Modulation of Pt → Ln Energy Transfer in PtLn2 (Ln = Nd, Er, Yb

Modulation of Pt f Ln Energy Transfer in PtLn2 (Ln ) Nd, Er, Yb) Complexes with 2,2′-Bipyridyl/2,2′:6′2′′-Terpyridyl Ethynyl Ligands Hai-Bing Xu,† Li-Yi Zhang,‡ Xiao-Ming Chen,† Xiu-Ling Li,‡ and Zhong-Ning Chen*,‡

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 569–576

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou 510275, China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed August 7, 2008; ReVised Manuscript ReceiVed October 13, 2008

ABSTRACT: Reaction of (CH3)3SiC≡Cbpy (bpy ) 2,2′-bipyridyl), (CH3)3SiC≡CC6H4tpy (tpy ) 2,2′:6′2′′-terpyridyl), or (CH3)3SiC≡ CC6H4C≡Cbpy with Pt(But2bpy)Cl2 in the presence of KF and CuI gave Pt(But2bpy)(C≡Cbpy)2 (1), Pt(But2bpy)(C≡CC6H4tpy)2 (5), or Pt(But2bpy)(C≡CC6H4C≡Cbpy)2 (9), respectively. Incorporating Ln(hfac)3 (Ln ) Nd, Er, Yb; hfac ) hexafluoroacetylacetonate) units with 1, 5, and 9 through 2,2′-bipyridyl/2,2′:6′2′′-terpyridyl chelation induces isolation of a series of PtLn2 heterotrinuclear complexes. Upon excitation of the PtLn2 complexes at 350 nm e λex e 500 nm, near-infrared (NIR) luminescence from lanthanide ions is indeed “lighting up” through effective Pt f Ln energy transfer from 3[d(Pt) f π*(But2bpy)] metal-to-ligand charge transfer transition excited-state of the Pt(But2bpy)bis(acetylide) antenna chromophore. By successive insertion of phenylene or ethynyl between acetylide and 2,2′-bipyridyl/2,2′:6′,2′′-terpyridyl in the bridging ligands, the rate and efficiency of Pt f Ln energy transfer are tunable. In contrast with quite effective Pt f Ln energy transfer in Pt-C≡Cbpy-Ln (Pt · · · Ln ) 8.6 Å) and Pt-C≡CC6H4tpy-Ln (Pt · · · Ln ) 14.1 Å) arrays, the long-range energy transfer across Pt-C≡CC6H4C≡Cbpy-Ln (Pt · · · Ln ) 14.9 Å) array becomes less efficient. Introduction Compared with the numerous studies on EuIII and TbIII complexes that emit red or green light,1-5 near-infrared (NIR) emitting lanthanide complexes have been less explored.6-8 The visible emission of EuIII and TbIII complexes arises primarily from UV irradiation, thus restricting the ranges of sensitizers available and limiting the sensitivity of assays in vivo.9 In contrast, the NIR emitting lanthanides such as NdIII, YbIII, and ErIII with low-energy emissive states can be achieved by energy transfer from sensitizing chromophores that absorb strongly in the visible region. As d-block organometallic chromophores frequently exhibit strong absorption in the visible region due to intense metal-to-ligand charge transfer transitions (MLCT), which closely match the receiving luminescence state of lanthanides, a new approach has been recently established to attain sensitized NIR lanthanide emission through effective d f f energy transfer from d-block chromophore sensitizers.10-22 In order to facilitate d f f energy transfer to occur from d-block chromophores to lanthanide ions, judicious selection of conjugated bridging ligands with suitable bonding sites for the d-block metal subunits and lanthanide(III) ions is critical for achieving NIR lanthanide luminescence. Among various classes of conjugated organic ligands, polypyridyl alkynes are particularly suitable for the design of d-f heterometallic and/ or multicomponent complexes.7,13c,21 The bifunctional character makes them favorable bridging ligands capable of associating d-block organometallic chromophores through metal-acetylide σ-coordination with lanthanide(III) centers through polypyridyl chelating. The rate and efficiency of energy transfer from d-block chromophores to lanthanide centers can thus be modu* To whom correspondence should be addressed. E-mail: czn@ ms.fjirsm.ac.cn. Fax: +86-591-379-2346. † Sun Yat-Sen University. ‡ Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.

lated by modification of the bridging ligands such as the length, π-conjugacy, and bonding character.7,12-14,21 In view of intense MLCT absorption in the visible region and bright luminescence in square-planar Pt(diimine)bis(acetylide) complexes arising primarily from 3[d(Pt)fπ*(diimine)] MLCT triple excited state, it is likely that they are favorable d-block antenna chromophores for achieving sensitized NIR lanthanide luminescence. By this consideration, 2,2′-bipyridyl (bpy) or 2,2′:6′2′′-terpyridyl (tpy) ethynes were utilized for the preparation of a series of PtLn2 heterotrinuclear complexes through both Pt-acetylide σ-coordination and Ln-bpy/tpy chelation.13c,21 Sensitized NIR lanthanide luminescence is successfully achieved through effective Pt f Ln intercomponent energy transfer from the Pt(diimine)bis(acetylide) antenna chromophores. By elaborately designed PtLn2 heterotrinuclear arrays with alterable intramolecular Pt · · · Ln separations through modification of the spacing length between 2,2′-bipyridyl/2,2′: 6′2′′-terpyridyl and acetylide, the rate of Pt f Ln energy transfer is tunable. With successive insertion of phenylene and/or ethynyl in the bridging spacer, the intramolecular Pt · · · Ln distances are extended from 8.6 Å in Pt-C≡Cbpy-Ln array to ca. 15 Å in Pt-C≡CC6H4C≡Cbpy-Ln array. For the purpose of systematic modulation of the rate and efficiency of Pt f Ln energy transfer, designed preparations of a series of PtLn2 complexes were carried out by modification of the spacing length between platinum(II) chromophore (energy donor) and lanthanide center (energy acceptor). Experimental Procedures Materials and Reagents. All manipulations were performed under dry argon atmosphere using the Schlenk technique and vacuum-line system. Solvents were dried by standard procedures and distilled prior to use. The reagents potassium tetrachloroplatinum (K2[PtCl4]), 4,4′dibutyl-2,2′-bipyridine (But2bpy), hexafluoroacetylacetone (Hhfac) were commercially available. 5-[2-(Trimethylsilyl)-1-ethynyl]-2,2′-bipyridine (Me3SiC≡Cbpy),23 4′-[4-{2-(trimethylsilyl)-1-ethynyl}-2,2′:6′,2′′-terpy-

10.1021/cg800866r CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

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Table 1. Crystallographic Data of 1 · 2CH2Cl2, 4 · CH2Cl2, 8 · CH2Cl2, and 9 · 3CH2Cl2 empirical formula fw space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Fcalcd, g/cm3 µ, mm-1 radiation (λ, Å) temp, K R1 (Fo)a wR2 (Fo2)b GOF a

1 · 2CH2Cl2

4 · CH2Cl2

8 · CH2Cl2

9 · 3CH2Cl2

C44H42Cl4N6Pt 991.73 P1j 9.765(3) 14.167(5) 17.435(7) 113.939(9) 94.527(18) 98.467(13) 2154.7(14) 2 1.529 3.542 0.71073 293(2) 0.0951 0.2657 1.179

C73H46Cl2F36N6O12PtYb2 2495.23 P1j 10.909(8) 20.639(14) 20.771(14) 90.362(11) 94.373(15) 92.239(11) 4659(5) 2 1.779 3.674 0.71073 293(2) 0.0746 0.1962 1.067

C95H60Cl2F36N8O12PtYb2 2801.58 P1j 18.490(6) 18.689(6) 21.547(9) 88.563(17) 83.632(14) 64.641(13) 6685(4) 2 1.392 2.57 0.71073 293(2) 0.0876 0.2476 1.059

C61H52Cl6N6Pt 1276.88 C2/c 19.445(6) 18.608(6) 16.675(5) 90.00 104.563(3) 90.00 5840(3) 4 1.452 2.721 0.71073 293(2) 0.0372 0.0976 1.045

R1 ) Σ|Fo - Fc|/ΣFo. b wR2 ) Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)]1/2. Table 2. Selected Bonding Distances (Å) and Angles (°) of 1 · 2CH2Cl2, 4 · CH2Cl2, 8 · CH2Cl2 and 9 · 3CH2Cl2 1 · 2CH2Cl2

4 · CH2Cl2

8 · CH2Cl2

Pt-Cl Pt-C3 Pt-N2 Pt-N1 C1-C2 C3-C4

1.96(2) 1.967(18) 2.049(14) 2.073(14) 1.20(3) 1.19(2)

Pt-C31 Pt-C19 Pt-N1 Pt-N2 C19-C20 C31-C32 Yb1-O10 Yb1-O11 Yb1-O12 Yb1-O8 Yb1-O7 Yb1-O9 Yb1-N3 Yb1-N4

1.924(11) 1.952(12) 2.057(8) 2.075(8) 1.203(15) 1.255(15) 2.248(8) 2.251(9) 2.285(9) 2.302(7) 2.309(8) 2.314(9) 2.457(9) 2.466(9)

C1-Pt-C3 C1-Pt-N2 C3-Pt-N2 C1-Pt-N1 C3-Pt-N1 N2-Pt-N1

88.9(7) 175.5(7) 95.5(7) 97.3(7) 173.8(7) 78.3(5)

C31-Pt-C19 C31-Pt-N1 C19-Pt-N1 C31-Pt-N2 C19-Pt-N2 O10-Yb1-O11 O10-Yb1-O12 O11-Yb1-O12 O10-Yb1-O8 O8-Yb1-N4 O7-Yb1-N4 O9-Yb1-N4 N3-Yb1-N4

86.7(4) 176.3(4) 96.9(4) 97.9(4) 174.8(4) 111.7(4) 73.0(4) 73.3(4) 150.4(3) 114.6(3) 74.2(3) 138.9(3) 66.0(3)

ridine (Me3SiC≡CC6H4tpy)23 were synthesized by the modified procedures described in the literature. The ligand (CH3)3SiC≡CC6H4C≡Cbpy was prepared as follows. 1-Bromo-4-iodobenzene (566 mg, 2 mmol), HC≡CSi(CH3)3 (0.35 mL, 2.4 mmol), CuI (4 mg, 0.02 mmol), and Pd(PPh3)2Cl2 (14 mg, 0.02 mmol) were added to triethylamine (30 mL) and the solution was refluxed with stirring for 1 day. The pale yellow crude product was purified by silica gel column chromatography using dichloromethane as an eluent to give the pure product (CH3)3SiC≡CC6H4Br. Yield: 78%. To an anhydrous THF (40 mL) solution containing (CH3)3SiC≡ CC6H4Br (253 mg, 1.0 mmol) and a small excess of HC≡Cbpy (230 mg, 1.28 mmol) was added Pd(PPh3)4 (115 mg, 0.1 mmol). After the reaction mixture was stirred under reflux for 1 day, the solvents were removed in vacuo. The white crude product was purified by chromatography on a silica gel column using dichloromethane as eluent to give the pure product (CH3)3SiC≡CC6H4C≡Cbpy. Yield: 77%. m.p: 152-153 °C. ESI-MS (CH3OH-CH2Cl2): m/z (%) 353 (100) [M + H]+. 1H NMR (500 MHz, CDCl3, ppm): 8.87 (s, 2H, bpy), 8.75 (s, 2H, bpy), 8.24 (s, 1H, bpy), 8.07 (s, 1H, bpy), 7.68 (s, 1H, bpy), 7.51 (m, 4H, J ) 9.9 Hz; C6H4), 0.27 (s, 9H, C3H9). IR (KBr, cm-1): 2148 m (C≡C), 1251s (SiC3H9).

Pt-C70 Pt-C19 Pt-N2 Pt-N1 C19-C20 C70-C71 Yb1-O4 Yb1-O3 Yb1-O6 Yb1-O1 Yb1-O5 Yb1-O2 Yb1-N4 Yb1-N5 Yb1-N3 C70-Pt-C19 C70-Pt-N2 C19-Pt-N2 C70-Pt-N1 C19-Pt-N1 N2-Pt-N1 O4-Yb1-O3 O4-Yb1-O6 O4-Yb1-O1 O3-Yb1-O1 O5-Yb1-N3 O2-Yb1-N3 N4-Yb1-N3 N5-Yb1-N3

9 · 3CH2Cl2 1.949(11) 1.966(11) 2.051(9) 2.068(9) 1.210(19) 1.191(17) 2.260(9) 2.296(9) 2.299(10) 2.315(12) 2.356(10) 2.439(12) 2.446(10) 2.470(11) 2.499(11) 86.0(5) 98.1(4) 175.9(5) 175.3(4) 98.7(5) 77.2(4) 76.8(4) 136.3(4) 76.2(4) 133.1(4) 134.0(4) 68.9(4) 66.0(3) 131.1(3)

Pt-C11

1.954(4)

Pt-N1

2.062(3)

C11-C12

1.199(5)

C11-Pt-C11 C11-Pt-N1 C11-Pt-N1 N1-Pt-N1

87.2(2) 175.06(12) 97.22(15) 78.45(17)

Ln(hfac)3(H2O)2 (Ln ) Nd, Eu, Yb) were prepared by addition of 3.3 equiv of Hhfac to aqueous solutions of lanthanide(III) acetates (pH 5-7) with stirring at room temperature for 3 h. The precipitates were filtered, washed with water, and dried under vacuum to afford the quantitative products. Pt(But2bpy)(C≡Cbpy)2 (1). To an anhydrous THF (50 mL) solution of Me3SiC≡Cbpy (1.1 mmol, 277 mg) and Pt(But2bpy)Cl2 (0.50 mmol, 267 mg) were added 2 mL of acetonitrile containing CuI (0.5 mg) and 5 mL of methanol containing KF (80 mg, 1.38 mmol). After the reaction was carried out at room temperature for 5 day, the solvents were removed in vacuo. The yellow crude product was purified by chromatography on a silica gel column using dichloromethane-methanol (v/v ) 100: 2) as eluent. Yield: 73%. Anal. Calcd. for C42H38N6Pt: C, 61.37; H, 4.66; N, 10.23. Found: C, 61.09; H, 4.53; N, 9.96. ESI-MS (CH3OHCH2Cl2): m/z 822 [M+H]+. 1H NMR (500 MHz, CDCl3): 9.66 (s, 2H, bpyC≡C), 8.86 (m, 4H, bpy), 8.48 (s, 2H, bpyC≡C), 8.39 (m, 2H, bpy), 7.99 (m, 4H, bpy and bpyC≡C), 7.87 (s, 2H, bpyC≡C), 7.68 (s, 2H, bpyC≡C), 7.33 (s, 2H, bpy), 1.47(s,18H,C4H9). IR (KBr, cm-1): 2115 m (C≡C). {Pt(But2bpy)(C≡Cbpy)2}{Ln(hfac)3}2 (Ln ) Nd, Er, Yb). The PtLn2 compounds were prepared by addition of 2.2 equiv of

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Scheme 1. The Synthetic Routes of PtII-LnIII2 (Ln ) Nd, Er, Yb) Arrays

Ln(hfac)3(H2O)2 to dichloromethane solutions of 1 with stirring for 1 h. After filtration, the concentrated dichloromethane solutions were layered

with n-hexane to afford the desired products as yellow crystals in 70-75% yields. 2 (Ln ) Nd). Anal. Calcd. for C72H44Nd2F36N6O12Pt: C, 36.76; H, 1.89; N, 3.57. Found: C, 36.95; H, 1.90; N, 3.76. IR (KBr, cm-1): 2121 m (C≡C), 1654 s (CdO). 3 (Ln ) Er). Anal. Calcd. for C72H44Er2F36N6O12Pt: C, 36.05; H, 1.85; N, 3.50. Found: C, 35.95; H, 1.91; N, 3.46. IR (KBr, cm-1): 2122 m (C≡C), 1652 s (CdO).

Figure 1. ORTEP drawing of 1 (30% thermal ellipsoid) with atomlabeling scheme.

4 (Ln ) Yb). Anal. Calcd. for C72H44F36N6O12PtYb2: C, 35.88; H, 1.84; N, 3.49. Found: C, 35.75; H, 1.80; N, 3.42. IR (KBr, cm-1): 2122 m (C≡C), 1653 s (CdO). Pt(But2bpy)(C≡CC6H4tpy)2 (5). This compound was prepared by the same synthetic procedure as that of 1 except for using Me3SiC≡CC6H4tpy instead of Me3SiC≡Cbpy. Yield: 56%. Anal. Calcd. for C64H52N8Pt · CH2Cl2: C, 64.39; H, 4.49; N, 9.25. Found: C, 64.30; H, 4.25; N, 9.21. ESI-MS (CH3OH-CH2Cl2): m/z 1129 [M + H]+. 1H NMR (500 MHz, CDCl3): 8.78 (m, 16H, bpy and tpy), 7.92 (m, 6H, bpy and tpy), 7.66 (d, 8H, J ) 8.0 Hz; C6H4), 7.40 (s, 4H, tpy), 1.46 (s, 18H, C4H9). IR (KBr, cm-1): 2110 m (C≡C).

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Figure 2. ORTEP drawing of 4 (30% thermal ellipsoid) with atomlabeling scheme. The F atoms on the trifluoromethyl are omitted for clarity. Figure 4. ORTEP drawing of 9 (30% thermal ellipsoid) with atomlabeling scheme.

Figure 3. ORTEP drawing of 8 (30% thermal ellipsoid) with atomlabeling scheme. The F atoms on the trifluoromethyl are omitted for clarity. {Pt(But2bpy)(C≡CC6H4tpy)2}{Ln(hfac)3}2 (Ln ) Nd, Er, Yb). These compounds were prepared by the same synthetic procedures as those of {Pt(But2bpy)(C≡Cbpy)2}{Ln(hfac)3}2 (2-4) except for using 5 instead of 1. Yields: 75-80%. 6 (Ln ) Nd). Anal. Calcd. for C94H58Nd2F36N8O12Pt: C, 42.46; H, 2.20; N, 4.21. Found: C, 42.49; H, 2.56; N, 4.18. IR (KBr, cm-1): 2113 m (C≡C), 1654 s (CdO). 7 (Ln ) Er). Anal. Calcd. for C94H58Er2F36N8O12Pt: C, 41.74; H, 2.16; N, 4.14. Found: C, 41.95; H, 2.26; N, 4.18. IR (KBr, cm-1): 2115 m (C≡C), 1657 s (CdO). 8 (Ln ) Yb). Anal. Calcd. for C94H58F36N8O12PtYb2: C, 41.56; H, 2.15; N, 4.12. Found: C, 41.29; H, 2.20; N, 4.20. IR (KBr, cm-1): 2113 m (C≡C), 1659 s (CdO). Pt(But2bpy)(C≡CC6H4C≡Cbpy)2 (9). To a dichloromethane (40 mL) solution containing Pt(But2bpy)Cl2 (0.50 mmol, 267 mg), (CH3)3SiC≡CC6H4C≡Cbpy (1.1 mmol, 387 mg), and CuI (5 mg) was added a methanol (5 mL) solution of KF (80 mg, 1.38 mmol). After the solution was stirred under refluxing for 1 day, the solvents were removed in vacuo. The crude product was purified by chromatography on a silica gel column using dichloromethane-methanol (v/v ) 100: 2) as an eluent. Yield: 55%. Anal. Calcd. for C58H46N6Pt · 3CH2Cl2: C,

57.38; H, 4.10; N, 6.58. Found: C, 57.30; H, 4.15; N, 6.71. ESI-MS (CH3OH-CH2Cl2): m/z 1023 [M + H]+. 1H NMR (500 MHz, CDCl3): 9.69 (s, 2H, bpyC≡C), 8.84 (d, 2H, J ) 10.4 Hz; bpy), 8.70 (s, 2H, bpyC≡C), 8.43 (m, 4H, bpyC≡C), 7.96 (m, 4H, bpyC≡C), 7.84 (s, 2H, bpy), 7.56 (m, 8H, C6H4), 7.34 (s, 2H, bpyC≡C), 7.31 (s, 2H, bpy), 1.49 (s,18H, C4H9). IR (KBr, cm-1): 2207w (C≡C), 2110s (C≡C). {Pt(But2bpy)(C≡CC6H4C≡Cbpy)2}{Ln(hfac)3}2 (Ln ) Nd, Gd, Er, Yb). These compounds were prepared by the same procedures as those of {Pt(But2bpy)(C≡Cbpy)2}{Ln(hfac)3}2 (2-4) except for using 9 instead of 1. Yields: 55-60%. 10 (Ln ) Nd). Anal. Calcd. for C88H52Nd2F36N6O12Pt · CH2Cl2: C, 40.52; H, 2.06; N, 3.19. Found: C, 40.49; H, 2.06; N, 3.38. IR (KBr, cm-1): 2219 w (C≡C), 2112 w (C≡C), 1651 s (CdO). 11 (Ln ) Gd). Anal. Calcd. for C88H52F36Gd2N6O12Pt: C, 40.98; H, 2.03; N, 3.26. Found: C, 40.79; H, 2.10; N, 3.18. IR (KBr, cm-1): 2218 w (C≡C), 2110 w (C≡C), 1653 s (CdO). 12 (Ln ) Er). Anal. Calcd. for C88H52Er2F36N6O12Pt: C, 40.67; H, 2.02; N, 3.23. Found: C, 40.45; H, 2.08; N, 3.22. IR (KBr, cm-1): 2219 w (C≡C), 2111 w (C≡C), 1655 s (CdO). 13 (Ln ) Yb). Anal. Calcd. for C88H52F36N6O12PtYb2: C, 40.49; H, 2.01; N, 3.22. Found: C, 39.99; H, 2.10; N, 3.20. IR (KBr, cm-1): 2217 w (C≡C), 2112 w (C≡C), 1651 s (CdO). Crystal Structural Determination. Single crystals of 1 · CH2Cl2, 4 · CH2Cl2, 8 · CH2Cl2, and 9 · 3CH2Cl2 suitable for X-ray diffraction were grown by layering n-hexane onto the corresponding dichloromethane solutions, respectively. The data collections were performed on a RIGAKU MERCURY CCD diffractometer by ω scan technique at room temperature using graphite-monochromated MoKR (λ ) 0.71073 Å) radiation. The Lp corrections were carried out in the reflection reduction process. The structures were solved by direct method. The remaining non-hydrogen atoms were determined from the successive difference Fourier syntheses. The non-hydrogen atoms were refined anisotropically except for the F atoms in 4 and 8, and the hydrogen atoms were generated geometrically with isotropic thermal parameters. The structures were refined on F2 by full-matrix least-squares methods using the SHELXTL-97 program package. For 4 and 8, the refinements were carried out by fixing the C-F distances (1.32 ( 0.01 Å) with the occupancy factors of F1-F36 and F1′-F36′ being 0.50, respectively. Crystallographic data of 1 · CH2Cl2, 4 · CH2Cl2, 8 · CH2Cl2, and 9 · 3CH2Cl2 are summarized in Table 1. Physical Measurements. Elemental analyses (C, H, N) were carried out on a Perkin-Elmer model 240C automatic instrument. Electrospray mass spectra (ES-MS) were recorded on a Finnigan LCQ mass spectrometer using dichloromethane-methanol as the mobile phase. UV-vis absorption spectra were measured on a Perkin-Elmer Lambda 25 UV-vis spectrometer. Infrared spectra were recorded on a Magna750 FT-IR spectrophotometer with KBr pellet. 1HNMR spectra were

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Table 3. Absorption and Emission Data of 1-13 at 298 K compound

medium

1

solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid CH2Cl2 solid

2 3 4 5 6 7 8 9 10

CH2Cl2 11 12 13

solid CH2Cl2 solid CH2Cl2 solid CH2Cl2

λabs/nm (ε/dm3mol-1cm-1) 338(58370), 397(11770) 306(92040), 363(42300) 295(89100), 366(44800) 294(87980), 366(43530) 286(78800), 321(60400), 392(10990) 289(107400), 304(106900), 369(56140) 289(127900), 302(123200), 366(62570) 289(116730), 301(109500), 367(57620) 288 (53120), 350 (91660), 420(14590) 293 (132200), 262 (83280) 293 (123820), 262(78830) 293(127000), 362(80900) 293 (136000), 362(86000)

λem/nm (τem/µs)a at 298 K 608(10.4) 553(1) 1061 1061 1540 1540 980(12.8) 980(11.5) 567(11.8) 552(1.1) 1066 1066 1534 1534 980(14.8) 980 (16.1) 660 (0.17) 546(0.78) 667(24 ns) 1061 550(4 ns) 1061 660(0.13) 556(0.72) 1534 1534 646(39 ns) 980(13.9) 548(34 ns) 980(12.7)

Φem (%)b,c 25.1

0.58 21

0.81 13

2.1

0.64

a The excitation wavelength in the lifetime measurement is 397 nm. b The quantum yield of mononuclear PtII complexes in degassed dichloromethane was determined relative to that of Ru(bpy)3(PF6)2 (Φ ) 0.062) in degassed acetonitrile. c The quantum yield of Yb complexes in dichloromethane solutions is estimated by the equation ΦLn ) τobs/τ0, in which τobs is the observed emission lifetime and τ0 is the radiative or ‘natural’ lifetime with τ0 ) 2 ms for YbIII. These values refer to the lanthanide-based emission process only and take no account for the efficiency of intersystem crossing and energy transfer processes.

Figure 5. Titration of 5 with Yb(hfac)3(H2O)2 in dichloromethane solution, showing a blue shift of the MLCT absorption band from PtII(But2bpy)(acetylide)2 chromophore. measured on a Varian UNITY-500 spectrometer using SiMe4 as the internal reference. Emission and excitation spectra in the UV-vis region were recorded on a Perkin-Elmer LS 55 luminescence spectrometer with a red-sensitive photomultiplier type R928. The steady-state NIR emission spectra were measured on an Edinburgh FLS920 fluorescence spectrometer equipped with a Hamamatsu R5509-72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with NIR grating blazed at 1000 nm. Corrected spectra were obtained via a calibration curve supplied with the instrument. The emission lifetimes above 10 µs were obtained by using an Edinburgh Xe900 450 W pulse xenon lamp as the excitation light source. The emission lifetimes below 10 µs were determined using an LED laser at 397 nm excitation. The emission quantum yields of 1, 5, and 9 were measured in degassed dichloromethane solutions at 298 K and estimated relative to [Ru(bpy)3](PF6)2 in acetonitrile as the standard (Φem ) 0.062) and calculated by Φs ) Φr(Br/Bs)(ns/nr)(Ds/Dr), where the subscripts s and r refer to the sample and reference standard solution, respectively, n is

Figure 6. Emission spectra of 5 (dot), 6 (solid), 7 (dash), and 8 (dash dot) in dichloromethane solutions at 298 K. the refractive index of the solvents, d is the integrated intensity, and Φ is the luminescence quantum yield. The quantity B is calculated by B ) 1 - 10-AL, where A is the absorbance at the excitation wavelength and L is the optical path length.

Results and Discussion Syntheses and Characterization. As shown in Scheme 1, reaction of Pt(But2bpy)Cl2 with Me3SiC≡Cbpy, Me3SiC≡ CC6H4tpy, or Me3SiC≡CC6H4C≡Cbpy in the presence of cuprous iodide and potassium fluoride via fluoride-catalyzed desilylation gives the mononuclear platinum(II) complex Pt(But2bpy)(C≡Cbpy)2 (1), Pt(But2bpy)(C≡CC6H4tpy)2 (5), or Pt(But2bpy)(C≡CC6H4C≡Cbpy)2 (9), respectively. These products were readily purified by chromatography on silica gel

574 Crystal Growth & Design, Vol. 9, No. 1, 2009

Figure 7. Titration of 5 with Yb(hfac)3(H2O)2 in dichloromethane solution, showing the quenching of the PtII(But2bpy)(acetylide)2-based emission.

columns using dichloromethane-methanol (v/v ) 100: 2) as the eluent. Addition of 2.2 equiv of Ln(hfac)3(H2O)2 (Ln ) Nd, Er, Yb) into the dichloromethane solutions of 1, 5, or 9 following crystallization by layering n-hexane afforded the corresponding PtLn2 heterotrinuclear complexes as pale-yellow crystals. These complexes were characterized by elemental analyses, ESI-MS spectrometry, IR and 1H NMR spectroscopy, and X-ray crystallography for 1 (Pt), 4 (PtYb2), 8 (PtYb2), and 9 (Pt). Microanalytical data coincide well with the calculated values for all of the complexes. The infrared spectra of these complexes show characteristic ν(C≡C) bands at 2110-2220 cm-1. For 9-13 with C≡CC6H4C≡Cbpy, two ν(C≡C) bands occur at ca. 2110 and 2218 cm-1, where the former is ascribed to the acetylide with Pt-C≡C coordination whereas the latter to the other ethynyl connected to bpy. Upon formation of PtLn2 complexes, the ν(CdO) frequency of hfac is observed at ca. 1655 cm-1, confirming that Ln(hfac)3 units are indeed bound to 1, 5, or 9 through 2,2′-bipyridyl or 2,2′:6′2′′-terpyridyl chelating. Crystal Structures. The ORTEP drawings of 1, 4, 8, and 9 were depicted in Figures 1-4, respectively. Selected atomic distances and bonding angles are summarized in Table 2. The mononuclear platinum(II) complexes 1 (Figure 1) and 9 (Figure 4) are characteristic of square-planar platinum(II) structure with C2N2 donors from bis(acetylide) and Bu2tbpy. The acetylide in C≡Cbpy or C≡CC6H4C≡Cbpy is σ-bonded to the platinum(II) center, whereas the 2,2′-bipyridyl lacks coordination. The Pt-N and Pt-C distances are comparable to those in other Pt(diimine)bis(acetylide) complexes described previously.13c,21b,c,24,25 The Pt-acetylide σ-bonding is quasi-linear since the Pt-C≡C angles deviate only slightly from 180°. The two C≡Cbpy ligands in 1 are not coplanar as indicated by the dihedral angle (26.3°) between their least-squares planes. For 9, the C6H4 and pyridyl planes in C≡CC6H4C≡Cbpy are severely distorted from the coplanarity with 21.2-40.2° of dihedral angles. Upon the formation of heterotrinuclear PtYb2 complex 4 (Figure 2), the dihedral angle between least-squares planes of the two C≡Cbpy ligands reaches to as high as 66.1° as a result of incorporating Yb(hfac)3 with 1 through 2,2′-bipyridyl chelation. Because of a steric requirement, the two Yb(hafc)3 units in the PtYb2 arrays (Figure 2) are oriented outward instead of inward.13c,21c The YbIII center is eight-coordinated with N2O6 donors to form a distorted square antiprism. The PtII · · · YbIII separations through a bridging C≡Cbpy are 8.62 and 8.63 Å.

Xu et al.

For the PtYb2 complex 8 (Figure 3), the whole molecular framework is nearly coplanar with the dihedral angle formed by two C≡CC6H4tpy only being 14.1°, which is strikingly smaller than that in 4 (66.1°). While the square-planar platinum(II) coordination geometry is composed of C2N2 donors, the YbIII center is nine-coordinated with N3O6 chromophore to afford a distorted capped square antiprism. The Pt · · · Yb separations through bridging C≡CC6H4tpy are 14.0 and 14.1 Å. The bonding distances and angles around platinum(II) center in the PtYb2 complexes 4 and 8 are all in the normal ranges and comparable to those in 1 and 9. The acetylide C-C lengths are in the range 1.19-1.26 Å, revealing the presence of typical C≡C bonds.24,25 Photophysical Properties. The UV-vis absorption spectral and luminescence data are listed in Table 3. The UV-vis spectra of mononuclear platinum(II) complexes 1 and 5 consist of UV absorption bands at 220-340 nm due to ligand-centered π f π* transition and low-energy broad bands with absorption maximum at ca. 400 nm, arising most likely from d(Pt) f π*(But2bpy) MLCT transition.24-28 For 9, the ligand-centered band due to π f π* transition occurs at 350 nm and the lowenergy broadband arising from dπ(Pt) f π*(But2bpy) MLCT transition is observed at ca. 420 nm. An obvious red-shift of the related absorptions for 9 compared with those for 1 and 5 is likely ascribed to the more extended π-system in 9 than that in 1 or 5, inducing the reduced HOMO-LUMO energy gaps for the corresponding electronic transitions in 9.21d-f Upon formation of the PtLn2 heterotrinuclear complexes by incorporating 1, 5, or 9 with Ln(hafc)3 units through 2,2′bipyridyl or 2,2′:6′,2′′-terpyridyl chelating, the absorptions from both π f π* (C≡C) and MLCT states are distinctly blue-shifted (15-35 nm) to the higher energy region (Figures S1-S3, Supporting Information) relative to those in 1, 5, or 9. This is easily understandable because incorporating Ln(hfac)3 unit with 1, 5, or 9 would reduce the electron density of the platinum(II) centers and lower the energy level of the d(Pt) orbital, thus raising the energy gap between d(Pt) (HOMO) and π*(But2bpy) (LUMO) orbitals and causing a blue shift of the MLCT absorption band. By addition of Ln(hfac)3(H2O)2 to a dichloromethane solution of 1 ( Figure 5), 5 (Figure S4, Supporting Information), or 9 (Figure S5, Supporting Information), the lowenergy MLCT absorption bands show an obvious blue-shift to higher energy, which is responsible for the color weakening during the reactions from yellow to pale yellow upon formation of the PtLn2 arrays. As shown in Figure 5, titration of 5 with Yb(hfac)3(H2O)2 in dichloromethane solution induced the MLCT maximum shift from ca. 400 to 370 nm when 2 equiv of Yb(hfac)3(H2O)2 was added. Upon irradiation at λex > 350 nm, mononuclear platinum(II) complexes 1, 5, and 9 emit brightly yellow-green to orange luminescence (Table 3) with microsecond range of lifetimes in fluid solutions at ambient temperature. The emission quantum yields in degassed dichloromethane at 298 K are 0.25, 0.21, and 0.13 for 1, 5, and 9, respectively. The emissive state is thus characteristic of 3[d(Pt) f π*(But2bpy)] 3MLCT state.24-28 Upon excitation of platinum(II)-based MLCT chromophore with 350 e λex e 500 nm, the PtLn2 complexes emit NIR luminescence that is typical for the corresponding lanthanide ions. The lifetimes of these PtLn2 complexes are in microsecond ranges in fluid dichloromethane solutions at ambient temperature. As depicted in Figure 6, three emission bands occur for PtNd2 complexes at ca. 870, 1060, and 1335 nm due to 4F3/2 f 4 I9/2, 4I11/2, 4I13/2 transitions, respectively, one for the PtEr2 complexes at ca. 1535 nm due to 4I13/2 f 4I15/2 transition, and

Pt f Ln Energy Transfer in PtLn2 Complexes

one for the PtYb2 complexes at ca. 980 nm due to 2F5/2 f 2F7/2 transition. In contrast, the platinum(II) chromophore-based 3 [MLCT] luminescence in the visible region disappears completely for PtLn2 species 2-4 and 6-8, demonstrating unambiguously that Pt(But2bpy)(acetylide)2 chromophore-based emission is entirely quenched because of quite fast (kET > 108 s-1) and effective energy transfer to occur from the d-block energy donors to the lanthanide centers across the bridging C≡Cbpy or C≡CC6H4C≡Ctpy with intramolecular Pt · · · Ln distances being ca. 8.6 Å for 2-4 and ca. 14.0 Å for 6-8. As indicated in Figure 7, titration of 5 with Yb(hfac)3(H2O)2 in dichloromethane induced rapid attenuation of the PtII(But2bpy)(acetylide)2 based emission so as to entire quenching of the emission when 2 equiv of Yb(hfac)3(H2O)2 was added. For PtLn2 complexes 10 (PtNd2) and 13 (PtYb2) with Pt f Ln energy transfer was transmitted through the Pt-C≡CC6H4C≡ Cbpy-Lnarray;however,theresidualPt(But2bpy)(acetylide)2chromophorebased emission was still observed, suggesting that the platinum(II) chromophore-based luminescence is only partially quenched because of the slower and less efficient energy transfer with Pt · · · Ln distance being as long as 14.9 Å. As indicated in Figure S9 (Supporting Infromation), the Pt(But2bpy)(acetylide)2 chromophore-based emission is rapidly attenuated upon titration of 9 with Yb(hfac)3(H2O)2 in dichloromethane solution, but not completely quenched even if more than 2 equiv of Yb(hfac)3(H2O)2 was added. The emissive lifetimes of unquenched Pt(But2bpy)(acetylide)2-based emission are 4 ns for PtNd2 (10), 720 ns for PtGd2 (11), and 34 ns for PtYb2 (13) species in fluid dichloromethane at ambient temperature. The luminescence lifetime of PtGd2 (11) complex can be regarded as the 3MLCT state lifetime in the absence of Pt f Ln energy transfer11 because it has no energy levels below 32000 cm-1 and cannot accept any energy from the Pt-based antenna triplet state. Using the equation kET ) (1/τ - 1/τ0)/2,11-14 where τ is the lifetime of residual Pt-based emission in the PtLn2 (Ln ) Nd 9, Yb 13) species, and τ0 (720 ns) is the lifetime in the reference PtGd2 (11) complex, the energy transfer rates (kET) can be estimated as kET ) (1/τPtNd - 1/τPtGd)/2 ) 1.24 × 108 s-1 for PtNd2 (10) and kET ) (1/τPtYb - 1/τPtGd)/2 ) 1.40 × 107 s-1 for PtYb2 (13) species. For the PtEr2 complex 12, the residual Pt-based emission at ca. 560 nm cannot be detected in our fluorescence spectrometer except that a weak emission occurs at ca. 460 nm due probably to ligand-centered transition of HC≡CC6H4C≡Cbpy. This suggests that PtfEr energy transfer in PtEr2 complex 12 is quite fast (kET > 108 s-1) and more effective than the corresponding Pt f Nd or Pt f Yb energy transfer in PtNd2 (kET ) 1.24 × 108 s-1) or PtYb2 (kET ) 1.40 × 107 s-1) species. The Pt f Ln energy transfer in the order PtYb2 (13) < PtNd2 (10) < PtEr2 (12) can be rationally elucidated by spectroscopic overlapping degree between the emission spectra of Pt(But2bpy)(acetylide)2 antenna chromophore and the f-f absorption spectra of lanthanide ions.11-14 For YbIII ion, a single f-f absorption at ca. 980 nm (10240 cm-1) can only overlap with the very weak low-energy tail of the PtII-based emission (centered ca. 546 nm). For NdIII ion, it has three f-f levels lying within 15400-20000 cm-1 (650-500 nm), causing better spectroscopic overlapping with the Pt-based MLCT emission band (546 nm). As for ErIII ion, however, there is a main absorption band at ca. 18500 cm-1 (540 nm) due to the 4S3/2 state which is quite close to the Pt-based MLCT emission (546 nm). Consequently, energy matching degree for Pt f Ln energy transfer in these PtLn2 complexes is PtEr2 (12) > PtNd2 (10)

Crystal Growth & Design, Vol. 9, No. 1, 2009 575

> PtYb2 (13), which induces the rates in the order kET (PtEr2) > kET (PtNd2) > kET (PtYb2). Conclusions A series of PtLn2 heterotrinuclear complexes were prepared in a stepwise synthetic approach using HC≡Cbpy, HC≡ CC6H4tpy, or HC≡CC6H4C≡Cbpy as a bridging ligand through Pt-acetylide σ-coordination as well as 2,2′-bipyridyl or 2,2′: 6′2′′-terpyridyl chelating the lathanide(III) centers. Upon irradiation of the PtLn2 complexes at 350 e λex e 500 nm, which is the platinum(II)-based MLCT absorption region, sensitized NIR luminescence from lanthanide ions is successfully achieved through effective Pt f Ln energy transfer from 3[d(Pt) f π*(But2bpy)] 3MLCT excited triplet state of the Pt(But2bpy)bis(acetylide) antenna chromophore. By changing the spacer between acetylide and 2,2′-bipyridyl or 2,2′:6′,2′′-terpyridyl in the bridging ligands, the rate and efficiency of Pt f Ln energy transfer can be controlled and modulated. With successive insertion of phenylene and/or ethynyl in the bridging spacer, the rates of Pt f Ln energy transfer become much slower in PtLn2 complexes containing a bridging C≡CC6H4C≡Cbpy with intramolecular Pt---Ln distances being ca. 15 Å. The rates of Pt f Ln energy transfer follow the order kET(PtEr2) > kET(PtNd2) > kET(PtYb2), which is rationally elucidated by spectroscopic overlapping between Pt-based MLCT emission and lanthanide(III) f-f absorption. Acknowledgment. This work was financially supported by the NSFC (Grants 20521101, 20625101, and 20773128), the 973 project (Grant 2007CB815304) from MSTC, the NSF of Fujian Province (Grants 2008I0027 and 2006F3131), the fund from the Chinese Academy of Sciences (Grant KJCX2-YWH01), and the Postdoctoral Foundation of China (Grant 200731000-4109337). Supporting Information Available: Additional UV-vis absorption spectra and emission spectra; X-ray crystallographic files in CIF format for the structure determination of compounds 1 · 2CH2Cl2, 4 · CH2Cl2, 8 · CH2Cl2, and 9 · 3CH2Cl2. This information is available free of charge via the Internet at http://pubs.acs.org.

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