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

Abstract

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 alpha(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.