Excited state energy dynamics of poly(2,5-diethynylthiophene) copolymers with different effective conjugation lengths

The relaxation dynamics of photoexcitation of poly(2,5-diethynylthiophene) copolymers (PDETs) with different repeating unit numbers (n = 12, 16, 18) were investigated by steady-state fluorescence spectroscopy and picosecond time-resolved fluorescence decay measurements. All three copolymers presented two peaks in a fluorescence spectrum; however, the position and the intensity of each peak changed, depending upon polymer size. These two peaks were assigned as radiative relaxations from a self-trapped (polaronic) exciton state and a conformational kink state, in comparison with the results of polydiacetylenes and polythiophenes. A bathochromic shift in the fluorescence spectra occurred between the PDET with n = 12 and the PDET with n = 16, but it was negligible for the PDET's with n between 16 and 18. The ratio of the populational relaxation at the conformational kink state to the relaxation at the self-trapped (polaronic) exciton state is larger in the PDET with n = 12 than in PDETs with n = 16 or n = 18. From the results of iterative least-squares fitting of time-resolved fluorescence decay data at different emission wavelengths, we obtained the relaxation lifetimes of 139 ± 5 psec and 205 ± 3 psec for each state and fluorescence build-up times of 18 ± 2 psec and 51 ± 5 psec, depending on the size of the copolymers; values which are attributed to charge carrier recombination times.