A new 4-(N,N-dimethylamino)benzonitrile (DMABN) derivative with tetrathiafulvalene unit: modulation of the dual fluorescence of DMABN by redox reaction of tetrathiafulvalene unit

Article abstract of DOI:10.1016/j.tetlet.2007.12.087

A new derivative of 4-(N,N-dimethylamino)benzonitrile (DMABN) (compound 1) with TTF unit is reported. Compound 1 exhibits dual fluorescence, and moreover the dual fluorescence behavior can be modulated by reversible oxidation and reduction of TTF unit of

Full text of DOI:10.1016/j.tetlet.2007.12.087

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Tetrahedron Letters 49 (2008) 1361–1364
A new 4-(N,N-dimethylamino)benzonitrile (DMABN) derivative
with tetrathiafulvalene unit: modulation of the dual fluorescence
of DMABN by redox reaction of tetrathiafulvalene unit
a,
Wei Tan a,b, Deqing Zhang , Hui Wu a,b, Daoben Zhu a,*
*
a Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
b Graduate School of Chinese Academy of Sciences, Beijing 100080, China
Received 6 October 2007; revised 14 December 2007; accepted 18 December 2007
Available online 23 December 2007
Abstract
A new derivative of 4-(N,N-dimethylamino)benzonitrile (DMABN) (compound 1) with TTF unit is reported. Compound 1 exhibits
dual fluorescence, and moreover the dual fluorescence behavior can be modulated by reversible oxidation and reduction of TTF unit of 1
either chemically or electrochemically. Accordingly, a new molecular fluorescence switch is realized by coupling the features of TTF and
DMABN.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: DMABN; TICT; Tetrathiafulvalene; Fluorescence switch
It is known that 4-(N,N-dimethylamino)benzonitrile
(DMABN) and its derivatives, as a class of organic
donor–acceptor compounds, exhibit dual fluorescence,
one related to the local excited state (‘B’ band), and the
other ascribed to the twisted intramolecular charge-transfer
(TICT) state (‘A’ band).1,2 Since the first observation
by Lippert et al.,1 a number of DMABN and its
ana-logues have been investigated regarding the TICT
mechanism.3–11 These studies clearly show that the dual
fluorescence behavior is influenced by the substituents
incorporated into DMABN. By making use of the feature
of DMABN, several ion-sensors based on DMABN as a
dual-fluorescent fluorophore have been described in recent
years.12–16 We have just recently reported DMABN deriv-
atives with boronic acid/boronate groups as ratiometric
fluorescence sensors for saccharides and fluoride ion.17,18
Herein, we will report a new derivative of DMABN with
one tetrathiafulvalene (TTF) unit (1, Scheme 1) with a view
Br
NH2
N
NH
a
b
CN
2
CN
4
CN
3
S
S
S
S
S
S
S
S
S
S
NC
S
S
S
S
N
c
1
CN
Scheme 1. Synthetic procedure for compound 1: (a) MeI, reflux, 45%; (b)
3 M H2SO4, bromoacetaldehyde diethyl acetal; NaBH4, 18%; (c)
CsOHꢀH2O, 4-(2-cyanoethylthio)-40,50-(dimethylthio)-tetrathiafulvalene,
75%.
*
Corresponding authors. Tel.: +86 10 62639355; fax: +86 10 62569349
(D.Z.).
E-mail address: dqzhang@iccas.ac.cn (D. Zhang).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.12.087

1362
W. Tan et al. / Tetrahedron Letters 49 (2008) 1361–1364
to building a new molecular fluorescence switch by cou-
pling the features of DMABN and TTF. The design ratio-
nale is described as follows: a neutral species of TTF can be
reversibly transformed into the corresponding species with
positive charges, radical cation (TTFÅ+), and dication
(TTF2+), by either chemical or electrochemical oxidation
at easily accessible potential.19–24 As a result, the electron
donating ability of TTF can be tuned by reversible redox
reactions. Moreover, TTF and TTFÅ+/TTF2+ show differ-
ent absorption spectra.25–28 Therefore, it is expected that
(1) oxidation of TTF unit may have an influence on the
ground/excited state conformation of DMABN unit of 1;
(2) oxidation of TTF unit may alter the intramolecular
energy/electron transfer process within compound 1;
accordingly, it may be possible to reversibly modulate the
dual fluorescence behavior of DMABN unit of compound 1.
The target compound 1 was synthesized through three
steps starting from 4-aminobenzonitrile (2, Scheme 1).
Compound 3 was yielded after reaction of compound 2
with MeI. After reaction with 3-bromo-1,1-diethoxypro-
pane, followed by treatment with NaBH4, compound 3
was converted to compound 4 in 18% yield, which was fur-
ther reacted with 4-(2-cyanoethylthio)-40,50-(dimethylthio)-
tetrathiafulvalene in the presence of CsOHꢀH2O leading
to compound 1 in 75% yield.29 The synthetic details are
provided in Supplementary data.
Figures 1 and 2 show the absorption and fluorescence
spectra of 1 and those after the addition of different
amounts of Fe3+. The absorption spectrum of 1 is almost
the superposition of those of DMABN and TTF units,
indicating that there is no detectable interaction between
DMABN and TTF units. As expected, compound 1 exhi-
bits dual fluorescence, showing two fluorescence bands
centered at 350 nm and 432 nm, which can be ascribed to
the corresponding B band (from the local excited state)
and A band (from the TICT state), respectively. The inten-
sity ratio of the two fluorescence bands at 350 nm for B
band and 432 nm for A band (IB/IA) was measured to be
0.74.
0.0 eq. Fe3+
1.2 eq. Fe3+
30
20
10
0
300
350
400
450
500
550
Wavelength (nm)
50
40
30
20
10
0
50 μM 1
50 μM 1+0.8eq Fe3+
50 μM 1+0.8eq Fe3++ Na2S2O3
300
350
400
450
500
550
Wavelength (nm)
Fig. 2. (A) The fluorescence spectrum of compound 1 (5.0 ꢁ 10ꢂ5 M in
CH2Cl2) and those in the presence of different amounts of Fe(ClO4)3. (B)
The reversible variation of the fluorescence spectrum of compound 1
(5.0 ꢁ 10ꢂ5 M in CH2Cl2) after oxidation by Fe3+ and further reduction
by Na2S2O3.
It is known that TTF and its analogues can be oxidized
stoichiometrically by Fe3+. New absorption bands around
460 nm and 800 nm emerge gradually after the addition of
Fe3+ to the solution of 1 as shown in Figure 1, where the
absorption spectra of 1 in the presence of different amounts
of Fe3+ are displayed. The appearance of absorption bands
around 460 nm and 800 nm indicates the formation of the
radical cation of TTF unit according to previous
reports.25–28 Simultaneously, variation of the fluorescence
spectrum of 1 is detected after the addition of Fe3+; the
fluorescence intensity of B band at about 300–400 nm
increases slightly while that of A band in the range of
400–500 nm decreases gradually (see Fig. 2A). For
instance, after reaction with 1.0 equiv of Fe3+ the fluores-
cence intensity of 1 at 350 nm within B band increases by
37%, and that at 432 nm within A band decreases by
88% compared to the corresponding initial intensities of 1
1.6
1.2
0.8
before the addition of Fe3+ 30
Accordingly, the intensity
.
1.2 eq. of Fe3+
0.0 eq. of Fe3+
ratio of the two fluorescence bands at 350 nm for B band
and 432 nm for A band (IB/IA = 8.3) increases by 11 times
compared to that before oxidation by Fe3+. As anticipated,
the radical cation of TTF unit of 1 can be transformed into
the neutral unit by treatment with excess sodium thiosul-
fate (Na2S2O3); consequently, both absorption and fluores-
cence spectra of 1 after oxidation by Fe3+ can be restored
0.4
0.0
400
600
800
1000
Wavelength (nm)
Fig. 1. Absorption spectrum of compound 1 (5.0 ꢁ 10ꢂ5 M in CH2Cl2)
and those in the presence of different amounts of Fe(ClO4)3.

W. Tan et al. / Tetrahedron Letters 49 (2008) 1361–1364
1363
by further reaction with Na2S2O3 (see Fig. 2B). Therefore,
modulation of the dual fluorescence behavior of 1 can be
realized by reversible chemical oxidation and reduction of
TTF unit of 1.
thus not all molecules of 1 in the solution were oxidized.
After the application of a reduction potential of 0.1 V (vs
Ag wire) to the electrochemically oxidized solution for
3.0 min, the fluorescence spectrum of 1 was restored (see
Fig. 3B). This is simply due to the reduction of the radical
cation of TTF unit into the neutral TTF unit.
It is also possible to modulate the dual fluorescence
behavior of 1 by reversible electrochemical oxidation and
reduction of TTF unit of 1. Oxidation of the solution of
1 containing n-Bu4NPF6 (28 mM), which was performed
by applying an oxidation potential of 0.80 V (vs Ag wire)
to the solution, also led to the increase for B band and
the decrease for A band of 1 as shown in Figure 3A. This
is similar to the fluorescence spectral variation observed for
1 after chemical oxidation as discussed above. Such fluores-
cence spectral change is obviously due to the transforma-
tion of the neutral TTF unit into the corresponding
cation radical, since the oxidation potential (0.8 V vs Ag
wire) employed for the electrochemical experiment is
higher than that of the first oxidation potential of TTF unit
of 1 [E11=2ðoxÞ (vs Ag/AgCl) = 0.57 V].31 But, the fluores-
cence intensity variation after electrochemical oxidation
was small compared to that observed after chemical oxida-
tion (see Fig. 2). This was likely due to the fact that electro-
chemical oxidation was carried out for a short period and
The above results clearly show that the dual fluorescence
behavior of 1 can be modulated by reversible oxidation and
reduction of TTF unit of 1. A quasi-reversible redox wave
at 1.41 V was detected for 1, which should be due to the
oxidation of DMABN unit. But, no redox wave at low
potential was observed. Therefore, both electron accepting
and donating abilities of DMABN unit are rather weak.
Accordingly, the corresponding photoinduced electron
transfer between TTF and DMABN units within 1 may
not occur. However, the intramolecular energy transfer
process may be responsible for this dual fluorescence modul-
ation. As discussed above, the DMABN unit of 1 exhibits
two emission bands, B band in the range of 300–400 nm
and A band in the range of 400–500 nm. As displayed in
Figure 1, the neutral TTF unit shows strong absorption
around 350 nm, while the corresponding radical cation
absorbs in the range of 400–550 nm besides in the range
of 600–1000 nm. Therefore, the B band of the fluorescence
spectrum of 1 with a neutral TTF unit would be quenched
to some extent due to the intramolecular energy transfer
since there is a spectral overlap between the fluorescence
B band and the absorption spectrum of TTF unit. On the
other hand, the A band of the fluorescence spectrum of 1
after oxidation (with TTFÅ+ unit) would decrease because
there is a spectral overlap between the fluorescence A band
and the absorption spectrum of the radical cation of TTF
unit and thus intramolecular energy transfer would take
place. An influence of oxidation of TTF unit on the
ground/excited state conformation of DMABN unit of 1
may also contribute to the dual fluorescence variation
observed for 1 after chemical/electrochemical oxidation
of TTF unit.
25
0.0 s
20
120 s
15
10
5
0
300
350
400
450
500
550
Wavelength (nm)
In summary, a new derivative of 4-(N,N-dimethyl-
amino)-benzonitrile (DMABN) (compound 1) with TTF
unit was designed and synthesized with a view to building
a new molecular fluorescence switch by coupling the
features of TTF and DMABN. Compound 1 exhibits dual
fluorescence, and moreover the dual fluorescence behavior
can be modulated by reversible oxidation and reduction of
TTF unit of 1 either chemically or electrochemically.
25
20
15
10
5
before
120 s oxidation
120 s oxidation
+ 180 s reduction
Acknowledgments
0
300
350
400
450
500
550
The present research was financially supported by
NSFC and State Key Basic Research Program.
Wavelength (nm)
Fig. 3. (A) Fluorescence spectra of the solution of compound
1
(5.0 ꢁ 10ꢂ5 M in CH2Cl2) containing n-Bu4NPF6 (28 mM) after applying
an oxidation of 0.8 V (vs Ag wire) for different periods; (B) fluorescence
spectrum of the solution of compound 1 (5.0 ꢁ 10ꢂ5 M in CH2Cl2)
containing n-Bu4NPF6 (28 mM) and those after applying an oxidation of
0.8 V (vs Ag wire) for 120 s and further by applying reduction potential of
0.1 V (vs Ag wire) for 180 s.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.12.087.

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W. Tan et al. / Tetrahedron Letters 49 (2008) 1361–1364
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29. Characterization data of compound 1: mp = 137–138 °C; Rf = 0.24
1
(CH2Cl2:petroleum = 1:1); H NMR (CDCl3): d 2.40 (s, 6H), 2.85 (t,
2H, J = 7.0 Hz), 3.03 (s, 3H), 3.63 (t, 2H, J = 7.1 Hz), 6.38 (s, 1H),
6.65 (d, 2H, J = 8.3 Hz), 7.46 (d, 2H, J = 8.2 Hz). 13C NMR (CDCl3):
d 19.3, 32.0, 38.7, 52.1, 98.4, 111.7, 113.3, 120.4, 123.3, 126.0, 127.7,
127.8, 133.8, 151.0. EI MS: m/z 486 (M+); HRMS (EI): Anal. Calcd
for ðC18H18N2S7þÞ: 485.9515; found: 485.9520.
30. When more than 2 equiv of Fe3+ were added, typical absorption
bands for the dication of TTF unit were observed. The fluorescence
intensities of both A band and B band decreased slightly.
31. Two reversible oxidation waves with E11=2ðoxÞ (vs Ag/AgCl) = 0.57 V
18. Tan, W.; Zhang, D. Q.; Zhu, D. B. Bioorg. Med. Chem. Lett. 2007, 17,
2629.
and E21=2ðoxÞ (vs Ag/AgCl) = 0.93 V were detected for compound 1.