Stimuli‐Responsive Dual‐Color Photon Upconversion: A Singlet‐to‐Triplet Absorption Sensitizer in a Soft Luminescent Cyclophane

: Reversible emission color switching of triplet-triplet annihilation-based photon upconversion (TTA-UC) is achieved by employing an Os complex sensitizer with singlet-to-triplet (S-T) absorption and an asymmetric luminescent cyclophane with switchable emission characteristics. The cyclophane contains the 9,10-bis(phenylethynyl)anthracene unit as an emitter and can assemble into two different structures, a stable crystalline phase and a metastable supercooled nematic phase. The two structures exhibit green and yellow fluorescence, respectively, and can be accessed by distinct heating/cooling sequences. The hybridization of the cyclophane with the Os complex allows near-infrared-to-visible TTA-UC. The large anti-Stokes shift is possible by the direct S-T excitation, which dispenses with the use of a conventional sequence of singlet-singlet absorption and intersystem crossing. The TTA-UC emission color is successfully switched between green and yellow by thermal stimulation.