The condensation of π-conjugated units: a new paradigm for designing high-performance optical crystals

The π-conjugated units condensation paradigm (π-CUCP) is defined as the formation of larger π-conjugated groups through the chemical bonding of small units possessing π-conjugated structures. Its core objective is to significantly enhance the macroscopic optical properties (birefringence or SHG response) by strengthening electron delocalization, increasing structural anisotropy, and optimizing band structures. Planar π-conjugated units like [BO₃]^3--[B₃O₆]^3- or [C(NH₂)₃]⁺-[C₃N₆H₇]⁺ serve as representative π-CUCP. π-CUCP effectively improves the microscopic optical properties of molecular structural units, such as polarizability anisotropy and hyperpolarizability, thereby enhancing the macroscopic optical performance of nonlinear optical and birefringent materials. This review systematically summarizes representative and emerging compounds of π-CUCP, primarily covering approximately 112 representative examples across three major categories: borates, carboxyl-C-N heterocycles and amino-C-N heterocycles. Further research indicates that incorporating π-CUCP into deep-ultraviolet optical compounds enhances birefringence and maintains relatively wide bandgaps, enabling phase matching within the short-wavelength spectrum. The introduction of π-CUCP into ultraviolet and deep ultraviolet birefringent optical materials provides a feasible and effective approach to enhance the birefringence. Through the integration of multiple exploratory examples, this paper proposes an innovative design strategy to elevate the birefringent properties of the compounds, paving a new direction for the development of high-performance optical materials.