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

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 (Φ ET ) of ca. 12% and rate constant ( k SET ) of ca. 1.1 × 10 7 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 (φ RIPISC ·φ RIPRC ) 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 (Φ ISC ) of ca. 56%. Intramolecular triplet energy transfer leading to the formation of the triplet norbornadiene chromophore is efficient (φ TT = ca. 65%, k TT = ca. 5.2 × 10 5 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.