Substituent Effects in the Photophysical and Electrochemical Properties of Meso-Tetraphenylporphyrin Derivatives

Abstract

Porphyrins were identified some years ago as a promising, easily accessible, and tunable class of organic photoredox catalysts, but a systematic study on the effect of the electronic nature and of the position of the substituents on both the ground-state and the excited-state redox potentials of these compounds is still lacking. We prepared a set of known functionalized porphyrin derivatives containing different substituents either in one of the meso positions or at a β-pyrrole carbon, and we determined their ground- and (singlet) excited-state redox potentials. We found that while the estimated singlet excited-state energies are essentially unaffected by the introduction of substituents, the redox potentials (both in the ground- and in the singlet excited-state) depend on the electron-withdrawing or electron-donating nature of the substituents. Thus, the presence of groups with electron-withdrawing resonance effects results in an enhancement of the reduction facility of the photocatalyst, both in the ground and in the excited state. We next prepared a second set of four previously unknown meso-substituted porphyrins, having a benzoyl group at different positions. The reduction facility of the porphyrin increases with the proximity of the substituent to the porphine core, reaching a maximum when the benzoyl substituent is introduced at a meso position.

Keywords: photoredox catalysis; photosensitizers; porphyrins; redox potential; substituent effects.

Scheme 1

Some applications of porphyrins as photoredox catalysts. A: α-alkylation of aldehydes with diazo esters [15]. B: arylation of heterocycles [16]. C: desulfonilative alkylation of alkynyl sulfones [17].

Figure 1

Porphyrin derivatives studied in this work: (A): previously known compounds. (B): new benzoyl-porphyrin derivatives.

Scheme 2

Synthesis of 5-(4′-benzoyl)-10,15,20-triphenylporphyrin 6.

Scheme 3

Synthesis of benzoyl-porphyrin derivative 7.

Scheme 4

Synthesis of porphyrin copper complex 8 and acid-promoted demetallation to 7.

Scheme 5

Synthesis of 5-(benzoyl)-10,15,20-triphenylporphyrin 9.