Quantum yield azoalkanes

The quantum yield of the selected azoalkanes photodecomposition are given in Table 3.18. The extinction coefficient s depends on the wavelength of light, and for AIBN in benzene solution (room temperature) has the values as given below [205] ... 

Quantum Yields of Azoalkanes Photolysis in Solution at Room Temperature RN2R + hv — N2 + Products ([205])... 

In the gas phase however decomposition can occur with unit efficiency Here electronically as well as vibrationally excited molecules are involved in the decomposition step. The azomethane photolysis in the gas phase leads to decomposition with s2 = 1 at lower pressures 55b>. With increasing pressure the quantum yield < N2 drops from 1 to lower values. This is attributed to partial intersystem crossing of the excited azoalkane to the triplet state which gives only dsjtrans isomerisation. 

In agreement with the theoretical results, the photoisomerization of simple azoalkanes is found to be rather effective. For azomethane in benzene at 25 C quantum yields of = 0.42 and = 0.45 have been observed (Thompson et al., 1979). Cis-azo compounds are moderately stable. Only tertiary ct -azoalkanes are thermally unstable and decompose to nitrogen and radicals. (See Section 7.2.2.)... 

The mechanism of the photochemical deazetation of these DBH derivatives (polycyclic azoalkane), which exhibit a weak (n, n ) ultraviolet (UV) absorption at about 350-370 nm [19], has been thoroughly studied and is believed to proceed through the intermediary diazenyl diradicals [20], with quantum yields near unity [21]. Significantly, even on direct excitation, the azoalkanes 8 undergo very effective intersystem crossing from the singlet [ , jr ] to the triplet 3[n, rc ] state [22] and form triplet diradicals. The photodeazetation constitutes a convenient source of such diradicals even at... 

The bicyclic derivatives 2,3-diazabicyclo[2.2.1]hept-2-ene, DBH (1), and 2,3-diazabicyclo[2.2.2]oct-2-ene, DBO (2), serve as instructive probes for the photochemical and photophysical properties of azoalkanes in view of their contrasting behavior. While derivatives of DBH (1) readily extrude molecular nitrogen from their excited states with quantum yield close to unity, - DBO (2) and most of its derivatives are photoreluctant, that is, they exhibit very low decomposition yields. - With a few exceptions, substituents in the DBH and DBO molecules do not significantly alter the photochemical properties of the parent molecules. Despite the contrasting photochemical reactivity of DBH (1) and DBO (2), their photophysical characteristics are similar. For both 1 and 2, the intersystem crossing is inefficient and triplet lifetimes are very short consequently, these azoalkanes do not phosphoresce, either in solution or even in matrix at 77K. ... 

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