Hydroxythiazole-Based Fluorescent Probes for Fluoride Ion Detection

New Twist on the Light-Switch Effect: Controlling the Fate of Excited States with pH in a 4-Hydroxy-thiazol-extended Ruthenium(II) Dppz Complex

We present a pH-dependent study of the excited state dynamics of a novel Ru complex bearing a 4-hydroxy thiazol-substituted dppz (dipyridophenazine) ligand (RuTzOH) and its deprotonated form (RuTzO-). We combine steady-state and time-resolved absorption and emission spectroscopy with electrochemical investigations to characterize the excited state relaxation, which upon photoexcitation at 400 nm is determined by a multitude of initially populated MLCT states for both complexes. Subsequently, for RuTzOH, two long-lived excited states are populated, leading to dual emission from the complexes, a feature that vanishes upon deprotonation. Upon deprotonation, the electron density on the dppz moiety increases significantly, leading to rapid energy populating ligand-centered states and thus deactivating the initially excited MLCT states.

A molecular design for a turn-off NIR fluoride chemosensor

We designed a turn-off near-infrared fluorescent fluoride chemosensor NIR-BODIPY-Si through the density functional theory/time-dependent functional theory calculations. In the designed sensor, the tert-butyldimethylsilyloxy moiety responses to the fluoride-triggered desilylation process, and the BODIPY dye serves as fluorophore. The molecular design firstly showed that the possibility of photoinduced electron transfer is low/high in NIR-BODIPY-Si/NIR-BODIPY-O (the desilylation product), thus referring that the fluorescence sensing mechanism is a photoinduced electron transfer mechanism that quenched the sensor's fluorescence after detection of fluoride anions. Absorption and emission spectra further demonstrated that the designed sensor is a near-infrared chemosensor. The largest binding energy between NIR-BODIPY-Si and F^- suggests that the sensor has an excellent selectivity to F^- and the low barrier of the desilylation reaction accounts for the sensor's rapid response speed to F^-. We also provided the synthetic routine for the molecule sensor, with the expectation that this molecular design can shed some light on the experimentally based design procedure.

Photophysics of Ruthenium(II) Complexes with Thiazole π-Extended Dipyridophenazine Ligands

Transition-metal-based donor-acceptor systems can produce long-lived excited charge-transfer states by visible-light irradiation. The novel ruthenium(II) polypyridyl type complexes Ru1 and Ru2 based on the dipyridophenazine ligand (L0) directly linked to 4-hydroxythiazoles of different donor strengths were synthesized and photophysically characterized. The excited-state dynamics were investigated by femtosecond-to-nanosecond transient absorption and nanosecond emission spectroscopy complemented by time-dependent density functional theory calculations. These results indicate that photoexcitation in the visible region leads to the population of both metal-to-ligand charge-transfer (¹MLCT) and thiazole (tz)-induced intraligand charge-transfer (¹ILCT) states. Thus, the excited-state dynamics is described by two excited-state branches, namely, the population of (i) a comparably short-lived phenazine-centered ³MLCT state (τ ≈ 150-400 ps) and (ii) a long-lived ³ILCT state (τ ≈ 40-300 ns) with excess charge density localized on the phenazine and tz moieties. Notably, the ruthenium(II) complexes feature long-lived dual emission with lifetimes in the ranges τ_Em,1 ≈ 40-300 ns and τ_Em,2 ≈ 100-200 ns, which are attributed to emission from the ³ILCT and ³MLCT manifolds, respectively.

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

Colorimetry is an advantageous method for detecting fluoride in drinking water in a resource-limited context, e. g., in parts of the developing world where excess fluoride intake leads to harmful health effects. Here we report a selective colorimetric chemosensor for fluoride that employs an azulene as the reporter motif and a pinacolborane as the receptor motif. The chemosensor, NAz-6-Bpin, is prepared using the Nozoe azulene synthesis, which allows for its rapid and low-cost synthesis. The chemosensor gives a visually observable response to fluoride both in pure organic solvent and also in water/alcohol binary solvent mixtures.

Assembly of T-Shaped Amphiphilic Thiazoles on the Air-Water Interface: Impact of Polar Chromophore Moieties, as Well as Dipolarity and π-Extension of the Chromophore on the Supramolecular Structure

The supramolecular structure essentially determines the properties of organic thin films. In this work, we systematically investigate the influence of the chromophore on the supramolecular structure formation at air-water interfaces by means of the Langmuir-Blodgett technique. Therefore, we focus on the recently introduced class of double-anchor T-shaped amphiphilic dyes, namely, 4-hydroxy-thiazole chromophores that are centrally equipped with an amphiphilicity-inducing hexanoic acid. The thiazoles contain hydrophilic subphase-anchor groups in the 2-position (4- N, N-dimethylaminophenyl (Am), 2-pyridyl (Py), and 4-nitrophenyl (Ni)), whereas the chromophores are systematically extended in the 5-position with various substituents. The combination of the Langmuir technique with online fluorescence measurements revealed that the π-π interactions that are pronounced in the case of 4-methoxybiphenyl derivatives yield the most distinct supramolecular structures. Whereas in the case of Py and Ni derivatives ordered J-type supramolecular structures in microdomains are formed, the Am derivative forms ordered supramolecular structures that are more homogeneous, which are, however, not stabilized by J-type dipolar interactions. Because of the synergetic π-π and dipolar stabilizations, the Ni derivative bearing the 4-methoxybiphenyl unit forms exceptionally stable quasi-two-dimensional Langmuir monolayers reaching very high surface pressures beyond 60 mN/m without any sign of disturbance of the Langmuir monolayer.

Azulene-boronate esters: colorimetric indicators for fluoride in drinking water

Low cost and in situ fluoride detection by non-experts is important for the determination of drinking water safety in developing countries. Colour reagents can provide results quickly without expensive equipment, but colorimetric fluoride indicators are often nonspecific, complex to use or do not work in water. Here we show that azulene-boronate indicators respond selectively to fluoride at concentrations relevant to the WHO limit of 1.5 mg L^-1.

Spectrometric and kinetics studies involving anionic chromogenic chemodosimeters based on silylated imines in acetonitrile or acetonitrile–water mixtures

The nucleophilic attack of F⁻ or CN⁻ on the silicon center of chemodosimeters, through an S_N2@Si mechanism, released colored phenolates as leaving groups. NMR, PGSE NMR, and UV-vis studies provided evidence on the mechanism of the reactions.

A novel pyrazoline-based fluorescent probe for detecting fluoride ion in water and its application on real samples

We have designed and synthesized a new fluorescent probe, pyrazoline-based probe 1, to detect fluoride ion with high selectivity and sensitivity in aqueous media.

π-Extension of a 4-ethoxy-1,3-thiazole via aryl alkyne cross coupling: synthesis and exploration of the electronic structure

π-Extension of 4-ethoxy-1,3-thiazolesviaaryl coupling resulted in a twofold increase of molar extinction coefficients and larger Stokes’ shifts.

Chromogenic chemodosimeter for highly selective detection of cyanide in water and blood plasma based on Si-O cleavage in the micellar system

A novel silylated imine was designed to act efficiently in a chemodosimeter approach for the selective detection of cyanide in an aqueous micellar CTABr solution. This simple system allows the detection of cyanide, with high sensitivity and specific selectivity, in water and in human blood plasma.

A small azide-modified thiazole-based reporter molecule for fluorescence and mass spectrometric detection

Molecular probes are widely used tools in chemical biology that allow tracing of bioactive metabolites and selective labeling of proteins and other biomacromolecules. A common structural motif for such probes consists of a reporter that can be attached by copper(I)-catalyzed 1,2,3-triazole formation between terminal alkynes and azides to a reactive headgroup. Here we introduce the synthesis and application of the new thiazole-based, azide-tagged reporter 4-(3-azidopropoxy)-5-(4-bromophenyl)-2-(pyridin-2-yl)thiazole for fluorescence, UV and mass spectrometry (MS) detection. This small fluorescent reporter bears a bromine functionalization facilitating the automated data mining of electrospray ionization MS runs by monitoring for its characteristic isotope signature. We demonstrate the universal utility of the reporter for the detection of an alkyne-modified small molecule by LC–MS and for the visualization of a model protein by in-gel fluorescence. The novel probe advantageously compares with commercially available azide-modified fluorophores and a brominated one. The ease of synthesis, small size, stability, and the universal detection possibilities make it an ideal reporter for activity-based protein profiling and functional metabolic profiling.

Investigation of desilylation in the recognition mechanism to fluoride by a 1,8-naphthalimide derivative

Desilylation based fluorescence sensor (AF-1) gives a dual signal for quantitative detection of F⁻ in MeCN.

A selective, sensitive, colorimetric, and fluorescence probe for relay recognition of fluoride and Cu(II) ions with "off-on-off" switching in ethanol-water solution

Anion to cation relay recognition was designed and realized for the first time with sequence specificity (F(-)→Cu(2+)) via a fluorescence "off-on-off" mechanism. Probe 1 was a highly selective, sensitive, and turn-on chemodosimeter for F(-) through a specific cyclization reaction triggered by the strong affinity of fluoride toward silicon with a significant change of fluorescence color in both ethanol and ethanol-water (1:1, v/v) solution. Fluorescence enhancement factors were dramatic: 833-fold in ethanol and 164-fold in ethanol-water (1:1, v/v) solution, respectively. The in situ system generated from the sensing of F(-) showed good relay recognition ability for Cu(2+) via fast fluorescence quenching by the formation of a 1:1 complex in ethanol-water (1:1, v/v) solution. The isolated pure compound 2 also exhibited high selectivity toward Cu(2+) in PBS buffer (pH = 7.0) solution. The origin of this sequence specificity of fluorescence recognition was disclosed through the crystal or optimized structures and DFT calculations of corresponding compounds.