Conjugated Polymer Aggregates Research Articles

A skeletal randomization strategy for high-performance quinoidal-aromatic polymers.

Enhancing the solution-processability of conjugated polymers (CPs) without diminishing their thin-film crystallinity is crucial for optimizing charge transport in organic field-effect transistors (OFETs). However, this presents a classic "Goldilocks zone" dilemma, as conventional solubility-tuning methods for CPs typically yield an inverse correlation between solubility and crystallinity. To address this fundamental issue, a straightforward skeletal randomization strategy is implemented to construct a quinoid-donor conjugated polymer, PA4T-Ra, that contains para-azaquinodimethane (p-AQM) and oligothiophenes as repeat units. A systematic study is conducted to contrast its properties against polymer homologues constructed following conventional solubility-tuning strategies. An unusually concurrent improvement of solubility and crystallinity is realized in the random polymer PA4T-Ra, which shows moderate polymer chain aggregation, the highest crystallinity and the least lattice disorder. Consequently, PA4T-Ra-based OFETs, fabricated under ambient air conditions, deliver an excellent hole mobility of 3.11 cm2 V-1 s-1, which is about 30 times higher than that of the other homologues and ranks among the highest for quinoidal CPs. These findings debunk the prevalent assumption that a random polymer backbone sequence results in decreased crystallinity. The considerable advantages of the skeletal randomization strategy illuminate new possibilities for the control of polymer aggregation and future design of high-performance CPs, potentially accelerating the development and commercialization of 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.

Fluorination-Induced Conversion of the Quasi- to the Normal Interchain Packing in Push–Pull Conjugated Polymer Aggregates

Fluorination-Induced Conversion of the Quasi- to the Normal Interchain Packing in Push–Pull Conjugated Polymer Aggregates

Nearly Isotropic Conjugated Polymer Aggregates with Efficient Local Exciton Diffusion

To bridge the gap between single-molecule and bulk thin film studies of organic semiconductors and understand the influence of molecular structure on photophysical properties across scales, we prepared and characterized aggregates containing a few to more than a thousand single chains via well-controlled solvent vapor swelling of an inert polymeric matrix containing the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). The single chains were initially immobilized in conformations with few interchain contacts, which can support exciton funneling on the single-molecule level. We show that aggregates prepared via solvent swelling and templated on such molecules result in largely isotropic aggregates. Analysis of aggregate polarization anisotropy, photoluminescence transient decay, and photoluminescence spectra as a function of aggregate size suggests that in these aggregates exciton diffusion is locally enhanced upon initial aggregation but does not reach extended length...

Fluorescent conjugated polymer nanoparticles and aggregates based on rapid precipitation and self-assembled π-conjugated systems

Conjugated copolymers containing fluorene and diphenylamine moieties with octyl and triethylene glycol side chains have been synthesized via Suzuki-Miyaura cross-coupling reaction and their structures have been characterized. Conjugated polymer nanoparticles and self-organization of the polymers have been prepared by rapid precipitation and solvent diffusion methods, respectively. The conjugated polymers exhibited well-defined spherical amorphous structures and crystalline microspheres with a diameter ranging from several nanometer to micrometer. Self-assembled conjugated polymer aggregates were obtained in a selective good solvent/poor solvent mixture. Alternating conjugated polymers are generally difficult to assemble into well-defined spheres due to their rigid and planar backbones, however, alternating polymers containing dioctyl and triethylene glycol side chains tends to form microspheres or sheet-like structures depending on the different solvent mixtures. The emission maximum of the dispersed polymers in water was significantly red-shifted with a dramatic reduction in the photoluminescence quantum yield. In addition, the emission maximum of the self-assembled copolymers was barely red shifted with a small reduction in the photoluminescence quantum yield.

Role of Disorder in the Extent of Interchain Delocalization and Polaron Generation in Polythiophene Crystalline Domains.

To understand how disorder within conjugated polymer aggregates influences the polaron generation process, we investigated poly(3-hexylthiophene) (P3HT) and a congeneric random copolymer incorporating 33 mol % substituent-free thiophene units (RP33). Steady-state absorption and fluorescence spectra showed that increasing the intrachain torsional disorder in aggregates increases the energy and breadth of the density of states (DOS). By extracting polaron dynamics in the transient absorption spectra, we found that an activation energy barrier of 0.05 eV is imposed on the charge separation process in P3HT, whereas that in RP33 is essentially barrierless. We also found that a significant amount of excitons in P3HT are deactivated by traps, while no trapped excitons are generated in RP33. This efficient polaron generation in RP33 was attributed to the excess energy and enhanced interchain delocalization of precursor states provided by the intrachain torsional disorder and the close-packing structure in the absence of hexyl substituents.

In Situ Optical Imaging of the Growth of Conjugated Polymer Aggregates.

Understanding the mechanisms that contribute to conjugated polymer aggregate formation and growth may yield enhanced control of aggregate morphology and functional properties on the mesoscopic scale. In situ optical imaging of the growth of MEH-PPV aggregates in real time in controlled swollen films shows that growth occurs through multiple mechanisms and is more complex than previously described. Direct evidence is provided for both Ostwald ripening and aggregate coalescence as operative modes of aggregate growth in solvent swollen films. These growth mechanisms have a distinct and strong impact on the evolution of morphological order of growing aggregates: while Ostwald ripening allows preservation of highly ordered morphology, aggregate coalescence occurs with no preferential orientation, leading to attenuation in degree of ordering.

In Situ Optical Imaging of the Growth of Conjugated Polymer Aggregates

AbstractUnderstanding the mechanisms that contribute to conjugated polymer aggregate formation and growth may yield enhanced control of aggregate morphology and functional properties on the mesoscopic scale. In situ optical imaging of the growth of MEH‐PPV aggregates in real time in controlled swollen films shows that growth occurs through multiple mechanisms and is more complex than previously described. Direct evidence is provided for both Ostwald ripening and aggregate coalescence as operative modes of aggregate growth in solvent swollen films. These growth mechanisms have a distinct and strong impact on the evolution of morphological order of growing aggregates: while Ostwald ripening allows preservation of highly ordered morphology, aggregate coalescence occurs with no preferential orientation, leading to attenuation in degree of ordering.

Switching between H- and J-type electronic coupling in single conjugated polymer aggregates

The aggregation of conjugated polymers and electronic coupling of chromophores play a central role in the fundamental understanding of light and charge generation processes. Here we report that the predominant coupling in isolated aggregates of conjugated polymers can be switched reversibly between H-type and J-type coupling by partially swelling and drying the aggregates. Aggregation is identified by shifts in photoluminescence energy, changes in vibronic peak ratio, and photoluminescence lifetime. This experiment unravels the internal electronic structure of the aggregate and highlights the importance of the drying process in the final spectroscopic properties. The electronic coupling after drying is tuned between H-type and J-type by changing the side chains of the conjugated polymer, but can also be entirely suppressed. The types of electronic coupling correlate with chain morphology, which is quantified by excitation polarization spectroscopy and the efficiency of interchromophoric energy transfer that is revealed by the degree of single-photon emission.

Highly Anisotropic Conjugated Polymer Aggregates: Preparation and Quantification of Physical and Optical Anisotropy

Controlling morphological order of conjugated polymers over mesoscopic and microscopic scales could yield critical improvements in the performance of organic electronics. Here, we utilize a multimodal apparatus allowing for controlled solvent vapor annealing and simultaneous wide-field epifluorescence microscopy to demonstrate bottom-up growth of morphologically ordered anisotropic aggregates prepared from single poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) chains, with length scales controllable from tens of nanometers to several micrometers. Preparation of micrometer-scale fiber aggregates that interconnect to form spanning networks is also demonstrated. We quantify aggregate physical and optical anisotropy, degree of quenching, and exciton diffusion characteristics as a function of aggregate size. The demonstration of controlled preparation of highly anisotropic aggregates provides a path for controlled postprocessing of organic thin films at length scales relevant to the operati...

Detection of Ultralow Concentrations of Non-emissive Conjugated Polymer Aggregates via Fluorescence Correlation Spectroscopy.

The aggregation of conjugated polymers in common organic solvents is investigated using fluorescence correlation spectroscopy (FCS), burst analysis, and microscopy. Poly(3-hexylthiophene) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] are both shown to form weakly bonded non-emissive aggregates in toluene that persist even at picomolar concentrations. These aggregates decrease the bulk emission intensity in solution but do not affect the fluorescence spectra or lifetimes, consistent with a static quenching mechanism. Passing the solutions through a syringe filter causes an increase in the number of emitters as measured by FCS, indicating that this process dissociates the aggregates. Films cast from solutions that have been filtered are more uniform and significantly more emissive than those made from unfiltered solutions. These results show that FCS is a highly sensitive probe of non-emissive aggregates in solution that have a deleterious effect on the emission properties and overall quality of spin-cast thin films, even at sub-nanomolar concentrations.

Interchain Charge-Transfer States Mediate Triplet Formation in Purified Conjugated Polymer Aggregates

The pathways and dynamics of converting spin-allowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly(3-hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show larg...

Liquid Crystalline Poly(3-hexylthiophene) Solutions Revisited: Role of Time-Dependent Self-Assembly

Poly(3-hexylthiophene) (P3HT) in trichlorobenzene solution self-assembles and exhibits liquid crystal ordering when confined to rectangular capillaries. The relative proportion of polymer assemblies increases with time, as determined by UV–vis spectroscopic analysis. Polarized optical microscopy (POM) reveals development of birefringence and monodomainlike long-range ordering. Micro-Raman spectroscopy was used to calculate the orientational order parameters, ⟨P2⟩ and ⟨P4⟩, of the liquid-crystalline P3HT solutions. The order parameter ⟨P2⟩ increased with time up to 0.35, indicating increased anisotropy. The calculated depolarization ratio (ρv) from depolarized dynamic light scattering measurements points to the time-dependent formation of highly ordered P3HT nanostructures, whereas cryogenic transmission electron microscopy was employed for the direct visualization of the rodlike assemblies. POM shows that the observed anisotropy can be preserved in P3HT films drawn from aged solutions. These results suggest that P3HT self-assembly leads to a liquid-crystalline solution of conjugated polymer aggregates, which may lead to a viable approach for optimization of processes for organic electronic device applications. Such ordered and oriented conjugated polymer assemblies have many desirable attributes for high-performance device applications, where the ability to control nano- through macroscale molecular ordering is required.

Circularly polarized luminescence from individual microstructures of conjugated polymer aggregates with solvent-induced chirality.

We report a microscopic study of circularly polarized luminescence (CPL) from R and S isomers of aggregates of the conjugated polymer F8BT, a copolymer containing fluorene and thiophene monomer units. Chirality in the aggregates is induced by addition of either (R)- or (S)-limonene during aggregation in solution. Solid-state samples are prepared either by drop-casting or spin-coating of the aggregate solution. CPL dissymmetry parameter g measured for individual aggregate microstructures shows a broad distribution of values ranging from positive to negative even for the same isomer. The difference in the centers of the g distribution between the R and S isomers reflects the intrinsic CPL from the chiral aggregates, and is found to depend on the size of the aggregate structures. On the other hand, the large widths of the distributions are ascribed to an optical effect arising from phase retardation along the optical path of the CPL inside the sample.

Emission Rate, Vibronic Entanglement, and Coherence in Aggregates of Conjugated Polymers

Here we have studied a dimer model of conjugated polymer aggregates based on the traditional J and H structures, with the extension in treating the electronic and vibrational degrees of freedom at par. We have considered various exchange symmetries corresponding to the parameters of the excited state Hamiltonian in assigning the symmetry of the vibronic states of the aggregate, going beyond the homodimer case. The emission rates are determined as a function of system parameters at low temperature for both types of aggregates. We have also determined the vibronic entanglement as a measure of the coupled electronic and vibrational motion as well as the exciton coherence number in the emitting state. As a function of interchain interaction strength, emission rate and entanglement grossly follow similar trends for the J-aggregate and opposite trends for the H-aggregate in totally symmetric as well as asymmetric cases. Variation of other system parameters, like electronic excitation energy and electron-vibration coupling parameter are also thoroughly investigated in governing these quantities. The role of symmetry of the wave function in governing the spectra and the exciton coherence are also analyzed thoroughly, which offers a way to realize the connection between such macroscopic and microscopic quantum features.

Designing J- and H-Aggregates through Wave Function Overlap Engineering: Applications to Poly(3-hexylthiophene)

A novel mechanism for J- and H-aggregate formation is presented on the basis of wave function overlap (WFO) coupling between neighboring chromophores which supplements the usual through-space (Coulombic) coupling. In cases where the latter is relatively small compared to the former, as might arise for excitons based on molecular transitions with low oscillator strengths, J- vs H-aggregation is determined by the sign of the product D(e)D(h), where D(e) (D(h)) is the coupling between a neutral Frenkel exciton and the charge transfer exciton created through the transfer of an electron (hole) to a neighboring chromophore. Adapting a sign convention based on translational symmetry in a linear array of chromophores, a positive (negative) sign for D(e)D(h) places the bright exciton on the bottom (top) of the exciton band, consistent with J- (H-) aggregation. The J- (H-) aggregates so formed behave as direct (indirect) bandgap semiconductors with vibronic signatures in absorption and photoluminescence that are identical to those displayed by conventional Coulomb coupled aggregates. WFO coupling leading to the mixing of intrachain Frenkel excitons and polaron pairs may be important in conjugated polymer aggregates where the Coulomb coupling practically vanishes with the (conjugation) length of the polymer. Calculations based on octathiophene (8T) dimers show that the eclipsed geometry yields a WFO coupling favoring H-aggregate behavior, although a longitudinal (long-axis) displacement by only 1.5 Å is enough to change the sign of the coupling, leading to J-aggregate behavior. Hence, it should be possible to design thiophene-based polymers which act as J-aggregates with respect to the interchain coupling.

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.

Self-assembly of highly ordered conjugated polymer aggregates with long-range energy transfer

Applications of conjugated polymers (CP) in organic electronic devices such as light-emitting diodes and solar cells depend critically on the nature of electronic energy transport in these materials. Single-molecule spectroscopy has revealed their fundamental properties with molecular detail, and recent reports suggest that energy transport in single CP chains can extend over extraordinarily long distances of up to 75 nm. An important question arises as to whether these characteristics are sustained when CP chains agglomerate into a neat solid. Here, we demonstrate that the electronic energy transport in aggregates composed of tens of polymer chains takes place on a similar distance scale as that in single chains. A recently developed molecular-level understanding of solvent vapour annealing has allowed us to develop a technique to control the CP agglomeration process. Aggregates with volumes of at least 45,000 nm(3) (molecular weight ≈ 21 MDa) maintain a highly ordered morphology and show pronounced fluorescence blinking behaviour, indicative of substantially long-range energy transport. Our findings provide a new lens through which the ordering of single CP chains and the evolution of their morphological and optoelectronic properties can be observed, which will ultimately enable the rational design of improved CP-based devices.

Role of positional disorder in the spectra of conjugated polymer aggregates: a conical intersection of potential energy surfaces

Here we have studied the effect of disorder on the luminescence spectra due to the orientational disposition of monomeric conjugated polymer chain units forming a dimer aggregate. The dipolar interchain interactions are described considering static as well as dynamic disorder created by the torsion or shearing degree of freedom. For static disorder, the shearing angle is treated parametrically for different frozen configurations of the positional defect. In the case of dynamical disorder the torsional motion is treated on an equal footing with the vibrational degree of freedom by including the shearing coordinate explicitly in the calculation. The inclusion of torsion angle dependence in the interchain interaction not only saves the 0–0 transition from total quenching but also generates a conical intersection in the adiabatic potential energy surfaces of the dimer aggregate. Due to the vanishing of the interchain coupling at the conical intersection point, the two chains become independent and the dimeric spectra pass through a monomeric pattern which can serve as an example of the presence of conical intersection in the aggregates of conjugated polymer chains. Also the presence of the low-frequency torsion mode explicitly considered in the case of dynamic disorder can be detected from a significant Stokes shift in the steady state spectra.

Water‐Soluble Conjugated Polymers for Fluorescent‐Enzyme Assays

Enzyme assays are receiving more and more research and application interest because of the rapidly increasing demands of clinical diagnosis, environmental analysis, drug discovery, and molecular biology. Water-soluble light-harvesting conjugated polymers (CPs) coordinate the action of a large number of absorbing units to afford an amplified fluorescence signal, which makes them useful as optical platforms in highly sensitive chemical and biological sensors. This Feature Article highlights recent developments of water-soluble CPs for fluorescent assays of enzymes. Different signal transduction mechanisms, such as electron transfer, fluorescence resonance energy transfer (FRET), and aggregation or conformation changes of CPs, are employed in these assays according to the dissimilar nature of enzymes. Potential challenges and future research directions in these approaches based on CPs are also discussed.