Novel thiocyanate complexes of cadmium(II) - Synthesis, spectroscopic characterization, X-ray studies and DFT calculations

Three novel thiocyanate cadmium(II) compounds [Cd(ind)₂(SCN)₂]_n (1), [Cd(dpa)Cl(SCN)]_n (2) and [Cd(terpy)(SCN)₂] (3) were synthesized and characterized by spectroscopic and crystallographic methods. Thiocyanate ions of 1 act as bridging ligands generating a one-dimensional chain constructed of Cd(SCN)₂Cd units and expanding along the crystallographic direction [1 0 0]. The Cd(II) ions of 2 are bridged by alternating di-μ-Cl and di-μ_1,3-NCS bridges to one-dimensional chain running along the crystallographic direction [0 1 0]. [Cd(terpy)(SCN)₂] (3) is a square pyramidal mononuclear compound.The fluorescence properties of the complexes 1 and 2 were examined in solid state, whereas fluorescence spectra of 3 were recorded in both solid state and solution. All they were compared with the fluorescence properties of the free ligands. Additionally, the electronic spectrum of 3 were investigated at the TDDFT level employing B3LYP functional in combination with LANL2DZ.     

F-Induced ‘turn-on’ fluorescent chemosensor based on 1,3-alt thiacalix[4]arene

A new thiacalix[4]arene based fluorescent sensor 1 bearing two naphthyl groups has been synthesized in 1,3-alternate conformation. In the absence of fluoride ion, the receptor 1 is in ‘off-state’ showing no fluorescence emission. The presence of fluoride ion triggered the fluorescence emission to ‘on-state’. The receptor shows pronounced selectivity for fluoride ions. In THF, the presence of F⁻ ions induces the formation of a 1:1 (G:H) complex.     

Hexabromide salt of a tiny octaazacryptand as a receptor for encapsulation of lower homolog halides: structural evidence on halide selectivity inside the tiny cage

Tiny azacryptand 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (L) upon reaction with 48% hydrobromic acid (containing <0.05% chloride contamination) forms hexabromide salt (1). Single crystal X-ray crystallographic investigation of the hexaprotonated bromide (1) shows no guest encapsulation inside the tiny cage. This bromide salt 1 with an empty proton cage has been utilized as the receptor for encapsulation of chloride (2) and fluoride (3). Crystallographic results of mixed chloride/bromide (2) and fluoride/bromide (3) complexes of L are examined, which show monotopic recognition of chloride in the case of 2 and fluoride in the case of 3 inside the proton cage with five bromide and three water molecules outside the cavity. Single crystals obtained from an experiment on mixed anionic system (chloride and fluoride), 1 shows selective encapsulation of fluoride, which supports the formation of complex 3 and crystals obtained upon treatment of 2 with tetrabutyl ammonium fluoride also yields complex 3. In a separate reaction between L and 49% hydrobromic acid containing higher chloride contamination (<0.2%) forms chloride/bromide salt (2). ¹H NMR studies of 1 with sodium chloride and fluoride support the encapsulation of the respective anions inside the proton cage.     

Bimodal fluorescence signaling based on control of the excited-state conformational twisting and the ground-state protonation processes

Amino-based fluoroionophores 1 and 2 can selectively sense alkaline earth metal ions in MeCN under both neutral and acidic conditions by different signaling mechanisms. The fluoroionophoric behavior for the neutral probes is characterized by an ‘off-on’ photoinduced electron transfer (PET)-like fluorescence intensity response due to a switching from a twisted internal charge transfer (TICT) to a planar internal charge transfer (PICT) state. For the protonated probes (i.e., 1/H⁺ and 2/H⁺), the fluorescing species is the localized stilbene fluorophores, but dual fluorescence is induced upon metal-ion recognition through a deprotonation process.     

A novel chromatism switcher with double receptors selectively for Ag in neutral aqueous solution: 4,5-diaminoalkeneamino-N-alkyl-l,8-naphthalimides

Two novel fluorosensors of 4,5-disubstituted-N-alkyl-l,8-naphthalimide derivatives (H1, H2, H3) with double ethylenediamino receptors, double propylenediamino receptors, or one methylpiperazine receptor were synthesized, respectively. Their fluorescence and absorption in the presence or absence of nine metal ions were studied. In the presence of Ag⁺, H1's absorption moved to long wavelength with color change from yellow-green to red, its quenching and red shift in fluorescence were also remarkable. Similarly, H1's fluorescence was also strongly quenched in the presence of Cu²⁺. In addition, H1 and H2 show high pH sensitively. There was 139-folds fluorescence enhancement for H1, 22-folds for H2, and 4-folds for H3 when pH was changed from 8 to 3, respectively.     

Fluorescence emission control and switching of oxymethylcrowned spirobenzopyrans by metal ion

Oxymethylcrowned spirobenzopyran 1 and pyrenylspirobenzopyran 2 were synthesized, and fluorescence emission of their corresponding merocyanine form was examined in the presence of metal ions. For 2, fluorescence emission derived from the pyrene moiety was completely quenched by photoinduced electron transfer (PET) of the nitrogen atom when the merocyanine form was not produced, namely, without metal ions. However, when 2 was converted to the merocyanine form by the complexation of its crown ether with a metal ion, fluorescence resonance energy transfer (FRET) from the pyrene to the merocyanine moieties took place to produce fluorescence emission. This result demonstrates that the spirobenzopyran isomerization can function as a fluorescence emission switch. Fluorescence quantum yield measurement for 1 and 2 showed that fluorescence emission depends on the binding metal ion in which the fluorescence quantum yield generally increased with the increase of metal ion radius.     

A rigid conjugated pyridinylthiazole derivative and its nanoparticles for divalent copper fluorescent sensing in aqueous media

A rigid conjugated pyridinylthiazole derivative (1) and two bithiazole derivatives with similar structures (2, 3) were synthesized and characterized. Their optical properties were investigated through spectral analysis. By applying the three compounds to Cu²⁺ ions detection, it was shown that compound 1 could be employed as a selective and sensitive Cu²⁺ ions fluorescent chemosensor. For aqueous assay, the nanoparticles of compound 1 were prepared in aqueous media. Compared to the monomer, 1 nanoparticles were more fluorescence sensitive to Cu²⁺ ions. Its binding mode with Cu²⁺ ions was correlated well with Langmuir equation. Compound 1 nanoparticles exhibit a dynamic working range for Cu²⁺ ions from 0.02 to 0.50 μM with a detection limit of 10 nM. The proposed chemosensor has been used for the direct measurement of Cu²⁺ content in drinking water samples with satisfying results.     

Anthracene-based ortho-phenylenediamine clefts for sensing carboxylates

Two ortho-phenylenediamine-based new receptors 1 and 2 with an anthracene-coupled benzimidazolium motif have been designed and synthesized. The directed hydrogen bonds (both conventional and unconventional) and charge-charge interactions allowed the open clefts of both 1 and 2 to bind carboxylate, fluoride and dihydrogenphosphate anions with moderate binding constant values. The selectivity and sensitivity were ascertained by ¹H NMR, UV-vis and fluorescence spectroscopic methods. The binding cleft of 2 is found to be more effective than that of 1.     

Heterocyclic quinol-type fluorophores. Part 9: Effect of forming a continuous intermolecular hydrogen bonding chain between fluorophores on the solid-state fluorescence properties

Three heterocyclic quinol-type fluorophores with benzo[c]carbazol-6-one skeleton or benzo[b]naphtho[1,2-d]furan-6-one skeleton, 9-dibutylamino-5-hydroxy-5-phenyl-5,7-dihydro-benzo[c]carbazol-6-one (5a), 7-butyl-9-dibutylamino-5-hydroxy-5-phenyl-5,7-dihydro-benzo[c]carbazol-6-one (5b) and 9-dibutylamino-5-hydroxy-5-phenyl-5H-benzo[b]naphtho[1,2-d]furan-6-one (6c) have been synthesized and their photophysical properties have been investigated in solution and in the solid state. The fluorescence quantum yield (Φ) increases in the order of 5b (0.35)<5a (0.41)<6c (0.74) in 1,4-dioxane. On the other hand, the Φ value in the solid state increases in the order of 5a<<6C (0.03)<5b (0.07), which are much smaller than those in 1,4-dioxane. To elucidate the effects of molecular and crystal structures on the solid-state fluorescence properties, we have performed the semi-empirical molecular orbital calculations (AM1 and INDO/S) and X-ray crystallographic analysis. It was found that the formation of a continuous intermolecular hydrogen bonding between adjacent fluorophores is observed in the crystal of 5a, which is considered to cause a drastic fluorescence quenching in the solid state.     

Reaction of quinidine acetate, epiquinidine and its acetate in superacid: formation of gem-difluoro derivatives with or without rearrangement

In HF-SbF₅, quinidine 1a or its dihydrochloride cyclises previously obtained with usual acids. A similar reaction is observed in the presence of CCl₄. Under similar conditions quinidine acetate 1b and epiquinidine acetate 2b dihydrochlorides both yield 10,10-difluoro derivatives epimeric at C-3, 6 and 7, and 9c and 10b, and a rearranged difluoro derivative 8b and 11b, respectively. Epiquinidine 2a leads to the expected analogues 10a and 11a and to a ketone 9a. Formation of gem-difluoro compounds implies chloro intermediates at C-10, precursors of α-chlorocarbenium ions, which are trapped by a fluoride ion and which lead by halogen exchange to the products.     

Highly selective and sensitive fluorescence turn-on probe for a catalytic amount of Cu(I) ions in water through the click reaction

A simple azide-functionalized coumarin (1) was utilized as a fluorescence turn-on probe for a catalytic amount of Cu(I) ions in HEPES buffer. The probe has shown a selective and sensitive response to the cuprous ions over other various cations through a Cu(I)-mediated click reaction of 1 to an alkyne. When a catalytic amount of copper sulfates was added in the presence of ascorbate, the prominent fluorescence ‘Off-On’ change of 1 was observed so that submicromolar concentration of copper ions was detectable by the naked eye.     

Solid-state photocycloaddition of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones with benzophenone

Photocycloaddition reactions of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones (4a-c) with benzophenone (2a) by mixing in the solid state (solid solution) afforded the corresponding oxetane derivatives (5a-c; 1:2 adducts) with high site- and regioselectivity across the C5-C6 and C5′-C6′ double bonds in 4 via the triplet excited state of benzophenone. The oxetane formation proceeded more effectively in the solid state than in solution. The reaction mechanism was inferred by MO methods to be initiated by electrostatic interaction between the C6 position of 4a-c and the carbonyl oxygen of 2a in their ground states. The solid-state interaction may be enhanced by the electron density at the carbonyl oxygen of the triplet 2a. The transition state (TS) analysis of the [2+2] cycloaddition reactions also suggested some triplet complexes and high regioselectivity. The hydrogen-bonding interaction between 2a and 4a-c and the triplet reaction mechanism were also explained by the IR analyses and the quenching experiments, respectively.     

Synthesis of the anionic fluororeceptors based on thiourea and amide groups and recognition property for α,ω-dicarboxylate

Three new neutral receptors (1, 2 and 3) containing thiourea and amide groups were synthesized by simple steps in good yields. The binding properties for anions of 1, 2 and 3 were examined by UV-vis, fluorescence, and ¹H NMR spectroscopy. Receptors 1, 2 and 3 all had a better adipate anion selectivity by comparison with other dicarboxylate anions. The association constants of 1, 2 and 3 with adipate were higher as compared to other anions (malonate, succinate, glutarate). In particular, a distinct color change was observed from light yellow to orange-red upon addition of adipate to the solution of 1 in DMSO. The UV-vis and fluorescence data indicate that a 1:1 stoichiometry complex is formed between compound 1, 2 or 3 and dicarboxylate anions through hydrogen bonding interactions.     

Coumarine–imino–C2-glucosyl conjugate as receptor for Cu in blood serum milieu,on silica gel sheet and in Hep G2 cells and the characterization of the species of recognition

A coumarine–imino–C2-glucosyl conjugate (L) was synthesized and characterized. The conjugate L is found to recognize Cu²⁺ in aqueous HEPES buffer by exhibiting a 95% fluorescence quenching in pH range 7–10 even in the presence of several biologically and ecologically relevant metal ions. Fluorescence on–off behavior has been clearly demonstrated on the basis of the binding variability of Cu²⁺ to L. The binding has been elicited through the changes observed in fluorescence, absorption, ESI-MS and ¹H NMR titrations. All the other thirteen metal ions studied did not show any change in the fluorescence emission. These ions do not interfere with the recognition of Cu²⁺ by L. The structural features of [CuL]₂ complex in both the isomeric forms were established by DFT computational calculations. The utility of L has been demonstrated by showing its sensitivity toward Cu²⁺ on a thin layer of silica gel. The L gives sensitive fluorescence signals for Cu²⁺ even in blood serum and exhibits appropriate fluorescence responses in living cells.     

Experimental and computational investigations of cadmium(II) complexes incorporating 2-benzoylpyridine

Three novel cadmium(II) complexes of 2-benzoylpyridine [Cd(bopy)₂Cl₂] (1), {[Cd(bopy)(μ_1,1-N₃)₂]·bopy}_n (2) and [Cd(bopy)₂(SCN)₂] (3) were synthesized and characterized by spectroscopic and crystallographic methods. Azide ions of 2 act as bridging ligands generating a 1D chain running along [0 1 0]. The zig-zag chains consisting of {Cd(bopy)(N₃)₂} units are separated by 2-benzoylpyridine molecules. The fluorescence properties of the complexes 1, 2 and 3 were examined both in solid state and solution, and compared with the free ligand. Additionally, the electronic spectrum of 3 were investigated at the TDDFT level employing B3LYP functional in combination with LANL2DZ.     

Cu and CN-selective fluorogenic sensors based on pyrene-appended thiacalix[4]arenes

Two new fluorescent sensors 1 and 2 based on thiacalix[4]arenes bearing pyrene moieties have been synthesized in cone conformation. The binding abilities of these sensors towards different cations such as lithium, sodium, potassium, nickel, cadmium, copper, zinc, lead, silver, mercury and anions like fluoride, chloride, bromide, iodide, cyanide, acetate, hydrogen sulfate and nitrate have been examined by UV-vis and fluorescence spectroscopies. These receptors show pronounced selectivity for copper and cyanide ions. In CH₂Cl₂/CH₃CN (1:1), the presence of Cu(II) ion induces the formation of 1:1 (H/G) complex with receptor 1 and 1:2 (H/G) complex with receptor 2. The cyanide ions form a 1:1 (H/G) complex with both receptors.     

Synthesis and optical/thermal properties of low molecular mass V-shaped materials based on 2,3-dicyanopyrazine

A novel series of luminescent low molecular mass materials containing a 2,3-dicyanopyrazine central core were synthesized through an esterification reaction between diphenol 10 and different aromatic carboxylic acids 1-6, containing terminal long alkyl chains. They have a similar V-shaped geometry with lack of planarity between the two arms, confirmed by the X-ray structure of the central core. The optical and thermal properties of these compounds were evaluated. They show blue fluorescence in solution ( 440-480 nm) with quantum fluorescence yields (Φ_F) from 0.003 to 0.1 and Stokes shifts of around 90 nm. In solid state, optical band gaps (E_g) were from 3.14 to 3.32 eV. Thin films of 11, 13, and 14 exhibited blue fluorescence ( 430-456 nm), and 12, 15, and 16 (more bulky) displayed green fluorescence ( 488-512 nm). Most of the materials exhibited good thermal stability, exhibiting an amorphous glassy state after melting. Transparent amorphous films were easily obtained through spin coating and characterized by AFM analysis.     

Effective receptors for fluoride and acetate ions: synthesis and binding study of pyrrole- and cystine-based cyclopeptido-mimetics

Two conformationally constrained pseudo-cyclopeptides (1, 2) consisting of pyrrole-, pyridine-, and cystine-moieties were designed and synthesized as neutral receptors for anionic guests. The anion recognition abilities of these two receptors were examined photometrically in acetonitrile solution. The UV-vis study revealed that the [1+1] receptor (1) formed 1:1 complexes with anions, whereas the [2+2] receptor (2) led to 1:2 mode binding with anions. Both receptors displayed good affinity and selectivity for fluoride and acetate ions.     

Zinc selective chemosensor based on pyridyl-amide fluorescence

Chemosensors are developed to image zinc ions. Fluorescence enhancement due to Zn²⁺ binding is an excellent way to detect its presence. A chemosensor for Zn²⁺ based on dipicolylamine (DPA) groups connected by a pyridyl amide backbone has been synthesized. Addition of 2-chloroacetyl chloride to 2,6-diaminopyridine affords 2,6-bis(chloroethylamido)pyridine, which is converted to the sensor BADPA-P by 2,2′-dipicolylamine displacement of chlorine. This compound along with two others, the mono-DPA, ADPA-P and the benzyl in place of pyridyl, BADPA-B, present three potential Zn²⁺ sensors. It was found that BADPA-P in the presence of Zn²⁺ shows a large increase in fluorescence, whether in polar organic or aqueous environments. Its fluorescence in the presence of Cd²⁺, unlike with Zn²⁺, is not enhanced when excited at longer wavelengths. Proton NMR measurements, indicate two Zn²⁺ ions bind to BADPA-P. Also, Zn²⁺ enhances fluorescence even when other metal ions are present.     

Synthesis, structural characterization, and anion binding ability of sterically congested adamantane-calix[4]pyrroles and adamantane-calixphyrins

Novel adamantane derivatives of calix[4]pyrroles 4-8 and calixphyrin 9 have been synthesized. The structure of 8 has been characterized by X-ray powder diffraction and the structure of 9 by single crystal X-ray analysis. Whereas calixphyrin 9 does not bind anions, analogous calyx[4]pyrrole 8 forms a complex with Cl⁻ in the DMSO solution and in the solid state. The solid state complexation has been accomplished on grinding in a mill, which is the first example of complex formation with an anion in the solid state.