EuIII Complexes Research Articles

Self-assembly aggregation-induced emission from Eu (III) complexes with o-hydroxy-benzophenone ligands

In this work, europium (III) complexes were synthesized by using different o-hydroxy-benzophenone ligands, namely 2-hydroxy-4-octyloxybenzophenone (HL1), 4-allyloxy-2-hydroxybenzophenone (HL2) and 2-hydroxy-4-methoxybenzophenone (HL3). The structures and optical properties of the EuIII complexes E1, E2 and E3 were characterized by elemental analysis, infrared/fluorescence spectroscopy and mass spectrometry. The aggregation-induced emission properties from E1, E2 and E3 were systematically investigated in the acetone/water binary mixtures. At the same time, the corresponding agglomeration behavior and self-assembly structures in the EuIII complex system was confirmed through scanning electron microscopy. Excellent luminescent performance was illustrated in sample of E1 (the luminescence quantum yield of 21.36% in solid state) which should be attributed to the aggregation-induced emission from better self-assembly behavior with longer carbon chain in ligand. The results of ultraviolet aging resistance from E1 reveal that the self-assembly behavior could not only improve the aggregation-induced emission performance, but also enhance the photostability of EuIII complex compared with the traditional β-diketone EuIII complex. This discovery may provide a facile strategy to design and prepare more promising rare earth complexes with good luminescence properties and strong photostability for application in solid state.

Unveiling the Versatility of Coumarin-Integrated with Phenanthroline/Thiabendazole-Based EuIII Complexes for Smart LEDs, Vapoluminescence, and Bioimaging Applications.

Narrow band red-emitting phosphors based on organo-Eu(III) complexes prove their energetic features with surprising performance in smart red/white LEDs, sensing, and biological fields. In this report, a series of unique Eu(III) complexes have been synthesized with coumarin integrated with a class of phenanthroline(Phen)/thiabendazole(TBZ) based ancillary ligands and dibenzoyl methane (DBM)/2-theonyl trifluoroacetone (TTA) as an anionic ligand. The computational study reveals that the TBZ/Phen-based neutral ligands are superior energy harvesters to those other reported analogue neutral ligands. All the Eu-complexes demonstrated outstanding red emission due to electric dipole (ED) transition (5D0 → 7F2) in solid, solution, and thin film with high quantum yield (QY). Theoretical analysis (TD-DFT) and experimental findings describe that the energy transfer (ET) from the ligand's triplet level to the Eu(III) ion is completely occurring. The Eu(III) complexes can potentially be used to fabricate intense hybrid white and red LEDs. All of the fabricated red LEDs revealed high luminous efficiency of radiation (LER) values. The fabricated blue LED based hybrid white LEDs displayed remarkable performance with a low correlated color temperature (5634 K), high color rendering index 88%, and CIE values (x = 0.33; y = 0.342) for 3Eu. By interaction with acid-base vapors, Eu-complexes displayed effectively alterable on-off-on luminescence. Further, cellular imaging shows that Eu-complexes can be a potential biomarker for cancer cell lines.

Aggregation induced emission dynamic chiral europium(III) complexes with excellent circularly polarized luminescence and smart sensors

The synthesis of dynamic chiral lanthanide complex emitters has always been difficult. Herein, we report three pairs of dynamic chiral EuIII complex emitters (R/S-Eu-R-1, R = Et/Me; R/S-Eu-Et-2) with aggregation-induced emission. In the molecular state, these EuIII complexes have almost no obvious emission, while in the aggregate state, they greatly enhance the EuIII emission through restriction of intramolecular rotation and restriction of intramolecular vibration. The asymmetry factor and the circularly polarized luminescence brightness are as high as 0.64 (5D0 → 7F1) and 2429 M−1cm−1 of R-Eu-Et-1, achieving a rare double improvement. R-Eu-Et-1/2 exhibit excellent sensing properties for low concentrations of CuII ions, and their detection limits are as low as 2.55 and 4.44 nM, respectively. Dynamic EuIII complexes are constructed by using chiral ligands with rotor structures or vibration units, an approach that opens a door for the construction of dynamic chiral luminescent materials.

Circularly Polarized Luminescence of Cholesteric Liquid Crystal Polymer Networks with a Europium(III) Complex as an Emitter.

A coordination complex, Eu(C12C12dbm)3(phen), with strong emission and a high quantum yield (QY ∼ 51.9%) was synthesized. The EuIII complex, as a fluorescent emitter, was embedded in cholesteric liquid crystal polymer networks (CLCNs). A series of free-standing EuIII-CLCN films were obtained, generating a typical sharp emission band corresponding to the EuIII complex. Tunable handedness of circularly polarized luminescence (CPL) with high |glum| values (up to 0.63) was observed. A series of CPL-active CLCN-coated PET films were also prepared (|glum| values up to 0.63), which can be used for large-area preparations. Moreover, by stacking an emitter-embedded PMMA layer and a CLCN layer, a composite system was built, and a large |glum| value (∼1.42) was achieved. Fluorescence patterns were prepared, and distinct images of CLCN films were recognized under both daylight and UV light. This work not only demonstrated that coordination compounds could be incorporated with CLCN films to prepare CPL-active materials with high |glum| values but also provided a new perspective for emissive CLCN materials used for anticounterfeiting and encryption.

Luminescent properties of Eu (III) complexes with carboxylate ligands and their light conversion films

This study reports the synthesis and characterization of four europium (III) complexes with different carboxylate ligands, namely 2-pyridine-acrylic acid (H2-PA), 2-pyridine-carboxylic acid (HPic), benzoic acid (HBen), and cinnamic acid (HCin), as well as a binuclear europium-lanthanum complex with HCin as the only ligand. The obtained complexes and the light conversion films prepared from them were extensively characterized by various techniques, including elemental analysis, infrared spectroscopy (IR), X-ray single crystal diffraction, thermogravimetric analysis (TGA), and fluorescence spectroscopy. Efforts were made to systemically investigate the structural, coordination, photophysical, and thermal properties of the luminescent materials, and elucidate the correlations between the ligand triplet energy levels and the luminescent properties of the complexes. The results reveal that the introduction of LaIII cannot only enhance the luminescent intensity of the light conversion agent and corresponding film, but also have obvious effect on enhancing the ultraviolet aging resistance of the light conversion film. Moreover, the proper doping concentration (0.5%) of the EuIII complex La-Eu-Cin in film may better balance the properties and cost.

Europium(III) and gallic acid ligand coordination compounds: synthesis, characterization, photophysical processes, and optimization of luminescent properties

We obtained EuIII, GdIII, and GdIII doped with EuIII (1 at.% and 5 at.%) complexes with the ligand derived from 3,4,5-trihydroxybenzoic acid (H4Gal). Synthetic routes are proposed from acetate and chloride solutions of the trivalent lanthanides. We demonstrated the decomposition of the ligands 3,4,5-trihydroxybenzoate (H3Gal ≡ L–) and 3-hydroxo-4,5-dihydroxybenzoate (H2Gal2– ≡ L’2–) in an acetic solution medium. [LnLL’(H2O)2] complexes are obtained in a metal chloride solution medium. We studied the complexes in the solid state. We described the photophysical processes in the complexes, and they resulted in an energy diagram of the EuIII compounds in which the position of the states related to the LMCT transition are intermediate to the ligand's singlet (S1) and triplet (T1) states. We obtained the position of the triplet state (T1) from the luminescence of the GdIII complex. We performed the calculations of the EuIII complex's electronic structure using the Lumpac software to confirm the propositions obtained through experiments. The proposition is that the excitation in the LMCT state rapidly populates the T1 state, and the L–L’2–→EuIII back energy transfer may occur. One of the possible disadvantages of the complexes is the enormous non-radioactive deactivation due to the oscillators (OH) and (CH). The EuIII characteristic luminescence is of little intensity. We obtained GdIII compounds doped with EuIII at 1.0 and 5.0 at.% to optimize luminescent properties. We obtained the emission and excitation spectra of the complexes at 77 and 17 K, showing ligands' phosphorescence superimposed on the characteristic emissions 5D0 →7FJ (J = 0–4) of the EuIII ion.

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 TbIII and EuIII 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 TbIII /EuIII emission, EuIII /SmIII emission, EuIII /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.

Bright LanthanideIII Triboluminescence despite Low Photoluminescence, and Dual Triboluminescence and Mechano‐Responsive Photoluminescence

AbstractIn pursuit of a new family of mechanically responsive luminescent materials, it is aimed to differentiate triboluminescence (TL) from photoluminescence (PL). A β‐diketonate ligand with tert‐butyl groups (2,2,6,6‐tetramethylheptane‐3,5‐dionate: tmh) is selected to quench EuIII‐centered PL via ligand‐to‐metal charge transfer, whereas tmh provides efficient photosensitization of TbIII ions. Bright TL is observed from the EuIII and TbIII homodinuclear complexes despite the fact that their PL quantum yields differed by a factor of >50. Nanomechanical tests reveal the ductility of the crystals, suggesting they are ideal for accumulating deformation energy before breakage. Furthermore, a TL/PL color difference is observed for a TbIII/EuIII heterodinuclear complex, and grinding results in mechanochromic luminescence (MCL); this is the first example of a dual TL‐ and MCL‐active lanthanideIII coordination compound. The photophysical properties before, during, and after grinding are investigated and correlated with powder and single‐crystal crystallographic data.

Interesting chemical and physical features of the products of the reactions between trivalent lanthanoids and a tetradentate Schiff base derived from cyclohexane-1,2-diamine.

The initial use of a tetradentate Schiff base (LH2) derived from the 2 : 1 condensation between 2-hydroxyacetophenone and cyclohexane-1,2-diamine in 4f-metal chemistry is described. The 1 : 2 reaction of Ln(NO3)3·xH2O (Ln = lanthanoid or yttrium) and LH2 in MeOH/CH2Cl2 has provided access to isostructural complexes [Ln(NO3)3(L'H2)(MeOH)] in moderate to good yields. Surprisingly, the products contain the corresponding Schiff base ligand L'H2 possessing six aliphatic -CH2- groups instead of the -CH-(CH2)4-CH- unit of the cyclohexane ring, i.e. an unusual ring-opening of the latter has occurred. A mechanism for this LnIII-assisted/promoted LH2 → L'H2 transformation has been proposed assuming transient LnII species and a second LH2 molecule as the H2 source for the reduction of the cyclohexane moiety. DFT calculations provide strong evidence for the great thermodynamic stability of the products in comparison with analogous complexes containing the original intact ligand. The structures of the PrIII, SmIII, GdIII, TbIII, and HoIII complexes have been determined by single-crystal X-ray crystallography. The 9-coordinate LnIII centre in the molecules is bound to six oxygen atoms from the three bidentate chelating nitrato groups, two oxygen atoms that belong to the bidentate chelating organic ligand, and one oxygen atom from the coordinated MeOH group. In the overall neutral bis(zwitterionic) L'H2 ligand, the acidic H atoms are clearly located on the imino nitrogen atoms and this results in the formation of an unusual 16-membered chelating ring. The coordination polyhedra defined by the nine donor atoms around the 4f-metal-ion centres can be best described as distorted, spherical capped square antiprisms. The EuIII, TbIII, and DyIII complexes exhibit LnIII-based luminescence in the visible region, with the coordinated L'H2 molecule acting as the antenna. Ac magnetometry experiments show that the DyIII member of the family behaves as an SIM at zero field and under external dc fields of 0.1 and 0.2 T without the enhancement of the peaks' maxima, suggesting that QTM is not the relaxation path. The GdIII complex behaves, rather unexpectedly, as a SIM with two different magnetic relaxation paths occurring at very close temperatures; this behaviour is tentatively attributed to a very small axial zero-field splitting (D ∼ 0.1 cm-1), which cannot be detected by magnetization or susceptibility experiments. The prospects of the present, first results in the lanthanoid(III)-LH2 chemistry are discussed.

Stress to distress: Triboluminescence and pressure luminescence of lanthanide diketonates

Triboluminescence and high-pressure luminescence are stress-induced phenomena with potential in stress sensing. However, the understanding of the former process and the correlation between the two processes are lacking. In this contribution, we study the influence of the crystal packing and high pressure compression on triboluminescence by using a new family of halogen-containing EuIII diketonate complexes. The size and position of halogens determine the crystal symmetry and packing due to the change in intermolecular interactions. The role of pressure is examined by a diamond anvil cell (DAC), in which the material compression induces phase transition, peak shifting and broadening in the luminescence spectrum of the complex. The experimental results show the triboluminescence of our EuIII complexes occurs at low pressure and is a fracture-induced (mechano stress) process. This work contributes to the understanding of triboluminescence and for the first time explores the possibility of applying lanthanide complexes as stress sensors.

Front Cover: Tribo‐Excited Chemical Reaction Using an EuIII Complex with a Stacked Anthracene Framework (Chem. Eur. J. 16/2022)

Tribo-excitation in a crystalline compound is an interesting phenomenon in which a mechanical stimulus drives the compound to its excited state. We demonstrate a tribo-excited chemical reaction using trivalent europium (EuIII) and gadolinium (GdIII) complexes with stacked anthracene ligands for the development of tribo-excitation chemistry. The tribo-oxidation and -dimerization reactions in the stacked anthracene ligands occur in the EuIII and GdIII complexes, respectively. More information can be found in the Research Article by Y. Kitagawa, Y. Hasegawa et al. (DOI: 10.1002/chem.202104401).

Tribo-Excited Chemical Reaction Using an EuIII Complex with a Stacked Anthracene Framework.

A spin-selective tribo-chemical reaction using a dinuclear lanthanide complex is demonstrated for the first time. The dinuclear complex is composed of two EuIII ions, hexafluoroacetylacetonato ligands, and anthracene-based phosphine oxide bridges. Single-crystal analysis revealed a face-to-face-type anthracene dimer structure in the dinuclear EuIII complex. Mechanical stimulus on the dinuclear EuIII complex induced selective formation of oxidized anthracene. The tribo-chemical reaction is based on a characteristic energy-transfer pathway for the selective formation of an excited triplet state.

Carbazole-functionalized dipicolinato LnIII complexes show two-photon excitation and viscosity-sensitive metal-centered emission

EuIII and YbIII complexes with the carbazole-dipicolinato ligand dpaCbz2−, namely K3[Eu(dpaCbz)3] and K3[Yb(dpaCbz)3], were isolated. The EuIII complex displayed metal-centered emission upon one-photon excitation with a sensitized emission efficiency ΦLLn of 1.8±0.3%, corresponding to an intrinsic emission efficiency ΦLnLn of 46% and a sensitization efficiency of ηsens 3.9%, with an emission lifetime of the emissive state τ of 1.087±0.005 ms. The YbIII complex displayed ΦLLn of 0.010±0.001%, and a τ of 2.32±0.06 μs The EuIII-centered emission was sensitized as well upon two-photon excitation and a two-photon absorption cross-section σ2PA of 63 GM at 750 nm was determined for the complex. The one- or two-photon sensitized emission intensity of the EuIII 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 YbIII 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.

Luminescent β-diketonate coordinated europium(III) sensor for rapid and sensitive detection of Bacillus Anthracis biomarker

The hazardous B. Anthracis are harmful pathogens causes anthrax could be potentially used as biothreat agents or bioweapons and have severe consequences to public health. These bacterial endospores contain dipicolinic acid (DPA) as calcium dipicolinate (CaDPA) as a primary cell wall constituent (1 M, 1015 molecules/spore). The anthrax attacks in USA by shipping spores in postal mails have triggered the hunt for a simple, rapid, sensitive, and selective detection of Bacillus Anthracis spores. Therefore, a fast and sensitive sensing platform for anthrax spores is critical to contain such biothreats and outbreaks of infection. Herein, we intend to develop a sensitive and selective optical detection strategy for anthrax spore biomarker based on DPA-sensitized lanthanide luminescence. The anionic hard DPA2− strongly binds with hard Ln3+ forming thermodynamically stable complex and sensitizing unique LnIII time-resolved luminescence originated from f-f transitions with a high S/N ratio. Here, we employed and repurposed a simple luminescent EuIII complex: [Eu(TTA)3(H2O)2] (1) (TTA: 3-thenoyltrifluoroacetonate) for the detection of the DPA2− biomarker. Complex 1, when titrated with DPA, displayed instant Switch-ON optical response by 10-fold enhancement in the time-resolved luminescence (TRL) intensity of the 5D0→7F2 transition, which is sensitive to the changes in the symmetry of the EuIII primary coordination sphere. Upon binding with DPA, we also observed shifting of the excitation wavelength (λex) from ∼345 nm to 275 nm in the presence of ∼3.0 mol equivalent of dipicolinic acid. After stoichiometric reaction with DPA, the resulting complex has a long-lived, highly stable excited state of the EuIII ion with a lifetime of 2100 μs in solution compared to only 260 μs for the original probe. We proposed possible displacement of the TTA− and H2O from the primary coordination sphere of the EuIII ion by potent chelating antenna ligand DPA2−. This reaction results in the formation of nine-coordinated [Eu(DPA)3]3- complex in solution confirmed by ESI-MS analysis. The luminescence response of the complex 1 is found highly selective for DPA in the presence of various interfering carboxylic acids and quantitatively able to detect up to 1.20 μM.

Chiral Dinuclear EuIII, TbIII, and YIII Complexes Supported by P‐Stereogenic Linear Tetraphosphine Tetraoxide

AbstractA P‐stereogenic linear tetraphosphine tetraoxide, (R,R)‐ or (S,S)‐dpmppm(=O)4, was synthesized to prepare C2 dinuclear M(hfa)3 complexes (M=Eu, Tb, Y) as the first example of lanthanide(III) complexes with P‐chiral multidentate phosphine oxides. The mononuclear M(hfa)3 complexes (M=Eu, Y) with a P‐chiral diphosphine dioxide, tpdpb(=O)2, were also prepared, and comparison of their photophysical properties for the EuIII complexes revealed that significant chiral induction from the P‐chiral centers arises on the achiral M(hfa)3 units through intramolecular π‐π stacking constraint in the dinuclear system.

Excited State Properties of Lanthanide(III) Complexes with a Nonadentate Bispidine Ligand.

EuIII, TbIII, GdIII and YbIII complexes of the nonadentate bispidine derivative L2 (bispidine=3,7‐diazabicyclo[3.3.1]nonane) were successfully synthesized and their emission properties studied. The X‐ray crystallography reveals full encapsulation by the nonadentate ligand L2 that enforces to all LnIII cations a common highly symmetrical capped square antiprismatic (CSAPR) coordination geometry (pseudo C4v symmetry). The well‐resolved identical emission spectra in solid state and in solution confirm equal structures in both media. As therefore expected, this results in long‐lived excited states and high emission quantum yields ([EuIIIL2]+, H2O, 298 K, τ=1.51 ms, ϕ=0.35; [TbIIIL2]+, H2O, 298 K, τ=1.95 ms, ϕ=0.68). Together with the very high kinetic and thermodynamic stabilities, these complexes are a possible basis for interesting biological probes.

Efficient Sensitized Luminescence of Binuclear Ln(III) Complexes Based on a Chelating Bis-Carbacylamidophosphate.

Binuclear rare earth complexes Ln2L3phen2 (LnIII = NdIII, SmIII, EuIII, TbIII, DyIII, YbIII and YIII) with bis-CAPh type ligand - tetramethyl N,N'-(2,2,3,3,4,4-hexafluoro-1,5-dioxopentane-1,5-diyl)bis(phosphoramidate) (H2L) and 1,10-phenanthroline (phen) were synthesized and characterized by elemental analysis, IR, NMR, absorption and luminescence spectroscopy. Luminescence measurements were performed for all the complexes in solid state and for the EuIII, TbIII and YIII complexes - in solution in DMSO as well. The effective energy transfer from organic ligands to LnIII ions strongly sensitizes the LnIII ions emission and under excitation by UV light, the complexes exhibited bright characteristic emission of lanthanide metal centers. It was found that the energy level of the ligands lowest triplet state in the complexes matches better to resonance level of EuIII rather than TbIII ion. Depending on temperature the emission decay times of solid europium and terbium complexes were in the range of 1.5-2.0ms. In solid state at room temperature the EuIII complex possess intense luminescence with very high intrinsic quantum yield 91% and decay time equal 1.88ms.

A Versatile Tripodal Ligand for Sensitizing Lanthanide (LnIII) Ions and Color Tuning

Lanthanide (LnIII) ions were successfully chelated and sensitized with a tripodal ligand. The absolute LnIII-centered emission efficiencies were ~3% for both the europium(III) (EuIII) and terbium (TbIII) complexes and up to 54% for the cerium(III) (CeIII) complex. The differences in emission quantum yields for the early lanthanides (CeIII) and the mid lanthanides (EuIII and TbIII) were attributed to their d–f and f–f nature, respectively. Despite the low quantum yield of the EuIII complex, the combination of the residual ligand fluorescence and the red EuIII emission resulted in a bluish-white material with the Commission Internationale de l’Eclairage (CIE) coordinates (0.258, 0.242). Thus, metal complexes of the ligand could be used in the generation of single-component white-light-emitting materials.

Two homochiral EuIII and SmIII enantiomeric pairs showing circularly polarized luminescence, photoluminescence and triboluminescence.

Two homochiral EuIII and SmIII tris(β-diketonate) enantiomeric pairs, based on fluorinated β-diketone (Hbtfa) and enantiopure asymmetric N,N'-donor ligands (LR and LS), Λ-Eu(btfa)3LR (R-1-Eu)/Δ-Eu(btfa)3LS (S-1-Eu) and Λ-Sm(btfa)3LR (R-2-Sm)/Δ-Sm(btfa)3LS (S-2-Sm) (btfa- = 4,4,4-trifluoro-1-phenyl-1,3-butanedionate and LR/LS = (-)/(+)-4,5-pineno-2,2'-bipyridine) were synthesized. The electronic circular dichroism (ECD) spectra confirmed their enantiomeric nature. R-1-Eu/S-1-Eu and R-2-Sm/S-2-Sm exhibit intense characteristic emissions of EuIII (red) and SmIII (orange-red) ions both in the solid state and in DCM with long lifetimes and high luminescence quantum yields. For example, the overall quantum yields reach up to 61% and 53% along with very high sensitization efficiency values of 82 and 79 for R-1-Eu in the solid state and in DCM, respectively. Notably, the corresponding values are determined to be 6.5% (solid state) and 3.1% (DCM) for R-2-Sm, which are among the highest quantum yields for rare SmIII tris(β-diketonate) luminescent complexes reported to date. Furthermore, R-1-Eu and R-2-Sm show a strong triboluminescence (TL) phenomenon visible with the naked eye in daylight. Moreover, R-1-Eu/S-1-Eu and R-2-Sm/S-2-Sm show circularly polarized luminescence (CPL) properties. Particularly, the luminescence dissymmetry factors (glum) for R-2-Sm/S-2-Sm are larger than those for R-1-Eu/S-1-Eu despite the fact that SmIII complexes usually show poorer emission than EuIII homologues, which is very rare in the reported EuIII and SmIII CPL-active complexes.

H-Bonding and intramolecular catalysis of proton exchange affect the CEST properties of EuIII complexes with HP-DO3A-like ligands.

Eu(HP-DO3A) is present in solution as a mixture of two diastereoisomers whose alcoholic groups are the source of the mobile protons for the CEST effect. The exchange is base catalyzed. Two novel EuIII complexes of HP-DO3A-like ligands containing an amino or a carboxylate functionality in the proximity of the -OH groups showed the occurrence of intramolecular catalysis of the prototropic exchange. New insights into the role of the intramolecular proton exchange on the CEST properties have been gained.