Institut de Physique de Rennes, UMR CNRS 6251, University of Rennes 1, 35042 Rennes (France).
Angew Chem Int Ed Engl. 2014 Apr 7;53(15):3863-7. doi: 10.1002/anie.201310884. Epub 2014 Mar 11.
Light-induced excited spin-state trapping (LIESST) in iron(II) spin-crossover compounds, that is, the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited (1) MLCT to the high-spin state via the (3) MLCT state within ca. 150 fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR.
三十年前，人们发现了铁(II)自旋交叉配合物中的光致激发自旋态捕获（LIESST）现象，即在低于热跃迁温度下，光诱导高自旋（S=2）态的占据。对于低自旋（S=0）物种的金属-配体电荷转移（MLCT）带的辐照，公认的顺序是系统在大约 150fs 内从最初的激发（1）MLCT 态通过（3）MLCT 态到达高自旋态，从而绕过低能配体场（LF）态。然而，这些态确实发挥了作用，这可以通过仅具有高能 MLCT 态的系统直接辐照 LF 带时 LIESST 和反向 LIESST 的观察得到证实。在此，我们通过确定最低能量 S=1 LF 态为中间态，并确定其寿命为 39ps，阐明了超快反向 LIESST 途径，这对于在近红外光照射到高自旋物种的自旋允许 LF 跃迁时，光诱导的高自旋到低自旋转换是有利的。
