Single-chain Level Research Articles

Energy transfer in PPV-based conjugated polymers: a defocused widefield fluorescence microscopy study

Both pendant and main chain conjugated MEH-PPV based polymers have been studied at the level of single chains using confocal and widefield fluorescence microscopy techniques. In particular, defocused widefield fluorescence is applied to reveal the extent of energy transfer in these polymers by identifying whether they act as single emitters. For main chain conjugated MEH-PPV, molecular weight and the surrounding matrix play a primary role in determining energy transport processes and whether single emitter behaviour is observed. Surprisingly in polymers with a saturated backbone but containing the same pendant MEH-PPV oligomer on each repeating unit, intra-chain energy transfer to a single emitter is also apparent. The results imply there is chromophore heterogeneity that can facilitate energy funneling to the emitting site. Both main chain conjugated and pendant MEH-PPV polymers exhibit changes in orientation of the emission dipole during a fluorescence trajectory of many seconds, whereas a model MEH-PPV oligomer does not. The results suggest that, in the polymers, the nature of the emitting chromophores can change during the time trajectory.

The unexpected flexibility of natural cellulose at a single-chain level and its implications to the design of nano materials.

At crystal or larger scales, natural cellulose was generally recognized to be a rigid material. Our single-chain mechanical measurements, however, reveal that the natural cellulose is as flexible as common synthetic polymers at the single-chain level, creating new opportunities in the designing of nano materials.

Single-Chain Mechanics of Poly(N,N-diethylacrylamide) and Poly(N-isopropylacrylamide): Comparative Study Reveals the Effect of Hydrogen Bond Donors

The single-chain mechanics of two similar thermosensitive polymers, poly(N,N-diethylacrylamide) (PDEAM) and poly(N-isopropylacrylamide) (PNIPAM), have been studied by atomic force microscopy-based single-molecule force spectroscopy (SMFS). In a typical nonpolar organic solvent, octane, both of the polymers show the same inherent elasticity, although they have different substitutional groups. However, the mechanics of the two polymers presents large differences in water. The energies needed for the rearrangement of the bound water during elongation at room temperature are estimated by the SMFS method at the single-chain level, which is ~1.13 ± 0.10 and ~5.19 ± 0.10 kJ/mol for PDEAM and PNIPAM, respectively. In addition, PNIPAM shows a temperature-dependent single-chain mechanics when the temperature is increased across the lower critical solution temperature (LCST), while PDEAM does not. These differences observed in aqueous solution originate from the different structures of the two polymers. With a hydrogen bond donor in the amide group, PNIPAM will be more hydrated when T < LCST. When T > LCST, PNIPAM will have larger changes in both conformation and hydration. These findings also suggest that PNIPAM is a good candidate for a thermo-driven single-molecule motor, while PDEAM is not.

Solvent Vapor Annealing of Single Conjugated Polymer Chains: Building Organic Optoelectronic Materials from the Bottom Up.

Optoelectronic devices based on organic materials show a strong relationship between the morphological structure of the material and the function of the device. One of the grand challenges in improving the efficiencies of these devices is hence achieving morphological control throughout the entire course of processing. One of the most important postprocessing methods is solvent vapor annealing, which has repeatedly demonstrated its utility in improving the efficiency of organic-material-based devices by changing bulk-film morphology. This Perspective discusses the recent impact of single-molecule spectroscopy techniques in unraveling morphological changes and molecular dynamics and presents solvent vapor annealing as a tool to build organic optoelectronic materials from the bottom up. In particular, we discuss examples of how solvent vapor annealing at the single-chain level can be split into two different regimes, (i) the solvation regime, in which intrachain interactions and molecular dynamics during solvent vapor annealing can be probed, and (ii) the aggregation regime, in which the influence of interchain interactions can be probed. Finally, it will be shown that solvent vapor annealing in the aggregation regime can be used to build highly ordered mesoscopic objects with distinct properties such as long-range energy transfer.

Is there a molecular signature of the LCST of single PNIPAM chains as measured by AFM force spectroscopy?

The molecular stretching behavior of temperature responsive poly(N-isopropylacrylamide) (PNIPAM) chains was studied by atomic force microscopy based single molecule force spectroscopy. Force–extension curves obtained in water below and above the lower critical solution temperature, in the co-nonsolvent water/methanol mixture, and in dimethyl sulfoxide (H-bond blocking) follow the same trajectory, independently whether the chain was pulled from a collapsed or from a solvated state. This result indicates that for a single PNIPAM chain the formation of intrachain H-bonds in the precipitated state does not cause measurable chain stiffening at the single chain level.

Synthetic polymers for solar harvesting

Synthetic polymers incorporating appropriate chromophores can act as light harvesting antennae for artificial photosynthetic systems. The photophysical processes occurring in a polymer based on phenylene vinylene have been investigated at the single chain level and in bulk solution to study energy transfer processes. Most single chains of an alternating copolymer of 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene and 1,4-phenylene vinylene (alt-co-MEH-PPV) dispersed in a transparent polymer matrix act as single chromophore emitters demonstrating that energy transfer is an efficient process in these polymers. However for individual polymer chains there are fluctuations in emission intensity ('blinking') and shifts in emission spectra, decay lifetimes and emission dipole orientation occurring on a time-scale of tens of seconds. Fluorescence blinking also occurs on a sub-millisecond time-scale and follows exponential kinetics, whereas the longer blinking is better described by a power law. These observations can be interpreted as arising from environmental relaxation processes and/or changes in the emitter and demonstrate the wide distribution of photophysical behaviours that can be observed among the individual molecules of a polymer sample. The relevance of these studies to the application of polymer materials for solar harvesting is highlighted.

Unmasking Bulk Exciton Traps and Interchain Electronic Interactions with Single Conjugated Polymer Aggregates

For conjugated polymer materials, there is currently a major gap in understanding between the fundamental properties observed in single molecule measurements and the bulk electronic properties extracted from measurements of highly heterogeneous thin films. New materials and methodologies are needed to follow the evolution from single chain to bulk film properties as multiple chains begin to interact. In this work, we used a controlled solvent vapor annealing process to assemble single chains of phenylene-vinylene conjugated polymers into aggregates that can be individually spectroscopically interrogated. This approach allowed us to probe the effects of interchain coupling in isolated conjugated polymer nanodomains of controlled size. By assembling these aggregates from building blocks of both pristine MEH-PPV and MEH-PPV derivatives containing structure-directing ortho- or para-terphenyl inclusions, we were able to control the ordering of these nanodomains as measured by single aggregate polarization anisotropy measurments. Depending on the individual chain constituents, these aggregates varied from highly anisotropic to nearly isotropic, respectively facilitating or inhibiting interchain coupling. From the single chain fluorescence lifetimes, we demonstrated that these structure directing inclusions effectively break the phenylene-vinylene conjugation, allowing us to differentiate interchain electronic effects from those due to hyper-extended conjugation. We observed well-defined bathochromic shifts in the fluorescence spectra of the aggregates containing extensive interchain interactions, indicating that low-energy exciton traps in MEH-PPV are the result of coupling interactions between neighboring chain segments. These results demonstrate the power of the synthetic inclusion approach to control properties at not just the single chain level, but as a comprehensive approach toward ground-up design of bulk electronic properties.

Direct Imaging of Polyethylene Films at Single-Chain Resolution with Torsional Tapping Atomic Force Microscopy

The physical properties of semicrystalline polymers depend on the organisation of chains within the crystal and amorphous regions, on the interface between the two, and on the location and nature of defects. Here, torsional tapping atomic force microscopy has been used to image crystalline lamellae and the crystal-amorphous-region interface at the single-chain level with resolution down to 3.7 Å. Defects within the crystalline phase, such as buried folds and chain ends, are revealed. Imaging at the chain level also allows direct measurement of crystalline stem lengths, providing a potential route to test theories of crystal thickness selection.

Towards polysaccharide handles for single molecule experiments: Spectroscopic evidence for the selective covalent coupling of terminal sugar residues to desired substrates

Single molecule force spectroscopy (SMFS) has recently given access to an unprecedented level of information regarding the stress-response of a host of biopolymers at the single chain level. However, the difficulty of designing handles for the specific attachment of the different ends of polysaccharide chains to substrates, such as piezoelectric scanners, cantilevers or microbeads; particularly in contrast to nucleotides and proteins, has meant that progress in more sophisticated single carbohydrate measurements has been relatively slow. Herein, methods for generating specific covalent attachments of the terminal sugar residue at one end of a pectin chain to polystyrene microspheres are demonstrated. Using FT-IR spectroscopy in combination with density functional theory (DFT) calculations the attachment was unequivocally shown to be mediated by the introduction of a C–N bond between the reducing end of the polysaccharide chain and a pre-aminated bead.

Affine deformation of single polymer chain in poly(methyl methacrylate) films under uniaxial extension observed by scanning near-field optical microscopy

The conformation of single poly(methyl methacrylate) chain in the uniaxially stretched film was observed by the combination of the fluorescence labeling technique and scanning near-field optical microscopy (SNOM). The dimension of the individual probe chain ( M w = 1.99 × 10 6) along the extension axis was evaluated from SNOM image. In the high molecular weight matrix ( M w = 1.89 × 10 6) the average extension ratio at the single chain level coincided with the macroscopic extension ratio. The distribution of the chain conformation was in good agreement with that of the freely jointed chain followed by affine deformation. On the other hand, the probe chain embedded in the low molecular weight matrix ( M w = 1.76 × 10 5) showed the smaller extension than that expected from the affine deformation. This suggests that the conformation of the probe chain is affected by the relaxation of the short surrounding chains through disentanglement.

Conformation of Single Block Copolymer Chain in Two-Dimensional Microphase-Separated Structure Studied by Scanning Near-Field Optical Microscopy

The localization and orientation of the symmetric diblock copolymer chain in a quasi-two-dimensional microphase-separated structure were studied by scanning near-field optical microscopy (SNOM). In the monolayer of poly(isobutyl methacrylate)-block-poly(octadecyl methacrylate) (PiBMA-b-PODMA), the individual PiBMA subchains were directly observed by SNOM, and the center of mass (CM) and orientational angle relative to the phase interface were examined at the single chain level. It was found that the position of the CM and the orientation of the PiBMA subchain in the lamellar structure were dependent on the curvature of the PiBMA/PODMA interface. As the interface was bent toward the objective chain, the block chain preferred the CM position closer to the domain center, and the conformation was strongly oriented perpendicularly to the domain interface. With increase of the curvature, the steric hindrance among the block chain increases, resulting in the stretched conformation.

A theory for heterogeneous states of polymer melts produced by single chain crystal melting.

We consider the polymer physics of a possible molecular mechanism for disentangled melt states observed recently. We find that the route from molten, but only marginally-overlapping single crystals to the equilibrium state must pass over a free-energy barrier that becomes large at high molecular weights. The penalty arises from elastic distortions of the entanglement network as the chains diffuse. A critical molecular weight arises naturally from the competition of elastic distortion and the free-energy of confinement. We calculate the barrier and critical molecular weight at both a simple single-chain level, and at a "one-loop" level of co-operative motion, finding that many-chain effects alter the physics quantitatively, but not qualitatively. Several new experiments are suggested.

Compaction of Single-Chain DNA by Histone-Inspired Nanoparticles

We elaborated a versatile experimental model of chromatin which consists of a single chain of long duplex DNA that interacts with well-defined cationic nanoparticles of various sizes. We found that the DNA compaction by nanoparticles is stepwise and progressive at the single-chain level. It is controlled by the ability of DNA to wrap nanoparticles, which is more efficient for larger particles and, similar to DNA-histone interaction, is optimal at a physiological salt concentration.

Dielectric Control of Counterion-Induced Single-Chain Folding Transition of DNA

In the presence of condensing agents, single chains of giant double-stranded DNA undergo a first-order phase transition between an elongated coil state and a folded compact state. To connect this like-charged attraction phenomenon to counterion condensation, we performed a series of single-chain experiments on aqueous solutions of DNA, where we varied the extent of counterion condensation by varying the relative dielectric constant epsilon(r) from 80 to 170. Single-chain observations of changes in the conformation of giant DNA were performed by transmission electron microscopy and fluorescence microscopy, with tetravalent spermine (SPM(4+)) as a condensing agent. At a fixed dielectric constant, single DNA chains fold into a compact state upon the addition of spermine, whereas at a constant spermine concentration single DNA chains unfold with an increase in epsilon(r). In both cases, the transition is largely discrete at the level of single chains. We found that the critical concentration of spermine necessary to induce the single-chain folding transition increases exponentially as the dielectric constant increases, corresponding to 87-88% of the DNA charge neutralized at the onset of the transition. We also observed that the toroidal morphology of compact DNA partially unfolds when epsilon(r) is increased.