An investigation of antenna-initiated photochemistry involving intramolecular singlet and triplet energy transfer in polyfunctional steroids

Shaojia Anna Jiang, Purdue University

Abstract

The work in this dissertation is a study of photochemistry resulting from intramolecular singlet-singlet energy transfer (intra-SSET) and intramolecular triplet-triplet energy transfer (intra-TTET) among functionalities in polyfunctional molecules. Steroids have been prepared which bear an antenna group (dimethylphenylsiloxy (DPSO)) with either C3 and/or C17 ketones or C3 olefin/C17 ketone functionalities. The DPSO group has been used to harvest 266 nm photons and selectively activate C17 through intra-SSET. Irradiation of 6$\beta$DPSO/3,17-dione gives two ring D-derived photoproducts, an epimer and an enal, both coming from the C17 excited singlet state. Direct excitation of the ketone functionalities with 308 nm light gives the same products but with somewhat higher quantum efficiencies. The monoketone analogs, 6$\beta$DPSO/17-one, and 6$\beta$DPSO/3-one also exhibit DPSO initiated chemistry at the distal carbonyl groups. Intra-SSET from the DPSO antenna to the ketones is evidenced by reduced DPSO fluorescence quantum efficiencies and singlet lifetimes, e.g. for 6$\beta$DPSO/3,17-dione $\Phi\sb{\rm intra-SSET}$ is ca. 88% efficient and occurs with a rate of ca. 6.5 $\times$ 10$\sp9$ s$\sp{-1}$. The data also are suggestive of facile intra-SSET from the C3 ketone to C17 in 6$\beta$DPSO/3,17-dione. The quenching and TEA data indicate that the C3 chemistry in 6$\beta$DPSO/3,17-dione and 6$\beta$DPSO/3-one is completely triplet derived and that there is no intra-TTET between C3 and C17. Intra-TTET from the aryl DPSO group to the C3 carbonyl group is confirmed based on E $\leftrightarrow$ Z isomerization of the C3 olefin activated by the antenna in the trifunctional systems: 6$\beta$DPSO/17-one/3-E and its Z isomer.

Degree

Ph.D.

Advisors

Morrison, Purdue University.

Subject Area

Organic chemistry|Chemistry

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