DaltonTransactions

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Cite this: Dalton Trans., 2013, 42, 5539

Received 19th November 2012,Accepted 6th February 2013

DOI: 10.1039/c3dt00003f

www.rsc.org/dalton

β-Substituted ferrocenyl porphyrins: synthesis,structure, and properties†

Rekha Sharma, Prabhat Gautam, Shaikh M. Mobin and Rajneesh Misra*

β-Substituted ferrocenyl porphyrins were designed and synthesized by the Pd-catalyzed Sonogashira

cross-coupling reaction. The UV-vis absorption, emission, and cyclic voltammetric results indicate strong

electronic communication between ferrocene and porphyrin. The porphyrin 4a is non-emissive in nature,

while 4b and 4c show reduced fluorescence quantum yield. The single crystal X-ray structure of 4b is

reported, which shows extensive C–H–π interactions.

Introduction

The design and synthesis of a molecular system containing achromophore and a redox active site is of great interest for avariety of applications, such as chemical sensors, electrontransfer, nonlinear optics, and solar energy conversion.1–5

Through-bond or through-space interactions between the chro-mophore and the redox active species may result in strong elec-tronic interactions.5 The electronic interaction can be tuned bythe introduction of a spacer group between the chromophoreand the redox active species. The literature reveals that thereare a number of reports where the redox groups are attached atthe meso-position of the porphyrin ring, directly or viadifferent spacer groups.3–16 The linkage of the ferrocene (redoxactive group) at the meso-position of the porphyrin ringinduces a strong electronic coupling between the two subunits,and the individual characteristic features of the porphyrin andthe ferrocenyl groups are altered significantly.17–22

There are a handful of reports where the redox active groupis attached at the β-position of the porphyrin.2,23–26

The β-substitution is of interest because such a substitutionis in direct conjugation with the 18π-molecular system, whichresults in substantial perturbation of the photophysical andelectrochemical properties of the porphyrin.25 Our group isinterested in the design and synthesis of chromophores withthe redox active group for various electronic and photonicapplications.27a,28–30a In this contribution, we designed the

molecular system, which has porphyrin as a chromophore,and ferrocene unit as a redox active group connected throughβ-linkage at the pyrrolic position, and explored its structural,photophysical, and electrochemical properties.

Results and discussion

The ferrocenyl substituted porphyrins 4a–4c were synthesizedby the Pd-catalysed Sonogashira cross-coupling reaction of theβ-bromo tetraphenyl porphyrin with the corresponding ethynylferrocenes (Scheme 1).

The tetraphenylporphyrin (H2TPP) 1 was synthesized by thecondensation reaction of pyrrole and benzaldehyde in propio-nic acid.12 The selective bromination of H2TPP using N-bromo-succinimide in refluxing CHCl3 resulted in β-bromotetraphenyl porphyrin 2, which was purified by columnchromatography using toluene–cyclohexane (30 : 70) as theeluent.9

The Sonogashira coupling reaction of β-bromo porphyrin 2with ethynylferrocene, 4-ferrocenylphenylacetylene, and 3-ferro-cenylphenylacetylene under the catalytic system Pd(dba)2/AsPh3 afforded compounds 4a–4c in 40–60% yield. TheSonogashira coupling reaction was performed under copperfree conditions to avoid the homocoupling reaction ofalkynes.10,11 The structures of all these compounds were wellcharacterized by NMR and HRMS techniques. The porphyrin4b was also characterized by single crystal X-ray analysis.

Single crystal X-ray diffraction study

The single crystal of the ferrocenyl substituted porphyrin 4bwas obtained by slow evaporation of a mixture of toluene andmethanol (3 : 7 ratio). The porphyrin 4b crystallizes in triclinic,P1̄ space group. Fig. 1 shows the single crystal X-ray structureof compound 4b, and the important crystallographic para-meters are listed in Table 1. In porphyrin 4b, there are two

†Electronic supplementary information (ESI) available: General experimentalmethods, and copies of 1H NMR, and HRMS spectra of all new compounds, crys-tallographic information file (CIF) for compound 4b. CCDC 903777. For ESI andcrystallographic data in CIF or other electronic format see DOI:10.1039/c3dt00003f

Department of Chemistry, Indian Institute of Technology Indore, Indore, 452017,

India. E-mail: rajneeshmisra@iiti.ac.in; Fax: +91 731 2361 482;

Tel: +91 731 2438 710

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independent molecules in the asymmetric unit, with a slightvariation in their bond lengths and bond angles.

The porphyrin core shows planar structure, with the fourmeso-phenyl rings in the porphyrin core oriented at a dihedralangle of 66.14°, 76.50°, 81.74°, and 89.45°. The cyclopenta-dienyl rings of the ferrocene unit show eclipsed conformation.The dihedral angle between the plane of the porphyrin coreand the plane of the cyclopentadienyl ring of the ferrocenylsubunit was found to be 87.61°. The study of non-bondinginteractions shows extensive C–H–π interactions between thetwo different porphyrins forming the asymmetric unit; theseC–H–π interactions are listed in Table 2 and shown in Fig. S1(see ESI† for details). The C–H–π interactions involving hydro-gen H90 with the meso-phenyl ring (C9, C10, C11, C6, C7, C8,2.935 Å) of another porphyrin, and hydrogen H28 with themeso-phenyl ring (C70, C71, C72, C73, C68, C69, 2.698 Å) ofanother porphyrin, lead to the formation of linear chains.These chains are interlinked through two C–H–π interactionsinvolving hydrogens H44 and H106 of the pyrrole rings withthe meso-phenyl rings (C101, C102, C103, C98, C99, C100,3.557 Å), and (C41, C36, C37, C39, C40, C38, 3.523 Å)

belonging to different chains, which results in the formationof layered structure shown in Fig. 2. The porphyrins of eachchain are also associated with the solvent molecules involvingH305 and H506 with cyclopentadienyl rings (C60, C61, C58,C62, C59, 2.816 Å), and (C123, C122, C12, C120, C124,2.760 Å).

Electrochemistry

The electrochemical behavior of the β-substituted ferrocenylporphyrins 4a–4c was investigated by cyclic voltammetricanalysis using tetrabutylammoniumhexafluorophosphate asthe supporting electrolyte in dichloromethane. The electro-chemical data of porphyrins 4a–4c are listed in Table 3, and arepresentative cyclic voltammogram (4b) along with the corre-sponding ethynyl ferrocene 3bH is presented in Fig. 3. Allpotentials are corrected to be referenced against FcH/FcH+, asrequired by IUPAC,39 and the electrochemical reversibility forone-electron transfer processes was characterised by peakpotential differences of Epa − Epc = ΔEp = 59 mV (pa = peakanodic, pc = peak cathodic) which is independent of the scanrate18,35–38 (Table 3). In general, porphyrins exhibit two

Scheme 1 Synthesis of ferrocenyl porphyrins (4a–4c).

Fig. 1 Single crystal X-ray structure of compound 4b. (a) ORTEP view of 4b. Thermal ellipsoids are plotted at the 50% level. (b) Side view.

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oxidation and two reduction waves corresponding to the for-mation of monocation, dication, and monoanion, dianions ofthe porphyrin ring in the solvent window, that dichloro-methane allows. In ferrocenyl porphyrins, an additional revers-ible oxidation wave corresponding to the oxidation offerrocene ring, is also anticipated.21,30b

The cyclic voltammograms of porphyrins 4a–4c exhibit tworing centred reduction waves labelled as 4, 5 and two oxidation

waves labelled as 2, 1 and ferrocenyl oxidation wave 3, withinthe potential window that dichloromethane allows.

The ferrocene based reversible oxidation wave labelled as 3for porphyrins 4a–4c was observed at Epa = 0.02–0.06 V vs. FcH/FcH+. In ferrocenyl porphyrins 4a–4c, the oxidation of the ferro-cenyl moiety became easier compared to the correspondingethynyl derivative 3aH–3cH, and this effect was most pro-nounced in the porphyrin 4a, where ferrocene is directlyattached to porphyrin by an acetylene bond.

The second and third reversible oxidation waves, labelled as2 and 1, originates from the oxidation of the porphyrin core,

Table 1 Crystal data and structure refinement parameter for compound 4b

Compound 4b

Empirical formula C152H115Fe2N8Formula weight 2165.22Temperature (K) 150(2)Wavelength (A) 0.71073Crystal system TriclinicSpace group P1̄a (Å) a = 15.9593(6)α (°) 67.580(3)b (Å) b = 18.1543(6)β (°) 85.727(2)c (Å) c = 21.9026(6)γ (°) 76.999(3)Volume (Å3) 5715.4(3)Z 2Calculated density (Mg m−3) 1.258Absorption coefficient (mm−1) 0.313F(000) 2270Crystal size (mm) 0.31 × 0.28 × 0.21θ range for data collection (°) 2.99 to 25.00Limiting indices −18 ≤ h ≤ 18, −21 ≤ k ≤ 20,

−25 ≤ l ≤ 26Total reflections measured 41 835/20 074 [R(int) = 0.0418]Completeness to θmax θ = 25.00; 99.8%Absorption correction Semi-empirical from equivalentsMax. and min. transmission 0.9371 and 0.9091Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 20 074/0/1479Goodness-of-fit on F2 1.024Final R indices [I > 2σ(I)] R1 = 0.0615, wR2 = 0.1431R indices (all data) R1 = 0.1051, wR2 = 0.1741Largest diff. peak and hole (e Å−3) 0.566 and −0.454

Table 2 Selected bond lengtha of intermolecular interactions in the crystalstructure (4b)

C–H–π interactions Bond length (Å)

C305–H305⋯C60, C61, C58, C62, C59 2.816 (3)C506–H506⋯C123, C122, C121, C120, C124 2.760 (9)C9–H9⋯C119, C118, C117, C116, C115 3.036 (17)C9–H9⋯C82, C81, C80, C79, C84, C83 3.480 (6)C90–H90⋯C9, C10, C11, C6, C7, C8 2.935 (10)C3–H3⋯C89, C90, C91, C92, C88, C87 3.589 (12)C95–H95⋯C48, C47, C51, C49, C50, C52 3.688 (11)C44–H44⋯C101, C102, C103, C98, C99, C100 3.557 (8)C106–H106⋯C41, C36, C37, C39, C40, C38 3.523 (5)C28–H28⋯C70, C71, C72, C73, C68, C69 2.698 (2)C65–H65⋯C28, C27, C26, C25, C30, C29 3.355 (6)C71–H71⋯C18, C17, C22, C21, C20, C19 3.411 (5)C33–H33⋯C113, C112, C111, C110, C109, C114 3.442 (8)C112–H112⋯C36, C40, C38, C41, C37, C39 2.843 (11)C71–H71⋯C54, C53, C55, C56, C57 3.342 (3)

a All bond lengths have been measured from the centroid and thelabelling of the carbon atom corresponds to the centroid.

Fig. 2 The supramolecular structure of compound 4b along the b axis. The sec-ondary interactions are shown by dashed lines.

Table 3 Electrochemical datac of ferrocenyl–porphyrins 4a–4c

Compound Wave E° (V) ΔEp (mV) ipc/ipa ΔE1/2 b (mV)

Ferrocene 0.00 80 0.923aH 0.14 70 0.943bH 0.05 68 0.95a

3cH 0.05 61 0.964a 3 0.06 57 0.94a 80

2 0.58 61 0.731 0.79 58 0.944 −1.57 70 0.70a

5 −1.86 68 0.89a

4b 3 0.02 59 0.89 302 0.53 62 0.741 0.77 60 0.804 −1.58 67 0.91a

5 −1.82 67 0.984c 3 0.03 60 0.91 20

2 0.53 70 0.701 0.77 58 0.784 −1.59 74 0.985 −1.83 64 0.97

a ipa/ipc.bΔE1/2 = ΔE1/2 3aH −ΔE1/2 4a, ΔE1/2 = ΔE1/2 3bH −ΔE1/2 4b,

ΔE1/2 = ΔE1/2 3cH −ΔE1/2 4c. c Cyclic voltammetry data (potentials vs.FcH/FcH+, scan rate = 100 mV s−1) of 1.0 × 10−4 M solutions ofanalytes in CH2Cl2 solutions containing a 0.1 M solution of Bu4NPF6at 25 °C.

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and was observed at Epa = 0.53–0.58 V and Epa = 0.77–0.79 V vs.FcH/FcH+ for porphyrins 4a–4c. The reduction potential ofporphyrins 4a–4c were shifted to less negative, compared toH2TPP, indicating that the porphyrin ring in ferrocenyl por-phyrins is easy to reduce compared to unsubstituted porphyr-ins (H2TPP)

25,27 (Table 3). All electron transfer processes wereelectrochemically reversible. The reversibility was alsoobserved with peak current ratios close to 1 for all processes,and the deviation from 1 is the result of baseline uncertaintydue to the onset of solvent decomposition at these lowpotentials.18

Electronic absorption and emission spectroscopy

The electronic absorption and emission spectra of the por-phyrins 4a–4c were recorded in toluene. The absorptionspectrum is shown in Fig. 4. The porphyrins 4a–4c showcharacteristic intense Soret bands at 425 nm, 426 nm, and427 nm respectively, and four Q-bands in the region

515–658 nm. The substitution of the ferrocenyl group on theporphyrin results in red shift of the Soret band and theQ-bands. The Soret band of porphyrins 4a–4c is red shifted by7 nm, 8 nm and 9 nm compared to H2TPP. The red shiftobserved in both the Soret band and the Q-band is due toenhanced π-conjugation. The protonation of the porphyrins4a–4c using trifluoroacetic acid (TFA) leads to red shift of theSoret band by approximately 20 nm, and the Q-bands disap-peared (see ESI† for details).

The emission properties of the β-substituted ferrocenyl por-phyrins were explored by steady state fluorescence techniques.The emission spectrum is shown in Fig. 5. The ferrocenyl por-phyrins 4a–4c show a substantial decrease in the fluorescenceintensity and the quantum yield compared to H2TPP, whichindicate that there is fast non radiative deactivation of theexcited state with intra molecular charge-transfer.30–32 The por-phyrin 4a exhibits non-emissive behaviour. The Fe(II)

Fig. 3 Cyclic voltammograms of (1.0 × 10−4 M) solutions of (a) 3bH, (b) 4b oxidation, and (c) 4b reduction, in CH2Cl2 containing 0.1 M Bu4NPF6 as the supportingelectrolyte, recorded at a scan speed of 100 mV s−1.

Fig. 4 Electronic absorption spectra of the porphyrins 4a–4c recorded intoluene. The concentration used was 1.0 × 10−6 M.

Fig. 5 Emission spectra of compounds 1, 4a–4c at 1.0 × 10−6 M concentration,the excitation wavelength was 418 nm for 1, 425 nm for 4a, 426 nm for 4b,and 427 nm for 4c in toluene.

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completely quenches the signal in porphyrin 4a, which con-firms the strong electronic communication between the ferro-cenyl moiety and porphyrin, whereas in porphyrins 4b and 4ccommunication was so poor that Fe(II) did not quench thesignal completely. Thus porphyrins 4b and 4c show two emis-sion bands located at 662, 728, and 660, 726 nm respectively.In porphyrin 4c, the meta substituted ferrocene disrupts theextended π-conjugation. Therefore the electronic communi-cation between ferrocene and porphyrin was less than in 4b.Thus, porphyrin 4c shows higher intensity fluorescence bandscompared to 4b. Compared to 1, porphyrins 4b and 4c alsoshow significantly reduced fluorescence quantum yields of0.01 and 0.06 respectively, while porphyrin 4a was completelynon-fluorescent. This further confirms the weaker electroniccommunication between porphyrin cores and redox active sidegroups, here ferrocenyl, in 4b and 4c compared to 4a as aresult of the large spacer between them. We conclude from theabove described electrochemical and fluorescence studies thatthe electronic communication between two particular centresmay be tuned by the introduction of differently sized spacergroups between them (Table 4).34

Conclusions

In summary we have synthesized a series of β-substituted ferro-cenyl porphyrins by the Pd catalyzed Sonogashira cross-coupling reaction. The photophysical and electrochemicalproperties of these porphyrins show considerable electronicinteraction between the porphyrin core and ferrocenyl sidegroups. This electronic communication can be tuned by theintroduction of a spacer group. The study of NLO properties ofthese molecules is currently ongoing in our group.

Experimental sectionGeneral methods

Chemicals were used as received unless otherwise indicated.All oxygen or moisture sensitive reactions were performed

under an argon atmosphere using the standard Schlenkmethod. Triethylamine (TEA) was received from a commercialsource, and distilled on KOH prior to use. 1H (400 MHz)spectra were recorded on a Bruker Avance (III) 400 MHz, usingCDCl3 as the solvent. Chemical shifts in 1H were reported inparts per million (ppm) with TMS (0 ppm) as the standard.The UV-vis absorption spectra of all compounds were recordedin toluene on a Carry-100 Bio UV-Visible Spectrophotometer.The fluorescence spectra of all compounds were recorded on aHoriba Jobin Yvon Floromax 4P spectrophotometer. HRMSwas recorded on a Brucker-Daltonics, micro TOF-Q II massspectrometer.

Cyclic voltammetry. Cyclic voltammograms (CVs) wererecorded on a CHI62OD electrochemical analyzer using a stan-dard three-electrode cell with glassy carbon as the workingelectrode. A 3 mm diameter glassy carbon working electrodefrom CH Instruments (CHI 104) was used. The electrode waspolished with two different Alpha alumina powder (1.0 and0.3 micron from CH Instruments) suspended in distilled wateron a Microcloth polishing pad; at the end of polishing, theelectrodes were thoroughly rinsed with distilled water. A plati-num wire was used as the counter electrode and saturatedcalomel as the reference electrode. The scan rate was 100mV s−1. A solution of tetrabutylammoniumhexafluorophos-phate (Bu4NPF6) in CH2Cl2 (0.1 M) was employed as the sup-porting electrolyte. CH2Cl2 was freshly distilled from CaH2

prior to use. All potentials were experimentally referencedagainst the saturated calomel electrode couple but were thenmanipulated to be referenced against Fc/Fc+ as recommendedby IUPAC.39 Under our conditions, the Fc/Fc+ couple exhibitedipc/ipa = 0.92, ΔEp = 80 mV and E° = 0.38 V versus SCE.

Fluorescence quantum yield. The fluorescence quantumyields (ΦF) of compounds 4a–4c were calculated (eqn (1))by the steady-state comparative method using H2TPP as astandard (Φst = 0.11).21,40

ΦF ¼ Φst � Su=Sst � Ast=Au � n 2Du=n2Dst … ð1Þwhere ΦF is the emission quantum yield of the sample, Φst isthe emission quantum yield of the standard, Ast and Au rep-resent the absorbance of the standard and the sample at theexcitation wavelength, respectively, while Sst and Su arethe integrated emission band areas of the standard andthe sample, respectively, and nDst and nDu are the solventrefractive index of the standard and the sample, and u and strefer to the unknown and the standard, respectively.

Single crystal X-ray diffraction studies

Single crystal X-ray structural studies of 4b were performed ona CCD Agilent Technologies (Oxford Diffraction) SUPER NOVAdiffractometer. Data were collected at 293(2) K using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å). The strategyfor the data collection was evaluated by using CrysAlisPro CCDsoftware. The data were collected by the standard phi–omegascan techniques, and were scaled and reduced usingCrysAlisPro RED software. The structures were solved by direct

Table 4 Absorption and emission data of ferrocenyl porphyrins (4a–4c)

Compound

λabsa (nm)

λem(nm) Φf

cSoretband

εb

(M−1 cm−1) Q-bands

1 418 — 515, 551,589, 647

650, 716 0.11

4a 425 1 190 000 520, 564,603, 658

— —

4b 426 650 000 523, 562,601, 657

662, 728 0.01

4c 427 910 000 522, 557,600, 656

660, 726 0.06

aMeasured in toluene. λabs (nm): absorption maximum of the Soretband. b ε, extinction coefficient. cDetermined by using H2TPP as astandard (Φst = 0.11).28

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methods using SHELXS-97, and refined by full matrix least-squares with SHELXL-97, refining on F2.1 The positions of allthe atoms were obtained by direct methods. All non-hydrogenatoms were refined anisotropically. The remaining hydrogenatoms were placed in geometrically constrained positions, andrefined with isotropic temperature factors, generally 1.2Ueq

of their parent atoms. The crystal and refinement data aresummarized in Table 1. The CCDC number 903777 containsthe supplementary crystallographic data for 4b.

Synthesis and characterization

The tetraphenylporphyrin (H2TPP) 1 was synthesized by follow-ing the earlier reported procedure.12 β-Bromotetraphenyl por-phyrin 2 was prepared using a procedure previously reported.9

The reactant 3aH was purchased from Sigma-Aldrich, andthe compounds 3bH–3cH were synthesized via diazotization of4-ethynylaniline, and 3-ethynylaniline respectively, accordingto known methods.33

General procedure for the preparation of compounds 4a–4c

A solution of 2-bromo-5,10,15,20-tetraphenylporphyrin(260 mg, 0.37 mmol) and the corresponding ethynyl ferrocene(70 mg, 0.54 mmol) in toluene–triethylamine 5 : 1 (60 mL) wasdeareated for 30 min with argon bubbling and then Pd(dba)2(40 mg, 0.07 mmol) and AsPh3 (170 mg, 0.55 mmol) wereadded. The solution was deareated for a further 5 min; afterthat, the reaction mixture was left under argon at 50 °C. Aftercompletion of the reaction, the mixture was cooled to roomtemperature and the solvent was evaporated. The crudeproduct was dissolved in dichloromethane and washed severaltimes with water. The organic solution was dried over anhy-drous Na2SO4 and the solvent evaporated under vacuum. Theproduct was purified by column chromatography on silica geleluting with CH2Cl2–hexane. Rf value = 0.5 (DCM–hexane 1 : 1).The first band eluted by hexane was a dimer of ethynyl ferro-cene. The second band eluted by DCM–hexane in a ratio of20 : 90 was bromo tetraphenyl porphyrin and the third bandeluted by DCM–hexane in a ratio of 30 : 70 was the desired por-phyrins 4a–4c which was further recrystallized from dichloro-methane/methanol to give compounds 4a–4c in 40–60% yield.

Synthesis of compound 4a: purple solid (168 mg, 50%),Rf value = 0.5 (DCM–hexane 1 : 1), eluted as the third bandfrom the column with CH2Cl2–hexane (20 : 70). 1H NMR(400 MHz, CDCl3): δ (ppm) 9.01 (s, 1H), 8.85 (m, 2H), 8.81 (d,1H, J = 5.03 Hz), 8.77 (s, 2H), 8.70 (d, 1H, J = 5.17 Hz), 8.21 (m,8H), 7.78 (m, 12H), 4.36 (s, 2H), 4.25 (s, 7H), −2.68 (s, 2H).HRMS (ESI) m/z, calcd for MH+ (C56H38Fe4N4): 823.2520;found: 823.2503. UV-vis (toluene): λmax: 425, 520, 564, 603,658 nm.

Synthesis of compound 4b: reddish purple solid (202 mg,60%). Rf value = 0.5 (DCM–hexane 1 : 1), eluted as the thirdband from the column with CH2Cl2–hexane (30 : 70). 1H NMR(400 MHz, CDCl3): δ (ppm) 9.07 (s, 1H), 8.87 (s, 2H), 8.82 (d,1H, J = 5.11 Hz), 8.77 (m, 2H), 8.74 (d, 1H, J = 5.11 Hz), 8.22(m, 8H), 7.77 (m, 12H), 7.43 (d, 2H, J = 8.22 Hz), 7.28 (d, 2H),4.70 (m, 2H), 4.37 (m, 2H), 4.08 (s, 5H), −2.66 (s, 2H). HRMS

(ESI) m/z, calculated for MH+ (C62H42FeN4): 899.2823; found:899.2900. UV-vis (toluene): λmax: 426, 523, 562, 601, 657 nm.

Synthesis of compound 4c: reddish purple solid (134 mg,40%). Rf value = 0.5 (DCM–hexane 1 : 1), eluted as the thirdband from the column with CH2Cl2–hexane (20 : 70). 1H NMR(400 MHz, CDCl3): δ (ppm) 9.10 (s, 1H), 8.87 (s, 2H), 8.82 (d,1H, J = 5.11 Hz), 8.76 (m, 3H), 8.22 (m, 8H), 7.75 (m, 13H),7.44 (d, 2H, J = 10.45), 7.19 (d, 1H, J = 7.58 Hz), 4.74 (m, 2H),4.39 (m, 2H), 4.11 (s, 5H), −2.66 (s, 2H). HRMS (ESI) m/z, calcu-lated for MH+ (C62H42FeN4): 899.2833; found: 898.2823. UV-vis(toluene): λmax: 427, 522, 557, 600, 656 nm.

Acknowledgements

This work was supported by DST and CSIR, Govt. of India, NewDelhi. We gratefully acknowledge Sophisticated Instrumenta-tion Centre (SIC), IIT Indore.

Notes and references

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Paper Dalton Transactions

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