Theoretical design of n-type organic semiconducting materials containing thiazole and oxazole frameworks

The characteristics of molecular structure and charge transport of some new n-type organic semiconductors containing thiazole 1a-6a and oxazole 1b-6b frameworks and trifluoromethylphenyl as terminal groups were predicted using density functional theory (DFT) methods. The energy levels of HOMO and LUMO of these compounds are decreased when thiophene and furan units are replaced by thiazole and oxazole units, respectively. The same trend was observed when benzo[1,2-d:4,5-d']bisthiazole groups were replaced with benzo[1,2-d:4,5-d']bisthiazole-4,8-diones. The reorganization energies for electron of compounds are computed in a range of 0.21-0.37 eV, which is comparable to the value of 0.25 eV of well-known n-type semiconductors such as perfluoropentacene. Some important trends can be pointed out as follows: (i) replacing the core thiazolothiazole unit of compounds 1a and 2a by the larger core benzo[1,2-d:4,5-d']bisthiazole units of 3a and 4a decreases both reorganization energies for electron (λ(e)); (ii) the λ(e) values of compounds containing thiazole 2a, 4a, and 6a are smaller than those of compounds containing thiophene 1a, 3a, and 5a, respectively; (iii) there is no clear trend when replacing benzene rings of compounds 3a and 4a by quinone rings of 5a and 6a. The λ(e) values of 5 and 6 are only somewhat larger. The same trend is also found for compounds containing oxazole 1b-6b. The intermolecular charge transports in solid state of these compounds mainly occur among molecules in the same molecular layer, whereas intermolecular interactions between molecules in different molecular layers are very small. Generally, beside some experimentally reported molecules 1a-4a, the remaining molecules designed here are good candidates for n-type organic semiconducting materials with small reorganization energies for electron and low energy levels of LUMO.