ULTRAFAST RELAXATION IN CONJUGATED POLYMERS WITH LARGE OPTICAL NONLINEARITY

The ultrafast relaxation of photoexcitations in conjugated polymers, polydiacetylenes (PDAs), and polythiophenes (PTs), was studied by a pump-probe experiment 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 polythiophenes (P3MT(poly[3-methylthiophene]) and P3DT (poly[3-dodecylthiophene])). Various spectral changes due to ultrafast nonlinear optical processes are observed to appear very rapidly after or even before the pump pulse. They are perturbed free-induction decay, coherent interaction between the pump and probe (pump polarization coupling), optical (a.c.) Stark shift, induced phase modulation, and hole burning. At slightly longer delay times (100-200 fs) the formation of a self-trapped (ST) exciton from a free exciton is observed. The exponential formation times of the ST excitons are in the range 70-100 fs in PTs and 140-150 fs in PDAs. The time dependence of the induced absorbance change due to the ST excitons in nonfluorescent polymers(PDA-3BCMU and PDA-4BCMU in the blue phase) is approximately represented by a single-exponential function after long components due to the triplet exciton, polaron, and/or bipolaron have been subtracted. The exponential decay time constants of nonfluorescent PDAs are between 1 and 3 ps in the temperature region 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) deviate significantly from the single-exponential functions. A model of the relaxation of photoexcitations in the conjugated polymers is proposed to explain systematically all the experimental results. The main mechanism of the ultrafast radiationless relaxation is tunneling from the ST exciton to the ground state in the adiabatic potential description.