A study of antenna-initiated energy transfer in polyfunctional steroids containing olefinic energy gates and ketonic switches

The steroids 3β-(dimethylphenylsiloxy)-17-Z-ethylidene-5α-androstane (1), 3β-(dimethylphenylsilyloxy)-17-Z-ethylidene-5-androstene (2), 3β-(dimethylphenylsilyloxy)-17-Z-ethylidene-6-methylene-5α-androstane (3a) and 3β-(dimethylphenylsilyloxy)-17-Z-ethylidene-6-methylene-5α-androstane (3b) have been prepared. The triplet-triplet energy transfer (TTET) that occurs from the C3 aryl “donor” to the C17 ethylidene “acceptor” has been studied in detail at 10mM steroid. Irradiation at 266 nm results in [special characters omitted] olefin isomerization of the C17 ethylidene group, a consequence of both intra- and interTTET. The [special characters omitted] and 0.004 for 1, 2, 3a, and 3b, respectively. Detailed kinetic analyses provide a complete set of rate constants relative to an assumed energy transfer rate constant to piperylene of [special characters omitted]. In particular, kintraTTET for 1 = 1.7 (±0.7) $ 106 s-1, which is explained as resulting from a TBI-mediated energy transfer process. Isomerization at C17 in 2, due to intraTTET, is reduced by 83% vs. 1 , 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 ([special characters omitted]). The higher level of gating that occurs in 3b is attributed to a shorter lifetime for the axial DPSO triplet, caused by an efficient through-space intraTTET from the axial DPSO group to the C6 exocyclic olefin. The compound 17β-(dimethylphenylsilyloxy)-3-Z-ethylidene-5α-androstane-6-one (11), and its 17α-dimethylphenylsilyloxy isomer ( 12) showed that the C6 ketones act as singlet-triplet switches, providing a 5x increase in the olefin isomerization yields vs. the non-ketonic models. In general, a 17β-DPSO donor gives a lower quantum efficiency for both intraSSET and intraTTET vs. a 17α-DPSO isomer. This is attributed to interaction with the C18 methyl group for the β-orientation, reducing the number of favorable conformations available for energy transfer