Unique Chiro-optical Properties of the Weakly-2D (R-/S-MBA)₂CuBr₄ Hybrid Material

Abstract

We establish that the formally 0D (R-/S-MBA)2CuBr4, containing R-/S-α-methyl benzylamine (R-/S-MBA) connected to highly distorted CuBr4 tetrahedral units in alternating layers, possesses extraordinary chiro-optical properties. The concentration and path length-independent chiral anisotropy factor, g CD, for this compound is the highest in the orange-red part of the visible spectrum reported so far from any hybrid material, arising from a chirality transfer from the organic component to the inorganic layer through the extensive asymmetric hydrogen bonding network and electronic coupling, driving the CuBr4 tetrahedral units to follow the 21-screw axis. This sensitivity in the orange-red part of the visible spectrum is achieved by incorporating bromine in the copper coordination sphere, which significantly red-shifts the band edge absorption to ∼710 nm compared to ∼490 nm reported for the chloride analogue. DFT/TDDFT calculations allow us to understand the underlying electronic structure responsible for its remarkable optical properties. We find that this compound gets a partial 2D character, crucial for its broadband chiro-optical properties, arising from Cu–Br···Br–Cu interactions connecting the otherwise isolated CuBr4 units.

Family of Chiral Ferroelectric Compounds with Widely Tunable Band Gaps

“…As we replace Cl (ionic radius, 181 pm) with the larger ionic-sized Br (ionic radius, 196 pm), we find an expected increase in the unit cell volume from 1054.98(10) to 1059.2(7) to 1063.9(2) Å 3 for the compositions x = 0, 1, and 2, respectively. On the other hand, x = 4 compounds, ( R -/ S -MBA) 2 CuBr 4 , reported earlier in ref , , crystallize into an orthorhombic crystal system having a nonpolar but chiral space group P 2 1 2 1 2 1 with Z = 4 (crystallographic details are given in Table S4). Upon complete replacement of Cl with Br, the unit cell volume of the Br compound has become almost double the [2240.6(4) Å 3 ] volume of the Cl compound.…”

“…Unlike in the cases of corner-sharing PbX 6 units (X = Cl, Br, and I) in lead halides, the connectivities between isolated copper halide arrangements, unlike the weak 2D character in the case of pure bromine compounds. 24 The phase purity and successive random replacement of Cl with Br have been confirmed from the powder X-ray diffraction (PXRD) pattern. The PXRD patterns collected from all of these samples are shown in Figures 2 and S4 for the R-and Schiral samples, respectively.…”

“…However, we prefer to use 25% of Hartree−Fock exchange in the HSE06 functional to remain consistent with our earlier publication on (R-MBA) 2 CuBr 4 . 24 Interestingly, the small mismatch between the calculated and the experimental band gaps has a systematic dependence on the composition, increasing with the Cl content of the sample or, equivalently, with a decrease in the Br content (x). This is a consequence of increasingly strong ionic bonding in going from Br to Cl that decreases the covalent screening effects, thereby increasing the impact of Coulomb interactions in the system.…”

“…After ∼2 h, platelike millimetersized single crystals were formed. These crystals were washed thoroughly with n-hexane and dried under reduced pressure for 24 h. The synthesis procedure for the x = 4 compositions has been described in ref 24.…”

Tellurium(IV) Halide Achieving Effective Nonlinear-Optical Activity: The Role of Chiral Ligands and Lattice Distortion

“…As shown in Figure a, the mirror CD signals can be observed in R / S -Cl , demonstrating that as-prepared products possess optical chirality. Moreover, the CD spectra of R / S -Cl are clearly different from the inducers (Figure S9), indicating the successful transfer of chirality from the chiral organic molecules to achiral inorganic frameworks of R / S -Cl . ,, Besides, anisotropic factor g CD values are calculated and analyzed in Table S9. The chiral space group promotes us to further investigate the SHG response of R / S -Cl .…”