The interactions between tetrapyridyl porphyrin and viologen units covalently linked to polymers

Abstract

The spectral properties of tetrapyridyl porphyrin covalently linked to polymer and copolymer of porphyrin–viologen in dimethylformamide solutions were compared. Many more viologen groups than tetrapyridyl chromophores were linked to polymer.The viologen is an electron acceptor. The processes of deactivation of the excitation energy with and without addition of electron donors (ethylendiaminetetracetic acid or triethanolamine) to the solution were investigated. In order to establish the fate of the absorbed energy the absorption, fluorescence, fluorescence excitation, as well as steady state and time-resolved photothermal spectra were measured. The yield of porphyrin triplet state generation was established. The excitation of reduced monomer of viologen (at 390nm) was followed by 530nm fluorescence due to some form of viologen. It is not excluded that this emission could be due to excimeric form of viologen. From absorption spectra of irradiated samples with the addition of electron donors it was shown that the electron transfer between porphyrin and methyl viologen units linked to polymer is not efficient. This transfer is more efficient between porphyrin and viologen added to solution. This shows that for electron transfer, shorter distance between porphyrin and viologen than that between those attached to polymer groups is necessary.In samples without viologen the singlet excitation energy can be transferred from the excited state of polymer or from a weakly bound to a more strongly bound porphyrin state which is responsible for fluorescence emission. Fluorescence emission competes with processes of intersystem (S1→T1) transition and thermal deactivation. The laser induced optoacoustic spectra (LIOAS) were measured at various temperatures in order to establish conditions in which the volume change of the illuminated samples could be neglected and measured signals were caused only by triplet states thermal deactivation. The addition of the electron donor to the solution has an influence on the yield and decay time of the triplet states of porphyrin units attached to polymer. Oxygen changed the decay time of triplet states. The schemes describing the fate of excitation in both types of samples were proposed. The data obtained can be useful in the construction of a system for conversion of light energy into electrical energy, in hydrogen production and the application of dyes in photodynamic therapy. The investigated systems can also be treated as a simplified model of photosynthetic apparatus of organisms.