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

Current nonlinear optical (NLO) materials encounter a classic dilemma in balancing the band gap and birefringence, particularly in the short wavelength (lambda PM <= 266 nm) region. Utilizing both pi-conjugated and non-pi-conjugated units to construct the short wavelength NLO crystals has emerged as the premier choice nowadays. Herein, the combination of pi-conjugated [CO3]/[C(NH2)3] moieties and non-pi-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 [CN4H7]H2PO2 and [CN4H7]HPO2(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, [CN4H7]HPO2(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.