Tetrathiafulvalene-flavin dyads: Electron transfer promoted by metal cations

Article abstract of DOI:10.1016/j.tetlet.2010.06.095

Tetrathiafulvalene-flavin dyads 1 and 2 are reported. Both absorption and ESR spectral studies show that the intramolecular electron transfer occurs from TTF to flavin units in dyads 1 and 2 in the presence of Pb²⁺/Sc ³⁺. But, the el

Full text of DOI:10.1016/j.tetlet.2010.06.095

Tetrahedron Letters 51 (2010) 4515–4518
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Tetrathiafulvalene–flavin dyads: electron transfer promoted by metal cations
a,
a
Lina Jia a,b, Guanxin Zhang a, , Deqing Zhang , Daoben Zhu
*
*
a Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100 190, China
b Graduate School of Chinese Academy of Sciences, Beijing 100 190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Tetrathiafulvalene–flavin dyads 1 and 2 are reported. Both absorption and ESR spectral studies show that
the intramolecular electron transfer occurs from TTF to flavin units in dyads 1 and 2 in the presence of
Pb2+/Sc3+. But, the electron transfer is more efficient for dyad 1 in the presence of Pb2+/Sc3+. Electrochem-
ical studies manifest that coordination of dyads 1 and 2 with Pb2+/Sc3+ play an important role in facilitat-
ing the electron transfer within dyads 1 and 2.
Received 24 April 2010
Revised 8 June 2010
Accepted 18 June 2010
Available online 22 June 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Tetrathiafulvalene
Flavin
Intramolecular electron transfer
In the past decades, a number of electron donor (D)–acceptor
(A) dyads with tetrathiafulvalene (TTF) unit or its derivatives as
the electron donating units have been reported.1–6 These TTF based
D–A molecules are interesting as models for studies of charge-trans-
fer interactions, photoinduced electron transfer processes, and
molecular level devices. For instance, fluorescence switches4 and
chemical sensors5 with D–A dyads or triads with TTF units have
been described. Recently, we have reported the metal ions pro-
moted electron transfer within TTF–quinone dyads in which the
TTF and quinone units are covalently linked by the oligoethylene
glycol chain.6 The investigations manifest that the electron transfer
cannot occur in the absence of metal ions and the coordination of
oligoethylene glycol chain and the radical anion of quinone with
metal ions is crucial for the electron transfer. Further studies show
that such metal ion-promoted electron transfer is dependent on
the electron accepting ability of the quinone unit.
flavin, it is interesting to examine whether electron transfer can oc-
cur for the TTF–flavin dyads in the presence of metal ions. Herein,
we report two TTF–flavin dyads 1 and 2, in which the TTF and fla-
vin units are linked by the glycol and alkyl chains, respectively.
The synthesis of dyads 1 and 2 started from compound 313 as
outlined in Scheme 1. Compound 4 was synthesized by removing
the cyanoethyl group in 3 in the presence of CsOH and further reac-
tion with tetraethyleneglycol monotoluenesulfonate. Compound 4
was converted to compound 5 via tosylation. Compound 6 was
prepared by the reaction of compound 5 with 4,5-dimethylben-
zene-1,2-diamine in the presence of K2CO3. Finally, the condensa-
tion of compound 6 with alloxan led to dyad 1 in reasonable yield.14
Dyad 2 was synthesized similarly as shown in Scheme 1. 5-Bromo-
1-pentanol was used for the preparation of compound 7. By
following the same procedures dyad 2 yielded compound 7 in three
steps. 15
Apart from quinone, isoalloxazine compounds, which are also
referred to as flavins, are important electron acceptors. As the
redox-active parts of flavoenzymes flavins enable flavoenzymes
to mediate a variety of electron transfer processes they play an
important role in redox reaction in biological organisms.7 Previous
studies indicate that the electron accepting abilities of flavins can
Figure 1A shows the absorption spectrum of dyad 1 and those in
the presence of different amounts of Pb2+. No absorption above
600 nm was detected for dyad 1 before the addition of Pb2+, and
the absorptions of 1 around 330, 367 and 450 nm were due to
the TTF and flavin units in 1 according to previous studies.12a,13
This implies that the intramolecular interaction between TTF and
flavin units within dyad 1 was negligible. However, new absorp-
tion around 780 nm emerged gradually after the addition of Pb2+
as depicted in Figure 1A. The absorption in the range of 450–
600 nm was also enhanced gradually. In comparison, direct oxida-
tion of compound 3 with Fe(ClO4)3 also led to two new absorptions
around 450 and 780 nm (see Supplementary Fig. S1). Therefore, the
appearance of these two absorptions indicates the formation of the
TTFÅ+ for dyad 1 in the presence of Pb2+. This is likely due to the
electron transfer within dyad 1 in the presence of Pb2+ based on
the previous investigations of TTF–quinone dyads.6
be modulated by hydrogen-bonding,7,8
p–p
stacking,9 donor–
acceptor interaction,10 and in particular coordination to metal
ions.11 The reduction potentials of flavins were positively shifted
in the presence of metal ions such as Mg2+ and Sc3+ and the oxida-
tion reactivity of singlet excited states of flavins could be signifi-
cantly enhanced by coordination with metal ions.11,12 Based on
the unique electron donating and accepting properties of TTF and
* 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 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.095

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L. Jia et al. / Tetrahedron Letters 51 (2010) 4515–4518
Scheme 1. The chemical structure and synthetic approach for dyad 1 and dyad 2: (a) CsOHÁH2O, tetraethyleneglycol monotoluenesulfonate, THF, rt, 60%; (b) p-tosyl chloride,
triethylamine, 0 °C, 84%; (c) 4,5-dimethylbenzene-1,2-diamine, K2CO3, DMF, 60 °C; (d) alloxan, H3BO3, CH3OH, rt, 19%; (e) CsOHÁH2O, 5-bromo-1-pentanol, THF, rt, 80%; (f) p-
tosyl chloride, triethylamine, 0 °C, 84%; (g) 4,5-dimethylbenzene-1,2-diamine, K2CO3, DMF, 60 °C; (h) alloxan, H3BO3, CH3OH, rt, 18%.
the absorption intensity at 780 nm due to the TTFÅ+ versus the mo-
1.0
A
lar fraction of Pb2+ in the solution as depicted in the inset of Figure
0.005
0.004
0.003
0.002
0.001
1A, it may be concluded one molecule of dyad 1 binds two Pb2+
.
0.8
0.6
0.4
0.2
0.0
Of interest is the observation of new absorptions around 450
and 780 nm for dyad 2 after the addition of Pb2+ as demonstrated
in Figure 1B. Compared to dyad 1, these new absorptions are rather
weak under the same condition. The variation of the absorption at
780 nm versus the molar fraction of Pb2+ (see the inset of Fig. 1B)
implies that dyad 2 and Pb2+ may form a complex with 1:1 stoichi-
ometry. As to be discussed below, ESR signal can be detected for
the solution of dyad 2 containing Pb2+ (see Supplementary
Fig. S7). These results manifest that the Pb2+ promoted electron
transfer occurs also for dyad 2, in which the TTF and flavin units
are linked by an alkyl chain.
0.1 0.2 0.3 0.4 0.5 0.6
2+
C
/C
+C
dyad 1 dyad 1 Pb
2+
Pb
400
600
800
1000
Wavelength (nm)
The solution of dyad 1 was ESR silent. However, strong ESR sig-
nal was observed for the solution of dyad 1 in the presence of Pb2+
as shown in Figure 2. The ESR signal intensity was found to be
dependent on the amount of Pb2+ in the solution. The ESR signal
was not well resolved and it might be due to the magnetic coupling
between the radical cation of TTF unit and the radical anion of
B
0.012
0.008
0.004
0.2
0.1
0.0
0.0
0.2
0.4
/C
0.6
+C
0.8
2+
C
dyad 2 dyad 2 Pb
g=2.0101
2+
Pb
400
600
800
1000
Wavelength (nm)
Figure 1. Absorption spectra of dyad 1 (1.0 Â 10À5 M in CH2Cl2) (A) and dyad 2
(1.0 Â 10À5 M in CH2Cl2) (B) upon addition of different amounts of Pb2+[Pb(ClO4)2];
the insets show Job’s plot for the binding of 1 (A)/2 (B) with Pb2+
.
3468
3474
3480
3486
Magnetic Field [G]
Additionally, the absorption around 330 nm due to the flavin
unit in dyad 1 became gradually weak and simultaneously red-
shifted. This may indicate the coordination of flavin unit with
Pb2+ based on the previous report.12a Based on the variation of
Figure 2. ESR spectrum of dyad 1 (1.0 Â 10À3 M) in CH2Cl2 in the presence of
4.0 equiv of Pb2+ [Pb(ClO4)2] recorded at room temperature; the solution was
degassed before measurement.

L. Jia et al. / Tetrahedron Letters 51 (2010) 4515–4518
4517
due to the formation of TTFÅ+ and TTF2+, respectively. The reduction
wave with E1re=d2 = À0.85 V is owing to the reduction of flavin unit in
dyad 1 according to previous study.8e In the presence of Pb2+, the
reduction wave was positively shifted (see the inset of Fig. 3). For
instance, the reduction peak potential was shifted from -0.85 V in
the absence of Pb2+ to À0.45 V in the presence of 10 equiv of Pb
2+. The reduction potential due to the flavin unit in dyad 1 was also
positively shifted in the presence of Sc3+ (see Supplementary
Fig. S5).
2+
no Pb
12
8
1 eq.
5 eq.
10 eq.
-1.0
4
-0.8
-0.4
E/V (vs.A g-/0A.6gC l)
0
-4
-8
Dyad 2 also shows three quasi-reversible redox waves, corre-
sponding to E1=2 = 0.50 V, E1=2 = 0.87 V and E1=2 = À0.86 V. Simi-
ox1
ox2
red
-1.0
-0.5
0.0
0.5
1.0
larly, the oxidation and reduction potentials are due to the TTF
and flavin units in dyad 2, respectively. The reduction potential
of flavin unit is also positively shifted in the presence of either
Pb2+ or Sc3+ (see Supplementary Fig. S8)
E/V (vs. Ag/AgCl)
Figure 3. Cyclic voltammograms of dyad 1 (5 Â 10À5 M) in CH2Cl2; the inset shows
partial (flavin unit) cyclic voltammograms of dyad 1 in CH2Cl2 in the presence of
different amounts of Pb2+; the scanning rate is 50 mV/s.
The above electrochemical investigations clearly indicate that
the reduction potential of flavin unit in dyads 1 and 2 becomes less
negative in the presence of Pb2+/Sc3+; thus, the electron accepting
flavin unit involving Pb2+. Nevertheless, the emergence of ESR sig-
nal confirms that electron transfer takes place for dyad 1 in the
presence of Pb2+. Similarly, ESR signal was also detected for the
solution of dyad 2 containing Pb2+ as depicted in Supplementary
Figure S7. This result provides further support for the occurrence
ability of flavin unit was enhanced in the presence of Pb2+
/
Sc3+ 11,2
Therefore, the electron transfer between TTF and flavins
.
units in dyads 1 and 2 would become more feasible in the presence
of these metal ions. By comparing the metal ion-promoted electron
transfer within TTF–quinone dyads, we assume that the sulfur
atom from TTFÅ+, oxygen atoms of oligoethylene glycol chain and
nitrogen/oxygen atoms of the radical anion of flavin unit in dyad
1 may coordinate with Pb2+/Sc3+ to stabilize the electron transfer
state by increasing the interaction between the corresponding cat-
ion and anion (see Scheme 2). The absorption spectral result indi-
cates that one molecule of dyad 1 binds two Pb2+. Therefore, it is
probable that binding of dyad 1 with metal ion to form a complex
with 1:2 stoichiometry in solution as illustrated in Scheme 2. For-
mation of such coordination complex would, on one hand, to en-
hance the electron accepting ability of flavin unit, and on the
other hand to stabilize the electron transfer state.
For dyad 2 the TTF and flavin units are linked with an alkyl
chain. Only the sulfur atom from TTFÅ+ and nitrogen/oxygen atoms
of the radical anion of flavin unit in dyad 2 may be involved in the
coordination with Pb2+/Sc3+. Therefore, the complex of dayd 2 with
Pb2+/Sc3+may be less stable compared to the corresponding com-
plex of dyad 1 with Pb2+/Sc3+. This is in agreement with the fact
that the absorption around 450 and 780 nm for dyad 2 in the pres-
ence of Pb2+/Sc3+ is weaker than that observed for dyad 1 under the
same condition.
of electron transfer within dyad 2 in the presence of Pb2+
.
The absorption and ESR spectra of dyads 1 and 2 were also mea-
sured in the presence of other metal ions including Sc3+, Zn2+, Mg2+
,
Cd2+ and Ba2+. Absorptions around 450 and 780 nm were observed
for dyad 1 in the presence of Sc3+ (see Supplementary Fig. S2). In
addition, strong ESR signal was also detected for the solution of
dyad 1 containing Sc3+. In comparison, dyad 1 exhibited rather
weak absorptions around 450 and 780 nm after introducing Zn2+
,
Mg2+, Cd2+ and Ba2+. Thus, it can be concluded that Pb2+/Sc3+ can
promote the electron transfer between TTF and flavin units within
dyad 1 more efficiently. Similar results were found for dyad 2. As
displayed in Supplementary Figure S6, the solution of dyad 2 con-
taining Sc3+ also showed absorptions around 450 and 780 nm due
to the TTFÅ+. Therefore, both Pb2+ and Sc3+ can trigger the electron
transfer within dyad 2. But, these absorptions were almost not
detectable for dyad 2 in the presence of Zn2+, Mg2+, Cd2+ and Ba2+
.
In order to understand the mechanism for Pb2+/Sc3+ promoted
electron transfer within dyads 1 and 2, the redox potentials of 1
and 2 in the absence and presence of Pb2+/Sc3+ were measured. Fig-
ure 3 depicts the cyclic voltammogram of 1 and those in the pres-
ence of Pb2+. Three quasi-reversible redox waves were detected for
In summary, TTF–flavin dyads 1 and 2 were synthesized and
characterized. Both absorption and ESR spectroscopic studies
clearly indicate that electron transfer occurs from TTF to the flavin
unit in the presence of metal ions (Pb2+ and Sc3+) for both dyad 1
dyad 1, corresponding to E1=2 = 0.53 V, E1=2 = 0.88 V and E1=2
=
ox1
ox2
red
À0.85 V. The two oxidation potentials are close of those of com-
pound 3 (see Scheme 1) (E1=2 = 0.47 V, E1=2 = 0.85 V).13 Thus, the
ox1
ox2
1=2
ox2
1=2
oxidation waves with E = 0.53 V and E = 0.88 V should be
ox1
S
S
O
S
S
S
S
O
O
O
N
N
N
O
S
NH
Mn+
S
S
S
S
S
S
S
O
Mn+
Mn+
O
N
O
HN
O
N
N
O
Scheme 2. The proposed mechanism for the metal ion-promoted electron transfer in dyad 1.

4518
L. Jia et al. / Tetrahedron Letters 51 (2010) 4515–4518
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important role in facilitating the electron transfer within dyads 1
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Acknowledgments
The present research was financially supported by the Chinese
Academy of Sciences, NSFC and State Key Basic Research Program.
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Supplementary data
Supplementary data associated (Synthesis and characterization;
absorption and ESR spectra; cyclic voltammogram) with this article
can be found, in the online version, at doi:10.1016/j.tetlet.2010.
06.095.
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14. Characterization data for dyad 1: 1H NMR (400 MHz, CDCl3): d 8.49 (1H, s), 8.03
(1H, s), 7.74 (1H, s), 6.40 (1H, br), 4.93 (2H, t, J = 5.26 Hz), 4.02 (2H, t,
J = 5.30 Hz), 3.65–3.60 (4H, m), 3.54 (6H, d, J = 2.73 Hz), 2.92 (2H, t, J = 6.44 Hz),
2.80 (4H, t, J = 6.96 Hz), 2.55 (3H, s), 2.45 (3H, s), 1.64–1.60 (4H, m), 1.44–1.36
(4H, m,), 1.33–1.26 (8H, m), 0.89 (6H, t, J = 6.52). 13C NMR (100 MHz, CDCl3): d
159.52, 154.98, 150.31, 148.00, 137.05, 135.94, 135.00, 132.30, 132.20, 127.86,
127.68, 126.47, 122.97, 116.91, 113.30, 70.91, 70.59, 70.55, 70.48, 69.73, 67.98,
45.81, 36.28, 35.24, 31.31, 29.71, 28.20, 22.53, 21.55, 19.53, 14.02. HR-MS(EI):
calcd for C38H52N4O5S7 868.1983; found, 868.1989.
15. Characterization data for dyad 2: 1H NMR (400 MHz, CDCl3): d 8.42 (1H, s), 8.07
(1H, s), 7.38 (1H, s), 4.70 (2H, br), 2.81–2.77 (6H, m), 2.57 (3H, s), 2.46 (3H, s),
1.93–1.85 (2H, m), 1.88–1.86 (2H, m), 1.64–1.62 (2H, m), 1.41–1.32 (4H, m),
1.29–1.25 (12H, m), 0.88 (6H, t, J = 6.56).13C NMR (100 MHz, CDCl3): 159.73,
155.54, 150.28, 148.60, 137.35, 136.34, 135.18, 133.11, 131.20, 115.46, 45.27,
36.54, 31.52, 29.91, 29.57, 29.29, 28.42, 28.23, 26.87, 25.71, 22.90, 22.75, 22.02,
19.76, 14.24, 14.06. HR-MS(EI): calcd for
778.1672.
C35H46N4O2S7, 778.1666; found,
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