- Email: [email protected]
Mechanism of 3,4-diarylpyrrole electrooxidation
Accepted Manuscript Title: Mechanism of 3,4-diarylpyrrole electrooxidation Author: M. Czichy P. Zassowski T. Jarosz E. Go´nka E. Janiga M. St˛epie´n M. Łapkowski PII: DOI: Reference:
S0013-4686(16)30604-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.066 EA 26896
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
17-12-2015 7-3-2016 10-3-2016
Please cite this article as: M.Czichy, P.Zassowski, T.Jarosz, E.Go´nka, E.Janiga, M.St˛epie´n, M.Lapkowski, Mechanism of 3,4-diarylpyrrole electrooxidation, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights: The ,'-coupling of pyrrole radical cations to -dimers is reversible and the deprotonation of the dimer is kinetically hindered. The addition of bases does not result in deprotonation of doubly charged –dimer in 2- and 2'-position. The slightly conducting poly(3,4-diarylpyrrole) is obtained via addition of neutral monomer to cation radical. The electrooxidation of 3,4-bis(3,4-dimethoxyphenyl)pyrroles is not produced expected poly(phenanthropyrrole) polymer.
1
Graphical abstract
2
Mechanism of 3,4-diarylpyrrole electrooxidation M. Czichya*, P. Zassowski a, T. Jarosz a, E. Gońkab, E. Janigab, M. Stępień b, M. Łapkowski a,c a
Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland
b
Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
c
Center of Polymer and Carbon Materials, Polish Academy of Science, M. Curie-Skłodowskiej 34, 41-819 Zabrze, Poland Corresponding author: Tel.: +48-322371736, E-mail: [email protected]
Abstract The purpose of this study was to investigate 3,4-diarylpyrrole electropolymerization and simultaneous or subsequent intramolecular oxidative coupling leading to poly(phenanthropyrrole). Small amounts of πconjugated products were obtained only under the conditions of increased concentration of the monomer and increased rate of potential sweep, which enabled both processes – monomer oxidation and deprotonation of the σ-dimer. The combined electrochemistry, UV-vis-NIR, ESR and TD-DFT results show, that the α,α'-coupling of 3,4-diarylpyrrole derivatives is inhibited by the stability of the σ-dimer dication, inhibition of the deprotonation and the reversal of the σ bond formation resulting in regeneration of the monomer. The addition of pyridine did not result in σ–dimer deprotonation in 2- and 2'-position.
3
1. Introduction
The synthesis of pyrrole derivatives is an important area of heterocyclic chemistry due to the ubiquity of pyrrolic motifs not only in natural products but also in materials chemistry. In the latter context, pyrrolebased polymers are of particular importance, with current research focused on increasing their solubility [1,2], and modification of their optical properties [3,4]. Both effects can be readily tailored through the use of substituted pyrroles. Electropolymerization of substituted pyrroles has been successfully performed, usually using N-substituted pyrroles [5], 3,4-alkylenedioxypyrroles [6], indoles or isoindoles [7]. The electrooxidation of 2,3-diphenylindole results in an almost quantitative transformation into the indoleindolenine dimer, involving linkage through a fused benzene ring [8]. Furthermore, electrochemically prepared polypyrroles suffer from the occurrence of undesired α–β and β–β couplings during polymerization [9], and the α–α coupling selectivity can be achieved by using 3,4disubstituted monopyrrole precursors [10]. The most common 3,4-disubstituted polypyrroles, prepared using electrochemical method are poly(3,4-dimethoxypyrrole)s [11] and poly(3,4-diethylenodioxypyrrole)s (PEDOP) [12,13]. In turn, 3,4-diarylpyrroles are interesting as precursors to physiologically and biologically active compounds [14]. An important property of these polymers is their ability to be doped through a partial oxidation or reduction. The use of electrochemical techniques allows to obtain polymers with specified doping degree in a controlled manner via partial oxidation or reduction, which may be implemented through the sign and value of applied potential [15]. The 3,4-diarylpyrrole moiety is a constituent of a wide array of natural products, such as 3-chloro-4-(3chloro-2-nitrophenyl)-1H-pyrrole (pyrrolnitrin) [16], Lamellarin [17] or Lycogarubin C [18]. Cyclization of α-amino carbonyl compounds and aldehydes is a simple method for the preparation of 1,3,4-triarylpyrroles [19]. 3,4-Diarylpyrroles have been developed using the Hinsberg reaction of benzil with dimethyl Nacetyliminodiacetate [20]. The Barton–Zard pyrrole synthesis using nitroalkenes [21], and the van Leusen TOSMIC method [22] are now well established for the preparation of β-arylpyrroles. Other synthetic methods include the reduction of β-nitrostyrene with TiCl3 [23], Knorr-type condensations of amino ketones [24], and palladium-catalyzed Suzuki cross-coupling of 3,4-dihydroxypyrrole bis-triflate derivatives [25]. The use of copper or nickel catalysis was recently found to yield highly selective denitrogenative annulations of vinyl azides with aryl acetaldehydes, leading to both 2,4- and 3,4-diaryl substituted pyrroles [26]. 3,4-Diarylpyrroles are important precursors to octaaryl- [27,28] and dodecaarylporphyrins [29,30]. Recently, some of us have shown that electron-rich 3,4-diarylpyrroles can undergo a tandem inter- and intramolecular oxidative coupling, providing access to bis(phenanthropyrroles) with extended π-conjugation. These bis(phenanthropyrroles) are characterized by restricted rotation around the α−α bond and exhibit strong blue fluorescence and large Stokes shifts [31]. This type of coupling reactivity was earlier employed in the synthesis of self-assembling porphyrin-based materials [32]. Literature reports detail various benzo-fused heterocyclic materials [33], characterized by high luminescence quantum yields [34]. Many such derivatives have two-dimensional self-assembled structures [35]. The supramolecular, self-assembly processes, occurring for these systems, are affected by a complex interplay of inter- and intramolecular interactions, non-covalent forces such as hydrogen-bonding, electrostatic or π–π stacking, which is also related to the luminous efficiency. This paper presents an attempt to control 3,4-diarylpyrrole electro-coupling, in order to obtain of oligo(phenanthropyrroles). Apparently, the intermolecular coupling is a rapid process and can compete with the intramolecular ring closure, suppressing the formation of monomeric phenanthropyrrole.
4
Mostly, electropolymerizations of aryl-substituted pyrroles were carried out using: N-aryl-pyrroles [36] or N-aryled and 3,4-substituted pyrroles [37]. Electropolymerization of 3,4dicarboranyl-functionalized pyrroles has also been reported [38]. 3-Aryl- and 3,4-aryl polymers are often obtained with other heterocyclic compounds such as thiophene derivatives [39,40] or their oligomeric precursors - e.g. 3',4'-diphenyl-2,2':5',2''-terthiophene [41]. Poly(benzothiophene)s can be obtained by electrocopolymerization with benzothiophene and thiophene or pyrrole derivatives [42]. In general, due to the steric hindrance in 3,4-substituted derivatives, straightforward electropolymerization was found to be unfeasible in compared to similar substances with aliphatic substituents [43]. Hovewer, poly(3,4-diphenylpyrroles) were grown electrochemically from solutions of acetonitrile containing diphenylpyrrole, tetrahexylammonium poly(styrenesulfonate) and sodium perchlorate, but no information was given concerning the possibility of coupling reactions between substituents [44]. In turn, poly(phenantro[9,10c]thiophene) can be prepared by electrochemical polymerization with Bu4NPF6 and it showed high conductivity (105 S∙cm-1) [45]. The mechanism of electropolymerization of aromatic heterocyclic monomers is still not fully understood and remains a subject of controversy. Tanaka et al. [46] carried out a study of coupling processes between two pyrrole radical cations. They have envisioned two possible routes of the pyrrole coupling involving on the one hand a σ-radical and on the other a π–radical [47]. Diaz proposed the classical route of conducting polymer formation which involved the dimerization of monomeric radical cations at their α-positions followed by deprotonation of the doubly charged σ-dimer, resulting in aromatic neutral dimer [48]. Other studies have shown that it is not the rate of coupling, but rather the elimination of protons from the σ-dimer, that is the rate-determining step [49]. The doubly charged σ-dimer and σ-intermediates of pyrrole oligomers are weak acids. Therefore, sufficiently strong bases are necessary to initiate proton elimination and ensure polymerization progress (e.g. water [50], pyridine [51]). In turn, Pletcher proposed another mechanism in which the cation radical formed by the loss of an electron reacts directly with a neutral molecule giving a cation dimer. The cation dimer then loses in following sequence: proton, electron, and second proton forming the neutral dimer [52]. Additionaly, Satoh established that this type of coupling reaction is affected by the monomer concentration [53]. Takakubo demonstrated on the basis on molecular orbital calculations, that the addition of a cation radical to a neutral molecule is symmetry forbidden and thus requires a high activation energy [54]. In some conjugated systems, such as α-capped bithiophenes and bipyrroles, monocation radicals were found to form σ-dimers and π-dimers [55,56]. The dimerization and deprotonation steps can be inhibited by too bulky substituents, and, in particular, by α-substitution. Thus, the anodic substitution and addition reactions of aromatic compounds via their π-radical cations are the object of interest to organic electrochemists.
2. Experimental 2.1. Chemicals and materials Tetrabutylammonium perchlorate (TBAP, 99.0%, Fluka, dried over night under vacuum before use), pyridine (Fluka), 2,6-di-tert-butyl-4-methyl-pyridine (Aldrich), dichloromethane (DCM, for HPLC, ≥99.8%, Sigma-Aldrich), acetonitrile (ACN, RPE ACS for analysis, Carlo Erba, ACN was stored over 4A molecular sieves), ferrocene (98%, Sigma-Aldrich), (BAHA, technical grade, Sigma), argon 6.0 (SIAD Group) were used.
5
2.2. Instrumentation and working conditions
Electrochemical investigations were performed using a CH Instruments Electrochemical Analyzer model 620 in a three-electrode system at room temperature. The experimental cell comprised a platinum disk (surface area: 2 mm2), used as the working electrode, a platinum coil counter electrode, and an Ag electrode as a o’ reference. The reported electrode potentials are given relative to formal redox potential (E ) of the ferrocene/ferrocenium (Fc/Fc+) couple. The potential of the Fc+/Fc couple is 0.37 V with respect to used reference electrode. The solutions in the electrochemical cell were purged with argon prior to the experiments. Cyclic voltammetric (CV) measurements were conducted at potential sweep rate of 50 or 250 mV∙s-1. Electrooxidation of the monomer was done at a concentration of 2.0, 5.5 or 7.5 mM, with 0.1 M TBAP electrolyte in DCM or ACN, without or with pyridine (1 v/v%) or 2,6-di-tert-butyl-4-methylpyridine (1 v/v%). The HOMO energy was graphically determined from the onset potential of the oxidation (Eox onset) using the Trasatti equations for non-aqueous electrolytes: HOMO = -([Eox onset − E1/2(Fc/Fc+)] + 5.1) (±0.10 eV), where E1/2(Fc/Fc+) is the half-wave potential of the Fc/Fc+ couple [57]. Spectroscopic changes accompanying electrooxidation were monitored by in-situ thin-layer UV-vis-NIR technique with the Ocean Optics QE6500 and NIRQuest apparatus equipped with DH-2000-BAL halogen and deuterium sources and detectors connected with a potentiostat AUTOLAB PGSTAT100N. The spectra between 200 – 1600 nm were recorded with a 1- and 10-second acquisition time. A thin-layer optical cell characterized by the optical path of 0.2 mm length was a quartz cuvette containing ITO/quartz electrode (20±5 Ω/sq, Praezisions Glas & Optic GmbH), Teflon separator (0.2 mm), silver wire (pseudoreference electrode) and platinum mesh (counter electrode). The experiments were carried out in a cuvette connected to the lamps and detector through fiber optics (400 μm, Ocean Optics). Electron spin resonance (ESR) spectra were acquired using a JEOL JES FA-200 X band spectrometer. A capillary quartz spectroelectrochemical cell was used, equipped with a Pt wire working electrode, Ag wire pseudoreference electrode (calibrated vs. Fc/Fc+) and a Pt coil counter electrode, as in the authors’ earlier works. Relative spin concentrations for each potential step have been obtained through manual double integration of the first derivative ESR spectrum. BAHA titration experiments were carried out using a Cary 500 SCAN UV-vis-NIR spectrophotometer. 1H NMR spectra were recorded on a high-field NMR spectrometer (1H frequency 600.13 MHz). Spectra were referenced to the residual solvent signal (dichloromethane-d2 5.32 ppm). The concentration of compound 1 was 1 mM in CD2Cl2. 1 was titrated with a CD2Cl2 solution of BAHA. DFT/TDDFT calculations were carried out with B3LYP or CAM-B3LYP [58] hybrid functionals combined with 6-31G(d,p) basis set. For investigated compounds ground state geometries were optimized with no symmetry constrains to a local minimum, which was followed by frequency calculations. In all cases no negative or imaginary frequencies were found. All calculations in this work were conducted with polarizable continuum model (PCM) using dichloromethane as solvent as implemented in Gaussian 09 software. Input files and molecular orbital plots were prepared with Gabedit 2.4.7 software [59]. All calculations had been carried with Gaussian 09 software [60]. The charge and multiplicity setting were set as follows: 0 and 1 for neutral molecules, 1 and 2 for radical cation, 2 and 1 for σ-dimers.
6
3. Results and discussion
The cyclic voltammograms (CVs) of considered compound were recorded in TBAP/DCM (1, 2, 3 compounds) and TBAP/ACN electrolyte (1, 2 compounds). The examples of appropriate CV curves and oxidation peak potentials are given in supporting information. In all cases, the occurrence of multi-step and irreversible processes was observed. The first oxidation peaks of 1 and 3 are located at similar potentials (1: +0.45 V, 3: +0.44 V, in 0.1 M TBAP/DCM). The first oxidation of 2, however, is shifted towards more positive potentials (+0.63 V), in comparison to the oxidation potential values of the other compounds. The HOMO energy levels determined via electrochemical experiments are -5.56, -5.74 and -5.55 eV for compounds 1, 2 and 3, respectively. In the main step, the maximum applied potential was limited to just beyond the first oxidation peak of 1 monomer with 2.0 mM concentration in TBAP/DCM (Fig. 1a). It shows an irreversible peak at +0.45 V corresponds to the formation of monomer radical cation (1)●+. It is commonly known, that the presence of products with extended π-conjugation length with increasing numbers of pyrrole units causes a decrease in the oxidation potential, relative to the oxidation potential of the monomer. However, subsequent CV curves have a similar shape, with no new redox behavior, which indicates the absence of a π-conducting dimer/oligomer. Next, there was an attempt to carry out the electropolymerization of 1 by anodic oxidation with increased monomer concentration. Increasing the monomer concentration (5.5 mM) (Fig. 1b) or using acetonitrile as the reaction medium (Fig. 1c) reveals the hysteresis loop with a crossover between the anodic and cathodic half-cycle curves, which can be explained by the formation of σ-dimer: [σ–(1)2]2+ (Scheme 3A – paths a0, a1) [61,62]. The elimination of protons at the level of the [σ–(1)2]2+ does not take place probably due to stability of the σ-dimer. The broad wave at negative potentials below 0.0 V can be originate from an irreversible reduction of [σ–(1)2]2+ with regeneration of 1 (Scheme 3A – path a2).
This behavior was confirmed by the ESR measurement shown in Figure 2a. ESR spectroelectrochemical has been used to study a variety of relative spin concentrations for each potential step of 1 oxidation in TBAP/ACN (Fig. 2a). There was observed an increase in the relative concentration of spinbearing species during staircasing the applied potential towards positive potentials, typical for cation radicals being generated during electrooxidation. When the applied potential is staircased back towards negative potentials, however, the spin concentration initially continues increasing, evidencing that spinless, dicationic species [σ–(1)2]2+, generated during oxidation are being transformed into cation radicals, while the existing cation radicals do not undergo any significant decay. This is evidence in favor of a significant restructuring of the cation-radical, with increase of its energetic stability and lowering its redox potential. Further lowering the applied potential eventually brings about a decrease in the relative spin concentration. Should the redox process generating the spin-bearing species be reversible, we would expect this decrease in spin concentration to occur exponentially with lowering the applied potential. This is not the case, however, as a significant amount of spins persist in the case of spin-bearing species generated through oxidation of (1) even at potentials as -0.4 V. Such a behavior is a testament to the relatively high stability of the cation radicals. Spectroscopic changes accompanying electrooxidation of 1 were monitored by in-situ UV-vis-NIR. Fig. 3a shows the successive UV-vis spectra of the 1 compound (2 mM), measured at the open circuit potential “oc” (neutral monomer in TBAP/CAN ▬), during the oxidation per 10 second at the potential of first oxidation peak (+0.48 V) (▬), again at “oc” (▬), and finally at -0.35 V (▬). Absorbance maximum of neutral 1 was found at 294 nm as shown in Figure 3a (insert). Vis-NIR spectra of anion radical observed at 407, 878, 976 and 1166 nm (▬). Next, we 7
observed a significant increase of bands at 456, 640, as well as those previously observed at 878, 976 and 1166 nm after switch-off the potential (▬). Apparently the increase in absorbance at 456, 640 and 878 nm can be attributed to the formation of [σ–(1)2]2+. In turn, the bands at 976 and 1166 nm indicate the presence of unreacted cation radical. Optical spectra measured at -0.35 V (▬) exhibited the reduced absorbance in a broad range of wavelengths (above 500 nm), indicating the backward transformation to 1 (Scheme 3A, a2). Simultaneously, weak bands in the 500–1200 nm range indicate the presence of trace quantities of (1)●+ and/or [σ–(1)2]2+. After this experiment any solid product was not observed on the ITO electrode. Simulated optical spectra of different forms of 1 are presented in Fig. 4. Both (1)●+ and [σ–(1)2]2+ showed a large number of transitions in the UV-vis range. The difference is in the Vis range, where this is especially seen peak of [σ–(1)2]2+ at 655 nm for simulated, clearly seen in the UV-Vis spectroelectrochemical experiments at the wavelength of 640 nm. In comparison, the absorption in experimental spectra in the 900-1400 nm range can be attributed mainly to (1)●+. Given the previously described experimental data, it seems to confirm that the stabilization of doubly charged σ-dimer caused by inhibited α-deprotonation (Scheme 3A – path a3) results in breaking of α,α-bond of the σ-dimer and restoration of 1 monomer during the negative potential scan (Scheme 3A – path a2). The above experiments were repeated with the addition of either of the two pyridine bases, in order to facilitate the deprotonation of [σ–(1)2]2+. The CVs show that addition of pyridine or hindered pyridine (1 v/v%) does not support the [σ–(1)2]2+ deprotonation (supplemented data). However, a change of the shape of the first peak, and an increase of peak intensity (▬), as well as an offset of E ox onset value can indicate changes in the structure, e.g. deprotonation. In turn, we observed the negative shift of the cathodic wave to -0.65 V, for example, as a result of reduction to radical of pyridine [63]. The presence of pyridine in the solution during electrooxidation causes a significant decrease of absorbance above 1000 nm, which may indicate the consumption of cation radicals. Nonetheless, the bands about 640 and 878 nm can be assigned to [σ–(1)2]2+ as well (Fig. 5b). A small amount of π-conjugated product was obtained on Pt electrode under the conditions of increased concentration of the monomer (7.5 mM) in ACN and increased rate of potential sweep (250 mV∙s-1) (Fig. 6). The current increases potential in the range of (-0.24) ─ +0.35 V) is very weakly, it can be concluded that only a small amount of polymer is deposited on the electrode during cycling. The shape of the curve and the onset potentials of oxidation at -0.24 V are both similar to the typical CVs of polypyrroles (Fig. 6b ▬). A trace amount of a solid π-conjugated product was confirmed by measurement of π–π* transition band at around 430 nm (λonset) (supplemented data). The increase in scan rate implies a shortening of the experimental time scale, which prevents a substantial generation of (1)●+. In the second - while also quickly we achieve -0.35 V potential, the doubly charged σ-dimer was electroreduced with regeneration of monomer, increasing the concentration of the latter. Low concentration of (1)●+ is to reduce the efficiency of path resulting in [σ–(1)2]2+. In this way, both (1)●+ and neutral 1 molecule are presented in the vicinity of the electrode and this factor can lead to their reaction between them to [σ–(1)2]●+. This indicates that besides the occurrence of main reaction to give [σ–(1)2]2+ (Scheme 3A – path a1), there is a path of electropolymerization via [σ–(1)2]●+ propagation (Scheme 3A – path a4). ESR study just confirms that the presence of spin-bearing species at -0.4 V can be attributed to [σ–(1)2] ●+ (Fig. 2a).
We examined by DFT/TDDFT method the possiblility of dimerization of (1), with two different mechanisms: as a reaction between two radical cations (1)●+ to form [σ–(1)2]2+ or as an attack of radical cation (1)●+ on neutral molecule (1), to form [σ–(1)2]●+. Therefore, we have calculated structures resulting 8
from these two cases, end-point geometries are presented in Fig. 5. As it can be seen, in case of [σ–(1)2]2+ geometry typical for σ–dimer has been obtained. Moreover, the middle benzene rings are lying in the same plane and thus are appropriately positioned for increase of π-π interactions, hydrogen-bond interactions between the methoxy groups, and finally, inhibition of the deprotonation in 2- and 2'-position of [σ–(1)2]2+. In turn, the covalent bond between pyrrole rings was not retained during geometry optimization of [σ–(1)2]●+. Furthermore, spin density of such optimalized geometry is placed exclusively on one of the molecules, which leads us to the conclusion that reaction between (1)●+ and (1) is less likely to occur, indicating that dominant intermolecular mechanism involves dimerization of two radical cations (1)●+. This is even more probable because of conditions present in the electrochemical oxidation – low concentration of neutral form of (1) near the vicinity of electrode surface, which would make the attack of (1)●+ on (1) with giving [σ–(1)2]●+ even less probable. As another possibility, we investigated the intramolecular coupling in the o-position of the phenyl ring. In contrast to formation of [σ–(1)2]●+, in this case optimization of geometry showed a formation of intramolecular cyclization product [σ-(1)]●+. However, the thermodynamics of such a process is far less favorable, when compared to the intermolecular dimerization of (1)●+. Calculated free energy of such processes are equal to 23 kcal/mol and 5 kcal/mol at the temperature of 298.15 K for intermolecular and intramolecular path, respectively. The total chemical oxidation process is dominated by the mechanism of coupling between (1)●+, what is evidenced by 1H NMR spectroscopy (supplementary data). The broadened signals were not observed in the chemical shift range corresponding to pyrrole ring, where the chemical exchange between (1) and (1)●+ would be lead to significant line broadening. The oxidation of monomer 2 was also carried out by applying polarization limited to just beyond the first oxidation peak (supplemented data). The similar shape of subsequent CVs can indicate the presence of renewed 2. UV-vis spectroelectrochemical of 2 oxidation was characterized by a slow increase of absorbance between 350 and 600 nm, with maxima at 356, 444 and 499 nm (Fig. 7a). There are also very weak bands at about 840 nm. This feature of the spectrum may indicate the presence of [σ–(2)2]2+ dimer with slight amount of (2)●+ cation-radical. The spectrum measured after the reduction at -0.01 V (▬) revealed the absorbance reduction in a broad range of wavelengths (above 350 nm), what can be assigned to regeneration of 2 monomer, but weak bands in the 350–500 nm range indicates the presence of trace quantities of stable [σ– (2)2]2+ too. Similar reasoning can be applied to the example of (2) (Fig. 4b), for which the simulated optical spectra support the formation of [σ–(2)2]2+ in the experimental spectra. ESR spectroelectrochemistry study is a testament to the relatively lower stability of the charged spacies of 2 derivative than in the case of 1 (Fig. 2). The 3 compound, which is a bis(phenanthro)-derivative of 2, is characterized by repetitive form, partial reversibility of the first peak, which occurs at much lower potential (+0.44 V) (▬) relative to 2. This process is identified as the formation of (3)●+ (supplementary data). The most characteristic changes in the spectra are observed at 364, 482, 576, 737, around 1044 and above 1600 nm (Fig. 7b). All the previously mentioned bands can be attributed to (3)●+ (▬). The small intensity of the bands follows from the fact that the experiment had to be carried out in a solution of the sparingly soluble compound. The previous conclusions indicate that it is not possible to obtain the 3 derivative by electrooxidation of 2 compound.
4. Conclusions The electrooxidation of 3,4-bis(3,4-dimethoxyphenyl)pyrroles was shown not to produce the expected poly(phenanthropyrrole) polymer. Apparently, the coupling of pyrrole radical cations to σ-dimers is reversible and the deprotonation of the dimer is kinetically hindered. The high stability of σ-dimer results in 9
the breaking of its 2,2'-bond and regeneration of the monomer during electroreduction process. The addition of bases does not result in σ–dimer deprotonation in 2- and 2'-position. The ESR, UV-vis-NIR study and DFT calculations indicate that the radical cation dimerization should be favored relative to the addition to the neutral pyrrole, although the latter process may contribute to the observed formation of spin-bearing species.
Acknowledgement The project was funded by the National Science Centre of Poland (to M.S., decision DEC2014/13/B/ST5/04394). This research was supported in part by PL-Grid Infrastructure. Appendix. Supporting information Supporting information related to this article can be found at [link].
References
[1] M. R. Nabid, A. A. Entezami, J. Appl. Poly. Science 94 (2004) 254 –258. [2] Min-Kyu Song, Young-Taek Kim, Bum-Seok Kim, Jinhwan Kim, Kookheon Char, Hee-Woo Rhee, Synth.Metals 141 (2004) 315–319. [3] F.S. Damo, R.C.S. Luz, L.T. Kubota, Electrochim. Acta 51 (2006) 1304–1312. [4] K. Dutta, S.K. De, Sol. State Comm. 140 (2006) 167–171. [5] S. Kumar, S. Krishnakanth, J. Mathew, Z. Pomerantz, J-P Lellouche, S. Ghosh, J. Phys. Chem. C, 118 (5) (2014) 2570–2579.
[6] R. M. Walczak, J. R. Reynolds, Adv. Mater. 18 (2006) 1121. [7] S.B. Rhee, M.-H. Lee, B. S. Moon, Y. Kang, Korea Polym. J. 1(1) 1993, 61-68. [8] V.-T. Truong, B. D. Turner, R. F. Muscat, M. S. Russo, Proceedings of the SPIE - The International Society for Optical Engineering 98 (1997) 3241. [9] D.V. Konev, O.I. Istakova, A. Sereda, A. Shamraeva, C.H. Devillers, M.A. Vorotyntsev, Electrochim. Acta 179 (2015), 315-325. [10] A. Smie, A. Synowczyk, J. Heinze, R. Alle, P. Tschuncky, G. Gӧtz, P. Bäuerle, J. Electroanal. Chem. 452 (1998) 87–95. [11] F. Gassmer, S. Graf, A. Merz, Synth. Met. 87 (1997) 75-79. [12] P. Schottland, G. Sonmez, J. R. Reynolds, Macromol. 33 (2000) 7051-7061. [13] B. N. Reddy, M. Deep, Amish G. Joshi, Chem. Phys. 16 (2014) 2062-2071. [14] A. Kraft, M. Rottmann, H.-D. Gilsing, H. Faltz, Electrochim. Acta 52 (2007) 5856–5862. 10
[15] M. Zhou, M. Pagels, B. Geschke, J. Heinze, J. Phys. Chem. B 106 (2002) 10065-10073. [16] M. D. Morrison, J. J. Hanthorn, D. A. Pratt, Org. Lett., 11(5) (2009) 1051-1054. [17] D. Pla, A. Marchal, C. A. Olsen, F. Albericio, M. Alvarez, J. Org. Chem. 70 (2005) 8231-8234.
[18] J. S. Oakdale, D. L. Boger, Org. Lett. 12(5) (2010) 1132–1134. [19] R. Yan, X. Kang, X. Zhou, X. Li, X. Liu, L. Xiang, Y. Li, G. Huang, J. Org. Chem. 79 (2014) 465–470. [20] M. Friedmann, J. Org. Chem. 30 (1965) 859. [21] D. H. R. Barton and S. Z. Zard, J. Chem. Soc., Chem. Commun. (1985) 1098 [22] A. M. van Leusen, H. Siderius, B. E. Hoogenboom 1, Daan van Leusen, Tetrahedron. Lett. No 52 (1972) 5337–5340.
[23] A. Sera, S. Fukumoto, T. Yoneda and H. Yamada, Heterocycles 24 (1986) 697. [24] A. Furstner, H. Weintritt, A. Hupperts, J. Org. Chem. 60 (1995) 6637. [25] M. Iwao, T. Takeuchi, N. Fujikawa, T. Fukuda, F. Ishibashi, Tetrahedron Lett. 44 (2003) 4443–4446. [26] F. Chen, T. Shen, Y. Cui, N. Jiao, Org. Lett. 14 (2012) 4926–4929. [27] M. Friedman, J. Org. Chem. 30 (1965) 859–863. [28] M. Stępień, B. Donnio, J. L. Sessler, Chem. Eur. J. 13 (2007) 6853–6863. [29] J. L. Retsek, C. J. Medforth, D. J. Nurco, S. Gentemann, V. S. Chirvony, K. M. Smith, D. Holten, J. Phys. Chem. B 105 (2001) 6396–6411.
[30] N. Ono, H. Miyagawa, T. Ueta, T. Ogawa, H. Tani, J. Chem. Soc., 1 (1998). [31] E. Gońka, D. Myśliwiec, T. Lis, P. J. Chmielewski, M. Stępień, J. Org. Chem. 78 (2013) 1260−1265. [32] D. Myśliwiec, B. Donnio, P. J. Chmielewski, B. Heinrich, M. Stępień, J. Am. Chem. Soc. 134 (2012) 4822–4833.
[33] T. D. Lash, B. H. Novak, Tetrahedron Lett., 36 (25) (1995) 4381–4384. [34] Z. Q. Gao, Z. H. Li, P. F. Xia, M. S. Wong, K. W. Cheah, C. H. Chen, Adv. Funct. Mater. 17 (2007) 3194. [35] C. Lõ, A. Adenier, K.I. Chane-Ching, F. Maurel, J. J. Aaron, B. Kosata, J. Svoboda, Synth. Met. 156 (2006) 256–269. [36] D. A. Walker, C. D’Silva, Electrochim. Acta 116 (2014) 175–182. [37] C. Mortier, T. Darmanin, F. Guittard, Macromolecules 48 (2015) 5188−5195. [38] E. Hao, B. Fabre, F. R. Fronczek, M. Graça, H. Vicente, Chem. Mater., 19 (2007) 6195-6205.
[39] D. J. Guerrero, X. Ren, J. P. Ferraris, Chem. Mater. 6 (1994) 1437-1443. 11
[40] J. P. Ferraris, M. M. Eissa, I.D. Brotherston, D. C. Loveday, A. A. Moxey, J. Electroanal. Chem. 459 (1998) 57–69.
[41] J. H. Vélez, F. R. Díaz, M. A. del Valle, J. C. Bernéde, J. P. Soto, J. Appl. Polym. Sci., Vol. 109 (2008) 1722–1729. [42] B. Ustamehmetoglu, F. Demir, E. Sezer, Prog. Org. Coat.76 (2013) 1515–1521. [43] M. A. Vorotyntsev, S. V. Vasilyeva, Adv. Colloid Interface Sci., 139 (2008) 99-151.
[44] D. L. Feldheim, S. M. Hendrickson, M. Krejcik, C. M. Elliott, Chem. Mater., 7(6) (1995) 1125. [45] P. Kathirgamanathan, M. K. Shepad, J. Electroanal. Chem. 354 (1993) 305. [46] K. Tanaka, T. Shichiri, M. Toriumi, T. Yamabe, Synth. Met., 30 (1989) 271. [47] K. Tanaka, T. Shichiri, M. Toriumi, T. Yamabe, Synth. Met. 33 (1989) 389.
[48] A. F. Diaz, J. I. Castillo, J. A. Logan, W. Y. Lee, J. Electroanal. Chem. 129 (1981) 115. [49] J. Heinze, H. John, M. Dietrich, P. Tschuncky, Synth. Met. 119 (2001) 49. [50] F. Beck, M. Oberst, R. Jansen, Electrochim. Acta 35 (1990) 1841. [51] N. J. Morse, D. R. Rosseinsky, R. J. Mortimer, D. J. Walton, J. Electroanal. Chem. 225 (1988) 119. [52] S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena, D. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [53] M. Satoh, K. Imanishi, K. Yoshino, J. Electroanal. Chem. 317 (1991) 139–151. [54] M. Takakubo, J. Electroanal. Chem. 258 (1989) 303.
[55] A. Merz, J. Kronberger, L. Dunsch, A. Neudeck, A. Petr, L. Parkanyi Angew. Chem. Int. Ed., 38 (10) (1999) 1442–1446.
[56] P. Tschuncky, J. Heinze, A. Smie, G. Engelmann, J. Koßmehl, Electroanal. Chem. 433 (1997) 223. [57] S. Trasatti, Appl. Chem. 58 (1986) 955. [58] T. Yanai, D.P. Tew, N.C. Handy, Chem. Phys. Lett. 393 (2004) 51–57. [59] A.-R. Allouche, Gabedit, J. Comput. Chem. 32 (2011) 174–82. [60] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zhe, Gaussian, Inc., Wallingford CT, 2009. [61] J. Heinze, A. Rasche, M. Pagels, B. Geschke, J. Phys. Chem. B 111 (2007) 989–997. [62] P. Hapiot, D. Lorcy, A. Tallec, R. Carlier, S. Robert, J. Phys. Chem. 100 (1996) 14823. [63] A. J. Lucio, S. K. Shaw, J. Phys. Chem. C 119 (2015) 12523−12530.
12
Scheme 1. Structures of 3,4-bis(3,4-dimethoxyphenyl)-1H-pyrrole (1); ethyl 3,4-bis(3,4-dimethoxyphenyl)1H-pyrrole-2-carboxylate (2) and 5,5',6,6',9,9',10,10'-octamethoxy-2H,2H-[1,1-bidibenzo[e,g]isoindole]-3,3dicarboxylate (3).
Scheme 2A. Structures of diarylpyrrole and phenanthropyrrole derivatives of 1 discussed in the text.
13
Scheme 2B. Structures of diarylpyrrole and phenanthropyrrole derivatives of 2 discussed in the text.
14
Scheme 3A. Proposed reaction mechanism of 1 electrooxidation.
15
Fig. 1. 1-10th CV plots of 1 with: 2.0 mM (a) and 5.5 mM concentration in 0.1 M TBAP/DCM (b) and 2.0 mM in 0.1 M TBAP/ACN (c); potential scan rate: 50 mV∙s−1.
16
Fig. 2. Relative spin concentrations (R.S.C.) of 1 (a) and 2 (b) plotted as a function of applied potential during oxidation and reduction half cycles.
17
Fig. 3. In-situ UV-vis-NIR spectra: of the neutral 1 (2.0 mM) in 0.1 M TBAP/ACN (▬) (insert); during electrooxidation of 1 at +0.48 V with 10- second acquisition time (▬) without pyridine (a) with addition of pyridine (1 v/v%) (b); after electrooxidation at “oc” potential (▬); after electrooxidation at -0.35 V (▬).
18
Fig. 4. TDDFT simulated optical spectra of various forms of 1, calculated at DFT/(u)CAMB3LYP/6-31G(d)/PCM(DCM). Vertical lines represent calculated transition energies. The lineshapes, provided for illustrative purposes were obtained by appyling a Gaussian lineshape with a half-width equal to 0.1 eV.
Fig. 5. Optimized geometries of assumed products of electrooxidation of 1, calculated at
DFT/(u)CAM-B3LYP/6-31G(d)/PCM(DCM). 19
Fig. 6. 1-10th CV plots of 1 (7.5 mM) in 0.1 M TBAP/ACN (▬) and, additionally, with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); insert - 1-10th CV plots of solid products deposited on Pt electrode during polarization in monomer-free electrolytic solution after deposition without (▬) and with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); potential scan rate: 250 mV∙s−1.
20
Fig. 7. In-situ UV-vis-NIR spectra of (a) 2 in 0.1 M TBAP/ACN and (b) 3 in 0.1 M TBAP/DCM.
21
Figure Captions Scheme 1. Structures of 3,4-Bis(3,4-dimethoxyphenyl)-1H-pyrrole (1); ethyl 3,4-bis(3,4-dimethoxyphenyl)1H-pyrrole-2-carboxylate (2) and 5,5',6,6',9,9',10,10'-octamethoxy-2H,2H-[1,1-bidibenzo[e,g]isoindole]-3,3dicarboxylate (3) compounds subjected to electrooxidation. Scheme 2A. Structures of diarylpyrrole and phenanthropyrrole derivatives of 1 discussed in the text. Scheme 2B. Structures of diarylpyrrole and phenanthropyrrole derivatives of 2 discussed in the text. Scheme 3A. Proposed reaction mechanism of 1 electrooxidation. Scheme 3B. Proposed reaction mechanism of 2 electrooxidation.
Fig. 1. 1-10th CV plots of 1 with: 2.0 mM (a) and 5.5 mM concentration in 0.1 M TBAP/DCM (b) and 2.0 mM in 0.1 M TBAP/ACN (c); potential scan rate: 50 mV∙s−1. Fig. 2. Relative spin concentrations (R.S.C.) of 1 (a) and 2 (b) plotted as a function of applied potential during oxidation and reduction half cycles. Fig. 3. In-situ UV-vis-NIR spectra: of the neutral 1 (2.0 mM) in 0.1 M TBAP/ACN (▬) (insert); during electrooxidation of 1 at +0.48 V with 10- second acquisition time (▬) without pyridine (a) with addition of pyridine (1 v/v%) (b); after electrooxidation at “oc” potential (▬); after electrooxidation at -0.35 V (▬). Fig. 4. TDDFT simulated optical spectra of various forms of 1, calculated at DFT/(u)CAM-
B3LYP/6-31G(d)/PCM(DCM). Vertical lines represent calculated transition energies. The lineshapes, provided for illustrative purposes were obtained by appyling a Gaussian lineshape with a half-width equal to 0.1 eV. Fig. 5. Optimized geometries of assumed products of electrooxidation of 1, calculated at
DFT/(u)CAM-B3LYP/6-31G(d)/PCM(DCM). Fig. 6. 1-10th CV plots of 1 (7.5 mM) in 0.1 M TBAP/ACN (▬) and, additionally, with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); insert - 1-10th CV plots of solid products deposited on Pt electrode during polarization in monomer-free electrolytic solution after deposition without (▬) and with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); potential scan rate: 250 mV∙s−1. Fig. 7. In-situ UV-vis-NIR spectra of (a) 2 in 0.1 M TBAP/ACN and (b) 3 in 0.1 M TBAP/DCM.
22
S0013-4686(16)30604-1 http://dx.doi.org/doi:10.1016/j.electacta.2016.03.066 EA 26896
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
17-12-2015 7-3-2016 10-3-2016
Please cite this article as: M.Czichy, P.Zassowski, T.Jarosz, E.Go´nka, E.Janiga, M.St˛epie´n, M.Lapkowski, Mechanism of 3,4-diarylpyrrole electrooxidation, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.03.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights: The ,'-coupling of pyrrole radical cations to -dimers is reversible and the deprotonation of the dimer is kinetically hindered. The addition of bases does not result in deprotonation of doubly charged –dimer in 2- and 2'-position. The slightly conducting poly(3,4-diarylpyrrole) is obtained via addition of neutral monomer to cation radical. The electrooxidation of 3,4-bis(3,4-dimethoxyphenyl)pyrroles is not produced expected poly(phenanthropyrrole) polymer.
1
Graphical abstract
2
Mechanism of 3,4-diarylpyrrole electrooxidation M. Czichya*, P. Zassowski a, T. Jarosz a, E. Gońkab, E. Janigab, M. Stępień b, M. Łapkowski a,c a
Faculty of Chemistry, Silesian University of Technology, Strzody 9, 44-100 Gliwice, Poland
b
Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
c
Center of Polymer and Carbon Materials, Polish Academy of Science, M. Curie-Skłodowskiej 34, 41-819 Zabrze, Poland Corresponding author: Tel.: +48-322371736, E-mail: [email protected]
Abstract The purpose of this study was to investigate 3,4-diarylpyrrole electropolymerization and simultaneous or subsequent intramolecular oxidative coupling leading to poly(phenanthropyrrole). Small amounts of πconjugated products were obtained only under the conditions of increased concentration of the monomer and increased rate of potential sweep, which enabled both processes – monomer oxidation and deprotonation of the σ-dimer. The combined electrochemistry, UV-vis-NIR, ESR and TD-DFT results show, that the α,α'-coupling of 3,4-diarylpyrrole derivatives is inhibited by the stability of the σ-dimer dication, inhibition of the deprotonation and the reversal of the σ bond formation resulting in regeneration of the monomer. The addition of pyridine did not result in σ–dimer deprotonation in 2- and 2'-position.
3
1. Introduction
The synthesis of pyrrole derivatives is an important area of heterocyclic chemistry due to the ubiquity of pyrrolic motifs not only in natural products but also in materials chemistry. In the latter context, pyrrolebased polymers are of particular importance, with current research focused on increasing their solubility [1,2], and modification of their optical properties [3,4]. Both effects can be readily tailored through the use of substituted pyrroles. Electropolymerization of substituted pyrroles has been successfully performed, usually using N-substituted pyrroles [5], 3,4-alkylenedioxypyrroles [6], indoles or isoindoles [7]. The electrooxidation of 2,3-diphenylindole results in an almost quantitative transformation into the indoleindolenine dimer, involving linkage through a fused benzene ring [8]. Furthermore, electrochemically prepared polypyrroles suffer from the occurrence of undesired α–β and β–β couplings during polymerization [9], and the α–α coupling selectivity can be achieved by using 3,4disubstituted monopyrrole precursors [10]. The most common 3,4-disubstituted polypyrroles, prepared using electrochemical method are poly(3,4-dimethoxypyrrole)s [11] and poly(3,4-diethylenodioxypyrrole)s (PEDOP) [12,13]. In turn, 3,4-diarylpyrroles are interesting as precursors to physiologically and biologically active compounds [14]. An important property of these polymers is their ability to be doped through a partial oxidation or reduction. The use of electrochemical techniques allows to obtain polymers with specified doping degree in a controlled manner via partial oxidation or reduction, which may be implemented through the sign and value of applied potential [15]. The 3,4-diarylpyrrole moiety is a constituent of a wide array of natural products, such as 3-chloro-4-(3chloro-2-nitrophenyl)-1H-pyrrole (pyrrolnitrin) [16], Lamellarin [17] or Lycogarubin C [18]. Cyclization of α-amino carbonyl compounds and aldehydes is a simple method for the preparation of 1,3,4-triarylpyrroles [19]. 3,4-Diarylpyrroles have been developed using the Hinsberg reaction of benzil with dimethyl Nacetyliminodiacetate [20]. The Barton–Zard pyrrole synthesis using nitroalkenes [21], and the van Leusen TOSMIC method [22] are now well established for the preparation of β-arylpyrroles. Other synthetic methods include the reduction of β-nitrostyrene with TiCl3 [23], Knorr-type condensations of amino ketones [24], and palladium-catalyzed Suzuki cross-coupling of 3,4-dihydroxypyrrole bis-triflate derivatives [25]. The use of copper or nickel catalysis was recently found to yield highly selective denitrogenative annulations of vinyl azides with aryl acetaldehydes, leading to both 2,4- and 3,4-diaryl substituted pyrroles [26]. 3,4-Diarylpyrroles are important precursors to octaaryl- [27,28] and dodecaarylporphyrins [29,30]. Recently, some of us have shown that electron-rich 3,4-diarylpyrroles can undergo a tandem inter- and intramolecular oxidative coupling, providing access to bis(phenanthropyrroles) with extended π-conjugation. These bis(phenanthropyrroles) are characterized by restricted rotation around the α−α bond and exhibit strong blue fluorescence and large Stokes shifts [31]. This type of coupling reactivity was earlier employed in the synthesis of self-assembling porphyrin-based materials [32]. Literature reports detail various benzo-fused heterocyclic materials [33], characterized by high luminescence quantum yields [34]. Many such derivatives have two-dimensional self-assembled structures [35]. The supramolecular, self-assembly processes, occurring for these systems, are affected by a complex interplay of inter- and intramolecular interactions, non-covalent forces such as hydrogen-bonding, electrostatic or π–π stacking, which is also related to the luminous efficiency. This paper presents an attempt to control 3,4-diarylpyrrole electro-coupling, in order to obtain of oligo(phenanthropyrroles). Apparently, the intermolecular coupling is a rapid process and can compete with the intramolecular ring closure, suppressing the formation of monomeric phenanthropyrrole.
4
Mostly, electropolymerizations of aryl-substituted pyrroles were carried out using: N-aryl-pyrroles [36] or N-aryled and 3,4-substituted pyrroles [37]. Electropolymerization of 3,4dicarboranyl-functionalized pyrroles has also been reported [38]. 3-Aryl- and 3,4-aryl polymers are often obtained with other heterocyclic compounds such as thiophene derivatives [39,40] or their oligomeric precursors - e.g. 3',4'-diphenyl-2,2':5',2''-terthiophene [41]. Poly(benzothiophene)s can be obtained by electrocopolymerization with benzothiophene and thiophene or pyrrole derivatives [42]. In general, due to the steric hindrance in 3,4-substituted derivatives, straightforward electropolymerization was found to be unfeasible in compared to similar substances with aliphatic substituents [43]. Hovewer, poly(3,4-diphenylpyrroles) were grown electrochemically from solutions of acetonitrile containing diphenylpyrrole, tetrahexylammonium poly(styrenesulfonate) and sodium perchlorate, but no information was given concerning the possibility of coupling reactions between substituents [44]. In turn, poly(phenantro[9,10c]thiophene) can be prepared by electrochemical polymerization with Bu4NPF6 and it showed high conductivity (105 S∙cm-1) [45]. The mechanism of electropolymerization of aromatic heterocyclic monomers is still not fully understood and remains a subject of controversy. Tanaka et al. [46] carried out a study of coupling processes between two pyrrole radical cations. They have envisioned two possible routes of the pyrrole coupling involving on the one hand a σ-radical and on the other a π–radical [47]. Diaz proposed the classical route of conducting polymer formation which involved the dimerization of monomeric radical cations at their α-positions followed by deprotonation of the doubly charged σ-dimer, resulting in aromatic neutral dimer [48]. Other studies have shown that it is not the rate of coupling, but rather the elimination of protons from the σ-dimer, that is the rate-determining step [49]. The doubly charged σ-dimer and σ-intermediates of pyrrole oligomers are weak acids. Therefore, sufficiently strong bases are necessary to initiate proton elimination and ensure polymerization progress (e.g. water [50], pyridine [51]). In turn, Pletcher proposed another mechanism in which the cation radical formed by the loss of an electron reacts directly with a neutral molecule giving a cation dimer. The cation dimer then loses in following sequence: proton, electron, and second proton forming the neutral dimer [52]. Additionaly, Satoh established that this type of coupling reaction is affected by the monomer concentration [53]. Takakubo demonstrated on the basis on molecular orbital calculations, that the addition of a cation radical to a neutral molecule is symmetry forbidden and thus requires a high activation energy [54]. In some conjugated systems, such as α-capped bithiophenes and bipyrroles, monocation radicals were found to form σ-dimers and π-dimers [55,56]. The dimerization and deprotonation steps can be inhibited by too bulky substituents, and, in particular, by α-substitution. Thus, the anodic substitution and addition reactions of aromatic compounds via their π-radical cations are the object of interest to organic electrochemists.
2. Experimental 2.1. Chemicals and materials Tetrabutylammonium perchlorate (TBAP, 99.0%, Fluka, dried over night under vacuum before use), pyridine (Fluka), 2,6-di-tert-butyl-4-methyl-pyridine (Aldrich), dichloromethane (DCM, for HPLC, ≥99.8%, Sigma-Aldrich), acetonitrile (ACN, RPE ACS for analysis, Carlo Erba, ACN was stored over 4A molecular sieves), ferrocene (98%, Sigma-Aldrich), (BAHA, technical grade, Sigma), argon 6.0 (SIAD Group) were used.
5
2.2. Instrumentation and working conditions
Electrochemical investigations were performed using a CH Instruments Electrochemical Analyzer model 620 in a three-electrode system at room temperature. The experimental cell comprised a platinum disk (surface area: 2 mm2), used as the working electrode, a platinum coil counter electrode, and an Ag electrode as a o’ reference. The reported electrode potentials are given relative to formal redox potential (E ) of the ferrocene/ferrocenium (Fc/Fc+) couple. The potential of the Fc+/Fc couple is 0.37 V with respect to used reference electrode. The solutions in the electrochemical cell were purged with argon prior to the experiments. Cyclic voltammetric (CV) measurements were conducted at potential sweep rate of 50 or 250 mV∙s-1. Electrooxidation of the monomer was done at a concentration of 2.0, 5.5 or 7.5 mM, with 0.1 M TBAP electrolyte in DCM or ACN, without or with pyridine (1 v/v%) or 2,6-di-tert-butyl-4-methylpyridine (1 v/v%). The HOMO energy was graphically determined from the onset potential of the oxidation (Eox onset) using the Trasatti equations for non-aqueous electrolytes: HOMO = -([Eox onset − E1/2(Fc/Fc+)] + 5.1) (±0.10 eV), where E1/2(Fc/Fc+) is the half-wave potential of the Fc/Fc+ couple [57]. Spectroscopic changes accompanying electrooxidation were monitored by in-situ thin-layer UV-vis-NIR technique with the Ocean Optics QE6500 and NIRQuest apparatus equipped with DH-2000-BAL halogen and deuterium sources and detectors connected with a potentiostat AUTOLAB PGSTAT100N. The spectra between 200 – 1600 nm were recorded with a 1- and 10-second acquisition time. A thin-layer optical cell characterized by the optical path of 0.2 mm length was a quartz cuvette containing ITO/quartz electrode (20±5 Ω/sq, Praezisions Glas & Optic GmbH), Teflon separator (0.2 mm), silver wire (pseudoreference electrode) and platinum mesh (counter electrode). The experiments were carried out in a cuvette connected to the lamps and detector through fiber optics (400 μm, Ocean Optics). Electron spin resonance (ESR) spectra were acquired using a JEOL JES FA-200 X band spectrometer. A capillary quartz spectroelectrochemical cell was used, equipped with a Pt wire working electrode, Ag wire pseudoreference electrode (calibrated vs. Fc/Fc+) and a Pt coil counter electrode, as in the authors’ earlier works. Relative spin concentrations for each potential step have been obtained through manual double integration of the first derivative ESR spectrum. BAHA titration experiments were carried out using a Cary 500 SCAN UV-vis-NIR spectrophotometer. 1H NMR spectra were recorded on a high-field NMR spectrometer (1H frequency 600.13 MHz). Spectra were referenced to the residual solvent signal (dichloromethane-d2 5.32 ppm). The concentration of compound 1 was 1 mM in CD2Cl2. 1 was titrated with a CD2Cl2 solution of BAHA. DFT/TDDFT calculations were carried out with B3LYP or CAM-B3LYP [58] hybrid functionals combined with 6-31G(d,p) basis set. For investigated compounds ground state geometries were optimized with no symmetry constrains to a local minimum, which was followed by frequency calculations. In all cases no negative or imaginary frequencies were found. All calculations in this work were conducted with polarizable continuum model (PCM) using dichloromethane as solvent as implemented in Gaussian 09 software. Input files and molecular orbital plots were prepared with Gabedit 2.4.7 software [59]. All calculations had been carried with Gaussian 09 software [60]. The charge and multiplicity setting were set as follows: 0 and 1 for neutral molecules, 1 and 2 for radical cation, 2 and 1 for σ-dimers.
6
3. Results and discussion
The cyclic voltammograms (CVs) of considered compound were recorded in TBAP/DCM (1, 2, 3 compounds) and TBAP/ACN electrolyte (1, 2 compounds). The examples of appropriate CV curves and oxidation peak potentials are given in supporting information. In all cases, the occurrence of multi-step and irreversible processes was observed. The first oxidation peaks of 1 and 3 are located at similar potentials (1: +0.45 V, 3: +0.44 V, in 0.1 M TBAP/DCM). The first oxidation of 2, however, is shifted towards more positive potentials (+0.63 V), in comparison to the oxidation potential values of the other compounds. The HOMO energy levels determined via electrochemical experiments are -5.56, -5.74 and -5.55 eV for compounds 1, 2 and 3, respectively. In the main step, the maximum applied potential was limited to just beyond the first oxidation peak of 1 monomer with 2.0 mM concentration in TBAP/DCM (Fig. 1a). It shows an irreversible peak at +0.45 V corresponds to the formation of monomer radical cation (1)●+. It is commonly known, that the presence of products with extended π-conjugation length with increasing numbers of pyrrole units causes a decrease in the oxidation potential, relative to the oxidation potential of the monomer. However, subsequent CV curves have a similar shape, with no new redox behavior, which indicates the absence of a π-conducting dimer/oligomer. Next, there was an attempt to carry out the electropolymerization of 1 by anodic oxidation with increased monomer concentration. Increasing the monomer concentration (5.5 mM) (Fig. 1b) or using acetonitrile as the reaction medium (Fig. 1c) reveals the hysteresis loop with a crossover between the anodic and cathodic half-cycle curves, which can be explained by the formation of σ-dimer: [σ–(1)2]2+ (Scheme 3A – paths a0, a1) [61,62]. The elimination of protons at the level of the [σ–(1)2]2+ does not take place probably due to stability of the σ-dimer. The broad wave at negative potentials below 0.0 V can be originate from an irreversible reduction of [σ–(1)2]2+ with regeneration of 1 (Scheme 3A – path a2).
This behavior was confirmed by the ESR measurement shown in Figure 2a. ESR spectroelectrochemical has been used to study a variety of relative spin concentrations for each potential step of 1 oxidation in TBAP/ACN (Fig. 2a). There was observed an increase in the relative concentration of spinbearing species during staircasing the applied potential towards positive potentials, typical for cation radicals being generated during electrooxidation. When the applied potential is staircased back towards negative potentials, however, the spin concentration initially continues increasing, evidencing that spinless, dicationic species [σ–(1)2]2+, generated during oxidation are being transformed into cation radicals, while the existing cation radicals do not undergo any significant decay. This is evidence in favor of a significant restructuring of the cation-radical, with increase of its energetic stability and lowering its redox potential. Further lowering the applied potential eventually brings about a decrease in the relative spin concentration. Should the redox process generating the spin-bearing species be reversible, we would expect this decrease in spin concentration to occur exponentially with lowering the applied potential. This is not the case, however, as a significant amount of spins persist in the case of spin-bearing species generated through oxidation of (1) even at potentials as -0.4 V. Such a behavior is a testament to the relatively high stability of the cation radicals. Spectroscopic changes accompanying electrooxidation of 1 were monitored by in-situ UV-vis-NIR. Fig. 3a shows the successive UV-vis spectra of the 1 compound (2 mM), measured at the open circuit potential “oc” (neutral monomer in TBAP/CAN ▬), during the oxidation per 10 second at the potential of first oxidation peak (+0.48 V) (▬), again at “oc” (▬), and finally at -0.35 V (▬). Absorbance maximum of neutral 1 was found at 294 nm as shown in Figure 3a (insert). Vis-NIR spectra of anion radical observed at 407, 878, 976 and 1166 nm (▬). Next, we 7
observed a significant increase of bands at 456, 640, as well as those previously observed at 878, 976 and 1166 nm after switch-off the potential (▬). Apparently the increase in absorbance at 456, 640 and 878 nm can be attributed to the formation of [σ–(1)2]2+. In turn, the bands at 976 and 1166 nm indicate the presence of unreacted cation radical. Optical spectra measured at -0.35 V (▬) exhibited the reduced absorbance in a broad range of wavelengths (above 500 nm), indicating the backward transformation to 1 (Scheme 3A, a2). Simultaneously, weak bands in the 500–1200 nm range indicate the presence of trace quantities of (1)●+ and/or [σ–(1)2]2+. After this experiment any solid product was not observed on the ITO electrode. Simulated optical spectra of different forms of 1 are presented in Fig. 4. Both (1)●+ and [σ–(1)2]2+ showed a large number of transitions in the UV-vis range. The difference is in the Vis range, where this is especially seen peak of [σ–(1)2]2+ at 655 nm for simulated, clearly seen in the UV-Vis spectroelectrochemical experiments at the wavelength of 640 nm. In comparison, the absorption in experimental spectra in the 900-1400 nm range can be attributed mainly to (1)●+. Given the previously described experimental data, it seems to confirm that the stabilization of doubly charged σ-dimer caused by inhibited α-deprotonation (Scheme 3A – path a3) results in breaking of α,α-bond of the σ-dimer and restoration of 1 monomer during the negative potential scan (Scheme 3A – path a2). The above experiments were repeated with the addition of either of the two pyridine bases, in order to facilitate the deprotonation of [σ–(1)2]2+. The CVs show that addition of pyridine or hindered pyridine (1 v/v%) does not support the [σ–(1)2]2+ deprotonation (supplemented data). However, a change of the shape of the first peak, and an increase of peak intensity (▬), as well as an offset of E ox onset value can indicate changes in the structure, e.g. deprotonation. In turn, we observed the negative shift of the cathodic wave to -0.65 V, for example, as a result of reduction to radical of pyridine [63]. The presence of pyridine in the solution during electrooxidation causes a significant decrease of absorbance above 1000 nm, which may indicate the consumption of cation radicals. Nonetheless, the bands about 640 and 878 nm can be assigned to [σ–(1)2]2+ as well (Fig. 5b). A small amount of π-conjugated product was obtained on Pt electrode under the conditions of increased concentration of the monomer (7.5 mM) in ACN and increased rate of potential sweep (250 mV∙s-1) (Fig. 6). The current increases potential in the range of (-0.24) ─ +0.35 V) is very weakly, it can be concluded that only a small amount of polymer is deposited on the electrode during cycling. The shape of the curve and the onset potentials of oxidation at -0.24 V are both similar to the typical CVs of polypyrroles (Fig. 6b ▬). A trace amount of a solid π-conjugated product was confirmed by measurement of π–π* transition band at around 430 nm (λonset) (supplemented data). The increase in scan rate implies a shortening of the experimental time scale, which prevents a substantial generation of (1)●+. In the second - while also quickly we achieve -0.35 V potential, the doubly charged σ-dimer was electroreduced with regeneration of monomer, increasing the concentration of the latter. Low concentration of (1)●+ is to reduce the efficiency of path resulting in [σ–(1)2]2+. In this way, both (1)●+ and neutral 1 molecule are presented in the vicinity of the electrode and this factor can lead to their reaction between them to [σ–(1)2]●+. This indicates that besides the occurrence of main reaction to give [σ–(1)2]2+ (Scheme 3A – path a1), there is a path of electropolymerization via [σ–(1)2]●+ propagation (Scheme 3A – path a4). ESR study just confirms that the presence of spin-bearing species at -0.4 V can be attributed to [σ–(1)2] ●+ (Fig. 2a).
We examined by DFT/TDDFT method the possiblility of dimerization of (1), with two different mechanisms: as a reaction between two radical cations (1)●+ to form [σ–(1)2]2+ or as an attack of radical cation (1)●+ on neutral molecule (1), to form [σ–(1)2]●+. Therefore, we have calculated structures resulting 8
from these two cases, end-point geometries are presented in Fig. 5. As it can be seen, in case of [σ–(1)2]2+ geometry typical for σ–dimer has been obtained. Moreover, the middle benzene rings are lying in the same plane and thus are appropriately positioned for increase of π-π interactions, hydrogen-bond interactions between the methoxy groups, and finally, inhibition of the deprotonation in 2- and 2'-position of [σ–(1)2]2+. In turn, the covalent bond between pyrrole rings was not retained during geometry optimization of [σ–(1)2]●+. Furthermore, spin density of such optimalized geometry is placed exclusively on one of the molecules, which leads us to the conclusion that reaction between (1)●+ and (1) is less likely to occur, indicating that dominant intermolecular mechanism involves dimerization of two radical cations (1)●+. This is even more probable because of conditions present in the electrochemical oxidation – low concentration of neutral form of (1) near the vicinity of electrode surface, which would make the attack of (1)●+ on (1) with giving [σ–(1)2]●+ even less probable. As another possibility, we investigated the intramolecular coupling in the o-position of the phenyl ring. In contrast to formation of [σ–(1)2]●+, in this case optimization of geometry showed a formation of intramolecular cyclization product [σ-(1)]●+. However, the thermodynamics of such a process is far less favorable, when compared to the intermolecular dimerization of (1)●+. Calculated free energy of such processes are equal to 23 kcal/mol and 5 kcal/mol at the temperature of 298.15 K for intermolecular and intramolecular path, respectively. The total chemical oxidation process is dominated by the mechanism of coupling between (1)●+, what is evidenced by 1H NMR spectroscopy (supplementary data). The broadened signals were not observed in the chemical shift range corresponding to pyrrole ring, where the chemical exchange between (1) and (1)●+ would be lead to significant line broadening. The oxidation of monomer 2 was also carried out by applying polarization limited to just beyond the first oxidation peak (supplemented data). The similar shape of subsequent CVs can indicate the presence of renewed 2. UV-vis spectroelectrochemical of 2 oxidation was characterized by a slow increase of absorbance between 350 and 600 nm, with maxima at 356, 444 and 499 nm (Fig. 7a). There are also very weak bands at about 840 nm. This feature of the spectrum may indicate the presence of [σ–(2)2]2+ dimer with slight amount of (2)●+ cation-radical. The spectrum measured after the reduction at -0.01 V (▬) revealed the absorbance reduction in a broad range of wavelengths (above 350 nm), what can be assigned to regeneration of 2 monomer, but weak bands in the 350–500 nm range indicates the presence of trace quantities of stable [σ– (2)2]2+ too. Similar reasoning can be applied to the example of (2) (Fig. 4b), for which the simulated optical spectra support the formation of [σ–(2)2]2+ in the experimental spectra. ESR spectroelectrochemistry study is a testament to the relatively lower stability of the charged spacies of 2 derivative than in the case of 1 (Fig. 2). The 3 compound, which is a bis(phenanthro)-derivative of 2, is characterized by repetitive form, partial reversibility of the first peak, which occurs at much lower potential (+0.44 V) (▬) relative to 2. This process is identified as the formation of (3)●+ (supplementary data). The most characteristic changes in the spectra are observed at 364, 482, 576, 737, around 1044 and above 1600 nm (Fig. 7b). All the previously mentioned bands can be attributed to (3)●+ (▬). The small intensity of the bands follows from the fact that the experiment had to be carried out in a solution of the sparingly soluble compound. The previous conclusions indicate that it is not possible to obtain the 3 derivative by electrooxidation of 2 compound.
4. Conclusions The electrooxidation of 3,4-bis(3,4-dimethoxyphenyl)pyrroles was shown not to produce the expected poly(phenanthropyrrole) polymer. Apparently, the coupling of pyrrole radical cations to σ-dimers is reversible and the deprotonation of the dimer is kinetically hindered. The high stability of σ-dimer results in 9
the breaking of its 2,2'-bond and regeneration of the monomer during electroreduction process. The addition of bases does not result in σ–dimer deprotonation in 2- and 2'-position. The ESR, UV-vis-NIR study and DFT calculations indicate that the radical cation dimerization should be favored relative to the addition to the neutral pyrrole, although the latter process may contribute to the observed formation of spin-bearing species.
Acknowledgement The project was funded by the National Science Centre of Poland (to M.S., decision DEC2014/13/B/ST5/04394). This research was supported in part by PL-Grid Infrastructure. Appendix. Supporting information Supporting information related to this article can be found at [link].
References
[1] M. R. Nabid, A. A. Entezami, J. Appl. Poly. Science 94 (2004) 254 –258. [2] Min-Kyu Song, Young-Taek Kim, Bum-Seok Kim, Jinhwan Kim, Kookheon Char, Hee-Woo Rhee, Synth.Metals 141 (2004) 315–319. [3] F.S. Damo, R.C.S. Luz, L.T. Kubota, Electrochim. Acta 51 (2006) 1304–1312. [4] K. Dutta, S.K. De, Sol. State Comm. 140 (2006) 167–171. [5] S. Kumar, S. Krishnakanth, J. Mathew, Z. Pomerantz, J-P Lellouche, S. Ghosh, J. Phys. Chem. C, 118 (5) (2014) 2570–2579.
[6] R. M. Walczak, J. R. Reynolds, Adv. Mater. 18 (2006) 1121. [7] S.B. Rhee, M.-H. Lee, B. S. Moon, Y. Kang, Korea Polym. J. 1(1) 1993, 61-68. [8] V.-T. Truong, B. D. Turner, R. F. Muscat, M. S. Russo, Proceedings of the SPIE - The International Society for Optical Engineering 98 (1997) 3241. [9] D.V. Konev, O.I. Istakova, A. Sereda, A. Shamraeva, C.H. Devillers, M.A. Vorotyntsev, Electrochim. Acta 179 (2015), 315-325. [10] A. Smie, A. Synowczyk, J. Heinze, R. Alle, P. Tschuncky, G. Gӧtz, P. Bäuerle, J. Electroanal. Chem. 452 (1998) 87–95. [11] F. Gassmer, S. Graf, A. Merz, Synth. Met. 87 (1997) 75-79. [12] P. Schottland, G. Sonmez, J. R. Reynolds, Macromol. 33 (2000) 7051-7061. [13] B. N. Reddy, M. Deep, Amish G. Joshi, Chem. Phys. 16 (2014) 2062-2071. [14] A. Kraft, M. Rottmann, H.-D. Gilsing, H. Faltz, Electrochim. Acta 52 (2007) 5856–5862. 10
[15] M. Zhou, M. Pagels, B. Geschke, J. Heinze, J. Phys. Chem. B 106 (2002) 10065-10073. [16] M. D. Morrison, J. J. Hanthorn, D. A. Pratt, Org. Lett., 11(5) (2009) 1051-1054. [17] D. Pla, A. Marchal, C. A. Olsen, F. Albericio, M. Alvarez, J. Org. Chem. 70 (2005) 8231-8234.
[18] J. S. Oakdale, D. L. Boger, Org. Lett. 12(5) (2010) 1132–1134. [19] R. Yan, X. Kang, X. Zhou, X. Li, X. Liu, L. Xiang, Y. Li, G. Huang, J. Org. Chem. 79 (2014) 465–470. [20] M. Friedmann, J. Org. Chem. 30 (1965) 859. [21] D. H. R. Barton and S. Z. Zard, J. Chem. Soc., Chem. Commun. (1985) 1098 [22] A. M. van Leusen, H. Siderius, B. E. Hoogenboom 1, Daan van Leusen, Tetrahedron. Lett. No 52 (1972) 5337–5340.
[23] A. Sera, S. Fukumoto, T. Yoneda and H. Yamada, Heterocycles 24 (1986) 697. [24] A. Furstner, H. Weintritt, A. Hupperts, J. Org. Chem. 60 (1995) 6637. [25] M. Iwao, T. Takeuchi, N. Fujikawa, T. Fukuda, F. Ishibashi, Tetrahedron Lett. 44 (2003) 4443–4446. [26] F. Chen, T. Shen, Y. Cui, N. Jiao, Org. Lett. 14 (2012) 4926–4929. [27] M. Friedman, J. Org. Chem. 30 (1965) 859–863. [28] M. Stępień, B. Donnio, J. L. Sessler, Chem. Eur. J. 13 (2007) 6853–6863. [29] J. L. Retsek, C. J. Medforth, D. J. Nurco, S. Gentemann, V. S. Chirvony, K. M. Smith, D. Holten, J. Phys. Chem. B 105 (2001) 6396–6411.
[30] N. Ono, H. Miyagawa, T. Ueta, T. Ogawa, H. Tani, J. Chem. Soc., 1 (1998). [31] E. Gońka, D. Myśliwiec, T. Lis, P. J. Chmielewski, M. Stępień, J. Org. Chem. 78 (2013) 1260−1265. [32] D. Myśliwiec, B. Donnio, P. J. Chmielewski, B. Heinrich, M. Stępień, J. Am. Chem. Soc. 134 (2012) 4822–4833.
[33] T. D. Lash, B. H. Novak, Tetrahedron Lett., 36 (25) (1995) 4381–4384. [34] Z. Q. Gao, Z. H. Li, P. F. Xia, M. S. Wong, K. W. Cheah, C. H. Chen, Adv. Funct. Mater. 17 (2007) 3194. [35] C. Lõ, A. Adenier, K.I. Chane-Ching, F. Maurel, J. J. Aaron, B. Kosata, J. Svoboda, Synth. Met. 156 (2006) 256–269. [36] D. A. Walker, C. D’Silva, Electrochim. Acta 116 (2014) 175–182. [37] C. Mortier, T. Darmanin, F. Guittard, Macromolecules 48 (2015) 5188−5195. [38] E. Hao, B. Fabre, F. R. Fronczek, M. Graça, H. Vicente, Chem. Mater., 19 (2007) 6195-6205.
[39] D. J. Guerrero, X. Ren, J. P. Ferraris, Chem. Mater. 6 (1994) 1437-1443. 11
[40] J. P. Ferraris, M. M. Eissa, I.D. Brotherston, D. C. Loveday, A. A. Moxey, J. Electroanal. Chem. 459 (1998) 57–69.
[41] J. H. Vélez, F. R. Díaz, M. A. del Valle, J. C. Bernéde, J. P. Soto, J. Appl. Polym. Sci., Vol. 109 (2008) 1722–1729. [42] B. Ustamehmetoglu, F. Demir, E. Sezer, Prog. Org. Coat.76 (2013) 1515–1521. [43] M. A. Vorotyntsev, S. V. Vasilyeva, Adv. Colloid Interface Sci., 139 (2008) 99-151.
[44] D. L. Feldheim, S. M. Hendrickson, M. Krejcik, C. M. Elliott, Chem. Mater., 7(6) (1995) 1125. [45] P. Kathirgamanathan, M. K. Shepad, J. Electroanal. Chem. 354 (1993) 305. [46] K. Tanaka, T. Shichiri, M. Toriumi, T. Yamabe, Synth. Met., 30 (1989) 271. [47] K. Tanaka, T. Shichiri, M. Toriumi, T. Yamabe, Synth. Met. 33 (1989) 389.
[48] A. F. Diaz, J. I. Castillo, J. A. Logan, W. Y. Lee, J. Electroanal. Chem. 129 (1981) 115. [49] J. Heinze, H. John, M. Dietrich, P. Tschuncky, Synth. Met. 119 (2001) 49. [50] F. Beck, M. Oberst, R. Jansen, Electrochim. Acta 35 (1990) 1841. [51] N. J. Morse, D. R. Rosseinsky, R. J. Mortimer, D. J. Walton, J. Electroanal. Chem. 225 (1988) 119. [52] S. Asavapiriyanont, G. K. Chandler, G. A. Gunawardena, D. Pletcher, J. Electroanal. Chem. 177 (1984) 229. [53] M. Satoh, K. Imanishi, K. Yoshino, J. Electroanal. Chem. 317 (1991) 139–151. [54] M. Takakubo, J. Electroanal. Chem. 258 (1989) 303.
[55] A. Merz, J. Kronberger, L. Dunsch, A. Neudeck, A. Petr, L. Parkanyi Angew. Chem. Int. Ed., 38 (10) (1999) 1442–1446.
[56] P. Tschuncky, J. Heinze, A. Smie, G. Engelmann, J. Koßmehl, Electroanal. Chem. 433 (1997) 223. [57] S. Trasatti, Appl. Chem. 58 (1986) 955. [58] T. Yanai, D.P. Tew, N.C. Handy, Chem. Phys. Lett. 393 (2004) 51–57. [59] A.-R. Allouche, Gabedit, J. Comput. Chem. 32 (2011) 174–82. [60] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zhe, Gaussian, Inc., Wallingford CT, 2009. [61] J. Heinze, A. Rasche, M. Pagels, B. Geschke, J. Phys. Chem. B 111 (2007) 989–997. [62] P. Hapiot, D. Lorcy, A. Tallec, R. Carlier, S. Robert, J. Phys. Chem. 100 (1996) 14823. [63] A. J. Lucio, S. K. Shaw, J. Phys. Chem. C 119 (2015) 12523−12530.
12
Scheme 1. Structures of 3,4-bis(3,4-dimethoxyphenyl)-1H-pyrrole (1); ethyl 3,4-bis(3,4-dimethoxyphenyl)1H-pyrrole-2-carboxylate (2) and 5,5',6,6',9,9',10,10'-octamethoxy-2H,2H-[1,1-bidibenzo[e,g]isoindole]-3,3dicarboxylate (3).
Scheme 2A. Structures of diarylpyrrole and phenanthropyrrole derivatives of 1 discussed in the text.
13
Scheme 2B. Structures of diarylpyrrole and phenanthropyrrole derivatives of 2 discussed in the text.
14
Scheme 3A. Proposed reaction mechanism of 1 electrooxidation.
15
Fig. 1. 1-10th CV plots of 1 with: 2.0 mM (a) and 5.5 mM concentration in 0.1 M TBAP/DCM (b) and 2.0 mM in 0.1 M TBAP/ACN (c); potential scan rate: 50 mV∙s−1.
16
Fig. 2. Relative spin concentrations (R.S.C.) of 1 (a) and 2 (b) plotted as a function of applied potential during oxidation and reduction half cycles.
17
Fig. 3. In-situ UV-vis-NIR spectra: of the neutral 1 (2.0 mM) in 0.1 M TBAP/ACN (▬) (insert); during electrooxidation of 1 at +0.48 V with 10- second acquisition time (▬) without pyridine (a) with addition of pyridine (1 v/v%) (b); after electrooxidation at “oc” potential (▬); after electrooxidation at -0.35 V (▬).
18
Fig. 4. TDDFT simulated optical spectra of various forms of 1, calculated at DFT/(u)CAMB3LYP/6-31G(d)/PCM(DCM). Vertical lines represent calculated transition energies. The lineshapes, provided for illustrative purposes were obtained by appyling a Gaussian lineshape with a half-width equal to 0.1 eV.
Fig. 5. Optimized geometries of assumed products of electrooxidation of 1, calculated at
DFT/(u)CAM-B3LYP/6-31G(d)/PCM(DCM). 19
Fig. 6. 1-10th CV plots of 1 (7.5 mM) in 0.1 M TBAP/ACN (▬) and, additionally, with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); insert - 1-10th CV plots of solid products deposited on Pt electrode during polarization in monomer-free electrolytic solution after deposition without (▬) and with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); potential scan rate: 250 mV∙s−1.
20
Fig. 7. In-situ UV-vis-NIR spectra of (a) 2 in 0.1 M TBAP/ACN and (b) 3 in 0.1 M TBAP/DCM.
21
Figure Captions Scheme 1. Structures of 3,4-Bis(3,4-dimethoxyphenyl)-1H-pyrrole (1); ethyl 3,4-bis(3,4-dimethoxyphenyl)1H-pyrrole-2-carboxylate (2) and 5,5',6,6',9,9',10,10'-octamethoxy-2H,2H-[1,1-bidibenzo[e,g]isoindole]-3,3dicarboxylate (3) compounds subjected to electrooxidation. Scheme 2A. Structures of diarylpyrrole and phenanthropyrrole derivatives of 1 discussed in the text. Scheme 2B. Structures of diarylpyrrole and phenanthropyrrole derivatives of 2 discussed in the text. Scheme 3A. Proposed reaction mechanism of 1 electrooxidation. Scheme 3B. Proposed reaction mechanism of 2 electrooxidation.
Fig. 1. 1-10th CV plots of 1 with: 2.0 mM (a) and 5.5 mM concentration in 0.1 M TBAP/DCM (b) and 2.0 mM in 0.1 M TBAP/ACN (c); potential scan rate: 50 mV∙s−1. Fig. 2. Relative spin concentrations (R.S.C.) of 1 (a) and 2 (b) plotted as a function of applied potential during oxidation and reduction half cycles. Fig. 3. In-situ UV-vis-NIR spectra: of the neutral 1 (2.0 mM) in 0.1 M TBAP/ACN (▬) (insert); during electrooxidation of 1 at +0.48 V with 10- second acquisition time (▬) without pyridine (a) with addition of pyridine (1 v/v%) (b); after electrooxidation at “oc” potential (▬); after electrooxidation at -0.35 V (▬). Fig. 4. TDDFT simulated optical spectra of various forms of 1, calculated at DFT/(u)CAM-
B3LYP/6-31G(d)/PCM(DCM). Vertical lines represent calculated transition energies. The lineshapes, provided for illustrative purposes were obtained by appyling a Gaussian lineshape with a half-width equal to 0.1 eV. Fig. 5. Optimized geometries of assumed products of electrooxidation of 1, calculated at
DFT/(u)CAM-B3LYP/6-31G(d)/PCM(DCM). Fig. 6. 1-10th CV plots of 1 (7.5 mM) in 0.1 M TBAP/ACN (▬) and, additionally, with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); insert - 1-10th CV plots of solid products deposited on Pt electrode during polarization in monomer-free electrolytic solution after deposition without (▬) and with 2,6-di-tert-but-4methyl-pyridine (1 v/v%) (▬); potential scan rate: 250 mV∙s−1. Fig. 7. In-situ UV-vis-NIR spectra of (a) 2 in 0.1 M TBAP/ACN and (b) 3 in 0.1 M TBAP/DCM.
22