Structural-Dynamic Synergy in Hydrogen-Bond Reconfiguration Enables Room-Temperature Tunable Nonlinear Optical Materials

Abstract Tunable Nonlinear Optical (NLO) materials, capable of modulating nonlinear effects under external stimuli, have garnered considerable research interest. A central challenge in this field is the design of materials that integrate NLO performance with thermally responsive structural dynamics, which is essential for achieving reversible tuning behavior. Although hydrogen-bonded organic–inorganic hybrids have been previously investigated, substantial opportunities remain for optimizing their thermal hysteresis characteristics through rational structural design and control. Herein, we report a metal-free organic–inorganic hybrid borate, [C(NH2)3]3(B3O3F6), which undergoes a reversible room-temperature phase transition accompanied by a moderate thermal hysteresis window and favorable optical figures of merit. Thermally driven structural changes are governed by a concerted order–disorder transition of the polar guanidinium cations and concomitant reorganization of the hydrogen-bond network. Synergistic coupling between these two processes gives rise to the observed hysteresis behavior, as corroborated by kinetic and thermodynamic analyses. Quantitative hydrogen-bond topology evaluation and birefringence contribution calculations further substantiate this cooperative mechanism across the transition boundary. The temperature-induced reversible phase transition endows the compound with pronounced tunability of its second-order NLO response. Collectively, our findings establish that the deliberate coupling of cationic ordering with hydrogen-bond network reconstruction offers a viable pathway toward the development of hybrid NLO tunable materials with tailored optical responses near ambient conditions.