Supramolecular organization of alpha,alpha'-disubstituted sexithiophenes

Bright Frenkel Excitons in Molecular Crystals: A Survey

We computed the optical properties of a large set of molecular crystals (∼2200 structures) composed of molecules whose lowest excited states are strongly coupled and generate wide excitonic bands. Such bands are classified in terms of their dimensionality (1-, 2-, and 3-dimensional), the position of the optically allowed state in relation with the excitonic density of states, and the presence of Davydov splitting. The survey confirms that one-dimensional aggregates are rare in molecular crystals highlighting the need to go beyond the simple low-dimensional models. Furthermore, this large set of data is used to search for technologically interesting and less common properties. For instance, we considered the largest excitonic bandwidth that is achievable within known molecular crystals and identified materials with strong super-radiant states. Finally, we explored the possibility that strong excitonic coupling can be used to generate emissive states in the near-infrared region in materials formed by molecules with bright visible absorption and we could identify the maximum allowable red shift in this material class. These insights with the associated searchable database provide practical guidelines for designing materials with interesting optical properties.

Chiroptical Properties in Thin Films of π-Conjugated Systems

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g_abs and g_lum) and on the application of ECD and CPL to derive structural information on aggregated states.

Hierarchy of Asymmetry in Chiral Supramolecular Polymers: Toward Functional, Helical Supramolecular Structures

The formation of helical structures through the supramolecular polymerization of a variety of self-assembling units is reviewed. These scaffolds are usually obtained by efficient transfer or amplification of chirality phenomena, in which the starting self-assembling molecules possess different elements of asymmetry, such as point or axial chirality. Relevant examples of helical supramolecular structures investigated under thermodynamic control are reviewed, and the helical outcome of remarkable examples of chiral entities obtained through kinetic control are also highlighted. Finally, selected examples of flexible macroscopic chirality and catalysis are described to illustrate the applicability of helical aggregates.

Organogel-Derived Covalent-Noncovalent Hybrid Polymers as Alkali Metal-Ion Scavengers for Partial Deionization of Water

We show that crown ethers (CEs) 1-5 congeal both polar and nonpolar solvents via their self-assembly through weak noncovalent interactions (NCIs) such as CH···O and CH···π interactions. Diisopropylidene-mannitol (6) is a known gelator that self-assembles through stronger OH···O H bonding. These two gelators together also congeal nonpolar solvents via their individual self-assembly. Gelator 6 self-assembles swiftly to fibers, which act as templates and attract CE to their surface through H bonding and thereby facilitate their self-assembly through weak NCI. Polymerization of styrene gels made from CE and 6, followed by the washing off of the sacrificial gelator 6, yields robust porous polystyrene-crown ether hybrid matrices (PCH), having pore-exposed CEs. These PCHs not only were efficient in sequestering alkali metal ions from aqueous solutions but also can be recycled. This novel use of organogels for making solid sorbents for metal-ion scavenging might be of great interest.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Chemical Events in Oligo(3-methoxythiophene) Coating Solutions and Their Effect on the Goldlike Coating Film Properties

Metal-free, metal-like lustrous films may find applications in a variety of fields, and a study of the factors affecting their stability is highly desirable. In particular, chemical events occurring in the coating solutions might affect the supramolecular organization of the films and therefore the metal-like luster. Herein, the chemical events occurring in acetonitrile and nitromethane coating solutions of oligo(3-methoxythiophene) and their effect on the optical properties of the films were investigated by X-ray diffraction, UV-vis absorption, and viscosity measurements. In acetonitrile, the oligomers underwent gradual dedoping with time, but only small changes in viscosity were observed. The solution was applied to a glass plate to yield a dark brown film, which turned into a goldlike lustrous film by rubbing. In nitromethane, the supramolecular structure of the oligomers changed with time from the nonaggregated state to π-dimers and then to π-stacks, and the viscosity increased. The properties of the goldlike films prepared from this solution were greatly affected by this chemical event. Remarkably, the π-dimer solution provided the film with the highest specular reflectance, yellowness, greenness, brightness, and crystallinity.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

In Situ Construction and Characterization of Chlorin-Based Supramolecular Aggregates in Tumor Cells

We demonstrate in situ construction and characterization of supramolecular aggregates from chlorin p6 (Cp6) molecules in tumor cells. Fully deprotonated Cp6 molecules in neutral condition were partially protonated inside the acidic lysosomes of cells and significantly increased the hydrophobicity of them that resulted in simultaneous formation of J-type aggregates. Importantly, the formation of J-aggregates was fully characterized in artificial tissues by UV-vis, circular dichroism (CD) and transmission electron microscope (TEM) techniques. Compared to the monomers, the J-aggregates exhibited 55-fold enhanced thermal conversion efficiency (η) at the optimal excitation wavelength (690 nm). The remarkably increased heat effect contributed to the stronger photoacoustic (PA) signals, leading to at least 2 orders of magnitude increase of the tumor-to-normal tissue ratio (T/N), which was defined as the PA signal ratio between tumor site and surrounding normal tissue. We envision that this proof-of-concept study will open a new way to develop tumor environment-induced self-assembly for variable biomedical applications.

Concentration Dependent Specific Rotations of Chiral Surfactants: Experimental and Computational Studies

Recent experimental studies have shown unexpected chiroptical response from some chiral surfactant molecules, where the specific rotations changed significantly as a function of concentration. To establish a theoretical understanding of this experimentally observed phenomena, a novel methodology for studying chiral surfactants via combined molecular dynamics (MD) and quantum mechanical (QM) calculations is presented. MD simulations on the +10 000 atom surfactant systems have been performed using MD and QM/molecular mechanics (MM) approaches. QM calculations performed on MD snapshots coupled with extensive analysis on lauryl ester of phenylalanine (LEP) surfactant system indicate that the experimentally observed variation of specific rotation with concentration may be due to the conformational differences of the surfactant monomers in the aggregates. Though traditional MM simulations did not show significant differences in the conformer populations, QM/MM simulations using the forces derived from the PM6 method did predict conformational differences between aggregated and nonaggregated LEP molecules, which is consistent with experimental data. Additionally the electrostatic environment of charged surfactants may also be important, since dramatic changes in the Boltzmann populations of surfactant monomers can be noted in the presence of an electric field generated by the chiral ionic aggregates.

Anti-cooperative supramolecular polymerization: a new K2-K model applied to the self-assembly of perylene bisimide dye proceeding via well-defined hydrogen-bonded dimers

A new perylene bisimide dye self-assembles in an anti-cooperative process predominently into even numbered aggregates via dimers which could be interpreted by a newly developed K₂–K model.

A perylene bisimide dye bearing amide functionalities at the imide positions derived from amino acid l-alanine and a dialkoxy-substituted benzyl amine self-assembles into tightly bound dimers by π–π-stacking and hydrogen bonding in chloroform. In less polar or unpolar solvents like toluene and methylcyclohexane, and in their mixtures, these dimers further self-assemble into extended oligomeric aggregates in an anti-cooperative process in which even numbered aggregates are highly favoured. The stepwise transition from dimers into oligomers can not be properly described by conventional K₂–K model, and thus a new K₂–K aggregation model has been developed, which interpretes the present anti-cooperative supramolecular polymerization more appropriately. The newly developed K₂–K model will be useful to describe self-assembly processes of a plethora of other π-conjugated molecules that are characterized by a favored dimer species.

In Situ Formation of Nanofibers from Purpurin18-Peptide Conjugates and the Assembly Induced Retention Effect in Tumor Sites

An assembly-induced retention effect for enhanced tumor photoacoustic (PA) imaging and therapeutics is described. A responsive small-molecule precursor is prepared that simultaneously self-assembles into nanofibers in tumor sites that exhibit an assembly-induced retention effect, which results in an improved PA imaging signal and enhanced therapeutic efficacy. This successful proof-of-concept study paves the way to develop novel supramolecular biomaterials for cancer diagnostics and therapeutics.

A theoretical investigation of one-dimensional lithium-bonded chain: enhanced first hyperpolarizability and little red-shift

We present a theoretical investigation of the electric properties of two kinds of one-dimensional lithium bonded chains: (NC-Li)n and (NC-CC-Li)n (n = 1-8). The resulting (NC-Li)n and (NC-CC-Li)n were found to exhibit enhanced first hyperpolarizabilities (β 0) with increasing n, and a slight change in the absorption maximum wavelength λmax at the crucial transition. Comparing with (NC-Li)n, (NC-CC-Li)n exhibited particularly drastically enhanced β 0 values due to clearly enhanced coupled oscillators and double-degenerated charge transitions. β 0 is known to be the microscopic origin of the second-order non-linear optical (NLO) property, and λmax is an approximate measure of the transparency achievable, thus both are important indices of high-performance NLO molecules. Therefore, our investigations into one-dimensional lithium bond chains will be beneficial to understanding the relationship between β 0 and λmax, thus aiding the design of one-dimensional NLO materials with excellent transparence-efficiency.

Synthesis and Self-Assembly of a Mikto-Arm Star Dual Drug Amphiphile Containing both Paclitaxel and Camptothecin

Self-assembly of anticancer therapeutics into discrete nanostructures provides an innovative way to develop a self-delivering nanomedicine with a high, quantitative drug loading. We report here the synthesis and assembly of a mikto-arm star dual drug amphiphile (DA) containing both a bulky paclitaxel (PTX) and a planar camptothecin (CPT). The two anti-cancer drugs of interest were stochastically conjugated to a β-sheet forming peptide (Sup35) and under physiologically-relevant conditions the dual DA could spontaneously associate into supramolecular filaments with a fixed 41% total drug loading (29% PTX and 12% CPT). Transmission electron microscopy imaging and circular dichroism spectroscopy studies reveal that the bulkiness of the PTX, as well as the π-π interaction preference between the CPT units, has a significant impact on the assembly kinetics, molecular level packing, and nanostructure morphology and stability. We found that the DA containing two PTX units assembled into non-filamentous micelle-like structures, in contrast to the filamentous structures formed by the hetero dual DA and the DA containing two CPTs. The hetero dual DA was found to effectively release the two anticancer agents, exhibiting superior cytotoxicity against PTX-resistant cervical cancer cells. The presented work offers a potential method to generate well-defined entwined filamentous nanostructures and provides the basis for a future combination therapy platform.

Hydrogen-bond-directed-linking solving transparence-efficiency tradeoff in nonlinear optical molecule

It is well known that settling transparency-efficiency tradeoff is important to design nonlinear optical (NLO) materials. In this work, we constructed one-dimensional polymeric cyanoacetylene (NCCCH)n by hydrogen-bond-directed-linking to understand this tradeoff from molecular level. Results show that the first hyperpolarizability of (NCCCH)n (n=2-8) gradually increased with the increase of n, and what is more important is that the red-shifts, associated with the increase of n, were very little. It is proposed that these polymeric structures possess double-degenerated charge transitions, which contribute to the hyperpolarizability in an additive fashion, and that the coupled oscillators are gradually improved, which lead to the increase of the first hyperpolarizability. Therefore, we propose the hydrogen-bond-directed-linking idea is helpful to develop the potential high-performance NLO materials.

α,ω-Dihexylsexithiophene self-assembled nanostructures on mica: atomic force microscopy study

Self-assembled nanostructures of α,ω-dihexylsexithiophene (DH6T) formed by spreading DH6T solutions onto freshly cleaved mica surface were studied by atomic force microscopy. The effects of solvent and concentration on the nanostructures of DH6T molecules were studied. Flat, well-ordered, and platelet-like domains were observed on mica surfaces after treatment with various polar solvent solutions of DH6T. These domains form a uniform film with height of 2.4 ± 0.2 nm, which is consistent with a 45° tilt in the molecular conformation of DH6T on mica surfaces. The formation mechanism of these multilayers is discussed in detail.

A quaterthiophene-based rotaxane: synthesis, spectroscopy, and self-assembly at surfaces

Threaded molecular wires are shown to feature tunable properties. A new rotaxane based on a quaterthiophene threaded through a single β-cyclodextrin exhibits delocalization of the aromatic system that is also extended onto the central phenyl rings of the m-terphenylene end-groups. The rotaxane can undergo self-assembly that is better than the analogous bithiophene derivative, due to the increased π-π interactions.

Multifunctional, biocompatible supramolecular hydrogelators consist only of nucleobase, amino acid, and glycoside

The integration of nucleobase, amino acid, and glycoside into a single molecule results in a novel class of supramolecular hydrogelators, which not only exhibit biocompatibility and biostability but also facilitate the entry of nucleic acids into cytosol and nuclei of cells. This work illustrates a simple way to generate an unprecedented molecular architecture from the basic biological building blocks for the development of sophisticated soft nanomaterials, including supramolecular hydrogels.

Green fluorescent organic nanoparticles by self-assembly induced enhanced emission of a naphthalene diimide bolaamphiphile

Naphthalene diimide (NDI) bolaamphiphilic molecules (1) self-assemble in water to form organic nanoparticles, which exhibit self-assembly induced preassociated excimer formation and hence an enhanced green fluorescence.

Rational construction of perylene bisimide columnar superstructures with a biased helical sense

Discotic supramolecular complexes bearing six perylene bisimide (PBI) chromophores were prepared by mixing monotopically triple-hydrogen-bonding melamines equipped with two PBI chromophores and two 3,7-dimethyloctyl chiral handles with tritopically triple-hydrogen-bonding cyanuric acid (CA). UV/Vis and fluorescence titration experiments demonstrated that the discotic complexes were formed in methylcyclohexane by the 3:1 complexation between the melamines and CA. TEM and AFM studies revealed that the complexes hierarchically organize into fibrous columnar assemblies, which eventually results in the formation of organogels. Circular dichroism (CD) and flash-photolysis time resolved microwave conductivity measurements revealed the presence of extended chiral stacks of PBI chromophores within the columns. The anisotropy factors of the columnar assemblies are remarkably high (g=1.5×10(-3)) when considering the presence of only one 3,7-dimethyloctyl chiral handle per perylene chromophore, suggesting that the columnar structures have a biased helical sense. The fact that the chiral centers are located inside the discotic complexes rather than at their peripheries might be unique structural property responsible for the rather strong optical activities for the assemblies of this chromophore. The effective transcription of the molecular chirality to the extended columnar assemblies through the formation of unique discotic complexes enables the expression of "majority-rules" chiral amplification effect, which is unprecedented for the supramolecular assemblies of PBIs.

Rigid dimers formed through strong interdigitated H-bonds yield compact 1D supramolecular helical polymers

Hierarchical self-assembly of small abiotic molecular modules interacting through noncovalent forces is increasingly being used to generate functional structures and materials for electronic, catalytic, and biomedical applications. The greatest control over the geometry in H-bond supramolecular architectures, especially in H-bonded supramolecular polymers, can be achieved by using conformationally rigid molecular modules undergoing self-assembly through strong H-bonds. Their binding strength depends on the multiplicity of the H-bonds, the nature of donor/acceptor pairs and their secondary attractive/repulsive interactions. Here a functionalized molecular module is described, which is capable of self-associating through self-complementary H-bonding patterns comprising four strong and two medium-strength H-bonds to form dimers. The self-association of these phenylpyrimidine-based dimers through directional H-bonding between two lateral pyridin-2(1H)-one units of neighboring molecules allows the formation of highly compact 1D supramolecular polymers by self-assembly on graphite. A concentration-dependent study by scanning tunneling microscopy at the solid-liquid interface, corroborated by dispersion-corrected density functional studies, reveals the controlled generation of either linear supramolecular 2D arrays, or long helical supramolecular polymers with a high shape persistence.