This molecule is composed of carbon, hydrogen and nitrogen, and when it is
hit by sunlight, it does something unusual: the bonds between its atoms are
rearranged and it turns into an energised new version
<https://www.nature.com/articles/s41467-018-04230-8> of itself, called an
isomer.
Like prey caught in a trap, energy from the sun is thus captured between
the isomer's strong chemical bonds, and it stays there even when the
molecule cools down to room temperature.
When the energy is needed - say at nighttime, or during winter - the fluid
is simply drawn through a catalyst that returns the molecule to its
original form, releasing energy in the form of heat.