Ultrasound and solvent-induced reversible chromism in a luminescent chalcogenidostannate

Solvothermal treatment of a mixture of Se, SnC12, CoC12, and dien (dien = diethylenetriamine) generated yellow crystals of [Co(dien)2]2Sn2Se6 (1), which could transform into black 2 with the same formula via rinsing with water. In the presence of water, ultrasound irradiation of black 2 went back to yellow 1. The reversible ultrasound and solvent-induced chromism has never been observed before. Compared with yellow 1, hyperchromism was apparent around 550 nm in the visible absorption spectrum of 2. The fluorescent properties for 1 and 2 were investigated and the mechanisms were validated by density functional theory calculations on the experimental geometries. In addition, electron paramagnetic resonance spectra of yellow 1 are similar to chromic black 2 in the signal at g = 4.22 and g = 2.19; the signal at g = 2.19 of 2 shows a sharp increase.