: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

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.

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials.

High crystallinity of covalent organic frameworks (COFs) with dual fluorescence emissions has not been reported so far. Here, we show the rational design and preparation of high-crystallinity COF TzDa via the synergetic interaction of docking sites and hydrogen bonds: 4,4′,4″-(1,3,5-Triazine-2,4,6-triyl)trianiline (Tz) with the docking site and 2,5-dihydroxyterephthalaldehyde (Da) with the OH group are employed to synthesize the imine-linked two-dimensional high-crystallinity layered structure TzDa. The prepared mesoporous TzDa (ca. 36 A) exhibits high thermal and chemical stability. The intramolecular charge transfer (ICT) and excited-state intramolecular proton transfer (ESIPT) effects bring TzDa two main fluorescence emissions around 500 and 590 nm. Water molecules can interfere with the ICT and ESIPT effects, allowing the development of a ratiometric fluorescent sensor for water in organic solvents. The proposed sensor shows high sensitivity to trace water in conventional organic solvents. The high st...

High-Crystallinity Covalent Organic Framework with Dual Fluorescence Emissions and Its Ratiometric Sensing Application

Recent progress on the development of glutathione (GSH) selective fluorescent and colorimetric probes

Recent progress in hydrogen sulphide (H2S) sensors by metal displacement approach

Copper complex based on a new class of fluorescence OFF-ON sensor 1.Cu has been reported for the detection of trace amounts of water in various organic solvents such as CH3OH, THF, CH3CN, and acetone by means of fluorescence emission intensity. The probe is highly responsive to water in THF (DL = 0.003 wt %). The dissociation of copper from probe 1.Cu in the presence of water is responsible for the fluorescence change and it was confirmed by electrospray ionization-mass spectrometry (ESI-MS), proton nuclear magnetic resonance (1H-NMR), and fluorescence lifetime studies. Real application of the probe was successfully applied for the detection of moisture content in commercial products such as salt, sugar, wheat, and washing powder.

Detection of Moisture by Fluorescent OFF-ON Sensor in Organic Solvents and Raw Food Products.

The simple off-the-shelf chemical 6,7-dihydroxycoumarin (1) based copper complex (1·Cu2+) has been used for the selective detection of toxic cyanide in aqueous medium. The DFT calculation confirms the binding behavior between 1 and Cu2+ (2:1) and the red shift in the UV–vis spectrum with copper ion was confirmed by the decrease in energy between HOMO–LUMO band gaps. The cyanide sensing in water was confirmed by both absorption and emission spectral studies. Cyanide ion showed 13-fold increments in fluorescent intensity in emission spectrum via displacement of copper from 1·Cu2+. The limit of detection of CN– in water is 5.77 μM; 1·Cu2+ also applicable for the detection of cyanide in fresh mouse serum with detection limit of 14.4 μM. The cell images showed that 1·Cu2+ could be used to detect intracellular CN–.

Selective Detection of Cyanide in Water and Biological Samples by an Off-the-Shelf Compound

A fluorescent probe for the monitoring of H2S levels in living cells and organisms is highly desirable. In this regard, near-infrared (NIR) fluorescent probes have emerged as a promising tool. NIR-I and NIR-II probes have many significant advantages; for instance, NIR light penetrates deeper into tissue than light at visible wavelengths, and it causes less photodamage during biosample analysis and less autofluorescence, enabling higher signal-to-background ratios. Therefore, it is expected that fluorescent probes having emission in the NIR region are more suitable for in vivo imaging. Consequently, a considerable increase in reports of new H2S-responsive NIR fluorescent probes appeared in the literature. This review highlights the advances made in developing new NIR fluorescent probes aimed at the sensitive and selective detection of H2S in biological samples. Their applications in real-time monitoring of H2S in cells and in vivo for bioimaging of living cells/animals are emphasized. The selection of suitable dyes for designing NIR fluorescent probes, along with the principles and mechanisms involved for the sensing of H2S in the NIR region, are described. The discussions are focused on small-molecule and nanomaterials-based NIR probes.

Sensing and Bioimaging of the Gaseous Signaling Molecule Hydrogen Sulfide by Near-Infrared Fluorescent Probes.

A new approach for the detection of hydrogen sulfide (H2S) was constructed within vesicles comprising phospholipids and amphiphilic copper complex as receptor. 1,2-Distearoyl-sn-glycero-3-phosphocholine (DSPC) vesicles with embedded metal complex receptor (1.Cu) sites have been prepared. The vesicles selectively respond to H2S in a buffered solution and show colorimetric as well as spectral transformation. Other analytes such as reactive sulfur species, reactive nitrogen species, biological phosphates, and other anions failed to induce changes. The H2S detection is established through a metal indicator displacement (MIDA) process, where Eosin-Y (EY) was employed as an indicator. Fluorescence, UV–vis spectroscopy, and the naked eye as the signal readout studies confirm the high selectivity, sensitivity, and lower detection limit of the vesicular receptor. The application of vesicular receptors for real sample analysis was also confirmed by fluorescence live cell imaging.

Rahul Kaushik

Papers

Recent progress in hydrogen sulphide (H2S) sensors by metal displacement approach

Detection of Moisture by Fluorescent OFF-ON Sensor in Organic Solvents and Raw Food Products.

Selective Detection of Cyanide in Water and Biological Samples by an Off-the-Shelf Compound

Sensing and Bioimaging of the Gaseous Signaling Molecule Hydrogen Sulfide by Near-Infrared Fluorescent Probes.

Selective Detection of H2S by Copper Complex Embedded in Vesicles through Metal Indicator Displacement Approach