Influence of the molecular structure on the refractive index of semiconducting di-β-alkylated sexithiophenes

The optical absorption of thin solid films of dialkylated sexithiophene derivatives (2A6T) has been measured at incident energies below the fundamental absorption edge, where these semiconducting organic materials are transparent. The absorption spectra present interference patterns that have been used for determining their refractive index n. A value of 1.904 was measured on unsubstituted sexithiophene. Alkyl substitution leads to a decrease of n, which is more pronounced as the alkyl chain length increases. This behavior is attributed to the corresponding decrease of the density of the material. Our data are compared to other experimental determinations of the optical parameters of thiophene oligomers and polymers published to date.