Solar thermal fuel
(STF) materials store energy through
light-induced
changes in the structures of photoactive molecular groups, and the
stored energy is released as heat when the system undergoes reconversion
to the ground-state structure. Solid-state STF devices could be useful
for a range of applications; however, the light-induced structural
changes required for energy storage are often limited or prevented
by dense molecular packing in condensed phases. Recently, polymers
have been proposed as effective solid-state STF platforms, as they
can offer the bulk properties of solid materials while retaining the
molecular-level free volume and/or mobility to enable local structural
changes in photoresponsive groups. Light-induced energy storage and
macroscopic heat release have been demonstrated for polymers with
photoisomerizable azobenzene side groups. However, the mechanism of
energy storage and the link between the polymer structure, energy
density and storage duration has not yet been explored in detail.
In this work, we present a systematic study of methacrylate- and acrylate-based
polymers with azobenzene side groups to establish the mechanism of
energy storage and release and the factors affecting energy density
and reconversion kinetics. For polymers with directly attached azobenzene
side groups, the energy storage properties are in line with previous
work on similar systems, and the photoisomerization and reconversion
properties of the azobenzene side groups mirror those of molecular
azobenzene. However, the inclusion of an alkyl linker between the
azobenzene side group and the backbone significantly increases the
photoswitching efficiency, giving almost quantitative conversion to
theZisomeric state. The presence of the alkyl linker
also reduces the glass transition temperature and leads to faster
spontaneous thermal reconversion to theEisomeric
form, but in all cases, half-lives of more than 4 days are observed
in the solid state, which provides scope for applications requiring
daily energy storage–release cycles. The maximum gravimetric
energy density observed is 143 J g–1, which represents
an increase of up to 44% compared to polymers with directly attached
azobenzene moieties.