Substituent effect induced the distinctive ESIPT reaction and photophysical property of N-salycilidene-5-chloroaminopyridine

Two novel molecules (SCLPY-MeO and SCLPY-CN) are designed based on the N-salycilidene-5-chloroaminopyridine (SCLPY) via introducing the methoxyl (–OCH3) and cyano (-CN) groups at the meta-position of the chlorine atom, respectively, to explore the effect of the substitution of electron-withdrawing and donating groups on the excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of SCLPY, employing the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The significant calculated structural parameters associated with the intramolecular hydrogen bonds (IHBs) have demonstrated that the IHBs intensity is enhanced in the excited state and SCLPY is endowed with the strongest IHB. The partial quenching of the short-wavelength fluorescence in SCLPY-CN has been satisfactorily explained by calculating the transition electric dipole moment and the long-wavelength fluorescence of that exhibits the largest Stokes shifts relative to the other molecules. Moreover, introducing the –OCH3 group has evidently enhanced the luminescent intensity of SCLPY. On account of the potential energy curves (PECs), scanning with gradually elongating the O1-H2 bond, it can be known that the forward proton transfer process is more readily to occur in the S1 state due to the relatively lower energy barriers compared with the ground state. Moreover, SCLPY is provided with the lowest energy barrier, implying that the ESIPT behavior is most likely inclined to proceed in SCLPY. Therefore, it is well illustrated that the introductions of –OCH3 and –CN groups indeed have an impact on the ESIPT process and photophysical properties of SCLPY.