Visualization of stress and strain in polymeric materials is essential for advanced material development, and mechanochromic mechanophores (MMs)—which reversibly change their optical properties under mechanical force—have attracted considerable attention. Here, we present a flag-hinged MM based on highly emissive boron complexes, covalently integrated into polyurethane (PU) elastomers. Upon stretching, the PU films induce a bent-to-planar conformational transformation of the MM, leading to distinct changes in its absorption and emission. The MM demonstrates excellent photophysical properties, achieving a photoluminescent quantum yield (PLQY) above 90% in dilute solution and maintaining a high PLQY of up to 78% even when embedded in PU films at very low concentrations (0.008–0.080 mol%). Repeated stretching aligns the polymer chains, enhancing the mechanochromic response and inducing an emission shift of over 45 nm, from yellow to orange. The emission wavelength directly follows the stress–strain behavior. Operando tensile testing combined with a streak-camera analysis generated two-dimensional emission–lifetime maps, providing insight into the underlying emission mechanism. In contrast to typical polymer stretching—which suppresses aggregation and results in a blue-shifted emission—this system promotes self-absorption, leading instead to a red shift. These results open new avenues for the design of high-brightness mechanochromic dyes with promising applications in stress sensing and advanced optoelectronic devices.
Maeda et al. (Thu,) studied this question.