Dipole‐Driven Polymorphism in Azobenzene: A Supramolecular Strategy for Photoresponsive Microresonators

ABSTRACT Although azobenzenes have the potential to possess both aggregation‐induced emission (AIE) and photoresponsivity, examples are still limited due to their low emission efficiency. Herein, we report a new supramolecular strategy utilizing a large molecular dipole moment to enhance emission efficiency in the aggregate state and to induce controllable crystal polymorphism, thereby demonstrating the first observation of azobenzene‐based whispering gallery mode (WGM) emission with multiple sharp peaks. While dipolar azobenzene monomers in toluene are non‐emissive, the drop‐cast film exhibits red emission under photoexcitation due to the AIE effect. Microscopic photoluminescence (PL) measurements revealed that the film comprises a mixture of crystals, with major red and minor near‐infrared (NIR) emissions. By tuning the recrystallization temperature in toluene, we successfully controlled the crystal polymorphism and revealed the PL properties of each form using time‐resolved PL spectroscopy and theoretical approaches. A direct transition from the NIR‐emissive crystal to the red‐emissive one was observed upon thermal annealing. Remarkably, the micro‐sized particles exhibited multiple sharp PL peaks due to the microresonator effect, attributed to WGM emission, and the WGM peak positions shifted upon photoirradiation. This work provides a fundamental principle for the creation of new photoresponsive WGM‐based materials.