Intersystem Crossing and Electron Spin Dynamics of Photoexcited Bodipy Dimers

Two Bodipy dimers were prepared (BDP-BDP andBDP-BDP-I2), with a phenyl unit as the linker between two Bodipymoieties. It was found that the triplet excited state of thefirst dimerdepopulates fast to the ground state, while the metastable tripletstate is well-detected in the second dimer. With two iodine atomsattached, a high singlet oxygen quantum yield (Phi Delta= 51.5%) and long intrinsic triplet state lifetime (Tau T= 210 mu s) of BDP-BDP-I2were observed in toluene, which are similar to those of diiodoBodipy monomer (Phi Delta= 54.9%,Tau T= 173 mu s). As determined byphosphorescence experiments, the T1-state of BDP-BDP-I2local-izes predominantly on the diiodo Bodipy subunit (BDP-I2, 92%)and a little on the diphenyl Bodipy subunit (BDP-Ph2, 8%). At lowtemperature (ca. 80-140 K), three types of triplet signals wereobserved for BDP-BDP-I2in time-resolved electron paramagnetic resonance spectroscopy (TREPR), which are assigned to two subunits (BDP-I2and BDP-Ph2) and the3CT state. Efficient energy transfer from the BDP-I2subunit to the BDP-Ph2subunit was also observed in BDP-BDP-I2. The local equilibrium between three metastable states observed within ca. 1 mu s after the laser flash is sensitive to the temperature and the polarity of the solvent. The observed properties imply the presence of the energy and electron transfer pathways between two moieties of the studied dimers