Photocontrolled electron transfer reaction between a new dyad, tetrathiafulvalene-photochromic spiropyran, and ferric ion

Article abstract of DOI:10.1021/jp031025y

Photocontrolled electron transfer reaction is important not only for understanding the complicated biological processes such as photosynthesis and respiration but also for the design and studies of molecular electronics. This paper presents the synthesis and spectral and electrochemical studies of a new dyad 1 containing an electroactive unit (tetrathiafulvalene, TTF) and a photochromic unit (spiropyran, SP). Spectral studies showed that the redox states of the TTF unit of dyad 1 in the presence of ferric ions were dependent on the photoswitching process of the spiropyran unit upon UV light irradiation. Electrochemical investigations indicated that the oxidation potential of ferrous ion was largely reduced after coordination with MC (the open form of SP). As a result, the electron-transfer reaction from MC?·Fe²⁺ to TTF^+ì?, which act as electron donor and acceptor, respectively, is thermodynamically favorable. Therefore, the electron-transfer reaction between the TTF unit and ferric ion can be photocontrolled in the presence of the SP unit. The present result shows the possibility to design new electron donor-acceptor supramolecules containing spiropyran units to photoregulate the electron-transfer reaction.

Full text of DOI:10.1021/jp031025y

212
J. Phys. Chem. B 2004, 108, 212-217
Photocontrolled Electron Transfer Reaction between a New Dyad,
Tetrathiafulvalene-Photochromic Spiropyran, and Ferric Ion
Xuefeng Guo,†,‡ Deqing Zhang,* and Daoben Zhu*
,†
,†
Organic Solids Laboratory, Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100080, China, and Graduate School, Chinese Academy of Sciences, Beijing 100080, China
ReceiVed: August 27, 2003
Photocontrolled electron transfer reaction is important not only for understanding the complicated biological
processes such as photosynthesis and respiration but also for the design and studies of molecular electronics.
This paper presents the synthesis and spectral and electrochemical studies of a new dyad 1 containing an
electroactive unit (tetrathiafulvalene, TTF) and a photochromic unit (spiropyran, SP). Spectral studies showed
that the redox states of the TTF unit of dyad 1 in the presence of ferric ions were dependent on the
photoswitching process of the spiropyran unit upon UV light irradiation. Electrochemical investigations indicated
that the oxidation potential of ferrous ion was largely reduced after coordination with MC (the open form of
2
+
+•
SP). As a result, the electron-transfer reaction from MC‚Fe to TTF , which act as electron donor and
acceptor, respectively, is thermodynamically favorable. Therefore, the electron-transfer reaction between the
TTF unit and ferric ion can be photocontrolled in the presence of the SP unit. The present result shows the
possibility to design new electron donor-acceptor supramolecules containing spiropyran units to photoregulate
the electron-transfer reaction.
Introduction
SCHEME 1: Illustrations of the Reversible Structural
Transformations of Nitrospiropyran in Response to
Light, Heat, and Metal Ions
Electron donor-acceptor supramolecules have been exten-
sively investigated in recent years not only for a better
understanding of many complicated biological processes such
1
as photosynthesis and respiration but also for the potential
applications in the construction of efficient and fast-responding
2
electronic/photonic devices at the molecular level. In particular,
much effort is currently devoted to regulating electronic
communication and thus controlling electron-transfer reactions
3
among electroactive components through photoisomerization
and redox reactions.4
Spiropyran molecules, as one of the promising families of
photochromic compounds, undergo reversible structural trans-
formations in response to external inputs such as light, heat,
5
proton, and metal ions (Scheme 1). By using the special
photoswitching properties of spiropyran molecules, complex
integrated logic gates and communication networks have been
metal ions such as the ferric and the ferrous ions, which show
redox behaviors, are used, it seems possible to alter their redox
potentials by light in the presence of photochromic spiropyran.
6
,7
constructed. We also reported previously new logic gates for
information processing and communication at the molecular
level, by taking the advantage of the photochromic behaviors
Tetrathiafulvalene (TTF) and its derivatives are good electron
donors for studies of organic conductors (as well as super-
8
of spiropyrans and the energy-transfer mechanism. However,
to the best of our knowledge, there has been no report that deals
with photoregulation of electron-transfer reaction for the electron
donor-acceptor (D-A) dyad with photochromic spiropyran as
one of the units. This is probably because both the closed form
10
conductors), and the construction of electron donor-acceptor
11
supramolecules. They can be oxidized by some metal ions
+•
2+ 12
such as the ferric ion to form TTF and even TTF .
Therefore, it would be interesting to investigate whether it is
possible to alter the redox states of the TTF unit of a new dyad
TTF-SP (dyad 1, Scheme 2) in the presence of ferric ion by
light. With these in mind, we initiated studies of photocontrolled
electron-transfer reaction for a new dyad 1 in the presence of
ferric ion. In this paper, we will report the synthesis and spectral
and electrochemical investigations of dyad 1. The results show
that the redox states of the TTF unit of dyad 1 are dependent
on the photoswitching process of the SP unit in the presence of
ferric ion. Namely, the electron-transfer reaction between the
(SP) of spiropyan and the corresponding open form (MC) are
weak electron donors and they have similar oxidation poten-
tials.7 It should be noted that the MC form is able to coordinate
with suitable metal ions and as a result the absorption spectrum
of MC form is hypsochromically shifted, while the correspond-
,9
5h,i,8a
ing SP form does not show this behavior.
Therefore, if the
*
Corresponding author. E-mail: dqzhang@iccas.ac.cn.
Institute of Chemistry.
Graduate School.
†
‡
1
0.1021/jp031025y CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/05/2003

Photocontrolled Electron-Transfer Reaction
J. Phys. Chem. B, Vol. 108, No. 1, 2004 213
SCHEME 2: Synthetic Route to Dyad 1, as Well as the Structures of Reference Compounds 2 and 4
TTF unit and ferric ion can be controlled by light in the presence
of the SP unit.
temperature. Similar experiments were performed for reference
compound 4, and the result was shown in the inset curve of
Figure 1A. Quantitatively, it took 95 s for the absorption
intensity at 580 nm to be reduced to half of the initial value for
the case of dyad 1 (τ1/2 ) 95 s), while it needed only 58 s for
the case of reference compound 4 under the same conditions
Results and Discussion
Synthesis. The preparation of dyad 1 was depicted in Scheme
2
. Compound 2 was prepared according to the procedures
(τ1/2 ) 58 s). Accordingly, the rate constants for the conversion
13
developed in our laboratory recently. Using the standard
deprotection/alkylation method,1 deprotection of cyanoethyl
group of compound 2 generated the corresponding thiolate,
3,14
13b
which further reacted with Br(CH2)12Br to afford compound 3.
Compound HO-SP was synthesized according to refs 8b and
5. Finally, the reaction of compound 3 with HO-SP in DMF
1
at room temperature in the presence of anhydrous K2CO3 yielded
the target compound 1 in 73% yield after separation with column
chromatography. The reference compound 4 was prepared in
the same way as that for dyad 1.8b
Spectral Studies. The absorption spectrum of dyad 1 (curve
a) is shown in Figure 1A, together with those of compounds 2
(curve c) and 4 (curve b). The absorption spectrum of dyad 1
is approximately the sum of the absorption spectra of compounds
2
al.,
and 4 (Figure 1A). As reported by Buncel and Raymo et
5h,6a
16
upon UV light irradiation at 365 nm, the SP unit of
dyad 1 was photoswitched to its open form (MC) with the
appearance of its characteristic absorption band with λmax )
5
80 nm (curve a, Figure 1B), and dyad 1 was transformed to
dyad TTF-MC (see Scheme 3). Similarly, reference compound
was photoswitched to its open form (curve b in Figure 1B)
4
under the same condition. As shown in Figure 1B, the absorption
peaks of the dyad TTF-MC are almost identical to those
observed for the reference compound 2 and the open form of
4
. On the basis of the above results, it can be concluded that
there exists no strong electronic interaction between the TTF
unit and the SP unit in dyad 1, and the SP unit also exhibits the
characteristic photochromic property in the presence of the TTF
unit.
But the presence of the TTF unit has a weak effect on the
photoswitching process of the SP unit of dyad 1. For example,
the absorption intensity at 580 nm (due to MC form) for dyad
-
4
Figure 1. Panel A: The absorption spectra of dyad 1 (1.0 × 10 M,
-4
THF, 25 °C) (a), reference compound 4 (1.0 × 10 M, THF, 25 °C)
-5
(
b), and reference compound 2 (5.0 × 10 M, THF, 25 °C) (c). Inset
1
is relatively low compared with that of the MC form of 4,
curve shows the decay of the absorption intensity at 580 nm with time
for the solution of 4 after irradiation with UV light for 5 min and then
being kept in the dark. Panel B shows the absorption spectra of dyad
probably because of the coabsorption of the TTF unit at 365
nm. In particular, the thermal cyclization of MC to SP in the
TTF-MC dyad in the dark is slowed compared with that of
the reference compound 4. The inset curve of Figure 1B shows
the variation of the absorption intensity at 580 nm (due to the
MC form) with time for the solution of dyad 1 that was first
irradiated by UV light and then kept in the dark at room
-4
1
after irradiation with ultraviolet light at 356 nm (1.0 × 10 M, THF,
2
5 °C) (a), reference compound 4 after irradiation with ultraviolet light
-
4
at 365 nm (1.0 × 10 M, THF, 25 °C) (b), and reference compound
-5
2
(5.0 × 10 M, THF, 25 °C) (c). Inset curve shows the decay of the
absorption intensity at 580 nm with time for the solution of 1 after
irradiation with UV light for 5 min and then being kept in the dark.

214 J. Phys. Chem. B, Vol. 108, No. 1, 2004
Guo et al.
SCHEME 3: Transformation of Dyad 1 under the Action
of UV Light and Ferric Ion.
-
4
Figure 2. The absorption spectra of dyad 1 (1.0 × 10 M, THF, 25
C) before (a) and after irradiation with ultraviolet light (b), upon
addition of 1 equiv of Fe(ClO immediately after irradiation with
ultraviolet light (c), and upon addition of 1 equiv of Fe(ClO
immediately after irradiation with ultraviolet light (d). Inset curves show
°
4 3
)
4 2
)
-
4
the absorption spectra of reference compound 4 (1.0 × 10 M, THF,
5 °C) before (a) and after irradiation with ultraviolet light (b), upon
addition of 1 equiv of Fe(ClO ) immediately after irradiation with
2
of MC to SP were determined to be 6.85 × 10 s-1 for dyad
-3
4
3
-
2 -1
1
and 1.20 × 10
s
for reference compound 4, respectively.
ultraviolet light (c), and upon addition of 1 equiv of Fe(ClO
4 2
)
immediately after irradiation with ultraviolet light (d).
The different behavior of dyad 1 from the reference compound
is likely due to the intimacy of the MC unit with the TTF
unit within a covalently linked dyad TTF-MC (see Scheme
), facilitating the π-π and the charge-transfer interactions by
4
3
folding the alkylchain (-(CH2)12-). Consequently, the MC form
in the TTF-MC dyad becomes more stable.
As described above, upon UV light irradiation at 365 nm,
the SP unit of dyad 1 was photoswitched to its open form (MC)
(Scheme 3) with the appearance of its characteristic absorption
band with λmax ) 580 nm (curve a in Figure 1B and curve b in
Figure 2). Further addition of 1 equiv of Fe(ClO4)3 to the
solution of dyad 1 after irradiation with UV light (dyad 1 was
converted to TTF-MC as shown in Scheme 3) led to the
emergence of a new strong absorption band with λmax ) 424
nm (curve c in Figure 2) with the concomitant disappearance
of the characteristic absorption band at 580 nm ascribed to the
MC form. Moreover, addition of Fe(ClO4)3 to the solution of
the TTF-MC dyad (Scheme 3) did not lead to oxidation of the
-
4
Figure 3. The absorption spectra of dyad 1 (1.0 × 10 M, THF, 25
C) before (a) and after addition of 1 equiv of Fe(ClO (b) and after
UV light irradiation of the solution that was first treated with 1 equiv
of Fe(ClO (c).
°
4 3
)
+
•
TTF unit because the characteristic absorption band of TTF
4 3
)
(
λmax ) 610 nm)17 was not observed. If Fe(ClO4)2 instead of
Fe(ClO4)3 was employed for the above reaction, only a much
weaker absorption band at about 424 nm (curve d in Figure 2)
was detected. On the basis of previous reports that the MC form
On the other hand, addition of 1 equiv of Fe(ClO4)3 to the
solution of 1 resulted in the appearance of the broad absorption
band with λmax ) 610 nm (curve b in Figure 3), which should
n+
+•
+• 17
can coordinate with suitable cations (M ) to induce the
be ascribed to radical cation TTF rather than SP . The
5h,5i,8a
+•
significant hypsochromic shift of the MC absorption band,
formation of TTF was due to the oxidation of the TTF unit
the new strong absorption band with λmax ) 424 nm was
ascribed to the coordination of the MC unit of the TTF-MC
by ferric ion, and as a result, ferric ion was reduced to ferrous
ion. The following experiment was performed to provide support
for this assumption: addition of the aqueous solution of K3Fe-
(CN)6 to the solution of dyad 1 that had been treated by 1 equiv
of Fe(ClO4)3 afforded a dark-blue precipitate immediately,
indicating the presence of ferrous ion in the mixture solution.
Interestingly, irradiation of the above solution with ultraviolet
light at 365 nm led to nearly the same absorption spectrum
(curve c in Figure 3) as that of the solution of dyad 1 that was
first treated by UV light followed by addition of Fe(ClO4)3
(curve c in Figure 2). As shown in Figure 3, the strong
absorption band of the radical cation TTF disappeared and
the distinct absorption band with λmax ) 580 nm ascribed to
the MC form was not observed. However, a new strong
absorption band with λmax ) 424 nm appeared in the meantime.
Furthermore, no dark-blue precipitate was formed after addition
of the aqueous solution of K3Fe(CN)6 to the solution of dyad 1
that had been treated by 1 equiv of Fe(ClO4)3 and UV light,
3+
dyad with Fe ion. As a result, dyad 1 was transformed to
TTF-MC‚Fe3 (see Scheme 3) after sequential reaction with
+
3+
UV light and Fe . Control experiments with the reference
compound 4 supported this assignment: UV light irradiation
of the solution of 4 induced the formation of MC with the
appearance of its characteristic absorption band with λmax )
5
80 nm (inset curve b in Figure 2). Further addition of 1 equiv
of Fe(ClO4)3 to the solution led to a strong absorption band
with λmax ) 424 nm with the complete disappearance of the
characteristic absorption band at 580 nm of MC (inset curve c
in Figure 2). Similarly, further addition of 1 equiv of Fe(ClO4)2
to the solution also led to a much weaker absorption band around
+
•
424 nm (inset curve d in Figure 2). Thus, it is clear that the
appearance of the strong absorption band with λmax ) 424 nm
after addition of Fe(ClO4)3 to the solution of MC was due to
the formation of MC‚Fe3 rather than MC‚Fe .
+
2+

Photocontrolled Electron-Transfer Reaction
J. Phys. Chem. B, Vol. 108, No. 1, 2004 215
Figure 4. Differential pulse voltammograms (left) of the THF solutions of 2 (a), 4 (b), dyad 1 (c), and 4 after consecutive treatment with UV light
and Fe(ClO (d). Potentials are relative to SCE. The potentials of all compounds are listed in the table (right).
4 2
)
indicating the absence of ferrous ion. As discussed above, the
new absorption band with λmax ) 424 nm was ascribed to the
formation of coordination complex between the MC unit and
ferric ion. These results indicated that UV light irradiation of
the solution of dyad 1 that had been treated by 1 equiv of Fe-
only one redox potential at 0.94 V (curve b in Figure 4), while
two redox waves for compound 2 were observed at 0.69 and
0.91 V (curve a in Figure 4). Under the same conditions,
irradiation of the solution of compound 4 with UV light at 365
nm also resulted in no observable change in its redox behavior.
By reference to those of reference compounds 2 and 4, the first
oxidation peak of dyad 1 at 0.66 V should be ascribed to the
(
ClO4)3 induced the reduction of the radical cation TTF+• unit
3
+
to the neutral TTF unit and the formation of TTF-MC‚Fe .
This is in excellent accordance with the disappearance of the
absorption band at about 610 nm and the appearance of the
absorption band at about 424 nm. Further evidence for the
reduction of the TTF unit to the neutral TTF unit came from
the following experiment: further addition of Fe(ClO4)3 to the
solution of dyad 1 that had been consecutively treated by 1 equiv
of Fe(ClO4)3 and UV light led to the emergence of the
+
•
oxidation of the TTF unit to TTF and the second oxidation
peak at 0.86 V of dyad 1 should be the result of two-electron
+
•
oxidation because TTF and SP possess similar oxidation
potentials.
+
•
Differential pulse voltammetric experiment with the solution
of reference compound 4 in the presence of Fe(ClO4)2 upon
UV light irradiation at 365 nm was also carefully performed.
As shown in Figure 4 (curve d), a new oxidation peak at about
+
•
characteristic absorption band of TTF again. This was
obviously due to the oxidation of the TTF unit in TTF-MC‚
0
.48 V for the solution of compound 4 after consecutive UV
3+
Fe by the additional ferric ion. In another approach, if the
light irradiation and addition of Fe(ClO4)2 was recorded, which
solution of dyad 1 was first treated by excess Fe(ClO4)3, the
2+
3+
should be ascribed to the oxidation of MC‚Fe to MC‚Fe ,
+
•
characteristic absorption band ascribed to TTF was still
observed (albeit with reduced intensity) after further UV light
irradiation.
and the second oxidation peak at about 0.94 V was due to the
+
+
oxidation of free SP (or MC) to SP (or MC ). Importantly,
after coordination with MC, the oxidation potential of iron(II)
1
8
+•
In addition, the intermolecular electron transfer between MC‚
was significantly reduced. The reduction of TTF to neutral
TTF occurred at about 0.66 V (see curve a in Figure 4). Thus,
it is thermodynamically favorable for the electron-transfer
Fe2 and TTF can also take place. Upon mixing the solution
+
+•
+
•
containing 2 formed by electrochemical oxidation of 2 and
the solution of MC‚Fe2 prepared by UV light irradiation of
the solution of 4 followed by addition of Fe(ClO4)2, the typical
absorption of TTF+ in the range of 500-800 nm disappeared
and a strong absorption band with λmax ) 424 nm due to MC‚
+
2+
+•
reaction from MC‚Fe (as electron donor) to TTF (as electron
1
9
acceptor ). This important result is crucial to interpret the
photocontrolled electron-transfer reaction between the TTF unit
and ferric ion in the presence of the SP unit as shown in Scheme
•
3+
3+
Fe was detected instead.
3. For the solution containing TTF-MC‚Fe , two oxidation
potentials at 0.68 and 0.89 V were recorded from differential
pulse voltammetric measurements, which were very close to
those of dyad 1 and 2, indicating that the TTF unit in TTF-
To conclude, upon either first reaction with UV light and
then with ferric ion or first with ferric ion and then with UV
3+
light, dyad 1 was transformed to TTF-MC‚Fe . The results
demonstrate that the electron-transfer reaction between the TTF
unit and ferric ion can be controlled by UV light irradiation in
the presence of the SP unit. Moreover, the results also showed
3
+
MC‚Fe was in the neutral state. This result is in accordance
with those of the spectral studies as discussed above.
The interesting behaviors of MC‚Fe2 and MC‚Fe3+ may be
qualitatively interpreted by considering the HOMO and LUMO
orbitals. The phenoxide group of the MC form as a good
electron-donating fragment may be involved in the formation
of the coordination bonds. Compared with the energies of the
+
2
+
that intermolecular electron transfer between MC‚Fe and
+
•
TTF could also take place efficiently. Differential pulse
voltammetric experiments detailed below indicated that the
+
•
2+
electron-transfer reaction between the TTF and MC‚Fe units
2
+
3+
was thermodynamically favorable, which was crucial for
understanding the transformation of dyad 1 to TTF-MC‚Fe
under the consecutive action of UV light and ferric ion.
frontier orbitals of the free Fe and Fe ions, the HOMO
3
+
2+
3+
energy and LUMO energy of MC‚Fe and MC‚Fe will be
increased, respectively, because of the coordination with MC
form. Consequently, compared with the free Fe , the oxidation
potential for the reaction of MC‚Fe to MC‚Fe will be
decreased. Similarly, the reduction potential for the reaction of
MC‚Fe to MC‚Fe will also be decreased.
2+
Differential Pulse Voltammetric Studies. Figure 4 shows
the differential pulse voltammograms of dyad 1, reference
2
+
3+
2+
compounds 2 and 4, and the solution containing MC‚Fe . Two
redox potentials of dyad 1 were found to be 0.66 and 0.86 V
3
+
2+
(curve c in Figure 4). The transformation of the SP unit to the
Mechanism Discussion. Based on the results of the above
MC form induced by UV light irradiation of the solution of
dyad 1 resulted in no observable change in its redox behavior,
consistent with the previous reports.7,9 Compound 4 possessed
electrochemical studies, mechanisms for the conversion of dyad
1 to TTF-MC‚Fe after first UV light irradiation and addition
of Fe(ClO4)3 (see Scheme 3) can be proposed as follows: (1)
3+

216 J. Phys. Chem. B, Vol. 108, No. 1, 2004
Guo et al.
UV light irradiation generated the TTF-MC dyad, followed
redox state of the TTF unit of dyad 1 in the presence of ferric
ion was dependent on the photoswitching process of the SP unit,
and thus dyad 1 provides a good example of photoregulation
of the electron transfer between an electroactive unit and an
oxidant in the presence of a photoactive unit. The present result
shows the possibility to design new electron donor-acceptor
supramolecules containing the spiropyran unit to regulate the
electron-transfer reaction by light.
3+
by direct coordination of the MC unit with Fe . Oxidation of
the TTF unit did not take place, probably because after
3
+
coordination with the MC form the oxidation potential of Fe
was largely reduced as discussed above. (2) Oxidation of the
3+
TTF unit by free Fe ion and coordination of MC form with
3+
Fe may occur simultaneously. Oxidation of the TTF unit
+
•
2+
would lead to the formation of TTF and Fe , which would
be coordinated by the MC form, and consequently, the
+
•
2+
intermediate dyad TTF -MC‚Fe was generated. Further
Experimental Section
2+
+•
intramolecular electron transfer from MC‚Fe to TTF led to
General. Melting points were measured with an XT4-100X
3+
the formation of TTF-MC‚Fe .
1
apparatus and uncorrected. H NMR spectra were recorded with
The sequential reaction of dyad 1 with first Fe3 and then
+
Bruker 300 MHz spectrometers. Infrared spectra were obtained
on a Perkin-Elmer System 2000 FT-IR spectrometer. Mass
spectra were determined with MALDI-TOF-MS. Elemental
analysis was performed on Carlo-Erba-1106 instrument. Ab-
sorption spectra were measured with Hitachi (model U-3010)
UV-vis spectrophotometer. Fluorescence measurements were
carried out with a Hitachi (model F-4500) spectrophotometer
in a 1-cm quartz cell. Differential pulse voltammetric experi-
ments were performed on an EGDG PAR 370 system at a scan
rate of 100 mV in THF using Bu4NPF6 as electrolyte, platinum
as counter and work electrodes, and SCE as reference electrode.
THF was dried over sodium/benzophenone before use. DMF
was predried by standing over molecular sieves (4 Å) for at
least 3 days before use. All other reagents and solvents (standard
grade) were used as received unless otherwise stated.
UV light can be described as follows (see Scheme 3): oxidation
+
•
of the TTF unit of dyad 1 led to TTF -SP. Because the
2
-1
absorption coefficient of dyad 1 (ꢀSP ) 10 973 cm ‚L ) at 365
3
+
3+
2
-1
nm was much larger than those of Fe (ꢀFe ) 497 cm ‚L )
2+
2+
2
-1
3+
and Fe ions (ꢀFe ) 480 cm ‚L ), the presence of Fe and
Fe ions in the solution was not able to block the photoisomer-
ization of the SP unit to the MC unit. Accordingly, upon UV
light irradiation TTF -SP was transformed to TTF -MC,
which would coordinate with Fe to generate the intermediate
dyad TTF -MC‚Fe . The further intramolecular electron
transfer between the TTF and MC‚Fe units led to TTF-
2+
+
•
+•
2+
+
•
2+
+
•
2+
MC‚Fe3 as discussed above.
+
As mentioned above, if the solution of dyad 1 was first treated
by excess Fe(ClO4)3, followed by UV light irradiation, the
+•
characteristic absorption band of TTF could still be observed.
This can be interpreted as follows: The MC unit in dyad
For the ultraviolet light irradiation experiments, a 140 W high-
pressure mercury lamp at 365 nm was used.
+
•
TTF -MC was first coordinated with iron(II) because iron-
III) complexes are usually thermodynamically favored but are
kinetically unstable. Consequently, electron transfer from MC‚
Compounds 2, 3, HO-SP, and 4 (see Scheme 2) were
(
synthesized on the basis of the reported procedures.8
b,13b
Dyad 1. To a solution of HO-SP (0.19 g, 0.56 mmol) in
DMF (20 mL) was added compound 3 (0.20 g, 0.28 mmol)
and anhydrous K2CO3 (0.81 g, 5.90 mmol). Then the mixture
was stirred at room temperature for 2 days before 150 mL of
H2O was added. The aqueous solution was extracted with
dichloromethane (3 × 70 mL), and the combined extracts were
washed with H2O (2 × 40 mL) and saturated aqueous NaCl
2
+
+•
+•
Fe to TTF took place, and then TTF was reduced to neutral
+
•
TTF, which can be oxidized back to TTF by excess Fe(ClO4)3
+
•
•
3+
quickly. As a result, TTF -MC ‚Fe was formed at last.
+
•
Another possible route was that iron(II) in the TTF -MC‚
2
+
Fe unit was first replaced by iron(III) to form iron(III)
+•
3+
complexes TTF -MC‚Fe because iron(III) is more strongly
bond than iron(II) before the electron-transfer reaction between
(20 mL), dried (MgSO4), and concentrated in vacuo. After
MC‚Fe2 to TTF took place. Thus, no matter which pathway
+
+•
column chromatography on silica gel with dichloromethane/
+
•
3+
took place, the ultimate state of dyad 1 was TTF -MC‚Fe
petroleum ether (60-90 °C, 1:2 v/v) as eluant, dyad 1 was
under this condition.
1
obtained as an orange oil in 73% yield. H NMR (CDCl3) δ
Finally, it’s interesting to note that the solution of MC‚Fe3+
0
.88 (6 H, t, J ) 6.8 Hz, 2CH3), 1.19 (3 H, s, CH3), 1.37 (29
H, br, 13CH2+CH3), 1.61 (8 H, m, 4CH2), 1.76 (2 H, m, CH2),
.68 (3 H, s, CH3), 2.77 (6 H, m, 3SCH2), 3.91 (2 H, t, J ) 6.5
Hz, CH2O), 5.83 (1 H, d, J ) 10 Hz, CH), 6.30 (1 H, s, olefinic),
.45 (1 H, d, J ) 9 Hz, Ar-H), 6.74 (3 H, m, Ar-H), 6.89 (1
H, d, J ) 10 Hz, CH), 7.99 (2 H, m, Ar-H). MALDI-TOF m/z
72.5; FT-IR(KBr) 2925 and 2853 (alkyl C-H), 1473 and 1337
NO2); Anal. Calcd for C49H68N2O4S7: H, 7.04; C, 60.46; N,
did not exhibit photochromism upon irradiation of visible light
(
λ > 460 nm) and even in the presence of Na2S2O3, while MC‚
2
2+
2+
Fe can be reversibly converted to SP and free Fe upon
3
+
irradiation with visible light. The solution of TTF-MC‚Fe
6
2
+
and TTF-MC‚Fe showed the same behaviors. This means
3
+
2+
that MC‚Fe is more stable than MC‚Fe . Such unique
9
(
3+
property of MC‚Fe makes the complete conversion of SP to
MC‚Fe3 possible. It should be noted that only an equilibrium
between SP and MC forms can be reached in the absence of
ferric ion under the same conditions.
+
2
.88. Found: H, 6.98; C, 60.29; N, 2.73.
Acknowledgment. The present research was financially
supported by NSFC (Grant 90101025), Chinese Academy of
Sciences, and State Key Basic Research Program. D.-Q. Zhang
thanks National Science Fund for Distinguished Young Scholars.
Conclusion
Spectral and electrochemical studies of a newly well-designed
dyad containing an electroactive TTF unit and a photoactive
2
+
Supporting Information Available: Electronic absorption
spiropyran unit were presented. Compared with the free Fe ,
its oxidation potential was largely reduced after it was coordi-
nated to MC. Consequently, the electron-transfer reaction from
+
•
+•
spectra of 2, 2 , 4, and 4 . This material is available free of
charge via the Internet at http://pubs.acs.org.
MC‚Fe2 to TTF can take place thermally. Owing to this
+
+•
2+
References and Notes
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between the TTF unit and ferric ion can be photocontrolled in
the presence of the photochromic SP unit. In other words, the
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