Electron Transfer Mechanism of Substituted Benzimidazoles: Dimer

Article pubs.acs.org/JPCC

Electron Transfer Mechanism of Substituted Benzimidazoles: Dimer Switching, Oscillations, and Search for Singlet Fission Properties Jan Plutnar,† Magdaléna Hromadová,‡ Nicolangelo Fanelli,§ Šárka Ramešová,‡ Zdeněk Havlas,† and Lubomír Pospíšil*,†,‡ †

Intstitue of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo 2, Prague, Czech Republic ‡ J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, Prague, Czech Republic § Institute of Chemistry of Organometallic Compounds, CNR Pisa, via Moruzzi 1, 56124 Pisa, Italy S Supporting Information *

ABSTRACT: Reduction and oxidation of 4,7-dimethyl-1,3-dimethoxybenzimidazolium cation (1) and the related dihydroxy analogue (2) were investigated by electrochemical and in situ spectroelectrochemical methods. Quantum chemical methods were applied to UV−vis spectra in native, reduced, and oxidized forms. Compounds were searched for possible formation of a cation radical and a dication radical suitable for the singlet fission effect. Indeed 1 yields by oxidation the target quinoidal structure. However, the reduction step for both compounds is coupled with very fast dimerization and prevents obtaining a stable target form. The complex mechanism of the reduction process yields electrochemical current oscillation. Estimation of the maximum Ljapunov exponent proves characteristics of the deterministic chaos.

1. INTRODUCTION Substituted benzimidazoles are primarily used in pharmacology as antimicrobial, antiviral, antidiabetic, and anticancer drugs.1 The benzimidazolinium derivatives were recently considered as intermediates in synthesis of compounds yielding, upon irradiation, the singlet fission effect,2,3 which is sought for higher yields in solar energy harvesting. Singlet fission may occur in compounds containing two chromophores.4 Irradiation yields a high energy excited singlet state of one chromophore, which interacts with an adjacent chromophore in its ground electronic state. Such interaction results in generation of two low-energy triplets, which could yield two electrons. Several small molecules with heteroatoms were theoretically and experimentally evaluated for estimation of the most probable synthetic strategy.5−9 Benzenimidazolinium cations could yield such a pair of chromophores upon reduction of their imidazolium ring and subsequent oxidation of the phenolic ring. At present the search for a suitable synthetic route involving oxidation and reduction steps of benzimdiazolium was not successful. Since substituted derivatives of benzimidazolinia were not electrochemically characterized we © 2017 American Chemical Society

have undertaken the present study with aim to identify the details of two-electron processes leading to reduced and to oxidized forms of these cations.

2. EXPERIMENTAL SECTION Synthesis and characterization of the studied compounds is described in Supporting Information (pages S2−S3 and Figures S1−S8). Methodology of the measurements was described in previous reports of our group and the details are given in the Supporting Information (page S12). Redox properties were investigated by dc polarography, cyclic voltammetry (CV), ac polarography, electrochemical impedance spectroscopy (EIS), exhaustive electrolysis and spectroelectrochemistry. The samples for electrochemical measurements were prepared in acetonitrile (AN) using 0.1 M tetrabutylammonium hexafluorophosphate ((TBA)PF6) as the indifferent electrolyte. The Received: March 2, 2017 Revised: April 20, 2017 Published: April 21, 2017 9963

DOI: 10.1021/acs.jpcc.7b02028 J. Phys. Chem. C 2017, 121, 9963−9969

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The Journal of Physical Chemistry C redox potential of the ferrocene/ferrocenium couple in acetonitrile against our reference electrode was 0.526 V. Quantum chemical calculations of optimized structures at the MP2/vtz10 level used program MOLPRO.11 The vertical electronic spectra were calculated at the CC2/d-aug-cc-pVTZ level12,13 using program TURBOMOLE.14 Analysis of current oscillations used the Kantz algorithm15 for estimation of possible transfer of periodicity to the deterministic chaos.

3. RESULTS AND DISCUSSION There are two redox active centers in the structure of compounds 1 and 2 (Chart 1), the aromatic ring with two Chart 1. Structure of Two Benzimidazolinium Triflates Figure 2. Dependence of the faradaic phase angle on frequency in solution of 0.2 mM 1 and 0.1 M (TBA)PF6 in acetonitrile using the Hg drop electrode.

evidenced by occurrence of two new anodic oxidation peaks at −0.2 and +0.1 V, which are absent during the negative potential scan. It is noticeable that these small anodic peaks have no associated cathodic counterparts. The mechanism of reduction therefore deserves a detailed analysis. Coupled chemical reactions could be suppressed by using higher voltage scan rates, which should allow reoxidation of the primary product of the electron transfer prior to its homogeneous reaction. Sufficiently fast voltage scan should restore the reversible character of the main reduction peak at −1.8 V. We applied fast voltammetry using microelectrodes, which permits application of the voltage scan rates in the range of kV·s−1. This is made possible using electrodes with the diameter of the order of ten microns, which largely diminishes the double layer capacity and substantially shortens the electronic time constant of the electrochemical cell.16 Here we explored the scan range from 100 V·s to 10 kV·s−1. We identified a transient species, which is characterized by a reoxidation current maximum near 0 V. The actual position of this peak shifts with the increasing scan rate toward higher positive potentials. A typical fast voltammogram measured at the scan rate 1.1 kV·s−1 is shown in Figure 1B. However, no sign of reversibility was found at scan rates in the range up to 30 kV·s−1. The chemical transformation of the primary product is therefore extremely fast. The ratio of peak currents ia/ic (where ia is the current of the anodic peak at negative potential) depends on the concentration of 1 and on

methoxy or hydroxy substituents and quaternary nitrogen on the imidazole ring. The reduction on the imidazole ring is identical for both compounds and only compound 1 will be described. Both compounds differ in their oxidations, which will be presented in the second section. 3.1. Reduction of Imidazolium Ring. The reduction of the imidazolium ring proceeds at rather negative potentials. Dc polarograms using the mercury drop electrode yield limiting currents at potentials more negative than −2.0 V, the height of which corresponds to the transfer of two electrons. Cyclic voltammetry at slow scan rates shows an irreversible cathodic peak located at −1.8 V (Figure 1A). A comparison of the peak current ic with the peak of ferrocene confirms the transfer of two-electrons, which is in agreement with charge consumption during the exhaustive electrolysis and with the values of limiting currents in dc polarography. The irreversible character of the redox process is evidently caused by a coupled chemical reaction of the primary product of the electron transfer. This is

Figure 1. (A) Cyclic voltammetry of 3 mM 1 and 0.1 M (TBA)PF6 in acetonitrile using glassy carbon electrode. The scan rate was 0.5 V·s−1. (B) Cyclic voltammetry of 7.7 mM 1 and 0.1 M (TBA)PF6 in acetonitrile using Au microelectrode of 25 μm diameter. The scan rate was 1100 V·s−1. 9964


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The Journal of Physical Chemistry C

Figure 3. Spectroelectrochemistry of 1.14 mM 1 and 0.1 M (TBA)PF6 in acetonitrile. Inserts show voltamograms in the OTTLE cell and red circles indicate potentials where the selected spectra were measured. The spectrum at the beginning of the voltage scan is a black thick line, whereas the red line is the final spectrum. The left panel (A) shows the reduction and the right panel (B) corresponds to the reoxidation of the reduction product. The voltage scan rate was 5 mV·s−1 and spectra were scanned every 7 or 10 s.

Figure 4. Cyclic voltammetry of 1 mM 1 (A) and 2 (B) and 0.1 M (TBA)PF6 in acetonitrile (A) and in DMSO (B) using the glassy carbon electrode. The scan rate was 0.5 V·s−1. The dashed line is the blank.

Figure 5. Spectroelectrochemistry of 1.14 mM 1 and 0.1 M (TBA)PF6 in acetonitrile. Inserts show voltammograms in the OTTLE cell and red circles indicate potentials where the selected spectra were measured. The spectrum at the beginning of the voltage scan is a black thick line, whereas the red line is the final spectrum. The voltage scan rate was 5 mV·s−1, and spectra were scanned every 8 s.

Figure 6. Experimental and calculated spectrum of 1 (starting material). The black line is the experimental spectrum, the green line is the simulated spectrum (Gaussian shape, line-width 3500 cm−1). The individual transitions (bars) are colored according to symmetry (C2v group): a1, red; b1, green; b2, blue; a2 is inactive. Heights of bars correspond to the calculated oscillator strength f.

Ac polarography indicates also certain anomalous features at potentials of the onset of the reduction (Figure S9). An estimation of the heterogeneous rate constant from the ac polarogram (Figure S10) yields a value 2.5 × 10−3 cm·s−1. We will show that this apparent value is influenced by a coupled chemical reaction and is meaningless. A correct value can be

the scan rate. This ratio changes from less than 0.2 to 0.5 at the scan rates from 0.5 V·s−1 to 10 kV·s−1. Since the peak ratio depends on the bulk concentration, the coupled chemical process is a bimolecular reaction and most likely involves a dimerization of the radical. 9965


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Figure 10. Current−time series recorded on the Hg drop electrode during the reduction of 1.92 mM 1 and 0.1 M (TBA)PF6 in acetonitrile at a constant applied potential −1.818 V.

Figure 7. Experimental and calculated spectrum of 1 in the reduced form. See caption of Figure 6.

Figure 8. Experimental and calculated spectrum of oxidized form. See caption of Figure 6 for details. Figure 11. Time-delay diagram of time series given in Figure 5. The delay constant τ = 4; current is in μA.

the electron transfer to oxidized form Ox and a follow-up chemical reaction of the reduced form Red to a product.17 k

Ox + ne− ⇄ Redn − ⎯→ ⎯ + product H

(1)

The linear asymptote at the highest frequencies in Figure 2 yields a correct estimate of the heterogeneous charge transfer rate constant k0 = 0.63 cm·s−1. We attempted to estimate the rate of a coupled chemical reaction initiated by the electron transfer. Simulation using a general treatment of coupled interfacial and diffusional impedances including chemical step enabled to fit the position of a maximum on the cot φ vs ω1/2 plot with a value of k ∼ 2 × 107 L·mol−1·s−1, however the fit of the entire shape of the curve in Figure 2 was not reached. Results of the fast voltammetry (new anodic peaks near 0 V) together with the impedance analysis indicate that the twoelectron reduction is a two-step mechanism, in which the product of a single electron transfer yields a neutral radical. The radical forms a dimer, which is discernible during voltammetric time scale experiments. The two-electron reduction yields the final product during the preparative electrolysis. Dimerization reactions of N-heterocyclic compounds are quite common. Recently we reported similar behavior in cases like alkylpyridinium18 and quinolinium cations,19 where many other

Figure 9. Dc polarograms of 1 and 0.1 M (TBA)PF6 in acetonitrile. Concentrations were 0.05 mM (●), 0.1 mM (○), and 0.2 mM (▼). The blank is the dashed line.

deduced only from the analysis of the electrode impedance on the frequency ω of sine wave perturbation. First, the solution resistance is subtracted from impedance data. The next step is the estimation of the double layer capacitance. A subtraction of these values from the impedance vector finally yields the dependence of the faradaic phase angle φ on the applied frequency (Figure 2) related directly to the heterogeneous rate constant k0 of the charge transfer step. The observed nonlinear dependence of cot φ vs ω1/2 clearly confirms the coupling of 9966


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Figure 12. Amplitude of oscillations obtained by FFT of current series (A) at −1.920 V and (B) at −1.925 V. The constant potentials correspond to the falling part of a maximum shown in Figure 9.

the original form by the electrochemical oxidation at the considerably less negative potential. 3.2. Oxidation of Substituents on the Benzene Ring. The oxidation of benzene substituents at +1.5 V is a twoelectron reversible process for compound 1 (Figure 4A), whereas the oxidation of 2 is irreversible (Figure 4B). Low solubility of 2 required to use DMSO as a solvent. Oxidation irreversibility disqualifies 2 for further consideration as a compound yielding a reversible chromophore, which possibly could give the singlet fission effect. Therefore, the behavior of 2 was not investigated in details. The oxidation of 1 is a very fast electron transfer process. The impedance measurements (Figure S12) estimate the value of the standard heterogeneous rate constant k0 = 4.1 ± 0.5 cm· s−1, which is an order of magnitude higher than k0 for the reduction step. Spectroelectrochemistry of 1 during the oxidation shows a decrease of the absorption band at 284 nm and a simultaneous increase of intensive maxima at 338, 436, and 459 nm. The new maxima in the spectrum of the oxidized form are composed of two overlapping bands (Figure 5). Spectroscopy in UV−vis region confirms the formation of a quinoidal structure,20 for which typical bands are located around 220, 330, 430, and 460 nm (confirmed also by the calculations). A quinone-type chromophore is the target structure required for the singlet fission effect. The spectroelectrochemistry confirmed that the oxidation process is fully reversible. The recorded spectrum resumes its original shape upon scanning the electrode potential in the OTTLE cell back to negative potentials (not shown). This is in contrast with the reduction process, during which the spectrum of the original form was obtained only at less negative potentials, where the dimer was reoxidized. 3.3. Calculation of UV−Vis Spectra. Experimental UV− vis spectra observed in the native form of 1, after its reduction and its oxidation were theoretically calculated using quantum chemical methods. Details of the optimized structures are given in Supporting Information (pages S15−S17). The spectrum of 1 is shown in Figure 6 together with the simulated spectrum. A similar comparison of the experimental and calculated spectra of 1 upon reduction or oxidation is given in Figures 7 and 8. In the initial form of the compound with A1 state symmetry is the lowest calculated transition at 32 500 cm−1, which has b2 symmetry. The transition is from 5a2 occupied orbital into three b1 virtual orbitals, 7b1 (amplitude 0.64), 8b1

relevant reports are quoted. Hence we consider the following reaction scheme where, for simplicity, we use symbol A for structure of the title compounds 1 and 2.

Reaction 2 describes the currentvoltage dependence observed at slow change of the applied potential yielding an irreversible peak at −1.8 V. The anodic redox step occurring at higher potentials (less negative) is taken into account by eq 3. The simulation of CV according to this reaction mechanism indeed reproduced the experimental features, namely the irreversibility and the presence of the anodic peak at the highest scan rates. Simulation yielded an estimate of the dimerization rate constant k ∼ 5 × 107 L·mol−1·s−1, which is in good agreement with the value evaluated from the EIS measurements. The final product of the reduction using NaBH4 as the reducing agent yields hydrogenation of the double bond in position between C2−N3 atoms of 1. Electrochemical reduction by an exhaustive electrolysis consumes charge corresponding to a transfer of two electrons per molecule leading to hydrogenation of the mentioned double bond. The reduction peak at −1.8 V completely disappears in fully electrolyzed solution (Figure S11). The electrolytic current during the electrolysis decays slowly down to the charge corresponding to a transfer of one electron. At the same time a white precipitate, most likely a dimer, is formed. The precipitated product is rapidly reoxidized to the starting material as was confirmed by NMR. This is in line with the mechanism of the electron transfer, eqs 2 and 3, involving monomer/dimer switching by the reduction/oxidation cycle. Further support gives in situ spectroelectrochemistry in optically transparent thin-layer cell (OTTLE) during the reduction and reoxidation. The voltage scan from −1.2 to −1.9 V reduces 1 and shifts the main absorption band from 221 to 228 nm (Figure 3A). The absorption at 288 nm slightly decreases. The back scan of the applied voltage in the same potential range does not restore the original spectrum. However, the subsequent voltage scan from −0.5 to +0.2 V restored the spectrum of the oxidized form of 1 (Figure 3B). This behavior confirms that the electrochemical reduction indeed leads to a product (the dimer), which is converted to 9967


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at potentials of the current maxima and on limiting currents of dc polarograms. The i−t curves at negative potentials of the flat limiting current decay smoothly without any enhanced noise or fluctuations. Quite different current decay is observed at the falling part of the dc current maximum. A typical i−t curves are shown in Figure 10. Current oscillates with the period of seconds. Higher frequency oscillations or irregularities are also present to a much smaller extent. These effects led to a hypothesis that homogeneous reaction of intermediates together with the mass transport is involved in the current enhancement and its periodicity. We described an important role of radical intermediates in our previous studies of oscillating heterogeneous redox systems. Quaternary N-heterocyclic systems yielded oscillations, period doubling and transfer to the deterministic chaos on the basis of cationic catalysis or multielectronic transfers.24−28 The present current instability is less intensive nevertheless it is quite distinct. Application of analysis of the system dynamics can confirm or disprove the irregularities as purely random events or deterministic fluctuations.29 The simplest way is a construction of time-delayed diagrams. Seemingly irregular time series i(t) governed by certain mechanistic laws (though not a priori known) yield a value i at any time instant t, which depends on a value at time t − τ, where τ is the delay constant. On the contrary, the random fluctuations show no correlation between the adjacent or distant points and fill the time delay diagram entirely. A typical 3D time delay diagram is shown in Figure 11 and clearly confirms the deterministic character of irregular i−t curves. The trajectory forms several smaller or larger orbits. Separation of the neighboring orbits in time and in space can distinguish possible change of periodicity to a deterministic chaos. The estimation of the Ljapunov exponent15 (Figure S13) yielded the value λ = 0.28. A positive value of λ indicates that the system dynamics has a certain tendency to enter into the deterministic chaos. The described dynamic property is in line with the existence of a short-lived intermediate, which can diffuse away from the electrode and dimerize. Alternatively, the radical can be further reduced at the interface. Interplay between the electron transfer, dimerization and the mass transfer of all species involved in the mechanism leads to the current periodicity or instability. The process depends on the rate of kinetics and the diffusion. The periodicity of the i−t curves can be seen also by application of the fast Fourier transform to data set, from which the average value was subtracted. The Fourier spectrum clearly shows an intensive low frequency lines (Figure 12A) changing the intensity with a small change of the applied potential (Figure 12B). High sensitivity of the system stability to small changes of external parameters, like the applied potential, is a typical feature of dynamic systems transferring from stability to periodicity or to a deterministic chaos.

(−0.45), and 9b1 (0.39). For the orbital shape see the Supporting Information (Figure S14). The transition is from π orbital, located mainly on 6-membered ring into π* orbitals located on NCH−N group of the imidazole ring. The next transition at 37 100 cm−1 of a1 symmetry is a transition from 5a2 π orbital into two diffuse (Rydberg) orbitals, 12a2 orbital (amplitude −0.62) and 10a2 orbital (0.52). The most intense transition of a1 symmetry at 46 200 cm−1 is a transition from 6b1 π orbital into three b1 π* orbitals, 7b1 (amplitude 0.61), 8b1 (−0.41), and 9b1 (0.35). This transition is followed by b2 one, from 5a2 orbital into Rydberg orbital 24b1. The reduced form has B1 ground state symmetry. The calculated spectrum shows a series of low intensity transitions below 40 000 cm−1, which is not shown in the experimental spectrum. On the contrary, the measured shoulder at around 34 000 cm−1 is not reproduced well in the calculation (only a superposition of many small peaks might resemble such a shoulder) and it might originate from a charge-transfer (CT) transition between the reduced form of the molecule and the original cationic form.21 Mixed CT complexes of parent and oxidized forms are already known.22 The most intense peak predicted at 43 000 cm−1 is the excitation from valence 6b1(β) orbital into mostly 4b1 orbitals of Rydberg type (see Supporting Information, Figure S16), 9b1(β) (amplitude 0.32), 15b1(β) (0.27), 25b1(β) −0.27), 29b1(β) (0.22). The oxidized form has A2 ground state symmetry. The electronic spectrum shows three intense peaks, both the experimental spectrum and the simulated spectrum. The calculations predict the first peak at 22 000 cm−1 of b2 symmetry (see Supporting Information, Figure S15). It represents excitation from 5b1(β) orbital into 5a2(β) orbital (amplitude 0.87) and from 5a2(α) into 7b1(α) orbital (0.41). Next peak is of the same symmetry at 29 600 cm−1. Excitation is similar to the first intense peak with opposite mixing of transitions, from 5a2(α) into 7b1(α) orbital (amplitude 0.86) and from 5b1(β) orbital into 5a2(β) orbital (−0.38). The third, most intense peak is of a1 symmetry and is predicted at 45 500 cm−1. The transition is from 6b1 orbitals α and β into 7b1 orbitals α and β, with amplitudes ±0.63. All the excitations are of π to π* type. 3.4. Analysis of Current Oscillations. This section aims to clarify the appearance of the unusual maxima on dc polarograms (Figure 9), which are most likely related to a complicated reaction mechanism. Dc polarograms using the mercury drop electrode yield limiting currents at potentials more negative than −2.0 V, height of which corresponds to the transfer of two electrons. The current−voltage curves (dc polarograms) do not have a regular shape of a polarographic wave. Instead, a broad current maximum followed by a limiting current is observed even at the lowest concentrations (Figure 9). The limiting currents at potentials more negative than −2.1 V are proportional to the bulk concentration whereas the current maxima increase with the concentration in a nonlinear manner. Such irregular features can be caused simply by the solution streaming at the interface or by reaction of intermediates.23 Current maxima on dc polarograms are known from days the polarography was discovered. Those classical maxima were caused by the solution streaming at high concentrations of redox active compounds on a free-flowing Hg drop electrode. The maximum found for the present system occurs even at very low concentrations and more importantly on a “static” valve-operated mercury drop electrode. A key finding was the registration of current−time series (i−t curves)

4. CONCLUSION Electrochemical and spectroelectrochemical characterization of two substituted benzimidazolium derivatives proved that a stable radical can be prepared only by oxidation of the methoxy-substituted compound. Similar electrochemical approach for reductive generation of a radical is hampered by its very fast homogeneous reaction. The reduction mechanism is consistent with a fast dimerization of the primary reduction product. Resulting dimer is easily reoxidized to the original form. A complex reaction sequence during the electron transfer 9968


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(11) Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M. WIREs. Comput. Mol. Sci. 2012, 2, 242−253. Werner, H.-J.; Knowles, P. J.; Knizia, G.; Manby, F. R.; Schütz, M.; Celani, P.; Györffy, W.; Kats, D.; Korona, T.; Lindh, R. et al. MOLPRO, version 2015.1, a package of ab initio programs; see http://www.molpro.net. (12) Christiansen, O.; Koch, H.; Jørgensen, P. The second-order approximate coupled cluster singles and doubles model CC2. Chem. Phys. Lett. 1995, 243, 409−418. (13) Woon, D. E.; Dunning, T. H., Jr. Gaussian basis sets for use in correlated molecular calculations. IV. Calculation of static electrical response properties. J. Chem. Phys. 1994, 100, 2975−2988. (14) TURBOMOLE V7.0 2015, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989−2007; TURBOMOLE GmbH: since 2007; available from http://www. turbomole.com. (15) Kantz, H. A robust method to estimate the maximal Ljapunov exponent of a time series. Phys. Lett. A 1994, 185, 77. (16) Amatore, C.; Bouret, Y.; Maisonhaute, E.; Abruna, H. D.; Goldsmith, J. I. Electrochemistry within molecules using ultrafast cyclic voltammetry. C. R. Chim. 2003, 6, 99−115. (17) Moreira, H.; De Levie, R. On the coupling of interfacial and diffusional impedances. II. J. Electroanal. Chem. Interfacial Electrochem. 1971, 29, 353−374. (18) Hromadová, M.; Pospíšil, L.; Sokolová, R.; Kolivoška, V. Electrochemical reduction of dodecylpyridinium bromide in aprotic solvents: mechanistic studies. Collect. Czech. Chem. Commun. 2011, 76, 1895−1908. (19) Teplý, F.; Č ížková, M.; Slavíček, P.; Kolivoška, V.; Tarábek, J.; Hromadová, M.; Pospíšil, L. Electron transfer triggers fast dimer/ monomer switching of pyridinium and quinolinium cations. J. Phys. Chem. C 2012, 116, 3779−3786. (20) Wallenfels, K.; Draber, W. Der Einfluss der Substituenten auf Elektronen- und Schwingungsspektren von Aminochinonen. Tetrahedron 1964, 20, 1889−1912. (21) Rathore, R.; Lindeman, S. V.; Kochi, J. K. Charge-transfer probes for molecular recognition via steric hindrance in donoracceptor pairs. J. Am. Chem. Soc. 1997, 119, 9393−9404. (22) Lipnická, Š.; Bělohradský, M.; Kolivoška, V.; Pospíšil, L.; Hromadová, M.; Pohl, R.; Chocholoušková, J. V.; Vacek, J.; Fiedler, J.; Stará, I. G.; Starý, I. Tetrathiafulvalene-oligo(p-phenyleneethynylene) conjugates: A formation of multiple mixed-valence complexes upon electrochemical oxidation. Chem. - Eur. J. 2013, 19, 6108−6121. (23) Pospíšil, L.; Hromadová, M.; Fanelli, N.; Valásě k, M.; Kolivoška, V.; Gál, M. Extended viologen as a source of electric oscillations. Phys. Chem. Chem. Phys. 2011, 13, 4365−4371. (24) Pospíšil, L.; Hromadová, M.; Sokolová, R.; Bulíčková, J.; Fanelli, N. Cationic catalysis and hidden negative differential resistance in reduction of radical anion of nitrobenzene. Electrochim. Acta 2008, 53, 4852−4858. (25) Hromadová, M.; Pospíšil, L.; Sokolová, R.; Fanelli, N. New electrochemical oscillator based on the cation-catalyzed reduction of nitroaromatic radical anions. Electrochim. Acta 2009, 54, 4991−4996. (26) Pospíšil, L.; Hromadová, M.; Gál, M.; Valásě k, M.; Fanelli, N.; Kolivoška, V. Irregular polarographic currents obey Feigenbaum universality route from order to chaos. Collect. Czech. Chem. Commun. 2009, 74, 1559−1570. (27) Pospíšil, L.; Hromadová, M.; Fanelli, N.; Valásě k, M.; Kolivoška, V.; Gál, M. Extended viologen as a source of electric oscillations. Phys. Chem. Chem. Phys. 2011, 13, 4365−4371. (28) Hromadová, M.; Valásě k, M.; Fanelli, N.; Randriamahazaka, H. N.; Pospíšil, L. Stochastic resonance in electron transfer oscillations of extended viologen. J. Phys. Chem. C 2014, 118, 9066−9072. (29) Peitgen, H.-O.; Jurgens, H.; Saupe, D. Chaos and Fractals. New Frontiers of Science, 2nd ed.; Springer-Verlag: Berlin, 2004.

leads to periodic electric phenomena observed at the onset of the reduction. The analysis by standard methods of the system dynamics confirmed that apparently random current fluctuations are of a deterministic nature. At present none of the investigated compounds suits fully to the requirements for observing the singlet fission effect, though a facile production of a cation radical is a promising feature of 1. Our study has shown that the major problem is the stability of the radical originating from the reduction of the heterocyclic ring. A future solution may be sought by blocking the dimer formation either by structural modification or by supramolecular protection.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02028. Synthesis and characterization, electrochemical methods and measurement details, determination of the Ljapunov exponent, and quantum chemical interpretation of spectroelectrochemistry (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(L.P.) E-mail: [email protected]. ORCID

Lubomír Pospíšil: 0000-0003-2543-2195 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Czech Science Foundation (1603085S and 15-19143S) and the Academy of Sciences of the Czech Republic (RVO: 61388963, 61388955) is gratefully acknowledged.

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REFERENCES

(1) Walia, R.; Hedaitullah, M.; Farha Naa, S.; Iqbal, K.; Lamba, H. S. Benzimidazole derivatives- an overview. Int. J. Res. Pharm. Chem. 2011, 1, 565−574. (2) Smith, M. B.; Michl, J. Singlet fission. Chem. Rev. 2010, 110, 6891−6936. (3) Smith, M. B.; Michl, J. Recent advances in singlet fission. Annu. Rev. Phys. Chem. 2013, 64, 361−386. (4) Johnson, J. C.; Nozik, A. J.; Michl, J. The role of chromophore coupling in singlet fission. Acc. Chem. Res. 2013, 46, 1290−1299. (5) Zeng, T.; Ananth, N.; Hoffmann, R. Seeking small molecules for singlet fission: a heteroatom substitution strategy. J. Am. Chem. Soc. 2014, 136, 12638−12647. (6) Piland, G. B.; Burdett, J. J.; Dillon, R. J.; Bardeen, C. J. Singlet fission: from coherences to kinetics. J. Phys. Chem. Lett. 2014, 5, 2312−2319. (7) Mirjani, F.; Renaud, N.; Gorczak, N.; Grozema, F. C. Theoretical investigation of singlet fission in molecular dimers: the role of charge transfer states and quantum Interference. J. Phys. Chem. C 2014, 118, 14192−14199. (8) Bhattacharyya, K.; Pratik, S. M.; Datta, A. Small organic molecules for efficient singlet fission: role of silicon substitution. J. Phys. Chem. C 2015, 119, 25696−25702. (9) Elfers, N.; Lyskov, I.; Spiegel, J. D.; Marian, C. M. Singlet fission in quinoidal oligothiophenes. J. Phys. Chem. C 2016, 120, 13901− 13910. (10) Dunning, T. H., Jr. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 1989, 90, 1007. 9969


Electron Transfer Mechanism of Substituted Benzimidazoles: Dimer

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