Exploring radical formation and ultrafast intersystem crossing in a heavy-atom-free thiophene derivative

The photophysical study of decarboxyranelic acid (DRAc), a thiophene derivative made from strontium ranelate, shows that it can form long-lived radical cation species through a pathway involving triplet states. Femtosecond transient absorption spectroscopy and computational modeling reveal a fast intersystem crossing (ISC) process in water, with a time constant of 1.1 ps and a quantum yield of 27%. The small energy gap between the S₁ and T₁ states, along with the mixed πσ* character of the S₁ state, contributes to this rapid ISC, in line with El-Sayed's rule. The results indicate that DRAc's photoreactivity is strongly influenced by the solvent: in water, the high hydrogen-bond donor capacity stabilizes the S₁ state, narrows the energy gap, and creates an almost barrier-free ISC pathway. Studies using deuterated solvents show that the ISC rate increases due to stronger stabilization of the S₁ state by deuterium bonds. Overall, these results make DRAc a useful model for studying heavy-atom-free molecules with efficient ISC and radical formation, with potential applications in photodynamic therapy and catalysis.