Ultrafast time-resolved spectroscopy of self-assembled cyclic Fe(II)-bisterpyridine complexes

Femtosecond time-resolved absorption spectroscopy has been used to study the excited-state charge transfer dynamics in a set of self-assembled cyclic Fe(II)-bisterpyridine compounds with different π-conjugated ligands. By analyzing the dynamics, the internal conversion process involving a ligand-centered π-π* state to a lower lying metal-to-ligand charge transfer (MLCT) state was investigated. This is followed by intersystem crossing to the lowest MLCT state, which was found to occur at the ∼100 fs time scale. Vibrational cooling in the lowest MLCT state was found to occur on the 10s of femtoseconds time scale. The lowest MLCT state had an excited-state lifetime longer than 5 ns, indicating the possibility of light-induced excited-state spin trapping (LIESST).