Dibutylquaterthiophene as Studied by in Situ ESR UV−Vis NIR

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Charged States of r,ω-Dicyano β,β′-Dibutylquaterthiophene as Studied by in Situ ESR UV-Vis NIR Spectroelectrochemistry Kinga Haubner,† Ja´n Tara´bek,†,‡ Frank Ziegs,† Vladimı´r Lukesˇ,§ Evelin Jaehne,| and Lothar Dunsch*,† Center of Spectroelectrochemistry, Department of Electrochemistry and Conducting Polymers, Leibniz Institute for Solid State and Materials Research, D-01069 Dresden, Germany, Institute of Organic Chemistry and Biochemistry, AS CR, V.V.i., FlemingoVo na´mestı´ 2, CZ-166 10 Prague, Czech Republic, Institute of Physical Chemistry and Chemical Physics, SloVak UniVersity of Technology BratislaVa, Radlinske´ho 9, SK-812 37 BratislaVa, SloVak Republic, and Macromolecular Chemistry, Chemistry Department, UniVersity of Technology Dresden, D-01069 Dresden, Germany ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: September 9, 2010

The influence of the molecular structure on the stabilization of charged states was studied in detail by in situ ESR UV-vis NIR spectroelectrochemistry at a novel R,ω-dicyano substituted β,β′-dibutylquaterthiophene (DCNDBQT) and the electrochemically generated cation and anion radicals have been proved for the first time. The voltammetry of DCNDBQT results in two separate oxidation steps with the reversible first one. The experimental absorption maxima at 646 and 1052 nm together with the calculated ones (by DFT method) as well as an ESR signal at the first anodic step prove the presence of a radical cation. Three additional optical bands (554, 906, and 1294 nm for CT-transition) can be attributed to the formation of cation radical dimer. The dicationic structure formed in the second oxidation step is not stable. The stabilization proceeds via a dimer formation in two chemical follow-up reactions. The existence of the dimeric structures was proved by ex situ MALDI TOF mass spectrometry. As the substitution by cyano groups opens the route to cathodic reductions, DCNDBQT shows a single quasi-reversible reduction step. Here, the in situ ESR UV-vis NIR spectroelectrochemical measurements and theoretical calculations let us confirm the electrochemical generation of an anion radical. As we found a low number of anion radicals by quantitative ESR spectroelectrochemistry and an appearance of additional bands in the UV-vis NIR absorption spectra, the formation of dimeric structures must be considered and was corroborated by mass spectrometry. The role of dimerization in the reaction mechanism of the DCNDBQT oxidation and reduction are discussed in general. The experimental results were interpreted using the quantum chemical calculations based on density functional theory. Introduction Oligothiophenes represent an attractive material for organic electronics due to their electronic tuning by structural variation.1,2 Mechanical, electronic, optical, and redox properties often change significantly with molecular design, which could be differentiated by changing the type, number, and position of substituents and the number of monomers in oligothiophens. Many studies were devoted to the preparation and characterization of oligothiophenes with electron donating substituents.1,2 Addition of electron-withdrawing groups or their combination with donating substituents give rise to new materials with unique properties.3,4 In addition, electron acceptor groups (e.g., cyano) promote the appearance of the reduction processes.5,6 The number and position of substituents play an important role in the formation of radical ions or dimers. It is known that short unsubstituted oligothiophenes form σ-dimers during the oxidation process.7,8 The introduction of substituents in R,ωposition improves the stability of oligothiophenes. In the 1990s the high stability of R,ω-cyano-endcapped oligothiophenes in charged states has been demonstrated by cyclic voltammetry.5 * To whom correspondence should be addressed. E-mail: L.Dunsch@ ifw-dresden.de. Fax: (+)49 351 4659 811. † Leibniz Institute for Solid State and Materials Research. ‡ Institute of Organic Chemistry and Biochemistry. § Slovak University of Technology. | University of Technology Dresden.

The understanding of redox processes of substituted oligothiophenes is not possible by voltammetric studies only. In situ spectroelectrochemistry provides a more precise way to characterize the structure of intermediates and final products in electrochemical reactions.9 The combination of UV-vis NIR and ESR spectroelectrochemistry opens a wide range to detailed studies of structures such as π and σ-dimers.10-17 Thus, reversible dimerization of end-capped oligothiophenes has been reported, and the formation π-dimers from R,ω-endcapped terthiophene was demonstrated based on UV-vis spectra at room temperature.12 Additionally it was shown that the substitution at the thiophene rings in the β-position inhibits a reversible π-dimerization.13,14 The formation of stable monoand dications from R- and R,ω-diphenylamino substituted oligothiophenes and a reversible dimerization in β-position have been shown by our group.15 The stability of monocation radicals of R-end-capped oligothiophenes has been shown to increase with length of thiophene chain.16 In a recent work we used in situ NMR spectroelectrochemistry of DCNDBQT to demonstrate the formation of a π-dimer via the cation generated in the first electron transfer as well as the formation of a σ-dimer by a follow-up reaction of the dication generated in the second oxidation step.18 As the detection and characterization of paramagnetic structures (such as cation and anion radicals) is not possible by in situ NMR spectroelectrochemistry, we applied in situ ESR UV-vis NIR spectroelec-

10.1021/jp106625m  2010 American Chemical Society Published on Web 10/08/2010

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Figure 1. Cyclic (a, b, d) and square-wave (c) voltammogramms of DCNDBQT (c ) 0.0005 M) in 0.1 M TBAPF6/CH3CN electrolyte solution (red line, first oxidation process), scan rate: (a,c) 0.05 V/s; (b,d) 100 V/s.

trochemistry as well as MALDI TOF mass spectrometry to get the structure and stability of primary products in charge transfer reactions at this oligothiophene. In the present work the electrochemically generated cation and anion radicals of the specially synthesized oligomer are characterized by cyclic voltammetry and in situ ESR/Vis-NIR spectroelectrochemistry as well as MALDI TOF mass spectrometry for the first time. The introduction of strong electron withdrawing cyano groups at the end-cap (R,ω-position) ensures, in general, the stability of the charged states. The combination of such kind of stabilization and the presence of alkyl chain in β-position, providing the structure solubility, promote the unique properties of this material. Organic devices based on semiconducting soluble materials are easy to process, and allow large area coverage and low cost production.1,2,19 Only the combination of techniques such as voltammetry at different scan rates, in situ ESR/Vis-NIR spectroelectrochemistry and MALDI TOF mass spectrometry can provide detailed insight into the structural changes of paramagnetic and diamagnetic molecules resulting in a electrochemical reaction mechanism on the both the cathodic as well as anodic site. To our knowledge, the existence of the trimeric structures of R,ω-endcapped oligothiophenes were proved by such a combination of

spectroelectrochemical techniques for the first time. Additionally, the electronic structure of the oligothiophene in the neutral and its charged states are described theoretically using density functional theory (DFT). The correlation of the experimental and calculated ESR isotropic hyperfine splitting constants as well as optical transitions are used to confirm the changes in electronic states upon charging. Experimental Section Commercially available acetonitrile (ACN, puriss.; absolute; w(H2O)e0.001%) (Aldrich), toluene (Aldrich), tetrahydrofuran (THF) from Acros, decamethylferrocene (p.a., g98.0%) purchased from Merck and 2,5-dihydroxy benzoic acid (2,5-DHB) from Aldrich were used without further purification. Tetrabutylammoniumhexafluorophosphate (TBAPF6) of puriss. quality (Fluka) was dried under reduced pressure at 70 °C for 24 h and stored in a glovebox. Cyclic voltammetry (CV) and SWV was carried out in acetonitrile with 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6) as supporting electrolyte using a one-compartment electrochemical cell with a platinum wire as working and counter electrodes and a chloride-covered Ag wire as a quasi-

Charged States of DCNDBQT

Figure 2. UV-vis NIR spectrolectrochemsitry at the first (a) oxidation and (b) reduction step of DCNDBQT (c ) 0.0001 M) in 0.1 M TBAPF6/ CH3CN electrolyte solution. Spectra were measured relative to the first spectrum at starting potential during the voltammetric scan.

reference electrode. DCNDBQT was dissolved at a concentration of 5 × 10-4 mol/L in acetonitrile. All electrochemical measurements were performed under inert nitrogen atmosphere in a glovebox. Data were recorded on an EG&G PAR 273A computer-controlled potentiostat. Decamethylferrocene was used as an internal reference. Cyclic voltammetry with scan rates higher than 50 V/s was recorded using an Autolab electrochemical analyzer equipped with a PGSTAT 100 potentiostat module and a fast analog scan generator (SCAN 250) in combination with a fast AD converter (ADC10M). The working electrode area was reduced to 0.25 mm2. The ESR spectra were recorded using an EMX X-Band ESR spectrometer (Bruker, Germany) and the optical spectra were measured by using a UV-vis NIR spectrometer system TIDAS (J&M, Aalen, Germany). Both the ESR spectrometer and UV-vis NIR spectrometer were triggered by a HEKA potentiostat PG 390, and the triggering was performed using the software package Potmaster v2 × 43 (HEKA Elektronik, Germany). The electronic ground state geometries of the selected molecules were optimized at the DFT level of theory employing Becke’s three parameter hybrid functional using the Lee, Yang, and Parr correlation functional for Gaussian 03 (B3LYP).20 The calculations of optimal geometries were performed using the 6-31G(d) basis set for C, N, and H atoms and the 6-31+G(d) basis set for S atoms21 (the energy cutoff of 4 × 10-3 kJ mol-1

J. Phys. Chem. A, Vol. 114, No. 43, 2010 11547 and the final root-mean-square energy gradient below 0.04 kJ mol-1 Å-1). The obtained optimal structures were confirmed by normal-mode analysis (no imaginary frequencies for all optimal geometries). On the basis of optimized geometries, the vertical transition energies and oscillator strengths between the initial and final states were computed by time dependent (TD)DFT method.22 The isotropic hyperfine splitting ESR constants were calculated using the EPR-III basis set,23 which was optimized for magnetic properties of radical species. The numerical integration of the DFT functional was performed using default fine integration grid. All calculations were done using the Gaussian 03 program package.24 For in situ ESR UV-vis NIR electrochemical experiments, special flat cell25 (Quarzglastechnik Bad Harzburg, Germany) and a laminated platinum mesh as the working electrode, a PTFE insulated silver wire chloride-covered at the open end as the quasi-reference electrode, and a platinum wire as the counter electrode were used. DCNDBQT was dissolved at a concentration 0.0001 mol/L in acetonitrile with 0.1 M TBAPF6 as supporting electrolyte and used in the in situ spectroelectrochemical measurements. MALDI-TOF measurements were carried out using a Bruker BIFLEX II spectrometer (Bruker Daltonic, Germany) equipped with a 337 nm nitrogen laser. The MALDI-TOF-spectra were measured in negative reflective mode. Samples were prepared by the sandwich method. First, a layer of matrix solution 2,5DHB in THF was added. After evaporation of the solvent, a second layer of oligothiophene solution in toluene was added, and then the last, third, layer as a further matrix was loaded on the target. The solvent was removed by evaporation in air, and the sample was transferred to the mass spectrometer for analysis. Mass spectra were calibrated by a fullerene mixture in a sulfur matrix. 3,3′′′-dibutyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DBQT) was synthesized from 3-butylsubstituted-2-bromthiophenes by Grignard reaction followed by a coupling with commercially available dithienyldibromide. Resulting DBQT was brominated by NBS to form the corresponding 5,5′′′-dibromo-3,3′′′-dibutyl2,2′:5′,2′′:5′′,2′′′-quaterthiophene (DBrDBQT).26 The DCNDBQT was prepared by reaction of DBrDBQT with copper(I) cyanide.27 Results and Discussion By cyclic voltammetry (Figure 1) it is shown that DCNDBQT undergoes a two-step oxidation (the first reversible and the irreversible second one) and a one-step reduction. Square-wave and fast scan voltammetry (Figure 1, panels b and c) indicate the presence of chemical follow-up reactions after the second oxidation step. The identification of products of these reactions was done by MALDI TOF mass spectrometry. The appearance of a mass peak at m/z ) 984 (dimer) in addition to that of m/z ) 492 (monomer) in both anodic (at the second oxidation step) and cathodic (at the first reduction step) scans manifests the formation of dimeric structures, although the voltammetry suggests the reversibility of the first reduction step (Figure 1, panels a, c, and d). Moreover, structures with higher m/z ratio (namely with 1476) pointing to the formation of trimers after the first reduction step were detected as well. Taking into account the in situ NMR-spectroelectrochemical analysis18 as well as DTF-calculated dimeric structure (Figure 5d), the presence of σ-bonds in dimeric and trimeric structures (between carbon atoms in position 9 and carbon atoms in position 15, see Figure 5c) was proved. We have a σ-dimer formation in spite of the fact that this thiophene oligomer is end-capped by

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Figure 3. Experimental and calculated (for details see Experimental section) spectra of the radical cation and radical anion of DCNDBQT (the same experimental conditions as for Figure 2) recorded at early stages of (a) oxidation (1.154 V vs DmFc/DmFc+ redox couple) and (b) reduction (-1.613 V vs DmFc/DmFc+ redox couple) of DCNDBQT.

SCHEME 1: Mechanism of the DCNDBQT Oxidationa cyano groups, thus the dimerization or trimerization is not possible at R- and ω-positions. Therefore, additional spectroelectrochemical techniques were used to complete the electrochemical redox mechanism of DCNDBQT in detail. As mentioned, the addition of electron-withdrawing (cyano-) groups promotes the stabilization of redox process and therefore the formation of anion radicals can be observed. In the cathodic scan one quasi-reversible reduction step at -1.59 V (Figure 1, panels a, c, and d) is detected, causing the formation dimeric and trimeric structures (Scheme 2) as shown by mass spectrometry (see Supporting Information). In the anodic voltammetric scans the first oxidation step at 1.15 V is pointing to the formation of a cation radical. The peakto-peak separation of 70 mV confirms the reversible behavior (Figure 1, panels a and c). However, under further oxidation (to the second charge transfer) both steps appear to be irreversible what as observed in square-wave voltammetry (Figure 1c) and cyclic voltammetry with increasing scan rate (in the range of 50-1000 V/s) where both steps become reversible (Figure 1b). Therefore, the irreversible second oxidation step is caused by chemical follow-up reactions to the electron transfer. To corroborate the formation of radical ions and dimeric structures, the in situ ESR and vis-NIR spectroelectrochemical analysis was applied. Figure 2 shows the UV-vis NIR spectra recorded simultaneously during the first charge transfer in anodic (Figure 2a) and cathodic (Figure 2b) direction. Due to the formation of dimeric structures (π- and σ-dimers) as proved by mass spectrometry and NMR spectroelectrochemistry,18 we tried to get the unchanged absorption features of the cation and anion radical in spectra of DCNDBQT at very early stages of the first oxidation and reduction steps, respectively, and to compare them with the calculated ones (Figure 3). The two bands at 646 and 1050 nm for DCNDBQT+• are assigned to SOMO-LUMO and HOMO-SOMO transitions, respectively (Figure 3a). For DCNDBQT-• the bands at 752 and 1506 nm can be assigned to HOMO-SOMO and SOMO-LUMO transitions, respec-

a Subscripts π and σ denote the formation of “pi” and “sigma” dimer, resp.

tively (Figure 3b). In addition to the characteristic bands of the radical ions, new features arise (indicated by arrows in Figure 2) at higher potentials, already in the potential range of the first oxidation or reduction step. As the mass spectrometry and in situ NMR spectroelectrochemistry18 provided the direct evidence of π- and σ-dimer formation at both the anodic and cathodic potential range at the first redox step (Schemes 1 and 2), theoretical calculations (not shown) and a comparison to previous studies12-14 let us conclude that the following bands to the dimer of two cation radicals (Figure 2a): 554 and 906 nm (1294 nm for CT-transition); and to the dimer of two anion radicals (Figure 2b): 492 and 908 nm (1260 nm for CTtransition). The optical spectra of the π-dimer present two optical π-π* transition blue-shifted in relation to the optical bands of the corresponding radical ion and the third band at longer wavelengths assigned charge transfer between the rings of the dimer.12,13,28 Direct evidence for the formation of radical species of DCNDBQT was supplied by the ESR spectroelectrochemical

Charged States of DCNDBQT SCHEME 2: Mechanism of the DCNDBQT Reductiona

a Subscript σ denotes the formation of sigma bonds. (Reduction to dianions is not considered due to experimental reasons).

Figure 4. ESR spectra obtained by the accumulation of 20 spectra during the potentiostatic (potentials vs DmFc/DmFc+ redox couple are shown) electrolysis in (a) anodic and (b) cathodic direction. Spectra were accumulated at starting potential (where no radical formation appears), at the potential of the first oxidation (reduction) step, and after rereduction (reoxidation) of the electrolyzed solution of DCNDBQT in 0.1 M TBAPF6/CH3CN.

measurements. The experimental ESR spectrum of the cation radical is given in Figure 4a, and Figure 4b shows the ESR spectrum of the anion radical. These spectra were recorded under potentiostatic conditions and an accumulation of 20 individual spectra was used to enhance the signal/noise ratio. Potential dependent measurements of the ESR signal point to the reversible formation of radicals. The ESR spectra measured under these conditions are the same as those resulting form the accumulations under potentiostatic conditions. The experimentally determined g-factors of DCNDBQT+• (2.0025) and DCNDBQT-• (2.0042) differ significantly. The first value is close to the g-factor of the free electron (2.0023). This g-factor of the

J. Phys. Chem. A, Vol. 114, No. 43, 2010 11549 DCNDBQT+• indicates the predominant contribution of carbon atom orbitals to the SOMO of the cation radical and thus reflecting the spin density involving primarily on the hydrocarbon backbone with the slight contribution of nitrogen atoms as underlined by the simulation of DCNDBQT+• ESR spectrum (Figure 5a, see the following hyperfine splitting constants) as well as by the representation of SOMO (Figure 6a). For the radical cation the following hyperfine splitting constants were obtained by ESR spectra simulations and DFT calculations: a(2H23,26) ) 0.2752 mT (0.1778 mT), a(2H24,25) ) 0.2056 mT (0.1549 mT), a(2H32,35) ) 0.0905 mT (0.0857 mT), a(2H33,36) ) 0.1959 mT (0.1121 mT), a(2H22,28) ) 0.0172 mT (0.0151 mT), and a(2N55,56) ) 0.0321 mT (0.0233 mT). The constants in parentheses are hyperfine splitting constants calculated by B3LYP/EPR-III method, and the numbering of atoms (in subscript) is shown in Figure 5c. On the other hand the higher g-factor of DCNDBQT-• points to a higher portion of sulfur atoms to SOMO of anion radical29 (see also Figure 6b) as compared to the cation radical. The hyperfine splitting constants obtained by DCNDBQT-• ESR spectrum simulations (see Figure 5b) and DFT calculations are the following: a(2H23,26) ) 0.1308 mT (0.0928 mT), a(2H24,25) ) 0.0222 mT (0.0172 mT), a(2H32,35) ) 0.0429 mT (0.0395 mT), a(2H33,36) ) 0.1659 mT (0.1023 mT), a(2H22,28) ) 0.0014 mT, and a(2N55,56) ) 0.0560 mT (0.0402 mT). Similarly to the cation radical, the constants in parentheses are hyperfine splitting constants calculated by B3LYP/EPR-III method, and the numbering of atoms (in subscript) are shown in Figure 5c. The values of the theoretical hyperfine splitting constants and the experimental ones obtained by the simulation of ESR spectra are proportional and the regression coefficients (0.98 for cation radical and 0.99 for anion radical) shows good consistency. Such an accordance of the hyperfine splitting constants calculated by DFT method and the experimental ones obtained by simulation demonstrates the unambiguous electronic state of radicals formed during the first oxidation and reduction step. As shown in Figure 6, the spin density is more localized in the center of the molecule backbone in both cases. The higher splitting constants for the cation radical in comparison with that of the anion radical do not indicate the higher spin density on carbon backbone for DCNDBQT+• than for DCNDBQT-• due to almost the same spin distribution for both radical ions (Figure 6). Such a phenomenon is rather explained by a different charge distribution as shown by Colpa et al.30 To describe the π- and σ-dimerization reaction at the first redox step in both the cathodic and anodic potential region quantitative ESR analysis of the redox processes was done. Referring the charge transferred to the spin number of the cation radical of DCNDBQT, a ratio charge/spin of 2.1 is found, that is, less than 50% of of the charge are used in the formation of a stable radical while the higher percentage is forming a diamagnetic structure obviously by dimerization. For the anion radical this ratio is even higher (3.0). The anion radical is therefore less stable at used experimental conditions and transformed into dimeric and trimeric structures or even polymers as shown by mass spectrometry and indicated by NMR spectroelectrochemistry18 (Scheme 2). The cation radical undergoes a reversible π-dimerization18 as given in Scheme 1. Therefore, quantitative ESR spectroelectrochemistry support the existence of follow-up reactions to diamagnetic structures for both the cation and the anion radical. Conclusion A novel R,ω-dicyano substituted β,β′-dibutylquaterthiophene (DCNDBQT) with the short alkyl side chains and improved

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Figure 5. Comparison of simulated and experimental ESR spectra of the (a) cation and (b) anion radical (see also Figure 4) (for parameters of the simulation see Results and Discussion). (c) Numbering of individual atoms as used in ESR simulations and calculation of hyperfine splitting constants by DFT. (d) B3LYP/6-31G (d) optimized structure of the neutral σ-dimer of DCNDBQT.

Figure 6. Single occupied molecular orbitals (SOMO) for the (a) cation and (b) anion radical as calculated by DFT method.

solubility was synthesized to study the influence of the molecular structure on the stabilization of charged states at short length oligomers. The redox processes of DCNDBQT were studied by voltammetry, in situ ESR UV-vis NIR spectroelectrochemistry, mass spectrometry, and quantum chemical calculations to follow both the paramagnetic and the diamagnetic species formed in redox reactions. The oxidation of DCNDBQT gives two separate redox steps with the first one being reversible. The UV-vis NIR absorption bands at 646 and 1052 nm and an ESR signal recorded at the first anodic step indicated the formation of a radical cation. From the optical data, NMR spectrolelectrochemistry18 and quantitative ESR analysis confirm a reversible π-dimerization of the radical. In the second oxidation step the formation of the σ-dimers18 by a follow-up reaction of the dication were observed (see Scheme 1). As substitution by cyano groups opens the route to cathodic reductions of thiophene structures DCNDBQT gave a single reduction step, which turned out to be quasi-reversible. By in situ ESR UV-vis NIR spectroelectrochemistry, the electrochemical generation of an anion radical was demonstrated. The anion radical of the quaterthiophene structure is not stable; therefore, further dimerization and trimerization stabilizes the charged structures (Scheme 2) as shown by ESR spectroscopy, NMR spectroelectrochemistry,18 and mass spectrometry as well. The experimental results were interpreted by quantum chemical calculations and show a good consistency of experiment and theory, which also strengthens the validity of our proposed reaction mechanisms.

Charged States of DCNDBQT To our knowledge this is the first detailed study of redox pathways of a thiophene oligomer that affords the study of both the radical cation and radical anion. The electronic structure of the species obtained experimentally are supported by quantum chemical calculations. Acknowledgment. For the financial support K. H. thanks the DFG GACR project 203/07/J067, J. T. the internal grant of the Institute of Organic Chemistry and Biochemistry (IOCB 820/ 82), and V. L. the Slovak Grant Agency VEGA (Project No 1/0774/08). Technical support by M. Senf and C. Malbrich (both IFW Dresden) is gratefully acknowledged. For the fruitful discussions on dimerization processes we thank Dr. Sabrina Klod (IFW Dresden). Supporting Information Available: Figure S1 presents MALDI-Tof mass spectra after electrolysis of DCNDBQT (c ) 0.0001 M) in 0.1 M TBAPF6/CH3CN electrolyte solution. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Mishra, A.; Ma, C.-Q.; Ba¨uerle, P. Chem. ReV. 2009, 109, 1141– 1276. (2) Facchetti, A. Mater. Today 2007, 10, 28–37. (3) Liu, Y.; Zhou, J.; Wan, X.; Chen, Y. Tetrahedron 2009, 65, 5209– 5215. (4) Ortiz, R. P.; Facchetti, A.; Marks, T. J.; Casado, J.; Zgierski, M. Z.; Kozaki, M.; Hernandez, V. J.; Navarrete, T. L. AdV. Funct. Mater. 2009, 19, 386–394. (5) Hapiot, P.; Demanze, F.; Yassar, A.; Garnier, F. J. Phys. Chem. 1996, 100, 397–8401. (6) Casado, J.; Ortiz, R. P.; Delgado, M. C. R.; Azumi, R.; Oakley, R. T.; Hernandez, V.; Navarrete, T. L. J. Phys. Chem. B 2005, 109, 10115– 10125. (7) Zotti, G.; Zecchin, S.; Vercelli, B.; Berlin, A.; Grimoldi, S.; Pasini, M. C.; Raposo, M. M. M. Chem. Mater. 2005, 17, 6492–6502. (8) Clarke, T. M.; Gordon, K. C.; Officer, D. L.; Grant, D. K. J.Phys.Chem. A 2005, 109, 1961–1973. (9) See for the different spectroscopic methods for example: (a) Spectroelectrochemistry; Gale, R. J., Ed.; London and New York: Plenum, 1988. (b) Spectroelectrochemistry; Kaim, W., Klein, A., Eds.; London Royal Society, 2008; (c) IR: Ashley, K.; Pons, S. Chem. ReV. 1988, 88, 673–695. (d) ESR: Petr, A.; Dunsch, L.; Neudeck, A. J. Electroanal. Chem. 1996, 412, 153–158. (e) UV-VIS: Toma, H. E.; Araki, K. Curr. Org. Chem. 2002, 6, 21–34. (f) Raman: Kavan, L.; Dunsch, L. ChemPhysChem 2007, 8, 975– 998. (10) Rapta, P.; Faber, R.; Dunsch, L.; Neudeck, A.; Nuyken, O. Spectrochimica Acta A. 2000, 56, 357–362.

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