Meso-tetra(4-fluorobenzoyl) porphyrin: synthesis, characterization and study of its applications as a visible-light photoredox catalyst

Abstract

Visible-light photoredox catalysis has become a powerful tool in organic synthesis, with an increasing interest in the development of efficient organophotocatalysts that avoid the use of scarce and/or toxic transition metals. Among these, porphyrins have attracted attention due to their tunable photophysical and redox properties. Previous studies by our research group demonstrated that both the electronic nature and the position of substituents on the porphyrin framework play a crucial role in modulating these properties and have led to the identification of meso-tetra(4-fluorobenzoyl)porphyrin as a particularly promising candidate due to the great facility of reduction of its excited state. In this context, the aims of this Bachelor’s thesis were to achieve a detailed characterization of this porphyrin, to investigate its photophysical behavior, and to evaluate its applicability as a visible-light photoredox catalyst. The target porphyrin was fully characterized by NMR spectroscopy, including HSQC experiments, allowing complete signal assignment (1H and 13C). Its singlet and triplet excited-state lifetimes were also determined . The compound was then applied as a photocatalyst in the addition of alkyl radicals, generated from 4-isopropyl-substituted Hantzsch esters, to p-substituted β-nitrostyrenes under visible-light irradiation. The reactions afforded mixtures of Giese addition and of nitro substitution products, whose distribution depends both on the irradiation wavelength and on the electronic nature of the styrene substituent. These results can be accounted for by a reductive quenching photoredox cycle involving initial oxidation of the Hantzsch ester by the excited porphyrin. While control experiments revealed that catalyst-free reactions can occur at wavelengths below 500 nm through direct excitation of the Hantzsch ester, at longer wavelengths the presence of the porphyrin photocatalyst becomes essential.