PCCP View Article Online PAPER View Journal | View Issue 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 pentacene 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 fullerene (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 fullerenes 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 Corannulene-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 molecules 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 coronene that any prospective n-channel material must have.23 Most compounds were compared with the bowl structures. single-molecule 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. benzene 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