Triplet states photobiology

The photochemical and photobiological behaviour of coumarins is well documented. It is considered that the lowest excited triplet state plays an important role in such processes and information about the nature of the Ti state of coumarin has been derived from magnetic and spectroscopic studies (76JA3460). 

The question of singlet and triplet stability is not only of importance for carbenes but in many other diradicaloid systems as well. Especially triplet states are increasingly studied by material scientists because of the possible ferromagnetism of the appropriate compounds. In photobiology triplets may be involved in the primary steps of the light-induced charge separation which takes place... 

The major interest in such radical EPR studies lies in identifying the role of the triplet states in photobiological processes. The establishment of triplet state intermediates has now been confirmed in a wide variety of photoionization and sensitization reactions 76,80,81) in frozen solutions of nucleic acid bases, porphyrins including chlorophyll and, of course, the aromatic amino acids 82). Comparisons of the photo-induced products formed in these systems with observations in vivo provides strong evidence that the triplet state is an essential intermediate in many photobiological processes. 

Substituted coumarins, such as psoralen and a number of its derivatives, exhibit strong photobiological effects on bactria [6,7], viruses [8], and mammalian skin [9,10] in presence of light. Coumarin itself is also knovm to show interesting photochemical behavior, particularly dimerization [11-17], in polar and non-polar solvents. In polar solvents such as ethanol, the cis head-to-head dimer is formed presumably via interaction of excited singlet coumarin with its groud state, while in non-polar solvents such as benzene, trans head-to-head dimer is formed via the triplet state. No detailed study on the photolysis of coumarin, however, has been done in the solid state [11,18] nor in the inclusion state. 

J. K. Thomas, Effect of Si02 and zeolite surfaces on the excited triplet state of benzophenone, BT a spectroscopic and kinetic study, Photochemical Photobiological Sciences, vol. 3, no. 5, pp. 483—488, 2004. 

Salet C, Bensasson R (1975) Studies on thymine and uracil triplet excited state in acetonitrile and water. Photochemistry and Photobiology 22 231—235. 

These triplet excited states are important in photobiology, being characteristic of biologically active molecules such as carotenes and visual pigments (Section 4.3.6.3.). 

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