Julien Alexandre Andres

Related publications (13)

Colorimetry of Luminescent Lanthanide Complexes

Julien Alexandre Andres, Anne-Sophie Chauvin

Europium, terbium, dysprosium, and samarium are the main trivalent lanthanide ions emitting in the visible spectrum. In this work, the potential of these ions for colorimetric applications and colour reproduction was studied. The conversion of spectral data to colour coordinates was undertaken for three sets of Ln complexes composed of different ligands. We showed that Eu is the most sensitive of the visible Ln ions, regarding ligand-induced colour shifts, due to its hypersensitive transition. Further investigation on the spectral bandwidth of the emission detector, on the wavelengths' accuracy, on the instrumental correction function, and on the use of incorrect intensity units confirm that the instrumental correction function is the most important spectrophotometric parameter to take into account in order to produce accurate colour values. Finally, we established and discussed the entire colour range (gamut) that can be generated by combining a red-emitting Eu complex with a green-emitting Tb complex and a blue fluorescent compound. The importance of choosing a proper white point is demonstrated. The potential of using different sets of complexes with different spectral fingerprints in order to obtain metameric colours suitable for anti-counterfeiting is also highlighted. This work answers many questions that could arise during a colorimetric analysis of luminescent probes.

MDPI2020

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Europium and terbium trisdipicolinate complexes are inkjet printed onto paper with commercially available desktop inkjet printers. Together with a commercial blue luminescent ink, the red-emitting luminescent ink containing europium and the green-emitting luminescent ink containing terbium are used to reproduce accurate full color images that are invisible under white light and appear under a 254 nm UV light. Such invisible luminescent images are attractive anti-counterfeiting security features. The luminescent prints have a color range (gamut) nearly as wide as the gamut of a standard sRGB display. The gamut of the luminescent prints is determined by relying on a simple model predicting the relative spectral radiant emittances of any printed luminescent color halftone. The model is also used to establish the correspondence between the surface coverages of the printed luminescent inks and the emitted color of these luminescent halftones. The accuracy of the spectral prediction model is very good and can be rationalized by the absence of quenching when the luminescent lanthanide complexes are printed in superposition with the other luminescent materials.

Wiley-Blackwell2014

We propose a method for creating authenticable color images under UV excitation backlight by printing an invisible luminescent white emissive ink layer on the verso side of a transmissive substrate and a classical cmy image on the recto side of the substrate. In order to obtain a backlit image whose colors are as close as possible to the original image colors, we map the input image sRGB color gamut into the gamut formed by the emissive white source attenuated by the classical ink halftones. The relationship between surface coverages of the classical cmy ink halftones and the resulting backlit colors is obtained by accounting for the transmission of the emissive white through the paper substrate and through the classical cmy ink halftones. The transmittance of the classical ink halftones is modeled by a new halftone absorbance prediction model. The lightness range of the luminescent backlit color gamut is expanded by printing a black and white UV-absorbing instance of the original color image in superposition with the luminescent white emissive layer and in registration with the cmy image printed on the recto side. Luminescent backlit color images provide a high anticounterfeiting security, as they combine a verso printed invisible luminescent ink layer, a verso printed black and white instance of the original image and a matching recto printed cmy instance of the original image. The resulting luminescent backlit image colors are close to the original image colors only when observed in transmission mode under UV light. (C) 2013 Wiley Periodicals, Inc.

Wiley-Blackwell2014

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Regeneration of dye sensitizer molecules by reducing species contained in the electrolyte is a key mechanism in liquid dye-sensitized solar cells because it competes kinetically with a detrimental charge recombination process. Kinetics of the reduction by iodide ions of the oxidized states (S+) of two RuII complex dyes and four organic π-conjugated bridged donor−acceptor sensitizers were examined as a function of the electrolyte concentration. Results show that two different cases can be distinguished. A sublinear behavior of the regeneration rate and a plateau value reached at high bulk iodide concentrations were found for N820 ruthenium dye and interpreted as being due to an associative interaction involving the formation of (S+, I−)···I− surface complexes prior to the reaction. On the other hand, feeble reaction rates at low electrolyte concentrations and a superlinear behavior are observed predominantly for the organic dyes, pointing to a repulsive interaction between the dyed surface and iodide anions. At higher iodide bulk concentration, a linear behavior is reached, providing an estimate of a second-order rate constant. A correlation of these two opposite behaviors with the structure of the dye is observed, emphasizing the role of sulfur atoms in the association of I− anions in the dye-sensitized layer. These findings allow for a better understanding of the dye−electrolyte interaction and of the effect of the iodide concentration on the photovoltaic performances of dye-sensitized solar cells.

Amer Chemical Soc2014

A method and computing system are proposed for producing an authenticable security device with two sides. The verso side is covered with an adjustable luminescent emissive layer formed by invisible luminescent ink halftones and possibly a UV absorbing printed layer. The recto side is covered with transmissive non-luminescent ink halftones. The backlit colors resulting from the emissions of the luminescent layer or resulting from illumination by normal white light through the transmissive non-luminescent ink halftones are predicted by a backlighting model. This model enables computing the surface coverages of the luminescent and/or non-luminescent ink halftones in order to obtain a desired color either under excitation light (UV light) or under normal white light. This enable creating authenticable backlit images substantially similar to pre-stored reference images, either under normal white light, under excitation light, or under both the normal white light and the excitation light.

2014

Energy transfer in coumarin-sensitised lanthanide luminescence: investigation of the nature of the sensitiser and its distance to the lanthanide ion

Julien Alexandre Andres, Anne-Sophie Chauvin

A series of lanthanide complexes Ln(dpxCy)(3) have been synthesised. The ligands are composed of a coordinating dipicolinic acid backbone decorated with a polyoxyethylene arm fitted with a coumarin moiety at its extremity. The nature of the coumarin as well as the length of the linker have been varied. Upon excitation at 320 nm, the coumarin exclusively acts as an antenna while the dipicolinic acid core is not excited. Upon excitation below 300 nm, both parts are excited. With europium as a metal centre, the relaxation of the europium ion (intrinsic quantum yield Phi(Eu)(Eu) and radiative lifetime tau(r)) is constant for all the studied ligands. Therefore, the observed differences in overall quantum yield (Phi(Eu)(L)) in such systems come exclusively from the variation of the terminal coumarin. The overall quantum yields of the studied complexes are low (Phi(Eu)(L) < 2% in aqueous solution). In order to rationalise the mechanism of the energy transfer and to improve the sensitisation efficiency (eta(sens)), the distance between the coumarin sensitiser and the lanthanide centre was explored in solution and compared to the solid state. In the solid state, a dramatic effect was confirmed, with an improvement of 80% in the quantum yield Phi(Eu)(L) for short linkers ((-CH2CH2O-)(n) with n = 1 compared to n = 3). By monitoring the lifetime decay of the excited state of the lanthanide cation with nanosecond vs. microsecond time-resolved spectroscopy at low temperature, the sensitisation of the lanthanide ions by coumarin derivatives was demonstrated to mainly occur through the singlet excited state of the coumarin and not via the usual triplet pathway. No evidence of a different behaviour at room temperature was found by transient triplet-triplet absorption spectroscopy.

Royal Society of Chemistry2013

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

Julien Alexandre Andres

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.

EPFL2012

Synthesis of authenticable halftone images with non-luminescent halftones illuminated by a luminescent emissive layer

Julien Alexandre Andres, Roger Hersch

A method and computing system are proposed for producing an authenticable security device with two sides. The verso side is covered with a luminescent emissive variable intensity layer formed for example by invisible luminescent ink halftones and the recto side is covered with transmissive non-luminescent ink halftones. The backlit colors resulting from the emissions of the luminescent layer or resulting from illumination by normal white light through the transmissive non-luminescent ink halftones are predicted by a backlighting model. This model enables computing the surface coverages of the luminescent and/or non-luminescent ink halftones in order to obtain a desired color either under excitation light (UV light) or under normal white light. This enable creating authenticable backlit images substantially similar to pre-stored reference images, either under normal white light, under excitation light, or under both the normal white light and the excitation light.

2012

Three 6-phosphoryl picolinic acid (6PPA) derivatives were synthesized and used as europium and terbium sensitizers. Two of the three ligands (6-diethoxyphosphoryl picolinic acid (Hdeppa) and 6-monoethoxyphosphoryl picofinic acid (H(2)meppa)) are water-soluble, once complexed to lanthanide ions, while the third (6-dihydroxyphosphoryl picolinic acid (H(3)dhppa)) forms a precipitate. The stability constants of the phosphoryl-based complexes were found to be higher than the carboxylate analogue (dipicolinic acid, H(2)dpa). The main species are the [LnL(3)] complexes under strict stoichiometric conditions, confirmed by P-31 NMR spectroscopy, mass spectrometry and lifetime analyses. The photophysical measurements reveal that the emission intensity of [Eu(deppa)(3)] is maximal at pH 4.8, whereas for Eu(meppa)(3), the optimum pH is observed at 9.0. The lifetimes are all in the millisecond range and have confirmed the absence of water molecules in the first coordination sphere. The emissions of the terbium are always brighter than the corresponding europium within this phosphoryl series. The quantum yields of the phosphoryl containing complexes are lower than the carboxylate analogue (Ln(dpa)(3)), except for [Tb(deppa)(3)], which exhibits an interesting quantum yield of 40% in aqueous solution.

2011

Europium Complexes of Tris(dipicolinato) Derivatives Coupled to Methylumbelliferone: A Double Sensitization

Julien Alexandre Andres, Anne-Sophie Chauvin

A para-poly(oxyethylene) dipicolinic acid derivative coupled to methylumbelliferone was synthesized and used as a ligand for coordination with europium ions. Characterization of the complex by mass spectrometry and spectrophotometry was performed in aqueous solution and points to the formation of the 1:3 Eu/L complex. Physicochemical properties are close to those of already reported dipicolinato complexes, confirming that the presence of a coumarin chromophore does not interfere with complex formation. Photophysical measurements result in observed lifetimes of 1.4 ms and intrinsic quantum yields, Phi(Eu)(Eu), ranging from 33 to 50% (depending on the solvent used). Selective excitation can be performed either mostly on the dipicolinate (DPA) backbone (at 280 nm) or exclusively on the coupled coumarin chromophore in the near visible range (320 nm). Despite the spatial distance between the antenna and the lanthanide ion, europium emission was observed upon 320 nm excitation together with the coumarin emission. The different sensitization pathways have been explored and rationalized. Finally, the complex has been used to probe the ratio of a binary mixture of solvent, establishing the potential interest of this new class of ligands in many fields of research, from photophysics to applied chemistry.

Wiley-Blackwell2010