Understanding Solvation: Comparison of Reichardt's Solvatochromic

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Understanding Solvation: Comparison of Reichardt’s Solvatochromic Probe and Related Molecular “Core” Structures Paulo A. R. Pires,† Omar A. El Seoud,*,† Vanderlei G. Machado,*,‡ Jeś sica C. de Jesus,† Carlos E. A. de Melo,‡ Jonatan L. O. Buske,‡ and Amanda P. Cardozo‡ †

Institute of Chemistry, University of São Paulo, Prof. Lineu Prestes Av., 748, 05508-000, São Paulo, São Paulo, Brazil Department of Chemistry, Federal University of Santa Catarina, P.O. Box 476, 88040-900, Florianópolis, Santa Catarina, Brazil

‡

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S Supporting Information *

ABSTRACT: The compound 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, p-RB, shows distinct colors in different solvents (solvatochromism). The compound 4(pyridinium-1-yl)phenolate, p-CB, represents the part of pRB which is responsible for this phenomenon. We compared the solvatochromism of both compounds and also the structurally related 2-(pyridinium-1-yl)phenolate, o-CB, and (2,4-dimethyl-6-(2,4,6-triphenyl-N-pyridinium-1-yl)phenolate, o-RB. In pure solvents, plots of the empirical solvent polarity parameter [ET(probe), kcal/mol] of the different probes correlate linearly with slopes close to unity. That is, these probes are similarly sensitive to specific and nonspecific interactions with the solvents. The solvatochromism of p-CB and o-CB was studied, for the first time, in binary mixtures of water with dimethyl sulfoxide (DMSO) and 1-propanol (1-PrOH). The dependence of ET(probe) on mixture composition was nonideal due to preferential solvation of the probe by one component of the binary solvent mixture. We treated our solvatochromic data using a solvent-exchange model that considers formation of the complex solvents [HOH···OS(CH3)2] and [HOH···O(H)-C3H7]. The model applies satisfactorily to our data and shows the importance to solvation of hydrogen-bonding and hydrophobic interactions. The preferential solvation of (more hydrophobic) p-RB is more pronounced than that of p-CB or o-CB. The solvent complex [OH2···O(H)-C3H7] is more efficient than [OH2··· OS(CH3)2] because of more possibilities of hydrogen bonding.

1. INTRODUCTION

property of these compounds is that they absorb light of different wavelengths (λ), hence acquiring distinct and vivid colors in different solvents, as shown in Figure 2. This phenomenon is called solvatochromism (effect of the solvent on the color of a compound, hereafter designated as “probe”). The probe p-RB is a member of a series of structurally related compounds introduced by Dimroth and co-workers; its number in the series synthesized was 30.2 Later, Reichardt et al. introduced structural modifications in these probes to make their use more general. Examples are substitution of the phenyl groups by a tert-butylphenyl counterpart to make the probe soluble in nonpolar solvents, e.g., aliphatic hydrocarbons, and introduction of a charged group (carboxylate) to make the probe water-soluble.3 The reason for the solvatochomism of these compounds is that absorption of light of the appropriate λ transforms the zwitterionic probe into a diradical. The time for this excitation (ca. 10−15 s) is, however, much shorter than the time (ca. 10−12

Figure 1 depicts the molecular structures of the compounds of interest in the present work. A very interesting and useful

Figure 1. Molecular structures of the solvatochromic probes of interest in the present work. They refer to Reichardt’s betaine dye (2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate), p-RB; 4(pyridinium-1-yl)phenolate, p-CB (CB = core betaine); 2-(pyridinium-1-yl)phenolate, o-CB; (2,4-dimethyl-6-(2,4,6-triphenylpyridinium-1-yl)phenolate, o-RB.1 © XXXX American Chemical Society

Special Issue: Latin America Received: December 11, 2018 Accepted: March 6, 2019

A

DOI: 10.1021/acs.jced.8b01192 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 2. Examples of solvatochromism of three probes dissolved in 1-propanol (1-PrOH) and dimethyl sulfoxide (DMSO).

from unity. In solvent mixtures, the dependence of ET(probe) on solvent composition is usually nonideal, i.e., nonlinear due to the so-called preferential solvation (PS) of the probe by one component of the mixture. There is no information on the solvation of p-CB and o-CB in binary solvent mixtures. Because the phenomenon of PS is probe- and solvent-dependent, we compared the solvatochromism of these two compounds in mixtures of water with protic and aprotic solvents, 1-propanol (1-PrOH) and dimethyl sulfoxide (DMSO), respectively. In aqueous mixtures of both organic solvents, the relatively hydrophobic p-RB shows more pronounced PS than o-CB and p-CB.

s) necessary for solvent dipoles/molecules to rearrange to solvate the excited state. Consequently, the medium essentially stabilizes the electronic ground state. The molar transition energy of the intramolecular charge-transfer (from phenolate oxygen → quaternary nitrogen) is given by eq 1: E T(probe) (kcal/mol) = 28591.5/ λmax (nm)

(1)

where λmax is the wavelength maximum of the charge-transfer band of the probe and ET(probe) is the empirical solvent polarity parameter. Dipolar solvents stabilize more the probe ground state, hence leading to a larger ET(probe), i.e., shorter λmax. This inverse relationship between empirical solvent polarity and the value of λ max is termed “negative solvatochromism.” ET(probe) is the sum of solvent descriptors as shown by eq 2: E T(dye) = E T(dye)0 + aSA + bSB + dSD + pSP

2. EXPERIMENTAL SECTION 2.1. Material. All solvents were purchased from Aldrich, Merck, or Synth (São Paulo) and were purified, when required, as recommended elsewhere;4,5 they were stored on activated type 3 Å molecular sieves. We used deionized water in all measurements. 2.2. Equipment. Infrared spectra were recorded with Shimadzu Prestige-21 and Bruker Vector 22 FTIR spectrophotometers; KBr pellets were used. UV−vis measurements were recorded with HP 8452A and Shimadzu UV-2550 UV−vis spectrophotometers equipped with thermostated cell compartments whose temperature was constant at ±0.1 °C (model 4000A digital thermometer, Yellow Springs Instrument). Unless otherwise stated, we used quartz cuvettes with PTFE stoppers and 1 cm optical path length. 1 H NMR spectra were recorded on a Bruker DRX500 NMR spectrometer operating at 500.13 MHz for 1H. All spectra were recorded at 25 °C. Data for p-CB and o-CB are reported as follows: chemical shift from TMS, peak multiplicity (d = doublet, t = triplet, dd = double doublet, tt = triple triplet, td = triple doublet, m = multiplet), coupling constants (in Hz), and integration. The attribution of these hydrogen atoms was based on COSY and HSQC spectra, see item SI-2 (item 2 of Supporting Information). We show the atom numbering in Figure 3. High-resolution mass spectra were obtained with an electrospray ionization-quadrupole time-of-flight mass spectrometer (HR ESI-MS QTOF). 2.3. Synthesis of p-CB and o-CB. We synthesized these probes as given elsewhere,6 with some modifications, according to Scheme 1. Details of the synthesis of p-CB and o-CB are given in section SI-1; we list below their spectroscopic data. 4-(Pyridinium-1-yl)phenolate, p-CB. IR (KBr, υ̅ max/cm−1): 3285 (O−); 1578 (CC); 1482 (CC); 1305 (N−C); 852

(2)

where SA, SB, SD, and SP refer to Lewis acidity, Lewis basicity, dipolarity, and polarizability of the solvent, respectively. The regression coefficients (a, b, d, and p) refer to the relative contribution of the solvent descriptors to the overall empirical solvent polarity. Accordingly, we use solvatochromism to get information about the relative contribution of solute−solvent interactions (e.g., hydrogen bonding and van der Waals interactions) and chemical phenomena (e.g., reactivity).3 In Figure 1, we marked in blue color the “core” part of p-RB, responsible for the intramolecular charge-transfer, i.e., for the observed solvatochromism. This part is identical to the probe 4-(pyridinium-1-yl)phenolate, hereafter designated p-CB (core betaine, phenolate oxygen in the para position relative to the quaternary nitrogen). This probe was synthesized and its solvatochromic response examined in a limited number of solvents.3 We decided to compare the solvatochromism of pCB and p-RB because the former is less sterically crowded. We were interested in assessing the effect of this difference on the solvatochromic response. Due to the insolubility of p-CB in several solvents, vide infra, we synthesized the structurally related compound o-CB of Figure 1. By analogy to p-RB and p-CB, we include in the present study published solvatochromic data of 2,4-dimethyl-6(2,4,6-triphenylpyridinium-1-yl)phenolate (o-RB). Note that oCB represents the “core structure” of o-RB; the latter probe is structurally related to p-RB, except that the phenolate oxygen ring carries two methyl groups instead of two phenyl rings. We correlated the solvatochromic data of these probes. All correlations were found to be linear, the slopes were not far B



Article

and C11), 133.69 (C4), 134.22 (C1), 146.17 (C10), 147.40 (C8 and C12), 160.78 (C2). HMRS m/z: 172.0759 [M + H]+. Calculated for C11H10NO: 172.0757. 2.4. 1 H NMR Study of the Solvation of the Synthesized Probes. The solvents used were CD3OD and CD3CN (Cambridge Isotope Laboratories) without TMS. The spectra were calibrated using the residual solvent 1H peak, whose chemical shifts relative to TMS are listed elsewhere.7 2.5. Sample Preparation for Solvatochromic Measurements. A 5.8 × 10−3 mol L−1 stock solution of each probe was prepared in anhydrous methanol. From this stock solution, 28 μL was transferred to 5 mL-volumetric flasks. After evaporation of methanol, the probes were dissolved in the pure solvent, yielding a solution with a final dye concentration of 8.0 × 10−5 mol L−1. The binary mixtures of DMSO-water and 1-PrOH/water were prepared by weight at 25 °C to cover a water mole fraction (χW) range from 0.10 to 0.95. The densities of these solutions were determined at 25 °C with a digital resonating density meter (Anton Paar model DMA 4500M). We recorded each spectrum three times at a resolution of 0.2 nm and determined the values of λmax from the first derivative of the absorption spectrum. The uncertainty in ET(probe) (calculated from eq 1) is 0.04 kcal mol−1 (spectral range = 400−500 nm). 2.6. Quantum Chemical Calculations. Initial optimization, by DFT, of the geometry of the probes was done using the B3LYP hybrid functional and 6-311++g(d,p) basis set. Gaussian-4 (G4) theory was used,8 as implemented in the Gaussian 09 program,9 to perform scans of the interplanar angle of C2−C1−N7−C8 of o-CB and p-CB. All quantumchemical calculations were done with the probes solvated in CH3OH and DMSO, using the Polarizable Continuum Model (PCM).10

Figure 3. Atom numbering of o-CB and p-CB.

Scheme 1. Synthesis of p-CB and o-CBa

3. RESULTS AND DISCUSSION 3.1. Comparison of the Solvatochromic UV−Vis Absorption Data of the Probes p-CB, p-RB, o-CB, and o-RB. Table 1 shows values of ET(probe); we added more solvents for p-CB and o-CB than previously reported.6 The correlations between values of the different ET(probe) are shown in Figure 4, and the results of the corresponding linear regression equations are listed in Table 2, after removing the “worst offender” points/solvents. The relevant points are that the solvatochromic data correlate linearly and the slopes are close to unity for structurally related probes. That is, the interactions of all these probes with solvents are qualitatively and practically quantitatively similar. The probe p-CB is more sensitive to solvent effect than o-CB presumably because of the position of the phenolate oxygen relative to the quaternary nitrogen. 3.2. Rationale for the unexpected limited solubility of p-CB. We suggest the following reason for the limited solubility of p-CB relative to o-CB in the solvents studied: the solvation free energy of p-CB is less favorable than its crystal lattice energy. This reasoning is the inverse of that advanced for explaining the liquid state of many ionic liquids (ILs); namely, the large size and conformational flexibility of the IL ions lead to small crystal lattice enthalpies and large entropy changes that favor the liquid state.12 Based on the thermodynamic cycle shown in section SI-3, we were able to calculate the free energy of dimerization (gas phase) and the free energy of solvation of the monomer and the dimer of p-CB and o-CB. The calculated values for

a (a) 80 °C, 10 min; (b) 4-aminophenol, anhydrous ethanol, reflux, 15 h, HClO4; (c) 2-aminophenol, anhydrous ethanol, reflux, 15 h, HClO4; (d) methanol, 65 °C, KOH.

(para-substituted ring). 1H NMR (500 MHz, CD3OD, δ/ ppm): 6.73 (d, J3,2 = 9.1 Hz, 2H, H3 and H5), 7.37 (d, J2,3 = 9.1 Hz, 2H, H2 and H6), 8.12 (t, J9,8 = 6.8 Hz, J9,10 = 7.4 Hz, 2H, H9 and H11), 8.55 (tt, J10,8 = 1.4 Hz, J10,9 = 7.4 Hz, 1H, H10), 9.05 (dd, J8,9 = 6.8 Hz, J8,10 = 1.4 Hz, 2H, H8 and H12). 13C NMR (125 MHz, CD3OD, δ/ppm): 121.31 (C3 and C5), 125.78 (C2 and C6), 129.34 (C9 and C11), 130.99 (C1), 144.97 (C8 and C12), 145.15 (C10), 172.98 (C4). HMRS m/z: 172.0758 [M + H]+. Calculated for C11H10NO: 172.0757. 2-(Pyridinium-1-yl)phenolate, o-CB. IR (KBr, υ̅max/cm−1): 3242 (O−); 1592 (CC); 1493 (CC); 1362 (N−C); 746 (ortho-substituted ring). 1H NMR (500 MHz, CD3OD, δ/ ppm): 6.64 (td, J5,6 = 7.9 Hz, J5,4 = 7.3 Hz, J5,3 = 1.3 Hz, 1H, H5), 6.92 (dd, J3,4 = 8.3 Hz, J3,5 = 1.3 Hz, J3,6 = 0.4 Hz, 1H, H3), 7.26−7.30 (m, J4,3 = 8.3 Hz, J4,5 = 7.3 Hz, J4,6 = 1.7 Hz, 1H, H4, partially overlapped with H6), 7.28−7.31 (m, J6,5 = 7.9 Hz, J6,4 = 1.7 Hz, J6,3 = 0.4 Hz, 1H, H6, partially overlapped with H4), 8.13 (t, J9,10 = 7.9 Hz, J9,8 = 6.9 Hz, 2H, H9 and H11), 8.59 (tt, J10,9 = 7.9 Hz, J10,8 = 1.4 Hz, 1H, H10), 8.95 (dd, J8,9 = 6.9 Hz, J8,10 = 1.4 Hz, 2H, H8). 13C NMR (125 MHz, CD3OD, δ/ppm): 115.28 (C5), 123.04 (C3), 126.24 (C6), 128.80 (C9 C



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Table 1. Values of ET(dye) in kcal mol−1 for Compounds pCB and o-CB in 30 Solvents entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

solvent acetone acetonitrile benzyl alcohol 1-butanol tert-butanol 2-(n-butoxy)ethanol chloroform 1-decanol diethyl ether 1,2-dimethoxyethane dimethylcarbonate 1,2-dichloroethane dichloromethane N,Ndimethylacetamide N,Ndimethylformamide dimethyl sulfoxide ethanol ethyl acetate 2-ethoxyethanol ethylene glycol methanol 2-methoxyethanol 1-methyl-2pyrrolidinone nitromethane 1-octanol 1-pentanol 4-picoline 1-propanol tetrahydrofuran water

ET(pRB)a

ET(pCB)

ET(oRB)c

ET(oCB)

42.2 45.6 50.4 49.7 43.3 50.0 39.1 47.7 34.5 38.2 38.2 41.3 40.7 42.9

60.4 61.6 68.2 66.2 63.4 66.3

45.0 46.0

56.0

60.2 59.9 67.9 66.8 63.4 68.2 56.0 67.3 58.7 56.0 56.1 59.9 56.5 57.2

43.2

56.4

58.1

45.1 51.9 38.1 51.0 56.3 55.4 52.0 42.2

58.8 68.9

46.3 48.1 49.1 39.5 50.7 37.4 63.1

b b b b b b b

43.9

44.7 53.9

b

67.7 73.9 71.8 69 69.2 b b

65.7 b

67.3 b

77.7

57.2

Table 2. Results of the Linear Correlations between the Different ET Values of the Four Probes equation

Na

α

β

R2

ET(o-CB) = α + β·ET (oRB) ET (p-CB) = α + β·ET (pRB) ET (p-CB) = α + β·ET (oCB) ET (o-RB) = α + β·ET (pRB)

6

12 ± 4

1.01 ± 0.09

0.95795

15

19 ± 4

0.96 ± 0.08

0.90111

15

−9 ± 4

1.15 ± 0.06

0.96426

11

15.8 ± 0.7

0.73 ± 0.05

0.95668

a

N = number of solvents considered in the correlation.

The favorable ΔGdimerization,gas‑phase raises the question whether this applies to the probes when they are solvated in polar solvents. We addressed this question experimentally, using the thermodynamic cycle shown in section SI-2. Using UV−vis cells with an optical path length from 0.5 to 10 cm, we recorded the spectra of p-CB in methanol (p-CB = 1 × 10−4 to 1 × 10−3 mol L−1) and DMSO (p-CB = 4 × 10−5 to 6 × 10−4 mol L−1). We observed no changes in the band-shape or the values of λmax (±0.5 nm) in both solvents. Unless that the dye dimer and monomer have the same spectral characteristics (unlikely), this result does not support a dimer formation of pCB in these two solvents. We corroborate this conclusion from the calculated values of ΔGdimerization,solvent and ΔGsolvation for both probes (see section SI-3 and Table 3). The former energy for both probes in the

59.3 68.1 58.8 68.4 72.5 69.5 68.8 67.0

Table 3. Calculated Dimerization Free Energies in Five Solvents (ΔGdimerization,solvent) and Free Energies of Solvation (ΔGsolvation) for the Monomeric and Dimeric Probes

65.7 65.4 66.8 55.1 67.2 55.3 75.6

ΔGdimerization,solven−1 t, kcal mol

a

Data taken from Reichardt and Welton.11 bProbe insoluble. cData taken from Paley et al.1

monomer ΔGsolvation, kcal mol−1

dimer ΔGsolvation, kcal mol−1

solvent

p-CB

o-CB

p-CB

o-CB

p-CB

o-CB

DMSO diethyl ether CH3CN CH3OH CH2Cl2

13.23 12.14

8.70 7.96

−12.23 −9.13

−7.14 −5.43

−7.07 −5.18

−4.64 −2.90

15.23 19.59 14.53

8.04 12.35 8.24

−8.20 −21.45 −11.08

−3.18 −14.67 −6.87

−1.17 −23.31 −7.63

1.68 −16.99 −5.50

solvents investigated are positive, i.e., unfavorable, more so for p-CB. Additionally, the order of |ΔGsolvation| is monomer > dimer; the only exception is methanol, in which dimer solubilization is slightly more favored. In summary, p-CB is not soluble in low polarity solvents probably because of its large crystal lattice energy. Once dissolved, p-CB and o-CB are present in their monomeric state. 3.3. Comparison of the Solvatochromic Data of the Probes p-CB, p-RB, and o-CB in Pure Solvents. Two equations are frequently employed to correlate ET(probe) with solvent descriptors, that of Kamlet et al.(not given)13−18 and that of Catalán et al.,19−22 eq 2. We use the latter because it separates solvent dipolarity from its polarizability. Table 4 shows the results of application of eq 2, where the worst offenders (solvents) were removed. The regression coefficients of Table 4 show that the solvatochromism of all probes is most sensitive to the solvent’s Lewis acidity. This result agrees with the 1H and 13C NMR data of the solvation of p-RB in CDCl3, DMSO-d6, and (CD3)2CO,23 with the result of an X-ray study of the p-RB hydrate,24 and with our 1H NMR data for p-

Figure 4. Correlations of ET(p-CB) (A) and ET(o-CB) (B) with ET(pRB).

ΔGdimerization,gas‑phase were −4.16 and −0.94 kcal mol−1, for p-CB and o-CB, respectively. If dimerization in the gas phase is taken as indication for the crystal lattice energy, then the latter is thermodynamically favorable for both probes; it is much largerin modulefor p-CB than for o-CB. D



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Table 4. Results of the Application of eq 2 to Solvatochromic Data of the Probesa probe

ET(dye)0

a

b

d

p

Nb

R2

p-RB p-CB o-CB

41.82 49.65 61.83

20.52 17.69 18.42

0.94 4.66 2.21

2.94 7.10 −2.09

−3.16 3.69 −2.83

24 10 21

0.93 0.98 0.85

a

See the text for the definition of the regression coefficients (a, b, d, and p). bN = number of solvents tested.

CB in CD3OD and CD3CN (Table 5). A change from the former to the latter solvent involves elimination of hydrogen Table 5. 1H NMR of p-CB Dissolved in CD3OD and CD3CN at 25 °C H atoms H2 and H6 H3 and H5 H8 and H12 H9 and H11 H10

δ/ppm, CD3OD

δ/ppm, CD3CN

Δδ/ppm, CD3OD

Δδ, ppm, CD3CN

7.368 6.734 9.048

7.404 6.920 8.818

1.680a

1.404a

8.118

8.069

1.384b

1.149b

8.548

8.509

Figure 5. Resonance forms electronic ground state (A,B), the excited state (C), as well as the molecular structure of the protonated form of p-CB (D).

calculated value for the phenolate ion (0.128 to 0.129 nm)28 and the X-ray based value for sodium phenolate (0.133 nm).29 Additionally, the C−N bond length is 0.146 ± 0.001 nm, similar to that of a C−N single bond (ca. 0.147 nm). Both results indicate that there is little, if any, conjugation in the ground state between the phenolate ring and the pyridinium ring, i.e., negligible contribution of form B of Figure 5. The (expected) resonance in the phenolate ring of p-CB and o-CB is shown by the calculated increase in the C−O bond length (by ca. 7%) on the protonation of these probes. We now address the C2−C1−N7−C8 interplanar angles, also listed in Table 6. These indicate (as expected) less twisting of the phenolate and pyridinium rings in p-CB relative to p-RB, and of o-CB relative to o-RB, as shown in Figure 6. The fact that this difference in geometry, e.g., between p-CB and p-RB, does not lead to differences in the response to solvent descriptors (Tables 1 and 2) is interesting and merits future investigation. 3.5. Solvatochromism of the Probes p-CB, p-RB, and o-CB in Binary Solvent Mixtures. As indicated above, the solvatochromism of p-CB and o-CB was not investigated previously in binary solvent mixtures. As shown repeatedly, the dependence of ET(probe) on the binary-solvent composition is usually not ideal, i.e., nonlinear because of PS of the probe by one component of the solvent mixture. The deviation from ideality depends on the probe and the solvent. For example, in aqueous binary mixtures of water with ILs, hydrophobic probes show more deviation from the ideal behavior than their less hydrophobic counterparts (in the same water−IL binary mixtures); solvatochromism of the same probe shows more deviation in binary mixtures of water with more hydrophobic ILs.30 We decided to assess this difference by studying the solvation of p-CB and o-CB in mixtures of water with a protic and an aprotic solvent, 1-PrOH and DMSO, respectively, and to compare these results with those of p-RB. Figure 7 shows the solvatochromic behavior of the probes in the aforementioned binary mixtures. The plots shown are for normalized ET(probe), ETN(probe), versus the mole fraction of water, χW, in the binary mixture. ETN(probe) is calculated using eq 3:

Δδ = δH8 − δH2. bΔδ = δH9 − δH3.

a

bonding to the phenolate oxygen, and introduction of shielding/deshielding of the probe’s hydrogens by the anisotropic triple bond of CD3CN. As a quantification of both factors is not feasible, we calculated the difference in δ between the hydrogens in the (electron-rich) phenolate ring and the (electron-deficient) pyridinium ring. As shown in Table 5, the orders of Δδ are ΔδCD3OD > ΔδCD3CN and ΔδH8−H2 > ΔδH9−H3. That is, the difference in chemical shifts (hence electron density) is larger in CD3OD than in CD3CN, certainly because of hydrogen bonding of the former solvent to the phenolate oxygen. Additionally, the hydrogen nuclei most affected by this (solvent) change are those close to both heteroatoms. A similar line of reasoning was used to explain the solvent influence on δ of p-RB.25 Theoretical calculations on the solvation of p-RB indicate that hydrogen bonding to the phenolate oxygen is strong, whereas there is little solvent interaction with the pyridinium nitrogen.26 Therefore, the susceptibility of these probes to solvent Lewis acidity is corroborated by our NMR data and theoretical calculations. 3.4. Further Discussion of Solvatochromism: Geometry and Resonance Forms of the Probes. As discussed above, the value of ET(probe) reflects the solvent influence on the energy of the intramolecular (phenolate oxygen → N+) charge transfer. This energy is affected by the geometry of the probe and the relative contributions of its resonance forms to the electronic ground state. In the following discussion, we dwell on the C−O and C−N bond lengths and then on the C2−C1−N7−C12 interplanar angles. Figure 5 depicts the resonance forms of the ground state of p-CB (A and B, respectively) and the diradical excited state (C) produced by UV−vis light absorption. Similar resonance forms can be drawn for o-CB. The contribution of form B to the groundstate energy can be assessed from the lengths of the C−O and C−N bonds shown in Table 6. For comparison, we also show the corresponding data of other probes. Focusing on bond lengths, the experimental and calculated values for C−O are 0.128 ± 0.001 nm, in agreement with the E



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Table 6. Calculated C2−C1−N7−C12 Interplanar Angles and C4−O and C2−O Bond Lengths of the Optimized Geometries of p-CB, o-CB, p-CB-H+, and o-CB-H+ in Methanol (MeOH) and Dimethylsulfoxide (DMSO)a o-RB

p-CB-H+

p-CB

o-CB-H+

Item

p-RB

o-CB

X-ray

X-ray

MeOH

DMSO

MeOH

DMSO

MeOH

DMSO

MeOH

DMSO

C−O (nm)b C−N (nm)b Angle (deg)c

0.129 0.147 63.47

0.129 0.147 90.0

0.128 0.144 41.3

0.127 0.144 41.6

0.136 0.145 55.3

0.136 0.145 55.3

0.127 0.144 47.2

0.127 0.144 47.5

0.136 0.145 64.2

0.136 0.145 61.1

a

Data for p-RB and o-RB are from X-ray diffraction. See Figure 3 for atom numbering. bCarbon−oxygen and carbon−nitrogen bond lengths in nanometers, from X-ray crystallographic data of brominated p-RB {p-RB-Br, 2,6-diphenyl-4-[2,6-diphenyl-4-(4-bromophenyl)pyridinium-1-yl] phenolate},3,27 and o-RB,1 or calculated by DFT in the gas phase (present work). cThe C2−C1−N7−C12 interplanar angle, obtained as given in footnote b of this table.

this nonideal behavior is due to PS of the probe by one component of the mixture. The extent of this PS depends on the components of the binary mixture and the hydrophobicity of the probe; more hydrophobic probes show more deviation from linearity.31−33 This agrees with the larger deviation of the more hydrophobic p-RB. (iii) We treated this PS according to a model discussed elsewhere, in which the medium is composed of a mixture of water (W), the organic solvent (S), and a “complex” one (S− W) formed by hydrogen bonding between the two pure solvents. This model was discussed in detail elsewhere;31−33 we give here the essentials, as summarized by eqs 4−7:

Figure 6. Quantum-chemically optimized geometries of (A) p-CB and (B) o-CB and (C) the experimentally determined conformation of p-RB-Br (by X-ray diffraction).

S+WFS−W

(4)

Probe(S)m + m(W) F Probe(W)m + mS

(5)

Probe(S)m + m(S − W) F Probe(S − W)m + mS

(6)

Probe(W)m + m(S − W) F Probe(S − W)m + m W (7)

Here, the concentrations are given on the mole fraction scale, (m) is the number of solvent molecules that perturb the intramolecular charge transfer within the probe, hence affecting the value of ET(probe). Note that m is not the solvation number of the probe. From the solvent-exchange equilibria in the probe solvation layer (see eqs 5−7), we calculated the corresponding solvent-exchange equilibrium constants, designated as solvent “fractionation factors, φ,” as detailed in section SI-4. Their values relative to unity indicate the magnitude of the PS by the appropriate solvent species. For example, φW/S < 1 means that PS is by the organic solvent S. In other words, S displaces W from the probe solvation layer; i.e., the latter is richer in S than the bulk solvent. Likewise, φ(S−W/S) > 1 means that the complex solvent S−W displaces S from the solvation layer of the probe. Finally, a solvent fractionation factor of unity indicates ideal solvation, i.e., the probe solvation layer and bulk solvent have the same composition. This was not observed in the cases studied here. (iv) We calculated φ as given in section SI-4. First, we calculated the ef fective concentrations of S, W, and S−W at each analytical solvent composition from accurate density data. As listed in Table 7, we calculated the following quantities by iteration of ET(probe) versus effective χW: φ(W/S) (water substituting organic solvent); φ(S−W/S) (complex solvent substituting organic solvent); φ(S−W/W) (complex solvent substituting water); and ET(Probe−S‑W), the empirical polarity parameter of the complex solvent. (v) The fit of the preferential solvation model to our data is shown by the excellent agreement between experimental and calculated ET(probe) of the pure solvents and the values of χ2

Figure 7. Dependence of ETN(probes) on the χW for aqueous mixtures in DMSO (left) and 1-PrOH (right). The black squares are for the probe p-CB, the red spheres for p-RB, and the green spheres for probe o-CB.

E T N(probe) = (E T(probe)smaller value − E T(probe)observed ) /(E T(probe)smaller value − E T(probe)larger value )

(3)

Regarding these results, the following is relevant: (i) The straight lines that we drew between χw = 0 and 1 represent ideal solvation behavior, i.e., when the empirical polarity in the probe solvation layer is equal to that of the bulk binary mixture. Clearly, the solvation observed is nonideal; the extent of nonideality is higher for p-RB. (ii) As discussed for probes of distinct chemical classes in binary mixtures of water with organic solvents and with ILs, F



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Table 7. Solvatochromism of p-RB, p-CB, and o-CB in binary mixtures of DMSO/W and 1-PrOH/W at 25 °C34,35 solvatochromism in dimethyl sulfoxide/water m p-RB p-CB o-CB

1.32 0.80 0.90 m

p-RB p-CB o-CB

1.40 1.22 1.37

ϕ(W/DMSO) 0.49 0.80 1.41 ϕ(W/1‑PrOH) 0.44 0.23 0.34

ϕ(DMSO‑W/DMSO) 3.29 1.70 2.40 ϕ(1‑PrOH‑W/1‑PrOH) 66.92 9.81 13.71

ϕ(DMSO‑W/W) 6.71 2.12 1.67 solvatochromism ϕ(1‑PrOH‑W/W) 152.09 42.65 40.32

ET(DMSO)

ET(W)

a

ET(DMSO/W)

χ2

R2

49.1 67.9 63.9

0.029 0.034 0.067

0.994 0.999 0.997

ET(1-PrOH/W)

χ2

R2

52.5 74.0 71.9

0.0023 0.006 0.004

0.999 0.999 0.999

a

45.3 (+ 0.2) 63.1 (0) 58.4 (−0.4)a 77.9 (+0.2)a a 59.1 (−0.2) 75.8 (+0.2)a in 1-propanol/water ET(1-PrOH) a

50.9 (+0.2) 67.2 (+0.1)a 67.6 (+0.4)a

ET(W) a

59.1 (+0.01) 77.7 (0)a 76.1 (−0.5)a

The numbers within parentheses refer to [calculated ET(probe) − experimentally determined ET(probe)].

a

(which is a measurement of how expectations compare to results) and R2 (which is a statistical measure of how close the data are to the fitted regression curve). (vi) Values of m are not far from unity, i.e., only a small number of solvent molecules interact with the probe, in particular, with the phenolate oxygen, leading to the observed nonideal dependence of ET(probe) on solvent composition. (vii) With one exception (o-CB in DMSO-W), all values for φW/S are less than unity, showing that the organic solvents are more efficient than water in the solvation of the probe. Likewise, the complex solvents are more efficient than the pure solvents, as shown by the corresponding values of φDMSO‑W/DMSO and φDMSO‑W/W (all >1). The reason for the efficiency of the complex solvent is that probe−solvent interactions include hydrogen bonding to the phenolate oxygen and hydrophobic interactions. Thus, 1-PrOH/W has more sites for hydrogen-bond donation/acceptance than water or 1-PrOH, while it is also capable of solvating the probe by hydrophobic effect due to the organic “end” of the solvent. A similar reasoning can be advanced for the efficiency of DMSO/ W relative to W and DMSO. (viii) All fractional factors are larger for 1-PrOH/W than DMSO/W, and for p-RB than p-CB and o-CB. The first result underlines the importance of hydrogen bonding to solvation, whereas the second one is in agreement with the dependence of PS (hence, the values of φ) on the hydrophobicity of the probe.30

well as hydrophobic solute−solvent effects to the overall solvation of the betaine probe dyes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b01192. Details of the procedures of the synthesis of the probes o-CB and p-CB, including intermediates; 1H, 13C, and 2D NMR spectra of o-CB and p-CB; equations for the calculation of Free Energy of dimerization in solvents; and equations for the calculation of solvent fraction factors (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Omar A. El Seoud: 0000-0003-1683-5953 Funding

We thank the Brazilian Federal Research Agencies CAPES (Finance code 001) and CNPq (grant 307022/2014-5 to O.A.E.S.) and the State Agencies FAPESC (Santa Catarina) and FAPESP (São Paulo; grant 2014/22136-4 to O.A.E.S.) for financial support and research fellowships. V. Machado thanks CEBIME/UFSC and UFSC.

4. CONCLUSIONS A large volume of information about solute−solvent interactions is gained from the study of solvatochromic probes, notably from p-RB. The probe p-CB represents a less crowded version of the part responsible for the solvatochromism of pRB. We extended the solvatochromic data of p-CB to 18 solvents and compared its solvatochromism in pure solvents and in two binary solvent mixtures (DMSO/W and 1-PrOH/ W) with that of p-RB. Because of the limited number of solvents in which p-CB is soluble, we synthesized (more soluble) o-CB, compared its data with p-RB and o-RB in pure solvents, and with p-RB in binary solvent mixtures. The relief of steric crowding in p-CB relative to p-RB did not lead to an enhanced solvatochromism (Table 2), probably because of the contribution of the phenyl rings of p-RB to the energy of its (solvent-sensitive) intramolecular charge transfer. We successively applied a preferential solvation model to our data measured in binary solvent mixtures. This shows the importance of hydrogen bonding to the phenolate oxygen as

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nicolas Keppler for help with the artwork. DEDICATION Dedicated to Prof. Christian Reichardt for his contribution to the theme of solvents and solvation effects. REFERENCES

(1) Paley, M. S.; Meehan, E. J.; Smith, C. D.; Rosenberger, F. E.; Howard, S. C.; Harris, J. M. Synthesis and Characterization of a Novel Betaine Dye: 2,4-Dimethyl-6-(2,4,6-Triphenyl-N-Pyridinio)Phenolate. J. Org. Chem. 1989, 54, 3432−3436. (2) Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Pyridinium N-Phenolbetaines and Their Use for the Characterization of the Polarity of Solvents. Justus Liebigs Ann. Chem. 1963, 661, 1−37. (3) Machado, V. G.; Stock, R. I.; Reichardt, C. Pyridinium NPhenolate Betaine Dyes. Chem. Rev. 2014, 114, 10429−10475.

G



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(4) Vogel, A. I.; Tatchell, A. R.; Furnis, B. S.; Hannaford, A. J.; Smith, P. W. G. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Longman: Essex, 1996. (5) Williams, D. B. G.; Lawton, M. Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants. J. Org. Chem. 2010, 75, 8351−8354. (6) Gonzalez, D.; Neilands, O.; Caroli Rezende, M. The Solvatochromic Behavior of 2- and 4-Pyridiniophenoxides. J. Chem. Soc., Perkin Trans. 2 1999, 713−718. (7) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. NMR Chemical Shifts of Common Laboratory Solvents as Trace Impurities. J. Org. Chem. 1997, 62, 7512−7515. (8) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K. Gaussian-4 Theory. J. Chem. Phys. 2007, 126, No. 084108. (9) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 09, revision A1; Gaussian, Inc.: Wallingford, CT, 2009. (10) Tomasi, J.; Mennucci, B.; Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 2005, 105, 2999−3094. (11) Reichardt, C.; Welton, T. Solvents and Solvent Effects in Organic Chemistry, 4th ed.; Wiley-VCH: Weinheim, 2011. (12) Krossing, I.; Slattery, J. M.; Daguenet, C.; Dyson, P. J.; Oleinikova, A.; Weingärtner, H. Why Are Ionic Liquids Liquid? A Simple Explanation Based on Lattice and Solvation Energies. J. Am. Chem. Soc. 2006, 128, 13427−13434. (13) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. Linear Solvation Energy Relationships. 23. A Comprehensive Collection of the Solvatochromic Parameters, π*, α, and β, and Some Methods for Simplifying the Generalized Solvatochromic Equation. J. Org. Chem. 1983, 48, 2877−2887. (14) Marcus, Y. The Properties of Organic Liquids That Are Relevant to Their Use as Solvating Solvents. Chem. Soc. Rev. 1993, 22, 409−416. (15) Trišović, N.; Valentić, N.; Ušcú mlić, G. Solvent Effects on the Structure-Property Relationship of Anticonvulsant Hydantoin Derivatives: A Solvatochromic Analysis. Chem. Cent. J. 2011, 5, 62. (16) El-Sayed, M.; Müller, H.; Rheinwald, G.; Lang, H.; Spange, S. Solvatochromism, Crystallochromism, and Solid State Structures of Hydrophilically Functionalized Aromatic Amino Ketones Containing Furan and Thiophene Rings. Chem. Mater. 2003, 15, 746−754. (17) Martins, C. T.; Lima, M. S.; El Seoud, O. A. Thermosolvatochromism of Merocyanine Polarity Indicators in Pure and Aqueous Solvents: Relevance of Solvent Lipophilicity. J. Org. Chem. 2006, 71, 9068−9079. (18) Jayabharathi, J.; Thanikachalam, V.; Devi, K. B.; Perumal, M. V. Kamlet-Taft and Catalán Studies of Some Novel Y-Shaped Imidazole Derivatives. J. Fluoresc. 2012, 22, 737−744. (19) Catalán, J.; López, V.; Pérez, P.; Martín-Villamil, R.; Rodriguez, J. G. Progress towards a Generalized Solvent Polarity Scale: The Solvatochromism of 2-(Dimethylamino)-7-nitrofluorene and Its Homomorph 2-Fluoro-7-nitrofluorene. Liebigs Ann. 1995, 1995, 241−252. (20) Catalán, J.; López, V.; Pérez, P. Solvent Dipolarity/Polarizability (SPP) of Alcoholic Solvents. Liebigs Ann. 1995, 1995, 793− 795. (21) Catalán, J.; Díaz, C.; López, V.; Pérez, P.; De Paz, J.-L. G.; Rodríguez, J. G. A Generalized Solvent Basicity Scale: The Solvatochromism of 5-Nitroindoline and Its Homomorph 1-Methyl5-nitroindoline. Liebigs Ann. 1996, 1996, 1785−1794. (22) Catalán, J. Toward a Generalized Treatment of the Solvent Effect Based on Four Empirical Scales: Dipolarity (SdP, a New Scale), Polarizability (SP), Acidity (SA), and Basicity (SB) of the medium. J. Phys. Chem. B 2009, 113, 5951−5960. (23) Dawber, J. G.; Williams, R. A. A Nuclear Magnetic Resonance Study of the Solvatochromism of a Pyridinium Betaine. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3097−3112. (24) Pike, S. J.; Bond, A. D.; Hunter, C. A. Solvatomorphism of Reichardt’s Dye. CrystEngComm 2018, 20, 2912−2915.

(25) Afri, M.; Gottlieb, H. E.; Frimer, A. A. Reichardt’s Dye: The NMR Story of the Solvatochromic Betaine Dye. Can. J. Chem. 2014, 92, 128−134. (26) Chiappe, C.; Pomelli, C. S. The First Solvation Shell of Reichardt’s Dye in Ionic Liquids: A Semiempirical Study. Theor. Chem. Acc. 2012, 131, 1195−2001. (27) Allmann, R. Die Kristallstruktur des 2.6-Diphenyl-4-(4bromphenyl)-N-(p-Oxy-m,m’-diphenyl)phenyl-pyridinium-betainmonoäthanolats. Z. Kristallogr. 1969, 128, 115−132. (28) Suter, H. U.; Nonella, M. Quantum Chemical Investigation of the C-O Bond Length and Stretching Mode of the Phenolate Anion. J. Phys. Chem. A 1998, 102, 10128−10133. (29) Sieler, J.; Pink, M.; Zahn, G. Zur Struktur von zwei Hydraten des Natriumphenolats: C6H5ONa H2O und C6H5ONa · 3H20. Z. Anorg. Allg. Chem. 1994, 620, 743−748. (30) Sato, B. M.; Martins, C. T.; El Seoud, O. a. Solvation in Aqueous Binary Mixtures: Consequences of the Hydrophobic Character of the Ionic Liquids and the Solvatochromic Probes. New J. Chem. 2012, 36, 2353−2360. (31) El Seoud, O. A. Solvation in Pure and Mixed Solvents: Some Recent Developments. Pure Appl. Chem. 2007, 79, 1135−1151. (32) El Seoud, O. A. Understanding Solvation. Pure Appl. Chem. 2009, 81, 697−707. (33) El Seoud, O. A. Quim. Nova 2010, 33, 2187−2192. (34) Silva, P. L.; Bastos, E. L.; El Seoud, O. A. Solvation in Binary Mixtures of Water and Polar Aprotic Solvents: Theoretical Calculations of the Concentrations of Solvent-Water HydrogenBonded Species and Application to Thermosolvatochromism of Polarity Probes. J. Phys. Chem. B 2007, 111, 6173−6180. (35) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Solvatochromism in Pure and Binary Solvent Mixtures: Effects of the Molecular Structure of the Zwitterionic Probe. J. Phys. Org. Chem. 2000, 13, 679−687.

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Understanding Solvation: Comparison of Reichardt's Solvatochromic

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