J. Phys. Chem. 1995,99, 13213-13216
13213
Semiempirical Calculations and Scanning Probe Microscopy Studies of Polythiophene Thin Films T. L. Porter* and D. Minore Department of Physics, Northem Arizona University, Flagstafi Arizona 8601I
D. Zhang Department of Chemistry, Northem Arizona University, Flagstafi Arizona 8601I Received: February 27, 1995; In Final Form: May 19, 1995@
Electrochemically prepared films of polythiophene were studied using the technique of noncontact scanning probe microscopy. Images of the film surfaces revealed many polymer strands atop the bulk film surface. The average diameter of these strands was 9-15 f 2 A, indicating a coil or helical structure. The substructure of the bulk film surface was not resolvable, likely indicating a close-packed structure of (planar) polymer chains. Semiempirical AM 1 calculations on isolated bithiophene and terthiophene oligomers indicate minima at torsional angles of 35" and 153" for bithiophene and combinations of these torsional angles for terthiophene. These calculations show the energetic feasibility of a nonplanar, syn-like thiophene molecular conformation on the film surface, where interactions with neighboring polythiophene are minimized. Geometry-optimized AM1 calculations were also performed for hydrated bithiophene. We show that partial orientation of the polar water molecules in the neighborhood of the bithiophene oligomer alters the energetics of torsion angle rotation for this molecule. Torsion angle minima were found to be at 0" and 180°, with the relative potential barrier between these conformations reduced with respect to the isolated case.
Introduction Polythiophene and polythiophene derivatives have been among the most intensively studied of the conducting polymers. Polythiophene films may be prepared chemically, electrochemically, or by vacuum e~aporation.'-~All of these forms are chemically stable and highly conductive upon oxidation (doping). The macroscopic properties of these materials (optical, electronic, and physical) are highly dependent on the microscopic structure of the various forms. Numerous experimental and theoretical studies have thus focused on the conformation of polythiophene molecules or oligomers. A more complete understanding of these basic properties should eventually lead to models that successfully deal with the more complex polymeric systems. X-ray diffraction5 and other experimental studies6 on unsubstituted polythiophene (PT) in the solid state show a planar (anti) structure for the molecule. In the gas or liquid phase, without strong solid-state nearest-neighbor interactions, other conformations are observed. Gas phase electron diffraction results on bithiophene indicate a dihedral angle of 146" (anti-like) in one study,' as well as 148" and 36" (synlike) in another.8 NMR results for bithiophene in liquid crystalline solvents indicate the existence of both anti-like and syn-like conformations and predict a rotational barrier of 5 f 2 kcal/m01.~ Experimental studies on the surface structure of polythiophene using the technique of scanning tunneling microscopy (STM) have also been performed. On unsubstituted PT films, simple helical structures of diameter 15 f 3 b; and superhelical structures of diameter 50-60 8, were observed.Io In another STM study by US,'^ evidence of both the anti- and syn-like conformations was observed for electrochemically prepared PT and substituted PT films. The STh4 technique, however, is subject to local variations in electronic structure and thus may not provide reliable measurements for these materials on this scale. @
Abstract published in Aduance ACS Abstracts, August 15, 1995.
Theoretical studies on the conformational states of polythiophene have also indicated the possibility of stable anti-like and syn-like structures. Both ~emiempirical'~~'~ and ab i n i t i ~ ' ~ . ' ~ calculations on bithiophene in vacuum show the presence of potential minima near the anti and syn limits. In this study, we have used the experimental technique of scanning force microscopy (SFM) in the true noncontact mode to investigate the surface structure of electrochemically prepared PT films. The advantages of this technique are that no contact between the sample surface and tip occurs, resulting in high-resolution images with no surface damage or modification, and additionally, local variations in the electronic structure of the sample surface will have no effect on the images obtained. We have combined these images with theoretical models of bithiophene and terthiophene oligomers on the basis of the semiempirical AM1 method.
Experimental Section
PT films were electrochemically prepared from 0.1 M solutions of thiophene in 0.1 M LiBF4 and acetonitrile. The films were deposited onto Pt substrates. Film thickness was kept at 1 p m or more to mask any substrate structural features. After deposition, films were rinsed in acetonitrile and dried in a vacuum oven for 24 h prior to SFM imaging. SFM images were obtained in true noncontact mode,I6 in which no contact whatsoever between the tip and the surface occurs. All scans were thus highly repeatable. Scan speeds ranged from only 200 &s for small area scans to over 1 pmts for larger area scans. Highly sharpened conical tips were used in all scans. These tips are generally capable of significantly higher lateral resolution than standard pyramidal tips. Theoretical Methods All geometry optimizations were performed using the AM1 semiempirical method as employed in both MOPAC and
0022-365419512099-13213$09.00/0 0 1995 American Chemical Society
Porter et al.
13214 J. Phys. Chem., Vol. 99, No. 35, 1995
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Figure 1. Large-scale 2 p m x 2 p m noncontact SFM image of an electropolymerized polythiophene surface. The surface is seen to contain many coil or helical polymer structures.
HyperChem.17 AM1 results for these two packages were identical. For bithiophene under vacuum, the dihedral angle between the two rings was varied in 5" increments from 0" (syn conformation) to 180" (anti conformation). At each increment, the dihedral angle was held fixed while the remainder of the molecule was fully optimized using AM1. An rms gradient in the energy of 0.01 kcal/&mol was used for "full optimization". Then, a single AM1 energy was calculated for that particular angle. For partially hydrated bithiophene, the oligomer was surrounded by 20 water molecules. At each increment, the dihedral angle was held fixed while a geometry optimization of the entire system (including water) was performed using the MM+ force field in HyperChem. This was done to include the effects of local variation in dielectric behavior due to the partial orientation of the water molecules in the presence of the bithiophene molecule. An energy gradient of 0.1 kcal/A*mol was used to reduce the very long computer times needed for these calculations. A minimum distance of 2.3 A was used between the solute and the water molecules. Finally, a single AM1 energy was calculated. For the calculations on terthiophene under vacuum, a similar procedure to that for bithiophene was followed, except both dihedral angles were varied from 0" to 180" in 5" increments.
Results and Discussion In Figure 1 an SFM scan of dimensions 2 pm x 2 pm is shown. In this noncontact image, the polythiophene surface is seen to be scattered with chainlike strands. It is worth noting that scans on the same surface in the more standard contact SFM mode produced no such results, indicating that the larger contact force used for standard SFM totally obscures these more delicate features. The appearance of polymer strands on an otherwise close-packed polymer surface has been previously documented by us using the technique of STM." The explanation given was that, during film growth in the electrochemical cell, the bulk of the material simply grows in a close-packed manner. The realized bulk structure is due to the energetics of strong nearest-neighbor interactions between adjacent polythiophene strands. This results in the bulk structure as seen in X-ray diffraction s t ~ d i e s . ~ . ~Growing J away from the surface, however, may be many polymer strands without the nearestneighbor interactions responsible for the planar-chain bulk structure. It is possible that these polymer molecules adopt a coil or helical conformation and then simply lie down on the film surface upon removal from the cell. We have noted that
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Figure 2. (a, top) SFM (1000 x 1000 imag? of a single x 300 A) image of a polythiophene coil. (b, bottom) SFM (300 polythiophene coil. The apparent diameter of these coils is 20 f 2 but these figures may be too large due to water layer and tip convolution effects.
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more intense washing of the films after polymerization generally results in fewer of these observed surface strands. In Figure 2a an image of dimensions 1000 A x 1000 8, is presented, while in Figure 2b a scan of dimensions 300 8, x 300 8, is shown. In Figure 2a a single strand is imaged, while in Figure 2b sections of two nearby strands are shown. The lateral dimension of the strands in all of the images is 20 f 2 8, as shown. What is not known, however, is the effect of SFM tip convolution on the apparent width of the strands. Without knowing the exact dimensions of the tip itself, this effect cannot be precisely calculated. We expect, however, that the polymer strand width is somewhat exaggerated by this effect (maybe by as much as 100%for strands of these small dimensions). Also, the effect of a water layer on the film surface would be to increase the apparent helical diameter by as much as 1 or 2 A. It would be more accurate then to say that the strands imaged have a width in the rangeo9-15 A, assuming a minimum tip convolution effect of 5 A. The noncontact SFM data thus obtained agree very well with previously published STM data and 15 f 3 A.1o indicating widths of 10 f 2 In Figure 3 the AM1 total energy calculations for bithiophene under vacuum are shown (the axes in Figures 2, 4, and 5 represent the relative energy as compared to the minimum energy obtained through optimization). In ode plot, the molecule was fully optimized geometrically at each dihedral (torsion) angle prior to the total energy calculation. In the
Studies of Polythiophene Thin Films 0.6 ,
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Figure 3. Results of AM1 total energy vs torsion angle calculations for bithiophene under vacuum. Local minima at 35" (syn-like) and 150" (anti-like) are obtained. Both fully optimized and partially optimized data are shown.
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Figure 4. Under vacuum total AM1 calculations as a function of two torsional angles for terthiophene. A total of four local minima are identified. 0.5 0.4
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Figure 5. Semiempirical and molecular mechanics calculation results for partially hydrated bithiophene. The effect of partial orientation of the water molecules in the vicinity of the bithiophene molecule is to reduce the relative barrier height between anti and syn conformations. Another effect is to make the true-anti and true-syn conformations more energetically favorable.
second plot, the torsional angle was simply varied starting with the fully optimized syn conformation. In the fully optimized case, minima occur at 35" and 153". For the nonoptimized case, the minima occur at 37" and 153". Also, the relative difference in rotational barrier (0.23 kcal/mol) is very small for the two cases. These data agree well with other recent AM1 calculations on bithiophene.l 3 Recent gas phase electron diffraction measurements on bithiophene confirm the existence of these two torsional conformations.* In this study, it was found that stable conformers representing torsional angles of 36" and 148" existed with relative abundances of 44% and 56%, respectively. The energy difference was calculated to be only 0.18 kcal/mol, with the anti-like case being the most energetically favorable. While the AM1 method accurately predicts the existence of these conformations, the predicted rotational barrier is low by greater than 1 order of magnitude. Experimental NMR results9.19as
J. Phys. Chem., Vol. 99, No. 35, 1995 13215 well as ab initio theoretical method^'^.'^ predict this barrier to be much larger, about 5 and 1.6-4.2 kcallmol, respectively. Ab initio methods do, however, predict the anti-like conformation (about 150") to be lower in energy than the the true-anti conformation (180") by only 0.5 kcallmol. In Figure 4 we show the results on similar AM1 calculations for terthiophene under vacuum. For this calculation, one torsional angle was incremented in 5" steps from 0" to 180". At each of these increments, the second torsional angle was then varied throughout the entire range 0-180". The plot indicates four energy minima, with the lowest energy occurring at 153" for both angles. The minimum highest in energy occurs at 35" for both angles, with the mixed syn-like and anti-like cases intermediate in energy. From these calculations, we may thus conclude that the anti-like and syn-like conformations of polythiophene may indeed exist at room temperature when solidstate nearest-neighbor interactions may be neglected. Finally, we investigate the effect of hydration or partial hydration on the conformational states of polythiophene. This is precisely the condition that is expected during ambient SFM imaging. For virtually all materials, a monolayer of water vapor is expected to cover entirely the sample surface being imaged. Furthermore, these water molecules may be partially oriented (even at room temperatures) in the presence of the polar polythiophene. This will result in possibly strong local variations in dielectric behavior in the vicinity of the polymer, which in turn may affect the polymer conformational energetics. To study these effects, we performed AM1 calculations on partially hydrated bithiophene as a function of torsional angle. At each torsional angle between 0" and 180°, the molecule was surrounded by 20 water molecules with random orientations. The minimum distance between water molecules and solute was 2.3 A. We chose to use 20 water molecules as a compromise between absolute accuracy and computational time. This system was then optimized using the MM+ force field, resulting in partial orientation of the H20 dipoles with respect to the bithiophene molecule. We note that a molecular mechanics optimization was chosen in this step to minimize computational time. After each MM+ optimization, an AM1 energy calculation was performed. Figure 5 represents the results of these calculations. The conformational minima now occur at the trueanti and true-syn geometries. There exists a tiny minimum at 30" which is reproducible, but this should have little or no effect at room temperature. The energy scale in this figure represents the total system energy, that of bithiophene and water. Direct comparisons between this figure and Figure 3 are thus impossible. What we can compare, however, is the relative rotational barrier between anti and syn conformations for the two cases. The effect of partial orientation of the polar solvent molecules has been to reduce the relative rotational barrier between anti and syn conformations by about 30%. Partial hydration also has the effect of making the true-syn (or true-anti) conformations more favorable with respect to the 35" syn-like or 150" antilike conformations. It is the possibility of stable syn or syn-like conformations that may lead to the formation of polymer coils or helices under vacuum or on surfaces. A syn or syn-like conformation will be nonplanar, with the molecule adopting a coil or helical conformation. A true-syn conformation adopts this helical conformation simply to avoid overlapping itself in a single plane. We would expect this conformation to have the greatest helical diameter, and possibly the lowest helical pitch. If we use the AM 1-optimized true-syn structure for bithiophene (or terthiophene) and construct from that a single coil, the diameter of this coil is about 23 A. This measurement would correspond
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13216 J. Phys. Chem., Vol. 99, No. 35, 1995
and water layer effects are considered. Semiempirical AM1 calculations on bithiophene, partially hydrated bithiophene, and terthiophene indicate the possibility of stable polythiophene helical structures. Undfr vacuum, a syn-like conformation with a helical radius of 11 A is predicted. A possible effect of the' presence of water molecules on the surface is to reduce the torsional angle more toward a true-syn molecular conformation. This would then tend to increase the helical diameter somewhat. The AM1 calculations provide good agreement with the experimental SFM measurements.
Acknowledgment. This research was funded by the National Science Foundation (DMR-9217525), Research Corporation, and the NAU Vice President for Research. Figure 6. Structure of the polythiophene molecule with a 35' torsion angle. The diameter of this molecule is 11 A.
to an upper limit for the theoretical helical diameter. If we then draw this loop into an extended helical structure, the helical diameter is reduced and the torsional angle between thiophene units is increased. The helical pitch is also increased during this process. If we draw the helix out until the torsional angle between neighboring thiophene rings is 35" (syn-like conformation), the resulting helical diameter is 11 A. This value serves as a lower theoretical limit on the helical diameter. Figure 6 shows a drawing of such a syn-like helical coil with a diameter of 11 A. It is likely that the presence of water molecules near the polythiophene coils serves to make the structure more truesyn, but maybe by only a small amount. This would then increase the helical diameter correspondingly. This prediction of an 11 A minimum size coil is in good agreement with the experimental SFM (and earlier STM) images. For surface-lying polythiophene coils, a layer of water molecules would serve to increase the SFM-measured diameter by a few angstroms and also possibly obscure the helical pitch (which is not resolved in SFM images).
Conclusions We have shown that the surfaces of electropolymerized polythiophene films contain helical or coil-like polymer structures. Noncontact SFM images of these coils show them to have a diameter in the range 10- 15 f 2 A when tip convolution
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