Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

In this work, we describe a protocol for a novel application of hyperspectral imaging (HSI) in the analysis of luminescent lanthanide (Ln3+)-based ...

University of Ottawa

Two homodinuclear and one heterodinuclear lanthanide (Ln)-based complexes of the general formula [Ln (bpm)(tfaa) ] (Ln=Eu (1), Tb (2), Eu-Tb (3), bpm=2,2'-bipyrimidine, tfaa =1,1,1-trifluoroacetylacetonate) were synthesized and characterized by single-crystal photoluminescence spectroscopy and hyperspectral imaging. Complexes 1 and 2 crystallize in two polymorphic structures, while three polymorphs were isolated for 3, namely having needle-, plate-, and block-like morphologies. Single-crystal photoluminescence spectroscopy and imaging on Eu -containing 1 and 3 revealed polymorph-dependent J-splitting of the hypersensitive  D → F Eu transition as well as electric-to-magnetic dipole emission intensity ratios. According to these observations, the lowest symmetry chemical environment was attributed to the Eu ions present in the needle-like polymorph, also in agreement with single-crystal X-ray diffraction analysis. More importantly, hyperspectral imaging on all three single-crystal polymorphs of 3 exhibits optical anisotropy with photoluminescence enhancement at specific crystallographic faces. This behavior was ascribed to the distinct molecular packing of the Ln-Ln dimers in each polymorphic crystal as well as to face-specific local symmetry of the Eu centers. Overall, opto-structural relationships of three Ln-Ln dimers and their single-crystal polymorphs were established as a particularly promising avenue for control of photoluminescence by chemical crystal engineering.

Probing Optical Anisotropy and Polymorph-Dependent Photoluminescence in [Ln ] Complexes by Hyperspectral Imaging on Single Crystals.

Growing attention toward optically active materials has prompted the development of novel synthesis methods for a more reliable and efficient access to these systems. In this regard, microwave-assisted approaches provide unique advantages over traditional solvothermal methods reliant on convectional heating: namely, significantly shorter reaction durations, more rigid reaction conditions, and thus a higher degree of reproducibility. Reported herein for the first time is a rapid synthesis of rare-earth (RE)-doped LiYF upconverting and downshifting microparticles with well-defined bipyramidal morphology and good size dispersion via a microwave-assisted solvothermal process. The suggested material growth mechanism identifies a suitable Li to RE ion ratio, an abundance of pH-sensitive acetate surface-capping ligands, and an appropriate reaction temperature/time profile as crucial for enabling a phase transformation of an intermediary yttrium ammonium fluoride phase into LiYF and subsequent particle ripening. The versatility of the reported method is highlighted by its extension toward the synthesis of other state of the art M(RE)F (M = alkali metal) optical materials: RE-doped LiYbF microparticles and β-NaGdF and α-NaYF nanoparticles. All of the obtained Yb/Er- and Yb/Tm-codoped M(RE)F materials exhibited characteristic upconversion emission, while downshifting capabilities were induced through Ce/Tb codoping of LiYF. Further attention was devoted to single-particle optical characterization via hyperspectral imaging of Yb/Er- and Yb/Tm-codoped LiYF microparticles to explore the spatial variability of upconversion emission within individual particles.

Microwave-Assisted Solvothermal Synthesis of Upconverting and Downshifting Rare-Earth-Doped LiYF Microparticles.

Sodium gadolinium fluoride (NaGdF4) nanoparticles are promising candidates as T1 shortening magnetic resonance imaging (MRI) contrast agents due to the paramagnetic properties of the Gd3+ ion. Effects of size and surface modification of these nanoparticles on proton relaxation times have been widely studied. However, to date, there has been no report on how T1 relaxivity (r1) is affected by the different polymorphs in which NaGdF4 crystallizes: cubic (α) and hexagonal (β). Here, a microwave-assisted thermal decomposition method was developed that grants selective access to NaGdF4 nanoparticles of either phase in the same size range, allowing the influence of host crystallinity on r1 to be investigated. It was found that at 3 T cubic NaGdF4 nanoparticles exhibit larger r1 values than their hexagonal analogues. This result was interpreted based on Solomon-Bloembergen-Morgan theory, suggesting that the inner sphere contribution to r1 is more pronounced for cubic NaGdF4 nanoparticles as compared to their hexagonal counterparts. This holds true irrespective of the chosen surface modification, i.e. small citrate groups or longer chain poly(acrylic acid). Key aspects were found to be a polymorph-induced larger hydrodynamic diameter and the higher magnetization possessed by cubic nanoparticles.

Cubic versus hexagonal - effect of host crystallinity on the T shortening behaviour of NaGdF nanoparticles.

We present a comprehensive investigation of the magnetic and optical properties of an ytterbium complex, which combines two desirable and practical features into a single molecular system. Based upon Yb ions that promote near-infrared optical activity and a chemical backbone that is ideal for an in-depth understanding of the magnetic behaviour, we have designed a multifunctional opto-magnetic species that operates as a luminescent thermometer and as a single-molecule magnet (SMM). Our magnetic investigations, in conjunction with  calculations, reveal one of the highest energy barriers reported for an Yb-based complex. Moreover, we correlate this anisotropic barrier with the emission spectrum of the compound, wherein we provide a complete assignment of the energetic profile of the complex. Such studies lay the foundation for the design of exciting multi-faceted materials that are able to retain information at the single-molecule level and possess built-in thermal self-monitoring capabilities.

Exploring the dual functionality of an ytterbium complex for luminescence thermometry and slow magnetic relaxation.

The development and integration of Single-Molecule Magnets (SMMs) into molecular electronic devices continue to be an exciting challenge. In such potential devices, heat generation due to the electric current is a critical issue that has to be considered upon device fabrication. To read out accurately the temperature at the submicrometer spatial range, new multifunctional SMMs need to be developed. Herein, we present the first self-calibrated molecular thermometer with SMM properties, which provides an elegant avenue to address these issues. The employment of 2,2'-bipyrimidine and 1,1,1-trifluoroacetylacetonate ligands results in a dinuclear compound, [Dy(bpm)(tfaa)], which exhibits slow relaxation of the magnetization along with remarkable photoluminescent properties. This combination allows the gaining of fundamental insight in the electronic properties of the compound and investigation of optomagnetic cross-effects (Zeeman effect). Importantly, spectral variations stemming from two distinct thermal-dependent mechanisms taking place at the molecular level are used to perform luminescence thermometry over the 5-398 K temperature range. Overall, these properties make the proposed system a unique molecular luminescent thermometer bearing SMM properties, which preserves its temperature self-monitoring capability even under applied magnetic fields.

A Luminescent Thermometer Exhibiting Slow Relaxation of the Magnetization: Toward Self-Monitored Building Blocks for Next-Generation Optomagnetic Devices.

Lanthanide-complex-based luminescence thermometry and single-molecule magnetism are two effervescent fields of research, owing to the great promise they hold from an application standpoint. The high thermal sensitivity achievable, their contactless nature, along with sub-micrometric spatial resolution make these luminescent thermometers appealing for accurate temperature probing in miniaturised electronics. To that end, single-molecule magnets (SMMs) are expected to revolutionise the field of spintronics, thanks to the improvements made in terms of their working temperature-now surpassing that of liquid nitrogen-and manipulation of their spin state. Hence, the combination of such opto-magnetic properties in a single molecule is desirable in the aim of overcoming, among others, addressability issues. Yet, improvements must be made through design strategies for the realisation of the aforementioned goal. Moving forward from these considerations, we present a thorough investigation of the effect that changes in the ligand scaffold of a family of terbium complexes have on their performance as luminescent thermometers and SMMs. In particular, an increased number of electron-withdrawing groups yields modifications of the metal coordination environment and a lowering of the triplet state of the ligands. These effects are tightly intertwined, thus, resulting in concomitant variations of the SMM and the luminescence thermometry behaviour of the complexes. Supported by ab initio calculations, we can rationally interpret the observed trends and provide solid foundations for the development of opto-magnetic lanthanide complexes.

Triplet-State Position and Crystal-Field Tuning in Opto-Magnetic Lanthanide Complexes: Two Sides of the Same Coin.

Lanthanide (Ln)-based nanoparticles (NPs) with light emission in the ultraviolet (UV), visible, and near-infrared (NIR) region under NIR excitation are emerging multifunctional players in the biomedical field. Excitation and emission wavelengths matching the NIR biological transparency windows (700-1870 nm) set the basis for deeper penetration depth into biological tissues due to less absorption, higher resolution as a result of reduced scattering, and improved imaging contrast due to tissue autofluorescence minimization; all being inevitable requirements for optical in vivo bioimaging. Moreover, frequency-conversion in Ln-based NPs is sensitive to their thermal environment, thus allowing for application as optical temperature sensors. Additional heating capability opens the pathway to self-monitored optical agents with potential for photothermal therapies (PTTs). UV-visible emitting upconverting nanoparticles (UCNPs) are of particular interest for applications in photodynamic therapies (PDTs) owing to their capability to trigger various photochemical processes upon NIR excitation. It is the aim of this review to highlight the most recent achievements and advances in the field of Ln-based NPs as frequency-converting materials for biomedical applications and to identify current trends as well as remaining challenges. Special focus will be set on the most recently presented Ln-based systems whose potential has been evaluated in vitro and in vivo as NIR-to-NIR bioimaging probes, NIR-operating nanothermometers and -heaters as well as photochemical agents.

Optical nanoprobes for biomedical applications: shining a light on upconverting and near-infrared emitting nanoparticles for imaging, thermal sensing, and photodynamic therapy.

Eva Hemmer

Department of Chemistry and Biomolecular Sciences

University of Ottawa

Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

Eva Hemmer

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Chemistry and Biomolecular Sciences

University of Ottawa

Hyperspectral Imaging as a Tool to Study Optical Anisotropy in Lanthanide-Based Molecular Single Crystals

Department of Chemistry and Biomolecular Sciences