Roles of ESIPT and TICT in the photophysical process of a Zn²⁺ sensor: Ratiometric or turn-on

The cis–trans isomerization of the C=N bond is generally believed to induce quenching in molecules containing a Schiff base. These molecules typically serve as turn-on sensors for cations, with only a few acting as ratiometric sensors. This study presents a comprehensive investigation into the photophysical processes of an unusual ratiometric sensor for Zn²⁺ based on Schiff base, utilizing density-functional theory (DFT) and time-dependent DFT (TDDFT). By examining the potential energy surface (PES) of the S₁ state, multiple dynamic processes including excited state intramolecular proton transfer (ESIPT), bond twisting, and C=N isomerization were analyzed. Energy barriers and rate constants for these processes were obtained and compared to evaluate their likelihood of occurrence. It was found that C=N isomerization can only take place after an ESIPT process and leads to a non-emissive twisted intramolecular charge transfer (TICT) state, turning the probe into a turn-on sensor. However, the electron-withdrawing nature of the cyano group induces strong intramolecular charge transfer during photoexcitation and leaves the Schiff base unexcited. As a result, the ESIPT process is unfavorable, and the subsequent C=N isomerization is prevented, making the probe a ratiometric sensor. Moreover, two additional sensors with electron-donating and electron-withdrawing groups were designed, and their photophysical processes were studied, providing further support for the proposed theory.