π-Extended nonplanar cobalt porphyrins immobilized on MWCNTs as efficient electrocatalysts for selective oxygen reduction reaction.

Two π-extended cobalt porphyrins are synthesized and one of them is crystallographically characterized. The nanocomposites of nonplanar (curved) porphyrin immobilized multi-walled carbon nanotubes were thoroughly characterized spectroscopically and microscopically, showing ∼200 mV positive shift in the O2 reduction peak potential in aqueous media and ∼100 mV shift in the onset potential of the O2 reduction relative to the control meso-tetraphenylporphyrinatocobalt(II) nanocomposite. Both the π-extended cobalt porphyrin immobilized nanocomposites efficiently catalyze selective 4e-/4H+ O2 reduction under ambient conditions with excellent methanol tolerance and high stability due to effective π-π interactions, and could be an alternative for expensive Pt-based cathode materials in fuel cells.

Unsymmetrically ß-Functionalized π-Extended Porphyrins: Synthesis, Spectral, Electrochemical Redox Properties, and Their Utilization as Efficient Two-Photon Absorbers.

Two new series of unsymmetrically ß-functionalized porphyrins, MTPP(NO2)MA (1M), (MA = methyl acrylate) and MTPP(NO2)MB (2M) (MB = mono-benzo) (where M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)), were synthesized and characterized by various spectroscopic techniques. The saddle shape conformation of ZnTPP(NO2)MAPy and ZnTPP(NO2)MB was confirmed by single-crystal X-ray analysis. Density functional theory (DFT) calculation revealed that NiTPP(NO2)MB has a severe nonplanar geometry possessing a high magnitude of ΔCß = ±0.727 Å and Δ24 = ±0.422 Å values among all other porphyrins. Synthesized ß-substituted porphyrins exhibited red-shifted B- and Q-bands corresponding to their parent molecule due to the electron-withdrawing peripheral substituents. Notable redshift (Δλmax = 50-60 nm) in electronic spectral features and with weak-intensity emission spectral features were observed for the free-base porphyrins and Zn(II) complexes compared to H2TPP and ZnTPP, respectively. The first-ring reduction potential of MTPP(NO2)MA (1M) exhibited 0.21-0.5 V anodic shift, whereas 0.18-0.23 V anodic shift was observed in the first-ring oxidation potential compared to the corresponding MTPPs due to the presence of electron-withdrawing ß-substituents at the periphery of the macrocycle. Interestingly, NiTPP(NO2)MA (1Ni) has shown an additional NiII/NiIII oxidation potential observed at 2.05 V along with two ring-centered oxidations. The first-ring reduction and oxidation potentials of MTPP(NO2)MB (2M) have shown 0.39-0.46 and 0.19-0.27 V anodic shifts with respect to their corresponding MTPPs. The nonlinear optical (NLO) properties of all of the porphyrins were investigated, and the extracted nonlinear optical parameters revealed intense reverse-saturable absorption (RSA) behavior and the self-focusing behavior with positive nonlinear refractive index in the range of (0.19-1.75) × 10-17 m2/W. Zn(II) complexes exhibited the highest two-photon absorption coefficient (ß) and cross section (σTPA) of ∼95 × 10-12 m/W and 19.66 × 104 GM, respectively, among all of the metal complexes.

Structural, Photophysical, and Electrochemical Properties of Doubly Fused Porphyrins and Related Fused Chlorins.

The electrochemical and physicochemical properties of tetraphenylporphyrins and tetraphenylchlorins with two fused indanedione (IND) or malononitrile (MN) groups and two antipodal Br, Ph, or H ß-substituents are investigated in nonaqueous media. These compounds were synthesized by oxidative fusion of free-base trans-chlorins, followed by metalation. The corresponding free-base di-fused chlorins were also isolated as intermediates and characterized for comparisons. The examined di-fused porphyrins (DFP) and di-fused chlorins (DFC) are represented as MDFP(Y)2(R)2 and H2DFC(Y)2(R)2, where M = 2H, CuII, NiII, ZnII, and CoII, Y is a fused indanedione (IND) or malononitrile group (MN), and R = H, Br, or Ph. The IND- and MN-appended compounds in both series exhibit the expected two one-electron oxidations but quite different redox behavior is observed upon reduction, where the free-base IND-appended chlorins show four reversible one-electron reductions, compared to only two for the related free-base MN-appended chlorins. Although porphyrin trianions and tetraanions have been recently described for derivatives with highly electron-withdrawing and/or π-extending substituents, this seems not to be the case for the doubly fused IND-chlorins, where the first two one-electron additions are proposed to be located at the conjugated macrocycle and the last two at the fused IND groups, each of which is reduced at a different potential, consistent with the behavior expected for two equivalent and interacting redox centers. Unlike the examined chlorins, which are all stable in their electroreduced forms, the electrogenerated anionic forms of the di-fused porphyrins are all highly reactive and characterized by cyclic voltammograms having reduction peaks not only for the synthesized compounds added to solution but also for one or more new redox active species formed at the electrode surface in homogeneous chemical reactions following electron transfer. Comparisons are made between electrochemical behavior of the structurally related porphyrins and chlorins and the sites of electron transfer assigned on the basis of known electrochemical diagnostic criteria. One of the compounds, ZnDFP(MN)2, was also structurally characterized as having a ruffled and twisted macrocyclic conformation.