The photophysics of phenyl disubstituted polyacetylenes: picosecond photoluminescence and femtosecond photoinduced absorption studies of PDPA-n-Bu

We have obtained picosecond (ps) photoluminescence (PL) decays of phenyl disubstituted polyacetylenes (PDPA) with an attached n-butyl group (PDPA-n-Bu) in solution and as thin films. In solution, we observe only a single exponential decay at all emission wavelengths over the range from 2.15 to 2.50 eV. The PL lifetime in solution is 190 ps. The PL decays in thin films cannot be fit well using a single exponential, and are significantly longer in thin films than in solution. In addition, the PL decay times vary with emission wavelength in thin films with longer PL decay times observed at lower emission energies. We have also obtained the femtosecond (fs) photoinduced absorption (PA) spectra of PDPA-n-Bu in solution over the range from 1.70 to 2.95 eV. These data exhibit fast ground state bleaching at high energy, along with the growth of a broad PA feature at low energies that decays with a time constant consistent with the PL decay time in solution. At long times (>200 ps) we observe the growth of a long-lived PA feature peaked at 2.3 eV that persists for hundreds of nanoseconds. We also observe a broad feature in the infrared PA at 0.23 eV whose dynamics are the same as the band at 2.3 eV. We assign these two long-lived bands to polaron transitions.