Gel Formed by Self-Assembly of a Urea-Modified Monopyrrolotetrathiafulvalene Derivative Displays Multi-Stimuli Responsiveness and Absorption of Rhodamine B

A new monopyrrolotetrathiafulvalene-based derivative containing a urea group was designed, synthesized and thoroughly characterized. It proved to be a non-gelator in a single solvent even when heated or sonicated. However, it could self-assemble in a CHCl₃ (CH₂ Cl₂ )/n-hexane mixture to form a thermo-responsive supramolecular organogel. SEM, FT-IR spectroscopy, UV/Vis absorption spectroscopy, and SAXS revealed that in the organogel system, the gelators self-assembled into supramolecular networks with a J-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen-bonding, and van der Waals forces. Interestingly, the gel phase was shown to undergo reversible gel-sol transformation induced by Fe³⁺ -Vitamin C (Vc), trifluoroacetic acid-triethylamine (TFA-TEA) and picric acid (PA)-NaOH. In particular, in the presence of picric acid, the experimental results proposed that charge transfer occurred from the electron-donor gelator to the electron-acceptor picric acid due to the possibility of complex formation. Furthermore, the formed organogel could behave as the matrix for encapsulating cationic fluorescent dye from wastewater, and the adsorption efficiency was directly proportional to the concentration of the gelator.

Large Synthetic Molecule that either Folds or Aggregates through Weak Supramolecular Interactions Determined by Solvent

Weak noncovalent interactions between large disclike molecules in poorly solvating media generally lead to the formation of fibers where the molecules stack atop one another. Here, we show that a particular chiral spacing group between large aromatic moieties, which usually lead to columnar stacks, in this case gives rise to an intramolecularly folded structure in relatively polar solvents, but in very apolar solvents forms finite aggregates. The molecule that displays this behavior has a C 3 symmetric benzene-1,3,5-tris(3,3'-diamido-2,2'-bipyridine) (BTAB) core with three metalloporphyrin units appended to it through short chiral spacers. Quite well-defined chromophore arrangements are evident by circular dichroism (CD) spectroscopy of this compound in solution, where clear exciton coupled bands of porphyrins are observed. In more polar solvents where the molecules are dispersed, a relatively weak CD signal is observed as a result of intramolecular folding, a feature confirmed by molecular modeling. The intramolecular folding was confirmed by measuring the CD of a C 2 symmetric analogue. The C 3 symmetric BTAB cores that would normally be expected to stack in a chiral arrangement in apolar solvents show no indication of CD, suggesting that there is no transfer of chirality through it (although the expected planar conformation of the 2,2'-bipyridine unit is confirmed by NMR spectroscopy). The incorporation of the porphyrins on the 3,3'-diamino-2,2'-bipyridine moiety spaced by a chiral unit leaves the latter incapable of assembling through supramolecular π-π stacking. Rather, modeling indicates that the three metalloporphyrin units interact, thanks to van der Waals interactions, favoring their close interactions over that of the BTAB units. Atomic force microscopy shows that, in contrast to other examples of molecules with the same core, disclike aggregates (rather than fibrillar one dimensional aggregates) are favored by the C 3 symmetric molecule. The closed structures are formed through nondirectional interlocking of porphyrin rings. The chiral spacer between the rigid core and the porphyrin moieties is undoubtedly important in determining the outcome in polar or less polar solvents, as modeling shows that this joint in the molecule has two favored conformations that render the molecule relatively flat or convex.

Supramolecular helical nanofibers formed by an achiral monopyrrolotetrathiafulvalene derivative: water-triggered gelation and chiral evolution

An achiral MPTTF-based gelator could form left- and right-handed supramolecular assemblies in pure DMF, whereas it turned into an opaque gel with right-handed nanofibers after adding small amounts of water.

Distinguishing between Mechanical and Electrostatic Interaction in Single Pass Multi Frequency Electrostatic Force Microscopy Measurements on a Molecular Material

Single-pass electrostatic force microscopy is postulated as one of the most advanced techniques in terms of spatial resolution and fastness in data acquisition for the study of electrostatic phenomena at the nanoscale. However, crosstalk anomalies, in which mechanical interactions combine with tip-sample electrostatic forces, are still a major issue to overcome, specifically in soft and biological samples. In this paper we propose a novel method based on bimodal-atomic force microscopy to distinguish mechanical crosstalk from electrostatic images. The method is based in the comparison of bimodal AFM images with electrostatic ones, where pure mechanical interaction can be discerned from a mixture of mechanical and electrostatic interactions. The proposed method is optimized and demonstrated using a supramolecular charge transfer material. Finally, the method is used as a tool to depict different crosstalk levels in tetrathiafulvalene-based (TTF) assemblies, discerning between electrical and mechanical interactions. This kind of observation is important for obtaining accurate descriptions of charge distribution in samples made from organic and molecular layers and materials.

Charge-Transfer Supra-Amphiphiles Built by Water-Soluble Tetrathiafulvalenes and Viologen-Containing Amphiphiles: Supramolecular Nanoassemblies with Modifiable Dimensions

In this study, multidimensional nanoassemblies with various morphologies such as nanosheets, nanorods, and nanofibers are developed via charge-transfer interaction and supra-amphiphile self-assembling in aqueous phase. The charge-transfer interactions between tetrathiafulvalene derivatives (TTFs) and methyl viologen derivatives (MVs) have been confirmed by the characteristic charger-transfer absorption. (1) H NMR and electrospray ionizsation mass spectrometry (ESI-MS) analyses also indicate supra-amphiphiles are formed by the combination of TTFs and MVs head group through charge-transfer interaction and Coulombic force. X-ray single crystal structural studies, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) reveal that both linkage pattern of TTFs in hydrophilic part and alkane chain structure in hydrophobic part have significant influence on nanoassemblies morphology and microstructure. Moreover, gold nanoparticles (AuNPs) are introduced in the above supramolecular nanoassemblies to construct a supra-amphiphile-driven organic-AuNPs assembly system. AuNPs could be assembled into 1D-3D structures by adding different amount of MVs.

Voltammetric studies on the inter-relationship between the redox chemistry of TTF, TTF+˙, TTF2+ and HTTF+ in acidic media

The chemistry of oxidized forms of TTF in acidified CH₃CN has been established using electrochemical and spectroscopic techniques.

Revisiting urea-based gelators: strong solvent- and casting-microstructure dependencies and organogel processing using an alumina template

Microstructures obtained from simple gelators prove to be strongly solvent- and casting-dependent. For the first time, a membrane template was used to process a xerogel-type material.

Self-assembly of a dendron-attached tetrathiafulvalene: gel formation and modulation in the presence of chloranil and metal ions

The self-assembly of a tetrathiafulvalene (TTF)-based molecular gelator with a dendron substituent into a gel is described. Scanning electron microscope and atomic force microscope studies show that the xerogels exhibit rope-like frameworks or interwoven nanofibers, depending on the polarity of solvent from which the gel is formed. Gels containing chloranil can also be successfully prepared. Interestingly, this two-component gel can be easily transformed into solution after the addition of either Sc³⁺ or Pb²⁺. Both absorption and electron spin resonance (ESR) spectroscopic investigations reveal that the TTF unit is oxidized into TTF+ by chloranil in the presence of either Sc³⁺ or Pb²⁺. Moreover, the gel phase can be restored by reduction of TTF+ into the neutral form using magnesium.

Self-assembled gelators for organic electronics

Nature excels at engineering materials by using the principles of chemical synthesis and molecular self-assembly with the help of noncovalent forces. Learning from these phenomena, scientists have been able to create a variety of self-assembled artificial materials of different size, shapes, and properties for wide ranging applications. An area of great interest in this regard is solvent-assisted gel formation with functional organic molecules, thus leading to one-dimensional fibers. Such fibers have improved electronic properties and are potential soft materials for organic electronic devices, particularly in bulk heterojunction solar cells. Described herein is how molecular self-assembly, which was originally proposed as a simple laboratory curiosity, has helped the evolution of a variety of soft functional materials useful for advanced electronic devices such as organic field-effect transistors and organic solar cells. Highlights on some of the recent developments are discussed.

Boosting electrical conductivity in a gel-derived material by nanostructuring with trace carbon nanotubes

An organogelator with two distinct π-functional units is able to incorporate carbon nanotubes into its mesh of fibres in the gel state. The morphology of the material derived from this nanocomposite after evaporation of the solvent is a complex mesh of fibres which is clearly different from the pure gelator. This feature indicates a role of the nanotubes in assisting the formation of a fibre structure in the gel thanks to their interaction with the pyrene units in the organogelator. The nanocomposite conducts electricity once the p-type gelator is doped with iodine vapour. The change in morphology caused by the carbon material increases the conductivity of the material compared with the purely organic conducting system. It is remarkable that this improvement in the physical property is caused by an extremely small proportion of the carbon material (only present at a ratio of 0.1% w/w). The practically unique properties of TTF unit allow measurements with both doped and undoped materials with conducting atomic force microscopy which have demonstrated that the carbon nanotubes are not directly responsible for the increased conductivity.

Organogelators based on TTF supramolecular assemblies: synthesis, characterization, and conductive property

A closely related family of organogelators 1-2 appended one or two electroactive tetrathiafulvalene (TTF) residues, has been designed and readily synthesized by Sonogashira reactions. These compounds can gelate a variety of organic solvents in view of multiple intermolecular interactions, and compounds 2 with two TTF subunits exhibit higher gelation ability than their corresponding 1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigation of the xerogels from toluene gave a visual image showing that fibrillar aggregates are entangled in three-dimensional network structures. The columnar TTF cores stacking in the fiber, evidenced by the mixed-valence states absorption at around 2000 nm in ultraviolet-visible-near-infrared (UV-vis-NIR) spectra, provide an efficient pathway for the electron conduction. Upon oxidized by iodine, these xerogels exhibit semiconductive behaviors with moderate levels of conductivity. Additionally, the electrical conductivity of doped-xerogels 2 is 1 order of magnitude higher than that of 1 under identical conditions.

A new ex-TTF-based organogelator: formation of organogels and tuning with fullerene

A new ex-TTF-based organogelator with L-glutamide-derived lipid unit (compound 1) was synthesized and characterized. The resulting transparent organogels can be obtained with several organic solvents such as toluene, DMSO, and ethanol. It is interesting that its gelation ability can be further enhanced by addition of C(60). The (1)H NMR and absorption spectral studies indicate that the intermolecular interaction between the ex-TTF unit in 1 and C(60) may be responsible for this phenomenon. Further studies imply that the assembly structures in the gel phases in the absence and presence of C(60) may be different. Additionally, organogels can also be prepared with compound 1 and PCBM, and the corresponding xerogel was used to modify ITO electrode. A stable and rapid photocurrent was generated after exposure of the modified ITO electrode to light. This may provide an alternative way for photocurrent generation with high energy conversion efficiency.