Triplet Pathways in Diarylethene Photochromism. A combined ab initio and density functional study.

A combined ab initio and DFT computational study has been carried out in the attempt to provide a working model for the recently discovered triplet reactivity of photochromic diarylethenes. The investigation of 1,2-bis(2-methylbenzothiophene-3-yl)maleimide type (DAE) by DFT methods provides a detailed description of the potential energy surfaces (PES) for ground state (S0) and lowest triplet state (T1) along the reaction coordinate for photocyclization/cycloreversion. The transition-state structures are alike on S0 and T1 and lean, along the reaction coordinate, toward the closed-ring form. At the transition-state geometry, the S0 and T1 PES are almost degenerate. On S0, a large isomerization barrier (ca. 45 kcal/mol) separates the open- and closed-ring minima, whereas on T1 the activation barriers are much smaller, that for cycloreversion (ca. 15-20 kcal/mol) being larger than that for cyclization (ca. 8 kcal/mol). These features account for the behaviour observed in Ru-DAE dyads, where the diarylethene is coupled to a Ru(II) polypyridine photosensitizer moiety: efficient sensitized photocyclization, in a microsecond time scale, inefficient sensitized cycloreversion. Triplet cyclization is viewed as a non-adiabatic process originating on T1 at open-ring geometry, proceeding via intersystem crossing at transition-state geometry, and completing on S0 at closed-ring geometry. Calculations on the model system 1,2-di(3-thienyl)ethene (mod-DAE) have been used to (i) validate the DFT results against ab initio (CASSCF and CASPT2) results and (ii) demonstrate the general value of the main topological features of the S0 and T1 PES obtained for DAE.