Remarkable solvatochromic color change via proton tautomerism of a phenol-linked imidazole derivative

Article abstract of DOI:10.1021/jp5007928

A unique solvatochromic 2-phenyl-4,5-diarylimidazole derivative linked with a phenol moiety and a p-quinonemethide moiety at the 4- and 5-positions of a imidazole ring, which shows remarkable color change via proton tautomerism, was synthesized and the mechanism of the solvatochromic color change was investigated. The yellow-colored OH tautomeric form (1-OH) exists as a dominant species in nonpolar solvents, whereas the blue-colored NH tautomeric form (1-NH) is stabilized in polar solvents. The molecular structures of these tautomers were determined by X-ray crystallographic analysis. The p-quinonemethide moiety and the imidazole ring of 1-OH are coplanar to one another and possess a planar quinoidal structure. On the other hand, 1-NH has a nonplanar twisted quinoidal structure causing large bathochromic shift in the visible absorption spectrum. Moreover, the X-ray crystallographic analysis and the DFT calculations support the closed-shell singlet character of 1-OH. In contrast 1-NH possesses partial single bond character that leads to the open-shell singlet biradical character and the decrease in the singlet-triplet energy gap. The twisting of the π-conjugated electron system induced by the proton tautomerization was found to be the origin of the open-shell biradical character of 1-NH and the enhanced solvatochromic color change.

Full text of DOI:10.1021/jp5007928

Article
pubs.acs.org/JPCA
Remarkable Solvatochromic Color Change via Proton Tautomerism
of a Phenol-Linked Imidazole Derivative
Hiroaki Yamashita and Jiro Abe*
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara,
Kanagawa 252-5258, Japan
S
* Supporting Information
ABSTRACT: A unique solvatochromic 2-phenyl-4,5-diarylimidazole derivative linked with a
phenol moiety and a p-quinonemethide moiety at the 4- and 5-positions of a imidazole ring,
which shows remarkable color change via proton tautomerism, was synthesized and the
mechanism of the solvatochromic color change was investigated. The yellow-colored OH
tautomeric form (1_OH) exists as a dominant species in nonpolar solvents, whereas the blue-
colored NH tautomeric form (1_NH) is stabilized in polar solvents. The molecular structures
of these tautomers were determined by X-ray crystallographic analysis. The p-
quinonemethide moiety and the imidazole ring of 1_OH are coplanar to one another and
possess a planar quinoidal structure. On the other hand, 1_NH has a nonplanar twisted
quinoidal structure causing large bathochromic shift in the visible absorption spectrum.
Moreover, the X-ray crystallographic analysis and the DFT calculations support the closed-
shell singlet character of 1_OH. In contrast 1_NH possesses partial single bond character
that leads to the open-shell singlet biradical character and the decrease in the singlet−triplet
energy gap. The twisting of the π-conjugated electron system induced by the proton
tautomerization was found to be the origin of the open-shell biradical character of 1_NH and the enhanced solvatochromic color
change.
the remarkable solvatochromic color change in the visible light
region between the oxime red tautomer and the nitroso blue-
green tautomer via proton tautomerism.14
INTRODUCTION
■
Proton tautomerism has been of sustained interest because of
its importance for understanding various chemical and
biochemical phenomena ever since Lapworth’s classic paper
on acetone enolization over a century ago.1−5 In the field of
organic synthesis and biochemistry, many reaction mechanisms
are explained by the proton tautomerism such as a keto−enol
tautomerism.6,7 From physicochemical point of view, proton
tautomerism can be described by an intramolecular proton
transfer reaction associated with a π-bond shifting. Besides, the
position of an equilibrium is generally sensitive to the local
environment of molecules. This implies that proton tautomer-
ism has potential for switching the molecular properties of π-
electron compounds by external stimuli such as light, heat, and
solvent polarity.8−15 It is noteworthy that π-bond shifting can
drastically change the π-electronic structure and the corre-
sponding physical properties such as dipole moment and
molecular polarizability of a molecule. However, there are only
a few examples of remarkable solvatochromic color change in
the visible light region resulting from switching of the π-
electronic structure by proton tautomerism. Furuta et al.
reported the solvatochromism via proton tautomerism between
the two types of tautomers for a N-confused porphyrin
(NCP).13 The significant solvatochromic color change of the
NCP was explained on the basis of the pronounced alteration
in the π-electronic structure by the proton tautomerization
between the aromatic tautomer and the less-aromatic tautomer.
4-Nitoroso-5-aminopyrazole derivatives are also known to show
Here we report a solvatochromic 2-phenyl-4,5-diarylimida-
zole derivative linked with a phenol moiety and a p-
quinonemethide moiety at the 4- and 5-positions of a imidazole
ring, that shows remarkable color change via proton
tautomerism. In nonpolar solvents, the OH tautomeric form,
1_OH, exists as a dominant species and shows yellow color.
However, the blue-colored NH tautomeric form, 1_NH, is
stabilized with remarkable color change in polar solvents (Chart
1). We investigated the structure−property relationship of the
solvatochromic imidazole to reveal the mechanism of the
remarkable solvatochromic color change and found that 1_NH
has some open-shell character caused by the bond twisting
despite purely closed-shell character of 1_OH. We report that
1_NH possess some open-shell singlet biradical character due
to the twisting of the π-conjugated electron system induced by
proton tautomerization and propose a new concept for
switching π-electronic structures by solvent polarity.
EXPERIMENTAL SECTION
■
Synthesis. All reactions were monitored by thin-layer
chromatography carried out on 0.2 mm E. Merck silica gel
Received: January 23, 2014
Revised: February 7, 2014
Published: February 10, 2014
© 2014 American Chemical Society
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2,6-Di-tert-butyl-4-[5-(3,5-di-tert-butyl-4-hydroxy-
phenyl)-2-phenyl-4H-imidazole-4-ylidene]cyclohexa-
2,5-dienone (1_OH). To a solution of 1_L (39.0 mg, 0.071
mmol) in benzene (4 mL) under nitrogen stirred at room
temperature was added PbO2 (44.0 mg, 0.18 mmol) and the
reaction mixture was stirred at room temperature for 2.5 h.
PbO2 was filtered off and the filtrate was concentrated in vacuo.
The crude product was purified by silica gel column
chromatography benzene as eluent to give a dark red solid
Chart 1
1
(1_OH), 28.1 mg (0.0510 mmol, 71%). H NMR (400 MHz,
CDCl3, δ): 8.56 (d, J = 8.0 Hz, 2H), 7.58−7.50 (m, 3H), 7.42
(s, 2H), 7.36 (s, 1H), 7.32 (d, J = 2.3 Hz, 1H), 5.57 (s, 1H),
1.47 (s, 18H), 1.41 (s, 9H), 1.04 (s, 9H). ESI−TOF−MS (m/
z): 551 [M + H]+.
1-(3,5-Di-tert-butyl-4-hydroxyphenyl)-2-(3,5-di-tert-
butyl-4-methoxyphenyl)ethane-1,2-dione (2_OMe). To a
suspension of 2 (50.0 mg, 0.107 mmol) and potassium
carbonate (15 mg, 0.108 mmol) in dry acetone (10 mL) was
added methyl iodide (10 μL, 0.161 mmol). After being refluxed
for 4 h, the reaction mixture was cooled to room temperature
and concentrated in vacuo. The reaction mixture was dissolved
in CH2Cl2 and washed with water. The organic layer was dried
over Na2SO4, filtered, and then concentrated in vacuo. The
crude product was purified by silica gel column chromatog-
raphy hexane/CH2Cl2 = 1:1 as eluent to give a white powder
(2_OMe), 20.4 mg (0.0428 mmol, 40%). 1H NMR (400 MHz,
CDCl3, δ): 7.89 (s, 2H), 7.85 (s, 2H), 5.90 (s, 1H), 3.74 (s,
3H), 1.45 (s, 18H), 1.42 (s, 18H). ESI−TOF−MS (m/z): 503
[M + Na]+.
plates (60F-254). Column chromatography was performed on
silica gel (Wakogel C-300). H NMR spectra were recorded at
1
400 MHz on a Bruker AVANCE III 400 NanoBay. Acetone-d6,
DMSO-d6, and CDCl3 were used as deuterated solvent. ESI−
TOF−MS spectra were recorded on a Bruker micrOTOF II-
AGA1. All glassware was washed with distilled water and dried.
Unless otherwise noted, all reagents and reaction solvents were
purchased from TCI, Wako Co. Ltd., Aldrich Chemical Co.,
Inc., and ACROS Organics and were used without further
purification.
1,2-Bis(3,5-di-tert-butyl-4-hydroxyphenyl)ethane-1,2-
dione (2). To a solution of 2,6-di-tert-butylphenol (1.0 g, 5.61
mmol) in dry dichloromethane (30 mL) under nitrogen stirred
at 0 °C was added oxalylchloride (0.29 mL, 3.38 mmol) and the
reaction mixture was stirred at 0 °C for 10 min. Then, AlCl3
(898 mg, 6.73 mmol) was added and the reaction mixture was
stirred at 0 °C for 30 min. The reaction mixture was allowed to
warm to room temperature and was stirred at room
temperature overnight. The reaction mixture was poured into
water and the organic layer was extracted with dichloro-
methane. The organic layer was dried over Na2SO4, filtered,
and then concentrated in vacuo. The crude product was
purified by recrystallization (dichloromethane/hexane) to give
a yellow crystal (2), 481 mg (1.03 mmol, 37%). 1H NMR (400
MHz, CDCl3, δ): 7.85 (s, 4H), 5.89 (s, 2H), 1.45 (s, 36H).
ESI−TOF−MS (m/z): 489 [M + Na]+.
4,4′-(2-Phenyl-1H-imidazole-4,5-diyl)bis(2,6-di-tert-
butylphenol) (1_L). Compound 2 (170.6 mg, 0.366 mmol),
benzaldehyde (38.6 mg, 0.364 mmol), and ammonium acetate
(130.0 mg, 1.69 mmol) were refluxed in acetic acid (5 mL) for
28 h. The reaction mixture was cooled to room temperature
and the slurry that precipitated by the neutralization the
mixture by aqueous NH3 was filtered off. The residue was
purified by silica gel column chromatography CHCl3 as eluent
to give a white powder (1_L), 168 mg (0.328 mmol, 90%). 1H
NMR (400 MHz, DMSO-d6, δ): 12.73 (s, 1H), 8.00 (d, J = 7.5
Hz, 2H), 7.44 (t, J = 7.3 Hz, 4H), 7.32 (s, 3H), 7.15 (s, 2H),
7.09 (s, 1H), 6.70 (s, 1H), 1.35 (s, 18H), 1.28 (s, 18H). ESI−
TOF−MS (m/z): 553 [M + H]+.
2,6-Di-tert-butyl-4-(5-(3,5-di-tert-butyl-4-methoxy-
phenyl)-2-phenyl-1H-imidazol-4-yl)phenol (1_L_OMe).
The compound 2_OMe (20.0 mg, 0.0416 mmol), benzalde-
hyde (25.0 mg, 0.236 mmol), and ammonium acetate (44.0 mg,
0.571 mmol) were refluxed in acetic acid (3 mL) for 24 h. The
reaction mixture was cooled to room temperature and the
slurry that precipitated by the neutralization the mixture by
aqueous NH3 was filtered off. The residue was washed with
hexane to give a white powder (1_L_OMe), 10.0 mg (0.328
1
mmol, 42%). H NMR (400 MHz, DMSO-d6) mixture of two
diastereomers δ: 12.48 (s, 1H, one diastereomer), 12.2 (s, 1H,
one diastereomer), 7.63 (s, 1H, one diastereomer), 7.48−7.42
(m, 4H, one diastereomer), 7.35 (s, 4H, one diastereomer),
7.28 (s, 2H, one diastereomer), 7.22 (s, 2H, one diastereomer),
7.16 (s, 2H, one diastereomer), 7.10 (s, 2H, one diastereomer),
1.36−1.28 (m, 72H, two diastereomers). ESI−TOF−MS (m/
z): 565 [M − H]−.
2,6-Di-tert-butyl-4-[5-(3,5-di-tert-butyl-4-methoxy-
phenyl)-2-phenyl-4H-imidazol-4-ylidene]cyclohexa-2,5-
dienone (1_OMe). To a solution of 1_L_OMe (23.1 mg,
0.0408 mmol) in benzene (3 mL) under nitrogen stirred at
room temperature was added PbO2 (98.6 mg, 0.412 mmol) and
the reaction mixture was stirred at room temperature for 6 h.
PbO2 was filtered off and the filtrate was concentrated in vacuo.
The crude product was purified by silica gel column
chromatography benzene as eluent to give a dark red solid
(1_OMe), 11.0 mg (0.0192 mmol, 47%). 1H NMR (400 MHz,
CDCl3, δ): 8.56 (d, J = 6.6 Hz, 2H), 8.44 (d, J = 1.2 Hz, 1H),
7.53−7.51 (m, 3H), 7.46 (s, 1H), 7.36 (s, 3H), 1.46 (s, 18H),
1.40 (s, 9H), 1.03 (s, 9H). ESI−TOF−MS (m/z): 565 [M +
H]+.
4,4′-(1-Methyl-2-phenyl-1H-imidazole-4,5-diylidene)-
bis(2,6-di-tert-butylcyclohexa-2,5-dienone) (1_NMe). To
a suspension of 1_NH (50.0 mg, 0.107 mmol) and potassium
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Scheme 1
carbonate (15 mg, 0.108 mmol) in dry acetone (10 mL) was
added dimethyl sulfate (10 μL, 0.161 mmol). After being
refluxed for 2 h, the reaction mixture was cooled to room
temperature and concentrated in vacuo. The reaction mixture
was dissolved in CH2Cl2 and washed with water. The organic
layer was dried over Na2SO4, filtered, and then concentrated in
vacuo. The crude product was purified by silica gel column
chromatography benzene as eluent to give a dark blue solid
(1_NMe), 20.4 mg (0.0428 mmol, 40%). 1H NMR (400 MHz,
DMSO-d6, δ): 8.20 (s, 1H), 7.91 (d, J = 7.5 Hz, 2H), 7.68−7.63
(m, 3H), 7.29 (s, 2H), 6.95 (s, 1H), 1.33 (s, 9H), 1.21 (s,
18H), 1.13 (s, 9H). ESI−TOF−MS (m/z): 565 [M + H]+.
X-ray Crystallographic Analysis. The diffraction data of
the single crystal was collected on the Bruker APEX II CCD
area detector (Mo Kα, λ = 0.71073 nm). The data refinement
was carried out by the Bruker APEXII software package with
SHELXT program.16,17 All non-hydrogen atoms were aniso-
tropically refined.
DFT Calculation. All calculations were carried out using the
Gaussian 09 program (Revision D.01).18 The molecular
structures were fully optimized at the (U)B3LYP/6-31+G(d,p)
and the PCM−DFT B3LYP/6-31+G(d,p) levels of the theory,
and analytical second derivatives were computed using
vibrational analysis to confirm each stationary point to be a
minimum. The TDDFT calculations were performed at the
B3LYP/6-31+G(d,p) level of the theory for the optimized
geometries obtained by the (U)B3LYP/6-31+G(d,p) method.
Figure 1. (a) The dichloromethane solution (left) and acetone
solution (right) in the tautomeric equilibrium between 1_OH and
1_NH. (b) UV−vis absorption spectra for the solutions in the
tautomeric equilibrium in the binary mixture of dichloromethane/
acetone. The volume ratios of two solvents are shown in legends. All
spectra were measured at the same concentration (2.4 × 10−4 M).
1. The UV−vis absorption spectra of 1_OH dissolved in a
series of dichloromethane/acetone binary mixture with differ-
ent volume ratio were measured to investigate the influence of
the solvent polarity. The pure dichloromethane solution shows
the absorption peak at 433 nm as shown in Figure 1b. With
increasing the solvent polarity, the absorption band around at
433 nm decreases, while the absorption band around at 656 nm
increases. These spectral changes are continuous and imply the
presence of the equilibrium between two species. To identify
RESULTS AND DISCUSSION
■
Solvatochromism and Molecular Structures of Tau-
tomers. The oxidation of the imidazole precursor 1_L with
PbO2 in benzene gives dark yellow solution of 1_OH (Scheme
1). However, 1_OH gives dark blue solutions when dissolved
in polar solvents such as methanol, acetone, and dimethylsulf-
oxide (DMSO) despite giving dark yellow color in nonpolar
solvents such as hexane, benzene, and dichloromethane
(Supporting Information Figure S2). As shown in Figure 1a,
the dichloromethane and acetone solutions of 1_OH show
typical solution colors in nonpolar and polar solvent,
respectively. The remarkable color change of the solutions
suggests a presence of a proton tautomeric equilibrium between
1_OH and another tautomer 1_NH, which is shown in Chart
1
each species in polar and nonpolar solvents, the H and 13C
NMR spectra were examined (Supporting Information Figures
S4−S7). In CDCl3 at room temperature, there is a character-
istic 1H NMR signal at 5.6 ppm assignable to the phenolic OH,
indicating 1_OH is the dominant species in nonpolar solvent. A
downfield 13C NMR signal at 187.0 ppm is attributable to the
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Scheme 2
carbonyl carbon of the p-quinonemethide moiety, which is
consistent with the structure of 1_OH. The 13C NMR signals at
178.3 and 173.0 ppm can be assigned to the two carbons of the
CN double bonds, which also support that 1_OH is
dominant species in nonpolar solvents. On the other hand,
1_NMe in hexane and acetone are shown in Figure 2a. The
absorption spectra of 1_OMe and 1_NMe, respectively, are
1
the H and 13C NMR spectra in acetone-d6 are completely
different from those in CDCl3. The broadening of the 1H NMR
signals are observed for all protons except for the protons in
phenyl rings at room temperature (Supporting Information
Figure S5a). In addition, only the 13C NMR signals attributable
to the carbons in the phenyl rings and tert-butyl groups were
observed at room temperature (Supporting Information Figure
S5b). The reason for these spectral behaviors will be discussed
later in detail. In order to prevent the signal broadening, the 1H
and 13C NMR spectra in acetone-d6 were measured at low
temperature. The 1H NMR signals became sharp with
decreasing the temperature, and a characteristic downfield
signal at 12.3 ppm assignable to the imidazole NH is observed
at −60 °C (Supporting Information Figure S5a). The two
downfield 13C NMR signals were observed at 186.6 and 185.5
ppm (Supporting Information Figure S5b). These signals
indicate the presence of two carbonyl carbons of the p-
quinonemethide moieties, which is consistent with the structure
of 1_NH. Therefore, we concluded that the dominant species
in polar solvents is 1_NH.
By comparing the UV−vis absorption spectrum of 1_OH in
basic methanol (Supporting Information Figure S7), the
absorption band around at 900 nm observed in acetone can
be assigned to an anionic intermediate (Supporting Information
Scheme S1). This band can be observed even though dry
acetone is used as a solvent, indicating that acetone is
protonated by 1_OH (Supporting Information Figure S3).
The origin of strong acidity of 1_OH is considered to be the
thermal stabilization of the anionic species by charge
delocalization.
Figure 2. (a) UV−vis absorption spectra for 1_OMe (dichloro-
methane solution, red dashed line; acetone solution, blue dashed line),
and 1_NMe (dichloromethane solution, red dotted line; acetone
solution, blue dotted line). Solid lines are UV−vis absorption spectra
for the solution in the tautomeric equilibrium between 1_OH and
1_NH in dichloromethane (red solid line) and acetone (blue solid
line). (b) Experimental and theoretical UV−vis absorption spectra for
1_OH and 1_NH. The calculated spectra (B3LYP/6-31+G(d,p)) are
shown by the perpendicular lines.
To obtain clear evidence for the presence of the proton
tautomeric reaction, O- and N-methylated derivatives (1_OMe
and 1_NMe), which are inactive against the proton tautomeric
reaction in both polar and nonpolar solvents, were synthesized
(Scheme 2). The UV−vis absorption spectra of 1_OMe and
similar to those of 1_OH and 1_NH, giving clear evidence that
the spectral changes by changing the solvent polarity are caused
by the proton tautomeric equilibrium between two tautomers.
We investigated the driving force of the proton tautomeric
reaction by using the DFT calculations including the bulk
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solvent effect by the polarized continuum model (DFT−PCM).
The molecular structures were fully optimized in heptane (ε =
1.92), acetone (ε = 20.7), and DMSO (ε = 46.7) at the PCM/
B3LYP/6-31+G(d,p) level of the theory. The changes in Gibbs
free energies ΔG° of the tautomeric reaction from 1_OH to
1_NH are +7.92, +6.25, −1.24, and −2.38 kJ/mol in gas phase,
heptane, acetone, and DMSO, respectively (Supporting
Information Figure S10). The ΔG° values show that 1_OH
is thermally more stable than 1_NH in gas phase or nonpolar
solvents. However, it is predicted that the relative thermal
stabilities would reverse in polar solvents and 1_NH becomes
more stable than 1_OH. This regular shift of the ΔG° values of
the tautomers predicted by the DFT−PCM calculations is
consistent with the experimental observations. The energetic
preferences of 1_OH in nonpolar solvent and 1_NH in polar
solvent can be explained as follows. In gas phase or nonpolar
solvents, 1_OH is more stable than 1_NH because of the
aromatic stabilization energy. However, in polar solvents,
1_NH is largely stabilized by solvent−solute electrostatic
interaction because the electric dipole moment of 1_NH is
larger than that of 1_OH. The calculated dipole moments in
gas phase (B3LYP/6-31+G(d,p)) of 1_NH and 1_OH are 8.22
and 4.89 D, respectively.
The theoretical absorption spectra of 1_OH and 1_NH
calculated by the TDDFT (B3LYP/6-31+G(d,p)) method are
shown in Figure 2b along with the experimental absorption
spectra in hexane and acetone. The calculation results explain
well the experimental observations and support the exper-
imental result that 1_OH and 1_NH are the dominant species
in nonpolar and polar solvents, respectively. The S0 → S3
transition (f = 0.4357) at 486 nm of 1_OH is attributable to
the HOMO−1 → LUMO transition which is described by the
transition from the phenol moiety with electron-donating
characteristics to the p-quinonemethide moiety with electron-
withdrawing characteristics (Figure 3). On the other hand, the
S0 → S1 transition (f = 0.2911) at 660 nm of 1_NH is
attributable to the HOMO → LUMO transition, described by
the π → π* transition of the two p-quinonemethide moieties
linked by the imidazole ring. These calculation results support
that the proton tautomeric reaction causes a large change in the
π-electronic structure and results in the drastic color change.
The tautomerization process from 1_OH to 1_NH loses the
aromatic phenol moiety and forms the p-quinonemethide
moiety. This tautomerization process leads to the breaking of
the p-quinoid structure composed of the p-quinonemethide
moiety and the imidazole ring in 1_OH and the formation of a
long π-conjugated moiety consisted of the two p-quinoneme-
thide moieties, resulting in the large bathochromic shift in the
visible absorption spectrum through the proton tautomeriza-
tion.
Moreover, we found that the TDDFT calculation (B3LYP/6-
31+G(d,p)) for a model compound, 1_NH_model in which all
tert-butyl groups are removed from 1_NH, predicts that the S0
→ S1 transition (f = 0.4822) is located at 575 nm, which is
much shorter than the S0 → S1 transition (660 nm, f = 0.2911)
of 1_NH calculated by the same level of the theory (Supporting
Information Figure S11). The interpretation of this hypso-
chromic shift of the absorption spectrum of 1_NH_model is as
follows. The calculated HOMO−LUMO energy gap of
1_NH_model is larger than that of 1_NH, implying that the
steric hindrance caused by the bulky tert-butyl groups affects
the energy levels of the frontier molecular orbitals. The HOMO
and LUMO, respectively, exhibit bonding and antibonding
Figure 3. Relevant molecular orbitals of 1_OH and 1_NH obtained at
the B3LYP/6-31+G(d,p) level of the theory.
characters with respect to the two CC double bonds between
the imidazole ring and the p-quinonemethide moieties. Thus, it
is expected that the resonance integral decreases with increasing
the twisting angles of the CC double bonds. Thus twisting of
the CC double bonds leads to raising the HOMO energy
and lowering the LUMO energy as illustrated in Figure 4.19,20
For example, in a twisted ethylene model the double bond
character decreases with increasing the twisting angle of the
CC double bond. In a 90° twisted ethylene, the HOMO−
LUMO energy gap vanishes and consequently two degenerate
orbitals localized on two carbon atoms are formed.21 The
ground electronic state of the 90° twisted ethylene is the triplet
biradical state according to Hund’s rule. Furthermore, the
oscillator strength of the HOMO → LUMO transition would
be getting close to zero with increasing the twisting angle of the
CC double bond because of decreasing the orbital overlap
between the adjacent π orbitals. Actually, the oscillator strength
of the S0 → S1 transition of 1_NH (f = 0.2911) is smaller than
that of 1_NH_model (f = 0.4822). The reason why the
chromophore of 1_NH shows the absorption band in near-
infrared region despite the short π-conjugated length is the
twisting of the π-conjugated chromophore. Thus the twisting of
a π-conjugated moiety is found to be effective to shift the
absorption maximum to the longer wavelength region though
the absorption intensity will be sacrificed.
The molecular structures of the 1_OH and 1_NH were
determined by single crystal X-ray crystallographic analysis
(Figure 5). The deep red crystals of 1_OH suitable for X-ray
crystallographic analysis were prepared by recrystallization from
benzene. The bond lengths and torsion angles are summarized
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Figure 4. The effect of twisting the double bond on the energy levels of the frontier molecular orbitals of a butadiene unit.
in Table 1. The C1−O1 bond length (1.381 Å) is consistent
with a typical bond length for the C−O single bond. The C7−
O2 (1.226 Å), C13−N1 (1.307 Å), and C15−N2 (1.308 Å) are
also consistent with the bond lengths for the usual CO and
CN double bonds, suggesting that 1_OH has planar
quinoidal structure as shown in Figure 5. The phenol ring is
almost orthogonal to the imidazole ring (the dihedral angle
N1−C13−C4−C3 = 91.9°) due to the formation of the cyclic
hydrogen-bonded dimer with neighboring molecules in which
two O−H−N hydrogen bonds between the phenol groups and
the imidazole rings are formed. On the other hand, the deep
blue crystals of 1_NH were obtained by recrystallization from
DMSO. The hydrogen bond between the imidazole NH and
the oxygen atom of DMSO was observed (2.874 Å), suggesting
that 1_NH is stabilized through the hydrogen bonding
formation in polar solvent compared with 1_OH. The
comparison of the bond lengths between 1_NH and 1_OH
reveals a notable structural feature of 1_NH. The CO double
bonds of 1_NH, C1−O1 (1.246 Å) and C7−O2 (1.248 Å) are
slightly longer than the C7−O2 bond of 1_OH (1.226 Å). The
bond lengths of the C4−C13 (1.404 Å) and C10−C14 (1.401
Å) are longer than the C10−C14 bond of 1_OH (1.384 Å).
Another notable feature is large torsion angle of the C4−C13
double bond (C5−C4−C13−C14 = 32.4°), which shows
Figure 5. ORTEP representation of the molecular structures of 1_OH
and 1_NH with thermal ellipsoids (50% probability) where nitrogen
and oxygen atoms are highlighted in blue and red, respectively. The
hydrogen atoms on the phenyl rings and tert-butyl groups, and the
solvent molecules are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Dihedral Angles (deg) for 1_OH and 1_NH Obtained by the X-ray Crystallographic
Analysis and the DFT Calculations
1_OH (X-ray)
1_OH (DFT)
1_NH (X-ray)
1_NH (DFT CS)
1_NH (DFT SBR)
N1−C13
C13−C14
C14−N2
N2−C15
C15−N1
C4−C13
C10−C14
O1−C1
1.307
1.478
1.386
1.308
1.426
1.485
1.384
1.381
1.226
82.7
1.313
1.495
1.380
1.320
1.405
1.468
1.395
1.371
1.237
43.2
1.385
1.455
1.393
1.314
1.373
1.404
1.401
1.246
1.248
32.4
1.390
1.460
1.389
1.310
1.385
1.403
1.408
1.243
1.241
21.44
15.18
1.388
1.446
1.385
1.314
1.381
1.416
1.422
1.246
1.244
24.0
O2−C7
C5−C4−C13−C14
C11−C10−C14−C13
9.9
9.6
0.1
18.2
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Article
marked distortion due to the steric repulsion between bulky
tert-butyl groups. These structural features indicate that the
CC and CO double bonds in 1_NH have partial single
bond character, implying a certain contribution of open-shell
singlet biradical (SBR) character. The SBR character generally
increases with decreasing the overlap integral between the two
adjacent π-orbitals.22,23 For comparison, the theoretical bond
lengths of the closed-shell (CS) and SBR states of 1_NH were
calculated at the (U)B3LYP/6-31+G(d,p) level of the theory.
The calculated result for the CS state well explains the
molecular geometry of 1_NH experimentally determined by
single crystal X-ray crystallographic analysis. Therefore, we
considered that the ground state electronic structure of 1_NH
is the CS singlet or the singlet state with small contribution
from the open-shell biradical resonance form. The energies of
each electronic structure will be discussed in the next section.
Open-Shell Character of 1_NH. Judging from the bond
lengths determined by the X-ray crystallographic analysis,
1_NH has weak double bonds due to the distortion by the
steric hindrance between the tert-butyl groups. The nature of
twisted double bonds has been intensely studied from the
standpoint of its nonlinear optical properties24 and importance
to understand the bond-breaking process of π-conjugated
electron systems.25 It is generally known that twisted double
bonds possess a certain contribution of SBR state because of
admixing of the doubly excited electronic configuration into the
ground state and result in small singlet−triplet energy gap
(ΔES−T) due to relatively small exchange integral.22,23
Figure 6. IR absorption spectra for the solid state 1_OH and 1_NH
and the theoretical IR spectra for 1_OH and 1_NH at (U)B3LYP/6-
31+G(d,p) level of the theory.
This value lies between the wavenumber for the typical
phenoxyl radical (1481 cm−1)29 and that for the CO double
bond of 1_OH (1622 cm−1), indicating that the twisting of the
CC double bonds weaken the double bond character of the
CO bonds of 1_NH. As shown in Figure 6, the experimental
IR spectra are in good agreement with the theoretical IR
spectra obtained by the DFT calculations for 1_OH and the CS
state for 1_NH at the B3LYP/6-31+G(d,p) level of the theory.
Thus, the ground state electronic structure of 1_NH can be
considered as the CS state rather than the pure open-shell
biradical state.
For the better understanding of the experimental results, we
performed theoretical investigation of the electronic structure
of 1_NH. The DFT calculations were conducted at the
(U)B3LYP/6-31+G(d,p) level of the theory for the CS, the
SBR, and the triplet biradical (TBR) states of 1_NH. The wave
function of the SBR state were obtained by using the broken-
symmetry DFT method.26−28 The SBR state of 1_NH was
calculated to be the lowest energy state with the spin-squared
expectation value ⟨S2⟩ = 0.3716 and the ΔE = E(CS) −
E(SBR) = 0.9 kcal mol−1 where E(CS) and E(SBR) are the
zero-point corrected total energies of the CS and SBR states,
respectively. The singlet−triplet energy gap ΔES−T of 1_NH
was estimated to be 5.7 kcal mol−1. However, the optimized
geometry of the CS state of 1_NH well explains the geometry
obtained by X-ray crystallographic analysis and the IR
spectrum, which means the ground-state electronic structure
of 1_NH can be described by the CS state with small
contribution from the SBR state. The total spin density
distribution of the SBR and TBR states of 1_NH are shown in
Supporting Information Figure S8. The singlet biradical
character (y) of the electronic ground singlet state of 1_NH
was calculated to be 22% from the LUMO occupation number
of 0.22 by the CASSCF(8,8)/6-31G(d)//RB3LYP/6-31+G-
(d,p) level of the theory. These results suggest that the ground
electronic state of 1_NH can be regarded as a singlet state with
small contribution of the open-shell biradical character.
The ESR spectroscopy gives direct information for the
thermally excited triplet state of 1_NH. The ESR spectra were
measured by using the quartz capillary tube containing a
DMSO solution since 1_NH is the dominant species in polar
solvent. The DMSO solution of 1_NH exhibits weak ESR
signal at room temperature (Supporting Information Figure
S9). The temperature dependence of the ESR spectra is shown
in Figure 7. The ESR signal intensities IESR, which were
Figure 7. Temperature dependence of the ESR signal intensity of
1_NH in DMSO. The solid line represents the fit to the Bleaney−
Bowers equation for the two-site Hisenberg Hamiltonian.
The FT−IR spectroscopy also provides helpful information
on the ground state electronic structures of 1_OH and 1_NH.
The IR absorption spectra in the fingerprint region of the
crystalline 1_OH and 1_NH are shown in Figure 6. The
intense absorption band at 1622 cm−1 in 1_OH corresponds to
the CO stretching vibrational mode of the p-quinonemethide
moiety. This indicates that the CO double bond of 1_OH is
regarded as a pure double bond. On the other hand, the CO
stretching vibrational mode of 1_NH is observed at 1558 cm−1.
obtained by using double integration of the first derivative
spectra, increases with heating the solution from room
temperature to 100 °C, indicating that the population of
thermally excited triplet state increases with raising the
temperature. The IESRT−T plot can be fitted by the
Bleaney−Bowers equation for the two-site Hisenberg Hamil-
tonian H = −2JS1·S2, giving the singlet−triplet energy gap
ΔES−T = 4.8 kcal mol−1.30
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The Journal of Physical Chemistry A
Article
3 exp − 2J
ACKNOWLEDGMENTS
■
(
)
RT
RT
IESRT = C1
+ C2
This work was partially supported by the Core Research for
Evolutional Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST), a Grant-in-Aid
for Scientific Research (A) (22245025) from the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT),
Japan.
1 + 3 exp − 2J
(
)
The reason for the NMR signal broadening can be explained
by the thermally excited TBR state of 1_NH. As in the case of
the 1_NH, 1H NMR signal broadening was observed except for
protons in the phenyl group at room temperature in acetone-d6
and CDCl3. Furthermore, the signal line widths decreases with
cooling from room temperature to −60 °C. These observations
give a definitive evidence for small ΔES−T of 1_NH. The total
spin density of the TBR state calculated at the UB3LYP/6-
31+G(d,p) level of the theory is mainly distributed over the
imidazole ring and two p-quinonemethide moieties but quite
small on the phenyl and tert-butyl groups. This result is
consistent with the behavior of the signal broadening observed
REFERENCES
■
(1) Lapworth, A. The Action of Halogens on Compounds
Containing the Carbonyl Group. J. Chem. Soc. 1904, 85, 30−42.
(2) Watson, H. B. Reactions of Halogens with Compounds
Containing the Carbonyl Group. Chem. Rev. 1930, 7, 173−201.
(3) Watson, J. D.; Crick, F. H. C. Genetical Implications of the
Structure of Deoxyribonucleic Acid. Nature 1953, 171, 964−967.
(4) Goodman, M. F. Mutations Caught in the Act. Nature 1995, 378,
237−238.
(5) Nissen, P.; Hansen, J.; Ban, N.; Moore, P. B.; Steitz, T. A. The
Structural Basis of Ribosome Activity in Peptide Bond Synthesis.
Science 2000, 289, 920−930.
(6) Beak, P.; Fry, F. S. The Equilibrium between 2-Hydroxypridine
and 2-Pyridone in the Gas Phase. J. Am. Chem. Soc. 1973, 95, 1700−
1702.
(7) Douhal, A.; Kim, S. K.; Zewail, A. H. Femtosecond Molecular
Dynamics of Tautomerization in Model Base Pairs. Nature 1995, 378,
260−263.
(8) Organic Photochromic and Thermochromic Compounds; Crano, J.
C., Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999; Vols. 1
and 2.
(9) Fujiwara, T.; Harada, J.; Ogawa, K. Hydrogen-Bonded Cyclic
Dimer Formation in Temperature-Induced Reversal of Tautomerism
of Salicylideneanilines. J. Phys. Chem. A 2009, 113, 1822−1826.
(10) Miura, M.; Harada, J.; Ogawa, K. Temperature-Induced Reversal
of Proton Tautomerism: Role of Hydrogen Bonding and Aggregation
in 7-Hydroxyquinolines. J. Phys. Chem. A 2007, 111, 9854−9858.
(11) Ogawa, K.; Harada, J.; Fujiwara, T.; Yoshida, S. Thermochrom-
ism of Salicylideneanilines in Solution: Aggregation-Controlled Proton
Tautomerization. J. Phys. Chem. A 2001, 105, 3425−3427.
(12) Reichardt, C. Solvatochromic Dyes as Solvent Polarity
Indicators. Chem. Rev. 1994, 94, 2319−2358.
(13) Furuta, H.; Ishizuka, T.; Osuka, A.; Dejima, H.; Nakagawa, H.;
Ishikawa, Y. NH Tautomerism of N-Confused Porphyrin. J. Am. Chem.
Soc. 2001, 123, 6207−6208.
(14) Holschbach, M. H.; Sanz, D.; Claramunt, R. M.; Infantes, L.;
Motherwell, S.; Raithby, P. R.; Jimeno, M. L.; Herrero, D.; Alkorta, I.;
Jagerovic, N.; et al. Structure of a 4-Nitroso-5-aminopyrazole and Its
Salts: Tautomerism, Protonation, and E/Z Isomerism. J. Org. Chem.
2003, 68, 8831−8837.
(15) Chakrasali, R. T.; Rao, C. S.; Ila, H.; Junjappa, H.
Cyclocondensation of α-Hydroxyimino-β-alkyl/arylimino-β-methyl-
thioketones with Hydrazine Hydrate: Synthesis of 3(5)-Aryl-5(3)-
alkyl/arylamino-4-nitroso (or amino) Pyrazoles. J. Heterocycl. Chem.
1993, 30, 129−134.
(16) Sheldrick, G. M. SHELXS-97 and SHELXL-97; University of
Gottingen: Germany, 1997.
(17) Sheldrick, G. M. SADABS; University of Gottingen: Germany,
1
in the variable temperature H NMR spectra of 1_NH in
acetone-d6.
CONCLUSION
■
In summary, we developed a novel solvatochromic molecule
that shows significant color change via proton tautomeric
reaction. The yellow-colored 1_OH exists as a dominant
species in nonpolar solvents, whereas the blue-colored 1_NH is
stabilized in polar solvents. We investigated the relationship
between the unique solvatochromic behavior and the electronic
structures of the tautomers. 1_OH has a planar quinoidal
structure and completely closed-shell electronic structure. On
the other hand, 1_NH consists of a nonplanar and twisted π-
electron system due to the steric hindrance between the bulky
tert-butyl groups, resulting in decreasing the double bond
character and increasing the SBR character. Moreover, 1_NH
shows the ESR signal at room temperature, indicating the small
energy gap between the singlet ground state and the thermally
accessible excited TBR state. On the other hand, the DFT and
TDDFT calculations for the CS state of 1_NH well explain the
experimental results obtained by the X-ray crystallographic
analysis and the IR and UV−vis absorption spectroscopy. Thus,
it can be concluded that the ground state of 1_NH is regarded
as the singlet state with small contribution of the biradical
character. This result is consistent with small value of the
singlet biradical character y = 0.22 calculated by the CASSCF
calculation. To our knowledge, this is the first report of
switching of the π-bond strength by solvent polarity. This work
would provide a strategy to develop the environmental stimuli-
responsive materials.
ASSOCIATED CONTENT
■
S
* Supporting Information
HPLC chromatogram, UV−vis absorption spectra in various
solvents, H NMR and 13C NMR spectra, ESR spectra, X-ray
1
crystallographic analysis data, details of the DFT calculations
(atomic coordinates and optimized structures). Complete
citations for refs 14, 18, and 24 and CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
1996.
(18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.:
Wallingford, CT, 2009.
(19) Okazaki, T.; Ogawa, K.; Kitagawa, T.; Takeuchi, K. trans-2,2′-
Bi(1-phenyladamantylidene): The Most Twisted Biadamantylidene. J.
Org. Chem. 2002, 67, 5981−5986.
(20) Piotrowiak, P.; Strati, G. Singlet Biradical → Singlet Zwitterion
Optical Transition in a Twisted Olefin. J. Am. Chem. Soc. 1996, 118,
8981−8982.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jiro_abe@chem.aoyama.ac.jp.
Notes
The authors declare no competing financial interest.
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The Journal of Physical Chemistry A
Article
(21) Salem, L.; Rowland, C. The Electronic Properties of Diradicals.
Angew. Chem., Int. Ed. 1972, 11, 92−111.
(22) Yamada, S.; Nakano, M.; Nagao, H.; Yamaguchi, K. Electron
Correlation and Structure Dependencies of the Second Hyper-
polarizability of Ethylene. Int. J. Quantum Chem. 1999, 71, 177−183.
(23) Nakano, M.; Kishi, R.; Nitta, T.; Kubo, T.; Nakasuji, K.;
Kamada, K.; Ohta, K.; Champagne, B.; Botek, E.; Yamaguchi, K.
Second Hyperpolarizability (γ) of Singlet Diradical System: Depend-
ence of γ on the Diradical Character. J. Phys. Chem. A 2005, 109, 885−
891.
(24) Kang, H.; Facchetti, A.; Jiang, H.; Cariati, E.; Righetto, S.; Ugo,
R.; Zuccaccia, C.; Macchioni, A.; Stern, C. L.; Liu, Z.; et al. Ultralarge
Hyperpolarizability Twisted π-Electron System Electro-Optic Chro-
mophores: Synthesis, Solid-State and Solution-Phase Structural
Characteristics, Electronic Structures, Linear and Nonlinear Optical
Properties, and Computational Studies. J. Am. Chem. Soc. 2007, 129,
3267−3286.
(25) Bruckmann, P.; Salem, L. Coexistence of Two Oppositely
Polarized Zwitterionic Forms on the Lowest Excited Singlet Surface of
Terminally Twisted Butadiene. Two-Funnel Photochemistry with
Dual Stereochemistry. J. Am. Chem. Soc. 1976, 98, 5037−5038.
(26) Noodleman, L. Valence Bond Description of Antiferromagnetic
Coupling in Transition Metal Dimers. J. Chem. Phys. 1981, 74, 5737−
5743.
(27) Norman, J. G.; Ryan, P. B.; Noodleman, L. Electronic Structure
of 2-Fe Ferredoxin Models by X.alpha. Valence Bond Theory. J. Am.
Chem. Soc. 1980, 102, 4279−4282.
(28) Noodleman, L.; Norman, J. G. The X Valence Bond Theory of
Weak Electronic Coupling. Application to the Lowlying States of
Mo2Cl84−. J. Chem. Phys. 1979, 70, 4903−4906.
(29) Spanget-Larsen, J.; Gil, M.; Gorski, A.; Blake, D. M.; Waluk, J.;
Radziszewski, J. G. Vibrations of the Phenoxyl Radical. J. Am. Chem.
Soc. 2001, 123, 11253−11261.
(30) Bleaney, B.; Bowers, K. D. Anomalous Paramagnetism of
Copper Acetate. Proc. R. Soc. London, Ser. A 1952, 214, 451−465.
1438
dx.doi.org/10.1021/jp5007928 | J. Phys. Chem. A 2014, 118, 1430−1438