Rational Design of a Super-Alkali Compound with Reversible Photoluminescence.

The zero-dimensional (0D) (H 5 O 2 )(C 4 H 14 N 2 S 2 ) 2 BiCl 8 : Sb 3+ single crystal is obtained by the cooling crystallization method. Surprisingly, this compound shows reversible photoluminescence (PL) upon H 5 O 2 + Cl - removal and insertion. To be specific, the release of H 5 O 2 + 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 [SbCl 6 ] 3- octahedron distortion induced by guest (H 5 O 2 + Cl - ) removal at the ground state, especially at the excited state, is responsible for the disparate PL performance. Encouragingly, we also found that (C 4 H 14 N 2 S 2 ) 2 BiCl 7 : Sb 3+ 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.