Synthesis and photophysical characterization of luminescent lanthanide complexes of nucleotide-functionalized cyclen- and dipicolinate-based ligands

New Coumarin Dipicolinate Europium Complexes with a Rich Chemical Speciation and Tunable Luminescence

Europium (III) luminescent chelates possess intrinsic photophysical properties that are extremely useful in a wide range of applications. The lack of examples of coumarin-based lanthanide complexes is mainly due to poor photo-sensitization attempts. However, with the appeal of using such a versatile scaffold as antenna, especially in the development of responsive molecular probes, it is worth the effort to research new structural motifs. In this work, we present a series of two new tris coumarin-dipicolinate europium (III) complexes, specifically tailored to be either a mono or a dual emitter, tuning their properties with a simple chemical modification. We also encountered a rich chemical speciation in solution, studied in detail by means of paramagnetic NMR and emission spectroscopy.

New 1,3-Disubstituted Benzo[h]Isoquinoline Cyclen-Based Ligand Platform: Synthesis, Eu3+ Multiphoton Sensitization and Imaging Applications

The development of lanthanide-based luminescent probes with a long emission lifetime has the potential to revolutionize imaging-based diagnostic techniques. By a rational design strategy taking advantage of computational predictions, a novel, water-soluble Eu³⁺ complex from a cyclen-based ligand bearing 1,3-disubstituted benzo[h]isoquinoline arms was realized. The ligand has been obtained overcoming the lack of reactivity of position 3 of the isoquinoline moiety. Notably, steric hindrance of the heteroaromatic chromophore allowed selective and stoichiometry-controlled insertion of two or three antennas on the cyclen platform without any protection strategy. The complex bears a fourth heptanoic arm for easy conjugation to biomolecules. This new chromophore allowed the sensitization of the metal center either with one or two photons excitation. The suitability as a luminescent bioprobe was validated by imaging BMI1 oncomarker in lung carcinoma cells following an established immunofluorescence approach. The use of a conventional epifluorescence microscope equipped with a linear structured illumination module disclosed a simple and inexpensive way to image confocally Ln-bioprobes by single photon excitation in the 350–400 nm window, where ordinary confocal systems have no excitation sources.

Lanthanide(III) Complexes of Cyclen Triacetates and Triamides Bearing Tertiary Amide-Linked Antennae

The coordination compounds of the trivalent lanthanide ions (Ln(III)) have unique photophysical properties. Ln(III) excitation is usually performed through a light-harvesting antenna. To enable Ln(III)-based emitters to reach their full potential, an understanding of how complex structure affects sensitization and quenching processes is necessary. Here, the role of the linker between the antenna and the metal binding fragment was studied. Four macrocyclic ligands carrying coumarin 2 or 4-methoxymethylcarbostyril sensitizing antennae linked to an octadentate macrocyclic ligand binding site were synthesized. Complexation with Ln(III) (Ln = La, Sm, Eu, Gd, Tb, Yb and Lu) yielded species with overall −1, 0, or +2 and +3-charge. Paramagnetic ¹H NMR spectroscopy indicated subtle differences between the coumarin- and carbostyril-carrying Eu(III) and Yb(III) complexes. Cyclic voltammetry showed that the effect of the linker on the Eu(III)/Eu(II) apparent reduction potential was dependent on the electronic properties of the N-substituent. The Eu(III), Tb(III) and Sm(III) complexes were all luminescent. Coumarin-sensitized complexes were poorly emissive; photoinduced electron transfer was not a major quenching pathway in these species. These results show that seemingly similar emitters can undergo very different photophysical processes, and highlight the crucial role the linker can play.

Colorimetry of Luminescent Lanthanide Complexes

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.

Spin Cross-Over (SCO) Complex Based on Unsymmetrical Functionalized Triazacyclononane Ligand: Structural Characterization and Magnetic Properties

The unsymmetrical ligand 1-(2-aminophenyl)-4,7-bis(pyridin-2-ylmethyl)-1,4,7-triazacyclononane (L6) has been prepared and characterized by NMR spectroscopy. The L6 ligand is based on the triazamacrocycle (tacn) ring that is functionalized by two flexible 2-pyridylmethyl and one rigid 2-aminophenyl groups. Reaction of this ligand with Fe(ClO4)2·xH2O led to the complex [Fe(L6)](ClO4)2 (1), which was characterized as the first Fe(II) complex based on the unsymmetrical N-functionalized tacn ligand. The crystal structure revealed a discrete monomeric [FeL6]2+ entity in which the unsymmetrical N-functionalized triazacyclononane molecule (L6) acts as hexadentate ligand. As observed in the few parent examples that are based on the symmetrical N-functionalized tacn ligands, the triazacyclononane ring is facially coordinated and the N-donor atoms of the three functional groups (two pyridine and one aniline groups) are disposed in the same side of the tacn ring, leading to a distorted FeN6 environment. The magnetic studies of 1 revealed the presence of an incomplete spin crossover (SCO) transition above 425 K, whose progress would be prevented by a very exothermic thermal decomposition at ca. 472 K, as shown by thermogravimetric and DSC measurements.