Flavin Charge Redistribution upon Optical Excitation Is Independent of Solvent Polarity

Abstract

Flavin absorption spectra encode molecular details of the flavin’s local environment through coupling of local electric fields with the chromophore’s charge redistribution upon optical excitation. Translating experimentally measured field-tuned transition energies to local electric field magnitudes and directions across a wide range of field magnitudes requires that the charge redistribution be independent of the local field. We have measured the charge redistribution upon optical excitation of the derivatized flavin TPARF in the non-hydrogen-bonding, nonpolar solvent toluene, with and without a tridentate hydrogen-bonding ligand, DBAP, using electronic Stark spectroscopy. These measurements were interpreted using TD-DFT finite field and difference density calculations. In comparing our present results to previous Stark spectroscopic analyses of flavin in more polar solvents, we conclude that flavin charge redistribution upon optical excitation is independent of solvent polarity, indicating that dependence of flavin transition energies on local field magnitude is linear with local field magnitude.

Electronic Structure Methods for Simulating Flavin’s Spectroscopy and Photophysics: Comparison of Multi-reference, TD-DFT, and Single-Reference Wave Function Methods

“…Multiple computational studies have investigated the excited-state electronic structure and photophysics of flavin. A nonexhaustive list includes refs , – , – . A more comprehensive list of computational studies can be found in a recent review article by Kar, Miller, and Mroginski .…”

“…Several methods, including semiempirical, TD-DFT, and multireference (often, MR-PT2) methods have been used already to calculate the excited-state properties of flavins. ,− ,− For multireference calculations, the issue of active space selection is a recurring theme and a decision is often made that balances computational cost and computational accuracy due to the steep increase in computing time with the size of the active space. Automated protocols to select the active space have been developed by several research groups. ,− Sayfutyarova and Hammes-Schiffer applied their automated protocol that is based on Hückel theory for the selection of a suitable active space for flavin’s π-conjugated orbitals .…”