New sensing mechanisms for design of fluorescent chemosensors emerging in recent years

During the past decade, fluorescent chemosensors have become an important research field of supramolecular chemistry and have attracted great attention because of their simplicity, high selectivity and sensitivity in fluorescent assays. In the design of new fluorescent chemosensors, exploration of new sensing mechanisms between recognition and signal reporting units is of continuing interest. Based on different photophysical processes, conventional sensing mechanisms including photo-induced electron transfer (PET), intramolecular charge transfer (ICT), metal-ligand charge transfer (MLCT), twisted intramolecular charge transfer (TICT), electronic energy transfer (EET), fluorescence resonance energy transfer (FRET), and excimer/exciplex formation have been investigated and reviewed extensively in the literature. This tutorial review will mainly focus on new fluorescent sensing mechanisms that have emerged in the past five years, such as aggregation-induced emission (AIE) and C=N isomerization, which can be ascribed to fluorescence changes via conformational restriction. In addition, excited-state intramolecular proton transfer (ESIPT) has not been well reviewed yet, although a number of chemosensors based on the ESIPT mechanism have been reported. Thus, ESIPT-based chemosensors have been also summarized in this review.     

激发态分子内质子转移荧光探针的研究进展

荧光探针凭借其选择性好、灵敏度高、响应时间快、易于操作和检测限低等优点得到了广泛的关注。 激发态分子内质子转移(ESIPT)化合物具有特殊的激发态光物理过程,其显著的光物理性质是有较高的荧光量子产率及大的斯托克斯位移。 对于荧光分子而言,较大的斯托克斯位移可以减少自吸收和由内滤效应产生的干扰,增强分子的耐光性,有利于荧光的发射。 本文对ESIPT荧光探针检测离子(包括金属阳离子和阴离子)、中性小分子和生物大分子的研究进展进行阐述,并对ESIPT荧光分子的存在问题和应用前景进行评述。     

Through bond energy transfer (TBET)-based fluorescent chemosensors

Excitation energy transfer is one of the crucial issues in photophysical and photochemical process of any muti-chromophoric molecular systems, such as energy harvester and fluorescent chemosensor. Through bond energy transfer (TBET)-based fluorescent chemosensors are composed of three main parts: energy donor, energy acceptor, and rigid linker. Comparing with the often used Förster resonance energy transfer (FRET) mechanism, TBET does not require spectral overlap, thus it may enable more possible combination of energy donors and acceptors to be employed and afford higher sensitivity toward targets through ratiometric fluorescence. In this review, we highlight the recent progress in the design and biological applications of the organic TBET-based fluorescent chemosensors during 2014–2019, which will provide profound guidance for designing powerful chemosensors as well as exploring further biological applications.     

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.     

Janus-Type ESIPT Chromophores with Distinctive Intramolecular Hydrogen-bonding Selectivity

Excited-state intramolecular proton transfer (ESIPT)-based solid luminescent materials with multiple hydrogen bond acceptors (HBAs) remain unexplored. Herein, we introduced a family of Janus-type ESIPT chromophores featuring distinctive hydrogen bond (H-bond) selectivity between competitive HBAs in a single molecule. Our investigations showed that the central hydroxyl group preferentially forms intramolecular H-bonds with imines in imine-modified 2-hydroxyphenyl benzothiazole (HBT) chromophores but tethers the benzothiazole moiety in hydrazone-modified HBT chromophores. Imine-derived HBTs generally exhibit higher fluorescence efficiency, while hydrazone-derived HBTs show a reduced overlap between the absorption and fluorescence bands. Quantum chemical calculations unveiled the molecular origins of the biased intramolecular H-bonds and their impact on the ESIPT process. This Janus-type ESIPT chromophore skeleton provides new opportunities for the design of solid luminescent materials.     

Excited state proton transfer and solvent relaxation of a 3-hydroxyflavone probe in lipid bilayers

The photophysics of a ratiometric fluorescent probe, N-[[4'- N, N-diethylamino-3-hydroxy-6-flavonyl]methyl]- N-methyl- N-(3-sulfopropyl)-1-dodecanaminium, inner salt (F2N12S), incorporated into phospholipid unilamellar vesicles is presented. The reconstructed time-resolved emission spectra (TRES) unravels a unique feature in the photophysics of this probe. TRES exhibit signatures of both an excited-state intramolecular proton transfer (ESIPT) and a dynamic Stokes shift associated with solvent relaxation in the lipid bilayer. The ESIPT is fast, being characterized by a risetime of approximately 30-40 ps that provides an equilibrium to be established between the excited normal (N*) and the ESIPT tautomer (T*) on a time scale of 100 ps. On the other hand, the solvent relaxation displays a bimodal decay kinetics with an average relaxation time of approximately 1 ns. The observed slow solvent relaxation dynamics likely embodies a response of nonspecific dipolar solvation coupled with formation of probe-water H-bonds as well as the relocation of the fluorophore in the lipid bilayer. Taking into account that ESIPT and solvent relaxation are governed by different physicochemical properties of the probe microenvironment, the present study provides a physical background for the multiparametric sensing of lipid bilayers using ESIPT based probes.     

Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors

Dicyanomethylene-4H-pyran (DCM) chromophores are typical donor-π-acceptor (D-π-A) type chromophores with a broad absorption band resulting from an ultra-fast internal charge-transfer (ICT) process. In 1989, Tang et al. firstly introduced a DCM derivative as a highly fluorescent dopant in organic electroluminescent diodes (OLEDs). Integration of ICT chromophore-receptor systems based on DCM chromophores with ion-induced shifts in absorption or emission is a convenient method to perform the logic expression for molecular logic gates. In recent years, various DCM-type derivatives have been explored due to their excellent optical-electronic properties and diverse structural modification. This feature article provides an insight into how the structural modification of DCM chromophores can be utilized for OLED emitters, logic gates and optical chemosensors. In addition, the aggregation-induced-emission (AIE) of DCM derivatives for further optical applications was also introduced.     

Chemical sensors based on π-conjugated organic molecules and gold nanoparticles

Scientists have developed techniques for synthesizing and characterizing many new materials including conjugated small molecules, polymers and gold particles protected by conjugated organic chromophores for testing specific sensing properties in the past decade. Still, the design and synthesis or supermolecular systems fabrication of novel materials with controlled sensing properties is a significant and ongoing challenge within nanoscience and nanotechnology. Recently, our group has successfully constructed a series of chemosensors using small organic molecules, conjugated polymers and gold nanoparticles for real-time detection of specific analytes. The chemosensors show high selectivity and sensitivity in the detection of cations and biologic analytes and thus are potentially promising for applications in sensing assay system. In this review, recent sutdies on the design, synthesis and photo-physical properties of novel materials and construct of chemosensors are summarized with an emphasis on the development in our groups in recent years. Supported by the National Natural Science Foundation of China (Grant Nos. 20531060, 20721061 & 20873155), and the National Basic Research 973 Programme of China (Grant No. 2007CB936401)     

Acid-base Vapor Sensing Enabled by ESIPT-attributed Cd(II) Coordination Polymer with Switchable Luminescence

Fluorescence materials based on excited state intramolecular proton transfer(ESIPT) have attracted great attention due to the unique four-level energy states. Herein, we report the assembly of a Cd-L_F coordination polymer from purposely designed L_F(H₂hpi2cf) ligand, which can present switchable luminescence behavior by gain or loss protons originated in uncoordinated ESIPT sites and serve as acid-base vapor sensors. Fabricated into in-situ grown film or transparent ink by simple methods, Cd-L_F presents facile and portable amine sensor for food spoilage detection and fluorescent anti-counterfeiting ink applications.     

Effect of biological confinement on the photophysics and dynamics of a proton-transfer phototautomer: an exploration of excitation and emission wavelength-dependent photophysics of the protein-bound drug

The present work demonstrates the effect of biological confinement on the photophysics and dynamics of a bio-active drug molecule viz., 5-chlorosalicylic acid (5ClSA). 5ClSA is a potential candidate exhibiting Excited-State Intramolecular Proton Transfer (ESIPT) reaction and thereby generating the phototautomer (i.e. proton transferred keto form) in the excited state. Given the pK(a) of 5ClSA (around 2.64), the anionic form of the drug molecule is expected to be the interacting species with the protein under the experimental conditions (buffered solution of pH 7.40). The ESIPT photophysics of the drug (5ClSA anion) is found to be remarkably modified within the confined bio-environment of a model transport protein Bovine Serum Albumin (BSA) in terms of remarkable emission intensity enhancement coupled with a discernible red-shift of the emission maximum wavelength. Such considerable modification of the ESIPT photophysics of the 5ClSA anion has been exploited to determine the drug-protein binding strength (as characterized by the binding constant K (±10%) = 6.11 × 10(2) M(-1)). The present work also delves into evaluation of the probable binding location of the drug within the biomacromolecular assembly of the protein by a blind docking simulation technique, which reveals hydrophobic subdomain IIA to be the probable binding site of the drug. Circular dichroism (CD) spectroscopy delineates the effect of drug binding on the protein secondary structure in terms of decrease of α-helical content of BSA with increasing drug concentration. Apart from this, the excitation-emission matrix fluorescence technique is found to hint at the effect on protein tertiary structure upon binding to the drug. Chaotrope-induced protein denaturation has been explored to complement the findings on the binding interaction process. The modulated dynamics of the proton transfer phototautomer of the 5ClSA anion within the biological confinement is also investigated in this context to explore the slower rate of solvent-relaxation dynamics.     

Bioinspired systems for metal-ion sensing: new emissive peptide probes based on benzo[d]oxazole derivatives and their gold and silica nanoparticles

Seven new bioinspired chemosensors (2-4 and 7-10) based on fluorescent peptides were synthesized and characterized by elemental analysis, (1)H and (13)C NMR, melting point, matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and IR and UV-vis absorption and emission spectroscopy. The interaction with transition- and post-transition-metal ions (Cu(2+), Ni(2+), Ag(+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), and Fe(3+)) has been explored by absorption and fluorescence emission spectroscopy and MALDI-TOF-MS. The reported fluorescent peptide systems, introducing biological molecules in the skeleton of the probes, enhance their sensitivity and confer them strong potential for applications in biological fields. Gold and silica nanoparticles functionalized with these peptides were also obtained. All nanoparticles were characterized by dynamic light scattering, transmission electron microscopy, and UV-vis absorption and fluorescence spectroscopy. Stable gold nanoparticles (diameter 2-10 nm) bearing ligands 1 and 4 were obtained by common reductive synthesis. Commercial silica nanoparticles were decorated at their surface using compounds 8-10, linked through a silane spacer. The same chemosensors were also taken into aqueous solutions through their dispersion in the outer layer of silica core/poly(ethylene glycol) shell nanoparticles. In both cases, these complex nanoarchitectures behaved as new sensitive materials for Ag(+) and Hg(2+) in water. The possibility of using these species in this solvent is particularly valuable because the impact on human health of heavy- and transition-metal-ion pollution is very severe, and all analytical and diagnostics investigations involve a water environment.     

Design and Development of Highly Efficient Light‐Emitting Layers in OLEDs with Dimesitylboranes: An Updated Review

With excellent luminescent properties and transport properties, triarylborane compounds containing two mesitylenes (Mes) have gained much attention for their application in OLEDs as light‐emitting layers. This study serves as an updated review summarizing recent developments in the design of fluorescent chromophores and phosphorescent host materials for OLEDs comprising small molecular compounds of dimesitylborane (BMes₂) as luminescent layers, with attention to the performance of different light‐emitting devices. Problems that need to be solved in the research and application of BMes₂ in OLEDs are presented and the application prospects of such materials are suggested.     

Accessing the long-lived triplet excited states in bodipy-conjugated 2-(2-hydroxyphenyl) benzothiazole/benzoxazoles and applications as organic triplet photosensitizers for photooxidations

Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7-10). The compounds show red-shifted UV-vis absorption (530-580 nm; ε up to 50000 M(-1) cm(-1)) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (Φ(F) up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and Φ(F) as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7-9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7-10 upon photoexcitation; the lifetimes of the triplet excited states (τ(T)) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet state. Different from the previous report, we demonstrated that population of the triplet excited states is not the result of ESIPT. The compounds were used as organic triplet photosensitizers for photooxidation of 1,5-dihydroxylnaphthalene. One of the compounds is more efficient than the conventional [Ir(ppy)(2)(phen)][PF(6)] triplet photosensitizer. Our result will be useful for design of new Bodipy derivatives, ESIPT compounds, and organic triplet photosensitizers, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.     

共轭聚合物离子荧光化学传感器

荧光传感器能够将分子识别的信息转换成荧光信号,荧光法在灵敏度、选择性和实时原位检测等方面优势突出,最近已引起了人们很大的兴趣。本文主要介绍以共轭聚合物为基础的离子荧光化学传感器的近期研究进展,重点综述了共轭聚合物的荧光化学传感器在阳离子识别检测中的分子设计、合成、作用机理和应用,并展望了该领域的发展方向。     

Control of Protonated Schiff Base Excited State Decay within Visual Protein Mimics: A Unified Model for Retinal Chromophores

Artificial biomimetic chromophore-protein complexes inspired by natural visual pigments can feature color tunability across the full visible spectrum. However, control of excited state dynamics of the retinal chromophore, which is of paramount importance for technological applications, is lacking due to its complex and subtle photophysics/photochemistry. Here, ultrafast transient absorption spectroscopy and quantum mechanics/molecular mechanics simulations are combined for the study of highly tunable rhodopsin mimics, as compared to retinal chromophores in solution. Conical intersections and transient fluorescent intermediates are identified with atomistic resolution, providing unambiguous assignment of their ultrafast excited state absorption features. The results point out that the electrostatic environment of the chromophore, modified by protein point mutations, affects its excited state properties allowing control of its photophysics with same power of chemical modifications of the chromophore. The complex nature of such fine control is a fundamental knowledge for the design of bio-mimetic opto-electronic and photonic devices.     

Acid‐Induced Shift of Intramolecular Hydrogen Bonding Responsible for Excited‐State Intramolecular Proton Transfer

The significant progress recently achieved in designing smart acid‐responsive materials based on intramolecular charge transfer inspired us to utilize excited‐state intramolecular proton transfer (ESIPT) for developing a turn‐on acid‐responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT‐active fluorophores, 2‐(2‐hydroxyphenyl)pyridine (HPP) and 2‐(2‐hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT‐HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady‐state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn‐on was caused by the acid‐induced shift of the ESIPT‐responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid‐responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.     

Excitation Wavelength Dependent Fluorescence of an ESIPT Triazole Derivative for Amine Sensing and Anti‐Counterfeiting Applications

Excitation wavelength dependent (Ex‐De) emission materials have potential applications in anti‐counterfeiting labels and bioimaging. Nevertheless, few purely organic chromophores are used in these areas. In this study, multiple excited states were incorporated into a molecule that was excited state intramolecular proton transfer (ESIPT) active, with the goal of manipulating the relaxation pathways of the excited states. The triazole derivative exhibits Ex‐De photoluminescence (PL), and the maximum PL wavelength is located at 526 nm and 593 nm under a series of excitation wavelengths. Spectral identification indicates that the excimer and ESIPT processes are responsible for the green (526 nm) and orange (593 nm) fluorescence, respectively. Importantly, the quick response code and test strip prepared with this triazole derivative can be used for anti‐counterfeiting and food spoilage detection applications, respectively. This research opens the door for developing novel Ex‐De materials for anti‐counterfeiting purposes.     

Optimized and far-red-emitting variants of fluorescent protein eqFP611

Fluorescent proteins (FPs) emitting in the far-red region of the spectrum are highly advantageous for whole-body imaging applications because scattering and absorption of long-wavelength light is markedly reduced in tissue. We characterized variants of the red fluorescent protein eqFP611 with bright fluorescence emission shifted up to 639 nm. The additional red shift is caused by a trans-cis isomerization of the chromophore. The equilibrium between the trans and cis conformations is strongly influenced by amino acid residues 143 and 158. Pseudo monomeric tags were obtained by further genetic engineering. For the red chromophores of eqFP611 variants, molar extinction coefficients of up to approximately 150,000 were determined by an approach that is not affected by the presence of molecules with nonfunctional red chromophores. The bright fluorescence makes the red-shifted eqFP611 variants promising lead structures for the development of near-infrared fluorescent markers. The red fluorescent proteins performed well in cell biological applications, including two-photon imaging.     

A novel AIEE and EISPT fluorescent probe for selective detection of cysteine

We designed and synthesized a simple and readily available fluorescent probe 3 for cysteine (Cys) based on naphthalene derivative. The probe is composed of a new class of aggregation-induced emission enhancement (AIEE) active dye 2 based on excited-state intramolecular proton transfer (ESIPT) and an acrylate group as the Cys recognition unit as well as the ESIPT blocking agent, which can be cut off by Cys from the probe in aqueous solution with mild conditions. The probe had great sensitivity and selectivity for the detection of Cys over homocysteine (Hcy) and glutathione (GSH) with a detection limit of 0.05 µM. In addition, we have successfully applied the probe for bioimaging studies of Cys in living cells, indicating that the probe holds great potential for biological applications.