KCS

Bithiophene, terthiophene, and their derivatives were optimized at the HF and MP2/6-311+G** levels using Gaussian 09 program. Optimized structure, geometric parameters, atomic charges, relative energies, tendency of HOMO and LUMO energies with increasing halosubstituent, and relative stability of isomers were investigated. Equilibrium structures of the halosubstituted bithiophene derivatives were optimized to anti- and syn-isomers. Anti-form was more stable. The stable structures of the halosubstituted terthiophene derivatives were optimized to (anti-,anti-), (syn-,anti-), and (syn-,syn-)isomer. (anti-,anti-)form was more stable than the others. With increasing halosubstituted group, dihedral angles of between two rings in the stable thiophene derivatives were approximately optimized to perpendicular. With increasing substituents, the HOMO energies of thiophene derivatives become to lower. HOMO-LUMO gaps of thiophene derivatives substituted with one to three halogen atoms were decreased, while the energies of thiophene derivatives substituted with four to six halogens were increased. The tendency of HOMO-LUMO gaps is similar to those of the UV-vis and PL spectra.