Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated.Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtos...

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Hydrogen Bonding in the Electronic Excited State

Coumarins are a very large family of compounds containing the unique 2H-chromen-2-one motif, as it is known according to IUPAC nomenclature. Coumarin derivatives are widely found in nature, especially in plants and are constituents of several essential oils. Up to now, thousands of coumarin derivatives have been isolated from nature or produced by chemists. More recently, the coumarin platform has been widely adopted in the design of small-molecule fluorescent chemosensors because of its excellent biocompatibility, strong and stable fluorescence emission, and good structural flexibility. This scaffold has found wide applications in the development of fluorescent chemosensors in the fields of molecular recognition, molecular imaging, bioorganic chemistry, analytical chemistry, materials chemistry, as well as in the biology and medical science communities. This review focuses on the important progress of coumarin-based small-molecule fluorescent chemosensors during the period of 2012-2018. This comprehensive and critical review may facilitate the development of more powerful fluorescent chemosensors for broad and exciting applications in the future.

Coumarin-Based Small-Molecule Fluorescent Chemosensors.

Metal cations and anions are essential for versatile physiological processes. Dysregulation of specific ion levels in living organisms is known to have an adverse effect on normal biological events. Owing to the pathophysiological significance of ions, sensitive and selective methods to detect these species in biological systems are in high demand. Because they can be used in methods for precise and quantitative analysis of ions, organic dye-based ratiometric fluorescent probes have been extensively explored in recent years. In this review, recent advances (2015-2019) made in the development and biological applications of synthetic ratiometric fluorescent probes are described. Particular emphasis is given to organic dye-based ratiometric fluorescent probes that are designed to detect biologically important and relevant ions in cells and living organisms. Also, the fundamental principles associated with the design of ratiometric fluorescent probes and perspectives about how to expand their biological applications are discussed.

Synthetic ratiometric fluorescent probes for detection of ions

Bioactive species, including reactive oxygen species (ROS, including O2˙-, H2O2, HOCl, 1O2, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO-, NO, NO2, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H2S, H2S n , SO2 derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

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Fluorescent probes for organelle-targeted bioactive species imaging

In this tutorial review, we will explore recent advances in the construction and application of Forster resonance energy transfer (FRET)-based small-molecule fluorescent probes. The advantages of FRET-based fluorescent probes include: a large Stokes shift, ratiometric sensing and dual/multi-analyte responsive systems. We discuss the underlying energy donor–acceptor dye combinations and emphasise their applications for the detection or imaging of cations, anions, small neutral molecules, biomacromolecules, cellular microenvionments and dual/multi-analyte responsive systems.

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Förster resonance energy transfer (FRET)-based small-molecule sensors and imaging agents.

A mitochondria-targeted fluorescent probe for ratiometric detection of endogenous sulfur dioxide derivatives in cancer cells

An effective colorimetric and ratiometric fluorescent probe for bisulfite in aqueous solution

We have developed a ratiometric fluorescent probe based on ICT mechanism for imaging intracellular sulfite (SO 3 2− ) in HeLa cells. The probe could selectively stain cells with scarce cytotoxicity and excellent photostability. The probe displayed a high selectivity for sulfite over other anions and active small biological molecules including cysteine (Cys), homocysteine (Hcy) and glutathione (GSH) at room temperature. The reaction could complete within 30 s and detection limit was as low as 76 nM, which is superior to some reported probes. The detecting mechanism was proved by MS, 1 H NMR and theoretical calculation. The probe can also detect endogenous sulfite in HepG2 cells.

An effective ICT-based and ratiometric fluorescent probe for sensing sulfite

A NBD-based simple but effective fluorescent pH probe for imaging of lysosomes in living cells.

A new imidazo[1,5-a]pyridine-based pH fluorescent probe has been synthesized and characterized. It is the first time that imidazo[1,5-a]pyridine is used as a fluorescent probe. This probe responds to acidic pH with fast response (within 3 min), high selectivity and sensitivity. It has good reversibility and nearly no interference from common metal ions. The probe is suitable for acidic conditions and can quantitatively detect pH value based on the equilibrium equation, pH = pKa − log[(Ia − Ix)/(Ix − Ib)], with a good linear relationship. Moreover, we verified that the protonation of the 2-nitrogen in the imidazole decreases the electron density on the fused ring by 1H NMR analysis and DFT calculation. We proposed that intramolecular charge transfer and solvation of the probe in aqueous solutions might be the mechanism of the fluorescence enhancement under strongly acidic conditions.

Xiang-Jian Cao

Papers

A mitochondria-targeted fluorescent probe for ratiometric detection of endogenous sulfur dioxide derivatives in cancer cells

An effective colorimetric and ratiometric fluorescent probe for bisulfite in aqueous solution

An effective ICT-based and ratiometric fluorescent probe for sensing sulfite

A NBD-based simple but effective fluorescent pH probe for imaging of lysosomes in living cells.

A new fluorescent pH probe for acidic conditions