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Статья Paper
Synthesis and Spectral-Luminescent Properties of Polynuclear Lanthanide Complexes with Functionalized Calix[4]arenes
Olga V. Snurnikova, Aleksandr P. Lukyanenko, Elena A. Alekseeva, Yuriy V. Korovin, and Natalya V. Rusakova@
A.V. Bogatsky Physico-Chemical Institute of National Academy of Sciences of Ukraine, 65080 Odessa, Ukraine ®Corresponding author E-mail: lanthachem@ukr.net
The mono- and heteronuclear lanthanide-containing complexes on the basis of 1,3-disubstituted p-tert-butylcalix[4] arenes modified with macrocyclic (porphyrin) and acyclic (benzimidazole) nitrogen-containing fragments were investigated. The optimal conditions of synthesis, compositions andstructures oflanthanide complexes withfunctionalized calix[4]arenes were determined. The influence of the structure of compounds on the spectral-luminescent properties of both the ligands and their complexes was analyzed.
Keywords: Calix[4]arenes, porphyrins, lanthanide-containing complexes, 4f-luminescence.
Introduction
Calix[4]arenes are macrocyclic compounds which are products of the cyclic oligomerization of phenol and formaldehyde.[1] Calix[4]arenes can form supramolecular "host-guest" complexes with metal ions or small organic molecules. The advantage of use of calix[4]arene platform in the structure of molecular receptors is their preorganization, which provides a favourable position of functional groups and facilitates the formation of complexes.[2] In particular, this principle is implemented in 1,3-disubstituted calix[4] arenes, in which the unsubstituted hydroxyl-groups fix the position of functional groups through hydrogen bonds. This type of ligands is called "molecular tweezers".[3] It is known[4,5] that diamide derivatives of calix[4]arenes form stable complexes with lanthanide ions, alkali and transition metals due to the presence of a cavity formed by donor atoms of phenol, ether and amide groups. The combination of calix[4]arene macrocycle and substituents containing additional coordination centers (for example, imidazole) leads to obtaining of polyfunctional compounds. In complexes with these ligands the metal ion, depending on its nature, can coordinate with oxygen atoms of phenolic, carbonyl, ether groups, or form complexes with nitrogen-containing substituents. At the same time, the porphyrin-substituted calix[4]arenes, maintaining the marked features, represent the ligands with specific complexation and spectral-luminescent properties. These properties are due to the fact that they contain two chromophoric macrocycles, which absorb energy in the ultraviolet and visible spectral regions, as well as two types of coordination centers and are characterized by unique spatial structure. Thus, functionalized calix[4]arenes can be used as effective "building" blocks in the homo- and heteronuclear assembles.[6]
The development of synthesis of mono- and heteronuclear lanthanide-containing complexes on the basis of 1,3-disubstituted p-tert-butylcalix[4]arenes modified with porphyrin and benzimidazole fragments (Figure 1), analysis
of the relationship between spectral-luminescent properties of these compounds and their structure were the purpose of this work.
\}Пг: X=2H; M^H;,: X=Cu (II), Zn (II)
o=c
NH L2H2-L5H2: R=—(CH2),
1 i л R n=l-4
л-v/u I
X-JO
Figure 1. Structures of ligands and complexes.
Experimental
Ytterbium(III), erbium(III) and lutetium(III) oxides (99.98% - 99.99%), copper(II) and zinc(II) chlorides were purchased from Aldrich (Gillingham, Dorset) and used without further purification. Chlorides of lanthanides were prepared from corresponding oxides. The rest of chemicals were analytically pure in quality. Syntheses of compounds L1H2-L5H2 were realized according to the known methods.[7,8] All ligands and complexes were isolated in solid state and characterized by elemental analysis, IR-, UV-vis-, 1H NMR-spectroscopy and mass-spectrometry.
Mass spectra MALDI were recorded on Autoflex II spectrometer (Varian BV, Middelburg) using method of laser activated matrix desorption/ionization (MALDI), and laser desorption/ionization (LDI). Mass spectra FAB were registered on Varian MAT CH-112 spectrometer (Varian BV, Middelburg) using m-nitrobenzyl alcohol as the matrix. 1H NMR spectra were recorded on Bruker Avance-300 (300 MHz) in CDCl3. UV-vis spectra were recorded on a Specord M40 UV/VIS (Carl Zeiss, Jena, Germany) spectrometer. Emission and excitation spectra were measured using Horiba Jobin Yvon Fluorolog 3-22 spectrofluorimeter equipped for measurements at both RT and 77 K (detectors Hamamatsu R928
and DSS-IGA020L Electro-Optical Systems, Inc. for visible and NIR domain, respectively). The excitation source was a 450 W Xe arc lamp. The relative quantum yields of 4/-luminescence (9) for the ytterbium complexes were calculated using Zn tetraphenylporphyrin as a standard, the quantum yield of which is 0.0315.[9]
General procedure for the synthesis of mononuclear lanthanide- containing complexes on the basis of ligands LnL1Cl-LnL5Cl: lanthanide chloride (0.11 mmol LnCl36H2O) was refluxing with 0.5 ml triethyl orthoformiate in dry acetonitrile (10 ml) for 30 min. Subsequently, a solution of ligand (0.10 mmol in 5 ml acetonitrile) was added and reflux was continued for 3.5-4 h. Then, the reaction mixture was cooled to the room temperature, and diethyl ether was added (50 ml). The precipitate was filtered, washed with water, and dried at a temperature of 110-120°C for 8-10 h. As an example, the data for the complexes LuL1Cl and LuL2Cl are given below.
LuL1Cl (0.200 g, 93 %). M.p. > 350°C (CH3CN). m/z (MALDI) (%) 2197.0 (95) calcd. for C136H116N10ClO6Lu 2196.86. UV-vis (DMF) Xmax (lge) nm: 280 (3.42), 312 (2.48), 418 (5.1), 516 (4.2), 552 (3.75),"591 (3.46), 648 (3.43). 1H NMR (CDCl3, 300 K) SH ppm: -2.81 (4H, s, NH^), 0.97 (18H, s, tBu), 1.26 (18H, s, tBu), 3.40 (4H, d, ArCHeqAr, J = 13.4), 3.75 (4H, d, ArCHAr, J = 13.4), 4.57 (4H, s, OCH2)eq 6.81 (4H, s, ArH), 7.05 (4H, s, ArH), 7.68-7.80 (20H, m, CHph_porph.), 8.12-8.18 (10H, m, CHph porph.), 8.25 (8 H, d,
СHPorph.), 8 73 (8H S, CHph.-Porph.), 8 86-8 88 (4H° СHPorph.), 8 93
(2H, d, NHCO), 9.17 (4H, d, CHpoiph).
LuL2Cl (0.105 g, 87%). M.p. 328-330°C (CH3CN). m/z (FAB) (%): 1233 (98) calcd. for C64H72N6ClO6Lu 1231.71. UV-vis (DMF) Xmax (lge) nm: 280 (4.19), 2646 (4.20), 316 (3.75). 1H NMR (CDCl3, 300 K) 5H ppm: 0.96 (18H, s, tBu), 1.26 (18H, s, tBu), 3.30 (4H, d, ArCHeqAr, J = 13.4), 4.00 (4H, d, ArCHAr, J = 13.4), 4.50 (4H, s, OCH2), 4.94 (4H, s, NCH2), 6.81 (4H, s" ArH), 7.03 (4H, s, ArH), 7.22-7.24 (4H, m, Hbzim), 7.54-7.56 (4H, m, Hbzim), 8.48 (2H, s, NHimid), 9.21 (2H, t, NH).
The binuclear complexes in which the lanthanide ion is coordinated by the nitrogen atoms of porphyrins (Ln2L1Cl2) were prepared similar to L1H2[7] from corresponding porphyrinates.
Lu2L1Cl2 (0.240 g, 70%). M.p. >350°C (CH2Cl2). m/z (MALDI) (%): 2404.9 (95) calcd. for C, H, N^CLOTu, 2405.26. UV-vis
136 114 10 2 6 2
(DMF) Xmax (lge) nm: 278 (3.82), 425 (4.90), 557 (3.52), 595 (3.12). 1H NMRm((CDCl3, 300 K) SH ppm: 1.18 (18H, s, tBu), 1.32 (18H, s, tBu), 3.49 (4H, d, ArCHeqAr, J = 13.1), 4.20 (4H, d, ArCH^Ar, J = 12.9), 4.92 (4H, s, OCH2), 7.15 (4H, s, ArH), 7.23 (4H, s, ArH),
7 51-7 70 № m CHph..porph), 8.12-8.18 (10H, m, CHph..porph), 8.27 (8 H, d, CHpoiph), 8.70 (8H, s, CHph_porph), 8.84 (2H, s, ArOH), 8.858.88 (4H, m, CH^J, 8.93 (2H, d, NHCO),9.27 (4H, d, CH^J.
Results and Discussion
The presence of substituents of different types has an influence on almost all physico-chemical and spectral properties of the compounds. The experimental data (elemental analysis, mass spectrometry, 1H NMR spectroscopy) suggest that lanthanide ions form 1:1 complexes with ligands L1H2-L5H2. In this case the complexes include an anion (Cl-) of starting salt. In complexes LnL1Cl-LnL5Cl the lanthanide coordination unit is formed by the replacement of the labile hydrogen atoms of phenolic groups and oxygen atoms of ether and carbonyl fragments, and an appropriate quantity of solvent molecules with the realization of the coordination numbers 7-9. The coordination units of complexes Ln2L'Cl2, as well as the starting lanthanide porphyrins, were formed by the interaction with the chloride anion (extra-ligand), four nitrogen atoms of the tetrapyrrole cycle and the corresponding amount of solvent molecules.
The analysis of 'H NMR spectra of complexes with diamagnetic lutetium ion, and their comparison with the spectra of the ligands reveal some structural changes, which take place upon complexation. The signals of the protons of aromatic rings (6.92, 7.10 and 6.89 ppm, 7.03 ppm) and methylene groups (3.52, 4.20 and 3.45 ppm, 3.95 ppm) of ligands L4H2 and L5H2 found in downfield in comparison with the L2H2 and L3H2. Such shift, along with large constants of spin-spin interaction between protons of Ar-CH2-Ar (13.3813.76), indicate the distortion of the "cone" conformation, which is connected with a change in dihedral angles between the tert-butylphenolic fragments. Thus, when the length of the alkyl radical increases the degree of distortion of the conformation of calix[4]arene increases also. The significant distortion of the conformation also occurs in the case of the bulky porphyrin substituents. The formation of complexes LuL*Cl-LuL5Cl caused an upfield shift of the signals of tert-butyl proton (A5 0.05-0.21 ppm) with the keeping of their amount, which indicates less distortion of conformation of the complex in comparison with the ligand. The disappearance of the signals of protons of phenolic groups and the downfield shift of signals of protons of O-CH2-CO groups of complexes (A5 = 0.11-0.20 ppm) confirm their participation in the complex formation. But in the case of complex Lu2L*Cl2 the signals of calix[4]arene subunit does not change practically in comparison with the ligand. At the same time, the singlet of NH-protons of the pyrrole fragments disappears that specifies on their replacement upon complex formation. Besides, the signals of protons in the ^-positions of porphyrinates are downfield shifted due to redistribution of electron density in the porphyrinate fragments. It should be noted that all ligands and complexes have the "cone" or "distorted cone" conformations.
The absorption spectra of all compounds (Table 1) are characterized by the broad bands with maxima at 270-290 nm (37000-38500 cm-1, lge = 4.1-4.5), which correspond to the n ^ n* - transitions of the calix[4]arene macrocycle. The absorption spectrum of calix-diporphyrin L1H2 contains the typical bands from the two macrocyclic fragments. The absorption band of calix[4]arene chromophore is observed in the UV region (Xmax = 274 nm, lge = 3.82); in contrast, the Soret band (X = 417 nm, lge = 4.91) and
v max j c /
four 0-bands of the porphyrin macrocycle are recorded in the visible region. The bands of porphyrin fragments exhibit blue shifts in comparison with the spectrum of the starting monoaminoporphyrin: AXSoret = 2 nm and 3-8 nm for 0-bands, that indicates the intramolecular interaction between two porphyrin fragments.[10] The absorption spectra of benzimidazole substituted ligands L2H2-L5H2 display two narrow bands at 278-281 and 284-286 nm, and the increasing number of methylene groups does not affect the band positions and the molar extinction coefficients. Hence, the ratio of the intensities of the bands remains constant.
The absorption spectrum of the complex LnL*Cl in the region 250-300 nm is similar to that of the ligand, but is characterized by the additional band at X = 312-315 nm
max
(lge = 2.48), indicating (according [1]) the participation of phenolic groups in the complex formation. The structure of bands of porphyrin fragments does not change practically in contrast to the binuclear complex Ln2L*Cl2. Soret bands in the absorption spectrum of Ln2L*Cl2 are red shifted
N. V. Rusakova et al.
Table 1. The spectral-luminescent properties of ligands (10-5 M, CHCL).
Ligand X nm (lg e) X„, nm n' ES, cm-1 X u, nm ph> ET, cm-1
l1h2 274 (3.82) 650 15400 846 11800
417 (4.91) 515 (3.63) 550 (3.35) 590 (3.12) 646 (2.91) 717
l2h2 278 (4.51) 416 24050 430 23250
284 (4.48)
l3h2 280 (4.27) 410 24400 427 23400
286 (4.31)
l4h2 281 (4.18) 408 24500 425 23500
286 (4.36)
l5h2 281 (4.15) 405 24700 425 23500
286 (4.21)
(AXSoret = 6-8 nm) and reduction of the number of "visible" bands up to two is observed (these changes are characteristic for metalloporphyrins). The absorption spectra of complexes of the same ligands with different lanthanides (Yb, Er, Lu) are identical, indicating that the type of coordination mode of the lanthanide ion does not change in the series. The appearance of the bands at 312-320 nm (lge = 3.75-3.85) in the absorption spectra of the complexes LnL2Cl-LnL5Cl, similar to the previous compounds, testifies to participation of OH groups of the ligands in the complex formation.
The fluorescence spectra of ligands and complexes also differ from each other. It should be noted that in the spectrum of L1H2 the fluorescence from calix[4]arene moiety is not observed due to the intense absorption of porphyrin substituents in this region. The fluorescence with maxima at 650 and 717 nm and low-temperature (77 K) phosphorescence with a maximum at 846 nm are registered for this ligand.
Table 2. The spectral-luminescent properties of lutetium
complexes (10-5 M, CHCl3).
Complex X, nm (lg e) Xfl,nm ES, cm-1 X ,nm E^ cm-1
LuL1Cl 280 (3.42) 312 (2.48) 418 (5.1) 516 (4.2) 552 (3.75) 591 (3.46) 638 (3.43) 6 46 710 15500 845 11800
Lu2L1Cl2 278 (3.82) 425 (4.90) 557 (3.52) 595 (3.12) 648 710 15400 852 11700
LuL2Cl 280 (4.19) 286 (4.20) 316 (3.75) 423 23700 435 23000
LuL3Cl 280 (4.20) 284 (4.22) 319 (3.85) 415 24100 430 23250
LuL4Cl 280 (4.20) 286 (4.36) 318 (3.65) 412 24300 430 23250
LuL5Cl 280 (4.20) 286 (4.37) 317 (3.70) 410 24400 427 23400
These maxima are blue shifted (14.6 and 16 nm, respectively) in comparison with the starting monoaminoporphyrin. The broad band of fluorescence of benzimidazole derivatives is registered only at 77 K in the region 416-405 nm (the maximum shifts to shorter wavelengths with increasing number of methylene groups of alkyl radical). The positions of the phosphorescence maxima (Xmax = 430-425 nm) are changed in a similar way, and can be observed only with the application of the time delay (50 ms) and the low temperature (77 K).
The molecular fluorescence and phosphorescence of lutetium complexes (Table 2) were recorded under similar conditions. The fluorescence of calix[4]arene component in the complex LuL'Cl is not observed; the fluorescence bands of porphyrin substituents exhibit blue shifts on 4 and 7 nm by comparison with the ligand. The fluorescence maxima at 648 and 710 nm and a band of phosphorescence (maximum at 852 nm) are observed for the binuclear complex Lu2L1Cl2. As well as for benzimidazole calix[4]arenes, the fluorescence and phosphorescence of complexes LuL2Cl-LuL5Cl were divided using the low-temperature conditions and time delay (50 ms). The observed red shifts of the bands of fluorescence and phosphorescence of the lutetium complexes show a slight decrease of energy of singlet ES (200-350 cm-1) and triplet ET (100-250 cm-1) levels in comparison with the ligands.
It is known[11] that the efficiency of 4/-luminescence of lanthanide complexes depends on the location of the lowest triplet states of ligands. The data (Table 1) clearly demonstrate that the luminescence, which realized by the mechanism of intramolecular energy transfer from the organic part of the complex to the resonance levels of ions Yb111 (2F5/2, 10300 cm-1) and Er111 (4I13/2, 6500 cm-1), is possible for all ligands, which triplet levels are above the excited energy levels of Ln111.
The 4/-luminescence of Er111 and Yb111 is realized in the homo- and dinuclear complexes (Table 3). The luminescence quantum yield of the complex YbL'Cl is less than 1.7 times compared with Yb2L'Cl2 that can be the result of energy transfer from the excited levels of calix[4]arene macrocycle to porphyrin subunits. The coincidence of the excitation spectra of 4/-luminescence and the absorption spectra of the complexes YbL2Cl - YbL5Cl indicates the sensitization of 4/-luminescence of ytterbium by organic chromophore. The growing number of methylene groups in the row YbL2Cl - YbL5Cl leads to the increase in quantum yield of 4/-luminescence up to n = 3 (YbL2Cl - YbL4Cl), but upon further elongation (n = 4, YbL5Cl) the luminescence signal notably decreases.
Table 3. Characteristics of 4/-luminescence of Yb111 in complexes (Error is ± 10%; CHCl3, 10-5 M).
Mononuclear complexes
Heteronuclear complexes
Compound 9*103
I, , %
lum?_
YbL'Cl Yb2L1Cl2 YbL2Cl YbL3Cl YbL4Cl YbL5Cl
2.8
4.7
3.8 4.2 4.4
2.9
Compound
Solid Solution
YbCu2L1Cl YbZn2L1Cl Yb(CuCl2)L3Cl Yb(ZnCl2)L3Cl
14 115 78 260
84 97 168
For the first time it was detected the 4f-luminescence of Er111 (4I13/2 ^ 4I15/2 transition) in the complexes ErL2Cl-ErL3Cl and Er2L'Q2. It was found that the changes of the intensity 4f-luminescence of ErIn in the investigated complexes are similar to the ones observed in a row of ytterbium complexes.
The formation of heteronuclear complexes is based on the polytopic nature of functionalized calix[4]arenes L'H2-L5H2, which is provided by substituents in the lower rim. The f-d-heteronuclear complexes were obtained based on 1,3-disubstituted p-tert-butylcalix[4]arenes (L'H2 and L3H2) modified by porphyrin and benzimidazole fragments. Copper and zinc (MCl2) have been selected as the d-metals (so called "test"-ions) due to different structure of the electron shell and, at the same time, almost identical ionic radii. In addition, the choice of d-metals was made taking into account the specific spectroscopic properties of their complex, in particular, with porphyrins.
To obtain the heteronuclear complexes based on the L'H2 and L3H2 were used their homocomplexes as "blocks": M2L'H2 and YbL3Cl (Figure 1). Heteronuclear complexes Conclusions LnM2L'Cl were synthesized based on calix[4]arene modified porphyrin fragments according to the method similar with mononuclear complex LnL'Cl. LnM2L3Cl was isolated from the CH3OH-solution containing complex LnL3Cl and an excess of d-metal chloride (1:5).
The absorption spectra of lanthanide-containing complexes with d-metals LnM2L'Cl compared with mononuclear complexes are characterized by the small blue shift (AX = 4-6 nm) of the bands (n ^ n*- transitions in calix[4]arene macrocycle) without change of intensity. This is due to the fact that additional complexation on the lower rim of the ligand changes the symmetry of the complexes, but the coordination unit of lanthanide remains unchanged. Only two g-bands in the visible region are observed in the spectra of complexes LnM2L'Cl instead of four absorption bands in the spectra of LnL'Cl that confirms the formation of d-metal porphyrins. The absorption spectra of heteronuclear complexes with LnL3Cl slightly differ from the absorption spectrum of the starting mononuclear complex. Namely, the ratio of intensities of the calix[4]arene bands slightly changes that can be attributed to the interaction of d-metal ions with benzimidazole groups of the lower rim. These deviations are insignificant and indicate that the formation of the heteronuclear complexes with d-metal do not affect the coordination unit of lanthanide in the "block"-complex.
The molecular fluorescence and phosphorescence spectra of heteronuclear complexes with calix[4]arenes L'H2 and L3H2 were recorded in solution. The bands of the calix[4] arene fragment are observed only for the complexes with benzimidazole derivative: fluorescence - at 420-450 nm and phosphorescence - at 450-470 nm. The obtained data show that notable decrease in energy of the singlet and triplet levels (T = 22630 cm-1, T = 21800 cm-1) is observed in both
v Cu ' Zn '
cases. The fluorescence spectra of heteronuclear complexes LuM2L'Cl contain two bands with maxima at the 600-605 nm (0-0) and 648-652 nm (0-1) with a different distribution of intensities. It was shown that the bands are of narrow form and blue shifted (2-5 nm) in frozen solutions.
The 4/-luminescence of Ybm ions in all heteronuclear complexes with L'H2 and L3H2 was observed in solution and in solid samples (Table 3). The presence of zinc does not affect on the 4/-luminescence intensity ytterbium-containing complexes with L'H2 both in solid state and in solution. But in Cu-Yb-complexes the quenching of the 4/-luminescence takes place. In Zn-Yb-complex with L3H2 the intensity of the 4/-luminescence is increased in 1.7 and 2.6 times in the solution and solid sample, respectively. Evidently, this fact is connected with the energy of the exited levels of d-metals: 5015 cm-1 and 17520-19510 cm-1 for copper and zinc compounds, respectively.[12] Thus, the 4/-luminescence intensity is changed little or is increased (complexes with zinc) if the excited level of the d-metal is located above the emitting level of lanthanides. Low-lying excited levels of copper are "acceptors" of 4/-luminescence of lanthanides, contributing to its effective role in process of quenching.
The mono- and heteronuclear lanthanide-containing complexes on the basis of 1,3-disubstituted p-tert-butylcalix[4]arenes modified with porphyrins and benzimidazole fragments were obtained. It was shown that in these complexes the 4f-luminescence of ErIn and Ybm ions is realized. The heteronuclear Yb-Cu and Yb-Zn complexes on the basis of calix[4]arenes L'H2 and L3H2 were also sensitized. It was established that the relative positions of excited levels of d- and f-metal have a significant influence on the 4f-luminescent characteristics of heteronuclear complexes.
Acknowledgements. The authors are grateful of the National Academy of Sciences of Ukraine for funding (Project No. 6.22.7.43).
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Received 30.04.2011 Accepted 11.05.2011
