Gollapalli R. Deviprasad

Electrocatalytic reduction of molecular oxygen using non-planar cobalt tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin

Abstract Synthesis of a new water soluble porphyrin, cobalt(II) tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin (Br8TPPS)Co and catalytic reduction of molecular oxygen to hydrogen peroxide using this porphyrin is described. The investigated porphyrin is highly non-planar and water soluble due to the presence of eight bromides on the β-pyrrole positions and four negative charges of the sulfonatophenyl groups on the meso positions. The first reversible reduction of (Br8TPPS)CoII is diffusion controlled one-electron transfer, located at E 1 2 = − 0.59 V vs. Ag|AgCl in a pH 8.9 solution and leads to the formation of a stable [(Br8TPPS)CoI]−. The synthesized cobalt(II) porphyrin binds dioxygen and catalytically reduces it to peroxide with an over voltage for dioxygen reduction of nearly 250 mV. Cyclic and rotating ring-disc voltammetric techniques are used to examine the mechanistic aspects of the catalytic process.

Self-assembled supramolecular triad composed of fulleropyrrolidine bearing two pyridine moieties axially coordinated to two zinc porphyrins

Spectroscopic, redox, and photochemical behavior of a self-assembled donor-acceptor triad formed by axial coordination of zinc meso-tetraphenylporphyrin, Zn(TPP) with fulleropyrrolidine bearing two pyridine moieties (C60(Py)2) was investigated. The UV-visible and 1H NMR spectral studies revealed supramolecular 1:2 triad formation between C60(Py)2 and Zn(TPP). The determined overall formation constant, K was as large as 1.45 × 104mol-2 in o-dichlorobenzene (o-DCB). The 1H NMR studies revealed axial coordination of both the pyridine entities of C60(Py)2 to the central metal ion of the zinc porphyrins. Electrochemical studies of the isolated self-assembled triad in o-DCB established the molecular stoichiometry of the supramolecular complex possessing one C60(Py)2 and two Zn(TPP) entities. The excited state electron transfer reactions were monitored by both steady-state emission as well as transient absorption techniques. In o-DCB, upon coordination of the pyridine entities of C60(Py)2 to two Zn(TPP), the main quenching pathway involved charge-separation from the singlet excited Zn(TPP) to the C60 moiety. However, in a coordinating solvent like PhCN, intermolecular electron-transfer takes place from the triplet excited state of Zn(TPP) to the C60 moiety.

Electrochemistry of Solutions as well as Simultaneous Cyclic Voltammetry and Piezoelectric Microgravimetry of Conducting Films of 2‐(n‐Alkyl)fulleropyrrolidines

Effects of alkyl chain length and protonation of the pyrrolidine nitrogen on the electrochemical behavior of 2-(n-alkyl)fulleropyrrolidines, C 60 pyr-C m (m = 4, 6, 8, 10, and 12), in solutions as well as in thin solid films are reported. Formal redox potentials of the first and second one-electron reversible electroreductions of C 60 pyr-C m in 0.1 mol dm -3 tetra(n-butyl)ammonium hexafluorophosphate in benzonitrile are shifted negatively by ca. 130 mV and that of the third electroreduction by ca. 240 mV with respect to the potentials of the corresponding C 60 redox couples. Protonation of the pyrrolidine nitrogen results in a positive shift of the formal redox potential of C 60 pyr-C m in solution by ca. 90 mV, indicating more pronounced electron deficiency of the C 60 cage. Simultaneous cyclic voltammetry and piezoelectric microgravimetry of the drop-coated thin solid films of C 60 pyr-C m in acetonitrile solutions reveals that the film solubility is larger the longer the alkyl chain and the more reduced the C 60 cage. Also, film stability with respect to dissolution is strongly dependent on the nature of the cation of the supporting electrolyte and acidity of solution. That is, the adduct films are more stable in the TBA + than Li + solutions, similar to that of pristine C 60 films. Moreover, electroreduction of the adduct films in acidified Li + solutions results in electrochemically inactive films.

Molecular recognition directed porphyrin chemosensor for selective detection of nicotine and cotinine

The first example of a metalloporphyrin based fluorescent chemosensor for selective detection of dinitrogen alkaloids such as nicotine and cotinine in solution by using a ‘two-point’ binding strategy and a modified fluorescence analysis procedure is reported.

Structural studies of a non-covalently linked porphyrin–fullerene dyad

X-ray structural and ab initio theoretical investigations of a non-covalently linked meso-tetraphenylporphyrinatozinc(II) and N-methyl-2-pyrid-4′-yl)-3,4-fullero-pyrrolidine complex is reported

Electrochemical and spectroelectrochemical­characterization of water-soluble, β-pyrrole-­brominated cobalt porphyrins

The electrochemical and spectroelectrochemical characterization of two water-soluble porphyrins, namely the tetrachloro salt of cobalt(II) tetrakis-(N-methyl pyridyl)-β-octabromoporphyrin, [(Br8TMPyP)CoII]Cl4, and the tetrasodium salt of cobalt(II) tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin, Na4[(Br8TPPS)CoII], under different pH conditions is reported. The investigated porphyrins are highly non-planar and electron-deficient owing to the presence of eight bromides at the β-pyrrole positions and the four meso-aryl substituents. The redox potentials corresponding to the first oxidation and first reduction are shifted positively compared with the redox potentials of the respective unbrominated porphyrin derivatives. Spectroelectrochemical studies reveal the formation of a cobalt(III) complex during the first oxidation of both cobalt porphyrins. The first reduction of Br8TPPS)CoII]4− results in a cobalt(I) complex, while involvement of the peripheral N-methyl pyridyl groups in the case of Br8TMPyP)CoII]4+ is suggested. The peak potentials of the first oxidation corresponding to a CoII/CoIII redox couple of both investigated compounds are found to be pH-dependent. Debromination of the β-pyrrole bromo substituents of Br8TMPyP)CoII]4+ at more negative potentials is observed. The UV-vis spectrum obtained after bulk electrolysis of Br8TMPyP)CoII]4+ at −1.0 V vs Ag/AgCl followed by reoxidation at 0.2 V indicates complete elimination of the bromo substituents of the porphyrin periphery.

Four-electron electrocatalytic reduction of dioxygen to water by an ion-pair cobalt porphyrin dimer adsorbed on a glassy carbon electrode

An ion-pair porphyrin dimer, formed by reacting the tetrachloro salt of [tetrakis(N-methylpyridyl)porphyrinato]- cobalt and the tetrasodium salt of [tetrakis(4-sulfonatophenyl)porphyrinato]cobalt, adsorbed electrochemically on a glassy carbon electrode is shown to catalytically reduce O2 to water by four electrons.

Crystal structure and intermolecular coordination behavior of a porphyrin bearing a hydroquinone receptor, meso-mono(hydroquinonyl) triphenylporphyrinatozinc

The structure of a porphyrin bearing a hydroquinone receptor unit, meso-mono(hydroquinonyl)triphenylporphyrinatozinc (1), is reported. The compound crystallizes from benzene in the monoclinic space group P21/c(#14) with a = 10.896(2) Å, b = 15.674(4) Å, c = 12.564(3) Å, β = 102.04(2)° and dcalc= 1.368 g/cm3 for Z = 2. The unit cell has two porphyrins and four benzene solvates. The structural study reveals no self-assembled aggregates due to intermolecular coordination of the hydroxy groups of the hydroquinone to the zinc ion in the porphyrin cavity, an observation that agrees with the results of 1H NMR studies performed in benzene-d6 solutions. On the contrary, the 1H NMR results in CDCl3 indicate the existence of self-assembled aggregates.

N-(2,4-Dinitrophenyl)-2-phenylfulleropyrrolidine: an electroactive organofullerene dyad

Using Sanger’s method, an electroactive organofullerene dyad, N-(2,4-dinitrophenyl)-2-phenylfulleropyrrolidine, is synthesized and its spectral and electrochemical characterization is reported.