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Jan 10, 2014 - School of Basic Sciences, Indian Institute of Technology Mandi, Academic Block, Mandi-175001, Himachal Pradesh, India. ‡ Nanosystem ...
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Effect of N‑α Substitution on the Electropolymerization of N‑Substituted Pyrroles: Structure−Reactivity Relationship Studies Sunil Kumar,†,∥ Sada Krishnakanth,†,∥ Jomon Mathew,‡ Zvika Pomerantz,§ Jean-Paul Lellouche,*,§ and Subrata Ghosh*,† †

School of Basic Sciences, Indian Institute of Technology Mandi, Academic Block, Mandi-175001, Himachal Pradesh, India Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, Umezono 1-1-1, Tsukuba 305-8568, Japan § Department of Chemistry, Nanomaterials Research Center, Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel ‡

S Supporting Information *

ABSTRACT: Structure−reactivity relationship (SRR) studies to understand the effect of N-α substitution toward electroactive poly(pyrrole) film formation have carefully been performed. To conduct these SRR studies, a selected chemical library of 21 different N-substituted pyrrole derivatives has been developed using the key well-known Clauson−Kass synthetic reaction from corresponding amine precursors. While investigating electropolymerization features of these novel N-substituted pyrroles, we observed that the steric factor due to α-substitution, one among other factors, plays the most significant role in preventing electropolymerization of these oxidizable chemical species. Interestingly, even a sterically small chemical group like a CH3 one present at α-position is large enough to prevent monomer electropolymerization. Density functional theory calculations were carried out to analyze the polymerization of substituted pyrrole molecules and to understand the effect of N-substituents on electropolymerization toward the formation of functional conductive film. This systematic study paves the way to effectively design the right N-substitution for the obtainment of corresponding modified functional electrodes for further related applications.

1. INTRODUCTION The electrosynthesis of functionalized conducting polymers (fCPs) has raised much interest1,2 because of the potential high utility of f-CPs in fields of chemical sensing,3 photoconductive materials, and light-emitting diodes.4 Indeed, this process provides several advantages such as better and simpler polymerization processes together with a better control of film thicknesses and morphologies as compared with oxidative chemical polymerizations. Among the numerous heterocyclic polymers known5 to date for the fabrication of a conducting interface, polypyrrole polymers (PPys) are very often used as they are environmentally stable being readily prepared from easily oxidizable commercially available Py monomers and providing good electrical conductivity (102 to 105 Sm−1).6 Therefore, PPy’s have fueled several quite significant applications7 being involved in drug-delivery systems and biosensing,8 antistatic coating,9 electromagnetic interference,10,11 batteries,12 supercapacitors,13−15 optical switching devices,16 electrochromic electrodes,17 transparent electrode materials, and electronic devices.18 Such known broad-range applications of conducting organic materials, as previously mentioned, are fully promoted by an effective organic synthesis of corresponding monomeric units and of subsequent structural variants. Powerful organic © 2014 American Chemical Society

synthesis tools enable the synthesis of structurally different substituted monomeric units by which one can easily control conducting properties of resulting polymers. To achieve this major goal, one has to resolve a critical issue, that is, how the chemical structure of any Py monomer can be effectively designed to prevent the blockage of its electropolymerization, thus establishing a clear relationship between the monomer chemical structure and the corresponding substitutions that prevent electropolymerization and PPy film formation. In this context, chemical modifications to the Py heterocycle leading to N-substituted derivatives19−21 and corresponding derivative electropolymerizations have been studied since the 1980s. It has been disclosed that bulky substituents (tert-butyl and cyclohexyl as N-substitution on pyrrole monomer) prevent electropolymerization processes. Similar polymerization blockage was observed for N-(hydroxyl alkyl)22 and N-(carboxyl alkyl) pyrrole-based monomers,23,24 and it was hypothesized that the prevention was due to internal cyclization (nucleophilic capture) of in situ generated cation radicals by chain OH/ CO2H nucleophilic groups.22 Several efforts have also been Received: November 11, 2013 Revised: January 4, 2014 Published: January 10, 2014 2570

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V at a scan rate of 100 mV/s (otherwise mentioned). 0.1 M TBAP (n-tetrabutyl ammonium perchlorate) was used as the supporting electrolyte. 2.4. Theoretical Calculations. Density functional theory calculations have been carried out to analyze the polymerization of substituted pyrrole molecules and to understand the effect of N-substituents on polymerization. All calculations have been performed at the B3LYP/6-31G(d,p) level27,28 of density functional theory using Gaussian 0929 suite of programs.

made to investigate the pyrrole electropolymerization mechanism18 and N-substituent effects to prevent/block polymerization processes. This has been widely discussed in many reports,22,24,26 but and to the best of our knowledge, there is still no qualitative report dealing with the N-α substitution (Figure 1) effect on the electropolymerization of such substituted pyrrole monomers.

3. RESULTS AND DISCUSSION To start with this investigation approach, all mentioned monomers (Table 1) have been synthesized using a wellknown Clauson−Kass reaction (Scheme 1) toward corresponding products 1−21 that were characterized using spectroscopic techniques (Supporting Information). N- Alkylpyrroles were studied utilizing a potential window operated from −1.0 to +1.0 V. In a first step of our studies and because it has been reported that monomers 1 (Figure S1, Supporting Information) and 4 (Figure 2) have been successfully polymerized, we also confirmed similar electropolymerization results. As expected, an irreversible oxidation peak around +1.0 V was observed in the first scan, and successive scans at the same potential window generated a reversible oxidation peak in the +0.2 to 0.5 V potential area. We assigned this former first scan peak to the monomer oxidation, while the later second peak system related to the polymer growth/oxidation, which was reversible in nature. Subsequently and for each tested monomer, a brownish polypyrrole coating was visually observed on the working electrode. In parallel and in each case, the working electrode was washed with excess of acetonitrile to remove nonpolymerized monomer from the electrode surface. This modified electrode was then used as the working electrode when recording CV curves of deposited polymers in a monomer free solution utilizing this similar electrode system. Careful examination of CV graphs revealed that no oxidation peak corresponding to monomer oxidation was ever observed. Similarly, CV graphs for monomers 2 and 3 have recorded under same conditions, but only a monomeric behavior was observed (Figure 3). No peak system has been ever observed that indicated corresponding polymer formation. So one can readily deduce that steric hindrance caused by both Me (2) and Et (3) alkyl substitutions at the N-α position might be the plausible reason to prevent the successful electropolymerization process of such N-alkyl substituted pyrroles. A report from the Bidan’s group22 demonstrated that selected N-hydroxyalkylpyrroles (with an appropriate chain length/methylene number) enabled the formation of corresponding five- or six-membered rings due to an bimolecular alkoxylation (trapping of a developing cation radical by nucleophilic OH groups of pending chains) of the pyrrole heterocycle during electropolymerization therefore behaving as very poorly polymerizable30 monomers. Similar results were recorded for monomers 5 (Figure 4) and 6 (Figure S3, Supporting Information). On the basis of such mechanistic considerations, we were quite curious to examine the electropolymerization behavior of the pyrrole monomer 7 (Figure 5), where the pending nucleophilic hydroxyl group was separated from the pyrrole unit by four methylene units to potentially provide a thermodynamically disfavored sevenmembered cycle. Thus, corresponding electropolymerization voltammetry data confirmed it to be poorly polymerizable because successive CV cycles disclosed a shift of oxidation

Figure 1. Comparison of classically studied N-substituted pyrrole unit together with the one with protected α-position of pyrrole unit used in this study.

Keeping this in mind, we systematically arranged our structure−reactivity relationship (SRR) study with the design and preparation of a library of 21 (Table 1) selected N-αsubstituted pyrrole monomers and tried to investigate the effect of N-α substitution of pyrrole on its electropolymerization. The results obtained have successfully explained that any type of αsubstitution causing steric hindrance will block the electropolymerization process.

2. EXPERIMENTAL SECTION 2.1. General Experimental. All chemicals required to synthesize pyrrole monomers were purchased from SigmaAldrich and used without further purification. Anhydrous sodium sulfate (Na2SO4), dichloromethane (CH2Cl2), sodium chloride (NaCl), and other solvents for column chromatography were received from Merck and used as is. HPLC-grade acetonitrile (CH3CN) having water content 0.03% and supporting electrolyte TBAP for electropolymerization were purchased from Merck and Sigma Aldrich, respectively. Acetonitrile was predried over 4A molecular sieves before further use, and all experiments were carried out under a nitrogen atmosphere. 1H and 13C NMR for pyrrole monomers were recorded on a JEOL ECX 500 spectrometer. TMS was used as internal standard for 1H NMR characterization. LCMS−ESI and HRMS−ESI spectra were recorded on QTOF micromass and WATERS-Q-Tof Premier spectrometer, respectively. FESEM imaging of the samples was carried out using FEI Quanta FEG450. AFM imaging of the samples was done using Agilent SPM5500 in noncontact mode analysis. 2.2. Synthesis. Detailed procedures used to synthesize the reported compounds in this work can be found in the Supporting Information along with characterization data (1H NMR, 13C NMR, IR, and mass spectroscopy). 2.3. Electrochemical Measurements. The redox behavior of monomeric pyrrole derivatives (5 mM in MeCN) was investigated using cyclic voltammetry on a Metrohm Autolab B.V. instrument. Each electrochemical reaction was carried out in a three-electrode electrochemical cell system utilizing a platinum disk (Pt disc) as the working electrode. Potentials were reported versus an Ag/AgCl reference electrode utilizing a Pt wire as the counter electrode. Electropolymerization behavior was studied in a potential window of −1.0 to +1.0 2571

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Table 1. Library of Selected N-α-Substituted Pyrrole Derivatives

values of polymers toward higher voltage (0.2 to 0.6 V). Similar behavior was observed for compounds 5 (Figure 4a, inset) and 6. So we can confirm that the cyclization reaction would also be favorable for unstable seven-membered rings and hence reason behind poor electropolymerizability. However, in this series and for compounds 5−7, cation radical trapping was found to be the only reason behind poor polymerization because the α-substitutent for these derivatives is hydrogen, which does not bring any electropolymerizationpreventing effect, as evidenced from the successful electropolymerization of monomer 1. To examine any effect of potential steric hindrance due to N-α substitution in this series, compound 8 (Figure S4, Supporting Information) with an αisopropyl substituent was synthesized and tentatively electropolymerized. Only the monomer oxidation peak was observed with no formation of any electropolymerized deposited film during successive scans. Hence it is clear that N-hydrox-

ypyrroles with a short chain separating the OH function from the pyrrole unit are poorly polymerizable due to cation radical neutralization by present nucleophilic OH-containing alkyl chains as N-substitution followed by dimerization/aromatization steps,30 but the effect of the steric hindrance caused by the N-α substitution is more prominent because it completely prevents electropolymerization of corresponding N-α substituted N-hydroxyalkylmonomers. Three more series of N-substituted pyrrole monomers, that is, N-benzyl pyrroles, N-(rigid substituent) pyrroles, and Ncarboxyalkylpyrroles, were synthesized and the electropolymerization behaviors were examined as discussed later to reinforce our survey concept of steric hindrance caused by N-α substitution. It is clear from corresponding CV data that only monomer 9 (Figure S5, Supporting Information) gave rise to an effective good electropolymerization reaction while monomers 10 to 11 2572

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Scheme 1. Synthetic Route to N-α-Substituted Pyrroles

Figure 2. Cyclic voltammograms of compound 4 recorded during electropolymerization of monomer (a) and CV curve of the poly-4 polymer (b) deposited on the working electrode (Pt disc).

(Figure S6, Supporting Information) failed to afford any conductive polypyrrole polymer during electrochemical oxidation/polymer growth. From this structurally related series of compounds that only differ from each other according to more or less hindering α-substituents, corresponding CV data fueled our proposed concept of steric hindrance due to N-α substitution that will significantly prevent pyrrole monomer polymerization. For the rigid and bulky phenyl-substituted monomer 13 (Figure S7, Supporting Information), a smooth electropolymerization reaction was observed that was in accordance with literature data.20,30 Similarly, even when increasing the bulkiness of such an aromatic substituent like, for example, in the case of the pyrene-substituted pyrrole monomer 14 (Figure 6), an easy electropolymerization was observed. A quite likely reason for this result could be the ring planarity of both Clinked phenyl/pyrenyl and pyrrole aromatic species that are

found to be out of plane with respect to the plane of pyrrole ring. This specific 3D bicyclic geometry likely generates less steric hindrance at positions C2,5 of the oxidizable pyrrole ring toward monomer electropolymerization via the formation of new C2/5-C bonds (polymer chain growth). Keeping this concept of N−C connected ring planarity in mind for other unsaturated substituents, electropolymerization studies were pursued with the specific monomer 12 (Figure S8, Supporting Information) that contained a rigid cyclopropyl ring. Again, both N−C connected cyclopropyl and pyrrole rings are also noncoplanar, thereby causing low/none steric hindrance at the pyrrole C2/5 level for successful electropolymerization, and hence it could be expected that such a substituted monomer will afford the corresponding polypyrrole polymer. Indeed, a successful electropolymerization was observed for monomer 12 filling such an expectation. Interestingly and to increase steric hindrance at the pyrrole α-position, the monomer 15 that 2573

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Figure 3. CV curves of electropolymerization of compounds 2 (a) and 3 (b).

Figure 4. Cyclic voltammograms recorded for the electropolymerization of monomer 5 (a) and CV of poly-5 polymer (b) deposited onto the working electrode (Pt disc).

Figure 5. CV curves of electropolymerization of 7 on Pt substrate in acetonitrile solution (a) and CV curve of the poly-7 polymer (b) deposited on the working electrode (Pt disc).

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Figure 6. CV curves of electropolymerization of 14 on a Pt substrate in an acetonitrile solution (a) and CV curve of the poly-14 polymer (b) deposited on the working electrode (Pt disc).

Figure 7. Cyclic voltammograms recorded for electropolymerization of monomer 17 (a) and CV curve of the poly-17 (b) polymer deposited onto the working electrode (Pt disc).

amino acids such as glycine, valine, and leucine were converted into corresponding pyrrole heterocyclic monomers 16, 18, and 19 using the modified Clauson−Kaas procedure previously mentioned. Monomer 17 was prepared from β-alanine. Similarly, monomers 20 and 21 were prepared from their corresponding ester of amino acid. Their electropolymerization behavior including the effect of free nucleophilic COOH groups at N-α substituents of the pyrrole unit was then investigated. Compound 16 was included into this monomer series because of its structural significance as it only possessed a COOH group at a position α to the pyrrole nitrogen with an expected simultaneous low steric influence to pyrrole electropolymerization. To better grasp the effect of α-substitution using carboxylated substituents, we included and examined monomer 17 in this series, where the COOH group was located at the β position. First, monomers 16 (Figure 8a, Figure S10 (Supporting Information)) and 17 (Figure 7) readily produced corresponding polymer films deposited onto the electrode surface, but compound 16 showed a poor electropolymerization behavior disclosing a small increase in current values during successive CV scans compared with monomers

possesses an adamantyl substitution was also tested. This adamantly substituent has a bowl-type 3D geometry with a potential much increased steric hindrance capability regarding pyrrole species electropolymerization. As expected, no electropolymerization was observed for this monomer (Figure S9, Supporting Information). A quite similar result has been reported for a N-cyclohexylpyrrole19 monomer, which was unable to afford the corresponding polymer due to maximal steric hindrance caused by the cyclohexyl group. Over the past few years, the attachment of biomolecules onto CP conducting surfaces has been examined provided that corresponding CP precursors/monomers will contain the appropriate pendant functionality. In this context, some significant examples have been described in the related literature,24,31,32 where CP polymeric coatings have been chemically modified utilizing outer functionalities toward the development of corresponding biosensor constructs33,34 and drug delivery25 systems. Therefore, poly(amino acid-derived pyrrole) polymers might be the best polymeric coating options toward chemically modified conductive electrodes enabling second-step well-known COOH group-related carbodiimide activation chemistries. Therefore, the amine group of typical 2575

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Figure 8. Cyclic voltammograms recorded for the electropolymerization of monomer 16 (a) and 21 (b).

Figure 9. SEM (a) and AFM (b,c) images of a poly-17 polymer film.

supposed to increase due to further α-alkyl substitution. As expected, monomers 18, 19 (Figure S11, Supporting Information), and 20 (Figure S12, Supporting Information) showed no polymerization behavior due to steric hindrance augmented by alkyl chains substituting the α-position. Similar results had also been observed for N-α-substituted carboxyalkyl monomers.35 This concluded that steric hindrance caused by αsubstitution is plausibly the major criterion behind preventing polymerization of monomers, but contributions from nucleophilic and electrophilic substitutions could not be excluded, too. It is well known that conducting polymer (CP) films serve as support for metal nanoparticles (MNPs)36 which are then utilized as nanocatalysts37 for various reactions (such as Heck, oxidation, and reduction reactions). These polymer-supported nanocatalysts have also found applications in fuel-cell technology.38,39 Physical properties like high surface area, conducting nature, and porosity of CPs allow these polymers to be utilized as support for MNPs,40 but of all of these properties, surface activation of polymer films is necessary to provide nucleation sites for metal ions. The simplest way to generate good nucleation sites on polymer films relates to the presence of surface functional groups, for example, carboxyl moieties on the polymer surface, as these can interact/capture MNPs effectively.41 Thus we assume that morphological studies of Ncarboxyalkyl series polypyrroles (poly-16, poly-17, and poly21) will be of real importance because these polymers contain

17. One plausible factor may be the electron-withdrawing property of carboxyl groups, which made the growing polymer oxidize at a slow rate. In this context, Govindaraji and colleagues24 have also proposed that the noneffective polymerization of selected N-carboxyl pyrrole monomers, that is, meaning presenting this type of N-substitution, was likely due to the intramolecular trapping of in situ formed cation radicals during the initial polymerization step by the nucleophilic OH group of the pending carboxyl functions present at the αposition of N-substituted pyrrole then rationalizing the poor polymerizability of monomer 16. In contrast, the presence of this same COOH group at any other position of the N-alkyl substituent (monomers 17) will not have any detrimental effect on corresponding polymerization processes due to less electron-withdrawing effect of the carboxyl group. On the basis of these results, one might expect that converting acid to ester will make the monomer more conductive, but it was observed that monomer 21 (Figure 8b) showed a similar type of polymeric behavior as we can see that current increase for both monomers (16 and 21) is almost similar. This indicated that there may be less contribution via internal trapping of cation radical by COOH group, but its high electronwithdrawing property and steric factor is playing a crucial role in preventing polymer (poly16 and poly21) conductive nature. Conductivity of polymers 16 and 21 was assumed to be further reduced even toward zero when steric factors were 2576

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Figure 10. Optimized geometries of the cation radicals of selected species along with important bond lengths and corresponding Mulliken spin densities at both C2 (N-α′) and C5 (N-α″) carbons of pyrrole rings. Bond lengths are in angstroms.

such free pending carboxyl moieties enabling further interactive chemistry. Moreover, SEM and AFM analyses were employed to study the morphology of the specific poly-17 polymer. Careful examination of corresponding images (Figure 9) revealed that the deposited polymer has a rough surface instead of a porous one, which also endues high surface area to this same deposited film being potentially useful as support for MNPs along with carboxyl-functionalized films. The average thickness of this polymer film was found to be about 50−80 nm. The theoretical analysis is focused on a representative set molecules consisting of 1, 2 (N-(alkyl) pyrroles), 12, 13, 15 (N-(rigid substituent) pyrroles), and 16, 19 (N-(carboxyalkyl) pyrroles). The most accepted Diaz’s mechanism42,43 for the electropolymerization of pyrrole proposes the formation of a cation radical by the oxidation of monomer as the first step of the reaction, which is followed by the coupling between two radicals to form a dimer. Because the resonance structure of the pyrrole cation radical shows greater unpaired electron density at the two equivalent α-positions, dimerization takes place by the Cα′−Cα′/Cα″ bond formation. In the case of substituted pyrrole molecules, it is important to look at the electron density at the N-α′/α″ positions, especially when the substituent is not symmetric with respect to the pyrrole ring because such substitutions can induce structural and electronic alterations to the pyrrole ring. Therefore, we analyzed the electron density at the N-α′/α″ positions of the pyrrole ring using Mulliken spin density (spin population).44,45 Figure 10 shows the optimized structure of the cation radicals of all systems along with the Mulliken spin density values at the C2 and C5 atoms of the pyrrole ring (N-α′/α″ positions) and important bond lengths (numbering of atoms in the pyrrole ring is shown in the structure of 1). It can be seen that the spin population at both C2 and C5 atoms of 1, 12, and 13 is 0.54, which means that both C2 and C5 atoms in these molecules are equivalent, but the spin density at C5 atom in 2, 15, 16, and 19 is higher than that of

their corresponding C2 atom. It should be noted that the Nsubstituent is not symmetric with respect to the pyrrole ring in these molecules and structural changes in the pyrrole ring are evident from different bond lengths for equivalent bonds. Because the electron density at C5 is higher than the C2 one in 2, 15, 16, and 19, dimerization of these monomers would take place by the bonding between C5 and C5′ atom of the second radical cation. On the basis of the spin density values, dimer, trimer, and tetramer of all molecules are optimized. Optimized structures along with structural parameters of dimer, trimer, and tetramer constructs of all of these molecules have been provided in the Supporting Information section (Figure S13 and S14 (Supporting Information)). Relative energy46 values of dimer, trimer, and tetramer species of all molecules are summarized in Table 2 along with the torsion angle between the pyrrole rings and the C5−C5′ bond lengths of dimer molecules. In the following discussion, dimer, trimer, and tetramer species of each molecule have been designated by subscripts di, tri, and tetra, respectively. Table 2. Relative Energies of Dimer, Trimer, and Tetramer of All Studied Molecules along with the Torsion Angle between the Pyrrole Ring Planes and C5−C5′ Bond Lengths in Dimer Molecules relative energy (kcal/mol)

molecule monomer 1 2 12 13 15 16 19 2577

0.0 0.0 0.0 0.0 0.0 0.0 0.0

dimer

trimer

tetramer

C5−C5′ distance in dimer (Å)

8.3 14.2 10.7 11.1 18.3 5.8 16.3

17.1 26.6 22.6 22.6 41.9 12.5 25.2

25.9 38.6 34.9 34.0 65.3 19.1 35.1

1.46 1.47 1.46 1.46 1.47 1.46 1.47

torsion angle in dimer (deg) 61.5 76.1 46.2 59.4 82.9 64.5 77.3

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revealed that N-α substitution is a determining factor for the successful electropolymerization of such pyrrole derivatives caused by α-substitution of N-(alkyl) pyrrole monomers, even when considering a small methyl group, enabling us to prevent a successful electropolymerization reaction. Substitution with hydroxy functionality was found to give poor polymerization results. A detailed theoretical calculation study helped us to better understand and rationalize our experimental findings. Hopefully this thorough study will help us to much improve the rational structural design of suitable Pyr-based monomers that will enable the formation of corresponding functional electrodeposited polypyrroles being thus a significant input to this CPrelated field.

Dimerization is endothermic for all molecules, and the endothermicity increases as the polymerization proceeds toward trimer and tetramer species. Dimer of 1 is 8.3 kcal/ mol less stable than its monomer, and the relative energy of 1tri and 1tetra is 17.1 and 25.9 kcal/mol, respectively. The torsion angle between the pyrrole rings in 1di is 61.5°, and the C5−C5′ bond length is 1.46 Å. Alkyl substitution at the N-α position of the substituent increases the steric bulkiness, which led to a higher torsion angle of 76.1° and C5−C5′ bond length of 1.47 Å in 2di. High relative energy values of 14.2, 26.6, and 38.6 kcal/ mol, respectively, for 2di, 2tri, and 2tetra can be understood from increased steric interactions in these molecules. The high endothermic nature for the polymerization of monomer 2 explains the experimental observation that N-α-substituted pyrroles do not undergo polymerization. Among N-(rigid substituent) pyrrole molecules (12, 13, and 15), relative energies of 12di and 13di are ∼11 kcal/mol, while that of 15di is 18.3 kcal/mol. Cyclopropyl and phenyl substituents in monomers 12 and 13 do not impose strong steric interactions on dimerization as torsion angles in 12di and 13di are 46.2 and 59.4°, respectively. An adamentyl substituent on N in 15 causes a strong steric interaction in 15di (torsion angle is 82.9°), which increases its relative energy and is hence unstable. Its stability decreases further when it goes to trimer (41.9 kcal/mol) and tetramer (65.3 kcal/mol). Therefore, monomer 15 cannot be polymerized. Compounds 16 and 19 are N-(carboxyalkyl) pyrrole molecules, and the possibility for the cyclization reaction should also be analyzed along with the polymerization. Relative energy of 16di, 16tri, and 16tetra is 5.8, 12.5 and 19.1 kcal/mol, respectively, while that of the cycloadduct of 16 is 30.4 kcal/mol, suggesting that the polymerization is the preferred pathway (Figure S15, Supporting Information). Monomer 19di experiences a strong steric interaction because the torsion angle and C5−C5′ bond length are 77.3° and 1.47 Å, respectively. Dimerization of compound 19 is endothermic by 16.3 kcal/mol as a result of strong steric interactions by the bulky N-substituents, which does not favor the polymerization reaction. The relative energy values calculated for dimer, trimer, and tetramer moieties of each polymer with respect to its corresponding monomer reflect the polymerization efficiency (p.e.) of each polymer. It can be seen that there exists a linear correspondence between the dimerization energy and the torsion angle between the pyrrole rings of the dimer, which is mainly decided by the steric nature of the N-substituent. The steric effect in trimer and tetramer cannot be described using a single parameter, but the energy change associated with the formation of trimer from dimer and tetramer from trimer is nearly that same as that of dimerization energy for all molecules, which suggests that the polymerization efficiency is largely controlled by the steric nature of N-substituents.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed synthetic procedures and characterization data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*J.-P.L.: E-mail: [email protected]. *S.G.: E-mail: [email protected]. Author Contributions ∥

Sunil Kumar and Sada Krishankanth contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was received from the Department of Science and Technology, India (Grant No. SERB/F/2408/2012-13). Sunil Kumar thanks the University Grant Commission for providing doctoral fellowship. We thankfully acknowledge the Director, IIT Mandi for research facilities. The support of Advanced Materials Research Center (AMRC), IIT Mandi, for sophisticated instrument facility is acknowledged. We thank Sudhir Saralch and Sacheen Kumar for recording SEM and AFM images, respectively, at NIT Hamirpur (H.P.). We thank Dr. Jaspreet Kaur Randhawa for her help during manuscript preparation.

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REFERENCES

(1) Roncali, J. Electrogenerated Functional Conjugated Polymers as Advanced Electrode Materials. J. Mater.Chem. 1999, 9, 1875−1893. (2) Vernitskaya, T. y. V.; Efimov, O. N. Polypyrrole: A Conducting polymer; its Synthesis, Properties and Applications. Russ. Chem. Rev. 1997, 66, 443−457. (3) Janata, J.; Josowicz, M. Conducting Polymers in Electronic Chemical Sensors. Nat. Mater. 2003, 2, 19−24. (4) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Santos, D. A. D.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Electroluminescence in Conjugated Polymers. Nature 1999, 397, 121−128. (5) Waltman, R. J.; Bargon, J. Electrically Conducting Polymers: a Review of the Electropolymerization Reaction, of the Effects of Chemical Structure on Polymer Film Properties, and of Applications Towards Technology. Can. J. Chem. 1986, 64, 76−95. (6) Kaiser, A. B. Systematic Conductivity Behavior in Conducting Polymers: Effects of Heterogeneous Disorder. Adv. Mater. 2001, 13, 927−941.

4. CONCLUSIONS To summarize, as electropolymerization of N-substituted pyrrole derivatives has been proven to be an efficient chemico-technological tool to develop materials/functionalmodified electrodes enabling a wide range applications, our efforts have been directed to better understand the structural requirements for these N-substituted pyrroles to provide successful electropolymerization reactions. Corresponding chemically modified electrodes thus presented corresponding conductive functional polypyrrole films based on a rational pyrrole monomer design. This present study effectively 2578

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