Synthesis, Characterization, Ion-Binding Properties, and Fluorescence

May 8, 2009 - Synthesis, Characterization, Ion-Binding Properties, and Fluorescence Resonance Energy Transfer Behavior of Rhenium(I) Complexes Contain...
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J. Phys. Chem. C 2009, 113, 11674–11682

Synthesis, Characterization, Ion-Binding Properties, and Fluorescence Resonance Energy Transfer Behavior of Rhenium(I) Complexes Containing a Coumarin-Appended 2,2′-Bipyridine† Vivian Wing-Wah Yam,*,‡,§ Hai-Ou Song,‡,§ Steve Tak-Wah Chan,§ Nianyong Zhu,§ Chi-Hang Tao,§ Keith Man-Chung Wong,§ and Li-Xin Wu‡ State Key Laboratory of Supramolecular Structure and Materials and College of Chemistry, Jilin UniVersity, Changchun 130012, P.R. China, and Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, P.R. China ReceiVed: February 20, 2009; ReVised Manuscript ReceiVed: April 9, 2009

A series of luminescent rhenium(I) complexes of 2,2′-bipyridine appended with a coumarin fluorophore has been successfully synthesized and characterized. Their photophysical behavior, ion-binding properties, and fluorescence resonance energy transfer (FRET) behavior from the coumarin donor to the rhenium(I) bipyridyl acceptor were studied. The FRET efficiencies were found to vary upon addition of various metal ions with different sizes due to conformational changes. Their stability constants have been determined by both UV-visible and luminescence spectrophotometric methods. Introduction Fo¨rster (or fluorescence) resonance energy transfer (FRET) is an excited-state energy transfer process from the initially excited donor molecule to an acceptor molecule without involving an emission step from the donor molecule. The efficiency of FRET depends on the degree of spectral overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor, and the distance and relative orientations between the donor and acceptor molecules. Typically, the rate decreases rapidly beyond ∼100 Å. As the rate of this type of energy transfer is inversely proportional to the sixth power of the distance between the donor and acceptor molecules,1 such strongly distance-dependent property can be utilized to provide useful information on the donor-acceptor distance, to determine the extent of association based on proximity and the distance of closest approach between the donor-acceptor pairs. On the basis of this, FRET is, in particular, an important and useful technique for the assessment of biological phenomena involving changes in molecular proximity that occur within similar distance regime. A number of biological and clinical applications have stemmed from this technique, which include immunoassays,2 automated DNA sequencing,3 protein and nucleic acid structure and conformation monitoring,4 receptor-ligand interactions,5 detection of singlebased mutations,6 chemosensing7 and others. Although there are a number of reports based on FRET, most of them are confined to systems containing two organic chromophores requiring relatively short wavelength, high-energy excitation sources. Until recently, luminescent semiconductor quantum dots (QDs) have proven to be potential substituents for traditional organic fluorophores in FRET-based sensing applications.8 Although f-block lanthanide-based FRET systems are known,9 it is surprising that the exploitation of the use of †

Part of the “Hiroshi Masuhara Festschrift”. * To whom correspondence should be addressed. E-mail: wwyam@ hku.hk(V.W.-W.Y.) Fax: (852) 2857-1586. Tel: (852) 2859-2153. ‡ Jilin University. § The University of Hong Kong.

luminescent d-block transition metal complexes with relatively long-lived excited states and low excitation energy used as the replacement of organic chromophores in FRET is rare and limited.10 Of particular interest is the d6 transition metal polypyridine complexes, especially those of ruthenium(II) and rhenium(I),11 which are well-known to show rich MLCT excited state properties. Recently there has been a growing interest in the study of FRET systems involving metal polypyridine complexes, especially that of ruthenium(II) and osmium(II).10 As a continuation of our previous studies on a series of rhenium(I) complexes containing a pyridine ligand with coumarin group,12 we report herein the synthesis, characterization and photophysical properties of several rhenium(I) complexes of 2,2′-bipyridine ligands with oligoether-tethered coumarin derivatives, [Re(CO)3(L1)Br] (1), [Re(CO)3(L2)Br] (2), [Re(CO)3(L3)Br] (3), [Re(CO)3(L4)Br] (4), and [Re(CO)3(L5)Br] (5) (Scheme 1). Perturbation of the FRET process by metal ions was explored. The interaction of the complexes with metal ions was investigated by UV-vis, luminescence, and NMR spectroscopic methods, and the binding constants of the complexes for various metal ions were determined. Experimental Section Materials and Reagents. Rhenium(I) pentacarbonyl bromide and thionyl chloride were purchased from Aldrich Chemical Co. Tetraethylene glycol, pentaethylene glycol, coumarin-3carboxylic acid, and anhydrous potassium carbonate were all of analytical grade and were purchased from Lancaster Synthesis Ltd. Tetra-n-butylammonium perchlorate, magnesium(II) perchlorate, calcium(II) perchlorate tetrahydrate, barium(II) perchlorate, lithium perchlorate, and sodium perchlorate were purchased from Aldrich Chemical Co. with purity over 99.0%. 4-Bromomethyl-4′-methyl-2,2′-bipyridine13 was synthesized according to the literature method. Safety Note. Caution! Metal perchlorate salts are potentially explosive. Only small amounts of these materials should be handled and with great caution. Synthesis. bpy-O3-OH. A mixture of 4-hydroxymethyl-4′methyl-2,2′-bipyridine (0.50 g, 2.50 mmol), diethylene glycol

10.1021/jp901561v CCC: $40.75  2009 American Chemical Society Published on Web 05/08/2009

Synthesis and Characterization Rhenium(I) Complexes

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SCHEME 1: Synthetic Routes for the Target Ligands and Complexes

(1.06 g, 10.0 mmol), and NaH (0.96 g, 40.0 mmol) in dry THF (70 mL) was heated to reflux for 24 h with stirring under N2. The mixture was cooled to room temperature, and the solvent was removed under vacuum after filtration. The residue was dissolved in CHCl3 and purified by column chromatography on silica gel with CHCl3-MeOH (100:1, v/v) as the eluent to give the product as a colorless oil. Yield: 0.44 g, 61%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.91 (s, 1H, OH), 2.37 (s, 3H, CH3), 3.57 (m, 2H, CH2O), 3.70 (m, 6H, CH2O), 4.61 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.07 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.23 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 8.17 (s, 1H, 3′-position of bipyridyl H), 8.32 (s, 1H, 3-position of bipyridyl H), 8.46 (d, J ) 5.0 Hz, 1H, 6′-position of bipyridyl H), 8.57(d, J ) 5.0 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 288 (M+). bpy-O4-OH. The procedure was similar to that described for the synthesis of bpy-O3-OH except triethylene glycol (1.50 g, 10.0 mmol) was used instead of diethylene glycol. Yield: 0.62 g, 75%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.67 (s, 1H, OH), 2.44 (s, 3H, CH3), 3.64 (m, 2H, CH2O), 3.71 (m, 10H, CH2O), 4.61 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.15 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.36 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 8.24 (s, 1H, 3′-position of bipyridyl H), 8.32 (s, 1H, 3-position of bipyridyl H), 8.53 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.65 (d, J ) 4.9 Hz, 6-position of bipyridyl H). Positive EI-MS: m/z 332 (M+). bpy-O5-OH. The procedure was similar to that described for the synthesis of bpy-O3-OH except tetraethylene glycol (1.94 g, 10.0 mmol) was used instead of diethylene glycol. Yield: 0.75 g, 80%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.67 (s, 1H, OH), 2.44 (s, 3H, CH3), 3.64 (m, 2H, CH2O), 3.71 (m, 14H, CH2O), 4.61 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.15 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.36 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 8.24 (s, 1H, 3′-position of bipyridyl H), 8.32 (s, 1H, 3-position of

bipyridyl H), 8.53 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.65 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 376 (M+). bpy-O6-OH. The procedure was similar to that described for the synthesis of bpy-O3-OH except pentaethylene glycol (2.38 g, 10.0 mmol) was used instead of diethylene glycol. Yield: 0.84 g, 80%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.67 (s, 1H, OH), 2.44(s, 3H, CH3), 3.65 (m, 4H, CH2O) 3.68 (m, 10H, CH2O), 3.72 (m, 6H, CH2O), 4.68 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.14 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.37 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 8.23 (s, 1H, 3′-position of bipyridyl H), 8.31 (s, 1H, 3-position of bipyridyl H), 8.53 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.65 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 420 (M+). L1. This was prepared by modification of a literature method for a related compound.12 Coumarin-3-carboxylic acid (0.23 g, 1.2 mmol) was heated to reflux with thionyl chloride (5 mL) for 2 h to give a clear solution. The excess of thionyl chloride was removed by distillation and the residue was dried in vacuum. The white solid residue was then dissolved in dry CH2Cl2 (5 mL), and this was added in a dropwise manner to the solution of bpy-O3-OH (0.28 g, 1.0 mmol) with Et3N (0.2 mL) in dry CH2Cl2 (10 mL) at 0 °C. The mixture was heated to reflux for overnight after the addition was complete. The mixture was cooled to room temperature, and the solvent was removed in vacuum. The residue was then purified by column chromatography on silica gel with CHCl3 as the eluent to give L1 as a yellow oil. Yield: 0.30 g, 65%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.43 (s, 3H, CH3), 3.77 (m, 6H, CH2O), 3.87 (m, 2H, CH2O), 4.54 (m, 2H, CH2O), 4.68 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.12 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.28 (m, 3H, 5′-position of bipyridyl H and coumarin H), 7.52 (d, J ) 6.3 Hz, 1H, coumarin H), 7.59 (t, J ) 7.9 Hz, 1H, coumarin H), 8.19 (s, 1H, 3′-position of bipyridyl H), 8.32 (s, 1H, 3-position of bipyridyl H), 8.51

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(m, 2H, 6′-position of bipyridyl H and coumarin H), 8.61 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 461 (M+). L2. This was prepared by the procedure similar to that for the synthesis of L1 except bpy-O4-OH (0.33 g, 1.0 mmol) was used instead of bpy-O3-OH and was isolated as a yellow oil. Yield: 0.35 g, 70%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.44 (s, 3H, CH3), 3.72 (m, 8H, CH2O), 3.86 (m, 2H, CH2O), 4.50 (m, 2H, CH2O), 4.66 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.13 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.32 (m, 3H, 5-position of bipyridyl H and coumarin H), 7.57 (d, J ) 7.8 Hz, 1H, coumarin H), 7.61 (t, J ) 7.1 Hz, 1H, coumarin H), 8.21 (s, 1H, 3′-position of bipyridyl H), 8.30 (s, 1H, 3-position of bipyridyl H), 8.51 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.54 (s, 1H, coumarin H), 8.62 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 504 (M+). L3. This was prepared by the procedure similar to that for the synthesis of L1 except bpy-O5-OH (0.38 g, 1.0 mmol) was used instead of bpy-O3-OH and was isolated as a yellow oil. Yield: 0.44 g, 80%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.44 (s, 3H, CH3), 3.72 (m, 12H, CH2O), 3.86 (m, 2H, CH2O), 4.50 (m, 2H, CH2O), 4.66 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.13 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.32 (m, 3H, 5-position of bipyridyl H and coumarin H), 7.52 (d, J ) 7.8 Hz, 1H, coumarin H), 7.61 (d, J ) 7.1 Hz, 1H, coumarin H), 8.21 (s, 1H, 3′-position of bipyridyl H), 8.30 (s, 1H, 3-position of bipyridyl H), 8.51 (s, 1H, 6′position of bipyridyl H), 8.54 (s, 1H, coumarin H), 8.62 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 548 (M+). L4. This was prepared by the procedure similar to that for the synthesis of L1 except bpy-O6-OH (0.42 g, 1.0 mmol) was used instead of bpy-O3-OH and was isolated as a yellow oil. Yield: 0.44 g, 75%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.43 (s, 3H, CH3), 3.66 (m, 16H, CH2O), 3.83 (m, 2H, CH2O), 4.49 (m, 2H, CH2O), 4.66 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.13 (d, J ) 4.9 Hz, 1H, 5′-position of bipyridyl H), 7.34 (m, 3H, 5-position of bipyridyl H and coumarin H), 7.35 (d, J ) 7.8 Hz, 1H, coumarin H), 7.61 (d, J ) 7.0 Hz, 1H, coumarin H), 8.21 (s, 1H, 3′-position of bipyridyl H), 8.29 (s, 1H, 3-position of bipyridyl H), 8.51 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.56 (s, 1H, coumarin H), 8.62 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 592 (M+). L5. To a solution of NaOH (0.05 g, 2.0 mmol) in H2O (5 mL) and THF (10 mL) was added bpy-O5-OH (0.38 g, 1.0 mmol) at 0 °C. p-TsCl (0.29 g, 1.5 mmol) in THF (10 mL) was then added dropwise. The reaction mixture was stirred at 0 °C for 3 h, and then stirred at room temperature for 12 h. THF was then removed under vacuum to give a yellow oil, which was dissolved in CHCl3 and washed with water. The organic phase was dried over anhydrous Na2SO4. The crude product was purified by column chromatography on silica gel using ethyl acetate-n-hexane (8:2, v/v) as the eluent to afford bpy-O5-OTs as an intermediate (0.60 g, 85%) for the next reaction step. A mixture of bpy-O5-OTs (0.60 g, 1.28 mmol), 7-hydroxy-4methylcoumarin (0.25 g, 1.4 mmol), and K2CO3 (0.35 g, 2.5 mmol) was heated to reflux in CH3CN (30 mL) for 48 h. The solvent CH3CN was then removed under vacuum, and the residual yellow oil was dissolved in CHCl3 and washed with H2O. The crude product was purified by column chromatography on silica gel using ethyl acetate:methanol (9:1, v/v) as the eluent to afford L5 as a yellow oil. Yield: 0.55 g, 80%. 1H NMR (400

Yam et al. MHz, CDCl3, 298 K): δ 2.39 (s, 3H, CH3 group on bipyridyl), 2.44 (s, 3H, CH3 group on coumarin), 3.70 (m, 12H, CH2O), 3.88 (m, 2H, CH2O), 4.17 (m, 2H, CH2O), 4.67 (s, 2H, CH2O group attached to 4-position of bipyridyl), 6.13 (s, 1H, coumarin H), 6.81 (d, J ) 2.4 Hz, 1H, coumarin H), 6.87 (dd, J ) 8.8 and 2.4 Hz, 1H, coumarin H), 7.13 (d, J ) 4.9 Hz, 1H, 5′position of bipyridyl H), 7.36 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 7.46 (d, J ) 8.8 Hz, 1H, coumarin H), 8.23 (s, 1H, 3′-position of bipyridyl H), 8.31 (s, 1H, 3-position of bipyridyl H), 8.52 (s, 1H, 6′-position of bipyridyl H), 8.63 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive EI-MS: m/z 534 (M+). [Re(CO)3(L1)Br] (1). L1 (115 mg, 0.25 mmol) was added to a solution of [Re(CO)5Br] (101 mg, 0.25 mmol) in benzene (35 mL) and the mixture was heated to reflux for overnight under N2, during which the clear colorless solution turned into a yellow suspension. The mixture was cooled to room temperature, and filtered to obtain the yellow residue. The residue was then dissolved in CHCl3 and purified by column chromatography on silica gel with CHCl3 as the eluent to give 1 as a yellow solid. The solid was washed several times with diethyl ether. Recrystallization from CH3CN-diethyl ether afforded the desired complex as yellow crystals. Yield: 172 mg, 85%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.58 (s, 3H, CH3), 3.82 (m, 4H, CH2O), 3.88 (m, 2H, CH2O), 4.57 (m, 2H, CH2O), 4.79 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.32 (m, J ) 6.3 Hz, 3H, 5′-position of bipyridyl H and coumarin H), 7.44 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 7.59 (d, J ) 7.6 Hz, 1H, coumarin H), 7.65 (m, J ) 7.6 Hz, 1H, coumarin H), 8.13 (s, 1H, 3′-position of bipyridyl H), 8.27 (s, 1H, 3-position of bipyridyl H), 8.55 (s, 1H, coumarin H), 8.87 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.94 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive FAB-MS: m/z 811 (M+). Elemental analysis calcd (%) for C29H24N2O9BrRe · 1/ 2CH3CH2OCH2CH3: C, 43.91; H, 3.42; N, 3.11. Found: C, 43.48; H, 3.40; N, 3.10. [Re(CO)3(L2)Br] (2). The procedure was similar to that described for the synthesis of 1 except L2 (126 mg, 0.25 mmol) was used instead of L1. Recrystallization from CH3CN-diethyl ether afforded the desired complex as yellow crystals. Yield: 181 mg, 85%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.56 (s, 3H, CH3), 3.52 (m, 8H, CH2O), 3.85 (m, 2H, CH2O), 4.50 (m, 2H, CH2O), 4.74 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.33 (m, J ) 6.3 Hz, 3H, 5′-position of bipyridyl H and coumarin H), 7.43 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 7.61 (m, 2H, coumarin H), 8.11 (s, 1H, 3′-position of bipyridyl H), 8.26 (s, 1H, 3-position of bipyridyl H), 8.55 (s, 1H, coumarin H), 8.84 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.92 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive FAB-MS: m/z 855 (M+). Elemental analysis calcd (%) for C31H28N2O10BrRe · 1/2H2O: C, 43.10; H, 3.36; N, 3.24. Found: C, 43.08; H, 3.78; N, 3.22. [Re(CO)3(L3)Br] (3). The procedure was similar to that described for the synthesis of 1 except L3 (137 mg, 0.25 mmol) was used instead of L1. Recrystallization from CH3CN-diethyl ether afforded the desired complex as yellow crystals. Yield: 191 mg, 85%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.59 (s, 3H, CH3), 3.78 (m,12H, CH2O), 3.83 (m, 2H, CH2O), 4.47 (m, 2H, CH2O), 4.77 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.33 (m, 3H, 5′-position of bipyridyl H and coumarin H), 7.39 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 7.63 (m, 2H, coumarin H), 8.08 (s, 1H, 3′-position of bipyridyl H), 8.25 (s, 1H, 3-position of bipyridyl H), 8.55 (s, 1H, coumarin H), 8.87 (d, 1H, 6′-position of bipyridyl H), 8.96 (d, J ) 4.9

Synthesis and Characterization Rhenium(I) Complexes Hz, 1H, 6-position of bipyridyl H). Positive FAB-MS: m/z 899 (M+). Elemental analysis calcd (%) for C33H32N2O11BrRe: C, 43.95; H, 3.91; N, 3.11. Found: C, 43.67; H, 3.30; N, 2.80. [Re(CO)3(L4)Br] (4). The procedure was similar to that described for the synthesis of 1 except L4 (148 mg, 0.25 mmol) was used instead of L1. Recrystallization from CH3CN-diethyl ether afforded the desired complex as yellow crystals. Yied: 200 mg, 85%. 1H NMR (300 MHz, CDCl3, 298 K): δ 2.59 (s, 3H, CH3), 3.76 (m,16H, CH2O), 3.82 (m, 2H, CH2O), 4.47 (m, 2H, CH2O), 4.78 (s, 2H, CH2O group attached to 4-position of bipyridyl), 7.34 (m, 3H, 5′-position of bipyridyl H and coumarin H), 7.45 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H and coumarin H), 7.65 (m, 2H, coumarin H), 8.10 (s, 1H, 3′-position of bipyridyl H), 8.27 (s, 1H, 3-position of bipyridyl H), 8.56 (s, 1H, coumarin H), 8.87 (d, J ) 4.9 Hz, 1H, 6′-position of bipyridyl H), 8.96 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive FAB-MS: m/z 944 (M+). Elemental analysis calcd (%) for C35H36N2O12BrRe: C, 44.45; H, 4.16; N, 2.96. Found: C, 44.37; H, 4.11; N, 2.62. [Re(CO)3(L5)Br] (5). The procedure was similar to that described for the synthesis of 1 except L5 (134 mg, 0.25 mmol) was used instead of L1. Recrystallization from CH3CN-diethyl ether afforded the desired complex as yellow crystals. Yield: 188 mg, 85%. 1H NMR (400 MHz, CDCl3, 298 K): δ 2.39 (s, 3H, CH3 group on bipyridyl), 2.59 (s, 3H, CH3 group on coumarin), 3.70 (m, 8H, CH2O), 3.77 (m, 4H, CH2O), 3.86 (t, J ) 4.4 Hz, 2H, CH2O), 4.15 (t, J ) 4.4 Hz, 2H, CH2O), 4.77 (s, 2H, CH2O group attached to 4-position of bipyridyl), 6.14 (s, 1H, coumarin H), 6.83 (d, J ) 2.4 Hz, 1H, coumarin H), 6.90 (dd, J ) 8.8 and 2.4 Hz, 1H, coumarin H), 7.33 (d, 1H, 5′-position of bipyridyl H), 7.39 (d, J ) 4.9 Hz, 1H, 5-position of bipyridyl H), 7.50 (d, J ) 8.8 Hz, 1H, coumarin H), 8.08 (s, 1H, 3′-position of bipyridyl H), 8.25 (s, 1H, 3-position of bipyridyl H), 8.87 (d, 1H, 6′-position of bipyridyl H), 8.96 (d, J ) 4.9 Hz, 1H, 6-position of bipyridyl H). Positive FAB-MS: m/z 885 (M+). Elemental analysis calcd (%) for C33H35N2O10BrRe: C, 44.74; H, 3.95; N, 3.16. Found: C, 44.72; H, 4.10; N, 3.11. Physical Measurements and Instrumentation. 1H NMR spectra were recorded on a Bruker DPX 300 (300 MHz) or Bruker DPX 400 (400 MHz) Fourier-transform NMR spectrometer with chemical shifts reported relative to tetramethylsilane, Me4Si. All positive ion fast atom bombardment (FAB) and electron impact (EI) mass spectra were recorded on a Finnigan MAT95 mass spectrometer. Elemental analyses of the complexes were performed on a Flash EA1112 at the Changchun Institute of Applied Chemistry or a Carlo Erba 1106 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences. The UV-visible absorption spectra were obtained using a Hewlett-Packard 8452A diode array spectrophotometer. Steady state excitation and emission spectra at room temperature were recorded on a Spex Fluorolog-2 model F111 fluorescence spectrofluorometer equipped with a Hamamatsu R-928 photomultiplier tube. Stability Constant Determination. The electronic absorption spectral titration for binding constant determination was performed with a Hewlett-Packard 8452A diode array spectrophotometer at 25 °C. Binding constants for 1:1 complexation were obtained by a nonlinear least-squares fit of the absorbance (X) versus the concentration of the metal ion added (Cm) according to eq 114

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X ) X0 +

Xlim - X0 {[C0] + [Cm] + 1/Ks 2[Co] [([C0] + [Cm] + 1/Ks)2 - 4[C0][Cm]]1/2} (1)

where X0 and X are the absorbance of rhenium(I) complexes at a selected wavelength in the absence and presence of the metal cation, respectively, C0 is the total concentration of rhenium(I) complexes, Cm is the concentration of the metal cation, Xlim is the limiting value of absorbance in the presence of excess metal ion, and Ks is the stability constant. From the emission data, binding constants were also obtained using eq 214 as follows:

I ) I0 +

Ilim - I0 {[C0] + [Cm] + 1/Ks 2[C0] [([C0] + [Cm] + 1/Ks)2 - 4[C0][Cm]]1/2} (2)

where I0 and I are the emission intensity of rhenium(I) complexes at a selected wavelength in the absence and presence of the metal cation, respectively, and Ilim is the limiting value of emission intensity in the presence of excess metal ion. For proton NMR titration experiments of 3 with Mg2+, a solution of 3 was prepared at a concentration of 1.2 × 10-3 mol dm-3 in CD3CN (1 mL). The initial 1H NMR spectrum was recorded and aliquots of Mg2+ were added using a microsyringe. After each addition and mixing, the spectrum was recorded and changes in the chemical shift of certain protons were noted. The results of the experiment were expressed as a plot of chemical shift as a function of the amount of cation added, which was subjected to analysis by curve fitting since the shape of the titration curve is indicative of the stability constant for complex formation. The computer program EQNMR15 was used, which requires a knowledge of the concentration of each component and the observed chemical shift for each data point. Crystal Structure Determination. A yellow crystal of complex 5 with dimensions of 0.30 mm × 0.20 mm × 0.15 mm mounted in a glass capillary was used for data collection at 28 °C on a MAR diffractometer with a 300 mm image plate detector using graphite monochromatized Mo KR radiation (λ ) 0.71073 Å). Data collection was made with 1.5 ° oscillation of φ steps, 10 min exposure time and a scanner distance of 120 mm. A total of 130 images were collected. The images were interpreted and intensities integrated using DENZO.16 The structure was solved by direct methods employing SHELXS97.17 The Re, Br atoms and most of the non-hydrogen atoms were located according to direct methods and successive leastsquares Fourier cycles. The positions of other non-hydrogen atoms were found after successful refinement by full-matrix least-squares using SHELXL-97.18 All 6087 independent reflections (Rint equal to 0.0319, 4691 reflections larger than 4σ(Fo)) from a total 12590 reflections participated in the full-matrix least-squares refinement against F,2 where Rint ) ∑|Fo2 Fo2(mean)|/∑[Fo2]. These reflections were in the range -14 e h e +14, -14 e k e +14, -16 e l e +16 with 2θmax equal to 51.32 °. One crystallographic asymmetric unit consists of one formula unit. Hydrogen atoms were generated using SHELXL-97 and their positions calculated based on the riding mode with thermal parameters equal to 1.2 times that of associated C atoms, and participated in the calculation of the final R-indices. Convergence ((∆/σ)max ) 0.001, av. 0.001) for 426 variable parameters by full-matrix least-squares refinement on F2 reaches to R1 ) 0.028 and wR2 ) 0.0728 with a goodnessof-fit of 0.525. Crystallographic and structural refinement data are given in Table 1.

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TABLE 1: Crystallographic and Structural Refinement Data for 5 empirial formula formula weight temperature wavelength crystal system space group unit cell dimensions

volume Z density (calculated) absorption coefficient F(000) crystal size data collection range index ranges reflections collected independent reflections completeness to θ ) 25.66° absorption correction refinement method data/restraints/parameters goodness-of-fitb on F2 final R indices [I > 2σ(I)]c R indices (all data) largest diff. peak and hole

C33H34BrN2O10Re 884.73 301(2) K 0.71073 Å triclinic P1j (no. 2) a ) 12.019(2) Å, b ) 12.068(2) Å, c ) 13.281(2) Å; R ) 109.45(3)°, β ) 105.58(3)°, γ ) 93.40(3)° 1726.1(5) Å3 2 1.702 g cm-3 4.735 mm-1 872 0.30 mm × 0.20 mm × 0.15 mm 1.99 to 25.66° -14 e h e +14, -14 e k e +14, -16 e l e +16 12590 6087 [R(int)a ) 0.0319] 93.0% none full-matrix least-squares on F2 6087/0/426 0.525 R1 ) 0.0280, wR2 ) 0.0728 R1 ) 0.0406, wR2 ) 0.0834 +0.586 and -0.908 eÅ-3

a Rint ) ∑|Fo2 - Fo2(mean)|/∑[Fo2]. b GooF ) {∑[w(Fo2 - Fc2)2]/ (n - p)}1/2, where n is the number of reflections and p is the total number of parameters refined. The weighting scheme is w ) 1/ [σ2(Fo2) + (aP)2 + bP], where P is [2Fc2 + Max(Fo2,0)]/3. c R1 ) ∑ ||Fo| - |F||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fc2)2] }1/2.

Results and Discussion Synthesis and Characterization. Ligands L1-L4 were prepared by modification of a literature method for the related compound.12 The synthetic routes for the ligands and complexes are summarized in Scheme 1. Conversion of coumarin-3carboxylic acid to coumarin-3-carboxylic chloride was achieved via heating to reflux for 2 h in thionyl chloride. The coumarin3-carboxylic chloride was then reacted with bpy-On-OH (n ) 3-6) and Et3N in CH2Cl2 to give L1-L4. On the other hand, the reaction of 7-hydroxy-4-methylcoumarin and the intermediate bpy-O5-OTs, generated by tosylation of bpy-O5-OH with p-toluenesulfonylchloride, afforded L5 in a slightly different reaction route. The crude products were then purified by column chromatography on silica gel to give L1-L5 as a yellow oil. Complexes 1-5 appended with a coumarin moiety were synthesized according to modification of a literature procedure for the related [Re(CO)3(bpy)Cl].11a,b Recrystallization from CH3CN-diethyl ether afforded the desired complexes as yellow crystals. The identities of complexes 1 - 5 were confirmed by satisfactory 1H NMR spectroscopy, FAB mass spectrometry and elemental analysis. The molecular structure of 5 has also been determined by X-ray crystallography. Crystal Structure Determination. The perspective drawing of 5 is shown in Figure 1 (left). Selected bond distances and bond angles are tabulated in Table 2. The structure of 5 shows a distorted octahedral geometry about rhenium(I) center with three carbonyl ligands in a facial arrangement, which is commonly observed in other related rhenium(I) tricarbonyl diimine complexes.11a-e,12 Similar to other related complexes, the N(1)-Re(1)-N(2) bond angle of 75.03°, which is smaller

Yam et al. than 90°, is due to the bite distances exerted by the steric requirements of the chelating bipyridyl ligand. The bond distances of Re-C and Re-N were 1.909-1.956 and 2.188-2.178 Å, respectively, which were typical of that found in rhenium(I) tricarbonyl diimine complexes.11a-e,12 The coumarin pendant is folded back toward the rhenium(I) diimine moiety, rather than directed away from each other, through the curving back of the flexible oligoether linkage. It is interesting to note that two molecules of 5 were found to exist together to form a dimeric structure, in which the bipyridyl plane in one of the molecules intercalates between the planes of another bipyridyl and the coumarin pendant (Figure 1 (right)). The two bipyridyl planes are essentially coplanar (interplanar angle of 0.0°) with interplanar distance of 3.38 Å, while the interplanar angle between one bipyridyl plane and the coumarin moiety is 17.40 ° with the planes separated by 3.67 Å, indicating the presence of π-π interactions. Although there is only a subtle change in the position of the coumarin pendant tethered to the oligoether linkage, the possibility of an essential difference on the molecular structures of 1 - 4 from the folded structure of 5 cannot be ruled out. Photophysical Properties. The electronic absorption spectra of 1-5 in acetonitrile solution showed intense absorption bands with molar extinction coefficients in the order of 104 dm3 mol-1 cm-1 at ca. 246-336 nm, which were assigned as intraligand (IL) π f π* transitions of the bipyridine and coumarin moieties. The absorption bands with molar extinction coefficients in the order of 103 dm3 mol-1 cm-1 at ca. 380-390 nm were ascribed to the dπ(Re) f π*(bpy-On-CM) metal-to-ligand charge transfer (MLCT) transitions of the respective complexes, typical of rhenium(I) tricarbonyl diimine complex systems.11a-e,12 The photophysical data of 1-5 are tabulated in Table 3. Upon excitation at λ e 440 nm in acetonitrile solution at room temperature, complexes 1-4 were found to exhibit two emission bands, a yellow low-energy emission band at ca. 620 nm and a blue high-energy emission band at ca. 440 nm. On the other hand, high-energy emission band at ca. 382 nm and a low-energy emission band at ca. 625 nm were observed in 5. The highenergy emission bands at ca. 440 and 382 nm were assigned as fluorescence from the derivatives of coumarin-3-carboxylic ester12 and 7-alkoxy-4-methylcoumarin19 in 1-4 and 5, respectively. With reference to other related Re(I) complexes,11a-e,12 the low-energy emission band was attributed to 3MLCT phosphorescence. In such system, the coumarin moiety with high-energy emission could be anticipated as energy donor (D) while the rhenium(I) diimine luminophore with low-energy emission could serve as energy acceptor (A). The ratios of the blue emission intensity at ca. 440 nm to that of the yellow emission at ca. 615 nm, ID/IA, in complexes 1-4 were found to be dependent on the lengths of the oligoether linkage. The ID/IA of complex 1 which contains the shortest separation of triethylene glycol linkage was found to be the smallest. In general, the shorter the separation between the coumarin donor and rhenium(I) diimine luminophore, the greater the efficiency of the energy transfer from the donor to the acceptor, leading to the smaller ID/IA ratio. The energy transfer efficiencies follow the order of 1 > 2 > 3 > 4, as reflected from their trend of ID/IA ratios: 1 (0.30) > 2 (0.40) > 3 (0.50) > 4 (0.55). These results demonstrated that the change in the length of the spacer connecting the donor and the acceptor played an important role in tuning the efficiency of the energy transfer. However, the changes of the energy transfer efficiency by the variation of the donor-acceptor separation is relatively small and this finding at the first glance does not seem to be in line with that predicted

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Figure 1. (Left) Perspective drawing of 5 with atomic numbering Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 50% probability level. (Right) Ball-and-stick model showing the dimeric arrangement of two molecules of 5.

TABLE 2: Selected Bond Distances (Å) and Bond Angles (deg) for 5 with Estimated Standard Deviations (e.s.d.s.) Given in Parentheses Re(1)-N(1) 2.188(4) Re(1)-C(1) 1.909(6) Re(1)-C(3) 1.956(6) C(1)-O(1) 1.165(6) C(3)-O(3) 1.059(6) N(1)-Re(1)-N(2) 75.03(14) Re(1)-C(2)-O(2) 177.6(5) N(1)-Re(1)-C(1) 170.80(17) Br(1)-Re(1)-C(3) 175.51(17)

Re(1)-N(2) Re(1)-C(2) Re(1)-Br(1) C(2)-O(2)

2.178(4) 1.925(6) 2.6178(10) 1.150(6)

Re(1)-C(1)-O(1) 177.5(5) Re(1)-C(3)-O(3) 174.6(6) N(2)-Re(1)-C(2) 173.69(18)

TABLE 3: Photophysical Data of 1-4 in CH3CN at 298 K absorption

emission

complex

λabs/nm (ε/dm3 mol-1 cm-1)

λem/nm

1

247 (22,185), 291 (268,75), 316 (15,430), 336 (9,530), 391 (3,120) 246 (23,930), 291 (30,640), 316 (16,105), 336 (9,765), 391 (3,185) 248 (22,580), 292 (29,300), 316 (15,610), 336 (9,395), 390 (3,100) 245 (21,925), 291 (25,850) 317 (15,510), 339 (8,915), 390 (3,065) 244 (24,565), 292 (25,190), 310 (24,455), 338

431,a 621b

2 3 4 5

a Emission due to the coumarin moiety. rhenium(I) bipyridyl luminophore.

b

Figure 2. UV-vis spectral changes of 3 (4.67 × 10-5 M) in CH3CN (0.2 M nBu4NClO4) upon addition of Mg(ClO4)2. Inset shows a plot of absorbance at 310 nm (9) as a function of [Mg2+] and its theoretical fit (s) according to a 1:1 binding model for 3 with Mg2+ ion in CH3CN (0.2 M nBu4NClO4).

429,a 623b 429,a 621b 429,a 621b 382,a 625b

Emission due to the

from Fo¨rster theory, in which the efficiency of energy transfer changes as a function of the inverse sixth power of the interchromophoric distance. However, with the nonrigid and flexible nature of the oligoether linkage as the spacer, it is not unreasonable to assume that the complex would adopt a folded conformation, with the distance between the donor and the acceptor being shorter than the geometrical length of the oligoether spacer. Such folded conformation would still lead to a larger separation between the donor and acceptor moieties, upon a lengthening of the oligoether spacer that links the coumarin moiety and the Re(I) luminophore, leading to a decrease in the energy transfer efficiencies even though a much less sensitive distance-dependent trend is anticipated due to the folded conformation that gave rise to small perturbation of separation distances upon an increase in the oligoether units. Cation-Binding Properties. Upon addition of metal ions to a solution of complexes 1-4 in CH3CN (0.2 M nBu4NClO4), spectral changes were observed in some of their electronic

Figure 3. UV-vis spectral changes of 3 (4.67 × 10-5 M) in CH3CN (0.2 M nBu4NClO4) upon addition of Ba(ClO4)2. Inset shows a plot of absorbance at 338 nm (9) as a function of [Ba2+] and its theoretical fit (s) according to a 1:1 binding model for 3 with Ba2+ ion in CH3CN (0.2 M nBu4NClO4).

absorption spectra, depending on the nature of the metal ions. For the alkali metal ions, such as Li+ and Na+ ions, only very small spectral changes were detected. In the case of alkaline earth metal ions such as Mg2+, Ca2+, and Ba2+, the low-energy band at about 320-360 nm was found to increase while the high-energy band at about 275 nm decreases with a well-defined isosbestic point at 292-297 nm. In general, Mg2+ with the smallest ionic radius was found to give the largest electronic absorption spectral changes, which could be rationalized by the largest electronic influence exerted by the highest charge density of Mg2+. The UV-visible spectral traces of 3 upon addition of magnesium perchlorate and barium perchlorate were shown in Figures 2 and 3, respectively. The insets show the plots of absorbance at 310 nm as a function of the added metal ion

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TABLE 4: Metal Ion-Binding Constant (log Ks) of 1-4 in CH3CN (0.2 M nBu4NClO4) log Ks metal ion Li

+

Na+ Mg2+ Ca2+ Ba2+

1 a,b 1.57 ( 0.05d a,b 1.64 ( 0.08d 1.88 ( 0.02b 1.96 ( 0.02,c1.96 ( 0.03d 2.46 ( 0.03b 2.34 ( 0.16,c 2.31 ( 0.04d 1.85 ( 0.03b 1.90 ( 0.21,c 1.97 ( 0.07d

2

3

4

a,b 1.81 ( 0.05d a,b 1.87 ( 0.42d 2.08 ( 0.02b 2.20 ( 0.09,c 2.12 ( 0.04d 2.87 ( 0.05b 2.08 ( 0.13,c 2.89 ( 0.05d 2.85 ( 0.020b 2.61 ( 0.20,c 2.89 ( 0.02d

1.54 ( 0.07 1.81 ( 0.02d 1.54 ( 0.07b 2.06 ( 0.07,c 2.13 ( 0.02d 2.15 ( 0.05b 2.27 ( 0.03,c 2.23 ( 0.02d 3.79 ( 0.10b 4.03 ( 0.05,c 4.16 ( 0.02d 4.36 ( 0.01b 4.68 ( 0.04,c 4.69 ( 0.03d

2.26 ( 0.04 1.92 ( 0.03d 2.26 ( 0.04b 2.43 ( 0.07d 2.69 ( 0.06b 2.28 ( 0.06,c 2.49 ( 0.05d 4.12 ( 0.08b 4.35 ( 0.04d 4.13 ( 0.09b 4.29 ( 0.19d

b

b

a

The spectral changes were too small for an accurate determination of the binding constant. b UV-visible spectrophotometric method. Emission method, calculated from the emission intensity of the coumarin moiety (∼440 nm). d Emission method, calculated from the emission intensity of the rhenium moiety (∼615 nm). c

Figure 4. Emission spectral traces of 3 (4.67 × 10-5 M) in CH3CN (0.2 M nBu4NClO4) upon addition of Mg(ClO4)2. Excitation at isosbestic wavelength of 298 nm. Inset shows a plot of emission intensity at 615 nm (9) as a function of [Mg2+] and its theoretical fit (s) for the 1:1 binding of 3 with metal ion in CH3CN (0.2 M nBu4NClO4).

Figure 5. Emission spectral traces of 3 (4.67 × 10-5 M) in CH3CN (0.2 M nBu4NClO4) upon addition of Ba(ClO4)2. Excitation at isosbestic wavelength of 297 nm. Inset shows a plot of emission intensity at 614 nm (9) as a function of [Ba2+] and its theoretical fit (s) for the 1:1 binding of 3 with metal ion in CH3CN (0.2 M nBu4NClO4).

concentration and their theoretical fits according to a 1:1 binding model. The close agreement of the experimental data to the nonlinear least-squares fit14 confirmed the 1:1 binding stoichiometry, from which the binding constants were determined. The binding constants of 1-4 for different metal ions are summarizied in Table 4. The binding studies of K+ and Cs+ ions were not made due to the low solubility of their perchlorate and hexafluorophosphate salts in the solution studied, while no significant changes were observed for the other transition metal ions such as Zn2+, Cd2+ and Hg2+. For 1 and 2 with relatively short oligoether linkages, the binding affinities toward Ca2+ were the largest among the metal ions studied. On the other hand, Ba2+ gave the largest binding constants for the cases of 3 and 4 comprising of longer oligoether linkages. The larger binding constants obtained for Ba2+ in 3 and 4 are ascribed to the longer oligother spacers that show a better fit for the larger Ba2+ ion. It is interesting to note that the

analogous complex 5, in which the coumarin pendant is linked at its 7-position via an ether linkage to the bipyridine unit, was found to give negligible electronic absorption spectral changes upon addition of various metal ions. The difference in the binding behaviors of 5, relative to 1-4 with ester linkage connecting at the 3-position of the coumarin moiety, suggests that the position of the carbonyl group on the coumarin moiety is crucial in determining the binding affinities for metal ions and may play a cooperative role with the oligoether oxygen atoms for the binding of metal ions. The cation-binding abilities of 1-4 were also investigated by emission spectrophotometric studies. With the excitation at the isosbestic wavelength of the corresponding electronic absorption spectra in acetonitrile (0.2 M nBu4NClO4), the emission due to the Re(I) complex acceptor decreases while the coumarin donor fluorescence intensity remains the same upon addition of Li+ and Na+ ions. On the contrary, addition of Mg2+ ions resulted in a large increase in the coumarin donor fluorescence intensity at around 440 nm, and a drop in the Re(I) acceptor emission intensity at around 615 nm. This demonstrated that binding of Mg2+ ions would lead to a poorer FRET efficiency. The binding constants were also obtained from the theoretical nonlinear least-squares fit of the experimental data to a 1:1 binding model. The binding constants are summarized in Table 4, which are consistent with those determined from the UV-visible spectrophotometric studies. The emission spectral changes of 3 upon addition of Mg2+ ions and the corresponding titration curve are shown in Figure 4. In the case of the Ca2+ ion, the emission of the coumarin donor also showed an increase in intensity, with a concomitant reduction in the intensity of the rhenium(I) complex acceptor emission, and the emission spectral changes were not as large as that of the Mg2+ ion, whereas for the Ba2+ ion, both the emission of the donor and the acceptor showed a decrease in intensity and the emission spectral changes of 3 upon addition of Ba2+ ion are shown in Figure 5. In view of the relatively large atomic mass of barium, such heavy atom is anticipated to facilitate the formation of triplet excited state of coumarin via facile intersystem crossing due to the large spin-orbit coupling. The population of such nonemissive triplet excited state may account for the decrease in coumarin emission as well as the MLCT emission. The binding constants of 3 followed the order of Ba2+ > Ca2+ > Mg2+, which has also been demonstrated using the UV-visible absorption method described. Among the metal ions studied, the ionic radius of Ba2+ (Ba2+ ∼1.36 Å, Ca2+ ∼1.00 Å, Mg2+ ∼0.72 Å, Na+ ∼1.02 Å, Li+ ∼0.76 Å)20 is the largest, and appears to fit well into the cavity or pocket created by the

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TABLE 5: Relative Ratios of the Donor to Acceptor Emission Intensity of 1-4 before and after Complexation with Metal Ions 1

2

3

4

metal ionic [(ID/IA)sat]/ [(ID/IA)sat]/ [(ID/IA)sat]/ [(ID/IA)sat]/ ion radius/Å (ID/IA)oa (ID/IA)sat.b [(ID/IA)o] (ID/IA)oa (ID/IA)sat.b [(ID/IA)o] (ID/IA)oa (ID/IA)sat.b [(ID/IA)o] (ID/IA)oa (ID/IA)sat.b [(ID/IA)o] Li+ Na+ Mg2+ Ca2+ Ba2+

0.76 1.02 0.72 1.00 1.35

0.30 0.30 0.30 0.31 0.30

0.64 0.45 1.25 0.90 0.80

2.13 1.50 4.17 2.90 2.67

0.41 0.41 0.40 0.40 0.41

0.90 0.65 1.86 1.64 0.96

2.20 1.59 4.65 4.10 2.34

0.47 0.51 0.50 0.50 0.50

1.79 1.15 2.98 1.85 1.62

3.81 2.25 5.96 3.70 3.24

0.54 0.55 0.53 0.55 0.55

1.14 0.89 1.89 1.82 1.73

2.11 1.67 3.57 3.31 3.15

a (ID/IA)o is the relative ratio of the donor to acceptor emission intensities of the complex before complexation with metal ions. b (ID/IA)sat is the relative ratio of the donor to acceptor emission intensities of the complex at the saturated complexation with metal ions.

Figure 6. Emission spectral traces of 4 (4.87 × 10-5 M) in CH3CN (0.2 M nBu4NClO4) upon addition of Mg(ClO4)2. Excitation at isosbestic wavelength of 296 nm. Inset shows a plot of emission intensity at 612 nm (9) as a function of [Mg2+] and its theoretical fit (s) for the 1:1 binding of 4 with metal ion in CH3CN (0.2 M nBu4NClO4).

polyether chain. It is interesting to note that upon binding of metal ions to the polyether chain, the FRET efficiencies, as reflected from the relative ratios of donor to acceptor emission intensities (ID/IA in Table 5), became poorer. Ba2+ with the largest ionic radius among the group of Mg2+, Ca2+, and Ba2+, showed the least effect on the FRET efficiency upon binding, with Mg2+ being the smallest in size, showing the largest decrease in the FRET efficiency. It is likely that binding of the smaller metal ions would cause the largest changes in the conformation of the molecule, leading to an increase in the donor-acceptor separation that gave rise to the drop in FRET efficiency. A proposed tight folding of the polyether chain upon the complexation of small metal ions may lead to an increase in the donor-acceptor distance. In general, the binding constants of 4, with a longer oligoether linkage were found to be larger than that of 3 for the same metal ion, indicating that the longer polyether chain between the donor and the acceptor showed a better binding affinity toward the metal ions, similar to results obtained from UV-visible spectrophotometric studies. The emission spectral traces of 4 upon addition of magnesium perchlorate and the change of emission intensity at 440 nm as a function of the added metal ion concentration are shown in Figure 6. On the contrary, no observable spectral change was found in 5 upon addition of various metal ions, similar to that obtained from electronic absorption spectrophotometric studies The remote position of the carbonyl group of the coumarin pendant in 5 is unable to function as an auxiliary group with the oligoether linkage for the binding of metal ions. Consequently, the lack of binding ability in 5 would not induce the conformational change and hence the influence to the FRET efficiency. The ion-binding properties of 3 have also been probed by 1H and 13C NMR spectroscopy. The assignment of protons was based on the chemical shifts, coupling patterns and twodimensional [1H-1H] COSY spectrum. In general, a decrease in the electron density at the oxygen atoms would be resulted upon complexation of the metal ions and hence downfield shifts of the signals were observed due to the weakening of the

Figure 7. 1H NMR spectral changes of 3 in CD3CN after addition of (a) Mg(ClO4)2 and (b) Ba(ClO4)2.

shielding effect on the neighboring protons. A control experiment was also carried out by addition of nBu4NClO4 to 3 in CD3CN, where no changes in the chemical shifts were observed, further confirming that the changes in the chemical shifts of 3 upon addition of metal ions were a result of the binding interaction between the complex and the metal ions. The 1H NMR spectra of 3 showing the changes in chemical shifts after addition of the selected Mg2+ and Ba2+ ions are shown in Figure 7. The aromatic coumarin proton adjacent to the ester group (H4) showed the largest downfield shift (4δ) of ca. 0.70 ppm, together with moderate downfield shift (4δ) of ca. 0.20 ppm at the methylene protons (-CH2-) adjacent to the coumarin unit, upon addition of Mg2+. On the contrary, different patterns of chemical shifts were observed in the case of Ba2+ complexation. A large upfield shift (4δ) of ca. 0.72 ppm was observed at the 9-position of coumarin proton (H9), indicating that the binding mode for Ba2+ is different from that for Mg2+. Unlike in the case of Mg2+, the protons of -CH2O- adjacent to the bipyridine ring (H20) were found to exhibit remarkable downfield shift (4δ) of ca. 0.37 ppm, indicating that the O atom of the -OCH2-

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unit adjacent to the bipyridine ring did not bind to the Mg2+ ion, probably due to the small size of the Mg2+ ion. The observation of larger downfield shifts of 13C NMR signal at 11-position upon addition of metal ions indicated that the carbonyl group at the 11-position of the coumarin unit was involved in the binding of metal ions. The strong binding interactions between the oligoether and the metal ions could lead to a large separation between coumarin and the rhenium(I) chromophore and subsequently alter the energy transfer efficiency of the complex. No significant shift of the resonance signals was observed for the 13C NMR spectra of 5 in the absence and in the presence of excess Mg2+ ions, indicating that there is no significant binding of metal ions to 5 as compared to 1-4, suggesting the importance of the position of the coumarin group in the binding process. This has further been supported by binding studies using electronic absorption and emission spectroscopy. In addition, 1H NMR titration study for the binding of Mg2+ ions by monitoring the chemical shift of H4 upon successive addition of Mg2+ ions was performed. The binding constant (log Ks) of 2.27 was obtained by the leastsquares fitting program EQNMR,15 which is consistent with the results from UV-visible absorption and emission studies. The satisfactory agreement of such theoretical fits is also indicative of a 1:1 complexation stoichiometry. Conclusion A series of rhenium(I) 2,2′-bipyridyl complexes with appended coumarin ligands, 1-5, has been synthesized and their photophysical properties have been studied. Excitation of the complexes at λ e 380 nm in acetonitrile solution at room temperature resulted in a strong yellow luminescence, attributed to the 3MLCT phosphorescence of the rhenium(I) diimine acceptor, and a weaker blue emission, assigned as the IL fluorescence of the coumarin donor. Upon addition of various alkali and alkaline-earth metal ions, their electronic absorption and emission spectra were found to vary. The complexation of the metal ions also induce conformational changes that resulted in a variation of the FRET efficiency. 1H and 13C NMR spectroscopy were also employed to probe the ion-binding properties of 3 with Mg2+ and Ba2+ ions. Downfield shifts of the 1H and 13C signals were observed due to the weakening of the shielding effect on the neighboring protons resulting from the decrease of the electron density at the oxygen atoms. Acknowledgment. V.W.-W.Y. acknowledges support under of the Distinguished Research Achievement Achievement Award Scheme from The University of Hong Kong. We acknowledge support from Jilin University and The University of Hong Kong. This work has been supported by the National Natural Science Foundation of China and the Research Grants Council of Hong Kong Joint Research Scheme (NSFC-RGC Project No. N_HKU 737/06). Supporting Information Available: UV-vis and emission spectral changes of 4 upon addition of Mg(ClO4)2 in CD3CN, 13 C NMR spectral changes of complex 3 in CD3CN after addition of Mg(ClO4)2 and Ba(ClO4)2, and cif file for complex 5. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fo¨rster, T. Ann. Phys. 1948, 2, 55. (2) (a) Morrison, L. E. Anal. Biochem. 1988, 174, 101. (b) Khanna, P. L.; Ullman, E. F. Anal. Biochem. 1980, 108, 156.

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