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, GALSO₃CH₃, 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, GALSO₃CH₃ 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 KH₂PO₄. 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.