Novel Photoactive Spirooxazine Based Switch@MOF Composite Materials

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. A possible approach for the development of such smart materials is the construction of hybrid systems that contain these photochromic molecules as well as a porous host matrix, which enables their switching process in the solid state. We herein present the first light‐responsive materials consisting of the photoswitchable spirooxazine 1,3,3‐trimethyl indolinonaphthospirooxazine (SP−O) and crystalline porous MOF (metal–organic framework) hosts, namely MOF‐5, MIL‐68(In), MIL‐68(Ga), and MIL‐53(Al), which were combined to form hybrid SP−O@MOF composites. These systems show a reversible photochromic response and host‐dependent absorption maxima of the incorporated guest molecules. Most remarkably, SP−O is extremely photostable upon repetitive and prolonged UV light exposure especially inside MOF‐5, making these composite materials attractive candidates for potential applications in data storage devices.

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

“…With respect to the embedment of photochromic molecules as non-covalently attached guest molecules, many functional materials with promising properties were obtained. Here, stilbenes [41] , 2-(phenylazo)pyridine [42,43] , azobenzenes and (fluorinated) derivatives [44][45][46][47][48][49] , diarylethenes [50] , spiropyrans [8][9][10][11] and spirooxazines [51] were applied and extrinsically introduced into different MOF host scaffolds.…”

“…For azobenzene@MOF [44,49] and spirooxazine@MOF [51] composites, it was possible to obtain IR spectroscopic data of both the ground and excited species in solid state for the first time.…”

“…Focusing on spirooxazines embedded inside different MOF matrices, the merocyanine form in its TTC [52] form was generated upon UV light irradiation [51] . Additionally, high fatigue resistance upon prolonged and repetitive UV light exposure was found, which was tracked via IR and UV/vis spectroscopic measurements.…”

“…Additionally, high fatigue resistance upon prolonged and repetitive UV light exposure was found, which was tracked via IR and UV/vis spectroscopic measurements. [51] Following the results of the IR spectroscopic investigations on azobenzenes and spirooxazines inside different MOF hosts, we present the first detailed IR-spectroscopic data of the nitro-substituted spiropyran, denoted as SP-Nitro in the following, non-covalently attached to MOF-5 [53] , MIL-68(In) [54] and MIL-68(Ga) [54] in this work. The vibrational changes of these SP-Nitro@MOF composite materials [8] were studied before and after irradiation with UV light (short and prolonged irradiation times) in order a) to give a complete assignment of all IR bands of the closed SP-form and the open MC form in the solid state and b) to probe the photostability of these hybrid systems.…”

“…Using the experimental and calculated data of Nakata and co-workers on SP-Nitro [52] inside an argon matrix at 8 K as a reference, we analyzed and assigned decreasing and increasing IR bands of both the closed and the open isomer of SP-Nitro we studied inside different MOF host matrices at ambient conditions (room temperature). For this purpose, the spectra of the non-irradiated species were subtracted from the irradiated ones to obtain extirrad-extground state difference spectra, which has successfully been applied for spirooxazine@MOF composite materials in a study prior to this work [51] . These difference spectra were then used to study IR band alterations upon UV light irradiation.…”