Arylazopyrazole-modulated stable dual-mode phototransistors

High-performance organic devices with dynamic and stable modulation are essential for building devices adaptable to the environment. However, the existing reported devices incorporating light-activated units exhibit either limited device stability or subpar optoelectronic properties. Here, we synthesize a new optically tunable polymer dielectric functionalized with photochromic arylazopyrazole units with a cis-isomer half-life of as long as 90 days. On this basis, stable dual-mode organic transistors that can be reversibly modulated are successfully fabricated. The trans-state devices exhibit high carrier mobility reaching 7.4 square centimeters per volt per second and excellent optical figures of merit, whereas the cis-state devices demonstrate stable but starkly different optoelectronic performance. Furthermore, optical image sensors are prepared with regulatable nonvolatile memories from 36 hours (cis state) to 108 hours (trans state). The achievement of dynamic light modulation shows remarkable prospects for the intelligent application of organic optoelectronic devices.

Arylazo-3,5-diphenylpyrazole Derivatives: Molecular Probes Exhibiting Reversible Light-induced Phase Transitions for Energy Storage and Direct Photolithographic Patterning

We report azopyrazole photoswitches decorated with variable N-alkyl and alkoxy chains (for hydrophobic interactions) and phenyl substituents on the pyrazoles (enabling π-π stacking), showing efficient bidirectional photoswitching and reversible light-induced phase transition (LIPT). Extensive spectroscopic, microscopic, and diffraction studies and computations confirmed the manifestation of molecular-level interactions and photoisomerization into macroscopic changes leading to the LIPT phenomena. Using differential scanning calorimetric (DSC) studies, the energetics associated with those accompanying processes were estimated. The long half-lives of Z isomers, high energy contents for isomerization and phase transitions, and the stability of phases over an extended temperature range (-60 to 80 °C) make them excellent candidates for energy storage and release applications. Remarkably, the difference in the solubility of the distinct phases in one of the derivatives allowed us to utilize it as a photoresist in photolithography applications on diverse substrates.

Robust Radical Cations of Hexabenzoperylene Exhibiting High Conductivity and Enabling an Organic Nonvolatile Optoelectronic Memory

Herein, we report robust π-conjugated radical cations resulting from the oxidation of hexabenzoperylene (HBP) derivatives, HBP-B and HBP-H, which have butyl and hexyl groups, respectively, attached to the same twisted double helicene π-backbone. The radical cation of HBP-B was successfully crystallized in the form of hexafluorophosphate, which exhibited conductivity as high as 1.32 ± 0.04 S cm^-1. Photochemical oxidation of HBP-H by molecular oxygen led to the formation of its radical cation in the solid state, as found with different techniques. This allowed the organic field effect transistor of HBP-H to function as a nonvolatile optoelectronic memory, with the memory switching contrast above 10³ and long-term stability without using a floating gate, an electret layer, or photochromic molecules.

High-Density Reconfigurable Synaptic Transistors Targeting a Minimalist Neural Network

Enormous synaptic devices are required to build a parallel, precise, and efficient neural computing system. To further improve the energy efficiency of neuromorphic computing, a single high-density synaptic (HDS) device with multiple nonvolatile synaptic states is suggested to reduce the number of synaptic devices in the neural network, although such a powerful synaptic device is rarely demonstrated. Here, a photoisomerism material, namely, diarylethene, whose energy level varies with the wavelength of illumination is first introduced to construct a powerful HDS device. The multiple synaptic states of the HDS device are intrinsically converted under UV-vis regulation and remain nonvolatile after the removal of illumination. More importantly, the conversion is reconfigurable and reversible under different light conditions, and the synaptic characteristics are comprehensively mimicked in each state. Finally, compared with a two-layer multilayer perceptron (MLP) architecture based on static synaptic devices, the HDS device-based architecture reduces the device number by 16 times to achieve a minimalist neural computing structure. The invention of the HDS device opens up a revolutionary paradigm for the establishment of a brain-like network.

Transition metal complexes incorporated with photoswitchable azo-based benzimidazole ligands: Photochromic and solvatochromic studies

Photochromic compounds are well-known for their promising applications in many areas. It attracts remarkable attention because of their potential ability for optical memory media and optical switching devices. Herein, we have synthesized azo-based benzimidazole ligand and their transition metal complexes for photochromic applications in the liquid state. The azo-based benzimidazole ligand exhibits trans-to-cis photoisomerization with highly tunable and excellent π-π* and n-π* band separation of ligand, whereas complexes show light-induced photo-dissociation as well as trans-to-cis photoisomerization of the ligand part. The reverse cis-to-trans isomerization can be driven by without using light or any external stimuli at room temperature by keeping the system in the dark condition.

Enlightening Materials with Photoswitches

Incorporating molecular photoswitches into various materials provides unique opportunities for controlling their properties and functions with high spatiotemporal resolution using remote optical stimuli. The great and largely still untapped potential of these photoresponsive systems has not yet been fully exploited due to the fundamental challenges in harnessing geometrical and electronic changes on the molecular level to modulate macroscopic and bulk material properties. Herein, progress made during the past decade in the field of photoswitchable materials is highlighted. After pointing to some general design principles, materials with an increasing order of the integrated photoswitchable units are discussed, spanning the range from amorphous settings over surfaces/interfaces and supramolecular ensembles, to liquid crystalline and crystalline phases. Finally, some potential future directions are pointed out in the conclusion. In view of the exciting recent achievements in the field, the future emergence and further development of light-driven and optically programmable (inter)active materials and systems are eagerly anticipated.

Theoretical investigation on radical anion promoted electrocyclization in photochromes

Rapid electrocyclization is proposed under radical anionic conditions in organic photochromes. DFT calculations have been performed to investigate the radical anion mediated electrocyclization in different organic photochromes. Furthermore, the activation barriers under radical anionic conditions are compared with those in neutral and radical cationic conditions. The nuclear independent chemical shift (NICS(0)) and synchronicity calculations have been performed for the confirmation of concerted nature and aromatic character of transition states, respectively. The activation barrier for thermal return of cyclophanediene (CPD) to dihydropyrene (DHP) under radical anionic conditions is very lower (ΔH = 5.92 kcal/mol, ΔG = 6.97 kcal/mol) than under neutral conditions, but higher than that in radical cationic conditions (ΔH = 3.13 kcal/mol, ΔG = 4.0 kcal/mol). Similarly, the other prominent classes of photochromes; dithienylethene (ΔH = 20.12 kcal/mol, ΔG = 21.55 kcal/mol) and vinylheptafulvene (ΔH = 23.72 kcal/mol, ΔG = 24.82 kcal/mol) have shown decreased activation barrier under radical anionic condition. However, activation barrier of fulgide under radical anionic conditions is not different than those under neutral and radical cationic conditions. Synchronicity and NICS(0) values for organic photochromes also show significant changes under radical anionic conditions.

Multilevel Photonic Transistor Memory Devices Using Conjugated/Insulated Polymer Blend Electrets

Photonic data storage has diverse optoelectronic applications such as optical sensing and recording, integrated image circuits, and multibit-storage flash memory. In this study, we employ conjugated/insulated polymer blends as the charge storage electret for photonic field-effect transistor memory devices by exploring the blend composition, energy level alignment, and morphology on the memory characteristics. The studied conjugated polymers included poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PF), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), poly[{2,5-di(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene}-co-{3-(4'-(3″,7″-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene}-co-{3-(3'-(3',7'-dimethyloctyloxy)phenyl)-1,4-phenylenevinylene}] (SY-PPV), and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT), and the insulated polymers were polystyrene (PS) and poly(methyl methacrylate) (PMMA). The photonic memory device using the PF/PS blend electret exhibited a dynamic switching behavior with light-writing and voltage-erasing processes both within only 1 s, along with a high contrast on the current on/off ratio between "Photo-On" and "Electrical-OFF" over 10⁶ and the decent retention time for more than 3 months. In addition, the multilevel memory behavior could be observed using different light sources of 405, 450, and 520 nm or energy intensity, which was supported by surface potential analysis. The characteristics were superior to those of the devices using PF/PMMA blend due to the higher charge storage behavior of PS supported by fluorescence analysis. The PF/PS blend film prepared from the chlorobenzene solvent exhibited mesh-like and aggregated PF domains in the PS matrix and enhanced the contact surface area between the semiconductor and blend electret, leading to a higher memory window. The photonic memory behavior was also observed in the blend electrets of PS with the low band gap polymer, MEH-PPV, SY-PPV, or F8BT, by changing the photoresponsive light sources. Our study demonstrated a new electret system to apply on the multilevel photonic memory devices.

High-performance optical memory transistors based on a novel organic semiconductor with nanosprouts

Here, we developed a novel organic semiconductor of 2,7-di(anthracen-2-yl)naphthalene (2,7-DAN), which not only exhibits outstanding hole-transport with a mobility of 3.3 cm2 V-1 s-1 but also shows a high photoresponsivity of 8000 A W-1 with detectivity as high as 1.2 × 1014 Jones. Most importantly, optical memory transistors (OMTs) based on it display an excellent memory effect due to the inhomogeneity (nanosprouts) of the 2,7-DAN film only, different from traditional strategies towards memory devices like the application of a floating gate layer, an electret layer or a photochromic molecule. The findings distinctly make 2,7-DAN an excellent candidate for high performance nonvolatile OMTs with a simpler structure.

Photocontrolled organic field effect transistors based on the fullerene C60 and spiropyran hybrid molecule

Photocontrolled organic field-effect transistors (OFETs) containing a hybrid compound of fullerene C₆₀ (n-semiconductor) with spiropyran (electrical conductivity photocontroller) as the active layer were fabricated for the first time.

Surface Modification of CdSe Quantum-Dot Floating Gates for Advancing Light-Erasable Organic Field-Effect Transistor Memories

Photoresponsive transistor memories that can be erased using light-only bias are of significant interest owing to their convenient elimination of stored data for information delivery. Herein, we suggest a strategy to improve light-erasable organic transistor memories, which enables fast "photoinduced recovery" under low-intensity light. CdSe quantum dots (QDs) whose surfaces are covered with three different organic molecules are introduced as photoactive floating-gate interlayers in organic transistor memories. We determine that CdSe QDs capped or surface-modified with small molecular ligands lead to efficient hole diffusion from the QDs to the conducting channel during "photoinduced recovery", resulting in faster erasing times. In particular, the memories with QDs surface-modified with fluorinated molecules function as normally-ON type transistor memories with nondestructive operation. These memories exhibit high memory ratios over 10⁵ between OFF and ON bistable current states for over 10 000 s and good dynamic switching behavior with voltage-driven programming processes and light-assisted erasing processes within 1 s. Our study provides a useful guideline for designing photoactive floating-gate materials to achieve desirable properties of light-erasable organic transistor memories.

Unexpected Nonresponsive Behavior of a Flexible Metal-Organic Framework under Conformational Changes of a Photoresponsive Guest Molecule

In this article, we describe the synthesis, characterization, and optical properties of a photochromic-guest-incorporated metal-organic framework (MOF). The photochromic guest molecule, 2-phenylazopyridine (PAP), was introduced into a pre-synthesized porous crystalline host MOF, [Zn₂(1,4-bdc)₂(dabco)] _n (1). The successful embedment of PAP has been confirmed by elemental analysis, powder X-ray diffraction measurements, IR spectroscopy, etc. The number of PAP molecules per unit cell of host was 1.0, as evidenced by elemental and thermogravimetric analyses of the host-guest composite, 1⊃PAP. The 1⊃PAP composite did not adsorb N₂, revealed by the adsorption isotherm of 1⊃PAP, which indicates the pore blockage by the close contact of the host framework with the guest PAP in the trans form. The light-induced trans/cis isomerization with partial reversibility of the guest molecule (PAP) in this hybrid host-guest compound (1⊃PAP) has been investigated by detailed IR spectroscopy and UV-vis spectroscopy. The structural transformation from tetragonal in 1 to orthorhombic in 1⊃PAP exhibits dynamic nature of the framework upon inclusion of guest in the framework, which remarkably becomes nonresponsive with the photoirradiation of guest PAP, retaining its orthorhombic structure in the photoirradiated complex, 1⊃PAP(UV).

Self-Assembled Photochromic Molecular Dipoles for High-Performance Polymer Thin-Film Transistors

The development of high-performance multifunctional polymer-based electronic circuits is a major step toward future flexible electronics. Here, we demonstrate a tunable approach to fabricate such devices based on rationally designed dielectric super-lattice structures with photochromic azobenzene molecules. These nanodielectrics possessing ionic, molecular, and atomic polarization are utilized in polymer thin-film transistors (TFTs) to realize high-performance electronics with a p-type field-effect mobility (μ_FET) exceeding 2 cm² V^-1 s^-1. A crossover in the transport mechanism from electrostatic dipolar disorder to ionic-induced disorder is observed in the transistor characteristics over a range of temperatures. The facile supramolecular design allows the possibility to optically control the extent of molecular and ionic polarization in the ultrathin nanodielectric. Thus, we demonstrate a 3-fold increase in the capacitance from 0.1 to 0.34 μF/cm², which results in a 200% increase in TFT channel current.

Novel Organic Phototransistor-Based Nonvolatile Memory Integrated with UV-Sensing/Green-Emissive Aggregation Enhanced Emission (AEE)-Active Aromatic Polyamide Electret Layer

A novel aggregation enhanced emission (AEE)-active polyamide TPA-CN-TPE with a high photoluminesence characteristic was successfully synthesized by the direct polymerization of 4-cyanotriphenyl diamine (TPA-CN) and tetraphenylethene (TPE)-containing dicarboxylic acid. The obtained luminescent polyamide plays a significant role as the polymer electret layer in organic field-effect transistors (OFETs)-type memory. The strong green emission of TPA-CN-TPE under ultraviolet (UV) irradiation can be directly absorbed by the pentacene channel, displaying a light-induced programming and voltage-driven erasing organic phototransistor-based nonvolatile memory. Memory window can be effectively manipulated between the programming and erasing states by applying UV light illumination and electrical field, respectively. The photoinduced memory behavior can be maintained for over 10⁴ s between these two states with an on/off ratio of 10⁴, and the memory switching can be steadily operated for many cycles. With high photoresponsivity ( R) and photosensitivity ( S), this organic phototransistor integrated with AEE-active polyamide electret layer could serve as an excellent candidate for UV photodetectors in optical applications. For comparison, an AEE-inactive aromatic polyimide TPA-PIS electret with much weaker solid-state emission was also applied in the same OFETs device architecture, but this device did not show any UV-sensitive and UV-induced memory characteristics, which further confirmed the significance of the light-emitting capability of the electret layer.

A High-Performance Optical Memory Array Based on Inhomogeneity of Organic Semiconductors

Organic optical memory devices keep attracting intensive interests for diverse optoelectronic applications including optical sensors and memories. Here, flexible nonvolatile optical memory devices are developed based on the bis[1]benzothieno[2,3-d;2',3'-d']naphtho[2,3-b;6,7-b']dithiophene (BBTNDT) organic field-effect transistors with charge trapping centers induced by the inhomogeneity (nanosprouts) of the organic thin film. The devices exhibit average mobility as high as 7.7 cm² V^-1 s^-1 , photoresponsivity of 433 A W^-1 , and long retention time for more than 6 h with a current ratio larger than 10⁶ . Compared with the standard floating gate memory transistors, the BBTNDT devices can reduce the fabrication complexity, cost, and time. Based on the reasonable performance of the single device on a rigid substrate, the optical memory transistor is further scaled up to a 16 × 16 active matrix array on a flexible substrate with operating voltage less than 3 V, and it is used to map out 2D optical images. The findings reveal the potentials of utilizing [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivatives as organic semiconductors for high-performance optical memory transistors with a facile structure. A detailed study on the charge trapping mechanism in the derivatives of BTBT materials is also provided, which is closely related to the nanosprouts formed inside the organic active layer.

Photoresponse modulation of reduced graphene oxide by surface modification with cardanol derived azobenzene

The covalent and non-covalent interaction of photochromic azobenzene with reduced graphene oxide (RGO) influences the photo induced electrical conductivity of RGO.

Unprecedented thermal condensation of tetracyanocyclopropanes to triazaphenalenes: a facile route for the design of novel materials for electronic applications

We report an unusual thermal condensation of readily available tetracyanocyclopropanes to tetracyanosubstituted triazaphenalenes, which revealed interesting optoelectronic properties such as strongly pronounced solvatochromism and bright photoluminescence. Optical memory elements and organic light emitting diodes with a deep red electroluminescence were designed using triazaphenalenes, thus highlighting the potential of these compounds as materials for electronic applications.

Photoinduced Recovery of Organic Transistor Memories with Photoactive Floating-Gate Interlayers

Optical memories based on photoresponsive organic field-effect transistors (OFETs) are of great interest due to their unique applications, such as multibit storage memories and flexible imaging circuits. Most studies of OFET-type memories have focused on the photoresponsive active channels, but more useful functions can be additionally given to the devices by using floating gates that can absorb light. In this case, effects of photoirradiation on photoactive floating-gate layers need to be fully understood. Herein, we studied the photoinduced erasing effects of floating-gate interlayers on the electrical responses of OFET-type memories and considered the possible mechanisms. Polymer/C₆₀ composites were inserted between pentacene and SiO₂ to form photoresponsive floating-gate interlayers in transistor memory. When exposed to light, C₆₀ generated excitons, and these photoexcited carriers contributed to the elimination of trapped charge carriers, which resulted in the recovery of OFET performance. Such memory devices exhibited bistable current states controlled with voltage-driven programming and light-driven erasure. Furthermore, these devices maintained their charge-storing properties over 10 000 s. This proof-of-concept study is expected to open up new avenues in information technology for the development of organic memories that exhibit photoinduced recovery over a wide range of wavelengths of light when combined with appropriate photoactive floating-gate materials.

Laser Patterning of Optically Reconfigurable Transistor Channels in a Photochromic Diarylethene Layer

Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. Here, we demonstrate laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. The main findings are (i) a number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams alternately between the source and drain electrodes, (ii) the conductivity (or resistivity) of the 1D channel can be controlled by the illumination conditions, such as the laser power density and the scan speed, and (iii) it is possible to draw an analogue adder circuit by optically writing 1D channels so that a portion of the channels overlaps and to perform optical summing operations by local laser illumination of the respective channels. These findings will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits, which are not possible with conventional thin film OFETs.

Photoswitchable metal organic frameworks: turn on the lights and close the windows

Progress and challenges in the development of photo-responsive metal organic frameworks.

High-performance organic broadband photomemory transistors exhibiting remarkable UV-NIR response

High-performance organic broadband photomemory transistors by photogenerated minority carrier trapping and accumulation kinetics.