The kinetics of the thermally induced cis-trans-isomerization of protonated azobenzene depend upon the amount of acid in a mixed H2SO4-H2O-solvent. This is rationalized by the assumption of a doubly protonated azobenzene molecule as an intermediate. Triplet sensitization of protonated azobenzene in 73% sulphuric acid (by weight) leads to cis-trans-isomerization and a photostationary state. This is similar to stilbene. With 1-aminonaphthalene as a sensitizer a photoreduction and cleavage of the azo group is an alternative reaction. It is shown that the cis-azobenzene molecule is the electron acceptor and that the photoreduction can be quenched by the addition of oxygen.
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