Synthesis, Structure, Optical, and Electrochemical Properties of Triple- and Quadruple-Decker Co-facial Tetrathiafulvalene Arrays

Spin and Structural Dynamics of Persistent π-Mer and π-Dimer of a Tethered S,C,C-Bridged Triphenylamine Dimer

One- and two-electron-oxidized species of two π-conjugated systems that adopt close π-stacking structures are referred to as π-mers and π-dimers, respectively. These cationic species exhibit intriguing physicochemical properties that are markedly distinct from those of their parent monomeric radical cations. In this study, we have achieved the chemical oxidation of a newly designed tethered dimer comprising two planar sulfur- and carbon-bridged triphenylamine (SCBT) units linked by a 1,8-naphthalene moiety to obtain the corresponding π-mer and π-dimer as crystalline materials. Single-crystal x-ray diffraction analyses revealed that these cationic species adopt almost perfectly overlapping face-to-face close π-stacking structures. As the degree of oxidation increases from the neutral form to the monocation and the dication, the interatomic S--S distance decreases drastically from 5.35 to 3.03 Å. The injection of electron spins into the neutral species effectively induces covalent-like bonding interactions between the sulfur atoms and changes the degree of structural freedom of the molecule. The salt of the π-mer exhibited an electrical conductivity of 1.9 × 10-6 S cm-1 at room temperature. The injection of spins by chemical oxidation also dramatically changed the dielectric response by generating dipole moments that affect the symmetry of the molecule.

A triad molecular conductor: simultaneous control of charge and molecular arrangements

Molecular and charge arrangements in the solid state were controlled by a new building block: a triad molecule. Owing to the appropriate flexibilities in both molecular structure and electron distribution of the triad, the apparently simple salt exhibits an unstable metallic phase, which is promising for superconducting transitions.

Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

Accordion-Like Motion in Electrochemically Switchable Crown Ether/Ammonium Oligorotaxanes

Reversible oxidation reactions in electrochemically switchable oligorotaxanes with tetrathiafulvalene (TTF) decorated 24-crown-8 ether wheels generate intramolecular mixed-valence and radical-cation interactions between the wheels. This induces shuttling of the wheels and a contraction of inter-wheel distances. Further oxidation generates repulsive forces between the TTFs and maximizes the inter-wheel distances instead. These interactions and co-conformational changes were not observed for structurally similar controls in which acetyl groups along the axle prevent translational motion of the wheels. This operation mode of oligorotaxanes, which is reminiscent of an accordion-like motion, is promising for functional materials and nanodevices such as piston-type rotaxane motors.

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.

Mixed-Valent Molecular Triple Deckers

Two phenothiazine (PTZ) moieties were connected via naphthalene spacers to a central arene to result in stacked PTZ-arene-PTZ structure elements. Benzene and tetramethoxybenzene units served as central arenes mediating electronic communication between the two PTZ units. Based on cyclic voltammetry, UV/Vis-NIR absorption, EPR spectroscopy, and computational studies, the one-electron oxidized forms of the resulting compounds behave as class II organic mixed-valence species in which the unpaired electron is partially delocalized over both PTZ units. The barrier for intramolecular electron transfer depends on the nature of the central arene sandwiched between the two PTZ moieties. These are the first examples of rigid organic mixed-valent triple-decker compounds with possible electron-transfer pathways directly across a stacked structure, and they illustrate the potential of oligo-naphthalene building blocks for long-range electron transfer and a future molecular electronics technology.

Tetrathiafulvalene - a redox-switchable building block to control motion in mechanically interlocked molecules

With the rise of artificial molecular machines, control of motion on the nanoscale has become a major contemporary research challenge. Tetrathiafulvalenes (TTFs) are one of the most versatile and widely used molecular redox switches to generate and control molecular motion. TTF can easily be implemented as functional unit into molecular and supramolecular structures and can be reversibly oxidized to a stable radical cation or dication. For over 20 years, TTFs have been key building blocks for the construction of redox-switchable mechanically interlocked molecules (MIMs) and their electrochemical operation has been thoroughly investigated. In this review, we provide an introduction into the field of TTF-based MIMs and their applications. A brief historical overview and a selection of important examples from the past until now are given. Furthermore, we will highlight our latest research on TTF-based rotaxanes.

Reversible conversion between a pleated oligo-tetrathiafulvalene radical foldamer and folded donor-acceptor [3]pseudorotaxane under redox conditions

Linear tetrathiafulvalene (TTF) oligomers were synthesized, which could not only form a pleated TTF˙⁺ radical cation foldamer under oxidation conditions, but also interlocked with CBPQT⁴⁺ to form folded donor-acceptor [3]pseudorotaxane in the neutral state of TTF. Moreover, switchable transformation between these two folded supramolecular structures was achieved under the alternative regulation of the redox states of TTF units.