What is the origin of color on metal complex dyes? Theoretical analysis of a Ni-coordinate azo dye

To analyze the ultraviolet and visible (UV/VIS) absorption spectrum on metal complex dyes, we have theoretically elucidated a Ni-coordinate azo, dye, bis(1-(2-thiazolylazo)-2-nathptholate)nickel(II) (Ni-TAN(2)), by using semi-empirical molecular orbital methods. The molecular structure was determined by the MSINDO method; then, the electronic transition properties were calculated by the INDO/S method. The MSINDO optimization well reproduces the X-ray structure, especially the Ni octahedral coordinate moiety. The INDO/S calculated result clearly shows that the intense visible band originates from the intraligand, TAN azo chromophore, transitions accompanied with the Ni coordinate perturbation. Although many Ni d-orbitals originated transitions lie in the lower energy region, these bands should not be observed by their negligible intensities. (C) 2004 Elsevier Ltd. All rights reserved.