Precise Manipulation of Excited-State Intramolecular Proton Transfer via Incorporating Charge Transfer toward High-Performance Film-Based Fluorescence Sensing

Utilizing excited-state proton transfer fluorescence quenching mechanism, layered rare earth hydroxides enable ultra-sensitive detection of nitroaromatic

Convenient, rapid, and accurate detection of nitroaromatic organic toxins and harmful substances is of great significance in research. In the present study, two-dimensional layered rare-earth hydroxides (LYH) were used as ion-exchange matrix materials, and the anionic fluorescent dye molecules (HPTS) were successfully introduced into the LYH structures in situ via a simple and effective "plug-and-play" strategy, which gave the compounds ultra-sensitive fluorescence sensing detection of nitrobenzene, p-nitrotoluene and p-nitrophenol (Fluorescence response time < 1 sec, and the LOD for nitrobenzene, p-nitrophenol and p-nitrotoluene reached an impressive 349 ppb, 22 ppb and 98 ppb, respectively). Combined with theoretical calculations, we elucidated in detail the fluorescence quenching response mechanism of the LYH-HPTS towards nitroaromatic. Additionally, we also constructed fluorescent paper sensor, which effectively transformed the LYH-HPTS from theoretical detection to device application. The LYH-HPTS material is not only simple to synthesize, cost-effective and stable, but also has the features of fast response, excellent sensitivity and selectivity, and good reproducibility, which provides a new approach for the rapid and accurate detection of nitroaromatic.

Full Electron Delocalization across the Cluster in 1,12-bisBMes2-p-carborane Radical Anion

Conjugation between three-dimensional (3D) carboranes and the attached substituents is commonly believed to be very weak. In this paper, we report that reducing 1,12-bis(BMes2)-p-carborane (B2pCab) with one electron gives a radical anion with a centrosymmetric semiquinoidal structure. This radical anion shows extensive electron delocalization between the two boron centers over the p-carborane bridge due to the overlap of carborane lowest unoccupied molecular orbital (LUMO) and the BMes2 LUMO. Unlike dianions of other C2B10H12 carboranes, which rearrange to a nido-form, two-electron reduction of B2pCab leads to a rearrangement into a basket-shaped intermediate.

Molecular design and architectonics towards film-based fluorescent sensing

The past few decades have witnessed encouraging progress in the development of high-performance film-based fluorescent sensors (FFSs) for detecting explosives, illicit drugs, chemical warfare agents (CWAs), and hazardous volatile organic chemicals (VOCs), among others. Several FFSs have transitioned from laboratory research to real-world applications, demonstrating their practical relevance. At the heart of FFS technology lies the sensing films, which play a crucial role in determining the analytes and the resulting signals. The selection of sensing fluorophores and the fabrication strategies employed in film construction are key factors that influence the fluorescence properties, active-layer structures, and overall sensing behaviors of these films. This review examines the progress and innovations in the research field of FFSs over the past two decades, focusing on advancements in fluorophore design and active-layer structural engineering. It underscores popular sensing fluorophore scaffolds and the dynamics of excited state processes. Additionally, it delves into six distinct categories of film fabrication technologies and strategies, providing insights into their advantages and limitations. This review further addresses important considerations such as photostability and substrate effects. Concluding with an overview of the field's challenges and prospects, it sheds light on the potential for further development in this burgeoning area.

Electronic Effect Driven Specific and Sensitive Recognition toward GHB

Precise detection of a trace substance that intrinsically possesses weak chemical activity and less-distinctive spatial structure is of great significance, but full of challenges, as it could not be effectively recognized via either an active covalent reaction process or multiple noncovalent interactions toward its simple structure. Here, the electronic-effect-driven recognition strategy was proposed to visually sense an illicit drug, γ-hydroxybutyric acid (GHB), which was treated as an analyte model due to its inherent simple structure. In particular, a sensing system composed of two probes substituted by the nitro (-NO2) and the hydrogen (-H), was constructed with the characteristic yellow coloring and blue fluorescence, as well as high sensitivity (0.586 ng/mL), fast response (0.2 s), and specific recognition, even in the presence of 22 interferents. In addition, a portable eyeshadow box-like sensing chip was fabricated and proven to be reliable and feasible in sensing GHB disguised in liquors for self-protection in a covert manner. Hence, this work developed an electronic-effect-driven modulation strategy of the recognition interaction between the probe and the analyte and, thus, would open up a new thought for detecting the analyte with weak activity and a simple structure, as well as propel the relevant application in real scenarios.

Novel Photocatalyst Based on Through-Space Charge Transfer Induced Intersystem Crossing Enables Rapid and Efficient Polymerization Under Low-Power Excitation Light

Currently, most photoredox catalysis polymerization systems are limited by high excitation power, long polymerization time, or the requirement of electron donors due to the precise design of efficient photocatalysts still poses a great challenge. Herein, we propose a new approach: the creation of efficient photocatalysts having low ground state oxidation potentials and high excited state energy levels, along with through-space charge transfer (TSCT) induced intersystem crossing (ISC) properties. A cabazole-naphthalimide (NI) dyad (NI-1) characterized by long triplet excited state lifetime (τT=62 μs), satisfactory ISC efficiency (ΦΔ=54.3 %) and powerful reduction capacity [Singlet: E1/2 (PC+1/*PC)=-1.93 eV, Triplet: E1/2 (PC+1/*PC)=-0.84 eV] was obtained. An efficient and rapid polymerization (83 % conversion of 1 mM monomer in 30 s) was observed under the conditions of without electron donor, low excitation power (10 mW cm-2) and low catalyst (NI-1) loading (<50 μM). In contrast, the conversion rate was lower at 29 % when the reference catalyst (NI-4) was used for photopolymerization under the same conditions, demonstrating the advantage of the TSCT photocatalyst. Finally, the TSCT material was used as a photocatalyst in practical lithography for the first time, achieving pattern resolutions of up to 10 μm.

A Self-Accelerating Naphthalimide-Based Probe Coupled with Upconversion Nanoparticles for Ultra-Accurate Tri-Mode Visualization of Hydrogen Peroxide

The design and development of ultra-accurate probe is of great significance to chemical sensing in complex practical scenarios. Here, a self-accelerating naphthalimide-based probe with fast response and high sensitivity toward hydrogen peroxide (H2O2) is designed. By coupling with the specially selected upconversion nanoparticles (UCNPs), an ultra-accurate colorimetric-fluorescent-upconversion luminescence (UCL) tri-mode platform is constructed. Owing to the promoted reaction process, this platform demonstrates rapid response (< 1 s), an ultra-low detection limit (4.34 nM), and superb anti-interferent ability even in presence of > 21 types of oxidants, explosives, metallic salts, daily compounds, colorful or fluorescent substances. In addition, the effectiveness of this design is further verified by a sponge-based sensing chip loaded with the UCNPs/probe in recognizing trace H2O2 vapor from interferents with the three characteristic colors existing simultaneously. The proposed design of probe and tri-mode visualization detection platform is expected to open up a brand-new methodology for ultra-accurate sensing.

Photochemical Mechanisms of Hydroxyquinoline Benzimidazole: Insights from Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

Excited-state intramolecular double proton transfer (ESIDPT) has received much attention because of its widespread existence in the life reactions of living organisms, and materials with this property are significant for their special luminescent properties. In this work, the complete active space self-consistent field (CASSCF) and OM2/multireference configuration interaction (OM2/MRCI) methods have been employed to study the static electronic structure calculations of the photochemistry and the possibility of ESIDPT process of hydroxyquinoline benzimidazole (HQB) molecule, along with the nonadiabatic dynamics simulations. The computational results show that the HQB molecule is relaxed to the S1-ENOL minimum after being excited to the Franck-Condon point in the S1 state. Subsequently, during the nonadiabatic deactivation process, the OH···N proton transfer and the twisting of benzimidazole occur before arriving at the single proton transfer conical intersection S1S0-KETO. Finally, the system can either return to the initial ground-state structure S0-ENOL or to the single proton transfer ground-state structure S0-KETO, both of which have almost the same probability. The dynamics simulations also show that no double proton transfer occurs. The excited-state lifetime of HQB is fitted to 1.1 ps, and only 64% of the dynamic trajectories return to the ground state within the 2.0 ps simulation time. We hope the detailed reaction mechanism of the HQB molecule will provide new insights into similar systems.

A ratiometric fluorescent indicator-displacement assay for on-site determination and intracellular imaging of nitroxinil

Nitroxinil (NIT) is a widely using veterinary medicine to protect cattle and sheep yet may threaten human health when ingested through food chain. Developing fluorescent analytical methods in ratiometric manners was essential for the on-site detection and in-situ monitoring of NIT but still challenging. Here, we improved the indicator-displacement assay (IDA)-based method and designed the first ratiometric fluorescent probe for NIT by using an albumin host and an Aggregation-induced emission (AIE) guest. This probe exhibited fast response (10 s), high sensitivity (limit of detection: 4.6 ppb), good selectivity (over twelve medicines) and eye-discriminable fluorescent color change (green-red) upon responding to NIT. Based on these properties, this probe enabled quantitative determination of NIT in real food samples, on-site analysis via a paper-based test strip, and fluorescence imaging of NIT in living cells.

Hydrogen-Bonded Thiol Undergoes Unconventional Excited-State Intramolecular Proton-Transfer Reactions

The chapter on the thiol-related hydrogen bond (H-bond) and its excited-state intramolecular proton-transfer (ESIPT) reaction was recently opened where compound 4'-diethylamino-3-mercaptoflavone (3NTF) undergoes ESIPT in both cyclohexane solution and solid, giving a 710 nm tautomer emission with an anomalously large Stokes shift of 12,230 cm-1. Considering the thiol H-bond to be unconventional compared to the conventional Pauling-type -OH or -NH H-bond, it is thus essential and timely to probe its fundamental difference between their ESIPT. However, thiol-associated ESIPT tends to be nonemissive due to the dominant nπ* character of the tautomeric lowest excited state. Herein, based on the 3-mercaptoflavone scaffold and π-elongation concept, a new series of 4'-substituted-7-diethylamino-3-mercaptoflavones, NTFs, was designed and synthesized with varied H-bond strength and 690-720 nm tautomeric emission upon ultraviolet (UV) excitation in cyclohexane. The order of their H-bonding strength was experimentally determined to be N-NTF < O-NTF < H-NTF < F-NTF, while the rate of -SH ESIPT measured by fluorescence upconversion was F-NTF (398 fs)-1 < H-NTF (232 fs)-1 < O-NTF (123 fs)-1 < N-NTF (101 fs)-1 in toluene. Unexpectedly, the strongest H-bonded F-NTF gives the slowest ESIPT, which does not conform to the traditional ESIPT model. The results are rationalized by the trend of carbonyl oxygen basicity rather than -SH acidity. Namely, the thiol acidity relevant to the H-bond strength plays a minor role in the driving force of ESIPT. Instead, the proton-accepting strength governs ESIPT. That is to say, the noncanonical thiol H-bonding system undergoes an unconventional type of ESIPT.

Interfacially Confined Dynamic Reaction Resulted to Fluorescent Nanofilms Depicting High-Performance Ammonia Sensing

Sensing materials innovation plays a crucial role in the development of high-performance film-based fluorescent sensors (FFSs). In our current study, we present the innovative fabrication of four fluorescent nanofilms via interfacially confined dynamic reaction of a specially designed fluorescent building block, a new boron-coordinated compound (NI-CHO), with a chosen one, benzene-1,3,5-tricarbohydrazide (BTH). The nanofilms as prepared are robust, uniform, flexible, and thickness tunable, at least from 40 to 1500 nm. The fabricated FFSs based on Film 3, one of the four nanofilms, shows highly selective and fully reversible response to NH3 vapor with an experimental detection limit of <0.1 ppm and a response time of 0.2 s. The unprecedented high performance of the nanofilm is ascribed to the specific quenching of its fluorescence emission owing to formation of an excited-state complex between the sensing unit and the analyte molecule. Efficient mass transfer also contributes to the high performance owing to the porous adlayer structure of the nanofilm. This work provides an example to show how to develop a high-performance sensing film via controlling the film's structure, especially the thickness.

Manipulation of excited-state intramolecular proton transfer by electron-donor substitution for high performance fluoride ions sensing

Excited-state intramolecular proton transfer (ESIPT) molecules has been using as a variety of functionalityled molecular systems. To investigate the relationship between the electron-donor substitution and luminescent properties of ESIPT luminogens, four 2-(2-hydroxyphenyl) benzothiazole derivatives with donor-π-acceptor (D-π-A)-structured were synthesized. The distinct fluorescence properties of them were found to be highly dependent on the electron-donor moiety (triphenylamine and anthracenyl), its substituent position (para and meta position) and solvent polarity. The M-TPA, P-En, and M-En showed ESIPT emission in organic solvents, while the P-TPA showed intramolecular charge transfer process (ICT) emission. It is due to the synergistic effect of the aggregation-induced emission (AIE) and ESIPT, that M-TPA and M-En exhibited high solid-state quantum yields and large Stokes shifts. They were used as a probe for detecting F-, which resulted in rapid colorimetric, high sensitivity and good selectivity. The M-TPA was a turn-on fluorescent probe, which had the best detection property, and the limit of detection was as low as 11 nM. Because M-TPA displayed phenol anion emission in DMSO and F- causes the deprotonation of the M-TPA, which led to significant red shift of the absorption band and enhancement of fluorescence emission. This work provides a reliable strategy for designing high-performance fluorescent sensor via ESIPT manipulation.

Intramolecular Charge Transfer of Gold Nanoclusters for pH Indicating

The investigation of the intramolecular charge transfer (ICT) process of gold nanoclusters (AuNCs) is critical to understand the unique features of the nanomaterials, which also benefits their further applications. Herein, 6-methyl-2-thiouracil (CH3-2-TU) and polyvinylpyrrolidone (PVP)-stabilized AuNCs are prepared, and the ICT behaviors are carefully studied. Protonation or deprotonation of the ligands around AuNCs could be used to regulate the ICT state, influencing the electron distribution and band gap. Shifted fluorescence emission phenomena are thus observed, which respond to external pH stimuli. In addition, the AuNCs are developed as color-switchable indicators for the highly sensitive detection of biogenic amines. As a proof of concept, the performance of this strategy in the evaluation of food spoilage by probing pH conditions is validated with satisfactory results. The discoveries in this work offer a convenient route to regulate the optical properties of AuNCs and the design of pH-based sensing applications.

Fast and Selective Luminescent Sensing by Langmuir-Schaeffer Films Based on Controlled Assembly of Perylene Bisimide Modified with A Cyclometalated AuIII Complex

Condensed films of functional luminophores dominated by the magnitude and dimensionality of the intermolecular interactions play important roles in sensing performance. However, controlling the molecular assembly and regulating photophysical properties remain challenging. In this study, a new luminophore, ortho-PBI-Au, was synthesized by anchoring a cyclometalated alkynyl-gold(III) unit at the ortho-position of perylene bisimide. An unprecedented T-type packing model driven by weak Au-π interaction and Au-H bonds was observed, laying foundation for striking properties of the luminophore. Controlled assembly of ortho-PBI-Au at the air-water interface, realized using the classical Langmuir-Schaeffer technique, afforded the obtained luminescent films with different packing structures. With an optimized film, sensitive, selective, and rapid detection of a hazardous new psychoactive substance, phenylethylamine (PEA), was achieved. The detection limit, response time, and recovery time were <4 ppb, <1 s, and <5 s, respectively, surpassing the performance of the PEA sensors known thus far. The relationship between the characters of films and the sensing performance was systematically examined by grey relational analysis (GRA). The present study suggests that designing novel molecular aggregation with definite adlayer structure is a crucial strategy to enhance the sensing performance, which could be favorable for the film-based fluorescent sensors.

Comparative Observation of Distinct Dynamic Stokes Shifts in Diaryl BODIPY Triads with Broadband Two-Photon Absorption

Time-resolved evolution of excited states in the twist-conjugated chromophores is of great fundamental interest for photoluminescent applications. The four diaryl BODIPY triads modified with diverse end-cappers at 2,6-positions were investigated properly, and considerable two-photon absorption capabilities in the first biological spectral window were obtained. Fast relaxations from the initially twisted conformation to the planarized conformation in the excited state were resolved spectrally and kinetically, accompanied by the discernible phenomenon of the fluorescence dynamic Stokes shift (DSS). Along with increasing electron donating capabilities and solvent polarities, the characteristics of structural rearrangement and intramolecular charge transfer have been estimated by enhanced DSS behaviors. Especially, the blue-shifted DSS was rationalized as the sequence conversion between the planarized state and the twisted charge transfer state. A molecular-level picture for relaxation pathways in different polarities was depicted and supported by the theoretical simulations. Significant and fast structural motions in this work contribute to the excited-state dynamics and rational development of versatile BODIPY chromophores.

Heterocyclic molecules with ESIPT emission: synthetic approaches, molecular diversities, and application strategies

Excited-state intramolecular proton transfer (ESIPT) is one of the most essential emission processes in most circumstances because of its dual emission band in most cases and its high Stokes shifts. These distinguishing properties make ESIPT-based probes more suitable for a variety of applications, including analyte sensors, solid-state sensing mechanisms, optical technologies, and biomarkers for endogenous or exogenous compounds in various settings. As a result, researchers around the world are working on ESIPT emissions and developing different scaffolds for various applications or industry demands. This field of study is rapidly expanding and there is a need for an up-to-date review of synthesis methodologies and applications. This paper provides the highlights of ESIPT-based heterocyclic scaffolds, synthesis strategies, and application scenarios in the literature from 2017 to 2023.

Tunable Non-Kasha Behaviors and Excited-State Dynamics of Quadrupolar Squaraine Aggregates

Versatile coupling theories have been developed for rationalizing unusual aggregation phenomena of multipolar chromophores. Here, diverse excitonic couplings of a quadrupolar squaraine dye protonated by trifluoroacetic acid could be achieved and tuned unprecedentedly in different solvation media. Subtle changes of the solvent and ion pair influenced the aggregation of the donor-acceptor-donor (D-A-D)-type SQC6 and led to significant variations in optical properties. In contrast to conventional H/J aggregates, strong spectroscopic evidence of nonfluorescent and red-shifted hJ aggregation was obtained. Assumptions of the excitonic interplay with variable strength stabilized by the synergic contributions of π-π stacking and electronic interaction were addressed. Comparative excited-state dynamics in the aggregates clarified the distinctive excitonic coupling of adjacent quadrupolar molecules and the nature of the excited state beyond the dimers. Meanwhile, dominant two-photon absorption transitions could be elucidated by a resonance-enhanced mechanism. The present unusual molecular interplay provides a strategy to fine tune the optical properties of multipolar aggregates.

Synthesis and Structural Characterization of p-Carboranylamidine Derivatives

In this contribution, the first amidinate and amidine derivatives of p-carborane are described. Double lithiation of p-carborane (1) with n-butyllithium followed by treatment with 1,3-diorganocarbodiimides, R-N=C=N-R (R = ⁱPr, Cy (= cyclohexyl)), in DME or THF afforded the new p-carboranylamidinate salts p-C₂H₁₀B₁₀[C(NⁱPr)₂Li(DME)]₂ (2) and p-C₂H₁₀B₁₀[C(NCy)₂Li(THF)₂]₂ (3). Subsequent treatment of 2 and 3 with 2 equiv. of chlorotrimethylsilane (Me₃SiCl) provided the silylated neutral bis(amidine) derivatives p-C₂H₁₀B₁₀[C{ⁱPrN(SiMe₃)}(=NⁱPr)]₂ (4) and p-C₂H₁₀B₁₀[C{CyN(SiMe₃)}(=NCy)]₂ (5). The new compounds 3 and 4 have been structurally characterized by single-crystal X-ray diffraction. The lithium carboranylamidinate 3 comprises a rare trigonal planar coordination geometry around the lithium ions.