Ultrafast optical response in polydiacetylenes and polythiophenes

Ultrafast optical response in the films of poly(3-dodecylthiophene) (P3DT) and blue-and red-phase polydiacetylenes (PDA-4BCMU) has been investigated by femtosecond absorption and picosecond luminescence spectroscopies. Several nonlinear optical processes, i.e., hole burning, Raman gain, inverse Raman scattering, and induced-frequency shift, have been observed. The relaxation processes from photoexcited free excitons to self-trapped excitons (STEs) has been observed. The time constant is estimated as 140±40 fs in the blue-phase PDA-4BCMU and 100±50 fs in P3DT. The generated unthermalized STEs thermalize with the time constant of about 1 ps. The STEs in the blue-phase PDA-4BCMU decay exponentially with lifetime of 1.6±0.1 ps at 290 K and 2.1±0.2 ps at 10 K. The decay curves in the red-phase PDA-4BCMU and P3DT are not single exponential but can be fitted to biexponential functions with time constants of slightly shorter than 1 ps and about 5 ps. These two decay time constants correspond to relaxations to the ground state, respectively, from the free exciton and unthermalized STE and from the thermalized STE.