Temperature-Dependent Spin-Driven Dimerization Determines the Ultrafast Dynamics of a Copper(II)-Bound Tripyrrindione Radical

Radicals and other open-shell molecules play a central role in chemical transformations and redox chemistry. While radicals are often highly reactive, stable radical systems are desirable for a range of potential applications ranging from materials chemistry and catalysis to spintronics and quantum information. Here we investigate the ultrafast properties of a stable radical system with temperature-dependent spin-tunable properties. This radical complex, Cu(II) hexaethyl tripyrrin-1,14-dione, accommodates unpaired electrons localized on both the copper metal center and on the tripyrrolic ligand. Two-dimensional electronic spectroscopy measurements of Cu(II) hexaethyl tripyrrin-1,14-dione were collected at room temperature and at 77 K. At room temperature, the molecules are present as monomers and have short, picosecond lifetimes. At 77 K, the molecules are present in a dimer form mediated by ferromagnetic and anti-ferromagnetic coupling. This reversible spin-driven dimerization changes the optical properties of the system, generating long-lived excitonic states.