Folding a Conjugated Chain: Oligo(o-phenyleneethynylene-alt-p-phenyleneethynylene)

Sharp Turns and Fluorescent Repeats: Modular Construction and Shape-Dependent Electronic Properties of π-Conjugated Chain Molecules

In search of the design rules for structural ordering of open-chain molecules, we have built a series of zig-zag shaped π-conjugated structures with ring-fused heteroaromatics as sharp turns and tolane-based linear fragments as light-emitting units. Using only a finite number of common building blocks, an efficient "double-elongation" strategy was implemented to construct a series of π-conjugated oligomers with precise length control (55-89 % yields). Our approach takes advantage of the modular nature of the bis(triazolo)benzene synthesis and the masked reactivity of the nitro group. A combination of photophysical and DFT computational studies revealed that the bis(triazolo)benzene-tolane repeat units behave as electronically decoupled light-absorbing/emitting units (λ_max,em = 408-422 nm; Φ_F = 20-25 % in THF). Such context-independent photophysical properties promise their potential applications in chemical sensing and switching.

The Diverse World of Foldamers: Endless Possibilities of Self-Assembly

Different classes of foldamers, which are synthetic oligomers that adopt well-defined conformations in solution, have been the subject of extensive studies devoted to the elucidation of the forces driving their secondary structures and their potential as bioactive molecules. Regardless of the backbone type (peptidic or abiotic), the most important features of foldamers are the high stability, easy predictability and tunability of their folding, as well as the possibility to endow them with enhanced biological functions, with respect to their natural counterparts, by the correct choice of monomers. Foldamers have also recently started playing a starring role in the self-assembly of higher-order structures. In this review, selected articles will be analyzed to show the striking number of self-assemblies obtained for foldamers with different backbones, which will be analyzed in order of increasing complexity. Starting from the simplest self-associations in solution (e.g., dimers of β-strands or helices, bundles, interpenetrating double and multiple helices), the formation of monolayers, vesicles, fibers, and eventually nanostructured solid tridimensional morphologies will be subsequently described. The experimental techniques used in the structural investigation, and in the determination of the driving forces and mechanisms underlying the self-assemblies, will be systematically reported. Where applicable, examples of biomimetic self-assembled foldamers and their interactions with biological components will be described.

Helical Folding Competing with Unfolded Aggregation in Phenylene Ethynylene Foldamers

The folding and aggregation behavior of a pair of oligo(phenylene ethynylene) (OPE) foldamers are investigated by means of UV/Vis absorption and circular dichroism spectroscopy. With identical OPE backbones, two foldamers, 1 with alkyl side groups and 2 with triethylene glycol side chains, manifest similar helical conformations in solutions in n-hexane and methanol, respectively. However, disparate and competing folding and aggregation processes are observed in alternative solvents. In cyclohexane, oligomer 1 initially adopts the helical conformation, but the self-aggregation of unfolded chains, as a minor component, gradually drives the folding-unfolding transition eventually to the unfolded aggregate state completely. In contrast, in aqueous solution (CH3 OH/H2 O) both folded and unfolded oligomer 2 appear to undergo self-association; aggregates of the folded chains are thermodynamically more stable. In solutions with a high H2 O content, self-aggregation among unfolded oligomers is kinetically favored; these oligomers very slowly transform into aggregates of helical structures with greater thermodynamic stability. The folded-unfolded conformational switch thus takes place with the free (nonaggregated) molecules, and the very slow folding transition is due to the low concentration of molecularly dispersed oligomers.

Helix–helix inversion of an optically-inactive π-conjugated foldamer triggered by concentration changes of a single enantiomeric guest leading to a change in the helical stability

An optically-inactive foldamer undergoes helicity induction and subsequent helix-inversion with the increasing amount of a single enantiomeric guest.

Hydrophobically driven twist sense bias of hollow helical foldamers of aromatic hydrazide polymers in water

Aromatic hydrazide polymers bearingS-chiral tetraethylene glycol chains can fold into M hollow helices in water driven by the hydrophobicity.

meta-Substituted benzamide oligomers that complex mono-, di- and tricarboxylates: folding-induced selectivity and chirality

meta-Substituted arylamide trimer, pentamer and heptamer have been prepared from simple benzene-1,3-diamine, benzene-1,3-dicarboxylic acid, and 3-aminobenzoic acid units. 2D NOESY (1)H NMR experiments reveal that these flexible oligomers form folded conformations to complex di- and tricarboxylate anions of varying sizes and shapes in DMSO of high polarity, which is driven by multiple intermolecular N-H···O=C and C-H···O=C hydrogen-bonds between the amide and aromatic hydrogens of the oligomers and the carboxylate oxygens of the anions. Generally, tricarboxylate anions display an increased binding affinity compared with the dicarboxylate anions and the complexes formed by 1,3-benzenedicarboxylate anion are more stable than those formed by 1,2- or 1,4-benzenedicarboxylate anions. Circular dichroism experiments show that chiral glutamic acid dianion can induce the oligomers to produce chiral bias, leading to the formation of chiral supramolecular complexes.

Synthetic, structural, photophysical and computational studies on 2-arylethynyl-1,3,2-diazaboroles

New 2-arylalkynyl benzo-1,3,2-diazaboroles, 2-(4'-XC(6)H(4)C[triple bond, length as m-dash]C)-1,3-Et(2)-1,3,2-N(2)BC(6)H(4) (X =Me ; MeO ; MeS ; Me(2)N ), were prepared from B-bromodiazaborole, 2-Br-1,3-Et(2)-1,3,2-N(2)BC(6)H(4), with the appropriate lithiated arylacetylene, ArC[triple bond, length as m-dash]CLi. Molecular structures of , and were determined by X-ray diffraction studies. UV-vis and luminescence spectroscopic studies on these diazaboroles reveal intense blue/violet fluorescence with very large quantum yields of 0.89-0.99 for . The experimental findings were complemented by DFT and TD-DFT calculations. The Stokes shift of only 2600 cm(-1) for , compared to Stokes shifts in the range of 5900-7300 cm(-1) for , is partly explained by the different electronic structures found in compared to (X = H). The HOMO is mainly located on the aryl group in and on the diazaborolyl group in whereas the LUMOs are largely aryl in character for all compounds. Thus, in contrast to other conjugated systems containing three-coordinate boron centers such as B(Mes)(2), (Mes = 2,4,6-Me(3)C(6)H(2)), in which the boron serves as a pi-acceptor, the 10-pi electron benzodiazaborole moiety appears to function as a pi-donor moiety.