From Single Chains to Aggregates, How Conjugated Polymers Behave in Dilute Solutions

Effect of solvent quality and sidechain architecture on conjugated polymer chain conformation in solution

Conjugated polymers (CPs) are solution-processible for various electronic applications, where solution aggregation and dynamics could impact the morphology in the solid state. Various solvents and solvent mixtures have been used to dissolve and process CPs, but few studies have quantified the effect of solvent quality on the solution behavior of CPs. Herein, we performed static light scattering and small-angle X-ray scattering combined with molecular dynamics (MD) simulation to investigate CP solution behaviors with solvents of varying quality, including poly(3-alkylthiophene) (P3ATs) with various sidechain lengths from -C4H9 to -C12H25, poly[bis(3-dodecyl-2-thienyl)-2,2'-dithiophene-5,5'-diyl] (PQT-12) and poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT-12). We found that chlorobenzene is a better solvent than toluene for various CPs, which was evident from the positive second virial coefficient A2 ranging from 0.3 to 4.7 × 10-3 cm3 mol g-2 towards P3ATs. For P3ATs in non-polar solvents, longer sidechains promote more positive A2, indicating a better polymer-solvent interaction, wherein A2 for toluene increases from -5.9 to 1.4 × 10-3 cm3 mol g-2, and in CB, A2 ranges from 1.0 to 4.7 × 10-3 cm3 mol g-2 when sidechain length increases from -C6H13 to -C12H25. Moreover, PQT-12 and PBTTT-12 have strong aggregation tendencies in all solutions, with an apparent positive A2 (∼0.5 × 10-3 cm3 mol g-2) due to multi-chain aggregates and peculiar chain folding. These solvent-dependent aggregation behaviors can be well correlated to spectroscopy measurement results. Our coarse-grained MD simulation results further suggested that CPs with long, dense, and branched sidechains can achieve enhanced polymer-solvent interaction, and thus enable overall better solution dispersion. This work provides quantitative insights into the solution behavior of conjugated polymers that can guide both the design and process of CPs toward next-generation organic electronics.

Elucidating the structure of donor-acceptor conjugated polymer aggregates in liquid solution

High-spin donor-acceptor conjugated polymers are extensively studied for their potential applications in magnetic and spintronic devices. Inter-chain charge transfer among these high-spin polymers mainly depends on the nature of the local structure of the thin film and π-stacking between the polymer chains. However, the microscopic structural details of high-spin polymeric materials are rarely studied with an atomistic force field, and the molecular-level local structure in the liquid phase remains ambiguous. Here, we have examined the effects of oligomer chain length, side chain, and processing temperature on the organization of the high-spin cyclopentadithiophene-benzobisthiadiazole donor-acceptor conjugated polymer in chloroform solvent. We find that the oligomers display ordered aggregates whose structure depends on their chain length, with an average π-stacking distance of 3.38 ± 0.03 Å (at T = 298 K) in good agreement with the experiment. Also, the oligomers with longer alkyl side chains show better solvation and a shorter π-stacking distance. Furthermore, the clusters grow faster at higher temperature with more ordered aggregation between the oligomer chains.

What is the significance of the chloroform stabilizer C5H10 and its association with MeOH in concentration-dependent polymeric solutions?

The understanding of excitonic transitions associated with polymeric aggregates is fundamental, as such transitions have implications on coherence lengths, coherence numbers and inter- and intra-chain binding parameters. In this context, the investigation of efficient solvents and other ways to control polymer aggregate formation is key for their consolidation as materials for new technologies. In this manuscript, we use Poly(3-hexothiophene) (P3HT) as a probe to investigate the significance of amylene (C5H10) and its association with methanol (MeOH) in both pure and C5H10-stabilized chloroform (CHCl3)-based polymeric solutions. Using the intensity ratio between the first and second vibronic transitions of the P3HT H-aggregates formed, values for their exciton bandwidths and interchain interactions are obtained and correlated with the presence of C5H10 and MeOH as agents determining the CHCl3 quality.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Molecular Structure and Conformational Design of Donor-Acceptor Conjugated Polymers to Enable Predictable Optoelectronic Property

Tuning the optoelectronic properties of donor-acceptor conjugated polymers (D-A CPs) is of great importance in designing various organic optoelectronic devices. However, there remains a critical challenge in precise control of bandgap through synthetic approach, since the chain conformation also affects molecular orbital energy levels. Here, D-A CPs with different acceptor units are explored that show an opposite trend in energy band gaps with the increasing length of oligothiophene donor units. By investigating their chain conformation and molecular orbital energy, it is found that the molecular orbital energy alignment between donor and acceptor units plays a crucial role in dictating the final optical bandgap of D-A CPs. For polymers with staggered orbital energy alignment, the higher HOMO with increasing oligothiophene length leads to a narrowing of the optical bandgap despite decreased chain rigidity. On the other hand, for polymers with sandwiched orbital energy alignment, the increased band gap with increasing oligothiophene length originates from the reduction of bandwidth due to more localized charge density distribution. Thus, this work provides a molecular understanding of the role of backbone building blocks on the chain conformation and bandgaps of D-A CPs for organic optoelectronic devices through the conformation design and segment orbital energy alignment.

Conjugated Polymers in Solution: A Physical Perspective

Excellent progress has been made in the optoelectronic properties of conjugated polymers by controlling solution-state aggregation. However, due to the wide variety and complex structures of conjugated polymers, it is still challenging to fully understand the complex aggregation process and microstructures both in solution and in the solid state. This Perspective focuses on the chain conformations and the aggregation of conjugated polymers in solution. We discuss the factors in detail which affect solution-state aggregation and microstructures from the perspective of polymer physics in solutions, including chemical structures and environmental conditions. Based on the understanding of multiple interactions of conjugated polymers in solution, strategies to regulate solid-state microstructures and obtain high-performance polymer-based devices from solution-state aggregation are summarized.

Competing single-chain folding and multi-chain aggregation pathways control solution-phase aggregate morphology of organic semiconducting polymers

Understanding the solution-phase behaviour of organic semiconducting polymers is important for systematically improving the performance of devices based on solution-processed thin films of these molecules. Conventional polymer theory predicts that polymer conformations become more compact as solvent quality decreases, but recent experiments have shown the high-performance organic-semiconducting polymer P(NDI2OD-T2) to form extended rod-like aggregates much larger than a single chain in poor solvents, with the formation of these extended aggregates correlated with enhanced electron mobility in films deposited from these solutions. We explain the unexpected formation of extended aggregates using a novel coarse-grained simulation model of P(NDI2OD-T2) that we have developed to study the effect of solvent quality on its solution-phase behaviour. In poor solvents, we find that aggregation through only a few monomers gives effectively inseparable chains, leading to the formation of extended structures of partially overlapping chains via non-equilibrium assembly. This behaviour requires that multi-chain aggregation occurs faster than chain folding, which we show is the case for the chain lengths and concentrations shown experimentally to form rod-like aggregates. This kinetically controlled process introduces a dependence of aggregate structure on concentration, chain length, and chain flexibility, which we show is able to reconcile experimental findings and is generalisable to the solution-phase assembly of other semiflexible polymers.

Molecular Encapsulation of Naphthalene Diimide (NDI) Based π-Conjugated Polymers: A Tool for Understanding Photoluminescence

Conjugated polymers are an important class of chromophores for optoelectronic devices. Understanding and controlling their excited state properties, in particular, radiative and non‐radiative recombination processes are among the greatest challenges that must be overcome. We report the synthesis and characterization of a molecularly encapsulated naphthalene diimide‐based polymer, one of the most successfully used motifs, and explore its structural and optical properties. The molecular encapsulation enables a detailed understanding of the effect of interpolymer interactions. We reveal that the non‐encapsulated analogue P(NDI‐2OD‐T) undergoes aggregation enhanced emission; an effect that is suppressed upon encapsulation due to an increasing π‐interchain stacking distance. This suggests that decreasing π‐stacking distances may be an attractive method to enhance the radiative properties of conjugated polymers in contrast to the current paradigm where it is viewed as a source of optical quenching.

Gel Trapping Enables Optical Spectroscopy of Single Solvated Conjugated Polymers in Equilibrium

Single-molecule studies have provided a wealth of insight into the photophysics of conjugated polymers in the solid and desolvated state. Desolvating conjugated chains, e.g., by their embedding in inert solid matrices, invariably leads to chain collapse and the formation of intermolecular aggregates, which have a pronounced effect on their properties. By contrast, the luminescent properties of individual semiconducting polymers in their solvated and thermodynamic state remain largely unexplored. In this paper, we demonstrate a versatile gel trapping technique that enables the chemistry-free immobilization and interrogation of individual conjugated macromolecules, which retain a fully equilibrated conformation by contrast to conventional solid-state immobilization methods. We show how the technique can be used to record full luminescence spectra of single chains, to evaluate their time-resolved fluorescence, and to probe their photodynamics. Finally, we explore how the photophysics of different conjugated polymers is strongly affected by desolvation and chain collapse.

Aggregation and Solubility of a Model Conjugated Donor-Acceptor Polymer

In conjugated polymers, solution-phase structure and aggregation exert a strong influence on device morphology and performance, making understanding solubility crucial for rational design. Using atomistic molecular dynamics (MD) and free-energy sampling algorithms, we examine the aggregation and solubility of the polymer PTB7, studying how side-chain structure can be modified to control aggregation. We demonstrate that free-energy sampling can be used to effectively screen polymer solubility in a variety of solvents but that solubility parameters derived from MD are not predictive. We then study the aggregation of variants of PTB7 including those with linear (octyl), branched (2-ethylhexyl), and cleaved (methyl) side chains, in a selection of explicit solvents and additives. Energetic analysis demonstrates that while side chains do disrupt polymer backbone stacking, solvent exclusion is a critical factor controlling polymer solubility.

Polymer nanoparticle sizes from dynamic light scattering and size exclusion chromatography: the case study of polysilanes

Dynamic light scattering (DLS) and size exclusion chromatography (SEC) are among the most popular methods for determining polymer sizes in solution. Taking dendritic and network polysilanes as a group of least soluble polymer substances, we critically compare and discuss the difference between nanoparticle sizes, obtained by DLS and SEC. Polymer nanoparticles are typically in poor solution conditions below the theta point and are therefore in the globular conformation. The determination of particle sizes in the presence of attractive interactions is not a trivial task. The only possibility to measure, aggregation-free, the true molecular size of polymer nanoparticles in such a solution regime, is to perform the experiment with a dilute solution of globules (below the theta point and above the miscibility line). Based on the results of our polysilane measurements, we come to a conclusion that DLS provides more reliable results than SEC for dilute solutions of globules. General implications for the size measurements of polymer nanoparticles in solution are discussed.

Controlled aggregation of DNA functionalized poly(phenylene-vinylene)

We show that aggregation of DNA-functionalized poly(phenylene-vinylene) can be controlled in solution through ion and DNA interactions.

Modeling intra- and intermolecular correlations for linear and branched polymers using a modified test-chain self-consistent field theory

A modified test-chain self-consistent field theory (SCFT) is presented to study the intra- and intermolecular correlations of linear and branched polymers in various solutions and melts. The key to the test-chain SCFT is to break the the translational symmetry by fixing a monomer at the origin of a coordinate. This theory successfully describes the crossover from self-avoiding walk at short distances to screened random walk at long distances in a semidilute solution or melt. The calculations indicated that branching enhances the swelling of polymers in melts and influences stretching at short distances. The test-chain SCFT calculations show good agreement with experiments and classic polymer theories. We highlight that the theory presented here provides a solution to interpret the polymer conformation and behavior under various conditions within the framework of one theory.

Influence of bile salt on vitamin E derived vesicles involving a surface active ionic liquid and conventional cationic micelle

This study has been actually performed with the aim to develop vitamin E derived vesicles individually from a surface active ionic liquid (1-Hexadecyl-3-Methylimidazolium chloride ([C₁₆mim]Cl)) and a common cationic amphiphile (benzyldimethylhexadecylammonium chloride (BHDC)) and also to investigate their consequent breakdown in presence of bile salt molecule. From this study, it is revealed that the rotational motion of coumarin 153 (C153) molecule is hindered as the vitamin E content is increased in the individual micellar solution of [C₁₆mim]Cl and BHDC. The extent of enhancement in rotational relaxation time is more pronounced in case of [C₁₆mim]Cl-vitamin E solutions than in the BHDC-vitamin E vesicular aggregates which confirms the greater rigidity of the former vesicular system than the later one. Moreover, the effect of bile salt in the vitamin E forming vesicular assemblies have also been unravelled. It is found that the large area occupancy by the steroidal backbone of the bile salt plays a crucial role towards the enlargement of the average surfactant head group area. This results in disintegration of the vesicles composed of vitamin E and consequently, vesicles are transformed into mixed micellar aggregates. From the anisotropy measurement it is found that the rotational motion of C153 is more hindered in the [C₁₆mim]Cl/BHDC-NaCh mixed micelles compared to that inside the individual vesicles. The fluorescence correlation spectroscopic (FCS) study also confirms that the mixed micelles have a more compact structure than that of the [C₁₆mim]Cl-vitamin E and BHDC-vitamin E vesicles. Altogether, the micelle to vesicle transition involving any vitamin and their disruption by bile salt would be an interesting investigation both from the view point of basic colloidal chemistry and towards the generation of new drug delivery vehicle due to their unique microenvironment. Therefore, in future, these systems can be utilised as vehicle for the transport and as well as delivery of drugs and as probable reactor in nanomaterial synthesis.

Different Gelation and Self-Sorting Properties of Two Isomeric Polyamides Owing to the Parallel versus Anti-Parallel Alignment of Backbone Dipoles

Two isomeric bottlebrush polyamides P-1 and A-1, with the same repeating monomer dipole units aligned along the polymer backbone in pseudo-parallel and pseudo-antiparallel fashion, respectively, were synthesized and characterized. Both polymers can form thermoreversible gels with aromatic solvents but P-1 was found to show inferior gelation strength compared with that of A-1. Furthermore, despite their close structural resemblance, a 1:1 mixture of the P-1 and A-1 polymers was shown to exhibit self-sorting in the gel state. Gel formation was found to be a kinetically trapped process through hydrogen bonding, π-π stacking interactions, and side chain interdigitation. The different gelation and self-sorting properties can be explained by the local dipole-dipole interactions originating from the different modes of backbone dipole alignment. In single gel systems, the antiparallel-aligned dipoles in A-1 facilitated a more compact molecular packing owing to the enthalpically more favorable polymer chain association. On the other hand, the parallel-aligned dipoles in P-1 gave rise to a less stable head-to-head packing, which had difficulties to convert to the more stable head-to-tail packing in a kinetically trapped environment. In the mixed gel system, it is the unfavorable hetero-polymer mismatch dipole-dipole interaction that inhibited the mixing of the A-1 and P-1 polymers and led to self-sorting.

Absorption and Quantum Yield of Single Conjugated Polymer Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Molecules

We simultaneously measured the absorption and emission of single conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) molecules in a poly(methyl methacrylate) (PMMA) matrix using near-critical xenon to enhance the photothermal contrast for direct absorption measurements. We directly measured the number of monomers and the quantum yield of single conjugated polymer molecules. Simultaneous absorption and emission measurements provided new insight into the photophysics of single conjugated polymers under optical excitation: quenching in larger molecules is more efficient than in smaller ones. Photoinduced traps and defects formed under prolonged illumination lead to decrease of both polymer fluorescence and absorption signals with the latter declining slower.

Ethylcellulose Colloids Incubated in Dilute Solution

This study revealed, for the first time, that dilute solutions made of a representative series of commercial ethylcellulose (EC; molecular weights 77-305 kDa, provided by the manufacturer) and four distinct organic solvents (α-terpineol, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TPIB), tetrahydrofuran (THF), and benzene) can be used to foster stabilized, nearly monodisperse, nanoscale (pure) polymer colloid, with no isolated chains present. Using combined light-scattering (dynamic light scattering, static form factor, and Zimm/Berry plots) and intrinsic viscosity (Tanglertpaibul-Rao, Huggins, and Kraemer plots) analyses, the structural features of colloidal EC aggregates, ρ = ⟨R_g⟩/⟨R_h⟩ = 0.67-0.83, were first shown to be described rather well by the theory on colloidal spheres (⟨R_g⟩ and ⟨R_h⟩ being the mean radius of gyration and the hydrodynamic radius, respectively). An empirical scaling law relating the intrinsic viscosity to the mean colloid size can thus be established: [η]_H = (1.7 ± 0.2) ×10^-3 ⟨R_h⟩^(2.1±0.3) ([η]_H and ⟨R_h⟩ in units of mL/g and nm, respectively), which may be contrasted with the Zimm model for isolated Gaussian coils, [η]_H ∼ ⟨R_h⟩¹, and the Einstein equation for isolated solid spheres, [η]_H ∼ ⟨R_h⟩⁰. Optical microscopy images of thin films cast from different EC solutions clearly revealed the abundance of micron EC agglomerates, contrary to the uniform thin-film morphology produced from a dilute polystyrene solution, which serves as a reference solution composed of isolated chains. These observations point to new features and applications of EC dispersions.

Aggregation and Gelation of Aromatic Polyamides with Parallel and Anti-parallel Alignment of Molecular Dipole Along the Backbone

The understanding of macromolecular structures and interactions is important but difficult, due to the facts that a macromolecules are of versatile conformations and aggregate states, which vary with environmental conditions and histories. In this work two polyamides with parallel or anti-parallel dipoles along the linear backbone, named as ABAB (parallel) and AABB (anti-parallel) have been studied. By using a combination of methods, the phase behaviors of the polymers during the aggregate and gelation, i.e., the forming or dissociation processes of nuclei and fibril, cluster of fibrils, and cluster-cluster aggregation have been revealed. Such abundant phase behaviors are dominated by the inter-chain interactions, including dispersion, polarity and hydrogen bonding, and correlatd with the solubility parameters of solvents, the temperature, and the polymer concentration. The results of X-ray diffraction and fast-mode dielectric relaxation indicate that AABB possesses more rigid conformation than ABAB, and because of that AABB aggregates are of long fibers while ABAB is of hairy fibril clusters, the gelation concentration in toluene is 1 w/v% for AABB, lower than the 3 w/v% for ABAB.

Threshold-like complexation of conjugated polymers with small molecule acceptors in solution within the neighbor-effect model

In some donor-acceptor blends based on conjugated polymers, a pronounced charge-transfer complex (CTC) forms in the electronic ground state. In contrast to small-molecule donor-acceptor blends, the CTC concentration in polymer:acceptor solution can increase with the acceptor content in a threshold-like way. This threshold-like behavior was earlier attributed to the neighbor effect (NE) in the polymer complexation, i.e., next CTCs are preferentially formed near the existing ones; however, the NE origin is unknown. To address the factors affecting the NE, we record the optical absorption data for blends of the most studied conjugated polymers, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene) (P3HT), with electron acceptors of fluorene series, 1,8-dinitro-9,10-antraquinone (), and 7,7,8,8-tetracyanoquinodimethane () in different solvents, and then analyze the data within the NE model. We have found that the NE depends on the polymer and acceptor molecular skeletons and solvent, while it does not depend on the acceptor electron affinity and polymer concentration. We conclude that the NE operates within a single macromolecule and stems from planarization of the polymer chain involved in the CTC with an acceptor molecule; as a result, the probability of further complexation with the next acceptor molecules at the adjacent repeat units increases. The steric and electronic microscopic mechanisms of NE are discussed.

Probing the Interaction between a DNA Nucleotide (Adenosine-5'-Monophosphate Disodium) and Surface Active Ionic Liquids by Rotational Relaxation Measurement and Fluorescence Correlation Spectroscopy

This article demonstrates the interaction of a deoxyribonucleic acid (DNA) nucleotide, adenosine-5'-monophosphate disodium (AMP) with a cationic surface active ionic liquid (SAIL) 1-dodecyl-3-methylimidazoium chloride (C₁₂mimCl), and an anionic SAIL, 1-butyl-3-methylimidazolium n-octylsulfate ([C₄mim][C₈SO₄]). Dynamic light scattering (DLS) measurements and ¹H NMR (nuclear magnetic resonance) studies indicate that substantial interaction is taking place among the DNA nucleotide (AMP) and the SAILs. Moreover, cryogenic transmission electron microscopy (cryo-TEM) suggests that SAILs containing micellar assemblies are transformed into larger micellar assemblies in the presence of DNA nucleotides. Additionally, the rotational motion of two oppositely charged molecules, rhodamine 6G perchlorate (R6G) and fluorescein sodium salt (Fl-Na), have been monitored in these aggregates. The rotational motion of R6G and Fl-Na differs significantly between SAILs micelles and SAILs-AMP containing larger micellar aggregates. The effect of negatively charged DNA nucleotide (AMP) addition into the cationic and anionic SAILs is more prominent for the cationic charged molecule R6G than that of anionic probe Fl-Na due to the favorable electrostatic interaction between the AMP and cationic R6G. Moreover, the influence of the anionic DNA nucleotide on the cationic and anionic SAIL micelles is monitored through the variation of the lateral diffusion motion of oppositely charged probe molecules (R6G and Fl-Na) inside these aggregates. This variation in diffusion coefficient values also suggests that the interaction pattern of these oppositely charged probes are different within the SAILs-nucleotide containing aggregates. Therefore, both rotational and translational diffusion measurements confirm that the DNA nucleotide (AMP) renders more rigid microenvironment within the micellar solution of SAILs.

5-Methyl Salicylic Acid-Induced Thermo Responsive Reversible Transition in Surface Active Ionic Liquid Assemblies: A Spectroscopic Approach

This article describes the formation of stable unilamellar vesicles involving surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium chloride (C16mimCl), and 5-methyl salicylic acid (5mS). Turbidity, dynamic light scattering (DLS), transmission electron microscopy (TEM), and viscosity measurements suggest that C16mimCl containing micellar aggregates are transformed to elongated micelle and finally into vesicular aggregates with the addition of 5mS. Besides, we have also investigated the photophysical aspects of a hydrophobic (coumarin 153, C153) and a hydrophilic molecule (rhodamine 6G (R6G) perchlorate) during 5mS-induced micelle to vesicle transition. The rotational motion of C153 becomes slower, whereas faster motion is observed for R6G during micelle to vesicle transition. Moreover, the fluorescence correlation spectroscopy (FCS) measurements suggest that the translational diffusion of hydrophobic probe becomes slower in C16mimCl-5mS aggregates in comparison to C16mimCl micelle. However, a reverse trend in translational diffusion motion of hydrophilic molecule has been observed in C16mimCl-5mS aggregates. Moreover, we have also found that the C16mimCl-5mS containing vesicles are transformed into micelles upon enhanced temperature, and it is further confirmed by turbidity, DLS measurements that this transition is a reversible one. Finally, temperature-induced rotational motion of C153 and R6G has been monitored in C16mimCl-5mS aggregates to get a complete scenario regarding the temperature-induced vesicle to micelle transition.

First Steps in the Aggregation Process of Copolymers Based on Thymine Monomers: Characterization by Molecular Dynamics Simulations and Atomic Force Microscopy

Atomistic molecular dynamic simulations were performed to study the structure of isolated VBT-VBA (vinylbenzylthymine-vinylbenzyltriethylammonium chloride) copolymer chains in water at different monomeric species ratios (1:1 and 1:4). The geometric parameters of the structure that the copolymers form in equilibrium together with the basic interactions that stabilize them were determined. Atomic force microscopy (AFM) measurements of dried diluted concentrations of the two copolymers onto highly oriented pyrolytic graphite (HOPG) substrates were carried out to study their aggregation arrangement. The experiments show that both copolymers arrange in fiber-like structures. Comparing the diameters predicted by the simulation results and those obtained by AFM, it can be concluded that individual copolymers arrange in bunches of two chains, stabilized by contra-ions-copolymer interactions for the 1:1 copolymerization ratio at the ionic strength of our samples. In contrast, for the 1:4 system the individual copolymer chains do not aggregate in bunches. These results remark the relevance of the copolymerization ratio and ionic strength of the solvent in the mesoscopic structure of these materials.

Accurate Diffusion Coefficients of Organosoluble Reference Dyes in Organic Media Measured by Dual-Focus Fluorescence Correlation Spectroscopy

Dual-focus fluorescence correlation spectroscopy (2fFCS) is a versatile method to determine accurate diffusion coefficients of fluorescent species in an absolute, reference-free manner. Whereas (either classical or dual-focus) FCS has been employed primarily in the life sciences and thus in aqueous environments, it is increasingly being used in materials chemistry, as well. These measurements are often performed in nonaqueous media such as organic solvents. However, the diffusion coefficients of reference dyes in organic solvents are not readily available. For this reason we determined the translational diffusion coefficients of several commercially available organosoluble fluorescent dyes by means of 2fFCS. The selected dyes and organic solvents span the visible spectrum and a broad range of refractive indices, respectively. The diffusion coefficients can be used as absolute reference values for the calibration of experimental FCS setups, allowing quantitative measurements to be performed. We show that reliable information about the hydrodynamic dimensions of the fluorescent species (including noncommercial compounds) within organic media can be extracted from the 2fFCS data.

Molecular Weight Determination by Counting Molecules

Molecular weight (MW) is one of the most important characteristics of macromolecules. Sometimes, MW cannot be measured correctly by conventional methods like gel permeation chromatography (GPC) due to, for example, aggregation. We propose using single-molecule spectroscopy to measure the average MW simply by counting individual fluorescent molecules embedded in a thin matrix film at known mass concentration. We tested the method on dye molecules, a labeled protein, and the conjugated polymer MEH-PPV. We showed that GPC with polystyrene calibration overestimates the MW of large MEH-PPV molecules by 40 times due to chain aggregation and stiffness. This is a crucial observation for understanding correlations between the conjugated polymer length, photophysics and performances of devices. The method can measure the MW of fluorescent molecules, biological objects, and nanoparticles at ultimately low concentrations and does not need any reference; it is conformation-independent and has no limitations regarding the detected MW range.