Tuning the Structure and Photoluminescence of [SbCl5]2−-Based Halides via Modification of Imidazolium-Based Cations

Structure-property relationships in imidazolium-based hybrid Sb(III) chlorides provide critical guidance for designing high-performance materials. Three zero-dimensional metal halides, namely, [C₃mmim]₂SbCl₅ (<b>1</b>, [C₃mmim]⁺ = 1-propyl-2,3-dimethylimidazolium), [C₅mmim]₂SbCl₅ (<b>2</b>, [C₅mmim]⁺ = 1-pentyl-2,3-dimethylimidazolium), and [C₅mim]₂SbCl₅ (<b>3</b>, [C₅mim]⁺ = 1-pentyl-3-methylimidazolium), are synthesized by ionothermal methods. These compounds exhibit markedly distinctly photophysical properties at their optimal excitation wavelengths. Structural analyses reveal that elongated alkyl chains in compounds <b>2</b> and <b>3</b> increase Sb-Sb distances compared to that in <b>1</b>, effectively isolating [SbCl₅]^2- units, suppressing inter-center energy transfer, and reducing non-radiative transitions, thereby enhancing the photoluminescence quantum yield (PLQY). Furthermore, methyl substitution at the C2-position of the imidazolium ring in compounds <b>1</b> and <b>2</b> induces asymmetric coordination environments around the [SbCl₅]^2- emission centers, leading to pronounced structural distortion. This distortion promotes non-radiative decay pathways and diminishes luminescent efficiency. Furthermore, temperature-dependent spectroscopy analysis and fitting of the Huang-Rhys factor (<i>S</i>) reveal significant electron-phonon coupling in compounds <b>1</b>-<b>3</b>, which effectively promotes the formation of self-trapped excitons (STEs). However, compound <b>1</b> exhibits extremely high <i>S</i>, which significantly enhances phonon-mediated non-radiative decay and ultimately reduces its PLQY. Overall, compound <b>3</b> has the highest PLQYs.