A2B2 type porphyrins with meso-donor groups: Synthesis, X-ray structures, DFT studies and photocatalytic application using sunlight

The trans-A<inf>2</inf>B<inf>2</inf> type porphyrins bearing thiophene linked N-butylcarbazole or N-butylphenothiazine groups at the meso-positions were synthesized. Donor−acceptor (D−A) type meso-substituted porphyrins and their Zn(II) and Pd(II) complexes were studied by UV–vis absorption, fluorescence and electrochemical techniques. These porphyrins exhibited efficient energy transfer (∼78%) from the meso-donors (thiophene linked N-butylcarbazole or N-butylphenothiazine groups) to the porphyrin core. Also, the Soret band of porphyrins was noticeably broad, indicating significant communication between the meso-donor groups and the porphyrin core. The donor−acceptor interaction between the porphyrin core and the meso-donors was supported by X-ray analysis and DFT studies of these D-A systems. The photo-catalytic application of Pd(II) porphyrin was demonstrated by efficient singlet oxygen generation (68%) and effective oxidation of aromatic aldehydes to their corresponding carboxylic acids (up to 99%) in the presence of white light or sunlight.