Toward a Mechanistic Understanding of the Photochromism of Dimethyldihydropyrenes

A mechanism accounting for the photochromic properties of dimethyldihydropyrene (DHP)/metacyclophanediene (CPD) is proposed based on high-level CASSCF/CASPT2 ab initio calculations. The very low ring-opening quantum yield observed upon irradiation of DHP is explained by the quenching of the photoisomerization channel by internal conversion of the initially excited zwitterionic state to the lowest excited state minimum, where the system can fluoresce or decay back to the ground state via another internal conversion. On the other hand, irradiation of CPD leads to the formation of a biradical excited state minimum along the ring-closure reaction path. Internal conversion back to the ground state can then occur at a well-characterized conical intersection, leading to the DHP photoproduct. Thus, our results point unambiguously toward a stepwise mechanism involving the formation of a biradical intermediate on a singlet excited state. The fact that this intermediate does not correspond to the lowest excited state minimum is mainly responsible for the inefficiency of this system.