Donor-acceptor molecules containing thiophene chromophore: synthesis, spectroscopic study and electrogenerated chemiluminescence

Article abstract of DOI:10.1016/j.tetlet.2006.04.154

Donor-acceptor molecules containing thiophene chromophore with remarkably large Stokes shift (>210 nm) have been found to exhibit strong and stable ECL emission via the singlet excited state without the addition of any co-reactant or a second compound.

Full text of DOI:10.1016/j.tetlet.2006.04.154

Tetrahedron Letters 47 (2006) 4961–4964
Donor–acceptor molecules containing thiophene
chromophore: synthesis, spectroscopic study and
electrogenerated chemiluminescence
a,
a
b
a
*
Xichuan Yang, Xiao Jiang, Changzhi Zhao, Ruikui Chen,
a
a,c,
*
Peng Qin and Licheng Sun
a
State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular Devices,
Dalian University of Technology(DUT), 158 Zhongshan Road, 116012 Dalian, PR China
b
College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road,
66042 Qingdao, PR China
2
c
Department of Chemistry, Organic Chemistry, Royal Institute of Technology(KTH),
Teknikringen 30, 10044 Stockholm, Sweden
Received 27 February 2006; revised 24 April 2006; accepted 25 April 2006
Available online 26 May 2006
Abstract—Donor–acceptor molecules containing thiophene chromophore with remarkably large Stokes shift (>210 nm) have been
found to exhibit strong and stable ECL emission via the singlet excited state without the addition of any co-reactant or a second
compound.
Ó 2006 Elsevier Ltd. All rights reserved.
3
a
It is well known that electrogenerated chemilumines-
cence (ECL) has been developed as an important and
valuable detecting technique in analytical chemistry. A
series of studies on ECL has been conducted in recent
years. Several review articles including different aspects
D–p–A molecule (BPQ–PTZ).
Recently, Ho and
co-workers have also reported the ECL of a series of
4
D–p–A compounds.
In this letter, we report the synthesis and photophysical
properties of a series of new multifunctional organic
fluorescent molecules, 1–5, with 1,2,2,4-tetramethyl-
1,2,3,4-tetrahydroquinolin-6-yl, N,N-dimethylamino or
1
a–d
of ECL have also been published.
During the past
two decades, most attention has been paid to the ECL
reaction based on the ruthenium polypyridine com-
2
þ
plexes, especially RuðbpyÞ3 and its derivatives, which
have shown broad applications.1e–k Many studies in this
field have been carried out by Bard and co-workers. By
comparison, however, organic molecules capable of
generating ECL have been reported relatively infre-
quently. There are a number of reports on the photo-
physics of the donor–acceptor p-conjugated (D–p–A)
systems, but only a few studies have been reported on
the generation of ECL through the direct attachment
N
N
S
PO(OEt)2
NC
PO(OEt)2
NC
1
3
4
NC
N
S
CHO
PO(OEt)2
S
2
of a donor to an acceptor. As examples, Bard and
N
2
co-workers reported detail studies on the ECL of the
S
N
Keywords: D–p–A; Thiophene; Electrogenerated chemiluminescence
PO(OEt)2
(ECL); Twist intramolecular charge transfer (TICT).
NC
*
Corresponding authors. Tel.: +86 411 88993886; fax: +86 411
3702185 (X.Y.); tel.: +46 8 7908127; fax: +46 8 7912333 (L.S.);
5
8
e-mail addresses: yangxc@dlut.edu.cn; lichengs@kth.se
Figure 1. The Structures of D–p–A Molecules 1–5.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.154

4962
X. Yang et al. / Tetrahedron Letters 47 (2006) 4961–4964
Table 1. Photophysical Characters of Compounds 1–5 in CH
3
OH
(eV)
the existence of the twist intramolecular charge transfer
4
a
(
TICT) states of 3. For a similar reason, compound 4
exhibits an even larger red shift. Both compounds 3 and
have the same electron-acceptor group. The larger red
Compound
kabs (nm)
kem (nm)
Dkst (nm)
E
g
1
2
3
4
5
408
433
487
507
444
464
651
710
734
485
56
218
223
227
41
2.73
2.39
2.10
1.97
2.58
4
shift of 4 than 3 is most likely attributable to the differ-
ence in their electron donor parts. The ring structure at
the end of the electron-donating group in 4 avoids free
rotation about the N–C bond forming a rigid planar
configuration, while 3 has the possibility for free rota-
tion. Compared to 1, the kmax of 5 is obviously red
shifted due to the effective delocalization of p-electrons
in the presence of the thiophene chromophore (Table
pyrrolidine as the donor moiety, a thienylvinyl unit as
the bridge, and diethyl cyanomethylphosphonate as
the acceptor moiety (Fig. 1). These molecules exhibit
chemiluminescence on a platinum electrode, with Ag/
1
). Furthermore, the energy gap, E (Table 1), between
g
+
the LUMO and the HOMO of these molecules was
decreases with the increasing strength of the D–p–A
system.
Ag as reference electrode in CH Cl , when an elec-
2
2
tronic-pulse was applied on the electrodes. In addition,
a twist intramolecular charge transfer (TICT) fluores-
cence with relatively larger Stokes shift (>210 nm) was
also observed, indicating the potential applications of
these molecules as fluorescence sensors, polarity sensors,
It is noteworthy to mention that remarkably large
Stokes shifts (>210 nm) were observed for compounds
5
2
, 3, and 4 (see Fig. 2b and Table 1). This also strongly
etc.
suggests the possible occurrence of a TICT state from
the donor moiety to the acceptor unit in these
The D–p–A molecules 1–5 are prepared according to a
general synthetic procedure. Reaction of 4-N,N-dimethyl-
amino-benzaldehyde with diethyl 2-thienylmethylphos-
phonate in the presence of t-BuOK via Horner–
Emmons Witting conditions leads to the formation of
3
,4a,6
molecules.
The redox potentials of compounds 1–5 are summarized
in Table 2, and the cyclic voltammograms of compounds
1
and 3 in CH Cl are shown in Figure 3. The CV of
compound
(E)-1-(4-N,N-dimethylaminophenyl)-2-(2-
2 2
compound 3 exhibited a reversible and an irreversible
one-electron oxidation wave, and a quasi-reversible
thienyl)ethylene. This compound was formylated in the
unsubstituted 5-position of the thiophene ring in the
presence of n-BuLi by Vilsmeier reaction to give com-
pound 2. Compound 3 was prepared by Knoevenagel
condensation of 2 with diethyl cyanomethylphospho-
nate in the presence of piperidine. Compound 4 was pre-
pared by a similar procedure to 3. Other compounds
were obtained according to the above corresponding
method.
Table 2. Electrochemical data (vs SCE), fluorescent and ECL charac-
ters of compounds 1–5 in CH Cl
2 2
Compound
E
pa(1)
E
pc
k
em
k
ECL
ꢀDH°ann
E
s
a
b
(
V)
(V)
(nm) (nm) (eV)
(eV)
1
2
3
4
5
1.07
0.66
0.66
0.55
0.86
ꢀ1.90 453
ꢀ1.74 584
ꢀ1.46 657
ꢀ1.46 687
ꢀ1.94 481
—
—
—
572
630
654
500
2.24
1.96
1.85
2.64
2.12
1.89
1.80
2.58
The absorption and emission maxima of compounds
1
–5 in CH OH are listed in Table 1. The absorption spec-
3
trum of 3 has a large red shift (more than 80 nm) and a
broad absorption band in comparison with 1, see Figure
a
Only the first oxidation potentials are listed in Table 2.
ꢀDHann=eV ¼ Epað1Þ ꢀ Epcð1Þ ꢀ 0:16eV.
b
o
7
2
a, due to the presence of a thiophene chromophore and
a
b
350 400 450 500 550 600 650 700 750 800 850
350
400
450
500
550
600
650
Wavelength / nm
Wavelength / nm
OH. (b) Absorption (left curve) and fluorescence (right
Figure 2. (a) Optical absorption of 1 (left curve), 3 (middle curve) and 4 (right curve) in CH
3
curve) spectra of 3 in CH OH.
3

X. Yang et al. / Tetrahedron Letters 47 (2006) 4961–4964
4963
a
b
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
Potential / V vs SCE
Potential / V vs SCE
ꢀ
3
Figure 3. Cyclic voltammograms of 1 (a) and 3 (b) with sample concentration of 10 M containing 50 mM TBAPF
6
as the supporting electrolyte in
ꢀ1
.
+
CH
2
2
Cl using a platinum electrode as a working electrode and Ag/Ag as reference electrode, scan rate 100 mVs
a
b
500
550
600
650
700
750
Wavelength / nm
Figure 5. ECL (left curve) and fluorescence (right curve) spectra of
ꢀ
3
1
0
M solution of 2 in CH Cl containing 50 mM TBAPF as an
2 2 6
electrolyte. The potential was stepped up from ꢀ1.8 to +1.2 V pulse
for 0.5 s.
The X-ray crystal structure of compound 2 (Fig. 4)
shows that the molecule has a planar structure, indicat-
8
ing an effective conjugation through the molecular axis.
From the crystal packing diagram, it was found that the
inter-planar distance between the two parallel benzene
˚
d,9
or thiophene planes of adjacent molecules was 2.63 A,
which is within the range of a typical p–p interaction.
4
In our electrochemical study, the first oxidation and first
reduction processes in these molecules are chemically
reversible in the cyclic voltammograms. So the electro-
generated radical ions are stable, and therefore an
efficient ECL could occur as a result of ion annihila-
Figure 4. X-ray crystal structure (a) and crystal packing diagram (b) of
compound 2 with thermal ellipsoid set at 30% probability.
4
b
tion. Figure 5 shows the ECL spectrum of compound
one-electron reduction wave. The CV of compound 2 is
very similar to that of 3. When comparing the Epc values
of 2 and 3, it is evident that the reduction potential of 2 is
more negative than that of 3 (Table 2). This indicates
that the reduction of 3 is easier than 2 due to the intro-
duction of a strong electron-withdrawing group.
2 in CH Cl containing 50 mM TBAPF as an electro-
2
2
6
lyte. The ECL maxima for compounds 1–5 are summa-
rized in Table 2. Compounds 2, 3, 4 and 5 exhibit ECL
character, while 1 does not. The ECL emission maxi-
mum of 2 is blue-shifted in comparison with the fluores-
cence emission maximum in the same solvent. This may

4964
X. Yang et al. / Tetrahedron Letters 47 (2006) 4961–4964
be attributed to the p–p interaction between the two
parallel molecules in solution (Fig. 4b). This type of
interaction formation may cause the ECL emission to
be blue-shifted when compared with the photolumines-
Foundation (Grant 20128005), the Ministry of Science
and Technology (MOST) (Grant 2001CCA02500), the
Ministry of Education (MOE) and the Swedish Energy
Agency and the Swedish Research Council.
4
c
cence values previously reported.
The ECL maximum of 2 in CH Cl is close to the photo-
References and notes
2
2
luminescence maximum, and only blue shifted by ca.
2 nm (Fig. 5). The narrow emission band of ECL,
1
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b,10
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during the ion annihilation process.
More impor-
tantly, the ECL intensity is not increased when the
fluorophore concentration is increased. Thus, we can
safely conclude that there is no excimer formation
during the ECL process. By comparison, the fluores-
cence spectrum of 2 was recorded in CH Cl in the same
2
2
ꢀ3
concentration (10 M) as used for the ECL measure-
ments. We found that the photoluminescence maximum
ꢀ5
is invariable as compared to that at 10 M concentra-
tion, although with drastically reduced intensity. There-
fore, it can be considered that the ECL emissive species
is different from that of the photoluminescent species,
and the ECL emission is from an aggregate which is
formed via the p–p interaction of two adjacent parallel
2
3
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1
1a
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4. (a) Chen, C.-Y.; Ho, J.-H.; Wang, S.-L.; Ho, T.-I.
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1b
tes,
and the blue shift can be explained as above.
Compounds 1 and 5 have simple and similar molecular
structure; however, an ECL emission was observed for 5
but not for 1. In addition to the delocalization of p-elec-
trons mentioned above for compound 5, under electro-
chemistry conditions, the charge separation is more
accessible due to the introduction of thiophene ring.
Here, the positive and negative charges are effectively
separated which allows for a longer life time. Thus,
ECL emission could be easily produced via the annihila-
8
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2
tion reaction of a radical anion and a radical cation. In
this case, the energy provided by the radical ions is
sufficient enough to populate the singlet excited state.
When the ECL is produced via the S-route, the follow-
1
2325–12335.
7
. Faulkner, L. R.; Tachikawa, H.; Bard, A. J. J. Am. Chem.
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1
2
ing equation must be satisfied:
8
. Crystal structure data for 2: C H NOS, M = 257.34,
1
4
15
r
o
ann
orthorhombic, space group: P2
1
2
1
2
1
,
a = 6.1356(4),
ꢀ
DH
P Es
ð1Þ
˚
b = 7.6377(5),
c = 28.6163(18) A,
3
a = b = c = 90°,
˚
o
ann
V = 1341.01(15) A , Z = 4, F(000) = 544, D
c
= 1.275 g
where ꢀDH is the annihilation reaction enthalpy and Es
ꢀ3
cm , R = 0.0465, R = 0.1175.CCDC-278807 contains
w
is the energy for the first excited singlet generation state.
the supplementary crystallographic data for this letter.
These data can be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB21EZ,
UK (fax: +44 1233 336 033; e-mail: deposit@ccdc.cam.
ac.uk or http://www.ccdc.cam.ac.uk).
As seen from Table 2, the energy provided by the ion-
annihilation reaction is larger than that required for
the direct production of the singlet state, hence it is en-
ough to produce a singlet state directly. Furthermore, in
these energy sufficient systems, all the stable ECL spec-
tra can be observed via the S-route without the addition
of any co-reactant or a second compound.
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Acknowledgements
Financial support of this work from the following sources
is gratefully acknowledged: China Natural Science
12. Fungo, F.; Wong, K.-T.; Ku, S.-Y.; Hung, Y.-Y.; Bard, A.
J. J. Phys. Chem. B 2005, 109, 3984–3989.