Sensitization of Near-Infrared-Emitting Lanthanide Cations in Solution by Tropolonate Ligands

摘要

The quest for luminescent reporters that emit in the near-infrared (NIR) domain is a target of prime importance, as NIR photons can penetrate deep into tissues without causing damage and without significant loss of intensity owing to the low absorption of NIR photons in such media.[1], [2] Furthermore, the use of NIR light is one of the best strategies to obtain high-resolution pictures of deep tissues, as NIR light diffracts much less than visible light (diffraction is proportional to 1/()SP, SP=scattering power).[3] NIR fluorescent organic dyes are the current candidates for such applications, but suffer from rapid photodecomposition when exposed to excitation light (photobleaching),[4] which prevents long exposure times or repetitive experiments and strongly limits the field of applications of these fluorophores. NIR luminescent lanthanide complexes are very promising reporters for detection in biological media for several reasons: 1) most lanthanide complexes do not photobleach,[5] 2) they display sharp emission bands that can be easily discriminated from background fluorescence,[6] and 3) they exhibit long luminescent lifetimes, which allow the removal of background fluorescence (autofluorescence) and increased assay sensitivity through time-resolved measurements.[7], [8] Recent advances in technology allow such complexes to be excited in the NIR domain by using multiphoton excitation techniques.[9], [10] So far, Nd and Yb complexes have been reported as NIR-luminescent reporters for these applications. However, there is still room for expanding the use of lanthanide complexes in biological applications. The molecules reported until now have limited luminescence output owing to low efficiency of intramolecular ligand-to-lanthanide energy transfer and/or insufficient protection of the metal ion against nonradiative deactivation processes. Furthermore, the sensitization of other lanthanide cations, such as HoIII or TmIII, is of high interest as supplementary emission wavelengths would then be also available. The narrow emission bandwidths of lanthanide cations allows for the simultaneous detection of several cations during the same experiment (multiplex assay). To obtain luminescent complexes for solution applications, the ligand(s) must harvest light and convert the resulting energy to the LnIII ion (antenna effect),[11], [12] as well as protect the LnIII ion from nonradiative deactivations.[13] Oxygen-donor ligands that can efficiently absorb light are preferred because they also form strong bonds with lanthanide cations, which are hard Lewis acids.