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Center for Nonlinear Studies |
Polarity sensitive, lipophilic dyes such as Laurdan report lipid packing in biomembranes,as the emission spectrum is red shifted in more polar environments. In simple membranes, the dye is more accessible to solvent in more disorderedmembranes, and the spectral shift is well-explained by dipolar relaxation of the solvent. However, in more complex systems other factors may contribute, especially hydrogen bonding between the environment and the chromophore. Disentangling the factors which control the spectral shift is complicated by the fact that polar environments likely stabilize a charge transfer-like state, and so predicting the emission requires accurate modeling of the relaxation of the environment around 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; (ii) prediction of the absorption and excited state by numerical quantum mechanics (QM); (iii) parameterization of an excited state MD model (iv) relaxation of the environment around the excited state by MD; (v)prediction of the emission by QM. The QM steps are computed using GW-BSE (as implemented in Versatile Object oriented Toolkit for Coarse grainingApplications: Exciton Transport Simulations(VOTCA-XTP)) with the environment modeled as fixed point charges, sampled in the MD simulation steps. Comparison to time dependent DFT shows that GW-BSE yields much more quantitative predictions for both absorption and emission, as expected for charge transfer states, because GW more accurately captures the effective electron-hole interactions following an excitation. The results reveal the mechanisms responsible for the spectral shift of Laurdan,and map an approach for similarapplications to other polarity sensitive probes.
Host: Christoph Junghans