ARTICLE pubs.acs.org/joc
Mono- and Dicarbonyl-Bridged Tricyclic Heterocyclic Acceptors: Synthesis and Electronic Properties Yulia A. Getmanenko,*,† Chad Risko,† Paul Tongwa,‡ Eung-Gun Kim,† Hong Li,† Bhupinder Sandhu,‡ Tatiana Timofeeva,‡ Jean-Luc Bredas,† and Seth R. Marder*,† †
School of Chemistry & Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, Georgia 30332-0400, United States ‡ Department of Chemistry, New Mexico Highlands University, Las Vegas, New Mexico 87701, United States
bS Supporting Information ABSTRACT:
A series of trialkylsilyl-substituted 2,20 -dithiophene, 4,40 -di-n-hexyl-2,20 -dithiophene, 5,50 -dithiazole, and 2,20 -diselenophene with carbonyl (2a-d) and R-dicarbonyl bridges (3a-d) were prepared from readily available dihalides, using double lithiation followed by trapping with N,N-dimethylcarbamoyl chloride or diethyl oxalate (or N,N-dimethylpiperazine-2,3-dione), respectively. Cyclic voltammetry reveals that the first half-wave reduction potentials for this series of compounds span a wide range, from -1.87 to -0.97 V vs the ferrocene/ferrocenium couple at 0 V (0.1 M nBu4NPF6 in THF). A significant increase of the first half-wave reduction potential (by 0.50-0.67 V) was observed on substitution of the monocarbonyl bridge with R-dicarbonyl. Adiabatic electron affinity (AEA, gas phase) trends determined via density functional theory (DFT) calculations are in good agreement with the electrochemical reduction potentials. UV-vis absorption spectra across the series show a weak absorption band in the visible range, corresponding to the HOMOfLUMO transition within a one-electron picture, followed by a more intense, high-energy transition(s). Single-crystal X-ray structural analyses reveal molecular packing features that balance the interplay of the presence of the bulky substituents, intermolecular π-stacking interactions, and S 3 3 3 O intermolecular contacts, all of which affect the DFT-evaluated intermolecular electronic couplings and effective charge-carrier masses for the crystals of the tricyclic cores.
’ INTRODUCTION Air-stable organic semiconductors with large electron (nchannel) mobilities are necessary for the continued development and future implementation of organic electronic devices. Over the past decade, different materials classes, e.g., perfluorinated copper phthalocyanines,1 rylene diimides,2,3 dicyanomethylenesubstituted terthienoquinoid oligomers,4 and dicyanomethylene derivatives of (bis)indenofluorenes,5 have demonstrated efficient air-stable performance in n-channel organic field-effect transistors (OFET). Recently, bis-heterocyclic electron-transport materials that contain a carbonyl-bridge with either a dithiophene core (Ia) (electron mobility, μe, up to 0.08 cm2 V-1 s-1)6,7 or a dithiazole core (Ib) (μe up to 0.06 cm2 V-1 s-1)8 have been r 2011 American Chemical Society
developed. Materials containing an R-dicarbonyl-bridged dithiophene, benzo[2,1-b:3,4-b0 ]dithiophene-4,5-dione (II), are still rare and only a few compounds have been reported.9 A recent attempt to isolate 2,7-bis(thiophene-2-yl)benzo[2,1-b:3,4-b0 ]dithiophene-4,5-dione was reported to be unsuccessful.10 Ia is the most studied core among the compounds depicted in Figure 1. Synthetic approaches for Ia typically involve several steps,11 starting from 3-bromothiophene and thiophene-3-carbaldehyde. Ia serves as a precursor for the preparation of polycyclopenta[2,1-b;3,4-b0 ]dithiophen-4-one derivatives,12 Received: December 31, 2010 Published: March 10, 2011 2660
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The Journal of Organic Chemistry 4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4-b0 ]dithiophene-4difluoromethyleneone, by reaction with malononitrile,13-17 0 bridged dithiophene derivatives,18 Δ4,4 -dicyclopenta[2,1-b;3,4b0 ]dithiophene by the reaction with Lawesson’s reagent,19 and poly-(4,4-dialkylcyclopenta[2,1-b;3,4-b0 ]dithiophene-2,6-diyls.20 A short and convenient method for the introduction of the carbonyl bridge was recently reported in the synthesis of 4Hcyclopenta[2,1-b;3,4-b0 ]dithiazole-4-one (TIPS derivative of Ib, 99% yield),8 when 2,20 -bis(triisopropylsilyl)-5,50 -dithiazole was lithiated with lithium diisopropylamide (LDA) followed by trapping with ethyl 1-piperidine carboxylate. A five-step synthesis of 4H-cyclopenta[2,1-b;3,4-b0 ]diselenophen-4-(1,3-dioxolane) (a derivative of Ic) was also reported in the literature.21 R-Dicarbonyl-bridged bis-heterocycles published to date are solely limited to dithiophene (II) derivatives to the best of our knowledge. To introduce the dicarbonyl bridge, an intramolecular benzoin condensation of 2,20 -dithiophene-3,30 -dicarbaldehyde22 or its 5,50 -disubstituted derivatives9 followed by air oxidation was used (23-30% yields). Condensation of benzo[2,1-b:3,4-b0 ]dithiophene-4,5-dione, II, with diamines was reported,23,24 as well as the preparation of 2,7-diiodobenzo[2,1-b:3,4b0 ]dithiophene-4,5-dione and its attempted application in the Suzuki coupling.10
Figure 1. Structures of 4H-cyclopenta[2,1-b;3,4-b0 ]dithiophen/dithiazole/diselenophen-4-one, I, and benzo[2,1-b:3,4-b0 ]dithiophene-4,5dione, II.
ARTICLE
Recently, we have developed a convenient method25 for the preparation of dibromo-bisarenes 1a,c-e, Scheme 1, suitable for cyclization reactions. Here, we report a one-pot synthesis of mono- (2a-d) and R-dicarbonyl-bridged (3a-d) dithiophene, dialkyl-dithiophene, dithiazole, and diselenophene structures. The electronic properties of these tricyclic cores were studied by cyclic voltammetry, UV-vis absorption spectroscopy, and density functional theory (DFT) calculations. Three materials, 2,6-bis(triisopropylsilyl)-4H-cyclopenta[2,1-b;3,4-b0 ]dithiazole4-one (2c), 2,7-bis(trimethylsilyl)benzo[2,1-b:3,4-b0 ]dithiophene-4,5-dione (3a), and 2,7-bis(triisopropylsilyl)benzo[2,1b:3,4-b0 ]dithiazole-4,5-dione (3c), were analyzed by single-crystal X-ray structural analysis. On the basis of these crystal structures, the intermolecular electronic couplings and charge-carrier effective masses were evaluated with DFT.
’ RESULTS AND DISCUSSION Synthesis. 2,6-Bis(trimethylsilyl)-4H-cyclopenta[2,1-b;3,4b0 ]dithiophen-4-one (2a) was prepared in 84-86% yields from 5,50 -bis(trimethylsilyl)-3,30 -dibromo-2,20 -dithiophene (1a) via double lithiation with n-butyllithium followed by reaction with dimethylcarbamoyl chloride26 and acidic workup with aqueous NH4Cl as described earlier.27 The cyclopenta[2,1-b;3,4-b0 ]dithiophen-4-one derivative 2b with two n-hexyl chains at the 3and 5-positions was synthesized with the intent of imparting improved solubility and processability for conjugated oligomers or polymers that may incorporate this core in the backbone. Compound 2b was prepared by a one-pot synthesis from 3,30 ,5,50 tetrabromo-4,40 -di-n-hexyl-2,20 -dithiophene (1e) through selective lithiation at the 5- and 50 -positions with 2 equiv of n-butyllithium followed by reaction with 2 equiv of chlorotrimethylsilane (TMSCl) to generate 3,30 -dibromo-4,40 -di-n-hexyl-5,50 -bis(trimethylsilyl)2,20 -dithiophene (1b), which was used in situ in the halogenlithium exchange reaction with 2 equiv of n-butyllithium followed by
Scheme 1. One-Pot Synthesis of Mono- and Dicarbonyl-Bridged Tricycles 2a-d and 3a-d
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The Journal of Organic Chemistry
ARTICLE
Table 1. Redox Properties of the Trialkylsilyl-Carbonyl-Containing Tricycles 2a-d and 3a-d compd
solvent for CVa
2a 2b 2c
E1/21-/2-,b V
E1/20/1-,c V vs SCE
AIP,d eV
AEA,d eV
n/a
-1.21
7.02
-1.48
-2.31
-1.18 6.81
-1.41
7.51
-1.82
E1/20/1þ, V
E1/20/1-, V
CH2Cl2
þ0.96
-1.67
THF
n/a
-1.74
CH2Cl2
þ0.83
-1.84
n/a
-1.38
THF
n/a
-1.87
-2.53
-1.31
CH2Cl2
n/a
-1.47
-2.14
-1.01
THF
n/a
-1.47
-2.12
-0.91
2d
CH2Cl2
þ0.89
-1.61
-2.18
-1.15
6.94
-1.61
3a
THF CH2Cl2
n/a n/a
-1.67 -1.10
-2.20 -1.73
-1.11 -0.64
7.43
-2.08
THF
n/a
-1.09
-1.68
-0.53 7.20
-1.98
7.76
-2.29
7.36
-2.17
3b 3c 3d
CH2Cl2
n/a
-1.26
-1.84
-0.80
THF
n/a
-1.20
-1.95
-0.64
CH2Cl2
n/a
-0.89
-1.58
-0.43
THF
n/a
-0.97
-1.73
-0.41
CH2Cl2
n/a
-1.00
-1.49
-0.54
THF
n/a
-1.04
-1.49
-0.48
CV was recorded in 0.1 M nBu4NPF6 in specified solvent (Cp2Fe0/1þ internal standard at 0 V, 50 mV 3 s-1 scan rate). b Second reduction was partially electrochemically reversible for all compounds and E1/21-/2- was estimated with large error. c E1/20/1-(vs SCE) = E1/20/1-(vs Cp2Fe) þ 0.56 V (for THF); E1/20/1-(vs SCE) = E1/20/1-(vs Cp2Fe) þ 0.46 (for dichloromethane).34 d AIP determined at the B3LYP/6-311G** level of theory, while AEA determined at the B3LYP/6-311þþG** level of theory; adiabatic ionization energies determined as E(ion) - E(neutral). a
reaction with N,N-dimethylcarbamoyl chloride. The reaction temperature after addition of N,N-dimethylcarbamoyl chloride must be kept below -30 to -25 °C in order to obtain 2,6bis(trimethylsilyl)-3,5-di-n-hexylcyclopenta[2,1-b;3,4-b0 ]dithiophen4-one (2b) in satisfactory yields (52-56%).28 Lower yields (2239%) were obtained for the 2,6-bis(triisopropylsilyl)cyclopenta [2,1-b;3,4-b0 ]dithiazole-4-one (2c) when 4,40 -dibromo-2,20 -bis(triisopropylsilyl)-5,50 -dithiazole (1c) was reacted with 2 equiv of n-butyllithium followed by cyclization using N,N-dimethylcarbamoyl chloride. Only a trace amount of desired product 2d was obtained from 5,50 -bis(trimethylsilyl)-3,30 -dibromo-2,20 -diselenophene (1d) using the same reaction conditions. Introduction of the R-dicarbonyl bridge was achieved by using either the diethyl oxalate or N,N-dimethylpiperazine-2,3-dione29,30 electrophiles after lithiation of the dihalides 1a-d with 2 equiv of n-butyllithium. The highest yield for 2,7-bis(trimethylsilyl)benzo[2,1-b:3,4-b0 ]dithiophene-4,5-dione (3a) was obtained when the dilithiated species generated from 5,50 -bis(trimethylsilyl)3,30 -dibromo-2,20 -dithiophene (1a) was transferred via cannula into a solution of diethyl oxalate followed by transfer of the reaction mixture via cannula into an aqueous solution of NH4Cl. Recrystallization of the crude product from ethanol produced 3a in 54-80% yields on multigram scales (the yields for 3a were improved from 54-61% to 76-80% when 1.3 equiv of diethyl oxalate were used instead of 1.05-1.1 equiv). N,N-Dimethylpiperazine-2,3-dione was also examined as the electrophile, and the best yield achieved for 3a was 44% after 18 h of stirring, but this yield was not readily reproducible and low yields (
