Excited-state reactions photoelimination

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

Diffusion of molecules in rigid glasses is negligible within the lifetimes of excited molecules, so that the main reactions are unimolecular dissociations and isomerization. These are rather similar to liquid state reactions, but the fragments cannot separate through diffusion and often recombine to restore the reactants. There are exceptions when the photoproducts are in fact more stable than the reactants, as in the case of photoeliminations. 

In addition to photosubstitution and photoelimination reactions, in the cases of some Ni(II) complexes, photoexcitation of square-planar complexes Ni(TP) and formation of the photoassociative ligand-field (LF) excited state 3Blg can lead to photoaddition reactions yielding hexacoordinate complexes Ni(TP)L2 [65, 66, 75-77], Such processes differ from the second step of photosubstitutions since an excited complex participates in them and the addition is conditioned by the electronic structure of the complex in its excited state (see Table 3). 

The isotope effect for the photoelimination ( h/ =18 and 1.6 in acetonitrile and pyridine solutions, respectively) supports a pathway where there is partial cleavage of the Pt-H bond in the transition state. It has been suggested that the reaction likely occurs from the first singlet excited state, but no photophysical data was obtainable to support this premise. 

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