New thermochromic materials are reported, which are useful for application as time–temperature indicators (TTIs) in temperature regimes that are complementary to those accessible with existing technologies. The design approach involves incorporating small amounts (0.3–2.9 wt.-%) of the excimer-forming, photoluminescent chromophores 4,4′-bis(2-benzoxazolyl)stilbene or a cyano-substituted oligo(p-phenylene vinylene) derivative into ethylene/norbornene copolymers with glass transition temperatures (T_g) between 131 and 149 °C. These dye/polymer blends were melt-processed and quenched from homogeneous melts to below their T_g to kinetically trap the dyes in dispersed states, which exhibit monomer fluorescence. Upon annealing the materials above T_g, self-assembly of the dye molecules into aggregates that allow for excimer formation occurs, concomitant with permanent and pronounced fluorescence color changes. This approach yielded TTI materials that demonstrate a predictable, Arrhenius-type behavior and which are useful in a temperature regime from ∼130 to 200 °C. The kinetics of the color change were tunable on time scales between seconds and days by changing dye concentration, dye structure, or host polymerT_g.