Rational Design of a Super-Alkali Compound with Reversible Photoluminescence

The zero-dimensional (0D) (H₅O₂)(C₄H₁₄N₂S₂)₂BiCl₈: Sb³⁺ single crystal is obtained by the cooling crystallization method. Surprisingly, this compound shows reversible photoluminescence (PL) upon H₅O₂⁺Cl^- removal and insertion. To be specific, the release of H₅O₂⁺Cl^- resulted in red-orange emission with a very low photoluminescence quantum yield (PLQY). While on the reuptake of it, a bright yellow emission with a nearly 10-fold increase of PLQY was observed. Density functional theory (DFT) calculations and temperature-dependent PL experiments reveal that significant [SbCl₆]^3- octahedron distortion induced by guest (H₅O₂⁺Cl^-) removal at the ground state, especially at the excited state, is responsible for the disparate PL performance. Encouragingly, we also found that (C₄H₁₄N₂S₂)₂BiCl₇: Sb³⁺ exhibits a fast response (<3 s) to dilute hydrochloric acid with naked-eye perceivable PL color changes, rendering it a potential sensing material for hydrochloric acid.