Photophysics of poly [p-(2,5-didodecylphenylene)ethynylene] in thin films

The optical properties of thin films of poly [p-(2,5-didodecylphenylene)ethynylene] (DPPE) have been investigated. In chloroform solution the DPPE exhibit structured blue emission with a lifetime of 0.4 ns. In contrast to the solution, pristine DPPE films show a broad featureless green fluorescence with a nonexponential decay with time constants of 1.15 ns (8%) and 5.9 ns (92%). Upon annealing, the emission spectrum returns to a structured blue emission similar to the solution, and the fluorescence decay is nonexponential with a component at 0.45 ns (38%) and 90 ps (62%). Both spin-cast and annealed films possess similar absorption spectra, suggesting that the lifetime difference is due to an excimer-like state in the pristine film and efficient fluorescence from isolated chains within the annealed films. The decrease of the fluorescence lifetime from 0.4 ns (DPPE in solution) to 90 ps (annealed DPPE films) is further attributed to the lack of conformational disorder found within the ordered solid state.