Magnetic molecules adsorbed on two-dimensional (2D) substrates have attracted broad attention because of their potential applications in quantum device applications. Experimental observations have demonstrated substantial alteration in the spin excitation energy of iron phthalocyanine (FePc) molecules when adsorbed on nitrogen-doped graphene substrates. However, the underlying mechanism responsible for this notable change remains unclear. To shed light on this, we employ an embedding method and ab initio quantum chemistry calculations to investigate the effects of surface doping on molecular properties. Our study unveils an unconventional chemical bonding at the interface between the FePc molecule and the N-doped graphene. This bonding interaction, stronger than non-covalent interactions, significantly modifies the magnetic anisotropy energy of the adsorbed molecule, consistent with experimental observations. These findings provide valuable insights into the electronic and magnetic properties of molecules on 2D substrates, offering a promising pathway for precise manipulation of molecular spin states.

Magnetic circular dichroism (MCD) is a widely used spectroscopic technique which reveals valuable information about molecular geometry and electronic structure. However, the weak signal and the necessary strong magnets impose major limitations on its application. We propose a novel protocol to overcome these limitations by using pulsed vector beams (VBs), which consist of nanosecond gigahertz pump and femtosecond UV-vis probe pulses. By virtue of the strong longitudinal electromagnetic fields, the MCD signal detected by using the pulsed VBs is greatly enhanced compared to conventional MCD performed with plane waves. Furthermore, varying the pump-probe time delay allows monitoring the ultrafast variation of molecular properties.

Fine-tuning the magnetic anisotropy energy (MAE) of a magnetic molecule with a high precision of sub-meV has been realized experimentally by manipulating the tip of a scanning tunneling microscope (STM). Understanding the mechanisms behind the observed evolution of spin excitation energy is essentially important for potential spintronic applications. In particular, it is crucial to unveil the influence of the surrounding environment on the molecular spin state. To this end, we carry out the first-principles simulation on the STM-tip control of an iron octaethylporphyrin chloride (Fe-OEP-Cl) molecule adsorbed on the Pb(111) substrate. By carefully taking into account the atomic structures of the tip and the substrate as well as the multireference feature of the Fe 3d electrons, the experimentally measured evolution of spin excitation energy, including a continuous increase, followed by a sudden drop in the MAE, is accurately reproduced by our simulation with a maximal discrepancy of less than 0.3 meV. Based on a comprehensive analysis of the change in geometric and electronic structures of the whole single-molecule junction, the exotic evolution of MAE is attributed to the variation of the ligand confinement effect and the resulting charge transfer. The unique role of the single-atomic Cl ligand is clarified by comparing the evolution under the STM-tip control with that of the tip/Fe-OEP/Pb(111) junction. The theoretical insights provided by this work would be valuable for the on-demand design of the mechanically controlled magnetic nanojunctions.