Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex

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 has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump–probe spectroscopy in the visible and mid‐infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization‐switching material realized using the metastable state. Moreover, the pump–probe data in the visible region reveal the 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. Full speed ahead: Ultrafast spectroscopy revealed that photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex proceeded with a time constant of 280 fs (see picture; LT/HT=low‐/high‐temperature, dhsq/dhbq= deprotonated dihydroxysemi‐/benzoquinone). The pronounced appearance of a low‐frequency coherent vibration and its mechanism suggest that the corresponding excited‐state vibrational mode facilitates the polarization switching.