Photorelaxation Pathways of 4-(N,N-Dimethylamino)-4′-nitrostilbene Upon S(1) Excitation Revealed by Conical Intersection and Intersystem Crossing Networks

Multi-state n-electron valence state second order perturbation theory (MS-NEVPT2) was utilized to reveal the photorelaxation pathways of 4-(N,N-dimethylamino)-4′-nitrostilbene (DANS) upon S(1) excitation. Within the interwoven networks of five S(1)/S(0) and three T(2)/T(1) conical intersections (CIs), and three S(1)/T(2), one S(1)/T(1) and one S(0)/T(1) intersystem crossings (ISCs), those competing nonadiabatic decay pathways play different roles in trans-to-cis and cis-to-trans processes, respectively. After being excited to the Franck–Condon (FC) region of the S(1) state, trans-S(1)-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, trans-S(1)-FC → ISC-S(1)/T(2)-trans → CI-T(2)/T(1)-trans → ISC-S(0)/T(1)-twist → trans- or cis-S(0). The singlet relaxation pathway mediated by CI-S(1)/S(0)-twist-c is hindered by the prominent energy barrier on S(1) surface and by the reason that CI-S(1)/S(0)-trans and CI-S(1)/S(0)-twist-t are both not energetically accessible upon S(1) excitation. On the other hand, the cis-S(1)-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S(1)/S(0)-twist-c and CI-S(1)/S(0)-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between α(C=C) twisting (cis-S(1)-FC → CI-S(1)/S(0)-twist-c → trans- or cis-S(0)) and DHP formation relaxation pathways (cis-S(1)-FC → CI-S(1)/S(0)-DHP → DHP-S(0)) on the S(1) surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, cis-S(1)-FC → ISC-S(1)/T(1)-cis → DHP-T(1) → DHP-S(0), however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for cis-S(1)-FC mediated by ISC-S(1)/T(2)-cis are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S(1) → T(2) → T(1) or S(1) → T(2) → S(1) 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.