Synthesis, structure and spectroscopic properties of Re(I) complexes incorporating 5-arylazo-8-hydroxyquinoline: a density functional theory/time-dependent density functional theory investigation.

Dinuclear rhenium(I) complexes having a fac-[Re(CO)3](+) moiety of general formula fac-[Re2(CO)6(hq)2] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with Hhq in a ratio of 1 : 1 in toluene in an argon atmosphere. Here hq(-) is the deprotonated form of 5-phenylazo-8-hydroxyquinoline (Hhq(1)), 5-(2-naphthylazo)-8-hydroxyquinoline (Hhq(2)) and 5-(2-fluorineazo)-8-hydroxyquinoline (Hhq(3)). The reaction of synthesized dinuclear complexes with imidazole (Im) and N-methylimidazole (N-MeIm) in dry dichloromethane under argon atmosphere afforded the mononuclear complexes of general formula fac-[Re(CO)3(hq)(Im)] and fac-[Re(CO)3(hq)(N-MeIm)] respectively in high yield. The elemental analysis and ESI mass spectroscopic measurements confirm the formation of the desired complexes. Molecular structures of fac-[Re(CO)3(hq(1))(Im)] and fac-[Re(CO)3(hq(1))(N-MeIm)] were confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. The complexes displayed bathochromically shifted intramolecular charge transfer (CT) bands as compared to complexes with unsubstituted 8-hydroxyquinoline complexes. The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of nine Re(i) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited is associated with the (3)IL excited state (ligand-localized) having a cis conformation of the pendant arylazo moiety. The emission-like transition is consistent with the strong (3)ILCT character.