Theoretical investigation on the fluorescence properties and ESIPT mechanism of the Al3+ ion sensor 1-((2-hydroxynaphthalen-1‐yl)methylene)urea(OCN)

We propose the double interaction mechanism between Al3+ ions and OCN or the isomer structure OCN-T. The absorption spectra of the sensors (OCN and OCN-T) and complexes with Al3+ (OCN-Al and OCN-T-Al) calculated by our theory agree well with the experimental values. For the fluorescence spectra, the OCN structure is at 429 nm, OCN-T emits a near-infrared fluorescence at 1380 nm, OCN-AL at 460 nm, and OCN-T-AL has no fluorescence. It shows that OCN and its isomer OCN-T are involved in the absorption, and the detected spectral signal is emitted from the OCN-AL complex. [Display omitted] •The double interaction mechanism between Al3+ ions and OCN or the isomer structure OCN-T is proposed.•It is found that the OCN-T molecule directly undergoes excited state proton transfer.•The hydrogen bonding of the OCN molecule is enhanced in the excited state. A new action mechanism for the fluorescent detection on the Al3+ ion of the sensitive 1-((2-hydroxynaphthalen-1-yl)methylene)urea(OCN) is theoretically studied. The extensive theoretical calculations on the OCN and the isomer structure OCN-T are performed. The emission and absorption spectra consistent with the experiment value. The absorption spectra peaks (362 nm and 326 nm) of OCN and OCN-T molecules are attributed to the experimentally observed absorption spectra at 356 nm and 314 nm, respectively. The calculated fluorescence value of the OCN-AL structure is 460 nm, while the OCN-T-AL structure has no fluorescence. These results better explain that OCN and its isomers OCN-T are involved in the absorption, and the detection spectrum signal is emitted from the OCN-AL complex. The OCN and OCN-T molecules are obvious hydrogen bonding systems. The excited state intramolecular proton transfer photochemical behaviors and detecting Al3+ ion photophysical changes were explained for the first time at the molecular level. As driving force of excited state intramolecular proton transfer (ESIPT) reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals. To further elucidate the proton transfer reactive paths, the scanned the potential energy curves (PECs) of OCN and OCN-T chemosensor in different electronic states are plotted. This work proposes a reasonable explanation for the detection mechanism of the OCN sensor.