Molecular solar thermal systems – control of light harvesting and energy storage by protonation/deprotonation

Molecular solar thermal (MOST) systems that undergo photoisomerizations to long-lived, high-energy forms present one approach of addressing the challenge of solar energy storage. For this approach to mature, photochromic molecules which can absorb at the right wavelengths and which can store a sufficient amount of energy in a controlled time period have to be developed. Here we show in a combined experimental and theoretical study that incorporation of a pyridyl substituent onto the dihydroazulene/vinylheptafulvene photo-/thermoswitch results in molecules whose optical properties, energy-releasing back-reactions and energy densities can be controlled by protonation/deprotonation. The work thus presents a proof-of-concept for using acid/base to control the properties of MOST systems.