Vibrational key bands and electrical conductivity of polythiophene

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 (N_dp), the average number of s-trans conjugated successive rings (N_c), and the amount of distorted part (kink) relative to that of conjugated part (A_k). It is possible to estimate N_dp, N_c, and A_k 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 N_c (>6) and the smallest A_k but an intermediate (5th) N_dp (20). The film with the largest N_dp (35) happened to show N_c smaller and A_k 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.