Development of a fluorescent scaffold by utilizing quercetin template for selective detection of Hg2+: Experimental and theoretical studies along with live cell imaging.

Quercetin is an important antioxidant with high bioactivity and it has been used as SARS-CoV-2 inhibitor significantly. Quercetin, one of the most abundant flavonoids in nature, has been in the spot of numerous experimental and theoretical studies in the past decade due to its great biological and medicinal importance. But there have been limited instances of employing quercetin and its derivatives as a fluorescent framework for specific detection of various cations and anions in the chemosensing field. Therefore, we have developed a novel chemosensor based on quercetin coupled benzyl ethers (QBE) for selective detection of Hg2+ with "naked-eye" colorimetric and "turn-on" fluorometric response. Initially QBE itself exhibited very weak fluorescence with low quantum yield (Φ = 0.009) due to operating photoinduced electron transfer (PET) and inhibition of excited state intramolecular proton transfer (ESIPT) as well as intramolecular charge transfer (ICT) within the molecule. But in presence of Hg2+, QBE showed a sharp increase in fluorescence intensity by 18-fold at wavelength 444 nm with high quantum yield (Φ = 0.159) for the chelation-enhanced fluorescence (CHEF) with coordination of Hg2+, which hampers PET within the molecule. The strong binding affinity of QBE towards Hg2+ has been proved by lower detection limit at 8.47 µM and high binding constant value as 2 × 104 M-1. The binding mechanism has been verified by DFT study, Cyclic voltammograms and Jobs plot analysis. For the practical application, the binding selectivity of QBE with Hg2+ has been capitalized in physiological medium to detect intracellular Hg2+ levels in living plant tissue by using green gram seeds. Thus, employing QBE as a fluorescent chemosensor for the specific identification of Hg2+ will pave the way for a novel approach to simplifying the creation of various chemosensors based on quercetin backbone for the precise detection of various biologically significant analytes.

Dual-mode chemosensor for the fluorescence detection of zinc and hypochlorite on a fluorescein backbone and its cell-imaging applications.

Fluorescein coupled with 3-(aminomethyl)-4,6-dimethylpyridin-2(1H)-one (FAD) was synthesized for the selective recognition of Zn2+ over other interfering metal ions in acetonitrile/aqueous buffer (1 : 1). Interestingly, there was a significant fluorescence enhancement of FAD in association with Zn2+ at 426 nm by strong chelation-induced fluorescence enhancement (CHEF) without interrupting the cyclic spirolactam ring. A binding stoichiometric ratio of 1 : 2 for the ligand FAD with metal Zn2+ was proven by a Jobs plot. However, the cyclic spirolactam ring was opened by hypochlorite (OCl-) as well as oxidative cleavage of the imine bond, which resulted in the emission enhancement of the wavelength at 520 nm. The binding constant and detection limit of FAD towards Zn2+ were determined to be 1 × 104 M-1 and 1.79 µM, respectively, and the detection limit for OCl- was determined as 2.24 µM. We introduced here a dual-mode chemosensor FAD having both the reactive functionalities for the simultaneous detection of Zn2+ and OCl- by employing a metal coordination (Zn2+) and analytes (OCl-) induced chemodosimetric approach, respectively. Furthermore, for the practical application, we studied the fluorescence imaging inside HeLa cells by using FAD, which demonstrated it can be very useful as a selective and sensitive fluorescent probe for zinc.

Tuning the Reactivity of Chromium(III)-Superoxo Species by Coordinating Axial Ligands.

Metal-superoxo species have attracted much attention recently as key intermediates in enzymatic and biomimetic oxidation reactions. The effect(s) of axial ligands on the chemical properties of metal-superoxo complexes has never been explored previously. In this study, we synthesized and characterized chromium(III)-superoxo complexes bearing TMC derivatives with pendant pyridine and imidazole donors, such as [Cr(III)(O2)(TMC-Py)](2+) (1, TMC-Py = 4,8,11-trimethyl-1-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane) and [Cr(III)(O2)(TMC-Im)](2+) (2, TMC-Im = 4,8,11-trimethyl-1-(2-methylimidazolmethyl)-1,4,8,11-tetraazacyclotetradecane). The reactivity of chromium(III)-superoxo complexes binding different axial ligands, such as 1, 2, and [Cr(III)(O2)(TMC)(Cl)](+) (3, TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was then investigated in C-H bond activation and oxygen atom transfer reactions. Kinetic studies revealed that the reactivity of the Cr(III)-superoxo complexes depends on the axial ligands, showing the reactivity order of 1 > 2 > 3 in those electrophilic oxidation reactions. It was also shown that there is a good correlation between the reactivity of the chromium(III)-superoxo complexes and their redox potentials, in which the redox potentials of the chromium(III)-superoxo complexes are in the order 1 > 2 > 3. DFT calculations reproduced the reactivity order between 1 and 3 in both C-H bond activation and oxygen atom transfer reactions, and the latter reaction is described using orbital interactions. The calculations are also in agreement with the experimentally obtained redox potentials. The present results provide the first example showing that the reactivity of metal-superoxo species can be tuned by the electron-donating ability of axial ligands bound trans to the metal-superoxo moiety.

Bis(2,1,3-benzoselenadiazole-κN)dibromidocopper(II).

In the title complex, [CuBr(2)(C(6)H(4)N(2)Se)(2)], the Cu(II) ion is tetra-coordinated by two bromide anions and two N atoms in a distorted square-planar geometry. The two essentially planar 2,1,3-benzoselenadiazole ligands [maximum deviations = 0.012 (2) and 0.030 (2) Å] are approximately coplanar [dihedral angle = 6.14 (6)°]. In the crystal, short inter-molecular Se⋯Br, Se⋯N and N⋯N inter-actions are observed. These short inter-actions and inter-molecular C-H⋯Br hydrogen bonds link the complex mol-ecules into two-dimensional arrays parallel to the ac plane.

(Z)-1,2-Dichloro-1,2-bis-(3-chloro-quinoxalin-2-yl)ethene.

The title compound, C(18)H(8)Cl(4)N(4), exists in a cis configuration with respect to the bridging C=C bond. The two essentially planar quinoxaline ring systems [maximum deviations = 0.012 (1) and 0.022 (1) Å] are inclined at an angle of 59.84 (3). In the crystal, adjacent mol-ecules are linked into chains propagating along [001] via inter-molecular C-H⋯N hydrogen bonds. Weak inter-molecular π-π [centroid-centroid distance = 3.6029 (7)°] and C-H⋯π inter-actions are also observed.

Adeninium perchlorate.

IN THE TITLE SALT (SYSTEMATIC NAME: 6-amino-9H-purin-1-ium perchlorate), C(5)H(6)N(5) (+)·ClO(4) (-), the adeninium cation is essentially planar, with a maximum deviation of 0.038 (1) Å. The whole of the perchlorate anion is disordered over two sets of sites with an occupancy ratio of 0.589 (13):0.411 (13). In the crystal, the adeninium cations are linked by pairs of N-H⋯N hydrogen bond into inversion dimers. The dimers and the anions are further inter-connected into a three-dimensional supra-molecular structure via inter-molecular N-H⋯O, C-H⋯O and C-H⋯N hydrogen bonds.

1,10-Phenanthroline-dithio-oxamide (2/1).

The asymmetric unit of the title compound, C(12)H(8)N(2)·0.5C(2)H(4)N(2)S(2), contains one 1,10-phenanthroline mol-ecule and a half-mol-ecule of dithio-oxamide, which lies across a crystallographic inversion center. The 1,10-phenanthroline unit is not strictly planar, with dihedral angles between the central benzene ring and the pyridine rings of 1.42 (10) and 1.40 (10)°. In the crystal structure, two 1,10-phenanthroline mol-ecules are linked together by one dithio-oxamide via inter-molecular N-H⋯N hydrogen bonds.

2,9-Bis(trichloro-meth-yl)-1,10-phenanthroline.

The asymmetric unit of the title compound, C(14)H(6)Cl(6)N(2), contains two crystallographically independent mol-ecules, each of which is slightly twisted from planarity. The dihedral angles between the central ring and the two outer rings are 3.81 (7) and 4.30 (7)° in one mol-ecule, and 4.13 (8) and 4.10 (7)° in the other. In the crystal structure, mol-ecules are inter-linked by C-Cl⋯Cl inter-actions into sheets parallel to the ac plane. These sheets are stacked along the b axis in such a way that the mol-ecules are anti-parallel; they are further connected into a supra-molecular network. There are no classical hydrogen bonds. C⋯Cl [3.637 (2) Å], Cl⋯Cl [3.5639 (5)-3.6807 (8) Å] and Cl⋯N [3.3802 (15)-3.4093 (15) Å] short contacts and π-π inter-actions, with centroid-centroid distances in the range 3.5868 (9)-3.7844 (9) Å, are observed.

2,7-Bis-(trichloro-meth-yl)-1,8-naphthyridine.

The complete mol-ecule of the title compound, C(10)H(4)Cl(6)N(2), is generated by crystallographic twofold symmetry, with two C atoms lying on the rotation axis; the 1,8-naphthyridine ring is almost planar with an r.m.s. deviation of 0.0002 Å. In the crystal structure, the mol-ecules are stacked in an anti-parallel manner along [001]. Short Cl⋯Cl [3.3502 (4)] and Cl⋯N [3.2004 (11)-3.2220 (10) Å] contacts are observed in the crystal structure.

Quinoxaline-2-carbonitrile.

In the title compound, C(9)H(5)N(3), the quinoxaline ring is essentially planar, with a maximum deviation of 0.012 (1) Å. Short inter-molecular distances between the centroids of the 2,3-dihydro-pyrazine and benzene rings [3.6490 (5) Å] indicate the existence of π⋯π inter-actions. In the crystal packing, the mol-ecules are linked via two pairs of inter-molecular C-H⋯N inter-actions, forming R(2) (2) (8) and R(2) (2) (10) ring motifs; these mol-ecules are further linked into a two-dimensional network parallel to (1 0 2) via another C-H⋯N inter-action.

7-Nitro-1,2,3,4-tetra-hydro-naphthalene-1-spiro-2'-(1,3-dithiane).

In the title compound, C(13)H(15)NO(2)S(2), the nitro group is coplanar with the benzene ring to which it is attached, forming a dihedral angle of 1.07 (14)°. The dithiane ring adopts a chair conformation. In the crystal structure, mol-ecules are linked through C-H⋯O and C-H⋯π [C⋯Cg = 3.7164 (15) Å] inter-actions. The crystal studied was an inversion twin with an 0.134 (5):0.866 (5) domain ratio.

2-(4-Nitro-phen-yl)-1,3-dithiane.

The nitro group in the title compound, C(10)H(11)NO(2)S(2), is almost coplanar with the benzene ring, making a dihedral angle of 3.42 (8)°. The 1,3-dithiane ring adopts a chair conformation. The crystal structure is stabilized by inter-molecular C-H⋯O and C-H⋯π [C⋯Cg = 3.4972 (10) Å] inter-actions.

1,2-Di-2-quinolylethene.

The title compound, C(20)H(14)N(2), comprises two crystallographically independent centrosymmetric mol-ecules (A and B) with different conformations due to the disorder of molecule B. The whole of mol-ecule B is disordered over two sets of positions, corresponding to a 180° rotation of the molecule, with a site-occupancy ratio of 0.780 (6):0.220 (6). The minor component of the disordered part in B has the same configuration as mol-ecule A, but the major component is different. The dihedral angle between the planes of mol-ecule A and mol-ecule B (major component) is 63.22 (3)°. The crystal structure is stabilized by inter-molecular C-H⋯π inter-actions.

N-[6-(Dibromo-meth-yl)-2-pyrid-yl]-2,2-dimethyl-propionamide.

In the mol-ecular structure of the title compound, C(11)H(14)Br(2)N(2)O, the dimethyl-propionamide substituent is twisted slightly with respect to the pyridine ring, the inter-planar angle being 12.3 (2)°. The dibromo-methyl group is orientated in such a way that the two Br atoms are tilted away from the pyridine ring. In the crystal structure, mol-ecules are associated into supra-molecular chains by weak C-H⋯O inter-actions. The crystal is further stabilized by weak N-H⋯Br and C-H⋯N inter-actions.

2-(Pyrene-1-yl)-1,3-dithiane.

In the title compound, C(20)H(16)S(2), the pyrene ring is planar [maximum deviation 0.0144 (15) Å] and the dithiane ring adopts a chair conformation. The crystal packing is stabilized by C-H⋯π inter-actions. An intra-molecular C-H⋯S hydrogen bond generates an S(5) ring motif.

Bis(2,1,3-benzoselenadiazole-κN)dichloridozinc(II).

In the title complex, [ZnCl(2)(C(6)H(4)N(2)Se)(2)], the Zn(II) center is tetra-coordinated by a Cl(2)N(2) donor set in a distorted tetrahedral geometry. Some of the distortion from the ideal tetrahedral geometry might be ascribed to two agostic Z⋯H interactions The two 2,1,3-benzoselenadiazole ligands are each essentially planar and form a dihedral angle of 35.06 (9)°. An interesting feature of the crystal packing is the observation of short intermolecular contacts between Se and Se, Se and N, and N and N atoms. These arise as a result of three-center bridging of adjacent molecules into chains along the b axis. The crystal structure is stablilized by π-π inter-actions [minimum centroid-centroid distance = 3.5694 (18) Å].

5-Bromo-5-bromo-methyl-2-phen-oxy-1,3,2-dioxaphospho-rinan-2-one.

In the title 1,3,2-dioxaphospho-rinane derivative, C(10)H(11)Br(2)O(4)P, the 1,3,2-dioxaphospho-rinane ring adopts a chair conformation, having the P=O bond equatorially oriented and the phen-oxy group axially oriented. The bromo substituent is in an axial position opposite to the phen-oxy group and the bromo-methyl group is in an equatorial position opposite to the P=O bond. In the crystal packing, mol-ecules are linked through weak C-H⋯O and C-H⋯Br inter-actions to form chains along the b axis. The chains are arranged into sheets parallel to the ab plane. In adjacent sheets, mol-ecules are arranged in an anti-parallel fashion. Inter-molecular C-H⋯π inter-actions are also observed.

2-Iodo-N-(6-methyl-2-pyrid-yl)benzamide.

The asymmetric unit of the title compound, C(13)H(11)IN(2)O, comprises two crystallographically independent mol-ecules. The dihedral angles between the ring planes are 53.56 (9) and 72.14 (19)° in the two molecules. Pairs of inter-molecular N-H⋯N hydrogen bonds and I⋯O inter-actions link neighbouring mol-ecules into two different pairs of dimers, those involving N-H⋯N interactions having R(2) (2)(8) ring motifs. Short inter-molecular I⋯O [3.1458 (15) Å] and I⋯N [3.4834 (16) Å] contacts are present. The crystal structure is further stabilized by inter-molecular C-H⋯π inter-actions [3.565 (2) and 3.629 (2) Å].

2-(1,3-Dithian-2-yl)-1,3-dithiane-2-carbaldehyde.

The asymmetric unit of the title compound, C(9)H(14)OS(4), comprises two crystallographically independent mol-ecules with similar conformations. In each mol-ecule, an intra-molecular C-H⋯O hydrogen bond generates a six-membered ring, producing an S(6) ring motif. All of the six-membered dithia-cyclo-hexane rings adopt chair conformations. The crystal structure is stabilized by four inter-molecular C-H⋯O and one C-H⋯S inter-action.

3,5-Dichloro-6-methyl-pyridin-2-amine.

In the title compound, C(6)H(6)Cl(2)N(2), intra-molecular N-H⋯Cl and C-H⋯Cl contacts generate five-membered rings, producing S(5) ring motifs. Pairs of inter-molecular N-H⋯N hydrogen bonds link neighbouring mol-ecules into dimers with R(2) (2)(8) ring motifs. In the crystal structure, these dimers are connected by N-H⋯Cl inter-actions and are packed into columns.