Multi-level aggregation of conjugated small molecules and polymers: from morphology control to physical insights

Aggregation of molecules is a multi-molecular phenomenon occurring when two or more molecules behave differently from discrete molecules due to their intermolecular interactions. Moving beyond single molecules, aggregation usually demonstrates evolutive or wholly emerging new functionalities relative to the molecular components. Conjugated small molecules and polymers interact with each other, resulting in complex solution-state aggregates and solid-state microstructures. Optoelectronic properties of conjugated small molecules and polymers are sensitively determined by their aggregation states across a broad range of spatial scales. This review focused on the aggregation ranging from molecular structure, intermolecular interactions, solution-state assemblies, and solid-state microstructures of conjugated small molecules and polymers. We addressed the importance of such aggregation in filling the gaps from the molecular level to device functions and highlighted the multi-scale structures and properties at different scales. From the view of multi-level aggregation behaviors, we divided the whole process from the molecule to devices into several parts: molecular design, solvation, solution-state aggregation, crystal engineering, and solid-state microstructures. We summarized the progress and challenges of relationships between optoelectronic properties and multi-level aggregation. We believe aggregation science will become an interdisciplinary research field and serves as a general platform to develop future materials with the desired functions.

Generic Model for Lamellar Self-Assembly in Conjugated Polymers: Linking Mesoscopic Morphology and Charge Transport in P3HT

We develop a generic coarse-grained model of soluble conjugated polymers, capable of describing their self-assembly into a lamellar mesophase. Polymer chains are described by a hindered-rotation model, where interaction centers represent entire repeat units, including side chains. We introduce soft anisotropic nonbonded interactions to mimic the potential of mean force between atomistic repeat units. The functional form of this potential reflects the symmetry of the molecular order in a lamellar mesophase. The model can generate both nematic and lamellar (sanidic smectic) molecular arrangements. We parametrize this model for a soluble conjugated polymer poly(3-hexylthiophene) (P3HT) and demonstrate that the simulated lamellar mesophase matches morphologies of low molecular weight P3HT, experimentally observed at elevated temperatures. A qualitative charge-transport model allows us to link local chain conformations and mesoscale order to charge transport. In particular, it shows how coarsening of lamellar domains and chain extension increase the charge carrier mobility. By modeling large systems and long chains, we can capture transport between lamellar layers, which is due to rare, but thermodynamically allowed, backbone bridges between neighboring layers.

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.

Effect of side-chain asymmetry on the intermolecular structure and order-disorder transition in alkyl-substituted polyfluorenes

We study relations among the side-chain asymmetry, structure, and order-disorder transition (ODT) in hairy-rod-type poly(9,9-dihexylfluorene) (PF6) with two identical side chains and atactic poly(9-octyl-9-methyl-fluorene) (PF1-8) with two different side chains per repeat. PF6 and PF1-8 organize into alternating side-chain and backbone layers that transform into an isotropic phase at T^{ODT}(PF6) and T_{bi}^{ODT}(PF1-8). We interpret polymers in terms of monodisperse and bidisperse brushes and predict scenarios T^{ODT}<T_{bi}^{ODT} and T^{ODT}∼T_{bi}^{ODT} for high and low grafting densities (the side-chain length above or below the average grafting distance). Calorimetry and x-ray scattering indicate the condition T^{ODT}(PF6)∼T_{bi}^{ODT}(PF1-8) following the low grafting prediction. PF6 side chains coming from the alternating backbone layers appear as two separate layers with thickness H(PF6), whereas PF1-8 side chains appear as an indistinguishable bilayer with a half thickness H_{bilayer}(PF1-8)/2≈H(PF6). The low grafting density region is structurally possible but not certain for PF6 and confirmed for PF1-8.

MORPHOLOGICAL PROPERTIES OF PYRROLE AND PHENYLENE ROD–COIL DIBLOCK COPOLYMERS BY DISSIPATIVE PARTICLE DYNAMICS

Poly (para-phenylene)s (PPP) and polypyrroles (PPy) are important members of the conducting polymers. Rod–coil type diblock copolymers formed by coupling of PPP and PPy rigid blocks with polycaprolactone (PCL), polystyrene (PS) and polymethylmethacrylate (PMMA) coil blocks were modeled and morphological properties have been studied by a coarse grained simulation method at the mesoscale. Geometry optimizations and the atomic charge calculations were done quantum mechanically to obtain the input parameters for the mesoscale dynamics simulations. The accurate mixing energies and the Flory–Huggins interaction parameters between the monomers of polymers were calculated and used to study the phase behaviors and the morphologies of the copolymers as a function of type and weight percentages of the blocks by Dissipative Particle Dynamics (DPD) simulations. We showed that the methodology employed took into account not only the interaction parameter and chain length of the blocks but also the chemical structure of the polymers and it could be used to produce the phase diagram of the copolymers which has importance for the industrial applications of such materials. Among the studied copolymers, the most suitable one for thin layer applications was predicted to be PPP–b–PCL in which PPP forms lamellar and cylindrical phases in the PCL matrix if amount of PPP rod block is below 50 wt%.

Organic semiconductors: impact of disorder at different timescales

The charge transport in organic materials, from molecular crystals to polymers, is determined by their degree of disorder. The dynamic disorder in ideal molecular crystals at room temperature and the static disorder in disordered polymers are just two limiting cases of the timescale of the fluctuations in the electronic Hamiltonian caused by nuclear motions. In fact, a very large number of important materials (e.g. liquid crystalline semiconductors) are actually in an intermediate regime where the disorder is neither purely static nor purely dynamic. This Minireview discusses the recent contribution of computational chemistry (molecular dynamics and quantum chemistry) to the characterization of these transport regimes and outlines the theoretical methods that can be used to relate the system characteristics to the measurable mobility.

Solvent-induced self-assembly of polymer-tethered nanorods

Self-assembly behaviors of polymer-tethered nanorods in the selective solvent are systematically investigated via a dissipative particle dynamics (DPD) simulation method. Three types of polymer-tethered nanorods are considered: one end tethered, both ends tethered, and middle tethered. The solvent-induced diverse morphologies and morphological transitions depend on the topology, rod/tether length ratio, solvent selectivity, and mixed solvent content. In the pure rod-selective solvent (solvent I) or the pure tether-selective solvent (solvent II), the ordered micellar structures include: cylinders, hexagonal cylinders, bilayer lamellae, lamellae/cylinder mixed phases, inverted hollow cylinders, and nematic bundles. These micelles are formed by the competition among the stretching of tethers, liquid crystalline of rods, interfacial energy, and solvent selectivity. In the I/II mixed solvent, with varying mixed solvent content in sequence (i.e., changing the solvent quality for the blocks), the reversible morphological transitions and fantastic intermediate phases (e.g., liquid crystalline phase) are observed, which correspond directly to the case of that induced by varying the rod/tether length ratio in the pure solvent. It is concluded that improving the selective solvent content is equivalent to increasing the soluble block ratio. The present study reveals that the morphology and morphological transition of polymer-tethered nanorods could be significantly manipulated through topology, block length, and solvent, especially the selectivity.