Quantum chemical elucidation of the turn-on luminescence mechanism in two new Schiff bases as selective chemosensors of Zn2+: synthesis, theory and bioimaging applications

Abstract

We report the synthesis and characterization of two new selective zinc sensors (S,E)-11-amino-8-((2,4-di-tert-butyl-1-hydroxybenzylidene) amino)-11-oxopentanoic acid (A) and (S,E)-11-amino-8-((8-hydroxybenzylidene)amino)-11-oxopentanoic acid (B) based on a Schiff base and an amino acid. The fluorescent probes, after binding to Zn²⁺ ions, presented an enhancement in fluorescent emission intensity up to 30 times (ϕ = A 50.10 and B 18.14%). The estimated LOD for compounds A and B was 1.17 and 1.20 μM respectively (mixture of acetonitrile : water 1 : 1). Theoretical research has enabled us to rationalize the behaviours of the two selective sensors to Zn²⁺ synthesized in this work. Our results showed that in the free sensors, PET and ESIPT are responsible for the quenching of the luminescence and that the turn-on of luminescence upon coordination to Zn²⁺ is mainly induced by the elimination of the PET, which is deeply analysed through EDA, NOCV, molecular structures, excited states and electronic transitions via TD-DFT computations. Confocal fluorescence microscopy experiments demonstrate that compound A could be used as a fluorescent probe for Zn²⁺ in living cells.

Scheme 1. Synthetic route of Schiff bases A and B.

Fig. 1. Fluorescence spectrum of compounds (10 μM) (a) A (excited at 370 nm) and (b) B (excited at 316 nm), in presence of various metal ions (50 μM).

Fig. 2. Histogram of compounds A and B in the presence of different metal ions (50 μM).

Fig. 3. Graphic of the fluorescence intensity of compound A (blue bars) and B (red bars) upon the addition of Zn²⁺ (1 equiv.), in presence of various metal ions (1 equiv.).

Fig. 4. Fluorescence spectra of (a) A and (b) B (10 μM) in presence of different concentrations of Zn²⁺ (0.625–50 μM).

Fig. 5. Job's plot for determining the binding stoichiometry of the compounds (above) A and (below) B, and Zn²⁺. The total concentration was kept 100 μM.

Fig. 6. Optimized structure of sensor A and coordination compounds (A/M), in S₀ and S₁ states.

Fig. 7. Optimized structure of sensor B and coordination compounds (B/M), in S₀ and S₁ states.

Fig. 8. Molecular orbital diagram based on the S₀ (absorption) and S₁ (emission) states in the free sensors (A and B). Where λ_a is the theoretical absorption wavelength (black), λ_ε is the theoretical wavelength of emission (blue) and f is the oscillator strength.

Fig. 9. Molecular orbital diagram based on the S₀ (absorption) and S₁ (emission) states in the sensors coordinated with metal ions. (a) A/Zn²⁺, (b) B/Zn²⁺, (c) A/Ni²⁺ and (d) B/Ni²⁺.

Fig. 10. Diagram showing (a) the quenching of the luminescence due to the MLCT in the sensor with Ni²⁺ ion and (b) the sensing mechanism due to the PET-blocking produced by Zn²⁺ ion in sensors.

Fig. 11. Confocal microscopic images of human epithelial cells Hs27 treated with (a) solution of Zn²⁺ (100 μM), (b) compound A (20 μM) in the presence of Zn²⁺ (100 μM) and (c) compound A (20 μM) in the absence of Zn²⁺. Incubation temperature is 37 °C.