Aromatic molecules, triplet states

Intersystem crossing from Si to and higher triplet states has also been reported in some aromatic molecules Li, R. Lim, E.C. Chem. Phys., 1972, 57, 605 Sharf, B. Silbey, R. 

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

In contrast to aromatic hydrocarbons, heavy-atom substitution onto carbonyl and heterocyclic molecules appears to have little effect on radiative and nonradiative intercombinational transitions. Wagner(138) has shown that as determined by the type II photoelimination, aliphatic ketones (n -> it excited states) are not sensitive to external heavy-atom perturbation. As seen previously in our discussion of type II photoelimination, aliphatic ketones undergo this cleavage from both the excited singlet and triplet states (in... 

It is a wide-spread belief that such reactions could not be relevant, since they are spin-forbidden. This need not be true. It appears that the reduced flavin is a soft molecule, which resists planarity in the singlet state because of an anti-aromatic number of delocalized 7r-electrons. Hence, the planar conformation of Flre(j might have an unusually low-lying triplet state, which favors the thermal spin relaxation in RX —... 

The most widely used nomenclature in the field of luminescence of aromatic molecules is that proposed by Birks. 3) The photophysical species and processes in this scheme which are encountered in rigid systems of aryl vinyl polymers are described in Tables 1-4. Triplet excimers have been omitted since it will be shown later that all triplet states play minor roles in the room-temperature, air saturated P2VN blends studied recently 2). 

Singlet excited state acid dissociation constants pK can be smaller or greater than the ground state constant pK by as much as 8 units. Phenols, thiols and aromatic amines are stronger acids upon excitation, whereas carboxylic acids, aldehydes and ketones with lowest >(71, ) states become much more basic. Triplet state constants pKr are closer to those for the ground state. Forster s cycle may be used to determine A pK =pK —pK) from fluorescence measurements if proton transfer occurs within the lifetime of the excited molecule. 

One example of an aromatic molecule with markedly different singlet and triplet-state photochemistry is provided by [2.2]paracyclo-phane.203 Irradiation at wavelengths above 2700 A or photosensitization with acetone yields p-ethylbibenzyl as the sole product. This must be a triplet product because the reaction is quenched by naphthalene. Direct irradiation of the paracyclophane at 2537 A and lower wavelengths produces other products. 

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