A new tetrathiafulvalene-quinone-tetrathiafulvalene triad: Modulation of the intramolecular charge transfer by the electron-transfer process promoted by metal ions

Article abstract of DOI:10.1021/jo9007332

(Chemical Equation Presented) Electron transfer can occur from the TTF units to the substituted quinone unit in a new TTF-quinone-TTF triad 1 containing the N,N-dialkylaniline-substituted quinone unit flanked by two TTF units, in the presence of metal ion

Full text of DOI:10.1021/jo9007332

of spiropyran.3a The electron transfer promoted by metal ions
can occur to TTF-quinone dyads in which the electron
accepting abilities of quinone units are rather weak.3b The results
also suggest that the synergic coordination of the oligoethylene
glycol chain and the radical anion of the quinone unit with metal
ion may contribute to stabilizing the corresponding charge-
separation state and thus facilitate the electron-transfer process.
In fact, electronic properties and even self-assembly morphology
were reported to be modulated for chromospheres connected
by oligoethylene glycol chain upon binding of metal cations.4
In this report, we describe a new TTF-quinone-TTF triad
1 (Scheme 1) in which the quinone unit is substituted with the
N,N-dialkylaniline unit. According to previous studies,5 in-
tramolecular charge transfer (ICT) should exist between the
quinone and N,N-dialkylaniline units. The absorption and ESR
spectral studies show that (1) electron transfer occurs within
triad 1 in the presence of metal ions (Pb2+, Zn2+, and Sc3+); (2)
simultaneously, the intramolecular charge transfer becomes weak
after addition of metal ions, indicating that the electron transfer
A New
Tetrathiafulvalene-Quinone-Tetrathiafulvalene
Triad: Modulation of the Intramolecular Charge
Transfer by the Electron-Transfer Process
Promoted by Metal Ions
Yan Zeng,†,‡ Guanxin Zhang,*,† Deqing Zhang,*,† and
Daoben Zhu†
Beijing National Laboratory for Molecular Sciences,
Organic Solids Laboratory, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, People’s Republic of
China, and Graduate School of Chinese Academy of
Sciences, Beijing 100049, People’s Republic of China
dqzhang@iccas.ac.cn
ReceiVed April 7, 2009
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Electron transfer can occur from the TTF units to the
substituted quinone unit in a new TTF-quinone-TTF triad
1 containing the N,N-dialkylaniline-substituted quinone unit
flanked by two TTF units, in the presence of metal ions
(Pb2+, Zn2+, and Sc3+). Simultaneously, the corresponding
charge transfer within the substituted quinone unit becomes
weak in the presence of metal ions. Moreover, the metal ion-
promoted electron transfer and the intramolecular charge
transfer can be tuned by alternating UV and visible light
irradiation in the presence of spiropyran.
Electron donor (D)-acceptor (A) compounds with TTF
(tetrathiafulvalene) as electron donor have been widely inves-
tigated for studies of intramolecular charge transfer/photoin-
duced electron-transfer processes and molecular level devices
as well as potential applications in solar cell systems.1 For
instance, fluorescence switches and chemical sensors with TTF
based D-A compounds have been reported.2
We have recently described the metal ions-promoted electron
transfer within TTF-quinone dyads in which the TTF and
quinone units are covalently linked by an oligoethylene glycol
chain.3 Moreover, the electron-transfer process can be modulated
by alternating UV and visible light irradiations in the presence
† Institute of Chemistry, Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
10.1021/jo9007332 CCC: $40.75
Published on Web 05/07/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 4375–4378 4375

SCHEME 1. The Chemical Structure and Synthetic
Approach for Triad 1
FIGURE 1. Absorption spectra of triad 1 recorded in CH2Cl2 (2.5 ×
10-5 M) in the presence of increasing amounts of Pb2+ [Pb(ClO4)2];
the inset shows the 518-640 nm part of the absorption spectra of triad
1 in the presence of increasing amounts of Pb2+; the solutions were
bubbled with N2 before measurements.
between TTF and quinone units influences the ICT within the
substituted quinone unit; and (3) both the electron transfer and
ICT processes can be modulated by UV and visible light
irradiations in the presence of the photochromic spiropyran.
The synthetic approach for triad 1 is shown in Scheme 1.
Removal of the cyanoethyl6 group in compound 2 and further
reaction with tetraethyleneglycol monotoluenesulfonate led to
compound 3, which was further transformed into compound 4
after reaction with succinic anhydride. Compound 5 was
separately synthesized from N,N-(dihydroxyethyl)aniline and
quinone in the presence of In(OTf)3 according to previous
report.7 The reaction of compound 5 and compound 4 in the
presence of dicyclohexylcarbodiimide (DCC) and dimethylami-
nopyridine (DMAP) led to triad 1. The detailed synthetic
procedures and characterization data are provided in the
Supporting Information.
Figure 1 shows the absorption spectrum of triad 1 in CH2Cl2.
A broad absorption around 542 nm was detected for triad 1.
By comparing with the absorption spectrum of 5 (see Figure
S1 in the SI), this broad absorption band should be due to the
intramolecular charge transfer (ICT) band associated with the
N,N-dialkylaniline-substituted quinone unit in triad 1.
After the addition of Pb2+ new absorptions around 450 and
850 nm emerged gradually and their intensities increased by
increasing the amounts of Pb2+ as shown in Figure 1. According
to previous studies,8 the absorption bands around 450 and 845
nm could be ascribed to the radical cation of TTF (TTF•+) units
in triad 1. Thus, the appearance of these two absorption bands
FIGURE 2. The ESR spectra of triad 1 (5.0 × 10-5 M) in CH2Cl2 in
the presence of 2.0 equiv of Pb2+ [Pb(ClO4)2] recorded at room
temperature; the solution was bubbled with N2 before measurement.
indicated the formation of the radical cation of the TTF unit
(TTF•+). This conclusion was supported by a direct oxidation
of the TTF unit in triad 1 by Fe3+, which also led to two
absorptions at 450 and 850 nm (see Figure S2 in the SI).
Therefore, it may be concluded that electron transfer occurs
between TTF and substituted quinone units of triad 1 in the
presence of Pb2+, being similar to the TTF-quinone dyads
reported by us recently.3
Moreover, ESR signals were detected for triad 1 after
introducing Pb2+ to the solution as displayed in Figure 2. The
solution of triad 1 in the presence of 2.0 equiv of Pb2+ exhibited
strong triplet signals (g ) 2.0098, aH ) 0.56 G). The triplet
signals were likely due to the splitting of the two H atoms of
the radical cations of the TTF units in triad 1. The ESR
spectroscopic studies are in agreement with the generation of
TTF•+ in the presence of Pb2+. Because of the easy dispropor-
tionation of radical anion of the quinone (Q•-) into the
corresponding Q2- and neutral Q in the presence of metal ions
at room temperature,9 the corresponding ESR signals due to
(6) (a) Jia, C. Y.; Zhang, D. Q.; Xu, W.; Zhu, D. B. Org. Lett. 2001, 3,
1941–1944. (b) Jia, C. Y.; Zhang, D. Q.; Guo, X. F.; Xu, W.; Zhu, D. B. Synthesis
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Matersteig, G.; Menzer, S.; Nielen, M. B.; Raymo, F. M.; Stoddart, J. F.; Venturi,
M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951–3957. (b) Spangaard,
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Q
•- were not observed. Accordingly, both absorption and ESR
spectroscopic investigations show that electron transfer takes
place between TTF and substituted quinone units of triad 1 in
the presence of Pb2+
.
As for the TTF-quinone dyads reported previously,3 the
metal ion-promoted electron transfer within triad 1 can be
(9) (a) Fukuzumi, S.; Hironaka, K.; Nishizawa, N.; Tanaka, T. Bull. Chem.
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4376 J. Org. Chem. Vol. 74, No. 11, 2009

Sc3+ (see Figures S8-2 and S8-3, SI). The binding constants
were calculated to be 3.27 × 109 and 4.19 × 109 M-2 for Zn2+
and Sc3+ complexes, respectively, by analyzing the absorbance
variation at 845 nm vs. the concentrations of Zn2+/Sc3+ with
the Benesi-Hildebrand equation (see Figures S9-2 and S9-3
in the SI). When the absorption and ESR spectra of triad 1 (the
SCHEME 2. The Proposed Mechanism for the Metal
Ion-Promoted Electron Transfer in Triad 1
same concentration) in the presence of equal amounts of Sc3+
/
Pb2+/Zn2+ are compared, the absorption intensity at 845 nm and
the ESR signal intensity varied in the following order: 1 + Sc3+
> 1 + Zn2+ > 1 + Pb2+. This leads us to conclude that Sc3+
can promote the electron transfer within triad 1 more efficiently.
This result can be understandable by considering the following
fact that Sc3+ shows a preference for oxygen coordination. The
presence of a nitrogen atom may increase the binding of triad
1 with Zn2+,11which may explain why Zn2+ facilitates the
electron-transfer process more efficiently than Pb2+
.
It is interesting to note that the charge transfer absorption
band at 542 nm, which is associated with triad 1, becomes weak
simultaneously in the presence of Pb2+, as shown in the inset
of Figure 1. Similar phenomena were observed for triad 1 in
the presence of Zn2+ or Sc3+ (see the insets of Figures S10-1
and S10-2 in the SI). This can be understood as follows: the
metal ion-promoted electron transfer from TTF to the quinone
unit would reduce the electron accepting ability of the quinone
uint. Accordingly, the degree of intramolecular charge transfer
would decrease within the N,N-dialkylaniline-substituted quino-
ne unit and thus the corresponding charge transfer band intensity
is reduced in the presence of metal ions (Pb2+, Zn2+, and Sc3+).
Thus, the intramolecular charge transfer can be modulated by
the metal ion-promoted electron transfer within triad 1. Ad-
ditionally, the coordination of the nitrogen atom with metal ions
would decrease the electron donating ability and as a result this
may also contribute to the weakening of the corresponding
charge transfer absorption band at 542 nm.
understood as follows: (1) It is known that the reduction
potentials of quinone units are positively shifted in the presence
of metal ions, and thus the electron transfer between TTF and
quinone units becomes thermodynamically more feasible in the
presence of these metal ions.10 As shown in Figure S3 of the
SI, the reduction potential of the quinone unit in triad 1 is
anodically shifted in the presence of 2.0 equiv of Pb2+. (2) The
synergic coordination of oligoethylene glycol chain4 and the
radical anion of quinone with metal ions as proposed in Scheme
2 would be able to stabilize the electron-transfer state by
increasing the interaction between the corresponding cations and
anion. This proposition was supported by the following finding:
addition of 2,2′-bipyridine to the solution of triad 1 containing
Pb2+ gradually led to the disappearance of the absorption bands
at 450 and 845 nm (seeing Figure S6 in the SI). 2,2′-Bipyridine
can bind Pb2+ and as a result the complex of 1 with Pb2+ would
be dissociated; accordingly, back electron transfer from the
quinone anion to TTF•+ would occur. Additionally, if polar
solvents such as CH3CN, THF, and DMF were used instead of
CH2Cl2, the absorptions around 450 and 850 nm due to TTF•+
could be neglected for the solution of 1 after introducing Pb2+
(see Figure S7-1 in the SI). This is probably because the
complex of 1 with Pb2+ becomes unstable in polar solvents.
On the basis of the corresponding Job’s plot (see Figure S8-1
in the SI), the stoichiometry of complex formed between 1 and
Pb2+ was estimated to be 1: 2. Furthermore, based on the
variation of the absorption intensity at 845 nm of 1 after addition
of different amounts of Pb2+, the binding constant of the 1:2
complex was calculated to be 1.38 × 109 M-2 by fitting the
data with the Benesi-Hildebrand equation (assuming a single
equilibrium process, see Figure S9-1 in the SI).
It was reported that the photochromic spiropyran can be
transformed into the corresponding merocyanine form (MC)
upon UV light irradiation and the MC form (in particular, the
spiropyran with the COOH group) can coordinate with metal
ions.12 When the SP compound such as compound 613 was
added to the solution of 1 containing metal ions, the MC form
generated by UV light irradiation would bind metal ions and
Besides Pb2+, Zn2+ and Sc3+ can also induce the intramo-
lecular electron-transfer process within triad 1. As shown in
Figures S10-1 and S10-2 in the SI, absorption bands at 450
and 845 nm due to the TTF radical cation of triad 1 were
detected in the presence of Zn2+ and Sc3+. Also, triplet ESR
signals were observed (see Figures S11-1 and S11-2 in the SI).
These results demonstrate that electron transfer occurs for triad
1 in the presence of Zn2+/Sc3+. According to the corresponding
(11) (a) Burdette, S. C.; Walkup, G. K.; Spingler, B.; Tsien, R. Y.; Lippard,
S. J. J. Am. Chem. Soc. 2001, 123, 7831–7841. (b) Maruyama, S.; Kikuchi, K.;
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Kim, J. S.; Yoon, J. J. Org. Chem. 2005, 70, 9603–9606. (e) Jose, D. A.; Mishra,
S.; Ghosh, A.; Shrivastav, A.; Mishra, S. K.; Das, A. Org. Lett. 2007, 9, 1979–
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H. E.; Seok, Y.-J.; Hong, J.-I. J. Am. Chem. Soc. 2008, 130, 784–785.
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120, 3237–3242. (b) Guo, X. F.; Zhang, D. Q.; Wang, T. X.; Zhu, D. B. Chem.
Commun. 2003, 914–915. (c) Guo, X. F.; Zhang, D. Q.; Tao, H. R.; Zhu, D. B.
Org. Lett. 2004, 6, 2491–2494. (d) Guo, X. F.; Zhang, D. Q.; Zhu, D. B. J.
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Job’s plots, 1:2 complexes were formed between 1 and Zn2+
/
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J. Org. Chem. Vol. 74, No. 11, 2009 4377

in the presence of metal ions (Pb2+, Zn2+, and Sc3+). Simulta-
neously, the corresponding charge transfer within the substituted
quinone unit becomes weak in the presence of metal ions.
Moreover, the metal ion-promoted electron transfer and the
intramolecular charge transfer can be tuned by alternating UV
and visible light irradiation in the presence of spiropyran, by
taking advantage of the unique features of photochromic
spiropyran. These results further confirm that the metal ion-
promoted electron transfer is general for TTF-quinone com-
pounds with glycol chain as the spacer.
Experimental Section
The synthesis and characterization of compounds 3-5 were
provided in the Supporting Information.
Synthesis of Triad 1. To a solution of 4 (0.47 g, 0.77 mmol),
5 (0.11 g, 0.38 mmol), and DMAP (25 mg, 0.2 mmol) in 30 mL of
dry CH2Cl2 at 0 °C was dropwise added a solution of DCC (0.22
g, 1.0 mmol) in 10 mL of dry CH2Cl2. Then, the mixture was
allowed to reach room temperature and stirred overnight. After the
white precipitate was removed by filtration, the residue was purified
by column chromatography with acetone/ petroleum ether (1:1, v/v)
FIGURE 3. Absorption spectra of the solution of triad 1 (1.0 × 10-5
M) and SP (1.0 × 10-4 M) before and after addition of Pb(ClO4)2 (2.0
× 10-5 M), followed by UV light (365 nm) irradiation for 100 s, and
further visible light irradiation for 100 s.
as a result the complex of 1 with metal ions would be partially
dissociated; the MC complex would be transformed into SP
under further visible light irradiation and triad 1 can bind metal
ions again. Therefore, it would be possible to tune the metal
ion-promoted electron transfer and the intramolecular charge
transfer within triad 1 in the presence of spiropyran by
alternating UV and visible light irradiation. As shown in Figure
3, the absorption intensity at 845 nm for the solution of triad 1
containing Pb2+ was reduced to 24% of that of the initial value
after UV light (365 nm) irradiation for 100 s. Simultaneously,
the corresponding charge transfer absorption band intensity was
enhanced. This is simply because the MC form of spiropyran
(SP) can coordinate with the metal ions such as Pb2+; as a result
the competing binding of metal ions between triad 1 and the
MC form occurs. Further visible light irradiation of the solution,
which had been treated by UV light irradiation, led to the
restoration of the absorption intensities at 450, 845, and 542
nm (due to ICT). Such reversible modulation of the absorption
spectrum of triad 1 containing Pb2+ in the presence of spiropyran
by light irradiation can be repeated for several cycles.
1
as eluent to give triad 1 as a purple oil (0.20 g, 17.6%). H NMR
(400 MHz, CDCl3) δ 7.47 (2H, d, J ) 9.0 Hz), 6.80-6.78 (5H,
m), 6.43 (2H, s), 4.28 (4H, t, J ) 6.2 Hz), 4.24 (4H, t, J ) 4.9
Hz), 3.70-3.63 (28H, m), 3.28, (8H, s), 2.92 (4H, t, J ) 6.4 Hz),
2.64-2.57 (8H, m); 13C NMR (100 MHz, CDCl3) δ 187.9, 187.7,
172.34, 172.33, 149.1, 145.1, 137.1, 136.4, 131.2, 129.1, 126.7,
123.1, 121.0, 118.0, 114.1, 114.06, 111.9, 106.8, 70.78, 70.75,
70.71, 70.6, 69.8, 69.2, 64.1, 61.6, 49.6, 35.4, 30.3, 29.8, 29.1, 29.0;
HR-MS (FAB) calcd for C56H67NO16S14 [M + 2H+] 1457.0544,
found 1457.0549.
Acknowledgment. The present research was financially
supported by NSFC, Chinese Academy of Sciences, and State
Key Basic Research Program. This work was partially supported
by the NSFC-DFG joint project (TRR61). The authors thank
the anonymous reviewers for their comments and suggestions
which enables us to greatly improve the paper.
Supporting Information Available: The synthesis details
and characterization of compounds 3-5, cyclic voltammo-
gramms in the presence of metal ion, and absorption spectral
studies. This material is available free of charge via the Internet
at http://pubs.acs.org.
In summary, a new TTF-quinone-TTF triad 1, in which
the N,N-dialkylaniline-substituted quinone unit is flanked by two
TTF units, was synthesized and investigated. Both absorption
and ESR spectroscopic studies clearly indicate that electron
transfer occurs from the TTF to the substituted quinone units
JO9007332
4378 J. Org. Chem. Vol. 74, No. 11, 2009