Photoinduced Intramolecular Charge Transfer in an Electronically Modified Flavin Derivative: Roseoflavin

Abstract

The photophysical properties of a push-pull flavin derivative, roseoflavin (RoF), are investigated in different surroundings at the molecular level, with focus on intramolecular charge transfer (ICT). Time-dependent density functional theory (TD-DFT, CAM-B3LYP functional) and DFT-based multireference configuration interaction (DFT/MRCI) are used to compute excited-state energies and one-electron properties of a truncated RoF model, roseolumiflavin (RoLF). Solvent effects are taken into account implicitly by the conductor-like polarizable continuum model and explicitly through a microsolvation scheme. In the gas phase, the calculations predict no crossing between the lowest locally excited (LE) and charge-transfer (CT) states upon twisting the dimethylamine donor group relative to the plane of the isoalloxazine acceptor moiety, whereas this crossing is found to be facile in solution (i.e., in water or benzene). Crossing of the LE and CT states facilitates ICT, which is the main cause of the fluorescence quenching and dual fluorescence character experimentally observed for roseoflavin in solution. The barrier for the ICT process is computed to be lower in water than in benzene, consistent with the enhanced ICT rates observed in more polar solvents. We present a detailed study of the molecular mechanism of the photoinduced ICT process in RoLF. For a typical donor-acceptor chromophore, three such mechanisms are discussed in the literature, which differ in the alignment of the donor and acceptor planes, namely, planar ICT (PICT), perpendicular-twisted ICT (TICT), and wagging ICT (WICT). Our theoretical results suggest that the TICT mechanism is favored in RoLF.

The DFT/MRCI method

“…[116][117][118] Also flavins have a rich photochemistry which depends critically on the solvent or protein environment. DFT/MRCI calculations engaging hybrid solvent models [119][120][121][122] or QM/MM models of protein environments [123][124][125] substantially added to the understanding of excited-state reactions of flavins in solution and in light-oxygen-voltage domains of blue-light receptors.…”

Radiative decay channel assessment to understand the sensing mechanism of a fluorescent turn‐on Al3+ chemosensor

“…Charge transfer is involved in many organic photochemical reactions and, in many cases, is responsible for the quenching of luminescence. This transfer can be established intramolecularly as photoelectron transfer (PET), twisted intramolecular charge transfer (TICT), intramolecular proton transfer from excited state, or ligand to metal charge transfer (LMCT) …”

“…[13,14] Charge transfer is involved in many organic photochemical reactions and, in many cases, is responsible for the quenching of luminescence. This transfer can be established intramolecularly as photoelectron transfer (PET), [15,16] twisted intramolecular charge transfer (TICT), [17,18] intramolecular proton transfer from excited state, [13,19] or ligand to metal charge transfer (LMCT). [20] In the last few years, the coordination chemistry of metal ion Schiff base complexes has especially developed because of their synthetic versatility.…”

Understanding flavin electronic structure and spectra

“…In the protein context, TD‐DFT was again found to overestimate the excitation energy 127 by amounts of 0.06 eV (B3LYP) and 0.54 eV (BHLYP) 126 . Application of the CAM‐B3LYP functional, which can describe charge transfer (CT) states reasonably well 25,128 results in excitation energies that are high by 0.03 eV in AppA BLUF protein and 0.25 eV in Slr1694 BLUF protein, albeit with different basis set sizes 129,130 . Meanwhile the CC2 method produces an overestimate of 0.24 eV.…”