Fluorescence enhancement mechanism of thymolphthalein-based probe by coordination interaction with zinc ion

Abstract

Although there are some proposed explanations for fluorescence enhancement of metal ions, a phenomenon with wide applications in biosensors and chemosensors, current understanding of the quantum–mechanical origin of this photophysical behaviour is limited. To address this issue, we selected thymolphthalein-based probe (T) reported recently as model system, and investigated the photochemical and photophysical properties of T and T-Zn(II) complex. The geometry structures of T and its Zn(II) complex were optimized, and the experimental absorption spectrum were well reproduced through density functional theory (DFT) methods. Furthermore, the excited state intramolecular proton transfer (ESIPT) mechanisms of T were systematically investigated. The constructed potential energy surfaces (PESs) of T at ground (S0) and first exited (S1) states indicate that, after photo-excitation, the intramolecular single-proton transfer reaction occurs more readily with the H atom removing from O to neighboring N. Furthermore, thermodynamic analysis of the T-Zn(II) complex shows that mononuclear complex (C1) is the most thermodynamically stable structure, and the intramolecular single proton transfer process of C1 is thermodynamically unfavorable at the S1 state. Time-dependent DFT calculations show that the fluorescence emission is from the S1 states of the single-proton transfer tautomer of T, as predicted from Kasha’s rule, in comparison, the fluorescence emission of Zn(II) complex is from the S1 states of T-Zn(II) rather than its single-proton transfer tautomer. The inhibitor of Zn(II) on the single proton transfer reaction plays an important role on fluorescence enhancement.