NMR analysis of photochromism of bisthiazolylindenols

The photochromic reaction of two derivatives of bisthiazolylindenols was investigated in three solvents of various polarities (C₆D₁₂, THF-d₈ and CD₃CN) by NMR spectroscopy. Photoirradiation can generate two diastereomeric closed forms. High conversion ratio and large diastereomer excess were obtained and the reasons of the excellent properties were discussed in terms of intramolecular hydrogen bonds, steric bulkiness of substituents, and solvent polarities. Two sets of intramolecular nitrogen–hydrogen interactions fix the conformation of the two investigated bisthiazolylindenols in favor of cyclization, which also lead to high diastereoselectivity. The N–H interactions are partially disrupted when polarity of the solvent increases.     

How to Control the Chemoselectivity of the Catalytic Formation of Chiral γ-Lactams or 2,3-Disubstituted Pyrroles by the Choice of Solvent

Summary. The reaction of the unsaturated imine methyl(3-phenylallylidene)amine with ethylene and carbon monoxide in the presence of catalytical amounts of Ru₃(CO)₁₂ leads to the formation of two heterocyclic products. One of the products is a chiral γ-lactam, the other one a 2,3-disubstituted pyrrole derivative, in which only the carbon atom from carbon monoxide is incorporated. The selectivity in the formation of the products may be controlled by the choice of solvent. In general, in nonpolar solvents the formation of the lactam is preferred whereas the use of more polar solvents enhances the yield of the pyrrole. For most of the solvents used there is a linear dependence of the product ratio on the relative permittivity of the corresponding solvent. Typically, polar aprotic solvents do not follow this rule.     

Rotational reorientation dynamics of ionic dye solutes in polar solvents with the application of a general model for the solvation shell

The rotational reorientation times of two cationic dyes, nile blue A and oxazine 720 have been determined in various protic and aprotic polar solvents from the picosecond decay of their transient absorption. The results have shown a good agreement with those obtained from theoretical calculations based on Stokes–Einstein–Debye theory, using a simple model for the rotating species. In this model these large closely planar solutes are treated as oblate symmetric tops, their solvent shells are represented by layers with uniform thickness of γ · 2r_S, where 2r_S is the diameter of the solvent molecule, and γ is a common fitting parameter for all types of solvent.     

Degradation of parathion methyl by reactive sorption on the cerium oxide surface: The effect of solvent on the degradation efficiency

A simple and easily scalable “wet” procedure was used to prepare nanocrystalline cerium oxide capable of destroying the toxic organophosphate pesticide parathion methyl. The synthetic procedure consists of the direct precipitation of cerous salt with aqueous ammonia in the absence of CO₂. The prepared cerium oxide was able to decompose the organophosphate compounds both in nonpolar (e.g., heptane) and polar aprotic (e.g., acetonitrile) solvents. However, in solvents with hydrogen-bond donating ability, the –OH groups on the cerium oxide surface were solvated and inactivated. The preferential solvation model was used to express the experimental dependencies of the cerium oxide degradation efficiency on the composition of the water-acetonitrile mixture. In certain solvent systems, some empirical polarity scales, such as the alpha-scale or the Dimrodth-Richardt parameter ET(30), may be correlated with the degradation efficiency of cerium oxide.     

13C NMR spectroscopic studies of the behaviors of carbonyl compounds in various solutions

¹³C NMR spectroscopic studies were performed for carbonyl compounds having a hydroxyl group, a carboalkoxy group, an acetoxy group, or a carboxyl group in various solvents with different polarities for observation of their behaviors of ¹³C NMR chemical shifts of carbonyl carbons in solutions. It was found that the chemical shifts of the carbonyl carbons in ¹³C NMR have good correlation with the empirical parameter for solvent polarities, E_T^N, depending on the structures. Inter- or intramolecular hydrogen bonding and dipolar-dipolar interactions appear to play a key role in this observation.     

Fluorescence properties of fluoranthene as a function of temperature and environment

The fluoranthene fluorescence properties were investigated in a variety of environments, i.e. in different solvents and temperatures, in the solid state, and in the vapor phase. The emission maximum was found to be independent of environment. The absorption spectrum in different solvents exhibits only minor changes. In solution, the fluorescence lifetime shows a slight inverse relationship to the solvent dielectric constant. With water/methanol mixtures of varying composition as the solvent, the lifetime decreases linearly with increasing mole fraction of water. At 77K, the fluoranthene fluorescence lifetime in frozen polar and nonpolar solvents are the same within experimental error. In hexane the fluorescence lifetime is independent of temperature (77±3 and 82±7 ns, at room temperature and 77 K, respectively). In methanol the lifetime is 64±3 ns at room temperature and increases linearly to 80±4 ns at 77 K. In the vapor phase the lifetime is 32±1 ns. No fluorescence quantum yield change was observed for either S₁ or S₂ manifold excitation.     

Hydrogen-bond-assisted syndiotactic-specific radical polymerization of N-isopropylacrylamide: The solvent effect on the stereospecificity

Radical polymerizations of N-isopropylacrylamide (NIPAAm) in several solvents at low temperatures in the absence or presence of hexamethylphosphoramide (HMPA) or 3-methyl-3-pentanol (3Me3PenOH) were examined. The isotacticities of the poly(NIPAAm)s obtained in the absence of HMPA and 3Me3PenOH at lower temperatures slightly increased as the polarities of the solvents used increased. The addition of HMPA significantly induced the syndiotactic-specificity even in polar solvents such as tetrahydrofuran and acetone, although the use of the solvents having proton-donating ability, such as chloroform, prevented the induction of the syndiotactic-specificity, even if their polarities are low. In the presence of 3Me3PenOH, a good correlation between the polarities of the solvents used and the syndiotacticities of the obtained poly(NIPAAm)s was observed, and poly(NIPAAm) with r = 73% was obtained using the toluene/methylcyclohexane mixed solvent.     

Thermal Behavior of VARIOUS sulfone-based diimid-diacids Solvated with Polar Organic Solvents

Thermal analysis techniques were performed to reveal ‘crystalline solvate’ behavior between organic compounds and polar solvents. Diimide-dicarboxylic acid (DIDA) was formed by reacting 3,3'-diaminodiphenylsulfone (3,3'-DPS) or 4,4'-diaminodiphenylsulfone (4,4'-DPS) with trimellitic anhydride (TMA) in some polar solvents (PSv). The products could crystallize upon cooling in a polar solvent media to form a solvate containing a finite quantity of solvents, leading to what can be termed as ‘crystalline solvates’ (CS). This study has demonstrated that sampling techniques in TG and DSC must be kept to be as similar as possible, which is a critical point in practices of thermal analysis techniques. DSC analysis revealed that there are two endothermic peaks in the CS, with the lower one being the de-solvate temperature of CS (T _d) at which the solvated solvent molecules were removed, and the higher peak being the melting point of the de-solvated DIDA (T _m). T _d was found to vary with the types of polar solvents and structures of DIDA. The TG result indicated that most of the sulfone-based DIDA-CS contained 2 moles of solvent per mole of solvate. X-ray analysis revealed that different crystalline structures were found for DIDA-CS solvated with different solvent molecules, but all de-solvated DIDA possessed the same crystal unit. This revised version was published online in July 2006 with corrections to the Cover Date.     

Photostability of the sunscreening agent 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone) in solvents of different polarity and proticity

The most widely used UVA absorber in broad-spectrum sunscreens is 4-tert-butyl-4′-methoxydibenzoylmethane (avobenzone). However, the photostability of avobenzone is solvent-dependent. The aim of this work was to investigate the photostability of avobenzone in solvents of different polarity and proticity. Four solvents were employed, namely, cyclohexane, ethyl acetate, dimethylsulfoxide and methanol. The cause of the instability of avobenzone in these solvents was determined by means of UV spectroscopy, high performance liquid chromatography, gas chromatography–mass spectrometry and nuclear magnetic resonance spectroscopy. The effect of oxygen on the photo-instability was also determined. Avobenzone was found to lose absorption efficacy as a result of photoisomerisation from the enol to the keto form and/or photodegradation to form photoproducts that absorb principally in the UVC region, depending on the solvent. It was found to be essentially photostable in the polar protic solvent methanol but photoisomerised in the polar aprotic solvent dimethylsulfoxide. In the nonpolar solvent cyclohexane, it photodegraded appreciably. Both photoisomerisation and photodegradation occurred to a similar extent in the moderately polar aprotic solvent ethyl acetate. Photoisomerisation occurred only in the presence of oxygen whereas photodegradation occurred irrespective of oxygen. This knowledge is important in order to achieve the correct formulation for sunscreens incorporating avobenzone.     

Intramolecular charge transfer effects on 3-aminobenzoic acid

Effect of solvents, buffer solutions of different pH and β-cyclodextrin on the absorption and fluorescence spectra of 3-aminobenzoic acid (3ABA) have been investigated. The solid inclusion complex of 3ABA with β-CD is discussed by UV–Vis, fluorimetry, semiempirical quantum calculations (AM1), FT-IR, ¹H NMR and Scanning Electron Microscope (SEM). The thermodynamic parameters (ΔH, ΔG and ΔS) of the inclusion process are also determined. The experimental results indicated that the inclusion processes is an exothermic and spontaneous. The large Stokes shift emission in solvents with 3ABA are correlated with different solvent polarity scales suggest that, 3ABA molecule is more polar in the S₁ state. Solvent, β-CD studies and excited state dipole moment values confirms that the presence of intramolecular charge transfer (ICT) in 3ABA. Acidity constants for different prototropic equilibria of 3ABA in the S₀ and S₁ states are calculated. β-Cyclodextrin studies shows that 3ABA forms a 1:1 inclusion complex with β-CD. β-CD studies suggest COOH group present in non-polar part and amino group present in hydrophilic part of the β-CD cavity. A mechanism is proposed to explain the inclusion process.     

Conformational properties of glucosyl-thioglucosides and their S-oxides in solution

A stereochemical study of a series of alkyl glucosyl-β-(1→6)-thioglucosides and their S-oxides by means of nuclear magnetic resonance and circular dichroism revealed that the populations around the thioglucosidic bond (ring I) as well as those of the interglycosidic linkage ω depend on the aglycone, the solvent and, in the S-oxides, on the absolute configuration of the sulfur atom. The results for the thio-disaccharides showed the strong influence of the solvent polarity on the conformational preferences of the interglycosidic bond. In polar solvents, the magnitudes of the rotamer populations, P_gg and P_gt, remained practically constant through the series, while in non-polar solvents a clear predominance of gt conformation was observed as well as the influence of the aglycone on the conformational equilibrium. The results for both (S_S)- and (R_S)-alkyl thiogentiobiosyl S-oxide series showed a clear predominance of the gt rotamer, P_gt always having a higher magnitude in the latter series than in the former. Both series exhibited linear correlations between interglycosidic P_gg and P_gt and Taft’s steric parameter (E_S) for the alkyl group attached to the sulfinyl group, especially in non-polar solvents. The stereochemical study around the C1–S bond established that the flexibility around this linkage depends on aglycone size, solvent polarity, and the absolute configuration of the sulfur, derivatives of the S_S series showing higher flexibility in both polar and non-polar media. The conformational properties of these compounds in solution are explained in terms of stereoelectronic, steric, and solvent effects.     

Solvophobically driven self-association of a butadiyne-bridged pyridine macrocycle

A butadiyne-bridged square-shaped pyridinophane 2b having ester groups derived from triethylene glycol monomethyl ether was prepared and its self-assembling properties were investigated to assess the repulsive dipole-dipole interaction in less polar solvent and solvophobic driving force in polar solvents. Concentration-dependent ¹H NMR spectra showed that whereas 2b did not self-assemble in chloroform-d, it did in polar solvents consisting of chloroform-d and methanol-d₄ (4:6 to 6:4) with increasing association constants with the increasing methanol-d₄ composition. Comparison of the free energies of association with those of the previously reported benzene macrocycle 1b indicates that solvophobically induced self-association of 2b is more promoted than 1b. The reasons for this difference are discussed from both enthalpy and entropy points of view.     

Solvent effect on radical homo- and copolymerization of di-2-[2-(2-methoxyethoxy)ethoxy]ethyl itaconate

Solvent effect on homo- and copolymerization of di-2-[2-(methoxyethoxy)ethoxy]ethyl itaconate (DMEI) was studied at 50 °C using dimethyl 2,2^′-azobisisobutyrate as radical initiator. The polymerization rate (R_p) highly depended on the kind of solvent; 19 solvents were used. The highest R_p (in 1-tetradecanol) is 13 times the smallest (in chloroform). On the other hand, the solvents did not exert as great an effect on the molecular weight of the resulting polymers. The propagation rate constant (k_p) was determined in 15 different solvents by means of ESR spectroscopy. The highest k_p (4.5 l/mol s in toluene) is 5.6 times the lowest (0.8 l/mol s in chloroform). A noticeable solvent effect was also observed in the copolymerization of DMEI (M₁) and styrene (M₂), where nine solvents were used. The highest r₁ (0.46 in 1-butanol) is about 6 times the lowest (0.08 in methanol). The r₂ value falls in the range of 0.2 (dimethyl sulfoxide) and 0.52 (benzene). The solvent effects thus observed were analyzed according to the linear solvation energy relationship proposed by Taft and co workers.     

Ultrasensitive fluorescent probe for the hydrophobic range of solvent polarities

The new 3-hydroxychromone derivative 2-(6-diethylaminobenzo[b]furan-2-yl)-3-hydroxychromone (FA) displays a dramatic solvent-dependent transformation of fluorescence spectra in the range of low-polarity solvents. The two well-separated emission bands change their relative intensities so that the short-wavelength band being of a very low intensity in hexane becomes dominant in the more polar ethyl acetate and trichloromethane. We suggest the participation in this effect of excited-state intramolecular proton transfer, which is characteristic for other 3-hydroxychromone and 3-hydroxyflavone derivatives, in the range of solvents of much higher polarities. Because of these unique properties, a number of spectroscopic parameters (positions of absorption and two fluorescence maxima, the ratio of their intensities and the fluorescence quantum yield) can be measured in this solvent range with multiparametric analysis of the data. In terms of solvent polarity, the shifts in both emission bands and their intensity ratio demonstrate a good correlation with empirical polarity scales E_T^N, P_y and SPP, while the absorption spectra reveal some deviations for the tested oxygen-containing solvent molecules. A good cross-correlation is observed between fluorescence spectral shifts and the ratio of band intensities. The latter provides the means for a dramatic amplification of solvent response. Thus, a new approach for ultrasensitive scaling and probing the solvent polarity in the low-polararity range can be suggested. It involves very simple ratiometric measurements at two emission bands and can be posed for a variety of applications. We present examples of these applications for distinguishing of polarities between methylated benzene derivatives, for quantitative assay of polar impurities in low-polar solvents and for detection of the changes of solvent polarity as a function of temperature.     

Solvent effects in the aza-Michael addition of anilines

The aza-Michael addition of functionally substituted anilines to enoates is significantly affected by the hydrogen bond donor ability of the solvent. Very polar protic fluorinated alcohols (hexafluoroisopropanol and trifluoroethanol) favor the addition of weak nucleophiles, whereas these solvents are no longer required for anilines bearing additional OH or NH₂ group.     

Spiropyran-azobenzene-DBU system as solvent indicator

A dyad bearing azobenzene and spiropyran units was synthesized and its applications in indicating the polarity and protic or aprotic properties of a solvent were explored. The spiropyran-azobenzene derivative (SPAB) can be induced to different forms in different miscellaneous solvents accompanied with different color changes and spectral characteristics at the presence of organic base DBU. In a nonpolar or low-polar solvent, SPAB exists in thermostable spiropyran form with yellow color output. While in an aprotic polar solvent, the spiropyran part isomerized to merocyanine form giving a blue color. When SPAB is subjected to a protic solvent, the alkylation reaction occurs at the oxygen generating the alkylated-SPAB with red color. This solvent-dependent property can be used for discriminating solvent type.     

Nucleophile-solvent isotope effects between methanol isotopomers during the interception of aziridinium imide-‘like’ closed intermediates

Deuterated methanol isotopomers were found to compete efficiently with normal methanol during the interception of an intermediate with structural characteristics of the aziridinium imide, formed in the reaction of N-phenyltriazolinedione with simple alkenes such as 2-methylpropene, 2-methyl-2-butene, and 2,3-dimethyl-2-butene. In general, a (trideuterio)methyl-group bearing methanol was found to add at the tertiary carbon atom of the intermediate more efficiently with regard to hydrogen isotopomeric methanol, and this result is explained in terms of the nucleophile-solvent isotope effect in an S_N2-‘like’ transition state of solvent addition to the intermediate.     

Unique solvent effects on selective electrochemical fluorination of organic compounds

Recent studies on solvent effects on electrochemical partial fluorination are reviewed. At first, the historical background and some problems of electrochemical fluorination in organic solvents like acetonitrile (MeCN) are briefly mentioned. Ethereal solvents like dimethoxyethane (DME) and a mixture of DME and MeCN were found to improve both the yield and current efficiency for electrochemical fluorination since these solvent systems effectively suppress anode passivation and overoxidation of fluorinated products once formed during the electrolysis. It was also found that DME stabilizes radical cationic intermediates of 4-arylthio-1,3-dioxolan-2-ones and 3-phenylthiophthalide leading to α-fluorination while dichloromethane (CH₂Cl₂) destabilizes them leading to fluorodesulfurization. On the other hand, imidazolium ionic liquids and liquid fluoride salts like Et₄NF·4HF and Et₃N·5HF exhibited similar effects to CH₂Cl₂. Selective fluorination of hardly oxidizable phthalide was also achieved using a combination of two kinds of ionic liquids (imidazolium triflate and liquid fluoride salts).     

Photochromic polypeptides

Two photochromic polypeptides were synthesized by reaction of 1-(4-iodobutyl)-3,3- dimethylindolindolino-6′-nitrobenzospiropyran with poly-L -tyrosine; their molar contents on photochromic units were 27.3 and 44.7%. The spectra of the photo-induced merocyanines and their decoloration kinetics were compared with these of the monomeric model compound, obtained by reaction of the same N-(4-iodobutyl)-indolinospiropyran derivative with N-acetyltyrosine methyl ester. Different types of solvents have been examined, mainly dimethylformamide and pyridine, acetone and tetrahydrofuran, and methanol and ethylene glycol. The polypeptides showed a much less pronounced solvatochromism than their model; on the other hand, their absorption spectra presented two absorption maxima instead of one for the model. These differences in photochromic behavior were interpreted on the basis of the solvatation of the polymeric chain. Inverse photochromism was observed for polypeptide P₂ as well as for the model in ethylene glycol solution; this effect is due to a higher merocyanine content at the thermal equilibrium spiropyran ? merocyanine in high polar solvent.