Spin-exchange quenching

A precedent to the spin-exchange quenching mechanism of olefin triplets by oxygen is provided by the interaction of stilbene triplets with the stable free radical di-rm-butyl nitroxide, (50). In this case triplet-sensitized ([t]/[c])j... 

Comparison of the enone and dienone data in Table 4 again leads to the conclusion that in the absence of structural constraints a p -type geometry is attained which undergoes spin-exchange quenching by O2 (62,63,64,67,68). 

Because of the spin exchange requirement, a close approach of the fluorescer and the quencher is required. Hence oxygen quenching is diffusion controlled. The increase in the rates of injersystem crossing has... 

Although the overall spin quantum number of the system M /Q rnust be kept in electron exchange, quenching can take place between singlet and triplet states in any combination ... 

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... 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

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