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Cationic group 4 metallocene complexes with pendant imine and pyridine donor groups were prepared as stable crystalline [B(C6F5)4]- salts either by protonation of the intramolecularly bound ketimide moiety in neutral complexes [(η5-C5Me5){η5-C5H4CMe2CMe2C(R)═N-κN}MCl] (M = Ti, Zr, Hf; R = t-Bu, Ph) by PhNMe2H+[B(C6F5)4]- to give [(η5-C5Me5){η5-C5H4CMe2CMe2C(R)═NH-κN}MCl]+[B(C6F5)4]- or by chloride ligand abstraction from the complexes [(η5-C5Me5)(η5-C5H4CMe2CH2C5H4N)MCl2] (M = Ti, Zr) by Li[B(C6F5)4]·2.5Et2O to give [(η5-C5Me5)(η5-C5H4CMe2CH2C5H4N-κN)MCl]+[B(C6F5)4]-. Solid state structures of the new compounds were established by X-ray diffraction analysis, and their electrochemical behavior was studied by cyclic voltammetry. The cationic complexes of Zr and Hf, compared to the corresponding neutral species, exhibited significantly enhanced luminescence predominantly from triplet ligand-to-metal (3LMCT) excited states with lifetimes up to 62 μs and quantum yields up to 58% in the solid state. DFT calculations were performed to explain the structural features and optical and electrochemical properties of the complexes.

A massive structural change accompanies electron capture by the 1,2-dicarba-closo-dodecaborane cage molecule (1). Bimolecular electron transfer (ET) by pulse radiolysis found a reduction potential of E0 = -1.92 V vs Fc+/0 for 1 and rate constants that slowed greatly for ET to or from 1 when the redox partner had a potential near this E0. Similarly, two electrochemical techniques could detect no current at potentials near E0, finding instead peaks or polarographic waves near -3.1 V, which is 1.2 V more negative than E0. Voltammetry could determine rate constants, but only near -3.1 V. DigiSim simulations can describe the irreversible voltammograms but require electrochemical rate constants near 1 × 10-10 cm/s at E0, a factor of 10-10 relative to molecules undergoing facile ET. This factor of 10-10 compared to ∼10-5 for bimolecular ET presents a puzzle. This puzzle can be understood as a manifestation of one of the "Frumkin Effects" in which only part of the applied voltage is available to drive ET at the electrode.

A sulphonamidic moiety was utilized as an electron-withdrawing group for enhancement of anion complexation features of urea-based receptors. A series of receptors varying in acidity of sulphonamidic and urea NH groups was synthesized and thoroughly tested. The individual complexation properties reflect deprotonation/complexation equilibrium in a given molecule as a function of the substitution. The receptors containing electron-donating groups in conjugation to the sulphonamidic moiety showed higher association constants towards H2 PO4- and carboxylate anions, while those containing electron-withdrawing groups inclined to deprotonation of sulphonamidic NH. The deprotonation issue can be avoided by alkylation at the early step of receptor synthesis or it can be utilized for insertion of suitable groups that enable its anchoring on various substrates to form more elaborated receptor structures.

A nitro group was used as an electrochemical binding/release switch in urea-based receptors for anions. The oxidized and reduced forms of the receptors were thoroughly studied, evaluating the impact of structural features on the binding affinity upon redox modification. In the case of a nitro group in the p- position to the urea moiety, the association constant of the oxidized form is up to 80 times higher than the value measured for the reduced form. The possible application of such an anion binding affinity switch was tested electrochemically in reduction of the nitro group up to the hydroxylamine stage. The presence of acidic urea protons induces a three-step auto-protonation mechanism, however, the complexation of dihydrogen phosphate prevents the urea proton dissociation, which leads to a standard two-step reduction process. Taking advantage of redox-sensitive NO2 chromophore, the whole process was also monitored using spectroelectrochemical measurements.

Very recently, substantial attention has been paid to search for suitable chromophores for singlet fission. Singlet fission (SF) is a photophysical process in which an organic chromophore in excited singlet state shares its excitation energy with a neighbouring ground-state chromophore and both are converted into two triplet states. [1,2] In this way, one photon can generate theoretically two electrons which should increase the efficiency of inexpensive single-junction solar cells. This spin-allowed process is very fast (in ps or fs time scale), therefore much slower radiative decay is out of competition. Nevertheless, its detailed mechanism is still unknown. Although up to now, SF has not found its way into practice, the synthetic, photophysical, electrochemical and computational research is running. When looking for new molecules – candidates for efficient SF, the following properties are crucial: their crystalline form and molecular packing, the relative energies of the S0 (ground state), T1 (lowest triplet), and S1 (first excited singlet) states, where energy of the triplet state should be lower than a half of the first excited singlet energy, and the potentials of one-electron reduction and one-electron oxidation. Redox properties of molecules for SF are thus critical for their use, therefore electrochemical approach is necessary. Nowadays two main promising groups of chromophores are investigated for SF: 1) functionalized polyacenes, mostly pentacenes, and 2) organic molecules which are able to form biradicaloids, on which our research is aimed. Derivatives of 1,3-diphenyl-isobenzofuran derivatives (DPIBF) were chosen as a first group of molecules suitable for SF. Our study was focused on a series of fluorinated derivatives of DPIBF where the influence of number and position of fluorine atoms in the molecule on the redox potentials and mechanism is followed. With increasing number of F atoms, two discontinuous changes of behavior (mechanisms) were found [3]. The second family of molecules which are currently under our investigation are derivatives of a very stable dye cibalackrot which are based on indigo structure. Our current work is focused on the study of oxidation and reduction of a series of differently substituted compounds. In all cases, electrochemical investigation was performed using classic voltammetric techniques (polarography, CV and RDE) and their combination with in situ UV-vis and EPR spectroscopy. The experimental results are correlated with quantum chemical calculations. Acknowledgement The financial support of the grant 19-22806S (Czech Science Foundation - GAČR) and the institutional support RVO: 61388955 are highly appreciated. The authors are grateful to dr. J. Kaleta and dr. M. Dudic for granting the substances and to prof. J. Michl for valuable discussions. References [1] M. B. Smith, J. Michl. J. Chem. Rev. 110 (2010) 6891. [2] M. B. Smith, J. Michl. Annu. Rev. Phys. Chem. 64 (2013) 361. [3] J. Kaleta, L. Šimková, A. Liška, D. Bím, J. Madridejos, R. Pohl, L. Rulíšek, J. Michl, J. Ludvík, - Electrochimica Acta 321 (2019) 134659. Figure 1

Anions play an important role in biological systems. At the same time, many of them are the major environmental pollutants and substances endangering living organisms [1]. For these purposes the anion recognition, elimination and sensing would find application in many branches including application in medicine or environmental chemistry. Suitable examples of anion pollutants are phosphates, which could cause eutrophication of water and they are also known for their limited removal from body fluids, which could lead to serious health problems. Therefore, the aim of our research group is the design and preparation of anionic receptors with suitable binding sites to bind these anions. In the first step of our synthesis a series of substituted nosylamidic intermediates (fig. a, b) was prepared. These substances are suitable precursors for the synthesis of novel receptors bearing ureido functions (fig. c). The sulfonamidic motif itself is known as a neutral donor of hydrogen bond. Hence for better understanding of the behaviour of the final urea receptors, the properties of intermediate molecules were studied by various physicochemical techniques including spectroscopic and electrochemical methods. Spectroscopic methods were used to evaluate the binding ability of this motif and to determine its acid-basic properties. Electrochemical methods (represented by polarography and cyclic voltammetry with mercury dropping or hanging electrodes in dimethylformamide with Bu4N PF6 as electrolyte) were chosen because the nitro groups in our intermediate molecules are not only important substituents enabling continuation in synthesis, but also electrochemical "probes" reflecting the properties of the parent molecule. Their reduction in aprotic media is generally known after the first one-electron reversible reduction step where a stable radical anion is formed, a three-electron irreversible process follows resulting in the corresponding hydroxylamine [2]. However, the reduction pattern of the series (b) is totally different: After the first irreversible step (theoretically involving 4/5 of electron) a one-electron reversible reduction takes place followed by a multielectron process. This behaviour can be explained by using autoprotonation mechanism [3] where the acidic hydrogen of the sulfonamidic group is involved. Then our attention focused on final receptors (fig c). It was shown, that sulfonamidic moiety can be utilized as an electron-withdrawing group useful to enhance the anion complexation properties of urea-based receptors. On the other hand, it brings an inherent predisposition for deprotonation in the presence of more basic anions. In order to shift the deprotonation/complexation equilibrium in favour of the complexation, the receptors were derivatized by electron-donating groups like –OCH3 at sulfonamidic moiety. The binding for this receptor was evaluated in a competitive solvent (DMSO) by using NMR titration experiments. The experimental results show a considerable selectivity of our receptor to phosphates compared to another biologically important anions such as Cl-, NO3 - and HSO4 - [4]. Based on these results, the prepared receptors appear to be suitable motifs for binding phosphate anions. In order to take advantage of their binding capabilities, these molecules need to be attached to the surface of a suitable molecular carrier. The formation of these functional materials (in our case especially dendrimers) is also associated with the need to design and optimize methods to describe anion binding ability, as well as the possibility to recycle the prepared materials. This was the reason for which it seems to be interesting to consider the presence of an electrochemical “probe” also in the final molecular receptors. The redox transformation of such a probe should fundamentally affect the complexing abilities of the receptor and allow targeted degradation of the complex. Acknowledgements: The work was supported by the grant number 18-12150S and 20-07833S. References: [1] A J. L. Sessler, P. A. Gale and W. S. Cho in Anion Receptor Chemistry, The Royal Soc. Of Chemistry, Cambridge, 2006. [2] a) O. Exner, A critical compilation of substituent constants, in: N.B. Chapman, J. Shorter (Eds.), Correlation Analysis in Chemistry, Plenum Press, N.Y. London, 1978, pp. 445. b) P. Zuman, Subsituent Effects in Organic Polarography, Plenum Press, New York, 1967. [3] E. Brillas, G. Farnia, M. G. Severin, E. Vianello; Electrochimica Acta 31 (1986) 759-766. [4] K. Salvadori, L. Šimková, I. Císařová, J. Sýkora, J. Ludvík, P. Cuřínová, Sulphonamidic groups as electron‐withdrawing units in ureido‐based anion receptors: enhanced anion complexation versus deprotonation. ChemPlusChem 2020, 85 (7), 1401–1411. Figure 1

A massive structural change accompanies electron capture by the 1,2-dicarba

The molecule of 2,2-dinitroethene-1,1-diamine (FOX-7) is one of the most interesting molecules with multiple redox centres stabilized by push-pull effect. To reveal the detailed mechanism of its electrochemical process radical intermediates formed in the course of its electrochemical reduction in organic aprotic media have been studied by in situ simultaneous electrochemical ESR measurements (SEESR). The radical generated on the second reduction step possesses an alternating line-width (AL) effect in the ESR spectra as a result of intramolecular dynamic processes in the timescale of ESR splitting constants. The spectra measured at different temperatures (230–335 K) were analysed with the help of a fitting program which includes a molecular dynamics. Observed dynamics describes well an asymmetric 2-site exchange model for the whole temperature range. With help of the optimized parameters and quantum chemical calculations this radical has been identified as 2,2-dinitroethane-1-amine-1-imine radical dianion, [(H2N)(HN)C=C(NO2)2] 2−. The dynamic process responsible for the AL effect consists of mutual turning (changing of dihedral angle) of the both nitro groups, resulting in an intramolecular spin-density (electron) transfer. The dynamic parameters of the process have been established.

UV–vis, IR and NMR spectroscopy were utilized as direct monitoring techniques for structure changes of a new promising energetic material FOX-7 in different protic and aprotic solvents. The dissolved FOX-7 indicates changes in molecular structure as compared with that in solid-state. The push–pull properties of the molecule together with the effects of solvents result in charge separated zwitterion formed in solution. By combination of different spectroscopic techniques, structural changes of charge-separated molecule of FOX-7 were described in dependence on the solvent properties, temperature, proton activity and concentration. From the analytical point of view, the lowest detectable concentration of FOX-7 using UV–vis spectra was determined to approximately 6 × 10−7 mol L−1.

The electrochemical reduction behaviour of a new promising energetic material 2,2-dinitroethene-1,1-diamine (FOX-7) was investigated in acetonitrile, dimethylformamide and dimethylsulfoxide at mercury, platinum, gold and glassy carbon electrodes. Instead of expected consumption of four to eight electrons necessary for reduction of nitro groups, only two electrons are consumed. In acetonitrile, the starting form of FOX-7 is the olefinic, nearly planar structure polarized by the push-pull effect, the first electron attacks one of the nitro groups. The electrochemical as well as in situ UV-vis-NIR and EPR spectroelectrochemical results are in agreement with autoprotonation reduction mechanism. The radical intermediate was intercepted and identified by EPR spectrometry where an alternation line-width (AL) effect was observed.

Molecules on the base of 1,3-diphenylisobenzofuran attract an interest because of their\npossible efficiency for singlet fission This study is focused on electrochemical behaviour of\nthe „quasi dimer“ of 1,3-diphenylisobenzofuran. For characterization of reduction mechanism\nin situ UV-vis and EPR spectroscopy were used. The monomer 1,3-diphenylisobenzofuran\nand its CH2 dimer were added to this study for comparison. The redox properties of both\ncompared molecules are similar. The location and the nature of the covalent bridging unit in\nCH2dimer have small effect onthe redox potentials. But the pi-conjugation in the molecule of\nquasi dimer causes approximately 0.2 V easier oxidization and reduction.

2,2-Dinitroethene-1,1-diamine (FOX-7) was studied by means of electrospray ionization (ESI) and chemical ionization (CI) mass spectrometry in both positive and negative ion mode. Detailed mechanisms of unimolecular fragmentations of protonated and deprotonated FOX-7 were investigated using high- and low-energy collision-induced dissociation (CID) mass spectrometry, neutral fragment reionization mass spectrometry and quantum chemistry calculations. In deprotonated FOX-7, elimination of the carbodiimide molecule was identified as the energetically most favored fragmentation channel, closely resembling the base hydrolysis of FOX-7. The dinitromethanide ion is formed during this fragmentation as revealed by comparison with CID mass spectra of an isobaric ion prepared by the ESI of authentic sodium dinitromethanide. The proton affinity of FOX-7 was estimated as 855 kJ mol−1 by high-accuracy quantum chemistry calculations. This value corresponds to protonation at the C-2 position, though the oxygen-protonated tautomer was found to be nearly isoenergetic in the gas phase. In acetonitrile, the nitro group-protonated FOX-7 was found to be significantly less stable then its C-2 tautomer. These theoretical findings are clearly reflected in differences in fragmentations of ESI- and CI-generated [M + H]+ ions. Interestingly, the consecutive losses of OH• and NO2• radicals instead of a whole HNO3 molecule were found to account for the most abundant fragment ion in the positive ESI CID mass spectra. In the CI-generated [M + H]+ and [M+D]+ ions, substantial internal energy effects upon the CID were observed.

Eleven fluorinated 1,3-diphenylisobenzofurans, materials that are promising to perform efficient singlet fission, have been synthesized and their electrochemical reduction and oxidation in non-aqueous dimethylformamide examined. The redox properties are not changed continuously with increasing number of fluorine substituents but there are some qualitative turns in behavior for specific types of substitution revealing a discontinuity requiring deeper investigation. The DFT calculations confirm the trends observed experimentally by correlation of experimental and calculated data. The differences between the geometries of the neutral species and their radical ions were computed and rationalized.

Redox properties of 2,2-dinitroethene-1,1-diamine (FOX-7) were electrochemically investigated by DC polarography, cyclic voltammetry and preparative electrolyses, and electroreduction products were analysed. The polarographic reduction of FOX-7 in aqueous buffered solutions exhibited a total sum of limiting currents corresponding to the expected 18 electrons. This result fits well with the chemical reduction of this energetic material resulting in ethan-1,2-diamine. Exhaustive electrolysis, however, did not yield this product and the passed charge for total degradation of this compound corresponds only to 4–7 F. As the main products, gaseous N2, N2O, NO and NH3 were proved by combination EC–GC–MS and, besides that, nitrite anions and ammonia ions were identified by other methods. All these observations suggest that instead of electrochemical reduction, an electrochemically initiated degradation process accompanying the exhaustive electrolysis occurs. This seems to be analogous to the chain of reactions during explosion, which are slowed down by a very low concentration and cooled by the solvent.

The recently synthesized energetic material 2,2-dinitroethene-1,1-diamine (FOX-7) is interesting not only as an explosive, but also as a molecule with multiple redox centers, where strong electron donors (amino groups) and acceptors (nitro groups) are close to each other. The present fundamental electrochemical study revealed that in aqueous solutions the reduction pathway during the exhaustive electrolysis does not follow the expected pattern but after the first electron transfer a chain of spontaneous intramolecular (eventually intermolecular) redox processes takes place leading not to reduction products, but to a total degradation of the title compound. Since evolution of gaseous products has been detected during the electrolysis, the electrochemically initiated decomposition of the FOX-7 seems to be analogous to the explosion process slowed down by the very low concentration and cooled by the solution.