Abstract:

We present a theoretical investigation of the electronic structure of some derivatives of poly-thiophene (PT), mainly poly (3-methylthiophene) (PMT), poly (3-ethylthiophene) (PET), poly(3-methoxythiophene) (PMOT), poly(3-cyanothiophene)(PCNT), poly (3,4-dimethyl thiophene) (PDMT), poly (3,4-dimethoxy thiophene)(PDMOT), poly (3,4-dicyanothiophene) (PDCNT), poly (3-methoxy-4-methylthiophene)(PMOMT), poly (3-methyl-4-cyanothiophene) (PMCNT) and poly (3-methoxy-4-cyanothiophene) (PMOCNT), by using SCF-CNDO/2-CO method. The goal is to discuss the influence of these substituents on the band structure of PT.
The calculated results showed that all of the 3-substituted PTs have coplanar configurations while the 3,4-disubstituted PTs will be nonplanar if the two substituents have a large steric hindrance (such as PDMT). From the band structures of these substituted PTs, we got the conclusions as follows:
1. The substitution of H atom by electron-donating or electron-withdrawing groups will make Eg narrower and the Ess of 3,4-disubstituted PTs are even narrower than those of 3-substituted PTs.
2. When the two substituents of the 3,4-disubstituted PTs have a large steric interaction with the S atoms of the neighbouring rings, such as poly (3,4-dimethylthiophene),its energy gap (Eg) will be much wider.
3. As the H atoms of 3 or 3, 4-position of PT are substituted by electron-donating groups, both of the I.P. and E.A. become lower, whereas by electron-withdrawing group, both of them become higher. So it is expected that the electron-donating group substituted PTs are favourable to p-type doping while the electron-withdrawing group substituted PTs to n-type doping.

Key words: Poly-substituted thiophene, Effect of substituents, Self consisitent field crystal orbital, Electronic structure