AZO PIGMENTS AND THEIR INTERMEDIATES - ON THE ANOMALOUS OPTICAL-ABSORPTION OF BISAZO PIGMENTS FROM 4,4'-DIAMINO-1,1'-DIANTHRIMIDE AND 2-HYDROXY-3-NAPHTHANILIDES

A number of bisazo pigments (2a-2o), which were designed to extend the optical absorption of azo pigments to longer wavelengths (greater than 650 nm), enabling these pigments to be useful in photoreceptors in light-emitting diode (LED) (680 nm) and diode laser (780 nm) printers, were synthesized by coupling a red-absorbing amine (1, 4,4'-diamino-1,1'-dianthrimide, lambda-max almost-equal-to 680 nm) and 2-hydroxy-3-naphthanilide couplers using the classic azotization-coupling procedure. All bisazo pigments synthesized are dark blue solids with melting points higher than 300-degrees-C. They absorb at lambda-max almost-equal-to 610 nm in the visible region in the solid state. Their wavelengths of absorption, which are insensitive to electronic and steric effects exerted by the substituents in the coupler moiety, are unexpectedly at shorter wavelengths than that of 1 (lambda-max almost-equal-to 680 nm). In order to facilitate our understanding of the hypsochromic shift of absorption from 1 to 2a-2o, the nature of the electronic transition of the long-wavelength absorption band of 1 was studied systematically by absorption spectroscopy. Results from structural effect and solvent effect experiments indicate that the long-wavelength absorption in 1 is a charge-transfer band resulting from an intramolecular charge-transfer interaction between the two aminoanthraquinonyl rings. In the bisazo pigment structure, owing to the steric repulsion between the two coupler moieties, such an intramolecular interaction is prohibited. This changes the conformation of the bisazo pigment and is shown to be responsible for the hypsochromic shift of the optical absorption from 1 to bisazo pigments 2a-2o.