From Luminescent Lanthanide Complexes to Color Reproduction and Optical Document Security with Invisible Luminescent Inks

The present thesis deals with luminescent compounds based on lanthanide complexes that are suitable for color reproduction and optical document security with invisible luminescent inks. This thesis is divided in two distinct fields: chemistry, which produces luminescent lanthanide complexes, and color science, which was required to characterize luminescent colors and use them for color reproduction and optical document security. Concerning luminescent lanthanide complexes, the investigation is focused on the derivatization of dipicolinic acid. A modification of the carboxylate coordinating moiety of the ligand is first undertaken in order to synthesize phosphorylated ligands, which often show interesting variations of the properties of the complexes among related structures. On another hand, a derivatization of the ligand by grafting a polyoxyethylene side chain with a terminal chromophore at the para position on the pyridine core of dipicolinic acid is carried out. The ability of this chromophore to act as a sensitizer for the distant lanthanide ion coordinated on the dipicolinate moiety is investigated. A series of coumarin fluorophores are tested as sensitizers and the best coumarin is then used to further examine the mechanisms of the sensitization of the lanthanide ion, for example by shortening the polyoxyethylene pendent and thus modifying the distance between the sensitizer and the lanthanide ion. The parent europium and terbium dipicolinate complexes are then used as red emitting and green emitting luminescent dyes in invisible luminescent inks together with a blue luminescent ink. An ink spreading enhanced spectral Neugebauer prediction model is used to predict the spectral radiant emittance of any combination of the luminescent inks. The good prediction accuracy of the model is then related to the photophysical properties of the luminescent inks, which are shown to be unquenched by superposition with the other inks. This trichromic system is finally used to reproduce luminescent color halftone images visible under UV light by relying on a gamut mapping algorithm and on the spectral prediction model. In a final part, a new spectral prediction model for transmittances and reflectances is developed. It is then used for building a backlighting model where the backlight source is a luminescent emission under UV light from printed luminescent inks, and where the backlit colors are obtained by the transmission of the backlight source through color halftones printed with classical cyan, magenta and yellow inks on the other side of the same substrate. Luminescent backlit color images can then be created, which produces a new security feature for optical document security combining recto classical images under normal light, with verso luminescent images and recto luminescent backlit images under UV excitation.