UltrafastCSpiro–O Dissociationvia a Conical IntersectionDrives Spiropyran to Merocyanine Photoswitching.

Themechanism of the photochemical conversion of spiropyran tomerocyanine is investigated theoretically. Calculations were performedat TD-DFT/ωB97XD/cc-pVDZ level of theory, which shows good agreementwith the reference RI-CC2 method. A two-dimensional scan of the potentialenergy surface has been performed along the C–O distance andthe central torsion angle in the ground state and in the first excitedstate, where the reaction takes place. Starting at the Franck–Condongeometry, the energy of the first excited state decreases in the directionof the C–O dissociation while the ground-state energy increases.This leads to a barrierless C–O bond dissociation in the firstexcited state. While relaxing on the S1PES toward longerC–O distances, the torsion angle hardly changes, but othercoordinates start to vary, leading to a conical intersection of theground state and the first excited state at a C–O distanceof about 3.4 Å. Passing the conical intersection, the reactioncontinues on the ground-state PES. At these large C–O distances,either barrierless Cspiro–O rebinding occurs thatquenches spiropyran isomerization or rotation around the central torsionangle occurs that leads to merocyanine. For the latter an energy barrierof 0.1 eV must be overcome explaining the low quantum yield of spiropyranto merocyanine photoswitching. [ABSTRACT FROM AUTHOR]