Solvent effect on the thermochromism of new betaine dyes

Article abstract of DOI:10.1016/j.dyepig.2016.08.034

In the present study, five betaine dyes, BT1, BT2, BT3, BT4 and BT5 were successfully synthesized and fully characterized by ¹H NMR, ¹³C NMR, ESI-MS, IR and UV–Visible spectroscopy. Except BT5, remaining betaine dyes are newly synthesized. The reversible thermochromic properties of these dyes were investigated at different temperatures in different solvents. The large thermochromic effect was observed for the dyes and it was attributed to a combination of conformational changes coupled with the relative permittivity of the respective solvent. The thermochromic properties of the betaines were evaluated by UV–Vis studies, and DFT calculations were conducted to rationalize the dye structures. Finally this is the first attempt of the study of color changes in numerous solvents at different temperatures.

Full text of DOI:10.1016/j.dyepig.2016.08.034

Dyes and Pigments 136 (2017) 458e466
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Solvent effect on the thermochromism of new betaine dyes
Rangaraju Satish Kumar, Jaemyeng Jeong, Naveen Mergu, Wooram Oh, Young-A. Son*
Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, five betaine dyes, BT1, BT2, BT3, BT4 and BT5 were successfully synthesized and
fully characterized by 1H NMR, 13C NMR, ESI-MS, IR and UVeVisible spectroscopy. Except BT5, remaining
betaine dyes are newly synthesized. The reversible thermochromic properties of these dyes were
investigated at different temperatures in different solvents. The large thermochromic effect was
observed for the dyes and it was attributed to a combination of conformational changes coupled with the
relative permittivity of the respective solvent. The thermochromic properties of the betaines were
evaluated by UVeVis studies, and DFT calculations were conducted to rationalize the dye structures.
Finally this is the first attempt of the study of color changes in numerous solvents at different
temperatures.
Received 11 June 2016
Received in revised form
15 August 2016
Accepted 16 August 2016
Available online 18 August 2016
Keywords:
Thermochromism
Betaines
Aminophenol
Hypsochromic
Reversible
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
induced differential desolvation of the highly zwitterionic ground
state relative to its less dipolar first excited state [16]. Betaine dye
Thermochromism refers to the reversible thermotransformation
of a chemical species among two forms that possess different ab-
sorption spectra. Recently, thermochromic materials have received
great interest due to the significant changes in their absorption
and/or fluorescent properties in reaction to external stimulation,
yielding numerous potential applications in a number of industries
such as inks, paints, plastics, and textiles. Because of their
temperature-sensitive nature, these materials are widely used in
devices such as thermal indicators, tunable light filters, optical
storage devices, chemo/biosensors, and other luminescent
switches [1e7]. Until now, fluorans [8,9], conjugated polymers [10],
betaines [11], spiropyrans [12,13], crystal violets [14] and inorganic
compounds [15] have been extensively used as thermochromic
dyes.
systems have been evaluated for use in thermochromic responsive
elastic polymer substrates [17], chromogenic materials [18] and
thermochromism in transparent hydrogels [19]. The thermosolva-
tochromism of pyridinium N-phenolates and other betaine dyes
has been applied in the design of optochemical devices for the
detection of ambient temperatures. Burt encapsulated betaine in a
stable organic silica sol-gel matrix, which exhibited changes in
color upon exposure to temperature changes. The response of the
device was reversible with respect to changes in temperature [20].
Betaine dyes are thermochromic, which means that the longest-
wavelength intramolecular charge transfer (ICT) visible absorption
band of these dyes depends on the solution temperature [21].
Several years ago, the temperature-dependent UVeVis spectrum,
or thermochromism, of Reichardt's dye was analyzed in acetonitrile
[22]; then, the thermochromism of 4-pyridiniophenolate was
analyzed by John O. Morley and co-workers [11]. Additional new
findings have been observed regarding the thermochromic prop-
erty of betaines [23e26]. To rationalize the behavior of the betaine
dye and its sensitivity against small changes in its molecular-
microscopic environment, this class of zwitterionic dyes has been
repeatedly and intensively studied. In the series of our thermo-
chromic studies [27], here we synthesized 5 betaine dyes (BT1 to
BT5) with the aim of changing the thermochromic property with
Among these dyes, betaine dyes are known as an interesting
family of zwitterion compounds, whose UVeVis spectra are rather
sensitive to their environment. A number of betaines have been
investigated for their different applications in thermochromism.
Thermochromic properties were observed due to a temperature-
* Corresponding author.
E-mail address: youngason@cnu.ac.kr (Y.-A. Son).
http://dx.doi.org/10.1016/j.dyepig.2016.08.034
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
459
the variation in temperature. Except for 5-(2,4,6-triphenylpyridin-
1-ium-1-yl)naphthalen-1-olate (BT4), remaining four betaine dyes
were newly synthesized (Fig. 1). The synthesized betaines were
fully characterized by 1H NMR, 13C NMR, IR and ESI-MS analysis. To
gain further insight into the geometry, electronic structure, and
optical properties of the dyes, we carried out thorough DFT and
TDDFT calculations. The thermochromic properties of these com-
pounds were studied in chloroform, ethanol, THF, toluene, benzene,
xylene, DMF and acetonitrile, solvents with different temperature
variations. Simply we discuss the thermochromic properties of all
the betaines with respect to electronic effects. To date except BT5,
no any reports are available for the synthesis of BT1, BT2, BT3 and
BT4.
2. Experimental section
2.1. Chemicals, materials and instruments
All solvents and reagents (analytical and spectroscopic grades)
were commercially obtained and used as received unless otherwise
noted. An AVANCE III 600 spectrometer (Akishima, Japan) was
operated at 600 MHz and 150 MHz for 1H and 13C NMR spectros-
copy, respectively. DMSO-d6 was used as the solvent, and Alice 4.0
Fig. 1. Structures of the betaine dyes.
Scheme 1. Synthetic route for the preparation of betaines BT1-BT5.

460
R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
Fig. 2. (a) Color change of BT1 at 25 ꢁC and ꢂ20 ꢁC in a chloroform solution; (b) UVeVis absorption spectra of BT1 in chloroform (1 ꢀ 10ꢂ3 M) at different temperatures.
software was used for analysis. The chemical shifts (
d
values) are
134.0,133.6,129.6,129.5,129.4,128.5,128.0,127.5,125.1,121.4,117.7.
IR nmax in cmꢂ1 (KBr): 3020, 1619, 1575, 1492, 1214, 745, 668; ESI-
MS: 433 [M]þ.
reported in ppm downfield from an internal standard (Me4Si). ESI-
Mass spectra were recorded on a 4000 Q TRAP mass spectrometer.
FT-IR spectra were recorded on an ALPHA-P spectrometer. UVeVis
absorption spectra were recorded using an Agilent 8453
spectrophotometer.
2.2.3. 5-chloro-2-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate
(BT2)
Yield: (0.757 g, 87.5%), mp 201e203 ꢁC. 1H NMR (600 MHz,
2.2. Synthesis of betaines
DMSO-d6): d 8.42 (s, 2H), 8.23e8.27 (m, 2H), 7.56e7.67 (m, 7H),
7.33e7.42 (m, 6H), 6.68 (d, J ¼ 8.2 Hz, 1H), 5.86e5.89 (m, 1H), 5.52
2.2.1. General procedure for the synthesis of the betaine dyes
The preparation was carried out by modifications in the pro-
cedure [26] to that reported by Kessler and Wolfbeis. A mixture of
2,4,6-triphenylpyrylium hydrogen sulfate (2 mmol), aminophenol
or aminonaphthol derivative (2 mmol), and anhydrous sodium
acetate (6 mmol) were added to 5 mL of ethanol. The mixture was
refluxed for 3 h and cooled. Next, 10 mL of a 5% aqueous sodium
hydroxide solution was added, and the mixture was stirred for
another 5 min. Then, 100 mL of water was added to the reaction
mixture, which was then extracted with DCM (2 ꢀ 50 mL). The
combined organic layers were dried over anhydrous Na2SO4. The
solvent was removed under reduced pressure. The resulting red
crystals were recrystallized from aqueous ethanol and dried over
P2O5 under vacuum for 24 h.
(d, J ¼ 8.2, 1H) ppm; 13C NMR (150 MHz, DMSO-d6):
d 158.1, 154.7,
135.0, 134.3, 134.2, 132.4, 130.2, 130.0, 129.8, 129.4, 129.1, 128.4,
128.1, 125.6, 120.0 ppm; IR nmax in cmꢂ1 (KBr): 3020, 1619, 1587,
1555, 1483, 1214, 745, 668; ESI-MS: 433 [M]þ.
2.2.4. 2-chloro-5-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate
(BT3)
Yield: (0.792 g, 91.5%), mp 199e201 ꢁC. 1H NMR (600 MHz,
DMSO-d6):
d 8.48 (s, 2H), 8.26e8.32 (m, 2H), 7.52e7.70 (m, 7H),
7.37e7.46 (m, 6H), 6.65 (d, J ¼ 8.1 Hz, 1H), 6.02e6.10 (m, 1H), 5.84
(d, J ¼ 8.1, 1H) ppm; 13C NMR (150 MHz, DMSO-d6):
d 156.4, 155.0,
138.6, 134.0, 132.6, 130.2, 130.0, 129.9, 129.2, 128.3, 127.8, 125.7,
119.2 ppm; IR nmax in cmꢂ1 (KBr): 3020,1618,1598,1578,1480,1412,
1214, 749, 667; ESI-MS: 433 [MþH]þ.
2.2.2. 2-chloro-4-(2,4,6-triphenylpyridin-1-ium-1-yl)phenolate
(BT1)
2.2.5. 3-(2,4,6-triphenylpyridin-1-ium-1-yl)naphthalen-2-olate
(BT4)
Yield: (0.779 g, 90%), mp 205e207 ꢁC. 1H NMR (600 MHz,
Yield: (0.776 g, 86.5%), mp 191e194 ꢁC. 1H NMR (600 MHz,
DMSO-d6):
d
8.48 (s, 2H), 8.24e8.32 (m, 2H), 7.60e7.69 (m, 3H),
DMSO-d6): d 8.45 (s, 2H), 8.33 (s, 2H), 8.26e8.31 (m, 2H), 7.62e7.73
7.37e7.49 (m, 10H), 6.90 (d, J ¼ 2.6 Hz, 1H), 6.54e6.60 (m, 1H), 5.84
(m, 6H), 7.24e7.31 (m, 6H), 7.10 (d, J ¼ 7.9 Hz, 1H), 7.02 (d, J ¼ 8.2 Hz,
(d, J ¼ 8.8, 1H) ppm; 13C NMR (150 MHz, DMSO-d6):
d 157.1, 153.9,
1H), 6.92 (t, J ¼ 7.4 Hz, 1H), 6.59 (t, J ¼ 7.4 Hz, 1H), 6.18 (s, 1H) ppm;
Fig. 3. (a) Color change of BT2 at 25 ꢁC and ꢂ20 ꢁC in a chloroform solution; (b) UVeVis absorption spectra of BT2 in chloroform (1 ꢀ 10ꢂ3 M) at different temperatures.

R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
461
Fig. 4. (a) Color change of BT3 at 50 ꢁC, 20 ꢁC and ꢂ20 ꢁC in a chloroform solution; (b) UVeVis absorption spectra of BT3 in chloroform (1 ꢀ 10ꢂ3 M) at different temperatures.
Fig. 5. (a) Color change of BT4 at 25 ꢁC and ꢂ20 ꢁC in a chloroform solution; (b) UVeVis absorption spectra of BT4 in chloroform (1 ꢀ 10ꢂ3 M) at different temperatures.
Fig. 6. (a) Color change of BT5 at 50 ꢁC and ꢂ10 ꢁC in a chloroform solution; (b) UVeVis absorption spectra of BT5 in chloroform (1 ꢀ 10ꢂ3 M) at different temperatures.
13C NMR (150 MHz, DMSO-d6):
130.2, 130.0, 129.6, 129.1, 128.1, 125.5, 124.1, 110.5 ppm; IR nmax in
cmꢂ1 (KBr): 3020, 1619, 1554, 1469, 1214, 745, 668; ESI-MS: 449
[M]þ.
d
157.2, 154.5, 134.3, 134.1, 132.4,
6.00 (d, J ¼ 6.5 Hz, 1H) ppm; 13C NMR (150 MHz, DMSO-d6):
d 156.9,
155.7, 133.7, 133.5, 132.7, 132.5, 130.6, 130.4, 129.9, 129.6, 129.0,
127.5, 126.8, 125.6, 119.0, 109.6 ppm; IR nmax in cmꢂ1 (KBr): 3021,
1618, 1544, 1214, 745, 668; ESI-MS: 449 [MþH]þ.
2.2.6. 5-(2,4,6-triphenylpyridin-1-ium-1-yl)naphthalen-1-olate
(BT5)
2.3. Computational details
Yield: (0.74 g, 82.5%), mp 196e198 ꢁC. 1H NMR (600 MHz,
Density functional theory (DFT) and time-dependent DFT
(TDDFT) calculations were performed on all of the betaine dyes. All
the calculations reported in this paper were carried out at the
B3LYP level using 6-31G** basis set using Gaussian 09 program [28].
DMSO-d6):
d
8.67 (s, 2H), 8.36e8.41 (m, 2H), 8.03 (d, J ¼ 8.2 Hz, 1H),
7.65e7.73 (m, 3H), 7.61 (d, J ¼ 7.1 Hz, 1H), 7.36e7.42 (m, 4H),
7.16e7.28 (m, 7H), 7.08 (t, J ¼ 8.2 Hz, 1H), 6.24 (d, J ¼ 6.5 Hz, 1H),

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R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
Fig. 7. Color change of BT1-BT5 at ꢂ20 ꢁC, 20 ꢁC and 60 ꢁC in different solvents (a) at 1 ꢀ 10ꢂ4 M and (b) at 1 ꢀ 10ꢂ3 M concentration.
The geometry optimization of the two dyes was carried out using
the M06 [29] hybrid meta exchange correlation density functional
at the 6-31G (d,p) level of theory. During the geometry optimiza-
tion, we did not impose any symmetry constraints. The frequency
analysis was carried out on optimized geometries to confirm that
each configuration is indeed a minimum on the potential energy
surface. We obtained the electron density distribution isosurfaces
of the dyes by performing a population analysis. To characterize the
photophysical properties of the betaine dyes, we performed TDDFT
simulations on the optimized ground state structures.
to the literature procedure [30]. The betaine dyes were prepared
using a previously reported procedure [26], with slight modifica-
tions. One equivalent of 2,4,6-triphenylpyrylium hydrogen sulfate,
aminophenol or amino naphthol and 3 equivalents of anhydrous
sodium acetate were dissolved in ethanol. Here, after completion of
the reaction, aqueous NaOH was added, and the solution was
directly extracted with DCM. All compounds were recrystallized in
aqueous ethanol. Finally, the synthesized dyes were fully charac-
terized by 1H NMR, 13C NMR, IR, and ESI-Mass spectrometry, as well
as UVeVis absorption spectrometry.
3. Results and discussion
3.2. Absorption and thermochromic studies in chloroform
3.1. Synthesis of the betaine dyes
The effects of solvent on electron transfer receive continuous
attention in the field of physical chemistry and are conveniently
studied spectroscopically [31]. Here, we conducted UVeVis studies
in chloroform at different temperatures. All the dyes showed a
reversible color change. Dye BT1 at ambient temperature was blue
in color, and its absorption maximum was located at 630 nm. When
We synthesized 5 betaine dyes, all of which were newly syn-
thesized except BT5. The synthetic protocol for the preparation of
the target dyes BT1, BT2, BT3, BT4 and BT5 is shown in Scheme 1.
2,4,6-Triphenylpyrylium hydrogen sulfate was prepared according

R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
463
Fig. 8. Optimized ground state (S0) geometries of the betaines obtained at the DFT level using a B3LYP/6-31G** basis set.
Table 1
maximum bathochromically shifted to 595 nm with a color change
to green when the temperature was increased to 50 ꢁC (Fig. 4). In
BT4, the absorption maximum changed from 602 nm at 25 ꢁC to
560 nm at ꢂ10 ꢁC with an associated color change from green to
reddish brown (Fig. 5). Finally, BT5 showed an absorption
maximum at 680 nm at 50 ꢁC accompanied by a yellow color, and
upon changing the temperature to ꢂ10 ꢁC hypsochromically shifted
the maximum to 550 nm with greenish yellow (Fig. 6). The expla-
nation for the hypsochromic shift of the all the dyes observed upon
cooling, or the thermochromic behavior, is attributable, at least in
part, to the increase in the relative permittivity of the solvent with
decreasing temperature also with combination of dielectric effect
and a hydrogen bonding effect [11]. In Figs. 2e6, we observed that
lower temperatures exhibit very high backgrounds in those
wavelength regions where no absorbance would be expected
Energies of the frontier molecular orbitals calculated at the DFT level using a B3LYP/
6-31G** basis set, along with the optical band gap (E0ꢂ0).
Dye
EHOMO (eV)
ELUMO (eV)
E0ꢂ0 (eV)a
BT1
BT2
BT3
BT4
BT5
ꢂ4.371
ꢂ4.367
ꢂ3.894
ꢂ3.668
ꢂ3.882
ꢂ2.666
ꢂ2.625
ꢂ2.579
ꢂ2.773
ꢂ2.884
1.705
1.742
1.315
0.895
0.998
the temperature was changed to ꢂ20 ꢁC, its absorption maximum
hypsochromically shifted (575 nm) in color to red (Fig. 2). The
original position of the absorption maximum and the blue color
were quickly restored upon warming to room temperature, and this
reversible thermochromic process was repeated a number of times
without significant changes in the UVeVis spectra. A hypsochromic
(blue) shift is observed for molecules whose dipole moment de-
creases in the excited state compared to the ground state, stabi-
lizing the ground state energy in polar solvents; this phenomenon
is called negative solvatochromism and is often observed for mol-
(l > 750 nm) and the backgrounds keep increasing with decreasing
temperature. This anomalous UVeVisible absorption could be due
to the scattered light effect.
This result reveals that the nature of the substituent present on
the betaine molecule has a significant role in the solution, and we
observed that the spectrum red shifts with increasing temperature.
ecules with n /
p* transitions [32,33]. The color transition with
temperature is explained by the small energy gap between the
conformers. The alteration in the color of the dye with varying
temperature is supportive, suggesting that the more stable con-
formers predominate at low temperatures [11].
In the case of BT2 in chloroform, the absorption maximum
changed from 575 nm at 25 ꢁC to 515 nm at ꢂ20 ꢁC with an asso-
ciated color change from purple to red (Fig. 3). The absorption
maximum of BT3 in chloroform moved from 570 nm at room
temperature to 520 nm when the solution was cooled to ꢂ20 ꢁC
with a change in color from red to blue, whereas the absorption
3.3. Thermochromic properties of BT1-BT5 in different solvents
Betaine dyes are solvatochromic in nature [31,34]. The S0-S1
electronic transition blue shifts with increasing solvent polarity as a
result of the much larger dipole moment of betaines in the ground
state compared to the excited state [35,36]. Here, all betaine dyes
showed different color properties, and these dyes were evaluated
for their thermochromic behavior in 10ꢂ3 and 10ꢂ4 M solutions of
acetonitrile, DMF, xylene, benzene, EtOH, CHCl3, toluene, and THF
(Fig. 7). The thermochromic behavior of these betaine dyes was

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R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
Fig. 9. Electron density distribution in HOMO-1, HOMO, LUMO, and LUMOþ1 for the betaines obtained at the DFT level using a B3LYP/6-31G** basis set.

R.S. Kumar et al. / Dyes and Pigments 136 (2017) 458e466
465
Table 2
Simulated absorption wavelengths (lcal), oscillator strengths (f), and coefficients of configuration interaction (CI) with the dominant contribution to each transition of the five
dyes.
Dominant contribution (%)b
a
Dye
BT1
Transition
lcal (nm)
lexp (nm)
f
CI coefficient
S0/S1
S0/S2
S0/S3
S0/S4
S0/S5
S0/S1
S0/S2
S0/S3
S0/S4
S0/S5
S0/S1
S0/S2
S0/S3
S0/S4
S0/S5
S0/S1
S0/S2
S0/S3
S0/S4
S0/S5
S0/S1
S0/S2
S0/S3
S0/S4
S0/S5
740
631
602
429
408
1171
668
586
466
432
2642
956
846
561
537
1713
734
703
554
466
1906
873
817
525
493
0.3097
0.0002
0.0253
0.0098
0.0309
0.0005
0.0040
0.0004
0.0065
0.0005
0.0085
0.0033
0.0390
0.0007
0.0332
0.0000
0.0040
0.0002
0.0033
0.0001
0.0566
0.0129
0.0226
0.0231
0.0009
0.55916
0.69375
0.69260
0.69145
0.69114
0.70305
0.70209
0.70205
0.70136
0.70530
0.64406
0.62953
0.61050
0.70397
0.68816
0.70367
0.70161
0.70279
0.70051
0.67569
0.50073
0.65949
0.66098
0.68488
0.69678
H / L (62)
630
H-1 / L (96)
H / Lþ1 (96)
H-2 / L (96)
H / Lþ2 (96)
H / L (99)
H-1 / L (99)
H / Lþ1 (99)
H-2 / L (98)
H-1 / Lþ1 (99)
H / L (83)
H-1 / L (79)
H / Lþ1 (75)
H-1 / Lþ1 (99)
H-2 / L (95)
H / L (99)
H-1 / L (98)
H / Lþ1 (99)
H-2 / L (98)
H / Lþ3 (91)
H / L (50)
BT2
BT3
BT4
BT5
575
570
602
680
H / Lþ1 (87)
H-1 / L (87)
H / Lþ2 (94)
H / Lþ3 (97)
a
Experimental absorption wavelengths in CHCl3.
H and L denote HOMO and LUMO, respectively.
b
assessed at three different temperatures (ꢂ20 ꢁC, 20 ꢁC, 60 ꢁC). All
five dyes showed good color variation in the different solvents. In
the 10ꢂ3 M solutions of the dyes, we obtained good thermochromic
color change properties compared to the 10ꢂ4 M solutions, and we
also observed a change in the thermochromic properties with
changes in the solvent polarity. Here, by changing from aprotic to
protic solvents, the completely different thermochromic colors
observed for all the dyes may have resulted from the strong
hydrogen bonding between the acidic hydrogen of the solvent and
the electronegative oxygen of betaine [37]. According to this figure,
we achieved almost all colors with thermochromic properties with
these five dyes. According to Morley and Padfield theoretical cal-
culations the molecular structure of betaine using the PM3/COSMO
method, a clockwise and anticlockwise conformational mobility of
the pendant phenyl groups relative to the pyridinium center, and
the thermosolvatochromic behavior of betaine in solvent is a result
of the combination of changes in the conformational profile of the
dye and of the alterations in the relative permittivity of the solvent
with changes in the temperature [11]. To the best of our knowledge,
this is the first attempt of a study that evaluates the color changes in
numerous solvents at different temperatures.
than from the unreliable Kohn-Sham LUMO eigenvalue. The ob-
tained LUMO energies of BT1, BT2, BT3, BT4 and BT5 are ꢂ2.66,
-2.62, -2.57, -2.77 and ꢂ2.88 eV, respectively.
The UVeVis spectra of all the dyes were simulated, and the
TDDFT simulations reproduced the main bands observed in the
experimental UVeVis spectra. The first five singlet vertical excita-
tion energies calculated along with their oscillator strengths are
given in Table 2. The calculated absorption maxima in the low-
energy region of the BT1, BT2, BT3 and BT4 dyes are located at
631, 586, 561 and 554, respectively, which agree well with the
experimental results. Whereas in the case of BT5, the large differ-
ence between experimental (680 nm) and theoretical (817 nm)
absorption maxima may be due to the intermolecular interactions
which the theoretical methods cannot take into account [38]. These
low-energy absorptions mainly occur from the transition from
HOMO to LUMO of the dyes. The excitation energies and oscillator
strengths were interpolated by a Gaussian convolution with an
FWHM of 2500 cmꢂ1
.
4. Conclusion
The outstanding sensitivity of the long-wavelength, intra-
molecular charge-transfer UVeVis absorption of the pyridinium-N
phenoxide betaine dyes to small changes in solvent polarity and
solution temperature was demonstrated. Here, small changes
induced greater chromic effects, showing that betaine dyes are very
sensitive towards small changes in molecular structure. Further-
more, this paper demonstrates that the pyridinium unit can func-
tion as a basic skeleton and phenyl moieties can be modified with
different substituents to produce promising compounds for new
functional betaine dyes. Substitution on the phenyl group affects
the development of the color formation reaction in the solution
state. We report herein color changes in chloroform from ambient
temperature to ꢂ20 ꢁC as follows: BT1, blue to red; BT2, purple to
red; BT3, red to blue; BT4, green to reddish-brown; and BT5, yellow
to greenish-yellow.
3.4. Computational studies
To gain further insight into the geometry, optical properties, and
electronic structure of these betaine dyes, we carried out in-depth
DFT and TDDFT calculations. The optimized ground-state geome-
tries of the dyes are shown in Fig. 8.
The calculated molecular orbital energy levels are given in
Table 1. As shown in Fig. 9, the simulated HOMO energies of BT1,
BT2, BT3, BT4 and BT5 are ꢂ4.37, -4.36, -3.89, -3.66 and ꢂ3.88 eV,
respectively. The TDDFT first transition energies (S0 / S1) were
considered the optical bandgaps of the dyes, which were 1.70, 1.74,
1.31, 0.89 and 0.99 eV for BT1, BT2, BT3, BT4 and BT5, respectively.
In the present study, we calculated the LUMO energy by taking the
sum of the HOMO energy and the TDDFT transition energy, rather

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reversible transparency and color control with temperature at a constant
Acknowledgement
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