Intramolecular Long Range Electronic Spin Exchange Coupling in Diradicals: A New Magnetic Coupling Unit

A phenyl diethynyl bridge is used as a magnetic coupling unit (MCU) between two iminonitroxide pi-radicals. The ethynyl moiety is attached to the central phenyl ring either in the para or in the meta position. The magnetic properties of the resulting bis-imino nitroxide diradicals (p-BIN and m-BIN) are investigated both at the molecular scale and in the condensed phases. An intramolecular through bond spin coupling is observed at an unusual long distance (about 20 Angstrom) in both derivatives. However, the ground state is found to be a singlet spin state in both compounds. The results suggest an interplay between the spin polarization and the spin delocalization (pi-conjugation). An intermolecular weak antiferromagnetic spin coupling is occuring in both compounds.