About Supramolecular Assemblies of π-Conjugated Systems

Self-assembly of amphiphilic imidazolium-based hexa-peri-hexabenzo-coronenes into fibreous aggregates

Imidazolium-based amphiphilic hexa-peri-hexabenzocoronenes were synthesized and shown to undergo ordered columnar self-assembly in solid-state as well as in solution to yield defined nanofibers upon solution drop casting onto solid substrate.

Thermotropic Liquid Crystals Based on Extended 2,5-Disubstituted-1,3,4-Oxadiazoles: Structure–Property Relationships, Variable-Temperature Powder X-ray Diffraction, and Small-Angle X-ray Scattering Studies

A class of extended 2,5-disubstituted-1,3,4-oxadiazoles R1-C6H4-{OC2N2}-C6H4-R2 (R1 = R2 = C10H21O 1 a, p-C10H21O-C6H4-C[triple bond]C 3 a, p-CH3O-C6H4-C[triple bond]C 3 b; R1 = C10H21O, R2 = CH3O 1 b, (CH3)2N 1 c; F 1 d; R1 = C10H21O-C6H4-C[triple bond]C, R2 = C10H21O 2 a, CH3O 2 b, (CH3)2N 2 c, F 2 d) were prepared, and their liquid-crystalline properties were examined. In CH2Cl2 solution, these compounds displayed a room-temperature emission with lambda(max) at 340-471 nm and quantum yields of 0.73-0.97. Compounds 1 d, 2 a-2 d, and 3 a exhibited various thermotropic mesophases (monotropic, enantiotropic nematic/smectic), which were examined by polarized-light optical microscopy and differential scanning calorimetry. Structure determination by a direct-space approach using simulated annealing or parallel tempering of the powder X-ray diffraction data revealed distinctive crystal-packing arrangements for mesogenic molecules 2 b and 3 a, leading to different nematic mesophase behavior, with 2 b being monotropic and 3 a enantiotropic in the narrow temperature range of 200-210 degrees C. The structural transitions associated with these crystalline solids and their mesophases were studied by variable-temperature X-ray diffractometry. Nondestructive phase transitions (crystal-to-crystal, crystal-to-mesophase, mesophase-to-liquid) were observed in the diffractograms of 1 b, 1 d, 2 b, 2 d, and 3 a measured at 25-200 degrees C. Powder X-ray diffraction and small-angle X-ray scattering data revealed that the structure of the annealed solid residue 2 b reverted to its original crystal/molecular packing when the isotropic liquid was cooled to room temperature. Structure-property relationships within these mesomorphic solids are discussed in the context of their molecular structures and intermolecular interactions.

Hydrogen-Bonded Oligo(p-phenylenevinylene) Functionalized with Perylene Bisimide: Self-Assembly and Energy Transfer

We describe the synthesis, supramolecular ordering on surfaces and in solution, and photophysical characterization of OPV4UT-PERY, an oligo(p-phenylenevinylene) (OPV) with a covalently attached perylene bisimide moiety. In chloroform, the molecule forms dimers through quadruple hydrogen bonding of the ureido-s-triazine array. This is supported by scanning tunneling microscopy (STM) studies, which reveal dimer formation at the liquid (1,2,4-trichlorobenzene)/solid (graphite) interface. Moreover, contrast reversal in bias-dependent STM imaging provides information on the ordering and different electronic properties of the oligo(p-phenylenevinylene) and perylene bisimide moieties. In dodecane, the molecule self-assembles into H-type aggregates that are still soluble as a result of the hydrophobic shell formed by the dodecyloxy wedges. The donor-acceptor molecule is characterized by efficient energy transfer from the photoexcited OPV to the perylene bisimide. Mixed assemblies with analogous OPVs lacking the perylene bisimide unit have been prepared in dodecane solution and energy transfer to the incorporated perylene bisimides has been studied by fluorescence spectroscopy.

Inorganic-Cored Photoactive Assemblies: Synthesis, Structure, and Photochemical Investigations on Stannoxane-Supported Multifluorene Compounds

Organostannoxanes were used as inert supports for the preparation of multichromophore assemblies. The synthesis involves a single-step procedure and allows the preparation of compounds in which the number of chromophore units can be varied from one to six. Thus, the reactions of LCOOH (1-fluorenecarboxylic acid) or L'COOH (9-fluorenecarboxylic acid) with various organostannoxane precursors afforded the fluorenyl derivatives [Ph(3)SnO(2)CL] (1), [Ph(3)SnO(2)CL'] (2), [{nBu(3)SnO(2)CL''}(n)] (3), [{nBu(3)SnO(2)CL'}(n)] (4), [{tBu(2)Sn(OH)O(2)CL}(2)] (5), [{tBu(2)Sn(OH)O(2)CL'}(2)] (6), [{[nBu(2)SnO(2)CL](2)O}(2)] (7) [{[nBu(2)SnO(2)CL'](2)O}(2)] (8), [{nBuSn(O)O(2)CL}(6)] (9), and [{nBuSn(O)O(2)CL'}(6)] (10). Interestingly, the formation of 3 is accompanied by an unusual oxo-transfer reaction. The ligand L is oxidized at the 9-position. Compounds 1, 3, 5, 7, and 8 were characterized by X-ray crystallography. The solid-state structures of these compounds reveal rich supramolecular structures owing to multiple intermolecular interactions between the various supramolecular synthons present in these molecules. The optical behavior of 1-10 is primarily dictated by the fluorenyl periphery. These compounds display strong blue fluorescent emission in solution and blue-green fluorescent emission in the solid state. Fluorescence lifetimes of all of these compounds are on the nanosecond timescale, and this suggests that the emission originates from the singlet excited state to the ground state. Intermolecular interactions in the solid state lead to considerable broadening of the emission bands.

Anions and Polyanions of Oligoindenopyrenes: Modes of Electron Delocalization and Dimerization

A series of pyrene-based polycyclic aromatic compounds, indeno[cd]pyrene, diindeno[cd,fg]pyrene, diindeno[cd,jk]pyrene, tris-(tert-butylindeno[cd,fg,jk])pyrene, and tetrakis-(tert-butylindeno[cd,fg,jk,mn])pyrene, were reduced with alkali metals in [D8]tetrahydrofuran, and the resulting anions were studied by NMR spectroscopy. It was found that the diatropic character of the dianions obtained depends on the number of annulated indeno groups. When one such group is present, a paratropic dianion is obtained, which is similar to the dianion of the parent pyrene; the effect, however, is weak. When more indeno groups are annulated, the dianions become diatropic owing to the greater number of five-membered rings that can acquire aromatic character as a result of reduction. The 1H NMR chemical shifts of tetrakis-(tert-butylindeno[cd,fg,jk,mn])pyrene in the neutral state show an interesting dependence on concentration that reflects an association of the molecules in solution by pi stacking. This phenomenon was not observed for the reduced species. The trianion radicals of tris-(tert-butylindeno[cd,fg,jk])pyrene and tetrakis-(tert-butylindeno[cd,fg,jk,mn])pyrene undergo reductive dimerization and form bilayered hexaanions.

Aggregation-Mediated Optical Properties of pH-Responsive Anionic Conjugated Polyelectrolytes

Conjugated polyelectrolyte copolymers containing 2,1,3-benzothiadiazole- (BT) and oligo(ethylene oxide)-substituted fluorene and phenylene units have been designed and synthesized. The phenylene pendent groups also have carboxylic acid functionalities, which allow probing the effect of pH on optical properties. The BT content in the backbone can be regulated at the synthesis stage. Dynamic light scattering studies show that polymers aggregate in water at low pH. Increased interchain contacts give rise to a lowering of the photoluminescence (PL) efficiency via self-quenching when the BT units are absent and increased levels of FRET from the phenylene-fluorene segments to BT. Furthermore, the PL efficiency of BT increases in the aggregated structures. Examination of solvent effects indicates that the increased BT efficiencies are likely due to decreased contact with water. The changes in PL efficiencies are reversible, showing that the aggregates are dynamic and not kinetically constrained.

Segregated assemblies in bridged electron-rich and electron-poor pi-conjugated moieties

We report a general strategy for the spontaneous segregation of electron-rich and electron-poor pi-conjugated moieties using mutually phobic aliphatic fluorocarbon-hydrocarbon interactions.

Supra-aggregates of fiber-forming anisotropic molecules

In this paper, the self-organization of fiber-forming anisotropic molecules is inspected both theoretically and experimentally. In the first part, a theoretical model which extends the de Gennes theory of thin films to assemblies of strongly anisotropic molecules is reported. The model predicts that solid supported thin films made up of fiber-forming discotic molecules can grow with both tangential and radial arrangement of the fibers, respectively leading to the formation of compact and holed supra-aggregates. These last systems form according to the following picture. The tangential growth minimizes the number of unfavorable free ends but introduces elastic strain especially in the central region of the aggregate. To reduce the elastic strain, some molecules are displaced from the central region toward the periphery of the growing aggregate, producing a localized well. In the second part of the paper, we experimentally face the above issue by depositing a strongly anisotropic disk-shaped molecule (rhodamine 123) onto different solid substrates through a spin coating procedure. By employing scanning force microscopy (SFM), the formation of thermodynamically favored fiberlike supramolecules as well as of compact and holed submicron-sized supra-aggregates has been demonstrated. The observed phenomena have been found to depend on the interplay of different parameters such as molecular concentration, evaporation time, and substrate composition. As main features, both theory and experiments show that holed supra-aggregates are more stable beyond a critical aggregation size and that the formation of holes is favored at high supersaturation. The theory seems valuable in extending previous dewetting models developed for fluid films with isotropic interaction forces.

Synthesis, Multiphase Characterization, and Helicity Control in Chiral DACH-Linked Oligothiophenes

A new class of chiral oligothiophenes is described. Mono-, bi-, ter-, and quarterthiophenes have been linked to enantiopure trans-1,2-cyclohexanediamine (DACH) via diamino or diimino moieties. The stereochemistry of DACH, the type of linker, and oligothiophene size determine the conformational flexibility of these molecules and consequently their molecular and supramolecular helicity in solution and in the solid state. The case of diaminobis(bithiophene), which inverts helicity and shows chiral amplification in the transition from solution to film, is described in detail. Based on the combined use of circular dichroism in solution and in the solid state, single-crystal/thin-film X-ray diffraction, and polarized optical microscopy, a working mechanism has been proposed to explain this unexpected behavior.

Fluorogenic 1,3-Dipolar Cycloaddition within the Hydrophobic Core of a Shell Cross-Linked Nanoparticle

Using either nitroxide mediated polymerization (NMP) or reversible addition fragmentation transfer (RAFT) techniques, novel block copolymers that present terminal acetylenes, in the side chain of the styrenic block, were obtained with narrow polydispersities and targeted molecular weights. For the conversion of these acetylene-functionalized polymers to amphiphilic block copolymers, RAFT techniques were preferred. Mild protection/deprotection chemistries were employed which were compatible with the incorporation of the acetylene functionality in the hydrophobic segment. These acetylene-functionalized, Click-readied amphiphilic block copolymers were then self-assembled and cross-linked to afford shell cross-linked knedel-like (SCK) nanoparticles that contained acetylene groups in the core domain. The hydrodynamic diameters (D(h)) of the block copolymer micelles and nanoparticles were determined by dynamic light scattering (DLS), and the dimensions of the nanoparticles were characterized using tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The chemical availability of the Click functionality within the core domain of the SCKs was investigated using the copper(I)-catalyzed 1,3-dipolar fluorogenic cycloaddition with a non-fluorescent 3-azidocoumarin profluorophore to afford intensely fluorescent nanoparticles.

Self-Assembly of Semifluorinated Dendrons Attached to Electron-Donor Groups Mediates Their π-Stacking via a Helical Pyramidal Column

Semifluorinated first-generation self-assembling dendrons attached via a flexible spacer to electron-donor molecules induce pi-stacking of the donors in the center of a supramolecular helical pyramidal column. These helical pyramidal columns self-organize in various columnar liquid crystal phases that mediate self-processing of large single crystal liquid crystal domains of columns and self-repair their intracolumnar structural defects. In addition, all supramolecular columns exhibit a columnar phase at lower temperatures that maintains the helical pyramidal columnar supramolecular structure and displays higher intracolumnar order than that in the liquid crystals phases. The results described here demonstrate the universality of this concept, the power of the fluorous phase or the fluorophobic effect in self-assembly and the unexpected generality of pyramidal liquid crystals.

Probing the solvent-assisted nucleation pathway in chemical self-assembly

Hierarchical self-assembly offers a powerful strategy for producing molecular nanostructures. Although widely used, the mechanistic details of self-assembly processes are poorly understood. We spectroscopically monitored a nucleation process in the self-assembly of p-conjugated molecules into helical supramolecular fibrillar structures. The data support a nucleation-growth pathway that gives rise to a remarkably high degree of cooperativity. Furthermore, we characterize a helical transition in the nucleating species before growth. The self-assembly process depends strongly on solvent structure, suggesting that an organized shell of solvent molecules plays an explicit role in rigidifying the aggregates and guiding them toward further assembly into bundles and/or gels.

Photoproduction of Proton Gradients with π-Stacked Fluorophore Scaffolds in Lipid Bilayers

Rigid p-octiphenyl rods were used to create helical tetrameric pi-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels.

Structural Factors Controlling the Self-Assembly of Columnar Liquid Crystals

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett sigma-values. The observed trends can be rationalized in terms of the strength of pi-pi interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.

Linearly Polarized Emission of an Organic Semiconductor Nanobelt

Linearly polarized emission has been observed for the nanobelts fabricated from a perylene diimide molecule through both solution-based and surface-supported self-assembling. The measurement of polarized emission was performed over single nanobelts with use of a near-field scanning optical microscope (NSOM) adapted with emission polarization (by putting a planar polarizer before the detector). Rotating the emission polarizer (from 0 degrees to 180 degrees) changed the emission intensity in a way depending on the relative angle between the long axis of the belt and the polarizer with a minimum of intensity detected at ca. 78 degrees, which is indicative of the tilted stacking of molecules along the belt direction.

Photoresponsive Self-Assembly and Self-Organization of Hydrogen-Bonded Supramolecular Tapes

Self-assembling building blocks that are readily functionalizable and capable of achieving programmed hierarchical organization have enabled us to create various functional nanomaterials. We have previously demonstrated that N,N'-disubstituted 4,6-diaminopyrimidin-2(1 H)-one (DAP), a guanine-cytosine hybridized molecule, is a versatile building block for the creation of tapelike supramolecular polymer species in solution. In the current study, DAP was functionalized with azobenzene side chains. 1H NMR, UV/Vis, and dynamic light scattering studies confirmed the presence of nanometer-scale tapelike supramolecular polymers in alkane solvents at micromolar regimes. At higher concentrations (millimolar regimes), the supramolecular polymers hierarchically organized into lamellar superstructures to form organogels, as shown by X-ray diffraction and polarized optical microscopy. Remarkably, the azobenzene side chains are photoisomerizable even in the supramolecular polymers, owing to their loosely packed state supported by the rigid hydrogen-bonded scaffold, enabling us to establish photocontrollable supramolecular polymerization and higher order organization of the tapelike supramolecular polymers into lamellar superstructures.

Pyrene-Stacked Nanostructures Constructed in the Recombinant Tobacco Mosaic Virus Rod Scaffold

The effect of pyrenes introduced into a tobacco mosaic virus (TMV) coat protein monomer on the formation and stability of the TMV assembly was investigated. The possible arrangement of the pyrenes in the inner cavity of the TMV rod was also estimated. The pyrene derivative was introduced to four specific amino acids in the cavity of the TMV rod structure. Rod-structure formation was examined by atomic force microscopy (AFM). Two pyrene-attached mutants (positions 99 and 100) assembled to increase the length of the rod structures by 2.5 microm at pH 5.5. The interaction of the pyrene moieties in the TMV cavity was investigated by steady-state and time-resolved spectroscopic analysis. Strong excimer emission with significantly short wavelength (465 nm) was observed from the two mutants mentioned above. Excitation and UV-visible spectra indicate that the pyrene moieties form pi-stacked structures in the TMV cavity. Details of the pyrene interaction were investigated by analyzing the fluorescence lifetime of the excimer. Results suggest that the pyrenes formed preassociated rigid structures with partially overlapped geometry in the restricted space of the TMV cavity. The pyrenes effectively stabilize the TMV rod through a pi-stacking interaction in a well-ordered way, and the single pyrene moiety introduced into the monomer affects the overall formation of the TMV rod structure.

EXCITONS IN CONJUGATED OLIGOMER AGGREGATES, FILMS, AND CRYSTALS

Recent experimental and theoretical investigations of excitons in conjugated oligomer nanoaggregates, thin films, and crystals are reviewed. The review focuses on the technologically important unsubstituted oligo-phenylene vinylenes (OPVn) and oligo-thiophenes (OTn), which exhibit side-by-side herringbone crystal packing. Many of the salient photophysical properties displayed by OPVn and OTn solid phases, including the large Davydov splitting, the rich variety of peaks due to vibronic coupling in both absorption and emission, and the unusual behavior of the emission origin, are accounted for in a model including excitonic coupling between molecules, linear exciton-phonon coupling, and disorder.

Synthesis of Fluorene-Based Oligomeric Organoboron Reagents via Kumada, Heck, and Stille Cross-Coupling Reactions

Boronic pinacol ester group is not reactive in Kumada, Heck and Stille coupling reaction conditions. Fluorene-based sophisticated organoboron compounds were synthesized by means of Palladium catalyzed Kumada, Heck and Stille cross-coupling reactions from halofluorenyl boronic esters.

Self-assembly of poly(9,9'-dihexylfluorene) to form highly ordered isoporous films via blending

Highly ordered hexagonal arrays of isoporous films prepared from poly(9,9'-dihexylfluorene) and polystyrene-grafted silica nanoparticles (Si-graft-PS) are presented. These close-packed arrays were formed in areas of many square millimeters. The pore size varied from 3.6 to 8.5 microm, depending on the concentration of Si-graft-PS and the processing conditions. Solid-state photoluminescence resulted in a significant red shift of up to 30 nm in these films compared to that in conventional processing techniques. These differences are attributed to enhanced aggregation of the polymers caused by polymer-solvent interactions. These highly ordered polymer films may find use in microelectronic and biological and/or chemical sensor applications.

Chiral Amphiphilic Self-Assembled α,α‘-Linked Quinque-, Sexi-, and Septithiophenes: Synthesis, Stability and Odd−Even Effects

The synthesis, characterization, and self-assembly in butanol of a series of well-defined alpha,alpha'-linked quinqui-, sexi-, and septithiophenes substituted, via ester links at their termini, by chiral oligo(ethylene oxide) chains carrying an alpha, beta, delta, and epsilon methyl, respectively, are reported. Studies of the self-assembly of these molecules using UV/visible absorption, luminescence, and circular dichroism spectroscopies reveal, for the sexithiophene case, that the magnitude of the observed Cotton effect in the aggregates diminishes progressively as the chiral substituent is moved away from the thiophene segment. The stability of the assemblies increases with the length of the oligothiophene and as the substituent chiral unit is moved away from the aromatic core, being greatest for the unsubstituted case. The sign of the Cotton effect alternates in an "odd/even" manner as the position of the chiral substituent is moved along the oligo(ethylene oxide) chain and on going from the quinquethiophene to the septithiophene having the same side chain. Atomic force microscopy on materials deposited from solution on an aluminum or glass surface and optical measurements show that capsules are formed from the oligothiophenes with H-type packing of the aromatic segments.

Polyelectrolyte Templated Polymerization in Langmuir Films: Nanoscopic Control of Polymer Chain Organization

Polyelectrolytes introduced in the aqueous subphase are shown to have a profound impact on the kinetics of polymerization of N-octadecylaniline at the air/water interface. This can be attributed to changes effected in molecular organization and reorientation behavior in the Langmuir film. The polyelectrolyte templates lead to considerable modification of the morphology of the monomer and polymer Langmuir films. Polyelectrolyte complexation is found to be an elegant and efficient methodology to achieve enhanced alignment of the polyaniline chains in the transferred Langmuir-Blodgett (Langmuir-Schafer) film.