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A simple method for measurement of quantum yields for the singlet→triplet crossing process in solution has been developed. The compound to be tested is used as a sensitizer for one or more cis—trans isomerization reactions and the quantum yield of the induced process is measured. The results are discussed briefly

brate rod cells of the retina has been shown to be the chromo-

The triplet excitation energies of a number of compounds have been determined by emission spectroscopy. Some of the principles involved in the choice of photosensitizers are discussed.

A critical examination of the conditions under which the formalism of the Forster singlet energy-transfer theory may be used to determine the donor-acceptor separation from the experimental energy-transfer efficiency is presented. In particular, the importance of multipole transitions, exchange interaction, "before-relaxation" transfer, and transla-