Storage of thermal energy using a piezochromic property of TADF materials

Thermally activated delayed fluorescence (TADF) requires a thermally activated step followed by light emission, which can be considered thermal energy harvesting and conversion to usable energy. However, the storage of the harvested thermal energy in this process is a challenging task because it is immediately converted and released. Here, by preparing a low-entropy state of NAI-DMAC, Ac-CNP, and 2-(3-(4-(diphenylamino)phenyl)-9H-carbazol-9-yl)isonicotinonitrile TADF samples through mechanical grinding, which provides a piezochromic effect, we show that a subsequent thermally induced reconstruction of these materials yields a twofold boosting of photoluminescence emission in a relatively narrow temperature interval between 70 and 90 °C, which then remains almost unchanged upon sample cooling and storage, being accessed later upon photoexcitation. The observed phenomenon is explained by the thermally induced conformation of the TADF molecules that adopt a reduced twisting, which is stabilized through increased intermolecular coupling and packing. The estimated value of the stored energy is in the range of 2–5 kJ/mol, being reached using a small charging energy of the same order of magnitude, which gives an excellent efficiency of the energy storage process.