Significantly enhanced NIR emission of solid-state clusters based on Cu₄Pt₂ triggered with volatile organic compounds

Near-infrared (NIR) emitting metal clusters, recognized for their low toxicity, large Stokes shift, and exceptional photostability, hold considerable promise as stimuli-responsive luminescent materials for applications including organic vapor sensing, pollutant detection, and photoluminescent thermometers. However, their limited quantum yield (QY) for NIR emission poses a challenge, highlighting the need for developing light-up sensors with NIR emitting metal clusters to broaden the scope of applications. Herein, the carbazole-alkyne ligand-incorporated novel bimetallic cluster, Cu₄Pt₂(CZ-PrA)₄(dppy)₄(PF₆)₂ (CZ-Cu4Pt2, CZ-PrAH = 9-(Prop-2-yn-1-yl)-9H-carbazole; dppy = diphenyl-2-pyridylphosphine), was synthesized, which exhibits NIR emission centered at 740 nm in the solid state and shows a significant blue shift compared to the previously reported analogues. Temperature-dependent luminescence tests demonstrated an increase in emission intensity with decreasing temperature and a blue shift in the emission peak. Remarkably, its photoluminescence (PL) intensity increased significantly upon exposure to solvents like ethyl acetate, formaldehyde (HCHO), and chlorobenzene, raising the QY from an initial 16.1% to a range of 46.7%-70.3%. HCHO, in particular, boosted the emission intensity by over 200 times. The emission enhancement mechanism, elucidated through UV-vis diffuse reflectance spectroscopy, powder X-ray diffraction, femtosecond transient absorption spectroscopy, and single-crystal X-ray diffraction, reveals that weak intermolecular interactions, particularly hydrogen bonding between solvent molecules and ligands, restrict intramolecular rotations and vibrations, thus promoting radiative transitions. The CZ-Cu4Pt2 cluster shows potential applications in non-contact fluorescence thermometry and organic vapor detection.