Investigation of the spin crossover behaviour of a sublimable Fe(II)-qsal complex: from the bulk to a submonolayer on graphene/SiO2

Abstract

We synthesized a sublimable molecular spin crossover Fe(II) complex based on the Schiff base tridentate ligand qsal-NEt2 (5-diethylamino-2-((quinolin-8-ylimino)methyl)phenol). The compound undergoes a transition in temperature with a thermally induced excited spin state-trapping (TIESST) for high temperature sweep rates, that can be suppressed by reducing the sweep rate. The X-ray absorption spectroscopy (XAS) studies on the microcrystalline powder confirms the TIESST effect. The molecules are deposited under ultra-high vacuum on a graphene/SiO2 substrate as a submonolayer. The investigation of the submonolayer by XAS reveals the molecules integrity and shows a spin crossover for the whole temperature range from 350 to 4 K, with residual HS species at low temperature and no TIESST effect. DFT calculations suggest a distribution of energetically similar adsorption configurations on graphene that is, with the smooth crossover behaviour and the absence of TIESST, consistent with very weak intermolecular interactions and the absence of large molecular islands within the submonolayer.