Π-conjugated Block Copolymers Research Articles

Helix-Induced Asymmetric Self-Assembly of π-Conjugated Block Copolymers: From Controlled Syntheses to Distinct Properties.

Conspectusπ-Conjugated polymers have gained significant interest because of their potential applications in optoelectronics, bioelectronics, and other domains. The controlled synthesis of π-conjugated block polymers optimizes their performance and enables novel properties and functions. However, precise control of the self-assembled architectures of π-conjugated polymers remains a formidable challenge. Inspired by the precise helical architectures of biomacromolecules, the helical polymers and the supramolecular helical assemblies have gained significant attention. Helical polymers with an excess of one-handed helicity can be optically active with a strong tendency toward self-assembly. Incorporating a helical polymer into a π-conjugated polymer can induce asymmetric helical assemblies, leading to novel chiral materials with unique functionalities.To control the self-assembly of architectures, π-conjugated polymers are usually synthesized into block copolymers by incorporating a polymer with self-assembling characteristics. Although various π-conjugated block copolymers have been produced, precise and asymmetric self-assembly is still challenging and has rarely been addressed. Incorporating helical polymers into the π-conjugated polymers can induce a precise and asymmetric self-assembly, which transfers the chirality of the helical polymer block to the π-conjugated polymer, resulting in chiral supramolecular architectures with unique chiroptical properties and functionalities. However, synthesizing hybrid block copolymers containing two distinct polymer blocks is complicated. Some general strategies such as connecting the chain ends of two preformed homopolymers and extending the chain of a prefabricated π-conjugated polymer with a second monomer are time-consuming and require complex synthetic protocols. Therefore, developing novel strategies for the facile synthesis of π-conjugated block copolymers with a predictable molar mass, low dispersity, and tunable composition is of practical importance.Recently, we investigated a controlled synthesis of helical polyisocyanides, helical polyallenes, and helical polycarbenes by developing advanced Pd(II) and Ni(II) catalysts. These helical polymers were successfully incorporated into π-conjugated polymers, including polythiophene, polyfluorene, and poly(phenyleneethynylene), via a one-pot sequential living block polymerization of the two distinct monomers using Pd(II)- or Ni(II)-complexes as catalysts. As a result, a variety of well-defined π-conjugated block copolymers containing helical polymeric blocks were readily synthesized. Although the copolymerized monomers possess different structures and polymerization mechanisms, the one-pot block copolymerization followed a living polymerization mechanism and provided the desired π-conjugated block copolymers in high yields with controlled molar mass, narrow size distribution, and tunable composition.Remarkably, the helical polymeric block induces the π-conjugated block copolymer asymmetric self-assembly into a supramolecular, one-handed helical architecture resulting in distinct optical properties. More interestingly, by utilizing the crystallization of conjugated blocks and one-handed helical blocks, the crystallization-driven and helix-induced precise asymmetric living self-assembly yielded a family of uniform and single-handed helical architectures with controlled dimensions, narrow distribution, and well-defined helicity. The transfer of helical chirality to the supramolecular architectures rendered the achiral π-conjugated blocks with unique chiroptical properties such as the emission of white light over a broad optical spectrum and the circularly polarized luminescence.

Controlled Synthesis, Chiral Assembly, and Circularly Polarized Luminescence of Poly(Phenyl Isocyanide)-Block-Polyfluorene Copolymers.

In this work, π-conjugated block copolymers consisting of poly(phenyl isocyanide) (PPI) and polyfluorene (PF) segments are facilely prepared by one-pot sequential polymerization of phenyl isocyanide (monomer 1) and 7-bromo-9,9-dioctylfluorene-2-boronic acid pinacol ester (monomer 2). The Pd(II)-terminated PPI is first prepared via polymerizing monomer 1catalyzed with phenyl alkyne-Pd(II) complex and then utilized to initiate the controlled Suzuki cross-coupling polymerization of monomer 2, yielding various PPI-b-PF copolymers possessing controlled molar mass and narrow dispersity. Owing to the helical conformation of PPI segment and π-conjugated structure of PF segment, PPI-b-PF copolymers present distinctive optical property and fascinating chiral self-assembly behavior. During the self-assembly process, chirality transfer from helical PPI block to the supramolecular aggregates of helical nanofibers occurs to afford optically active helical nanofibers with high optical activity. Furthermore, the self-assembled helical nanofibers exhibit excellent circularly polarized luminescence performance.

Polyfluorene-block-poly(phenyl isocyanide) Copolymers: One-Pot Synthesis, Helical Assembly, and Circularly Polarized Luminescence

Optically active π-conjugated block copolymers are of considerable significance and interest, but their controlled synthesis and helical self-assembly remain challenging. In this study, π-conjugated block copolymers consisting of polyfluorene (PF) and poly(phenyl isocyanide) (PPI) segments were facilely prepared in one pot through two sequential polymerizations. First, a Pd(II)-terminated PF precursor was synthesized via the Suzuki cross-coupling polymerization of 7-bromo-9,9-dihexylfluorene-2-boronic acid pinacol ester, which was catalyzed using a Pd(0)/t-Bu3P system. Then, PF-b-PPI copolymers with controlled molar mass and narrow dispersities were fabricated via controlled polymerization of phenyl isocyanide monomers utilizing Pd(II)-terminated PF as the catalyst. Benefiting from the π-conjugated structure of the PF block and the helicity of the PPI block, the resulting PF-b-PPI copolymers could self-assemble into single-handed helical nanofibers. In the self-assembly process, chirality was transferred from the helical PPI block to the self-assembled helical nanofibers, thereby endowing them with optical activity and excellent circularly polarized luminescence properties.

Precise Synthesis of π-Conjugated Block Copolymers and Polymerization-Induced Chiral Self-Assembly toward Helical Nanofibers with Circularly Polarized Luminescence.

Precise synthesis and efficient self-assembly of semiconducting polymers are of great interest. Herein, we report the controlled synthesis of π-conjugated poly(phenyl isocyanide)-b-poly(phenyleneethylene) (PPI-b-PPE) copolymers via chain extension of ethynyl 4-iodobenzene initiated by Pd(II)-terminated helical poly(phenyl isocyanide) (PPI). The in-situ-generated block copolymers self-assembled into various supramolecular architectures depending on the PPE length. The helical PPI segment induced the block copolymers with an appropriate PPE length self-assemble into helical nanofibers with a controlled size and defined helicity. Interestingly, the chiral assemblies of the block copolymers exhibit intense optical activity and emit clear circularly polarized luminescence.

Precisely Controlled Two-Dimensional Rhombic Copolymer Micelles for Sensitive Flexible Tunneling Devices

Nanoscale two-dimensional (2D) organic materials have attracted significant interest on account of their unique properties, which result from their ultrathin and flat morphology. Supramolecular 2D ...

Long-term thermally stable nanoconfining networks for efficient fully conjugated semicrystalline/amorphous diblock copolymer photovoltaics

Abstract In this study, a new fully π-conjugated diblock copolymer comprising of major semicrystalline poly(3-butyl thiophene) (P3BT) and minor amorphous poly(2,5-dihexyloxy-p-phenylene) (PPP) block chains was rationally designed and further used as synergistic charge-transporting networks for PCBM based solar cells. The light-harvesting P3BT block was carefully chosen due to its high melting temperature and greater hole mobility to afford better thermal stability. Despite lower molecular weight P3BT block in the copolymer compared with that of a P3BT homopolymer counterpart, the copolymer exhibited long-range ordered fibrils with larger P3BT crystals and higher total crystallinity, indicating that the minor PPP chains promote the ordering and crystallization of P3BT. In addition, the strong immiscibility and difference in the crystallographic kinetics among the two main-chain moieties allow the copolymer/PCBM hybrid to spontaneously self-assemble into an interpenetrating nanostructure, while permitting PCBM nanoparticles to preferentially locate in the amorphous PPP domains to form bicontinuous conduits for efficient dual-charge transport. Notably, the copolymer/PCBM hybrids exhibited superior long term thermal stability against degradation and PCBM aggregation upon accelerated aging of the devices. At stringently high aging temperature of 180 °C for 4 h for a thermal stress test, the copolymer devices retain nearly 90% of their maximum PCE, whereas those of the reference P3BT/PCBM solar cells decrease severely to be less than 50% of their maximum. These results demonstrate the advantage of using thermally stable all π-conjugated block copolymers to provide substantial enhancement in charge dissociation, transport as well as extended thermal stability due to the nanoconfining effect of its self-assembled copolymer structure.

Uniform electroactive fibre-like micelle nanowires for organic electronics

Micelles formed by the self-assembly of block copolymers in selective solvents have attracted widespread attention and have uses in a wide variety of fields, whereas applications based on their electronic properties are virtually unexplored. Herein we describe studies of solution-processable, low-dispersity, electroactive fibre-like micelles of controlled length from π-conjugated diblock copolymers containing a crystalline regioregular poly(3-hexylthiophene) core and a solubilizing, amorphous regiosymmetric poly(3-hexylthiophene) or polystyrene corona. Tunnelling atomic force microscopy measurements demonstrate that the individual fibres exhibit appreciable conductivity. The fibres were subsequently incorporated as the active layer in field-effect transistors. The resulting charge carrier mobility strongly depends on both the degree of polymerization of the core-forming block and the fibre length, and is independent of corona composition. The use of uniform, colloidally stable electroactive fibre-like micelles based on common π-conjugated block copolymers highlights their significant potential to provide fundamental insight into charge carrier processes in devices, and to enable future electronic applications.

Co-crystallization phase transformations in all π-conjugated block copolymers with different main-chain moieties

Driven by molecular affinity and balance in the crystallization kinetics, the ability to co-crystallize dissimilar yet self-crystallizable blocks of a block copolymer (BCP) into a uniform domain may strongly affect its phase diagram. In this study, we synthesize a new series of crystalline and monodisperse all-π-conjugated poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-(2-ethylhexyl)thiophene) (PPP-P3EHT) BCPs and investigate this multi-crystallization effect. Despite vastly different side-chain and main-chain structures, PPP and P3EHT blocks are able to co-crystallize into a single uniform domain comprising PPP and P3EHT main-chains with mutually interdigitated side-chains spaced in-between. With increasing P3EHT fraction, PPP-P3EHTs undergo sequential phase transitions and form hierarchical superstructures including predominately PPP nanofibrils, co-crystalline nanofibrils, a bilayer co-crystalline/pure P3EHT lamellar structure, a microphase-separated bilayer PPP-P3EHT lamellar structure, and finally P3EHT nanofibrils. In particular, the presence of the new co-crystalline lamellar structure is the manifestation of the interaction balance between self-crystallization and co-crystallization of the dissimilar polymers on the resulting nanostructure of the BCP. The current study demonstrates the co-crystallization nature of all-conjugated BCPs with different main-chain moieties and may provide new guidelines for the organization of π-conjugated BCPs for future optoelectronic applications.

Solution self-assembly and phase transformations of form II crystals in nanoconfined poly(3-hexyl thiophene) based rod-coil block copolymers

Solution processing of π-conjugated polymers constitutes a major low-cost manufacturing method for the fabrication of many new organic optoelectronic devices. The solution self-assembly kinetics of π-conjugated rod-coil block copolymers of symmetric poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT-P2VP) during drying and the phase transformations of the subsequently dried samples were studied by using a combination of TEM, SAXS, WAXS and DSC measurements. During solution drying in chlorobenzene, a good solvent for the copolymer, P3HT-P2VP first formed nanoseed aggregates followed by the directional growth of nanofibrils driven by the formation of prevailing form II P3HT crystals within its nanofibril core confined by the surrounding domain of P2VP blocks. This result was in sharp contrast when a similar molecular weight P3HT homopolymer was solution self-assembled in chlorobenzene, nearly free from confinement, in which case the resulting nanofibrils consisted of a mixture of majority form I and form II crystals. Solvent-cast films of P3HT-P2VP nanofibrils with form II crystals were heat-/cold-treated and showed solid-state phase transformations from form II crystals to form I crystals, both within nanofibrils with annealing, indicating the metastability of the form II crystals with temperature. A disordered state followed with increasing temperatures which, when cooled, induced the formation of a thermodynamically stable lamellar phase with only form I P3HT crystals. Correspondingly, the study provides new strategies for controlling polymorphs and nanostructures of π-conjugated block copolymers for future applications using solution processing and subsequent heat treatment.

Organicπ-Conjugated Copolymers as Molecular Charge Qubits

We propose a design for molecular charge qubits based on π-conjugated block copolymers and determine their electronic structure as well as their vibrational active modes. By tuning the length of the oligomers, the tunnel coupling in the charge qubit and its decoherence properties due to molecular vibrations can be chemically engineered. Coherent oscillations result with quality factors of up to 10(4) at room temperature. In turn, the molecular vibrational spectrum induces strong non-Markovian electronic effects which support the survival of quantum coherence.

Self-assembly and phase transformations of π-conjugated block copolymers that bend and twist: from rigid-rod nanowires to highly curvaceous gyroids

In this study, a series of π-conjugated block copolymers of regioregular poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT–P2VP) was synthesized and their self-assembly behavior and the detailed thermodynamic phase diagram were explored. By a combination of TEM, SAXS, WAXS, and UV-VIS measurements, it was found that the π-conjugated P3HT in their various self-assembled nanodomains could be a rigid rod, or a semi-rigid chain, or even a fully flexible chain. With the P2VP volume fraction, ϕ, = 0.20, the P3HT–P2VP displays a nanowire structure with a fully extended all-transP3HT rod structure across the width of the nanowires, indicating a prevailing rod–rod interaction between P3HT blocks over the microphase separation interaction between the constituent blocks. With ϕ = ∼0.30 to 0.6, the P3HT–P2VPs show a highly ordered lamellar structure with the P3HT block exhibiting as a semirod-like chain composed of shorter rods connected by twisted 3HT units. With ϕ > 0.68, the π-conjugated block copolymers display self-assembling nanostructures of hexagonal close packed cylinders and spheres, indicating that P3HT adopts a fully coil-like structure that favors interfacial curvatures. In particular, for the P3HT–P2VP with ϕ = 0.68, a gyroid phase, the first of its kind for π-conjugated block copolymers, was observed upon heating. For the nanowire structured P3HT–P2VP, a liquid crystalline phase transition from the smectic-like crystalline state to a nematic structure was observed at ∼200 °C. The observed microstructures and transformations reveal the importance of the semirigid nature of π-conjugated P3HT chains and provide new guidelines for the organization of π-conjugated block copolymers for future optoelectronic applications.

Molecular architectures based on π-conjugated block copolymers for global quantum computation

We propose a molecular setup for the physical implementation of a barrier global quantum computation scheme based on the electron-doped π-conjugated copolymer architecture of nine blocks PPP-PDA-PPP-PA-(CCH-acene)-PA-PPP-PDA-PPP (where each block is an oligomer). The physical carriers of information are electrons coupled through the Coulomb interaction, and the building block of the computing architecture is composed by three adjacent qubit systems in a quasi-linear arrangement, each of them allowing qubit storage, but with the central qubit exhibiting a third accessible state of electronic energy far away from that of the qubits' transition energy. The third state is reached from one of the computational states by means of an on-resonance coherent laser field, and acts as a barrier mechanism for the direct control of qubit entanglement. Initial estimations of the spontaneous emission decay rates associated to the energy level structure allow us to compute a damping rate of order 10-7 s, which suggest a not so strong coupling to the environment. Our results offer an all-optical, scalable, proposal for global quantum computing based on semiconducting π-conjugated polymers.

Poly(3-hexylthiophene) Based Block Copolymers Prepared by “Click” Chemistry

π-Conjugated block copolymers have been prepared from terminal azide functionalized polystyrenes (PS) and alkyne functionalized poly(3-hexylthiophene)s (P3HT) via a copper(I) catalyzed Huisgen [3 + 2] dipolar cycloaddition reaction. The functionalized α-azido-PS homopolymer was prepared by atom transfer radical polymerization from a specifically designed initiator bearing the azide function, whereas ω-ethynyl-P3HT and α,ω-pentynyl-P3HT were synthesized by a modified Grignard metathesis polymerization using alkynyl Grignard derivatives. The electronic environment of the alkynyl end groups was shown to be decisive in determining triazole ring formation.

Alternating π-conjugated block copolymers with precise block length

A Sonogashira polycondensation reaction has been used to synthesize copolymers consisting of alternating oligo(p-phenyleneethynylene) with a precise block length as an electron-rich component and 1,4-bis(2-phenylene-2-cyanovinylene)benzene or 2,6-bis(2-pyridinylene-ethynylene)pyridine as an electron-poor component. The copolymers differ in the length of the phenyleneethynylene block (trimer or pentamer) and the content of the electron-poor component. The length of the phenyleneethynylene block has no influence on the maximum wavelength. The electron-poor cyano-block component lowers the optical band-gap energy of the copolymers. The value is equivalent to that of poly(cyano-phenylenevinylene) (CN-PPV) (2.3–2.4 eV). © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3574–3587, 2005