Photoluminescence study of crosslinked reactive mesogens for organic light emitting devices

We study the spectroscopic properties of luminescent liquid crystals which show a glassy nematic phase at room temperature and then form polymer networks by polymerization using ultraviolet light. The reactive mesogens possess fluorene-based aromatic cores with either diene or acrylate photoreactive groups at the end of aliphatic spacers. The photoluminescence quantum efficiency is enhanced when a rigid polymer backbone is formed by crosslinking of the diene endgroups. Spectral shifts of the vibronic transitions confirm an increase in the viscosity of the matrix following photopolymerization. Continuous and time-resolved photoluminescence measurements show that the quantum efficiency is limited by exciton diffusion to traps. Either the diffusion constant or the density of traps is reduced by photopolymerization.