Switchable photoluminescence of europium(III) complexes with chromonylhydrazones

Abstract

Europium(III) complexes bearing 4-hydroxy- or 4-methyl-N′-((6-methyl-4-oxo-4H-chromen-3-yl)methylene)benzohydrazide (HL¹ or HL²) showed characteristic Eu^{III 5}D₀ → ⁷F_J (J = 0–4) luminescence both in acetonitrile and in solid states with relatively high Φ_tot values. The luminescence was quenched not only by adding triethylamine in acetonitrile, but also by heating the solid sample, and recovered by adding perchloric acid in solution or by diffusion of HCl vapor to the resulting solid sample.

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Certain lanthanoid(III) complexes exhibit intense luminescence with high color purity when compared to luminescent organic compounds or transition-metal complexes and can be applied to bioprobes,^1–3 optical devices, and electroluminescent materials.^4,5 However, photoluminescent materials for switching devices using photo-,^6–9 thermo-^10–12 and acidochromic behaviors^13,14 have also been studied extensively over the past decade. In particular, lanthanoid(III) complexes showing similar responsiveness to external stimuli usually require a complicated molecular structure,^15–18 and it is often difficult to design and prepare these ligands to obtain stimuli-responsive chromic properties.

Hydrazone compounds are widely used as convenient ligands in transition-metal complexes, because they are easily prepared in high yields through condensation reactions of the corresponding hydrazine and carbonyl compounds under relatively mild reaction conditions. Within these hydrazones, a large number of transition-metal complexes having characteristic chromic behaviors have been reported to date.^19–23 For example, an acetonitrile solution of a platinum(II) complex with 2-(diphenylphosphino)benzaldehyde-2-pyridyl-hydrazone showed absorption and emission color changes by the addition of base (i.e., deprotonation) or acid (i.e., reprotonation).²⁴ More interestingly, this compound showed reversible changes following heating and exposure to HCl vapor in the solid state. Considering the lanthanoid(III) complexes of hydrazones, some europium(III) complexes bearing 2,6-diformylpyridine-bis(benzoylhydrazone)^25,26 or 7-methoxychromone-3-carbaldehyde-(4′-hydroxy) benzoylhydrazone^27,28 were synthesized and their luminescent properties reported. Apart from hydrazone complexes, highly intriguing europium(III) complexes, which exhibit switchable photoluminescence property in response to exposure of acidic/basic vapor, have recently been reported.^29,30 However, examples of lanthanoid(III) complexes which showed reversible color changes or photoluminescence properties by external stimuli both in solution and in solid states are very limited. Therefore, it would be interesting if some lanthanoid(III) complexes with certain hydrazones exhibit a switchable (on/off) sharp and intense luminescence in response to external stimuli both in solution and in solid states.

Herein, we report the synthesis and structural and spectroscopic characterization of europium(III) complexes with two kinds of chromonylhydrazones, 6-methylchromone-3-carbaldehyde-(4′-hydroxy or 4′-methyl)benzoylhydrazone (i.e., HL¹ or HL²) (Scheme 1). The isolated complex salts were [Eu(HL¹)₃]Cl₃ (1Cl), [Eu(HL¹)₃](OTf)₃ (1OTf) and [Eu(HL²)₃](OTf)₃ (2OTf). These complexes exhibited reversible color change and turn-on/off behavior in the Eu^{III 5}D₀ → ⁷F_J (J = 0–4) luminescence by addition of acids or bases in acetonitrile, as well as by heating or diffusion of acidic vapor in the solid states. This switching was induced by deprotonation of only one N–H proton in the tris(hydrazone)-type complexes.

Scheme 1 Structures of the hydrazones, HL¹ and HL².

The hydrazones, HL¹ and HL², were prepared by a reaction of 6-methyl-3-formylchromone with p-hydroxybenzohydrazide and p-methylbenzohydrazide, respectively, in methanol at room temperature (see: supplementary information (SI)). The complex salt, 1Cl, was prepared by a reaction of EuCl₃·6H₂O and HL¹ in a 1 : 3 molar ratio in 2-methoxyethanol and obtained as a crystalline product by addition of diisopropyl ether into the reaction mixture. The corresponding trifluoromethanesulfonate (OTf⁻) salt, 1OTf, was similarly prepared using a mixed acetonitrile and chloroform solution of Eu(OTf)₃ and HL¹ in a 1 : 3 molar ratio. The other hydrazone compound, 2OTf, was also obtained from an acetone solution of Eu(OTf)₃ and HL² in a 1 : 3 molar ratio. All complexes were crystallized with some solvent molecules of crystallization (see: Table S1). Single-crystal X-ray diffraction (SC-XRD) analysis revealed that the molecular structures of the Eu^III complex cations in 1Cl, 1OTf and 2OTf are similar. In particular, the Eu^III ion was coordinated by three HL¹ or HL² ligands having the O,N,O tridentate mode (Fig. 1 and Fig. S2). In the crystal structures of these complexes, all N–H protons formed hydrogen bonds with the Cl⁻ or OTf⁻ anions. Although the crystal structures were successfully analyzed at 188 or 100 K, these crystals were highly efflorescent due to the loss of the solvating molecules at ambient condition. Therefore, characterization of the products by elemental analysis and FT-IR spectroscopy (Fig. S1) were performed with the samples dried and then left in air for a while at room temperature, confirming the purity of the bulk samples. In this study, the corresponding gadolinium(III) (1Gd-Cl, 1Gd-OTf and 2Gd-OTf) and yttrium(III) (2Y-OTf) complexes were also prepared and fully characterized by elemental analysis, FT-IR and ESI-MS spectroscopy (Fig. S4–S8) as well as the SC-XRD study for 1Gd-Cl and 2Gd-OTf, which revealed that their crystal structures were isomorphous to those of the corresponding europium(III) complexes (Table S1).

Fig. 1 An ORTEP drawing (the thermal ellipsoids at 50% probability level) of the Eu^III cationic complex and the surrounding Cl⁻ anions those bound to the cation with hydrogen bonds in 1Cl.

To confirm the possibility of deprotonation from the hydrazone N–H moiety in 1OTf, titration of triethylamine (TEA) in acetonitrile was performed by monitoring its UV-vis absorption spectrum. To ensure full protonation at the N–H moiety of three hydrazone ligands in 1OTf, an equivalent amount (1.0 eq.) of HClO₄ was added, prior to the titration. As shown in Fig. 2a, the spectrum of an acetonitrile solution of 1OTf with a 1.0 eq. HClO₄ exhibited an intense absorption band at 310 nm. Following addition of TEA, the absorption gradually decreased and the absorption shoulder around 380 nm became larger. After addition of 4.0 eq. of TEA, the solution was inversely titrated with HClO₄, resulting in the recovery of the original spectrum (Fig. 2b). The 2OTf complex showed a similar behavior in spectral changes to 1OTf (Fig. S9). Proton NMR measurement was conducted in acetonitrile-d₃ using analogous 2Y-OTf: addition of TEA to the solution caused the N–H signal to disappear (Fig. S10). Therefore, deprotonation and reprotonation reversibly occurred at the N–H moiety.

Fig. 2 Absorption spectral change of (a) 1OTf and 1.0 eq. HClO₄ in acetonitrile {solution (A)} by addition of TEA and that of (b) 1OTf, 1.0 eq. HClO₄ and 4.0 eq. TEA in acetonitrile {solution (B)} by addition of HClO₄.

Photoluminescence spectra of 1Cl and 1OTf in acetonitrile and the solid states were examined, which showed five sharp peaks derived from the Eu^III-centered excited states at 583.5, 593, 616.5, 654.5 and 699 nm, attributed to the ⁵D₀ → ⁷F_J (J = 0–4) transitions (Fig. S11a–c) and those of 2OTf (see Fig. S13d and e). Photoluminescence quantum yield (Φ_tot), luminescence lifetime (τ_obs) and photosensitization efficiency (η_sens) are summarized in Table 1. These europium(III) complexes in solid states showed relatively high Φ_tot, indicating that the hydrazone ligands (i.e., HL¹ and HL²) are good photosensitizers. The energy gap between the S₀ and T₁ states of HL¹ and HL² were estimated by the phosphorescence spectra of the corresponding gadolinium(III) complexes (1Gd-Cl and 2Gd-OTf) in the solid states (the 0–0 vibronic bands at 514 and 520 nm) as 19 500 cm⁻¹ and 19 200 cm⁻¹, respectively (Fig. S14), which is enough to transfer the energy to the ⁵D₀ excited state of the Eu^III center.

Table 1 Photophysical properties of the Eu^III complexes in acetonitrile and in the solid state

Complex	Sample states	Φtot	ΦEu	τobs/ms	k^Eur/s^−1	k^Eunr/s^−1	ηsens
1Cl	Solid	0.56	0.56	0.76	76.9	546	>0.99
1OTf	Solid	0.57	0.57	0.53	71.3	140	>0.99
	In CH3CN	0.03	0.32	0.70	45.0	974	0.09
2OTf	Solid	0.40	0.57	0.74	77.2	587	0.70
	In CH3CN	0.01>	0.34	0.74	46.5	888	0.02

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The luminescence spectral change of 1OTf in acetonitrile by addition of TEA were examined (Fig. 3). Similar to the above-mentioned titration with absorption spectra, 1.0 eq. HClO₄ was added to the sample solution beforehand (Fig. S13), ensuring full protonation at three hydrazone N–H moieties of 1OTf. At 616.5 nm, the luminescence intensity remained nearly unchanged until 1.0 eq. TEA was added to the mixed sample solution. However, addition of more than 1 eq. TEA gradually decreased the ⁵D₀ → ⁷F_J luminescence, and by 2.0 eq. TEA, the luminescence was nearly quenched (Fig. 4 and Table S2). Hence, only one deprotonation forming [Eu(L¹)(HL¹)₂]²⁺ from [Eu(HL¹)₃]³⁺ was enough to quench the ⁵D₀ → ⁷F_J luminescence. When the hydrazonato ligand was reprotonated (by addition of HClO₄), the luminescent bands were recovered (Fig. S14). Therefore, the deprotonation and reprotonation process, which induces a switching of photoluminescent behavior, is fully reversible. An analogous hydrazone complex of 2OTf exhibited a similar reversible photoluminescent property (Fig. S15–S17). Furthermore, the stability tests toward the addition of acid and base were conducted on 1OTf and 2OTf in acetonitrile. No significant changes in luminescence intensities were observed upon the repeated deprotonation/reprotonaion for up to 10 cycle times (Fig. S18), which indicated the high stabilities of 1OTf and 2OTf in solution.

Fig. 3 (a) Photoluminescence (λ_ex = 360 nm) spectral change of 1OTf (9.7 × 10⁻⁵ M) with 1.0 eq. HClO₄ in acetonitrile by titration of TEA. (b) The plots of the luminescence intensity at 616.5 nm during the titration of TEA (0–2.25 eq.).

Fig. 4 Schematic representation of the successive deprotonation/re-protonation for 1OTf and 2OTf. Inserted pictures show the luminescence color change with an acetonitrile solution of 1OTf on irradiation by UV light (λ_ex = 365 nm, c = 9.7 × 10⁻⁵ M).

To explain the mechanism of quenching by deprotonation, phosphorescence spectra of the analogous gadolinium(III) complexes, 1Gd-OTf and 2Gd-OTf, were examined. In acetonitrile, however, both complexes gave no phosphorescence spectra even at 77 K. We assume that the excitation energy, Δ(S₀–S₁), and the phosphorescent energy, Δ(S₀–T₁), of the neutral hydrazone complexes in the solid states and in acetonitrile are nearly the same, because the excitation peak maximum of 1OTf in the solid state and in acetonitrile are almost the same (Fig. S11f). However, an acetonitrile solution of 1Gd-OTf or 2Gd-OTf with 1.0 eq. HClO₄ and 2.25 eq. TEA showed a broad phosphorescence band around 640 nm (Fig. S21). In particular, these bands were remarkably red shifted from those of the above-mentioned 1Gd-Cl and 2Gd-OTf in the solid states, and the phosphorescent energy in the deprotonated complexes, are lower than the ⁵D₀ → ⁷F_J luminescent energy (Fig. 5). This finding suggests that deprotonation from one HL¹ in 1OTf (or one HL² in 2OTf) induced stabilization of the triplet excited state in the complex, being 15 570 cm⁻¹. Then, the energy transfer from the deprotonated ligand triplet state to the Eu^{III 5}D₀ excited state resulted in up-conversion. Considering 1OTf and 2OTf, the quenching of the Eu^{III 5}D₀ → ⁷F_J luminescence can be controlled by reversible deprotonation/reprotonation through the S₁ and T₁ excited states of the hydrazone ligands.

Fig. 5 Proposed excited energy diagram of the hydrazone (S₁, T₁) and hydrazonato europium(III) complex.

Quenching of luminescence in the Eu^III complexes by removing the hydrazone N–H proton in the solid state has also been examined with 1Cl. After heating the powdered sample of 1Cl at 140 °C for 5 minutes, the diffuse reflectance spectrum showed a clear redshift in the band (Fig. 6a). To confirm the origin of this redshift, which may related to the deprotonation from the N–H group as a release of HCl molecule, the electrical conductivity of the sample solution in methanol was measured. Titration with an methanolic AgNO₃ solution gave, owing to the precipitation of AgCl by the dissolved Cl⁻ anions in solution, an increase of the electric conductivity of the sample solution just after the stoichiometric point. The results for the original and the heated samples indicated that approximately one-thirds of the original amount of Cl⁻ ion in 1Cl was released as HCl gas on heating (Fig. S20). In the heated sample, the emission bands corresponding to the Eu^{III 5}D₀ → ⁷F_J (J = 0–4) disappeared completely (Fig. 6b and Table S2). Interestingly, vapor diffusion of HCl gas in the heated (and quenched) sample recovered both the diffuse reflectance spectrum (Fig. 6a) and the luminescence bands. Although the recovery of luminescence intensities in the solid state was not complete as illustrated in Fig. 6b, the recovered luminescence bands were disappeared, again, on the second heating (Fig. S18c). Currently, the thermal stability of the sample in the solid state for repeated HCl release and recombination was insufficient, but this problem could likely be improved by doping the Eu^III hydrazone complex into the structurally related and thermally more stable compounds or fablicating the organic film containing the sample. In conclusion, the deprotonation of the N–H moiety from one of the hydrazone ligands in 1Cl, as a result of heating the solid sample, induced quenching of the Eu^{III 5}D₀ → ⁷F_J (J = 0–4) luminescence bands (Fig. 6b). In addition, the reprotonation of the partial hydrazonato complex by diffusion of HCl vapor recovered the luminescence, as seen in the case of the acetonitrile solution of 1OTf above.

Fig. 6 (a) Diffuse reflectance spectra and (b) photoluminescence spectra (λ_ex = 365 nm) of 1Cl before (black) and after (blue) heating the sample and, then, exposure to HCl vapor (red).

In summary, we have succeeded in synthesizing Eu^III complexes with chromonylhydrazone, which can be prepared easily from the chromonyl aldehyde and benzolyl hydrazine. These complexes showed reversible switching of the ⁵D₀ → ⁷F_J (J = 0–4) luminescence by the addition of acid or base in solution or by heating or acid vapor diffusion in the solid state. The luminescence gave a relatively high quantum yield in acetonitrile and the solid state. The reason for the switching of the luminescence of these complexes resulted from the stabilization of the S₁ and T₁ excited states, as compared to the Eu^III-centered ⁵D₀ excited state, by deprotonation of the hydrazone N–H moiety from one of three ligands in the complexes. This finding of switching behavior of the simply prepared Eu^III complexes both in the solid states and in solution can be applied to the future creation of new luminescent devices with high color purity.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. see DOI: https://doi.org/10.1039/d6cc01278g.

CCDC 2532337–2532341 contain the supplementary crystallographic data for this paper.^31a–e

Acknowledgements

This research was partly supported by JSPS KAKENHI, Grant Number 24K01498 and JP24K23108, Hyogo Science and Technology Assosication, Grant Number 7022 (for K. T.). The authors show a great gratitude to performing the synchrotron radiation experiments at the BL02B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research institute (JASRI) (Proposal No. 2025A1698) and Y. Nakamura as a person-in-change for this beamline.

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