Intramolecular Proton Transfer Process Research Articles

Switching of photoinduced proton transfer from one six-membered hydrogen-bonded ring to other: a molecule of hydrazine and pH sensor.

The present study describes photophysical properties of 3-(benzo[d]oxazol-2-yl)-5-bromo-2-hydroxybenzaldehyde (HBOB) and (E)-2-(benzo[d]oxazol-2-yl)-4-bromo-6-(hydrazonomethyl)phenol (HBON) molecules with asymmetric two-way proton transfer sites. The purpose of this study is to know the direction of ESIPT out of the two-way proton transfer pathways in these molecules in both the solid and solution state. The steady state and time-resolved spectral behaviour of HBOB and HBON and a comparison of the spectral features with the two distinct control compounds 2-(benzo[d]oxazol-2-yl)-4-bromophenol (HBO) and (E)-4-bromo-2-(hydrazonomethyl)phenol (HBN) having single 6-membered hydrogen bonded network reveal that HBOB undergoes imine-amine photoisomerisation by proton transfer towards the oxazole side and HBON undergoes towards the hydrazone side with characteristic Stokes' shifted emission. Proton transfer forms with the red shifted emission of these molecules shows fast decay than the locally excited state. In the solid state, extremely high fluorescence intensity was observed, following a similar type of ESIPT pattern. Calculated ground (S0) and excited state (S1) energy barriers for the PT process obtained using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) corroborate the unidirectional excited state intramolecular proton transfer (ESIPT) process for HBOB and HBON, and the theoretical spectral features validate our experimental absorption and emission spectra well. Interestingly, the unique unidirectional ESIPT behaviour of HBOB was utilised to detect hydrazine both in solution and solid phases. On the other hand, HBON was found to be a good fluorescence pH sensor with a ratiometric color change from yellow to green in acidic and basic media.

Effects of extended π-conjugated bridge on the ESIPT process and electronic spectra of 1,8-dihydroxynaphthalene substitute

The effect of extended π-conjugated bridge on the excited state intramolecular proton transfer (ESIPT) process and photophysical property of (E)-1-(1-hydroxynaphthalen-2-yl)-3-(2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1ij]quinolin-9yl)prop-2-en-1-one (NHP) was investigated using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Theoretical analyses demonstrate that the intramolecular hydrogen bonds (IHBs) with the studied molecules are strengthened to varying extents in the excited state. Specifically, as the π-conjugated bridge is extended, the ESIPT process becomes increasingly challenging due to the elevated energy barrier. Meanwhile, the fluorescence peak prominently intensifies and red-shifts to 720 nm. The study is expected to provide new insights into ESIPT-type red-emitting materials.

Red‐Shifting ESIPT Fluorescence by Site‐Specific Functionalization in 2‐(2’‐hydroxyphenyl)benzazole Derivatives

We describe the synthesis, full photophysical study, and ab initio calculations of 2‐(2’‐hydroxyphenyl)benzazole (HBX) fluorophores substituted, at the meta position of the phenol group, by pyridine derivatives. HBX are commonly used as model dyes to study the stimuli‐induced modulation of the Excited State Intramolecular Proton Transfer (ESIPT) process. The meta‐substituted fluorophores reported herein, display a photophysical profile different from the previously reported ortho‐ and para‐substituted HBO pyridine isomers. Indeed, while all dyes undergo spontaneous deprotonation in neutral conditions, leading to highly emissive anionic species; upon protonation, ortho‐ and para‐pyridine substitution leads to resonance‐stabilized keto isomers, formed after ESIPT. Protonated meta derivatives, unable to stabilize their excited structure by such electronic delocalization process, display sizable intramolecular charge transfer (ICT) processes, translating into significantly redshifted emission. In addition, all dyes present a strong emission intensity, not only in neutral and acidic solutions, but also in the solid‐state. The nature of the emissive transitions was confirmed in each case by theoretical calculations combining Time‐Dependent Density Functional Theory (TD‐DFT) and second‐order Coupled Cluster (CC2) methods.

Theoretical Insights into the Effect of Different Numbers of Thiophene Groups on Hydrogen Bond Interaction and Excited-State Intramolecular Proton-Transfer Process for Flavonoid Derivatives.

In this study, we systematically explored the impact of varying the number of thiophene groups on the hydrogen bond interaction and excited-state intramolecular proton-transfer (ESIPT) processes in flavonoid derivatives (STF, DTF, and TTF) using the density functional theory and time-dependent density functional theory methods. Initially, a thorough analysis of the optimized geometric structures revealed that the intramolecular hydrogen bond in the S1 state is enhanced and gradually weakened as the number of thiophene groups increases. To gain a deeper understanding of the hydrogen bond interaction, topological analysis, interaction region indicator scatter plots, and isosurface plots were employed. These images provide further insights that align with the structural analysis. Additionally, we observed a red-shift in the electronic spectra (absorption and fluorescence spectra), which is primarily attributed to the narrowing of the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, as elucidated by the frontier molecular orbitals. Furthermore, a combined analysis between the hole-electron distribution and the transition density matrix heat map shows that electron excitation involves the unidirectional charge-transfer mechanism. In the end, by conducting relaxed potential energy curve scans, we found that an increase in the number of thiophene groups elevates the energy barrier for ESIPT, making it more challenging for the reaction. In summary, our study underscores the vital effect of thiophene-substituted numbers in modulating the ESIPT process, which is able to provide valuable insights for the design and synthesis of desired organic fluorescent probes in the future.

A near-infrared fluorescent probe with thiadiazole unit as key skeleton for ICT and ESIPT mechanism and effective detection of Cu2+

Fluorescent probe L-I was synthesized to demonstrate that 1,3,4-thiadiazole is an attractive moiety and could be utilized as positive hydrogen bond acceptor for excited state intramolecular proton transfer (ESIPT) processes, guider of electrons movement for intramolecular charge transfer (ICT) process and identify group for mental ions. Furthermore, dicyanoisophorone framework was employed to improve the fluorescence characteristics and near-infrared (NIR) fluorescent emission at 695 nm accompanied by a Stoke's shift as large as 260 nm was obtained. L-I could selectively detect Cu2+ over other analytes taking advantages of high sensitivity, fast response within 30 s and low detection limit (0.026 μM). More important, L-I exhibited good performance for detection of Cu2+ in actual water samples, food products, traditional Chinese medicine and for cell imaging which demonstrates practical significance in the fields of environmental monitor, food safety and biotechnology.

Mechanistic Insights into the Excited-State Intramolecular Proton Transfer (ESIPT) Process of 2-(2-Aminophenyl)naphthalene.

The 2-(2-aminophenyl)naphthalene molecule attracted much attention due to excited-state intramolecular proton transfer (ESIPT) from an amino NH2 group to a carbon atom of an adjacent aromatic ring. The ESIPT mechanisms of 2-(2-aminophenyl)naphthalene are still unclear. Herein, the decay pathways of this molecule in vacuum were investigated by combining static electronic structure calculations and nonadiabatic dynamics simulations. The calculations indicated the existence of two stable structures (S0-1 and S0-2) in the S0 and S1 states. For the S0-1 isomer, upon excitation to the Franck-Condon point, the system relaxed to the S1 minimum quickly, and then there exist four decay pathways (two ESIPT ones and two decay channels with C atom pyramidalization). In the ESIPT decay pathways, the system encounters the S1S0-PT-1 or S1S0-PT-2 conical intersection, which funnels the system rapidly to the S0 state. In the other two pathways, the system de-excited from the S1 to the S0 state via the S1S0-1 or S1S0-2 conical intersection. For the S0-2 structure, the decay pathways were similar to those of S0-1. The dynamics simulations showed that 75 and 69% of trajectories experienced the two ESIPT conical intersections for the S0-1 and S0-2 structures, respectively. Our simulations showed that the lifetime of the S1 state of S0-1 (S0-2) is estimated to be 358 (400) fs. Notably, we not only found the detailed reaction mechanism of the system but also found that the different ground-state configurations of this system have little effect on the reaction mechanism in vacuum.

Rapid sequential detection of Al3+ and glyphosate using an “Off-On-Off” fluorescent probe based on salicylate modified layered double hydroxides

A fluorescent probe based on salicylate modified layered double hydroxide (LDH-SA) is presented, enabling the swift sequential detection of Al3+, fosetyl-Al and glyphosate in aqueous environment. The probe was synthesized using a simple co-precipitation procedure, and its properties and synthesis conditions were thoroughly characterized and optimized. A unique “off-on-off” fluorescent response was observed when the probe sequentially interacted with Al3+ and glyphosate, and the detection method based on this phenomenon was established. The limits of detection for Al3+ and glyphosate were determined as 0.03 μmol/L and 0.03 mg/L, respectively, with rapid detection periods of one minute and four minutes. The LDH-SA/Al3+ complex requires Al3+ to generate a chelation-gathered fluorescence effect, which is the mechanism by which it quenches LDH-SA. This is possible due to the inhibition of excited-state intramolecular proton transfer and photoinduced electron transfer processes within LDH-SA after incorporating Al3+. Upon interaction with glyphosate, competitive complexation between glyphosate and Al3+ is initiated, which leads to a recovery of the fluorescence spectrum of LDH-SA and demonstrating the “off-on-off” behavior. An “INHIBIT” logic gate system was devised utilizing the response, indicating potential applications in fluorescence-based devices. Such a rapid, sequential detection capacity is impressive. It attests to the utility of LDH-SA as a probe for Al3+ or glyphosate, and suggests promise for applications in pollutant analysis or environmental monitoring applications.

Amination and Protonation Facilitated Novel Isoxazole Derivative for Highly Efficient Electron and Hole Separation.

It is of great importance to understand the intrinsic relationship between phototautomerization and photoelectric properties for the exploration of novel organic materials. Here, in order to chemically control the protonation process, the aminated isoxazole derivative (2,2'-(isoxazolo[5,4-d]isoxazole-3,6-diyl)dibenzenaminium, DP-DA-DPIxz) with -N═ as the proton acceptor was designed to achieve the twisted intramolecular charge transfer (TICT) state which was triggered by an excited-state intramolecular proton transfer (ESIPT) process. This kind of protonation enhanced the intramolecular hydrogen bonding, conjugative effect, and steric hindrance effects, ensuring a barrierless spontaneous TICT process. Through the intramolecular proton transfer, the configuration torsion and conjugation dissociation of the DP-DA-DPIxz molecule was favored, which led to efficient charge separation and remarkable variations in light-emitting properties. We hope the present investigation will provide a new approach to design novel optoelectronic organic materials and shine light on the understanding of the charge transfer and separation process in molecular science.

Dual channel chemosensor for successive detection of environmentally toxic Pd2+ and CN− ions and its application to cancer cell imaging

BackgroundDetecting and neutralizing Pd2+ ions are a significant challenge due to their cytotoxicity, even at low concentrations. To address this issue, various chemosensors have been designed for advanced detection systems, offering simplicity and the potential to differentiate signals from different analytes. Nonetheless, these chemosensors often suffer from limited emission response and complex synthesis procedures. As a result, the tracking and quantification of residual palladium in biological systems and environments remain challenging tasks, with only a few chemosensing probes available for commercial use. ResultsIn this paper, a straightforward approach for the selective detection of Pd2+ ions is proposed, which involves the design, synthesis, and utilization of a propargylated naphthalene-derived probe (E)-N'-((2-(prop-2-yn-1-yloxy)naphthalen-1-yl)methylene)benzohydrazide (NHP). The NHP probe exhibits sensitive dual-channel colorimetry and fluorescence Pd2+ detection over other tested metal ions. The detection process is performed through a catalytic depropargylation reaction, followed by an excited state intramolecular proton transfer (ESIPT) process, the detection limit is as low as 11.58 × 10−7 M under mild conditions. Interestingly, the resultant chemodosimeter adduct (E)-N'-((2-hydroxynaphthalen-1-yl)methylene)benzohydrazide (NHH) was employed for the consecutive detection of CN− ions, exhibiting an impressive detection limit of 31.79 × 10−8 M. Validation of both detection processes was achieved through 1H nuclear magnetic resonance and density functional theory calculations. For real-time applications of the NHP and NHH probes, smartphone-assisted detection, and intracellular detection of Pd2+ and CN− ions within HeLa cells were studied. SignificanceThis research presents a novel naphthalene derivative for visually detecting environmentally toxic Pd2+ and CN− ions. The synthesized probe selectively binds to Pd2+, forming a chemodosimeter. It successfully detects CN− ions through colorimetry and fluorimetry, offering a low detection limit and quick response. Notably, it's the first naphthalene-based small molecule to serve as a dual probe for toxic analytes - palladium and cyanide. Moreover, it effectively detects Pd2+ and CN− intracellularly in cancer cells.

A thermochromic salicylaldehyde Schiff bases derivative with AIE properties due to twisted structure

Great attention has been attracted to the salicylaldehyde Schiff base due to its thermochromic and photochromic characteristics in the solid state. However, the boundary between photochromic and thermochromic salicylaldehyde Schiff base structures is still unclear, and few studies have been conducted to investigate their thermochromic behavior and structures in depth and systematically. In the present work, we have synthesized a novel salicylaldehyde Schiff base derivative (2,2′-((1E,1′E)-((2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diyl)-bis(azaneylylidene))bis(methaneylylidene))diphenol, BTB) with twisted structure, which possesses both thermochromic and aggregation-induced emission (AIE) characteristics, and systematically investigated its thermochromic properties, enol-keto isomerization, AIE properties, aggregated state, and excited state intramolecular proton transfer (ESIPT) process. Interestingly, in our study, we found that the color of the BTB molecule changed gradually during the heating process, and it can be found that the thermochromism of BTB is produced by the reversible equilibrium shift of the enol form and the keto forms due to the temperature change by the color difference test, the in-situ IR spectra and the DSC curves. Finally, the pathway of BTB proton transfer was analyzed through quantum chemical calculations, and it was found that enol ketone isomerization is achieved through single excited state intramolecular proton transfer, rather than the expected double excited state intramolecular proton transfer.

Spectral, crystal structure of novel Pyran-2-one Zwitterion Schiff derivative: Thermal, Physicochemical, DFT/HSA-interactions, enol↔imine tautomerization and anticancer activity

A novel Schiff base (SB) derivative, (E)-3-(1-((2,6-diisopropylphenyl)-imino)-ethyl)-4‑hydroxy-6-methyl-2H-pyran-2-one, was synthesized in a high yield through the direct condensation of 2,6-diisopropylaniline with 3-acetyl-4‑hydroxy-6-methyl-2H-pyran-2-one in ethanol under reflux conditions. The synthesized ligand was identified using different elemental analysis (CHN-EA) and spectroscopic techniques (2D-nuclear magnetic resonance (1H- & 13C- NMR), Fourier transform infrared (FT-IR) and mass (MS) spectroscopies), and their structural analysis was performed by single crystal X-ray diffraction (XRD). In addition, the thermogravimetric behavior of the ligand was evaluated using thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The DFT calculations on the gaseous state and in chloroform as solvent using B3LYP-D3/6–311++G(2d,p) and the HSA calculations were performed to examine the stability of the tautomeric forms of the ligand and to understand its Zwitterionic behaviors. The results showed that the ligand exhibits a zwitterionic form stabilized by an intramolecular single proton transfer process between the enol and imine tautomerism. The enol↔imine tautomerization was determined by XRD and computed by DFT/HSA calculations, which proved the stabilization of the [N+−−H⋯O−−] zwitterionic form via S6 intra-H-bond. In addition, the Colorimetric [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay showed that the ligand exhibits high activity against cancer cells, specifically HeLa cells, at low concentrations.

Study on ESIPT process of fluorescence probe BT-ITC for detecting H2S in DMF/H2O mixed solution

A probe molecule named BT-ITC with an isothiocyanate group was synthesized and evaluated for its H2S sensing ability based on the excited state intramolecular proton transfer (ESIPT) process J K Kim, S Y Bong, R Park, J Park, D Ok Jang 2022 An ESIPT-based fluorescent turn-on probe with isothiocyanate for detecting hydrogen sulfide in environmental and biological systems, Spectrochimica Acta Part A 278 121333. The BT-ITC molecule has a large Stokes shift, short response time, and low detection limit. The end result of the reaction between the molecules BT-ITC and H2S is the BTC-NH2 molecule. The hydrogen bond of the Enol structure BTC-NH2 molecule in excited-state (S1 state) is strengthened, compared to the ground-state (S0 state). The calculated absorption and fluorescence spectra were in good agreement with the experimental values. The frontier molecule orbitals (FMOs), the potential energy curve (PEC), and scatter plots of reduced density gradient (RDG) were calculated. According to aforementioned, it was discovered that the charges of BTC-NH2 molecule were rearranged via an electron transition brought on by photoexcitation. This changes the acidity and alkaline of the proton donor and acceptor, and thus promotes the ESIPT process.

Photophysical Properties of (E)-1-(4-(Diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide Compound and Its Triple Fluorescence Emission Mechanism: A Theoretical Perspective.

In view of the application prospects in biomedicine of (E)-1-(4-(diethyla-mino)-2-hydroxybenzylidene)-4,4-dimethylthiosemicarbazide (DAHTS), the behavior of excited-state dynamics and photophysical properties were studied using the density functional theory/time-dependent density functional theory method. A series of studies indicated that the intramolecular hydrogen-bond (IHB) intensity of DAHTS was enhanced after photoexcitation. This was conducive to promoting the excited-state intramolecular proton-transfer (ESIPT) process. Combining the analysis of the IHB and hole-electron, it revealed that the molecule underwent both the ESIPT process and the twisted charge-transfer (TICT) process. Relying on exploration of the potential energy surface, it was proposed that the different competitive mechanisms between the ESIPT and TICT processes were regulated by solvent polarity. In acetonitrile (ACN) solvent, the ESIPT process occurred first, and the TICT process occurred later. In contrast, in the CYH solvent, the molecule first underwent the TICT process and then the ESIPT process. Furthermore, we raised the possibility that the TICT behavior was the cause of weak fluorescence emission for the DAHTS in CYH and ACN solvents. By the dimer correlation analysis, the corresponding components of triple fluorescence emission were clearly assigned, corresponding to the monomer, dimer, and ESIPT isomer in turn. Our work precisely elucidated the photophysical mechanism of DAHTS and the attribution of the triple fluorescence emission components, which provided valuable guidance for the development and regulation of bioactive fluorescence probes with multiband and multicolor emission characteristics.

Specific detection of sarin gas Mimicking, diethylchlorophosphate employing solvent mediated in-situ generated hydroxyl functional group in a phthalimide-chromone coupled fluorogenic probe

Nerve agents are particularly hazardous substances and can result in fatality due to suppressing acetylcholinesterase function. This article introduced a fluorescent probe, (E)-5-(((4-hydroxy-2-methoxy-2H-chromen-3-yl)methylene)amino)isoindoline-1,3-dione (CMAD), which was exceptionally effective at detecting the sarin replacement diethylchlorophosphate (DCP), both in solution and vapor phase, respectively which exhibited quick response, excellent sensitivity and specificity. As a result of the gradual addition of DCP into the solution of the probe, the photoluminescence intensity at 476 nm is markedly increased, exhibiting bright cyan color fluorescence because of inhibition of the excited state intramolecular proton transfer (ESIPT) process, which is also clearly reflected in the estimation of photoluminescence quantum yield of phosphorylated CMAD (0.55 relatives to 4-aminophalimide) and the color chromaticity (CIE) diagram. The lowest detection limit and limit of quantification are 3.25 µM and 10.8 µM, respectively, within the stockpiles of other analogous analytes. Additionally, for on-the-spot detection, the probe may also be utilized in the test strips with superior selectivity for DCP. The ability to identify and measure DCP in the gas phase has also been established using a dip-stick and dipped-vial-conical-flask analysis. Moreover, we also performed the smartphone-based technique for the practical utility of CMAD for the on-spot quantification and detection of sarin gas in real threat situations. The effective demonstrations of practical usage, including soil analysis, inspire additional applications in different fields. Therefore, probe CMAD is a reliable, on-site monitoring fluorescence probe for precisely identifying DCP in the presence of additional competing nerve agents.

Dual detection of Hg2+ and Pb2+ by a coumarin-functionalized Schiff base in environmental and biosystems

Fluorescent chemosensors are often preferred for tracking toxic ions because of their non-destructive measurement and ease of use in environmental real samples and biosystems. Exploring high selectivity, great sensitivity, and biocompatible fluorophores with facile, accessible and dual-responsive features is currently highly demanding. A coumarin-based naphthol hydrazone Schiff base chemosensor, NaChro, is designed and synthesized in a two-step process to detect toxic metal ions with strong emission. Fluorescence spectra analysis demonstrates that the probe binds to Hg2+ and Pb2+ ions with a 1:1 and a 2:1 stoichiometry, respectively, with high sensitivity, short response time and minimal interference from other metal ions. The observed reversible turn-on reaction was attributed to the inhibition of C = N isomerization and excited-state intramolecular proton transfer (ESIPT) processes once the ions were introduced. The practical applications of NaChro are successfully addressed in paper strips, various water samples, HeLa cells and Zebrafish, demonstrating that the probe can detect and track Hg2+ and Pb2+ ions in environmental samples and biosystems.

Theoretical study on the ESIPT and photophysical properties for molecules with different extension π conjugation

In a recent study, Jincheng Si and coworkers designed four candidate linker units with different extension π conjugation sizes and directions. These molecules were used for the synthesis of metal–organic framework (MOF) structures known as Olympic rings, which can detect the Cr2O72- in the aqueous phase. However, the experimental work did not provide a qualitative understanding of the excited-state dynamic mechanism. Therefore, this research focused on investigating the excited state intramolecular proton transfer (ESIPT) process and the photophysical properties of these molecules. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were employed to conduct an in-depth analysis. The molecule structure properties were investigated by analyzing the hydrogen bond parameters and the infrared (IR) vibrational spectra. The calculated absorption and emission spectra were in good agreement with the experimental values, it was proved that the correctness of the optimized structure and the rationality of our calculation method, and we can judge their detection sensitivity by the sizes of the Stokes shift. The Hirshfeld method was used to study the enhancement mechanism of hydrogen bond interaction. Furthermore, the reaction mechanism and ESIPT reactivity of molecules with different extension π conjugation sizes and directions were explained by calculating and plotting the relaxed potential energy curve, electrostatic potentials (ESPs) and frontier molecular orbitals (FMOs). Our research found that extension π conjugation in different sizes and directions can either promote or inhibit the occurrence of the ESIPT reactions and the characteristic of intramolecular charge transfer (ICT), thereby altering the fluorescence performance. Based on our findings, we concluded that the ESIPT activity and photophysical properties of the H4BIHQC were higher than the other three molecules. This discovery enhances the potential application value of MOF as a fluorescence detection tool.

Theoretical investigation on regulating photophysical properties and proton transfer behavior by electronegativity for near-infrared emitting styryl dyes.

Our main focus is to explore the atomic electronegativity-dependent photoinduced behavior of styryl derivatives (HBO, HBS, and HBSe). The results of structural parameter calculation by the DFT method show that the intramolecular hydrogen bonds of normal and tautomer form are strengthened and weakened, respectively, in an excited state (S1), which is conducive to the excited intramolecular proton transfer (ESIPT) process. The enhancement of excited hydrogen bond is beneficial to the ESIPT process from the aspects of infrared vibration frequency (IR), Mulliken's charge analysis, and density gradient reduction (RDG). Additionally, by determining the bond energy with the band critical point (BCP) parameter, we found that the lower the electronegativity of the atom, the larger the hydrogen bond strength at the excited state and the more likely ESIPT reaction occurs. Meanwhile, the intramolecular H-bonds O-H…N in HBO, HBS, and HBSe are enhanced with the weakened electron-withdrawing capacity of the atom (from O to S and Se). Subsequently, frontier molecular orbital (FMOs) and charge density difference (CDD) analyses essentially revealed that electron redistribution induces the ESIPT process. Low atomic electronegativity exhibits the high chemical activity of the excited state. Furthermore, to demonstrate the electronegativity-dependent ESIPT behavior of the system, we built potential energy curves (PECs) and located the transition states (TS) of proton transfer processes.

A Fluorescent Probe with Zwitterionic ESIPT Feature for Ratiometric Monitoring of Peroxynitrite In Vitro and In Vivo.

Peroxynitrite (ONOO-), as a short-term reactive biological oxidant, could lead to a series of effects in various physiological and pathological processes due to its subtle concentration changes. In vivo monitoring of ONOO- and relevant physiological processes is urgently required. Herein, we describe a novel fluorescent probe termed HBT-Fl-BnB for the ratiometric detection of ONOO- in vitro and in vivo. The probe consists of an HBT core with Fl groups at the ortho and para positions responding to the zwitterionic excited-state intramolecular proton-transfer (zwitterionic ESIPT) process and a boronic acid pinacol ester with dual roles that block the zwitterionic ESIPT and recognize ONOO-. Thanks to the specificity as well as low cytotoxicity, success in imaging of endogenous and exogenous ONOO- in living cells by HBT-Fl-BnB was obtained. Additionally, the applicability of HBT-Fl-BnB to tracking the abnormal expression of ONOO- in vivo induced by inactivated Escherichia coli was also explored. This is the first report of a fluorescent probe for ONOO- sensing via a zwitterionic ESIPT mechanism.

Crystal structure, tautomerism and photostability of 2-(2-pyridyl)-phenalene-1,3-dione

Tautomerism of cyclic β-diketones has been of scientific interest for decades due to the practical applications it can render, from sensors to photostable materials. In this work the tautomerism of 2-(2-pyridyl)-phenalene-1,3-dione (PPD) was studied in solid state, in solution, and in gas phase through a combination of experimental and theoretical methods. Single-crystal X-ray diffraction analysis of PPD revealed its existence in diketo form in the solid state with protonated pyridyl nitrogen that is stabilized by the intramolecular hydrogen bond of N-H···O type. A very large Stokes shift was observed in the emission spectra in DMSO (ca. 177 nm) and in ethanol (ca. 160 nm), indicating on possible excited-state intramolecular proton transfer (ESIPT) process. DFT calculations in ground state (gas phase and water medium) and in the excited singlet state (B3LYP-6311++G(d,p)) agree very well with the experimental data and suggest reverse order of the possible OH and NH tautomers in the ground and the excited singlet state, namely the diketo NH tautomer is the most stable one in the ground state whereas in the excited singlet state the enol OH form is more stable. The photostability of PPD was estimated by irradiation experiments that were performed in comparison with 2-acetylindan-1,3-dione (2AID), which is a compound with proven high photostability in ethanol and fast ESIPT. The data showed that unlike 2AID, the 2-(2-pyridyl)-phenalene-1,3-dione exhibits lower photostability in ethanol. These results could suggest that either the ESIPT process is not the only transformation that takes place in the excited state of PPD, or it is not as fast as the one of 2AID.

DFT/TDDFT based study to decipher the excited state intramolecular proton transfer process for 3-benzothiazol-2-yl-2-hydroxy-5-methyl-benzaldehyde-oxime

In this study, we theoretically investigate the interactions of hydrogen bonding in excited states and the mechanism of intramolecular proton transfer (ESIPT) for the novel compound 3-benzothiazol-2-yl-2-hydroxy-5-methyl-benzaldehyde-oxime (HBT-phos) theoretically. Herein, the effects of atomic electronegativity of chalcogen (O, S and Se) are also focused. Firstly, employing quantum theory of atoms-in-molecules (QTAIM) method, we confirm intrinsic quality of O-H···N within HBT-phos derivatives (HBT-phos-O, HBT-phos-S and HBT-phos-Se). By astutely examining changes in geometries and infrared (IR) spectra, we elegantly demonstrate O-H···N is remarkably strengthened in S1 state, indicating a pronounced tendency towards ESIPT. Computational hydrogen bonding energies further magnificently support this conclusion. Probing into photo-induced excitation, we discover heightened electronic densities surrounding N atom that play pivotal roles in attracting protons, thereby facilitating ESIPT. Ultimately, upon comparing potential energy curves in excited states, we deduce ESIPT process is expected to be exceptionally rapid with low atomic electronegativity (O → S → Se), which elucidates why detecting fluorescence peak of HBT-phos-S itself proves challenging in experiment. This study not only fills the void in understanding ESIPT mechanism within HBT-phos derivatives and authentically reveals its unambiguous mechanism, but also presents atomic-electronegativity-dependent ESIPT behaviour and the explanation for fluorescence quenching observed in prior experiment.