Direct observation of slow photochemical changes in a molecular crystal: energies, spins, and structures

Abstract Photoinduced phenomena occurring in materials are inhomogeneous and non-equilibrium processes. They proceed too rapidly to observe without ultrafast time-resolved spectroscopy. We have discovered a material exhibiting slow photo-induced changes, enabling the direct observation of structural and physical properties using standard laboratory equipment. Upon UV irradiation, a crystalline charge-transfer complex, MV[Au(dmit)2]2 (MV = methyl viologen, dmit = 1,3-dithole-2-thione-4,5-ditholate), undergoes a change toward a different but closely related stable state. Because the atomic movement was highly restricted in the single crystals at ambient temperature and pressure, and because the Gibbs energy difference between the initial and final states was small, the entire changes required months to complete, as if the process were quasistatic. Thus, during the changes, we could observe the real-time crystal structures, energies, and electronic states using various types of laboratory equipment with similar accuracy and details to the structures, energies, and electronic states in steady-state observations. Real-time electron spin resonance, UV-Vis-NIR, and Raman spectra in addition to X-ray structural analyses revealed the dynamics in this material after UV irradiation. Theoretical energy calculations of the observed transient structure revealed how part of the absorbed photon energy was distributed and exchanged between neighboring molecules. The findings provide insight into the chemical and physical processes beginning with photoexcited states.