Directional Construction of New Nonlinear Optical Bifunctional Units through Molecular Engineering Design Inspired by the B₃O₇-Typed Configuration

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the “structure-analogue” strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC2N5O2 and B(C2N5)2 units. The BC2N5O2 unit with a highly analogous structure to the B3O7 group and its derivate B(C2N5)2 unit with a configuration of B5O10 group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC2N5H6(OH)2·H2O (I) and B(C2N5H6.5)2(NO3)2 (II), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (ΔnI = 0.181 @ 546.1 nm and ΔnII = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC2N5O2 and B(C2N5)2 units make great contributions to the SHG effects and birefringence, which confirms that the BC2N5O2 and B(C2N5)2 units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.