Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state

The novel vacuum-evaporable complex [Fe(pypypyr)(2)] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

Automated summary

The article reports a compound with unprecedented properties, which can maintain its high-spin state at high temperatures for a long period of time under light irradiation. The researchers attribute this behavior to the large structural difference between the two spin states and the four distortion coordinates associated with the spin transition, which drastically reduce the relaxation rate of the metastable high-spin state. These findings open up new strategies for developing compounds that exhibit light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature.