Benzobisoxazole cruciforms as fluorescent sensors.

CONSPECTUS: Cross-conjugated molecular cruciforms are intriguing platforms for optoelectronic applications. Their two intersecting π-conjugated arms allow independent modulation of the molecules' HOMO and LUMO levels and guarantee a well-defined optical response to analyte binding. In addition, the rigid cross-conjugated geometries of these molecules allow their organization in two- and three-dimensional space with long-range order, making them convenient precursors for the transition from solution-based to the more practical solid-state- and surface-based devices. Not surprisingly, a number of molecular cruciform classes have been explored because of these appealing properties. These include tetrakis(arylethynyl)benzenes, tetrastyrylbenzenes, distyrylbis(arylethynyl)benzenes, tetraalkynylethenes, biphenyl-based "swivel" cruciforms, and benzobisoxazole-based cruciforms. In this Account, we summarize our group's work on benzobisoxazole molecular cruciforms. The heterocyclic central core of these molecules forces their HOMOs to localize along the vertical bisethynylbenzene axis; the HOMO localization switches to the horizontal benzobisoxazole axis only in cases when that axis bears electron-rich 4-(N,N-dimethylamino)phenyl substituents and the vertical axis does not. In contrast, the LUMOs are generally delocalized across the entire molecule, and their localization occurs only in cruciforms with donor-acceptor substitution. Such spatially isolated frontier molecular orbitals (FMOs) of the benzobisoxazole cruciforms make their response to protonation very predictable. Benzobisoxazole cruciforms are highly solvatochromic, and their fluorescence quantum yields reach 80% in nonpolar solvents. Solutions of cruciforms in different solvents change emission colors upon addition of carboxylic and boronic acid analytes. These changes are highly sensitive to the analyte structure, and the emission color responses permit qualitative discrimination among structurally closely related species. In self-assembled complexes with boronic acids, benzobisoxazole fluorophores switch their analyte preferences and become responsive to Lewis basic species: phenoxides, amines, ureas, and small organic and inorganic anions. These sensing complexes allow the decoupling of the sensor's two functions: a nonfluorescent boronic acid does the chemistry through the exchange of its labile B-O bonds for other nucleophiles, and it can be optimized for solubility and analyte specificity; the benzobisoxazole fluorophore senses the electronic changes on the boron and reports them to the operator through changes in its emission colors, allowing this sensing element to be kept constant across a broad range of analytes. We have recently expanded our studies to benzimidazole-based "half-cruciforms", which are L-shaped rigid fluorophores that maintain most of the spatial separation of FMOs observed in benzobisoxazole cruciforms. Unlike benzobisoxazoles, benzimidazoles are acidic on account of their polar N-H bonds, and this feature allows them to respond to a broader range of pH values than their benzobisoxazole counterparts. The deprotonated benzimidazolate anions maintain their fluorescence, which makes them promising candidates for incorporation into solid-state sensing materials known as zeolithic imidazolate frameworks.

Basic apoptotic and necrotic cell death in human liver carcinoma (HepG2 ) cells induced by synthetic azamacrocycle.

Treatment of diseases with synthetic materials has been an aspiration of mankind since the dawn of human development. In this research, three complex compounds of azamacrocycle (TD1, TD2, and TD3) were synthesized, and experiments were conducted to determine whether their toxicity to human liver carcinoma (HepG2 ) cells is associated with apoptotic and/or necrotic cell death. Cell survival was determined by MTT assay. Apoptosis and necrosis were measured by annexin V FITC/PI assay using the flow cytometry and by propidium iodide (PI) assay using the cellometer vision. HepG2 cells were treated with different concentrations of azamacrocycles for 48 h. Results from MTT assay indicated that all the three azamacrocycles significantly (p < 0.05) reduce cell viability in a dose-dependent manner, showing 48 h-LD50 values of about 37.97, 33.60, and 19.29 µM, for TD3, TD1 and TD2, respectively. Among the three compounds tested, TD2 showed the most pronounced cytotoxic activity against HepG2 cells, being about twofold more potent than TD3. The order of toxicity was TD2 > TD1 > TD3. Because TD2 exerted the most cytotoxic activity against HepG2 cells, it was used in the subsequent apoptosis and necrosis-related experiments. The flow cytometry assessment showed a strong dose-response relationship with regard to TD2 exposure and annexin V/PI positive cells. PI assay data indicated that TD2 exposure increased the proportion of fluorescence positive cells. Overall, our results indicate that azamacrocycle toxicity to HepG2 cells is associated with apoptotic and necrotic cell death resulting from phosphatidylserine externalization and loss of membrane integrity.

A self-assembled fluoride-water cyclic cluster of [F(H2O)]4(4-) in a molecular box.

We present an unprecedented fluoride-water cyclic cluster of [F(H(2)O)](4)(4-) assembled in a cuboid molecular box formed by two large macrocycles. Structural characterization reveals that [F(H(2)O)](4)(4-) is assembled by strong H-bonding interactions [OH···F = 2.684(3)-2.724(3) Å], where a fluoride anion plays the topological role of a water molecule in the classical cyclic water octamer. The interaction of fluoride was further confirmed by (19)F NMR and (1)H NMR spectroscopies, indicating the encapsulation of the anionic species within the cavity in solution. High-level DFT calculations and Bader topological analyses fully support the crystallographic results, demonstrating that the bonding arrangement in the fluoride-water cluster arises from the unique geometry of the host.

Phosphate binding with a thiophene-based azamacrocycle in water.

Structural characterization of the phosphate complex with a thiophene-based macrocycle suggests that two dihydrogen phosphates in a dimeric form are encapsulated in the cavity via several hydrogen bonds from NH···O and CH···O interactions. In the lattice framework, the two dimers are linearly hydrogen-bonded to form a tetramer. 1H NMR titrations suggest that the host forms a 1:1 complex with phosphate, showing an association constant of 120 M-1 in D2O at pH = 5.5. The host guest complexation was further confirmed by ESI-MS in a gas phase.

Anion recognition and sensing by a new macrocyclic dinuclear copper(II) complex: a selective receptor for iodide.

A macrocyclic dinuclear copper complex, [Cu(2)(II)(1)Br(3)(H(2)O)]Br, has been synthesized and characterized by X-ray crystallography, in which the macrocycle is folded to form a bowl-shaped cavity. The sensing ability of the receptor has been studied for halides by UV-vis spectroscopy in water-acetonitrile (1:3, v/v) and water. The results indicate that the new receptor exhibits a strong affinity and selectivity for iodide.

Unusual complexes of trapped methanol with azacryptands.

Structural analysis of sulfate complexes of two different azacryptands crystallized under identical conditions in the presence of methanol and water reveals that one methanol is selectively trapped in each cavity, assisted by the specific arrangements of three external sulfates close to one tren unit.

Fluorescent detection of phosphate anion by a highly selective chemosensor in water.

A macrocyclic dinuclear copper complex, [Cu(2) (II)(1)Br(4)]·2H(2)O has been synthesized and characterized by X-ray crystallography, in which each metal ion is pentacoordinated in a square pyramidal environment and the macrocycle is folded to form a boat-shaped empty cavity. As studied by an indicator displacement assay, the dinuclear complex shows strong selectivity for phosphate over sulfate, nitrate, perchlorate and halides in water at physiological pH.

Cooperative NH⋯O and CH⋯O interactions for sulfate encapsulation in a thiophene-based macrocycle.

A thiophene-based macrocycle containing four secondary and two tertiary amines has been synthesized and its binding affinity has been investigated toward sulfate anion in solution and solid states. Structural analysis of the sulfate salt suggests that the ligand in its hexaprotonated form, is capable of encapsulating one sulfate within the cavity through cooperative NH⋯O and CH⋯O interactions. As investigated by (1)HNMR titrations, the lignad forms a 1:1 complex with sulfate in water at pH 2.1, showing a binding constant (K) of 3200 M(-1). The formation of the complex has been further confirmed by ESI-MS, indicating that the complex can exist in solution with a considerable stability.

3,3'-Di-tert-butyl-1,1'-[1,3-phenyl-enebis(methyl-ene)]diurea.

The title compound, C(18)H(30)N(4)O(2), contains two tert-butyl urea groups, each connected to a benzene ring though a methyl-ene group. One of the groups occupies a position almost normal to the aromatic plane with a C-N-C-C torsion angle of -94.4 (4)°, while the other is considerably twisted from the ring with a C-N-C-C torsion angle of -136.1 (4)°. In the crystal, pairs of mol-ecules are connected to each other, forming centrosymmetric dimers in which two NH groups of one mol-ecule act as hydrogen-bond donors to one carbonyl O atom of the other mol-ecule. The dimers are linked into sheets parallel to (100) by N-H⋯O hydrogen bonds involving the remaining N-H and C=O groups.

1,3-Phenyl-enediammonium dinitrate.

In the title compound, C(6)H(10)N(2) (2+)·2NO(3) (-), the dication lies on a crystallographic twofold rotation axis. The nitrate ions are linked to the dications though N-H⋯O hydrogen bonds, forming a three-dimensional network.

3,6,9,16,19,22-Hexaazatricyclo-[22.2.2.2]triaconta-1(27),11 (30),-12,14(29),24(28),25-hexaene hexakis(p-toluenesulfonate) dihydrate.

In the title compound, C(24)H(44)N(6) (6+).6C(7)H(7)O(3)S(-).2H(2)O, the macrocycle crystallizes in its hexaprotonated form, accompanied by six p-toluenesulfonate ions and two water molecules, and lies on an inversion center. The three independent p-toluenesulfonate anions and their inversion equivalents at (1 - x, 1 - y, 1 - z) are linked to the macrocyclic cation through N-Hcdots, three dots, centeredO hydrogen bonds. Of these, two p-toluenesulfonate ions are located on opposite sides of the macrocyclic plane and are linked to bridgehead N atoms via N-Hcdots, three dots, centeredO hydrogen bonds. The remaining four p-toluenesulfonate ions bridge two adjacent macrocyclic cationic units through N-Hcdots, three dots, centeredO hydrogen bonding involving other N atoms, forming a chain along the a axis. The water molecules, which could not be located and may be disordered, do not interact with the macrocycle; however, they form hydrogen bonds with anions.

Experimental and Theoretical Aspects of Anion Complexes with a Thiophene-Based Cryptand.

Selective recognition of anions has received a tremendous attention in recent years because of their significant importance in biology and environment. This article highlights our recent research on a thiophene-based azacryptand that has been shown to effectively bind anions including iodide, bromide, chloride, nitrate and sulfate. Structural studies indicate that the ligand forms inclusion complexes with chloride and iodide. On the other hand, it forms cleft-like complexes with nitrate and sulfate, where three anions are bound between the cyclic arms. The ligand binds each anion with a 1:1 binding mode in water, exhibiting strong selectivity for sulfate; which is further supported by ESI-MS and DFT calculations.

Alkyne-substituted diminazene as G-quadruplex binders with anticancer activities.

G-quadruplex ligands have been touted as potential anticancer agents, however, none of the reported G-quadruplex-interactive small molecules have gone past phase II clinical trials. Recently it was revealed that diminazene (berenil, DMZ) actually binds to G-quadruplexes 1000 times better than DNA duplexes, with dissociation constants approaching 1 nM. DMZ however does not have strong anticancer activities. In this paper, using a panel of biophysical tools, including NMR, FRET melting assay and FRET competition assay, we discovered that monoamidine analogues of DMZ bearing alkyne substitutes selectively bind to G-quadruplexes. The lead DMZ analogues were shown to be able to target c-MYC G-quadruplex both in vitro and in vivo. Alkyne DMZ analogues display respectable anticancer activities (single digit micromolar GI50) against ovarian (OVCAR-3), prostate (PC-3) and triple negative breast (MDA-MB-231) cancer cell lines and represent interesting new leads to develop anticancer agents.

Anion Cluster: Assembly of Dihydrogen Phosphates for the Formation of a Cyclic Anion Octamer.

Structural characterization of a dihydrogen phosphate complex of triprotonated tris[2-(2-thienylmethylamino)ethyl] amine shows that eight dihydrogen phosphate anions are assembled around the host by strong interactions of H-bond donors and acceptors to form a new type of cyclic anion octamer as (H(2)PO(4) (-))(8), an analogy of cyclic water octamer. The presence of an anion cluster has also been identified by electrospray ionization mass spectrometry and (31)P NMR experiments.

Self-assembly of ordered water tetramers in an encapsulated [Br(H2O)12]- complex.

An unanticipated anion-water cluster is assembled by one bromide and three highly-ordered "water tetramers" within the cavity of a receptor, providing a perfect C(3) symmetric propeller-shaped bromide-water cluster of [Br(H(2)O)(12)](-).

Application of capillary electrophoresis in anion binding studies: Complexation and separation of nitrate and nitrite by an azacryptand.

Capillary electrophoresis (CE) was employed for studying the complexation of an azacryptand with nitrate and nitrite in aqueous solution. CE separation of a mixture of nitrate and nitrite with 10 mM acetate buffer (pH 3.3) showed two peaks at the retention times of 2.8 and 3.1 min for nitrate and nitrite, respectively. However, when the ligand (2 mM) was added to the running buffer, the peaks emerged in the reverse order and at shorter retention times of 2.7 and 2.5 min for nitrate and nitrite, respectively. The longer retention time for nitrate compared with nitrite indicates a stronger complex formation between the ligand and nitrate, that reduces the migration speed of nitrate as compared with the less strongly bound nitrite. The (1)H NMR titrations of L with these two anions at the pH 3.3, gave the binding constants (log K), 3.75 and 4.23, for nitrite and nitrate, respectively which were in consistence with the results obtained from the CE method.

A C Symmetric Nitrate Complex with a Thiophene-Based Tripodal Receptor.

A thiophene-based tripodal receptor has been synthesized and its complexes with nitrate and iodide have determined by single-crystal X-ray analysis. In the nitrate complex, one nitrate is encapsulated in a selective orientation forming a C(3) symmetric complex, which is bonded to three protonated secondary amines with six NH···O bonds. The anion is coordinated in a plane perpendicular to the principal rotation axis passing through the tertiary nitrogen of the receptor and the nitrogen of the encapsulated nitrate. High-level DFT calculations support the crystallographic results demonstrating that an adduct with trigonal binding of three oxygen atoms is more stable than that of one oxygen atom of the encapsulate nitrate. On the other hand, in the structure of the iodide complex, all three iodides lie outside the cavity. (1)H NMR titration studies indicate that the receptor forms a 1:1 complex with nitrate with a binding constant of K = 315 M(-1) in chloroform, showing a moderate selectivity over halides and perchlorate.

Unusual bridging of three nitrates with two bridgehead protons in an octaprotonated azacryptand.

Structural analysis of the nitrate complex of a thiophene-based azacryptand suggests that three nitrates are bridged with two bridgehead protons which play the topological role of two transition metal ions in a classical Werner type coordination complex bridging three anions.

An anthracene-based tripodal chemosensor for anion sensing.

An anthracene-based tripodal ligand was synthesized from the condensation of tren with 9-anthraldehyde, and the subsequent reduction with sodium borohydride. The neutral ligand was protonated from the reaction with p-toluenesulfonic acid to give a triply charged chemosensor that was examined for its anion binding ability toward fluoride, chloride, bromide, sulfate and nitrate by the fluorescence spectroscopy in DMSO. The addition of an anion to the ligand resulted in an enhancement in fluorescence intensity at the excitation of 310 nm. Analysis of the spectral changes suggested that the ligand formed a 1:1 complex with each of the anions, showing strong affinity for fluoride and sulfate in DMSO. The unsubstituted tren was reacted with sulfuric acid to form a sulfate complex and the structure was determined by the X-ray crystallography. Analysis of the complex revealed that three sulfates are held between two ligands by multiple hydrogen bonding interactions with protonated amines.