Triplet-Sensitized 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

Abstract

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 [2pia + 2pia + 2sigmas] 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-Buchi reaction. The quenching rate constant (k(q)) of triplet BP by photoisomer 6 (3.4 x 10(8) dm(3) mol(-)(1) s(-)(1)) was found to be 1 order of magnitude smaller than that for XT (5.0 x 10(9) dm(3) 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 approximately 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.