Optical Characteristics of Spiropyran@MOF Composites as a Function of the Metal-Organic Framework Linker Substitution

In functional materials, the understanding of the interactions between the individual components is essential for further development. One example for such materials are guest@host systems, with a metal-organic framework (= MOF) as the host matrix. These porous compounds consist of metal nodes and organic linker molecules and are therefore inorganic-organic hybrid materials. Combining MOFs with photoswitchable dyes such as spiropyrans (SPs) resulted in new functional materials with fascinating properties. To obtain such materials with technologically relevant properties, knowledge of the individual adjusting screws to modulate, for example, the optical response is obligatory to further push material's design. Within this work, we systematically studied the optical properties of a nitro-substituted SP inside an MIL-68(In) series as a function of linker substitution. In particular, 1,3,3-trimethylindolino-6'-nitrobenzopyrylospiran (= SP-Nitro) was non-covalently attached into the pores of MIL-68(In) with the terephthalate linker molecule being functionalized either with a Br-, NH2-, or a NO2- group. Three different hybrid systems were obtained, exhibiting both photochromic and solvatochromic response. The observed optical characteristics were found to strongly depend on the substitution pattern of the linker molecule. Based on the resulting structural and dielectric properties of the substituted terephthalic acid molecules calculated at B3LYP and MP2 levels, the observed UV/vis reflectance spectra were directly correlated with the out-of-plane rotation of the carboxyl group in the vicinity of the respective substituent.