A thermodynamic study on phthalide-type dyes in aqueous alcohol solutions

Article abstract of DOI:10.1002/bkcs.10381

Phthalide-type dyes in aqueous variable alcohol solution were studied by UV-vis absorption spectroscopy. In this study, 7-[4-(diethylamino)-2-ethoxyphenyl]-7-(1-ethyl-2-methyl-1H-indol-3-yl)furo[3,4-b]pyridin-5(7H)-one ("Blue-63") was used. Generally, a blue shift is observed from dyes in higher-carbon-number-alcohol solvents. The results indicated the dye equilibrium constant in aqueous alcohol solutions. The equilibrium constant (K), Gibbs energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) of the dye, depending on temperature change, were also calculated using UV absorbance.

Full text of DOI:10.1002/bkcs.10381

Article
DOI: 10.1002/bkcs.10381
BULLETIN OF THE
J. U. Kim et al.
KOREAN CHEMICAL SOCIETY
A Thermodynamic Study on Phthalide-Type dyes in Aqueous
Alcohol Solutions
*
Jae-Uk Kim, Jeong-Yeol Yoo, and Jong-Gyu Kim
Department of Chemistry, Dankook University, Cheonan 330-714, Korea. *E-mail: jkim16@dankook.ac.kr
Received February 26, 2015, Accepted April 7, 2015, Published online July 24, 2015
Phthalide-type dyes in aqueous variable alcohol solution were studied by UV–vis absorption spectroscopy. In
this study, 7-[4-(diethylamino)-2-ethoxyphenyl]-7-(1-ethyl-2-methyl-1H-indol-3-yl)furo[3,4-b]pyridin-5
(7H)-one(“Blue-63”)wasused. Generally, ablueshiftisobservedfromdyesinhigher-carbon-number-alcohol
solvents. The results indicated the dye equilibrium constant in aqueous alcohol solutions. The equilibrium con-
stant (K), Gibbs energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) of the dye, depending on
temperature change, were also calculated using UV absorbance.
Keywords: Thermochromism, Phthalide-type color, Thermodynamic property, Leuco dye,
Equilibrium constant
Introduction
for both physical chemistry and introductory chemistry
laboratories.
Thermochromism is a reversible color change resulting from
variation in temperature. Thermochromism has been observed
in many substances: inorganic, organic, metal-organic, and
macromolecular compounds. It can appear both in solution
and solid state.1
LeucodyeBlue-63exhibitsseveralinterestingequilibria. In
solution, the spectrum of the dye depends on temperature, sol-
vent, and concentration.17 In alcohol solvents, Blue-63 exists
as an equilibrium mixture of a colorless lactone [L] and a col-
ored zwitterion [Z] (Scheme 1).
Phthalide-type dyes, also known as leuco dyes, are thermo-
chromic dyes. These dyes, being intermolecular donor-
acceptor systems, are capable of changing their spectral and
luminescent properties considerably under electronic excita-
tion. For this reason, these dyes are widely used in various
fields of science and engineering2–4 with applications that
include biological stains and water-tracing agents,5 laser
dyes,6 pH indicators,7 photosensitizers,8 quantum counters,9
and anticancer agents.10
Reaction of the electron-donating colorless phthalide (color
former of the dye precursor) with an electron-accepting
color developer results in reversible opening of the lactone
ring, yielding the resonance-stabilized cationic dye. Thus, it
is obvious that the properties of the color former are also
dependent on those of the developer, but this is a complex
problem and beyond the scope of this article. However, where
important differences arise, the developer dependence will
be referred to; it is necessary to briefly mention the various
types of developers used in present-day carbonless copying
papers.11
The formation of the equilibrium depends on both solvent
hydrogen bond donating ability and solvent dielectric/polariz-
ability characteristics.18,19
In this study, structural changes in the lactone/zwitterion
form in variable alcohol solutions were investigated to
clarify the lactone/zwitterion reaction of phthalide dyes. The
synthesis of 7-[4-(diethylamino)-2-ethoxyphynyl]-7-(1-
ethyl-2-methyl-1H-indol-3-yl)furo[3,4-b]pyridin-5(7H)-one
(Blue-63) is reported as the condensation of quinolinic anhy-
dride, N-ethyl-2-methylindoline, N,N-diethyl-3-phenetidine,
acetic anhydride, acetic acid, ammonium hydroxide, and
zinc chloride as a Friedel-Crafts catalyst. Blue-63 was studied
by means of UV–vis spectroscopy methods, and the equilib-
rium constant (K), enthalpy change (ΔH), entropy change
(ΔS), and Gibbs energy change (ΔG) were determined in var-
iable alcohol solutions (concentration range 10−3 to 10−5 M) at
20.0-60.0 ꢀC. The effects of the dye molecular structure on
(K), (ΔG), (ΔH), and (ΔS) are discussed.
Experimental
Thermochromic color change provides an excellent
opportunity to study thermodynamic properties because the
color changes involved allow one to follow the progress of
the transformation with temperature. Earlier reports have
described thermochromic systems.12–15 An experiment to
derive thermodynamic values from a thermochromic equilib-
rium has also been reported.16 In the present report, an exper-
iment that uses a phthalide-type dye (Blue-63) and a simple,
double-beam spectrophotometer is described; it is suitable
Synthesis of 7-[4-(diethylamino)-2-ethoxyphynyl]-7-(1-
ethyl-2-methyl-1H-indol-3-yl)furo[3,4-b]pyridin-5(7H)-
one (Blue-63). There are several reports on the synthesis of
Blue-63.20 As described in scheme 2, quinolinic anhydride
1.02 g (149.10 g/mol, 7.5 mM, 1.1 eqiv), N-ethyl-2-
methylindoline 1 g (161.24 g/mol, 6.20 mM), and zinc chlo-
ride 0.14 g (136.29 g/mol, 1.03 mM) in 5 mL acetic acid were
mixedatroomtemperature for3 h. N,N-Diethyl-3-phenetidine
Bull. Korean Chem. Soc. 2015, Vol. 36, 1980–1984
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A Thermodynamic Study on Phthalide-Type Dyes
KOREAN CHEMICAL SOCIETY
N
N
N
N
H+
–H+
N
N
O
O
O
H
O
O
O
Zwitterion [Z]
Lactone [L]
Scheme 1. Reverse thermochromic behavior of phthalide-type dye in aqueous alcohol solutions.
H
N
O
N
O
O
N
N
N
Acetic anhydride
N
Zinc chloride
Acetic acid
O
N
N
O
O
O
O
O
Scheme 2. Preparation of phthalide-type dye (Blue-63).
1.32 g (193.29 g/mol, 6.83 mM) in acetic anhydride was
added and heated at 65 ꢀC for 2 h. The reaction mixture was
poured into 20 mL isopropyl alcohol, 7 mL ammonium
hydroxide was added, and the mixture was neutralized. The
precipitate was collected by filtration and washed with isopro-
pyl alcohol. The solid intermediate was then added to isopro-
pyl alcohol and heated to 85 ꢀC for 5 h. Filtration of the
solution afforded a white crystalline product.
Instrumentation. FT-IR (Fourier transform infrared spec-
troscopy) were obtained using a Thermo Fisher Nicolet 850
spectrophotometer (Waltham, MA, USA). 1H and 13C
nuclear magnetic resonance (NMR) was recorded on an
Avance 500 (Bruker 500 MHz) instrument. Absorption spec-
tra were recorded on a UV-1601PC UV–vis spectrophotome-
ter (Shimadzu Corp, Kyoto, Japan) in which the specimen
chamber temperature was controlled by a CPS-240A
(Shimadzu Corp, Kyoto, Japan).
Blue-63: Yield 72%. mp 358.3 ꢀC. FT-IR (KBr, cm−1)
2973, 2932, 2894 (aromatic-C-H), 1752 (carbonyl-C O),
1610, 1560 (aromatic-C C). 1H-NMR (500 MHz DMSO) δ
8.78 (d, 1H), 8.26 (d, 1H), 7.67 (d, 1H), 7.55 (d, 1H), 7.40
(d, 1H), 7.05 (d, 1H), 6.88 (m, 2H), 6.09 (m, 2H), 4.18 (m,
2H), 3.72 (d, 1H), 3.69 (d, 1H), 3.33 (m, 4H), 2.44 (m, 3H),
1.22 (m, 3H), 1.06 (m, 6H), 0.61 (m, 3H). 13C-NMR
(DMSO) δ 173.701, 168.951, 158.661, 154.670, 150.137,
135.557, 134.924, 134.094, 132.246, 126.420, 123.860,
121.904, 120.812, 119.996, 119.392, 111.728, 109.615,
109.102, 102.939, 95.943, 92.022, 62.911, 44.132, 40.458,
40.292, 40.125, 39.958, 39.791, 39.624, 39.458, 37.572,
15.433, 14.214, 13.137, 12.929.
Sample Preparation. The various alcohols (methyl-, ethyl-,
propyl-, butyl-) used were of spectrometric grade from Sigma-
Aldrich. Stock solutions (1.0 × 10−3 M) were prepared by
dissolving these solid dyes in alcohol solutions. The concen-
tration of the reagent was measured at 1.0 × 10−3 M in order to
observe the absorbance of the lactone form. Acid was added in
the case of zwitterion form and the concentration was con-
trolled at 1.0 × 10−5 M ~ 5.0 × 10−5 M, and the concentration
of the test piece was measured. About 1 mL of 1 N HCl acid
was added to 100 mL solution to make the acid solution.
Results and Discussion
Characterization. The chemical structures and composition
1
of the resulting compound were characterized by H-NMR,
13C-NMR, and FT-IR.
UV–Vis Spectrum Properties. Figure 1 shows the absorp-
tion spectra of the lactone and zwitterion forms of Blue-63
in butyl alcohol solution at room temperature. The Blue-63
lactone and zwitterion spectrum confirms the two λmax at
584 nm and 270 nm. The lactone form concentration in the
sample was measured at 1.0 × 10−3 M and the zwitterion con-
centration was measured at 1.0 × 10−5 M. The concentration
difference between the lactone form was stable. It was not pos-
sible to measure the zwitterion at the same concentration.
Figure 2 shows the different absorption spectra of Blue-63
showing the same concentration at different temperatures. The
results show that Blue-63 may exist in equilibrium with the
zwitterion in methyl alcohol solution between 20.0 and
60.0 ꢀC. In all cases, the visible absorption peak decreased
with increasing temperature. The absorption spectrum was
changed according to temperature, and thus it could be
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BULLETIN OF THE
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A Thermodynamic Study on Phthalide-Type Dyes
KOREAN CHEMICAL SOCIETY
confirmed that the equilibrium between the lactone and zwit-
terion was tilted to one side by the temperature change.
This slope can be confirmed by the change in absorption
spectra.
between the zwitterion and -COO− of the solvent. Thus, the
solvent zwitterion and -COO− conjugation of the dye are inter-
fered with by a greater interaction of the two positive charges
of carbon between -COO−, causing the red shift. Figure 3
shows the red shift based on the difference of polarity of the
solvent, by which it can be confirmed that as the polarity of
the solvent reduces, the absorbance also reduces.
Figure 4 shows the absorption spectra of lactone of Blue-63
in butyl alcohol at various concentrations. Peaks were
obtained at 278 nm. The absorption intensity of the lactone
form relative to Blue-63 was enhanced with increasing dye
concentration. It is possible to calculate the molar extinction
coefficient (ε) based on the change in absorbance due to con-
centration change. This is shown in Table 1.
Spectral resultsfor Blue-63 in various alcohols are shown in
Figure 3. A steady decrease in the intensity of the visible
absorption of Blue-63 was observed with progressively longer
carbon chain lengths of the alcohol. The maximum absorption
was obtained at different wavelengths with different alcohols
(methyl = 577.6 nm, ethyl = 579.1 nm, propyl = 581.9 nm,
butyl = 583.8 nm). A red shift was observed in the dye with
higher-carbon-number-alcohol solvents. In contrast to Blue-
63 results, the zwitterion absorption spectrum was relatively
insensitive to solvent changes. The phenyl ring containing
the +N(Et)2 and benzene ring containing -COO− are main-
tained by steric hindrance from each other. The zwitterion will
maintain a strong conjugation by substitution of -COO−.
Using zwitterions as the solvent results in an interaction
The plot of ln A versus 1/T is a straight line, as shown in
Figure 5. From the slope, the enthalpy change was calculated
to be 21.77 kJ/mol for Blue-63 at 20.0–60.0 ꢀC in methyl alco-
hol solution.
0.8
1.0
Zwitterion form
1.0 x 10–5 M
Methyl alcohol
Ethyl alcohol
Propry alcohol
0.6
0.4
0.2
0.0
0.8
Lactone form
1.0 x 10–3
Butyl alcohol
M
0.6
270nm
584nm
0.4
0.2
0.0
200
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 3. Absorption spectra of the zwitterion form of 1.0 × 10−5
M
Figure 1. Absorption spectra of the lactone and zwitterion forms of
Blue 63 in butyl alcohol solution at room temperature.
Blue-63 with HCl in different solvents at 30 ꢀC.
3.0
2.5
2.0
1
2
3
4
5
20oC
30oC
40oC
50oC
60oC
1
0.5
0.4
0.3
0.2
0.1
0.0
1
5
5
5.0 x 10–5
4.0 x 10–5
M
M
M
M
1.5
1.0
0.5
0.0
3.0 x 10–5
2.0 x 10–5
1.0 x 10–5 M
200
300
400
500
600
700
800
200
300
400
500
Wavelength (nm)
600
700
800
Wavelength (nm)
Figure 2. Absorption spectra of the zwitterion form of 1.0 × 10−5
M
Figure 4. Absorption spectra of the lactone form of Blue-63 in butyl
alcohol solution at various concentrations.
Blue-63 with HCl in methyl alcohol solution.
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A Thermodynamic Study on Phthalide-Type Dyes
KOREAN CHEMICAL SOCIETY
Table 1. Thermodynamic parameters of Blue-63.
Blue-63
T (Kelvin)
T1
T2
ε (1/Mꢂcm)
K
ΔH (KJ/mol)
ΔG (KJ/mol)
ΔS (KJ/molꢂK)
MeOH
EtOH
PrOH
BuOH
293.15
293.15
293.15
293.15
333.15
333.15
333.15
333.15
17125
23600
25750
30750
2.72
1.46
1.28
0.68
21.77
21.26
21.74
21.36
−2.44
−0.92
−0.61
0.94
−82.57
−75.67
−76.23
−69.67
ΔG = − RTlnK
ð4Þ
Blue-63
–0.76
–0.78
–0.80
–0.82
Also, the entropy (ΔS) can be obtained by using a calculated
Gibbs energy change and enthalpy change.
ΔG = ΔH −TΔS
ð5Þ
Values obtained for , K, enthalpy change, entropy change,
and Gibbs energy change are shown in Table 1. Variation of
the difference between the enthalpy of the polar solvents
and the interaction between solvent and sample were
confirmed.
Conclusions
3.0
3.1
3.2
3.3
3.4
3.5
Temp. (1/T, 1x10–3 K)
We have demonstrated the synthesis of phthalide-type Blue-
63 mediated by a Friedel-Crafts catalyst and investigated
its structure. Absorption spectra with a blue shift were
observed for Blue-63 with higher-carbon-number-alcohol sol-
vents. The spectrometric determination of the thermodynamic
parameters of Blue-63 dye in various alcohol solutions is
reported. The enthalpy change (ΔH), entropy change (ΔS),
Gibbs energy change (ΔG), and equilibrium constant (K) were
obtained.
Figure 5. Plot of ln A versus 1/T for the open form of Blue-63 in
methyl alcohol solution (λmax = 577.6 nm).
Thermodynamics of Phthalide-Type Dyes. The strong vis-
ible absorption of the zwitterion provides a convenient handle
to follow the L,Z equilibrium. To determine the concentra-
tion of zwitterion, it is necessary to have a reference standard.
From this, it is possible to calculate the equilibrium constant
(K) for the L, Z equilibrium,
Acknowledgments. The present research was supported by
the research fund of Dankook University in 2013.
½Zꢁ
K =
ð1Þ
½Lꢁ
References
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ꢀ
ꢁ
ꢀ
ꢁ
KeqðT2Þ
KeqðT1Þ
ΔH
1
1
ln
= −
−
ð2Þ
R
T2 T1
The equilibrium constant is proportional to absorption as:
ꢀ
ꢁ
ꢀ
ꢁ
AzðT2Þ
AzðT1Þ
ΔH
1
1
ln
= −
−
ð3Þ
R
T2 T1
7. M. Ramaiah, Chemistry and Application of Leuco Dyes,
Springer, New York, 2002, vol. 1, chapter 4.
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© 2015 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A Thermodynamic Study on Phthalide-Type Dyes
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Bull. Korean Chem. Soc. 2015, Vol. 36, 1980–1984
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www.bkcs.wiley-vch.de 1984