Fe(II), Ru(II) and Re(I) complexes of endotopic, sterically non-hindering, U-shaped 8,8'-disubstituted-3,3'-biisoquinoline ligands: syntheses and spectroscopic properties

The redox behaviour, optical-absorption spectra and emission properties of U-shaped and elongated disubstituted biisoquinoline ligands and of derived octahedral Fe(II), Ru(II), and Re(I) complexes are reported. The ligands are 8,8'-dichloro-3,3'-biisoquinoline (1), 8,8'-dianisyl-3,3'-biisoquinoline (2), and 8,8'-di(phenylanisyl)-3,3'-biisoquinoline (3), and the complexes are [Fe(2)(3)](2+), [Fe(3)(3)](2+), [Ru(1)(phen)(2)](2+), [Ru(2)(3)](2+), [Ru(3)(3)](2+), [Re(2)(py)(CO)(3)](+), and [Re(3)(py)(CO)(3)](+). For the ligands, the optical properties as observed in dichloromethane are in line with expectations based on the predominant (1)pi pi* nature of the involved excited states, with contributions at lower energies from (1)n pi* and (ILCT)-I-1 (intraligand charge transfer) transitions. For all of the Fe(II), Ru(II), and Re(I) complexes, studied in acetonitrile, the transitions associated with the lowest-energy absorption band are of (MLCT)-M-1 (metal-to-ligand charge transfer) nature. The emission properties, as observed at room temperature and at 77 K, can be described as follows: (i) the Fe(II) complexes do not emit, either at room temperature or at 77 K; (ii) the room-temperature emission of the Ru(II) complexes (phi(em) > 10(-3), tau in the mu s range) is of mixed (MLCT)-M-3/(LC)-L-3 character (and similarly at 77 K); and (iii) the room-temperature emission of the Re(I) complexes (phi(em) similar to 3 x 10(-3), tau < 1 ns) is of (MLCT)-M-3 character and becomes of (LC)-L-3 (ligand-centered) character (tau in the ms time scale) at 77 K. The interplay of the involved excited states in determining the luminescence output is examined.