Cyclic azocompounds

In recent years several cyclic structures have been shown to give rise to a more or less intense emission when irradiated in their first (n, n ) absorption region. It 

EMISSION YIELDS AND FLUORESCENCE LIFETIMES IN THE PHOTOLYSIS OF 2,3-diazabicyclo-[2.2.1]heptene-2 (BDH) and 2,3-diazabicyclo[2.2.2]octene-2 

pressure has little effect on either decomposition or on fluorescence. Molecular oxygen suppresses the decomposition and fluorescence of BDO by the same extent. It appears that crossing of the dissociative state with the potential energy surface of BDO occurs at higher vibrational energies than in the case of BDH which fact manifests itself in the different pressure dependence of the two reactions. 

Andrews and Day observed fluorescence from methylene-1-pyrazolines. They also showed that energy transfer from the lowest excited singlet state to dienes (piperylene) occurs with moderate efficiency. The decomposition products from methylene pyrazolines have been postulated to be the resonance stabilized diradicals, 

The triplet benzophenone sensitized decomposition of several pyrazolines has been shown to lead to significantly different product distributions from those obtained in direct photolysis This seems to suggest that direct photolysis involves only singlet species and not concurrent singlet-triplet reactions and the benzophenone sensitized decomposition only triplet species. In agreement with this, the direct photolysis is not affected by triplet quenchers. 

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The lowest members of cyclic azocompounds are the diazirines. Photofragmentation of diazirines affords carbenes which can react in the normal way 63>. 

Some additional recent studies on cyclic azocompounds include those on 3-cyclopropyl 1-pyrazoline derivatives and on the 1-pyrazolines derived from 9-diazofluorene and bicyclic[2.2.1]olefins . ... 

In these cases the agreement is quite good. Landy et al. in their very accurate work [27] come to the concJusion that in the case of substituted phenols, NMR and UV titrations give mucdi more accurate results than ICD, due to the quite unfavorable g fador for these aromatic systems. In general, ICD and CD have more favorable g factors, if magnetically allowed but weak transitions are involved. This holds true for the n n transition of carbonyl compounds, and also for the n —> 71 transition of the azo chromophore in diazirines and other cyclic azocompounds. Indeed, in the latter cases exceptionally good results were obtained by ICD [17, 21, 23, 28, 29]. 

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