Article pubs.acs.org/joc
Structures and Optical Properties of Tris(trimethylsilyl)silylated Oligothiophene Derivatives Hikaru Inubushi, Yohei Hattori, Yoshinori Yamanoi,* and Hiroshi Nishihara* Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan S Supporting Information *
ABSTRACT: The structures and optical properties of tris(trimethylsilyl)silylated oligothiophenes were examined by spectroscopies, theoretical calculations, and singlecrystal X-ray measurements. Bathochromic shift from the original oligothiophenes was observed in the tris(trimethylsilyl)silylated ones, confirming the σ−π conjugation between Si−Si σ bonds and π-orbital. 5,5′-Bis(tris(trimethylsilyl)silyl)-2,2′-bithiophene (Si-T2) showed the highest fluorescence quantum yield (ΦF) both in solution (0.67, excited at 350 nm) and the solid state (0.74, excited at 371 nm). The introduction of tris(trimethylsilyl)silyl groups led to the small nonradiative rate constant of Si-T2, resulting in the high ΦF in the solution state. Si-T2 also exhibited effective σ−π conjugation and poor molecular interaction, which reflected its high ΦF in the solid state. On the contrary, lower ΦF (0.13, excited at 331 nm) in the solid state was observed in the longest oligothiophene examined, 5,5‴-bis(1,1,1,3,3,3-hexamethyl-2(trimethylsilyl)trisilan-2-yl)-2,2′:5′,2″:5″,2‴-quaterthiophene (Si-T4). Single-crystal Xray measurement clarified that this compound adopted a zigzag packing structure and a rare syn-anti-syn conformation, which led to the poor σ−π conjugation and the decrease of π-orbital overlap in the solid state.
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INTRODUCTION
Oligothiophenes are an interesting class of conjugated organic compound because they are widely applicable in devices such as organic light-emitting devices, organic field-effect transistors, and photovoltaic cells.1 Various studies have been reported to tune their properties by ring fusion2 or by introduction of substituents.3 The use of substituents is more attractive, because the optical and electronic properties of oligothiophenes can be concomitantly controlled using proper substituents, which facilitates the evolution of new classes of oligothiophenebased materials. The incorporation of oligosilyl groups into π-conjugated frameworks allows their photophysical and electrochemical properties to be tuned via hyperconjugation.4 Previously, we found that hydrosilanes can be easily coupled with organic halides containing various functional groups in the presence of a transition metal catalyst and a base.5 During the course of our experiments, 5,5′-bis(tris(trimethylsilyl)silyl)-2,2′-bithiophene (Si-T2) was found to exhibit good fluorescence in the solid state.6 We were motivated to demonstrate the reason for such good fluorescence of Si-T2. As an extensive study in this area, we would like to describe here the incorporation of tris(trimethylsilyl)silyl group into α-linked or fused oligothiophenes to evaluate their effects on the optical properties, because the systematic study on oligomers allows the elucidation of correlation of physical properties with chemical structures and enable the generation of useful structure− property relationships. © 2014 American Chemical Society
RESULTS AND DISCUSSION
According to the strategies, the effects of tris(trimethylsilyl)silyl groups were investigated by introducing them into various oligothiophenes with different skeletons. The structures of investigated compounds are depicted in Figure 1. These compounds were synthesized in moderate yields according to our previous method.6 The optical properties of the original
Figure 1. Structures of oligothiophenes (left) and doubly tris(trimethylsilyl)silylated ones (right) investigated in the present work. Received: January 7, 2014 Published: March 10, 2014 2974
dx.doi.org/10.1021/jo500029f | J. Org. Chem. 2014, 79, 2974−2979
The Journal of Organic Chemistry
Article
Table 1. Photochemical Properties of the Original and Doubly Tris(trimethylsilyl)silylated Oligothiophenes λem (nm)d compound
a
λabs (nm) CH2Cl2
λex (nm) solid
Si-T1 T1 Si-T2 T2 Si-T3 T3 Si-T4 T4 Si-F2 F2 Si-F3 F3
285 230i 350 304 394 355 417 393 309 270 334 291
308 j 371 349 368 453 331 393 330 250 363 379
ε (10 M
b
c
4
−1
−1
cm )
ΦFe
CH2Cl2
solid
CH2Cl2
solid
τf (ns)
kFg (ns−1)
kNRh (ns−1)
355 247i 410 359 471 432 511 480 354 293 381 335
353 j 409 389 471 508 522 580 356 471 386 429
0.07 0.03i 0.67 0.01 0.09 0.05 0.21 0.16 0.06 0.01 0.36 0.01
0.09 j 0.74 300 °C; 1H NMR (500 MHz, CDCl3) δ 7.21 (s, 2H), 0.26 (s, 54H); 13C NMR (125 MHz, CDCl3) δ 143.3 (Cq), 135.4 (Cq), 135.0 (Cq), 128.3 (CH), 0.97 (CH3); MS (FAB) m/z 688 (M+). Anal. Calcd for C26H56S3Si8: C 45.28, H 8.19. Found: C 45.55, H 8.40. Crystal data for C26H56S3Si8, crystal system triclinic, space group P-1 (No. 2), a = 8.827(2) Å, b = 15.669(3) Å, c = 16.195(3) Å, α = 105.691(2)°, β = 101.137(3)°, γ = 97.772(2)°, V = 2073.6(7) Å3, Z = 2, dcalc = 1.104 g cm−3, F(000) = 744.00, reflections/parameters/constraints = 8901/ 334/0, goodness of fit = 1.039, μ = 0.425, R1 (I > 2σ(I)) = 0.0308, wR2 (all data) = 0.0854.
these compounds. From the viewpoint of the structure− property relationship, 2,2′-bithiophene can be regarded as a very promising and interesting oligothiophene unit for further investigations. We believe that the present study provides valuable information for the design and synthesis of new oligothiophene-based materials.
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EXPERIMENTAL SECTION
General. All experiments were carried out under an argon atmosphere in oven-dried glassware. Unless otherwise noted, all solvents and commercially available reagents were used as received. HRMS was performed on a double-focusing sector mass spectrometer in FAB mode. The ground state geometries were fully optimized by DFT calculations at the B3LYP/6-31G(d) level. Single crystals of SiT2, Si-T4, and Si-F3 suitable for X-ray crystallography were obtained by vapor diffusion of acetonitrile into toluene. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 957554 (Si-T2), 957553 (Si-T4), and 957555 (Si-F3). Typical Procedure for the Pd-Catalyzed Synthesis of Doubly (Trimethylsilyl)silylated Oligothiophenes. To a solution of Pd(P(t-Bu)3)2 (0.05 equiv), diisopropylethylamine (3.0 equiv), and appropriate diiodoligoothiophene14 (1.0 equiv) in anhydrous THF (1.0 M) was added tris(trimethylsilyl)silane (3.0 equiv). After stirring for 3 d at room temperature, the reaction mixture was quenched with water, extracted four times with CH2Cl2, and dried over Na2SO4. The solvent was evaporated under reduced pressure, and fractionated column chromatography was carried out on silica gel (eluent hexane) to afford desired doubly tris(trimethylsilyl)silylated oligothiophene. All analytically pure samples for optical measurements were obtained by recrystallization from CH3CN or CH3CN/toluene more than two times. 2,5-Bis(1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilan-2-yl)thiophene (Si-T1). White solid (971 mg, 42%); mp 167.0−168.0 °C; 1 H NMR (400 MHz, CDCl3) δ 7.14 (s, 2H), 0.22 (s, 54H); 13C NMR (100 MHz, CDCl3) δ 137.4 (Cq), 136.7 (CH), 0.9 (CH3); MS (EI) m/z 576 (M+); HR-FAB-MS (NBA, positive) m/z for C22H56Si8S calcd 576.2257, found 576.2228. 5,5′-Bis(tris(trimethylsilyl)silyl)-2,2′-bithiophene (Si-T2). This compound was synthesized according to the literature.6 The analytical data were consistent with those previously reported. Crystal data for C26H58S2Si8, crystal system triclinic, space group P-1 (No. 2), a = 8.774(2) Å, b = 9.091(2) Å, c = 14.938(4) Å, α = 86.627(11)°, β = 76.940(9)°, γ = 61.217(7)°, V = 1015.5(5) Å3, Z = 1, dcalc = 1.078 g cm−3, F(000) = 358.00, reflections/parameters/constraints = 3460/ 172/0, goodness of fit = 1.073, μ = 0.382, R1 (I > 2σ(I)) = 0.0289, wR2 (all data) = 0.0766. 5,5″-Bis(1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilan-2-yl)2,2′:5′,2″-terthiophene (Si-T3). Yellow solid (778 mg, 17%); mp 134.0−135.0 °C; 1H NMR (500 MHz, CDCl3) δ 7.18 (d, 2H, J = 3.4 Hz), 7.05 (s, 2H), 7.02 (d, 2H, J = 3.7 Hz), 0.25 (s, 54H); 13C NMR (125 Hz, CDCl3) δ 141.8 (Cq), 136.7 (CH), 135.9 (Cq), 132.7 (Cq), 124.9 (CH), 124.0 (CH), 0.93 (CH3); MS (EI) m/z 740 (M+). Anal. Calcd for C30H60S3Si8: C 48.58, H 8.15. Found: C 48.35, H 8.29. 5,5‴-Bis(1,1,1,3,3,3-hexamethyl-2-(trimethylsilyl)trisilan-2-yl)2,2′:5′,2″:5″,2‴-quaterthiophene (Si-T4). Yellow solid (578 mg, 8%); mp 165.0−166.5 °C; 1H NMR (500 MHz, CDCl3) δ 7.19 (d, 2H, J = 3.4 Hz), 7.06 (dd, 4H, J = 3.9, 8.6 Hz), 7.03 (d, 2H, J = 3.5 Hz), 0.25 (s, 54H); 13C NMR (125 Hz, CDCl3) δ 141.6 (Cq), 136.7 (CH), 136.3 (Cq), 135.6 (Cq), 133.0 (Cq), 125.0 (CH), 124.1 (CH), 124.1 (CH), 0.91 (CH3); MS (EI) m/z 822 (M+). Anal. Calcd for C34H62S4Si8: C 49.57, H 7.59. Found: C 49.43, H 7.66. Crystal data for C34H62S4Si8, crystal system monoclinic, space group C2/c (No. 15), a = 15.679(3) Å, b = 12.993(2) Å, c = 24.253(4) Å, α = 90°, β = 102.693(2)°, γ = 90°, V = 4819.9(12) Å3, Z = 4, dcalc = 1.135 g cm−3, F(000) = 1768.00, reflections/parameters/constraints = 5501/208/0, goodness of fit = 1.050, μ = 0.418, R1 (I > 2σ(I)) = 0.0375, wR2 (all data) = 0.0919.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
* Supporting Information S
X-ray analysis data and cif files for Si-T2, Si-T4, and Si-F3, UV− vis absorption spectra, fluorescence spectra, table of atom coordinates and absolute energies for theoretical calculations, and copies of 1H and 13C NMR of all new compounds. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Ms. Kimiyo Saeki and Dr. Aiko Kamitsubo of the Elemental Analysis Center of The University of Tokyo for their assistance with the elemental analysis measurements. We also thank Mr. Hideki Waragai for the measurements of absolute fluorescence quantum yields in the solid state. This work was financially supported by Grant-in-Aids for Scientific Research (C) (no. 24550221) and Scientific Research on Innovative Areas “Coordination Programming” (area 2107, no. 21108002).
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