Synthesis and solvatofluorochromism behaviors on intramolecular charge transfer system of novel D - A dyes

Article abstract of DOI:10.1080/15421406.2012.689733

Solvatochromic donor - acceptor (D - A) charge transfer dyes have been great attention from the viewpoint of their applicability as probes for the determination of solvent polarity and colorimetric chemosensors. In this work, novel solvatochromic fluorophore dyes based on D - A charge transfer system were designed and synthesized containing a strong electron-donating unit as a carbazole moiety and three different strong electron-withdrawing units. The corresponding molecular design and the optical properties of the D - A dyes were examined in various solutions with widely different polarity. The prepared dyes were found to exhibit a positive fluorescence solvatochromism. Furthermore, the energy properties of these dyes were also computationally optimized and calculated by computational simulation approaches using Material Studio 4.3 and electrochemical measurement. Taylor and Francis Group, LLC.

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Synthesis and Solvatofluorochromism
Behaviors on Intramolecular Charge
Transfer System of Novel D-π-A Dyes
Byung-Soon Kim a c , Hyungwook Yu a b & Young-A Son b
a Graduate School of Environmental and Information Sciences,
Yokohama National University, Hodogaya-ku , Yokohama , 240-8501 ,
Japan
b Department of Advanced Organic Materials and Textile System
Engineering , Chungnam National University , Daejeon , 305-764 ,
Korea
c Seoul Regional Center, Defense Agency for Technology and
Quality , Seoul , 130-010 , Korea
Published online: 02 Aug 2012.
To cite this article: Byung-Soon Kim , Hyungwook Yu & Young-A Son (2012) Synthesis and
Solvatofluorochromism Behaviors on Intramolecular Charge Transfer System of Novel D-π-A Dyes,
Molecular Crystals and Liquid Crystals, 563:1, 257-271, DOI: 10.1080/15421406.2012.689733
To link to this article: http://dx.doi.org/10.1080/15421406.2012.689733
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Mol. Cryst. Liq. Cryst., Vol. 563: pp. 257–271, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.689733
Synthesis and Solvatofluorochromism Behaviors
on Intramolecular Charge Transfer System
of Novel D-π-A Dyes
BYUNG-SOON KIM,1,3 HYUNGWOOK YU,1,2
AND YOUNG-A SON2,∗
1Graduate School of Environmental and Information Sciences, Yokohama
National University, Hodogaya-ku, Yokohama 240-8501, Japan
2Department of Advanced Organic Materials and Textile System Engineering,
Chungnam National University, Daejeon 305-764, Korea
3Seoul Regional Center, Defense Agency for Technology and Quality,
Seoul 130-010, Korea
Solvatochromic donor-π-acceptor (D-π-A) charge transfer dyes have been great atten-
tion from the viewpoint of their applicability as probes for the determination of solvent
polarity and colorimetric chemosensors. In this work, novel solvatochromic fluorophore
dyes based on D-π-A charge transfer system were designed and synthesized containing
a strong electron-donating unit as a carbazole moiety and three different strong electron-
withdrawing units. The corresponding molecular design and the optical properties of the
D-π-A dyes were examined in various solutions with widely different polarity. The pre-
pared dyes were found to exhibit a positive fluorescence solvatochromism. Furthermore,
the energy properties of these dyes were also computationally optimized and calculated
by computational simulation approaches using Material Studio 4.3 and electrochemical
measurement.
Keywords Cyclic voltammetry; D-π-A charge transfer dye; molecular orbital calcu-
lations; optical property; positive fluorescence solvatochromism.
1. Introduction
Solvatochromism effect is characterized by color changing behaviors in a strong depen-
dence of absorption or emission spectra for the dye chromophores with different solvents
[1, 2]. The described behaviors of the solvatochromism depend on the differential stabiliza-
tion of the ground and the excited states of the chromophore in the solvent polarity change.
The differential stabilization of the ground and the excited states is corresponded with a
change in the energy gap between these electronic states and consequently, with variations
in Homo/Lumo position, intensity and shape of absorption or emission spectra can be
∗Address correspondence to Prof. Young-A Son, Department of Advanced Organic Materials
and Textile System Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Dae-
jeon 305-764, Republic of Korea (ROK). Tel: (+82)42-821-6620; Fax: (+82)42-823-3736. E-mail:
yason@cnu.ac.kr
257

258
B.-S. Kim et al.
determined. A significant spectral change in absorption or emission has attracted consid-
erable attention as probes for the understanding of solvent polarity effects, colorimetric
chemosensors and complex biological system [3–7].
Recently, many studies have focused on the solvatochromism effect and solvatochromic
dyes such as pyrifinium betain [8] and stilbazolium [9] have been synthesized and stud-
ied. This effect was based on intramolecular charge transfer (ICT) system of molecular
structure having donor-π-acceptor (D-π-A), and thus, the electron flowing within their
molecular structures are considered to play a major role for the solvatochromism effect.
These electron flowing effects of D-π-A charge transfer chromophoric system have also
gained considerable importance from the viewpoint of their applications in optoelectronics
materials [10–14]. There are many research activities [10–16] focused on the development
for the power conversion efficiency, photo-stability and absorption or emission properties
within their D-π-A structural systems. One of the key problems associated with the devel-
opment for above mentioned application materials is to design the D-π-A structural system
with high-performance properties.
In this study, we have designed and synthesized novel D-π-A charge transfer dyes
by using carbazole moiety as a strong electro-donating unit and three different electro-
withdrawing units such as malononitrile, 1-indanone, 1,3-indanedione. Cabazole derivative
(1), namely 4-(9H-carbazol-9-yl)benzaldehyde, was easily synthesized by the reported
procedure [17–19]. The designed novel D-π-A dyes (2–4) were synthesized by the one-
step condensation of 4-(9H-carbazol-9-yl)benzaldehyde as a electron-donating unit with
malononitrile for 2, 1-indanone for 3, and 1,3-indanedione for 4 as a electron-withdrawing
unit, respectively. The solvatochromic behaviours on the effect of electro-withdrawing unit
were investigated by UV-Vis absorption and fluorescent properties in several solvents with
different polarity parameter in details. Their electronic states were also characterized by
theoretical and electrochemical measurements.
2. Experimental
2.1 Materials and equipment
Cabazole (95%), potassium tert-butoxide (97%), malononitrile (99%), 1-indanone (99%)
and 1,3-indanedione (97%) were purchased from Sigma-Aldrich Co. 4-fluorobenzaldehyde
(98%) was obtained from Acros Organics Inc. Silica gel 60 (230-340 mesh, ASTM) was
used for the column chromatography. The 1H NMR spectra were recorded on a JNM-AL
400 MHz NMR instrument (JEOL Ltd.) with TMS as the internal standard. The elemen-
tal analyses were recorded on a Flash EA 1112 series (Thermo Fisher Scientific). The
absorption and fluorescence spectra of the dyes were determined using an Agilent 8453
UV-Vis spectrophotometer and a Varian AT/Cary Eclipse Luminescence spectrophotome-
ter, respectively. The concentration of the solutions used for absorption and fluorescence
measurements was 1.0 × 10−5 M. Electron distributions and energy potentials were com-
putationally calculated and optimized with Material Studio 4.3. The electrochemistry prop-
erties of these dyes were examined and determined on a Versa STAT3 with three-electrode
conventional electro chemical cell (a platinum wire served as a working electrode, an
Ag/Ag+ electrode served as a reference electrode, and a carbon served as a counter elec-
trode). Cyclic voltammetry (CV) test was conducted in an CH2Cl2 solution containing
tetrabutlyammonium hexafluorophosphate electrolyte. The scan rate was set to 100mV/s.
All measurements were taken at room temperature at approximately 298 K.

Synthesis and Solvatofluorochromism Behaviors
259
Scheme 1. Synthetic routes.
2.2 Synthesis and Characterization
As illustrated in Scheme 1, D-π-A dyes 2–4 were synthesized according to a previously
described method with some modifications [20–23], and their chemical structure was
confirmed by 1H NMR and elemental analysis.
2.2.1 Donor unit 1. 4-(9H-carbazol-9-yl)benzaldehyde was synthesized by the reported
procedure [17–19]. To the mixture of cabazole 50 mmol (8.40 g) and potassium tert-
butoxide 50 mmol (5.60 g) were stirring in 200 ml DMF at 110◦C. After 30 min, 4-
fluorobenzaldehyde 50 mmol (6.20g) was added dropwise during the reaction. The resulting
mixture was stirred for 36 h. After 36 h, the reaction was cooled to room temperature and
the mixture was quenched into 1000 ml ice water. The mixed solution was stirring until the
precipitate to be the enough. The solid was filtered and purified by recrystalization with
acetone/ water (acetone:water = 10:1 v/v). Yield: 40.0% (4.00 g)
2.2.2 D-π-A dyes 2-4. Dye 2, to 13 ml of 1-butanol was added a mixture of 2 mmol 4-(9H-
carbazol-9-yl)benzaldehyde (0.54 g) and 2 mmol of the malononitrile (0.13 g) the resulting
mixture was stirred. Reflux was continued for 8 h. After 8 h, the reaction mixture was
cooled to room temperature. The precipitated crude product was filtered and then purified
by recrytallization from 1-butanol. 2, Yield: 67.0% (0.22 g); Anal. Calcd. For C22H13N3:
C, 82.74; H, 4.10; N, 13.16; Found: C, 82.99; H, 4.22; N, 12.41. 1H NMR (CD3COCD3):
8.46(s, 1H), 8.38-8.36(d, 2H), 8.26-8.24(d, 2H), 8.01-7.98 (d, 2H), 7.64-7.61(t, 2H), 7.50-
7.46(t, 2H), 7.37-7.35(t, 2H).
Dye 3, 1-indanone 2 mmol (0.27 g) was dissolved in 20ml methanol. 4-(9H-carbazol-
9-yl)benzaldehyde 2 mmol (0.54 g) and NaOMe (0.04g) dissolved in 10 ml of methanol was
added and the reaction mixture was stirred at 40◦C for 5 h. After 5 h, the reaction mixture
was cooled to room temperature. The precipitated crude product was filtered and washed
with distilled water. 3, Yield: 58.0% (0.45 g); Anal. Calcd. For C28H19NO: C, 87.25; H,
1
4.97; N, 3.63; Found: C, 86.92; H, 5.01; N, 3.69. H NMR (CD3Cl3): 8.26-8.25(d, 2H),
8.24-8.23(d, 2H), 8.15-7.81(m, 3H), 7.74-7.71(m, 3H), 7.55-7.45(m, 5H); 7.34-7.30(t, 2H),
4.28-4.27(s, 2H).
Dye 4, 1.4 mmol of 1,3-indanedione (0.21 g) and 4-(9H-carbazol-9-yl)benzaldehyde
(0.38 g) were dissolved in 15ml of 1-butanol. Reflux was continued for 3 h. After 3 h,

260
B.-S. Kim et al.
the reaction mixture was cooled to room temperature. The precipitated crude product was
filtered and then purified by column chromatography using ethyl acetate/ n-hexane (ethyl
acetate:n-hexane = 1:4 v/v). 4, Yield: 37.0% (0.28 g); Anal. Calcd. For C28H17NO2: C,
84.19; H, 4.29; N, 3.51; Found: C, 83.42; H, 4.53; N, 3.84. 1H NMR (CD3Cl3): 8.69-8.67(d,
2H), 8.09-8.07(d, 2H), 8.00-7.97(m, 2H), 7.91(s, 1H), 7.80-7.76(m, 2H), 7.73-7.71 (d, 2H),
7.52-7.50(d, 2H), 7.40-7.35(t, 2H), 7.28-7.24(t, 2H).
2.3 Crystal Growth and Data Collection
Among the three D-π-A dyes, single yellow prism crystal of 2 and 3 suitable for X-ray
diffraction measurement was grown by solvent diffusion method and solved by a direct
method SIR 92 [24] and SIR 2004 [25] using the CrystalStructure [26] crystallographic
software package. Details on the crystal structure of 2 will be discussed in elsewhere [27]
and the experimental method and crystal structure of 3 have been published [28].
3. Results and Discussion
3.1 Solvatochromic Behaviours
We designed and synthesized the solvatochromic dyes by using the D-π-A charge transfer
dye system. Among the different electron-donating units, carbazole was examined in this
study. Carbazole was well known compound as a strong electron donor. All the prepared
dyes 2-4 were successfully obtained by a one-step reaction condensing 4-(9H-carbazol-9-
yl)benzaldehyde (1) with various electron-withdrawing units, and their chemical structures
were confirmed by 1H NMR, and elemental analysis. All the prepared dyes were obtained
in yields of 37–67%.
The absorption spectra in different polarity solvents are given in Fig 1. Optical proper-
ties for 2 and 4 were measured in CCl4, toluene, CH2Cl2, 1,2-dichloroethane, THF and ethyl
acetate (EA), whereas 3 have a different solubility in which it showed good solubility with
CH2Cl2, 1,2-dichloroethane, THF, EA, acetonitrile and DMSO, and thus these solvents are
used. The synthesized 2-4 show strong absorption around 420 nm, 370 nm and 450 nm in
various solvent polarities. All the dyes have a strong absorption band within the spectral
range of 20 nm, despite having various solvent polarities. It is difficult to assure that this
spectral change in various solvent polarities is the major reason for solvatochromism. This
spectral shift within the range of 20 nm is might be caused by their different solubility
and stability features in different solvents and consequently the spectral change holds a no
remarkable consideration.
However, the spectral change of the fluorescence bands of all dyes is noteworthy. The
fluorescence bands in various solvent polarities for 2-4 are illustrated in Fig. 2, where
solvents are used that for their absorption spectra. In order to define the relation and
dependence of the emission values of 2-4 versus ET(30) [29, 30] solvent polarity parameter,
the corresponding patterns were also determined (Fig. 3). The fluorescence spectra were
measured by using their maximum absorption peaks such as 420 nm for 2, 370 nm for
3, and 450 nm for 4, respectively. In the case of fluorescence emission spectra, they are
exhibited with a solvent effect where the emission maximum showed shifts with solvent
polarity. These spectra extended from 472 nm (CCl4) to 565 nm (CH2Cl2) for 2, 486 nm
(THF) to 544 nm (Acetonitrile) for 3, and 489 nm (CCl4) to 570 nm (1,2-dichloroethane)
for 4, respectively, which confirms a strongly allowed π–π∗ transition energy with charge

Synthesis and Solvatofluorochromism Behaviors
261
Figure 1. Absorption spectra of 2 (a), 3 (b), and 4 (c) in several solutions. The concentration of dyes
is ca. 1.0 × 10−5 M.

262
B.-S. Kim et al.
Figure 2. Fluorescence emission spectra of 2 (a), 3 (b), and 4 (c) in several solutions. The concen-
tration of dyes is ca. 1.0 × 10−5 M.

Synthesis and Solvatofluorochromism Behaviors
263
Figure 3. Plot of emission maxima of 2 (a), 3 (b), and 4 (c) versus solvent polarity parameter ET(30).

264
B.-S. Kim et al.
Table 1. λmax,abs and λmax,em values of 2-4 in various solvents and ET(30) values.
2
3
4
ET(30)/
λmax/nm λem/nm λmax/nm λem/nm λmax/nm λem/nm kcal mol−1
Solvent
CCl4
Toluene
THF
436
423
409
406
420
419
—
472
504
558
557
565
564
—
—
—
—
—
457
448
430
423
440
441
—
489
517
561
559
570
570
—
32.4
33.9
37.4
38.1
40.7
41.3
45.1
45.6
375
372
376
376
376
368
486
477
514
511
543
544
EA
CH2Cl2
1.2-dichloroethane
DMSO
Acetonitrile
—
—
—
—
transfer characters because of differential solvation of ground and excited states of the
probe [7].
The observed bathochromic shifted fluorescence bands with increasing solvent polarity
for 2-4 can be regarded as a positive fluorescence solvatochromism. These finding char-
acteristics also clearly indicated that a bathochromic shift was observed with increasing
application solvent polarity, which was a positive fluorescence solvatochromism. Fluores-
cence emission photographs of 2-4 in several solvents are shown in Fig. 4. The various
fluorescent emissions in different polarity solvents for 2-4 can be easily observed by the
naked eyes. In all dyes, however, their fluorescence intensity was decreased with increasing
solvent polarity, which causes by the fluorescence quenching of aromatic hydrocarbons
corresponded to have via radical ion formation with solvent polarity [31, 32]. As above
mentioned spectral behavior results in various solvents and ET(30) values for all dyes are
listed in Table 1.
Among the all dyes, 3 exhibited the strong fluorescence intensity. In the case of 3, it has
an one electron-withdrawing unit as a carbonyl group on its 2,3-dihydro-1H-indene ring and
this reflects a slight difference between other dyes within their electron distribution. This
result clearly indicates that it has a strong electron-pushing and -pulling effect between
electron-donating and electron-withdrawing units and it consequently shows the strong
fluorescence intensity compared to other dyes. This electron charge distribution in 2-4
according to a simple model is illustrated in Fig. 5, which show a strong migration of
electron charge distribution.
3.2 Theoretical and Electrochemical Calculations
To gain further insight into the interpretation of the ICT process of 2-4, their molecular
geometry and electro properties such as HOMO and LUMO energy values were com-
puted by the density function theory (DFT) with exchange correction functional of local
density approximation (LDA) based on the Perdew-Wang (PWC) set. All the calculation
approaches were performed with a Material Studio 4.3 program package [33]. The op-
timized molecular geometry and electron distribution of the HOMO and LUMO energy
level for 2-4 is illustrated in Fig. 6. The molecule of 2-4 has a D-π-A conjugation system
from the electro-donating carbazole group to the electro-withdrawing malononitrile for 2,

Synthesis and Solvatofluorochromism Behaviors
265
Figure 4. Fluorescence emission photographs of 2 (a), 3 (b), and 4 (c) in several solvents.
1-indanone for 3 and 1,3-indanedione for 4, respectively. The electron distribution from
HOMO to LUMO levels indicated a strong ICT character in these chromophoric dye sys-
tems upon excitation. In the case of 3, HOMO-LUMO excitation moves from the amino
nitrogen atom including carbazole moiety to an oxygen atom including 1-indanone moiety,

266
B.-S. Kim et al.
Figure 5. Electron-charge distribution in 2-4 according to a simple model.
Figure 6. Optimized molecular geometry and electron distribution of the HOMO and LUMO energy
levels for 2-4.

Synthesis and Solvatofluorochromism Behaviors
267
whereas other dyes were mainly distributed over the two electro-withdrawing groups. This
electron distribution strongly implies that a strong fluorescence intensity exhibits in 3 as
described in solvatofluorchromism behaviors. This theoretical calculation result shows in
good agreement with the obtained results from their optical properties in which 3 showed
strong fluorescence intensity compared to other dyes.
Molecular orbital calculations were also performed with the electrochemical oxidation
behavior by CV in CH2Cl2 solution containing tetrabutlyammonium hexafluorophosphate
electrolyte at 100 mV/s of scan rate. Using this measurement, we can be determined the
HOMO energy level using the electrochemical oxidation onset peak. The HOMO energy
levels were calculated by the following formula (1) [34]:
HOMO (eV) = − 4.8 − [Epeak or onset − E1/2(Ferrocene)]
(1)
where potential of Epeak and Eonset is the highest oxidation peak and rising starting peak
potential, respectively. E1/2(Ferrocene) denotes half-wave potential of Ferrocene (0.42 V).
Among the two different approaches, we estimated the energy levels of 2-4 by onset
approach. In addition, LUMO energy level was estimated with energy gap (ꢀE) by using
onset approach on their absorption spectra. The following equation (2) can be used at ꢀE
determination [34].
ꢀE(eV) = 1240/λoneset
(2)
As shown in Fig. 7, all dyes exhibited oxidation peak around and thus we have
successfully found the oxidation onset peak at 0.61 V for 2, 0.59 V for 3, and 0.60 V for 4,
respectively. From the above equation (1), the ionization potentials (HOMO levels) were
estimated to be -4.99 eV for 2, -4.97 eV for 3, and -4.98 eV for 4, respectively. In addition,
their LUMO energy levels were also estimated by the equation (2) to be -2.32 eV for 2,
-2.04 eV for 3, and -2.48 eV for 4, respectively. Dyes 2-4 caused ICT in their molecular
conformation. Especially 3 had a strong effect within the D-π-A conjugation system and
consequently strong fluorescence intensity was exhibited in 3. Electrochemical results
were also in good agreement with their observed optical and computational simulation
features. The estimated potential electronic states (HOMO/ LUMO energy levels) by using
electrochemical behavior and absorption spectra were also in similar characteristic pattern
with their theoretical results and these compare results are also listed in Table 2.
Table 2. Optimized and calculated molecular orbital by theoretical and electrochemical
method for 2–4
Computationally calculated
Determined with cyclic voltammetry
Dye
HOMO/eV
LUMO/eV
ꢀE/eV
HOMO/eV
LUMO/eV
ꢀE/eV
2
3
4
−5.493
−5.067
−5.201
−3.694
−3.070
−3.489
1.799
1.997
1.712
−4.99
−4.97
−4.98
−2.32
−2.04
−2.48
2.62
2.93
2.50

268
B.-S. Kim et al.
Figure 7. Electrochemical properties and absorption edge of 2 (a), 3 (b), and 4 (c) in CH2Cl2.
3.3 Molecular Structure of 2 and 3
Among the three D-π-A dyes, yellow crystal of 2 and 3 was successfully grown by
solvent diffusion method and their details of crystal data collection and structure refinement
parameters is listed in Table 3.

Synthesis and Solvatofluorochromism Behaviors
269
Table 3. Crystallographic and structure refinement data
Crystal data
dye 2
dye 3
Crystal size (mm)
Chemical formula
0.40,0.10,0.04
C22H13N3
0.30,0.10,0.10
C28H19NO
385.46
296.1
Monoclinic
P21/a
7.7107(3)
32.422(1)
8.3715(4)
90
105.095(3)
90
2020.6(1)
4
1.267
0.596
808
Formula weight (g mol−1
)
319.36
296.1
Temperature (K)
Crystal system
Space group
a (Å)
Orthorhombic
P212121
8.8010(2)
22.9945(6)
24.7542(6)
90
90
90
5009.6(2)
12
1.270
b (Å)
c (Å)
α, (◦)
β, (◦)
γ , (◦)
V (Å3)
Z
DX (g cm−3
)
µ (mm−1
F (000)
Tmin
)
0.596
808
0.721
0.713
Tmax
0.976
0.942
No. of refln measured
No. of unique refrections
No. of parameters
θmin (◦)
47716
9135
676
68.14
18287
3674
290
68.23
R1, wR
GOF
ꢀρmax, ꢀρmin (e Å−3
0.0789,0.2179
0.929
0.15, −0.20
0.0355,0.0711
0.862
0.32, −0.36
)
Molecular conformation of 2 and 3 has almost planar structure between the electron-
donating amine group and the electron-withdrawing malononitrile for 2 [27], 1-indanone for
3 [28]. Furthermore, their effective π-conjugation system developed between the electro-
donating and the electro-withdrawing groups clearly recognized by their small bond alter-
nation in the electro-withdrawing groups [35]. This finding indicated that 2 and 3 can move
of electron smoothly in molecules and consequently these dyes are expected to exhibit the
solvatochromism effect.
4. Conclusions
For gaining novel D-π-A charge transfer chromophoric dyes with the high-performance
properties, we designed and synthesized based on a cabazole moiety as a strong electron-
donating unit and three different electron-withdrawing units such as malononitrile, 1-
indanone, 1,3-indanedione. In various solvent polarities, all the dyes significantly can be
monitored in fluorescence emission bands within the spectral range of 70 nm, whereas
no significant difference was observed in absorption bands. The fluorescence spectra ex-
hibited a solvent effect where the emission maximum showed bathochromic shift with

270
B.-S. Kim et al.
solvent polarity. All dyes can be characterized as a positive fluorescence solvatochromism.
Among the all dyes, 3 has strong fluorescence intensity due to its electron charge distri-
bution in molecular structure. This electron distribution feature was confirmed their ICT
by theoretical simulation approach and the calculation result were in good interpretation
of their theoretical results. Furthermore, effective D-π-A conjugation system between the
electron-donating and the electron-withdrawing groups of 2 and 3 was clearly recognized
in molecular structure.
Acknowledgement
This research was supported by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and
Technology (No. 20110022326). This research was supported by a grant from the Funda-
mental R&D Program for Core Technology funded by the Ministry of Knowledge Economy,
Republic of Korea.
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