π-Conjugated Trigonal Planar [C(NH₂)₃]⁺ Cationic Group: A Superior Functional Unit for Ultraviolet Nonlinear Optical Materials

Abstract

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.

Sharp Enhancement of Birefringence in Antimony Oxalates Achieved by the Cation–Anion Synergetic Interaction Strategy

“…Recently, [C­(NH 2 ) 3 ] + (Gu) cationic groups with a planar π-conjugated triangular structure drew our attention. , It is found that the Gu cation has a wide range of UV transmission, which could replace alkali metal/alkaline earth metal cations to ensure the transmission of materials in the UV region. Moreover, its unique planar π-conjugated structure not only exhibits a large anisotropic polarizability, close to that of the classical BO 3 , NO 3 , and CO 3 anionic groups, but also acts as a good structure-directing template to tune the structure, because the linkage with the Gu cation could only extend along the C–N plane.…”

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation

“…Very recently, an organic π-conjugated [C­(NH 2 ) 3 ] group, as a new type of NLO-active FBUs, has attracted a great deal of research interest. , The [C­(NH 2 ) 3 ] (guanidine cation) group is analogous to the inorganic [BO 3 ] group, in which the central C atom links with three N atoms to create a planar configuration. Hence, it can be predicted that guanidine-based crystals may show a large band gap and optical anisotropic polarizability, and some experimental results have been reported, such as C­(NH 2 ) 3 ClO 4 , [C­(NH 2 ) 3 ] 3 PO 4 ·2H 2 O, BaGuCl 2 , and C­(NH 2 ) 3 SO 3 F . It has shown that the rational combination of two or more types of NLO-active FBUs can produce promising NLO crystals. ,, Therefore, it is interesting to develop guanidine-based NLO crystals by combining different functional moieties.…”

Unprecedented boat-shaped [Mo2O5(IO3)4]2− polyanions induced a strong second harmonic generation response

“…Recently, the introduction of organic cation groups with structural diversities into material systems with inorganic anion groups has produced many superior organic–inorganic hybrid NLO materials, such as C(NH 2 ) 3 ClO 4 (3 × KDP), 40 [C(NH 2 ) 3 ] 6 (PO 4 ) 2 ·3H 2 O (3.8 × KDP) 41 and (C 5 H 6 ON) + (H 2 PO 4 ) − (3 × KDP) etc. 42 To our knowledge, only one case of an organic–inorganic hybrid NLO iodate has been studied, that is [ o -C 5 H 4 NHOH] 2 [I 7 O 18 (OH)]·3H 2 O (8.5 × KDP).…”