Laser photolysis studies of endothermic triplet energy transfer in solution

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A bimolecular rate constant (κ_q) and an efficiency (α_<TET>) of an endoergonic triplet energy transfer from triplet α-naphthyl phenyl ketone(NPK) to α-naphthylmethyl phenyl sulfide(NMPS) in solution, where the energy difference (ΔE_T) is 1.7 kcal mol^<-1>, have been determined by means of nanosecond laser flash photolysis. Base on the Arrhenius plots of the κ_q value for the NPK-NMPS system, an activation energy (ΔE_a) for the endoergonic triplet energy transfer is found to be 2.6 kcal mol^<-1>, which is larger than the endothermic energy gap (ΔE_T=1.7 kcal mol^<-1>). The endoergonic triplet energy transfer in solution is demonstrated to proceed with the aid of thermal activation for the first time.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. Upon XT-sensi … More tized irradiation of the photoisomer 6, a cyclophane 5 and a novel polycyclic product 7 were obtained via an adiabatic cycloreversion and a formal [2πa+2πa+ 2σs] rearrangement. The maximum quantum yield for formation of the cyclophane 5 (0.69) and the upper-limit efficiency for formation of the polycycle 7 (0.31) were determined by laser photolysis techniques. For BP-sensitized photolysis of the photoisomer 6, an oxetane 8 was formed via a Paterno-Buchi reaction in addition to cyclophane 5 and polycycle 7. Quenching rate constant (κ_q) of the triplet sensitizers by photoisomer 6 was smaller by one order of magnitude for BP (3.4x 10^8 dm^3 mol^<-1> s^<-1>) than for XT (5.0x10^9 dm^3 mol^<-1> s^<-1>) and the triplet energy level of photoisomer 6 was estimated as 71 kcal mol^<-1> based on the relationship between κ_q and triplet donor-acceptor energy gap. The photochemical and the photophysical processes involved in the sensitized photolyses are summarized in an energetic reaction diagram and discussed in detail. Less