Proton Transfer Dynamics of 4'-N,N-Dimethylamino-3-hydroxyflavone Observed in Hydrogen-Bonding Solvents and Aqueous Micelles

Anti-cancer drug axitinib: a unique tautomerism-induced dual-emissive probe for protein analysis

A commercial anti-cancer drug, axitinib, exhibits very stable dual emissions for discrimination of human serum albumin.

Heterocyclic molecules with ESIPT emission: synthetic approaches, molecular diversities, and application strategies

Excited-state intramolecular proton transfer (ESIPT) is one of the most essential emission processes in most circumstances because of its dual emission band in most cases and its high Stokes shifts. These distinguishing properties make ESIPT-based probes more suitable for a variety of applications, including analyte sensors, solid-state sensing mechanisms, optical technologies, and biomarkers for endogenous or exogenous compounds in various settings. As a result, researchers around the world are working on ESIPT emissions and developing different scaffolds for various applications or industry demands. This field of study is rapidly expanding and there is a need for an up-to-date review of synthesis methodologies and applications. This paper provides the highlights of ESIPT-based heterocyclic scaffolds, synthesis strategies, and application scenarios in the literature from 2017 to 2023.

The two-pronged approach of heteroatoms and substituents to achieve a synergistic regulation of the ESIPT process in amino 2-(2'-hydroxyphenyl)benzoxazole derivatives

Amino 2-(2'-hydroxyphenyl)benzazole derivatives are a class of molecules with excellent photophysical properties. Most of them can be applied as a fluorescent probe via the excited-state intramolecular proton transfer (ESIPT) process. In this work, we focus on the effects of heteroatoms (O, S) and substituents (acetylacetone, hydrogen) in the derivatives. Using DFT/TDDFT methods with the B3LYP-D3BJ functionals, the absorption and emission peaks are in good agreement with the experimental data. Results of optimized structures, infrared vibrational spectra, and reduced density gradient present the existence of the ESIPT process in the S₁ state in these molecules, it also indirectly shows that the heteroatom S is more than O, and the substituent acetylacetone is more than hydrogen has stronger hydrogen bonds. The proton transfer (PT) potential energy curves (PECs) qualitatively show that it is easier for the heteroatom S to induce ESIPT than that of O. The same for the substituent acetylacetone than that of hydrogen. Under the joint influence of the simultaneous stacking of heteroatom S and acetylacetone substituent, the energy barrier of the PT process can be effectively lowered, realizing a synergistic strategy, which can provide some guidance for the design of fluorescent materials.

An insight into the dual fluorescence of 3,6-dihydroxybenzene-1,2,4,5-tetracarboxylic acid tetraethyl ester - An experimental and theoretical study

The Excited State Intramolecular Proton Transfer (ESIPT) phenomenon involving photo-induced keto-enol tautomerization is known to cause significant variations in the excited state structures and photophysical properties of certain molecules. Here, the dual emission exhibited by 3,6-dihydroxybenzene-1,2,4,5-tetracarboxylic acid tetraethyl ester has been studied both experimentally and theoretically and it is concluded that the second emission is due to ESIPT in polar protic solvents, while it is due to dianion formation in solvents like DMSO and DMF.

Proton-Coupled Electron Transfer in the Aqueous Nanochannels of Lyotropic Liquid Crystals: Interplay of H-Bonding and Polarity Effects

A molecular-level description of the aqueous nanochannels in lyotropic liquid crystals (LLCs) is crucial for their widespread utilization in diverse fields. Herein, the polarity and hydrogen bonding effects of LLC water molecules have been simultaneously explored using a single probe, 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF), by the unique multiparametric sensitivity of the excited state proton-coupled electron transfer (PCET) phenomenon. The decreased ESIPT efficiency and the significantly retarded ESIPT dynamics (>20 times) of DMA3HF in the LLC phases suggests the dominant influence of strong hydrogen-bonded solute-solvent complexes that leads to a high activation barrier for ESIPT in the mesophases. The effects of hydrogen bonding on ESIPT have been elucidated by enhanced sampling techniques based on classical MD simulations of DMA3HF in explicit water. ESIPT via an extended hydrogen-bonded water wire is associated with a significantly high ESIPT activation barrier, substantiating the experimentally observed slow ESIPT dynamics inside the LLCs.

Effect of Water Microsolvation on the Excited-State Proton Transfer of 3-Hydroxyflavone Enclosed in γ-Cyclodextrin

The effect of microsolvation on excited-state proton transfer (ESPT) reaction of 3-hydroxyflavone (3HF) and its inclusion complex with γ-cyclodextrin (γ-CD) was studied using computational approaches. From molecular dynamics simulations, two possible inclusion complexes formed by the chromone ring (C-ring, Form I) and the phenyl ring (P-ring, Form II) of 3HF insertion to γ-CD were observed. Form II is likely more stable because of lower fluctuation of 3HF inside the hydrophobic cavity and lower water accessibility to the encapsulated 3HF. Next, the conformation analysis of these models in the ground (S₀) and the first excited (S₁) states was carried out by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, respectively, to reveal the photophysical properties of 3HF influenced by the γ-CD. The results show that the intermolecular hydrogen bonding (interHB) between 3HF and γ-CD, and intramolecular hydrogen bonding (intraHB) within 3HF are strengthened in the S₁ state confirmed by the shorter interHB and intraHB distances and the red-shift of O–H vibrational modes involving in the ESPT process. The simulated absorption and emission spectra are in good agreement with the experimental data. Significantly, in the S₁ state, the keto form of 3HF is stabilized by γ-CD, explaining the increased quantum yield of keto emission of 3HF when complexing with γ-CD in the experiment. In the other word, ESPT of 3HF is more favorable in the γ-CD hydrophobic cavity than in aqueous solution.

A Flavone-Based Solvatochromic Probe with A Low Expected Perturbation Impact on the Membrane Physical State

The study of the cell membrane is an ambitious and arduous objective since its physical state is regulated by a series of processes that guarantee its regular functionality. Among the different methods of analysis, fluorescence spectroscopy is a technique of election, non-invasive, and easy to use. Besides, molecular dynamics analysis (MD) on model membranes provides useful information on the possibility of using a new probe, following its positioning in the membrane, and evaluating the possible perturbation of the double layer. In this work, we report the rational design and the synthesis of a new fluorescent solvatochromic probe and its characterization in model membranes. The probe consists of a fluorescent aromatic nucleus of a 3-hydroxyflavone moiety, provided with a saturated chain of 18 carbon atoms and a zwitterionic head so to facilitate the anchoring to the polar heads of the lipid bilayer and avoid the complete internalization. It was possible to study the behavior of the probe in GUV model membranes by MD analysis and fluorescence microscopy, demonstrating that the new probe can efficiently be incorporated in the lipid bilayer, and give a color response, thanks to is solvatochromic properties. Moreover, MD simulation of the probe in the membrane supports the hypothesis of a reduced perturbation of the membrane physical state.

Excited state intramolecular proton transfer (ESIPT) luminescence mechanism for 4-N,N-diethylamino-3-hydroxyflavone in propylene carbonate, acetonitrile and the mixed solvents

In this work, density functional theory (DFT) and time density functional theory (TDDFT) methods were employed to investigate the nature of the double fluorescence emission of DEAHF in these three solvents. We analyzed the geometric structures, vibrational frequencies, frontier molecular orbitals (MOs), molecular electrostatic potential surface (MEPS), calculated absorption and fluorescence spectra and the potential-energy curves for DEAHF. All the results show that the intramolecular hydrogen bond of DEAHF is strengthened from S₀ to S₁ and the electron density redistribution occurs between the proton acceptor and donor, which can facilitate ESIPT. Moreover, the geometric structures, absorption and emission spectra, MEPS and potential-energy curve of DEAHF are identical. It reveals theoretically that ACN and PC can maintain the polarity of the solvent with 1:1 mixing, which is consistent with the experimental results.

Peculiar hydrogen bonding behaviour of water molecules inside the aqueous nanochannels of lyotropic liquid crystals

The hydrogen bonding abilities of the LLC water molecules and their effects on intramolecular hydrogen bonds of the target probe molecules.

Modulation of Excited-State Proton-Transfer Dynamics inside the Nanocavity of Microheterogeneous Systems: Microenvironment-Sensitive Förster Energy Transfer to Riboflavin

The excited-state proton-transfer efficiency of a tetraarylpyrene derivative, 1,3,6,8-tetrakis(4-hydroxy-2,6-dimethylphenyl)pyrene (TDMPP), was investigated thoroughly in the presence of various surfactant assemblies, such as micelles and vesicles. The confined microheterogeneous environments can significantly retard the extent of the excited-state proton-transfer process, resulting in a distinguishable optical signal compared to that in the bulk medium. Physical characteristics of the surfactant assemblies, such as order, interfacial hydration, and surface charge, influence the proton transfer process and allow multiparametric sensing. A higher degree of interfacial hydration facilitates the proton-transfer process, while the positively charged head groups of the surfactants specifically stabilize the anionic form of the probe (TDMPP-O*). Furthermore, Forster energy transfer from the probe to riboflavin was studied in a phospholipid membrane, wherein the relative ratio of the neutral versus anionic forms (TDMPP-OH/TDMPP-O*) was found to influence the extent of energy transfer. Overall, we demonstrate how an ultrafast photophysical process, that is, the excited-state proton transfer, can be influenced by the microenvironment.

Time-dependent emission stokes shift in Au, Ag and Au/Ag fluorescent nanoclusters: evidence of multiple emissive states

Au, Ag and bimetallic Au/Ag nanoclusters were prepared under similar microwave-irradiated synthesis conditions in aqueous solutions, using BSA as a stabilizer. The nanoclusters exhibited strong fluorescence with characteristic emission spectral peak features that suggested the presence of multiple emissive states in each case. Time-resolved emission studies revealed the very rare incidence of the time-dependent emission Stokes shift in metal nanoclusters over a time-scale of ∼10 ns. The shifts were of varying degrees: ∼200 cm-1 for Ag, ∼750 cm-1 for Au and ∼1400 cm-1 for Au/Ag nanoclusters. Application of the Time-Resolved Area Normalized Emission Spectra (TRANES) method confirmed the existence of multiple emissive species in each of the nanoclusters, which also helped to explain the time-dependent emission Stokes shift.

Hydrogen bonding effects on the fluorescence properties of 4'-diethylamino-3-hydroxyflavone in water and water-acetone mixtures

The fluorescence properties of 4'-diethylamino-3-hydroxyflavone (FET), a dye probe sensitive to the polarity as well as the hydrogen bonding ability of its environment, have been studied in acetone-water mixtures by measuring spectra and decay curves over the whole composition range and analyzing the results on the basis of theoretical calculations. In acetone, like in most of organic solvents, the dye showed dual fluorescence, due to an excited state intramolecular proton transfer (ESIPT), in which a quasi-equilibrium between the two excited species, N* and T*, was reached. In acetone-water mixtures with lower molar fractions of water, where the water molecules are largely dispersed, only one type of hydrate could be detected, a complex with 1:1 composition, showing only N* emission, but with a high (0.45) fluorescence quantum yield. At higher water concentrations, the interaction of FET with the hydrogen-bonded water clusters resulted in fluorescence quenching. In neat water the fluorescence quantum yield fell to ~0.001. Theoretical calculations on a FET-acetone complex, a FET-water complex and a FET-water-acetone triple complex (the latter as model for the samples with low water concentrations) concluded that ESIPT was energetically favored in all the models, but the E(N*)-E(T*) energy difference for the water complexes was much lower. The kinetic barrier of ESIPT was found greatly higher in the FET-water complex than in the isolated solute. The intermolecular hydrogen bonds in the water complexes became significantly stronger following the excitation, stabilizing the N* form of the hydrated dye.

Ultrafast Ground-State Intramolecular Proton Transfer in Diethylaminohydroxyflavone Resolved with Pump-Dump-Probe Spectroscopy

4'- N, N-Diethylamino-3-hydroxyflavone (DEAHF), due to excited-state intramolecular proton transfer (ESIPT) reaction, exhibits two solvent-dependent emission bands. Because of the slow formation and fast decay of the ground-state tautomer, its population does not accumulate enough for its detection during the normal photocycle. As a result, the details of the ground-state intramolecular proton-transfer (GSIPT) reaction have remained unknown. The present work uses femtosecond pump-dump-probe spectroscopy to prepare the short-lived ground-state tautomer and track this GSIPT process in solution. By simultaneously measuring femtosecond pump-probe and pump-dump-probe spectra, ultrafast kinetics of the ESIPT and GSIPT reactions are obtained. The GSIPT reaction is shown to be a solvent-dependent irreversible two-state process in two solvents, with estimated time constants of 1.7 ps in toluene and 10 ps in the more polar tetrahydrofuran. These results are of great value in both fully describing the photocycle of this four-level proton transfer molecule and for providing a deeper understanding of dynamical solvent effects on tautomerization.

Tuning the emission of a water-soluble 3-hydroxyflavone derivative by host-guest complexation

3-Hydroxyflavone derivatives have great potential as fluorescent probes for bio-labeling in aqueous medium. They were extensively studied in various organic solvents for the "excited state intramolecular proton transfer" process, but seldom addressed in aqueous solution due to the poor water solubility. Herein, an amphiphilic molecule bearing 3-hydroxyflavone and oligo(ethylene oxide) (denoted as 3HF-EO) was designed and synthesized. Different from the fluorescence in organic solvents, 3HF-EO in aqueous solution showed a remarkable single fluorescence emission, which is ascribed to the fluorescence of its anionic species. We found that the fluorescence intensity could be efficiently tuned via host-guest complexation. α-CD has little effect on the emission, while β-CD and γ-CD lead to enhanced and reduced emissions of 3HF-EO, respectively. The 1H NMR and 2D NOESY NMR spectra indicate that α-CD barely had any interaction with 3HF-EO, while β-CD and γ-CD formed complexes with one and two 3HF-EO molecules, respectively. These results provide a sound explanation for the modulated fluorescence intensity.

Excited-state intramolecular proton transfer and conformational relaxation in 4'-N,N-dimethylamino-3-hydroxyflavone doped in acetonitrile crystals

The effect of intermolecular interactions on excited-state intramolecular proton transfer (ESIPT) in 4'-N,N-dimethylamino-3-hydroxyflavone (DMHF) doped in acetonitrile crystals was investigated by measuring its temperature dependence of steady-state fluorescence excitation and fluorescence spectra and picosecond time-resolved spectra. The relative intensity of emission from the excited state of the normal form (N*) to that from the excited state of the tautomer form (T*) and spectral features changed markedly with temperature. Unusual changes in the spectral shift and spectral features were observed in the fluorescence spectra measured between 200 and 218 K, indicating that a solid-solid phase transition of DMHF-doped acetonitrile crystals occurred. Time-resolved fluorescence spectra suggested conformational relaxation of the N* state competed with ESIPT after photoexcitation and the ESIPT rate increased with temperature in the low-temperature phase of acetonitrile. However, the intermolecular interaction of N* with acetonitrile in the high-temperature phase markedly stabilized the potential minimum of the fluorescent N* state and slowed the ESIPT. This stabilization can be explained by reorganization of acetonitrile originating from the strong electric dipole-dipole interaction between DMHF and acetonitrile molecules.

Excited-state intramolecular proton-transfer (ESIPT) based fluorescence sensors and imaging agents

We review recent advances in the design and application of excited-state intramolecular proton-transfer (ESIPT) based fluorescent probes. These sensors and imaging agents (probes) are important in biology, physiology, pharmacology, and environmental science.

Antioxidant flavone analog functionalized fluorescent silica nanoparticles: Synthesis and exploration of their possible use as biomolecule sensor

For the first time, a synthetic fluorescent antioxidant flavone analog was successfully anchored onto the surface of the APTES-modified mesoporous silica nanoparticles (NPs) through sulfonamide linkage. The surface chemistry and morphology of the flavone modified fluorescent silica (FMFS) NPs were studied in detail. The flavone moiety when attached onto the FMFS NP surface, imparted its characteristic fluorescence and antioxidant activities to these NPs. Moreover, the NPs are highly biocompatible as evidenced from their cytotoxicity assay on normal lung cell (L132). The fluorescence activity of these biocompatible NPs was further utilized to study their interaction with a biomolecule, BSA (Bovine Serum Albumin). It was interesting to note that the fluorescence behavior of FMFS NPs completely changed on their binding with BSA. On the other hand, the intrinsic fluorescence activity of BSA was also significantly modified due to its interaction with FMFS NPs. Thus, the sensing and detection of biomolecules like BSA in presence of FMFS NPs can be accomplished by monitoring changes in the fluorescence behavior of either FMFS NPs or BSA. Furthermore, these FMFS NPs retained their intrinsic fluorescence behavior in the cellular medium which opens up their possible use as biocompatible cell imaging agents in future.

Temperature dependent spectroscopic and excited state dynamics of 3-hydroxychromones with electron donor and acceptor substituents

We have studied the photophysical and photochemical behavior of three compounds derived from 3-hydroxychromone (3-HC), capable of undergoing excited state proton transfer (ESIPT). The compounds have two substituents, located in positions 2 and 7, one on each ring of the 3-HC heterocycle. The substituent pattern shows different electron donating and acceptor features. The compounds were studied by absorption and emission spectroscopy, steady state anisotropy, and time resolved emission spectroscopy (TRES) as a function of temperature. Results were interpreted using time dependent density functional theory calculations. Compared to reference compounds of 3-HC substituted only in the 2 position, the compounds show similar absorption and emission spectra, shifted 20-30 nm to higher wavelengths due to extended conjugation. TRES shows the existence of ESIPT in the thermodynamic equilibrium regime. This process is endothermic in all three compounds. The different behavior compared to monosubstituted 3-HC is attributed to the extended conjugation and to the electron donor acceptor character of the substituents, which has a more pronounced effect when the electron acceptor is located in position 2.

Theoretical study on the photooxygenation and photorearrangement reactions of 3-hydroxyflavone

The mechanisms of three photodegradation reactions of 3-hydroxyflavone – its photosensitized oxygenation, photooxygenation with ³O₂ and photorearrangement into an indanedione derivative – have been investigated by computing the free energy profiles.

Excited-state intramolecular proton-transfer (ESIPT)-inspired solid state emitters

Solid state emitters based on excited state intramolecular proton transfer (ESIPT) have been attracting considerable interest since the past few years in the field of optoelectronic devices because of their desirable unique photophysical properties. The photophysical properties of the solid state ESIPT fluorophores determine their possible applicability in functional materials. Less fluorescence quantum efficiencies and short fluorescence lifetime in the solid state are the shortcomings of the existing ESIPT solid state emitters. Designing of ESIPT chromophores with high fluorescence quantum efficiencies and a long fluorescence lifetime in the solid state is a challenging issue because of the unclear mechanism of the solid state emitters in the excited state. Reported design strategies, detailed photophysical properties, and their applications will help in assisting researchers to overcome existing challenges in designing novel solid state ESIPT fluorophores for promising applications. This review highlights recently developed solid state ESIPT emitters with focus on molecular design strategies and their photophysical properties, reported in the last five years.

The kinetics and mechanism of photooxygenation of 4'-diethylamino-3-hydroxyflavone

The photolysis reactions of 4'-diethylamino-3-hydroxyflavone (D), a versatile fluorescent probe showing excited-state intramolecular proton transfer (ESIPT), and the magnesium chelate of D (MgD(2+)) have been studied in acetonitrile solution. Upon UV irradiation both species were oxidized into O-4-diethylaminobenzoyl salicylic acid, differently from the photoreaction of the parent compound 3-hydroxyflavone (3HF) which was described to undergo rearrangement to 3-hydroxy-3-phenyl-indan-1,2-dione. The photooxygenation of the Mg(2+) complex was found to be significantly faster than the reaction of the pure dye. As the kinetic analysis of the absorption spectra of samples under irradiation showed, the rate coefficients for the oxygenations of the excited state dye and complex have close values, kox(D*) = 2.4 × 10(7) min(-1), kox(MgD(2+)*) = 3.9 × 10(7) min(-1); the difference arises from the higher photooxygenation quantum yield of the complex, Φ(MgD(2+)) = 2.3 × 10(-3), than the respective value for the pure dye, Φ(D) = 1.5 × 10(-4). The potential energy surface of the photooxygenation of D was calculated assuming a reaction path in which the phototautomer formed from Dvia ESIPT, reacts in its triplet state with triplet molecular oxygen O2, a mechanism similar to that suggested for the photoreaction of the parent 3HF. The moderate values for the transition state energies confirmed the plausibility of the hypothetical mechanism.

3-Hydroxyflavone derivatives synthesized by a new simple method as chemosensors for cyanide anions

Two 3-hydroxyflavone derivatives as chemosensors for cyanide were synthesized by one step simple condensation, cyclization and subsequent oxidation reaction catalyzed by pyrrolidine, which shows great convenience compared with traditional method.

Unusually slow intramolecular proton transfer dynamics of 4′-N,N-dimethylamino-3-hydroxyflavone in high n-alcohols: involvement of solvent relaxation

The S₁ energy surface of DMA3HF as a function of solvation and ESIPT coordinates, viewed down the energy axis as a 2-dimensional projection.