Structural and Electronic Characterization of Chemical and Conformational Defects in Conjugated Polymers

Accessing the electronic structure of liquid crystalline semiconductors with bottom-up electronic coarse-graining

Understanding the relationship between multiscale morphology and electronic structure is a grand challenge for semiconducting soft materials. Computational studies aimed at characterizing these relationships require the complex integration of quantum-chemical (QC) calculations, all-atom and coarse-grained (CG) molecular dynamics simulations, and back-mapping approaches. However, these methods pose substantial computational challenges that limit their application to the requisite length scales of soft material morphologies. Here, we demonstrate the bottom-up electronic coarse-graining (ECG) of morphology-dependent electronic structure in the liquid-crystal-forming semiconductor, 2-(4-methoxyphenyl)-7-octyl-benzothienobenzothiophene (BTBT). ECG is applied to construct density functional theory (DFT)-accurate valence band Hamiltonians of the isotropic and smectic liquid crystal (LC) phases using only the CG representation of BTBT. By bypassing the atomistic resolution and its prohibitive computational costs, ECG enables the first calculations of the morphology dependence of the electronic structure of charge carriers across LC phases at the ∼20 nm length scale, with robust statistical sampling. Kinetic Monte Carlo (kMC) simulations reveal a strong morphology dependence on zero-field charge mobility among different LC phases as well as the presence of two-molecule charge carriers that act as traps and hinder charge transport. We leverage these results to further evaluate the feasibility of developing mesoscopic, field-based ECG models in future works. The fully CG approach to electronic property predictions in LC semiconductors opens a new computational direction for designing electronic processes in soft materials at their characteristic length scales.

Implications of relaxation dynamics of collapsed conjugated polymeric nanoparticles for light-harvesting applications

The mechanism of the formation of nanoparticles (collapsed state) from the extended state of polymers and their ultrafast excited state relaxation dynamics are illustrated.

Defects and defect engineering in Soft Matter

Soft matter covers a wide range of materials based on linear or branched polymers, gels and rubbers, amphiphilic (macro)molecules, colloids, and self-assembled structures. These materials have applications in various industries, all highly important for our daily life, and they control all biological functions; therefore, controlling and tailoring their properties is crucial. One way to approach this target is defect engineering, which aims to control defects in the material's structure, and/or to purposely add defects into it to trigger specific functions. While this approach has been a striking success story in crystalline inorganic hard matter, both for mechanical and electronic properties, and has also been applied to organic hard materials, defect engineering is rarely used in soft matter design. In this review, we present a survey on investigations on defects and/or defect engineering in nine classes of soft matter composed of liquid crystals, colloids, linear polymers with moderate degree of branching, hyperbranched polymers and dendrimers, conjugated polymers, polymeric networks, self-assembled amphiphiles and proteins, block copolymers and supramolecular polymers. This overview proposes a promising role of this approach for tuning the properties of soft matter.

The correspondence between the conformational and chromophoric properties of amorphous conjugated polymers in mesoscale condensed systems

For π-conjugated polymers, the notion of spectroscopic units or "chromophores" provides illuminating insights into the experimentally observed absorption/emission spectra and the mechanisms of energy/charge transfer. To date, however, no statistical analysis has revealed a direct correspondence between chromophoric and conformational properties-with the latter being fundamental to polymer semiconductors. Herein, we propose a "persistence length" calculation to re-evaluate chain conformation over a full conjugation length. The mesoscale condensed systems of MEH-PPV and MEH-PPV/C₆₀ hybrid (system size ∼10 × 10 × 10 nm³) are utilized as two prototypical model systems, along with a full range of segmental lengths (2-20-mer) and five lowest singlet excited states to hint at the generality of the features presented. We demonstrate, for the first time, that two properly re-defined conformational factors that characterize chain folding and planarity, respectively, capture excellently the population distribution of chromophores in both systems investigated. In contrast, the conventional strategy of utilizing two adjacent monomer units to characterize (local) chain conformation results in only an inconspicuous correlation between the two, as previously reported. It is further shown that chain folding-and not chain planarity-is more relevant in capturing the associated oscillator strength for the first excited state, where the transient dipole moments are known to align with the chain conformation, although the corresponding excitation energy and exciton size seem relatively unaffected. The observed effects of C₆₀ on the MEH-PPV adsorption spectra also agree with recent experimental trends. Overall, the present findings are expected to aid future multiscale computer simulations and spectroscopy-data interpretations for polymer semiconductors and their hybrid systems.

Chain conformations and phase behavior of conjugated polymers

Conjugated polymers may play an important role in various emerging optoelectronic applications because they combine the chemical versatility of organic molecules and the flexibility, stretchability and toughness of polymers with semiconducting properties. Nevertheless, in order to achieve the full potential of conjugated polymers, a clear description of how their structure, morphology, and macroscopic properties are interrelated is needed. We propose that the starting point for understanding conjugated polymers includes understanding chain conformations and phase behavior. Efforts to predict and measure the persistence length have significantly refined our intuition of the chain stiffness, and have led to predictions of nematic-to-isotropic transitions. Exploring mixing between conjugated polymers and small molecules or other polymers has demonstrated tremendous advancements in attaining the needed properties for various optoelectronic devices. Current efforts continue to refine our knowledge of chain conformations and phase behavior and the factors that influence these properties, thereby providing opportunities for the development of novel optoelectronic materials based on conjugated polymers.

Naphthalene Tetracarboxydiimide-Based n-Type Polymers with Removable Solubility via Thermally Cleavable Side Chains

Multilayer solution-processed devices in organic electronics show the tendency of intermixing of subsequently deposited layers. Here, we synthesize naphthalene tetracarboxydiimide (NDI)-based n-type semiconducting polymers with thermally cleavable side chains which upon removal render the polymer insoluble. Infrared and photoelectron spectroscopy were performed to investigate the pyrolysis process. Characterization of organic field-effect transistors provides insight into charge transport. After the pyrolysis homogeneous films could be produced which are insoluble in the primary solvent. By varying curing temperature and time we show that these process parameters govern the amount of side chains in the film and influence the device performance.

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.

Coarse-Grained Simulations of the Effects of Chain Length, Solvent Quality, and Chemical Defects on the Solution-Phase Morphology of MEH-PPV Conjugated Polymers

A mesoscale coarse-grained model of the conjugated polymer poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in implicit solvent is developed. The model is parametrized to reproduce the local structure and dynamics of an atomistic simulation model and accounts for the effects of solvent quality and saturation chemical defects on the polymer structure. Polymers with defect concentrations of 0 to 10 % are simulated using Langevin dynamics in tetrahydrofuran (THF) and in a model poor solvent for chain lengths and solution concentrations used experimentally. The polymer chains are extended in THF and collapse into compact structures in the poor solvent. The radius of gyration decreases with defect content in THF and agrees quantitatively with experiment. The structures formed in poor solvent by chains with 300 monomer units change from toroidal to cylindrical with increasing defect content, while chains containing 1000 monomers form cylinders regardless of defect content. These results have implications for energy transfer in MEH-PPV.

Coherent Excitation Transfer Driven by Torsional Dynamics: a Model Hamiltonian for PPV Type Systems

A model Hamiltonian is proposed for systems of poly-phenylene-vinylene (PPV) type, which describes excitation energy transfer (EET) in the presence of torsional and bond-length-alternation coordinates that couple neighboring fragments. This model is employed to describe coherent EET, starting from a partially delocalized exciton state which is initially separated from the rest of the chain by a defect due to a pronounced torsional displacement at one of the inter-fragment junctions. EET is found to be driven by the restoring force of the torsions towards the planar geometry. Quantum dynamical calculations are carried out for a minimal five-site model, using the multiconfiguration time-dependent Hartree (MCTDH) method.

Prediction of Excitation Energies for Conjugated Oligomers and Polymers from Time-Dependent Density Functional Theory

With technological advances, light-emitting conjugated oligomers and polymers have become competitive candidates in the commercial market of light-emitting diodes for display and other technologies, due to the ultralow cost, light weight, and flexibility. Prediction of excitation energies of these systems plays a crucial role in the understanding of their optical properties and device design. In this review article, we discuss the calculation of excitation energies with time-dependent density functional theory, which is one of the most successful methods in the investigation of the dynamical response of molecular systems to external perturbation, owing to its high computational efficiency.

π-conjugation and conformation in a semiconducting polymer: small angle x-ray scattering study

Small angle x-ray scattering (SAXS) in a poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) solution has shown the important role of π-electron conjugation in controlling the chain conformation and assembly. By increasing the extent of conjugation from 30 to 100%, the persistence length (l(p)) increases from 20 to 66 Å. Moreover, a pronounced second peak in the pair distribution function has been observed in a fully conjugated chain, at larger length scales. This feature indicates that the chain segments tend to self-assemble as the conjugation along the chain increases. Xylene enhances the rigidity of the PPV backbone to yield extended structures, while tetrahydrofuran solvates the side groups to form compact coils in which the l(p) is much shorter.

Electron and hole transport to trap groups at the ends of conjugated polyfluorenes

Polyfluorenes (pF) were synthesized having anthraquinone (AQ) or naphtylimide (NI) end caps that trap electrons or di- p-tolylaminophenyl (APT2) caps that trap holes. The average lengths of the pF chains in these molecules varied from 7 to 30 nm. End capping was found not to be complete in these molecules so that some were without caps. Electrons or holes were injected into these polymers in solution by pulse radiolysis. Following attachment, the charges migrated to the end cap traps in times near 2 ns in pF12AQ or 5 ns in pF35NI. From these observations, electron mobilities for transport along single chains to the end caps in THF solution were determined to be smaller by a factor of 100 than those observed by microwave conductivity. Despite this, the mobilities were sufficiently large to provide encouragement to the use of such single chains in solar photovoltaics. Most charges were observed to transport over substantial distances in these polymers, but 23, 18, and 37% of the charges attached to pFNI, pFAQ, and pFAPT2, respectively, were trapped in the pF chains and decayed by slower bimolecular reactions. For pFAQ and pFAPT2, all of the trapped charges were accounted for by estimates of the fraction of molecules having no end cap traps. For pF35NI, 23% of the attached electrons were found to be trapped in the chains, but only 4% of chains were expected to have no end caps. This could indicate some trapping by kinks or other defects but may just reflect uncertainties in the capping of this long polymer. When the charges reach the trap groups, their spectra have no features of pF(*-) or pF(*+), nor do the principal bands of the trapped ions resemble spectra of the radical ions of isolated trap molecules. The optical absorption spectra are rather dominated by new bands identified as charge-transfer transitions, which probably reinject electrons or holes into the pF chains. The energies of those bands correlate well with measured redox potentials.

Size-Dependent Spectroscopic Properties of Conjugated Polymer Nanoparticles

This paper is focused on how the spectroscopic properties of conjugated polymers evolve in the size range between single polymer chains and the bulk material. The measurements used single-particle spectroscopy techniques and include both static and dynamic measurements. The main observation of this work is that the spectroscopic properties of MEH-PPV evolve rapidly as a function of nanoparticle size and achieve bulk-like properties for nanoparticles greater than 10 nm in size. Nanoparticles were assembled by a reprecipitation technique and characterized by fluorescence emission spectroscopy. The physical origin of the size-dependent spectroscopic properties is assigned to the distance dependence of four main processes: electronic energy transfer between blue and red sites, triplet-triplet annihilation, singlet exciton quenching by triplets, and singlet exciton quenching by hole polarons.

A Class of Nonplanar Conjugated Compounds with Aggregation-Induced Emission: Structural and Optical Properties of 2,5-Diphenyl-1,4-distyrylbenzene Derivatives with All Cis Double Bonds

We have studied the structural and optical properties of four 2,5-diphenyl-1,4-distyrylbenzene derivatives with all cis double bonds. These compounds belong to a class of nonplanar conjugated compounds possessing a typical Aggregation-Induced Emission (AIE) property that has no emission in solution but intense emission in crystal. The four molecules are packed in different stacking modes with different intermolecular interactions, resulting in different crystalline state photoluminescence (PL) efficiency. The torsional molecular configuration increases the intermolecular distances effectively in the crystalline state, which decreases the difference of the optical properties from the frozen isolated molecules to the crystalline state. The Stokes shifts of these compounds are very large and the PL spectra have only one broad emission band with poor structure, due to the relatively large configuration difference between the ground state and the first singlet excited state, and the abundant vibration energy levels of the torsional molecule with changeable conformation.

Quantum Simulation of Solution Phase Intramolecular Electron Transfer Rates in Betaine-30

Mixed quantum-classical atomistic simulations have been carried out to investigate the mechanistic details of excited state intramolecular electron transfer in a betaine-30 molecule in acetonitrile. The key electronic degrees of freedom of the solute molecule are treated quantum mechanically using the semiempirical Pariser-Parr-Pople Hamiltonian, including the solvent influence on electronic structure. The intramolecular vibrational modes are also treated explicitly at a quantum level, with the remaining elements treated classically using empirical potentials. The electron-transfer rate, corresponding to S1 --> S0 relaxation, is evaluated via time-dependent perturbation theory with the explicit inclusion of the dynamics of solvation and intramolecular conformation. The calculations reveal that, while solvation dynamics is critical to the rate, the intramolecular torsional dynamics also plays an important role. The importance of the use of multiple high-frequency quantum modes is also discussed.

Chromophores in phenylenevinylene-based conjugated polymers: Role of conformational kinks and chemical defects

The influence of chemical defects and conformational kinks on the nature of the lowest electronic excitations in phenylenevinylene-based polymers is assessed at the semiempirical quantum-chemical level. The amount of excited-state localization and the amplitude of through-space (Coulomb-like) versus through-bond (charge-transfer-like) interactions have been quantified by comparing the results provided by excitonic and supermolecular models. While excitation delocalization among conjugated segments delineated by the defects occurs in the acceptor configuration, self-confinement on individual chromophores follows from geometric relaxation in the excited-state donor configuration. The extent of excited-state localization is found to be sensitive to both the nature of the defect and the length of the conjugated chains. Implications for resonant energy transfer along conjugated polymer chains are discussed.

Single Molecule Nanoparticles of the Conjugated Polymer MEH−PPV, Preparation and Characterization by Near-Field Scanning Optical Microscopy

We have developed a straightforward method for producing a stable, aqueous suspension of hydrophobic, fluorescent pi-conjugated polymer nanoparticles consisting primarily of individual conjugated polymer molecules. Features of the method are the facile preparation, purity, unique optical properties, and small size (approximately 5-10 nm) of the resulting nanoparticles. The results of TEM, scanning force microscopy, and near-field scanning optical microscopy of particles cast from the suspension indicate that the particles are single conjugated polymer molecules. The NSOM results yield estimates of the optical cross-sections of individual conjugated polymer molecules. The UV-vis absorption spectra of the nanoparticle suspensions indicate a reduction in conjugation length attributed to deformations of the polymer backbone. Fluorescence spectra of the aqueous nanoparticle suspensions indicate interactions between segments of the polymer chain and intramolecular energy transfer.

Measurement and dynamics of the spatial distribution of an electron localized at a metal–dielectric interface

The ability of time- and angle-resolved two-photon photoemission to estimate the size distribution of electron localization in the plane of a metal-adsorbate interface is discussed. It is shown that the width of angular distribution of the photoelectric current is inversely proportional to the electron localization size within the most common approximations in the description of image potential states. The localization of the n=1 image potential state for two monolayers of butyronitrile on Ag(111) is used as an example. For the delocalized n=1 state, the shape of the signal amplitude as a function of momentum parallel to the surface changes rapidly with time, indicating efficient intraband relaxation on a 100 fs time scale. For the localized state, little change was observed. The latter is related to the constant size distribution of electron localization, which is estimated to be a Gaussian with a 15+/-4 A full width at half maximum in the plane of the interface. A simple model was used to study the effect of a weak localization potential on the overall width of the angular distribution of the photoemitted electrons, which exhibited little sensitivity to the details of the potential. This substantiates the validity of the localization size estimate.