Versatility of Terpyridine-Functionalised Aryl Tetrazoles: Photophysical Properties, Ratiometric Sensing of Zinc Cations and Sensitisation of Lanthanide Luminescence
Graphical Abstract
Tetrazole functional groups, in conjugation with terpyridine systems, exhibit intrinsic luminescence properties that can exploited in areas such as pH sensing and ratiometric and efficient sensing of Zn2+ cations, with discrimination of Zn2+ from Cd2+ interference, as well as sensitisation of visible and near-infrared emission from lanthanide ions.
Abstract
Four new tetrazole-containing species, in conjugation with terpyridine moieties through phenyl or pyridyl linkers, have been synthesised and characterised. In the series, the tetrazole functional groups are either in their acidic form or are alkylated at the N2 position with a methyl group. The photophysical properties of the species reveal moderate UV or efficient blue fluorescent emission, with photoluminescence quantum yields of around 30 % and 80 % for UV and blue emission, respectively. The spectral profiles can be reversibly modulated through protonation/deprotonation, with changes being consistent with an increase of the electron density on the tetrazole rings upon deprotonation or a decrease of the electron density on the terpyridines through protonation. The tetrazole-containing species were also investigated for fluorescence sensing of biologically and environmentally important metal ions, highlighting a ratiometric response to the presence of Zn2+. Furthermore, this ratiometric response could be well discriminated from that of interfering Cd2+ ions. Lastly, the species have been investigated as ligands for Eu3+ and Yb3+ cations, revealing efficient sensitisation, to give typical red and near-infrared emissions, respectively.
Introduction
As a functional group, tetrazoles have found widespread use in the design of molecular species for diverse applications, including medicinal chemistry,1 coordination polymers,2 crystal growth inhibition3 and luminescent materials.4-6 Tetrazoles are five-membered aromatic heterocycles possessing four adjacent N atoms. In terms of acidity, tetrazoles have pKa values similar to carboxylic acids, so negatively charged tetrazolates are easily accessible through deprotonation reactions.7, 8 For this reason, tetrazoles are considered to be more metabolically stable bioisosteres of carboxylic acids.9 The presence of four sp2-hybridised lone pairs on the nitrogen atoms allows the construction of complex molecular architectures by bridging multiple metal ions,2 and many of these examples have been reported for species containing both transition metals and lanthanide elements.10-15
In the area of luminescent metal complexes, tetrazoles have proven to very versatile ligands, both from the point of view of coordination modes and the modulation of photophysical properties. We have been interested in this area for some time, highlighting the construction of multinuclear luminescent assemblies with metallic cations such as ReI,16 IrIII,5 RuII,12 CuI[17] and PtII.18 Furthermore, we have shown how coordinated tetrazoles can undergo facile reaction with electrophilic reagents, thus modulating the photophysical properties of the complex.19 These complexes have proven to be useful in applications such as organic light-emitting devices20 and molecular probes for live-cell imaging.21 In these studies, the luminescence properties mainly originate from the final metal complex, rather than the tetrazole-containing ligands.
In trying to expand the scope of tetrazole-containing species for luminescence applications, we have decided to investigate molecular architectures composed of a tetrazole functional group conjugated to a terpyridine moiety by either a phenyl or pyridyl linker. The rationale for the design of this series (Figure 1) was to access ligands with intrinsic photophysical properties that could be modulated by reversible interaction with electrophiles and Lewis acids, as well as complexation with transition-metal and lanthanide cations. In the newly synthesised compounds, the tetrazole functional groups are either in their acidic form or are alkylated with a methyl substituent at the N2 position. This work reports the synthetic protocols for the construction of the terpyridine-functionalised tetrazoles shown in Figure 1, along with the investigation of their intrinsic photophysical properties and their sensitivity to protonation/deprotonation reactions. Furthermore, the versatility of these species for luminescence applications are highlighted in the ratiometric sensing of Zn2+ cations, with discrimination for interference from Cd2+ cations, as well as sensitisation of visible red emission and near-infrared emission through complexation with Eu3+ and Yb3+, respectively.
Results and Discussion
Synthesis and Spectroscopic Characterisation
Following a published procedure,22 the precursor NCPhTpy was prepared by reaction of 4-cyanobenzaldehyde and pyridine-2-carboxyaldehyde in ethanol and in the presence of potassium hydroxide and aqueous ammonia. The targeted species HTzPhTpy was then prepared by 1,3-dipolar cycloaddition between sodium azide and NCPhTpy in toluene (Figure 2), following slight modifications of a reported procedure.23
HTzPhTpy was characterised by IR spectroscopy and 1H and 13C NMR spectroscopy. The IR spectrum displays a band at 1603 cm–1, corresponding to the CN stretch of the tetrazole moiety, and the spectrum is missing the nitrile peak at 2225 cm–1, indicating absence of the starting material.
Due to the low solubility of this compound in common deuterated solvents and the presence of multiple overlapping peaks, the 1H NMR spectrum of HTzPhTpy was difficult to interpret (Figure S1). The spectrum in deuterated DMSO shows broad signals in the ranges 8.80–8.95 ppm, 8.20–8.35 ppm and 7.67–7.76 ppm, with an integration ratio of 6:6:2, matching the proposed structure. Excess triethylamine was therefore added to the deuterated DMSO solution to promote the formation of the more soluble triethylammonium tetrazolate salt [HNEt3][TzPhTpy]. The resulting 1H NMR spectrum (Figure S1) becomes more defined, showing, as expected, seven peaks in the aromatic region between 9.00 and 7.00 ppm; these belong to the phenyl and pyridine rings. The 13C NMR spectrum for this compound exhibits thirteen signals, corresponding to those expected for the product. In particular, the formation of the tetrazole ring is supported by the presence of a peak at δ = 160.3 ppm.24, 25
An alternative sequence was followed for the preparation of HTzPyTpy (Figure 3). Firstly, 2-cyanopyridine-5-carboxyalhedyde, NCPyCHO, was prepared by reaction of 2-bromopyridine-5-carboxyalhedyde with copper(I) cyanide in DMF, according to a previously published procedure.26 A 1,3-dipolar cycloaddition reaction was then performed on NCPyCHO for the formation of the tetrazole ring in HTzPyCHO.23 The species HTzPyCHO was analysed by IR spectroscopy and 1H NMR and 13C NMR spectroscopy. The IR spectrum for this compound has key signals at 1704 cm–1, corresponding to the aldehyde carbonyl group, and 1605 cm–1 for the tetrazole CN stretch.
The 1H NMR spectrum matches the desired product, showing a singlet at δ = 10.20 ppm, corresponding to the H atom of the CHO substituent, along with one singlet and two doublets belonging to the three H atoms of the pyridine ring. The 13C NMR spectrum displays seven signals consistent with the proposed structure, with noteworthy resonances at δ = 191.8 ppm, corresponding to the aldehyde C atom, and 154.6 ppm, corresponding to the tetrazolic C atom.24, 25
The terpyridine moiety was formed by reaction of HTzPyCHO with pyridine-2-carboxyaldehyde in ethanol and in the presence of potassium hydroxide and aqueous ammonia.22 The product was characterised by IR spectroscopy and showed a signal at 1597 cm–1, corresponding to the tetrazole CN stretch. After the addition of excess triethylamine, the product was also characterised by 1H NMR spectroscopy (Figure S2), displaying better resolved peaks, as in the previous case of HTzPhTpy.
Alkylation of the tetrazole ring in HTzPhTpy was first attempted by reaction with trifluoroacetic acid and sulfuric acid in tert-butanol. This procedure was chosen because its regioselectivity for the N2 position of the tetrazole ring was previously demonstrated.27, 28 However, this reaction failed to yield the target alkylated species. Therefore, HTzPhTpy was treated with methyl iodide and potassium carbonate in acetonitrile at reflux. These conditions allowed the isolation of a mixture of methylated regioisomers at the N1 and N2 positions of the tetrazole ring, with an approximate ratio of 4:6, respectively, as evidenced by NMR spectroscopic analysis. The two regioisomers could be easily separated by using column chromatography. The identity of MeTzPhTpy was confirmed by the appearance of a singlet corresponding to three H atoms at δ = 4.42 ppm in the 1H NMR spectrum, belonging to the methyl substituent (Figure S3). Furthermore, the resonance at δ = 165.0 ppm, belonging to the tetrazolic C atom in the 13C NMR spectrum, confirmed the methylation at the N2 atom of the tetrazole ring.24, 25
To install a consistent alkylating functional group bound to the tetrazole ring, the same methylation protocol was applied to HTzPyTpy. At the end of the reaction, the analysis, by NMR spectroscopy, revealed the expected mixture of regioisomers in a 6:4 ratio for the N1 and N2 methylated products, respectively. Unfortunately, these could not be separated by column chromatography. A new synthetic strategy was therefore adopted for the preparation of MeTzPyTpy (Figure 4). Firstly, the previously prepared 2-(1H-tetrazol-5-yl)pyridine-5-carboxyaldehyde, HTzPyCHO, was methylated by reaction with methyl iodide and potassium carbonate. The methylation yielded a 4:6 mixture of N1/N2 alkylated regioisomers, which could be easily purified by column chromatography. Then, the isolated MeTzPyCHO was treated with pyridine-2-carboxyaldehyde, potassium hydroxide and aqueous ammonium hydroxide to yield the targeted terpyridine functionalised species MeTzPyTpy.22 The presence of a singlet at δ = 4.49 ppm for the methyl substituent in the 1H NMR spectrum confirmed the identity of the target compound (Figure S4), whereas the N2 isomer was determined by the resonance of the tetrazolic C atom at δ = 164.8 ppm in the 13C NMR spectrum.24, 25
Photophysical Properties
A summary of the photophysical properties of HTzPhTpy, HTzPyTpy, MeTzPhTpy and MeTzPyTpy, recorded in acetonitrile, is reported in Table 1. The absorption profiles of the four species (Figure 5) highlight the presence of intense and structured bands in the region between 200 and 350 nm. These bands are mainly ascribed to spin-allowed π–π* transitions with possible admixtures of n–π* transitions in the lower energy region of the broad bands. The lowest energy shoulders appear redshifted for HTzPhTpy and HTzPyTpy when compared with the two methylated species MeTzPhTpy and MeTzPyTpy.
| Compound | Absorption | Emission (298 K) | ||
|---|---|---|---|---|
| λ [nm] | λ [nm] | τ [ns] | Φ | |
| (ε [× 10–4 m–1 cm–1]) | ||||
| HTzPhTpy | 295 (2.06) | 426 | 2.62 | 0.77 |
| 253 (1.15) | ||||
| 231 (1.51) | ||||
| HTzPyTpy | 295 (3.32) | 400 | 3.36 | 0.85 |
| 253 (2.31) | ||||
| 230 (2.16) | ||||
| MeTzPhTpy | 283 (3.93) | 356 | 2.84 | 0.32 |
| 255 (2.51) | ||||
| 232 (1.66) | ||||
| MeTzPyTpy | 282 (2.76) | 362 | 2.76 | 0.28 |
| 253 (2.44) | ||||
The emission profiles, shown in Figure 5, display broad bands in the late-UV region, with similar maxima at 356 and 362 nm for the methylated species MeTzPhTpy and MeTzPyTpy, respectively. On the other hand, HTzPhTpy and HTzPyTpy display redshifted profiles in the blue region, peaking at 426 and 400 nm, respectively. This redshifted emission might be explained by the more electron-rich nature of the unmethylated tetrazole rings, which could also exist in solution as a zwitterionic form, displaying donor–acceptor behaviour between the anionic tetrazolate ring and cationic protonated form of the terpyridine moiety. This conclusion is also corroborated by the slight blueshift of HTzPyTpy with respect to HTzPhTpy, as the presence of the pyridine linker for the former causes a reduction of the electron density of the tetrazole ring by charge delocalisation. HTzPhTpy and HTzPyTpy display strong emission intensities, with quantum yield (Φ) values measured at 0.77 and 0.85, respectively. The quantum yield reduces to moderate upon methylation, with values of 0.32 for MeTzPhTpy and 0.28 for MeTzPyTpy. In all cases, the excited-state lifetime decay (τ) is very short, of the order of few nanoseconds, which is consistent with spin-allowed fluorescent emission.
Photophysical Changes upon Addition of Acids and Bases
The relative energy of the excited states for the synthesised species can be readily influenced by acid–base equilibria occurring on the tetrazole and terpyridine substituents. To investigate the effect of these equilibria on the luminescence properties, we measured the changes in the absorption and emission spectra upon sequential addition of camphorsulfonic acid in acetonitrile solutions. The methylated species MeTzPhTpy and MeTzPyTpy were investigated first, as this system is absent from the added protonation equilibrium occurring on the tetrazole substituent.
Sequential absorption and emission plots for acetonitrile solutions of MeTzPhTpy and MeTzPyTpy are shown in Figure 6, where each sequential spectrum is recorded upon addition of 0.33 equiv. of acid. The trend in the absorption plots is analogous and highlights the progressive appearance of a redshifted band peaking around 330 nm and the presence of two isosbestic points in each spectrum. The corresponding emission profiles also show an analogous trend, where the UV emission band progressively disappears, with the concomitant appearance of a broader redshifted band of aqua-coloured emission for MeTzPhTpy and blue emission for MeTzPyTpy. This trend is rationalised by protonation occurring at the terpyridine substituent causing stabilisation of the π* orbitals. The effective result is a reduction of the HOMO–LUMO gap. The blueshifted emission for the protonation of MeTzPyTpy, with respect to MeTzPhTpy, is rationalised by the fact that the protonation can also occur at the 2-pyridyltetrazole position, hence stabilising the π orbitals to a greater extent, with consequent widening of the HOMO–LUMO gap.
Under similar conditions, the changes in the absorption and emission profiles for HTzPhTpy and HTzPyTpy were monitored (Figure S5). In this case, the initial spectra were recorded from a solution containing each species and tetrabutylammonium hydroxide (1 equiv.) to completely deprotonate the tetrazole ring. The profiles highlight changes upon addition of acid, but the resulting spectra resemble a more complex superimposition of bands. The superimposition could originate from protonation occurring on the terpyridine moiety and on the tetrazole ring for HTzPhTpy and on the 2-pyridyltetraole site for HTzPyTpy. Unlike the methylated derivatives, a clear trend could not be easily established, although significant variations of the absorption and emission spectra are clearly highlighted.
Ratiometric Fluorescence Sensing of Zn2+ and Discrimination from Cd2+ Interference
The pH-induced fluorescence changes of the synthesised tetrazole species and the presence of well-established terpyridine metal-binding motifs prompted us to assess the use of these species for the fluorescence sensing of metal ions. For this investigation, the emission properties were measured in buffered aqueous media using an excitation wavelength of 290 nm (Figure 7). Similar to the ligands in acetonitrile, the fluorescence spectra in buffered aqueous media highlight broad bands between 350 and 500 nm. The trends in emission maxima are analogous to the previously performed measurements, with a more blueshifted emission originating from the methylated MeTzPhTpy with respect to HTzPhTpy and HTzPyTpy. The emission profile of MeTzPyTpy appears more complex, with the presence of multiple maxima. However, notably, MeTzPyTpy exhibits reduced solubility in aqueous solvent, compared with the other three species; hence, the spectral profile was measured under nonideal conditions (nontransparent solution).
The screen of fluorescence responses of the ligands to a wide range of biologically relevant metal ions revealed that most first-row transition-metal ions cause quenching of fluorescence (Figure 8). On the other hand, spectroscopically silent alkali and alkali-earth cations produce little change in the fluorescent emission of the ligands. In contrast, a unique response of each ligand to Zn2+ was observed. A Job's plot analysis of the emission intensity at 380 nm against the molar fraction of Zn2+ cations revealed a 2:1 ligand-to-metal binding stoichiometry in the case of MeTzPyTpy (Figure S6). This result suggests that the main mode of interaction is through coordination at the terpyridine site. A study of the other ligands was attempted through Job's plots, but the responses were less straightforward to interpret. Zn2+ induces a change in the spectral profile of HTzPhTpy, with the fluorescence maximum shifting from 410 to 480 nm (Figure 8a). By monitoring the ratio of intensity for the emission at 480 nm and 410 nm, it is possible to distinguish Zn2+ from all other investigated metal ions. Similarly, the addition of Zn2+ to HTzPyTpy induces a fluorescence increase, as well as the appearance of shoulders at 360 and 480 nm (Figure 8b), which can be used to identify the presence of Zn2+ over other metal ions. For MeTzPhTpy, addition of Zn2+ causes a marked ratiometric change, with a new emission maximum at 500 nm (Figure 8c). The integrated emission intensity at this new peak increases uniquely for Zn2+. The general appearance of redshifted peaks is analogous to the results obtained from protonation of the species and can be ascribed to the coordination of Zn2+ to the terpyridine moiety, therefore lowering the energy of delocalised π* orbitals. Finally, Zn2+ alone is found to induce increased emission of the 380 nm peak of MeTzPyTpy (Figure 8d).
These results demonstrate the potential of these ligands to act as selective sensors of Zn2+, whether alone or incorporated into more elaborate molecular systems. Furthermore, all ligands exhibit a ratiometric response to Zn2+, which is a particularly valuable mode of fluorescence sensing, as emission ratios are independent of fluorophore concentration and other background parameters.29 Zinc is an essential metal ion in biology, performing a plethora of roles, including structure stabilisation, Lewis acid catalysis,30 and putative roles in signalling.31 As a result, there is great interest in detecting this metal ion in cellular studies, and to this end, a number of Zn2+-selective sensors have been reported,32, 33 the majority of which are based on ligands bearing pyridyl coordinating motifs, consistent with the findings presented in this work.
A longstanding challenge in the fluorescence sensing of Zn2+ is the potential interference from Cd2+, which can induce similar fluorescence changes, due to its similar chemical nature.34 While basal cellular Cd2+ levels are not expected to be sufficiently high to interfere with Zn2+ sensing in biological studies, Cd2+ is itself a highly toxic metal, and the measurement of cellular Cd2+ levels in systems potentially subjected to Cd2+ exposure is therefore important.35 Furthermore, since cadmium is widely used in industry, its unambiguous detection in environmental samples (which are likely to contain considerable background Zn2+) is also essential. We therefore compared the responses of the ligands to Cd2+ and Zn2+ (Figure 9). When we monitored the response to Cd2+ according to the same parameters reported above, we found similar fluorescence changes to those observed for Zn2+ (Figure S7). However, Cd2+ induced unique spectral changes for HTzPhTpy, HTzPyTpy and MeTzPhTpy (Figure 9a–c). In contrast, MeTzPyTpy showed the same spectral form in the presence of both Zn2+ and Cd2+ (Figure 9d). For the former set of ligands, we have therefore been able to identify parameters that enable the unambiguous discrimination of Zn2+ from Cd2+. This unique feature of these ligands may enable the development of fluorescence assays for distinguishing Cd2+ from Zn2+ in complex mixtures, whether industrial, environmental or biological samples.
Sensitisation of Eu3+ and Yb3+ Luminescence
The capacity of the synthesised species to sensitise lanthanide luminescence was then explored. As a first step, the relative energy of the triplet 3π–π* excited states of the ligands was estimated by adding excess Gd3+ in acetonitrile solution.36 [Gd(NO3)3(DMSO)n] was chosen, due to its higher solubility in organic solvents.37 Figure 10 shows all four absorption profiles after the addition of Gd3+, highlighting the appearance of redshifted bands around 340 nm, which are ascribed to the complexation of the terpyridine moiety.4 The emission profiles of these solutions were measured after freezing to 77 K, to enhance the phosphorescence decay from the 3π–π* excited states. All of the complexes exhibit intense structured bands between 400 and 700 nm, with some residual peaks at wavelengths shorter than 400 nm attributed to residual fluorescence decay from 1π–π* excited states (Figure 10). From each of the phosphorescent emission bands, the energies of the 0–0 transitions could be estimated; the corresponding values are reported in Table 2. From these values, it can be seen that four species could be potential sensitisers for the visible red emission of Eu3+ and the NIR emission of Yb3+. For the latter, while the difference in energy between the 3π–π* and the Yb3+ 2F5/2 accepting state is above 10000 cm–1, a sensitisation mechanism involving a ligand-to-metal charge-transfer state with formation of Yb2+ could facilitate energy transfer.38-40
| Compound | E0–0 (3π–π*) | ΔE (3π–π* – 5D0) | ΔE (3π–π* – 7F5/2) |
|---|---|---|---|
| [cm–1] | [cm–1] | [cm–1] | |
| HTzPhTpy | 21052 | 3552 | 10652 |
| HTzPyTpy | 22222 | 4722 | 11822 |
| MeTzPhTpy | 21052 | 3552 | 10652 |
| MeTzPyTpy | 22000 | 4722 | 11822 |
The addition of [Eu(NO3)3(DMSO)n] to acetonitrile solutions of the four species showed the typical red emission upon excitation at 310 nm. The four emission spectra are virtually superimposable, demonstrating that the Eu3+ complexes in solution are very similar (Figure 11).41 The profiles are composed of the typical line-like peaks corresponding to the 5D0 → 7FJ (J = 0–4) transitions. No residual fluorescent or phosphorescent emission from the ligand was observed in the spectra. The 5D0 → 7F0 is visible and has a calculated full width at half maximum below 60 cm–1, again supporting the fact that the emissive species have a unique and similar coordination environment.41 The quantum yield values are in the 0.20–0.25 range. From the spectral profiles, the radiative decay (τr) values, reported in Table 3, can be calculated by means of the equation:
(τr)–1 = AMD,0 × n3 × (Itot/IMD)
where AMD,0 is a constant of value 14.65 s–1, n is the refractive index of the solvent and Itot/IMD is the ratio of the integrated intensity of the whole spectrum (Itot) versus the integrated intensity of the magnetic-dipole transition 5D0 → 7F1 (IMD). From the values of τr, the intrinsic quantum yield (Φi) can be calculated as the ratio of τ/τr and the sensitisation efficiency (η) can be found from the ratio of Φ/Φi. The data, reported in Table 3, indicate that all the four species are good sensitisers of Eu3+ luminescence, with efficiencies ranging from 0.48 to 0.63. These data are consistent with previously reported luminescence properties of Eu3+ centres bound to terpyridine-type ligands.11, 42
| Compound | Ln3+ | Emission (298 K) | |||||
|---|---|---|---|---|---|---|---|
| λ [nm] | τ [µs] | Φ | τr [µs] | Φ i | η | ||
| HTzPhTpy | Eu3+ | 618a | 1225 | 0.239 | 2802 | 0.437 | 0.545 |
| HTzPyTpy | Eu3+ | 618a | 1194 | 0.208 | 2749 | 0.434 | 0.478 |
| MeTzPhTpy | Eu3+ | 618a | 1087 | 0.249 | 2747 | 0.396 | 0.630 |
| MeTzPyTpy | Eu3+ | 618a | 1263 | 0.214 | 2806 | 0.450 | 0.476 |
| HTzPhTpy | Yb3+ | 980 | 10.07 | – | 1200b | 0.008 | – |
| HTzPyTpy | Yb3+ | 980 | 10.62 | – | 1200b | 0.009 | – |
| MeTzPhTpy | Yb3+ | 980 | 9.64 | – | 1200b | 0.008 | – |
| MeTzPyTpy | Yb3+ | 980 | 8.48 | – | 1200b | 0.007 | – |
- a Only the more intense maximum belonging to the hypersensitive 5D0 → 7F2 transition is reported.
- b Literature value.
The addition of [Yb(NO3)3(DMSO)n] to acetonitrile solutions of the four species also caused the quenching of the ligand-centred emission, with concomitant sensitisation of the Yb3+ luminescence (Figure 11). A broad band in the NIR region, with a characteristic peak at 980 nm, is evident in each case, and it is ascribed to the radiative decay occurring from the 2F5/2 → 2F7/2 transition. The excited-state lifetime decay is similar in all cases and it ranges between 8.48 and 10.62 µs, which are typical values for Yb3+ NIR emission. From these values, assuming a radiative decay for Yb3+ at 1200 µs, the intrinsic quantum yields of these complexes can be estimated to be just below 0.01. These values are typical for Yb3+ complexes sensitised by terpyridine ligands.42
Conclusion
Four new tetrazole-containing species were synthesised, characterised and investigated in relation to their photophysical properties. The species are composed of protonated or methylated tetrazole moieties conjugated to terpyridine functional groups and they are spaced by either a phenyl or pyridyl ring. The compounds are emissive in the UV–blue region of the spectrum and exhibit high (77–85 %) to moderate (28–32 %) quantum yields for the pronated and methylated species, respectively. Significant variations in the emission profiles of the methylated species could be detected upon addition of acids and bases, consistent with protonation of the terpyridine functional group, causing a lowering of the HOMO–LUMO gap. The remaining protonated tetrazole species also showed variations, albeit with more complicated trends, due to the added protonation site on the tetrazolate ring. The species were studied as sensors for important biological and environmental metal cations, highlighting a selective and ratiometric response to Zn2+ compared with other transition-metal, alkali and alkali-earth cations. Furthermore, the response of the species to the detection of Zn2+ could be well discriminated from Cd2+ cations. Lastly, the synthesised species were investigated as sensitisers for lanthanide luminescence, highlighting efficient sensitisation for red-emitting Eu3+ and near-infrared-emitting Yb3+ cations. This work has highlighted terpyridine-functionalised tetrazole species as very versatile molecules with useful luminescence properties and potential application in fields such as sensing and optical materials.
Experimental Section
General Considerations: All reagents and solvents were purchased from Sigma Aldrich and were used as received, without further purification. The [Ln(NO3)3(DMSO)n] (Ln3+ = Gd3+, Eu3+, Yb3+),37 NCPhTpy22 and NCPyCHO26 precursors were prepared according to previously published procedures. Nuclear magnetic resonance spectra, consisting of 1H NMR and 13C NMR, were recorded with a Bruker Avance 400 spectrometer (400.1 MHz for 1H, 100 MHz for 13C) at 300 K. The 1H NMR and 13C NMR chemical shifts were referenced to residual solvent signals. Infrared spectra were recorded in the solid state, using an attenuated total reflectance Perkin–Elmer Spectrum 100 FTIR, equipped with a diamond stage. Compounds were scanned from 4000 to 650 cm–1. The intensities of the IR spectroscopic bands are reported as strong (s), medium (m) or weak (w). Melting points were determined with a BI Barnstead Electrothermal 9100 apparatus. Elemental analyses were obtained at the Central Science Laboratory, University of Tasmania, using a Thermo Finnigan EA 1112 Series Flash.
Synthetic Details
Synthesis of HTzPhTpy: Triethylamine (0.8 mL, 5.8 mmol) and HCl (0.70 mL, 5.8 mmol) were combined with toluene (50 mL) in an ice bath and were stirred until fuming subsided. NCPhTpy (1.93 g, 5.8 mmol) and NaN3 (0.4 g, 5.8 mmol) were added and the mixture was stirred overnight at reflux. The solvent was evaporated under reduced pressure and the solid was recrystallised from water (50 mL) and then washed with (3× 20 mL) DCM. Yield: 0.750 g (35 %). M.p. 280 °C dec. IR: ν̃ν̃ = 3046 (m), 2444 (m), 1683 (w), 1605 (w), 1593 (s), 1581 (s), 1542 (s), 1470 (m), 1393 (s), 1295 (w), 1113 (w), 1076 (w), 1008 (s), 890 (w), 842 (w), 793 (s) cm–1. 1H NMR ([D6]DMSO): δ = 8.78–8.75 (m, 4 H), 8.66 (d, J = 8.1 Hz, 2 H), 8.26 (d, J = 8.8 Hz, 2 H), 8.02 (app t, J = 8.6 Hz, 2 H), 7.97 (d, J = 8.4 Hz, 2 H), 7.54–7.49 (m, 2 H) ppm. 13C NMR ([D6]DMSO): δ = 160.3 (N4C), 155.7, 155.0, 149.4, 149.2, 137.4, 135.8, 133.6, 127.0, 126.7, 124.5, 120.9, 117.6 ppm. HTzPhTpy·0.2(H2O) (377.41): calcd. C 69.35, H 4.02, N 25.73; found C 69.54, H 3.85, N 25.60.
Synthesis of HTzPyCHO:, Triethylamine (0.52 mL, 3.7 mmol) and HCl (0.41 mL, 3.7 mmol) were combined with toluene (40 mL) in an ice bath and stirred until fuming subsided. Reagents 2-cyanopyridine-5-carboxyalhedyde (0.5 g, 3.7 mmol) and NaN3 (0.24 g, 3.7 mmol) were added and the mixture was stirred overnight at reflux. The mixture was extracted with water (30 mL), the aqueous phase was acidified and the formed precipitate was filtered. The remaining aqueous solution was extracted with ethyl acetate (3× 15 mL). The organic phase was collected and dried with magnesium sulfate and the solvent was removed under reduced pressure, giving an off-white solid. Yield: 0.32 g (40 %). M.p. 220 °C dec. IR: ν̃ = 2592 (m br.), 1705 (s), 1605 (s), 1575 (m), 1370 (m), 1263 (m), 1205 (w), 1178 (w), 1116 (w), 1039 (w), 1016 (w), 845 (w) cm–1. 1H NMR ([D6]DMSO): δ = 10.20 (s, 1 H, CHO), 9.27 (s, 1 H), 8.50 (d, J = 6.4 Hz, 1 H), 8.42 (d, J = 8.2 Hz, 1 H) ppm. A peak at 1.90 indicated the presence of acetic acid, probably resulting from the extraction of the acidic aqueous solution with ethyl acetate. 13C NMR ([D6]DMSO): δ = 191.8 (CHO), 154.6 (N4C), 151.7, 147.6, 138.3, 132.3, 123.0 ppm. HTzPyCHO·0.1(CH3COOH) (175.15): calcd. C 47.74, H 3.00, N 38.66; found C 47.49, H 2.76, N 38.76.
Synthesis of HTzPyTpy: The compound 2-acetylpyridine (0.37 mL, 3.32 mmol) was added to a solution of HTzPyCHO (0.2 g, 1.1 mmol) in EtOH (20 mL). KOH pellets (0.2 g, 3.3 mmol) and NH4OH (28.0–30.0 % NH3 aqueous solution, 0.5 mL, 3.3 mmol) were then added to the solution, which was stirred overnight at room temp. The mixture was filtered and the solid was left to dry under vacuum for 5 min. The solid was then dissolved in minimal water. The water was acidified to yield an off-white solid, which was filtered and dried in air. Yield: 0.13 g (40 %). M.p. 282 °C dec. IR: ν̃ = 3370 (m), 2701 (m), 1618 (w), 1597 (s), 1563 (w), 1530 (s), 1497 (w), 1435 (w), 1338 (w), 1300 (w), 1278 (w), 1239 (w), 1172 (w), 1007 (w), 993 (w), 791 (w) cm–1. 1H NMR ([D6]DMSO): δ = 9.18 (s, 1 H), 8.79–8.78 (m, 4 H), 8.68 (d, J = 8.2 Hz, 2 H), 8.39 (d, J = 8.2 Hz, 1 H), 8.24 (d, J = 8.4 Hz, 1 H), 8.05 (app t, J = 7.8 Hz, 2 H), 7.57–7.53 (m, 2 H) ppm. 13C NMR ([D6]DMSO): δ = 161.1 (N4C), 156.0, 155.0, 151.8, 149.5, 147.6, 146.9, 137.8, 135.3, 131.6, 124.8, 121.9, 121.2, 118.0 ppm. This compound was found to be very hygroscopic and reproducible elemental analysis could not be obtained.
Synthesis of MeTzPhTpy: HTzPhTpy (0.25 g, 0.66 mmol) was combined with K2CO3 in acetonitrile (15 mL) and stirred for 1 min. CH3I (44 µL, 0.66 mmol) was added and the mixture was stirred overnight at reflux. The mixture was filtered and the solvent was removed under reduced pressure. The leftover solid was loaded onto Brockmann II basic alumina and eluted with DCM. The first fraction was recovered and the target compound was obtained as a white solid after removal of the solvent. Yield: 0.075 g (30 %). M.p. 227 °C dec. IR: ν̃ = 3062 (w), 1601 (w), 1582 (w), 1565 (m), 1543 (m), 1479 (m), 1465 (m), 1423 (m), 1388 (w), 1263 (w), 1139 (w), 918 (w), 845 cm–1. 1H NMR (CDCl3): δ = 8.79 (s, 2 H), 8.74 (d, J = 8.1 Hz, 2 H), 8.68 (d, J = 8.1 Hz, 2 H), 8.28 (d, J = 8.4 Hz, 2 H), 8.04 (d, J = 8.4 Hz, 2 H), 7.92–7.87 (m, J = 2.4 Hz, 2 H), 7.39–7.35 (m, 2 H), 5.29 (s, 3 H) ppm. 13C NMR (CDCl3): δ = 165.0 (N4C) 156.2, 156.1, 149.5, 149.2, 140.4, 137.1, 128.0, 128.0, 127.5, 124.1, 121.6, 119.0, 39.7 (CH3) ppm. MeTzPhTpy·0.2(CH2Cl2) (391.44): calcd. C 68.23, H 4.29, N 24.01; found C 68.27, H 3.98, N 24.14.
Synthesis of MeTzPybCHO: HTzPyCHO (0.45 g, 2.5 mmol) was combined with K2CO3 in acetonitrile (25 mL) and stirred for 1 min. CH3I (0.20 mL, 3 mmol) was added and the mixture was stirred at reflux overnight. The mixture was filtered and the solvent was removed under reduced pressure. The solid was loaded onto silica and eluted with a 1:1 mixture of hexane and ethyl acetate. The first fraction was recovered and the target compound was obtained as a white solid after removal of the solvent. Yield: 0.157 g (33 %). M.p. 178–179 °C. IR: ν̃ = 2865, 1692 (s), 1677 (s), 1592 (s), 1525 (w), 1443 (w), 1391 (m), 1206 (m), 1051 (m), 840 (m) cm–1. 1H NMR (CDCl3): δ = 10.18 (s, 1 H, CHO), 9.20 (s, 1 H, H6), 8.41 (d, J = 6.4 Hz, 1 H, H3), 8.33 (d, J = 6.4 Hz, 1 H, H4), 4.48 (s, 3 H, CH3) ppm. 13C NMR (CDCl3): δ = 190.1 (CHO), 164.2 (N4C), 152.5, 151.2, 137.3, 131.9, 122.7, 40.0 (CH3) ppm. MeTzPyCHO·0.1(CH3CO2CH2CH3) (189.18): calcd. C 50.90, H 3.89, N 35.91; found C 51.27, H 3.96, N 35.65.
Synthesis of MeTzPyTpy: The compound 2-acetylpyridine (0.2 mL, 1.6 mmol) was added to a solution of MeTzPyCHO (0.15 g, 0.8 mmol) in EtOH (10 mL). KOH pellets (0.1 g, 1.8 mmol) and NH4OH(aq.) (28.0–30.0 % NH3 aqueous solution, 0.24 mL, 2.2 mmol) were then added to the solution, which was stirred overnight at room temperature. The mixture was filtered and the solid was washed with EtOH (4 mL) and dried in air. Yield: 0.1 g (31 %). M.p. 273 °C dec. IR: ν̃ = 3054 (w), 1603 (m), 1585 (s), 1566 (s), 1467 (s), 1440 (w), 1404 (s), 1120 (w), 1041 (w), 1015 (w), 836 (m) cm–1. 1H NMR (CDCl3): δ = 9.26 (s, 1 H), 8.79 (s, 2 H), 8.74 (d, J = 4.4 Hz, 2 H), 8.68 (d, J = 8.2 Hz, 2 H), 8.41–8.34 (m, 2 H), 7.92–7.88 (m, 2 H), 7.40–7.37 (m, 2 H), 4.49 (s, 3 H) ppm. 13C NMR (CDCl3): δ = 164.8 (N4C) 156.5, 155.9, 149.3, 149.0, 147.1, 146.7, 137.2, 136.0, 135.6, 124.3, 122.5, 121.6, 119.0, 39.9 (CH3) ppm. MeTzPyTpy·(CH2Cl2) (392.15): calcd. C 63.76, H 3.99, N 26.63; found C 63.15, H 3.47, N 26.59.
Fluorescence Sensing of Metal Ions: The fluorescence properties of the ligands and their response to metal ions were analysed for solutions of the ligands (10 µm) in HEPES buffer (20 mm, pH 7.4) containing 1 % DMSO (HTzPhTpy, HTzPyTpy and MeTzPhTpy) or 5 % DMSO and 0.5 % of acetic acid (MeTzPyTpy). To enable high-throughput screening of the fluorescent response, the emission spectra for all the ligands alone, and in the presence of the metal ions (10 equiv.), were collected with a PerkinElmer Enspire Plate Reader at the same settings (λexc = 290 nm, emission from 310 nm to 560 nm with a 1 nm step size and 100 flashes per each sample).
Acknowledgements
This work was supported by the Australian Research Council (FT130100033 to M. M., DP150100649 to E. J. N.). P. J. W. wishes to thank the Curtin University for the Australian Postgraduate Award. E. J. N. thanks the Westpac Bicentennial Foundation for a Research Fellowship.
