Halide-Regulated Dual Emission and Thermochromism in Zero-Dimensional Phosphonium Metal Halides

Abstract

Temperature-responsive luminescent materials have attracted considerable attention for optical sensing and dynamic anticounterfeiting. However, designing thermochromic luminescent materials with pronounced color variation and well-defined emission mechanisms remains challenging. A series of zero-dimensional dual-emissive complexes, (2-NpPPh2Bn)2M2X6 (2-NpPPh2Bn = benzyl(naphthalen-2-yl)diphenylphosphonium; M = Zn, X = Cl (1); M = Zn, X = Br (2); M = Cd, X = Cl (3); M = Cd, X = Br (4)), are constructed. At room temperature, these complexes exhibit single-band blue emission at 399 nm, which evolves into dual-band white emission centered at 417 and 557 nm upon cooling to 80 K. Variable-temperature lifetime measurements and theoretical calculations reveal the 417 nm band originates from halide-to-ligand charge transfer (XLCT) dominated self-trapped exciton luminescence (1.479 μs), while the 557 nm band is attributed to intraligand charge transfer (ILCT)-mediated phosphorescence (17.249 ms). Halide substitution from Cl to Br reverses the relative intensity of the two emission bands. Complex 3 exhibits the pronounced yellow afterglow at 80 K. The orthogonal responses to temperature, UV excitation, and afterglow decay time enable a multilevel dynamic information encryption system. Functionalized phosphonium countercation and halide substitution cooperatively regulate XLCT/ILCT competition, offering a strategy for designing multiresponsive luminescent materials with tunable emission mechanisms.