Steroids as molecular photonic wires. Z -> E olefin photoisomerization by intramolecular triplet-triplet energy transfer with and without interveningolefinic gates

The steroids 3 beta-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5 alpha-an drostane (1), 3 beta-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5-androst ene (2), 3 beta-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5 alpha-androstane (3a), and 3 alpha-((dimethylphenylsilyl)oxy)-17-(Z)-ethyli dene-6-methylene-5 alpha-androstane (3b) have been prepared. The triplet-tr iplet excited-state energy transfer (TTET) that occurs from the C3 aryl "do nor" group to the C17 ethylidene "acceptor" has been studied in detail at 1 0 mM steroid concentration. Irradiation with 266 nm light results in Z --> E olefin isomerization of the C17 ethylidene group, a consequence of both i ntra- and interTTET. Phi(Z-->E) = 0.037, 0.018, 0.028, and 0.004 for 1, 2, 3a, and 3b, respectively. Detailed kinetic analyses of these compounds and appropriate models, with and without added olefin quenchers, provide a comp lete set of rate constants which are determined relative to an assumed ener gy transfer rate constant to piperylene of 7.0 x 10(9) M-1 s(-1). In partic ular, k(intraTTET) for 1 = [1.7 (+/-0.7)] x 10(6) s(-1). Isomerization at C 17 in 2, due to intraTTET, is reduced (83% vs 1) but not completely elimina ted by the endocyclic alkene in ring B, which functions as a "triplet gate" . The exocyclic methylene group in 3b is more efficient in gating the intra TTET than it is in 3a (Phi(TET) = 0.08 VS 0.71, respectively). This higher level of gating that occurs in 3b is attributed to a much shorter lifetime of the axial DPSO triplet, caused by an efficient through-space intraTTET f rom the axial DPSO group to the C6 exocyclic olefin.