Systems Approach of Photoisomerization Metrology for Single-Crystal Optical Actuators: A Case Study of [Ru(SO₂)(NH₃)₄Cl]Cl

Chemicals that undergo photoisomerization in their single-crystal form may act as optical actuators or sensors. It is important to determine the molecular-scale characteristics of this photoisomerization as they govern the macroscopic nature of the optical phenomenon. In situ single-crystal X-ray diffraction has led the way in this field of characterization, with photoinduced crystal structures being realized via technical developments that have become known as “photocrystallography”. Single-crystal optical spectroscopy methods are less well developed in this research area, even though they provide synergistic information that is crucial for optimizing photoconversion levels in single-crystal optical actuators and enabling their comprehensive photoisomerization analysis. This paper demonstrates the importance of characterizing photoisomerism in crystalline media using a systems approach to photoisomerization metrology. Therefore, single-crystal optical spectroscopy, single-crystal Raman spectroscopy, and photocrystallography are concerted. The case study on [Ru(SO₂)(NH₃)₄Cl]Cl (1) shows how this approach optimizes its photoconversion levels, using key parameters that are established by single-crystal optical absorption spectroscopy. These parameters furnish the design of single-crystal Raman spectroscopy and photocrystallography experiments which are needed for a comprehensive photoisomerization analysis of 1. Raman shifts identify photostructural changes in isolated bonds as a function of temperature. One such shift leads to the realization of a previously unidentified photoisomer in 1, which is then confirmed by photocrystallography that renders its 3-D light-induced crystal structure. Both optical-spectroscopy methods are concerted with imaging to evidence photochromic changes in single crystals that occur upon photoisomerization and upon their thermal decay, whose characteristics are realized via single-crystal Raman spectroscopy. Overall, an end-to-end operational pipeline is showcased for more optimally characterizing the light-induced optical properties and structures of single-crystal optical actuators.