Coordination Geometrical Effect on Ligand-to-Metal Charge Transfer-Dependent Energy Transfer Processes of Luminescent Eu(III) Complexes

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

Enhancing the photodegradation of tetracycline in aquaculture wastewater via iron(III)-alginate: Degradation pathway transformation and toxicity reduction

Tetracycline's (TC) incomplete self-photolysis by light irradiation generally produces toxic intermediate products, which posing serious harm to the aqueous environment. In order to diminish the environmental risks of TC self-photolysis, an iron(III)-alginate (Fe-SA) hydrogel assisted photocatalytic method was developed and the underlying mechanisms was also analyzed in this work. Under simulated sunlight, the photo-degradation efficiency of TC was 61.1% at pH 7.0 within 2 h. Importantly, four of the seven intermediate products that identified during the self-photolysis of TC were found toxic based on QSAR analysis. In contrast, the removal efficiency of TC could be improved to 87.4% by adding Fe-SA under the same conditions. Moreover, only two relatively weakly toxic intermediate products were detected after exposing to the Fe-SA photocatalytic system, indicating a significant reduction of the potential ecological risks caused by TC self-photolysis. Furthermore, the determination of reactive oxidation species (ROS) demonstrated that the addition of Fe-SA primarily facilitated the degradation of TC and the related toxic intermediate products through assisting the free radical (∙OH and ∙O2-) photocatalytic degradation pathway. Additionally, the photocatalytic application under actual sunlight conditions and the reusability experiments of Fe-SA further confirmed its effectiveness and low cost in removing TC. This study revealed the photodegradation mechanisms of TC from the perspective of the self-photolysis process, and also offering new insights into the removal of TC pollution in the environment.

Pure red-light emitting europium based complexes as efficient UV light converters: synthesis, crystal structure and photoluminescence properties

This paper reports three new crystallographically characterized europium complexes with composition as follows: [Eu(fod)3(L1)] (1), [Eu(fod)3(L2)] (2) and [Eu(fod)3(L3)] (3) {L1 = benzimidazole (bzi), L2 = 4,7-diphenyl-1,10-phenanthroline (bath), L3 = 2-(2-pyridyl) benzimidazole (py-im) and fod = anion of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione (Hfod)}. The single crystal (SC) XRD analysis shows that complex 1 is seven-coordinated while complexes 2 and 3 are eight-coordinated with the geometrical structures of a mono-capped octahedron and a trigonal dodecahedron, respectively. The NMR spectra of the complexes validate the SC-XRD results in solution. The complexes are stable in solution as no dissociation of any ligand was observed in the NMR spectra of the complexes. The photophysical properties of the complexes in solution, solid state, and PMMA thin films were studied. The hypersensitive transition 5D0 → 7F2 dominates the emission spectra in all phases, showing the highly asymmetric environment around the Eu(III) ion. The bath ligand is found to be the best sensitizer of the Eu ion and hence complex 2 shows the strongest luminescence properties with the highest absolute quantum yield among the three complexes. The CIE coordinate analysis shows that pure red-luminescence is emitted by the Eu complexes in the solid state since the coordinates found in this phase are closer to the standard NTSC 1987 values. The optical band gaps were determined for the complexes and the observed values suggest that the complexes can have possible applications in the field of semiconductor materials.

Competing excitation paths in luminescent heterobimetallic Ln-Al complexes: Unraveling interactions via experimental and theoretical investigations

The interest for heterometallic lanthanide-d or-p metal (Ln-M) complexes is growing because of a potential cooperative or synergistic effect related to the proximity of two different metals in the same molecular architecture affording special tunable physical properties. To exploit the potentiality of Ln-M complexes, suitable synthetic approaches, and the in-depth understanding of the effect of each building block on their properties are mandatory. Here, we report the study on a family of heterometallic luminescent complexes [Ln(hfac)₃Al(L)₃], Ln= Eu³⁺ and Tb³⁺. Using different L ligands, we investigated the effect of the steric and electronic properties of the Al(L)₃ fragment, highlighting the general validity of the employed synthetic route. A marked difference in the light emission of [Eu(hfac)₃Al(L)₃] and [Tb(hfac)₃Al(L)₃] complexes has been observed. Thanks to photoluminescence experiments and Density Functional Theory calculations, Ln³⁺ emissions are explained with a model involving two non-interacting excitation paths through hfac or Al(L)₃ ligands.

Luminescent Lanthanide Complexes for Effective Oxygen-Sensing and Singlet Oxygen Generation

Oxygen quantification using luminescence has attracted considerable attention in various fields, including environmental monitoring and clinical analysis. Among the reported luminophores, trivalent lanthanide complexes have displayed characteristic narrow emission bands with high brightness. This bright emission is based on photo-sensitized energy transfer via organic triplet states. The organic triplet states in lanthanide complexes effectively react with the triplet oxygen, enabling oxygen quantification by lanthanide luminescence. Some Tb^III and Eu^III complexes with slow deactivation processes have also formed the excited state equilibrium, thus resulting in the emission-lifetime based oxygen sensing property. The combination of Tb^III /Eu^III emission, Eu^III /Sm^III emission, Eu^III /ligand phosphorescence, and ligand fluorescence/ligand phosphorescence provide the ratiometric oxygen-sensing properties. Moreover, the reaction generates singlet oxygen species which exhibit numerous applications in the photo-medical field. The ligands with large π-conjugated aromatic systems, such as porphyrin, phthalocyanine, and polyaromatic compounds, induces highly efficient oxygen generation. The combination of effective luminescence with singlet-oxygen generation by the lanthanide complexes render them suitable for photo-driven theranostics. This review summarizes the research progress of lanthanide complexes with efficient oxygen-sensing and singlet-oxygen generation properties.

Synthesis and Characterization of Coordination Compound [Eu(µ2-OC2H5)(btfa)(NO3)(phen)]2phen with High Luminescence Efficiency

A high-luminescent, blue-light excitable europium(III) coordination complex, [Eu(µ₂-OC₂H₅)(btfa)(NO₃)(phen)]₂phen (1) {btfa = benzoyl trifluoroacetone, phen = 1,10-phenantroline}, has been synthesized and investigated. The complex was characterized by infrared (IR) and photoluminescence (PL) spectroscopy. The PL emission spectra of powder samples registered in a range of 10.7-300 K exhibit characteristic metal-centered luminescence bands, assigned to internal radiative transitions of the Eu³⁺ ion, ⁵D₁→⁷F_j and ⁵D₀→⁷F_j (j = 0-4). The high-resolution spectrum of the transition ⁵D₀→⁷F₀ shows that it consists of two narrow components, separated by 0.96 meV, which indicates the presence in the matrix of two different sites of the Eu³⁺ ion. The splitting pattern of ⁵D₀→⁷F_j (j = 0-4) transitions indicates that europium ions are located in a low-symmetry environment. The absolute quantum yield and the sensitization efficiency were determined to be 49.2% and 89.3%, respectively. The complex can be excited with low-cost lasers at around 405 nm and is attractive for potential applications in optoelectronics and biochemistry.

Nontoxic Tb3+-induced hyaluronic nano-poached egg aggregates for colorimetric and luminescent detection of Fe3+ ions

This study demonstrates that a luminescent Tb³⁺ complex with green emission can be complexed with hyaluronic (hya) to form nanoparticles. The structure of complexation is composed of a Tb(acac)₂phen core with a hya surface, similar to those of the nano-poached eggs. What makes the structure unique is that Tb(acac)₂phen and hya are connected by chemical bonds. To confirm their utility, we illustrate that the luminescence is rapidly and selectively quenched in the presence of Fe³⁺. Initial cytotoxicity experiments with human liver carcinoma cells show that the luminescent lanthanide complexes are cytotoxic, however, complexing lanthanides to hya renders them cytocompatible. The new complex integrates the advantages of superior lanthanide luminescence, the unique shape of nano-poached eggs, compatibility with aqueous systems, and cytocompatibility. Tb³⁺-induced hyaluronic nano-poached eggs (THNE) can, therefore, be used for Fe³⁺ detection in aqueous systems.

The original Tb³⁺-induced hyaluronic nano-poached eggs (THNE) integrates the advantages of superior lanthanide luminescence, the unique shape of nano-poached eggs, and non-toxicity, for the sensing of Fe³⁺ in aqueous surroundings.

Preparation of photonic molecular trains via soft-crystal polymerization of lanthanide complexes

Soft-crystals are defined as flexible molecular solids with highly ordered structures and have attracted attention in molecular sensing materials based on external triggers and environments. Here, we show the soft-crystal copolymerization of green-luminescent Tb(III) and yellow-luminescent Dy(III) coordination centers. Soft-crystal polymerization is achieved via transformation of monomeric dinuclear complexes and polymeric structures with respect to coordination number and geometry. The structural transformation is characterized using single-crystal and powder X-ray diffraction. The connected Tb(III) crystal-Dy(III) crystal show photon energy transfer from the Dy(III) centre to the Tb(III) centre under blue light excitation (selective Dy(III) centre excitation: 460 ± 10 nm). The activation energy of the energy transfer is estimated using the temperature-dependent emission lifetimes and emission quantum yields, and time-dependent density functional theory (B3LYP) calculations. Luminescence-conductive polymers, photonic molecular trains, are successfully prepared via soft-crystal polymerization on crystal media with remarkable long-range energy migration.

Soft-crystals are molecular solids with highly ordered structures. Here, authors report the soft-crystal copolymerization of green-luminescent Tb(III) and yellow-luminescent Dy(III) complexes, and study the long-range energy transfer from one crystal to the other.

Thermally Activated Photophysical Processes of Organolanthanide Complexes in Solution

The effect of temperature upon the lanthanide luminescence lifetime and intensity has been investigated in toluene solution for the complexes LnPhen(TTA)3 (Ln = Eu, Sm, Nd, Yb; Phen = 1,10-phenanthroline; TTA = thenoyltrifluoroacetonate). Thermally excited back-transfer to a charge transfer state was found to occur for Ln = Eu and can be explained by lifetime and intensity back-transfer models. The emission intensity and lifetime were also quenched with increasing temperature for Ln = Sm, and the activation energy for nonradiative decay is similar to that for the thermal population of Sm3+ excited states. Unusual behavior for lifetime and intensity was found for both Ln = Nd, Yb. The usually assumed equivalence of τ/τ0 = I/I0 (where τ is lifetime and I is intensity) does not hold for these cases. We infer that for these lanthanide systems the intensity decreases with temperature in the stage prior to population of the luminescent state. The lifetime changes are discussed.

Asymmetric Lumino-Transformer: Circularly Polarized Luminescence of Chiral Eu(III) Coordination Polymer with Phase-Transition Behavior

A chiral Eu(III) coordination polymer with phase-transition behavior, [Eu(+tfc)₃(m-dpeb)]_n, (+tfc: (+)-3-trifluoroacetylcamphorato, m-dpeb: 1,3-bis(diphenylphosphorylethynyl)benzene) was reported for understanding the effect of polymer chain arrangement (orientation effect) on the circularly polarized luminescence (CPL) in a solid system. The phase-transition behavior of the transformable Eu(III) coordination polymer was characterized using differential scanning calorimetry and powder X-ray diffraction. The Eu(III) coordination polymer exhibited phase transition at approximately 180 °C. The magnitude of the CPL intensity was drastically changed because of the phase transition, without coordination geometrical change around the Eu(III) ion. In this study, the orientation effect of a chiral Eu(III) coordination polymer on the CPL properties in crystalline solid is demonstrated.

Sensitization of Nd3+ Luminescence by Simultaneous Two-Photon Excitation through a Coordinating Polymethinic Antenna

We have designed and synthesized two new cyaninic Nd³⁺ complexes where the lanthanide emission can be induced from simultaneous two-photon absorption followed by energy migration. These complexes correspond to a molecular design that uses an antenna ligand formed by the functionalization of a heptamethine dye with 5-ol-phenanthroline or 4-phenyl-terpyridine derivatives. These complexes employ the important nonlinear optical properties of symmetric polymethines to sensitize the lanthanide ion. We verified that simultaneous biphotonic excitation indirectly induces the ⁴F_3/2 → ⁴I_11/2 Nd³⁺ emission using femtosecond laser pulses tuned below the first electronic transition of the antenna. The simultaneous two-photon excitation events initially form the nonlinear-active second excited singlet of the polymethine antenna, which rapidly evolves into its first excited singlet. This state in turn induces the formation of the emissive Nd³⁺ states through energy transfer. The role of the first excited singlet of the antenna as the donor state in this process was verified through time resolution of the antenna's fluorescence. These measurements also provided the rates for antenna-lanthanide energy transfer, which indicate that the phenanthroline-type ligand is approximately five times more efficient for energy transfer than the phenyl-terpyridine derivative due to their relative donor-acceptor distances. The simultaneous two-photon excitation of this polymethine antenna allows for high spatial localization of the Nd³⁺excitation events.

Charge-transfer excited states of π- and 4f-orbitals for development of luminescent Eu(III) complexes

Transition metal complexes provide photofunctional properties through the charge transfer excited states of their metal ion and organic ligand components. Recently, there are increasing reports on the charge transfer excited states of the ligand (π)- and 4f-orbitals of lanthanide complexes, where the latter are shielded by filled 5s² and 5p⁶ orbitals. This area of research is relatively unestablished; thus, the study of photo-excited organic-lanthanide charge transfer would lead to the construction of next-generation photofunctional metal complexes. In this review, we summarize the latest research progress in photofunctional materials using the charge transfer excited states of lanthanide complexes, and discuss the photophysical/theoretical analyses of these charge transfer excited states.

Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents

Hydrocalumite structured layered double hydroxides (LDHs) with various Fe3+ ratios were prepared through a coprecipitation method. In order to control the Fe3+ content in LDH, binary Ca–Fe LDHs were first synthesized with various Ca/Fe ratios. The X-ray diffraction pattern showed that only a limited Ca/Fe ratio resulted in LDH formation. The Fe3+ content in LDH was controlled by applying Al3+ while the divalent and trivalent metal ratio was set to 2. Through X-ray diffraction patterns, ternary LDHs with Ca–Al–Fe composition were successfully synthesized without significant impurities, with the Al increasing crystallinity. Quantification showed that Al moiety participated in the formation of the LDH framework more than Ca and Fe, implying a structural stabilization in the presence of Al. In order to investigate the global and local structure of Fe moiety in the LDH, both solid state UV-vis and X-ray absorption spectroscopies were carried out. Both spectroscopies revealed that the existence of Al induced slight local distortion in coordination but global crystal stabilization.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes

Nine-coordinate [ErN₉] or [ErN₃O₆] chromophores found in triple helical [Er(L)₃]³⁺ complexes (L corresponds to 2,2′,6′,2′′-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)₃]³⁻ (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)₃]⁹⁺ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10⁻¹¹ ≤ ϕ^up_tot(ESA) ≤ 1.7(2) × 10⁻⁹ appear to be 1–3 orders of magnitude larger than those predicted by the accepted single-center excited-state absorption mechanism (ESA). Switching to the alternative energy transfer upconversion mechanism (ETU), which operates in multi-centers [CrErCr(bpb-bzimpy)₃]⁹⁺, leads to an improved quantum yield of ϕ^up_tot(ETU) = 5.8(6) × 10⁻⁸, but also to an even larger discrepancy by 4–6 orders of magnitude when compared with theoretical models. All photophysical studies point to Er(⁴I_13/2) as being the only available ‘long-lived’ (1.8 ≤ τ ≤ 6.3 μs) and emissive excited state, which works as an intermediate relay for absorbing the second photon, but with an unexpected large cross-section for an intrashell 4f → 4f electronic transition. With this in mind, the ETU mechanism, thought to optimize upconversion via intermetallic Cr → Er communication in [CrErCr(bpb-bzimpy)₃]⁹⁺, is indeed not crucial and the boosted associated upconversion quantum yield is indebted to the dominant contribution of the single-center erbium ESA process. This curious phenomenon is responsible for the successful implementation of light upconversion in molecular coordination complexes under reasonable light power intensities, which paves the way for applications in medicine and biology. Its origin could be linked with the presence of metal–ligand bonding.

Near-infrared to visible molecular upconversion exhibits quantum yields which are 2–6 orders of magnitude larger than those modeled with the accepted linear excited state absorption (ESA) or energy transfer (ETU) mechanisms: we have had a problem!

Fluorescent multi-component polymer sensors for the sensitive and selective detection of Hg2+/Hg+ ions via dual mode fluorescence and colorimetry

Fluorescent multi-component polymers, which are sensitive and selective to Hg2+/Hg+ through fluorescence and colorimetry, were synthesized by the Heck coupling reaction.