Excited-state Dissociation Constants Research Articles

Emission properties of fluorescein in strongly acidic solutions

Experimental determination of the fluorescence quantum yield and spectral profile for a variety of protolytic forms of fluorescein is challenged by their simultaneous co-existence in aqueous solutions as well as the excited-state proton transfer (ESPT). Different methods of fluorescence spectroscopy (both steady-state and time-resolved) were applied to investigate aqueous solutions of fluorescein containing sulfuric and hydrochloric acids (pH < 3.5). The obtained emission spectra were decomposed by Alentsev-Fok method to separate spectral contributions for cationic and neutral quinoid forms. The resolved individual fluorescence spectra were used for the estimation of the excited-state dissociation constant of the cation-quinoid equilibrium (pka*=-0.5) as well as fluorescence quantum yield for the quinoid form (0.42). An almost total fluorescence quenching of the cationic fluorescein was observed in the solutions with high concentration of the hydrochloric acid whereas quantum yield for this form remains considerable (0.92) in the solutions containing sulfuric acid at similar pH. The obtained results bring new insights to the understanding of the role of the ESPT and counter-ion of the acid in the relaxation of the excited states of fluorescein in acidic media.

Effect of bulky aryl group on the optical properties of Cypridina oxyluciferin analogues

Because of high bioluminescence efficiency and independent cofactors, Cypridina bioluminescence has been already developed as an indispensable analytical tool. However, Cypridina only emits blue light which may be absorbed by some biomaterials. To address this problem, we design three series of oxyluciferin derivatives, replacing moiety at three positions, 3-indolyl group at C5 (R1), 3-(1-guanidino) propyl group at C3 (R2) and an (S)-2-butyl group at the acyl part (R3), with phenyl, naphthyl and phenanthryl groups, respectively. The results found that Cypridina oxyluciferin analogues can emit violet, blue, green and yellow light. The maximum wavelength was observed at 562 nm, which achieved only with extending the conjugation effect by phenanthryl group. Moreover, replacing with naphthyl group at the 5-position have more influential to emission wavelength than phenyl and phenanthryl groups. The calculation of the excited state dissociation constant indicates that 2-acetamido-5-naphthyl-3-naphthylpyrazine (B2) can be potential candidate for fluorescent pH probe.

Solvation and protonation of coumarin 102 in aqueous media: a fluorescence spectroscopic and theoretical study.

The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.

Methodological keys for accurate simulations

Photoacids have a stronger propensity to give protons in their excited state than in their ground state which is a key feature for developing new material properties. Experimentally the determination of the excited state dissociation constants (pKa*) remains challenging as the lifetime of the photoacid, in its excited state, is too small. The present article establishes several protocols using the latest developments of the PCM-TD-DFT formalism e.g. both the corrected linear response (cLR) and state specific (SS) approaches. Equilibrium (eq) and non-equilibrium (neq) limits of the implicit solvent have been compared and we highlight that the SS-TD-DFT formalism provides figures in good agreement with experimental data once the eq limit is combined with the Born-Haber cycle or when the neq is used with the Föster cycle using absorption rather than emission transition energies.

Spectroscopy and Photophysics of Indoline and Indoline-2-Carboxylic Acid

This paper presents a detailed description of the fluorescence and photophysical behavior of indoline and indoline-2-carboxylic acid (I2CA). Indoline is an analogue of indole lacking the C2C3 double bond, but unlike indole, indoline possesses well-separated 1La and 1Lb states. I2CA, which displays similar fluorescence properties, is an amino acid and can be regarded as a fluorescent analogue of proline. In view of the potential for indoline and I2CA as fluorescent probes in peptides and proteins, we have undertaken an in-depth study of the spectroscopic and photophysical properties of these molecules. A companion paper (Slaughter, B.D.; Allen, M. W.; Lushington, G. H.; Johnson, C. K. J. Phys. Chem. A 2003, 107, 5670) presents a theoretical treatment of the spectroscopic transitions of indoline and I2CA. The pH-dependent absorption and fluorescence characteristics are investigated for both indoline and I2CA. Ground- and excited-state dissociation constants are determined spectroscopically. Quantum yields a...

Micelle effect on ground and excited state proton transfer reactions involving the 4-methyl-7-hydroxyflavylium cation

In aqueous solution, the 4-methyl-7-hydroxyflavylium ion (AH+) undergoes a ground state dissociation reaction to form the quinoidal base A and, when excited, an efficient excited state proton transfer (ESPT) leading to *A. The effect of micelles on the ground and excited state proton transfer reactions has been investigated. As far as the acid–base properties of the ground state system are concerned, negatively charged SDS micelles stabilize the acidic form AH+, while the positively charged CTAB and, to a lesser extent, the neutral Triton X 100 micelles destabilize AH+. Moreover, the neutral form A exhibits a strong decrease in molar absorption coefficient that cannot be explained by electrostatic reasons only, suggesting that specific interactions of A with the hydrophobic part of the surfactants may play some role. Even the excited state dissociation constant (Ka*) is affected by the micelles, and the pKa* is positively or negatively shifted with respect to that in water by negatively or positively charged micelles, respectively. It was also observed that the excited states can be intrinsically affected by the micelles. In particular, the excited AH+ is not only stabilized by the negative SDS micelle, but also exhibits a luminescence lifetime longer than that in water, most probably because of the electrostatic interaction exerted on it by the negative charge of the micelles.

Kinetics of Excited-State Proton Transfer of Doubly Protonated 2-Aminoacridine

2-Aminoacridine has been used to extend the quantitation of excited-state proton-transfer kinetics in concentrated acid media to ≃13 M sulfuric acid. The reasons for the large discrepancy between the excited-state dissociation constant determined from kinetic measurements and that calculated from a thermodynamic cycle are discussed.

Correlation of ground and excited state dissociation constants oftrans-para- and ortho-substituted cinnamic acids

The ground and excited state dissociation constants oftrans- para- and ortho-substituted cinnamic acids have been determined in 50% (v/v) dioxan-water mixture at 30°C using the Forster cycle. The measured dissociation constants are analysed in the light of single and dual substituent parameter (DSP) equations. Excited state dissociation constants ofp-substituted cinnamic acids correlate well with the exalted substituent constants. The DSP method of analysis shows that resonance effect is predominant relative to inductive effect in the excited state than in the ground state for the para-substituted acids. The single parameter equation gives poor correlation for the ortho-substituted acids. However, the DSP analysis shows fairly good correlation. The inductive effect is predominant relative to the resonance effect in the excited state than in the ground state

Internal hydrogen bonding in benzo[a]pyrene diol and diol epoxide metabolites

Fluorescence, excitation, and emission spectra of some benzo[a]pyrene diols and diol epoxides in aqueous and non-aqueous media have been studied and ground- and excited-state dissociation constants have been determined. Spectral shifts due to changes in salvation, Stokes shifts, and pKa and pKa* values are employed to show that the diols and diol epoxides possess an intramolecular hydrogen bond between ortho hydroxy groups when in non-aqueous media. In aqueous media no internal hydrogen bonding occurs either between ortho hydroxy groups or hydroxy and epoxide groups. The latter point is significant with regard to the carcinogenic reactivity of anti- and syn-7,8-dihydroxy 9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene.

Solvent effects on electronic absorption and fluorescence spectra of salicylaldehyde 2-pyridylhydrazone and indolaldehyde 2-salicylhydrazone: Ground and excited state dissociation constants

The absorption and fluorescence spectra of salicylaldehyde 2-pyridyl-hydrazone (SAPH) and indolaldehyde 2-salicylhydrazone (IASH) were studied in various solvents. The relative fluorescence intensity and efficiency ratio are reported. Highly polar solvents cause red shifts in SAPH and blue shifts in IASH which indicate that the long-wavelength bands of the neutral molecule arise from π → π* and n → π* transitions respectively. The ground state dissociation constants p K a of SAPH and IASH were measured spectrophotometrically and spectrofluorometrically in 40 vol.% ethanol—water solutions. The corresponding p K a reveal that the compounds have an increased acidity in the excited state.

Proton transfer in the lowest excited singlet state of 4-methoxyacridine: extraction of kinetic parameters from an incomplete fluorimetric titration curve

The fluorimetric pH titration curve of 4-methoxyacridine cannot be completed because of quenching of the neutral molecule above pH 14. Normalization of the points in the curve with respect to some point, corresponding to φ φ o < 1, can be employed to determine the correction factor for conversion to normalization, with respect to the unattainable fluorescence intensity of the neutral molecule, in the absence of excited-state protonexchange or quenching. The excited-state dissociation constant (pK a *) for protonated 4-methoxyacridine, calculated from the rate constants for excited-state proton exchange determined by this approach, is in excellent agreement with the value of p K a * calculated from a Förster cycle.

Electronic absorption spectra and protolytic equilibria of doxorubicin: direct spectrophotometric determination of microconstants.

The ground- and excited-state dissociation constants and the electronic absorption and fluorescence spectra of doxorubicin were investigated by spectrophotometry. A general method for the direct calculation of individual microscopic dissociation constants was derived using the spectrophotometric data obtained. It was concluded that the protonated amino sugar group is slightly more acidic than the phenolic group. The spectrophotometric data were analyzed, and the macro- and microconstants for the various equilibria was reported.

An electronic spectral study of the influence of thermal and electronic processes on the determination of the excited singlet-state dissociation constants of 1- and 2-naphthoic acid

The electronic absorption and fluorescence spectra of 1-naphthoic acid and 2-naphthoic acid were studied as functions of prototropic form, solvent and temperature. The dissociation constants for the prototropic equilibria between the cations and neutral molecules were determined titrimetrically for ground and excited singlet states with respect to the revised Hammett acidity scale of Jorgenson and Hartter. Calculation of excited-state dissociation constants from the Förster cycle is critically evaluated with respect to thermal relaxation processes in electronically excited and ground states and their effects on the ordering of electronic states. The interpretation of excited-state prototropism in the 2-isomer is straightforward. However, in the 1-isomer there is strong indication of inversion of states which introduces complications into the interpretation of p K * a values obtained from fluorimetric titrations as well as calculations from shifts of electronic spectra.

Fluorescence studies on some hydroxypyridines including compounds of the vitamin B6 group.

1. The variations with pH (from 36n-sulphuric acid to 10n-sodium hydroxide) of the excitation and fluorescence wavelengths and fluorescence intensity of 2-, 3- and 4-hydroxypyridine and their O- and N-methyl derivatives were investigated. 2. 4-Hydroxy- and 4-methoxy-pyridine were non-fluorescent at all pH values. 3. The cations and dipolar ions of the 3-hydroxypyridine derivatives and the anion of 3-hydroxypyridine were fluorescent, but the neutral forms were not. 4. All the forms of the 2-hydroxypyridine derivatives were fluorescent. 5. Pyridoxol, pyridoxal and its 5-phosphate, pyridoxamine and pyridoxic acid and its lactone were studied similarly. All these compounds, except pyridoxal 5-phosphate, were more fluorescent than 3-hydroxypyridine. 6. The most fluorescent forms of these compounds are the anions, except for pyridoxol, where the dipolar ion was the most fluorescent form. The least fluorescent forms are the neutral molecules. The dipolar ions were appreciably fluorescent in all cases. 7. The most fluorescent form examined was the dianion of pyridoxic acid lactone. 8. The cations were all fluorescent except the cations of 2- and 3-methoxypyridine. All the cations showed excited-state ionization. The excited pK(a) values of these cations were determined and the results are discussed with reference to Weller's (1952) equation relating ground- and excited-state dissociation constants. 9. The pK(a) values for all ionizations undergone by the compounds examined were determined from fluorescence data. 10. Stokes shifts for the various ionic and neutral species of the compounds examined were calculated and are discussed.