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[BPDMT]* for the photoinduced oxetane cleavage to generate 3BP* and DMT through an adiabatic photochemical reaction. The experimental signature of 3[BPDMT]* is the appearance of an absorption band at ca. 400 nm, detected by femtosecond transient absorption spectroscopy. Its formation is markedly regioselective, as it is more efficient and proceeds faster for HH-1 (∼2.8 ps) than for HT-1 (∼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.