Ultrafast Relaxation in Conjugated Polymers with Large Optical Nonlinearity

Abstract

Abstract The ultrafast relaxation of photoexcitations in conjugated polymers by pump-probe experiment was studied using a 100 fs amplified CPM (colliding pulse mode-locked) laser to clarify the detailed mechanism and temporal response of the optical nonlinearity of conjugated polymers with large third-order susceptibility. Samples used are 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 polythiophenes (P3MT (poly[3-methyl-thiophene]) and P3DT (poly[3-dodecylthiophene])). The time dependence 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 exciton, polaron, and/or bipolaron being subtracted. The exponential formation and decay time constants of nonfluorescent PDAs ranges between 70 and 150 ts and 1 and 3 ps, respectively, in the temperature region of 10 −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) are significantly deviated from the single-exponential functions. These dynamical behaviors of ST excitons were explained using an adiabatic potential model. The formation of ST exciton corresponds to the spontaneous geometrical relaxation of the free exciton in one-dimensional system. The decay channel of the ST exciton is the tunneling from the adiabatic potential curve of the ST exciton to that of the ground state.