Triplet states identification

The lifetimes and kinetic behavior of many triplet states of molecules, particularly of aromatic compounds, have been studied by Porter and his coworkers. The identification of the absorption spectra of methyl radicals, of HCO radicals and of NH2 radicals, mainly in Herzberg s laboratory at Ottawa, has permitted flash photolysis to be used directly to measure the rates of radical reactions. 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

Several standard tests have been established to aid in the identification of the transient species. Solvated electrons generated by photoionization in a nitrogen-gassed solution have a characteristic broad structureless absorption peak at about 700 nm, depending on the solvent (720 nm in aqueous solution). Oxygen quenches this absorption and also quenches the triplet state, while nitrous oxide gassing can be used to quench the solvated electron only, thereby gaining an indication of any transient absorption that arises from the triplet state. 

Carotenoids function in photosynthesis as quenchers of chlorophyll triplet states to prevent their harmful reaction with oxygen. Current research has mainly focused on their detection and identification, the determination of kinetic parameters, and the elucidation of the triplet energy transfer pathways in both photosynthetic antenna and reaction centers. Since carotenoids do not take part in the photosynthetic electron transfer reactions, their paramagnetic radical species occur to a lesser extent in vivo, although they may play arole in the photoprotection of Photosystem II. 

Independently, a monomeric triplet state was observed in RCs of the carotenoid-less mutant Rb. sphaeroides R26, and later identified with BJby its characteristic microwave induced absorption (MIA) spectrum (Angerhofer and Aust, 1993 Hartwich et al., 1995). The identification was confirmed by the interaction band which is visible in the MIA spectrum of the spheroidene triplet state (Hartwich et al.,... 

However, the satisfactory explanation of phosphorescence as a radiative transition from the lowest triplet state had to wait till the decisive paper of G.N. Lewis and M. Kasha in 1944. Moreover, considering that the characteristics of the phosphorescence phenomenon, in terms of wavelengths, triplet lifetime, quantum yield, etc., are specific of the emitting molecule, these authors also suggested the potential of phosphorescence for the individual identification of organic compounds in complex mixtures. 

Thus, the rigid cage did not affect the rate of phosphorescence but rather protected the metastable state from the non-radiative decay, which reinforced the identification of this as the triplet state for organic molecules. Normal absorptiOTi... 

The use of triplet state acceptors has generally two roles. (1) Where the triplet state is to be removed and the transferred energy degraded as heat. Any passive absorber with lower triplet state energy than the donor will act in this way. Such a triplet state acceptor is also a photochemical stabiliser. (2) Where the triplet energy is to be trapped in a triplet state to be used for measurement or some specific function, e.g. energy transfer to an acceptor of known transient triplet absorption spectrum or emission. Here the acceptor triplet state photophysics and photochemistry must be known. Identification of energy transfer to such another known triplet state is often used as confirmatory evidence that a triplet state species is involved in the reaction under study. 

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