Cyclometalated Iridium(III) Complexes as Photosensitizers for Long‐Range Electron Transfer: Occurrence of a Coulomb Barrier

Abstract

AbstractSix cyclometalated iridium(III) complexes were investigated to assess their potential as photosensitizers for long‐range electron transfer, and two of them were incorporated directly into covalent donor–bridge–acceptor molecules. The influence of ligand substitutions on the excited‐state properties and the photoredox behavior of the iridium complexes was explored by optical absorption, steady‐state and time‐resolved luminescence spectroscopy, as well as by electrochemical methods. Bimolecular electron transfer between the photoexcited complexes and 10‐methylphenothiazine and methylviologen was found to be only weakly dependent on the ligand substitutions. Intramolecular long‐range electron transfer from phenothiazine to photoexcited iridium(III) in the dyads is slow due to the occurrence of a Coulomb barrier. Consequently, an electron‐transfer photoproduct is only observable in the transient absorption spectrum of a donor–bridge–acceptor molecule with a fluorinated photosensitizer that exhibits a very long excited‐state lifetime. A flash‐quench technique is necessary for detection of an electron‐transfer product in the dyad with a non‐fluorinated photosensitizer. The occurrence of a Coulomb barrier associated with intramolecular (excited‐state) long‐range electron transfer in the dyads with cyclometalated iridium(III) photosensitizers represents an important difference to previously investigated similar donor–bridge–acceptor molecules with photosensitizers based on d6 metal diimine complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)