Electrochemistry and Spectroelectrochemistry of Cobalt Porphyrins with π‑Extending and/or Highly Electron-Withdrawing Pyrrole Substituents. In Situ Electrogeneration of σ‑Bonded Complexes

A series of cobalt porphyrins with π-extending or highly electron-withdrawing β-pyrrole substituents were investigated as to their electrochemistry, spectroscopic properties, and reactivity after electroreduction or electroxidation in nonaqueous media. Each porphyrin, represented as PorCo (where Por = TPP­(NO₂)­Y₂ or TPP­(NO₂)­Y₆ and Y = phenyl, phenylethynyl, Br, or CN) was shown to undergo multiple redox reactions involving the conjugated π-ring system or central metal ion which could exist in a Co­(III), Co­(II), or Co­(I) oxidation state under the application of an applied oxidizing or reducing potential. Thermodynamic half-wave potentials for the stepwise conversion between each oxidation state of [PorCo]^<i>n</i> (where <i>n</i> ranged from +3 to −3) were measured by cyclic voltammetry and analyzed as a function of the compound structure and properties of the electrochemical solvent. UV–visible spectra were obtained for each oxidized or reduced porphyrin in up to six different oxidation states ranging from [PorCo]^3– to [PorCo]³⁺ and analyzed as a function of the compound structure and utilized electrochemical solvent. Chemically or electrochemically generated Co­(I) porphyrins are known to be highly reactive in solutions containing alkyl or aryl halides, and this property was utilized to in situ generate a new series of methyl carbon-bonded cobalt­(III) porphyrins with the same π-extending or highly electron-withdrawing substituents as the initial Co­(II) derivatives. The electrosynthesized carbon-bonded Co­(III) porphyrins were then characterized as to their own electrochemical and spectroscopic properties after the addition of one, two, or three electrons in nonaqueous media

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 = Cu^II, Ni^II, Zn^II, Co^II, 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

Synthesis and Spectroscopic Investigation of a Series of Push–Pull Boron Dipyrromethenes (BODIPYs)

A series of push–pull BODIPYs bearing multiple electron-donating and electron-acceptor groups were synthesized regioselectively from 2,3,5,6,8-pentachloro-BODIPY, and characterized by NMR spectroscopy, HRMS, and X-ray crystallography. The influence of the push–pull substituents on the spectroscopic and electrochemical properties of BODIPYs was investigated. Bathochromic shifts were observed for both absorbance (up to 37 nm) and emission (up to 60 nm) in different solvents upon introduction of the push–pull moieties. DFT calculations, consistent with the spectroscopic and cyclic voltammetry studies, show decreased HOMO–LUMO energy gaps upon the installation of the push–pull moieties. BODIPY <b>7</b> bearing thienyl groups on the 2 and 6 positions showed the largest λ_max for both absorption (635–653 nm) and emission (706–707 nm), but also the lowest fluorescence quantum yields. All BODIPYs were nontoxic in the dark (IC₅₀ > 200 μM) and showed low phototoxicity (IC₅₀ > 100 μM, 1.5 J/cm²) toward human HEp2 cells. Despite the relatively low fluorescence quantum yields, the push–pull BODIPYS were effective for cell imaging, readily accumulating within cells and localizing mainly in the ER and Golgi. Our structure–property studies can guide future design of functionalized BODIPYs for various applications, including bioimaging and in dye-sensitized solar cells