Dynamic redox systems based on bis(spiroxanthene)-type donors with a polycyclic aromatic hydrocarbon: preparation, X-ray structures, and electrochromic response with fluorescence change

Five shades of green: substituent influence on the (spectro-) electrochemical properties of diferrocenyl(phenyl)methylium dyes

Five new, intensely green diferrocenylphenylmethylium complexes 1+-5+ with electron donating (EDG: 4-MeO, 4-Me, 4-Br) or withdrawing (EWG: 3,5-CF₃, 4-ⁿC₆F₁₃) substituents were synthesized and fully characterized. The substituent influence on their electrochemical and spectroscopic properties was investigated by cyclic voltammetry, UV/Vis/NIR and T-dependent EPR spectroscopy of the cationic as well as the oxidized (12+-52+) and reduced (1˙-5˙) species. The reduced forms equilibrate with their corresponding dimers (65-83%) with a clear substituent influence as expressed by their Hammett parameters in an ordering 4+ > 5+ > 3+ > 2+ > 1+. The structures of all five precursor carbinols 1-OH-5-OH and those of three of the diferrocenylphenylmethylium cations (1+, 4+-5+) were established by X-ray crystallography.

Electrochemical, Spectroelectrochemical, Mößbauer, and EPR Spectroscopic Studies on Ferrocenyl-Substituted Tritylium Dyes

Four monoferrocenyl tritylium derivatives with donor-substituted (OMe, NMe₂ ) aryl rings are reported, along with their spectroscopic and electrochemical properties. All the complexes show a one-electron reduction and a quasi-reversible ferrocene oxidation at a very positive potential. Small quadrupole splittings, ΔE_Q , in Mößbauer spectra agree with highly electron-deficient ferrocenes. Comparison of the experimental half-wave potentials for ferrocene oxidation, E_1/2 (Fc/Fc⁺ ), with those estimated from established correlations of E_1/2 (Fc/Fc⁺ ) with ΔE_Q indicates that the E_1/2 values of the anisyl-substituted congeners FcOMe⁺ and FcMeOMe⁺ are affected by Coulombic repulsion between the positive charges at the Fe ion and the neighboring methylium site. Electronic spectra are recorded and interpreted with the aid of quantum chemical calculations. UV/Vis spectroelectrochemical measurements as well as chemical reduction provide insight into the redox-induced color changes upon ferrocene oxidation or upon reduction to the neutral trityl radicals. The neutral radicals reversibly form EPR-silent dimers. This process is studied by temperature-dependent EPR spectroscopy, and thermodynamic data for their dimerization are determined. Experimental and quantum chemical data suggest that the dimers assume classical hexaarylethane structures as opposed to normal or "offset" Jacobson-Nauta-type structures.

Modern Spin on the Electrochemical Persistence of Heteroatom-Bridged Triphenylmethyl-Type Radicals

Herein we present a clarification of the ambiguous persistence of the 10-methyl-9-phenylacridanyl, 9-phenylxanthenyl, and 9-phenylthioxanthenyl radicals in electrochemical experiments. Each of these radicals has separately been the subject of conflicting literature results for decades with publications claiming both their chemical inertness and propensity to dimerize. We assert that each radical is persistent at conventional electrochemical time scales up to several minutes based on reversible redox couples and cyclic voltammogram simulations of the radicals and their respective cations. All three radicals are rapidly consumed by aerial O₂, which lends irreversibility to the redox couples after fewer than 20 s of exposure to air. With appreciation for the O₂ sensitivity of these radicals, their electrochemically generated UV-visible absorption spectra have been acquired and matched to predictions made by TD-DFT calculations. Further, we propose that previous claims to have electrochemically measured radical-radical dimerizations have only observed reaction of these radicals with dissolved O₂.

Bis(diarylethenyl)-thiophenes, -bithiophenes, and -terthiophenes: a new series of electrochromic systems that exhibit a fluorescence response

2,5-Bis(diarylethenyl)thiophenes (1) and their bithiophene (2) and terthiophene (3) homologues were designed as a new series of violene/cyanine-hybrid-type electrochromic materials. Alkoxyphenyl and dialkylaminophenyl groups are used as the aryl group in the cyanine parts of 1/2/3O and 1/2/3N, respectively, which emit fluorescence in the neutral state but not in the oxidized state. Voltammetric analyses of 18 newly prepared electron donors show that they all undergo two reversible one-electron oxidations. Redox potentials, UV–vis spectral features, and fluorescence quantum yields are important parameters for characterizing the present electrochromic behavior with a fluorescence response, and can be finely tuned by changing aryl groups in the cyanine part, the number of thiophene rings in the violene part, and the alkyl-chain length of the alkoxyphenyl or diaklylaminophenyl groups (R = CH3, C8H17, C16H33), which makes this a versatile platform for the design of novel electrochromic materials.

Ultralong carbon-carbon bonds in dispirobis(10-methylacridan) derivatives with an acenaphthene, pyracene, or dihydropyracylene skeleton

Acenapthalene, pyracene, and dihydropyracylene attached to two units of spiroacridan are a novel class of hexaphenylethane (HPE) derivatives that have an ultralong Csp3-Csp3 bond (1.77-1.70 A). These sterically challenged molecules were cleanly prepared by C-C bond formation through two-electron reduction from the less-hindered dications. These ultralong bonds were realized based on several molecular-design concepts including enhanced "front strain" through "multiclamping" by means of fusing or bridging aryl groups in the HPE molecule. The lengths of these ultralong bonds and their relation to the conformation (torsional angle) were also validated by means of theoretical calculations. Bond-fission experiments revealed that the bonds are more easily cleaved than standard covalent bonds to produce the corresponding dication upon oxidation with an increase in the length of the C-C bond.

Ultralong C-C bonds in hexaphenylethane derivatives

The longer C-C bond than the standard (1.54 Å) is so weakened that it is cleaved easily, as found in the parent hexaphenylethane (HPE). However, the compounds with an ultralong C-C bond (1.75 Å) can be isolated as stable solids when the bond-dissociated species does not undergo any reactions other than bond reformation. This is the central point in designing the highly strained HPEs, which were obtained by two-electron reduction of the corresponding dications. Steric repulsion of "front strain" is the major factor to expand the central C-C bond of HPEs. During the detailed examination of the ultralong C-C bond, the authors discovered the intriguing phenomenon of "expandability": the C-C bond length can be altered over a wide range by applying only a small amount of energy (1 kcal mol-1) supplied by crystal packing force. This observation indicates that the much longer C-C bond than the shortest nonbonded contact (1.80 Å) will be realized under the rational molecular design concept.

Multi-input–Multi-output Molecular Response System Based on Dynamic Redox Behavior of Hexaphenylethane-type Electron Donors with the Tetrahydrophenanthrazepine Skeleton: Strong Chiroptical Signals through the Transmission of Point Chirality to Helicity

The title heterocyclic donors undergo reversible C-C bond formation/cleavage upon electron transfer (dynamic redox behavior). The helical sense in both neutral and cationic states is interconvertible by facile ring flipping. The pi-type asymmetric center on the azepine nitrogen atom induces a significant degree of diasteromeric preference, thus endowing strong CD activity based on exciton coupling. Chiroptical properties could be modified not only by redox reactions but also by heat and protonation. The present redox pairs can serve as unprecedented three-way-input (e, H+, delta) and two-way-output (UV/Vis, CD) response systems.

Butane-1,4-diyl dications stabilized by steric factors: electrochiroptical response systems based on reversible interconversion between dihydro[5]helicene-type electron acceptors and electron-donating 1,1′-binaphthyls

Incorporation in the dihydro[5]helicene framework prevents deprotonation of the title dications by steric factors, thus allowing their isolation as deeply colored stable salts. Based on the reversible interconversion with the electron-donating binaphthylic diolefins, they constitute a new class of electrochromic systems, in which C-C bond making/breaking is accompanied by two-electron transfer. Optically pure (R)-binaphthylic donors are interconvertible with the 1,4-dications with the R,R-configuration. The very large molar ellipticity makes it possible for them to be used as electrochiroptical response systems, by which the electrochemical input is transduced into two spectral outputs, i.e. UV-Vis and circular dichroism. Structurally related push-pull-type bis(quinonemethide)s also exhibit a similar multi-output electrochemical response.