Electronic relaxation in benzaldehyde evaluated via TD-DFT and localized diabatization: Intersystem crossings, conical intersections, and phosphorescence

The photophysics of benzaldehyde are analyzed through the lens of TD-DFT adiabatic excited states and Boys or Edmiston-Ruedenberg localized diabatic states. We predict rate constants for two processes in excited benzaldehyde: (i) the intersystem crossing from S₁ → T₂ and (ii) the phosphorescence from T₁ → S₀. We also study (iii) the conical intersection between T₂ and T₁ that is putatively responsible for an ultrafast internal conversion process, T₂ → T₁. In agreement with Ohmori et al. (J. Phys. Chem. 1988, 92 (5), 1086-1093), our results suggest that the S₁ → T₂ intersystem crossing in benzaldehyde is rapid not only because of a large spin-orbit matrix element (i.e., El-Sayed's rule) but also because of a fortuitously small energy barrier. Furthermore, when studying the T₂ → T₁ internal conversion, we find that both Boys and Edmiston-Ruedenberg localization give remarkably stable and accurate diabatic states which will be useful for ongoing studies of dynamics near conical intersections. To our knowledge, this is the first example whereby localized diabatization techniques have been tested and have successfully recovered the topology of a conical intersection.