Synthesis, electrochemical, and photophysical study of covalently linked porphyrin dimers with two different macrocycles

Four unsymmetrical covalently Linked porphyrin dimers were synthesized, and their electrochemical, spectroelectrochemical, and/or photophysical properties were examined. The investigated compounds are represented as M[p-(TPP-DEHMP)]M and M[m-(TPP-DEHMP)]M (M = H-2, Zn), where p-(TPP-DEHMP) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,18-diethyl-3,7,8,12,13,17-hexamethylporphyrinyl)]benzene and m-(TPP-DEHMP) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-3-[10-(2,18-diethyl-3,7,8,12,13,17-hexamethylporphyrinyl)]benzene. The oxidation and reduction potentials of Zn[p-(TPP-DEHMP)]Zn are virtually identical to combined oxidation and reduction potentials of (TPP)Zn and (OEP)Zn, i., cyclic voltammograms of the heterodimers resemble those of the superposed monomeric units under the same experimental conditions. The UV-visible spectra of neutral, singly oxidized, and/or singly reduced Zn[p-(TPP-DEHMP)]Zn also resemble superposed spectra of the respective neutral, oxidized, or reduced monomeric (TPP)Zn and (OEP)Zn derivatives. Hence, both the electrochemical and spectroelectrochemical data suggest that there is little electronic interaction between the two subunits of the heterodimers in their neutral, singly oxidized, or singly reduced forms. The photophysical properties of four related homoleptic dimers, M[p-(TPP-TPP)]M and M[m-(TPP-TPP)]M, where M = H-2 or Zn, p-(TPP-TPP) = the tetraanion of 1,4-bis[5'-(10',15',20'-triphenylporphyrinyl)]benzene, and m-(TPP-TPP) = the tetraanion of 1,3-bis[5'-(10',15',20'-triphenylporphyrinyl)]benzene, were also examined, and their data were compared to the data for the above heteroleptic dimers. irradiation of H-2[p-(TPP-DEHMP)]H-2 and M[m-(TPP-DEHMP)]M (M = H-2, Zn) with excitation wavelengths in the visible spectrum of the DEHMP subunit, results in an efficient energy transfer between the DEHMP and TPP moieties. Calculated energy transfer rate constants agree well with the experimental data and suggest that the energy transfer processes are best described by a Faster mechanism. H-2[p-(TPP-DEHMP)]H-2 and H-2[m-(TPP-DEHMP)]H-2 thus demonstrate highly efficient and directional energy transfer processes rather than charge separation between the two subunits after irradiation.