Understanding Solvation: Comparison of Reichardt's Solvatochromic Probe and Related Molecular "core" Structures

Article abstract of DOI:10.1021/acs.jced.8b01192

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 p-RB 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 [E_T(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 E_T(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.

Full text of DOI:10.1021/acs.jced.8b01192

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,* Jessica C. de Jesus,
́
Carlos E. A. de Melo,‡ Jonatan L. O. Buske,‡ and Amanda P. Cardozo‡
†Institute of Chemistry, University of Sao
̃
Paulo, Prof. Lineu Prestes Av., 748, 05508-000, Sao
̃
Paulo, Sao
̃
Paulo, Brazil
‡
́
Department of Chemistry, Federal University of Santa Catarina, P.O. Box 476, 88040-900, Florianopolis, Santa Catarina, Brazil
S
* Supporting Information
ABSTRACT: The compound 2,6-diphenyl-4-(2,4,6-triphe-
nylpyridinium-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 p-
RB 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.
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
1. INTRODUCTION
Figure 1 depicts the molecular structures of the compounds of
interest in the present work. A very interesting and useful
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. 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-(pyridi-
nium-1-yl)phenolate, o-CB; (2,4-dimethyl-6-(2,4,6-triphenylpyridi-
nium-1-yl)phenolate, o-RB.1
Special Issue: Latin America
Received: December 11, 2018
Accepted: March 6, 2019
© XXXX American Chemical Society
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).
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:
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.
ET(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 λmax is termed “negative
solvatochromism.” ET(probe) is the sum of solvent descriptors
as shown by eq 2:
2. EXPERIMENTAL SECTION
2.1. Material. All solvents were purchased from Aldrich,
Merck, or Synth (Sao 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.
ET(dye) = ET(dye)0 + aSA + bSB + dSD + pSP
(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 p-
CB 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 o-
CB 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
2.2. Equipment. Infrared spectra were recorded with
Shimadzu Prestige-21 and Bruker Vector 22 FTIR spectropho-
tometers; KBr pellets were used. UV−vis measurements were
recorded with HP 8452A and Shimadzu UV-2550 UV−vis
spectrophotometers equipped with thermostated cell compart-
ments 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.
1H 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 spec-
trometer (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, υmax/cm−1):
̅
3285 (O−); 1578 (CC); 1482 (CC); 1305 (N−C); 852
B
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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. 1H NMR Study of the Solvation of the
Synthesized Probes. The solvents used were CD3OD and
CD3CN (Cambridge Isotope Laboratories) without TMS. The
1
spectra were calibrated using the residual solvent H peak,
whose chemical shifts relative to TMS are listed elsewhere.7
2.5. Sample Preparation for Solvatochromic Meas-
urements. 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).
Figure 3. Atom numbering of o-CB and p-CB.
a
Scheme 1. Synthesis of p-CB and o-CB
2.6. Quantum Chemical Calculations. Initial optimiza-
tion, 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 quantum-
chemical calculations were done with the probes solvated in
CH3OH and DMSO, using the Polarizable Continuum Model
(PCM).10
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
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.
1
(para-substituted ring). H 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, υmax/cm−1):
̅
3242 (O−); 1592 (CC); 1493 (CC); 1362 (N−C); 746
1
(ortho-substituted ring). H 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),
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
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
DOI: 10.1021/acs.jced.8b01192
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Table 1. Values of ET(dye) in kcal mol−1 for Compounds p-
CB and o-CB in 30 Solvents
Table 2. Results of the Linear Correlations between the
Different ET Values of the Four Probes
a
ET(p-
a
ET(p-
ET(o-
c
ET(o-
equation
N
α
β
R2
entry
solvent
RB)
CB)
RB)
CB)
ET(o-CB) = α + β·ET (o-
6
12
19
4
4
4
1.01 0.09
0.95795
1
acetone
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
45.0
46.0
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
RB)
2
acetonitrile
ET (p-CB) = α + β·ET (p- 15
0.96 0.08
1.15 0.06
0.73 0.05
0.90111
0.96426
0.95668
RB)
3
4
benzyl alcohol
1-butanol
ET (p-CB) = α + β·ET (o- 15
−9
CB)
5
tert-butanol
ET (o-RB) = α + β·ET (p- 11
15.8 0.7
6
7
2-(n-butoxy)ethanol
chloroform
66.3
b
RB)
a
N = number of solvents considered in the correlation.
b
b
b
b
b
b
8
1-decanol
9
diethyl ether
The favorable ΔGdimerization,gas‑phase raises the question
10
11
12
13
14
1,2-dimethoxyethane
dimethylcarbonate
1,2-dichloroethane
dichloromethane
N,N-
dimethylacetamide
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 p-
CB in these two solvents.
43.9
56.0
56.4
58.8
15
N,N-
dimethylformamide
43.2
58.1
16
17
18
19
20
21
22
23
dimethyl sulfoxide
ethanol
ethyl acetate
2-ethoxyethanol
ethylene glycol
methanol
2-methoxyethanol
1-methyl-2-
pyrrolidinone
45.1
51.9
38.1
51.0
56.3
55.4
52.0
42.2
44.7
53.9
59.3
68.1
58.8
68.4
72.5
69.5
68.8
67.0
68.9
b
67.7
73.9
71.8
69
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
57.2
69.2
Table 3. Calculated Dimerization Free Energies in Five
Solvents (ΔGdimerization,solvent) and Free Energies of Solvation
(ΔGsolvation) for the Monomeric and Dimeric Probes
b
b
24
25
26
27
28
29
30
nitromethane
1-octanol
46.3
48.1
49.1
39.5
50.7
37.4
63.1
65.7
65.4
66.8
55.1
67.2
55.3
1-pentanol
4-picoline
1-propanol
tetrahydrofuran
water
65.7
b
ΔGdimerization,solven-
monomer ΔGsolvation
,
dimer ΔGsolvation,
t, kcal mol−1
kcal mol−1
kcal mol−1
67.3
b
solvent
DMSO
diethyl
ether
p-CB
o-CB
p-CB
o-CB
p-CB
o-CB
77.7
75.6
c
13.23
12.14
8.70
7.96
−12.23
−9.13
−7.14
−5.43
−7.07
−5.18
−4.64
−2.90
a
b
Data taken from Reichardt and Welton.11 Probe insoluble. Data
taken from Paley et al.1
CH3CN
CH3OH
CH2Cl2
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
19−22
Figure 4. Correlations of ET(p-CB) (A) and ET(o-CB) (B) with ET(p-
RB).
́
that of Catalan et al.,
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-
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
a
Solvatochromic Data of the Probes
b
probe ET(dye)0
a
b
d
p
N
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,
b
and p). N = 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
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).
δ/ppm,
CD3OD
δ/ppm,
CD3CN
Δδ/ppm,
Δδ, ppm,
H atoms
CD3OD
CD3CN
H2 and H6
H3 and H5
H8 and
H12
H9 and
H11
H10
7.368
6.734
9.048
7.404
6.920
8.818
a
a
1.680
1.404
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
b
b
8.118
8.069
8.509
1.384
1.149
Additionally, the C−N bond length is 0.146
0.001 nm,
8.548
a
b
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.
Δδ = δH8 − δH2. Δδ = δH9 − δH3
.
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 ΔδCD OD > ΔδCD CN and ΔδH8−H2
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.
3
3
> 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: Geom-
etry 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 ground-
state 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.
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:
Focusing on bond lengths, the experimental and calculated
values for C−O are 0.128 0.001 nm, in agreement with the
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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
a
of p-CB, o-CB, p-CB-H+, and o-CB-H+ in Methanol (MeOH) and Dimethylsulfoxide (DMSO)
Item
p-RB
o-RB
p-CB
p-CB-H+
o-CB
o-CB-H+
X-ray
X-ray
MeOH
DMSO
MeOH
DMSO
MeOH
DMSO
MeOH
DMSO
b
b
C−O (nm)
C−N (nm)
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
c
Angle (deg)
a
b
Data for p-RB and o-RB are from X-ray diffraction. See Figure 3 for atom numbering. Carbon−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). The C2−C1−N7−C12 interplanar angle, obtained as given in
c
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).
(4)
(5)
(6)
S + W F S − W
Probe(S)m + m(W) F Probe(W)m + mS
Probe(S)m + m(S − W) F Probe(S − W)m + mS
Probe(W)m + m(S − W) F Probe(S − W)m + mW
(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, des-
ignated 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 effective 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.
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.
ETN(probe) = (ET(probe)smaller value − ET(probe)observed
/(ET(probe)smaller value
)
− ET(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,
(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
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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
ϕ(W/DMSO)
ϕ(DMSO‑W/DMSO)
ϕ(DMSO‑W/W)
ET(DMSO)
ET(W)
ET(DMSO/W)
χ2
R2
a
a
p-RB
p-CB
o-CB
1.32
0.80
0.90
0.49
0.80
1.41
3.29
1.70
2.40
6.71
2.12
1.67
45.3 (+ 0.2)
58.4 (−0.4)
59.1 (−0.2)
63.1 (0)
49.1
67.9
63.9
0.029
0.034
0.067
0.994
0.999
0.997
a
a
a
77.9 (+0.2)
75.8 (+0.2)
a
solvatochromism in 1-propanol/water
m
ϕ(W/1‑PrOH)
ϕ(1‑PrOH‑W/1‑PrOH)
ϕ(1‑PrOH‑W/W)
ET(1-PrOH)
ET(W)
ET(1-PrOH/W)
χ2
R2
a
a
p-RB
p-CB
o-CB
1.40
1.22
1.37
0.44
0.23
0.34
66.92
9.81
13.71
152.09
42.65
40.32
50.9 (+0.2)
67.2 (+0.1)
67.6 (+0.4)
59.1 (+0.01)
77.7 (0)
76.1 (−0.5)
52.5
74.0
71.9
0.0023
0.006
0.004
0.999
0.999
0.999
a
a
a
a
a
The numbers within parentheses refer to [calculated ET(probe) − experimentally determined ET(probe)].
(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).
well as hydrophobic solute−solvent effects to the overall
solvation of the betaine probe dyes.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jced.8b01192.
(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.
■
S
Details of the procedures of the synthesis of the probes
1
o-CB and p-CB, including intermediates; H, 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)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: elseoud.usp@gmail.com.
*E-mail: vanderlei.machado@ufsc.br.
■
ORCID
Omar A. El Seoud: 0000-0003-1683-5953
(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
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 (Sao Paulo; grant 2014/22136-4 to O.A.E.S.) for
financial support and research fellowships. V. Machado thanks
CEBIME/UFSC and UFSC.
Notes
4. CONCLUSIONS
The authors declare no competing financial interest.
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 p-
RB. 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 succes-
sively 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
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.
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