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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Helv Chim Acta. Author manuscript; available in PMC 2010 November 1.
Published in final edited form as:
Helv Chim Acta. 2009 November 1; 92(11): 2439–2460.
doi:  10.1002/hlca.200900161
PMCID: PMC2794132
NIHMSID: NIHMS161692

A [Cyclentetrakis(methylene)]tetrakis[2-hydroxybenzamide] Ligand That Complexes and Sensitizes Lanthanide(III) Ions

Abstract

The synthesis of the cyclen derivative H4L1·2 HBr containing four 2-hydroxybenzamide groups is described. The spectroscopic properties of the LnIII conplexes of L1 (Ln=Gd, Tb, Yb, and Eu) reveal changes of the UV/VIS-absorption, circular-dichroism-absorption, luminescence, and circularly polarized luminescence spectra. It is shown that at least two metal-complex species are present in solution, whose relative amounts are pH dependent. At pH > 8.0, an intense long-lived emission is observed (for [TbL1] and [YbL1]), while at pH < 8.0, a weaker, shorter-lived species predominates. Unconventional LnIII emitters (Pr, Nd, Sm, Dy, and Tm) were sensitized in basic solution, both in the VIS and in the near-IR, to measure the emission of these ions.

Introduction

Luminescent lanthanide complexes have attracted much recent attention because of their use and potential in a wide variety of applications such as biofluoroimmunoassays [1] [2], sensors [37], light-emitting diodes [810], and waveguide amplifiers for lasers [1117]. In most of the cases, luminescent lanthanide complexes consist of a lanthanide ion attached to a chelating chromophore which transfers the excitation energy to the lanthanide ion, which must be protected from H2O coordination to avoid quenching. The presence of this chromophore overcomes the limitation of an intrinsically small molar absorption coefficient (ε) for the metal by using a strongly absorbing organic ligand. This light-harvesting phenomenon is known as the antenna effect and, compared to the bare cations, increases the brightness (defined as the product of the luminescence quantum yield and of the molar absorption coefficient) and the luminescence lifetimes of the lanthanide ions, which are highly sensitive to the local environment and quenched by overtones of X–H vibrations, where X=O, N, C.

At the molecular level, the strong paramagnetism of these ions has led to important applications in NMR shift reagents applicable in 3D-protein-structure determination, for example [18]. The ligand-field splitting of LnIII can also be used to determine changes in coordination geometry around the lanthanide ion [1923]. In such cases, the luminescence pattern can be used to determine or indicate a change in the geometry of the binding atoms about the lanthanide center. In addition, chiral ligands allow the use of techniques such as circular-dichroism (CD) absorption or circularly polarized luminescence (CPL) that provide insight into the chiral-ligand conformations (number of species present as well as differentiation of those species).

We report herein the synthesis of a ligand H4L1·2 HBr composed of a cyclen (=1,4,7,10-tetraazacyclododecane) scaffold with four appended 2-hydroxybenzamide groups (Scheme). The free ligand H4L1·2 HBr was complexed to a series of LnIII (Ln=Gd, Tb, Yb, Eu, Pr, Nd, Sm, Dy, and Tm). A detailed study on the spectroscopic properties of the uncomplexed ligand as well as of its Gd, Tb, Yb, and Eu complexes shows the presence of two emissive complex species whose relative amounts depend on the pH of the base/buffer used. The dominant species at high pH luminesces intensely and exhibits a long luminescence lifetime, while the dominant species at mildly acidic pH is less emissive and has a shorter lifetime. Assignment of the emissive transitions for other unconventional LnIII (Ln = Pr, Nd, Sm, Dy, and Tm) is also presented.

Scheme
Preparation of the Ligand H4L1·2 HBr

Experimental. –1. General

Thin-layer chromotography (TLC): precoated silica gel 60 F254 plates. Flash chromatography (FC): EM-Science silica gel 60 (SiO2; 230–400 mesh). NMR Spectra: Bruker-AM-300 or -DRX-500 spectrometers, at 300 (75) and 500 (125) MHz for 1H (or 13C), resp.; δ(H) or δ(C) in ppm rel. to the solvent resonances, taken as δ(H) 7.26 (δ(C) 77.0) and δ(H) 2.49 (δ(C) 39.5), resp., for CDCl3 and (D6)DMSO, coupling constants J in Hz. Fast-atom bombardment mass spectra (FAB-MS): 3-nitrobenzyl alcohol (NBA) or thioglycerol/glycerol (TG/G) as the matrix; in m/z (rel. %). Elemental analyses were performed by the Microanalytical Laboratory, University of California, Berkeley, CA.

2. Syntheses. Methyl 2-Methoxy-3-methylbenzoate (1)

To a mixture of 3-methylsalicylic acid (200 g, 1.32 mol), anh. K2CO3 (500 g, 3.6 mol), and dry acetone (3.5 l) (5 l round-bottom flask), dimethyl sulfate ((MeO)2SO2; 210 ml, 2.2 mol) was added in several portions. The mixture was stirred for 2 d (TLC monitoring) and then heated under reflux overnight. The mixture was then filtered and the filtrate concentrated: 1 (215 g, 91%). Pale yellow thick oil: 1H-NMR (500 MHz, CDCl3, 25°): 2.26 (s, Me); 3.78 (s, MeO); 3.85 (s, MeO); 6.98 (t, 3J = 7.5, 1 arom. H); 7.27 (d, 3J = 7.5, 1 arom. H); 7.58 (d, 3J = 7.5, 1 arom. H). 13C-NMR (125 MHz, CDCl3, 25°): 15.7; 51.8; 61.2; 123.3; 124.4; 128.9; 132.5; 134.88; 158.2; 166.6.

Methyl 3-(Bromomethyl)-2-methoxybenzoate (2)

A 1000-watt halogen-tungsten lamp was placed in an upright position about 3 cm from a 500 ml flask containing 1 (0.05 mol) and benzene (80 ml). The soln. was gently refluxed and irradiated while 0.05 mol of Br2 in 80 ml of benzene was added at such a rate that a red color persisted at all times. The reaction was judged complete when all Br2 was added and the red Br2 color had disappeared: 2 (92%). 1H-NMR (500 MHz, CDCl3, 25°): 3.92 (s, MeO); 3.96 (s, MeO); 4.58 (s, CH2Br); 6.86 (t, 3J = 7.8, 1 arom. H); 7.55 (dd, 3J = 7.5, 1.8, 1 arom. H); 7.81 (dd, 3J 7.5, 4J = 1.8, 1 arom. H). 13C-NMR (125 MHz, CDCl3, 25°): 27.4; 52.3; 62.8; 123.9; 124.7; 132.4; 132.8; 135.19; 158.5; 166.1.

Tetramethyl 3,3′,3′′,3′′′-[1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayltetrakis(methylene)]tetrakis[2-methoxybenzoate] (3)

To a soln. of cyclen (300 mg, 1.74 mmol) in dry MeCN (30 ml) were added N,N-diisopropylethylamine (3.1 ml, 18 mmol) and 2 (2.1 g, 8.1 mmol) consecutively. The mixture was warmed to 60° under N2 for 24 h. The white precipitate was filtered, washed with MeCN (3 × 5 ml), and dried in vacuo: 3 (81%). Beige solid. 1H-NMR (500 MHz, CDCl3, 25°): 2.73 (s, 16 H, cyclen-CH2); 3.55 (s, 4 benzylic CH2); 3.72 (s, 4 MeO); 3.98 (s, 4 MeO); 6.87 (t, 3J = 7.5, 4 arom. H); 7.63 (d, 3J = 7.5, 4 arom. H); 7.91 (d, 3J = 7.5, 4 arom. H). 13C-NMR (125 MHz, CDCl3, 25°): 52.1; 53.2; 53.5; 62.0; 123.4; 123.9; 134.3; 134.5; 158.2; 166.7. FAB-MS (pos.): 885.5 ([M + H]+, C48H61N4O12+; calc. 885.4).

3,3′,3′′,3′′′-[1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayltetrakis(methylene)]tetrakis[2-methoxybenzoic Acid] (4)

To a stirred soln. of 3 (445 mg, 0.5 mmol) in MeOH (30 ml) was added 10% aq. KOH soln. (1 ml). The mixture was stirred for 16 h. The volatiles were evaporated, and the residue was dissolved in H2O. The soln. was neutralized with 6n HCl, and the solid product was collected and dried in vacuo: 4 (91%). Beige solid. 1H-NMR (300 MHz, D2O/NaOD, 25°): 2.80 (br. s, 8 H, cyclen-CH2); 3.67 (s, 4 MeO); 2.75 (br. s, 4 benzylic CH2); 7.15 (t, 3J = 7.2, 4 arom. H); 7.51 (d, 3J = 6.9, 4 arom. H); 7.65 (d, 3J = 7.2, 4 arom. H); 13.04 (br. s, 4 COOH). FAB-MS (pos.): 773 ([M + H]+, C40H45N4O12+; calc. 773.3).

3,3′,3′′,3′′′-[1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayltetrakis(methylene)]tetrakis[2-methoxy-N-[(1R)-1-phenylethyl]carboxamide] (5)

To a stirred soln. of 4 (77 mg, 1 mmol) in dry DMF (5 ml), which was cooled in an ice bath, (+)-(1R)-1-phenylethanamine (0.5 ml, 4.1 mmol) was added under N2. The cold mixture was treated with HATU (= O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate; 0.77 g, 2 mmol) and N,N-diisopropylethylamine (1 ml, 5.7 mmol) successively. The mixture was stirred for 7 h, then the solvent was evaporated. The residue was partitioned between CH2Cl2 (20 ml) and 1m aq. HCl (2 × 20 ml), the org. phase concentrated, and the residue purified by FC (SiO2, gradient 3 → 7% MeOH/CH2Cl2): 5 (71%). White foam. 1H-NMR (300 MHz, CDCl3, 25°): 1.56 (s, 4 Me); 2.83 (br. s, 8 cyclen-CH2); 3.46 (s, 4 MeO); 3.65 (s, 4 benzylic CH2); 7.03 (t, 3J = 8.0, 4 arom. H); 7.20–7.40 (m, 20 arom. H); 7.48 (d, 3J = 8.1, 4 arom. H); 7.84 (d, 3J = 7.8, 4 arom. H). 13C-NMR (125 MHz, CDCl3, 25°): 21.2; 39.4; 49.7; 51.2; 62.4; 124.2; 125.9; 127.1; 128.4; 128.8; 131.2; 134.1; 142.9; 156.5; 164.1; 170.4. FAB-MS (pos.): 1241.6 ([M + H]+, C76H89N8O8+; calc. 1241.7).

3,3′,3′′,3′′′-[1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetrayltetrakis(methylene)]tetrakis[2-hydroxy-N-[(1R)-1-phenylethyl]carboxamide] Hydrogenbromide (1 : 2) (H4L12·HBr)

To a stirred soln. of 5 (445 mg, 0.5 mmol) in dry CH2Cl2 (30 ml) was added an excess of BBr3 (1 ml) with cooling under N2.The mixture was stirred for 48 h. The volatiles were evaporated, and the residue was quenched with MeOH (150 ml). The MeOH soln. was refluxed gently in an open round-bottom flask overnight. The MeOH soln. was then boiled with H2O for 6 h. The product precipitated upon cooling and was dried in vacuo: H4L1·2 HBr (85%). Beige solid. 1H-NMR (300 MHz, CDCl3, 25°): 1.46 (s, 4 Me); 2.80–3.30 (br. s, 8 cyclen-CH2); 3.98 (s, 4 benzylic CH2); 5.18 (m, 4 CH); 6.18 (t, 3J = 7.5, 1 arom. H); 6.99 (t, 3J = 8.5, 3 arom. H); 7.20–7.40 (m, 20 arom. H); 7.58 (d, 3J = 7.2, 4 NH); 7.95 (d, 3J = 8.1, 1 arom. H); 8.09 (d, 3J = 7.8, 3 arom. H). 9.20 (d, 3J = 7.8, 1 arom. H); 9.29 (d, 3J = 8.4, arom. H). FAB-MS (pos.): 1185.6 ([M + H]+, C72H81N8O8+; calc. 1185.6). Anal. calc. for C72H80N8O8·2 HBr (1347.28): C 64.19, H 6.13, N 8.32; found: C 64.23, H 6.43, N 8.14.

{Cyclenetetrakis(methylene)tetrakis[2-hydroxybenzamidato(1–)}lanthanide Pyridinium Dibromide Complex

Ligand H4L1·2 HBr (1 equiv.) was dissolved in MeOH (3 ml) with three drops of pyridine. Lanthanide(III) chloride hexahydrate (1 equiv.) in MeOH (3 ml) was added. The soln. was heated to reflux temp. during 4 h, and then left to cool to r.t. Slow evaporation of the MeOH at r.t. for one night afforded a precipitate of the complex, which was collected by filtration.

[GdH3(L1)]Br2. Yield 17%. Anal. calc. for C81H90GdN9O8·2 HBr·9 H2O (1798.85): C 51.98, H 5.88, N 6.74; found: C 51.98, H 5.57, N 6.84.

[TbH3(L1)]Br2. Yield 23%. Anal. calc. for C81H90N9O8Tb·2 HBr·9 H2O (1800.52): C 51.96, H 5.81, N 6.73; found: C 52.07, H 5.98, N 6.71.

[YbH3(L1)]Br2. Yield 22%. Anal. calc. for C81H90N9O8Yb·2 HBr·15 H2O (1922.73): C 48.38, H 6.16, N 6.27; found: C 48.09, H 5.99, N 6.33.

3. Optical Spectroscopy

UV/VIS Absorption spectra: Varian-Cary-300 double beam absorption spectrometer. Circular-dichroism spectra: Jasco-J-810 spectropolarimeter. Circularly polarized luminescence and total luminescence spectra were recorded on an instrument described previously [24] [25] operating in a differential photon-counting mode. The light source for indirect excitation was a continuous-wave 450 W Xe arc lamp from a Spex-FluoroLog-2 spectrofluorometer, equipped with excitation and emission monochromators with dispersions of 4 nm/mm (Spex, 1681B). Selective excitation of TbIII was accomplished with either a Coherent Innova-70 or Coherent Sabre TSM 15. The optical-detection system consisted of a focusing lens, long pass filter, and 0.22 m monochromator. The emitted light was detected by a cooled EMI-9558B photomultiplier tube operating in photon-counting mode. All measurements were performed with quartz cuvettes with a path length of 0.4 or 1.0 cm. Emission spectra were acquired with a Horiba-Jobin-Yvon-IBH-FluoroLog-3 spectrofluorimeter, equipped with 3-slit double-grating excitation and emission monochromators (2.1 nm/mm dispersion, 1200 grooves/mm). Spectra were reference-corrected for both the excitation-light-source variation (lamp and grating) and the emission-spectral response (detector and grating). Luminescence lifetimes were determined with a Horiba-Jobin-Yvon-IBH-FluoroLog-3 spectrofluorimeter, adapted for time-correlated single-photon-counting (TCSPC) and multichannel scaling (MCS) measurements. A sub-microsecond Xe flashlamp (Jobin Yvon, 5000XeF) was used as the light source, with an input pulse energy (100 nF discharge capacitance) of ca. 50 mJ, yielding an optical-pulse duration of less than 300 ns at FWHM. Spectral selection was achieved by passage through the same double-grating excitation monochromator. Emission was monitored perpendicular to the excitation pulse, again with spectral selection achieved by passage through the double-grating emission monochromator (2.1 nm/mm dispersion, 1200 grooves/mm). A thermoelectrically cooled single-photon-detection module (Horiba Jobin Yvon IBH, TBX-04-D) incorporating a fast-rise-time photomultiplier tube (PMT), a wide-bandwidth preamplifier, and a picosecond constant-fraction discriminator was used as the detector. Signals were acquired with an IBH-DataStation-Hub photon-counting module, and data analysis was performed with the commercially available DAS-6 decay-analysis software package from Horiba Jobin Yvon IBH. Goodness of fit was assessed by minimizing the reduced chi squared function, χ2, and a visual inspection of the weighted residuals. Each trace contained at least 10000 points, and the reported lifetime values resulted from at least three independent measurements. Typical sample concentrations for both absorption and fluorescence measurements were ca. 10–5–10–6 m and 1.0 cm cells in quartz Suprasil or equivalent were used for all measurements. Quantum yields were determined by the optically dilute method (with optical density < 0.1) with Eqn. 1, where A is the absorbance at the excitation wavelength (λ), I is the intensity of the excitation light at the same wavelength, n is the refractive index, and D is the integrated luminescence intensity. The subscripts ‘x’ and ‘r’ refer to the sample and reference, resp. For quantum-yield calculations, an excitation wavelength of 340 nm was utilized for both the reference and sample, hence the I(λr)/I(λx) term is removed. Similarly, the refractive indices term, nx2/nr2, was taken to be identical for the aq. reference and sample solns. Hence, a plot of integrated emission intensity (i.e., Dr) vs. absorbance at 340 nm (i.e., Ar(λr)) yields a linear plot with a slope which can be equated to the reference quantum yield Φr. Quinine sulfate in 0.5m (1.0n) H2SO4 was used as the reference (Φr = 0.546). By analogy, for the sample, a plot of integrated emission intensity (i.e., Dx) vs. absorbance at 340 nm (i.e., Ax(λx)) yields a linear plot, and Φx can then be evaluated. The values reported in this paper are the average of four independent measurements. For YbIII, since there is no reliable NIR reference, the luminescence quantum yield were measured one resp. to the other fixing, arbitrary, the value of the lowest pH measured to 1.

ΦxΦr=[Ar(λr)Ax(λx)][I(λr)I(λx)][nx2nr2][DxDr]
(1)

Synthesis

The ligand was synthesized by a six-step synthesis starting from commercially available 3-methylsalicylic acid (Scheme). First, the phenolic and carboxylic acid groups were protected by Me groups with (MeO)2SO2 (→1). Subsequently, the Me group was oxidized to give the bromomethyl derivative 2. This synthon readily reacted with cyclen in basic medium yielding 3. This protected ester was then saponified (→4), and the chiral amino group was introduced by amidation in the presence of HATU (→5). Finally, the phenolic methyl ether functions were deprotected with BBr3 yielding the desired ligand H4L1·2 HBr in rather good yield (32% overall).

The metal complexes were prepared in the presence of an excess of pyridine as base by refluxing 1 equiv. of ligand with one equiv. of LnCl3·6 H2O in MeOH for 12 h. The resultant complexes were precipitated by adding Et2O, washed thoroughly with pentane and cold Et2O, and dried before submission to elemental analysis. All complexes analyzed fit with a [LnH3(L1)]Br2 complex formula with the bromide counterion introduced from the ligand deprotection strategy. The Yb complex of H2L2 (Fig. 1) was made in situ by adding 0.5 equiv. of YbCl3·6 H2O in MeOH soln. as described for TbIII derivatives [21]. The full characterization of the ligands and complexes and synthetic details are reported in the Exper. Part.

Fig. 1
Studied ligand and its model H2L2

The free ligand H4L1 contains twelve coordinating atoms (four O-atoms from the phenol moiety, four C=O groups from the benzamide moiety, and the four N-atoms from the cyclen backbone), and possesses eight distinct pKa values, four belonging to the cyclen scaffold and four belonging to the 2-hydroxybenzamide (phenol/phenolate) moieties. In the presence of lanthanide ions, H4L1·2 HBr is expected to form [ML] complexes with a coordination number of eight (CN=8) as confirmed by elemental analysis of the isolated lanthanide complexes. Furthermore, the presence of sterically hindered groups at the amide groups can be useful to protect the metal center from the local environment.

Results and Discussion

Since the complexes were not stable in aqueous solution, the spectroscopic properties of L1,[Gd(L1)], [Tb(L1)], [Yb(L1)], and [Eu(L1)]1) were carried out in MeOH solution where the stability was found to be higher. To control the basicity of the MeOH solutions, different organic bases of known pKa values were used.

1. Ligand

The UV/VIS absorption spectra of the ligand L1 in MeOH in the presence of various bases with pKas ranging from 0.2 to 14.0 are plotted in Fig. 2, a, whereas the data are reported in Table 1. As can be seen, the molar absorption coefficients vary from ca. 19000 to 23500 m –1·cm–1 and are in good agreement with those of [Tb(L2)2] (19300–26500 m–1·cm–1) [21]. This is consistent with the four benzamide units branched via a CH2 linker on the cyclen moiety. Of special importance is that the absorption maximum is blue-shifted from 335 nm in highly basic media (pH from 8.3 to 14.0) to 305 nm (pH below 6.0) with an isosbestic point at ca. 320 nm. By plotting the pH dependence of the molar absorption coefficients at 305 and 335 nm (Fig. 2, b), one can see the presence of a long transition between 4.8 and 11.2, which strongly suggests the presence of numerous protonations of L1.

Fig. 2
a) UV/VIS Absorption spectra of L1 as a function of the pKa of the base used (CF3COOH (violet line), ClCH2COOH (blue line), pyridine (brown line), N(CH2CH(Me)OH)3 (orange line), sym-collidine (red line), Et3N (green line), piperidine (black line)). b) ...
Table 1
Optical Properties of L1 in MeOH at Room Temperature and at 77 K in the Presence of Different Acids or Bases

The luminescence spectra of L1 were measured in MeOH solutions. The quantum yields were determined with quinine hydrogensulfate in 1n H2SO4 as the standard and corrected for the different refractive indexes of the solvents [26]. Selected spectra are shown in Fig. S1a of the Supplementary Information (S. I.)2), and all the data are reported in Table 1. In acidic medium, only one broad emission band is observed, with a maximum at ca. 435 nm. This emission maximum is red-shifted compared to the maximum observed in the absorption spectra (100–130 nm Stoke shift). In basic medium, the same emission profile is observed with a shoulder on the blue side of the emission. This shoulder can be attributed to the emission band of the singlet excited state (with a small Stokes shift), while the main emission band at ca. 435 nm corresponds to the triplet excited state of L1. It should be noted that the observation of the unconventional emission from the triplet excited state at room temperature is due to the heavy-atom effect induced by the bromide salt of the ligand. This was confirmed by recording the emission spectra at 77 K (Fig. S2 of the S. I.2)).

2. Gadolinium

Trivalent Gd-ion was chosen because of its similar electronic structure and size with Eu3+ (4f7 vs. 4f6), but lacking an accessible metal-based low-energy electronic excited state (the lowest one, 6P7/2, lies at 32224 cm–1). As a result, one can expect the same kind of information found for the free ligand, but it will correspond to the positions of the excited singlet and triplet states of the ligand complexed to the Gd3+ ion. For instance, the molar absorption coefficients range from 19000 to 23500 m–1·cm–1, whereas the absorption maxima are found between 305 and 325 nm in basic medium, with an isosbestic point at 313 nm (Fig. S3a of the S. I.2)). These changes in the position of the absorption maxima between the free and complexed ligands confirm the formation of the complex.

The complex formation is also corroborated by the changes observed in the pH dependence of the UV/VIS absorption molar coefficient values (Fig. S3b of the S. I.2)). Unlike the free ligand (one protonation step, Fig. S1b of the S. I.2)), the complexation of L1 to GdIII showed two different protonation steps, observed between 4–5 and 7–8, respectively. In addition to the isosbestic point at 313 nm, there is also one at 341 nm that can be associated to the higher pH species.

As observed for the ligand, the emission spectra of [Gd(L1)] are dependent on the pH determined by the bases used (Fig. S4a of the S. I.2)). The main difference between L1 and its GdIII complex was that the emission spectrum has a broad emission band at ca. 375 nm, corresponding to the singlet excited state, with a shoulder growing in at 435 nm (attributed to the triplet excited state) when the pH is decreased. It should be noted that the variation of the quantum yield cannot be taken into account since two different states are involved and the intersystem crossing rate changes with the conditions (Fig. S4b of the S. I.2)).

At 77 K, despite a small residual emission arising from the singlet excited state, the emission spectra are mainly composed of a structured emission band located at ca. 430 nm (Fig. 3). The deconvolution of this triplet-excited-state emission band into a vibronic progression [2729] of several overlapping Gaussian functions with separations of ca. 1000–1100 cm–1 led to the determination of a value of 24690 cm–1 for the triplet-excited-state energy. Such a high-energy triplet excited state should allow sensitization of many lanthanide ions, including EuIII, TbIII, PrIII, NdIII, SmIII, DyIII, or TmIII (vide infra).

Fig. 3
Luminescence spectra of [Gd(L1)] in solid matrix (77 K in a mixture MeOH/EtOH 1 : 4) with selected bases or acids (λex 320 nm) (piperidine (black line), Et3N (red line), morpholine (blue line), sym-collidine (orange line))

Finally, it must be emphasized that the GdIII complex is unstable under acidic conditions, since identical absorption spectra and molar absorption coefficient values (within the experimental errors) were obtained for L1 and its GdIII complex at room temperature (see Tables 1 and and2).2). A similar observation was noticed for the emission spectra recorded at 77 K (the two emission bands broaden to give similar spectra as for L1).

Table 2
Optical Properties of [Gd(L1)] in MeOH at Room Temperature and at 77 K in the Presence of Different Acids or Bases

3. Terbium

Since the 5D4 level is at 20500 cm–1 (TbIII emitting excited state) and its next excited state (5G6) is 6000 cm–1 higher, TbIII appears to be the ideal lanthanide to be sensitized by L1, based on the good match between the TbIII and triplet-excited-state energies. The photophysical and chiroptical properties of [Tb(L1)] were studied by UV/VIS-absorption, CD-absorption, luminescence (luminescence quantum yield and luminescence lifetimes), and CPL spectroscopy at various pH values between 2.8 and 14.0. All the results are summarized in Table 3, and selected spectra are presented hereafter.

Table 3
Optical Properties of [Tb(L1)] in MeOH at Room Temperature and at 77 K in the Presence of Different Acids or Bases

The UV/VIS profile of [Tb(L1)] is identical to the one of [Gd(L1)], with a shift of the absorption maximum from 305 nm to 325 nm when going from acidic to basic conditions (Fig. 4, a). The molar absorption coefficient values are ca. 20000 m–1·cm–1, as observed for L1 and [Gd(L1)]. As for [Gd(L1)], the UV/VIS spectra of [Tb(L1)] show two isosbestic points, and the pH dependence of the molar absorption coefficient values (Fig. 5, a and b) reveals the presence of two different pKas (between 4–5 and ca. 7–8). The photophysical properties of the GdIII and TbIII complexes are then in good agreement and, more importantly, three species are present in solution. The change between these species seems to occur at pH values between 4–5 and 7–8.

Fig. 4
a) UV/VIS Absorption spectra of [Tb(L1)] in MeOH as a function of the pKa of the base used. b) Luminescence spectra of [Tb(L1)] in MeOH as a function of the pKa of the base used (λex 320 nm) (piperidine (black line), Et3N (red line), morpholine ...
Fig. 5
a) and b) Evolution of the UV/VIS molar absorption coefficient for [Tb(L1)] in MeOH at 306 nm (An external file that holds a picture, illustration, etc.
Object name is nihms-161692-ig0007.jpg) and 325 nm (An external file that holds a picture, illustration, etc.
Object name is nihms-161692-ig0008.jpg) as a function of the pKa of the base used (grey bars: changes between species). c) Evolution of the luminescence quantum yield for [Tb(L1)] ...

As shown in Fig. 4, b, the luminescence spectra of [Tb(L1)] are different, depending on the base used and the resultant pH. In basic medium, the typical luminescence spectrum of the TbIII ion can be observed with sharp line emission bands arising from the 5D4 excited state on the 7FJ manifold ground state. In an acidic medium, only the triplet-excited-state luminescence band can be observed, which suggests that the complex is not formed under these conditions. This is confirmed by the observation of the Tb- and ligand-centered emissions in the luminescence spectra of [Tb(L1)] under mild basic conditions (pH 5–6).

The luminescence quantum yield of the Tb complex can be calculated and separated from that of the triplet excited state. Thus, the dependence of the quantum yield of both excited states as a function of the pH of the base used can be followed (Figs. 5c and d). In the case of the Tb-centered emission, one can see that the luminescence quantum yield is low (3–4%) at low pH. A considerable increase is observed at pH ca. 4–5 (14–20%) and continues until it reaches its maximum (ca. 60%). On the other hand, the opposite effect can be observed when looking at the triplet-excited-state emission (i.e., large and low quantum yields in acidic and basic media, resp.). This quenching of the triplet-excited-state emission as the Tb-centered luminescence increases reflects the efficiency of the energy transfer (poor and good efficiencies under acidic and basic conditions, resp.), and that the separation of these two species is possible in the presence of bases of appropriate strength.

This luminescence study allows us to conclude that there are at least two different species depending on the pH of the solution, resulting in changes in the emission efficiency.

Another way to observe the number of species is to measure the luminescence lifetime of the Tb-centered emission. At room temperature, one monoexponential decay of ca. 2.1 ms can be found above pH ca. 6, while below this pH, a second component can be detected with a shorter luminescence lifetime of ca. 800 μs (Fig. S5a of the S. I.2) and Table 3). The intensity of the latter emission decreases by increasing the pH, as the long-lived, deprotonated species dominates the species distribution.

Measurements in deuterated solvent enable calculation of ‘q’ the number of inner-sphere-bound solvent molecules [30]. These experiments were performed in the presence of piperidine, Et3N, pyridine, and pyridin-3-ol as buffer bases in CD3OD and showed a monoexponential decay with identical luminescence lifetimes at high pH (piperidine and Et3N) and at lower pH (pyridine and pyridin-3-ol). Application of Beeby's equation [31] to MeOH gave 0.0 and 0.3 inner-sphere H2O molecules (the latter value reflects the protonation of a nearby atom in the complexing ligand when decreasing the pH). This result establishes that there is no MeOH molecule in the inner sphere of the TbIII complexes.

Since L1 and its complexes are chiral, CD absorption spectra were measured. As shown in Fig. 6, a, different spectra were obtained, depending on the pH. In basic medium, the CD spectra reveal two CD exciton bands located at 310 and 335 nm, with positive and negative values, respectively. At neutral pH, the CD spectra only show one main CD exciton band at 335 nm with a positive value. It is interesting to note that, in the case of AcOH or CF3COOH, a different pattern can be seen with a residual ‘complex’ absorption at 335 nm for AcOH, while with CF3COOH (pH 0.2), no remaining transition can be observed at this wavelength (Fig. 6, a). In addition, the pH dependence of the δdeg also suggests the presence of two different species (pH ca. 7–8; Fig. 6, b).

Fig. 6
a) CD Absorption spectra of [Tb(L1)] in MeOH as a function of the pKa of the base used (piperidine (black line), Et3N (red line), morpholine (green line), sym-collidine (blue line), 2-methyl-1H-imidazol (cyan line), N(CH2CH(Me)OH)3 (royal blue line), ...

Also, measurements of circularly polarized luminescence (CPL) to study the chiroptical properties of the TbIII-containing compound were performed. Generally speaking, CPL spectroscopy is the emission analog of CD absorption spectroscopy. CD allows one to detect the differential absorption of left and right circularly polarized lights, while CPL measures the difference in the emission intensity of left and right circularly polarized light. With TbIII complexes, the studied transition is the magnetic-dipole-allowed transition 5D47F5 for which one predicts that the CPL would be large [32]. In such measurements, the luminescence dissymmetry ratio, glum, is defined by Eqn. 2, where IL and IR refer, respectively, to the intensity of left and right circularly polarized lights.

glum=ΔI12I=ILIR12(IL+IR)
(2)

As can be seen from Fig. 7, the CPL spectra of the magnetic-dipole-allowed transition 5D47F5 of [Tb(L1)] display several peaks corresponding to crystal-field splitting of the electronic level. However, differences in the magnitude, sign, and shape of the CPL signals indicate that the chiral structure of the TbIII complex is dependent on the nature of the base present in solution (Fig. 7). It is interesting to note that the CPL activity exhibited by the [Tb(L1)] complex in the presence of Et3N (Fig. 7, a) is comparable when either a direct excitation of the TbIII ion (λexc 488 nm) or an indirect excitation through the ligand absorption bands (λexc 331 nm) is used (Table 4). Observation of similar CPL spectra following direct and indirect excitations indicates that the same species in solution is responsible for the CPL activity detected [25]. A useful experiment to determine whether or not the solution contains a mixture of species is to excite directly the Tb3+ ion with circularly polarized light. These experiments showed that the glum values obtained for [Tb(L1)] in MeOH solution in the presence of Et3N are independent of the polarization of the excitation beam (i.e., right-, left-, or plane-polarized light). This is consistent with the presence of only one species in solution. If the solution would have contained a mixture of diastereoisomers or enantiomers, the CPL should have been dependent on the excitation polarization [25].

Fig. 7
a) Circularly polarized luminescence (upper curve) and total luminescence (lower curve) spectra of the 5D47F5 transition of the [Tb(L1)] complex in a 2.45·10–5 m MeOH solution (O.D.=0.6335) with 1% of Et3N at 295 K, upon excitation ...
Table 4
Summary of the CPL Data for [Tb(L1)] in MeOH Solutions

On the other hand, the glum values obtained for the [Tb(L1)] complex solution in the presence of pyridine were dependent of the polarization of the excitation beam. In addition, different glum values were obtained when a direct excitation of the TbIII ion (λexc 488 nm) and an indirect excitation through the ligand absorption bands (λexc 330 nm) were used (Table 4). These results would suggest that the solution of the [Tb(L1)] complex contains more than one species responsible for the CPL activity measured when the solution is prepared with pyridine as a base. This is confirmed by the observation of two different CPL spectra when direct and indirect excitations are used (Fig. 7, b).

4. Ytterbium

Luminescence experiments were also performed with the YbIII ion. Yb is interesting since it is a near-infrared (NIR) emitter which possesses only one excited state (2F5/2). Because of the latter point, the luminescence quantum yield is relatively high for a lanthanide ion emitting in the NIR, thus making the measurement easier than for other NIR emitters. Indeed, the energy gap between the triplet excited state and 2F5/2 is large (δE = 14290 cm–1) which makes the energy transfer much less efficient than for the VIS emitters. On the other hand, the absence of excited states between the triplet and 2F5/2 avoids any back-transfer-deactivation pathway. The UV/VIS absorption spectra are identical to the ones of the TbIII and GdIII complexes in the presence of piperidine or pyridin-3-ol.

As can be seen in Fig. 8, a, under the most basic conditions (piperidine), the emission spectrum of [Yb(L1)] is composed of mainly four splittings between 950 and 1150 nm with the most intense at 997 nm. In the presence of a less basic base such as pyridin-3-ol (Fig. 8, b), a strikingly different spectrum is obtained with the first splitting as the dominant one (at 975 nm). The spectrum obtained under the latter condition leads to the ‘usual’ Yb emission as often reported for Yb-based compounds [3337]. It should be indicated that the profile of the emission spectrum shown in Fig. 8, a was already observed for phosphor [38] and also in glass or solid state [33] [39]. This suggests an important change of the geometry around the metal as the crystal field changes.

Fig. 8
Luminescence spectra a) of the [Yb(L1)] complex in MeOH in the presence of piperidine in the NIR at room temperature (black line) and at 77 K (red line) and b) of the [Yb(L1)] complex in MeOH in presence of pyridin-3-ol in the NIR at room temperature ...

The dependence of the VIS luminescence quantum yield and the Yb emission area as a function of the pH (Table 5 and Fig. 8) were investigated as for the TbIII complex. In the VIS region of the spectrum (Fig. 8, a), it can be seen that there is a considerable decrease in luminescence quantum yield at pH > 7.4 (Fig. 8, c). Unfortunately, the study of the sensitization process with YbIII is precluded due to the changes observed in the emission profile (emission shifting from the triplet-excited-state to the singlet-excited-state bands). The same phenomenon was observed for the GdIII complex (observation of the emission from the singlet and triplet excited states when basic and acidic conditions were used, see Table 2). However, the NIR relative luminescence quantum yield trend (slight increase until 7.4 followed by a considerable jump and then again a slightly increase until reaching a plateau, Fig. S6 of the S. I.2)) clearly supports the presence of two species that certainly possess different geometries around the metal. This hypothesis is also corroborated by the important emission-pattern change observed in the Yb-centered luminescence spectra. It was also confirmed by the pH dependence of the luminescence lifetime of the Yb in the NIR (Fig. 8, d and Table 5). As for the TbIII complex, a monoexponential decay is observed under basic conditions, whereas two lifetimes can be found when the pH is lower than 7.0. This clearly shows the presence of a second species in solution.

Table 5
Photophysical Properties of [Yb(L1)] in MeOH with O.D. = 0.11

To understand the striking difference of emission patterns, the YbIII complex of the isophthalamide-based ligand H2L2 (Fig. 1) was prepared. As can be seen in Fig. 9, the emission of [Yb(L2)2] is composed of a main splitting at 977 nm followed by a broad shoulder (much less intense). This emission profile looks like the emission of [Yb(L1)] in the presence of pyridin-3-ol as a base. This suggests that at low pH, the complex existing in solution is based on the 2-hydroxybenzamide units (no involvement of the cyclen unit) while, at high pH, the different spectrum seems to indicate that not only the 2-hydroxybenzamide moieties but all or part of the cyclen moiety would participate to the complexation.

Fig. 9
Luminescence spectrum of [Yb(L1)] with piperidine as a base (black line) and [Yb(L1)] with pyridin-3-ol as a base (red line) and of [Yb(L2)2]with piperidine or pyridin-3-ol as a base (blue line) (in MeOH at room temperature, λex 320 ...

To ensure that the result is not just coincidential, measurements in methanol (CD3OD, MeOD, and MeOH) were performed, and all the results are reported in Table 6. As already discussed, the decay found for [Yb(L1)] is biexponential in the presence of pyridin-3-ol as a base with a component at ca. 2 μs and another one at ca. 1 μs in undeuterated MeOH. The longer luminescence lifetime possesses a small weight as compared to the other one. It can also be found that the value of the relative luminescence quantum yield is identical (within the experimental error) to the one of [Yb(L1)] in the presence of piperidine as a base. Interestingly, the shorter luminescence lifetime (1 μs) is close to that of [Yb(L2)2] (Table 6). In MeOD and CD3OD, the same kind of trend can be found. These results seem to confirm the fact that the complex formed mainly involves the 2-hydroxybenzamide chromophores at low pH, whereas stronger bases certainly allow involvement of the cyclen moiety in lanthanide coordination. Thus, this yields different properties than for the ‘pure’ isophthalamide-based complexes.

Table 6
Luminescence Lifetime of [Yb(L1)] and [Yb(L2)2] Emission in MeOH, MeOD, and CD3OD

The number of MeOH molecules in the inner sphere (n) can be calculated with the formula developed for YbIII by Beeby et al. [40] adapted to MeOH. This formula was defined in H2O with different kinds of ligand. Since the measurements were made in H2O, no C–H vibrations are brought by the solvent and, in consequence, this kind of vibration has not been taken into account. For [Yb(L1)], it can be found that, in piperidine, the complex does not have any solvent molecules in the inner sphere, while, at lower pH, the calculation suggests that there would be one molecule of solvent bound to the metal (n=0.8) which is in contradiction with the data obtained for [Tb(L1)]. This result certainly reflects the larger influence of the medium (OH, NH, and CH vibrations) on the luminescence properties for Yb-based emission vs. Tb-based emission (VIS emitter).

5. Europium

A reasonable level of sensitization was expected with EuIII because the triplet excited state is higher than the 5D2 state (too high to be optimum) [41] [42]. Instead, as illustrated in Fig. 10, a, only the triplet-excited-state emission could be recorded. Therefore, at 77 K, the ‘typical’ and characteristically split spectrum of Eu was observed (Fig. 10).

Fig. 10
a) Luminescence spectrum of [Eu(L1)] in the VIS in MeOH containing 1% of piperidine at room temperature (λex 340 nm; black line) and at 77 K (λex 320 nm; red line). b) Focus on the J = 0 and J = 1 of the luminescence spectrum of [Eu(L ...

The recovery of the sensitization occurring at 77 K strongly suggests that there is a transfer–back-transfer interaction between the triplet excited state of the ligand and the 5D2 of the Eu3+ ion with a much faster triplet-excited-state relaxation (radiative and nonradiative) than the 5D25D0 relaxation.

At 77 K in solid matrix, a thorough study of the basicity dependence reveals two different patterns if one examines the J = 0 and J = 1 transitions (Fig. 10, b). Interestingly, only one J = 0 transition can be seen under all conditions, suggesting the presence of only one species. Also, no spectral differences are observed at pH higher or lower than 5.8, again suggesting the presence of only one species. Noticeably, a luminescence-lifetime experiment at 77 K confirmed the presence of only one species in every case: at high pH, a luminescence lifetime of 720 μs can be observed, while under mildly basic conditions, a luminescence lifetime of 650 μs is measured.

These experiments on [Eu(L1)] also establish the presence of two emissive species: one existing at high pH, the other one present at pH lower than 7. In the following part, the best conditions of sensitization were used to sensitize other lanthanide ions.

6. Pr, Nd, Sm, Dy, and Tm

Because the ligand triplet-excited-state energy seems well suited to populate most of the LnIII cations, all the lanthanide complexes of L1 emitting from the f–f orbitals were prepared, yielding sensitization of Pr, Nd, Sm, Dy, and Tm. With the exception of Nd, which only possesses transitions in the NIR region, all these lanthanide ions emit in both the NIR and VIS regions. All the relevant luminescence spectra of the complexes in the VIS and in the NIR regions are presented in Fig. 11, and a brief description of these luminescence spectra is provided below.

Fig. 11
Luminescence spectrum in MeOH containing 1% of piperidine of a) [Pr(L1)] in the VIS, b) [Pr(L1)] in the NIR, c) [Sm(L1)] in the VIS, d) [Sm(L1)] in the NIR, e) [Dy(L1)] in the VIS, f) [Dy(L1)] in the NIR, g) [Tm(L1)] in the VIS, and h) [Tm(L1)] in the ...

For PrIII, the 3P0 (21750 cm–1) is populated yielding transitions attributed to the 1D23H4 (Fig. 11, a), 1D23F3, and 1D23F4 transitions (Fig. 11, b).

NdIII is also sensitized by ligand L1 resulting in the typical emission feature of this ion (Fig. S7 of the S. I.2)) with the 4F3/24I11/2 and 4F3/24I13/2 transitions in the NIR.

For SmIII, the 4G5/2 excited state emits in the VIS (Fig. 11, c) and in the NIR region where three transitions can be observed (Fig. 11, d), and arising from the 4G5/2 level to the 6F5/2, 6F7/2, and 6F9/2, respectively.

DyIII emits from the 4F9/2 excited state giving rise to four transitions to the 6H15/2, 6H13/2, 6H11/2, and 6H9/2/6F11/2 in the VIS (Fig. 11, e) and three transitions in the NIR (Fig. 11, f) attributed to the 4F9/27F7/2, 4F9/27F5/2, and 4F9/27F3/2.

TmIII emission from the 1G4 level is observed with the 1G43H6 and the 1G43H4 transitions in the VIS (Fig. 11, g) and the 1G43F4 in the NIR (Fig. 11, h).

Conclusions

In conclusion, the synthesis of a cyclen derivative containing four 2-hydroxybenzamide groups, H4L1·2 HBr, is described. The spectroscopic properties of the LnIII complexes of L1 (Ln = Gd, Tb, Yb, and Eu) were studied and reveal that at least two species are present in solution whose relative populations are dependent on the pH. In basic medium, an intense long-lived emission is observed (for [Tb(L1)] and [Yb(L1)]), while in mildly basic medium, a weaker emission of a shorter-lived species is present. A study of the J = 0 and J = 1 transition patterns of the [Eu(L1)] emission also supports the change in species upon protonation of the complexing ligand. These two different complexes change their coordination geometry upon protonation/deprotonation of one of the twelve coordinating atoms (eight O-atoms and four N-atoms), but in both cases still exclude solvent molecules from the inner coordination sphere. This was also supported by the lifetime and CPL measurements with some of the LnIII complexes of interest.

In addition, under the more basic conditions (the most efficient conditions for sensitization of TbIII and YbIII), other lanthanides(III) (PrIII, NdIII, SmIII, DyIII, and TmIII) were sensitized showing emission in the NIR region (and in the VIS in some cases). All emissive transitions on the spectra were attributed.

Supplementary Material

ESI

Acknowledgments

This work was partially supported by the NIH (Grant HL69832) and supported by the Director, Office of Science, Office of Basic Energy Sciences, and the Division of Chemical Sciences, Geosciences, and Biosciences of the U.S. Department of Energy at the Lawrence Berkeley National Laboratory (LBNL) under Contract No. DE-AC02-05CH11231. This technology is licensed to Lumiphore, Inc. in which some of the authors have a financial interest. G. M. thanks the National Institute of Health, Minority Biomedical Research Support (2 S06 GM008192-27) and the Henry Dreyfus Teacher Scholar Award for financial support.

Footnotes

Dedicated to Professor Jean-Claude Bünzli on the occasion of his 65th birthday

1)For convenience, the short forms [Ln(L1)] are used for [LnH3(L1)]Br2.H4L1·2 HBr in solution is denoted by L1.

2)Supplementary Information (S. I.) is available upon request from K. N. R.

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