Ordered Solid-State Microstructures of Conjugated Polymers Arising from Solution-State Aggregation

Controlling the solution-state aggregation of conjugated polymers for producing specific microstructures remains challenging. Herein, a practical approach is developed to finely tune the solid-state microstructures through temperature-controlled solution-state aggregation and polymer crystallization. High temperature generates significant conformation fluctuation of conjugated backbones in solution, which facilitates the polymer crystallization from solvated aggregates to orderly packed structures. The polymer films deposited at high temperatures exhibit less structural disorders and higher electron mobilities (up to two orders of magnitude) in field-effect transistors, compared to those deposited at low temperatures. This work provides an effective strategy to tune the solution-state aggregation to reveal the relationship between solution-state aggregation and solid-state microstructures of conjugated polymers.     

共轭高分子链构象和溶液聚集结构研究进展

共轭高分子具有优异的光电性质和可加工性,被广泛用于有机光电器件的制备。共轭单元的存在使得此类高分子具有更刚性的主链结构。由于较强的分子间相互作用,共轭高分子容易在溶液中形成组装结构。共轭高分子的链构象、组装体结构、薄膜形貌和光电性能之间的联系研究成为了本领域的研究热点。然而,共轭高分子在可见光区存在较强的吸收效应,用传统的光散射技术对共轭高分子溶液的研究充满挑战。本文总结了近年来对于共轭高分子链刚性的研究,并从分子尺度上讨论了链结构与光学、电学性能间可能存在的关联;进一步阐述了共轭高分子溶液聚集的形成和演化,总结了溶液聚集与成膜过程中影响场效应迁移率的因素。试图在不同尺度上讨论共轭高分子的微观结构与宏观性能之间的关系。     

Controlling the Microstructure of Conjugated Polymers in High‐Mobility Monolayer Transistors via the Dissolution Temperature

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (T_dis) is investigated as a powerful strategy for morphology control. Low T_dis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field‐effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high T_dis with a value approaching 1 cm² V^?1 s^?1. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low T_dis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on T_dis is consistent with the aggregate size in solution. The generalizability of the T_dis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high‐mobility monolayer transistors.     

Conformation Control of Conjugated Polymers

In the past few decades, conjugated polymers have aroused extensive interest in organic electronic applications. The electrical performance of conjugated polymers has a close relationship with their backbone conformation. The conformation of the polymer backbone strongly affects the πelectron delocalization along polymer chains, the energy band gap, interchain interactions, and further affects charge transport properties. To realize a rigid coplanar backbone that usually possesses efficient intrachain charge transport properties and enhanced π–π stackings, such conformation control becomes a useful strategy to achieve high-performance (semi)conducting polymers. This minireview summarizes the most important polymer structures through conformation control at the molecular level, and then divides these rigid coplanar conjugated polymers into three categories: 1) noncovalent interactions locked conjugated polymers; 2) double-bond linked conjugated polymers; 3) ladder conjugated polymers. The effect of the conformation control on physical nature, optoelectronic properties, and their device performance is also discussed, as well as the challenges of chemical synthesis and structural characterization.     

Spiropyran Main‐Chain Conjugated Polymers

The first main‐chain conjugated copolymers based on alternating spiropyran (SP) and 9,9‐dioctylfluorene (F8) units synthesized via Suzuki polycondensation (SPC) are presented. The reaction conditions of SPC are optimized to obtain materials of type P(para‐SP‐F8) with appreciably high molecular weights up to M _w ≈ 100 kg mol⁻¹. ¹³C NMR is used to identify the random orientation of the non‐symmetric SP unit in P(p‐SP‐F8). Ultrasound‐induced isomerization of P(p‐SP‐F8) to the corresponding merocyanine form P(p‐MC‐F8) yields a deep‐red solution. This isomerization reaction is followed by ¹H NMR in solution using sonication, whereby the color increasingly changes to deep red. The possibility to incorporate multiple SP units into main‐chain polymers significantly broadens existing SP‐based polymeric architectures.     

Planar Conjugated Polymers Containing 9,10‐Disubstituted Phenanthrene Units for Efficient Polymer Solar Cells

Four novel conjugated polymers ( P1‐4 ) with 9,10‐disubstituted phenanthrene (PhA) as the donor unit and 5,6‐bis(octyloxy)benzothiadiazole as the acceptor unit are synthesized and characterized. These polymers are of medium bandgaps (2.0 eV), low‐lying HOMO energy levels (below −5.3 eV), and high hole mobilities (in the range of 3.6 × 10⁻³ to 0.02 cm² V⁻¹ s⁻¹). Bulk heterojunction (BHJ) polymer solar cells (PSCs) with P1‐4 :PC₇₁BM blends as the active layer and an alcohol‐soluble fullerene derivative (FN‐C60) as the interfacial layer between the active layer and cathode give the best power conversion efficiency (PCE) of 4.24%, indicating that 9,10‐disubstituted PhA are potential donor materials for high‐efficiency BHJ PSCs.

     

Electrochemical Post‐Functionalization of Conducting Polymers

This article summarizes recent progress in the post‐functionalization of conjugated polymers by electrochemical methods. These electrochemical polymer reactions typically proceed via electrochemical doping of a conjugated polymer film, followed by chemical transformation. Examples include the quantitative oxidative fluorination of polyfluorenes and oxidative halogenation of polythiophenes, as well as the reductive hydrogenation of polyfluorenones. The degree of functionalization, otherwise known as the reaction ratio, can be controlled by varying the charge passed through the polymer, allowing the optoelectronic properties of the conjugated polymers to be tailored. Wireless bipolar electrodes with an in‐plane potential distribution are also useful with regard to the electrochemical doping and reaction of conjugated polymers and allow the synthesis of films exhibiting composition gradients. Such bipolar electrochemistry can induce multiple reaction sites during electrochemical polymer reactions.

     

Aromatic Ring‐Fused Carborane‐Based Luminescent π‐Conjugated Polymers

A series of π‐conjugated polymers linked by benzocarborane (1,2‐(buta‐1′,3′‐diene‐1′,4′‐diyl)‐1,2‐dicarbadodecaborane) were synthesized via Sonogashira–Hagihara polycondensation reaction. The opened molecular structure of diiodo monomer containing benzocarborane resulted in fast polymerization and high molecular weights. The obtained polymers were fully characterized by ¹H, ¹³C, and ¹¹B NMR spectroscopies. UV‐vis absorption and photoluminescence studies revealed the acceptor‐profile of benzocarborane. Unlike the polymers linked by o‐carborane, these polymers exhibited strong luminescence in the solution state, presumably because the inductive effect of carborane is dominant, rather than cage‐π interactions.

     

Amphiphilic Hybrid π‐Conjugated Polymers Containing Polyhedral Oligomeric Silsesquioxanes

Amphiphilic hybrid π‐conjugated polymers that have polyhedral oligomeric silsesquioxanes on their side chains have been successfully synthesized by the Sonogashira–Hagihara polycondensation reaction. The obtained polymers were studied with ultraviolet‐visible absorption and photoluminescence spectra. In these polymers, the π‐conjugation length was extended along the poly(p‐phenylene‐ethynylene) backbone. Furthermore, the content of the POSS substituents can influence the aggregation behavior of the polymers and subsequent luminescent properties.

     

Orienting Semi‐Conducting π‐Conjugated Polymers

The present review focuses on the recent progress made in thin film orientation of semi‐conducting polymers with particular emphasis on methods using epitaxy and shear forces. The main results reported in this review deal with regioregular poly(3‐alkylthiophene)s and poly(dialkylfluorenes). Correlations existing between processing conditions, macromolecular parameters and the resulting structures formed in thin films are underlined. It is shown that epitaxial orientation of semi‐conducting polymers can generate a large palette of semi‐crystalline and nanostructured morphologies by a subtle choice of the orienting substrates and growth conditions.

     

Fluorescence‐Amplifying Assay for Irradiated DNA Lesions Using Water‐Soluble Conjugated Polymers

A new platform has been developed for DNA lesion detection using a cationic conjugated polymer (CCP). DNA that contains two adjacent thymine bases is irradiated with ultraviolet light to allow for the formation of cyclobutane pyrimidine dimers and pyrimidine–pyrimidone dimers. The DNA lesions block the primer extension, and the base labeled with fluorescein cannot be incorporated into the DNA strand. Addition of the CCP leads to inefficient fluorescence resonance energy transfer (FRET) from CCP to fluorescein. For the case without DNA lesions, successful primer extension allows for efficient FRET between them. In view of the FRET signal changes, the DNA lesions can be detected. This new protocol offers a convenient detection for DNA lesions in aqueous solution without any isolation and washing steps.

     

Metal Free Acid‐Mediated Solid‐State Polymerization and Processability of Polymers Based on 3,4‐Alkylenedioxythiophenes

The present article reports metal free acid‐mediated polymerization of 3,4‐alkylenedioxythiophenes in a solvent free medium and processability of insoluble polymers. It also describes structural variants and functional aqueous dispersions based on 3,4‐alkylenedioxythiophenes. Polymers were processed by dispersing the insoluble polymer powders in aqueous media using polymeric aromatic surfactant (polystyrenesulfonate, PSS) and aliphatic surfactant (sodiumdodecylsulfonate, SDS), respectively. The effect of surfactant on particle size, stability, and conductivity of the dispersions was investigated in detail. Polymer particles in SDS‐based dispersions tend to agglomerate which resulted enhanced conductivity of the thin films. Electrochemical studies revealed that the polymers are electroactive in nature and are transparent in oxidized state. The electrochromic contrast between the oxidized and reduced states of the polymers was in the range of 40–46%. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 783–791     

Diketopyrrolopyrrole‐Porphyrin Based Conjugated Polymers for Ambipolar Field‐Effect Transistors

Porphyrin‐based molecules have been widely used in dye‐sensitized solar cells and bulk heterojunction solar cells, but their application in field‐effect transistors (FETs) is limited. In this work, two conjugated polymers based on diketopyrrolopyrrole and porphyrin units were developed for FETs. The polymers exhibit extra‐low band gap with energy levels close to −4.0 eV and −5.0 eV due to the strong electron‐donating and withdrawing ability of porphyrin and diketopyrrolopyrrole. With additionally high crystalline properties, ambipolar charge carrier transports with a hole mobility of 0.1 cm² V⁻¹ s⁻¹ in FETs were realized in these polymers, representing the highest performance in solution‐processed FETs based on porphyrin unit.     

Novel Conjugated Polymers Based on Dithieno[3,2‐b:6,7‐b]carbazole for Solution Processed Thin‐Film Transistors

Two conjugated polymers (CPs) P‐tCzC12 and P‐tCzC16 comprising alternating dithieno[3,2‐b:6,7‐b]carbazole and 4,4′‐dihexadecyl‐2,2′‐bithiophene units have been designed and synthesized. Upon thermal annealing, they can form ordered thin films in which the polymer backbones dominantly adopted an edge‐on orientation respective to the substrate with a lamellar spacing of ≈24 Å and a π‐stacking distance of ≈3.7 Å. Organic thin‐film transistors (OTFTs) were fabricated by solution casting. A hole mobility of 0.39 cm² V⁻¹s⁻¹ has been demonstrated with P‐tCzC16. This value is the highest among the CPs containing heteroacenes larger than 4 rings.     

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.

     

Thermally Cross‐Linkable Hole Transport Polymers for Solution‐Based Organic Light‐Emitting Diodes

Two thermally cross‐linkable hole transport polymers that contain phenoxazine and triphenylamine moieties, X‐P1 and X‐P2, are developed for use in solution‐processed multi‐stack organic light‐emitting diodes (OLEDs). Both X‐P1 and X‐P2 exhibit satisfactory cross‐linking and optoelectronic properties. The highest occupied molecular orbital (HOMO) levels of X‐P1 and X‐P2 are −5.24 and −5.16 eV, respectively. Solution‐processed super yellow polymer devices (ITO/X‐P1 or X‐P2/PDY‐132/LiF/Al) with X‐P1 or X‐P2 hole transport layers of various thicknesses are fabricated with the aim of optimizing the device characteristics. The fabricated multi‐stack yellow devices containing the newly synthesized hole transport polymers exhibit satisfactory currents and power efficiencies. The optimized X‐P2 device exhibits a device efficiency that is dramatically improved by more than 66% over that of a reference device without an HTL.

     

A Fluorescence Ratiometric Protein Assay Using Light‐Harvesting Conjugated Polymers

Summary: A highly selective protein assay was created which combines the fluorescent ratiometric technique based on FRET with the light‐harvesting properties of conjugated polymers. The cationic poly[(9,9‐bis(6′‐N,N,N‐trimethylammonium)‐hexyl)‐fluorene phenylene] bromide (PFP‐NMe) and the negatively charged biotinylated fluorescein probe (Fl‐B) were used to detect the target protein streptavidin optically. The strong electrostatic interactions between PFP‐NMe and fluorescein result in efficient FRET from PFP‐NMe to fluorescein. In the presence of streptavidin, however, the biotin moiety of Fl‐B specifically associates with streptavidin and the fluorescein molecule is buried deeply in the adjacent vacant binding sites. This separates the fluorescein spatially from the PFP‐NMe moiety, resulting in inefficient FRET from PFP‐NMe to fluorescein. Although a nonspecific protein, such as BSA, shows nonspecific interactions with PFP‐NMe, it does not affect the fluorescent ratio value of PFP‐NMe to fluorescein. Hence, the charged neutral complex of two oppositely charged conjugated polymers can eliminate the nonspecific interactions, and thus optimize their application in protein assays.

A schematic representation of the protein assay operation.     

Click Functionalization of a Dibenzocyclooctyne‐Containing Conjugated Polyimine

A conjugated poly(phenyl‐co‐dibenzocyclooctyne) Schiff‐base polymer, prepared through polycondensation of dibenzocyclooctyne bisamine (DIBO‐(NH₂)₂) with bis(hexadecyloxy)phenyldialdehyde, is reported. The resulting polymer, which has a high molecular weight (M_n>30 kDa, M_w>60 kDa), undergoes efficient strain‐promoted alkyne–azide cycloaddition reactions with a series of azides. This enables quantitative modification of each repeat unit within the polymer backbone and the rapid synthesis of a conjugated polymer library with widely different substituents but a consistent degree of polymerization (DP). Kinetic studies show a second‐order reaction rate constant that is consistent with monomeric dibenzocyclooctynes. Grafting with azide‐terminated polystyrene and polyethylene glycol monomethyl ether chains of varying molecular weight resulted in the efficient syntheses of a series of graft copolymers with a conjugated backbone and maximal graft density.     

Multifunctional Non‐Viral Delivery Systems Based on Conjugated Polymers

Multifunctional nanomaterials with simultaneous therapeutic and imaging functions explore new strategies for the treatment of various diseases. Conjugated polymers (CPs) are considered as novel candidates to serve as multifunctional delivery systems due to their high fluorescence quantum yield, good photostability, and low cytotoxicity. Highly sensitive sensing and imaging properties of CPs are well reviewed, while the applications of CPs as delivery systems are rarely covered. This feature article mainly focuses on CP‐based multifunctional non‐viral delivery systems for drug, protein, gene, and cell delivery. Promising directions for the further development of CP‐based delivery systems are also discussed.