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Conjugate acene fused buckybowls: evaluating their

Cite this: Phys. Chem. Chem. Phys., 2013, suitability for p-type, ambipolar and n-type air stable 15, 5039 organic semiconductors†

Uppula Purushotham and G. Narahari Sastry*

Elaborate and exhaustive first principles calculations were carried out to screen the novel properties of a series of acene fused buckybowls. The acene fused compounds exhibit hole transport property due to their higher electron injection and lower hole transport barrier relative to the work function potential of Au electrodes. The higher HOMO and lower LUMO energy levels suggest lower hole and electron injection barriers of F and CN substituted and boron doped bowls which indicates ambipolar property of these bowls. The dicyano substituted fused bowls show only electron transport property with lower LUMO (À4.26 eV to À4.27 eV) and higher HOMO (À5.56 eV to À5.90 eV) energy levels. Received 25th December 2012, High electron affinity (42.80 eV) and low LUMO energy (o À4.00 eV) attributes air stability to these Accepted 22nd January 2013 bowls. Curvature decreased frontier orbital energies and increased ionization energy and electron

DOI: 10.1039/c3cp44673e affinity of bowls. This study reveals substitution of electron withdrawing groups and boron doped acene fused bowls can be a prominent materials for ambipolar and electron transport organic www.rsc.org/pccp semiconductors.

1. Introduction The chemistry of buckybowls has a twofold objective: (a) they can serve as precursors for synthesis of (b) they may mimic Carbon, besides being the central element in organic chemistry, is the novel properties that are exhibited by fullerene.5 The recent the most fascinating of all elements in material science, electronics, discovery of graphene and its successful applications in a wide optical, medical and biological sciences. Coal, graphite and range of fields has also triggered enthusiastic efforts by synthetic diamond are the traditional allotropic forms of carbon and have chemists to synthesize finite size molecular entities which can 6 Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. had dramatic influence on science, medicine and engineering mimic graphene. This also gave a boost to the chemistry of areas.1 The discovery of about two decades ago and graphdiyne, graphyne and dehydroannulene types of materials.7,8 the discovery of graphene about a decade back have dramatically The feature which is common to fullerenes, buckybowls, carbon impacted not only on physics and chemistry but also biology, nanotubes, and graphene is that they consist exclusively of sp2 material science and medicine.2,3 From a chemist’s point of hybridized carbons, while graphene is supposedly a flat surface, view the preparation of all the allotropic forms coal, graphite, however recent studies has shown that its propensity to distort out diamonds, fullerenes and carbon nanotubes are all from the ofplaneisfairlyhigh.9 Obviously fullerenes, carbon nanomaterials, traditional carbons. graphene and buckybowls are exceptional compounds. The discovery of fullerene has given deep thirst to research It is interesting to explore the potential of the curved on buckybowl chemistry. Predictive computing is very important polycyclic aromatic hydrocarbons as organic semiconducting to trigger the effective interplay between theory and experiment. materials, and we found that studies in this direction are Buckybowls, the easily identifiable carbon skeletons which can scarce. The bowl to bowl inversion barrier, the effect of hetero- map on to a fullerene surface, are interesting in their own right.4 atoms on buckybowls, the curvature and synthetic feasibility of buckybowls have been studied very extensively by many groups Center for Molecular Modeling, Indian Institute of Chemical Technology, Tarnaka, including ours.10–19 -bowl based systems offer Hyderabad 500607, AP, India. E-mail: [email protected] notionally a large p–p overlap if aligned in a convex–concave † Electronic supplementary information (ESI) available: Tables containing the 20 cartesian coordinates of the optimized geometries at B3LYP/6-31G level of theory. stacked fashion. A dense arrangement in the solid state could Energies at B3LYP/6-31G(d) and B3LYP/6-311+G(d)//B3LYP/6-31G(d) level of the- prevent the intrusion of oxygen and moisture into the channel ory. See DOI: 10.1039/c3cp44673e region and might be important in the design of air-stable

This journal is c the Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 5039--5048 5039 View Article Online Paper PCCP

21 n-channel type semiconductors. Further study of these curved A small reorganization energy lÀ for electron transport is only polycyclic aromatic hydrocarbons to design organic thin film one molecular property that an n-channel semiconductor transistors is interesting in its own right. should possess. To allow for efficient electron injection from In last few decades there has been significant progress in the common metal electrodes, the electron affinity (EA) should be development and rational design of organic thin film transistors in excess of 3.00 eV.23 (OTFTs).22 Most efforts have been devoted to p-type semiconductors, In this manuscript we propose to enumerate the properties whereas n-channel materials in which the majority carriers are of nano size carbon structures obtained by fusing two proto- electrons attracted the material chemists more recently. The design typical buckybowl corannulene and sumanene with of high-performance n-type materials is desirable for the fabrication a series of acenes. We have explored electronic and molecular of p–n junctions and complementary logic circuits. Frisbie and properties of acene fused structures, boron and nitrogen doped co-workers summarized recent progress in the development of bowls by using density functional theory (DFT). To gauge new n-channel organic semiconductors, and critical properties the effect of curvature on various properties, flat that any prospective n-channel material must have.23 Most compounds were compared with the bowl structures. single- organic semiconductors are composed of extended p-conjugated systems. The availability of high-energy occupied and low-energy unoccupied molecular orbitals (HOMO 2. Computational details and LUMO respectively) in these systems facilitates injection of The current study considers two prototypical buckybowls holes or electrons from the source metal electrode into the sumanene and corannulene fused with a series of acenes from semiconductor and subsequent extraction at the drain electrode. to pentacene (B1 to B5) depicted in Scheme 1. In order In addition, extended p-conjugated systems allow for effective to see the effect of doping and substitution of electron with- intra and intermolecular delocalization and transport of 24 drawing groups on structure and properties, we have under- charge. While many factors, such as thin-film formation, taken the study of boron and nitrogen doped bowls as well as crystal growth, and molecule alignment within the bulk media electron withdrawing F and CN substituted bowls. Since these play a role in increasing the charge mobility in the semiconductor, bowls have more than one possible combination we have the crux of the performance at the molecular level is the control of 25 considered homo dimers corannulene–corannulene (1), sumanene– energy levels and band gaps. In molecular electronic devices, sumanene (2) and hetero dimer sumanene–corannulene (3). As oneimportantissueiswhetherchargecarrierscanbeinjected sumanene and corannulene have bowl shapes, they can exhibit efficiently into the organic semiconductors. For example, the concave and convex orientations. Two possible concave–concave HOMO level of many organic semiconductors is in the range of À4.8 eV to À5.3 eV, which aligns well with the work function of gold (4.8 eV to 5.1 eV), thus the injection of holes into the HOMO level is easily achieved using gold electrodes. On other hand, the LUMO level often lies much higher at around À3.00 eV to À4.00 eV. When gold electrodes are used, n-type transport is much harder to achieve due to an extremely high injection barrier of 2 to 3 eV.26 However, to construct organic field-effect transistor (OFET) devices for applications, it is very important

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. to search for new types of materials, especially n-type or ambipolar organic semiconductors. Pentacene is well known as an excellent hole transferring material. Recently, a hole mobility of 45.0 cm2 VÀ1 sÀ1 has been obtained.27 When all hydrogen atoms of pentacene are substituted by fluorine atoms, the compound is turned into perfluoropentacene, and the fabricated OFET transports electrons more efficiently than holes, with a high electron mobility of 0.11 to 0.22 cm2 VÀ1 sÀ1.28,29 As of 2000, the highest reported electron field effect mobility in an 30 organic thin film was for C60 on an amine modified surface (m E 0.3 cm2 VÀ1 sÀ1), but the performance of these films degraded rapidly upon exposure to air. Recently, Kobayashi 2 À1 À1 et al. reported mobilities as high as 0.56 cm V s for C60 films fabricated by molecular beam deposition, but these mobilities could only be achieved by performing device fabrication and electrical characterization without breaking the vacuum.31 The high charge carrier mobilities of oligoacenes, especially tetracenes and , the archetypical p-type semiconductors, have Scheme 1 The systems considered and there nomenclature used in the present 22 been attributed largely to the small l+ of these materials. study.

5040 Phys. Chem. Chem. Phys., 2013, 15, 5039--5048 This journal is c the Owner Societies 2013 View Article Online PCCP Paper

(CC) and concave–convex (CV) orientations of these bowls have been considered for initial optimizations. The nomenclature used in this study is X1Bn, X2Bn and X3Bn for unsubstituted bowls, where X represents C for undoped bowls, B or N for boron or nitrogen doped bowls respectively. The corannulene–corannulene, sumanene–sumanene, and sumanene–corannulene bowls are termed as X1, X2 and X3 respectively and the Bn (n =1,2,3,4 Fig. 1 Definition of the internal reorganization energy for (a) hole transfer + - + À - À and 5) for the number of acene rings present in a given bowl. The R reaction M + M M + M and (b) electron-transfer reaction M + M M +M. The cation (anion) reorganization energy l (l ) is equal to the difference and R1 represent the substituents presented on the bowls. For 2 4 between the vertical and adiabatic ionization energy (electron affinity): l2 = initial geometries and vibrational frequencies of neutral systems, IEv À IEa (l4 = EAv À EAa). radical cations and radical anions we have employed the B3LYP method using the 6-31G basis set. The calculated properties such as frontier molecular orbital energies, ionization energy, electron l+ = l1 + l2 = E0(Q+) À E0(Q0)+E+(Q0) À E+(Q+) (2) affinity and reorganization energy of CC and CV bowls are similar lÀ = l3 + l4 = E0(QÀ) À E0(Q0)+EÀ(Q0) À EÀ(QÀ) (3) at B3LYP/6-31G level of theory. Based on the above results we have considered only the CC form of bowls for further studies. Single point calculations were carried out using more flexible valence 3. Results and discussion triple zeta 6-311+G(d) basis set on B3LYP/6-31G optimized geometries. The results of B3LYP/6-311+G(d)//B3LYP/6-31G Frontier molecular orbital energies and HOMO/LUMO energy level of theory were considered for further discussion. In gap (HLG) are significant criteria which affects the electronic, addition, to the geometry optimizations we have also estimated optical, and conducting properties of materials. On a direct various properties such as reorganization energy, ionization metal–semiconductor junction, the barrier height at the metal- energy (IE) and electron affinity (EA) of these systems. The IE semiconductor interface is given by the difference between and EA values have been calculated by taking the energy metal work function and semiconductor HOMO/LUMO energy. differences between the ground and ionized states (non-relaxed For organic field-effect transistors (OFETs) with Au source- ionized states give vertical IE/EA and relaxed ionized states give drain electrodes, the p-type semiconductors should possess adiabatic IE/EA).32 All the calculations were carried out using high HOMO energies to ensure effective charge injection from the Gaussian 09 package.33 the source electrode. On the contrary, n-type materials should In this section we briefly review the method employed to possess low LUMO energies, such that they lie close to the work calculate the reorganization energy. Apart from potential external function potential of the Au electrode.37–39 Bao group under- traps (defects, impurities, grain boundaries) self-trapping can took an impressive study of the correlation between charge account for intermolecular charge transfer when the average carrier types in the OTFT geometry and experimentally calculated residence time of a charge carrier on a specific molecule is similar molecular HOMO/LUMO levels. A very general observation in magnitude to the relaxation time of that molecule to the derived from this study involves the transition from p-type to optimumgeometryofchargedstate.Onbasisofthatthecharge ambipolar to n-type behavior for transistor applications. Using Au transfer can be described as a self-exchange electron-transfer electrodes, materials with LUMO 4 À3.15 eV and HOMO 4 reaction between a neutral molecule and a neighboring radical À5.60 eV tended to exhibit only p-type behavior, probably due to 34 Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. anion (n-type materials) or radical cation (p-type materials). large barriers to electron injection. Once the LUMO dropped Classical Marcus theory was employed to calculate charge transfer below À3.15 eV, both p-type and n-type (i.e., ambipolar) behavior rates for organic materials35,36 could be observed. Materials with LUMO o À3.15 eV and HOMO o À5.60 eV exhibited strictly n-type behavior, which was attrib- 40 ket =(t2/h)(p/lÆkBT)1/2 exp(ÀlÆ/4kBT) (1) uted to large barriers to hole injection. These observations correlate well with the observation of ambipolar behavior in both 41 42 Where t is the transfer integral, lÆ is the reorganization energy pentacene (LUMO = À3.2 eV) and rubrene (LUMO = À3.15 eV).

(l+ or lÀ is used for hole or electron transfer, respectively), kB is In this section we have started the discussion with a detailed the Boltzmann constant, and T is the temperature. To achieve description about the hole transport materials based on various high charge carrier mobility in organic semiconductors, the properties, it has been extended to ambipolar and electron reorganization energy needs to be minimized, and the inter- transport materials also. To observe the air stability of these molecular charge transfer integral needs to be maximized. The bowls, LUMO energy and EA values are discussed. The effect of inner reorganization can be defined as showed in Fig. 1. The curvature in bowl and flat structures has been addressed.

reorganization energy l+ (lÀ) for hole (electron) transport is calculated as the sum of the energy required for reorganization 3.1 Hole transport materials of the vertically ionized neutral to the cation (anion) geometry, In this section we have been evaluated hole transport properties

l2 (l4), plus the energy required to reorganize the cation (anion) of acene fused buckybowls. Table 1 depicts the HOMO/LUMO geometry back to the neutral equilibrium structure on the orbital energies and HLG of various acene fused buckybowls

ground state potential energy surface, l1 (l3) as showed below. at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory. The HOMO

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Table 1 Energies of HOMO, LUMO orbital’s and HOMO–LUMO energy gap

(HLG) and the reorganization energies l+ (hole transport), lÀ (electron transport) of various acene fused buckybowls at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory (all values are given in eV)

Name HOMO LUMO HLG l+ lÀ Cor-B1-Cor À5.87 À2.30 À3.57 0.13 0.04 Sum-B1-Cor À5.39 À2.30 À3.09 0.14 0.04 Sum-B1-Sum À5.02 À2.37 À2.65 0.15 0.01 Cor-B2-Cor À5.58 À2.46 À3.11 0.12 0.02 Sum-B2-Cor À5.15 À2.54 À2.62 0.13 0.02 Sum-B2-Sum À4.85 À2.61 À2.23 0.13 0.03 Cor-B3-Cor À5.31 À2.69 À2.61 0.12 0.01 Sum-B3-Cor À4.96 À2.77 À2.19 0.12 0.03 Sum-B3-Sum À4.71 À2.83 À1.88 0.12 0.02 Cor-B4-Cor À5.08 À2.91 À2.17 0.11 0.01 Sum-B4-Cor À4.79 À2.96 À1.83 0.12 0.01 Sum-B4-Sum À4.59 À3.00 À1.59 0.11 0.02 Cor-B5-Cor À4.89 À3.09 À1.80 0.11 0.01 Fig. 2 Correlation of carrier type with frontier molecular orbital energy levels of Sum-B5-Cor À4.66 À3.12 À1.54 0.11 0.01 acene fused buckybowls and there boron and nitrogen doped bowls at B3LYP/ Sum-B5-Sum À4.49 À3.14 À1.35 0.10 0.02 6-311+G(d)//B3LYP/6-31G level of theory. Cor-B5-Cor-F À5.13 À3.33 À1.80 0.15 0.03 Sum-B5-Cor-F À4.91 À3.36 À1.54 0.15 0.01 Sum-B5-Sum-F À4.72 À3.38 À1.35 0.15 0.02 Cor-B5-Cor-CN À5.50 À3.74 À1.76 0.10 0.00 hole transport property due to their small barrier to hole Sum-B5-Cor-CN À5.23 À3.70 À1.52 0.11 0.02 Sum-B5-Sum-CN À5.00 À3.66 À1.35 0.11 0.01 injection and large barrier to electron injection in to the Cor-B5-Cor-CN2 À5.96 À4.27 À1.30 0.11 0.04 conduction band of Au electrode. The sparsely filled area of Sum-B5-Cor-CN2 À5.74 À4.26 À1.48 0.12 0.05 Fig. 2 represents the compounds which showed hole transport Sum-B5-Sum-CN2 À5.56 À4.26 À1.30 0.12 0.04 property. The C1B1 bowl with HOMO À5.87 eV and LUMO À2.30 eV energy levels showed high electron and hole injection and LUMO orbital energies of acene fused bowls clustered in barrier with Au source electrode. As a result this bowl cannot the range of À4.49 eV to À5.87 eV and À2.30 eV to À3.14 eV exhibit either electron transport or hole transport properties. respectively. It is clear from the Table 1 that the corannulene The above analysis based on frontier molecular orbital energies fused bowls showed lower HOMO energies than sumanene reveals that the HOMO and LUMO energy levels decrease with fused bowls. On the contrary, sumanene fused bowls showed size of acenes and make these bowls prominent hole transport lower LUMO energies than corannulene fused bowls. It is materials. obvious from the above discussion that the corannulene com- The performance of organic light-emitting diodes (OLEDs) pounds showed higher HOMO energies and lower LUMO device depends on the charge injection, transfer, and balance energies than sumanene fused bowls. The reason may be the as well as the exciton confinement in a device. So it is important

difference in conjugation, particularly, the CQC bonds have to investigate the IE, EA, and reorganization energy (lÆ), which impact on the resonance effect, and the resonance effects can be used to evaluate the energy barrier for the injection of usually increased with an increase in the number of conjugated holes and electrons, and charge transfer (or transport) rate.

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. CQC bonds in these compounds. Thus, it is understandable From previous studies it is clear that the B3LYP method with that the more number of CQC bonds in corannulene increased double zeta basis set is a reliable theory to reproduce the EAs of HOMO energy and decreased LUMO energy. With increase in polycyclic aromatic hydrocarbons.43,44 In this study we have the size of acenes from B1 to B5, LUMO energy levels and HLG used triple zeta 6-311+G(d) basis set to calculate the EA of acene decreased consistently and HOMO orbital energies also fused buckybowls to get the more accurate results. Recently Liu decreased with the size of acenes however, the decrease is not and co-workers showed the adiabatic ionization energy (AIE) of consistent. The lower LUMO orbitals decreased the electron air-stable p-channel OFET materials are in the range of 5.68 eV injection barrier with source drain Au electrode and facilitate to 6.78 eV, and the adiabatic electron affinity (AEA) of air-stable efficient electron transport property in semiconducting materials. n-channel OFET materials have 2.41 eV to 3.14 eV.45 We have plotted 54 molecules in the Fig. 2 as the function of Chang et al. have established a correlation between calcu- carrier type with frontier molecular orbital energy levels. The lated adiabatic EA and air stability and they observed the HOMO and LUMO energy represented as black and crossed threshold value for air stability was ca. 2.80 eV.46 Table 2 and squares respectively. A dotted line at À5.60 eV and another line Fig. 3 depicts the adiabatic and vertical ionization potentials at À3.15 eV show the barrier for HOMO and LUMO energy levels and electron affinities of acene fused buckybowls at B3LYP/6- respectively to group these bowls in to various charge carrier type 311+G(d)//B3LYP/6-31G level of theory. Cursory look at the materials. From Fig. 2 it is obvious that acene fused sumanene Table 2 shows that the AIE and VIE values are clustered in and corannulene (except C1B1) bowls showed higher HOMO the range of 6.89 eV to 5.34 eV and 6.93 eV to 5.38 eV energies than À5.60 eV and LUMO energies higher than respectively. The VIE values are slightly higher (B0.05 eV) than À3.15 eV. With Au electrodes, these compounds can exhibit AIE values. The ionization energies decreased with increase in

5042 Phys. Chem. Chem. Phys., 2013, 15, 5039--5048 This journal is c the Owner Societies 2013 View Article Online PCCP Paper

Table 2 Adiabatic electron affinity and ionization energy (AEA and AIE respec- tively) and vertical electron affinity and ionization energy (VEA and VIE respec- tively) of acene fused buckybowls at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory (all values are given in eV)

Name AIE VIE AEA VEA Cor-B1-Cor 6.89 6.93 1.29 1.26 Sum-B1-Cor 6.42 6.47 1.25 1.24 Sum-B1-Sum 6.03 6.08 1.35 1.34 Cor-B2-Cor 6.55 6.60 1.50 1.47 Sum-B2-Cor 6.14 6.18 1.56 1.53 Sum-B2-Sum 5.81 5.86 1.63 1.60 Cor-B3-Cor 6.25 6.30 1.75 1.72 Sum-B3-Cor 5.90 5.94 1.82 1.79 Fig. 4 The graph shows the relationship between air stability and adiabatic Sum-B3-Sum 5.63 5.67 1.92 1.90 electron affinity of acene fused buckybowls and there boron and nitrogen doped Cor-B4-Cor 6.00 6.04 1.99 1.96 bowls at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory. Sum-B4-Cor 5.75 5.80 2.02 1.99 Sum-B4-Sum 5.47 5.51 2.08 2.06 Cor-B5-Cor 5.78 5.82 2.19 2.17 Sum-B5-Cor 5.54 5.57 2.23 2.21 Sum-B5-Sum 5.34 5.38 2.25 2.23 Cor-B5-Cor-F 6.00 6.07 2.42 2.39 Sum-B5-Cor-F 5.76 5.82 2.45 2.42 Sum-B5-Sum-F 5.56 5.62 2.47 2.45 Cor-B5-Cor-CN 6.38 6.42 2.90 2.87 Sum-B5-Cor-CN 6.10 6.14 2.85 2.81 Sum-B5-Sum-CN 5.86 5.90 2.80 2.76 Cor-B5-Cor-CN2 6.84 6.88 3.48 3.49 Sum-B5-Cor-CN2 6.59 6.63 3.46 3.47 Sum-B5-Sum-CN2 6.38 6.43 3.44 3.45

Fig. 5 The reorganization energy, hole transport (l+) and electron transport (lÀ) size of acene ring from B1 to B5 fused bowls. Based on the of acene fused buckybowls at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory. small difference and similar trends between AIE and VIE energies, we have considered AIE values for further discussions. Quick observation of these values shows that B4, B5 fused in the charge-carrier transporting layers can be portrayed as compounds showed slightly lower AIE values than B1, B2 and the electron-transfer or hole-transfer reactions between the B3 fused compounds. The hole transport materials except neighboring molecules. The lower reorganization energy shows C2B4, C3B5 and C2B3 showed higher IE values than hole higher charge transfer rate. Table 1 and Fig. 5 depict the transport air stable threshold IE (5.70 eV). The higher ioniza- electron transport (lÀ) and hole transport (l+) values of acene tion values of these compounds are essential for the efficient fused buckybowls. Reorganization energies of these acene hole transport property and to increase the stability of radical fused bowls are clustered in the range of 0.10 eV to 0.15 eV cations. and 0.01 eV to 0.04 eV of l+ and lÀ values respectively. The The two widely used theories for describing charge mobility reorganization energy of these bowls is clustered in the small

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. 47–49 in organic materials are band theory and hopping model. range and from B1 to B5 fused bowls lÆ reorganization energies The hole and electron transport process at the molecular level decreased consistently. The electron transport lÀ values are less than hole transport l+ values in these bowls. It is obvious from the Table 1 that as the acene size increases from B1 to B5

the lÆ values are decreased. The small reorganization energies of these bowls facilitate efficient electron and hole transport properties between neighboring materials. The above analysis reveals that the acene fused bowls with higher HOMO orbital B energies (4 À5.60), lower l+ reorganization energies ( 0.10 eV) and higher ionization energies can exhibit efficient hole trans- port semiconductor properties.

3.2 Ambipolar materials Most organic semiconductors (OSC) exhibit only p-channel properties under ambient conditions when they are fabricated as OFETs. For the purpose of constructing a device with a large Fig. 3 Adiabatic and vertical ionization energies (AIE and VIE respectively) and adiabatic and vertical electron affinity (AEA and VEA respectively) of acene fused noise margin and low power dissipation, both p- and n-channel buckybowls and there boron and nitrogen doped bowls at B3LYP/6-311+G(d)// transistors are necessary for a complementary circuit. The best B3LYP/6-31G level of theory. way to achieve such devices is to fabricate the complementary

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circuit using a single ambipolar OSC (i.e., transporting both Table 3 Energies of HOMO, LUMO orbital’s and HOMO–LUMO energy gap holes and electrons). (HLG) of boron and nitrogen doped acene fused buckybowls at B3LYP/6- 311+G(d)//B3LYP/6-31G level of theory (all values are given in eV) In terms of both injection barrier and charge-transfer mobility,

molecular design towards ambipolar organic semiconductors Str. HOMO LUMO HLG l+ lÀ should aim for compounds with both low LUMO and high HOMO Cor-B1-Cor-B À5.17 À4.90 À0.27 0.01 0.19 energies, high electron affinity and low ionization potential, Cor-B2-Cor-B À5.11 À4.87 À0.24 0.01 0.07 and small intrinsic electron and hole reorganization energies in Cor-B3-Cor-B À5.06 À4.83 À0.23 À0.03 0.04 Cor-B4-Cor-B À4.98 À4.77 À0.21 À0.06 0.09 nature. Functionalizing p-type semiconductors, including Cor-B5-Cor-B À4.93 À4.73 À0.20 0.02 0.06 replacing the functional atoms and substituting the hydrogen Sum-B1-Cor-B À4.98 À4.61 À0.37 0.01 0.09 atoms with electron withdrawing groups, has been revealed as Sum-B1-Sum-B À4.81 À4.35 À0.46 0.00 0.08 Sum-B2-Cor-B À4.91 À4.58 À0.32 À0.01 0.08 one of the most effective methods in tuning the semiconducting Sum-B2-Sum-B À4.73 À4.33 À0.40 0.00 0.07 property from p-type to n-type or to ambipolar semiconductors. Sum-B3-Cor-B À4.84 À4.55 À0.29 0.01 0.07 In this study we have used one of the most effective Sum-B3-Sum-B À4.67 À4.31 À0.36 0.01 0.06 Sum-B4-Cor-B À4.77 À4.44 À0.33 0.01 0.05 strategies to tune the semiconducting property of hole trans- Sum-B4-Sum-B À4.61 À4.29 À0.32 0.01 0.06 port semiconductors such as substituting the hydrogen atom Sum-B5-Cor-B À4.74 À4.38 À0.36 0.03 0.06 with electron withdrawing group. We have used electron with- Sum-B5-Sum-B À4.55 À4.27 À0.28 0.00 0.10 Cor-B1-Cor-N À3.34 À3.01 À0.34 À0.04 0.09 drawing F and CN groups as substituent on B5 fused bowls, Cor-B2-Cor-N À3.34 À3.05 À0.29 À0.05 0.06 which showed higher HOMO and lower LUMO values than Cor-B3-Cor-N À3.35 À3.09 À0.26 À0.06 0.03 other bowls to gauge the effect of substituting hydrogen atom Cor-B4-Cor-N À3.38 À3.14 À0.24 À0.05 0.03 Cor-B5-Cor-N À3.40 À3.18 À0.22 À0.03 0.14 with electron withdrawing groups. We have substituted two Sum-B1-Cor-N À3.13 À2.71 À0.42 0.03 0.22 electron withdrawing groups on each bowl of B5 fused bowls. Sum-B1-Sum-N À2.95 À2.44 À0.52 À0.05 0.11 Table 1 depicts HOMO, LUMO orbital energies, HLG and Sum-B2-Cor-N À3.15 À2.77 À0.38 À0.05 0.11 Sum-B2-Sum-N À3.00 À2.51 À0.49 À0.05 0.12 reorganization energies of electron withdrawing F and CN Sum-B3-Cor-N À3.19 À2.83 À0.36 À0.06 0.06 substituted B5 fused bowls. In F substituted bowls HOMO/ Sum-B3-Sum-N À3.05 À2.59 À0.46 À0.05 0.12 LUMO energies are clustered in the range of À4.47 eV to À5.13 eV Sum-B4-Cor-N À3.30 À2.91 À0.39 À0.05 À0.24 Sum-B4-Sum-N À3.10 À2.69 À0.41 À0.05 0.12 and À5.00 eV to À5.50 eV respectively. The HOMO/LUMO Sum-B5-Cor-N À3.25 À2.93 À0.31 À0.03 À0.25 energies of CN substituted compounds are clustered in range of Sum-B5-Sum-N À3.14 À2.77 À0.37 0.05 0.10 À3.33 eV to À3.38 eV and À3.66 eV to À3.74 eV respectively. The HOMO orbital energies are slightly lowered in F substituted bowls

than CN substituted buckybowls (À5.60 eV). However these bowls 0.15 eV and 0.01 eV to 0.03 eV for l+ and lÀ respectively. In can show efficient hole transport property with a Au source this study the CN substituted bowls have lower reorganization electrode. As we expected LUMO energies are decreased signifi- energies than F substituted bowls and reorganization energy cantly, which is lower than À3.16 eV. The lower LUMO energies values are smaller than unsubstituted bowls. The electron lower the electron injection barrier and make these molecules withdrawing substituted B5 fused bowls with HOMO energy 4 electron transport (n-type) semiconductors. The above analysis À5.60 eV, LUMO energy o À3.16 eV, lower reorganization shows that substitution of F and CN groups lowered frontier energiesandhigherelectronaffinityvaluesthesebowlscanbe molecular orbital energies. As a result electron and hole transport prominent materials for electron and hole transport properties

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. barriers lowered with source drain electrode. Adiabatic and (i.e. ambipolar). vertical ionization energies of electron withdrawing F and CN To gauge the effect of doping on acene fused buckybowls we substituted bowls are clustered in the range of 5.56 eV to 6.38 eV have undertaken an impressive study of boron and nitrogen and 5.62 eV to 6.42 eV respectively (Table 2). Upon doping of doped bowls. Table 3 depicts the energy of HOMO, LUMO bowls ionization energies are increased. However, the increase in orbitals and HLG of boron and nitrogen doped buckybowls. It F substituted bowls is less than CN substituted bowls. To allow is clear from Table 3 that the HOMO orbital energies of boron efficient electron injection from metal electrodes, it has been doped bowls lower than undoped bowls and these energies are suggested that the electron affinity value should lie within the clustered in the range of À5.17 eV to À4.55 eV. The LUMO rangeof3.00eVto4.00eV.50 It is obvious from the Table 2 that orbital energies of boron doped bowls are decreased drastically the substitution of electronegative groups (CN and F) on B5 fused and fall in the range of À4.50 eV to À5.00 eV. Doped bowls have bowls increased the electron affinity than unsubstituted bowls. higher HOMO orbital energies and lower LUMO energies than Theelectronaffinityvaluesofthesebowlsareclusteredinthe undoped bowls. range of 2.42 eV to 2.90 eV (AIE). The CN substitution increased On the other hand the nitrogen doped bowls have shown the EA values in these compoundswheretheEAvaluesarealmost remarkably higher HOMO orbital energies and higher LUMO reached up to B2.90 eV. The higher EA values of these com- orbitals (À2.95 eV to 3.40 eV and À2.44 eV to 3.18 eV respectively). pounds allow the electron transport efficiently from the metal As explained in above sections Fig. 2 depicts the correlation of electrodes. Substitutions of electron withdrawing F and CN HOMO and LUMO orbital levels with transport properties of groups on B5 fused bowls have lower reorganization energy values doped bowls with source Au electrode. It is obvious from Fig. 2 than unsubstituted bowls. They are in the range of 0.10 eV to that the boron doped bowls show ambipolar properties with lower

5044 Phys. Chem. Chem. Phys., 2013, 15, 5039--5048 This journal is c the Owner Societies 2013 View Article Online PCCP Paper

LUMO (o À3.16 eV) and higher HOMO (4 À5.16 eV) orbital The EA values are increased with the size of acenes from B1 energies. to B5 in nitrogen and boron doped bowls. The boron doped Compared to undoped bowls these compounds can show structures with higher EA values more than B3.00 eV allow an efficient electron and hole transport properties with lower efficient electron transport in to the metal electrode. Unlike EA, HOMO and LUMO values which lowers the electron and hole IE of doped bowls lesser than undoped bowls and the IEs of transport injection barrier with the metal electrode. The above these bowls are clustered in the range of 3.69 eV to 6.10 eV. results show that doped bowls, in particular boron doped Here the VIE values are slightly higher than the AIE values bowls, can be promising materials for ambipolar organic by B0.04 eV. In nitrogen doped bowls IE values decreased semiconductors with their higher HOMO and lower LUMO gradually, they are in the range of 3.87 eV to 4.28 eV. Nitrogen energies. Here we can observe exceptionally lower HLG in doped bowls showed lower IE values than undoped and boron boron and nitrogen doped bowls (o0.5 eV). doped values. In all doped bowls the IE values are decreased as In recent years molecules with exceptionally small (o0.5 eV) we go from B1 to B5 fused bowls. The higher IE values increase HLG have reported a variety of unusual optoelectronic properties the stability of radical cations and facilitates the efficient hole such as intrinsic conductivity, infrared electrochromic displays, transport property in these bowls. charge-storage capability and electron-transfer phenomena have The reorganization energies of boron and nitrogen doped already been demonstrated for these compounds, making them compounds are showed in Fig. 5 and Table 3 depicts very desirable targets for further physical studies and electronics the reorganization energy of boron and nitrogen doped com- applications.51 Doping increased the EA values drastically in pounds. A cursory look at these values shows that electron

boron doped bowls as compared to undoped bowls. Table 4 shows transport (lÀ) values are slightly higher than the hole transport

the AEA and VEA values of boron and nitrogen doped buckybowls. (l+) values. The l+ values clustered in the range of 0.01 eV to

TheborondopedEAvaluesclusteredintherangeof3.37eVto 0.03 eV and lÀ values in the range of 0.04 eV to 0.19 eV. The l+ 5.43 eV. All boron doped bowls showed the EA values in the range values are lower in boron doped compounds by 0.08 eV to of 3.00 eV to 4.00 eV. Unlike boron doped bowls, nitrogen doped 0.18 eV compared to undoped bowls. The lower reorganization bowls have EA values which are clustered below B2.60 eV. Here values of boron doped compounds make these compounds an the boron doped bowls showed increase in EA values by 1.28 eV to efficient hole and electron transport (i.e. ambipolar) materials. 3.87 eV compared to undoped acene fused bowls. The above results shows that boron doped bowls with HOMO energy (4 À5.60 eV), LUMO energy (o À3.16 eV), lower reorga- nization energies and higher electron affinity (43.00 eV) Table 4 Adiabatic electron affinity and ionization energy (AEA and AIE respec- values, they can behave as efficient ambipolar semiconductors. tively) and vertical electron affinity and ionization energy (VEA and VIE respec- tively) of boron and nitrogen doped acene fused buckybowls at the B3LYP/6- 3.3 Electron transport materials 311+G(d)//B3LYP/6-31G level of theory In terms of frontier molecular orbital energy, high electron Str. AEA VEA AIE VIE affinities and small reorganization energies, CN substituted Cor-B1-Cor-B 3.94 3.85 6.10 6.13 pentacene fused bowls are the most promising candidates for Cor-B2-Cor-B 3.99 3.26 5.85 5.88 hole and ambipolar semiconducting materials in this study. Cor-B3-Cor-B 4.04 4.01 5.52 5.56 However, for efficient electron transport (n-type) materials Cor-B4-Cor-B 4.08 4.06 5.24 5.28 Cor-B5-Cor-B 5.43 5.41 4.69 3.72 electron affinity should be higher than 3.00 eV and HOMO

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. Sum-B1-Cor-B 3.65 3.62 5.82 5.86 energies lower than À5.60 eV. To increase the electron affinities Sum-B1-Sum-B 3.37 3.36 5.65 5.68 and lower LUMO orbital energies of acene fused bowls we have Sum-B2-Cor-B 3.68 3.65 5.63 5.69 Sum-B2-Sum-B 3.42 3.40 5.47 5.50 substituted additional two CN groups on each bowl of B5 fused Sum-B3-Cor-B 3.70 3.68 5.40 5.43 bowls. The substitution of additional CN groups increased Sum-B3-Sum-B 3.45 3.43 5.31 5.34 electron affinities up to 3.48 eV. The LUMO energies also Sum-B4-Cor-B 3.62 3.60 5.34 5.37 Sum-B4-Sum-B 3.48 3.46 5.16 5.19 decreased from À3.14 eV to À4.27 eV of these bowls by Sum-B5-Cor-B 3.61 3.59 5.30 5.30 substitution of additional CN groups on B5 fused bowls. The Sum-B5-Sum-B 3.53 3.49 5.02 5.06 dicyano substituted C1B5CN2 and C2B5CN2 have lower LUMO Cor-B1-Cor-N 2.05 2.01 4.28 4.37 Cor-B2-Cor-N 2.17 2.14 4.20 4.29 values À4.27 eV and À4.26 eV respectively and higher HOMO Cor-B3-Cor-N 2.30 2.29 4.15 4.23 values À5.96 eV and À5.70 eV respectively. The C1B5CN2 and Cor-B4-Cor-N 2.45 2.44 4.15 4.22 C2B5CN2 compounds exhibit only electron transport (n-type) Cor-B5-Cor-N 2.62 2.50 4.14 4.20 Sum-B1-Cor-N 1.90 1.82 4.01 4.12 property, which was attributed to a lower electron injection Sum-B1-Sum-N 1.74 1.67 3.87 3.96 barrier and higher hole injection barrier. As we expected the Sum-B2-Cor-N 1.98 1.92 3.99 4.08 substitution of electron withdrawing CN group further lowered Sum-B2-Sum-N 1.94 1.85 3.87 3.95 Sum-B3-Cor-N 2.08 2.04 3.97 4.05 LUMO energy level and tune the barrier of electron transport Sum-B3-Sum-N 2.11 2.02 3.88 3.95 mechanism which behave as electron transport materials. The Sum-B4-Cor-N 2.01 2.28 4.07 4.14 small reorganization energy and higher electron affinity are Sum-B4-Sum-N 2.28 2.18 3.88 3.95 Sum-B5-Cor-N 2.25 2.52 4.01 4.06 essential properties for a material to be an n-type semiconductor. Sum-B5-Sum-N 2.42 2.32 3.90 3.96 To allow efficient electron injection from metal electrodes, the

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electron affinity should be in excess of 3.00 eV and an upper limit for the EA value of 4.00 eV has been suggested. Devices constructed from systems with higher EA will hardly be stable towards environmental reductants under ambient conditions.50 The C1B5CN2 and C2B5CN2 compounds showed EA values 3.49 eV and 3.47 eV respectively, which are higher than 3.00 eV. The higher electron affinity of these bowls facilitates electron injection easily from the metal electrodes. The above results shows that lower LUMO energies, higher EA and lower

lÀ reorganization energies make these bowls efficient electron transport materials. Fig. 6 This graph shows the comparison of various properties of bowl and flat 3.4 Air stability structures at B3LYP/6-311+G(d)//B3LYP/6-31G level of theory. Air stability of organic semiconductors is an important issue. The charge carrier of organic semiconductors should be stable structures are showed the lower HOMO values by 0.11 eV, 0.17 eV

enough against ambient oxidants such as O2 and H2O. It has and0.16eVinC50H22,C54H24 and C58H26 respectively. It is clear been suggested that organic semiconductors (OSCs) with large from above results that as molecular weight increases the HOMO electron affinity will possess air stable charge carriers. The orbital energies are increased. The HOMO orbital energies are correlation between EA and air stability has been established the important criteria in the organic semiconductors, which is by DFT and the threshold value observed as 2.80 eV.46 The responsible for hole transport properties. On other hand the relative positions of frontier molecular orbitals with respect LUMO orbital energies are remarkably lower in bowl structure

to H2O and O2-oxidation–reduction reactions determine the than flat structure. The LUMO energies are lowered by 0.24 eV to ambient stabilities of the electrons–holes during the charge 0.53 eV compared to flat structures. The lower LUMO orbital transport process. The LUMO energy of À4.00 eV is essential to energies are the important property to achieve lower electron stabilize electrons during charge transport in electron transport injection barrier. As a result lower LUMO energy compounds materials. Fig. 4 shows the correlation between air stability of become efficient electron transport materials. The above analysis bowls with adiabatic electron affinity and LUMO energy. It is reveals that introduction of curvature (flat to bowl) decreases the obvious from the Fig. 4 that the CN and dicyano substituted B5 frontier molecular orbital energy levels and make these com- fused compounds showed air stability with EA values greater pounds better hole and electron transport materials. This can be than 2.80 eV. On the basis of LUMO energy dicyano substituted important criteria to lower the HOMO and LUMO values in bowls shows air stability with LUMO energies higher than organic semiconductors. The performance of OLED device À4.00 eV. The above analysis shows that CN and dicyano depends on the charge injection, transfer, and balance as well substituted B5 fused buckybowls which showed higher EA as the exciton confinement in a device. Thus, the IE, EA, and

energies and lower LUMO energies can be stable in ambient reorganization energies (lÆ) are important properties, which can conditions. It is obvious from the Fig. 4 that boron doped be used to evaluate the energy barrier for the injection of holes compounds shows good air stability with higher EA values and electrons, and the charge transfer (or transport) rate and more than 2.8 eV and LUMO energy lower than À4.00 eV. The balance. Herein we have also compared the IE, EA and reorga-

Published on 23 January 2013. Downloaded by Indian Institute of Chemical Technology (IICT), Hyderabad 22/01/2014 09:32:17. nitrogen doped compounds have EA values lower than 2.80 eV nization energy of coronene and bowl structures. Interestingly and LUMO orbital energies also higher than À4.00 eV. The the IE and EA values are higher in bowl structure. With increas- above analysis reveals that the higher EA and lower LUMO ing molecular weight EA values are increased and IE values are values make CN, dicyano substituted bowls and boron and decreased. The IE values are higher in bowls by 0.15 eV to 0.75 eV. nitrogen doped bowls air stable n-channel and ambipolar The EA values are higher in bowls by 0.16 eV to 0.51 eV. The bowl

semiconductors. structures showed higher hole transport (l+)valuesandlower

electron transport (lÀ) values. Overall the above analysis shows 3.5 Bowl vs. flat structure that curvature lowers the frontier molecular orbital energies and To gauge the effect of curvature of bowls on various properties, reorganization energies as well as it increase the EA and IE values. we have undertaken an intensive study to compare bowls with Lower orbital energies, reorganization energies and higher IE, EA flat coronene structures. We have compared bowl and flat values are important criteria to make good hole and electron structures which have same molecular formula such as transport materials. Size of the system seems to be important in 52,53 C50H22 (C1B3 and COB2CO), C54H24 (C1B4 and COB2CO) and modulating the properties of the system.

C58H26 (C1B5 and COB3CO). Fig. 6 shows the comparison of various properties of bowl and flat structures. We have compared various important properties such as frontier mole- 4. Conclusions cular orbital energies, electron affinity, ionization energy and reorganization energy of bowls. The HOMO orbital energies are Density functional theory calculations were carried out to higher in bowl compounds by 0.10 eV to 0.29 eV. The flat investigate a series of acene fused buckybowls. The dependence

5046 Phys. Chem. Chem. Phys., 2013, 15, 5039--5048 This journal is c the Owner Societies 2013 View Article Online PCCP Paper

of HOMO and LUMO energy levels, ionization potential, 13 T. C. Dinadayalane and G. N. Sastry, Tetrahedron, 2003, electron affinity, reorganization energy for both hole and 59, 8347. electron transport mobility has been systematically studied at 14 D. Umadevi and G. N. Sastry, J. Phys. Chem. C, 2011, B3LYP/6-311+G(d)//B3LYP/6-31G level of theory. The acene 115, 9656. fused compounds (except C1B1) are revealed to act as hole 15 D. Umadevi and G. N. Sastry, J. Phys. Chem. Lett., 2011, transport organic semiconductors due to their high electron 2, 1572. injection barrier relative to the work function potential of Au 16 T. C. Dinadayalane and G. N. Sastry, Tetrahedron Lett., 2011, electrodes. The electron withdrawing F and CN substituted B5 42, 6421. fused compounds showed both hole and electron injection 17 G. N. Sastry, E. D. Jemmis, G. Mehta and S. R. Shah, J. Chem. barriers with their high HOMO energy level and low LUMO Soc., Perkin Trans. 2, 1993, 1867. energy level. The C1B5CN2 and C2B5CN2 compound showed 18 D. Vijay, H. Sakurai, V. Subramanian and G. N. Sastry, Phys. lower LUMO (À4.26 eV and À4.27 eV) and HOMO (À5.72 eV Chem. Chem. Phys., 2012, 14, 3057. and À5.96 eV) energy levels which make these compounds as 19 U. D. Priyakumar and G. N. Sastry, J. Org. Chem., 2001, efficient n-type semiconductors. The higher ionization poten- 66, 6523. tials and electro negative values allow efficient electron injec- 20 D. Eisenberg, A. S. Filatov, E. A. Jackson, M. Rabinovitz, tion and hole transport in ambipolar compounds. The lower M. A. Petrukhina and L. T. Scott, J. Org. Chem., 2008, reorganization values show that these compounds are efficient 73, 6073. electron and hole transport materials. The higher electron 21 A. S. Filatov, L. T. Scott and M. A. Petrukhina, Cryst. Growth affinity and lower LUMO energy of CN substituted compounds Des., 2010, 10, 4607. make these bowls air-stable. Boron doped bowls have shown 22 M. Bendikov, F. Wudl and D. F. Perepichka, Chem. Rev., extremely lower HOMO and LUMO energy levels and exceptionally 2004, 104, 4891. small HOMO–LUMO gap revealed to act as ambipolar semi- 23 C. R. Newman, C. D. Frisbie, D. A. da Silva Filho, conductors. All the above results conclude that acene fused J. L. Bredas, P. C. Ewbank and K. R. Mann, Chem. Mater., buckybowls are good p-type and ambipolar materials. Particu- 2004, 16, 4436. larly the C1B5CN2 and C2B5CN2 bowls appear to be prominent 24 J. L. Bredas, D. Beljonne, V. Coropceanu and J. Cornil, materials for air stable n-type semiconductors. Chem. Rev., 2004, 104, 4971. 25 M. L. Tang, A. D. Reichardt, P. Wei and Z. Bao, J. Am. Chem. Soc., 2009, 131, 5264. Acknowledgements 26 A. Murphy and J. Frechet, Chem. Rev., 2007, 107, 1066. 27 T. W. Kelley, D. V. Muyres, P. F. Baude, T. P. Smith and We thank CSIR for financial support. UP thanks CSIR, New T. D. Jones, MRS Online Proc. LIbr., 2003, 771, 169. Delhi for senior research fellowship. 28 Y. Sakamoto, T. Suzuki, M. Kobayashi, Y. Gao, Y. Fukai, Y. Inoue, F. Sato and S. Tokito, J. Am. Chem. Soc., 2004, References 126, 8138. 29 Y. Sakamoto, T. Suzuki, M. Kobayashi, Y. Gao, Y. Inoue and 1 A. Hirsch, Nat. Mater., 2010, 9, 868. S. Tokito, Mol. Cryst. Liq. Cryst., 2006, 444, 225. 2 A. A. Correa, S. A. Bonev and G. Galli, Proc. Natl. Acad. Sci. 30 R. C. Haddon, A. S. Perel, R. C. Morris, T. T. M. Palstra,

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