Kinetically Stable Lanthanide Complexes Displaying Exceptionally

Article pubs.acs.org/IC

Kinetically Stable Lanthanide Complexes Displaying Exceptionally High Quantum Yields upon Long-Wavelength Excitation: Synthesis, Photophysical Properties, and Solution Speciation Jack D. Routledge, Michael W. Jones, Stephen Faulkner, and Manuel Tropiano* University of Oxford, Chemistry Research Laboratory, 12 Mansfield Road, Oxford OX1 3TA, United Kingdom S Supporting Information *

ABSTRACT: We demonstrate how highly emissive, kinetically stable complexes can be prepared using the macrocyclic scaffold of DO3A bearing coordinating aryl ketones as highly effective sensitizing chromophores. In the europium complexes, high quantum yields (up to 18% in water) can be combined with long-wavelength excitation (370 nm). The behavior in solution upon variation of pH, studied by means of UV−vis absorption, emission, and NMR spectroscopies, reveals that the nature of the chromophore can give rise to pH-dependent behavior as a consequence of deprotonation adjacent to the carbonyl group. Knowledge of the molecular speciation in solution is therefore critical when assessing the luminescence properties of such complexes.

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INTRODUCTION Lanthanide-containing compounds find application in magnetic resonance imaging (MRI), single-molecule magnets, and optical imaging as luminescent probes for fluoroimmunoassays and light microscopy.1 The luminescence properties displayed by almost all lanthanide ions are due to the forbidden nature of f−f transitions, while the narrow emission lines are a consequence of the core-like nature of f-type orbitals. These properties result in lanthanide complexes that show long-lived luminescence and distinctive emission profiles that have often been used for multiplexed probes through the formation of heteromultimetallic compounds.2 Given the very low molar absorption coefficients for direct excitation of the trivalent metal ions, sensitized luminescence using a light-harvesting “antenna” chromophore is often exploited. Organic chromophores are typically used, although transition metal complexes have been widely exploited over the past few years.3 The overall efficiency of this stepwise energy transfer process determines the quantum yield of a lanthanide complex. In particular, the lanthanide’s intrinsic quantum yield and the sensitization efficiency are two important parameters to consider in the design of highly emissive lanthanide compounds, since the overall quantum yield will be the product of the quantum yields for the individual steps in the energy transfer cascade from the excited singlet state.4 The intrinsic quantum yield of emission from a complex can be enhanced by elimination of quenching pathways, for instance, by reducing the number of nearby X−H oscillators or by using low-symmetry lanthanide centers to reduce the radiative lifetime of the lanthanide itself.5 Such an approach has favored the use of nonadentate or bulky octadentate ligands that exclude solvent from the inner © 2015 American Chemical Society

coordination sphere while also increasing the overall kinetic stability.6 The sensitization efficiency can be increased by improving the efficiency of the energy transfer between the chromophore and the excited state of the lanthanide ion. This can be achieved by fine tuning the energy overlap between the excited states involved in the process, which translates in the design of organic chromophores with optimal photophysical properties for the lanthanide ion of interest. In addition, given the distance dependence of the Förster and Dexter energy transfer processes, the proximity of the chromophore and lanthanide ion is also an important parameter to consider, with the Dexter mechanism typically operating at short distances while the Forster mechanism dominates over a much longer range.7 Triplet-mediated energy transfer generally dominates the energy transfer cascade, though Yb(III) luminescence has been demonstrated to occur via a double electron transfer mechanism when such a mechanism is thermodynamically feasible.8 The mechanistic dichotomy for ytterbium also has consequences for europium complexes in which the LMCT state competes with europium-centered luminescence, reducing the overall quantum yield. In the Eu(III) case, a clear correlation between the energy gap from the triplet state of the ligand to the emissive state of the lanthanide ion and the quantum yield was indeed observed, with an optimal energy gap for Eu(III) found to be ca. 2000−5000 cm−1 above the 5Do Eu(III) state located at 17 200 cm−1.9 Given the energy requirements needed for lanthanide sensitization, the preparaReceived: December 20, 2014 Published: March 9, 2015 3337

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Inorganic Chemistry Scheme 1. Synthetic Route toward the Preparation of Complexes Ln.L1−3 Studied in This Worka

a

Reagents and conditions: (a) K2CO3, MeCN, RT, 18 h. (b) TFA, DCM, RT, 48 h. (c) Ln(OTf)3, MeOH, 60 °C, 48 h.

development of imaging methods for millisecond lifetimes imaging using commercially available microscopes has recently been demonstrated, thus adding importance to the development of visible and, in the near future, NIR long-lived luminescent probes.12 We have undertaken a different approach to prepare highly emissive Eu(III) complexes based on the use of chromophores containing a ketone moiety. Here, the small singlet−triplet energy gap allows long excitation wavelengths and increases the sensitization efficiency as a result of the high yield of triplet formation.13 A similar strategy has been used previously affording visible light sensitization of Eu(III) albeit with a low emission quantum yield, while we have recently shown that azidophenacyl DO3A derivatives can be used to exploit these principles to generate sulfide-mediated switching of lanthanide luminescence, through reduction of the azide group.10b,14 Here we report on the synthesis and photophysical study of highly emissive, kinetically stable Eu(III) complexes that

tion of highly emissive Eu(III) complexes, particularly those that can be excited with near visible light, can be very challenging, with indeed only a few examples present in the literature.10 In particular, in the case of highly emissive complexes of potential use for light microscopy or, in general, for fluorescence-based assays, kinetic stability is an essential parameter to be considered during the molecular design and aqueous solubility is obviously advantageous. Recent developments in this field have afforded bright, nine-coordinate Eu(III) complexes based on the triazacyclononane macrocycle, where careful design of the organic antenna yielded highly emissive, long-lived Eu(III)-based luminophores that are ideally suited for light microscopy, with the added advantage of very low photobleaching compared to commercially available organic dyes.11 In this case, the exclusion of solvent from the metal coordination sphere and the rigidity of the ligand together with a highly efficient sensitization process are essential for the strong emission displayed by these compounds. In addition, the 3338

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four axial ring proton signals between 25 and 40 ppm suggest that the square antiprismatic (SAP) geometry is predominant in solution, with the twisted square antiprismatic (TSAP) diastereoisomer present in much lower abundance (less than 20%), giving rise to a broader, less shifted set of resonances. This is not observed in the case of Yb.L1, where the SAP isomer dominates with no detectable TSAP isomer by NMR, hence displaying a behavior similar to that reported for Ln(III) DOTA complexes.15 From these spectra it is clear that the oxygen atom of the ketone carbonyl is coordinated to the Ln(III) ion, similar to previously reported complexes based on octadentate DO3A ligands, including DOTA monoamide, triazolyl, and triazole DO3A complexes.16 The 1H NMR spectra of Eu.L2, Eu.L3 and Yb.L2, Yb.L3 show similar behavior, indicating that the metal ion is encapsulated in the cavity of an octadentate ligand with one bound water molecule to complete the coordination sphere of the metal. The SAP isomer dominates with both ligands, with a minor TSAP isomer present in Eu.L3 but not in Yb.L3, as expected from previous findings with an analogous acetophenone derivative.17

incorporate arylketone chromophores close to the metal center, with long sensitization wavelengths (up to 370 nm and extending into the visible) and quantum yields as high as 18% in water. The analogous complexes of Gd(III), Lu(III), and Yb(III) have also been prepared and used to obtain an in-depth understanding of the photophysical properties and of the speciation of this class of compounds in solution.

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RESULTS AND DISCUSSION In order to ensure high kinetic and thermodynamic stability of the target metal complexes, the rigid macrocyclic scaffold based on the triacid derivative of cyclen (DO3A) was employed. First, the triester of DO3A was obtained in good yield following trialkylation of cyclen using tert-butyl bromoacetate as previously reported.10a The synthesis of precursor S3 involved Friedel−Crafts acylation of carbazole using chloroacetyl chloride in the presence of a Lewis-acid catalyst. A crystal structure of this molecule is reported in the Supporting Information. In the case of the phenanthrene derivative, S5, bromination of 3-acetylphenanthrene afforded 2-bromo-3′acetophenanthrone (S5), while the naphthalene derivative, S1, is commercially available (see the Supporting Information for details). The pro-ligands were obtained by alkylation of triester (the tris-tert-butyl ester of DO3A) with the corresponding chromophores (see Scheme 1). Deprotection of the tert-butyl esters was achieved over 48 h in acidic conditions using TFA. The lanthanide complexes of Eu(III), Gd(III), Yb(III), and Lu(III) were obtained upon reaction of the ligands with the corresponding lanthanide triflates in methanol. Semipreparative HPLC was carried out in order to obtain analytically pure samples. All of the complexes were readily soluble in water up to millimolar concentrations, and all of the characterization carried out in this work was conducted in aqueous solution. The 1H NMR spectrum of Eu.L1 is shown in Figure 1. Due to the presence of the paramagnetic ion, the ethylene protons

Figure 2. 1H NMR spectrum of Yb.L1 (D2O, pH 4, 298 K).

Photophysical Properties. The electronic absorption and emission spectra of the Eu(III) complexes are reported in Figure 3, while the main photophysical properties of Eu.L1−3 are reported in Table 1. All compounds display strong absorption in the UV region, with maxima of the lowest energy bands between 346 and 369 nm. The values of the molar absorption coefficient displayed by the complexes (see Table 1) suggest that these transitions are π−π* in nature with the n−π* transition centered on the carbonyl group presumably subsumed under the intense π−π* band in each complex.13 In particular, Eu.L2 displays an absorption spectrum that extends well into the visible. Intense Eu(III) luminescence was observed for all compounds upon excitation into the lowest energy bands. Indeed, in the case of Eu.L1 and Eu.L3, emission quantum yields of, respectively, 0.18 and 0.16 were observed in H2O, among the highest reported for kinetically stable Eu(III) complexes in aqueous solution.11b,18 These are exceptional values for a Eu(III) complex, particularly when considering the octadentate nature of the ligand employed and the long excitation wavelength used. These values increased to 0.48 and 0.40 for Eu.L1 and Eu.L3, respectively, when measured in deuterium oxide, where the

Figure 1. 1H NMR spectrum of Eu.L1 (D2O, pH 4, 298 K).

on the macrocycle and on the arms display a substantial shift, giving rise to resonances ranging from −20 to +40 ppm. Of the expected 24 signals, the spectrum displays 21 resonances outside the 0−10 ppm region, with the remaining 3 signals presumably subsumed under the unshifted resonances of the protons on the aromatic group. In the 1H NMR spectrum of Yb.L1, where the lanthanide-induced shift is more significant, all 24 nonequivalent resonances are clearly resolved. In Eu.L1 the 3339

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Figure 3. Absorption and emission spectra of Eu.L1−3 upon excitation into the lower energy band (H2O, pH 7, 295 K, 1 nm emission slits). Emission spectra are normalized to the intensity of the ΔJ0 band.

Table 1. Photophysical Properties of Eu.L1−3 Measured at T = 298 K (5 < pH < 8) λex (nm) τH2O (ms)a τD2O (ms)a q ΦH2Ob ΦD2Ob ε (M−1 cm−1) a

Eu.L1

Eu.L2

Eu.L3

362 0.58 1.44 0.9 0.18 0.48 5100

369 0.52 1.32 1.1 0.01 0.03 15 600

346 0.55 1.59 1.1 0.16 0.40 19 600

Uncertainty ±10%. bUncertainty ±15%.

absence of O−H oscillators eliminates the main quenching pathway of luminescence from the first coordination sphere of the lanthanide ion. It is worth noting that, despite the low quantum yield value of Eu.L2, which will be discussed in detail below, the absorption profile of the chromophore employed makes it amenable to excitation in the visible range using wavelengths of up to 450 nm. Measurement of the excited state lifetimes in water and deuterium oxide allowed the determination of the number of water molecules directly coordinated to the metal using the modified Horrocks’ equation5a

Figure 4. Phosphorescence spectra of Gd.L1−3 measured at 77 K in EtOH (excitation at 360, 370, and 345 nm; delay time 0.5 ms; gate time 2 ms; 5 nm emission slits).

profile was found to be independent of wavelengths of excitation with emission lifetimes of 40, 5, and 21 ms for Gd.L1, Gd.L2, and Gd.L3 respectively, thus suggesting that phosphorescence originates from a 3(n−π*) excited state for all Gd(III) complexes (as opposed to thousands of milliseconds for 3(π−π*) emission). While Gd.L1 and Gd.L3 show similar phosphorescence profiles with clear vibronic structure and 0−0 transition energies of 19 900 and 19 500 cm−1, respectively, the phosphorescence spectrum of Gd.L2 is significantly different, with the 0−0 transition located at 21 300 cm−1. From these values we can deduce that while for complexes Eu.L1 and Eu.L3 there is an ideal energy overlap between the antenna chromophore and the 5D0 state of Eu(III) at 17 240 cm−1, the same is not true for Eu.L2, where the energy difference is in excess of 4000 cm−1. This observation explains the different quantum yield values recorded for Eu.L1−3 and clearly demonstrates the major role donor−acceptor energy overlap plays in determining the ligand-to-metal energy transfer efficiency and, consequently, the overall quantum yield of luminescence. Analysis of the emission profiles of Eu.L1−3 (Figure 3) highlights the low-symmetry environment of the metal ions, reflected in three distinct peaks in the 5D0−7F1 transition. In particular, the fine splitting of this band is indicative of an almost identical coordination for Eu.L1 and Eu.L3, while the somewhat different emission profile of Eu.L2 can be ascribed to a different ligand field experienced by the metal ion in this

qEu = 1.2(kH − kD − 0.25)

The hydration number was found to be close to one for all Eu(III) complexes, thus confirming the coordination of the chromophore carbonyl to the lanthanide ion as observed by 1H NMR, with a single solvent molecule coordinating the metal in the ninth axial position. While direct coordination of the chromophore to the metal ion is certainly a prerequisite for ensuring high sensitization efficiency, a good energy overlap between donor and acceptor states is also a key factor for strong overall emission.19 Given that the energies of the accepting states for lanthanide ions are known, the energy gap can be estimated from the determination of the energy of the chromophore-centered triplet state. Thus, the synthesis of the “model” compounds Gd.L1−3 was also undertaken. The 6P7/2 excited state of Gd(III) lies higher in energy (E = 31 750 cm−1) than the ligands, thus preventing ligand-to-metal energy transfer. As a result, the observed luminescence in Gd.L1−3 is to be ascribed solely to the organic chromophore. Indeed, the Gd(III) ion enhances spin−orbit coupling but does not quench the luminescence of the triplet state of the ligand.20 The phosphorescence spectra obtained from organic glasses of Gd.L1−3 at 77 K are shown in Figure 4. Each phosphorescence 3340

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ties displayed by the Eu(III) complexes, in particular, and hence their potential for widespread use in microscopy or fluorescence assays, it was deemed important to fully understand their behavior in solution. In particular, due to the presence of a ketone moiety, we were interested in understanding their acid−base properties, especially considering its direct coordination to the electron-withdrawing metal ion. Indeed, we observed deprotonation of the carbon in the α position in basic solutions to form the corresponding enolate anion (see Scheme 2).

complex, likely due to the strong electron-donating ability of the carbazole chromophore employed. Additionally, due to the hypersensitivity of this transition, the ΔJ2 band of Eu.L2 also displays a relatively stronger emission compared to Eu.L1 and Eu.L3. The only excited state of Yb(III) at ca. 10 200 cm−1 is amenable to sensitization from the excited states of all chromophores used in this work. Indeed, the characteristic near-infrared luminescence from Yb(III) was observed in all compounds upon excitation into the π−π* transition centered on the ligand. All of the emission spectra feature a distinctive profile that is common to all Yb(III) complexes reported here (see Figure 5), suggesting a similar ligand field around the

Scheme 2. Acid Base Properties of Ln.L1

In the case of Eu.L1 the equilibrium was followed by UV−vis absorption and luminescence spectroscopies, monitoring the emission of Eu(III) upon increasing the pH of the solution (Figure 6), revealing a pKa of 10.5 ± 0.1 (0.1 M NaCl, 295 K). In this case, only the ketone form is emissive with a gradual decrease of emission intensity observed upon formation of the enolate species [Eu.L1]−. This hypothesis was confirmed by the unchanged emission and excitation profiles (see Supporting Information) and lifetime of the solution from pH 4 to 12, indicating that there is indeed only one emissive species in this pH range, namely, Eu.L1, and that the enolate form [Eu.L1]− is not emissive, most likely due to the formation of an LMCT state that quenches the 5D0 state of Eu(III). This is, together with O−H oscillator overtones, one of the main quenching pathways of Eu(III) emission.8 A similar behavior was observed for Eu.L3 with a pKa of 10.2 ± 0.1 determined by both UV−vis absorption and emission spectroscopies (see Supporting Information). In this case, the marginally lower pKa value observed is likely to be a consequence of the extended aromaticity of this system and hence the higher stabilization of the negative charge formed upon deprotonation. Interestingly, the absorption and emission spectra of Eu.L2 did not show any significant changes upon increasing the pH of the solution (see Supporting Information for spectra), consistent with the electron-donating properties of the N-ethyl group on the carbazole chromophore, with a less stabilized deprotonated form and a consequently lower acidity, with deprotonation not observed in the pH range investigated. Additionally, at high pH values the speciation in solution of Eu.L2 as revealed by 1H NMR studies remained unchanged (see below for discussion of pH-dependent NMR studies). The relatively low value of pKa observed for Eu.L1 and Eu.L3 confirms the role played by the Lewis-acidic lanthanide ion in enhancing the acidity of the ketone CH2 through its electronwithdrawing ability upon direct coordination to the carbonyl group.22 It is worth noticing that in both Eu.L1 and Eu.L3 the profile of the emission spectra and the luminescence lifetimes did not change upon increasing pH. This is indicative of a single Eu(III) emissive species in solution and suggests that the newly formed enolate is not luminescent, likely due to the formation of a LMCT state lower in energy than the emissive 5 D0 state of Eu(III). Indeed, the greatly different ligand field

Figure 5. Emission spectra of Yb.L1−3 recorded in D2O (5 < pH < 8.5; λex = 370 nm for Yb.L2, 320 nm for Yb.L1 and Yb.L3; 5 nm emission slits).

lanthanide ion in all cases, probably due to the stronger electrostatic interactions between Yb(III) and the ketone carbonyl relative to the Eu(III) case as a consequence of the lanthanide contraction. Emission lifetimes of the 2F5/2−2F7/2 transition at 980 nm were determined by pulsed excitation at 337 nm. The values of the lifetimes obtained were used to derive the hydration number of the metal ion using the modified Horrock’s equation for Yb(III) qYb = (kH − kD − 0.1)

The hydration values obtained (see Table 2) are in line with the ones determined for the Eu.L1−3 complexes, although generally Table 2. Emission Lifetimes of Yb.L1−3 a pH < 7.5

Yb.L1 Yb.L2 Yb.L3 a

pH > 11

τH2O (μs)

τD2O (μs)

q

τH2O (μs)

τD2O (μs)

q

1.23 1.01 1.00

7.70 8.26 8.04

0.6 0.8 0.8

1.67 1.02 1.50

7.72 8.14 7.69

0.4 0.8 0.4

Experimental uncertainties on lifetimes are ±10%.

lower, consistent with the lanthanide contraction and the change in structure across the lanthanide series upon decreasing ionic radius, as also observed in the NMR solution studies discussed earlier.21 pH-Dependent Behavior in Solution. Given the remarkable emission quantum yields and photophysical proper3341

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Figure 6. UV−vis absorption (left) and emission spectra (right) of Eu.L1 upon increasing the pH of the solution (8.9 × 10−6 M, 0.1 M NaCl(aq), 295 K).

lifetimes, likely as a consequence of the electron-donating properties of the carbazole group (see Supporting Information for spectra of Yb.L2). The pKa found for the Yb(III) complexes did not differ significantly from the corresponding Eu(III) complexes displaying values of 10.5 ± 0.1 and 10.3 ± 0.1 for Yb.L1 and Yb.L3, respectively (in 0.1 M NaCl, 295 K, see Supporting Information for spectra). In order to support these observations, the speciation in solution was further studied by 1H NMR spectroscopy both on a diamagnetic and on a paramagnetic complex. The complex Lu.L1 was chosen as a representative compound for the pHdependent solution studies, and its 1H NMR spectrum was registered at two different values of pH and over time. Upon increasing the pH from 5 to 12, the overall spectrum displayed significant differences in the aromatic and macrocyclic regions with a new single species dominating, combined with the disappearance of the proton resonances on the α carbon resulting from exchange with deuterium (see Supporting Information). This new spectrum was assigned to the deprotonated enolate species [Lu.L1]−. The slow proton exchange with the solvent, due to the keto−enol tautomerism, meant that, at pH 5, disappearance of the proton resonances on the α carbon could be observed, with the overall form of the remaining resonances left unchanged (see Supporting Information). Paramagnetic NMR lends itself to the study of the speciation in solution, particularly in cases where spectral resolution is required. In this instance, the complex Yb.L1 was used to follow the deuterium exchange of the proton alpha to the carbonyl of the ketone moiety. Indeed, the paramagnetically shifted 1H NMR spectrum of Yb.L1 shows two markedly different resonances attributed to the protons on the α carbon to the ketone carbonyl, where the difference in shift results from the different positions of the two nuclei with respect to the lanthanide center. Upon equilibration over 48 h of a solution of Yb.L1 in deuterium oxide at pH 5, gradual disappearance of the α protons was observed, as expected from deuterium exchange following keto−enol tautomerization (see Figure 8). Evidence of the formation of the enolate complexes at high pH was also observed in the 1H NMR spectra for the other paramagnetic lanthanide complexes Eu.L1, Eu.L3, and Yb.L3. Figure 9 shows the 1H NMR spectrum recorded for Eu.L1 at pH 12 where [Eu.L1]− species dominates, highlighting the very different solution structure for the enolate complex (compare with Figure 1), characterized by broad resonances assigned to the SAP isomer of [Eu.L1]− in intermediate exchange with the

(see NMR discussion below) would be reflected in a change of the emission spectrum and in particular of the splitting of the 5 D0−7F1 transition and lifetime values. We verified this hypothesis by studying the changes in the photophysical properties upon changing the pH in the analogous Yb(III) complexes, Yb.L1 and Yb.L3, where formation of a LMCT state would not lead to quenching of luminescence. Indeed, in the case of Yb(III), a double electron transfer has been demonstrated to be an alternative sensitization mechanism where the LMCT Yb(II) state acts as energy donor to the lower Yb(III) 2F5/2 state following MLCT back electron transfer.3,8 As expected, basification of a solution of Yb.L1 led to a drastic change in the emission profile with a new species dominating at more basic pH as shown in Figure 7. Here, the

Figure 7. Emission spectrum of Yb.L1 and [Yb.L1]− at pH 8 and 12.5, respectively, in D2O.

emission profile of the deprotonated species [Yb.L1]− is very different from Yb.L1 as a consequence of the change in the metal coordination brought about by deprotonation of the ligand.23 This is in contrast with the case of Eu(III) complexes, where only the ketone form is emissive. Measurements of the excited state lifetimes at 980 nm following pulsed excitation at 337 nm confirmed the change in the coordination with markedly longer emission lifetimes in basic solutions in H2O and a lower hydration number (see Table 2). As observed for Eu.L2, the analogue Yb.L2 complex did not show significant changes in UV−vis absorption and NIR emission spectra or 3342

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CONCLUSION A family of kinetically stable lanthanide complexes bearing a coordinating ketone-containing chromophore has been synthesized, and their photophysical and NMR properties in solution were characterized. In particular, we demonstrated that the use of aryl ketones as sensitizers afford highly emissive complexes when the chromophore is directly coordinated to the lanthanide. The Eu(III) and Yb(III) compounds show strong emission upon near UV light excitation with Eu.L1 and Eu.L3 displaying very high quantum yields in water (0.18 and 0.16, respectively). In addition, the direct coordination of the aryl chromophore to the lanthanide ion changes the pKa of the proton alpha to the ketone, allowing facile proton exchange. This behavior is highly dependent on the local structure and on the nature of the aryl chromophore, implying that the pHdependent behavior can be tuned by changing the electronic demands of the chromophore. The motif studied here therefore lends itself to exploitation as a pH probe. We are currently investigating prospects for the use of these compounds as luminescent and magnetic imaging probes.

Figure 8. Portion of the 1H NMR of Yb.L1 at pH 5 upon equilibration over 48 h at T = 298 K in D2O. Asterisk (*) indicates the resonances for the alpha protons.

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EXPERIMENTAL SECTION

General Procedures. Commercially available solvents and chemicals (including S1) were used without further purification unless otherwise stated. Cyclen was purchased from Chematech, lanthanide salts were purchased from Strem Chemicals, while solvents were purchased from Sigma-Aldrich. NMR spectra were recorded on a Bruker AVIII 400 or AVIII 500 spectrometer as specified. All chemical shift (δ) values are given in parts per million. Low-resolution mass spectra were recorded on a Waters Micromass LCT Premier XE spectrometer. Accurate masses were determined to four decimal places using Bruker μTOF and Micromass GCT spectrometers at the Chemistry Research Laboratory of the University of Oxford. A Hanna pH meter was used for pH measurements of a solution of complexes in water and deuterium oxide. In the case of deuterium oxide (particularly for NMR studies), the value reported here is the pH value given by the instrument with pD = pH + 0.45 as reported in the literature.24 Synthesis of S2. To a solution of 1,4,7-tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (triester) (0.98 g, 1.65 mmol) in anhydrous acetonitrile (50 mL) was added potassium carbonate (0.46 g, 3.29 mmol) followed by 2-bromo-2′-acetonaphthone (0.41 mg, 1.65 mmol). This suspension was left stirring at RT under nitrogen atmosphere for 16 h, monitoring the reaction using TLC (DCM/MeOH 9:1). The reaction mixture was filtered, and the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography (DCM/MeOH from 0% to 4% MeOH with the product eluting at 3%) to obtain a light yellow colored glassy solid (790 mg, 70%). 1H NMR (400 MHz; CDCl3): δ 8.09 (s, 1H), 7.99 (d, J = 4.0 Hz, 2H), 7.94−786 (m, 3H), 7.64−7.56 (m, 2H), 4.17 (s, br, 2H), 3.73−2.18 (m, br, 22H), 1.47 (s, 27H). 13C NMR (101 MHz; CDCl3): δ 199.5, 172.8, 135.7, 133.0, 132.3, 129.6, 129.4, 128.7, 128.5, 127.7, 126.9, 123.1, 81.94, 81.87, 60.3, 55.6, 52.9 (br), 50.6, 48.6 (br), 27.9. LR-ESMS: m/z calcd for [M + H]+ 683.4, found 683.5. HR-ESMS: m/z calcd for C38H59O7N4 683.436, found 683.438. Synthesis of S4. To a solution of 1,4,7-tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclododecane (triester) (0.180 g, 0.30 mmol) in anhydrous acetonitrile (25 mL), potassium carbonate was added (0.084 g, 0.61 mmol) followed by S3 (0.080 g, 0.30 mmol). This suspension was left stirring at RT under nitrogen for 18 h, monitoring the reaction using TLC (DCM/MeOH 9:1). The reaction mixture was filtered, and the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography (DCM/MeOH from 0% to 4% MeOH with the product eluting at 3%) to obtain a light yellow colored solid (155 mg, 69%). 1H NMR (400 MHz; CDCl3): δ 8.67 (s, 1H), 8.11 (d, J = 4.0

Figure 9. 1H NMR spectrum of [Eu.L1]− (top) and [Yb.L1]− (bottom) (D2O, pH 12, 298 K).

TSAP isomer. A similar behavior was also observed for the analogue Yb.L1 (Figure 9), Eu.L3, and Yb.L3 complexes (see Supporting Information) where the difference in the coordination environment resulted in an overall broadening of NMR resonances and increase in paramagnetic shift, while the 1H NMR of Eu.L2 and Yb.L2 remained unchanged. 3343

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Inorganic Chemistry Hz, 1H), 8.05 (d, J = 8 Hz, 1H), 7.55−7.50 (m, 1H), 7.46 (d, J = 8 Hz, 1H), 7.42 (d, J = 8 Hz, 1H), 7.31 (t, J = 8 Hz, 1H), 4.41 (m, 3H), 4.14−2.20 (m, br, 26 H), 1.46 (s, 18H), 1.38 (s, 9H). 13C NMR (101 MHz; CDCl3): δ 198.3, 172.8, 169.8, 143.0, 140.7, 121.2, 126.7, 125.7, 123.1, 122.7, 121.0, 120.5, 120.1, 109.2, 108.3, 81.9, 60.0, 55.8, 55.7, 53.5, 52.7 (br), 49.0 (br), 37.9, 27.9, 27.8, 13.8. LR-ESMS: m/z calcd for [M + Na]+ 772.5, found 772.5. HR-ESMS: m/z calcd for C42H63N5O723Na 772.460, found 772.462. Synthesis of S6. To a solution of S1 (0.211 g, 0.35 mmol) in dry acetonitrile (25 mL) was added potassium carbonate (0.100 g, 0.70 mmol) followed by S5 (0.106 g, 0.35 mmol). This suspension was left stirring at RT under nitrogen for 18 h, monitoring the reaction using TLC (DCM/MeOH 9:1). The reaction mixture was filtered, and the solvent was removed under reduced pressure. The crude product was purified by silica gel column chromatography (DCM/MeOH from 0% to 4% MeOH with the product eluting at 3%) to obtain a light yellow colored solid (85 mg, 33%). 1H NMR (400 MHz; CDCl3): δ 9.28 (s, 1H), 8.76 (d, J = 8 Hz, 1H), 8.06 (d, J = 8 Hz, 1H), 7.94 (d, J = 8 Hz, 2H), 7.88 (d, J = 8 Hz, 1H), 7.77−7.72 (m, 2H), 7.67 (t, J = 8 Hz, 1H), 4.25 (s, br, 2H), 3.40−2.22 (m, br, 22H), 1.47 (s, 18H), 1.26 (s, 9H). 13C NMR (101 MHz; CDCl3): δ 199.8, 173.1, 172.9, 135.3, 133.5, 132.2, 130.5, 130.0, 129.8, 129.1, 128.9, 127.4, 126.2, 124.6, 123.2, 122.6, 82.0, 81.9, 60.6, 55.9, 55.7, 53.4, 52.6 (br), 49.1 (br), 27.9, 27.8. LR-ESMS: m/z calcd for [M + Na]+ 755.4, found 755.4. HR-ESMS: m/z calcd for C42H60N4O723Na 755.435, found 755.435. Synthesis of H3L1. TFA (3 mL) was added dropwise to a solution of S2 (0.480 g, 0.70 mmol) in DCM (3 mL). The reaction was stirred at RT for 48 h, the solvents were removed under reduced pressure, and the product was obtained quantitatively from methanol by trituration with diethyl ether. 1H NMR (400 MHz; D2O): δ 8.54 (s, 1H), 8.06 (d, J = 8.0 Hz, 2H), 8.02−7.92 (m, 2H), 7.70 (t, J = 8.0 Hz, 1H), 7.63 (t, J = 8.0 Hz, 1H), 5.08 (s, br, 2H), 3.96−1.97 (m, br, 22H). 13C NMR (101 MHz; D2O): δ 163.3, 163.0, 162.6, 162.3, 135.8, 131.9, 129.7, 128.7, 127.8, 127.5, 123.0, 120.7, 117.8, 114.9, 112.0, 66.0, 58.7 (br), 53.5 (br), 51.4 (br), 48.4 (br). LR-ESMS: m/z calcd for [M + H]+ 515.3, found 515.8. HR-ESMS: m/z calcd for C26H35O7N4 515.250, found 515.249. Synthesis of H3L2. TFA (3 mL) was added dropwise to a solution of S4 (0.150 g, 0.20 mmol) in DCM (3 mL). The reaction was stirred at RT for 48 h, the solvents were removed under reduced pressure, and the product was obtained quantitatively from methanol by trituration with diethyl ether. 1H NMR (400 MHz; D2O): δ 8.49 (s, 1H), 8.02 (s, br, 1H), 7.85 (s, br, 1H), 7.40 (m, br, 3H), 7.23 (s, br, 1H), 4.18−3.19 (m, br, 29H). 13C NMR (101 MHz; D2O): δ 190.4 (br), 174.6, 174.1 (br), 169.1 (br), 140.0 (br), 140.1, 126.6, 125.8, 123.9 (br), 122.3, 122.0, 121.5, 120.5, 109.3 108.9, 59.5 (br), 55.1 (br), 53.6 (br), 53.2, 51.7 (br), 49.3, 48.1 (br), 47.8, 42.4, 37.4 (br), 34.4, 31.7, 13.2. LRESMS: m/z calcd for [M + H]+ 582.3, found 582.3. HR-ESMS: m/z calcd for C30H40O7N5 582.292, found 582.291. Synthesis of H3L3. TFA (2 mL) was added dropwise to a solution of S6 (0.53 g, 0.07 mmol) in DCM (2 mL). The reaction was stirred at RT for 48 h, the solvents were removed under reduced pressure, and the product was obtained quantitatively from methanol by trituration with diethyl ether. 1H NMR (400 MHz; D2O): δ 8.71 (s, br, 1H), 8.40 (s, br, 1H), 7.90−7.18 (m, br, 7H), 4.15−2.70 (m, br, 24H). 13C NMR (101 MHz; D2O): δ 174.3, 169.0 (br), 165.7, 135.3 (br), 131.6, 129.7, 128.8, 127.6, 126.0, 124.5, 122.6, 71.7, 69.6, 69.5, 69.4, 60.3, 55.1 (br), 53.4 (br), 53.1, 51.80, 51.4 (br), 48.8, 48.0 (br), 42.2. LR-ESMS: m/z calcd for [M + H]+ 565.3, found 565.3. HR-ESMS: m/z calcd for C30H36O7N423Na 587.24762, found 587.24554. General Method for Synthesis of Lanthanide Complexes. To a solution of the ligand in methanol the appropriate lanthanide triflate salt was added (1.05 mol equiv), and the reaction mixture was stirred at 40 °C for 30 min. The pH was adjusted to 4 by dropwise addition of an aqueous NaOH solution. The reaction was left stirring at 40 °C for 48 h. The methanol was removed under reduced pressure, leaving an oil that was dissolved in water. The pH of this solution was adjusted to 10 by addition of aqueous NaOH to remove excess lanthanide as its insoluble hydroxide. The resulting precipitate was centrifuged, and the supernatant was filtered. The solution was neutralized to pH 7 using

diluted hydrochloric acid, and the solvent was removed under reduced pressure to obtain the product as a white solid. Eu.L1. Yield: 89%. 1H NMR (500 MHz; D2O; pH 7): δ 37.6 (s), 29.8 (s), 28.4 (s), 27.5 (s), 10.4 (s), 8.7 (s), 8.2 (s), 8.0 (s), 7.8 (s), 7.6 (s), 5.6 (s), 4.2 (s), −1.9 (s), −3.3 (s), −3.9 (s), −4.7 (s), −5.9 (s), −8.1 (s), −10.6 (s), −11.2 (s), −13.2 (s), −13.6 (s), −14.1 (s), −14.6 (s), −17.4 (s), −17.6 (s), −19.7 (s). LR-ESMS: m/z calcd for [M + H]+ 665.15, found 665.14. HR-ESMS: m/z calcd for C26H30O7N4151Eu 661.131, found 661.132. UV−vis (H2O) λmax (ε/M−1 cm−1): 207 (42 000), 260 (66 300), 305 (28 200), 362 nm (5100). Lifetimes: 0.58 (H2O), 1.44 ms (D2O); q = 0.9. HPLC. tr: 10.2 min. Yb.L1. Yield: 95%. 1H NMR (500 MHz; D2O; pH 5): δ 134.1 (s), 105.8 (s), 99.5 (s), 93.0 (s), 48.1 (s), 37.2 (s), 19.4 (s), 18.4 (s), 15.2 (s), 14.8 (s), 11.6 (s), 9.9 (s), 9.5 (s), 9.1 (s), 8.6 (s), 8.5 (s), 8.1 (s), 5.6 (s), 3.3 (s), 0.3 (s), −5.3 (s), −20.7 (s), −22.2 (s), −42.1 (s), −48.1 (s), −61.7 (s), −62.0 (s), −62.7 (s), −63.2 (s), −65.6 (s), −74.0 (s). LR-ESMS: m/z calcd for [M + Na+] 707.14, found 708.2. HR-ESMS: m/z calcd for C26H30O7N4174Yb 684.151, found 684.151. UV−vis (H2O) λmax: 207, 260, 305, 363 nm. Lifetimes (4 < pH < 8): 1.23 (H2O), 7.70 μs (D2O); q = 0.6. Lu.L1. Yield: 92%. 1H NMR (400 MHz; D2O; pH 6): δ 8.77 (s), 7.99 (m, 4H), 7.72 (t, J = 8 Hz, 1H), 7.62 (t, 8 Hz, 1H), 4.95 (d, J = 16 Hz, 1H), 4.47 (d, J = 16 Hz, 1H), 3.66−3.24 (m, br, 10H), 2.96−2.39 (m, br, 12H). LR-ESMS: m/z calcd for [M + H]+ 687.17, found 687.21. HR-ESMS: m/z calcd for C26H30O7N4175Lu 687.167, found 687.166. UV−vis (H2O) λmax: 207, 227, 260, 305, 362 nm. Gd.L1. Yield: 91%. LR-ESMS: m/z calcd for [M + H]+ 670.15, found 670.21. HR-ESMS: m/z calcd for C30H31O7N4158Gd23Na 692.133, found 692.132. ET ≈ 19 900 cm−1. Eu.L2. Yield: 95%. 1H NMR (500 MHz; D2O; pH 7): δ 34.9 (s), 28.7 (s), 27.9 (s), 27.4 (s), 6.9 (s), 1.4 (s), 0.5 (s), −2.0 (s), −4.1 (s), −4.8 (s), −7.8 (s), −8.9 (s), −10.1 (s), −12.0 (s), −12.8 (s), −13.2 (s),-14.4 (s), −14.9 (s), −16.1 (s), −17.9 (s). LR-ESMS: m/z calcd for [M + H]+ 732.19, found 732.25. HR-ESMS: m/z calcd for C30H35O7N5153Eu 730.175, found 730.176. UV−vis (H2O) λmax (ε/ M−1 cm−1): 233 (21 700), 279 (22 600), 309 (16 800), 369 nm (15 600). Lifetimes: 0.52 (H2O), 1.04 ms (D2O); q = 0.9. HPLC tr: 11.4 min. Yb.L2. Yield: 95%. 1H NMR (500 MHz; D2O; pH 4): δ 135.9 (s), 112.5 (s), 108.1 (s), 102.7 (s), 43.4 (s), 33.8 (s), 24.4 (s), 19.4 (s), 17.2 (s), 14.2 (s), 12.6 (s), 10.4 (s), −12.6 (s), −27.5 (s), −37.7 (s), −41.1 (s), −45.3 (s), −57.4 (s), −66.4 (s), −67.2 (s), −68.7 (s), −70.0 (s), −82.5 (s) ppm. Only resolved peaks outside the 0−10 ppm region are reported. LR-ESMS: m/z calcd for [M + Na+] 775.19, found 775.25. HR-ESMS: m/z calcd for C30H36O7N523Na174Yb 775.190, found 775.189. UV−vis (H2O) λmax: 233, 279, 308, 368 nm. Lifetimes (4 < pH < 8): 1.01 (H2O), 8.26 μs (D2O); q = 0.8. Gd.L2. Yield: 94%. LR-ESMS: m/z calcd for [M + Na]+ 759.18, found 759.24. HR-ESMS: m/z calcd for C30H36O7N5158Gd23Na 759.175, found 759.175. UV−vis (H2O) λmax: 234, 277, 296, 365 nm. ET ≈ 21 300 cm−1. Lu.L2. Yield: 95%. 1H NMR (500 MHz; D2O; pH 4): δ 8.63 (s, br, 1H), 8.02 (s, br, 1H), 7.94 (s, br, 1H), 7.28 (s, br, 1H), 7.24 (s, br, 1H), 7.09 (s, br, 2H), 4.05−1.25 (m, br, 24H), 1.30 (s, br, 2H), 1.09 (s, br, 3H). LR-ESMS: m/z calcd for [M + H]+ 754.21, found 754.22. HR-ESMS: m/z calcd for C30H37O7N5175Lu 754.210, found 754.208. UV−vis (H2O) λmax: 234, 279, 308, 368 nm. Eu.L3. Yield: 95%. 1H NMR (500 MHz; D2O; pH 7): δ 35.4 (s), 28.2 (s), 26.8 (s), 26.0 (s), 9.7 (s), 8.1−7.8 (m), 3.3 (s), 1.1 (s), −1.7 (s), −3.2 (s), −4.5 (s), −5.6 (s), −7.5 (s), −10.4 (s), −11.8 (s), −12.5 (s), −15.8 (s), −16.3 (s), −18.3 (s). LR-ESMS: m/z calcd for [M + H]+ 715.16, found 715.22. HR-ESMS: m/z calcd for C30H34O7N4153Eu 715.163, found 715.163. UV−vis (H2O) λmax (ε/M−1 cm−1): 242 (58 800), 254 (45 600), 267 (42 600), 346 nm (19 600). Lifetimes: 0.55 (H2O), 1.59 ms (D2O); q = 1.1. HPLC tr: 11.2 min. Yb.L3. Yield: 95%. 1H NMR (500 MHz; D2O; pH 7). δ: 138.4 (s), 105.0 (s), 98.7 (s), 92.9 (s), 48.1 (s), 37.2 (s), 18.0 (s), 15.9 (s), 11.2 (s), 10.5 (s), 9.7 (s), 9.1 (s), 8.5 (s), 8.1 (s), 7.7 (s), 5.3 (s), 0.5 (s), −4.7 (s), −20.8 (s), −42.3 (s), −48.1 (s), −61.2 (s), −62.2 (s), −62.5 (s), −65.0 (s), −74.2 (s). LR-ESMS: m/z calcd for [M + H]+ 736.18, 3344

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Inorganic Chemistry found 736.27. HR-ESMS: m/z calcd for C30H33O7N423Na174Yb 758.163, found 758.162. UV−vis (H2O) λmax: 242, 256, 266, 346 nm. Lifetimes (4 < pH < 8): 1.00 (H2O), 8.04 μs (D2O); q = 0.8. Gd.L3. Yield: 95%. LR-ESMS: m/z calcd for [M + H]+ 720.17, found 720.25. HR-ESMS: m/z calcd for C30H33O7N4158Gd23Na 742.148, found 742.148. UV−vis (H2O) λmax: 242, 254, 267, 346 nm. ET ≈ 19 500 cm−1. Lu.L3. Yield: 94%. 1H NMR (500 MHz; D2O; pH 4): δ 9.12 (s, br, 1H), 8.57 (s, br, 1H), 8.01 (s, br, 1H), 7.76 (s, br, 2H), 7.67 (s, br, 2H), 7.59 (s, br, 1H), 7.51 (s, br, 1H), 3.81−2.12 (m, br, 24H). LRESMS: m/z calcd for [M + Na]+ 759.16, found 759.20. HR-ESMS: m/ z calcd for C30H33O7N4175Lu23Na 759.165, found 759.164. UV−vis (H2O) λmax: 242, 254, 267, 346 nm.

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ASSOCIATED CONTENT

S Supporting Information *

Details of photophysical studies, HPLC traces, NMR spectra, luminescence and UV−vis spectra, fitted lifetimes decays and accurate ESMS profiles, syntheses of S3 and S5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christ Church Oxford for a Junior Research Fellowship (M.T.) and the Chemistry Research Laboratory for research support (Faulkner’s lab). M.T. thanks Dr. Octavia Blackburn for helpful discussions.

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REFERENCES

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DOI: 10.1021/ic503049m Inorg. Chem. 2015, 54, 3337−3345