Ultrafast Intersystem Crossing of 4-Thiothymidine in Aqueous Solution

Ultrafast intersystem crossing of UVA-sensitive 4-thiothymidine in aqueous solution was investigated by pump?probe transient absorption. 4-Thiothymidine in the excited singlet state exhibited a distinctively different photophysical pathway from that of thymidine, with intersystem crossing to the triplet manifold being dominant and the triplet formation being completed within approximately 10 ps. This was further supported by quantum chemical calculations. We propose that the unique behavior of photoexcited 4-thiothymidine results from its distinctive molecular and electronic structures and that the triplet form is the initial toxic source to DNA and other biomolecules.