Comparison of Alternant and Nonalternant Aromatic Bridge Systems

J. Phys. Chem. C 2007, 111, 3197-3204

3197

Comparison of Alternant and Nonalternant Aromatic Bridge Systems with Respect to Their ET-Properties Gilbert No1 ll,*,†,‡ Manuele Avola,§ Michelle Lynch,§ and Jo1 rg Daub§ Institut fu¨r Physikalische und Theoretische Chemie and Institut fu¨r Organische Chemie, UniVersita¨t Regensburg, UniVersita¨tsstrasse 31, D-93053 Regensburg, Germany and Department of Analytical Chemistry, Lund UniVersity, P. O. Box 124, SE-221 00 Lund, Sweden ReceiVed: October 11, 2006; In Final Form: NoVember 29, 2006

The bis(triarylamine) systems 1-5 were synthesized and investigated by spectroscopic and electrochemical methods. They all have an aromatic five-membered ring system in common as a central part of their π-electron bridge. The absorption spectra are presented. All compounds undergo five oxidations whereupon only the first two are reversible under semi-infinite cyclic voltammetry conditions. The spectra of the radical cations and dications of 1-5 were collected upon stepwise titration with SbCl5. All monoradical cations exhibit rather intense absorption bands in the NIR region that are assigned to optically induced charge transfer between the amine redox centers or between the amine redox center and aromatic bridge. It is suggested that with CH2Cl2 as solvent the charge in 1+ and 2+ is localized mainly at the peripheral amine redox centers whereas 3+ and 4+ are symmetrically delocalized systems with the highest charge density at the bridge. Upon increasing the solvent polarity, solvent induced symmetry breaking occurs as previously reported for the anthracene derivative 7+. Less clear is the situation in 5+. The nonalternant azulene derivative 4 behaves entirely different with respect to its optical and electrochemical properties if compared with the alternant naphthalene compound 6. The 1,3-azulene bridging unit turns out to mediate a strong electronic coupling combined with a lowoxidation potential.

Introduction Mixed valence systems based on bis(triarylamines) turned out to be almost perfect model compounds to study the electron transfer (ET) properties of conjugated π-electron systems with respect to their substitution pattern.1,2 The molecules of interest are arranged between a neutral (donor) and an oxidized triarylamine redox center (acceptor). These monoradical cations exhibit rather intense intervalence charge transfer (IV-CT) bands in the NIR-region which may be assigned to an optically induced ET.1-6 A detailed band shape analysis of the IV-CTband by applying the Marcus-Hush theory allows the calculation of the electronic coupling V between the redox centers and furthermore the ET-rate constant.1,2,5-15 In strongly coupled bis(triarylamine) radical cations bearing relatively short π-electron systems as bridging units, the charge is either localized at the redox centers (class II) or delocalized over the whole molecule (class III).5-7,16-19 When more extended bridging units are incorporated, additional bridge states come into play and ET may occur via a superexchange or hopping mechanism.1,2,20-24 In our previous work, we have shown that the ET properties of p-di(ethinyl)phenylene bridged systems may be controlled solely by the substituents at the phenylene subunit in such a way that either hopping or superexchange is favored.1,2 In the special case of a bis(ethinyl)anthracene bridged radical cation 7+ solvent induced symmetry breaking was observed.1 In weakly polar media, the charge is “valence delocalized” with highest charge density at the bridge, whereas in polar MeCN the charge is * Corresponding author. Phone: +46 46 222 0103. Fax: +46 46 222 4544. E-mail: [email protected]. † Institut fu ¨ r Physikalische und Theoretische Chemie. ‡ Lund University. § Institut fu ¨ r Organische Chemie.

asymmetrically localized at the redox centers. In extension of these investigations to other significant π-electron systems, we synthesized the compounds 1-5. All compounds have a fivemembered ring in common as a central part of their π-electron bridge. The furan derivative 1 is well suited for comparison with other electron-rich systems. We choose compounds 2 and 3 because thiophene and its derivatives are very important not only as building blocks of molecular wires but also for their use in molecular devices, organic light emitting diodes (OLED), and photovoltaic cells.19,25-32 Polyethylene-dioxythiophene (PEDOT) is one of the most widely used conducting polymers. Additionally to 1, 2, and 3, we synthesized the azulene derivatives 4 and 5 that allow comparison with the recently studied naphthalenylene derivative 6.33 Azulene is a conjugated, aromatic, nonalternant, and planar 10-π-electron hydrocarbon with characteristic features such as blue color, small highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap, emission from S2 violating Kasha’s rule, and a polarized ground state (tropylium cation/cyclopentadienyl anion, dipole moment around 1D) that differs significantly in its properties from the isomeric colorless nonpolar naphthalene.34-38 For instance, the electrochemical oxidation of azulene in CH2Cl2 is followed by electropolymerization34 whereas the oxidation of naphthalene leads to electrocrystallization.39 It is also worth mentioning the different topology (connectivity) in 1,3-azulenylene substituted and 1,4- naphthalenylene substituted systems. In the class of meta-bridged benzenoid donor-bridge-donor (acceptor) systems, the bridge acts electronically as a decoupler in the ground state and shows increased coupling in the excited state.40 The calculation of the electronic coupling V in strongly coupled bis(triaryl)amines with extended bridging units such

10.1021/jp066681n CCC: $37.00 © 2007 American Chemical Society Published on Web 01/26/2007

3198 J. Phys. Chem. C, Vol. 111, No. 7, 2007 CHART 1

as 1 - 7 (Chart 1) is highly dependent upon the applied level of theory and on the determination of the ET-distances.1,2,5,8,11,41-43 Furthermore, the symmetry of the diabatic energy surfaces and the curves applied for fitting the CT bands will affect the results.1 It has been pointed out that the common two-level model leads to poor results if the charge is not mainly localized but delocalized.1 The more accurate Mulliken-Hush three-level approach is not applicable solely on experimental data. It depends strongly on the approximation of various parameters by quantum chemical calculations.1,8 Here, we concentrate on a qualitative discussion of the IV-CT bands collected for 1+5+. Experimental Section Synthesis. 2,5-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenylethynyl]furan 1. N,N-Di(4-methoxyphenyl)-N-(4-ethynylphenyl)amine (150 mg, 0.455 mmol, 2 eq), 2,5-dibromofuran (52 mg, 0.228 mmol 1 eq), tris-tbutylphosphine (37 mg, 0.054 mL, 40 mol %), bis(benzonitrile)palladium chloride (7.0 mg, 4 mol %), and CuI (1.7 mg, 2 mol %) were stirred under N2 inert gas atmosphere in dry triethylamine (7 mL) at 60 °C for 4 h. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:1). The product was recrystallized from MeOH/CH2Cl2. Yield: 32 mg, 0.044 mmol, 20%, C48H38N2O5, 722.84 g/mol, mp 77-80 °C, yellow crystals. 1H NMR (600 MHz, acetone-d6): δ ) 7.33 (m, AA’, 4H, AmPh); 7.13 (m, AA’, 8H MeOPh); 6.96 (m, BB’, 8H, MeOPh); 6.77 (m, BB’, 4H, AmPh); 6.72 (s, 2H, furan-H); 3.80 (s, 12H, MeO). 13C NMR (150 MHz, acetone-d6): δ ) 158.0, 150.8, 140.4, 138.6, 133.2, 128.6, 118.7, 116.6, 115.9, 112.3, 95.6, 78.5, 55.8. ESIMS(PI) : MH+, 723 (100%); M+ 722(41%). EIMS (high resolution, PI) calcd: 722.2781. Found: 722.2781, δ ) 0 ppm. 2,5-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl]thiophene 233. N,N-Di(4-methoxyphenyl)-N-(4-ethynylphenyl)amine (150 mg, 0.455 mmol, 2 eq), 2,5-dibromothiophene (55 mg, 0.228 mmol, 0.025 mL, 1 eq), tris-tbutylphosphine (37 mg, 0.054 mL, 40 mol %), bis(benzonitrile)palladium chloride (7.0 mg, 4 mol %), and CuI (1.7 mg, 2 mol %) were stirred under N2 inert gas atmosphere in dry triethylamine (7 mL) at 60 °C for 4 h. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:1). The product was recrystallized from MeOH/CH2Cl2. Yield: 65

No¨ll et al. mg, 0.088 mmol, 39%, C48H38N2O4S, 738.90 g/mol, mp 8990 °C, bright yellow crystals. EA calcd: C, 78.03; H, 5.18; N, 3.79. Found: C, 77.52; H, 5.26; N, 3.77. 1H NMR (400 MHz, CDCl3): δ ) 7.28 (m, AA’, 4H, AmPh); 7.07 (m, AA’, 8H, MeOPh); 7.06 (s, 2H, thiophene-H); 6.87-6.80 (m, BB’, 8H, MeOPh, BB’, 4H, AmPh); 3.80 (s, 12H, MeO). 13C NMR (100 MHz, CDCl3): δ ) 156.4, 149.1, 140.1, 132.3, 131.1, 127.2, 124.7, 119.0, 114.9, 113.1, 94.7, 81.2, 55.5. EIMS(PI): M+ 739 (50%), 738 (100%); M2+ 369.5 (18%), 369 (34%). EIMS (high resolution, PI) calcd: 738.2552. Found: 738.2546, δ ) 1 ppm. 2,5-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl](3,4-ethylenedioxy)thiophene 3. N,N-Di(4-methoxyphenyl)-N(4-ethynylphenyl)amine (150 mg, 0.455 mmol, 2 eq), 2,5dibromo-3,4-ethylenedioxythiophene44 (68 mg, 0.228 mmol, 1 eq), tris-tbutylphosphine (37 mg, 0.054 mL, 40 mol %), bis(benzonitrile)palladium chloride (7.0 mg, 4 mol %) and CuI (1.7 mg, 2 mol %) were stirred under N2 inert gas atmosphere in dry triethylamine (7 mL) at 60 °C for 4 h. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:1). The product was recrystallized from MeOH/CH2Cl2. Yield: 83 mg, 0.104 mmol, 46%, C50H40N2O6S, 796.94 g/mol, mp 109-111 °C, bright yellow crystals. EA calcd: C, 75.36; H, 5.06; N, 3.52. Found: C, 74.99; H, 4.94. N: 3.83. 1H NMR (600 MHz, acetone-d6): δ ) 7.29 (m, AA’, 4H, AmPh); 7.12 (m, AA’, 8H MeOPh); 6.94 (m, BB’, 8H, MeOPh); 6.77(m, BB’, 4H, AmPh); 4.33(s, 4H, CH2); 3.80 (s, 12H, MeO). 13C NMR (150 MHz, acetone-d6): δ ) 157.9, 150.4, 144.2, 140.6, 133.0, 128.5, 118.9, 115.9, 113.6, 99.7, 97.9, 79.3, 65.7, 55.8. ESIMS(PI): MH+ 797 (77%); M+ 796 (100%). 1,3-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl]azulene 4. N,N-Di(4-methoxyphenyl)-N-(4-ethynylphenyl)amine (150 mg, 0.455 mmol, 2 eq), 1,3-dibromoazulene45 (65 mg, 0.228 mmol, 1 eq), tris-tbutylphosphine (37 mg, 0.054 mL, 40 mol %), bis(benzonitrile)palladium chloride (7.0 mg, 4 mol %), and CuI (1.7 mg, 2 mol %) were stirred under N2 inert gas atmosphere in dry triethylamine (7 mL) at 60 °C for 4 h. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:1). The product was recrystallized from MeOH/CH2Cl2. Yield: 58 mg, 0.074 mmol, 33%, C54H42N2O4, 782.93 g/mol, mp 97-100 °C, olive green crystals. 1H NMR (400 MHz, CDCl3): δ ) 8.52 (d, 2H, J ) 9.6 Hz); 8.06 (s, 1H); 7.62 (t, 1H, J ) 9.8 Hz); 7.39 (m, AA’, 4H, AmPh); 7.24 (pt, 2H, J ) 9.7 Hz); 7.08 (m, AA’, 8H MeOPh); 6.89-6.84 (m, BB’, 8H, MeOPh, BB’, 4H, AmPh); 3.80 (s, 12H, MeO). 13C NMR (100 MHz, CDCl3): δ ) 156.2, 148.5, 141.5, 141.0, 140.4, 139.8, 137.0, 132.2, 127.0, 125.1, 119.6, 114.8, 114.8, 111.2, 94.6, 83.6, 55.5. EIMS(PI): M+ 783 (19%), 782 (49%); M2+ 391.5 (10%), 369 (18%); EIMS (high resolution, PI) calcd: 782.3145. Found: 782.3146, δ ) 0.2 ppm. 1-[4-{N,N-Di(4-methoxyphenyl)amino}phenylethynyl]-3-bromoazulene 4a. N,N-Di(4-methoxyphenyl)-N-(4-ethynylphenyl)amine (150 mg, 0.455 mmol, 2 eq), 1,3-dibromoazulene45 (65 mg, 0.228 mmol, 1 eq), PdCl2(PPh3)2 (6.4 mg, 4 mol %), and CuI (1.7 mg 2 mol %) were stirred under N2 inert gas atmosphere in dry triethylamine (7 mL) at 60 °C for 4 h. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:1). The product was recrystallized from MeOH/CH2Cl2. Further purification was carried out by HPLC. Yield: 52 mg, 0.097 mmol, 43%, C32H24BrNO2, 534.45 g/mol, mp 146-148 °C, green crystals. 1H NMR (600 MHz, CDCl3): δ ) 8.54 (d, 1H, J ) 9.8 Hz); 8.28 (d, 1H, J ) 9.6 Hz); 7.93 (s, 1H); 7.64 (pt, 1H, J ) 9.6); 7.38 (m, AA′, 2H, AmPh); 7.22-7.26 (m, 2H); 7.09 (m, AA’, 4H,

ET through Different Aromatic Bridge Systems

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Figure 1. Absorption spectra of 1-4, 4a, and 5 in CH2Cl2. For the azulene derivatives 4, 4a, and 5, the absorption between 500 and 800 nm is shown also with an enlargement factor of 10.

MeOPh); 6.90-6.86 (m, BB′, 2H, AmPh); 6.86-6.82 (m, BB′, 4H, MeOPh); 3.81 (s, 6H, MeO). 13C NMR (150 MHz, CDCl3): δ ) 156.3, 148.6, 140.6, 140.3, 140.0, 139.6, 136.9, 136.7, 136.6, 132.2, 127.0, 124.7, 124.4, 119.5, 114.8, 114.6, 110.9, 104.0, 95.2, 83.0, 55.5. ESIMS(PI): M+ 536 (62%), 535 (100%), 534 (60%), 533 (88%). EIMS (high resolution, PI) calcd: 533.0991. Found: 533.0983, δ ) 1.4 ppm. 1,3-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl]azulene-2-carboxylic acid methyl ester 5. N,N-Di(4-methoxyphenyl)-N-(4-tributylstannylethynyl-phenyl)amine (220 mg, 0.357 mmol), 1,3-dibromoazulene-2-methylcarboxylate (41 mg, 0.119 mmol, 1 eq), Pd2(dba)3CHCl3 (10 mg, 10 µmol), tris-tbutylphosphine (10 µL, 0.033 mmol), and CuI (2 mg, 4 mol %) were stirred under N2 inert gas atmosphere in 5 mL dry toluene at 80 °C over night. The solvent was removed in vacuo, and the residue was purified by chromatography on silica gel (PE/CH2Cl2, 1:2 f CH2Cl2). The product was recrystallized from MeOH/CH2Cl2. Yield: 40 mg, 0.0476 mmol, 40%, C56H44N2O6, 840.97 g/mol, mp 113-115 °C, green crystals. 1H NMR (400 MHz, CDCl3): δ ) 8.64 (d, 2H, J ) 9.2 Hz); 7.66 (t, 1H, J ) 9.8 Hz); 7.42 (m, AA’, 4H, AmPh); 7.23 (pt, 2H, J ) 9.8 Hz); 7.08 (m, AA’, 8H MeOPh); 6.90-6.83 (m, 4H, AmPh BB’, 8H, MeOPh, BB’); 3.05 (s, 3H, OMe); 3.81 (s, 12H, MeO).

NMR (100 MHz, CDCl3): δ ) 164.9, 156.3, 148.7, 142.4, 141.3, 140.5, 140.3, 137.5, 132.4, 127.0, 126.0, 119.5, 114.8, 114.7, 112.2, 97.9, 82.9, 76.7, 55.5. EIMS(PI): M+ 842 (30%), 841 (60%), 840 (100); M2+ 420.5 (11%), 420 (18%). EIMS (high resolution, PI) calcd: 840.3199. Found: 840.3195, δ ) 0.5 ppm. The yields of the reactions have not been optimized. Electrochemical Setup. Cyclic voltammetry measurements were performed at room temperature using an undivided electrochemical cell with a three-electrode arrangement and a computer controlled EG & G Potentiostat/Galvanostat model 283 A. As a working electrode, we used a platinum disk electrode together with an Ag/AgCl pseudoreference- and a platinum-counter electrode. As a supporting electrolyte, tetrabutylammonium hexafluorophosphate (TBAH) was used. Ferrocene was used as internal standard. For electrochemical experiments, 5 mg of the compounds were dissolved in 5-8 mL of the solvent.

13C

Results and Discussion All molecules were synthesized as described in the experimental section by Pd-catalyzed cross coupling reactions of N,N-

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Figure 2. CVs of 3 (left) and 4 (right) in CH2Cl2 under semi-infinite conditions collected with a scan rate of V ) 250 mV s-1 (measured data, black; fitted data, red). Only the reversible first oxidative wave (first and second oxidation) is depicted.

di-(4-methoxyphenyl)-N-(4-ethynylphenyl)amine or N,N-di(4methoxyphenyl)-N-(4-tributylstannylethynylphenyl)amine and the corresponding aromatic dibromo derivative serving as central part of the bridge. Substitution was possible only once or even twice (referring to 4 and 4a) depending on the phosphine ligand applied in the catalytic circle. Figure 1 shows the absorption spectra of the compounds 1-5 in CH2Cl2 respectively. All compounds show an intense absorption around 400 nm. The absorption maximum of 5 ( ) 38 000 M-1 cm-1 at 392 nm) is only about half of the value for the more symmetric azulene derivative 4 ( ) 71 000 M-1 cm-1 at 372 nm), which may be because of electronic effects. Similar absorption bands have been detected for 7 and related donor-π-bridge-donor systems.46 The corresponding excited states were described as mixed-valence states that show, depending on the chemical nature of the π-bridge, a varying amount of interactions (couplings). In the absorption spectra of 4, 4a, and 5, a broad absorption band of low intensity in the long wavelength region between 500-800 nm with maxima at 645, 640, and 710 nm is detected (HOMO-LUMO transition), which is typical for azulene derivatives.38,47 The bathochromic shift of this transition compared to unsubstituted azulene (λmax ) 575 nm in dichloromethane, data not shown) underlines the strong influence of substituents at the 1,3-positions on the azulene HOMO-LUMO gap. Cyclic voltammetry experiments were carried out under semiinfinite conditions at a scan rate of V ) 250 mV s-1 in CH2Cl2 with an electrochemical setup described in the experimental section (supporting electrolyte: TBAH, 0.2 M). Compounds 1-5 could be oxidized five times, whereupon only the first two oxidations turned out to be completely reversible. The first two oxidations appear as one single broaded oxidative wave. The first three oxidations are assigned to the first oxidations of the two triarylamines and to the oxidation of the bridge. The fourth and fifth oxidations are due to the irreversible second oxidations of the triarylamine groups.48 The splitting of the redox potentials for first and second oxidation (∆E) was determined by fitting the cyclic voltammograms (CVs) with digisim.49 For comparison, compound 4a could be oxidized three times whereupon only the first oxidation was found to be reversible. Representatively, the measured and fitted voltammograms (first and second oxidation) for 3 and 4 are depicted in Figure 2. Only the reversible first oxidative wave is depicted. The redox potentials for the reversible oxidations of 1-7 are summarized in Table 1. Because of the electron donating character of the ethylenedioxy group, the oxidation potential of the first oxidation of 3 (240 mV vs Fc/Fc+) is less positive than those of 1 and 2 (270 and 260 mV vs Fc/Fc+). The redox splitting ∆E is about 60

TABLE 1: Redox Potentials of First and Second Oxidation of 1-7 and the Corresponding Differences of the Standard Redox Potential (∆E, Redox Splitting)a

compound

E1/2/mV vs Fc/Fc+ first ox

E1/2/mV vs Fc/Fc+ second ox

∆E/mV

1 2 3 3 in MeCN 4 4a 5 5 in MeCN 6 7

270 260 240 290 185 235 200 255 255b 255c

330 320 300 350 285

60 60 60 60 100

280 335 320b 320c

80 80 65 65

a Measurements have been carried out under semi-infinite conditions in CH2Cl2 unless otherwise stated (scan rate V ) 250 mV s-1; supporting electrolyte, TBAH, 0.2 M). b See ref 33. c See ref 1.

mV for 1, 2, and 3. This value also has been observed for a series of bis(triarylamine) homologues with p-diethynylphenylene derivatives as bridging units.1,2 The first oxidation of the azulene derivative 4 (185 mV vs Fc/Fc+) appears at less positive potential than the first oxidation of the EDOT derivative 3 indicating that the azulene exerts a stronger electron-donating effect to the system. Furthermore, the redox splitting ∆E is significantly larger in 4 (100 mV) than in 1-3 (60 mV) and in the isomeric naphthalene derivative 6 (∆E ) 65 mV). Also, for compound 5 a relatively high redox splitting (80 mV) could be observed. The spectra of the radical cations and dications of 1-5 as depicted in Figure 3 have been collected upon stepwise titration with SbCl5. The spectra of the radical cations have been corrected as described in ref.5 We assign the lowest-energy absorption band of 1+ and 2+ which is completely absent in the dication to the regular IV-CT transition. The second band is characteristic for a charge transfer from triarylamine to bridge (CTbr). In both spectra a third band appears with a maximum at about 13 450 cm-1, which is typical for a π-π*-excitation of a triarylamine radical cation.1,5 These “radical bands” show only a weak bathochromic shift when 1+ and 2+ are further oxidized to the dications. To fit the two low energy absorption bands of 1+ and 2+, three Gaussian-functions were necessary to reproduce (i) the IV-CT band, (ii) the CTbr band, and (iii) the low-energy side of the radical band, respectively (data not shown). The fit results in two maxima at 6200 cm-1 (IV-CT,  ) 15 500 M-1 cm-1) and 11 000 cm-1 (CTbr,  ) 10 200 M-1 cm-1) for 1+ and 6600 cm-1 (IV-CT,  ) 15 000 M-1 cm-1) and 11 100 cm-1 (CTbr,

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Figure 3. Spectra of the radical cations (black) and dications (red) of 1-5 in CH2Cl2 and of the radical cations (blue) and dications (green) of 3 and 5 in MeCN. The oxidation was carried out by stepwise titration with SbCl5.

 ) 11 800 M-1 cm-1) for 2+. The spectra of 1+ and 2+ do not differ significantly. In contrast, the low energy absorption of 3+, 4+, and 5+ in CH2Cl2 could not sufficiently be fitted with three Gaussianshaped bands (i.e., two CT-transitions and the low-energy side of the next band higher in energy) indicating that additional transitions have to be taken into account. If the radical cations 3+, 4+, and 5+ are bridge-localized in their ground state, the main contribution to the low energy absorption band would be a charge transfer between triarylamine and bridge (CTbr), whereupon this time the hole is transferred from the bridge to the triarylamine. The maxima of the low energy absorptions of 3+, 4+, and 5+ in CH2Cl2 at 5200, 4700, and 4700 cm-1, respectively, are significantly lower in energy than the IV-CT bands of 1+ and 2+. Also, the unusual band shape of 3+, 4+, and 5+ (in CH2Cl2) resembling that of 7+ (in CH2Cl2, maximum at 4640 nm) are indications that these radical cations might be bridge-localized, or to be more precise “valence-delocalized”, systems with the highest charge density at the bridge.

Upon having a closer look at the spectra of 3+ and 32+, it turns out that the typical triarylamine π-π*-excitation at 13 300 cm-1 in the spectrum of 32+ is missing for 3+. Instead of this band, two bands appear at 14 650 and 16 300 cm-1 that look similar to bands observed for oligomeric EDOT radical cations in CH2Cl2.50 These findings confirm the assumption that in 3+ in CH2Cl2 the positive charge is mainly localized at the bridge. The same conclusion is made for 4+. While in the spectrum of the monoradical cation 4+ no maximum was found between 10 000 and 15 000 cm-1 at all, there is an intense triarylamine π-π*-excitation band at 13 450 cm-1 (sh. at 16 500 cm-1) in the dication 42+. In the spectrum of 4a+, there is an intense absorption band at 7100 cm-1 that could be assigned to a CT from triarylamine to azulene. Beside this absorption, two bands at 14 300 cm-1 and 16 450 cm-1 were found. The band at 14 300 cm-1 could be a triarylamine π-π*-excitation. In contrast to 4a+ a triarylamine π-π*-excitation band is missing in 4+. Therefore, we come to the conclusion that 4+ (as well 3+) is a valence delocalized system in CH2Cl2.

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TABLE 2: Maxima of IV-CT, CTbr, and Further Absorption Bands Together with Their Molar Absorptivity E between 4000 and 20 000 cm-1 for the Radical Cations and Dications of 1-7a

1+

V˜ IV-CT/cm-1 (/M-1 cm-1)

V˜ CTbr/cm-1 (/M-1 cm-1)

6200 (15500)

11000 (10200) 9000 (31100) 11100 (11800)

12+ 2+

6600 (15000)

22+

9950 (34600) 5200 (23500)

3+

32+ 3+ in MeCN 32+ in MeCN 4+

8900 (16500)

42+ 4a+

5+ 52+ 5+ in MeCN 52+ in MeCN 6+ 62+ 7+ 72+

7900 (5800) 6970b (12200)

8450 (53000) 12050 (3300) 9600 (33600) 4700 (30500) 7300 (19300) 7100 (16500) 4700 (15800) 8000 (11900) 12030 (3400) 9500 (13500) 11170b (9800) 10500b (31000) 4640c (45500) 7700c (25000)

TABLE 3: Electronic Couplings V Calculated by the Two-level Model Together with the Applied Transition Moments µeg and Band Maxima W˜ IV-CT

V˜ max3/cm-1 /M-1 cm-1) 13450 (30500) 13150 (39100) 13450 (32200) 13250 (53900) 14650 (26400) 16300 (21700) 13300 (33800) 13650 (28000) 13600 (39800) 13450 (32500) 14300 (13900) 16450 (10200) 14400 (12200) 13450 (20800) 14000 (14200) 13450 (27200) 13500b (35000) 13300b (55000) ca. 12000c, d (30000) 13300c (60000)

1+ 2+ 6+ 8+ 9+ a

V/cm-1

µeg/D

V˜ IV-CT/cm-1

720 710 690 620 540 660 750

10.3 9.5 9.1a 8.2a 6.2a 9.5a 10.3b

6200 6600 6710a 6970a 8060a 6470a 6780b

See ref 33. b See ref 51.

shifting to a charge localized configuration in MeCN as it has been previously reported for 7+. Upon increasing the solvent polarity, solvent induced symmetry breaking occurs. The same behavior is expected for 4+ that turns out to be a delocalized system in CH2Cl2. It also is suggested that the radical cation 5+ behaves basically in the same way. However, in contrast to 3+ and 4+, CH2Cl2 as a solvent might be not apolar enough to shift 5+ completely into a charge-delocalized configuration. The spectrum of 5+ in CH2Cl2 could be interpreted as the special case where both species are present at a certain ratio. It is highly interesting that the azulene derivatives 4+ and 5+ behave entirely different than the isomeric naphthalene derivative 6+ that was described as a “usual” asymmetric mixed valence compound.2,33 The electronic coupling V was calculated from the absorption spectra in CH2Cl2 by the two-level model5 with

V ) µeg/er ν˜ IV-CT for the localized compounds 1+ and 2+ and compared with those of 6+, and the radical cations of 1,4-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl]-benzene, and 1,4-Bis[4-{N,N-di(4-methoxyphenyl)amino}phenyl-ethynyl]-(2,5-dimethoxy)benzene thereafter labeled as 8+ and 9+, respectively.

a If possible, the peak maxima have been determined by a fitting procedure as described in the main text. b See ref 33. c See ref 1. d With fine structure typical for an anthracene radical cation.

Less clear is the situation in 5+. Because of the electron withdrawing carboxymethyl substituent, the azulene bridge is expected to be shifted to higher energies. Nevertheless, the maximum of the low-energy absorption is at the same value as found for 4+ (4700 cm-1) but with almost half of its intensity. Additionally, there is a second maximum in 5+ at 14400 cm-1 that could be caused by a triarylamine π-π*-excitation. The radical cations and dications of 3 and 5 also were measured in MeCN (Figure 3). The spectrum of 4+ in MeCN was not detected because 4 did not show reversible CV behavior in MeCN. The spectra of 3+ and 5+ in MeCN could be successfully fitted with the usual fitting procedure leading to two maxima at 8900 cm-1 ( ) 16 500 M-1 cm-1) and 12 050 cm-1 ( ) 3300 M-1 cm-1) for 3+ and 8000 cm-1 ( ) 7800 M-1 cm-1) and 12 050 cm-1 ( ) 3400 M-1 cm-1) for 5+, respectively. The spectral data of the mono and diradical cations of 1-7 are summarized in Table 2. According to the experimental data, we come to the conclusion that 3+ represents a valence-delocalized system in CH2Cl2

As a rough approximation, the AM1-calculated N-N distance of r ) 18.4 Å for 1+ and 2+ and r ) 19.3 Å for 6+, 8+, and 9+ was applied.5,33 The calculated values for V together with the applied transition moments µeg and band maxima V˜ IV-CT are summarized in Table 3. As it can be deduced from Table 3, V is almost the same in 1+ and 2+ and slightly higher than in 6+. The coupling in all three radical cations is larger than in the phenylene bridged system 8+. Depending on the experimental values applied for the p-dimethoxyphenylene bridged 9+, V in 1+, 2+, and 9+ is in the same order of magnitude. However, it has to be mentioned that the rather simple twolevel model comparing linear and nonlinear ET systems in which an additional bridge state comes into play can only show a rough tendency. The bridge state as such, the bridge energy ∆G0 and reorganization energy λ, as well as the couplings between redox centers and bridge are neglected. At high bridge-energy-levels, the ET will occur by a superexchange mechanism. On lowering the bridge energy, the charge is less localized at the redox centers and more at the bridge. Thereby ET via a hopping mechanism becomes more

ET through Different Aromatic Bridge Systems favorable. It was demonstrated by a generalized three-level analysis that in 9+ the ET is possible by a hopping and by a superexchange mechanism.1,2 If the bridge energy is further decreased, the charge will be delocalized over the whole system with the highest charge density at the bridge. Obviously, the latter is the situation in 3+, 4+, and 7+. If we assume equal values for the electronic couplings and the reorganization energies, we may consider the first ionization potential of the bridge unit as a means to compare the bridge energies relative to each other. Furan and thiophene have almost the same ionization potentials (IE ) 8.88 and 8.86 eV, respectively52), which is in line with the similarities between 1+ and 2+. The ionization potentials of azulene (IE ) 7.42 eV52) and anthracene (IE ) 7.44 eV52) are almost the same. The ionization potential of naphthalene (IE ) 8.14 eV52) is significantly higher. The couplings and the reorganization energies in 4+, 6+, and 7+ are not expected to be equal if one compares the pseudometa coupling in 4+ and the “paracoupling” in 6 and 7. Nevertheless, the trend shown by the ionization potentials might be one reason why 4+ and 7+ are delocalized systems whereas 6+ is not. One could argue that using a bridge unit that is more easily oxidized than the terminal redox centers will lead to a symmetrical radical cation that is not valence delocalized as such but has to be termed bridge localized. The isolated bridge units applied in 3+, 4+, and 7+ itself have significant higher oxidation potentials than the triarylamines attached: EDOT, Epa ) 630 mV53 vs Fc/Fc+ in MeCN; azulene, Epa ) 600 mV vs Fc/Fc+ in CH2Cl2 (own results) and Epa ) 670 mV34 vs Fc/Fc+ in MeCN; anthracene, E1/2 ) 900 mV1 vs Fc/Fc+ in MeCN; and trianisylamine, E1/2 ) 109 mV48 vs Fc/Fc+ in CH2Cl2 and E1/2 ) 160 mV54 vs Fc/Fc+ in MeCN. If only one donor is attached to azulene, as in 4a+, it is the triarylamine redox center that is oxidized to yield the radical cation (see above). The same was demonstrated for anthracene.1 Summary and Conclusion A series of alternant and nonalternant organic π-electron systems have been used as bridging and ET mediating central units between two triarylamine redox active centers. All compounds can be oxidized five times whereupon only the first two oxidations are reversible under semi-infinite conditions. The reversible oxidations of the azulene derivatives occur at lower redox potential with higher potential separation ∆E between the first and second redox process than determined for the other systems. The monoradical cations of all derivatives exhibit strong absorption bands in the NIR region. The spectra of 1+ and 2+ look almost identical. The electronic couplings in 1+, 2+, and 6+ were calculated by the simple two-state model and compared with those of a phenylene- and a p-dimethoxyphenylene-bridged system 8+ and 9+. All these derivatives (1+, 2+, 6+, 8+, and 9+) are “usual” mixed valence compounds exhibiting unsymmetrical charge distribution independent of the solvent. The electronic coupling decreases in the following order, 9+ g 1+ g 2+ > 6+ > 8+, derived from the respective optical data. A qualitative comparison with the optical spectra of 3+, 4+, 5+, and 7+ in CH2Cl2 leads to the conclusion that in the EDOT, the azulene, and the anthracene systems the positive charge is symmetrically delocalized with the highest charge density at the bridge. Upon increasing the solvent polarity, solvent-induced symmetry breaking occurs as it has been previously reported for the analogue anthracene derivative 7+.1 As shown by the present study, the azulene and the naphthalene moiety behave entirely different, whereas the azulene and the anthracene derivatives 4+ and 7+ possess similar ET properties. It turned

J. Phys. Chem. C, Vol. 111, No. 7, 2007 3203 out that not only phenylene1,2,55 or thiophene but also azulene derivatives are able to mediate a strong electronic coupling accompanied by a very low redox potential. Thus the expected decoupling effect in 1,3-disubstituted azulenes (pseudometa effect) is compensated by the specific electronic structure of azulene.56 Therefore, the compounds studied have to be taken into account as attractive building blocks for molecular wires. Similar to phenylene and thiophenylene, also the ET properties of azulenes may be tuned by the substitution pattern. Because of the nonuniform distribution of the electron density in azulene, it is highly sensitive against substitution at varying positions. If the substitution pattern is chosen carefully, novel and powerful bridge-based multiswitching systems could be obtained. Acknowledgment. This work was supported by the DFG (GRK 640 “Sensory Photoreceptors in Natural and Artificial Systems”). G.N. thanks Dr. Bernhard Dick for kind support at Regensburg. References and Notes (1) Lambert, C.; Amthor, S.; Schelter, J. J. Phys. Chem. A. 2004, 108, 6474. (2) Lambert, C.; No¨ll, G.; Schelter, J. Nat. Mater. 2002, 1, 69. (3) Bonvoisin, J.; Launay, J.-P.; Van der, Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1994, 98, 5052. (4) Bonvoisin, J.; Launay, J.-P.; Verbouwe, W.; Van der, Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 17079. (5) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434. (6) Barlow, S.; Risko, C.; Chung, S.-J.; Tucker, N. M.; Coropceanu, V.; Jones, S. C.; Levi, Z.; Bredas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 16900. (7) Nelsen, S. F. Chem.sEur. J. 2000, 6, 581. (8) Amthor, S.; Lambert, C. J. Phys. Chem. A. 2006, 110, 1177. (9) Coropceanu, V.; Boldyrev, S. I.; Risko, C.; Bredas, J.-L. Chem. Phys. 2006, 326, 107. (10) Nelsen, S. F. Electron Transfer Reactions in Organic Chemistry. In Electron Transfer in Chemistry 1. Principles and Theories Methods and Techniques; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 1; pp 342. (11) Newton, M. D. Electron Transfer: Theoretical Models and Computational Implementation. In Electron Transfer in Chemistry 1. Principles and Theories Methods and Techniques; Balzani, V., Ed.; WileyVCH: Weinheim, Germany, 2001; Vol. 1; pp 3. (12) Sun, D.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2004, 126, 1388. (13) Sun, D.; Rosokha, S. V.; Kochi, J. K. Angew. Chem., Int. Ed. 2005, 44, 5133. (14) Sun, D.-L.; Rosokha, S. V.; Lindeman, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003, 125, 15950. (15) Zheng, S.; Barlow, S.; Risko, C.; Kinnibrugh, T. L.; Khrustalev, V. N.; Jones, S. C.; Antipin, M. Y.; Tucker, N. M.; Timofeeva, T. V.; Coropceanu, V.; Bredas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2006, 128, 1812. (16) Coropceanu, V.; Gruhn, N. E.; Barlow, S.; Lambert, C.; Durivage, J. C.; Bill, T. G.; No¨ll, G.; Marder, S. R.; Bredas, J.-L. J. Am. Chem. Soc. 2004, 126, 2727. (17) No¨ll, G.; Avola, M. J. Phys. Org. Chem. 2006, 19, 238. (18) Szeghalmi, A. V.; Erdmann, M.; Engel, V.; Schmitt, M.; Amthor, S.; Kriegisch, V.; No¨ll, G.; Stahl, R.; Lambert, C.; Leusser, D.; Stalke, D.; Zabel, M.; Popp, J. J. Am. Chem. Soc. 2004, 126, 7834. (19) Lacroix, J. C.; Chane-Ching, K. I.; Maquere, F.; Maurel, F. J. Am. Chem. Soc. 2006, 128, 7264. (20) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (21) Pourtois, G.; Beljonne, D.; Cornil, J.; Ratner, M. A.; Bredas, J. L. J. Am. Chem. Soc. 2002, 124, 4436. (22) Rosokha, S. V.; Sun, D.-L.; Kochi, J. K. J. Phys. Chem. A. 2002, 106, 2283. (23) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 5577. (24) Weiss, E. A.; Sinks, L. E.; Lukas, A. S.; Chernick, E. T.; Ratner, M. A.; Wasielewski, M. R. J. Phys. Chem. B. 2004, 108, 10309. (25) Ba¨uerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217. (26) Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481. (27) Groenendaal, L. B.; Zotti, G.; Aubert, P.-H.; Waybright, S. M.; Reynolds, J. R. AdV. Mater. 2003, 15, 855.

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