A highly birefringent metal-free crystal assembled by cooperative non-covalent interactions.

Birefringent crystals can manipulate the phase and polarization of light, so they are widely used as essential components in various optical devices. Common strategies to construct birefringent crystals are introducing metal cations that are either able to realize favorable coordination with functional anionic units or are susceptible to polarizability anisotropy. Herein, we report a metal-free crystal, NH4(H2C6N7O3)·2H2O, synthesized using the facile solution method. In the crystal structure of NH4(H2C6N7O3)·2H2O, (H2C6N7O3)- functional units are assembled in an optimal manner by cooperative non-covalent interactions, i.e., hydrogen bonding and π-π interactions. As a result, this metal-free crystal possesses exceptional birefringence up to 0.54@550 nm, which is larger than those of most metal-containing birefringent crystals. In addition, the interference color of this crystal does not change obviously from 243 K to 313 K, indicating that the birefringence is robust at different temperatures. This work will inspire useful insights into the role of non-covalent interactions in designing outstanding birefringent crystals for efficient polarized optical devices.

Deep-Ultraviolet Bialkali-Rare Earth Metal Anhydrous Sulfate Birefringent Crystal.

Birefringence is an important linear optical property of anisotropic crystals that plays a significant role in regulating light polarization. A new bialkali-rare earth metal sulfate, NaRbY2(SO4)4 compound, consisting of non-π-conjugated alkali metals and rare earth metal-centered dodecahedral YO8 has been synthesized. The structure analysis suggests that the three-dimensional (3D) structure of the compound is found to be attributable to the combination of dodecahedral YO8 and tetrahedral SO4 groups with Na+ and Rb+ located in the cavities. The ultraviolet, visible, and near-infrared (UV-vis-NIR) spectra reveal that the compound exhibits transparency at a wavelength of less than 200 nm. The observed birefringence of the compound is 0.045@550 nm, which is comparatively larger than that of most deep-ultraviolet (DUV) birefringent crystals. The birefringence mainly originated from the YO8 dodecahedron, which is suggested by first-principles calculations. This research work can provide a useful perspective to explore new DUV sulfates with excellent birefringence.

A Second-Order Nonlinear Optical Material Consisting of Two π-Conjugated Groups.

This work reports a new second-order nonlinear optical (NLO) material [C(NH2 )3 ]3 C3 N3 S3 (GU3 TMT), consisting of π-conjugated planar (C3 N3 S3 )3- and triangular [C(NH2 )3 ]+ groups. Interestingly, GU3 TMT exhibits a large NLO response (2.0×KH2 PO4 ) and moderate birefringence 0.067 at wavelength 550 nm, even though (C3 N3 S3 )3- and [C(NH2 )3 ]+ do not exhibit the most favorable arrangement in the structure of GU3 TMT. First-principles calculations suggest that NLO properties mainly originate from the highly π-conjugated (C3 N3 S3 )3- rings, and the π-conjugated [C(NH2 )3 ]+ triangles contribute much less to the overall NLO response. This work will inspire new thoughts with in-depth on the role of π-conjugated groups in NLO crystals.

Variable Asymmetric Chains in Transition Metal Oxyfluorides: Structure-Second-Harmonic-Generation Property Relationships.

Four novel transition metal oxyfluorides, [Zn(pz)3][MoO2F4]·0.1H2O (1), [Zn(pz)2F2][Zn(pz)3]2[WO2F4]2 (2), [Cd(pz)4][Cd(pz)4(H2O)][MoO2F4]2·0.625H2O (3), and [Zn(mpz)3]2[MoO2F4]2 (4) (pz = pyrazole; mpz = 3-methyl pyrazole) have been synthesized. Compounds 1 and 4 contain helical chains. Compound 2 accommodates zigzag chains, and compound 3 has quasi-one-dimensional linear chains. The variable chain structures are found to be attributable to the different structure-directing anionic groups and hydrogen bonding interactions. Compound 4 crystallized in the noncentrosymmetric (NCS) polar space group, Pna21, is nonphase-matchable (Type I), and reveals a moderate second-harmonic-generation (SHG) efficiency (10 × α-SiO2). The observed SHG efficiency of compound 4 is due to the small net polarization occurring from the arrangement of ZnN3F2 trigonal bipyramids. Spectroscopic and thermal characterizations along with calculations for the title materials are reported.