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Intramolecular Long-Distance Electron Transfer and Triplet Energy Transfer. Photophysical and Photochemical Studies on a Norbornadiene−Steroid−Benzidine System

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journal contribution
posted on 11.06.1997, 00:00 by Chen-Ho Tung, Li-Ping Zhang, Yi Li, Hong Cao, Yoshifumi Tanimoto
A bichromophoric compound 3β-((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 its photochemistry was examined by using both steady-state and time-resolved techniques. Intramolecular electron transfer from the singlet excited state of benzidine to the norbornadiene chromophore in N−S−B occurs with efficiency (ΦET ) of ca. 12% and rate constant (kSET) of ca. 1.1 × 107 s-1, resulting in the singlet radical ion pair 1(N•-−S−B•+) followed by intersystem crossing to the triplet state 3(N•-−S−B•+). Recombination of 3(N•-−S−B•+) yields triplet norbornadiene group. The efficiency (φRIPISC·φRIPRC) of the formation of the triplet norbornadiene from the radical ion pair is ca. 17%. The singlet excited state of benzidine group in N−S−B undergoes intersystem crossing to its triplet state with efficiency (ΦISC) of ca. 56%. Intramolecular triplet energy transfer leading to the formation of the triplet norbornadiene chromophore is efficient (φTT = ca. 65%, kTT = ca. 5.2 × 105 s-1). Selective excitation of the benzidine chromophore results in isomerization of the norbornadiene group to quadricyclane. This isomerization proceeds either via intramolecular triplet sensitization or radical-ion pair recombination mechanism. The long-distance intramolecular triplet energy transfer and singlet electron transfer are proposed to proceed via a through-bond mechanism.

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