Solvent Polarity Scale Research Articles

Deciphering the Photophysical Role of Conjugated Diyne in Butadiynyl Fluorophores: Synthesis, Photophysical and Theoretical Study

The present work focuses on the current interest in diyne bridged chromophores necessitating a clearer understanding of the photophysics of such molecules. The significance of the diyne moiety in the photophysics has been investigated by synthesizing simple substituted diphenyl butadiynyl derivatives following a quick and efficient microwave assisted Eglinton coupling of terminal alkynes. Emission of the fluorophores is observed from the usual locally excited (LE) state and intramolecular charge transfer (ICT) state. Separation of pure ICT emission from pure LE emission has been carried out by Gaussian/Lorentzian curve fitting. The vibronic coupling in the local transitions appears to be confined to the normal mode involving the C-C triple bond stretching of the diyne moiety. This implies that the LE transition involves the diyne moiety, a conclusion supported by quantum chemical calculations. The resolved ICT emission follows double linear dependence on ET(30) solvent polarity scale. The important role of the diyne moiety in the photophysics of this class of molecules is clearly discernible in this study.

Stability constants of lithium complexes with 15-crown-5 and 18-crown-6 in a hydrophobic ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulphonyl]imide

Thermodynamic data for lithium complexes formation with 15-crown-5 (15C5, L) and 18-crown-6 (18C6, L) in a hydrophobic room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulphonyl]imide ([BMIM][N(Tf)2]) are measured with NMR 7Li technique at 27–57°C. For 15C5 and 18C6 only LiL+ complexes are found. The stability of lithium complexes in RTIL is estimated to be in the range between water and acetonitrile. The following values for logK(Li+L) and ΔНo(Li+L) at 25°C are determined: 1.66 (0.03), ∼0kJ/mol for 15C5 and 2.0 (0.1), −17.0 (0.5)kJ/mol for 18C6, respectively. Unlike the situation with molecular solvents the lithium complex with 15C5 in [BMIM][N(Tf)2] is entropy stabilized. The polarity of [BMIM][N(Tf)2] fits well the existing empirical solvent polarity scale for molecular solvents characterizing the polarity of this RTIL as rather normal instead of “superpolar”.

Estimating viscosity and polarity in the microenvironment of polymeric gels—introducing a microviscosity parameter

Estimation of micropolarity and microfluidity at the water-polymer interface is important in understanding the physicochemical behavior of polymeric gels in the context of their wide biomedical applications. In this work, fluorescence studies of 8-Anilino-1-naphthalene-sulphonate (ANS) were used to introduce a new microviscosity parameter. Z and ET(30) solvent polarity scales were found to work well in resolving the polarity differences in various polyacrylamide gel compositions. A microviscosity parameter τη is an easy-to-estimate parameter that was used to follow the changes in microviscosity with changes in polymer composition. The microviscosity parameter could efficiently estimate the increase of microviscosity with increase in the crosslinker, as well as initiator concentrations at the same monomer concentration.

Spectral Investigations of Preferential Solvation and Solute–Solvent Interactions of Free Base and Protonated 5,10,15,20-Tetrakis(4-trimethyl-ammonio-phenyl)-porphine Tetratosylate in Aqueous Organic Mixed Solvents

The solvatochromic properties of the free base and the protonated 5,10,15,20-tetrakis(4-trimethyl-ammonio-phenyl)-porphine tetratosylate (TTMAPP) were studied in pure water, methanol, ethanol, 2-propanol, and their corresponding aqueous mixtures. The correlation of the empirical solvent polarity scale (E T) values of TTMAPP with composition of the solvents were analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition effects in preferential solvation of the solute dyes were investigated in terms of both solvent–solvent and solute–solvent interactions and also the local mole fraction of each solvent composition was calculated in the cybotactic region of the probe. The effective mole fraction variation may provide significant physicochemical insights in the microscopic and molecular level of interactions between TTMAPP species and the solvent components and, therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TTMAPP.

Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an FeII Complex Revealed by 2D IR Spectroscopy

The vibrational dephasing dynamics of the nitrosyl stretching vibration (ν(NO)) in sodium nitroprusside (SNP, Na2[Fe(CN)5NO]·2H2O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The ν(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the ν(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the ν(NO) exhibits a large vibrational solvatochromic shift of 52 cm(-1), ranging from 1884 cm(-1) in dimethyl sulfoxide to 1936 cm(-1) in water. The vibrational anharmonicity of the ν(NO) varies from 21 to 28 cm(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the ν(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the ν(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the ν(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the π-back-bonding network along the Fe-NO bond in SNP.

N,N-dialkyl perfluoroalkanesulfonamides: Synthesis, characterization and properties

Twenty derivatives of N,N-dialkyl perfluoroalkanesulfonamides, n-CmF2m+1SO2NRR′ (m=1, 2, 4, 6, 8; R, R′=CH3, CH2CH3 or CH2CH2OCH3), have been prepared and characterized. Their fundamental physicochemical properties, including melting point, density, viscosity and dielectric constant, are determined. The influences of structural variations in both the perfluoroalkyl and alkyl chains on the above properties are extensively studied. Among these N,N-dialkyl perfluoroalkanesulfonamides, 14 compounds are liquid at room temperature, and some of them exhibit low melting points, even lower than −40°C. The empirical solvent polarity scales for these liquid perfluoroalkanesulfonamides, such as ‘energy of transition’ET(30) values and Kamlet–Taft solvent parameters (π*, α, β), are also determined by solvatochromic measurements, and show good linear temperature dependence. The ET(30) values are in the range of 45.36–55.19kcalmol−1 at 25°C, comparable to those of short chain alcohols. The π* parameters for the compounds containing alkyl chain are in the range of 0.550–0.651 at 25°C, being a little higher than those of short chain alcohols. All these prepared compounds show strong hydrogen-bond donating ability (α values ranging from 0.865 to 1.204 at 25°C), comparable to that of short chain alcohols. Meanwhile, those containing alkoxyalkyl chain display strong hydrogen-bond accepting ability (β values ranging from 0.71 to 0.82 at 25°C), comparable to that of short chain alcohols.

Induced morphology control of Ln–asparagine coordination polymers from the macro to nanoscopic regime in polar solvent–water mixtures

A bottom-up approach is presented here for morphology control of Ln-asparagine (Asn) coordination polymers (CPs) from the macro to nanoscopic regime by tuning solvent polarity based on the π* solvent polarity scale in polar solvent-water mixed solutions. By a simple hydrothermal treatment, large macroporous spheres with sizes up to millimeters were obtained in ethanol (π*: 0.54)-water mixtures, and their formation mechanism was proved to be particle aggregation together with a hollowing process based on Ostwald ripening. Other solvents with increasing polarity were also used, and submicrometer spheres as well as fine nanoparticles were prepared in acetone (π*: 0.71) and DMF (π*: 0.88), respectively. When dioxane (π*: 0.55) was used here, whose π* value is similar to ethanol, large macroporous spheres were also prepared. Furthermore, with Ce- and La-Asn CPs large spheres as precursors, after calcination, ceria and lanthana large spheres with hierarchical structures were fabricated, respectively. Considering that many growth media of CPs are single solvent systems, designing certain solvents mixtures and tuning their polarity would bring us new opportunities to achieve morphology and composition control of CPs materials.

Environment Reaction Fields for Lipophilic Fluorophores using Solvatochromic Shifts

Biological activities such as permeability, and the energetics of protein insertion are governed, in part, by differences in polarity across the phospholipid membrane. Environment polarity can also induce changes in absorption or emission maxima, for a given fluorophore. This is termed solvatochromism. In this study we investigated the relationship between solvatochromic shifts and the membrane polarity for curcumin and aluminum phthalocyanine disulfonic acid (AlPcS2). From the Lippert equation, fluorophore solvatochromism was analyzed using the reaction field of Onsager and later iterations. Each reaction field model predicted the solvatochromic shift based on the solvent dielectric constant, solvent refractive index, and allowed for polarizability of the solute. In addition, the models were extended to consider the effect of dispersion forces. For curcumin, the reaction field of Block and Walker gave the strongest agreement between experimental and predicted values (r = 0.977, p<0.0001). For AlPcS2, the reaction field of Wertheim, based on statistical mechanics, gave the best agreement (r = 0.951, p = 0.001), only when dispersion forces and solute polarizability were considered. These results allowed identification of fluorophore environment polarity upon binding to lipid bilayers. In addition, we correlated the model predictions to the Dimroth-Reichardt ET (30) solvent polarity scale used by Frimer and colleagues. This technique can qualitatively estimate the location of a fluorophore in the lipid membrane. Using the models, curcumin was estimated to be in the acyl chain region of the lipid bilayer, compared to AlPcS2, at the fatty acid carbonyl. This investigation provides a general method to link easily obtained absorption and emission spectra of a given lipophilic fluorophore to its location in a lipid bilayer via classically derived reaction field models.

Determination of solvatochromic solvent polarity parameters for the characterisation of some nematic liquid crystal in anisotropic and isotropic phases

Characterisation of liquid crystals (LCs) as solvents is needed, to obtain the polarity and solvatochromic polarity parameters of these media. Polarity parameters demonstrate the effects of LC media on the photo-physical behaviour of solute molecules in an anisotropic medium. The practical limitations in determining solvent polarity scale parameters for LCs can overcome the overlapping absorption band of LCs and solvent-sensitive standard compounds or their insolubility in LCs. In this work, we report Kamlet–Abboud–Taft polarity functions of some nematic LCs in different temperatures and phases, isotropic and anisotropic, with the solvatochromic method, using the Reichardt's dye and 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-phenolate standard probe. In addition, a new azo and coumarin dye were used as probes to obtain some solvatochromic polarity parameters. Finally, a new polarity parameter, the LC anisotropic matrix, is introduced.

ChemInform Abstract: Molecular Probes of Solvation Phenomena

AbstractReview: 54 refs.

Ketocyanine dyes as sensors for proticity and pH of a medium

Photophysical properties of two structurally similar ketocyanine dyes have been studied in aprotic+protic (dioxane+water, tetrahydrofuran+water) and protic+protic (methanol+water) solvent mixtures. The observed photophysical properties of these dyes show different trends with an increase in solvent polarity (in the ET(30) scale) in aprotic and protic media. The H-bonding at the carbonyl centres of the dyes in the excited state presumably plays a role in modifying the emitting state and hence the photophysical properties show similar trend in a protic solvent or in a mixed aprotic+protic solvent at higher percentage of the later. Thus the dyes are supposed to act as useful probes for studying H-bonding ability of a medium. Absorption and fluorescence spectral characteristics of these dyes have been found to get modified in the presence of strong acid. The presence of H+ ions persists the dyes to exist in equilibrium with the protonated forms having different absorption and fluorescence characteristics. Reversible change of the equilibrium with pH of the medium makes the dyes, particularly the symmetric one, good probes for indicating the pH of the medium.

Molecular probes of solvation phenomena

The properties of the molecules present in any chemical or biological system are dependent on interactions with the environment, and a quantitative understanding of solvation phenomena remains a major challenge. Molecular recognition probes provide a new approach to quantitatively measure the properties of solvents. Traditionally, solvent polarity scales have been based on spectroscopic probes that provide insight into the nature of solvent-solute interactions. This review compares the solvent polarity parameters obtained from the wavelengths of UV/Visible absorption maxima with solute H-bond parameters obtained from the free energies of solution equilibria. The similarity of the solvent and solute H-bond scales leads to a general H-bond scale that uses the same parameters to describe both solvent and solute. The general H-bond scale provides a framework for understanding the relationship between local intermolecular interactions and the properties of the bulk medium. Intermolecular interactions are sensitive to solvation equilibria, so molecular recognition probes provide fundamentally different information from spectroscopic probes that are sensitive to the populations of different solvation states of the solute. Studies of mixed solvents demonstrate the potential of molecular recognition probes for providing new insights into solvation phenomena.

Solvent and substituent effects on aggregation constants of perylene bisimide π-stacks – a linear free energy relationship analysis

A series of six perylene bisimides (PBIs) with hydrophilic and hydrophobic side chains at the imide nitrogens were applied for a comparative study of the solvent and structural effects on the aggregation behaviour of this class of dyes. A comparison of the binding constants in tetrachloromethane at room temperature revealed the highest binding constant of about 10(5) M(-1) for a PBI bearing 3,4,5-tridodecyloxyphenyl substituents at the imide nitrogens, followed by 3,4,5-tridodecylphenyl and alkyl-substituted PBIs, whereas no aggregation could be observed in the accessible concentration range for PBIs equipped with bulky 2,6-diisopropylphenyl substituents at the imide nitrogens. The aggregation behaviour of three properly soluble compounds was investigated in 17 different solvents covering a broad polarity range from nonpolar n-hexane to highly polar DMSO and water. Linear free energy relationships (LFER) revealed a biphasic behaviour between Gibbs free energies of aggregation and common empirical solvent polarity scales indicating particularly strong π-π stacking interactions in nonpolar aliphatic and polar alcoholic solvents whilst the weakest binding is observed in dichloromethane and chloroform. Accordingly, PBI aggregation is dominated by electrostatic interactions in nonpolar solvents and by solvophobic interactions in protic solvents. In water, the aggregation constant is increased far beyond LFER expectations pointing at a pronounced hydrophobic effect.

Preferential solvation and solvation shell composition of free base and protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin in aqueous organic mixed solvents

The solvatochromic properties of the free base and the protonated 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) were studied in pure water, methanol, ethanol (protic solvents), dimethylsulfoxide, DMSO, (non-protic solvent), and their corresponding aqueous-organic binary mixed solvents. The correlation of the empirical solvent polarity scale ( E T) values of TPPS with composition of the solvents was analyzed by the solvent exchange model of Bosch and Roses to clarify the preferential solvation of the probe dyes in the binary mixed solvents. The solvation shell composition and the synergistic effects in preferential solvation of the solute dyes were investigated in terms of both solvent–solvent and solute–solvent interactions and also, the local mole fraction of each solvent composition was calculated in cybotactic region of the probe. The effective mole fraction variation may provide significant physico-chemical insights in the microscopic and molecular level of interactions between TPPS species and the solvent components and therefore, can be used to interpret the solvent effect on kinetics and thermodynamics of TPPS. The obtained results from the preferential solvation and solvent–solvent interactions have been successfully applied to explain the variation of equilibrium behavior of protonation of TPPS occurring in aqueous organic mixed solvents of methanol, ethanol and DMSO.

Synchronous fluorescence spectroscopic study of solvatochromic curcumin dye

Curcumin, the main yellow bioactive component of turmeric, has recently acquired attention by chemists due its wide range of potential biological applications as an antioxidant, an anti-inflammatory, and an anti-carcinogenic agent. This molecule fluoresces weakly and poorly soluble in water. In this detailed study of curcumin in thirteen different solvents, both the absorption and fluorescence spectra of curcumin was found to be broad, however, a narrower and simple synchronous fluorescence spectrum of curcumin was obtained at Δ λ = 10–20 nm. Lippert–Mataga plot of curcumin in different solvents illustrated two sets of linearity which is consistent with the plot of Stokes’ shift vs. the E T30. When Stokes's shift in wavenumber scale was replaced by synchronous fluorescence maximum in nanometer scale, the solvent polarity dependency measured by λ SFS max vs. Lippert–Mataga plot or E T30 values offered similar trends as measured via Stokes’ shift for protic and aprotic solvents for curcumin. Better linear correlation of λ SFS max vs. π* scale of solvent polarity was found compared to λ abs max or λ em max or Stokes’ shift measurements. In Stokes’ shift measurement both absorption/excitation as well as emission (fluorescence) spectra are required to compute the Stokes’ shift in wavenumber scale, but measurement could be done in a very fast and simple way by taking a single scan of SFS avoiding calculation and obtain information about polarity of the solvent. Curcumin decay properties in all the solvents could be fitted well to a double-exponential decay function.

Estimation of ground- and excited-state dipole moments of oxazine 1 in liquid and liquid crystalline media

The absorption and fluorescence spectra of a laser dye, oxazine-1 (OX1), in liquid and liquid crystalline media were studied at room temperature. The solvatochromic method was used to determine the ground- and excited-state dipole moments by means of Lippert–Mataga, Bakshiev, Kawski–Chamma–Viallet polarity functions. The solvent polarity has no large and regular effect on the spectral behavior of OX1, and thus it might be considered as a poor solvatochromic indicator dye. In addition, applicability of solvatochromic method for this dye in anisotropic media was investigated. On the other hand, nature and degree of the solute–solvent interactions were characterized using correlation of multi-parameter solvent polarity scales. Due to the theoretical restrictions and the dye molecular structure, deviations from the solvatochromic correlation were observed.

Photophysical properties of an asymmetrical 2,5-diarylidene-cyclopentanone dye possessing electron donor and acceptor substituents

Spectroscopic and photophysical properties have been measured in a variety of solvents for (2E,5E)-2-(p-cyanobenzylidene)-5-(p-dimethylaminobenzylidene)-cyclopentanone ( I), an asymmetrically substituted 2,5-diarylidene-cyclopentanone known to have potential for applications utilizing two-photon absorption (TPA). Compound I was synthesized by a simple procedure utilizing DIMCARB catalyst. Absorption and fluorescence maxima have been correlated with the E T(30) solvent polarity scale. Theoretical TD-DFT calculations and Lippert–Mataga analysis demonstrate the internal charge transfer (ICT) nature of the S 0 → S 1 excitation. Radiative and nonradiative decay constants have been determined from fluorescence quantum yields and lifetimes. Substantial variation with solvent in the rate of nonradiative decay is interpreted in terms of a competition between internal conversion and intersystem crossing.

Solvent and temperature induced switching between structural isomers of Rh(I) phosphinoalkyl thioether (PS) complexes.

To develop functional systems based on the weak-link approach (WLA), it is important to understand how solvent and ligand binding strength alter the coordination geometry of complexes formed from this method. A series of phosphinoalkyl thioether (PS) hemilabile ligands with varying electron donating abilities were synthesized and incorporated into homoligated Rh(I)(PS)2Cl complexes to help understand the effects of solvent and ligand binding strength on the preferred coordination modes. The switching between closed and semiopen structural isomers of these Rh(I)(PS)2Cl complexes was studied by variable temperature 31P NMR spectroscopy in different solvent mixtures of CH2Cl2 and tetrahydrofuran (THF) to obtain thermodynamic parameters (DeltaG(o), DeltaH(o), TDeltaS(o), and K(eq)). The isomers differ in the position of the chloride counterion. In the closed isomer, the Cl- anion occupies the outer coordination sphere, while in the semiopen isomer, the Cl- has moved inner sphere and displaced one of the Rh-S bonds. The closed isomer is favored in CH2Cl2 and the semiopen isomer is favored in THF. The preference for either isomer at equilibrium depends on the solvent polarity, based upon the E(T)(N) solvent polarity scale, as was determined from 15 different solvents, with more polar solvents favoring the closed isomer. The isomer preference also depends on the electron donating ability of the group attached to the sulfur of the PS ligand, with electron donating groups favoring the closed isomers and electron withdrawing groups favoring the semiopen isomers. The formation of the semiopen isomer from the closed isomer is entropically favored but enthalpically disfavored under all conditions studied. Elucidation of the principles and environments that determine the equilibrium between the two isomers will aid in the design of functional complexes prepared by the WLA.

On the Molecular Structure and UV/vis Spectroscopic Properties of the Solvatochromic and Thermochromic Pyridinium-N-Phenolate Betaine Dye B30

Quantum chemical calculations as well as vis absorption and fluorescence measurements of the pyridinium-N-phenolate betaine dye B30, dissolved in 1-chlorobutane at temperatures between 343 and 77 K, shed more light on the solvatochromism, thermosolvatochromism, and photophysical behavior of this probe dye, formerly used to establish an empirical scale of solvent polarity, called E(T)(30) or E(T)(N) scale. A new calculated gas-phase E(T)(30) value is reported. Complementary to recent work of Kharlanov and Rettig (J. Phys. Chem. A 2009, 113, 10693-10703), it is shown that fluorescence of B30 in 1-chlorobutane solution is observable already at temperatures just below the solvent's melting point and not only at 77 K. Analogous to increasing solvent polarity, decreasing solvent temperature leads to a large hypsochromic shift of the vis absorption band of B30, dissolved in 1-chlorobutane (Deltalambda = -245 nm from 797 nm at 343 K to 552 nm at 77 K). This thermosolvatochromism can be easily seen: the solution color changes from greenish yellow (343 K) to magenta-violet (77 K).

Photophysical Properties and Two-Photon Polymerization Ability of a Nitroalkoxystilbene Derivative

The photophysical behavior of the trans-4-propoxy 4′-nitrostilbene (PNS) as well as its one- and two-photon polymerization properties are reported. A detailed solvatochromic analysis of the steady-state and time-resolved fluorescence indicates that the locally excited state of PNS (LE) undergoes multiple deactivation pathways which are sequentially triggered by the solvent polarity. In low polar solvent, an efficient intersystem crossing (ISC) identified as a S1→Tn process leads to the population of the lowest triple state from which the trans−cis photoisomerization is mainly proceeding. From ns-laser flash photolysis measurements, it also appears that the T1 state which exhibits a nπ* character can efficiently abstract a hydrogen from aliphatic amines and subsequently produces α-aminoalkyl radicals, highly reactive species for the photoinitiation of free-radical polymerization. As a consequence, PNS constitutes a good photoinitiating system. However, the further increase of the solvent polarity opens a new relaxation pathway which populates a highly polar TICT level to the detriment of ISC and trans−cis photoisomerization. A strong increase of the fluorescence quantum yield with a concomitant decrease of the trans−cis photoisomerization one is then observed. At the high end of the solvent-polarity scale, an adding non-radiative deactivation process associated with the twisting of the nitro group leads to a noticeable decrease of the fluorescence process. The photoinduced charge transfer process which largely impacts the photophysical feature of such a D−π−A system also promotes the two-photon absorption properties of PNS. The chromophore exhibits a sizable two-photon absorption spectrum in the 700−900 nm region with δ of about 180 ± 15 GM at 750 nm. In conjunction with its photoinitiating properties, the two-photon polymerization ability of PNS was finally demonstrated. A systematic comparison with a 2,7 diaminofluorene derivative used as reference for two-photon polymerization confirms the substantial improvement of the two-photon polymerization efficiency.