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Toward a Mechanistic Understanding of the Photochromism of Dimethyldihydropyrenes
journal contribution
posted on 2007-06-08, 00:00 authored by Martial Boggio-Pasqua, Michael J. Bearpark, Michael A. RobbA 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.