Chromophore-Functionalized Phenanthro-diimine Ligands and Their Re(I) Complexes

A series of diimine ligands has been designed on the basis of 2-pyridyl-1 H-phenanthro[9,10- d]imidazole (L1, L2). Coupling the basic motif of L1 with anthracene-containing fragments affords the bichromophore compounds L3-L5, of which L4 and L5 adopt a donor-acceptor architecture. The latter allows intramolecular charge transfer with intense absorption bands in the visible spectrum (lowest λ_abs 464 nm (ε = 1.2 × 10⁴ M^-1 cm^-1) and 490 nm (ε = 5.2 × 10⁴ M^-1 cm^-1) in CH₂Cl₂ for L4 and L5, respectively). L1-L5 show strong fluorescence in a fluid medium (Φ_em = 22-92%, λ_em 370-602 nm in CH₂Cl₂); discernible emission solvatochromism is observed for L4 and L5. In addition, the presence of pyridyl (L1-L5) and dimethylaminophenyl (L5) groups enables reversible alteration of their optical properties by means of protonation. Ligands L1-L5 were used to synthesize the corresponding [Re(CO)₃X(diimine)] (X = Cl, 1-5; X = CN, 1-CN) complexes. 1 and 2 exhibit unusual dual emission of singlet and triplet parentage, which originate from independently populated ¹ππ* and ³MLCT excited states. In contrast to the majority of the reported Re(I) carbonyl luminophores, complexes 3-5 display moderately intense ligand-based fluorescence from an anthracene-containing secondary chromophore and complete quenching of emission from the ³MLCT state presumably due to the triplet-triplet energy transfer (³MLCT → ³ILCT).