Twisting-based spectroscopic measure of solvent polarity: The PT scale

Article abstract of DOI:10.1021/jo400586w

The P_T scale, which is correlated well with the E _T(30), Y, and Z scales, is the first twisting-based spectroscopic measure of solvent polarity. It is based on two combined mechanisms: (1) the solvent-dependent intramolecular charge-transfer absorption that displays a regular hypsochromic band shift in polar solvents and (2) overcoming the intramolecular hydrogen-bond of o-1 by its differentiated solvation in polar solvents, causing a C-C bond to twist that leads to a regular hypsochromic shift of its lowest energy electronic absorption band.

Full text of DOI:10.1021/jo400586w

Article
pubs.acs.org/joc
Twisting-Based Spectroscopic Measure of Solvent Polarity: The PT
Scale
Wei-Jen Lo, Yi-Hui Chen, and Kuangsen Sung*
Department of Chemistry, National Cheng Kung University, No. 1 University Road, Tainan 700, Taiwan
S
* Supporting Information
ABSTRACT: The PT scale, which is correlated well with the ET(30), Y, and Z scales, is the first
twisting-based spectroscopic measure of solvent polarity. It is based on two combined mechanisms:
(1) the solvent-dependent intramolecular charge-transfer absorption that displays a regular
hypsochromic band shift in polar solvents and (2) overcoming the intramolecular hydrogen-bond
of o-1 by its differentiated solvation in polar solvents, causing a C−C bond to twist that leads to a
regular hypsochromic shift of its lowest energy electronic absorption band.
which was based on absorption of 2,6-diphenyl-4-(2,4,6-
INTRODUCTION
■
triphenylpyridinium-1-yl)phenolate in various solvents, empiri-
cally measures the ionizing power of solvents in terms of
solvent influence on an intramolecular charge-transfer
absorption. The ET(30) scale is available for a wide range of
solvents and it is one of the most useful and widely accepted
empirical parameters of solvent polarity. The χR scale,5a χB
scale,5a π* scale,5b π*azo scale,5c and E*LMCT scale5d are useful
empirical parameters of solvent polarity that are based on
intramolecular charge-transfer absorption.
Fluorescent dyes with conjugated donor−acceptor systems
are another kind of molecules that may sense solvent polarity
through solvent-dependent fluorescence emissions in a variety
of solvents.6 This type of molecule undergoes a photoinduced
intramolecular charge transfer (PCT), so that their charge-
transfer excited states are usually much more polar than the
corresponding ground states. According to the Franck−
Condon principle, light absorption occurs with a vertical
transition of an electron within femtoseconds, followed by
solvent reorganization in the range of picoseconds before
fluorescent light emits in nanoseconds.7 Before fluorescent light
emits, the charge-transfer excited-states have sufficient time to
interact with the surrounding solvent molecules to reach the
solvent-stabilized excited-states. Hence, fluorescent emission of
this type of molecules provides one of the best ways to sense
solvent polarity. The most popular application for this type of
fluorescent dyes is the visualization of cell and protein
structures.8
Solvents can significantly influence reaction rates, the position
of chemical equilibria, and absorption and emission spectra
involving electronic, vibrational, and rotational transitions.
These solvent effects have been usually understood in terms of
solvent polarity. Solvent polarity can be defined as the overall
solvation capability (solvation power).1 The solvation power
involves nonspecific dipole/dipole and dipole/induced dipole
solute/solvent interactions, as well as specific hydrogen-bond
and electron-pair-acceptor (EPA)/electron-pair-donor (EPD)
solute/solvent interactions at a molecular microscopic level.
Therefore, macroscopic physical solvent parameters such as
relative permittivity (εr) and refractive index (n) and molecular
microscopic physical solvent parameters such as permanent
dipole moment (μ) fail to accurately describe the polarity of the
solute-surrounding local solvation shell.1 Thus, some empirical
parameters of solvent polarity have been developed as follows.
The desmotropic constants (L),2 based on a solvent-
dependent chemical equilibrium derived from the tautomeriza-
tion of ethyl acetoacetate as a reference, empirically measure
the solvents’ enolization capability. The Y scale,3 based on
solvent-dependent reaction rates with the solvolysis of 2-
chloro-2-methylpropane as a standard reaction, empirically
measures the solvent ionizing power in terms of the solvent
influence on build-up of positive charge of a solute during
solvolysis. However, the Y scale can be measured only for a
limited number of solvents. The Z scale,4 which was based on
electronic absorption of 1-ethyl-4-(methoxycarbonyl)-
pyridinium iodide in various solvents, empirically measures
the ionizing power of solvents in terms of the influence of the
solvent on an intermolecular charge-transfer absorption.
However, because of the low solubility of the salt, the Z values
for nonpolar solvents are difficult to get. The ET(30) scale,1
In this paper, we designed as a prototype molecule the
merocyanine dye o-1 of twisting-based spectroscopic indicator
Received: March 20, 2013
Published: April 23, 2013
© 2013 American Chemical Society
5925
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
for solvent polarity (Figure 1). This merocyanine dye o-1 is
composed of two rings, an o-acetaminophenyl electron-donor
RESULTS AND DISCUSSION
■
Preparation of o-1 was done by treating (Z)-4-(2-amino-
benzylidene)-1,2-dimethyl-(1H,4H)-imidazol-5-one (o-
ABDI)10 with 2 equiv of acetic anhydride (Scheme 1).
Scheme 1. Synthesis of o-1 from o-ABDI
According to the crystal structure of o-1 obtained through X-
ray diffraction, the imidazolone and the o-acetaminophenyl ring
almost stay in a planar conformation with the interplanar angle
of ∠N(2)−C(8)−C(2)−C(1) = 0.19° (Figure 2). The distance
Figure 1. Brief description of the solvent influence on the electronic
absorption of planar o-1 and its twisted conformation and definition of
the molar transition energy, PT (twisting-based polarity).
Figure 2. X-ray single-crystal molecular structure of o-1 with thermal
ellipsoids at the 50% probability level.
of H(1)···N(2) between the o-acetaminophenyl and the
imidazolone ring is 185 pm, which corresponds to a hydrogen
bonding distance, with ∠N(1)−H(1)−N(2) = 170.7° and
N(1)−N(2) = 275 pm. Hence, in the crystal lattice, a seven-
membered ring with an intramolecular hydrogen-bond through
N(1)−H(1)···N(2) is formed, which is strongly supported by
its 1H NMR spectrum in CDCl3, where the signal at δ 12.34 for
H(1) on N(1) is significantly downfield-shifted.
and an imidazolone electron-acceptor moiety, with an addi-
tional intramolecular hydrogen bond as a special feature. These
two rings are connected together in two ways by conjugated
bonding and intramolecular hydrogen bonding. In solvents with
various polarities, the solvation of this indicator influences its
intramolecular hydrogen bonding for better stabilization. If the
solvation overcomes the intramolecular hydrogen bonding, the
two rings of the indicator get twisted around the C2−C7 bond
with an interplanar angle of τ, leading to a decrease of
resonance within the conjugated π-system. Hence, we expect
that the lowest energy electronic absorption band of this
indicator dye is blue-shifted in polar solvents. In this paper, we
show that the blue-shift degree of this indicator in polar
solvents is proportional to their corresponding solvent polarity.
We do not expect that this empirical parameter of solvent
polarity would be better than the ET(30) scale, but we hope it
can trigger the generation of even better twisting-based
spectroscopic indicators for solvent polarity in the future.
The lowest energy electronic absorption band of o-1
measured in acetonitrile is located at λ = 365 nm (ε = 1.29
× 104 M−1 cm−1), which is hypsochromically shift in polar
solvents. We did calculations on the electronic transitions of o-1
at the B3LYP-TD/cc-PVDZ//B3LYP/6-31+G* level to under-
stand whether the lowest energy electronic absorption band
involves a charge-transfer transition. The calculated lowest
energy singlet electronic transition of o-1 is contributed by
0.59[HOMO → LUMO] − 0.27[HOMO-1 → LUMO] and
located at λ = 397 nm with an oscillator strength (f) of 0.2658
and a transition electric dipole moment of −1.79 au (−4.55 D)
along the x axis and −0.51 au (−1.30 D) along the y axis, which
is in good agreement with the experimental data (λmax = 383.5
nm in n-pentane with ε = 1.3 × 104 M−1 cm−1, which
corresponds to an oscillator strength (f) of 0.28 according to
the equation7 of f = 4.3 × 10−9∫ εdν = 4.3 × 10−9εmaxΔν1/2).
The magnitude of this transition electric dipole moment is
larger than those of the charge-transfer transitions of nitro-
benzene and peridinin (3.2 D for nitrobenzene11a and 3.4 D for
peridinin11b). Hence, we suggest that the lowest energy
electronic absorption of o-1 involves a charge-transfer
transition. The negative sign of this transition electric dipole
moment of o-1 indicates that a substantial electron transfer
COMPUTATIONAL DETAILS
■
All calculations reported here were performed with the Gaussian 03
program.9 Geometry optimization of o-1 was carried out at the
B3LYP/6-31+G* level without any symmetry restriction. After the
geometry optimization was performed, an analytical vibration
frequency was calculated at the same level to determine the nature
of the located stationary point. Time-dependent density functional
theory (TDDFT) was used to calculate the first five vertical electronic
transitions of o-1 in the gas phase and the electron-density surfaces of
its molecular orbital at the level of B3LYP-TD/cc-PVDZ//B3LYP/6-
31+G*.
5926
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
from the imidazolone moiety to the o-acetaminophenyl moiety
occurs during excitation (Figure 3). This causes the dipole
from its hydrogen-bonded imidazolone moiety by solvation, so
the intramolecular hydrogen bond should remain intact in n-
pentane and its structure in n-pentane is supposed to be very
similar to its single-crystal molecular structure with a fully
extended and conjugated π-system across both o-acetamino-
phenyl and imidazolone moieties, resulting in a red-shift of its
lowest energy electronic absorption band to a longer
wavelength of λ = 383.5 nm. This lowest energy electronic
absorption band of o-1 is regularly blue-shifted as the polarity of
the solvent increases (Table 1). It reaches the largest blue-shift
in water with the maximum blue-shift of Δλ = 35.5 nm, as
compared to n-pentane.
Figure 3. Electron-density surfaces of HOMO-1, HOMO, and LUMO
of o-1.
The hypsochromic band shift also happens to the emission
maximum of o-1 in polar solvents, but the largest blue-shift is
only Δλ = 11 nm, which is much less than that found for its
electronic absorption spectra (Table 1). This is because the
excited state from which emission of o-1 occurs is not a
Franck−Condon excited state but a solvent-stabilized equili-
brium excited state.7 The Franck−Condon ground state to
which emission of o-1 leads is not solvent-stabilized,7 but it is
destabilized as compared to the solvent-stabilized equilibrium
ground state, from which the electronic absorption of o-1
occurs. In addition to the smaller blue-shift degree, the
fluorescence quantum yield of o-1 is quite low, so the emission
maximum of o-1 is not a suitable indicator for solvent polarity.
The lowest energy electronic absorption of o-1 has been
measured in 51 common solvents at a concentration of 3.0 ×
10−5 M, which include alkanes, alkenes, cycloalkanes, cyclo-
alkenes, haloalkanes, alkylarenes, heteroarenes, aliphatic mono-
alcohols, an aliphatic polyalcohol, aliphatic and cycloaliphatic
ethers, aliphatic ketones, carboxylic acid and anhydride,
aliphatic and aromatic esters, amides, aliphatic nitriles, aliphatic
and aromatic amines, phosphorus compounds, sulfur com-
pound, water, and heavy water. The 51 solvents are evenly
distributed from nonpolar to polar solvents. The solubility of o-
1 in all 51 solvents is good and sufficient for the electronic
absorption measurement. The molar absorption coefficients of
o-1 in these 51 solvents are all very similar, and their values are
around ε = 1.3 × 104 M−1cm−1. The lowest energy electronic
absorption maximum of o-1 can be converted into the
corresponding molar transition energy, PT (twisting-based
moment of the first singlet Franck−Condon excited state to be
smaller than that of the ground state.12 This might be one of
reasons that make the lowest energy absorption band of o-1
display a regular hypsochromic band shift in polar solvents.
However, the lowest energy electronic absorption of o-ABDI,
whose structure is very close to that of o-1, does not display a
regular hypsochromic band shift in polar solvents, even though
it also involves a charge-transfer transition that has a substantial
electron transfer from the imidazolone moiety to the aniline
moiety during excitation.10 In addition, the lowest energy
electronic absorption band of p-1 is not shifted with increasing
solvent polarity.13 This implies that an additional mechanism
may be applied to the regular hypsochromic shift of the lowest
energy electronic absorption band of o-1 in polar solvents.
We propose that the additional mechanism for the regular
hypsochromic shift of the lowest energy electronic absorption
band of o-1 in polar solvents is based on the first-order
perturbation theory that describes the influence of bond
twisting on the HOMO and LUMO energies of a conjugated π-
system.14 If the HOMO is antibonding and the LUMO is
bonding in the twisted bond, the bond twisting stabilizes the
HOMO but destabilizes the LUMO. On the other hand, if the
HOMO is bonding and the LUMO is antibonding in the
twisted bond, the bond twisting destabilizes the HOMO but
stabilizes the LUMO. According to the DFT calculations of o-1
at the level of B3LYP/cc-pVDZ//B3LYP/6-31+G*, its HOMO
and HOMO-1 are antibonding and its LUMO is bonding in the
C2−C7 bond, so the bond twisting around the C2−C7 bond
stabilizes its HOMO and HOMO-1 but destabilizes its LUMO,
leading to the hypsochromic shift of the intramolecular charge-
transfer π−π* transition.
polarity), with the equation of PT = hcυmaxNA = [28591/λ
̅
max(nm)] (kcal/mol) with c = light speed and NA
=
What conditions would make o-1 twisted around the C2−C7
bond? To answer this question, we need to know what the
forces are to drive o-1 to be a flat molecule. Obviously, these
forces are intramolecular hydrogen bonding according to
N(1)−H(1)···N(2) and through-molecule π-conjugation.
Hence, to twist o-1 around the C2−C7 bond, the intramolecular
hydrogen bond must be weakened and the through-molecule π-
conjugation must be reduced, and both of them cost energy. It
is known that intramolecular hydrogen bonding within peptides
can be overcome by intermolecular hydrogen bonding with
water15 and ionic bonding within ionic salts can be overcome
by solvation with water.16 Hence, we suggest that solvation of
o-1 may weaken its intramolecular hydrogen bonding and
through-molecule π-conjugation to a certain degree, and that
causes o-1 twisted around the C2−C7 bond to a certain
interplanar angle of τ. According to the first-order perturbation
theory, the twist angles (τ) of o-1 in various solvents might be
detected by its electronic absorptions in various solvents.
According to the chemical principle of “like dissolves like”, n-
pentane is unlikely to separate the o-acetamino moiety of o-1
Avogadro’s number. As the solvent polarity increases from n-
pentane to water, the PT values regularly increase from 74.55 to
82.16. (ΔPT = 7.61 kcal/mol).
Both the PT and the ET(30)1c scale register nonspecific
dipole/dipole and dipole/induced dipole solute/solvent inter-
actions very well. For example, replacement of an alkyl group
by the corresponding alkenyl group usually increases its
polarity, such as, for example, cyclohexane (PT = 74.65,
ET(30) = 30.9) versus cyclohexene (PT = 76.04, ET(30) =
32.2); 1-propanol (PT = 79.22, ET(30) = 50.7) versus allyl
alcohol (PT = 80.31, ET(30) = 51.9). It is likely that the
polarizability of the electron cloud of a π bond is higher than
that of a σ bond. Replacement of a bromo atom by a chloro
atom usually increases the polarity, such as dibromomethane
(PT = 76.04, ET(30) = 39.04) versus dichloromethane (PT =
76.86, ET(30) = 40.07); 1,2-dibromoethane (PT = 75.04,
ET(30) = 38.3) versus 1,2-dichloroethane (PT = 76.45, ET(30)
= 41.3). It is likely because a bromo atom is more polarizable
and less electronegative than a chloro atom.
5927
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
Table 1. Electronic Absorption and Emission Maxima of o-1 Measured in 51 Solvents at Room Temperature and the
Corresponding PT, ET(30), Z, and δΔG⧧ Values of the Solvents
c
δΔG⧧
a
b
solvent
λabs (nm)
λem (nm)
PT (kcal/mol)
ET(30) (kcal/mol)
Z (kcal/mol)
(kcal/mol)
Alkanes, Alkenes, Cycloalkanes, and Cycloalkenes
n-pentane
n-hexane
383.5
383.0
383.0
376.0
74.55
74.65
31.0
31.0
30.9
32.2
cyclohexane
cyclohexene
436(1)
74.65
76.04
60.1
Haloalkanes
Alkylarenes
dichloromethane
372.0
376.0
377.0
385.0
374.0
381.0
76.86
76.04
76.84
74.26
76.45
75.04
40.7
39.4
39.1
34.8
41.3
38.3
64.2
62.8
63.2
2.59
dibromomethane
trichloromethane
bromotrichloromethane
1,2-dichloroethane
1,2-dibromoethane
63.4
60.0
1.35
4.92
benzene
toluene
381.0
383.7
385.9
382.0
75.04
74.51
74.09
74.85
34.3
33.9
54.0
m-xylene
p-xylene
n/a
33.1
Heteroarenes
76.86
76.22
pyridine
372.0
375.1
40.5
41.9
64.0
83.6
78.3
77.7
2-chloropyridine
Aliphatic Monoalcohols
methanol
354.2
353.9
352.0
360.9
367.0
356.0
367.0
367.0
367.0
80.72
55.4
52.6
54.1
50.7
48.5
51.9
49.7
47.1
49.1
−3.34
−1.66
−1.4
ethanol (95% v/v)
2,2,2-trichloroethanol
1-propanol
80.79
81.22
79.22
3-phenyl-1-propanol
allyl alcohol
77.90
80.31
1-butanol
77.90
(+/−)-2-butanol
1-pentanol
77.90
77.90
Aliphatic Polyalcohol
(+/−)-1,2-propanediol
355.0
80.54
54.1
Aliphatic and Cycloaliphatic Ethers
diethyl ether
376.1
376.6
372.9
374.0
372.0
76.02
75.92
76.67
34.5
34.1
38.6
37.4
36.0
5.72
diisopropyl ether
diethylene glycol dimethyl ether
tetrahydrofuran
434(1)
76.45
76.86
58.8
3.34
3.08
1,4-dioxane
64.55
Aliphatic Ketones
acetone
368.0
369.0
372.0
77.69
77.48
76.86
42.2
41.3
39.4
65.7
1.84
2-butanone
4-methyl-2-pentanone
Carboxylic Acids and Anhydrides
acetic acid
353.0
367.0
80.99
51.7
43.9
79.2
64.0
−2.52
acetic anhydride
77.90
Aliphatic and Aromatic Esters
ethyl acetate
372.2
374.0
371.7
370.0
358.0
367.0
76.82
38.1
38.5
38.8
42.2
55.8
43.2
4.02
n-butyl acetate
76.45
76.92
methyl acrylate
N-methyl(pyrrolidin-2-one) (NMP)
formamide
77.27
0.57
83.3
68.5
79.86
−5.66
0.0
N,N-dimethylformamide (DMF)
77.90
Aliphatic Nitriles
acetonitrile
367.0
376.0
432(1)
77.90
76.04
45.6
40.0
71.3
0.18
trichloroacetonitrile
Aliphatic and Aromatic Amines
diethylamine
373.0
386.0
376.0
76.65
74.07
76.04
35.4
32.1
36.5
triethylamine
N,N-dimethylaniline
5928
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
Table 1. continued
c
δΔG⧧
a
b
solvent
λabs (nm)
λem (nm)
PT (kcal/mol)
ET(30) (kcal/mol)
Z (kcal/mol)
(kcal/mol)
Phosphorus Compounds
phosphoric acid
dimethyl sulfoxide (DMSO)
water
355.0
80.54
77.90
Sulfur Compounds
367.00
432(3)
45.1
63.1
70.2
0.62
Water and Heavy Water
348
348
425(0.7)
82.16
91.8
−9.56
deuterium oxide
82.16
62.8
a
b
c
Emission wavelength with the fluorescence quantum yield (ϕf × 10−3) inside the parentheses. References 4c and 17. Reference 18.
Figure 4. Linear correlation between the PT and ET(30) values, measured in 48 solvents of different polarity at 25 °C with the correlation equation of
PT = (0.2411)[ ET(30)] + 67.083.
Figure 5. Linear correlation between the PT scale and the Gibbs energies of activation of the solvolysis of 2-chloro-2-methylpropane in the 18
solvents of different polarity, taken from ref 18; δΔG⧧ = ΔG⧧ (DMF as reference solvent); the correlation equation: PT = (−0.4477)(δΔG⧧) +
78.065.
The π* scale has a problem to accurately measure the
polarity of hydrogen-bonding-donor (HBD) solvents.19 On the
contrary, the PT scale registers the specific hydrogen-bonding
solute/solvent interactions in HBD solvents as well as the
ET(30) scale1c does. For example, the polarity of alkyl alcohol
decreases as the alkyl chain length increases, such as methanol
(PT = 80.72, ET(30) = 55.4), 1-propanol (PT = 79.22, ET(30) =
50.7), 1-butanol (PT = 77.90, ET(30) = 49.7), and 1-pentanol
(PT = 77.90, ET(30) = 49.1). A HBD solvent is usually more
polar than the corresponding non-HBD solvent, such as 1-
pentanol (PT = 78.12, ET(30) = 49.1) versus diethyl ether (PT =
76.02, ET(30) = 34.5); diethylamine (PT = 76.65, ET(30) =
35.4) versus triethylamine (PT = 74.07, ET(30) = 32.1);
formamide (PT = 79.86, ET(30) = 55.8) versus N,N-
dimethylformamide (DMF) (PT = 77.90, ET(30) = 43.2).
It is practically impossible to find a solvent scale that is
universally useful for all kinds of solute−solvent interactions.1c
However, many empirical parameters of solvent polarity
derived from similar probe molecules are linearly correlated
with each other.1c Hence, one may use the correlation between
two empirical solvent polarity scales as a tool to know if the two
empirical parameters of solvent polarity register similar probe/
solvent interactions. For example, the π* scale19 is poorly
correlated with the ET(30) scale1c with a correlation coefficient
r2 of 0.71. This is because the π* scale is useful for nonspecific
dipole/dipole and dipole/induced dipole solute−solvent
interactions only, but it cannot be used for hydrogen-bonding
solute−solvent interactions.19 For the ET(30) scale, it is also
useful for nonspecific dipole/dipole and dipole/induced dipole
solute/solvent interactions. In addition to that, the probe of the
ET(30) scale can sense hydrogen-bonding-donor (HBD) and
electron-pair-acceptor (EPA) solvents but it senses electron-
pair-donor (EPD) to a much lesser extent.1c The PT scale is also
poorly correlated with the π* scale with a correlation coefficient
r2 of 0.54, but it is well correlated with the ET(30) scale with a
correlation coefficient r2 of 0.90 (Figure 4). It implies that the
PT scale registers nonspecific dipole/dipole and dipole/induced
dipole as well as specific hydrogen-bonding and EPA/EPD
5929
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
Figure 6. Linear correlation between the PT and the Z scale,4c,17 measured in 21 solvents of different polarity at 25 °C, with the correlation equation
of PT = (0.193)(Z) + 64.233.
solute/solvent interactions much closer to the ET(30) scale
than the π* scale.
CONCLUSION
■
This twisting-based spectroscopic measure of solvent polarity,
the PT scale, correlates well with the ET(30), Y, and Z scale. It is
caused by two combined mechanisms. The first is the solvent-
dependent intramolecular charge-transfer absorption that
displays a regular hypsochromic band shift in polar solvents.
The second is that the solvation of the probe molecule, o-1, in
polar solvents overcomes its intramolecular hydrogen bonding,
causing a bond twisting in the C2−C7 bond, where both
HOMO and HOMO-1 are antibonding but LUMO is bonding.
The bond twisting stabilizes both the HOMO and HOMO-1
but destabilizes the LUMO, leading to the hypsochromic shift
of the lowest energy electronic absorption band (π−π* charge-
transfer transition). The PT scale successfully provides an
empirical measurement of the solvent polarity and registers
nonspecific dipole/dipole and dipole/induced dipole solute/
solvent interactions, as well as specific hydrogen-bond and
EPA/EPD solute/solvent interactions.
The PT scale is correlated well with the Gibbs free energy of
activation18 of the solvolysis of 2-chloro-2-methylpropane,
which is a kinetically derived empirical solvent polarity
parameter and has been used to introduce the Y scale of
solvent ionizing power (Figure 5). This implies that the PT
scale registers the solute/solvent interactions that are similar to
those of the Y scale,18 which can sense solvent dipolarity and
solvent hydrogen-bond donor acidity.
The PT scale is also correlated well with the Z scale,4c,17
which was based on the intermolecular charge-transfer
absorption of 1-ethyl-4-(methoxycarbonyl)pyridinium iodide
in various solvents (Figure 6). This implies that the PT scale
registers the solute/solvent interactions that are similar to those
of the Z scale,4c,17 which also sense dipolarity and hydrogen-
bond acidity of solvents.
The question is: is it really necessary to introduce a new
solvent polarity scale in addition to the many already existing
scales, with the respect to the fact that the new PT scale
correlates well with the Z and ET(30) scale? There is one new
aspect which possibly leads to a positive answer to this
question: the new probe dye contains an intramolecular
hydrogen bond, which can be broken in hydrogen-bond-
accepting (HBA) solvents, giving a new intermolecular solute/
solvent hydrogen bond, with corresponding changes in the
electronic absorption spectrum. Therefore, the PT scale includes
also to a certain extent the HBA-properties of a solvent, which
is not measured by the Z and ET(30) values. Actually, the major
structural difference between the probe molecule of the PT scale
and the probe molecules of the ET(30), Y, and Z scale is that
the probe molecule of the PT scale is a hydrogen-bond donor,
which allows the PT scale to sense HBA solvents. The PT scale
cannot get a better fit with the ET(30), Y, or Z scale probably
because of this major structural difference.
The probe molecule of the ET(30) scale is a betaine dye,
whose charge-transfer absorption disappears in stronger acidic
solvents like acetic acid because of protonation, so it is not a
good probe for acidic solvents.1c The ET(30) values of acidic
solvents were deduced from the Z scale, which can be measured
in acidic solvents and shows good linear correlation with the
ET(30) scale.1c On the contrary, our probe molecule, o-1, of the
PT scale can be used to measure the solute/solvent interactions
with acidic solvents, such as acetic acid and phosphoric acid.
The probe molecule of the PT scale still has some room to
get improved in order for the lowest energy electronic
absorption band to appear at a longer wavelength and to be
shifted hypsochromically with increasing solvent polarity to as
large an extent as possible. We hope the probe molecule, o-1, of
the PT scale can trigger the generation of even better twisting-
based spectroscopic indicators for solvent polarity in the future.
EXPERIMENTAL SECTION
■
Materials and Methods. The o-ABDI is known and was prepared
according to the literature method.10 All solvents were ordered from
fine chemical companies and were of spectrophotometric grade or had
the highest purity in the market. Before the measurement of the
electronic absorption spectrum of o-1, all solvents were dried with
activated 4 Å molecular sieves. The concentration of o-1 in all 51
solvents used for electronic absorption measurement was 3.0 × 10−5
M. High-resolution mass (HRMS) measurements were obtained with
a magnetic-electric sector mass analyzer.
(Z)-4-[2-(Acetamino)benzylidene]-1,2-dimethyl-(1H, 4H)-imida-
zol-5-one (o-1). To o-ABDI (215 mg, 1 mmol) dissolved in 3 mL
of dried THF was added acetic anhydride (204 mg, 2 mmol). The
mixture was stirred in a nitrogen atmosphere at room temperature for
15 h. The solution was quenched by 10 mL of water. The quenched
solution was extracted with ethyl acetate. The organic layer was
separated from the aqueous layer, dried with anhydrous sodium sulfate
and concentrated by a rotary evaporator. The crude product was
purified by column chromatography with hexane/ethyl acetate (1:1) as
a mobile phase to get o-1 (219 mg, 0.85 mmol), and the yield was
1
85%: H NMR (CDCl3, 300 MHz) δ 2.21 (s, 3H, CH3), 2.40 (s, 3H,
CH3), 3.22 (s, 3H, CH3), 7.09 (t, J = 7.3 Hz, 1H, PhH), 7.15 (s, 1H,
CH), 7.38−7.46 (m, 2H, PhH), 8.36 (d, J = 8.3 Hz, 1H, PhH), 12.34
(br s, 1H, NH); 13C NMR (CDCl3, 75 MHz) δ 15.5, 24.5, 26.9, 122.9,
123.6, 123.7, 128.3, 132.2, 135.1, 135.9, 138.0, 160.4, 169.0, 169.2;
HRMS (EI, M+) m/z calcd for C14H15N3O2 257.1164, found
257.1169.
ASSOCIATED CONTENT
■
S
* Supporting Information
Energy, dipole moment, redundant internal coordinates, and
1H and 13C NMR and HMQC spectra of o-1, as well as the 1H
and 13C NMR spectra of o-ABDI. This material is available free
of charge via the Internet at http://pubs.acs.org.
5930
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931

The Journal of Organic Chemistry
Article
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision A.1;
Gaussian, Inc.: Pittsburgh, PA, 2003.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kssung@mail.ncku.edu.tw.
Notes
■
(10) Chen, Y.-H.; Lo, W.-J.; Sung, K. J. Org. Chem. 2013, 78, 301−
310.
The authors declare no competing financial interest.
(11) (a) Zhu, X.-M.; Zhang, S.-Q.; Zheng, X.; Phillips, D. L. J. Phys.
Chem. A 2005, 109, 3086−3093. (b) Vaswani, H. M.; Hsu, C.-P.;
Head-Gordon, M.; Fleming, G. R. J. Phys. Chem. B 2003, 107, 7940−
7946.
ACKNOWLEDGMENTS
■
We thank the National Science Council of Taiwan for financial
support (NSC101-2113-M-006-001-MY3).
(12) The dipole moment for the optimized structure of the electronic
ground-state of o-1 at the B3PW91/6-311++G(3df,2p)// B3LYP/6-
31+G* level is 5.1248 D. The dipole moment for the optimized
structure of the first singlet excited state of o-1 at the B3LYP/def-
SV(P) level is 3.1338 D, which was calculated with the Turbomole
version 6.0.
REFERENCES
■
(1) (a) Reichardt, C. Pure Appl. Chem. 2008, 80, 1415−1432.
(b) Reichardt, C. Pure Appl. Chem. 2004, 76, 1903−1919. (c) Reich-
ardt, C. Chem. Rev. 1994, 94, 2319−2358. (d) Reichardt, C.; Asharin-
Fard, S.; Blum, A.; Eschner, M.; Mehranpour, A.-M.; Milart, P.; Niem,
T.; Schafer, G.; Wilk, M. Pure Appl. Chem. 1993, 65, 2593−2601.
(e) Reichardt, C.; Dimroth, K. Z. Anal. Chem. 1966, 215, 344−350.
(2) Meyer, K. H. Ber. Dtsch. Chem. Ges. 1914, 47, 826−832.
(3) (a) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17,
121−158. (b) Grunwald, E.; Winstein, S. J. Am. Chem. Soc. 1948, 70,
846−854.
(4) (a) Larsen, J. W.; Edwards, A. G.; Dobi, P. J. Am. Chem. Soc.
1980, 102, 6780−6783. (b) Kosower, E. M. An Introduction to Physical
Organic Chemistry; Wiley: New York, 1968. (c) Kosower, E. M. J. Am.
Chem. Soc. 1958, 80, 3253−3260. (d) Kosower, E. M. J. Am. Chem. Soc.
1958, 80, 3261−3266. (e) Kosower, E. M. J. Am. Chem. Soc. 1958, 80,
3267−3270. (f) Kosower, E. M.; Skorcz, J. A.; Schwarz, W. M.; Patton,
J. W. J. Am. Chem. Soc. 1960, 82, 2188−2191.
(5) (a) Brooker, L. G. S.; Craig, A. C.; Heseltine, D. W.; Jenkins, P.
W.; Lincoln, L. L. J. Am. Chem. Soc. 1966, 87, 2443−2450. (b) Kamlet,
M. J.; Abboud, J.-L. M.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027−
6038. (c) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. J. Am. Chem. Soc.
1977, 99, 8325−8327. (d) Buncel, E.; Rajagopal, S. J. Org. Chem. 1989,
54, 798−809. (e) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1983, 22,
3825−3828. (f) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1986, 25,
3212−3218.
(13) Chen, Y.-H. Ph.D. Thesis; National Cheng Kung University:
Tainan, Taiwan, 2012.
(14) Klessinger, M.; Michl, J. Excited States and Photochemistry of
Organic Molecules; VCH Publishers: New York, 1995.
(15) (a) Soriano-Correa, C.; del Valle, F. J. O.; Munoz-Losa, A.;
Galvan, I. F.; Martin, M. E.; Aguilar, M. A. J. Phys. Chem. B 2010, 114,
8961−8970. (b) Chang, J.; Lenhoff, A. M.; Sandler, S. I. J. Phys. Chem.
B 2007, 111, 2098−2106. (c) Wong, S. E.; Bernacki, K.; Jacobson, M.
J. Phys. Chem. B 2005, 109, 5249−5258.
(16) Galamba, N. J. Phys. Chem. B 2012, 116, 5242−5250.
(17) (a) Griffiths, T. R.; Pugh, D. C. Coord. Chem. Rev. 1979, 29,
129−211. (b) Medda, K.; Chatterjee, P.; Chandra, A. K.; Bagchi, S. J.
Chem. Soc., Perkin Trans. 2 1992, 343−346. (c) Medda, K.; Chatterjee,
P.; Pal, M.; Bagchi, S. J. Solution Chem. 1990, 19, 271−287.
(18) Abraham, M. H.; Grellier, P. L.; Nasehzadeh, A.; Walker, R. A.
C. J. Chem. Soc., Perkin Trans. 2 1988, 1717−1719.
(19) Laurence, C.; Nicolet, P.; Dalati, M. T.; Abboud, J.-L.; Notario,
R. J. Phys. Chem. 1994, 98, 5807−5816.
(6) (a) Kucherak, O. A.; Richert, L.; Mely, Y.; Klymchenko, A. S.
Phys. Chem. Chem. Phys. 2012, 14, 2292−2300. (b) Dsouza, R. N.;
Pischel, U.; Nau, W. M. Chem. Rev. 2011, 111, 7941−7980. (c) Saito,
R.; Matsumura, Y.; Suzuki, S.; Okazaki, N. Tetrahedron 2010, 66,
8273−8279. (d) Lopez, M.; Velasco, D.; Lopez-Calahorra, F.; Julia, L.
Tetrahedron Lett. 2008, 49, 5196−5199. (e) Xing, Y.; Lin, H.; Wang,
F.; Lu, P. Sens. Actuators B 2006, 114, 28−31. (f) Kosower, E. M. Acc.
Chem. Res. 1982, 15, 259−266. (g) Scherer, T.; Hielkema, W.; Krijnen,
B.; Hermant, R. M.; Eijckelhoff, C.; Kerkhof, F.; Ng, A. K. F.; Verleg,
R.; van der Tol, E. B.; Brouwer, A. M.; Verhoeven, J. W. Recl. Trav.
Chim. Pays-Bas 1993, 112, 535−548.
(7) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Modern Molecular
Photochemistry of Organic Molecules; University Science Publishers:
New York, 2010.
(8) (a) Kudo, K.; Momotake, A.; Kanna, Y.; Nishimura, Y.; Arai, T.
Chem. Commun. 2011, 47, 3867−3869. (b) Kudo, K.; Momotake, A.;
Tanaka, J. K.; Miwa, Y.; Arai, T. Photochem. Photobiol. Sci. 2012, 11,
674−678.
(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.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J.
B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.;
Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman,
J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
5931
dx.doi.org/10.1021/jo400586w | J. Org. Chem. 2013, 78, 5925−5931