Proton-induced fluorescent switching of a new 2D-π-A type vinylcyanoacetate-pyran dye

Article abstract of DOI:10.1016/j.saa.2008.10.035

A new 2D-π-A type vinylcyanoacetate-pyran dye, ethyl 2-(2,6-bis(2-butoxy-4-(diethylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate (BDPC) was prepared by condensation reaction. The chemical structures of all the intermediates and the BDPC dye are characterized by ¹H NMR, MS. It showed interestingly solvatochromic and solvatofluorchromic properties in various solvents. Its LUMO and HOMO values were obtained by the theoretical calculation. The fluorescent intensity of BDPC dye can be reversibly selected by protonation/deprotonation of the amine moiety via control of intramolecular charge transfer (ICT), leading to a fluorescent molecular switch with two distinguished "on" and "off" states.

Full text of DOI:10.1016/j.saa.2008.10.035

Spectrochimica Acta Part A 72 (2009) 677–681
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Short communication
Proton-induced fluorescent switching of a new 2D-␲-A type
vinylcyanoacetate-pyran dye
Sheng Wanga,b, Sung-Hoon Kima,∗
a Department of Textile System Engineering, Kyungpook National University, Daegu 702-701,
Republic of Korea
b School of Chemistry Science & Technology, Zhanjiang Normal University,
Zhanjiang 524048, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2008
Accepted 26 October 2008
A new 2D-␲-A type vinylcyanoacetate-pyran dye, ethyl 2-(2,6-bis(2-butoxy-4-(diethylamino)styryl)-4H-
pyran-4-ylidene)-2-cyanoacetate (BDPC) was prepared by condensation reaction. The chemical structures
of all the intermediates and the BDPC dye are characterized by 1H NMR, MS. It showed interestingly
solvatochromic and solvatofluorchromic properties in various solvents. Its LUMO and HOMO values were
obtained by the theoretical calculation. The fluorescent intensity of BDPC dye can be reversibly selected
by protonation/deprotonation of the amine moiety via control of intramolecular charge transfer (ICT),
leading to a fluorescent molecular switch with two distinguished “on” and “off” states.
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Fluorescence
Switch
Solvatochromism
Vinylcyanoacetate-pyran dye
Intramolecular charge transfer
1. Introduction
transfer (ICT) [16] between fluorophore and receptor systems is
one of the most frequently used pathways for building useful
The design and creation of molecular systems that show chem-
ical and physical changes in responses to external stimuli is a topic
of current interest because of their potential application as sens-
ing and switching devices [1–5]. In particular, multifunctional dyes
that are sensitive toward both external physical and chemical trig-
gers provide a versatile basis for the construction of sophisticated
switches that communicate, for instance, through changes in flu-
orescence, refractive index, polarizability, electrical conductivity,
and magnetism properties. Of particular interest is the design of
fluorescent molecular switches, as they can precisely control and
detect the fluorescence signal by simple chemical input, even down
to the level of single molecules or ions under ambient conditions.
Various fluorescent molecular switches have been proposed based
on systems whose emission properties can be modulated by exter-
nal inputs, such as temperature [6], light [7], redox potential [8],
and ions [9]. Protons are often used as input chemicals because
the proton binding often triggers several photoinduced processes,
such as electron transfer [10–11], charge transfer [12–13], and
energy transfer [14], thus enabling the modulation of emission
properties. In principle, fluorescent molecule construction with
photoinduced electron transfer (PET) [15] or intramolecular charge
sensors and switches. Aromatic amines are typical examples of
pH fluorescent sensors and switches because their fluorescent
spectra can be changed reversibly by protonation/deprotonation
[17–18]. Meanwhile, it is a challenge to design new molecules
that switch with modulation of intramolecular charge trans-
fer (ICT). Here, we reported a new proton induced fluorescent
molecular switch based on 2D-␲-A type vinylcyanoacetate-pyran
dye, named thyl 2-(2,6-bis(2-butoxy-4-(diethylamino)styryl)-4H-
pyran-4-ylidene)-2-cyanoacetate (BDPC) as shown in Scheme 1.
The fluorescent intensity of BDPC dye can be reversibly selected
by protonation/deprotonation of the amine moiety via control of
intramolecular charge transfer (ICT), leading to a fluorescent molec-
ular switch with two distinguished “on” and “off” states.
2. Experiment
2.1. Materials and methods
Most of chemicals were purchased from Aldrich Chemical Co.
and TCI. Solvents were purified by normal procedures and han-
dled under moisture free atmosphere. The other materials were
commercial products and were used without further purification.
The intermediates 2-butoxy-4-(diethylamino) benzaldehyde 4 was
synthesized according to the literature method with a high yield of
90% [19].
∗
Corresponding author. Tel.: +82 53 950 5641; fax: +82 53 950 6617.
E-mail address: shokim@knu.ac.kr (S.-H. Kim).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.10.035

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S. Wang, S.-H. Kim / Spectrochimica Acta Part A 72 (2009) 677–681
Scheme 1. The chemical switching process and proton modulation mechanism of 2D-␲-A vinylcyanoacetate-pyran BDPC dye.
2.2. Instruments and spectral measurements
2.4. Synthesis of ethyl 2-(2,6-bis(2-butoxy-4-
(diethylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate dye
(BDPC)
Melting points were determined using an Electrothermal IA 900
and are uncorrected. UV–visible absorption spectra were measured
on an Agilent 8453 spectrophotometer. Fluorescence spectra were
measured on a Shimadzu RF-5301PC Fluorescence spectropho-
tometer. Mass spectra were recorded on a Shimadzu QP-1000
spectrometer using electron energy of 70 eV and the direct probe
EI method. 1H NMR spectra were recorded using a Varian Inova
400 MHz FT-NMR spectrometer with TMS as internal standard.
Spectrophotometric titrations were performed on 2.0 × 10−5mol/L
solutions of 2D-␲-A type vinylcyanoacetate-pyran BDPC dye in
C2H5OH. The different concentration of acid and base standard
solution of the required protonation/deprotonation were added,
respectively. The UV–vis absorption and fluorescence spectra of the
samples were recorded.
A
solution
of
2-cyano-2-(2,6-dimethyl-4H-pyran-4-
ylidene)acetate 3 (1 g,4.6 mmol), 2-butoxy-4-(diethylamino)benzal
dehyde 4 (2.3 g, 9.1 mmol,) and piperidine (0.45 ml) in 1-propane
(30 ml) was refluxed for 8 h using a Dean–Stark trap. The reaction
solution was cooled to room temperature, and the solid product
was isolated and washed well with n-propanol, and dried. The
crude product was recrystallized from methanol.
Yield: 21%, m.p.: 128–130 ◦C; Ms (m/z) 681(M+); 1H NMR
(400 MHz, CDCl3): ı (ppm) 0.98 (t, J = 7.04, 6H), 1.21 (t, J = 7.0, 12H),
1.34 (t, J = 7.04, 3H), 1.54 (m, J = 7.52, 4H), 1.89 (m, J = 7.52, 4H), 3.40
(m, J = 7.0, 8H), 4.05 (m, J = 6.52, 4H), 4.24 (m, J = 7.0, 2H), 6.14 (s,
2H), 6.28 (d, J = 8.8, 2H), 6.59 (s, 1H), 6.75 (d, J = 13.52, 2H), 7.67(d,
J = 7.52, 2H), 7.91 (s, 1H). Anal. Cald. for C42H55N3O5: C, 73.98; H,
8.13; N, 6.16 Found; C, 73.75; H, 8.39; N, 6.24%.
2.3. Synthesis of ethyl
2-cyano-2-(2,6-dimethyl-4H-pyran-4-ylidene)acetate
3. Results and discussion
A mixture of 2,6-dimethylpyran-4-pyrone 1 (3.1 g, 25 mmol),
ethyl cyanoacetate 2 (2.7 ml, 25 mmol), and acetic acid (12.5 ml)
was refluxed for 6 h. The pale, yellow solid obtained was filtered,
washed with methanol, and recrystallized from methanol.
Yield: 58%, m.p.: 184–186 ◦C; mass (m/z) 219(M+); 1H NMR
(400 MHz, CDCl3): ı (ppm) 7.81 (s, 1H), 6.59 (s, 1H), 4.14 (m, 2H),
2.33 (s, 6H), 1.22 (t, J = 7.52, 3H); Anal. Cald. for C12H13NO3: C, 65.74;
H, 5.98; N, 6.39. Found; C, 65.61; H, 6.12; N, 6.20%.
3.1. The design and synthesis of 2D-ꢀ-A type
vinylcyanoacetate-pyran BDPC dye
In our synthesis strategy, a 2D-␲-A type vinylcyanoacetate-
pyran dye was prepared by condensation reaction as showed
in Scheme 2. The vinyl-dicyano-pyran dye is the typical elec-
tron donor-␲-conjugate-electron acceptor (2D-␲-A) compounds,
Scheme 2. Synthesis of vinylcyanoacetate-pyran BDPC dye.

S. Wang, S.-H. Kim / Spectrochimica Acta Part A 72 (2009) 677–681
679
Table 1
Photophysical properties of vinylcyanoacetate-pyran BDPC dye.
Solvent
ꢁmax (nm)
ꢁem (nm)
ꢂꢃa (cm−1
)
ET (kcal mol−1
)
Hexane
Toluene
THF
Ethyl acetate
CHCl3
460
474
486
482
488
485
490
486
496
500
492
486
524
562
600
596
605
614
635
647
650
653
645
648
2655
3303
3909
4768
3963
4332
4660
5120
4777
4686
4821
5144
30.1
33.9
37.4
38.1
39.1
40.7
42.2
45.6
43.8
45.1
51.9
55.4
CH2Cl2
Acetone
Acetonitrile
DMF
DMSO
C2H5OH
CH3OH
a
Stokes shift
Fig. 1. The Normalized UV–vis absorption and fluorescence emission spectra of
vinylcyanoacetat-pyran BDPC dye in hexane (1), toluene (2), ethyl acetate (3), ace-
tone (4) and DMSO (5), respectively (1–5; absorption, 1ꢀ–5ꢀ; emission).
2D-␲-A type vinylcyanoacetate-pyran BDPC dye are characterized
by 1H NMR, MS. Elementary analysis.
3.2. Photophysical properties of vinylcyanoacetate-pyran BDPC
dye
which showed interesting photochemical and photophysical prop-
erties due to their intrinsic intramolecular charge transfer (ICT)
character [20]. In addition, pyran dye is highly charge-separated
at a photoexcited state and shows a remarkably large Stokes
shift. The dicyano-pyran based dyes have been intensively
developed for applications using as photo- (PL) and electro-
luminescent (EL) materials in the fields of dye laser [21], fluorescent
sensor and logic memory [22–23], and organic light-emitting
device (OLED) [24]. At first, the intermediates ethyl 2-cyano-
2-(2,6-dimethyl-4H-pyran-4-ylidene)acetate 3 was synthesized
by reacting 2,6-dimethylpyran-4-pyrone 1 and ethyl cyanoac-
The normalized UV–vis spectra, fluorescence emission of
vinylcyanoacetate-pyran BDPC dyein several solvents having differ-
ent polarity are shown in Fig. 1. The highly intense long-wavelength
absorption band of BDPC dye undergoes evident red-shifts with
increasing solvent polarity (ca. 40 nm on going from hexane to
*
DMSO). These features indicate a strongly allowed ␲–␲ transition
with charge transfer characters. The intramolecular charge trans-
fer (ICT) interaction, that is from the diethylaminobenzene group
to 2-cyanoacetate fragment is strongly enhanced upon excitation
as evidence from the extreme bathochromic shift of the fluores-
cence maximum in polar solvents. The solvent dependence of
fluorescence spectra has sometimes called solvatofluorchromism.
We found that the BDPC dye showed the interesting solvatoflu-
orchromic property in various solvents. The emission maximum
etate
2 in acetic anhydride solution refluxed for 6 h. Then
2D-␲-A type vinylcyanoacetate-pyran BDPC dye was obtained by
condensation reaction with the ethyl 2-cyano-2-(2,6-dimethyl-4H-
pyran-4-ylidene)acetate 3 and compound 4 using piperidine as
catalyst. The chemical structures of all the intermediate 3, 4 and the
Fig. 2. Electron distribution of the HOMO and LUMO energy level of BDPC dye and BDPC + H+.

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S. Wang, S.-H. Kim / Spectrochimica Acta Part A 72 (2009) 677–681
Fig. 5. The fluorescent monitoring of cycles on and off (by alternate titration
with acid and base) conversion of vinylcyanoacetate-pyran BDPC dye in C2H5OH
(2.0 × 10−5mol L−1) solution (ꢁem = 645 nm; ꢁex = 410 nm).
Fig. 3. The UV–vis absorption spectra changes of vinylcyanoacetate-pyran
BDPC dye in C2H5OH (2.0 × 10−5mol L−1
) solution upon addition of protona-
tion/deprotonation.
3.4. Absorption spectra titration of vinylcyanoacetate-pyran
BDPC dye with protonation/deprotonation
of BDPC dye is bathochromic shifted 129 nm changing the solvent
from hexane to DMSO. Such behavior indicates stabiliza-
a
The interaction of the 2D-␲-A type vinylcyanoacetate-pyran
BDPC dye with acid/base was investigated in C2H5OH solution
through spectrophotometric titration experiments. In particular, a
standard solution of HCl was added stepwise to a (2.5 × 10−4 M)
solution of BDPC dye at room temperature. Fig. 3 showed the UV–vis
absorption spectra changes of BDPC dye in C2H5OH solution with
protonation/deprotonation. Upon addition of proton, the band at
492 nm progressively decreases, while a new band with a peak at
410 nm forms and develops. The presence of an isosbestic points
at 445 nm indicates that only two species coexist at the equilib-
rium. Upon addition of base, the above yellow solution of BDPC dye
became gradually red and recovered the initial state. The charac-
ter absorption shifts hypsochromically by 80 nm. As demonstrated
above, the absorption at 410 nm is slight ‘switched on’ upon grad-
ual addition of proton for BDPC dye, and the 492 nm absorption is
‘switched off’.
tion of the highly dipolar excited state in polar solvents. The
maxmium Stokes shift is 5120 cm−1 in the acetontrile solution.
The photophysical data of BDPC dye are also summarized in
Table 1.
3.3. Theoretical calculation of vinylcyanoacetate-pyran BDPC dye
In order to gain a deeper insight into the intramolecular charge
transfer process of BDPC dye, we performed by theory calcula-
tion based on DMol3 program in the Materials Studio 4.2 package
[25–26]. Fig. 2 shows the electron distribution of the HOMO and
LUMO energy level of BDPC dye and its bonding the proton. Accord-
ingly, the HOMO and LUMO energy level of BDPC dye before and
after protonation were calculated to be −4.466, −2.541 eV and
−4.826, −2.601 eV, respectively. Comparison of the electron dis-
tribution in the frontier MOs reveals the HOMO–LUMO excitation
moved the electron distribution from diethylaminobenzene moiety
to the 2-cyanoacetate moiety, which showed a strong migration of
ICT character of BDPC dye.
3.5. Fluorescence switching of vinylcyanoacetate-pyran BDPC dye
with protonnation/deprotonation
Corresponding to the difference between the Uv–vis absorption
with protonation and deprotonation, the fluorescent response of
BDPC dye was also shown as Fig. 4. The BDPC dye produced strong
fluorescence emission at 650 nm (excitation at 410 nm) at room
temperature. Upon addition of proton, the fluorescent emission
peak at 650 nm progressively decreases and absolutely quenched.
Upon addition of base, the fluorescent intensity gradually recov-
ered the initial state. The switching signal transduction mechanism
here is based on modulation of intramolecular charge transfer
(ICT) by the interaction of the proton and the electron of nitro-
gen atom of the aminobenzene moiety. The modulation in the
electron-donating capabilities of the amide group in the presence
and absence of proton directly influences the ICT from donor to
acceptor moiety. In the presence of proton, a positively charged
aminobenzene moiety of BDPC dye was formed, which causes a
significant decrease in the charge density on the aminobenzene
nitrogen, with associated reducing in the push–pull effect of the ICT
transition. However, in the absence of proton, the extent of ICT from
the donor amide moiety to the acceptor moiety is enhanced, so the
fluorescent intensity gradually increased and recovered the initial
state. In addition, the Fig. 5 showed that the fluorescence intensity
reversibly changed at 650 nm by alternate addition with acid and
Fig. 4. The fluorescence emission spectra changes of vinylcyanoacetate-pyran
BDPC dye in C2H5OH (2.0 × 10−5mol L−1
tion/deprotonation (ꢁex = 410 nm).
) solution upon addition of protona-

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681
Fig. 6. The color change and fluorescence emission response photographs of vinylcyanoacetate-pyran BDPC dye in C2H5OH on addition of 10 equiv. HCl. Left to right: BDPC
dye, BDPC dye and HCl, the emission of BDPC dye, and emission of BDPC dye and HCl.
base and this cycle could be repeated more than 5 times, which is
an excellent proton induced fluorescence switch. Fig. 6 shows the
UV–vis absorption and fluorescence emission photographs of BDPC
dye in C2H5OH on addition of 10 equiv. proton.
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4. Conclusions
In conclusion, we designed and synthesized a new new 2D-␲-A
type vinylcyanoacetate-pyran dye. It exhibited the evidently posi-
tive solvatochromic and solvatofluorchromic properties in different
polarity solvents. An acid-induced fluorescent molecular switch
was demonstrated by modulation of intramolecular charge transfer
with protonation/deprotonation. This fluorescent molecular switch
system showed promising application in the fields of optonic and
electronic smart materials, logic gate, nanomachines, fluorescence
sensors and other molecular photonic devices.
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Acknowledgement
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This research was supported by Kyungpook National University
Research Fund, 2008.
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