Time-resolved EPR study of exciton hopping in porphyrin dimers in their photoexcited triplet state

We have used time-resolved electron-paramagnetic resonance (EPR) spectroscopy to investigate the triplet energy transfer in various covalently linked porphyrin dimers. For all these dimers the fine structure parameters are rather similar to the monomer values; thus according to the field-swept EPR spectra the triplet state appears to be localized on one monomeric subunit. However, for particular conformations, the time evolution of the EPR spectra reveals dynamic properties specific to a dimer. For these cases the hopping rates of the triplet exciton can be determined from the measured spin-lattice relaxation times. The values obtained for the different dimers can be related to structural parameters of the dimers and to the overlap of their electronic wavefunctions. Such overlap is essential for energy transfer via the exchange mechanism.