Fluorescent response mechanism based on ESIPT and TICT of novel probe H₂Q_JI: A TD-DFT investigation

Recently, synthesized N-linked-disalicylaldehyde H₂Q_JI probes have been used to detect heavy metal ions in the experiment conveniently. Nevertheless, there needs to be a more in-depth examination of the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of the probe. This work remedied it based on quantum chemistry calculations. We contained due hydrogen bond (O1-H2 ⋯ N3 and O4-H5 ⋯ O6) and then analyzed bond parameters, IR vibration spectra, and non-covalent interaction. The bond strength is enhanced under photoexcitation, and the former is significantly stronger. The calculated electron spectra are in agreement with the experimental values. The results of the S₀ and S₁ potential energy curves and IRC calculations also confirm the unique ESIPT behavior, which isan excited stated stepwise double proton transfer. The fluorescence, internal conversion, and intersystem crossing rate of KD molecules (twisted-, double proton transfer) were calculated respectively to reveal the radiative and non-radiative pathways. It proved that the corresponding spectra are not obtained since the electrons are mainly deactivated by the ISC (S₁->T₁). Furthermore, the interfragment charge transfer (IFCT) approach indicates that the molecule possesses twisted intramolecular charge transfer (TICT) characteristics, which lead to the quenching of fluorescence introduction.