Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

A new HBT-quinolinium platform for optical detection of biogenic amines and its application in food quality monitoring

Biogenic amines (BAs) are critical biomolecules that play key roles in physiological processes and serve as important indicators in food safety, clinical diagnostics, and environmental monitoring. Therefore, sensitive, selective, and rapid tools are required for BA detection. This study explores the synthesis and applications of a new fluorescent probe (DBQ) for detecting BAs, focusing on their fluorescence response mechanisms. DBQ offers a promising alternative due to its high selectivity, sharp color change, low detection limit (0.057 μM for cadaverine), long lifetime (τ ≈ 2.40 ns), rapid response (15 min), low cytotoxicity (over 90 % cell viability in the presence of 10.0 μM of DBQ) and favorable photophysical properties, including large Stokes shift (213 nm in CHCl3). In addition, DBQ displays solvent-dependent intramolecular charge transfer (ICT), resulting in solvatochromism. The developed smartphone sensing system was applied to the detection of BAs. The developed sensing test kit responds quickly to the presence of volatile biogenic amines, with notable visible response and high selectivity. In on-site analysis, we were able to successfully use these test strips for non-destructive evaluation of chicken and cheese freshness with the use of a smartphone. Therefore, the current study will contribute to improving food safety and reduce food loss and waste by developing new bioanalytical technologies that offer chemical information about the composition of food, which is extremely valuable for enhancing traceability and extending food shelf life.

ESDPT induced dual-tautomer fluorescence of newly designed 1,8-dihydroxy-2-naphthaldehyde analogue with different solvent polarity

Excited-state intramolecular double proton transfer (ESDPT) has long been a subject of attention due to its crucial role in both fundamental exploration and designing related functional materials. In this work, the static and dynamical characterization from first-principles are performed to reveal the ESDPT mechanism of DHNA-2, a molecule designed based on 1,8-dihydroxy-2-naphthaldehyde (DHNA). The modification could provide easier ESDPT with favorable thermodynamics. More importantly, the DHNA-2 possesses enhanced absorption and fluorescence intensity (Δf > 100 %) due to the additional π-conjunction. Meanwhile, the distinct dual-tautomer emission (Δλ > 90 nm) is observed from the first and second PT products in excited states. The corresponding PT paths are demonstrated by constructing energy profiles and potential energy surfaces in both the ground and excited states. Moreover, we examined the influence of solvents with different polarities on conformational energies and spectra properties. Inspection of infrared spectroscopy, visualization of weak interactions, and bond order calculations indicate that photoexcitation could strengthen the intramolecular hydrogen bonds and promote the ESDPT. The H-bond strengthening mechanism caused by electron rearrangement during photoexcitation is also confirmed through the analyses of frontier molecular orbitals and charge population calculations. Additionally, the dynamic trajectories are shown to be in good agreement with static calculations, revealing the conformational changes associated with PT. Considering the improved photoluminescence and optical sensitivity for solvent polarity, the newly designed DHNA-2 is expected to provide multi-perspective reference for understanding the essence of ESDPT and inspiration for polarity-sensitized molecular design.

Tail-Assisted Excited-State Intramolecular Proton Transfer (ta-ESIPT) Fluorophores: A Universal Ratiometric Platform for Hydration-Sensitive Biomolecular Imaging and Sensing

Excited-state intramolecular proton transfer (ESIPT) fluorophores are valuable for ratiometric bioimaging due to their microenvironmental sensitivity, but traditional enol-keto systems suffer from poor biocompatibility and reduced efficiency in polar, protic environments. Here, we introduce a tail-assisted ESIPT (ta-ESIPT) strategy in which proton transfer occurs from an amide donor to an amino nitrogen acceptor. This mechanism applies to biocompatible charge-transfer fluorophores, such as naphthalimide, coumarin, NBD, and acedan. ta-ESIPT fluorophores exhibit broad environmental stability and a hydration-gated response─proton transfer is inhibited in aqueous environments but restored in nonaqueous microenvironments, yielding ratiometric red-shifted emission. This property enables the precise visualization of biomolecular interactions. By conjugating ta-ESIPT fluorophores with protein ligands, we achieve precise, ratiometric imaging of targets like SNAP-tag, hCA, avidin, and HaloTag in live cells, with fluorescence signals that directly correlate with binding affinities. This correlation enables real-time monitoring of protein interactions and evaluation of inhibitors while minimizing nonspecific interference in the complex cellular environment, ensuring dynamic and accurate protein recognition.

Carbazole-Based Colorimetric and Fluorescent Chemosensors for Metal Ions Detection : A Comprehensive Review ( 2012 to till date )

This review focuses on the development of carbazole based colorimetric and fluorescent chemosensors for the detection of metal ions including, mercury (Hg), iron (Fe), aluminium (Al) chromium (Cr) zinc (Zn), cobalt (Co) and copper (Cu) ions detection. Traditional analytical methods for detecting these metal ions, while widely used, present several draw backs such as high cost, low sensitivity, time consuming and the requirement for skilled technician. To overcome these limitations, more efficient alternatives such as colorimetric and fluorescent chemosensors have been developed. These sensors offer advantages like cost effectiveness, high sensitivity, rapid detection, and ease of use without requiring specialized technical expertise. In this review, we emphasize the applications of carbazole based colorimetric and fluorescence chemosensors. Carbazole and its derivatives are well-suited for this purpose due to their unique properties including excellent solubility, a highly conjugated structure, chemical stability, intramolecular charge transfer capabilities, and sensitivity to structural changes. These features make them ideal candidates for use as optical materials and fluorescence chemosensors in metal ion detection. The applications of carbazole- based colorimetric and fluorescent chemosensors. are summarized in tabular format for clarity.

Ultrafast Proton-Coupled Electron-Transfer Dynamics in Amino-Type Indole-Triazolopyrimidine Derivatives

Excited-state intramolecular proton transfer (ESIPT) can be accompanied by electron transfer within a molecular skeleton, leading to a proton-coupled electron transfer (PCET) process. The kinetics of ESIPT are influenced by the concurrent electron transfer, which in turn affects the overall photophysical properties of the system. In this study, we elucidate the photoinduced PCET mechanism in an ESIPT system based on 7-(indol-2-yl)-triazolopyrimidine (In-TAP) derivatives, which feature an N-H···N-type intramolecular hydrogen bond between adjacent heteroaromatic rings. Chemical modifications introduce short-range and long-range charge-transfer characteristics, facilitating an ultrafast ESIPT reaction on a time scale of ∼150 fs, prior to the system reaching its equilibrium polarization of the solvent field. Strongly solvatochromic T* emission is attributed to the solvation-associated ESIET process following the ultrafast ESIPT reaction. This work proposes a novel and relatively rare N-H···N-type molecular framework that approximately follows the case-B PCET mechanism (PT occurs prior to ET), extending insights from the well-established systems such as 2-((2-(2-hydroxyphenyl)benzo-[d]oxazol-6-yl)methylene)-malononitrile (diCN-HBO), 2-((2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl)-methylene)-cyanoacetic acid (HBODC), and related analogs.

An excitation-wavelength-dependent organic photoluminescent molecule with high quantum yield integrating both ESIPT and PCET mechanisms

Excitation wavelength-dependent (Ex-De) chromophores, which exhibit changes in spectral composition with varying excitation wavelengths, have garnered significant interest. However, the pursuit of novel photoluminescence (PL) mechanisms and high luminescence quantum yields is facing huge challenges. Here, we discover that the introduction of a spinacine moiety to 2-(2-hydroxy-5-methylphenyl)benzothiazole, a traditional excited-state intramolecular proton transfer (ESIPT) fluorophore, results in a novel Ex-De PL molecule. The luminescent color of this compound can be effectively modulated from greenish-blue to yellow-green by adjusting either the excitation wavelength or temperature. Transient absorption and spectroelectrochemistry spectra elucidate the underlying mechanism, demonstrating the roles of ESIPT and proton-coupled electron transfer (PCET). When embedded in a poly(vinyl alcohol) film, the composite exhibits remarkable Ex-De PL behavior, achieving absolute fluorescence quantum yields of 55.6% (λ ex: 396 nm) and 69.6% (λ ex: 363 nm), as well as phosphorescence at room temperature. These properties highlight its potential for multiple encryption features, enhancing its application in anti-counterfeiting technologies.

Tuning the Excited-State Intramolecular Proton Transfer in Carbon Dots via Coordination with Metal Ion

Although carbon dots (CDs) are widely used in the detection of heavy metal ions, rationally designing high-performance CDs for metal ion detection remains a significant challenge due to the limited understanding of their interaction mechanism. Here, we reveal the excited-state intramolecular proton transfer (ESIPT) in CDs and provide an effective strategy for switching the ESIPT by coordinating with metal ions. This work demonstrates that Zn2+ has strong coordination capacity toward CDs, while Mn2+ has the mildest coordination mode that can effectively sense other metal ions. Reasonable design of CDs with unique responses to heavy metal ions is achieved by doping Zn2+ or Mn2+ to regulate the ESIPT process. Therefore, a fluorescent sensor array for the sensitive identification of 10 kinds of heavy metal ions in real samples is constructed, which shows consistent detection results with ICP-OES. This work deepens the understanding of the fundamental fluorescence mechanism of CDs, opens new avenues for the rational tuning the emissive process, and provides a simple yet reliable platform to design nanoprobes for heavy metal ions as well as other molecules.

Polyimides with Excited-State Intramolecular Proton Transfer and Room-Temperature Phosphorescence Properties by End-capping

Organic polymers possessing excited-state intramolecular proton transfer (ESIPT) significantly impact their electronic and optical properties through stimulus response, leading to unique photoluminescence behaviors such as large Stokes shifts, dual emission, and environmental sensitivity. In this work, polyimides (PIs) exhibiting both ESIPT and room-temperature phosphorescence (RTP) properties were designed and synthesized by end-capping with 5-amino-2-(2-hydroxyphenyl) benzothiazole (HBTA) and 5-amino-2-(2-hydroxyphenyl) benzimidazole (HBIA), respectively. HBTA-PI and HBIA-PI exhibit bright yellow-green and sky-blue fluorescence at 510 nm and 470 nm, with a relatively high photoluminescence quantum yields of about 27.2% and 25.1%, respectively. Importantly, HBTA-PI and HBIA-PI show excellent RTP emissions at 520 nm and 485 nm, with decay lifetimes of 76.5 ms and 53.2 ms. The theoretical results indicate that the effective and large spin-orbit coupling (SOC) constants from their keto forms in the excited state can enhance the intersystem crossing (ISC) process to produce triplet excitons. The rigid polyimide networks further inhibit the nonradiative decay of triplet excitons and enhance phosphorescence emission. Additionally, the excellent ESIPT properties of HBTA-PI and HBIA-PI enable the detection of HCl vapor, which induces multiple fluorescent and phosphorescent color changes. This work provides good examples of polyimides for organic polymers with ESIPT properties and multi-stimulus response emissions.

N-Aryl or N-Alkyl Pyridinium-Substituted Excited-State Intramolecular Proton Transfer Fluorophores

In this article, it describes the synthesis of a series of fluorophores consisting of N-alkyl or N-aryl pyridinium groups connected at different positions of a 2-(2'-hydroxyphenyl)benzoxazole scaffold and the exploration of the photophysical properties in solution (dichloromethane) and in the solid state, as amorphous powders. All dyes display a bathochromically shifted fluorescent transition from an excited keto state, formed after excited-state intramolecular proton transfer process. A full chemical engineering study was performed by changing the nature of the substitution at the pyridinium site (alkyl or aryl), the position of the pyridinium substitution and the nature of the counterion (six examples). The nature of the radiative transitions observed in these fluorescent dyes was confirmed by Time-dependent density functional theory (TD-DFT) calculations.

Substituent-induced modulation of competing dual-acceptor hydrogen bond in ESIPT-type HBT derivatives

The 2-(2'-hydroxyphenyl) benzothiazole derivatives (HBT-R) featuring a dual-acceptor hydrogen bond (H-bond) have attracted our attention due to their unique structure and potential applications. There are two distinct acceptor sites located on either side of the central H-bond donor, thereby giving rise to the formation of inter-acceptor competition. In this study, quantum chemical calculations are employed to elucidate that substituents possessing distinct electronic properties modulate H-bond orientations, photophysical characteristics, and excited-state intramolecular proton transfer (ESIPT) in a dual-acceptor H-bond system. The origins of the competition in H-bonds and their effect on the distinct photophysical phenomena were revealed. The relaxed potential energy curves and molecular dynamics conducted for HBT-R indicate that electron-donating groups improve the efficiency of the ESIPT process in molecules with O-H···N H-bond. However, these groups hinder the ESIPT process in molecules with an opposite H-bond orientation. This work is expected to provide comprehensive insights into the mechanisms of substituent-induced modulation of competing dual-acceptor H-bond to inspire the design of more advanced luminescent materials.

Conjugated Iminodibenzyl Dyes Incorporating Phenolic Hydroxyl Group and Strong Electron Donating or Accepting Groups for Facilitating ESIPT and Proton Transfer in Six- or Seven-Membered Cycles

In this study, a range of conjugated iminodibenzyl derivatives D1-D6 including phenolic group and strong electro donating/accepting groups (N,N-diethylamino/nitro groups) were prepared as the target dyes, which could process internal proton transfer in excited state via six-/seven membered ring transition state (TS). Furthermore, reference iminodibenzyl dyes D7-D12, devoid of the phenolic segment, were synthesized for comparative analysis of their absorption and emission spectral properties. The internal H-bond of the target dyes was analyzed by the 1H-NMR spectroscopy and ultraviolet/visible absorption spectroscopy. As compared with the reference dyes, the steady and transient fluorescence measurements suggest that the target dyes D1-D3 CAN undergo excited state intramolecular proton transfer occurring through a six membered ring TS in various organic solvents. However, the target dyes D4-D6 CAN NOT process internal proton transfer in the excited state through a seven membered ring TS. Furthermore, the strong electron donating/accepting substituted groups show an effect on internal hydrogen bond in the target dyes, and thus they display influence on intramolecular proton transfer in the excited state of the target dyes D1-D3. The molecular modeling of the target dyes was further performed to reveal ESIPT through a six membered ring TS of the target dyes D1-D3 can occur, but ESIPT through a seven membered ring TS of the target dyes D4-D6 may not happen.

Chemistry of 2-(2'-Aminophenyl)benzothiazole Derivatives: Syntheses, Photophysical Properties and Applications

2-(2'-aminophenyl)benzothiazole is a readily tunable fluorescent core with widespread applications in coordination chemistry, sensing, light-emitting processes, medicinal chemistry, and catalysis. This review provides an overview of the synthetic methodologies to access 2-(2'-aminophenyl)benzothiazole and its organic derivatives, including various phosphorous and silane pincer ligands. The luminescent properties will be discussed, with a special focus on ESIPT and AIE processes. The coordination of transition metals and lanthanides is presented, as well as their influence on biological and light-emitting properties. 2-(2'-aminophenyl)benzothiazole derivatives have also been employed as sensors for a range of cations and anions due to their various binding modes, as well as for bioimaging purposes. Recently, the first application in photocatalysis has emerged, showing one of the many openings for these organic building blocks in the future.

Theoretical insights into fluorescent properties and ESIPT behavior of novel flavone-based fluorophore and its thiol and thione derivatives

For the typical ESIPT process, the proton transfer process is often completed via the intramolecular hydrogen bond (IHB) with oxygen or nitrogen as proton donor or proton acceptor. In recent years, the ESIPT process for sulfur-containing hydrogen bonds has received more and more attention, but it has been rarely reported. We systematically studied the ESIPT processes and photophysical properties of 2-(benzothiophene-2-yl)-3-hydroxy-4H-chromen-4-one (BTOH), 2-(benzothiophene-2-yl)-3-mercapto-4H-chromen-4-one (BTSH) and 2-(benzothiophen-2-yl)-3-hydroxy-4H-chromene-4-thione (BTS) at the HISSbPBE/6-31+G(d,p) and TD-HISSbPBE/6-31+G(d,p) computational level. The IHBs were investigated by analyzing structural parameters, infrared (IR) spectra and electron densities. All the results showed that the IHBs become stronger in the excited state. Among these three molecules, the ESIPT energy barrier of BTSH is the lowest, while that of BTS is the highest. By calculating the natural population analysis (NPA) charge, we found that SH group as a proton donor is easier to provide protons than OH group, and the S group as a proton acceptor is more difficult to obtain protons than O group. The simulated electronic spectra showed that the absorption and fluorescence wavelengths of BTSH and BTS have more or less red-shift compared with BTOH.

Copillar[5]arene Appended Pyrene Schiff Base: Photophysics, Aggregation Induced Emission and Picric Acid Recognition

Herein, we report the synthesis of copillar[5]arene-based pyrene Schiff base 1 and its characterization by using 1H, 13C NMR, FT-IR and mass spectrometry. UV-vis absorption, steady-state fluorescence and time-resolved fluorescence are done to elucidate the photophysical behaviors of 1. To understand the electronic structure of 1, density functional theory (DFT) calculations are performed. Owing to the presence of pyrene via a Schiff base linkage, compound 1 exhibits aggregation-induced emission (AIE) characteristics. It shows aggregation in aqueous THF and DMF. The aggregation behavior is successfully demonstrated by steady-state fluorescence, dynamic light scattering (DLS) and time-correlated single-photon counting (TCSPC) experiments. Experimental findings reveal that hydrophobic effect is the driving force in the formation of aggregates. As application, the aggregated state of 1 in aqueous THF fluorimetrically recognizes picric acid (PA) selectively over a series of nitro- and nonnitroaromatics with a detection limit of 1.62×10-7 M. The emission of the aggregated state is fully quenched upon interaction with PA.

Modified Imidazole-Phenol-Based ESIPT Fluorophores as Self-Absorption Free Emitters for Efficient Electroluminescent Devices

Excited-state intramolecular proton transfer (ESIPT) molecules are promising fluorophores for various applications including bioimaging, sensing, and optoelectronic devices. Particularly, their self-absorption-free fluorescence properties would make them a perfect choice as emissive materials for organic light-emitting diodes (OLEDs). Nevertheless, to become effective emitters some of their properties need to be altered by structural modifications. Herein, we design and synthesize a series of new ESIPT molecules (2PImBzP, 2ImBzP, and 2FImBzP) by functionalization of imidazole-phenol-based ESIPT cores with electron-deficient benzo[d]thiazole and various ambipolar imidazole moieties (1-phenyl-1H-phenanthro[9,10-d]imidazole (PIm), 1,4,5-triphenyl-1H-imidazole (Im), and (4,5-bis(4-fluorophenyl)-1-phenyl-1H-imidazole (FIm)), respectively. Each molecule displays a complete ESIPT process with intense green emissions from a pure keto form and high solid-state photoluminescence quantum yields (ΦPL) of 65-80 %. These fluorophores with superior thermal stability and balanced charge carrier mobility are effectively employed as non-doped emitters in OLEDs. The non-doped devices emit greenish lights with high brightness, high current efficiency (CE) (10.95-17.66 cd A-1), and low turn-on voltages (2.8-2.9 V). The electroluminescence purely originates from the emission of the keto tautomer of the emissive layers. Specifically, the 2PImBzP-based non-doped OLED stands out by achieving a remarkable brightness of 56,220 cd m-2, a CE of up to 17.66 cd A-1, and an impressive external quantum efficiency (EQE) of 5.65 % with a slight efficiency roll-off.

pH-Activated NIR fluorescent probe for sensitive mitochondrial viscosity detection

Understanding mitochondrial viscosity is crucial for comprehending cellular health and function. Therefore, accurate and sensitive measurement of mitochondrial viscosity is essential for advancing medical diagnostics and treatment strategies. In this study, a novel near-infrared (NIR) fluorescent probe SSN was developed, based on the dual mechanisms of Excited-State Intramolecular Proton Transfer (ESIPT) and Twisted Intramolecular Charge Transfer (TICT). SSN incorporates a thiophene-enhanced HBT structure and a hemicyanine moiety for effective mitochondrial targeting. It exhibits a large Stokes shift (240 nm) and high sensitivity to viscosity changes, enabling accurate detection under physiological conditions. The probe was successfully applied in HepG2 cells, zebrafish, and glucose-treated systems, demonstrating its potential for real-time viscosity monitoring in complex biological environments.

Small-Molecule Fluorescent Probes for Butyrylcholinesterase

Butyrylcholinesterase plays an indispensable role in organisms, and its abnormal expression poses a significant threat to human health and safety, covering various aspects including liver-related diseases, diabetes, obesity, cardiovascular and cerebrovascular diseases, and neurodegenerative diseases. In addition, toxic substances such as organophosphorus and carbamate pesticides markedly inhibit BChE activity. BChE activity serves as a critical parameter for the clinical diagnosis of acute organophosphorus pesticide poisoning and the evaluation of organophosphorus and carbamate pesticide residues. Therefore, the accurate and reliable detection of butyrylcholinesterase activity is particularly urgent and important for in-depth analysis of its biological function, diagnosis and therapy of related diseases, drug screening and sensitive detection of pesticide residues. Fluorescent probes have become a promising tool for sensing and imaging of butyrylcholinesterase, due to its advantages of high spatio-temporal resolution, high selectivity, non-invasive, high sensitivity, and tailored molecule structures. Here, this paper provides a comprehensive overview of the research progress in the sensing, imaging and therapy of butyrylcholinesterase utilizing fluorescent probes. This paper might be a useful guideline for researchers to design new high-performance fluorescence probes for BChE, and making further contributions to this intriguing field.

Benzazole-Based ESIPT Fluorophores: Proton Transfer from the Chalcogen Perspective. A Combined Theoretical and Experimental Study

In this study, we present a comprehensive photophysical investigation of ESIPT-reactive benzazole derivatives in both solution and the solid state. These derivatives incorporate different chalcogen atoms (O, S, and Se) into their structures, and we explore how these variations impact their electronic properties in both ground and excited states. Changes in the UV-Vis absorption and fluorescence emission spectra were analyzed and correlated with the chalcogen atom and solvent polarity. In general, the spectral band of the benzazole derivative containing selenium was redshifted in both the ground and excited states compared to that of its oxygen and sulfur counterparts. Furthermore, we observed that the solvent played a distinctive role in influencing the ESIPT process within these compounds, underscoring once again the significant influence of the chalcogen atom on their photophysical behavior. Theoretical calculations provided a deeper understanding of the molecular dynamics, electronic structures, and photophysical properties of these compounds. These calculations highlighted the effect of chalcogen atoms on the molecular geometry, absorption and emission characteristics, and intramolecular hydrogen bonding, revealing intricate details of the ESIPT mechanism. The integration of experimental and computational data offers a detailed view of the structural and electronic factors governing the photophysical behavior of benzazole derivatives.

Fluorogenic Crystallization via Enzyme-Instructed Excited-State Intramolecular Proton Transfer for Dynamic Microenvironment Profiling

Understanding dynamic changes within cellular microenvironments is crucial for elucidating biological processes and developing targeted therapies. Here, we present a rapid and straightforward strategy for profiling tumor microenvironments via fluorogenic crystallization driven by enzyme-instructed excited-state intramolecular proton transfer (ESIPT). By engineering ESIPT-based fluorophores, we achieve selective crystallization with strong dual-emission ratiometric fluorescence signals that are easily visualized, offering a real-time readout of tumor microenvironmental variations. We demonstrate that this method enables the efficient detection of tumor microenvironmental features, including enzymatic activity and pH heterogeneity, across different cellular models. This approach provides a simple, efficient, and versatile tool for studying microenvironment dynamics with broad potential applications in disease diagnosis, drug discovery, and personalized medicine.

Amine-Imine Tautomeric Excited State Intramolecular Proton Transfer in Metal-Organic Frameworks: Alcohol and Anion Recognition

To study the amine-imine tautomeric alteration through excited state intramolecular proton transfer (ESIPT), three amine-functionalized metal-organic frameworks (MOFs) have been synthesized by using two different metal ions and neutral linkers along with same ESIPT-enable anionic linker through the slow diffusion process. All the MOFs have 2D structure with asymmetric units of {[Mn(2,6-dip)(2-atp)](H2O)(CH3OH)}n (1), {[Mn(3,5-dip)(2-atp)](solvent)x}n (2) and {Zn2(3,5-dip)2(2-atp)(μ2-O2-)](solvent)x}n (3) (where 2,6-dip=2,6-di(1H-imidazol-1-yl)pyridine, 3,5-dip=3,5-di(1H-imidazol-1-yl)pyridine, 2-atp=2-amino terephthalic acid). The 2D networks of desolvated form of compound 1, 2, and 3 show water influencing proton transfer for amine-imine tautomerism in excited state through the ionic interaction with water molecules to the frameworks. However, in case of polar small chain aliphatic alcohols like methanol, ethanol and isopropanol; the compounds do not show any dual emissive ESIPT but exhibit three different intensified single peaks for each of the compounds. This different emission intensity in presence of different alcohols are helpful to detect these alcohols selectively. In addition to that in case of all three compounds, the water assisted-ESIPT is interrupted by some strong oxidizing agents like CrO4 2-, Cr2O7 2- and MnO4 - ions. This phenomenon allows the method of detection for the aforesaid oxidizing ions in water by interruption of the dual emissive fluorescence.

A Phototautomeric 3D Covalent Organic Framework for Ratiometric Fluorescence Humidity Sensing

Photoinduced proton transfer is an essential photochemical process for designing photocatalysts, white light emitters, bioimaging, and fluorescence sensing materials. However, deliberate control of the excited/ground states and meticulous manipulation of the excited state intramolecular proton transfer (ESIPT) pathway constitute a significant challenge in liquids and dense solids. Here, we present the integration of a hydronaphthoquinone fluorophore into a crystalline, porous, phototautomeric dynamic 3D covalent organic framework (COF) to show guest-induced fluorescence turn-on, emission redshift enhancement, and shortened lifetimes for ratiometric fluorescence humidity sensing. Theoretical and spectroscopic studies provide mechanistic insights into the conformational dynamics, charge transfer coupled with local excitation, and ground-state uphill regulation for the multiple tautomers. We illustrate the sensitive, rapid, steady, and self-calibrated ratiometric fluorescence sensing for a wide range of humidity benefiting from the architectural and chemical robustness and crystallinity of such a phototautomeric 3D COF. These findings provide molecular insights into the design of functional porous materials that integrate host-guest mutual recognition and photoelectronic response for multiplex molecular sensing for environmental monitoring and biomedical diagnostics applications.

Multi-Stimuli-Responsive Circularly Polarized Luminescence with Handedness Inversion and Near-Infrared Phosphorescence in Chiral Metal-Organic Framework Platform for White Light Emission and Information Encryption

Preparing multi-color and multi-stimuli-responsive circularly polarized luminescence (CPL) materials and understanding the evolution of chirality through the visualized mode is still a challenge. Here, an encapsulation engineering approach of chiral metal-organic frameworks (MOFs) is proposed to confine guest emitters to realize multi-color and multi-stimuli-responsive CPL. Based on triplet-triplet energy transfer (TTET), white CPL and near-infrared circularly polarized room temperature phosphorescence (NIR-CPRTP) can be obtained by introducing the pyrene derivatives. With the introduction of the guest containing vinylpyrene group, the light- and thermal-responsive CPL with the signal inversion can be realized through the reversible [2+2] cycloaddition reaction between the ligand and guest triggered by visible light/ultraviolet light or heating. Furthermore, the excitation-dependent CPL is successfully achieved with the incorporation of excited state intramolecular proton transfer (ESIPT) molecules into nanopores. Importantly, the chirality magnification can be greatly enhanced through the chiral spatial confinement, the accurate host-guest single crystal structures of FLT@DCF-12 and FLT@LCF-12 provide the visualized mode to understand the mechanism of chirality transfer, amplification and responsiveness. White LED and multiple information display and encryption are further demonstrated. This breakthrough provides a new perspective to guest-encapsulated chiral MOFs and contributes to the construction of stimuli-responsive CPL-active materials.

Electronic Substitution Effect on ESIPT-Driven pH and Amine Sensing: Exploring Mechanism

It is required to have a more straightforward and easier way to check the quality of food to ensure the safety of the public health. The decomposition of meat protein results in ammonia and biogenic amines (BAs). Consequently, to evaluate the safety and quality of meat products throughout the storage, transit, and consumption depends on the sensitive detection of the released BAs. Here, we have designed and synthesized three luminescent-based probe molecules, which originated from 2-(2-hydroxyphenyl) benzothiazole (HBT) derivatives and showed the excited state-induced proton transfer (ESIPT) phenomenon. The two substituents (OMe and NO2) were used rationally at the para position of HBT, and the electronic properties were evaluated using Hammett substituent constants. The proton donating ability of the O-H to the acceptor is largely facilitated by the presence of a strong electron-withdrawing group, which in this case is NO2. The proton transfer rate can be controlled, and in this case, to a slower rate with the influence of the electron donating group OMe. The controllability of proton transfer led us to use it in pH sensing. A prominent and multi-color change with pH variation was observed in the case of the OMe substituted compound. These probes were further employed for amine sensing, and the limit of detection (LOD) was determined to be 28.6 μM and 61.34 nM for ammonia and hydrazine, respectively. In addition, strip-based detection of spoilage of chicken meat was studied for real-world applications via both contact and non-contact modes.

Exploration of ESIPT in 4,6-O-Ethylidene-N-(2-hydroxybenzylidene)-β-D-glucopyranosylamine and Its Application in Selective Turn-on Response Toward Lu(III) ion

4,6-O-Ethylidene-N-(2-hydroxybenzylidene)-β-D-glucopyranosylamine (PL1) exhibits excited state intramolecular proton transfer (ESIPT) mechanism, which has been established by experimental and theoretical calculations. Further, PL1 selectively interacts with Lu3+ ion yielding a turn-on response in methanol, where the emission intensity of the former enhances (~15 fold) after metal ion interaction. The complexation has been investigated using various analytical tools like UV-visible absorption/emission, nuclear magnetic resonance spectroscopy and lifetime studies. The relative quantum yield of PL1 (0.0043) enhances to 0.1025 upon interaction with the Lu3+ ion. The studies on stoichiometry by Job's plot, ESI mass spectrometry and density functional theory calculations revealed the ligand-to-metal interaction in 2 : 1 ratio.

Effectively Controlling NIR Emissive Property and the ESIPT Behavior of Modified Styryl Dyes by Atomic Substituent: DFT/TD-DFT Approach

Recent literature on biosensing and bioimaging has explored excited state intramolecular proton transfer (ESIPT) cyanide dyes. These classes of fluorescence dyes generally use the classical pyridinium or indolium cations acceptor units' styrene with the ESIPT core. This work studied the photophysical and ESIPT kinetics of novel flavylium cation as an acceptor unit styrene with an ESIPT core using DFT/TD-DFT calculations. Two new ESIPT cyanine dyes, namely (E)-4-(3-(benzo[d]thiazol-2-yl)-2-hydroxystyryl)-7-(dimethylamino)-2-phenyl chromenylium (PSS) and (E)-4-(3-(benzo[d]oxazol-2-yl)-2-hydroxystyryl)-7-(dimethylamino)-2-phenylchromenylium (PSO) were designed and fully studies. This is concerned with studying changes in intramolecular hydrogen bonds, molecular orbitals at the frontier of the ESIPT process, absorption and fluorescence spectra, and excited state energy barriers. As a result, both the systems considered here can undergo an ultrafast ESIPT reaction with PSS and then PSO. Furthermore, ESIPT is more accessible in the normal enol-form first excited singlet (S1) state, with shorter hydrogen bonds. The intersystem crossing between the S1 state and the triplet (T1) state greatly influences the fluorescence efficiency of PSO and PSS. The potential energy curve and transition state energy profiles of PSS and PSO show that ultrafast ESIPT occurs in the state. Furthermore, the PSS shows less energy barriers, which leads to faster proton transfer than PSO. The current study will advance knowledge of the mechanism behind the ESIPT process and help enhance the qualities of the cyanine dye used in ESIPT.

A novel multifunctional fluorescent probe with ESIPT and AIE effects for the detection of Co2+ and HClO

We developed a novel fluorescent probe featuring excited-state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) effects, which displayed dual-channel fluorescence emission. The probe detected both Co2+ and HClO with naked eye under daylight as well as through a fluorescence spectrophotometer. The probe exhibited a low detection limit for Co2+ at 2.823 μM, while the detection limit for HClO was 11.55 μM. When the probe (10 μM) was mixed with Co2+, the fluorescence intensity at 556 nm rapidly decreased within 10 minutes and stabilized after 40 minutes, while for HClO, it took 960 min to observe the same decrease in intensity within 960 min. The probe (10 μM) achieved naked-eye detection of Co2+ recognition under daylight; however, achieving naked-eye detection of HClO under daylight necessitated higher concentrations (500 μM). Thus, this probe shows promising potential for environmental monitoring and water quality detection.

Deciphering the Photophysical Properties of Nonplanar Heterocyclic Compounds in Different Polarity Environments

Nonplanar (butterfly-shaped) phenothiazine (PTZ) and its derivative's (M-PTZ) photophysical and spectral properties have been tuned by varying the solvents and their polarity and investigated employing spectroscopic techniques such as UV-Vis, steady-state and time-resolved fluorescence, and TDDFT calculations. The UV-Vis absorption studies and TDDFT calculations reveal two distinct bands for both compounds: a strong π-π* transition at shorter wavelengths and a weaker n-π* transition, which displays a little bathochromic shift in polar solvents. The detailed emission studies reveal that such dual emission is a result of the photoinduced excited-state conjugation enhancement (ESCE) process. The band at a shorter wavelength corresponds to the locally excited (LE) state, while the longer wavelength band arises from the planarized excited state resulting from ESCE. With the increase in solvent polarity, the LE band is less affected, whereas strong positive solvatochromism is observed for the ESCE band. As the solvent polarity increases, the ESCE band demonstrates significant positive solvatochromism, while emission intensity decreases with higher solvent polarity, suggesting stabilization of the excited state. The biexponential decay of fluorescence lifetimes further corroborates the dual emission behavior of PTZ and M-PTZ. PTZ exhibits a higher photoluminescence quantum yield (PLQY) than that observed for M-PTZ, and the solvent viscosity influences the PLQY, indicating that nonradiative decay is activated during the planarization of the excited state, also known as excited-state conjugation enhancement. Furthermore, the (time-dependent) density functional theory (TD) DFT calculations performed to understand the geometrical parameters and the electronic transitions of these model molecules further corroborate experimental findings. These findings underscore the significant influence of solvent polarity and molecular structure on the dual emission and excited-state dynamics of PTZ and M-PTZ, which eventually hold substantial implications for advanced photophysical applications.

Hydrazone fluorescent sensors for the monitoring of toxic metals involved in human health from 2014-2024

Hydrazone-based fluorescent sensors have been instrumental for the detection of toxic metals over the past decade due to their ease of synthesis and unique properties. This review summaries the diverse range of sensors reported for toxic metals (Al3+, Fe3+, Cu2+, Zn2+ and Hg2+) highlighting the key role this class of sensors will play in the foreseeable future.

Advances in Organic Fluorescent and Colorimetric Probes for The Detection of Cu2+ and Their Applications in Cancer Cell Imaging (2020-2024)

Organic fluorescence and colorimetric probes have emerged as vital tools for detecting metal ions, due to their high sensitivity, selectivity, and rapid response times. Copper, an essential trace element, plays a critical role in biological systems, yet its imbalance can lead to severe disorders such as neurodegenerative diseases, cancer, and Wilson's disease. Over the past few years, advancements in probe design have unlocked innovative avenues for not only detecting Cu2+ in environmental and biological samples but also for visualizing its distribution through fluorescence imaging. These probes offering robust performance under diverse conditions. Fluorescence imaging using these probes plays a pivotal role in cancer diagnosis, prognosis, and treatment monitoring by offering real-time visualization of tumor morphology and biomolecular interactions at cellular and tissue levels. This review aims to explore the diversity of organic fluorescence and colorimetric probes developed for the detection of Cu2+, with a particular focus on their applications in fluorescence imaging from 2020 to 2024. The discussion highlights the use of these probes in visualizing Cu2+ in various cancer cells such as SiHa, HCT 116, GES-1, RAW 264.7, HepG2, HeLa, MCF-7 and DrG cell lines, tissues, and small living organisms. By targeting cancer-specific pathways and monitoring copper-related physiological changes, these probes have significantly advanced the fields of cancer diagnostics and therapeutics. This comprehensive analysis emphasizes the potential of fluorescence imaging as a powerful tool for elucidating the roles of Cu2+ in health and disease, paving the way for future advances in biomedical research.

Novel Perylene Diimide-Benzothaizole Hybrid: A Reaction-Based Probe for the Detection and Discrimination of H2S, Cys and DTT

The reaction-based probe perylene diimide-hydroxyphenyl benzothiazole (PR) can be used for the detection and discrimination of H2S, DTT and Cys in 20% HEPES buffer-DMSO and DMSO. The H2S induced radical anion formation of PR in 20% HEPES buffer and thiolysis of the ether bond of PR in DMSO. However, the addition of DTT showed only a decrease in the absorbance intensity and Cys showed insignificant behaviour towards PR in DMSO. The optical, AFM and DLS studies along with isolation of the reaction product in the model reaction support the interaction of the PR with bio thiols.

Deciphering Design of Aggregation-Induced Emission Materials by Data Interpretation

This work presents a novel methodology for elucidating the characteristics of aggregation-induced emission (AIE) systems through the application of data science techniques. A new set of chemical fingerprints specifically tailored to the photophysics of AIE systems is developed. The fingerprints are readily interpretable and have demonstrated promising efficacy in addressing influences related to the photophysics of organic light-emitting materials, achieving high accuracy and precision in the regression of emission transition energy (mean absolute error (MAE) ∼ 0.13eV) and the classification of optical features and excited state dynamics mechanisms (F1score ∼ 0.94). Furthermore, a conditional variational autoencoder and integrated gradient analysis are employed to examine the trained neural network model, thereby gaining insights into the relationship between the structural features encapsulated in the fingerprints and the macroscopic photophysical properties. This methodology promotes a more profound and thorough comprehension of the characteristics of AIE and guides the development strategies for AIE systems. It offers a solid and overarching framework for the theoretical analysis involved in the design of AIE-generating compounds and elucidates the optical phenomena associated with these compounds.

Coassembly of Dual-Modulated AIE-ESIPT Photosensitizers and UCNPs for Enhanced NIR-Excited Photodynamic Therapy

Aggregation-induced emission (AIE) photosensitizers are promising for photodynamic therapy, yet their short excitation wavelengths present a limitation. In this study, we develop a series of organic photosensitizers with dual modulation capabilities based on excited-state intramolecular proton transfer (ESIPT) and AIE. Notably, we synthesize near-infrared (NIR)-excited photosensitive nanoparticles through a coassembly strategy utilizing upconversion nanoparticles (UCNPs) and amphiphilic polymers. The spectroscopic analysis and theoretical calculations elucidate the significant impact of additional or π-spacer groups on the conformational change and the energy barrier of the ESIPT process. An efficient Förster resonance energy transfer between the photosensitizer and UCNPs is achieved through the coassembly strategy. Both in vitro and in vivo experiments demonstrate the antitumor efficacy of these nanoparticles under NIR excitation. This work not only introduces a novel approach for simultaneously modulating AIE properties and the ESIPT process but also provides a robust solution for overcoming the excitation wavelength limitations of many organic photosensitizers.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Visible-Light-Driven Solid-State Fluorescent Photoswitches for High-Level Information Encryption

Developing visible-light-driven fluorescent photoswitches in the solid state remains an enormous challenge in smart materials. Such photoswitches are obtained from salicylaldimines through excited-state intramolecular proton transfer (ESIPT) and subsequent cis-trans isomerization strategies. By incorporating a bulky naphthalimide fluorophore into a Schiff base, three photoswitches achieve dual-mode changes (both in color and fluorescence) in the solid state. In particular, the optimal one generates triple fluorescence changing from green, to yellow and finally to orange upon visible-light irradiation. This switching process is fully reversible and can be repeated at least 10 times without obvious attenuation, suggesting its good photo-fatigue resistance. Mechanism studies reveal that the naphthalimide group not only enables the tuning of multicolor with an additional emission, but also induces a folded structure, reducing molecular stacking and facilitating ESIPT and cis-trans isomerization. As such, photopatterning, ternary encoding and transient information recording and erasing are successfully developed. The present study provides a reliable strategy for visible-light-driven fluorescent photoswitches, showing implications for advanced information encryption materials.

Uncovering Integrated Dual-State ESIPT-Conductivity, Redox-Capacity, and Opto-Electronic Responses Toward Hg(II)/ Cr(III) of Aliphatic Fluorescent Polymers

Excited-state intramolecular proton transfer (ESIPT)-associated dual-state emissive aliphatic dual-light emitting conducting polymers (DLECPs) having oxidation-reduction capacities are prepared polymerizing 2-acrylamido-2-methylpropane-1-sulfonic acid, methacrylic acid, and 2-methyl-3-(N-(2-methyl-1-sulfopropan-2-yl)acrylamido)propanoic acid monomers. Of as-synthesized DLECPs, nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopies, fluorescent enhancements (I/I0), and computational investigation indicate intriguing photophysical features in DLECP3 (optimum composition). In DLECP3, ─CONH─, ─CON<, and ─COOH subluminophores are recognized by density-functional theory (DFT)/time-dependent-DFT calculations and experimental investigations. ESIPT-associated dual-state emission/conductivity, aggregation-enhanced emissions, selective opto-electronic responses toward Hg(II)/Cr(III) at 437/574 nm, and redox properties of DLECP3 are supported by solid-state/solution spectroscopies, time-correlated single photon counting (TCSPC) measurements, dual-state excitation dependent emissions, microscopic images, electrochemical measurements, and DFT calculations. Here, preferential interaction of Hg(II)/Cr(III) with DLECP3 (amide)/DLECP3 (imidol) and reduction/oxidation of Hg(II)/Cr(III) to Hg(I)/Cr(VI) are substantiated by UV-vis, FTIR, and X-ray photoelectron spectroscopies; TCSPC measurements; NMR-titration; electrochemical studies; alongside computational calculations. The proton-electrical conductivities of DLECP3, Hg(II/I)-DLECP3, and Cr(III/VI)-DLECP3 in solids/solutions are 15.27 × 10-5/6.16 × 10-5, 19.60 × 10-5/25.52 × 10-5, and 26.69 × 10-5/27.60 × 10-5 S cm-1, respectively.

Highly Potent Fluorenone Azine-based ESIPT Active Fluorophores for Cellular Viscosity Detection and Bioimaging Applications

Herein, synthesizes of fluorenone azine-based Schiff fluorescence probes: (E)-2-(((9H-fluoren-9-ylidene)hydrazineylidene)methyl)-5-(diethylamino)phenol (3a), (E)-9-(((9H-fluoren-9-ylidene)hydrazineylidene) methyl)-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol (3b), and (E)-1-(((9H-fluoren-9-ylidene)hydrazineylidene)methyl) naphthalen-2-ol (3c). The probes were structurally characterized using Fourier-transform infrared spectroscopy (FTIR), 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS) analysis. The probes exhibit hydrogen bonding between phenolic -OH and imine nitrogen, enabling excited state intramolecular proton transfer (ESIPT) and free rotation in the azine (> C = N-N = C <) functional, facilitating twisted intramolecular charge transfer (TICT), and a positive solvatochromism in solvent-dependent emission studies. Further, density functional theory (DFT) based calculations accounted for the observed photophysical TICT and ESIPT processes, revealing a non-covalent interaction between phenolic -OH and imine nitrogen. Furthermore, the fluorescence intensity (log I) showed good linearity (R2 = 0.999) with the viscosity (log η) with Förster-Hoffmann coefficient (X) values of 2.238, 1.405 and 3.121 for 3a, 3b and 3c, respectively. The study established the probes toxicity and fluorescence imaging in the Caenorhabditis elegans model. Probe 3a, the first azine-based probe for micro viscosity detection, demonstrated exceptional efficacy in detecting intercellular viscosity and facilitating bioimaging applications.

Bis(tricyclic) Aromatic Enes That Exhibit Efficient Fluorescence in the Solid State

We report herein that bis(tricyclic) aromatic enes (BAEs) consisting of 6-6-6-membered frameworks such as acridine, xanthene, thioxanthene, and thioxanthene-S,S-dioxide act as a new class of organic luminophores that exhibit blue-to-green fluorescence in the solid state and in polymer film with good to excellent quantum yields. The BAEs were prepared by the palladium-catalyzed double cross-coupling reaction of phenazastannines or 10,10-dimethyl-10H-phenothiastannin with 9-(dibromomethylene)xanthene, 9-(dibromomethylene)thioxanthene, or 9-(dibromomethylene)-9H-thioxanthene-10,10-dioxide. Microcrystals or powder samples of the BAEs exhibited brilliant fluorescence with good to high quantum yields (Φ = 0.45-0.88). Furthermore, more efficient emission of blue-to-green light (Φ = 0.59-0.91) was observed for the BAEs dispersed in the poly(methyl methacrylate) (PMMA) films. Density functional theory (DFT) calculations suggest that the photo-absorption of the (thio)xanthene moiety-containing BAEs proceeds via π-π* transitions, whereas the optical excitation of 10,10-dioxido-9H-thioxanthene moiety-containing BAEs involves an intramolecular charge transfer from the acridine/thioxanthene part to the electron-accepting 10,10-dioxido-9H-thioxanthene moiety.

Excited-state intramolecular proton transfer (ESIPT) active interwoven polycatenated coordination polymer for selective detection of Al3+ and Ag+ ions along with water detection in less polar solvents

The external stimuli-responsive excited-state intramolecular proton transfer (ESIPT) on/off mechanism is a unique and expedient sensing method that offers easy monitoring through the transition between dual and single-peak emissions. To avail this advantage of ESIPT-based sensing for selective metal ion detection and trace water detection, we have synthesized a 2,5-dihydroxyterephthalate (dht)-based interwoven polycatenated coordination polymer (1). The synthesized compound has been thoroughly characterized using single-crystal and powder X-ray diffraction techniques, along with other physicochemical methods. The synthesized compound exhibits a visual luminescence color change from faint yellow to bright green under UV irradiation in the presence of Al3+ ions. This change is attributed to a blue shift in fluorescence maxima of the keto form of the dht ligand in contact with Al3+ ions. Additionally, the material detects Ag+ ions through an ESIPT-off mechanism. These significant changes in ESIPT - blue shifting for Al3+ and ESIPT-off for Ag+ - start in just 1 mM aqueous solutions of these ions. Significantly, the ESIPT-off for Ag+ is evident even in the presence of other interfering ions. Beyond metal ion detection, this material also offers both qualitative and quantitative sensing of trace amounts of water in various polar organic solvents, such as ethanol (EtOH), tetrahydrofuran (THF), isopropanol (IPA), acetone, and acetonitrile (ACN), through the ESIPT-on/off phenomenon. The activated framework of compound 1 (1') can detect 2%, 4%, 4%, 3%, and 3% water in acetone, ACN, EtOH, IPA, and THF, respectively; through the conversion from a single to dual hump emission alteration. The respective ESIPT peak shift and ESIPT-on/off in the presence of metal ions and water is explained by the interaction between the host coordination polymer and guest analytes.

Thermo-controlled Water Microenvironment Inducing Fluorescence Enhancement of Chalcone Nanohydrogels for Mitochondrial Temperature Sensing

Developing aggregation-induced emission (AIE)-based hydrogels that exhibit fluorescence enhancement as to thermal properties is an interesting and challenging task. In this work, we employed the fluorophore 2'-hydroxychalcone (HC), fluorescence properties of which are easily influenced by the excited-state intramolecular proton transfer and twisted intramolecular charge transfer (TICT) effects, to develop a novel type of temperature-sensitive polymers, hydroxychalcone-based polymers (HCPs). By controlling the temperature-dependent water microenvironments in HCPs, the intramolecular hydrogen bonds between water and HCPs can be regulated, thereby influencing the TICT process and leading to thermo-induced fluorescence enhancement, which shows a contrary tendency compared to typical AIEgens that always exhibit fluorescence attenuation as the thermal energy accelerates the molecular motion. Following the decoration with triphenylphosphine, the resulting polymer P-HCP assembled into nanohydrogels and served as a fluorescent probe for intracellular mitochondrial temperature sensing.

Catalyst-free and wavelength-tuned glycosylation based on excited-state intramolecular proton transfer

The chemoselectivity of organic reactions is a fundamental topic in organic chemistry. In the long history of chemical synthesis, achieving chemoselectivity is mainly limited to thermodynamic conditions by an exogenous activation strategy. Here, we design an endogenous activation method, which can be used to control the chemoselectivity of phenol and naphthol through the photo-induced excited-state intramolecular proton transfer (ESIPT). A wavelength-tuned glycosylation is developed to showcase the penitential of this new strategy. Traditionally, an exogenous activator (electrophilic promoters) is essential to induce the cleave of a polar single bond, and this strategy has been extensively studied and used in the glycosylation chemistry, for the formation of oxocarbenium cation intermediate. In our systems, the oxocarbenium cation intermediates can be selectively formed from glycosyl donors bearing tunable chromophoric groups under mild conditions of acid-base free and redox neutrality, which enables continuous synthesis of oligosaccharides.

Tunable Emission and Structural Insights of 6-Arylvinyl-2,4-bis(2'-hydroxyphenyl)pyrimidines and Their O∧N∧O-Chelated Boron Complexes

In this study, we present the synthesis and photophysical characteristics of a novel series of 6-arylvinyl-2,4-bis(2'-hydroxyphenyl)pyrimidines. These compounds exhibit nonemissive properties attributed to the potential occurrence of a excited-state intramolecular proton transfer process from the OH groups to the nitrogen atoms of the pyrimidine ring. The introduction of an acid for protonation of the pyrimidine ring results in a significant enhancement of the fluorescence response, easily perceptible to the naked eye. Notably, these molecules serve as intriguing rigid O∧N∧O ligands for boron chelation. The incorporation of boron atoms promotes structural planarity, increases rigidity, and successfully restores fluorescence in both solution and the solid state. Moreover, the photoluminescence was found to be strongly influenced by the nature of the end groups on the arylvinylene fragment, allowing for the modulation of the emission color and covering the optical spectrum from blue to red. Strong emission solvatochromism was observed in various solvents, a finding that supports the formation of intramolecular charge-separated emitting states, particularly when terminal electron-donating groups are present in the structure. X-ray diffraction analysis enables the determination of inter- and intramolecular interactions, as well as molecular packing structures, aiding in the rationalization of distinct luminescent behaviors in the solid state. All experimental findings are elucidated through extensive density functional theory (DFT) and time-dependent DFT calculations.

A Schiff-Base Molecular Probe for Selective Fluorescence Sensing of Maleic Acid with Recognition of Maleic Acid in Food Additives and Cell Imaging

The 2,4-dinitrophenylhydrazine based Schiff base (L) acts as an effective fluorescence sensor for the selective detection of maleic acid. The detection limit of L towards maleic acid is observed to be 1.29 × 10-7 M. A 1:1 binding stoichiometry between L and maleic acid was obtained using Bensi-Hilderbrand method. The binding constant (Ka) was measured to be 5.17 × 106 M-1. The sensing behavior of L was confirmed through analysis using FT-IR, DLS and SEM analysis, alongside DFT calculations. Theoretical assessments clearly suggest that the L's mono-protonation and complexation in the solvent medium are the primary mechanisms in the sensing process. Additionally, L is used to imaging the maleic acid in living cells, demonstrating its potential biological uses. In addition, recognition of maleic acid in food additives was reported.

Recent advances in fluorescent probes for fatty liver imaging by detecting lipid droplets

Fatty liver, a major health problem worldwide, is closely associated with aberrant accumulation and alteration of energy storage organelles, lipid droplets (LDs), in the disease process. Fluorescent probes with excellent optical performance, high sensitivity/selectivity and real-time monitoring have emerged as an attractive tool for the detection of LDs used in the diagnosis of fatty liver. In this review, we summarize various probes based on different response mechanisms to image LDs in the fatty liver process using different excitation imaging modes and emission wavelengths, including the visible to the near-infrared, two/three-photon, and the second near-infrared region. The perspectives and barriers associated with the reported lipid droplet (LD) probes for future development are also discussed.

Substituent effects on the ESIPT process and the potential applications in materials transport field of 2'-aminochalcone derivatives

This work investigates the different charge transfer characteristics and excited state intramolecular proton transfer process (ESIPT) of 2'-aminochalcones derivatives carrying different electron-withdrawing groups. Four new molecules are designed in the experiment and named as 2c, 3c, 4c and 5c, respectively. (Dyes and Pigments, 2022, 202.) Based on these four molecules, the effect of substituents on the ESIPT process and the charge transfer process are discussed in detail in our work. According to the study of the related parameters at the hydrogen bond site, infrared vibration spectrum, interaction region indicator isosurface (IRI) and scatter plots, it is concluded that the hydrogen bond interaction is enhanced under photoexcitation, and the descending order of the excited state hydrogen bond strength is 3c > 5c > 4c > 2c. The hydrogen bond energy is calculated by atoms in moleculs (AIM) topological analysis and core-valence bifurcation (CVB) index. The potential energy curve reveals the ESIPT mechanism. Frontier molecular orbital and electron-hole analyses explain the reasons for the changes in the ESIPT process at the electronic level. In addition, the ionization potentials (IPa and IPv), affinity energies (EAa and EAv) and reorganization energies are calculated to evaluate the potential application value of organic molecules in material transport field.

Stimuli-Induced Fluorescence Switching in Azine-Containing Fluorophores Displaying Resonance-Stabilized ESIPT Emission

This article reports the synthesis, along with structural and photophysical characterization of 2-(2'-hydroxyphenyl)benzazole derivatives functionalized with various azaheterocycles (pyridine, pyrimidine, terpyridine). These compounds show dual-state emission properties, that is intense fluorescence both in solution and in the solid-state with a range of fluorescent color going from blue to orange. Moreover, the nature of their excited state can be tuned by the presence of external stimuli such as protons or metal cations. In the absence of stimuli, these dyes show emission stemming from anionic species obtained after deprotonation (D* transition), whereas upon protonation or metal chelation, ESIPT process occurs leading to a stabilized and highly emissive K* transition. With the help of extensive ab initio calculations, we confirm that external stimuli can switch the nature of the transitions, making this series of dyes attractive candidates for the development of stimuli-responsive fluorescent ratiometric probes.

Proton transfer induced excited-state aromaticity gain for chromophores with maximal Stokes shifts

Excited state aromaticity (ESA) offers a fascinating route for driving photophysical and photochemical processes but is challenging to harness fully due to its inherent association with unstable antiaromatic ground states. Here, we propose to circumvent this problem via the introduction of a new class of photophysical processes, the generation of ESA via an excited-state intramolecular proton transfer. We select twelve candidate molecules based on the cyclobutadiene and pentalene scaffolds and investigate their ground and excited state properties using computation. The study highlights the feasibility of proton transfer induced ESA gain and shows that it gives rise to pronounced excited-state relaxation producing Stokes shifts in excess of 2 eV. The underlying electronic structure properties are analysed in terms of the orbitals involved as well as aromaticity descriptors illustrating the pronounced changes these molecules undergo upon both excitation and proton transfer. In summary, we believe that the present work will pave the way toward a new class of chromophores with maximal Stokes shifts and excited-state relaxation.

Flexible organic crystals with multi-stimuli-responsive CPL for broadband multicolor optical waveguides

Flexible organic crystals, capable of transmitting light and responding to various external stimuli, are emerging as a new frontier in optoelectronic materials. They hold immense potential for applications in molecular machines, sensors, displays, and intelligent devices. Here, we report on flexible organic crystals based on single-component enantiomeric organic compounds, demonstrating multi-stimuli-responsive circularly polarized light (CPL). These crystals exhibit remarkable elasticity, responsiveness to light and acid vapors, and tunable circularly polarized optical signals. Upon exposure to acid vapors, the fluorescence of the crystals shifts from initial yellow emission to green emission, attributable to the protonation-induced inhibition of excited-state intramolecular proton transfer. Under UV irradiation, the fluorescence emission undergoes a red-shift, resulting from the molecular transformation from an enol configuration to a ketone configuration. Notably, both processes are reversible and can be restored under daylight. The integration of reversible fluorescence changes under light and acid vapors stimuli, CPL signals, and flexible optical waveguides within a single crystal paves the way for the application of organic crystals as all-organic chiral functional materials.

2D Conjugated Polymer Thin Films for Organic Electronics: Opportunities and Challenges

2D conjugated polymers (2DCPs) possess extended in-plane π-conjugated lattice and out-of-plane π-π stacking, which results in enhanced electronic performance and potentially unique band structures. These properties, along with predesignability, well-defined channels, easy postmodification, and order structure attract extensive attention from material science to organic electronics. In this review, the recent advance in the interfacial synthesis and conductivity tuning strategies of 2DCP thin films, as well as their application in organic electronics is summarized. Furthermore, it is shown that, by combining topology structure design and targeted conductivity adjustment, researchers have fabricated 2DCP thin films with predesigned active groups, highly ordered structures, and enhanced conductivity. These films exhibit great potential for various thin-film organic electronics, such as organic transistors, memristors, electrochromism, chemiresistors, and photodetectors. Finally, the future research directions and perspectives of 2DCPs are discussed in terms of the interfacial synthetic design and structure engineering for the fabrication of fully conjugated 2DCP thin films, as well as the functional manipulation of conductivity to advance their applications in future organic electronics.

Geometrically twisted intra-inter-molecular cooperative interactions for an enhanced photo-response in an ORMOSIL-based host-guest system

In situ encapsulation of the 2-(2'-hydroxylphenyl)benzothiazole (HBT) fluorophore into organically modified silica (ORMOSIL) is recognized as a potential approach to increase the photocurrent conversion efficiency. Encapsulation resulted in a twisted geometrical configuration of HBT with an efficient charge transfer and increased carrier density, resulting in a 246% photo-response enhancement.

Handedness-Inverted and Stimuli-Responsive Circularly Polarized Luminescent Nano/Micromaterials Through Pathway-Dependent Chiral Supramolecular Polymorphism

The precise manipulation of supramolecular polymorphs has been widely applied to control the morphologies and functions of self-assemblies, but is rarely utilized for the fabrication of circularly polarized luminescence (CPL) materials with tailored properties. Here, this work reports that an amphiphilic naphthalene-histidine compound (NIHis) readily self-assembled into distinct chiral nanostructures through pathway-dependent supramolecular polymorphism, which shows opposite and multistimuli responsive CPL signals. Specifically, NIHis display assembly-induced CPL from the polymorphic keto tautomer, which become predominant during enol-keto tautomerization shifting controlled by a bulk solvent effect. Interestingly, chiral polymorphs of nanofiber and microbelt with inverted CPL signals can be prepared from the same NIHis monomer in exactly the same solvent compositions and concentrations by only changing the temperature. The tunable CPL performance of the solid microbelts is realized under multi external physical or chemical stimuli including grinding, acid fuming, and heating. In particular, an emission color and CPL on-off switch based on the microbelt polymorph by reversible heating-cooling protocol is developed. This work brings a new approach for developing smart CPL materials via supramolecular polymorphism engineering.