Photorelaxation Pathways of 4-(,-Dimethylamino)-4'-nitrostilbene Upon S Excitation Revealed by Conical Intersection and Intersystem Crossing Networks.

Multi-state -electron valence state second order perturbation theory (MS-NEVPT2) was utilized to reveal the photorelaxation pathways of 4-(-dimethylamino)-4'-nitrostilbene (DANS) upon S excitation. Within the interwoven networks of five S/S and three T/T conical intersections (CIs), and three S/T, one S/T and one S/T intersystem crossings (ISCs), those competing nonadiabatic decay pathways play different roles in -to- and -to- processes, respectively. After being excited to the Franck-Condon (FC) region of the S state, -S-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, -S-FC → ISC-S/T- → CI-T/T- → ISC-S/T- → - or -S. The singlet relaxation pathway mediated by CI-S/S- is hindered by the prominent energy barrier on S surface and by the reason that CI-S/S- and CI-S/S- are both not energetically accessible upon S excitation. On the other hand, the -S-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S/S- and CI-S/S-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between twisting (-S-FC → CI-S/S- → - or -S) and DHP formation relaxation pathways (-S-FC → CI-S/S-DHP → DHP-S) on the S surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, -S-FC → ISC-S/T- → DHP-T → DHP-S, however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for -S-FC mediated by ISC-S/T- are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S → T → T or S → T → S processes. The present study reveals photoisomerization dynamic pathways via conical intersection and intersystem crossing networks and provides nice physical insight into experimental investigation of DANS.