Christelle Freund, William Porzio, Umberto Giovanella, Francesco Vignali, Mariacecilia Pasini, and Silvia Destri*
Istituto per lo Studio delle Macromolecole-CNR, Via E. Bassini 15, 20133 Milano, Italy
Agnieszka Mech
Dipartimento di Scienza dei Materiali, Universita di Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy
Sebastiano Di Pietro and Lorenzo Di Bari
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
Placido Mineo
Dipartimento di Scienze Chimiche, Universita di Catania, Viale A. Doria 6, 95125 Catania, Italy
b
S Supporting Information’ INTRODUCTION
The photophysical properties of lanthanide (Ln) complexes, based on π-conjugated ligands, acting as light antennae, have been fully assessed.1Interestingly the emission of ions spans from visible to IR according to the Ln(III) chosen,13allowing for applications in disparatefields such as optoelectronic devices,1,2 sensors,4bioassays,5and telecommunication.6Recent years have witnessed a growing interest in their use both alone and as a component in blends with polymers imparting mechanical processing properties to the material, to fabricate organic light- emitting diodes (OLEDs) of different colors and even white OLEDs,7 luminescent solar concentrators, lasers, and plastic opticalfibers for data transmission.8
One of the most studied class of ligands is constituted by β-diketonate derivatives (β-DK) in view of their chemical
stability, ease of preparation, and noticeable emission properties due to the effectiveness of the energy transfer (ET) from this ligand to the Ln(III) ion.9
Particularly, europiumβ-DK complexes have attracted more interest in optoelectronic applications because of their strong and narrow red emission.8,10The intensity of this emission depends on the type ofβ-diketone and on the type of complex. Con- sidering ternary complexes ofβ-DK ligands and Lewis bases to complete the coordination sphere only, the majority of the highly luminescent complexes contain fluorinated groups which in- crease volatility and improve thermal and oxidative stability of the compounds.11 However, a combination of aromatic and
Received: October 20, 2010
ABSTRACT:The synthesis and the molecular and photophysical characteriza- tion, together with solid state and solution structure analysis, of a series of europium complexes based on β-diketonate ligands are reported. The Eu(III) complex emission, specifically its photoluminescence quantum yield (PL-QY), can be tuned by changing ligands whichfinely modifies the environment of the metal ion. Steady-state and time-resolved emission spectroscopy and overall PL- QY measurements are reported and related to geometrical features observed in crystal structures of some selected compounds. Moreover, paramagnetic NMR, based on the analogous complexes with other lanthanides, are use to demonstrate that there is a significant structural reorganization upon dissolution, which justifies the observed differences in the emission properties between solid and solution states. The energy of the triplet levels of the ligands and the occurrence of nonradiative deactivation processes clearly account for the luminescence efficien- cies of the complexes in the series.
5418 dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50,5417–5429
Inorganic Chemistry ARTICLE
fluoro-aliphatic substituents on the diketone gives europium(III) complexes with a more intense luminescence because of the more efficient ET from the ligand to the lanthanide ion.9b
As a matter of fact, europium tris(2-thienoyltrifluoroacetonate) 3 phe- nantroline [Eu(TTA)3Phen] is one of the most efficient euro-
pium complexes in theβ-DK series. To date, the largest solid state photoluminescence quantum yield (PL-QY) measured for a Eu(III) complex, [Eu(TTA)3DBSO] where DBSO = dibenzyl
sulfoxide, is 85%.12It is well-known that complex PL-QY upon ligand excitation results from a balance between the ligands-to- Eu(III) ET rates and the5D0radiative and nonradiative decay
rates. The nonradiative decay rates may have contributions from several processes: multiphonon relaxation, thermally assisted back-ET (BT) from lanthanide ion to ligand excited levels, relaxa- tion to the ground state via crossover to another excited state, for example, the ligand-to-metal charge-transfer state of the Eu(III) ion, or ET between the lanthanide ions themselves.13In this con- tribution a series constituted by thienoyl ligands only is considered to maximize the intersystem crossing process (ISC) between S1
and T1levels of the ligand (Figure 1 below). A complete character-
ization of theseβ-DK complexes, particularly the study of the PL properties in the series related to photophysical considerations, crystal structures, where available, and to complexes geometry determined by NMR studies is presented. The ligands consid- ered in the present study are reported in Scheme 1 where Ln(III) is Eu(III) for the complexes actually discussed or Gd(III) for energy triplet determination, and Pr, Tb, Tm, Yb, Lu for paramagnetic NMR (Lu provides the necessary diamagnetic reference).
Chemical and magnetic properties of lanthanides offer a unique tool to investigate in detail the structure of these complexes in solution and to get further insight into the relation between geometry and luminescence properties. 1H and 13C spectra of the Eu(III) complexes do not lend themselves to an accurate analysis because of the limited pseudocontact shifts
induced by Eu(III). Fortunately, taking advantage also of nuclear relaxation rates, we can demonstrate isostructurality in solution upon substitution of Eu(III) with late lanthanides, notably Tb- (III) and Yb(III), which induce much larger paramagnetic shifts and derive a set of reliable pseudocontact terms, which allows us to put forward an alternative structure and dynamic picture for the complexes in solution, accounting for the PL-QY differences observed between solid and solution states.
As a consequence, the relevance of emission quenching provoked by high energy CH oscillators in this series, although its effectiveness is lesser than that observed in Nd(III), Er(III) complexes,14has been recognized.
’ EXPERIMENTAL SECTION
Materials. Sodium amide, sodium methoxide, methanol, tetrahy- drofuran (THF), ethanol, hexane, dichloromethane (CH2Cl2), chloro-
form (CHCl3), ethyl ether (Et2O), ethyl acetate, sodium hydroxide,
anhydrous sodium sulfate, hydrochloric acid, and phosphoric acid were purchased from Aldrich and Merck, and deuterochloroform (CDCl3)
and dideuteromethylenchloride (CD2Cl2) from Cambridge Isotope
Laboratories-Inc. Methanol, ethanol, and THF were purified by stan- dard distillation methods.
4,4,4-Trifluoro-1-(2-thienyl)-1,3-butanedione (HTTA), 2-acetyl-3- bromothiophene, 2-acetylthiophene, 2-bromothiophene, ethyltrifluor- oacetate, 2-thiophenecarboxylic acid ethyl ester, 5-(2-methylthiophene)- 4,4,5,5-tetrametyl-1,3,2-dioxoborolane, 1,8-dibromooctane, acetic anhy- dride, tetrakis(triphenylphosphine)palladium [Pd(PPh3)4], 1,10-phe-
nantroline (Phen), and europium(III) chloride hexahydrate (EuCl33
6H2O) were purchased from Aldrich and used without any further purifica-
tion. The synthesis of ligands is reported in the Supporting Information.
Apparatus and Procedure. Crystals suitable for X-ray diffrac- tion (XRD) analysis were obtained by slow crystallization from THF for Eu(DTDK)3Phen and from CHCl3for Eu(Br-TTA)3Phen and
5419 dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50,5417–5429
Inorganic Chemistry ARTICLE
Eu(MeT-TTA)3Phen, respectively. XRD experiments were carried out
using an Enraf Nonius CAD4 instrument for single crystal analysis, while films and powder were examined using a computer controlled Siemens D-500 diffractometer equipped with Soller slits and an Anton-Paar camera for variable temperature experiments under nitrogen atmosphere.
Crystal structure resolution was performed according to the condi- tions shown in Table 1 and Supporting Information, Table S2, using WINGX package.15
NMR spectra were recorded on Bruker Advance 400 and 270 spectrometers and on a Varian Inova 600 (14.1 T) instrument equipped with triple resonance gradient probe.
FTIR spectra were taken on a Perkin-Elmer 2000 FTIR spectrometer. Positive MALDI-TOF mass spectra were acquired by a Voyager DE- STR (PerSeptive Biosystem) using a delay extraction procedure (25 kV applied after 2600 ns with a potential gradient of 454 V/mm and a wire voltage of 25 V) and detection in linear mode. The instrument was equipped with a nitrogen laser (emission at 337 nm for 3 ns) and aflash AD con- verter (time base 2 ns). trans-2[3-(4-tert-Butylphenyl)-2-methyl-2-pro- penylidene]-malononitrile (DCTB) or dithranol were used as a matrix. Mass spectrometer calibration was performed using 5,15-bis(p-dodecan-
oxyphenyl)-10,20-bis(p-hydroxyphenyl)porphyrin (C68H78N4O4,
1014 Da) and tetrakis(p-dodecanoxyphenyl) porphyrin (C92H126N4O4,
1350 Da).
UVvis absorption spectra for both solutions and films were measured with a Lambda 900 Perkin-Elmer spectrometer. Steady-state photoluminescence (PL) spectra were recorded using a SPEX 270 M monochromator equipped with a N2-cooled CCD detector, by exciting
with a monochromated Xe lamp. The overall PL-QY were measured in CH2Cl2solution [105Mol L1] at room temperature with respect to a
reference solution of quinine sulfate in 1 N H2SO4and exciting at
350 nm (PL-QY = 54.6%). The overall PL-QY for thefilms obtained by spin-coating from CH2Cl2 solutions, was determined under ligand
excitation (360 nm) and is based on the absolute method using a calibrated integrating sphere.16The estimated error for the solid-state
PL-QY is 10%.
The emission lifetimes were collected while monitoring 615 nm Eu(III)5D0-7F2hypersensitive transition. All the samples were excited
with a beam of 355 nm Nd:YAG laser, with the power of 100μW and the repetition rate of 200 Hz (for the sample with the shorter lifetime the repetition rate was 3000 Hz), a PCI plug-in multichannel scaler ORTEC
Table 1. Crystallographic Details of Eu(Br-TTA)3Phen and Eu(DTDK)3Phen Complexes
chemical formula C36H17Br3EuF9N2O6S3 C53H45EuN2O8S6
formula weight 1232.39 1182.23
space group P1 (No.2) P21/c (No.14)
a/nm 0.9804(1) 1.2039(8) b/nm 1.3673(2) 3.0280(2) c/nm 1.6067(2) 1.4916(11) R (deg) 95.118(3) 90.0 β (deg) 95.479(3) 93.89(4) γ (deg) 98.792(3) 90.0 V/nm3 2106.9(5) 5425.0(15) Z 2 4 Dcalc/g cm3 1.943 1.447 radiation Mo KR (0.71073 Å) Mo KR (0.71073 Å) μ/cm1 4.568 1.44 F(000) 1184 2400 temperature/ K 293 293
anomalous dispersion all non-H atoms all non-H atoms
parameters refined in full-matrix least-squares 541 632
unweighted agreement factor 0.057 0.058
weighted agreement factor 0.156 0.122
goodness offita 1.016 0.965
least-squares weights b c
largest shifts 0.00 0.05σ
high and low peaks infinal diff. map/e Å 2.77 and1.09 0.75 and0.53
crystal color yellow yellow
crystal shape prism prism
crystal dimensions/mm3 0.44 0.25 0.19 0.50 0.23 0.20
instrument Bruker-AXS SMART-APEX Bruker-AXS SMART-APEX
empirical absorption 0.240.48 0.530.76
maximumθ/deg 25 25
reflections included Fo> 4σ(Fo) 5711 over 7406 4495 over 9524
monochromator graphite crystal, attenuator Zr foil, factor 17.0
scan typeω ω ω
scan rate 10 s/imaged 10 s/imaged
scan width/deg 0.2 0.2
a
Minimization function∑w(Fo2 Fc2).bw = 1/[\σ2(Fo2)þ (0.1125P)2þ 2.6021P] where P = (Fo2þ 2Fc2)/3.cw = 1/[\σ2(Fo2)þ (0.0723P)2þ
5420 dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50,5417–5429
Inorganic Chemistry ARTICLE
9353 100-ps Time Digitizer/MCS has been used in a photon counting acquisition mode, with a time resolution better than 100 ns. The estimated error for the solution emission lifetimes is 2%.
Synthesis of Complexes: General Procedure. Eu(DTDK)3-
Phen. 1,3-Di(thien-2-yl)propane-1,3-dione (HDTDK) (708.9 mg, 3 mmol) and Phen (180.2 mg, 1 mmol) were dissolved in hot EtOH (7 mL). To this solution cooled to room temperature was added an aqueous NaOH solution (3 mL, 1 mol L1, 3 mmol). After stirring for 20 min, EuCl33 6H2O (366.2 mg, 1 mmol) in water (7 mL) was added to the
solution. The addition was accompanied by a large precipitation. The mixture was then heated for 3 h at 60 C. After cooling to room temperature, the yellow precipitate was collected by filtration and dried under vacuum to afford 91% of the complex (0.9437 g).
FT-IR [film from CH2Cl2,ν (cm1)]: 1585, 1560, 1545, 1530, 1515,
1480, 14301410 (br), 1345, 1295 (br), 1235, 1195, 1150, 1110, 1075, 1030, 855, 840, 875, 750, 730710 (br), 660, 620, 590. UVvis absorption: solution (CH2Cl2) λmax= 272 and 373 nm;film λmax=
280 and 400 nm. MALDI-TOF (matrix DCTB, m/z): main peak, [2 L Phen Eu]þ802.47; [2 L 2 Phen Eu]þ982.54; the ions were detected as Mþspecies because of loss of a DTDKanion fragment from the corresponding molecular species. Taking into account all the peaks present in the spectrum and their intensity, a DTDK:Phen:Eu ratio of 3:1:1 could be calculated.1H NMR (CD2Cl2, 400 MHz,δppm, JHz):
major form (∼ 85%): 12.95 (s br, 2H, Phen), 10.71 (d, 2H, J3,4= 7.6,
Phen), 10.06 (s, 2H, Phen), 9.29 (d, 2H, J4,3= 7.6, Phen), 6.68 (d, 6H,
J5,4= 4.8, Th), 6.20 (dd br, 6H, J4,3= 3.6, Th), 5.59 (d, 6H, J3,4= 2.8, Th),
2.96 (s, 3H, CHβ-diketonate); minor form (∼15%): 12.05 (s br, 2H, Phen), 10.63 (d, 2H, J3,4= 7.6, Phen), 9.99 (s, 2H, Phen), 9.07 (d, 2H,
J4,3= 7.6, Phen), 6.55 (d, 6H, J5,4= 4.4, Th), 6.07 (dd br, 6H, J4,3= 3.6,
Th), 5.23 (s br, 6H, Th), 2.59 (s, 3H, CHβ-diketonate).13C NMR (CD2Cl2, 100 MHz, δppm, JHz): 181.9 (CdO), 164.4, 162.7, 149.9,
131.3, 128.1, 123.1, 122.7, 115.1, 109.3, 94.0 (CHβ-diketonate), 57.9. Elemental analysis was performed for the C, H, S atoms: calc. for EuC53H45O8N2S6using crystal for XRD: C, 53.84; H, 3.84; S, 16.27.
Found C, 53.72; H, 3.92; S, 16.13.
Eu(TTA)3Phen. Starting products: Phen and HTTA as ligands, and
EuCl33 6H2O as the metal precursor. The complex was obtained as a
pale rose powder with a yield of 65% after crystallization from CHCl3.
MALDI-TOF (matrix dithranol, m/z): [2 L Phen Eu]þ773.4, main peak; [2 L 2Phen Eu]þ953.7; the ions were detected as Mþspecies because of loss of a TTA fragment from the corresponding molecular species. Taking into account all the peaks present in the spectrum and their intensity a TTA: Phen: Eu ratio of 3:1:1 could be calculated.1H NMR (CD2Cl2, 400 MHz,δppm, JHz): 10.89 (s br, 2H, Phen), 10.17 (d,
2H, J4,3= 8.0, Phen), 9.44 (s, 2H, Phen), 8.56 (d, 2H, J3,4= 7.6, Phen),
7.04 (d, 3H, J5,4= 4.8, Th), 6.50 (dd, 3H, J4,5= 4.2, Th), 6.02 (s large, 3H,
Th), 3.13 (s, 3H, CHβ-diketonate).13C NMR (CD2Cl2, 100 MHz,
δppm, JHz): 179.5 (CO-Th), 168.8 (COCF3), 162.9, 150.2, 135.5,
127.1, 127.0, 123.7, 112.2, 106.3, 95.6 (CHβ-diketonate). Elemental analysis was performed for C, H, S, atoms. Calc. for EuC36H20F9O6N2S3: C,
43.43 ; H, 2.02; S, 9.66. Found: C, 43.70; H, 1.92; S, 9.48.
Eu(Br-TTA)3Phen. Starting products: Phen and Br-HTTA as ligands,
and EuCl33 6H2O as the metal precursor. The complex was obtained as a
light orange powder which was crystallized from CHCl3(yield 70%).
MALDI-TOF (matrix dithranol, m/z): [2 L Phen Eu]þ931.3, main peak; [2 L 2Phen Eu]þ1111.4; the ions were detected as Mþspecies, because of loss of a BrTTAanion fragment from the corresponding molecular species. Taking into account all the peaks present in the spectrum and their intensity a Br-TTA:Phen:Eu ratio of 3:1:1 could be calculated. NMR1H (CD2Cl2, 400 MHz,δppm, JHz): 10.94 (s large, 2H,
Phen), 10.12 (s large, 2H, Phen), 9.35 (s, 2H, Phen), 8.53 (s large, 2H, Phen), 6.50 (s br, 3H, Th), 5.66 (s br, 3H, Th), 3.24 (s, 3H, CH β-diketonate).1H NMR (acetone d
6, 270 MHz,δppm, JHz): 11.82 (s large,
2H, Phen), 10.21 (d, J4,3= 7.8, 2H, Phen), 9.28 (s, 2H, Phen), 8.69 (d,
J3,4= 7.0, 2H, Phen), 6.50 (d, J4,3= 4.1, 3H, Th), 5.64 (d, J3,4= 4.1, 3H,
Th), 3.29 (s, 3H, CHβ-diketonate). 13C NMR (CD2Cl2, 100 MHz,
δppm, JHz): 179.5 (CO-Th), 168.3 (COCF3), 162.8, 150.4, 127.1,
126.7, 124.8, 111.8, 106.2, 97.5 CHβ-diketonate. Elemental analysis was performed for C, H, S, atoms: Calc. for EuBr3S3F9O6N2C36H17: C,
35.09; H, 1.39; S, 7.81. Found: C, 34.87; H, 1.43; S, 7.58.
Eu(TTA)2(Br-TTA)Phen. Starting products: Phen, HTTA, and Br-
HTTA as ligands and EuCl33 6H2O as the metal precursor, in the
respective stoichiometry of 1, 2, 1, and 1. Yield 85%. The complex was obtained as a light pink beige powder, which emits strongly under excitation at 360 nm, in the solid state and in solution as well. No attempts for purification were performed, to avoid any decomposition. MALDI-TOF (matrix dithranol, m/z): [2L1 Phen Eu]þ773.6; [L1 L2 Phen Eu]þ851.5; [2L2 Phen Eu]þ931.3; [2L1 2Phen Eu]þ953.6; [L1 L2 2Phen Eu]þ 1031.6; [2L2 2Phen Eu]þ 1111.4; the ions were detected as Mþspecies because of loss of a TTAor Br-TTAanion fragment from the corresponding molecular species.1H NMR (CD2Cl2,
400 MHz,δppm, JHz): 10.8810.82 (2H, Phen), 10.1210.02 (2H,
Phen), 9.379.25 (2H, Phen), 8.498.45 (2H, Phen), 6.96 (d, 2H, J5,4= 4.6, Th TTA), 6.476.40 [m, 3H, Th TTA (2H) and Th Br-TTA
(1H)], 5.94 and 5.86 (2d, 2H, J3,4= 3.6, Th TTA), 5.71 and 5.63 (2d,
1H, J3,4= 3.8, Th Br-TTA), 3.50 and 3.28 (2s, 1H, CHβ-diketonate of
Br-TTA), 3.05 and 2.88 (2s, 2H, CHβ-diketonate of TTA).13C NMR (CD2Cl2, 100 MHz,δppm, JHz): 179.5, 168.6, 162.8, 150.2, 135.6, 135.5,
127.2, 127.1, 127.0, 126.9, 126.5, 124.5, 123.7, 123.6, 112.0, 106.2, 95.6, and 95.1 (CHβ-diketonate). Elemental analysis was performed for C, H, S, atoms: Calc. for EuC36H19BrF9N2O6S3: C, 40.24; H, 1.78; S, 8.95.
Found: C, 40.04; H, 1.88; S, 8.78.
Eu(BrC8-TTA)3Phen. Starting products: Phen and BrC8-HTTA as
ligands, and EuCl33 6H2O as the metal precursor Yield 48%. The
complex was obtained as a yellowish solid after precipitation from hexane solution. MALDI-TOF (matrix dithranol, m/z): main peak [2 L Phen Eu]þ1154.9; [2 L 2Phen Eu]þ1333.7; the ions were detected as Mþ species, because of loss of a BrC8-TTAanion fragment from the corresponding molecular species. Taking into account all the peaks present in the spectrum and their intensity a BrC8TTA:Phen:Eu ratio of 3:1:1 could be calculated.1H NMR (CD2Cl2, 270 MHz,δppm, JHz) 10.35
(s br, 2H, Phen), 10.15 (d, 2H, J4,3= 8.1, Phen), 9.47 (s, 2H, Phen), 8.48
(d br, 2H, J3,4= 8.1, Phen), 6.14 (d, J3,4= 3,4, 3H, Th), 5.83 (d J4,3, 3H,
Th), 3.50 (t, 6H, CH2Th), 3.42 (t, 6H, CH2Br), 2.47 (s, 3H, CH β-
diketonate), 1.891.26 (36H, alkyl chain). Elemental analysis was performed for C, H, S, atoms: Calc. for EuC60H65F9N2O6S3Br3: C,
45.93 ; H, 4.18; S, 6.13. Found: C, 46.03 ; H, 4.01; S, 6.00.
Eu(MeT-TTA)3Phen. Starting products: Phen and MeT-HTTA as
ligands, and EuCl33 6H2O as the metal precursor. Solvent EtOH:THF
(1:5). The complex was obtained as a yellow powder with a yield of 72% after crystallization from CHCl3. MALDI-TOF (matrix dithranol, m/z):
[2 L Phen Eu]þ965.57; [3 L Eu]Hþ1102.26; the ions were detected as Mþspecies, because of loss of a MeT-TTAanion fragment from the corresponding molecular species, and as MHþ species, respectively. Taking into account all the peaks present in the spectrum and their intensity a MeT-TTA:Phen:Eu ratio of 3:1:1 could be calculated.1H NMR (CD2Cl2, 270 MHz,δppm, JHz): 11.09 (s br, 2H, Phen), 10.06
(d, 2H, J4,3= 7.8, Phen), 9.31 (s, 2H, Phen), 8.53 (d, 2H, J3,4= 7.6, Phen), 7.26
(d, 3H, J = 3.2, Th), 6.90 (d, 3H, J = 2.4, Th), 6.45 (d, 3H, J = 3.8, Th), 5.76 (d, 3H, J = 3.8, Th), 2.97 (s, 3H, CHβ-diketonate), 2.73 (s, 9H, Me). Elemental analysis was performed for C, H, S, atoms: Calc. for EuC51H32F9N2O6S6: C,
47.70; H, 2.51; S, 14.98. Found C, 47.82; H, 2.55; S, 14.80.
The same synthetic procedures were used in the preparation of the corresponding Ln(TTA)3Phen, Ln = Gd, Pr, Tb, Tm, Yb, Lu, as well as
for the systems of empirical formula Gd(Br-TTA)3Phen, Gd(TTA)2-
(Br-TTA)Phen, Gd(BrC8-TTA)3Phen, Gd(DTDK)3Phen, Yb (TTA)2-
(Br-TTA)Phen, and Yb(BrC8-TTA)3Phen. Elemental analyses were
5421 dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50,5417–5429
Inorganic Chemistry ARTICLE
Calc. for GdC36H20F9N2O6S3: C, 43.20; H, 2.01; S, 9.61. Found C,
43.47; H, 2.12; S, 9.44.
Calc. for GdBr3S3F9O6N2C36H17: C, 34.97; H, 1.39; S, 7.78. Found:
C, 34.80; H, 1.43; S, 7.55.
Calc. for GdC36H19BrF9N2O6S3: C, 40.06; H, 1.77; S, 8.91. Found:
C, 39.93; H, 1.85; S, 8.72.
Calc. for GdC60H65F9N2O6S3Br3: C, 45.81; H, 4.16; S, 6.11. Found:
C, 45.93; H, 4.21; S, 6.00.
Calc. for GdC53H45O8N2S6: C, 53.60; H, 3.82; S, 16.20. Found C,
53.49; H, 3.94; S, 16.01.
Calc. for PrC36H20F9N2O6S3: C, 43.91; H, 2.05; S, 9.77. Found C,
44.12; H, 2.15; S, 9.58.
Calc. for TbC36H20F9N2O6S3: C, 43.12; H, 2.01; S, 9.59. Found C,
43.30; H, 2.13; S, 9.40.
Calc. for TmC36H20F9N2O6S3: C, 42.70; H, 1.99; S, 9.50. Found C,
42.85; H, 2.07; S, 9.37.
Calc. for YbC36H20F9N2O6S3: C, 42.52; H, 1.98; S, 9.46. Found C,
42.72 ; H, 2.09; S, 9.26.
Calc. for LuC36H20F9N2O6S3: C, 42.44; H, 1.98; S, 9.44. Found C,
42.63; H, 2.08; S, 9.27.
Calc. for YbC36H19BrF9N2O6S3: C, 39.46; H, 1.75; S, 8.78. Found: C,
39.60; H, 1.88; S, 8.59.
Calc. for YbC60H65F9N2O6S3Br3: C, 45.32; H, 4.12; S, 6.05. Found:
C, 45.54; H, 4.01; S, 5.91.
’ RESULTS AND DISCUSSIONS
Synthesis of Ligands and Complexes.5-Bromo-2-thienoyl- trifluoroacetone Br-HTTA was synthesized by the Claisen con- densation of 2-acetyl-5-bromothiophene and ethyltrifluoroacetate with sodium methoxide as the base and MeOH/Et2O mixture as
the solvent.17The pure product was obtained in a 65% yield after sublimation. In the synthesis of ligand HDTDK, sodium amide replaced sodium methoxide as the base, and THF was used as the solvent.18To obtain MeT-HTTA the strategy of modifying the acetylthiophene reagent via Suzuki coupling was adopted. Spe- cifically 5-bromo-2-acetylthiophene reacted with the 5-(2-methyl- thiophene)-boronic acid pinacol ester to afford the intermediate ketone, which was later transformed by a Claisen type condensa- tion with ethyltrifluoroacetate, into the corresponding diketone MeT-HTTA. 8-Bromooctyl-2-thienoyltrifluoroacetone (BrC8- HTTA) was prepared by acylation with acetic anhydride and phosphoric acid19of 2-(8-bromooctyl)thiophene, obtained fol- lowing the procedure reported in the literature,20and subsequent Claisen condensation of ethyltrifluoroacetate on the terminal ketonic group.17NMR spectroscopy of theβ-DK ligands in CDCl3
as the solvent shows the presence of both ketonic and enolic tautomers for HDTDK, BrC8-HTTA, and Br-HTTA, although in a different ratio (15% for the first, 10% for the second, and less than 1% for the third one, this last percentage slightly increases in
a more polar solvent as deuterated acetone); while for MeT- HTTA as well as for HTTA only the signal of enolic form is apparent. The stronger pushpull character of the latter group of molecules can be related to the enolic form stabilization.21
The complexes Eu(Ligand)3Phen were synthesized from
EuCl33 6H2O and the corresponding diketonate in EtOH/water
with phenanthroline as the ancillary ligand as described by Melby et al.22and reported in Scheme 2.
In the case of 5-methylthien-2-yl, higher yields of complex were attained only by using 1:5 EtOH:THF as the solvent, to maintain the ligand enolic anion in solution. The complexes are poorly soluble in most organic solvents except for acetone. They are partially soluble in CH2Cl2, CHCl3, and EtOH. The com-
plexes were characterized by1H,13C NMR spectroscopy, (see also below results and discussion on paramagnetic NMR).13C NMR spectrum was carried out only in the case of adequate solubility of the complex. FTIR and UVvis spectroscopy, cyclovoltammetry (presented in Supporting Information, Table S1 and Figure S1), elemental analysis, MALDI-TOF mass spectrometry determination together with XRD analysis, when single crystals could be obtained, were performed to complete the characterization. The two last analyses and NMR integrals are in agreement with a Ligand:Phen:Ln ratio of 3:1:1. In the MALDI-TOF experiments, in some cases, exchanges among the ligands was observed, and also the matrix taking part in this process. Deeper investigations are in progress with different matrixes to clarify the matrix effect and will be the object of a forthcoming paper.
Photophysical Characterization. The optical properties of Eu(III) complexes have been studied for both CH2Cl2diluted
solutions and solid-state (thin film) samples by absorption, steady-state and time-resolved emission spectroscopy, and over- all PL-QY measurements.
In Figure 1, the absorption spectra of the ligands in solution (a) and of all the complexes in solution and as thinfilm (b) are shown. The broad bands of the ligand absorption show their maximum (λmax) ranging from 250 to 375 nm and absorption
edges up to 430 nm. The exception being MeT-HTTA whose λmaxis equal to 415 nm. The absorption profiles of the complexes
dominated by the ligand absorption are almost identical for the samples measured in solution as well as in thinfilms. As expected the edge of the absorption band for the thinfilms is slightly red- shifted.
The sensitization pathway in Ln(III) complexes generally consists of excitation of the ligand into its singlet excited state, subsequent ISC to its triplet state, and ET from the triplet state of the ligand to the Ln(III) ion.1,5a,9b
To determine the energy of the triplet state of the ligands versus energy states of Eu(III) ions, the phosphorescence spectra of the frozen solution of the corresponding Gd3þ complexes, Scheme 2
5422 dx.doi.org/10.1021/ic1021164 |Inorg. Chem. 2011, 50,5417–5429
Inorganic Chemistry ARTICLE
considering the 00 transition, were measured7b
(Supporting Information, Figure S2). The energy values of the triplet levels of the ligands in complexes were as follows: TTA (502 nm, 19920 cm1), BrC8-TTA (511 nm, 19569 cm1), and Br- TTA (521 nm, 19194 cm1), which places them between the
5
D2 (465 nm, 21500 cm1) and5D1 (524 nm, 19070 cm1)
excited states of the metal ion.23 As already shown by Chen et al.,24the introduction of an electron donor substituent on the TTA ligand provokes a bathochromic shift of the triplet state energy level. The energy of the triplet state of DTKT (531 nm, 18832 cm1) is close to the5D0(578 nm, 17300 cm1) level of
Eu(III).2The energy of the triplet state of the ligand MeT-TTA was hard to determine because the ligandfluorescence in the corresponding Gd(MeT-TTA)3Phen frozen solution overlaps
the phosphorescence, indicating that the triplet state was scarcely populated. A value around 526 nm, 19011 cm1, too close to5D1
to prevent metalligand BT, was estimated. A diagram of the most probable states involved in the photophysics of the Eu(III)