SIZE-DEPENDENT PROPERTIES OF OLIGOTHIOPHENES BY PICOSECOND TIME-RESOLVED SPECTROSCOPY

Thiophene oligomers nT with n = 2-6 repeat units were studied in solution upon excitation with UV pulses (349 nm, 100 mu J, 25 ps). Each nT showed transient absorption spectra S-n <-- S-I, induced fluorescence, and delayed appearing triplet absorption bands. The transient bands shift bathochrom with increasing repeat units n. Band shapes and extinction coefficients of the transient singlet and triplet bands were determined. The transient S-n <-- S-I absorption bands decay with a single-exponential function. The decay times agree with the corresponding fluorescence lifetimes of the nT. They increase from 51 ps for 2T to 1100 ps for 6T with the oligomer size. The triplet bands appear during the decay of the S-n <-- S-I bands. The spectral shift of transitions from the ground and excited singlet and triplet states can be excellently described by the extended FEMO model. Even average bond lengths of the conjugated chain of oligothiophenes at ground and excited states could be estimated using the FEMO model.