Coumarin Derivative Directly Coordinated to Lanthanides Acts as an

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Coumarin Derivative Directly Coordinated to Lanthanides Acts as an Excellent Antenna for UV−Vis and Near-IR Emission Ó scar Guzmán-Méndez,† Federico González,‡ Sylvain Bernès,§ Marcos Flores-Á lamo,† Javier Ordóñez-Hernández,† Héctor García-Ortega,† Joselin Guerrero,† Wenjie Qian,∥ Nuria Aliaga-Alcalde,⊥,∥ and Laura Gasque*,† †

Facultad de Química, National Autonomous University of Mexico, Avenida Universidad 3000, CDMX 04510, Mexico Ingeniería de Procesos e Hidráulica, UAM Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, CDMX 09340, Mexico § Instituto de Física, Benemérita Universidad Autónoma de Puebla, San Claudio y 18 Sur s/n Puebla, Puebla 72570, Mexico ⊥ Institució Catalana de Recerca i Estudis Avançats. Passeig Lluis Companys 23, 08010 Barcelona, Spain ∥ Institute of Material Science of Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, Spain ‡

S Supporting Information *

recrystallization in EtOH or MeOH yielded single crystals suitable for X-ray diffraction. The four complexes share many structural features, although they are not isotypic, because of the alcohol coordinated to the metal, which also serves as a lattice solvent. For Ln = Sm and Eu, [LnCum3(MeOH)(H2O)]·MeOH was obtained, while with Ln = Tb and Dy, we obtained [LnCum3(EtOH)(H2O)]·EtOH. This small difference is enough to change the Laue symmetry from −1 to 2/m. In all cases, the Ln is coordinated by eight O atoms, in a geometry best described with the polyhedral symbol TPRS-8 in the IUPAC nomenclature (trigonal prism, square-face-bicapped; see Figure 1). Six of the coordination sites are occupied by three

ABSTRACT: A chelating coumarin-derived ligand sensitizes all emitting lanthanide ions in the solid state and gives high absolute quantum yields for ethanol solutions of complexes of Sm, Eu, Tb, and Dy, above 20% for the last two. Crystal structures of these four complexes are [Ln(Cum)3(H2O)(X)]·X where X = MeOH or EtOH.

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oumarins are a family of 1,2-benzopyrones widely present in plants that have motivated interest because of their applications in the life sciences based on their optical properties.1−3 These optical properties have been acknowledged for decades; although the nonsubstituted parent coumarin exhibits almost zero fluorescence, properly substituted derivatives yield intense emission.4,5 In recent years, studies have been published dealing with their application as laser dyes or fluorescent probes,6−8 and in the field of materials science, coumarins have recently been used as components of organic light-emitting devices and dye-sensitized solar cells.9,10 In the realm of coordination chemistry, however, the presence of coumarin ligands is rather scarce, and the existing literature is mostly focused on the biological activity of the complexes.11,12 Only recently have several chelating ligands containing coumarin as pendant moieties been described as luminescence sensitizers for lanthanide (Ln) ions,13−16 but to our knowledge, there are no reports, including crystal structures and fluorescence studies of coordination complexes, in which the coumarin moiety is directly bonded to Ln ions. We describe here the Ln complexes of a rather simple coumarin-derived ligand (Cum) in which an acetyl and a hydroxy group have been introduced respectively to the 3 and 4 positions of the basic coumarin skeleton. This creates a single charged ligand that chelates through two O donors, forming a sixmembered ring with the metal ion, in a fashion similar to βdiketonates, which for a long time have been known to be good sensitizing ligands for Ln ions.17,18 The complexes presented here have the general formula [Ln(Cum)3(H2O)(X)]·X, where X can be methanol (MeOH) or ethanol (EtOH), and were obtained similarly (Supporting Information). For the Sm, Eu, Tb, and Dy complexes, © XXXX American Chemical Society

Figure 1. Molecular structures for the EuIII (left) and DyIII (right) complexes. The coordination polyhedra are represented with green sticks, the Ln−O bonds have been omitted for clarity, and the lattice solvent is omitted.

Cum ligands, and the remaining two are occupied by water and the alcohol used as the solvent for crystallization. All of Ln−O bond lengths, ranging from 2.292(3) to 2.457(2) Å, are unexceptional if compared by means of MOGUL19 to Ln−O bonds found in other eight-coordinate complexes. More interesting is the geometry of the coordinating groups in the coumarin derivative: bond lengths are consistent with a negative charge fully delocalized over the O−C−C−C−O fragment, allowing the formation of a strongly stabilizing six-membered Received: November 8, 2017

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DOI: 10.1021/acs.inorgchem.7b02861 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

to the metal ion, one from a water molecule and another from EtOH or MeOH, which are known to quench Ln emissions. As can be appreciated in Figure 3, for the Pr, Nd, Ho, Er, Tm, and Yb

metallacycle. The coumarin thus gathers two attractive features for its application as a sensitizer: the ligand is tightly bonded to the Ln metal, favoring energy transfer by exchange interaction, and, moreover, the π-electron system of the coumarin moiety is suitable for providing the antenna effect in the sensitizer. The only significant difference between the molecular structures is the orientation of one coumarin, which depends on the alcohol coordinated to the metal (Figure 1). The crystal structure is also solvent-dependent because the network of hydrogen bonds is modified. A common feature is, however, that all potential donor and acceptor groups participate in the formation of the supramolecular structure; the most efficient bonds are formed between a coordinated water molecule, the lattice alcohol, and the uncoordinated carbonyl. Although there is not an infallible way of predicting whether an organic compound can act as a sensitizer once coordinated to a Ln ion, most authors agree on the importance of a high absorptivity and emission of the sensitizer in the near-UV, a small singlet−triplet energy gap, and finally the position of the latter at least 2000 cm−1 above the emitting level of the metal ion.20−22 Experimental absorption and emission spectra of the Cum ligand, backed up by density functional theory calculations, point to the fulfillment of these conditions by this ligand. Experimental spectral data for the ligand are shown in Figure 2 and Table S2; it

Figure 2. Optical spectra of the ligand in EtOH and THF: (left) absorption; (right) emission.

is important to notice that while in EtOH no absorption at all is observed beyond 355 nm, the absorption spectrum in a tetrahydrofuran (THF) solution has a weak but noticeable absorption at 367 nm. When the ligand is excited at 325 nm, either in EtOH or in THF, the emission observed is very similar; however, when excited at 365 nm, the THF solution gives an additional emission signal at 410 nm. Moreover, the room temperature emission spectrum obtained on the Gd tris complex of this ligand displayed a band at 430 nm with a lifetime τ = 15 μs, which supports the T1 → S0 assignment for this band. Theoretical calculations were performed on the ligand and could reproduce its absorption maxima in solution within a few nanometers of the experimental data, assigned to S0 → S1 (318 nm) and S0 → S2 (303 nm) (Figure S6). The calculation yielded four triplet states, the highest of which (T4) has an energy value of 326 nm, very close in energy to S1, allowing it to act as an intersystem crossing path toward energy transfer. Internal conversion can then take place from T4 to T1. From this state, at 407 nm, energy could be emitted from the free ligand or transferred to a suitable acceptor Ln ion. There are only a few examples of ligands able to sensitize all emitting Ln ions like Cum does.23−26 This is rather notable because crystal structures show two OH groups directly bonded

Figure 3. Emission spectra in the solid state for the [Ln(Cum)3(H2O)X] complexes. λexc = 330 nm.

complexes, commonly reported emissions become clearly visible, but the ligand-centered emission is a prominent feature in the spectra. However, for the Sm, Eu, Tb, and Dy complexes, the virtual disappearance of this band is indicative of an efficient energy transfer. Figure S8 shows assigned transitions and wavelengths. For the complexes of these four Ln ions, substantial emission in solution was obtained in EtOH and THF (Figure 4). Table 1 summarizes the information on the observed transitions for these four complexes, as well as the absolute quantum yields (QYs) measured in EtOH. Compared to Tb and Eu, reports on the Sm complexes and their photophysical properties are sparse probably because of the weaker luminescence intensity commonly found for this ion. In the solid state, [Sm(Cum)3(H2O)(MeOH)] exhibited all commonly observed emissions for Sm3+, both in the visible region and in the near-IR, while in solution, only the former were observed. The luminescence QY obtained for this complex in EtOH was 0.5%, which is significant for a protic solvent; to our knowledge, the highest QYs reported for Sm complex solutions are 2.7% in pyridine, 4.9% in benzene, and 1.4% in MeOH.27,28,25 B

DOI: 10.1021/acs.inorgchem.7b02861 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

be appreciated on the inset photograph in Figure 4, this complex gives an intense white light. The reports on the photostability of Ln complexes are scarce, and the methodologies employed vary significantly.31,32 We made an estimation of the photostability of 0.2 mM EtOH solutions of the Sm, Eu, Tb, and Dy complexes. Emission intensity versus irradiation time plots show that all of the complexes maintain more than 97% of the initial intensity after 120 min and 93% after 16 h (Figure S10). In conclusion, we have found a coumarin-derived ligand that sensitizes all emitting Ln ions in the solid state. The good solubility found for these complexes in a wide variety of solvents, together with the foreseeable biocompatibility of the ligand and their good emission properties and photostability, leads to the presumption that they could be valuable in a wide variety of applications. At present, we are extending our research in two directions: studying an extended number of solvents and their deuterated analogues toward a detailed calculation of the inner-sphere solvation numbers (q)33 and obtaining other coumarin-derived ligands to broaden the findings and understanding of this work.

Figure 4. Emission spectra for EtOH (solid line) and THF (dotted line) solutions of tris coumarin chelates of Sm, Eu, Tb, and Dy. λexc = 340 nm. Insets show photographs of the solutions.

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Table 1. Photophysical Data for Ln Tris Chelates of Cum Ln

Sm

G5/2 → 6H5/2 G5/2 → 6H7/2 4 G5/2 → 6H9/2 4 G5/2 → 6H11/2 5 D0 → 7F0 5 D0 → 7F1 5 D0 → 7F2 5 D0 → 7F3 5 D0 → 7F4 5 D4 → 7F6 5 D4 → 7F5 5 D4 → 7F4 5 D4 → 7F3 5 D4 → 7F2 5 D4 → 7F1 4 F9/2 → 6H15/2 4 F9/2 → 6H13/2 4 F9/2 → 6H11/2 4 4

Eu

Tb

Dy

% of total emission

transition, λ (nm)

3+

562 599 645 708 578 592 611 660 702 490 544 582 620 657 681 480 574 663

8.2 24.7 60.2 6.9 1.0 5.4 77.9 3.5 12.1 15.0 63.9 10.4 7.4 2.8 0.5 24.6 67.8 7.6

lifetimes (μs) of solid, EtOH, THF 15, 21, 19

441, 458, 566

ASSOCIATED CONTENT

S Supporting Information *

% QY in EtOH

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02861. Synthesis and characterization of the ligand and complexes, crystal data, optical spectra for the ligand and complexes, fluorescence measurements, experimental details, and a photostability plot (PDF)

0.5

11.6

Accession Codes

398, 874, 748

CCDC 1582364−1582367 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

29.2

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26, 22, 18

AUTHOR INFORMATION

Corresponding Author

21.9

*E-mail: [email protected]. ORCID

Sylvain Bernès: 0000-0001-9209-7580 Laura Gasque: 0000-0001-9013-4771

For the Eu, Tb, and Dy complexes in solution, all of the expected peaks could be identified. The relative intensity of the hypersensitive transition in all of them is consistent with the asymmetric coordination environment around the Ln ion. In the case of Eu, the emission lifetime in EtOH is lower than that in THF, in contrast to its emission intensity in Figure 3. Such a fact exposes the solvent dependency of the photophysical properties of the systems. The effect of small variations on hydration and/or the coordinative role of the solvent, together with the displacement of one or two molecules of H2O and EtOH/ MeOH in the initial compounds, may dictate the final photophysical parameters.29 The QY of 21.9% found for [Dy(Cum)3(H2O)(EtOH)] in an ethanolic solution is, to our knowledge, the highest reported to date.22 Dy compounds are valued for their white emission, which results from the combination of mainly the first two of its three bands.30 As can

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.G. thanks DGAPA-UNAM for Grant PAPIIT IN222615. O.G.M. thanks Conacyt for a scholarship. N.A.-A. acknowledges economic support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496), and Project MAT2016-77852-C2-1-R. S.B. thanks Conacyt for Project 268178.

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DOI: 10.1021/acs.inorgchem.7b02861 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02861 Inorg. Chem. XXXX, XXX, XXX−XXX