Synthesis and photoluminescence properties of rhenium(i) complexes based on 2,2′:6′,2′′-terpyridine derivatives with hole-transporting units

Abstract

Based on 2,2′:6′,2′′-terpyridine ligands (L1), five terpyridine derivatives, namely 4′-carbazol-9-yl-2,2′:6′,2′′-terpyridine (L2), 4′-diphenylamino-2,2′:6′,2′′-terpyridine (L3), 4′-bis(4-tert-butylphenyl)amino-2,2′:6′,2′′-terpyridine (L4), 4′-[naphthalen-1-yl-(phenyl)amino]-2,2′:6′,2′′-terpyridine (L5), 4′-[naphthalen-2-yl(phenyl)amino]-2,2′:6′,2′′-terpyridine (L6) and their corresponding Re(I) complexes ReLⁿnn(CO)₃3Cl (n = 1–6) have been synthesized and characterized by elemental analysis and ¹H NMR spectroscopy. The X-ray crystal structure of ReL3(CO)₃3Cl has also been obtained. The luminescence spectra of ReL2(CO)₃3Cl–ReL5(CO)₃3Cl, obtained in CH₂Cl₂ solution at room temperature, show strong dπ (Re) → π* (diimine) MLCT character (λ_max ∼ 600 nm) and a small red shift relative to ReL1(CO)₃3Cl. This, confirmed by the study of the triplet energy levels of the L1–L6 ligands at low temperature (77 K rigid matrix), indicates that the introduction of electron-donating moieties on the terpyridine unit decreases the triplet levels of the ligands, leading to a reduction of the energy gap between d and π* orbitals. In the solid state, upon MLCT excitation, all the complexes show an even stronger emission and a blue spectral shift (λ_max ∼ 550 nm) compared to those obtained in solution.