The Remarkable Hyperchromicity of Ketohydrazone Dyes and Pigment Lakes Derived from 4-Morpholino-2-naphthol

The syntheses of a series of new ketohydrazone dyes and pigment lakes with a unique structural motif derived from 4-morpholino-2-naphthol are described. The morpholine substituent is responsible for the hypsochromically shifted absorption maxima with significantly enhanced molar extinction coefficients of the new dyes; this conclusion is supported by time-dependent DFT (TD-DFT) simulations. The location of the morpholine substituent at a sterically demanding site in the pigment lake obstructs further coordination of the bridging sulfonate groups in the solid state and prevents the formation of a 1-dimensional polymeric system, which is common for other pigment lakes.