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

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 ReLn(CO)3Cl (n = 1-6) have been synthesized and characterized by elemental analysis and 1H NMR spectroscopy. The X-ray crystal structure of ReL3(CO)3Cl has also been obtained. The luminescence spectra of ReL2(CO)3Cl-ReL5 (CO)3Cl, obtained in CH2Cl2 solution at room temperature, show strong d? (Re) -> ?* (diimine) MLCT character (?max ~ 600 nm) and a small red shift relative to ReL1(CO)3Cl. 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 ?* orbitals. In the solid state, upon MLCT excitation, all the complexes show an even stronger emission and a blue spectral shift (?max ~ 550 nm) compared to those obtained in solution.