9036
J. Phys. Chem. B 2002, 106, 9036-9043
Polymer Conversion Measurement of Diacetylene-Containing Thin Films and Monolayers Using Soft X-ray Fluorescence Spectroscopy Christine E. Evans,† Amethyst C. Smith,† Daniel J. Burnett,† Anderson L. Marsh,‡ Daniel A. Fischer,§ and John L. Gland*,† Chemical Engineering Department and Chemistry Department, UniVersity of Michigan, Ann Arbor, Michigan 48109, and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: July 16, 2001; In Final Form: January 15, 2002
Polymer formation within ordered monolayer and thin-film assemblies is measured using fluorescence yield near-edge spectroscopy (FYNES). This spectroscopic method measures soft X-ray absorption in the 270330-eV energy range, probing core-electron transitions to unfilled molecular orbitals by detecting the resultant fluorescence emission. By observing transitions in the carbon 1s-π* region (280-286 eV), the diminution of monomer and formation of polymer are measured directly and simultaneously. In contrast with spectroscopic methods measuring partial electron yield, film damage caused by neutralization of sample charging is eliminated with FYNES. Using FYNES, monomer-to-polymer conversion efficiencies for ultrathin films and monolayer assemblies are successfully measured for the first time. Measured conversion efficiencies for diacetylenecontaining assemblies is near 80%, demonstrating remarkable agreement among ultrathin films, surfaceattached configurations, and previous studies of thicker film structures. Likewise, polymerization kinetics for these assemblies are also similar, exhibiting at least two distinct rate regimes. These results indicate that the polymerization properties of diacetylenes are not significantly dependent on molecular architecture and that induced-dipole interactions between neighboring diacetylenes predominate.
Introduction The combination of spontaneous self-assembly strategies with polymer chemistry provides numerous possibilities for the fabrication of robust interfacial materials. Polydiacetylenes, with their highly conjugated polymer backbone structure, are of particular interest for their optoelectronic, sensor, and frictional properties.1-3 First reported over thirty years ago by Wegner,4 diacetylene-containing compounds have been shown to polymerize within a wide variety of architectures ranging from thick and thin films to crystals, bilayers, and single molecular layers.5-19 Cross-linking of adjacent diacetylene-containing molecules into a linear polymer likely occurs via a 1,4-diradical mechanism after exposure to UV radiation, heat, or γ radiation. As illustrated in Figure 1, the spatial constraints for successful cross-linking are considerable.9,10 Nonetheless, diacetylenes are successfully polymerized within a wide range of architectures. Previous studies of monolayer assemblies indicate that the interaction between adjacent, electron-rich diacetylenes creates a noncovalent molecular scaffolding within a single molecular layer.3,16 When unhindered, such interactions lead to fortuitous diacetylene alignment, allowing polymerization within widely varying structural environments. Clearly, this alignment can be disrupted by simple steric constraints of pendant groups, but less evident mechanisms can also limit polymer formation. For example, recent monolayer studies demonstrate that the odd/ even nature of the methylene chain attached to the surface plays a major role in polymer formation.17 * Corresponding author. E-mail:
[email protected]. † Chemical Engineering Department, University of Michigan. ‡ Chemistry Department, University of Michigan. § National Institute of Standards and Technology.
Figure 1. Schematic of polymerization reaction highlighting the rigorous spatial constraints for polymer formation. For the reaction to proceed d1 ) d2 ) 4.7-5.2 Å.
As might be expected, characterization of these thin-film and monolayer polymers is a difficult and enduring challenge. The limited material that is present severely constrains the use of conventional polymer characterization methods. Electron delocalization within the polydiacetylene backbone has been evaluated by visible5-7,18 and resonance Raman spectroscopy.6,10,15,19 These measurements reveal the presence of several chromatic phases with varying degrees of electron delocalization. Among these phases, the interconversion between the visibly blue and red phases is perhaps the most interesting. Estimates based on Kuhn’s model20 show a minimum delocalization length of 2550 monomer units for the longer conjugation length, blue-phase polymer.15 This measurement provides only a minimum value for the conjugation length, with no indication of the actual polymer length. Likewise, the resonance nature of the Raman measurement precludes determination of the monomer content, hindering evaluation of the monomer-to-polymer conversion efficiency. In contrast with resonance Raman spectroscopy that detects only delocalized states, core-electron measurements are localized on individual atoms, allowing simultaneous observation of both monomer and polymer phases. These methods allow the determination of the specific types of bonds as well as the measurement of their relative populations. In characterizing
10.1021/jp0127216 CCC: $22.00 © 2002 American Chemical Society Published on Web 08/14/2002
Polymer Conversion Measurements of Thin Films polydiacetylene (PDA) thin films, only electron energy loss spectroscopy (EELS)21 and X-ray absorption near-edge spectroscopy (XANES)22,23 studies are documented in the literature. These studies suffer from structural and bonding perturbations during the measurement, and as a result, difficulties with reproducibility were observed. Likewise, our preliminary measurement attempts using electron yield detection for near-edge X-ray absorption fine structure spectroscopy (NEXAFS) measurements with charge neutralization were not reproducible. At first glance, it is unclear whether difficulties arise from damage induced by the incident X-ray beam or from the low-energy electron gun used to neutralize sample charging. Fluorescence yield near-edge spectroscopy (FYNES) at the carbon K edge provides a powerful core-electron method for characterizing polymeric materials, which are generally susceptible to electron beam damage. As with NEXAFS, significant bonding information is obtained by probing the core-electron transitions to unfilled molecular orbitals. The outstanding sensitivity of FYNES enables coverages below 1013 molecules/ cm2 to be detected,24 ensuring that fundamental bonding configurations can be determined even within single monolayers. Not only can the type of bond be assessed, but the quantitative nature of FYNES allows direct measurement of the relative populations of specific bond types. Moreover, in contrast to electron-yield measurements, the 1000-fold increase in the mean free path of the detected soft X-rays (1-10 µm) provides nearuniform response throughout the bulk of these films. These unique capabilities of FYNES afford direct comparison of polymer conversion within thin-film and monolayer assemblies. Using this measurement scheme, the extent of polymerization in diacetylene-containing thin films and monolayer assemblies is successfully demonstrated for the first time. Experimental Methods Thin Film Fabrication. Platinum films were prepared using an electron beam, vapor deposition system (Kurt J. Lesker Co., Enerjet model). Prior to platinum deposition, Si(100) wafer substrates were cleaned with piranha solution (1:3 30% H2O2/ concentrated H2SO4). Caution: Piranha solution is a powerful oxidizing agent and reacts violently with organic compounds. Immediately after cleaning, approximately 2000 Å of Pt was deposited onto Ti-primed (∼50-100 Å) wafers. The wafers were then diced into 1 cm × 1 cm pieces and cleaned again with piranha solution. Thin organic films were prepared by rinsing the platinum substrates with pure chloroform (Aldrich, 99.9%), followed by vertical flow deposition of a 1-mM chloroform solution of 10,12-pentacosadiynoic acid (CH3(CH2)11CtC-CtC(CH2)8COOH, GFS Chemicals, 99%). The final thickness of the films was estimated by atomic force microscopy (Nanoscope IIIA; Digital Instruments) to be approximately 0.1 µm. Strict light control was maintained during the preparation and storage of the diacetylene solution and thin films. Films were polymerized using a low-intensity pen lamp (Ultra-Violet Products Inc., model 11SC-1) held approximately 2 cm from the film surface. The intensity at 254 nm specified by the manufacturer at this distance is 4.4 mW/cm2. The chromatic phase of each thin film was monitored by reflection UV-vis absorbance spectroscopy, with the unpolymerized film as the reference. UV spectra obtained for these films are very similar to those reported previously in the literature.5-7,18 The maximum levels of blue- and red-phase polymer occurred reproducibly at 4 and 30 min polymerization time, respectively. Monolayer Fabrication. Gold films were prepared using a custom-built UHV deposition system. Mica (ASTM V-2;
J. Phys. Chem. B, Vol. 106, No. 35, 2002 9037 Asheville-Schoonmaker Mica Co.) was cleaved on both sides immediately before insertion into the UHV chamber. The mica substrates were suspended 21 cm above the source and approximately 8 cm above two 500-W halogen lamps. Before deposition, the lamps and heat tapes were used to bake out the chamber for 12-24 h at a substrate temperature of 435 °C to pressures that were less than 1 × 10-8 Torr. After turning off the heat tapes and lowering the power to the lamps, the substrate temperature cooled to 380 °C, and gold (99.99%) was vapordeposited at 1300 °C from a K cell (Oxford Instruments) at a rate of 2 Å/s to a final thickness of ∼4000 Å (Leybold Inficon Inc.). Substrates were annealed for 4 h at the deposition temperature and then were allowed to cool to room temperature (P ) 2 × 10-10 Torr). The chamber was then back-filled with nitrogen, and the substrates were removed. The resultant gold films were immediately immersed in a 1-mM chloroform solution (Aldrich, >99.9%) of dinonacosa-10,12-diyndisulfide, [CH3(CH2)15CtC-CtC(CH2)9S-]2. Synthesis of this diyne, designated 15,9-DA, proceeded via a Cadiot-Chodkiewicz coupling and has been previously reported.25 After monolayers formed from the solution that was held at room temperature for 40-48 h, substrates were removed from the solution, rinsed with chloroform and deionized water (Millipore, >18 MΩ), and dried with nitrogen. The resulting diacetylene monolayers were polymerized under nitrogen with a low-intensity UV lamp (model UVG-11, Ultra-Violet Products Inc.; λ ) 250-260 nm) at a distance of 2 cm. Lamp intensity at this distance is estimated to be 4 mW/cm2 by the manufacturer. Strict light control was maintained during the preparation and storage of the diacetylene solutions and monolayer films, and no photooxidation was observed. Fourier Transform Infrared Spectroscopy. External reflection FTIR experiments were performed using a nitrogen-purged spectrometer with a liquid nitrogen-cooled MCT detector (Nicolet 550 Magna IR). Spectra were obtained using a grazingincidence angle of 77° (Spectra-Tech Inc.) and p-polarized light. Using a resolution of 2 cm-1, 1024 and 256 scans were collected for the monolayer and thin films, respectively. Absorbance spectra were determined using an unmodified gold surface as a reference for the monolayers and a platinum surface as a reference for the thin films. To minimize reference film contamination, the reference spectra for the monolayers were obtained immediately after removing the substrates from the deposition system. Platinum substrates were rinsed with pure chloroform immediately before obtaining the reference spectrum. The primary features observed in this C-H stretching region of the spectrum are the symmetric methylene stretch at 2851 cm-1, Fermi resonance with the methyl symmetric stretch at 2878 cm-1, asymmetric methylene stretch at 2919 cm-1 with a shoulder at higher energy (∼2928 cm-1) corresponding to reduced methylene chain-chain interaction in the region immediately adjacent to polymer,16 and the out-of-plane asymmetric methyl stretch at 2963 cm-1. All monolayer and thinfilm samples show that alkyl chains reside in a highly constrained and ordered environment. Fluorescence Yield Near-Edge Spectroscopy. FYNES experiments were conducted at the U7A beamline at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory. A toroidal-mirror, spherical grating monochromator was utilized along with a fluorescence-yield detector optimized for carbon fluorescence.26,27 Preliminary studies using electron-yield NEXAFS measurements were limited by significant charging problems, and all attempts at neutralization resulted in varying degrees of damage to the films. Damage
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TABLE 1: FYNES Curve-Fitting Parameters (eV) and Peak Assignmentsa thin films
monolayers
peak energy fwhm peak energy fwhm (1) 283.93 (2) 284.63 (3) 285.21 (4) 285.50 (5) 287.37 (6) 292.29 (7) 299.45
0.86 0.57 0.77 0.73 1.48 4.67 8.40
283.93 284.63 285.21 285.50 287.23 291.81 298.63
0.86 0.57 0.77 0.73 1.12 5.00 9.03
assignment conjugated, polymer, π* outer C, monomer, π* inner C, monomer, π* nonconjugated, polymer, π* C-H σ* C-C σ* C-C σ*
a Significant figures have been truncated to represent measurement resolution more accurately. As a result, these data should not be used to reproduce the spectral curve fits.
was evidenced by dramatic changes in both the energy and intensity of the resultant electron-yield spectra together with substantial peak broadening. In addition, FTIR spectra taken after neutralization exposure show large increases in alkyl chain disorder as well as a significant loss of material. This damage occurred even when using a relatively low-energy (15 V) electron flood gun to minimize surface charging. Recent papers characterize this damage in further detail for saturated and unsaturated alkyl interfaces.28-30 In contrast, fluorescence-yield measurements do not require neutralization because photons are not affected by surface charging.26 Repeated FYNES measurements at the same location on the films shows no energy or intensity changes and also no peak broadening. To ensure that the incident beam itself was not causing damage to the films, four spectra were taken in succession without moving the sample. For the monolayer films, no changes were observed from the first to the last spectrum. The thin films, however, were polymerized to a small extent by the X-ray beam after multiple spectra were taken at the same location. In practice, no more than three spectra were obtained from each location on a monolayer, and only one spectrum was taken from each location on a thin film. Throughout these studies, we were careful to minimize X-ray exposure to the films, and X-rayinduced polymerization was eliminated. For the monolayer films, the total spectral acquisition time was approximately 15 and 8 min for the thin films. An incident E vector of 55° from the surface normal (magic angle) was used for all spectra, thus allowing direct spectral comparison without molecular orientation considerations. All spectra were taken with 30-µm entrance and exit slits, resulting in a spectroscopic resolution of 0.1 eV. For all spectra reported here, the fluorescence intensity is normalized to the ring current Io, which is measured at a gold grid upstream of the experimental chamber. An evaporator is located on the Io chamber to allow evaporation of fresh gold on the grid prior to FYNES measurements. A second gold grid, located upstream from the Io chamber, was used for energy calibration. This grid is coated with a thin film of graphite that acts as an energy standard, with the incident photon energy calibrated on the basis of the C π* transition from graphite at 284.38 eV. Data were collected at 0.2-eV intervals from 270 to 330 eV, with an integration time of 4 s for monolayers and 2 s for thin films. After subtracting the carbon-edge function for the detector at 288 eV,31 peaks were fit using a Gaussian function. In choosing the constraints for peak fitting, it is assumed that the local environment determines the transition energies and peak widths (vide infra). This local model provides the conceptual basis for constraining the energy and peak width of each local transition. Curve fitting was constrained by fitting identical peak widths and energies for all thin films at all polymerization times (Table 1). Spectral fitting for all monolayer
Figure 2. External reflection FTIR spectrum of the CH stretching region of a 15,9-DA monolayer. Methylene asymmetric and symmetric stretching transitions at 2919 and 2851 cm-1, respectively, indicate a highly ordered alkyl chain structure within the monolayer. Spectra taken on the same films before and after FYNES measurements confirm that no alteration in film structure is caused by the measurement process.
films was also constrained with these identical peak widths and energies in the π* region (Table 1). Significant figures in Table 1 have been truncated to represent the measurement resolution more accurately. As a result, these data should not be used to reproduce the spectral curve fits. In general, the fluorescence intensity for the monolayer films was less than 1/10 the signal of the thin films. As a result, the background for the monolayer spectra was corrected for modest scattered light interference by adjusting the baseline between 270 and 283 eV to match the lowest scattered light level. This data treatment is valid as an approximation because the majority of scattered light is encountered below ∼283 eV, where the detector window is transparent to scattered light.31 Results and Discussion This paper focuses on highly ordered assemblies of diacetylene-containing molecules and the resultant polymeric films formed upon UV irradiation. Polymerization is evaluated here for both thin-film (R′ ) CH3(CH2)11; R ) (CH2)8COOH) and monolayer (R′ ) CH3(CH2)15; R ) (CH2)9S) assemblies (see Figure 1). Methylene chain order within these structures is evaluated using FTIR. As shown for the monolayer polymers in Figure 2, FTIR spectra of the CH stretching region show prominent methylene asymmetric and symmetric transitions at 2919 and 2851 cm-1, respectively. These transition frequencies are indicative of a highly constrained, crystalline structure of alkyl chains in an all-trans configuration.32 As previously shown, no significant perturbation in this high degree of order is observed upon polymerization.15 This result is consistent with the minimal spatial perturbation and absence of residual products expected from the diradical polymerization process (Figure 1).10,11,33 Likewise, the thin films examined here also exhibit highly crystalline alkyl chains with asymmetric and symmetric stretching transitions at 2918 and 2850 cm-1, respectively. Thus, FTIR spectroscopy shows that the pendant alkyl chains in both architectures are tightly constrained and very highly structured. However, symmetry within the polymer backbone structure leads to no dipole change upon excitation. As a result, no significant information about bonding within the polymer
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J. Phys. Chem. B, Vol. 106, No. 35, 2002 9039
Figure 3. Magic angle FYNES spectrum of a polymerized 10,12-pentacosadiynoic acid thin-film with peak assignments. Polymerized for 30 min, this thin film shows four distinct π* transitions together with C-H and C-C σ* transitions. Gaussian fits for each transition are illustrated with dotted lines, with the exact energies for each transition given in Table 1. In addition to the molecular transitions, the atomic-like K-edge transitions are shown with a dashed line.
structure is accessible with FTIR. The limited sensitivity of most conventional polymer characterization methods makes determination of bonding within the polymer backbone of these ultrathin films intractable. High-sensitivity, core-level spectroscopies can provide chemical bonding information that is inaccessible with conventional methods. Most of these core-level measurement methods are based on electron emission. For insulating samples, neutralization of accumulated charge is required to minimize differential charging and enhance detection sensitivity. Typically accomplished using low-energy electrons, significant changes in bonding are caused by bombardment even with the 5-15-eV electrons that are necessary for neutralization.28-30 Preliminary electron-yield studies of these ultrathin polymers clearly indicate substantial damage during the measurement process (see Experimental Methods). For this reason, all spectra reported here are fluorescence-yield measurements, where no electron neutralization is necessary. These fluorescence-yield, soft X-ray absorption measurements directly probe the energy and number of π bonds within the polymer backbone. As a result, FYNES provides key bonding information without compromising the structure of these ultrathin samples. Indeed, FTIR measurements indicate no disruption in these highly ordered monolayer and thin-film assemblies after the FYNES measurements (including exposure to UHV conditions, X-ray exposure, and general handling). Such consistency in film structure throughout the experiment is essential for the reliability of these core-electron measurements. For the first time, we take advantage of the unique capabilities of FYNES to characterize bonding changes upon polymerization within ultrathin film assemblies. FYNES Peak Assignments. A representative FYNES spectrum of a polymerized thin film is illustrated in Figure 3 for the energy range from 270 to 330 eV, with peak assignments and fitting parameters given in Table 1. All spectra were recorded for an incident E vector 55° from the surface normal (magic angle) to allow direct spectral comparison without molecular orientation considerations. General assignment of the
peaks is straightforward, with transitions below ∼286 eV assigned to π* absorptions. On the basis of near-edge X-ray absorption spectroscopy studies of simple hydrocarbon monolayers, the peak at 287.4 eV is attributed to a C-H σ* transition, and the peaks at 292.3 and 299.5 eV are attributed to C-C σ* transitions.34-36 Consistent with the commonly accepted practice, specific peak assignments are based on spectra of similar materials. In this study, acetylene multilayers were chosen because they are identical in structure to two of the bonds in the films of interest here. The monomer in both the thin-film and monolayer samples (zero polymerization time) acts as an additional standard. Energy calibration was identical for the standards and the polymerized films. Indeed, all measurement conditions were identical and were conducted on the same apparatus with the same detector. Accurate determination of peak energies and intensities requires curve fitting for the overlapped peaks in the π* region. For such complex spectra, choosing the appropriate fitting constraints is particularly important. In this study, a continuous shift in π* transition energies with polymer formation might be expected from delocalized behavior or low measurement resolution. However, this behavior was not observed. Instead, new peaks appeared upon polymerization, and existing peak energies and peak widths measured for the unpolymerized and polymerized standards were identical. From this observation, it can be surmised that the transitions appear to be sufficiently localized such that the local environment determined the transition energies and peak widths. This local model provides the conceptual basis for constraining the energy and peak width of each local transition. The consistency and quality of fitting provides further evidence for the validity of this simple localized approach. The localized nature of these transitions is further highlighted by the identical behavior of thin films and monolayers that have thicknesses that differ by a factor of ∼50. In assessing the π* region, it is important to consider the two primary perspectives utilized in evaluating core-hole transitions. In the initial-state framework, the conditions present
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Figure 4. Polymerization of thin films. The effect of UV polymerization time on the magic angle FYNES spectra of 11,8-DA/COOH thin films is shown. From the inset, monomer transitions are observed at 284.6 and 285.2 eV with new transitions appearing at 283.9 and 285.5 eV upon polymerization. Spectra above correspond to films polymerized for 0 (s), 4 (- - -), and 30 (‚‚‚) min.
before core-hole formation predominate and are presumed not to change upon excitation.37 For polyenes, this so-called “frozenorbital” picture predicts a band structure that broadens as the conjugation length increases. Specific predictions for polydiacetylenes place this energy broadening in the π* region at g3 eV.38-40 In contrast, the final-state framework considers conditions after core-hole formation. That is, creation of a positive, partially charged state upon core-hole formation directly affects the energetics of the unoccupied orbitals. In the simplest terms, Coulombic attraction acts to decrease the energy of the π* transition. This transition-energy decrease is mediated by electron screening between orbitals, with higher electron density leading to more screening and higher transition energies. As a result, predictions for polyenes based on this final-state framework indicate variations in transition energies with atomic position along the polyene chain.41 In contrast to the initialstate framework, no broadening is predicted for transition energies in the π* region. Although the limited predictive literature for polydiacetylenes primarily emphasizes the initialstate framework, wide-ranging studies in interaction and reaction chemistry point to the validity of the final-state model.42 The reality of core-hole transitions is, of course, somewhere between these two extremes, and theoretical studies are focused on this challenging problem.41,43,44 Together with the theoretical framework, specific assignments in the π* region are facilitated by a time study of thin films polymerized for 0, 4, and 30 min (Figure 4). These polymerization times correspond to a diacetylene film (unpolymerized), a blue-phase polymer, and a red-phase polymer, respectively. Because spectra collected at the magic angle are independent of orientational changes, spectral differences with polymerization time directly reflect changes in the bonding structure. As expected from the bonding changes that accompany polymerization (see Figure 1), only the π* region of the spectrum shows significant changes upon polymer formation. The dominant transitions in the π* region of the unpolymerized film (0 min)
Figure 5. Schematic illustration of the conjugated and unconjugated π components of the polymerized diacetylene backbone. Because these two components are orthogonal, they are treated independently in the FYNES analysis.
are at 284.6 and 285.2 eV and serve as a reference for the assignment of the conjugated triple bond in the diacetylene monomer. On the basis of a final-state model, energy differences can be discussed in terms of core-hole screening relaxation effects.41,42,45 In this case, the two π* transitions observed for the monomer likely arise from electron density differences of the inner and outer carbons of the diacetylene unit. That is, upon formation of a core hole, the energy of the π* transition decreases because of Coulombic attraction. This energy contraction, however, is mediated by electron screening, with the higher electron density inner carbons screening to a greater extent than the outer carbons. As a result, the inner and outer carbons are expected to be inequivalent, with the transitions on the inner carbons at higher energy (285.2 eV) and those on the outer carbons at lower energy (284.6 eV). The measured inner/outer carbon energy splitting of 0.6 eV is in good agreement with 0.66 eV observed for butadiene.45 As cross-linking occurs and polymer is formed, the intensities of the monomer transitions decrease, and peaks appear simultaneously at lower (283.9 eV) and higher (285.5 eV) energy. The π* transition attributed to the conjugated part of the polymer backbone is expected to occur at lower energy than that of the diacetylene monomer as a result of decreased screening in the more delocalized polymer. In fact, the conjugated portion of
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J. Phys. Chem. B, Vol. 106, No. 35, 2002 9041
Figure 6. Polymerization of monolayers. The effect of UV polymerization time on the magic angle FYNES spectra of 15,9-DA/PDA monolayers is shown. From the inset, monomer transitions are observed at 284.6 and 285.2 eV with new transitions appearing at 283.9 and 285.5 eV upon polymerization. Spectra above correspond polymerization times of 0 (s), 7 (- - -), 30 (‚‚‚), and 60 (_ ‚ _ ‚ _) min.
polydiacetylene (Figure 5) is directly analogous to polyacetylene because the nonconjugated p orbitals in PDA are orthogonal and are not expected to contribute, that is, the conjugated part of the PDA polymer backbone comprises alternating single and double bonds, consistent with polyacetylene. The π* transition for PDA at 283.9 eV is consistent with the measured polyacetylene transition of 284.1 eV.46 As expected, the inequivalence of inner and outer carbons disappears upon polymerization. Moreover, energy broadening that is expected on the basis of the initial-state model is not observed, further supporting the predominate nature of the final-state model. The higher-energy transition measured for PDA at 285.5 eV is likely attributed to the localized, unconjugated acetylenic orbitals. Because of the isolated nature of the nonconjugated p orbitals, this transition energy is expected to be consistent with the π* transition energy of acetylene. Using acetylene as a standard, FYNES measurements of cryogenically condensed acetylene multilayers were made in a separate experiment with the same detector and energy calibration. Further details of this measurement are given in ref 47. The resultant acetylene π* transition energy of 285.4 eV is in excellent agreement, confirming the unconjugated, acetylenic assignment. As shown in Figure 6, FYNES spectra of the surface-attached monolayers are remarkably similar to the thin-film spectra. In the π* region, monomer assemblies show two overlapped peaks at 284.6 and 285.2 eV that are identical in energy to the thinfilm measurements. Likewise, upon photopolymerization, the polymer transitions in the π* region evolve identically, appearing at 283.9 and 285.2 eV. (Table 1) Even with the considerable constraint of fitting this region with parameters identical to those for the thin films, the curve fitting is excellent. Minor deviations in the C-C σ* transition energies between the thin films and monolayers are insignificant on the basis of the large width of these transitions. In both monolayer and thin-film spectra, no energy broadening is observed with polymer formation in the π* region. Conjugation Length. Using theoretical predictions based on static exchange calculations (STEX) and electronic relaxation
using a ∆SCF process,41 a lower limit can be estimated for the conjugation length of the polymer backbone. Short polyenes are expected to have a range of π* transition energies as a function of position along the polymer chain arising primarily from differences in electron screening from electron density variations. For a polyene with five ethylene units, five transitions (one for each chemically distinct carbon) ranging from 283.5 to 285.1 eV are expected, with the mid-atom transition at 284.2 eV. The end atom and its nearest neighbor serve to define the range of energies. As the polymer length approaches 10 ethylene units, transition energies converge near 283.3 and 284.0 eV for end atoms and mid atoms, respectively, but no value is given for the end-atom nearest neighbor. Because the end and mid atoms converge to 0.2 eV below the five ethylene polyene energies, it is reasonable to assume that the energy shift for the nearest-neighbor transition is similar. This convergence leads to an ensemble of transitions ranging from 283.3 to 284.9 eV for polyene chains g10 ethylene units in length, with a midatom energy of 284.0 eV. In the monolayer and thin-film systems studied here, the measured π* transition for the conjugated polyene in polydiacetylene is centered at 283.9 eV. This value is in excellent agreement with the predicted mid-atom convergence to 284.0 eV and indicates that the polymers measured here have g10 ethylene units. In addition, the measured width at half-height of the conjugated π* transition is 0.86 eV. It is clear from this modest width that the end carbons and their nearest neighbors are not represented in the observed transition. From a population ensemble perspective, the end atoms and nearest neighbors that define the energy extremes can be presumed to contribute less than ∼10% of the measured intensity. This constraint is consistent with a polyene chain of at least 20 ethylene units or a diacetylene chain length of at least 10 monomers. Previous resonance Raman measurements, together with Kuhn’s electronic delocalization model,20 provide a lower-limit estimate for the blue-phase polymer conjugation length of 25-50 monomer units.15 Unfortunately, these are truly lower-limit estimates that,
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Evans et al. approximate polymerization time associated with the chromatic transition from blue- to red-phase polymer. This observation is in general agreement with previous multilayer measurements.7 This agreement indicates similar rate-limiting processes for polymerization over a surprisingly wide range of molecular architectures. The monomer-to-polymer conversion measurements demonstrated here represent the first that have been successfully accomplished for thin films and monolayer assemblies. Conclusions
Figure 7. Polymerization efficiency of a diacetylene monolayer on the basis of the integrated intensities of π* peaks 2 and 3. An initially high rate of polymer formation is observed while the blue-phase polymer dominates. After the blue-to-red phase transition at 7 min of UV exposure, the monomer-to-polymer conversion rate slows. The lines shown are point-to-point and are added only to clarify the trend. Shown here for the monolayer, the diacetylene thin-film behavior is similar.
although in nominal agreement, do not provide insight into the average conjugation length of the polymer. Monomer-to-Polymer Conversion. The efficiency of converting monomer to polymer is a primary figure of merit in characterizing polymeric materials. Conversion efficiency, the fraction of monomer reacted to form polymer, is commonly measured using chromatographic and gravimetric methods. The minimal amount of material present within monolayers and thin films precludes the implementation of these methods, and to our knowledge, there is presently no alternative method for determining the monomer-to-polymer conversion within monolayers or thin films. Using the sensitive measurement of monomer transitions in the presence of polymer provided by FYNES, conversion efficiency can be determined directly. The integrated intensity of each monomer transition (at 284.6 and 285.2 eV) is used individually to assess conversion. In these determinations, the unpolymerized monolayer defines 100% monomer (0% conversion), and 0% monomer is expected at 100% conversion. Using this method, the measured thin-film conversion of monomer to polymer at 30 min of UV exposure is approximately 80and 90% on the basis of peaks 2 and 3, respectively. This thin-film conversion is in agreement with the ∼80% conversion for bulk phase 9,8-PDA/COOH6 but is slightly greater than the ∼70% conversion previously reported for multilayer assemblies of cadmium salts of 11,8-PDA/COO-.5 Monolayer studies of conversion percentage with polymerization time are illustrated in Figure 7. The lines in this Figure are point-to-point and are added only to clarify the trends. The highest measured conversion for the monolayer assemblies of ∼70% is in good agreement with multilayer studies. Both monomer transitions exhibit an initially high conversion rate (4-6.5%/min) followed by a slower conversion rate (0.2-0.7%/min), which is consistent with general bulk-phase measurements of diacetylene polymerization.7 This trend is also observed in the thin-film measurements, which show initial rates of 8-10%/min followed by slower rates of ∼2%/min. (Although not shown here, the polymer transition represented by the integrated intensity of peak 4 exhibits the same trend with polymerization time, further validating peak assignments). The range of measured conversion rates between peaks 2 and 3 is well within the expected measurement precision. For both monolayer and thin-film architectures, the decrease in conversion rate occurs at the
Characterization of polymer formation within ultrathin films and single molecular layers has been previously hindered by the limited amount of material in these interfacial assemblies. Studies here demonstrate the unique capabilities of FYNES for measuring polymer conversion within these ultrathin layers. Using the molecular specificity and quantitative nature of this core-level spectroscopic method, we can identify monomer and polymer transitions and determine polymer conversion efficiency directly. Moreover, these measurements are accomplished without significant film damage or bond disruption, leading to highly reproducible determinations and allowing this method to be implemented in conjunction with other methodologies. Polymer conversion results for these diacetylene-containing films show remarkable similarities in total conversion and general kinetic trends over a wide range of architectures. These observations indicate that interactions between neighboring diacetylenes predominate within widely varying molecular structures and appear to be the determining factor in polymer conversion efficiency and kinetics. More detailed, in situ kinetic studies will be required to elucidate the universality of this polymerization mechanism fully. Nonetheless, these studies demonstrate the untapped potential of FYNES for molecular characterization of polymer thin films and monolayers. Acknowledgment. This work was supported in part by the National Institutes of Health, General Medicine Sciences (GM52555-01 A1). Research was performed, in part, at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Material Sciences and Division of Chemical Sciences. References and Notes (1) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207. (2) Pan, J.; Charych, D. Langmuir 1997, 13, 1365. (3) Mowery, M. D.; Kopta, S.; Ogletree, D. F.; Salmeron, M.; Evans, C. E. Langmuir 1999, 15, 5118. (4) Wegner, G. Z. Naturforsch., B: Chem. Sci. 1969, 24, 824. (5) Tieke, B.; Lieser, G. J. Colloid Interface Sci. 1981, 83, 230. (6) Tieke, B.; Bloor, D.; Young, R. J. J. Mater. Sci. 1982, 17, 1156. (7) Tieke, B.; Lieser, G. J. Colloid Interface Sci. 1982, 88, 471. (8) Day, D.; Lando, J. B. Macromolecules 1980, 13, 1478. (9) Enkelmann, V. AdV. Polym. Sci. 1984, 63, 91. (10) Polydiacetylenes; Bloor, D., Chance, R. R., Eds.; Nijhoff: Dordrecht, The Netherlands, 1985. (11) Schott, M.; Wegner, G. In Nonlinear Optical Properties of Organic Molecules and Crystals; Chemla, D. S., Zyss, J., Eds.; Academic Press: Orlando, FL, 1987. (12) Plant, A. L.; Benson, D. M.; Trusty, G. L. Biophys. J. 1990, 57, 925. (13) Batchelder, D. N.; Evans, S. D.; Freeman, T. L.; Haussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (14) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (15) Mowery, M. D.; Menzel, H.; Cai, M.; Evans, C. E. Langmuir 1998, 14, 5594. (16) Menzel, H.; Mowery, M. D.; Cai, M.; Evans, C. E. Macromolecules 1999, 32, 4343.
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