Binaphthalene Molecules with Tetrathiafulvalene Units: CD Spectrum Modulation and New Chiral Molecular Switches by Reversible Oxidation and Reduction of Tetrathiafulvalene Units

Harnessing Radicals: Advances in Self-Assembly and Molecular Machinery

Radicals, with their unpaired electrons, exhibit unique chemical and physical properties that have long intrigued chemists. Despite early skepticism about their stability, the discovery of persistent radicals has opened new possibilities for molecular interactions. This review examines the mechanisms and applications of radically driven self-assembly, focusing on key motifs such as naphthalene diimides, tetrathiafulvalenes, and viologens, which serve as models for radical assembly. The potential of radical interactions in the development of artificial molecular machines (AMMs) are also discussed. These AMMs, powered by radical-radical interactions, represent significant advancements in non-equilibrium chemistry, mimicking the functionalities of biological systems. From molecular switches to ratchets and pumps, the versatility and unique properties of radically powered AMMs are highlighted. Additionally, the applications of radical assembly in materials science are explored, particularly in creating smart materials with redox-responsive properties. The review concludes by comparing AMMs to biological molecular machines, offering insights into future directions. This overview underscores the impact of radical chemistry on molecular assembly and its promising applications in both synthetic and biological systems.

Chiral Spiro-Tetrathiafulvalenes: Synthesis, Chiroptical Properties, Conformational Issues and Charge Transfer Complexes

Within this work we have investigated spiro-based tetrathiafulvalenes (TTFs) obtained as mixtures of stereoisomers from racemic spiro[5.5]undeca-1,8-dien-3-one. Compared to previously described spiro-TTFs, enantiomeric and diastereoisomeric forms have been here separated by chiral HPLC and fully characterized both experimentally and theoretically. The two types of spiro-based chiral derivatives contain either one (2) or three (1) chiral centres out of each one is spiro-type. Experimental CD, supported by TD-DFT calculations, shows differences in the optical activity between the 1 and 2 and their intermediates. The low optical activity of 2 and 3 (spiro alone chirality) was attributed to the presence of two conformers in the solution (ax and eq) of opposite Cotton effect whereas in the case of 1 and 5 (spiro and stereogenic centres) the spiro chirality seems to be responsible of the Cotton effect in the high energy region whereas the R and S chirality in the low energy region. Racemic and enantiopure forms have been successfully used for the synthesis of charge transfer complexes with tetracyanoquinodimethane (TCNQ) based acceptors.

Regioselective Substitution of BINOL

1,1'-Bi-2-naphthol (BINOL) has been extensively used as the chirality source in the fields of molecular recognition, asymmetric synthesis, and materials science. The direct electrophilic substitution at the aromatic rings of the optically active BINOL has been developed as one of the most convenient strategies to structurally modify BINOL for diverse applications. High regioselectivity has been achieved for the reaction of BINOL with electrophiles. Depending upon the reaction conditions and substitution patterns, various functional groups can be introduced to the specific positions, such as the 6-, 5-, 4-, and 3-positions, of BINOL. Ortho-lithiation at the 3-position directed by the functional groups at the 2-position of BINOL have been extensively used to prepare the 3- and 3,3'-substituted BINOLs. The use of transition metal-catalyzed C-H activation has also been explored to functionalize BINOL at the 3-, 4-, 5-, 6-, and 7-positions. These regioselective substitutions of BINOL have allowed the construction of tremendous amount of BINOL derivatives with fascinating structures and properties as reviewed in this article. Examples for the applications of the optically active BINOLs with varying substitutions in asymmetric catalysis, molecular recognition, chiral sensing and materials are also provided.

Chiroptical inversion of a planar chiral redox-switchable rotaxane

A tetrathiafulvalene (TTF)-containing crown ether macrocycle with C s symmetry was designed to implement planar chirality into a redox-active [2]rotaxane. The directionality of the macrocycle atom sequence together with the non-symmetric axle renders the corresponding [2]rotaxane mechanically planar chiral. Enantiomeric separation of the [2]rotaxane was achieved by chiral HPLC. The electrochemical properties - caused by the reversible oxidation of the TTF - are similar to a non-chiral control. Reversible inversion of the main band in the ECD spectra for the individual enantiomers was observed after oxidation. Experimental evidence, conformational analysis and DFT calculations of the neutral and doubly oxidised species indicate that mainly electronic effects of the oxidation are responsible for the chiroptical switching. This is the first electrochemically switchable rotaxane with a reversible inversion of the main ECD band.

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.

New unsymmetrically benzene-fused bis (tetrathiafulvalene): synthesis, characterization, electrochemical properties and electrical conductivity of their materials

The synthesis of new unsymmetrically benzene-fused bis (tetrathiafulvalene) has been carried out by a cross-coupling reaction of the respective 4,5-dialkyl-1,3-dithiole-2-selenone 6-9 with 2-(4-(p-nitrophenyl)-1,3-dithiole-2-ylidene)-1,3,5,7-tetrathia-s-indacene-6-one 5 prepared by olefination of 4-(p-nitrophenyl)-1,3-dithiole-2-selenone 3 and 1,3,5,7-tetrathia-s-indacene-2,6-dione 4. The conversion of the nitro moiety 10a-d to amino 11a-d then dibenzylamine 12a-d groups respectively used reduction and alkylation methods. The electron donor ability of these new compounds has been measured by cyclic voltammetry (CV) technique. Charge transfer complexes with tetracyanoquino-dimethane (TCNQ) were prepared by chemical redox reactions. The complexes have been proven to give conducting materials.

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

SIGNIFICANCE

This review relates to artificial redox-driven molecular devices. The advantages of using very simple chemical building blocks for the bottom-up design of nanoleveled functional motors and the importance of the unidirectionality of a switching process for the development of redox-driven molecular motors are discussed. Furthermore, the crucial difference between artificial molecular switches and motors is explained.

RECENT ADVANCES

This review discusses few selected examples of redox-driven devices exhibiting partially complex-coupled movement sequences, which, however, due to the lack of an overall directionally controlled movement are not able to perform mechanical work on a molecular scale. Recent examples for redox-driven devices with at least one directionally controlled switching process as well as the proof for the unidirectionality of the switching process are presented.

CRITICAL ISSUES

The challenge in designing directionally controlled switches is the fact that during the switching process, a configuration (or conformation) must be changed reversibly. This crucial process can be a flip caused by the change of the coordination sphere of a metal ion, a rotation around a C-C single bond, or around a C-C double bond.

FUTURE DIRECTIONS

For future developments, we suggest designing artificial redox-based molecular motors in which the motion process of the presented directionally controlled switches are coupled with another switchable unit. The latter could also be switchable in a nondirected way.

Chiroptical, linear, and second-order nonlinear optical properties of binaphthol derivatives

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure-property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV-vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10(-30) esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (6(2+) and 6(4+)), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.

New substituted tetrathiafulvalene-quinone dyads: the influences of electron accepting abilities of quinone units on the metal ion-promoted electron-transfer processes

The metal ion-promoted electron transfer occurs to all new dyads 1, 2, 3, and 4, even one of them, dyad 4, which has a rather weak electron acceptor unit. The results also indicate that the metal ion-promoted electron transfer within the dyads is influenced by the electron accepting abilities of quinone units; dyad 2 with the strongest electron acceptor among the four dyads shows the strongest absorption and ESR signals attributed to TTF.+ in the presence of metal ions.