Electron Transfer Mechanism of Substituted Benzimidazoles: Dimer Switching, Oscillations, and Search for Singlet Fission Properties Jan Plutnar, † Magdale ́ na Hromadova ́ , ‡ Nicolangelo Fanelli, § S ̌ a ́ rka Rames ̌ ova ́ , ‡ Zdeně k Havlas, † and Lubomír Pospís ̌ il* ,†,‡ † Intstitue of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo 2, Prague, Czech Republic ‡ J. Heyrovsky ́ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ̌ kova 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 Irradi- ation 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 benzimdiazo- lium was not successful. Since substituted derivatives of benzimidazolinia were not electrochemically characterized we 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 hexafluor- ophosphate ((TBA)PF 6 ) as the indifferent electrolyte. The Received: March 2, 2017 Revised: April 20, 2017 Published: April 21, 2017 Article pubs.acs.org/JPCC © 2017 American Chemical Society 9963 DOI: 10.1021/acs.jpcc.7b02028 J. Phys. Chem. C 2017, 121, 9963-9969