J. Phys. Chem. C 2007, 111, 1993-1998
1993
Fluorescence Properties of Si-Linked Oligothiophenes Mamoru Fujitsuka,† Dae Won Cho,†,‡ Joji Ohshita,§ Atsutaka Kunai,§ and Tetsuro Majima*,† The Institute of Scientific and Industrial Research (SANKEN), Osaka UniVersity, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan, Department of Chemistry, Chosun UniVersity, Gwangju 501-759, Korea, and Department of Applied Chemistry, Graduate School of Engineering, Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan ReceiVed: September 27, 2006; In Final Form: NoVember 20, 2006
The fluorescence properties of Si-linked oligothiophenes were investigated by using various spectroscopic methods. From the molecular orbital calculation, it was indicated that Si-linked oligothiophenes tend to take structures with a smaller repulsion between oligothiophene entities and methyl groups on the Si-atom. Steady state absorption spectra showed red-shifted absorption peaks due to the σ-π conjugation when compared with the corresponding oligothiophenes. Peak shift to the longer wavelength side was also confirmed in the fluorescence spectra. Si-linked terthiophenes showed excimer formation upon excitation with a pulsed laser, although its formation yield was as small as ∼0.1-0.2. On the other hand, Si-linked pentathiophenes do not form the excimer probably because of the lower mobility of larger oligothiophenes. From the anisotropy decay, excitation energy migration among the oligothiophenes was confirmed. The excitation energy migration was discussed on the basis of the incoherent energy hopping mechanism. The effect of σ-π conjugation on the present energy transfer was indicated.
Introduction Conjugated polymers and their oligomers have been applied to various optical devices such as light-emitting diodes and so on.1 From this viewpoint, excitation and deactivation processes of these materials attract much attention from a wide range of researchers. It is well-known that the deactivation process of these materials largely depends on molecular structures and morphology, because a lower energy site can be easily produced upon formation of irregular structures such as stacking.2 Fast energy migration along a polymer chain enables efficient energy deactivation through these lower energy sites.3-6 Thus, the formation process of these sites and the energy migration process are important subjects to be cleared. Recently, we have investigated the energy migration process along the polymer chain in a series of alternating block copolymers of oligothiophene and oligosilylene by observing the fluorescence anisotropy decay in the picosecond region.7 It was revealed that the energy migration is governed by the incoherent energy hopping mechanism. Furthermore, it became clear that the singlet excited energy deactivated to the relaxed singlet excited state which can be attributed to some conformational changes such as planarization and stacking of oligothiophenes in the polymer chain. In order to elucidate further details in the deactivation process of these materials, investigation on the structurally well-defined compounds is essential. In the present study, we investigated the fluorescence properties of a series of oligothiophenes connected by Si-atoms (nTm, n and m denote the number of thiophene rings and oligothiophenes, respectively; see Figure 1). 3T3 and 5T3 have a branched structure with three oligothiophenes. One of the * To whom correspondence should be addressed. E-mail: majima@ sanken.osaka-u.ac.jp. † Osaka University. ‡ Chosun University. § Hiroshima University.
oligothiophenes of 3T3 was changed to a methyl group in the case of 3T2, in order to investigate the effect of the number of chromophores. Although 5T of 5T2 in the present study is not connected by the three-silicon-atom linkage, the energy transfer rate in 5T2 is useful to discuss the conformation dependence of the energy transfer rate. The molecular structures of these compounds were estimated by molecular orbital calculation. From the time-resolved fluorescence measurements, the relaxation process of the singlet excited state was investigated. Furthermore, the excitation energy migration in Si-linked oligomers was confirmed by the anisotropy decay, which was discussed on the basis of the molecular structure and σ-π conjugation. Experimental Materials. 5T2 was prepared as reported in the literature.8 Other Si-linked oligothiophenes were synthesized according to the procedure summarized in the Supporting Information. All other chemicals were of the best commercial grades available. Apparatus. The time-resolved fluorescence spectra were measured by the single photon counting method using a streakscope (Hamamatsu Photonics, C4334-01) equipped with a polychromator (Acton Research, SpectraPro150). The ultrashort laser pulse was generated by a Ti:sapphire laser (Spectra-Physics, Tsunami 3941-M1BB, 80 fs fwhm) pumped with a diode-pumped solid state laser (Spectra-Physics, Millennia VIIIs). For excitation of the sample, the output of the Ti:sapphire laser was converted to the second harmonic oscillation (390 or 430 nm) with a harmonic generator (SpectraPhysics, GWU-23FL). The fluorescence lifetime in the sub-picosecond regime was estimated using the fluorescence up-conversion method. The second harmonic oscillation (390 or 430 nm) of the output of the femtosecond laser (780 or 860 nm) was used to excite the sample in a cell with a 1.0 mm optical path length. The residual fundamental and the fluorescence were focused in a BBO type
10.1021/jp066336y CCC: $37.00 © 2007 American Chemical Society Published on Web 01/11/2007
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Figure 1. Molecular structures of Si-linked oligothiophenes (nTm) and oligothiophenes.
Figure 2. Optimized structures of 3T3 and 5T2 (side view (a, d) and top view (b, e)) at the B3LYP/3-21G* level. A possible structure for the excimer of 3T3 (c). In the theoretical calculation, ethyl groups at the R position of oligothiophenes were reduced to methyl groups for the simplicity of the calculation.
I crystal to generate a sum-frequency oscillation, which was detected by a photomultiplier tube (Hamamatsu Photonics, H8259) and the photon counter (Stanford Research Systems, SR400) after passing through the monochromator (Nikon G250). The cross-correlation time of the apparatus was 300 fs fwhm. The steady state absorption and fluorescence spectra were measured using Shimadzu UV-3100PC and Hitachi 850 instruments, respectively. All of the spectroscopic measurements were carried out under the Ar-saturated condition in order to avoid possible sample degradation.
Optimized structures of Si-linked oligothiophenes were estimated at the B3LYP/3-21G* level using the Gaussian 03 package.9 Results and Discussion Optimized Structures. Figure 2a and b show the energyminimized structure of 3T3 calculated at the B3LYP/3-21G* level. All oligothiophenes take a planar structure, which enhances the π-electron delocalization over the oligothiophene entity. The central Si-atom takes a tetrahedron structure, in
Fluorescence of Si-Linked Oligothiophene
J. Phys. Chem. C, Vol. 111, No. 5, 2007 1995 TABLE 1: Absorption and Fluorescence Properties of nTm in Toluene 3T1 3T2 3T3 3Tb 5T2 5T3 5T b
λabs/nm
λf/nm
Φfa
τf/ns
368 370 370 354 425 423 416
423, 445 429, 454 433, 456 406, 426 492, 528 499, 527 482, 514
0.098 0.12 0.094 0.066 0.46 0.39 0.36
0.24 (100%) 0.21 (96%), 2.66 (4%) 0.19 (92%), 2.48 (8%) 0.21 (100%) 0.85 (100%) 0.88 (100%) 0.82 (100%)
a The Φ values were estimated by using Φ values of 3T and 5T as f f references (ref 10). b Data from ref 10.
Figure 3. Normalized absorption and fluorescence spectra in toluene: (a) 3T3 (red), 3T2 (blue), and 3T1 (green); (b) 5T3 (red) and 5T2 (blue).
which the angle of (3TSi)-Si-(Si3T) is 111.2°, which is larger than 109.5° for an ideal tetrahedron structure. From the molecular orbital calculation on CH3Si(Si(CH3)3)3, the angle of (CH3Si)-Si-(SiCH3) was estimated to be 111.0°, which is close to that of 3T3. The repulsion between the methyl groups attached to the Si-atom enlarged the Si-Si-Si angle from the ideal value. This same result was also confirmed for 5T3, in which the angle of (5TSi)-Si-(Si5T) is 111.3° (Supporting Information Figure S1). In the case of 3T2, the angle of (3TSi)Si-(Si3T) is 114.9°, indicating that the absence of one 3T group makes it possible to take a structure with lower repulsion between oligothiophene entities (Supporting Information Figure S1). Thus, the repulsion between the oligothiophenes is another important factor determining the energy-minimized structure of Si-linked oligothiophenes. In the case of 5T2 (Figure 2d and e), oligothiophenes are forwarded to the opposite direction to minimize the repulsion. Steady State Absorption. Figure 3a shows the absorption spectra of the Si-linked terthiophenes. An absorption peak (λabs) of 3T1 appeared at 368 nm, which is red-shifted when compared to terthiophene with a λabs value of 354 nm.10 The shift of the absorption peak can be attributed to the σ-π conjugation due to the Si-atom connected to terthiophene. This tendency was also confirmed with 3T2 and 3T3, which showed further shift of λabs compared to 3T1, indicating the effect of further conjugation of oligosilylene. The difference between 3T2 and 3T3 is rather small. The absorption tail of 3T3 showed a small shift compared with that of 3T2. On the other hand, an increase of absorption shorter than 350 nm can be attributed to an increase of σ conjugation of oligosilylene.11 It should be stressed that the quite small change of absorption spectra of 3T1, 3T2, and 3T3 supports the molecular structures discussed in the above section, in which it was revealed that the Si-linked oligothiophenes tend to take a structure minimizing the repulsion between oligothiophenes and methyl groups on Si. Thus, the stacking structure is inhibited in the ground state. In the case of 5T2 and 5T3 (Figure 3b, Table 1), absorption peak shift due to σ-π conjugation was confirmed when compared with the λabs value of pentathiophene,10 while the difference between 5T2 and 5T3 was small. These results can be explained on the basis of the molecular structure as in the case of the Si-linked terthiophenes. Fluorescence Spectra. The effect of the σ-π conjugation was also observed in the fluorescence peak (λf) shift of the Si-
Figure 4. Time-resolved fluorescence spectra integrated between 0 and 250 ps (black) and 1.0 and 4.0 ns (red) after the laser pulse (400 nm, 80 fs fwhm) of 3T3 in toluene at room temperature. Inset: semilog plot of fluorescence decay profiles estimated at room temperature (black) and 77 K (blue).
linked oligomers compared with the reference compounds (Figure 3, Table 1). It is interesting to note that the fluorescence spectrum of 3T3 shows a substantial enhancement around 510 nm, when compared with that of 3T1. The possible reason for the observed enhancement will be an intramolecular excimer formation of a terthiophene moiety, because a Si-(Si3T) bond allows a rotation which places terthiophene moieties with rather close proximity to another, as shown in Figure 2c. The molecular structure of Figure 2c was obtained by energy minimization after rotating around the Si-Si bond and was found to be 11.2 kcal mol-1 higher in energy than the energy-minimized structure (Figure 2a and b). In the molecular structure of Figure 2c, the distance between terthiophenes is about 4.2-6.2 Å, which is sufficiently close to form an excimer. In the present case, intermolecular excimer formation can be ruled out because of low concentration (∼10-6 M). Furthermore, 3T1 did not show the excimer fluorescence. In order to confirm the intramolecular excimer formation process, time-resolved fluorescence spectra of 3T3 were measured, as shown in Figure 4. The fluorescence spectrum observed immediately after the laser excitation (0-250 ps) is close to the monomer-like fluorescence. As shown in the inset of Figure 4, the fluorescence decay profile exhibits two-component decay, of which the slow component showed a peak around 510 nm with a quite broad spectral feature. The observed fluorescence spectrum with the peak around 510 nm in Figure 4 explains the enhancement around 510 nm in the steady state fluorescence spectrum of 3T3 (Figure 3a). Intramolecular excimer formation was also supported by the fact that the almost single exponential fluorescence decay was confirmed for the sample in low temperature glass at 77 K, in which the molecular motion is inhibited (inset of Figure 4). The lifetime (τf) of monomer-like fluorescence (i.e., fast decay component of 3T3) at room temperature is slightly shorter than
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the τf value of 3T1. The observed difference in the τf values can be attributed to the formation of the intramolecular excimer. Thus, the excimer formation rate (kex) can be estimated to be 1.1 × 109 s-1 from the relation
kex ) 1/τf(3T3) - 1/τf(3T1)
(1)
where τf(3T3) and τf(3T1) are the τf values of 3T3 and 3T1, respectively. Similar intramolecular excimer formation was observed for 3T2 in the time-resolved fluorescence spectra measurements. The kex value was estimated to be 5.7 × 108 s-1 from the τf value of 3T2. The kex value of 3T2 is about one-half of the kex value of 3T3. This difference in the kex value will be related to the possibility of finding the partner of the excimer formation upon rotation of excited terthiophene around the Si-Si bond: Twice the possibility is expected for 3T3 when compared with 3T2. The kex value is not so large in comparison with the deactivation rate of terthiophene. The yield of the excimer formation can be calculated to be 0.20 and 0.12 for 3T3 and 3T2, respectively. Thus, the excimer formation is a minor process in the deactivation pathway of the singlet excited state of Silinked terthiophenes. This will be the reason for the inefficient formation of the excimer. The lifetime of the excimer was 2.48 and 2.66 ns for 3T3 and 3T2, respectively, and almost independent of the number of oligothiophenes. It is interesting to note that the excimer formation was not observed with the Si-linked pentathiophenes, as evidenced from the almost identical fluorescence lifetimes of these compounds (Table 1). A possible reason for the absence of intramolecular excimer formation will be a larger volume of pentamer, which slows down the rotation of the Si-Si bond to inhibit excimer formation within the lifetime of the singlet excited state. In the previous paper on the deactivation process of the alternating block copolymer of oligothiophene and oligosilylene, we found that the polymer with longer oligothiophene did not show fluorescence spectral change due to conformational change with time evolution.7 Therefore, the present observation of no excimer formation of the Si-linked pentamer will be another example indicating the lower mobility of longer oligothiophenes. For various aromatic hydrocarbons, it has been reported that the introduction of Si-including substituents often increases the fluorescence quantum yield.12 As shown in Table 1, the increase in fluorescence quantum yield was also observed with the present Si-linked oligothiophenes. Especially in the case of 3T2, the quantum yield is almost double when compared with terthiophene. Fluorescence Anisotropy. The intramolecular excimer discussed above was formed by structural relaxation after the excitation to the singlet excited state. Thus, immediately after the excitation, Si-linked oligothiophenes are expected to take the same structure as the energy-minimized one discussed on the basis of the molecular orbital calculation. Upon excitation to the singlet excited state, the excitation energy is expected to move to other oligothiophene entities by the energy migration. In order to obtain the evidence and kinetic data of the energy migration in the present Si-linked oligothiophenes, anisotropy of the fluorescence was measured by using the fluorescence upconversion method. The parallel (I|) and perpendicular (I⊥) components of the fluorescence with respect to the pump pulse were obtained, as indicated in Figure 5. The time course of the anisotropy (r(t)) in Figure 5 was estimated by eq 2
r(t) ) (I| - GI⊥)/(I| + 2GI⊥)
(2)
Figure 5. Fluorescence (upper panel) and anisotropy (r, lower panel) profiles of 3T2 (a), 3T3 (b), 5T2 (c), and 5T3 (d) in toluene. The black and gray lines are parallel and perpendicular components, respectively. The dashed line is the irf. The thick lines in the lower panels are the fitted curves.
where G is a constant. Applying the single exponential function, the time constants for the anisotropy decay were estimated to be (4.4 ps)-1, (5.2 ps)-1, (8.0 ps)-1, and (9.4 ps)-1 for 3T2, 3T3, 5T2, and 5T3, respectively. Since 5T did not show any anisotropy decay in this time region, the anisotropy decay observed for the Si-linked oligothiophenes can be attributed to the singlet energy migration among the oligothiophene entities. The incoherent energy hopping mechanism is usually employed to explain the energy migration process in various supramolecular systems such as porphyrin arrays.5 To obtain a better understanding, the Fo¨rster theory (eqs 3 and 4)13 has been employed to evaluate the singlet energy migration rate (k) by the hopping mechanism
k)
9000 ln 10κ2Φ J 128π5nD4NAR6τ
(3)
J)
∫F(ν) (ν)ν-4 dν
(4)
where κ, Φ, nD, NA, R, J, F, and are the orientation factor, fluorescence quantum yield, refractive index, Avogadro’s number, distance, spectral overlap, donor-fluorescence spectrum, and acceptor-absorption spectrum, respectively. From the optimized structure discussed above and absorption and fluorescence spectra, J, R, and k values were calculated, as summarized in Table 2. It should be noted that the R values were estimated assuming that the excitation is delocalized on the oligothiophene moiety. Although the calculated values are 22-30 times smaller than the observed values (kobsd in Table 2), the observed tendency can be explained on the basis of the theoretical calculation. The calculated energy transfer rate
Fluorescence of Si-Linked Oligothiophene
J. Phys. Chem. C, Vol. 111, No. 5, 2007 1997
TABLE 2: Spectral Overlap (J), Distance (R), and Calculated and Observed Energy Transfer Rates (k and kobsd, Respectively) 3T2 3T3 5T2 5T3
R/Å
J/M-1 cm3 a
k/s-1
kobsd/s-1
14.7 14.1 22.6 20.7
2.9 × 10-15 2.9 × 10-15 2.2 × 10-14 1.9 × 10-14
7.7 × 109 (1.0) 7.1 × 109 (0.92) 6.0 × 109 (0.78) 4.1 × 109 (0.53)
2.3 × 1011 (1.0) 1.9 × 1011 (0.85) 1.3 × 1011 (0.55) 1.1 × 1011 (0.47)
a
The J values were estimated using the extinction coefficients of the corresponding monomers (26 000 and 40 000 M-1 cm-1 at absorption peaks of 3T and 5T, respectively).
constants are in the order of 3T2 > 3T3 > 5T2 > 5T3 which is in the same order with the observed one. Furthermore, the relative rate constants (values in parentheses in Table 2) are similar to the observed ones. Therefore, the present energy migration process can be attributed to the incoherent hopping mechanism. Goodson and co-workers reported that the energy migration rate in the N-centered dendrimer is faster than those of the Cand P-centered dendrimers.6 They attributed the fast energy migration rate to the coherent mechanism because of a strong interaction in the N-centered dendrimer, in which chromophores are connected by only one N-atom. In the Si-linked oligothiophenes in the present study, the number of silicon atoms connecting oligothiophenes is three. The longer linker seems to diminish the interaction to give the incoherent hopping. The energy transfer rates of the Si-linked pentathiophenes are slower than those of the Si-linked terthiophenes, in spite of larger J values. The J values of pentathiophenes are 1 order larger than those of terthiophenes because of the larger fluorescence quantum yields and larger extinction coefficients of pentamers. The slower energy transfer rates of the Si-linked pentathiophenes can be attributed to the longer distance between the two oligothiophenes, in which excitation energy should move. Therefore, the R-6 dependence of the energy transfer rate (eq 3) resulted in the slower energy transfer rate of pentathiophenes. In the case of 3T2 and 3T3, the energy transfer rates are almost the same because of the relatively similar conformations. In the case of 5T2 and 5T3, on the other hand, the energy transfer in 5T3 is slower than that in 5T2. This difference can be attributed to the conformation of oligothiophenes, because the parallel dipole moments in 5T2 resulted in a larger κ2 value than that of 5T3 with dipole moments in a bent conformation. As indicated above, the calculated k values were estimated by assuming that the excitation delocalized on the oligothiophene moiety. If the excitation is delocalized over the oligothiophene and Si-atom at the R position of oligothiophene, the R values become smaller than the listed values in Table 2 by about 1 Å. On the basis of this assumption, the k values became about 2 times larger than the listed values. Therefore, one of the reasons for the discrepancy between the kobsd and k values should be the delocalization of the excitation due to the enhanced conjugation of the present compounds due to the σ-π conjugation. Conclusion In the present study, we revealed various interesting fluorescence properties of the Si-linked oligothiophenes. For the Si-linked terthiophenes, the excimer formation was confirmed although it was not observed with the Si-linked pentathiophenes, indicating the large difference in the mobility of oligothiophenes. The energy migration in the Si-linked oligothiophenes was confirmed by the observation of the fluorescence anisotropy
decay. The decay rate was explained qualitatively on the basis of the molecular structure. These results indicate that the studies on the well-defined oligomers give useful information on the excitation process of these interesting functional materials. Acknowledgment. This work has been partly supported by a Grant-in-Aid for Scientific Research (Project 17105005, Priority Area (417), 21st Century COE Research, and others) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government. Supporting Information Available: Synthetic methods and spectroscopic data of Si-linked oligothiophenes and the energyminimized structures of 5T3 and 3T2 and XYZ coordinates. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Baldo, M. A.; Thompson, M. E.; Forest, S. R. Nature 2000, 403, 750. (b) Brown, T. M.; Kim, J. S.; Friend, R. H.; Cacialli, F.; Daik, R.; Feast, W. J. Appl. Phys. Lett. 1999, 75, 1679. (c) Cao, Y.; Yu, G.; Heeger, A. J. AdV. Mater. 1998, 10, 12. (d) Kido, J.; Matsumoto, T. Appl. Phys. Lett. 1998, 73, 2886. (e) Barbarella, G.; Favaretto, L.; Zambiannchi, M.; Pudova, O.; Arbizzani, C.; Bongini, A.; Mastragostino, M. AdV. Mater. 1998, 10, 551. (f) Gigli, G.; Lomascolo, R.; Cingolani, R. Babarella, G.; Zambianchi, M.; Antolini, L.; Della Sala, F.; Di Carlo, A.; Lugli, P. Appl. Phys. Lett. 1998, 73, 2414. (2) (a) Samuel, I. D. W.; Crystall, B.; Rumbles, G.; Burn, P. L.; Holmes, A. B.; Friend, R. H. Chem. Phys. Lett. 1993, 213, 472. (b) Hayes, G. R.; Samuel, I. D. W.; Phillips, R. Phys. ReV. B 1995, 52, 11569. (c) Watanabe, A.; Kodaira, T.; Ito, O. Chem. Phys. Lett. 1997, 273, 227. (d) Sato, T.; Fujitsuka, M.; Segawa, H.; Shimidzu, T.; Tanaka, K. Synth. Met. 1998, 95, 107. (e) Theander, M.; Ingana¨s, O.; Mammo, W.; Olinga, T.; Svensson, M.; Andersson, M. R. J. Phys. Chem. B 1999, 103, 7771. (3) (a) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: Cambridge, U.K., 1985. (b) Webber, S. E. Chem. ReV. 1990, 90, 1469. (4) (a) Bradforth, S. E.; Jimenez, R.; van Mourik, F.; van Grondelle, R.; Fleming, G. R. J. Phys. Chem. 1995, 99, 16179. (b) Koolhaas, M. H. C.; van der Zwan, G.; Frese, R. N.; van Grondelle, R. J. Phys. Chem. B 1997, 101, 7262. (c) Jimenez, R.; van Mourik, F.; Young, Yu, J.; Fleming, G. R. J. Phys. Chem. B 1997, 101, 7350. (d) Scholes, G. D.; Fleming, G. R. J. Phys. Chem. B 2000, 104, 1854. (5) Cho, H. S.; Rhee, H.; Song, J. K.; Min, C.-K.; Takase, M.; Aratani, N.; Cho, S.; Osuka, A.; Joo, T;, Kim, D. J. Am. Chem. Soc. 2003, 125, 5849. (6) (a) Varnavski, O.; Samuel, I. D. W.; Pålsson, L.-O.; Beavington, R.; Burn, P. L.; Goodson, T., III. J. Chem. Phys. 2002, 116, 8893. (b) Ranasinghe, M. I.; Wang, Y.; Goodson, T., III. J. Am. Chem. Soc. 2003, 125, 5258. (c) Wang, Y.; Ranasinghe, M. I.; Goodson, T., III. J. Am. Chem. Soc. 2003, 125, 9562. (d) Rnanasinghe, M. I.; Murphy, P.; Lu, Z.; Huang, S. D.; Twieg, R. J.; Goodson, T., III. Chem. Phys. Lett. 2004, 383, 411. (e) Ranasinghe, M. I.; Hager, M. W.; Gorman, C. B.; Goodson, T., III. J. Phys. Chem. B 2004, 108, 8543. (f) Varnavski, O.; Goodson, T., III; Sukhomlinova, L.; Twieg, R. J. Phys. Chem. B 2004, 108, 10484. (g) Yan, X. Y.; Pawlas, J.; Goodson, T., III; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 9105. (h) Wang, Y.; He, G. S.; Prasad, P. N.; Goodson, T., III. J. Am. Chem. Soc. 2005, 127, 10128. (i) Goodson, T., III. Acc. Chem. Res. 2005, 38, 99. (7) Fujitsuka, M.; Cho, D. W.; Ohshita, J.; Kunai, A.; Majima, T. J. Phys. Chem. B 2006, 110, 12446. (8) Kim, D.-H.; Ohshita, J.; Kunugi, Y.; Kunai, A. Chem. Lett. 2006, 35, 266. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
1998 J. Phys. Chem. C, Vol. 111, No. 5, 2007 Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (10) Becker, R. S.; de Melo, J. S; Mac¸ anita, A. L.; Elisei, F. Pure Appl. Chem. 1995, 67, 9. (11) Tsuji, H.; Michl, J.; Tamao, K. J. Organomet. Chem. 2003, 685, 9. (12) (a) Kyushin, S.; Ikarugi, M.; Goto, M.; Hiratsuka, H.; Matsumoto, H. Organometallics 1996, 15, 1067. (b) Maeda, H.; Inoue, Y.; Ishida, H.;
Fujitsuka et al. Mizuno, K. Chem. Lett. 2001, 1224. (c) Maeda, H.; Nishimura, K.; Mizuno, K.; Yamaji, M.; Oshima, J.; Tobita, S. J. Org. Chem. 2005, 70, 9693. (d) Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K.; Shimizu, H.; Inouye, M. Chem.sEur. J. 2006, 12, 824. (13) (a) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7. (b) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/ Plenum Publishers: New York, 1999.
