Monitoring the formation of TTF dimers by Na+ complexation

Photoinduced asymmetric charge trapping in a symmetric tetraazapyrene-fused bis(tetrathiafulvalene) conjugate

In fused donor-acceptor (D-A) ensembles, rapid charge recombination often occurs because the D and A units are spatially close and strongly coupled. To the best of our knowledge, a long-lived charge separated (CS) state is still elusive in such systems. The results presented here show that symmetric annulation of two tetrathiafulvalene (TTF) donors to a central tetraazapyrene (TAP) acceptor via two quinoxaline units leads to a CS state lifetime of a few ns. A detailed study of the electronic interactions between TTF and TAP units in the ground and excited states was performed and compared with the asymmetric counterpart by cyclic voltammetry, optical absorption and ultrafast transient absorption spectroscopy. The results demonstrate that the photoinduced asymmetric charge trapping between two TTFs significantly stabilizes the CS state, which is also verified theoretically.

Redox-Active Metal-Organic Frameworks with Three-Dimensional Lattice Containing the m-Tetrathiafulvalene-Tetrabenzoate

Metal-organic frameworks (MOFs) constructed by tetrathiafulvalene-tetrabenzoate (H₄TTFTB) have been widely studied in porous materials, while the studies of other TTFTB derivatives are rare. Herein, the meta derivative of the frequently used p-H₄TTFTB ligand, m-H₄TTFTB, and lanthanide (Ln) metal ions (Tb³⁺, Er³⁺, and Gd³⁺) were assembled into three novel MOFs. Compared with the reported porous Ln-TTFTB, the resulted three-dimensional frameworks, Ln-m-TTFTB ([Ln₂(m-TTFTB)(m-H₂TTFTB)_0.5(HCOO)(DMF)]·2DMF·3H₂O), possess a more dense stacking which leads to scarce porosity. The solid-state cyclic voltammetry studies revealed that these MOFs show similar redox activity with two reversible one-electron processes at 0.21 and 0.48 V (vs. Fc/Fc⁺). The results of magnetic properties suggested Dy-m-TTFTB and Er-m-TTFTB exhibit slow relaxation of the magnetization. Porosity was not found in these materials, which is probably due to the meta-configuration of the m-TTFTB ligand that seems to hinder the formation of pores. However, the m-TTFTB ligand has shown to be promising to construct redox-active or electrically conductive MOFs in future work.

Enhancing the photothermal conversion of tetrathiafulvalene-based MOFs by redox doping and plasmon resonance

Near-infrared (NIR) photothermal materials hold great promise for use in several applications, particularly in photothermal therapy, diagnosis, and imaging. However, current NIR responsive materials often show narrow absorption bands and low absorption efficiency, and have long response times. Herein, we demonstrate that the NIR absorption of tetrathiafulvalene-based metal–organic frameworks (MOFs) can be tuned by redox doping and using plasmonic nanoparticles. In this work, a MOF containing redox-active tetrathiafulvalene (TTF) units and Dy-carboxylate chains was constructed, Dy-m-TTFTB. The NIR absorption of the as-synthesized Dy-m-TTFTB was further enhanced by Ag⁺ or I₂ oxidation, transforming the neutral TTF into a TTF˙⁺ radical state. Interestingly, treatment with Ag⁺ not only generated TTF˙⁺ radicals, but it also formed Ag nanoparticles (NPs) in situ within the MOF pores. With both TTF˙⁺ radicals and Ag NPs, Ag NPs@Dy-m-TTFTB was shown to exhibit a wide range of absorption wavelengths (200–1000 nm) and also a high NIR photothermal conversion. When the system was irradiated with an 808 nm laser (energy power of 0.7 W cm⁻²), Ag NPs@Dy-m-TTFTB showed a sharp temperature increase of 239.8 °C. This increase was higher than that of pristine Dy-m-TTFTB (90.1 °C) or I₂ treated I₃⁻@Dy-m-TTFTB (213.0 °C).

The photo-response of the redox-active metal–organic framework has been systematically tuned by incorporating plasmonic Ag nanoparticles and tetrathiafulvalene radicals, resulting in efficient near-infrared photothermal conversion materials.

A self-assembled tetrathiafulvalene box

A M₈L₂ metalla-cage constructed through coordination-driven self-assembly from a quinonato bis-ruthenium complex and an electron-rich tetrathiafulvalene (TTF) tetrapyridyl ligand is depicted.

Toward Redox-Active Indenofluorene-Extended Tetrathiafulvalene Oligomers-Synthesis and Studies of Dimeric Scaffolds

The promotion of mixed-valence interactions between redox-active, π-conjugated scaffolds is of interest when developing new conducting or electrochromic materials as well as in the construction of redox-controlled supramolecular assemblies. In this work, dimeric structures of the redox-active indenofluorene-extended tetrathiafulvalene (IF-TTF) unit were synthesized in a stepwise protocol. The synthesis relied on the development of a new unsymmetrical IF-TTF building block by a combination of phosphite-mediated and Horner-Wadsworth-Emmons reactions for introduction of the dithiafulvene units. The redox properties were studied by cyclic voltammetry, where it was observed that a first one-electron oxidation, corresponding to a mixed-valence state, occurs at a significantly lower potential when the IF-TTF unit is incorporated into a dimer, compared to a monomer analogue. This result indicates that locking the redox-active IF-TTF units in close proximity promotes intramolecular mixed-valence interactions. A computational study was also conducted, supporting the involvement of intramolecular interactions.

Tuning Redox Properties and Self-Assembly of Thienoacene-Extended Tetrathiafulvalenes

Turning on and off associations between molecules by a reversible change in their redox states is a convenient way of controlling self-assembly and hence for advancing supramolecular chemistry. Here we present systematic studies on a selection of extended tetrathiafulvalenes with thienoacene spacers. By cyclic and differential pulse voltammetry and in situ EPR/UV-Vis-NIR spectroelectrochemistry, in combination with computations, we have elucidated how the number and orientations of thiophene rings in the spacer between the two dithiafulvene rings influence both the donor strength and association properties. The radical cations and their associates were found to cover a remarkable large region of the UV-Vis-NIR spectrum, but the appearance of the absorption spectrum of the radical cations as well as of the unassociated dications also depended strongly on the exact molecular structure.

An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)˙+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF˙+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole π-surface in comparison to pristine tetrathiafulvalene.

Redox-controlled hybridization of helical foldamers

Redox stimulations allow controlling the hybridization equilibrium of foldamers.

Understanding room-temperature π-dimerisation of radical ions: intramolecular π-[TTF]22+ in functionalised calix[4]arenes

Long, multicentre π-dimers of radical ions are weakly bound and can only be observed in solution at low temperature. However, recent supramolecular approaches induce the extra stabilisation required to preserve them at room temperature, by different means depending on the approach. In particular, π-[TTF]₂²⁺ dimers (TTF = tetrathiafulvalene) were detected upon oxidation of a TTF-based calix[4]arene in acetonitrile solution at room temperature, manifesting intramolecular [R-TTF]˙⁺[R-TTF]˙⁺ interactions (Chem. Commun. 2006, 2, 2233). In this work, the reasons behind the remarkable formation of these π-dimers in the calix[4]arene, [calix], molecule are unravelled by means of DFT calculations. We first demonstrate that the properties of the π-[R-TTF]₂²⁺ dimers are preserved in the [calix]²⁺. Most importantly, our results show that the π-dimerised and non-dimerised forms of the [calix]²⁺ are isoenergetic at room temperature, and that the activation energy for this process is ca. 9.5 kcal mol^-1. Hence, both forms coexist in equilibrium at 298 K, as the intramolecular nature of the interaction ensures a high reaction rate. The role of the Na⁺ cation in preventing the π-[R-TTF]₂²⁺ dimerisation of the [calix]²⁺ receptor is also examined, unveiling that this effect is mostly due to the electrostatic repulsion induced by the cation. Finally, we provide a revision on room-temperature stable supramolecular long, multicentre π-dimers of radical ions, a class of systems with great potential as molecular switches.

Functionalised tetrathiafulvalene- (TTF-) macrocycles: recent trends in applied supramolecular chemistry

Tetrathiafulvalene- (TTF-) based macrocyclic systems, cages and supramolecularly self-assembled 3D constructs have been extensively explored as functional materials for sensing and switching applications.

Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

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.

Coordination Chemistry of 2,2'-Bipyridyl- and 2,2':6',2″-Terpyridyl-Substituted BEDT-TTFs: Formation of a Supramolecular Capsule Motif by the Iron(II) Tris Complex of 2,2'-Bipyridine-4-thiomethyl-BEDT-TTF

Molecules of tris(2,2'-bipyridine-4-thiomethyl-BEDT-TTF)iron(II) (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) assemble in pairs to form a novel supramolecular capsular structure in the solid state. Three BEDT-TTF residues from one complex lie in the three grooves between coordinated bipyridines of the other complex, and vice versa, to form a capsule with 3-fold rotational symmetry and an internal volume of ca. 160 Å(3). Further aspects of the coordination chemistry of this ligand, its 6-substituted isomer, and the 2,2':6'2″-terpyridyl-4'-thiomethyl-BEDT-TTF analogue are described.

On the association of neutral and cationic tris(tetrathiafulvaleno)dodecadehydro[18]annulenes

Here, we report the first X-ray crystal structure of a tetrathiafulvalene-fused dehydroannulene with peripheral ethylthio substituents. In addition, we have subjected this compound to electrochemical and UV-Vis-NIR/ESR spectroelectrochemical studies to elucidate the degree to which the oxidised species associate.

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).

Enhancement of electron-donating character in alkylated monopyrrolo-tetrathiafulvalene derivatives

Bis-alkyl-substituted monopyrrolo-tetrathiafulvalene (MPTTF) derivatives display stronger affinity to electron-deficient molecular guests in comparison with their non-substituted MPTTF analogues.

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.

Preparation of a responsive carbohydrate-coated biointerface based on graphene/azido-terminated tetrathiafulvalene nanohybrid material

A one-step method for the reduction of graphene oxide (GO) to reduced graphene oxide (rGO) is reported taking advantage of the electron-donor properties of an azido-terminated tetrathiafulvalene (TTF-N(3)). The resulting graphene/TTF-N(3) nanohybrid material is characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) spectroscopy, and by electrical and electrochemical means. The accessibility of the azide function to chemoselective modification by any alkyne-terminated partner molecule via Cu(I)-catalyzed "click" chemistry is demonstrated. In a proof of principle and motivated by the importance of glycan-modified materials, many alkynyl-terminated mannose units were grated onto graphene/TTF-N(3). The TTF-mannose units could be released efficiently from the graphene matrix by chemical oxidation of TTF-mannose surface units to TTF(2+)-mannose, using Fe(ClO(4))(3) or the electron-deficient tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)).

A novel redox-active calix[4]arene-tetrathiafulvalene dyad

A novel redox-active calix[4]arene-TTF 5 was prepared by the reaction of p-tert-butylcalix[4]arene 4 with the tosylated TTF 3 in the presence of cesium fluoride. The structure of the dyad 5 was identified by X-ray diffraction analysis, and the preliminary electrochemical properties of 5 were investigated by cyclic voltammetry (CV), for which two reversible one-electron waves were observed. Moreover, the UV-vis absorption spectra studies show that the dyad 5 undergoes progressive oxidation at the TTF moiety in presence of increasing amounts of Cu2+ or Hg2+.