EXCITED-STATE ENERGY DYNAMICS OF POLY(2,5-DIETHYNYLTHIOPHENE) COPOLYMERS WITH DIFFERENT EFFECTIVE CONJUGATION LENGTHS

The relaxation dynamics of photoexcitation of poly(2,5-diethynylthioph ene) copolymers (PDETs) with different repeating unit numbers (n = 12, 16, 18) were investigated by steady-state fluorescence spectroscopy a nd picosecond time-resolved fluorescence decay measurements. All three copolymers presented two peaks in a fluorescence spectrum; however, t he position and the intensity of each peak changed, depending upon pol ymer size. These two peaks were assigned as radiative relaxations from a self-trapped (polaronic) exciton state and a conformational kink st ate, in comparison with the results of polydiacetylenes and polythioph enes. A bathochromic shift in the fluorescence spectra occurred betwee n 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 populationa l 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 dif ferent emission wavelengths, we obtained the relaxation lifetimes of 1 39 +/- 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 recombi nation times. (C) 1998 Elsevier Science Ltd. All rights reserved.