Ultrafast relaxation in conjugated polymers

Abstract

Femtosecond pump-probe spectroscopy has been studied for polydiacetylenes (PDA-3BCMU (poly[4,6-decadiyne-1,10-diol-bis(n-butoxycarbonylmethylurethane)] (blue phase)) and PDA-4BCMU (poly[5,7-dodecadiyne-1,12-diol-bis(n-butoxycarbonylmethylurethane)] (red and blue phases)), and PDA-DFMP (poly[1,4-bis(2.5-bistrifluoromethylphenyl)-1.3-butadiyne])) and poly-thiophenes (P3MT (poly(3-methylthiophene)) and P3DT (poly(3-dodecylthiophene))) using a CPM (colliding-pulse mode-locked) laser with pulse width of 100 fs. The decay kinetics of the induced absorption intensity due to the self-trapped (ST) excitons in nonfluorescent polymers (PDA-3BCMU and PDA-4BCMU in the blue phase) is approximately represented by a single-exponential function after long-life components due to triplet excitons, polarons and/or bipolarons are subtracted. The exponential formation and decay time constants of ST excitons in nonfluorescent PDAs range between 70 and 150 fs and 1 and 3 ps, respectively, at any temperature between 10 and 290 K. Both the formation and decay times are only weakly temperature dependent. The decay kinetics of the ST exciton in fluorescent polymers (PDA-4BCMU in the red phase, P3MT and P3DT) substantially deviates from the single-exponential function. An adiabatic potential curve model is proposed to explain this dynamic behavior of ST excitons. The ST excition formation corresponds to the spontaneous geometrical relaxation of the free exciton in the one-dimensional system. The decay channel of the ST exciton is the tunneling process from the potential curve of the ST exciton to that of the ground state.