Cyanated Pentaceno[2,3-c]chalcogenophenes for Potential

Dec 1, 2010 - Koichi Mitsudo , Jyunji Harada , Yo Tanaka , Hiroki Mandai , Chie Nishioka , Hideo Tanaka , Atsushi Wakamiya , Yasujiro Murata , and Sei...
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Cyanated Pentaceno[2,3-c]chalcogenophenes for Potential Application in Air-Stable Ambipolar Organic Thin-Film Transistors Chia-Chun Liu,† Shih-Wei Mao,‡ and Ming-Yu Kuo*,† Department of Applied Chemistry, National Chi Nan UniVersity, Puli, Nantou, Taiwan and Department of Mechanical Engineering, R.O.C Military Academy, Fengshan, Kaohsiung, Taiwan ReceiVed: October 17, 2010; ReVised Manuscript ReceiVed: NoVember 16, 2010

The current study investigates the effects of conjugation length and heteroatom substitutions on the internal reorganization energy (λ), adiabatic IP, and adiabatic EA of a series of oligoaceno[2,3-c]chalcogenophenes using density functional theory. The calculated IP and EA values of cyanated pentaceno[2,3-c]chalcogenophenes indicate that these compounds have high potential for use as air-stable ambipolar OFET materials. Their λ+ and λ- values are markedly smaller than those of well-known ambipolar air-stable DCMST and BTIFDMT, indicating that they are promising materials for use in high-performance ambipolar air-stable OFETs. The calculated results show that attaching electron-withdrawing groups on the compounds with extended conjugation length is an effective strategy to achieve ambipolar air-stable OFETs. Introduction Organic semiconductors have received considerable attention in recent decades owing to their potential applications in inexpensive, lightweight, flexible, and large-area electronic devices, such as organic field-effect transistors (OFET),1 lightemitting diodes (OLED),2 and solar cells.3 Among these devices, the OFET is the most technologically interesting because it can function as the main logic components in an electronic circuit. To create a large noise margin and low power dissipation device, both p- and n-channel transistors are necessary for a complementary circuit. In recent years, air-stable p-channel OFETs with hole mobilities exceeding 1 cm2 V-1 s-1 have been manufactured, such as 2,7-diphenyl[1]benzothieno[3,2-b][1]benzothiophene (DPh-BTBT), 2,6-dipheny-benzo[1,2-b:4,5-b′]diselenophene (DPh-BDS), and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PENT) OFETs.4 The literature has not fully explored n-channel transistors. Although remarkable progress has recently been made in developing new n-channel organic semiconductors, only a few n-channel OFETs can operate under ambient conditions because of unstable radical anions.1b Naphthalene-1,4:5,8-bis(dicarboximides) (NDI) and perylene-3,4: 9,10-bis(dicarboximides) (PDI) derivatives are the most commonly used air-stable n-channel semiconductors because of their high electron affinities (EAs) that stabilize their radical anions under ambient conditions.5 Among these materials, N,N′di(heptafluorobutyl)-PDI (PDI-F) has a high electron mobility of 1.24 cm2 V-1 s-1 in air,5a comparable to that of air-stable p-channel transistors (1 cm2 V-1 s-1). Scheme 1 lists several high-performance air-stable p- and n-channel organic semiconductors and lists one air-unstable n-channel organic semiconductor, N,N′-dioctyl-NDI (NDI-C8),5b for comparison. To simplify fabrication of a complementary circuit, the best method is to use a single ambipolar semiconductor to fabricate a device. However, studies have only developed a few ambipolar transistors based on a single molecule due to the high injection * To whom correspondence should be addressed. Fax: +886-49-2917956. E-mail: [email protected]. † National Chi Nan University. ‡ R.O.C Military Academy.

SCHEME 1: Chemical Structures of Selected High-Performance Air-Stable p- (left) and n-Channel (right) Organic Semiconductorsa

a

Air-unstable n-channel NDI-C8 is also given for comparison.

barrier for either hole or electron into the organic semiconductor from the same electrode.6,7 In 2006 and 2007, Anthopoulos et al. demonstrated that bis(4-dimethylaminodithiobenzyl)nickel (BDMADTN) and (diphenylethylenedithiolato)(1,3-dithiol-2thione-4,5-dithiolato)nickel (DPEDTDTTDTN) with a low band gap exhibited ambipolar transfer properties, even under ambient conditions.6a,b Marks and Takimiya et al., respectively, showed that 2,8-di-4,4′-didodecyl-2,2′-bithiophene-indeno[1,2-b]fluorene-6,12-dimalononitrile (BTIFDMT) and 5,5′′-bis(dicyanomethylene)-5,5′′-dihydro-∆2,1′;3′,2′′-diselenyl-5′,5′-bis(hexyloxymethyl)cyclopenta[c]thiophene (DCMST) can be applied in airstable ambipolar OFETs due to substitution of the nitrile group (-CN). This substitution stabilizes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) to balance the injection barrier of both the hole and the electron from the commonly used Au electrode.6c,d In a theoretical study, Jiang et al. also demonstrated that substituting sulfur atoms in p-channel pentathienoacenes and hexathienoacenes with electron-withdrawing boron atoms can convert them into potential ambipolar materials.8 The aforementioned results

10.1021/jp1099464  2010 American Chemical Society Published on Web 12/01/2010

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SCHEME 2: Chemical Structures of Selected Air-Stable Ambipolar Organic Semiconductors

indicate that attaching electron-withdrawing groups, such as the nitrile group (-CN), onto the framework with a low band gap is an effective strategy for synthesizing ambipolar OFET materials that can operate in air. Recently, Bao et al. showed a crucial relationship between carrier types and frontier molecular orbital energy levels in OFET devices.6e Scheme 2 shows the chemical structures of the selected air-stable ambipolar semiconductors. For comparison, 7,8,9,10-tetrafluoro-5,12-bis(triisopropylsilylethynyl)-tetraceno[2,3-b]thiophene (TIPSF4TbTH), which showed ambipolar behavior in nitrogen but only p-channel properties in air, was also listed.6f At room temperature, the charge transfer for organic materials involves a thermally activated hopping between a neutral molecule (M) and a neighboring radical cation (M+) or anion (M-). The rate constant for charge transfer can be described by Marcus theory as expressed in eq 1.9

ket )

(

1 4π2 λ t2exp h 4πλk T 4kBT √ B

)

(1)

Two key parameters govern eq 1: the reorganization energy (λ) due to the geometric relaxation accompanying charge transfer, and the electronic coupling (t) between two molecules, dictated largely by the orbital overlap. Achieving a high charge transfer rate requires minimizing λ and maximizing t values. The aforementioned mechanism is valid only in single-crystalline samples. The disordered model should describe the charge transfer in amorphous systems, since it does not apply to the discussion above.10 Theoretical Basis. The reorganization energy for charge transfer (λ) corresponds to the sum of geometric relaxation energies following vertical ionization of a neutral molecule and vertical neutralization of a charged geometry, illustrated in Scheme 3. Equations 2 and 3 express the reorganization energy for hole (λ+) and electron (λ-) transfer, respectively.

+ λ+ ) λ+ 1 + λ2 ) [EN(Q+) - EN(QN)] + [E+(QN) - E+(Q+)] (2)

SCHEME 3: Illustration of the Internal Reorganization Energy (λ) for Charge Transfer between M and M+ (or M-)

λ- ) λ1 + λ2 ) [EN(Q-) - EN(QN)] + [E-(QN) - E-(Q-)] (3)

where EN(QN), E+(Q+), and E-(Q-) are the lowest energies of the neutral state, cation state, and anion state and EN(Q+) and E+(QN) represent the energies of neutral and cation species at the optimal cation and neutral geometries. EN(Q-) and E-(QN) represent the energies of neutral and anion species at the optimal anion and neutral geometries. The geometry optimization, frequency, and reorganization energy were calculated at the B3LYP/6-31G** level with the Gaussian 09 Program.11 The calculated expectation values of the spin operator 〈S2〉 for the open-shell ionic states are very close to 0.75 for doublet electronic states at the UB3LYP/6-31G** level except DCMST (0.8083 at cationic state). Results and Discussion As tetraceno[2,3-c]thiophene (TcTH) and cyanated TcTH (DCN-TcTh) have promising potential for p-type organic semiconductors due to their appropriate adiabatic ionization potential (IP) and ultrasmall λ+ values,12 we believe that attaching nitrile groups onto materials with extended conjugation length, such as pentaceno[2,3-c]thiophene (PcTH), can achieve organic semiconductors to be utilized as ambipolar air-stable OFETs. This study investigated the effects of conjugation length, heteroatom substitutions, position of substitution, and number

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Figure 1. Chemical structures and abbreviated notations of oligoaceno[2,3-c]chalcogenophenes and their cyanated analogues.

TABLE 1: Internal Reorganization Energies of the Hole Transport (λ+), Electron Transport (λ-), Adiabatic Ionization Potentials (IP), and Adiabatic Electron Affinities (EA) Calculated at the B3LYP/6-31G** Levela compounds DPh-BTBT DPh-BDS TIPS-PENT NDI-C8 NDI-F PDI-F PDI-PE DCMST BTIFDMT TIPS-F4TbTH

λ+ (meV)

λ- (meV)

b

EA (eV)

6.530 6.406 5.680d

236 (2.0) 251 (1.5)c 144d (1.8)e

104 (7 × 10-3)k 278 (1 × 10-4)l 161 (0.12)m

IP (eV)

349 388 297 278 332 219 225

(0.16)f (0.01)h (1.24)I (0.11)j (0.016)k (1 × 10-3)l (0.37)m,n

6.786 6.127 6.005

d-IP (eV)

d-EA (eV)

6.743 6.636 5.905 2.052g 2.437g 2.635g 2.411g 3.141 2.912 1.848

7.026 6.349 6.285

2.461g 2.916g 3.069g 2.797g 3.479 3.231 2.201

a The d-IP and d-EA express the adiabatic ionization potentials and adiabatic electron affinities, respectively, calculated at the B3LYP/ 6-31+G*//B3LYP/6-31G** levels. The experimental charge mobilities are also given in parentheses (in cm2 V-1 s-1). b From ref 4a. c From ref 4b. d From ref 14. e From ref 4c. f From ref 5b. g From ref 5k. h From ref 5b. I From ref 5a. j From ref 5c. k From ref 6d. l From ref 6c. m From ref 6f. n Measured in nitrogen.

of substitutions on the internal reorganization energy (λ), adiabatic IP, and adiabatic EA. The latter two parameters are apparently important when determining carrier polarity and air stability of materials. Figure 1 lists the chemical structures and abbreviations of oligoaceno[2,3-c]chalcogenophenes and their cyanated analogues. Table 1 lists the calculated internal reorganization energies (λ), adiabatic IPs, and adiabatic EAs of a series of well-known air-stable p- and n-channel and ambipolar OFET materials based on the B3LYP/6-31G** level. The dodecyl and hexyloxy groups of BTIFDMT and DCMST simplify by treating them as ethyl and ethoxy groups, respectively. The adiabatic IPs of selected air-stable p-channel OFET materials are 5.680-6.786 eV, and the adiabatic EAs of selected air-stable n-channel OFET materials are 2.411-3.141 eV. Previous research groups have shown that the B3LYP/6-31+G** level is a reliable theory level to reproduce the EAs of polycyclic aromatic hydrocarbons.13 Chao et al. recently showed that the differences of EAs between B3LYP/6-31+G* and B3LYP/6-31+G*//B3LYP/6-31G** levels are within 0.002 eV for several oligoacenes and their analogues.5k Therefore, the following discussion on the relationship between air stability and energy levels of OFET materials

is based on the B3LYP/6-31+G*//B3LYP/6-31G** level. The calculated results indicate that the criteria of IP and EA values for designing air-stable ambipolar OFET materials are 5.905-7.026 and 2.797-3.479 eV, respectively. This is why TIPS-F4TbTH just exhibited p-channel properties in air. The calculated λ values are 104-388 meV, most of which are markedly larger than that of high hole mobility pentacene (PENT, λ+ ) 94 meV).14 Among these compounds, the charge mobilities of air-stable ambipolar OFETs are 0.016-0.001 cm2 V-1 s-1, which are much lower than those of PENT and commercial amorphous silicon. Therefore, it is necessary to increase the charge mobilities of air-stable ambipolar OFETs. According to Marcus theory, reducing λ values and increasing t values enhances the charge mobility of OFET materials. Table 2 presents the calculated internal λ values, adiabatic IPs, and adiabatic EAs of a series of oligoaceno[2,3-c]chalcogenophenes and their cyanated analogues. The λ+ and λ- values of anthraceno[2,3-c]chalcogenophenes are 74-84 and 146-173 meV, respectively, which are comparable to those of the aforementioned air-stable p- and n-channel OFET materials. The adiabatic IP values of anthraceno[2,3-c]chalcogenophenes are 6.101-6.201 eV, located within the criterion for IP values of

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TABLE 2: Internal λ+, Internal λ-, Adiabatic IP, and Adiabatic EA Values of Our Model Compounds Calculated at the B3LYP/6-31G** Levela compounds λ+ (meV) λ- (meV) IP (eV) EA (eV) d-IP (eV) d-EA (eV) AcTH AcFU AcSE DCN-AcTH DCN-AcFU DCN-AcSE TcTH TcFU TcSE DCN-TcTH DCN-TcFU DCN-TcSE PcTH PcFU PcSE DCN-PcTH DCN-PcFU DCN-PcSE

83 74 84 74 67 79 66 59 65 50 43 50 53 49 52 40 37 39

158 173 146 153 170 147 127 136 118 128 137 122 103 110 98 109 115 104

5.970 5.980 5.887 6.725 6.747 6.634 5.632 5.643 5.560 6.352 6.374 6.275 5.385 5.397 5.322 6.049 6.072 5.985

0.946 0.840 0.981 2.060 2.034 2.066 1.315 1.249 1.338 2.306 2.295 2.307 1.590 1.549 1.605 2.475 2.473 2.474

6.178 6.201 6.101 6.947 6.981 6.861 5.845 5.869 5.778 6.580 6.613 6.508 5.601 5.621 5.542 6.280 6.312 6.219

1.292 1.217 1.340 2.410 2.411 2.423 1.642 1.601 1.676 2.644 2.657 2.652 1.904 1.884 1.927 2.803 2.823 2.808

a The d-IP and d-EA express the adiabatic ionization potentials and adiabatic electron affinities, respectively, calculated at the B3LYP/6-31+G*//B3LYP/6-31G** level.

the aforementioned air-stable p-channel OFET materials; however, the adiabatic EA values (1.217-1.340 eV) of these compounds are much smaller than those of the aforementioned air-stable n-channel OFET materials. Therefore, these compounds can only be employed as air-stable p-channel OFET materials. Previous studies have demonstrated that functionalizing p-channel materials with electron-withdrawing groups is an effective strategy to increase their EA values and convert them into n-channel materials.15 The cyanation of anthraceno[2,3-c]chalcogenophenes increases their EA values from 1.217-1.340 to 2.410-2.423 eV, which is still smaller than those of the aforementioned air-stable n-channel OFET materials. Therefore, cyanated analogues can only function as airstable p-channel OFET materials. Most λ values of cyanated analogues are slightly smaller than those of their parent compounds, a finding consistent with previous studies.12,14 Previous studies have shown that substituting electronwithdrawing groups for small band gap materials is an effective strategy for synthesizing air-stable ambipolar OFET materials.6c,d Thus, this study attempts to extend the conjugation length to achieve air-stable ambipolar OFET. However, EA values (1.6.01-2.657 eV) of tetraceno[2,3-c]chalcogenophenes and their cyanated analogues are still less than EA threshold values of air-stable n-channel OFET materials. Therefore, this study increased the conjugation length again. Finally, the IP (6.219-6.312 eV) and EA (2.803-2.823 eV) values of cyanated

pentaceno[2,3-c]chalcogenophenes meet the needed energy levels for designing air-stable ambipolar OFET materials. The differences in IP and EA values between cyanated pentaceno[2,3-c]chalcogenophenes are within 0.1 eV, indicating the small impact of the substituent of heteratoms on the IP and EA values. Their λ+ and λ- values are 37-40 and 104-109 meV, respectively, much smaller than those of air-stable DCMST and BTIFDMT. These small λ+ and λ- values are likely due to the nonbonding characteristics of the HOMOs and LUMOs at first two rings next to thiophene, respectively. Figure 2a shows the HOMO and LUMO distributions of one cyanated pentaceno[2,3-c]chalcogenophene, 5,14-dicarbonitrile-pentaceno[2,3c]thiophene (DCN-PcTH). Previous studies have shown that a small λ value is a prerequisite for high-mobility OFETs such as PENT.14,16 Among these materials, cyanated pentaceno[2,3c]furan (DCN-PcFU) and cyanated pentaceno[2,3-c]selenophene (DCN-PcSE) have the smallest λ+ and λ-values, respectively. However, the chalcogen-chalcogen interactions between sulfur atoms and selenium atoms may provide appropriate electronic coupling for DCN-PcTH and DCN-PcSE.17 Therefore, we believe that DCN-PcTH and DCN-PcSE are promising materials for fabricating high-mobility ambipolar OFETs that can operate under ambient conditions. The current research next investigated the influence of position and number of substitutions on charge transfer and air stability. Figure 3 and Table 3 present the chemical structures and calculated results, respectively. The IP and EA values of cyanated PcTH are both larger than those of PcTH. The aforementioned result shows that DCN-PcTH is an ambipolar air-stable OFET material; however, the EA value of DCNPcTH-2 (2.614 eV) is under 2.797 eV. Accordingly, DCNPcTH-2 is not an ambipolar air-stable OFET material. The result indicates that substituent positions can have different impacts on IP and EA values of materials. The Mulliken-type population analysis of the HOMO and LUMO of unsubstituted PcTH on carbons 5 and 14 are larger than those on carbons 9 and 10 (Figure 2b). Therefore, cyanation at carbons 5 and 14 stabilizes the HOMO and LOMO level more effectively than that at carbons 9 and 10. This explains why the IP and EA values of DCN-PcTH are larger than those of DCN-PcTH-2. Most of their λ+ and λ- values are smaller than those of unsubstituted PcTH. The λ+ and λ- values of DCN-PcTH are smaller and larger than those of DCN-PcTH-2, respectively, consistent with the more and less nonbonding characteristic of the HOMO and LUMO of DCN-PcTH on carbons 4a, 5a, 13a, and 14a after cyanation, respectively. The IP and EA values of TCN-PcTH and HCN-PcTH also meet the needed energy levels for designing air-stable ambipolar OFET materials. However, the IP and EA values of HCN-PcTH are as large as 7.168 and 3.973

Figure 2. HOMO (up) and LUOM (bottom) distributions of (a) DCN-PcTH, (b) PcTH, (c) DCN-PcTH-2, and (d) TCN-PcTH. The numbers in b are the Mulliken-type population analysis of the HOMO and LUMO of PcTH on carbons 5, 9, 10, and 14.

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Figure 3. Chemical structures and abbreviated notations of cyanated PcTH and TcTH.

TABLE 3: Internal λ+, Internal λ-, Adiabatic IP, and Adiabatic EA Values of Our Model Compounds Calculated at the B3LYP/6-31G** Levela compounds DCN-PcTH DCN-PcTH-2 TCN-PcTH HCN-PcTH TIPS-DCN-TcTH TIPS-TCN-TcTH

λ+ (meV) λ- (meV) IP (eV) EA (eV) d-IP (eV) d-EA (eV) 40 54 39 46 121 125

109 93 96 90 158 130

6.049 5.926 6.559 6.931 5.970 6.215

2.475 2.308 3.076 3.652 2.488 3.036

6.280 6.146 6.795 7.168 6.198 6.433

2.803 2.614 3.396 3.973 2.775 3.326

a d-IP and d-EA are the adiabatic ionization potentials and adiabatic electron affinities, respectively, calculated at the B3LYP/ 6-31+G*//B3LYP/6-31G** level.

eV, respectively. Such values might create a high injection barrier and impose an “air-dope” problem for p- and n-channel OFETs, respectively, when using these compounds to fabricate devices. For the purpose of solution-processable OFETs, one also can extend the conjugation length along the side chain, such as TIPS-DCN-TcTH and TIPS-TCN-TcTH. The calculated results indicate that TIPS-TCN-TcTH is an air-stable ambipolar OFET material but TIPS-DCN-TcTH is not. Notably, the materials that have energy levels agreeing with the criteria for IP and EA values do not guarantee OFET air stablility. For instance, the IP value of PENT (6.115 eV at the B3LYP/631+G*//B3LYP/6-31G** level) is within this preferred range; however, the OFET based on PENT cannot operate under ambient conditions because of its electron-rich central ring, which is subject to a Diels-Alder reaction with oxygen.18 Bao et al. have shown that unsubstited tetraceno[2,3-b]thiophene (TbTH) is more environmentally stable than PENT.19 Our previous study showed that the better stability of TbTH may due to its asymmetrical structure and higher calculated IP (dIP ) 6.214 eV) than that of PENT (d-IP ) 6.115 eV).12 For the same reason, we believe that our proposed structures with appropriate IP and EA values are ambipolar air-stable OFET materials. Conclusion This study investigated the effects of conjugation length and heteroatom substitutions on internal reorganization energy (λ), adiabatic ionization potential (IP), and adiabatic electron affinity (EA). The calculated IP and EA values of cyanated pentaceno[2,3-c]chalcogenophenes indicate that these compounds have high potential for use as air-stable ambipolar OFET materials. Their λ+ and λ- values are markedly smaller than those of well-known ambipolar air-stable DCMST and BTIFDMT. Calculated results strongly indicate that cyanated pentaceno[2,3-c]chalcogenophenes are promising materials for use

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