Conical Intersection Mechanism for Photochemical Ring Opening in Benzospiropyran Compounds

Abstract

The photoinduced ring-opening process in spiropyran compounds has been modeled through CASSCF calculations on the lowest excited state (S1) of benzopyran. The documented S1 reaction coordinate is characterized by a ring-opening transition state connecting a cyclic intermediate to a much more stable acyclic structure. Remarkably this structure does not correspond to a real S1 intermediate but to a crossing point (i.e., a conical intersection) between the excited- and ground-state energy surfaces. At this crossing the excited-state system can undergo fully efficient decay to S0. Using the parent pyran molecule as a model, we have characterized two ground-state valleys which develop from the crossing point. The first valley leads to reactant (pyran) regeneration. The second valley leads to cZc-penta-2,4-dienal indicating that the primary ring-opened photoproduct is formed in an unstable conformation. The benzopyran/pyran reaction coordinates suggest that the first step in the benzospiropyran photochromic reaction must correspond to the excited-state ring opening of the benzopyran moiety. It is shown that while this process can be controlled by a small energy barrier, it ultimately leads to ultrafast radiationless decay at a structure where the C−O bond distance is ca. 2.3 Å. Upon ground-state relaxation this structure can generate either a cZc precursor of merocyanine (via full ring opening) or the starting benzospiropyran (via ring closure). This mechanistic picture seems to be consistent with the present experimental knowledge.