Benzophenone-Initiated Photoisomerization of the Norbornadiene Group in a Benzophenone−Steroid−Norbornadiene System via Long-Distance Intramolecular Triplet Energy Transfer
journal contributionposted on 14.03.1996 by Chen-Ho Tung, Li-Ping Zhang, Yi Li, Hong Cao, Yoshifumi Tanimoto
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Bichromophoric compound 3β-((2-(methoxycarbonyl)bicyclo[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17β-yl benzophenone-4-carboxylate (NBD-S-BP) was synthesized, and its photochemistry was examined. The phosphorescence of the benzophenone (BP, donor) chromophore is efficiently quenched by the remote norbornadiene (NBD, acceptor) group. Time-resolved spectroscopic measurements indicate that the lifetime of the triplet state of the BP chromophore is shortened by the NBD group. Selective excitation of the BP chromophore results in isomerization of the NBD group to quadricyclane. All these observations suggest that a long-distance intramolecular triplet energy transfer occurs in the NBD-S-BP molecule. The efficiency and rate constant for this triplet energy transfer were determined to be ca. 22% and 1.5 × 105 s-1, respectively, by steady-state photolysis and laser flash photolysis. This long-distance intramolecular triplet energy transfer is proposed to proceed via a through-bond exchange mechanism.