Acid-base responsive photoelectric conversion of a hydroxyazobenzene- appended bipyridine-copper complex system

Article abstract of DOI:10.1246/cl.2010.204

We report the synthesis of a new 4-hydroxyazobenzene-appended bipyridine complex of copper, [Cu(oAB-2OH)₂]BF₄, which exhibits a reversible acid-base responsive redox-active photoisomerization reaction. The acid-base modulation of the photoisomerization reaction permits ON/OFF toggling of the photoelectric conversion in devices containing this complex by the addition of small quantities of acid or base in the presence of 4,4′-bis(ethoxycarbonyl)-2,2′-bipyridine, bpy-2COOEt.

Full text of DOI:10.1246/cl.2010.204

doi:10.1246/cl.2010.204
204
Published on the web January 30, 2010
Acid-Base Responsive Photoelectric Conversion of a Hydroxyazobenzene-appended
Bipyridine-Copper Complex System
Satoshi Umeki, Shoko Kume, and Hiroshi Nishihara*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received December 8, 2009; CL-091088; E-mail: nisihara@chem.s.u-tokyo.ac.jp)
We report the synthesis of a new 4-hydroxyazobenzene-
Cu
Cu
appended bipyridine complex of copper, [Cu(oAB-2OH)2]BF4,
which exhibits a reversible acid-base responsive redox-active
photoisomerization reaction. The acid-base modulation of the
photoisomerization reaction permits ON/OFF toggling of the
photoelectric conversion in devices containing this complex by
the addition of small quantities of acid or base in the presence of
4,4¤-bis(ethoxycarbonyl)-2,2¤-bipyridine, bpy-2COOEt.
-
-
OH
OH
-
-
OH
OH
UV
Vis
HO
-
-
-
OH
OH
Interligand
π-π stacking
Cu
Cu
Cu
HO
-
Base Acid
-
O
O
-
O
O
O
O
-
O
O
-
-
UV
Cu
-
-
-
Cu
HO
Cu
Molecular machines have attracted much attention in
nanoscale research, and their development has been accompa-
nied by the introduction of new technologies for handling and
assembling single molecules. The construction of molecular
machines via combination and synchronization of molecular
modules with well-characterized responses constitutes an effi-
cient design strategy.1 Previously, we developed a photoelectric
conversion system based on the ligand-exchange reaction
between a copper complex containing azobenzene-appended
bipyridine ligands and free bipyridines. The ligand exchange
was modulated by the reversible photoisomerization of the
azobenzene moieties.2 The next synthetic goal was to expand the
functionalities of this system in novel ways by introducing
appropriate substituents to this copper complex system.
The present study aims to toggle the photoelectric response
of the copper complex system using solution pH changes. For
this purpose, we synthesized a 4-hydroxyazobenzene-appended
bipyridine complex of copper, [Cu(oAB-2OH)2]BF4, and stud-
ied its physical properties in acidic and basic solutions. The
reversible acid-base response and the photochromic properties
of [Cu(oAB-2OH)2]BF4 were characterized, permitting the
controllable ON/OFF switching of the photoelectric response
through introduction of acids or bases in the presence of 4,4¤-
bis(ethoxycarbonyl)-2,2¤-bipyridine (bpy-2COOEt).
The [Cu(oAB-2OH)2]BF4 system, shown in Chart 1, con-
tains interligand ³-³ stacking interactions that stabilize the
coordination of azobenzene-containing ligands. The ³-stacking
stabilization is lost by the trans-to-cis isomerization of the
azobenzene moieties upon UV irradiation, and ligand exchange
with bipyridine derivatives is favored. This chemical process
modulates the reduction potential of the CuII/CuI redox couple.
Because the original state can be regenerated by visible light
irradiation, the photoresponse of the CuII/CuI redox couple can
be cycled repeatedly. The addition of acid or base respectively
lowers or raises the barrier to photoisomerization, modulating
the photoelectric response.
4-Hydroxyazobenzene-appended bipyridine, oAB-2OH,
was synthesized from 6,6¤-bis(4-aminophenyl)-4,4¤-bis(4-tert-
butylphenyl)-2,2¤-bipyridine3 and 4-nitrophenyl tosyl ether.4
[Cu(oAB-2OH)2]BF4 was synthesized by reaction of oAB-
2OH and [Cu(CH3CN)4]BF4, and characterized by NMR and
OH
N
N
N
N
EtOOC
N
N
N
N
N
N
Cu+
Cu
=
=
EtOOC
N
N
N
N
HO
OH
[Cu(oAB-2OH)2]+
bpy-2COOEt
Chart 1. ON/OFF toggling of the photoelectric response.
elemental analysis.
The UV-vis absorption spectrum of [Cu(oAB-2OH)2]BF4 in
THF showed an intense band at 370 nm, ascribed to the ³-³*
transition of the azobenzene moieties. The n-³* and d-³*
transition (MLCT) bands overlapped in the visible region from
450 to 550 nm.3 When excessive potassium t-butoxide was
added to this solution, the band at 370 nm decreased and a new
band appeared at 480 nm.3 This indicates the occurrence of
deprotonation of hydroxy groups causing further delocalization
of ³ electrons. The original spectrum was recovered by addition
of trifluoroacetic acid,3 confirming the reversibility of deproto-
nation of [Cu(oAB-2OH)2]BF4.
Figure 1 shows UV-vis absorption spectral changes of
[Cu(oAB-2OH)2]BF4 upon irradiation. The intensity of the
³-³* transition band decreased and the n-³* transition band
increased under 365 nm light irradiation, indicating a trans-to-
cis isomerization. The isomerization in the reverse direction was
observed under 436 nm light irradiation. The solution reached
a photostationary state by irradiation with 365 or 436 nm light
for 5 min. It was reversible between these two photostationary
states (Figure 1a). When 0.2 equivalents (equiv) of 1,8-diaza-
bicyclo[5.4.0]undec-7-ene (DBU) were added to this solution,
photoisomerization was not observed (Figure 1b). A small
quantity of base was sufficient to suppress the photoisomeriza-
tion of [Cu(oAB-2OH)2]BF4 because the thermally driven cis-
to-trans isomerization of the deprotonated form is much more
rapid than the photochemically driven trans-to-cis isomerization
of the protonated form and the cis form hardly existed.5 The
photoresponse was recovered, and reversible photoisomerization
was observed again after the addition of TFA (Figure 1c). In
Chem. Lett. 2010, 39, 204-205
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

205
1
0.2
0
(a)
(c)
(b)
(a)
(b)
(c)
UV Vis UV Vis
UV Vis UV Vis
UV Vis UV Vis
0.5
0
1
300
400
500
600
700
Wavelength / nm
-0.2
0
60
Time / s
1200 0
600
Time / s
1200
0
600
Time / s
1200
365 nm
0.5
Cu
Cu
436 nm
[Cu(trans2-oAB-2OH)2]+
[Cu(cis2-oAB-2OH)2]+
Acid : ON
Base : OFF
Figure 3. Rest potential changes of [Cu(oAB-2OH)2]BF4 (1.0 ©
10¹5 mol dm¹3), bpy-2COOEt (2.0 © 10¹5 mol dm¹3), and [Cu(bpy-
0
300
400
500
600
700
¹3
Wavelength / nm
2COOEt)2](BF4)2 (1 © 10¹6 mol dm¹3) in acetone with 0.1 mol dm
Bu4NClO4 (a) initially, (b) after addition of 0.2 equiv DBU, and (c)
after addition of 0.2 equiv TFA.
Figure 1. Absorption spectral changes of [Cu(oAB-2OH)2]BF4
(6.4 © 10¹6 mol dm¹3) in THF (a) initially, (b) after addition of 0.2
equiv DBU, and (c) after addition of 0.2 equiv TFA. Blue line: prior
to irradiation; red line: irradiation at 365 nm for 5 min; green line:
irradiation at 436 nm for 5 min.
formation of [CuI(bpy-2COOEt)2]+, with a less positive CuII/CuI
redox potential, through the ligand exchange coupled to photo-
isomerization. Upon addition of 0.2 equiv DBU, the rest potential
response to irradiation was silenced (Figure 3b) due to the
suppression of the ligand exchange caused by the lack of the cis
form. Addition of TFA recovered the reversible rest potential
response to UV and visible light (Figure 3c), because the barrier
to photoisomerization was lowered by protonation. The rest
potential changes in Figures 3a and 3c were different, caused by
the difference of the ratio of the trans form to the cis form in the
photostationary states in Figures 1a and 1c. These results indicate
that the photoelectric response switched between OFF and ON
states by the addition of small amounts of DBU and TFA, respec-
tively. Further investigation of this response is now ongoing.
In conclusion, [Cu(oAB-2OH)2]BF4 undergoes reversible
photoisomerization coupled to reversible protonation-deproto-
nation and redox reactions. The photoisomerization of [Cu(oAB-
2OH)2]BF4 can be switched ON or OFF by adding small
quantities of acid or base, respectively. Facile ligand exchange
between [Cu(oAB-2OH)2]BF4 and bpy-2COOEt yields
[Cu(bpy-2COOEt)2]BF4. A reversible rest potential redox
response is obtained under alternating UV and visible photo-
irradiation of the [CuI(oAB-2OH)2]BF4/bpy-2COOEt/[CuII-
(bpy-2COOEt)2](BF4)2 system. The photoelectric response
could be toggled by the addition of acid or base.
6
+
[Cu(oAB-2OH)2
]
4
2
+
[Cu(bpy-2COOEt)2
]
0
-2
-4
-0.6
-0.4 -0.2
0
0.2
0.4
0.6
0.8
Potential / V vs. Fc+/Fc
Figure 2. Cyclic voltammograms of [Cu(bpy-2COOEt)2]BF4 (2.4 ©
¹3
10¹4 mol dm¹3) in acetone with 0.1 mol dm Bu4NClO4 (black line)
and [Cu(oAB-2OH)2]BF4 (2.8 © 10¹4 mol dm¹3) in acetone with 0.1
mol dm¹3 Bu4NClO4. Blue line: prior to addition; red line: after addi-
tion of 0.2 equiv DBU; green line: after addition of 0.2 equiv TFA.
summary, photoisomerization could be toggled using acid or
base addition.
An acetone-d6 solution containing [Cu(oAB-2OH)2]BF4 and
1
bpy-2COOEt presented H NMR signals of [Cu(oAB-2OH)2]-
BF4, oAB-2OH, and bpy-2COOEt.3 The signals from [Cu(bpy-
2COOEt)2]BF4 were not observed due to broadening attributed
to ligand self-exchange. In contrast, the signals of [Cu(oAB-
OH)2]BF4 were sharp due to stabilization by interligand ³-³
stacking. These results indicate the presence of an equilibrium
between the copper coordination of oAB-2OH and bpy-2COOEt.
Figure 2 shows the cyclic voltammograms of [Cu(oAB-
2OH)2]BF4. The CuII/CuI couple showed a reversible redox
wave at 0.31 V vs. ferrocenium (Fc+)/ferrocene (Fc), which is a
typical potential for 6,6¤-disubstituted bipyridine complexes.6
The redox potential was not significantly affected by the addition
of acid or base (Figure 2). A cyclic voltammogram of [Cu(bpy-
2COOEt)2]BF4 is shown in Figure 2. A reversible redox wave for
the CuII/CuI couple was observed at ¹0.06 V vs. Fc+/Fc, a
typical potential for bipyridine derivative complexes.7 The large
difference between the redox potentials of [Cu(oAB-2OH)2]BF4
and [Cu(bpy-2COOEt)2]BF4 can be harnessed to achieve a
photoelectric response via the ligand exchange reaction.
This work was supported by Grants-in-Aid for Scientific Re-
search from MEXT, Japan (Nos. 20245013 and 21108002, area
2107), and the Global COE Program for Chemistry Innovation.
References and Notes
1
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Int. Ed. 2006, 45, 4793. c) D. J. Cárdenas, A. Livoreil, J.-P.
Sauvage, J. Am. Chem. Soc. 1996, 118, 11980. d) R. A. Bissell, E.
Córdova, A. E. Kaifer, J. F. Stoddart, Nature 1994, 369, 133.
S. Kume, M. Murata, T. Ozeki, H. Nishihara, J. Am. Chem. Soc.
2005, 127, 490.
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.html.
A. Sakamoto, A. Hirooka, K. Namiki, M. Kurihara, M. Murata, M.
Sugimoto, H. Nishihara, Inorg. Chem. 2005, 44, 7547.
M. Kojima, S. Nebashi, K. Ogawa, N. Kurita, J. Phys. Org. Chem.
2005, 18, 994.
2
3
4
5
6
7
The
[CuI(oAB-2OH)2]BF4/bpy-2COOEt/[CuII(bpy-
2COOEt)2](BF4)2 system yielded a reversible rest potential
response under alternating photoirradiation at 365 nm (UV) and
436 nm (visible) (Figure 3a). The response first exhibited a
negative potential shift accompanying the trans-to-cis isomer-
ization under 365 nm irradiation. This shift was caused by
P. Federlin, J.-M. Kern, A. Rasteder, C. Dietrich-Buchecker, P. A.
Marnot, J.-P. Sauvage, New J. Chem. 1990, 14, 9.
D. B. Rorabacher, Chem. Rev. 2004, 104, 651.
Chem. Lett. 2010, 39, 204-205
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/