Regiochemical memory in the adiabatic photolysis of thymine-derived oxetanes. A combined ultrafast spectroscopic and CASSCF/CASPT2 computational study.

The photoinduced cycloreversion of oxetanes has been thoroughly investigated in connection with the photorepair of the well-known DNA (6-4) photoproducts. In the present work, the direct photolysis of the two regioisomers arising from the irradiation of benzophenone (BP) and 1,3-dimethylthymine (DMT), namely the head-to-head (HH-1) and head-to-tail (HT-1) oxetane adducts, has been investigated by combining ultrafast spectroscopy and theoretical multiconfigurational quantum chemistry analysis. Both the experimental and computational results agree with the involvement of an excited triplet exciplex(3)[BPMIDLINE HORIZONTAL ELLIPSISDMT]* for the photoinduced oxetane cleavage to generate(3)BP* and DMT through an adiabatic photochemical reaction. The experimental signature of(3)[BPMIDLINE HORIZONTAL ELLIPSISDMT]* is the appearance of an absorption band atca.400 nm, detected by femtosecond transient absorption spectroscopy. Its formation is markedly regioselective, as it is more efficient and proceeds faster for HH-1 (similar to 2.8 ps) than for HT-1 (similar to 6.3 ps). This is in line with the theoretical analysis, which predicts an energy barrier to reach the triplet exciplex for HT-1, in contrast with a less hindered profile for HH-1. Finally, the more favorable adiabatic cycloreversion of HH-1 compared to that of HT-1 is explained by its lower probability to reach the intersystem crossing with the ground state, which would induce a radiationless deactivation process leading either to a starting adduct or to a dissociated BP and DMT.