Colloidal Quantum Dots as an Emerging Vast Platform and Versatile Sensitizer for Singlet Molecular Oxygen Generation.

Singlet molecular oxygen (1O2) has been reported in wide arrays of applications ranging from optoelectronic to photooxygenation reactions and therapy in biomedical proposals. It is also considered a major determinant of photodynamic therapy (PDT) efficacy. Since the direct excitation from the triplet ground state (3O2) of oxygen to the singlet excited state 1O2 is spin forbidden; therefore, a rational design and development of heterogeneous sensitizers is remarkably important for the efficient production of 1O2. For this purpose, quantum dots (QDs) have emerged as versatile candidates either by acting individually as sensitizers for 1O2 generation or by working in conjunction with other inorganic materials or organic sensitizers by providing them a vast platform. Thus, conjoining the photophysical properties of QDs with other materials, e.g., coupling/combining with other inorganic materials, doping with the transition metal ions or lanthanide ions, and conjugation with a molecular sensitizer provide the opportunity to achieve high-efficiency quantum yields of 1O2 which is not possible with either component separately. Hence, the current review has been focused on the recent advances made in the semiconductor QDs, perovskite QDs, and transition metal dichalcogenide QD-sensitized 1O2 generation in the context of ongoing and previously published research work (over the past eight years, from 2015 to 2023).

Exploring the intra-4f and the bright white light upconversion emissions of Gd2O3:Yb3+,Er3+-based materials for thermometry.

Upconversion broadband white light emission driven by low-power near-infrared (NIR) lasers has been reported for many materials, but the mechanisms and effects related to this phenomenon remain unclear. Herein, we investigate the origin of laser-induced continuous white light emission in synthesized nanoparticles (Gd0.89Yb0.10Er0.01)2O3 and a mechanical mixture of commercial oxides with the same composition 89% Gd2O3, 10% Yb2O3, and 1% Er2O3. We report their photophysical features with respect to sample compactness, laser irradiation (wavelength, power density, excitation cycles), pressure, temperature, and temporal dynamics. Despite the sensitizer (Yb3+) and activator (Er3+) being in different particles for the mechanical mixture, efficient discrete and continuous upconversion emissions were observed. Furthermore, the synthesized nanoparticles were developed as primary luminescent thermometers (upon excitation at NIR) in the 299-363 K range, using the Er3+ upconversion 2H11/2 → 4I15/2/4S3/2 → 4I15/2 intensity ratio. They were also operating as secondary ones in the 1949-3086 K, based on the blackbody distribution of the observed white light emission. Our findings provide important insights into the mechanisms and effects related to the transition from discrete to continuous upconversion emissions with potential applications in remote temperature sensing.

Eu3+ and Tb3+ coordination compounds with phenyl-containing carbacylamidophosphates: comparison with selected Ln3+ ß-diketonates.

Materials based on Eu3+ and Tb3+ coordination compounds are of great interest due to their strong red and green luminescence. Appropriate selection of ligands plays a huge role in optimizing their photophysical properties. Another very helpful instrument for such optimization is theoretical modelling, which permits the prediction of the emissive properties of materials through intramolecular energy transfer analysis. The ligands that allow for achieving high efficiency of Eu3+ and Tb3+ emissions include carbacylamidophosphates (CAPh, HL). In this brief review, we summarize recent research for lanthanides CAPh-based coordination compounds of general formulas Cat[LnL]4, [LnL3Q] and [Ln(HL)3(NO3)3], where Cat+ = Cs+, NEt4+, PPh4 + and Q = 1,10-phenanthroline, 2,2-bipyridine or triphenylphosphine oxide, involving the use of thermal gravimetric analysis, X-ray analysis, and absorption and luminescence spectroscopy. We carried out a comparison with selected Ln3+ ß-diketonates. Possibilities and developments of theoretical calculations on energy transfer rates are also presented.

Dynamics of the Energy Transfer Process in Eu(III) Complexes Containing Polydentate Ligands Based on Pyridine, Quinoline, and Isoquinoline as Chromophoric Antennae.

In this work, we investigated from a theoretical point of view the dynamics of the energy transfer process from the ligand to Eu(III) ion for 12 isomeric species originating from six different complexes differing by nature of the ligand and the total charge. The cationic complexes present the general formula [Eu(L)(H2O)2]+ (where L = bpcd2- = N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate; bQcd2- = N,N'-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate; and bisoQcd2- = N,N'-bis(2-isoquinolinmethyl)-trans-1,2-diaminocyclohexane N,N'-diacetate), while the neutral complexes present the Eu(L)(H2O)2 formula (where L = PyC3A3- = N-picolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate; QC3A3- = N-quinolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate; and isoQC3A3- = N-isoquinolyl-N,N',N'-trans-1,2-cyclohexylenediaminetriacetate). Time-dependent density functional theory (TD-DFT) calculations provided the energy of the ligand excited donor states, distances between donor and acceptor orbitals involved in the energy transfer mechanism (RL), spin-orbit coupling matrix elements, and excited-state reorganization energies. The intramolecular energy transfer (IET) rates for both singlet-triplet intersystem crossing and ligand-to-metal (and vice versa) involving a multitude of ligand and Eu(III) levels and the theoretical overall quantum yields (ϕovl) were calculated (the latter for the first time without the introduction of experimental parameters). This was achieved using a blend of DFT, Judd-Ofelt theory, IET theory, and rate equation modeling. Thanks to this study, for each isomeric species, the most efficient IET process feeding the Eu(III) excited state, its related physical mechanism (exchange interaction), and the reasons for a better or worse overall energy transfer efficiency (ηsens) in the different complexes were determined. The spectroscopically measured ϕovl values are in good agreement with the ones obtained theoretically in this work.

New Luminescent Lanthanide Tetrakis-Complexes NEt4 [LnL4 ] Based on Dimethyl-N-Benzoylamidophosphate.

New lanthanide dimethyl-N-benzoylamidophosphate (HL) based tetrakis-complexes NEt4 [LnL4 ] (Ln3+ =La, Nd, Sm, Eu, Gd, Tb, Dy) are reported. The complexes are characterized by means of NMR, IR, absorption, and luminescent spectroscopy as well as by elemental, X-Ray, and thermal gravimetric analyses. The phenyl groups of the four ligands of the complex anion are directed towards one side, while the methoxy groups are directed in the opposite side, which makes the complexes under consideration structurally similar to calixarenes. The effect of changing the alkali metal counterion to the organic cation NEt4+ on the structure and properties of the tetrakis-complex [LnL4]- is analyzed. The complexes exhibit bright characteristic for respective lanthanides luminescence. Rather high intensity of the band of 5 D0 →7 F4 transition, observed in the luminescence spectrum of NEt4 [EuL4 ], is discussed based on theoretical calculations.

Spectroscopic aspects for the Yb3+ coordination compound with a large energy gap between the ligand and Yb3+ excited states.

We present the experimental and theoretical results that made it possible to propose the energy transfer mechanism for a Yb complex with a large energy gap between the ligand and Yb excited states using a theoretical model and experimental data. Absorption and emission spectroscopy in the 300-4 K range is used for the study of the Yb3+ compound with N-phosphorylated sulfonamide (Na[YbL4]), which, despite the large energy gap, is characterized by high emission sensitization efficiency (ηsens = 40%) and relatively long Yb3+ emission lifetime (27 µs). The crystal structure of Na[YbL4], radiative lifetime (930 µs), refractive index (1.46), intrinsic (3.0%), and overall (1.3%) emission quantum yield were determined. To obtain the electronic properties of the Na[YbL4], a time-dependent density functional theory (TD-DFT) was performed. The intramolecular energy transfer (IET) rates from the excited states S1 and T1 to the Yb3+ ion as well as between the ligand and the ligand-to-metal charge transfer (LMCT) states were calculated. Once the intersystem crossing S1 â†’ T1 is not so effective due to a large energy gap between S1 and T1 (≈10000 cm-1), it has been shown that the LMCT state acts as an additional channel to feed the T1 state. Then, the T1 can transfer energy to the Yb3+ 2F5/2 energy level (WT), where WT is dominated by the exchange mechanism. Based on IET and a rate equation model, the overall emission quantum yield QLLn was simulated with and without the LMCT, this also confirmed that the pathway S1 â†’ LMCT â†’ T1 â†’ Yb3+ is more likely than the S1 â†’ T1 â†’ Yb3+ one.

Surface Plasmon-Photon Coupling in Lanthanide-Doped Nanoparticles.

Lanthanide-doped nanoparticles have great potential for energy conversion applications, as their optical properties can be precisely controlled by varying the doping composition, concentration, and surface structures, as well as through plasmonic coupling. In this Perspective we highlight recent advances in upconversion emission modulation enabled by coupling upconversion nanoparticles with well-defined plasmonic nanostructures. We emphasize fundamental understanding of luminescence enhancement, monochromatic emission amplification, lifetime tuning, and polarization control at nanoscale. The interplay between localized surface plasmons and absorbed photons at the plasmonic metal-lanthanide interface substantially enriches the interpretation of plasmon-coupled nonlinear photophysical processes. These studies will enable novel functional nanomaterials or nanostructures to be designed for a multitude of technological applications, including biomedicine, lasing, optogenetics, super-resolution imaging, photovoltaics, and photocatalysis.

Overlap properties of chemical bonds in generic systems including unusual bonding situations.

Chemical bond is a ubiquitous and fundamental concept in chemistry, in which the overlap plays a defining role. By using a new approach based on localized molecular orbitals, the overlap properties, e.g., polarizability [Formula: see text], population pOP, intra [Formula: see text], and inter [Formula: see text] repulsions, and density ρOP, of polyatomic systems were calculated, analyzed, and correlated. Several trends are shown for these properties, which are rationalized by the balance of some well-known effects, such as, electron donor/withdrawing character and electronegativity. The overlap properties of unusual bonds are also analyzed, revealing an OZn4(OOCH)6 structure with four equivalent Zn-O chemical bonds with overlap properties like the O-O bond in H2O2, while in protonated methane [Formula: see text], it is observed that a CH3⋯[Formula: see text] bond pattern at the equilibrium structure changes to a [Formula: see text]⋯H2 pattern upon dissociation. Charge-shift resonance energies, atom-in-molecule properties, and the lone-pair-bond-weakening effects are related to the overlap properties, which can provide alternative views and insights into chemical bonds. Graphical abstract A chemical bond analysis approach based on its overlap properties is presented for the first time. The model was applied directly to 25 diatomics and for 28 bonds in polytomics employing localized molecular orbitals. Correlations of the overlap properties with the charge-shift resonance energies and with atom-in-molecule (AIM) properties were uncovered. In addition, it provided insights into the Zn-O bonds in the unusual OZn4(OOCH)6 system as well as in the bonding patterns of [Formula: see text] at equilibrium and upon dissociation.

Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes.

The odd-even effect in luminescent [Eu2 (L)3 (H2 O)x ]⋅y(H2 O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2-6 and y=0-4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu3+ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand-metal coordination via the mixed mode of bridge-chelate coordination, except for the Eu3+ -oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd-even effect on the 4 f-4 f luminescence intensity parameters (Ω2 and Ω4 ) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( α* ) were simulated by a ghost atom located at the middle of each ligand chain. The values of α* were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( QLnLn ) of the Eu3+ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω2 and Ω4 intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.

Polarized Luminescence of Anisotropic LaPO4:Eu Nanocrystal Polymorphs.

Lanthanide elements exhibit highly appealing spectroscopic properties that are extensively used for phosphor applications. Their luminescence contains precise information on the internal structure of the host materials. Especially, the polarization behavior of the transition sublevel peaks is a fingerprint of the crystal phase, symmetry, and defects. However, this unique feature is poorly explored in current research on lanthanide nanophosphors. We here report on a detailed investigation of the evolution of Eu3+ luminescence during the thermally induced phase transition of LaPO4 nanocrystal hosts. By means of c-axis-aligned nanocrystal assemblies, we demonstrate a dramatic change of the emission polarization feature corresponding to the distinct Eu3+ site symmetries in different LaPO4 polymorphs. We also show that changes of the nanocrystal structure can be identified by this spectroscopic method, with a much higher sensitivity than the X-ray diffraction analysis. This new insight into the nanostructure-luminescence relationship, associated with the unprecedented polarization characterizations, provides a new methodology to investigate phase transitions in nanomaterials. It also suggests a novel function of lanthanide emitters as orientation-sensing nanoprobes for innovative applications such as in bioimaging or microfluidics.

Contribution of Energy Transfer from the Singlet State to the Sensitization of Eu3+ and Tb3+ Luminescence by Sulfonylamidophosphates.

A series of stable lanthanide complexes Na[Ln(L)4 ] (Ln=La3+ , Eu3+ , Gd3+ , Tb3+ , with L=dimethyl(4-methylphenylsulfonyl)amidophosphate and dimethyl-2-naphthylsulfonylamidophosphate) were synthesized. The compounds were characterized by single-crystal X-ray diffraction, IR, absorption, and emission spectroscopy at 293 and 77 K. In contrast to the usual and well-known dominant role of the ligand triplet state in intramolecular energy transfer processes in Ln complexes, in this particular new class of Ln compounds with sulphonylamidophosphate ligands, strong experimental and detailed theoretical evidence suggest a dominant role is played by the ligand first excited singlet state. The importance of the role played by the 7 F5 level in the case of the Tb3+ compound in this process is shown. The theoretical approach for the energy transfer rates was successfully applied to the rationalization of the experimental data. The higher-lying excited levels of Eu (5 DJ , 5 LJ , 5 GJ ) and Tb (5 DJ , 5 GJ , 5 LJ , 5 HJ , 5 FJ , 5 IJ ) were included in the calculations for the first time. Both the multipolar and exchange mechanisms were taken into account. The experimental intensity parameters (Ωλ ), emission lifetimes (τ), radiative (Arad ) and non-radiative (Anrad ) decay rates, and quantum yields (theoretical and experimental) were determined and are discussed in detail.

Highly-sensitive Eu(3+) ratiometric thermometers based on excited state absorption with predictable calibration.

Temperature measurements ranging from a few degrees to a few hundreds of Kelvin are of great interest in the fields of nanomedicine and nanotechnology. Here, we report a new ratiometric luminescent thermometer using thermally excited state absorption of the Eu(3+) ion. The thermometer is based on the simple Eu(3+) energy level structure and can operate between 180 and 323 K with a relative sensitivity ranging from 0.7 to 1.7% K(-1). The thermometric parameter is defined as the ratio between the emission intensities of the (5)D0 → (7)F4 transition when the (5)D0 emitting level is excited through the (7)F2 (physiological range) or (7)F1 (down to 180 K) level. Nano and microcrystals of Y2O3:Eu(3+) were chosen as a proof of concept of the operational principles in which both excitation and detection are within the first biological transparent window. A novel and of paramount importance aspect is that the calibration factor can be calculated from the Eu(3+) emission spectrum avoiding the need for new calibration procedures whenever the thermometer operates in different media.

Boosting the sensitivity of Nd(3+)-based luminescent nanothermometers.

Luminescence thermal sensing and deep-tissue imaging using nanomaterials operating within the first biological window (ca. 700-980 nm) are of great interest, prompted by the ever-growing demands in the fields of nanotechnology and nanomedicine. Here, we show that (Gd1-xNdx)2O3 (x = 0.009, 0.024 and 0.049) nanorods exhibit one of the highest thermal sensitivity and temperature uncertainty reported so far (1.75 ± 0.04% K(-1) and 0.14 ± 0.05 K, respectively) for a nanothermometer operating in the first transparent near infrared window at temperatures in the physiological range. This sensitivity value is achieved using a common R928 photomultiplier tube that allows defining the thermometric parameter as the integrated intensity ratio between the (4)F5/2 → (4)I9/2 and (4)F3/2 → (4)I9/2 transitions (with an energy difference between the barycentres of the two transitions >1000 cm(-1)). Moreover, the measured sensitivity is one order of magnitude higher than the values reported so far for Nd(3+)-based nanothermometers enlarging, therefore, the potential of using Nd(3+) ions in luminescence thermal sensing and deep-tissue imaging.

Features of chemical bonds based on the overlap polarizabilities: diatomic and solid-state systems with the frozen-density embedding approach.

The chemical bond overlap properties were obtained for alkali halides NaY (Y = F, Cl, Br), alkaline-earth chalcogenides MX (M = Ca, Mg and X = O, S, Se) and alkali and alkali-earth metals (Li, Na, and Mg) in diatomic and solid-state systems using an embedding approach based on the frozen density functional theory to simulate the crystalline effects. The computational protocol established provides errors for bond distances smaller than 1%. The results indicate that larger chemical bond covalency leads to larger absorption or scattering by the overlap region. The ionic specific valence and overlap polarizability are closely related to the valence orbital compactness measured by the sum of Mulliken electronegativities. The embedding approach used in this work makes it possible to quantify the effects of the crystalline environment on the chemical bond overlap properties. In the solid-state, the bond overlap charges are less polarizable, in cases of well-known ionic systems (provided by electronegativity differences), leading to smaller chemical bond covalency in solids than in diatomics. The spectroscopic properties of the polarizability of the electron density in the overlap region of a chemical bond could be measured in the 1-20 eV spectral region and could be used to characterize some bands in several spectra whose assignments are ambiguous or not available.

Red-green emitting and superparamagnetic nanomarkers containing Fe3O4 functionalized with calixarene and rare earth complexes.

The design of bifunctional magnetic luminescent nanomaterials containing Fe3O4 functionalized with rare earth ion complexes of calixarene and ß-diketonate ligands is reported. Their preparation is accessible through a facile one-pot method. These novel Fe3O4@calix-Eu(TTA) (TTA = thenoyltrifluoroacetonate) and Fe3O4@calix-Tb(ACAC) (ACAC = acetylacetonate) magnetic luminescent nanomaterials show interesting superparamagnetic and photonic properties. The magnetic properties (M-H and ZFC/FC measurements) at temperatures of 5 and 300 K were explored to investigate the extent of coating and the crystallinity effect on the saturation magnetization values and blocking temperatures. Even though magnetite is a strong luminescence quencher, the coating of the Fe3O4 nanoparticles with synthetically functionalized rare earth complexes has overcome this difficulty. The intramolecular energy transfer from the T1 excited triplet states of TTA and ACAC ligands to the emitting levels of Eu(3+) and Tb(3+) in the nanomaterials and emission efficiencies are presented and discussed, as well as the structural conclusions from the values of the 4f-4f intensity parameters in the case of the Eu(3+) ion. These novel nanomaterials may act as the emitting layer for the red and green light for magnetic light-converting molecular devices (MLCMDs).

Efficient and tuneable photoluminescent boehmite hybrid nanoplates lacking metal activator centres for single-phase white LEDs.

White light-emitting diodes (WLEDs) are candidates to revolutionize the lighting industry towards energy efficient and environmental friendly lighting and displays. The current challenges in WLEDs encompass high luminous efficiency, chromatic stability, high colour-rending index and price competitiveness. Recently, the development of efficient and low-cost downconverting photoluminescent phosphors for ultraviolet/blue to white light conversion was highly investigated. Here we report a simple route to design high-efficient WLEDs by combining a commercial ultraviolet LED chip (InGaAsN, 390 nm) and boehmite (γ-AlOOH) hybrid nanoplates. Unusually high quantum yields (ηyield=38-58%) result from a synergic energy transfer between the boehmite-related states and the triplet states of the benzoate ligands bound to the surface of the nanoplates. The nanoplates with ηyield=38% are able to emit white light with Commission International de l'Eclairage coordinates, colour-rendering index and correlated colour temperature values of (0.32, 0.33), 85.5 and 6,111 K, respectively; overwhelming state-of-the-art single-phase ultraviolet-pumped WLEDs phosphors.

Unusual photoluminescence properties of the 3D mixed-lanthanide-organic frameworks induced by dimeric structures: a theoretical and experimental approach.

The present work describes a complementary experimental and theoretical investigation of the spectroscopic properties of the four isostructural 3D Ln-MOFs (wherein PDC = pyrazole-3,5-dicarboxylate, [La2(PDC)3(H2O)4]·2H2O (1), [(La0.9Eu0.1)2(PDC)3(H2O)4]·2H2O (2), [(La0.9Tb0.1)2(PDC)3(H2O)4]·2H2O (3) and [(La0.9Eu0.5Tb0.5)2(PDC)3(H2O)4]·2H2O (4)). The experimental data and theoretical calculations show that the singular photophysical properties presented by these Ln-MOFs are induced by strong interaction between the Ln(3+) ions.

Energy-transfer from Gd(III) to Tb(III) in (Gd,Yb,Tb)PO4 nanocrystals.

The photoluminescence properties of (Gd,Yb,Tb)PO4 nanocrystals synthesized via a hydrothermal route at 150 °C are reported. Energy-transfer from Gd(3+) to Tb(3+) is witnessed by the detailed analyses of excited-state lifetimes, emission quantum yields, and emission and excitation spectra at room temperature, for Tb(3+) concentrations ranging from 0.5 to 5.0 mol%. Absolute-emission quantum yields up to 42% are obtained by exciting within the (6)I7/2-17/2 (Gd(3+)) manifold at 272 nm. The room temperature emission spectrum is dominated by the (5)D4 → (7)F5 (Tb(3+)) transition at 543 nm, with a long decay-time (3.95-6.25 ms) and exhibiting a rise-time component. The (5)D3 → (7)F6 (Tb(3+)) rise-time (0.078 ms) and the (6)P7/2 → (8)S7/2 (Gd(3+)) decay-time (0.103 ms) are of the same order, supporting the Gd(3+) to Tb(3+) energy-transfer process. A remarkably longer lifetime of 2.29 ms was measured at 11 K for the (6)P7/2 → (8)S7/2 (Gd(3+)) emission upon excitation at 272 nm, while the emission spectrum at 11 K is dominated by the (6)P7/2 → (8)S7/2 transition line, showing that the Gd(3+) to Tb(3+) energy-transfer process is mainly phonon-assisted with an efficiency of ~95% at room temperature. The Gd(3+) to Tb(3+) energy transfer is governed by the exchange mechanism with rates between 10(2) and 10(3) s(-1), depending on the energy mismatch conditions between the (6)I7/2 and (6)P7/2 levels of Gd(3+) and the Tb(3+ 5)I7, (5)F2,3 and (5)H5,6,7 manifolds and the radial overlap integral values.

Luminescent material based on the [Eu(TTA)3(H2O)2] complex incorporated into modified silica particles for biological applications.

Amino-functionalized luminescent silica particles were investigated for use in immunoassays. The particles were prepared by the Stöber method where the [Eu(TTA)3(H2O)2] complex (TTA: 3-thenoyltrifluoroacetonate) was incorporated into silica particles during the hydrolysis and condensation of TEOS: tetraethylorthosilicate. Then, the amino groups were introduced in the particle surface using APTS: 3-aminopropyltriethoxisilane. The resulting particles were characterized by scanning electron microscopy (SEM), X ray diffraction (XRD) and photoluminescence spectroscopy. In order to demonstrate the viability of the use of luminescent particles as optical markers, an enzyme-substrate reaction was performed using HRP: horseradish peroxidase. It was possible to verify the binding of HRP-oxidized LDL (low density lipoprotein) and anti-oxLDL antibody-luminescent silica particles through the evaluation of the presence of HRP. The bioassay data open a broad field for the development of protein-tagged luminescent particles for use in biomedical sciences.