Exploring the Photophysical Processes of an Al³⁺ Sensor Based on Schiff Base

Schiff bases are commonly used as building blocks in the development of turn-on sensors for Al³⁺ detection. The isomerization of the CN bond in Schiff bases is generally believed to induce fluorescence quenching. Inhibiting this isomerization process through interactions with the target ion, Al³⁺, enhances fluorescence, enabling its detection. This mechanism is widely used to explain turn-on signals in similar sensors. However, the photophysical processes of such sensors may be more complex, necessitating a deeper understanding of their underlying sensing mechanisms. This study presents a comprehensive investigation into the photophysical processes and sensing mechanism of a turn-on sensor for Al³⁺ featuring a Schiff base moiety. Multiple excited-state intramolecular proton transfer (ESIPT) processes are observed, all closely associated with the Schiff base structure. These ESIPT processes trigger CN isomerization, leading to the formation of two nonemissive twisted intramolecular charge transfer (TICT) states. In addition to CN isomerization, two bond rotation processes with lower energy barriers are identified. These rotational processes generate two additional nonemissive TICT states and play a dominant role in the weak fluorescence of the sensor. This elucidation of photophysical processes provides a clearer understanding of the Al³⁺ sensing mechanism.