A Strategy to Protect and Sensitize Near-Infrared Luminescent Nd³⁺ and Yb³⁺:  Organic Tropolonate Ligands for the Sensitization of Ln³⁺-Doped NaYF₄ Nanocrystals

Abstract

A strategy to sensitize and protect near-infrared (NIR) emitting Nd 3+ and Yb 3+ is presented. Combining protection provided by the inorganic matrix of NaYF 4 nanocrystals and sensitization from tropolonate ligands capped on their surface, the lanthanide cation centered luminescence was observed through the ligand excitation. The extended lanthanide luminescence lifetimes indicate the success of this strategy.Lanthanide-based near-infrared (NIR) emitters have a great potential to serve as bioanalytical reporters for several reasons: (i) NIR photons scatter less than visible photons, improving image resolution. 1 (ii) Biological systems have low native autofluorescence in the NIR energy domain, 2 resulting in higher detection sensitivity due to improved signal-to-noise ratio. (iii) Most luminescent lanthanide complexes are not susceptible to photodecomposition, allowing long or repeated experiments and simplifying sample storage and preparation procedures. Since f−f transitions are Laporte forbidden, 3 free Ln 3+ have low extinction coefficients resulting in low luminescence intensity. Therefore, it is necessary to sensitize these cations through a suitable chromophore ("antenna effect"), 4 an area of research that has been highly active in recent years. 5 However, this approach has intrinsic limitations because lanthanide luminescence is easily quenched through nonradiative routes when the cations are in close proximity to the vibrational overtones of -OH, -NH, and -CH groups present in the sensitizing ligand and/or solvent. 6 This effect is particularly dramatic for NIR emitting Ln 3+ because of relatively small energy gaps between ground and excited electronic states. 6 12 These materials protect lanthanide cations from sources of nonradiative deactivation; however, they have either limited (e.g., LnVO 4 ) or no absorbance in the UV range. Thus, these inorganic materials are not able to sensitize lanthanide luminescence with the efficiency of organic sensitizers.Here we introduce a strategy to overcome the limited lanthanide sensitization by binding organic tropolonate chromophoric groups to the surface of NaYF 4 nanocrystals (NCs), doped with NIR emitting Nd 3+ or Yb 3+ (Scheme 1). Tropolonate (Trop -) was chosen as a capping ligand since it has been previously demonstrated to be a suitable sensitizer for several lanthanide cations emitting in the NIR range when coordinated in KLn(Trop) 4 complexes. 13 These novel systems use the NaYF 4 matrix to protect Ln 3+ from nonradiative deactivations, while a chromophoric coating sensitizes their luminescence.A synthetic method to prepare Nd 3+ or Yb 3+ doped NaYF 4 NCs was developed on the basis of a recently reported synthesis of NaYF 4 NCs (see Supporting Information). 14 The Trop -capped NCs were synthesized using the following procedure. Tropolone was dissolved in methanol, then deprotonated with an equimolar amount of KOH in methanol. Chloroform was added to obtain a 1/1 (v/v) MeOH/CHCl 3 solvent mixture. This solution was added to a purified solution...

Novel antennae for the sensitization of near infrared luminescent lanthanide cations

“…In order to sensitize lanthanides, several molecules of organic chromophore, tropolonate, were attached on the surface of the Nd 3+ or Yb 3+ -doped NaYF 4 nanocrystal (Fig. 15) [70]. Tropolonate was chosen as sensitizer to achieve this goal as it has been previously tested for the sensitization of NIR emitting cation in molecular complexes (see above) [12,59].…”

Enhanced Ultraviolet Photon Capture in Ligand-Sensitized Nanocrystals

“…Routes previously explored to address this challenge include the relaxation of Laporte selection rules through the embedding of lanthanide ions in low-symmetry crystal hosts and the employ of parityallowed, d → f charge transfer in divalent lanthanides such as Eu(II) 15 . By contrast, the possibility of photosensitizing nanocrystalline lanthanides with organic ligands remains a relatively novel method of enhancing f-block nanoparticle light absorption, though a recent study has implemented such schemes in light upconversion 16 , and some work exploring the photochemical effects surface ligands have on lanthanide nanoparticle luminescence has been conducted [17][18][19][20][21][22][23] .…”

Visible Discrimination of Broadband Infrared Light by Dye-Enhanced Upconversion in Lanthanide-Doped Nanocrystals

“…FRET has been reported for nanocomposites made with dyes attached by physisorption [40] by ligand exchange [25,41,42] and by covalent bonds [3,15,27]. Although all three techniques yield the desired short separation distance between dye and nanoparticle, the strong covalent bond is best able to withstand subsequent processing steps.…”