J. Chem. Theory Comput. (accepted)
The effects of the biomolecular embedding on the photo-induced relaxation process of the DNA minor-groove binder berenil, diminazene aceturate, is studied with quantum mechanics/molecular mechanics, QM/MM, calculations that employ the algebraic diagrammatic construction through second-order, ADC(2), for the quantum mechanical and an atomistic polarizable embedding for the classical part. The lowest singlet excitation to the S$_{1}$ state is a bright transition with a $\pi\pi^{\ast}$ character and a perichromatic red-shift, due to the interactions with the solvent and DNA. The excited-state relaxation pathway is a two-step mechanism, an N=N \textit{azo-}bond stretch followed by a volume-conserving bicycle-pedal twist. The DNA confinement and the coupling to solvent molecules via hydrogen bonds lead, for the excited-state relaxation process, only to small deviations from the ideal bicycle-pedal relaxation. Because of its volume-conserving character, the S$_{1}$ excited-state relaxation proceeds almost unhindered, even in a fully rigid minor-groove confinement. With a fully frozen DNA minor groove and solvent, the energy gap for deexcitation from S$_1$ to the ground-state increased to 2.0 eV compared to 0.16 eV in aqueous solution. When the relaxation of the first solvation shell is included, the relaxation process on the S$_{1}$ potential energy surface proceeds to a region on the potential energy surface, where only a small gap to the ground-state potential energy surface remains, 0.43 eV. These results show that the solvent relaxation has a significant effect in controlling the energy gap between the ground and S$_1$ electronically excited state, which explains experimental observations of the fluorescence characteristics of berenil in DNA confinement.
Electronic mail: Alireza.Marefatkhah@rub.de
Electronic mail: christof.haettig@rub.de
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