Following Flavin's Vibrational Modes to Probe Anharmonicities and Low-Lying Conical Intersections

Flavins (e.g., riboflavin, FMN, and FAD) are biologically ubiquitous molecules widely studied using spectroscopic techniques such as ultraviolet-visible (UV/vis), Fourier-transform infrared (FTIR), and Raman spectroscopies. Those spectroscopic methods typically involve the excitation of flavin's vibrational modes either on the ground or excited state potential energy surfaces. In the case of UV/vis spectroscopy, this vibrational excitation is coupled to an electronic excitation and leads to a Franck-Condon progression that gives flavin its UV/vis absorption profile. To support experimental spectroscopy measurements, it is desirable to use computations that can accurately predict the vibrational frequencies for flavin's ground and excited electronic states. However, commonly used computational approaches often rely on a harmonic approximation, which may lead to errors for modes that are not harmonic. By using time-dependent density functional theory (TD-DFT) on lumiflavin, a simplified model system to represent flavins, we mapped the ground- and excited-state potential energy surfaces computed along 3N-6 vibrational modes near the ground equilibrium structure and computed the corresponding excited-state energies along those modes. We fitted these computed potentials with second-order polynomials and used goodness-of-fit statistics as indicators of the harmonicity of vibrational modes. Among the modes that display anharmonic behavior, several are Franck-Condon active modes, which can help explain why flavin's vertical excitation energies reported in the literature do not typically match well with the experimental wavelength of maximum absorption (λ_max). These calculations help inform future computational and experimental UV/vis, FTIR, and resonance Raman studies of flavins. Some of those potential scans also revealed low-lying conical intersections between the first and second singlet excited states. These intersections constitute potential photophysical deactivation channels that can compete with fluorescence, intersystem crossing, or photoredox chemistry when flavin is in a nonpolar environment.