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 (TP) isomers in solution. Our study used ultrafast transient X-ray absorption spectroscopy (XAS) at the sulfur K-edge, 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. For 2-TP, the photoexcited S (ππ*) state rapidly undergoes IC to the S (nπ*) state below the instrument response time, followed by ISC to the T (ππ*) state on a timescale of 600 fs in acetonitrile. For 4-TP, the timescale of S to S IC increases to 330 fs and the timescale of ISC increases to more than 10 ps. The differences between isomers are rationalized by considering the key role of the, 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 the increase in ISC rate in the isomer as compared to the isomer, while changing the solvent from acetonitrile to water had minimal effect on the electronic excited state relaxation mechanism.