White light and colour-tunable emission from a single component europium-1,8-naphthalimide thin film

Abstract

The synthesis and fabrication of spin coated films of a new Eu³⁺ complex [Eu(1)₃] derived from the 1,8-naphthalimide containing ligand 1H is presented. The complex is multi-emissive displaying blue emission from the 1,8-naphthalimide fluorophore and red emission from the Eu³⁺ centre in both solution-state and solid-state. This allows the overall emission to be tuned by changing the excitaton wavelength, where varing degrees of red and blue emission intensity alter the overall emission colour from blue, to red and including white-light emission. The complex was spin-coated onto quartz slides giving 134 nm thick coatings that retained the multi-emissive and colour tunable properties. Overall, resulting in a colour-tunable system which in solution, solid, and thin film states can alter the overall colour from deep red to dark blue.

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Introduction

Multi-emissive materials continue to attract significant attention from the chemistry research community. Their propensity towards advanced applications (ratiometric sensors,^1–3 molecular barcodes,⁴ logic gates,⁵ imaging agents,⁶ and white emitting materials)^7–11 means they are high value targets. Various approaches to generating multi-emissive materials exist,^12–15 including the formation of supramolecular materials from trivalent lanthanide ions (Ln³⁺).^7,11,16 Advantages of the supramolecular approach include combining both Ln³⁺ and ligand functionalities easily into a single component (i.e. a luminescent metal and a luminescent ligand to give multi-emission), inherently soluble systems that allow for solution processibility,¹⁷ and modular design meaning the systems can be easily tuned or improved. Ln³⁺ are particularly attractive for the aforementioned applications owing to their excellent photophysical properties (long-lived excited states, solid-state emission, line-like emission bands and tunable quantum yields).^7,18 Whilst many multi-emissive Ln³⁺ containing systems are Metal Organic Frameworks (MOFs),³ we are interested in discrete systems as these are typically more soluble and processable. Choice of ligand fluorophore plays an important role – it must be emissive in the correct wavelength range, easily tuned, and not reduce solubility. 1,8-Naphthalimides (Nap) are excellent candidates,¹⁹ as they emit across a broad range of wavelengths,²⁰ their solubility is tunable, they are synthetically easy to include into ligands, and have proven layer forming abilities.^21–23 Naphthalimides have previously been used to generate Ln³⁺ complexes^21–30 including in white emissive lanthanide systems.^8,10 Ward and co-workers showed that single component multi-emission was possible by combining Eu³⁺ and Nap in a macrocyclic complex, achieving white emission in solution (CIE coordinates of x,y = 0.27, 0.25) and blue emission in thin films prepared by evaporation.¹⁰ In comparison Yan and co-workers achieved white emission in the solid-state using Eu³⁺ and Nap carboxylate ligands (CIE coordinates x,y = 0.34, 0.31–pure white is considered x,y = 0.33).^8,10,31 However, to the best of our knowledge the naphthalimide–lanthanide combination has not been used to generate white-emissive thin-films. Herein, we report our approach to generating multi-emissive lanthanide thin-films by combining 1,8-naphthalimides and pyridine-2-carboxylate-6-amide (PDC) Ln³⁺ binding groups (Fig. 1). PDC is a relatively under-utilised motif for lanthanide assemblies but has both benefits of improved coordination of the more common dipicolinic acid (DPA) from the free carboxylate, and easy modification of the 2,6-pyridinedicarboxylate amide (PDA) through the amide side arm. PDC systems have shown to be effective sensitizers for a range of visibly emitting lanthanides.³² With this in mind, we have utilised our previously developed 1,2,3-triazole “click” synthetic strategy^32–34 to functionalise PDC with a blue emissive Nap and complex with the visibly emissive lanthanide ion Eu³⁺.

Fig. 1 Two stage synthesis of Ligand 1H with CuACC “click” reaction between precursors A and B followed by deprotection of benzyl ester resulting in ligand 1H.

Results and discussion

Ligand synthesis and characterisation

Ligand 1H was synthesised in a converging seven-step synthesis starting from commercially available DPA and 1,8-naphthalic anhydride (Fig. 1).^32,33,35,36 1,8-Naphthalic anhydride was converted into N-(3-azidopropyl)-1,8-naphthalimide (B) in a three step process following literature procedures^35,36 and then underwent a CuAAC reaction with our previously reported mono-alkyne (A)³² at 80 °C in 4 : 1 DMF : H₂O forming the benzyl protected intermediate (1Bz).³² Removal of the benzyl protecting group was carried out using LiOH following reported procedures³² and resulted in 1H as a off-white solid in 59% yield. All compounds were characterised using ¹H-NMR, ¹³C-NMR, FTIR and Mass Spectrometry. Successful formation of 1H was confirmed by LRMS with peaks at 485.10 m/z and 507.00 m/z corresponding to the [1H + H]⁺ species (calc. for C₂₅H₂₁N₆O₅⁺, 485.16) and [1H + Na]⁺ species (calc. for C₂₅H₂₀N₆O₅Na⁺, 507.14) respectively. ¹H-NMR also indicated successful formation of 1H, particularly when comparing the spectra of 1H with intermediate 1Bz and precursors A and B (Fig. S21†). The appearance of a singlet resonance peak at ≈8.0 ppm assigned to the 1,2,3-triazole proton along with the corresponding disappearance of the terminal alkyne proton signal at 3.2 ppm of precursor A and downfield shifts observed in linking methylene groups bonded to the 1,2,3-triazole ring (t) (3.5 to 4.1 ppm for N₃–CH₂ to t–CH₂ and 4.2 to 4.6 ppm for NH–CH₂–C CH to NH–CH₂–t) post CuAAC reaction is evidence of successful 1,2,3-triazole formation in intermediate 1Bz. Deprotection of the carboxylate, forming 1H, is evident from the disappearance of the associated benzyl aromatic protons (Bz, 7.3–7.5 ppm) and the CH₂–Bz linker (5.4 ppm). ¹³C NMR also confirmed formation of 1H with the correct number of carbon environments and appearance of 1,2,3-triazole ring signals (144.8 (C) and 123.5 (CH) ppm).

Lanthanide complexation and characterisation

Ln(1)₃ complexes (Ln = Eu³⁺ and La³⁺) were prepared by heating 1H with 0.3 equivalents of Ln(CF₃SO₃)·xH₂O and 1 equivalent of NEt₃, in DCM : MeOH : CHCl₃ (3 : 3 : 2). Vapour diffusion of diethyl ether resulted in off-white solids in good yields (68–83%). Complexes were fully characterised by elemental analysis, FTIR, HRMS and ¹H-NMR (see ESI†). Lanthanide coordination within the PDC NO₂ pocket is suggested by the notable shift to lower energy for both the C=O stretches from 1739 cm⁻¹ (carboxyl) and 1656 cm⁻¹ (amide) to a merged peak at 1632 cm⁻¹, a shift commonly associated with coordination to Ln³⁺ ions (Fig. S22†).³⁷ While the carbonyl stretch associated with the Nap is seen to remain unchanged at 1693 cm⁻¹, indicating the lack of interaction between the Ln³⁺ and the Nap as expected. ¹H NMR of La(1)₃ (Fig. S28†) shows a significant downfield shift in the NH signal (0.71 ppm) and a corresponding upfield shift in the methylene linker (CH₂–NH, 0.18 ppm), previously observed in similar PDC systems and indicative of La³⁺ coordination in the NO₂ pocket.³⁸ While proton signals associated with the central pyridyl ring of the PDC motif (which overlapped as one signal in the 1H spectrum) now separate into three distinct proton signals, while all other signals observed no significant shift, further indicative of lanthanide coordination occurring only within the PDC NO₂ pocket. HRMS (Fig. S24–S27†) showed ions with the expected masses and isotropic distributions: [Eu(1)₃ + 2Na]²⁺ 824.1980 m/z (calc. for C₇₅H₅₇N₁₈O₁₅EuNa₂ 824.1630 m/z), and [La(1)₃ + 2Na]²⁺ 817.1818 m/z (calc. for C₇₅H₅₇N₁₈O₁₅LaNa₂ 817.1549 m/z).

Photophysical properties

UV-visible absorption properties of 1H and Ln(1)₃ complexes were measured in MeOH (Fig. S23†). 1H showed a broad absorbance spectrum from 210 to 360 nm, significant absorptions are observed at 220 nm and 370 nm associated with n → π* and π → π* transitions from the Nap,³⁹ and weaker absorptions from 260–280 nm assigned to the n → π* and π → π* transitions from the central pyridyl unit.^33,40–42 Upon complexation the absorption spectra of all Ln³⁺ complexes underwent a slight red shift in the pyridyl associated bands while the Nap bands remained unshifted (Fig. S23†).

Photoluminescence studies of 1H (Fig. S29†) showed the expected Nap monomer emission ranging from 350 to 450 nm, with λ_max = 388 nm in both the solution and solid states,²⁰ when excited into the Nap based absorption bands (240 or 340 nm). The fluorescence quantum yield in MeOH was 12.4%, similar to other 1,8-naphthalimide containing systems.^20,25,43

Upon coordination, the Nap monomer emission at 388 nm was retained in all complexes (when excited at λ_ex = 240, 274 or 340 nm). However, an additional new broad peak at 490 nm was observed (Fig. 2 and S30–S31†). This is likely from the three Nap moieties being in close proximity within the complex, resulting in an excimer emission band – a known phenomenon in Nap systems.^20,44 Excitation spectra showed the excimer bands associated with the Nap motif.

Fig. 2 Fluorescence spectra of 1H, Eu(1)₃ and La(1)₃ showing excimer formation upon complexation (0.01 mM, MeOH, λ_ex = 300 nm).

Eu(1)₃ displayed strong Eu³⁺ centred emission (λ_ex between 210 and 290 nm) with (J = 0–4) transitions in both steady state and time delayed measurements, including the 581 nm ⁵D₀ → ⁷F₀ transition indicative of an overall C₃ symmetry (Fig. S31 and 32†).⁴⁵ Emission lifetimes of the ⁵D₀ → ⁷F₂ transition were found to be 1.175 ms and were best fit to a single exponential decay, indicative of a single species in solution (Fig. S33†). The overall quantum yield was determined to be 7.5% which is midrange for PDC based Eu³⁺ complexes.^{32,38,46–48} Excitation plots indicated the primary antenna for Eu³⁺ centred emission is the pyridyl group with a major peak at 278 nm (Fig. S31†). Interestingly, there is a much smaller band from 300 to 350 nm associated with the Nap group, which can act as a secondary antenna. This is expected due to the Nap triplet state energy matching the Eu³⁺ excited state (18 500 cm⁻¹ and 17 500 cm⁻¹ respectively).¹⁰

Self-assembly studies

To further confirm that excimer formation was a result of complexation, concentration vs. emission intensity, and self-assembly studies of 1H were carried out. At high concentrations of 1H (10 mM) aggregation induced excimer emission was present. Upon reducing the concentration from 10 mM to 1 × 10⁻⁴ mM excimer emission disappeared with monomer emission becoming more intense (Fig. 3a). Fluorescence self-assembly studies between Eu³⁺ and 1H (0.02 mM) showed that on addition of Eu³⁺ from 0 to 0.35 equiv. excimer (and Eu³⁺ centred) emission increased and monomer emission decreased. Excimer emission (and Eu³⁺ centred emission) peaked at 0.35 equivalents of Eu³⁺ while monomer emission reached a minimum, signifying the formation of the Eu(1)₃ complex and intramolecular interaction between neighbouring Nap, resulting in excimer emission (Fig. 3b and c).⁴⁴ Further addition of Eu³⁺ resulted in the formation of Eu(1)₂ and Eu(1) species (as seen in other PDC systems)³² which was accompanied by decreased excimer emission and increased monomer emission, suggestive of all three Nap motifs situated on the same side of the Eu(1)₃ complex, thus maximising intramolecular interaction between neighbouring Nap (Fig. 3b and c).

Fig. 3 (a) Fluorescence emission spectra (λ_ex = 340 nm) of 1H at different concentrations showing aggregation induced excimer formation at higher concentrations in MeOH. (b) Fluorescence titration of 1H with Eu(CF₃SO₃)₃ from 0 to 4.5 equivalents (λ_ex = 279 nm, 0.02 mM, MeOH). (c) Changes in fluorescence emission of 1H (0.02 mM, MeOH) when titrated with Eu(CF₃SO₃)·6H₂O: Nap monomer emission at λ = 380 nm, Nap excimer emission at λ = 480 nm and Eu³⁺ emission at λ = 616 nm.

Colour-tunability

With two well separated luminophores with different excitation wavelengths, the overall emission profile of Eu(1)₃ in solution (0.01 mM, MeOH) is dependent on the excitation wavelength (Nap emission at λ_ex = 200–240 nm and 290–350 nm, and Eu³⁺ emission at λ_ex = 245–290 nm, Fig. 4a, b and S39†). As a result, Eu(1)₃ is a rare example of a single component compound displaying colour-tunable emission. By simply altering the excitation wavelength, a large range of colour profiles were achieved. At excitation wavelengths below 240 nm blue emission was observed with Nap-based emission dominating the spectrum. Excitation at 245 nm resulted in increased Eu³⁺ emission and a colour-profile in the white emission region (CIE x,y = 0.35, 0.26). On increasing the excitation wavelength to 275 nm, where Eu³⁺ emission was maximised, bright red emission (CIE x,y = 0.54, 0.31) was observed. Further increasing the excitation to >290 nm, where Eu³⁺ emission reduced and Nap increased, gave the expected blue emission profile.

Fig. 4 (a) Eu(1)₃ fluorescent emission (0.01 mM, MeOH), showing Eu³⁺ centred emission vs.Nap centred emission at critical wavelengths. (b) CIE chromaticity diagram showing the colour-tunable emission range of Eu(1)₃ in solution (0.01 mM, MeOH). (c) Different concentrations of Eu(1)₃ in MeOH ranging from 10, 5, 1, 0.01 mM left to right, under shortwave UV light (λ_ex = 254 nm).

The colour-tunable emission can be further modified by changing the concentration to bring about aggregation-induced intermolecular excimer emission between complexes. Increasing the concentration from 0.01 mM to 1 mM, 5 mM and 10 mM significantly increased excimer emission (Fig. S40–42†). As a result, the colour-tunable window was reduced with the 1 mM solution only able to obtain a light red emission (Fig. S40†), and the 5 mM (Fig. S41†) and 10 mM (Fig. S42†) solutions unable to achieve red emission. The reduction of the colour-tunable window coincides with improved white emission where the 1 mM solution achieved close to pure-white emission when excited at 250 nm (CIE x,y = 0.33, 0.28). The 5 mM solution appeared white under shortwave UV irradiation (Fig. 4c) and achieved emission with CIE coordinates of x,y = 0.31, 0.27 when excited at 270 nm. A further increase to 10 mM caused overall emission to appear blue-white under shortwave UV (Fig. 4c) and excitation at 270 nm gave white emission (CIE x,y = 0.28, 0.26) similar to Ward and co-workers system.¹⁰

Solid state emission properties

The colour-tunable emission was also observed in the solid state. As expected, with only a Nap luminophore, solids of 1H and La(1)₃ gave bright blue emission under both shortwave (λ_ex = 254 nm) and longwave (λ_ex = 365 nm) irradiation. While Eu(1)₃ displayed colour-tunable emission (Fig. 5), switching from bright blue to white under long- and short-wave UV respectively. Solid state fluorescence showed increased excimer emission, and a colour-tunable window similar to that observed in the Eu(1)₃ 5 mM solution (Fig. S41†). Excitation outside of the 260–288 nm range resulted in overall blue emission while between these excitation wavelengths, overall emission was white with CIE coordinates ranging within x = 0.28–0.32 and y = 0.29. Nearest to pure white was achieved when λ_ex = 275 nm with CIE coordinates x,y = 0.32, 0.29.

Fig. 5 Eu(1)₃ solid powder, shown (left to right) under ambient light, 254 nm UV irradiation and 365 nm UV irradiation.

Thin film fabrication

With Eu(1)₃ displaying colour-tunable and white emission observed in both solid- and solution-states we next investigated the ability of Eu(1)₃ to form thin films through spin coating deposition. A 10 mg ml⁻¹ solution of Eu(1)₃ in DCM : MeOH : DMF (4 : 4 : 2) was spin coated onto 15 × 20 mm glass and 20 × 20 mm quartz substrates. 50 μL was dynamically deposited onto the substrates at 500 rpm and then spun up to 4000 rpm, resulting in a uniform film to the eye (Fig. 6). AFM measurements showed a stippled coating with an average particle height of 133.5 nm, while also containing some larger aggregates (see Fig. S45 and 46†). Importantly, the film retained the properties exhibited in the solution and bulk solid states. The film appeared bright white under short-wave and light blue under long-wave UV irradiation (Fig. 6). Photophysical measurements were carried out on the film showing that it retained the same absorption profile of Eu(1)₃ as measured in solution (Fig. S44†). Fluorescence emission showed a similar colour-tunable emission profile to the solid state (and high concentration solution) with strong excimer emission (Fig. S47†). Excitation wavelengths outside of the 260–288 nm range resulted in overall blue emission as it favours Nap excimer emission. Excitation between 260–288 nm (where Eu³⁺ centred emission increases) resulted in overall emission within the white region with CIE coordinates ranging within x = 0.28–0.34 and y = 0.26–0.28 (Fig. 6). Closest to pure white emission is achieved over a range of excitation wavelengths (λ_ex = 270–272, 279–283 nm) with CIE coordinates equalling x,y = 0.33, 0.28, close to the emission of the original solid. While λ_ex of 273–278 nm emission is still close to pure white with CIE coordinates x,y = 0.34, 0.28.

Fig. 6 (Top) Spin coated films of Eu(1)₃ on a 15 × 20 mm glass substrate, under 365 nm, ambient light and 254 nm UV irradiation, respectively. (Bottom) CIE coordinates showing the colour-tunable emission range for the Eu(1)₃ spin coated film (see ESI† for CIE coordinates).

Conclusions

The discrete lanthanide complex Eu(1)₃ resulted in the formation of a multi-emissive assembly capable of colour-tunable emission dependent on the excitation wavelength ranging from blue to red and close to white emission in solution and solid states. With Eu(1)₃ being a discrete system, it is easily solubilised in a range of solvents. This allowed Eu(1)₃ to form thin films by spin coating. The thin films retained both colour-tunability and white emission, which to the best of our knowledge is the first example of a discrete Ln³⁺-1,8-naphthalimide complex processed into thin films that exhibit such properties. Additionally, the luminophores were observed to act almost independently from each other at most excitation wavelengths. Such multi-emission is ideal for sensor development, as it allows for self-calibrating ratiometric systems.^1,49,50 We are currently investigating the potential for self-calibrating ratiometric oxygen sensors both in solution and as films, in which the organic emission is quenched in the presence of oxygen, while the Ln³⁺ signal self-calibrates and regulates signal change.

Experimental

General experimental details

All reagents, solvents and starting materials were purchased from Sigma-Aldrich. Precursors A³² and B^35,36 were synthesised using literature methods and showed the expected characterisation. NMR spectra were recorded using a Bruker Ultrashield 300, with chemical shifts recorded in parts per million (ppm) downfield from the standard. UV-visible absorption was recorded on a Shimadzu UV-1800 using CH₂Cl₂ and MeOH solvents in a 3 mL quartz cuvette with a 1 cm path length. Steady state–fluorescence measurements were recorded on a Shimadzu RF-6000 Spectro fluorophotometer and time resolved phosphorescence measurements were carried out using an Agilent Technologies Cary Eclipse fluorescence spectrophotometer, with CH₂Cl₃ and MeOH solvents in a capped 3.5 mL quartz cuvette with a 1 cm path length. Self-assembly titrations: The formation of 1 : 1, 1 : 2 and 1 : 3 (M : L) species was monitored by measuring changes in the UV-visible absorption and fluorescence spectra (λ_ex = 279 nm) of 1H (2 × 10⁻⁵ M in 3 mL volume in MeOH diluted from 0.001 M stock solution in DCM : MeOH (1 : 1)) when titrated with Eu(CF₃SO₃)₃·6H₂O solution (1 mM, MeOH) from 0→4.5 equivalents, with 3 μL additions for 0 to 1 equivalents, 6 μL additions for 1 to 2 equivalents, and 30 μL additions for 2 to 4.5 equivalents. Commission Internationale de l'Eclairage (CIE) plots were generated in OSRAM LED ColorCalculator software. Melting points were obtained on a Electrothermal IA9000 Series Melting Point Apparatus. Fourier-transform infrared spectroscopy (FTIR) of solids were recorded on a Bruker Alpha platinum-ATR. Mass spectrometry of ligands was carried out in HPLC grade solvents, on a Shimadzu LCMS-2020 for ligands and a Bruker Daltonics MicrOTOF™ Spectrometer for complexes. Spin coated films were fabricated on an Ossila spin coater using 1 mm thick quartz substrates (10 mm × 15 mm). Atomic force microscopy (AFM) measurements were performed in tapping mode, on an Asylum Cypher ES (Oxford Instruments, US). AFM tip parameters with silicon probes (nominal spring constant 5 N m⁻¹, resonance frequency 150 kHz, from Budget Sensors, Bulgaria), with a set point at 80% amplitude and measurements run in attractive mode. Safety Note: whilst no issues were encountered here, low molecular weight organic azides are potential explosives and care must be taken during their handling.

Quantum yield measurements

Quantum yield measurements were determined by the dilute comparison method⁵¹ using relative standards Cs₃[Eu(dpa)₃]·8H₂O,^52,53 complex in a 0.1 M tris-HCl buffer solution (pH ≈ 7.45) and quinine sulfate in 0.5 M H₂SO₄,⁵⁴ with known quantum yields of Φ_ref = 24 ± 2.5%, and Φ_ref = 54.6% respectively. Cs₃[Eu(dpa)₃]·9H₂O was used for Eu(1)₃ and quinine sulfate was used for the 1,8-naphthalimide emission. Barrier slit widths remained the same between measurements for different compounds with 1.5, 3 nm excitation and emission widths. Excitation wavelength was the same for all measurements with the standard 279 nm excitation wavelength being used for Eu³⁺ emissions and 366 nm for quinine sulfate and Nap emissions. Complexes were dissolved in a 1 : 1 MeOH : CH₂Cl₂ and then diluted into MeOH.

Estimated overall quantum yields Φ^L_Ln = Φ_x were calculated according to eqn (1).^51,52 Here grad refers to the slope of plotted emission area vs. absorbance (emission area was taken from specific emission peaks, Eu(1)₃ (⁵D₀ → ⁷F₂), and 1,8-naphthalimide excimer and monomer broad emissions vs. quinine sulfate), n refers to refractive index of the solution (a refractive index of n = 1.3295 was found for MeOH : CH₂Cl₂ solution), and subscripts are ref for reference and x for sample.^51,52

Synthesis of N₂-bis((1-(N-(3-propyl)-1,8-naphthalimide)-1H-1,2,3-triazol-4-yl)methyl)-O₆-((benzyloxy)carbonyl)pyridine-2 carboxamide-6-carboxylate (1Bz)

A (0.400 g, 1.4 mM) was combined with sodium ascorbate (0.269 g, 1.4 mM), CuSO₄·5H₂O (0.170 g, 0.7 mM) and B (0.381 g, 1.4 mM) in 30 mL of DMF : H₂O (4 : 1). The solution was heated to 80 °C overnight, changing from a light yellow to a dark green. Water was slowly added to the solution until a cloudy precipitate formed, which was filtered and then washed with 0.1 M NaOH/0.5 M EDTA solution yielding 1Bz as an off-white solid (0.558 g, 71%). Melting point = 143.3 °C. LRMS m/z = 575.15 [1Bz + H]⁺ (calc. for C₃₂H₂₇N₆O₅⁺, 575.20). ¹H NMR (300 MHz, DMSO-d₆, ppm), δ = 9.04 (t, J = 6.0 Hz, 1H, NH), 8.49 (t, J = 7.0, 4H, Nap–H), 8.27 (m, 3H, Py–H), 8.05 (s, 1H, t–H), 7.89 (m, 2H, Nap–H), 7.41 (m, 5H, Bz–H), 5.45 (s, 1H, CH₂–Bz), 4.59 (d, J = 5.5 Hz, 2H, NH–CH₂–t), 4.48 (t, J = 7.0 Hz, 2H, t–CH₂), 4.15 (t, J = 7.0 Hz, 2H, CH₂–Nap), 2.23 (m, 2.23, CH₂–CH₂–CH₂); ¹³C NMR (75 MHz, DMSO-d₆, ppm): δ = 164.3 (C=O–Py), 164.1 (C=O–Nap), 163.6 (C=O–Py), 150.6 (CN–Py), 146.9 (CN–Py), 144.8 (C–t), 140.0 (CH–Py), 136.2 (C–Bz), 134.7 (CH–Nap), 131.7 (C–Nap), 131.1 (CH–Nap), 129.0 (CH–Bz), 128.7 (CH–Bz), 128.5 (CH–Bz), 127.9 (C–Nap), 127.9 (CH–Py), 127.6 (CH–Nap), 125.9 (CH–Py), 123.5 (CH–t), 122.6 (C–Nap), 67.3 (CH₂–Bn), 48.0 (CH₂), 37.8 (CH₂), 35.2 (CH₂–NH), 29.0 (CH₂). FTIR (cm⁻¹) 1779, 1734, 1695, 1651, 1585, 1531, 1438, 1348, 1305, 1305, 1232, 1171, 1124, 1049, 1002, 907, 843, 773, 745, 675, 643, 541, 514.

Synthesis of N₂-((1-(N-(3-propyl)-1,8-naphthalimide)-1H-1,2,3-triazol-4-yl)methyl)pyridine-2-carboxamide-6-carboxylate (1H)

1Bz (0.858 g, 1.5 mM) was combined with LiOH (0.143 g, 6.0 mM) in 20 mL of THF : MeOH : H₂O (3 : 1 : 1). This was stirred overnight under a nitrogen atmosphere, where the solid slowly dissolved into a light-yellow solution. The solvent was then removed resulting in an oily residue. 1 M HCl was then added causing an off-white precipitate to form, which was filtered and washed with water and acetone to remove the 1,8-naphthalimide by-product, yielding 1H as an off-white solid (0.428 g, 59%). Melting point = 148.2 °C. LRMS m/z = 485.10 [1H + H]⁺ (calc. for C₂₅H₂₁N₆O₅⁺, 485.16) and m/z = 507.00 [1H + Na]⁺ (calc. for C₂₅H₂₀N₆O₅Na⁺, 507.14). ¹H NMR (300 MHz, DMSO-d₆, ppm), δ = 9.71 (t, J = 6.0 Hz, 1H, NH), 8.46 (t, J = 6.0, 4H, Nap–H), 8.23 (m, 3H, Py–H), 8.05 (s, 1H, t–H), 7.87 (m, 2H, Nap–H), 4.58 (d, J = 6.0 Hz, 2H, NH–CH₂–t), 4.45 (t, J = 7.0 Hz, 2H, t–CH₂), 4.11 (t, J = 7.0 Hz, 2H, CH₂–Nap), 2.20 (m, 2H, CH₂–CH₂–CH₂); ¹³C NMR (75 MHz, DMSO-d₆, ppm): 165.4 (C=O–Py), 164.1 (C=O–Nap), 163.3 (C=O–Py), 149.5 (CN–Py), 146.4 (CN–Py), 144.8 (C–t), 140.4 (CH–Py), 134.8 (CH–Nap), 131.8 (C–Nap), 131.2 (CH–Nap), 127.9 (C–Nap), 127.7 (CH–Nap), 127.1 (CH–Py), 125.9 (CH–Py), 123.6 (CH–t), 122.8 (C–Nap), 48.0 (CH₂), 37.8 (CH₂), 34.9 (CH₂–NH), 29.0 (CH₂). FTIR (cm⁻¹) 3234, 3138, 2925, 2853, 2394, 1893, 1739 (COOH–Py), 1695 (CO–Nap), 1657 (CONH–Py), 1625, 1587, 1530, 1454, 1440, 1387, 1345, 1306, 1235, 1170, 1147, 1121, 1074, 1054, 1016, 1001, 963, 907, 880, 847, 801, 780, 765, 747, 682, 643, 628, 563, 546, 516, 489, 423.

Synthesis of Eu(1)₃

1H (0.0640 g, 0.132 mM) was combined with Eu(CF₃SO₃)₃·6H₂O (0.0308 g, 0.044 mM) and 1 equivalent of triethylamine in 5 mL of DCM : MeOH : CHCl₃ (3 : 3 : 2). The complex precipitated as a white powder, which was filtered giving Eu(1)₃ as a white solid (0.058 g, 83%). Melting point = 205–226 °C. HRMS m/z = 824.1980 [Eu(1)₃ + 2Na]²⁺ (calc. for (C₇₅H₅₇N₁₈O₁₅EuNa₂)²⁺, 824.1630), m/z = 1625.3545 [Eu(1)₃ + Na]⁺ (calc. for (C₇₅H₅₇N₁₈O₁₅EuNa)⁺, 1625.3367). FTIR (cm⁻¹) 3233, 3073, 2965, 1698, 1633, 1587, 1565, 1462, 1430, 1347, 1277, 1235, 1155, 1082, 1048, 1029, 1018, 890, 848, 801, 779, 761, 724, 687, 660, 637, 573, 540, 517, 410. Elemental analysis for C₇₅H₅₇N₁₈O₁₅Eu·3.5H₂O·2.5CH₂Cl₂ (1878.7389 g mol⁻¹) calculated: C 49.55, H 3.70, N 13.42%. Found C 49.74, H 4.00, N 13.45%.

Synthesis of La(1)₃

1H (0.0622 g, 0.128 mM) was combined with La(CF₃SO₃)₃·9H₂O (0.0317 g, 0.044 mM) and 1 equivalent of triethylamine in 5 mL of DCM : MeOH : CHCl₃ (3 : 3 : 2). The complex precipitated as a white powder, which was filtered giving La(1)₃ as a white solid (0.046 g, 68%). HRMS m/z = 817.1818 [La(1)₃ + 2Na]²⁺ (calc. for (C₇₅H₅₇N₁₈O₁₅LaNa₂)²⁺, 817.1549). FTIR (cm⁻¹) 3270, 3080, 2967, 2337, 2318, 1697, 1624, 1587, 1566, 1436, 1347, 1275, 1235, 1169, 1084, 1049, 1030, 890, 847, 801, 778, 759, 723, 687, 637, 573, 540, 516, 407. Elemental analysis for C₇₅H₅₇N₁₈O₁₅La·2·CHCl₃ (1860.0489 g mol⁻¹) calculated: C 49.72, H 3.20, N 13.55%. Found C 49.76, H 3.46, N 13.37%. ¹H NMR (300 MHz, DMSO-d₆, ppm), δ = 10.43 (t, J = 6.0 Hz, 1H, NH), 8.45 (t, J = 6.0, 4H, Nap–H), 8.37 (m, 1H, Py–H), 8.22 (m, 1H, Py–H), 8.1 (s, 1H, t–H), 7.91 (m, 1H, Py–H), 7.83 (m, 2H, Nap–H), 4.48 (d, J = 6.0 Hz, 2H, CH₂–Nap), 4.40 (t, J = 7.0 Hz, 2H, NH–CH₂–t), 4.11 (t, J = 7.0 Hz, 2H, t–CH₂), 2.20 (m, 2H, CH₂–CH₂–CH₂).

Author contributions

Alex O'Neil: conceptualisation, investigation, methodology, analysis, writing – original draft. Anaïs Chalard: AFM investigation, writing – review & editing. Jenny Malmström: supervision, AFM investigation, writing – review & editing. Jonathan Kitchen: conceptualisation, writing – review & editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2dt03644d

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