Nonradiative Deactivation of Lanthanoid Excited States by Inner-Sphere Carboxylates

The vibrational deactivation of metal-centered excited states is one of the fundamental processes that governs the luminescence of inorganic luminophores. In molecular lanthanoid luminescence, the most reliable way to modulate and systematically investigate these processes is deuteration of X–H stretching modes (X = O, N, C). Apart from the effect of these high-energy vibrational motifs, very little is known about the impact of other oscillator fragments present in lanthanoid complexes. We have developed a synthetic protocol to efficiently and selectively label the popular chelator motif “pyridine-2-carboxylic acid” with stable ¹³C/¹⁸O isotope at the carboxylate group. The corresponding isotopologic lanthanoid complexes (Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching frequency of up to 5% after isotopic substitution. While this does not seems to have any effect on the luminescence of lanthanoids with medium- to high-energy gaps (Sm and Eu), we have found the first example of a quantifiable luminescence isotope effect for one of the near-IR transitions of holmium (³K₈ → ⁵I₅) that only involves the isotopic editing of the vibrational environment at the four carbonyl oscillators

Computational Estimation of Lanthanoid−Water Bond Lengths by Semiempirical Methods

Over 650 lanthanoid complexes with Ln−OH2 motifs have been modeled by the three semiempirical methods (AM1, PM3, or PM6)/SPARKLE. The geometrical deviations from the corresponding crystal structures can be described by normal distributions. Statistical inference analysis shows that AM1/SPARKLE is surprisingly accurate for the estimation of the average bond lengths Ln−OH2 for the technologically important central lanthanoids (Ln = Eu−Tb) in complexes with pyridine-like ligands with a worst-case error of only 4.9%

Perdeuterated 2,2′-Bipyridine-6,6′-dicarboxylate: An Extremely Efficient Sensitizer for Thulium Luminescence in Solution

Lanthanoid luminescence has become an important pillar for many modern photonics applications such as bioanalytical research or functional material science. So far, however, thulium despite having one of the most interesting photophysics among the lanthanoids has suffered from extremely low luminescence efficiencies in molecular complexes with organic sensitizer ligands. This has greatly hampered the investigation and application of thulium emission in solution. Here, the discovery of a powerful sensitizer for thulium photoluminescence is reported. The corresponding thulium complex exhibits emission efficiencies (quantum yield Φ > 0.12%; lifetime τ_obs = 4.6 μs; brightness εΦ > 30 M^–1 cm^–1) and can even be detected at low micromolar concentrations in high-phonon solvents like water without the need for laser excitation

Nonradiative Deactivation of Lanthanoid Excited States by Inner-Sphere Carboxylates

The vibrational deactivation of metal-centered excited states is one of the fundamental processes that governs the luminescence of inorganic luminophores. In molecular lanthanoid luminescence, the most reliable way to modulate and systematically investigate these processes is deuteration of X–H stretching modes (X = O, N, C). Apart from the effect of these high-energy vibrational motifs, very little is known about the impact of other oscillator fragments present in lanthanoid complexes. We have developed a synthetic protocol to efficiently and selectively label the popular chelator motif “pyridine-2-carboxylic acid” with stable ¹³C/¹⁸O isotope at the carboxylate group. The corresponding isotopologic lanthanoid complexes (Ln = Sm, Eu, Ho) show a decrease of the local-mode carbonyl stretching frequency of up to 5% after isotopic substitution. While this does not seems to have any effect on the luminescence of lanthanoids with medium- to high-energy gaps (Sm and Eu), we have found the first example of a quantifiable luminescence isotope effect for one of the near-IR transitions of holmium (³K₈ → ⁵I₅) that only involves the isotopic editing of the vibrational environment at the four carbonyl oscillators

Computational Estimation of Lanthanoid−Water Bond Lengths by Semiempirical Methods

Over 650 lanthanoid complexes with Ln−OH2 motifs have been modeled by the three semiempirical methods (AM1, PM3, or PM6)/SPARKLE. The geometrical deviations from the corresponding crystal structures can be described by normal distributions. Statistical inference analysis shows that AM1/SPARKLE is surprisingly accurate for the estimation of the average bond lengths Ln−OH2 for the technologically important central lanthanoids (Ln = Eu−Tb) in complexes with pyridine-like ligands with a worst-case error of only 4.9%

Synthesis of Inert Homo- and Heterodinuclear Rare-Earth Cryptates

A new ditopic cryptand based on two tris­(biaryl)-based binding pockets bridged by a 2,2′-bipyrimidine unit enables the selective synthesis of homo- and heterodinuclear rare-earth cryptates, which are kinetically inert under challenging conditions and can even be purified by preparative high-performance liquid chromatography

Perdeuterated 2,2′-Bipyridine-6,6′-dicarboxylate: An Extremely Efficient Sensitizer for Thulium Luminescence in Solution

Lanthanoid luminescence has become an important pillar for many modern photonics applications such as bioanalytical research or functional material science. So far, however, thulium despite having one of the most interesting photophysics among the lanthanoids has suffered from extremely low luminescence efficiencies in molecular complexes with organic sensitizer ligands. This has greatly hampered the investigation and application of thulium emission in solution. Here, the discovery of a powerful sensitizer for thulium photoluminescence is reported. The corresponding thulium complex exhibits emission efficiencies (quantum yield Φ > 0.12%; lifetime τ_obs = 4.6 μs; brightness εΦ > 30 M^–1 cm^–1) and can even be detected at low micromolar concentrations in high-phonon solvents like water without the need for laser excitation

1,2-HOIQOa Highly Versatile 1,2-HOPO Analogue

A cyclic, bidentate hydroxamic acid binding unit based on an isoquinoline scaffold has been utilized for the synthesis of a hexadentate tripodal ligand based on the TREN backbone. This prototype for a new class of multidentate chelators forms mononuclear iron(III) complexes and one-dimensional coordination polymers with lanthanide(III) cations. The latter has been determined by single-crystal X-ray analysis of the cerium species. The solid-state structure in the monoclinic space group P21/c (C36H34CeN7O11, a = 12.341(2) Å, b = 26.649(4) Å, c = 10.621(2) Å, α = γ = 90°, β = 96.753(3)°, V = 3468.6(9) Å3, Z = 4) exhibits a trigonal-dodecahedral environment around the cerium cation. The proof of concept for the versatility of the new scaffold has been shown by the modification of the crucial precursor 3-carboxyisocoumarin through electrophilic aromatic substitutions to yield the corresponding chlorosulfonated and nitrated analogues

1,2-HOIQOa Highly Versatile 1,2-HOPO Analogue

A cyclic, bidentate hydroxamic acid binding unit based on an isoquinoline scaffold has been utilized for the synthesis of a hexadentate tripodal ligand based on the TREN backbone. This prototype for a new class of multidentate chelators forms mononuclear iron(III) complexes and one-dimensional coordination polymers with lanthanide(III) cations. The latter has been determined by single-crystal X-ray analysis of the cerium species. The solid-state structure in the monoclinic space group P21/c (C36H34CeN7O11, a = 12.341(2) Å, b = 26.649(4) Å, c = 10.621(2) Å, α = γ = 90°, β = 96.753(3)°, V = 3468.6(9) Å3, Z = 4) exhibits a trigonal-dodecahedral environment around the cerium cation. The proof of concept for the versatility of the new scaffold has been shown by the modification of the crucial precursor 3-carboxyisocoumarin through electrophilic aromatic substitutions to yield the corresponding chlorosulfonated and nitrated analogues

Dependence of the Photophysical Properties on the Number of 2,2′-Bipyridine Units in a Series of Luminescent Europium and Terbium Cryptates

The luminescence properties of a series of lanthanoid cryptates with an increasing number of 2,2′-bipyridine units have been investigated for the lanthanoids Eu and Tb in aqueous solution. The trends in important parameters that influence the photophysics in these complexes have been determined. With increasing bipyridine content, an increase is observed for the intersystem crossing efficiencies and the number of inner-sphere water molecules. In contrast, a decrease is found in the same direction for overall quantum yields, triplet energies, and sensitization efficiencies