Gelation-Assisted Control over Excitonic Interaction in Merocyanine Supramolecular Assemblies

Supramolecular Polymer Polymorphism: Spontaneous Helix-Helicoid Transition through Dislocation of Hydrogen-Bonded π-Rosettes

Polymorphism, a phenomenon whereby disparate self-assembled products can be formed from identical molecules, has incited interest in the field of supramolecular polymers. Conventionally, the monomers that constitute supramolecular polymers are engineered to facilitate one-dimensional aggregation and, consequently, their polymorphism surfaces primarily when the states of assembly differ significantly. This engenders polymorphs of divergent dimensionalities such as one- and two-dimensional aggregates. Notwithstanding, realizing supramolecular polymer polymorphism, wherein polymorphs maintain one-dimensional aggregation, persists as a daunting challenge. In this work, we expound upon the manifestation of two supramolecular polymer polymorphs formed from a large discotic supramolecular monomer (rosette), which consists of six hydrogen-bonded molecules with an extended π-conjugated core. These polymorphs are generated in mixtures of chloroform and methylcyclohexane, attributable to distinctly different disc stacking arrangements. The face-to-face (minimal displacement) and offset (large displacement) stacking arrangements can be predicated on their distinctive photophysical properties. The face-to-face stacking results in a twisted helix structure. Conversely, the offset stacking induces inherent curvature in the supramolecular fiber, thereby culminating in a hollow helical coil (helicoid). While both polymorphs exhibit bistability in nonpolar solvent compositions, the face-to-face stacking attains stability purely in a kinetic sense within a polar solvent composition and undergoes conversion into offset stacking through a dislocation of stacked rosettes. This occurs without the dissociation and nucleation of monomers, leading to unprecedented helicoidal folding of supramolecular polymers. Our findings augment our understanding of supramolecular polymer polymorphism, but they also highlight a distinctive method for achieving helicoidal folding in supramolecular polymers.

Crystallization-Driven Controlled Two-Dimensional (2D) Assemblies from Chromophore-Appended Poly(L-lactide)s: Highly Efficient Energy Transfer on a 2D Surface

A rational approach towards precision two-dimensional (2D) assemblies by crystallization-driven self-assembly (CDSA) of poly(L-lactides) (PLLAs), end-capped with dipolar dyes like merocyanine (MC) or naphthalene monoimide (NMI) and hydrophobic pyrene (PY) or benzene (Bn) is described. PLLA chains crystallize into diamond-shaped platelets in isopropanol, which forces the terminal dyes to assemble into a 2D array on the platelet surface by either dipolar interactions or π-stacking and exhibit tunable emission. Dipolar dyes play a critical role in imparting colloidal stability and structural uniformity to the 2D crystals, which is partly compromised for hydrophobic ones. Co-crystallization between NMI- and PY-labeled PLLAs yields similar diamond-shaped co-platelets with highly efficient (≈80 %) Förster Resonance Energy Transfer on the 2D surface. Further, the "living" CDSA method confers enlarged, segmented block co-platelets using one of the homopolymers as "seed" and the other as "unimer".

Photo-modulation of supramolecular polymorphism in the self-assembly of a scissor-shaped azobenzene dyad into nanotoroids and fibers

Recent advances in the research field of supramolecularly engineered dye aggregates have enabled the design of simple one-dimensional stacks such as fibers and of closed structures such as nanotoroids (nanorings). More complex and advanced supramolecular systems could potentially be designed using a molecule that is able to provide either of these distinct nanostructures under different conditions. In this study, we introduced bulky but strongly aggregating cholesterol units to a scissor-shaped azobenzene dyad framework, which affords either nanotoroids, nanotubes, or 1D fibers, depending on the substituents. This new dyad with two trans-azobenzene arms shows supramolecular polymorphism in its temperature-controlled self-assembly, leading to not only oligomeric nanotoroids as kinetic products, but also to one-dimensional fibers as thermodynamic products. This supramolecular polymorphism can also be achieved via photo-triggered self-assembly, i.e., irradiation of a monomeric solution of the dyad with two cis-azobenzene arms using strong visible light leads to the preferential formation of nanotoroids, whereas irradiation with weak visible light leads to the predominant formation of 1D fibers. This is the first example of a successful light-induced modulation of supramolecular polymorphism to produce distinctly nanostructured aggregates under isothermal conditions.

Fluorescent supramolecular polymers of barbiturate dyes with thiophene-cored twisted π-systems

Because supramolecular polymerization of emissive π-conjugated molecules depends strongly on π-π stacking interaction, the formation of well-defined one-dimensional nanostructures often results in a decrease or only a small increase of emission efficiency. This is also true for our barbiturate-based supramolecular polymers wherein hydrogen-bonded rosettes of barbiturates stack quasi-one-dimensionally through π-π stacking interaction. Herein we report supramolecular polymerization-induced emission of two regioisomeric 2,3-diphenylthiophene derivatives functionalized with barbituric acid and tri(dodecyloxy)benzyl wedge units. In CHCl3, both compounds are molecularly dissolved and accordingly poorly emissive due to a torsion-induced non-radiative decay. In methylcyclohexane-rich conditions, these barbiturates self-assemble to form crystalline nanofibers and exhibit strongly enhanced emission through supramolecular polymerization driven by hydrogen-bonding. Our structural analysis suggests that the barbiturates form a tape-like hydrogen-bonding motif, which is rationalized by considering that the twisted geometries of 2,3-diphenylthiophene cores prevend the competing rosettes from stacking into columnar supramolecular polymers. We also found that a small difference in the molecular polarity originating from the substitutional position of the thiophene core influences interchain association of the supramolecular polymers, affording different luminescent soft materials, gel and nanosheet.

Folding-induced Fluorescence Enhancement in a Series of Merocyanine Hetero-Folda-Trimers

Many dyes suffer from fast non-radiative decay pathways, thereby showing only short-lived excited states and weak photoluminescence. Here we show a pronounced fluorescence enhancement for a weakly fluorescent merocyanine (MC) dye by being co-facially stacked to other dyes in hetero-folda-trimer architectures. By means of fluorescence spectroscopy (lifetime, quantum yield) the fluorescence enhancement was explained by the rigidification of the emitting chromophore in the defined foldamer architecture and the presence of a non-forbidden lowest exciton state in H-coupled hetero-aggregates. This folding-induced fluorescence enhancement (FIFE) for specific sequences of π-stacked dyes points at a viable strategy toward improved fluorophores that relates to the approach used by nature in the green fluorescent protein (GFP).

Solvatochromic Fluorescence of a GFP Chromophore-Containing Organogelator in Solutions and Organogels

Solvatofluorochromism, a solvation effect on the fluorescence color of an organic dye, is a property generally limited to fluid solutions. We demonstrate herein the concept of solid-state solvatofluorochromism by using an organogelator (1-SG), which consists of a solvatofluorochromic green fluorescence protein (GFP) chromophore (1) and a sugar gelator (SG). While 1-SG could be located in the liquid phase or in the fibrous solid matrix of the SG gel, our results show that the one in the solid matrix but near the liquid interface has superior fluorescence stability and quantum efficiency as well as solvatofluorochromicity than the one in the liquid phase. In addition, the phenomenon of fluorescence turn-on occurs when the gel is formed in protic solvents. These features have been applied to perform multicolor fluorescence patterning, chemical vapor sensing, data encryption and decryption, and real-time fluorescence cell imaging.

Aggregation-Induced Emission-Active Gels: Fabrications, Functions, and Applications

Chromophores that exhibit aggregation-induced emission (i.e., aggregation-induced emission luminogens [AIEgens]) emit intense fluorescence in their aggregated states, but show negligible emission as discrete molecular species in solution due to the changes in restriction and freedom of intramolecular motions. As solvent-swollen quasi-solids with both a compact phase and a free space, gels enable manipulation of intramolecular motions. Thus, AIE-active gels have attracted significant interest owing to their various distinctive properties and promising application potential. Herein, a comprehensive overview of AIE-active gels is provided. The fabrication strategies employed are detailed, and the applications of AIEgens are summarized. In addition, the gel functions arising from the AIE moieties are revealed, along with their structure-property relationships. Furthermore, the applications of AIE-active gels in diverse areas are illustrated. Finally, ongoing challenges and potential means to address them are discussed, along with future perspectives on AIE-active gels, with the overall aim of inspiring research on novel materials and ideas.

K. AK, Sagar S, Das S, Varghese R. Blue-emissive two-component supergelator with aggregation-induced enhanced emission

Two-component organogels offer several advantages over one-component gels, but their design is highly challenging. Hence, it is extremely important to design new approaches for the crafting of two-component organogels with interesting optical and mechanical properties. Herein, we report the design of a new class of two-component supergelators obtained from the assembly between acid functionalized tetraphenylethylene (TPE)-based dendrons and alkylated melamine. No gelation behaviour is observed for the individual components, but interestingly, remarkable gelation behaviour is observed for their hydrogen-bonded complex. The primary driving force responsible for the gelation is the strong π–π stacking interaction of TPE units. Because of the strong π-stacking of TPEs in the gel state, the C(sp²)–C(sp²) bond rotation of the TPE segment is completely arrested in the gel state, which results in intense fluorescence emission of the gels. Furthermore, excellent elastic response is observed for the gels as evident from their high storage modulus compared to loss modulus values. Our results clearly demonstrate that by the appropriate selection of the molecular components, this approach can be applied for the creation of functional nanomaterials with emergent properties absent in the individual blocks.

Design of a novel class of two-component, highly emissive, low molecular weight supergelator is reported.

Fast Response Organic Supramolecular Transistors Utilizing In-Situ π-Ion Gels

Despite their remarkable charge carrier mobility when forming well-ordered fibers, supramolecular transistors often suffer from poor processability that hinders device integration, resulting in disappointing transconductance and output currents. Here, a new class of supramolecular transistors, π-ion gel transistors (PIGTs), is presented. An in situ π-ion gel, which is an unprecedented composite of semiconducting nanofibers and an enclosed ionic liquid, is directly employed as an active material and internal capacitor. In comparison to other supramolecular transistors, a PIGT displays a high transconductance (133 µS) and output current (139 µA at -6 V), while retaining a high charge-carrier mobility (4.2 × 10-2 cm2 V-1 s-1 ) and on/off ratio (3.7 × 104 ). Importantly, the unique device configuration and the high ionic conductivity associated with the distinct nanosegregation enables the fastest response among accumulation-mode electrochemical-based transistors (<20 µs). Considering the advantages of the absence of dielectric layers and the facile fabrication process, PIGT has great potential to be utilized in printed flexible devices. The device platform is widely applicable to various supramolecular assemblies, shedding light on the interdisciplinary research of supramolecular chemistry and organic electronics.

Light-Emitting Electrochemical Cells Based on Conjugated Ion Gels

π-conjugated gels are potentially useful for organic electronic applications. We present a π-conjugated ion gel, composed of substituted poly(para-phenyleneethynylene) (PPE) and an ionic liquid. This combination is well suited as an active material in a light-emitting electrochemical cells (LECs). The nanosegregated structure of the gels achieves a large interface between the polymer and ionic liquid (IL) and allows-by nature of its structure-facile ion conduction and continuous electrical conduction paths. Efficient doping significantly improves the response time. This concept should be applicable to other π-conjugated gels, and it allows the construction of gel-LECs.

Slow Anion-Exchange Reaction of Cesium Lead Halide Perovskite Nanocrystals in Supramolecular Gel Networks

Cesium lead halide perovskite nanocrystals are widely studied as among the most attractive emissive nanomaterials because of their high photoluminescence quantum yield and tunable emission wavelengths over the whole visible-light region by the halide ion-exchange reaction. However, the reactions were often observed in solution and generally very fast, which interferes with the fine-tuning capability of the emission properties. Here, we report a novel nanocrystal-organogel hybrid soft material in which the perovskite nanocrystals in a supramolecular gel exhibit extremely slow and inhomogeneous anion-exchange reactions that are different from those in solution. Furthermore, the inhomogeneous emission in the gel became homogeneous over several days due to a slow diffusion.

Supramolecularly cross-linked amphiphilic block copolymer assembly by the dipolar interaction of a merocyanine dye

Dipolar interaction driven dimerization of a merocyanine (MC) dye has been exploited to achieve non-covalently crosslinked stable micelles in water and reverse micelles in toluene with emissive properties from a MC-pendant amphiphilic block copolymer.

Supramolecularly Cross-Linked Nanogel by Merocyanine Pendent Copolymer

Directional dipole-dipole interaction mediated antiparallel dimerization of merocyanine dye (MD) has been explored for maneuvering supramolecular assembly of MD-conjugated flexible macromolecules leading to a cross-linked nanogel. The MD-functionalized copolymer was synthesized by a newly developed organocatalytic transesterification strategy for postpolymerization functionalization of poly(pentafluorophenyl acrylate) (polyPFPA)-based reactive copolymer. Presence of ∼35% pendant MD attached to a coil-like polymer chain leads to spontaneous formation of highly emitting cross-linked nanogel with efficient container property and appreciable stability in toluene owing to strong dimerization propensity among the MD. Considering the significance of MD in the context of nonlinear optics and photovoltaics, these results not only enrich the toolbox for engineering macromolecular assembly, but also open up new possibilities for future organic materials.

Tuning soft nanostructures in self-assembled supramolecular gels: from morphology control to morphology-dependent functions

Supramolecular gels are one kind of important soft material, in which small low-molecular weight compounds self-assemble into various nanostructures through non-covalent interactions to immobilize the solvents. While there are many important fundamental issues related to the gelation process, such as the design of the gelator, synergism of various non-covalent interactions between gelators, gelator-solvents, the balances between gelation and crystallization and so on, the self-assembled nanostructures forming during gelation are very interesting. These nanostructures have many unique features, such as the flexibility to respond to external stimuli, morphological diversity, ease of fabrication in large quantities, and so on. This review highlights some important features in tuning the nanostructures in the supramolecular gels from their morphological diversity, morphology control, morphology conversion, and morphology-depended functions.

Cooperative supramolecular polymerization: comparison of different models applied on the self-assembly of bis(merocyanine) dyes

Three new molecular building blocks 1 a-c for supramolecular polymerization are described that feature two dipolar merocyanine dyes tethered by p-xylylene spacers. Concentration- and temperature-dependent UV/Vis spectroscopy in chloroform combined with dynamic light scattering, capillary viscosimetry and atomic force microscopy investigations were applied to elucidate the mechanistic features of the self-assembly of these strongly dipolar dyes. Our detailed studies reveal that the self-assembly is very pronounced for bis(merocyanines) 1 a,b bearing linear alkyl chains, but completely absent for bis(merocyanine) 1 c bearing sterically more bulky ethylhexyl substituents. Both temperature- and concentration-dependent UV/Vis data provide unambiguous evidence for a cooperative self-assembly process for bis(merocyanines) 1 a,b, which was analyzed in detail by the Meijer-Schenning-Van-der-Schoot model (applicable to temperature-dependent data) and by the Goldstein-Stryer model (applicable to concentration-dependent data). By combining both methods all parameters of interest to understand the self-assembly process could be derived, including in particular the nucleus size (8-10 monomeric units), the cooperativity factor (ca. 0.006), and the nucleation and elongation constants of about 10(3) and 10(6)  M(-1) in chloroform at room temperature, respectively.

Hydrogen-bonding induced alternate stacking of donor (D) and acceptor (A) chromophores and their supramolecular switching to segregated states

This paper reports comprehensive studies on the mixed assembly of bis-(trialkoxybenzamide)-functionalized dialkoxynaphthalene (DAN) donors and naphthalene-diimide (NDI) acceptors due the cooperative effects of hydrogen bonding, charge-transfer (CT) interactions, and solvophobic effects. A series of DAN as well as NDI building blocks have been examined (wherein the relative distance between the two amide groups in a particular chromophore is the variable structural parameter) to understand the structure-dependent variation in mode of supramolecular assembly and morphology (organogel, reverse vesicle, etc.) of the self-assembled material. Interestingly, it was observed that when the amide functionalities are introduced to enhance the self-assembly propensity, the mode of co-assembly among the DAN and NDI chromophores no longer remained trivial and was dictated by a relatively stronger hydrogen-bonding interaction instead of a weak CT interaction. Consequently, in a highly non-polar solvent like methylcyclohexane (MCH), although kinetically controlled CT-gelation was initially noticed, within a few hours the system sacrificed the CT-interaction and switched over to the more stable self-sorted gel to maximize the gain in enthalpy from the hydrogen-bonding interaction. In contrast, in a relatively less non-polar solvent such as tetrachloroethylene (TCE), in which the strength of hydrogen bonding is inherently weak, the contribution of the CT interaction also had to be accounted for along with hydrogen bonding leading to a stable CT-state in the gel or solution phase. The stability and morphology of the CT complex and rate of supramolecular switching (from CT to segregated state) were found to be greatly influenced by subtle structural variation of the building blocks, solvent polarity, and the DAN/NDI ratio. For example, in a given D-A pair, by introducing just one methylene unit in the spacer segment of either of the building blocks a complete change in the mode of co-assembly (CT state or segregated state) and the morphology (1D fiber to 2D reverse vesicle) was observed. The role of solvent polarity, structural variation, and D/A ratio on the nature of co-assembly, morphology, and the unprecedented supramolecular-switching phenomenon have been studied by detail spectroscopic and microscopic experiments in a gel as well as in the solution state and are well supported by DFT calculations.

Reversible phase transitions within self-assembled fibrillar networks of (R)-18-(n-alkylamino)octadecan-7-ols in their carbon tetrachloride gels

The CCl(4) gel phases of a series of low-molecular-mass organogelators, (R)-18-(n-alkylamino)octadecan-7-ols (HSN-n, where n = 0-5, 18 is the alkyl chain length), appear to be unprecedented in that the fibrillar networks of some of the homologues undergo thermally reversible, gel-to-gel phase transitions, and some of those transitions are evident as opaque-transparent changes in the appearance of the samples. The gels have been examined at different concentrations and temperatures by a wide variety of spectroscopic, diffraction, thermal, and rheological techniques. Analyses of those data and data from the neat gelators have led to an understanding of the source of the gel-to-gel transitions. IR and SANS data implicate the expulsion (on heating the lower-temperature gel) or the inclusion (on cooling the higher-temperature gel) of molecules of CCl(4) that are interspersed between fibers in bundles. However, the root cause of the transitions is a consequence of changes in the molecular packing of the HSN-n within the fibers. This study offers opportunities to design new gelators that are capable of behaving in multiple fashions without entering the sol/solution phase, and it identifies a heretofore unknown transformation of organogels.

Self-assembly and semiconductivity of an oligothiophene supergelator

A bis(trialkoxybenzamide)-functionalized quaterthiophene derivative was synthesized and its self-assembly properties in solution were studied. In non-polar solvents such as cyclohexane, this quaterthiophene π-system formed fibril aggregates with an H-type molecular arrangement due to synergistic effect of hydrogen bonding and π-stacking. The self-assembled fibres were found to gelate numerous organic solvents of diverse polarity. The charge transport ability of such elongated fibres of quaterthiophene π-system was explored by the pulse radiolysis time resolved microwave conductivity (PR-TRMC) technique and moderate mobility values were obtained. Furthermore, initial AFM and UV-vis spectroscopic studies of a mixture of our electron-rich quaterthiophene derivative with the electron acceptor [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) revealed a nanoscale segregated assembly of the individual building blocks in the blend.

Facile preparation of superhydrophobic biomimetic surface based on octadecyltrichlorosilane and silica nanoparticles

The present paper describes the simple and low-cost process for the production of a superhydrophobic surface with micronano hierarchical structure from the chemisorptions of SiO(2) nanoparticles onto polymerized n-octadecylsilane. The process was carried out under ambient conditions without the use of expensive equipment. The as-prepared micronano-binary films exhibited a very high contact angle of 179.9 degrees and a low contact hysteresis of 2.5 degrees . On the basis of the results of the characterization techniques, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and contact angle (CA) measurements, a formation mechnism of the superhydrophobic micronano structure was proposed. Drop impact experiments on the modified-glass substrate showed that the as-prepared films possess a high-impalement threshold.