SurfaceâEnhanced Polymerization via Schiff-Base Coupling at the

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The Journal of Physical Chemistry

Surface-Enhanced Polymerization via Schiff-base Coupling At the Solid-Water Interface under pH-Control

Marco Di Giovannantonio,a,b Tomasz Kosmala,a,c Beatrice Bonanni,a Giulia Serrano,a Nicola Zema,c Stefano Turchini,b Daniele Catone,b Klaus Wandelt,c Dario Pasini,d Giorgio Contini,b,a,* and Claudio Golettia

a

Physics Department, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Roma, Italy

b

Istituto di Struttura della Materia, CNR, Via Fosso del Cavaliere 100, 00133 Roma, Italy c

Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstraße 12, D-53115 Bonn, Germany

d

Department of Chemistry and INSTM Research Unit, University of Pavia, Viale Taramelli, 10, 27100 Pavia, Italy

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ABSTRACT: On-surface polymerization realized at the solid-liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of π-conjugated polymer formation at the interface between an iodinemodified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and πconjugated final product. The I/Au(111) substrate enhances the formation of π-conjugated polymers, as established comparing their production on the surface and in the bulk solution.

KEYWORDS: π-conjugated polymers, low-dimensional systems, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM)


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1. INTRODUCTION The request for realizing materials with reduced size in view of technological applications is driving the research of the last decade toward miniaturization.1 Nanostructures based on organic materials have received great interest because of their possible employment as active media in organic electronic devices and their advantages (cheap, easy to process on large scale, flexible and transparent) compared to inorganic materials.2-3 Efforts in obtaining molecular-based devices based on ordered organic structures at the nanoscale with improved performances are ongoing. Self-assembly is a well-established mechanism to obtain ordered molecular layers,4-8 but the weak (i.e. non-covalent) intermolecular interactions limit the electron transport and mechanical resistance of such systems. On the other hand, polymers exhibit higher mechanical stability and their extended π-conjugated structures allow more efficient electron transport compared to self-assembled layers.3-4,

9-10

A variety of polymers can be

synthesized in solution, but they are typically disordered.11 As an alternative method, on-surface synthesis of polymers confines the growth to a 2-dimensional (2D) plane and produces highly ordered structures. Several studies have been reported on surface polymerization under ultra-high vacuum (UHV) conditions, obtaining 1-dimensional (1D) and 2D ordered polymeric structures; the substrate is often both a template and a catalyst for the polymer growth.10,

12-22

However, there is a strong

interest in realizing polymeric networks under less-demanding reaction conditions, which is more appealing for technological applications. Different methods have been used to produce polymers under ambient conditions23-25 and at the solid-liquid interface26-27 but most of them require an external input to initiate the reaction (such as UV irradiation, heating or voltage application), unfavorable in view of large scale production. One of the proposed polymerization methods at the solid-liquid interface, easy and of low cost, utilizes the formation of Schiff-base functionalities in a condensation polymerization approach. 3 ACS Paragon Plus Environment


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Schiff-bases are obtained in a two-step reaction involving complementary functionalities, amino and aldehyde (Scheme 1a).28-29 This type of surface-confined condensation has been studied in either organic solvents30-35 or aqueous solution:36-40 in the second case the reactivity is known to be pH-dependent, offering a control on the reaction evolution. At low pH values the majority of the amino groups are in ammonium form (-NH3+) and they are prevented to react with the aldehydes; by increasing the pH, the ammonium ions are deprotonated (to -NH2) and can react forming Schiffbases (i.e. imines) through nucleophilic addition at the aldehyde. π-conjugated 1D or 2D polymers can be grown on suitable surfaces depending on the degree of branching of multifunctional molecular building blocks possessing amino and aldehyde groups.

Scheme 1. (a) Reaction scheme for Schiff-base coupling. (b) Molecules used as precursors (terephthalaldehyde, TPA, and 4,4’-diaminostilbene dihydrochloride, ASB). (c) Intermediate state of the reaction. (d) Final product of the reaction (π-conjugated polymer).

Schiff-base coupling is one of the most interesting and most often studied reactions since the imine bond (C=N) can participate in three types of equilibrium-controlled reactions: (i) hydrolysis, (ii) transimination and (iii) imine-imine exchange.41 In these processes imines may undergo disconnection-reconnection cycles in a dynamic exchange,42 which is of fundamental importance in 4 ACS Paragon Plus Environment

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driving specific biological reactions43 or self-repair of defects in polymeric networks leading to highly ordered organic structures.9 This approach belongs to the Constitutional Dynamic Chemistry (CDC), which implies a change in the nature, number and arrangement of the components of molecular or supramolecular entities under pressure of internal factors or external stimuli.43 An implementation of such approach opens important routes toward (i) the study of synthetic systems, (ii) the search for bioactive substances (molecular key for a biological lock) and (iii) the development of dynamic materials like adaptive materials and dynamers.43 The polymers synthesized on surface in the present work can be classified as dynamers and combine both the covalent character of molecular dynamers and the spatial extension of supramolecular dynamers;44 their applications, beyond those in organic electronic devices, extend to self-healing and responsive materials, time-delayed release of drugs or fragrancies and biodegradability, merging chemistry, biology and physics.43 The choice of Schiff-base coupling as leading reaction to obtain πconjugated polymers on surfaces is also justified by the broad range of biological processes imines take part in, as antimicrobial activities45 and rhodopsin photoisomerization in the retina of the human eye.46 A deeper understanding of the reaction details, especially on surfaces, is of general interest for biological applications in scaffold engineering, biocompatible active coatings and drugdelivery. Surface-confined polymerization upon Schiff-base coupling has been mainly studied by local observation of the product formed on surfaces, using scanning tunneling microscopy (STM).30-34, 38 Only information about the local morphology is reported, from which it is neither possible to extract the chemical composition of the organic layer nor to verify the formation of extended π-conjugated structures: this is feasible by spectroscopic measurements, as shown by X-ray photoelectron spectroscopy (XPS) investigation on the condensation products obtained at the interface between highly-oriented pyrolytic graphite (HOPG) and tetrahydrofuran33 or between silicon wafer and toluene.47 However, the use of aqueous solution instead of organic solvents enables the control of the reaction evolution as a function of the pH. The intermediate and final states of the Schiff-base 5 ACS Paragon Plus Environment


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coupling do not differ significantly in their morphology (Scheme 1c,d) and might be undistinguishable by STM only, unlike the case of other types of reactions, such as by Ullmann coupling.13, 20 Moreover, the influence of the substrate on the formations of π-conjugated polymers via Schiff-base coupling has not been investigated in detail. We report herein a study identifying the chemical states of the molecules at the various steps of the Schiff-base coupling and monitoring the polymerization at the solid-water interface as a function of the pH. 4,4’-diaminostilbene dihydrochloride (ASB) and terephthalaldehyde (TPA), able to form 1D π-conjugated polymers (Scheme 1b-d), have been used as precursors on iodinemodified Au(111). The analysis of the molecular layer on the surface, performed by XPS and STM, combined with UV-Vis measurements on the bulk solution, reveals the presence of the intermediate state of the Schiff-base coupling at the surface together with the final π-conjugated phase. The production of the latter is enhanced by the I/Au(111) surface.

2. EXPERIMENTAL METHODS The Au(111) single crystal (Mateck) was cleaned before each STM experiment under argon flux by flame annealing for 5 minutes and transferred into an home-made electrochemical (EC) STM.48 For XPS measurements, it was cleaned in UHV by repeated cycles of Ar+ sputtering at 1 keV and annealing up to 800 K; the clean sample was transferred into a pre-chamber filled with argon to avoid contact with air. The as-prepared Au(111) surface was immersed in 1 mM KI solutions to form the I/Au(111) and then in the solution containing the precursor molecules at controlled pH. For the XPS experiments, the “hanging meniscus” configuration (Figure S1, Supporting Information) has been adopted and the immersion time of Au(111) in each solution was 90 s. After each emersion from the solution containing KI or the molecules, a rinsing step with ultrapure water was performed, to remove all the ions and molecules not directly adsorbed on the surface and eliminate the presence of artefacts in the measurements. The pH of the water in the last step was the 6 ACS Paragon Plus Environment

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same of the solution with the molecules. The two different cleaning procedures adopted before the STM and the XPS experiments produce surfaces with the same characteristics: the clean samples exhibit the 22 × √3 reconstruction of the Au(111) surface, while the I/Au(111) shows the

√3 × √330 superstructure of the iodine, as evaluated comparing low-energy electron diffraction (LEED) patterns and STM images (Figure S2, Supporting Information). The precursor molecules (4,4’-diaminostilbene

dihydrochloride,

ASB,

C14H14N2·2HCl,

purity

95%,

and

terephthalaldehyde, TPA, C8H6O2, purity 99%) were purchased from Sigma Aldrich and used without further purification to prepare 0.1 mM solutions, whose pH was adjusted by adding HCl or NaOH. All the solutions were prepared using ultrapure water (resistivity larger than 18 MΩcm). In situ STM measurements were performed in constant current mode using tungsten tips electrochemically etched and coated (except the apex) with glue to minimize Faradaic current collection from the solution. All the quoted bias voltages (Vb) refer to the grounded sample (images acquired with negative bias correspond to tunneling into empty states of the sample). STM images were analyzed by WSxM49 and are reported after processing (flatten filter and color equalization). XPS measurements were carried out at the Circular Polarization beamline50 at the Elettra synchrotron radiation facility (Trieste, Italy). Photoemitted electrons are collected in normal emission geometry with a 5 channeltron hemispherical electron analyzer; the linearly polarized incident photon beam forms an angle of 40° with the surface normal. UV-Vis measurements of the bulk solution were performed in quartz cuvettes, using a double-beam spectrophotometer (equipped with W-halogen and Deuterium lamps). FT-IR measurements were conducted on sample powders by measuring the intensity of the diffuse radiation (diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS). All the experiments were conducted at room temperature.

3. RESULTS AND DISCUSSION



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STM images of a I/Au(111) immersed in aqueous solution containing TPA and ASB at different pH are shown in Figure 1a-f. The presence of underlying iodine is confirmed by ex-situ XPS measurements reported in Figure S3 (Supporting Information).

Figure 1. (a-f) STM images of a I/Au(111) surface immersed in an aqueous solution of 0.1 mM TPA and 0.1 mM ASB. (a) pH 2.1, 97.6×97.6 nm2, Vb=-410 mV, It=1 nA. (b) pH 3.6, 97.6×97.6 nm2, Vb=-413 mV, It=1 nA. (c) pH 2.1, 20.4×20.4 nm2, Vb=-410 mV, It=1 nA. (d) pH 3.6, 20.4×20.4 nm2, Vb=-413 mV, It=1 nA. (e) pH 2.1, 8.7×8.7 nm2, Vb=-410 mV, It=1 nA. (f) pH 3.6, 8.7×8.7 nm2, Vb=-413 mV, It=1 nA. (g, h) Height profiles along the lines in panel e and f.


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Ordered nanostructure domains are present at the surface: both the domain size and degree of order increase as a function of increasing pH, from 2.1 to 3.6. A closer inspection of these structures (Figure 1c-f) reveals that they consist of adjacent chains: the distances between the protrusions along the chains are 0.65 ± 0.05 nm at pH 2.1 and 3.6 (evaluated from height profiles reported in Figure 1g,h). This value is larger than the expected spacing between iodine atoms in the √3 ×

√330 superstructure formed on Au(111) (which is 0.50 nm, as observed in Figure S2d, Supporting Information); in turn, it is in agreement with the expected spacing between the aromatic rings in the polymeric phase,38 suggesting that the chains may be attributed to the π-conjugated polymers at both pH values. However, also the intermediate state of the reaction could be in a polymeric phase (Scheme 1c), although not π-conjugated. The intra-chain distance between the protrusions does not differ significantly in the two cases to be appreciated with the experimental accuracy of the STM images. At pH 2.1 a part of the surface is not covered by molecular domains and presents noisy streaks, typical of diffusing material, probably unreacted monomers and short oligomers. This is consistent with the complete lack of an ordered overlayer at pH values lower than 2.1, where the adsorbed molecules remain free to diffuse on the surface and cannot be imaged by STM at room temperature. The images are of better quality for pH 3.6 where the surface area appears fully covered by immobilized domains. At this pH the chains (with length up to 30 aromatic rings) are adsorbed on the substrate and meander over the surface mimicking the threefold symmetry of the substrate (as indicated by the dashed black line with angle of 120° ± 5° in Figure 1d); a contrast modulation, that can be attributed to a Moiré effect, is observed. The I/Au(111) surface acts as template for the ordering of such chains, suggesting that the Schiff-base coupling occurs at the solid-water interface: in fact, the chains would most likely not mimic the symmetry of the substrate if formed in the bulk solution and then just precipitated on the surface.



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STM images are not conclusive in proving that π-conjugated polymers have been formed and do not provide detailed information on the chemical steps of the process. To add this information, synchrotron radiation XPS measurements were carried out.

Figure 2. (a) Synchrotron radiation N 1s XPS spectra of a I/Au(111) surface after emersion from a solution containing ASB or ASB+TPA at the reported pH. Spectra are collected using an incident photon energy of 500 eV. Experimental data after Shirley background subtraction,51 fitting Voigt functions and best fit are shown as dots, colored filled areas and solid black line, respectively. The 10 ACS Paragon Plus Environment

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color of each Voigt function refers to the corresponding chemical structure in panel b. (b) Chemical formulas of the molecules in the different reaction steps. (c) Relative amount of the dominant chemical states observed in the N 1s signal, as evaluated from the areas of the three fit components in panel a.

N 1s XPS signals are reported in Figure 2a and refer to the I/Au(111) surface immersed from a solution containing ASB or a mixture of ASB and TPA at different pH values. The preparation of the surface for XPS measurements has been conducted not to induce any artifact to the spectra, as described in the experimental section. We focus on the N 1s level since nitrogen is involved in all the reaction steps (Scheme 1a) and its signal is not affected by possible contaminants, as might happen for carbon and oxygen. When ASB is present in an aqueous solution at pH values below 1.5 all amino groups are protonated (to ammonium ions, ASB2+) and the UV-Vis absorption spectrum shows a band with maximum at about 290 nm (Figure 3). Increasing the pH, ASB progressively deprotonates (to ASB0) and the maximum of the absorption band moves to about 335 nm. Above pH 5.5 ASB is fully in amino form.

Figure 3. (a) UV-Vis absorption spectra for ASB dissolved in aqueous solution at the pH indicated in the legend. Bands with maxima at 290 nm and at 335 nm correspond to ammonium ions and 11 ACS Paragon Plus Environment


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deprotonated amines (NH3+ and NH2, respectively). (b) Absorption at 335 nm as a function of the pH, representing the relative amount of deprotonated amines in aqueous solution.

Different from what is observed in the solution, Figure 2a shows that on I/Au(111) at pH 1.0 20% of ASB is in ammonium state (as evaluated from the relative areas), whereas the majority (80%) is deprotonated (-NH3+ and -NH2 giving N 1s binding energies (BEs) of 401.2 eV (violet) and 399.4 eV (orange), respectively, in agreement with studies on gold and oxidized silicon surfaces functionalized with aliphatic and aromatic aminosilanes and aminothiols).52-53 At pH 4.0 93% of the ASB on the surface is in an amino state (Figure 2a). This effect can be explained in terms of competition between solvation and adsorption of ASB. The ASB2+ is solvated by water (polar solvent) through electrostatic interactions. When the pH increases, the amount of ASB2+ decreases in favor of ASB0, as reported in Figure 3. The ASB0 is less solvated by the water than ASB2+ and its adsorption on the I/Au(111) surface is favored. The preference of ASB0 for adsorption leads to a higher ASB0/ASB2+ ratio at the I/Au(111) surface with respect to that in the bulk solution. The N 1s spectra of the I/Au(111) surface emersed from a solution containing ASB and TPA change as a function of the pH value. At pH 1.7 two new N 1s components are present beside the NH2 state (Figure 2a). One, at BE 400.6 eV (blue peak in Figure 2a), belongs to nitrogen atoms in a secondary amine type structure (similar to amide state observed in aging of amine functionalities on oxidized silicon),52 which corresponds to the carbinolamine functionality, intermediate state of the Schiff-base coupling (Scheme 1a,c). Increasing the pH, the intensity of this component decreases along with the amine component (orange peak), as reported in Figure 2c. Up to date, no spectroscopic evidence of the intermediate state of the Schiff-base coupling has been detected on surfaces, although this has been identified in solution by nuclear magnetic resonance (NMR), Raman spectroscopy and mass spectrometry.54-56 The other component at BE 398.5 eV (green peak in Figure 2a) is related to nitrogen atoms in an imine functionality, distinctive of the π-conjugated 12 ACS Paragon Plus Environment

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polymer expected as a final product of Schiff-base coupling (in agreement with studies on the coupling of 4,4’-methylenebis(2,6-diethylaniline) and pentafluorobenzaldehyde47 and of 1,3,5tris(4-aminophenyl)benzene and terephthalaldehyde).33 This component increases in intensity and becomes dominant at pH 3.6, where more than 50% of the molecules are converted into πconjugated polymers (Figure 2c). At high pH, a small amount of the component at 399.4 eV (due to the ASB end-monomer of polymers and unreacted molecules) is present, together with a shake-up signal from the π-conjugated polymeric chains at BE 401.4 eV (red peak in Figure 2a). The intensity decrease of the O 1s signal (Figure S4, Supporting Information) at pH 4.0 compared to that at pH 1.7, where oxygen is still present in unreacted TPA and intermediate state (Scheme 1), is a further demonstration that the polymerization has occurred. These XPS data demonstrate that it is possible to distinguish the different states of the molecules, which evolve as a function of the pH, during the Schiff-base coupling. At low pH both the polymers in the intermediate state and the final π-conjugated polymers are present at the surface: their relative amount progressively changes when increasing the pH, leading to a predominance of π-conjugated polymers above pH 2.6, becoming the only polymeric state present at pH 4.0. It is worth noting that the π-conjugated polymers are stable in air as shown by the lack of significant changes in the N 1s signal after exposure to air (Figure S5, Supporting Information): this is a necessary requirement for such materials if to be used for devices. To evaluate the effect of the surface in driving the polymerization reaction we have studied for comparison the Schiff-base coupling in the bulk solution. UV-Vis absorption spectra of the solution containing ASB and TPA at different pH values are reported in Figure 4a.



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Figure 4. (a) UV-Vis absorption spectra for ASB and TPA dissolved in aqueous solution at the pH indicated in the legend; curve intensity has been normalized between 0 and 1; not-normalized spectra are reported in Figure S6 (Supporting Information). (b) Normalized absorbance at 400 nm as a function of pH, representing the amount of π-conjugated polymers produced in solution (gray points, from the data in panel a), and the relative amount of π-conjugated polymers produced on the I/Au(111) surface (green squares, from the XPS data of Figure 2). Data at pH 4.0 are coincident for the two curves (see text for details).

As a function of increasing pH, a new wide band arises in the absorption spectra with a maximum at about 400 nm (Figure 4a). FT-IR and UV-Vis absorption measurements on the precipitate that is formed in solution above pH 4.0 (visible by naked eye) reveal that this band can be attributed to π-conjugated polymers of Scheme 1d, final product of the Schiff-base coupling (Figure S7, Supporting Information). The intensity of this band is due to the amount of πconjugated polymers in solution at a certain pH value, since neither ASB nor TPA significantly absorb at this wavelength for all the investigated pH values (Figure 3a and S7b, Supporting Information). To compare the polymer production on the surface with that in the bulk solution in all the range of investigated pH values, we would need to know the amount of the π-conjugated polymers in the two cases. While this quantity is known on the surface (from the XPS data), it cannot be evaluated 14 ACS Paragon Plus Environment

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for the bulk solution because of the unknown equilibrium constants and of the precipitation of the polymers at pH values higher than 4.0 (in fact, our polymers are no longer completely soluble in water when their length increases). However, at low pH values no polymers are produced in the bulk solution (since no significant absorbance increase has been observed at 400 nm up to pH 2.0, see Figure 4a and gray dots in Figure 4b), whereas on the surface 13% of precursor molecules are already converted into π-conjugated polymers (green squares in Figure 4b) demonstrating the active role of the substrate. Therefore, we can set an upper limit for the relative amount of π-conjugated polymers in solution assuming that the concentration of polymers on the surface, due to its active role, cannot be lower than that in the bulk solution. Following this assumption, in Figure 4b we rescale the UV-Vis absorbance signal to match at pH 4.0 the relative amount found from the XPS. The results shown in Figure 4b demonstrate that the I/Au(111) surface enhances the production of π-conjugated polymers. Beyond the mere confinement in 2D which, in some cases, may facilitate the reaction,42 this enhancement is related to several factors. Molecules adsorbed on surfaces have typically larger density than those in the bulk solution (in our case we estimate three orders of magnitude); in addition, it has been reported that the hydrophobic I/Au(111) surface, pushing away water molecules, produces a higher molecular concentration near the surface.38 These effects facilitate molecular coupling. In fact, the reaction rate depends upon the concentration of reagents, the activation energy and the temperature. Since, at room temperature, the I/Au(111) surface is stable and rather inert (i.e. it is a bad catalyst in terms of lowering the activation energy) we mainly attribute the enhancement in the production of π-conjugated polymers to the molecule densification occurring at the surface. The removal of water also changes the equilibrium of the Schiff-base reaction toward the formation of the products (Scheme 1a).38 Our results show an additional effect, related to the competition between solvation by water molecules and adsorption strength (as previously discussed for XPS data): the majority of ASB is deprotonated on the I/Au(111) even at low pH values whereas it is protonated in the bulk solution (Figure 2 and 3). Since the availability of deprotonated amino groups is a necessary condition for the Schiff-base coupling, this leads to an 15 ACS Paragon Plus Environment


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enhanced production of π-conjugated polymers on the I/Au(111) surface. This active role of the substrate is of great importance in terms of cheap and large-scale fabrication of polymeric networks. In fact, having a reservoir of unreacted precursor molecules which polymerize only if in contact with a substrate immersed in the solution would enable the use of roll-to-roll methods, avoiding the formation of clusters and precipitates in solution occurring when the pH value is too high.

4. CONCLUSIONS A study of ASB and TPA undergoing Schiff-base coupling on a I/Au(111) surface in aqueous solution as a function of the pH is reported. STM images show ordered 1D chains formed at the solid-liquid interface. We have monitored all the molecular states of the reaction by synchrotron radiation XPS measurements, showing that both intermediate and π-conjugated polymers, undistinguishable by STM, are on the surface. Their ratio progressively changes increasing the pH, in favor of π-conjugated polymers. The surface enhances the polymer production, compared to the bulk solution, due to the densification of the molecules at the hydrophobic I/Au(111) surface and to the interplay between solvation and adsorption of ASB. These results provide new insight into surface-confined Schiff-base coupling and may help in designing new active materials with specific functions and tunable properties for electronics and biology.

Supporting Information Available: “Hanging meniscus” configuration adopted before XPS investigations, comparison between the two different procedures adopted for preparation of I/Au(111) before XPS and STM measurements, XPS characterization of the adsorbed iodine, additional evidence of the formation of π-conjugated polymers from O 1s XPS data, air stability of the π-conjugated polymers, not-normalized UV-Vis absorption spectra on aqueous solution containing ASB and TPA, FT-IR and UV-Vis absorption measurements on the precipitate obtained


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from aqueous solution containing ASB and TPA at pH greater than 4.0. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information *Address correspondence to: [email protected]

Acknowledgements We acknowledge beamtime access and support from ELETTRA. We are grateful to Dr. F. De Matteis (University of Rome “Tor Vergata”, Italy) for giving us full access to the spectrophotometer for the UV-Vis absorption measurements. Dr. G. Pennesi (Istituto di Struttura della Materia, CNR, Italy) is acknowledged for the FT-IR.



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