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Fluorescent cyclopentadithiophene derivatives having phenyl-substituted silyl moieties
Tetrahedron Letters 52 (2011) 4039–4041
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Fluorescent cyclopentadithiophene derivatives having phenyl-substituted silyl moieties Hitoshi Hanamura, Ryoko Haneishi, Nobukatsu Nemoto ⇑ Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, Tamura-machi, Koriyama, Fukushima 963-8642, Japan
a r t i c l e
i n f o
Article history: Received 28 April 2011 Revised 20 May 2011 Accepted 27 May 2011 Available online 2 June 2011 Keywords: Cyclopentadithiophene Fluorescence Substituent effect Silyl group Phenyl group
a b s t r a c t 4,4-Dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives bearing trimethyl-, dimethylphenyl-, diphenylmethyl-, or triphenyl-silyl moieties were synthesized. The introduction of the silyl moieties onto cyclopenta[2,1-b:3,4-b0 ]dithiophene induced fluorescent emission as well as the bathochromic shift of wavelength at the maximum absorption and fluorescence. It was found that the larger number of phenyl group on silyl moiety resulted in the higher fluorescence quantum yield. Ó 2011 Elsevier Ltd. All rights reserved.
Conjugated polymers1 obtained by polymerization of fused ring derivatives have emerged as attractive materials for flexible, low cost and low power electro-optic devices. Polyfluorenes are representatives of conjugated polymers which are suitable for applications in electronics and optoelectronics, such as organic light emitting diodes (OLEDs);2 while thiophene-based polymers and oligomers, including fused ring bithiophene derivatives3 such as dithienothiophene4 derivatives, exhibit efficient charge transport in organic field-effect transistors (OFETs).5 On the other hand, cyclopenta[2,1-b:3,4-b0 ]dithiophene (CPDT)6 is composed of a fused ring bithiophene derivative and regarded as analogous structure of fluorene where the benzene rings are replaced by thiophene rings. CPDT derivatives have been rigid precursors of polymeric semiconducting materials for the development of organic photovoltaic devices;7 however, there have been few reports on the fluorescent properties of CPDT derivatives or conjugated polymers based on CPDT.8 In the meantime, the incorporation of silyl substituent onto aromatic species has resulted in the high fluorescence quantum yield,9 as the photochemistry of organosilicon compounds has been reported in the recent review.10 In addition, Karatsu et al.11 reported the fluorescent properties of silyl substituted anthracene derivatives and revealed that the introduction of bulky phenyl group onto the silyl moieties induces the high fluorescence quantum yield as well as the pure-blue electroluminescence. From these points of view, we describe here the synthesis of CPDT derivatives having silyl substituents and their optical proper⇑ Corresponding author. Tel./fax: +81 24 956 8812. E-mail address: [email protected] (N. Nemoto). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.05.128
ties investigated by absorption and emission spectroscopy as well as quantum calculation using density functional theory (DFT) method. It will be revealed that the introduction of plural phenyl groups onto the silyl moieties remarkably increases the fluorescence quantum yield. Scheme 1 describes the synthesis of CPDT derivatives having silyl substituents (CPDT1–CPDT4) from 2,6-dibromo-4,4-dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene (1), which was prepared by the modified method of literature (see Scheme S1 in the Supplementary data).6a,b Namely, an addition of chlorosilane reagent having different substituents after the lithiation reaction of 1 using n-butyllithium in the presence of N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) in diethyl ether afforded CPDT1–CPDT4 with moderate yields (see Experimental details described in the Supplementary data).
Scheme 1. Synthesis of 4,4-dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives having silyl substituents.
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H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
Next the optical properties of CPDT1–CPDT4 were investigated. Figure 1 shows the absorption spectra of CPDT1–CPDT4 and 4,4dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene (DMCPDT). Table 1 summarizes the optical properties of CPDT1–CPDT4 and DMCPDT. In the absorption spectra of CPDT1–CPDT4, bathochromic and hyperchromic effects were observed by the introduction of silyl groups onto CPDT skeletons, presumably because of r–p and r⁄– p⁄ conjugations between the silyl groups and aromatic moieties.9,10 The bathochromic effect has been known to be induced by lowering the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states because of both destabilization of HOMO state through r-p conjugation and stabilization of LUMO state through r⁄–p⁄ conjugation. The hyperchromic effects would be caused by the enhancement of transition moment based on the increase in the dipole moments of the HOMO and LUMO states owing to the r–p conjugation in the HOMO and the r⁄–p⁄ conjugation in the LUMO. In addition, the bathochromic shift of the maximum absorption and the increase of the molar extinction coefficient (e) were observed by the introduction of phenyl group onto the silyl moieties. Namely, the introduction of phenyl group onto silyl moieties was also revealed to induce the bathochromic and hyperchromic effects. We calculated the HOMO and LUMO energy levels of CPDT1– CPDT4 and DMCPDT with density functional theory (DFT) method at the B3LYP/6-31G(d) level of theory12 for the confirmation of the effects of the introduction of silyl moieties on the absorption spectra. Figure 2 depicts the energy diagrams for the HOMO and LUMO energy levels as well as the energy gap between LUMO and HOMO. The difference in the LUMO energy level between DMCPDT (1.01 eV) and CPDT1 (1.16 eV) and that in the HOMO energy level between DMCPDT (5.18 eV) and CPDT1 (5.11 eV) means that stabilization of LUMO state through r⁄–p⁄ conjugation and destabilization of HOMO state through r–p conjugation are induced by the introduction of trimethylsilyl moieties. Thus, the introduction of trimethylsilyl moieties caused the decrease in the energy gap between LUMO and HOMO, resulting in the bathochromic shift of the maximum absorption. The replacement of methyl group by phenyl one on silyl moieties was found to decrease both HOMO and LUMO energy levels as shown in Figure 2. The degree of decrease of LUMO energy level was rather higher than that of HOMO energy level with increase in the number of phenyl group on silyl moieties, inducing the rather bathochromic shift of the maximum absorption with increase in the number of phenyl group on silyl moieties.
Figure 3 depicts the fluorescence spectra of CPDT1–CPDT4 and DMCPDT in CHCl3 at ambient temperature. The wavelength at maximum emission (kem) and fluorescence quantum yield (UF) of CPDT1–CPDT4 and DMCPDT are also summarized in Table 1. In the fluorescence spectra of CPDT1–CPDT4, bathochromic effects were observed by the introduction of silyl groups onto CPDT skeletons. The UFs of unsubsitituted CPDT (DMCPDT) and CPDT1 were 0.01 and 0.03, respectively, indicating that the introduction of trimethylsilyl moieties onto CPDT skeleton rather increased UF. Namely, the fluorescent ability could be
Figure 1. Absorption spectra of CPDT derivatives in CHCl3 at ambient temperature (Conc.: 2.0 106 mol/L).
Figure 3. Fluorescence spectra of CPDT derivatives in CHCl3 at ambient temperature (kex: 317 nm, conc.: 2.0 106 mol/L).
Table 1 Optical properties of CPDT derivatives Compound
kabs/nm (e/L mol1 cm1)
kem/nm
UFa
DMCPDT CPDT1 CPDT2 CPDT3 CPDT4
319 334 337 341 344
378 382 385 387 389
0.01 0.03 0.08 0.23 0.70
(15000) (22000) (26600) (29900) (31500)
329 345 349 352 355
(11800) (17500) (20900) (23900) (26100)
a Fluorescence quantum yields (UFs) were determined by using pyrene (UF: 0.19)9e as a standard in CHCl3.
Figure 2. Energy diagrams of CPDT derivatives. Calculated using DFT method at the B3LYP/6-31G(d) level.
H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
attached by the introduction of silyl moieties onto CPDT skeleton, which exhibits very low fluorescent ability. In addition, the UF of CPDT2 was 0.08 and 2.7 times as large as that of CPDT1, indicating that the introduction of a phenyl group onto silyl moieties drastically increase UF. It should be still remarkable that the increase in the number of phenyl group on silyl moieties enhances the fluorescent efficiency, as it was observed that the UFs of CPDT3 and CPDT4 were 0.23 and 0.70, respectively. Further detailed analysis of photochemical processes including the determination of the rate constants as radiative rate constant, intersystem crossing rate constant, and non-radiative rate constant would be necessary for the clarification of the reason for the increase in UF; however, the plausible reasons for the increase in UF are as follows.10 In the case of the present CPDT1–CPDT4, the larger number of phenyl groups on the silyl moieties resulted in the larger molar extinction coefficient (e) as mentioned above. It would be likely that the increase in UF was induced by the increase in e because the radiative rate constant has linear relation with e. Another reason for the increase in UF would be that the intersystem crossing rate constant becomes negligible owing to the energy stabilization of the first singlet excited state (S1) by silyl substitution to result in the change of the relative energy position against the second triplet excited state (T2) as observed in the case of silylsubstituted certain aromatic compounds.9a,c,10 It would be also likely that the increase in UF was induced by the energy stabilization of the first singlet excited state (S1) by the introduction of phenyl groups on the silyl moieties. In summary, we achieved four cyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives having silyl moieties. The introduction of the silyl moieties onto cyclopenta[2,1-b:3,4-b0 ]dithiophene induced the bathochromic shift of wavelength at the maximum absorption and fluorescence because of both destabilization of HOMO state through r–p conjugation and stabilization of LUMO state through r⁄–p⁄ conjugation. It should be remarkable that the increase in the number of phenyl group on silyl moieties enhances the fluorescent efficiency, and the highest UF in the present series of CPDT derivatives was 0.70.
Acknowledgments The authors would like to appreciate Ms. Satoko Tokiwa and Ms. Nami Sugashima, Nihon University College of Engineering Worldwide Research Center for Advanced Engineering and Technology (NEWCAT) for performing NMR measurements and Dr. Yoshio Saito, Associate Professor of Nihon University, for performing HR-MS measurements.
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Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2011.05.128. References and notes 1. (a) Andrew, C. G.; Klaus, M. Adv. Polym. Sci. 2006, 199, 1–82; (b) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897. 2. Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867; (b) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365; (c) Scherf, U.; List, E. J. W. Adv. Mater. 2002, 14, 477. 3. Ohshita, J.; Matsui, S.; Yamamoto, R.; Mizumo, T.; Ooyama, Y.; Harima, Y.; Murafuji, T.; Tao, K.; Kuramochi, Y.; Kaikoh, T.; Higashimura, H. Organometallics 2010, 29, 3239. 4. Ozturk, T.; Ertas, E.; Mert, O. Tetrahedron 2005, 61, 11055. 5. (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953; (b) Zhang, X.; Zhu, D. Polym. Chem. 2010, 1, 409. 6. (a) Kraak, A.; Wiersema, A. K.; Jordens, P.; Wynberg, H. Tetrahedron 1968, 24, 3381; (b) Coppo, P.; Turner, M. L. J. Mater. Chem. 2005, 15, 1123. 7. (a) Beyer, R.; Kalaji, M.; Kingscote-Burton, G.; Merphy, P. J.; Pereria, V. M. S. C.; Taylor, D. M.; Williams, G. O. Synth. Met. 1998, 92, 25; (b) Chochos, C. L.; Economopoulos, S. P.; Deimede, V.; Gregoriou, V. G.; Lloyd, M. T.; Malliaras, G. G.; Kallitsis, J. K. J. Phys. Chem. C 2007, 111, 10732; (c) Li, K.-C.; Huang, J.-H.; Hsu, Y.-C.; Huang, P.-J.; Chu, C.-W.; Lin, J.-T.; Ho, K.-C.; Wei, K.-H.; Lin, H.-C. Macromolecules 2009, 42, 3681; (d) Chen, C.-H.; Hsieh, C.-H.; Dubosc, M.; Cheng, Y.-J.; Hsu, C.-S. Macromolecules 2010, 43, 697; (e) Ko, S.; Mondal, R.; Risko, C.; Lee, J. K.; Hong, S.; McGehee, M. D.; Bredas, J.-L.; Bao, Z. Macromolecules 2010, 43, 6685. 8. Pal, B.; Yen, W.-C.; Yang, J.-S.; Chao, C.-Y.; Hung, Y.-C.; Lin, S.-T.; Chuang, C.-H.; Chen, C.-W.; Su, W.-F. Macromolecules 2008, 41, 6664. 9. (a) Shizuka, H.; Sato, Y.; Ueki, Y.; Ishikawa, M.; Kumada, M. J. Chem. Soc., Faraday Trans. 1984, 80, 341; (b) Declercq, D.; Delbeke, P.; De Schryver, F. C.; Van Meervelt, L.; Miller, R. D. J. Am. Chem. Soc. 1993, 115, 5702; (c) Kyushin, S.; Ikarugi, M.; Goto, M.; Hiratsuka, H.; Matsumoto, H. Organometallics 1996, 15, 1067; (d) Maeda, H.; Inoue, Y.; Ishida, H.; Mizuno, K. Chem. Lett. 2001, 1224; (e) Imai, K.; Sasaki, T.; Abe, J.; Kimoto, A.; Tamai, Y.; Nemoto, N. Polym. J. 2009, 41, 584; (f) Imai, K.; Hatano, S.; Kimoto, A.; Abe, J.; Tamai, Y.; Nemoto, N. Tetrahedron 2010, 66, 8012; (g) Imai, K.; Kihara, Y.; Kimoto, A.; Abe, J.; Tamai, Y.; Nemoto, N. Polym. J. 2011, 43, 58; (h) Ponomarenko, S. A.; Kirchmeyer, S. Adv. Polym. Sci. 2011, 235, 33. 10. Karatsu, T. J. Photochem. Photobiol. C: Photochem. Rev. 2008, 9, 111. 11. Karatsu, T.; Hazuku, R.; Asuke, M.; Nishigaki, A.; Yagai, S.; Suzuri, Y.; Kita, H.; Kitamura, A. Org. Electron. 2007, 8, 357. 12. Calculated using Spartan ’08 for Windows (Wavefunction, Inc., Irvine, CA, USA); Shao, Y.; Molnar, L. F.; Jung, Y.; Kussmann, J.; Ochsenfeld, C.; Brown, S. T.; Gilbert, A. T. B.; Slipchenko, L. V.; Levchenko, S. V.; O’Neill, D. P.; DiStasio, Jr., R. A.; Lochan, R. C.; Wang, T.; Beran, G. J. O.; Besley, N. A.; Herbert, J. M.; Lin, C. Y.; Van Voorhis, T.; Chien, S. H.; Sodt, A.; Steele, R. P.; Rassolov, V. A.; Maslen, P. E.; Korambath, P. P.; Adamson, R. D.; Austin, B.; Baker, J.; Byrd, E. F. C.; Dachsel, H.; Doerksen, R. J.; Dreuw, A.; Dunietz, B. D.; Dutoi, A. D.; Furlani, T. R.; Gwaltney, S. R.; Heyden, A.; Hirata, S.; Hsu, C. –P.; Kedziora, G.; Khalliulin, R. Z.; Klunzinger, P.; Lee, A. M.; Lee, M. S.; Liang, W. Z.; Lotan, I.; Nair, N.; Peters, B.; Proynov, E. I.; Pieniazek, P. A.; Rhee, Y. M.; Ritchie, J.; Rosta, E.; Sherrill, C. D.; Simmonett, A. C.; Subotnik, J. E.; Woodcock III, H. L.; Zhang, W.; Bell, A. T.; Chakraborty, A. K.; Chipman, D. M.; Keil, F. J.; Warshel, A.; Hehre, W. J.; Schaefer III, H. F.; Kong, J.; Krylov, A. I.; Gilla, P. M. W.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2006, 8, 3172.
Contents lists available at ScienceDirect
Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Fluorescent cyclopentadithiophene derivatives having phenyl-substituted silyl moieties Hitoshi Hanamura, Ryoko Haneishi, Nobukatsu Nemoto ⇑ Department of Chemical Biology and Applied Chemistry, College of Engineering, Nihon University, Tamura-machi, Koriyama, Fukushima 963-8642, Japan
a r t i c l e
i n f o
Article history: Received 28 April 2011 Revised 20 May 2011 Accepted 27 May 2011 Available online 2 June 2011 Keywords: Cyclopentadithiophene Fluorescence Substituent effect Silyl group Phenyl group
a b s t r a c t 4,4-Dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives bearing trimethyl-, dimethylphenyl-, diphenylmethyl-, or triphenyl-silyl moieties were synthesized. The introduction of the silyl moieties onto cyclopenta[2,1-b:3,4-b0 ]dithiophene induced fluorescent emission as well as the bathochromic shift of wavelength at the maximum absorption and fluorescence. It was found that the larger number of phenyl group on silyl moiety resulted in the higher fluorescence quantum yield. Ó 2011 Elsevier Ltd. All rights reserved.
Conjugated polymers1 obtained by polymerization of fused ring derivatives have emerged as attractive materials for flexible, low cost and low power electro-optic devices. Polyfluorenes are representatives of conjugated polymers which are suitable for applications in electronics and optoelectronics, such as organic light emitting diodes (OLEDs);2 while thiophene-based polymers and oligomers, including fused ring bithiophene derivatives3 such as dithienothiophene4 derivatives, exhibit efficient charge transport in organic field-effect transistors (OFETs).5 On the other hand, cyclopenta[2,1-b:3,4-b0 ]dithiophene (CPDT)6 is composed of a fused ring bithiophene derivative and regarded as analogous structure of fluorene where the benzene rings are replaced by thiophene rings. CPDT derivatives have been rigid precursors of polymeric semiconducting materials for the development of organic photovoltaic devices;7 however, there have been few reports on the fluorescent properties of CPDT derivatives or conjugated polymers based on CPDT.8 In the meantime, the incorporation of silyl substituent onto aromatic species has resulted in the high fluorescence quantum yield,9 as the photochemistry of organosilicon compounds has been reported in the recent review.10 In addition, Karatsu et al.11 reported the fluorescent properties of silyl substituted anthracene derivatives and revealed that the introduction of bulky phenyl group onto the silyl moieties induces the high fluorescence quantum yield as well as the pure-blue electroluminescence. From these points of view, we describe here the synthesis of CPDT derivatives having silyl substituents and their optical proper⇑ Corresponding author. Tel./fax: +81 24 956 8812. E-mail address: [email protected] (N. Nemoto). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.05.128
ties investigated by absorption and emission spectroscopy as well as quantum calculation using density functional theory (DFT) method. It will be revealed that the introduction of plural phenyl groups onto the silyl moieties remarkably increases the fluorescence quantum yield. Scheme 1 describes the synthesis of CPDT derivatives having silyl substituents (CPDT1–CPDT4) from 2,6-dibromo-4,4-dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene (1), which was prepared by the modified method of literature (see Scheme S1 in the Supplementary data).6a,b Namely, an addition of chlorosilane reagent having different substituents after the lithiation reaction of 1 using n-butyllithium in the presence of N,N,N0 ,N0 -tetramethylethylenediamine (TMEDA) in diethyl ether afforded CPDT1–CPDT4 with moderate yields (see Experimental details described in the Supplementary data).
Scheme 1. Synthesis of 4,4-dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives having silyl substituents.
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H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
Next the optical properties of CPDT1–CPDT4 were investigated. Figure 1 shows the absorption spectra of CPDT1–CPDT4 and 4,4dimethylcyclopenta[2,1-b:3,4-b0 ]dithiophene (DMCPDT). Table 1 summarizes the optical properties of CPDT1–CPDT4 and DMCPDT. In the absorption spectra of CPDT1–CPDT4, bathochromic and hyperchromic effects were observed by the introduction of silyl groups onto CPDT skeletons, presumably because of r–p and r⁄– p⁄ conjugations between the silyl groups and aromatic moieties.9,10 The bathochromic effect has been known to be induced by lowering the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) states because of both destabilization of HOMO state through r-p conjugation and stabilization of LUMO state through r⁄–p⁄ conjugation. The hyperchromic effects would be caused by the enhancement of transition moment based on the increase in the dipole moments of the HOMO and LUMO states owing to the r–p conjugation in the HOMO and the r⁄–p⁄ conjugation in the LUMO. In addition, the bathochromic shift of the maximum absorption and the increase of the molar extinction coefficient (e) were observed by the introduction of phenyl group onto the silyl moieties. Namely, the introduction of phenyl group onto silyl moieties was also revealed to induce the bathochromic and hyperchromic effects. We calculated the HOMO and LUMO energy levels of CPDT1– CPDT4 and DMCPDT with density functional theory (DFT) method at the B3LYP/6-31G(d) level of theory12 for the confirmation of the effects of the introduction of silyl moieties on the absorption spectra. Figure 2 depicts the energy diagrams for the HOMO and LUMO energy levels as well as the energy gap between LUMO and HOMO. The difference in the LUMO energy level between DMCPDT (1.01 eV) and CPDT1 (1.16 eV) and that in the HOMO energy level between DMCPDT (5.18 eV) and CPDT1 (5.11 eV) means that stabilization of LUMO state through r⁄–p⁄ conjugation and destabilization of HOMO state through r–p conjugation are induced by the introduction of trimethylsilyl moieties. Thus, the introduction of trimethylsilyl moieties caused the decrease in the energy gap between LUMO and HOMO, resulting in the bathochromic shift of the maximum absorption. The replacement of methyl group by phenyl one on silyl moieties was found to decrease both HOMO and LUMO energy levels as shown in Figure 2. The degree of decrease of LUMO energy level was rather higher than that of HOMO energy level with increase in the number of phenyl group on silyl moieties, inducing the rather bathochromic shift of the maximum absorption with increase in the number of phenyl group on silyl moieties.
Figure 3 depicts the fluorescence spectra of CPDT1–CPDT4 and DMCPDT in CHCl3 at ambient temperature. The wavelength at maximum emission (kem) and fluorescence quantum yield (UF) of CPDT1–CPDT4 and DMCPDT are also summarized in Table 1. In the fluorescence spectra of CPDT1–CPDT4, bathochromic effects were observed by the introduction of silyl groups onto CPDT skeletons. The UFs of unsubsitituted CPDT (DMCPDT) and CPDT1 were 0.01 and 0.03, respectively, indicating that the introduction of trimethylsilyl moieties onto CPDT skeleton rather increased UF. Namely, the fluorescent ability could be
Figure 1. Absorption spectra of CPDT derivatives in CHCl3 at ambient temperature (Conc.: 2.0 106 mol/L).
Figure 3. Fluorescence spectra of CPDT derivatives in CHCl3 at ambient temperature (kex: 317 nm, conc.: 2.0 106 mol/L).
Table 1 Optical properties of CPDT derivatives Compound
kabs/nm (e/L mol1 cm1)
kem/nm
UFa
DMCPDT CPDT1 CPDT2 CPDT3 CPDT4
319 334 337 341 344
378 382 385 387 389
0.01 0.03 0.08 0.23 0.70
(15000) (22000) (26600) (29900) (31500)
329 345 349 352 355
(11800) (17500) (20900) (23900) (26100)
a Fluorescence quantum yields (UFs) were determined by using pyrene (UF: 0.19)9e as a standard in CHCl3.
Figure 2. Energy diagrams of CPDT derivatives. Calculated using DFT method at the B3LYP/6-31G(d) level.
H. Hanamura et al. / Tetrahedron Letters 52 (2011) 4039–4041
attached by the introduction of silyl moieties onto CPDT skeleton, which exhibits very low fluorescent ability. In addition, the UF of CPDT2 was 0.08 and 2.7 times as large as that of CPDT1, indicating that the introduction of a phenyl group onto silyl moieties drastically increase UF. It should be still remarkable that the increase in the number of phenyl group on silyl moieties enhances the fluorescent efficiency, as it was observed that the UFs of CPDT3 and CPDT4 were 0.23 and 0.70, respectively. Further detailed analysis of photochemical processes including the determination of the rate constants as radiative rate constant, intersystem crossing rate constant, and non-radiative rate constant would be necessary for the clarification of the reason for the increase in UF; however, the plausible reasons for the increase in UF are as follows.10 In the case of the present CPDT1–CPDT4, the larger number of phenyl groups on the silyl moieties resulted in the larger molar extinction coefficient (e) as mentioned above. It would be likely that the increase in UF was induced by the increase in e because the radiative rate constant has linear relation with e. Another reason for the increase in UF would be that the intersystem crossing rate constant becomes negligible owing to the energy stabilization of the first singlet excited state (S1) by silyl substitution to result in the change of the relative energy position against the second triplet excited state (T2) as observed in the case of silylsubstituted certain aromatic compounds.9a,c,10 It would be also likely that the increase in UF was induced by the energy stabilization of the first singlet excited state (S1) by the introduction of phenyl groups on the silyl moieties. In summary, we achieved four cyclopenta[2,1-b:3,4-b0 ]dithiophene derivatives having silyl moieties. The introduction of the silyl moieties onto cyclopenta[2,1-b:3,4-b0 ]dithiophene induced the bathochromic shift of wavelength at the maximum absorption and fluorescence because of both destabilization of HOMO state through r–p conjugation and stabilization of LUMO state through r⁄–p⁄ conjugation. It should be remarkable that the increase in the number of phenyl group on silyl moieties enhances the fluorescent efficiency, and the highest UF in the present series of CPDT derivatives was 0.70.
Acknowledgments The authors would like to appreciate Ms. Satoko Tokiwa and Ms. Nami Sugashima, Nihon University College of Engineering Worldwide Research Center for Advanced Engineering and Technology (NEWCAT) for performing NMR measurements and Dr. Yoshio Saito, Associate Professor of Nihon University, for performing HR-MS measurements.
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