Femtosecond Polarization Switching of [CrCo] Dinuclear Complex Crystals.

: Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types.  However, application of such molecular-level switching schemes has been often limited by its speed and thus efficiency.  Here, we demonstrate the ultrafast, photoinduced polarization switching in the crystal of the [CrCo] dinuclear complex by ultrafast pump-probe spectroscopy in the visible and mid-infrared regions.  Our data clearly show that the photoinduced polarization switching is an ultrafast process with a time constant of 280 fs, demonstrating itself as the fastest polarization switching material realized using the metastable state.  Moreover, the pump-probe data in the visible region reveal pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm-1, which we attribute to a lattice vibrational mode.  The pronounced non-Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.