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

A series of diimine ligands has been designed on the basis of 2-pyridyl-1H-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 λabs 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 (Φem = 22–92%, λem 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).