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