Observation of a New TICT State during the Photophysical Process of an Al³⁺ Sensor

Schiff base with a C═N bond is widely used in the fabrication of turn-on sensors for cations. The isomerization of C═N is generally believed to induce a dark state and quenches the sensor's fluorescence. With the aid of time-dependent density functional theory (TDDFT), this contribution performs a comprehensive investigation on the photophysical process of a turn-on sensor for Al³⁺. The isomerization of C═N leads to a non-emissive twisted intramolecular charge transfer (TICT) state, which is initiated by an early stage excited state intramolecular proton transfer (ESIPT) process. However, this isomerization process has a very large energy barrier and low reaction rate that cannot effectively quench the sensor's fluorescence. Interestingly, a brand new non-emissive TICT state is observed which is not induced by the isomerization of C═N but by the rotation of a neighboring C-C bond. Due to the low rotation energy barrier, this new TICT state can be attained easily and opens up an effective channel for non-emissive decays. This observation implies that the excited state potential energy surface for sensors based on a Schiff base should be much more complicated than expected. Based on the photophysical process, the sensing mechanism for Al³⁺ as well as its selectivity in the face of interfering cations are uncovered.