ELECTRONIC-STRUCTURE OF RETINAL AND RELATED POLYENALS IN THE LOWEST TRIPLET-STATE - AN ELECTRON-SPIN ECHO STUDY

We have observed transitions between the spin sublevels of the lowest triplet state T0 of the all-trans isomers of retinal and methyl-substituted polyenals by means of electron spin echo spectroscopy in a magnetic field and in zero field. This triplet state appears to be pi-pi* in nature. The frequencies of the zero-field transitions have been determined. For retinal we have observed the T(z)-T(x) transition at 2370 MHz and the T(z)-T(y) transition at 1990 MHz. The upper spin sublevel T(z) dominates the populating of T0 and decays fastest, but the selectivity in the decay is clearly less than in the populating. The lifetimes of the individual sublevels of the lowest triplet state of retinal are 71-mu-s for T(x), 17-mu-s for T(y), and 12-mu-s for T(z) at 1.2 K. The data for retinal correspond to those for a linear polyenal with six double bonds. This shows that the carbon-carbon double bond in the cyclohexene ring of retinal is also involved in the excitation, which suggests that the chromophoric part of retinal is approximately planar in the lowest triplet state.