Fluorescent deactivation behaviors based on ESIPT and TICT of novel double target fluorescent probe and its sensing mechanism for Al3+/Mg2+: A TD-DFT study

Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods with integral equation formula polarized continuum model (IEFPCM), the fluorescent behavior and recognizing mechanism of probe N'-((1-hydroxynaphthalen-2-yl)methylene)isoquinoline-3-carbohydrazide (NHMI) for Al³⁺/Mg²⁺ ion were investigated in more detail. Excited state intramolecular proton transfer (ESIPT) process in probe NHMI occurs in the stepwise pattern. The proton H₅ of enol structure (E1) firstly moves from O₄ to N₆ to form single proton-transfer (SPT2) structure, and then the proton H₂ of SPT2 transfers from N₁ to N₃ to form the stable double proton-transfer (DPT) structure. Subsequently, the transformation from DPT to its isomer (DPT1) induces the twisted intramolecular charge transfer (TICT) process. Two non-emissive TICT states (TICT1 and TICT2) were obtained, and TICT2 state quenches the fluorescence observed in the experiment. With the addition of aluminum (Al³⁺) or magnesium (Mg²⁺) ion, TICT process is prohibited by the coordination interaction between NHMI and Al³⁺/Mg²⁺, and the strong fluorescent signal is turned on. For probe NHMI, the twisted C-N single bond of acylhydrazone part leads to the TICT state. This sensing mechanism may inspire researchers to develop new probes from a different direction.

Investigation on non-radioactive behavior of an acylhydrazone-based fluorescent probe: Coexistence of PET and TICT mechanisms

A fluorescent probe (E)-((2,4-dihydroxybenzyl)diazenyl)(pyridin-2-yl)methanone (HL) to effectively and selectively detect Al³⁺ was designed and synthesized in the experiment. Herein, we explained the excited state dynamics mechanism of HL by using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The potential energy surfaces (PESs) proved that the excited-state intramolecular proton transfer (ESIPT) process hardly occurs due to the high reaction barriers, so the fluorescence quenching behavior of HL was not based on ESIPT. The frontier molecular orbitals (FMOs) and spectral properties were analyzed to better understand the origination of fluorescence quenching. It was found that an electron on C = N in HL could be transferred to the fluorophore during excitation in the absence of Al³⁺, accompanied by the PET process. The excited state could undergo a twisted intramolecular charge transfer (TICT) process, releasing non-radiative decay. After binding to Al³⁺, the photo-induced electron transfer (PET) process has no longer occurred, and the TICT process is eliminated, resulting in a significant fluorescence enhancement. Therefore, the calculation results well explain the quenching and enhancement behaviors of fluorescence before and after the reaction with Al³⁺.

A colorimetric and turn-on fluorescent sensor for cyanide and acetate-based Schiff base compound of 2,2'-((1E,11E)-5,8-dioxa-2,11-diazadodeca-1,11-diene-1,12-diyl)bis(4-((E)-phenyldiazenyl)phenol)

A novel Schiff base-based sensor (L) has been designed, synthesized, and developed as a fluorescent and colorimetric sensor for cyanide and acetate. This L exhibited a quick response with rapid sensitivity to CN^- and AcO^- through a remarkable color change from yellow to red which was detectable by the naked eyes. It also sensed CN^- and AcO^- in a fluorescent way via an enhancement in fluorescence intensity. The interaction between L and anions (CN^- and AcO^-) was investigated by using UV-Vis studies, and ¹H NMR titration. The theoretical DFT calculations were also employed to support the results, which displayed good agreement with the experimental value acquisition. As the detection limit for cyanide and acetate were 2.1 × 10^-9 M and 1.7 × 10^-9 M; respectively, low concentrations of these anions could be detectable in the proposed L sensor. In addition, L showed significant reversibility of CN^- detection by using Cu²⁺ as a proper reagent with two different sensing methods including color change and UV-Vis. Last but not least, L could be applied to rapidly detect CN^- in a wide range of pH. As a result, the proposed sensor is promising to identify cyanide and acetate in practical applications in medical, biological, and chemical fields.

Computational Insights into Sensing Mechanism for Al3+ in a New Acylhydrazone Fluorescent Probe Based on Excited-State Intramolecular Proton Transfer (ESIPT) and Twisted Intramolecular Charge Transfer (TICT)

The work explored the fluorescent properties of probe N'-(2, 4-dihydroxy-benzylidene)pyridine-3-carbohydrazide (HL) and its sensing mechanism for the Al³⁺ ion in detail. HL has two competing deactivation processes: ESIPT and TICT. Upon light-excitation, only one proton can transfer, and the SPT1 structure is generated. The SPT1 form is highly emissive, which is inconsistent with the colorless emission observed in the experiment. Then a nonemissive TICT state was obtained by rotating the C-N single bond. The energy barrier of the TICT process is lower than that of the ESIPT process, which indicates that probe HL will decay to the TICT state and quench the fluorescence. When Al³⁺ is recognized by probe HL, strong coordinate bonds are formed between HL and Al³⁺, and then the TICT state is prohibited, and the fluorescence of HL is turned on. Al³⁺ as a coordinated ion can effectively remove the TICT state but cannot influence the photoinduced electron transfer (PET) process of HL.

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.

N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn²⁺ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λ_ex = 300 nm and 27% at λ_ex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S₁ state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL₂][Zn(OAc)₂]₂·2H₂O and [ZnL₂][ZnCl₂]₂·4H₂O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Backbone extension via peptidomimetics at N-terminal; self-assembled nanofibrous cluster and application to selective progesterone detection in an aqueous medium

Despite the adequacy of the endogenous steroid (progesterone) levels in biological functioning, elevated levels of progesterone hormone have several physiological effects that are amplified due to its direct and indirect uptake from the environment, food products, and medical therapy. So, it is much needed to evaluate the progesterone levels in environmental samples as well as for biological fluids. In this work, we focused on the development of the nano sensing probe for the selective detection of progesterone among the library of steroid hormones belonging to the class of female sex hormones. Herein, functionalization of dipeptide is carried out at N-terminal to produce N-functionalized dipeptide (SS3), and simultaneously, its self-assembly properties are explored. Furthermore, HR-TEM imaging was also performed to examine the morphology of the self-assembled architectures before and after the addition of the steroid hormone. To investigate the binding mechanism of the sensing probe, Fluorescence spectroscopy, Circular Dichroism (CD), MD-Simulation, and DFT studies were performed and studied in detail. Moreover, to check the potency of the real-time application of the developed nanoprobe, we have successfully determined the spiked concentration of progesterone levels in pharmaceutical and biological fluid samples with functional percentage recovery. Also, the stability and other competitive binding studies of the probe with the coexisting substances are performed to check the rationality of the sensing probe at physiological conditions.

Mitochondria- and nucleolus-targeted copper(i) complexes with pyrazole-linked triphenylphosphine moieties for live cell imaging

The labelling and imaging of mitochondria and nucleolus have attracted great attention because of the involvement of these cellular organelles in critical cellular activities. Therefore, a large number of mitochondria- or nucleolus-labelling probes have been developed throughout the world. However, in the current study, we successfully developed two pyrazole-based, copper-linked triphenylphosphine-coupled emissive metallo-complexes (C1 and C2) for the simultaneous visualization of mitochondria and nucleolus in a single run. These complexes were very inexpensive and could be synthesized by a simple one-pot multicomponent reaction scheme. The complexes were very specific, and only a small concentration of 5 μM was found to be sufficient to probe both the organelles efficiently. Additionally, even under a shorter incubation period (half hour), the fluorescence intensity from the cells was appreciable. Also, both the compounds were found to be photostable when torched with 10% of a 100 mW laser for up to 10 min. All these results indicate that both the complexes may contribute towards the future development of cell imaging tools. To the best of our knowledge, this is the first report on the development of multifunctional live cell imaging tools for simultaneous mitochondria and nucleolus imaging and within the shortest incubation time of about 30 minutes.

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.

A biscoumarin scaffold as an efficient anti-Zika virus lead with NS3-helicase inhibitory potential: in vitro and in silico investigations

Competitive NTPase inhibition and the potential binding to the RNA binding pocket of Zika NS3-helicase were observed using biscoumarin derivatives. The SAR was established, and MN-9 and MN-10 were identified as potent anti-Zika leads.