Radiative Lifetime, Non-Radiative Relaxation, and Sensitization Efficiency in Luminescent Europium and Neodymium Cryptates - The Roles of 2,2'-Bipyridine-N,N'-dioxide and Deuteration.

Five luminescent tris(bipyridine)-based cryptates with the lanthanoids Eu and Nd have been prepared with a systematic increase in the number of 2,2'-bipyridine-N,N'-dioxide units and with different deuteration levels in the complexing cryptands for the europium species. Careful analysis of the radiative lifetime τrad in these systems reveals that an increase in N-oxide units around the metal centers uniformly lowers τrad by about 30-40 %. The potential involvement of nephelauxetic effects is discussed. Exchange of 30 C-D for C-H oscillators around the europium centers does not affect the radiative lifetimes but decreases non-radiative deactivation and increases the overall luminescence quantum yield in D2 O by 45 %.

The Radiative Lifetime in Near-IR-Luminescent Ytterbium Cryptates: The Key to Extremely High Quantum Yields.

A powerful strategy for the improvement of near-IR lanthanoid luminescence has been successfully employed for the first time, which involves the rational and deliberate shortening of the radiative luminescence lifetimes τ(rad) in molecular ytterbium complexes. In this context, the bidentate chelating unit 2,2'-bipyridine-N,N'-dioxide has been identified as being responsible for decreasing τ(rad) substantially in macrobicyclic Yb cryptates. This strategy, when combined with conventional approaches, yields unprecedented absolute near-IR quantum yields of up to 12%. This extraordinary efficiency represents the highest value measured for any molecular lanthanoid near-IR emitter. The proof-of-concept for the implementation of the new strategy opens up entirely new prospects for the field of lanthanoid luminescence.

Breakdown of the energy gap law in molecular lanthanoid luminescence: the smallest energy gap is not universally relevant for nonradiative deactivation.

For several decades, the energy gap law has been the prevalent theoretical framework for the discussion of nonradiative deactivation of lanthanoid luminescence in molecular coordination chemistry. Here we show experimentally on samarium and dysprosium model complexes that the size of the energy gap ΔE between a lanthanoid emitting state and the next-lower electronic state cannot be considered a reliable and accurate predictor of the quantitative extent of nonradiative deactivation by aromatic C-H and C-D oscillator overtones. Because the energy gap is the central pillar for the entire conceptual framework of the energy gap law, this finding amounts to largely invalidating this theory for the quantitative description of molecular multiphonon relaxation.

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.

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.

The first enantiopure lanthanoid cryptate.

The first enantiomerically pure 2,2'-bipyridine-based cryptand is reported. Complexation of this macrobicyclic ligand with the trivalent lanthanoid cations Yb and Lu produces enantiomerically and diastereomerically pure cryptates in solution. The Yb cryptate shows near-IR photoluminescence with lifetimes of up to τ = 29.8 µs in CD3OD.