Triplet-sensitised photolysis of the photoisomer of a 2,11-diaza[3,3](9,10) anthracenoparacyclophane: An adiabatic cycloreversion and a [2πa + 2πa + 2σs] rearrangement in a triplet state of the biplanophane system

The triplet-sensitized photoreactions of the title biplanophane system 6, the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane derivative 5, were investigated by stationary and laser-flash photolyses using xanthone (XT) and benzophenone (BP) as triplet sensitizers. When photoisomer 6 underwent XT-sensitized irradiation, a triplet cyclophane 5 and a novel polycyclic product 7 were obtained via an adiabatic cycloreversion and a formal [2πa + 2πa + 2σs] rearrangement, respectively. The maximum quantum yield for the formation of cyclophane 5 (0.69) and the upper-limit efficiency for the formation of polycycle 7 (0.31) were determined by laser photolysis techniques. For BP-sensitized photolysis of photoisomer 6, oxetane 8, in addition to triplet cyclophane 5 and polycycle 7, was formed by a Paterno-Büchi reaction. The quenching rate constant (k_q) of triplet BP by photoisomer 6 (3.4 × 10⁸ dm³ mol^-1 s^-1) was found to be 1 order of magnitude smaller than that for XT (5.0 × 10⁹ dm³ mol^-1 s^-1). On the basis of the relationship between k_q and the triplet donor-acceptor energy gap, the triplet energy level of photoisomer 6 was estimated to be ∼71 kcal mol^-1. The photochemical and the photophysical processes involved in the sensitized photolyses are summarized in an energetic reaction diagram and discussed in detail.