Single Conjugated Polymer Chains Research Articles

Stochastic Electrical Detection of Single Ion-Gated Semiconducting Polymers.

Semiconducting polymer chains constitute the building blocks for a wide range of electronic materials and devices. However, most of their electrical characteristics at the single-molecule level have received little attention. Elucidating these properties can help understanding performance limits and enable new applications. Here, coupled ionic-electronic charge transport is exploited to measure the quasi-1D electrical current through long single conjugated polymer chains as they form transient contacts with electrodes separated by ≈10nm. Fluctuations between internal conformations of the individual polymers are resolved as abrupt, multilevel switches in the electrical current. This behavior is consistent with the theoretical simulations based on the worm-like-chain (WLC) model for semiflexible polymers. In addition to probing the intrinsic properties of single semiconducting polymer chains, the results provide an unprecedented window into the dynamics of random-coil polymers and enable the use of semiconducting polymers as electrical labels for single-molecule (bio)sensing assays.

Electronic energy transfer in single conjugated polymer molecules revealed by phase-modulated pulse-pair controlled single molecule spectroscopy

Detailed understanding of the electronic energy transfer dynamics in conjugated polymer molecules and their conformation dependence is central for improving the photophysical properties as well as the performance of devices based on conjugated polymers. In this work, we demonstrate simultaneous measurement of the absorption and emission sites in single conjugated polymer poly[2,7-(9,9-dioctyluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) molecules based on polarization-resolved confocal fluorescence microscopy with excitation of phase-modulated ultrashort pulse pairs. The evolution of absorbing chromophores can be derived by modulating the relative phase between ultrashort pulse pairs and extracting modulation information in phase-dependent fluorescence. Meanwhile, the emitting chromophore can be measured by polarization-resolved emission. Simultaneous absorption and emission measurements give new insights into the evolution of energy transfer pathways in individual conjugated polymer molecules. The results suggest that the conformation of single conjugated polymer chains can be influenced by solvents. Single PFO-DBT conjugated molecules spin-cast from toluene solution have relatively fixed absorption and emission dipole moments. In contrast, single conjugated polymer molecules prepared with chloroform show multichromophore behavior that is responsible for distribution of absorption and emission in a single chain. The proposed scheme paves the way for further understanding of conformation dependent photophysical properties and the possible role of quantum effects in the energy transfer pathway in both natural and artificial light harvesting systems in the nanoscale.

Population dynamics of multiple triplet excitons revealed from time-dependent fluorescence quenching of single conjugated polymer chains

The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency.

Resolving Anomalous Heavy Atom Effects from Discrete Triplet Mediated Photochemistry Events on Single Conjugated Polymer Chains

Spin-forbidden triplet excited states of conjugated polymers have important ramifications for material performance and stability, yet triplet processes are difficult to understand and control at th...

Solution for the Trivial Crossing Problem in Surface Hopping Simulations by the Classification on Excited States

In this article, we study excited-state relaxation dynamics in single conjugated polymer chains with surface hopping simulations and propose a solution for the encountered “trivial crossing problem”. By analyzing the transition density matrices, we find that all excited states can be divided into two types. The surface hopping simulations show that the two types of excited states do not interact each other (the nonadiabatic couplings between them are negligible), indicating that the trivial crossings only occur between different types of excited state. The two types of excited state separately span two uncoupled subspaces, and we can let the system evolve only in one of the subspaces. In such a way, the trivial crossings between different types of excited states are completely eliminated. Moreover, this method greatly reduces the computational cost of surface hopping simulations.

Real-time observation of conformational switching in single conjugated polymer chains.

Conjugated polymers (CPs) are an important class of organic semiconductors that combine novel optoelectronic properties with simple processing from organic solvents. It is important to study CP conformation in solution to understand the physics of these materials and because it affects the properties of solution-processed films. Single-molecule techniques are unique in their ability to extract information on a chain-to-chain basis; however, in the context of CPs, technical challenges have limited their general application to host matrices or semiliquid environments that constrain the conformational dynamics of the polymer. We introduce a conceptually different methodology that enables measurements in organic solvents using the single-end anchoring of polymer chains to avoid diffusion while preserving polymer flexibility. We explore the effect of organic solvents and show that, in addition to chain-to-chain conformational heterogeneity, collapsed and extended polymer segments can coexist within the same chain. The technique enables real-time solvent-exchange measurements, which show that anchored CP chains respond to sudden changes in solvent conditions on a subsecond time scale. Our results give an unprecedented glimpse into the mechanism of solvent-induced reorganization of CPs and can be expected to lead to a new range of techniques to investigate and conformationally manipulate CPs.

Crosslinked polymer nanoparticles containing single conjugated polymer chains

Conjugated polymer nanoparticles are widely used in fluorescent labeling and sensing, as they have mean radii between 5 and 100 nm, narrow size dispersion, high brightness, and are photochemically stable, allowing single particle detection with high spatial and temporal resolution. Highly crosslinked polymers formed by linking individual chains through covalent bonds yield high-strength rigid materials capable of withstanding dissolution by organic solvents. Hence, the combination of crosslinked polymers and conjugated polymers in a nanoparticulated material presents the possibility of interesting applications that require the combined properties of constituent polymers and nanosized dimension. In the present work, F8BT@pEGDMA nanoparticles composed of poly(ethylene glycol dimethacrylate) (pEGDMA; a crosslinked polymer) and containing the commercial conjugated polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) were synthesized and characterized. Microemulsion polymerization was applied to produce F8BT@pEDGMA particles with nanosized dimensions in a ∼25% yield. Photophysical and size distribution properties of F8BT@pEDGMA nanoparticles were evaluated by various methods, in particular single particle fluorescence microscopy techniques. The results demonstrate that the crosslinking/polymerization process imparts structural rigidity to the F8BT@pEDGMA particles by providing resistance against dissolution/disintegration in organic solvents. The synthesized fluorescent crosslinked nanoparticles contain (for the most part) single F8BT chains and can be detected at the single particle level, using fluorescence microscopy, which bodes well for their potential application as molecularly imprinted polymer fluorescent nanosensors with high spatial and temporal resolution.

Understanding the Structural Evolution of Single Conjugated Polymer Chain Conformers.

Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure–function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known “Missing Mode Effect” (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting “blue” MEH-PPV form exhibits PL maxima at ~18,200 cm−1 with characteristic MIME progression intervals of ~1200–1350 cm−1, whereas the lower energy emitting “red” form peaks at ~17,100 cm−1 with intervals in the range of ~1350–1450 cm−1. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C–H wag of the MEH-PPV vinylene group (~966 cm−1) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)—indicating lower intra-chain order—lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities.

Excitonic linewidth of organic quantum wires generated in reduced dimensionality matrices.

Luminescent organic quantum wires are generated in diacetylene crystalline ultra-thin films grown on orientation-inducing surfaces obtained by poly-tetrafluoroethylene (teflon) deposition. The films are characterized by atomic force microscopy showing that quasi-two-dimensional surroundings are achieved. In this particular environment, pure dephasing processes still determine the wires' homogeneous emission widths, measured using micro-photoluminescence. Coherence times that are slightly shorter in the films also exhibit a distinctive temperature dependence. A model inspired by semiconductor physics for exciton-phonon coupling accounts for the observed behaviour and evidences the role of matrix dimensionality on the coherence properties.

Chromophore Bending Controls Fluorescence Lifetime in Single Conjugated Polymer Chains.

Single-molecule spectroscopy of conjugated polymers offers unique insight into the interplay between the spatial arrangement of monomer units-twisting and bending-and the characteristics of the primary excitonic photoexcitation, provided that a single conjugated segment can be isolated. β-phase polyfluorene constitutes an ideal model to study variations in intermonomeric coupling, determined by nanoscale molecular shape, on the fundamental optical transition. If structural relaxation in the excited state is weak, exciton self-trapping occurs stochastically along the conjugated segment. Bending of the π-system is then revealed by a reduction in single-photon polarization anisotropy, correlating directly with increased fluorescence lifetime. Strong relaxation raises emission anisotropy because of deterministic exciton localization, decelerating fluorescence decay due to a decrease in exciton coherence length.

Conformational Effect on Energy Transfer in Single Polythiophene Chains

Herein we describe the use of regioregular (rr-) and regiorandom (rra-) P3HT as models to study energy transfer in ordered and disordered single conjugated polymer chains. Single molecule fluorescence spectra and excitation/emission polarization measurements were compared with a Förster resonance energy transfer (FRET) model simulation. An increase in the mean single chain polarization anisotropy from excitation to emission was observed for both rr- and rra-P3HT. The peak emission wavelengths of rr-P3HT were at substantially lower energies than those of rra-P3HT. A simulation based on FRET in single polymer chain conformations successfully reproduced the experimental observations. These studies showed that ordered conformations facilitated efficient energy transfer to a small number of low-energy sites compared to disordered conformations. As a result, the histograms of spectral peak wavelengths for ordered conformations were centered at much lower energies than those obtained for disordered conformations. Collectively, these experimental and simulated results provide the basis for quantitatively describing energy transfer in an important class of conjugated polymers commonly used in a variety of organic electronics applications.

Conformation and Energy Transfer in Single Conjugated Polymers

In contrast to the detailed understanding of inorganic materials, researchers lack a comprehensive view of how the properties of bulk organic materials arise from their individual components. For conjugated polymers to eventually serve as low cost semiconductor layers in electronic devices, researchers need to better understand their functionality. For organics, traditional materials science measurements tend to destroy the species of interest, especially at low concentrations. However, fluorescence continues to be a remarkably flexible, relatively noninvasive tool for probing the properties of individual molecules and allows researchers to carry out a broad range of experiments based on a relatively simple concept. In addition, the sensitivity of single-molecule spectroscopy allows researchers to see the properties of an individual component that would be masked in the bulk phase. In this Account, we examine several photophysical properties of different conjugated polymers using single-molecule spectroscopy. In these experiments, we probed the relationship between the conformation of single conjugated polymer chains and the distance scale and efficiency of energy transfer within the polymer. Recent studies used polarization anisotropy measurements on single polymer chains to study chain folding following spin-casting from solution. This Account summarizes the effects of monomer regioregularity and backbone rigidity, by comparing a regiorandom phenylene vinylene (MEH-PPV) with both a regiorandom and regioregular thiophene (P3HT). Synthesis of novel polymers allowed us to explore the role of different conformation-directing inclusions in a PPV backbone. We showed that these inclusions control the conformation of individual chains and that molecular dynamics can predict these structural effects. In situ solvent vapor annealing studies explored the dynamics of polymer chains as well as the effect of solvent evaporation on the structural equilibrium of the polymer. We observed that a slower rate of solvent evaporation results in a narrow population of highly ordered polymer chains. These highly ordered single chains serve as a model system to probe the effect of conformation on energy transfer following excitation in single MEH-PPV polymer chains in two distinct experiments. In the first, we correlated the anisotropy of the fluorescence emission of individual chains with the anisotropy of their fluorescence excitation. Using this data, we derived a model for energy transfer in a conjugated polymer, simulating chromophores along a chain, coupled via Förster energy transfer. In the second experiment, super-resolution measurements demonstrated the ability of single-molecule spectroscopy to directly visualize energy transfer along a polymer chain embedded in a model device environment. A capacitive device allowed for controlled localization of hole polarons onto the polymer chain. These positive charges subsequently quenched local excitations, providing insight into the range of energy transfer in these single polymer molecules. As researchers continue to characterize conjugated polymer films and develop methods for creating multichain systems, single-molecule techniques will provide a greater understanding of how polymer morphology influences interchain interactions and will lead to a richer description of the electronic properties of bulk conjugated polymer films.

A multiconfigurational time-dependent Hartree-Fock method for excited electronic states. II. Coulomb interaction effects in single conjugated polymer chains

Conjugated polymers have attracted considerable attention in the last few decades due to their potential for optoelectronic applications. A key step that needs optimisation is charge carrier separation following photoexcitation. To understand better the dynamics of the exciton prior to charge separation, we have performed simulations of the formation and dynamics of localised excitations in single conjugated polymer strands. We use a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. We show the relaxation of electron-hole pairs to form excitons and oppositely charged polaron pairs and discuss the modifications to the relaxation process predicted by the inclusion of the Coulomb interaction between the carriers. The issue of charge photogeneration in conjugated polymers in dilute solution is also addressed.

Highly Ordered Single Conjugated Polymer Chain Rod Morphologies

We have reexamined the fluorescence polarization anisotropy of single polymer chains of the prototypical conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) isolated in a poly(methyl methacrylate) (PMMA) matrix employing improved synthetic samples that contain a much smaller number of tetrahedral chemical defects per chain. The new measurements reveal a much larger fraction of highly anisotropic MEH-PPV chains, with >70% of the chains exhibiting polarization anisotropy values falling in the range of 0.6-0.9. High anisotropy is strong evidence for a rod-shaped conformation. A comparison of the experimental results with coarse grain, beads on a chain simulations reveals that simulations with the usual bead-bead pairwise additive potentials cannot reproduce the observed large fraction of high polarization values. Apparently, this type of potential lacks some yet to be identified molecular feature that is necessary to accurately simulate the experimental results.

Dependence of Single-photon Emission from Single Conjugated Polymer Chains on Their Spatial Size as Determined by Photon-correlation Measurements in Fluid Solution

Abstract Photon-correlation measurements were applied to single chains of poly[2-methoxy-5-(2-ethylhexyloxy)-p-phenylene vinylene] of various molecular weights in a fluid solvent in order to investigate the relationship between the probability of single-photon emission and spatial size of the chains. We clearly observed that polymer chains with faster translational diffusion time exhibited a high probability of single-photon emission. This result indicates that the photon antibunching behavior of single conjugated polymer chains is strongly correlated with their spatial size, and that the polymer chains can be made to behave as single-photon sources by controlling the size.

Electrothermal Manipulation of Individual Chromophores in Single Conjugated Polymer Chains: Controlling Intrachain FRET, Blinking, and Spectral Diffusion

Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized pi-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices.

Factors Controlling Hole Injection in Single Conjugated Polymer Molecules

New insights on the molecular level details of the recently reported light-assisted injection of positive charge into single conjugated polymer chains are reported. Extensive new fluorescence-voltage single molecule spectroscopy (FV-SMS) measurements were performed on single chains of the archetypical conjugated polymer MEH-PPV embedded in a capacitor device to complement previous studies of the influence of the bias scan rate and optical excitation intensity. The use of a vacuum microscope allowed for the precise control of the device atmosphere, demonstrating the influence of triplet states in the MEH-PPV on the FV-SMS modulation. For identical device conditions, little variation was observed in the rate and yield of charging from molecule to molecule. Through the use of thicker supporting matrices and insulating polymer "blocking layers", it was determined that good electrical contact between the hole transport layers and the single molecules was necessary for charge injection. The results demonstrate the complexity of charge transfer processes at the interface of organic semiconductors and highlight the ability of single molecule methods to advance the understanding of such processes at the nanoscale.

Single-photon emission from a single nanoparticle consisting of a single conjugated polymer chain

Single-photon behavior in the emission from a single nanoparticle consisting of a single poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV, molecular weight: 2,600,000 amu) chain has been investigated. Photon correlation measurements of a number of single nanoparticles revealed that the probability of single-photon emission from the single nanoparticles clearly increases, compared with that of single chains embedded in host polymer matrices, poly(methyl methacrylate) (PMMA) and polystyrene (PS). The result suggested that even a single MEH-PPV chain with high molecular weight, which does not exhibit single-photon emission in PMMA and PS, is forced to exhibit single-photon emission by adopting “compact” chain conformation like nanoparticle. Efficient exciton migration and exciton annihilation processes in the compact chain conformation most probably result in the efficient single-photon emission from the single nanoparticle. The present results indicate that multi-quantum systems consisting of a large number of chromophores, such as the MEH-PPV, can be made to behave as single-photon sources by appropriately controlling their size.

Light-assisted deep-trapping of holes in conjugated polymers.

The injection of positive charge carriers (holes) into a single conjugated polymer chain was observed to be light-assisted. This effect may underlie critical, poorly understood organic electronic device phenomena such as the build-up of functional deeply trapped charge layers in polymer light emitting diodes. The charging/discharging dynamics were investigated indirectly by a variety of single molecule electro-optical spectroscopic techniques, including an "image-capture" approach.

Detailed Investigation of Light Induced Charge Injection into a Single Conjugated Polymer Chain

Detailed single molecule spectroscopic results are presented for the recently reported oxidation of single-chain conjugated polymers of MEH-PPV at the HTL/insulator interface of an MIS device. The charge injection process was determined to be light dominated with no charging occurring in the dark at any applied bias. The nature of the optical preparation of the injected charge is explored by a range of single molecule fluorescence voltage modulation techniques. The observed charging dynamics are highly consistent with a cooperative charging effect. It is also observed that analogous charging occurs for the conjugated polymer F8BT.