Solid state thermosalience-a sudden exertion of an expansive or contractive physical force following a temperature change in a solid state compound-is rare, few are reversible systems, and most of these are limited to a dozen or so cycles before the crystal degrades or they reverse slowly over the course of many minutes or even hours. In this work, we show a fully reversible actuator that is stable at room temperature for multiple years and is capable of actuation for more than two hundred cycles at near ambient temperature. Specifically, the crystals shrink to 90% of its original length instantaneously upon heating beyond 45 °C and expands back to its original length upon cooling below 35 °C. This temperature regime is important because it occurs around physiologically important temperatures. Furthermore, the phase transition occurs instantaneously, with little obvious hysteresis, allowing us to create real-time actuating thermal fuses that cycle between on and off rapidly.

Alteration of the number of carbon atoms on the alkoxyphenyl substituent in naphthalene diimides results in tunable thermo-salient behavior.

π-stacking in ground-state dimers/trimers/tetramers of N-butoxyphenyl(naphthalene)diimide (BNDI) exceeds 50 kcal ⋅ mol−1 in strength, drastically surpassing that for the *3[pyrene]2 excimer (∼30 kcal ⋅ mol−1; formal bond order = 1) and similar to other weak-to-moderate classical covalent bonds. Cooperative π-stacking in triclinic (BNDI-T) and monoclinic (BNDI-M) polymorphs effects unusually large linear thermal expansion coefficients (αa, αb, αc, β) of (452, −16.8, −154, 273) × 10−6 ⋅ K−1 and (70.1, −44.7, 163, 177) × 10−6 ⋅ K−1, respectively. BNDI-T exhibits highly reversible thermochromism over a 300-K range, manifest by color changes from orange (ambient temperature) toward red (cryogenic temperatures) or yellow (375 K), with repeated thermal cycling sustained for over at least 2 y.