Influence of substitution pattern on the dynamics of internal conversion and intersystem crossing in thiopyridone isomers

We report a combined experimental and theoretical investigation of the ultrafast internal conversion (IC) and intersystem crossing (ISC) dynamics of two thiopyridone isomers in solution. Our study used sulfur K-edge ultrafast transient x-ray absorption spectroscopy (XAS), in conjunction with electronic excited state surface hopping molecular dynamics and simulations of the excited state XAS, to investigate the impact of the functional group substitution pattern and solvent on the dynamics of IC and ISC. The combination of the localized x-ray probe and the simulation results enables, in part, the differentiation between ππ∗ and nπ∗ character excited states, as well as singlet and triplet states. Access to nπ∗ character excitations has particular value since they often prove challenging to assess with optical spectroscopy. The results point to the key role of these nπ∗ intermediates in mediating the intersystem crossing of these systems. Varying the substitution pattern of the molecule can stabilize or destabilize these intermediates leading to an increase in the rate of ISC in the ortho isomer as compared to the para isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.