Photocatalytic Activity of the Molecular Complexes of meso‐Tetraarylporphyrins with Lewis Acids for the Oxidation of Olefins: Significant Effects of Lewis Acids and meso Substituents

Abstract

In continuing our studies on the photosensitizing ability of the 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) molecular complex of meso‐tetraphenylporphyrin, we have studied the photocatalytic activity of tetracyanoethylene (TCNE) molecular complexes of a series of para‐ or ortho‐substituted meso‐tetraarylporphyrins (aryl = phenyl, 2‐methylphenyl, 4‐methylphenyl and 4‐methoxylphenyl) for the aerobic oxidation of olefins and compared the results with those of the DDQ counterparts and the free‐base porphyrins. Significantly higher catalytic performances were observed for the TCNE molecular complexes over a much shorter reaction time. This observation is in accord with the remarkably higher singlet‐oxygen quantum yield of the TCNE complexes (φΔ = 0.57–0.98) relative to that of the DDQ complexes (φΔ = 0.03–0.57) and the free‐base porphyrins (0.32–0.63). Also, the photocatalytic activity of the molecular complexes was found to be greatly influenced by the meso substituents. The involvement of a singlet‐oxygen mechanism in the oxidation reactions was confirmed by using 1,3‐diphenylisobenzofuran and 1,4‐benzoquinone as the singlet‐oxygen quencher and superoxide anion radical scavenger, respectively.

Significantly Increased Stability of Donor–Acceptor Molecular Complexes under Heterogeneous Conditions: Synthesis, Characterization, and Photosensitizing Activity of a Nanostructured Porphyrin–Lewis Acid Adduct

“…In continuation of previous studies performed concerning the photocatalytic activity of the molecular complexes and diacids of porphyrins, − ,− , in the present study, the molecular complex of a cationic porphyrin (H 2 TMPyP) supported on the neutralized nanoparticles of Amberlyst 15 (nanoAmbSO 3 Na) with BF 3 was synthesized and characterized (Figure ). In contrast to the corresponding nonimmobilized analogue, − , the heterogenized molecular complex was found to be very stable toward dissociation into its components in solvents as polar as water. The nanostructured photosensitizer was used as a photosensitizer in aerobic oxidation of organic compounds under aqueous and nonaqueous conditions.…”

“…46 The procedure was described elsewhere. [10][11][12][13][17][18][19]41 3. RESULTS AND DISCUSSION 3.1.…”

“…25−27,32−39 The molecular complexes of porphyrins possessing substantially red-shifted absorption bands have been used as efficient alternatives of structurally complicated porphyrins in photooxidation reactions. 10,11 Furthermore, the electron-withdrawing effects of the attached acid molecules and their steric hindrance provide evidence to improve the stability of the aromatic macrocyclic ligand toward electrophilic attack by ROS.…”

“…Singlet molecular oxygen ( 1 O 2 , 1 Δ g ) is a reactive oxygen species (ROS) with extensive applications in organic syntheses, wastewater treatment, and biological therapies. − This species can be prepared by chemical and photochemical methods. , The presence of a photosensitizer with intense absorption bands in the visible wavelength region is among the main requirements for light-induced generation of 1 O 2 . Porphyrins and related aromatic macrocyclic compounds are among the main photosensitizers commonly used for photosensitization of molecular oxygen because of the intense absorption of visible light, , high photooxidative stability, − various positions for functionalization of the aromatic macrocycle, their ability for S 1 to T 1 intersystem crossing, and straightforward immobilization on different supports. − Although most of these properties stem from the aromaticity of the porphyrin core, computational studies show that the aromatic macrocycle can easily accept nonplanar conformations with little or no decrease in the porphyrin ring current. − The wavelength and molar absorptivity of the absorption bands of these aromatic macroheterocycles are determined by the substituents introduced at the porphyrin periphery as well as the central nitrogen atoms. In spite of the observation of similar bands in the case of meso -tetra­(aryl)- and meso -tetra­(alkyl)­porphyrins, bearing a planar or nearly planar conformation, the longer wavelength band of the latter, known as the Q (0,0) band, appears at relatively longer wavelengths compared to that of the former .…”

“…Furthermore, there are limitations in the introduction of groups at the pyrrole ring of porphyrins. As shown by different authors, porphyrins form stable molecular complexes with various weak and strong Brønsted and molecular Lewis acids. − ,− The molecular complexes of porphyrins possessing substantially red-shifted absorption bands have been used as efficient alternatives of structurally complicated porphyrins in photooxidation reactions. , Furthermore, the electron-withdrawing effects of the attached acid molecules and their steric hindrance provide evidence to improve the stability of the aromatic macrocyclic ligand toward electrophilic attack by ROS.…”