Synthesis and Photophysical Properties of 2-Donor-7-acceptor-9

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Synthesis and Photophysical Properties of 2-Donor-7-acceptor-9-silafluorenes: Remarkable Fluorescence Solvatochromism and Highly Efficient Fluorescence in Doped Polymer Films Masaki Shimizu,* Kenji Mochida, Masaki Katoh, and Tamejiro Hiyama† Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto UniVersity Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed: April 15, 2010; ReVised Manuscript ReceiVed: April 25, 2010

Recently, π-conjugated molecules containing a 9-silafluorene moiety have attracted considerable attention owing to their potential applications in organic electronics. While most studies focus on the oligomers and (co)polymers of 9-silafluorenes, functionalized 9-silafluorenes themselves remain unexplored. In this paper, we describe the preparation, photophysical properties, and theoretical calculations of 2-amino-7-acceptor-9silafluorenes and we show that these silafluorenes can potentially be used as novel chromophores for functional organic materials. The D-π-A type silafluorenes were prepared by Pd-catalyzed intramolecular coupling of 2-(3-aminophenyldiisopropylsilyl)aryl triflates and the subsequent functional group conversion through reduction of the cyano group to a formyl and Knoevenagel condensation of the formyl group with malonitrile. The UV-visible absorption and fluorescence spectra of the D-π-A type silafluorenes exhibited a red-shift when the electron-withdrawing nature of the acceptor increased. The emission maxima of the fluorescence were highly dependent on the solvent. In the cases of formyl- and dicyanoethenyl-substituted silafluorenes, the emission colors ranged from blue to yellow for the formyl derivatives and from green to red for the dicyanoethenyl derivatives. The noticeable fluorescence solvatochromism suggests the intramolecular chargetransfer character of the excited states. The silafluorenes also exhibited fluorescence in the solid state (e.g., a neat thin film and a doped polymer film), and the emission color was dependent on the polarity of the polymer. In the solid state, the quantum yields of diphenylamino derivatives were generally higher than those of dimethylamino derivatives, presumably because the bulky diphenylamino group prevented chromophores from assembling close to each other. A comparison of the photophysical properties and theoretical calculations of D-π-A type silafluorenes with those of the corresponding fluorenes revealed that the silicon bridge contributed to the extension of the effective conjugation length of the biphenyl moiety when the acceptor was either hydrogen, trifluoromethyl, or a cyano group, whereas there was no contribution of the silicon bridge to the π-extension in silafluorenes substituted by strong electron acceptors such as formyl and dicyanoethenyl groups. White photoluminescence was demonstrated with an excellent quantum yield of 0.81 in the solid state from the poly(methyl methacrylate) (PMMA) film doped with Ph2N/CHO- and Ph2N/CHd(CN)2-substituted silafluorenes. This work reveals the potential of D-π-A type silafluorenes as versatile organic emitting materials. Introduction Bridging a π-conjugated framework that consists of two connected π-modules such as alkenyl moieties and aromatic rings by a diorganosilylene moiety imparts coplanarity to the framework and perturbs the conjugation via the interaction between the π*-orbital of the π-modules and the σ*-orbitals of the exocyclic silicon-carbon bonds.1-3 Hence, the incorporation of a silicon bridge across π-conjugated systems is a versatile strategy for tuning the molecular and electronic structures of π-conjugated systems; the electronic and optical properties of π-conjugated systems are determined by their molecular and electronic structures. Indeed, various silylene-bridged 1,3butadienes (siloles),4-20 2,2′-bithiophenes,21-31 and oligo(pphenylenevinylene)s32-38 have been designed and characterized so far. In recent times, 9-silafluorene, which can be regarded as a silylene-bridged biphenyl, is being increasingly used as a key component of functional materials39,40 for organic lightemitting diodes,41-49 organic thin-film field effect transistors,50-52 * To whom correspondence should be addressed. E-mail: m.shimizu@ hs2.ecs.kyoto-u.ac.jp. † Present address: Graduate School of Science and Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.

organic solar cells,53-55 and organic chemical sensors.56 Most of the functional materials consist of oligomers or (co)polymers of 9-silafluorenes. The representative examples are shown in Figure 1. However, studies on molecular design and development of functionalized 9-silafluorenes are relatively limited. Some of the functionalized 9-silafluorenes that were studied include 9,9′-spiro-9-silabifluorenes,57,58 4,5-dimethylsilylene- or 4,5tetramethyldisilylene-bridged 9-silafluorenes,59 perfluorinated 2,7-bis(phenylethynyl)-9-silafluorenes,60 and bis(4-aminophenylethenyl)-9-silafluorenes (Figure 2).61,62 Considering that functionalized siloles exhibit excellent electronic and optical properties due to their unique electronic structures induced by the silicon bridge,4-8 the design, synthesis, and characterization of functionalized 9-silafluorenes will be useful for the development of superior silicon-based organic materials. The introduction of an electron donor (D) at one end and an electron acceptor (A) at the other end of a π-conjugated system can induce the contribution of a zwitterionic resonance structure that originates from intramolecular charge transfer within the conjugated system.63-65 Thus, the construction of a D-π-A structure exhibiting quinoid character is beneficial for tuning a

10.1021/jp103410x  2010 American Chemical Society Published on Web 05/13/2010

2-Donor-7-acceptor-9-silafluorenes

J. Phys. Chem. C, Vol. 114, No. 21, 2010 10005

Figure 1. Silafluorene-based oligomeric/polymeric materials recorded previously.

Figure 3. Chemical structures of 2-amino-7-acceptor-9-silafluorenes 1-5. Figure 2. Functionalized 9-silafluorenes recorded previously.

Experimental Methods

HOMO-LUMO gap, which is essential to attain the desired electronic and optical properties. For example, 2-donor-7acceptor-substituted fluorenes have been shown to exhibit superior solid-state luminescence,66,67 two-photon absorption,68 and nonlinear optical polarizability.69,70 Hence, 2-donor-7acceptor-9-silafluorenes are very attractive candidates for the core of functional organic materials. However, the D-π-A type 9-silafluorenes remain unexplored probably because unsymmetrical 9-silafluorenes cannot be synthesized by conventional synthetic methods such as double lithiation of the corresponding 2,2′-dihalobiphenyls followed by silylation with dichlorodiorganosilanes.71-76 We recently invented a new synthetic method for silicon bridged biaryls including symmetrical and unsymmetrical 9-silafluorenes; the method involves Pd-catalyzed intramolecular coupling of 2-(arylsilyl)aryl triflates.77,78 During a further study on the scope of the coupling, we found that the reaction of 2-(3dimethylaminophenyldiisopropylsilyl)phenyl triflate proceeded regiospecifically at the para-position of the amino group to give 2-amino-9-silalfluorene 1 as the sole product in an excellent yield (eq 1). This perfect regiochemical outcome of cyclization provided a synthetic route to 2-amino-7-acceptor-9-silafluorenes and thus allowed us to scrutinize its properties in detail. We report herein the preparation, photophysical and electrochemical properties, and theoretical study of 2-amino-7-acceptor-9silafluorenes 1-5 (Figure 3).

General Information. Melting points were determined using a Stanford Research Systems MPA100 instrument. 1H NMR spectra were measured on Varian Mercury 300 (300 MHz) and 400 (400 MHz) spectrometers. The chemical shifts of 1H NMR are expressed in parts per million downfield relative to the internal tetramethylsilane (δ ) 0 ppm) or chloroform (δ ) 7.26 ppm). Splitting patterns are indicated as s, singlet; d, doublet; t, triplet; q, quartet; hep, heptet; m, multiplet. 13C NMR spectra were measured on Varian Mercury 300 (75 MHz) and 400 (100 MHz) spectrometers with tetramethylsilane as an internal standard (δ ) 0 ppm) or chloroform-d (δ ) 77.0 ppm). 19F NMR spectra were measured on a Varian Mercury 300 (282 MHz) spectrometer with CFCl3 as an internal standard (δ ) 0 ppm). Chemical shift values are given in parts per million downfield relative to the internal standards. Infrared spectra (IR) were recorded on a Shimadzu FTIR-8400 spectrometer. EI-MS analyses were performed with a JEOL JMS-700 spectrometer by electron ionization at 70 eV. FAB-MS analyses were performed with a JEOL-HX110A spectrometer. Elemental analyses were carried out with a YANAKO MT2 CHN CORDER machine at Elemental Analysis Center of Kyoto University. TLC analyses were performed by means of Merck Kieselgel 60 F254. Silica gel column chromatography was carried out using Merck Kieselgel 60 (230-400 mesh). Dichlorodiisopropylsilane was purchased from Tokyo Chemical Industry Co., Ltd. Dimethylacetamide (DMA) was purchased from Wako, Inc. Reagent-grade dichloromethane, diethyl ether, and tetrahydrofuran were passed through two packed columns of neutral

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SCHEME 1: Synthesis of 2-Amino-7-acceptor-9-silafluorenes 1-5

alumina and copper oxide under a nitrogen atmosphere before use. All reactions were carried out under an argon atmosphere. General Procedure for Pd-Catalyzed Intramolecular Coupling Reaction. An oven-dried 3 mL vial equipped with a magnetic stir bar was charged with 6, 7, or 8 (1.0 mmol) and DMA (1.0 mL). To the solution was added a solution of Pd(OAc)2 (5.5 mg, 0.025 mmol) and PCy3 (14 mg, 0.05 mmol) in DMA (1.0 mL) and then Et2NH (200 µL, 2.0 mmol). The reaction mixture was stirred at 100 °C until Pd-black precipitated. The resulting mixture was cooled to room temperature and diluted with CH2Cl2 (10 mL). Saturated aq. NH4Cl (5 mL) was added to the solution, and the aqueous layer was extracted with hexane (20 mL × 3). The combined organic layer was washed with H2O (15 mL × 3) and saturated aq. NaCl (15 mL), dried over anhydrous MgSO4, and concentrated by rotary evaporation. The residue was purified by column chromatography on silica gel to give 1, 2, or 3. 2-Dimethylamino-9,9-diisopropyl-9-silafluorene (1). Purification: silica gel column chromatography (hexane/AcOEt 20:

Shimizu et al. 1). Yield: 97%, colorless solid. Mp: 89.5-90.2 °C. TLC: Rf 0.40 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ 1.05 (d, J ) 7.5 Hz, 6H), 1.06 (d, J ) 7.5 Hz, 6H), 1.37 (qq, J ) 7.5, 7.5 Hz, 2H), 3.02 (s, 6H), 6.83 (dd, J ) 8.6, 2.6 Hz, 1H), 7.01 (d, J ) 2.6 Hz, 1H), 7.16 (dd, J ) 7.3, 7.1 Hz, 1H), 7.40 (dd, J ) 7.5, 7.3 Hz, 1H), 7.58 (d, J ) 7.1 Hz, 1H), 7.71 (d, J ) 7.5 Hz, 1H), 7.72 (d, J ) 8.6 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 11.3, 18.3, 40.9, 113.9, 117.2, 119.3, 121.4, 125.1, 129.7, 133.4, 134.3, 134.7, 137.4, 137.8, 149.5. IR (KBr): ν ) 3000, 2850, 1749, 1734, 1717, 1699, 1684, 1559, 1541, 1506, 1489, 1456, 1417, 1339, 1223, 1175, 1057, 883, 770, 725, 673, 609 cm-1. MS (FAB) m/z: 309 (100, M+), 266 (11), 224 (7). Anal. Calcd for C20H27NSi: C, 77.61; H, 8.79. Found: C, 77.55; H, 8.67. 2-Dimethylamino-7-trifluoromethyl-9,9-diisopropyl-9-silafluorene (2). Purification: silica gel column chromatography (hexane/AcOEt 10:1). Yield: 95%, colorless solid. Mp: 124.9-125.8 °C. TLC: Rf 0.38 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ 1.05 (d, J ) 7.3 Hz, 6H), 1.06 (d, J ) 7.3 Hz, 6H), 1.40 (qq, J ) 7.3, 7.3 Hz, 2H), 3.05 (s, 6H), 6.76-6.84 (m, 1H), 6.94-7.00 (m, 1H), 7.59 (d, J ) 8.2 Hz, 1H), 7.71-7.73 (m, 3H). 13C NMR (100 MHz, CDCl3): δ 11.2, 18.18, 18.21, 40.7, 113.8, 116.8, 119.1, 122.3, 124.8 (q, J ) 271.2 Hz), 126.7 (q, J ) 30.7 Hz), 127.0 (q, J ) 3.8 Hz), 129.6 (q, J ) 3.8 Hz), 135.4, 136.1, 138.0, 150.0, 153.0. 19F NMR (282 MHz, CDCl3): δ -62.3. IR (KBr): ν ) 2942, 2862, 1587, 1557, 1489, 1460, 1354, 1327, 1287, 1263, 1136, 1111, 1074, 880, 812, 725, 635 cm-1. MS (FAB) m/z: 377 (100, M+), 334 (12), 292 (6). Anal. Calcd for C21H26F3NSi: C, 66.81; H, 6.94. Found: C, 66.53; H, 6.77. 2-Cyano-7-dimethylamino-9,9-diisopropyl-9-silafluorene (3a). Purification: silica gel column chromatography (hexane/AcOEt 10:1). Yield: 85%, a colorless solid. Mp: 113.6-114.4 °C. TLC: Rf 0.18 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ 1.04 (d, J ) 7.5 Hz, 6H), 1.05 (d, J ) 7.3 Hz, 6H), 1.39 (qq, J ) 7.5, 7.3 Hz, 2H), 3.06 (s, 6H), 6.78-6.84 (m, 1H), 6.93-6.97 (m, 1H), 7.62 (dd, J ) 8.0, 1.5 Hz, 1H), 7.69 (d, J

Figure 4. Molecular and crystal structures of 3b: (a) top view, (b) side view, (c) packing diagram along the b-axis, and (d) packing diagram with two adjacent molecules. Hydrogen atoms are omitted for clarity.



TABLE 1: UV-Vis Absorption Properties of 1-5 in Various Solventsa solvent (ε)b

1

2

c-C6H12 (2.02) C6H6 (2.28) Et2O (4.34) CH2Cl2 (9.08) EtOH (25.07) CH3CN (38.8) DMSO (48.9)

323 (17 100) 323 (23 800) 318 (32 100) 323 (17 100) 319 (23 900) 321 (23 400) 324 (22 700)

331 (24 600) 338 (23 400) 334 (26 700) 338 (24 300) 335 (22 500) 337 (23 100) 344 (22 400)

a

absorption maximum (nm) (molar absorption coefficient/M-1 cm-1) 3a 4a 5a 3b 357 (28 700) 360 (16 100) 358 (28 900) 368 (28 200) 360 (26 300) 360 (26 500) 375 (19 600)

390 (32 900) 392 (32 300) 384 (35 200) 397 (27 100) 395 (25 400) 391 (28 200) 400 (25 900)

480 (54 800) 480 (50 100) 469 (40 700) 489 (38 300) 478 (32 200) 475 (33 700) 488 (28 200)

391 (29 300) 389 (27 900) 383 (28 900) 388 (28 600) 385 (26 200) 381 (29 400) 387 (26 500)

4b

5b

403 (35 700) 398 (31 700) 393 (30 500) 401 (27 800) 396 (27 400) 392 (28 000) 398 (27 300)

484 (44 600) 477 (36 800) 467 (35 300) 481 (30 500) 467 (36 800) 461 (31 300) 466 (30 800)

Measured at 1 × 10-5 mol/L. b ε: dielectric constants cited from ref 81.

) 8.0 Hz, 1H), 7.70 (d, J ) 8.6 Hz, 1H), 7.76 (d, J ) 1.5 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 11.1, 18.10, 18.15, 40.6, 107.8, 113.7, 116.5, 119.4, 120.1, 122.8, 133.8, 135.8, 136.6, 138.4, 150.3, 153.8, 154.6. IR (KBr): ν ) 2938, 2861, 2216, 1582, 1560, 1491, 1456, 1356, 1177, 882, 808, 721, 665, 638 cm-1. MS (FAB) m/z: 334 (100, M+), 291 (10), 248 (3). Anal. Calcd for C21H26N2Si: C, 75.40; H, 7.83. Found: C, 75.20; H, 7.99. 2-Cyano-7-diphenylamino-9,9-diisopropyl-9-silafluorene (3b). Purification: silica gel column chromatography (hexane/AcOEt 10:1). Yield: 81%, a colorless solid. Mp: 178.8-179.8 °C. TLC: Rf 0.24 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ 0.97 (d, J ) 7.6 Hz, 6H), 1.01 (d, J ) 7.6 Hz, 6H), 1.34 (qq, J ) 7.6, 7.6 Hz, 2H), 7.03-7.08 (m, 2H), 7.09-7.15 (m, 5H), 7.25-7.30 (m, 4H), 7.33 (d, J ) 2.4 Hz, 1H), 7.64-7.69 (m, 2H), 7.74 (d, J ) 7.8 Hz, 1H), 7.80 (dd, J ) 2.6, 0.7 Hz, 1H). 13 C NMR (100 MHz, CDCl3): δ 11.0, 18.1, 18.2, 109.1, 119.8, 120.2, 122.7, 123.2, 124.4, 124.8, 127.8, 129.2, 133.8, 136.7, 136.9, 138.2, 140.7, 147.2, 148.1, 152.8. IR (KBr): ν ) 2937, 2862, 2218, 1581, 1492, 1282, 1267, 881, 825, 748, 729, 694 cm-1. MS (FAB) m/z: 458 (49, M+), 415 (2), 373 (2). Anal. Calcd for C31H20N2Si: C, 81.18; H, 6.59. Found: C, 81.32; H, 6.50. Preparation of 2-Amino-7-formylsilafluorenes 4. An ovendried 20 mL Schlenk tube equipped with a magnetic stir bar and a rubber septum was charged with 2-amino-7-cyanosilafluorene 3 (0.9 mmol) and toluene (5 mL). To the solution was added DIBAL-H (1.5 M toluene solution, 0.9 mL, 1.35 mmol) dropwise at 0 °C. The resulting solution was warmed to room temperature and then stirred for 12 h before addition of CHCl3 (5 mL) and 1 M HCl (5 mL). The mixture was stirred for 1 h, and then the aqueous layer was extracted with CHCl3 (20 mL × 3). The combined organic layer was washed with H2O (15 mL) and saturated aq. NaCl (15 mL), dried over anhydrous MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel to give 2-amino-7-formylsilafluorene 4. 2-Dimethylamino-7-formyl-9,9-diisopropyl-9-silafluorene (4a). Purification: silica gel column chromatography (hexane/AcOEt 10:1). Yield: 79%, a yellow-green solid. Mp: 85.6-86.0 °C. TLC: Rf 0.33 (hexane/AcOEt 5:1). 1H NMR (400 MHz, CDCl3): δ 1.06 (d, J ) 7.3 Hz, 12H), 1.42 (hep, J ) 7.3 Hz, 2H), 3.07 (s, 6H), 6.76-6.90 (m, 1H), 6.98-7.00 (m, 1H), 7.77 (d, J ) 8.4 Hz, 1H), 7.78 (d, J ) 8.0 Hz, 1H), 7.86 (dd, J ) 8.0, 1.6 Hz, 1H), 8.03 (d, J ) 1.5 Hz, 1H), 9.96 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 11.1, 18.2, 18.3, 40.5, 113.6, 116.6, 119.4, 123.1, 132.7, 133.1, 134.4, 135.3, 135.6, 139.2, 150.2, 156.0, 191.9. IR (KBr): ν ) 2943, 2862, 1684, 1576, 1560, 1493, 1445, 1356, 1288, 1180, 1051, 899, 816, 725, 679 cm-1. MS (FAB) m/z: 337 (100, M+), 294 (14), 266 (6), 252 (6). Anal. Calcd for C21H27NOSi: C, 74.73; H, 8.06. Found: C, 74.46; H, 8.02.

2-Diphenylamino-7-formyl-9,9-diisopropyl-9-silafluorene (4b). Purification: silica gel column chromatography (hexane/AcOEt 10:1). Yield: 54%, a yellow solid. Mp: 143.6-144.5 °C. TLC: Rf 0.22 (hexane/AcOEt 10:1). 1H NMR (400 MHz, CDCl3): δ 0.98 (d, J ) 7.6 Hz, 6H), 1.02 (d, J ) 7.6 Hz, 6H), 1.36 (qq, J ) 7.6, 7.6 Hz, 2H), 7.04-7.10 (m, 2H), 7.10-7.15 (m, 6H), 7.25-7.28 (m, 3H), 7.35 (d, J ) 2.4 Hz, 1H), 7.73 (d, J ) 8.4 Hz, 1H), 7.82 (d, J ) 8.0 Hz, 1H), 7.89 (dd, J ) 8.0, 1.6 Hz, 1H), 8.06 (d, J ) 1.6 Hz, 1H), 10.00 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 11.1, 18.21, 18.24, 120.3, 122.9, 123.1, 124.4, 124.8, 127.9, 129.2, 132.6, 133.9, 134.5, 136.4, 139.0, 141.2, 147.2, 148.0, 154.8, 192.0. IR (KBr): ν ) 2955, 2937, 2862, 1688, 1582, 1491, 1277, 1195, 875, 826, 754, 696, 629 cm-1. MS (FAB) m/z: 461 (100, M+), 418 (14), 376 (3). Anal. Calcd for C31H31NOSi: C, 80.65; H, 6.77. Found: C, 80.93; H, 6.96. Preparation of Dicyanoethenyl-Substituted Silafluorenes 5. An oven-dried 3 mL vial equipped with a magnetic stir bar was charged with 4 (0.1 mmol), malononitrile (13 mg, 0.2 mmol), basic Al2O3 (47 mg), and toluene (0.5 mL). The reaction mixture was stirred at 70 °C for 20 h. The resulting mixture was cooled to room temperature and diluted with CH2Cl2 (10 mL). Saturated aq. NH4Cl (5 mL) was added to the solution, and the aqueous layer was extracted with CH2Cl2 (20 mL × 3). The combined organic layer was washed with H2O (15 mL × 3) and saturated aq. NaCl (15 mL), dried over anhydrous MgSO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel or recrystallization to give 5. 2-(2,2-Dicyanoethenyl)-7-dimethylamino-9,9-diisopropyl-9silafluorene (5a). Purification: silica gel column chromatography (hexane/AcOEt 1:1). Yield: 86%, a red solid. Mp: 169.4-170.8 °C. TLC: Rf 0.39 (hexane/AcOEt 2.5:1). 1H NMR (400 MHz, CDCl3): δ 1.05 (d, J ) 7.3 Hz, 6H), 1.07 (d, J ) 7.3 Hz, 6H), 1.42 (qq, J ) 7.3, 7.3 Hz, 2H), 3.09 (s, 6H), 6.79-6.85 (m, 1H), 6.93-6.99 (m, 1H), 7.68 (s, 1H), 7.74 (dd, J ) 8.3, 6.7 Hz, 2H), 7.92 (dd, J ) 8.3, 1.9 Hz, 1H), 8.06 (d, J ) 1.9 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 11.1, 18.13, 18.18, 40.6, 78.0, 113.6, 113.7, 114.8, 116.6, 119.9, 123.7, 127.8, 133.4, 136.0, 140.0, 150.5, 150.6, 156.6, 159.2. IR (KBr): ν ) 2938, 2861, 2220, 1566, 1545, 1462, 1402, 1354, 1217, 1167, 1055, 810, 725, 677, 611 cm-1. MS (FAB) m/z: 385 (100, M+), 342 (6), 300 (6). Anal. Calcd for C24H27N3Si: C, 74.76; H, 7.06. Found: C, 74.73; H, 7.00. 2-(2,2-Dicyanoethenyl)-7-diphenylamino-9,9-diisopropyl-9silafluorene (5b). Purification: recrystallization (hexane/CH2Cl2). Yield: quantitative, a red solid. Mp: 144.9-145.9 °C. TLC: Rf 0.28 (hexane/AcOEt 2.5:1). 1H NMR (400 MHz, CDCl3): δ 0.98 (d, J ) 7.6 Hz, 6H), 1.02 (d, J ) 7.6 Hz, 6H), 1.36 (qq, J ) 7.6, 7.6 Hz, 2H), 7.06-7.14 (m, 3H), 7.14-7.16 (m, 4H), 7.27-7.33 (m, 5H), 7.70-7.72 (m, 2H), 7.79 (d, J ) 8.3 Hz, 1H), 7.95 (dd, J ) 8.3, 1.9 Hz, 1H), 8.09 (d, J ) 1.9 Hz, 1H).

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13

C NMR (100 MHz, CDCl3): δ 11.1, 18.18, 18.21, 79.4, 113.3, 114.4, 120.7, 123.3, 123.5, 124.3, 124.7, 127.3, 128.6, 129.3, 133.2, 136.0, 137.3, 139.2, 140.4, 147.0, 148.7, 155.3, 159.2. IR (KBr): ν ) 2940, 2861, 2360, 2224, 1568, 1489, 1273, 1220, 883, 754, 696 cm-1. MS (FAB) m/z: 509 (25, M+), 460 (4), 438 (1). Anal. Calcd for C34H31N3Si: C, 80.12; H, 6.13. Found: C, 80.01; H, 6.20. UV-Vis Absorption and Fluorescent Measurement. The spectroscopic-grade solvents for UV-vis absorption and fluorescence measurements were purchased from Kanto Chemical Co., Inc. and degassed with argon before use. Neat thin films and doped polymer films were prepared by a spin-coating method with a MIKASA MS-A-100 spincoater. UV-vis absorption spectra were measured with a Shimadzu UV-2550 spectrometer. Fluorescence spectra and absolute quantum yields were recorded with a Hamamatsu Photonics C9920-02 Absolute PL Quantum Yield Measurement System. The fluorescence lifetime was measured with a Horiba Jobin Yvon TemPro instrument (time-correlated single-photon counting, fluorescence lifetime system, Horiba Jobin Yvon IBH). Preparation of Neat Thin Films. In a tube glass, a sample (EXSTAR 6000 TG/DTA, Seiko Instruments Inc., was used for weighing a sample) was dissolved in toluene with concentration of 0.1 mg/mL. The resulting solution was dropped onto a quartz plate (10 mm ×10 mm) and spin-coated at 100 rpm for 20 s and then at 1000 rpm for 100 s. The deposited film was dried under reduced pressure at 50 °C for 1 h. Preparation of Doped Polymer Films. In a tube glass, a sample was dissolved in a saturated solution of polymer with a concentration of 0.1 mg/mL. The resulting solution was dropped onto a quartz plate (10 mm ×10 mm) and spin-coated at 100 rpm for 20 s and then at 1000 rpm for 100 s. The deposited film was dried under reduced pressure at 50 °C for 1 h. Solvents for dissolving polymer are as follows: benzene for polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(ethylene glycol) (PEG) and dimethylformamide (DMF) for poly(acrylonitrile) (PAN). Preparation of PMMA Film Doped with 4b and 5b. In a glass tube, silafluorene 4b (1.372 mg, 8.5 µmol) and 5b (0.224 mg, 0.4 µmol) was dissolved in a saturated benzene solution of PMMA (3 mL). The resulting solution was dropped onto a quartz plate (10 mm ×10 mm), and the plate was dried at room temperature for 12 h and then at 50 °C for 1 h under reduced pressure. CV Measurement. Cyclic voltammetry (CV) was performed on a BAS ALS610c electrochemical analyzer. The CV cell consisted of a Pt disk electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The measurement was carried out under argon atmosphere using a CH2Cl2 solution of a sample with a concentration of 1 mM and 0.1 M tetrabutylammonium perchlorate (Bu4NClO4) as a supporting electrolyte at a scan rate of 100 mV/s. Results and Discussion Synthesis of 2-Amino-7-acceptor-9-silafluorenes. As shown in Scheme 1, amino-substituted 9-silafluorenes 1, 2, and 3 were synthesized by Pd-catalyzed intramolecular coupling of 2-(3aminophenyldiisopropylsilyl)aryl triflates 6, 7, and 8, respectively, in high to excellent yields under the standard conditions [Pd(OAc)2 (2.5 mol %), PCy3 (5 mol %), Et2NH (2 equiv), and DMA at 100 °C for 24 h]. It should be noted that the regioisomeric product of 1 was not produced at all.79 DIBAL-H reduction of the cyano group in 3a and 3b gave 7-formyl-2amino-9-silafluorenes 4a and 4b, respectively. Dicyanoethenyl-

Figure 5. UV-visible absorption spectra of 1-5 in cyclohexane.

Figure 6. UV-visible absorption spectra of 4a in various solvents.

substituted silafluorenes 5a and 5b were obtained in high to quantitative yields by the Knoevenagel condensation reaction of 4a and 4b with malononitrile, respectively. Single crystals of 3b suitable for X-ray crystal structure analysis were obtained by recrystallization from the hexane/ dichloromethane solution.80 The silafluorene moiety is almost flat with large deformation of the silicon atom from the ideal geometry of an sp3-hybridized silicon atom; the bond angles of C(1)-Si(1)-C(13) and C(13)-Si(1)-C(16) are significantly expanded to 112.5° and 113.0°, respectively, and the C(1)-Si(1)C(12) angle is narrowed to 90.8°, which is much smaller than the standard value of 109.5° (Figures 4a and b). The isopropyl groups are oriented perpendicular to the silafluorene skeleton and the two phenyl rings adopt a noncoplanar conformation to the π-plane. The conformational characteristics of 3b in the crystal retard close packing such as π-π stacking, as illustrated in Figure 4c. The shortest distance between the π-planes of the adjacent 3b molecule is 6.522 Å (Figure 4d). UV-Vis Absorption Properties in Solvents. The UV-visible absorption spectra of 1-5 were measured in various solvents whose dielectric constants were largely distinct; the results are summarized in Table 1.81 Figure 5 shows the absorption spectra of 1-5 measured in cyclohexane. The absorption maxima of the D-π-A type silafluorenes red-shifted significantly, and the molar absorption coefficients increased considerably as the electronwithdrawing nature of the acceptor was enhanced. The replacement of a dimethylamino group with a diphenylamino group induced a bathochromic shift of the absorption spectra; however, the degree of the shift reduced when the acceptor was strengthened further. Similar phenomena were observed in other solvents such as benzene, diethyl ether, dichloromethane, ethanol, acetonitrile, and dimethyl sulfoxide (see the Supporting Information). As expected, these results indicate that donor and acceptor substitutions at the 2- and 7-positions of 9-silafluorene, respectively, are effective for tuning the HOMO-LUMO energy gap and observing the absorption behavior of 9-silafluorenes.



TABLE 2: Fluorescence Properties of 1-5 in Various Solventsa solvent (ε)b

1

2

3a


397 (0.26) 404 (0.22) 406 (0.22) 410 (0.13) 405 (0.11) 418 (0.28) 424 (0.12)

402 (0.37) 410 (0.29) 410 (0.35) 423 (0.33) 422 (0.28) 440 (0.40) 451 (0.17)

410 (0.60) 422 (0.62) 423 (0.66) 442 (0.79) 456 (0.74) 464 (0.65) 473 (0.51)

emission maximum (nm) (Φf)c 4a 5a 414 (0.08) 450 (0.75) 449 (0.79) 501 (0.71) 565 (0.40) 528 (0.81) 541 (0.57)

509 (0.08) 566 (0.36) 584 (0.56) 644 (0.56) 691 (0.16) 694 (0.32) 714 (0.23)

3b

4b

5b

413 (0.64) 432 (0.75) 436 (0.74) 470 (0.80) 482 (0.72) 500 (0.67) 500 (0.81)

425 (0.50) 456 (0.77) 459 (0.76) 524 (0.80) 482 (0.11) 555 (0.48) 557 (0.58)

515 (0.46) 564 (0.69) 586 (0.80) 674 (0.43) -d -d -d

a Measured at 1 × 10-5 mol/L. Excitation was effected by UV light at 290 (for 1 and 2), 320 (3a and 4a), 390 (5a, 3b, and 4b), and 430 (5b) nm. b ε: dielectric constants cited from ref 81. c Absolute quantum yield determined by a calibrated integrating sphere system. d No fluorescence was observed.

Figure 7. Fluorescence spectra of 1-5 in cyclohexane.

The absorption spectra of 4a in various solvents are shown in Figure 6. The shortest wavelength of the absorption maxima was 390 nm recorded in cyclohexane, and the longest wavelength was 400 nm observed in dimethyl sulfoxide (DMSO), indicating that the effect of solvent polarity on the absorption spectra of the D-π-A type silafluorenes was quite small. Fluorescence Properties in Solvents. The photoluminescence properties of 1-5 in various solvents are summarized in Table 2. As can be seen in the normalized spectra measured in cyclohexane (Figure 7), the emission maxima of 1-5 red-shifted in line with the electron-withdrawing nature of the acceptor. In addition, the emission maxima were highly dependent on the dielectric constant of the solvent; the more polar the solvent, the longer the wavelength of the emission maxima. The difference between the shortest emission maxima observed in cyclohexane and the longest one in DMSO increased in the order of 1 (27 nm), 2 (49 nm), 3a (63 nm), 4a (127 nm), and 5a (205 nm). The remarkable solvatochromism of 4a and 5a is demonstrated with the fluorescence spectra and photographs in Figures 8 and 9. The fluorescence colors ranged from blue to yellow for 4a and from green to the near-infrared region for 5a. In the case of 4a, the emission maxima measured in EtOH appeared at longer wavelengths than those measured in acetonitrile and DMSO. This is probably due to the effect of the hydrogen bonding between the formyl group and EtOH.82 Considering the fact that the absorption spectra have a low dependency on solvent polarity, the strong fluorescence solvatochromism suggests the intramolecular charge-transfer character of the excited states. The fluorescence quantum yields (Φf) of 1 and 2 were not high in any of the solvents, whereas 3-5 exhibited fluorescence in moderate to high quantum yields up to 0.81 in specific solvents. The fluorescence lifetimes (τs) of 1-5 in cyclohexane and dichloromethane were measured at room temperature. Table 3 summarizes the quantum yields (Φf), lifetimes (τs), and radiative (kr) and nonradiative (knr) decay rate constants of the excited states.83 The radiative constants ranging from 3.3 × 107 to 31

Figure 8. Fluorescence spectra of 4a in various solvents (λex ) 320 nm) and the photographs of the luminescence upon irradiation with an UV lamp (365 nm): (a) cyclohexane, (b) benzene, (c) diethyl ether, (d) dichloromethane, (e) ethanol, (f) acetonitrile, and (g) dimethyl sulfoxide.

Figure 9. Fluorescence spectra of 5a in various solvents (λex ) 390 nm) and the photographs of the luminescence upon irradiation with an UV lamp (365 nm): (a) cyclohexane, (b) benzene, (c) diethyl ether, (d) dichloromethane, (e) ethanol, (f) acetonitrile, and (g) dimethyl sulfoxide.

× 107 s-1 were slightly enhanced when the acceptor became stronger, but they were almost independent of the solvents. On the other hand, the nonradiative constants of 4a (270 × 107 s-1) and 5a (290 × 107 s-1) in cyclohexane were much larger

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TABLE 3: Fluorescence Lifetimes (τs), Radiative Rate Constants (kr), and Nonradiative Decay Rate Constants (knr) of Dimethylamino-Substituted 9-Silafluorenes 1-5 in Cyclohexane and Dichloromethane excitation (nm)

solvent

Φf

τs (ns)

kr (107 s-1)a

knr (107 s-1)b

1

340

2

340

3a

388

4a

388

5a

474

c-C6H12 CH2Cl2 c-C6H12 CH2Cl2 c-C6H12 CH2Cl2 c-C6H12 CH2Cl2 c-C6H12 CH2Cl2

0.26 0.13 0.37 0.33 0.60 0.79 0.08 0.71 0.08 0.56

6.5 4.0 3.3 2.5 2.6 2.6 0.3 3.0 0.3 3.2

4.0 3.3 11 13 23 31 24 24 25 18

11 22 19 27 16 8.1 270 9.7 290 14

compound

a

kr ) Φf/τs. b knr ) (1 - Φf)/τs.

TABLE 4: Fluorescence Properties of 1-5 in the Solid Statea solid state (ε)b

1

2

3a

Neat thin film PS film (2.5) PMMA film (3.3) PAN film (5.5) PEG film (14)

403 (0.30) 402 (0.47) 404 (0.45) 404 (0.10) 418 (0.27)

417 (0.39) 409 (0.26) 416 (0.37) 423 (0.13) 417 (0.39)

457 (0.25) 422 (0.65) 434 (0.66) 446 (0.34) 460 (0.28)

emission maximum (nm) (Φf)c 4a 5a 515 (0.06) 455 (0.76) 472 (0.76) 501 (0.31) 512 (0.21)

-d 573 607 644 671

(0.48) (0.61) (0.14) (0.08)

3b

4b

5b

461 (0.39) 447 (0.83) 446 (0.82) 465 (0.48) 467 (0.71)

503 (0.31) 454 (0.82) 469 (0.80) 518 (0.37) 523 (0.57)

665 (0.33) 573 (0.78) 594 (0.80) 627 (0.08) 656 (0.22)

a Excitation was effected by UV light at 290 (for 1 and 2), 320 (3a and 4a), 390 (5a, 3b, and 4b), and 430 (5b) nm. b ε: dielectric constants cited from two publications in ref 94 for PS, PMMA, PAN, and PEG. c Absolute quantum yield determined by a calibrated integrating sphere system. d No fluorescence was observed.

Figure 10. Fluorescence spectra of 1-5 dispersed in PMMA film.

than those of 1-3 (8.1 × 107-27 × 107 s-1). The nonradiative constants of 4a and 5a in cyclohexane were also larger than their values in dichloromethane (9.7 × 107 and 14 × 107 s-1). Thus, the low quantum yields of 4a and 5a in cyclohexane are ascribed to the accelerated nonradiative decay process in nonpolar media. Fluorescence Properties in the Solid States. Silafluorenes 1-5 also exhibited fluorescence in the solid states, for example, neat thin film and doped polymer films. The properties are summarized in Table 4. In a series of dimethylamino-substituted silafluorenes, the emission maxima showed a bathochromic shift in the neat thin film as well as in doped polymer films, in the order of 1, 2, 3a, 4a, and 5a as observed in the fluorescence spectra in solution. In other words, the fluorescence color can be tuned by the acceptor. The spectra of doped poly(methyl methacrylate) (PMMA) films are shown in Figure 10 as a representative example. Diphenylamino-substituted silafluorenes 3b-5b exhibited a slight blue-shift of the emission maxima compared with the corresponding dimethylamino derivatives 3a-5a. The quantum yields of 1-5 in the neat film were low to moderate with the exception of 5b. Since red fluorophores in the solid state generally have a strong tendency to cause aggregation that results in severe luminescence quenching,84 it is remarkable that 5b exhibited red fluorescence with a quantum

Figure 11. Fluorescence spectra of 4a dispersed in polymer film [PS, polystyrene; PMMA, poly(methyl methacrylate); PAN, polyacrylonitrile; PEG, poly(ethylene glycol)] and photographs of the films upon irradiation with an UV lamp (365 nm): (a) PS, (b) PMMA, (c) PAN, and (d) PEG.

yield of 0.33.66,85-92 Meanwhile, the quantum yields of 1-5 in polymer films ranged from moderate to high, depending on both the silafluorenes and the host polymers. Generally, 1-5 dispersed in polystyrene (PS) and PMMA films exhibited fluorescence with high to excellent quantum yields. The quantum yields of diphenylamino derivatives 3b-5b were generally higher than those of dimethylamino derivatives 3a-5a in both neat and polymer films. These observations are reasonable by assuming that the bulkier diphenylamino group arranges the chromophores far from each other for a molecule in the solid state, as evidenced by the crystal structure of 3b, thereby preventing intermolecular interaction such as π-π stacking that causes luminescence quenching.66 The emission colors of the silafluorene-doped films are also dependent on the polarity of the polymers.93,94 Especially in the cases of 4a, 4b, 5a, and 5b, the differences between the wavelength of the emission maxima



Figure 14. Fluorescence spectra of 9-11 along with 2, 4a, and 5a in benzene.

Figure 12. Fluorescence spectra of 5a dispersed in polymer film [PS, polystyrene; PMMA, poly(methyl methacrylate); PAN, polyacrylonitrile; PEG, poly(ethylene glycol)] and photographs of the films upon irradiation with an UV lamp (365 nm): (a) PS, (b) PMMA, (c) PAN, and (d) PEG.

Figure 15. Cyclic voltammograms of 1-5 measured in dichloromethane at a scanning rate of 100 mV/s.

Figure 13. UV-visible absorption spectra of 9-11 along with 2, 4a, and 5a in benzene.

in the least polar PS film and the most polar poly(ethylene glycol) film were 57, 69, 98, and 83 nm, respectively. The fluorescence spectra of 4a and 5a are shown as representative examples in Figures 11 and 12, respectively. When the polarity of the polymer media was increased, the fluorescence color of 4a in the polymer film changed from blue to green, as shown in Figure 11, and the emission of 5a dispersed in the polymer film varied from orange to red, as shown in Figure 12. Comparison with Carbon Analogues. To examine the effect of the silicon bridge on the properties of the D-π-A type silafluorenes, we prepared the corresponding D-π-A type fluorenes 9-11 and examined their photophysical properties.95,96 The results are summarized in Table 5. The UV-visible absorption spectra and the fluorescence spectra of 9-11 along with the silicon counterparts 2, 4a, and 5a in benzene are shown in Figures 13 and 14, respectively. The absorption edges of 9 (378 nm) and 10 (434 nm) exhibited a hypsochromic shift of 21 and 7 nm compared with those of 2 (399 nm) and 4a (441 nm), respectively, while no shift was observed in the cases of 11 and 5a. The emission maxima of fluorescence behaved similarly as illustrated in Figure 14. These observations indicate that the silicon bridge can contribute more than the carbon bridge toward the slight extension of the effective conjugation length of the donor- and acceptor-substituted biphenyl moiety due to the σ*-π* conjugation. However, the contribution appears to be smaller as the electron-withdrawing character of the acceptor becomes stronger, and the contribution almost reaches zero in the case of 5a.

Figure 16. Oxidation potential of 1-5 along with 9-11 measured in dichloromethane.

Figure 17. HOMO and LUMO energies of 1-5 (the values between the HOMO and LUMO levels are the energy gaps of the HOMOs and LUMOs).

Electrochemical Properties. The redox properties of 1-5 were measured by cyclic voltammetry (CV), which was carried out in a 0.1 M Bu4NClO4 solution in dichloromethane with a scanning rate of 100 mV/s using Pt as the working electrode

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TABLE 5: Photophysical Properties of Fluorenes 9-11 in Various Solventsa

emission maximum (nm) (Φf)c

absorption maximum (nm) (molar absorption coefficient) solvent (ε)b

9

10

11

9

10

11


341 (27 600) 346 (25 700) 342 (30 200) 344 (25 200) 342 (25 400) 302 (13 200) 350 (27 700)

385 (40 700) 388 (31 600) 379 (35 300) 392 (33 000) 388 (20 400) 385 (33 700) 395 (30 600)

480 (58 800) 480 (39 300) 470 (39 300) 490 (39 300) 478 (37 900) 474 (37 500) 490 (17 600)

360 (0.67) 374 (0.59) 373 (0.63) 387 (0.44) 388 (0.58) 337 (0.04) 414 (0.53)

416 (0.02) 437 (0.82) 436 (0.71) 491 (0.91) 562 (0.64) 521 (0.81) 532 (0.92)

528 (0.04) 560 (0.22) 579 (0.39) 631 (0.62) 677 (0.18) 688 (0.26) 711 (0.24)

a Measured at 1 × 10-5 mol/L. Excitation was effected by UV light at 300 (for 9), 360 (10), and 390 (11) nm. b ε: dielectric constants cited from ref 81. c Absolute quantum yield determined by a calibrated integrating sphere system.

Figure 18. HOMO and LUMO diagrams of 1-5.

and Ag/AgCl as the reference electrode. All the D-π-A type silafluorenes exhibited reversible oxidation waves as shown in Figure 15; the reduction waves were structureless, and therefore, the reduction potentials could not be detected. Considering that spirobisilafluorenes showed irreversible single-electron oxidation and reduction,58 the reversibility of the oxidation process in 1-5 is attributed presumably to the D-π-A electronic structure. The oxidation potentials of 1-5 together with those of 9-11 are illustrated in Figure 16. As the electron-withdrawing nature of the acceptor is enhanced, the oxidation potential increases further. The replacement of a dimethylamino group with a diphenylamino group also resulted in the enhancement of the oxidation potentials. Thus, the radical cations produced by the single-electron oxidation should be definitely stabilized by the presence of both amino and acceptor groups, described as the capto-dative effect.97 Theoretical Study. Molecular orbital calculations of 1-5 were carried out by the density functional theory (DFT) method at the B3LYP/6-31G*//B3LYP/6-31G* level using the Gaussian 03 package.98 The results are shown in Figures 17 and 18. The HOMO-LUMO gaps became smaller as the electron-withdrawing nature of the acceptor was enhanced, and this is mainly ascribed to the lowering of the LUMOs. The replacement of the dimethylamino group with a diphenylamino group in 3-5 also resulted in the reduction of the HOMO-LUMO gaps caused by both lowering of the LUMOs and rising of the HOMOs. These results are consistent with the observed redshift of the absorption spectra shown in Figure 5. The HOMOs of 1-5 are extended over the biphenyl and amino moieties (Figure 18). The LUMOs of 1-3 are delocalized on the acceptor, biphenyl, and silicon bridge, whereas no lobe is spread over the silicon bridges of 4 and 5. Thus, when the acceptor became stronger, the silicon bridge did not participate in the delocalization of the π-system consisting of the donor-biphenyl-

Figure 19. Fluorescence spectra of 4b, 5b and the mixture dispersed in PMMA with the image of the white emission (upon irradiation with an UV lamp) from the mixture.

acceptor skeleton. These results are well consistent with observations made during the comparison of the photophysical properties of silafluorenes and those of the corresponding fluorenes. White Emission. To demonstrate the use of the D-π-A type silafluorenes as solid-state emitting materials, we prepared a PMMA film doped with 4b and 5b in the ratio 19:1.99-105 The polymer film showed intense white emission (CIE coordinate: x ) 0.30, y ) 0.34) with an excellent quantum yield of 0.81 upon irradiation with UV light at 390 nm (Figure 19). As shown in Figure 19, the spectra had two broad bands that originated from 4b and 5b.106 Conclusions By means of the Pd-catalyzed intramolecular coupling of the corresponding 2-(3-aminophenyldiisopropylsilyl)aryl triflates, we have synthesized 2-amino-7-acceptor-9-silafluorenes with different acceptor groups (hydrogen, trifluoromethyl, and cyano groups) in high yields. Donor-π-acceptor type 9-silafluorenes

2-Donor-7-acceptor-9-silafluorenes containing a formyl group as the acceptor and those containing a dicyanoethenyl group as the acceptor were synthesized by the reduction of the cyano derivative by DIBAL-H and by the Knoevenagel condensation of the formyl derivative with malonitrile, respectively. As the electron-withdrawing nature of the acceptor was enhanced further, the UV-visible absorption and fluorescence spectra of the D-π-A type silafluorenes exhibited a larger bathochromic shift in organic solvents. Remarkable solvatochromism of the fluorescence maxima of formyl- and dicyanoethenyl-substituted silafluorenes was observed and understood in the sense that the excited states of the silafluorenes were polarized by intramolecular charge transfer. The emission colors drastically changed from blue to yellow for the formyl derivatives and from green to red for the dicyanoethenyl derivatives. The silafluorenes in a neat film and a polymer film generally showed fluorescence with moderate to excellent efficiency, and the emission color was dependent on the polarity of the host polymer, which was consistent with the solvatochromism of the fluorescence spectra. DFT calculations support that a silicon bridge is effective for conjugation extending over the donor-biphenyl-acceptor framework when the acceptor is less electron-withdrawing. The PMMA film doped with Ph2N/ CHO- and Ph2N/CHd(CN)2-substituted silafluorenes exhibited white photoluminescence with an excellent quantum yield. These results clearly shed light on the high potential of D-π-A type 9-silafluorenes as a versatile fluorophore that can be used not only in solution but also in the solid state. Moreover, the practical applications of D-π-A type 9-silafluorenes to optoelectronic devices appear promising. Acknowledgment. This work was supported by Grant-inAid for Creative Research, No. 16GS0209, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. One of the authors (M.S.) acknowledges the financial support from Iketani Science and Technology Foundation and General Sekiyu Research & Development Encouragement & Assistance Foundation. The authors thank Dr. Suguru Inoue and Ms. Ikuko Tsuda of Horiba Co., Ltd. for their measurement of fluorescence lifetimes. Supporting Information Available: Experimental procedures and characterization data for 6-11, and UV-vis absorption and fluorescence spectra of 1-5 and 9-11. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Khabashesku, V. N.; Balaji, V.; Boganov, S. E.; Nefedov, O. M.; Michl, J. J. Am. Chem. Soc. 1994, 116, 320. (2) Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69, 2327. (3) Chen, J.; Cao, Y. Macromol. Rapid Commun. 2007, 28, 1714. (4) Dubac, J.; Laporterie, A.; Manuel, G. Chem. ReV. 1990, 90, 215. (5) Colomer, E.; Corriu, R. J. P.; Lheureux, M. Chem. ReV. 1990, 90, 265. (6) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 3693. (7) Hissler, M.; Dyer, P. W.; Reáu, R. Coord. Chem. ReV. 2003, 244, 1. (8) Zhan, X.; Barlow, S.; Marder, S. R. Chem. Commun. 2009, 1948. (9) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974. (10) Yamaguchi, S.; Endo, T.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Tamao, K. Chem.sEur. J. 2000, 6, 1683. (11) Uchida, M.; Izumizawa, T.; Nakano, T.; Yamaguchi, S.; Tamao, K.; Furukawa, K. Chem. Mater. 2001, 13, 2680. (12) Tang, B. Z.; Zhan, X.; Yu, G.; Lee, P. P. S.; Liu, Y.; Zhu, D. J. Mater. Chem. 2001, 11, 2974. (13) Chen, J.; Law, C. C. W.; Lam, J. W. Y.; Dong, Y.; Lo, S. M. F.; Williams, I. D.; Zhu, D.; Tang, B. Z. Chem. Mater. 2003, 15, 1535. (14) Palilis, L. C.; Murata, H.; Uchida, M.; Kafafi, Z. H. Org. Electron. 2003, 4, 113.

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JP103410X

Synthesis and Photophysical Properties of 2-Donor-7-acceptor-9

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