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

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(NH₂)₂]H₂PO₄ (I) and [CH(NH₂)₂]₂HPO₄ (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.