An electron–spin–echo study of the nonradiative triplet state of polyenals

Ros, M.; Groenen, E.J.J.

doi:doi:10.1063/1.460151

The lowest triplet state T0 of the all-trans isomers of dodecapentaenal, decatetraenal, octatrienal, and hexadienal dissolved in polyethylene films has been investigated by electron-spin-echo spectroscopy in combination with pulsed laser excitation. Microwave transitions between the spin sublevels have been observed both in magnetic field and in zero field. For these polyenals T0 is found to be of pi-pi* character. The direction of the principal axes of the fine-structure tensors and the frequencies of the zero-field transitions have been determined. The largest zero-field splitting varies from 2496 MHz for dodecapentaenal to 5260 MHz for hexadienal. The zero-field splitting turns out to be inversely proportional to the number of double bonds in the conjugated chain, which is found to be compatible with a description of the triplet excitation as the promotion of an electron from the highest occupied to the lowest unoccupied molecular orbital. The upper spin sublevel is...Show moreThe lowest triplet state T0 of the all-trans isomers of dodecapentaenal, decatetraenal, octatrienal, and hexadienal dissolved in polyethylene films has been investigated by electron-spin-echo spectroscopy in combination with pulsed laser excitation. Microwave transitions between the spin sublevels have been observed both in magnetic field and in zero field. For these polyenals T0 is found to be of pi-pi* character. The direction of the principal axes of the fine-structure tensors and the frequencies of the zero-field transitions have been determined. The largest zero-field splitting varies from 2496 MHz for dodecapentaenal to 5260 MHz for hexadienal. The zero-field splitting turns out to be inversely proportional to the number of double bonds in the conjugated chain, which is found to be compatible with a description of the triplet excitation as the promotion of an electron from the highest occupied to the lowest unoccupied molecular orbital. The upper spin sublevel is preferentially populated in the intersystem crossing process and decays fastest with a lifetime of 14-mu-s for dodecapentaenal, of 28-mu-s for decatetraenal, and of 77-mu-s for octatrienal. This variation is adequately described by an energy-gap law, which indicates that the Franck-Condon factors play a dominant role in the decay process.Show less

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An electron–spin–echo study of the nonradiative triplet state of polyenals