Halochromism in the UV-Vis-NIR Region of Polymers Incorporating Overcrowded Bistricyclic Aromatic Enes

Bistricyclic aromatic enes (BAEs) are known for their fascinating ability to change color in response to stimuli like heat or mechanical force. However, as this phenomenon is typically limited to the solid state, such as crystals, their incorporation into versatile polymer materials has remained to be investigated. To explore their potential in soft materials, we incorporated a BAE unit into the flexible main chains of poly(methyl acrylate) (PMA) and polyurethane (PU). We then investigated the chromic behavior of these polymers to mechanical and chemical stimuli. Although they did not change color when stretched, they revealed a striking halochromism. The polymer films, initially absorbing dominantly in the UV region, remained unchanged with acetic acid but underwent a dramatic transformation upon exposure to trifluoroacetic acid, causing their significant absorption band shifts into the visible and near-infrared (NIR) regions. DFT calculations suggest this halochromism originates from the protonation of the BAE unit. Furthermore, this process was found to be reversible, as the addition of an amine restored the original color. These findings highlight the potential of these BAE-containing polymers for advanced applications like sensors or NIR-light-responsive materials.