Pinky Yadav

Synthesis, spectroscopic, electrochemical redox, solvatochromism and anion binding properties of β-tetra- and -octaphenylethynyl substituted meso-tetraphenylporphyrins

β-Tetra/octa-phenylethynyl substituted porphyrins and their metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes have been synthesized and characterized. H2TPP(PE)8 exhibited a remarkable red shift in the Soret (Δλmax = 92 nm) band and longest wavelength band, Qx(0,0) (Δλmax = 117 nm), as compared to H2TPP. Interestingly, MTPP(PE)8 exhibited a 450–500 mV anodic shift in the first ring reduction potentials as compared to MTPP which is ascribed to the electron accepting nature of phenylethynyl groups and extended π-conjugation whereas the first ring oxidation potentials remain unaltered. Hence, we observed a significant reduction in the HOMO–LUMO gap. The fluorescence quantum yields and lifetime values of the free base and Zn(II) porphyrins are found to be 10–20 and 2–6 times, respectively lower than MTPPs. These porphyrins display a strong solvatochromism that is reflected by a large red-shift in their absorption and emission maxima upon increasing the solvent polarity. These porphyrins exhibited lower radiative rate constants (kr) and enhanced nonradiative rate constants (knr) as compared to MTPPs. The decrement in fluorescence lifetime values, quantum yields, radiative rate constants (kr) and profound solvatochromism with enhanced nonradiative rate constants (knr) have been interpreted in terms of intramolecular charge transfer (ICT) from the porphyrin core to phenylethynyl moieties and extended π-conjugation. Further, ZnTPP(PE)8 was utilized for the colorimetric “naked-eye” sensing of CN−, F−, Cl−, Br−, H2PO4− and CH3COO− ions through axial coordination of anions to Zn(II) metal centre. The electron deficient nature of the Zn(II) centre of ZnTPP(PE)8 was further proved by axial ligation studies with anions in toluene. ZnTPP(PE)8 exhibited extremely high binding constants (1012 to 107 M−2) with anions.

Facile and Reversible Electrogeneration of Porphyrin Trianions and Tetraanions in Nonaqueous Media

The first examples for the facile, reversible, and stepwise electrogeneration of triply ring-reduced porphyrin macrocycles are presented. The investigated compounds are represented as MTPP(NO2)(PE)6, MTTP(PE)8, NiTPP(NO2)(Ph)4, and MTPP(CN)4, where TTP and TPP are the dianions of tetratolylporphyrin and tetraphenylporphyrin, respectively, NO2, phenylethynyl (PE), and CN are substituents at the β-pyrrole positions of the macrocycle, and M = CuII, NiII, ZnII, CoII, or 2H. Each porphyrin undergoes three or four reductions within the negative potential limit of the electrochemical solvent. The UV-visible spectra of the first three reduction products were characterized by means of thin-layer UV-vis spectroelectrochemistry, and the generation of multianionic porphyrins is interpreted in terms of extensive stabilization of the LUMOs due to the electron-withdrawing and/or extended π-conjugation of the β-substituents.

Facile Generation of A2B Corrole Radical Using Fe(III) Salts and Its Spectroscopic Properties

A carboxyphenyl-substituted corrole, 5,15-dimesityl-10-(4′-carboxyphenyl)corrole (1), has been synthesized and characterized by UV–vis, fluorescence, 1H NMR spectroscopy, and electrospray ionization (ESI)-mass spectrometry (MS) techniques. An air-stable corrole radical (1•) was obtained with the addition of the Fe(III) salt to 1 in dimethyl sulfoxide (DMSO) and characterized by UV–vis, fluorescence, electron paramagnetic resonance (EPR), ESI-MS techniques, and density functional theory studies. The neutral corrole radical (1•) exhibited a sharp EPR signal at g = 2.006 in DMSO. The reduced bipyrrolic (N–C–C–N) dihedral angle (χ) of 1 from 19.11 to 7.07° leads to the release of angle strain, which is the driving force for the generation of 1•. Notably, trans-dimesityl groups prevent the dimerization or aggregation of the corrole radical. Further, 1• was converted to 1 by excess addition of Fe(II) salts in DMSO at 298 K.