Meta-linked poly(phenylene ethynylene) conjugated polyelectrolyte featuring a chiral side group: helical folding and guest binding

Emergence of Chiroptical Properties in Molecular Assemblies of Phenyleneethynylenes: The Role of Quasi-degenerate Excitations

Chiroptical properties of supramolecular assemblies originate through the asymmetric coupling of molecular transition dipole moments. Herein, we report a joint experimental and theoretical investigation to understand the influence of intermolecular interactions on chiroptical properties, particularly during the early stages of self-assembly. In this regard, phenyleneethynylene-based molecular systems appended with d- and l-isomers of phenylalanine have been synthesized with one as well as two electronic transitions in the spectral region of interest. When self-assembled, these molecules show distinctly different chiroptical properties with the right- and left-handed organizations, guided by the chirality of phenylalanines. The standard exciton approach explains the observation of a bisignated electronic circular dichroism signal in systems with a single transition but fails when applied to systems with two nearby transitions. Here, we present a generalized exciton approach that addresses the unusual chiroptical properties of systems with multiple transitions.

Polymeric Tubular Structures

Nature makes use of tubular structures for the spatial separation of matter on many different length scales, ranging from the nanometer scale (selective channels based on folded proteins) up to the centimeter scale (blood vessels). Today, polymer chemists and engineers can prepare polymeric tubular structures via a variety of different methods also covering many lengthscales, from nanometers to meters. The synthetic approaches described in this chapter vary significantly from the folding of single polymer chains via the self-assembly of DNA fragments to coordinative metal-organic nanotubes to tubes engineerd from bulk polymers using a range of porous or fibrous templates. While all examples reported in this chapter form tubular structures and thereby mimic their naturally occuring counterparts, it is mainly the engineered tubes that are more straightforward to prepare that also show some bio-inspired function.

Parallel folding topology-selective label-free detection and monitoring of conformational and topological changes of different G-quadruplex DNAs by emission spectral changes via FRET of mPPE-Ala-Pt(ii) complex ensemble

The formation of supramolecular assemblies between [Pt(bzimpy-Et){C[triple bond, length as m-dash]CC₆H₄(CH₂NMe₃-4)}]Cl₂ (1) and mPPE-Ala and the FRET properties of the ensemble have been revealed from the UV-vis absorption, steady-state emission and time-resolved emission decay studies. The two-component mPPE-Ala-1 ensemble has been employed in a "proof-of-principle" concept for label-free detection of G-quadruplex DNAs with the intramolecular propeller parallel folding topology, such as c-myc, in aqueous buffer solution. By the modulation of the aggregation/deaggregation of the polymer-metal complex aggregates and hence the FRET from the mPPE-Ala donor to the aggregated 1 as acceptor, the ensemble has been demonstrated for sensitive and selective label-free detection of c-myc via the monitoring of emission spectral changes of the ensemble. Ratiometric emission of the ensemble at 461 and 662 nm has been shown to distinguish the intramolecular propeller parallel G-quadruplex folding topology of c-myc from other G-quadruplex-forming sequences of different folding topologies, owing to the strong and specific interactions between c-myc and 1 as suggested by the UV-vis absorption and UV melting studies. In addition, the formation of high-order intermolecular multimeric G-quadruplexes from c-myc under molecular crowding conditions has been successfully probed by the ratiometric emission of the ensemble. The conformational and topological transition of human telomeric DNA from the mixed-hybrid form to the intramolecular propeller parallel form, as observed from the circular dichroism spectroscopy, has also been monitored by the ratiometric emission of the ensemble. The ability of the ensemble to detect these conformational and topological transitions of G-quadruplex DNAs has been rationalized by the excellent selectivity and sensitivity of the ensemble towards the intramolecular propeller parallel G-quadruplex DNAs and their high-order intermolecular multimers, which are due to the extra stabilization gained from Pt···Pt and π-π interactions in addition to the electrostatic and hydrophobic interactions found in the polymer-metal complex aggregates.

"Light Switch" Effect Upon Binding of Ru-dppz to Water-Soluble Conjugated Polyelectrolyte Dendrimers

We report the "light switch" effect of [Ru(bpy)2(dppz)](2+) (where bpy = 2,2'-bipyridine and dppz = dipyrido[3,2-a:2',3'-c] phenazine, Ru-dppz) in the presence of anionic conjugated polyelectrolyte dendrimers (CPDs). The metal-to-ligand charge-transfer luminescence from Ru-dppz is efficient in the presence of CPD because the complex is shielded from water by binding to the hydrophobic dendrimer core.

Helical Conjugated Polyelectrolyte Aggregation Induced by Biotin-Avidin Interaction

Fluorescence correlation spectroscopy (FCS) is applied to demonstrate avidin-induced cross-linking in a system consisting of a helical anionic conjugated polyelectrolyte (P1) and a biotin-tetramethylrhodamine (TMR) conjugate (2). In a previous study, we used fluorescence spectroscopy to demonstrate that 2 binds to P1 via intercalation of the TMR chromophore into the P1 helix. Addition of avidin to the P1/2 complex induces little change in the fluorescence of the system; however, FCS reveals a remarkable increase in the diffusion time of the P1/2 complex in the presence of avidin. This change is attributed to supramolecular polymer aggregates produced by cross-link formation between the biotin unit of intercalated 2 and avidin. Atomic force microscopy imaging provides evidence supporting the existence of these aggregates. The highly sensitive FCS method is used to develop a novel sensor for the biotin-avidin interaction, with a detection limit of <100 pM for avidin.

Reversible helix-random coil transition of poly(m-phenylenediethynylene) by a rotaxane switch

Pendant rotaxane switch-tethering poly(m-phenylene diethynylene) was synthesized by the polyoxidative coupling of a rotaxane containing an axle-terminal m-diethynylbenzene group and an optically active crown ether. The reversible helix-random coil transition of the polymer was successfully performed by the positional switching of the rotaxane wheel.

Aggregation-induced amplified quenching in conjugated polyelectrolytes with interrupted conjugation

A pair of anionic conjugated polyelectrolytes that contain three-ring (phenylene ethynylene) units linked by a single -CH(2)- or -O- tether (P1 and P2, respectively) are studied. The linkers serve to interrupt the π conjugation along the polymer backbone. Fluorescence spectroscopy reveals that P2 forms a fluorescent aggregate in methanol and water; however, the fluorescence of P1 is much weaker in water, and P1 exhibits only weak aggregate fluorescence. Fluorescence quenching of the polymers was examined using methyl viologen (MV(2+)) as a cationic quencher. P1 shows only a weak amplified quenching effect, with a Stern-Volmer quenching constant of K(SV) ≈ 6 × 10(5) M(-1) in methanol. Interestingly, for P2 in methanol, the aggregate emission is strongly quenched with K(SV) ≈ 5 × 10(6) M(-1), which is comparable to the highest quenching efficiency observed for fully π-conjugated polyelectrolytes. By contrast, the monomer emission is quenched much less efficiently, with K(SV) ≈ 2 × 10(5) M(-1). The results are explained by a model in which -O- linked polymer P2 is able to fold into a helical conformation in solution, which facilitates the formation of extended π-stacked aggregates allowing long-distance exciton transport.

Intercalation-FRET biosensor with a helical conjugated polyelectrolyte

A biotin-tetramethylrhodamine (biotin-TMR) quencher-ligand interacts with a (phenylene-ethynylene) based helical conjugated polyelectrolyte (poly-1) via intercalation of the TMR unit into the helix. The interaction is signaled by efficient fluorescence resonance energy transfer (FRET) from the polymer to the TMR chromophore. Avidin addition to the poly-1/biotin-TMR intercalation complex does not interrupt FRET, instead resulting in the formation of avidin-biotin "cross-links". Mixing of biotin-TMR with avidin prior to addition of the polymer efficiently disrupts the FRET signal, giving rise to a sensor with a detection limit of 100 pM for avidin. Study of the FRET response as a function of biotin-TMR and avidin concentration affords insight into the interaction of the protein with the poly-1/biotin-TMR intercalation complex.

Conjugated polyelectrolytes: synthesis, photophysics, and applications

Organic optoelectronic polymers have evolved to the point where fine structural control of the conjugated main chain, coupled with solubilizing and property-modifying pendant substituents, provides an entirely new class of materials. Conjugated polyelectrolytes (CPEs) provide a unique set of properties, including water solubility and processability, main-chain-controlled exciton and charge transport, variable band gap light absorption and fluorescence, ionic interactions, and aggregation phenomena. These characteristics allow these materials to be considered for use in applications ranging from light-emitting diodes and electrochromic color-changing displays, to photovoltaic devices and photodetectors, along with chemical and biological sensors. This Review describes the evolution of CPE structures from simple polymers to complex materials, describes numerous photophysical aspects, including amplified quenching in macromolecules and aggregates, and illustrates how the physical and electronic properties lead to useful applications in devices.