Polyenes triplet-state photochemistry

Nonconjugated dienes and polyenes have triplet photochemistry which may be considered to arise from intramolecular interaction of one excited double bond with an isolated ground-state double bond. For example, the photocyclization of enrfo-dicyclopentadiene can be effected using acetone as a sensitizer.286 Other more flexible 1,5-dienes, when sensitized to triplet states, cross couple to yield bicyclo[2.1.1]-hexane structures. For instance, triplet mercury atoms convert both 1,5-hexadiene and 1,5-cyclooctadiene to such structures.267 Irradiation of the cyclooctadiene in the presence of cuprous chloride produces the tricyclo derivative in good yield266 but recent evidence again indicates that this latter reaction may proceed via free-radical intermediates.269... 

Truscott TG, Land EJ, Sykes A. The in vitro photochemistry of biological molecules III. Absorption spectra, lifetimes and rates of oxygen quenching of the triplet states of beta-carotene, retinal and related polyenes. Photochem Photobiol 1973 17 43-51. 

Collisional energy transfer is frequently called sensitization. It is an important tool in photochemistry because it provides access to excited triplet states of compounds whose singlets do not intersystem cross efficiently (such as alkenes and conjugated polyenes). It also provides access directly to the triplet states of compounds for determination of the multiplicity of the state responsible for a photochemical reaction. 

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