Synthetic progress of organic thermally activated delayed fluorescence emitters via C-H activation and functionalization.

Thermally activated delayed fluorescence (TADF) emitters have become increasingly prominent due to their promising applications across various fields, prompting a continuous demand for developing reliable synthetic methods to access them. This review aims to highlight the progress made in the last decade in synthesizing organic TADF compounds through C-H bond activation and functionalization. The review begins with a brief introduction to the basic features and design principles of TADF emitters. It then provides an overview of the advantages and concise development of C-H bond transformations in constructing TADF emitters. Subsequently, it summarizes both transition-metal-catalyzed and non-transition-metal-promoted C-H bond transformations used for the synthesis of TADF emitters. Finally, the review gives an outlook on further challenges and potential directions in this field.

Highly Sensitive, Low-Energy-Consumption Biomimetic Olfactory Synaptic Transistors Based on the Aggregation of the Semiconductor Films.

Artificial olfactory synaptic devices with low energy consumption and low detection limits are important for the further development of neuromorphic computing and intelligent robotics. In this work, an ultralow energy consumption and low detection limit imitation olfactory synaptic device based on organic field-effect transistors (OFETs) was prepared. The aggregation state of poly(diketopyrrolopyrrole-selenophene) (PTDPP) semiconductor films is modulated by adding unfavorable solvents and annealing treatments to obtain excellent charge transfer and gas synaptic properties. The regulated OFET device can execute basic biological synaptic functions, including excitatory postsynaptic currents (EPSCs), paired-pulse facilitation (PPF), and the transition from short-term to long-term plasticity, at an ultralow operating voltage of -0.0005 V. The ultralow energy consumption during the biomimetic simulation is in the range of 8.94-88 fJ per spike. Noteworthily, the gas detection limit of the device is as low as 50 ppb, well below normal human NO2 gas perception limits (100-1000 ppb). Additionally, high-pass filtering, Pavlovian conditioned reflexes, and decoding of "Morse code" were simulated. Finally, a grid-free conformal device with outstanding flexibility and stability was fabricated. In conclusion, the control of semiconductor thin-film aggregation provides effective guidance for preparing low-energy-consumption, highly sensitive olfactory nerve-mimicking devices and promoting the development of wearable electronics.

Helical Nanofiber Photoelectric Synaptic Devices for an Artificial Vision Nervous System.

Inspired by the helical structure and the resultant exquisite functions of biomolecules, helical polymers have received increasing attention. Here, a series of poly(3-hexylthiophene)-block-poly(phenyl isocyanide) (P3HT-b-PPI) copolymers were prepared using a simple one-pot living polymerization method. Interestingly, the P3HT80-b-PPI30 films were found to have a helical nanofiber structure. The corresponding device has superior optoelectronic properties, such as a broadened spectral response range from the visible band to the deep ultraviolet (DUV) and an approximately 5-fold longer carrier decay time after DUV light stimulation. An energy consumption of 1.44 fJ per synaptic event was obtained, which is the lowest energy consumption achieved so far with DUV light stimulation. The encryption and decryption of images are implemented using an array of devices. Finally, a photoreceptor neural pathway was constructed to achieve early warning for the recognition of the display of harmful light. This research provides an effective strategy for the development of a novel optoelectronic synaptic device.

Multi-stable dye-doped dual-frequency twisted nematic liquid crystal smart window.

In this study, a novel, to the best of our knowledge, dye-doped dual-frequency liquid crystal multi-stable smart window is proposed. A chiral dopant with appropriate content was introduced into dye-doped dual-frequency liquid crystals to achieve an initial 180° twisted state. These liquid crystals can be switched to a nematic phase or a 360° twisted state by controlling the magnitude and frequency of the applied voltage. These nematic phase and 360° twisted states can exist stably for a long time because of the backflow effect and the anisotropic nature of the dual-frequency liquid crystal material. Due to the optical waveguide effect of dye-doped liquid crystals in the long-pitch state, the transmittance was different in nematic phase, 180°, and 360° twisted three zero-field stabilized absorption states. Finally, a multi-stable smart window is developed to switch between three zero-field stabilized absorption and scattering states.

Achieving High Performance Stretchable Conjugated Polymers via Donor Structure Engineering.

A backbone engineering strategy is developed to tune the mechanical and electrical properties of conjugated polymer semiconductors. Four Donor-Acceptor (D-A) polymers, named PTDPPSe, PTDPPTT, PTDPPBT, and PTDPPTVT, are synthesized using selenophene (Se), thienothiophene (TT), bithiophene (BT), and thienylenevinylenethiophene (TVT) as the donors and siloxane side chain modified diketopyrrolopyrrole (DPP) as acceptor. The influences of the donor structure on the polymer energy level, film morphology, molecular stacking, carrier transport properties, and tensile properties are all examined. The films of PTDPPSe show the best stretchability with crack-onset-strain greater than 100%, but the worst electrical properties with a mobility of only 0.54 cm2  V-1  s-1 . The replacement of the Se donor with larger conjugated donors, that is, TT, BT, and TVT, significantly improves the mobility of conjugated polymers but also leads to reduced stretchability. Remarkably, PTDPPBT exhibits moderate stretchability with crack-onset-strain ≈50% and excellent electrical properties. At 50% strain, it has a mobility of 2.37 cm2 V-1  s-1 parallel to the stretched direction, which is higher than the mobility of most stretchable conjugated polymers in this stretching state.

Enhancing the Elasticity of Conjugated Polymers through Precise Control of the Spacing between the Backbone and Siloxane Side-Chains.

Intrinsically stretchable conjugated polymers (CPs) have extensively been studied for the development of novel flexible electronic devices. In this work, a method to control the elastic properties of CPs has been proposed via regulation of spacer length between the siloxane side-chain and the backbone. The target polymers were CP films with the structure P(mC-Si) for four different numbers of the spacer methylene groups, namely, m = 5, 6, 7, and 8. The effect of spacer length on the aggregation state as well as on electrical and elastic properties of the prepared films was then investigated. An adjustable lamellar spacing (dL-L), in addition to improved elastic properties, was achieved as the spacer length was changed in the prepared polymer films. Moreover, P(7C-Si) has a sufficient dL-L value of 35.77 Å, which provides enough space for inter-chain sliding to dissipate stress. This facilitated the dissipation of stress during the straining process. At a strain value of 100% in the vertical direction, the mobility of the P(7C-Si) film was 0.79 cm2 V-1 s-1 and reduced to 84.0% of the initial value without any applied strain. The study provides clear evidence that tuning the spacer length between the silicone endgroup and backbone is an effective way to improve the intrinsic stretchability of CPs with siloxane side chains.

Amplification of Dissymmetry for Circularly Polarized Photodetection by Cooperative Supramolecular Polymerization.

Circularly polarized photodetectors require chiral light absorption materials with high sensing efficiency and low costs. Here readily accessible point chirality has been introduced to dicyanostilbenes as the chiral source, facilitating remote chirality transfer to the π-aromatic core by cooperative supramolecular polymerization. The single-handed supramolecular polymers display powerful circularly polarized photodetection capability with a dissymmetry factor value as high as 0.83, superior to those of π-conjugated small molecules and oligomers. Strong chiral amplification occurs between the enantiopure sergeants and the achiral soldiers. The resulting supramolecular copolymers exhibit comparable photodetection efficiency to those of the homopolymeric ones, with a 90 % decrease in the enantiopure compound consumption. Therefore, cooperative supramolecular polymerization provides an effective yet economical avenue toward circularly polarized photodetection applications.

Optically Readable Electrochromic-Based Microfiber Synaptic Device for Photonic Neuromorphic Systems.

Optical synaptic devices possess great potential in both artificial intelligence and neuromorphic photonics. In this work, an optically readable electrochromic-based microfiber synaptic device was designed by the combination of a multimode fiber and an electrochromic device and using an external voltage to control the transmission of light in the fiber. The proposed synaptic device has the ability to imitate various basic functions of the biological synapses, such as synaptic plasticity, and paired-pulse facilitation (PPF), as well as the transition from short-term memory to long-term memory. Moreover, the proposed device decodes the output optical signal with the international Morse code to express the signal "HFUT" in two ways, and a 3 × 3 array composed of this device can simulate the perceptual learning process. The device can be easily prepared for a wide range of applications, and the incorporated microfibers can be replaced by planar optical waveguides, making it easy to be integrated into a more complex and versatile photonic neuromorphic system.

One-Step Preparation of Semiconductor/Dielectric Bilayer Structures for the Simulation of Flexible Bionic Photonic Synapses.

Flexible synaptic devices with information sensing, processing, and storage functions are indispensable in the development of wearable artificial intelligence electronic systems. Here, a semiconductor/dielectric bilayer structure was prepared by a one-step deposition method and used for the first time in a flexible biomimetic photonic synaptic transistor device. Specifically, poly(3-hexylthiophene)-block-poly(phenyl isocyanide) with pentafluorophenyl ester (P3HT-b-PPI(5F)) was prepared as the device active layer, where the P3HT segment served as a carrier transport channel and optical gate and the PPI(5F) segment was used for charge trapping. Various biomimetic synaptic behaviors, such as excitatory postsynaptic currents, paired-pulse facilitation, and short-term/long-term memory, were successfully simulated under green light stimulation. An ultra-low energy consumption of 1.82 fJ was achieved with a greatly reduced operating voltage. Further, the "Morse-code" optical decoding was simulated using the excellent synaptic plasticity of the device. In addition, flexible synaptic devices were prepared by a one-step deposition method and can be well-affixed to arbitrary substrates. This has promising applications in the field of wearable bionic electronics.

Optically Programmable Circularly Polarized Photodetector.

The detection of circularly polarized light (CPL) has aroused wide attention from both the scientific and industrial communities. However, from the optical activity of the chiral layer in the conventional CPL photodetectors, the sign inversion property is difficult to be achieved. As a result, great challenges arise during the preparation of miniaturized and integrated devices for tunable CPL detection applications. Along these lines, in this work, by taking advantage of the CPL-induced chirality characteristics of the achiral poly(9,9-di-n-hexylfluorene-alt-benzothiadiazole) (F6BT) and the good crystalline and electrical properties of the poly(3-hexylthiophene) (P3HT) film, an optically programmable CPL photodetector was fabricated. Interestingly, the device exhibited excellent discrimination between left- and right-handed CPL, while the maximum anisotropy factor of responsivity was 0.425. On top of that, the rigorously controlled chirality of the F6BT and the capability to be switched by the handedness of CPL was leveraged to realize the switchable detection of both L-CPL and R-CPL. Furthermore, a CPL photodetector array was fabricated, and the image processing and cryptographic characteristics were demonstrated. The proposed device configuration can find application in various scientific fields, including photonics, emission, conversion, or sensing with CPL but also is anticipated to play a key role for imaging and anticounterfeiting applications.

Role of Molecular Weight in the Mechanical Properties and Charge Transport of Conjugated Polymers Containing Siloxane Side Chains.

The molecular weight is a key factor affecting the properties of conjugated polymers. To determine the critical molecular weights of conjugated polymers modified with siloxane side chains, poly-diketo-pyrrolopyrrole-selenophene (PTDPPSe-5Si) samples with molecular weights ranging from 20 to 350 kDa are synthesized. The critical molecular weight of the polymer is determined in the range of 60-100 kDa by testing the viscosity of the solution. When the molecular weight of the 27-60 kDa polymers is below the critical molecular weight, they exhibit a high crystallinity and low ductility. When the molecular weight of the 100 kDa polymer reaches the critical molecular weight, the crystallinity decreases, and the ductility increases. As the molecular weight increases, the polymer film also gradually changes from brittle to ductile. Furthermore, when the molecular weight of the 315 kDa polymer is much higher than the critical molecular weight, the film exhibits a significant ductility, which results in the polymer films showing no pronounced cracks after high-percentage stretching. Additionally, due to the oriented alignment of the molecular chains caused by stretching, the carrier mobility in the parallel direction becomes 2.14-fold of the initial film.

Stable and tunable single-mode lasers based on cholesteric liquid crystal microdroplets.

Although many studies on cholesteric liquid crystal (CLC) microdroplet single-mode lasers are available, it has been shown that the stability and tunability of such microdroplets are difficult to achieve simultaneously. In this paper, a new, to the best of our knowledge, method is proposed for the mass and rapid preparation of stable and tunable monodisperse CLC microdroplet single-mode lasers. This is based on the formation of polymer networks on the surface of the microdroplet via interfacial polymerization and a disruption of the orderliness of the polymer networks by increasing the temperature during polymerization, which results in a single pitch inside the microdroplets. This approach enables CLC microdroplet single-mode lasers to achieve improved environmental robustness, while maintaining the same temperature tunability as the unpolymerized sample. Our method has promising future applications in integrated optics, flexible devices, and sensors.

Deep Ultraviolet Light Stimulated Synaptic Transistors Based on Poly(3-hexylthiophene) Ultrathin Films.

Deep ultraviolet (DUV)-light-stimulated artificial synaptic devices exhibit potential applications in various disciplines including intelligent military monitoring, biological and medical analysis, flame detection, etc. Along these lines, we report here a DUV-light-stimulated synaptic transistor fabricated on a poly(3-hexylthiophene) (P3HT) ultrathin film that responds selectively to DUV light. Significantly, our devices have the ability to successfully simulate various synapse-like behaviors including excitatory postsynaptic currents (EPSCs), paired-pulse facilitation (PPF), short-term memory (STM), long-term memory (LTM), STM-to-LTM transition, and learning and forgetting behaviors. Moreover, the proposed artificial synaptic structures were also fabricated on flexible poly(ethylene terephthalate) (PET) substrates and also successfully simulated typical synaptic behaviors, which could be of great importance for wearable applications.

Variable-focus liquid lens based on electrically responsive fluid.

In this work, an adaptive liquid lens using a novel transparent electrically responsive fluid, dibutyl adipate (DBA), is demonstrated. The DBA liquid lens with a hemispherical plano-convex shape can change its curvature according to the application of various input voltages. More specifically, when an external direct current (DC) electric field is applied to the DBA liquid, the charges that are injected from the cathode move along with the DBA molecules toward the anode and accumulate on the surface of the anode. When the DC electric field is removed, the shape of the DBA liquid is recovered to its original state. This electrostatic force induces the deformation of the DBA liquid lens within a concentric annular anode electrode. In addition, the focal length of our system is increased from a value of approximately 7.5 mm to 13.1 mm when the voltage is changed from 0 to 100 V. Interestingly, the resolution of our DBA liquid lens can reach a value of ∼28.5 lp/mm. The proposed DBA liquid lens exhibits high optical transmittance (∼95%), good thermal stability (20-100°C), simple structure, and an excellent imaging property, which implies that the DBA liquid is a promising candidate for fabricating novel adaptive liquid lenses.

Asymmetric Hybrid Siloxane Side Chains for Enhanced Mobility and Mechanical Properties of Diketopyrrolopyrrole-Based Polymers.

High performance organic field effect transistor devices based on intrinsically scalable materials are of great significance in wearable electronics. In this work, an exclusive approach is reported to rationale the carrier mobility and stretchability of the conjugate polymers (CPs) by modifying the symmetry of the side chains species. Semiconductor CPs with symmetrical alkyl side chains (P(C-C)), symmetrical siloxane side chains (P(Si-Si)), and asymmetrical silicon-carbon side chains (P(C-Si)) are synthesized to investigate the influence of these side chains on the carrier mobility and mechanical behavior. The result shows that silicon-carbon asymmetric side chains can modulate the aggregation degree of polymer chains with a coherence length of 134 Å and maintain the mobility at 0.90 cm2 V-1 s-1 . P(C-Si) exhibits superior tensile properties that even elongation up to 100% the value of mobility retains a majority properties. The main reason is that the lowest coherence length of P(C-Si) polymer leads to an increased proportion of amorphous zones in its polymer film, which efficiently dissipates mechanical stresses. This study provides an efficient strategy for the design and synthesis of the CPs with high carrier transport properties-mechanical stability.

Tunable multi-mode laser based on robust cholesteric liquid crystal microdroplet.

To date, various studies have been dedicated to the development of cholesteric liquid crystal (CLC) microdroplet omnidirectional lasers. In this work, a stable and tunable multi-mode laser emission is achieved by designing a dye-doping CLC microdroplet. In such a structure, the polymer network only exists on the surface, maintaining stability while providing tunability, and due to the uneven distribution of the pitch, it leads to multi-mode laser emission. A large number of microdroplets are produced quickly via a new method based on ultrasonic separation. During the reaction, we introduce interfacial polymerization where monomers and photoinitiator are respectively distributed inside and outside the microdroplets through mutual diffusion, which enables one to make the polymer network exist on the surface instead of the interior. The obtained microdroplet-based multi-mode laser is shown to possess stability and tunability, demonstrating a great potential for flexible devices and 3D displays.

Circularly Polarized Photodetectors Based on Chiral Materials: A Review.

Circularly polarized light (CPL) plays an important role in many photonic techniques, including tomographic scanning based on circular polarization ellipsometry, optical communication and information of spin, and quantum-based optical calculation and information processing. To fully exploit the functions of CPL in these fields, integrated photoelectric sensors capable of detecting CPL are essential. Photodetectors based on chiral materials can directly detect CPL due to their intrinsic optical activity, without the need to be coupled with polarizers and quarter-wave plates as in conventional photodetectors. This review summarizes the recent research progress in CPL photodetectors based on chiral materials. We first briefly introduce the CPL photodetectors based on different types of chiral materials and their working principles. Finally, current challenges and future opportunities in the development of CPL photodetectors are prospected.

Polyvinyl alcohol microlens array obtained by solvent evaporation from a confined droplet array.

In this study, polyvinyl alcohol (PVA) microlens arrays (MLAs) were prepared, and the dynamics of contact lines and contact angles during confined PVA solution droplet evaporation were investigated by in situ optical microscopy. First, hydrophobic layers patterned with hydrophilic microholes array modified substrates were prepared by photolithography and coating methods. The flowing of PVA solution on the substrates formed droplets in each microhole self-assembly. The substrate was then heated to allow evaporation of the solvent. The results showed the contact line of confined droplets pinned at the junction between the hydrophilic and hydrophobic areas during the whole evaporation process. The apparent contact angle decreased nonlinearly during evaporation. The evaporation of PVA solution droplet in each microhole followed a constant contact radius mode, meaning constant contact area and declined contact angle during evaporation. After complete solvent evaporation, PVA formed a convex shape with convergent lens character in each microhole. In sum, the obtained PVA convex arrays with uniform sizes and good focusing properties would have potential applications in wavefront sensing, infrared focal plane detection or CCD array light accumulation, laser array scanning, laser display, optical fiber coupling, and many other optical systems.

Linear hybrid siloxane-based side chains for highly soluble isoindigo-based conjugated polymers.

Three isoindigo-based conjugated polymers modified with linear hybrid siloxane-based side chains were synthesized (PIID-Cm-Si7, m = 5-7). All polymers showed good solubilities in halogenated hydrocarbons, aromatic hydrocarbons, ethers, alkanes, and esters. The polymer films of PIID-C5-Si7, PIID-C6-Si7, and PIID-C7-Si7 achieved mobilities of 0.32, 0.82, and 1.58 cm2 V-1 s-1, respectively.

Polyvinyl chloride gels microlens array with a well-controlled curvature obtained by solvent evaporation under DC electric fields.

In this paper, polyvinyl chloride (PVC) gels microlens arrays (MLAs) with controllable curvatures were prepared by evaporation of the solvent under DC electric fields. In order to obtain these arrays, the PVC gel solution was first injected into the cofferdam of a ring array patterned electrode substrate. Upon polarization under DC electric field, the electric charge injected from the cathode was carried by the plasticizers towards the anode to accumulate on its surface. After complete evaporation of the solvent, the PVC gels formed stable MLAs. The focal length of the formed MLAs obtained after evaporation of the 100 µL PVC gel solvent under 30 V DC field was 8.68 mm. The focal length of the as-obtained PVC gel-based MLAs can be well-controlled by merely tuning the strength of the electric field or by changing the volume of the PVC gel solution. Thus, it can be concluded that the proposed methodology looks very promising for future fabrication of MLAs with uniform size in larger areas.