Naphthalene singlet excited quenching

The stable nitroxyl radicals can be used for quenching the singlet excited naphthalene through an electron exchange mechanism156. 

The confusion generated in the initial report185 on photoracemization of sulfoxides has recently been removed with the postulate that naphthalene singlet forms an excited complex with sulfoxides.186 Thus, despite the fact that the singlet state of 36 lies at 113 kcal, some 23 kcal above that of naphthalene, 36 quenches the fluorescence of this hydrocarbon with kes = 3.2 x 107Af -1 sec-1. From the dependence of the quantum yield of racemization on sulfoxide concentration (Eq. 34), a value of kes = 2.3 x 107Af-1 sec-1 was deduced. Since these values are the same within experimental error, it follows that the singlet state of naphthalene is responsible for photoracemization. 

Since quenching experiments with piperylene and naphthalene showed no effect, it was suggested that the triplet state of the thiatriazole is not involved in the reactions.22 It was later shown that the photoreactions leading to nitrile, isothiocyanate, sulfur, and nitrogen actually take place from a singlet excited state of the thiatriazole.20 Thus the photoreduction of benzophenone in isopropyl alcohol is effectively quenched by addition of 5-phenylthiatriazole, but analysis revealed that all thiatriazole could be recovered from the photolysis mixture, indicating the lack of photoreactivity of the triplet state. 

Response theory describes the S-T transition probabilities in unsaturated hydrocarbons quite well more than 99 % of the So - Xi transition intensity is out-of-plane polarized in agreement with experiment for aromatics in ethylene, butadiene and naphthalene the y spin-sublevel of the T state is the most active one, where y is the long in-plane axis of the molecules [134,132]. The main difference between the triplet states of aromatic and aliphatic compounds is the lack of phosphorescence for the latter. We have related this to the fact that polyenes also lack fluorescence (or have very weak fluorescence). This have been explained from the effective quenching of singlet excited (tr r ) states, which is an inherent property for the short polyenes. Our results suggest that this situation also prevails for the lowest triplet states. 

Calculate the concentration required to quench 99% of singlet excited naphthalene, given that the lifetime of the naphthalene singlet state is 100 ns in the absence of dioxygen and A. /V-di methyl aniline and that the rate constant of diffusion /cdill is 5 x 109m 1 s [0.2m]... 

This reaction is not quenched by 2,5-dimethyl-2,4-hexadiene or naphthalene. C(3)- and C(5)-alkylsubstituted 2(5//)-thiophenones behave similarly, but alkyl groups on C(4) prevent this photosolvolysis [18]. The rates for these reactions are similar in water, methanol, ethanol, 2-propanol or t. butanol but about ten times slower in 2,2,2-trifluoroethanol. All this suggests that the (singlet) excited thiolactone is attacked at C(4) by nucleophilic reagents to give an intermediate ketene which then affords the -configured ester (or acid) selectively (Scheme 19). 

These authors observed phosphorescence from 4,4-diphenylcyclohexadie-none at 77°K (0-0 band at 68.8 kcal per mole) and pointed out that the similarity between the phosphorescence spectrum of the dienone and that of benzophenone indicates that the lowest triplet state of the former compound is n — IT. This evidence suggests that a triplet state of the dienone may be involved in the unsensitized rearrangement but is not sufficient to justify Zimmerman and Swenton s conclusion that such a state is demanded. Interestingly, naphthalene (triplet 0-0 band at 61 kcal per mole) sensitizes rather than quenches Reaction (110). Such sensitization has also been observed by Caspar and Hammond (unpublished results) in a similar system and can be best interpreted as involving singlet excitation transfer from naphthalene to the dienone. 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

Photoreduction of m-dinitrobenzene is suppressed by nitric oxide and atmospheric oxygen Quenching by naphthalene, which also has been observed, does not necessarily imply a triplet excited reacting state, since quenching of the excited singlet by naphthalene would be an alternative possibility. 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

An internal conversion process recently discovered in our laboratory may shed light on the subject. Murovla found that quadricyclene, 6, is a powerful quencher of the excited singlet states of naphthalene and other aromatic hydrocarbons. In the course of the quenching reaction, the quencher was extensively converted to bicyclo [2.2.1] hepta-2,5-diene, 7. The following mechanism was suggested. 

In the mechanism above, two excited naphthalene dimers are included similar to those mentioned earlier in Section V-C. This is because in certain solutions, e.g., pyrene at room temperature and naphthalene at — 105°C., delayed emission is observed from two excited states, the excited monomer and presumably an excited dimer.68 62 These results have been interpreted as being due to a mechanism in which triplet-triplet quenching produces a system of dimers, one of which emits and/or dissociates into an excited and an unexcited singlet state. The term excimer has been proposed to distinguish this type of excited dimer from those produced by direct absorption from a stable ground-state dimer.87 The role of excimers in energy transfer processes is still... 

New anionic photoreductions initiated by excited / -naphtholate anions have been presented recently. According to measurements of electron transfer quenchings of the jS-naphtholate excited singlet state [96], it was shown that this powerful reducing species is able to reduce all the benzenic compounds bearing two or more chlorine substituents and all naphthalenes and biphenyls bearing at least monochlorinated. This means of course that this process is applicable to the polychlorobiphenyls, well known for their undesired persistence in the environment. When applied to chloronaphthalene or monochlorobiphenyl, this anionic photodechlorination presents characteristics comparing favorably with other sensitized processes [175]. 

---