Synergistic Stimulation of Metal-Organic Frameworks for Stable Super-cooled Liquid and Quenched Glass.

Metal-organic framework (MOF) glasses are a fascinating new class of materials, yet their prosperity has been impeded by the scarcity of known examples and limited vitrification methods. In the work described in this report, we applied synergistic stimuli of vapor hydration and thermal dehydration to introduce structural disorders in interpenetrated dia-net MOF, which facilitate the formation of stable super-cooled liquid and quenched glass. The material after stimulus has a glass transition temperature (Tg) of 560 K, far below the decomposition temperature of 695 K. When heated, the perturbed MOF enters a super-cooled liquid phase that is stable for a long period of time (>104 s), across a broad temperature range (26 K), and has a large fragility index of 83. Quenching the super-cooled liquid gives rise to porous MOF glass with maintained framework connectivity, confirmed by EXAFS and PDF analysis. This method provides a fundamentally new route to obtain glassy materials from MOFs that cannot be melted without causing decomposition.

An Exceptional Thermally Induced Four-State Nonlinear Optical Switch Arising from Stepwise Molecular Dynamic Changes in a New Hybrid Salt.

Switching materials in channels of nonlinear optics (NLOs) are of particular interest in NLO material science. Numerous crystalline NLO switches based on structural phase transition have emerged, but most of them reveal a single-step switch between two different second-harmonic-generation (SHG) states, and only very rare cases involve three or more SHG states. Herein, we report a new organic-inorganic hybrid salt, (Me3 NNH2 )2 [CdI4 ], which is an unprecedented case of a reversible three-step NLO switch between SHG-silent, -medium, -low, and -high states, with high contrasts of 25.5/4.3/9.2 in a temperature range of 213-303 K. By using the combined techniques of variable-temperature X-ray single-crystal structural analyses, dielectric constants, solid-state 13 C nuclear magnetic resonance spectroscopy, and Hirshfeld surface analyses, we disclose that this four-state switchable SHG behavior is highly associated with the stepwise-changed molecular dynamics of the polar organic cations. This finding demonstrates well the complexity of molecular dynamics in simple hybrid salts and their potential in designing new advanced multistep switching materials.