Photophysical and structural characterization of a ruthenium complex–pyrene Coulombic dyad

Herein, we thoroughly investigate the (photo)physical properties and the geometry of the first reported Coulombic dyad, composed of dicationic tris(1,10-phenanthroline)ruthenium(II) and tetraanionic 1,3,6,8-pyrenetetrasulfonate. The mixed dynamic and static quenching behavior of the photoexcited ruthenium complex by the pyrene derivative in water was investigated by time-resolved (laser flash photolysis as well as time-correlated single photon counting) and steady-state emission spectroscopy, highlighting their differences and the importance of accurate models to determine the association constant. Temperature-dependent measurements revealed a decrease in dynamic and static quenching efficiency at elevated temperatures and provided the enthalpy and entropy of ion-pair formation in water. The time constant for intra-ion-pair Dexter energy transfer from the triplet excited state of the metal complex to the organic chromophore was determined to be ∼87 ps using femtosecond transient absorption spectroscopy. Structural insights were obtained from single-crystal X-ray crystallography and molecular dynamics simulations. The simulations revealed a high persistence of the ion-pair in the ground-state and a dynamic yet geometrically well-defined association. A structural similarity was found between solution and solid-state arrangements, indicating π-interactions besides Coulombic interactions between the ions. These interactions enable sufficient orbital overlap, rationalizing the observed efficient Dexter energy transfer, which is as fast as in many covalently linked donor−acceptor systems. These in-depth investigations provide a comprehensive picture of structure–property relationships in Coulombic dyads, offering valuable insights for their future design and applications.