The fluorescence and absorption spectra of two series of retinyh polyenes have been measured. One series consists of the retinyl-carbon moiety with different end groups, these being the alcohol, acetate, acid, oxime, and methyl-amine. The second group comprises the Irans, 11-cis, 13-cis, and 9-cis stereoisomers of retinol (vitamin A). The spectra of the first series have been measured at room temperature and 77°K in both hydrocarbon and methanol solvents. The acetate and acid have also been studied as crystals. The relative shapes and the degree of overlap of the absorption and fluorescence spectra are discussed in terms of simple potential-energy diagrams. It is shown that the bands are severely Franck-Condon forbidden on two counts; first, by virtue of the bond alternation in the ground state that leads to a gross change in geometry of the nuclear framework on excitation, and second, from steric hindrance that occurs at various points in the retinyl-carbon framework. The spectra reported here show how an end group or a bend in the chain can affect the nature of the vibrational envelope. The effect of an environment is shown to be particularly pronounced upon Franck-Condon forbidden bands. The quantum yields of fluorescence at room temperature have been determined for the Irans, ll-cis, 13-cis, and 9-cis retinols, and trans-retinyl acetate. The yields at room temperature are given in both hydrocarbon and polar solvents. The intensity of the fluorescence as a function of temperature has been estimated in some cases. Fluorescence could not be detected from retinyl acid and retinylidene methylamine at room temperature. The emission yield is shown to be correlated with the relative shapes of the ground- and excited-state potential-energy surfaces. It is suggested that vibrational relaxation in the excited state is the major process deactivating excited polyenes in fluid solution. The efficiency of this deactivating process is much reduced by increasing the viscosity of the environment. This process may be of great significance when polyenes are employed in photobiology. The radiative lifetime of /rare-retinol in methanol calculated from the data is 2.35 nsec.
