Efficient emission of Zn(II) and Cd(II) complexes with nopinane-annelated 4,5-diazafluorene and 4,5-diazafluoren-9-one ligands: how slight structural modification alters fluorescence mechanism.

Zinc(II) and cadmium(II) chlorido complexes with an N,N-chelating nopinane-annelated 4,5-diazafluoren-9-one ligand (LO) were synthesized. While the zinc(II) complex is mononuclear and adopts a tetrahedral ZnN2Cl2 coordination geometry, its cadmium(II) analogue features a 1D polymeric structure due to the bridging coordination of chlorido ligands with Cd2+ ions having an octahedral CdN2Cl4 coordination geometry. The photophysical properties of the oxygen-containing LO ligand and its zinc(II) and cadmium(II) complexes were studied in solution and in the solid state and matched against the properties of its oxygen-free 4,5-diazafluorene congener L and its complexes of the same metal ions. Comprehensive experimental and theoretical studies revealed the impact of the oxygen atom in the ligand core on the luminescence of the ligands and the complexes. For the oxygen-free L ligand and L-based complexes, the structural differences between the S0 and S1 geometries are small, which leads to fluorescence with extraordinarily small Stokes shifts. The emission of these compounds is of locally excited character for L and of mixed locally excited + ligand-to-halide charge transfer character for the L-based complexes. The introduction of the oxygen atom in the ligand core results in a drastic red-shift of the emission band due to short-range charge transfer. The differences between the S0 and S1 geometries are much more pronounced for LO and LO-based compounds than those of their oxygen-free analogues, leading to an order of magnitude larger Stokes shifts. On going from solution to the solid state, LO and its complexes exhibit aggregation-induced emission (AIE) behaviour with photoluminescence quantum yields (PLQYs) reaching tens of percent.

Luminescent Zn(ii) and Cd(ii) complexes with chiral 2,2'-bipyridine ligands bearing natural monoterpene groups: synthesis, speciation in solution and photophysics.

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 NM2+ and C[double bond, length as m-dash]OM2+ contacts and N-HCl 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.

Ln(iii) complexes (Ln = Eu, Gd, Tb, Dy) with a chiral ligand containing 1,10-phenanthroline and (-)-menthol fragments: synthesis, structure, magnetic properties and photoluminescence.

A series of lanthanide(iii) complexes based on the new chiral ligand L, which contains 1,10-phenanthroline and (-)-menthol fragments, namely [LnL2(NO3)3] (Ln = Eu (1), Gd (2), Tb (3), Dy (4)), have been synthesized and structurally characterized. Complexes 1-4 are isostructural and crystallize in the non-centrosymmetric space group P41212. The mononuclear complexes comprise a 10-coordinate Ln3+ ion with two bidentate N,N-donor ligands (L) and three bidentate chelating nitrate groups. The magnetic properties of complexes 1-4 are determined mainly by the Ln3+ ions. In the case of complexes 3 and 4, significant anisotropy results in nonlinear field dependences of magnetization at low temperature. Complexes 1, 3 and 4 exhibit metal-centered red (Eu3+), green (Tb3+) and yellow (Dy3+) luminescence, respectively, whereas complex 2 displays blue ligand-based luminescence in the solid state at room temperature. The luminescence quantum yield for the solid samples increases in the order 4 < 2 ≈ 3 < 1. The europium(iii) complex shows long luminescence lifetimes (up to 1750 µs) and a very high quantum yield (φf = 0.87); these make this compound promising for application in sensing and optoelectronics.