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

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, gCD, 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.