Photophysical and photochemical properties of manganese complexes with cationic porphyrin ligands: Effects of alkyl substituents and micellar environment

• Cationic manganese(III) porphyrins display characteristic fluorescence.
• They proved to be efficient photocatalysts in generation of MV●+.
• Long-lived triplet states play key roles in these photocatalytic systems.
• Both emission and photocatalytic features depend on their alkyl substituents.
• Micelles also considerably affect the photoinduced behavior of these complexes.

Although Mn(III) porphyrins were considered earlier to be very weakly emissive, the fluorescence displayed by Mn(III) complexes with the cationic TMPyP2+ ligand (H2TMPyP4+ = 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin) as well as with its other alkyl (such as hexyl and dodecyl) derivatives proved to be strong enough for a comparative study. Elongation of the alkyl substituent increased both the quantum yield and the lifetime of the emission for the Mn(III) porphyrins, while resulted in an opposite effect for the corresponding free bases in homogeneous solutions. The presence of cationic micelles, however, reversed this tendency regarding the emission lifetime of the complexes. These cationic metalloporphyrins were applied in a photocatalytic system involving triethanolamine (TEOA) as a sacrificial electron donor and methylviologen (MV2+) as an acceptor. In the first step of the catalytic process outer-sphere photoinduced reduction of the metal center took place via quenching of the triplet excited state of these metalloporphyrins by TEOA. The corresponding manganese(II) porphyrins formed in this way were also photoactive; they forwarded an electron to MV2+ upon irradiation, regenerating the starting complex. Elongation of the alkyl substituents increased the quantum yield of the formation of methylviologen radical (MV●+) in this system, but considerably decreased the durability of the photocatalyst. Anionic micelles totally hindered the photoinduced generation of Mn(II) porphyrins, while cationic micellar environment appreciably increased the efficiency of the accumulation of MV●+.