Hierarchical lanthanide-based self-assembly complexes that undergo reversible luminescent response to both temperature changes and chemical reactions within competitive media

Abstract

Here, we report hierarchical self-assembly formation of a luminescent Eu³⁺-1-DPA complex in mixed aqueous–organic (THF or ethanol) solutions. Its reversible responsive behaviour to temperature change and in situ chemical reactions were investigated. The ligands 1-DPA and 1-DPA-acetate as well as the Eu(III) complex of 1-DPA, Eu³⁺-1-DPA were synthesised, and their X-ray crystal structures were determined. The photophysical and material (morphological) properties of the ligands and Eu(III) complexes formed in situ of both 1-DPA and 1-DPA-acetate were probed in mixed aqueous–organic (THF or ethanol) solutions. The ligands as well as the complexes were shown to aggregate into hierarchical and homogenously formed microspheres and microrods, as demonstrated by carrying out SEM of the drop-cast solutions of these complexes in ethanol–water mixtures. The Eu³⁺-1-DPA complex gave rise to Eu(III)-centred emission [due to deactivation of the ⁵D₀ → ⁷F_J (J = 1–4)]. It showed reversible, thermo-responsive lanthanide-centred emission, while in situ esterification using acidic anhydride (forming Eu³⁺-1-DPA-acetate, which was not emissive), and the subsequent NaOH-mediated hydrolysis of the Eu³⁺-1-DPA-acetate assembly induced both Eu(III)-emission and morphological changes, as demonstrated using both spectroscopic and SEM studies.

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Introduction

Developing self-assembly structures from synthetic organic building blocks with the view of generating higher order, or hierarchical self-assemblies, has become an active area of research within supramolecular chemistry.¹ Such an approach can both mimic and help in understanding the properties and the function of naturally occurring hierarchical structures and materials. Non-covalent interactions such as hydrogen bonding,² host–guest interactions,³ donor–acceptor interactions,⁴ and metal coordination,⁵ have been used (individually or in combination) in the development of a range of functional self-assembly materials.⁶ [Ln(III)] is an attractive means to generate such materials, not only because of its unique coordination requirements but also because of its physical properties, such as luminescence and magnetism, which can be transferred to the resulting bulk material. Hence, ions such as Eu(III), Tb(III), and Sm(III) that emit at long wavelengths with line-like emission bands and long excited state lifetimes have been used in the preparation of optoelectronic devices, in up-conversion emission, in chemo- and bio-sensing, probing of chemical processes, in counterfeiting, and in the generation of bio-inspired materials.⁷ We reported their use in the development of tuneable luminescent sensors, probes and supramolecular and polymeric gels,^7b,8 as well as in the formation of nanomaterials,⁹ and MOFs,^10a to name just a few. We recently started investigating the use of such ions more generally in the formation of hierarchical assemblies^8,10 that included the use of triple-stranded di-metallic lanthanide helicates. Here, we show that the 3 : 1 (ligand : metal ion) complex Eu³⁺-1-DPA can be employed in the formation of such structures and functional materials. We show that Eu³⁺-1-DPA,¹² in a reversible manner, can respond to both changes in temperature and chemical composition, or a chemical reaction, where Eu³⁺-1-DPA is converted to the corresponding Eu³⁺-1-DPA-acetate, (Scheme 1) upon reacting it with acetic anhydride under mild catalytic conditions, with the reaction being monitored in real time by probing delayed Eu(III) emission. Hence, this work establishes the use of such lanthanide-based hierarchical self-assembly systems as a responsive material in solution.

Scheme 1 Reversible conversion of Eu³⁺-1-DPA and Eu³⁺-1-DPA-acetate by in situ esterification and hydrolysis reaction.

Results and discussion

Synthesis and characterisation

The Eu³⁺-1-DPA complex was synthesised from the ligand 4-hydroxy-N²,N⁶-bis((S)-1-(naphthalen-1-yl)ethyl)pyridine-2,6-dicarboxamide (1-DPA), as previously reported by us; in this work, we used 1-DPA as a starting point for the preparation of several functional lanthanide complexes through functionalisation at the hydroxy position,¹² (see Experimental section and ESI Fig. S1–S3†), and the characterization matched the reported data.^11,12 The 1-DPA-acetate ligand was also derived from 1-DPA, by reacting it with acetic anhydride in the presence of pyridine in CH₃CN solution at room temperature for 2 h. The resulting product was fully characterised (ESI Fig. S4–S6†). The ¹H NMR (ESI Fig. S4†) showed the characteristic splitting corresponding to the stereogenic centre.

X-ray diffraction analysis was performed for both 1-DPA and 1-DPA-acetate ligand crystals (see ESI Fig. S7 and S8,† respectively). The crystal of 1-DPA was grown from a 1 : 1 DMSO–water solution and was solved in the space group P2₁2₁2₁. The asymmetric unit contained two ligands and two DMSO molecules, with π–π stacking interactions between the pyridine unit and naphthalene unit of two ligands, as well as intramolecular hydrogen bonding between amine N–H and C=O of the amide group of ligands. Each DMSO was H-bonded to the 4-hydroxyl group of 1-DPA. The crystal structure of 1-DPA-acetate ligand was obtained from a crystal grown by slow evaporation of a CH₂Cl₂ solution and was solved in the space group P2₁2₁2₁. The asymmetric unit contained two ligands and a disordered solvent: a CH₂Cl₂ molecule at 0.7 occupancy, and a CH₃CN molecule at 0.3 occupancy. There were similar π–π stacking and hydrogen bonding interactions compared with the 1-DPA ligand.

The corresponding Eu³⁺ complex of 1-DPA, Eu³⁺-1-DPA was formed by microwave reaction of the ligand in the presence of Eu(III) perchlorate salt in methanol. The crystals were grown by slow diffusion of diethyl ether into a saturated methanol solution of Eu(1-DPA)₃(ClO₄)₃ complex. The solid-state X-ray crystal structure of Eu³⁺-1-DPA was resolved and demonstrated that the complex was formed in 3 : 1 stoichiometry (Fig. 1). The structure showed each pyridine moiety sandwiched between two naphthalene units, one from each of the other two ligands, resulting in ‘forced’ π–π stacking, which was absent in the ligand structure and could only be ‘forced’ upon Eu(III) coordination (see Fig. S9, ESI† for the space filling showing the complete ‘engulfing’ of the Eu(III) ion by the three ligands). The extended packing is also shown in Fig. 1c, demonstrating highly ordered packing, which was helical in nature. Recently, we showed that such helicity is directly related to the stereochemistry of the ligand, which is transferred to the overall extended structures.^12b In the case of Eu³⁺-1-DPA, the (S,S) ligand stereochemistry resulted in a Δ stereochemistry around the Eu(III) ion.

Fig. 1 (a) Eu³⁺-1-DPA complex. (b) Symmetry-generated unit cell, showing one disordered conformation of 1-DPA's and ClO₄⁻ anions. Four unique ClO₄⁻ sites shown with Cl1 and Cl4 half-occupied. (c) Extended structure and X-ray packing of the Eu³⁺-1-DPA complex showed a wavy layer-like arrangement (see also ESI Fig. S9†).

Photophysical and morphological studies of free ligand 1-DPA and in the presence of Eu(III)

Having synthesised the Eu complex, we next investigated the luminescent and morphological properties of 1-DPA in the absence and presence of Eu(III).¹² The absorption, fluorescence and excitation spectra of 1-DPA were recorded in a 50% ethanol/water mixture at room temperature. The ground state displayed a typical absorption band seen for such DPA-based systems with two main bands at 220 nm, 280 nm and a hump at ∼320 nm (see ESI Fig. S10† for details). Excitation into 280 nm and 320 nm bands resulted in fluorescence emission centred at ca. 395 nm (see also ESI Fig. S11†).

The potential aggregation of this ligand was also investigated by carrying out dilution studies, observing both the changes in the absorption and fluorescence emission spectra. In contrast to what was seen in absorption spectra where, with increasing concentration two bands at 280 nm and 320 nm obeyed the Beer–Lambert Law, the emission intensity at 395 nm increased then decreased for λ_ex = 280 nm, while the emission intensity at the same wavelength (395 nm) steadily increased for λ_ex = 320 nm (see ESI Fig. S11a–d†) with increasing concentration. The fluorescence excitation also reflected a significant change as a function of ligand concentration. Such a redshift in the excitation band is typically caused by aggregation (see ESI Fig. S11e†). We have previously seen such a phenomenon for the formation of microspheres that are formed in mixed aqueous–organic solutions.^1a,b Hence, we also carried out an investigation into the material properties of this ligand.

The microscopic self-assembly property of 1-DPA was first investigated in two solvent compositions: water/ethanol (EtOH) and water/tetrahydrofuran (THF) mixtures. The 1-DPA ligand (1 mM) formed a clear solution in up to 50% water in EtOH and up to 60% water in THF. Scanning electron microscopy (SEM) of the drop-cast and dried samples from these solutions showed the formation of microspheres with average diameters of 0.63 nm and 0.75 nm, respectively, as shown in Fig. 2a (see also ESI: Fig. S12 and S13†), demonstrating their highly uniform morphology.

Fig. 2 SEM images of drop-casted and dried (a) 1-DPA, (b) 1-DPA with Eu(OAc)₃ (0.1 eq.), (c) 1-DPA with Eu(OAc)₃ (0.2 eq.) and (d) 1-DPA with Eu(OAc)₃ (0.3 eq.) solution in 50% H₂O/EtOH.

Next, the assembly was repeated in the presence of 0.1 equivalent of Eu(III) (as Eu(OAc)₃) with respect to 1-DPA which, with the SEM image obtained of the drop-casted solution shown in Fig. 2b, also gave rise to microsphere morphology (see also ESI Fig. S14†). Upon increasing the Eu(III) concentration to 0.2 equivalent, the microspheres underwent association and form a 2D layer-like morphology (Fig. 2c and ESI Fig. S15†). On further increasing Eu(III) to 0.3 equivalent, the spheres gave rise to a close-packing and homogenous morphology (Fig. 2d and see also ESI Fig. S16†). Given that the DPA ligand motif forms 1 : 3 complexes with Eu(III), this suggests that the presence of Eu(III) with 1-DPA gave rise to the formation of uniformly formed self-assembled microspheres that are highly ordered and homogenous in their hierarchical structures.

Next, we investigated the photophysical properties of the Eu³⁺-1-DPA complex and self-assembly material similar to that described above. In EtOH solution, the UV–Vis absorption spectrum gave rise to bands arising from both pyridine as well as naphthalene units (ESI Fig. S17†). Upon excitation into these bands, the Eu(III)-centred emission was apparent due to deactivation of the Eu(III) excited state; ⁵D₀ → ⁷F_J [J = (0) 1–4], demonstrating the sensitisation of the Eu(III) excited state by a ligand (recorded in the phosphorescence mode, cf.Fig. 3a). Eu(III) emission could also be observed in the (total) fluorescence emission spectra, as shown in Fig. S23a and S27a.† The Eu(III) electronic-dipole J = 0 transition was also observed (though low in intensity, cf.Fig. 3a), signifying that in this complex, the Laporte selection rules were relaxed.^7a,8d

Fig. 3 (a) Variation of emission spectra of the Eu³⁺-1-DPA complex (3.3 μM) for λ_ex 281 nm with an increase in water percentage in H₂O/EtOH mixture. (b) Emission change of the Eu³⁺-1-DPA complex (3.3 μM) at 595 nm, 616 nm, 620 nm, and 696 nm with variation in water percentage. (c) Emission change of Eu³⁺-1-DPA (0.33 mM) with heating and cooling, and (d) repeatability of the heating–cooling cycle.

Upon addition of water, a decrease in the absorption centred at 223 nm and 281 nm, respectively, was observed (Fig. S17, ESI†). Concomitantly, Eu(III) emission was also affected, showing a sharp drop upon increasing the percentage of water from 0.45% to 10%, thereafter it showed a gradual increase with increasing water percentage (Fig. 3a and b).

This quenching we assigned to non-radiative deactivation via vibrational coupling with water (which is commonly seen for Eu-complexes) as well as a possible contribution from hydrogen bonding involving the three hydroxy moieties in Eu³⁺-1-DPA. Afterwards, emission intensity increased, likely due to increased aggregation of the Eu³⁺-1-DPA complexes and possible removal of water from the aggregated spheres.

Above 90% water, however, a reduction in the emission intensity was again observed. This was most likely due to the formation of larger higher-order aggregates of Eu³⁺-1-DPA in solution, though we cannot say so for certain, because the inner-filter effect can also have a role at such higher concentrations of aggregates. Therefore, from the water percentage-dependent study, we confirmed that 50–60% water/ethanol solvent composition was optimum for a self-assembly study of the Eu-1-DPA complex. In this solvent composition range, Eu(III) emission intensity showed a good rise, and the formation of larger aggregates could be omitted. However, prior to that, the changes in Eu(III)-centred emission was also investigated as a function of temperature.

Probing the thermometric luminescence of Eu³⁺-1-DPA

Many examples of luminescent Eu(III) complexes and polymers have been reported for their thermometric luminescence property.¹³ Therefore, having formed the above higher-order systems, we were interested in investigating if indeed Eu³⁺-1-DPA would give rise to such thermo-responsive changes using a 50% H₂O/EtOH solution. Gratifyingly, Eu(III) emission was found to be highly dependent upon temperature, being quenched as a function of gradual heating from 20° C up to 60° C. Moreover, the emission properties were fully reversible, being restored upon cooling back to 20° C (Fig. 3c, see also Fig. S18 ESI†). Furthermore, this ‘on–off’ emission ‘switching’ or ‘heating–cooling cycle’ was repeated five times, as demonstrated in Fig. 3d, with good emission recovery, signifying high stability. This, combined with the changes discussed above in range of aqueous–organic solvent combinations, demonstrate that the complex was stable under these competitive conditions.

Esterification on Eu³⁺-1-DPA and the concomitant effect on Eu(III) luminescent and morphological properties

Having demonstrated the thermo-responsive emission changes for Eu³⁺-1-DPA, we then investigated if Eu³⁺-1-DPA emission properties could be monitored to observe chemical modification at the 4-hydroxy moiety in Eu³⁺-1-DPA. To demonstrate this, we chose to employ simple esterification and de-esterification reactions, but such reactions have very crucial roles in many (specific and non-specific) biological processes.¹⁴ Moreover, in situ esterification and hydrolysis reactions are employed for developing non-equilibrium supramolecular hydrogel formation and self-assembly, which lanthanide ions can also have a major role in.¹⁵ The in situ esterification responsive behaviour has, to the best of our knowledge, not been shown for such lanthanide hierarchical self-assemblies before.

For this study, the Eu(III) complex of 1-DPA and 1-DPA-acetate were prepared in situ in a mixture of 60% H₂O : THF (THF was used to ensure solubility during the acylation reaction) by the addition of aq. Eu(OAc)₃ solution, and the Eu(III) emission properties were monitored. The absorption and (total) fluorescence emission spectra (encompassing both 1-DPA ligand emission and Eu(III) centred emission) of Eu³⁺-1-DPA in this solvent system are shown in Fig. 4a and b, being similar to that observed above.

Fig. 4 (a) Absorption and (b) (total) fluorescence emission spectra of the Eu³⁺-1-DPA (33.3 μM) complex and Eu³⁺-1-DPA-acetate (33.3 μM) complex in 60% H₂O/THF for λ_ex 281 nm. SEM image of the Eu³⁺-1-DPA-acetate (0.33 mM) complex (c) in 60% H₂O/THF and (d) in 40% H₂O/EtOH.

Therefore, in the case of the Eu³⁺-1-DPA complex formation, the emerging characteristic Eu(III) emission was monitored at 595 nm, 616 nm, and 695 nm in 60% H₂O/THF solution along with the emission band for 1-DPA ligand at 390 nm (Fig. 4b). However, in the case of the Eu³⁺-1-DPA-acetate, the acetate ligand was found not to sensitize the Eu(III) excited state, and characteristic peaks for Eu(III) were not observed, Fig. 4b (see also ESI Fig. S19† for phosphorescence spectra). The Eu³⁺-1-DPA-acetate complex (1 mM) was fully soluble in 60% H₂O/THF solution and, upon drop-casting and imaging with SEM, showed a well-defined spherical morphology (ca. 0.7 nm) (Fig. 4c) whereas, in 40–50% H₂O/EtOH solution, only a suspension was formed. However, when SEM was done of the drop-casted and dried samples from 50% H₂O/EtOH immediately, and at 30 min, 50 min and 16 h after sample preparation, the Eu³⁺-1-DPA-acetate complex showed a morphological transformation from an entangled particle network to a micro rod-like morphology as the suspension precipitated out of solution (Fig. 4d, see also Fig. S20, ESI†).

Such time-dependent morphological transformations are not commonly reported, but we have recently reported systems based on amino acid-derived BTA ligands,^1a where such a transformation from particles to fibrous networks was observed with time. These often begin due to formation of thermodynamically driven assembly, though we did not carry out an in-depth investigation into these processes.¹⁶

Reversible chemical modification on Eu³⁺-1-DPA/Eu³⁺-1-DPA-acetate, and the concomitant effect on Eu(III) luminescent and morphological properties

Having investigated the morphology of Eu³⁺-1-DPA-acetate assembly, we next studied its transformation to Eu³⁺-1-DPA, focusing on base-catalysed ester hydrolysis. As stated above, the Eu³⁺-1-DPA acetate system was non-emissive in 50% H₂O/EtOH solution. However, upon base-catalysed hydrolysis, the Eu(III)-centred emission became apparent over time (upon excitation into the ligand absorption) signifying the formation of the product Eu³⁺-1-DPA (Fig. 5a, see also ESI Fig. S21†). Moreover, morphological investigation of this sample showed the reverse transformation from microrods to aggregated microspheres as the suspension turned into a clear solution (Fig. 5b and Fig. S22, ESI†), further demonstrating the formation of Eu³⁺-1-DPA.

Fig. 5 (a) Increase in the phosphorescence emission of Eu³⁺-DPA-acetate (0.33 mM) complex solution in 50% H₂O/EtOH with time for λ_ex 330 nm after addition of NaOH. (b) SEM image of the drop-casted and dried hydrolysis reaction product in 50% H₂O/EtOH.

Having achieved the hydrolysis reaction in solution, we next carried out esterification on the luminescent Eu³⁺-DPA complex, in the same manner as described above, using acetic anhydride and pyridine in 50% H₂O : EtOH. However, esterification did not occur, most likely due to the presence of EtOH.

However, when the reaction was performed in 50% H₂O : THF solution, and the changes in the Eu(III) emission were monitored, a sharp quenching was observed within 1 h (Fig. S23, ESI†). Furthermore, slight changes were observed in the UV–Vis absorption spectra as the reaction proceeded, which were consistent with that observed for Eu³⁺-DPA-acetate complex (Fig. S24, ESI†). This indicated that indeed the esterification had been successful.

To further confirm this, the product was isolated, and characterized by ¹H NMR spectroscopy (Fig. S25, ESI†) and high-resolution mass spectrometry (HRMS) (Fig. S26, ESI†). Gratifyingly, both techniques confirmed the successful in situ formation of the ester. Moreover, carrying out base-catalysed hydrolysis of this product in situ resulted in the reformation of Eu³⁺-1-DPA (see Fig. S27, ESI†), with concomitant luminescence enhancements in Eu(III) emission. This was also supported by monitoring the lifetimes of the Eu(III) emission upon these transformations (Fig. S27c and Table S4, ESI†).

As stated above, the products from these reactions were also analysed by SEM (Fig. 6), which showed the expected morphology for both Eu³⁺-1-DPA acetate and Eu³⁺-1-DPA complexes. To study the reversibility of these reactions further, the esterification of Eu-1-DPA and concomitant hydrolysis cycles were investigated over three consecutive times in 60% H₂O : THF solutions (Fig. 7). In each case, the addition of pyridine and acetic anhydride to a solution of Eu³⁺-1-DPA resulted in Eu(III) emission quenching (Fig. 7), with the base-catalysed hydrolysis restoring the Eu(III) luminescence. These results further demonstrated the stability of the Eu(III) assembly Eu³⁺-1-DPA in solution, that the in situ chemical modification of the system to yield Eu³⁺-1-DPA acetate was possible, and the process was fully reversible over consecutive cycles.

Fig. 6 SEM images of drop-casted and dried samples of (a) the Eu³⁺-1-DPA complex in 60% H₂O/THF, (b) Eu³⁺-1-DPA-acetate complex in 60% H₂O/THF, (c) Eu³⁺-1-DPA complex + pyridine in 60% H₂O/THF and (d–f) Eu³⁺-1-DPA complex + pyridine + acetic anhydride in 60% H₂O/THF solution.

Fig. 7 (a) Emission spectra of the Eu³⁺-1-DPA (0.33 mM) complex upon in situ esterification by Ac₂O and pyridine, and hydrolysis by NaOH repeated three times in 60% H₂O/THF. (b) Emission at 595 nm, 616 nm, 620 nm and 695 nm was plotted with each esterification and hydrolysis cycle.

Conclusions

We demonstrated a highly ordered packing arrangement of Eu-1-DPA complex crystal, which was reflected in the microscopic dimension as well. Hence, microspheres formed from the self-assembly of the 1-DPA ligand underwent 2D assembly on Eu(III) complexation with higher uniformity in spheres size. The Eu³⁺-1-DPA complex demonstrated both water-responsive and thermo-responsive emission properties. The in situ hydrolysis of Eu³⁺-1-DPA acetate showed morphological transformation from microrods to spheres in EtOH : H₂O solution, which was accommodated with changes in the photophysical properties of these complexes. All these processes were found to be fully reversible. This work is directed towards developing responsive luminescent lanthanide-based hierarchical self-assembled materials for advanced functional investigations.

Experimental

Materials and methods

All solvents and chemicals/reagents were purchased from commercial sources and used without further purification.

Synthesis

4-Hydroxy-N²,N⁶-bis((S)-1-(naphthalen-1-yl)ethyl)pyridine-2,6-dicarboxamide (1-DPA). A solution of chelidamic acid monohydrate (1 g, 4.9 mmol) in anhydrous tetrahydrofuran (50 mL) was kept at 0 °C and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl, 2.5 g, 13.0 mmol), 1-hydroxybenzotriazole hydrate (HOBt hydrate, 1.6 g, 11.8 mmol) and N,N-diisopropylethylamine (DIPEA, 3.8 mL, 21.8 mmol) were added and stirred for 30 min. (S)-1-(1-Naphthyl) ethylamine (1.93 mL, 12.0 mmol) was subsequently added and stirred at room temperature for 48 h. THF was removed, and the compound was re-dissolved in dichloromethane (50 mL), washed with 0.1 N HCl (50 mL), aqueous saturated sodium bicarbonate (30 mL) and brine (30 mL). The organic layer was dried over Na₂SO₄, and after removing the solvent under vacuum, the crude product was purified by a flash silica column (24 g, 0–5% MeOH in DCM) to afford 1-DPA as an off-white solid (1.9 g, 79% yield). ¹H NMR (600 MHz, DMSO-D₆) δ ppm: ¹H NMR (600 MHz, DMSO) δ 11.41 (s, 1H), 9.39 (d, J = 8.3 Hz, 2H), 8.23 (d, J = 8.4 Hz, 2H), 7.96 (d, J = 8.0 Hz, 2H), 7.86 (d, J = 8.1 Hz, 2H), 7.69 (d, J = 7.1 Hz, 2H), 7.62–7.46 (m, 8H), 5.99 (p, J = 7.1 Hz, 2H), 1.70 (d, J = 7.0 Hz, 6H); ¹³C NMR(150 MHz, DMSO-D₆) δ ppm: 166.40, 162.77, 151.13, 139.63, 133.46, 130.40, 128.75, 127.52, 126.31, 125.66, 125.44, 123.88, 122.85, 111.95, 44.97, 21.50; HRMS (m/z) calculated for C₃₁H₂₈N₃O₃ [M + H] 490.2131, observed for C₃₁H₂₈N₃O₃ [M + H] 490.2129; elemental analysis calculated for C₃₁H₂₇N₃O₃. 0.5H₂O, C 74.68, H 5.66, N 8.43, observed C 74.38, H 5.45, O 8.40.

2,6-Bis(((S)-1-(naphthalen-1-yl)ethyl)carbamoyl)pyridin-4-yl acetate (1-DPA-acetate). Acetic anhydride (344 μL, 3.67 mmol) and pyridine (295 μL, 3.67 mmol) were added to an acetonitrile solution (3 mL) of 1-DPA (180 mg, 0.367 mmol) and stirred for 2 h at room temperature. Acetonitrile was removed by evaporation. The reaction mixture was diluted with DCM (30 mL) and washed with water (2 × 30 mL). Dichloromethane was evaporated by rotary evaporation, and the compound was recrystallized from dichloromethane to obtain 1-DPA-acetate as a crystalline white solid (113 mg, 58% yield). ¹H NMR (600 MHz, DMSO) δ ppm: 9.49 (d, J = 8.3 Hz, 1H), 8.23 (d, J = 8.3 Hz, 1H), 8.03–7.99 (m, 1H), 7.97 (d, J = 7.8 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 7.70 (d, J = 7.2 Hz, 1H), 7.62–7.49 (m, 3H), 6.01 (p, J = 7.0 Hz, 1H), 2.33 (s, 1H), 1.73 (d, J = 6.9 Hz, 3H). ¹³C NMR (150 MHz, DMSO-D₆) δ ppm: 168.17, 162.00, 159.75, 151.46, 139.30, 133.44, 130.38, 128.74, 127.59, 126.32, 125.66, 125.40, 123.13, 122.82, 118.21, 54.84 (CH₂Cl₂), 45.15, 21.38, 20.95; IR v_max(cm⁻¹): 3287.64, 3048.34, 2978.82, 2932.63, 2874.21, 1777.13, 1656.28, 1596.82, 1509.28, 1448.17, 1366.96, 1175.65, 1120.66, 1017.40, 1000.32, 975.98, 907.68, 871.92, 800.44, 774.64, 729.23; HRMS (m/z) calculated for C₃₃H₃₀N₃O₄ [M + H] 532.2231, observed for C₃₃H₃₀N₃O₄ [M + H] 532.2239; elemental analysis calculated for C₃₃H₂₉N₃O₄. 0.45CH₂Cl₂, C 70.51, H 5.29, N 7.37, observed C 70.48, H 5.25, O 7.35.

Data availability

All data are available upon reasonable request. ESI† is also included with this submission.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Science Foundation Ireland (SFI PI Awards 13/IA/1865; SSPC SFI Research Centres Phase 2: 12/RC/2275_P2 to TG), the Irish Research Council (IRC GOIPD/2019/400 to TG and IRC GOIPD/2020/585 to ETL) for financial support. We thank Drs Feeney, Hessman, O'Brien and Ruether for their help with the recording of HRMS and NMR samples.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, supporting figures, and crystallography data. CCDC 2427466–2427468. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5dt00831j

‡ Present address: Department of Polymers and Functional Materials, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India.

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