Observation of the triplet energy transfer in orthogonal photoexcited iodinated-BODIPY dimers

Intramolecular charge and energy transfer processes initiated by light absorption can change the photosensitization properties of molecular conjugates. Transient optical and electron paramagnetic resonance (TREPR) spectroscopies are well suited for the study of these processes. In the TREPR spectra of the triplet state of an iodinated BODIPY dimer, we have observed the effect of the averaging of the zero-field (ZFS) parameter E , which becomes more efficient with increasing temperature. This property is associated with triplet energy transfer from one chromophore in the dimer to another, implying the presence of a dynamic equilibrium between the two chromophores of the dimer. From the comparison of the ZFS parameters of the monomer and the dimer, the rates of the reversible hopping are derived within the framework of a two-site model. The obtained data indicate that the triplet states are separated by an energy barrier of ca. 70 K ( ca. 0.006 eV) and that below this temperature energy transfer occurs via tunneling. The reversible intramolecular energy transfer process between the two triplet states localized on the highly identical chromophores in Bodipy dimer was observed with TREPR spectroscopies.