Breaking the π‐Conjugation‐Dependence: Enhanced Birefringence and SHG Response in Densely Packed Deep‐UV Phosphates

ABSTRACT Optimizing birefringence and SHG in deep‐UV NLO crystals based on tetrahedral oxyanions remains challenging as extending π‐conjugation often sacrifices UV transparency. In this study, we propose a synergic design strategy that correlates π‐conjugation degree, packing density, and optical performance, resulting in two formamidinium phosphate crystals, enabling two deep‐UV NLO crystals, [CH(NH 2 ) 2 ]H 2 PO 4 ( I ) and [CH(NH 2 ) 2 ] 2 HPO 4 ( II ), with simultaneously enhanced birefringence (0.10@550 nm of I and 0.15@550 nm of II ) and SHG responses (1.1 × KDP of I and 2.0 × KDP of II ). Notably, birefringence of compound II outperforms a number of guanidinium‐based and phosphate‐based NLO crystals, endowing it with exceptional comprehensive performance, as evidenced by its highest quality factor ( Q = 4.4 for II) . Due to the superior birefringence and SHG intensity, both I and II realize phase‐matching at fundamental wavelength of 1064 nm and 532 nm. Structure–property relationship studies reveal that formamidinium's small steric size facilitates dense packing of building blocks within lattice, thereby enhancing birefringence and SHG. This work elucidates the critical influence of π‐conjugation degree on packing density and overall performance (SHG intensity and birefringence). Based on these findings, a novel design pathway for deep‐UV materials might be proposed, linking π‐conjugation degree and packing density with SHG and birefringence.