An excited state intramolecular proton transfer induced phosphate ion targeted ratiometric fluorescent switch to monitor phosphate ions in human peripheral blood mononuclear cells

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(II) complexes with 1-hydroxy-1H-imidazole-based ligands

The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1H-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal₂ with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (λ_max = 600-650 nm). The coordination of HLq by Zn²⁺ ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S₁ state, leading to the only possible fluorescence channel in the tautomeric form (T), S₁^T → S₀^T. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH₂Cl₂ solutions, the compounds demonstrate dual emission in the UV (λ_max = 358 nm) and green (λ_max = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S₁^N → S₀^N and S₁^T → S₀^T, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S₀^T, resulting in the possibility of the S₀^T → S₁^T photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction.

A new ratiometric switch "two-way" detects hydrazine and hypochlorite via a "dye-release" mechanism with a PBMC bioimaging study

A new ratiometric fluorescent probe (E)-2-(benzo[d]thiazol-2-yl)-3-(8-methoxyquinolin-2-yl)acrylonitrile (HQCN) was synthesised by the perfect blending of quinoline and a 2-benzothiazoleacetonitrile unit. In a mixed aqueous solution, HQCN reacts with hydrazine (N₂H₄) to give a new product 2-(hydrazonomethyl)-8-methoxyquinoline along with the liberation of the 2-benzothiazoleacetonitrile moiety. In contrast, the reaction of hypochlorite ions (OCl^-) with the probe gives 8-methoxyquinoline-2-carbaldehyde. In both cases, the chemodosimetric approaches of hydrazine and hypochlorite selectively occur at the olefinic carbon but give two different products with two different outputs, as observed from the fluorescence study exhibiting signals at 455 nm and 500 nm for hydrazine and hypochlorite, respectively. A UV-vis spectroscopy study also depicts a distinct change in the spectrum of HQCN in the presence of hydrazine and hypochlorite. The hydrazinolysis of HQCN exhibits a prominent chromogenic as well as ratiometric fluorescence change with a 165 nm left-shift in the fluorescence spectrum. Similarly, the probe in hand (HQCN) can selectively detect hypochlorite in a ratiometric manner with a shift of 120 nm, as observed from the fluorescence emission spectra. HQCN can detect hydrazine and OCl^- as low as 2.25 × 10^-8 M and 3.46 × 10^-8 M, respectively, as evaluated from the fluorescence experiments again. The excited state behaviour of the probe HQCN and the chemodosimetric products with hydrazine and hypochlorite are studied by the nanosecond time-resolved fluorescence technique. Computational studies (DFT and TDDFT) with the probe and the hydrazine and hypochlorite products were also performed. The observations made in the fluorescence imaging studies with human blood cells manifest that HQCN can be employed to monitor hydrazine and OCl^- in human peripheral blood mononuclear cells (PBMCs). It is indeed a rare case that the single probe HQCN is found to be successfully able to detect hydrazine and hypochlorite in PBMCs, with two different outputs.