Lipophilic dyes such as Laurdan and PRODAN report lipid packing in biomembranes, via a red shift of emission in more polar environments. Disentangling the factors which control the spectral shift is complicated by the stabilization of a charge transfer-like state in more polar environments. Predicting the emission therefore requires modeling both the relaxation of the environment and the corresponding evolution of the excited state. An approach has been developed in which (i) the local environment is sampled by classical molecular dynamics (MD) simulation of the dye and solvent; (ii) prediction of the absorption and excited state of PRODAN by numerical quantum mechanics (QM); (iii) relaxation of the environment around the excited state by MD; (iv)prediction of the emission by QM. The QM steps are computed using GW-BSE (as implemented in Versatile Object oriented Toolkit for Coarse graining Applications: Exciton Transport Simulations(VOTCA-XTP)) with the environment modeled as fixed point charges, sampled in the MD simulation steps. Comparison to ultrafast time resolved transient absorption measurements demonstrates that the iterative MD/QM approach agrees quantitatively with both the polarity dependent shift in emission and the timescale over which the charge transfer state is stabilized.

Solvent-dependent relaxation of PRODAN: Quantitative simulation and ultrafast spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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