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Tavarekere K Chandrashekar

Publications and source records attributed to Tavarekere K Chandrashekar.

At least 19 recordsLinked to original sources
PubMed2007

meso-meso-Linked corrole dimers with modified cores: synthesis, characterization, and properties.

The synthesis and characterization of the first examples of core-modified corrole dimers linked through the meso positions are described. The dimers are obtained by a simple Ag(I)- or Fe(III)-catalyzed coupling reaction in near-quantitative yields. The corroles obtained are metalated with metal ions, such as Cu(II) and Ni(II). The electronic-absorption spectral studies reveal weak electronic interaction between the two subunits and the exciton coupling observed for the free-base corrole dimer (1717 cm(-1)) is lower than that for the corresponding protonated derivative (4081 cm(-1)). The solution-state structure derived from (1)H and two-dimensional NMR spectral studies reveals a noncoplanar arrangement of two corrole units. Geometry optimization at the B3LYP-631G level also confirms the noncoplanar arrangement of corroles with a dihedral angle of 64.8 degrees between two corrole planes. The electron paramagnetic resonance (EPR) and magnetic characterization studies on the paramagnetic copper-metalated dimer indicate that both copper ions behave as independent spins without any noticeable interaction. Results of fluorescence studies reveal a bathochromic shift of about 60 nm upon dimerization. The first hyperpolarizability (beta) measured by using the hyper Rayleigh scattering (HRS) method reveals doubling of the beta values on progressing from monomer to dimer, attributed to enhanced pi conjugation. The use of copper dimers in the photocleavage of DNA is also explored. It is shown that the bimetallic copper dimer selectively cleaves the nucleic acids without affecting the proteins, suggesting a possible application of the copper complex in the removal of nucleic acid contaminants from protein extracts through a simple photolytic pathway.

Catalysis↗
PubMed2006

22pi smaragdyrin molecular conjugates with aromatic phenylacetylenes and ferrocenes: Syntheses, electrochemical, and photonic properties.

Syntheses, spectroscopic, electrochemical, and third-order nonlinear optical susceptibilities of a series of 22pi smaragdyrins and their corresponding Rh(I) derivatives bearing phenylacetylene substituents and ferrocene-containing substituents are reported. The synthetic strategy involved a [3 + 2] acid-catalyzed oxidative coupling reaction of the appropriate dipyrromethane and oxatripyrrane. The desired meso substituents, such as phenylacetylenylphenyl and the ferrocenes, were incorporated to the dipyrromethane unit prior to the oxidative coupling reaction. The optical absorption, emission characteristics, and the quantum yield of the smaragdyrin conjugates depends on the nature of the substituent, nature of linker group, and the spacer length. Theoretical studies at the DFT level suggest high delocalization of electrons confined to only four of the five available heterocyclic rings for the free bases. However, upon Rh(I) metalation, the pi-electron delocalization is extended to all the heterocyclic rings. The two-photon absorption cross section (TPA) values sigma(2) measured through the open aperture Z-scan method, increases linearly with enhanced pi-electron delocalization for the smaragdyrins containing phenylacetylene substituents. The meta branching of substituents decreases sigma(2) values. Introduction of Rh(I) to the smaragdyrin cavity enhances the sigma(2) values by about 3-10 orders of magnitude, attributed to the increased aromatic character upon Rh(I) insertion. The calculated molecular electrostatic potential (MESP) and harmonic oscillator model of aromaticity (HOMA) for the free bases and the Rh(I) derivatives justifies such a conclusion. A linear correlation observed for the second oxidation potential of Rh(I) derivatives and corresponding free bases also support the increased aromaticity upon Rh(I) insertion. The electrochemical data for ferrocene-containing smaragdyrins reveal easier ring oxidation by about 50-130 mV and harder ferrocene oxidation by 40-180 mV suggesting electron-donating nature of the ferrocene upon linking with the smaragdyrin system. The TPA cross section value of 88782 GM observed for 5g represents one of the highest values known for a metalloexpanded porphyrin derivative.

Acetylene↗
PubMed2005

Modified expanded corrole-ferrocene conjugates: syntheses, structure and properties.

Syntheses, characterization and properties of expanded corrole-ferrocene conjugates are reported. Ferrocenyl group are covalently linked to the corrole macrocycle through three different spacers groups. The synthetic strategy involved prior insertion of ferrocene with spacers to the dipyrromethane unit followed by a "3+2" acid-catalyzed oxidative coupling methodology. The optical and emission data of the expanded corrole-ferrocene conjugates depend on the nature and length of the spacer groups and the maximum effects are seen where ferrocene is directly linked to the meso carbon of macrocycle. The single crystal X-ray structure of two expanded corrole-ferrocene conjugates; [22]pentaphyrin (1.1.0.1.0) with different meso substituents, clearly reveal shortening of the C-C bond length linking the meso carbon and the aryl substituent containing the ferrocene moiety relative to meso aryl substituents without ferrocene. The results suggest that an electronic interaction between the two pi systems. Electrochemical data reveal harder oxidation for the ferrocene unit in the conjugates relative to free ferrocene; this suggests the electron donating nature of the ferrocene. The first corrole ring oxidation shows easier oxidation relative to 1 and the magnitude of shifts in potential is inversely proportional to the length of spacer. The molecular first hyperpolarizabilities (beta) measured at 1064 nm by HRS method vary in the range 20-32x10(-30) esu and imply that the beta values can be increased by enhancing the number of mobile electrons in the conjugation. The conjugates form 1:1 metal complex with the Rh(I) where rhodium is coordinated to one amino and one imino nitrogen of the dipyrromethane unit.

Crystallography, X-Ray↗
PubMed2005

Core-modified expanded porphyrins with large third-order nonlinear optical response.

The third-order nonlinear optical response through measurement of two-photon absorption cross-sections (TPACS) for aromatic core-modified expanded porphyrin analogues by a femtosecond open aperture Z-scan method is reported. The values reported here are among the largest known so far in the literature for any organic molecules.

Molecular Structure↗
PubMed2004

Syntheses of core-modified corroles by three different [3 + 1] methodologies.

Three new methods for syntheses of modified oxa corroles bearing one meso free carbon in reasonably good yields are reported. The formation of the meso carbon bridge and the direct pyrrole-pyrrole linkage occur in a single step by a simple condensation and coupling with TFA as a catalyst with appropriate precursors. The reactions are optimized with different conditions by varying the meso substituents, acid catalyst concentration, and the nature of the solvent to afford corroles in good yields.

Journal Article↗
PubMed2004

Oxasmaragdyrin-ferrocene and oxacorrole-ferrocene conjugates: synthesis, structure, and nonlinear optical properties.

Ferrocenyl macrocyclic conjugates involving 22 pi oxasmaragdyrins and 18 pi oxacorroles have been synthesized and characterized. The direct covalent linkage of the ferrocenyl moiety to the meso position of the macrocycle is achieved by simple oxidative coupling of appropriate precursors with trifluoroacetic acid as catalyst. The electronic coupling between the ferrocenyl moiety and the macrocyclic pi system is apparent from: a) the red shifts (293-718 cm(-1)) of the Soret and Q-bands in the electronic absorption spectra of ferrocenyl conjugates; b) the shift of oxidation potentials (50-130 mV) of both the ferrocene and the corrole rings to the positive potentials; and c) considerable shortening of the C-C bond which connects the ferrocene and the meso-carbon atom of the macrocycle. The single-crystal X-ray structure of oxasmaragdyrin-ferrocene conjugate 9 reveals the planarity of the 22 pi skeleton with very small deviations of the meso-carbon atoms. The meso-ferrocenyl substituent has a small dihedral angle of 38 degrees, making way for mixing of the molecular orbitals of the ferrocene and the macrocycle. However, the other two meso substituents are almost perpendicular to the mean plane, defined by the three meso carbon atoms. Classical C-H...O and nonclassical C-H...pi interactions lead to a two-dimensional supramolecular network. Ferrocene-smaragdyrin conjugate 9 bonds to a chloride ion in the protonated form and a rhodium(i) ion in the free base form. Nonlinear optical measurements reveal a larger nonlinear refractive index (-5.83 x 10(-8)cm(2)W(-1)) and figure of merit (2.28 x 10(-8)cm(3)W(-1)) for the rhodium smaragdyrin-ferrocene conjugate 19 than for the others, suggesting its possible application in optical devices.

Journal Article↗
PubMed2003

meso-substituted aromatic 34pi core-modified octaphyrins: syntheses, characterization and anion binding properties.

Modified octaphyrins with 34pi electrons have been synthesized and characterized following a simple synthetic methodology. An acid-catalyzed alpha,alpha coupling of tetrapyrranes containing furan, thiophene and selenophene rings resulted in the formation of the respective octaphyrins in relatively good yield. Solution studies by (1)H NMR and 2D NMR methods and single crystal Xray structural characterization reveal an almost flat structure with two heterocyclic rings inverted. Specifically, in 14 two selenophene rings (one on each biselenophene unit) are inverted while in 15 two furan rings (one on each bifuran unit) are inverted when the meso substituent are mesityl groups. On changing the mesityl substituent to m-xylyl group as in 19, the location of ring inversion shifts to pyrrole rings (one on each bipyrrole unit) indicating the dependence of structure on the meso substituents. UV/Vis studies, both in freebase and protonated forms reveal typical porphyrinic character and the aromatic nature of the octaphyrins. The Deltadelta values evaluated by (1)H NMR spectroscopy also support their aromatic nature. The protonated forms of octaphyrins bind TFA anion in a 1:2 ratio. The TFA anions are located one above and below the plane of the octaphyrin macrocycle and they are held by weak electrostatic NH-O interactions similar to that observed for protonated rubyrins. However, in the present case, there is an additional non-electrostatic CH-O interaction involving beta-CH of the inverted heterocyclic ring and the carbonyl oxygen of the TFA. Furthermore, inter molecular interactions between the Cbond;H of the meso-mesityl group and the fluorine of CF(3) group of bound TFA leads to the formation of one-dimensional supramolecular arrays with interplanar distance of 13 A between two octaphyrins.

Journal Article↗
PubMed2003

Core-modified expanded porphyrins: new generation organic materials.

Even though the first expanded porphyrin was reported in the mid-1960s, the advances in its chemistry are more recent. New and powerful synthetic methods have facilitated the availability in affordable quantities of expanded porphyrins in general and core modified systems in particular. This has stimulated interest in studying various properties pertaining to their potential applications in biomedicine and materials chemistry. In this Account, we not only summarize the details of the synthetic methodologies reported, but we also highlight studies that focus on the structural diversity, aromaticity, and anion and cation binding abilities of expanded porphyrins.

Journal Article↗
PubMed2002

30pi aromatic meso-substituted heptaphyrin isomers: syntheses, characterization, and spectroscopic studies.

The syntheses of new aromatic 30pi heptaphyrins either through a [5 + 2] or a [4 + 3] acid-catalyzed condensation and oxidative coupling reactions of easily available and air-stable precursors are reported. The methodology followed is not only simple and efficient but also allows synthesis of a range of heptaphyrins with different heteroatoms in the core. The oxidative coupling reactions of modified tripyrranes 11 and tetrapyrranes 12 were found to be dependent on the acid concentration used and as well as the substituents present on the meso position. The change of meso aryl substituents in 11 and 12 to meso mesityl substituents gave a new heptaphyrin 18. The structural characterization has been done with extensive 1H and 2D NMR studies. The heptaphyrins reported here show rich structural diversity when the connections of the heterocyclic rings are altered, and accordingly, one ring and two ring inversions have been observed. By a judicious choice of the precursors it has been possible to control the site of ring inversion either in the bithiophene unit or in the tripyrrane unit. Theoretical calculations performed on three different heptaphyrins, 4, 5, and 17, also reveal that the inverted structures are approximately 35-40 kJ lower in energy relative to the corresponding noninverted structures. Furthermore, one of the heptaphyrins 10c shows the presence of two conformers in solution in the ratio 1:2 and no interconversion between the conformers have been observed in the temperature range of 343-228 K. On protonation, the aromaticity and the ring inversions are retained and the deltadelta values vary in the range 10.07-20.59 ppm. The energies of the Soret maxima and the HOMO-LUMO gap vary linearly with the increase in pi electrons further justifying the aromatic nature of the heptaphyrins.

Journal Article↗