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ARTICLES Pressure-Induced Phase Transitions on a Liquid Crystalline Europium(III) Complex Yuetao Yang,† Xiaojun Liu,*,† Arao Nakamura,‡ Koen Binnemans,§ and Jing Liu| Key Laboratory of Modern Acoustics of MOE, Institute of Acoustics, Nanjing UniVersity, Nanjing 210093, China, Department of Applied Physics, Nagoya UniVersity, Nagoya 8603, Japan, Katholieke UniVersiteit LeuVen, Department of Chemistry, Celestijnenlaan 200F, B-3001 LeuVen, Belgium, and Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ReceiVed: December 21, 2007; In Final Form: February 17, 2008
The effect of pressure on the phase behavior of the liquid crystalline complex [Eu(bta)3L2] (bta is benzoyltrifluoroacetonate, and L is the Schiff base 2-hydroxy-N-octadecyl-4-tetradecyloxybenzaldimine) was studied by X-ray diffraction, Raman spectroscopy, and luminescence spectroscopy. The pressure was varied between ambient pressure and 8.0 GPa. [Eu(bta)3L2] exhibits a smectic A (SmA) phase at room temperature. The complex undergoes a transition from the SmA phase to a solid lamellar structure around 0.22 GPa and another transition from the solid lamellar phase to an amorphous state from 1.6 to 3.5 GPa. At low pressures, the smectic layer spacing increases, and the intermolecular distance decreases. Above 3.5 GPa, both the interlamellar and the intermolecular spacings hardly change, but the intensity of X-ray reflections exhibits a remarkable decrease and eventually vanishes. An interpretation of the changes in the molecular structure is given. It was found that less interdigitation of the alkyl chains situated in adjacent layers and/or a full extension of the alkyl chains occurred at low pressures and that the second phase transition was accompanied by a transfer of the hydrogen atom from the nitrogen atom of the imine group to the oxygen atom of the Schiff base ligand. The effect of applying pressure equals that of the lanthanide contraction on the phase behavior.
1. Introduction Because of the desire to combine the properties of liquid crystals and metal complexes, a substantial amount of effort has been devoted in the last two decades to the design of liquid crystalline-metal complexes (metallomesogens).1-8 The first discovered classes of metallomesogens were designed in such a way that they mimicked the shape of organic liquid crystals, such as linear complexes (Ag+ and Hg2+) and square-planar complexes (Cu2+, Ni2+, Pd2+, and Pt2+). Because of strong geometrical constraints, it is a challenge to design metallomesogens with coordination geometries other than linear or squareplanar orientations.9-12 The first metallomesogens with an octahedral coordination sphere were the 1,4,7-triazacyclononane tricarbonyl and trichlorometal metal complexes.13-15 Bruce and co-workers prepared octahedrally coordinated metallomesogens with a rod-like shape.16-18 It is even more difficult to obtain metallomesogens with coordination numbers higher than six. The driving forces for the development of new metallomesogens with high coordination numbers are not only the desire to build molecular edifices with geometries that are impossible to achieve by all-organic liquid-crystals but also the unique luminescence and magnetic properties of lanthanide ions.7 Galyametdinov et al. and Binnemans et al. described lanthanide-containing liquid crystals with coordination numbers of eight or nine.19-24 Other * Corresponding author. E-mail:
[email protected]. † Nanjing University. ‡ Nagoya University. § Katholieke Universiteit Leuven. | Chinese Academy of Sciences.
types of lanthanide-containing liquid crystals then were studied by the research groups of Piguet et al. and Bu¨nzli et al.25-29 and Serrano et al.30 High coordination numbers also were found for liquid crystalline ferrocene derivatives, in which the iron atom was ten-coordinate.31-33 For the design of high coordination number metallomesogens with useful properties, one of the most important aspects is to understand the relation between molecular structures and liquid crystalline behavior. Although the crystal structures of some analogous nonmesomorphic complexes have been determined, much less is known about the actual structure of the mesomorphic lanthanide complexes themselves. It is well-known that high-pressure studies can give complementary information to the conventional chemical composition approach.34 By the application of high pressure, we can continuously vary the structure and coordination environment for a given chemical composition.35 A liquid crystal phase is a fragile molecular assembly and very sensitive to pressure changes.36-40 The study of pressure-induced structural modifications in metallomesogens is essentially an unexplored research field. To the best of our knowledge, only one report on this topic is available. Guillon and co-workers described the effect of pressure on the structures of liquid crystalline copper(II) carboxylates.41 They found that the area of the two-dimensional lattice decreases with increasing pressure and, at sufficiently high pressure, that the columnar mesophase transformed into a crystalline lamellar phase. In this work, we studied the effect of high pressure on the mesophase structure of the metallomesogen [Eu(bta)3L2] (bta is benzoyltrifluoroacetonate, and L is 2-hydroxy-N-octadecyl-
10.1021/jp7119763 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/04/2008
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4-tetradecyloxybenzaldimine).42 We carried out XRD, Raman, and luminescence studies up to a pressure of 8 GPa to search for the existence of any structural changes in the complex. We report in this paper the existence of a transition from the SmA phase to a solid lamellar structure at a pressure of 0.22 GPa, as well as another transition from the solid lamellar phase to an amorphous state at pressures from 1.6 to 3.5 GPa. An interpretation of the corresponding changes in the molecular structure is given. 2. Experimental Procedures The synthesis and characterization of the [Eu(bta)3L2]complex have been reported elsewhere.42 The europium(III) complex exhibits an enantiotropic smectic (SmA) phase at ambient temperature, with a melting point at 18 °C and a clearing point at 40 °C. Angle dispersive X-ray diffraction experiments were performed at the 4W2 High-Pressure Station of the Beijing Synchrotron Radiation Facility. A diamond-anvil cell (DAC) was used to generate the high pressures. The culet of the diamond was 0.8 mm in diameter, and the sample was loaded into a hole that was drilled into a pre-indented stainless steel gasket. The pressure was calibrated using a linear ruby fluorescence scale. Monochromatic synchrotron X-rays (λ ) 0.6199 Å) from the 4W2 beamline were used for X-ray diffraction measurements. An image plate detector (IP, MAR3450) was used to collect the diffraction patterns. A CeO2 standard was used to calibrate the sample to the detector distance and the detector tilt. Typical IP exposure times were 5 min. All the measurements were performed at ambient temperature. High-pressure Raman and luminescence measurements were performed in an unpolarized configuration at 300 K using a DAC and liquid paraffin as a pressure medium. A tiny sample was placed in the gasket hole (0.3 mm in diameter). The applied pressure was monitored using the luminescence peak of a small piece of ruby placed in the gasket hole. At 300 K, the pressure inhomogeneity was estimated to be 0.2 GPa at 6 GPa according to the pressure-induced boarding of the luminescence peak of the ruby. Raman scattering light was detected with a Renishaw spectrometer at 632.8 nm. Extreme care was taken to avoid sample damage or laser-induced heating. Measurements were performed from 0.5 to 2.4 mW for incident laser power. No significant spectral change was observed in this range. The emission was excited with the 325 nm line of a HeCd laser. Luminescence spectra for the complex were detected with a grating monochromator and a standard CCD detection system, and luminescence decay curves were recorded with a redsensitive photomultiplier. 3. Results and Discussion 3.1. Transition from SmA Phase to Solid Lamellar Phase. The [Eu(bta)3L2] complex exhibits an enantiotropic SmA phase at ambient temperature; therefore, the pressure effect on the mesophase can be studied at room temperature. X-ray diffractograms of the complex are shown in Figure 1. In the smallangle region of the X-ray patterns, sharp reflections with reciprocal spacings in the ratio 1:2:3, ... were observed, corresponding to the indexation (00l) ) (001), (002), (003), ... of a lamellar phase. In most of the measurements, only the firstorder peak and a weak second-order peak were observable. However, higher orders could be detected after longer measuring times. The broad halo in the wide-angle region shows that the alkyl chains are in a molten state. These results supported the identification of the phase as a SmA phase. The dependence of
Figure 1. X-ray diffractograms of the [Eu(bta)3L2] complex at different pressures.
Figure 2. Variation of layer distance and intermolecular distance of [Eu(bta)3L2] as a function of pressure.
the layer distance and intermolecular distance (lateral spacing within the layer) on the pressure is shown in Figure 2. The values of lateral spacing were calculated from the peak of the wide-angle broad reflection with an accuracy of (5%. The layer spacing increases, and the intermolecular distance rapidly decreases with increasing pressure up to 0.22 GPa. At higher pressures, both distances decrease. The inflection around 0.22 GPa suggests a phase transition for the complex. During the transition, a sudden pressure drop was observed. This is strong evidence for crystallization of the complex accompanying a large decrease in volume. The fact that the solid lamellar state is stable at 0.02 GPa indicates that the pressure hysteresis of the transition is approximately 0.2 GPa. The increase of layer spacing with increasing pressure at low pressures up to 0.22 GPa is interesting, as it is in contrast to the common observed pressure effect. It may be explained as follows. The layer spacing (ca. 26.0 Å) of the complex is much shorter than the calculated length (45.5 Å) of the Schiff base ligand in the fully extended (all-trans) conformation. The most probable mechanism is an interdigitation of the alkyl chains of the molecules situated in adjacent layers, as this is an efficient way of space filling. At low pressure, less interdigitation of the alkyl chains may occur due to the fact that the compressibility perpendicular to the layers is smaller than that parallel to the layers, as clearly is shown in Figure 2. In fact, molecules in the SmA phase can be tilted in a random way with a zero-tilt average value. Another possible reason is that applying pressure may force the core of the europium complex to be parallel, resulting in the increase of layer spacing. It should be noted that although for short linear alkanes a transformation from a trans to a gauche conformation is favored under pressure, the pressure-induced trans/gauche transformation
Liquid Crystalline Eu(III) Complex Phase Transition
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Figure 4. Variation of CdN and CdO Raman vibrations as a function of pressure. The squares and triangles represent the CdO and CdN vibration modes, respectively.
Figure 3. Raman spectra of the [Eu(bta)3L2] complex at different pressures.
Figure 5. Schematic structural change of the [Eu(bta)3L2] complex.
does not occur in long alkyl chains at low pressure because it is prevented by the lateral compression that causes a full extension of the alkyl chains.43 The Raman spectra were divided into two regions for clarity (Figure 3). The CdO stretching vibration of the complex at 1595 cm-1 hardly changes as compared to that of Eu(bta)3(H2O)2. The CdN stretching vibration of the complex is at 1644 cm-1. The Raman peaks around 1550 and 1610 cm-1 can be attributed to the stretching vibrations of substituted benzyl rings.44 Since the CdN or CdO group is polar, their Raman resonance indicates that there is a strong coupling of the CdN or CdO group with the CdC bonds of the benzyl rings. The CdN vibration shifts to higher wavenumbers as compared to the corresponding value (1622 cm-1) in the free ligand, which indicates that the nitrogen atom is not involved in the complex formation and that a CdN+ group is present. The best piece of evidence is the single-crystal X-ray structure of a homologous complex formed by a ligand with short alkyl chains.42 In [La(bta)3L′2] (L′ is N-butyl-2-hydroxy-4-methoxy benzaldimine), the oxygen atom of the Schiff base’s OH group is deprotonated, and the hydrogen atom is transferred to the nitrogen atom. As a consequence, a zwitterionic structure is generated. Intramolecular NsH‚‚‚O hydrogen bonds appear between the protonated nitrogen and the deprotonated oxygen atoms. Spectroscopic data indicate that the first coordination sphere around the lanthanide ion is not changed by elongation of the alkyl chains. Although the CH stretching region is complex as a result of numerous spectral bands and Fermi resonance splitting, conformation order information can be obtained empirically from the peak intensity ratio of νas (CH2) at 2880 cm-1 to νs (CH2) near 2850 cm-1.45 At ambient conditions, the intensity ratio of νas to νs is 0.8, while this ratio increases to 1.6 after 0.22 GPa, which is consistent with the change of the Raman spectra for the long normal alkyl chains C-16 or C-18 undergoing a transition from the liquid to the solid state under pressure or upon cooling,43,45 indicating that alkyl chains of the Schiff base transfer from the liquid to the solid state. At pressures above 3.1 GPa, the CH stretching vibrations overlap into one broad Raman peak.
3.2. Transition from Solid Lamellar Phase to Amorphous Phase. As seen in Figure 2, the layer spacing of the mesomorphic complex decreases rapidly from 0.22 to 1 GPa and decreases slowly with increasing pressure. The intermolecular spacing decreases smoothly above 0.22 GPa. When the pressure is higher than 3.5 GPa, both the interlamellar and the intermolecular spacing hardly change, while the intensity of X-ray reflections decreases remarkably, indicating a great loss of the order in the structure. Above 5.8 GPa, the complex becomes completely amorphous, as demonstrated by the fact that all sharp X-ray diffraction peaks disappear. In Figure 3, obvious changes are found in the 1500-1700 cm-1 region of the Raman spectra. One major aspect is that the CdN vibration at 1644 cm-1 for the complex shows a blue shift with pressure and disappears above 3.5 GPa. On the other hand, a new Raman vibration emerges at 1.6 GPa, which is about 20 cm-1 red shifted as compared to the 1644 cm-1 peak. The new peak shows a blue shift and can be observed as a shoulder peak at a pressure up to 7.3 GPa. The Raman shifts of CdN and new Raman vibrations are shown in Figure 4. We also note that the coupling of CdO and CdC vibrations at 1595 cm-1 exhibits a continuous blue shift as the pressure increases. The corresponding change in the molecular structure, which is related to the phase transition to the amorphous phase, is difficult to interpret. However, closer inspection of the molecular structure permits us to obtain an idea about the possible mechanism involved. For the complex, there exists an intramolecular NsH‚‚‚O hydrogen bond at ambient conditions. Under high pressure, the average O‚‚‚H distance may become shorter due to the compression of the molecule, and eventually, the hydrogen atom may be transferred from the nitrogen to the phenolic oxygen atom of the Schiff base ligand, and the N‚‚‚HsO hydrogen bond thus forms (Figure 5), resulting in the disappearance of the 1644 cm-1 peak and the emergence of the new Raman peak. In Figure 3, one Raman vibration separates from the broad Raman peak and shows a red shift from 1570 to 1550 cm-1 at pressures higher than 3.1 GPa. This may be attributed to the fact that in a stronger conjugated system containing the benzyl ring, the
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Figure 8. Luminescence decay curves of the [Eu(bta)3L2] complex at different pressures.
Figure 6. Luminescence spectra of the [Eu(bta)3L2] complex at different pressures.
Figure 7. Variation of emission wavelength of the 5D0 f 7F0 line as a function of pressure.
CdN and C-OH groups of the Schiff bases will form accompanying the transition, due to the deprotonation of the CdN+ groups.46 To further clarify the structural change, the luminescence properties of the europium(III) complex with pressure were examined (Figure 6). The fine structure reveals that the site symmetry of the Eu3+ ion is low, C2V or even lower. This is evident from the observation of three crystal field levels for the 7F1 level at 595 nm. The 5D0 f 7F0 transition (580 nm) consists of one peak at ambient pressure, indicating that all the Eu3+ ions occupy a site of the same symmetry and/or crystal field strength. As the pressure increases, the emission bands of Eu3+ exhibit red shifts. This is quite natural because of the shortening of Eu-O bonds with increasing pressure, indicated by the blue shift of the corresponding Raman vibration around 440 cm-1.47 When the pressure is higher than 2.8 GPa, a blue shift of the 5D0 f 7F0 emission line is observed (Figure 7). This is unusual for the luminescence behavior of Eu3+ under pressure because a blue shift indicates a decrease of the nephelauxetic effect and an increase of ionicity of the Eu-O bonds. However, it is consistent with the previous prediction based on the Raman spectra. The proton transfer from the nitrogen to the phenolic oxygen atom of the Schiff base ligands will lead to a weaker interaction between the coordinated oxygen
atom of the Schiff base’s OH group and the Eu3+ ion. Consequently, this results in a higher ionicity of the bonding. The delay between the blue shift of the luminescence and the emergence of the new Raman peak is due to a balance effect between the protonation of the phenolic oxygen atom of the Schiff base and the shortening of Eu-O bonds with increasing pressure. Luminescence decay curves of the 5D0 state at different pressures are shown in Figure 8. It was found that the luminescence decay is single exponential at ambient pressure. The decay curve could be fitted by a biexponential function in the pressure region of the phase transition, which is evidence of the existence of two different coordination spheres for the europium(III) ion. This result is in agreement with the Raman spectra, revealing that the transition takes place progressively through a two-phase regime. Moreover, a marked decrease of the luminescence decay time was observed during the phase transition. The phase transition shifts the -OH groups closer to the europium(III) ion, so an increase in the nonradiative decay rate is expected, and correspondently, there is a decrease in the luminescence decay time. Another reason is due to the pressure effect demonstrated by Hayes and Drickamer,48 that is, a high pressure decreases the energy barrier for triplet quenching of the 5D0 state of europium(III) β-diketonate complexes, correspondingly leading to a decrease of both the luminescence intensity and the luminescence decay time. The X-ray diffractograms, the Raman spectra, and the luminescence spectra observed at ambient pressure were recovered upon release of the pressure, indicating that the proton transfer of the Schiff base is reversible. When the pressure was lowered to ambient pressure, the complex readopted the SmA phase. For the lanthanide complex [Ln(bta)3L2] (Ln ranges from La to Lu), it is interesting to make a comparison between the pressure effect and the lanthanide contraction effect on the molecular structure.42 The clearing point decreases continuously from the lanthanum(III) to the erbium(III) complex. The thulium(III), ytterbium(III), and lutetium(III) complexes, however, are not mesomorphic and melt directly to the isotropic state. The Raman peak of the CdN stretching vibration for the thulium(III), ytterbium(III), and lutetium(III) complexes is at 1620 cm-1 instead of 1644 cm-1 for the other lanthanide(III) complexes. For Eu(bta)3L2 under high pressure, a new CdN vibration with a similar red shift was observed. This co-incident Raman shift may be understood from the fact that both the pressure and the lanthanide contraction resulted in a more congested coordination environment. Both a small lanthanide ion or high pressure can induce changes in the orientation of the aromatic groups and alkyl chains of the Schiff base ligands. However, even subtle structural changes are sufficient to destroy the mesomorphism in this type of compound.
Liquid Crystalline Eu(III) Complex Phase Transition 4. Conclusion A high-pressure study of the metallomesogen [Eu(bta)3L2] was performed. A phase transition from the SmA phase to a solid lamellar structure was observed around 0.22 GPa. Another transition to an amorphous state was observed at higher pressure, accompanying a structural change of the Schiff base ligand. This is the first example of a pressure-induced change in the chemical bonding of a metallomesogen. It was found that a high pressure can induce phase transitions in the lanthanidomesogens but that the application of high pressure also can give additional structural information on this type of compound. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grants 10574071 and 10374041, the Key Project of Chinese Ministry of Education under Grant 107051, and the Program for New Century Excellent Talents in Chinese Universities under Grant NECT04-0456. B.K. acknowledges FWO-Flanders (Project G0508.07) for financial support. J.L. acknowledges the Chinese Academy of Sciences (Grant KJCX2-SW-N20). The authors are grateful to Prof. Y. Moritomo from Tsukuba University for fruitful discussions on high-pressure X-ray diffraction experiments. References and Notes (1) Giroud-Godquin, A. M.; Maitlis, P. M. Angew. Chem., Int. Ed. Engl. 1991, 30, 375. (2) Espinet, P.; Esteruelas, M. A.; Oro, L. A.; Serrano, J. L.; Sola, E. Coord. Chem. ReV. 1992, 117, 215. (3) Hudson, S. A.; Maitlis, P. M. Chem. ReV. 1993, 93, 861. (4) Serrano, J. L. Metallomesogens, Synthesis, Properties, and Applications; VCH: Weinheim, Germany, 1996. (5) Bruce, D. W. In Inorganic Material, 2nd ed.; Bruce, D. W., O’Hare, D., Eds.; Wiley: Chichester, 1996; Ch. 8, p 429. (6) Donnio, B.; Bruce, D. W. Struct. Bonding (Berlin) 1999, 95, 193. (7) Binnemans, K.; Go¨rller-Walrand, C. Chem. ReV. 2002, 102, 2303. (8) Donnio, B.; Guillon, D.; Deschenaux, R.; Bruce, D. W. In ComprehensiVe Coordination Chemistry II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Oxford, 2003; Vol. 7, Ch. 7.9, pp 357-627. (9) Bruce, D. W. AdV. Mater. 1994, 6, 699. (10) Neve, F.; Ghedini, M.; De Munno, G.; Levelut, A. M. Chem. Mater. 1995, 7, 688. (11) Ziessel, R.; Douce, L.; Elghayoury, A.; Harriman, A.; Skoulios, A. Angew. Chem., Int. Ed. 2000, 39, 1489. (12) Gime´nez, R.; Manrique, A. B.; Uriel, S.; Barbera, J.; Serrano, J. L. Chem. Commun. (Cambridge, U.K.) 2004, 2064. (13) Lattermann, G.; Schmidt, S.; Kleppinger, R.; Wendorff, J. H. AdV. Mater. 1992, 4, 30. (14) Schmidt, S.; Lattermann, G.; Kleppinger, R.; Wendorff, J. H. Liq. Cryst. 1994, 16, 693. (15) Walf, G. H.; Benda, R.; Litterst, F. J.; Stebani, U.; Schmidt, S.; Lattermann, G. Chem.sEur. J. 1998, 4, 93. (16) Bruce, D. W.; Liu, X. H. J. Chem. Soc., Chem. Commun. 1994, 729. (17) Morrone, S.; Harrison, G.; Bruce, D. W. AdV. Mater. 1995, 7, 665. (18) Morrone, S.; Guillon, D.; Bruce, D. W. Inorg. Chem. 1996, 35, 7041.
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