An Excellent Deep‐Ultraviolet Nonlinear Optical Crystal Realized via a Functional Group Based on a Two‐in‐One Strategy

Deep‐ultraviolet (DUV) nonlinear optical (NLO) crystals requires the simultaneous realization of strong second harmonic generation (SHG) intensity, short ultraviolet cutoff edge, and large birefringence, yet these critical properties are often mutually constrained. Herein, based on a “two‐in‐one” design strategy integrating guanidinium and carboxyl groups, a new functional group, C 4 H 9 N 3 O 2 (GPA), was developed by employing non‐π‐conjugated electronic isolation to effectively confine electron delocalization. Guided by theoretical calculations showing that GPA possesses a large hyperpolarizability, polarizability anisotropy, and HOMO‐LUMO gap, the centrosymmetric C 24 H 56 N 18 O 21 S 2 Zn 2 (Zn‐GPA(I)) was synthesized, which exhibits a short ultraviolet cutoff edge of 191 nm and a refractive index difference (0.213@546 nm). Moreover, owing to the separation of distinct bonding sites within the GPA groups, a new non‐centrosymmetric C 12 H 27 N 9 O 10 SZn (Zn‐GPA(II)), was successfully obtained. As an excellent DUV NLO crystal, Zn‐GPA(II) features a strong SHG intensity (2.1 × KH 2 PO 4 ), a short UV cutoff edge of 189 nm (6.13 eV), and a refractive index difference (0.206@546 nm), thereby maintaining a balance among these three fundamental optical properties. These results indicate that this two‐in‐one strategy for new functional group provides a new avenue for the development of DUV NLO crystals.