Photorelaxation Pathways of 4-(N,N-Dimethylamino)-4'-nitrostilbene Upon S1 Excitation Revealed by Conical Intersection and Intersystem Crossing Networks.

Ziyue He,Le Yu,Ruidi Xue,Yibo Lei,Chaoyuan Zhu

doi:10.3390/molecules25092230

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 S1 excitation. Within the interwoven networks of five S1/S0 and three T2/T1 conical intersections (CIs), and three S1/T2, one S1/T1 and one S0/T1 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 S1 state, trans-S1-FC firstly encounters an ultrafast conversion to quinoid form. Subsequently, the relaxation mainly proceeds along the triplet pathway, trans-S1-FC → ISC-S1/T2-trans → CI-T2/T1-trans → ISC-S0/T1-twist → trans- or cis-S0. The singlet relaxation pathway mediated by CI-S1/S0-twist-c is hindered by the prominent energy barrier on S1 surface and by the reason that CI-S1/S0-trans and CI-S1/S0-twist-t are both not energetically accessible upon S1 excitation. On the other hand, the cis-S1-FC lies at the top of steeply decreasing potential energy surfaces (PESs) towards the CI-S1/S0-twist-c and CI-S1/S0-DHP regions; therefore, the initial twisting directions of DN and DAP moieties determine the branching ratio between αC=C twisting (cis-S1-FC → CI-S1/S0-twist-c → trans- or cis-S0) and DHP formation relaxation pathways (cis-S1-FC → CI-S1/S0-DHP → DHP-S0) on the S1 surface. Moreover, the DHP formation could also take place via the triplet relaxation pathway, cis-S1-FC → ISC-S1/T1-cis → DHP-T1 → DHP-S0, however, which may be hindered by insufficient spin-orbit coupling (SOC) strength. The other triplet pathways for cis-S1-FC mediated by ISC-S1/T2-cis are negligible due to the energy or geometry incompatibility of possible consecutive stepwise S1 → T2 → T1 or S1 → T2 → S1 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.

Highlights

The definition of important internal coordinates correlating with the photorelaxation processes and atomic numbering are given in Scheme 1
The SA6-complete active space self-consistent field (CASSCF)(18,12)/6-31G* optimized internal coordinates for the minimum energy geometries, such as trans, cis, twist-DANS, dihydrophenanthrene (DHP) states and ground state transition states (TS) are listed in Table 1 and those for the conical intersections (CIs) and intersystem crossings (ISCs) are given in Molecules 2020, 25, x FOR PEER REVIEW
We investigated the photorelaxation mechanisms of DANS upon S1 excitation by constructing the interwoven conical intersection and intersystem crossing networks at the

Summary

Introduction

Upon excitation to the S1 state, both trans- and cis-stilbene evolve along the C=C torsion coordinate and decay via twisted S1 /S0 conical intersections (CIs) [12], while for substituted stilbene, the triplet route mediated by intersystem crossing (ISC) may open [13,14], and the formation of stable intramolecular charge transfer (ICT) states affect the fluorescence efficiency [13,14,15,16,17,18,19]. The optical properties of stilbenes can be controlled by introducing suitable substitution groups [8]. The nitro, cyano and halogen substitution on the phenyl ring promotes the triplet pathway [20,21,22]; the amino group substitution raises the C=C torsion barrier and slow down the isomerization process [23,24].

Methods

Results

Conclusion