Periodic Reporting for period 1 - LIDOS (Light-Induced Spin Switch using Dynamic Organic Species)
Période du rapport: 2018-10-01 au 2020-09-30
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.
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.
We studied the combination of photoswitches, radicals and linker units to achieve the photo-magnetic switches with the desired properties. We identified five-membered azoheteroarenes (AHs) as the most promising P units since they preserve the significant structural change of azobenzene, but displayed a better thermal stability under the type of architecture we envisaged in the DoA. Also, AHs are becoming an important class of azo-dyes due to their larger structural and electronic diversity than AB, which suited perfectly our needs. Being AHs less well-known than AB, a necessary step in the project has been the rationalization of their optical properties depending on the substituents that are attached to them. This was tackled in the paper entitled “Exploring chemical space in the search for improved azoheteroarene-based photoswitches”, published in 2019 (DOI: 10.1039/C9CP03831K). Also, durign the screening of PM architectures we observed that the radical unit tends to act as a very strong electron-withdrawing unit, which has an important effect on the optical properties of the photoswitch. This effect was indirectly studied in the paper entitled “The Photoisomerization Pathway(s) of Push-Pull Phenyl-azoheteroarenes” (DOI: 10.1002/chem.202002321) in which we evaluated the energy changes in the conical intersection landscape upon push-pull substitution, and also the change in the photoisomerization kinetics. Also, we discovered that the cyclic nature of the PM switch according to our design principle, had also an impact on the characteristics of the photoswitch. One effect was on the thermal stability, the other on the photo-isomerization pathway and kinetics. Both points have been investigated in the third paper of the action, entitled “Tuning the Thermal Stability and Photoisomerization of Azoheteroarenes through Macrocycle Strain” (DOI: 10.1002/chem.202003926). Overall, these papers have provided important information on the photoisomerization of azoheteroarenes, not only in the context of this MSCA, but for the general application of AHs as molecular switches.
While we could not find suitable PM switches, which was one of the main goals of the action, our research contributed to clarify the type of Photoswitches, Radicals and Linkers necessary to achieve the desired PM switches, thus reducing the chemical space one would need to screen to find suitable candidates. Indeed, an important aspect is that we can now rely more on molecular design rather than on trial and error, since we know much better how the different pieces need to be combined. We have thus made one step forward towards the realization of an all-organic alternative to the LIESST effect typically observed in inorganic complexes based on Iron.