Light-Switchable One-Dimensional Photonic Crystals Based on MOFs with Photomodulatable Refractive Index

Photonic crystals are solids with regular structures having periodicities comparable to the wavelength of light. Here, we showcase the photomodulation of the refractive index of a crystalline material and present a quasi-one-dimensional photonic crystal with remote-controllable optical properties. The photonic material is composed of layers of TiO₂ and films of a nanoporous metal-organic framework (MOF) with azobenzene side groups. While the rigid MOF lattice is unaffected, the optical density is reversibly modified by the light-induced trans-cis-azobenzene isomerization. Spectroscopic ellipsometry and precise DFT calculations show the optical-density change results from the different orbital localizations of the azobenzene isomers and their tremendously different oscillator strengths. The photomodulation of the MOF refractive index controls the optical properties of the quasi-one-dimensional photonic crystal with Bragg reflexes reversibly shifted by more than 4 nm. This study may path the way to photoswitchable photonic materials applied in advanced, tunable optical components and lens coatings and in light-based information processing.