Visible-Light-Mediated Excited State Relaxation in Semi-Synthetic Genetic Alphabet: d5SICS and dNaM

The excited state dynamics of an unnatural base pair (UBP) d5SICS/dNaM were investigated by accurate ab-initio calculations. Time-dependent density functional and high-level multireference calculations (MS-CASPT2) were performed to elucidate the excitation of this UBP and its excited state relaxation mechanism. After excitation to the bright state S₂ (ππ*), it decays to the S₁ state and then undergoes efficient intersystem crossing to the triplet manifold. The presence of sulfur atom in d5SICS leads to strong spin-orbit coupling (SOC) and a small energy gap that facilitates intersystem crossing from S₁ (n_s π*) to T₂ (ππ*) followed by internal conversion to T₁ state. Similarly in dNaM, the deactivation pathway follows analogous trends. CASPT2 calculations suggest that the S₁ (ππ*) state is a dark state below the accessible S₂ (ππ*) bright state. During the ultrafast deactivation, it exhibits bond length inversion. From S₁ state, significant SOC leads the population transfer to T₃ due to a smaller energy gap. Henceforth, fast internal conversion occurs from T₃ to T₂ followed by T₁ . From time-dependent trajectory surface hopping dynamics, it is found that excited state relaxation occurs on a sub-picosecond timescale in d5SICS and dNaM. Our findings strongly suggest that there is enough energy available in triplet state of UBP to generate reactive oxygen species and induce phototoxicity with respect to cellular DNA.