Tautomerism and Geometric Isomerism in Arylazo‐Phenols and Naphthols. Part III. Orthohydroxy Derivatives and their Reversible Photochemical Reactions

Abstract Solutions of o‐hydroxy or o‐methoxy azobenzene and phenylazonaphthalenes were studied spectroscopically and photochemically in a wide range of temperatures and solvents. All the hydroxy derivatives exist in solution in the azo form, except for (I) and (IV) which show an equilibrium between the azo and hydrazone forms. The latter predominates in (IV), and in (I) and (IV) is responsible for the so‐called luminescence of o‐hydroxy azo compounds. Luminescence was absent in the other compounds. All compounds except (IV) undergo distinct spectral changes when irradiated with ultraviolet or visible light at temperatures sufficiently low to prevent thermal reversion. In the methoxy derivatives the changes are due to cis‐trans isomerization, as shown also by the activation energy of 23 kcal/mole for the thermal reversion. The same explanation is assumed for the hydroxy compounds. The detailed involvement of the hydrazonic form of (I) in these spectral changes is not clear. In rigid media at −187°C, irradiation of (I) with visible light causes a different spectral change, tentatively ascribed to a “hydrazone” → “azo” conversion. Heating the solutions reverts the changes caused by irradiation. The kinetics of this reversion in the hydroxy compounds are complex, showing that the reactions are either catalyzed or else pass through a hydrazonic form, thereby proceeding with much smaller activation energies and frequency factors. It is suggested that several of the eight possible configurations of the “hydroxy” compounds serve as intermediates in the photochemical and thermal conversions, while others coexist in thermal equilibrium.