Considerable attention has been focused on Cu(I)–diimine–phosphine complexes because of their good room-temperature phosphorescence since 1979, but no attention has been focused on the exploration and exploitation by using them as smart acid–base responsive switching luminescent materials. Herein, the reaction of [Cu(MeCN)₄]PF₆, 2-(2-naphthyl)-1H-imidazo[4,5-f][1,10]phenanthroline (nimpH) and chelating diphosphine ligands bis[(2-diphenylphosphino)phenyl] ether (POP) and 9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos) or (±)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (BINAP) in a 1 : 1 : 1 ratio afforded three ionic [Cu(nimpH)(PP)]PF₆ complexes (PP = POP, 1a; xantphos, 2a; BINAP, 3a) and three neutral [Cu(nimp)(PP)] complexes (PP = POP, 1b; xantphos, 2b; BINAP, 3b) in neutral and strong alkaline media, respectively. The non-emissive complexes 1a, 2a and 3a exhibit turn-on luminescence upon exposure to the strong base NaOH and tetrabutylammonium fluoride (TBAF), and the intensely emissive complexes 1b, 2b and 3b display turn-off luminescence upon exposure to trifluoroacetic acid (TFA) in CH₂Cl₂ solutions. Naked-eye repeatable off–on–off or on–off–on luminescence behaviors of these complexes are observed in CH₂Cl₂ with reversible base–acid or acid–base responses. On the other hand, the addition of TBAF to neutral complexes 1b, 2b and 3b results in obvious luminescence enhancement, especially for the diluted solutions and complexes with the flexible diphosphine ligand POP. The plausible mechanisms of the above results are discussed.

Four half-sandwich type mononuclear lanthanide crown ether complexes with the formula [Ln(12-crown-4)(NO₃)₃] (Ln³⁺ = Dy³⁺ (1), Tb³⁺ (2), Ho³⁺ (3), and Er³⁺ (4); 12-crown-4 = 1,4,7,10-tetraoxacyclododecane) were successfully synthesized and structurally characterized. Ac magnetic susceptibility measurements reveal that dysprosium complex 1 behaves as a typical single-ion magnet, while solid state photoluminescence studies show that complexes 1 and 2 have the respective lanthanide(III) luminescence characteristics.

α-Diimine-Cu(I)–diphosphine complexes are good phosphorescence materials at room temperature; however, so far, most reported systems have been mononuclear and homonuclear Cu(I) complexes, and little attention has been paid to heteronuclear Cu(I) complexes. On the other hand, some of the heteronuclear Cu(I)–Ag(I) complexes have displayed enhanced luminescence properties compared to homonuclear analogues, and this inspires us to explore the synthesis and luminescence behaviour of hetronuclear α-diimine-Cu(I)–Ag(I)–diphosphine complexes. Herein, 2-(6-bromo-2-pyridyl)-1H-imidazo[4,5-f][1,10]phenanthroline (bpipH) and its four homobinuclear Cu(I)–Cu(I) complexes (1a, 2a, 1b and 2b) and two heterobinuclear Cu(I)–Ag(I) complexes (1c and 2c) were synthesized. All the Cu(I) and Ag(I) centers show typical distorted tetrahedral configurations. Both homobinuclear complexes (1b and 2b) and heterobinuclear analogues (1c and 2c) of deprotonated bpipH ligands show better luminescence than the corresponding homobinuclear complexes (1a and 2a) of the neutral bpipH. Most importantly, compared with the corresponding homobinuclear complexes, heteronuclear Cu(I)–Ag(I) complexes distinctly show further enhanced luminescence and dual emission bands (501 and 601 nm for complex 1c, and 506 and 603 nm for complex 2c), which probably result from ³(LL + ML + IL)CT transitions. Here, the better emission behaviour of heteronuclear α-diimine-Cu(I)–Ag(I)–diphosphine complexes proves that introducing Ag(I) ions to the α-diimine-Cu(I)–diphosphine systems is worth exploring to enhance the luminescence intensity and tune the luminescence colors.

Understanding how solvents influence the luminescence behavior of Cu(I) complexes is crucial for designing advanced optical sensors. This study reports the synthesis, structures and photophysical investigation of an 8-hydroxyquinoline-functionalized 1H-imidazo[4,5-f][1,10]phenanthroline ligand, ipqH₂, and its four Cu(I) complexes with diphosphine co-ligands. Photoluminescence studies demonstrated distinct solvent-dependent excited-state mechanisms. In DMSO/alcohol mixtures, free ipqH₂ exhibited excited-state proton transfer (ESPT) and enol-keto tautomerization, producing dual emission at about 447 and 560 nm, while the complexes resisted ESPT due to hydrogen bond blocking by PF₆⁻ anions and Cu(I) coordination. In DMSO/H₂O, aggregation-caused quenching (ACQ) and high-energy O–H vibrational quenching dominated, but complexes 1 and 2 showed a significant red-shifted emission (569–574 nm) with high water content due to solvent-stabilized intra-ligand charge transfer and metal-to-ligand charge transfer ((IL+ML)CT) states. In DMSO/DMF, hydrogen bond competition and solvation-shell reorganization led to distinct responses: complexes 1 and 3, with flexible bis[(2-diphenylphosphino)phenyl]ether (POP) ligands, displayed peak splitting and (IL + ML)CT redshift emission (501 ⟶ 530 nm), whereas complexes 2 and 4, with rigid 9,9-dimethyl-4,5-bis(diphenylphosphino)-9H-xanthene (xantphos), showed weaker responses. The flexibility of the diphosphine ligand dictated DMF sensitivity, while the coordination, the hydrogen bonds between PF₆⁻ anions and ipqH₂, and water solubility governed the alcohol/water responses. This work elucidates the multifaceted solvent-responsive mechanisms in Cu(I) complexes, facilitating the design of solvent-discriminative luminescent sensors.