Supramolecular ferroelectrics.

Supramolecular chemistry uses non-covalent interactions to coax molecules into forming ordered assemblies. The construction of ordered materials with these reversible bonds has led to dramatic innovations in organic electronics, polymer science and biomaterials. Here, we review how supramolecular strategies can advance the burgeoning field of organic ferroelectricity. Ferroelectrics - materials with a spontaneous and electrically reversible polarization - are touted for use in non-volatile computer memories, sensors and optics. Historically, this physical phenomenon has been studied in inorganic materials, although some organic examples are known and strong interest exists to extend the search for ferroelectric molecular systems. Other undiscovered applications outside this regime could also emerge. We describe the key features necessary for molecular and supramolecular dipoles in organic ferroelectrics and their incorporation into ordered systems, such as porous frameworks and liquid crystals. The goal of this Review is to motivate the development of innovative supramolecular ferroelectrics that exceed the performance and usefulness of known systems.

Heteroleptic silver(I) complexes prepared from phenanthroline and bis-phosphine ligands.

The heteroleptic coordination scenario of silver(I) with various phenanthroline ligands (NN) and different bis-phosphine (PP) derivatives has been investigated. In addition to the X-ray crystal structural characterization of the resulting mixed ligand Ag(I) complexes, detailed NMR studies have been performed to disclose the behavior of the prepared silver(I) complexes in solution. The results obtained with silver(I) have been also systematically related to the one obtained for copper(I) with the same combination of PP and NN ligands. Starting from an equimolar mixture of AgBF4, bis[(2-diphenylphosphino)phenyl] ether (POP), and 1,10-phenanthroline (phen), the mononuclear complex [Ag(POP)(phen)](+) has been obtained as the tetrafluoroborate salt. By following the same experimental procedure starting from bis(diphenylphosphino)methane (dppm) or 1,3-bis(diphenylphosphino)propane (dppp) as the PP ligand, dinuclear complexes with two bridging PP ligands, i.e., [Ag2(NN)2(µ-dppm)2](2+) and [Ag2(NN)2(µ-dppp)2](2+) with NN = phen or Bphen (bathophenanthroline), have been isolated as the tetrafluoroborate salts. Surprisingly, by using an equimolar ratio of AgBF4, phen or Bphen, and 1,2-bis(diphenyl-phosphino)ethane (dppe), the corresponding monobridged diphosphine dinuclear complexes [Ag2(NN)2(µ-dppe)](2+) have been obtained as the tetrafluoroborate salts. These compounds have been also prepared in excellent yield by using a more appropriate 2:1:2 (phen:dppe:Ag) stoichiometry. These results prompted us to also perform the reactions with dppm and dppp using a 1:2:2 (PP:NN:Ag) stoichiometry. Under these conditions, [Ag2(NN)2(µ-dppm)](BF4)2 (NN = phen or Bphen) and [Ag2(NN)2(µ-dppp)](BF4)2 (NN = phen or Bphen) have been obtained upon crystallization. When compared to their copper(I) analogues, the complexation scenario becomes more complex with silver(I) as the system tolerates also coordinatively frustrated metal ligand assemblies, i.e., with a trigonal coordination geometry. Depending on the stoichiometry or on the nature of the PP partner, silver(I) shows an adaptive capability leading to various complexes with different coordination geometries and composition. However, as in the case of copper(I), their solution behavior is highly dependent on the relative thermodynamic stability of the various possible complexes. In most of the cases, a single Ag(I) complex is observed in solution and the NMR data are in a perfect agreement with their solid state structures. The dppp-containing complexes are the only notable exception; both [Ag2(NN)2(µ-dppp)2](BF4)2 and [Ag2(NN)2(µ-dppp)](BF4)2 are stable in the solid state but a dynamic mixture is observed as soon as these compounds are dissolved. Finally, whereas both dppe and dppp are chelating ligands for copper(I), it is not the case anymore with silver(I) for which a destabilization of species with chelating dppe and dppp ligands is clearly suggested by our results.

Heteroleptic copper(I) complexes prepared from phenanthroline and bis-phosphine ligands.

Preparation of [Cu(NN)(PP)](+) derivatives has been systematically investigated starting from two libraries of phenanthroline (NN) derivatives and bis-phosphine (PP) ligands, namely, (A) 1,10-phenanthroline (phen), neocuproine (2,9-dimethyl-1,10-phenanthroline, dmp), bathophenanthroline (4,7-diphenyl-1,10-phenanthroline, Bphen), 2,9-diphenethyl-1,10-phenanthroline (dpep), and 2,9-diphenyl-1,10-phenanthroline (dpp); (B) bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,2-bis(diphenylphosphino)benzene (dppb), 1,1'-bis(diphenylphosphino)ferrocene (dppFc), and bis[(2-diphenylphosphino)phenyl] ether (POP). Whatever the bis-phosphine ligand, stable heteroleptic [Cu(NN)(PP)](+) complexes are obtained from the 2,9-unsubstituted-1,10-phenanthroline ligands (phen and Bphen). By contrast, heteroleptic complexes obtained from dmp and dpep are stable in the solid state, but a dynamic ligand exchange reaction is systematically observed in solution, and the homoleptic/heteroleptic ratio is highly dependent on the bis-phosphine ligand. Detailed analysis revealed that the dynamic equilibrium resulting from ligand exchange reactions is mainly influenced by the relative thermodynamic stability of the different possible complexes. Finally, in the case of dpp, only homoleptic complexes were obtained whatever the bis-phosphine ligand. Obviously, steric effects resulting from the presence of the bulky phenyl rings on the dpp ligand destabilize the heteroleptic [Cu(NN)(PP)](+) complexes. In addition to the remarkable thermodynamic stability of [Cu(dpp)2]BF4, this negative steric effect drives the dynamic complexation scenario toward almost exclusive formation of homoleptic [Cu(NN)2](+) and [Cu(PP)2](+) complexes. This work provides the definitive rationalization of the stability of [Cu(NN)(PP)](+) complexes, marking the way for future developments in this field.

Fluorescent π-conjugated polymer dots versus self-assembled small-molecule nanoparticles: what's the difference?

Fluorescent nanoparticles based on π-conjugated small molecules and polymers are two different classes of π-conjugated systems that have attracted much interest. To date, both emerging classes have only been studied separately and showed no clear differences in their properties. Herein these nanoparticles are compared on the basis of a fluorene co-polymer and its corresponding small molecule. Both systems formed nanoparticles with the same diameter, whereas the fluorescence properties clearly differed. In case of the polymer the fluorescence diminished, whereas for the small molecules the fluorescence increased. In addition, the capability of encapsulation and release of a hydrophobic dye from the fluorescent nanoparticles was studied. For the polymer system, encapsulation was highly efficient and no release was observed, whereas for the small molecule system encapsulation was less efficient and release of the dye was observed. These studies show a clear difference between small molecules and polymers which has important implications for the design of fluorescent nanoparticles.

A stable and strongly luminescent dinuclear Cu(I) helical complex prepared from 2-diphenylphosphino-6-methylpyridine.

Treatment of 2-diphenylphosphino-6-methylpyridine (dpPyMe) with Cu(CH(3)CN)(4)BF(4) afforded the stable dinuclear Cu(I) complex [Cu(2)(µ-dpPyMe)(3)(CH(3)CN)](BF(4))(2). This compound is a weak emitter in solution, however a remarkably high emission quantum yield (46%) has been found in a rigid matrix at room temperature.

Side chains control dynamics and self-sorting in fluorescent organic nanoparticles.

To develop fluorescent organic nanoparticles with tailored properties for imaging and sensing, full control over the size, fluorescence, stability, dynamics, and supramolecular organization of these particles is crucial. We have designed, synthesized, and fully characterized 12 nonionic fluorene co-oligomers that formed self-assembled fluorescent nanoparticles in water. In these series of molecules, the ratio of hydrophilic ethylene glycol and hydrophobic alkyl side chains was systematically altered to investigate its role on the above-mentioned properties. The nanoparticles consisting of π-conjugated oligomers containing polar ethylene glycol side chains were less stable and larger in size, while nanoparticles self-assembled from oligomers containing nonpolar pendant chains were more stable, smaller, and generally had a higher fluorescence quantum yield. Furthermore, the dynamics of the molecules between the nanoparticles was enhanced if the number of hydrophilic side chains increased. Energy transfer studies between naphthalene and benzothiadiazole fluorene co-oligomers with the same side chains showed no exchange of molecules between the particles for the apolar molecules. For the more polar systems, the exchange of molecules between nanoparticles took place at room temperature or after annealing. Self-assembled nanoparticles consisting of π-conjugated oligomers having different side chains caused self-sorting, resulting either in the formation of domains within particles or the formation of separate nanoparticles. Our results show that we can control the stability, fluorescence, dynamics, and self-sorting properties of the nanoparticles by simply changing the nature of the side chains of the π-conjugated oligomers. These findings are not only important for the field of self-assembled nanoparticles but also for the construction of well-defined multicomponent supramolecular materials in general.

Morphology-dependent energy transfer dynamics in fluorene-based amphiphile nanoparticles.

Nanoparticles are interesting systems to study because of their large range of potential uses in biological imaging and sensing. We investigated molecular nanoparticles formed by fast injection of a small volume of molecularly dissolved fluorene-derivative amphiphilic molecules into a polar solvent, which resulted in solid spherical particles of ∼80 nm diameter with high stability. Energy transfer studies were carried out on two-component nanoparticles that contained mixtures of donor and acceptor amphiphiles of various fractions. We conducted time-resolved photoluminescence measurements on the two-component nanoparticles in order to determine whether the fundamental donor-acceptor interaction parameter (the Förster radius) depends on the acceptor concentration. The Förster radius was found to be large for very low incorporated acceptor fractions (<0.1%), but it declined with increasing concentration. These changes were concomitant with shifts in the acceptor emission and absorption circular dichroism spectra that indicated an increasing clustering of acceptors into domains as their fraction was raised. In addition, for acceptor fractions below 2% the extracted Förster radii were found to be significantly larger than predicted from donor-acceptor spectral overlap calculations, in accordance with efficient excitation diffusion within the donor matrix, aiding the overall transfer to acceptors. We conclude that energy transfer in two-component nanoparticles shows a complex interplay between phase segregation of the constituent donor and acceptor molecules and excitation diffusion within their domains.

Pre- and postfunctionalized self-assembled π-conjugated fluorescent organic nanoparticles for dual targeting.

There is currently a high demand for novel approaches to engineer fluorescent nanoparticles with precise surface properties suitable for various applications, including imaging and sensing. To this end, we report a facile and highly reproducible one-step method for generating functionalized fluorescent organic nanoparticles via self-assembly of prefunctionalized π-conjugated oligomers. The engineered design of the nonionic amphiphilic oligomers enables the introduction of different ligands at the extremities of inert ethylene glycol side chains without interfering with the self-assembly process. The intrinsic fluorescence of the nanoparticles permits the measurement of their surface properties and binding to dye-labeled target molecules via Förster resonance energy transfer (FRET). Co-assembly of differently functionalized oligomers is also demonstrated, which enables the tuning of ligand composition and density. Furthermore, nanoparticle prefunctionalization has been combined with subsequent postmodification of azide-bearing oligomers via click chemistry. This allows for expanding ligand diversity at two independent stages in the nanoparticle fabrication process. The practicability of the different methods entails greater control over surface functionality. Through labeling with different ligands, selective binding of proteins, bacteria, and functionalized beads to the nanoparticles has been achieved. This, in combination with the absence of unspecific adsorption, clearly demonstrates the broad potential of these nanoparticles for selective targeting and sequestration. Therefore, controlled bifunctionalization of fluorescent π-conjugated oligomer nanoparticles represents a novel approach with high applicability to multitargeted imaging and sensing in biology and medicine.

DNA-templated assembly of dyes and extended π-conjugated systems.

This feature article reports on the use of DNA as a template to assemble dyes and π-conjugated systems with the aim to construct functional multicomponent nanostructures that have a well-defined size, shape and sequence.

Fluorescent nanoparticles based on self-assembled pi-conjugated systems.

pi-Conjugated molecules are interesting components to prepare fluorescent nanoparticles. From the use of polymer chains that form small aggregates in water to the self-assembly of small chromophoric segments into highly ordered structures, the preparation of these materials allows to develop systems with applications as sensors or biolabels. The potential functionalization of the nanoparticles can lead to specific probing. This progress report describes the recent advances in the preparation of such emittive organic nanoparticles.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands.

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.

Modular construction and hierarchical gelation of organooxotin nanoclusters derived from simple building blocks.

Mixtures of an appropriate carboxylic acid and n-butylstannoic acid constitute modular gelation systems, in which the formation of a well-defined 'tin-drum' nanocluster subsequently underpins the hierarchical assembly of nanostructured fibres, which form self-supporting gel-phase networks in organic solvents.