Transfer of Excitation Energy Sensitisation and Quenching

Only the donor molecule absorbs the incident light, and the triplet excited state energy of donor is at least 3 kcal/mol greater than that of triplet excitation energy of Acceptor [ 7 = )] Light absorption by donor produces singlet 

Let us discuss some specific examples showing photochemical sensitisation. Direct irradiation of 1, 3-butadiene in solution gives cyclobutene and bicyclobutane with minor amount of dimers. 

Intersystem crossing efficiency approaches zero in 1, 3-butadiene and triplet derived products are not formed. Triplet excited 1, 3-butadiene is produced by photosensitisation, in which triplet excited benzophenone is used as sensitiser. Triplet excited 1, 3-butadiene produces only dimers. 

Benzophenone is used as sensitiser for 1,3-butadiene because triplet energy of benzophenone (69 k cal mor ) is slightly greater than the triplet energy of 1, 3-butadiene (60 k cal mol ). Now overall reaction mechanism is as follows. 

In any transfer of electronic energy between an excited species and a ground state species, the overall spin angular momentum does not change. Thus, energy transfer is subjected to the Wigner spin conservation rule. In photosensitisation, there are following allowed spin possibilities. 

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