Direct Observation of Mixed-Valence and Radical Cation Dimer States of Tetrathiafulvalene in Solution at Room Temperature: Association and Dissociation of Molecular Clip Dimers Under Oxidative Control

Switchable molecular tweezers: design and applications

Switchable molecular tweezers are a unique class of molecular switches that, like their macroscopic analogs, exhibit mechanical motion between an open and closed conformation in response to stimuli. Such systems constitute an essential component of artificial molecular machines. This review will present selected examples of switchable molecular tweezers and their potential applications. The first part will be devoted to chemically responsive tweezers, including stimuli such as pH, metal coordination, and anion binding. Then, redox-active and photochemical tweezers will be presented.

Mixed-Valence BN-Doped Corannulene Trimer Radical Cations

Mixed-valence (MV) dimers have been extensively investigated, however, the structure and properties of purely organic MV trimers based on open-shell polycyclic aromatic hydrocarbons remain elusive. Herein, unprecedented MV BN-doped corannulene radical cations [BN-Cor1]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- and [BN-Cor2]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- were synthesized via chemical oxidation, and their structures were unambiguously confirmed by single-crystal X-ray diffraction. These uncommon radical cations consist of three corannulene cores and two [BArylF 4 ]- anions, and three corannulene motifs [BN-Cor1]3 ⋅⋅2+ and [BN-Cor2]3 ⋅⋅2+ in the unit cell exhibit a trimer structure with a slipped π-stacking configuration. Detailed structural analyses further revealed that the corannulene cores exhibit an infinite layered self-assembly configuration, allowing their potential applications as building blocks for molecular conductors. The detection of a forbidden transition (Δms =±2) by electron paramagnetic resonance (EPR) spectroscopy further confirmed the existence of two unpaired electrons in the π-trimers and the MV characteristic of these two species. Variable-temperature EPR and conductivity measurements suggested that the BN-doped π-trimers exhibited antiferromagnetic coupling and conductivity properties.

Assembling Molecular Clips To Build π-Stacks

Nature utilizes an intimate stacking of aromatic motifs to construct functional structures, as demonstrated in protein folding and polynucleotide assembly. However, organized π-stacks of artificial molecules are difficult to build, primarily due to the weak, non-directional, and context-sensitive nature of van der Waals forces. To overcome these challenges, chemists have invented ingenious architectural designs to construct π-stacked supramolecular assemblies using clip-like molecules. This Concept article focuses on molecular clips that enable precise spatial control over assembly patterns, beyond the scope of simple host-guest chemistry. Different design strategies are analyzed and compared that leverage non-covalent interactions to create multi-layer π-stacks. Particular emphasis is placed on the choice of spine units as they play a crucial role in controlling the (i) spacing, (ii) orientation, and (iii) conformational pre-organization of linked aromatics to achieve long-range spatial ordering.

Superhelicenes in the gas phase: experimental and computational evidence of stable radical cation dimers

Electrospray-ionization mass spectrometry (ESI-MS) readily produces stable radical cation π-dimers of the superhelicenes. Energy-resolved collision experiments reveal the dissociation of the dicationic dimer into two singly charged superhelicenes. DFT calculations indicate that open-shell dications composed of two radical cations are thermochemically more attractive than the closed-shell dimer formed by a doubly charged and a neutral superhelicene.

A Self-Assembled Homochiral Radical Cage with Paramagnetic Behaviors

Condensation of an inherently C₃ -symmetric polychlorotriphenylmethyl (PTM) radical trisaldehyde with tris(2-aminoethyl)amine (TREN) yields a [4+4] tetrahedral radical cage as a racemic pair of homochiral enantiomers in 75 % isolated yield. The structure was characterized by X-ray crystallography, confirming the homochirality of each cage framework. The homochirality results from intramolecular [CH⋅⋅⋅π] and hydrogen-bonding interactions within the cage framework. The four PTM radicals in a cage undergo weak through-space coupling. Magnetic measurements demonstrated that each cage bears 3.58 spins.

Switchable synchronisation of pirouetting motions in a redox-active [3]rotaxane

In this study, the crown/ammonium [3]rotaxane R2 is reported which allows a switchable synchronisation of wheel pirouetting motions. The rotaxane is composed of a dumbbell-shaped axle molecule with two mechanically interlocked macrocycles which are decorated with a redox-active tetrathiafulvalene (TTF) unit. Electrochemical, spectroscopic, and electron paramagnetic resonance experiments reveal that rotaxane R2 can be reversibly switched between four stable oxidation states (R2, R2˙⁺, R2²⁽˙⁺⁾, and R2⁴⁺). The oxidations enable non-covalent, cofacial interactions between the TTF units in each state-including a stabilised mixed-valence (TTF₂)˙⁺ and a radical-cation (TTF˙⁺)₂ dimer interaction-which dictate a syn (R2, R2˙⁺, and R2²⁽˙⁺⁾) or anti (R2⁴⁺) ground state co-conformation of the wheels in the rotaxane. Furthermore, the strength of these wheel-wheel interactions varies with the oxidation state, and thus electrochemical switching allows a controllable synchronisation of the wheels' pirouetting motions. DFT calculations explore the potential energy surface of the counter-rotation of the two interacting wheels in all oxidation states. The controlled coupling of pirouetting motions in rotaxanes can lead to novel molecular gearing systems which transmit rotational motion by switchable non-covalent interactions.

TTF–PTM dyads: from switched molecular self assembly in solution to radical conductors in solid state

Dyads formed by tetrathiafulvalene (TTF) linked to perchlorotriphenylmethyl (PTM) radicals exhibit interesting physical properties such as bistability in solution or conductivity in solid state.

Confinement Effects of Metal-Organic Framework on the Formation of Charge-Transfer Tetrathiafulvalene Dimers

Three transition metal coordination polymers (CPs) based on the redox-active dimethylthio-tetrathiafulvalene-bicarboxylate (L) and 1,3-bi(4-pyridyl)propane (bpp) ligands, formulated as [MnL(bpp)]_n (1), [CdL(bpp)]_n (2), and [Cd₂L(bpp)₂(H₂O)(C₂O₄)_0.5]_n·n(ClO₄)·n(H₂O) (3), are crystallographically characterized. Complexes 1 and 2 are isostructural 2-D polymers, and 3 features an unusual 3-D metal-organic framework (MOF). The 3-D MOF is constructed from tetranuclear cluster nodes built through the μ₂-O bridge of the TTF ligand, which is first found for TTF coordination polymers. It is found that the channel generated by the 3-D MOF exerts a confinement effect on the formation of TTF dimers. The TTF dimers show strong intradimer interaction with partial electron transfer or charge transfer, and hence, the Cd compound 3 has relatively good photocurrent response property in comparison with that of 2-D Cd compound 2.

Formation of Long, Multicenter π-[TCNE]22- Dimers in Solution: Solvation and Stability Assessed through Molecular Dynamics Simulations

Purely organic radical ions dimerize in solution at low temperature, forming long, multicenter bonds, despite the metastability of the isolated dimers. Here, we present the first computational study of these π-dimers in solution, with explicit consideration of solvent molecules and finite temperature effects. By means of force-field and ab initio molecular dynamics and free energy simulations, the structure and stability of π-[TCNE]₂^2- (TCNE=tetracyanoethylene) dimers in dichloromethane have been evaluated. Although the dimers dissociate at room temperature, they are stable at 175 K and their structure is similar to the one in the solid state, with a cofacial arrangement of the radicals at an interplanar separation of approximately 3.0 Å. The π-[TCNE]₂^2- dimers form dissociated ion pairs with the NBu₄⁺ counterions, and their first solvation shell comprises approximately 20 CH₂ Cl₂ molecules. Among them, the eight molecules distributed along the equatorial plane of the dimer play a key role in stabilizing the dimer through bridging C-H⋅⋅⋅N contacts. The calculated free energy of dimerization of TCNE^.- in solution at 175 K is -5.5 kcal mol^-1 . These results provide the first quantitative model describing the pairing of radical ions in solution, and demonstrate the key role of solvation forces on the dimerization process.

Three Redox States of a Diradical Acceptor-Donor-Acceptor Triad: Gating the Magnetic Coupling and the Electron Delocalization

The diradical acceptor-donor-acceptor triad 1(••), based on two polychlorotriphenylmethyl (PTM) radicals connected through a tetrathiafulvalene(TTF)-vinylene bridge, has been synthesized. The generation of the mixed-valence radical anion, 1(•-), and triradical cation species, 1(•••+), obtained upon electrochemical reduction and oxidation, respectively, was monitored by optical and ESR spectroscopy. Interestingly, the modification of electron delocalization and magnetic coupling was observed when the charged species were generated and the changes have been rationalized by theoretical calculations.

Tetrathiafulvalene-containing polymers for simultaneous non-covalent modification and electronic modulation of MoS2 nanomaterials

Polymers with pendent tetrathiafulvalene groups for solubilization and electronic modification of MoS₂ nanosheets.

Transition metal dichalcogenides (TMDCs) such as MoS₂ comprise an important class of 2D semiconductors with numerous interesting electronic and mechanical features. Full utilization of TMDCs in materials and devices, however, necessitates robust functionalization methods. We report well-defined tetrathiafulvalene (TTF)-based polymers, exploiting synthetic routes that overcome challenges previously associated with these systems. These platforms enable basal plane coordinative interactions with MoS₂, conceptually in parallel with pyrene-containing platforms for graphene and carbon nanotube modification. Not yet reported for TMDCs, these non-covalent interactions are universal and effective for MoS₂ irrespective of the lattice structure, affording significantly enhanced solution stabilization of the nanosheets. Additionally, the TTF-functionalized polymers offer electronic structure modulation of MoS₂ by ground state charge transfer and work function reduction, demonstrated using Kelvin probe force microscopy (KPFM). Notably, coordination and electronic effects are amplified for the TTF–polymers over TTF itself. Experiments are supported by first-principles density functional theory (DFT) calculations that probe polymer–TTF surface interactions with MoS₂ and the resultant impact on electronic properties.

Long, Multicenter Bonding--A New Concept for Supramolecular Materials

A new concept for constructing supramolecular architectures is discussed. In addition to van der Waals and hydrogen-bonding intermolecular interactions that primarily account for supramolecular structures for all materials lacking extended 3D network structures, directional, long, multicenter bonding that can occur for anionic, cationic, neutral, and zwitterionic radicals and can direct intermolecular recognition through π interactions and form extended network supramolecular structural motifs.

Glycoluril-tetrathiafulvalene molecular clips: on the influence of electronic and spatial properties for binding neutral accepting guests

Glycoluril-based molecular clips incorporating tetrathiafulvalene (TTF) sidewalls have been synthesized through different strategies with the aim of investigating the effect of electrochemical and spatial properties for binding neutral accepting guests. We have in particular focused our study on the spacer extension in order to tune the intramolecular TTF···TTF distance within the clip and, consequently, the redox behavior of the receptor. Carried out at different concentrations in solution, electrochemical and spectroelectrochemical experiments provide evidence of mixed-valence and/or π-dimer intermolecular interactions between TTF units from two closed clips. The stepwise oxidation of each molecular clip involves an electrochemical mechanism with three one-electron processes and two charge-coupled chemical reactions, a scheme which is supported by electrochemical simulations. The fine-tunable π-donating ability of the TTF units and the cavity size allow to control binding interaction towards a strong electron acceptor such as tetrafluorotetracyanoquinodimethane (F4-TCNQ) or a weaker electron acceptor such as 1,3-dinitrobenzene (m-DNB).

Electronic excitation energies in dimers between radical ions presenting long, multicenter bonding

The formation of long, multicenter dimers between radical ions is usually monitored through UV-vis spectroscopy given the characteristic low-energy absorption band that they exhibit, not observed for the parent monomers. In this work, the performance of CASPT2, RASPT2, and TD-DFT methods for obtaining excitation energies of the long, multicenter bonded π-[TCNE]2(2-) and π-[TTF]2(2+) dimers has been addressed (TCNE = tetracyanoethylene; TTF = tetrathiafulvalene). The impact of the active space on the vertical electronic transitions computed at the RASPT2 and CASPT2 levels has been tested against experimentally observed absorption bands. Analogous tests have been carried out for a wide variety of density functionals within the TD-DFT formalism. Our calculations show that whereas CASPT2 predicts very accurate excitation energies for the π-[TCNE]2(2-), the mean absolute error for π-[TTF]2(2+) is higher for CASPT2 than for TD-DFT calculations, whenever pure density functionals or low % HF exchange hybrid functionals are used. Hybrid functionals with high % HF exchange (and thus RSH functionals) conduct to large errors on the excitation energies in both dimers. Furthermore, vertical electronic transitions are also obtained for 100 configurations extracted from a 45 ps molecular dynamics (CPMD) simulation aimed at providing an accurate description of the thermal fluctuation effects of a π-[TCNE]2(2-) dimer in dichloromethane. These thermal effects explain the shape of the experimental UV-vis spectrum, where the lowest HOMO → LUMO absorption presents a broad band and the following HOMO-1 → LUMO absorption exhibits a narrower band.

Metastable oxidation states of tetrathiafulvalenes on the surface of liposomes

Redox-active liposomes are prepared by the incorporation of tetrathiafulvalene–cholesterol conjugate 1 in phospholipid vesicles.

Diindenothienoacene–tetrathiafulvalene redox systems

The redox properties and cation associations of extended tetrathiafulvalenes with diindenothienoacene cores are strongly dependent on the number and regioisomeric structural orientation of the central thiophene units.

Tetrathiafulvalene terminal-decorated PAMAM Dendrimers for triggered release synergistically stimulated by redox and CB[7]

A series of polyamidoamine (PAMAM) dendrimers with tetrathiafulvalene (TTF) at the periphery (Gn-PAMAM-TTF), generation 0-2, were synthesized. These functionalized dendrimers exist as nanospheres with diameters around 80-100 nm in aqueous phase, which can encapsulate hydrophobic molecules. The terminal TTF groups can go through a reversible redox process upon addition of the oxidizing and reducing agents. Each terminal TTF(+•) group of the oxidized Gn-PAMAM-TTF assembled with cucurbit[7]uril (CB[7]) forming a 1:1 inclusion complex with association constants of (3.14 ± 0.36) × 10(5), (1.29 ± 0.12) × 10(6), and (1.79 ± 0.24) × 10(6) M(-1) for generation 0-2, respectively, even at the aggregate state. The formation of the inclusion complex loosened the structure of the nanospheres and initiated the release of cargo, and the release mechanism was validated by dynamic light scattering (DLS), cryo-transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR) experiments. This study provides a potential strategy for the development of drug delivery systems synergistically triggered by redox and supramolecular assembly.

Controlling charge separation in a novel donor–acceptor metal–organic framework via redox modulation

Donor–acceptor charge transfer interactions in a tetrathiafulvalene–naphthalene diimide-based metal–organic framework (MOF) are interrogated using a complementary suite of solid state spectroscopic, electrochemical and spectroelectrochemical methods along with computational calculations.