STEROIDS AS PHOTONIC WIRES - Z-]E OLEFIN PHOTOISOMERIZATION INVOLVINGKETONE SINGLET-]TRIPLET SWITCHES BY THROUGH-BOND ENERGY-TRANSFER

A series of steroids has been studied in which there is a dimethylphen ylsiloxy (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'' o r as a ''singlet-triplet switch'', and an olefin at C17 to serve as a triplet energy acceptor. These include 3 alpha-(dimethylphenylsiloxy)- 17-(Z)-ethylidene-5 alpha-androstan-11-one (2), its C6 carbonyl analog (3), and the C6 ketone 3 beta isomer (4). The nonketonic steroid 3 al pha-(dimethylphenylsiloxy)-17-(Z)-ethylidene-5 alpha-androstane (1) se rves as a reference. Excitation of all four compounds with light absor bed 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 D PSO 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-b ond) 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) x 10(9) s(-1). The C3 DPSO antenna;transfers both singlet and triplet energy more eff iciently when it is alpha (axial) than when it is beta (equatorial). I ntraTTET from C6 and C11 to C17 is ca. 80% efficient; k(C6-->17Z), det ermined from triplet quenching experiments, equals 8.3 x 10(8) 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 b y the proximity of the C19 angular methyl group.