• V Krishnan

      Articles written in Journal of Chemical Sciences

    • Schiff bases of nickel(II), copper(II) porphyrins and dibenzo-18-crown-6 interspersedbis-metal porphyrins. Protonation studies

      G Bhaskar Maiya V Krishnan

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      Schiff-base (SB) derivatives of Ni(II) and Cu(II) porphyrins endowed with various amine functions (R−NH2),n-butylamine,p-anisidine andm-nitroaniline have been prepared from corresponding formyl porphyrins. Protonation studies of these SB derivatives reveal a marked red shift of the optical absorption bands in the visible region relative to the unprotonated imines. The magnitude of the observed red shifts in the protonated derivatives, (SBH+) are found to depend on the electron-withdrawing or electron-donating nature of the R group of the amines. The results of the optical absorption,1H NMR, EPR, and cyclicovoltammetric studies are illustrative of the fact that protonation of the SB derivatives results in a localized positive charge,$$P - C(H) = \bar N(H) - R$$ in the periphery of the porphyrin (p) system. The dibenzo-18-crown-6 interspersedbisporphyrin schiff bases have been prepared fromtrans 4,4′-diamino dibenzo-18-crown-6 and formyl porphyrins. The protonation of these SB derivatives is found to proceed in a concerted fashion. The cation complexation studies by the crown ether entity in thebisporphyrin systems have been investigated using optical absorption, magnetic resonance and electrochemical methods. The redox characteristics of the protonated dimeric SB porphyrins reveal that the first oxidation step involves a two-electron transfer reaction. This is important in view of their possible usage in multielectron transfer reactions of biological and catalytic interest.

    • Singlet quenching of tetraphenylporphyrin and its metal derivatives by iron(III) coordination compounds

      Francis D'souza V Krishnan

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      The quenching of fluorescence of the free-base tetraphenylporphyrin, H2TPP, and its metal derivatives, MgTPP and ZnTPP by diverse iron(III) complexes, [Fe(CN)6]3−, Fe(acac)3, [Fe(mnt)2], Fe(Salen)Cl, [Fe4S4(SPh)4]2−·, FeTPPCl and [Fe(Cp)2]+ has been studied both in homogeneous medium (CH3CN) and micellar media, SDS., CTAB and Triton X-100. The quenching efficiencies are analysed in terms of diffusional encounters and it has been possible to separate static quenching components. The quenching constants are dependent on the nature of the ligating atoms around iron(III) and also on the extent of π-conjugation of the ligands. The quenching mechanism has been investigated using steady-state irradiation experiments. Evidence for oxidative quenching by iron(III) complexes was obtained, though the spin multiplicities of the excited electronic states of iron(III) complexes permit both energy and electron transfer mechanisms for quenching of the singlet excited state of the porphyrins.

    • Editor’s note

      V Krishnan

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    • Molecular structure of electron donor-acceptor complexes of metallotetraphenylporphyrins with trinitrobenzene

      M Padmanabhan M Damodara Poojari V Krishnan

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      The metallotetraphenylporphyrins, MTPPs, where M=Co(II), Cu(II) and Ag(II) form one to one molecular complexes in solution with 1,3,5-trinitrobenzene (TNB). The crystal structure of CoTPP.TNB.2CH3OH revealed that the mean separation between the porphyrin and TNB planes is 3·27 Å and the centre of the aryl ring of TNB is displaced by 0·90 Å from the centre of the porphyrin plane. Extended Huckel molecular orbital calculations suggest that acceptor orbitals are predominantly nitro-group based rather than an aryl π framework. The contribution of the metal orbitals in the donation is also seen as predicted by the structural data. Electrochemical redox measurements in solution provide evidence for charge-transfer stabilisation in these complexes.

    • Editor’s note

      V Krishnan

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    • Axial ligation properties ofβ-substituted tetraphenylporphyrin-metal derivatives

      Avijit Sen V Krishnan

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    • Foreword

      V Krishnan

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    • Isomers of β-substituted di and tri-nitrotetraphenylporphyrins and their copper(II) derivatives: Structure, optical and electrochemical redox properties

      S Dahal V Krishnan M Nethaji

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      Controlled nitration ofmeso-tetraphenylporphyrinatocopper(II) using fuming nitric acid leads to the formation of different β-substituted nitro derivatives. Five isomers of dinitro and three isomers of trinitrotetraphenylporphyrins have been isolated and characterised using FAB/MS,1H NMR, UV-VIS and IR spectroscopies. The single crystal X-ray data on the copper(II) derivative of one of the isomers of trinitrotetraphenylporphyrin revealed ‘saddle’ conformation of the porphyrin core resulting from the nitro group substitution at theβ-pyrrole carbons. The electrochemical redox behaviour of the free-base porphyrins and their copper(II) derivatives revealed that successive substitution of nitro groups at the pyrrole carbons shifts the one-electron ring oxidations anodically while the ring reduction occurs at a less cathodic potential relative to the unsubstituted porphyrin. The isomer specific shifts in the redox potentials bear a direct relationship with the relative energy levels of HOMO/LUMO obtained from AM1 calculations.

    • Foreword

      V Krishnan

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    • Editor’s note

      V Krishnan

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    • Meso-functionalized octamethoxyporphyrins: A new class of nonasubstituted porphyrins

      Pradeepta K Panda V Krishnan

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      Octamethoxyporphyrin containing multiple-donor substituents has been functionalized for the first time. A large number of its mono-meso-substituted derivatives with substituents such as nitro, amino, N-methylamino, formyl, hydroxymethyl, oxime, cyano and carboxy functional groups have been synthesized and characterized. They form a new class of nonasubstituted porphyrins. Crystallographic studies on the cyano derivative show that the -C N group is in conjugation with the prophyrin π-system. The calculated optical transition energies and the electron densities on the imino nitrogens of the synthesised porphyrins using AMI calculations correlate well with the experimentally observed data. Meso-substituted porphyrins are found to be essentially planar.

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