| Abstract
| - A bichromophoric compound3β-((2-(methoxycarbonyl)bicyclo-[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17β-yl-2,2‘,6,6‘,N,N,N‘,N‘heptamethylbenzidine (N−S−B) was synthesized, and itsphotochemistry was examinedby using both steady-state and time-resolved techniques.Intramolecular electron transfer from the singletexcitedstate of benzidine to the norbornadiene chromophore in N−S−B occurswith efficiency (ΦET ) of ca. 12% and rateconstant (kSET) of ca. 1.1 × 107s-1, resulting in the singlet radical ionpair1(N•-−S−B•+)followed by intersystemcrossing to the triplet state3(N•-−S−B•+).Recombination of3(N•-−S−B•+)yields triplet norbornadiene group.The efficiency (φRIPISC·φRIPRC) ofthe formation of the triplet norbornadiene from the radical ion pair isca. 17%.The singlet excited state of benzidine group in N−S−Bundergoes intersystem crossing to its triplet state withefficiency(ΦISC) of ca. 56%. Intramolecular triplet energytransfer leading to the formation of the tripletnorbornadienechromophore is efficient (φTT = ca. 65%,kTT = ca. 5.2 × 105s-1). Selective excitation of thebenzidine chromophoreresults in isomerization of the norbornadiene group to quadricyclane.This isomerization proceeds either viaintramolecular triplet sensitization or radical-ion pair recombinationmechanism. The long-distance intramoleculartriplet energy transfer and singlet electron transfer are proposed toproceed via a through-bond mechanism.
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