Ambipolar charge transport in a donor–acceptor–donor‐type conjugated block copolymer and its gate‐voltage‐controlled thin film transistor memory

We demonstrate a fully conjugated donor–acceptor–donor (D–A–D) triblock copolymer, PBDTT–PNDIBT–PBDTT, which contains PBDTT as the donor block and PNDIBT as the acceptor block. The polymer was synthesized by end‐capping each block with a reactive unit, followed by condensation copolymerization between the two blocks. The physical, optical, and electrochemical properties of the polymer were investigated by comparing those of donor‐ and acceptor‐homopolymers (i.e., PBDTT and PNDIBT), which are the oligomeric monomers, and their blends. On using the newly synthesized block copolymer, ambipolar charge transport behavior was observed in the corresponding thin‐film transistor, and the behavior was compared to that of blend film of donor‐ and acceptor‐homopolymers. Owing to the presence of donor and acceptor blocks in a single polymer chain, it was found that the triblock copolymer can store two‐level information; the ability to store this information is one of the most intriguing challenges in memory applications. In this study, we confirmed the potential of the triblock copolymer in achieving distinct two‐stage data storage by utilizing the ambipolar charge trapping phenomenon, which is expected in donor and acceptor containing random and block copolymers in a thin‐film transistor. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3223–3235     

Combining Light Harvesting and Electron Transfer in Silica–Titania‐Based Organic–Inorganic Hybrid Materials

A convenient protocol to fabricate an organic–inorganic hybrid system with covalently bound light‐harvesting chromophores (stilbene and terphenylene–divinylene) and an electron acceptor (titanium oxide) is described. Efficient energy‐ and electron‐transfer processes may take place in these systems. Covalent bonding between the acceptor chromophores and the titania/silica matrix would be important for electron transfer, whereas fluorescence resonant energy transfer (FRET) would strongly depend on the ratio of donor to acceptor chromophores. Time‐resolved spectroscopy was employed to elucidate the detailed photophysical processes. The coupling of FRET and electron transfer was shown to work coherently to lead to photocurrent enhancement. The photocurrent responses reached a maximum when the hybrid‐material thin film contained 60 % acceptor and 40 % donor.     

Controlled self‐assembly of porphyrin/fullerene donor–acceptor complex in a polymer thin film

Thin films composed of polycyclohexane (PCHE), zinc(II)‐5,10,15,20‐tetra‐(2‐naphthyl)porphyrin (ZnTNpP), and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) blends are prepared to investigate their potential for the controlled self‐assembly of a porphyrin/fullerene donor–acceptor complex in a polymer thin film. The compatibilities of PCHE/PCBM (p), PCHE/ZnTNpP (q), and ZnTNpP/PCBM (r) in these blends have a significant effect on the dispersion of the ZnTNpP/PCBM donor–acceptor complex in the PCHE thin film. When the compatibilities are p << q, r, and q ≈ r, the ZnTNpP/PCBM donor–acceptor complex is formed between the PCHE and PCBM phases. This concept to form a controlled self‐assembly of the ZnTNpP/PCBM donor–acceptor complex may be applied to various combinations of porphyrin/fullerene systems in polymer thin film solar cells to achieve excellent performance. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 743–746     

Direct arylation synthesis of thienoisoindigo‐based low‐band‐gap polymer from asymmetric donor–acceptor monomer

Thienoisoindigo (TIG) moiety has been paid numerous attentions as an excellent acceptor building block in low‐band‐gap polymers. Herein, a new TIG‐dithiophene alternating copolymer (PTIG2T) was successfully synthesized from an asymmetric TIG‐based donor–acceptor (D‐A) monomer via the self‐condensation‐type direct arylation polymerization. PTIG2T exhibited the light absorption over 1000 nm owing to the intramolecular charge transfer in the thin film state, which corresponded to an optical band gap of 1.24 eV. The HOMO and LUMO levels of PTIG2T were determined to be −5.08 and −3.60 eV, respectively. Furthermore, the organic photovoltaic (OPV) with a PTIG2T/PC₆₁BM active layer achieved a power conversion efficiency (PCE) of 3.19%, which is one of the highest PEC achieved by OPVs with TIG‐based materials. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 430–436     

Recent Advances in Isoindigo‐Inspired Organic Semiconductors

Over the past decade, isoindigo has become a widely used electron‐deficient subunit in donor‐acceptor organic semiconductors, and these isoindigo‐based materials have been widely used in both organic photovoltaic (OPV) devices and organic field effect transistors (OFETs). Shortly after the development of isoindigo‐based semiconductors, researchers began to modify the isoindigo structure in order to change the optoelectronic properties of the resulting materials. This led to the development of many new isoindigo‐inspired compounds; since 2012, the Kelly Research Group has synthesized a number of these isoindigo analogues and produced a variety of new donor‐acceptor semiconductors. In this Personal Account, recent progress in the field is reviewed. We describe how the field has evolved from relatively simple donor‐acceptor small molecules to structurally complex, highly planarized polymer systems. The relevance of these materials in OPV and OFET applications is highlighted, with particular emphasis on structure‐property relationships.     

Photoconductivity in Metal–Organic Framework (MOF) Thin Films

Photoconductivity is a characteristic property of semi‐conductors. Herein, we present a photo‐conducting crystalline metal–organic framework (MOF) thin film with an on–off photocurrent ratio of two orders of magnitude. These oriented, surface‐mounted MOF thin films (SURMOFs), contain porphyrin in the framework backbone and C₆₀ guests, loaded in the pores using a layer‐by‐layer process. By comparison with results obtained for reference MOF structures and based on DFT calculations, we conclude that donor–acceptor interactions between the porphyrin of the host MOF and the C₆₀ guests give rise to a rapid charge separation. Subsequently, holes and electrons are transported through separate channels formed by porphyrin and by C₆₀, respectively. The ability to tune the properties and energy levels of the porphyrin and fullerene, along with the controlled organization of donor–acceptor pairs in this regular framework offers potential to increase the photoconduction on–off ratio.     

Synthesis and properties of the [2.2]paracyclophane‐containing conjugated polymer with benzothiadiazole as an electron acceptor

A novel donor–acceptor type conjugated polymer based on PPE with [2.2]paracyclophane and benzothiadiazole units in the main chain was synthesized by the Sonogashira coupling reaction. The obtained polymer was quite soluble in common organic solvents, and the transparent and uniform thin film of the polymer was obtained easily by casting or spin‐coating from a toluene solution. The polymer showed an extension of π‐delocalization via the through‐space with π‐π stacking according to the UV–visible (UV–vis) absorption spectra in comparison with that of the model compound. The polymer exhibited orange photoluminescence in solution (fluorescence quantum efficiency = 0.13) and in the solid state. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5891–5899, 2004     

Luminescent copolymers for applications in multicolor‐light‐emitting devices

Light‐emitting diodes based on organic materials [organic light‐emitting diodes (OLEDs)] have attracted much interest over the past decade. Several different attempts have been made to realize multicolor OLEDs. This article describes a new approach based on energy transfer in a donor/acceptor system. A copolymer containing both donor and acceptor compounds as comonomer units is prepared. The polymer consists of a derivative of a luminescent dye [4‐dicyanmethylene‐2‐methyl‐6‐4H‐pyran (DCM); acceptor compound], which is copolymerized with fluorene (donor compound) to combine the properties of an electroactive polymer with a highly luminescent dye. Photochemical processing is achieved by UV irradiation of this copolymer in the presence of gaseous trialkylsilanes. This reagent selectively saturates the C?C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue‐green electroluminescence from the polyfluorene units is recovered. Compared with previous approaches based on polymer blends, this copolymer approach avoids problems associated with phase‐separation phenomena in the active layer of OLEDs. © 2006Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4317–4327, 2006     

Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence Switching of Tetraphenylethene‐Dithienylethene‐Perylenemonoimide Triad with Donor–Bridge–Acceptor Structure

Photochromic 1,2‐dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor–photochromic bridge–acceptor tetraphenylethene‐dithienylethene‐perylenemonoimide (TPE‐DTE‐PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE‐DTE‐PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE‐DTE‐PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.     

Synthesis and Characterization of a Soluble A–D–A Molecule Containing a 2D Conjugated Selenophene‐Based Side Group for Organic Solar Cells

A new acceptor–donor–acceptor (A–D–A) small molecule based on benzodithiophene (BDT) and diketopyrrolopyrrole (DPP) is synthesized via a Stille cross‐coupling reaction. A highly conjugated selenophene‐based side group is incorporated into each BDT unit to generate a 2D soluble small molecule (SeBDT‐DPP). SeBDT‐DPP thin films produce two distinct absorption peaks. The shorter wavelength absorption (400 nm) is attributed to the BDT units containing conjugated selenophene‐based side groups, and the longer wavelength band is due to the intramolecular charge transfer between the BDT donor and the DPP acceptor. SeBDT‐DPP thin films can harvest a broad solar spectrum covering the range 350–750 nm and have a low bandgap energy of 1.63 eV. Solution‐processed field‐effect transistors fabricated with this small molecule exhibit p‐type organic thin film transistor characteristics, and the field‐effect mobility of a SeBDT‐DPP device is measured to be 2.3 × 10⁻³ cm² V⁻¹ s⁻¹. A small molecule solar cell device is prepared by using SeBDT‐DPP as the active layer is found to exhibit a power conversion efficiency of 5.04% under AM 1.5 G (100 mW cm⁻²) conditions.

     

A Novel Bat‐Shaped Dicyanomethylene‐4H‐pyran‐Functionalized Naphthalimide for Highly Efficient Solution‐Processed Multilevel Memory Devices

Small‐molecule‐based multilevel memory devices have attracted increasing attention because of their advantages, such as super‐high storage density, fast reading speed, light weight, low energy consumption, and shock resistance. However, the fabrication of small‐molecule‐based devices always requires expensive vacuum‐deposition techniques or high temperatures for spin‐coating. Herein, through rational tailoring of a previous molecule, DPCNCANA (4,4′‐(6,6′‐bis(2‐octyl‐1,3‐dioxo‐2,3‐dihydro‐1H‐benzo[de]isoquinolin‐6‐yl)‐9H,9′H‐[3,3′‐bicarbazole]‐9,9′‐diyl)dibenzonitrile), a novel bat‐shaped A‐D‐A‐type (A‐D‐A=acceptor–donor–acceptor) symmetric framework has been successfully synthesized and can be dissolved in common solvents at room temperature. Additionally, it has a low‐energy bandgap and dense intramolecular stacking in the film state. The solution‐processed memory devices exhibited high‐performance nonvolatile multilevel data‐storage properties with low switching threshold voltages of about −1.3 and −2.7 V, which is beneficial for low power consumption. Our result should prompt the study of highly efficient solution‐processed multilevel memory devices in the field of organic electronics.     

Organic Photovoltaic Devices Based on a New Class of Oligothienylenevinylene Derivatives as Donor Materials

A new class of donor–acceptor‐containing oligothienylenevinylenes with a triphenylamine donor and a dicyanovinyl group as acceptor has been synthesized and characterized. By extending the oligothiophene backbone, both the optical bandgaps and the charge‐transport properties can be tuned. These oligothienylenevinylene derivatives show intense charge‐transfer absorption bands that cover the entire visible spectrum, with low optical bandgaps of approximately 1.64 eV. In addition, electrochemical studies reveal that these compounds possess relatively large ionization potentials of approximately 5.5 eV. On the basis of these newly developed dicyanovinyl‐substituted chromophores as donor materials and C₆₀ as acceptor material, bilayer organic photovoltaic devices have been fabricated, with the best device showing a high power conversion efficiency (PCE) of 2.0 %, with an open‐circuit voltage of 0.68 V and a fill factor of 0.60 after thermal annealing. The obvious morphology change with the formation of small domains in thin films and the reduction of series resistance are believed to be responsible for the dramatic performance improvement upon thermal annealing.     

Advances in Organic Near‐Infrared Materials and Emerging Applications

Much progress has been made in the field of research on organic near‐infrared materials for potential applications in photonics, communications, energy, and biophotonics. This account mainly describes our research work on organic near‐infrared materials; in particular, donor‐acceptor small molecules, organometallics, and donor‐acceptor polymers with the bandgaps less than 1.2 eV. The molecular designs, structure‐property relationships, unique near‐infrared absorption, emission and color/wavelength‐changing properties, and some emerging applications are discussed.     

Functionalization of a Ruthenium–Diacetylide Organometallic Complex as a Next‐Generation Push–Pull Chromophore

The design and preparation of an asymmetric ruthenium–diacetylide organometallic complex was successfully achieved to provide an original donor–π–[M]–π–acceptor architecture, in which [M] corresponds to the [Ru(dppe)₂] (dppe: bisdiphenylphosphinoethane) metal fragment. The charge‐transfer processes occurring upon photoexcitation of the push–pull metal–dialkynyl σ complex were investigated by combining experimental and theoretical data. The novel push–pull complex, appropriately end capped with an anchoring carboxylic acid function, was further adsorbed onto a semiconducting metal oxide porous thin film to serve as a photosensitizer in hybrid solar cells. The resulting photoactive material, when embedded in dye‐sensitized solar cell devices, showed a good spectral response with a broad incident photon‐to‐current conversion efficiency profile and a power conversion efficiency that reached 7.3 %. Thus, this material paves the way to a new generation of organometallic chromophores for photovoltaic applications.     

Controlling Singlet–Triplet Energy Splitting for Deep‐Blue Thermally Activated Delayed Fluorescence Emitters

The development of efficient metal‐free organic emitters with thermally activated delayed fluorescence (TADF) properties for deep‐blue emission is still challenging. A new family of deep‐blue TADF emitters based on a donor–acceptor architecture has been developed. The electronic interaction between donor and acceptor plays a key role in the TADF mechanism. Deep‐blue OLEDs fabricated with these TADF emitters achieved high external quantum efficiencies over 19.2 % with CIE coordinates of (0.148, 0.098).     

In‐line coupled single drop liquid–liquid–liquid microextraction with capillary electrophoresis for determining fluoroquinolones in water samples

A simple in‐line single drop liquid–liquid–liquid microextraction (SD‐LLLME) coupled with CE for the determination of two fluoroquinolones was developed. The method is capable to quantify trace amount of analytes in water samples and to improve the sensitivity of CE detection. For the SD‐LLLME, a thin layer of organic phase was used to separate a drop of 0.1 M NaOH hanging at the inlet of the capillary from the aqueous donor phase. By this way, the analytes were extracted to the acceptor phase through the organic layer based on their acidic/basic dissociation equilibrium. The drop was immersed into the organic phase during 10 min for extraction and then it is directly injected into the capillary for the analysis. Parameters such as type and volume of organic solvent phase, aqueous donor, and acceptor phases and extraction time and temperature were optimized. The enrichment factor was calculated, resulting 40‐fold for enrofloxacin (ENR) and sixfold for ciprofloxacin (CIP). The linear range were 20–400 μg/L for ENR and 60–400 μg/L for CIP. The detection limits were 10.1 μg/L and 55.3 μg/L for ENR and CIP, respectively, and a good reproducibility was obtained (4.4% for ENR and 5.6% for CIP). Two real water samples were analysed applying the new method and the obtained results presented satisfactory recovery percentages (90–100.3%).     

Synthesis and electroluminescent performance of thermally activated delayed fluorescence‐conjugated polymers with simple formylphenyl as pendant acceptor

Formylphenyl has been demonstrated to act as an acceptor to construct thermally activated delayed fluorescence (TADF) emitter, and therefore a series of the TADF‐conjugated polymers with formylphenyl as pendant acceptor and carbazole/acridine as backbone donor are designed and synthesized. All polymers involve the twisted donor/acceptor structural moieties with the sufficiently spatial separation between the highest occupied molecular orbital and the lowest unoccupied molecular orbital as well as a small singlet/triplet splitting, and exhibit the legible TADF features confirmed by theoretical calculation and their transient decay spectra. The solution‐processed organic light‐emitting diodes using neat film of the polymers as emissive layer achieve excellent performance with the maximum external quantum efficiency (EQE) of up to 10.6%, the maximum current efficiency of up to 35.3 cd A⁻¹ and the low turn‐on voltage of 2.7 V. Moreover, the EQE still remains 10.3% at the luminance of 1000 cd m⁻² with the low driving voltage of 4.4 V and extremely small efficiency roll‐off. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1989–1996     

A Solution‐Processable Molecule using Thieno[3,2‐b]thiophene as Building Block for Efficient Organic Solar Cells

A solution‐processed acceptor‐π‐donor‐π‐acceptor (A‐π‐D‐π‐A) type small molecule, namely DCATT, has been designed and synthesized for the application as donor material in organic solar cells. The fused aromatic unit thieno[3,2‐b]thiophene (TT) flanked with thiophene is applied as π bridge, while 4,8‐bisthienyl substituted benzodithiophene (BDT) and 2‐ethylhexyl cyanoacetate are chosen as the central building block and end group, respectively. Introduction of fused ring to the small molecule enhances the conjugation length of the main chain, and gives a strong tendency to form π–π stacking with a large overlapping area which favors to high charge carrier transport. Small‐molecule organic solar cells based on blends of DCATT and fullerene acceptor exhibit power conversion efficiencies as high as 5.20 % under the illumination of AM 1.5G, 100 mW cm^?2.     

Solution‐Grown Organic Single‐Crystalline Donor–Acceptor Heterojunctions for Photovoltaics

Organic single crystals are ideal candidates for high‐performance photovoltaics due to their high charge mobility and long exciton diffusion length; however, they have not been largely considered for photovoltaics due to the practical difficulty in making a heterojunction between donor and acceptor single crystals. Here, we demonstrate that extended single‐crystalline heterojunctions with a consistent donor‐top and acceptor‐bottom structure throughout the substrate can be simply obtained from a mixed solution of C₆₀ (acceptor) and 3,6‐bis(5‐(4‐n‐butylphenyl)thiophene‐2‐yl)‐2,5‐bis(2‐ethylhexyl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione (donor). 46 photovoltaic devices were studied with the power conversion efficiency of (0.255±0.095) % under 1 sun, which is significantly higher than the previously reported value for a vapor‐grown organic single‐crystalline donor–acceptor heterojunction (0.007 %). As such, this work opens a practical avenue for the study of organic photovoltaics based on single crystals.     

Efficient Light‐Harvesting Systems with Tunable Emission through Controlled Precipitation in Confined Nanospace

Light harvesting is a key step in photosynthesis but creation of synthetic light‐harvesting systems (LHSs) with high efficiencies has been challenging. When donor and acceptor dyes with aggregation‐induced emission were trapped within the interior of cross‐linked reverse vesicles, LHSs were obtained readily through spontaneous hydrophobically driven aggregation of the dyes in water. Aggregation in the confined nanospace was critical to the energy transfer and the light‐harvesting efficiency. The efficiency of the excitation energy transfer (EET) reached 95 % at a donor/acceptor ratio of 100:1 and the energy transfer was clearly visible even at a donor/acceptor ratio of 10 000:1. Multicolor emission was achieved simply by tuning the donor/acceptor feed ratio in the preparation and the quantum yield of white light emission from the system was 0.38, the highest reported for organic materials in water to date.