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Synthesis and Characterization of Perfluoroaryl-Substituted Siloles and Thiophenes: A Series of Electron-Deficient Blue Light Emitting Materials Katharine Geramita,† Jennifer McBee,† Yulong Shen,‡ Nora Radu,§ and T. Don Tilley*,† Department of Chemistry, UniVersity of California at Berkeley, Berkeley California 94720, Dupont Displays, Chestnut Run Plaza 708/141, Wilmington, Delaware 19808, and Dupont Displays, Experimental Station 328/106, Wilmington, Delaware 19808 ReceiVed February 10, 2006. ReVised Manuscript ReceiVed April 21, 2006
Perfluoroaryl (Arf)-derivatized siloles and thiophenes have been synthesized via nucleophilic aromatic substitutions (SNAr) involving reactions of hexafluorobenzene, octafluoronaphthalene, and decafluorobiphenyl with the appropriate dilithiosilole or dilithiothiophene intermediate. These compounds are of interest as electron-transport layers and/or blue light emitters, as they possess relatively low LUMO energy levels while maintaining high HOMO-LUMO gaps. Siloles and thiophene were modified in the 2- and 5-positions, while bithiophene substitution occurred in the 5- and 5′-positions. The HOMOLUMO gaps, as determined by UV-vis spectroscopy, range between 2.79 and 3.56 eV, while photoluminescence emission spectra reveal λmax,ems values from 396 to 506 nm (corresponding to violet to blue/green emission). Dilute solution-state quantum yields varied from 0.01 to 0.10 for the silole compounds and from 0.25 to 0.71 for the thiophene-based compounds. The experimentally determined LUMO levels (ca. -2.6 to -2.9 eV, as determined by cyclic voltammetry) suggest that these compounds are good candidates for electron-transport layers. DFT calculations were used to investigate the electronic properties of the compounds, and a preliminary assessment of charge transport and electroluminescent behavior was made.
Introduction In recent years, the use of organic semiconducting materials for applications in “plastic electronics”, such as field effect transistors (FETs), organic light emitting diodes (OLEDs), and photovoltaics (PVs), has gained increasing interest.1 Organic systems offer the possibility for cheap raw materials and processing costs and are readily tailored to access a wide range of physical, optical, and electrical properties in the final device.2-4 In general, conjugated organic π-systems are effective hole-transporting (p-type) materials;5 however, the development of improved devices (e.g., FET and PV4,6) should be facilitated by the introduction of efficient electron-conducting (n-type) materials. An additional challenge for the development of organic components in electronics devices concerns light emitting materials. Full color displays require materials that emit over a broad range * To whom correspondence should be addressed. E-mail:
[email protected]. † University of California at Berkeley. ‡ Dupont Displays, Chestnut Run Plaza 708/141. § Dupont Displays, Experimental Station 328/106.
(1) Hwang, D. H.; Lee, J. M.; Lee, S.; Lee, C. H.; Jin, S. H. J. Mater. Chem. 2003, 13, 1540-1545. (2) Hwang, D. H.; Kim, S. K.; Park, M. J.; Koo, B. W.; Kang, I. N.; Kim, S. H.; Zyung, T. Chem. Mater. 2004, 16, 1298-1303. (3) Schwartz, M.; Srinivas, G.; Yeates, A.; Berry, R.; Dudia, D. Synth. Met. 2004, 143, 229-236. (4) Zhu, Y.; Alam, M. M.; Jenekhe, S. A. Macromoledules, 2003, 36, 8958-8968. (5) Renak, M. L.; Bartholomew, G. P.; Wang, S. J.; Ricatto, P. J.; Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc. 1999, 121, 77877799. (6) Tonzola, C. J.; Alam, M. M., Kaminsky, W.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13548-13558.
of the color spectrum, but stable blue light emitters have proven illusive.7,8 Thus, two areas of major research focus are the development of improved n-type materials9 and the development of stable and pure blue light emitters.10 Useful n-type organic materials should possess a low barrier to electron injection and a high propensity for electron transport. While it is generally accepted that a low-energy LUMO should facilitate charge injection, the factors resulting in high electron mobility are not well understood. Electron deficient π-systems are thought to be linked to high electron mobility, and pyridines,4 oxadiazoles,9,11,12 metalloles,13,14 and fluorinated aromatics3,15,16 have all been identified as potential n-type components. Metalloles possess low-energy LUMOs as a result of stabilization of the diene π-system via (7) Cho, N. S.; Hwang, D.-H.; Jung, B.-J.; Oh, J.; Chu, H. Y.; Shim, H. K.: Synth. Met. 2004, 143, 277-282. (8) Leung, L. M.; Lo, W. Y.; So, S. K.; Lee, L. M.; Choi, W. K. J. Am. Chem. Soc. 2000, 122, 5640-5641. (9) Jin, S.-H.; Kim, M.-Y.; Kim, J. Y.; Lee, K. Gal, Y.-S. J. Am. Chem. Soc. 2004, 126, 2472-2480. (10) Jacob, J.; Sax, S.; Piok, T.; List, E.; Grimsdale, A.; Mullem, K. J. Am. Chem. Soc. 2004, 126, 6987-6995. (11) Huang, W.; Yu, W. L.; Meng, H.; Pei, J.; Li, S. F. Y. Chem. Mater. 1998, 10, 3340-3345. (12) Kim, J. H.; Lee, H. Synth. Met. 2004, 144, 169-176. (13) Tamao, K.; Uchinda, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974-11975. (14) Yamaguchi, S.; Endo, T.; Uchinda, M.; Izumizawa, T.; Furukawa, K.; Tamao, K. Chem. Eur. J. 2000, 6, 1683-1692. (15) Facchetti, A.; Yoon, M. H.; Stern, C. L.; Katz, H. E.; Marks, T. J. Angew. Chem., Int. Ed. 2003, 42, 3900-3903. (16) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Mori, T.; Tokito, S.; Taga, T. J. Am. Chem. Soc. 2000, 122, 10240-10241.
10.1021/cm060346u CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006
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interaction with the silicon-based σ* orbitals,17,18 and work by Tang et al. has produced silole-based derivatives with high external electroluminescence (EL) efficiencies.19 Recently, work by Facchetti et al. has demonstrated control over the nature of the charge carrier in thiophene-based systems by coupling oligothiophenes with perfluoroalkyl or perfluoroaryl groups.15,20,21 Such hybrid compounds represent promising materials for which physical and electronic properties can be fine-tuned. One difficultly associated with development of stable blue light emitting materials originates from the fact that a relatively large HOMO-LUMO gap is necessary to achieve blue emission,22 and this property often presents a barrier for electron injection.8 Doping an organic with elemental B or Al has improved electron injection, but often results in a decrease in the HOMO-LUMO energy gap, leading to poor color purity for the emitted light.8 A general strategy for achieving pure and efficient blue light emission involves stabilization of the LUMO level, to facilitate electron injection, while maintaining a high HOMO-LUMO energy gap.23 The development of both n-type conducting and blue light emitting organic materials should benefit from the incorporation of electron-deficient moieties into conjugated systems. Previous work in our laboratories has shown that nucleophilic aromatic substitution (SNAr) is a highly effective mode for carbon-carbon bond formation involving perfluoroaromatic compounds,24 and high conversion to a single product was demonstrated for a variety of substrates. In this report we describe the synthesis and physical, optical, and electrochemical characterizations of two series of compounds, prepared via nucleophilic aromatic substitution (SNAr), that may have uses as n-type conductors and/or blue light emitters. The 2,5-perfluoroaryl siloles 1a-c result from the combination of two n-type conducting components, while the second set of compounds (2a-c, 3a-c) represent hybrid materials built from p- and n-type subunits. Results and Discussion Synthesis of Perfluoroaryl-Substituted Compounds. Transition metal catalyzed coupling reactions are currently among the most popular methods for the coupling of sp or sp2 carbons. However, the final reductive elimination step in such transformations may prove difficult when electrondeficient species are used. An alternate method for carboncarbon bond formation involving electron-deficient aromatic compounds is a direct nucleophilic substitution. Perfluoro(17) Tamao, K.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc. 1994, 116, 11715-11722. (18) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 36933702. (19) Chen, H.-Y.; Lam, W. Y.; Luo, J. D.; Ho, Y. L.; Tang, B. Z.; Zhu, D. B.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574-576. (20) Facchetti, A.; Letizia, J.; Yoon, M.-H.; Mushrush, M.; Katz, H. E.; Marks, T. J. Chem. Mater. 2004, 16, 4715-4727. (21) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13859-13874. (22) Ahn, T.; Song, S. Y.; Shim, H. K. Macromolecules 2000, 33, 67646771. (23) Kim, K. D.; Park, J. S.; Kim, H. K.; Lee, T. B.; No, K. T. Macromolecules 1998, 31, 7267-7272. (24) Nitschke, J.; Tilley, T. D. J. Am. Chem. Soc. 2001, 123, 10183-10190.
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arenes represent exceptionally good substrates for nucleophilic aromatic substitution (SNAr),25 and this relatively straightforward carbon-carbon coupling pathway may provide a number of advantages with respect to traditional metal catalyzed carbon-carbon bond formations. The general synthetic route to silole-based compounds is outlined in Scheme 1. Preparation of the dilithiosilole intermediate, as described by Tamao and co-workers, proceeds via one-electron reductions of the phenyl ethynyl groups of a diphenyl ethynyl silane.17 Reaction of the resulting dilithiosilole species with hexafluorobenzene proceeded readily in THF at -78 °C to give 1a. Compound 1a, isolated via flash column chromatorgraphy in 54% yield, is highly soluble in common organic solvents. Preparation of the perfluorobiphenyl derivative 1b via a method analogous to that for 1a produced the desired product in 50% yield, although 19F NMR analysis of the crude reaction mixture suggests that 1b is formed in greater than 90% yield. Preparation of the heptafluoronaphthalyl derivative 1c via a synthetic method similar to that used for 1a and 1b resulted in very low yields of isolated product. Investigation of the reaction mixture by 1H NMR spectroscopy confirmed complete consumption of the starting silane; however, the 19F NMR spectrum of the crude reaction mixture revealed only minor consumption of the octafluoronaphthalene. With hexanes as solvent, the yield of 1c (by 19F NMR spectroscopy) increased to ∼75%, and this allowed isolation of the product in 25% yield. Substitution on the octafluoronaphthalene ring occurred predominantly at the 2-position, as evidenced by the large peri coupling constants observed for four of the fluorine atoms (4JF1F8,peri ) 67.4 Hz and 4JF4F5,peri ) 57.0 Hz).26 This conclusion was confirmed by assignment of the 19F NMR spectrum via 2D 19F correlation NMR spectroscopy (COSY). The synthetic route to the thiophene derivatives (Scheme 2) involves addition of a solution of 2,5-dilithiothiophene, generated in situ from the 2,5-dihaolothiophene, to a solution of excess fluoroarene. The desired products were purified via sublimation and isolated in 25% (2a), 40% (2b), and 25% (2c) yield. The synthetic route to the bithiophene derivatives (3a-c; Scheme 3) involves generation of dilithiobithiophene, generated in situ via deprotonation of bithiophene at the 5,5′positions, followed by addition of this species to a solution (25) Burdon, J. Tetrahedron 1965, 21, 3373-3380. (26) Burdon, J.; Childs, A. C.; Parsons, I. W.; Tatlow, J. C. J. Chem. Soc., Chem. Commun. 1982, 10, 534-535.
Perfluoroaryl-Substituted Siloles and Thiophenes Scheme 1
Chem. Mater., Vol. 18, No. 14, 2006 3263 Table 1. Select Optical and Electrochemical Characterization Data for Compounds 1-3 λmax,abs nm (log()) 1a 1b 1c
Scheme 2
Scheme 3
of excess perfluoroarene. In the syntheses of 3a and 3b, a significant amount of insoluble byproduct was removed by filtration. The products were purified via sublimation to remove excess perfluoroarene and monosubstituted byproduct and were isolated in 40% (3a) and 47% (3b) yield. The low solubility of 3c (in THF and dichloromethane) facilitated its isolation by filtration in 51% yield. Very recently, the synthesis of compounds 2a and 3a (in 72% and 66% yield, respectively) via a Suzuki coupling of either dibromothiophene or dibromobithiophene and pentafluoroboronic acid was reported by Takimiya and co-workers.27 Spectroscopic Characterization of 1-3. The solution UV-vis data collected for compounds 1-3 are summarized in Table 1. The HOMO-LUMO energy gaps (Egopt) were calculated from the absorption onset wavelength, which was taken to be the wavelength at 10% of the most red-shifted λmax absorbance. The UV-vis spectra of 1a-c exhibit at least two intense peaks. For previously reported silole compounds the peak at 240-250 nm is generally assigned to π f π* transitions of the aryl groups in the 3,4-positions, while the lower energy absorption is assigned to the π f π* transition of the silole ring.13,24 While the lower energy λmax values for compounds 1a-c are similar, all compounds exhibit a shoulder that extends as far as 400 nm. Differences in the energies of these shoulders account for differences in the calculated Egopt values. For other silole-containing compounds it has been shown that the HOMO and LUMO (27) Takimiya, K.; Niihara, N.; Otsubo, T. Synthesis 2005, 10, 1589-1592.
2a 2b 2c 3a 3b 3c
240 (4.23) 298 (3.87) 242 (5.30) 296 (4.38) 244 (4.22) 296 (3.73) 306 (4.23) 328 (4.61) 341 (3.72) 358 (4.50) 380 (4.69) 390
Egopt (eV)
λmax,ems nm, soln
ΦPL soln
λmax,ems nm, film
LUMO eV
3.44
451
0.01
485
-2.62
3.30
484
0.10
491
-2.84
3.19
501
0.07
3.56 3.33 3.17 3.04 2.86 2.79
393, 411 402 420 444 448, 468 454, 480
0.66 0.59 0.71 0.26 0.37 0.45
-2.87 431 404-407 481 505 503
-2.58 -2.92 -2.93 -2.7 -2.86 -2.81
orbitals extend significantly onto the 2,5-aryl substiuents.28 The λmax values for 1a-c are blue-shifted by 40-80 nm relative to those of nonfluorinated analogues reported in the literature,13 suggesting that the 2,5-perfluoroaryl-substituted derivatives may possess less silole ring-aryl conjugation. The UV-vis spectra of the thiophene-based compounds 2-3 exhibit a single dominant absorbance with λmax values ranging from 306 to 390 nm. In general, the heptafluoronaphthyl-containing compounds exhibit lower Egopt values than the corresponding nonafluorobiphenyl- or pentafluorophenyl-containing compounds, and the bithiophene-based compounds exhibited lower Egopt values than their thiophene or silole counterparts. These trends can be explained by the increase in conjugation length afforded by the bithiophene and naphthalene moieties. This conclusion is supported by the computational studies presented in the following section. Solution-state photoluminescence data for 1-3 are also summarized in Table 1. With excitation by 350 nm light, the emission colors ranged from violet to blue-green. Emission color and intensity are primarily dependent on the core ring structure, while the affect of the perfluoroaryl group is minimal (Figures 1a and 1b). Emission of the silole compounds 1a-c ranged from 450 to 500 nm (blue-cyan), which is greatly red-shifted with respect to their thiophene analogues 2-3, even though the silole compounds possess a significantly higher energy Egopt. The large Stokes shifts for compounds 1a-c (150-200 nm) are much larger than those for nonperfluorinated 2,5-aryl-substituted siloles reported in the literature (95-120),14,29 suggesting a significant difference between ground- and excited-state geometries. Emission energies for the thiophene-based compounds 2-3 range from 393 to 480 nm (violet-blue), with emission of the bithiophene-based derivatives (3) being red-shifted by about 50 nm with respect to their monothiophene-based analogues (2). The notably smaller Stokes shifts for the thiophene-based compounds (65-105 nm) suggest a more planar structure for the thiophene (vs silole) derivatives and is consistent with reported emission energies for other thiophene systems.19,30 (28) Lee, J.; Liu, Q.-D.; Motala, M.; Dane, J.; Gao, J.; Kang, Y.; Wang, S. Chem. Mater. 2004, 16, 1869-1877. (29) 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, 15351546. (30) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 13480-13501.
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Figure 1. Emission spectra for (a) nonafluorobiphenyl derivatives. 2b, 3b, and 1b; (b) heptafluoro-naphthyl derivatives 2c, 3c, and 1c, in THF solvent with an excitation wavelength of 350 nm.
Dilute solution state (optical density 0.1) photoluminescence quantum yields (ΦPL, Table 1) varied significantly among the compounds investigated. The dilute solution fluorescence quantum yields of 0.06 for the heptafluoronaphthyl silole derivative and 0.10 for the nonafluorobiphenyl derivative are among the highest values reported for silole compounds in dilute solution.31,32 The ΦPL values ranged from 0.26 to 0.45 for the bithiophene-based systems and 0.59-0.71 for the monothiophene-based systems. Reported ΦPL quantum yields for thiophene derivatives exhibit considerable variation,28,33-37 and the ΦPL values for 2a-c are among some of the highest. Electrochemical Studies of 1-3. The solution-state redox behavior for 1-3 was investigated by cyclic voltammetry, and the resulting calculated LUMO levels are listed in Table 1. All measurements were performed in a dry THF electrolyte solution (0.1 mol/L solution of tetrabutylammonium hexafluorophosphate in THF) with a silver wire as the pseudoreference electrode. All compounds exhibited one or two reduction peaks in the range investigated (-1.8 to +1 V) but no oxidation peaks were observed. The LUMO energies, calculated with reference to ferrocene,38 ranged from -2.58 (31) Boydston, A. J.; Yin, Y.; Pagenkopf, B. L. J. Am. Chem. Soc. 2004, 126, 3724-5. (32) Hissler, M.; Dyer, P. W.; Reau, R. Coord. Chem. ReV. 2003, 244, 1-44. (33) Pappenfus, T. M.; Mann, K. R. Org. Lett. 2002, 4, 3043-3046. (34) Su, Y. Z.; Lin, J. T.; Tao, Y.-T.; Ko, C.-W.; Lin, S.-C.; Sun, S.-S. Chem. Mater. 2002, 14, 1884-1890. (35) Pei, J.; Yu, W.-L.; Ni, J.; Lai, Y.-H.; Huang, W.; Heeger, A. Macromolecules 2001, 34, 7241-2748. (36) Pepitone, M. F.; Eaiprasersak, K.; Hardaker, S.; Gregory, R. Org. Lett. 2003, 5, 3229-3232. (37) Su, F. K.; Hong, J. L.; Lin, L. L. Synth. Met. 2004, 142, 63-69. (38) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler, H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7, 551.
Figure 2. Representative cyclic voltammetry curves demonstrating the differences in reduction behavior: (a) irreversible reduction (1a); (b) reversible reduction (3b); (c) quasirreversible reduction (2c).
to -2.93 eV. Representative CV curves are shown in Figure 2. These values are among the lowest LUMOs energies reported for n-type conducting materials,4,14,15,39 suggesting that this series of compounds may be useful in this context. Previous work by Tamao and co-workers has shown that the redox behavior of siloles is sensitive to the electronwithdrawing/electron-donating nature of the 2,5 substituents.14 The measured LUMO levels of -2.53 to -2.71 eV for 1a-c, which are about 0.5 eV lower than those for the nonfluorinated analogues,14 are consistent with these observations. Additionally, all silole compounds exhibit irreversible reduction characteristics, which is also consistent with observations reported for a number of 2,5-biaryl(3,5-biphenyl)siloles.14,31 The thiophene and bithiophene compounds described here demonstrate irreversible, quasirreversible, or reversible behavior depending on the perfluoroaryl substituent. Although there are few published electrochemical studies on nonfluorinated analogues of 2a-3c, there are a number of studies on the redox behavior of related donor/acceptor compounds. For example, a copolymer of thiophene and pyrido[3,4-b]pyrazine exhibits multiple reversible reduction peaks with an onset potential of ca. -1.6 V (vs Ag/Ag+).40 For regioregular dialkylbithiophene/bis(phenylquinoline) co(39) Lee, B. L.; Yamamoto, T. Macromolecules 1999, 32, 1375-1382. (40) Tonzola, C. J.; Alam, M. M., Bean, B. A.; Jenekhe, S. A. Macromolecules 2004, 37, 3554-3563.
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Figure 4. Optimized HOMO-LUMO orbital density for thiophene 2a.
Figure 3. Optimized HOMO-LUMO orbital density for silole 1a.
polymers, the LUMO energy levels ranged between -2.88 and -2.97 eV and quasirreversible reduction properties were observed.39 Systems highly related to 2a-3c are sets of oligothiophenes capped with perfluoroalkyl groups or possessing thiophene-co-perfluorobenzene oligomeric structures.15,29 The perfluoroalkyl-capped oligomers have slightly lower LUMO energy values (-3.25 to -3.42 eV) while the thiophene-co-perfluorobenzene oligomers exhibit LUMO energy values of -2.85 eV. The latter oligomers exhibited reversible reduction characteristics.15,29 Computational Studies. To further investigate the electronic structures of compounds 1-2, 3a, and 3c, density functional calculations using the Gaussian 98 suite of programs were performed. Minimum energy geometries were determined using the B3LYP/6-311G level of theory and orbital energy levels were further minimized using the B3LYP/6-31G**(dp) level of theory (see Supporting Information for pictures of optimized geometries and HOMO/ LUMO orbital densities for all compounds). Results from the geometry minimization predicted a much larger dihedral angle between the Arf group and the core moiety for the silole compounds (∼70 °) as compared to the thiophene derivatives (13-18°). This difference in dihedral angles is expected due to steric interactions of the Arf group with the phenyl groups in the 3,4-positions of the silole ring and is supported by crystallographic data for 1a and 2b (see following section). As illustrated in Figure 3, both the HOMO and LUMO of 1a have significant orbital contributions from all four aryl groups. This feature is similar for 1b and 1c (not shown). The aryl rings in the 3,4-positions contribute more to the HOMO than the perfluoroaryl rings in the 2,5-positions. This
was unexpected, as orbital analyses of various 2,5-arylderivatized siloles indicated very little participation of the 3,4-substituents in the HOMO.28 The perfluoroaryl rings of 1a contribute more significantly to the LUMO, as might be expected. These results suggest that it should be possible to independently vary the HOMO and the LUMO energy levels for silole derivatives. For the pentafluorophenyl- and heptafluorophenyl-derivatized thiophenes (2a,c and 3a,c) the HOMO and the LUMO orbital density is evenly distributed over the thiophene core and the perfluoroaryl substituents, as illustrated in Figure 4. In this respect, the electronic structures of these hybrid, thiophene-based compounds appear to resemble those of purely p- or n-type systems,41,42 rather than those of donoracceptor compounds, which possess HOMOs and LUMOs that are concentrated in the electron-rich and electron-poor regions, respectively.43 For the nonafluorobiphenyl thiophene (2b) the HOMO and LUMO orbital density was evenly distributed over the thiophene and adjacent C6F4 rings, but significantly reduced on the outer C6F5 rings. This is likely a consequence of the large dihedral angle between the two perfluoroaryl rings. This distribution of electron density is consistent with observed trends in the UV-vis data for compounds 2-3. The predicted and experimental LUMO energy levels decrease with increasing size of the Arf substituent, as illustrated by Table 2. For the thiophene-based compounds, the experimentally determined LUMO energy level is ∼0.5 eV lower than the theoretically predicted value, a difference that may be attributed to the differences between gas-phase properties (corresponding to the theoretical calculations) and solution-phase properties (used for experimental determinations). For the silole compounds there is very close agreement between theoretically predicted and experimentally determined LUMO energy levels. Solid-State Structures. Many attempts to grow X-ray quality crystals of 1-3, including cooling, slow evaporation, (41) Radke, K. R.; Ogawa, K.; Rasmussen, S. Org. Lett. 2005, 7, 52535256. (42) Hutchison, G. R.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 16866-16881. (43) Kulkarni, A. P.; Wu, P.-T.; Kwon, T. W.; Jenekhe, S. A. J. Phys. Chem. B 2005, 109, 19584-19594.
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Table 2. Comparison of Experimentally Observed and Theoretically Predicted LUMO Energy Levels
1a 1b 1c 2a 2b 2c 3a 3b 3c
Experimental LUMO eV
Calculated LUMO eV
-2.62 -2.84 -2.87 -2.58 -2.92 -2.93 -2.7 -2.86 -2.81
-2.56 -2.75 -2.69 -2.94 -3.29 -3.26 -3.05 -3.12
and vapor diffusion, resulted in fine powders or platelets that were unsuitable for structure determinations. Acceptable crystals were grown for 1a (by slow evaporation of a dichloromethane solution) and for 2b (by slow evaporation of a benzene solution). Weak diffraction was observed for both sets of crystals, resulting in fairly large errors associated with the data sets. Based upon the estimated standard deviations, the bond distances are accurate to 0.1 Å, and torsion angles are accurate to 1-2°. Silole 1a crystallized in the triclinic system space group P1h (Figure 5), with two molecules in the asymmetric unit cell. Steric repulsion between neighboring aryl groups prevents 1a from adopting a planar structure, and dihedral angles associated with the silole and pentafluorophenyl groups (66-85°) are larger than those between the silole and phenyl rings (55-65°). These results differ from those reported for a number of other silole compounds, which have torsion angles of 20-40° associated with groups in the 2,5positions and approximately 55-75° for phenyl substituents in the 3,4-positions.28,29 Despite the abundance of -C6H5 and -C6F5 groups in the structure of 1a, there are no apparent π-stacking interactions between these groups, of the type that have been observed in related compounds.44,45 The calculated points of closest contact (2.7-3.2 Å) are between the orthofluorine atoms and the idealized hydrogens on the silole methyl group of an adjacent molecule. Thiophene 2b cocrystallized with 1 equiv of benzene in the triclinic system space group P1h. As predicted by theoretical calculations, the three internal rings of 2b are almost coplanar, while the terminal pentafluorophenyl rings are twisted out of this central plane (dihedral angles of 1830° and 48-52°, respectively). As illustrated in Figure 6, 2b‚C6H6 packs in a cofacial fashion rather than in a herringbone or tilted motif. This packing geometry appears to result from the slightly twisted nature of the molecule, and to the π-stacking interactions involving molecules of 2b and benzene solvate. The C6F5 and C6H6 rings have a vertical interplanar distance of 3.5 Å and lateral, intermolecular H‚‚‚F distances ranging from 2.7 to 3.0 Å. These distances are typical of π-stacking and hydrogen-bonding interactions, respectively, and may indicate that the benzene molecules are “templating” the packing of 2b. Marder et al. have reported perfluoroarene-perhydroarene distances of (44) Dai, C.: Nguyen, P.; Marder, T.: Scott, A.; Clegg, W.; Viney, C. Chem. Commun. 1999, 24, 2493-2494. (45) Coates, G.; Dunn, A.; Henling, L.; Ziller, J.; Lobkovsky, E.; Grubbs, R. J. Am. Chem. Soc. 1998, 120, 3641-3649.
Figure 5. Unit cell of silole 1a. Hydrogen atoms removed for clarity.
Figure 6. Crystal structures of thiophene 2b.
Figure 7. AFM analysis on a thin film of 3b.
3.77-3.79 Å and H‚‚‚F distances of 2.50-2.67 Å for 1:1 cocrystals of 1,4-bis(phenylethynyl)-tetrafluorobenzene and 1,4-bis(pentafluorophenylethynyl)-benzene,44 while Grubbs et al. have reported molecular separations of 3.4-3.78 Å for cocrystals of trans-stilbene and trans-decafluorostilbene.45 Thin films of 3b and 3c were produced via high vacuum evaporation, and the film morphology was probed by atomic force microscopy (AFM) and X-ray diffraction (XRD). The presence of distinct domains or “terraces” in thin film samples of 3b was clearly visible by AFM (Figure 7) and section analysis of the sample revealed that the step heights between terrace levels measured between 2.5 and 3.0 nm. The close correspondence of this distance with the approximate length of the molecule (∼2.6 nm) suggests that these molecules pack perpendicular to the substrate. Additionally, XRD analysis supports the presence of crystalline domains in the films, as 2θ peaks corresponding to dspacings of 13.0, 8.8, and 6.8 Å for 3b and 9.1 Å for 3c were observed. This evidence of stacking and ordering suggests that these compounds might work well as semi-
Perfluoroaryl-Substituted Siloles and Thiophenes
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measurements from being made, and alternative device constructions are being pursued to alleviate this problem. Concluding Remarks
Figure 8. Gate-modulated source-drain current behavior for 3b and 3c.
conductors for field effect transistors.46-48 Preliminary Device Studies. Preliminary device studies included the evaluation of thiophene compounds 3b and 3c for use in field effect transistor (FET) applications, and of siloles 1a and 1b, and thiophenes 2b, 3a, and 3b, as electrontransport or EL layers for direct current applications. Unoptimized FETs were made from thin films of 3b and 3c produced via high vacuum evaporation. These devices exhibited the gate modulated source-drain current behavior illustrated in Figure 8. Threshold voltages of ∼22 V were observed for 3b and 3c (VD ) 40 V); however, significant device hysteresis prevented definitive determinations of field effect mobilities. Time-of-flight (ToF) analysis of compound 1a resulted in an observed electron mobility of 9 × 10-5 cm2 /V‚s (at 0.4 MV/cm),49 which is approximately 2 orders of magnitude higher than the most widely used electron-transport material, Alq3.50 However, this value is slightly lower than the only other reported ToF mobility for a silole-based compound, 2,5-bis(6′-(2′,2′′-bipyridyl))-1,1-dimethyl-3,4-diphenyl silole (PyPySPyPy), of 1-3 × 10-4 cm2/V‚s (at 0.4 MV/cm).51,52 This evidence of high electron mobility for two significantly different silole derivatives is encouraging, as it indicates that silole-based compounds may maintain high electron mobility, a key factor for brightness and power efficiency in OLEDs, as other physical properties are varied to improve overall device performance and lifetime. The EL properties of 2b, 3a, and 3b were investigated with a two-layer OLED with the following structure: glassITO (anode)/PEDOT-PSS(hole injection)/2b, 3a or 3b /Al (cathode). All organic layers were spin-coated, and the perfluoroaryl-substituted materials fulfilled the role of both electron-transport and emission layers. Initial results, for all three compounds, indicated pale blue emission with turn-on voltages below 15 V. Rapid crystallization of the films made from compounds 1a and 1b prevented meaningful EL (46) Garnier, F.; Horowitz, G.; Fichou, D.; Yasser, A. Synth. Met. 1996, 81, 163-171. (47) Katz, H. E.; Bao, Z. J. Phys. Chem. B 2000, 104, 671-678. (48) Horowitz, G. AdV. Mater. 1998, 10, 365-377. (49) Melnyk, A. R.; Pai, D. M. In Determination of Electronic and Optical Properties, Physical Methods of Chemistry Series, 2nd ed.; Rossiter, B. W., Baetzold, R. C., Eds.; Wiley: New York, 1993; Vol. VIII. (50) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556-4573. (51) Murata, H.; Malliaras, G. G.; Uchida, M.; Shen, Y.; Kafai, Z. H. Chem. Phys. Lett. 2001, 339, 161-166. (52) Tabatake, S.; Naka, S.; Okada, H.; Onnagawa, H.; Uchida, M.; Nakano, T.; Furukawa, K. Jpn. J. App. Phys. Pt. 1 2002, 41, 6582-6585.
A series of perfluoroaryl-derivatized siloles and thiophenes have been prepared via nucleophilic aromatic substitution. All samples emitted light in the violet to blue-green range upon photoexcitation and have solution-state LUMO energy levels around -2.7 eV. Preliminary solid-state device studies indicate that these materials are effective electron transporters in both field effect and direct current devices, which demonstrates the versatility of perfluoroaryl-derivatized materials in electro-optical applications. Future work will focus on the optimization of these molecular systems for electron-transport applications. Experimental Section General. All reactions were performed under an inert atmosphere of nitrogen using standard Schlenk techniques. Tetrahydrofuran (THF) and hexanes were distilled from sodium/benzophenone ketyl and stored under nitrogen. Phenylacetylene, diiodothiophene (Aldrich), and hexafluorobenzene (Oakwood Products Inc.) were used as received. Naphthalene (Aldrich), bithiophene (Aldrich), octafluoronaphthalene (Oakwood Products Inc.), and decafluorobiphenyl (Avocado) were purified via sublimation under vacuum. Lithium metal (wire, stored under mineral oil) was purchased from Aldrich. Butyllithium was used as-received (Aldrich, 1.6 M in hexanes). Tetrabutylammonium hexafluorophosphate (Aldrich) was used asreceived. UV-vis spectra were measured with an HP 8452A Series 3000 spectrometer and all samples were measured in THF. Emission spectra were measured with an Instrument SA/Jobin Yvon-Spex Fluoromax photon-counting fluorometer equipped with an Xe arc lamp excitation source and a Hamamatsu R928P photomultiplier tube operating at -900 V dc. All fluorescence quantum yields (Φ) were calculated with respect to freshly sublimed 9,9-diphenylanthracene in THF (Φ ) 0.90). Unless otherwise stated, 1H NMR and 19F NMR spectra were measured in CDCl3 with a Bruker AVQ 400 MHz spectrometer. All chemical shifts are reported in ppm units. For 19F NMR spectra C6F6 was used as internal reference at -163 ppm and 1H NMR chemical shifts were referenced to the residual peak of the deuterated solvent. Melting point determinations were performed by a Melt Temp II and are uncorrected. Cyclic voltammetry was performed on a Bioanalytical Systems CV-50W Voltammetric Analyzer with a C-3 Cell Stand. The potentials were measured vs a Ag wire reference electrode, with a Pt disk (PTE) as the working electrode and a Pt wire axial electrode, in dry THF containing 0.1 mol/L tetrabutylammoniumhexafluorophosphate, with ferrocene as the external standard (HOMO ) -4.8 eV) and potential sweep rate of 100 mV/s. Unless otherwise stated all gas chromatographs and mass spectra were obtained with a Agilent Technologies GC-MS system using the following temperature ramp profile: 50 °C - 3 min, 15 °C/min, 300 °C - 20 min. All elemental analyses were obtained at the Micro-Mass Facility of the University of California, Berkeley. Tapping mode atomic force microscopy (AFM) was done on a DI Nanoscope Dimension 3100 equipped with a Nanoscope IV controller. The field effect transistor measurements were performed in a nitrogen atmosphere using an Agilent 4156C semiconductor parameter analyzer. Lithium phenylacetylene and dimethylbis(phenylethynyl) silane were prepared as described in the literature. 2,5-Bis(pentafluorobenzene)-1,1-dimethyl-3,4-diphenylsilole (1a). A solution of lithium naphthalenide was prepared by stirring
3268 Chem. Mater., Vol. 18, No. 14, 2006 a mixture of naphthalene (0.26 g, 2.0 mmol) and lithium (0.014 g, 2.0 mmol) in dry THF (3 mL) for 14 h at room temperature under a nitrogen atmosphere. To the solution of lithium naphthalenide was added a THF (5 mL) solution of dimethylbis(phenylethynyl)silane (0.50 mmol) dropwise at room temperature, and then the mixture was stirred for 15 min. The resulting solution of 1,1dimethyl-2,5-dilithiosilole (0.50 mmol) was cooled to -78 °C and to this was added a solution of chlorotriphenylsilane (0.30 g, 1.0 mmol) in dry THF (4 mL) to quench the excess lithium naphthalenide. The mixture was stirred for 25 min and was then added dropwise to a solution of excess hexafluorobenzene (0.6 mL, 2.0 mmol) in dry THF (20 mL) at -78 °C. The mixture was stirred for 9 h at -78 °C and then warmed to room temperature. The light brown/yellow reaction mixture was quenched with H2O and extracted with hexanes (3 × 50 mL). The combined extract was dried over anhydrous MgSO4, filtered, and evaporated. The residue was subjected to column chromatography on alumina gel (hexane: diethyl ether) to give 160 mg (0.27 mmol, 54% yield) of 1a as a white powder. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 0.39 (s, 6H), 6.76-6.78 (m, 4H), 6.99-7.08 (m, 6H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -141.08 (m, 2F), -158.26 (m, 1F), -163.80 (m, 2F). MS: 594 (M+), 517, 478, 77. Anal. Calcd for C30H16F10Si: C, 60.61; H, 2.71. Found: C, 60.24; H, 2.98. mp 148-153 °C. 2,5-Bis(nonafluorobipheny)-1,1-dimethyl-3,4-diphenylsilole (1b). Silole 1b (pale yellow powder, 50% isolated yield) was prepared in a method analogous to 1a. Details are provided in the Supporting Information. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 0.51 (s, 6H), 6.82-6.83 (m, 4H), 7.02-7.09 (m, 6H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -138.61 (m, 2F), -140.23 (m, 2F), -140.45 (m, 2F), -151.96 (m, 1F), -162.11 (m, 2F). MS: 890.1 (M+) Anal. Calcd for C42H16F18Si: C, 56.64; H, 1.81. Found: C, 56.83; H, 2.00. mp 157-167 °C. 2,5-Bis(heptafluoronaphthyl)-1,1-dimethyl-3,4-diphenylsilole (1c). The procedure for 1a was employed for the formation of the dilithiosilole intermediate. The mixture of 2,5-dilithiosilole was stirred for 25 min and the THF was removed under vacuum. The resulting reaction mixture was redispersed in dry hexanes and then was added dropwise to a solution of excess octafluoronaphthalene (0.544 g, 2.0 mmol) in dry hexanes (20 mL) at room temperature. The light brown/yellow reaction mixture was stirred for 9 h, then quenched with H2O, and extracted with hexanes (3 × 50 mL). The combined extract was dried over anhydrous MgSO4, filtered, and evaporated to dryness. The resulting residue was subjected to column chromatography on alumina gel (hexane:diethyl ether) to give 96 mg (0.13 mmol, 25% yield) of 1c as a pale yellow powder. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 0.46 (s, 6H), 6.826.83 (m, 4H), 6.97-7.02 (m, 6H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -119.47 (m, 1F), -135.16 (m, 1F), -145.70 (m, 1F), -147.80 (m,1F), -150.66 (m, 1F), -156.05 (m, 1F), -157.46 (m, 1F). MS: 766 (M+), 281, 208, 147, 77. Anal. Calcd for C38H16F14Si: C, 59.54; H, 2.10. Found: C, 59.87; H, 2.09. mp 274-275 °C. 2,5-Bis(pentafluorophenyl)-thiophene (2a). To a solution of 2,5-diiodothiophene (0.204 g, 0.6 mmol) in diethyl ether (30 mL) at -78 °C was added tert-butyllithium (1.5 mL, 1.5 M in hexanes) dropwise via syringe. This solution was allowed to warm to room temperature over 5 h, resulting in a white suspension. This mixture was then added dropwise to a solution of hexafluorobenzene (0.28 mL, 2.4 mmol) in diethyl ether:hexanes (1:1, 50 mL) at -30 °C. The reaction mixture was allowed to warm to room temperature over 6 h. The resulting light yellow suspension was quenched with water and the organics were extracted with CH2Cl2. The extracts were dried over MgSO4 and concentrated via rotary evaporation. The disubstituted product was isolated via sublimation to give 61 mg (0.1 mmol, 25% yield) of 2a as a white powder. 1H NMR
Geramita et al. (CDCl3, 400 MHz, 25 °C): δ 7.60 (s, 2H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -140.49 (m, 2F), -155.62 (m, 1F), -162.71 (m, 2F). MS: 416 (M+), 211. Anal. Calcd for C16H2F10S: C, 46.17; H, 0.48. Found: C, 46.52; H, 0.65. mp 102-104 °C. 2,5-Bis(nonafluorbiphenyl)-thiophene (2b). Thiophene 2b (white needles, 40% isolated yield) was prepared in a method analogous to 2a. Details are provided in the Supporting Information. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.82 (s, 2H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -138.35 (m, 2F), -139.35 (m, 2F), -139.60 (m, 2F), -151.15 (m, 1F), -161.60 (m, 2F). Anal. Calcd for C28H2F18S: C, 47.21; H, 0.28. Found: C, 47.49; H, 0.13. mp 166167 °C. 2,5-Bis(heptafluoronaphthyl)-thiophene (2c). Thiophene 2c (pale yellow flakes, 25% isolated yield) was prepared in a method analogous to 2a. Details are provided in the Supporting Information. 1H NMR (CDCl , 400 MHz, 25 °C): δ 7.82 (s, 2H). 19F NMR 3 (CDCl3, 400 MHz, 25 °C): δ -117.54 (m, 1F), -136.02 (m, 1F), -144.45 (m, 1F), -147.04 (m,1F), -149.24 (m, 1F), -153.39 (m, 1F), -155.80 (m, 1F). MS 588 (M+), 297. Anal. Calcd for C24H2F14S: C, 49.00; H, 0.34. Found: C, 49.2; H, 0.37. mp 203209 °C. 5,5′-Bis(pentafluorophenyl)-2,2′-bithiophene (3a). To a solution of 2,2′-bithiophene (0.083 g, 0.5 mmol) in dry THF (10 mL) was added butyllithium (0.55 mL, 1.6 M solution in hexanes) and the resulting white suspension was mixed for 1 h. The 5,5′-dilithio2,2′-bithiophene suspension was then added dropwise to a solution of excess hexafluorobenzene (0.6 mL, 2.0 mmol) in THF at -78 °C. The resulting mixture was allowed to warm to room temperature with stirring (9 h) and was then quenched with H2O and extracted with CH2Cl2. The mixture was filtered to remove large amounts of insoluble material, dried over anhydrous MgSO4, filtered over Celite, and evaporated. The residue was purified via sublimation, to remove the monosubstituted product to give 97 mg (0.2 mmol, 40% yield) of 3a as a pale yellow powder. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.36 (d, 2H), 7.52 (d, 2H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -140.84 (m, 2F), -156.59 (m, 1F), -163.00 (m, 2F). MS: 498 (M+), 211. Anal. Calcd for C20H4F10S2: C, 48.20; H, 0.81. Found: C, 48.30; H, 0.93. mp 173-178 °C. 5,5′-Bis(nonafluorbiphenyl)-2,2′-bithiophene (3b). Bithiophene 3b (bright yellow powder, 47% isolated yield) was prepared in a method analogous to 3a. Details are provided in the Supporting Information. 1H NMR (CDCl3, 400 MHz, 25 °C): δ 7.41 (d, 2H), 7.71 (d, 2H). 19F NMR (CDCl3, 400 MHz, 25 °C): δ -138.42 (m, 2F), -139.60 (m, 2F), -140.20 (m, 2F), -151.37 (m, 1F), -161.72 (m, 2F). Anal. Calcd for C32H4F18S2: C, 48.38; H, 0.51. Found: C, 48.13; H, 0.14. mp 210-212 °C. 5,5′-Bis(heptafluoronaphthyl)-2,2′-bithiophene (3c). Bithiophene 3c (orange powder, 50% isolated yield) was prepared in a method analogous to 3a. Details are provided in the Supporting Information. 1H NMR (400 MHz, 25 °C): δ 7.70 (d, 2H), 7.38 (d, 2H). 19F NMR (400 MHz, 25 °C): δ -118.10 (m, 1F), -136.18 (m, 1F), -144.75 (m, 1F), -147.22 (m,1F), -149.45 (m, 1F), -154.15 (m, 1F), -156.05 (m, 1F). MS: 670 (M+) Anal. Calcd for C28H4F14S2: C, 50.16; H, 0.6. Found: C, 49.69; H, 1.38. The results from combustion analysis of a given sample were somewhat variable, for unknown reasons. However, the purity and identity of compound 3c is supported by the NMR and GC/MS data provided above. mp 225-230 °C (dec). Device Fabrication and Testing. FET DeVices. FETs were fabricated using heavily doped n-type silicon wafers as the gate, and thermally oxidizing the surface to form a 1000 Å silicon dioxide dielectric layer. Gold source and drain electrodes, 100 nm thick, were deposited using thermal evaporation and patterned with a liftoff technique. A 2.5 nm chrome adhesion layer was used for the
Perfluoroaryl-Substituted Siloles and Thiophenes gold. Finally, organic layers of 3b or 3c, 100-200 nm, were deposited via high vacuum deposition (substrate temperature 30 °C). ToF (Time-of-Flight) DeVices. Patterned indium tin oxide (ITO) coated glass substrates from Thin Film Devices, Inc. were used, with 1400 Å of ITO, a sheet resistance of 30 Ω/square and 80% light transmission. Substrates were cleaned ultrasonically in aqueous detergent solution, rinsed with distilled water, cleaned ultrasonically in acetone, rinsed with 2-propanol, and dried in a stream of nitrogen. A 50 nm charge generation layer of CuPc was vacuum-deposited onto the cleaned ITO substrate onto which a 1.1 µm layer of 1a was vacuum-deposited at the rate of around 5 Å/s. This was followed by the deposition of another 50 nm of CuPc and then by 15 nm of Al as the second electrode. Devices were then encapsulated in glass for testing. The ToF mobility measurements involved the application of a negative voltage to ITO electrode, followed by irradiation with 337 nm nitrogen laser (Laser Science Inc.) light with pulse duration of about 10 ns. The transient voltage signal is amplified via a preamplifier and recorded by an oscilloscope. The mobility was
Chem. Mater., Vol. 18, No. 14, 2006 3269 calculated from the relationship µ ) L2/(tTRV), where L is the sample thickness, V is the applied voltage, and tTR is the transit time.
Acknowledgment. This work was supported by the National Science Foundation research grant CHE0314709. The authors would also like to thank Dr. Gao Liu of Lawrence Berkeley National Laboratory for the electroluminescence data, the A. M. Stacy research group at University California Berkeley for the use of their XRD, and the V. Subramanian group for FET fabrication and testing equipment (patterned wafers, vapor deposition chambers, and probe station), AFM analysis, as well as numerous insightful discussions. Supporting Information Available: Pictures of optimized geometries and HOMO/LUMO orbital densities for all compounds, output files for computational calculations, XRD data, and crystallographic data (1a and 2b only). This material is available free of charge via the Internet at http://pubs.acs.org. CM060346U
