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Highly luminescent Eu(III) complexes with 2,4,6-tri(2-pyridyl)-1,3,5-triazine ligand: Synthesis, structural characterization, and photoluminescence studies
Polyhedron 25 (2006) 3449–3455 www.elsevier.com/locate/poly
Highly luminescent Eu(III) complexes with 2,4,6-tri(2-pyridyl)-1,3,5-triazine ligand: Synthesis, structural characterization, and photoluminescence studies Channa R. De Silva a, Ruiyao Wang b, Zhiping Zheng b
a,*
a Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA Department of Chemistry, Queen’s University, Kingston, Ont., Canada K7L 3N6
Received 4 April 2006; accepted 28 June 2006 Available online 14 July 2006
Abstract Two new Eu(III) complexes featuring 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) and a rigid Lewis base ligand 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz), ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2), were synthesized. Their structures were established by single crystal X-ray diffraction. The europium ion in each of these complexes is nona-coordinate with six oxygen and three nitrogen atoms forming a coordination polyhedron best describable as a monocapped square antiprism. The difference in the composition and structure between these two complexes is caused by simply reversing the order of ligand (hfac and tptz) addition during the complex synthesis, and is rationalized in terms of the structural and electronic properties of the ligands and the overall steric bulk of the coordination sphere. Both complexes display characteristic Eu(III)-originated red emission upon UV excitation. The high quantum yields observed, 52% (1) and 60% (2), are rationalized in terms of the strong absorptions of both hfac and tptz ligands near the excitation wavelength (295 nm), an interpretation consistent with the well-established mechanism of ligand-mediated energy transfer for lanthanide-based light emission. 2006 Elsevier Ltd. All rights reserved. Keywords: Europium complexes; 2,4,6-Tri(2-pyridyl)-1,3,5-triazine; Crystal structures; Absorption; Photoluminescence; Synthesis
1. Introduction The study of luminescent lanthanide complexes remains an active area of research, due largely to the applications of such materials in biomedical and optical technologies [1–3]. Besides the unique luminescence characteristics originated from the f-electronic structure of the lanthanide elements, the usefulness of these materials depends critically on the molecular structure of the complexes. This dependence is reflected in a number of aspects, the saturation of coordination sphere of these rather bulky ions to minimize potentially luminescence–quenching solvent interactions, the use of ligands capable of promoting ligand-mediated energy transfer, and the avoidance of ligands with possibility of *
Corresponding author. Tel.: +1 520 626 6495; fax: +1 520 621 8407. E-mail address: [email protected] (Z. Zheng).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.032
luminescence–quenching vibrational coupling with the metal center [4–6]. The stability of the complex and its facility in device fabrication are also part of the consideration when it comes to the design of practically useful materials. Arguably the most common lanthanide complexes based on the above consideration are lanthanide b-diketonates [7]. Both anionic complexes featuring four diketonate ligands and electrically neutral complexes containing both diketonate and other ligands have been studied [8–13]. Their photophysical properties, thermal and chemical stabilities, and processibility can be modified and fine-tuned by judiciously chosen ligand combinations. We have been interested in developing new lanthanide complexes for potential applications in organic light-emitting devices [14–17]. Of particular interest are complexes featuring three diketonate ligands and one or more neutral
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ligands (L) of the general formula Ln(b-diketonate)3L, for which impressive photoluminescence properties have been demonstrated [18]. For the diketonate ligand, we have chosen 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) because of its low vibrational coupling with Eu(III) emission [9,19–21], enhanced thermal stability and volatility of the resulting lanthanide complexes when compared with its non-fluorinated analogs [22]. Our choice of ligand L is 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz), a bulky aromatic compound featuring three 2-pyridyl rings fixed on a central 1,3,5-triazine platform [23]. Lanthanide complexes with tptz ligand have first been introduced by Durham et al. [24]. Recently a number of structurally characterized lanthanide complexes were reported [16,25–28], including the very first tptz adduct of a lanthanide b-diketonate reported by us [16]; tptz acts as a terdentate ligand in these structurally characterized complexes, coordinating the lanthanide ion with one of the three pyrazine N atoms and the N atoms on the ortho-substituted pyridyl groups. The remaining N atoms allow for further coordination to other metals, transition metals in particular [29]. Thus, structurally sophisticated multinuclear heterometallic complexes may be envisioned, whose magnetic and photophysical properties are of fundamental interest and practical significance. By adopting a procedure used previously for the synthesis of Eu(dbm)3(tptz) (dbm = dibenzoylmethanate) [16], in which a tptz complex with the starting EuCl3 hydrate was prepared followed by the substitution of Cl for the diketonate ligands, an unexpected complex formulated as ½Eu ðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) was obtained. The complex planned for, Eu(hfac)3(tptz) (2), was subsequently produced by using the more traditional route of making the hydrated lanthanide diketonate first, followed
by the replacement of the aqua ligands by tptz. Here the synthesis, structural characterization by single-crystal Xray diffraction, and photoluminescence studies of these two new Eu(III) complexes are reported (Scheme 1). 2. Experimental EuCl3 Æ 6H2O (99.9%), 2,4,6-tri(2-pyridyl)-1,3,5-triazine (98%), 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (98%), and potassium tert-butoxide (95%) were purchased from Aldrich and used without further purification. A slightly modified procedure for the recently reported Eu(dbm)3(tptz) was adopted [16]. Elemental analysis (CHN) was performed by Numega Resonance Laboratory, San Diego, California. 2.1. Synthesis of [Eu(hfac)2 (H 2 O)(EtOH )(tptz)][CF 3 CO2 ] (1) A mixture of tptz (0.312 g, 1.00 mmol) and EuCl3 Æ 6H2O (0.366 g, 1.00 mmol) in 20 mL of absolute ethanol was stirred at 60 C for 10 min to afford a clear solution. To this solution was added over 10 min a solution of KOBut (0.336 g, 3.00 mmol) and Hhfac (0.624 g, 3.00 mmol) in 10 mL of absolute ethanol, and the resulting mixture was stirred at 60 C for 1 h under nitrogen, and then at room temperature for 3 h. The mixture thus obtained was filtered, and the solid was washed with copious de-ionized water to remove KCl. The crude product was dissolved in acetone, and the solution was dried over anhydrous MgSO4. The filtrate was collected, from which a pale yellow solid was obtained upon removal of the solvent. Recrystallization from ethanol:acetone (v/v 1:1) produced a crystalline solid as the analytically pure product (0.64 g, 61%). Anal. Calc.
N N
N N
N
N
N
+
N
(i)
N
EuCl3 . 2H2O
N N
N
N N
N
(ii)
N
F3C
N
O
N
EuCl3.6H2O
O
Eu
OH2
O F3C
(i) EtOH, 60oC (ii) KOBu t, Hhfac, 60 oC
.CF3COO-
O
O F3C
CF3
N N
N H2O
Hhfac
OH2 CF3
F3C O
(i)
+ EuCl3 .6H2O
N
O
Eu
F3C
O F3C
CF3
F3C
O
O
N
N
(ii) O CF3
O CF3
(i) H2O, KOBut, 60oC (ii) EtOH / acetone, 60 oC
O
Eu
O
O F3C
O F3C
CF3
O CF3
Scheme 1. Synthesis of ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2).
C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455
for C32H22N6O8F15Eu: C, 36.41; H, 2.10; N, 7.96. Found: C, 36.33; H, 2.06; N, 8.09%. 2.2. Synthesis of Eu(hfac)3(tptz) (2) Using a modified literature [11], Eu(hfac)3(H2O)2 was first prepared. Briefly, Hhfac (0.624 g, 3.0 mmol) was added to a solution of KOBut (0.336 g, 3.0 mmol) in H2O (10 mL). The resulting clear solution was stirred for 10 min at room temperature and then added to an aqueous solution of EuCl3 Æ 6H2O (0.366 g 1.0 mmol in 10 mL H2O). The mixture was stirred at 60 C for 30 min under nitrogen and then for 2.5 h at room temperature. The mixture was filtered, and the precipitate was washed with cold de-ionized water (2 · 100 mL) and dried under vacuum at room temperature for 12 hr. If necessary, the product can be recrystallized from acetone:ethanol (v/v 1:1). To a solution of Eu(hfac)3(H2O)2 (0.808 g, 1.0 mmol) in acetone (15 mL) was added tptz (0.312 g, 1.00 mmol) in ethanol (15 mL). The mixture was stirred at 60 C for 0.5 h and then overnight at room temperature. The mixture was filtered, and the filtrate was allowed to evaporate at room temperature to afford analytically pure product (0.73 g, 67%). Anal. Calc. for C33H15N6O6F18Eu: C, 36.52; H, 1.39; N, 7.74. Found: C, 36.65; H, 1.36; N, 7.80%.
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2.3. X-ray structure determinations Single crystals of both compounds suitable for X-ray diffraction studies were obtained by slow evaporation of their saturated solutions in ethanol:acetone (v/v 1:1) at room temperature over several days. The crystals were mounted on a glass fiber in a random orientation and cooled to 93 C in a stream of nitrogen gas controlled with Cryostream Controller 700. Data collection was performed on a Bruker SMART CCD 1000 X-ray diffractometer with graphite-monochromated Mo ˚ ), operating at 50 kV and Ka radiation (k = 0.71073 A 30 mA over 2h ranges of 2.58–50.00. No significant decay was observed during the data collection. Data were processed on a Pentium PC using the Bruker AXS Crystal Structure Analysis Package, Version 5.10 [30]. Neutral atom scattering factors were taken from Cromer and Waber [31]. The raw intensity data were integrated using the program SAINT-Plus and absorption corrections were applied using program SADABS. The structure was solved by direct methods. Full-matrix least-square P refinements minimizing the function W ðF 2o F 2c Þ were applied to the compound. All non-hydrogen atoms were refined anisotropically. All of the water hydrogen atoms and the hydroxyl hydrogen atoms were located gradually
Table 1 Crystal data and structure refinement for ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) Æ EtOH (2 Æ EtOH) Crystal data
½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2
[Eu(hfac)3(tptz)] Æ EtOH
Empirical formula Formula weight Temperature (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Density (calculated) (mg/m3) Absorption coefficient (mm1) F(0 0 0) h Range for data collection () Index ranges
C34H30EuF15N6O10 1119.60 180(2) triclinic P 1 10.3854(15) 13.3089(18) 16.452(2) 93.673(3) 104.942(2) 103.719(2) 2115.3(5) 2 1.758 1.610 1108 1.29–25.00 12 6 h 6 12, 15 6 k 6 15, 19 6 l 6 18 12 200 7389 [0.0473] 99.1 0.4642 and 0.3449 full-matrix least-squares on F2 7389/6/668 1.052 R1 = 0.0414, wR2 = 0.1118 R1 = 0.0446, wR2 = 0.1145 2.803 and 1.652
C35H21EuF18N6O7 1131.54 180(2) triclinic P 1 10.7819(14) 13.4118(17) 14.8428(19) 98.559(2) 95.931(2) 97.830(2) 2085.8(5) 2 1.802 1.638 1108 1.40–25.00 12 6 h 6 12, 15 6 k 6 15, 17 6 l 6 17 12 328 7309 [0.0156] 99.6 1.0000 and 0.8472 full-matrix least-squares on F2 7309/0/664 1.034 R1 = 0.0298, wR2 = 0.0829 R1 = 0.0325, wR2 = 0.0850 0.873 and 0.702
Reflections collected Independent reflections [Rint] Completeness to h = 25.00 (%) Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference peak and hole (e A
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in difference Fourier map, while the rest hydrogen atoms were calculated, and their contributions were included in the structure factor calculations. Details on data collection and structure refinements are given in Table 1. 2.4. Photophysical studies Electronic absorption spectra in dichloromethane solutions were recorded on a Perkin Elmer Lambda 10 spectrophotometer. Photoluminescence studies were carried out using a Fluorolog-3 fluorometer. The measured fluorescence in the visible range was excited by the light from a Xe-Arc lamp and detected with a photo multiplier tube at an angle of 90 to the incident beam. Photoluminescent quantum yields were measured and calculated using cresyl violet perchlorate (U = 0.54 in methanol) [32] or rhodamin 6G (95% in ethanol) [33] as the standards. Corrections were made for the instrumental parameters and differing refractive indices of the solvents used [34,35]. The experimental uncertainty for the quantum efficiency calculations is 10%.
approach. EuCl3 Æ 6H2O was reacted with three equivalents of Hhfac/base to form Eu(hfac)3(H2O)2, whose aqua ligands were then substituted for tptz under mild conditions. Both 1 and 2 are air-stable and readily soluble in solvents such as acetone, chloroform, and dichloromethane, but sparingly soluble in alcohols. Product purification was facile by recrystallization from a mixture of acetone and ethanol. Satisfactory microanalysis results (CHN) were obtained for both compounds. The molecular structures of 1 and 2 Æ EtOH were established by single crystal X-ray diffraction, and are shown in Figs. 1 and 2, respectively. The Eu(III) of 1 is coordinated by two hfac, one tptz, one H2O, and one EtOH, whereas the coordination of 2 features three hfac and one tptz ligands. In both cases, the coordination polyhedron may be best described as a distorted square antiprism monocapped by the middle coordinating N atom [N(1)]. For complex 1, the charge-balancing CF3 CO2 is hydrogen-
3. Results and discussion Most of the lanthanide b-diketonates reported were synthesized either by an one-pot synthesis in which the lanthanide starting material, a b-diketone, and a selected neutral ligand are reacted in the presence of a suitable base or by first making the electrically neutral b-diketonate hydrates, followed by substitution of the aqua ligand(s) for neutral ligands of a different type [8,36]. However, adventitious hydrolysis producing unexpected lanthanide oxo/hydroxo complexes has been observed in both syntheses [37]. We noted recently that introducing the neutral Lewis base ligand prior to the introduction of the b-diketonate ligand is an effective approach to suppressing the formation of the hydrolysis products [16]. Subsequently, this procedure was utilized for the successful production of a number of tptz adducts with various lanthanide b-diketonates [17]. Surprisingly, when the same procedure was applied for the synthesis of Eu(hfac)3(tptz), a complex formulated as ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) was instead isolated in good yield. Only two diketonate ligands are incorporated into the coordination sphere, although the initial molar ratios of Eu(III) to HFA is 1:3. Charge balancing is provided by CF3 CO2 whose formation is also worth noting since it was not present in the starting materials. This in situ formation of CF3 CO2 is not unprecedented; it has been observed by other researchers as the result of hfac decomposition via the reverse reaction of Claissen condensation, a process facilitated by a base in the presence of trace amount of water [38,39]. In the present case, the uncoordinated N atoms of tptz may serve as the base catalyst for the decomposition of the third HFA that is used for Eu(III) coordination. The tptz adduct initially planned for, Eu(hfac)3(tptz) (2), was subsequently synthesized by using the more traditional
Fig. 1. An ORTEP view of the crystal structure of ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1). Thermal ellipsoids are drawn at the 50% probability level.
Fig. 2. An ORTEP view of the crystal structure of Eu(hfac)3(tptz) Æ EtOH (2). Thermal ellipsoids are drawn at the 50% probability level.
C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455
bonded with the OH group of the Eu(III)-bound EtOH. For 2 Æ EtOH, there is a molecule of EtOH, hydrogenbonded to the metal free N3 and N6 of the tptz ligand. ˚ to The Eu–O(hfac) distances of 1 range from 2.390(3) A ˚ ˚ 2.450(3) A, with an average of 2.420(8) A. For 2 Æ EtOH, ˚ to the corresponding values are from 2.354(2) A ˚ ˚ 2.503(2) A, with an average of 2.411(4) A. These distances are within the range reported for similar complexes [9,13,40,41]. The tptz ligand is nearly coplanar in both ˚ and complexes. The Eu–N(1) bond distance is 2.534(3) A ˚ 2.539(2) A for 1 and 2, respectively. These values, indicated in bold face in Table 2, are the shortest of the three, consistent with the findings made with previously reported lanthanide-tptz complexes [16,25,28]. As compared with previously reported tptz adducts of europium b-diketonates, a number of structural features of 1 and 2 merit further discussions. The Eu–N1 distances ˚ for 1 and 2.539(2) A ˚ for 2] are noticeably shorter [2.534(3) A than those of Eu(b-diketonate)3(tptz). The corresponding ˚ (tta, thinoyltrifluoroacetone), values are 2.580(7) A ˚ 2.607(3) A (btfa, 4,4,4-trifluoro-1-phenyl-1,3-butanedione), ˚ (ba, 1-benzoylacetone), and 2.676(2) A ˚ (dbm, 2.618(11) A dibenzoylmethane) [16,17]. They are even shorter than ˚ ] and Eu(tptz)those of Eu(tptz)Cl3(MeOH)2 [2.555(4) A ˚ ] whose Eu(III) ions are situated (NO3)3(H2O) [2.576(2) A in a less sterically congested environment [25,28]. This observation may be rationalized in terms of the electron withdrawing power of the fluorinated ligands; hfac is less reactive toward the Eu(III) center due to the relatively low electron density on its O atoms. As a result, the net positive charge of the lanthanide ion with hfac coordination is probably more than when non-fluorinate ligands are utilized. A corollary is that tptz ligand is more strongly bound
Table 2 ˚ ) and angles () of ½EuðhfacÞ ðH2 OÞðEtOHÞSelected bond distances (A 2 ðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) Æ EtOH (2 Æ EtOH) ½EuðhfacÞ2 ðH2 OÞ ðEtOHÞðtptzÞ½CF3 CO2
O(1)–Eu(1)–O(2) O(1)–Eu(1)–O(4) O(4)–Eu(1)–O(3) O(6)–Eu(1)–O(3) O(2)–Eu(1)–O(5) O(6)–Eu(1)–N(4) O(5)–Eu(1)–N(5) N(1)–Eu(1)–N(4) N(1)–Eu(1)–N(5) N(4)–Eu(1)–N(5)
to Eu(III) in the case of hfac coordination, resulting in a shorter Eu–N distance. The preoccupation of part of the coordination sphere by the bulky tptz ligand in the synthesis of 1 may have prevented the coordination of three hfac ligands, particularly so if the relatively weak coordinating ability of the hfac ligand is considered. A similar observation was made in the synthesis of Yb(hfac)2(NO3)(terpy) (terpy = 2,2 0 :6 0 ,200 terpyridine) where Yb(III) ion is coordinated to two hfac ligands instead of three due to the preoccupation of the metal center by terpy, which has a similar steric bulk of coordination to that of tptz [42]. 3.1. Electronic spectroscopic and photoluminescence studies The electronic absorption spectra obtained for 2 and its ligands (Hhfac and tptz) in dichloromethane are shown in Fig. 3. Tptz shows absorption bands at 247 nm and 282 nm, whereas a strong absorption is shown at 275 nm for Hhfac. The spectrum of 2 (kmax = 295 nm) contains essentially the combined ligand absorptions, but with slight red shifts due to metal complexation. Complex 1 displays almost identical ligand-based absorption in the range of 225–350 nm. The electronic excitation and photoluminescence of 1 and 2 were studied in dichloromethane solution at room temperature, and they exhibit very similar properties. Only the spectra of 2 are shown in Fig. 4. The excitation spectrum resembles its absorption spectrum, confirming that the energy transfer occurs from the ligands to the Eu(III) ion. Five narrow emission peaks are observed in the range 570–715 nm, characteristic of Eu(III)-originated luminescence. The photoluminescence quantum yields of 1 and 2 were determined to be 52% and 60%, respectively. The maximum excitation wavelength of the title complexes (295 nm) is very close to the maximum absorption wave1.0
Eu(hfac)3(tptz) Æ EtOH 2.390(3) 2.435(3) 2.437(3) 2.411(3) 2.450(3) 2.400(3) 2.534(3) 2.570(3) 2.581(3)
69.92(11) 101.00(11) 69.96(10) 69.85(10) 73.56(11) 82.63(10) 81.34(11) 63.64(10) 62.73(10) 126.34(10)
Eu(1)–O(1) Eu(1)–O(2) Eu(1)–O(3) Eu(1)–O(4) Eu(1)–O(5) Eu(1)–O(6) Eu(1)–N(1) Eu(1)–N(4) Eu(1)–N(5) O(1)–Eu(1)–O(2) O(2)–Eu(1)–O(3) O(3)–Eu(1)–O(4) O(1)–Eu(1)–O(5) O(5)–Eu(1)–O(6) O(6)–Eu(1)–N(5) O(4)–Eu(1)–N(4) N(1)–Eu(1)–N(4) N(1)–Eu(1)–N(5) N(4)–Eu(1)–N(5)
2.354(2) 2.374(2) 2.503(2) 2.392(2) 2.438(2) 2.406(2) 2.539(2) 2.581(3) 2.576(3) 74.83(9) 72.57(8) 67.79(8) 75.57(8) 69.00(8) 67.84(8) 67.41(8) 62.87(8) 63.20(8) 126.03(8)
HFA TPTZ Eu(HFA)3TPTZ
0.8
Absorbance
Eu(1)–O(1) Eu(1)–O(2) Eu(1)–O(3) Eu(1)–O(4) Eu(1)–O(5) Eu(1)–O(6) Eu(1)–N(1) Eu(1)–N(4) Eu(1)–N(5)
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0.6
0.4
0.2
0.0 250
300
350
Wavelength (nm) Fig. 3. Absorption spectra of Hhfac, tptz, and 2 in dichloromethane solution.
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C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455
between these two complexes is caused by simply reversing the order of ligand (Hhfac and tptz) addition during the complex synthesis. Both complexes display bright photoluminescence characteristic of Eu(III) ion. Both ligands are believed to be responsible for the high quantum yields observed.
Excitation Emission
1.0
Intensity (a.u.)
0.8
0.6
Acknowledgements 0.4
0.2
0.0 200
300
400
500
600
700
This work was supported by NSF CAREER Grant No. CHE-0238790. Acknowledgment is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Professor K. Miranda for the use of fluorometer. The CCD-based X-ray diffractometer was purchased through an NSF Grant (CHE-96103474, USA).
Wavelength (nm) Fig. 4. Excitation (—) and emission (–) spectra of 2 in dichloromethane solution.
lengths of both Hhfac (275 nm) and tptz (282 nm). As such, both ligands are presumably responsible for the ligandmediated energy transfer, resulting in effective population of the Eu(III) 5D0 emissive state and high quantum yields of light emission. Thus, tptz ligand not only saturates the coordination sphere but also enhances the ligand-mediated energy transfer. This may be the reason why the quantum yield observed for 1 is still high despite the presence of luminescence–quenching water and ethanol ligands. In fact, the two complexes reported here possess the highest quantum yields among all the reported Eu(III)-hfac complexes, lending further support to the significant role(s) of tptz. In comparison, the photoluminescence quantum yields for Eu(dbm)3(tptz), Eu(ba)3(tptz), Eu(tta)3(tptz), and Eu(btfa)3(tptz) were found to be 17.4, 15.5, 40.2, and 69.7%, respectively [17]. In these cases, the energy transfer was believed to be mainly mediated by the b-diketonate ligands as the excitation wavelengths match the absorption spectra of the corresponding b-diketonate ligands. The comparably high values for Eu(tta)3(tptz) and Eu(btfa)3(tptz) may be the result of a better energy match between the ligand singlet and triplet excited states and/or a better energy match between the ligand triple excited state and the metal ion’s resonance levels [7,14], with the tptz ligand providing additional advantages of site-protection. 4. Summary Two new europium b-diketonates featuring 1,1,1,5,5,5hexafluoro-2,4-pentanedionate and 2,4,6-tri(2-pyridyl)1,3,5-triazine, a rigid neutral ligand were synthesized. These two complexes, formulated as ½EuðhfacÞ2 ðH2 OÞ ðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2), were structurally characterized using single crystal X-ray diffraction. The difference in the composition and structure
Appendix A. Supporting information available Crystallographic data of ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ ½CF3 CO2 and Eu(hfac)3 (tptz) Æ EtOH(CIF) have been deposited with the Cambridge Crystallographic Data Centre, CCDC 603042 and 603043. This material is available free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; email: [email protected] or http://www.ccdc.cam.ac.uk/ deposit). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2006.06.032. References [1] N. Sabbatini, M. Guardogi, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201. [2] J.G. Bunzli, C. Piguet, Chem. Soc. Rev. 34 (2005) 1048. [3] J. Kido, Y. Okamoto, Chem. Rev. 102 (2002) 2357. [4] M.L. Bhaumik, M.A. EI-Sayed, J. Chem. Phys. 42 (1965) 787. [5] S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura, F. Barigelletti, Inorg. Chem. 44 (2005) 529. [6] F.S. Richardson, Chem. Rev. 82 (1982) 541. [7] K. Binnemans, Rare-earth b-diketonates, in: K.A. GschneidnerJr., J.-C.G. Bu¨nzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Rare Earths, vol. 35, Elsevier B.V., 2005. [8] L.R. Melby, N.J. Rose, E. Abramson, J.C. Caris, J. Am. Chem. Soc. 86 (1964) 5117. [9] S. Kang, Y.S. Jung, Y.S. Sohn, Bull. Korean Chem. Soc. 18 (1997) 75. [10] H.J. Batista, A.V.M. de Andrade, R.L. Longo, A.M. Simas, G.F. de Sa, N.K. Ito, L.C. Thompson, Inorg. Chem. 37 (1998) 3542. [11] P.C. Christidis, I.O. Tossidis, D.G. Paschalidis, L.C. Tzavellas, Acta Crystallogr. Sect. C54 (1998) 1233. [12] Z. Zheng, J. Wang, H. Liu, M.D. Carducci, N. Peyghambarian, G.E. Jabbourb, Acta Crystallogr. Sect. C58 (2002) m50. [13] Y. Hasegawa, M. Yamamuro, Y. Wada, N. Kanehisa, Y. Kai, S. Yanagida, J. Phys. Chem. A 107 (2003) 1697. [14] C.H. Huang, F.Y. Li, W. Huang, Introduction to Organic LightEmitting Materials and Devices, Fudan Press, Shanghai, China, 2005 (chapter 8). [15] R. Wang, R. Wang, J. Yang, Z. Zheng, M.D. Carducci, T. Cayou, N. Peyghambarian, G.E. Jabbour, J. Am. Chem. Soc. 123 (2001) 6179.
C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455 [16] C.R. De Silva, J. Wang, M.D. Carducci, S.A. Rajapakshe, Z. Zheng, Inorg. Chim. Acta 357 (2004) 630. [17] C.R. De Silva, J.R. Maeyer, A. Dawson, Z. Zheng, Inorg. Chem. submitted to publication. [18] Y. Hasegawa, H. Kawai, K. Nakamura, N. Yasuda, Y. Wada, S. Yanagida, J. Alloy Compd. 408 (2006) 669. [19] L.J. Nugent, J.L. Burnett, R.D. Baybarz, G.K. Werner, S.P. Tanner, J.R. Tarrant, O.L. Keller, J. Phys. Chem. 73 (1969) 1540. [20] G. Malandrino, M. Bettinelli, A. Speghini, I.L. Fragala, Eur. J. Inorg. Chem. (2001) 1039. [21] S. Kang, Y.S. Jung, Y.S. Sohn, Bull. Korean Chem. Soc. 18 (1997) 266. [22] J. Yu, L. Zhou, H. Zhang, Y. Zheng, H. Li, R. Deng, Z. Peng, Z. Li, Inorg. Chem. 44 (2005) 1611. [23] G. Ionova, C. Raber, R. Guillaumont, S. Ionov, C. Madic, J.C. Krupa, D. Guillaneux, New J. Chem. 26 (2002) 234. [24] D.A. Durham, G.H. Frost, F.A. Hart, J. Inorg. Nucl. Chem. 31 (1969) 571. [25] R. Wietzke, M. Mazzanti, J. Latour, J. Pecaut, Inorg. Chem. 38 (1999) 3581. [26] M.G.B. Drew, M.J. Hudson, P.B. Iverson, C. Madic, Acta Crystallogr. Sect. C 56 (2000) 434. [27] G.Y.S. Chan, M.G.B. Drew, M.J. Hudson, N.S. Isaacs, P. Byers, Polyhedron 15 (1996) 3385. [28] S.A. Cotton, V. Franckevicius, M.F. Mahon, L.L. Ooi, P.R. Raithby, S.J. Teat, Polyhedron 25 (2006) 1057.
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[29] R. Zibaseresht, R.M. Hartshorn, Aust. J. Chem. 58 (2005) 345. [30] Bruker AXS Crystal Structure Analysis Package, Version 5.10 (SMARTNT (Version 5.053), SAINT-Plus (Version 6.01), SHELXTL (Version 5.1)); Bruker AXS Inc., Madison, WI, 1999. [31] D.T. Cromer, J.T. Waber, International Tables for Xray Crystallography, vol. 4, Kynoch Press, Birmingham, UK, 1974, Table 2.2 A. [32] F.R.G. Silva, O.L. Malta, C. Reinhard, H. Gudel, C. Piguet, J.E. Moser, J. Bunzli, J. Phys. Chem. A 106 (2002) 1670. [33] R.F. Kubin, A.N. Fletcher, J. Lumin. 27 (1982) 455. [34] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991. [35] J.V. Caspar, T.J. Meyer, J. Am. Chem. Soc. 105 (1983) 5583. [36] J.A. Fernandes, R.A. Sa Fereira, M. Pillinger, L.D. Carlos, J. Jepsen, A. Hazell, P. Ribeiro-Claro, I.S. Goncalves, J. Lumin. 113 (2005) 50. [37] R. Wang, D. Song, S. Wang, Chem. Commun. (2002) 368. [38] S.R. Drake, A. Lyons, D.J. Otway, D.J. Williams, Inorg. Chem. 33 (1994) 1230. [39] S. Wang, Z. Pang, K.D.L. Smith, Y. Hua, C. Deslippe, M.J. Wagner, Inorg. Chem. 34 (1995) 908. [40] C. Benelli, A. Caneschi, A.C. Fabretti, D. Gatteschi, L. Pardi, Inorg. Chem. 29 (1990) 4153. [41] W.J. Evans, D.G. Giarikos, M.A. Johnston, M.A. Greci, J.W. Ziller, J. Chem. Soc., Dalton Trans. (2002) 520. [42] K. Hayashi, N. Nagao, K. Harada, M. Haga, Y. Fukuda, Chem. Lett. (1998) 1173.
Highly luminescent Eu(III) complexes with 2,4,6-tri(2-pyridyl)-1,3,5-triazine ligand: Synthesis, structural characterization, and photoluminescence studies Channa R. De Silva a, Ruiyao Wang b, Zhiping Zheng b
a,*
a Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA Department of Chemistry, Queen’s University, Kingston, Ont., Canada K7L 3N6
Received 4 April 2006; accepted 28 June 2006 Available online 14 July 2006
Abstract Two new Eu(III) complexes featuring 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) and a rigid Lewis base ligand 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz), ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2), were synthesized. Their structures were established by single crystal X-ray diffraction. The europium ion in each of these complexes is nona-coordinate with six oxygen and three nitrogen atoms forming a coordination polyhedron best describable as a monocapped square antiprism. The difference in the composition and structure between these two complexes is caused by simply reversing the order of ligand (hfac and tptz) addition during the complex synthesis, and is rationalized in terms of the structural and electronic properties of the ligands and the overall steric bulk of the coordination sphere. Both complexes display characteristic Eu(III)-originated red emission upon UV excitation. The high quantum yields observed, 52% (1) and 60% (2), are rationalized in terms of the strong absorptions of both hfac and tptz ligands near the excitation wavelength (295 nm), an interpretation consistent with the well-established mechanism of ligand-mediated energy transfer for lanthanide-based light emission. 2006 Elsevier Ltd. All rights reserved. Keywords: Europium complexes; 2,4,6-Tri(2-pyridyl)-1,3,5-triazine; Crystal structures; Absorption; Photoluminescence; Synthesis
1. Introduction The study of luminescent lanthanide complexes remains an active area of research, due largely to the applications of such materials in biomedical and optical technologies [1–3]. Besides the unique luminescence characteristics originated from the f-electronic structure of the lanthanide elements, the usefulness of these materials depends critically on the molecular structure of the complexes. This dependence is reflected in a number of aspects, the saturation of coordination sphere of these rather bulky ions to minimize potentially luminescence–quenching solvent interactions, the use of ligands capable of promoting ligand-mediated energy transfer, and the avoidance of ligands with possibility of *
Corresponding author. Tel.: +1 520 626 6495; fax: +1 520 621 8407. E-mail address: [email protected] (Z. Zheng).
0277-5387/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.06.032
luminescence–quenching vibrational coupling with the metal center [4–6]. The stability of the complex and its facility in device fabrication are also part of the consideration when it comes to the design of practically useful materials. Arguably the most common lanthanide complexes based on the above consideration are lanthanide b-diketonates [7]. Both anionic complexes featuring four diketonate ligands and electrically neutral complexes containing both diketonate and other ligands have been studied [8–13]. Their photophysical properties, thermal and chemical stabilities, and processibility can be modified and fine-tuned by judiciously chosen ligand combinations. We have been interested in developing new lanthanide complexes for potential applications in organic light-emitting devices [14–17]. Of particular interest are complexes featuring three diketonate ligands and one or more neutral
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ligands (L) of the general formula Ln(b-diketonate)3L, for which impressive photoluminescence properties have been demonstrated [18]. For the diketonate ligand, we have chosen 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac) because of its low vibrational coupling with Eu(III) emission [9,19–21], enhanced thermal stability and volatility of the resulting lanthanide complexes when compared with its non-fluorinated analogs [22]. Our choice of ligand L is 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz), a bulky aromatic compound featuring three 2-pyridyl rings fixed on a central 1,3,5-triazine platform [23]. Lanthanide complexes with tptz ligand have first been introduced by Durham et al. [24]. Recently a number of structurally characterized lanthanide complexes were reported [16,25–28], including the very first tptz adduct of a lanthanide b-diketonate reported by us [16]; tptz acts as a terdentate ligand in these structurally characterized complexes, coordinating the lanthanide ion with one of the three pyrazine N atoms and the N atoms on the ortho-substituted pyridyl groups. The remaining N atoms allow for further coordination to other metals, transition metals in particular [29]. Thus, structurally sophisticated multinuclear heterometallic complexes may be envisioned, whose magnetic and photophysical properties are of fundamental interest and practical significance. By adopting a procedure used previously for the synthesis of Eu(dbm)3(tptz) (dbm = dibenzoylmethanate) [16], in which a tptz complex with the starting EuCl3 hydrate was prepared followed by the substitution of Cl for the diketonate ligands, an unexpected complex formulated as ½Eu ðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) was obtained. The complex planned for, Eu(hfac)3(tptz) (2), was subsequently produced by using the more traditional route of making the hydrated lanthanide diketonate first, followed
by the replacement of the aqua ligands by tptz. Here the synthesis, structural characterization by single-crystal Xray diffraction, and photoluminescence studies of these two new Eu(III) complexes are reported (Scheme 1). 2. Experimental EuCl3 Æ 6H2O (99.9%), 2,4,6-tri(2-pyridyl)-1,3,5-triazine (98%), 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (98%), and potassium tert-butoxide (95%) were purchased from Aldrich and used without further purification. A slightly modified procedure for the recently reported Eu(dbm)3(tptz) was adopted [16]. Elemental analysis (CHN) was performed by Numega Resonance Laboratory, San Diego, California. 2.1. Synthesis of [Eu(hfac)2 (H 2 O)(EtOH )(tptz)][CF 3 CO2 ] (1) A mixture of tptz (0.312 g, 1.00 mmol) and EuCl3 Æ 6H2O (0.366 g, 1.00 mmol) in 20 mL of absolute ethanol was stirred at 60 C for 10 min to afford a clear solution. To this solution was added over 10 min a solution of KOBut (0.336 g, 3.00 mmol) and Hhfac (0.624 g, 3.00 mmol) in 10 mL of absolute ethanol, and the resulting mixture was stirred at 60 C for 1 h under nitrogen, and then at room temperature for 3 h. The mixture thus obtained was filtered, and the solid was washed with copious de-ionized water to remove KCl. The crude product was dissolved in acetone, and the solution was dried over anhydrous MgSO4. The filtrate was collected, from which a pale yellow solid was obtained upon removal of the solvent. Recrystallization from ethanol:acetone (v/v 1:1) produced a crystalline solid as the analytically pure product (0.64 g, 61%). Anal. Calc.
N N
N N
N
N
N
+
N
(i)
N
EuCl3 . 2H2O
N N
N
N N
N
(ii)
N
F3C
N
O
N
EuCl3.6H2O
O
Eu
OH2
O F3C
(i) EtOH, 60oC (ii) KOBu t, Hhfac, 60 oC
.CF3COO-
O
O F3C
CF3
N N
N H2O
Hhfac
OH2 CF3
F3C O
(i)
+ EuCl3 .6H2O
N
O
Eu
F3C
O F3C
CF3
F3C
O
O
N
N
(ii) O CF3
O CF3
(i) H2O, KOBut, 60oC (ii) EtOH / acetone, 60 oC
O
Eu
O
O F3C
O F3C
CF3
O CF3
Scheme 1. Synthesis of ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2).
C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455
for C32H22N6O8F15Eu: C, 36.41; H, 2.10; N, 7.96. Found: C, 36.33; H, 2.06; N, 8.09%. 2.2. Synthesis of Eu(hfac)3(tptz) (2) Using a modified literature [11], Eu(hfac)3(H2O)2 was first prepared. Briefly, Hhfac (0.624 g, 3.0 mmol) was added to a solution of KOBut (0.336 g, 3.0 mmol) in H2O (10 mL). The resulting clear solution was stirred for 10 min at room temperature and then added to an aqueous solution of EuCl3 Æ 6H2O (0.366 g 1.0 mmol in 10 mL H2O). The mixture was stirred at 60 C for 30 min under nitrogen and then for 2.5 h at room temperature. The mixture was filtered, and the precipitate was washed with cold de-ionized water (2 · 100 mL) and dried under vacuum at room temperature for 12 hr. If necessary, the product can be recrystallized from acetone:ethanol (v/v 1:1). To a solution of Eu(hfac)3(H2O)2 (0.808 g, 1.0 mmol) in acetone (15 mL) was added tptz (0.312 g, 1.00 mmol) in ethanol (15 mL). The mixture was stirred at 60 C for 0.5 h and then overnight at room temperature. The mixture was filtered, and the filtrate was allowed to evaporate at room temperature to afford analytically pure product (0.73 g, 67%). Anal. Calc. for C33H15N6O6F18Eu: C, 36.52; H, 1.39; N, 7.74. Found: C, 36.65; H, 1.36; N, 7.80%.
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2.3. X-ray structure determinations Single crystals of both compounds suitable for X-ray diffraction studies were obtained by slow evaporation of their saturated solutions in ethanol:acetone (v/v 1:1) at room temperature over several days. The crystals were mounted on a glass fiber in a random orientation and cooled to 93 C in a stream of nitrogen gas controlled with Cryostream Controller 700. Data collection was performed on a Bruker SMART CCD 1000 X-ray diffractometer with graphite-monochromated Mo ˚ ), operating at 50 kV and Ka radiation (k = 0.71073 A 30 mA over 2h ranges of 2.58–50.00. No significant decay was observed during the data collection. Data were processed on a Pentium PC using the Bruker AXS Crystal Structure Analysis Package, Version 5.10 [30]. Neutral atom scattering factors were taken from Cromer and Waber [31]. The raw intensity data were integrated using the program SAINT-Plus and absorption corrections were applied using program SADABS. The structure was solved by direct methods. Full-matrix least-square P refinements minimizing the function W ðF 2o F 2c Þ were applied to the compound. All non-hydrogen atoms were refined anisotropically. All of the water hydrogen atoms and the hydroxyl hydrogen atoms were located gradually
Table 1 Crystal data and structure refinement for ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) Æ EtOH (2 Æ EtOH) Crystal data
½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2
[Eu(hfac)3(tptz)] Æ EtOH
Empirical formula Formula weight Temperature (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Volume (A Z Density (calculated) (mg/m3) Absorption coefficient (mm1) F(0 0 0) h Range for data collection () Index ranges
C34H30EuF15N6O10 1119.60 180(2) triclinic P 1 10.3854(15) 13.3089(18) 16.452(2) 93.673(3) 104.942(2) 103.719(2) 2115.3(5) 2 1.758 1.610 1108 1.29–25.00 12 6 h 6 12, 15 6 k 6 15, 19 6 l 6 18 12 200 7389 [0.0473] 99.1 0.4642 and 0.3449 full-matrix least-squares on F2 7389/6/668 1.052 R1 = 0.0414, wR2 = 0.1118 R1 = 0.0446, wR2 = 0.1145 2.803 and 1.652
C35H21EuF18N6O7 1131.54 180(2) triclinic P 1 10.7819(14) 13.4118(17) 14.8428(19) 98.559(2) 95.931(2) 97.830(2) 2085.8(5) 2 1.802 1.638 1108 1.40–25.00 12 6 h 6 12, 15 6 k 6 15, 17 6 l 6 17 12 328 7309 [0.0156] 99.6 1.0000 and 0.8472 full-matrix least-squares on F2 7309/0/664 1.034 R1 = 0.0298, wR2 = 0.0829 R1 = 0.0325, wR2 = 0.0850 0.873 and 0.702
Reflections collected Independent reflections [Rint] Completeness to h = 25.00 (%) Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) ˚ 3) Largest difference peak and hole (e A
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in difference Fourier map, while the rest hydrogen atoms were calculated, and their contributions were included in the structure factor calculations. Details on data collection and structure refinements are given in Table 1. 2.4. Photophysical studies Electronic absorption spectra in dichloromethane solutions were recorded on a Perkin Elmer Lambda 10 spectrophotometer. Photoluminescence studies were carried out using a Fluorolog-3 fluorometer. The measured fluorescence in the visible range was excited by the light from a Xe-Arc lamp and detected with a photo multiplier tube at an angle of 90 to the incident beam. Photoluminescent quantum yields were measured and calculated using cresyl violet perchlorate (U = 0.54 in methanol) [32] or rhodamin 6G (95% in ethanol) [33] as the standards. Corrections were made for the instrumental parameters and differing refractive indices of the solvents used [34,35]. The experimental uncertainty for the quantum efficiency calculations is 10%.
approach. EuCl3 Æ 6H2O was reacted with three equivalents of Hhfac/base to form Eu(hfac)3(H2O)2, whose aqua ligands were then substituted for tptz under mild conditions. Both 1 and 2 are air-stable and readily soluble in solvents such as acetone, chloroform, and dichloromethane, but sparingly soluble in alcohols. Product purification was facile by recrystallization from a mixture of acetone and ethanol. Satisfactory microanalysis results (CHN) were obtained for both compounds. The molecular structures of 1 and 2 Æ EtOH were established by single crystal X-ray diffraction, and are shown in Figs. 1 and 2, respectively. The Eu(III) of 1 is coordinated by two hfac, one tptz, one H2O, and one EtOH, whereas the coordination of 2 features three hfac and one tptz ligands. In both cases, the coordination polyhedron may be best described as a distorted square antiprism monocapped by the middle coordinating N atom [N(1)]. For complex 1, the charge-balancing CF3 CO2 is hydrogen-
3. Results and discussion Most of the lanthanide b-diketonates reported were synthesized either by an one-pot synthesis in which the lanthanide starting material, a b-diketone, and a selected neutral ligand are reacted in the presence of a suitable base or by first making the electrically neutral b-diketonate hydrates, followed by substitution of the aqua ligand(s) for neutral ligands of a different type [8,36]. However, adventitious hydrolysis producing unexpected lanthanide oxo/hydroxo complexes has been observed in both syntheses [37]. We noted recently that introducing the neutral Lewis base ligand prior to the introduction of the b-diketonate ligand is an effective approach to suppressing the formation of the hydrolysis products [16]. Subsequently, this procedure was utilized for the successful production of a number of tptz adducts with various lanthanide b-diketonates [17]. Surprisingly, when the same procedure was applied for the synthesis of Eu(hfac)3(tptz), a complex formulated as ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1) was instead isolated in good yield. Only two diketonate ligands are incorporated into the coordination sphere, although the initial molar ratios of Eu(III) to HFA is 1:3. Charge balancing is provided by CF3 CO2 whose formation is also worth noting since it was not present in the starting materials. This in situ formation of CF3 CO2 is not unprecedented; it has been observed by other researchers as the result of hfac decomposition via the reverse reaction of Claissen condensation, a process facilitated by a base in the presence of trace amount of water [38,39]. In the present case, the uncoordinated N atoms of tptz may serve as the base catalyst for the decomposition of the third HFA that is used for Eu(III) coordination. The tptz adduct initially planned for, Eu(hfac)3(tptz) (2), was subsequently synthesized by using the more traditional
Fig. 1. An ORTEP view of the crystal structure of ½EuðhfacÞ2 ðH2 OÞðEtOHÞðtptzÞ½CF3 CO2 (1). Thermal ellipsoids are drawn at the 50% probability level.
Fig. 2. An ORTEP view of the crystal structure of Eu(hfac)3(tptz) Æ EtOH (2). Thermal ellipsoids are drawn at the 50% probability level.
C.R. De Silva et al. / Polyhedron 25 (2006) 3449–3455
bonded with the OH group of the Eu(III)-bound EtOH. For 2 Æ EtOH, there is a molecule of EtOH, hydrogenbonded to the metal free N3 and N6 of the tptz ligand. ˚ to The Eu–O(hfac) distances of 1 range from 2.390(3) A ˚ ˚ 2.450(3) A, with an average of 2.420(8) A. For 2 Æ EtOH, ˚ to the corresponding values are from 2.354(2) A ˚ ˚ 2.503(2) A, with an average of 2.411(4) A. These distances are within the range reported for similar complexes [9,13,40,41]. The tptz ligand is nearly coplanar in both ˚ and complexes. The Eu–N(1) bond distance is 2.534(3) A ˚ 2.539(2) A for 1 and 2, respectively. These values, indicated in bold face in Table 2, are the shortest of the three, consistent with the findings made with previously reported lanthanide-tptz complexes [16,25,28]. As compared with previously reported tptz adducts of europium b-diketonates, a number of structural features of 1 and 2 merit further discussions. The Eu–N1 distances ˚ for 1 and 2.539(2) A ˚ for 2] are noticeably shorter [2.534(3) A than those of Eu(b-diketonate)3(tptz). The corresponding ˚ (tta, thinoyltrifluoroacetone), values are 2.580(7) A ˚ 2.607(3) A (btfa, 4,4,4-trifluoro-1-phenyl-1,3-butanedione), ˚ (ba, 1-benzoylacetone), and 2.676(2) A ˚ (dbm, 2.618(11) A dibenzoylmethane) [16,17]. They are even shorter than ˚ ] and Eu(tptz)those of Eu(tptz)Cl3(MeOH)2 [2.555(4) A ˚ ] whose Eu(III) ions are situated (NO3)3(H2O) [2.576(2) A in a less sterically congested environment [25,28]. This observation may be rationalized in terms of the electron withdrawing power of the fluorinated ligands; hfac is less reactive toward the Eu(III) center due to the relatively low electron density on its O atoms. As a result, the net positive charge of the lanthanide ion with hfac coordination is probably more than when non-fluorinate ligands are utilized. A corollary is that tptz ligand is more strongly bound
Table 2 ˚ ) and angles () of ½EuðhfacÞ ðH2 OÞðEtOHÞSelected bond distances (A 2 ðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) Æ EtOH (2 Æ EtOH) ½EuðhfacÞ2 ðH2 OÞ ðEtOHÞðtptzÞ½CF3 CO2
O(1)–Eu(1)–O(2) O(1)–Eu(1)–O(4) O(4)–Eu(1)–O(3) O(6)–Eu(1)–O(3) O(2)–Eu(1)–O(5) O(6)–Eu(1)–N(4) O(5)–Eu(1)–N(5) N(1)–Eu(1)–N(4) N(1)–Eu(1)–N(5) N(4)–Eu(1)–N(5)
to Eu(III) in the case of hfac coordination, resulting in a shorter Eu–N distance. The preoccupation of part of the coordination sphere by the bulky tptz ligand in the synthesis of 1 may have prevented the coordination of three hfac ligands, particularly so if the relatively weak coordinating ability of the hfac ligand is considered. A similar observation was made in the synthesis of Yb(hfac)2(NO3)(terpy) (terpy = 2,2 0 :6 0 ,200 terpyridine) where Yb(III) ion is coordinated to two hfac ligands instead of three due to the preoccupation of the metal center by terpy, which has a similar steric bulk of coordination to that of tptz [42]. 3.1. Electronic spectroscopic and photoluminescence studies The electronic absorption spectra obtained for 2 and its ligands (Hhfac and tptz) in dichloromethane are shown in Fig. 3. Tptz shows absorption bands at 247 nm and 282 nm, whereas a strong absorption is shown at 275 nm for Hhfac. The spectrum of 2 (kmax = 295 nm) contains essentially the combined ligand absorptions, but with slight red shifts due to metal complexation. Complex 1 displays almost identical ligand-based absorption in the range of 225–350 nm. The electronic excitation and photoluminescence of 1 and 2 were studied in dichloromethane solution at room temperature, and they exhibit very similar properties. Only the spectra of 2 are shown in Fig. 4. The excitation spectrum resembles its absorption spectrum, confirming that the energy transfer occurs from the ligands to the Eu(III) ion. Five narrow emission peaks are observed in the range 570–715 nm, characteristic of Eu(III)-originated luminescence. The photoluminescence quantum yields of 1 and 2 were determined to be 52% and 60%, respectively. The maximum excitation wavelength of the title complexes (295 nm) is very close to the maximum absorption wave1.0
Eu(hfac)3(tptz) Æ EtOH 2.390(3) 2.435(3) 2.437(3) 2.411(3) 2.450(3) 2.400(3) 2.534(3) 2.570(3) 2.581(3)
69.92(11) 101.00(11) 69.96(10) 69.85(10) 73.56(11) 82.63(10) 81.34(11) 63.64(10) 62.73(10) 126.34(10)
Eu(1)–O(1) Eu(1)–O(2) Eu(1)–O(3) Eu(1)–O(4) Eu(1)–O(5) Eu(1)–O(6) Eu(1)–N(1) Eu(1)–N(4) Eu(1)–N(5) O(1)–Eu(1)–O(2) O(2)–Eu(1)–O(3) O(3)–Eu(1)–O(4) O(1)–Eu(1)–O(5) O(5)–Eu(1)–O(6) O(6)–Eu(1)–N(5) O(4)–Eu(1)–N(4) N(1)–Eu(1)–N(4) N(1)–Eu(1)–N(5) N(4)–Eu(1)–N(5)
2.354(2) 2.374(2) 2.503(2) 2.392(2) 2.438(2) 2.406(2) 2.539(2) 2.581(3) 2.576(3) 74.83(9) 72.57(8) 67.79(8) 75.57(8) 69.00(8) 67.84(8) 67.41(8) 62.87(8) 63.20(8) 126.03(8)
HFA TPTZ Eu(HFA)3TPTZ
0.8
Absorbance
Eu(1)–O(1) Eu(1)–O(2) Eu(1)–O(3) Eu(1)–O(4) Eu(1)–O(5) Eu(1)–O(6) Eu(1)–N(1) Eu(1)–N(4) Eu(1)–N(5)
3453
0.6
0.4
0.2
0.0 250
300
350
Wavelength (nm) Fig. 3. Absorption spectra of Hhfac, tptz, and 2 in dichloromethane solution.
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between these two complexes is caused by simply reversing the order of ligand (Hhfac and tptz) addition during the complex synthesis. Both complexes display bright photoluminescence characteristic of Eu(III) ion. Both ligands are believed to be responsible for the high quantum yields observed.
Excitation Emission
1.0
Intensity (a.u.)
0.8
0.6
Acknowledgements 0.4
0.2
0.0 200
300
400
500
600
700
This work was supported by NSF CAREER Grant No. CHE-0238790. Acknowledgment is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Professor K. Miranda for the use of fluorometer. The CCD-based X-ray diffractometer was purchased through an NSF Grant (CHE-96103474, USA).
Wavelength (nm) Fig. 4. Excitation (—) and emission (–) spectra of 2 in dichloromethane solution.
lengths of both Hhfac (275 nm) and tptz (282 nm). As such, both ligands are presumably responsible for the ligandmediated energy transfer, resulting in effective population of the Eu(III) 5D0 emissive state and high quantum yields of light emission. Thus, tptz ligand not only saturates the coordination sphere but also enhances the ligand-mediated energy transfer. This may be the reason why the quantum yield observed for 1 is still high despite the presence of luminescence–quenching water and ethanol ligands. In fact, the two complexes reported here possess the highest quantum yields among all the reported Eu(III)-hfac complexes, lending further support to the significant role(s) of tptz. In comparison, the photoluminescence quantum yields for Eu(dbm)3(tptz), Eu(ba)3(tptz), Eu(tta)3(tptz), and Eu(btfa)3(tptz) were found to be 17.4, 15.5, 40.2, and 69.7%, respectively [17]. In these cases, the energy transfer was believed to be mainly mediated by the b-diketonate ligands as the excitation wavelengths match the absorption spectra of the corresponding b-diketonate ligands. The comparably high values for Eu(tta)3(tptz) and Eu(btfa)3(tptz) may be the result of a better energy match between the ligand singlet and triplet excited states and/or a better energy match between the ligand triple excited state and the metal ion’s resonance levels [7,14], with the tptz ligand providing additional advantages of site-protection. 4. Summary Two new europium b-diketonates featuring 1,1,1,5,5,5hexafluoro-2,4-pentanedionate and 2,4,6-tri(2-pyridyl)1,3,5-triazine, a rigid neutral ligand were synthesized. These two complexes, formulated as ½EuðhfacÞ2 ðH2 OÞ ðEtOHÞðtptzÞ½CF3 CO2 (1) and Eu(hfac)3(tptz) (2), were structurally characterized using single crystal X-ray diffraction. The difference in the composition and structure
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