The present invention provides a monoazo compound represented by formula (1) or a salt thereof:
wherein, Y1 and Y2 represent a group selected from hydrogen atom, formula (2) and formula (3);
-CO-E-X (3)
provided that at least one of Y1 and Y2 is a group represented by formula (2);
Z is a group selected from formulae (4), (5) and (6);
Crystal structures of two black monoazo (hydrazone) pigments have been determined from powder X-ray diffraction data combined with DFT calculations. The DFT calculations suggested the molecules are in hydrazone forms but not in azo forms in both crystal structures. The molecular arrangements in the crystal structures suggested Davydov splitting may occur by excitonic interactions and it causes the red absorption band shift in the crystalline state leading to the characteristic black colors.