Artificial redox-driven directionally controlled switches as a basis for redox-driven molecular motors

Reversible Stereoisomerization of 1,3-Diphosphetane Frameworks Revealed by a Single-Electron Redox Approach

The discovery of pyramidal inversion has continued to impact modern organic and organometallic chemistry. Sequential alkylation reactions of an N-heterocyclic carbene (NHC) ligated dicarbondiphosphide 1 with RI (R = Me, Et, or iBu) and ZnMe₂ give rise to the highly stereoselective synthesis of cis-1,3-diphosphetanes 3. cis-3 is conformationally favorable at room temperature, whereas inversion to trans-3 is observed at 110 °C. One-electron oxidation of cis-3 with Fc⁺(BAr^F) (Fc = [Fe(C₅H₅)₂]; BAr^F = [B(3,5-(CF₃)₂C₆H₃)₄)]^-) leads to the stereoselective formation of trans-1,3-diphosphetane radical cation salts 3^•+(BAr^F), which can be reversibly transformed to cis-3 upon one-electron reduction. Salts 3^•+(BAr^F) represent the first examples of 1,3-diphosphetane radical cations. These results provide a potential application of planar four-membered heterocycle-based building blocks for electrically fueled molecular switches.

Reversible Redox Switching of Chromophoric Phenylmethylenepyrans by Carbon-Carbon Bond Making/Breaking

Electrochromic organic systems that can undergo substantial variation of their optical properties upon electron stimulus are of high interest for the development of functional materials. In particular, devices based on radical dimerization are appropriate because of the effectiveness and speed of carbon-carbon bond making/breaking. Phenylmethylenepyrans are organic chromophores which are well suited for such purposes since their oxidation leads to the reversible formation of bispyrylium species by radical dimerization. In this paper, we show that the redox and spectroscopic properties of phenylmethylenepyrans can be modulated by adequate variation of the substituting group on the para position of the phenyl moiety, as supported by DFT calculations. This redox switching is reversible over several cycles and is accompanied by a significant modification of the UV-vis spectrum of the chromophore, as shown by time-resolved spectroelectrochemistry in thin-layer conditions.

Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2-substituted dibenzofulvene molecular actuators based on (2,2,2-triphenylethylidene)fluorene (TEF) are reported. The 2-substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X-ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2-substituents on steric gearing. The addition-elimination reaction of Z-9 with trityl anion to form Z-10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266-355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near-UV, making it a particularly promising drive rotor moiety for light-powered molecular devices.

From eight-membered 10π electron sulfur-nitrogen cycles to bicycles and cages: a theoretical approach

A simple way of rationalizing the structures of cyclic, bicyclic, and tricyclic sulfur-nitrogen species and their congeners is presented. Starting from a planar tetrasulfur tetranitride with 12π electrons, we formally derived on paper a number of heterocyclic eight-membered 10π electron species by reacting the 3p orbitals of two opposite sulfur centers with one radical each, or by replacing these centers by other atoms with five (P) or four (Si, C) valence electrons. This led to planar aromatic 10π electron systems, nonplanar bicyclic structures with a transannular S-S bond, and tricyclic structures by bridging the planar rings with an acceptor or donor unit. The final structures depend on the number of π electrons in the bridges. Intermediate biradicals are stabilized by Jahn-Teller distortion, giving transannular S-S bonds between the NSN units. This procedure may be summarized by two rules, which provide a rationale for the structures of a large number of sulfur-nitrogen-based molecules. The long bonds between the NSN units show a p character of >95 %. The qualitative results have been compared with known molecular structures and the results of B3LYP/cc-pVTZ calculations as well as CASSCF and CASVB calculations. B3LYP/cc-pVTZ calculations have also provided the UV/Vis spectra and the NICS values of the planar 10π systems.