Initial steps of photosynthetic water splitting by associates of porphyrin

Abstract

Photochemical and spectral properties of dimeric forms of meso-tetraphenylporphine (TPP), associated meso-tetra( p-aminophenyl)porphine (TAPP) and meso-tetra( N,N,N-trihexadecyl-4-aminophenyl)porphine (TTAP), which produce donor–acceptor complexes, are investigated by absorption, luminescence, and Raman spectroscopy. Three different protonated dimers of TPP observed in the ground state, according to fluorescence spectra, reveal only two different porphyrin dimers in the excited state and have maxima of emission bands at 693 and 730 nm. Low-temperature measurements of TPP dimers solutions at 77 K after illumination show EPR signal with g-factor of free electron and Δ H pp = 0.7 mT. The differential absorption spectrum of the solution is somewhat similar to spectrum of π-cation radical of TPP. Visible light illumination of associated TTAP in water containing small amount of dimethylformamide (DMF) produces irreversible changes at room temperature. The corresponding differential spectrum of the photoinduced changes exhibits mainly 692 nm band overlapping with broad band in the 460–800 nm region with the maximum at about 620 nm. According to fluorescence spectra of the TPP dimers, interaction between low- and high-energy dimeric forms of porphyrin (with the maxima of Soret bands at 465 and 403 nm, respectively), takes place in the excited state. Similar behavior can be possible for aminoporphyrins but interpretation of their spectra in this respect meets difficulties because of donor–acceptor interactions in the association complexes. Most probably the interactions bring about electron transfer photoreaction between TPP dimers and in the case of TTAP dimeric (or associated) similar forms too. It is proposed that the electron transfer photoreaction involves water and utilizes it as an electron donor in the case of TPP dimers and a sacrificial electron donor in the case of associated TTAP. Unordinary vibrational bands observed in the resonance Raman spectra of TAPP and TTAP support this unusual behavior of the aminoporphyrins.