Perhalogenated porphyrins as a sink of excitation energy in porphyrin heterodimers

The photophysics of a new family of free-base and zinc derivatives of meso- tetraphenylporphyrin heterodimers have been studied by UV-vis absorption, fluorescence and nanosecond flash photolysis techniques. An almost complete (≈99%) and directionally controlled transfer of excitation energy from a donor porphyrin moiety was obtained by multiple bromination (four and eight Br substituents in the two series of compounds investigated) on the β-pyrrole positions of the acceptor porphyrin molecule. The covalently linked porphyrin dimers populate almost exclusively low energy triplet states because of the extremely efficient intramolecular singlet-to-triplet inter system crossing (ISC) process which is enhanced by the multiple heavy atoms substitutions. The nature of the electronic interactions determining the actual relaxation pathway followed by the porphyrin donor-acceptor pair is discussed.