The spectroscopic properties of a number of important metal–gossypol complexes have been studied and compared with those of 2-hydroxy-1-naphthaldehydes. Gossypol and its derivatives exhibit three bands, α, β, and p. The α- and β-bands are assigned to π → π* transitions and the p-band to electronic transition from highest-occupied to lowest-unoccupied molecular orbital. In the complexes, the α-band is shifted to longer wavelengths and the p-band disappears. The infrared spectra of gossypol exhibits a strong band at 6.1 μ (C—O stretching); but in the complexes, this band disappears and a new band at 5.7 μ appears. The infrared data on 2-hydroxy-1-naphthaldehyde-anthranilic acid indicates that carbonyl groups are involved in a salt-type linkage (dimer) with copper atoms. Electronic absorption spectra of several Schiff-base complexes of 2-hydroxy-l-naphthaldehyde, in conjunction with magnetic susceptibility data, are evaluated to draw conclusions on the structure of gossypol complexes.
H. N. Ramaswamy and R. T. O'Connor, "Spectroscopic Properties of Some Metal Complexes of Gossypol," Appl. Spectrosc. 24, 50-52 (1970)