Publications

A set of conjugated oligo- and polytluorene-tethered fac-Ir(ppy)(3) complexes were synthesized, In addition to steady-state absorption and emission, time-resolved emission spectroscopy was used to systematically study the correlation of photophysical properties with chemical structures. A chain length dependency study showed that both radiative and nonradiative triplet decay rates, as well as the phosphorescence quantum yield, decreased with increasing chain length of the appended oligofluorene. Notably, the complex with oligofluorene tethered to the pyridine tiara to phenyl ring possessed a substantially higher phosphorescence quantum efficiency and shorter lifetime than those of an isomeric complex with the oligofluorene linked to the phenyl ring para to pyridine. Nonetheless, both these two oligomer complexes exhibited an excited state of mixed MLCT. (metal-to-ligand charge transfer) and LC (ligand-centered) transitions, whereas another isomeric complex having an oligofluorene appended to the phenyl ring pant to the iridium ion exhibited a particularly long triplet lifetime (> 100 mu s), indicative of a (LC)-L-3 excited state. A moderately high quantum yield (similar to 0.5) was displayed by this (LC)-L-3-featured phosphor. DEL calculations substantiated the proposition that the attachment of oligofluorene to Ir(ppy)(3) at different positions resulted in varied molecular orbitals, with different relative contribution of MLCT to the emissive excited state. Hence, photophysical properties such as radiative decay rate, lifetime, and quantum yield, etc., were all influenced by the substitution isomerism. As these results indicated that if short lifetime and fast radiative decay were desired, among different substitution patterns appending the conjugated chain to the pyridine unit was the most favorable. Thus, star-shaped complexes with an oligo- or polyfluorene tethered to each of the three pyridine units of Ir(ppy)(3) were prepared. In such a structure, the tris-cyclometalated iridium effected nearly complete intersystem crossing (ISC) in all three ligands across three fluorene units, without compromising the phosphorescence quantum yield. But the study showed that further extending the conjugated ligand resulted in partial ISC or even complete loss of capacity for ISC beyond a certain distance.

Arylene ethynylene macrocycles containing 9,10-anthrylene or 1,4-naphthylene units were synthesized. In chloroform, significant resonance upfield shifting was observed with beta-protons of anthrylene and naphthylene in NMR spectra. This was considered to result from partial stacking of these aromatic units intramolecularly, driven by attractive pi-pi interactions. DFT calculations supported the proposed intramolecular stacking motif. Moreover, a liquid-crystal phase was exhibited by the anthrylene-containing macrocycle, by virtue of the unique discotic shape.

Well-defined 3-dimensional architectures constitute the indispensable structural basis of the versatile, mind-boggling functions of biological macromolecules, such as proteins and nucleic acids. In the past few decades, diversified synthetic systems have been designed to mimic these biological entities in their capability of adopting such specific, higher order structures The relevant research field presents one of the most rapidly developing areas related to supramolecular chemistry The current contribution will focus on the most recent progress related to foldamers consisting of arylene ethynylene building blocks Some of the work features developing novel functions based on previously established arylene ethynylene folding systems, and others have designed and synthesized new arylene ethynylene foldable structures that aim to realize previously uncharted properties (C) 2010 Elsevier B.V. All rights reserved

Crystalline microwires of a phenyleneethynylene (PE) macrocycle self-assembled from solution exhibited superior photoconductive properties. Photoswitches fabricated with single wires afforded nA-scale photocurrents with on/off ratios of ca. 10(3). At a bias of 30 volts highest gain value achieved was up to 4.5. The stable and rapid responses to light qualify these microwire-based devices for excellent photoswitches or photodetectors.

Dimeric and trimeric molecules comprising perylenediimide units conjugatively linked by phenylene, ethynylene, or a butadiynylene spacer via the bay positions were prepared. Electrochemical and photophysical characterizations showed that oligomers connected by C-C triple bond(s) exhibited effectively lowered LUMO compared to the monomer. Molecular modeling confirmed that the C-C triple bond realized efficient delocalization of frontier orbitals, while phenylene was less competent in extending the conjugation, partially due to steric interactions.

An oligo(o-phenyleneethynylene-alt-p-phenyleneethynylene) was synthesized to create a conjugated molecule capable of adopting a helical secondary structure. A special feature of such a folded molecule is that the effective directions of energy/charge transport via covalent conjugation and through pi-pi stacking are converged to be along the helix axis. The transition from random conformations to the helix, driven by solvophobic and aromatic stacking interactions, was controlled by solvent properties. UV-vis and fluorescence spectroscopies gave supportive evidence for the folding process.

Three different models are developed to calculate the thermodynamic product size distribution in a nucleation-elongation polymerization between a pair of A-A and B-B typed comonomers. These monomers are designed to undergo a single step of nucleation prior to an isodesmic chain elongation, namely, a cooperative, step-growth polymerization with dimerization being an energetically less favored process. Particularly, emphasis is laid on analyzing product distribution under conditions of imbalanced functionality stoichiometry. Consistent results are obtained from independent approaches, mechanistic and statistical, demonstrating that when the mole ratio of the comonomers deviates from unity, at polymerization equilibrium such a nucleation-elongation polymerization generates products of substantially higher molecular weights than those from a corresponding isodesmic system having an identical energetics for chain propagation yet without the nucleation process. This higher molecular weight is shown achieved by retaining a large portion of the excess monomer unreacted at equilibrium and selectively compose product chains with comonomers at a roughly stoichiometric ratio. Essentially, such a polymer-monomer coexisting bimodal distribution is a result from destabilization of the oligomeric species due to the nucleation effect.

A series of monodispersed oligo(p-phenyleneethynylene)s were synthesized bearing intramolecular hydrogen bonds between side chains of adjacent phenylene units in the backbone. Thus, all repeating units of the molecules are constrained in a coplanar orientation. Such planarized conformation is considered favorable for single-molecule conductance. Photophysical characterization results show narrowed bandgaps and extended conjugation lengths, consistent with a rigid, planar backbone framework as a result of intramolecular hydrogen bonding.

graph An efficient synthesis of large lptycenes appended with alkoxy and ethynyl substituents is reported. The rigid shape-persistent iptycene scaffold prevents interactions between the polymer backbones and can be used to solubilize polymers containing less soluble but readily accessible comonomers to prepare functional, solution-processible poly(p-phenyleneethynylene) (PPE)-conjugated polymers. These polymers are highly emissive in thin films without significant excimer/exciplex formation as a result of the effective chain isolation enforced by the iptycene units.

A new series of poly(p-phenylenebutadiynylene)s has been synthesized with unique polymer structural features. In these systems each of the p-phenylene units in the conjugated backbone is the core of a rigid three-dimensional iptycene scaffold. The fluorescence quenching properties of these polymers in response to a series of electron-deficient aromatic compounds have been investigated in both solution and the solid state. It was found that in solution these polymers displayed higher quenching sensitivity toward studied quenchers compared to a more open-structure iptycene-containing poly(p-phenyleneethynylene). The quenching behaviors of the conjugated polymer were shown to be strongly influenced by the configuration of the incorporated iptycences. The thin films investigations revealed differences in both the fluorescence quenching and the recovery processes. Distinct behaviors indicated that the fluorescence quenching in the solid state is dictated by different factors than those in solution. Our results further suggest that poly(p-phenylenebutadiynylene)s containing large iptycene scaffolds that introduce porosity have the ability to efficiently sequester the quencher molecules within thin films as these materials display slow fluorescence recoveries.

As a result of the helical structure of the polymeric product, the folding-driven polymerization of oligo(m-phenyleneethynylene) imines in solution should inherently show nucleation-elongation in chain growth. Here, we present evidence for this behavior based on results of polymerizations conducted under conditions of imbalanced stoichiometry. Because the polymerization proceeds via imine metathesis between a pair of bifunctional monomers of types A-A and B-B, the molar ratio of the polymerizing functional groups can be arbitrarily varied. Alternatively, stoichiometry can be controlled by the addition of a monofunctional oligomer. Similar results were obtained in both cases whereby the molecular weight distribution was significantly different from that expected for classical step-growth polymerizations. At equilibrium, high molecular weight polymers were observed to coexist with the monomer in excess. Thermodynamic equilibrium was established by showing that the same distribution was reached starting either from a monomer mixture or from high polymers to which one monomer was added. These results are in great contrast to the low molecular weight oligomers that were produced when the reaction was conducted by melt condensation of bifunctional aldehyde and amine monomers, a polymerization that apparently proceeds without the nucleation event. An equilibrium model that captures the features of nucleation-elongation under conditions of imbalanced stoichiometry qualitatively supports the monomer-polymer distribution observed experimentally.

Bis(imino) end-functionalized oligo(m-phenylene ethynylene)s were equilibrated in a closed system under conditions that promote reversible imine metathesis. The metathesis reaction joins two oligomers and produces a small molecule byproduct. In polar solvents, equilibration gave high molecular weight polymers while equilibration in chloroform produced only low molecular weight oligomers. This polymerization is hypothesized to be driven by the free energy gained from the folding of the long polymer chains directed by the noncovalent, intramolecular aromatic stacking and solvophobic interactions. This polymerization was also conducted in a series of solvents in which m-phenylene ethynylene oligomers have previously shown varied, intermediate folding stabilities. These experiments revealed a good correlation of the product molecular weight with the stability of the m-phenylene ethynylene helix. The equilibrium state of the metathesis reaction was also demonstrated to depend on the chain length of the starter sequences. With a pair of trimeric precursors, macrocyclization instead of polymerization takes place. Consistent with the notion that the polymerization is a consequence of the intramolecular solvophobic chain association, higher degrees of polymerization followed from enhanced solvophobicity of the m-phenylene ethynylene backbone, achieved by appending a methyl substituent to half of the repeat units. The considerably longer equilibration time required by these more stabilized sequences suggests that the elongation process may involve unfolding or partial unfolding of the chain; alternatively, intermolecular association may be responsible for the slow chain growth.

The kinetic and thermodynamic characteristics of polymerizations following a cooperative, nucleation-elongation mechanism are discussed in comparison to those of non-cooperative, isodesmic polymerizations. Nucleation-elongation polymerization is a relatively unexplored avenue of synthetic polymer chemistry and offers some unique and interesting thermodynamic and kinetic attributes not found in the more classical mechanisms of polymer chemistry.

This article describes recent developments in the synthesis of macrocycles having rigid, monocyclic skeletons composed of arylene and ethynylene units and the studies on their self-assembling behavior.

The purpose of this study was to test the suitability of the imine bond as a structural unit within the backbone of phenylene ethynylene macrocycles and oligomers by determining the ability of m-phenylene ethynylene macrocycle 1 to form pi-stacked aggregates in both solution and the solid state. Macrocycle 1, with two imine bonds, was synthesized in high yield from diamine 4 and dialdehyde 5. The imine-forming macrocyclization step was carried out under a variety of conditions, with the best yield obtained simply by refluxing the reactants in methanol. The self-association behavior of 1 in various solvents was probed by H-1 NMR. The association constants (K-E) in acetone-d(6) and tetrahydrofuran-d(8) were determined by fitting the concentration-dependent chemical shifts with indefinite self-association models. The results showed that solvophobically driven intermolecular pi-pi stacking could be preserved in the imine-containing m-phenylene ethynylene macrocycles. Interestingly, in acetone macrocycle 1 exhibited a stronger tendency to form a dimer rather than higher aggregates. We postulate that this behavior may be due to electrostatic attraction between dipolar imine groups. The solid-state packing of 1 was studied by wide- and small-angle X-ray powder diffraction (WAXD and SAXD). Bragg reflections of 1 were consistent with a hexagonal packing motif similar to our previous studies on m-phenylene ethynylene macrocycles that formed columnar liquid crystal phases.