Multiple excited-state intramolecular proton transfer (ESIPT) processes and twisted intramolecular charge transfer (TICT) states are observed on the S₁ state potential energy surface of an Al³⁺ sensor. The ESIPT process induces an ultrafast CC rotation process and leads to a non-emissive TICT state. By contrast, the CN isomerization has a large energy barrier and is not likely to take place.

Schiff bases are commonly used as building blocks in the development of turn-on sensors for Al³⁺ detection. The isomerization of the CN 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 CN isomerization, leading to the formation of two nonemissive twisted intramolecular charge transfer (TICT) states. In addition to CN 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.