Synthesis, crystal structure, DFT calculation and trans → cis isomerisation studies of bipyridyl ruthenium(II) complexes bearing 8-oxyquinolate azo ligands

Abstract: Two stable Ru(II) bipyridyl complexes were synthesized with the deprotonated forms of the azo ligands of 8-hydroxyquinoline (hq) as analogues and they were chromatographically separated. The extended azo ligands coordinated as a bidentate ligand and chelates to ruthenium(II) through 8-quinolinolate moiety, leaving the azo part free from coordination. The general formula of the complexes are [Ru(bpy)₂(q)]⁺. Here, q− is the deprotonated form of 5-phenylazo-8-hydroxyquinoline (Hq¹) and 5-(2-naphthylazo)-8-hydroxyquinoline (Hq²). The complexes were verified by ¹H NMR, ESI-mass, absorption-emission spectra, cyclic voltammetry and single-crystal X-ray structure determination. UV light-induced trans → cis isomerization and reverse isomerism i.e. cis → trans around -N=N- bond at room temperature were proposed from the UV-Vis spectral changes as well as the changing of the colour of the solution of the complexes. In aid of understanding the electronic charge distribution and charge-transfer properties, computational studies employing DFT and TDDFT method have been executed. Graphic abstract: Significant alteration in absorption spectra and colour change was observed for trans-cis photo-isomerization of the complexes upon UV light irradiation due to presence of free rotating azo groups[Figure not available: see fulltext.].