π-Conjugated Trigonal Planar [C(NH(2))(3)](+) Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials

[Image: see text] Employing π-conjugated anionic groups in molecular construction has been proven to be an effective strategy to find superior ultraviolet (UV) nonlinear optical (NLO) crystals over the decades. Herein, unlike the traditional π-conjugated anionic groups, we identify that a π-conjugated cationic group, viz., [C(NH(2))(3)](+), is also an excellent UV NLO-active functional group in theory. Furthermore, we identify a [C(NH(2))(3)](+)-containing compound, C(NH(2))(3)ClO(4), as a promising UV NLO candidate due to its short UV cutoff edge (200 nm), remarkable second-harmonic generation effect (∼3 × KDP), and moderate birefringence of 0.076@1064 nm. Additionally, C(NH(2))(3)ClO(4) has excellent ferroelectric properties and reversal of domains, which also enables it to produce ultraviolet coherent light as short as 200 nm by a quasi-phase matching technique with a periodically poling method. Our study may provide not only a promising UV NLO crystal but also a new π-conjugated functional unit, [C(NH(2))(3)](+), which will open a path to finding new classes of high-performance UV NLO crystals.