Fluorescence yield and lifetime of isolated polydiacetylene chains: Evidence for a one-dimensional exciton band in a conjugated polymer

The fluorescence lifetime τ and quantum yield ${\eta }_{\mathrm{f}}$ of isolated red polydiacetylene chains dispersed in their monomer single crystal matrix were measured between 10 and 100 K. τ increases up to 120 ps at 40 K, then rapidly decreases at higher T. ${\eta }_f$ is a continuously decreasing function of T. The radiative lifetime ${\tau }_{\mathrm{r}}$ is proportional to $\sqrt{T}$ in the whole T range studied. This is the expected behavior for a purely one-dimensional system without localization, a behavior which is not usually observed even in semiconductor quantum wires. An order of magnitude of the exciton effective mass $m^{*}\approx 0.3$ is inferred. The nonradiative lifetime ${\tau }_{\mathrm{nr}}$ is constant up to 50 K, then decreases exponentially, indicating the opening of a thermally activated decay channel.