P-Methoxy Azobenzene Terpolymer as a Promising Energy-Storage Liquid Crystal System

Light-responsive materials capable of undergoing photoinduced molecular transformation are excellent candidates for energy storage. Herein, we report a promising new liquid crystalline terpolymer that is capable of trapping the absorbed photon energy upon exposure to UV light through trans → cis isomerization and molecular aggregation. MeOAzB-T (contains p-methoxyazobenzene, 2-acrylamido-2-methyl-1-propanesulfonic acid, and methyl(methacrylate) monomeric units) shows a trans cis normal thermal recovery in solution (THF) after UV exposure, whereas in the glassy state (at 22 °C) and smectic phase (at 75 °C), the recovery process indicates an unusual increase in absorbance of the trans isomer. Surprisingly, when the smectic phase was cooled down to the glassy state while maintaining UV exposure, the increase in absorbance of the trans isomer was maintained at the same level in the dark (monitored for 20 days) until the sample was heated to 75 °C and then cooled to 22 °C in which a full recovery was achieved. Enhancement of H- and J-aggregations of the trans-MeOAzB units was found to be responsible for the absorbance increase. Both forms of aggregation have absorbance signatures and were reproduced by time-dependent density functional theory (TD-DFT). A long lifetime component was detected in the femtosecond transient absorption (TA) spectra in thin films (215 ps in the glass state and 97 ps in the smectic phase) and was assigned to a long-lived intermediate state as a result of aggregation. The stability of the excited state leads to more aggregation as a consequence of the larger dipole moment, compared to the ground state, which is manifested in the TD-DFT calculations as an increase in electron density at the central azo bond. The current results suggest that aggregation after UV irradiation can be maintained for a long time at room temperature and can be relaxed to the original configuration by heating above the glass transition, showing the suitability of MeOAzB-T as a solar thermal fuel.