Several 2,2′:6′,2′′-terpyridines substituted in the 4′-position were synthesized and their photophysical properties were investigated by absorption and photoluminescence spectroscopy in dilute solutions and solid state. The studies confirmed that the absorption and emission wavelengths, fluorescence quantum yields and lifetimes of 1-R^1–16 are strongly structure-related, demonstrating a decisive role of the nature of the substituent in determining the photophysical properties of 4′-functionalized terpyridines. Additionally, the density functional theory (DFT) calculations were performed for 1-R^1–16 to get insight into their electronic structure and spectroscopic properties.

Structural, spectroscopic and electrochemical properties of six complexes [AuCl(L¹)](PF₆)₂·CH₃CN (1), [AuCl(L²)](PF₆)₂ (2), [PtCl(L¹)](BPh₄)·CH₃CN (3), [PtCl(L²)](SO₃CF₃) (4), [CuCl₂(L¹)] (5) and [CuCl₂(L²)]·CH₃CN (6) with modified 2,2′:6′,2′′-terpyridine ligands, 4′-(4-methoxyphenyl)-2,2′:6′,2′′-terpyridine (L¹) and 4′-(4-methoxynaphthalen-1-yl)-2,2′:6′,2′′-terpyridine (L²) were thoroughly investigated and a significant role of the substituent (4-methoxyphenyl or 4-methoxynaphthalen-1-yl) and the metal center was demonstrated. The naphthyl-based substituent was found to increase the emission quantum yield of the luminescent Au(III) and Pt(II) complexes. Furthermore, the antiproliferative potential of the reported complexes was examined towards human colorectal (HCT116) and ovarian (A2780) carcinoma cell lines as well as towards normal human fibroblasts. The Au(III) complex 2 and Cu(II) complex 5 were found to have a higher antiproliferative effect on HCT116 colorectal and A2780 ovarian carcinoma cells when compared with the Pt(II) complex with the same ligand (4). The order of cytotoxicity in both cell lines is 2 > 6 > 1 > 3 > 4. Complex 2 seems to be more cytotoxic towards HCT116 and A2780 cancer cell lines with IC₅₀ values 300× and 130× higher in normal human fibroblasts compared to the respective cancer cells. The viability loss induced by the complexes agrees with Hoechst 33258 staining and the typical morphological apoptotic characteristics like chromatin condensation and nuclear fragmentation and flow cytometry assay. The induction of apoptosis correlates with the induction of reactive oxygen species (ROS). Fluorescence microscopy analysis indicates that after 3 h of incubation, complexes 1–4 are localized inside HCT116 cells and the high levels of internalization correlate with their cytotoxicity.

Three Re(I) carbonyl complexes [ReCl(CO)₃(Lⁿ)] bearing 2,2′-bipyridine, 2,2′:6′,2′′-terpyridine, and 1,10-phenanthroline functionalized with diphenylamine/or triphenylamine units (L¹–L³) were synthesized to explore the impact of highly electron donating units appended to the imine ligand on the thermal and optoelectronic properties of Re(I) systems. Additionally, for comparison, the ligands L^1–3 and parent complexes [ReCl(CO)₃(bipy)], [ReCl(CO)₃(phen)] and [ReCl(CO)₃(terpy-κ²N)] were investigated. The thermal stability was evaluated by differential scanning calorimetry. The ground- and excited-state electronic properties of the Re(I) complexes were studied by cyclic voltammetry and differential pulse voltammetry, absorption and emission spectroscopy, as well as using density-functional theory (DFT). The majority of the compounds form amorphous molecular materials with high glass transition temperatures above 100 °C. Compared to the unsubstituted complexes [ReCl(CO)₃(bipy)], [ReCl(CO)₃(phen)] and [ReCl(CO)₃(terpy-κ²N)], the HOMO–LUMO gap of the corresponding Re(I) systems bearing modified imine ligands is reduced, and the decrease in the value of the ΔE_H–L is mainly caused by the increase in HOMO energy level. In relation to the parent complexes, all designed Re(I) carbonyls were found to show enhanced photoluminescence, both in solution and in solid state. The investigated ligands and complexes were also preliminarily tested as luminophores in light emitting diodes with the structures ITO/PEDOT:PSS/compound/Al and ITO/PEDOT:PSS/PVK:PBD:compound/Al. The pronounced effect of the ligand chemical structure on electroluminescence ability was clearly visible.