From polyenals to retinal: an electron-spin-echo study of the triplet state

Abstract

The lowest triplet state T0 of all-trans polyenals dissolved in polyethylene films has been investigated by pulsed laser excitation in combination with electron-spin-echo spectroscopy, both in a magnetic field and in zero field. It is found to be a 3-pi-pi* state and the orientation of the fine-structure axes, the zero-field energies of the spin sublevels and their relative populating rates and decay rates have been determined for a number of unsubstituted polyenals, methyl-substituted analogues and retinal. Variations of these properties with the number of carbon-carbon double bonds and the substitution pattern will be discussed. As compared to the unsubstituted conjugated chains, methyl substitution hardly affects the zero-field splitting and the relative populating rates but accelerates the radiationless decay. The near equivalence of the triplet data for retinal and 3,7,11-trimethyl-2,4,6,8,10- dodecapentaenal shows that the triplet excitation in retinal extends over all six double bonds which suggests that the chromophore of retinal is more or less planar in T0.