Theoretical study on electronic properties of trans-oligomers of thiophenes, siloles and alternate thiophene and siloles

In this paper, the geometrical structures, electronic structures, absorption and emission spectra of three series of oligomers containing thiophenes, siloles as well as both thiophene and siloles alternately, respectively, are comparatively studied by ab initio, B3LYP and TDDFT methods with basis set 6-31G(d).The results show that the highest occupied orbitals (HOMOs), the lowest unoccupied orbitals (LUMOs) of these oligomers increase and decrease, respectively, as the molecular chain lengths increase. The HOMO-LUMO energy gaps and the lowest excitation energies of these oligomers are proportional to reciprocal of repeat units. Better descriptions of between the lowest excitation energies and molecular chains for studied oligomers are obtained by Kuhn equation. Their linear relationships are obtained, so the HOMO-LUMO energy gaps and the lowest excitation energies of corresponding polymers are obtained. The HOMO and LUMO energies, the vertical excited energies for the ground states and the first excited states of hybrid oligomers lie between thiophene and silole oligomers while oligomers contain the same number of units.