Direct Nanopatterning Into Conjugated Polymers Using Interference Crosslinking

Nanoscale photonic structures are patterned directly by exposing a thin film of conjugated polymer to the interference pattern of UV laser beams, thus inducing crosslinking between the polymer molecules in the bright fringes. The crosslinked polymer molecules become indissolvable in organic solvent due to the localized photochemical modification, enabling successful lift‐off of the soft grating structures. The resultant nanodevice exhibits excellent chemical and physical stability. This demonstrates the high spatial resolution of the crosslinking process and introduces a direct nanofabrication technique, which may be utilized in the design of optoelectronic and laser devices.     

Molecular‐Recognition‐Induced Phase Transitions of Two Thermo‐Responsive Polymers with Pendent β‐Cyclodextrin Groups

PNIPAM‐based thermo‐responsive polymers with pendent β‐cyclodextrin groups were synthesized and the molecular‐recognition‐induced phase‐transition behavior of fabricated polymers was investigated. The results showed that the thermo‐sensitive PNG‐ECD and PNG‐HCD polymers could significantly recognize the guest ANS molecules, and their LCSTs in ANS aqueous solutions were lower than those in blank aqueous solution. The more ANS could be recognized by the polymer, the lower was the LCST of the polymer. The guest NS molecules had an opposite influence on the LCSTs of the PNG‐ECD and PNG‐HCD polymers, because the complexation between CD and NS slightly enlarged the hydrophilic moiety of the polymers.

     

Synthesis of Ruthenium Complex Containing Conjugated Polymers and Their Applications in Dye‐Sensitized Solar Cells

The synthesis of two ruthenium terpyridine complexes containing conjugated polymers is reported and their application as photosensitizers in dye‐sensitized solar cells (DSSC) was studied. The polymers were synthesized by the palladium‐catalyzed coupling reaction between the ruthenium complex monomer and the fluorene‐based comonomer. One polymer was functionalized with carboxylic acid groups on the main chain, which strongly facilitate the anchorage of polymer dye molecules on the electrode surface. DSSCs based on TiO₂ nanoparticles and nanotubes were fabricated. It was found that the ruthenium complexes played an important role in the photosensitization process, and the power conversion efficiencies measured were in the order of 0.1%. Annealing of TiO₂ nanotubes in ammonia gas flow did not result in significant improvement in cell performance.

     

“Heavy‐Atom” Donor–Acceptor Conjugated Polymers

Heavy‐atom substitution chemistry in donor–acceptor (D–A) conjugated polymers has gained increasing attention in the past few years. Overall, the introduction of “heavy” atoms (below C in group 14; below N in group 15; below S in group 16) into D–A conjugated polymers allows control over properties through relatively straightforward synthetic chemistry, and produces materials with high molecular weights (>20 000 g mol^?1), strong absorption (ε ≈ 40 000 L mol^?1 cm^?1), narrow highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps (<1 eV), as well as high charge‐carrier mobility (>10^?2 cm² V^?1 s^?1). This Trend article aims to describe heavy‐atom substitution effects in D–A polymers, and their applications, by systematically examining representative polymer structures.

     

Synthesis of New Blue Anthracene‐based Conjugated Polymers and Their Applications in Polymer Light‐Emitting Diodes

A series of anthracene‐based conjugated copolymers containing 9,10‐bis(6‐bromonaphthalen‐2‐yl)‐2‐tert‐butylanthracene (BNA) and 2,7‐diphenyl substituted fluorene (DPPF) moieties are prepared via a palladium‐catalyzed Suzuki polymerization. All of the synthesized polymers emit blue light at around 450 nm and show good thermal and color stability. Their electroluminescence spectra remain unchanged at high driving voltage. The double‐layer polymer light‐emitting diode (PLED) fabricated with ITO/PEDOT:PSS/DPPFBNA3/CsF/Al, produces a maximum brightness of 1 650 cd · m^?2 and has a luminance efficiency of 0.39 cd · A^?1. The ITO/PEDOT:PSS/TFB/DPPFBNA3/CsF/Al multilayer PLED, incorporating a TFB layer to facilitate hole transportation, produces a maximum brightness of 5 371 cd · m^?2 and a luminance efficiency of 1.18 cd · A^?1.

     

One‐Step Route to Ladder‐Type C–N Linked Conjugated Polymers

A one‐step synthetic method is demonstrated to construct ladder‐type conjugated polymers without the resource to post‐cyclization procedures. The ladder‐type C–N linked conjugated polymers ( P1 and P2 ) and model compounds ( 1 and 2 ) are achieved in high yields. The obtained model compounds and polymers display desirable solubility in commonly used solvents and high thermal stability, which show a promising application in photoelectric material field.     

π‐Conjugated Polymers Consisting of 9,10‐Dihydrophenanthrene Units

π‐Conjugated polymers consisting of 9,10‐disubstituted 9,10‐dihydrophenenthrene, with the substituents octyl, 2‐ethylhexyl, ‐OSiBu₃, etc., units are prepared by organometallic polycondensation. Homopolymers ( PH₂Ph(9,10‐R) ) have a π‐conjugation system similar to that of polymers of 9,9‐dialkylfluorene and show UV‐Vis peaks at ≈380 nm. In addition to the peak at ≈380 nm, some homopolymers give rise to a peak at a longer wavelength, suggesting molecular assembly of the polymers. X‐ray diffraction data support the molecular assembly. The homopolymers show photoluminescence (PL) with PL peaks at ≈430 nm, and PL spectrum of the polymer film is essentially unchanged after heating the polymer film at 150 °C in air. The homopolymers undergo electrochemical p‐doping at about 1.5 V versus Ag⁺/Ag.     

Conjugated Main Chain Azo‐Polymers Based on Polycyclic Aromatic Hydrocarbons

A reductive coupling reaction employing sodium bis(2‐methoxyethoxy) aluminum hydride is used to prepare main chain azo‐polymers comprising of polycyclic aromatic hydrocarbons (naphthalene, anthraquinone, or fluorenone) from their dinitro‐derivatives. The azo‐bridges act as effective means of conjugation and all polymers exhibit differences in the ultra‐violet–visible light absorption and photoluminescence emission spectra depending on the degree of polymerization. Furthermore, in the case of poly(azofluorenone)s and poly(azoanthraquinone)s, these spectra may be modified by changes in the protonation state of the polymers. The lowest unoccupied molecular orbital and highest occupied molecular orbital energy levels and the band gap of poly(azoanthraquinone) are estimated from cyclic voltammetry data and UV–visible absorption of films.     

High‐Mobility Conjugated Polymers Based on Fused‐Thiophene Building Blocks

Organic field‐effect transistors (OFETs) are a promising cost‐effective alternative to silicon‐based field‐effect transistors, and possess low‐cost, light‐weight, and flexibility advantages. Conjugated polymers based on fused‐thiophene building blocks have received considerable attention in the emerging field of organic electronics. In this review the most recent developments in conjugated polymers based on fused‐thiophene rings for high‐performance OFETs are summarized. The focus is on correlations of polymer chemical structures with properties, such as energy levels, film‐forming property, film morphology, and OFET performance. This structure–property relationship analysis may guide rational structural design and evaluation of organic semiconductors.

     

4,5‐Ethylene‐2,7‐Carbazole‐Based Medium‐Bandgap Conjugated Polymers with Low‐Lying HOMO Levels Toward Efficient Polymer Solar Cells with High Open‐Circuit Voltage

Three medium‐bandgap polymers based on a 4,5‐ethylene‐2,7‐dithienyl carbazole as the electron‐donating unit and different 5,6‐dialkoxy‐2,1,3‐benzothiadiazoles as the electron‐accepting units, are synthesized as polymer donors for photovoltaic applications. The three copolymers possess highest occupied molecular oribital (HOMO) levels around ?5.47 eV and medium bandgaps of about 1.94 eV. The solar cells with polymer:[6,6]‐phenyl C71‐butyric acid methyl ester (PC₇₁BM) = 1:4 as the active layer, show an especially high open‐circuit voltage (V_oc) of 0.95 V and attain good power conversion efficiency up to 5.91%. The hole mobilities of the active layer films, measured by space‐charge‐limited current (SCLC), are up to 3.5 × 10^?4 cm² V^?1 s^?1. Given the favorable medium bandgaps, low‐lying HOMO levels, and good hole mobilities, these copolymers are promising candidates for the construction of a highly efficient front cell to harvest the shorter wavelength band of the solar radiation in a tandem solar cell with high V_oc.

     

Conjugated Polymers Featuring Oxacyclophane‐Scaffolded π‐Stacking Interactions

An oxacyclophane framework is modified to include π‐conjugated segments positioned so as to allow an optimized face‐to‐face π‐stacking interaction between them. This unique architecture is first employed for the preparation of model compounds with defined interaction between two small molecular organic chromophores. The interaction allows facile through‐space energy transfer between the chromophores when they are held in the appropriate geometry. The oxacyclophane is thus employed in a polymer with consecutive short stretches of phenylene ethynylene such that polymer chain conjugation requires through‐space interaction between segments rather than the through‐bond π‐conjugation that typifies traditional conjugated polymers. The extent of the through‐space interactions along the polymer backbone is explored through photophysical measurements, calculations, X‐ray diffraction, and comparison with a variety of model and control materials.

     

Investigation of Bimetallic Nickel Catalysts in Catalyst‐Transfer Polymerization of π‐Conjugated Polymers

A comparative study involving bimetallic nickel catalysts designed from disubstituted N,N,N′,N′‐tetra(diphenylphosphanylmethyl)benzene diamine bridging ligands is reported. Catalyst behavior is explored in the Kumada catalyst‐transfer polymerization (KCTP) using poly(3‐hexylthiophene) (P3HT) as the model system. The success of a controlled polymerization is monitored by analyzing monomer conversion, degree of polymerization, end‐group identity, and molecular weight distribution. The characterization of P3HT obtained from KCTP initiated with the bimetallic catalysts shows chain‐growth behavior; however, the presence of Br/Br end‐groups and broader molecular weight distribution reveals a reduced controlled polymerization compared to the commonly employed Ni(dppp)Cl₂. The observed increase in intermolecular chain transfer and termination processes in KCTP initiation with the bimetallic catalysts can be attributed to a weaker Ni(0)‐π‐aryl complex interaction, which is caused by increased steric crowding of the coordination sphere.     

Solid‐State Dye‐Sensitized Solar Cells with Conjugated Polymers as Hole‐Transporting Materials

DSCs are a promising alternative to conventional silicon‐based solar cells owing to their low cost and relatively high efficiency. However, the utilization of a liquid electrolyte containing the iodine/iodide redox couple in traditional DSCs brings practical problems for their long‐term application, which leads to rapid development of SDSCs based on inorganic p‐type semiconductors or organic HTMs. In this review, we summarize the current research on SDSCs using conjugated polymer as HTM. Special attention is paid to understand the effects of polymer HTM structure and deposition process on SDSC performance. The limiting factors for SDSC energy conversion efficiency are discussed and strategies to improve device performance are proposed.

     

Enhanced Fluorescence Quenching of 2‐Thiohydantoin‐Containing Conjugated Polymers: Applications for Ion Sensing

A novel poly(phenylene ethynylene) containing 2‐thiohydantoin [poly( 1 )] is synthesized. The addition of F^? quenches the fluorescence, while the fluorescence of the solution only changes slightly upon addition of other anions, indicating the sensing ability of poly( 1 ) toward F^?. The addition of Ag⁺ quenches the fluorescence of the polymer, whereas the addition of other metal ions results in only slight changes to the fluorescence. Compared with its small‐molecule counterpart, the Stern–Volmer quenching constants of poly( 1 ) toward F^? and Ag⁺ are 165 and 105 times greater, respectively. This result indicates the amplified quenching effect of poly( 1 ).

     

Synthesis and Optical Properties of π‐Conjugated Polymers Composed of Benzo[1,2‐b:4,5‐b′]dithiophene and Thiophenes Bearing Electron‐Deficient Ethenyl Groups in the Side Chains

π‐Conjugated polymers composed of dialkoxybenzo[1,2‐b:4,5‐b′]dithiophene and thiophenes bearing cyano‐alkoxycarbonylethenyl [? CH?C(CO₂R)CN] and bis(alkoxycarbonyl)ethenyl groups [? CH?C(CO₂R)₂] were prepared. The optical properties of the obtained polymers were investigated by UV‐vis and photoluminescence spectroscopy to demonstrate that the absorption maxima of the polymers were tunable by varying the substituent of thiophene unit. The photoluminescence wavelengths and intensities of the polymer in solution were significantly dependent on the polymer side chain. The HOMO energy level of the polymer was lowered by up to ?5.51 eV by introducing electron‐deficient cyano groups. Polymer solar cells based on the new polymers were fabricated to achieve a PCE of 1.90%.

     

Bisfuran‐s‐Tetrazine‐Based Conjugated Polymers: Synthesis,Characterization, and Photovoltaic Properties

Bisfuran‐s‐tetrazine (FTz) and its copolymers with cyclopenta[2,1‐b:3,4‐b′]dithiophene (CPDT) and benzo[1,2‐b:4,5‐b′]dithiophene (BDT) (PCPDTFTz and PBDTFTz) are prepared with the alternating s‐tetrazine and CPDT or BDT units bridged by a furan ring. Their optical and electrochemical properties are studied and compared with their thiophene analogs (PCPDTTTz‐out and PCPDTTTz‐in), in which the bridging unit is 4‐hexylthiophene or 3‐hexylthiophene, respectively. Bulk heterojunction (BHJ) solar cells fabricated from these polymers with PC₇₁BM have a power conversion efficiency (PCE) of 0.8%, which is much lower than that from the PCPDTTTz‐out analog (3.2%), due to the low steric hindrance of the furan polymers in the absence of alkyl substitution on the furan ring.     

Direct Sub‐Micrometer‐Patterning of Conjugated Polymers and Polymer Light‐Emitting Devices by Electron Beam Lithography

The lateral pattering of polymer light emitting devices (PLEDs) on the micro‐ and even sub‐micrometer level is a challenging task. Being able to fabricate devices with sub‐micrometer active device dimensions will, however, open new possibilities for fundamental studies as well as enable improvements in device performance. Therefore, in this study an electron beam‐based patterning method for conjugated polymers is evaluated, where the structuring is achieved by deliberate degradation and alteration of the molecular structure accompanied by a change in the emission properties. We find that the typical feature size accomplished with this method is approx. 2 µm for the active line width in PLEDs, while the in‐depth investigation of the structures shows that the lateral resolution is found to be approx. 500 nm.

     

Realizing Long‐Range Orientational Order in Conjugated Polymers via Solventless Polymerization Strategy

Long‐range orientational order in polymers, especially in conjugated polymers (CPs), is of significance in soft matter: from in‐depth understanding of their intrinsic states to fabrication of high‐performance CP‐based devices. However, this field is under‐explored due to lack of approaches to realizing such long‐range order in CPs. Here a universal rule for the synthesis of highly oriented CPs is presented: in a very stable environment with controllable growth rate and size. In order to verify this rule, the growth of highly oriented PPy is realized within the semi‐closed micropores of a graphene foam using a solventless polymerization method at low temperature. These findings provide directions for the construction of ordered CPs. Further explorations of the physical/chemical properties and high‐performance CP‐based device fabrication are anticipated.     

Screening of Film‐Formation Qualities of Various Solvent Systems for π‐Conjugated Polymers Via Combinatorial Inkjet Printing

A systematic investigation of the relationships between the choice of solvent system and the film quality of inkjet printed π‐conjugated polymer films is presented. Solution properties, such as surface tension and viscosity, have no effect on the final film quality, whereas the boiling point of the solvent and, more specifically, the difference in boiling point between the main and the co‐solvent have a strong influence on the final film quality. New solvent systems are developed based on non‐chlorinated, aromatic solvents that reveal homogeneous films when the difference in boiling point between the main and the co‐solvent is between 90 and 100 °C.     

Conjugated Polymer‐Based Nanoparticles for Cancer Cell‐Targeted and Image‐Guided Photodynamic Therapy

Multifunctional materials with cell imaging and therapeutic functions are anticipated to provide promising agents in cancer treatment. Herein, folic acid modified conjugated polymer nanoparticles (FA‐CPNs) are exploited to target cancer cells and destroy the neighboring cancer cells. Folic acid in FA‐CPNs enables the strong binding with high affinity to folate receptor, a cancer‐cell‐associated protein, to target cancer cells. FA‐CPNs are able to produce cytotoxic reactive oxygen species upon irradiation 600 nm light, and therefore cause the cell death of folate receptor‐overexpressed cancer cells effectively. Therefore, the conjugated polymer‐based nanoparticles provide a multifunctional system for targeting cancer cells and photodynamic therapy.