Diketopyrrolopyrrole Organic Thin‐Film Transistors: Impact of Alkyl Substituents and Tolerance of Ethylhexyl Stereoisomers

Bis(thiophen‐2‐yl)‐diketopyrrolopyrrole (DPP) dyes bearing various alkyl substituents at the amide positions (n‐butyl, n‐pentyl, n‐hexyl, n‐heptyl, n‐octyl, 2‐ethylhexyl) and chlorine (Cl), bromine (Br), or cyano (CN) substituents at the thiophene positions have been synthesized and investigated with regard to their molecular and semiconducting properties. Intense absorption, strong fluorescence, and reversible oxidation and reduction processes are common to all of these dyes. Their characterization as organic semiconductors in vacuum‐processed thin‐film transistors reveals p‐channel operation with field‐effect mobilities ranging from 0.01 to 0.7 cm² V^?1 s^?1. The highest mobility is found for the DPP dyes bearing the 2‐ethylhexyl substituents, which is surprising, considering that as a result of the chiral substituents, this material is a mixture of (R,R), (S,S), and (R,S) stereoisomers. The high carrier mobility in the films of the DPPs bearing stereoisomerically inhomogeneous ethylhexyl groups is rationalized here by single‐crystal X‐ray diffraction (XRD) analysis in combination with XRD and atomic force microscopy studies on thin films, which reveal the presence of slightly different 2D layer arrangements for the n‐alkyl and the 2‐ethylhexyl derivatives. For the cyano‐substituted DPPs possessing the lowest LUMO levels, ambipolar transport characteristics are observed.     

Electroactive Polymeric Networks Created by Oxidation of Oligothiophene‐Functionalized Perylene Bisimides

A series of fourfold oligothienyl‐functionalized perylene bisimides, N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐[2,2′]bithien‐5‐yl)‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7a ), N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐[2,2′;5′,2′′]terthien‐5‐yl)‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7b ), and N,N′‐bis(2,6‐diisopropylphenyl)‐1,6,7,12‐tetra(4‐(7‐(5″‐Methyl‐[2,2′;5′,2″]terthien‐5‐yl))‐heptanoyloxyphenoxy)perylene‐3,4:9,10‐tetracarboxylic acid bisimide ( 7c ), have been synthesized. Oligothienyl and perylene bisimide chromophores in these dyads display their characteristic optical UV/vis absorption properties. Upon excitation of the oligothiophene subunits, fluorescence resonance energy transfer (FRET) occurs to the perylene bisimide core. Cyclic voltammetric studies revealed that the reduction of the perylene bisimide moiety is not affected by the presence of oligothiophenes, showing two waves at around ‐0.7 and ‐1.0 V versus Ag/AgCl, respectively. On the other hand, the oxidation of the oligothienyl moieties leads to oxidative coupling for 7a and 7b , providing electroactive sexithiophene‐ and quaterthiophene‐perylene bisimide networks, respectively. Electrochemical deposition of compounds 7a , b was performed and the films were characterized using cyclic voltammetry and in situ conductance, which reveal remarkable p‐type conductivity. Significantly, two separate regimes of electrical conductance have been observed for the films generated from 7b .     

Supramolecular Assembly of Perylene Bisimide with β‐Cyclodextrin Grafts as a Solid‐State Fluorescence Sensor for Vapor Detection

A nanoscopic supramolecular aggregate is constructed from perylene bisimide‐bridged bis‐(permethyl‐β‐cyclodextrins) 1 via π–π stacking interactions. Its self‐assembly behavior in organic and aqueous solutions is investigated by UV–Vis, fluorescence, and ¹H NMR spectroscopy. Transmission electron microscopy and scanning electron microscopy images show the 1D nanorod aggregation of 1 , which is birefringent under crossed polarizer conditions and strongly fluorescent as depicted in the fluorescence microscopy image. X‐ray powder diffraction measurements indicate that 1 forms a well‐ordered crystalline arrangement with a π–π stacking distance of 4.02 Å. Furthermore, the solid‐state fluorescence sensing is explored by utilizing the poly(vinylidene fluoride) membrane‐embedded 1 , giving that 1 , as a novel vapor detecting material, can probe several kinds of volatile organic compounds and, especially, exhibits high sensitivity to organic amines.     

Nanostructured semiconductor block copolymers: π–π Stacking,optical and electrochemical properties

The structural and optical properties of semiconductor block copolymers containing triphenylamine as hole transport material and perylene bisimide as dye and electron transport material are reported. The polymers were prepared by nitroxide mediated controlled radical polymerisation and characterized with GPC, DSC, and TGA. The electrochemical properties as determined by cyclic voltammetry show the HOMO and LUMO values of the block copolymers to be −5.23 eV and −3.65 eV, respectively. The perylene bisimide units aggregate by π–π stacking which could be analyzed with wide-angle X-ray scattering. The absorption and fluorescence properties of the perylene bisimide polymers and monomers in solution and film were investigated. It could be shown that they are strongly influenced by intramolecular coupling between different perylene bisimide units in polymers. The block copolymers exhibit a microphase separation on a nanometer scale with a constant perylene bisimide domain width of 13 nm and lengths of up to several micrometers.     

Composites of Perylene Chromophores and Layered Double Hydroxides: Direct Synthesis,Characterization, and Photo‐ and Chemical Stability

Composites of the tetra‐anion of the perylene dye N,N′‐di(phenyl‐3,5‐disulfonic acid)perylene‐3,4:9,10‐tetracarboxydiimide (PBITS) with layered double hydroxides (LDHs) were formed by direct synthesis (co‐precipitation at constant pH). The LDHs were of the hydrotalcite (Mg–Al–OH and Zn–Al–OH compositions). During synthesis of the hydrocalumite type (Ca–Al–OH), partial destruction of the dye occurs, being more pronounced at higher pH values. The composites were characterized with regard to their composition by elemental and thermal analysis. From UV‐vis spectroscopic data and powder X‐ray diffraction, a structural model is developed for the composites. In the galleries between the hydroxide layers, the chromophore molecules are stacked in an J‐type arrangement. The compounds have brilliant colors and are insoluble in common solvents. With regard to a possible application as pigments, their photostability and their chemical resistance against a typical application environment was tested. The photostability of the dye molecules and their chemical resistance against setting cement are slightly raised by the occlusion within the LDH structure; however, the photostability of the LDH–cement is lower than that of cement colored with the pure perylene dye.     

Highly Conductive Organosilica Hybrid Films Prepared from a Liquid‐Crystal Perylene Bisimide Precursor

Significant anisotropic electrical conduction in organosilica films is achieved by long‐range orientation of electroactive perylene bisimide (PBI) moieties in the silica scaffold. A new PBI‐based organosilane precursor is designed with lyotropic liquid‐crystalline properties. The PBI precursor with triethoxysilylphenyl groups exhibits a hexagonal columnar phase in the presence of organic solvents. The lyotropic liquid‐crystalline behavior of the precursor enables the preparation of dip‐coated films consisting of uniaxially aligned columnar aggregates of the PBI precursor on the centimeter scale. The oriented structure is successfully fixed by in situ polycondensation, which yields insoluble, thermally stable PBI–silica hybrid films. The oriented organosilica films doped with hydrazine exhibit high electrical conductivities on the order of 10^?2 S cm^?1, which are at the highest level for organosilica materials, and are comparable to those of all‐organic PBI assemblies. Definite anisotropy of conductivities is also found for these films. The present results suggest that the induction of significant electrical properties in organic molecular assemblies is compatible with the structural stabilization by inorganic–organic hybridization.     

Photoswitching of Ferroelectric Liquid Crystals Using Unsymmetrical Chiral Thioindigo Dopants: Photoinduced Inversion of the Sign of Spontaneous Polarization

A new unsymmetrical chiral thioindigo dopant 6‐[(R,R)‐2,3‐difluorooct‐1‐yloxy]‐5′‐nitro‐6′‐[(R)‐2‐octyloxy]thioindigo ( 4 ) designed to photoinvert the sign of spontaneous polarization (P_S) in a ferroelectric chiral smectic C (SmC*) liquid crystal was prepared using a synthetic approach previously developed in our laboratory. In this new “ambidextrous” design, the (R)‐2‐octyloxy side‐chain is sterically coupled to the thioindigo core and induces a positive P_S, whereas the (R,R)‐2,3‐difluorooctyloxy side‐chain is decoupled from the core and induces a larger negative P_S. In the trans form, this dopant induces a negative polarization in the SmC host (+)‐4‐(4‐methylhexyloxy)phenyl 4‐decyloxybenzoate ( PhB ). Irradiation of a 1 mol‐% mixture of 4 in PhB at λ = 510 nm caused a sign inversion of P_S, from –0.88 to +0.42 nC cm^–2 at T – T_C = –5 °C, which is consistent with an increase in the polarization power of the coupled 2‐octyloxy/thioindigo unit over that of the 2,3‐difluorooctyloxy unit, due to the increase in transverse dipole moment of the thioindigo core upon trans–cis photoisomerization. The P_S sign inversion was confirmed by a surface‐stabilized ferroelectric liquid crystal photoswitching experiment. Spectroscopic measurements on films of the doped liquid crystal mixtures showed that trans–cis photoisomerization is gradually suppressed with increasing dopant mole fraction, possibly as a result of increased dopant aggregation.     

Novel Conjugated Organic Dyes for Efficient Dye‐Sensitized Solar Cells

Novel conjugated organic dyes that have N,N‐dimethylaniline (DMA) moieties as the electron donor and a cyanoacetic acid (CAA) moiety as the electron acceptor were developed for use in dye‐sensitized nanocrystalline‐TiO₂ solar cells (DSSCs). We attained a maximum solar‐energy‐to‐electricity conversion efficiency (η) of 6.8 % under AM 1.5 irradiation (100 mW cm^–2) with a DSSC based on 2‐cyano‐7,7‐bis(4‐dimethylamino‐phenyl)hepta‐2,4,6‐trienoic acid (NKX‐2569): short‐circuit photocurrent density (J_sc) = 12.9 mA cm^–2, open‐circuit voltage (V_oc) = 0.71 V, and fill factor (ff) = 0.74. The high performance of the solar cells indicated that highly efficient electron injection from the excited dyes to the conduction band of TiO₂ occurred. The experimental and calculated Fourier‐transform infrared (FT‐IR) absorption spectra clearly showed that these dyes were adsorbed on the TiO₂ surface with the carboxylate coordination form. A molecular‐orbital calculation indicated that the electron distribution moved from the DMA moiety to the CAA moiety by photoexcitation of the dye.     

High‐Performance Solution‐Processable Poly(p‐phenylene vinylene)s for Air‐Stable Organic Field‐Effect Transistors

The influence of the substitution pattern (unsymmetrical or symmetrical), the nature of the side chain (linear or branched), and the processing of several solution processable alkoxy‐substituted poly(p‐phenylene vinylene)s (PPVs) on the charge‐carrier mobility in organic field‐effect transistors (OFETs) is investigated. We have found the highest mobilities in a class of symmetrically substituted PPVs with linear alkyl chains (e.g., R¹, R² = n‐C₁₁H₂₃, R³ = n‐C₁₈H₃₇). We have shown that the mobility of these PPVs can be improved significantly up to values of 10^–2 cm² V^–1 s^–1 by annealing at 110 °C. In addition, these devices display an excellent stability in air and dark conditions. No change in the electrical performance is observed, even after storage for thirty days in humid air.     

Tunable Self‐Assembled Micro/Nanostructures of Carboxyl‐Functionalized Squarylium Cyanine for Ammonia Sensing

Orderly molecular self‐assembly for tunable micro/nanostructures is an effective way to prepare novel functional materials with desired properties. Squarylium cyanine (SCy) dyes have received great attention in the fields of laser, imaging, and optoelectronic device. However, the detailed self‐assembly behavior of SCy has rarely been investigated. In the present work, two SCy derivatives, D1 and D2 , respectively, bearing four and two carboxylic acid groups at different positions are prepared and used as a model system to investigate the molecular self‐assembly, morphology, and optical properties of SCy dyes. The hydrogen‐bonding interactions between the carboxylic acid groups in D1 and D2 are determined with X‐ray diffraction, 2D nuclear magnetic resonance, and Fourier transformation infrared spectroscopy. The two types of hydrogen bonds in D1 cooperating with inherent π–π stacking interaction result in tunable molecular aggregations, which further leads to the transformation between J‐aggregation and H‐aggregation of D1 in the solid state in response to ammonia gas. In all, this work provides a feasible and effective way to study the self‐assembled aggregates of SCy dyes at both molecular and supramolecular levels, and has developed a reversible sensor for ammonia gas detection.     

Bioinspired Anisotropic Hydrogel Actuators with On–Off Switchable and Color‐Tunable Fluorescence Behaviors

An effective approach to develop a novel macroscopic anisotropic bilayer hydrogel actuator with on–off switchable fluorescent color‐changing function is reported. Through combining a collapsed thermoresponsive graphene oxide‐poly(N‐isopropylacrylamide) (GO‐PNIPAM) hydrogel layer with a pH‐responsive perylene bisimide‐functionalized hyperbranched polyethylenimine (PBI‐HPEI) hydrogel layer via macroscopic supramolecular assembly, a bilayer hydrogel is obtained that can be tailored and reswells to form a 3D hydrogel actuator. The actuator can undergo complex shape deformation caused by the PNIPAM outside layer, then the PBI‐HPEI hydrogel inside layer can be unfolded to trigger the on–off switch of the pH‐responsive fluorescence under the green light irradiation. This work will inspire the design and fabrication of novel biomimetic smart materials with synergistic functions.     

Significant Improvement of Dye‐Sensitized Solar Cell Performance by Small Structural Modification in π‐Conjugated Donor–Acceptor Dyes

Two donor‐π‐acceptor (D‐π‐A) dyes are synthesized for application in dye‐sensitized solar cells (DSSC). These D‐π‐A sensitizers use triphenylamine as donor, oligothiophene as both donor and π‐bridge, and benzothiadiazole (BTDA)/cyanoacrylic acid as acceptor that can be anchored to the TiO₂ surface. Tuning of the optical and electrochemical properties is observed by the insertion of a phenyl ring between the BTDA and cyanoacrylic acid acceptor units. Density functional theory (DFT) calculations of these sensitizers provide further insight into the molecular geometry and the impact of the additional phenyl group on the photophysical and photovoltaic performance. These dyes are investigated as sensitizers in liquid‐electrolyte‐based dye‐sensitized solar cells. The insertion of an additional phenyl ring shows significant influence on the solar cells' performance leading to an over 6.5 times higher efficiency (η = 8.21%) in DSSCs compared to the sensitizer without phenyl unit (η = 1.24%). Photophysical investigations reveal that the insertion of the phenyl ring blocks the back electron transfer of the charge separated state, thus slowing down recombination processes by over 5 times, while maintaining efficient electron injection from the excited dye into the TiO₂‐photoanode.     

Unprecedented J‐Aggregated Dyes in Pure Organic Solvents

The design and synthesis of the first organic dyes enabling spontaneous formation of stable J‐aggregates in common organic solvents without additives is described. The new dyes are O‐BODIPYs with a B‐spiranic 4,4‐diacyloxyl substitution pattern. Key to the effectiveness of the J‐aggregation process is the high conformational rigidity of the B‐spiranic molecular design as well as the orthogonal disposition of the B‐diacyloxyl substituent and the meso‐aryl group with respect to the mean plane of the boradiazaindacene. Atomistic simulations, both in vacuum and in a solvent cage, support the dynamics of the J‐aggregation process as well as its dependence on the alkylation pattern of the BODIPY chromophore. A detailed analysis of the photophysical and laser properties of the new dyes provides convincing evidence for the unambiguous assignment of these J‐aggregates and their dependence on the environmental conditions.     

Self‐Assembly of m‐Diethynylbenzene Macrocycles Containing Exoannular Chiral Side Chains

Induced circular dichroism (CD) spectra of the m‐diethynylbenzene macrocycles (S)‐ 2 and (R)‐ 2 that have exoannular chiral side chains are observed in a methanol/chloroform (8:2) solution, indicating the formation of chiral, helical aggregates in solution. Solid films prepared on the surface of quartz substrates by spin‐coating solutions of (S)‐ 2 also exhibit CD signals that are remarkably dependent on the solvent used for the spin‐coating. The relationship between the CD spectra and the morphology of the solid films observed by atomic force microscopy is discussed.     

Rational Design of Perylenediimide‐Substituted Triphenylethylene to Electron Transporting Aggregation‐Induced Emission Luminogens (AIEgens) with High Mobility and Near‐Infrared Emission

Organic materials with both high electron mobility and strong solid‐state emission are rare although for their importance to advanced organic optoelectronics. In this paper, triphenylethylenes with varying number of perylenediimide (PDI) unit (TriPE‐nPDIs, n = 1?3) are synthesized and their optical and charge‐transporting properties are systematically investigated. All the molecules exhibit strong solid‐stated near infrared (NIR) emission and some of them exhibit aggregation‐enhanced emission characteristics. Organic field‐effect transistors (OFETs) using TriPE‐nPDIs are fabricated. TriPE‐3PDI shows the best performance with maximum quantum yield of ≈30% and optimized electron mobility of over 0.01 cm² V^?1 s^?1, which are the highest values among aggregation‐induced emission luminogens with NIR emissions reported so far. Photophysical property investigation and theoretical calculation indicate that the molecular conformation plays an important role on the optical properties of TriPE‐nPDI, while the result from film microstructure study reveals that the film crystallinity influences greatly their OFET device performance.     

Nanostructured Crystalline Comb Polymer of Perylenebisimide by Directed Self‐Assembly: Poly(4‐vinylpyridine)‐pentadecylphenol Perylenebisimide

Well defined nanostructured polymeric supramolecular assemblies are formed when an asymmetric perylenebisimide substituted with ethylhexyl chains on one end and functionalized with 3‐pentadecylphenol at the other termini ( PDP‐UPBI ) is complexed with poly(4‐vinylpyridine) (P4VP) via a non‐covalent specific interaction such as hydrogen‐bonding. The resulting P4VP(PDP‐UPBI) _n complexes are fully solution processable. The bulk structure and morphologies of the supramolecular film studied using small angle and wide angle X‐ray scattering reveals highly crystalline nature of the complex. Thin film morphology of the 1:1 complex analyzed using transmission electron microscopy shows uniform lamellar structures in the domain range of 5–10 nm. A clear trend of improved electrical parameters in P4VP(PDP‐UPBI) system compared to pristine ( PDP‐UPBI ) is observed from space charge limited current measurements. In short, a simple and facile method to obtain spatially defined organization of n‐type semiconductor perylenebisimide molecules using hydrogen bonding interactions with P4VP as the structural motif is showcased herein.     

NixSy Nanowalls/Nitrogen‐Doped Graphene Foam Is an Efficient Trifunctional Catalyst for Unassisted Artificial Photosynthesis

Solar‐to‐hydrogen (STH) conversion through unassisted artificial photosynthesis (APS) devices is one of the promising and environmentally friendly strategies for sustainable development. However, the practical large‐scale application of the unassisted APS devices is impeded by the need for expensive noble metal‐based catalysts in photovoltaics and/or electrolyzers. Herein, well‐aligned 2D Ni_xS_y nanowalls (2D Ni_xS_y NWs) on a 3D nitrogen‐doped graphene foam (3D NGF) are synthesized and further employed it in unassisted APS. Due to the positive synergistic effect between the highly electrocatalytic activity of Ni_xS_y NW and excellent conductivity of NGF, this low cost material of (2D/3D) Ni_xS_y NW/NGF is highly efficient as a multifunctional catalyst in various applications: a counterelectrode for dye‐sensitized solar cell (DSSC) and a “bifunctional” electrocatalyst for oxygen and hydrogen evolution for electrocatalytic overall water splitting. Furthermore, three Ni_xS_y NW/NGF‐based DSSCs as a tandem cell for unassisted solar‐driven overall water splitting is connected, using Ni_xS_y NW/NGF itself on nickel foams as the anode and cathode. Impressively, such integrated photovoltaic‐electrolyzer APS device can achieve an STH efficiency of 3.2% with an excellent stability and low cost. This work opens an avenue to advanced multifunctional materials for the low‐cost and unassisted solar‐driven overall water splitting.     

Diversity order analysis of orthogonal space–time block coding over keyhole fading channels with antenna selection

Although there are many research results for the keyhole fading channels in the current literature, fewer results have investigated the antenna selection scheme and quantified the diversity order. In this letter, we first derive some simple statistics of the output signal‐to‐noise ratio for the orthogonal space–time block coding over keyhole fading channels. On the basis of these results, we derive an approximate BER expression for the keyhole MIMO channels with receive antenna selection scheme. Results show that the diversity order with receive antenna selection is min{n_R,n_T}for n_R ≠ n_T, which means that the full diversity order with antenna selection is maintained. Finally, numerical results demonstrate the accuracy of our analytical expressions and the tightness of approximate formulas. Copyright © 2012 John Wiley & Sons, Ltd.     

XBi4S7 (X = Mn,Fe): New Cost‐Efficient Layered n‐Type Thermoelectric Sulfides with Ultralow Thermal Conductivity

A new class of cost‐efficient n‐type thermoelectric sulfides with a layered structure is reported, namely MnBi₄S₇ and FeBi₄S₇. Theoretical calculations combined with synchrotron X‐ray/neutron diffraction analyses reveal the origin of their electronic and thermal properties. The complex low‐symmetry monoclinic crystal structure generates an electronic band structure with a mixture of heavy and light bands near the conduction band edge, as well as vibrational properties favorable for high thermoelectric performance. The low thermal conductivity can be attributed to the complex layered crystal structure and to the existence of the lone pair of electrons in Bi³⁺. This feature combined with the relatively high power factor lead to a figure of merit as high as 0.21 (700 K) in undoped MnBi₄S₇, making this material a promising n‐type candidate for low‐ and intermediate‐temperature thermoelectric applications.     

Dye Encapsulated Metal‐Organic Framework for Warm‐White LED with High Color‐Rendering Index

A strategy by encapsulating organic dyes into the pores of a luminescent metal‐organic framework (MOF) is developed to achieve white‐light‐emitting phosphor. Both the red‐light emitting dye 4‐(p‐dimethylaminostyryl)‐1‐methylpyridinium ( DSM ) and the green‐light emitting dye acriflavine ( AF ) are encapsulated into a blue‐emitting anionic MOF ZJU‐28 through an ion‐exchange process to yield the MOF?dye composite ZJU‐28?DSM/AF . The emission color of the obtained composite can be easily modulated by simply adjusting the amount and component of dyes. With careful adjustment of the relative concentration of the dyes DSM and AF , the resulting ZJU‐28?DSM/AF (0.02 wt% DSM , 0.06 wt% AF ) exhibits a broadband white emission with ideal CIE coordinates of (0.34, 0.32), high color‐rendering index value of 91, and moderate correlated color temperature value of 5327 K. Such a strategy can be easily expanded to other luminescent MOFs and dyes, thus opening a new perspective for the development of white light emitting materials.