Direct Experimental Determination of Spectral Densities of Molecular Complexes

TL;DR

This paper introduces a straightforward method to extract spectral densities of molecular complexes from experimental data, enabling comprehensive understanding of their dynamics without artificial separation of processes.

Contribution

It presents a robust strategy based on fluorescence Stokes shift data to determine spectral densities, applicable to various molecular systems and solvent environments.

Findings

Spectral densities are well described by a three-parameter sub-Ohmic family.

Experimental data for dyes, proteins, and photochemical systems fit the model.

The spectral density exhibits a Gaussian decay followed by algebraic decay.

Abstract

Determining the spectral density of a molecular system immersed in a proteomic scaffold and in contact to a solvent is a fundamental challenge in the coarse-grained description of, e.g., electron and energy transfer dynamics. Once the spectral density is characterized, all the time scales are captured and no artificial separation between fast and slow processes need be invoked. Based on the fluorescence Stokes shift function, we utilize a simple and robust strategy to extract the spectral density of a number of molecular complexes from available experimental data. Specifically, we show that experimental data for dye molecules in several solvents, amino acid proteins in water, and some photochemical systems (e.g., rhodopsin and green fluorescence proteins), are well described by a three-parameter family of sub-Ohmic spectral densities that are characterized by a fast initial Gaussian-like decay followed by a slow algebraic-like decay rate at long times.