Designing optically switchable multifunctional materials using photochromic spirooxazine ligands




Paquette, Michelle Marie

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Photoswitchable molecular materials are of interest for optical data storage, optically controlled electronics, and light-controlled molecular machines or ‗smart‘ surfaces. A promising way to incorporate optical switchability into materials is by using organic photochromic molecules—which convert reversibly between two forms with light—as ligands in coordination complexes. This design allows for the intimate communication between ligand and metal such that light-induced photoisomerization may be used to modulate metal-based properties. Spirooxazines, photochromic systems that photochemically isomerize between nonconjugated ring-closed spirooxazine (SO) and highly conjugated ring-opened photomerocyanine (PMC) forms, were derivatized with a phenanthroline moiety to enable the binding of transition-metal ions. Two phenanthroline–spirooxazines, an indolyl derivative and an azahomoadamantyl derivative, were investigated in the context of chemical substitution and medium effects. The ring-opened PMC forms of the spirooxazines were characterized by solid- and/or solution-state methods to extract the relative contributions of the canonical quinoidal and zwitterionic resonance forms to their molecular structure. The PMC form of the azahomoadamantyl derivative was found to exhibit significant zwitterionic character, with demonstrated sensitivity to medium polarity. The pronounced zwitterionic character was correlated with the high stability of the PMC form, high photoresponsivity, and slow thermal relaxation rates in this class of spirooxazines. The relative ligand field strengths of the SO and PMC forms of the two phenanthroline–spirooxazines were analyzed using the FT-IR and 13C NMR carbonyl signals of their molybdenum–tetracarbonyl– spirooxazine complexes. Differences in metal–ligand bonding in the SO and PMC forms were also investigated by a density functional theory fragment molecular orbital analysis. The SO form was found to be a better π-acceptor both empirically and theoretically. Lastly, the spirooxazine ligands were incorporated into electronically bistable cobalt– dioxolene redox isomers, where the low-spin-CoIII/high-spin-CoII equilibrium is sensitive to ligand field strength. Using solution-state spectroscopic methods, it was shown that the redox state of the cobalt centre could be modulated through photoisomerization of the spirooxazine ligand. As changes in cobalt redox state are associated with changes in magnetic spin state, this system forms the basis for a room-temperature photomagnetic material and highlights the powerful role of photochromic phenanthroline–spirooxazine ligands in developing photoswitchable multifunctional materials.



Photoswitchable, ligands, metal