Photophysical characterization of a ruthenium-based tetrameric pentacene complex

The synthesis of a ruthenium(II) tetramer (Ru(L-pc)(4)Cl-2) bearing pyridyl pentacene-based ligands and its corresponding model complex lacking pentacene moieties (Ru(Lref)(4)Cl-2) is presented. The formation of these tetramers is corroborated by H-1, C-13 NMR, and IR spectroscopies, as well as X-ray crystallography and mass spectrometry. Photophysical properties of the tetramers are probed by steady-state and time-resolved transient absorption spectroscopy. Steady-state absorption and fluorescence assays suggest weak ground-state interactions between the pentacene moieties of Ru(L-pc)(4)Cl-2 and sizeable metal-to-ligand interactions, which are ruthenium-to-pentacene based. Time-resolved transient absorption measurements reveal an ultrafast singlet-to-triplet transition that occurs on the femtosecond timescale and that yields vibrationally hot and solvent unrelaxed triplet excited states. After relaxation, these triplet excited states deactivate back to the ground state in less than 3 mu s. These observations lead us to conclude that intersystem crossing outperforms photophysical processes such as singlet fission.