Highly luminescent Tb(III) macrocyclic complex based ...

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Oct 24, 2013 - hosting unit and an appended carboxylated N,C-pyrazolylpyridine antenna ..... 77 K of the free ligand and its gadolinium complex were carried out in a Tris .... domain at 259 and 286 nm, that according to their molar absorp-.
Journal of Photochemistry and Photobiology A: Chemistry 274 (2014) 124–132

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Highly luminescent Tb(III) macrocyclic complex based on a DO3A hosting unit and an appended carboxylated N,C-pyrazolylpyridine antenna Isabelle Nasso a,b , Neri Geum c , Ghassan Bechara a,b , Béatrice Mestre-Voegtlé a,b , Chantal Galaup a,b,∗ , Claude Picard a,b,∗ a CNRS, Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Biologique, SPCMIB, UMR-5068, 118 Route de Narbonne, F-31062 Toulouse cedex 9, France b Université de Toulouse, UPS, Laboratoire de Synthèse et Physico-Chimie de Molécules d’Intérêt Biologique, SPCMIB, 118 route de Narbonne, F-31062 Toulouse cedex 9, France c Department of Chemistry, Dankook University, 119, Dandae-ro, Dongnam-gu,Cheonan-si, Chungnam 330-714, Republic of Korea

a r t i c l e

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Article history: Received 18 September 2013 Received in revised form 10 October 2013 Accepted 13 October 2013 Available online 24 October 2013 Keywords: N,C-pyrazolylpyridine Tb(III) Luminescent probe Photophysical studies

a b s t r a c t The synthesis of a new ligand based on a DO3A moiety (DO3A = 1,4,7,10 tetraaza cyclododecane 1,4,7-triacetic acid) as a hosting agent for the lanthanide ions and an appended chromophoric unit N,C-pyrazolylpyridine core functionalized by a carboxylic acid in the 3-position of the pyrazole ring was reported. The luminescence properties of the 1/1 Eu(III) and Tb(III) corresponding complexes were investigated in air-equilibrated water and deuterated water (Tris buffer, pH 7.4) at 298 and 77 K. High luminescence lifetime ( = 2.30 ms), quantum yield (˚ = 45%) and efficiency for the sensitized emission (sens = 60%) were observed for the terbium complex. A detailed study of the steps originating from light absorption at the chromophore unit and leading to lanthanide-centred emission was reported. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The use of lanthanide luminescent bioprobes based primarily on Eu(III) and Tb(III) ions which can overcome several drawbacks of conventional organic fluorophores (e.g. re-absorption, large emission signal, short-emission lifetime, photobleaching) is nowadays well established. As a matter of fact, these Ln(III) luminescent probes display under ambient conditions (i) a large Stokes shift (>200 nm) allowing multilabelling, (ii) narrow emission peaks ( 1020 M−1 ) [27]. Consequently, DO3A derivatives are suitable in satisfying requirement for water-stable lanthanide complexes and their structure and spectroscopic properties are well researched [28–31]. In the designed ligand L6 (DO3A-PyPzCOOH), it is expected that the PyPzCOOH sensitizer linked to the DO3A core by a single methylene spacer should be suitably positioned to bind the lanthanide, expelling residual water molecules from the coordination sphere of the metal. In this report, the photophysical properties of this ligand and its corresponding Eu(III), Tb(III) and Gd(III) complexes have been investigated in various aqueous conditions. This detailed study allow us: (i) to determine the hydration number of the complexes in solution, (ii)

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to bring up the crucial role of the triplet energy of the ligand in the ligand to Ln(III) energy transfer process, and (iii) to demonstrate that the Tb(III) complex is attractive as an efficient luminescent probe. 2. Experimental 2.1. General procedures 3-(Ethoxycarbonyl)pyrazole and DO3A(tBuO)3 .HBr (3) were prepared according to literature procedures [32,33]. Thin-layer chromatography was performed on Merck silica or alumina plates with a fluorescence indicator. Column chromatography was carried ˚ and on alumina out on silica gel (Merck, 60–200 ␮m, porosity 60 A) (Macherey-Nagel, activity IV, 50–200 ␮m). Infrared spectra were recorded on a Perkin-Elmer FTIR 1725× spectrophotometer. 1 H and 13 C NMR spectra were recorded on a Bruker Avance 300 spectrometer; chemical shifts are given in ppm according to the solvent peak. Microanalysis was performed by the microanalytical department of the Laboratory of Coordination Chemistry (LCC, Toulouse, France). Electrospray (ESI) mass spectra were obtained on a Xevo G2 Q TOF waters and Q TRAP Applied Biosystems spectrometers. DCI (NH3 ) mass spectra were obtained on a DSQ II Thermo Fisher spectrometer. Absorption measurements were done with a Hewlett Packard 8453 temperature-controlled spectrophotometer. Ligand L6 (DO3A-PyPzCOOH) and its Ln(III) complexes were analyzed by RP-HPLC using a Waters Alliance 2695 system with a PDA 2996 detector and using a RP C8 column (Phenomenex Luna C8(2), 5 ␮m, 150 × 4.6 mm). The flow rate was 1 mL/min with UV monitoring at 290 nm. HPLC solvents were a 10 mM pH 4 ammonium formate buffer (solvent A) and acetonitrile (solvent B). The compounds were analyzed using the HPLC gradient system beginning with a solvent composition of 100% A and following a linear gradient up to 90% A: 10% B from 0 to 18 min. 2.2. Synthesis of ligand L6 (DO3A-PyPzCOOH) 2.2.1. Synthesis of 6-methyl-2-(3-ethoxycarbonyl pyrazol-1-yl)pyridine (1) To a mixture of 3-(ethoxycarbonyl)pyrazole (1 g, 7.14 mmol), CuI (1.12 g, 5.88 mmol) and K2 CO3 (1.97 g, 14.29 mmol) in dioxane (20 mL) was added 2-bromo-6-methylpyridine (1.02 g, 5.96 mmol) and trans-1,2-cyclohexanediamine (0.68 g, 5.96 mmol). The mixture was heated at reflux under an atmosphere of argon for 48 h,

Scheme 1. Ligands based on the N,C-pyrazolylpyridine chromophore.

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after which it was cooled to room temperature, diluted with ethyl acetate (80 mL) and filtered through a plug of Celite. The filtrate was washed with saturated EDTA aqueous solution (2 × 25 mL) and was then dried with Na2 SO4 . The solvents were evaporated under reduced pressure, and the residue was purified by column chromatography (silica, CH2 Cl2 as eluent) to give desired product 1 (739 mg, 54%) as a yellow oil. Rf (CH2 Cl2 –petroleum ether, 98:02) = 0.50. IR (NaCl plates):  1738 cm−1 (CO ester). 1 H NMR (CDCl3 ): ı 1.37 (3H, t, J = 6 Hz), 2.51 (3H, s), 4.39 (2H, q, J = 6 Hz), 6.90 (1H, d, J = 3 Hz), 7.04 (1H, d, J = 9 Hz), 7.66 (1H, t, J = 9 Hz), 7.86 (1H, d, J = 9 Hz), 8.57 (1H, d, J = 3 Hz). 13 C NMR (CDCl3 ): ı 14.35 (CH3 ), 24.2 (CH3 ), 61.2 (CH2 ), 109.85 (CH), 110.0 (CH), 121.9 (CH), 128.3 (CH), 138.9 (CH), 145.7 (Cq), 150.3 (Cq), 157.5 (Cq), 162.3 (CO). MS (ESI+): m/z (%) 270 (49) [M+K]+ , 254 (100) [M+Na]+ , 232 (36) [M+H]+ . 2.2.2. Synthesis of 6-bromomethyl-2-(3-ethoxycarbonyl pyrazol-1-yl)pyridine (2) To a solution of diheterocycle 1 (100 mg, 0.43 mmol) in CCl4 (15 mL) was added N-bromosuccinimide (NBS, 77 mg, 0.43 mmol) and benzoyl peroxide (5 mg, 0.02 mmol). The solution was irradiated and heated at reflux by using a halogen lamp (150 W) until its fading (∼3 h). The hot mixture was then filtered and evaporated under reduced pressure and the residue was purified by column chromatography (silica, petroleum ether–AcOEt 80:20) to give bromo compound 2 (55 mg, 41%) as a yellow oil. Rf (silica, petroleum ether–AcOEt, 80:20) = 0.28. 1 H NMR (CDCl3 ): ı 1.47 (3H, t, J = 6 Hz), 4.48 (2H, q, J = 6 Hz), 4.58 (2H, s), 7.01 (1H, d, J = 3 Hz), 7.42 (1H, d, J = 6 Hz), 7.88 (1H, t, J = 6 Hz), 8.10 (1H, d, J = 6 Hz), 8.67 (1H, d, J = 3 Hz). 13 C NMR (CDCl3 ): ı 14.4 (CH3 ), 33.0 (CH2 ), 61.3 (CH2 ), 110.15 (CH), 112.5 (CH), 122.0 (CH), 128.6 (CH), 139.9 (CH), 146.1 (Cq), 150.5 (Cq), 155.6 (Cq), 162.1 (CO). MS (DCI, NH3 ): m/z (%) 327 and 329 (100) [M+NH3 +H]+ , 310 and 312 (5) [M+H]+ . 2.2.3. Synthesis of fully-protected ligand DO3A-PyPzCOOH (4) To a mixture of DO3A(tBuO)3 .HBr 3 (101 mg, 0.170 mmol) and K2 CO3 (117 mg, 0.85 mmol) in acetonitrile (10 mL) was added bromo compound 2 (52.7 mg, 0.170 mmol). The mixture was heated to reflux for a period of 20 h, and then the solvent was evaporated under reduced pressure. To the residue 20 mL of CH2 Cl2 was added, the potassium salts were filtered and the filtrate was evaporated under reduced pressure to dryness. The oily residue was purified by chromatography on alumina eluting with a gradient of CH2 Cl2 –methanol (100:0–96:04) to give the expected macrocycle 4 (118 mg, 93%) as a yellow oil. Rf (silica, CH2 Cl2 –methanol, 96:04) = 0.26. 1 H NMR (CDCl3 ): ı 1.36–1.45 (30H, m), 2.14–2.94 (24H, m), 4.40 (2H, q, J = 6 Hz), 6.76 (1H, d, J = 3 Hz), 7.29 (1H, d, J = 9 Hz), 7.86 (1H, t, J = 9 Hz), 8.06 (1H, d, J = 9 Hz), 9.12 (1H, d, J = 3 Hz). 13 C NMR (CDCl3 ): ı 14.35 (CH3 ), 27.7 (CH3 ), 27.9 (CH3 ), 28.05 (CH3 ), 50.2 (CH2 ), 55.65 (CH2 ), 55.9 (CH2 ), 56.4 (CH2 ), 56.6 (CH2 ), 60.2 (CH2 ), 61.2 (CH2 ), 81.9 (Cq), 82.1 (Cq), 82.4 (Cq), 110.2 (CH), 112.3 (CH), 124.4 (CH), 129.7 (CH), 139.8 (CH), 145.7 (Cq), 151.7 (Cq), 157.0 (Cq), 162.1 (CO), 170.6 (CO), 172.6 (CO), 172.8 (CO). MS (ESI+): m/z (%) 766.7 (100) [M+Na]+ , 744.8 (13) [M+H]+ . 2.2.4. Synthesis of ligand DO3A-PyPzCOOH as its sodium salt (5) To a solution of compound 4 (116 mg, 0.156 mmol) in methanol (1.5 mL) was added a solution of NaOH (25 mg, 0.625 mmol) in a mixture of 1 mL of methanol and 1 mL of water. The solution was heated at reflux for 18 h and was cooled to room temperature. The insolubles were removed by centrifugation and the supernatant was evaporated under reduced pressure to dryness. The residue was dissolved in methanol (10 mL) and diethyl ether was added to precipitate the desired ligand (as its sodium salt) which was isolated by centrifugation and dried under vacuum (94 mg, 90%). HPLC analysis: tR = 6.21 min. 1 H NMR (D2 O): ı 2.01–2.76 (18H, m), 2.94 (4H, s), 3.68 (2H, s), 6.79 (1H, d, J = 3 Hz), 7.34 (1H, d, J = 6 Hz), 7.75

(1H, d, J = 6 Hz), 7.87 (1H, t, J = 6 Hz), 8.42 (1H, d, J = 3 Hz). 13 C NMR (D2 O): ı 52.0 (CH2 ), 53.3 (CH2 ), 61.0 (CH2 ), 61.1 (CH2 ), 61.25 (CH2 ), 61.5 (CH2 ), 111.8 (CH), 114.5 (CH), 126.0 (CH), 132.1 (CH), 143.0 (CH), 152.7 (Cq), 153.85 (Cq), 159.7 (Cq), 171.9 (CO), 182.45 (CO), 182.5 (CO), 182.9 (CO). MS (ESI+): m/z (%) 570.2 (100) [M-3Na+4H]+ , 548.2 (33) [M-4Na+5H]+ . HRMS (ESI+): m/z ([M−4Na+5H]+ ), calculated for C24 H34 N7 O8 : 548.2469; found, 548.2478. Anal. Calcd for C24 H29 N7 O8 Na4 .2H2 O: C, 42.93; H, 4.95; N, 14.60. Found: C, 42.66; H, 4.90; N, 14.32. abs (Tris buffer, pH 7.4): ␭/nm (ε/M−1 cm−1 ) 259 (7060), 286 (11200). 2.3. Preparation of the Ln(III) complexes General procedure: An equimolar amount of LnCl3 ·6H2 O in water (2 × 10−3 M) was added to a solution of 5 (2 × 10−3 M) in water. The solution was stirred for 24 h at room temperature and then adjusted to a final concentration of 1 × 10−6 M in Tris buffer (50 mM, pH 7.4). Eu(III) complex: HPLC analysis: tR = 7.62 min. MS (ESI+): m/z (%) 740.1 (82) and 742.1 (94) [M−H+2Na]+ , 718.1 (48) and 720.1 (55) [M+Na]+ , 696.1 (85) and 698.1 (100) [M+H]+ . abs (Tris buffer, pH 7.4): /nm (ε/M−1 cm−1 ) 258 (7090), 297 (7960). em (Tris buffer, pH 7.4, exc = 297 nm)/nm 580 (relative intensity, corrected spectrum 5), 592 (35), 615 (100), 652 (7) and 690 (99). Tb(III) complex: HPLC analysis: tR = 7.61 min. MS (ESI+): m/z (%) 748.1 (66) [M−H+2Na]+ , 726.1 (24) [M+Na]+ , 704.1 (100) [M+H]+ . abs (Tris buffer, pH 7.4): /nm (ε/M−1 cm−1 ) 260 (7090), 298 (7960). em (Tris buffer, pH 7.4, exc = 298 nm)/nm 488 (relative intensity, corrected spectrum 44), 544 (100), 586 (34) and 621 (16). Gd(III) complex: HPLC analysis: tR = 7.62 min. MS (ESI+): m/z (%) 745.1, 746.1, 747.1 (100), 748.1 and 749.1 (100) [M−H + 2Na]+ ; 701.1, 702.1,703.1 (51), 704.1 and 705.1 [M+H]+ . abs (Tris buffer, pH 7.4): /nm (ε/M−1 cm−1 ) 260 (7250), 297 (8300). 2.4. Luminescence measurements Fluorescence and phosphorescence spectra were obtained with a LS-50B Perkin-Elmer and a Cary Eclipse spectrofluorimeters equipped with a Xenon flash lamp source and a Hamamatsu R928 photomultiplier tube. The measurements were carried out at pH 7.4 in Tris buffer (50 mM) and all samples were prepared with an absorbance between 0.01 and 0.05 at the excitation wavelength in order to prevent the inner-filter effect. Phosphorescence spectra at 77 K of the free ligand and its gadolinium complex were carried out in a Tris buffer (pH 7.4)/glycerol (4/1) mixture and recorded with the LS-50B Perkin-Elmer spectrofluorimeter equipped with the low-temperature accessory No. L2250136. The triplet state energy levels were determined from the shortest-wavelength phosphorescence bands, which were assumed to be the 0–0 transitions. Spectra were corrected for both the excitation light source variation and the emission spectral response. Lifetimes  (uncertainty ≤5%) are made by monitoring the decay at a wavelength corresponding to the maximum intensity of the emission spectrum, following pulsed excitation. They are the average values from at least five separate measurements covering two or more lifetimes. The luminescence decay curves were fitted by an equation of the form I(t) = I(0) exp(-t/) by using a curve-fitting programme. High correlation coefficients were observed in each cases (higher than 0.999). The luminescence quantum yields (uncertainty ± 15%) were determined by the method described by Haas and Stein [34], using as standards [Ru(bpy)3 ]2+ in aerated water (˚ = 0.028 [35]) for the Eu(III) complexes or quinine sulfate in 1 N sulfuric acid (˚ = 0.546 [36]) for the Tb(III) complexes and corrected for the refractive index of the solvent. They were measured according to conventional procedures with diluted solutions (optical density < 0.05).

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Scheme 2. Synthesis of Ligand L6 as its sodium salt.

3. Results and discussion 3.1. Preparations of ligand and complexes Ligand L6 was obtained according to the synthetic methodology depicted in Scheme 2. Alkylation of DO3A(tBuO)3 (3) with bromo N-pyrazolyl derivative, 2, was carried out in CH3 CN at reflux in the presence of Na2 CO3 as the base and afforded macrocyclic compound 4 in 93% yield after purification by column chromatography on alumina. Saponification of the tert-butyl and ethyl ester functions was achieved in a methanol–water solution in the presence of NaOH (4 equiv.) and afforded ligand L6 as its sodium salt. The structure and the purity of this ligand were assessed by NMR, HR-MS, elemental and HPLC analyses. Starting material 3 was prepared according to the literature by reaction of commercially available cyclen with 3.3 equivalents of tert-butylbromoacetate at room temperature and was readily purified by crystallization from toluene [33]. For the preparation of 2, the key step involves an N-arylation of pyrazole which includes functional substituent with heteroaryl bromide. For that, we have taken advantage of reports claiming that species derived from copper iodide and simple diamines promote efficiently the N-arylation of nitrogen heterocycles such as imidazole and pyrazole [37]. Thus the reaction between 3-ethyl ester pyrazole, obtained in two steps from 3-methylpyrazole according to literature procedures [32], and 2-bromo-6-methyl-pyridine was carried out in the presence of K2 CO3 , CuI and commercial trans-1,2-cyclohexanediamine in refluxing dioxane and afforded biheterocycle 1 in a 54% isolated yield. The isolated compound 1 exhibits chemical shifts (ı = 6.90 and 8.57 ppm) and coupling constants (J = 3 Hz) of the hydrogen atoms present in the pyrazole ring which are aligned with a structure in which the pyrazole ring is linked with the ␣-nitrogen atom of the heterocyclic unit [38]. As expected, the coupling occurred preferentially at the less-hindered N atom. For targeting the bromomethyl derivative 2, we opted for a free-radical bromination of the benzylic methyl group. The methyl group of 1 was monobrominated via classical radical conditions (NBS, benzoyl peroxide, and under irradiation) in CCl4 , affording compound 2 in 41% yield after purification by column chromatography on silica. Complexation reactions were carried out in aqueous solutions, typically by the addition of equimolar quantities of the appropriate lanthanide chloride to the aqueous solution of macrocycle 5. These solutions were then adjusted in Tris buffer (50 mM, pH 7.4) at a final concentration of 3 × 10−5 M for absorption and 10−6 M for emission spectroscopies. The complexation was detected by changes in the ligand absorption spectrum upon addition of

the lanthanide salt but also by changes in the europium emission spectrum (vide infra). The stoichiometry of the association between the ligand and the lanthanide ions was 1:1 as evaluated by ESI+ -MS analyses. The Ms spectra revealed in all cases the pseudomolecular peak or sodium adducts as the major species, with, in the case of the Eu(III) and Gd(III) complexes, the expected isotopic distributions of these ions. The reversed-phase (RP) – HPLC chromatograms of these complexes formed in situ showed only one single peak corresponding of the desired complex. Interestingly, The Eu(III), Tb(III) and Gd(III) complexes showed identical retention times, suggesting that the Ln(III) ions share the same coordination sphere in these complexes. All the photophysical properties (, ˚) of solutions of L6 .Eu and L6.Tb in aerated Tris buffer solutions remained unchanged after several days at room temperature, which highlights the kinetic inertness of these complexes in aqueous media. Moreover, when the Tb(III) complex was challenged in the presence of a 100-fold excess of chelating agent such as diethylenetriaminepentaacetic acid (DTPA), a competing ligand which binds strongly to Tb(III) (log Kcond DTPA.Tb = 18.3 at pH 7.4 [39]), luminescent experiments showed that no dissociation (transchelation reaction) occurs after four days. This is an improvement over the behaviour of Tb(III) complexes derived from acyclic and macrocyclic ligands L2 and L3 , earlier reported in our laboratory [24,25]. These results are in agreement with the high kinetic stability of Ln(III) complexes derived from DO3 A ligands. 3.2. Ligand-centred transitions As above mentioned, the energy levels of the ligand excited states play a leading role in the intramolecular energy transfer process (L* → Ln*). Consequently, this study was performed on the free ligand L6 and its Gd(III) complex. It was well established that Gd(III) complexes allow the identification of energies of the ligand-centred (LC) excited states in a structure identical with those of the corresponding Eu(III) and Tb(III) complexes, on both sides of Gd(III) in the lanthanide series [40]. The Gd(III) excited levels have energies (>31,000 cm−1 ) much higher than those typical of heteroaromatic ligand triplet states, preventing any ligand-to-metal energy transfer process. Consequently, in Gd(III) complexes, the fluorescence and phosphorescence spectra will be composed by transitions arising from the singlet and triplet ligands levels. The ligand-centred absorption and emission properties are collected in Table 1. The UV/vis absorption spectrum of the free ligand in Tris buffer solution (pH 7.4) displays two absorption bands in the UV domain at 259 and 286 nm, that according to their molar absorption coefficients (ε > 7000 M−1 cm−1 ) are attributed to ␲ → ␲*

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Table 1 Ligand-centred absorption and emission maxima of ligand L6 and its corresponding Gd(III) complex. Compound

␲ → ␲*a max (nm) (log ε)

1

␲␲*a,b max (nm)

L6

259 (3.85) 286 (4.05)

330

400

800

L6 .Gd

260 (3.86) 297 (3.92)

335 (32,050)

410 (26,200)

21

3

␲␲*c max (nm)d

 (ms)

a

In Tris buffer (0.05 M, pH 7.4) solutions at 298 K. From fluorescence data in solutions at 298 K, max are given from the maximum of the band envelope, and the energies of the 0–0 transition (within brackets) are given in cm−1 . c From phosphorescence data in frozen solutions at 77 K, in Tris buffer (pH 7.4)/glycerol (4/1). d max are given from the maximum of the band envelope, and the energies of the 0–0 transition (within brackets) are given in cm−1 . b

transitions centred on the PyPzCOOH moiety. At room temperature, UV excitation in the ␲–␲* absorption bands of the ligand results in a broad emission band in the range 300 − 400 nm with a maximum around 330 nm, and which disappears upon enforcement of a 50 ␮s time delay. Upon decreasing the temperature at 77 K, a second emission band is observed in the range 370 − 500 nm with a maximum around 400 nm and a long luminescence lifetime (800 ms). In these spectra, the high-energy band is attributed to a singlet state emission of the ligand, whereas the band at lower energy upon time resolution is attributed to a triplet state of the ligand. Upon Gd(III) complexation, the lowest-energy absorption band undergoes a red shift (11 nm) and a hypochromic effect (∼25%) (Fig. 1). This bathochromic shift is higher than those observed in lanthanide complexes with other ligands containing N-pyrazolylpyridine chromophore (∼5 nm) [24,25]. On the other hand, the ligand-centred luminescence of Gd complex displays essentially the same features as those of the free ligand. The energies of both the singlet and triplet LC states are slightly red shifted (450 and 600 cm−1 , respectively). The effect from the lanthanide ion is more apparent on the fluorescence intensity and the rate of deactivation of the triplet excited state. In Gd complex, the total fluorescence intensity of the ligand at room temperature is decreased by a factor 2, while the lifetime of the triplet state at 77 K is decreased by an order of magnitude of about 1.6 relative to that of the free ligand. It seems likely that these results are due to an increase of the rate of 1 ␲␲* → 3 ␲␲* intersystem crossing, caused by the heavy atom effect of the paramagnetic metal ion [41]. From the spectra of Gd(III) complex (Fig. 2), the energy of the 0–0 transition of the first excited singlet state of the ligand was estimated to be 32,050 cm−1 (intercept of the absorption and fluorescence spectra) and the value of the 0–0 energy for the ligand triplet state was estimated to be 26,200 cm−1 (highest energy peak of the structured phosphorescence emission). This latter value is

close to those reported for L2 .Gd and L3 .Gd complexes (26,400 and 25,950 cm−1 , respectively). In L6 .Gd the energy gap between the 0-phonon transitions of the singlet and triplet states (5850 cm−1 ) is slightly above the ideal value (5000 cm−1 ) generally accepted for an efficient intersystem crossing, which plays a significant role in the overall sensitizing process [42]. In the following, these values will be taken for the discussion on the photophysical properties of the Eu(III) and Tb(III) complexes. On the other hand, we can already note that the ligand emission in the region 300 − 500 nm could not be detected in the emission spectra of Eu(III) and Tb(III) complexes suggesting that efficient energy transfer occurs between the ligand excited states and the emissive level of these two ions. 3.3. Eu- and Tb-centred luminescence Table 2 summarizes the main electronic and photophysical properties of Eu(III) and Tb(III) complexes obtained in air equilibrated Tris buffer (50 mM, pH 7.4). The absorption properties of aqueous solutions of these complexes are very similar to those observed for the corresponding Gd complex. At room temperature and upon excitation into the ligand levels, Eu(III) complex exhibits five bands in the 570 − 750 nm range, resulting from deactivation of the 5 D0 excited state to the corresponding ground state 7 F (J = 0 − 4) (Fig. 3). The hypersensitive 5 D → 7 F and the sensiJ 0 2 tive 5 D0 → 7 F4 transitions are dominant and account for ∼80% of the total emitted intensity. The intensity of the 5 D0 → 7 F2 transition (electric dipole) being stronger than that of the 5 D0 → 7 F1 transition (magnetic dipole), I(7 F2 )/I(7 F1 ) = 2.9, this implies that the coordination environment of the Eu(III) ion is asymmetric. On the other hand, the intensity repartition of these bands is very different

100

80

(a)

(b)

(c)

Intensity (a.u.)

0.7

Absorbance

0.6 0.5 0.4 0.3

60

40

20

0.2 0

0.1 0 220

240 260 280 300 320 340 360 380 400 420 440 460 480

240

260

280 300 Wavelength (nm)

320

340

360

Fig. 1. Absorption spectra of free ligand L6 (- - -) and its corresponding Gd(III) complex (––) in Tris buffer solution (pH 7.4).

Wavelength (nm) Fig. 2. Normalized (a) absorption (- - -), (b) fluorescence (––) and (c) time-resolved phosphorescence (-.-.-) spectra of L6 .Gd complex. The absorption and fluorescence spectra were measured at 298 K in Tris buffer (pH 7.4) and the phosphorescence spectrum at 77 K (time-delay 0.1 ms) in Tris buffer-glycerol (4:1 v/v) glassy matrix.

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Table 2 Metal luminescence data of L6 ·Eu and L6 ·Tb in aerated Tris buffer (50 mM, pH 7.4) solutions. Compound L6 .Eu L6 .Tb

max (nm) (log ε) 258 (3.85) 297 (3.90) 260 (3.85) 298 (3.90)

em (nm) 615

 H2O a (ms) (298 K) 0.84

 H2O a (ms) (77 K) 1.08

 D2O a (ms) (298 K) 1.63

 D2O a (ms) (77 K) 1.68

qb (298 K) 0.30

qb (77 K) 0.02

544

2.30

2.49

3.00

3.05

0.21

0.07

a Determined by excitation into the lowest-energy ligand-centred absorption band and recording the intensity of the 5 D0 → 7 F2 (615 nm), 5 D4 → 7 F5 (544 nm) for Eu(III), Tb(III), respectively. b Number of coordinated H2 O molecules calculated using the Horrocks equation (Eu complex) [46] and parker equation (Tb complex) [47].

100 90 80 Intensity (a.u.)

70 60 50 40 30 20 10 0 220

300

400

500

600

700

800

Wavelength (nm) Fig. 3. Normalized and corrected excitation (—, em = 615 nm) and emission ( , ex = 297 nm) spectra of the L6 .Eu complex at 298 K in Tris buffer solution (pH 7.4).

from the spectrum of EuCl3 in Tris buffer (I(7 F2 )/I(7 F1 ) = 0.7), reflecting the substitution around the metal ion by coordinating atoms of the ligand of water molecules. Similarly, through ligand sensitization, the Tb complex leads to green emission from the 5 D4 → 7 FJ (J = 6 − 3) transitions in the range 450 − 650 nm (Fig. 4). The sensitive 5 D4 → 7 F5 transition at 544 nm is the most prominent and accounts for ∼50% of the total emitted intensity. Recording the intensity of the 5 D0 → 7 F2 or 5 D4 → 7 F5 transitions as a function of excitation wavelength confirms that the PyPz-COOH unit acts as light-absorbing centre for collecting UV photons and transferring them to the lanthanide centre. As is visible in Figs. 3 and 4, the excitation spectra closely resemble the absorption spectrum of the

100 90 80 Intensity (a.u.)

70 60 50 40 30 20 10 0 220

300

400

500

600

700

800

Wavelength (nm) Fig. 4. Normalized and corrected excitation (—, em = 544 nm) and emission ( , ex = 298 nm) spectra of the L6 .Tb complex at 298 K in Tris buffer solution (pH 7.4).

incorporated antenna chromophore. In these spectra, the absence of additional features due to intra -Eu3+ or -Tb3+ absorbencies confirms the sensitized character of the observed emissions. The luminescence lifetimes of the complexes were measured at both ambient (298 K) and low (77 K) temperatures by timeresolved luminescence spectroscopy. In all cases, the measurement of luminescence lifetimes led to monoexponential fittings of the decay curve, suggesting that both lanthanide ions are coordinated in identical binding sites. The metal luminescence lifetime of the Tb complex is 2.30 ms at room temperature, a remarkably high value in comparison with those usually found in Tb complexes of DO3A derivatives containing a diazaheterocyclic chromophore, such as bipyridine, phenanthroline or tetraazatriphenylene (0.31 <  < 1.56 ms [43–45]). The luminescence lifetimes of L6 .Eu and L6 .Tb are also longer than those reported for L1 –L3 .Eu and L1 –L3 .Tb complexes, suggesting that Ln(III) ions are better protected by ligand L6 towards the interaction with water molecules. In this direction, lifetime measurements in H2 O and D2 O give usual information regarding the hydration number q (i.e. the number of water molecules directly coordinated to the Ln(III) ion) because of the differing abilities of the O H and O D oscillators to quench the Ln(III) – based excited states [9]. From these lifetime measurements it is possible to evaluate q by using empirical relationships (Eqs. (1) and (2)) which take into account the effect of outer sphere water molecules [46,47]. qEu = 1.11(1/ H2O − 1/ D2O − 0.31)

(1)

qTb = 5(1/ H2O − 1/ D2O − 0.06)

(2)

The values obtained for Eu and Tb complexes are in good agreement with one another at ∼0.25 at room temperature and ∼0.05 at 77 K, with a generally accepted uncertainty of ±0.2. These results are in agreement with a coordination sphere of the lanthanide ion perfectly shielded from the water environment. For both complexes, lifetimes in D2 O solutions at 298 and 77 K are similar, indicating that temperature-dependent non-radiative decay processes do not occur in these complexes. This shows that a backenergy transfer from the metal-centred level to the ligand-centred triplet level, which is observed particularly for Tb(III) complexes does not seem to be operative in the investigated complexes. These results are consistent with the energy value of the lowest ligand triplet excited state, experimentally observed in the Gd complex. The main emitting level of 5 D4 for Tb(III) ion is ∼20,500 cm−1 and thus the energy gap 3 ␲␲*–5 D4 (5700 cm−1 ) is widely above the limiting value of 1850 cm−1 proposed by Latva and co-workers to prevent such a Tb(III)-ligand reversible process [40]. On the other hand, looking at the energy of the ligand triplet state, we can conclude that an energy transfer to the Eu(III) ion is feasible to the 5 DJ (J = 0–3) levels of the metal. The energy gaps between the triplet and the 5 D0 (8900 cm−1 ), 5 D1 (7200 cm−1 ) and 5 D2 (4700 cm−1 ) levels are so large to prevent back-energy transfer. The temperature independence of the lifetime of the Eu complex indicates also that no upper-lying ligand-to-metal charge transfer (LMCT) state is thermally accessible from the 5 D0 emitting state. Quenching

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Table 3 Observed quantum yield (˚) and calculated luminescence parameters (˚Ln , sens , kr and knr ) for the two investigated complexes using the experimental quantities ˚,  298K (H2 O) and  77K (D2 O) in Tris buffer solution (pH 7.4). Compound

˚ (%)

˚Ln (%)

sens (%)

kr (s−1 )

knr (s−1 )

L6 .Eu L6 .Tb

4 45

50 75

8 60

595 328

595 107

mechanisms induced by the presence of LMCT state is also known to be of great importance for Eu(III) complexes [10,48,49]. The overall luminescence quantum yields (˚) of these complexes were determined experimentally by excitation of the ligand (exc = 297 nm) and are strongly metal dependent. The quantum yield of the Tb(III) complex is eleven fold higher than the corresponding Eu(III) one, reaching a substantial value of 45% in aqueous solution, in line with those reported for Tb(III) complexes derived from podand, macrocyclic or cryptand ligands L1 –L3 [23–25]. This ˚ value is quite competitive with those reported for Tb(III) complexes derived from DO3A ligand and sensitized by other diazaheterocycles [43–45]. It is also interesting to note that this quantum yield is higher or close to the ones reported for terbium-based commercial luminescent probes. DTPA-Cs124-Tb (Invitrogen) displays a 32% overall quantum yield, while Lumi4TM -Tb cryptate (CISBIO International) based on 2-hydroxyisophthalamide chromophoric and chelating units is characterized by a 50% overall quantum yield [20,50]. As far as the brightness parameter is concerned, that is the product of the extinction coefficient at the excitation wavelength and the overall quantum yield, the calculated ˚ × ε values are 320 and 3580 M−1 cm−1 for L6 .Eu and L6 .Tb, respectively. The terbium complex provides appealing brightness properties at 300 nm, but for a system to be employed for biological purposes, a longer excitation wavelength maximum must be available, as underlined by a recent report in this area [51]. To gain a better understanding of the photophysical properties of the designed ligand, it was appropriate to determine the efficiency of the ligand-to-metal energy transfer (sens ). To simplify the discussion, we assume that sens involves the inter-system crossing efficiency of the chromophoric antenna (˚isc ) and the subsequent energy migration from the ligand triplet state to the excited levels of the lanthanide ion (˚ET ). Thus we analyzed the luminescent behaviour of the Eu(III) and Tb(III) complexes in terms of Eq. (3) [52]. In this equation ˚ represents the ligand-sensitized quantum yield as determined experimentally and ˚Ln represents the quantum yield for metal-centred luminescence (also termed intrinsic quantum yield), i.e. upon direct f–f excitation. The ˚Ln value is difficult to determine experimentally owing to the weak absorption cross-section of f–f transitions (ε < 10 M−1 cm−1 ), but can be calculated from Eq. (4), where  is the experimentally observed luminescence lifetime at room temperature, which is influenced by non-radiative processes ( = (kr + knr )−1 ) and  rad is the natural or pure radiative lifetime, which is not affected by these processes ( rad = kr −1 ). With the assumption that the decay process at 77 K in a deuterated solvent is purely radiative, it was assumed in several reports that the value of  determined in D2 O at 77 K may be taken for  rad [10,25,53–55]. This is obviously a conscientious simplification and the thus obtained decay times are probably shorter than the real radiative lifetimes. However, this way of working allows a direct comparison between Eu(III) and Tb(III) complexes. ˚ = sens × ˚Ln = ˚isc × ˚ET × ˚Ln

(3)

˚Ln = kr /(kr + ˙knr ) =  298K / rad =  298K (H2 O)/ 77K (D2 O)

(4)

Using the above equations and this assumption, Table 3 summarizes ˚, ˚Ln , sens , radiative (kr ) and non radiative (knr ) decays rates. The intrinsic quantum yields of Ln(III) estimated from the

ratio  obs / rad are high (>50%) for the two complexes, reflecting a similar coordination environment and the absence of inner water molecules. In contrast, the sensitization efficiency (sens ) is 7.5 fold lower for the Eu(III) complex. Assuming than ˚isc is similar in L6 .Eu and L6 .Tb, the difference observed for these two complexes in ˚ET may be attributed to a poor match between the ligand triplet energy and the acceptor lanthanide state. Looking at this energy value of 26,200 cm−1 , it is likely that the excitation energy is primarily transferred to the 5 D3 (24,500 cm−1 ), 5 D2 (21,500 cm−1 ) or 5 D1 (19,000 cm−1 ) levels of Eu(III) ion which decay nonradiatively to the lower-lying 5 D0 level. During this transfer from upper 5 DJ levels to the emitting 5 D0 level, quite significant energy losses can consequently take place. Since the energy of the 5 D3 level of the Tb(III) ion is about 26,250 cm−1 , the energy is directly transferred to the lowest excited resonance level of Tb(III) 5 D4 in L6 ·Tb. On the other hand, although no water molecule is coordinated to the metal ion, the overall quantum yield noted for the Tb(III) complex in the present study (˚ = 45%) is similar to those of monoaquo Tb(III) complexes derived from ligands L2 and L3 . Moreover, the sensitization efficiency of L6 ·Tb (sens = 60%) is found somewhat inferior as compared with those of L2 ·Tb and L3 ·Tb complexes (sens = 80 and 81%, respectively) [25,56]. As the efficiency of Förster energy transfer from the antenna to the lanthanide follows an r6 dependence, this suggests that geometric parameters, that is the chromophore-lanthanide separation but also the angle of orientation of the chromophore with respect to the Ln(III) ion, have to be taken into account to explain the results obtained in terms of ligand-to-metal energy transfer for these three complexes [57]. Finally, a comment has to be made regarding the high knr value. Decadentate ligand L6 provides higher number of donor atoms than that usually required by Eu(III) and Tb(III) ions in aqueous solutions (generally, CN = 9). So, we cannot exclude that a dynamic process, implying a reversible partial de-coordination of the carboxylic acid moiety for instance, contributes to this high knr value. 4. Conclusion The preparation and emission properties of 1:1 lanthanide (Eu(III) and Tb(III)) complexes of the new water-soluble ligand DO3A-PyPzCOOH are reported. These complexes are characterized by the absence of water molecules in the first coordination sphere of the metal ion, and are stable in aqueous media and in the presence of strongly binding competing ligands such as DTPA. In addition to a long emission lifetime (2.30 ms), the Tb(III) complex exhibits a bright green luminescence in aqueous solutions (Tris buffer pH 7.4) with a quantum yield of 45%, which favourably compare with those reported for other antenna/DO3A systems. This high quantum yield is attributable to the good match between the 5 D4 level of Tb(III) and the lowest triplet state energy-level of the ligand (E (3␲␲* − 5 D4 ) = 5700 cm−1 ) leading to an efficient ligand-to-metal energy transfer (sens = 60%). By contrast the poor sensitization and luminescence efficiency (sens = 8% and ˚ = 4%) observed for the Eu(III) complex can be explained on the basis of the larger energy gap between the ligand triplet state and the 5 D0 level of Eu(III) (E (3␲␲* − 5 D0 ) = 8900 cm−1 ). References [1] A. Thibon, V.C. Pierre, Principles of responsive lanthanide-based luminescent probes for cellular imaging, Analytical and Bioanalytical Chemistry 394 (2009) 107–120. [2] A.K. Hagan, T. Zuchner, Lanthanide-based time-resolved luminescence immunoassays, Analytical and Bioanalytical Chemistry 400 (2011) 2847–2864. [3] S.J. Butler, D. Parker, Anion binding in water at lanthanide centres: from structure and selectivity to signalling and sensing, Chemical Society Reviews 42 (2013) 1652–1666. [4] J.-C. Bunzli, Lanthanide luminescence for biomedical analyses and imaging, Chemical Reviews 110 (2010) 2729–2755.

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