No abstract

Cholesteric liquid crystal (CLC) droplets exhibit nontrivial topological features, which are controlled by the ratio between the cholesteric pitch and the droplet radius. The radial spherical structure (RSS) is of particular interest, as it reveals an onion-like concentric organization of the cholesteric helices, leading to the expression of spherical Bragg microcavities. Using an overcrowded alkene-based unidirectional molecular motor as a dopant, we show that the topological defect structure in the droplet can be activated by illumination. By using appropriate molecular motor concentrations, light can either break the symmetry of topological defects (as observed for the bent-twisted bipolar structure), or it can induce inversion of handedness in an onion-like organization (in the case of RSS). This latter feature may pave the way toward alternative activation modes of lasers based on cholesteric droplets. By also studying CLC droplets once they have reached full photoconversion at photostationary state (PSS), we highlight that the strong influence of confinement on the droplets structure occurs to the same extent after the helix inversion event. Our results are interpreted in terms of numerical simulations of the droplets' structure, which shed light on the major role played by curvature close to the droplets' center, this latter one becoming dominant when the droplet radius is small.

A hybrid disk-like/rod-like molecule comprising central triphenylene core symmetrically substituted with six azobenzene moieties (C-12) has been adsorbed at the 1,2,4trichlorobenzene/Au(111) interface, revealing the potential of azobenzene moieties for the control of 2D chiral networks. The C-12, which due to its complex molecular structure possesses a relatively large number of degrees of freedom, surprisingly forms monolayers of only one kind of structure, namely a hexagonal network of large period, 3.5 nm. By combining Scanning Tunneling Microscopy (STM) and DFT calculations, we evidence that this specific 2D-ordering is due to cooperative weak hydrogen bonds between neighboring azobenzenes and azobenzene-Au(111) interactions. The crystallographic network is hexagonal, but azobenzene-azobenzene pairing, associated with hydrogen bonding renders the network chiral with a chirality spanning all ranges, from the molecular C-12 configuration, the configuration of the azobenzene dimers and the rosette-like azobenzene network, to the C-12 network orientation on Au(111) rotated by ±8° from the main crystallographic direction Au<110>, depending on the handedness of the molecular network. This chiral 2D system thus paves the way for the formation of macroscopic 2D molecular crystals of unique handedness, if additional enantiomeric chiral dopants can be used.

We report on an improved melt-processing method to prepare organic semiconducting layers with large crystal size.

We investigate the expression of chirality in a monolayer formed spontaneously by 2,3,6,7,10,11-pentyloxytriphenylene (H5T) on Au(111). We resolve its interface morphology by combining scanning tunneling microscopy (STM) with theoretical calculations of intermolecular and interfacial interaction potentials. We observe two commensurate structures. While both of them belong to a hexagonal space group, analogical to the triangular symmetry of the molecule and the hexagonal symmetry of the substrate surface, they surprisingly reveal a 2D chiral character. The corresponding breaking of symmetry arises for two reasons. First it is due to the establishment of a large molecular density on the substrate, which leads to a rotation of the molecules with respect to the molecular network crystallographic axes to avoid steric repulsion between neighboring alkoxy chains. Second it is due to the molecule-substrate interactions, leading to commensurable large crystallographic cells associated with the large size of the molecule. As a consequence, molecular networks disoriented with respect to the high symmetry directions of the substrate are induced. The high simplicity of the intermolecular and molecule-substrate van der Waals interactions leading to these observations suggests a generic character for this kind of symmetry breaking. We demonstrate that, for similar molecular densities, only two kinds of molecular networks are stabilized by the molecule-substrate interactions. The most stable network favors the interfacial interactions between terminal alkoxy tails and Au(111). The metastable one favors a specific orientation of the triphenylene core with its symmetry axes collinear to the Au⟨110⟩. This specific orientation of the triphenylene cores with respect to Au(111) appears associated with an energy advantage larger by at least 0.26 eV with respect to the disoriented core.

Organic electronics became an attractive alternative for practical applications in complementary logic circuits due to the unique features of organic semiconductors such as solution processability and ease of large-area manufacturing. Bulk heterojunctions (BHJ), consisting of a blend of two organic semiconductors of different electronic affinities, allow fabrication of a broad range of devices such as light-emitting transistors, light-emitting diodes, photovoltaics, photodetectors, ambipolar transistors and sensors. In this work, the charge carrier transport of BHJ films in field-effect transistors is switched from electron to hole domination upon processing and post-treatment. Low molecular weight n-type N,N′-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN2) was blended with p-type poly[2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene] (PBTTT-C14) and deposited by spin-coating to form BHJ films. Systematic investigation of the role of rotation speed, solution temperature, and thermal annealing on thin film morphology was performed using atomic force microscopy, scanning electron microscopy, and grazing incidence wide-angle X-ray scattering. It has been determined that upon thermal annealing the BHJ morphology is modified from small interconnected PDI8-CN2 crystals uniformly distributed in the polymer fraction to large planar PDI8-CN2 crystal domains on top of the blend film, leading to the switch from electron to hole transport in field-effect transistors.

Free-standing and flexible field-effect transistors based on 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene)/polystyrene bilayers are obtained by well-controlled phase separation of both components. The phase separation is induced by solvent vapor annealing of initially amorphous blend films, leading to crystallization of TIPS-pentacene as the top layer. The crystallinity and blend morphology strongly depend on the molecular weight of polystyrene, and under optimized conditions, distinct phase separation with a well-defined and trap-free interface between both fractions is achieved. Due to the distinct bilayer morphology, the resulting flexible field-effect transistors reveal similar charge carrier mobilities as rigid devices and additionally pronounced environmental and bias stress stabilities. The performance of the flexible transistors remains stable up to a strain of 1.8%, while above this deformation, a close relation between current and strain is observed that is required for applications in strain sensors.

Simple solution-processed structures of organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) have been demonstrated, but their efficiency and roll-off are still problematic, mainly due to the difficulty in optimizing such device structures. For the first time, solution-processed fabrication of efficient TADF green OLEDs with a simple structure is demonstrated. The emitter 2PXZ-OXD (2,5-bis­(4-(10H-phenoxazin-10-yl)­phenyl)-1,3,4-oxadiazole) was dispersed in a poly­(N-vinylcarbazole)/1,3-bis­[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]­benzene (OXD-7) host matrix. Different emitter concentrations (1–10% wt) and different layer thicknesses were applied. Photophysical analysis supports the device structure configuration. The results show a balanced electrical carrier transport, a low turn-on voltage (5 V), an external quantum efficiency (EQE) of 7.5%, and a current efficiency of 16.2 cd/A with a brightness of 7240 cd/m2. The device exhibited a suppressed roll-off at 1000 cd/m2 (EQE: 7.15%). Simulations of charge transport properties allowed to explain the results and to optimize, opening interesting frameworks for simple solution-deposited large-area OLEDs.