Cation‐Dominated Second‐Order Nonlinear Optical Response Enabled by π‐Conjugated Guanidinium Derivatives

Disentangling structure–property relationships and identifying high‐performance functional building units are essential for the rational design of ultraviolet (UV) nonlinear optical (NLO) crystals. Herein, we report a cation‐dominated NLO response in a new hybrid UV crystal, GALSO 3 CH 3 , in which the NLO activity of guanidino‐acetic lactam (GAL + ) cation was activated by a stepwise molecular orbital regulation engineering. The GAL cation, belonging to the guanidinium family, is obtained through intramolecular cyclization of guanidino‐acetic acid, leading to a systematic enhancement of π‐electron delocalization, molecular polarity, and polarizability anisotropy while preserving a wide electronic band gap. Consequently, GALSO 3 CH 3 exhibits a short UV cutoff edge (∼214 nm), moderate birefringence (∼0.06), and a large and phase‐matchable second‐harmonic generation (SHG) response approximately four times that of KH 2 PO 4 . First‐principles calculations reveal that over 90% of the SHG response originates from the π‐conjugated GAL cations. This work identifies guanidinium‐derived cations as NLO‐active building units and introduces a π‐conjugation‐enhanced strategy for UV NLO materials.