Self-Assembly of Tripodal Squaraines: Cation-Assisted Expression of Molecular Chirality and Change from Spherical to Helical Morphology

Helicity Control in the Aggregation of Achiral Squaraine Dyes in Solution and Thin Films

Squaraine dyes are well known for their strong absorption in the visible regime. Reports on chiral squaraine dyes are, however, scarce. To address this gap, we here report two novel chiral squaraine dyes and their achiral counterparts. The presented dyes are aggregated in solution and in thin films. A detailed chiroptical study shows that thin films formed by co-assembling the chiral dye with its achiral counterpart exhibit exceptional photophysical properties. The circular dichroism (CD) of the co-assembled structures reaches a maximum when just 25 % of the chiral dye are present in the mixture. The solid structures with the highest relative CD effect are achieved when the chiral dye is used solely as a director, rather than the structural component. The chiroptical data are further supported by selected spin-filtering measurements using mc-AFM. These findings provide a promising platform for investigating the relationship between the dissymmetry of a supramolecular structure and emerging material properties rather than a comparison between a chiral molecular structure and an achiral counterpart.

Thin-film-based nanoarchitectures for soft matter: controlled assemblies into two-dimensional worlds

Controlling the organization of molecular building blocks at the nanometer level is of utmost importance, not only from the viewpoint of scientific curiosity, but also for the development of next-generation organic devices with electrical, optical, chemical, or biological functions. Self-assembly offers great potential for the manufacture of nanoarchitectures (nanostructures and nanopatterns) over large areas by using low-energy and inexpensive spontaneous processes. However, self-assembled structures in 3D media, such as solutions or solids, are not easily incorporated into current device-oriented nanotechnology. The scope of this review is therefore to introduce the expanding methodology for the construction of thin-film-based nanoarchitectures on solid surfaces and to try to address a general concept with emphasis on the availability of dynamic interfaces for the creation and manipulation of nanoarchitectures. In this review, the strategies for the construction of nanostructures, the control and manipulation of nanopatterns, and the application of nanoarchitectures are described; the construction strategies are categorized into three classes: i) π-conjugated molecular assembly in two dimensions, ii) bio-directed molecular assembly on surfaces, and iii) recent thin-film preparation technologies.

Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

The self-assembly of triangular-shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X-ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ∼20° (λ=Cu(Kα)), the higher crystallinity of OPE 3, endowed with three linear decyl chains, results in a diffractrogram with a number of well-resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)-1 a and (R)-1 b-endowed with (S)- and (R)-3,7-dimethyloctyloxy chains-transfer their chirality to the supramolecular structures formed upon their self-assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)-2 decorated with (S)-2-methylnonyloxy chains which forms right-handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)-1 a that self-assembles into left-handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self-assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)-1 a and (R)-1 b self-assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)-2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.

Self-assembly of discrete homochiral, helical, hydrogen-bonded nanocages: from vesicles to microspheres and tubules capable of gelating solvents

The chiral tris-monodentate imidazolinyl ligands 1 a-c exhibit a strong tendency to form the discrete, helical [2+3] nanocages 3 ([1(2).2(3)]) with tartaric acids 2. Circular dichroism (CD) spectra and theoretical calculations reveal that supramolecular handedness of capsulelike architectures is determined only by the chirality of the imidazolinyl ligands rather than tartaric acids. The chirality of imidazolinyl ligands is transferred to the helicity of the complexes through the directed hydrogen bonds between the N3 atom of imidazoline rings and the carboxyl of tartaric acids. These hydrogen-bonded nanocages can spontaneously self-assemble into spherical vesicles, during which the hydrogen bonding that arises from the hydroxyl groups of tartaric acids plays a crucial issue. The vesicles formed by [{(S,S,S)-1 a}(2)(2(L))(3)] (3 a) may further evolve into microspheres that gelate organic solvents after being aged at -20 degrees C for 24 h, and can also be unprecedentedly transformed to tubular assemblies capable of rigidifying the solvents when subjected to ultrasound irradiation.

Dendronized triangular oligo(phenylene ethynylene) amphiphiles: nanofibrillar self-assembly and dye encapsulation

Triangular-shaped oligo(phenylene ethynylene) amphiphiles 1a and 1b decorated in their periphery with two- and four-branched hydrophilic triethyleneglycol dendron wedges, have been synthesized and their self-assembling properties in solution and onto surfaces investigated. The steric demand produced by the dendritic substituents induces a face-to-face rotated pi stacking of the aromatic moieties. Studies on the concentration and temperature dependence confirm this mechanism and provide binding constants of 1.2 x 10(5) and 1.7 x 10(5) M(-1) in acetonitrile for 1a and 1b, respectively. Dynamic and static light scattering measurements complement the study of the self-assembly in solution and demonstrate the formation of rod-like supramolecular structures in aqueous solution. The nanofibers formed in solution can be efficiently transferred onto surfaces. Thus, TEM images reveal the presence of strands of various thickness, with the most common being several micrometers long and with diameters of around 70 nm. Some of these nanofibers present folded edges that are indicative of their ribbon-like nature. Interestingly, compound 1b can also form thick filaments with a rope-like appearance, which points to a chiral arrangement of the fibers. AFM images under highly diluted conditions also reveal long fibers with height profiles that fit well with the molecular dimensions calculated for both amphiphiles. Finally, we have demonstrated the intercalation of the hydrophobic dye Disperse Orange 3 within the filaments and its subsequent release upon increasing the temperature.

Tunable gel formation by both sonication and thermal processing in a cholesterol-based self-assembly system

A family of asymmetric cholesterol-based fluorescent organogelators containing naphthalimide, connected by two acylamines and different alkyl-chain spacers, have been designed and prepared. These compounds can gelate a variety of organic solvents with both ultrasound stimuli and general sol-gel processes. The self-assembly and gelling properties of the compounds depend on the length of the alkyl chains and can be controlled by ultrasound stimuli and renewed by a thermodynamic process. The morphologies and surface wetabilities of the xerogels prepared from these gelators are strongly affected by environmental stimuli. The mechanism of the process was investigated by confocal laser scanning microscopy, transmission or scanning electron microscopy, wide-angle X-ray scattering analysis, and rheological experiments. The studies reveal that the cooperation and relative competition of multiple intermolecular interactions, influenced by the sonication or thermal stimulus, are the main contributors for the aggregation or nucleation processes; this results in the macrodifferences in morphology and surface properties. These results provide a deeper understanding of the intermediate transition state of the gel during use of an ultrasound stimulus.

Modulated morphology in the self-organization of a rectangular amphiphile

The rectangular oligo(phenylene ethynylene) amphiphile 1 has been synthesized to investigate its self-assembling features in solution and onto surfaces. Concentration-dependent and variable-temperature NMR experiments firstly demonstrate the influence of the solvent in the stabilization of the non-covalent forces involved in the association of 1, namely, pi-pi stacking interactions between the aromatic fragments and van der Waals, hydrogen-bonding and/or solvophobic forces between the triethyleneglycol chains. This subtle balance of non-covalent interactions also conditions the thermodynamics of the self-assembly process and concentration-dependent UV/Vis investigations show a linear correlation between the polarity of the solvent and the K(a) values (K(a) approximately 5.2 x 10(5) M(-1) for CH3CN/H2O mixtures and 4.4 x 10(4) M(-1) for benzene). Moreover, these UV/Vis studies prove the organization of this compound following the indefinite self-association model. Microscopy techniques reveal that the morphology and dimensionality of the assemblies formed from 1 can be finely modulated. Although polar solvents yield hollow vesicles or toroidal 3D objects, depending upon concentration, the utilization of non-polar benzene results in the formation of unimolecular wires that can grow to form networks upon increasing concentration. These findings support the direct relationship existing between the self-assembling features of this amphiphile in solution and onto surfaces.

Construction of helical J-aggregates self-assembled from a thymidylic acid appended anthracene dye and DNA as a template

The thymidylic acid appended anthracene dye 2,6-bis[5-(3'-thymidylic acid)pentyloxy]anthracene (1) was synthesized, and the self-assembly of 1 and the binary self-assembly of 1 with a complementary single-stranded 20-meric oligodeoxyadenylic acid (dA(20)) were performed in 0.1 x TE buffer solution (i.e., 1.0 x 10(-3) M Tris-HCl, 1.0 x 10(-4) M ethylenediaminetriacetic acid (EDTA)). The characteristic J-band, small Stokes shift (6 nm), Cotton effect, and helical nanofibers 5.1 nm in diameter are observed in UV/Vis, fluorescence, and circular dichroism (CD) spectroscopies and atomic force microscopy (AFM) measurements for the binary self-assembly of 1 and dA(20) in aqueous solution. These observations revealed that the helical J-aggregates, in which the short-axis transition dipoles of the anthracene moieties are aligned in a head-to-tail fashion, are formed from the binary self-assembly of 1 and dA(20). The UV/Vis absorption and CD band of the anthracene L(a) region were found to be strongly dependent on temperature and showed cooperative changes for the binary self-assembly of 1 and dA(20). The self-assembly of the single-component 1 produced right- and left-handed helical nanofibers with diameters ranging from 4.0 to 10 nm. In contrast, for the binary self-assembly, the UV/Vis and fluorescence spectra showed no J-band and the Stokes shift was at approximately 107 nm for the single-component 1 in aqueous solution. In addition, the binary self-assembly of 1 and noncomplementary single-stranded 20-meric oligothymidylic acid (dT(20)) showed a small J-band and the J-band disappeared at 50 degrees C upon heating. On the basis of these observations, we concluded that thymine-adenine base-pair formation induced supramolecular helical J-aggregates in the binary self-assembly of 1 and dA(20) in aqueous solutions.