Excited state evolution in polydiacetylene studied by sub-10-fs time resolved spectroscopy

Polydiacetylenes are a well-known class of one-dimensional /spl pi/-conjugated semiconducting polymers. They display optical transitions with high oscillator strength and low photoluminescence efficiency (typically 10/sup -4/). This is consistent with ultrafast relaxation of the primary photoexcited state 1/sup 1/B/sub u/ into one or more dark states, i.e. states which are energetically below the 1/sup 1/B/sub u/ state and not optically coupled to the ground state. The recent availability of sub-10-fs pulses in the visible range enables direct monitoring of this process. In this work we study solutions of polycarbazolyldiacetylene in benzene, which show a steep onset of ground state absorption. The differential transmission signal is shown as a function of pump-probe delay at the probe wavelength of 590 nm. Because of the negligible ground state absorption at this wavelength, the increased transmission is assigned to stimulated emission from the photoexcited 1/sup 1/B/sub u/ state. The decay of the emission corresponds to the quick internal conversion to the lower energy, optically dark 2/sup 1/A/sub g/ state. The same dynamics are observed over the whole stimulated emission region. Superimposed on the transient signal we observe coherent molecular oscillations. The lack of oscillations different from those identified in the Raman spectrum suggests that ground state vibrations are probed. The absence of excited state modes can be explained by the short lifetime of the excited 1/sup 1/B/sub u/ state, which is rapidly depopulated towards the dark state.