Relaxation Dynamics of the Triazene Compound Berenil in DNA-Minor-Groove Confinement after Photoexcitation

The effects of biomolecular embedding on the photoinduced relaxation process of the DNA-minor-groove binder berenil, diminazene aceturate, are studied with quantum mechanics/molecular mechanics, QM/MM, calculations that employ the algebraic diagrammatic construction through second-order, ADC(2), for the quantum mechanical part 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* 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 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 states, which explains the experimental observations of the fluorescence characteristics of berenil in DNA confinement.