The synthesis and spectral properties of a stimuli-responsive D-π-A charge transfer dye

Article abstract of DOI:10.1016/j.saa.2010.09.027

A new donor-π-acceptor (D-π-A) type isophorone dye was synthesized by the condensation reaction between 2-(3,5,5-trimethylcyclohex-2-enylidene)- malononitrile and indole-3-carboxaldehyde. The chemical structure of the dye was characterized by ¹

Full text of DOI:10.1016/j.saa.2010.09.027

Spectrochimica Acta Part A 78 (2011) 234–237
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The synthesis and spectral properties of a stimuli-responsive D-␲-A charge
transfer dye
Sung-Hoon Kima,b,∗, Seon-Yeong Gwona, Jin-Seok Baea, Young-A. Sonc,∗∗
a Department of Textile System Engineering, Kyungpook National University, Daegu 702-701, South Korea
b School of Chemistry Science & Technology, Zhanjiang Normal University, Zhanjiang 524048, PR China
c Department of Advanced Organic Materials and Textile Systmem Engineering, Chungnam National University, 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:
Received 13 July 2010
Received in revised form 28 August 2010
Accepted 29 September 2010
A new donor-␲-acceptor (D-␲-A) type isophorone dye was synthesized by the condensation reac-
tion between 2-(3,5,5-trimethylcyclohex-2-enylidene)-malononitrile and indole-3-carboxaldehyde. The
chemical structure of the dye was characterized by 1H NMR, EA and MS. A novel, chromogenic, fluo-
rescent dye based on indol as donor unit and isophorone as acceptor unit displayed marked UV–visible
absorption changes and highly selective fluorescence quenching in the presence of fluoride ion. The dye
also exhibited sizeable colour changes when used as a pH-induced molecular switch and as a detector
for volatile organic compounds. The absorption and fluorescent intensity of the dye can be reversibly
selected by protonation/deprotonation of the amine moiety via control of intramolecular charge transfer
(ICT), leading to a molecular switch with “on” and “off” states.
Keywords:
Fluoride ion sensing
D-␲-A
Absorption changes
Fluorescent receptor
pH stimulation
VOC
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
paper concerns the synthesis of a chemosensor for F ion based on
a donor–acceptor, intramolecular charge transfer system. In this
The sensing of anions using dyes has attracted much attention
owing to its application in both chemical and environmental pro-
cesses [1–4]. The synthesis of chemical sensors or receptors capable
of recognizing anions selectively continues to attract research
interest. In addition, the ability of a molecular switching chemosen-
sor to detect volatile organic compounds (VOC’s) is an interesting
research subject. The design of chemical sensors that are sensitive
towards a specific target anion is a great challenge for dye chemists
[5]. Of various anions, fluoride is of especial interest because of its
well-known role in dental care [6,7] and osteoporosis [8]; how-
ever, a high concentration of fluoride can result in the potentially
damaging side effect, fluorosis [1].
Of various types of D-␲-A charge transfer dye,
4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-
4H-pyran (DCM) is a well-known, representative, which undergoes
ultrafast intramolecular charge transfer [9–11]. D-␲-A isophorone
dyes are DCM analogues which display strong red emission,
moderately high fluorescence efficiency and good chemical and
thermal stability [12]. As such dyes contain an amide (NH) group,
they can act as H-bond donors for fluoride ions [5,13–16]. This
context, both the ability of dye 3 (Fig. 1) to recognize anions and
the changes in its absorption/emission properties imparted by pH
and acid/amine VOC’s, were determined
2. Experimental
All reagents and solvents were purchased from Aldrich and were
used without further purification.
Elemental analyses were recorded on a Carlo Elba Model 1106
analyzer, mass spectra were obtained using a Shimadzu QP-1000
and 1H NMR spectra were recorded on a Varian Inova 400 MHz
FT-NMR using TMS as internal standard. UV–vis absorption spec-
tra were measured on an Agilent 8453 spectrophotometer whilst
fluorescecne spectra were measured on a Shimadzu RF-5301PC
fluorescence spectrophotometer. Melting points were determined
using an Electrothermal IA900. 1 was prepared using the previously
described procedure [13].
The interaction of dye 3 with various anions (Br−, Cl−, F−, NO3
,
−
H2PO4−, ClO4 and HSO4−) was investigated using spectrophoto-
metric and spectrofluorimetric titration, by adding an acetonitrile
solution of the corresponding tertabutylammonium salt to an ace-
tonitrile solution of dye 3 at room temperature.
−
To fabricate a VOC sensor for acids and amines, a dye-coated
silica sheet was prepared by dipping a TLC plate into a 1.00 × 10−5 M
CH3CN solution of dye for 10 s. The TLC plate was then allowed to
∗
Corresponding author. Tel.: +82 53 950 5641; fax: +82 53 950 6617.
Corresponding author. Tel.: +82 42 821 6620; fax: +82 42 823 3736.
E-mail addresses: shokim@knu.ac.kr (S.-H. Kim), yason@cnu.ac.kr (Y.-A. Son).
∗∗
1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.09.027

S.-H. Kim et al. / Spectrochimica Acta Part A 78 (2011) 234–237
235
NC
CN
O
H
NC
CN
1-propanol
+
N
reflux
H
1
2
3
HN
Fig. 1. Synthetic routes for dye 3.
dry at room temperature for 48 h. The coated sheet was exposed to
a saturated acetic acid and dimethylamine gas flow to determine
whether or not it could detect the VOC. The photograph was taken
using digital camera (Panasonic DMC-FX 38, jpeg format) under
daylight condition.
2.0
1.5
1.0
0.5
0.0
a
10 equiv.
2.0
1.5
1.0
0.5
0.0
[F-]
0 equiv.
0
2
4
6
8
10 12 14 16
2.1. Synthesis of dye 3
-
[F ] / [Dye 3]
2-(3,5,5-Trimethylcyclohex-2-enylidene)-malononitrile
1
(0.5 g, 2.7mmol) and indole-3-carboxaldehyde 2 (0.4 g, 2.7mmol)
were dissolved in 25 ml of 1-propanol. Piperidine was added
dropwise and the ensuing mixture was refluxed for 10 h, after
which time, the desired solid compound was removed by filtration
and washed with 1-propanol and hexane (60% yield).
400
500
600
700
1H NMR (400 MHz, DMSO-d6): ı 1.03 (s, 1H, –(CH3)2), 2.57(br S,
4H), 6.85 (s, 1H), 7.15 (m, 3H), 7.45 (d, J = 7.44,1H), 7.58 (d, J = 16.04,
1H), 7.93 (s, 1H), 8.12 (d, J = 7.4, 1H), 11.81 (s, 1H, –NH). EA: anal.
calcd. C21H19N3: C 80.45, H 6.11, N 13.41, Found C 79.74, H 6.30, N
13.16%. M+ = 313. mp: 250 ◦C.
Wavelength(nm)
250
200
150
100
50
b
200
10 equiv.
180
160
140
120
100
80
[F-]
3. Results and discussion
60
0
2
4
6
8
10 12 14 16
0 equiv.
-
[F ] / [Dye 3]
Dye 3 was synthesized readily from commercially available
starting materials using a one-step reaction (Fig. 1). Owing to
its well-defined, intramolecular charge transfer, donor–acceptor
structure that contains an amide (NH) hydrogen atom, it was
anticipated that dye 3 could function as a host molecule for the
recognition of anions via H-bonding interaction. This was investi-
gated by adding various anions (Br−, Cl−, F−, NO3−, H2PO4−, ClO4
−
and HSO4−) to a 1.00 × 10−5 M solution of dye 3 in CH3CN and
recording the changes in absorption and emission spectra that
occured during the titration of the solution of dye 3 with between
2 and 10 equiv. of fluoride ion.
0
500
550
600
650
Wavelength(nm)
Fig. 2. Effect of fluoride ion concentration on the absorbance and intensity of a
1.00 × 10−5 M solution of dye 3 in CH3CN.
As Fig. 2 shows, upon the addition of fluoride ion to the solution
of dye 3, the absorption band at 462 nm progressively decreased in
intensity and a new peak at 623 nm appeared; an isobestic point
at 530 nm also developed. The appearance of this isobestic point
suggests that at least one stable dye-fluoride ion species is present
in solution and is indicative that a stable complex formed between
dye 3 and fluoride ion. However, none of the−other anions investi-
2.0
1.5
1.0
0.5
0.0
gated, namely Br−, Cl−, NO3−, H2PO4−, ClO4 and HSO4 had any
−
noticeable effect on absorption, as shown in Fig. 3.
The UV–vis absorption and fluorescence emission changes
(Fig. 2) imparted by the fluoride ion can be attributed to its greater
negative charge and superior H-bond acceptor properties com-
pared to the other anions examined [5]. When fluoride ions and
the dye chemosensor formed a complex via the –NH protons of
the indole, the large intramolecular charge transfer system within
dye 3, namely the donor–acceptor electron stream, was changed,
resulting in the observed changes in UV–vis absorption and fluo-
rescence emission. The colour changes imparted to a 1.00 × 10−5 M
solution of dye 3 in CH3CN by the addition of 10 equiv. of various
anions are shown in Fig. 4 from which the distinctive colour change
obtained for fluoride ion is clearly evident.
HSO4-
Cl-
NO3- H2PO4- ClO4-
F-
Br-
Fig. 3. Comparison of absorption ratio for various anions (A and A0 are the
absorbance in the presence and the absence of metal ions, respectively at 630 nm).

236
S.-H. Kim et al. / Spectrochimica Acta Part A 78 (2011) 234–237
Fig. 4. Colour change of a 1.00 × 10−5 M solution of dye 3 in CH3CN imparted by the addition of 10 equiv. of various anions.
In the context of the ability of dye 3 to act as a fluorescence anion
acid then caused the absorption peak at 468 nm to re-appear. The
corresponding fluorescence emission spectra (Fig. 5(b)) showed
similar results insofar as the addition of acetic acid to a colourless
solution of dye 3 resulted in a return to the initial emission state.
Hence, fluorescence intensity was changed, reversibly, at 570 nm
by the alternate addition of diethylamine and acetic acid.
sensor, the marked quenching of fluorescence emission imparted
by fluoride ion (Fig. 2) can be explained on the basis that, as a result
of the formation of an –NH, H-bonded complex between fluoride
and dye 3, both the intramolecular charge transfer system and the
excited state were greatly modified, resulting in reduced fluores-
cence emission. This observation is in agreement with the UV–vis
absorption behavior (Fig. 2) insofar as, colorimetric changes due to
the deprotonation of –NH, which is directly influenced by the chro-
mophore and the fluorophore, have been employed in the design
of a chemosensor for fluoride ion [17,18].
The potential of dye 3 to undergo molecular switching imparted
by pH change was investigated by adding acid and alkali to a
1.00 × 10–5 M solution of 1 in DMSO. The reversible absorption and
fluorescence emission changes obtained are displayed in Fig. 5,
from which it is apparent that upon the addition of diethylamine
to a solution of dye 3 in CH3CN, the peak ꢀmax at 468 nm decreased
and a new peak at 624 nm appeared (Fig. 5(a)); the addition of acetic
For interpretation of the intramolecular charge transfer system
of dye 3, the quantum chemical DMol3 approach was considered.
All theoretical calculations were performed by DMol3 program of
Materials Studio 4.3 package, which has the quantum mechani-
cal code using density functional theory. Perdew–Burke–Ernzerhof
(PBE) function of generalized gradient approximation (GGA) level
with double numeric polarization basis set was used to calculate
the energy level of the frontier molecular orbital. Fig. 6 shows the
electron distribution states in the frontier MOs. Electron distribu-
tion in HOMO was presented in indole moiety of donor and was
moved to acceptor moiety in LUMO after energy excitation. This
observation can be well agreed with a strong migration character-
istic of electrons in dye 3, namely intramolecular charge-transfer
system. The HOMO–LUMO excitation moves the electron distribu-
tion from indole moiety to the acceptor, which shows the charge
migration of ICT character of dye 3. The energy potential values of
dye 3 in both HOMO and LUMO states were compared. As shown in
Fig. 5, deprotonation shifted the absorption band bathochromically,
the corresponding energy levels being: HOMO (−5.136 eV)/LUMO
(−3.328 eV) in dye 3 and HOMO (−1.628 eV)/LUMO (−0.099 eV) in
3a. The low excitation energy gap of 1.529 eV for the 3a fitted well
with the observed maximum absorption wavelengths in terms of
the decrease in the peak at 468 nm and the increase in the peak at
624 nm.
1.0
a
base
acid
0.8
0.6
0.4
0.2
0.0
In the context of the ability of dye 3 to function as a VOC detector,
when a TLC plate which had been coated with dye 3 was exposed
to acid and amine vapour, marked changes in UV–vis absorption
and fluorescence emission occurred (Fig. 7). It is apparent that
upon exposure to dimethylamine vapour, the changes in colour
400
500
600
700
Wavelength(nm)
b 160
120
80
40
0
base acid
500
550
600
650
700
Wavelength(nm)
Fig. 5. Effect of deprotonation/protonation on UV–vis absorption (a) and fluores-
cence emission switching (b) of a 1.00 × 10−5 M solution of dye 3 in DMSO.
Fig. 6. Electron distribution of HOMO and LUMO energy levels in dye 3 and 3a.

S.-H. Kim et al. / Spectrochimica Acta Part A 78 (2011) 234–237
237
the Korea government (MEST) (No. 2010-0001885). This research
was supported by a grant from the Fundamental R&D Program for
Core Technology of Materials funded by the Ministry of Knowledge
Economy, Republic of Korea (2010).
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Acknowledgements
This work was supported by the Basic Science Research Program
through the National Research Foundation (NRF) grant funded by