctioning a project. E. R thanks CSIR,India for awarding the Senior Research Fellowship. The authorsthank Prof. P. Ramamurthy, NCUFP, University of Madras andNCFRR, University of Pune for flash photolysis and TCSPCmeasurements.

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