The LIDOS project aimed at utilizing the versatility of molecular chemistry, coupled with state-of-the-art computational tools, to advance the science of nano-sized switches. Overall, switchable materials (or switches) are molecular materials that can be switched ‘on’ and ‘off’ with respect to a physicochemical property (or properties) of interest by applying an external stimulus. Several electronic, magnetic, optical and mechanical switches have been designed over the last decade. Amongst these, magnetic switches are particularly interesting as they are candidates for use in high-density magnetic data storage, molecular electronics and spintronics, communication networks, displays, and photosensors. The magnetic switch is generally induced by a change in temperature, however, a switch utilizing light presents multiple advantages including: it is high-tuneability, being non-invasive, fast-acting and highly-selective. Today, the most common non-redox approach for combining light and magnetism is the Light-Induced Excited Spin-State Trapping (LIESST), in which the magnetic unit, typically an FeII or FeIII ion, is directly irradiated. Despite numerous advantages, this approach still has one major drawback: the magnetically-active (S=2) metastable excited-state (ES) decays back to the magnetically-silent (S=0) ground state (GS) far below room temperature, which limits the potential applications in devices. An underexplored alternative would be to combine a magnetic center (or radical, R) and a light-reactive molecular photochrome or photoswitch (P, see Fig. 1). These molecules can reversibly isomerize between two states upon irradiation. Prototypical examples include the E/Z isomerization of Azo-dyes and the open-/closed- form of Diarylethene molecules and their derivatives.
Reversible photoswitchable systems incorporating DAE or AB moieties have been exploited to trigger changes in multiple properties including dipole moment, current, charge transfer/transport, photoconductivity, fluorescence and morphology. In contrast, combining P and R units to switch the magnetic response has proven to be more difficult. The objective of this project is to design purely-organic photo-magnetic (PM) switches with a significant change in magnetism at room temperature, and with optimal photoswitch characteristics. In the LIDOS project, we proposed to rely upon reversible π-dimerization to trigger a significant change in the intermolecular magnetic interaction between adjacent π-stacked PR dyads (see Fig. 3). This dimerization mechanism typically occurs when two open-shell π-stacked molecules lie near van der Waals distance from one another. Suitable SOMO-SOMO overlap allows the unpaired electrons to form a weak delocalized bond that is associated with a very-strong AFM interaction that weakens as overlap decreases. This simple mechanism is at the heart of the magnetic switching in organic radicals, and represents the best strategy for achieving a reversible spin-change in this class of molecules. Our objective is to achieve reversible pi-dimerization using a supramolecular approach, by the smart combination of radical molecules and photoswitches.
