The effect of thermal expansion on photoisomerisation in the crystals of [Co(NH3)5NO2]Cl(NO3): different strain origins – different outcomes

The effect of anisotropic structural strain resulting from thermal expansion on linkage photoisomerisation in crystals of [Co(NH3)5NO2]Cl(NO3) was studied. The quantum yield of the photoisomerisation was qualitatively correlated with the bending of needle-shaped crystals across a series of temperatures from 80–363 K. The thermal evolution of the crystal structure was also studied across the same temperature range by single-crystal X-ray diffraction. A qualitative correlation was found between the photo-reaction rate and both the cell volume and the volume of void space around the nitro groups. However, this correlation was found to be non-linear. The effect of the anisotropic structural strain resulting from temperature variations on the rate of photoisomerisation is qualitatively different from the effect of strain that is generated by elastic external loading. If strain is induced by external load, the reaction rate correlates linearly with compression along the crystallographic b direction. In contrast, if strain results from temperature variations, then the reaction rate correlates linearly with changes along the crystallographic a direction. This effect is measured quantitatively and its rationalisation is attempted.