Rigidified malononitrile- and ketone-merocyanines in rigid environments

Two merocyanine dyes containing a malononitrile or a ketone functional group as electron-acceptors, and a piperidine group as electron-donor were synthetized and crystallized as pigments. The electron-donor and -acceptor moieties are linked via an octahydroanthracene skeleton, forming an electronic push-pull molecular system. The crystal structure of the malononitrile compound was solved ab initio from X-ray powder diffraction data, complementing the reported structure of the ketone pigment. Both compounds show similar molecular structures in the solid state, yet with completely different crystal packing schemes. The crystal structures were analysed with inspecting the Hirshfeld surfaces. IR spectroscopy was applied to complement the crystallographic study. The absorption characteristics of both pigments emerge from the push-pull chemical structure, which was visualized by plotting the electrostatic potentials, calculated using molecular geometries as observed in the solid state. The solid state UV-vis spectra showed peak broadening and bathochromic spectral shift as compared to the spectra recorded in solution, depending on the polarity of the solvent molecules: The largest shifts of the spectra of solid state pigments were observed with respect to the spectra recorded in toluene solution, whether the smallest to those in ethanol.