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.

Single-Probe-Based Sensor Array for Fingerprint Recognition of Trivalent Metal Ions and Application in Water Identification.

Abnormal concentration levels of trivalent metal ions (M3+) might hinder their natural biological activities in physiological processes and cause severe health hazards. Herein, a dual-chromophore probe (RhB-TPE) composed of rhodamine and tetraphenylethene (TPE) units was synthesized and explored for discriminating M3+ ions. It exhibited special aggregation and AIE properties in aqueous media. Its ensemble with anionic surfactant SDBS assemblies (RhB-TPE/SDBS) could be utilized as fluorescent sensors for selective and sensitive detection of M3+ ions such as Fe3+, Al3+, and Cr3+ by illustrating quenched TPE emission and switched-on rhodamine emission. Moreover, the use of SDBS assemblies at two concentrations could provide a single-probe-based sensor array and realize four-signal pattern recognition of different concentrations of the three M3+ ions and identify M3+ mixtures or unknown samples. The cross-reactive fluorescence variation was attributed to the M3+ influence on the FRET process from TPE to open-ring form rhodamine in the two ensemble sensors. With the coexistence of Al3+, the optimized RhB-TPE/SDBS ensemble sensor array was successfully applied to differentiate commercially available brand mineral water and purified water, as well as tap water. The present work provides a novel strategy to generate a single-probe-based sensor array and realizes fingerprint recognition of three trivalent metal ions and efficient discrimination of different types of water. The modulation FRET process of a dual chromophore in different surfactant ensembles inspires the future construction of novel and effective sensing platforms.

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

Excited-state intramolecular proton transfer (ESIPT) has been widely employed for the design of a variety of functionality-led molecular systems. However, precise manipulation of the excited-state reaction is challenging. Herein, we report a new tactic for tuning ESIPT via incorporating an excited-state intramolecular charge transfer (ESICT) process. Specifically, three o-carborane derivatives, NaCBO, PaCBO, and PyCBO, were designed, where the 2-(2'-hydroxyphenyl)-benzothiazole is a typical ESIPT unit functioning as an electron acceptor, and the electron-donating units are naphthyl-(Na), phenanthrenyl-(Pa), and pyrenyl-(Py), respectively. The architectures of the molecules are featured with a face-to-face alignment of the two units. Spectroscopy and theoretical calculation studies revealed that the electron-donating capacity of the donors and solvent polarity continuously modulate the ESIPT/ESICT energetics and dynamics, resulting in distinct emissions. Moreover, the molecules depicted not only highly porous structures but also very different fluorescent colors in the solid state, enabling highly selective film-based fluorescence sensing of mustard gas simulant, 2-chloroethyl ethyl sulfide, with a detection limit of 50 ppb and a response time of 5 s. This work thus provides a reliable strategy for the creation of high-performance sensing fluorophores via ESIPT manipulation.

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.

Fast and Selective Detection of Trace Chemical Warfare Agents Enabled by an ESIPT-Based Fluorescent Film Sensor.

Reliable detection of airborne chemical warfare agents (CWAs) at the site and in real-time remains a challenge due to the rarity of miniaturized analytical tools. Herein, an o-carborane-functionalized benzothiazole derivative (PCBO) with excited-state intramolecular proton transfer (ESIPT) and AIE characteristics was synthesized. The PCBO-based film sensor showed a highly sensitive response to representative simulants of CWAs, and detection limits were found to be 1.0 mg·m-3 for triphosgene, 6.0 mg·m-3 for chloroethyl ethyl sulfide, and 0.2 mg·m-3 for diethyl chlorophosphite. Moreover, the sensor showed great reusability (>100 cycles) and unprecedented response speed (<0.5 s). The excellent sensing performance was ascribed to the microenvironmental sensitivity of the sensing fluorophore, the porous adlayer structure of the film, and the specific binding of the fluorophore to the analytes. Furthermore, discrimination and identification of the examined CWA simulants were realized via the introduction of another fluorophore (HCBO)-based film. Importantly, a portable fluorescent CWA detector was built with the sensor as the key component, and its applicability was demonstrated by the successful detection of a typical CWA sample (Sarin). The present study indicates that fluorescent film sensors could satisfy reliable onsite and real-time detection of harmful chemicals.

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

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

Through-Space Charge Transfer: A New Way to Develop a High-Performance Fluorescence Sensing Film towards Opto-Electronically Inert Alkanes.

New strategies are in high demand for fast, sensitive, selective, on-site and real-time detection of the important but challenging alkane vapors owing to their opto-electronic inertness. Herein, we report, for the first time, a high-performance fluorescent film sensor (FFS) for the alkanes with a rationally designed through-space charge transfer (TSCT) molecule as the sensing fluorophore. Steady-state fluorescence, femto-second transient absorption spectroscopy and theoretical studies revealed continuous TSCT dynamics in the excited U-shaped molecule with increasing medium polarity. Furthermore, the interlocked, face-to-face alignment between the donor and acceptor favors mass transport of the analyte molecules in the film state. As anticipated, the compound-based FFS showed an experimental detection limit of ≈10 ppm for n-pentane, less than 5 s for a full detection, negligible interference and super-stability, revealing the effectiveness of the design strategy. Notably, the sensor is small (≈3.7 cm3 ), power-saving, and workable at room temperature.

Single-Fluorophore-Based Organic Crystals with Distinct Conformers Enabling Wide-Range Excitation-Dependent Emissions.

Achieving wide-range tunable emission colors, especially in the solid state of single-fluorophore materials, remains a significant challenge. Herein, we report a molecular design strategy that affords wide-range excitation-dependent emissions spanning over ≈230 nm in crystalline states. Under the donor-π-acceptor configuration, we judiciously choose a rotatable acceptor fragment, o-carborane, to enrich conformational diversities in the crystalline state and generate conformation-dependent multicolor emissions. We further show that this molecular platform is generalizable in creating crystalline materials with multicolor emissions. Based on these materials, a high-capacity information storage device and a finite-state machine were fabricated to showcase multicolor displays and information storage.

Dual-Phase Emission AIEgen with ICT Properties for VOC Chromic Sensing.

In a film-based fluorescence sensor, luminogens are of vital importance since they play the role of probes or indicators. Traditional organic luminogens like pyrene show high luminescence quantum yields in dilute solutions, but their applications are usually limited by the aggregation-caused quenching (ACQ) effect and bad photochemical stability. Thus, this paper reports a novel aggregation-induced emission luminogen (AIEgen) containing both pyrene and o-carborane (CB-PY), which possesses unique dual-phase emission both in solution and solid state and intramolecular charge transfer (ICT) properties, fulfilling the gap between ACQ and AIE compounds. Importantly, the fluorophore presents extraordinary stability that there was almost no attenuation in the emission intensity of CB-PY in the solid state after 4 months of exposure at ambient conditions. It is these merits that make CB-PY exhibit outstanding sensing performances for volatile organic compounds (VOCs), where the fluorescence test strip shows fast, reversible, and visual discrimination of four organic solvents with varied polarities. Moreover, 92#, 95#, and 98# gasolines could be discriminated with CB-PY, showing different colors under UV illumination.

Perylene Bisimide-Cored Supramolecular Coordination Complexes: Interplay between Ensembles, Excited State Processes, and Aggregation Behaviors.

Manipulating the optical properties of fluorescent species is challenging owing to complicated and tedious synthetic works. Herein, the photophysical properties of perylene bisimide (PBI) were effectively tuned by varying the geometrical arrangement of PBI moieties within supramolecular coordination complexes (SCCs), where a PBI-based dicycle (2) and a trigonal prism (3) were generated via using a typical 90° Pt(II) reagent, cis-(PEt3 )2 Pt(OTf)2 -based coordination-driven self-assembly approach. The ligand, an ortho-tetrapyridiyl-PBI (1), exhibits a moderate fluorescence quantum yield (∼13 %) and efficient inter-system crossing (ISC). 2, however, is much more emissive with a fluorescence quantum yield of ∼41 %, and the relevant ISC process is significantly hindered. The fluorescence quantum yield of 3 is merely ∼6 % due to the observed symmetry-breaking charge separation (SB-CS), which turns to triplet state upon charge recombination. Interestingly, 3 could be fully transformed into 2 by simply adding a suitable amount of a 90° Pt(II)-based neutral triangle. Moreover, 2 tends to form discrete dimers both in crystal and solution states, but 3 does not show the property. Therefore, controlling geometrical arrangement of fluorophores through coordination-driven self-assembly could be taken as another effective way to tune their excited state relaxation pathways and construct high-performance optical molecular materials, which generally have to be prepared via organic synthesis.

Perylene Bisimide Derivative-Based Fluorescent Film Sensors: From Sensory Materials to Device Fabrication.

Film-based fluorescent sensors have become an important field of sensor research due to abundant acquirable signals, real-time monitoring, and ease of miniaturization and integration, where chemically sensitive films are the most vital component of the sensor devices. In this feature article, we introduce hardware structures of film-based fluorescent sensors following the examination/investigation of the recent progress of such sensors with perylene bisimide (PBI) derivatives as sensing fluorophores in the films. PBI derivatives were specially chosen because of their outstanding chemical, photochemical, and thermal stabilities as well as their unusual high-fluorescence quantum yields. And finally, we provide a prediction for the future developments and challenges of this emerging field.

Marriage of Aggregation-Induced Emission and Intramolecular Charge Transfer toward High Performance Film-Based Sensing of Phenolic Compounds in the Air.

Film-based fluorescence sensing is recognized as one of the most optimized techniques for trace analysis of chemicals in the air after the invention of ion mobility spectrometry. The performance of the technique is highly dependent on the design of the film. This paper reports a new fluorescent film which shows unprecedented and discriminative sensing performance to the presence of phenol, o-cresol, m-cresol, and p-cresol in the air with an ultralow detection limit as low as 0.4, 0.3, 10, and 0.8 ppt, respectively. The film was designed via combination of the advantages of aggregation-induced emission (AIE) and those of intramolecular charge transfer (ICT), where the former provides the opportunity to avoid the widely encountered aggregation-caused quenching (ACQ) effect and the latter allows sensitive sensing of the microenvironment change of the film. The biggest challenge of the design is to find a fluorophore possessing both AIE and ICT effects. Fortunately, a newly synthesized biphenyl derivative of o-carborane capped with azetidine moiety (BZPCarb) shows the properties as expected. Importantly, the fluorophore is photochemically stable, a prerequirement for multiple uses of a film device. In addition, the nonplanar structure of the fluorophore is also favorable for film sensing as it could form porous films owing to screening of dense stacking of the molecules. It is the merits that make BZPCarb-based film show outstanding sensing and discriminative performances. Based on the fluorophore and the design, a conceptual high-performance fluorescent vapor sensor for phenolic compounds was developed.

Film-Based Fluorescent Sensor for Monitoring Ethanol-Water-Mixture Composition via Vapor Sampling.

In situ, on line, noncontact, and fast monitoring of the compositions of ethanol-water mixtures via vapor-phase sampling has remained a challenge for years. In this work, we report for the first time a film-based fluorescent sensor showing unprecedented ability to discriminate the compositions of ethanol-water mixtures. Importantly, ethanol contents in the mixtures can vary from 0 to 100% (v/v), the response time is less than 2 s, and the sensing is fully reversible. More importantly, the monitoring was performed via vapor-phase sampling, avoiding sample contamination. The principle behind it is ascribed to the big difference in the fluorescent quantum yield of the sensing unit, a newly designed and synthesized monosubstituted fluorescent o-carborane derivative (ZPCarb), in the two solvents. In addition, the sensor as developed was successfully used for the determination of ethanol contents in four commercial liquors, suggesting its potential application in the quality control of beverages, in monitoring fermentation processes, and in other processes.

Structure-activity relationships of aniline-based squaraines for distinguishable staining and bright two-photon fluorescence bioimaging in plant cells.

An organelle-selective vision provides insights into the physiological response of plants and crops to environmental stresses in sustainable agriculture ecosystems. Biological applications often require two-photon excited fluorophores with low phototoxicity, high brightness, deep penetration, and tuneable cell entry. We obtained three aniline-based squaraines (SQs) tuned from hydrophobic to hydrophilic characteristics by modifying terminal pendant groups and substituents, and investigated their steady-state absorption and far-red-emitting fluorescence properties. The SQs exhibited two-photon absorption (2PA) ranging from 750 to 870 nm within the first biological spectral window; their structure-property relationships, corresponding to the 2PA cross sections (δ2PA), and structure differences were demonstrated. The maximum δ2PA value was ∼1220 GM at 800 nm for hydrophilic SQ3. Distinct biological staining efficiency and selective SQ bioimaging were evaluated utilizing the onion epidermal cell model. Contrary to the hydrophobic SQ1 results in the onion epidermal cell wall, amphiphilic SQ2 tagged the vacuole and nucleus and SQ3 tagged the vacuole. Distinguishable staining profiles in the roots and leaves were achieved. We believe that this study is the first to demonstrate distinct visualisation efficiency induced by the structure differences of two-photon excited SQs. Our results can help establish the versatile roles of novel near-infrared-emitting SQs in biological applications.

Integrated Sensing Platform Validated for the Efficient and On-Site Screening of Amine-Containing Illicit Drugs.

Efficient and reliable technologies for the on-site detection of illicit drugs are important in drug-facilitated crime investigations. However, the development of such technologies is challenging. Based on the synthetic optimization, introducing a boron ester functional group to the two furanic indicators endows the stimulus-responsive properties synergistically. The ring-opening reaction of the indicators in the presence of amine-containing illicit drugs generated well-known donor-acceptor Stenhouse adducts, accompanied by strong color changes. A small-size and lightweight laminated sensor was integrated based on the outstanding ratiometric variations of the two active furanic indicators. A prototype platform was fabricated equipped with a circuit control, a mini pump, and a signal processing system. A user-friendly detection and efficient screening of amine-containing illicit drugs, including phenethylamines, amphetamines, cathinones, and tryptamines in the liquid states were conducted. The ratiometric response of the sensor was linear in the concentration range of 2.1-10.6 µg·mL-1 for methamphetamine·HCl and methcathinone ·HCl. The detection limits for the two illicit drugs at the sublevel (ng·mL-1) were found to be 8.4 and 9.0 ng·mL-1, respectively. Double-blind field tests and different illicit drugs were evaluated with good screening capability. Successful trials showed the potential applications of the developed prototype platform for efficient and on-site analytical determination.

Advances in Molecular Design and Photophysical Engineering of Perylene Bisimide-Containing Polyads and Multichromophores for Film-Based Fluorescent Sensors.

Film-based fluorescent sensors (FFSs) represent an important chemistry technology for meeting the urgent needs of on-site and real-time analysis, thereby enabling significant applications in environmental and health monitoring. As the core of FFSs, innovative design of sensing fluorophores and their intrinsic excited-state-related response nature endow FFSs with superior sensing performances in an endless expansion. In this Perspective, we specifically focus on perylene bisimide (PBI)-containing polyads and multichromophores with rigid configuration and notable photochemical stability for developing high-performance FFSs. These nonplanar structures mitigate aggregation and create abundant gaps for the sake of mass transfer and availability of the sensing units in the adlayer of the sensing films. We also comprehensively discuss how to adjust electronic coupling governing the excited-state events by appropriate functionalization strategies, thus providing a plethora of valuable insights for the exploration of the structure-property relationships in these orchestrated molecular systems. Throughout this Perspective, we also identify opportunities for FFSs in the future developments.

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.

Velocity field distribution control in antisolvent flow realizing highly stable and efficient perovskite nanocrystals.

Achieving highly stable and efficient perovskite nanocrystals (NCs) without applying functional additives or encapsulation, particularly sustaining the stability in ultra-dilute solution, is still a formidable challenge. Here, we show the FAPbI3 perovskite NCs with achieved ∼100 % photoluminescence quantum yield (PLQY) and low defect density (∼0.2 cm-3 per NCs), which is obtained by controlling the velocity field distribution of antisolvent flow in ligand-assisted reprecipitation process. The NCs show incredible reproducibility with narrow deviation of PLQY and linewidth between batch by batch, as well as remarkable stability of maintaining over 80 % PLQY, either in an ultra-diluted solution (9.3 × 10-6 mg/mL), or storing in ambient condition after 90 days with concentration of 0.09 mg/mL. The results in this work demonstrate the interplay of fluid mechanics and crystallization kinetics of perovskite, which pioneers a novel and unprecedent understanding for improving the stability of perovskite NCs for efficient quantum light source.

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.

Control of n-Phase Distribution in Quasi Two-Dimensional Perovskite for Efficient Blue Light-Emitting Diodes.

Pure-bromide quasi-2D perovskite (PBQ-2DP) promises high-performance light-emitting diodes (LEDs), while a challenge remains on control over its n-phase distribution for bright true-blue emission. Present work addresses the challenge through exploring the passivation molecule of amino acid with reinforced binding energy, which generates narrow n-phase distribution preferentially at n = 3 with true blue emission at 478 nm. Consequently, a peak external quantum efficiency of 5.52% and a record brightness of 512 cd m-2 are achieved on the PBQ-2DP-based true blue PeLED, these both values located among the top in the records of similar devices. We further reveal that the electron-phonon coupling results in the red-shifted emission in the PBQ-2DP film, suggesting that the view of n-phase distribution dominated true-blue emission in PBQ-2DP needs to be revisited, pointing out a guideline of electron-phonon coupling suppression to relieve the strait of realizing true blue or even deep blue emission in the PBQ-2DP film.