Self‐Assembly of Bis(merocyanine) Tweezers into Discrete Bimolecular π‐Stacks

Dipolar aggregation : Examples of a novel class of tweezer molecules have been constructed through the tethering of two dipolar merocyanine chromophores. The electrostatic‐interaction‐directed self‐assembly of these tweezers affords centrosymmetric bimolecular complexes with a unique aggregate geometry of four π‐stacked chromophores (see picture) with an alternating arrangement of their dipole moments and very high dimerization constants, even in the good solvating solvent chloroform.

     

Synthesis,Photophysics, and Self‐Assembly of Furan‐Embedded Heteroarenes

A series of pyrene/phenanthrene‐fused furan derivatives ( 1 – 8 ) were synthesized by a simple condensation reaction between pyrene‐4,5‐diketone/phenanthrenequinone and substituted phenol/naphthol in the presence of trifluoromethanesulfonic acid in 1,2‐dichlorobenzene heated at reflux. The formed compounds can emit strong blue light in organic solvents. Additionally, the self‐assembly behaviors of two of the compounds ( 3 and 5 ) were studied through re‐precipitation method and the resulting nanostructures were characterized by UV/Vis, fluorescence spectra, and field‐emission scanning electron microscopy (FESEM). The findings showed that the shape and size of compounds 3 and 5 could be tuned by the ratio of THF and hexadecyl trimethyl ammonium bromide (CTAB) solution in water.     

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 10³ and 10⁶ M ^?1 in chloroform at room temperature, respectively.     

Self‐Assembly and Aggregate‐Induced Enhanced Emission of Amphiphilic Fluorescence Dyes in Water and in the Solid State

1‐Cyano‐1,2‐bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self‐assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate‐induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.     

Non‐covalent Versus Covalent Control of Self‐Assembly and Chirality of Nile Red‐modified Nucleoside and DNA

A DNA‐based covalent versus a non‐covalent approach is demonstrated to control the optical, chirooptical and higher order structures of Nile red ( Nr ) aggregation. Dynamic light scattering and TEM studies revealed that in aqueous media Nr ‐modified 2′‐deoxyuridine aggregates through the co‐operative effect of various non‐covalent interactions including the hydrogen bonding ability of the nucleoside and sugar moieties and the π‐stacking tendency of the highly hydrophobic dye. This results in the formation of optically active nanovesicles. A left‐handed helically twisted H‐type packing of the dye is observed in the bilayer of the vesicle as evidenced from the optical and chirooptical studies. On the other hand, a left‐handed helically twisted J‐type packing in vesicles was obtained from a non‐polar solvent (toluene). Even though the primary stacking interaction of the dye aggregates transformed from H→J while going from aqueous to non‐polar media, the induced supramolecular chirality of the aggregates remained the same (left‐handed). Circular dichroism studies of DNA that contained several synthetically incorporated and covalently attached Nr ‐modified nucleosides revealed the formation of helically stacked H‐aggregates of Nr but—in comparison to the noncovalent aggregates—an inversed chirality (right‐handed). This self‐assembly propensity difference can, in principle, be applied to other hydrophobic dyes and chromophores and thus open a DNA‐based approach to modulate the primary stacking interactions and supramolecular chirality of dye aggregates.     

Coordination‐Driven Self‐Assembly of PEO‐Functionalized Perylene Bisimides: Supramolecular Diversity from a Limited Set of Molecular Building Blocks

A limited number of poly(ethylene oxide)‐substituted perylene bisimides, some of which are equipped with terpyridine ligands for transition‐metal coordination (see structure), combine different types of noncovalent interactions to yield optoelectronically active organic materials with different types of supramolecular morphologies.

     

Diketopyrrolopyrrole J‐Aggregates Formed by Self‐Organization with DNA

Bis(2‐thienyl)diketopyrrolopyrrole with two Zn^II‐cyclens (ZnCyc‐DPP) was designed and synthesized to evaluate the selective binding of Zn^II‐cyclen with thymine base in single‐strand DNA as a tool for the construction of a highly ordered multichromophore system on DNAs. Through UV/Vis titrations, gel filtration chromatography, and circular dichroism spectroscopy, ZnCyc‐DPP formed J‐type DPP aggregates with oligo‐dT_n DNAs. The DPP aggregates absorbed on a gold electrode exhibited good photocurrent responses. The present results show that binding Zn^II‐cyclen–chromophore conjugates and thymine bases together is a powerful tool for preparing DNA‐templated multichromophoric systems with specific functions.     

Construction of Helical J‐Aggregates Self‐Assembled from a Thymidylic Acid Appended Anthracene Dye and DNA as a Template

Driven round the twist by DNA : One‐dimensional helical J‐aggregates are formed by the self‐assembly of thymidylic acid appended anthracene dye (shown in red and yellow) in the presence of complementary single‐stranded oligoadenylic acid (shown in green and blue) in an aqueous solution.

     

Light‐Induced Self‐Assembly and Decay of J Aggregates of Thiamonomethinecyanine Dyes

Formation of J aggregates, that is, one‐dimensional supramolecular self‐organizations in which the transition moments of individual molecules are aligned parallel to the line joining their centers through a “head‐to‐tail” arrangement, normally proceed via electrostatic interactions between oppositely charged molecular groups; this is facilitated by an electrolyte medium. Here, we show that J aggregates of thiamonomethinecyanine dyes in a solution can be assembled from dye dimers by illuminating the solution with light of the appropriate wavelength to induce excitation of the dye dimers. The reverse process is also demonstrated by application of light of the correct wavelength to induce excitation of the J aggregates. Our results indicate that spontaneous J aggregation in the dark and formation of J aggregates through illumination proceed through different mechanisms; the former resulting in an increase in the number of the aggregates and the latter in an increase in the size of the aggregates.     

Control of H‐ and J‐Type π Stacking by Peripheral Alkyl Chains and Self‐Sorting Phenomena in Perylene Bisimide Homo‐ and Heteroaggregates

The synthesis, self‐assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functionalities and π–π stacking between the PBI units directs the formation of the self‐assembled structure in solution, which beyond a certain concentration results in gelation. Effects of different peripheral alkyl substituents on the self‐assembly were studied by solvent‐ and temperature‐dependent UV‐visible and circular dichroism (CD) spectroscopy. PBI derivatives containing linear alkyl side chains in the periphery formed H‐type π stacks and red gels, whereas by introducing branched alkyl chains the formation of J‐type π stacks and green gels could be achieved. Sterically demanding substituents, in particular, the 2‐ethylhexyl group completely suppressed the π stacking. Coaggregation studies with H‐ and J‐aggregating chromophores revealed the formation of solely H‐type π stacks containing both precursor molecules at a lower mole fraction of J‐aggregating chromophore. Beyond a critical composition of the two chromophores, mixed H‐aggregate and J‐aggregate were formed simultaneously, which points to a self‐sorting process. The versatility of the gelators is strongly dependent on the length and nature of the peripheral alkyl substituents. CD spectroscopic studies revealed a preferential helicity of the aggregates of PBI building blocks bearing chiral side chains. Even for achiral PBI derivatives, the utilization of chiral solvents such as (R)‐ or (S)‐limonene was effective in preferential population of one‐handed helical fibers. AFM studies revealed the formation of helical fibers from all the present PBI gelators, irrespective of the presence of chiral or achiral side chains. Furthermore, vortex flow was found to be effective in macroscopic orientation of the aggregates as evidenced from the origin of CD signals from aggregates of achiral PBI molecules.     

Circularly Polarized Luminescence in Supramolecular Assemblies of Chiral Bichromophoric Perylene Bisimides

Chiral bichromophoric perylene bisimides are demonstrated as active materials of circularly polarized emission. The bichromophoric system exhibited circularly polarized luminescence with dissymmetry factors typical of that of similar organic chiral chromophoric systems in the monomeric state. Variation in solvent composition led to the formation of stably soluble helical aggregates through intermolecular interactions. A large enhancement in the dissymmetry of circularly polarized luminescence was exhibited by the aggregated structures both in the solution and solid states. The sum of excitonic couplings between the individual chromophoric units in the self‐assembled state results in relatively large dissymmetry in the circularly polarized luminescence, thereby giving rise to enhanced dissymmetry factors for the aggregated structures. The spacer between chiral center and chromophoric units played a crucial role in the effective enhancement of chiroptical properties in the self‐assembled structures. These materials might provide opportunities for the design of a new class of functional bichromophoric organic nanoarchitectures that can find potential applications in the field of chiroptical memory and light‐emitting devices based on supramolecular electronics.