Electronic and geometric structures of oligothiophenes

Abstract

The semi-empirical MNDO (modified neglect of diatomic overlap) method was applied to investigate ultraviolet photoelectron spectra (UPS) and molecular geometries of some oligothiophenes: α 7, which is an α-linked septithiophene, α 3βα 3, in which the middle thiophene ring of septithiophene has β-linkages, and α 3Vα 3, in which the fourth thiophene ring of α 7 is replaced by a vinyl group. The reliability of the MNDO calculation was carefully investigated by a comparison with the observed molecular geometries and the observed UPS spectra of thiophene and 2,2′-bithiophene. The results of the MNDO calculations were also compared with those of the ab initio HF/STO-3G calculations. The observed UPS spectra of oligothiophenes were well reproduced by using Koopmans' theorem ionization energies with a Gaussian function of width of δ=0.6 eV. The β-linkage and the vinyl group act as an impurity in the polythiophene system. In the molecular geometry of α 3βα 3 optimized by MNDO, where a β-linked ring is perpendicular to the plane of normal α-linked parts (α 3), the π-band spectrum of α 3βα 3 could be reproduced by the superposition of the spectra of two α 3s and thiophene. Therefore, it is confirmed that the β-linkage prohibits the delocalization of π-electrons and divides the polymer chain into short π-segments. In the case of α 3Vα 3, in which two α 3s are connected by a vinyl having a trans double bond conformation, the molecular geometry according to the MNDO calculation was almost planar and the UPS spectrum was almost the same as that of normal α 7. The mixing of the vinyl group into the polythiophene system does not cause any drastic change in the π-electron system but changes its σ-electron system. Simulated UPS spectra accurately reproduced the difference between α 3Vα 3 and α 7. Since these molecules are good models for polythiophene (α n), the features of the UPS spectra and the molecular geometry proposed by the theoretical calculation can explain several experimental observations on polythiophene. Further, it is suggested that the method of polymerization strongly affects the electronic and electric properties of polythiophene, and the control of the polymerization process is very important for achieving good conductivity.