Infrared and Raman spectra of neutral polythiophene were analyzed on the basis of the data of oligomers (α-bithiophene, α-terthiophene, α-quaterthiophene, and α-sexithiophene) and deuterated and 2,5-13C-substituted polythiophenes. Vibrational assignments were confirmed including those of key bands useful in deriving the degree of polymerization (Ndp), the average number of s-trans conjugated successive rings (Nc), and the amount of distorted part (kink) relative to that of conjugated part (Ak). It is possible to estimate Ndp, Nc, and Ak from the vibrational spectra of a neutral film. Among ten films prepared under different conditions, the one which showed the highest electrical conductivity (2.0×102 Scm-1) on maximum iodine doping exhibited the largest Nc (>6) and the smallest Ak but an intermediate (5th) Ndp (20). The film with the largest Ndp (35) happened to show Nc smaller and Ak larger than those of the above-mentioned film and the conductivity was 5.0×10 Scm-1. Hence, a small number of kinks (i.e., abundance of long conjugated segments) is indispensable for a polymer film to gain high electrical conductivity at the doped state.
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