226714-54-1Relevant articles and documents
Steroids as molecular photonic wires. Z → E olefin photoisomerization by intramolecular triplet-triplet energy transfer with and without intervening olefinic gates
Timberlake, Larry D.,Morrison, Harry
, p. 3618 - 3632 (1999)
The steroids 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5α- androstane (1), 3β((dimethyl-phenylsilyl)oxy)-17-(Z)-ethylidene-5-androstene (2), 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6-methylene-5α- androstane (3a), and 3α((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-6- methylene-5α-androstane (3b) have been prepared. The triplet-triplet excited-state energy transfer (TTET) that occurs from the C3 aryl 'donor' group to the C17 ethylidene 'acceptor', has been studied in detail at 10 mM steroid concentration. Irradiation with 266 nm light results in Z → E olefin isomerization of the C17 ethylidene group, a consequence of both intra- and interTTET. Φ(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 complete set of rate constants which are determined relative to an assumed energy transfer rate constant to piperylene of 7.0 x 109 M-1 s-1. In particular, k(intra)TTET for 1 = [1.7 (±0.7)] x 106 s-1. Isomerization at C17 in 2, due to intraTTET, is reduced (83% vs 1) but not completely eliminated 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 intraTTET than it is in 3a (Φ(TTET) = 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 from the axial DPSO group to the C6 exocyclic olefin.
