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 pi-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. (C) 2012 Elsevier Ltd. All rights reserved.