Detection of the Photoreversibility of NO₂-ONO Linkage Isomerization in [Co(NH₃)₅NO₂]Cl(NO₃) Crystals by the Photomechanical Response Method

The linkage isomerization of NO₂-ONO (nitro-nitrito) in the complex cation [Co(NH₃)₅NO₂]²⁺ is a well-studied classical reaction. It is believed that the photoisomerization of the nitro form to the nitrito form in the crystalline phase reaches complete transformation at a low temperature, while the reverse transformation proceeds as a first-order thermal intramolecular reaction upon the heating of the crystals. To date, there is no information on the possibility of reverse photoisomerization. In this study, photoisomerization in [Co(NH₃)₅NO₂]Cl(NO₃) crystals is investigated by the analysis of the crystal deformation caused by the transformation under irradiation at different wavelengths. A change in the lattice parameters during the transformation leads to reliably measurable elongation and bending of acicular crystals. It is shown that the limiting elongation of the crystal under prolonged irradiation depends on the wavelength, which proves the reversibility of photoisomerization. The quantum yield of the reverse reaction is estimated to be 0.04 of the quantum yield of the direct reaction.