Slater-Condon Rules and Spin-Orbit Couplings: 2‑(2-(2,5-Dimethoxybenzylidene)hydrazineyl)-4-(trifluoromethyl)thiazole a Test Case.

Light-atom chromophores can display properties often associated with heavy-atom compounds, such as intersystem crossing, leading to phosphorescence and singlet oxygen generation, yet their use remains comparatively underexplored. Here, we report the synthesis of HM610, a derivative of the benzylidenehydrazinylthiazole light-atom chromophore backbone. Spin-orbit couplings (SOCs), computed with the sf-X2C-S-TDDFT method, follow Slater-Condon rules and predict moderate values. Trajectory surface hopping simulations further illustrate the role of dynamical effects in promoting ISC, yet these results together establish that HM610 has only limited potential as a triplet sensitizer without further structural modification, such as heavy atom substitution. Based on the benchmarked (TD)-DFT protocol, a computational set studying six systematic analogues allowed us to study the influence of electron-donating (-OMe) and electron-withdrawing (-CF3) substituents on the common backbone, revealing the impact of substitution on the geometry and photophysics of light atom analogues of HM610 and paving the way for future studies where the introduction of heavy atoms and their impact on triplet sensitization by this family of chromophores can be probed.