Observation of a New TICT State during the Photophysical Process of an Al 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 Al. 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 Al as well as its selectivity in the face of interfering cations are uncovered.