Aromatic triplet states

Furrer R, Fujara F, Lange C, Stehlik D, Vieth H and Vollmann W 1980 Transient ESR nutation signals in excited aromatic triplet states Chem. Rhys. Lett. 75 332-9... 

Photochromism Based on Triplet Formation. Upon absorption of light, many polycycHc aromatic hydrocarbons and their heterocycHc analogues undergo transitions to their triplet state which has an absorption spectmm different from that of the ground state (24). In rigid glasses and some plastics, the triplet state, which may absorb in the visible, has a lifetime of up to 20 seconds. 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

Oxepin has also been converted photochemically to phenol in 74% yield. This reaction occurs under irradiation conditions by which benzene oxide is excited to a triplet state, e.g. by irradiation in acetone as solvent.207 A rare example for a nucleophilic catalysis of the aromatization of an oxepin/benzene oxide to a phenol has been reported for /err-butyl oxepin-4-carboxylate which undergoes a rearrangement reaction in the presence of trimethylamine to give a mixture of /m-butyl 3-hydroxybenzoate (94%) and 4-hydroxybenzoate (6%).243... 

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. 

The sensitizer in our experiments is benzophenone (BP) which reacts as shown in Scheme 2. UV light of 300 to 400 nm is absorbed and excites the aromatic ketone group to a singlet state which by intersystem crossing (ISC) reverts to a triplet state, abstracts a... 

With aromatic carbonyls, oxetane formation appears to arise from the carbonyl triplet state, as evidenced by quenching studies. For example, benzaldehyde irradiated in the presence of cyclohexene yields products indicative of hydrogen abstraction reactions and an oxetane ... 

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 ... 

With the exception of the a-naphtho position, bridging to aromatic sites is very unfavorable in the barrelene-semibullvalene rearrangements. If we exclude anthraceno-vinyl bridging, there is insufficient energy available (43 kcal/mole) for vinyl-vinyl bridging (58 kcal/mole) in the lowest anthrabarrelene triplet state. Consequently, it is not surprising that the 7 state is unreactive. In contrast, (76 kcal/mole) and T2 (74 kcal/mole) are not subject to this limitation. Evidence in favor of one or the other of these two possible electronic states is not available. 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

This method can be successfully applied to any aromatic hydrocarbon whose lowest excited singlet and triplet states are lower in energy than the substrate... 

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 —... 

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