Thermotropic Structure Phase Transitions and Two Types of Thermochromic Behaviors in a Bromoargentate Cluster...

Organic-inorganic silver halide hybrids show abundant phase transitions and thermochromism. However, it is very rare that silver halides exhibit thermochromism related to thermotropic structure phase transition. Herein, a bromoargentate hybrid, [Pr-dabco]₂Ag₄Br₆ (1) (Pr-dabco⁺ = 1-propyl-1,4-diazabicyclo-[2.2.2]octan-1-ium), with tetranuclear [Ag₄Br₆]^2- clusters was prepared and characterized by microanalysis, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, and thermogravimetric (TG) and differential scanning calorimetry (DSC) techniques. Interestingly, 1 undergoes an irreversible structure phase transition at ∼436 K in the first heating process, which is accompanied by an abrupt color change from colorless to yellow; however, a reversible color change between pale yellow and yellow is observed in the next heating-cooling cycles. Notably, DSC measurement revealed that a reversible phase transition is associated with the change in color between pale yellow and yellow, while the powder X-ray diffraction (PXRD) patterns corresponding to pale yellow and yellow phases are quite similar to each other. These observations demonstrate that thermochromism in the next heating-cooling runs is associated with a reversible structure phase transition, which perhaps concerns the disorder-order transformation of alkyl chains in the cationic ligand [Pr-dabco]⁺, and relevant to the anharmonic fluctuations of the Ag-Br and Ag-N bonds, a strong electron-phonon coupling effect is seen within the bromoargentate cluster.