TDDFT study on the sensing mechanism of a fluorescent sensor based on Si O bond for fluoride anion

TDDFT study on the photophysical properties of coumarinyl chalcones and sensing mechanism of a derived fluorescent probe for hydrogen sulfide

Coumarin-chalcone hybrids have attracted much attention in recent years due to their important optical properties. Herein, the photophysical properties of a series of coumarinyl chalcones and the sensing mechanism for H₂S of a related fluorescent probe CC-DNP were investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The predicted spectral properties agree well with the experimental results, which allowed an assignment of the spectra. Our calculations successfully clarified the experimental observed fluorescence "off-on" effect and the fluorescent quenching mechanism of the probe. The results revealed that the first excited state (S₁) of the probe CC-DNP is a dark state with obvious charge transfer from coumarin unit to 2,4-dinitrophenyl (DNP) moiety, which results in the fluorescence quenching via the nonradiative photoinduced electron transfer (PET) process. On the other hand, the excited state S₁ in the thiolysis product CC-OH decayed directly to S₀, and thus the fluorescence is recovered.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

TDDFT study on aluminum and fluoride dual-sensing mechanism of a Schiff-Base sensor

The aluminum and fluoride dual-sensing mechanism of a previously reported sensor with a Schiff-base moiety (Spectrochim. Acta A, 2017, 183, 267-274) has been investigated by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. The present calculations reproduce the photoproperties of the sensor as well as its aluminum and fluoride complexes, which illustrates that DFT and TDDFT constitute a reliable tool for uncovering detailed fluorescence-based sensing mechanisms in diverse electronic states. Theoretical results indicate that there are two OH⋯N hydrogen bonds in the sensor and two OH⋯F hydrogen bonds in its F¯ complex. Different degrees of coplanarity caused by these hydrogen bonds are responsible for their distinct absorption wavelengths. However, excited-state geometry optimization and a scan of the potential-energy surface show that there is twisted intramolecular charge transfer about the CN bond in the sensor molecule and an excited-state proton-transfer process from the fluoride anion to the neighboring N atom in the fluoride-sensor complex, whereby the fluorescence is quenched. A chelation-enhanced fluorescence effect associated with the aluminum-sensor complex shows a different excited-state process. The local excitation and emission occur exclusively within the planar fluorophore, and negligible structural change upon excitation of the aluminum-sensor complex leads to its strong fluorescence. Therefore, it is theoretically explained why the sensor may be successfully used to analyze the fluoride anion by absorption spectroscopy and the aluminum cation by emission spectroscopy.

Colorimetric and fluorimetric response of salicylaldehyde dithiosemicarbazone towards fluoride, cyanide and copper ions: Spectroscopic and TD-DFT studies

The sensing mechanism of salicylaldehyde phenyldithiosemicarbazone (SDTSC) chemosensor has been investigated by spectroscopic and TD-DFT methods. The SDTSC shows colourimetric and spectral changes towards fluoride, cyanide and copper ions. The interaction between SDTSC with fluoride, cyanide and copper ions was examined through their absorption and fluorescence behaviour, and found that SDTSC has more sensing ability towards Cu²⁺ ion than CN^- and F^- ions. The ¹H NMR titration with SDTSC and F^- gives the structural changes in the sensing process. The reversibility of SDTSC was also evaluated and thus it is confirmed as a reusable chemosensor which can be clarified by the "Read-Erase-Read-Write" logic system. The DFT and TD-DFT calculations give the detailed sensing mechanism of SDTSC towards fluoride ion. The potential energy surface (PES) analysis confirms the excited state electron transfer mechanism.