Room-Temperature Phase Transition Material with Switchable Second-Order Nonlinear Optical Properties

Abstract

Phase transition materials with switchable second-order nonlinear optical (NLO) properties have attracted extensive attention because of their great application potential in photoelectric switches, sensors, and modulators, while metal-free organics with NLO switchability near room temperature remain scarce. Herein, we report a hydrogen-bonded metal-free organic crystal, 2-methylpropan-2-aminium 2,2-dimethylpropanoate (1), exhibiting a room-temperature phase transition and favorable NLO switchability. Through investigations on its thermal anomalies, dielectric properties, and crystal structures, we uncover that 1 holds a near-room-temperature phase transition at 303 K from noncentrosymmetric point group C 2v to centrosymmetric one D 2h , which is attributed to the order− disorder transformations of both tert-butylamine cations and dimethylpropionic acid anions. Accompanied by symmetry change during the phase transition, 1 exhibits reversible and repeatable NLO "on−off" switchability with a desirable switching contrast ratio of ca. 19 between high and low NLO states. This discovery demonstrates a metal-free organic crystal with NLO switching behavior near room temperature, serving as a promising candidate in smart and ecofriendly photoelectric functional materials and devices.

Second-Order Nonlinear Switching and Photoluminescence Properties of Cd-Based Hybrid Perovskite with High-Temperature Phase Transition

“…Responsive switching materials are indispensable in modern technology due to their capability to rapidly and reversibly alter their physical, chemical, or biological properties when exposed to external factors like temperature, light, pressure, or electromagnetic fields. − These unique attributes render them highly promising for various applications, including sensors, actuators, energy conversion and storage, and biomedicine. , Nonlinear optical (NLO) switching materials, in particular, have attracted considerable attention in current research, owing to their substantial potential in optical communication, laser technology, and optoelectronic displays. − Second-harmonic generation (SHG) is a fundamental NLO process that involves the interaction of two photons with a nonlinear material to produce a new photon with twice the energy and thus twice the frequency of the original photons. This process is highly sensitive to the symmetry properties of the material, typically requiring noncentrosymmetric structures apart from those crystallizing in the D 4 (422), D 6 (622), and O (432) point groups. − The ability to modulate the NLO effect by adjusting the composition and structure of the material enables precise control over the optical signal, facilitating the on–off switching mechanism. A particularly effective strategy for developing NLO switching materials is leveraging solid-to-solid structural phase transitions between noncentrosymmetric and centrosymmetric phases, allowing for reversible control of the SHG signal. − …”