Highly Polarizable Phosphorus‐Centered Tetrahedra and π‐Conjugated Carbon‐Centered Triangular Moieties Inducing Large Anisotropy and Short‐Wavelength Phase‐Matching

Abstract Current nonlinear optical (NLO) materials encounter a classic dilemma in balancing the band gap and birefringence, particularly in the short wavelength (λ PM ≤ 266 nm) region. Utilizing both π‐conjugated and non‐π‐conjugated units to construct the short wavelength NLO crystals has emerged as the premier choice nowadays. Herein, the combination of π‐conjugated [CO 3 ]/[C(NH 2 ) 3 ] moieties and non‐π‐conjugated phosphorus‐based tetrahedra generates nine new crystals, whose birefringence obtains a significant gain compared to short wavelength phosphate crystals. Experimental results demonstrate that the compounds have achieved a balance between bandgap and birefringence. Especially, the large birefringence induces the shortest phase‐matching wavelength of [CN 4 H 7 ]H 2 PO 2 and [CN 4 H 7 ]HPO 2 (OH) to be about 226 and 200 nm, respectively, indicating that they have potential applications as short wavelength NLO crystals. It is worth mentioning that, [CN 4 H 7 ]HPO 2 (OH) has the shortest phase‐matching wavelength among crystals composed of guanidine and P‐based tetrahedral groups, to the best of the knowledge. Even more, this study provides a novel and essential design strategy for short‐wavelength NLO crystals.