Sequence-Switchable MOF-on-MOF Photonic Crystals for Broadband Visible Structural Color Modulation

Abstract

Structural colors generated by photonic crystals (PCs) have attracted interest in optical sensing. However, the sensing behavior of porous PCs is commonly tailored through material composition, pore structure, or surface chemistry. Here, we show that the spatial configuration of sequence-switchable MOF-on-MOF Bragg stacks regulates molecular access and adsorption while preserving the constituent metal–organic frameworks (MOFs). The reflection wavelength is continuously tuned across the visible spectrum by controlling layer thickness and refractive-index contrast, producing blue-to-red structural colors with a peak reflectance of approximately 90%. Reversing the layer sequence generates distinct optical response amplitudes and kinetics toward volatile organic compounds (VOCs). Combined optical analysis and molecular dynamics simulations further show that the sensing responses depend not only on molecular uptake but also on spatial adsorption distribution and framework–guest interactions. These findings establish molecular transport pathway control through spatial configuration as an additional strategy for regulating the sensing behavior of porous photonic materials.