Trans-enol Form Research Articles

Thermal-Induced Twisting and Photoinduced Planarization of Salicylideneaniline in Alcohols.

Thermofluorochromism and photochromism of salicylideneaniline (SA) in alcohol were investigated using steady-state and time-resolved fluorescence and absorption spectroscopy. The planar trans-enol form of SA in alcohols is converted into the twisted trans-enol form on heating. This conversion results change in the emission maximum from the 530 to the 440 nm region with an increase in fluorescence intensity, which confirms the absence of intramolecular hydrogen bonding between imine nitrogen and phenolic hydrogen in the twisted trans-enol form. The activation barrier for thermal-induced formation of the twisted trans-enol form in methanol was determined experimentally and was found to be 20.15 ± 2.22 kcal/mol. The rotation of the phenolic C7-C8 and C7-N1 bond followed by breaking of the intramolecular hydrogen bond and formation of an intermolecular hydrogen bond with alcohol solvent molecules results in the thermally stable twisted trans-enol form in alcohol solvents. The biexponential nature of the fluorescence decay of the twisted trans-enol form of SA confirms that the emission originates from multiple (π-π* and n-π*) excited states. On photolysis under UV light, the twisted trans-enol form is converted back into the planar trans-enol form. The time-resolved absorption and excitation-resolved fluorescence spectrum of SA in methanol confirm the existence of the twisted cis-keto form as a transient photochromic intermediate in the light-induced planarization of SA in alcohols. In alcohols, an interplay between the intra- and intermolecular hydrogen-bonding controls excited-state reaction dynamics and conformational relaxation of SA, which are responsible for the photochromism of salicylideneaniline.

Negative differential resistance and stable conductance switching behaviors of salicylideneaniline molecular devices sandwiched between armchair graphene nanoribbon electrodes

By applying nonequilibrium Green's functions in combination with the density-functional theory, we investigate the electron transport properties of the salicylideneaniline molecule sandwiched between two armchair graphene nanoribbon (AGNR) electrodes. It shows that the width of the AGNR electrodes plays a significant role in the transport properties of the salicylideneaniline molecular junctions. The current–voltage characteristics of the salicylideneaniline molecule sandwiched between 8AGNR electrodes can perform a stable conductance switching behavior at the bias region [1.2 V, 1.6 V] when the molecule translates between the trans-enol form and the trans-keto form. When the electrodes change to 7AGNR, a remarkable negative differential resistance behavior can be found in the salicylideneaniline molecular device at the trans-keto form. That means the salicylideneaniline molecule can perform different functions when it connects to the different AGNR electrodes.

Physico-chemical studies on some fluorescence sensors: DFT based ESIPT process

A group of novel heterocyclic Schiff base derivatives were synthesized and characterized by NMR spectra, mass and CHN analysis. Excited state intramolecular proton transfer (ESIPT) processes in o-hydroxy Schiff base have been studied using electronic spectral studies. Experimental and FT studies support that trans enol form predominates over the cis enol form. The fluorescence of Schiff bases quenched markedly with the gradual addition of Cu2+. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the Schiff base derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state. The charge distribution has been calculated from the atomic charges by non-linear optical (NLO) and natural bond orbital (NBO) analysis. The energies of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels and the molecular electrostatic potential (MEP) energy surface studies evidenced the existence of intramolecular charge transfer (ICT) within the molecule.

DFT based ESIPT process of luminescent chemosensor: Taft and Catalan solvatochromism

Amino acid containing o-hydroxy Schiff base derivative was synthesized and an excited state intramolecular proton transfer (ESIPT) process in o-hydroxy Schiff base (HBMBA) has been studied using solution spectral studies. It was detected that the two distinct ground state isomers I and II are responsible for the emission. By comparing the emission wavelength in hydrocarbon solvent, it was predicted that the trans enol form predominates over the cis enol form. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the Schiff base derivative were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than in the ground state. The fluorescence of HBMBA quenched with the gradual addition of Cu2+. Therefore the prepared Schiff base HBMBA can be used as a new fluorescent sensor to detect the quantity of Cu2+ ions in any sample solution depending on the relative intensity change. Measurements of absorption solvatochromism have been interpreted with Marcus and Reichardt–Dimroth solvent functions to estimate the transition dipoles. Good correlation exists between absorption as well as fluorescence wavenumbers obtained by the multi-component linear regression employing the Taft and Catalan solvent parameters with the experimental values.

A physiochemical study of azo dyes: DFT based ESIPT process

Azo linked dye derivatives were synthesized and characterized by NMR, mass and elemental analysis. An excited state intramolecular proton transfer (ESIPT) in hydroxy Schiff base has been analyzed, and found that two distinct ground state isomers of I and II are responsible for the observed dual emission. DFT calculation on energy, dipole moment, charge distribution of the rotamers in the ground and excited states support the ESIPT process. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state. By varying the addition of base concentration to hydroxy Schiff base, two isobestic points were found which confirm the equilibrium among the trans enol form, anion and the cis enol form. Fluorescence quenching with metal ions reveal that hydroxy Schiff base can be used as a new fluorescence sensor to detect the Cu 2+ ion.

A physiochemical study of excited state intramolecular proton transfer process: Luminescent chemosensor by spectroscopic investigation supported by ab initio calculations

A group of novel Schiff base derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy Schiff base (SB 4) has been studied using emission spectroscopy and it was detected that the two distinct ground state isomers of I and II are responsible for the emission. The comparison of the emission wavelength in hydrocarbon solvent strongly supports that trans enol form predominates over the cis enol form for Schiff base (SB 4). With increasing base concentration of the solutions of hydroxy substituted Schiff bases (SB 4 and SB 5), two isobestic points are found which confirm the equilibrium among the trans enol form, anion and the cis enol form. The fluorescence of (SB 4) quenched markedly with the gradual addition of Cu 2+ but the fluorescence properties of (SB 5) was influenced by other metal ions. Therefore Schiff base (SB 5) can be used as a new fluorescence sensor to detect the quantity of Cu 2+ ion in any sample solution depending on the relative intensity change. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the Schiff base derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.

Structure and Spectroscopy of Oxyluciferin, the Light Emitter of the Firefly Bioluminescence

The crystal structures of the pure, unsubstituted firefly emitter oxyluciferin (OxyLH(2)) and its 5-methyl analogue (MOxyLH(2)) were determined for the first time to reveal that both molecules exist as pure trans-enol forms, enol-OxyLH(2) and enol-MOxyLH(2), assembled as head-to-tail hydrogen-bonded dimers. Their steady-state absorption and emission spectra (in solution and in the solid state) and nanosecond time-resolved fluorescence decays (in solution) were recorded and assigned to the six possible trans chemical forms of the emitter and its anions. The spectra of the pure emitter were compared to its bioluminescence and fluorescence spectra when it is complexed with luciferase from the Japanese firefly (Luciola cruciata) and interpreted in terms of the intermolecular interactions based on the structure of the emitter in the luciferase active site. The wavelengths of the emission spectral maxima of the six chemical forms of OxyLH(2) are generally in good agreement with the theoretically predicted energies of the S(0)-S(1) transitions and range from the blue to the red regions, while the respective absorption maxima range from the ultraviolet to the green regions. It was confirmed that both neutral forms, phenol-enol and phenol-keto, are blue emitters, whereas the phenolate-enol form is yellow-green emitter. The phenol-enolate form, which probably only exists as a mixture with other species, and the phenolate-enolate dianion are yellow or orange emitters with close position of their emission bands. The phenolate-keto form always emits in the red region. The concentration ratio of the different chemical species in solutions of OxyLH(2) is determined by several factors which affect the intricate triple chemical equilibrium, most notably the pH, solvent polarity, hydrogen bonding, presence of additional ions, and pi-pi stacking. Due to the stabilization of the enol group of the 4-hydroxythiazole ring by hydrogen bonding to the proximate adenosine monophosphate, which according to the density functional calculations is similar to that due to the dimerization of two enol molecules observed in the crystal, the phenolate ion of the enol tautomer, which is the predominant ground-state species within the narrow pH interval 7.44-8.14 in buffered aqueous solutions, is the most probable emitter of the yellow-green bioluminescence common for most wild-type luciferases. This conclusion is supported by the bioluminescence/fluorescence spectra and the NMR data, as well the crystal structures of OxyLH(2) and MOxyLH(2), where the conjugated acid (phenol) of the emitter exists as pure enol tautomer.

Thermochromism of N-Triphenylmethylsalicylideneimine Activity of the Trans-Open Enol Conformer at Low Temperature

N-triphenylmethylsalicylideneimine (MS1) represents a reversible thermal and photo-coloration system. The initial stage of the system studied was defined by means of X-ray structure analysis. Crystal structures were done at 295 and 230 K, taking into account the results of differential scanning calorimetry; basing on the DSC measurements, a pseudo-first order phase transition at 241 K was identified. According to the analysis of the luminescence spectra of MS1, low temperature favors a benzenoid-type trans-enol open form of the molecule under study. However, after UV excitation, the photochemical reaction (proton transfer and/or photoisomerization) generates the quinoid form. So, thermochromy has been interpreted as due to geometry changes of a chelate ring, and creating an open enol molecular form rather than a shift of the tautomeric equilibrium benzenoid-to-quinoid form. Below 240 K, the light-induced generation of unstable photochromic species is manifested as the sample color change (colorless-to-orange). That newly created species is of a trans-keto-type isomer of MS1 molecule with breaking hydrogen bond, and the open trans-enol form existing in the ground state is a source of that orange species.

Excited state proton transfer reaction of two new intramolecularly hydrogen bonded Schiff bases at room temperature and 77K

Two new orthohydroxy Schiff bases, 7-phenylsalicylidene benzylamine (PSBA) and 7-ethylsalicylideneaniline (ESA) have been synthesized. The excited state intramolecular proton transfer (ESIPT) and the structure of PSBA and ESA in its crystalline form and in the solvents n-hexane, n-heptane and 1,4-dioxane have been investigated by means of absorption, emission and nanosecond spectroscopy at room temperature and 77K. One ground state species has been detected both in neutral and basic solutions of both PSBA and ESA: the cis-enol form with an intramolecular hydrogen bond. The ESIPT and formation of keto tautomer are evidenced by a large Stokes shifted emission (∼12 000 cm −1) at room temperature only in the case of ESA. On the other hand the keto tautomer is the predominant species at 77K in a solid matrix and as a solid sample at room temperature both in the case of ESA and PSBA. In the case of both ESA and PSBA the more intense, higher energy emission is due to the species which has not undergone ESIPT and attributed mainly due to cis-enol form. The trans-enol form is also observed by changing the excitation wavelength. Both the compounds are found to undergo a structural change to a zwitterionic and intermolecular hydrogen bonded form in the presence of a strong base like triethylamine. From the nanosecond measurements and quantum yield of fluorescence we have estimated the decay rates of proton transfer reaction in the case of PSBA. Our theoretical calculation at the AM1 level of approximation shows that the ground singlet state has a rather large activation barrier both in the case of PSBA and ESA. The barrier height is much lower on the corresponding excited singlet surface only in the case of ESA. The process is predicted to be endothermic in the ground state and exothermic in the excited singlet state.

Ground-state tautomerism and rotational isomerization in 4,5-dimethyl-2-(2-hydroxyphenyl)imidazole in the gas phase and in polar solvents: a theoretical study of the aromaticity, intramolecular hydrogen-bond strength and differential solute–solvent interactions

Ground-state tautomerism and rotational isomerization in 4,5-dimethyl-2-(2-hydroxyphenyl)imidazole in the gas phase and in solution have been investigated by means of quantum mechanical calculations, NMR and steady-state fluorescence spectroscopy. In the gas phase, the cis-enol form is the most stable species, followed by the trans-enol and the keto forms. Several theoretical approaches were employed to characterize the electronic structure of the different isomers in the gas phase at the RB3LYP/6-31 + G* level of theory. The observed behavior could be rationalized according to the different weights of the resonance and the hydrogen-bond energies in the overall energy of each isomer. The hydroxyphenyl rings of the trans-enol and the cis-enol forms exhibit a nearly aromatic structure, whereas the electronic structure of the keto-form shows a higher degree of localization. In turn, it was found that the intramolecular hydrogen bond shows similar strength in the cis-enol and keto-forms, whereas this interaction is very weak in the trans-enol form. Polar solute–solvent non-specific interactions were modeled through the Onsager, SCI-PCM and COSMO methods based on ab initio and semiempirical Hamiltonians. In solution, the keto-form is stabilized by its much greater solute–solvent electrostatic interaction through its dipolar term and the aromatization of its phenyl ring. The trans-enol form is mainly stabilized by electrostatic interactions through higher multipolar terms than dipolar and specific solute–solvent interactions through the lone pair of the imidazole N3.

Ground-State Tautomerism and Excited-State Proton-Transfer Processes in 4,5-Dimethyl-2-(2‘-hydroxyphenyl)imidazole in Solution: Fluorescence Spectroscopy and Quantum Mechanical Calculations

Ground-state tautomerism and excited-state proton transfer of 4,5-dimethyl-2-(2′-hydroxyphenyl)imidazole (DMHI) was investigated by means of UV - vis absorption spectroscopy, by steady-state and time-resolved fluorescence spectroscopy, and by quantum mechanical calculations. The behavior of DMHI in ethanol and water was studied in neutral, acidic, and basic conditions. Three ground-state species were detected in neutral solutions of DMHI: the cis-enol form with an intramolecular hydrogen bond, the trans-enol form that is hydrogen bonded to the solvent, and the keto tautomer. The relative proportions of these species depend strongly on the solvent: the keto tautomer was not detected in ethanol, whereas this tautomer is the predominant species in neutral water, where negligible amounts of the cis-enol form exist. These experimental findings could be reproduced by quantum mechanical calculations up to the B3LYP/6-311+G** level of theory combined with semiempirical calculations of solvation free energies, which allowed the energy diagram in the gas phase and in solution to be known. A combination of the supermolecular approach and the SMx methodology was necessary to account for the differential solvation of the different isomers in neutral aqueous solution. In the excited state, the cis-enol form always undergoes the fast excited-state intramolecular proton transfer to yield the keto tautomer. Upon photoexcitation of the trans-enol form and the keto tautomer, no excited-state reaction takes place. In acidic media, the photoexcited cation suffers deprotonation of the hydroxyl group, yielding the excited keto tautomer. © 1998 American Chemical Society.

Reaction Enthalpy and Reaction Volume Changes upon Photoenolization: 2-Methylbenzophenone

The transient grating (TG) signal after photoexcitation of 2-methylbenzophenone is fully analyzed in detail in terms of the thermal contribution by the nonradiative transition from the excited states, enthalpy change (ΔH) by the photoisomerization from the cis- and trans-enol forms to the keto form, molecular volume change (ΔV) by the reaction, and population grating contribution due to the absorption change. From a quantitative measurement of the TG intensity combined with that of the photoacoustic intensity, ΔH and ΔV are determined simultaneously without changing any external properties such as solvents, pressure or temperature. The intermolecular hydrogen bonding species (trans-enol) diffuses more slowly than the keto form, which provides clear evidence of the enol formation. The molecular volume of the enol form is smaller than that of the keto form, and the contraction is explained by the intermolecular hydrogen bonding. The obtained physical quantities, ΔH, ΔV, and diffusion constant of the keto and enol forms, are discussed relating to the molecular structure.

Photochemical behavior of poly(ethylacryloylacetate) and poly(acryloylacetone) films

Films of poly(ethylacryloylacetate) (PEAA) and poly(acryloylacetone) (PAA) were subjected to UV irradiation (λ = 254 nm) at room temperature. The photoinduced structure transfer from cis-enol onto a diketo forms has been investigated. The structure transfer caused by UV light was found to be slower than for the corresponding process in solution. The spectral investigations (UV, IR) showed reversible process of photoketonization. The results were analyzed in terms of the model for the participation of the trans-enol form in the process of the ketonization. Based on the results obtained, some general conclusions were made about the organization of the units in the polymer chain. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3683–3688, 1997

Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2‘-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State

Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2‘-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV−vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited keto form, but this process is faster in water than in ethanol, in which fluorescence by the cation is also observed.

Investigation of triplet states of 2-(2-hydroxyphenyl)benzothiazole and 2-(2-hydroxyphenyl)benzoxazole by transient absorption spectroscopy and ab initio calculations

Properties of the 3 (π,π * ) states of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and 2-(2'-hydroxyphenyl)-benzoxazole (HBO) were investigated by means of transient absorption spectroscopy and ab initio calculations. When the volume fraction of ethanol in a mixed solvent of toluene and ethanol is small, the intramolecularly proton-transferred cis-keto form of HBT predominates over the intermolecularly hydrogen-bonded trans-enol form in the triplet state. However, when the volume fraction of ethanol becomes large, the relative composition is reversed. The triplet species of HBO at 10 μs after photoexcitation contain a cis-enol form in both nonpolar and alcoholic solvents

Investigation of excited-state intramolecular proton transfer in the lowest triplet states of 2-(2'-hydroxyphenyl)benzoxazole by time-resolved electron paramagnetic resonance

Intramolecular proton-transfer process in the lowest triplet (T 1 ) state of 2-(2'-hydroxyphenyl)benzoxazole (HBO) has been investigated by means of time-resolved EPR method at 77 K. In nonpolar solvents, three different species of the T 1 states of HBO are observed. They are attributed to the cis-enol, cis-keto, and trans-enol forms. A back proton-transfer process in the T 1 states from the keto to the enol form is observed as the rise of the signal in nonpolar solvents. The proton-transfer rate constants are estimated from the time dependence of the signal. In ethanol, signals due to both the keto and enol forms are observed, but the proton transfer process in the T 1 states does not appear clearly

Spectroscopic study of hydrogen bonding in the enol form of β-diketones—I. Vibrational assignment and strength of the bond

The i.r. and Raman spectra of the trans-enol form of some β-diketones and their deuterated analogues in α position are examined in the 4000-50cm −1 range and an approximate assignment is given for the fundamental vibrations. The ν as (OH) and ν(CO) values suggest that the intramolecular H-bond strength increases in the order: hexafluoroacetylacetone < trifluoroacetylacetone < acetylacetone < α - Cl - acetylacetone < benzoylacetone < dibenzoylmethane < tetraacetylethane. The observed relative Raman intensities of the ν(CC) and ν(CC) bands also support this feature. The spectroscopic results show a great change in the internal modes of the β-diketones upon deuteration; this behaviour indicates a greater localization of the π-electron system of the chelate ring and a weakening of the H-bond on deuteration. The ν s (OH) vibration seems to be involved in one of the in-plane ring deformational modes.

Stability of certain ?-dicarbonyl enol forms with intramolecular hydrogen bonding

1. The energy of intermolecular hydrogen bonding between the trans-enol forms of β- ketoaldehydes and proton-acceptor compounds has been determined. 2. The energy required for breakdown of the intramolecular hydrogen bonding in cisenols with passage to a hydrogen bond free trans-enol form has also been determined.

The influence of structural factors and the solvent on the nature of the enolization of the formyl group

1. O,O-Diethylformylchloro-, O,O-diethylformylbromomethylphosphonates, ethoxymethylenephosphoneacetone, and formylphosphoneacetone were synthesized. 2. The enolization of a number of substituted O,O-diethylformylmethylphosphonates in solutions was studied by the PMR method, and a comparison was made with their carboxylic analogs. 3. The position of the cis-aldo-trans-enol equilibrium in solutions is determined not so much by steric as by inductive and mesomeric influences of the substituents in the molecule. The substituents, which lower the basicity of the C=O and P=O groups, reduce the content of cis-enol forms and increase the content of trans-enol forms in the solutions. 4. The formation of intermolecular hydrogen bonds stabilizes the trans-enol forms, which is especially characteristic of compounds containing a diethoxyphosphinyl group. 5. The substituents are arranged in the following series with respect to their ability to form an intramolecular hydrogen bond with the oxymethylene group: CH3CO>P(O)(OC2H5)2>COOC2H5.

Enolization of phosphorylated derivatives of formylacetic acid

1. A quantitative evaluation of the position of equilibrium of diethoxyphosphinylformylacetonitrile, diethoxyphosphinylformylethyl acetate, and formylcyanoacetic ester in various solvents was performed by the method of PMR spectroscopy. 2. In not one case was the presence of the aldo-form detected in the tautomeric mixture. For diethoxy-phosphinylformylacetonitrile and formylcyanoacetic ester, a tautomeric equilibrium of the cis- and trans-enol forms is observed; the content of the latter increases with increasing polarity of the solvent. For diethoxy-phosphinylformylethylacetate, two cis-enol forms exist at equilibrium, with predominance of the structure with an intramolecular hydrogen bond with the P=O group.