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

• Published in: Polymer (Guilford) 2012-07, Vol.53 (16), p.3421-3425
• Main Authors: Trohalaki, Steven, Kedziora, Gary S., Pachter, Ruth
• Format: Article
• Language: English
• Subjects: Applied sciences; dimerization; Electrical, magnetic and optical properties; Exact sciences and technology; molecular dynamics; Molecular dynamics simulation; Organic polymers; Physicochemistry of polymers; polymers; Polymimides; Properties and characterization; Two-photon absorption
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2012.05.061

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. [Display omitted]