Intermolecular Charge Transfer Processes Research Articles

D-π-A-π-D-Configured Imidazole-Tethered Benzothiadiazole-Based Sensor for the Ratiometric Discrimination of Picric Acid: Applications in Latent Fingerprint Imaging.

A tetraphenyl imidazole-appended benzothiadiazole-based fluorogenic probe (4,7-bis(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenyl)benzo[c][1,2,5]thiadiazole (BIPT)) has been successfully synthesized and characterized by NMR and high-resolution mass spectrometry (HRMS) spectral analyses. A low limit of detection (LOD) can be achieved to detect picric acid (PA; 7.89 nM). When benzothiadiazole acceptors are incorporated in the D-A-D probe, it can produce a large Stokes shift (206 nm) as a result. Fascinatingly, the fluorescence signals of BIPT were ratiometrically induced by the interaction with PA and exhibited an apparent emission shift from pink to green. The detection process of BIPT is triggered by an intermolecular charge transfer process, as the charge transfer takes place from the electron-rich imidazole to the electron-deficient PA. Moreover, fluorescence detection of PA has been employed in paper strips. Advantageously, sensor BIPT can potentially be applied to contact mode and real-time detection of PA in environmental water samples. Additionally, the BIPT sensor has been successfully employed for latent fingerprint imaging. The study provides clear insights into the rational design of chemosensors for sensing and real-time applications.

Fluorimetric Detection of Insulin Misfolding by Probes Derived from Functionalized Fluorene Frameworks.

A group of functionalized fluorene derivatives that are structurally similar to the cellular prion protein ligand N,N'-(methylenedi-4,1-phenylene)bis [2-(1-pyrrolidinyl)acetamide] (GN8) have been synthesized. These compounds show remarkable native fluorescence due to the fluorene ring. The substituents introduced at positions 2 and 7 of the fluorene moiety are sufficiently flexible to accommodate the beta-conformational folding that develops in amyloidogenic proteins. Changes in the native fluorescence of these fluorene derivatives provide evidence of transformations in the amyloidogenic aggregation processes of insulin. The increase observed in the fluorescence intensity of the sensors in the presence of native insulin or amyloid aggregates suggest their potential use as fluorescence probes for detecting abnormal conformations; therefore, the compounds can be proposed for use as "turn-on" fluorescence sensors. Protein-sensor dissociation constants are in the 5-10 μM range and an intermolecular charge transfer process between the protein and the sensors can be successfully exploited for the sensitive detection of abnormal insulin conformations. The values obtained for the Stern-Volmer quenching constant for compound 4 as a consequence of the sensor-protein interaction are comparable to those obtained for the reference compound GN8. Fluorene derivatives showed good performance in scavenging reactive oxygen species (ROS), and they show antioxidant capacity according to the FRAP and DPPH assays.

Polarization engineering of conjugated microporous polymers to boost exciton dissociation by dielectric constant regulation for photocatalytic degradation of antibiotics

Photocatalytic degradation of antibiotics in wastewater by polymeric semiconductors represents a promising strategy for alleviating environmental pollution and ecological damage. However, due to the relatively low dielectric constants, polymeric semiconductors are subject to high exciton binding energies, and their exciton dissociation efficiency is substantially inferior to expectation. Herein, a polarization-triggered strategy was developed in conjugated microporous polymers (CMPs) to boost exciton dissociation via the regulation of dielectric constants. After polarization by acceptors with high dipole moment, the resulting SpiroCMP-4 exhibited a greatly enhanced dielectric constant, which endowed it with an exciton binding energy of 99 meV, underperforming those of SpiroCMP-1(132 meV), SpiroCMP-2 (126 meV), and SpiroCMP-3 (106 meV) and thus facilitating the intermolecular charge transfer process. Coupling the fabricated CMPs with sepiolite (SEP), the composite SpiroCMP-4@SEP delivered the highest photocatalytic removal efficiency of ofloxacin (97.7 % in 100 min), obviously exceeding those of its analogues. Briefly, this work cultivates a state-of-the-art polarization engineering strategy to regulate dielectric constant targeting for minimizing exciton binding energy, and thus enhancing photocatalytic efficiency for environmental purification.

Boosting Exciton Dissociation and Charge Transfer in Triazole-Based Covalent Organic Frameworks by Increasing the Donor Unit from One to Two for the Efficient Photocatalytic Elimination of Emerging Contaminants.

As novel photocatalysts, covalent organic frameworks (COFs) have potential for water purification. Insufficient exciton dissociation and low charge mobility in COFs yet restricted their photocatalytic activity. Excitonic dissociation and charge transfer in COFs could be optimized via regulating the donor-acceptor (D-A) interactions through adjusting the number of donor units within COFs, yet relevant research is lacking. By integrating the 1,2,4-triazole or bis-1,2,4-triazole unit with quinone, we fabricated COF-DT (with a single donor unit) and COF-DBT (with double donor units) via a facile sonochemical method and used to decontaminate emerging contaminants. Due to the stronger D-A interactions than COF-DT, the exciton binding energy was lower for COF-DBT, facilitating the intermolecular charge transfer process. The degradation kinetics of tetracycline (model contaminant) by COF-DBT (k = (12.21 ± 1.29) × 10-2 min-1) was higher than that by COF-DT (k = (5.11 ± 0.59) × 10-2 min-1) under visible-light irradiation. COF-DBT could efficiently photodegrade tetracycline under complex water chemistry conditions and four real water samples. Moreover, six other emerging contaminants, both Gram-negative and Gram-positive strains, could also be effectively eliminated by COF-DBT. High tetracycline degradation performance achieved in a continuous-flow system and in five reused cycles in both laboratory and outdoor experiments with sunlight irradiation showed the stability and the potential for the practical application of COF-DBT.

Intermolecular Charge Transfer Induced Fragmentation of Formic Acid Dimers.

We investigate the intermolecular nonradiative charge transfer process in a double hydrogen-bonded formic acid (FA) dimer, initiated by electron-collision induced double ionization of one FA molecule. Through fragment ions and electron coincident momentum measurements and abinitio calculations, we obtain direct evidence that electron transfer from the neighboring FA molecule to fill one of the two vacancies occurs by a potential energy curve crossing of FA^{++}+FA with FA^{+}+FA^{+*} curves, forming an electronic excited state of dicationic dimers. This process causes the breaking of two hydrogen bonds and subsequently the cleavage of C─H and C─O covalent bonds in the dimers, which is expected to be a general phenomenon occurring in molecular complexes and can have important implications for radiation damage to biological matter.

Pt-N Coordination Rendering the Chemotherapeutic Agent with Photoactivated ROS Generation and Self-Reporting Cell Uptake.

The performance of chemotherapeutic agents has been largely restrained by the dose-dependent toxic side effects. In this work, cisplatin (CDDP) was endowed with the capability of photoactivated reactive oxygen species (ROS) generation and self-reporting cell uptake via coordination with a small organic molecule MSN. In the resultant MSN-Pt, the Pt-N coordination could obviously enhance the intermolecular charge transfer (ICT) process that allows the integration of fluorescence imaging, photogenerated ROS, and chemotherapeutic performance. The resultant MSN-Pt can recognize between normal and cancer cells and quickly penetrate the cancer cell membrane, self-reporting the cell uptake. Upon light illumination, mitochondria and nuclei were severely damaged. An in vivo mouse model demonstrated that MSN-Pt completely inhibited the tumor growth, exhibiting a higher efficacy compared with that of CDDP. This work provides a facile strategy to develop chemotherapy (CT) drugs for drug-resistant cancers.

Nonfused Dimethoxybenzene Electron Acceptors in Organic Solar Cells: From Molecular Design to Structure–Performance Relationship

The photovoltaic mechanism of asymmetric non-fullerene acceptors (NFAs) has not been sufficiently understood, although it is developing as a forceful method to design effective organic solar cells. This article systematically investigated and designed several configurations of nonfused acceptors at the micro level. Skeletons were constructed by a central 1,4-dimethoxybenzene core and flanked by different dithieno-pyrroles and thiophene derivates as electron-donating groups; the end-capped groups were composed of two 1,1-dicyanomethyl-3-indanone. Through theoretical calculation, one found that the designed DBPT series possessed narrower gaps, broadened the absorption to the near-infrared region, larger electron-accepting power, and separated the distance of holes and electrons. They also yielded enhancement mobility, which may relate to the significant dipole moment and electronic coupling. Furthermore, we constructed the device models by using PBDB-T as the donor. The heterojunction model results show that the optimized blends of PBDB-T/DBPT-4F achieved more intermolecular fragment charge amounts, more CT states, and smaller binding energy, showing a significant charge separation/recombination ratio. Then, under the macro-scale model, the device PBDB-T/DBPT-4F has a better power conversion efficiency of 14.69%, owing to a large simulated JSC of 18.82 mA cm–2, consistent with the trend of better interfacial transfer property. This work provided the global description of the molecular light response and the microscopic physicochemical mechanism of the intermolecular charge transfer process, which also sheds light on the structure–property relationship of unfused asymmetric NFAs and enables the strategy to develop valuable OSCs.

Synthesis of crystallinity covalent organic framework and its ratiometric fluorescent detection of trace water

The sensitive and accurate detection of trace water in solid food products is of great significance for the protection of their stability and quality. Herein, a novel crystalline COF TAPT-BPTA with fluorescence was prepared via Schiff-base condensation reaction between 4,4′,4″-(1,3,5-Triazine-2,4,6-triyl)trianiline (TAPT) and 2,5-bis(2-propyn-1-yloxy)-1,4-benzenedicarboxaldehyde (BPTA). The fluorescence of TAPT-BPTA was sensitive to trace water and can be quenched due to the disruption of intermolecular charge transfer (ICT) process by water molecules. Hence, COF TAPT-BPTA was successfully applied for sensing of trace water in common organic solvents including acetone, acetonitrile, THF and DMF with rapid response (less than 3 s) and good sensing ranges (0.05–20 v/v% for acetone, 0.10–25 v/v% for THF, 0.05–15 v/v% for ACN, and 0.05–10 v/v% for DMF). The limit of detection of water in acetone, ACN, DMF, THF were 0.0585%, 0.0631%, 0.0185% and 0.0718% (v/v), respectively. Additionally, using TAPT-BPTA as signal probe and tris(2,2′-bipyridyl)ruthenium (II) dichloride with red fluorescent as reference, a ratiometric fluorescent sensor was prepared and successfully used for the sensing of trace water in milk powder samples.

Aggregation-Induced Intermolecular Charge Transfer Emission for Solution-Processable Bipolar Host Material via Adjusting the Length of Alkyl Chain.

Molecules with donor-spacer-acceptor configuration have been developed rapidly given their peculiar properties. How to utilize intermolecular interactions and charge transfers for solution-processed organic light-emitting diodes (OLEDs) greatly relies on molecular design strategy. Herein, soluble luminophores with D-spacer-A motif were constructed via shortening the alkyl chain from nonane to propane, where the alkyl chain was utilized as a spatial linker between the donor and acceptor. The alkyl chain blocks the molecular conjugation and induces the existence of aggregation-induced intermolecular CT emission, as well as the improved solubility and morphology in a solid-state film. In addition, the length of the alkyl chain affects the glass transition temperature, carrier transport and balance properties. The mCP-3C-TRZ with nonane as the spacer shows better thermal stability and bipolar carrier transport ability, so the corresponding solution-processable phosphorescent organic light-emitting diodes exhibit superior external quantum efficiency of 9.8% when using mCP-3C-TRZ as a host material. This work offers a promising strategy to establish a bipolar host via utilizing intermolecular charge transfer process in an aggregated state.

Experimental and theoretical studies of linear and non-linear optical properties of novel fused-triazine derivatives for advanced technological applications

Controlling photophysical properties is critical for the continued development of electroluminescent devices and luminescent materials. The preparation and study of novel molecules suitable as luminescent for the development of optoelectrical devices have recently received a lot of attention. Even though the as-triazine unit is a good building block for organic active substances, it is rarely used in this context. We created here novel bis-triazine derivative dyes in the far UV–Vis range by alkylation of triazine-thione derivatives with appropriate dibromo compounds. At the B3LYP/6-311**G(d,p) basis set, their optimal molecular structures were obtained. DFT technique confirmed that the new triazine derivatives are in noncoplanar with one of the two phenyl rings and the triazine plane rotating out by 102.09. Also, depending on the energy gap difference between HOMO and LUMO, some important parameters including chemical potential (π), electronegativity (χ), and chemical hardness (η) were calculated. The compounds may be readily polarized and have significant NLO characteristics, as seen by the tiny HOMO–LUMO energy gap. The calculated values for the polarizability (α) of the two new triazine derivatives have the range 6.09–10.75 × 10–24 (esu). The emission peaks seemed to move to the long-wavelength (redshift), with a rise in the fluorescence band, suggesting that the singlet excited state is more polar than the ground state. The influence of solvent polarity and the intermolecular charge transfer (ICT) processes are reflected in the photophysical properties of new fused triazine derivatives. These properties such as extinction coefficient, absorption and emission cross-sections, fluorescence quantum yield, fluorescence lifetime, oscillator strength, the dipole moment, radiative decay rate constant, the energy yield of fluorescence, and the attenuation length were assessed and discussed.

Umbelliferone-Based Fluorescent Probe for Selective Recognition of Hydrogen Sulfide and Its Bioimaging in Living Cells and Zebrafish

Hydrogen sulfide (H2S) plays a crucial role in a variety of physiological and pathological processes, similar to other gaseous signaling molecules. The significant pathophysiological functions of H2S have sparked a great deal of interest in the creation of fluorescent probes for H2S monitoring and imaging. Using 3-cyanoumbelliferone as the push–pull fluorophore and a dinitrophenyl substituent as the response site, herein we developed a umbelliferone-based fluorescent probe 1 for H2S, which exhibited a remarkable turn-on fluorescence response with a low detection limit (79.8 nM), high sensitivity and selectivity. The H2S-sensing mechanism could be attributed to the cleavage of the ether bond between the dinitrophenyl group and the umbelliferone, leading to the recovery of an intermolecular charge transfer (ICT) process. Moreover, the probe had negligible cytotoxicity and good cell membrane permeability, which was successfully applied to image H2S in MCF-7 cells and zebrafish.

Determination of water in organic solvents and raw food products by fluorescence quenching of a crystalline vinyl-functionalized COF.

Covalent organic frameworks (COFs) with good chemical stability, flexible chemical functionalization, tunable pore sizes, and high specific surface areas have been increasingly employed in the field of fluorescence sensing. In this work, a crystalline vinyl-functionalized COF TzDa-V was facilely prepared through a room-temperature synthetic method via condensation reaction between 4,4',4″-(1,3,5-triazine-2,4,6-triyl)trianiline (Tz) and 2,5-diallyloxyterephthalaldehyde (Da-V). The intermolecular charge transfer (ICT) effect endowed the TzDa-V with fluorescence characteristic, and it was sensitive to trace water and can be quenched due to the disruption of ICT process by water. On this base, the prepared COF TzDa-V with excellent chemical/thermal stability was applied to sensing of trace water in common organic solvents such as DMF, acetone, THF, and ethyl acetate with rapid response (less than 10s), satisfactory sensing range (0.5-18% water in DMF, 0.5-15% water in acetone, 0.5-16% water in THF, 0.5-5% in ethyl acetate, v/v), and high sensitivity. The limits of detection for water in DMF, acetone, THF, and ethyl acetate were 0.0497%, 0.0590%, 0.0502%, and 0.0766% (v/v), respectively. The proposed probe was successfully used for the detection of trace water in food products such as salt and sugar. The COF TzDa-V would be a good candidate for application in water sensing.

A Ni(II) Metal-Organic Framework with Mixed Carboxylate and Bipyridine Ligands for Ultrafast and Selective Sensing of Explosives and Photoelectrochemical Hydrogen Evolution.

We report a Ni-MOF (nickel metal-organic framework), Ni-SIP-BPY, synthesized by using two linkers 5-sulfoisophthalic acid (SIP) and 4,4'-bipyridine (BPY) simultaneously. It displays an orthorhombic crystal system with the Ama2 space group: a = 31.425 Å, b = 19.524 Å, c = 11.2074 Å, α = 90°, β = 90°, γ = 90°, and two different types of nickel(II) centers. Interestingly, Ni-SIP-BPY exhibits excellent sensitivity (limit of detection, 87 ppb) and selectivity toward the 2,4,6-trinitrophenol (TNP)-like mutagenic environmental toxin in the pool of its other congeners via "turn-off" fluorescence response by the synergism of resonance energy transfer, photoinduced electron transfer, intermolecular charge transfer, π-π interactions, and competitive absorption processes. Experimental studies along with corroborated theoretical experimentation, vide density functional theory studies, shed light on determining the plausible mechanistic pathway in selective TNP detection, which is highly beneficial in the context of homeland security perspective. Along with the sensing of nitroaromatic explosives, the moderately low band gap and the p-type semiconducting behavior of Ni-SIP-BPY make it suitable as a photoanode material for visible-light-driven water splitting. Highly active surface functionalities and sufficient conduction band minima effectively reduce the water and result in a seven times higher photocurrent density under visible-light illumination.

A radical photochromic metal-organic framework for boosting NIR-II photothermal conversion and therapy.

This work demonstrates for the first time that a photochromic metal organic framework (pMOF) can be employed as a promising class of NIR-II photothermal material based on the photoinduced donor-acceptor intermolecular charge transfer process. After further surface-modification, such UV-activated pMOF-a nanoparticles allow the strong inhibition of 4T1 cancer cells under 1064 nm laser irradiation.

Aggregation induced intermolecular charge transfer in simple nonconjugated donor–acceptor system

The exploration of exciplex for organic light-emitting diodes (OLEDs) has been fleetly developed. However, many of them confront with the problems like phase separation and poor solubility, hampering their utilization in solution process. Hence, a series of soluble exciplex luminophores with the simple architecture of D-spacer-A (mCP-6C-TRZ, phCz-6C-TRZ and 2phCz-6C-TRZ) are synthesized and characterized, in which, the alkyl chain as ample spacer breaks the molecular backbone conjugation, induces intermolecular charge transfer process instead of intramolecular charge transfer in solid state. These materials are endowed with narrowed singlet−triplet splitting energy (ΔE ST ), efficient reverse intersystem crossing (RISC) process, and distinct thermally activated delayed fluorescence (TADF) characteristics. In view of their high triplet energy level (E T ) and bipolar carrier transport ability, where efficient exciplexes are applied as the host, the solution-processed phosphorescence devices realize a low efficiency roll-off of 7.0% at 1000 cd m −2 , high luminance, current efficiency (CE) and external quantum efficiency (EQE) of 25,990 cd m −2 , 20.0 cd A −1 and 6.7%, respectively. These results offer a promising tactic to the establishment of exciplex with TADF feature as host for fabricating efficient solution processed OLEDs. Three exciplex materials with D-spacer-A configuration, were synthesized and characterized, where the long alkyl chain as spacer provided a platform for intermolecular charge transfer instead of intramolecular charge transfer, induced TADF characteristic, as well as the better EL performance of doped solution processed OLEDs obtained based on mCP-6C-TRZ as bipolar host. • Simple exciplex hosts with spacer linker, were synthesized and characterized. • The alkyl chain as spacer deters intra-CT, induces inter-CT and distinct TADF trait. • The solution-processed OLED based on mCP-6C-TRZ realized a low roll-off. • A luminance of 25,990 cd m −2 , CE and EQE of 20.0 cd A −1 and 6.7%, were also realized.

Two-Channel Space Charge Transfer-Induced Thermally Activated Delayed Fluorescent Materials for Efficient OLEDs with Low Efficiency Roll-Off.

Enhancing the reverse intersystem crossing (RISC) process of thermally activated delayed fluorescent (TADF) emitters is an effective approach to realize efficient organic light-emitting diodes (OLEDs) with low efficiency roll-off. In this work, we designed two novel TADF emitters, SAT-DAC and SATX-DAC, via a spiro architecture. Efficient maximum external quantum efficiencies (EQEs) of 22.6 and 20.9% with reduced efficiency roll-off (EQEs of 17.9 and 17.0% at 1000 cd m-2) were achieved via a "two-RISC-channel" strategy. X-ray diffraction shows close donor (D)/acceptor (A) spacing and suitable D/A orientation in crystals of the two emitters favoring both intra- and intermolecular through-space charge transfer (TSCT) processes. Transient photoluminescence decay measurements show that both emitters have two RISC channels leading to kISCT exceeding 106 s-1. These results suggest that the "two-RISC-channel" design can be a novel approach for enhancing performance of TADF emitters, in particular at high excitation densities.

Tetraphenylethylene-Substituted Bis(thienyl)imidazole (DTITPE), An Efficient Molecular Sensor for the Detection and Quantification of Fluoride Ions

Fluoride ion plays a pivotal role in a range of biological and chemical applications however excessive exposure can cause severe kidney and gastric problems. A simple and selective molecular sensor, 4,5-di(thien-2-yl)-2-(4-(1,2,2-triphenylvinyl)-phenyl)-1H-imidazole, DTITPE, has been synthesized for the detection of fluoride ions, with detection limits of 1.37 × 10−7 M and 2.67 × 10−13 M, determined by UV-vis. and fluorescence spectroscopy, respectively. The variation in the optical properties of the molecular sensor in the presence of fluoride ions was explained by an intermolecular charge transfer (ICT) process between the bis(thienyl) and tetraphenylethylene (TPE) moieties upon the formation of a N-H---F− hydrogen bond of the imidazole proton. The sensing mechanism exhibited by DTITPE for fluoride ions was confirmed by 1H NMR spectroscopic studies and density functional theory (DFT) calculations. Test strips coated with the molecular sensor can detect fluoride ions in THF, undergoing a color change from white to yellow, which can be observed with the naked eye, showcasing their potential real-world application.

Photo-physical study of coumarins in aqueous organic solvents: An experimental and theoretical approach

In the present work, we have studied the spectroscopic behavior of four structurally similar coumarin dyes and ground (μg) and excited state (μe) dipole moments in aqueous DMSO as well as aqueous DMA using absorption and fluorescence emission spectroscopy. The μg and μe have been determined using the Bilot-Kawski method. The experimental results show that the dipole moments of solutes are influenced by solute-solvent interaction. The increase in solvent polarity leads to an increase in Stokes shift in both the aqueous mixtures while a decrease in Stokes shift has been observed for C519 and C523 in aqueous DMA. The μe is observed to be higher than that of the μg in all the solvents indicating the intramolecular charge transfer (ICT). To support experimental results, the electronic structure and spectroscopic properties of Coumarins have been investigated in gas and different solvent phases by DFT/ TD-DFT calculations. The HOMO-LUMO, molecular electrostatic potential map (MEP), and Natural Bond Orbital (NBO) analysis have been carried out to study the intermolecular charge transfer process.

Intermolecular charge transfer in donor–acceptor substituted triarylmethane dye for NLO and optical limiting applications

The present work reports the intermolecular charge transfer (ICT) between donor and acceptor substituted ethyl violet (EV) (Color Index: 42600) dye for nonlinear optical (NLO) and optical limiting (OL) applications. The functional groups of EV dye have been analyzed by FT-IR spectra and suggest that the ICT process occurs within the dye molecules. Third-order NLO characteristics of EV dye in aqueous solution were investigated by a 5 mW power continuous wave laser operating at 635 nm wavelength. The nonlinear index of refraction (n2), nonlinear coefficient of absorption (β) and third-order NLO susceptibility [χ(3)] of EV dye were found to be in the order of 10−7 cm2 W−1, 10−3 cm W−1 and 10−6 esu, respectively. The ground and excited-state absorption cross-sections of the experimental dye were calculated for ensuring the existence of reverse saturable absorption. The OL behavior of aqueous solutions of EV dye was studied, and the limiting threshold was substantially reliant on dye concentration. The obtained results revealed that the EV dye is a possible NLO material for future photonics and optoelectronics applications.

Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes.

Owing to their natural thermally activated delayed fluorescence (TADF) characteristics, the development of exciplex emitters for organic light-emitting diodes (OLEDs) has witnessed booming progress in recent years. Formed between electron-donating and electron-accepting molecules, exciplexes with intermolecular charge transfer processes have unique advantages compared with unimolecular TADF materials, offering a new way to develop high-performance TADF emitters. In this review, a comprehensive overview of TADF exciplex emitters is presented with a focus on the relationship between the constituents of exciplexes and their electroluminescence performance. We summarize and discuss the latest and most significant developments of TADF exciplex emitters. Notably, the design principles of efficient TADF exciplex emitters are systematically categorized into three systems within this review. These progressive achievements of TADF exciplex emitters point out future challenges to trigger more research endeavors in this growing field.