Controllable synthesis and spontaneous phase transition of photonic coordination polymer to produce a strong second-harmonic generation response

Herein, a novel photonic coordination polymer material was constructed by aggregation-induced emission luminogen (AIEgen) containing a tripyridyl moiety used as the linking ligand. It displayed a spontaneous direct centrosymmetric to noncentrosymmetric phase transition in a single crystal. The two crystals, before and after the phase transition, were both controllably synthesized and characterized by single-crystal X-ray diffraction. After being exposed to air, the centrosymmetric metastable phase (1-alpha) transitioned to a new stable phase with a noncentrosymmetric structure (1-beta). Interestingly, the 1-beta structure exhibited a strong phase-matching second-harmonic generation (SHG) response, about 4.5 times higher than that of KH2PO4 (KDP). In order to better understand the relationship between the structure and the nonlinear optical properties, the dipole moments were calculated and discussed. Remarkably, the noncentrosymmetric phase with high thermal stability for 1-beta retained and improved the initial photoluminescent properties of the AIEgen ligand after the structural phase transition from 1-alpha, and simultaneously produced the excellent SHG property, which are beneficial for the design and construction of excellent optical materials.