Heteroatom Substitution of Oligothienoacenes: From Good p-Type

Mar 7, 2008 - All the results indicate that heteroatom substitution of sulfur atoms in oligothienoacenes is a rational way toward good ambipolar OFET ...
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J. Phys. Chem. C 2008, 112, 5148-5159

Heteroatom Substitution of Oligothienoacenes: From Good p-Type Semiconductors to Good Ambipolar Semiconductors for Organic Field-Effect Transistors Yuexing Zhang,† Xue Cai,†,‡ Yongzhong Bian,† Xiyou Li,† and Jianzhuang Jiang*,† Department of Chemistry, Shandong UniVersity, Jinan 250100, China, and Department of Chemistry, Mudanjiang Teacher’s College, Mudanjiang 157012, China ReceiVed: October 18, 2007; In Final Form: January 14, 2008

Density functional theory calculations were carried out to investigate the effect of oligomer length, halogen substitution, and heteroatom substitution on the organic field-effect transistor (OFET) performance of a series of oligothienoacenes (1-5 for oligothienoacene with thiophene units’ number from two to six). Compounds 1-5 are revealed to act only as p-type semiconductors due to their very high electron injection barrier relative to the work function potential of Au source-drain electrodes. Heteroatom substitution of the thiophene sulfur atom in particular with boron in the fused-ring thiophene oligomer 5 was revealed to elevate the HOMO energy level and lower the LUMO energy level and therefore lower both the hole and electron injection barriers. However, halogen substitution cannot effectively improve the electron injection barrier, but significantly increased the reorganization energy, therefore leading to decreased transfer mobility. The appropriate ionization potential and electron affinity, balanced charge injection barrier for both hole and electron relative to the work function potential of Au source-drain electrodes, low hole and electron reorganization energy, and good intrinsic transfer mobility for both hole and electron of both the boron-substituted hexathienoacenes 5BH and 5BH-2F-a make these two compounds good potential semiconductors for ambipolar OFET devices, with calculated intrinsic charge-transfer mobilities achieving 3.74 and 5.07 cm2 V-1 s-1 for hole and 4.77 and 5.76 cm2 V-1 s-1 for electron, respectively. The high intrinsic mobilities of 5BH and 5BH-2F-a are rationalized in terms of their frontier orbitals, molecular structure variation upon oxidation and reduction, and electron coupling between two neighboring molecules. All the results indicate that heteroatom substitution of sulfur atoms in oligothienoacenes is a rational way toward good ambipolar OFET semiconducting materials.

Introduction Organic field-effect transistors (OFETs) have attracted increasing research interest due to their potential applications in the field of flexible displays, integrated circuits, and low-cost electronic devices since their first report in 1986.1,2 In recent years, great progress has been made on developing new molecular semiconductors and device fabrication techniques to improve OFET performance of both p-type and n-type semiconductors.3 More recently, ambipolar OFETs have become the research focus due to their potential practical application in organic integrated circuits.4 One of the efficient and convenient methods to construct ambipolar OFETs is to combine n-type and p-type organic semiconductors with high charge mobility into a device as the active layer. However, the lack of enough air-stable n-type semiconductors with comparable high mobility as p-type counterparts limits the development of ambipolar OFETs fabricated by this method. The most ideal way toward ambipolar OFETs is to fabricate devices with a single semiconductor material which is of high charge mobility for both hole and electron transfer. However, despite the intrinsic ambipolar nature for both hole and electron transfers of the organic materials in terms of theory,5 few single-component organic semiconductors have been found to exhibit ambipolar * To whom correspondence should be addressed. E-mail: jzjiang@ sdu.edu.cn. † Shandong University. ‡ Mudanjiang Teacher’s College.

transfer property in practical OFET devices due to the high injection barrier for at least one charge carrier, either hole or in particular electron, of general organic OFET materials relative to the work function potential of Au source-drain electrodes.6 Designing and synthesizing new materials with high chargetransfer mobility, high ambient stability, and good solubility in common organic solvents as the active layer of OFETs is still a great challenge in the manufacture of OFET devices, especially for those made to prepare ambipolar devices. Theoretically, five factors have influence on the intrinsic semiconducting property of OFET materials: HOMO and LUMO energy, ionization potential (IP) and electron affinity (EA), reorganization energy for hole and electron, transfer integral for hole and election, and corresponding charge-transfer distance. The last three factors combine together to determine the charge-transfer mobility of the organic seminconductors, while the first two factors determine the ease of charge injection from the electrodes. The semiconductors with high charge-transfer mobility usually are of small reorganization energy and/or large transfer integral even for long transfer distances. Furthermore, the p- and n-type semiconductors should possess high HOMO and low LUMO energies, respectively, which in turn results in small ionization potential and large electron affinity (at least 3.0 eV). Both of which should actually be close to the work function potential of the source-drain electrodes (5.1 eV for Au electrode), yielding smaller charge injection barriers for hole and electron, respectively, to ensure the effective charge injection from the source

10.1021/jp710123r CCC: $40.75 © 2008 American Chemical Society Published on Web 03/07/2008

Series of Oligothienoacenes for Transistors electrode. As expected, the ambipolar semiconductors are those materials with simultaneously high HOMO and low LUMO energies as well as small ionization potentials and large electron affinities. Among the five factors mentioned above that take effect on semiconductor nature, the former three ones are clearly determined by the intrinsic properties of single molecules while the later two ones are determined by the interaction between neighboring molecules and thus closely associated with the solid-state structure of the corresponding compound. As a result, in terms of both injection barrier and charge-transfer mobility, molecular design toward ambipolar organic semiconductors should aim for compounds with both low LUMO and high HOMO energies, high electron affinity and low ionization potential, and small intrinsic electron and hole reorganization energies in nature. Functionalizing p-type semiconductors, including replacing the functional atom(s) and substituting the hydrogen atom(s) with electron-withdrawing group(s), has been revealed as one of the most effective methods in tuning the semiconducting property from p-type to n-type or to ambipolar semiconductors.4f,7,8 Phthalocyanines, perylenebisimides, and π-conjugated organic oligomers as well as polymers are among the most widely studied semiconductors. Despite the high charge mobility,9 acenes oligomeric semiconducting materials, with pentacene as a representative, suffer from hard peripheral functionalization, low ambient instability, and extreme insolubility in common organic solvents.10 Thiophenes, another promising class of organic oligomeric semiconductor materials, appear to display an advantage over acenes in terms of easy functionalization, high ambient stability, and good solubility in common organic solvents.11 In particular, the fused-ring thiophene oligomers (oligothienoacenes) were proposed to have good OFET performance comparable with acenes due to various intra- and intermolecular interactions including weak hydrogen bonding, π-π stacking, and sulfur-sulfur interactions originating from the high polarizability of sulfur electrons in the thiophene rings.11c-f Actually, oligothienoacenes with up to seven thiophene units have been synthesized by Matzger and co-workers,12 and heteroatom-substituted dithieno[2,3-b:2′,3′-d]thiophene (2) of sulfur atom in the central thiophene unit with element of group IIIA (B), IVA (C, Si), VA (N, P) and group VIA (S, Se) were also reported.13a Furthermore, a series of thiophene-, selenophene-, and mixed-thio(seleno)phene-based heteroacenes have been synthesized and characterized very lately.13b-e Among the series of oligothienoacenes and corresponding substituted derivatives, the pentathienoacene (4) was revealed to be good p-type OFET material with the hole transfer mobility of 0.045 cm2 V-1 s-1,14a while the phenyl-substituted dithieno[2,3-b:2′,3′d]thiophene was found to exhibit hole transfer mobility of 0.42 cm2 V-1 s-1,14b dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene displays hole transfer mobility of 0.50 cm2 V-1 s-1,14c and single-crystal-based OFETs of benzoannulated pentathienoacene and its selenium analogue show hole transfer mobilities as high as 0.50 and 1.1 cm2 V-1 s-1, respectively.13e However, theoretical modeling and exploration on the OFET performance of fused-ring thiophene oligomers has not yet been carried out, and n-type or ambipolar OFETs fabricated with oligothienoacenes and its functionalized derivatives are not reported experimentally so far, to the best of our knowledge. It is worth noting that during the final stage of preparation of the present paper, Kim and co-workers reported the charge transport parameters of the pentathienoacene (4) crystal calculated using density functional theory (DFT) and gas-phase ultraviolet photoelectron spectroscopy.17 However, report and the in turn

J. Phys. Chem. C, Vol. 112, No. 13, 2008 5149 discussion of the charge-transfer mobility as a function of oligomer length are not mentioned in their work. In this paper, on the basis of our research interests in OFETs15 and accumulation in the theoretical studies,16 we tried to investigate the effects of oligomer length, heteroatom substitution, and halogen substitution on the charge-transfer properties of organic semiconductors of a series of fused-ring thiophene oligomers, 1-5, by DFT calculations. In line with the experimental findings, the calculation data on thiophene oligomer, actually pentathienoacene 4, also reveal its good charge mobility with p-type semiconductivity. Nevertheless, the results indicate that heteroatom substitution of the thiophene sulfur atom with boron in the fused-ring thiophene oligomers 4 and 5 leads to the formation of ambipolar semiconductors from the p-type thiophene oligomer. The present work, representing the first systematic theoretical study on the heteroatom substitution effect to the OFET properties of fused-ring thiophene semiconductors, is helpful toward the rational design of organic semiconducting materials with ambipolar nature for OFET devices. It is noteworthy here that despite the different calculation methods employed, our calculation results for both hole and electron reorganization energies of several unsubstituted oligothienoacenescorrespond well with those obtained by Kim and coworkers.17 Methodology and Computational Details Charge transfer can be described as a self-exchange electrontransfer reaction between a neutral molecule and a neighboring radical anion (n-type) or radical cation (p-type). The rate constant for charge transfer (W) and, hence, the mobility can be modeled by classical Marcus theory,18

W ) (t2/p)(π/λ(kBT)1/2 exp(-λ(/4kBT) where t is the transfer integral, λ( is the reorganization energy, kB is the Boltzmann constant, and T is the temperature. The reorganization energy λ+ or λ- for hole or electron transfer, respectively, is calculated as the sum of the energy required for reorganization of the vertically ionized neutral to the cation or anion geometry, plus the energy required to reorganize the cation or anion geometry back to the neutral equilibrium structure on the ground-state potential energy surface. To achieve high charge carrier mobility, the reorganization energy needs to be minimized.2a,19 The transfer integral t depends on the relative arrangement of the molecules in the solid state and describes the intermolecular electronic coupling which needs to be maximized to achieve high charge carrier mobility. The quantity has been approximated as half of the splitting of the HOMO levels for hole transfer or LUMO levels for electron transfer induced by interaction of two stacked molecules according to Koopmans’ theorem.2a,19 It must be pointed out that although the site-energy correction due to the crystal environment should be taken into account in calculating transfer integrals when the dimer is not cofacially stacked according to Valeev et al.,20 the fact that the HOMO and HOMO - 1 as well as LUMO and LUMO + 1 molecular orbitals are simple linear combinations of the HOMOs or LUMOs of the charge-localized isolated molecules in our predicted crystals rationalizes the present calculation of transfer integrals using Koopmans’s theorem.5b Furthermore, the transfer integral of route 5 in the predicted crystal of 5BH (typical herringbone packing mode) for hole and electron is also calculated using the site-energy-corrected energy splitting method of Valeev et al.20 to study the influence of site

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SCHEME 1: Molecular Structures of Oligothienoacenes and Their Substituted Derivatives

energy (Table S6, Supporting Information). The calculated results reveal that Koopmans’s theorem method only overestimates the transfer integral of 5BH by less than 2 × 10-6 eV, which is negligible. Given the rate constant for charge transfer (W) between two neighboring molecules, the diffusion coefficient can be evaluated as19j,21

D)

2 1 〈x(t) 〉

2d

t

≈

1 2d

∑i ri2WiPi

In the above equation i represents a specific transfer path way with ri being the transfer distance (intermolecular center to center distance), 1/Wi the transfer time, and d the spatial dimension, which is equal to 3 for the crystal. P is the relative probability for the ith pathway:

Pi ) W i /

∑i Wi

The basic assumption is that the charge transfer is a slow process in which the molecules have enough time to reach equilibrium. This is pertinent for the soft organic system. The

drift charge-transfer mobility, µ, is then evaluated from the Einstein relation:

µ)

e D kBT

Molecular structure optimization and frequency calculation for all the compounds were carried out using the Gaussian 03 program22 in the IBM P690 system at the Shandong Province High Performance Computing Centre. For all cases, the hybrid density functional B3LYP (Becke-Lee-Young-Parr composite of exchange-correlation functional) method23 and 6-31G(d) basis set24 were used. On the basis of the optimized structures, single point energy was calculated with 6-31G(d) basis set to provide the reorganization energy. Reorganization energies of 3 and 5BH were also calculated with 6-31G(d,p) and 6-31++G(d,p) basis sets to examine the basis set dependence. Crystal prediction was carried out using the Polymorph module in a Cerius2 software package from Accelrys, Inc. (see note in the Supporting Information for detailed settings and the efficiency of Polymorph module in predicting crystal structures).25 On the basis of the predicted crystal structure, the splitting of the HOMO and LUMO molecular orbitals of two stacked molecules

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Figure 1. HOMO and LUMO energies (a), ionization potentials and electron affinities (b), hole and electron reorganization energies (c), and hole and electron-transfer mobilities (d) of 1-5 with 1/N (the number of thiophen units).

was calculated using B3LYP method and 6-31G(d) basis set in the Gaussian 03 program.22 Results and Discussion The molecular structures of oligothienoacenes (1-5) and their substituted derivatives (1Se-5Se, 2R, 2-nX, 4BH, 5BH, 5BHnF) are shown in Scheme 1. Since the molecule for all the compounds can hardly keep its symmetry in the solid state as it can in vacuum conditions, no symmetry restriction is added during the structure optimization. The calculated HOMO and LUMO energies, HOMO-LUMO gaps, vertical and adiabatic ionization potentials, vertical and adiabatic electronic affinities, internal reorganization energies, and intrinsic charge-transfer mobilities for all the compounds are tabulated in Tables 1-4 and S1 (Supporting Information). Tables S2-S6 (Supporting Information) list the calculated transfer integral t and transfer distance r on the basis of predicted crystal structures obtained using the Polymorph module in the Cerius2 software package.25 The products of the largest transfer integrals and corresponding distances as a function of different oligothienoacenes are shown in Figures S14-S18 (Supporting Information). It is worth noting that the reorganization energies for compounds 3 and 5BH calculated using the basis set 6-31G(d) correspond well with those obtained using either the 6-31G(d,p) or 6-31++G(d,p) basis sets, demonstrating that the 6-31G(d) basis set is big enough for calculating the reorganization energies of all the compounds (Tables 1 and 4). As a result, all the calculations are conducted on the basis of the 6-31G(d) basis set hereafter. Effect of Oligomer Length. Parts a and b of Figure 1 show the energies of frontier molecular orbitals and calculated vertical

ionization potentials and electronic affinities of 1-5 as a function of 1/N (N: the number of thiophene units). In line with the previous findings for conjugated systems,26 the HOMO energy increases while the LUMO energy decreases along with the increase of thiophene unit number. When N is large enough (1/N ≈ 0), the HOMO and LUMO energies are estimated to be ca. -4.7 and -2.5 eV, respectively, according to the fitted line. As expected from the change of frontier molecular orbital energy levels, the vertical ionization potential decreases along with the increase of oligomer length, while the vertical electronic affinity increases in the same order. It is well-known that the barrier for hole or electron injection corresponds to the energy difference between the ionization potential or electron affinity, respectively, of the semiconducting material and the work function of the electrode (Au electrode in the present case). The decrease of vertical ionization potential and increase of vertical electronic affinity along with the increase of oligomer length therefore reveal the decrease in the barrier for both hole and electron injection along the same order. As clearly shown in both Figure 1b and Table 1, the vertical ionization potential of oligothienoacenes (7.86-6.56 eV for 1-5) is closer to the work function potential of common gold electrode, ca. 5.1 eV, than to the vertical electronic affinity (-1.21 to 0.35 eV) is, rendering this series of oligothienoacenes p-type rather than n-type semiconducting material (it is worth pointing out that according to Newman and co-workers,19a materials with the electronic affinity located in the range of 3.0-4.0 eV can ensure both efficient electron injection from common gold electrode and enough ambient stability and are therefore good n-type semiconductors for OFETs). Even for 1/N ) 0, the estimated

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TABLE 1: HOMO and LUMO Energies, HOMO-LUMO Gaps (Egap), Adiabatic and Vertical Ionization Potentials and Electron Affinities (IPadia, EAadia, IPvert, and EAvert), Hole and Electron Reorganization Energies (λ+ and λ-), and Hole and Electron-Transfer Mobilities (µ+ and µ-) of 1-5 (all in eV for Energies and cm2 V-1 s-1 for Mobilities) molecule

HOMO

LUMO

Egap

IPadia

EAadia

IPvert

EAvert

λ+

λ-

µ+

µ-

1 2 3 3a 3b 4 5

-5.85 -5.60 -5.42 -5.42 -5.66 -5.30 -5.20

-0.72 -1.09 -1.34 -1.35 -1.62 -1.53 -1.67

5.13 4.52 4.08 4.08 4.04 3.77 3.53

7.65 7.19 6.85 6.85 7.03 6.60 6.41

-0.99 -0.47 -0.07 -0.06 0.30 0.23 0.47

7.86 7.37 7.01 7.02 7.20 6.75 6.56

-1.21 -0.63 -0.21 -0.21 0.17 0.10 0.35

0.41 0.35 0.33 0.33 0.32 0.31 0.29

0.68 0.32 0.29 0.29 0.28 0.27 0.25

0.13 0.34 0.23

0.006 0.37 0.29

0.55 0.48

0.80 0.22

a

B3LYP/6-31G(d,p) values. b B3LYP/6-31++G(d,p) values.

electronic affinity of ca. 1.7 eV cannot satisfy the standard for n-type semiconductors. This actually is in good accordance with the experimental result that pentathienoacene (4) displayed quite good OFET performance as p-type semiconductors,14a but no n-type oligothienoacenes-based OFET device with Au electrodes has been reported so far. Table 1 organizes the internal reorganization energies of the series of compounds 1-5 for both hole and electron transfer, and Figure 1c exhibits the change in the internal reorganization energies of 1-5 for both hole and electron as a function of 1/N. The results reveal that the reorganization energies for both hole and electron decrease with increasing thiophene unit number, which in turn indicates the increasing charge-transfer mobility for both holes and electrons of oligothienoacenes. In line with the point of Cornil et al.,5a the calculated reorganization energies of oligothienoacenes 1-5 for holes are in the same order of magnitude as their reorganization energies for electrons (Table 1 and Figure 1c). Nevertheless, the fact that the reorganization energies of oligothienoacenes 2-5 for holes are a bit larger than those for electrons actually suggests the possibly greater charge-transfer mobility of 2-5 for electrons than that for holes if only in terms of the reorganization energy, despite their p-type semiconductor nature revealed by charge injection barrier relative to the work function potential of source Au electrode, as detailed above. To finally calculate the charge mobility of these oligothienoacene compounds, molecular packing modes in the semiconductor materials need to be clarified to calculate the transfer integral between neighboring molecules and corresponding distance. Molecular packing in the single-crystal form usually reflects the most advantageous molecular arrangement of molecular materials in the solid-state form. As a result, singlecrystal-based OFETs should exhibit the largest charge-transfer mobility comparable with the intrinsic property of the material. As a consequence, the transfer integrals for the series of compounds 1-5 are calculated on the bases of their crystal structures hereafter. Among the whole series of oligothienoacenes 1-5, the crystal structures for 1, 3, and 4 were determined by X-ray diffraction technique,12,27 while those of 2 and 5 remain unresolved experimentally. As a consequence, the crystal structures for compounds 2 and 5 were predicted using the Polymorph module in the Cerius2 software package.25 For the purpose of comparison, crystal structures for compounds 1, 3, and 4 have also been predicted with the same method. The calculated results are given in the Supporting Information. With these obtained reorganization energies, transfer integrals, and transfer distances, the charge-transfer mobilities of 1-5 for both hole and electron were calculated according to Marcus and Einstein theory (Table 1 and Figure 1d). As can be observed, the series of oligothienoacene compounds display quite good semiconducting properties for both hole and electron, with the hole mobilities in the range of 0.13-0.55 cm2 V-1 s-1 and

electron mobilities of 0.006-0.80 cm2 V-1 s-1. It is worth noting again that despite the same order of magnitude of the charge mobilities for electrons as their charge mobilities for holes of this series of compounds 1-5, their very high injection barrier for electrons relative to the work function of Au source electrode prevents them from showing n-type semiconducting property when fabricated into OFETs with Au electrodes. However, application of asymmetrical source-drain metal electrodes should render it possible to use these compounds as good ambipolar semiconducting materials. Among the whole series, compound 4 with five thiophene units displays the highest charge mobility for both hole and electron, 0.55 and 0.80 cm2 V-1 s-1, respectively, while compound 5 with six thiophene units shows a bit smaller but still comparable charge mobility with that of 4, 0.48 and 0.22 cm2 V-1 s-1, due to the much smaller product of largest transfer integral and corresponding transfer distance for 5 than that for 4. This appears in line with the fact that only the semiconducing property of pentathienoacene (4) among the whole series of existed unsubstituted oligothienoacenes was studied and reported thus far.14a It is worth noting that the mobilities obtained here are proposed intrinsic mobilities, which cannot be directly compared to experimental ones. As a consequence, only the calculated mobility trend along with different compounds and the semiconductor nature are compared with the experimental results. Effect of Heteroatomic Substitution. To aim for new organic semiconductors with improved molecular electronic properties, chemical stability, and crystal structure, heteroatomic substitutions of the oligomers have been studied and verified useful in tuning semiconducting properties such as charge mobility.7,26a In the present work, the effect of heteroatomic substitution of the sulfur atom in the thiophene unit of oligothienoacene with selenium, boron, carbon, nitrogen, oxygen, silicon, and phosphorous on the semiconducting properties was systematically studied on the basis of theoretical calculations. Additionally, heteroatomic substitution of the thiophene peripheral proton(s) in dithieno[2,3-b:2′,3′-d]thiophene (2) with halogen atom(s) was also investigated in a similar manner. The HOMO and LUMO energies, vertical and adiabatic ionization potentials, vertical and adiabatic electronic affinities, internal reorganization energies, and charge-transfer mobilities for the series of selenium-substituted oligothienoacenes 1Se5Se were similarly calculated, and the data are summarized in Table S1 (Supporting Information). Figure S11 (Supporting Information) shows their change as a function of 1/N. Comparison with corresponding data of unsubstituted oligothienoacenes reveals that selenium substitution of the thiophene sulfur atom in oligothienoacene induces a slight increase in the HOMO energy and vertical and adiabatic electronic affinities and a decrease in the LUMO energies and vertical and adiabatic ionization potentials but without changing their trend as a function of the oligomer length. Unsurprisingly, the difference in the

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Figure 2. HOMO and LUMO energies, ionization potentials and electron affinities, hole and electron reorganization energies, and hole and electrontransfer mobilities of heteroatom-substituted compounds of 2 and 2-R.

reorganization energies between 1-5 and 1Se-5Se is very small, indicating the very small influence of selenium substitution on the reorganization energy. However, the electronic affinity of the selenium-substituted oligothienoacenes 1Se-5Se is still far from the work function potential of common gold electrode. As a result, selenium-substituted oligothienoacenes 1Se-5Se can act only as potential good p-type rather than n-type semiconducting materials if fabricated into OFETs with sourcedrain Au electrodes. Due to the increased product of largest transfer integral and corresponding transfer distance for both hole and electron of these selenium-substituted oligothienoacenes 1Se-5Se in comparison with that of unsubstituted oligothienoacenes 1-5 (see the Supporting Information), the former series of compounds are revealed to show higher charge-transfer mobility for both hole and electron in comparison with the latter series on the basis of the calculation results, revealing the effect of selenium substitution on the semiconducting property of oligothienoacenes. It must be pointed out that these calculated results are in good accordance with the experimental findings. As revealed by Yamada and co-workers, substitution of the sulfur atoms in the central three thiophene units of the benzoannulated pentathienoacene leads to obvious increase in the hole transfer mobility from 0.50 cm2 V-1 s-1 for benzoannulated pentathienoacene to 1.1 cm2 V-1 s-1 for tri(selenium)-substituted benzoannulated pentathienoacene.13e These results indicate that selenium-substituted oligothienoacenes are better semiconductors than unsubstituted oligothienoacenes for p-type OFETs with common Au electrodes and even for ambipolar OFETs in cases where asymmetric source-drain metal electrodes can be employed.

For economic reasons and to simplify the calculations, the effect of the heteroatomic substitution of sulfur with boron, carbon, nitrogen, oxygen, silicon, and phosphor on the semiconducting properties was checked only on a relatively simple and experimentally completed system among the series of oligothienoacenes, dithieno[2,3-b:2′,3′-d]thiophene (2), by substituting the sulfur atom only in the medium thiophene unit with the heteroatom.13 This is also true for the peripherally halogensubstituted systems as detailed below. As shown in Figure 2 and Table 2, the substitution of sulfur atom with all the heteroatoms mentioned above induces an increase in the HOMO energy level and a decrease in the vertical ionization potential. As a result, all the heteroatom-substituted compounds 2-R (R ) BH, CH2, NH, O, SiH2, PH) show improved charge injection for hole compared with their unsubstituted counterpart 2. However, heteroatomic substitution of the medium thiophene unit sulfur atom of 2 with only boron, silicon, and phosphorous atoms is revealed to lower the LUMO energy level of oligothienoacene (Figure 2). This, in line with the increase in the vertical electron affinity calculated for 2-SiH2, 2-PH, and 2-BH, indicates that heteroatomic substitution with boron, silicon, and phosphorous atoms can effectively decrease the injection barrier for electron and therefore is a potential way toward ambipolar semiconductors from p-type semiconducting oligothienoacenes. Nevertheless, our calculation results indicate that among the three different heteroatomic substitutions only the boron substitution leads to a decrease in the reorganization energy for electron, suggesting the best effect of boron substitution on tuning the semiconducting property for electron transfer of oligothienoacene in terms of the electron reorganization energy. The calculated product of the largest transfer integral and

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TABLE 2: HOMO and LUMO Energies, HOMO-LUMO Gaps (Egap), Adiabatic and Vertical Ionization Potentials and Electron Affinities (IPadia, EAadia, IPvert, and EAvert), Hole and Electron Reorganization Energies (λ+ and λ-), and Hole and Electron-Transfer Mobilities (µ+ and µ-) of Heteroatom-Substituted Compounds of Dithieno[2,3-b:2′,3′-d]thiophene 2-R (R ) BH, CH2, NH, O, SiH2, PH) (all in eV for Energies and cm2 V-1 s-1 for Mobilities) molecule

HOMO

LUMO

gap

IPadia

EAadia

IPvert

EAvert

λ+

λ-

µ+

µ-

2-BH 2-CH2 2-NH 2-O 2-SiH2 2-PH 2

-5.35 -5.22 -5.14 -5.48 -5.49 -5.53 -5.60

-2.52 -0.99 -0.56 -0.93 -1.48 -1.35 -1.09

2.83 4.23 4.58 4.55 4.01 4.18 4.52

6.95 6.83 6.79 7.15 7.01 7.07 7.19

0.90 -0.58 -0.95 -0.65 0.00 -0.17 -0.47

7.13 7.00 6.95 7.32 7.22 7.26 7.37

0.77 -0.75 -1.17 -0.83 -0.21 -0.35 -0.63

0.36 0.34 0.32 0.34 0.41 0.37 0.35

0.25 0.33 0.65 0.52 0.40 0.38 0.32

0.31 0.53 1.74 0.43 0.19 0.25 0.34

1.92 0.67 0.042 0.051 0.085 0.23 0.37

TABLE 3: HOMO and LUMO Energies, HOMO-LUMO Gaps (Egap), Adiabatic and Vertical Ionization Potentials and Electron Affinities (IPadia, EAadia, IPvert, and EAvert), Hole and Electron Reorganization Energies (λ+ and λ-), and Hole and Electron-Transfer Mobilities (µ+ and µ-) of Halogen-Substituted Compounds of Dithieno[2,3-b:2′,3′-d]thiophene 2-nX (X ) F, Cl; n ) 1-4) (all in eV for Energies and cm2 V-1 s-1 for Mobilities) molecule

HOMO

LUMO

gap

IPadia

EAadia

IPvert

EAvert

λ+

λ-

µ+

µ-

2-1F-a 2-1F-b 2-2F-a 2-2F-b 2-2F-c 2-2F-d 2-3F-a 2-3F-b 2-4F 2-1Cl-a 2-1Cl-b 2-2Cl-a 2-2Cl-b 2-2Cl-c 2-2Cl-d 2-3Cl-a 2-3Cl-b 2-4Cl 2

-5.77 -5.60 -5.75 -5.60 -5.94 -5.77 -5.91 -5.75 -5.89 -5.83 -5.69 -5.87 -5.78 -6.04 -5.90 -6.06 -5.94 -6.09 -5.60

-1.27 -1.12 -1.31 -1.14 -1.45 -1.29 -1.49 -1.33 -1.53 -1.34 -1.32 -1.52 -1.54 -1.57 -1.55 -1.74 -1.72 -1.90 -1.09

4.51 4.48 4.44 4.46 4.49 4.47 4.42 4.41 4.36 4.49 4.37 4.35 4.24 4.47 4.35 4.32 4.22 4.19 4.52

7.35 7.16 7.29 7.14 7.51 7.32 7.45 7.27 7.41 7.35 7.20 7.32 7.21 7.51 7.36 7.47 7.33 7.44 7.19

-0.28 -0.34 -0.07 -0.29 -0.09 -0.17 0.10 -0.02 0.21 -0.16 0.20 0.43 0.33 0.11 0.35 0.57 0.55 0.64 -0.47

7.53 7.37 7.51 7.37 7.70 7.53 7.67 7.51 7.65 7.53 7.38 7.50 7.39 7.69 7.54 7.65 7.51 7.62 7.37

-0.45 -0.60 -0.41 -0.58 -0.28 -0.43 -0.24 -0.39 -0.20 -0.34 -0.33 -0.10 -0.06 -0.06 -0.06 0.16 0.16 0.37 -0.63

0.37 0.41 0.43 0.46 0.38 0.42 0.44 0.48 0.49 0.36 0.36 0.36 0.37 0.36 0.36 0.36 0.37 0.36 0.35

0.42 0.92 1.02 1.02 0.47 0.92 1.01 1.09 1.24 0.42 2.94 2.85 2.72 0.40 2.75 2.60 2.59 1.05 0.32

0.19 0.060 0.22 6.8 10-3 0.18 0.025 0.11 0.095 0.022

0.11 2.3 × 10-4 5.9 × 10-4 6.9 × 10-5 0.038 7.5 × 10-5 2.8 × 10-4 1.3 × 10-4 2.0 × 10-5

0.34

0.37

corresponding transfer distance shown in Figure S16 (Supporting Information) also reveals that 2-BH has the second largest product value (just a little smaller than that of 2-NH) among all the heteroatom-substituted derivatives of dithieno[2,3-b:2′3′d]thiophene (2). As a total result, the charge-transfer mobility for electrons for the boron-substituted 2-BH is calculated to be as high as 1.92 cm2 V-1 s-1, the highest value among the series of heteroatomic-substituted oligothienoacenes, which is about twice that of 4, which had the highest value among the unsubstituted oligothienoacenes 1-5. Briefly summarizing the above statements, boron substitution appears to be the most effective way to improve the semiconducting properties of oligothienoacenes for both hole and in particular electron transfer, and therefore, a potential method for tuning the p-type oligothienoacenes into ambipolar semiconductors (vide infra). It is worth noting that our calculated results for heteroatom substitutional effect are in line with the ones of Lagowski and co-workers.26a Effect of Halogen Substitution. It has been verified that the introduction of electron-withdrawing substituents such as fluorine and chlorine onto the p-type semiconductors is useful in improving their semiconducting properties for electrons. Incorporating electron-withdrawing groups onto the p-type semiconductors was a common step toward the production of ambipolar or n-type semiconducing materials.4f,8 In the present case, similar methodology is employed to improve the electron injection barrier of oligothienoacenes. Table 3 and Figure 3 organize the calculated HOMO and LUMO energy levels, vertical ionization potentials and electron affinities, reorganization energies for both hole and electron, and charge-transfer mobilities for both hole and electron of the halogen-substituted derivatives of dithieno-

[2,3-b:2′,3′-d]thiophene (2). As expected, halogen substitution of the thiophene peripheral proton(s) with fluorine or chlorine induces a slight decrease in the HOMO and LUMO energy level and an increase in the ionization energy and electron affinity, which therefore leads to a slight increase in the injection barrier for holes but a decrease in the injection barrier for electrons relative to the common Au source-drain electrodes. Along with the slight increase in injection barrier for holes, the reorganization energy for holes shows a small increase, suggesting the decreased transfer mobility for holes. However, in contrast to the decrease in injection barrier for electrons, thiophehene peripheral halogen substitution of dithieno[2,3-b:2′,3′-d]thiophenes has been found to significantly elevate the reorganization energy for electrons (Table 3 and Figure 3), resulting in significantly decreased charge mobility for electrons. In addition, the calculation results reveal that the effect of halogen substitution differs along with the positions, numbers, and electron-withdrawing properties of substituents. Substituents at the meta position of the sulfur atom in the thiophene unit add more significant effect on the electron affinity and therefore injection barrier for electron of corresponding compounds than those at the ortho position (Table 3 and Figure 3). This is also true for increasing the halogen substituent number and changing the fluorine substituents to chlorines. Among all the different type of halogen-substituted dithieno[2,3-b:2′,3′-d]thiophenes, difluronation appears more effective to improve the semiconducting properties of dithieno[2,3-b:2′,3′-d]thiophenes in terms of both injection barrier and charge-transfer mobility for electrons, due to the relatively larger reorganization energy for electrons of perhalogenated and trihalogenated compounds (24X, 2-3X-a, and 2-3X-b) and chlorine-substituted dithieno[2,3-

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Figure 3. HOMO and LUMO energies, ionization potentials and electron affinities, hole and electron reorganization energies, and hole and electrontransfer mobilities of fluorine- and chlorine-substituted compounds of 2 and 2-nX.

TABLE 4: HOMO and LUMO Energies, HOMO-LUMO Gaps (Egap), Adiabatic and Vertical Ionization Potentials and Electron Affinities (IPadia, EAadia, IPvert, and EAvert), Hole and Electron Reorganization Energies (λ+ and λ-), and Hole and Electron-Transfer Mobilities (µ+ and µ-) of Boron-Substituted Pentathienoacene (4BH) and Hexathienoacene (5BH) and the Halogen-Substituted Derivatives of 5BH (5BH-nF; n ) 2, 4, 10) (all in eV for Energies and cm2 V-1 s-1 for Mobilities) molecule

HOMO

LUMO

gap

IPadia

EAadia

IPvert

EAvert

λ+

λ-

µ+

µ-

4BH 5BH 5BHa 5BHb 5BH-2F-a 5BH-2F-c 5BH-4F 5BH-10F

-5.49 -5.50 -5.50 -5.77 -5.67 -5.79 -5.55 -5.48

-4.37 -4.64 -4.64 -4.92 -4.74 -4.98 -5.14 -4.87

1.12 0.87 0.86 0.85 0.93 0.80 0.41 0.61

6.84 6.70 6.71 6.92 6.85 6.87 6.32 6.79

3.04 3.41 3.42 3.75 3.54 3.75 4.36 3.84

6.93 6.77 6.77 7.00 6.94 7.03 6.81 6.81

2.94 3.35 3.35 3.69 3.45 3.68 3.85 3.55

0.18 0.15 0.14 0.14 0.19 0.34 -0.09 0.31

0.19 0.13 0.13 0.13 0.19 0.15 -0.06 0.40

0.42 3.74

1.92 4.77

5.07 0.22 0.55 0.070

5.76 0.68 0.49 0.035

a

B3LYP/6-31G(d,p) values. b B3LYP/6-31++G(d,p) values.

b:2′,3′-d]thiophenes and the poor effect of monohalogenation on dithieno[2,3-b:2′,3′-d]thiophene in tuning the semiconductor nature. Figure S17 (Supporting Information) also shows that both 2-2F-a and 2-2F-c have relatively larger products of transfer integral and corresponding transfer distances than those of the other two difluronated substituted dithieno[2,3-b:2′,3′d]thiophenes. As a result, trials toward the design of ambipolar OFET semiconducting materials from oligothienoacenes are conducted on boron-substituted pentathienoacene and hexathieno-

acene together with the difluronated derivatives of hexathienoacene, 4BH, 5BH, 5BH-2F-a, and 5BH-2F-c. Toward Ambipolar Semiconductors with High ChargeTransfer Mobilities. On the basis of the aforementioned results, investigation of the HOMO and LUMO energies, vertical ionization potential and electron affinity, reorganization energy for both hole and electron, transfer integral for hole and electron, and charger transfer mobility for hole and electron was first conducted on boron-substituted pentathienoacene 4BH and

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Figure 4. HOMO and LUMO energies, ionization potentials and electron affinities, hole and electron reorganization energies, and hole and electrontransfer mobilities of hexathienoacene (5) and the boron-substituted derivatives 4BH, 5BH, and 5BH-nF.

Figure 5. HOMOs and LUMOs of 5 and 5BH.

hexathienoacene 5BH, for a targeted design of semiconducting materials with ambipolar properties. Corresponding data obtained for these two compounds are compared with those of 5 in Table 4 and Figure 4. The predicted crystal structure of 5BH and all the possible transfer routes in the predicted crystal structure are shown in Figure S12 (Supporting Information). In line with the findings from monoboron-substituted dithieno[2,3-b:2′,3′-d]thiophene 2-BH, substitution of all the thiophenene sulfur atoms in pentathienoacene 4 and hexathienoacene 5 with boron atoms does not change their semiconducting property for hole very much in terms of both the injection barrier and chargetransfer mobility. In other words, both 4BH and 5BH still remain as good p-type semiconducting layers of OFETs with common Au electrodes. Nevertheless, perboron substitution of sulfur atoms in the thiophene units of pentathienoacene 4 and hexathienoacene 5 significantly improves their semiconducting property for electrons by greatly lowering the LUMO energy level and increasing their electronic affinity. As tabulated in

Table 4, both the vertical electron affinities of 4BH and 5BH and their ionization potentials, 6.93 and 6.77 eV, respectively, are very close to the work function potential of gold electrode, 5.1 eV, indicating their great potential as good p-type semiconducting materials for OFET devices with common Au source-drain electrodes. More importantly, the electronic affinity of 5BH is only located in the range of 3.0-4.0 eV, while the affinity for 4BH is very close to this range, which agrees well with the standard of good n-type as well as ambipolar semiconductors in terms of efficient electron injection from common gold electrodes and enough ambient stability.19a This is also true for the difluronated counterparts 5BH-2F-a and 5BH-2F-c. In addition, calculation results reveal the decrease in the reorganization energy for hole and electron following the boron substitution of sulfur atoms. This, in combination with the fact that the boron substitution of sulfur atoms in compounds 4 and 5 leads to a very small degree of decrease in the largest transfer integral for hole but significant increase in the largest transfer integral for electron in 4BH and 5BH rationalizes the at least comparable intrinsic mobilities for holes of 4BH and 5BH, 0.42 and 3.74 cm2 V-1 s-1, versus those of 4 and 5, 0.55 and 0.48 cm2 V-1 s-1, and the increased charge mobilities for electrons of 4BH and 5BH, 1.92 and 4.77 cm2 V-1 s-1 versus those of 4 and 5, 0.80 and 0.22 cm2 V-1 s-1. All these results indicate that the boron substitution of sulfur atoms in the thiophene units of pentathienoacene and hexathienoacene significantly improves their semiconducting properties for electrons, but at the same time keeps their semiconducting properties for holes almost unchanged, and is therefore an effective method toward the creation of good ambipolar

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Figure 6. Bond lengths (in Å) of neutral, oxidized, and reduced forms of 5 and 5BH.

semiconducting materials for OFET devices with common source-drain Au electrodes. Difluronation of boron-substituted oligothienoacenes is expected to further improve the ambipolar semiconducting performance of corresponding compounds, also on the basis of the aforementioned results. Figure S13 (Supporting Information) shows the predicted crystal structure of 5BH-2F-a and all the possible charge-transfer routes in the crystal. However, calculations on the difluronated boron-substituted hexathienoacenes 5BH-2F-a and 5BH-2F-c (Table 4 and Figure 4), indicate that difluronation at the meta positions of the two terminal thiophene units of 5BH leads to a 1 order of magnitude decrease in the charge-transfer mobility for both hole and electron of 5BH2F-c, due to the larger reorganization energy and especially the smaller product of largest transfer integral and corresponding transfer distance of 5BH-2F-c in comparison with that of 5BH (Table 4 and Figures 4 and S18 (Supporting Information)). In contrast, difluronation of the two protons of one terminal thiophene unit of 5BH leads to a slight increase in the chargetransfer mobility for both hole and electron of 5BH-2F-a (Table 4 and Figure 4). To rationalize the much smaller reorganization energies for both hole and electron of 5BH in comparison with those of 5, their HOMO and LUMO maps are shown in Figure 5. Figure 6 organizes the bond lengths of neutral, oxidized, and reduced forms of 5 and 5BH. It has been shown that the HOMOs and LUMOs of the neutral compounds correspond to the spin-density distribution of the radical cations and anions, respectively, and the frontier orbitals are very useful for understanding the trends of reorganization energies.8b,h It can be seen from Figure 5 that the LUMO of 5 is distributed on all the carbon and sulfur atoms, resulting in large bond length variation for all the C-C and C-S bonds upon reduction. In addition to the small contribution from the sulfur atoms of the two terminal thiophene units of 5, the HOMO of 5 is distributed on all the edge-tip carbon atoms with almost no contribution from the other sulfur atoms. This leads to very small variation in the length of C-S bonds but much large variation in the length of C-C bonds upon oxidation. In contrast, the HOMOs and LUMOs of 5BH are more delocalized than those for 5, resulting in relative smaller bond

length variation upon both oxidation and reduction. These results well rationalize the much smaller reorganization energies for both hole and electron of 5BH over those of 5. Comparison of the calculated transfer integrals and transfer distances between 5BH and 5 gives additional support for the much larger transfer mobility for both hole and electron of the former compound over that of the latter one. As organized in Tables S2 and S6 (Supporting Information), the largest transfer integral in the crystal of 5BH for hole is only a little smaller while that for electron is much larger than that of 5. However, the corresponding transfer distances in 5BH for hole and electron are much longer than those in 5. As shown in Figure S18 (Supporting Information), the combined effect of transfer integral and transfer distance, that is, their product for 5BH, is much larger than that for 5 for both hole and electron transfer. As a consequence, charge transfer for both hole and electron in 5BH is more mobile than that in 5 in terms of both reorganization energy and transfer integral. Furthermore, despite the much greater reorganization energies for both hole and electron of 5BH-2F-a than those of 5BH, the greater charge-transfer mobility of the former compound over the latter one can be explained in terms of greater transfer integral and transfer distance of 5BH-2F-a than those of 5BH. Nevertheless, as shown in Figures S12 and S13 (Supporting Information), molecules in the crystal of 5BH take a herringbone packing while those of 5BH-2F-a take a face-to-face π-stacking packing model, giving further evidence for the greater chargetransfer mobility of 5BH-2F-a over that of 5BH.8h,28 Conclusion A series of oligothienoacenes and their heteroatomic and halogen-substituted derivatives have been investigated computationally to elucidate their potential as semiconducting materials for OFETs. The dependence of HOMO and LUMO energy levels, ionization potential, electron affinity, reorganization energy, transfer integral, charge-transfer mobility for both hole and electron, and in particular the nature of semiconductor on the oligomer length and heteroatomic and halogen substitution was systematically studied on the basis of calculation results.

5158 J. Phys. Chem. C, Vol. 112, No. 13, 2008 Unsubstituted oligothienoacenes are revealed to work only as p-type semiconductors due to their high electron injection barrier relative to the work function of Au electrodes. Heteroatomic substitution of the sulfur atoms in the thiophene units of oligothienoacenes, in particular boron substitution, significantly improves their semiconducting properties for electrons through a decrease of the injection barrier and an increase of the chargetransfer mobility for electrons but without lowering their semiconducting performance for holes, leading to the conversion of p-type oligothienoacenes to ambipolar boron-substituted oligothienoacenes as active semiconductor layers for OFETs with common gold electrodes. Difluronation of the two protons of one terminal thiophene unit of boron-substituted hexathienoacene induces a slight increase in the charge-transfer mobility for both hole and electron. These results indicate that heteroatom substitution of the p-type oligothienoacenes is a rational step toward the production of good ambipolar OFET materials and justifies the wait for forthcoming experimental demonstrations. Acknowledgment. Financial support from the Natural Science Foundation of China, Ministry of Education of China, and Shandong University is gratefully acknowledged. We are also grateful to the Shandong Province High Performance Computing Centre for a grant of computer time. Supporting Information Available: Structures and supercells of the predicted crystals of 1-5 and 5BH(-2F-a), arithmetic product of the largest transfer integral and its corresponding transfer distance of all the compounds, OFET performance of 1Se-5Se, the transfer integral and corresponding transfer distance for all the possible transfer routes of all the compounds studied, and full list of authors of ref 22. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tsumura, A.; Koezuka, H.; Ando, T. Appl. Phys. Lett. 1986, 49, 1210-1212. (2) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. Adv. Mater. 2002, 14, 99-117. (b) Horowitz, G. Adv. Mater. 1998, 10, 365-377. (3) For example: (a) Nelson, S. F.; Lin, Y. Y.; Gundlach, D. J.; Jackson, T. N. Appl. Phys. Lett. 1998, 72, 1854. (b) Sundar, V. C.; Zaumseil, J.; Podzorov, V.; Menard, E.; Willett, R. L.; Someya, T.; Gershenson, M. E.; Rogers, J. A. Science 2004, 303, 1644. (c) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. J. Am. Chem. Soc. 2007, 129, 15259. (d) Ando, S.; Nishida, J. I.; Tada, H.; Inoue, Y.; Tokito, S.; Yamashita, Y. J. Am. Chem. Soc. 2005, 127, 5336. (e) Panzer, M. J.; Frisbie, C. D. AdV. Funct. Mater. 2006, 16, 1051. (f) Xia, Y.; Kalihari, V.; Frisbie, C. D.; Oh, N. K.; Rogers, J. A. Appl. Phys. Lett. 2007, 90, 162106. (g) Kim, C.; Facchetti, A.; Marks, T. J. AdV. Mater. 2007, 19, 2561. (h) Facchetti, A.; Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, S.; Marks, T. J.; Friend, R. H. Angew Chem., Int. Ed. 2000, 39, 4547. (i) Jones, B. A.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R. Chem. Mater. 2007, 19, 2703. (j) Pappenfus, T. M.; Chesterfield, R. J.; Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. J. Am. Chem. Soc. 2002, 124, 4184. (k) Yoon, M.-H.; Kim, C.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 12851. (l) Chua, L.-L.; Zaumseil1, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194. (4) (a) Dodabalapor, A.; Katz, H. E.; Torsi, L.; Haddon, R. C. Science 1995, 1560, 269. (b) Babel, A.; Wind, J. D.; Jenekhe, S. A. AdV. Funct. Mater. 2004, 14, 891. (c) Ye, R.; Suzuki, K. Appl. Phys. Lett. 2005, 86, 253505. (d) Wang, J.; Wang, H.; Yan, X.; Huang, H.; Yan, D. Appl. Phys. Lett. 2005, 87, 093507. (e) Wang, J.; Wang, H.; Yan, X.; Huang, H.; Yan, D. Chem. Phys. Lett. 2005, 407, 87. (f) Rost, C.; Gundlach, D. J.; Karg, S.; Riess, W. J. Appl. Phys. 2004, 95, 5782. (g) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.; Lnoue, Y.; Seto, F.; Tokito, S. J. Am. Chem. Soc. 2004, 126, 8138. (h) Chesterfield, R. J.; Newman, C. R.; Pappenfus, T. M.; Ewbank, P. C.; Haukaas, M. H.; Mann, K. R.; Miller, L. L.; Frisbie, C. D. AdV. Mater. 2003, 15, 1278. (i) Jung, T.; Yoo, B.; Wang, L.; Dodabalapur, A.; Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. Appl. Phys. Lett. 2006, 88, 183102. (j) Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 1348. (k) Dinelli, F.; Capelli, R.; Loi, M. A.; Murgia, M.; Muccini, M.; Facchetti, A.; Marks, T. J. AdV. Mater. 2006, 18, 1416.

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