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 (1, [C3mmim]+ = 1-propyl-2,3-dimethylimidazolium), [C5mmim]2SbCl5 (2, [C5mmim]+ = 1-pentyl-2,3-dimethylimidazolium), and [C5mim]2SbCl5 (3, [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 2 and 3 increase Sb–Sb distances compared to that in 1, 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 1 and 2 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 (S) reveal significant electron–phonon coupling in compounds 1–3, which effectively promotes the formation of self-trapped excitons (STEs). However, compound 1 exhibits extremely high S, which significantly enhances phonon-mediated non-radiative decay and ultimately reduces its PLQY. Overall, compound 3 has the highest PLQYs.