Colour and constitution relationships in CI Acid Red 138 and its homologues

Abstract The potentially tautomeric acid azo dye CI Acid Red 138 and its homologues substituted by different alkyl groups were investigated by UV-Vis and PPP-MO techniques. The calculated π-electronic transition for the hydrazone form corresponds very closely to the experimental absorption spectrum. The observed negative solvatochromic effect is explained using the calculated dipole moments of the ground and first excited singles states. The calculated transition moments are in opposite directions for both tautomeric structures, suggesting a specific substituent effect depending on which tautomeric form predominates. The observed low intensity fluorescence is assigned to the fluorescence ability of the hydrazone structure. The long wavelength band determining the colour is composed of discrete vibronic transitions, the relative intensities of which can change when particular chemical interactions between a dye molecule and its environment occur.