Theoretical Study of the Photophysics of 8-Vinylguanine, an Isomorphic Fluorescent Analogue of Guanine

Paving the way for the application of the algebraic-diagrammatic construction scheme of second-order (ADC(2)) to systems based on the guanine chromophore, we demonstrate the this excited-state electronic structure method provides a realistic description of the photochemistry of 9H-guanine, in close agreement with the benchmark provided by the CASPT2 method. We then proceed to apply the ADC(2) method to the photochemistry of 8-vinylguanine (8vG), a minimally modified analogue of guanine which, unlike the naturally occurring nucleobase, displays intense fluorescence, indicative of a much longer-lived excited electronic state. The emissive electronic state of 8vG is identified as an $\pi\pi$*-type intramolecular charge transfer (ICT) state, in which a charge of roughly −0.2 e is transferred from the guanine moiety onto the vinyl substituent. The main radiationless deactivation pathway competing with fluorescence is predicted to involve the molecule leaving the minimum on the ICT $\pi\pi$* state, and reaching a region of the $S_{1}$ adiabatic state where it resembles the $L_{a}$ $\pi\pi$* state of unmodified 9H-guanine. The topology of the $L_{a}$ $\pi\pi$* region of the $S_{1}$ state favors subsequent internal conversion at a crossing seam with the ground electronic state. The sensitivity of this process to environment polarity may explain the experimentally observed fluorescence quenching of 8vG upon incorporation in single- and double-stranded DNA