PICOSECOND SPECTROSCOPIC STUDIES ON THIOPHENE OLIGOMERS IN SOLUTION - SIZE-DEPENDENT EFFECTS

Thiophene oligomers with different chain length (n = 3 - 5) solved in CH2Cl2 have been excited by UV light pulses (351 nm, 6 ps, 100 muJ). Transient absorption spectra of different kinetic behaviour were observed between 400 and 900 nm by picosecond excite and probe spectroscopy. Moreover, the fluorescence kinetics and spectra have been studied. The oligomers exhibit transient absorption bands which follow the same kinetics as the fluorescence with lifetimes in the ps-region, increasing with the chain lengths. These bands we assign to an absorption from the primary excited state. Moreover, at oligomers with 4 and 5 rings bands with long lifetimes compared with the studied time scale of < 1 ns were found which shift to longer wavelengths with increasing number of rings in the oligomer, too. Similar spectra have been observed in doped thiophene oligomers [1] and, therefore, may be assigned to charged species.