Conical Intersection Mechanism for Photochemical Ring Opening in Benzospiropyran Compounds

The photoinduced ring-opening process in spiropyran compounds has been modeled through CASSCF calculations on the lowest excited state (S-1) of benzopyran, The documented S-1 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 S-1 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 S-0, 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 Angstrom. 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.