Schwartz, Heidi Annemarie
(2018).
Metal-Organic Frameworks as Crystalline Porous Hosts for Photoactive Molecules.
PhD thesis, Universität zu Köln.
Abstract
Molecules, which reversibly transform between two structural configurations upon excitation with electromagnetic radiation, are attractive candidates for the design of smart materials e.g. memory devices. An essential prerequisite for the development of such smart materials is the construction of hybrid systems that contain these photochromic molecules on the one and a porous host matrix, which enables their switching process in solid state, on the other hand. In the present thesis such light responsive materials consisting of switchable guests and crystalline porous hosts were synthesized to form hybrid systems. These systems show photochromic response and host dependent absorption maxima of the incorporated guest molecule, which is known as solvatochromism. Spiropyrans and spirooxazines were selected as guest molecules and different Metal-Organic Frameworks (MOFs) as suitable host matrices.
The starting point of this thesis was the nitro-substituted spiropyran 1,3,3- trimethylindolino-6´-nitro-benzopyrylospiran (SP-Nitro) as photochromic guest molecule. For the first time, the embedment into different MOFs as host matrices allowed the light induced isomerization to
convert from its closed SP form to the open MC form in solid state. Moreover, the host scaffolds provide varying chemical environments within their pores, which caused MOF dependent absorption maxima of the incorporated SP-Nitro. This solvatochromic response highly resembles the behavior of SP-Nitro when dissolved in solvents of varying polarity.
Thus, the term of solvatochromism is expanded to MOFs, which can be classified as “solid solvents” for spiropyrans. Based on these results, the structurally related spirooxazine 1,3,3-trimethylindolino-naphthospirooxazine (SP-O) was embedded. In contrast to spiropyrans, SPO exhibits enhanced resistance towards photodegradation. Analogous to SP-Nitro, SP-O converts to its open MC-O form upon UV light exposure inside the MOF pores, but with suppressed photodegradation. Furthermore, solvatochromism was perceived in dependence of
the used host scaffold as well. The solvatochromic response of both dyes, spiropyrans and spirooxazines, clearly indicates different polarities of the host lattices, which were finally determined by the embedment of the polarity sensor 4,5-dimethoxy-N-butylphthalimide. Here, the light emission also diverged relying on the surrounding host matrix. Consequently, MOFs behave analogously to solvents as both interact with and influence the electronic structure of the guest molecule. Moreover, MOFs enable switching processes of spiropyrans and spirooxazines in solid state. Therefore, MOFs function as “solid solvents” for dye molecules. This fundamental knowledge is indispensable for the systematic design and development of functional smart composite materials.
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