This work describes interaction of a family of [Pt(N ∧ C)(PR 3 )Cl] complexes with imidazole (Im), possible application of this chemistry for regioselective labeling of proteins through imidazole rings of histidine residues and employment of the resulting phosphorescent products in bioimaging. It was found that the complexes containing aliphatic phosphines display reversible substitution of chloride ligand for imidazole function that required considerable excess of imidazole to obtain full conversion into the substituted [Pt(ppy)(PR 3 )(Im)] product, whereas the substitution in the complexes with aromatic phosphines readily proceeds in 1:1.5 mixture of reagents. Rapid, selective, and quantitative coordination of imidazole to the platinum complexes enabled regioselective labeling of ubiquitin. Xray protein crystallography of the {[Pt(ppy)(PPh 3 )]/ubiquitin} conjugate revealed direct bonding of the platinum center to unique histidine-68 residue through the nitrogen atom of imidazole function, the coordination being also supported by noncovalent interaction of the ligands with the protein secondary structure. The variations of the cyclometalating N ∧ C ligands gave a series of [Pt(N ∧ C)(PPh 3 )Cl] complexes (N ∧ C = 2-phenylpyridine, 2-(benzofuran-3-yl)pyridine, 2-(benzo[b]thiophen-3-yl)pyridine, methyl-2-phenylquinoline-4-carboxylate), which were used to investigate the impact of N ∧ C-ligand onto photophysical properties of the imidazole complexes and conjugates with human serum albumin (HSA). The chloride ligand substitution for imidazole and formation of the conjugates results in ignition of the platinum chromophore luminescence with substantially higher quantum yield in the latter case. Variation of the metalating N ∧ C-ligand made possible the shift of the emission to the red region of visible spectrum for both types of the products. Cell-viability tests revealed low cytotoxicity of all {[Pt(N ∧ C)(PPh 3 )Cl]/HSA} conjugates, while PLIM experiments demonstrated their high potential for oxygen sensing.

A 6-membered P-heterocycle has been fused with a polyaromatic framework through intramolecular transformation of anthracene-based phosphine induced by CuCl. The resulting water soluble phosphonium fluorophores exhibit extremely high quantum efficiency along with prominent stability and low toxicity that make them suitable for one- and two-photon imaging purposes.

A series of novel diimine (NN) ligands containing developed aromatic [2,1- a]pyrrolo[3,2- c]isoquinoline system have been prepared and used in the synthesis of Ir(III) luminescent complexes. In organic solvents, the ligands display fluorescence which depends strongly on the nature of solvents to give moderate to strong orange emission in aprotic solvents and shows a considerable blue shift and substantial increase in emission intensity in methanol. Insertion of electron-withdrawing and -donating substituents into peripheral phenyl fragment has nearly no effect onto emission parameters. The ligands were successfully used to prepare the metalated [Ir(NC)(NN)] complexes (where NC = phenylpyridine (NC-1), p-tolylpyridine (NC-2), 2-(benzo[ b]thiophen-2-yl)pyridine (NC-3), 2-benzo[ b]thiophen-3-yl)pyridine (NC-4), and methyl 2-phenylquinoline-4-carboxylate (NC-5)) using standard synthetic procedures. The complexes obtained display moderate to strong phosphorescence in organic solvents; the emission characteristics is determined by the nature of emissive triplet state, which varies substantially with the variations in the structure and donor properties of the C- and N-coordinating functions in metalating ligands. TD-DFT calculations show that for complexes 1, 2, and 4 the emission originates from the mixed MLCT/LLCT excited states with the major contribution from the aromatic moiety of the diimine ligand, whereas in 3 the emissive triplet manifold is mainly located at the NC ligand to give structured emission band typical for the ligand centered (LC) excited state. In the case of 5, the phosphorescence may be also assigned to the mixed MLCT/LLCT excited state; however, the major contribution is attributed to the aromatic moiety of the metalating NC ligand.