Giant Birefringence and Strong Third-Order Nonlinear Optical Response in a Hybrid Crystal Templated by a Push–Pull π-Conjugated System

High-performance birefringent and third-order nonlinear optical (NLO) crystals are indispensable components of advanced photonic devices owing to their excellent ability to precisely modulate light polarization. The conventional hybrid organic–inorganic (HOI) crystals often suffer from intrinsic limitations that hinder their performance. In contrast, our novel strategy employs a distinctive π-conjugated organic functional unit with an optimized push–pull configuration further integrated into a robust metal-halide framework. The novel crystal, namely, (C 5 H 5 N 3 O 2 ) 2 ZnCl 2 (2A5NPZC), represents a novel paradigm in HOI design and exhibits remarkable properties, including bulk crystal (15 × 7 × 4 mm 3 ), giant refractive index difference (0.40 at 514 nm (exp)), a high third-order NLO susceptibility (χ (3) = 4.442 × 10 –9 esu), excellent optical transmittance range (415–1400 nm), and superior optical limiting capability (6.3 mW/cm 2 at 532 nm). Our findings highlight the broader potential of push–pull-type π-conjugated units as a versatile platform for the rational design of high-performance materials well-suited for advanced photonic technologies.