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

The photophysical properties of a pushpull 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 donoracceptor 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.