Mangalampalli Ravikanth

Boron-dipyrromethene based reversible and reusable selective chemosensor for fluoride detection.

We synthesized benzimidazole substituted boron-dipyrromethene 1 (BODIPY 1) by treating 3,5-diformyl BODIPY 2 with o-phenylenediamine under mild acid catalyzed conditions and characterized by using various spectroscopic techniques. The X-ray structure analysis revealed that the benzimidazole NH group is involved in intramolecular hydrogen bonding with fluoride atoms which resulted in a coplanar geometry between BODIPY and benzimidazole moiety. The presence of benzimidazole moiety at 3-position of BODIPY siginificantly altered the electronic properties, which is clearly evident in bathochromic shifts of absorption and fluorescence bands, improved quantum yields, increased lifetimes compared to BODIPY 2. The anion binding studies indicated that BODIPY 1 showed remarkable selectivity and specificity toward F(-) ion over other anions. Addition of F(-) ion to BODIPY 1 resulted in quenching of fluorescence accompanied by a visual detectable color change from fluorescent pink to nonfluorescent blue. The recognition mechanism is attributed to a fluoride-triggered disruption of the hydrogen bonding between BODIPY and benzimidazole moieties leading to (i) noncoplanar geometry between BODIPY and benzimidazole units and (ii) operation of photoinduced electron transfer (PET) from benzimidazole moiety to BODIPY unit causing quenching of fluorescence. Interestingly, when we titrated the nonfluorescent blue 1-F(-) solution with TFA resulted in a significant enhancement of fluorescence intensity (15-fold) because the PET quenching is prevented due to protonation of benzimidazole group. Furthermore, the reversibility and reusability of sensor 1 for the detection of F(-) ion was tested for six cycles indicating the sensor 1 is stable and can be used in reversible manner.

Smaragdyrins: Emeralds of Expanded Porphyrin Family

Porphyrins are tetrapyrrolic 18 π electron conjugated macrocycles with wide applications that range from materials to medicine. Expanded porphyrins, synthetic analogues of porphyrins that contain more than 18 π electrons in the conjugated pathway, have an increased number of pyrroles or other heterocyles or multiple meso-carbon bridges. The expanded porphyrins have attracted tremendous attention because of unique features such as anion binding or transport that are not present in porphyrins. Expanded porphyrins exhibit wide applications that include their use in the coordination of large metal ions, as contrasting agents in magnetic resonance imaging (MRI), as sensitizers for photodynamic therapy (PDT) and as materials for nonlinear optical (NLO) studies. Pentaphyrin 1, sapphyrin 2, and smaragdyrin 3 are expanded porphyrins that include five pyrroles or heterocyclic rings. They differ from each other in the number of bridging carbons and direct bonds that connect the five heterocyclic rings. Sapphyrins were the first stable expanded porphyrins reported in the literature and remain one of the most extensively studied macrocycles. The strategies used to synthesize sapphyrins are well established, and these macrocycles are versatile anion binding agents. They possess rich porphyrin-like coordination chemistry and have been used in diverse applications. This Account reviews developments in smaragdyrin chemistry. Although smaragdyrins were discovered at the same time as sapphyrins, the chemistry of smaragdyrins remained underdeveloped because of synthetic difficulties and their comparative instability. Earlier efforts resulted in the isolation of stable β-substituted smaragdyrins and meso-aryl isosmaragdyrins. Recently, researchers have synthesized stable meso-aryl smaragdyrins by [3 + 2] oxidative coupling reactions. These results have stimulated renewed research interest in the exploration of these compounds for anion and cation binding, energy transfer, fluorescent sensors, and their NLO properties. Recently reported results on smaragdyrin macrocycles have set the stage for further synthetic studies to produce stable meso-aryl smaragdyrins with different inner cores to study their properties and potential for various applications.

3,5-Diformylboron dipyrromethenes as fluorescent pH sensors.

A series of boron dipyrromethene (BODIPY) dyes containing two aldehyde functional groups at the 3 and 5 positions have been synthesized in low-to-decent yields in two steps. In the first step, the meso-aryl dipyrromethanes were treated with POCl(3) in N,N-dimethylformamide to afford 1,9-diformylated dipyrromethanes. In the second step, the diformylated dipyrromethanes were first in situ oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and then reacted with BF(3)·OEt(2) to afford 3,5-diformylboron dipyrromethenes. The X-ray structural analysis indicated that the aldehyde groups are involved in intramolecular hydrogen bonding with fluoride atoms, which may be responsible for the stability of the diformylated BODIPY compounds. The presence of two formyl groups significantly alters the electronic properties, which is clearly evident in downfield shifts in the (1)H and (19)F NMR spectra, bathochromic shifts in the absorption and fluorescence spectra, better quantum yields, and increased lifetimes compared to 3,5-unsubstituted BODIPYs. Furthermore, 3,5-diformylboron dipyrromethenes are highly electron-deficient and undergo facile reductions compared to unsubstituted BODIPYs. These compounds exhibit pH-dependent on/off fluorescence and thus act as fluorescent pH sensors.

Synthesis of sterically crowded polyarylated boron-dipyrromethenes.

A rapid synthetic route for polyarylated boron-dipyrromethenes using hexabromo boron-dipyrromethene as the key synthon is described. The X-ray structure revealed that the polyarylated BODIPY adopts a propeller-like conformation. These compounds exhibit red-shifted absorption and fluorescence bands with decent quantum yields and reversible oxidation and reduction waves when compared to unsubstituted boron-dipyrromethenes.

Sensing Hg(II) in Vitro and in Vivo Using a Benzimidazole Substituted BODIPY

A multisignaling Hg(II) sensor based on a benzimidazole substituted BODIPY framework was designed, which displays excellent selectively toward Hg(II) in vitro and in vivo. Optical and fluorogenic measurements in solution reveal that the sensor can detect mercury ions at submicromolar concentrations, with high specificity. The detection of Hg(II) is associated with a blue-shift in optical spectra and a simultaneous increase in the fluorescence quantum yield of the sensor, which is attributed to a decrease in charge delocalization and inhibition of photoinduced electron transfer upon binding to Hg(II). Using several spectroscopic measurements, it is shown that the binding mechanism involves two sensor molecules, where lone pairs of the benzimidazole nitrogen coordinate to a single mercury ion. The utility of this BODIPY sensor to detect Hg(II) in vivo was demonstrated by fluorescence imaging and spectroscopy of labeled human breast adenocarcinoma cells. While average emission intensity of the sensor over a large number of cells increases with incubated mercury concentrations, spatially resolved fluorescence spectroscopy performed on individual cells reveals clear spectral blue-shifts from a subensemble of sensors, corroborating the detection of Hg(II). Interestingly, the emission spectra at various submicrometer locations within cells exhibited considerable inhomogeneity in the extent of blue-shift, which demonstrates the potential of this sensor to monitor the local (effective) concentration of mercury ions within various subcellular environments.

Boron complexes of oxasmaragdyrin, a core-modified expanded porphyrin.

We report the synthesis of first examples of BF(2) and B(OR)(2) complexes of oxasmaragdyrin, the expanded core-modified porphyrin, in decent yields under very simple reaction conditions at room temperature. The boron complexation of oxasmaragdyrin alters the electronic properties of the macrocycle significantly as evident by various spectroscopic techniques. Our preliminary studies indicated that the B(OH)(2)-smaragdyrin complex can act as a selective neutral fluoride ion sensor.

Synthesis of 3,5-Bis(acrylaldehyde) Boron-dipyrromethene and Application in Detection of Cysteine and Homocysteine in Living Cells

Synthesis, characterization, and spectral and electrochemical properties of 3,5-bis(acrylaldehyde) BODIPY are described. The compound exhibited higher selectivity toward cysteine/homocysteine than toward other amino acids and thiol-containing compounds as shown by absorption and emission titration experiments and by experiments in living cells.

Cyclotriphosphazene Ring as a Platform for Multiporphyrin Assemblies

A simple method has been employed to synthesize a cyclotriphosphazene appended with six porphyrins. The reaction of one equivalent of hexachlorocyclotriphosphazene with six equivalents of 5-(4-hydroxyphenyl)-10,15,20-tri(p-tolyl)porphyrin or -21-thiaporphyrin in tetrahydrofuran in the presence of cesium carbonate, followed by simple column-chromatography purification, afforded the hexaporphyrinato cyclotriphosphazene systems as the single products in 80-90 % yields. The metal(II) derivatives (Cu(II), Zn(II)) of the hexaporphyrinato cyclotriphosphazene systems were prepared by treating the hexaporphyrin assemblies with the appropriate metal salts under standard metallation conditions. The compounds are freely soluble in common organic solvents and were characterized by mass spectrometry, NMR, absorption, and fluorescence spectroscopy, and electrochemical techniques. Material studies carried out by using fluorescence microscopy, SEM, and AFM techniques showed that the hexaporphyrinato cyclotriphosphazene systems form ringlike architectures. Preliminary biological studies indicate that the hexacopper(II) porphyrin assembly can be used as an artificial nuclease.

Explosive vapor sensor using poly (3-hexylthiophene) and CuII tetraphenylporphyrin composite based organic field effect transistors

Organic field effect transistors based on poly(3-hexylthiophene) and CuII tetraphenylporphyrin composite were investigated as sensors for detection of vapors of nitrobased explosive compounds, viz., 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), 2,4,6-trinitrotoluene (TNT), and dinitrobenzene, which are also strong oxidizing agents. Significant changes, suitable for sensor response, were observed in transistor “on” current (Ion) and conductance (S) after exposure. A similar device response was, however, not observed for oxidizing agents such as benzoquinone and benzophenone. The Fourier transform infrared spectrometry experiments supported the results, where exposure to RDX and TNT vapors resulted in a significant shift in IR peaks.

Synthesis of BF2 Complexes of Prodigiosin Type Oligopyrroles

We developed a simple, facile route for the synthesis of BF(2) complexes of prodigiosin type oligopyrroles and their cholesterol conjugates. This route gives an access to synthesize any desired meso-aryl-substituted 3-pyrrolyl BODIPYs which were not easily accessible earlier.