Two-Photon Antenna Effect Induced in Octupolar Europium Complexes

Carbazole-Functionalized Dipicolinato LnIII Complexes Show Two-Photon Excitation and Viscosity-Sensitive Metal-Centered Emission

Eu^III and Yb^III complexes with the carbazole-dipicolinato ligand dpaCbz^2-, namely K₃[Eu(dpaCbz)₃] and K₃[Yb(dpaCbz)₃], were isolated. The Eu^III complex displayed metal-centred emission upon one-photon excitation with a sensitized emission efficiency Φ L Ln of 1.8±0.3 %, corresponding to an intrinsic emission efficiency Φ Ln Ln of 46% and a sensitization efficiency of η_sens 3.9%, with an emission lifetime of the emissive state τ of 1.087±0.005 ms. The Yb^III complex displayed Φ L Ln of 0.010±0.001 %, and a τ of 2.32±0.06 μs. The Eu^III-centred emission was sensitized as well upon two-photon excitation and a two-photon absorption cross-section σ_2PA of 63 GM at 750 nm was determined for the complex. The one- or two-photon sensitized emission intensity of the Eu^III complex changes by more than two-fold when the solvent viscosity is varied in the range 0.5 - 200 cP and the emission is independent of dissolved oxygen. The Yb^III complex displays a change in emission intensity as well. However, in this case, a dependence of the emission intensity on dissolved oxygen content was observed.

Luminescence, chiroptical, magnetic and ab initio crystal-field characterizations of an enantiopure helicoidal Yb(iii) complex

The electronic structure of a chiral Yb(iii)-based complex is fully determined by taking advantage of experimental magnetic, luminescence, and chiroptical (NIR-ECD and CPL) characterizations in combination with ab initio wavefunction calculations.

Photophysical and Chiroptical Properties of the Enantiomers of N,N'-Bis(1-phenylpropyl)-2,6-pyridinecarboxamide and their Chiral 9-Coordinate Ln3+ Complexes

The R,R and S,S enantiomers of N,N'-bis(1-phenylpropyl)-2,6-pyridinedicarboxamide, L(Et), react with Ln3+ ions (Ln = La, Eu, Gd, and Tb) to give stable [Ln((R,R)- and (S,S)-L(Et))3]3+ in anhydrous acetonitrile solution, as evidenced by various spectroscopic measurements, including NMR and luminescence titrations. In addition to the characteristic Eu3+ and Tb3+ luminescence bands, the steady-state and time-resolved luminescence spectra of the aforementioned complexes show the residual ligand-centered emission of the 1ππ* to 3ππ* states, indicating an incomplete intersystem crossing (ISC) transfer from the 1ππ* to 3ππ* and ligand-to-Ln3+ energy transfer, respectively. The high circularly polarized luminescence (CPL) activity of [Eu(L(Et))3]3+ confirms that using a single enantiomer of L(Et) induces the preferential formation of one chiral [Eu(L(Et))3]3+ complex, consistent with the [EuL 3]3+ complexes formed with other ligands derived from a 2,6-pyridine dicarboxamide moiety. Furthermore, the CPL sign patterns of complexes with (R,R) or (S,S) enantiomer of L(Et) are consistent with the CPL sign pattern of related [LnL 3]3+ complexes with the (R,R) or (S,S) enantiomer of the respective ligands in this family.

Luminescent Carbazole-Based EuIII and YbIII Complexes with a High Two-Photon Absorption Cross-Section Enable Viscosity Sensing in the Visible and Near IR with One- and Two-Photon Excitation

The newly synthesized Eu^III and Yb^III complexes with the new carbazole-based ligands CPAD^2- and CPAP^4- display the characteristic long-lived metal-centered emission upon one- and two-photon excitation. The Eu^III complexes show the expected narrow emission bands in the red region, with emission lifetimes between 0.382 and 1.464 ms and quantum yields between 2.7% and 35.8%, while the Yb^III complexes show the expected emission in the NIR region, with emission lifetimes between 0.52 and 37.86 μs and quantum yields between 0.028% and 1.12%. Two-photon absorption cross sections (σ_2PA) as high as 857 GM were measured for the two ligands. The complexes showed a strong dependence of the one- and two-photon sensitized emission intensity on solvent viscosity in the range of 0.5-200 cP in the visible and NIR region.

Preparation, photoluminescence, semiconductor properties, and theoretical calculations for a novel zinc zero-dimensional structure complex

The complex [Zn(CH3OH)4(MCA)2] (3-hydroxy-2-methylquinoline-4-carboxylic acid) has been synthesized through hydrothermal reactions and characterized by single-crystal X-ray diffraction. Its solid-state photoluminescence spectrum shows an emission in the blue region, which is assigned to ligand-to-metal charge transfer on the basis of time-dependent density functional theory calculations. Solid-state diffuse reflectance measurements show the existence of a narrow optical band gap of 1.73 eV.

Excitation modulation of Eu:BPEPC based complexes as low-energy reference standards for circularly polarised luminescence (CPL)

The enantiomers of [EuL₃]·3Cl serve as effective reference complexes for the calibration of circularly polarised luminescence (CPL) spectrometers.

Synthesis, structure, photoluminescent, optical and magnetic properties of a novel thulium p-hydroxybenzenesulfonate complex

A novel thulium p-hydroxybenzenesulfonate complex [Tm(C6H5O4S)2(H2O)6](C6H5O4S)·3H2O (1) was solvothermally synthesized and characterized by elemental analyses, photoluminescence, solid-state UV/vis diffuse reflectance, magnetic measurements and single-crystal X-ray diffraction. Complex 1 features an ionic structure with the thulium ion possessing a square antiprismatic geometry. Complex 1 crystallized in the monoclinic system with space group P21. Photoluminescent measurements with solid-state sample demonstrate that the anti-Stokes emission bands in the red/NIR spectral region 710 and 812 nm are observed from the Tm3+ 4f intrashell transitions from the 3F2,3 excited states to the 3H6 and 1G4 excited state to the 3H5 state, respectively. Solid-state UV/vis diffuse reflectance spectra of complex 1 show the existence of a wide optical band gap of 3.56 eV. Variable-temperature magnetic susceptibility and field dependence magnetization measurements are also studied and the magnetic susceptibility obeys the Curie-Weiss law (χ m =c/(T−θ)) with the value C being of 8.6 K and a negative Weiss constant θ being of −0.2 K.

Stereocontrolled self-assembly and photochromic transformation of lanthanide supramolecular helicates

Stereocontrolled self-assembly of a dinuclear triple-stranded europium helicate (Eu2L3) based on DTE-functionalized ligands has been achieved via the chiral-induction strategy. The point-chirality of the ligands is transferred to give either Δ or Λ metal-centers and hence leads to the overall P or M helical senses. CD spectroscopy and NMR enantiomeric differentiation experiments have confirmed the formation of enantiomers. Moreover, the helicates in solution feature reversible photocyclization and cycloreversion, offering an opportunity to develop as chiroptical switches.

3D water-stable europium metal organic frameworks as a multi-responsive luminescent sensor for high-efficiency detection of Cr2O72−, MnO4−, Cr3+ ions and SDBS in aqueous solution

A 3D water-stable europium metal organic framework has been synthesized. Fluorescence measurements show that the MOF can selectively and sensitively detect Cr₂O₇²⁻, MnO₄⁻, Cr³⁺ ions and SDBS in aqueous solutions.

Design of Novel, Water Soluble and Highly Luminescent Europium Labels with Potential to Enhance Immunoassay Sensitivities

To meet the continual demands of more-sensitive immunoassays, the synthesis of novel luminescent Eu(III) chelate labels having similar substituted 4-(phenylethynyl)pyridine chromophores in three different chelate structure classes are reported. Significantly enhanced luminescence intensities were obtained, evidently caused by the intra-ligand charge transfer (ILCT) mediated sensitization, but the alternative ligands triplet state process cannot be ruled out. Based on the present study, even quite small changes on the chelate structure, and, especially, on the substituents' donor/acceptor strength on both ends of 4-(phenylethynyl)pyridine subunits have an unpredictable effect on the luminescence. The highest observed brightness was 16,400 M^-1cm^-1 in solution and 69,500 M^-1cm^-1 on dry surface, being 3.4 and 8.7 fold higher compared to the reference chelate. The new label chelates provide solutions for improved assay sensitivity up-to tenfold from the present concepts.

Enhanced DNA sensitized Tb3+ luminescence in organic solvents for more sensitive detection

DNA-sensitized Tb³⁺ luminescence spectroscopy is a powerful method for probing nucleic acids and developing biosensors. Its performance in organic solvents has yet to be explored. In this study, Tb³⁺ luminescence with nucleosides, nucleotides and DNA oligonucleotides in various organic solvents is studied. Tb³⁺ emission with single nucleotides is quenched up to 88% in dimethyl formamide (DMF), while its emission with nucleosides is enhanced. For the four 15-mer DNA homopolymers, the strongest absolute emission enhancement was achieved with C₁₅. Similar emission properties are observed in other solvents including DMF, DMSO, acetonitrile methanol, ethanol, isopropanol and ethylene glycol. A few DNAzymes are tested as random DNA sequences all showing 1.4-6.9-fold emission enhancement in ethanol. A previously reported optimized sequence in water (G₃T)₅ is further enhanced by the solvents. Using this sequence, a detection limit of 5.5 nm Hg²⁺ is achieved in 25% ethanol solution. A similar Hg²⁺ sensitivity is also observed in a lake water mixed with ethanol. Luminescence lifetime is longer in DMF than in water. This study indicates that DNA-sensitized Tb³⁺ luminescence can be measured in water miscible solvents and most likely, with even stronger emission than that in water.

Lighting the Way to See Inside Two-Photon Absorption Materials: Structure-Property Relationship and Biological Imaging

The application of two-photon absorption (2PA) materials is a classical research field and has recently attracted increasing interest. It has generated a demand for new dyes with high 2PA cross-sections. In this short review, we briefly cover the structure-2PA property relationships of organic fluorophores, organic-inorganic nanohybrids and metal complexes explored by our group. (1) The two-photon absorption cross-section (δ) of organic fluorophores increases with the extent of charge transfer, which is important to optimize the core, donor-acceptor pair, and conjugation-bridge to obtain a large δ value. Among the various cores, triphenylamine appears to be an efficient core. Lengthening of the conjugation with styryl groups in the D-π-D quadrupoles and D-π-A dipoles increased δ over a long wavelength range than when vinylene groups were used. Large values of δ were observed for extended conjugation length and moderate donor-acceptors in the near-IR wavelengths. The δ value of the three-arm octupole is larger than that of the individual arm, if the core has electron accepting groups that allow significant electronic coupling between the arms; (2) Optical functional organic/inorganic hybrid materials usually show high thermal stability and excellent optical activity; therefore the design of functional organic molecules to build functional organic-inorganic hybrids and optimize the 2PA properties are significant. Advances have been made in the design of organic-inorganic nanohybrid materials of different sizes and shapes for 2PA property, which provide useful examples to illustrate the new features of the 2PA response in comparison to the more thoroughly investigated donor-acceptor based organic compounds and inorganic components; (3) Metal complexes are of particular interest for the design of new materials with large 2PA ability. They offer a wide range of metals with different ligands, which can give rise to tunable electronic and 2PA properties. The metal ions, including transition metals and lanthanides, can serve as an important part of the structure to control the intramolecular charge-transfer process that drives the 2PA process. As templates, transition metal ions can assemble simple to more sophisticated ligands in a variety of multipolar arrangements resulting in interesting and tailorable electronic and optical properties, depending on the nature of the metal center and the energetics of the metal-ligand interactions, such as intraligand charge-transfer (ILCT) and metal-ligand charge-transfer (MLCT) processes. Lanthanide complexes are attractive for a number of reasons: (i) their visible emissions are quite long-lived; (ii) their absorption and emission can be tuned with the aid of appropriate photoactive ligands; (iii) the accessible energy-transfer path between the photo-active ligands and the lanthanide ion can facilitate efficient lanthanide-based 2PA properties. Thus, the above materials with excellent 2PA properties should be applied in two-photon applications, especially two-photon fluorescence microscopy (TPFM) and related emission-based applications. Furthermore, the progress of research into the use of those new 2PA materials with moderate 2PA cross section in the near-infrared region, good Materials 2017, 10, 223 2 of 37 biocompatibility, and enhanced two-photon excited fluorescence for two-photon bio-imaging is summarized. In addition, several possible future directions in this field are also discussed (146 references).

Selectively switching on europium emission in drug site one of human serum albumin

A luminescent europium probe has been discovered that binds selectively to drug-site I in human serum albumin, signalled by a ‘switching on’ of europium emission, and accompanied by strong induced circularly polarised luminescence.

Carbazole functionalized new bipolar ligand for monochromatic red light-emitting europium(iii) complex: combined experimental and theoretical study

Complete energy transfer observed from carbazole functionalized bipolar ligand as well as β-diketonate to Eu(iii) ion and Eu(iii) complex shows efficient PLQE (41.4%) with appropriate CIE color coordinates (x = 0.64, y = 0.36).

Versatile pyridine-2,6-bis-tetrazolate scaffolds for the formation of highly luminescent lanthanide complexes

Here we report the straightforward synthesis of the ligands L(tz), L(tzC8), L(tzPEG), L(tzAnis) and L(≡Anis) presenting the same pyridine-bistetrazolate (pytz, L) chelating scaffold but different substituents on the para position of the pyridine ring. Its substitution allows the tuning of the solubility of the LnIII complexes [Ln(Li)3]3(-). These new ligands form homoleptic nona-coordinated LnIII complexes of analogous structure and comparable stability in water and methanol. The derivatization of the para position with triazole and substituted triazole groups does not lead to a significant shift of the absorption window, but a shift of 40 nm is observed in ligand L(≡Anis) due to the presence of the p-ethynyl anisole fragment. The L(tzX) series sensitize well, both the Eu and the Tb ions with quantum yields up to 98% for Tb and 43% for Eu, while the ligand L(≡Anis) sensitizes well Eu (QY of 48%) but cannot sensitize Tb due to the position of its triplet state.

Time-resolved confocal microscopy using lanthanide centred near-IR emission

Time-resolved NIR imaging of lanthanide coated silica particles using Photon Arrival Time Imaging allows fast acquisition of high contrast images based on the probe luminescence lifetime.

EuroTracker® dyes: design, synthesis, structure and photophysical properties of very bright europium complexes and their use in bioassays and cellular optical imaging

The creation of the most emissive series of europium complexes is traced and examples given of their use in imaging.

Self-assembly of Terbium(III)-based metal-organic complexes with two-photon absorbing active

Hybrid complexes based on D-π-A type dyes p-aminostyryl-pyridinum and Terbium(III) complex anion (1, 2) have been synthesized by ionic exchange reaction. Meanwhile two different alkyl-substituted amino groups were used as electron donors in organic dyes cations. The synthesized complexes were characterized by element analysis. In addition, the structural features of them were systematic studied by single crystal X-ray diffraction analysis. Their linear properties have been systematically investigated by absorption spectra and fluorescence, the results show that the energy transfer takes place from the trans-4-[4'-(N,N-diethylamino)styryl]-N-methyl pyridinium (2') cation to Tb(III). In addition, complex 2 exhibit a large two-photon absorption coefficient β: 0.044cm/GW at 710nm.

Supramolecular self-assembly enhanced europium(III) luminescence under visible light

In this paper, we report on the luminescence of europium by directly exciting europium ions with visible light in aqueous medium. Upon replacing all the water molecules that coordinate around a central europium ion with a ditopic ligand 1,11-bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane (L2EO4), the quenching from water molecules is efficiently eliminated, offering considerable europium emission. By stoichiometrically mixing with a positively charged block polyelectrolyte, the negatively charged L2EO4-Eu coordinating complex can be transformed into a coordination 'polymer', which simultaneously forms electrostatic micelles with further enhanced europium fluorescence emission, owing to the increased fraction of L2EO4-coordinated Eu(III) as revealed by the fluorescence lifetime measurements. This approach avoids the use of the antenna effect that often utilizes UV light as the irradiation source. We further use those micelles for bio-imaging, and for the first time demonstrate the use of directly excited Eu-containing nano-probes for in vivo fluorescence imaging in small animals under visible excitation. Although literature results have shown that the direct excitation of europium ions in water may lead to emissions in the presence of coordinating ligands, those emissions were too weak to be applied due to the remaining water molecules in the coordination sphere. Our work points out that the direct excitation of europium can generate considerable europium emission given that all the water molecules in the coordination sphere are excluded, which does not only greatly reduce tedious lab work in synthesizing antenna molecules, but also facilitates the application of europium in aqueous medium under visible light.

Rational design of phosphorescent chemodosimeter for reaction-based one- and two-photon and time-resolved luminescent imaging of biothiols in living cells

A selective phosphorescent biothiols probe is synthesized based on Ir(III) complex 1, which has 2,2'-biquinoline as the N^N ligand for realizing the satisfied two-photon absorption cross-section and two-functionalized 2-phenylpyridine ligands with an α,β-unsaturated ketone moiety as the thiol reaction site. The one- and two-photon optical properties of 1 are investigated through UV-vis absorption spectrum and photoluminescence spectrum. This Ir(III) complex can act as an excellent one- and two-photon excited "OFF-ON" phosphorescent probe for biothiols based on the 1,4-addition of biothiol to α,β-unsaturated ketones. Moreover, one- and two-photon-induced luminescent imagings of biothiols in living cells are also realized. Furthermore, the experiments of time-resolved photoluminescence technique and fluorescence lifetime imaging microscopy demonstrate that 1 is able to detect biothiols in the presence of strong background fluorescence. In addition, probe 1 is adsorbed into the shell of mesoporous silica nanoparticles with core-shell structure to form a nanoprobe, which can realize the ratiometric detection of biothiols in absolute water solution and living cells based on two phosphorescent signals.