An Exceptional Thermally-Induced Four-State Nonlinear-Optical Switch Arising from Stepwise Molecular Dynamic Changes in a New Hybrid Salt
The switch materials in channel of nonlinear optics (NLO) are of particular interest in NLO material science. Numerous crystalline NLO switches emerged based on structural phase transition, but most of them reveal single-step switch between two different second-harmonic-generation (SHG) states, very rare case involves three or more SHG states. Herein, we report a new organic-inorganic hybrid salt, (Me 3 NNH 2 ) 2 [CdI 4 ], which is an unprecedented instance of reversible three-step NLO switches between SHG-silent, -medium, -low, and -high states, with high contrasts of ~25.5/4.3/9.2, respectively, 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 disclosed that this four-state switchable SHG behavior is highly associated with the stepwise changed molecular dynamics of the polar organic cations. This finding well demonstrates a complexity of molecular dynamics in simple hybrid salts and its potential in designing new advanced multi-step switching materials.