arXiv2019
Photo-switching the physical properties of molecular systems opens large possibilities for driving materials far from equilibrium toward new states. Moreover, ultra-short pulses of light make it possible to induce and to record photo-switching on a very short time-scale, opening the way to fascinating new functionalities. Among molecular materials, Fe(II) complexes exhibit an ultrafast spin-state transition during which the spin state of the complex switches from a low spin state (LS, S=0) to a high spin state (HS, S=2). The latter process is remarkable: it takes place within ~100 fs with a quantum efficiency of ~100%. Moreover, the spin state switching induces an important shift of the broad metal to ligand absorption band of the complex and it results in large modifications of the physical and chemical properties of the compounds. But, because most of the Fe(II) complexes crystallize in centrosymmetric space groups, this prevents them from exhibiting piezoelectric, ferroelectric as well as second-order nonlinear optical properties such as second harmonic generation (SHG). This considerably limits their potential applications. We have recently synthesized [Fe(phen)3] [$Δ$-As2(tartrate)2] chiral complexes that crystallize in a noncentrosymmetric 32 space group. Hereafter, upon the excitation of a thin film of these complexes by a femtosecond laser pulse and performing simultaneously transient absorption (TRA) and time-resolved SHG (TRSH) measurements, we have recorded the ultrafast LS to HS switching. Whereas a single TRA measurement only gives partial information, we demonstrate that TRSH readily reveals the different mechanisms in play during the HS to LS state relaxation. Moreover, a simple model makes it possible to evaluate the relaxation times as well as the hyperpolarizabilities of the different excited states through which the system travels during the spin-state transition