Photoluminescence enhancement of Re(i) carbonyl complexes bearing D–A and D–π–A ligands

Abstract

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.