Molecular-Level Control of Exciton and Nonlinear Optical Properties in 1D Hybrid Antimony Halides via Cation Steric Design

Hybrid antimony halides with one-dimensional (1D) architectures have gained increasing attention due to their structural tunability and versatile optoelectronic properties. In this study, six 1D organic–inorganic metal halides (OIMHs), including (C7H18N2)SbX5 and (C6H16N2)SbX5 (X = Cl, Br, I), were synthesized using two structurally related organic cations: 1-methyl-4-(methylamino)piperidine (C7H16N2) and 4-(methylamino)piperidine dihydrochloride (C6H16Cl2N2). Comparative analysis reveals that the introduction of a methyl group significantly alters the steric environment and electronic structure, leading to reduced octahedral distortion and modified hydrogen bonding interactions. Among the synthesized compounds, (C7H18N2)SbCl5 exhibits strong broadband self-trapped exciton emission (STEs) with long lifetime and stable excitation behavior. In contrast, (C6H16N2)SbCl5 exhibits no detectable photoluminescence but demonstrates significant second-harmonic generation (SHG) activity, with an intensity approximately 1.5 times that of KH2PO4. These findings demonstrate that subtle changes in organic cation structure can effectively modulate excitonic dynamics and nonlinear optical responses in hybrid halide systems, providing a viable strategy for designing multifunctional optoelectronic materials.