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

[Image: see text] 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 λ(abs) 464 nm (ε = 1.2 × 10(4) M(–1) cm(–1)) and 490 nm (ε = 5.2 × 10(4) M(–1) cm(–1)) in CH(2)Cl(2) for L4 and L5, respectively). L1–L5 show strong fluorescence in a fluid medium (Φ(em) = 22–92%, λ(em) 370–602 nm in CH(2)Cl(2)); 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)(3)X(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 (1)ππ* and (3)MLCT 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 (3)MLCT state presumably due to the triplet–triplet energy transfer ((3)MLCT → (3)ILCT).