Formation Of Charge Transfer State Research Articles

Two-Dimensional Electronic Spectroscopy Reveals Vibrational Modes Coupled to Charge Transfer in a Julolidine-BODIPY Dyad.

Understanding charge transfer (CT) dynamics in molecular donor-acceptor (D-A) dyads can provide insight into developing efficient D-A molecules for capturing solar energy. Here, we characterize the excited-state evolution of a julolidine-BODIPY (Jul-BD) D-A system with an emissive CT state using time-resolved fluorescence, femtosecond transient absorption, and two-dimensional electronic spectroscopies. Comparison of these results with those from phenyl-BODIPY (Ph-BD) allows us to identify the dynamics at play during CT state formation and its subsequent conversion to either a fully charge-separated or triplet state. Photoexcitation of Jul-BD in tetrahydrofuran results in the formation of an initial emissive CT state that relaxes before fully charge-separating. In contrast, Jul-BD in toluene exhibits similar CT state dynamics, albeit at slower timescales, before decaying to a terminal triplet species. Quantum beat analysis at early times in both solvents shows several vibronic modes, which are corroborated using density functional theory (DFT) calculations. For Ph-BD, a single 220 cm-1 compression mode about the single bond linking the phenyl to BODIPY modulates their orbital overlap. Three active vibronic modes, 147, 174, and 214 cm-1, are found in Jul-BD, regardless of the dielectric constant of the medium. These motions correspond to compression and torsional motions along the single bond joining Jul to BD and are responsible for the evolution of the spontaneous and stimulated emission features in the time-resolved spectroscopic data, which is further supported by time-dependent DFT calculations of the steady-state absorption spectrum of the Jul-BD as a function of increasing D-A dihedral core angle. These findings show how torsional and compression motions can play a pivotal role in intramolecular CT between a D and an A linked by a single bond.

Enhanced photothermal therapy performance of D-A conjugated polymers based on [1,2,3]triazolo[4,5-g]quinoxaline by manipulating molecular motion.

Donor-acceptor (D-A) conjugated polymers can favor the nonradiative thermal dissipation process, due to the formation of an intramolecular charge transfer (ICT) state resulting from the electron cloud delocalization of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). Thus, to realize a high extinction coefficient and excellent photothermal conversion ability for a single photothermal agent, donor-acceptor type conjugated polymers PBDT-QTz and PCDT-QTz, comprising a new electron-deficient unit 2-(2-decyltetradecyl)-6,7-dimethyl-2H-[1,2,3]triazolo [4,5-g] quinoxaline (QTz) as the acceptor and 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene (BDT) or 4H-cyclopenta[2,1-b:3,4-b'] dithiophene (CDT) as the donor, are designed and synthesized by manipulating intramolecular motion. The high extinction coefficient of 28.5 L g-1 cm-1 at 850 nm and the optimal photothermal conversion efficiency of 64.3% under an 808 nm laser are achieved based on PBDT-QTz. Consequently, PBDT-QTz nanoparticles can be successfully used for both in vitro and in vivo experiments. After intravenous administration and 808 nm laser irradiation, HeLa tumor-bearing mice achieve complete tumor remission without recurrence. The results provide an efficient photothermal agent by manipulating molecular motion.

Synthesis and photophysical investigations of pyridine-pyrazolate bound boron(III) diaryl complexes

This study presents the design and synthetic pathway of unsymmetric ligands based on pyridine-pyrazolate scaffold with Donor–Acceptor (D–A) molecular arrays and their boron complexes to achieve a large Stokes shift. Intermolecular charge transfer (ICT) triggered by the uneven molecular charge distribution from electronically dense pyrazolate (donor) part of the ligands to electron-deficient boron centre (acceptor) resulted in a mega Stokes shift up to 263 nm for selected compounds while retaining the characteristic quantum efficiency and chemical stability. The photophysical properties of derivatization of pyrazolate group in the pyridine-pyrazolate scaffold of diaryl boron complexes were explored based on UV–Visible, steady-state and time-resolved fluorescence spectroscopy. An interesting dual emission along with quenching behaviour was also observed for 2-(6-methoxynaphthelene) 5-(2-pyridyl) pyrazolate boron complex (P5) due to the formation of a twisted intermolecular charge transfer (TICT) state from a locally excited (LE) state rendering it a potential candidate for sensing applications based on H-Bond quenching. In addition, the extended excited state lifetime of the reported compounds compared to classical boron-dipyrromethene (BODIPY) makes them suitable as potential probes for analytical applications requiring a longer excited state lifetime.

Interplay between charge separation and hole back transfer determines the efficiency of non-fullerene organic solar cells with low energy level offset

Organic bulk heterojunction solar cells with electron acceptors based on small donor-acceptor type molecules show record efficiencies mainly due to their long wavelength absorption, which enables efficient harvesting of solar light and, thus, causes high current density. Meanwhile, relative positions of HOMO and LUMO levels of donor and acceptor materials determine the open circuit voltage. Here, we apply ultrafast transient absorption and transient luminescence techniques together with specially-designed modelling technique to address charge carrier generation and recombination dynamics in detail. We demonstrate the importance of careful adjustment of the HOMO and LUMO levels, as their positions determine formation and recombination rates of interfacial charge transfer (CT) states. An insufficient donor and acceptor LUMO level offset, lower than ∼300 meV, leads to slow and inefficient CT state formation, while an offset of the HOMO level below ∼100 meV leads to fast CT state recombination, which we attribute to the back transfer of a hole from the donor to the acceptor. • CT state formation was investigated by ultrafast transient techniques and modelling. • ∼300 meV LUMO offset is required for efficient CT state generation. • > 100 meV HOMO offset is sufficient for ultrafast hole transfer. • Low HOMO offset opens a loss channel via the back transfer of an electron.

Water-Soluble Single-Benzene Chromophores: Excited State Dynamics and Fluorescence Detection.

Two water-soluble single-benzene-based chromophores, 2,5-di(azetidine-1-yl)-tereph- thalic acid (DAPA) and its disodium carboxylate (DAP-Na), were conveniently obtained. Both chromophores preserved moderate quantum yields in a wide range of polar and protonic solvents. Spectroscopic studies demonstrated that DAPA exhibited red luminescence as well as large Stokes shift (>200 nm) in aqueous solutions. Femtosecond transient absorption spectra illustrated quadrupolar DAPA usually involved the formation of an intramolecular charge transfer state. Its Frank–Condon state could be rapidly relaxed to a slight symmetry-breaking state upon light excitation following the solvent relaxation, then the slight charge separation may occur and the charge localization became partially asymmetrical in polar environments. Density functional theory (DFT) calculation results were supported well with the experimental measurements. Unique pH-dependent fluorescent properties endows the two chromophores with rapid, highly selective, and sensitive responses to the amino acids in aqueous media. In detail, DAPA served as a fluorescence turn-on probe with a detection limit (DL) of 0.50 μM for Arg and with that of 0.41 μM for Lys. In contrast, DAP-Na featured bright green luminescence and showed fluorescence turn-off responses to Asp and Glu with the DLs of 0.12 μM and 0.16 μM, respectively. Meanwhile, these two simple-structure probes exhibited strong anti-interference ability towards other natural amino acids and realized visual identification of specific analytes. The present work helps to understand the photophysic–structure relationship of these kinds of compounds and render their fluorescent detection applications.

AIEgens-NLOphores coumarin-triphenylamine chalcone derivatives: Synthesis, photophysical properties and DFT computational study

Three coumarin-triphenylamine chalcone derivatives (5a-c) with push-pull structures of type A-π-D and D-π-A-π-D were synthesized, varying the substituent at position 7 of coumarin. The characterization of the final compounds as for the intermediates was conducted using 1H and 13C NMR, FT-IR, and HRMS. The study of the photophysical properties of 5a-c revealed a strong solvatochromism, mainly in fluorescent emission, due to the formation of a twisted intramolecular charge transfer (TICT) state. Moreover, aggregation-induced emission (AIE) in MeCNH2O mixtures was notably observed, as well as a strong red emission in the solid state. Computational studies by DFT methods allowed to elucidate the molecular geometries of the ground (S0) and excited states (S1), assign vibrational modes and electronic transitions of IR and UV–Vis spectra, respectively, to describe the reactivity and the electronic distribution from the molecular electrostatic potential (MEP) and the HOMO-LUMO orbitals. Computationally calculated first and second-order hyperpolarizabilities show chalcones 5a-c as potential NLO materials.

Efficient Charge Dissociation of Triplet Excitons in Bulk Heterojunction Solar Cells

Understanding the sources of energy loss in bulk heterojunction (BHJ) solar cells is of outstanding importance for increasing power conversion energy. Herein, we employ a full quantum mechanics approach to determine the rates of charge transfer processes at the acceptor–donor interface for a prototypical BHJ blend. It is shown that charge recombination of a singlet charge transfer (CT) state at the acceptor/donor (A/D) interface is very fast, being the most effective process which prevents efficient charge migration toward the electrodes. Triplet CT states can also undergo charge recombination by fast decay to a low lying triplet state of the donor and/or of the acceptor, but the backward process is also fast enough to allow efficient charge dissociation. Slowing down the fast decay process of singlet CT states appears to be hardly practicable, since it would reflect either on a reduced spectral absorption window or on a slow rate of photoinduced electron transfer, so that an alternative and possibly more viable route for increasing organic solar cell efficiency could be that of favoring the formation of triplet CT states over singlet ones.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria.

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

Supramolecular modulation in photophysical features of berberine and its application towards ATP sensing

In this study we report supramolecular interaction of the cationic dye berberine (BBR) with the highly substituted polyanionic cyclodextrin derivative, sulfobutylether beta cyclodextrin (SBE10βCD). Though interaction of BBR with parent β-cyclodextrin (βCD) host is very weak, its interaction with SBE10βCD host is significantly strong with an association constant Ka as large as ∼2.67 × 105 M−1. Strong binding of BBR with SBE10βCD is governed by cooperative effect of electrostatic attraction and enhanced hydrophobic interaction caused by extended SBE10βCD cavity for bound BBR dye. In the BBR-SBE10βCD complex, lower micropolarity and rigid microenvironment of host cavity cause a large decrease both in structural agility and intramolecular charge transfer (ICT) state formation of bound BBR. Accordingly, the non-radiative de-excitation of excited BBR in the dye-host complex is reduced significantly, causing its emission intensity to increase quite largely. The BBR-SBE10βCD complex is found to be highly sensitive towards ionic strength of the medium. This property is further explored to form a ternary complex, BBR-SBE10βCD-Cd2+, with largely reduced fluorescence intensity. Interestingly, this ternary complex exhibits selective fluorescence turn-on in response to an important bio-analyte, adenosine triphosphate (ATP). Present study, thus, demonstrates a very strong supramolecular complex, BBR-SBE10βCD, which is found to be very useful in the ATP detection in the presence of Cd2+, involving the ATP responsive fluorescence.

Phototoxicity of BODIPY in long-term imaging can be reduced by intramolecular motion

For long-term live-cell fluorescence imaging and biosensing, it is crucial to work with a dye that has high fluorescence quantum yield and photostability without being detrimental to the cells. In this paper, we demonstrate that neutral boron-dipyrromethene (BODIPY)-based molecular rotors have great properties for high-light-dosage demanding live-cell fluorescence imaging applications that require repetitive illuminations. In molecular rotors, an intramolecular rotation (IMR) allows an alternative route for the decay of the singlet excited state (S1) via the formation of an intramolecular charge transfer state (CT). The occurrence of IMR reduces the probability of the formation of a triplet state (T1) which could further react with molecular oxygen (3O2) to form cytotoxic reactive oxygen species, e.g., singlet oxygen (1O2). We demonstrate that the oxygen-related nature of the phototoxicity for BODIPY derivatives can be significantly reduced if a neutral molecular rotor is used as a probe. The studied neutral molecular rotor probe shows remarkably lower phototoxicity when compared with both the non-rotating BODIPY derivative and the cationic BODIPY-based molecular rotor in different light dosages and dye concentrations. It is also evident that the charge and localization of the fluorescent probe are as significant as the IMR in terms of the phototoxicity in a long-term live-cell imaging.Graphical abstract

Intricacies and Mechanism of p‐Doping Spiro‐MeOTAD Using Cu(TFSI)2

AbstractCopper salts are a popular choice as p‐dopants for organic semiconductors, particularly in N2,N2,N2′,N2′,N7,N7,N7′,N7′‐octakis(4‐methoxyphenyl)‐9,9′‐spirobi[9H‐fluoren]‐2,2′,7,7′‐tetramine (Spiro‐MeOTAD) hole transport material for solar cells. While being exceptionally effective, no scientific consensus about their doping mechanism has been established so far. This study describes the thermodynamic equilibria of involved species in copper(II) bis(trifluoromethanesulfonyl)imide (Cu(TFSI)2) doped, co‐evaporated Spiro‐MeOTAD. A temperature‐independent formation of charge transfer states is found, followed by an endothermic release of free charge carriers. Impedance and electron paramagnetic resonance spectroscopy unravel low activation energies for hole release and hopping transport. As a result, (52.0 ± 6.4)% of the total Cu(TFSI)2 molecules form free, dissociated holes at 10 mol% and room temperature. CuI species arising out of doping are stabilized by formation of a [CuI(TFSI)2]‐ cuprate, inhibiting elemental copper formation. This CuI species presents a potent hole trap reducing their mobility, which can be averted by simple addition of a bathocuproine complexing agent. A nonlinear temperature‐dependent conductivity and mobility that contradicts current charge transport models is observed. This is attributed to a combination of trap‐ and charge transfer state freeze‐out. These insights may be adapted to other metal salts, providing guidelines for designing next‐generation ultra‐high efficiency dopants.

Long-Lived Charge-Transfer State in Spiro Compact Electron Donor-Acceptor Dyads Based on Pyromellitimide-Derived Rhodamine: Charge Transfer Dynamics and Electron Spin Polarization.

We observed a long‐lived charge transfer (CT) state in a novel orthogonal compact electron donor–acceptor dyads, with closed form of rhodamine (Rho) as electron donor and pyromellitimide (PI),or thionated PI, as electron acceptor. The two parts in the dyads are connected via a spiro quaternary carbon atom, thus the torsion between the donor and acceptor is completely inhibited, which is beneficial to reduce the reorganization energy and to exploit the Marcus inverted region effect to prolong the CT state lifetime. Femtosecond transient absorption spectra show that the charge separation is rather fast, while nanosecond transient absorption spectra confirmed the formation of long‐lived CT state (2.6 μs). Time‐resolved electron paramagnetic resonance (TREPR) spectra determined the spin multiplicity of the long living state and assigned it to a 3CT state. Replacement of an oxygen atom in the PI part with a sulfur atom favoring classical intersystem crossing processes, causes a consistently shortening of the lifetime of the 3CT state (0.29 μs).

Thermally Activated Reverse Electron Transfer Limits Carrier Generation Efficiency in PM6:Y6 Non‐Fullerene Organic Solar Cells

Transient absorption and time‐resolved fluorescence measurements in a wide temperature range are used to investigate the mechanism of charge carrier generation in efficient organic solar cells based on a PM6:Y6 donor–acceptor blend. The generation mechanisms differ significantly under excitation of a donor or acceptor. The investigations reveal a temperature‐dependent interplay between the formation of interfacial charge transfer (CT) states and intra‐moiety CT states of the acceptor, their separation into free charge carriers and carrier recombination. The efficient charge carrier generation is ensured by the carrier separation over a small energy barrier, which is easily surmountable at room temperature. However, the overall yield of charge carrier generation at room temperature is reduced by the recombination of charge carriers due to the thermally activated back transfer of electrons from the acceptor to the donor via the highest occupied molecular orbit (HOMO) levels, which is enabled by the small energy offset between HOMO levels of the donor and the acceptor.

Activating charge-transfer state formation in strongly-coupled dimers using DNA scaffolds.

Strongly-coupled multichromophoric assemblies orchestrate the absorption, transport, and conversion of photonic energy in natural and synthetic systems. Programming these functionalities involves the production of materials in which chromophore placement is precisely controlled. DNA nanomaterials have emerged as a programmable scaffold that introduces the control necessary to select desired excitonic properties. While the ability to control photophysical processes, such as energy transport, has been established, similar control over photochemical processes, such as interchromophore charge transfer, has not been demonstrated in DNA. In particular, charge transfer requires the presence of close-range interchromophoric interactions, which have a particularly steep distance dependence, but are required for eventual energy conversion. Here, we report a DNA-chromophore platform in which long-range excitonic couplings and short-range charge-transfer couplings can be tailored. Using combinatorial screening, we discovered chromophore geometries that enhance or suppress photochemistry. We combined spectroscopic and computational results to establish the presence of symmetry-breaking charge transfer in DNA-scaffolded squaraines, which had not been previously achieved in these chromophores. Our results demonstrate that the geometric control introduced through the DNA can access otherwise inaccessible processes and program the evolution of excitonic states of molecular chromophores, opening up opportunities for designer photoactive materials for light harvesting and computation.

Molecular self-assembly and fluorescence change of donor-acceptor cocrystal based on naphthalimide derivatives

<p indent="0mm">Organic cocrystals formed by two or more components through π-π interactions, charge transfer, and hydrogen/halogen bondings have attracted extensive attention because of their applications in the field of organic optoelectronics. Here, we prepared three charge-transfer cocrystals by solution-process self-assembly method which used Coronene and pyrene as donors, <italic>N</italic>,<italic>N</italic>′-dimethyl-1,4,5,8-naphthalenetetracarboxylic diimide (NDI-1) and <italic>N</italic>,<italic>N</italic>′-bis(ethyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NDI-2) as acceptors. Detailed structural characterizations clarified the mixed stacking mode in the charge-transfer cocrystals, and the supramolecular network was stabilized by C–H···O hydrogen bonds and π<italic>-</italic>π interactions. Interestingly, compared with the original donors, three cocrystals show different fluorescence emissions, which is related to the formation of charge-transfer states between donors and acceptors. This cocrystal strategy provides a new opportunity for the design of new organic light-emitting charge-transfer complexes.

An Edaravone-Guided Design of a Rhodamine-Based Turn-on Fluorescent Probe for Detecting Hydroxyl Radicals in Living Systems.

The hydroxyl radical (·OH), one of the reactive oxygen species (ROS) in biosystems, is found to be involved in many physiological and pathological processes. However, specifically detecting endogenous ·OH remains an outstanding challenge owing to the high reactivity and short lifetime of this radical. Herein, inspired by the scavenging mechanism of a neuroprotective drug edaravone toward ·OH, we developed a new ·OH-specific fluorescent probe RH-EDA. RH-EDA is a hybrid of rhodamine and edaravone and exploits a ·OH-specific 3-methyl-pyrazolone moiety to control its fluorescence behavior. RH-EDA itself is almost nonfluorescent in physiological conditions, which was attributed to the formation of a twisted intramolecular charge transfer (TICT) state upon photoexcitation and the acylation of its rhodamine nitrogen at the 3' position. However, upon a treatment with ·OH, its edaravone subunit was converted to the corresponding 2-oxo-3-(phenylhydrazono)-butanoic acid (OPB) derivative (to afford RH-OPB), thus leading to a significant fluorescence increase (ca. 195-fold). RH-EDA shows a high sensitivity and selectivity to ·OH without interference from other ROS. RH-EDA has been utilized for imaging endogenous ·OH production in living cells and zebrafishes under different stimuli. Moreover, RH-EDA allows a high-contrast discrimination of cancer cells from normal ones by monitoring their different ·OH levels upon stimulation with β-Lapachone (β-Lap), an effective ROS-generating anticancer therapeutic agent. The present study provides a promising methodology for the construction of probes through a drug-guided approach.

Restriction of Twisted Intramolecular Charge Transfer Enables the Aggregation-Induced Emission of 1-(N,N-Dialkylamino)-naphthalene Derivatives.

Understanding the mechanisms of aggregation-induced emission (AIE) is essential for the rational design and deployment of AIEgens toward various applications. Such a deep mechanistic understanding demands a thorough investigation of the excited-state behaviors of AIEgens. However, because of considerable complexity and rapid decay, these behaviors are often not experimentally accessible and the mechanistic comprehension of many AIEgens is lacking. Herein, utilizing detailed quantum chemical calculations, we provide insights toward the AIE mechanism of 1-(N,N-dialkylamino)-naphthalene (DAN) derivatives. Our theoretical analysis, corroborated by experimental observations, leads to the discovery that modulating the formation of the twisted intramolecular charge transfer (TICT) state (caused by the rotation of the amino groups) and managing the steric hindrance to minimize solid-state intermolecular interactions provides a plausible explanation for the AIE characteristics of DAN derivatives. These results will inspire the deployment of the TICT mechanism as a useful design strategy toward AIEgen development.

“Turn-on” unsymmetrical azine based fluorophore for the selective detection of diethylchlorophosphate via photoinduced electron transfer to intramolecular charge transfer pathway

The detection of chemical warfare agents (CWAs) in a highly selective, sensitive and speedy manner is essential for public safety in the case of terrorist attacks and achieving this is a challenging task. This study involves in developing a new unsymmetrical azine based fluorophore 4-((E)-(((E)-2-methoxybenzylidene)hydrazono)methyl)benzonitrile[A1] which shows high selectivity and sensitivity to the nerve agent mimic molecule, diethylchlorophosphate (DCP) through fluorescence switch on mechanism. In a fascinating manner, DCP sensing by A1 operates via solvent dependent optical output mechanisms. In the absence of DCP, the fluorescence of A1 was in the off state through photoinduced electron transfer process. In the presence of DCP, a nucleophilic substitution reaction occurs at the imine nitrogen is closer to the anisole moiety that results in the formation of a new intramolecular charge transfer state along with fluorescence enhancement. In acetonitrile, A1 shows 1763-fold fluorescence enhancement in the presence of DCP with a detection limit of 9.86 nM. In Acetonitrile/water (2:8) mixture, protonation at the imine nitrogen leads to 1188-fold fluorescence enhancement. The sensing mechanisms are confirmed by both experimental and time dependent density functional theoretical studies.

Structural effects on the emission dynamics of oxide crystals activated with Tb3+

Flux grown crystals of YPO4, GdPO4, LaPO4, CaWO4, CaMoO4, KLa(MoO4)2, GdAl3(BO3)4, and GdTa7O19 activated with Tb3+ have been comparatively characterized by Raman and luminescence spectroscopy in order to investigate the relation between structural and emission properties. Processes like host sensitization, Tb3+- Tb3+ energy transfer, formation of charge transfer states, etc., as well as their effects on the emission dynamics have been examined in the light of the spatial distribution of the involved centers. The analysis of the experimental results allows to evidence the role of the site symmetry of the active ions, of their distribution inside the lattice, of their interactions with the vibrational and electronic structure of the host, and provide useful information for understanding the luminescence properties and for the design of new Tb3+-doped materials.

Effect of Light and Heat on Polymer‐Based Resistive Random Access Memory

Highly repeatable, photosensitive nonvolatile resistive random access memory (ReRAM) made of a Si/SiO2/Cu/PRPC/Ta stack with a photoresponsive polymer composite (PRPC) as the active medium is presented. The device shows errorless bipolar resistive switching with more than 100 cycles repeatability and stable SET and RESET voltages. It changes its resistive states in response to white light and recovers completely after removal of the light source. It also changes its state in response to heat and recovers randomly after the removal of the heat source. The phenomenon is explained by valance charge transfer along with a space‐charge‐limited conduction (SCLC) mechanism. The initial state of the device is low resistance state (LRS) because of electric‐field‐induced formation of valance charge transfer states. It switches to a high resistance state (HRS) due to electric‐field‐induced dissociation of the charge transfer states.