J. Phys. Chem. B 2006, 110, 26081-26088
26081
Infrared Spectroscopy of Competitive Interactions between Liquid Crystals, Metal Salts, and Dimethyl Methylphosphonate at Surfaces Katie D. Cadwell, Mahriah E. Alf, and Nicholas L. Abbott* Department of Chemical and Biological Engineering, UniVersity of Wisconsin-Madison, 1415 Engineering DriVe, Madison, Wisconsin 53706-1607 ReceiVed: May 25, 2006; In Final Form: September 28, 2006
We report the use of Fourier transform polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) to characterize the influence of dimethyl methylphosphonate (DMMP) on the molecular interactions occurring within thin films of nitrile-containing liquid crystals supported on surfaces presenting metal perchlorate salts. Infrared spectra obtained using thin films of 4′-octyl-4-biphenylcarbonitrile (8CB) supported on copper(II) perchlorate salts reveal the nitrile groups of 8CB to be coordinated to the copper(II) on these surfaces, and subsequent exposure of the system to DMMP to result in the elimination of these coordinated nitrile groups. Concurrently, evidence of coordination of the phosphoryl group of DMMP with copper(II) is provided by measurement of a shift of the phosphoryl stretch from 1246 to 1198 cm-1. In contrast, surfaces presenting nickel(II) perchlorate salts only weakly coordinate with DMMP [the phosphoryl peak shifts from 1246 to 1213 cm-1 in the presence of nickel(II)], and exposure of 8CB to DMMP results in only partial loss of coordination of the nitrile groups of 8CB with nickel(II). These PM-IRRAS measurements and others reported in this article provide insights into the molecular origins of macroscopic ordering transitions that are observed when micrometer-thick films of nitrile-containing liquid crystals supported on copper(II) or nickel(II) perchlorate are exposed to DMMP: Upon exposure to DMMP, nematic phases of 4′-pentyl-4biphenylcarbonitrile (5CB) supported on copper(II) perchlorate salts undergo ordering transitions, whereas 5CB supported on nickel(II) perchlorate salts do not. Our IR results support the hypothesis that these ordering transitions reflect the relative strengths of coordination interactions occurring between the 5CB, DMMP, and the metal salts at these interfaces.
1. Introduction The orientational ordering of liquid-crystalline phases at surfaces is known to be influenced by the chemical functionality of the surfaces.1-3 Recent advances in surface chemistry now permit the preparation of surfaces with well-defined chemical functionality with which to manipulate interactions between liquid crystals and surfaces.4-10 These interactions include dipolar interactions associated with electrical double layers formed within the liquid-crystalline phase as a result of fixed charges at the interface,4 hydrogen bonding between chemical groups presented at the surface and the mesogens comprising the liquid crystal,5,6 and coordination interactions between surface-immobilized metal ions and mesogens.7-10 The study reported in this article addresses the last class of interaction described above and seeks to advance our understanding of the orientational ordering of nematic liquid crystals on surfaces that are decorated with metals salts. This investigation is motivated by the past observations that (i) the nematic liquid crystal 4′-pentyl-4-biphenylcarbonitrile (5CB) will order on surfaces presenting metal salts in a manner that depends on the identity of the metal ions and (ii) exposure of nematic 5CB supported on certain metal salts to an organophosphonate compound can lead to ordering transitions in the liquid crystal that are readily visible to the naked eye. For example, exposure of micrometer-thick films of nematic 5CB supported on surfaces of copper(II) salts to parts-per-billion vapor concentrations of * To whom correspondence should be addressed. Fax: +1-608-2625434. E-mail:
[email protected].
dimethyl methylphosphonate (DMMP) leads to ordering transitions within the liquid crystal in tens of seconds. In contrast, exposure of these films to 300 ppm of water, hexanes, acetone, and ethanol does not produce such an ordering transition.8,9 The experiments described herein aimed to provide insights into the intermolecular interactions involved in the abovedescribed ordering transition triggered in films of liquid crystals by DMMP. This research builds on a previous investigation in which we characterized the interactions involved in the ordering of 5CB on metal salt surfaces prior to exposure of DMMP. In that report, it was demonstrated that surfaces presenting metal ions with an electron affinity at or below 15.64 eV [Cs(I), Na(I), Ag(I), Mg(II), Mn(II)] resulted in a planar or tilted orientation of nematic 5CB, whereas metal ions with higher electron affinities [Cd(II), Co(II), Zn(II), Ni(II), Cu(II), La(III), Al(III), Eu(III)] promoted homeotropic (perpendicular) anchoring of 5CB. These observations and others led to the proposition that coordination of the nitrile group of 5CB and the metal ions on these surfaces was influencing the ordering of the liquid crystal. Support for this proposition was obtained by infrared measurements involving the related liquid crystal 4′-octyl-4-biphenylcarbonitrile (8CB) supported on surfaces presenting metal salts. In addition to the free nitrile stretch of 8CB at 2227 cm-1, a blue-shifted nitrile peak corresponding to the metal-coordinated nitrile was observed. Infrared evidence revealed a close correlation between homeotropic orientation of 5CB and coordination of the nitrile group of 8CB to the metal ions.8
10.1021/jp063211k CCC: $33.50 © 2006 American Chemical Society Published on Web 12/02/2006
26082 J. Phys. Chem. B, Vol. 110, No. 51, 2006 Whereas the inquiry described above characterized the interactions of metal salts and nitrile-containing mesogens that lead to the ordering of the liquid crystals on surfaces presenting metal salts, the investigation reported here sought to understand the mechanism by which DMMP perturbs the ordering of the liquid crystals (LCs) imposed by the metal salts. Specifically, we sought to test the hypothesis that the metal-nitrile coordination interactions giving rise to the homeotropic orientation of 5CB described above are perturbed by competitive coordination interactions of DMMP for the metal ion. The competitive interactions are proposed to underlie the orientational transition of the liquid crystal from homeotropic to tilted or planar upon exposure to DMMP. The experimental approach used to test this proposition builds on past studies that used in situ reflective IR techniques to investigate the molecular interactions between surfaces and molecules adsorbed from vapor phases.11-22 These studies demonstrated that reflective IR techniques are sufficiently sensitive to permit characterization of molecular vibrations from a single monolayer of molecules adsorbed to a surface.11-22 They also demonstrated the utility of reflective IR spectroscopy to monitor in situ the interactions between organic thin films formed on gold films and adsorbates from a vapor.11-15 Of particular relevance to our work is a report by Crooks et al. in which Fourier transform polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS) was used to monitor the interactions of diisopropyl methylphosphonate (DIMP) adsorbed from the vapor phase onto SAMs presenting metal carboxylate terminal groups (prepared from 11-mercaptoundecanoic acid) on films of gold.15 The use of PM-IRRAS eliminated contributions to the spectra from the DIMP present in the vapor and thus permitted characterization of the DIMP adsorbed at the surface.23-25 This study found that the DIMP coordinated to the metal ions immobilized on the surface and simultaneously perturbed the coordination of the metal ions with the carboxyl groups of the 11-mercaptoundecanoic acid (MUA) monolayer. 15 The specific goals of the investigation reported in this article were four-fold. First, we sought to characterize the orientational ordering of 5CB supported on surfaces presenting excess copper(II) and nickel(II) perchlorate salts and to observe the presence or absence of an orientational transition of 5CB upon exposure to DMMP. We note here that this step was necessary because, as described in more detail below, the methods used to prepare the surfaces employed in these experiments differ from those described previously. We characterized the orientational ordering of the nematic liquid crystal 5CB supported on copper(II) or nickel(II) perchlorate salts by using polarized light microscopy and confirmed the presence (with copper) or absence (with nickel) of an orientational transition when the LC was exposed to DMMP. The second goal of the study reported in this article was aimed at testing our hypothesis that coordination of the nitrile groups of 5CB to copper(II) perchlorate salts is perturbed in the presence of DMMP, thus leading to an ordering transition in the LC. Following exposure to DMMP, we sought to use PMIRRAS to observe the loss of the infrared adsorption corresponding to the nitrile group in a coordination complex with copper(II). Simultaneously, we sought to find spectroscopic evidence for coordination of DMMP to the copper ion and, thus, provide support for the postulated competitive interaction between the LC and DMMP for coordination with copper(II). To this end, we characterized films of 8CB supported on copper(II) perchlorate salts by PM-IRRAS before, during, and after exposure to vapor-phase DMMP. 8CB was used in these spectroscopic measurements because 5CB does not form stable
Cadwell et al. thin films on metal perchlorate salts. We note here that 8CB supported on surfaces decorated with copper(II) ions has also been reported to undergo an ordering transition upon exposure to DMMP.10 The third goal of the inquiry described herein was to compare the infrared spectra of 8CB and DMMP on surfaces decorated with copper(II) perchlorate salts (which trigger the ordering transition upon exposure to DMMP) to infrared spectra obtained using surfaces decorated with nickel(II) perchlorate salts (which do not trigger the ordering transition upon exposure to DMMP). We sought to confirm that perturbation of the metal-nitrile coordination complex by DMMP is a necessary condition for an ordering transition to be observed. In brief, the fourth goal of our investigation was to determine whether the reVersibility of the orientational transition of 5CB on copper(II) perchlorate surfaces upon exposure to DMMP is mirrored in the infrared spectra. Past research has demonstrated that the orientational transition of 5CB on copper(II) perchlorate induced by DMMP is reversible. 5CB assumes a planar or tilted orientation in the presence of DMMP, but reverts to its original homeotropic orientation upon purging of the headspace above the LC film with air or nitrogen. This reversibility was observed over several cycles of exposure and purging.8,9 2. Experimental Section Materials. 11-Mercaptoundecanoic acid (MUA) and the liquid crystal 4′-octyl-4-biphenylcarbonitrile (8CB) were purchased from Sigma Aldrich (Milwaukee, WI). The liquid crystal 4′-pentyl-4-biphenylcarbonitrile (5CB) was purchased from EMD Chemicals (Hawthorne, NY). Titanium (99.999%) and gold (99.999%) were purchased from Advanced Materials (Spring Valley, NY). Hydrated perchlorate salts of copper(II) (98%) and nickel(II) (99.9%) were purchased from Sigma Aldrich and Alfa Aesar (Ward Hill, MA), respectively. Sodium perchlorate (>99%) was purchased from Acros Organics (Geel, Belgium) through Fisher Scientific (Pittsburgh, PA). Dimethyl methylphosphonate (DMMP) in nitrogen at a nominal concentration of 10 ppm was obtained from Linweld Inc. (Des Moines, IA) and used as received. Glass slides were Fisher’s Finest purchased from Fisher Scientific. Silicon wafers were purchased from Silicon Sense (Nashua, NH). Methods. Fabrication of Polyurethane Substrates with Micrometer-Scale Topography. Polyurethane surfaces with patterned topography were fabricated to provide a substrate on which to prepare thin films of nematic 5CB for measurements of anchoring transitions. These substrates (i) provide a surface topography that prevents dewetting of thin films of 5CB, (ii) are optically transparent and nonbirefringent, and (iii) can be chemically functionalized (see Preparation of Substrates for Anchoring Studies) to achieve the desired orientation of 5CB. To prepare the polyurethane substrates, we first used photolithography to pattern silicon wafers with arrays of wells, each having a lateral size of 500 µm and a depth of ∼2.8 µm. Titanium (100 Å) and gold (2000 Å) were evaporated onto the silicon surface at normal incidence with an electron beam evaporator (Tek-Vac Industries, Brentwood, NY) prior to molding poly(dimethyl siloxane) (PDMS, Sylgard 184, Dow Corning, Midland, MI) on the surface. The rates of deposition of titanium and gold were 0.2 and 1.0 Å/s, respectively. The pressure in the evaporator was maintained below 2 × 10-6 Torr throughout evaporation of the metal. Although bare gold is a high-energy surface, exposure of gold to atmosphere for several hours leads to a low-energy surface as a result of the formation of a layer of adventitious adsorbates. These adsorbates reduce
Competitive Interactions between LCs, Metal Salts, and DMMP the adhesion between the PDMS and the gold surface. The PDMS replica was prepared by pouring a 10:1 by weight mixture of PDMS prepolymer and curing agent over the patterned wafer and then curing overnight at 60 °C. The polyurethane (PU, NOA61, Norland Products Inc., Cranbury, NJ) substrates were molded from the PDMS replicas as described in an earlier publication.26 Profilometry measurements (Tencor Alphastep 200, KLA-Tencor Corp., San Jose, CA) confirmed that the feature sizes of the PU replicas were the same as those of the lithographically patterned wafer. Preparation of Microfabricated Substrates for Anchoring Studies. Titanium (thickness of 80 Å) and gold (thickness of 200 Å) were deposited sequentially at a rate of 0.2 Å/s onto the polyurethane replicas using an electron beam evaporator. The gold films were used within 1 week of deposition. SAMs were formed on the surfaces of the gold films by immersion into 2 mM ethanolic solutions of 11-mercaptoundecanoic acid for ∼1 h. Longer immersion times led to swelling of the PU replicas and disruption of the gold films. Upon removal, the samples were rinsed with copious amounts of ethanol to remove residual thiols and dried under a stream of nitrogen. Metal ions were coordinated to the carboxylic acid-terminated SAMs by soaking the samples in 25 mM ethanolic solutions of metal perchlorates for 5 min and then rinsing them thoroughly in ethanol. Excess metal perchlorate salts were then deposited onto the surface by spin-coating 30 µL of 4.5 mM metal perchlorate salts in ethanol onto each sample at 3000 rpm for 60 s (WS400A-6NPP/Lite, Laurell Technologies, North Wales, PA). The functionalized PU wells were filled with 5CB by drawing 5CB into glass microcapillary tubes and then depositing the 5CB into the PU wells by touching the surface of the wells with the filled tubes. Optical Imaging of Ordering Transitions in Micrometer-Thick Films of LC. An Olympus C-765 Ultra Zoom digital camera was used to image the wells of the PU substrates filled with 5CB. All images were obtained with the samples in an aluminum flow cell with glass windows for transmission of light. Polarizing plastic sheets (Edmund Industrial Optics, Barrington, NJ) were taped to the bottom and top windows and oriented at 90° from each other for maximum extinction of light. The optical textures of 5CB were also examined using plane-polarized light in transmission mode on an Olympus BX60 microscope with crossed polars. Homeotropic alignments were determined by inserting a condenser below the stage and a Bertrand lens above the stage for conoscopic examination and observing a pattern of two crossed isogyres.27 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. The gold-coated silicon wafers were then cut into ∼25 mm × 25 mm pieces, rinsed with ethanol, and dried under a nitrogen stream. SAMs were formed from 11-mercaptoundecanoic acid (MUA) on these surfaces, and then metal perchlorate salts were applied in the same manner as for the preparation of substrates for anchoring studies (see above). The ellipsometric thickness of the salt film was found to be ∼6 nm prior to application of 8CB to the substrate. The 8CB was applied to these surfaces by spinning 30 µL of ∼1.0 wt % 8CB in toluene at 3000 rpm for 60 s. The ellipsometric thickness of the resulting 8CB and metal perchlorate film was measured to be ∼30 nm. Ellipsometry. Ellipsometric measurements were performed with a Gaertner LSE ellipsometer, λ ) 632.8 nm, ψ ) 70°. Although 8CB films prepared in the above manner are birefringent, we calculated an ellipsometric thickness by modeling
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26083 the film as an isotropic material with n ) 1.6.28-30 We used this approximation because we sought only to confirm that the films were less than 200 nm in thickness (optimal for surfaceselective PM-IRRAS measurements24) and to ensure that all of the samples used for IR studies had similar thicknesses. The optical thicknesses of the SAMs formed from MUA and the metal perchlorate salts deposited onto the SAMs were determined assuming a refractive index of n ) 1.5.31-34 Fourier Transform Polarization-Modulation IR ReflectanceAbsorbance Spectroscopy (PM-IRRAS). IR spectra of 8CB on surfaces were obtained using a Nicolet Magna-IR 860 FT-IR spectrometer with a photoelastic modulator (PEM-90, Hinds Instruments, Hillsboro, OR), a synchronous sampling demodulator (SSD-100, GWC Technologies, Madison, WI), and a liquidN2-cooled mercury cadmium telluride (MCT) detector. All spectra (1000-4000 cm-1) were recorded at an incident angle of 83° with the modulation centered at either 1400 or 2200 cm-1. All spectral data reported within this article were recorded within 500 cm-1 of the modulation center. For each sample, 1000 scans were recorded at a resolution of 4 cm-1. Data were collected in terms of differential reflectance vs wavenumber, and spectra were normalized and converted to absorbance units via the method outlined in Frey et al.24 During data collection, samples were held in a gastight homemade aluminum vapor cell with sodium chloride windows (Thermo Electron Spectroscopy, Madison, WI), built in-house to fit within the PM-IRRAS instrument. The vapor cell had inlet and outlet valves that could be opened to allow vapor-phase DMMP to flow into the cell (10 ppm DMMP in nitrogen at 500 sccm for 5 min) and then closed to place the cell into the PM-IRRAS instrument. 3. Results Anchoring Transitions of 5CB upon Exposure to DMMP. Our first goal was to verify the homeotropic orientation of nematic 5CB supported on surfaces presenting copper(II) and nickel(II) perchlorate salts and to characterize changes in the orientational ordering of the 5CB upon exposure to DMMP. We note here that our past experiments did not use microfabricated substrates to support the films of 5CB (see the Methods section for details).8 After the wells of the microfabricated substrates had been filled with 5CB, the optical appearance of the LC was observed between crossed polars. The dark appearance of the LC shown in Figure 1a, when combined with use of conoscopy, led us to conclude that 5CB assumed a homeotropic orientation on both the copper(II) and nickel(II) perchlorate salt-treated surfaces. Next, we exposed the 5CB hosted within the microfabricated wells to 10 ppm DMMP by flowing a gaseous stream of nitrogen containing DMMP over the 5CB for 5 min (this time corresponds to that used for the infrared experiments described below). During this process, we observed the optical appearance of the LC (Figure 1b). The change in optical appearance of the 5CB supported on the copper(II) perchlorate salts from dark to bright indicates that the 5CB underwent a transition from a homeotropic orientation to either a planar or tilted orientation upon exposure to DMMP. We noted that the transition from dark to bright occurred within 15 s of the onset of exposure to DMMP and was nearly complete after 1 min of exposure to 10 ppm DMMP (data not shown). After purging the flow cell with air, we observed the 5CB to return to an optical appearance consistent with the initial homeotropic orientation of the LC (data not shown). In contrast, the 5CB supported on the nickel(II) perchlorate salts remained dark after 5 min of exposure to 10 ppm DMMP, indicating that the 5CB on the nickel(II)
26084 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Figure 1. Optical images (crossed polars) of 5CB supported on metal perchlorate salts hosted within arrays of wells (depth of ∼2.8 µm, width of 500 µm) (a) before and (b) during exposure to 10 ppm DMMP in nitrogen for 5 min. White outlines indicate the location of a single row of LC-filled wells in image a.
perchlorate surfaces did not undergo an ordering transition upon exposure to DMMP (Figure 1a,b). These results are qualitatively similar to those reported previously by us using a different experimental setup.8 Effect of DMMP on Infrared Spectra of 8CB on Copper(II) Perchlorate Salts. Next, we sought to characterize the coordination of the nitrile groups of 8CB to copper(II) ions on surfaces using PM-IRRAS and to determine whether this coordination was perturbed by the presence of DMMP. These experiments were performed using copper(II) perchlorate salts deposited onto SAMs formed from 11-mercaptoundecanoic acid on gold films. As described in detail in the Methods section, thin films of 8CB (ellipsometric thickness of ∼30 nm) were deposited onto the metal perchlorate-decorated surfaces by spincoating. Using ellipsometry and assuming that no metal perchlorate salt is displaced from the surface during deposition of 8CB, we calculate the ratio of 8CB molecules to copper ions on these surfaces to be approximately 2.5:1. We placed the samples into a gastight chamber built to fit within the PMIRRAS instrument and obtained IR spectra with modulation centered at 2200 cm-1 (nitrile stretching region). For convenience, a summary of spectral assignments used in this study can be found in Table 1.34-46 Examination of the middle-wavenumber spectrum prior to exposure of the supported film of 8CB to DMMP (Figure 2a) reveals two main peaks corresponding to the free (uncoordinated) nitrile stretch at 2227 cm-1 and the copper-coordinated nitrile at 2287 cm-1. We note that a broad shoulder is observed at ∼2240 cm-1. The origin of this peak is unknown. We also note that the relative magnitude of the absorbance of the coppercoordinated nitrile peak at 2287 cm-1 as compared to that of the free nitrile at 2227 cm-1 is larger than reported in our previous study.8 This difference is due to the relative amounts of salts present on the surfaces in the two studies. We adopted the procedures reported in this article so that the surfaces used for the PM-IRRAS measurements closely resembled the surfaces used for the observations of the ordering transitions. Finally, we point out that the copper-coordinated nitrile peak ranged in position from 2279 to 2287 cm-1 (previously reported at 2280
Cadwell et al. cm-1) in a manner that depended on the amount of copper(II) perchlorate salt on the surface. The wavenumber and magnitude of the peak were observed to increase with the ellipsometric thickness of the copper(II) perchlorate deposited onto the surface prior to contact with the LC. After contacting the sample for 5 min with a gaseous stream of 10 ppm DMMP in nitrogen, the copper-coordinated peak seen in Figure 2a at 2287 cm-1 largely disappeared (Figure 2b). Simultaneously, the magnitude of the peak corresponding to the free nitrile group was observed to increase. Both observations are consistent with the proposition that DMMP perturbs the copper-nitrile coordination at these interfaces. We note that the loss of peak area in the copper-coordinated nitrile absorbance is not reflected proportionally in the gain in peak area of the free nitrile absorbance. A prior vibrational and molecular orbital study of acetonitrile coordination by Purcell and Drago notes an increase of infrared adsorption intensity upon coordination of the nitrile nitrogen due to a change of the normal vibrational mode of the nitrile moiety.47 If the copper-coordinated nitrile of 8CB in our study similarly absorbs infrared radiation more strongly than the free nitrile group, the decrease in area of the copper-coordinated nitrile peak would not be compensated exactly by the gain of the free nitrile absorbance. We also note that the film of 8CB might undergo reorganization upon exposure to DMMP. As PM-IRRAS on gold substrates reports only the surface normal component of transition dipole moments, any reorganization that increases the tilt of the 8CB molecules away from the surface normal will lead to a decrease in absorbance of the nitrile stretch, which lies nominally along the long axis of the 8CB molecule.46 After the IR spectrum had been acquired in the presence of DMMP, the holder containing the sample was purged with air for 30 min, and a third spectrum was obtained (Figure 2c). In this third spectrum, we observe a return of the coppercoordinated nitrile peak and a concurrent decrease in magnitude of the free nitrile peak, suggesting that the copper-nitrile coordination is reestablished as DMMP partitions out of the 8CB film. This behavior mirrors the reversibility of the orientational transition seen in anchoring studies with 5CB (Figure 1). We note, however, that the magnitude of the coppercoordinated nitrile peak at 2287 cm-1 is larger after the transient exposure to DMMP (Figure 2c) as compared to that measured prior to exposure (Figure 2a). We speculate that spin-coating of the 8CB leads to molecular orientations within the 8CB thin film that are relaxed upon introduction and removal of DMMP. After 24 h in air, the copper-coordinated and free nitrile peaks are qualitatively similar to those prior to DMMP exposure; that is, the copper-coordinated nitrile peak is larger than the free nitrile peak. However, the exact magnitudes of the peaks vary from sample to sample after 24 h in air (data not shown). The results above provide evidence that DMMP perturbs the interaction between the copper ion and the nitrile group of 8CB. To obtain evidence of the interaction of DMMP molecules with copper(II) perchlorate salts, we measured the PM-IRRAS spectra in the 1000-1800 cm-1 region, where perchlorate and DMMP vibrations are located. Prior to exposure to DMMP (Figure 3a), two stretches visible in the IR spectrum are the coppercoordinated perchlorate peaks at 1057 cm-1 (shoulder) and 1122 cm-1. Upon exposure to DMMP (Figure 3b), we find evidence for absorption of DMMP into the 8CB via the appearance of strong bands corresponding to P-OC stretches at 1045 and 1063 cm-1 and the P-CH3 deformation at 1317 cm-1. An additional peak is observed in Figure 3b at 1198 cm-1, which we assigned to the copper-coordinated phosphoryl stretch (OdP) of DMMP.
Competitive Interactions between LCs, Metal Salts, and DMMP
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26085
TABLE 1: Vibrational Frequency (cm-1) Assignmentsa for 8CB on a Carboxylic Acid-Terminated SAM Surface Supporting Copper(II) Perchlorate Salts, before and during Exposure to 10 ppm DMMP pre-DMMP exposure assignment ν(OClO3)c
a
10 ppm DMMP frequency
ClO4-
assignment
frequency
ν(P-CO)
DMMP
1045 (sh)b
ν(P-CO) ν(ClO4)d
DMMP ClO4-
1063 1107
δ(CH2), F(CH) ν(PdO--Cu) δ(PCH3) F(CH3) ν(CdC) δ(PCH3) F(CH2) ν(CdC) ν(CdC) νa(COO-) ν(CdO) ν(CtN) ν(CtN--Cu)
MUA, 8CB DMMP DMMP 8CB 8CB DMMP MUA 8CB 8CB MUA MUA 8CB 8CB
1186 (sh) 1198 1317 1379 1402 1416 1465 (d) 1495 1606 1638 (br) 1738 2227 2278
1057 (sh)
ν(OClO3) δ(CH2), F(CH)
ClO4MUA, 8CB
1122 1186
F(CH3) ν(CdC)
8CB 8CB
1379 1402
F(CH2) ν(CdC) ν(CdC) νa(COO-) ν(CdO) ν(CtN) ν(CtN--Cu)
MUA 8CB 8CB MUA MUA 8CB 8CB
1465 (d) 1495 1606 1643 (br) 1738 2227 2287
Analysis based in part on refs 34-46. b sh, shoulder; br, broad; d, doublet. c Coordinated perchlorate. d Ionic perchlorate.
Figure 2. Middle-wavenumber IR spectra of a thin film of 8CB on a carboxylic acid-terminated SAM surface supporting copper(II) perchlorate salts (a) before and (b) during exposure to 10 ppm DMMP and (c) after a 30-min air purge.
Figure 3. Lower-wavenumber IR spectra of a thin film of 8CB on a carboxylic acid-terminated SAM surface supporting copper(II) perchlorate salts (a) before and (b) during exposure to 10 ppm DMMP and (c) after a 30-min air purge.
The shift of this peak from the uncoordinated condensed-phase phosphoryl stretch at about 1246 cm-1 is taken as evidence for coordination of DMMP to the copper atom via its phosphoryl oxygen. The observed downshift is consistent with previous studies that reported a shift in the phosphoryl peak from ∼1250 cm-1 (uncoordinated) to values between 1166 and 1212 cm-1 upon coordination of DMMP to metal ions or upon adsorption of DMMP onto metal oxide surfaces.35-43 Inspection of Figure 3b also reveals that exposure to DMMP results in a loss of the copper-coordinated perchlorate peaks at 1057 and 1122 cm-1, replaced by the ionic (uncoordinated) perchlorate peak at 1107 cm-1sa further indication of changes in coordination around the copper center. Furthermore, we observed a decrease in the asymmetric carboxylate stretch of the 11-mercaptoundecanoic acid SAM at 1643 cm-1 upon exposure to DMMP. This change in the spectrum might be due to a loss of copper coordination by the carboxyl groups of the SAM as DMMP competes for coordination of the copper ion.15 Alternately, it might reflect changes in the orientation of the carboxyl groups of the SAM (away from the surface normal), leading to a decreased absorbance of the carboxyl vibrations.
After the holder containing the sample had been purged with air for 30 min (Figure 3c), the DMMP peaks in the lowerwavenumber region were largely eliminated, although some spectral differences relative to the spectrum measured prior to exposure to DMMP (Figure 3a) were apparent, such as remaining evidence of the ionic perchlorate peak at 1107 cm-1 and the absence of the asymmetric carboxylate stretch of the SAM at 1643 cm-1. However, after 24 h of equilibration in air, the original spectrum was recovered (data not shown). Effect of DMMP on Infrared Spectra of 8CB on Nickel(II) Perchlorate Salts. Whereas 5CB supported on copper(II) perchlorate undergoes a transition from a homeotropic orientation to a tilted orientation upon exposure to DMMP, 5CB supported on nickel(II) perchlorate salts remains homeotropic when exposed to DMMP (Figure 1). We next sought to characterize the differences in the interactions of 8CB and DMMP with nickel(II) perchlorate and copper(II) perchlorate surfaces by using IR spectroscopy to provide insights into the origins of these different behaviors. IR measurements with nickel(II) perchlorate surfaces revealed [similar to copper(II) perchlorate] a peak corresponding to the free nitrile stretch at
26086 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Figure 4. Middle-wavenumber IR spectra of a thin film of 8CB on a carboxylic acid-terminated SAM surface supporting nickel(II) perchlorate salts (a) before and (b) during exposure to 10 ppm DMMP and (c) after a 30-min air purge.
2227 cm-1, as well as a nickel-coordinated nitrile peak at 2274 cm-1 (Figure 4a). The shift of the latter peak is smaller than that for the copper-coordinated nitrile at 2287 cm-1, indicating a weaker metal-nitrile interaction for nickel. Following exposure to 10 ppm DMMP (Figure 4b), we measured a decrease in the nickel-coordinated nitrile absorbance and a slight downshift in the peak position from 2274 to 2270 cm-1. Although our results indicate that the coordination interaction of nickel(II) and the nitrile group of 8CB is perturbed by the presence of DMMP, the peak corresponding to the nickel-coordinated nitrile does not decrease in magnitude to the same extent as observed for copper(II) (Figure 2a,b). After a 30-min purge in air (Figure 4c), the peak corresponding to the nickel-coordinated nitrile was observed to recover, as was also the case with the coppercoordinated nitrile (Figure 2c). We note that spectral assignments for 8CB supported on nickel(II) perchlorate salts are listed in Table 2.34-46 In the lower-wavenumber region, prior to DMMP exposure (Figure 5a), there is evidence of nickel-coordinated perchlorate at 1039 and 1134 cm-1, as well as ionic perchlorate at 1101 cm-1. Upon exposure to DMMP, we note the presence of DMMP in the thin film of 8CB by the appearance of P-OC stretches at 1043 and 1066 cm-1 and a P-CH3 deformation at 1315 cm-1 (Figure 5b). The phosphoryl stretch of DMMP is observed at 1213 cm-1, which we attribute to the nickelcoordinated phosphoryl group. These data, taken together with the observation of a decrease in the nickel-coordinated nitrile of 8CB, provide evidence that the phosphoryl group of DMMP competes with the nitrile group of 8CB for nickel coordination. The shift from the free PdO stretch at 1246 cm-1 to 1213 cm-1 on nickel(II) perchlorate surfaces, however, is smaller than the shift on the copper(II) perchlorate surface from 1246 to 1198 cm-1. This result implies that DMMP does not coordinate as strongly with nickel ions as with copper ions. In Figure 5c, we observe that, after 30 min of purging of the sample with air, the spectrum is similar to that measured prior to exposure to DMMP, in contrast to the copper(II) surface, for which the loss of DMMP from the 8CB thin film was observed to be slower. We sought to determine whether the amount of DMMP present in the thin film of LC was influenced by the identity of the metal ion. By comparing the magnitude of the 8CB phenyl stretches at 1495 and 1606 cm-1 in Figures 3b and 5b (the magnitudes of which are dependent on the 8CB film thickness
Cadwell et al. and are similar in the two spectra) to the DMMP absorption peaks in each of these figures, it is apparent that the DMMP absorbance peaks are much greater on the copper(II) perchlorate surfaces than on the nickel(II) perchlorate surfaces. In Figure 6, we compare the magnitude of the absorbance of the DMMP P-CH3 deformation at 1315-1317 cm-1 in 8CB thin films supported on copper(II), nickel(II) and sodium perchlorate salts. This peak was chosen as a measure of the total amount of DMMP present in the 8CB thin film because (i) it is relatively isolated in the spectra so that other absorptions do not interfere with its shape or magnitude and (ii) it does not shift appreciably upon coordination of the DMMP molecule via the phosphoryl group. Comparison of the peak magnitudes shows that less DMMP is present in the 8CB thin film hosted on nickel(II) perchlorate salts than for 8CB on copper(II) perchlorate salts. This suggests that the stronger coordination between copper and DMMP shifts the equilibrium between DMMP in the 8CB film and DMMP in the vapor phase such that more DMMP is absorbed into the 8CB thin film. It might also be noted that surfaces decorated with sodium perchlorate salts, which do not coordinate to the 8CB nitrile groups or DMMP, also do not show appreciable absorption of DMMP into the 8CB thin film at 10 ppm vapor concentration. At saturation levels of DMMP in the vapor phase (∼900 ppm), we obtained evidence that DMMP is present on all of these surfaces by measuring absorptions of the P-CH3 deformation at 1317 cm-1 and the uncoordinated PdO stretch at 1244 cm-1 (data not shown). 4. Discussion The results described above are consistent with our hypothesis that the coordination of nitrile-containing liquid crystals by copper(II) perchlorate salts is perturbed by DMMP, leading to the ordering transition observed with 5CB supported on copper(II) perchlorate salts. The loss of the copper-coordinated nitrile peak of 8CB in the IR spectrum at 2287 cm-1 coincident with growth in the free nitrile stretch at 2227 cm-1 upon exposure to DMMP indicates perturbation of the copper-nitrile coordination by DMMP (Figure 2). The observation of a shift of the DMMP phosphoryl stretch from 1244 to 1198 cm-1 provides evidence of phosphonate coordination to the copper ion (Figure 3). These results, along with observations of perchlorate and carboxylate coordination changes, support the proposal that DMMP molecules alter the coordination chemistry about the copper center in the copper(II) perchlorate salts and thus trigger the anchoring transition shown in Figure 1. Infrared experiments also reveal differences between the interactions of 8CB and DMMP supported on copper(II) and nickel(II) perchlorate surfaces. Whereas the infrared absorption peak of the copper-coordinated nitrile is lost almost entirely upon exposure to DMMP (Figure 2), the nickel-coordinated nitrile peak decreases only by roughly one-half of its original magnitude (Figure 4), suggesting that DMMP does not compete with 8CB as strongly for nickel as for copper. The smaller shift of the phosphoryl peak of DMMP on the nickel(II) perchlorate surface than on the copper(II) perchlorate surface provides additional evidence for a weaker coordination of the DMMP molecules to nickel(II) than to copper(II). Finally, more DMMP is found to be adsorbed into the LC thin film on the copper surfaces than on the nickel surfaces, as evidenced by the magnitude of the DMMP adsorption at ∼1317 cm-1 (Figure 6). This result also suggests that the stronger complexation of DMMP by copper changes the equilibrium between vapor-phase DMMP and DMMP adsorbed into the LC phase. We end by remarking that the molecular interactions evidenced via IR
Competitive Interactions between LCs, Metal Salts, and DMMP
J. Phys. Chem. B, Vol. 110, No. 51, 2006 26087
TABLE 2: Vibrational Frequency (cm-1) Assignmentsa for 8CB on a Carboxylic Acid-Terminated SAM Surface Supporting Nickel(II) Perchlorate Salts, before and during Exposure to 10 ppm DMMP pre-DMMP exposure assignment b
ν(OClO3)
a
10 ppm DMMP frequency
-
ClO4
assignment
frequency
1039
ν(ClO4)c ν(OClO3) δ(CH2), F(CH)
ClO4ClO4MUA, 8CB
1101 1134 1186
F(CH3) ν(CdC) νs(COO-) F(CH2) ν(CdC) ν(CdC) νa(COO-) ν(CdO) ν(CtN) ν(CtN--Ni)
8CB 8CB MUA MUA 8CB 8CB MUA MUA 8CB 8CB
1379 1402 1431 1465 (d)d 1495 1606 1640 (br) 1738 2227 2274
ν(P-CO) ν(P-CO) ν(ClO4) ν(OClO3) δ(CH2), F(CH) ν(PdO--Ni) δ(PCH3) F(CH3) ν(CdC) νs(COO-) F(CH2) ν(CdC) ν(CdC)
DMMP DMMP ClO4ClO4MUA, 8CB DMMP DMMP 8CB 8CB MUA MUA 8CB 8CB
1043 1066 1101 1136 1188 1213 1315 1379 1402 1431 1465 (d) 1495 1606
ν(CdO) ν(CtN) ν(CtN--Ni)
MUA 8CB 8CB
1738 2227 2270
Analysis based in part upon refs 34-46. b Coordinated perchlorate. c Ionic perchlorate. d sh, shoulder; br, broad; d, doublet.
Figure 5. Lower-wavenumber IR spectra of a thin film of 8CB on a carboxylic acid-terminated SAM surface supporting nickel(II) perchlorate salts (a) before and (b) during exposure to 10 ppm DMMP and (c) after a 30-min air purge.
spectroscopy appear to mirror the LC orientational transitions reported here and elsewhere8-10 upon exposure to DMMP. In the case of copper(II) perchlorate surfaces, the loss of the 8CB copper-coordinated nitrile stretch upon DMMP exposure coincides with a loss of homeotropic orientation of 5CB supported upon similar surfaces exposed to DMMP. 5. Conclusions This report provides evidence that DMMP-triggered orientational ordering transitions of nitrile-containing LCs supported on metal perchlorate surfaces are the result of competitive interactions between the nitrile groups of the LCs and phosphoryl groups of DMMP for metal ion coordination. In situ infrared experiments demonstrate that coordination of copper(II) with the nitrile groups of 8CB is largely eliminated upon exposure to DMMP, whereas coordination of nickel(II) with the nitrile groups of 8CB was reduced by only 50% upon exposure to the same DMMP concentration. IR experiments also revealed a shift of the frequency of the phosphoryl stretch of DMMP of -48 cm-1 on copper(II) perchlorate surfaces, versus
Figure 6. Magnitude of PM-IRRAS absorbance at 1315-1317 cm-1 [δ(PCH3) of DMMP] when thin films of 8CB supported on metal perchlorate salts are exposed to 10 ppm DMMP. Error bars indicate the standard deviation between samples.
a shift of -33 cm-1 on nickel(II) perchlorate surfaces, consistent with stronger coordination of DMMP by copper ions than by nickel ions. Finally, the extent of partitioning of DMMP from the vapor phase into the 8CB thin film was measured to be greater when using copper(II) perchlorate than when using nickel(II) perchlorate. We conclude that DMMP competes more effectively with the nitrile groups of LCs for copper coordination than for nickel coordination, leading to an orientational ordering transition in thin films of the liquid crystals supported on copper(II) but not nickel(II) perchlorate salts. These results also reveal PM-IRRAS to be a useful technique for in situ investigations of competitive molecular interactions in thin films of LCs, thus providing insights that can be used to guide the design and evaluation of liquid-crystal-based sensors for reporting targeted chemical analytes. Acknowledgment. This research was supported by funding from the U.S. Army Research Office (DAAD19-02-1-0299 and
26088 J. Phys. Chem. B, Vol. 110, No. 51, 2006 W911NF-06-1-0314) and NSF(CTS-0428027). The authors thank John Cannon for his assistance in constructing the flow cells used for optical observations and PM-IRRAS measurements. References and Notes (1) de Gennes, P.-G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.; Clarendon Press/Oxford University Press: New York, 1993. (2) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391. (3) Cognard, J. Mol. Cryst. Liq. Cryst., Suppl. Ser. 1982, 1, 1. (4) Shah, R. R.; Abbott, N. L. J. Phys. Chem. B 2001, 105, 4936. (5) Shah, R. R.; Abbott, N. L. J. Am. Chem. Soc. 1999, 121, 11300. (6) Luk, Y.-Y.; Yang, K.-L.; Cadwell, K.; Abbott, N. L. Surf. Sci. 2004, 570, 43. (7) Yang, K.-L.; Cadwell, K.; Abbott, N. L. AdV. Mater. 2003, 15, 1819. (8) Yang, K.-L.; Cadwell, K.; Abbott, N. L. J. Phys. Chem. B 2004, 108, 20180. (9) Shah, R. R.; Abbott, N. L. Science 2001, 293, 1296. (10) Yang, K.-L.; Cadwell, K.; Abbott, N. L. Sens. Actuators B 2005, 104, 50. (11) Evans, C. R.; Spurlin, T. A.; Frey, B. L. Anal. Chem. 2002, 74, 1157. (12) Hierlemann, A.; Ricco, A. J.; Bodenhofer, K.; Gopel, W. Anal. Chem. 1999, 71, 3022. (13) Wells, M.; Dermody, D. L.; Yang, H. C.; Kim, T.; Crooks, R. M.; Ricco, A. J. Langmuir 1996, 12, 1989. (14) Xu, C.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1993, 65, 2102. (15) Crooks, R. M.; Yang, H. C.; McEllistrem, L. J.; Thomas, R. C.; Ricco, A. J. Faraday Discuss. 1997, 107, 285. (16) Urakawa, A.; Burgi, T.; Schlapfer, H.-P.; Baiker, A. J. Chem. Phys. 2006, 124, 054717/1. (17) Wadayama, T.; Hanata, Y.; Sue¨taka, W. Surf. Sci. 1985, 158, 579. (18) Wadayama, T.; Monma, K.; Sue¨taka, W. J. Phys. Chem. 1983, 87, 3181. (19) Ozensoy, E.; Meier, D. C.; Goodman, D. W. J. Phys. Chem. B 2002, 106, 9367. (20) Humblot, V.; Methivier, C.; Pradier, C.-M. Langmuir 2006, 22, 3089. (21) Hess, C.; Ozensoy, E.; Yi, C.-W.; Goodman, D. W. J. Am. Chem. Soc. 2006, 128, 2988. (22) Beitel, G. A.; Laskov, A.; Oosterbeek, H.; Kuipers, E. W. J. Phys. Chem. 1996, 100, 12494. (23) Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of Infrared Spectroscopy of Ultrathin Films; John Wiley & Sons: New York, 2003. (24) Frey, B. L.; Corn, R. M.; Weibel, S. C. Polarization-Modulation Approaches to Reflection-Absorption Spectroscopy. In Handbook of
Cadwell et al. Vibrational Spectroscopy; Chalmers, J., Griffiths, P. R., Eds.; John Wiley & Sons: New York, 2001; Vol. 2, p 1042. (25) Golden, W. G. Fourier Transform Infrared Reflection-Absorption Spectroscopy. In Fourier Transform Infrared Spectroscopy: Applications to Chemical Systems; John, R., Ferraro, L. J. B., Eds.; Academic Press: New York, 1985; Vol. 4, p 315. (26) Kim, S. R.; Teixeira, A. I.; Nealey, P. F.; Wendt, A. E.; Abbott, N. L. AdV. Mater. 2002, 14, 1468. (27) Bloss, F. D. An Introduction to the Methods of Optical Crystallography; Holt, Rinehart and Winston: New York, 1961. (28) Simple average of the ordinary and extraordinary indices of refraction for 8CB near room temperature.29,30 (29) Matsuhashi, N.; Okumoto, Y.; Kimura, M.; Akahane, T. Jpn. J. Appl. Phys. 2002, 41, 4615. (30) Dunmur, D. A.; Manterfield, M. R.; Miller, W. H.; Dunleavy, J. K. Mol. Cryst. Liq. Cryst. 1978, 45, 127. (31) The refractive indices for cobalt(II) perchlorate hexahydrate and nickel(II) perchlorate hexahydrate crystals have been reported as n ) 1.55.32 The refractive index for magnesium perchlorate hexahydrate crystals has been reported as n ) 1.482.33 (32) Washburn, E. W., Ed. International Critical Tables of Numerical Data, Physics, Chemistry and Technology, 1st electronic ed.; Knovel: Norwich, NY, 2003; Vol. 2006. http://www.knovel.com/knovel2/toc.jsp? BookIC=735&verticalID=0. (33) Patnaik, P. Handbook of Inorganic Chemicals; McGraw-Hill: New York, 2003. (34) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (35) Karayannis, N. M.; Mikulski, C. M.; Strocko, M. J.; Pytlewski, L. L. Inorg. Chim. Acta 1974, 8, 91. (36) Mikulski, C. M.; Karayannis, N. M.; Pytlewski, L. L. J. LessCommon Met. 1977, 51, 201. (37) Karayannis, N. M.; Mikulski, C. M.; Strocko, M. J.; Pytlewski, L. L.; Labes, M. M. Inorg. Chim. Acta 1974, 8, 91. (38) Kim, C. S.; Lad, R. J.; Tripp, C. P. Sens. Actuators B 2001, 76, 442. (39) Kanan, S. M.; Lu, Z.; Tripp, C. P. J. Phys. Chem. B 2002, 106, 9576. (40) Aurian-Blajeni, B.; Boucher, M. M. Langmuir 1989, 5, 170. (41) Karayannis, N. M.; Mikulski, C. M.; Gelfand, L. S.; Pytlewski, L. L. J. Inorg. Nucl. Chem. 1978, 40, 1513. (42) Scheide, E. P.; Guilbault, G. G. J. Inorg. Nucl. Chem. 1971, 33, 1689. (43) Li, Y. X.; Schlup, J. R.; Klabunde, K. J. Langmuir 1991, 7, 1394. (44) Bowen, J. M.; Powers, C. R.; Ratcliffe, A. E.; Rockley, M. G.; Hounslow, A. W. EnViron. Sci. Technol. 1988, 22, 1178. (45) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.; Winograd, N. J. Phys. Chem. B 2000, 104, 3267. (46) Hayashi, S.; Kurita, K.; Kimura, N.; Umemura, J.; Takenaka, T. Bull. Inst. Chem. Res., Kyoto UniV. 1985, 63, 276. (47) Purcell, K. F.; Drago, R. S. J. Am. Chem. Soc. 1966, 88, 919.
