Synthesis of novel tert-butyl substituted fluorans and an investigation of their thermochromic behavior

Article abstract of DOI:10.1016/j.dyepig.2016.01.036

In the present study, six tert-butyl substituted fluorans have been synthesized, and their reversible thermochromic properties have been investigated. This work is the first direct demonstration of the role of the substituent on the lactone concerning the thermochromic reversibility of the fluoran. The electron donating substituents induced greater chromic effects than the electron withdrawing substituents. Furthermore, the three-component mixtures that contained fluorans, bisphenol-A, and methyl stearate or 1-dodecanol were used to examine the thermochromic behavior of the fluorans as bulk samples with steady heating and cooling process. The thermochromic properties of the fluorans were evaluated using solid-state UV-Vis, reflectance and FT-IR spectroscopic techniques. DFT calculations were conducted to rationalize the optical behavior of the fluoran in spirolactam form and cationic form. Finally, one of the synthesized fluoran has been successfully converted into the form of a test paper similar to pH paper and acrylic fiber tape for use as thermal indicators.

Full text of DOI:10.1016/j.dyepig.2016.01.036

Dyes and Pigments 128 (2016) 235e245
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of novel tert-butyl substituted fluorans and an investigation
of their thermochromic behavior
Wooram Oh 1, Satheshkumar Angupillai 1, Palanisamy Muthukumar, Hee-Soo So,
Younga 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, six tert-butyl substituted fluorans have been synthesized, and their reversible
thermochromic properties have been investigated. This work is the first direct demonstration of the role
of the substituent on the lactone concerning the thermochromic reversibility of the fluoran. The electron
donating substituents induced greater chromic effects than the electron withdrawing substituents.
Furthermore, the three-component mixtures that contained fluorans, bisphenol-A, and methyl stearate
or 1-dodecanol were used to examine the thermochromic behavior of the fluorans as bulk samples with
steady heating and cooling process. The thermochromic properties of the fluorans were evaluated using
solid-state UVeVis, reflectance and FT-IR spectroscopic techniques. DFT calculations were conducted to
rationalize the optical behavior of the fluoran in spirolactam form and cationic form. Finally, one of the
synthesized fluoran has been successfully converted into the form of a test paper similar to pH paper and
acrylic fiber tape for use as thermal indicators.
Received 22 November 2015
Received in revised form
25 January 2016
Accepted 30 January 2016
Available online 9 February 2016
Keywords:
Fluorans
Thermochromic
Substituent effect
DFT calculation
Thermal indicator
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
through thermochromic formulations [15,16]. Such formulations
integrate three component systems, which include a fluoran (color
Recently, thermochromic materials have been attracting great
interest due to the significant changes in their absorption and/or
fluorescent properties in response to external stimulation, giving
them numerous potential applications such as thermal indicators,
optical storage devices and other luminescent switches [1e4]. Up
until now, many leuco dyes (fluorans and crystal violet lactone),
spiropyrans, inorganic complexes and conjugated polymers were
known to act as thermochromic materials [5e8]. Among these
materials, fluorans have been investigated with a large amount of
research focused on their commercial applications in recent years,
particularly in the last decade. Studies on fluoran dye systems have
been directed toward dyes designed for sensors and probes for a
variety of analytes and thermochromic systems [9e11] as well as
pressure sensitive materials [12]. In addition, the dyes, which have
been derived from a xanthene core, can be utilized in electro-
chromic displays [13,14].
former) and a Bronsted acid (acid color developer) in a passive low
melting point medium, such as methyl stearate [17]. In addition,
reversible thermal sensitive fluorans are themselves thermochro-
mic, and they are also sensitive to some external stimuli other than
temperature, for example, several colorants are sensitive to pH. In
the solid state, these types of functional dyes undergo a ring-
opening reaction and change from a colorless to a colored state
when placed in contact with acid. In general, this thermochromic
effect is achieved by the combination of the following three com-
ponents into a thermochromic system: (i) a color former; (ii) a color
developer; and (iii) a low melting co-solvents, which controls the
interaction between (i) and (ii). When the three component mix-
tures outlined above are heated together in the control proportions,
the color formers and developers are dissolved in the low melting
co-solvent. The solution is then cooled, and the cooled composite
turns an intense red color. However, warming these components up
to the melting point of the co-solvent results in the complete loss of
color (Scheme 1) [18e20]. Because the proportions of the three
components in the system must be maintained in the phase be-
tween the colored solid and colorless liquid phase, therefore the
thermochromic properties do not suffer when the ratio of the
components maintain in microencapsulation [21,22].
A large number of fluoran {spiro[isobenzofuran-l(3H), 90-(9H)
xanthene]-3-one} compounds can act as thermochromic materials
* Corresponding author. Tel.: þ82 42 821 6620; fax: þ82 42 821 8870.
E-mail address: youngason@cnu.ac.kr (Y. Son).
1
These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.dyepig.2016.01.036
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

236
W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
Scheme 1. Thermochromic response of fluorans-bisphenol A system.
Fluorans have great utility as leuco dyes (color formers) for use
and bisphenol-A were performed on a Shimadzu SolidSpec-3700.
FT-IR spectra were recorded using a Perkin Elmer Spectrum One
spectrophotometer equipped with a diamond probe ATR attach-
ment (neat sample). ESI-mass spectra were recorded using a
Thermo Scientific LTQ Orbitrap XL Fourier transform mass spec-
trometer. The key intermediates and fluorans (FL1-FL6) for the
present study were prepared according to the literature procedures
(Scheme 2) [30].
in pressure- and thermo-sensitive recording materials [23e25]. In
general, fluoran dyes are often designed which was containing a
N,N-dialkylamino group at the meta position of the xanthene ox-
ygen with the introduction of substituent groups at the 2- and 3-
positions. In such a manner, Shen Meiqin and coworkers dis-
cussed the thermochromic properties of fluoran red and fluoran
black dyes [5]. Masashi Hojo et al. have developed an aniline-
substituted fluoran and investigated the reversible color changing
properties of corresponding fluorans with different acid developers
including Mg2þ and macrocyclic polyamine [26]. In recent years, to
achieve smart thermochromic properties, the fluoran itself must
possess thermochromic characteristics or thermochromic fillers
must be added into the polymer matrix [27,28]. R. G. Patel et al. [29]
developed six fluorans substituted with substituted 4(3H)-quina-
zolinone. This result reveals that the color of the thermochromic
material depends on the substituent present on the quinazolinone
ring. Likewise, in this present investigation, we synthesized six tert-
butyl substituted fluorans that possess different substituents on the
xanthene ring, such as methyl (electron donating), chloro and
bromo (electron withdrawing substituents). Furthermore, we
discuss the thermochromic properties of the fluorans (FL1-FL6)
with respect to electronic effects. To the best of our knowledge, this
is the first example of a study that evaluates the electronic effect of
the substituent on the lactone unit. Finally, we discussed the
thermochromic properties of FL1-FL6 in the solid state.
2.2. General method for the preparation of intermediates (1e6,
except 4)
A
solution of 3-dimethylamino phenol or 3-diethylamino
phenol (1.0 equiv) and substituted phthalic anhydride (1.0 equiv)
in toluene was refluxed under N2 for 5 h and then allowed to 60 ꢀC
within 1 h. The 35% aqueous NaOH was slowly added to the reac-
tion mixture at 60 ꢀC and heated at 90 ꢀC for 12 h. The resulting
mixture was poured into distilled water, acidified with conc. HCl at
0
ꢀC, and allowed to stir at room temperature for 2 h. The sus-
pension was then filtered. The solid was recrystallized from a
mixture of water and methanol and was then dried to afford a crude
pink solid. The obtained crude material was purified by column
chromatography with gradient enhancement of ethylacetate in
petroleum ether.
2.2.1. 2-(4-(diethylamino)-2-hydroxybenzoyl)benzoic acid (1)
The solid filtrate was recrystallized from a mixture of 20% water
and methanol and was then dried to afford the crude pink solid. The
obtained crude material was purified by column chromatography
with 15% ethylacetate in petroleum ether to afford the title com-
pound as a pale pink solid in a 68.7% yield. 1H NMR (DMSO-D6,
2. Experimental section
2.1. Equipment and reagents
300 MHz),
d
(ppm): 1.09 (t, 6H, J ¼ 6.9 Hz), 3.37 (m, 4H), 6.08 (d, 1H,
All reagents were purchased from either Aldrich or TCI chem-
icals and were used as received. Methanesulfonic acid (99.5%,
analytical grade) was purchased from Aldrich and low melting
solvents were obtained from TCI chemicals. NMR spectra were
recorded on a Bruker Avance 300 MHz instrument (1H NMR
300 MHz, 13C NMR 100 MHz) with tetramethylsilane as an internal
reference. All new compounds were homogeneous by TLC per-
formed on either Merck TLC aluminum or silica gel 60 F254 sheets
using a range of eluent systems with differing polarities. Flash
column chromatography was performed on chromatography silica
gel (230e400 mesh). Melting points were determined using a TA
Instrument DSC 2010. Elemental analysis was performed on a Carlo
Elba Model 1106 analyzer. Solid state UVeVis and reflectance
spectra of the new thermochromic compounds in methyl stearate
J ¼ 2.1 Hz), 6.20 (m, 1H), 6.84 (d, 1H, J ¼ 9.3 Hz), 7.39 (d, 1H,
J ¼ 6.6 Hz), 7.66 (m, 2H), 7.97 (d, 1H, J ¼ 6.6 Hz), 12.57, (s, 1H), 13.08
(bs, 1H). 13C NMR (DMSO-D6, 75 MHz),
d(ppm): 12.94, 44.49, 96.79,
104.45, 109.74, 128.14, 129.86, 130.16, 130.34, 132.47, 134.71, 140.51,
153.96, 165.15, 167.39, 198.78. ESI-MS (m/z) calcd. 313.3, found
314.3 (M þ Hþ).
2.2.2. 2-(4-(dimethylamino)-2-hydroxybenzoyl)benzoic acid (2)
The solid filtrate was recrystallized from a mixture of 20%
water and methanol and was then dried to afford the crude pink
solid. The obtained crude material was purified by column
chromatography with gradient solution of ethylacetate in pe-
troleum ether to afford the title compound as pale pink solid in a

W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
237
R1
O
O
OH
R1
R
N
R2
R3
OH
O
(i) Toluene, 90° C
(ii) 35% NaOH
OH
R
O
+
R
R4
O
N
R
R4
R2
R3
1-6
1: R=CH2CH3, R1=R2=R3=R4=H
2: R=CH3, R1=R2=R3=R4=H
3a: R=CH2CH3, R1=R2=R4=H;R3=CH3 and 3b: R=CH2CH3, R1=R3=R4=H;R2=CH3
4a: R=CH3, R1=R2=R4=H;R3=CH3 and 4b:R=CH3, R1=R2=R4=H;R3=CH3
5: R=CH2CH3, R1=R2=R3=R4=Cl
6: R=CH2CH3, R1=R2=R3=R4=Br
R
N
O
O
OH
R
(i) CH3SO3H,
60-90° C
OH
O
R1
HO
O
R1
R2
+
R
(ii) NaOH
0° C-RT
O
N
R4
R2
R
R3
R4
R3
1-6
FL1-FL6
FL1: R=CH2CH3, R1=R2=R3=R4=H
FL2: R=CH3, R1=R2=R3=R4=H
FL3a: R=CH2CH3, R1=R2=R4=H;R3=CH3 and FL3b: R=CH2CH3, R1=R2=R4=H;R3=CH3
FL4a: R=CH3, R1=R2=R4=H;R3=CH3 and FL4b: R=CH3, R1=R2=R4=H;R3=CH3
FL5: R=CH2CH3, R1=R2=R3=R4=Cl
FL6: R=CH2CH3, R1=R2=R3=R4=Br
Scheme 2. Synthetic route for preparation of fluorans FL1-FL6.
51% yield. 1H NMR (DMSO-D6, 300 MHz),
d
(ppm): 3.00 (s, 6H),
2.2.4. 2-(4-(dimethylamino)-2-hydroxybenzoyl)-4-methylbenzoic
acid (4a, 77%) & 2-(4-(dimethylamino)-2-hydroxybenzoyl)-5-
methylbenzoic acid (4b, 23%) (4)
6.11 (d, 1H, J ¼ 2.4 Hz), 6.22 (m, 1H), 6.83 (d, 1H, J ¼ 9.0 Hz), 7.40
(d, 1H, J ¼ 7.5 Hz), 7.69 (m, 2H), 7.98 (d, 1H, J ¼ 7.5 Hz), 12.55, (bs,
1H). 13C NMR (DMSO-D6, 75 MHz),
d
(ppm): 97.50, 104.78, 110.14,
A solution of 3-(dimethylamino)phenol (1.0 equiv) and 4-
methylphthalic anhydride (1.0 equiv) in toluene was refluxed un-
der N2 for 3e6 h and was then cooled to 60 ꢀC. The 35% aqueous
NaOH was slowly added to the reaction mixture at 60 ꢀC and heated
at 90 ꢀC for 12 h. The resulting mixture was poured into distilled
water, acidified with concentrated HCl at 0 ꢀC, and allowed to stir at
room temperature for 2 h. The acidified solution was extracted with
ethylacetate, and the organic layer was evaporated. The obtained
crude material was purified by column chromatography with 30%
ethylacetate in petroleum ether to afford the title compound as a
pale yellow solid in a 41.0% yield. According to the 1H NMR spec-
trum, the formed pure product exhibited two positional isomers in
128.09, 129.91, 130.21, 130.34, 132.49, 134.35, 140.49, 156.16,
164.76, 167.37, 199.23. ESI-MS (m/z) calcd. 285.3, found 308.3
(M þ Na).
2.2.3. 2-(4-(diethylamino)-2-hydroxybenzoyl)-5-methylbenzoic
acid (3a, 62.5%) & 2-(4-(diethylamino)-2-hydroxybenzoyl)-4-
methylbenzoic acid (3b, 37.5%) (3)
The solid filtrate was recrystallized from a mixture of 25% water
and methanol and was then dried to afford the crude pink solid. The
obtained crude material was purified by column chromatography
with 30% ethylacetate in petroleum ether to afford the title com-
pound as a pale yellow solid in an 85.8% yield. According to the 1H
NMR spectrum, the formed pure product exhibits two positional
isomers in a ratio of 62.5:37.5%.
a percentage ratio of 77:23. 1H NMR (DMSO-D6, 300 MHz),
d(ppm):
2.39 (s, 3H), 2.42 (s, 0.9H), 2.99 (s, 7.8H), 6.09 (d, 1.3H, J ¼ 2.4 Hz),
6.22 (m, 1.3H), 6.81 (t, 1.3H, J ¼ 9 Hz), 7.18 (s, 1.0H), 7.27 (d, 0.3H,
J ¼ 7.5 Hz), 7.42 (m, 1.0H), 7.50 (m, 0.3H), 7.76 (s, 0.3H), 7.87 (d. 1H,
J ¼ 7.8 Hz), 12.54 (s, 1H), 12.59 (s, 0.4H).
1H NMR (DMSO-D6, 300 MHz),
d(ppm): 1.09 (t, 9.8H,
J ¼ 6.9 Hz), 2.39 (s, 3H), 2.42 (s, 1.8H), 3.39 (m, 6.5H), 6.06 (d,
1.6H, J ¼ 2.1 Hz), 6.19 (d, 1.6H, J ¼ 9.3 Hz), 6.80 (t, 1.6H,
J ¼ 9.3 Hz), 7.18 (s, 1.0H), 7.25 (d, 0.6H, J ¼ 7.5 Hz), 7.42 (d, 1.0H,
J ¼ 8.1 Hz), 7.49 (d, 0.6H, J ¼ 7.8 Hz), 7.762 (s, 0.6H), 7.87 (d, 1H,
J ¼ 7.8 Hz), 12.56 (s, 1H), 12.61 (s, 0.5H), 12.92 (bs, 1.3H). 13C NMR
2.2.5. 2,3,4,5-tetrachloro-6-(4-(diethylamino)-2-hydroxybenzoyl)
benzoic acid (5)
The solid filtrate was recrystallized from a mixture of 25% water
and methanol and was then dried to afford the crude pink solid. The
obtained crude material was purified by column chromatography
with 30% ethylacetate in petroleum ether to afford the title com-
pound as a pale yellow solid in an 18.0% yield. 1H NMR (DMSO-D6,
(DMSO-D6, 75 MHz),
d(ppm): 12.94, 21.14, 21.36, 44.48, 96.76,
104.35, 104.41, 109.80, 109.86, 127.16, 128.19, 128.49, 130.28,
130.35, 130.48, 130.67, 132.79, 134.74, 137.82, 139.60, 140.76,
142.95, 153.91, 165.08, 165.14, 167.19, 167.55, 198.99, 199.04. ESI-
MS (m/z) calcd. 327.1, found 351.1 (M þ Na).
300 MHz),
d(ppm): 1.09 (t, 6H, J ¼ 7.2 Hz), 3.39 (m, 4H), 6.01 (s, 1H),

238
W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
6.21 (m, 1H), 6.92 (m, 1H), 11.82, (s, 1H), 12.27 (bs, 1H). 13C NMR
(DMSO-D6, 75 MHz), (ppm): 12.96, 44.60, 79.66, 96.58, 105.06,
43.45, 83.16, 96.56, 104.19, 107.24, 112.60, 115.38, 119.75, 123.07,
123.78, 126.25, 126.53, 127.89,128.34,133.63, 148.57,150.49,152.03,
152.21, 153.13, 168.63. ESI-MS (m/z) calcd. 399.1, found 400
(M þ Hþ), Anal. Calc. for C26H25NO3: C, 78.17; H, 6.31; N, 3.51.
Found: C, 78.31; H, 6.30; N, 3.50. (White solid; Yield ¼ 68.0%).
d
128.83, 154.60, 165.01. ESI-MS (m/z) calcd. 451.1, found 452.2
(M þ Hþ).
2.2.6. 2,3,4,5-tetrabromo-6-(4-(diethylamino)-2-hydroxybenzoyl)
benzoic acid (6)
2.3.3. Mixture of 30-(tert-butyl)-6'-(diethylamino)-5-methyl-3H-
spiro[isobenzofuran-1,9'-xanthen]-3-one (62.5%) (FL3a) and 30-
(tert-butyl)-60-(diethylamino)-6-methyl-3H-spiro[isobenzofuran-
1,90-xanthen]-3-one (37.5%) (FL3b)
The solid filtrate was recrystallized from a mixture of 25% water
and methanol and was then dried to afford the crude pink solid. The
obtained crude material was purified by column chromatography
with 30% ethylacetate in petroleum ether to afford the title com-
pound as a pale yellow solid in a 46.2% yield. 1H NMR (DMSO-D6,
m.p.: 196e198 ꢀC. IR (ʋmax in cmꢁ1): 2961,1755,1619,1552,1512,
1410, 1345, 1268,1195, 1117, 930, 815. 1H NMR (CDCl3, 300 MHz),
300 MHz),
6.22 (m, 1H), 6.96 (s, 1H), 12.16 (bs, 1H). 13C NMR (CDCl3, 75 MHz),
(ppm): 12.79, 37.78, 68.11, 84.75, 119.86, 129.47, 133.81, 135.35,
d(ppm): 1.09 (t, 6H, J ¼ 7.2 Hz), 3.37 (m, 4H), 6.03 (s, 1H),
d
(ppm): 1.10 (t, 9.7 H, J ¼ 6.9 Hz), 1.29 (s, 14.4H), 2.37 (s, 3H), 2.49 (s,
1.8H), 3.35 (m, 6.3H), 6.47 (m, 5H), 6.66 (m, 1.6H), 7.13 (m, 3.3H),
7.30 (s, 1.6H), 7.53 (d, 1.0H, J ¼ 7.8 Hz), 7.60 (d, 0.6 H, J ¼ 7.5 Hz), 7.80
(s, 0.6 H), 7.89 (d, 1.0H, J ¼ 7.8 Hz). 13C NMR (CDCl3, 75 MHz),
d
171.21.
d(ppm): 12.54, 14. 14, 22.07, 22.66, 31.13, 31.60, 34.82, 44.48, 83.69,
2.2.7. Isomeric mixture analysis of intermediate 3 and 4 by 1H NMR
According to the 1H NMR spectra of intermediate 3 (Fig. S1), the
percentage ratio of positional isomeric mixtures (3a) and (3b) were
obtained as 62.5:37.5%. In major isomer (3a), the singlet proton (a)
was appear at 7.18 ppm whereas in the case of minor isomer (3b)
the corresponding (a') (ortho to COOH group) proton was appear at
7.70 ppm. The deshielding effect of a' in minor isomer (3b) is mainly
due to the electron withdrawing effect of COOH group. In addition,
the doublet proton (b) was more deshilded in major isomer (3a)
(Fig. S1). According to the 1H NMR spectra of intermediate 4
(Fig. S2), the percentage ratio of positional isomeric mixtures (4a)
and (4b) were obtained as 77:23%. The 1H NMR spectral behaviors
of intermediate 4 has also been reflecting the resemblance of in-
termediate 3 (Fig. S2).
84.07, 97.52, 105.41, 108.21, 113.59, 116.56, 120.77, 123.78, 124.28,
124.58, 124.69, 124.75, 127.57, 127.64, 129.01, 130.59, 135.86, 139.68,
145.96, 149.54, 150.64, 151.43, 152.96, 153.91, 154.07. ESI-MS (m/z)
calcd. 441.3, found 442.4 (M þ Hþ), Anal. Calc. for C29H31NO3: C,
78.88; H, 7.08; N, 3.17. Found: C, 78.89; H, 7.10; N, 3.16. (White solid;
Yield ¼ 55.0%).
2.3.4. Mixture of 30-(tert-butyl)-60-(dimethylamino)-5-methyl-3H-
spiro[isobenzofuran-1,90-xanthen]-3-one (81%)(FL4a) and 30-(tert-
butyl)-60-(dimethylamino)-6-methyl-3H-spiro[isobenzofuran-1,90-
xanthen]-3-one (19%) (FL4b)
m.p.: 189e195 ꢀC. IR (ʋmax in cmꢁ1): 2955, 1759, 1615, 1514,
1459, 1409, 1366, 1289, 1228, 1107, 990, 953, 875, 817, 772, 756. 1H
NMR (CDCl3, 300 MHz),
0.7H), 2.91 (s, 7.3H), 6.42 (m, 2.7H), 6.61 (m, 2.8H), 6.97 (m, 2.8H),
7.32 (m, 1.4H), 7.82 (m, 1.3H). 13C NMR (CDCl3, 75 MHz),
(ppm):
d(ppm): 1.24 (s, 11.5H), 2.30 (s, 3H), 2.44 (s,
2.3. General method for the preparation of FL1-FL6
d
22.07, 31.13, 34.82, 40.25, 83.43, 98,46,106.44,108.77,113.61,116.49,
120.86, 124.52, 124.62, 127.63, 128.81, 130.63, 146.04, 151.35, 152.06,
152.61, 154.04, 154.13, 169.80. ESI-MS (m/z) calcd. 413.3, found
414.4 (M þ Hþ), Anal. Calc. for C27H27NO3: C, 78.42; H, 6.58; N, 3.39.
Found: C, 78.44; H, 6.58; N, 3.38. (White solid; Yield ¼ 52.0%).
A solution of intermediates (1e6) and 3-tert-butylphenol in
methanesulfonic acid (99.5%, analytical grade) was heated up to
60e90 ꢀC for 2 h. Then the reaction mixture was cooled to room
temperature and 30% of NaOH (20 mL) solution was slowly added
into the reaction mixture. The resulting alkaline mixture was stir-
red at room temperature for 1 h. The formed solid was filtered
through the filter paper and the obtained crude material was pu-
rified by column chromatography with gradient enhancement of
ethylacetate in petroleum ether.
2.3.5. 30-(tert-butyl)-4,5,6,7-tetrachloro-60-(diethylamino)-3H-
spiro[isobenzofuran-1,90-xanthen]-3-one (FL5)
m.p.: 223e226 ꢀC. IR (ʋmax in cmꢁ1): 2962, 1755, 1614, 1512,
1408, 1320, 1266, 1222, 1108, 1019, 872, 817, 789, 694. 1H NMR
2.3.1. 30-(tert-butyl)-6'-(diethylamino)-3H-spiro[isobenzofuran-
1,9'-xanthen]-3-one (FL1)
(CDCl3, 300 MHz),
d
(ppm): 1.11 (t, 6H, J ¼ 7.2 Hz), 1.25 (s, 9H), 3.30
(m, 4H), 6.32 (m, 1.0H), 6.36 (d, 1H, J ¼ 0.8 Hz), 6.57, (d, 1H, J ¼ 9 Hz),
6.70 (d, 1H, J ¼ 8.1 Hz), 7.00 (m, 1H), 7.18 (d, 1H, J ¼ 2.1 Hz). 13C NMR
m.p.: 185e188 ꢀC. FT-IR (ʋmax in cmꢁ1): 2960, 1760, 1615, 1511,
1463, 1407, 1316, 1268, 1106, 876, 770. 1H NMR (CDCl3, 300 MHz),
(CDCl3, 75 MHz), d(ppm): 12.50, 31.09, 34.90, 44.48, 82.45, 97.58,
101.60, 108.40, 113.02, 113.83, 120.99, 123.45, 126.44, 127.70, 128.59,
130.47, 135.63, 140.07, 149.63, 149.93, 151.58, 153.02, 154.87, 164.38.
ESI-MS (m/z) calcd. 565.3, found 566.3 (M þ Hþ), Anal. Calc. for
d
(ppm): 1.10 (t, 6H, J ¼ 6.9 Hz),1.24 (s, 9H), 3.30 (m, 4H), 6.3 (m,1H),
6.39 (d, 1H, J ¼ 2.4 Hz), 6.52 (d, 1H, J ¼ 9.0 Hz), 6.62 (d, 1H,
J ¼ 8.4 Hz), 6.96 (m, 1H), 7.14 (d, 1H, J ¼ 7.8 Hz), 7.19 (d, 1H,
J ¼ 1.5 Hz), 7.55 (m, 2H), 7.95 (m, 1H). 13C NMR (CDCl3, 75 MHz),
C28H25Cl4NO3: C, 59.49; H, 4.46; N, 2.48. Found: C, 59.46; H, 4.47; N,
2.49. (Pale pink solid; Yield ¼ 16.0%).
d(ppm): 30.08, 33.78, 39.19, 82.90, 97.47, 105.22, 107.78, 112.62,
115.31, 119.84, 123.81, 126.09, 126.52, 127.70, 128.38, 133.69, 150.42,
151.09, 151.68, 152.32, 153.19, 168.62. ESI-MS (m/z) calcd. 427.5,
found 450 (M þ Na), Anal. Calc. for C28H29NO3: C, 78.66; H, 6.84; N,
3.28. Found: C, 78.47; H, 6.83; N, 3.29. (White solid; Yield ¼ 44.0%).
2.3.6. 4,5,6,7-tetrabromo-30-(tert-butyl)-60-(diethylamino)-3H-
spiro[isobenzofuran-1,90-xanthen]-3-one (FL6)
m.p.: 213e220 ꢀC. IR (ʋmax in cmꢁ1): 2956, 1755, 1615, 1515,
1448, 1366, 1310, 1266, 1232, 1106, 986, 953, 868, 813, 694. 1H NMR
2.3.2. 30-(tert-butyl)-6'-(dimethylamino)-3H-spiro[isobenzofuran-
1,9'-xanthen]-3-one (FL2)
(CDCl3, 300 MHz),
d
(ppm): 1.11 (t, 9.7H, J ¼ 9 Hz), 1.25 (s, 9H), 2.37
(s, 3H), 3.31 (m, 4.0H), 6.32 (m, 1.0H), 6.36 (d, 1H, J ¼ 2.4 Hz), 6.54,
m.p.: 204e211 ꢀC. IR (ʋmax in cmꢁ1): 2962, 1766, 1620, 1515,
(d, 1H, J ¼ 8.7 Hz), 6.67 (d, 1H, J ¼ 8.4 Hz), 6.98 (m, 1H), 7.17 (d, 1H,
1410, 1372, 1272,1229, 1122, 930, 813, 799. 1H NMR (DMSO-D6,
J ¼ 1.8 Hz). 13C NMR (CDCl3, 75 MHz),
d(ppm): 12.50, 31.10, 34.89,
300 MHz),
d(ppm): 1.29 (s, 9H), 2.95 (s, 6H), 6.52 (m, 3H), 6.67 (d,
44.48, 97.48, 108.42, 113.02, 113.68, 120.96, 122.70, 126.53, 127.79,
149.91, 151.84, 153.30, 154.83, 164.57. ESI-MS (m/z) calcd. 743.4,
found 744.1 (M þ Hþ), Anal. Calc. for C28H25Br4NO3: C, 45.26; H,
1H, J ¼ 8.4 Hz), 7.16 (m, 1H), 7.30 (m, 2H), 7.76 (m, 2H), 8.02 (d, 1H,
J ¼ 6.9 Hz). 13C NMR (CDCl3, 75 MHz),
d(ppm): 11.49, 30.08, 33.78,

W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
239
3.39; N, 1.88. Found: C, 45.12; H, 3.40; N, 1.89. (Pale pink solid;
3.3. Thermochromic properties of FL1-FL6 in the solid state
Yield ¼ 33.8%).
The stated thermochromic materials herein form a composite
with three components including the following: (i) an electron-
3. Results and discussion
donating molecules (FL1-FL6); (ii)
a
compound such as
bisphenol-A (acid activator) that is capable of reversibly accepting
an electron from the electron-donating component; and (iii) a
compound such as methyl stearate or 1-dodecanol that is capable
of controlling the temperature and sensitivity of the coloration/
decoloration of the thermochromic material. Intrinsically, the
thermochromic mixtures are generally dark red when cool, but
they become decolorized when warmed (Fig. 4). The compositions
and the characteristics of the thermochromic materials are shown
in Table 1. As shown in Table 1 in the methyl stearate system, all
fluorans (FL1-FL6) demonstrated color reversibility at 40 ꢀC. When
1-dodecanol was used as the solvent, the three component systems
reversibly produce the thermochromic color at 10 ꢀC. The tem-
perature control of these three component composite systems is
dependent on the solvent system used. The ongoing studies indi-
cate that the colorless semisolid (FL1-FL6) systems turn to a deep
red color as a consequence of the ring-opening of the fluoran sys-
tems, presumably through the intermolecular association between
the electron acceptor phenolic moiety and the spirolactone unit.
However, in actuality, the main reason for the color change of FL1-
FL6 is due to the following reason: FL1-FL6 are colorless (proton
acceptors) because the central carbon atom of a spiro molecule
forms a bond with other carbon atoms through sp3 hybrid orbitals,
forming the colorless lactone ring. When FL1-FL6 are in contact
with bisphenol (proton donor), the lactone ring opens, and the sp3
hybridized orbital of the central carbon atom is converted to a sp2
hybrid orbital (Fig. 5). Due to the formation of sp2 hybridized or-
3.1. Color properties in different solvents
When dissolved in aprotic solvents such as toluene, benzene,
tetrahydrofuran, and acetonitrile, compounds FL1-FL6 showed no
visible color changes. This indicates that in aprotic solvents, these
compounds substantially exist as their colorless lactone structures.
Meanwhile, in the case of an acetic acid solution, the color of the
solution changed to deep orange due to the ring-opening of the
lactone unit to generate the conjugated zwitterionic species.
Additionally, the color of the fluorans in acetic acid medium is
mainly dependent on the substituent present on the fluoran ring.
As shown in Fig. 1, the electron donating substituted fluorans FL1,
FL2, FL3 and FL4 are a deep orange red color, whereas the electron
withdrawing substituted fluorans FL5 and FL6 are pink in color.
3.2. Electronic spectral studies
To ascertain the mechanism and electronic effect of the fluoran
ring, the UVeVis absorption spectra of FL1-FL6 were recorded in
toluene and acetic acid. The electronic spectra of FL1-FL6 exhibit
peaks at approximately 310e330 nm in toluene due to the colorless
ring-closed lactone structure (Fig. 2a). In the case of acetic acid
medium, all fluorans showed two new broad peaks in the range of
472e531 nm. Notably, the lmax values of the compounds are mainly
dependent on the substituent present on the fluoran ring (Fig. 2b).
Nevertheless, the result in Fig. 2c indicated that the unsubstituted
fluoran (FL1) and electron donating substituted fluorans (FL2-FL4)
have lmax values at relatively longer wavelengths. Meanwhile, the
lmax values of electron withdrawing substituted fluorans are at
shorter wavelengths than that of the unsubstituted fluoran (Fig. 3).
We hypothesize that the reason behind this observation concerning
zwitterionic structure may be due to the electronic effect because
bitals, the p-conjugation was extended and the lmax was shifted to
red region [5].
3.4. Solid state spectroscopic studies
To obtain further insight into the electronic structures of these
systems, we employed solid state UVeVis spectroscopic analysis at
different temperatures. The electronic effect of the substituent has
also been reflected in solid state UVeVis spectroscopic studies
similar to solution studies (Fig. 6). The thermochromic systems
(FL1-FL6) do not exhibit any peaks at approximately 500 nm at
40 ꢀC, whereas, when cooled to RT, all of these systems exhibit two
absorption peaks at approximately 500 and 550 nm. This is mainly
due to the ring-opening of the fluoran systems presumably through
the intermolecular association between the electron acceptor
phenolic moieties and the spirolactone unit [18e20,31].
for electron donating substituted fluorans (FL1-FL4), the n-p*
transition is relatively easier than for electron withdrawing
substituted fluorans. This result reveals that the nature of the
substituent present on the fluoran has a significant role in the so-
lution state.
The differences in the reflectance of the fluorans (FL1-FL6) in
methyl stearate and bisphenol at low and high temperatures are
shown by the reflectance spectra (Fig. 7). The reflectance spectra
(Fig. 7) shows that at 24 ꢀC, the ring opened form of the fluorans
(FL1-FL6) absorbs at approximately 450e600 nm. Meanwhile,
when the temperature is increased up to 40 ꢀC, the lactone ring was
regenerated and concomitantly the color of the mixture was van-
ished. To explain the role of the color developer (bisphenol-A), we
cross-verified the aforementioned experiment with a two compo-
nent system using FL1 (5 mg) and methyl stearate (10 mg) in the
absence of bisphenol-A. As expected, it did not display intrinsic
thermochromism [27] when heated to 40 ꢀC due to the lack of
acidity in the system. The typical thermochromism of FL1 was
rebuilt by the addition of 1 mg of bisphenol A to the mixture, which
generated a red color that faded to a colorless mixture upon
heating.
Fig. 1. The color of FL1-FL6 in toluene (Top) and acetic acid (bottom).

240
W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
Fig. 2. (a) UVeVis absorption spectra of (FL1-FL6) in toluene, (b) UVeVis absorption spectra of (FL1-FL6) in acetic acid, (c) lmax of (FL1-FL6) in acetic acid.
3.5. FT-IR spectroscopic studies
To obtain more information about the structures of the ther-
mochromic materials, the FT-IR spectra of the aforementioned
thermochromic systems (FL1-FL6) were recorded at different
temperatures (Fig. 8 and Fig. S1). The FT-IR spectra of FL1-FL6 at
24 ꢀC and 40 ꢀC were recorded, and the peak assignments for
important peaks are given in Table S1. The results indicated that
the shifts in positions of several peaks could be attributed to the
electronic structure modifications in the three component sys-
tems. For a representative case, in the FL1 system, the peak found
at 1761 cmꢁ1 in the colorless sample (at 40 ꢀC) shifted to
approximately 1738 cmꢁ1 in the cool red sample. The value of
1761 cmꢁ1 is due to the C]O stretching frequency [32,33]. This
result reveals that the decrease in the frequency of the C]O
stretch in the cooling state is due to the ester ring opening,
producing a carboxylate group that can be protonated [20,32].
Similar FT-IR spectral studies were also performed using FL2-FL6.
The results of the temperature variation studies indicated that
FL2-FL6 systems behaved in a similar manner to that of FL1
Fig. 3. Substituent effect on the lactone ring.
(Fig. S1).

W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
241
Fig. 4. Solid state color change of Fluorans FL1-FL6 (at warm and cool) with bisphenol-A and methylsterate.
Table 1
Thermochromic color change of FL1-FL6 in methyl stearate and 1-dodecanol.
Thermochromic materials
Temperature of coloration/decoloration (ꢀC)
Fluorans
FL1
Color developer
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Bisphenol A
Solvents
ꢀ
Red 40 C Colorless
Methyl stearate
1-dodecanol
Methyl stearate
1-dodecanol
Methyl stearate
1-dodecanol
Methyl stearate
1-dodecanol
Methyl stearate
1-dodecanol
Methyl stearate
1-dodecanol
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL1
ƒ!ƒ
ꢀ
Red 40 C Colorless
FL2
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL2
ƒ!ƒ
ꢀ
Red 40 C Colorless
FL3
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL3
ƒ!ƒ
ꢀ
Red 40 C Colorless
FL4
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL4
ƒ!ƒ
ꢀ
Red 40 C Colorless
FL5
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL5
ƒ!ƒ
ꢀ
Red 40 C Colorless
FL6
ƒ!ƒ
ꢀ
Red 20 C Colorless
FL6
ƒ!ƒ
Fig. 5. Changes of orbital hybridization at fluorans spiro ring with proton donors.

242
W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
Fig. 6. Solid state UVeVis spectra of FL1-FL6 at 24 ꢀC (cool) and 40 ꢀC (heating).
3.6. Theoretical study
molecular orbitals (MOs) are shown in Figs. 9 and 10. In FL1, the
HOMO is localized on the xanthene unit, and the LUMO is
localized on the lactone unit. While in the case of FL1þ, the
HOMO was retained on the xanthene ring, and the LUMO was
uniformly distributed throughout the entire molecule. This sup-
ports the hypothesis that the cationic form is more extensively
delocalized than the lactone form. The energies of the MOs and
the energies corresponding to the electron transfer transitions
To gain further insight into the mechanism of thermo-
chromism of fluoran FL1, molecular modeling was performed
using the density functional theory with B3LYP using 6-31G basic
sets implemented in a Gaussian 03 program. The optimized ge-
ometries of FL1 (lactone form) and its cationic form FL1þ along
with the calculated electron distributions in the frontier

W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
243
Fig. 7. Solid state reflactance spectra of FL1-FL6 at 24 ꢀC (cool) and 40 ꢀC (heating).
(
D
E ¼ EHOMO ꢁ ELUMO) in FL1 are shown in Fig. 10. The results
containing bisphenol A, and monitor the color of the mixture on
paper and acrylic fiber (as a thermal indicator). In the present
work, a color former composite thermal indicator containing FL1
(5 wt.%), bisphenol A (2.5 wt.%) and methyl stearate (92.5 wt.%)
was prepared as mentioned in the following procedure. The FL1
and bisphenol-A, which were highly soluble in hot methyl stea-
rate, were dissolved in hot (90 ꢀC) methyl stearate. The filter
paper or acrylic fiber was dipped into the colorless solution and
then cooled to ambient temperature. During the cooling process,
FL1 developed an intense red color as a consequence of ring-
opening of the lactone ring through intermolecular association
indicate that the
D
E value of FL1þ is considerably smaller than
that of the FL1, which is in good agreement with the red shift
observed in the absorption spectra upon the addition of the weak
acid (bisphenol-A) to a fluoran. This result reveals that the
cationic form is more delocalized than the lactone form.
3.7. Thermal-induced reversible color switching
It is interesting to evaluate a typical fluoran color former FL1,
in which the formulation contains FL1 in methyl stearate

244
W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
Fig. 8. FT-IR spectra of FL1 at heat (40 ꢀC) and cool (24 ꢀC).
between the phenolic moieties and the spirolactone units. The
color change of thermal indicators as a thin film material is
clearly indicated (Fig. 11), and it can also be used for its reversible
color switching property (Fig. 11). When a thermal indicator
paper was kept on top of a glass vessel containing warm water at
40 ꢀC, the tape turned colorless, as shown in Fig. 11a. The
colorless tape returned to red when the tape was taken out and
cooled down. This color change was shown to be reversible by
repeatedly keeping it in or taking it out of the warm water plate.
The acrylic fiber-based tape has also been observed to act as an
extreme thermal indicator. As depicted in Fig. 11b, the LHS side is
colorless in warm conditions (40 ꢀC), and the RHS side is an
intense red color at ambient temperature. To prove the sensi-
tivity of the thermal indicator tape, a different temperature
environment was applied to the thermal indicator tape as
depicted in Fig. S2. Depending on the time and contact temper-
ature, the color of the tape changed from a red color to colorless
and vice-versa (Fig. S2).
Fig. 10. HOMOeLUMO orbitals of the FL1 and FL1þ and their energy level diagram.
Fig. 11. (a) Color change of FL1 coated thermal indicator paper at 20 ꢀC and 40 ꢀC, (b)
color change of FL1 coated acrylic fiber at 20 ꢀC and 40 ꢀC.
under acidic conditions. The development of color in the three
component color formation reaction depends on the substitution
on the fluoran compound. The substitution of the lactone moi-
eties also affects the development of the color formation reaction
in the solid state. For example, FL1-FL4 are red, whereas FL5-FL6
are pink. The color change of FL1 with bisphenol-A in methyl
stearate has been demonstrated in the form of a thin film on both
test paper and acrylic tape, which can both be used as thermal
indicators.
4. Conclusion
In conclusion, this paper has described the role of the sub-
stituent effect on new fluorans with a tert-butyl moiety at the 3-
position of the xanthene ring. Furthermore, this paper demon-
strates that the lactone unit can function as a basic skeleton that
can incorporate different substituents to produce promising
compounds for new functional fluoran and leuco dyes. The role
of the substituent on the lactone ring has been demonstrated
Fig. 9. Optimized geomentry of FL1 and FLþ
.

W. Oh et al. / Dyes and Pigments 128 (2016) 235e245
245
[14] Kanazawa K, Nakamura K, Kobayashi N. High-contrast electroswitching of
emission and coloration based on single-molecular fluoran derivatives. J Phys
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[15] Yamamoto S, Furuya H, Tsutsui K, Ueno S, Sato K. In situ observation of
thermochromic behavior of binary mixtures of phenolic long-chain molecules
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2256e63.
Acknowledgment
This study was supported by the National Research Foundation
of Korea (NRF) funded by the Ministry of Science, ICT and Future
Planning (Grant no. 2015064440, 2015063131).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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