Nonadiabatic Dynamics of Photoexcited <i>cis</i>-Stilbene Using Ab Initio Multiple Spawning

Abstract

The photochemistry of <i>cis</i>-stilbene proceeds through two pathways: <i>cis</i>-<i>trans</i> isomerization and ring closure to 4a,4b-dihydrophenanthrene (DHP). Despite serving for many decades as a model system for photoisomerization, the photodynamics of <i>cis</i>-stilbene is still not fully understood. We use <i>ab initio</i> multiple spawning on a SA-2-CASSCF(2,2) potential energy surface to simulate the nonadiabatic dynamics of isolated <i>cis</i>-stilbene. We find the cyclization (to DHP and <i>cis</i>-stilbene) and isomerization (to <i>trans</i>- and <i>cis</i>-stilbene) reaction coordinates to be orthogonal; branching between the two pathways is determined on the S₁ excited state within 150 fs of photoexcitation. Trajectory basis functions (TBFs) undergoing cyclization decay rapidly to the ground state in 250 fs, while TBFs moving along the isomerization coordinate remain on the excited state longer, with the majority decaying between 300 and 500 fs. We observe three avoided crossing regions in the dynamics: two along the isomerization coordinate (displaying pyramidalization and migration of an ethylenic hydrogen or phenyl group), and one DHP-like conical intersection along the cyclization coordinate. The isomeric form of the vibrationally hot photoproducts (as determined by measurement 2 ps after photoexcitation) is determined within less than 50 fs of decay to the ground state mediated by passage through a conical intersection. Excess vibrational energy of ground state <i>cis</i>- and <i>trans</i>-stilbene is channelled into phenyl torsions (with mostly opposing directionality). Our simulations are validated by direct comparison to experiment for the absorption spectrum, branching ratio of the three photoproducts (44:52:4 <i>cis</i>-stilbene:<i>trans</i>-stilbene:DHP), and excited state lifetime (520 ± 40 fs).