EMISSION PROPERTIES OF PARA-HEXAPHENYL POLYCRYSTALLINE FILMS

The emission properties of para-hexaphenyl polycrystalline films are investigated by combining time-resolved photoluminescence and pump-probe measurements with site-selective spectroscopy. Photoinduced absorption, gain, and photoluminescence exhibit the same temporal behavior, showing that the same emitting species are involved. The photoluminescence decay is relatively long, featuring a lifetime of $400\ifmmode\pm\else\textpm\fi{}10\mathrm{ps}$ at 300 K. Absorption and photoluminescence excitation profiles, as well as the relaxation processes and the photoinduced dynamics, can be rationalized assuming that photoexcitations are intramolecular species. Dichroic and site-selective photoluminescence measurements point to an important short-range orientational and energetic disorder. Both kinds of disorder do not favor the molecular exciton coupling (H aggregates), and this agrees with the high external photoluminescence quantum yield 0.3 of para-hexaphenyl in the solid state.