Aromatic triplet states nature

Aromatic ketones have lowest Si(n,7t ) states and the (n,7t ) and (71,71 ) triplet states are close together in energy. Which of these is the lower in energy depends on the nature of the substituents on the aromatic rings and on external factors such as the nature of the solvent. 

Hutchison reported the first ESR spectrum of a metastable phosphorescent state by study of naphthalene oriented in durene crystals.4 Since then, similar spectra have been recorded for several other polynuclear aromatics both oriented in host crystals and randomly suspended in glassy matrices. D values for all these ir,n excited states are quite low, indicating little interaction between the unpaired electrons. Interestingly, D for the quinolines equals 0.10cm"1 just as in naphthalene,197 indicating that the presence of a heteroatom does not necessarily change the ir,w nature of the lowest triplet state very much. A similar conclusion has been reached from a comparison of the ESR spectra of fluorene, carbazole, dibenzofuran, and dibenzothiophene.198... 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

For most carbonyl compounds, we expect to have two near-lying excited states in the triplet manifold, which are either n-n or tc-tc in character. The n-7T states frequently show radical-like behavior. Benzophenone is an example of such a molecule which has an n-7t triplet state in which we see occurrences of hydrogen abstraction and very efficient intersystem crossing. When the lowest state is the 71-71 state, largely centered on the aromatic part of the molecule, as in the case of p-methoxyacetophenone, the reactivity decreases significantly (8,9). With the nature of lignin and the nature of the model we have chosen, we are mostly interested in molecules which have this type of behavior. 

The environmental sensitivity of the fluorescence and phosphorescence of phenylalanine, tryptophan and tyrosine, and their side chains, is often examined when considering the macromolecular luminescence of natural peptides and proteins. Therefore, lower-lying singlet and triplet states of toluene, aniline and phenol have been extensively studied as the simplest models of the proteins mentioned above, respectively. Knowledge of the various aspects of electronic spectra of the corresponding aromatic amino acids is often exploited to probe those of the proteins137. In other words, accurate information on both... 

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