Spectroelectrochemical Measurement of Charge Transport Properties of Electropolymerized Tetrakis(hydroxyphenyl)porphyrins

Hydroxyphenyl-substituted metalloporphyrins have been electropolymerized as films onto ITO substrates. An ether bond between the phenyl rings of two monomers links the porphyrin units, leaving the porphyrin macrocycles undisturbed. During electrochemical oxidation of the films using a 0.0−0.9 V potential window, a single, semireversible reaction is observed, with n ≅ 0.5 electron per porphyrin molecule. The spectroelectrochemical characteristics of these films can be adequately described by assuming that two neighboring porphyrin units are involved in the progressive oxidation process, in agreement with the changes in the absorption spectrum of the film. To characterize charge carrier conduction, diffusion coefficients for chemically different films have been determined. From the optical changes upon oxidation of the film, the diffusion coefficients for hole transport were found to be (1−2) × 10 -10 cm 2 s 1 . No correlation was found between the value of the diffusion coefficient and the number of cross links per porphyrin unit. The average distance between vicinal porphyrin macrocycles, therefore, most likely determines charge carrier diffusion.