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J. Phys. Chem. C 2007, 111, 3512-3516
Charge Resonance Excitations in 1,3-Bis[di(4-methoxyphenyl)amino]azulene Radical Cations Gilbert No1 ll,*,†,‡ Stephan Amthor,§ Manuele Avola,| Christoph Lambert,§ and Jo1 rg Daub| Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Regensburg, UniVersita¨tsstrasse 31, D-93053 Regensburg, Germany, Department of Analytical Chemistry, Lund UniVersity, P. O. Box 124, SE-221 00 Lund, Sweden, Institut fu¨r Organische Chemie, Bayerische Julius-Maximilians-UniVersita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany, and Institut fu¨r Organische Chemie, UniVersita¨t Regensburg, UniVersita¨tsstrasse 31, D-93053 Regensburg ReceiVed: September 28, 2006; In Final Form: NoVember 28, 2006
The 1,3-bis[di(4-methoxyphenyl)amino]azulene 1 and 1,3-bis[di(4-methoxyphenyl)amino]azulene-2-carboxylic acid methyl ester 2 were synthesized and investigated by spectroscopic and electrochemical methods in order to estimate the electronic coupling between the attached amine redox centers. We found that the electronic coupling V which is mediated by a nonalternant 1,3-azulene bridge in the radical cation 2+ (V ) 3900 cm-1) is almost the same as in the isomeric alternant 1,4-naphthalene system 3+ (V ) 4000 cm-1). The electrochemical stability of azulene derivatives is drastically increased by substituents at the 2-position: whereas the oxidation of compound 1 is irreversible, the methylcarboxylate derivative 2 undergoes four reversible oxidation processes in CH2Cl2 under semi-infinite conditions. The different redox states of the corresponding radical cations are well separated. However, under finite diffusion conditions only the first three oxidation processes are reversible. The absorption spectra of the radical cations of 2+, 22+, and 23+ show intense absorption bands in the NIR region. The analysis of the optical spectra as well as DFT calculations indicate that in 2+ the charge is symmetrically distributed similar to the naphthalene isomer 3+. Thus, the 1,3-azulene unit and its derivatives are useful bridging units due to their ability to mediate a strong electronic coupling similar to naphthalene but with a less positive redox potential.
Introduction
CHART 1:
While electron transfer (ET) through alternant π-electron bridge systems like phenylene, naphthalene, or anthracene derivatives has thoroughly been investigated,1 only few studies on pure nonalternant hydrocarbons are known.2,3 In order to understand the nature of charge transfer and charge localization in azulene based mixed valence systems, we have already studied 1,2,3-tris-{4-[N,N-bis(4-methoxyphenyl)amino]phenyl}6-[N,N-bis(4-methoxyphenyl)amino]azulene concerning its nonlinear optical and ET properties.4 Unfortunately, a detailed analysis of the multidimensional ET pathways failed in the latter case because we have not been able to assign the four oxidations to definite redox centers. In extension of these investigations model compounds 1 and 2 were synthesized in which two arylamine redox moieties are connected via an 1,3-azulene bridging unit. Triarylamine radical cations bearing two or more redoxcenters are known to exhibit rather intense intervalence charge transfer (IV-CT) bands in the NIR-region which can be assigned to an optically induced hole transfer (HT).1,5-9 A band shape analysis of the IV-CT-band using the Mulliken-Hush concept allows the calculation of the electronic coupling V between states with differently charged redox centers mediated * Corresponding author. Department of Analytical Chemistry, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden. Telephone: +46 46 222 0103. Fax: +46 46 222 4544. E-mail:
[email protected]. † Institut fu ¨ r Physikalische und Theoretische Chemie, Universita¨t Regensburg. ‡ Department of Analytical Chemistry, Lund University. § Institut fu ¨ r Organische Chemie, Bayerische Julius-Maximilians-Universita¨t Wu¨rzburg. | Institut fu ¨ r Organische Chemie, Universita¨t Regensburg.
by the bridging unit.10-17 A detailed description of the Mulliken-Hush theory and its application to triarylamines is given elsewhere.5,6,18 Methoxy groups in the para-positions of the arylamine moieties were introduced in order to induce a low redox potential and to avoid dimerization at unprotected para-positions of triarylamine moieties.19 The azulene core serves as the bridge mediating the electronic coupling between the degenerate redox centers. The methylcarboxylate substituent at the 2-position of the azulene subunit in 2 serves for studying the influence of substituents at this position. The radical cations of 1 and 2 allow the direct comparison of their optical and electronic properties with those of its isomeric naphthalene derivative 3+ which was published recently.1 To the best of our knowledge, this is the first comparison of the magnitude of
10.1021/jp066385a CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007
Charge Resonance Excitations in Azulenes
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3513
electronic couplings mediated by isomeric alternant and nonalternant hydrocarbons respectively. Experimental Section Synthesis. 1,3-Bis[di(4-methoxyphenyl)amino]azulene, 1. 1,3-Dibromoazulene (1 equiv) was obtained by bromination of azulene with NBS (2.1 equiv) in Et2O and purified upon chromatography.20 1,3-Dibromoazulene (194 mg, 0.678 mmol), dianisylamine (261 mg, 1.14 mmol), NaOtBu (250 mg, 2.2 mmol), Pd2(dba)3‚CHCl3 (35 mg, 33.8 mmol) and tris(tertbutyl)phosphine (30 mL, 0.1 mmol) were stirred over night in 5 mL toluene (abs.) under nitrogen atmosphere. The solvent was removed in vacuo and the product was purified by crystallization from CH2Cl2/MeOH. Yield: 130 mg, 33%, C38H34N2O4, 582, 70 g/mol, mp ) 80 °C, brown crystals. EIMS(PI): M+, 583(40%), 582(100%); M2+, 291.5(3%), 291(8%). EI-MS (high resolution, PI): calcd, 582.2519; found, 582.2508; ∆ ) 1.1 ppm. 1H NMR (400 MHz, CDCl3): δ ) 7.80 (2H, m, br, H4/8), 7.44 (1H, s, H2), 7.23 (1H, m, br H6), 6.92 (8H, m, AA′,), 6.74 (8H, m, BB′), 6.59 (2H, m, br, H5/7), 3.76 (12H, s, methoxy). 13C NMR (100.6 MHz, CDCl3): δ ) 154.4, 142.8, 139.2, 134.5, 133.6, 132.5, 129.2, 123.0, 121.1, 114.4, 55.5 ppm. 1,3-Bis[di(4-methoxyphenyl)amino]azulene-2-carboxylic Acid Methyl Ester, 2. Compound 2 was prepared as an analogue to compound 1. Purification was done by chromatography (SiO2, PE/CH2Cl2, 2:1). Further purification by HPLC was carried out. Yield: 30 mg, 28%, C40H36N2O6, 640.73 g/mol, mp ) 85 °C, dark green crystals. EI-MS(PI): M+, 641(40%), 640(100%); M2+, 320.5 (4%), 320(10%). EI-MS (high resolution, PI): calcd, 640.2573; found, 640.2575; ∆ ) 0.1 ppm. 1H NMR (600 MHz, CDCl3): δ ) 8.08 (2H, d, H4/8, J ) 9.2 Hz), 7.38 (1H, t, H6, J ) 9.5 Hz), 6.92 (8H, m, AA′,), 6.78 (2H, pt, H5/7, J ) 9.5 Hz), 6.74 (8H, m, BB′), 3.75 (12H, s, methoxy), 3.24 (3H, s, methyl). 13C NMR (150.9 MHz, CDCl3): δ ) 164.4, 154.1, 142.5, 141.3, 137.2, 133.8, 132.2, 131.0, 123.2, 122.2, 114.8, 55.5, 51.2 ppm. 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 working electrode we used a platinum disk electrode together with an Ag/AgCl pseudo-reference and a platinum counter electrode. As supporting electrolyte tetrabutylammonium hexafluoro-phosphate (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. The spectroelectrochemical cell applied in this work has been described already in detail.21 DFT Calculations. Time dependent density functional theory (TD-DFT) calculations were carried out using the unrestricted B3LYP functional with the SV(P) basis set implemented in Turbomole V5.6 program.22 The geometry optimization of the radical cation 1+ was done with symmetry restrictions (C2). The excitation energy and the transition dipole moment were calculated using time dependent functional theory (TD-DFT). The orbitals were plotted using Molekel 4.3.23 Results and Discussion The synthesis of 1 was carried out from 1,3-dibromoazulene20,24 and dianisylamine by a Pd-catalyzed Hartwig-Buchwald coupling reaction in toluene as described in the Experimental Section. Compound 2 was prepared in a manner similar to 1 from the corresponding 1,3-dibromoazulene-2-methylcarboxylate (prepared analogue to 1,3-dibromoazulene)20,24 and
Figure 1. UV/vis spectra of 1 and 2 in CH2Cl2.
dianisylamine. The UV/vis spectra of 1 and 2 in CH2Cl2 are shown in Figure 1. Besides strong absorption bands between 250 and 500 nm, both compounds show a weak absorption between 500 and 850 nm with a maximum at 667 nm which is typical of azulene derivatives (HOMO-LUMO transition).25 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, or, more precisely, on the specific electron repulsion terms in the azulene electronic transitions.26 Compounds 1 and 2 were studied by cyclic voltammetry. Already the first oxidation of 1 at a redox potential of -310 mV vs Fc/Fc+ was not completely reversible as indicated by a decrease in the cathodic compared to the anodic peak current (data not shown). Under finite diffusion (thin-layer) conditions this oxidation was totally irreversible. When the full oxidative potential range was scanned (semi-infinite conditions, scan rate V ) 250 mV s-1), five oxidative waves were collected in total (data not shown). In contrast to compound 1 the first oxidation of 2 turned out to be fully reversible at an oxidation potential of E1/2ox1 ) -150 mV vs Fc/Fc+ (see Figure 2, left). The less negative redox potential of 2 compared to 1 is due to the electron withdrawing influence of the methylcarboxylate acceptor group. In total compound 2 undergoes four reversible oxidations (see Table 1). The first redox splitting ∆E of 2 (375 mV) is almost as high as found for tetraanisyl-p-phenylenediamine (485 mV in CH2Cl2 and 340 mV in DMSO) which has been classified as a valence delocalized Robin/Day class III compound,1,27,28 and the redox splitting is even higher than that of 3 (in CH2Cl2, E1/2ox1 ) 60 mV, E1/2ox2 ) 320 mV, ∆E ) 260 mV). In spite of the electron-withdrawing methylcarboxylate substituent the first oxidation potential of 2 is 210 mV lower and the irreversible oxidation of the isomeric system 1 is even at 370 mV lower potential than the first oxidation of 3. This effect might be due to the significantly lower ionization potential of azulene (IE ) 7.42 eV29) compared to naphthalene (IE ) 8.14 eV29). Obviously the electronic properties of azulene mix into the arylamine character which lowers the overall redox potentials of 1 and 2 considerably. Thin layer multi-cycle CVs (which excludes diffusional effects) of 2 were carried out in order to prove the long-term stability of the radical cations of 2. Under these conditions only the first three oxidations of 2 are reversible in CH2Cl2 (Figure 2, right). The redox potentials of 1, 2, and 3 in CH2Cl2 are summarized in Table 1. Spectroelectrochemical measurements of 2 were carried out in transmission by a thin-layer cell with a transparent gold minigrid working electrode (solvent: CH2Cl2, supporting electrolyte: TBAH, 0.2 M) as described elsewhere.21 The spectra
3514 J. Phys. Chem. C, Vol. 111, No. 8, 2007
No¨ll et al.
Figure 2. CVs of 2 in CH2Cl2 under semi-infinite conditions measured with a scan rate of V ) 250 mVs-1 (left) and under finite diffusion (thin-layer) conditions with V ) 20 mVs-1 (right).
TABLE 1: Redox Potentials of 1-3 in Dichloromethane/ TBAH, 0.2 M vs Fc/Fc+ Collected at a Scan Rate of W ) 250 mV s-1 E1/2ox1/mV -310 -150 60c
E1/2ox2/mV
E1/2ox3/mV
E1/2ox4/mV
225 320c
1000 1210a
1210b
2+
V˜ max1/cm-1 (/M-1cm-1) 7850 (10050)
22+ 23+
9700 (36350) 5350 (3900)
3+
8000 (19100) a
32+
12150 (72000) b
∆E/mV
a
1 2 3
TABLE 2: Absorption Maxima W˜ max between 4000 and 20000 cm-1, Molar Absorptivity E, and Electronic Coupling V of the Radical Cations 2+, 22+, 23+, 3+, and 32+ in CH2Cl2
375 260c
Peak potential of an irreversible process, this work. b Irreversible process under finite diffusion (thin-layer) conditions. c See ref 1. a
a
V˜ max2 /cm-1 (/M-1cm-1) 18000 (17400) sh at 16000 16950 (21600) 11050 (46000); 3rd max, V˜ max3 at 15450 (34150) 17900 (19700) a sh at 16000 16000 (24400) b
V/cm-1 3900
4000a
See ref 1. b See ref 31.
TABLE 3: Experimental (2+) and Calculated Transition Energies (1+) exp (2+) DFT (1+)
Figure 3. Spectra of the radical cation, dication, and trication of 2 measured by spectroelectrochemistry in CH2Cl2 (supporting electrolyte: TBAH, 0.2 M).
of the mono radical cation, dication, and trication of 2 in the range 4000-20000 cm-1 are shown in Figure 3. The spectrum of the mono radical cation 2+ in Figure 3 shows two absorption maxima at 7850 cm-1 ( ) 10050 M-1 cm-1) and 18000 cm-1 ( ) 17400 M-1 cm-1, sh at 16000 cm-1) similar to the tetraanisylnaphthylenediamine radical cation 3+ (maxima at 8000 cm-1, ) 19100 M-1 cm-1, and 17900 cm-1, ) 19700 M-1 cm-1, sh at 16000 cm-1).1 An absorption band with a maximum between 12000 and 15000 cm-1 in the spectrum of 2+ as it is typical of the πfπ*excitation in localized triarylamine radical cations belonging to the Robin/Day class II5,6,30 is not observed. The band shape of the broad low energy absorption band of 2+ at 7850 cm-1 deviates remarkably from a Gaussian-shaped curve and shows a stronger decrease at the low-energy side. According to this behavior, this band has some similarities to IV-CT bands of strongly coupled triarylamine based mixed-valence systems that are close to the class-II/class-III borderline6,27,28 such as tetraanisylnaphthylenediamine radical cation 3+1. A spectrum of
V˜ /cm-1
µ/D
7850 8300
6.7 6.9
2+ has also been recorded in acetonitrile without any remarkable solvatochromic shift (maximum of the low energy absorption 8050 cm-1, data not shown). The asymmetric band shape and the negligible solvatochromism observed for 2+ suggests that, at least in relatively apolar media, 2+ is approaching a fully delocalized class III structure. Therefore, the electronic coupling element V ) 3900 cm-1 can be estimated from the maximum of the “IV-CT” band in CH2Cl2 with V ) V˜ max/2. The absorption maxima V˜ max between 4000 and 20000 cm-1, molar absorptivity , and electronic coupling V of the radical cations 2+, 22+, 23+, 3+, and 32+ in CH2Cl2 are summarized in Table 2. While the absorption spectra of 2+ and 3+ are very similar, those of 22+ and 32+ are entirely different. The low energy absorption band of 22+ is not in the typical energy range for πfπ*-excitations of localized triarylamine radical cations. This is in contrast to the spectrum of 32+ which shows an intense absorption at 12150 cm-1 ( ) 72000 M-1 cm-1).31 In the spectrum of 23+, the lowest absorption band at 5350 cm-1 is clearly Gaussian-shaped. However, the nature of this absorption remains obscure. After complete oxidation to the trication, the potential was switched back to negative values. A spectrum that equals the initial spectrum (collected before the oxidation was
Charge Resonance Excitations in Azulenes
J. Phys. Chem. C, Vol. 111, No. 8, 2007 3515 these observations into account we come to the conclusion that an 1,3-azulene bridging unit has the ability to mediate strong electronic coupling between attached redox centers. Due to their extremely low first oxidation potential that minimizes the energy barrier for hole injection,33 1,3-diaminoazulenes may be good candidates for novel systems of hole transporting materials in organic light emitting diodes (OLEDs). Work on 1,3-disubstituted azulenes, related to the present study, has been recently published.34-38
Figure 4. Representation of B3LYP/SV(P)-derived HOMOβ and SOMOβ orbitals of 1+.
started) was detected indicating complete reversibility of all three investigated redox processes. Time dependent density functional theory (TD-DFT) B3LYP/ SV(P) calculations were carried out in order to gain insight into the electronic nature of the optical transitions. The DFToptimization of the electronic ground state of the mono radical cation 1+ was done with symmetry restrictions within C2. Although the ester substituent was neglected the calculated energy of the first doublet excitation (D0 f D1) as well as the computed transition dipole moment of this excitation are in good agreement with the experiment of the mono radical cation 2+ (see Table 3). The empty SOMOβ orbital of the mono radical cation 1+ clearly indicates a symmetric structure. The positive charge is localized at the azulene and both nitrogen centers (see Figure 4). According to the TD-DFT computations the first electronic transition mainly consists of a Koopman type HOMOβSOMOβ excitation.32 The nitrogen center orbitals show alternating phase behavior (the nitrogen orbital coefficients are in phase in the HOMO and out of phase in the SOMO); this transition consists of a mixture of a dianisylamine to azulene charge transfer with a charge transfer within the azulene bridge. This HOMOβ-SOMOβ transition can be conceived as the Hush type intervalence transition but like that in other delocalized class III species may also be termed a charge resonance type transition because of the absence of a dipole moment change along the N-N vector. We also calculated the electron spin density of 1+ (data not shown). The highest spin density is located at the nitrogen centers and the C-5/C-7 positions whereas no spin density was found at C-2. Summary and Conclusion In the present study, we show that the electrochemical stability of the 1,3-bis[di(4-methoxyphenyl)amino]azulene 1 is clearly increased by a methylcarboxylate substituent at the C-2-position of the azulene subunit as demonstrated by compound 2. In contrast to the naphthalene derivative 3 that shows only two reversible oxidations in the CV, compound 2 undergoes four reversible oxidations in CH2Cl2 under semi-infinite conditions. The first oxidation potential of 1 and even of 2 is significantly lower than that of 3 which is due to the much lower ionization potential of the azulene moiety. The vis/NIR spectra of the radical cations 2+, 22+, and 23+ exhibit rather intense absorption bands in the NIR region. The NIR absorption of 2+ is typical for a valence delocalized cationic species. DFT calculations of 1+ yield a SOMOβ-orbital indicating a symmetric structure with the positive charge and the spin density mainly at the nitrogen centers and the azulene bridge. The lowest energy excitation and the electronic coupling in 2+ is similar to the one in the tetraanisylnaphthylenediamine radical cation 3+ whereas the absorption spectra of 22+ and 32+ are entirely different. Taking
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. References and Notes (1) Lambert, C.; Risko, C.; Coropceanu, V.; Schelter, J.; Amthor, S.; Gruhn, N. E.; Durivage, J. C.; Bredas, J.-L. J. Am. Chem. Soc. 2005, 127, 8508. (2) Schneider, J. J.; Spickermann, D.; Labahn, T.; Magull, J.; Fontani, M.; Laschi, F.; Zanello, P. Chem.sEur. J. 2000, 6, 3686. (3) Barybin, M. V.; Chisholm, M. H.; Dalal, N. S.; Holovics, T. H.; Patmore, N. J.; Robinson, R. E.; Zipse, D. J. J. Am. Chem. Soc. 2005, 127, 15182. (4) Lambert, C.; No¨ll, G.; Kriegisch, V.; Zabel, M.; Hampel, F.; Schma¨lzlin, E.; Meerholz, K.; Bra¨uchle, C. Chem.sEur. J. 2003, 9, 4232. (5) Lambert, C.; Amthor, S.; Schelter, J. J. Phys. Chem. A 2004, 108, 6474. (6) Lambert, C.; No¨ll, G. J. Am. Chem. Soc. 1999, 121, 8434. (7) Bonvoisin, J.; Launay, J.-P.; Verbouwe, W.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 17079. (8) 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. (9) Noell, G.; Avola, M. J. Phys. Org. Chem. 2006, 19, 238. (10) 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. (11) Newton, M. D. AdV. Chem. Phys. 1999, 106, 303. (12) Bailey, S. E.; Zink, J. I.; Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 5939. (13) Nelsen, S. F.; Newton, M. D. J. Phys. Chem. A 2000, 104, 10023. (14) Sun, D.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2004, 126, 1388. (15) Sun, D.; Rosokha, S. V.; Kochi, J. K. Angew. Chem., Int. Ed. 2005, 44, 5133. (16) 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. (17) 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. (18) Amthor, S.; Lambert, C. J. Phys. Chem. A 2006, 110, 1177. (19) Lambert, C.; No¨ll, G. Synth. Met. 2003, 139, 57. (20) Hafner, K.; Patzelt, H.; Kaiser, H. Liebigs Annal. Chem. 1962, 656, 24. (21) Salbeck, J. Anal. Chem. 1993, 65, 2169. (22) Ahlrichs, R.; Ba¨r, M.; Baron, H.-P.; Bauernschmitt, R.; Bo¨cker, S.; Ehrig, M.; Eichkorn, K.; Elliott, S.; Furche, F.; Haase, F.; Ha¨ser, M.; Ha¨ttig, C.; Horn, H.; Huber, C.; Huniar, U.; Kattannek, M.; Ko¨hn, A.; Ko¨lmel, C.; Kollwitz, M.; May, K.; Ochsenfeld, C.; O ¨ hm, H.; Scha¨fer, A.; Schneider, U.; Treutler, O.; Tsereteli, K.; Unterreiner, B.; von Arnim, M.; Weigend, F.; Weis, P.; Weiss, H. TURBOMOLE V5.6; Quantum Chemistry Group, University of Karlsruhe: Karlsruhe, Germany, 2002. (23) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. F. W. MOLEKEL 4.3; Swiss Center for Scientific Computing: Manno, Switzerland; 20002002. (24) Mitchell, R. H.; Chen, Y.; Zhang, J. Org. Prep. Proc. Int. 1997, 29, 715. (25) Redl, F. X.; Ko¨the, O.; Ro¨ckl, K.; Bauer, W.; Daub, J. Macromol. Chem. Phys. 2000, 201. (26) Michl, J.; Thulstrup, E. W. Tetrahedron 1976, 32, 205. (27) 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.
3516 J. Phys. Chem. C, Vol. 111, No. 8, 2007 (28) 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. (29) Linstrom, P. J., Mallard, W. G., Eds. NIST Chemistry WebBook; NIST Standard Reference Database Number 69; National Institute of Standards and Technology: Gaithersburg MD (http://webbook.nist.gov), June 2005. (30) Amthor, S.; Noller, B.; Lambert, C. Chem. Phys. 2005, 316, 141. (31) Schelter, J. Dissertation, University of Wu¨rzburg, 2003. Available online: http://opus.bibliothek.uni-wuerzburg.de/opus/volltexte/2004/837. (32) Nelsen, S. F.; Weaver, M. N.; Zink, J. I.; Telo, J. P. J. Am. Chem. Soc. 2005, 127, 10611.
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