Thermochromism and optical absorption in Langmuir–Blodgett films of alkyl-substituted polythiophenes

Thermochromism and optical absorption in mono- and multilayers of Langmuir–Blodgett films of poly(3-alkylthiophenes), poly(3-octyl-2,2′-bithiophene), and poly(3′-octyl-2,2′;5′,2″-terthiophene) were studied. In sparsely alkylated polythiophenes the magnitude of the thermochromic shift was smaller than in poly(3-alkylthiophenes) and roughly proportional to the sidechain concentration. Results of Valence Effective Hamiltonian calculations were compared with the experimental results of thermochromism. A vibronic structure was found in the absorption spectra of Langmuir–Blodgett films at room temperature. The vibronic splitting in poly(3-hexylthiophene) was approximately 0.18 eV as previously has been observed in poly(3-alkylthiophenes) but in poly(3′-octyl-2,2′;5′,2″-terthiophene) it was 0.20–0.25 eV. The vibronic peaks stay approximately at constant energies and vanish at elevated temperatures.