Improving the luminescent properties of a dysprosium complex in the presence of Zn II : an emissive Zn 2 Dy compound

The mononuclear complex [Dy(H3L)(H2O)(NO3)](NO3)2 (Dy) and the heterotrinuclear compound [Zn2Dy(L)(NO3)3(OH)]·3H2O (Zn2Dy·3H2O) can be obtained with the same H3L compartmental ligand. The single X-ray crystal structure of both complexes shows a DyO9 core but with different geometries: a muffin-like disposition in Dy and a spherical capped square antiprism geometry in Zn2Dy. The luminiscent characterisation of the metal compounds in methanol at 298 K shows a notable increase in the fluorescent emission of the heterotrinuclear complex respect to the mononuclear one, indicating that the presence of zinc improves significantly the fluorescent character of Zn2Dy·3H2O respect to Dy. Accordingly, Zn2Dy·3H2O could be a potential fluorescent probe for imaging applications.


Introduction
One relevant aspect of the lanthanide (Ln) metals is their unusual spectroscopic properties.Ln 3+ cations display absorption and emission bands that correspond to Laporte-forbidden f-f transitions.
Because the 4f orbitals are relatively insensitive to the ligand field, these bands are line-like and characteristic of each metal [1].Besides, lanthanide ions have other unique features, such as high luminescence quantum yield, long-lived emission, and large Stokes shifts [2].These properties make them potential candidates for fluorescent probes, light-emitting diodes, and conversion or amplification of light [3].Nevertheless, lanthanides have low molar absorptivities, making the direct excitation of the metals very inefficient.This problem is overcome by using a strongly absorbing chromophore to sensitize Ln(III) emission, in a process known as the antenna effect.Thus, luminescent emission in lanthanide complexes is usually enhanced by the use of appropriate light-harvesting units, such as organic ligands or metal complexes typically based on d-metal ions (e.g., Zn(II) and Ir(III)) [4][5].The use of Zn(II) is particularly interesting because Zn 2+ can be encapsulated in small ligands and has a preference for low coordination numbers (4-6) while Ln ions are more bulky and tend to achieve higher coordination numbers (usually higher than 8).These differences in size and coordination preferences allow predesigning ligands that can allocate both kinds of metal ions in predetermined pockets.
With these considerations in mind, the reported ligand H3L (Scheme 1) [6] was revised under the light of its compartmental character.The reactivity of H3L towards Dy(III) and towards Dy(III) and Zn(II) was studied.The results achieved, which include the study and comparison of the luminescent properties of the isolated complexes, are described herein.The introduction should briefly place the study in a broad context and define the purpose of the work and its significance.

Materials and general methods
All chemical reagents and solvents were purchased from commercial sources and used as received without further purification.Elemental analyses of C, H and N were performed on a FISONS EA 1108 analyser.Infrared spectra were recorded in the ATR mode on a Varian 670 FT/IR spectrophotometer in the range 4000-500 cm -1 .

Spectrophotometric and spectrofluorimetric measurements
Absorption spectra were recorded on a JASCO V-630 spectrophotometer and fluorescence emission spectra on a JASCO FP-8300 spectrofluorimeter.The linearity of the fluorescence emission vs concentration was checked in the concentration range used M).
The spectrophotometric characterisation was made by preparing stock solutions of Dy, and Zn2Dy•3H2O in methanol (ca. 10 -3 M).The studied solutions were prepared by appropriate dilution of the stock solutions up to 10 -5 M. Fluorescence quantum yields were determined using a 0.1 M solution of quinine sulphate in 0.5 M H2SO4 as standard (ɸ = 0.546) and all values were corrected taking into account the solvent refraction index [7].All the measurements were taken at 298 K.
Drying of the crystals leads to loss of the acetonitrile solvate, generating Dy.Single crystals of Zn2Dy•3H2O were isolated by diffusion of diethyl ether into a methanol/acetonitrile solution of the powder sample, with a few drops of cyclohexane.
The formulations of both compounds were confirmed by microanalysis, IR spectroscopy and by single X-ray diffraction studies.Their luminescence properties were also analysed.
The IR spectra of Dy and Zn2Dy•3H2O show a sharp band at ca. 1645 cm -1 , assigned to ν(C=O)aldehyde.When the aldehyde group of this ligand remains uncoordinated, it gives rise to an IR band at 1673 cm -1 [6], and, accordingly, the low wavenumbers observed for this vibration in these complexes suggests the involvement of the Oaldehyde atom in the coordination to the metal ions.In addition, sharp bands at ca. 1635 and 1300 cm -1 are also present, indicating the existence in both compounds of imine [6,8] moieties and nitrate groups [8], respectively.O1-Dy-O3 147.5(2)O10-Dy-O11 50.77 (13) In this complex, H3L acts as hexadentate, using all its oxygen atoms (three Ophenol In this structure, all the distances and angles about the metal ion are within their usual range and do not merit further consideration [10][11].
Zn2Dy can be understood as a [Zn2L(NO3)2(OH)] 2-fragment that acts as a metalloligand towards a Dy 3+ ion.Thus, in this fragment, the Schiff base, which is fully deprotonated, allocates a zinc(II) ion in each one of its internal N2O compartments (O1N1N2).In addition, both zinc centres are bridged by an endogenous phenolate oxygen atom (O3) of the central ligand arm, the carbonyl oxygen atoms remaining uncoordinated.The coordination spheres of the zinc centres are completed by two nitrate anions, coordinated each one to a Zn II ion as a monodentate terminal donor, and by a hydroxide ligand, acting as a bridge between the zinc atoms.This gives rise to a coordination number of 6 for the zinc atoms, with distorted octahedral geometry.

Photophysical properties
All spectrophotometric measurements for Dy and Zn2Dy•3H2O were taken in methanol solution at 298 K.The photophysical characterisation and the main photophysical data are reported in Table 3 and the absorption and emission spectra both compounds are depicted in Figure 5.
The electronic absorption spectra show three absorption bands at ca. 245, 260, 400 nm, which are attributed to π-π* electronic transitions of the phenol rings and the imine bonds present (Table 3) [12,13].In both cases, the perfect match between the absorption and the excitation spectra rules out the presence of any emissive impurity, in agreement with the high purity of the complexes.The fluorescence spectra of both compounds display a broad band with a maximum at ca. 480 nm, which seems to be a ligand-based emission, given the broad nature of these signals.In addition, the low fluorescence observed for Dy can be ascribed to the absence of coordination between the lanthanide ions and the imine groups of the ligand skeleton.This effect promotes that the Photo-induced Electron Transfer (PET) process takes place from the Nimine lone pair to the excited state of the fluorophore (phenol rings) [14].The coordination of metal ions to these nitrogen atoms prevent PET quenching from the lone pair of electrons of each nitrogen atom to the fluorophore moieties.This effect is observed in Zn2Dy•3H2O, where the Nimine lone pairs are bound to the Zn 2+ , thus favouring the fluorescence emission [15].Thus, a remarkable increase of fluorescence intensity was observed for Zn2Dy•3H2O (19-fold, ɸFlu = 0.019) compared with Dy (1-fold, ɸFlu < 0.001), at peak wavelength λmax = 476 nm and λmax = 483 nm, respectively.Accordingly, the presence of Zn in Zn2Dy•3H2O converts it in a fluorescent compound.

Conclusions
New Dy and Zn2Dy complexes have been obtained from related H3L.The characterisation by single X-ray diffraction shows that the complexes present DyO9 cores, with structure between muffin-like and spherical capped square antiprism.The study and comparison of the luminescent properties of both complexes show that the presence of zinc produces an outstanding increase of the fluorescence in Zn2Dy•3H2O respect to the Dy, in such a way that Zn2Dy•3H2O can be consider as a molecular fluorescent material.

3. 2
Figure1shows an ellipsoid diagram for Dy and selected bond distances and angles are recorded in Table1.

Figure 2 .
Figure 2. Coordination environments for the Dy ion in Dy, showing the muffin structure.

Table 3 .
Photophysical data for the metal complexes in methanol at 298 K