Molecular Design for Luminescent Azobenzene‐Containing Conjugated Polymers and Their Application to Film Sensors for Recording Acid Exposure History

It has been found that azobenzene structures show luminescence by suppressing their photoisomerization and alternating electronic states through molecular modification and metal complexation. Herein, we propose a three-step design strategy for creating luminescent azobenzene derivatives from quantum chemical calculations. First, we introduced intramolecular hydrogen bonding between o-hydroxy groups and the azobenzene moiety, not only for stabilizing the n-orbital of azobenzene but also for suppressing photoisomerization. Second, the expansion of the π-conjugation was employed to convert the S_0-S₁ transition to the allowed π-π^* transition. Third, we derived the fast excited-state intramolecular proton transfer (ESIPT) process, which is suitable for observing emission from azobenzene. Through these steps, we designed and synthesized azobenzene derivatives and succeeded in observing intense deep-red to NIR emission from conjugated polymers including azobenzene moieties in the main chain. Furthermore, we applied the synthesized polymers to film-type acid vapor sensors. In addition to the real-time detection ability toward acid vapor observed in our previous work, we found that the current material can discriminate whether the film was once exposed to acid vapor from the changes in luminescence and film colors. Our findings reveal that azobenzene is a potential platform for creating luminescent materials with stimuli-responsiveness according to rational molecular design.