Direct Observation of Crystal Structures and Raman Spectra during Photochemical Changes of Organic Charge-Transfer Complexes: Thermodynamic Aspect of Heavy Atom Effects

For insight into photochemical processes, it is required to observe structures and spectra with the corresponding thermodynamic functions and electronic states. We have compared the photochemical processes between MV[NiL2]2 (L = 1,3-dithole-2-selone-4,5-dithiolate, the selone derivative, or L = 1,3-dithiole-2-thione-4,5-dithiolate, the thione derivative), which exhibited sufficiently slow photochemical changes to observe them by various methods, to study heavy-atom effects on photoluminescence. X-ray single-crystal structural analyses, first-principles calculations, and Raman spectra indicated that, in both derivatives after cessation of photoirradiation, their molecular orbitals, charge distributions, and thermodynamic quantities remained changing for weeks to months. Triggered by a 532 nm laser, the selone derivative more efficiently emitted infrared light than did the thione derivative. Comparison between the two derivatives revealed structural and thermodynamic aspects of heavy-atom effects. The findings bridge macroscopic and microscopic viewpoints of photoinduced phenomena in materials, providing experimental grounds for theories.