Molecular dynamics simulation of two photon-absorbing polyimides: Evidence for the formation of intra- and inter-chain dimers

The two photon-absorbing (2PA) chromophore diphenylamino-diethylfluorene-benzothiazole was recently incorporated into a new series of polyimides that form clear, heat-resistant films. Although the chromophore is fluorescent, the polyimides are not fluorescent when in solution or in the solid state. While the two-photon response remains undiminished for both nanosecond and femtosecond timescales in solution, nearly an order of magnitude increase in nanosecond two-photon response was observed in the solid state. In order to better understand 2PA in excited-state dimers of this class of polyimide, we explored dimerization in the ground-state, either within the polymer chain or inter-molecularly among chains. We built models of polyimides at experimental density and performed classical molecular dynamics simulations. Analyses using spatial orientation correlation functions suggest that ground state dimers composed of π-stacked chromophore pairs or chromophore-phthalimide moieties form within chains and between chains. Dimer conformations obtained from the molecular dynamics trajectories are also consistent with our analyses.