Mechanistic photophysics of tellurium‐substituted cytosine: Electronic structure calculations and nonadiabatic dynamics simulations1.

Previously, the MS‐CASPT2 method was performed to study the static and qualitative photophysics of tellurium‐substituted cytosine (TeC). To get quantitative information, we used our recently developed QTMF‐FSSH dynamics method to simulate the excited‐state decay of TeC. The CASSCF method was adopted to reduce the calculation costs, which was confirmed to provide reliable structures and energies as those of MS‐CASPT2. A detailed structural analysis showed that only 5% trajectories will hop to the lower triplet or singlet state via the twisted (S2/S1/T2)T intersection, while 67% trajectories will choose the planar intersections of (S2/S1/T3/T2/T1)P and (S2/S1/T2/T1)P but subsequently become twisted in other electronic states. By contrast, ~28% trajectories will maintain in a plane throughout dynamics. Electronic population revealed that the S2 population will ultrafast transfer to the lower triplet or singlet state. Later, the TeC system will populate in the spin‐mixed electronic states composed of S1, T1 and T2. At the end of 300 fs, most trajectories (~74%) will decay to the ground state and only 17.4% will survive in the triplet states. Our dynamics simulation verified that tellurium substitution will enhance the intersystem crossings, but the very short triplet lifetime (ca. 125 fs) will make TeC a less effective photosensitizer. [ABSTRACT FROM AUTHOR]