Electron Donor-acceptor Systems Research Articles

Disaggregation-driven far-red BODPIY dye for selective G4 DNA structures detection

Push-pull solvatochromic dyes undergoing intramolecular charge transfer (ICT) have emerged as useful tools for imaging target biomolecules directly in living cells and monitoring interactions of biomolecules by changing the signal of their fluorescence. However, the currently reported solvatochromic fluorescent dyes frequently suffer from the problem of inherit background noise, and cause unspecific fluorescence that masks the specific signal from biomolecules. Herein, based on the combined the effect of aggregation-caused quenching (ACQ), a D-π-A structural far-red BODIPY dye BSK was designed and developed as a G-quadruplex (G4) DNA structures probe. In organic solvents, the spectra results underlined the excellent characteristics of BSK, such as high polarity sensitivity and strong solvatochromic effect due to the electron donor–acceptor system. In aqueous solution, BSK underwent aggregation to form nonfluorescent amorphous aggregates in which the dye kept low background fluorescence in a solution. Upon binding of BSK with c-MYC G4 DNA, an intense fluorescence was trigged by the synergistic properties of disaggregation-induced emission and inhibited twisted intramolecular charge transfer, with a low detection limit of 16 nM. In addition, BSK exhibited a selective ability to target G4 DNA structures and effectively differentiated from duplex and single-strand DNAs. Moreover, BSK has the potential to be used in visualizing the cellular nucleus in cells.

Tuning the Absorption, Fluorescence, Intramolecular Charge Transfer, and Intersystem Crossing in Spiro[fluorene]acridinone.

In this work, we prepared a series of electron donor-acceptor systems based on spiro[fluorene-9,7'-dibenzo[c,h]acridine]-5'-one (SFDBAO). Our SFDBAOs consist of orthogonally positioned fluorenes and aromatic ketones. By fine-tuning the substitution of electron-donating pyrenes, the complex interplay among different excited-state decay channels and the overall impact of solvents on these decay channels were uncovered. Placing pyrene, for example, at the aromatic ketones resulted in a profound solvatochromism in the form of a bright charge-transfer (CT) emission spanning from yellow to red-NIR. In contrast, a dark non-emissive CT was noted upon pyrene substitution at the fluorenes. In apolar solvents, efficient triplet-excited state generation was observed for all SFDBAOs. Either charge transfer was concluded to mediate the intersystem crossing (ISC) in the case of pyrene substitution or the El-Sayed rule was applicable when lacking pyrene substitution as in the case of SFABAO. In polar solvents, charge separation is the sole decay upon pyrene substitution. Moreover, competition between ISC and CT lowered the triplet-excited state generation in SFDBAO.

Construction of a redox pathway through a polyoxometalate and covalent organic framework for H2O2 photosynthesis.

A novel type of electron donor-acceptor system was built from a nitrogen-rich covalent organic framework (PC) and a polyoxometalate (BW12), fabricating a composite material (BW12@PC-250), which shows significantly improved photocatalytic H2O2 yield (56.4 μM h-1) under full spectrum illumination in pure water, being about 30 times higher than that of PC. This is due to the opening of the electron and proton transport pathway between PC and BW12, which paves a new way for POMs to modulate the photocatalytic reactions of COFs.

Bisphenol-A-diglycidyl ether modified chitosan/nano-SiO2 via hydrothermal process: A statistical modeling and adsorption mechanism for reactive orange 16 dye removal

In this study, chitosan/nano SiO2 (CTS/NS) was chemically modified with bisphenol A diglycidyl ether (BADGE) cross-linker-assisted hydrothermal process to create an effective adsorbent, CTS-BADGE/NS, for the removal of reactive orange 16 (RO16) dye from aquatic systems. Box-Behnken design (BBD) was used to optimize the adsorption process by varying the adsorbent dose (0.02–0.1 g/100 mL), pH (4–10), and time (20–360 min). The adsorption isotherm results indicated that the Langmuir model fits the experimental data well, suggesting that the adsorption process involves a monolayer formation of RO16 on the surface of CTS-BADGE/NS. The kinetic modeling of RO16 adsorption by CTS-BADGE/NS demonstrated that the pseudo-first-order model fits the adsorption data. CTS-BADGE/NS achieved an adsorption capacity of 97.8 mg/g for RO16 dye at optimum desirability functions of dosage 0.099 g/100 mL, solution pH of 4.44, and temperature of 25 °C. Overall, the π-π electron donor-acceptor system significantly improved the adsorption performance of the CTS-BADGE/NS. The results of the regeneration investigation demonstrate that the CTS-BADGE/NS exhibits effective adsorption of RO16, even after undergoing five consecutive cycles. The results of this study suggest that the developed CTS-BADGE/NS composite can be a promising adsorbent for water purification applications.

Development of a chitosan/nanosilica biocomposite with arene functionalization via hydrothermal synthesis for acid red 88 dye removal

Herein, the polymer nanomatrix of chitosan/SiO2 (CHI/n-SiO2) was enriched with a π-π electron donor-acceptor system using diaromatic rings of benzil (BEZ) assisted via a hydrothermal process to obtain an effective adsorbent of chitosan-benzil/SiO2 (CHI-BEZ/n-SiO2). The polymer nanomatrix (CHI/n-SiO2) and the resulting adsorbent (CHI-BEZ/n-SiO2) were applied to remove the anionic acid red 88 (AR88) dye from aqueous media in a comparative mode. Box-Behnken design (BBD) was adopted to optimize AR88 adsorption onto CHI/n-SiO2 and CHI-BEZ/n-SiO2 with respect to variables that influence AR88 adsorption (adsorbent dose: 0.02–0.1 g/100 mL; pH: 4–10; and time: 10–90). The adsorption studies at equilibrium were conducted with a variety of initial AR88 dye concentrations (20–200 mg/L). The adsorption isotherm results reveal that the AR88 adsorption by CHI/n-SiO2 and CHI-BEZ/n-SiO2 are described by the Langmuir model. The kinetic adsorption profiles of AR88 with CHI/n-SiO2 and CHI-BEZ/n-SiO2 reveal that the pseudo-first-order model provides the best fit results. Interestingly, CHI-BEZ/n-SiO2 has a high adsorption capacity (261.2 mg/g), which exceeds the adsorption capacity of CHI/n-SiO2 (215.1 mg/g) that relates to the surface effects of SiO2 and the functionalization of chitosan with BEZ. These findings show that CHI-BEZ/n-SiO2 represents a highly efficient adsorbent for the removal of harmful pollutants from water, which outperforming the CHI/n-SiO2 system.

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor-Acceptor Systems.

Charge transfer (CT) in molecular electron donor-acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

Pyridine-linked covalent triazine frameworks with bidirectional electron donor-acceptor for efficient organic pollution removal.

Simultaneous regulation of adsorption and photocatalytic performance of covalent triazine frameworks (CTFs) to achieve efficient control of organic pollution in water is a promising strategy, but remains a formidable challenge. Herein, pyridine linkers were innovatively introduced into pristine CTF (p-CTF) and the bidirectional electron donor-acceptor (EDA) system of contaminant-to-pyridine and pyridine-to-triazine was constructed inside. Experimental results combined with theoretical calculations revealed that pyridine units with π-deficient properties performed as electron acceptors and electron donors in the adsorption and photocatalytic processes, respectively. This special structure provided a directional pathway for electron transfer, which endowed CTFs with excellent adsorption and photocatalytic properties. Compared to p-CTF, pyridine-linked CTF (M-CTF) showed a 16-fold increase in adsorption capacity for naphthalene (973.4 μmol·g-1). Benefiting from the optimized light absorption and electron transfer form (n → π*transition), M-CTF exhibited high regeneration efficiency after adsorption of both bisphenol A (94% after 4 cycles) and naphthalene (95% after 4 cycles). Besides, the removal performance of organic micropollutants from natural water showed a great advantage thanks to the bidirectional EDA system. Overall, the present study provides new insights into the optimization of electronic structures for carbon-based environmental functional materials applied to organic pollution control in water.

Meso-Carbazole decorated BODIPYs – an electron donor–acceptor system with excellent fluorosolvato/vapochromic behavior, aggregation-induced emission, and antileishmanial activity

meso-Carbazole BODIPY generates an electron donor–acceptor system with multiple photo responses and has excellent anti-leishmanial activity.

Photoinduced interfacial electron transfer from perovskite quantum dots to molecular acceptors for solar cells.

Bandgap-engineered inorganic and hybrid halide perovskite (HP) films, nanocrystals, and quantum dots (PQDs) are promising for solar cells. Fluctuations of photoinduced electron transfer (PET) rates affect the interfacial charge separation efficiencies of such solar cells. Electron donor- or acceptor-doped perovskite samples help analyze PET and harvest photogenerated charge carriers efficiently. Therefore, PET in perovskite-based donor-acceptor (D-A) systems has received considerable attention. We analyzed the fluctuations of interfacial PET from MAPbBr3 or CsPbBr3 PQDs to classical electron acceptors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 1,2,4,5-tetracyanobenzene (TCNB) at single-particle and ensemble levels. The significantly negative Gibbs free energy changes of PET estimated from the donor-acceptor redox potentials, the donor-acceptor sizes, and the solvent dielectric properties help us clarify the PET in the above D-A systems. The dynamic nature of PET is apparent from the decrease in photoluminescence (PL) lifetimes and PL photocounts of PQDs with an increase in the acceptor concentrations. Also, the acceptor radical anion spectrum helps us characterize the charge-separated states. Furthermore, the PL blinking time and PET rate fluctuations (108 to 107 s-1) provide us with single-molecule level information about interfacial PET in perovskites.

Construction of 3D porous BiOBr/MIL-101(Cr) Z-scheme heterostructure for boosted photocatalytic degradation of tetracycline hydrochloride

The challenge for efficient photocatalysis lies in the adsorption, reuse capacity and charge separation. Herein, we constructed a Büchner funnel-like three-dimensional (3D) porous BiOBr/MIL-101(Cr) Z-scheme heterostructure with large specific surface area by immobilizing BiOBr on MIL-101(Cr) framework. The BiOBr/MIL-101(Cr) hybrids exhibited efficient charge separation and increasing adsorption capacity. Importantly, Bi and Cr atoms formed an electron donor-acceptor system, which benefits to induce internal electric field (IEF) to construct Z-scheme heterostructures for promoting charge transfer and separation. In addition, MIL-101(Cr) allows the continued growth of BiOBr in the presence of (001) direction, thus exposing more (001) facet of BiOBr, which helps the electron transfer from (110) to (001) and accelerates the carrier separation. Meanwhile, the formation of 3D pore structure endowed the BiOBr/MIL-101 (BM-35, which contained 35 wt% MIL-101) with twice higher adsorption capacity than pure BiOBr. Tetracycline hydrochloride (TC-HCl) was chosen as the target pollutant to evaluate the photocatalytic performance of the as-prepared BM-35 under visible light irradiation, 94 % degradation rate of TC-HCl was achieved over BiOBr/MIL-101(Cr) after 100 min and maintained at 84 % after 5 cycles. The small pores of MIL-101(Cr) only enable water molecules to pass through, while TC-HCl cannot enter its inner pores. Moreover, the formation of BiOBr by sheet stacking has large pores that capture target pollutants onto the highly active surface and decompose them. Benefiting from the distinctive 3D structure, an adsorption-degradation cycle system was formed, the difficulty of MOF desorption was effectively overcome.

Mesityl‐Appended 1,4‐Bis(β‐acrylonitrile)‐2,5‐dimethoxybenzene: Blue and Green Fluorescent Crystals from a Soluble Donor–Acceptor Molecular System

AbstractBecause acrylonitrile‐phenylene‐based π‐conjugated molecules have highly rigid structures (or low structural flexibility), these molecules have low solubility in various organic solvents, including nonpolar and polar solvents such as hexane, ethanol, and dimethyl sulfoxide. Alkylation or PEGulation strategies are useful approaches for designing highly soluble acrylonitrile‐phenylene compounds, but the number of good solvents is limited. Herein, we report mesityl‐appended 1,4‐bis(β‐acrylonitrile)‐2,5‐dimethoxybenzene including an electron donor–acceptor system. The molecule shows good solubility in various organic solvents, such as hexane, toluene, chloroform, dichloromethane, acetone, ethanol, N,N‐dimethylformamide, acetonitrile, and dimethyl sulfoxide, owing to the appended bulky mesityl groups. Spectroscopic measurements were performed in various solutions. The solvatochromic behaviour was investigated according to the polarity and affinities by absorption and photoluminescence (PL) spectroscopic analyses in various solvents. The acrylonitrile unit in the π‐conjugated structure allows rotation under ambient light. E−Z isomerisation was observed owing to its structural flexibility. This feature gave two fluorescent molecular crystals: blue‐PL Z,Z‐molecular crystal BC (minor material, CCDC 2108373, λem=473 nm) and green‐PL E,E‐molecular crystal GC (major material, CCDC 2108365, λem=508 nm). It is reasonable that the more planar donor–acceptor core structure of the E,E‐crystal shows lower energy fluorescence than the slightly twisted structure of the Z,Z‐crystal.

Synergistic Effects of Mechanical, Thermal, and Optical Properties of Polyimide Composite Films Reinforced with WO3 Nanorods

AbstractIn this study, WO3 nanorods are directly blended with polyamic acid (PAA) solutions to improve the electrochromic capacity of WO3/PAA composite films. Specifically, the hexagonal WO3 nanorods are prepared as inorganic reinforcements using a hydrothermal method and modified by γ‐aminopropyl triethoxysilane. After curing at 300 °C, the WO3 nanorods/polyimide (PI) composite films demonstrate excellent mechanical and thermal properties. The tensile strength of the 1 wt% WO3/PI composite film is ≈110.27 MPa. The thermal stability of the 5 wt% WO3/PI composite film is better than that of the others, and the T5 (5% thermal weight loss temperature) value is ≈560.05 °C. Moreover, the WO3/PAA composite films demonstrate electrochromic behaviors. The electrochromic range of the 5 wt% WO3/PAA composite film exceeds to 68%. For the WO3/PAA samples, the oxidation peak of the WO3 nanorods is evident in the cyclic voltammetry curves. The color contrast of WO3/PI nanocomposites can be enhanced owing to the significant synergistic effects of the electron donor–acceptor system.

Spin Orbit Coupling in Orthogonal Charge Transfer States: (TD-)DFT of Pyrene-Dimethylaniline.

The conformational dependence of the matrix element for spin–orbit coupling and of the electronic coupling for charge separation are determined for an electron donor–acceptor system containing a pyrene acceptor and a dimethylaniline donor. Different kinetic and energetic aspects that play a role in the spin–orbit charge transfer intersystem crossing (SOCT-ISC) mechanism are discussed. This includes parameters related to initial charge separation and the charge recombination pathways using the Classical Marcus Theory of electron transfer. The spin–orbit coupling, which plays a significant role in charge recombination to the triplet state, can be probed by (TD)-DFT, using the latter as a tool to understand and predict the SOCT-ISC mechanism. The matrix elements for spin–orbit coupling for acetone and 4-thio-thymine are used for benchmarking. (Time Dependent-) Density Functional Theory (DFT and TD-DFT) calculations are applied using the quantum chemical program Amsterdam Density Functional (ADF).

Photophysical properties regulation and applications of BODIPY-based derivatives with electron donor-acceptor system

As a class of important fluorescence dyes, BODIPY and its derivatives have the advantages of high quantum yield, good biocompatibility and easy structure adjustment, etc. In view of the structural characteristics of BODIPY, through the methods of introducing electron donors and electron acceptors into BODIPY core, the photophysical properties can be adjusted effectively through the push–pull electron effect. In this review, based on different fluorescence regulation mechanisms and the BODIPY electronic roles as donating, withdrawing or linkage parts in the molecular structures, we summarized the research progress and application directions of BODIPY-based derivatives with donor–acceptor electron system.

In-situ construction of N-doped carbon nanosnakes encapsulated FeCoSe nanoparticles as efficient bifunctional electrocatalyst for overall water splitting

The development of bifunctional electrocatalysts with high activity and stability for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial for efficient overall water splitting but still challenging. Herein, we propose a facile and effective polymerization–pyrolysis–selenization (PPS) strategy for in-situ synthesis of N-doped carbon nanosnakes (NCNSs) encapsulated Fe-doped CoSe nanoparticles (NPs) derived from predesigned trimetallic Zn/Fe/Co polyphthalocyanine conjugated polymer networks. Benefiting from the synergistic effect between the regulation of Fe atoms and CoSe NPs as well as the confinement effect of in situ formed porous conductive carbon nanosnakes, the FeCoSe@NCNSs catalyst exhibited the excellent electrocatalytic activity for HER with small overpotentials (142 and 99 mV in 0.5 M H2SO4 and 1 M KOH) and OER (320 mV in 1 M KOH) at the current density of 10 mA cm−2. Particularly, it also can be used as an efficient bifunctional electrocatalyst with a cell voltage of 1.66 V to achieve a current density of 10 mA cm−2 and superior stability for overall water splitting. Density functional theory study reveals that the doping of Fe atoms on CoSe enhanced the splitting and delocalization of metal-d orbitals close Fermi level, and modifies the distribution of Se-p orbitals close Fermi level, which improved the flexibility of electron donor-acceptor system and the hydrogen adsorption free energy change on metal-metal bridge sites in FeCoSe@NCNSs. Additionally, beneficial from the accepting of Fe-Se bridge site, the overpotential of OER which following intramolecular oxygen coupling mechanism is also decreased, thus accelerating the electrocatalytic performance. This work presents a novel strategy to regulate the activity and stability of transition metal selenides and facilitating the rational design of bifunctional electrocatalysts for overall water splitting applications.

Janus luminogens with bended intramolecular charge transfer: Toward molecular transistor and brain imaging

<h2>Summary</h2> The ingenious construction of electron donor-acceptor (D-A) systems has been proven to be the major trend for advanced performance optoelectronic materials. However, the related development is undiversified and has become stereotyped in recent years, and the explorations of innovative architecture with both prominent optoelectronic properties and innovatively coined optoelectronic mechanisms are appealing, yet significantly challenging tasks. Here, we exploit a series of unique Janus luminogens, namely TAOs, with unique charge separation in a simple five-membered mesoionic ring. TAOs, having low molecular weight (∼329 g mol⁻¹), present efficient aggregation-induced red/near-infrared emission (550–850 nm) with up to 21.5% of fluorescence quantum yield. An original mechanism, termed bended intramolecular charge transfer (BICT), is proposed to understand the fluorescence behavior. It is experimentally demonstrated that TAOs exhibit great potential for use as molecular transistors and can be efficiently utilized in living cells, bacteria, and brain imaging in a straightforward manner by using intravenous postinjection with outstanding photostability and biocompatibility.

Optoelectrical switching behavior based on a composite of a fullerene acceptor and carbazole donor

An electron donor–acceptor system composed of a C60 fullerene acceptor and poly(9-vinylcarbazole) (PVK) as the donor has been constructed. This material can respond to both electrical and optical stimuli. Using the C60:PVK blends as the active layer, a solution-processed active layer with a configuration of indium tin oxid (ITO)/C60:PVK/Al is fabricated. Interestingly, when the applied voltages varied from 0 to −0.8 V, the as-fabricated device exhibits both photo-induced resistive state changes and volatile photo-response characteristics in the broadband visible region. The light illumination gives rise to a significant decrease in the device resistance. Furthermore, it is also found that, when the sweep-voltage applied to the device is changed from 0 to ±4 V, this device shows a typical nonvolatile rewritable memory performance in the dark. Upon illumination with different wavelengths of light, both the switching-on voltage and the ON/OFF ratio of the ITO/C60:PVK/Al device are found to be greatly decreased. This work opens up a pathway to the integration of information storage and modulating and demodulating functions in an optoelectronic device.

Controlling and Fine-Tuning Charge-Transfer Emission in 2,6-Dicyanoaniline Multichromophores Prepared through Domino Reactions: Entry to a Potentially New Class of OLEDs.

Substituted 2,6-dicyanoanilines are versatile electron donor-acceptor compounds, which have recently received considerable attention, since they exhibit strong fluorescence and may have utility in the synthesis of fluorescent materials, non-natural photosynthetic systems, and materials with nonlinear optical properties. The majority of known synthetic procedures are, however, "stop-and-go" reaction processes involving time-consuming and waste-producing isolation and purification of product intermediates. Here, we present the synthesis of substituted 2,6-dicyanoanilines via atom-economical and eco-friendly one-pot processes, involving metal-free domino reactions, and their subsequent photochemical and photophysical measurements and theoretical calculations. These studies exhibit the existence of an easily tunable radical ion pair-based charge-transfer (CT) emission in the synthesized 2,6-dicyanoaniline-based electron donor-acceptor systems. The charge-transfer processes were explored by photochemical and radiation chemical measurements, in particular, based on femtosecond laser photolysis transient absorption spectroscopy and time-resolved emission spectroscopy, accompanied by pulse radiolysis and complemented by quantum chemical investigations employing time-dependent density-functional theory. This chromophore class exhibits a broad-wavelength-range fine-tunable charge recombination emission with high photoluminescence quantum yields up to 0.98. Together with its rather simple and cost-effective synthesis (using easily available starting materials) and customizable properties, it renders this class of compounds feasible candidates as potential dyes for future optoelectronic devices like organic light-emitting diodes (OLEDs).

Amphiphilic Zinc Porphyrin Single-Walled Carbon Nanotube Hybrids: Efficient Formation and Excited State Charge Transfer Studies.

Herein, the microscopic and spectroscopic characterization of a novel non-covalent electron donor-acceptor system, in which three different metalloporphyrins (1, 2, and 3) play the dual role of light harvester and electron donor with SWCNTs as electron acceptor, is described. To this end, microscopy, that is, atomic force microscopy (AFM) and transmission electron microscopy (TEM) corroborate the formation of 1-SWCNT, 2-SWCNT, and 3-SWCNT. Spectroscopy by means of Raman, fluorescence, and transient absorption spectroscopy confirmed efficient charge-transfer interaction from photoexcited metalloporphyrins to SWCNTs in the ground and excited state of 1-SWCNT, 2-SWCNT, and 3-SWCNT. The complementary use of spectroelectrochemical and transient absorption measurements substantiates the formation of one-electron oxidized metalloporphyrins after photoexcitation. Multiwavelength global analysis provides insights into the charge-separation and recombination processes in 1-SWCNT, 2-SWCNT, and 3-SWCNT upon photoexcitation. Notably, both the charge-separation and recombination dynamics are fastest in 2-SWCNT. Importantly, the strongest interactions in the steady-state experiments are associated with the fastest excited state decay in the time-resolved measurements.

Long-range light-modulated charge transport across the molecular heterostructure doped protein biopolymers.

Biological electron transfer (ET) across proteins is ubiquitous, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via intramolecular cofactors within the protein. Far from biology, carbon dots (C-Dots) with their unique optoelectronic properties can be considered as game-changers for next-generation advanced technologies. Here, we use C-Dots for making heterostructure (HS) configurations by conjugating them to a natural ET mediator, the hemin molecule, thus making an electron donor–acceptor system. We show by transient absorption and emission spectroscopy that the rapid intramolecular charge separation happens following light excitation, which can be ascribed to an ultrafast electron and hole transfer (HT) from the C-Dot donor to the hemin acceptor. Upon integrating the HS into a protein matrix, we show that this HT within the HS configuration is 3.3 times faster compared to the same process in solution, indicating the active role of the protein in supporting the rapid light-induced long-range intermolecular charge separation. We further use impedance, electrochemical, and transient photocurrent measurements to show that the light-induced transient charge separation results in an enhanced ET and HT efficiency across the protein biopolymer. The charge conduction across our protein biopolymers, reaching nearly 0.01 S cm−1, along with the simplicity and low-cost of their formation promotes their use in a variety of optoelectronic devices, such as artificial photosynthesis, photo-responsive protonic–electronic transistors, and photodetectors.