Development of photoswitchable charge-transfer materials with photochromic spirooxazines: from molecular systems to surfaces




Kurimoto, Aiko

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Optical modulation of the physical properties of materials is important for future development of optical memories and switches, optoelectronics, and smart surfaces. Incorporation of an optically bistable photochromic compound into an electronically bifunctional material is a promising strategy for a development of photoswitchable materials. Photochromic spirooxazine ligands undergo light-induced ring-opening and closure between the closed-spirooxazine (SO) and open-photomerocynanine (PMC) forms. The structural reorganization leads to accompanying changes in electronic structure which can lead to a change in the oxidation/reduction potentials and spin state of a bound metal center. Changes in the ligand field about a metal center in turn can lead to “non-classical” photoinduced magnetic (PIM) effects. The “non-classical” PIM effect is an effect that occurs through ligand-centered processes via the metal center, rather than direct excitation at the metal center. The structural change of the photochromic compounds also results in a change in the frontier orbital energies and donor-acceptor character, which may lead to optically-gated charge-transfer and energy-transfer processes. In this dissertation, the structural factors that govern thermal relaxation of spirooxazines, as optical control units, was investigated toward controlling the photostationary states of this important class of photochromes. The electronic structure of the PMC form of azahomoadamantyl-based spirooxazines was found to control the thermal coloration/decoloration rates of photochromic spirooxazines. A significant charge-separated character of the PMC form was correlated with the slow thermal coloration/decoloration rates in spirooxazines. This concept was then extended to an investigation of the effect of Lewis-acidic metal complexation. Solution study of the charge-separated character of the PMC form via metal complexation of the photochromic spirooxazines supported the correlation between the charge-separated character of the PMC form and the rate of the thermal coloration/decoloration. The studies provide a potential pathway for modulating PMC thermal relaxation rates through optimization of the structure of the spirooxazines and metal complexation. The studies were then extended to an investigation of the photomodulation of charge-transfer processes in cobalt multinuclear clusters by photoisomerization of photochromic spirooxazines. Incorporation of optically bistable phenanthroline-spirooxazine ligands into a magnetically bistable cobalt-dioxolene valence tautomeric cluster resulted in large magnetic moments in the solid and solution states. This study suggests that the redox-isomeric behavior of the cobalt dioxolenes can be coupled to isomerization of the photochromic ligand in the solution state when the π-acceptor ability of the photochromic ligands align with the direction of charge transfer of the cobalt dioxolene components. The potential of these cobalt multinuclear clusters to enhance the relaxivity of water in MRI for biological imaging was investigated. A cobalt tetranuclear cluster was prepared and found to exhibit high magnetic moments in solution at room temperature, and large relaxivities relative to commercially available gadolinium based MRI contrast agents. Lastly, the photomodulation of ionic doping of graphene organic field-effect transistors (OFETs) by photochromic spirooxazines was investigated. The electron donor or acceptor nature of the photochromic isomers modulates the direction and magnitude of ionic doping of graphene, and in turn the gate voltages of graphene OFETs, leading to optical modulation of OFET gate voltages for data processing and memory technologies.



photoswitching, photochromism, spirooxazines, magnetism, MRI contrast agents, graphene, organic field effect transistors