Dioxygen activation at mononuclear nonheme iron active sites: enzymes, models, and intermediates.

Exposure to even very low levels of lead, cadmium, and mercury ions is known to cause neurological, reproductive, cardiovascular, and developmental disorders, which are more serious problems for children particularly. Accordingly, great efforts have been devoted to the development of fluorescent and colorimetric sensors, which can selectively detect lead, cadmium, and mercury ions. In this critical review, the fluorescent and colorimetric sensors are classified according to their receptors into several categories, including small molecule based sensors, calixarene based chemosensors, BODIPY based chemosensors, polymer based chemosensors, DNA functionalized sensing systems, protein based sensing systems and nanoparticle based sensing systems (197 references).

Fluorescent and colorimetric sensors for detection of lead, cadmium, and mercury ions

Matrix Differential Calculus with Applications in Statistics and Econometrics

: 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

Design strategies for water-soluble small molecular chromogenic and fluorogenic probes.

A molecular probe was prepared that selectively responds to cyanide in aqueous solutions by fluorescence enhancement. Using the peptide β-turn as a structural template, we designed a series of diphenylacetylene derivatives in which the π-conjugated backbone was functionalized with an aldehyde group to render the molecule nonfluorescent. The N−H···O hydrogen bond across the 2,2′-functionalized diphenylacetylene turn motif activates the carbonyl group toward nucleophilic attack, and chemical transformation of this internal quencher site by reaction with CN− elicits a rapid (k = 72 M−1 s−1) enhancement in the emission at λmax = 375 nm. Tethering of an ammonium group to the hydrogen bond donor fragment significantly increased both the response kinetics and the intensity of the fluorescence signal. In addition to providing electrostatic attraction toward the CN− ion, this positively charged R-NH3+ fragment can engage in a secondary hydrogen bond to facilitate the formation of the cyanohydrin adduct responsible...

Turn-On Fluorescence Detection of Cyanide in Water: Activation of Latent Fluorophores through Remote Hydrogen Bonds That Mimic Peptide β-Turn Motif

An oxidative cyclization reaction transforms nonemissive azoanilines into highly fluorescent benzotriazoles. We have found that introduction of multiple electron-donating amino groups onto a simple o-(phenylazo)aniline platform dramatically accelerates its conversion to the emissive polycyclic product. Notably, this chemistry can be effected by μM-level concentrations of copper(II) ion in water (pH = 6–8) at room temperature to elicit >80-fold enhancement in the green emission at λem = 530 nm. Comparative kinetic and electrochemical studies on a series of structural analogues have established that the accelerated reaction rates correlate directly with a systematic cathodic shift in the oxidation onset potential of the azo precursors. In addition, single-crystal X-ray crystallographic analysis on the most reactive derivative revealed the presence of a five-membered ring intramolecular hydrogen-bonding network. An enhanced contribution of the quinoid-type resonance in such conformation apparently facilitate...

Reactivity-Based Detection of Copper(II) Ion in Water: Oxidative Cyclization of Azoaromatics as Fluorescence Turn-On Signaling Mechanism

Hydrogen-bonding promoted covalent modifications are finding useful applications in small-molecule chemical synthesis and detection. We have designed a xanthene-based fluorescent probe 1, in which tightly held acylguanidine and aldehyde groups engage in multiple intramolecular hydrogen bonds within the concave side of the molecule. Such an interdigitated hydrogen bond donor–acceptor (HBD–HBA) array imposes significant energy barriers (ΔG‡ = 10–16 kcal mol–1) for internal bond rotations to assist structural preorganization and effectively polarizes the electrophilic carbonyl group toward a nucleophilic attack by CN– in aqueous environment. This covalent modification redirects the de-excitation pathways of the cyanohydrin adduct 2 to elicit a large (>7-fold) enhancement in the fluorescence intensity at λmax = 440 nm. A remarkably faster (> 60-fold) response kinetics of 1, relative to its N-substituted (and therefore “loosely held”) analogue 9, provided compelling experimental evidence for the functional rol...

Interdigitated hydrogen bonds: electrophile activation for covalent capture and fluorescence turn-on detection of cyanide.

An efficient triple azo coupling reaction between anilines and phloroglucinol furnished a series of C3-symmetric molecules 7−9 supporting multiple conjugation pathways that converge at the molecular core. A combination of 1H/13C NMR spectroscopy, X-ray crystallography, and density functional theory computational studies provided a coherent picture of the [n,π]-conjugated molecular core, which is best described as the tris(hydrazone) [rather than tris(azo)] tautomer stabilized by resonance-assisted hydrogen bonding. For a homologous series of compounds, an increase in the torsional angles between the planar molecular core and the peripheral aryl groups results in a systematic blue shift in the low-energy electronic transitions (7, 523 nm; 8, 505 nm; 9, 445 nm in CHCl3) that qualitatively correlates with the shrinkage of effective conjugation through structural distortion. Similar spectral shifts could also be induced by amine substrates that interact with the intramolecular hydrogen-bonding network to trig...

Torsionally responsive C3-symmetric azo dyes: azo-hydrazone tautomerism, conformational switching, and application for chemical sensing.

An efficient 2 + 2 cyclocondensation with dihydroxysilane converted simple arylboronic acids to bifunctional borasiloxane cage molecules, which were subsequently electropolymerized to furnish air-stable thin films. The extended [p,pi]-conjugation that defines the rigid backbone of this new conjugated polymer (CP) motif gives rise to longer-wavelength UV-vis transitions upon oxidative doping, the spectral window and intensity of which can be modified by interaction with Lewis basic reagents. Notably, this boron-containing CP undergoes a rapid and reversible color change from green to orange upon exposure to volatile amine samples under ambient conditions. This direct naked-eye detection scheme can best be explained by invoking the reversible B-N dative bond formation that profoundly influences the p-pi* orbital overlap.

Dongwhan Lee

Papers

Turn-On Fluorescence Detection of Cyanide in Water: Activation of Latent Fluorophores through Remote Hydrogen Bonds That Mimic Peptide β-Turn Motif

Reactivity-Based Detection of Copper(II) Ion in Water: Oxidative Cyclization of Azoaromatics as Fluorescence Turn-On Signaling Mechanism

Interdigitated hydrogen bonds: electrophile activation for covalent capture and fluorescence turn-on detection of cyanide.

Torsionally responsive C3-symmetric azo dyes: azo-hydrazone tautomerism, conformational switching, and application for chemical sensing.

Conjugated polymer sensors built on pi-extended borasiloxane cages.