Spin-state switching at the single-molecule level by distortion of the coordination sphere: validation based on quantum-chemistry calculations†

Different strategies have been proposed to trigger spin switching in single-molecule junctions based on spin-crossover complexes. Here, we report on a computational study aimed to validate one of the hypothesized mechanisms consisting of the distortion of the coordination sphere of the molecule. We focus on a series of heteroleptic [Fe^II(tpy)₂]⁺² complexes in a mechanically controlled break junction setup, displaying voltage-dependent bistabilities, related to the switching of the Fe^II centre between the LS and HS states. Our model for the molecular junction can explain the hysteretic behaviour found in some of the junctions, with a mechanism close to the hypothesized mechanism, but without the requirement of long-range interactions between the electrodes and different parts of the molecule. Our results predict the existence of a switching field able to foster the required distortion driving the switching between the LS and HS states.