Solvation by Glycerol at Temperatures from 353 to 77 K: Its

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Solvation by Glycerol at Temperatures From 353 to 77 K: Its Solvatochromic Characterization and Use to Block the Molecular Structure of Conformationally Flexible Structures Javier Catalan, and Christian Reichardt J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b06027 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 1, 2017

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Solvation by Glycerol at Temperatures from 353 to 77 K: Its Solvatochromic Characterization and Use to Block the Molecular Structure of Conformationally Flexible Structures Javier Catalán*† and Christian Reichardt‡ †

Departamento de Química Física Aplicada, Universidad Autónoma de Madrid, 28049 Madrid, Spain.

‡

Fachbereich Chemie, Philipps-Universität, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany

S Supporting Information ______________________________________________________________________ ABSTRACT: Glycerol is UV/Vis spectroscopically characterized by using suitable solvatochromic polarity probes spanning a wide temperature range of 353 to 77 K. For the first time we find experimental evidence that, when the solvent preserves its internal structure in a broad range of temperatures, all solvatochromically derived solvent parameters (i.e. SP, SdP, SA, and SB) also maintain their values unchanged. Based on these solvatochromic measurements, it is shown that below 180 K glycerol efficiently blocks the molecular structure of conformationally flexible solutes. ______________________________________________________________________

1 INTRODUCTION Propane-1,2,3-triol, also known as glycerol and glycerin, is an easily polarizable solvent in solvatochromic terms; it is also highly dipolar, substantially acidic, but only moderately basic.1 Hence, as solvent it behaves quite similar to methanol.1 However, glycerol possesses some unique physical properties not present in common molecular solvents, namely: (1) it has an unusually high melting (291 K 2) and boiling point [563 K/1013 hPa (dec.) 2]; (2) it occurs as a less volatile (vapor pressure p = 0.33 Pa at 323 K) and non-flammable liquid (flash point = 433 K) 2; (3) it is colorless and transparent with a very low UV cut-off point (λ = 205 nm); (4) it is a liquid of highly variable viscosity ranging from η = 31.9 mPa · s (at 353 K) to 1412 mPa · s (at 293 K) 3, but 1 ACS Paragon Plus Environment


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below 291 K it is a glassy solid the viscosity of which reaches η ≈ 1013 mPa · s at 184 K 4

; (5) it has a comparatively high surface tension of σair/glycerol = 63.4 mN · m‒1 (at 20 °C)

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; and (6) it has a relatively large permanent dipole moment of µ = 2.56 D at 313 K (in

1,4-dioxane) and a comparatively large relative permittivity of εr = 42.5 at 298 K.2 Glycerol also has exceptional chemical and biochemical properties. Thus, it is non-toxic [LD50 (oral rat) = 12 600 mg/kg] and biodegradable, and an excellent reaction medium suitable for its use as a green solvent.6–11 Together with its low cost and ready availability from the vegetable oil industry, the properties of glycerol make it a highly attractive choice for biodiesel and for pharmaceutical productions. The high viscosity of glycerol and its temperature-sensitivity can undoubtedly facilitate the spectroscopic examination of solutes. Thus, a glycerol solution prepared at room temperature ‒ or at a higher temperature if required by the particular solute ‒ can be cooled down in such a way that the solute will hardly precipitate; rather, it will remain dissolved because the viscosity of the medium increases as the temperature is lowered. This property is especially important considering that, according to Schulz,12 the density of glycerol increases linearly with decreasing temperature from 313 to 200 K, but remains virtually constant below 200 K. Therefore, glycerol must become increasingly compact, and the space between its molecules shrinks in going from 313 to 200 K. Below 200 K, however, volume contraction stops and glycerol remains in a crystalline state where its molecules are completely blocked. One can therefore wonder, if lowering the temperature of a dilute glycerol solution below 200 K, would also lead to a blocking of the solute molecules within the crystal lattice adopted by the solvent. Does the outcome depend on the molecular structure and conformational flexibility of the solute? The high spectroscopical interest in the use of glycerol as solvent at temperatures from 353 to 77 K warrants examination of its general (polarizability and dipolarity) and specific interactions with solutes (HBD acidity and HBA basicity) in this temperature range. As now widely accepted,13 solvents can be characterized by solvatochromically derived parameters in terms of specific properties such as their HBD 2 ACS Paragon Plus Environment

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acidity (SA)14,15 and HBA basicity (SB),16 and of general properties such as polarizability (SP)17 and dipolarity (SdP).1 The corresponding values for glycerol at 293 K are as follows: SA = 0.653, SB = 0.309 (a low value), SP = 0.828 (an intermediate value), and SdP = 0.921 (a high value). One should bear in mind that these scales were constructed with media spanning values from 0.000 for the gas phase (i.e., in the absence of solvation) to 1.000 for a solvent with a high level of the property concerned, i.e., dipolarity of DMSO (SdP), basicity of tetramethylguanidine (SB), and polarizability of carbon disulfide (SP). The values of these solvent properties can be determined by using appropriate solvatochromic molecular probe dyes. A solvent with the particular acid/ base properties of glycerol can be expected to hinder the dissolution of a widely used aromatic polarity probe such as 2-(dimethylamino)-7-nitrofluorene (DMANF).18 This requires using an alternative probe of similar electronic characteristics but of smaller size (e.g., 1-methyl5-nitroindoline, MNI) to facilitate its dissolution in glycerol in order to measure its dipolarity at variable temperatures; anthracene1 to measure its polarizability; 3,6diethyltetrazine (DETZ)15 for its HBD acidity; and the 5-nitroindoline(NI)/1-methyl-5nitroindoline (MNI) probe/homomorph couple for its HBA basicity.16 Scheme 1 shows the molecular structures of these probes. In this work, we determined the polarizability (SP), dipolarity (SdP), HBD acidity (SA), and HBA basicity (SB) of glycerol at decreasing temperatures from 353 to 77 K (∆T = 276 K). We also examined the UV/Vis spectroscopical behavior of conformationally more flexible chromophores such as the negatively solvatochromic pyridinium-N-phenolate betaine dye B30 (sometimes also called “Reichardt’s betaine dye”) and the α-amino acid L-tryptophan. Scheme 2 shows the molecular structures of these chromophores.

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2 EXPERIMENTAL SECTION Glycerol (99.9% pure), L-tryptophan (>99% pure), and anthracene (refined by zone melting) were purchased from Sigma-Aldrich. 5-Nitroindoline (NI) was also obtained from Sigma-Aldrich and carefully purified by silica-gel column chromatography using dichloromethane/n-hexane (6:4 by volume) as eluent. 1-Methyl-5-nitroindoline (MNI) was synthesized as described elsewhere18 and purified by silica-gel column chromatography using an n-hexane/dichloromethane/ethyl acetate mixture (55:30:15 by volume) as eluent. 3,6-Diethyltetrazine (DETZ) was also obtained as described elsewhere.21 2,6-Diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (betaine dye B30) is available from Sigma-Aldrich. The spectroscopic measurements with temperature control were carried out as described earlier in ref. 20.

3 RESULTS AND DISCUSSION We initially assessed the four solvatochromic parameters of glycerol (i.e., for its polarizability, dipolarity, HBD acidity, and HBA basicity) from 353 to 77 K in steps of 10 K, and then examined their changes, with provision of the fact that below 200 K it is a crystalline solid with temperature-independent density. The resulting values will be anchored, taking as reference the values at 293 K for consistency with our solvatochromic scales.1 Finally, we examined whether two conformationally highly flexible compounds (i.e., the betaine dye B30 and the α-amino acid L-tryptophan) retain their molecular fine structure in glycerol throughout the same temperature range. 3.1 Solvatochromic parameters of glycerol. The polarizability (SP) of glycerol was calculated from the frequency of the maximum of the 0–0 component of the first UV/Vis transition of anthracene, dissolved in glycerol, at each temperature studied, by means of eq. 11 in ref. 1. The results are shown in Table 1. The frequency of the absorption maximum of the first UV/Vis transition of MNI was used to calculate the value for DMANF, using eq. (1) in ref.

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The previously

calculated SP value, and that obtained from eqs. 3 and 4 in ref. 1, were used to calculate 4 ACS Paragon Plus Environment

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the dipolarity (SdP) of glycerol at each temperature level. The results are also shown in Table 1. The frequency of the absorption maximum of the first UV/Vis transition of NI and MNI, as calculated from the expression given in a caption to Table 1 in ref. 16, was used to calculate the HBA basicity (SB) of the solvent at each temperature studied. The results are given in Table 1. Finally, the frequency of the absorption maximum of the first UV/Vis transition of the probe DETZ, as calculated from equations in ref.

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, was used to calculate the

HBD acidity (SA) of glycerol at each temperature level. These results are collected in Table 1. 3.2 Solvatochromic behavior in glycerol as solvent.

Figures 1a-d show the

variation of the polarizability, dipolarity, HBD acidity, and HBA basicity of glycerol over the temperature range of 353 to 77 K. As can be seen, all four parameters exhibit a plateau below 180 K where they remain nearly constant, in addition to two segments where they change linearly with temperature. However, the slope of the segment where the solvent is still a liquid is different from that where it is a solid. Interestingly, lowering the temperature increases the polarizability, acidity, and basicity of glycerol, but decreases its dipolarity. In any case, the changes are quite small: only 0.03 SP units, 0.07 SA units, 0.14 SdP units, and 0.17 SB units. The fact that the solvatochromic parameters for glycerol remain practically constant below 180 K confirms the previous finding of Schulz12 that its density is constant below 200 K. Clearly, this evidence indicates that, below 200 K, glycerol molecules cannot further approach to other ones, so the solvation behavior of the solvent can hardly change below that point. It would be interesting to see whether this behavior can also be found with other suitable chromophores dissolved in glycerol. The finding that the values of SP, SdP, SA, and SB are practically constant below 180 K clearly indicates that, if the solute/solvent distances do not change over a wide range of temperatures, the corresponding values of the solvatochromic parameters are also kept unchanged. 5 ACS Paragon Plus Environment


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3.3 Solvatochromic behavior of structurally rigid chromophores. Figure 2 shows the temperature dependence of the frequency of the 0–0 component of the first UV/Vis transition for a structurally rigid chromophore such as anthracene (see Scheme 1). The results are unequivocal: below 180 K, the energy of the electronic transition remains constant, i.e., anthracene retails its molecular structure and so does its environment consisting of glycerol molecules. As can be seen from Fig. 3, the results obtained for 1methyl-5-nitroindoline (MNI) afford a similar, albeit less distinct conclusion. Would this also be the case with structurally more flexible chromophores?

3.4

Solvatochromic behavior of structurally more flexible chromophores:

Pyridinium-N-phenolate Betaine dye B30 and L-Tryptophan. 3.4.1 Betaine dye B30. B30 bears five peripheral phenyl groups on its central pyridinium-N-phenolate chromophore which can easily change their torsional angle in both the electronic ground (S0) and first excited state (S1) (Scheme 2). The torsion of the five phenyl rings on the betaine chromophore of B30 and the flexible interplanar angle between pyridinium and phenolate moiety testify its structural flexibility.20,23 According to Kharlanov and Rettig,23 the electronic excitation of B30 leads to an S1 structure in which phenolate and pyridinium moiety adopt an orthogonal arrangement, the energy of which is so close to that of the Franck-Condon ground state that it triggers very efficiently the radiationless processes preventing B30 from fluorescing at room temperature. However, B30 dissolved in 1-chlorobutane (ClB) emits fluorescence at lower temperatures such as 77 K, where the phenolate and pyridinium rings cannot lay regularly to each other.23 Our group showed that B30 in its electronic ground state in the gas phase exists with three different conformational structures constituting true energy minima.20 Electronic excitation dramatically alters their geometry leading to pyramidalization at the nitrogen atom. The structural deformation is so large that this pyramidalization is 6 ACS Paragon Plus Environment

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highly sensitive to the viscosity of the medium. We showed that B30 dissolved in ClB exhibits fluorescence not only at 77 K but also at any temperature below the melting point of ClB, i.e.,

Solvation by Glycerol at Temperatures from 353 to 77 K: Its

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