Electronic absorption and fluorescence of cinchophen, cinchoninic acid and their methyl esters; biprotonic phototautomerism of the singly-protonated species

The pH and Hammett acidity dependences of the absorption and fluorescence spectra of cinchoninic acid (quinoline-4-carboxylic acid), cinchophen (2-phenylquinoline-4-carboxylic acid) and their methyl esters, were studied. The predominant uncharged ground-state species derived from the free acids are zwitterions. Prototropic equilibria are too slow to compete with fluorescence for deactivation of the excited state at hydrogen ion concentrations represented by the pH scale. However, fluorescence shifts accompanying protonation indicate that the carboxyl group is more basic than the ring nitrogen atom in the excited state. In the Hammett acidity range the singly-charged cations of all the compounds studied undergo phototautomerism in the lowest excited singlet state. The rate of this process is acidity dependent. In very concentrated sulphuric acid solutions doubly-charged cations are formed in the excited state but not in the ground state. The intense emissions of these compounds in moderately concentrated sulphuric acid may be suitable for quantitative analysis if great care is taken to control solution acidity.     

Intramolecular Proton Transfer in 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine: Evidence for Tautomer in the Ground State

The intramolecular proton transfer in a newly synthesized molecule, 2‐(2′‐hydroxyphenyl)oxazolo[4,5‐b]pyridine (HPOP) is studied using UV‐visible absorption, fluorescence emission, fluorescence excitation and time‐resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis‐ and trans‐enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited‐state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis‐enol‐trans‐enol‐keto equilibrium and the nonradiative deactivation from the excited state are also investigated.     

Photodissociation dynamics of hydroxybenzoic acids

Aromatic amino acids have large UV absorption cross-sections and low fluorescence quantum yields. Ultrafast internal conversion, which transforms electronic excitation energy to vibrational energy, was assumed to account for the photostability of amino acids. Recent theoretical and experimental investigations suggested that low fluorescence quantum yields of phenol (chromophore of tyrosine) are due to the dissociation from a repulsive excited state. Radicals generated from dissociation may undergo undesired reactions. It contradicts the observed photostability of amino acids. In this work, we explored the photodissociation dynamics of the tyrosine chromophores, 2-, 3- and 4-hydroxybenzoic acid in a molecular beam at 193 nm using multimass ion imaging techniques. We demonstrated that dissociation from the excited state is effectively quenched for the conformers of hydroxybenzoic acids with intramolecular hydrogen bonding. Ab initio calculations show that the excited state and the ground state potential energy surfaces change significantly for the conformers with intramolecular hydrogen bonding. It shows the importance of intramolecular hydrogen bond in the excited state dynamics and provides an alternative molecular mechanism for the photostability of aromatic amino acids upon irradiation of ultraviolet photons.     

UV‐Dissipation Mechanisms in the Eumelanin Building Block DHICA

The UV‐dissipative mechanisms of the eumelanin building block 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA) and the 4,7‐dideutero derivative (DHICA‐d₂) in buffered H₂O or D₂O have been characterized by using ultrafast time‐resolved fluorescence spectroscopy. Excitation of the carboxylate anion form, the dominating state at neutral pH, leads to dual fluorescence. The band peaking at λ=378 nm is caused by emission from the excited initial geometry. The second band around λ=450 nm is owed to a complex formed between the mono‐anion and specific buffer components. In the absence of complex formation, the mono‐anion solely decays non‐radiatively or by emission with a lifetime of about 2.1 ns. Excitation of the neutral carboxylic acid state, which dominates at acidic pH, leads to a weak emission around λ=427 nm with a short lifetime of 240 ps. This emission originates from the zwitterionic state, formed upon excitation of the neutral state by sub‐ps excited‐state intramolecular proton transfer (ESIPT) between the carboxylic acid group and the indole nitrogen. Future studies will unravel whether this also occurs in larger building blocks and ESIPT is a built‐in photoprotective mechanism in epidermal eumelanin.     

Inter- and intramolecular excited state proton transfer in 2-hydroxy-9H-carzole-1-carboxylic acid

Absorption, fluorescence and fluorescence excitation spectroscopy and single photon counting time dependence spectrofluorimetry have been used to study the inter- and intramolecular excited state proton transfer (ESIPT) reactions in 2-hydroxy-9H-carbazole-1-carboxylic acid (2-HCA). Except in cyclohexane and water (pH 5) dual fluorescence is observed in rest of the solvents used. Normal Stokes shifted band seems to originate from 2-HCA-1-c and tautomer emission band from the tautomer formed by ESIPT in 2-HCA-1-c followed by structural reorganization. Both these emission band systems originate from the same ground state species. AM1 and CNDO/S-CI calculations have been carried out to establish the identity of the species. Different prototropic equilibria have been determined and discussed.     

Electronic spectroscopy and photochromic mechanism of bis-Schiff bases, N, N′-bis(2-hydroxy-l-naphthylidene)-l, 4-phenyldiamine

The steady state and transient state absorption spectra and fluorescence spectra of N, Ń-bis(2-hydroxy-1-naphthylidene)-1, 4-phenyldiamine (BNP) in cyclohexane and acetonitrile were determined. The photochromic mechanism was discussed. In nonpolar solvents, BNP exists mainly in the enol form and has the absorption maximum in the UV region. In polar solvents, however, both the enol and proton transfer tautomer are formed, but the former is the main one. Fluorescence emissions result from the excited state of proton transfer product. Project supported by the National Natural Science Foundation of China and the Foundation of Chinese Academy of Sciences.     

REINVESTIGATION OF THE PHOTOPHYSICS OF 2-(2'-HYDROXY-4'-DIETHYLAMINOPHENYL)BENZOTHIAZOLE

Abstract— The photophysical properties of 2-(2'-hydroxy-4'-diethylaminophenyl) benzothiazole (HABT) have been investigated by steady-state and time-resolved spectroscopies. In n-heptane HABT exhibits both normal and tautomer emissions with ∼equal fluorescence intensity at room temperature, in contrast to a previous report in which negligible tautomer emission was observed. The normal/tautomer (400/500 nm) ratio of emission intensity increases as the temperature decreases. Two possible excited-state intramolecular proton transfer (ESIPT) mechanisms are proposed, which cannot be resolved at the present stage. One proposed mechanism incorporates state mixing between -OH and -N(C₂H₅)₂ charge transfer states, resulting in a significant energy barrier for ESIPT. An alternative mechanism is also proposed in which fast proton tunneling may take place between enol and keto forms, which are in equilibrium in the excited singlet state.     

Electronic spectroscopy and photochromic mechanism of bis-Schiff bases, N, N''''-bis(2-hydroxy-1-naphthylidene)-1,4-phenyldiamine

The steady state and transient state absorption spectra and fluorescence spectra of N,N'-bis(2-hy-droxy-1-napbthylidcne)-1,4-phenyldiamme ( BNP ) in cyclohexane and acetonitrile were determined.The pho-tochromic mechanism was discussed In nonpolar solvents,BNP exists mainly in the enol form and has the absorption maximum in the UV region In polar solvents,however,both the enol and proton transfer tautomer are formed,but the farmer is the main one Fluorescence emissions result from the excited state of proton transfer product.     

EXCITATION WAVELENGTH DEPENDENCE OF PROTOTROPIC DISSOCIATION AND TAUTOMERISM OF SALICYLAMIDE IN THE LOWEST EXCITED SINGLET STATE

Abstract—The fluorescence spectra of salicylamide in cyclohexane, ethanol and at different pH in water were studied. The short and long wavelength fluorescences observed in the organic solvents originate from emissions of a weakly or non-intramolecularly hydrogen bonded conformer and from phototautomerization of a strongly intramolecularly hydrogen bonded conformer, respectively. Evidence for at least 2 conformers in the ground state exists in the excitation wavelength dependence of the ratio of short wavelength to long wavelength emission. In water, prototropic dissociation of the phenolic group of salicylamide in the lowest excited singlet state also shows an excitation wavelength dependence, indicating that the weakly or non-intramolecularly hydrogen bonded conformer in water is predominately responsible for photodissociation.     

Photophysics of 6-(2′-hydroxy-4′-methoxyphenyl)-s-triazine photostabilizers

The room temperature photophysical properties of several sulphonated and unsulphonated 6-(2′-hydroxy-4′-methoxyphenyl)-s-triazines were investigated in a range of solvents by means of steady state and picosecond fluorescence spectroscopy. Compounds possessing phenyl or p-tolyl groups in the s-triazinyl ring exhibit only a very weak normal Stokes-shifted fluorescence, arising from the initially excited chromophore. Substitution of phenoxy groups into the s-triazinyl ring results in the appearance of an additional longer-wavelength fluorescence which is assigned to the keto tautomer, formed following excited state intramolecular proton transfer (ESIPT). The rate constant for the (ESIPT) process that occurs in sodium 3-(3′,5′-diphenoxy-2′,4′,6′-triazinyl)-4-hydroxy-2-methoxybenzene sulphonate in water is estimated to be greater than 10¹¹ s⁻¹.     

Solvent‐ and concentration‐sensitive fluorescence of 2‐(3,4,5,6‐tetrafluoro‐2‐hydroxyphenyl)benzoxazole

2‐(3,4,5,6‐Tetrafluoro‐2‐hydroxyphenyl)benzoxazole ( 2 ) emits the long wavelength fluorescence around 500 nm in nonpolar solvent via the intramolecular proton transfer process in the excited state of 2 (enol‐form) and also emits the intermediate wavelength fluorescence around 440 nm in polar solvent, which is assumed to originate from the excited state of 2 (anion). The ease of formation of 2 (anion), compared to 2‐(2‐hydroxyphenyl)benzoxazole ( 1 ), is explained by the strongly inductive fluorine atoms. In a solvent with the intermediate polarity, 2 emits both fluorescences and their relative intensity is dependent on the concentration of 2 , which is supposed to be caused by the high sensitivity of the intermediate wavelength emission to the concentration quenching.     

KINETICS OF PHOTODIMERIZATION OF OROTIC ACID IN AQUEOUS MEDIUM

Abstract— Ultraviolet irradiation of orotic acid in aqueous medium leads to the formation of dimers with a cyclobutane ring between the 5,6 bonds of two monomers. These photodimers in turn undergo photodissociation, with the result that an equilibrium is established between photodimerization and photodissociation. The equilibrium point is independent of dose rate, but is appreciably dependent on the irradiation wavelength, pH of the medium (i.e. whether the monomer is in the neutral or anionic form), and initial monomer and oxygen concentrations; the effect of oxygen apparently being to deactivate excited monomers. The kinetics of photodimerization and photodissociation have been examined at different wavelengths and with varying oxygen concentrations at different initial monomer concentrations. The results have been compared with a theoretical model in which it is assumed that a dimer is formed between an excited monomer and second molecule in the ground state, and that oxygen can also react with, and deactivate, an excited monomer. The agreement between the proposed model and the experimental data is reasonably good, and has made possible an estimate of the lifetime of the excited state of orotic acid in aqueous medium, which is about 10^-6 sec.     

Carbazole as an excited state proton transfer fluorescent probe for lipid bilayers in alkaline medium

In our effort to look for novel excited state proton transfer (ESPT) fluorescent probes in alkaline pH range, we have examined carbazole as a possible candidate because of its high extinction coefficient, high quantum yield and a larger difference in ionization constant between the ground and excited state (pKa - pKa*). The photodissociation of carbazole was studied in liposome membrane by steady state fluorescence measurements at alkaline pH ranges. The neutral form and the anionic form of carbazole emit at 362 and 417 nm, respectively. This large shift in emission makes it convenient to monitor the physical properties of liposomes. The neutral form fluorescence intensity of carbazole is sensitive to phase changes in the membrane and also shows a maximum at phase transition temperature. This variation of intensity can be explained in terms of redistribution of probe between the surface and interior of the liposomes. Cholesterol induced phase changes of liposomes were also sensed by the ESPT of carbazole.     

Absorption and emission spectroscopic characterization of lumichrome in aqueous solutions

The spectroscopic behavior of lumichrome (7,8-dimethyl-alloxazine, LC) in aqueous solutions in a pH range from -1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground-state LC changes with rising pH from the cationic form (LCH(2)(+)) to the neutral form (LCH) with a mid-point pH of pK(c) ≈ -0.53, and to the anionic form (LC(-)) with a mid-point pH of pK(a) ≈ 12.5. Above pH 7 a partial ground-state tautomerization of LCH to 7,8-dimethyl-isoalloxazine (IAH) occurs by N1-N10 intra-molecular proton transfer. For pH > pK(a) ≈ 12.5 LCH and IAH change to the anionic forms LC(-) and IA(-), and above pH 14 LC(-) tautomerizes completely to IA(-). In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH(2)(+)) at low pH by proton transfer from H(3)O(+) to LCH*. No photoinduced excited-state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC(-) and IA(-) are lower than those of LCH and IAH. The fluorescence of LCH(2)(+) is strongly quenched likely by intra-molecular diabatic charge transfer and excited-state relaxation by potential surface touching with the ground state.     

Spectroscopic Properties of 4‐Pyridoxic Acid as a Function of pH and Solvent

The photophysical properties and acid/base equilibria of 4‐pyridoxic acid (=3‐hydroxy‐5‐(hydroxymethyl)‐2‐methylpyridine‐4‐carboxylic acid), the final product of the catabolism of vitamin B₆, have been studied in aqueous solutions. The ground state of 4‐pyridoxic acid exhibits the different protonated forms A – D in the range of H₀=?6 to pH 11.5. HMQC‐ and HMBC‐NMR Studies allowed the pH‐dependent assignment of the different C‐atoms, and the evaluation of the deprotonation sequence. The 3‐OH group in the ground state has a ‘pK_a’ of H₀=?0.64, which is much lower than that found for other vitamin B₆ related compounds. The pK_a value of the 4‐COOH group is 5.4. Fluorescence studies showed that the same species exist at the lowest excited singlet state, but in different pH ranges. The 3‐OH group is four pH units more acidic in the lowest excited singlet state than in the ground state. Excitation spectra and emission decays in the pH range of 8 to 11.5 indicate that the pyridine N‐atom is more basic in the excited singlet state than in the ground state. The emission spectra are red‐shifted in protic solvents, in agreement with an intramolecular H‐bond between the ionized 3‐OH group and the nonionized 4‐COOH group.     

Ionic photodissociation in arylallyl acetates

We have synthesized several 3-arylallyl acetates 1, 2, 3, 5 and 6, and E-3-(1-naphthyl)-2-propene-1-ol 4 for studying ionic photodissociation. Compounds 1, 2 and 3 underwent an efficient ionic photodissociation in polar solvents like acetonitrile and methanol leading to the formation of rearranged acetate and methyl ether products, as well as undergoing an E-Z isomerization. The arylallyl alcohol 4 and the two arylallyl acetates 5 and 6 did not undergo ionic photodissociation. Quantum yields of product formation, quantum yields of fluorescence, solvent polarity effects and triplet-sensitization studies indicated that a highly polarized excited singlet state is responsible for the ionic photodissociation. Both the singlet- and triplet-excited states are effective in displaying E-Z isomerization in 1, 2, 3 and 4. Compounds 5 and 6 are highly fluorescent, and the fluorescence may be the excited state deactivation pathway along with internal conversion.     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Intramolecular excited-state proton-transfer studies on flavones in different environments

The absorption and fluorescence spectra of some biologically active flavones have been studied as a function of the acidity (pH/H0) of the solution. Dissociation constants have been determined for the ground and first excited singlet states. The results are compared with those obtained from Forster-Weller calculations. The acidity constants obtained by fluorimetric titration method are in complete agreement (in most of the systems) with ground state data indicating a excited state deactivation prior to prototropic equilibration. Compared to umbelliferones, flavones are only weakly fluorescent in alkaline solution. This behaviour is explained by the small energy difference between the singlet excited state and triplet excited state giving rise to more efficient intersystem crossing. Most of the flavones studied here undergo adiabatic photodissociation in the singlet excited state indicating the formation of an exciplex or a phototautomer.     

The fluorescent oxidation products of dihydroxyphenylalanine and its esters

Dihydroxyphenylalanine (DOPA), its methyl ester (DOPAM) and the N-acetylated derivative of the ester (DOPAMNA) are found to undergo rapid oxidation in air-saturated alkaline solution. Some of the products of oxidation exhibit fluorescent emission in the 300-500 nm spectral range and their excitation-emission spectra have been determined in acidic and alkaline aqueous solutions. The spectral distributions and positions of the maxima depend on the pH of the solution. Excitation-emission maxima associated with the protonated phenolic form of the compounds occur at shorter wavelengths than those of the conjugate base. At some pH values the phenolic forms of these molecules are excited and undergo rapid deprotonation in the excited state; as a consequence, emission is observed from the phenolate anion. The fluorescence excitation-emission spectrum of an authentic sample of 3,4-dihydroxycinnamic (caffeic) acid has also been determined and features of the fluorescence spectra of the principal oxidation products are consistent with the presence of 3,4-hydroxycinnamoyl compounds in solutions of oxidized DOPAM and DOPAMNA.     

Photoluminescence of 2-aminofluorene: a relook

The absorption and fluorescence spectra of 2-aminofluorene (2-AF) have been studied as a function of solvent polarity and acid concentration. Using the multiparametric approach of Taft et al., it is clear that 2-AF is a better proton acceptor in the S₀ state and a proton donor in the S₁ state. Excitation of 2-AF to three electronically excited states has shown that fluorescence is always observed from the lowest excited singlet state, but fluorescence quantum yield increases with the increase of λ_exc. The decrease in fluorescence quantum yield with increase in solvent polarity and hydrogen bonding is due to solvent-induced quenching. A correspondence is observed between the decrease and increase in fluorescence intensities of the neutral and monocation, respectively, in the pH range from 6 to 3. Proton-induced fluorescence quenching of neutral 2-AF is noticed in the pH range 3 to 1. pK_a and values were determined for different prototropic equilibria and are discussed.