THE POPULATION AND DECAY OF THE LOWEST TRIPLET STATE IN RETINENE
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Date
1962
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Ohio State University
Abstract
Population of the lowest triplet state in linear polyenes by flash exciation is negligible because the rediative and radiationless depopulation of the first excited $\pi, \pi^{\ast}$ singlet state to the ground state is faster by at least two orders of magnitude than the competing radiationless population of the lowest triplet by intersystem crossing. However, when an heteroatom is conjugated with the $\pi$ electron system as it is in the polyene aldehyde, retinenc, measurable population of the triplet is achieved by rapid radiationless internal conversion of the $\pi, \pi^{\ast}$ singlet to a lower lying $n, \pi^{\ast}$ singlet which then undergoes intersystem crossing to the triplet. When flash exciation of retinene is carried out in the mixed solvent methylcyclohexanemethanol a very sharp reduction in triplet population occurs with increasing concentration of methanol. The later supports the assumption of the formation of a hydrogen bonded complex between the methanol and the carbonyl oxygen of the retinene which removes the intermediate singlet pathway to the triplet. The formation constant for the retinene-methanol complex on a mole fraction basis is 7.4 while the constant for the water-retinene complex obtained in the mixed solvent system methanol-water is 30. Decay of the retinene triplet is strictly first order but the rate constant decreases sharply with methanol concentration in the methylcyclohexane-methanol system. Formation of a triplet retinene-methanol complex is supported by the data and a formation constant of about 5 is calculated. Because of the small difference in the formation constants of the ground state and triplet methanol complexes it is felt that the lowest triplet is $\pi, \pi^{\ast}$ rather than $n, \pi^{\ast}$. The triplet-triplet absorption spectrum of retinene is methanol shows two distinct peaks which support the existence of two retinene triplets---one a methanol complex---which are in equilibrium
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Author Institution: Department of Chemistry, Case Institute of Technology