Light harvesting complexes contain tightly packed chlorophyll molecules that lie in different protein environments. Knowledge of how these different chlorophylls couple to their local protein environment, and how this changes upon aggregation, is important for understanding and modeling energy transfer in light harvesting complexes. Studying chlorophyll a in different solvent environments, and under different aggregation states, could serve as a model system to investigate how chlorophylls interact with different local environments and how this manifests in their spectral properties. In principle, two-dimensional electronic spectroscopy (2DES) can provide this information through analysis of the inhomogeneity index. The inhomogeneity index is directly related to the frequency fluctuation correlation function (FFCF) and reports on how molecules interact with their local environment. We have applied 2DES to chlorophyll a in various solvent environments and in different aggregated states. We extracted the inhomogeneity index from the 2DES spectra and verified it with the ellipticity and center line lope methods. For monomeric chlorophyll a, our analysis reveals multiple timescales associated with solvation dynamics, an ultrafast ~50-100 fs component, followed by slower ~1-3 ps and ~25-50 ps components, and a long-lived inhomogeneity that increases with increased hydrogen bonding and viscosity. For aggregated chlorophyll a we observe an ultrafast 80 fs time component and a ~3-4 ps component followed by a long-lived inhomogeneity that decreases more rapidly in more concentrated chlorophyll samples. This indicates that the aggregated chlorophylls interact less with the bath, which could arise from an increase in the number of states over which energy can be dissipated.

Analyzing the inhomogeneity index of monomeric and aggregated chlorophyll a with two-dimensional electronic spectroscopy

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

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.