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

Area of Science:

• Photochemistry
• Supramolecular Chemistry
• Chemical Sensing

Background:

• Schiff bases are widely used in turn-on sensors for Al³⁺ detection.
• The C=N bond isomerization is typically thought to cause fluorescence quenching, with Al³⁺ inhibition leading to a turn-on signal.
• Existing models may oversimplify the complex photophysical processes governing sensor behavior.

Purpose of the Study:

• To comprehensively investigate the photophysical processes and sensing mechanism of a Schiff base-based turn-on sensor for Al³⁺.
• To elucidate the detailed mechanisms responsible for fluorescence modulation upon Al³⁺ binding.
• To provide a deeper understanding beyond the commonly accepted C=N isomerization model.

Main Methods:

• Computational investigation of photophysical processes.
• Analysis of excited-state intramolecular proton transfer (ESIPT).
• Identification of bond rotation pathways and twisted intramolecular charge transfer (TICT) states.

Main Results:

• Multiple ESIPT processes linked to the Schiff base structure were identified.
• ESIPT triggers C=N isomerization, forming nonemissive TICT states.
• Lower-energy bond rotations generate additional nonemissive TICT states, dominating weak fluorescence.

Conclusions:

• The Al³⁺ sensing mechanism is more complex than simple C=N isomerization inhibition.
• ESIPT and multiple bond rotations significantly contribute to the sensor's photophysics.
• This detailed understanding is crucial for designing more efficient Al³⁺ sensors.