The photo-switching study of guest 2-(phenylazo)pyridine (PAP) embedded in solid host material MOF-5

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

Understanding the interactions between the single components of hybrid systems is essential to drive the development of advanced functional materials. A prerequisite for this is the systematic variation of the building blocks of such compounds. Focusing on spiropyran@metal-organic framework (MOF) composite materials with noncovalently attached spiropyran dyes, both the host scaffold and the dye molecules can be systematically tuned. In this work, a broad substitution pattern was applied to systematically elucidate the characteristics of the resulting hybrid materials as a function of the supplemental substitution on spiropyran. The newly developed 12 composites exhibit substitution and host-dependent optical characteristics, which are particularly affected by the substitution of the 6'-position on the chromene ring. Through the favorable combination of the MOF host's polarity and an adequate strength of the spiropyran's indoline_donor-chromene_acceptor pair, reversible conversion between photoisomers is efficiently accomplished, especially for nitro-substituted spiropyrans inside MIL-68(In).

Structural, dynamical, and photochemical properties of ortho-tetrafluoroazobenzene inside a flexible MOF under visible light irradiation

Considering porous materials as host matrices is an elegant way to enable photoswitching of non-covalently attached organic dyes even in the solid state. By focusing on the resulting optical properties as a function of loading degree and synthesis procedure, the occurring host-guest and guest-guest interactions can be determined and further exploited. In the course of this study, the photochromic behavior of ortho-tetrafluoroazobenzene (tF-AZB) inside flexible DMOF-1 was investigated from these points of view. It was found that depending on the loading degree and temperature, tF-AZB shows varying E/Z ratios and switching efficiency. For systems with low loading, reversible visible light induced isomerization was observed over ten switching cycles: Upon violet light exposure, formation of 100% E isomer was generated, while green light irradiation resulted in ∼60% Z-tF-AZB. Complementary molecular dynamics simulations at DFTB (density functional tight binding)-level revealed changing binding sites for Z-tF-AZB inside DMOF-1. For the E isomer, only low oscillations have been found, which in turn display a rare T-stacking interaction. Although the interaction strengths of the E and Z isomers with DMOF-1 are in the same range, the different mobility of both isomers due to varying binding sites explains the preference of the E isomer even upon green light exposure.

Transition metal complexes incorporated with photoswitchable azo-based benzimidazole ligands: Photochromic and solvatochromic studies

Photochromic compounds are well-known for their promising applications in many areas. It attracts remarkable attention because of their potential ability for optical memory media and optical switching devices. Herein, we have synthesized azo-based benzimidazole ligand and their transition metal complexes for photochromic applications in the liquid state. The azo-based benzimidazole ligand exhibits trans-to-cis photoisomerization with highly tunable and excellent π-π* and n-π* band separation of ligand, whereas complexes show light-induced photo-dissociation as well as trans-to-cis photoisomerization of the ligand part. The reverse cis-to-trans isomerization can be driven by without using light or any external stimuli at room temperature by keeping the system in the dark condition.

Advanced Photoresponsive Materials Using the Metal-Organic Framework Approach

When fabricating macroscopic devices exploiting the properties of organic chromophores, the corresponding molecules need to be condensed into a solid material. Since optical absorption properties are often strongly affected by interchromophore interactions, solids with a well-defined structure carry substantial advantages over amorphous materials. Here, the metal-organic framework (MOF)-based approach is presented. By appropriate functionalization, most organic chromophores can be converted to function as linkers, which can coordinate to metal or metal-oxo centers so as to yield stable, crystalline frameworks. Photoexcitations in such chromophore-based MOFs are surveyed, with a special emphasis on light-switchable MOFs from photochromic molecules. The conventional powder form of MOFs obtained using solvothermal approaches carries certain disadvantages for optical applications, such as limited efficiency resulting from absorption and light scattering caused by the (micrometer-sized) powder particles. How these problems can be avoided by using MOF thin films is demonstrated.

Tracking the light-induced isomerization processes and the photostability of spiropyrans embedded in the pores of crystalline nanoporous MOFs via IR spectroscopy

Difference IR spectroscopy of spiropyran@MOF systems to obtain the characteristic signatures of the spiropyran and merocyanine form at ambient conditions.