ULTRAFAST RELAXATION IN CONJUGATED POLYMERS

Ultrafast relaxation of photoexcitations in polydiacetylenes (PDA-3BCMU (poly[4,6-decadiyne-1,10-diol-bis(n-butoxycarbonylmethylurethane)] (blue phase), 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 polythiophenes (P3MT(poly[3-methylthiophene]) and P3DT(poly[3-dodecylthiophene])) was studied. The formation time of self-trapped (ST) exciton from free exciton ranges between 70 and 100 fs in PTs and between 120 and 150 fs in PDAs. The decay function of the induced absorption intensity due to the ST excitons in nonfluorescent polymers (PDA-3BCMU, PDA-4BCMU, and PDA-DFMP all in the blue phase) is nearly single exponential with decay time constant ranging between 1 and 3 ps between 10 and 300 K after long-lived components due to triplet exciton, polaron, and/or bipolaron being subtracted. Both the formation and decay times are only weakly temperature dependent. The decay kinetics of the ST exciton in fluorescent polymers (PDA-4BCMU in red phase, P3MT, and P3DT) are significantly deviated from the single-exponential functions. A model of the relaxation in the conjugated polymers is proposed to explain systematically all the experimental results, and the decay kinetics were explained by tunneling from the ST exciton potential to the ground state potential in the configuration space.