Intramolecular energy transfer in steroid analogues

The compounds 3β-p-methoxybenzene-5α-androstane-17-ethylidene (1), 3-methoxy-1, 3, 5 [10]-estratrien-17-ethylidene ( 2) and 4′-methoxy-17Z-ethylidene [benzo-1 ′, 2′:2, 3-androstane] (3) have been prepared. Irradiation of these compounds with 254 nm light resulted in the absorption of photons by the donor chromophore, followed by triplet-triplet excited-state energy transfer (TTET) to the ethylidene acceptor. The result is Z → E isomerization of the ethylidene group. This isomerization occurs both intra- and intermolecularly at the 5 mM concentration used in the study of these compounds. The quantum efficiencies of energy transfer (ΦZ→E) for compounds 1, 2, and 3 are 0.17 ± 0.00, 0.18 ± 0.004 and 0.20 ± 0.00, respectively. A complete set of rate constants were obtained by kinetic analyses of these compounds using model compounds. The rate constants for intramolecular energy transfer (kintraTTET→C17) for 1, 2 and 3 as (1.6 ± 0.4) × 106, (1.7 ± 0.1) × 105 and (5.1 ± 0.6) × 105 s−1, respectively. Energy transfer for steroids 1 and 3 is attributed to a through-bond (TB) mechanism. The value of k intraTTET→C17 for steroid 1 is in a good agreement with the value of kintraTTET→C17 for the steroid 3β-((dimethylphenylsilyl)oxy)-17-(Z)-ethylidene-5α-androstane, (1.7 ± 0.7) × 106 s−1, previously reported from our group. The value of kintraTTET→C17 for compound 2 is considerably lower than might have been expected for either through-space or TB interaction between the two chromophores. This is attributed to an improper alignment of the p-orbitals and/or distorting of the σ-bond skeleton in this system away from the all-trans arrangement known to be ideal for through-bond interaction