Solvent- and pH-induced self-assembly of cationic meta-linked poly(phenylene ethynylene): effects of helix formation on amplified fluorescence quenching and Förster resonance energy transfer

We reported here the synthesis and characterization of a novel water-soluble, meta-linked poly(phenylene ethynylene) (m-PPE-NEt(2)Me(+)) featuring quaternized side groups. We studied the solvent-induced self-assembly of m-PPE-NEt(2)Me(+) in MeOH/H(2)O solvent mixtures by using UV-vis absorption and fluorescence spectroscopies. The results showed that the polymer folded into a helical conformation and that the extent of helical folding increased with the volume % water in the solvent. This cationic polymer also exhibited unique pH-induced helix formation, which was attributed to the partial neutralization of quaternized side groups at high pH and the meta-links in the main chain of the polymer. Studies on the fluorescence quenching of m-PPE-NEt(2)Me(+) by anthraquinone-2,6-disulfonate (AQS) and Fe(CN)(6)(4-), two small-molecule anionic quenchers with different typical structures, revealed more efficient quenching of helical conformation by AQS than by Fe(CN)(6)(4-). We proposed that the two quenchers most likely interacted with the polymer helix in two different modes; that was, AQS featuring large planar aromatic ring could intercalate within adjacent π-stacked phenylene ethynylene units in the polymer helix, whereas Fe(CN)(6)(4-) mainly bound to the periphery of polymer helix through ion-pair formation. Finally, the results of FRET from the helical polymer to the fluorescein (C*)-labeled polyanions, ssDNA-C* (ssDNA: single-stranded DNA) and dsDNA-C* (dsDNA: double-stranded DNA) also suggested two different modes of interactions. As compared with the FRET to dsDNA-C*, the FRET to ssDNA-C* was slightly more efficient, which was believed to arise from the additional binding of ssDNA-C* with the polymer via intercalation of its exposed hydrophobic bases into the π stack of adjacent phenylene ethynylene units in the polymer helix.     

Quantifying the efficiency of förster‐assisted optical gain in semiconducting polymer blends by excitation wavelength selective amplified spontaneous emission

We study the correlation between Förster resonance energy transfer (FRET) and optical gain properties in conjugated polymer blends based on regioregular poly(3‐hexylthiophene) (P3HT) and poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT). First, FRET dynamics are investigated with femtosecond transient absorption spectroscopy observing a sub‐picosecond energy transfer from F8BT to P3HT (550 fs) at medium doping levels (40% wt P3HT in F8BT). Amplified spontaneous emission (ASE) is then characterized in blends upon exciting predominantly the host and guest polymers, respectively. The corresponding density of absorbed photons at threshold conditions is compared upon host or guest photoexcitation as a method to quantitatively determine the FRET‐assisted ASE efficiencies. We observe a reduction in ASE efficiency upon host photoexcitation of 20%, in the best case, respect to guest photoexcitation. Our results confirm that even in strongly coupled host:guest mixtures delayed exciton population by energy transfer is subtle to losses ascribed to exciton–exciton annihilation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2311–2317.     

Solvent-assisted optical modulation of FRET-induced fluorescence for efficient conjugated polymer-based DNA detection

The solvent effects were studied in fluorescence resonance energy transfer (FRET) from a cationic polyfluorene copolymer (FHQ, FPQ) to a fluorescein (Fl)-labelled oligonucleotide (ssDNA-Fl). Upon addition of dimethyl sulfoxide (DMSO), the optical properties of polymers and the probe dye were substantially modified and the FRET-induced PL signal was enhanced 3.8-37 times, relative to that in phosphate buffer solution (PBS). The hydrophobic interaction between polymers and ssDNA-Fl is expected to decrease in the presence of DMSO, which induces the weaker polymer/ssDNA-Fl complexation with longer intermolecular donor-acceptor separation and perturbs the competition between the FRET and PL quenching processes such as photo-induced charge transfer. The gradual decrease in Fl PL quenching with increasing the DMSO content was investigated by measuring the Stern-Volmer quenching constants (3.3-4.2 × 10(6) M(-1) in PBS, 0.56-1.1 × 10(6) M(-1) in 80 vol% DMSO) and PL lifetime of the excited Fl* in polymer/ssDNA-Fl (600 ps in PBS and 2120 ps in 80 vol% DMSO for FHQ/ssDNA-Fl) in PBS/DMSO mixtures. The substantially reduced PL quenching would amplify the resulting FRET Fl signal. The signal amplification in real DNA detection was also demonstrated with fluorescein-labelled PNA (probe PNA) in the presence of a complementary target DNA and noncomplementary DNA in aqueous DMSO solutions. This approach suggests a simple way of modifying the fine-structure of polymer/ssDNA-Fl and improving the detection sensitivity in conjugated polymer-based FRET bioassays.     

Optimization of interactions between a cationic conjugated polymer and chromophore-labeled DNA for optical amplification of fluorescent sensors

Cationic conjugated polymers (CCPs) have been widely utilized as signal amplifiers in biosensors to improve the detection sensitivity through fluorescence resonance energy transfer (FRET) from CCPs to dye-labeled probes or targets. This paper investigates the effect of sodium dodecyl sulfate (SDS) on energy transfer between a cationic polyfluoreneethynylene copolymer (P1) and Texas Red labeled single-stranded DNA (ssDNA-TR). The presence of SDS in solution affects both the optical properties of P1 and TR emission within P1/ssDNA-TR complexes, which provides basic information on the role of SDS in FRET between P1 and ssDNA-TR. Although the quantum yield of P1 decreases in the presence of low concentrations of SDS, the presence of SDS reduces TR fluorescence quenching within P1/ssDNA-TR complexes and increases the number of optically active polymer repeat units within the proximity of TR, which are beneficial to P1-sensitized TR emission. In the absence of SDS, FRET from P1 to ssDNA-TR provides a 2.6-fold enhancement in TR emission intensity as compared to that upon direct excitation of TR at 595 nm. At the optimum SDS concentration (5 microM), P1-sensitized TR signal output increases to 11.3-fold relative to direct excitation of TR. This study highlights the importance of modulation of the CCP/ssDNA-dye interaction in improving the signal output of dye-labeled DNA by CCP through FRET.     

Fluorescence Study of Energy Transfer in PMMA Polymers with Pendant Oligo‐Phenylene‐Ethynylenes

A spectroscopic characterization of polymers containing rigid π‐conjugated oligo(phenyleneethynylene) chromophores as well as oligo(phenyleneethynylene) and methyl methacrylate is presented. The polymers exhibit molar masses of up to 15 000 g mol^?1 and a degree of polymerization between 22 and 80. Emission measurements of the monomeric and polymeric species show that radiative as well as nonradiative rates are influenced by the degree of polymerization due to intramolecular interactions of chromophores pendant to the polymer backbone. Time‐resolved emission anisotropy measurements suggest that energy migrates within the polymers. Steady‐state emission anisotropy measurements also point to energy migration. Additionally, two oligo(phenyleneethynylene)s with different sizes of the conjugated system are copolymerized in order to enable energy trapping due to energy transfer. The shortened energy‐donor fluorescence lifetime within the donor–acceptor copolymers suggest energy transfer. Depending on the degree of polymerization, dispersion of the donor fluorescence lifetime is observed.     

Tetrahydrofuran Activates Fluorescence Resonant Energy Transfer from a Cationic Conjugated Polyelectrolyte to Fluorescein‐Labeled DNA in Aqueous Media

A cationic water‐soluble conjugated polyelectrolyte, poly[9,9‐bis(6′′‐(N,N,N‐trimethylammonium)hexyl)fluorene‐co‐alt‐2,5‐bis(6′‐(N,N,N‐trimethylammonium)hexyloxyphenylene) tetrabromide], was synthesized. Fluorescence resonant energy transfer (FRET) experiments between the polymer and fluorescein‐labeled single‐stranded DNA (ssDNA‐Fl) were conducted in aqueous buffer and THF/buffer mixtures. Weak fluorescence emission in aqueous buffer was observed upon excitation of the polymer, whereas addition of THF turned on the fluorescence. Fluorescence self‐quenching of ssDNA‐Fl in the ssDNA‐Fl/polymer complexes as well as electron transfer from the polymer to fluorescein may account for the low fluorescence emission in buffer. The improved sensitization of fluorescence by the polymer observed in THF/buffer could be attributed to the weaker binding between the polymer and ssDNA‐Fl and a decrease in dielectric constant of the solvent mixture, which disfavors electron transfer. THF‐assisted signal sensitization was also observed for the polymer and fluorescein‐labeled double‐stranded DNA (dsDNA‐Fl). These results indicate that the use of cosolvent provides a strategy to improve the detection sensitivity for biosensors based on the optical amplification provided by conjugated polymers.     

Characterization of tectoRNA assembly with cationic conjugated polymers

Association between RNAs with preprogrammed molecular recognition units can be quantified by using cationic, water-soluble conjugated polymers. The method uses a fluorophore-labeled probe RNA (RNA-F*), which is treated with a target structure (RNA-T). Heterodimer formation, (RNA-T/RNA-F*), increases the total negative charge on the F*-bearing macromolecule and reduces the number of negatively charged molecules (relative to unbound RNA-T+ RNA-F*). On the basis of electrostatic interactions, we anticipated more effective binding between CCP and (RNAT/RNA-F*), a reduction of the average CCP- - -F* distance, and more effective FRET upon excitation of the conjugated polymer. The resulting signals benefit from the optical amplification characteristic of emissive conjugated polymers. Solution dissociation constants can be determined by monitoring F* intensity changes as a function of [RNA-F*] and the ratio: [I(T) - I(NB)]/I(NB), where I(T) and I(NB) are the F* intensities in the presence of the target RNA (RNA-T) and a nonbinding RNA (RNA-NB), respectively, while keeping the concentration of the conjugated polymer constant. By focusing on [I(T) - I(NB)]/I(NB) as a function of RNA concentration, one can detect the concentration range wherein increased fluorescence is the result of dimerization.     

Innenrücktitelbild: Developing Conjugated Polymers with High Electron Affinity by Replacing a C?C Unit with a B←N Unit (Angew. Chem. 12/2015)

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a C C unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.     

Developing Conjugated Polymers with High Electron Affinity by Replacing a CC Unit with a B←N Unit

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a C? C unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.     

荧光共轭聚合物在生物大分子检测中的应用*

共轭聚合物因其具有π-电子体系及共轭离域结构,一般都具有优异的发光性能,其发光强度和发射波长会随被检测化合物结构的不同而发生特异性响应,特别是在与被检测物相互作用过程中所产生电荷和能量能够沿共轭分子链进行有效传递,成倍放大这种作用,从而有效提高了检测灵敏度,这比相应的小分子化合物更具有优越性。目前共轭聚合物已被用于开发新型化学、生物传感器,尤其是在生物分子检测方面的应用得到迅速的发展。本文总结了近年来荧光共轭聚合物在生物传感方面的研究进展,主要讨论共轭聚合物在蛋白质、核酸及毒素检测中的应用。     

Quantifying interactions between cell receptors and adhesion ligand-modified polymers in solution

Specific interactions between cells and cell-interactive polymers in solution were investigated by the fluorescence resonance energy transfer (FRET) technique and rheological measurements. The green fluorescence emission was dramatically reduced when rhodamine-stained cells were mixed with a fluorescein-labeled RGD-alginate solution, compared with those mixed with no RGD-containing alginate solution, which indicated an occurrence of FRET and existence of specific interactions between the cells and the polymers in solution. Rheological measurements also confirmed the formation of ordered structures of cell/polymer mixtures, caused by specific cell-polymer interactions. The FRET method was able to provide a useful means of investigating cell-polymer interactions, both in a qualitative and quantitative manner, and this approach to monitoring and controlling specific interactions between cells and polymers could be useful in the design and tailoring of polymeric carriers for cells, as well as for biological drugs, especially for tissue engineering applications.     

Conjugation enhancement of intramolecular exciton migration in poly(p-phenylene ethynylene)s

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper understanding of the nature of this process. We make use of specially designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition, we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The ability to improve the exciton migration efficiency through precise control of the polymer structure with liquid crystalline solutions demonstrates the importance of a polymer's conformation for energy transfer, and provides a way to improve the energy transporting performance of conjugated polymers.     

Fluorescence blinking, exciton dynamics, and energy transfer domains in single conjugated polymer chains

In order to understand exciton migration and fluorescence intensity fluctuation mechanisms in conjugated polymer single molecules, we studied fluorescence decay dynamics at "on" and "off" fluorescence intensity levels with 20 ps time resolution using MEH-PPV [poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] dispersed in PMMA. Two types of intensity fluctuations were distinguished for single chains of conjugated polymers. Abrupt intensity fluctuations (blinking) were found to be always accompanied by corresponding changes in fluorescence lifetime. On the contrary, during "smooth" intensity fluctuations no lifetime change was observed. Time-resolved data in combination with data on fluorescence emission and excitation anisotropy lead to a picture where a single polymer molecule is seen as consisting of several energy transfer domains. Exciton migration is efficient within a domain and not efficient between domains. Each domain can have several emitting low-energy sites over which the exciton continuously migrates until it decays. Emission of individual domains is often highly polarized. Fluorescence from a domain can be strongly quenched by Forster energy transfer to a quencher (hole polaron) if the domain overlaps with the quenching sphere.     

Simple Conjugated Polymers with On‐Chain Phosphorescent Iridium(III) Complexes: Toward Ratiometric Chemodosimeters for Detecting Trace Amounts of Mercury(II)

For the development of excellent optical probes for mercury(II), a series of simple conjugated polymers that contain phosphorescent iridium(III) complexes as receptors for mercury(II) were designed and synthesized. These conjugated polymers showed energy transfer from the polymer host to iridium(III) complex guest in both solution and the solid state. Unexpectedly, they can work as excellent polymer chemodosimeters for mercury(II) by utilizing the mercury(II)‐induced decomposition of iridium(III) complex. They exhibit a pronounced optical signal change with switchable phosphorescence and fluorescence, even when the concentration of a solution of mercury(II) in THF was as low as 0.5 ppb. With the addition of mercury(II), the phosphorescent emission intensity of iridium(III) complexes was quenched completely. As the emission from polymer backbones increased, the emission wavelength was redshifted simultaneously, thereby realizing ratiometric detection. Excellent selectivity toward mercury(II) over other potentially interfering cations was also realized. In addition, an obvious emission color change of polymer solution from red to yellow‐green was observed, thus realizing a “naked‐eye” detection of mercury(II). More importantly, the solid films of these polymer chemodosimeters also exhibited high sensitivity and rapid response to mercury(II), thereby demonstrating the possibility of the fabrication of sensing devices with fast and convenient detection of mercury(II). The sensing mechanism was also investigated in detail. This is the first report on chemodosimeters based on conjugated polymers with phosphorescent iridium(III) complexes.     

Combinatorial Energy Transfer between an End‐Capped Conjugated Polyelectrolyte and Chromophore‐Labeled PNA for Strand‐Specific DNA Detection

We report a macromolecular end‐capping approach to improve the detection sensitivity of cationic conjugated polymer (CCP) based DNA detection. A phenylethynyl anthracene (PEA) end‐capped cationic polyfluorene (PF) derivative ( P1 ) is synthesized via Suzuki coupling. Due to efficient fluorescence resonance energy transfer (FRET) from the polymer backbone to the end‐capper PEA units, the polymer ( P1 ) fluorescence is dominated by the emission from PEA even in dilute aqueous solution. P1 emission has a better spectral overlap with fluorescein (Fl) absorption compared to that for uncapped PF ( P2 ). In addition, the intra and intermolecular energy transfer for P1 is more efficient in the presence of DNA due to complexation‐induced polymer aggregation. These impart a combinatorial FRET between P1 and an Fl‐labeled probe which is more efficient than that between P2 and the same probe. P1 thus offers a better DNA detection sensitivity relative to P2 and opens up new opportunities to improve the performance of CCP based biosensors involving FRET.

     

Structure—Property relationships for exciton transfer in conjugated polymers

The ability of conjugated polymers to function as electronic materials is dependent on the efficient transport of excitons along the polymer chain. Generally, the photophysics of the chromophore monomer dictate the excited state behavior of the corresponding conjugated polymers. Different molecular structures are examined to study the role of excited state lifetimes and molecular conformations on energy transfer. The incorporation of rigid, three‐dimensional scaffolds, such as iptycenes and cyclophanes, can encourage an oblique packing of the chromophore units of a conjugated polymer, thus allowing the formation of electronically‐coupled aggregates that retain high quantum yields of emission. Rigid iptycene scaffolds also act as excellent structural directors that encourage complete solvation of PPEs in a liquid crystal (LC) solvent. LC‐PPE mixtures display both an enhanced conformational alignment of polymer chains and extended effective conjugation lengths relative to isotropic solutions, which leads to enhanced energy transfer. Facile exciton migration in poly(p‐phenylene ethynylene)s (PPEs) allows energy absorbed over large areas to be funneled into traps created by the binding of analytes, resulting in signal amplification in sensory devices. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Fluorescein provides a resonance gate for FRET from conjugated polymers to DNA intercalated dyes

Fluorescence spectra show that excitation of the cationic water-soluble conjugated polymer poly[(1,4-phenylene)-2,7-[9,9-bis(6'-N,N,N-trimethylammonium)-hexyl]fluorene diiodide] (1) results in inefficient fluorescence resonance energy transfer (FRET) to ethidium bromide (EB) intercalated within double-stranded DNA (dsDNA). When fluorescein (Fl) is attached to one terminus of the dsDNA, there is efficient FRET from 1 through Fl to EB. The cascading energy-transfer process was examined mechanistically via fluorescence decay kinetics and fluorescence anisotropy measurements. These experiments show that the proximity and conformational freedom of Fl provide a FRET gate to dyes intercalated within DNA which are optically amplified by the properties of the conjugated polymer. The overall process provides a substantial improvement over previous homogeneous conjugated polymer based DNA sensors, namely, in the form of improved selectivity.     

Construction of new cross-linked supramolecular polymer with FRET phenomenon based on oligo(p-phenylenevinylene)-linked pillar[5]arene dimer

Cross-linked supramolecular polymers generally show distinct mechanical properties with intriguing functions, which have become one of the hot research topics in recent years. However, the cross-linked supramolecular polymers functionalised with fluorescence resonance energy transfer (FRET) properties have been rarely reported yet. Herein, a new cross-linked supramolecular polymer equipped with efficient FRET property was constructed successfully. The oligo(p-phenylenevinylene)-linked pillar[5]arene dimer (PA-OPV) functioned as a host and energy donor, while four pentanenitrile groups coupled porphyrin (CN-Por) acted as guest with energy acceptor. The supramolecular polymerisation and FRET properties were investigated by using NMR, viscosity, Fluorescence, UV–vis and SEM.