Relating Excited States to the Dynamics of Macroscopic Strain in Photoresponsive Crystals

Exposure of a photoreactive single crystal to light with a wavelength offset from its absorption maximum can have two distinct effects. The first is the direct effect, wherein the excited state generated in individual chemical species is influenced. The second is the indirect effect, which describes the penetration of light into the crystal and hence the spatial propagation and completeness of transformation. We illustrate using the nitro-nitrito isomerization of [Co(NH3)(5)NO2]Cl(NO3) as an example that the direct and indirect effects can be independently determined. This is achieved by comparing the dynamics of macroscopic crystal deformation (bending curvature and crystal elongation) induced by the photochemical reaction when irradiating a crystal at the absorption maximum and at different band edges (above or below the maximum) of the same band. Quantitative description of the macroscopic strain dynamics in comparison with experiments allowed us to suggest that irradiation at different tails of the same absorption band causes isomerization to proceed via different excited states and an additional photochemical reaction (presumably, reverse nitrito-nitro isomerization) can occur on irradiation at the ligand-field band edges.

Automated summary

This study investigates the direct and indirect effects of exposing a photoreactive single crystal to light at wavelengths offset from its absorption maximum. By comparing the dynamics of macroscopic crystal deformation under different wavelength irradiation, it reveals that different excited states are involved in isomerization when irradiated at different tails of the same absorption band.