State Intramolecular Proton Transfer Reaction Research Articles

Optical properties of 1-(4,5-diphenyl-1-p-tolyl-1H-imidazol-2-yl)naphthalen-2-ol – ESIPT process

This article presents optical, electrochemical, and thermal properties of novel class of green fluorescent 1-(4,5-diphenyl-1-p-tolyl-1H-imidazol-2-yl)naphthalen-2-ol. Detailed photo physical and quantum chemical studies have been performed to elucidate the excited state intramolecular proton transfer (ESIPT) reaction leading a large stokes shifted fluorescence emission from the phototautomer. The results of quantum chemical investigations confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers. The high photoluminescence quantum yield is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the imidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition without self quenching.

Structural, Electronic and Charge Transfer Studies of Highly Sensitive Fluorescent Probe 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenol: Quantum Chemical Investigations

This article presents a facile synthesis of novel class of bluish-green fluorescent 2-((E)-2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenol [PPIVP] and their optical, electrochemical and thermal properties. Detailed photophysical and quantum chemical studies have been performed to elucidate the origin of the dual emission shifts. PPIVP undergo excited state intramolecular proton transfer (ESIPT) reaction leading a large Stoke's shifted fluorescence emission from the phototautomer. The results of quantum chemical investigations not only confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers but also provided a rational for the observed high fluorescence quantum efficiency in the solid state. The high photoluminescence quantum yield in the solid state is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the phenanthroimidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition without self quenching.

Excitation Wavelength Dependence of Excited State Intramolecular Proton Transfer Reaction of 4′-N,N-Diethylamino-3-hydroxyflavone in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement

Excited state intramolecular proton transfer reactions (ESIPT) of 4'-N,N-diethylamino-3-hydroxyflavone (DEAHF) in ionic liquids have been studied by steady-state and time-resolved fluorescence measurements at different excitation wavelengths. Steady-state measurements show the relative yield of the tautomeric form to the normal form of DEAHF decreases as excitation wavelength is increased from 380 to 450 nm. The decrease in yield is significant in ionic liquids that have cations with long alkyl chains. The extent of the decrease is correlated with the number of carbon atoms in the alkyl chains. Time-resolved fluorescence measurements using optical Kerr gate spectroscopy show that ESIPT rate has a strong excitation wavelength dependence. There is a large difference between the spectra at a 200 ps delay from different excitation wavelengths in each ionic liquid. The difference is pronounced in ionic liquids having a long alkyl chain. The equilibrium constant in the electronic excited state obtained at a 200 ps delay and the average reaction rate are also correlated with the alkyl chain length. Considering the results of the steady-state fluorescence and time-resolved measurements, the excitation wavelength dependence of ESIPT is explained by state selective excitation due to the difference of the solvation, and the number of alkyl chain carbon atoms is found to be a good indicator of the effect of inhomogeneity for this reaction.

Comments on the paper “Excited state intramolecular proton transfer reaction revisited: S1 state or general reversibility?” by A.O. Doroshenko, A.Yu. Matsakov, O.V. Nevskii, O.V. Grygorovych [J. Photochem. Photobiol. A: Chem. 250 (2012) 40–49

We cannot agree with the point of view [1] that in characteristics of dual fluorescence of flavones ESIPT reverse reaction is not actual and “simple test (suggested in our paper [2]) for the mechanism of excited-state reaction” does not confirm the ESIPT reaction reversibility. We present here our arguments and experimental results in favor of our position.

Active Role of Proton in Excited State Intramolecular Proton Transfer Reaction

Proton transfer is one of the most important elementary reactions in chemistry and biology. The role of proton in the course of proton transfer, whether it is active or passive, has been the subject of intense investigations. Here we demonstrate the active role of proton in the excited state intramolecular proton transfer (ESIPT) of 10-hydroxybenzo[h]quinoline (HBQ). The ESIPT of HBQ proceeds in 12 ± 6 fs, and the rate is slowed down to 25 ± 5 fs for DBQ where the reactive hydrogen is replaced by deuterium. The results are consistent with the ballistic proton wave packet transfer within the experimental uncertainty. This ultrafast proton transfer leads to the coherent excitation of the vibrational modes of the product state. In contrast, ESIPT of 2-(2'-hydroxyphenyl)benzothiazole (HBT) is much slower at 62 fs and shows no isotope dependence implying complete passive role of the proton.

Excited state intramolecular proton transfer reaction revisited: S1 state or general reversibility?

Abstract The excited state intramolecular proton transfer (ESIPT) reaction reversibility was reexamined both experimentally and theoretically on the example of a series of 3-hydroxychromone (3HC) derivatives. The experimental reversibility criteria for the excited state adiabatic photoreactions based on dynamic quenching of two-banded 3HC fluorescence were re-deduced. Quenching experiments in the series of 3HC with the specially selected quencher, which does not absorb on the excitation wavelength, 2,4-dinitrotoluene, did not reveal the excited state reversibility in any of the examined dry aprotic solvents of different polarity. Quantum-chemical modeling including the excited state molecular geometry analysis, estimation of the ground and excited state H-bonds energies with the application of the “atoms in molecules” (AIM) approach and calculation of energetic profiles of the forward and back proton transfer reactions in the S0 and S1 states allows us to conclude there are no serious prerequisites for the excited state reversibility of the intramolecular proton transfer reaction.

2-(2′-Hydroxyphenyl)-4(3H)-quinazolinone derivatives based fluorescent probes for mercury(II) via an intramolecular proton transfer mechanism

2-(2′,5′-Dihydroxy-phenyl)-4(3H)-quinazolinone (DHPQ), a new fluorescent dye that exhibits excited state intramolecular proton transfer (ESIPT) reaction and possesses good photophysical properties, is synthesised and used as fluorescent probe for detection of Hg2+. Mercuric ions can be detected and quantitated by measuring the fluorescent intensity decrease of the probe. The decrease of fluorescence intensity of DHPQ upon the addition of Hg2+ was attributed to the blocking of ESIPT reactions of DHPQ and quenching its fluorescence. The analytical performance characteristics of the proposed Hg2+ probe were investigated. The probe can be applied to the quantification of Hg2+ with a concentration range covering from 8.0 × 10−7 to 2.0 × 10−4 mol L−1, with a working pH range of 5.5–6.5. It shows excellent selectivity for Hg2+ over other transition metal cations. The proposed method was testified for the Hg2+ assay in river water samples with satisfying recoveries.

Synthesis, characterization and optical properties of aryl and diaryl substituted phenanthroimidazoles

Abstract This article presents a facile synthesis of novel class of blue-green fluorescent phenanthroimidazoles and report on their optical, electrochemical, and thermal properties. Detailed photophysical and quantum chemical studies with a series of hydroxy-, methoxy-, nitro-, amino- and tosylaminophenyl substituted derivatives of 2-phenyl-1 H -phenanthro[9,10- d ]imidazole and 1,2-diphenyl-1 H -phenanthro[9,10- d ]imidazole have been performed to elucidate the origin of the surprisingly divergent emission shifts. Out of these, three dyes (HPhI, HPPhI, and TsPPhI) undergo excited state intramolecular proton transfer (ESIPT) reaction leading a large Stokes’ shifted fluorescence emission from the phototautomer. The results of quantum chemical investigations not only confirmed the intramolecular charge transfer characteristics of the ESIPT tautomers but also provided a rational for the observed high fluorescence quantum efficiency in the solid state. The high photoluminescence quantum yield ( Φ PL ∼ 39–68%) in the solid state is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the phenanthroimidazole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition without self quenching.

Molecular dynamics of excited state intramolecular proton transfer: 3-hydroxyflavone in solution

The ultrafast enol-keto photoisomerization in the lowest singlet excited state of 3-hydroxyflavone is investigated using classical molecular dynamics in conjunction with empirical valence bond (EVB) potentials for the description of intramolecular interactions, and a molecular mechanics and variable partial charge model, dependent on transferring proton position, for the description of solute-solvent interactions. A parallel multi-level genetic program was used to accurately fit the EVB potential energy surfaces to high level ab initio data. We have studied the excited state intramolecular proton transfer (ESIPT) reaction in three different solvent environments: methylcyclohexane, acetonitrile, and methanol. The effects of the environment on the proton transfer time and the underlying mechanisms responsible for the varied time scales of the ESIPT reaction rates are analyzed. We find that simulations with our EVB potential energy surfaces accurately reproduce experimentally determined reaction rates, fluorescence spectra, and vibrational frequency spectra in all three solvents. Furthermore, we find that the ultrafast ESIPT process results from a combination of ballistic transfer, and intramolecular vibrational redistribution, which leads to the excitation of a set of low frequency promoting vibrational modes. From this set of promoting modes, we find that an O-O in plane bend and a C-H out of plane bend are present in all three solvents, indicating that they are fundamental to the ultrafast proton transfer. Analysis of the slow proton transfer trajectories reveals a solvent mediated proton transfer mechanism, which is diffusion limited.

Exploring hydrogen bond in the excited state leading toward intramolecular proton transfer: detailed analysis of the structure and charge density topology along the reaction path using QTAIM

Excited state intramolecular proton transfer (ESIPT) reaction along the O-H[Symbol: see text][Symbol: see text][Symbol: see text][Symbol: see text]O hydrogen bond of o-hydroxy benzaldehyde (OHBA), methyl salicylate (MS) and salicylic acid (SA) was investigated by ab-initio quantum chemical calculation and theory of atoms and molecules (QTAIM) for the first time. Variation in several geometric as well as QTAIM parameters along the reaction coordinate was monitored in the fully relaxed excited state potential energy curve (PEC) obtained from intrinsic reaction coordinate (IRC) analysis. Although, the excited state barrier height for the forward reaction (∆E (0) (#) ) reduces substantially in all the systems, MS and SA do not show any obvious asymmetry for proton transfer. For MS and SA, the crossover of the bond index as well as the lengths of the participating bonds at the saddle point is assigned due to this symmetry in accordance with bond energy - bond order (BEBO) model, which does not hold true in OHBA both in the ground and excited states. Bond ellipticity, covalent and metallic character were examined for different structures along the reaction path within the QTAIM framework. The QTAIM analysis was found to be able to uniquely distinguish between the ground and excited states of the OHBA molecule as well as both determining the effects on the bonding character of adding different substituent groups and differentiating between the ESIPT reactions in the SA and MS molecules.

Selective anion recognition by inhibition of excited state intramolecular proton transfer process via hydrogen bonding interaction and efficient deprotonation: Spectroscopic and theoretical investigation

Abstract Anion (X) recognition fluorescent chemosensor 5-(4-Fluoro-phenyl)-2-hydroxy-nicotinonitrile (FP2HN) having both –NH and –OH groups in its two tautomeric forms has been synthesized and its excited state intramolecular proton transfer (ESIPT) reaction has been investigated by UV–vis, fluorescence, 1 H NMR spectroscopy in combination with computational calculations. The experimental findings show that FP2HN selectively recognizes F − , AcO − and H 2 PO 4 − ions through the formation of –N(O)H⋯X hydrogen bonding (HB) complex. The binding constant obtained by non-linear fit predicts the binding order of F − > AcO − > H 2 PO 4 − > Cl − . The ESIPT process of FP2HN is inhibited either by the fluoride induced deprotonation of acidic proton of FP2HN or by the formation of a strong –N(O)H⋯X intermolecular hydrogen bonding complex. Further insights into the recognition mechanism of the receptor–anion complexes using ab initio calculations have been performed which support the experimental binding affinity order of F − > AcO − > H 2 PO 4 − > Cl − .

A computational investigation on the intramolecular hydrogen bonding interaction and excited state intramolecular proton transfer process in 2-quinolin-2-yl-phenol

In the present work, we report computational investigation on the photophysics of 2-quinolin-2-yl-phenol (2Q2P) which can function as the basic unit responsive to excited state intramolecular proton transfer (ESIPT) reaction in the polymer framework composed of polyquinoline derivatives. The operation of ESIPT in 2Q2P has been most critically addressed on the lexicon of potential energy surface (PES) across the reaction coordinate and cross-validated from frontier molecular orbital (MO) analysis. Evolution of other geometrical parameters during the course of the ESIPT process in 2Q2P has been crucial to delve into the mechanistic details of the process. Major emphasis of the work is rendered on the analysis of the intramolecular hydrogen bonding (IMHB) interaction in 2Q2P explored by calculation of electron density ρ( r) and the Laplacian ∇ 2 ρ( r) at the bond critical point (BCP) using Atoms-In-Molecule (AIM) theory. Topological features, energy densities provided by AIM theory are calculated with ρ( r) for a number of intramolecular H-bond distances. The results suggest that at equilibrium geometry the IMHB interaction in the system (2Q2P) develops certain characteristics typical of covalent interaction. Integrated AIM properties have been applied to delve into the phenomenon of resonance-assisted hydrogen bonding (RAHB). The inter-correlation between aromaticity and RAHB is also discussed in this context.

Dual Fluorescence of Ellipticine: Excited State Proton Transfer from Solvent versus Solvent Mediated Intramolecular Proton Transfer

Photophysical properties of a natural plant alkaloid, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), which comprises both proton donating and accepting sites, have been studied in different solvents using steady state and time-resolved fluorescence techniques primarily to understand the origin of dual fluorescence that this molecule exhibits in some specific alcoholic solvents. Ground and excited state calculations based on density functional theory have also been carried out to help interpretation of the experimental data. It is shown that the long-wavelength emission of the molecule is dependent on the hydrogen bond donating ability of the solvent, and in methanol, this emission band arises solely from an excited state reaction. However, in ethylene glycol, both ground and excited state reactions contribute to the long wavelength emission. The time-resolved fluorescence data of the system in methanol and ethylene glycol indicates the presence of two different hydrogen bonded species of ellipticine of which only one participates in the excited state reaction. The rate constant of the excited state reaction in these solvents is estimated to be around 4.2-8.0 × 10(8) s(-1). It appears that the present results are better understood in terms of solvent-mediated excited state intramolecular proton transfer reaction from the pyrrole nitrogen to the pyridine nitrogen leading to the formation of the tautomeric form of the molecule rather than excited state proton transfer from the solvents leading to the formation of the protonated form of ellipticine.

On the Photophysics of 3,5,6-Trichlorosalicylic Acid: Spectroscopic Study Combined with Hartree-Fock and Density Functional Theory Calculations

The present contribution reports a detailed photophysical study of a simple salicylic acid derivative viz., 3,5,6-Trichlorosalicylic acid (TCSA) based on steady state absorption, emission and time-resolved emission spectroscopy. Anomalous "dual" emission coupled with a large Stokes shift and negligible solvent polarity dependence marks the spectroscopic signature for Excited State Intramolecular Proton Transfer (ESIPT) reaction. Variation of medium polarity and pH of the medium have been implemented as fruitful tools to decipher the photophysics of TCSA. Quantum chemical calculation by ab initio Hartree-Fock and Density Functional Theory methods yields consistent results to follow experimental findings with distinct illustration of the inoperativeness of GSIPT reaction as well as occurrence of ESIPT process. A rigorous comparison of our experimental and theoretical measurements of TCSA with the parent compound salicylic acid, 5-chlorosalicylic acid and 3,5-dichlorosalicylic acid reveals the impact of chlorine substitution on the photophysics of the studied molecular systems with simultaneous exploration of the complexities induced in TCSA with respect to salicylic acid.

Inequivalence of substitution pairs in hydroxynaphthaldehyde: A theoretical measurement by intramolecular hydrogen bond strength, aromaticity, and excited‐state intramolecular proton transfer reaction

The inequivalence of substitution pair positions of naphthalene ring has been investigated by a theoretical measurement of hydrogen bond strength, aromaticity, and excited state intramolecular proton transfer (ESIPT) reaction as the tools in three substituted naphthalene compounds viz 1-hydroxy-2-naphthaldehyde (HN12), 2-hydroxy-1-naphthaldehyde (HN21), and 2-hydroxy-3-naphthaldehyde (HN23). The difference in intramolecular hydrogen bond (IMHB) strength clearly reflects the inequivalence of substitution pairs where the calculated IMHB strength is found to be greater for HN12 and HN21 than HN23. The H-bonding interactions have been explored by calculation of electron density ρ(r) and Laplacian ∇(2) ρ(r) at the bond critical point using atoms in molecule method and by calculation of interaction between σ* of OH with lone pair of carbonyl oxygen atom using NBO analysis. The ground and excited state potential energy surfaces (PESs) for the proton transfer reaction at HF (6-31G**) and DFT (B3LYP/6-31G**) levels are similar for HN12, HN21 and different for HN23. The computed aromaticity of the two rings of naphthalene moiety at B3LYP/6-31G** method also predicts similarity between HN12 and HN21, but different for HN23.

Lead chloride-based layered perovskite incorporated with an excited state intramolecular proton transfer dye

We successfully fabricated thin films of organic–inorganic layered perovskite-type compound (AEHBA)2PbCl4, where AEHBA stands for N-(2-aminoethyl)-2-hydroxybenzamide, a blue fluorescent dye that shows excited state intramolecular proton transfer (ESIPT) reaction. The formation of its highly ordered structure was readily achieved by the spin-coating of N,N-dimethylformamide solution dissolving PbCl2 and the AEHBA hydrogen chloride salt. X-ray diffraction analysis of the films revealed that organic AEHBA and inorganic PbCl4 perovskite layers were alternately stacked parallel to the substrate. In the absorption spectrum, exciton formation within the perovskite layers was confirmed by the appearance of a characteristic sharp band, while the photoluminescence spectrum showed a large Stokes-shifted band of AEHBA, indicating that it underwent an ESIPT.

Excited State Intramolecular Proton Transfer Reaction of 4′-N,N-Diethylamino-3-hydroxyflavone and Solvation Dynamics in Room Temperature Ionic Liquids Studied by Optical Kerr Gate Fluorescence Measurement

Fluorescence dynamics of 4'-N,N-diethylamino-3-hydroxyflavone (DEAHF) and its methoxy derivative (DEAMF) in various room temperature ionic liquids (RTILs) have been studied mainly by an optical Kerr gate method. DEAMF showed a single band fluorescence whose peak shifted with time by the solvation dynamics. The averaged solvation time determined by the fluorescence peak shift was proportional to the viscosity of the solvent except for tetradecyltrihexylphosphonium bis(trifluoromethanesulfonyl)amide. The solvation times were consistent with reported values determined with different probe molecules. DEAHF showed dual fluorescence due to the normal and tautomer forms produced by the excited state intramolecular proton transfer (ESIPT), and the relative intensities were dependent on the time and the solvent cation or anion species. By using the information of the fluorescence spectrum of DEAMF, the fluorescence spectrum of DEAHF at each delay time after the photoexcitation was decomposed into the normal and the tautomer fluorescence components, respectively. The normal component showed a very fast decay simulated by a biexponential function (2-3 and 20-30 ps) with an additional slower decay component. The tautomer component showed a rise with the time constants corresponding to the faster decay of the normal form with an additional instantaneous rise. The faster dynamics of the normal and tautomer population changes were assigned to the ESIPT process, while the slower decay of the fluorescence was attributed to the population decay from the excited state through the radiative and nonradiative processes. The average ESIPT time was much faster than the averaged solvation time of RTILs. Basically, the ESIPT kinetics in RTILs is similar to those in conventional liquid solvents like acetonitrile (Chou et al. J. Phys. Chem. A 2005, 109, 3777). The faster ESIPT is interpreted in terms of the activation barrierless process from the Franck-Condon state before the solvation of the normal state in the electronic excited state. With the advance of the solvation in the excited state, the normal form becomes relatively more stable than the tautomer form, which makes the ESIPT become an activation process.

Influence of chlorine substitution on intramolecular hydrogen bond energy and ESIPT barrier: Experimental and theoretical measurements on the photophysics of 3,5-dichlorosalicylic acid

The effect of chlorine atom on the intramolecular hydrogen bond strength and excited state proton transfer barrier in pharmaceutically important chloro-substituted derivative of salicylic acid viz., 3,5-dichlorosalicylic acid (3,5DCSA) has been explored through steady-state absorption, emission and time-resolved fluorescence spectroscopy. Stokes shifted emission band with negligible solvent polarity dependency corresponds to the spectroscopic signature of excited state intramolecular proton transfer (ESIPT) reaction. The spectral signature was compared with its parent molecule salicylic acid (SA) and 5-chlorosalicylic acid (5ClSA). Quantum chemical calculations by ab initio Hartree–Fock (HF) and Density Functional Theory (DFT) methods have been fruitfully employed to correlate experimental findings. Calculated S 0 and S 1 states potential energy surfaces across the proton transfer co-ordinate substantiates the experimental evidence for the occurrence of ESIPT process and negates the ground state intramolecular proton transfer (GSIPT) reaction. Weakening of intramolecular hydrogen bond (IMHB) energy and subsequent enhancement of barrier to ESIPT reaction in 3,5DCSA as compared to SA and 5ClSA appears to be a reflection of conjugate impact of electron withdrawing inductive and electron donating resonance effects of chlorine substitutions depending on its location on the aromatic benzene nucleus.

Interactions of therapeutically active plant flavonols with biological targets : Insights from fluorescence spectroscopic studies

Plant flavonols have attracted much recent attention in view of their novel therapeutic properties (effective against various free radical mediated and other diseases) which make them promising alternatives to conventional therapeutic drugs. However, till date, not much is known, regarding their mode of interactions and binding affinities with relevant biological targets. This article presents perspectives highlighting the usefulness of the exquisitely sensitive 'two color' fluorescence behavior of flavonols (which arise due to highly efficient photoinduced excited state intramolecular proton transfer (ESIPT) reactions) for exploring their interactions, at the molecular level, with biomembranes and proteins, which are the principal biological targets of such drug molecules. In this context, we made exploratory studies on the interactions of some representative therapeutically important flavonols with model and natural membranes (composed of phosphatidylcholine liposomes and red blood cell ghost membranes respectively) and serum albumin proteins. Since the ESIPT process is highly sensitive to external hydrogen bonding perturbation effects, the relative contribution between the two colors is strongly modulated by the local environment of the fluorophore, with dramatic changes in the emission yield, energy, anisotropy (r), lifetime (τ) and related parameters of both ESIPT tautomer and normal fluorescence bands. This provides multiparametric fluorescence probing opportunities, revealing salient details about the nature and location of binding sites as well as quantitative estimates of partition coefficients/binding constants. This promising new approach may be expected to open up new avenues for the 'screening' of the most appropriate flavonoid derivatives, from among numerous structural variants found in nature, as well as the design of relevant synthetic derivatives with improved features.

Pyrazolyc 3-hydroxychromones: Regulation of ESIPT reaction by the “flavonol-like” intramolecular hydrogen bonding to carbonyl group oxygen, which dominates over the “alternative” H-bond to heterocyclic nitrogen

Two isomeric pyrazole derivatives of 3-hydroxychromone (3HC) with and without the possibility of the multiple intramolecular hydrogen bonds formation were compared theoretically and experimentally with the aim to find out whether the excited state intramolecular proton transfer (ESIPT) reaction follows the traditional to the most of 3HCs “flavonol-like” direction towards the C O group oxygen or an “alternative” direction towards the heterocyclic nitrogen atom. Quantum-chemical modeling and comparative study of the experimental spectral parameters of the title compounds indicated the preferential realization of “flavonol-like” ESIPT to oxygen channel. The 3HC systems with the “alternative” intramolecular hydrogen bond to nitrogen were characterized as low fluorescent and practically unable to ESIPT with participation of the nitrogen containing heterocyclic unit.