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

• Published in: Molecules (Basel, Switzerland) Switzerland), 2020-05, Vol.25 (9)
• Main Authors: He, Ziyue, Xue, Ruidi, Lei, Yibo, Yu, Le, Zhu, Chaoyuan
• Format: Article
• Language: English
• Subjects: Isomerism; Molecular Structure; Photochemical Processes; Stilbenes - chemistry
• ISSN: 1420-3049
• DOI: 10.3390/molecules25092230

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.