Fluorescence Enhancement/Quenching Based on Metal Orbital Control: Computational Studies of a 6-Thienyllumazine-Based Mercury Sensor

Abstract

To understand the sensing behaviors of molecular fluorescent probes, lumazine (Lm) and 6-thienyllumazine (TLm) and their complexation with metal(II) ions ([(L)nM(H2O)m](2+), M = Cd(2+) and Hg(2+)) were examined by scalar relativistic density functional theory (DFT). A red shifting from L to [(L)nM(H2O)m](2+) was found. This is due to the metal affinity that stabilizes the LUMOs of [(L)nM(H2O)m](2+) greater than the HOMOs. Singlet excited-state structures of L and [(L)nM(H2O)m](2+) (M = Cd(2+) and Hg(2+)) were fully optimized using time-dependent DFT (TDDFT). Their fluorescent emissions in aqueous solution were calculated to be 371 nm (Lm), 439 nm (cis-TLm), and 441 nm (trans-TLm), agreeing with experimental values of 380 nm for Lm and 452 nm for TLm. Theoretical support is presented for a sensing mechanism of photoinduced charge transfer of the L probe. The mechanism of the chelation-enhanced fluorescence (CHEF) and the chelating quenched fluorescence (CHQF) is explained. Fluorescence amplification (for Cd(2+)) is due to blocking of the nitrogen lone pair orbital due to the stabilizing interaction with the vacant s-orbital of the metal ion, while fluorescence quenching (Hg(2+)) results from the energy of the LUMO of the metal ion being between HOMO and LUMO of the sensor. Effects of structure rearrangements on the fluorescence spectra of the sensors are insignificant. This proposed mechanism of metal orbital controlled fluorescence enhancement/quenching suggests a development concept in the future design of fluorescent turn-on/off sensors.

Two Different Acid Oxidation Syntheses to Open C60 Fullerene for Heavy Metal Detection

“…Both types of aggregation allow the chelation of metal ions, with a consequence of PL quenching effect. In the case of cadmium ions, the PL enhanced could be due to the Chelation-Enhanced Fluorescence (CHEF) mechanism [40,41], Cd(II) ions might coordinate with the basal surface of the carbon layers trough cation-π interaction [41][42][43] using the oxygen lone pair electrons of the carbonyl and epoxy groups by immobilizing the lone pair electrons, Cd(II) cation increases the recombination rate and the fluorescence intensity of UFNPs.…”

Metalloproteinek fémkötőhelyein alapuló oligopeptidek, mint potenciális toxikus fémion érzékelők

“…[5][6][7]9,12 A fluoreszcenciás elven mûködõ fémion-szenzorokat a fémion-kötõhely és a fluorofór csoport(ok) egymáshoz viszonyított helyzete alapján három nagyobb csoportba sorolhatjuk. 13 Az ún. I. Típusú.…”

Silver Nanoparticles with Different Thiol Functionalization: An Opposite Optical Behaviour in Presence of Hg(II)

“…Also, in these cases, photoemitting material have been used to detect the presence of various species of pollutants [23][24][25][26][27][28]. The interaction mechanism would explain the opposite behaviour in the PL intensity change [29].…”