Multifunctional Förster Resonance Energy Transfer Materials as Thermo‐ and Mechanochromic Time Integrators

Abstract Optical integrators are receiving much attention for the monitoring and tracking of exposure conditions of materials to enhance their quality and safety, and to reduce waste. Currently, there is no fluorescent material that can simultaneously track mechanical and thermal histories, which is key to predicting failure modes of materials, and offer programmable response selectivity and sensitivity that provides on‐demand tailorability to various applications. Here, a fluorescent sensor that can simultaneously detect strain and thermal history whilst exhibiting memorability is reported. This optical integrator consists of binary droplets containing fluorescent donors and acceptors, respectively. The fluorescent coating showcases an optical‐time response via Förster resonance energy transfer (FRET) in which the exchange of fluorophores among binary droplets over time leads to a decrease in the average intermolecular distance between a donor–acceptor pair. The irreversible thermochromic response can be programmed by both the construction of the coating and the concentration of the dye‐doped liquid crystal (LC) droplets. The fluorescent coating can further be utilized as a strain sensor. The programmable mechanochromic response depends on the duration and degree of strain. The integrator provides an interface that enables visual detection of both the strain and temperature history of materials.