THERMOCHROMISM AND OPTICAL ADSORPTION IN LANGMUIR-BLODGETT-FILMS OF ALKYL-SUBSTITUTED POLYTHIOPHENES

Thermochromism and optical absorption in mono- and multilayers of Lang muir-Blodgett films of poly(3-alkylthiophenes), poly(3-octyl-2,2'-bith iophene), and poly(3'-octyl-2,2';5',2-terthiophene) were studied. In s parsely alkylated polythiophenes the magnitude of the thermochromic sh ift was smaller than in poly(3-alkylthiophenes) and roughly proportion al to the sidechain concentration. Results of Valence Effective Hamilt onian calculations were compared with the experimental results of ther mochromism. A vibronic structure was found in the absorption spectra o f 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 approxi mately at constant energies and vanish at elevated temperatures.