Three different fluorescent responses to transition metal ions using receptors based on 1,2-bis- and 1,2,4,5-tetrakis-(8-hydroxyquinolinoxymethyl)benzene

The dipod 1,2-bis(8-hydroxyquinolinoxymethyl)benzene (3) and tetrapod 1,2,4,5-tetrakis(8-hydroxyquinolinoxymethyl)benzene (5) have been synthesized through nucleophilic substitution of respective 1,2-bis(bromomethyl)benzene (2) and 1,2,4,5-tetra(bromomethyl)benzene (4) with 8-hydroxyquinoline (1). For comparison, 1,3,5-tris(8-hydroxyquinolinoxymethyl)benzene derivatives (7a and 7b) have been obtained. The complexation behavior of these podands towards Ag⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ metal ions has been investigated in acetonitrile by fluorescence spectroscopy. The sterically crowded 1,2,4,5-tetrapod 5 displays unique fluorescence ‘ON-OFF-ON’ switching through fluorescence quenching (λ_max 395 nm, switch OFF) with <1.0 equiv of Ag⁺ and fluorescence enhancement (λ_max 495 nm, switch ON) with >3 equiv Ag⁺ and can be used for estimation of two different concentrations of Ag⁺ at two different wavelengths. The addition of Cu²⁺, Ni²⁺, and Co²⁺ metal ions to tetrapod 5 causes fluorescence quenching, i.e., ‘ON-OFF’ phenomena at λ_max 395 nm for <10 μM (1 equiv) of these ions but addition of Zn²⁺ and Cd²⁺ to tetrapod 5 results in fluorescence enhancement with a gradual shift of λ_em from 395 to 432 and 418 nm, respectively. Similarly, dipod 3 behaves as an ‘ON-OFF-ON’ switch with Ag⁺, an ‘ON-OFF’ switch with Cu²⁺, and an ‘OFF-ON’ switch with Zn²⁺. The placement of quinolinoxymethyl groups at the 1,3,5-positions of benzene ring in tripod 7a-b leads to simultaneous fluorescence quenching at λ_max 380 nm and enhancement at λ_max 490 nm with both Ag⁺ and Cu²⁺. This behavior is in parallel with 8-methoxyquinoline 8. The rationalization of these results in terms of metal ion coordination and protonation of podands shows that 1,2 placement of quinoline units in tetrapod 5 and dipod 3 causes three different fluorescent responses, i.e., ‘ON-OFF-ON’, ‘ON-OFF’, and ‘OFF-ON’ due to metal ion coordination of different transition metal ions and 1, 3, and 5 placement of three quinolines in tripod 7, the protonation of quinolines is preferred over metal ion coordination. In general, the greater number of quinoline units coordinated per metal ion in 5 compared with the other podands points to organization of the four quinoline moieties around metal ions in the case of 5.     

A unique ‘ON-OFF-ON’ switch with two perturbations at two different concentrations of Ag

The dipod 1,2-bis(8-quinolinoxymethyl)benzene 3 and tetrapod 1,2,4,5-tetrakis(8-quinolinoxymethyl)benzene 4 show two perturbations in fluorescence with Ag⁺, (i) fluorescence quenching with <1.0 equiv of AgNO₃ at λ_max 395 nm and (ii) fluorescence enhancement at λ_max 500 nm with >3 equiv of AgNO₃. This ‘ON-OFF-ON’ switching of 3 and 4 in comparison with simultaneous fluorescence quenching and enhancement in the case of 8-methoxyquinoline 1 and the tripod 1,3,5-trimethyl-2,4,6-tris(8-quinolinoxymethyl)benzene 2 point to the unique role of molecular architectures arising due to the number and spatial positions of quinoline units in the fluorescence behaviour of an 8-alkoxyquinoline moiety towards Ag⁺.     

Synthesis and luminescent properties of Zn complex based on 8-hydroxyquinoline group containing 3,5-bis(trifluoromethyl) benzene unit with unique crystal structure

The use of a novel 2-substituted-8-hydroxyquinoline ligand (E)-2-{2-[3,5-bis (trifluoromethyl)phenyl]ethenyl}-8-hydroxyquinoline(4, BFHQ) characterized by EIMS, ¹H NMR spectroscopy, elemental analysis, and FTIR spectroscopy enabled the isolation of a trimeric Zn(II) complex with the formula Zn₃(BFHQ)₆ (5, Scheme 1). X-ray structural analysis shows that 5 exhibits a trinuclear core, which is bridged by four 8-hydroxyquinoline rings. The trinuclear core is surrounded by three pairs of BFHQ ligands with offset π-π stacking, showing propeller-like molecular structure. The aggregation behavior of Zn(AcO)₂ and the ligand 4 in solution was investigated by UV-vis. The luminescence properties of compound 5 were investigated by UV-vis and fluorescence spectra at room temperature. The experimental results show that the complex 5 emits yellow luminescence at 553 nm (λ_{em, max}) in DMSO solution and at 610 nm (λ_em, max) in solid state. The thermogravimetric analysis was carried out to examine the thermal stability.     

Cyclam‐Cored Dendrimers Appended with Four Dendrons of Two Different Types: Intradendrimer Energy Transfer

We have synthesized two cyclam‐cored dendrimers appended with dendrons of two different types by proper protection/deprotection of the cyclam unit. The resulting dendrimers contain six naphthyl and two dansyl units ( N6 D2 ) or two dansyl and six naphthyl units ( N2 D6 ) at the periphery. Their photophysical properties have been compared to those of a dendrimer containing 8 dansyl units ( D8 ) and a previously investigated dendrimer containing 8 naphthyl units ( N8 ). The absorption spectra are those expected on the basis of the number of chromophores, demonstrating that no ground state interaction takes place. The emission spectra of N2 D6 and N6 D2 show naphthalene localized and naphthalene excimer emission similar to those observed in the case of N8 , together with a much stronger dansyl emission with maximum at 525 nm. Addition of CF₃SO₃H to dendrimer solutions in CH₃CN/CH₂Cl₂ 1:1 (v/v) leads to protonation of the aliphatic amine units of the cyclam core at first and then of the aromatic amine of each dansyl chromophores. Cyclam can be diprotonated and this affects dansyl absorption and, most significantly, emission bands by a charge perturbation effect. Each dansyl unit is independently protonated in both dendrimers. The most interesting photophysical feature of these heterofunctionalized cyclam‐cored dendrimers is the occurrence of an intradendrimer photoinduced energy transfer from naphthyl to dansyl chromophores of two different dendrons (interdendron mechanism). The efficiency of this process is 50 % for N6 D2 and it can be increased up to 75 % upon protonation of the cyclam core and formation of N6 D2 (2H⁺). This arises from the fact that protonation of the amine units of the cyclam prevents formation of exciplexes upon naphthyl excitation, thus shutting down one of the deactivation processes of the fluorescent naphthyl excited state.     

New star-shaped triarylamines: synthesis, mesomorphic behavior, and photophysical properties

A total of 18 compounds 1-6 derived from triphenylamine as core group were prepared and characterized, and their mesomorphic properties were also investigated. Compounds 1-4 and 5,6 were prepared from p,p′,p″-triformyltriphenylamine and p,p′-diformyltriphenylamine with appropriate alkoxyphenylamines. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry, polarized optical microscopy, and powder XRD diffraction. Compounds 1-3 exhibited columnar mesophase, however, compounds 4-6 were nonmesogenic. The mesophases observed in compounds 1-3 were found to be side dependent. Compounds 1a, 2a, and 3a appended with one, two, or three side chains exhibited lamellar columnar (Col_L) phases, and compounds 2b and 3b with four or six side chains formed hexagonal columnar (Col_h) phases. The formation of the mesophases, lamellar or columnar mesophases, was probably induced by H-bonding formed between -CH₂NH groups. The oxidation process determined by cyclic voltammetry showed two redox waves, one appeared at 220-255 mV and the other one at 503-677 mV, which gave energy to HOMOs range of 5.02-5.36 eV. The fluorescent properties of the compounds were examined. All λ_max peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 307-392 nm and 368-456 nm, respectively. Compound 4a has a larger red shift due to a better conjugation linked by CC double bonds instead of -CH₂NH in other compounds.     

Syntheses, crystal structures and photoluminescence properties of three novel organically bonded indium selenates or selenites

Three new organically bonded indium(III) selenates or selenites, namely, In₂(2,2′-bipy)₂(SeO₄)₃(H₂O)₂·2H₂O (1), In₂(2,2′-bipy)₂(SeO₄)(C₂O₄)₂·0.5H₂O (2) and In₂(2,2′-bipy)₂(SeO₃)₂(C₂O₄) (3) (2,2′-bipy=2,2′-bipyridine) have been synthesized by hydro/solvothermal reactions by using the in situ ligand synthesis technique (2 and 3) and structurally characterized. The structure of compound 1 features a dimer composed of two InO₄N₂ octahedra bridged by three SeO₄ tetrahedra via cornering sharing. The structure of compound 2 shows a 2D inorganic-organic hybrid layer in which the indium (III) ions are bridged by both selenate and oxalate anions, whereas the structure of compound 3 possesses a 2D inorganic-organic hybrid layer composed of the indium (III) ions interconnected by both selenite and oxalate anions. The adjacent layers are further interconnected via π…π interactions between the bipy ligands. Compound 1 displays an intra-ligand luminescent emission band at λ_max=360 nm whereas compounds 2 and 3 both exhibit intra-ligand luminescent emission bands at λ_max=389 nm.     

Polymer-supported calix[4]arenes for sensing and conversion of NO2/N2O4

The use of simple calix[4]arenes 1a,b for NO₂/N₂O₄ sensing and conversion is demonstrated, both in solution and in the solid state. Upon reacting with these gases, compounds 1a,b encapsulate reactive NO⁺ cations within their cavities with the formation of deeply colored (λ_max∼570 nm) charge-transfer complexes 2a,b. Further functionalization of the calix[4]arene platform is reported for attachment to solid supports. Polymer-supported calixarene material 3 was prepared, which reversibly traps NO₂/N₂O₄ with the formation of nitrosonium storing polymer 4. Material 4 was effectively used for nitrosation of amides.     

Diamido/diamine cyclam-based zirconium and hafnium complexes: Synthesis and characterization

The synthesis and characterization of amido-amine cyclam based metal complexes is described. A novel tetraazamacrocycle ligand precursor (Li₂[1,8-Bn₂-1,4,8,11-tetraazacyclotetradecane], Li₂Bn₂cyclam, 2) is reported. Reactions of 2 with MCl₄(THF)₂ afforded M(Bn₂cyclam)Cl₂ (M = Zr 3, Hf 4). The two complexes show trigonal prismatic metal coordination geometries in the solid-state molecular structures. The cross-bridged cyclam 1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-H₂cyclam 5) was used to prepare the lithiated ligand precursor (CB-Li₂cyclam 6) and (CB-(Me₃Si)₂cyclam 7). M(CB-cyclam)Cl₂ (M = Zr 8, Hf 9) were synthesized from reactions of MCl₄(THF)₂ with 6. The structures of 3 and 4 are compared with those of zirconium and hafnium complexes derived from cyclam and unsaturated tetraazamacrocyclic ligands.     

Synthesis, characterization and ion transportation studies of some novel cyclophane amides

Various novel cyclophane amides with a large cavity have been synthesized. The structures of cyclophane amides 14 and 15 were resolved using XRD studies. Cyclophane amide 28 shows a shift in λ_max in the UV/Vis. spectra when treated with Cu (II) ion as well as with Pb (II) ion. Ion transportation studies were carried out with cyclophane amide 14 which proved that the Na⁺ ion passes through the cavity while K⁺ ions are retained.     

Synthesis and photoresponsive study of azobenzene centered polyamidoamine dendrimers

A new series of novel polyamidoamine (PAMAM) dendrimers 4, 5 and 6 possessing azobenzene units specifically at the core were prepared and their reversible trans/cis photoisomerization properties were studied. PAMAM dendritic wedges as well as azo-based PAMAM dendrimers were fully characterized by means of FT-IR, NMR (¹H and ¹³C), mass spectrometry (MALDI-MS), thermogravimetric and elemental analysis.     

Hydrogen bonding interaction of CpCo(Dithiolene) complex with monocyclic 2-pyridonyl substituent and unexpected formation of dithiolene-fused tricyclic pyridone derivative

One-pot reaction of [CpCo(CO)₂], elemental sulfur with some heterocycle-substituted alkynes (R-CC-HET) produced [CpCo(dithiolene)] complexes with ²PyOBn (2), with both ²PyOBn and 2-hydroxy-2-propyl groups (C(OH)Me₂) (5), both ²Py and C(OH)Me₂ (8), both ⁴Py and C(OH)Me₂ (11), and with ⁴Py substituent (13). A deprotection of benzyl group (Bn) from 2 with trimethylsilyl iodide formed [CpCo(dithiolene)] with 2-pyridonyl substituent (3). Heating reaction of 8 without any base resulted in the C(OH)Me₂ group elimination to form the 2-pyridylethylenedithiolate complex (9), but 11 underwent only dehydration at the C(OH)Me₂ under heating. While the preparation of 5, the benzyl free complex (6) was obtained as a main product. 6 has a dithiolene-fused tricyclic pyridone skeleton. The structures of 3, 5, 6, 8, and 11 were determined by X-ray diffraction studies. Intramolecular OH?N(²Py) hydrogen bondings are found in 5 and 8, and an intermolecular OH?N(⁴Py) one is found in 11 at solid state. In the 2-pyridonyl complex 3, intermolecular NH?O and CH(dithiolene)?O hydrogen bondings are observed. 8 showed an intermolecular Cp?Cp face-to-face interaction. The tricyclic complex 6 exhibited lower energy electronic absorption (λ_max = 668 nm) compared with the others (λ_max = 562-614 nm), due to an extended π-conjugation of aromatic cobaltadithiolene ring.     

Synthesis and photophysical properties of novel push-pull-type solvatochromic 7-deaza-2′-deoxypurine nucleosides

We have synthesized two novel push-pull-type fluorescent 7-deazapurine nucleosides, ^CNZA and ^CNZG, and investigated their photophysical properties. In particular, ^CNZA was found to exhibit a remarkable solvatofluorochromicity (Δλ_fl.max = 60 nm). We incorporated ^CNZA into oligonucleotides and found that ^CNZA can form a stable base pair with both thymine and cytosine. Such environmentally sensitive fluorescent nucleosides have a potential as a fluorescence sensor for structural studies of nucleic acids.     

Synthesis of the first representative of dicarbothiacyanine dyes with completely fluorinated polymethine chain

2-Iodobenzothiazole was reacted with tributyl(pentafluorobutadienyl)tin in the Stille reaction conditions to give 2-pentafluorobutadienylbenzothiazole 1. The quaternary salt of 1 via interaction with 2-fluoromethylbenzothiazole methylene base, obtained in situ, forms two cyanine dyes 8 and 9 as a result of nucleophilic attack of two different positions of perfluorobutadiene chain. The pure 2-fluoromethylbenzothiazole methylene base (in a dimer form) was obtained by deprotonation of the corresponding salt by NaH and on reacting with 1 forms the dye base 11 that underwent electrocyclization and subsequent HF addition to form the cyclohexadiene 12 identified by X-ray analysis data. Upon quaternization and HF abstraction by toluidine the recyclization occurred and dye 8 - the first representative of dicarbothiacyanine dyes with perfluorinated polymethine chain - was obtained. It has λ_max = 691 nm, that is 41 nm more than in non fluorinated dye, vinylene shift is equal to 111 nm.     

Direct observation and kinetic characterization of o-quinodimethane and its radical cation variant generated in a photoinduced electron-transfer reaction of 1,2-bis(α-styryl)benzene

Nanosecond time-resolved absorption spectroscopy on the laser flash photolysis of 1,2-bis(α-styryl)benzene (1) under N-methylquinolinium tetrafluoroborate-toluene-sensitized conditions in acetonitrile confirmed that an o-quinodimethane radical cation (2⁺, λ_max = 569 nm) decayed and the corresponding neutral prototype (2, λ_max = 444 nm) rose with rate constants of 5.6 and 5.9 × 10⁵ s⁻¹, respectively, showing the first agreement in kinetics between a reactive radical cation intermediate intervening in chemical reaction and the corresponding neutral species formed by back electron transfer.     

Unsymmetric 1,3,4-oxa(thia)diazoles of quinoxaline–naphthalene conjugates

Two new series of unsymmetric 1,3,4-oxa(thia)diazoles 1a,b containing both quinoxaline and naphthalene moieties were prepared and their mesomorphic properties were investigated. The mesomorphic behavior of compounds 1a,b and 2 was studied by DSC analysis and polarized optical microscopy. All compounds 1a and 2 exhibited hexagonal columnar phases (Col_h), which were also confirmed by powder XRD diffractometer. N_cell and R_ar values equal to 5.23 and 22.73 Å² within a slice of 9.0 Å thick were also obtained for 1a (n=16), indicating that a more disc-like structure constructed by two molecules lying side-by-side was correlated in Col_h phases. In contrast, all compounds 1b were not mesogenic, and the lack of mesomorphic properties in 1b might be due to their unfavorable conformations. The PL spectra of all compounds 1a,b showed one intense peak at λ_max=509–512 nm, and these photoluminescent emissions originated from quinoxaline moiety.     

Selective turn-on fluorescence for Zn and Zn + Cd metal ions by single Schiff base chemosensor

Chemosensor based on Schiff base molecules (1, 2) were synthesized and demonstrated the selective fluoro/colorimetric sensing of multiple metal ions (Mn²⁺, Zn²⁺ and Cd²⁺) in acetonitrile–aqueous solution. Both 1 and 2 showed a highly selective naked-eye detectable colorimetric change for Mn²⁺ ions at 10⁻⁷ M. Fluorescence sensing studies of 1 and 2 exhibited a strong fluorescence enhancement (36 fold) selectively upon addition of Zn²⁺ (10⁻⁷ M, λ_max = 488 nm). Fluorescence titration and single crystal X-ray analysis confirmed the formation of 1:1 molecular coordination complex between 1 and Zn²⁺. Interestingly, a rare phenomenon of strong second turn-on fluorescence (190 fold, λ_max = 466 nm) was observed by the addition of Cd²⁺ (10⁻⁷ M) into 1 + Zn²⁺ or Zn²⁺ (10⁻⁷ M) into 1 + Cd²⁺. Importantly both 1 and 2 exhibited different fluorescence λ_max with clearly distinguishable color for both Zn²⁺ and Cd²⁺.     

Photoactive building blocks for coordination complexes: Gilding 2,2′:6′,2″-terpyridine

The alkyne unit of 4′-ethynyl-2,2′:6′,2″-terpyridine has been functionalized with Ph₃PAu, (2-tolyl)₃PAu or Au(dppe)Au units to produce compounds 1-3, respectively. These derivatives have been characterized by electrospray mass spectrometry, solution ¹H and ¹³C NMR, UV-Vis and emission spectroscopies, and single crystal X-ray diffraction. In the solid state, molecules of 1 or 2 pack with separated domains of tpy and R₃PAu units; the tpy units in 2 (but not 1) exhibit face-to-face π-stacking. Compound 3 crystallizes as 2(3)^.CHCl₃, and the folded conformation of the dppe backbone results in a short (2.9470(8) Å) aurophilic interaction. Folded molecule 3 captures CHCl₃, preventing intramolecular face-to-face π-interactions between the tpy units. In CH₂Cl₂ solution, 1-3 are emissive when excited between 230 and 300 nm, but over minutes when λ_ex = 230 nm, the emission bands decay as the compounds photodegrade.     

Synthesis of oligophenylenes containing hydroxyl group and their solvatochromic behavior

Oligo(p-phenylene)s containing hydroxyl group(s), namely, OPP(n)-OH (n=3, 4, and 5; n denotes the number of benzene rings), HO-OPP(3)-OH, and 1,3,5-tri(4-biphenyl)phenol TBP-OH were synthesized in high yields by the Suzuki coupling reaction. Absorption maxima (λ_maxs) of OPP(n)-OHs shifted progressively toward long wavelengths due to the expansion of the π-conjugation system with an increase in the number of benzene rings. Deprotonation of the OH group of OPP(n)-OHs by treatment with NaH caused a bathochromic shift of λ_max. The bathochromic shift of the deprotonated species increased with the donor numbers (DNs) of the solvents. The emission peak positions of OPP(n)-OH and OPP(n)-ONa depended on the DN of the solvents; that is, the emission color could be tuned by changing the solvent.     

Single stranded helical supramolecular architecture with a left handed helical water chain in ternary copper(II) tryptophan/diamine complexes

The ternary copper(II) complexes [Cu(l-trp)(bpy)](ClO₄) (1) and [Cu(l-trp)(phen)] (ClO₄) · 3H₂O (2) (where l-trp = l-tryptophan, bpy = bipridyl, phen = phenanthroline) have been synthesized. The single crystal X-ray structures for these complexes revealed that the monocationic Cu^II-units are interlinked through Cu–OCO–Cu connectivity and exist as helical coordination polymers. The two different helical strands composed with Cu1 and Cu2 independently, possess a similar pitch distance of 7.713 Å in complex 1. For complex 2, existing in the hydrated form, the Cu(II) polymeric strand and the hydrated water molecules have gained a supramolecular helical architecture with a similar pitch distance of 8.133 Å. The two helical strands in complex 1 are associated with right handed (PP) supramolecular chirality, while the helical water chain and the Cu^II-strand in 2 are self assembled into left handed (MM) helicity in the solid state. The solid state CD recorded for 1 and the dehydrated form of 2 exhibit a positive optical sign at their respective d–d band [λ_max = 667 nm, 1; λ_max = 630 nm, 2], the solution state CD for both these complexes are found to be inverted into a negative optical sign, which could be attributed to inversion of their associated supramolecular helicity. The TGA curve illustrates two distinct weight losses at 60 °C and 87 °C, equivalent to one and two water molecules, respectively. The PXRD pattern for the hydrated and dehydrated forms of 2 indicated a change, on comparison with the simulated diffractograph. The fluorescence properties of both these complexes, possessing tryptophan and bipy/phen, were investigated.