Supramolecular Cation-π Interaction Enhances Molecular Solar Thermal Fuel

Molecular solar thermal fuels (MOSTs), especially azobenzene-based MOSTs (Azo-MOSTs), have been considered as ideal energy-storage and conversion systems in outer or confined space because of their "closed loop" properties. However, there are two main obstacles existing in practical applications of Azo-MOSTs: the solvent-assistant charging process and the high molar extinction coefficient of chromophores, which are both closely related to the π-π stacking. Here, we report one efficient strategy to improve the energy density by introducing a supramolecular "cation-π" interaction into one phase-changeable Azo-MOST system. The energy density is increased by 24.7% (from 164.3 to 204.9 J/g) in Azo-MOST with a small loading amount of cation (2.0 mol %). Upon light triggering, the cation-π-enhanced Azo-MOST demonstrates one gravimetric energy density of about 56.9 W h/kg and a temperature increase of 8 °C in ambient conditions. Then the enhanced mechanism is revealed in both molecular and crystalline scales. This work demonstrates the huge potential of supramolecular interaction in the development of Azo-MOST systems, which could not only provide a universal method for enhancing the energy density of solar energy storage but also balance the conflicts between molecular design and the condensed state for phase-changeable materials.