Smart Molecular Crystals With Dynamic Photonic Modulation and Integrated Waveguiding

Stimuli‐responsive materials capable of dynamically tuning their optical outputs promise new architectures for smart photonics, yet simultaneously achieving rapid response kinetics and low loss optical waveguiding within a single crystal phase remains challenging. Here, we present two Excited‐state intramolecular proton transfer (ESIPT) active Schiff base crystals where supramolecular hydrogen bonding engineering resolves this trade‐off. By strategically introducing intermolecular hydrogen bonding networks alongside intrinsic intramolecular proton transfer channels, these crystals deliver highly sensitive, fully reversible thermofluorochromic switching coupled with efficient waveguiding performance. Single‐crystal X‐ray diffraction and theoretical calculations reveal that the subtle interplay between molecular conformation and supramolecular packing topology governs both the thermal emission color gradation and the quenching dynamics. Leveraging these dual photonic functions, we demonstrate a multilevel anti‐counterfeiting platform that pairs static visual features with dynamic thermal responsiveness. Overall, this work highlights the potential of tailor‐made supramolecular architectures in converting organic molecular crystals into versatile building blocks for advanced micro‐/nanophotonics and information encryption.