Intramolecular long-distance electron transfer and triplet energy transfer. Photophysical and photochemical studies on a norbornadiene-steroid-benzidine system

A bichromophoric compound 3 beta-((2-(methoxycarbonyl)bicyclo-[2.2.1]hepta-2,5-diene-3-yl)carboxy)androst-5-en-17 beta-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 (Phi(ET)) of ca. 12% and rate constant (k(SET)) of ca. 1.1 x 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 (phi(RIPISC).phi(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 (Phi(ISC)) of ca. 56%. Intramolecular triplet energy transfer leading to the formation of the triplet norbornadiene chromophore is efficient (phi(TT) ca. 65%, k(TT) = ca. 5.2 x 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.