Mechanistic Study of the Anchoring Behavior of Liquid Crystals

This paper reports an investigation of the orientational behavior of films of nematic liquid crystal (LC) 4'-pentyl-4-biphenyl-carbonitrile (5CB) supp...
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20180

J. Phys. Chem. B 2004, 108, 20180-20186

Mechanistic Study of the Anchoring Behavior of Liquid Crystals Supported on Metal Salts and Their Orientational Responses to Dimethyl Methylphosphonate Kun-Lin Yang, Katie Cadwell, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, UniVersity of WisconsinsMadison, 1415 Engineering DriVe, Madison, Wisconsin 53706-1607 ReceiVed: July 5, 2004; In Final Form: September 18, 2004

This paper reports an investigation of the orientational behavior of films of nematic liquid crystal (LC) 4′-pentyl-4-biphenyl-carbonitrile (5CB) supported on a variety of metal perchlorate salts and their orientational response to dimethyl methylphosphonate (DMMP). It is found that 5CB supported on perchlorate salts comprised of metal ions with high electron affinities (>15.64 eV), such as Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+, and Fe3+, assumes a homeotropic orientation whereas 5CB supported on salts of metal ions with low electron affinities, including Mn2+, Mg2+, Ag+, Cs+, and Na+, assumes a planar or tilted orientation. The role of the nitrile group of 5CB in determining the orientational behavior of the LC via its coordination with the metal ions is supported by polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). A new nitrile stretching peak is measured with Cu2+, Ni2+, and Cd2+ (which cause homeotropic anchoring) but not with Ag+, Na+, and Mg2+ (which cause tilted or planar orientations of 5CB). Upon exposure to 100 ppb of DMMP, the 5CB supported on the perchlorate salts of Al3+, Eu3+, and Cu2+ undergoes a homeotropic-to-planar anchoring transition while 5CB supported on the other metal ions shows no measurable orientational response to DMMP. These results, when combined, support the view that the orientational response of 5CB to DMMP on metal perchlorate salts results from a competitive binding of 5CB and DMMP to the cations of the metal salts.

1. Introduction It is well known that the orientations assumed by liquid crystals (LCs) near surfaces are sensitive to the molecular details of the surfaces.1-4 Recently, this sensitivity was reported to provide principles for chemical sensing of low molecular weight analytes from vapor phases.5 In particular, micrometer-thick films of nematic 4′-pentyl-4-biphenyl-carbonitrile (5CB) supported on surfaces presenting copper carboxylate groups were observed to assume a homeotropic (perpendicular) orientation.6 The homeotropic anchoring of 5CB on the copper-treated surfaces was attributed to binding of 5CB to the copper ions. When the films of 5CB were exposed to dimethyl methylphosphonate (DMMP), the films of LCs underwent surface-driven orientational transitions (homeotropic to planar). Because the nematic phase of 5CB is birefringent,7 the orientational transitions were readily visualized between crossed polars. Anchoring transitions driven by vapor concentrations of DMMP in the parts-per-billion range were reported. These transitions were proposed to result from competitive binding of the nitrile groups of 5CB and DMMP to the copper ions on the surfaces.6,8,9 In this paper, we report the results of an experimental study that sought to better understand the nature of the interactions between metal salts and 5CB that give rise to the orientations of 5CB at surfaces presenting metal salts as well the orientational response of 5CB to DMMP. The recognition-driven orientational transitions in LCs, as described above, were postulated to reflect the relative strengths of the copper(II) ion complexes formed with DMMP and 5CB.6,10,11 In this paper, we test this proposition by performing * To whom correspondence should be addressed. Fax: +1-608-2625434. E-mail: [email protected].

several experiments, including an investigation of the influence of the metal ion type on the orientational behavior of the liquid crystals. Past studies have established that for alkali and alkalineearth metal ions, such as sodium and magnesium, metal-ligand bonds are ionic in nature and the strength of such metal ion complexes is largely determined by electrostatic effects.10,12 Therefore, the metal-ligand bond strength increases with the charge density at the metal center and increases with decreasing ionic radius of the metal ions.10,12 For transition metal ions, however, the strength of metal ion complexes also depends on the ligand field stabilization energy. This contribution to the bond energy results from the stabilization of the d electrons of the metal in the metal-ligand environment. The ligand field stabilization energy makes an important contribution to the stability of complexes formed by Fe2+, Co2+, Ni2+, and Cu2+.12 Past experimental studies have also led to the ranking of metal ions according to the stability of their complexes with ligands. For example, the Irving-Williams series13,14 ranks the relative stability of complexes with divalent metal ions as Ba2+ < Sr2+ < Ca2+ < Mg2+ < Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+. This ranking, which is largely independent of the nature of the ligands, correlates closely with either (depending on the type of ligand) the second ionization energy (E2) or the sum of the first two ionization energies (E1 + E2) of the divalent metal ions10

M f M ++ e- , E1

(1a)

M+ f M2++ e- , E2

(1b)

The correlation between the ionization energy and the thermodynamic stability of the metal complex suggests that the

10.1021/jp0470391 CCC: $27.50 © 2004 American Chemical Society Published on Web 12/07/2004

Study of the Anchoring Behavior of Liquid Crystals

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lowest unoccupied molecular orbital (LUMO) is involved in the formation of the metal complex (e.g., via the hybridization of the d2sp3 orbital for metal ions that form octahedral complexes). A convenient index with which to characterize the orbital energy is the so-called “electron affinity” (A), which is defined as the negative of the orbital energy of the LUMO, LUMO.15,16

A ) - LUMO

(2)

For divalent metal ions, the value of the electron affinity is the same as the second ionization energy E2. Thus, the electron affinities correlate closely with the Irving-Williams series Sr2+ (11.03 eV) < Ca2+ (11.87 eV) < Mg2+ (15.04 eV) < Mn2+ (15.64 eV) < Fe2+ (16.18 eV) < Co2+ (17.06 eV) < Ni2+ (18.17 eV) < Cu2+ (20.29 eV) > Zn2+ (17.96 eV). A central hypothesis of the study reported in this paper was that the orientational behavior of 5CB on surfaces presenting metal salts would correlate with the relative thermodynamic stabilities of complexes formed by the metal ions, as predicted by electron affinities (and the Irving-William series). The study reported in this paper also sought to obtain spectroscopic evidence of the interactions of the nitrile group of 5CB with surfaces presenting metal ions. The formation of complexes between metal ions and nitrile groups has been investigated in past studies of bulk solutions, mostly with acetonitrile.17-19 In these studies the vibrational frequencies (stretch) of nitrile groups coordinated with metal ions were found to correlate closely with the Irving-Williams series. The formation of metal ion complexes with organophosphonate compounds has also been confirmed in past studies using IR spectroscopy.20-22 These studies conclude that organophosphonates form a thermodynamically stable complex with most transition metal ions through PdO or P-O-C groups.20-22 Moreover, the shift of the PdO stretching frequency after complexation with metal ions was also found to be proportional to the electron affinities of the metal ions.23 The remainder of this paper is structured as follows. First, we describe an experimental study that sought to determine if the anchoring of 5CB on surfaces presenting metal perchlorate salts correlates with the electron affinities of the metal ions in the salt. Second, we describe measurements using PM-IRRAS that aimed to obtain spectroscopic evidence of the formation of complexes between metal ions and the nitrile groups of the 5CB. Finally, we report an experimental investigation which demonstrates that the orientational response of 5CB to DMMP correlates with the electron affinities of the metal ions. 2. Experimental Section Materials. 11-Mercaptoundecanoic acid and the liquid-crystal 4′-octyl-4-biphenyl-carbonitrile (8CB) were purchased from Sigma Aldrich (Milwaukee, WI). Liquid-crystal 4′-pentyl-4biphenyl-carbonitrile (5CB) was purchased from EMD Chemicals (Hawthorne, NY). Titanium (99.999%) and gold (99.999%) were purchased from Advanced Materials (Spring Valley, NY). Perchlorate salts of metal ions (Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+ Mn2+, Fe3+, Mg2+, Ag+, Cs+, Na+) in their highest purity form were purchased from either Sigma Aldrich or Alfa Aesar (Ward Hill, MA). DMMP in nitrogen at a nominal concentration of 10 ppmv was obtained from Quality Standards & Research Gases (Pasadena, TX) and used as received. Gold specimen grids with thicknesses of ∼20 µm and hole sizes of 297 µm were obtained from Electron Microscopy Sciences (Fort Washington, PA).

Methods. Preparation of Substrates for Anchoring Studies. Eighty Ångstroms of titanium and 200 Å of gold were deposited sequentially onto piranha-cleaned glass slides (Fisher’s Finest) by using an electron beam evaporator (Tek-Vac Industries, Brentwood, NY). Details regarding piranha cleaning and evaporation of gold can be found elsewhere.24 The gold films were used within 1 week of deposition. Self-assembled monolayers (SAMs) were formed on the surfaces of the gold films overnight by immersion of the films into ethanolic solutions containing 1 mM of 11-mercaptoundecanoic acid. The gold films were rinsed with copious amounts of ethanol to remove residual thiols and then dried under a stream of nitrogen. Metal perchlorate salts were deposited onto the gold films decorated with 11-mercaptoundecanoic acid by spin coating. Five microliters of an ethanolic solution containing 5 mM of the metal perchlorate salt was applied to each surface (∼12 mm × 12 mm) mounted on a spinner (WS-400A-6NPP/Lite, Laurell Technologies, North Wales, PA), and the substrate was spun at 3000 rpm for 1 min. Preparation of Substrates for IR Studies. Substrates used in the IR experiments were prepared by sequential deposition of 100 Å of titanium and 2000 Å of gold onto silicon wafers (Silicon Sense, Nashua, NH). The gold-coated silicon wafers were then cut to ∼25 mm × 25 mm pieces, rinsed with ethanol, and dried under a nitrogen stream. Self-assembled monolayers were formed from 11-mercaptoundecanoic acid on these surfaces, as described above. The SAMs (no metal salt) were immersed in ∼0.1 M aqueous HCl for 1 min and dried with nitrogen to ensure protonation of the acid. Sodium carboxylate monolayers were prepared by immersion of the SAMs in 0.01 M aqueous NaOH for 1 min and dried under nitrogen. All other metal carboxylate monolayers were prepared by immersion of the carboxyl-terminated SAMs into 100 mM ethanolic solutions of the corresponding metal perchlorate salt for 5 min. The samples were lightly rinsed with ethanol and dried under nitrogen. For IR studies, 10 µL of 0.5 wt % 8CB in toluene was then spin coated onto each sample at 3000 rpm for 60 s. Ellipsometry. The optical thicknesses of the metal salts deposited onto the SAMs were measured by using a Rudolph Auto EL ellipsometer (Flanders, NJ) at a wavelength of 632 nm and an angle of 70°. Measurements were performed on gold films with thicknesses of 500 Å. Ellipsometric constants of the gold films were determined to be n ) 0.18 and k ) 3.54 by using a SAM formed from hexadecanethiol with a known optical thickness of 2.3 nm. Five spots were measured on each of five samples for each sample type. Polarization-Modulation IR Reflectance-Absorbance Spectroscopy (PM-IRRAS). IR spectra of 8CB supported on carboxylterminated SAMs were obtained using a Nicolet Magna-IR 860 FT-IR spectrometer with photoelastic modulator (PEM-90, Hinds Instruments, Hillsboro, OR), synchronous sampling demodulator (SSD-100, GWC Technologies, Madison, WI), and a liquid N2-cooled mercury cadmium telluride (MCT) detector. All spectra (1000-4000 cm-1) were taken at an incident angle of 83° with the modulation centered at 2200 cm-1. For each sample, 500 scans were taken at a resolution of 4 cm-1. Data were collected as differential reflectance vs wavenumber, and spectra were normalized and converted to absorbance units via the method outlined in Frey et al.25 X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy was performed with a Perkin-Elmer PhiX 5400. The X-ray source was Mg KR, and the scanning window was 2 mm × 1 mm. Survey scans were performed for 10 cycles with a pass energy of 89.45 eV to identify elements present on

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Yang et al. TABLE 1: Composition of Metal Salt Surfaces Obtained by Using XPS multiplex analysis by XPS metal ion

Au

O

Cl

M

O/metala

Cu2+

39.9 26.3

47.0 61.5

4.1 4.9

9.0 7.3

4.4 17.5

Ag+

a

After subtraction of the contribution from the metal perchlorate

salt.

Figure 1. Ellipsometric thicknesses of metal perchlorate salts spin coated onto gold surfaces decorated with carboxyl-terminated SAMs.

the surface. The survey scan was then followed by elementspecific acquisitions for the elements of interest (i.e., Au, Cu, or Ag, O, and Cl) at 44.75 eV for another five cycles. The major peaks for these elements are O(1s), Cl(2p), Au(4f7/2), Ag (3d), and Cu(2p3/2), respectively.26 After baselines were established, the area under each peak was integrated, corrected for the sensitivity factor of each element, and the percentage of each element was calculated. Preparation of Optical Cells of 5CB. Gold grids were cleaned sequentially in methylene chloride, ethanol, and methanol, dried under nitrogen, and then heated at 110 °C for 24 h. These grids were then placed onto the metal salt surfaces to prevent 5CB from dewetting the surfaces. Approximately 1 µL of 5CB was dispensed onto each gold grid, and the excess 5CB was removed by touching the 5CB with a capillary tube. Gas Handling System for the Dilution of DMMP. We used a home-built gas handling system to dilute 10-ppmv DMMP with pure nitrogen by using mass flow controllers. Details of this gas handling system can be found elsewhere.27 The diluted stream of DMMP was delivered at a flow rate of 100 cm3/min to a flow cell in which each optical cell was held. Imaging System. A digital camera (Olympus 5050 Zoom) with a polarizing lens was used to image the LC. A second polarizer, obtained from Edmund Optics (Barrington, NJ), was placed between the white light source and flow cell. All images were obtained using crossed polars and converted to gray-scale intensities, and their luminosities were quantified by using Photoshop (Adobe, San Jose, CA). The optical textures of 5CB were also examined by using plane-polarized light in transmission mode on an Olympus BX60 microscope with crossed polars. The modulation in the intensity of light transmitted through the sample was recorded while rotating the stage from 0° to 45°. Homeotropic alignments were determined by inserting a condenser below the stage and a Bertrand lens above the stage for conoscopic examination. 3. Results Characterization of Metal Salt Surfaces. The first goal of our study was to prepare surfaces presenting metal perchlorate salts and to characterize them. The surfaces were prepared by spin coating 5-µL of ethanolic solutions containing 5 mM of metal perchlorate salt at 3000 rpm for 1 min onto gold surfaces decorated with carboxyl-terminated SAMs. Ellipsometric measurements performed before and after spin coating were used to determine the ellipsometric thickness of the metal salts deposited by spin coating (Figure 1). Inspection of Figure 1 reveals that the metal ions with the same valence have

Figure 2. Optical images (crossed polars) of gold grids impregnated with 5CB supported on metal perchlorate salts before and after exposure to 100 ppbv of DMMP for 1 h.

comparable ellipsometric thicknesses (except for Cs+ which has a very small thickness) and that the thickness correlates with the valence of metal ions. This result suggests that the film thickness may be influenced by the amount of perchlorate associated with the metal ions. To confirm the presence of perchlorate on the surfaces, XPS was employed. The elemental compositions of the perchlorate salts of Cu2+ and Ag+ are shown in Table 1. From this table we make two principal observations. First, because the only source of chlorine on the surface is perchlorate, the presence of chlorine on the surface leads us to conclude that perchlorate salts were present on both surfaces. Second, the ratio of metal ion to perchlorate on the surface is larger than the stoichiometric ratio of the perchlorate salts (i.e., Cu:Cl ) 1:2 and Ag:Cl ) 1:1). This result is consistent with formation of a complex between the metal ions and carboxylate functional groups on the surface. After subtracting the contributions from the perchlorate salt, the oxygen-to-metal ratios are 4.4 and 17.5 for Cu2+ and Ag+, respectively. The value of 4.4 suggests that most surface carboxylate groups are coordinated with copper ions. In contrast, the high oxygen-to-Ag ratio suggests that only 11% of the carboxylate groups on the surface are complexed with Ag+. Anchoring of 5CB on Surfaces Presenting Metal Perchlorate Salts. Next, we studied the anchoring behavior of 5CB supported on the metal salt surfaces. To avoid the dewetting of 5CB from these surfaces, gold grids were placed on the metal salt surfaces and 5CB was dispensed into the gold grids (see Methods section for details). When observed between crossed polars in transmission mode, the optical images of 5CB confined within the gold grids were either dark or bright (Figure 2). The dark optical appearance is consistent with homeotropic alignment of 5CB, which was confirmed by conoscopic examination of the samples. Past studies have established that 5CB aligns homeotropically at the 5CB-air interface.28 Therefore, we concluded that the orientation of 5CB at the metal-salt interface was also homeotropic for those samples with a dark optical appearance between crossed polars (Figure 3a). The samples

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Figure 3. Schematic illustration of the orientations of 5CB hosted within gold grids supported on metal salt surfaces: (a) homeotropic anchoring of 5CB at both air and metal salt interfaces; (b) planar (or tilted) orientation at metal salt interfaces and homeotropic at the airLC interface.

Figure 5. PM-IRRAS of films of 8CB spin coated onto carboxylterminated SAMs complexed to metal ions. The peak at 2227 cm-1 corresponds to the stretch vibration of the nitrile groups of 8CB in bulk.

Figure 4. Intensity of light (crossed polars) transmitted through 5CB supported on various metal perchlorate salts as a function of the electron affinities of the metal ions.

in Figure 2 with a bright optical appearance possessed a strong in-plane birefringence under crossed polars. Because 5CB assumes a homeotropic orientation at the 5CB-air interface, we concluded that the in-plane birefringence was caused by a planar or tilted orientation of 5CB at the interface with the metal salt surface. These boundary conditions lead to a splay and bend distortion within the film of LC, as illustrated in Figure 3b. By examining the optical appearance of 5CB in Figure 2, we concluded that 5CB supported on Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+, and Fe3+ assumed a homeotropic orientation whereas 5CB supported on Mn2+, Mg2+, Ag+, Cs+, and Na+ assumed planar or tilted orientations. We sought to determine if the orientational behavior described above correlated with the electron affinities of the metal ions on the surfaces. In Figure 4 the intensity of light transmitted through each film of 5CB is plotted as a function of the electron affinities of the metal ions on which the 5CB was supported. Figure 4 shows that 5CB supported on the metal ions with an electron affinity higher than 15.64 eV (Mn2+) transmitted little light (due to homeotropic anchoring) whereas 5CB supported on metal ions with an electron affinity lower than 15.64 eV (Mn2+) led to high intensities of transmitted light because of the planar or tilted anchoring of the 5CB at the metal salt interface. The correlation between the anchoring behavior of 5CB and the electron affinities of the metal ions supports our proposition that the anchoring of 5CB is controlled by the thermodynamic stability of a complex formed between the metal ions and mesogens.

Characterization of the Metal Ion Complexation with Nitrile Groups Using PM-IRRAS. To provide further characterization of the nature of the complex formed between the metal ions and the mesogens, we measured the IR spectra of nanometer-thick films of 8CB supported on the metal salt surfaces. We used 8CB instead of 5CB because it readily formed a stable thin film and did not dewet the surfaces.29 The small thickness of the 8CB film (∼20 nm) allowed us to probe the interactions between the mesogens and metal ions at the interface without obscuring the signal by the bulk phase of 8CB. The IR spectra of 8CB supported on carboxyl-terminated SAMs pretreated with either 0.1 M HCl or one of five perchlorate salts are shown in Figure 5a. We make several observations regarding this figure. First, for 8CB supported on carboxyl-terminated SAMs, there is a peak in the spectrum at 2227 cm-1, corresponding to the stretching of the nitrile groups of the 8CB molecules.30 All other 8CB samples supported on the surfaces presenting metal ions also show the same peak. Second, we observed the presence of blue-shifted secondary peaks when 8CB was supported on surfaces with Cu2+, Ni2+, and Cd2+ (at ∼2280, 2274, and 2260 cm-1, respectively). Past studies have established that these peaks are associated with nitrile groups coordinated to metal ions.31,32 The coordination of the metal ion to the nitrile group via the lone pair of electrons on the nitrogen causes a rehybridization that strengthens the σ-component of the C-N bond and thus leads to a vibration shift to higher wavenumber.33,34 Although the molecular orbital of the nitrile group permits the back-bonding from transition metal ions that have electrons in the d-orbital, the back-bonding of metal ions with nitrile groups, which would weaken the bonding of the nitrile groups, appears to be offset by rehybridization of the nitrogen atoms. A destabilizing of the CN bonds due to the π-back-bonding would cause the shifting of the 2230 cm-1 peak corresponding to nitrile stretching to a lower wavenumber in the IR spectrum, whereas our experimental measurements reveal

20184 J. Phys. Chem. B, Vol. 108, No. 52, 2004 shifts to higher wavenumbers. It is noted that the position of this secondary nitrile peak correlates closely with the electron affinities of Cu2+ (20.29 eV), Ni2+ (18.17 eV), and Cd2+(16.91 eV) as shown in Figure 5b. Third, there is no secondary peak for Mg2+ (15.04 eV), Ag+(7.58 eV), and Na+ (5.14 eV). This result suggests the absence of a complex between the nitrile groups and the metal ions with low electron affinities. Fourth, 5CB supported on the metal ions exhibiting secondary peaks in the IR spectra (Cu2+, Ni2+, and Cd2+) assume homeotropic orientations whereas 5CB supported on metal ions that exhibit no secondary peaks (Mg2+, Ag+, and Na+) assume planar or tilted orientations (see Figure 2). The correlation between the secondary nitrile peak and the anchoring behaviors of 5CB and the correlation between the secondary nitrile peak and the electron affinities leads us to conclude that the formation of the complex between metal ion and nitrile groups of 5CB underlies the anchoring behavior of 5CB. We also conclude that the orientation of 5CB on these surfaces can be predicted from knowledge of the electron affinities of the metal ions. We also considered our experimental results within the context of the hard and soft acid-base theory (HSAB). According to the definition of HSAB theory, hard acids include Na+, Mg2+, Mn2+, Al3+, and Fe3+, borderline acids include Cu2+, Co2+, Zn2+, and Ni2+, and soft acids include Ag+ and Cd2+. Eu3+ and La3+ are either hard or borderline acids. Because the nitrile group is a soft base and because a soft base should according to HSAB theory preferentially bind to a soft acid, HSAB theory predicts strong binding of nitrile groups to Ag+ and Cd2+. We do not see this prediction reflected either in our PM-IRRAS results or in the orientation of the cyanobiphenyl mesogens supported on metal salt surfaces. Influence of DMMP on Orientation of 5CB. Next, we sought to understand the influence of DMMP on the orientation of 5CB anchored on the metal salts described above. To this end, we exposed 5CB (confined in gold grids) to a nitrogen stream containing 100 ppb DMMP vapor for 1 h. Optical images of 5CB supported on various metal salts after exposure to DMMP are shown in Figure 2. Inspection of Figure 2 reveals that the optical appearance of 5CB supported on the Cu2+, Eu3+, and Al3+ perchlorate surfaces changed from dark to bright in response to exposure to DMMP. In contrast, 5CB supported on the other metal salts did not show an orientational response to DMMP. The change in optical appearance of 5CB in response to 100-ppb DMMP indicates an orientational transition of the 5CB at the surface of the metal salts. The binding of DMMP at the metal-5CB interface caused the 5CB to tilt away from its homeotropic orientation at this interface, thus causing a distortion of the liquid crystal within the grid (Figure 3b). Because liquid-crystal 5CB is birefringent, the change in the orientation in 5CB was accompanied by an easily visualized change in the intensity of light transmitted through the LC, as shown in Figure 2. We note that all samples that responded to DMMP are samples supported on metal salts that caused a homeotropic orientation of 5CB before exposure to DMMP. Metal salts that caused a planar or titled orientation of 5CB prior to exposure to DMMP did not cause 5CB to change orientation over the period of exposure to DMMP. This observation is consistent with our conclusion that the homeotropic anchoring of 5CB is caused by complexation of nitrile groups to metal ions. In the absence of a complex between the metal ion and 5CB, competitive binding of DMMP to metal ions cannot displace the nitrile groups from a complex and thus drive an orientational transition of the LC at the metal salt surfaces. We also note

Yang et al.

Figure 6. Intensity of light (crossed polars) transmitted through 5CB supported on various metal salts as a function of the electron affinities of the metal ions. The system was exposed to 100 ppbv of DMMP for 1 h prior to measurement of the intensity of the transmitted light.

that 5CB assumes a homeotropic orientation on five metal ions (Cd2+, Zn2+, Ni2+, Co2+, and Fe3+) but did not respond to the presence of DMMP on these surfaces. As discussed below, the lack of response to DMMP is attributed to weaker binding of DMMP than nitrile groups of 5CB to these metal ions. Figure 6 shows the relationship between the orientational response of 5CB to DMMP on the metal salt surfaces and electron affinities of the metal ions. The results in Figure 6 suggest that metal ions with high electron affinities bind DMMP more strongly than 5CB and thus give rise to an orientation response of the LC to DMMP. We wish to emphasize that 5CB supported on Fe3+ did respond to 1 ppm of DMMP, consistent with the correlations based on electron affinity. However, the response was very slow, taking 80 min to be observed. Therefore, the lack of response of 5CB supported on Fe3+ to 100 ppb of DMMP may reflect very slow kinetics of this interaction. We also considered the above-described results within the context of HSAB theory. The response of the liquid crystal to binding of DMMP does appear to follow the predictions of HSAB. Because PdO of DMMP is a harder base than the nitrile group of 5CB, HSAB theory predicts that DMMP will bind Na+, Mg2+, Mn2+, Al3+, and Fe3+ more strongly than 5CB will bind the same set of metal ions. HSAB also predicts that DMMP should also prefer binding to Cu2+ over Ni2+ because the former is harder. In our experiments, we observed 5CB that was anchored on surfaces with Al3+, Eu3+, and Cu2+ did respond to DMMP. Interestingly, all metal ions that provided a response to DMMP are hard acids or borderline acids. We also note that we do not expect an orientational response of 5CB to DMMP when using hard acids such as Na+, Mg2+, and Mn2+ because the interaction of 5CB with these metals is sufficiently weak to not cause homeotropic anchoring of 5CB. 4. Discussion The results described above are consistent with the proposition that the orientational behavior of 5CB on metal perchlorate surfaces results from the formation of a complex between the metal ions and the nitrile group of 5CB. Metals with high electron affinities form stable complexes with 5CB and thus cause homeotropic anchoring of the liquid crystal. This conclusion is supported by PM-IRRAS measurements that show a close correlation between the orientational behavior of the 5CB and the appearance and position of the nitrile stretch of 8CB on the metal salt surfaces. The experimental results also support the view that the orientational response of 5CB to DMMP is the result of

Study of the Anchoring Behavior of Liquid Crystals

Figure 7. Plot of the stability constants (K) of complexes formed between metal ions and either ethylenediamine or salicylaldehyde. The abscissa shows the electron affinity of the metal ions. Data from ref 10.

competitive interactions between the DMMP and 5CB for metal ions on these surfaces. For example, in the absence of a homeotropic orientation of 5CB, we observed no orientational response of 5CB to the DMMP. We also observed, however, that some metal ions that caused a homeotropic orientation of 5CB did not respond to DMMP. Here, we discuss a possible explanation for this latter observation. Past studies have established that the thermodynamic stability constant (K) of metal-ligand complexes depends on both the electron affinity (A) of metals and the nature of the ligands. For example, Figure 7 shows the stability constant of a series of metal ions with salicylaldehyde and ethylenediamine as the ligands.10 Although the stability constants increase with electron affinity for both salicylaldehyde and ethylenediamine, the slopes of the two plots are different. As a result, salicylaldehyde binds more strongly to Mn2+ than ethylenediamine whereas ethylenediamine binds more strongly to Cu2+ than salicylaldehyde. We speculate that the orientational response of 5CB to DMMP on surfaces exhibiting homeotropic orientation may be explained by consideration of the relative thermodynamic stabilities of the complexes formed by 5CB and DMMP with metal ions. In particular, we offer a possible explanation of the experimental observation that only metal ions with higher electron affinities respond to DMMP whereas metal ions with lower electron affinities do not. In short, we postulate that the dependence of the stability constant of the metal complexes on the electron affinity of the metals is different for 5CB and DMMP. Our results suggest that metal-nitrile complexes are more stable than DMMP-metal complexes for metals with lower electron affinities (Ni2+, Cd2+) whereas DMMP-metal complexes are more stable than metal-nitrile complexes for metals with higher electron affinities (Cu2+, Al3+). Past studies have established that the dependence of the stability constant on electron affinity is determined by the relative basicity of a ligand (i.e., the ability to donate electrons to the metal site).10 Although we do not know the basicities for DMMP and 5CB, gas-phase basicity data is available for benzonitrile and trimethyl phosphate. The basicity of trimethyl phosphate is 860.8 kJ/mol, which is greater than that of benzonitrile, 780.9 kJ/mol.35 These data support our proposition outlined above. To provide further support to our proposition that the orientational transitions reported in this paper are caused by the competitive binding of DMMP to metal ions, we used transmission FTIR spectroscopy to characterize solutions of benzonitrile containing 20 mM of metal perchlorate, before and after dissolving DMMP into the solution. Prior to exposure to

J. Phys. Chem. B, Vol. 108, No. 52, 2004 20185 DMMP, we observed a secondary peak at 2280 cm-1 corresponding to nitrile groups complexed with copper ions (similar to Figure 5a). After addition of 1 mM DMMP, this peak was observed to diminish, consistent with a model in which the interactions of the nitrile groups with the metal ions are displaced by DMMP. We also wish to point out that the coordination numbers and geometries of metal complexes do not appear to correlate with the orientational behavior of 5CB and its response to DMMP, as described above. Past studies indicate that Cu2+ has a coordination number of four and assumes a square planar geometry when it complexes with acetonitrile, whereas Mn2+, Ni2+, Zn2+, and Co2+ have a coordination number of six and assume an octahedral geometry with acetonitrile.36 In our study, however, 5CB assumes a homeotropic orientation on Co2+, Ni2+, Zn2+, and Cu2+ and assumes planar anchoring on Mn2+. 5. Conclusions In conclusion, this paper provides insights into the orientational behavior of 5CB supported on metal salt surfaces relevant to chemical sensing. It is concluded that 5CB supported on the perchlorate salts of metal ions with high electron affinities (>15.64 eV), including Cu2+, Zn2+, Cd2+, Ni2+, Co2+, La3+, Al3+, Eu3+, and Fe3+, assume homeotropic anchoring whereas 5CB supported on perchlorate salts of metal ions with low electron affinities, including Mn2+, Mg2+, Ag+, Cs+, and Na+, assume planar or tilted orientations. These results, as well as spectroscopic evidence obtained by using PM-IRRAS, support our proposition that the homeotropic anchoring is caused by binding of the nitrile group of the mesogens to the cations of the metal salts. We also conclude that electron affinities of metals are useful in predicting the orientational response of 5CB to DMMP. A correlation between the orientational response of 5CB to DMMP and the electron affinities of metal ions supports a competitive binding model and offers a possible explanation for why 5CB supported on metal ions with high electron affinities responds to DMMP. Acknowledgment. This research was supported by funding from the U.S. Army Research Office (DAAD19 02-1-0299) and the National Science Foundation (CTS-0428027). References and Notes (1) Abbott, N. L. Curr. Opin. Colloid Interface Sci. 1997, 2, 76. (2) Cognard, J. J. Mol. Cryst. Liq. Cryst. 1982, 1 (Suppl.), 1. (3) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391. (4) Goossens, W. J. A. Mol. Cryst. Liq. Cryst. 1985, 124, 305. (5) Shah, R. R.; Abbott, N. L. J. Am. Chem. Soc. 1999, 121, 11300. (6) Shah, R. R.; Abbott, N. L. Science 2001, 293, 1296. (7) Collings, P. J.; Hird, M. Introduction to Liquid Crsytals; Taylor & Francis Ltd.: London, U.K., 1997. (8) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (9) Crooks, R. M.; Yang, H. C.; McEllistrem, L. J.; Thomas, R. C.; Ricco, A. J. Faraday Discuss. 1997, 107, 285. (10) Martell, A. E.; Calvin, M. Chemistry of the Metal Chelate Compounds; Prentice Hall: New York, 1952. (11) Sillen, L. G.; Martell, A. E. Stability Constants of Metal-Ion Complexs; The Chemical Society: London, 1964. (12) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 2nd ed.; Prentice Hall: Upper Saddle River, NJ, 1998. (13) Irving, H.; Williams, R. J. P. Nature 1948, 162, 746. (14) Liu, Z. S.; Erhan, S. Z. J. Appl. Polym. Sci. 2002, 84, 2386. (15) Pearson, R. G. Inorg. Chem. 1988, 27, 734. (16) Moore, C. E. Ionization Potentials and Ionization Limits; U.S. National Bureau of Standards, 1970; Vol. NSRDS-NBS 34. (17) Inada, Y.; Sugata, T.; Ozutsumi, K.; Funahashi, S. Inorg. Chem. 1998, 37, 1886. (18) Funahashi, S.; Inada, Y. Bull. Chem. Soc. Jpn. 2002, 75, 1901. (19) Bertran, J. F.; Ruı´z, E. R. Spectrochim. Acta 1993, 49A, 43. (20) Karayannis, N. M.; Owens, C.; Pytlewski, L. L.; Labes, M. M. J. Inorg. Nucl. Chem. 1969, 31, 2767.

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