Steroids as Photonic Wires. Z → E Olefin Photoisomerization Involving Ketone Singlet → Triplet Switches by Through-Bond Energy Transfer1,2

Abstract

A series of steroids has been studied in which there is a dimethylphenylsiloxy (DPSO) group at the 3 position to serve as a light-absorbing antenna, a ketone group at C6 or C11 to serve as an energy “relay” or as a “singlet−triplet switch”, and an olefin at C17 to serve as a triplet energy acceptor. These include 3α-(dimethylphenylsiloxy)-17-(Z)-ethylidene-5α-androstan-11-one (2), its C6 carbonyl analog (3), and the C6 ketone 3β isomer (4). The nonketonic steroid 3α-(dimethylphenylsiloxy)-17-(Z)-ethylidene-5α-androstane (1) serves as a reference. Excitation of all four compounds with light absorbed by the DPSO chromophore leads to Z → E isomerization of the C17 ethylidene group. For the ketonic steroids this isomerization involves intramolecular singlet−singlet energy transfer (intraSSET) from the DPSO group to the carbonyl group, intersystem crossing to the carbonyl triplet, and intramolecular triplet−triplet energy transfer (intraTTET) to the alkene. For compound 1 there is modestly efficient (through-bond) intraTTET directly from C3 to C17. For 2−4 intraSSET is ca. 75−90% efficient and occurs with rate constants of (1.1−1.7) × 109 s-1. The C3 DPSO antenna transfers both singlet and triplet energy more efficiently when it is α (axial) than when it is β (equatorial). IntraTTET from C6 and C11 to C17 is ca. 80% efficient; kC6→17Z, determined from triplet quenching experiments, equals 8.3 × 108 s-1. The C11 ketone has an unusually short singlet lifetime (0.4 ns), which can be attributed to an enhanced rate of radiationless decay caused by the proximity of the C19 angular methyl group.