Deconvoluting the Role of Charge in a Supramolecular Catalyst.

We have demonstrated that the microenvironment of a highly anionic supramolecular catalyst can mimic the active sites of enzymes and impart rate accelerations of a million-fold or more. However, these microenvironments can be challenging to study, especially in the context of understanding which specific features of the catalyst are responsible for its high performance. We report here the development of an experimental mechanistic probe consisting of two isostructural catalysts. When examined in parallel transformations, the behavior of these catalysts provides insight relevant to the importance of anionic host charge on reactivity. These two catalysts exhibit similar host-substrate interactions, but feature a significant difference in overall anionic charge (12- and 8-). Within these systems, we compare the effect of constrictive binding in a net neutral aza-Cope rearrangement. We then demonstrate how the magnitude of anionic host charge has an exceptional influence on the reaction rates for a Nazarov cyclization, evidenced by an impressive 680-fold change in reaction rate as a consequence of a 33% reduction in catalyst charge.

Understanding the quenching effects of aromatic C-H- and C-D-oscillators in near-IR lanthanoid luminescence.

Several series of selectively deuterated 2,2'-bipyridine-based cryptates with the near-IR emissive lanthanoids Pr, Nd, Er, and Yb are reported. The structural and luminescence properties of these complexes have been comprehensively investigated. A combination of experimental techniques (X-ray crystallography, lanthanoid-induced NMR shift analysis, luminescence, vibrational near-IR absorption) and theoretical concepts has been applied with a focus on nonradiative deactivation through multiphonon relaxation of lanthanoid excited states by aromatic, high-energy C-(H/D) oscillators. It is shown that the characteristics for the overtones of these vibrational modes deviate substantially from harmonic oscillators and that anharmonicity within a local-mode Morse model is an essential parameter for any accurate description. The spectral overlap integrals (SOIs) of lanthanoid electronic states with aromatic C-(H/D) overtones are evaluated quantitatively for different lanthanoid/oscillator combinations and the implications for luminescence enhancement through deuteration is discussed. Simple Gaussian functions are proposed as appropriate mathematical forms for the empirical approximation of SOIs.

Anomalous reversal of C-H and C-D quenching efficiencies in luminescent praseodymium cryptates.

A series of selectively deuterated praseodymium cryptates has been synthesized. Their luminescence lifetimes in solution range from 150 to 595 ns for the (1)D(2) → (3)F(4) transition. Global fitting of the nonradiative deactivation rate differences of the isotopologic C-(H/D) oscillators revealed that aromatic C-D overtones anomalously quench the luminescence more than C-H vibrations. This is explained by the dominance of Franck-Condon overlap factors that greatly favor C-D oscillators, which are in almost ideal resonance with the relevant energy gap (1)D(2)-(1)G(4) of praseodymium.

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.

Rigid, perdeuterated lanthanoid cryptates: extraordinarily bright near-IR luminophores.

Near-IR emissive lanthanoid cryptates have been developed with the lanthanoids Yb, Nd, Er, and Pr by designing a fully deuterated ligand environment that greatly suppresses multiphonon nonradiative deactivation pathways through avoidance of high-energy oscillators and rigidification of the ligand backbone. Strong luminescence is observed in CD(3)CN for all four lanthanoids. Luminescence lifetimes in CD(3)CN are among the highest values for molecular complexes in solution reported so far (Yb, τ(obs) = 79 µs; Nd, τ(obs) = 3.3 µs). For the ytterbium cryptate, the highest luminescence lifetime can be obtained using CD(3)OD (τ(obs) = 91 µs) and even in nondeuterated CH(3)CN the lifetime is still unusually high (τ(obs) = 53 µs). X-ray crystallography and (1)H NMR analysis of the corresponding nondeuterated lutetium cryptate suggest that the inner coordination sphere in solution is completely saturated by the octadentate cryptand and one chloride counterion. All lanthanoid cryptates remarkably show complete stability during reversed-phase HPLC measurements under strongly acidic conditions.

Computational estimation of lanthanoid-water bond lengths by semiempirical methods.

Over 650 lanthanoid complexes with Ln-OH(2) 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-OH(2) for the technologically important central lanthanoids (Ln = Eu-Tb) in complexes with pyridine-like ligands with a worst-case error of only 4.9%.