More rigid μ₂-N,S coordination of 2-(alkylsulfanyl)pyrimidine ligands in copper(i) iodide coordination polymers leads to a much more efficient emission in comparison with N-monodentate coordination.

Silver(I) complexes with 2-amino-4-phenyl-6-methylpyrimidine (L), [Ag₃L₂(H₂O)_0.55(NO₃)₃]_n (1) and [Ag₂L₂(PPh₃)₂(NO₃)₂] (2), have been synthesized and structurally characterized. In the solid state the free ligand L demonstrates dual fluorescence: a high energy band at 375 nm is the emission of the monomer L while a low-energy band at 450 nm can be attributed to the emission of the tautomeric dimer, L₂-imino, formed in the excited state double proton transfer (ESDPT) process. In the solid state the complexes demonstrate excitation wavelength dependent photoluminescence originating from three emission mechanisms (hereafter abbreviated as A, B and C for complex 1; A′, B′ and C′ for complex 2). The spectroscopic and computational results reveal that these emission mechanisms can be assigned to metal + nitrate anion to ligand L charge transfer phosphorescence (mechanisms A and A′), excimer ¹π–π* fluorescence (mechanism B), fluorescence of L₂-imino dimeric units formed due to ESDPT (mechanism B′) and ligand-centered ¹π–π*/¹n–π* fluorescence (mechanisms C and C′). The luminescence of complex 1 at λ_ex = 340 nm shows thermochromic behavior; the emission color dramatically changes from blue-green at 77 K to yellowish green at 175 K and white at 300 K.

Mononuclear zinc(II) and cadmium(II) complexes, ZnLCl₂ (1), CdLCl₂ (2), ZnL¹Cl₂·2H₂O (3), and CdL¹Cl₂·2H₂O (4), with chiral ligands containing a 2,2′-bipyridine moiety and natural terpene (+)-limonene (L) or (+)-3-carene (L¹) moieties were synthesized. In these complexes the L and L¹ ligands are shown to coordinate Zn²⁺ and Cd²⁺ ions through the 2,2′-bipyridine moiety. The acetamide group of the ligands interacts with M²⁺ ions by forming N⋯M²⁺ and CO⋯M²⁺ contacts and N–H⋯Cl hydrogen bonds with coordinated Cl⁻ ions. In solutions the complexes have several conformers differing by the degree of the turn of the acetamide moiety relative to the ligand core and the type of its interaction with the coordination core. The ligands and complexes exhibit luminescence with the quantum yield increasing in the order: ligand < cadmium(II) complex < zinc(II) complex. The complexes 3 and 4 demostrate excitation wavelength independent single-channel fluorescence. As opposed to 3 and 4, the complexes 1 and 2 demonstrate excitation wavelength dependent emission with nanosecond and microsecond lifetimes of the excited states. According to our TD-DFT calculations, an interplay of ligand centered and halide to ligand transitions facilitates two deactivation channels in 1 and 2: S₁–S₀ and T₁–S₀.

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HL^q) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HL^q afford ESIPT-capable [Zn(HL^q)Hal₂] (Hal = Cl, Br, I) complexes. In the solid state HL^q and [Zn(HL^q)Hal₂] luminesce in the orange region (λ_max = 600–650 nm). The coordination of HL^q by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁ ^T → S₀ ^T. The emission of [Zn(HL^q)Hal₂] in the solid state is blue-shifted as compared with HL^q due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁ ^N → S₀ ^N and S₁ ^T → S₀ ^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀ ^T, resulting in the possibility of the S₀ ^T → S₁ ^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.