Hydroxyphenyl Benzimidazole Research Articles

Ratiometric fluorescence sensing ability of 2-(2′-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues towards metal ions

Excited state intramolecular proton transfer (ESIPT) based ratiometric fluorescence sensing of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues viz. 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2′-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c) towards different metal ions was studied. The normal to tautomer fluorescence intensity ratio of HPBI increases as high as 143 and 92 folds in presence of Cu2+ and Fe3+, respectively. The ratio of HPIP-b reaches maximum up to 4.7 fold in presence of Co2+ and that of HPIP-c up to 11.6 fold in presence of Pd2+. The increase in intensity ratio of the normal emission in presence of metal ions is attributed to inhibition of ESIPT process. The crystal structure of 1:2 Ni2+ complex of HPIP-b indicates that the metal ion coordinates through imidazole nitrogen and phenolic oxygen of the fluorophores.

2-(2′-Hydroxyphenyl)benzimidazole and 9,10-Phenanthroimidazole Chelates and Borate Complexes: Solution- and Solid-State Emitters

The synthesis, structural, and optical properties of a series of luminescent N-alkylated 2-(2'-hydroxyphenyl)benzimidazole (HBI) or N-arylated 9,10-phenanthroimidazole (HPI) borate complexes are described. The optical properties of these complexes as well as their corresponding ligands were evaluated in solution and the solid state. Efficient emission in the blue-green region was obtained with quantum yields up to 91% in CH2Cl2 and 27% in the solid state. These emissions originate from excited state intramolecular proton transfer (ESIPT) for the ligands and from a singlet excited state for the borate complexes.

Enhancing Excited State Intramolecular Proton Transfer in 2-(2′-Hydroxyphenyl)benzimidazole and Its Nitrogen-Substituted Analogues by β-Cyclodextrin: The Effect of Nitrogen Substitution

Excited state intramolecular proton transfer (ESIPT) in nitrogen-substituted analogues of 2-(2'-hydroxyphenyl)benzimidazole (HPBI), 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b), and 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-c]pyridine (HPIP-c) have been investigated in a β-cyclodextrin (β-CD) nanocavity and compared with that of HPBI. The stoichiometry and the binding constants of the complexes were determined by tautomer emissions. Both pKa and NMR experiments were employed to determine the orientation of the molecules inside of the β-CD cavity. Huge enhancement in the tautomer emission of HPIP-b and HPIP-c compared to that of HPBI in β-CD suggests that not only is the ESIPT favored inside of the cavity, but also, the environment reduces the nonradiative decay through the formation of an intramolecular charge-transfer (ICT) state. Unlike HPBI, the tautomer emission to normal emission ratio of HPIP-b increases from 0.9 to 2.6, and that of HPIP-c increases from 4.9 to 7.4 in 15 mM β-CD. The effect of dimethylsulfoxide (DMSO) on complexation was also investigated for all three guest molecules. In DMSO, HPBI is present in neutral form, but the nitrogen-substituted analogues are present in both neutral and monoanionic forms. However, in DMSO upon encapsulation by β-CD, all three molecules are present in both neutral and monoanionic forms in the nanocavity. The monoanion is stabilized more inside of the β-CD cavity. The studies revealed that the ESIPT of nitrogen-substituted analogues is more susceptible to the environment than HPBI, and therefore, they are more promising probes.

Spectroscopy of hydroxyphenyl benzazoles in solution and human serum albumin: detecting flexibility, specificity and high affinity of the warfarin drug binding site

The complex photophysics of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole (HBI), and 2-(2′-hydroxyphenyl)benzothiazole (HBT) in different media makes them suitable as fluorescent probes to study the nature of binding sites in macromolecular systems. In this work, we investigate the spectroscopy of the three benzazole derivatives (HBXs) in different solvents and in human serum albumin (HSA) in order to understand the binding mechanism in subdomain IIA of HSA which has the ability to host a large variety of natural and pharmaceutical compounds. The three probes are found to specifically bind close to W214, the sole tryptophan residue in HSA, in a mode similar to that of the binding of the anticoagulant drug warfarin. The current results show that the structural differences between the three HBX molecules did not produce any measurable effects when binding with HSA. In particular, the change in planarity of the molecular backbone, from a perfectly planar and more rigid structure (HBO) to a twisted structure (HBI) and a flexible structure (HBT) has no effect on the mode of binding. Also, the strength of the intramolecular hydrogen bonds in HBXs (HBO > HBT > HBI) is shown not to intervene with the ability of HSA to ionize the ligands via through-space interaction with polar amino acid residues, similar to enzymatic reactions. The results emphasize the nature of HSA as a versatile and indiscriminate receptor, capable of binding a variety of ligands by adapting its binding pockets. In this regard, binding of HBXs, and other structurally similar ligands, in subdomain IIA is best described by the induced-fit model in which considerable flexibility of the binding site is necessary for molecular recognition. The results also point to the high affinity of the warfarin binding pocket (within subdomain IIA) for binding versatile molecular structures including several drugs. This affinity stems from the flexibility of the amino acids forming the binding pocket.

Excited-state intramolecular proton transfer and charge transfer in 2-(2′-hydroxyphenyl)benzimidazole crystals studied by polymorphs-selected electronic spectroscopy

Crystal structures of polymorphs of 2-(2'-hydroxyphenyl)benzimidazole (HPBI), Forms α and β, are analyzed by X-ray crystallography. The fluorescence excitation (FE) and fluorescence spectra of the polymorphs are separately observed at temperatures 77-298 K. It has been found that the electronic spectra of the two crystal forms are significantly different from each other. Photo-excitation of the enol forms in Forms α and β induces the excited-state intramolecular proton transfer (ESIPT) to produce the S(1) state of the keto forms. In the FE spectra of Forms α and β, the S(1) ← S(0) (ππ*) transition of the keto form is observed in the 360-420 nm region in addition to that of the enol form in the 250-420 nm region. In the FE spectrum of Form β a new band peaking at 305 nm is observed, which is assigned to the S(1) ← S(0) transition of a non-planar enol form based on the observation of dual fluorescence in the UV and visible regions and quantum chemical calculation on the transition energy against the twisted angle between the benzimidazole and hydroxyphenyl rings. The fluorescence quantum yield (φ(T)) for the keto form is remarkably dependent on polymorphs at room temperature; φ(T) = 0.53 for Form α is much larger than φ(T) ≤ 0.23 for Form β. At 77 K the φ(T) values for Forms α and β increase to 0.67 and ≤0.57, respectively. The changes in the φ(T) values are associated with the intramolecular charge transfer (ICT) state. The potential barrier height between the S(1)-keto and S(1)-ICT states is significantly lower for Form β than for Form α. At 77 K the S(1)-keto → S(1)-ICT process followed by S(1)-ICT → S(0)-keto internal conversion is significantly suppressed in Form β. We compare difference in the dynamics between Forms α and β in the electronic ground and excited states.

NIR luminescence of 2-(2′-hydroxyphenyl)benzimidazole lanthanide (Ln=Nd and Yb) complexes

Abstract Reactions of 2-(2′-hydroxyphenyl)benzimidazole (HL) with Nd(NO3)3 ⋅ 6H2O and YbCl3 ⋅ 6H2O affords two lanthanide complexes [H2L]2[Nd(HL)2(NO3)4]2 ⋅ 2CH3OH ⋅ 2H2O (1) and [H2L][Yb(HL)2Cl4] ⋅ 2CH3OH (2), respectively. X-ray crystallographic analysis reveals that the two crystallographically distinct Nd3+ ions are ten-coordinated to two O atoms from two ligands and eight O atoms from four bidentate nitrate groups in 1. In the case of 2, the crystallographically distinct Yb3+ ion is hexa-coordinated to four Cl− ions and two O atoms in an octahedral geometry. Structural analysis indicates the importance of H-bondings and π–π stacking in the construction of 1 and 2. Complexes 1 and 2 exhibit essential NIR luminescence of Nd3+ and Yb3+ ions, respectively.

Two-photon-induced intramolecular excited-state proton transfer process and nonlinear optical properties of HBI in ethanol solution

The two-photon-induced excited state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl) benzimidazole (HBI) in ethanol solution has been investigated. We focused on the calculation of TPA coefficient and nonlinear refraction index of HBI, and found that the theoretical calculated results were in good agreement with the experimental ones. By establishing the two-photon-induced ESIPT kinetic model for HBI, the TPA cross section was determined to be 2.09 × 10 4 GM. For their large TPA cross section, HBI is a promising TPA candidate material for their potential application in many fields.

Role of nitrogen substitution in phenyl ring on excited state intramolecular proton transfer and rotamerism of 2-(2′-hydroxyphenyl)benzimidazole: A theoretical study

A comparative study of 2-(2'-hydroxy-3'-pyridyl)benzimidazole (2',3'-HPyBI), 2-(3'-hydroxy-4'-pyridyl)benzimidazole (3',4'-HPyBI), 2-(4'-hydroxy-3'-pyridyl)benzimidazole (4',3'-HPyBI), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (3',2'-HPyBI), and 2-(5'-hydroxy-4'-pyrimidinyl)benzimidazole (5',4'-HPymBI) with 2-(2'-hydroxyphenyl)benzimidazole (HPBI) was performed theoretically to evaluate the effect of nitrogen substitution in the phenolic ring on the photophysics and rotamerism of HPBI. Density functional theory (DFT) and configuration interaction singles (CIS) combined with time-dependent DFT were employed for ground and excited state studies, respectively. Different possible molecular forms were considered for each molecule viz., cis-enol, trans-enol, open-enol, and keto forms. The computational results revealed that cis-enol is the most stable form in the ground state for all the molecules except in 2',3'-HPyBI. In 2',3'-HPyBI, K-2 keto is the most stable form. Water molecule assisted interconversions between different forms of 2',3'-HPyBI were examined theoretically. Excitation and emission energies for all the forms have been calculated theoretically and the values are in good agreement with the available experimental data. The calculations show that intramolecular proton transfer (ESIPT) is endothermic in the ground state while it is exothermic in the first excited singlet state (except 5',4'-HPymBI). The barrier for the excited state ESIPT reaction increases with nitrogen substitution. Torsional rotation between the benzimidazole and the pyridinyl∕pyrimidinyl rings in the S(1) state depicts that twisted-keto structures involve charge transfer from the hydroxypyridinyl∕hydoxypyrimidinyl to the benzimidazole ring. However, the formation of twisted-keto is not energetically favored in these systems.

A fluorescent probe for biothiols based on the conjugate addition of thiols to α,β‐unsaturated ester

A sensitive fluorogenic probe 1 for biothiols was developed based on the Michael addition reaction. The probe 1 was readily synthesized via the reaction of 2-(4'-hydroxyphenyl) benzimidazole (HPBI) with acryloyl chloride and shows weak fluorescence emission. Upon mixing with biothiols, the fluorescence of 1 is significantly enhanced due to the conjugate addition of thiols to the α,β-unsaturated carbonyl moiety, thus eliminating the photoinduced electron transfer (PET) quenching of the fluorophore by the intramolecular carbon-carbon double bond. Cysteine (Cys) was selected as the representative thiol in the spectral experiment. A good linear relationship was obtained from 1.0 to 30.0 µmol L(-1) for Cys and the detection limit was 0.17 µmol L(-1). Furthermore, probe 1 was highly selective for biothiols without the interference of some biologically relevant analytes and has been applied to detecting biothiols in human urine samples.

TD-DFT Study of the Excited-State Potential Energy Surfaces of 2-(2′-Hydroxyphenyl)benzimidazole and its Amino Derivatives

In this study, we used TD-PBE0 calculations to investigate the first singlet excited state (S(1)) behavior of 2-(2'-hydroxyphenyl)benzimidazole (HBI) and its amino derivatives. We employed the potential energy surfaces (PESs) at the S(1) state covering the normal syn, tautomeric (S(1)-T(syn)), and intramolecular charge-transfer (S(1)-T(ICT)) states in ethanol and cyclohexane to investigate the reaction mechanisms, including excited-state intramolecular proton transfer (ESIPT) and intramolecular charge-transfer (ICT) processes. Two new S(1)-T(ICT) states, stable in ethanol and cyclohexane, were found for HBI and its amino derivatives; they are twisted and pyramidalized. The flat PES of the ICT process makes the S(1)-T(ICT) states accessible. The S(1)-T(ICT) state is effective for radiationless relaxation, which is responsible for quenching the fluorescence of the S(1)-T(syn) state. In contrast to the situation encountered conventionally, the S(1)-T(ICT) state does not possess a critically larger dipole moment than its precursor, S(1)-T(syn) state; hence, it is not particularly stable in polar solvents. On the basis of the detailed PESs, we rationalize various experimental observations complementing previous studies and provide insight to understand the excited-state reaction mechanisms of HBI and its amino derivatives.

Modulation of Excited‐State Proton Transfer of 2‐(2′‐Hydroxyphenyl)benzimidazole in a Macrocyclic Cucurbit[7]uril Host Cavity: Dual Emission Behavior and pKa Shift

The effect of the macrocyclic host, cucurbit[7]uril (CB7), on the photophysical properties of the 2-(2'-hydroxyphenyl)benzimidazole (HPBI) dye have been investigated in aqueous solution by using ground-state absorption and steady-state and time-resolved fluorescence measurements. All three prototropic forms of the dye (cationic, neutral, and anionic) form inclusion complexes with CB7, with the largest binding constant found for the cationic form (K approximately 2.4x10(6) M(-1)). At pH approximately 4, the appearance of a blue emission band upon excitation of the HPBI cation in the presence of CB7 indicates that encapsulation into the CB7 cavity retards the deprotonation process of the excited cation, and hence reduces its subsequent conversion to the keto form. Excitation of the neutral form (pH approximately 8.5), however, leads to an increase in the keto form fluorescence, indicating an enhanced excited-state intramolecular proton-transfer process for the encapsulated dye. In both the ground and excited states, the two pK(a) values of the HPBI dye show upward shifts in the presence of CB7. The prototropic equilibrium of the CB7-complexed dye is represented by a six-state model, and the pH-dependent changes in the binding constants have been analyzed accordingly. It has been observed that the calculated pK(a) values using this six-state model match well with the values obtained experimentally. The changes in the pK(a) values in the presence of CB7 have been corroborated with the modulation of the proton-transfer process of the dye within the host cavity.

Comparative Theoretical Study of Rotamerism and Excited State Intramolecular Proton Transfer of 2-(2′-Hydroxyphenyl)benzimidazole, 2-(2′-Hydroxyphenyl)imidazo[4,5-b]pyridine, 2-(2′-Hydroxyphenyl)imidazo[4,5-c] pyridine and 8-(2′-Hydroxyphenyl)purine

The effect of nitrogen substitution in the benzene ring of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) on the photophysics and rotamerization were examined theoretically by a comparative study of HPBI with 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (HPIP-b), 2-(2'-hydroxyphenyl)imidazo[4,5-c]pyridine (HPIP-c), and 8-(2'-hydroxyphenyl)purine (HPP). Density functional theory (DFT) was used for ground state calculations. Restricted configuration interaction singles (RCIS) combined time dependent DFT (TDDFT) was used for excited state calculations. The calculations reveal in the ground state all of the molecules have two stable rotameric forms, but their relative population is strongly affected by nitrogen substitution. The excitation and emission bands have been calculated theoretically for the rotamers and tautomers. Fluorescence emission and excitation spectra were recorded for HPBI in dioxane and compared with the theoretical results. Theoretical excitation and emission data are in good agreement with the available experimental data. The potential energy surface simulated for the proton transfer processes reflect that it is not favorable in S(0) state, but it is feasible in S(1) state in all of the molecules. Except in HPIP-b, HPIP-b', and HPP', in all other nitrogen substituted molecules, the energy difference between the keto and enol form along the excited state proton transfer coordinates decreases compared to that in HPBI. The study also reveals that torsional relaxation of tautomer to twisted state competes with radiative transitions and leads to fluorescence quenching. Nitrogen substitution enhances this torsional induced nonradiative process and it follows the order HPBI < HPIP-b < HPIP-c < HPP.

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm ( I 454/ I 360) increases linearly with fluoride ion concentration in the range 0.3–8.0 μmol L −1 and the detection limit is 0.19 μmol L −1. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.

Reversible Core-Interconversion of an Iron(III) Dihydroxo Bridged Complex

Complexes [Fe(Hhbi)(2)(NO(3))].2EtOH (1.2EtOH) and [Fe(2)(mu-OH)(2)(Hhbi)(4)].2H(2)O.8EtOH (2.2H(2)O.8EtOH) crystallize in the orthorhombic Fdd2 and P4(2)2(1)2 space groups, respectively (Hhbi(-) = the monoanion of 2-(2'-hydroxyphenyl benzimidazole). Complex 1 exhibits paramagnetic relaxation as evidenced by Mossbauer spectroscopy, and significant axial zero-field splitting (1.5 cm(1) <D < 3 cm(-1)), along with a rhombic distortion (E = 0.205 cm(-1)), as shown by X-band electron paramagnetic resonance (EPR) spectroscopy. Complex 2 undergoes, upon drying, a core conversion toward an oxo-bridged diferric species and, to a smaller extent, decomposition toward a mononuclear ferric complex, as evidenced by Mossbauer, magnetic, EPR, and mass spectrometry data. The core conversion is reversible through recrystallization from wet EtOH, leading to the original complex, which crystallizes in the initial space group.

Excited-State Intramolecular Proton Transfer in 2-(3‘-Hydroxy-2‘-pyridyl)benzoxazole. Evidence of Coupled Proton and Charge Transfer in the Excited State of Some o-Hydroxyarylbenzazoles

The influence of solvent, temperature, and viscosity on the phototautomerization processes of a series of o-hydroxyarylbenzazoles was studied by means of ultraviolet-visible (UV-vis) absorption spectroscopy and steady-state and time-resolved fluorescence spectroscopy. The compounds studied were 2-(2'-hydroxyphenyl)benzimidazole (HBI), 2-(2'-hydroxyphenyl)benzoxazole (HBO), 2-(2'-hydroxyphenyl)benzothiazole (HBT), 2-(3'-hydroxy-2'-pyridyl)benzimidazole (HPyBI), and the new derivative 2-(3'-hydroxy-2'-pyridyl)benzoxazole (HPyBO), this one studied in neutral and acid media. All of these compounds undergo an excited-state intramolecular proton transfer (ESIPT) from the hydroxyl group to the benzazole N3 to yield an excited tautomer in syn conformation. A temperature- and viscosity-dependent radiationless deactivation of the tautomer has been detected for all compounds except HBI and HPyBI. We show that this radiationless decay also takes place for 2-(3-methyl-1,3-benzothiazol-3-ium-2-yl)benzenolate (NMeOBT), the N-methylated analog of the tautomer, whose ground-state structure has anti conformation. In ethanol, the radiationless decay shows intrinsic activation energy for HPyBO and HBO; however, it is barrierless for HBT and NMeOBT and controlled instead by the solvent dynamics. The relative efficiency of the radiationless decay in the series of molecules studied supports the hypothesis that this transition is connected with a charge-transfer process taking place in the tautomer, its efficiency being related to the strength of the electron donor (dissociated phenol or pyridinol moiety) and electron acceptor (protonated benzazole). We propose that the charge transfer is associated with a large-amplitude conformational change of the tautomer, the process leading to a nonfluorescent charge-transfer intermediate. The previous ESIPT step generates the structure with the suitable redox pair to undergo the charge-transfer process; therefore, an excited-state intramolecular coupled proton and charge transfer takes place for these compounds.

Amplified spontaneous emission of proton transfer dyes in polymers

We compare the Amplified Spontaneous Emission (ASE) of the proton transfer dyes 2-(2’-hydroxyphenyl) benzimidazole (HPBI) and 2-(2’-hydroxy-5’-chlorphenyl)benzimidazole (Cl-HPBI) in three different polymeric hosts under UV pulsed laser excitation. Pulseform and degradation of the dyes in the solid polymers were analysed. Spectroscopic data were used in a simple four-level kinetic rate equation system, in the attempt to explain the observed results, i.e. first order exponential decay of maximum output energy and pulse shortening. The model includes transitions for the normal and tautomer form of the molecule as well as intersystem crossing and triplet-triplet transitions.

ASE and photodegradation of two benzimidazole derivatives proton transfer dyes in polymeric matrices

Abstract We report the preparation and spectral characteristics of solid polymeric hosts doped with proton-transfer dyes 2-(2′-hydroxyphenyl)benzimidazole (HPBI) and 2-(2′-hydroxy-5′-chlorphenyl)benzimidazole (Cl-HPBI). The material shows amplified stimulated emission (ASE) under 337 nm pulse excitation. In comparing with PMMA, PS and a 1:1 PS/PMMA copolymer as host material the latter shows improved properties relating to efficiency and photo stability. For all samples we observed a shortening in ASE pulse width during emission due to photo degradation.

Theoretical Modelling for the Ground State Rotamerisation and Excited State Intramolecular Proton Transfer of 2-(2’-hydroxyphenyl)oxazole, 2-(2’-hydroxyphenyl)imidazole, 2-(2’-hydroxyphenyl)thiazole and Their Benzo Analogues

Two series of compounds, one comprising of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole (HBI), 2-(2′-hydroxyphenyl)benzothiazole (HBT), and the other of 2-(2′-hydroxyphenyl)oxazole (HPO), 2-(2′-hydroxyphenyl)imidazole (HPI) and 2-(2′-hydroxyphenyl)thiazole (HPT) are susceptible to ground state rotamerization as well as excited state intramolecular proton transfer (ESIPT) reactions. Some of these compounds show experimental evidence of the existence of two ground state conformers. Out of these two one undergoes ESIPT reaction leading to the formation of the tautomer. The two photophysical processes, in combination, result in the production of a number of fluorescence bands each one of which corresponding to a particular species. Semiempirical AM1-SCI calculations have been performed to rationalize the photophysical behaviour of the compounds. The calculations suggest that for the first series of compounds, two rotational isomers are present in the ground state of HBO and HBI while HBT has a single conformer under similar circumstances. For the molecules of the other series existence of rotamers depends very much on the polarity of the environment. The potential energy curves (PEC) for the ESIPT process in different electronic states of the molecules have been generated theoretically. The simulated PECs reveal that for all these systems the IPT reaction is unfavourable in the ground state but feasible, both kinetically and thermodynamically, in the S1 as well as T1 states.

Synthesis of ruthenium(III) complexes with benzimidazole derivatives

Complexes of ruthenium with biologically important benzimidazole derivatives, viz. 2-(2'-hydroxyphenyl)benzimidazole (HOPBI), 2-(2'-mercaptophenyl)benzimidazole (HSPBI), 2-(2'-hydroxynaphthyl)benzimidazole (HONBI) have been synthesized and characterized.

Principal Component Global Analysis of Fluorescence and Absorption Spectra of 2-(2'-Hydroxyphenyl)Benzimidazole

Principal component analysis (PCA) of a series of absorption and emission spectra of 2-(2'-hydroxyphenyl)benzimidazole (HBI) in aqueous solutions in the pH range from 1.11 to 12.3 and 3.85 to 9.62, respectively, reveals the presence of three contributing components both in the ground and in the excited state. The combination of PCA with global analysis (PCGA) confirms the previously proposed reaction mechanism and allows the determination of p Ka values, pure component spectra, and concentration profiles of the ground- and excited-state species in spite of their strong spectral overlap. PCGA is proved to be a powerful complement to standard nonlinear least-squares analysis of physicochemical systems.