Function Of Solvent Polarity Research Articles

A comparison of chemical shift sensitivity of trifluoromethyl tags: optimizing resolution in ¹⁹F NMR studies of proteins.

The elucidation of distinct protein conformers or states by fluorine ((19)F) NMR requires fluorinated moieties whose chemical shifts are most sensitive to subtle changes in the local dielectric and magnetic shielding environment. In this study we evaluate the effective chemical shift dispersion of a number of thiol-reactive trifluoromethyl probes [i.e. 2-bromo-N-(4-(trifluoromethyl)phenyl)acetamide (BTFMA), N-(4-bromo-3-(trifluoromethyl)phenyl)acetamide (3-BTFMA), 3-bromo-1,1,1-trifluoropropan-2-ol (BTFP), 1-bromo-3,3,4,4,4-pentafluorobutan-2-one (BPFB), 3-bromo-1,1,1-trifluoropropan-2-one (BTFA), and 2,2,2-trifluoroethyl-1-thiol (TFET)] under conditions of varying polarity. In considering the sensitivity of the (19)F NMR chemical shift to the local environment, a series of methanol/water mixtures were prepared, ranging from relatively non-polar (MeOH:H2O = 4) to polar (MeOH:H2O = 0.25). (19)F NMR spectra of the tripeptide, glutathione ((2S)-2-amino-4-{[(1R)-1-[(carboxymethyl)carbamoyl]-2-sulfanylethyl]carbamoyl}butanoic acid), conjugated to each of the above trifluoromethyl probes, revealed that the BTFMA tag exhibited a significantly greater range of chemical shift as a function of solvent polarity than did either BTFA or TFET. DFT calculations using the B3LYP hybrid functional and the 6-31G(d,p) basis set, confirmed the observed trend in chemical shift dispersion with solvent polarity.

Solvent-Controlled Excited State Relaxation Path of 4-Acetyl-4'-(dimethylamino)biphenyl.

Stationary and picosecond time-resolved fluorescence (TRF) and absorption spectra were compared in different aprotic solvents at various temperatures for 4-acetyl-4'-(dimethylamino)biphenyl (ADAB). A large value of the excited state dipole moment, 18-25 D, was estimated from the plot of solvatochromic shift. TRF spectra of ADAB recorded as a function of solvent polarity and temperature show unusual temporal evolution (shift and decay) of the fluorescence bands. In some cases, the dynamic Stokes shifts occur on a time scale much shorter than expected on the basis of literature data on solvent relaxation. In order to investigate variations in the energy of electronic transitions, oscillator strengths, and dipole moments upon changing the molecular geometry, quantum chemical modeling (DFT, TD-DFT, CIS) was performed for ADAB and its ground-state pretwisted derivative, 4-acetyl-4'-dimethylamino-2,2'-dimethylbiphenyl (ADAB-Me). Combination of spectroscopic data and computational results leads to the model of excited state relaxation which involves dynamic solvent-dependent interaction between two close-lying (1)nπ* and (1)ππ* excited electronic states.

Experimental and computational approaches on dipole moment of 5-aminoisoquinoline

The ground and excited state (S1) dipole moments of 5-aminoisoquinoline (5-AIQ) are determined from solvatochromic shift method as a function of different solvent polarity functions. Dipole moment calculations using DFT and ab initio methods with different basis sets are performed. The experimental results show that the dipole moment in the excited state is higher than the ground state.

Unusually high fluorescence quantum yield of a homopolyfluorenylazomethine--towards a universal fluorophore.

The absolute fluorescence quantum yield (Φfl) of a polyfluorenyl azomethine homopolymer was measured as a function of solvent polarity. The solvent induced and temperature dependent fluorescence of the homopolymer were also investigated and they were compared to the corresponding monomer and copolymer. The Φfl of the homopolymer was consistent (45-70%), regardless of solvent polarity with Stokes shifts up to 7460 cm(-1) in ethanol. In contrast, the Φfl of its corresponding monomer decreased from 60% in ethanol to 1% in toluene, whereas a Φfl < 5% for its analogous copolymer was measured. Moderate fluorescence yields (Φfl ≈ 25%) were also possible in thin film when co-depositing the homopolymer with PMMA. Cryofluorescence was used to probe the excited state deactivation modes. Deactivation by internal conversion was found to compete with fluorescence. The fluorescence deactivation pathways of the homopolymer and its corresponding monomer could be suppressed at 77 K, resulting in fluorescence turn-on. Both fluorophores were found to detect nitroaromatics.

Amphiphilic copolymer of poly(ethylene glycol)-block-polypyridine; synthesis, physicochemical characterization, and adsorption onto silica nanoparticle.

In this study, we newly synthesized amphiphilic block copolymers composed of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic pyridine segments (PEG-b-Py). Chain transfer agent terminated PEG was subsequently chain-extended with 3-(4-pyridyl)-propyl acrylate to obtain PEG-b-Py by reversible additional-fragmentation chain transfer (RAFT) polymerization. Particularly, the effect of varying PEG molecular weight (M(n)) of the block copolymers (M(n) = 2000 (2k), and 5000 (5k)) was investigated in terms of critical micelle concentration (cmc), pyrene solubilization, micelle size distribution, and association number per micelle. Based on the amphiphilic balance, PEG-b-Pys formed core-shell type polymer micelle. The cmc value of PEG2k-b-Py was lower than that of PEG5k-b-Py, suggesting the degree of phase separation was strongly depended on PEG M(n). Furthermore, the adsorption of PEG-b-Py copolymer onto silica nanoparticles as dispersant was studied to estimate the effect of PEG M(n) in the copolymers and their solubility in the medium on the adsorption. Adsorbed density of PEG2k-b-Py copolymer onto silica nanoparticle was higher than that of PEG5k-b-Py, which was significantly correlated with the degree of phase-separation based on the amphiphilic balance. The adsorbed amount of copolymer was further changed as a function of solvent polarity, phase separation predicting the presence of the acid-base interaction between Py and silanol group existed on silica nanoparticles. The resultant dispersion stability was highly correlated with the graft density of copolymer onto silica surface. As a result, PEG2k-b-Py coated silica nanoparticles in aqueous media (with high solvent polarity) showed high dispersion stability. These fundamental investigations for the surface modification of the nanoparticle provide the insight into the highly stable colloidal dispersion as well as the design of dispersant molecular structure.

Ion pairing in protic ionic liquids probed by far-infrared spectroscopy: effects of solvent polarity and temperature.

The cation-anion and cation-solvent interactions in solutions of the protic ionic liquid (PIL) [Et3NH][I] dissolved in solvents of different polarities are studied by means of far infrared vibrational (FIR) spectroscopy and density functional theory (DFT) calculations. The dissociation of contact ion pairs (CIPs) and the resulting formation of solvent-separated ion pairs (SIPs) can be observed and analyzed as a function of solvent concentration, solvent polarity, and temperature. In apolar environments, the CIPs dominate for all solvent concentrations and temperatures. At high concentrations of polar solvents, SIPs are favored over CIPs. For these PIL/solvent mixtures, CIPs are reformed by increasing the temperature due to the reduced polarity of the solvent. Overall, this approach provides equilibrium constants, free energies, enthalpies, and entropies for ion-pair formation in trialkylammonium-containing PILs. These results have important implications for the understanding of solvation chemistry and the reactivity of ionic liquids.

A solvatochromic study of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide hydrochloride: An experimental and theoretical approach

The effects of a solvent on the position of the long-wavelength electronic absorption band of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide hydrochloride [m-AMSA.HCl], an antitumor drug, were investigated. To assess the nature of molecular interactions of protonated m-AMSA (1) with various organic solvents the solvatochromic shifts of absorption maxima (ν‾A) with (i) traditionally used bulk solvent polarity functions (Δf and F(ε, n)) and (ii) empirical scales of solvent polarity (Z, χB and ETN) were analyzed. Additionally, in order to investigate the influence of non-specific and specific solute–solvent interactions on absorption band shifts in protic solvents the multicomponent linear regression with two Kamlet–Taft’s solvatochromic parameters (π* and α) was used. The ν‾A in solvents were also obtained using theoretical calculations with the AM1-SM5.4 method and compared with experimental values. Finally, all the results show that in aprotic solvents there are general dipolarity/polarizability effects, while in protic solvents specific interactions connected with the formation of hydrogen bonds are additionally observed.

Effect of solvent polarity and temperature on the spectral and thermodynamic properties of exciplexes of 1-cyanonaphthalene with hexamethylbenzene in organic solvents

Study of the effect of solvent polarity and temperature is done on the exciplex emission spectra of 1-cyanonaphthalene with hexamethylbenzene. Exciplex system is studied in the range of partially polar solvents and in solvent mixture of propyl acetate and butyronitrile. The unique feature of this solvent mixture is that only the solvent polarity changes (6.0≤εs≤24.7) with the change in the mole fraction of solvents whereas the solvent viscosity and refractive index remains unaffected. Thermodynamic properties are calculated according to the models developed by Weller and Kuzmin. Fluorescence lifetimes for both the fluorophore as well as the exciplex are evaluated in all used solvents. Exciplex energetics as a function of solvent polarity and temperature are also discussed. Kuzmin model of self-consistent polarization is used for the explanation of the exciplex emission spectra. The effects of solvent polarity and temperature on energy of zero–zero transitions (hv0/), Huang–Rhys factor (S), Gauss broadening of vibronic level (σ) and the dominant high-frequency vibration (hνν) are investigated. The strong dependence of exciplex stability and energetics upon the solvent polarity and temperature are observed. Full charge transfer exciplexes were observed in solvents of all polarities and stronger exciplex with large emission intensities were found in solvents of low polarities but with the increase in solvent polarity the exciplex becomes weak and they dissociate fastly into radical ion pairs. The kinetic model of Kuzmin was observed to reduce into the Weller kinetic model for this exciplex system with ∆GET = −0.22eV and the spectral shift, h∆ν>0.2eV.

Design of a New Optical Material with Broad Spectrum Linear and Two-Photon Absorption and Solvatochromism

A fluorene-bridged squaraine dimer (SD-FLU-SD) was designed with the purpose of combining various chromophores in one molecule and enhancing its two-photon absorption properties using intra- and interchromophore transitions. Linear and nonlinear absorption properties of SD-FLU-SD were investigated with the goals of understanding the nature of one- and two-photon absorption spectra, determining the molecular optical parameters, and performing modeling of the photophysical processes. The optical behavior of this new SD-FLU-SD “hybrid” molecule was compared with its separate squaraine constituent moiety. Linear spectroscopic characterization includes absorption, fluorescence, excitation and emission anisotropy, and quantum yield measurements in solvents of different polarity and viscosity. Spectral positions of the absorption–fluorescence peaks and quantum yields of SD-FLU-SD and its separate squaraine moiety exhibited complex and nontrivial behavior as a function of solvent polarity. Comprehensive study of ...

Ground and excited state properties of chalcogenol esters: a combined theoretical and experimental study

In this article, the synthesis and characterization of a series of chalcogenol esters 1a, 1b, 1c, 1d, 1e, 1f are described. The photophysical behavior of the esters in solution was studied using UV–vis absorption and steady‐state fluorescence emission spectroscopies. These chalcogenol esters present absorption maxima located around 332 nm and fluorescence emission maxima in the UV‐violet‐blue region. The obtained values for the Stokes' shift and the relation of the fluorescence maxima versus the solvent polarity function (Δf) from the Lippert–Mataga correlation indicate that charge transfer in the excited state occurs only for esters 1c and 1f. Theoretical calculations were also performed in order to study the geometry and charge distribution of these compounds in their ground and excited electronic states, as well as to clarify the role of the chalcogen atom in the photophysics of these compounds. Time‐dependent density functional theory calculations were performed using the CAM‐B3LYP functional with the 6‐31+G(d) basis set for geometrical optimizations and 6‐311++(2d,p) basis set for single points (absorbing and emitting structures). Solvent effects were included by the integral equation formalism of the polarizable continuum model. The large solvatochromic effect observed in the experimental emission spectra indicates that these dyes are more polar in the excited state, which was confirmed by the theoretical calculations. The computationally predicted properties are in good agreement with the experimental results and provide confirmation that the chalcogen atom plays a key role in the photophysical characteristics of the chalcogenol esters. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Synthesis of Aliphatic Esters of Cinnamic Acid as Potential Lipophilic Antioxidants Catalyzed by Lipase B from Candida antarctica

Immobilized lipase from Candida antarctica (Novozyme 435) was tested for the synthesis of various phenolic acid esters (ethyl and n-butyl cinnamate, ethyl p-coumarate and n-butyl p-methoxycinnamate). The second-order kinetic model was used to mathematically describe the reaction kinetics and to compare present processes quantitatively. It was found that the model agreed well with the experimental data. Further, the effect of alcohol type on the esterification of cinnamic acid was investigated. The immobilized lipase showed more ability to catalyze the synthesis of butyl cinnamate. Therefore, the process was optimized for the synthesis of butyl cinnamate as a function of solvent polarity (logP) and amount of biocatalyst. The highest ester yield of 60.7 % was obtained for the highest enzyme concentration tested (3 % w/w), but the productivity was for 34 % lower than the corresponding value obtained for the enzyme concentration of 1 % (w/w). The synthesized esters were purified, identified, and screened for antioxidant activities. Both DPPH assay and cyclic voltammetry measurement have shown that cinnamic acid esters have better antioxidant properties than cinnamic acid itself.

Ultrafast excited state dynamics of the green fluorescent protein chromophore and its kindling fluorescent protein analogue

Fluorescent proteins exhibit a very diverse range of photochemical behaviour, from efficient fluorescence through photochromism to photochemical reactivity. Remarkably this diverse behaviour arises from chromophores which have very similar structures. Here we describe measurements and modelling of the excited state dynamics in the chromophores of GFP (HBDI) and the kindling fluorescent protein, KFP (FHBMI). The methods are ultrafast fluorescence spectroscopy with sub 50 fs time resolution and the modelling is based on the Smoluchowski equation. The excited state decays of both chromophores are very fast, longer for their anions than for the neutral form and independent of wavelength. Detailed studies show the mean fluorescence wavelength to be independent of time. The excited state decay times are also observed to be a very weak function of solvent polarity and viscosity. These results are modelled utilising recently calculated potential energy surfaces for the ground and excited states as a function of the twist coordinates about the two bridging bonds of the chromophore. For FHBMI and the scarce data on the neutral HBDI the calculations are not successful suggesting the need for refinement of these potential energy surfaces. For HBDI in methanol the simulation is successful provided a strong dependence of the radiationless decay rate on the coordinate is assumed. Such dependence should be included in future calculations of excited state dynamics. When the simulations are extended to more viscous solvents they fail to reproduce the observed weak viscosity dependence. The implications of these results for the nature of the coordinate leading to radiationless decay in the chromophore and for the photodynamics of fluorescent proteins are discussed.

Tuning the Förster overlap integral: energy transfer over 20 Ångstroms from a pyrene-based donor to borondipyrromethene (Bodipy)

A linear molecular dyad was synthesised comprising a pyrene-thiophene energy donor linked via a triazole unit to a borondipyrromethene (Bodipy) energy acceptor. The donor to acceptor separation distance is around 20 Å. Emission from the donor originates from a mixed π-π* and partial charge-transfer state and overlaps favourably with the absorption profile for the acceptor. The level of spectral overlap is dependent on the solvent polarity. Rates for electronic energy transfer were measured by transient absorption spectroscopy in solvents of varying polarity and refractive index. The measured rates for Förster energy transfer (k(EET)) correlate fairly well with the calculated overlap integrals (J(F)). A sigmoidal relationship is observed between k(EET) and the solvent polarity function ΔF.

Solvent dependent photophysical properties of dimethoxy curcumin

Dimethoxy curcumin (DMC) is a methylated derivative of curcumin. In order to know the effect of ring substitution on photophysical properties of curcumin, steady state absorption and fluorescence spectra of DMC were recorded in organic solvents with different polarity and compared with those of curcumin. The absorption and fluorescence spectra of DMC, like curcumin, are strongly dependent on solvent polarity and the maxima of DMC showed red shift with increase in solvent polarity function (Δf), but the above effect is prominently observed in case of fluorescence maxima. From the dependence of Stokes’ shift on solvent polarity function the difference between the excited state and ground state dipole moment was estimated as 4.9D. Fluorescence quantum yield (ϕf) and fluorescence lifetime (τf) of DMC were also measured in different solvents at room temperature. The results indicated that with increasing solvent polarity, ϕf increased linearly, which has been accounted for the decrease in non-radiative rate by intersystem crossing (ISC) processes.

Synthesis, Photophysical and Photochemical Properties of Photoacid Generators Based on N-Hydroxyanthracene-1,9-dicarboxyimide and Their Application toward Modification of Silicon Surfaces

We have introduced a series of nonionic photoacid generators (PAGs) for carboxylic and sulfonic acids based on N-hydroxyanthracene-1,9-dicarboxyimide (HADI). The newly synthesized PAGs exhibited positive solvachromatic emission (λ(max)(hexane) 461 nm, λ(max)(ethanol) 505 nm) as a function of solvent polarity. Irradiation of PAGs in acetonitrile (ACN) using UV light above 410 nm resulted in the cleavage of weak N-O bonds, leading to the generation of carboxylic and sulfonic acids in good quantum and chemical yields. Mechanism for the homolytic N-O bond cleavage for acid generation was supported by time-dependent density functional theory (TD-DFT) calculations. More importantly, using the PAG monomer N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide (VBSADI), we have synthesized N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-methyl methacrylate (VBSADI-MMA) and N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-ethyl acrylate (VBSADI-EA) copolymer through atom transfer radical polymerization (ATRP). Finally, we have also developed photoresponsive organosilicon surfaces using the aforementioned polymers.

Effect of Molecular Symmetry on the Spectra and Dynamics of the Intramolecular Charge Transfer (ICT) State of Peridinin

The spectroscopic properties and dynamics of the excited states of two different synthetic analogues of peridinin were investigated as a function of solvent polarity using steady-state absorption, fluorescence, and ultrafast time-resolved optical spectroscopy. The analogues are denoted S-1- and S-2-peridinin and differ from naturally occurring peridinin in the location of the lactone ring and its associated carbonyl group, known to be obligatory for the observation of a solvent dependence of the lifetime of the S(1) state of carotenoids. Relative to peridinin, S-1- and S-2-peridinin have their lactone rings two and four carbons more toward the center of the π-electron system of conjugated carbon-carbon double bonds, respectively. The present experimental results show that as the polarity of the solvent increases, the steady-state spectra of the molecules broaden, and the lowest excited state lifetime of S-1-peridinin changes from ∼155 to ∼17 ps which is similar to the magnitude of the effect reported for peridinin. The solvent-induced change in the lowest excited state lifetime of S-2-peridinin is much smaller and changes only from ∼90 to ∼67 ps as the solvent polarity is increased. These results are interpreted in terms of an intramolecular charge transfer (ICT) state that is formed readily in peridinin and S-1-peridinin, but not in S-2-peridinin. Quantum mechanical computations reveal the critical factors required for the formation of the ICT state and the associated solvent-modulated effects on the spectra and dynamics of these molecules and other carbonyl-containing carotenoids and polyenes. The factors are the magnitude and orientation of the ground- and excited-state dipole moments which must be suitable to generate sufficient mixing of the lowest two excited singlet states.

4‐alkoxyethoxy‐N‐octadecyl‐1,8‐naphthalimide fluorescent sensor for human serum albumin and other major blood proteins: design, synthesis and solvent effect

A series of 4-alkoxyethoxy-N-octadecyl-1,8-naphthalimides with intense blue fluorescence were designed and synthesized as polarity and spectrofluorimetric probes for the determination of proteins. In solvents of different polarities, the Stokes shifts of two dyes increased with increasing solvent polarity and fluorescence quantum yields decreased significantly, suggesting that electronic transiting from ground to excited states was π-π(*) in character. Dipole moment changes were estimated from solvent-dependent Stokes shift data using a solvatochromic method based on bulk solvent polarity functions and the microscopic solvent polarity parameter (E(T)N). These results were generally consistent with semi-empirical molecular orbital calculations and were found to be quite reliable based on the fact that the correlation of the solvatochromic Stokes shifts with E(T)N was superior to that obtained using bulk solvent polarity functions. Fluorescence data revealed that the fluorescence quenching of human serum albumin (HSA) by dyes was the result of the formation of a Dye-HSA complex. The method was applied to the determination of total proteins (HSA + immunoglobulins) in human serum samples and results were in good agreement with those reported by the research institute.

Thermosolvatochromism of Nitrospiropyran and Merocyanine Free and Bound to Cyclodextrin

The thermosolvatochromism of nitrospiropyran free and bound to cyclodextrin was studied in dimethylsulfoxide (DMSO)-water binary solvent systems. Spiropyran was interconverted to merocyanine by heating the sample to 55 °C. The merocyanine (MC) was converted back to spiropyran (SP) either by cooling the sample to room temperature or irradiating the sample with a visible light emitting diode. Steady state absorption spectra of SP and MC samples in the free state and bound to cyclodextrin were obtained in several DMSO-water binary solutions. Emission spectra of MC both free and cyclodextrin-bound were also acquired. Blue-shifted absorption and emission spectra of the studied molecules with increasing solvent polarity suggest that the dipole moments of free and bound merocyanines are higher in the ground state compared to the excited state. Merocyanine dipole moments in the ground and excited states were determined using thermosolvatochromism measurements and the Lippert-Mataga, Bakhshiev, and Kawski-Chamma-Viallet polarity functions. A large change in the dipole moment (ca. 16 D) of the merocyanine in aqueous DMSO was obtained upon electronic excitation, S(1) ← S(0). Analysis of the merocyanine Stokes' shifts as a function of solvent polarity indicates that both general solvent effects and specific solvent effects are present in all systems studied. These findings reveal that merocyanine could be utilized as a polarity sensor for DMSO-water binary solutions.

Photophysical properties of a series of 4‐aryl substituted 1,4‐dihydropyridines

In this article, a series of Hantzsch 1,4‐dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher Stokes’ shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation of the fluorescence maxima (νmax) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation. A comparison between the experimental data and time‐dependent density functional theory‐polarizable continuum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics of these compounds. For these calculations, the S0 and S1 states were optimized using Becke, three‐parameter, Lee–Yang–Parr/6‐31 G* and Configuration Interaction Singles/6‐31 G*, respectively. The predicted absorption maxima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not match the experimental, which means that the excited specie cannot be related to neither a locally excited state nor to an aromatized structure. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Photophysical study of some 3-benzoylmethyleneindol-2-ones and estimation of ground and excited states dipole moments from solvatochromic methods using solvent polarity parameters

► Photophysical studies of 3-benzoylmethyleneindol-2-ones in a series of different solvents. ► Ground and excited states dipole moments. ► Analysis of the ground and excited state processes. ► Study of pH effect on 3-benzoylmethyleneindol-2-ones. 3-Benzoylmethyleneindol-2-ones, isatin based chalcones containing donor and acceptor moieties that exhibit excited-state intramolecular charge transfer, have been studied in different solvents by absorption and emission spectroscopy. The excited state behavior of these compounds is strongly dependent on the nature of substituents and the environment. These compounds show multiple emissions arising from a locally excited state and the two states due to intramolecular processes viz. intramolecular charge transfer (ICT) and excited state intramolecular proton transfer (ESIPT). Excited-state dipole moments have been calculated using Stoke-shifts of LE and ICT states using solvatochromic methods. The higher values of dipole moments obtained lead to support the formation of ICT state as one of the prominent species in the excited states of all 3-benzoylmethyleneindol-2-ones. The correlation of the solvatochromic Stokes-shifts with the microscopic solvent polarity parameter ( E T N ) was found to be superior to that obtained using bulk solvent polarity functions. The absorption and florescence spectral characteristics have been also investigated as a function of acidity and basicity (Ho/pH) in aqueous phase.