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Molecular Design toward High Hole Mobility Organic Semiconductors: Tetraceno[2,3-c]thiophene Derivatives of Ultrasmall Reorganization Energies Ming-Yu Kuo* and Chia-Chun Liu Department of Applied Chemistry, National Chi Nan UniVersity; Puli, Nantou, Taiwan, R.O.C ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: August 18, 2009
The internal reorganization energies (λ+) associated with the transfer of a hole in a series of p-type five-ring heteroarenes were investigated using density functional theory (DFT). The λ+ value of the first model compound, tetraceno[2,3-c]thiophene (TcTH), is 31 meV less than that of its analogue, the well-known tetraceno[2,3b]thiophene (TbTH), because of the fusing of thiophene in a nonbonding fashion. The λ+ value of cyanated TcTH (DCN-TcTH) is as low as 50 meV. For a given degree of electronic coupling (t), the electron exchange rate (ket) of DCN-TcTH is 2.2 times that of TbTH. This study strongly indicates that TcTH and its derivatives are promising materials for fabricating high-mobility p-type organic field effect transistors. Organic field effect transistors (OFETs) are highly promising for use in flexible, inexpensive, and large-area devices.1 The most well-known p-type organic semiconductor, pentacene (PENT), has received considerable attention owing to its hole mobility, which exceeds 1.0 cm2/(V s), and its high on/off ratio.2 Despite its potential, PENT decomposes rapidly under ambient conditions because it has an electron-rich central ring, which is subject to a Diels-Alder reaction with oxygen.3 To eliminate this issue, two five-ring heteroarenes with only slightly varying long and plate-like molecular shapes of PENT were synthesized. They were anthradithiophene (ADT) and pentathienoacene (PTA), both of which have better environmental stability than PENT.4 However, ADT was obtained in a mixture of syn and anti isomers that are not easily separable. Such a blending can create disorder in the solid state. Two independent research groups developed a new asymmetrical oligoacene, tetraceno[2,3-b]thiophene (TbTH), which is tetracene fused with a terminal thiophene ring.5 TbTH and its derivatives were more environmentally stable than PENT.5,6 Among TbTH and its derivatives, 5,12-bis(triisopropylsilylethynyl)tetraceno[2,3-b]thiophene (TIPS-TbTH) has a hole mobility of 1.25 cm2/(V s).6a However, only a few asymmetrical p-type organic semiconductors have been developed because of the lack of efficient synthetic strategies.6,7 Therefore, strategies for designing and synthesizing asymmetrical p-type organic semiconductors are still sought. This work systematically investigates the adiabatic ionization potentials (IPs) and internal reorganization energies (λ+’s for hole transfer) of a series of p-type five-ring heteroarenesssix extensively studied compounds and three asymmetric model compounds, tetraceno[2,3-c]thiophene (TcTH), 5,12-dicarbonitrile-tetraceno[2,3-c]thiophene (DCN-TcTH), and 5,12-bis(triisopropylsilylethynyl)-tetraceno[2,3-c]thiophene(TIPS-TcTH). The λ+’s of TcTH (66 meV) and DCN-TcTH (50 meV) are lower than that of TbTH (97 meV) by 31 and 47 meV, * To whom correspondence should be addressed. E-mail: mykuo@ ncnu.edu.tw.
10.1021/jp9065423 CCC: $40.75
respectively. These calculations indicate that TcTH and DCNTcTH have significant potential for use as high-mobility p-type organic semiconductors. Figure 1 presents the chemical structures of the materials employed herein and their abbreviations. Charge transfer in p-type OFET materials is believed to be governed by thermally activated hopping between a neutral molecule (M) and neighboring radical cation (M+); this process can be modeled using the Marcus theory.8 The two key parameters that dominate the charge-transfer rate are the reorganization energy (λ) and the electronic coupling (t). The λ+ and t+ values must be minimized and maximized, respectively, to ensure high hole mobility. The electronic coupling (t) can be evaluated only when a single crystal structure is available. Conversely, the internal reorganization energy (λ) of a single molecule can be determined theoretically before the molecule is synthesized. Additionlly, the ionization potential (IP) is apparently important for the determination of the carrier polarity of materials. Hence, the λ+ values and adiabatic IPs of a series of p-type five-ring heteroarenes are evaluated based on the density functional theory (DFT) with the B3LYP/6-31G** level.9 The adiabatic IPs of the model compounds (5.439-6.352 eV) are comparable to those of p-type OFETs, PENT (5.888 eV) and PTA (6.605 eV), indicating that these compounds can function as p-type semiconductors (Table 1). The calculated λ+ values of PENT and its analogues, ADT and TbTH, are 94-97 meV. Chao and Bre´das et al. demonstrated that PENT and ADT have small λ+ values because of the nonbonding characteristics of the highest occupied molecular orbital (HOMO) in the central aromatic ring.10 The nonbonding characteristics of the central aromatic ring are also responsible for the small λ+ value of TbTH (Figure 2a). Therefore, changes in the length of the C-C bonds of TbTH in the central aromatic ring upon oxidation are generally negligible. These three materials have been widely investigated in the literature since they have high hole mobilities, which are consistent with their small λ+ values.2a,d,4a,b,5 However, the λ+ value of PTA is as large as 308 meV, which is 3.3 times that of PENT. The large λ+ value of PTA is attributable to the very large change in the length of the C-C bonds upon 2009 American Chemical Society
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Figure 1. Chemical structures and abbreviated notations of five-ring heteroarenes.
TABLE 1: Internal Reorganization Energies of the Hole Transport (λ+), Adiabatic Ionization Potentials (IP) Calculated at the B3LYP/6-31G** Level, and Experimental Hole Mobilities (µ+exptl)
e
compounds
λ+ (meV)
IP (eV)
µ+exptl (cm2/(V s))
PENT TIPS-PENT TbTH TIPS-TbTH PTA PF-PENT TcTH DCN-TcTH TIPS-TcTH
94 144 97 143 308 225 (λ-) 66 50 113
5.888 5.680 6.013 5.748 6.605
5.5a 1.8b 0.47c 1.25d 0.045e 0.11 (µ-exptl)f
a From ref 2d. b From ref 11a. From ref 4d. f From ref 11b.
5.632 6.352 5.439 c
From ref 5a.
d
From ref 6a.
oxidation, especially that of the central thiophene (Figure 2b). According to Marcus theory, the large λ+ value of PTA indicates low hole mobility, which agrees well with the experimental value of 0.045 cm2/(V s) obtained when the PTA is manufactured as the active layer of an OFET.4d For comparison, the λvalue of a common n-type organic semiconductor, perfluoropentacene (PF-PENT), was also calculated.10c,11b Again, the large λ- value (225 meV) of PF-PENT results in a relatively low electron mobility of 0.11 cm2/(V s).11b The above discussion indicates that the internal reorganization energy (λ) can be used crudely to screen promising candidates for high-performance organic semiconductors. The λ+ value of the first model compound, TcTH, was as small as 66 meV, which is 28 and 31 meV smaller than those of PENT and TbTH, respectively. The only difference between TbTH and TcTH is the position at which tetracene fuses with the terminal thiophene (Figure 1). Unexpectedly, the small change in configuration has an enormous impact on the λ+ value of TcTH. Thiophene is fused with tetracene in an antibonding form in TbTH but in a nonbonding form in TcTH (cf. Figure 2a and c). Therefore, most changes in the bond lengths in thiophene and the adjacent aromatic ring in TbTH upon oxidation exceed 0.01 Å; however, those in TcTH are only 0.001-0.006 Å. To determine the reason for the nonbonding of thiophene in TcTH, a computer-aided composition of atomic orbitals (CACAO) calculation, based on extended Hu¨ckel molecular orbital theory, is utilized.12 The basic characteristics of the Hu¨ckel- and DFT-calculated frontier orbitals are very similar to each other (Figure 3). For CACAO analysis, both the bonding and the antibonding orbitals of the fragmental thiophene participate in the HOMO of TcTH and thus cancel out the population of carbon atoms 3 and 4 in thiophene. We have already demonstrated that cyanating organic
semiconductors is an effective strategy for reducing their λ+ and λ- values.10a,13 Correspondingly, the λ+ value of DCN-TcTH was further reduced to an extremely small value of 50 meV, which is roughly half of and one-sixth of those of TbTH (97 meV) and PTA (308 meV), respectively. On the basis of a given degree of electronic coupling (t), the electron exchange rate (ket) of DCN-TcTH becomes 2.2 and 30.1 times those of TbTH and PTA, respectively. For the solution processing of OFETs, Bao and Anthony et al. developed two soluble p-type organic semiconductors, TIPS-TbTH and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PENT), respectively, each with hole mobilities of more than 1.0 cm2/(V s).6a,11a The calculated λ+ values of alkynyl compounds are generally 50 meV larger than those of their parent compounds. The λ+ value of TIPS-TcTH is only 113 meV, which is 30 meV smaller than that of TIPSTbTH. Molecules with similar chemical structures may exhibit similar molecular packing in the solid state and similar degrees of electronic coupling (t). For example, the molecular packing and electronic coupling (t) of TIPS-PENT and 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT) are similar to those of their analogues, 6,13-bis(triisopropylsilylethynyl)-5,7,12,14tetraazapentacene (TIPS-TAPENT) and 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (DF-TES-ADT), respectively (Figure S1 and Table S1, Supporting Information).4c,14 If TIPS-TbTH and TIPS-TcTH also exhibit these similarities, then the electron exchange rate (ket) of TIPS-TcTH can be increased to 1.5 times that of TIPS-TbTH. Notably, the small λ value does not guarantee high carrier mobility, which also depends on other factors such as electronic coupling and film quality; likewise, a large λ value does not guarantee low carrier mobility when the electronic coupling is sufficiently large. For instance, the electron mobility of N-fluoroalkylated dicyanoperylene-3,4:9,10-bis(dicarboximides) (PDI-FCN2) can reach 6.0 cm2/(V s), even with a large calculated λ- of 277 meV.10a,15 For thin-film OFETs, grain boundaries and charge traps should not be negligible for charge transport. However, many studies have proven that a small λ value is an important prerequisite for the high-mobility OFETs such as PENT.10,16 To support this viewpoint, the λ values of several OFET materials with charge mobilities of over 0.5 cm2/(V s) were calculated.7f,17 Again, the result shows that such materials with small λ values (85-167 meV) are highly likely to be highmobility (0.52-4.30 cm2/(V s)) OFETs (Figure S2, Supporting Information). Hence, we believe that the λ value may be a useful tool for screening hopeful candidates for high-performance organic semiconductors. Notably, the calculations of λ+ values in this study are based only on a single molecule. For flexible dithiophene-tetrafulvalene (DT-TTF), the effect of the environ-
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Figure 2. The isosurface plots of HOMOs (left) and bond length differences (in 10-3 Å) upon oxidation (right) of (a) TbTH, (b) PTA, and (c) TcTH.
TES-ADT, TIPS-TAPENT, and DF-TES-ADT, as well as calculated reorganization energies and experimental charge mobilities of several well-known p-type OFET materials. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 3. CACAO molecular orbital diagram of HOMO for TcTH and the percentage of orbital contribution from the fragments (mesh). The DFT-optimized geometry was used for the CACAO analysis. The DFT frontier orbitals are also shown for comparison (solid).
ment on the λ+ value is dramatic because of both geometric restrictions and electronic polarization. This effect is less important for the relatively rigid naphthalene and PENT.18 Therefore, this study considers only the contribution of a single molecule to the reorganization energy. In conclusion, the internal λ+ values of a series of p-type five-ring heteroarenes were investigated, including six wellknown and three model organic semiconductors. The λ+ value of the first model compound, TcTH (66 meV), is 31 meV smaller than that of its analogue, TbTH (97 meV), due to the fusing of thiophene in a nonbonding fashion. For the same reason, the λ+ value of DCN-TcTH is as small as 50 meV. On the basis of the same degree of electronic coupling (t), the electron exchange rate (ket) of DCN-TcTH is 2.2 and 30.1 times those of TbTH and PTA, respectively. This study strongly indicates that TcTH and its derivatives are promising materials for use in high-mobility p-type organic field effect transistors, which awaits experimental demonstration. The model compounds can be synthesized from 3,4-diformylthiophene and 1,4anthracenedione using the method developed by Tao et al.5b Acknowledgment. This work was supported by the National Science Council of Taiwan (NSC-96-2113-M-260-006-MY2). We thank the National Center for High-performance Computing for providing computational resource. Supporting Information Available: Computational details, molecular packings, and electronic couplings of TIPS-PENT,
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