Quenchers azulene

More recently, photochemical reactions of 138 with cyclic and acyclic olefins have been described. When 138 is irradiated (Pyrex) with cyclo-heptatriene, [4 + 4]- and [4 + 6]-adducts (247-250, Scheme 16) are obtained in addition, photodimer 242 and o-dibenzoylbenzene (140) were isolated. The ratio of the [4 + 6]-adducts to the [4 + 4]-adducts [(249 + 250)/(247 + 248)] is increased in air-saturated benzene solutions compared with oxygen-free benzene solutions, and enhanced in heavy atom solvents (e.g., chloroform compared with cyclohexane) furthermore, this ratio is decreased in the presence of the triplet quencher azulene. These observations suggest that [4 + 6]- and [4 + 4]-adducts are formed by different mechanisms. 

Pham W, Weissleder R, Tung CH (2002) An azulene dimer as a near-infrared quencher. Angew Chem Int Ed Engl 41 3659-3662... 

Using azulene as a triplet quencher it has been shown that triplet states are not involved in the cis-trans isomerization of stilbene on direct excitation. Triplet-sensitized cis-trans isomerization, however, is observed and proceeds in both directions through a minimum in the triplet potential energy surface at a twisted geometry, often referred to as the triplet phantom state P. ... 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

Attempts to study further some mechanistic aspects of the triplet decay by using additives with low triplet energies led initially to confusion. It turned out that two groups of quenchers exist. One group of classical triplet quenchers (e.g., azulene, ferrocene, and /i-carotene) quench preferentially 3t to 11 3c is not accessible because of its very short lifetime. Molecules of the second group (e.g., oxygen and di-tert-butyl nitroxide) quench preferentially 3p to p via a spin-exchange mechanism (Section... 

A kinetic analysis of the behaviour of the styrene (40) upon irradiation using oxygen and azulene as quenchers has shown that the lifetime of the triplet state is... 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

Fig. 5 Tuning the absorbance wavelength of azulene-based quenchers...

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