Broad Photoluminescence and Second-Harmonic Generation in the Noncentrosymmetric Organic–Inorganic Hybrid Halide (C6H5(CH2)4NH3)4MX7·H2O (M = Bi, In, X = Br or I)

Recent discoveries in organic−inorganic metal halides reveal superior semiconducting and polarization properties. Herein, we report three organic–inorganic metal halides, (PBA)4BiBr7·H2O, (PBA)4BiI7·H2O, and (PBA)4InBr7·H2O [(PBA)+ = C6H5(CH2)4NH3+], with band gaps of ∼3.52, ∼2.29, and ∼4.05 eV, respectively. They possess zero-dimensional structures containing the inorganic octahedra [MX6]3– (M = Bi, In, X = Br, I) and unbound X– ions and crystallize in the C2 space group. (PBA)4BiI7·H2O shows a second-harmonic-generation (SHG) response in the infrared region, approximately 1.3 times that of AgGaS2; (PBA)4BiBr7·H2O and (PBA)4InBr7·H2O show SHG responses in the ultraviolet region, approximately 0.4 and 0.6 times that of KH2PO4, respectively. The large SHG responses are attributed to the synergistic contribution of the octahedral distortion of [MX6]3– (M = Bi, In, X = Br, I) and the ordered arrangement of the benzene ring-containing organic cation PBA+. Upon ultraviolet and visible-light excitations at room temperature, (PBA)4BiBr7·H2O, (PBA)4BiI7·H2O, and (PBA)4InBr7·H2O exhibit broad red-light luminescence with large Stokes shifts of 290, 237, and 360 nm, respectively, due to self-trapped exciton emission. All of these properties demonstrate that this series of metal halides are potential multifunctional optoelectronic materials.