Bis(zinc porphyrin) bridged by benzo orthocarbonates as a conformational switch under regulation of DABCO and a Cu+ Ion

Article abstract of DOI:10.1021/ol802976h

A tweezer-type bis(zinc porphyrin) bridged by benzo orthocarbonates was synthesized and applied as a molecular conformational switch under regulation of DABCO and a Cu⁺ ion. The switch property has been confirmed by ¹H NMR, UV-vis sp

Full text of DOI:10.1021/ol802976h

ORGANIC
LETTERS
2009
Vol. 11, No. 8
1781-1784
Bis(zinc porphyrin) Bridged by Benzo
Orthocarbonates as a Conformational
Switch under Regulation of DABCO and
a Cu+ Ion
Zaichun Zhou, Chenzhong Cao,* Zhiqing Yin, and Qiuhua Liu
School of Chemistry and Chemical Engineering, Hunan UniVersity of Science and
Technology, Xiangtan 411201, China, Key Laboratory of Theoretical Chemistry and
Molecular Simulation of Ministry of Education, Hunan UniVersity of Science and
Technology, China, and Hunan ProVincial UniVersity Key Laboratory of
QSAR/QSPR, China
czcao@hnust.edu.cn
Received February 14, 2009
ABSTRACT
A tweezer-type bis(zinc porphyrin) bridged by benzo orthocarbonates was synthesized and applied as a molecular conformational switch
under regulation of DABCO and a Cu+ ion. The switch property has been confirmed by 1H NMR, UV-vis spectral titration, and HR-MS spectra
method.
Achieving specific or multifunctional properties is always
one of the main goals in designing molecular devices.1 The
development of molecular systems that can change their state,
both reversibly and irreversibly, is of interest because they
conceptually provide a route for the development of molec-
ular scale devices.2,3 These devices are interesting because
switching at the single-molecule (or complex) level4,5 arouses
the possibility of molecular-scale information processing. The
practical issues, however, in terms of both positioning and
interfacing the molecules are considerable.6,7
Many compounds are able to adapt their state (e.g.,
conformation, shape, luminescence, etc.) in response to
external stimuli including pH,8,9 temperature,10 redox,8,11
light,12 ion-pairing,13 etc. Many reports show that metal-
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10.1021/ol802976h CCC: $40.75
Published on Web 03/23/2009
2009 American Chemical Society

loporphyrin dimers bridged by soft chains can fold and form
stable sandwich complexes with bifunctional ligands,14-16
but no report has shown that a bis(metalloporphyrin) linked
to a rigid component can be distorted and form a 1:1
inclusion complex.
from the condensation of 2,2-dichlorobenzo[1,3]dioxole 3
with 5-(o-hydroxyphenyl)-10,15,20-triphenylporphyrin 4 un-
der a molar ratio of 1:2 (Scheme 1). Compounds 3 and 4
An inspection of the structure of 2,2-di(phenoxy)-
benzo[1,3]dioxole 5 (Figure 1) reveals that the two phenoxy
Scheme 1. Synthesis of Bis(zinc porphyrin) 1
Figure 1. (Left) 2,2-Di(phenoxy)benzo[1,3]dioxole and its deriva-
tives. (Right) X-ray structure of 5.
groups are distributed on both sides of the benzodioxole
plane. If two large-size substituents (e.g., zinc porphyrinyl)
were linked at the ortho-position of the phenoxy groups, it
is expected that they would adopt a reverse arrangement to
the axis through the two O atoms of the phenoxy groups
due to steric hindrance. If some driving force exists, the
molecule would be folded such that its two substituents
would be in a parallel arrangement, which implies the
formation of a tweezer-type molecular conformational switch.
Here, we demonstrate that (i) the bis(zinc porphyrin) 1
linked to a rigid benzo orthocarbonate adopts an open tweezer
like arrangement that can be quantitatively closed under
regulation of 1,4-diazobicyclo[2,2,2]octane (DABCO) due
to the formation of a stable, inclusive complex 1 ⊃ DABCO
in chloroform solution; (ii) the closed tweezers can be
reopened after removal of the complexed DABCO. Both of
the above processes practically display a molecular confor-
mational switch property. Direct measurement of the switch
was achieved using 1H NMR and UV-vis spectral titration
methods and high-resolution mass spectrometry (HR-MS)
technique. The experimental data fully prove that the
molecule 1 takes on the well-defined “OFF” and “ON”
reversible conformational switch behavior in response to the
introduction and removal of DABCO.
were prepared according to the methods of Endo17 and
Lindsey,18 respectively.
The 1H NMR spectrum of compound 1 indicates an open
conformation. The appearance of five protonic signals shifted
downfield of the nonaromatic region (6.1-4.0 ppm, Figure
2a) is not in agreement with the theoretical positions of
Compound 1 was obtained by metallization of its precur-
sor, free base porphyrin dimer 2. Compound 2 was obtained
Figure 2.
Signal changes of molecule 1 during a 1H NMR titration
with DABCO at 293 K; a, b, c, d, and e, respectively, represent
the 1H NMR spectra of the mixture of DABCO and molecule 1 in
molar ratios of: 0, 0.3, 0.7, 1, and 2; $ and # denote the methylene
signal of DABCO in the complexed and free states, respectively.
The numbering colored is described in Scheme 1 and Figure 3.
(13) Barrell, M. J.; Leigh, D. A.; Lusby, P. J.; Slawin, A. M. Z. Angew.
Chem., Int. Ed. 2008, 47, 8036.
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512. (b) Marois, J. S.; Cantin, K.; Desmarais, A.; Morin, J. F. Org. Lett.
2008, 10, 33. (c) Yang, Q.; Olmsted, C.; Borhan, B. Org. Lett. 2002, 4,
3423
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Chem. Commun. 2005, 1276. (b) Kieran, A. L.; Pascu, S. I.; Jarrosson, T.;
Sanders, J. K. M. Chem. Commun. 2003, 2674
.
aromatic protons not influenced by a magnetic shielding
effect on the corresponding protons. A possible reason for
this lies in the fact that one of the meso-phenyl groups linked
to the benzo orthocarbonates is shielded by the other
porphyrin π-system (Figure 3a). Four (two two-proton
(16) (a) Ballester, P.; Costa, A.; Castilla, A. M.; Dey, P. M.; Frontera,
A.; Gomila, R. M.; Hunter, C. A. Chem.-Eur. J. 2005, 11, 2196. (b)
Schuster, D. I.; Li, K.; Guldi, D. M.; Ramey, J. Org. Lett. 2004, 6, 1919
.
(17) Komatsu, S.; Takata, T.; Endo, T. Macromolecules 1993, 26, 7009
.
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D. F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391
.
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Org. Lett., Vol. 11, No. 8, 2009

difficulty of ligand axial exchange. The experiment showed
that the open conformation of compound 1, however, cannot
be closed under coordination of the bifunctional 4,4′-
bipyridyl (Bpy). The reason is probably that the distance
between the two nitrogen atoms of Bpy is larger than that
of DABCO and so there is no suitable size-matching between
1 and Bpy (see Supporting Information, S-14).
The inclusion behavior of compound 1 with DABCO was
also studied by the HR MS technique. The formation of the
stable 1:1 inclusion complex of 1 ⊃ DABCO was confirmed
by the HR-MS spectral result (Figure 4). In contrast, the HR-
Figure 3. Relative position change of two porphyrin components
after addition of DABCO, Bn are not distorted sigma bonds (a), Bd
are distorted ones (b).
doublets and two two-proton triplets) of the diagnostic signals
are derived from the four protons in the two meso-phenyl
groups, and all four signals show a distinct upfield shift. A
signal at ∼4.1 ppm results from one of the protons in the
benzo component located at the apex. The ꢀ-position signals
of the porphyrin units are displayed at three isolated positions
due to the π-system influence of the opposing porphyrin.
Therefore, free molecule 1 can be considered to adopt an
open tweezer like arrangement. That is, the molecule is in
the “ON” state of the conformational switch.
Figure 4
.
HR-MS spectrum of the inclusion complex 1 ⊃ DABCO,
calcd for [1 ⊃ DABCO]+: 1614.4162, found: 1614.4159.
The open conformation of compound 1 can be quantita-
tively closed following coordination of the bifunctional
ligand, DABCO. The assembling behavior of compound 1
MS spectral study of 1 ⊃ Bpy demonstrated that a stable
1:1 inclusion complex could not be formed under the same
conditions.
1
with DABCO was studied by H NMR titration in CDCl3
The inclusion behavior of compound 1 is also reflected in
the spectrophotometric titration experiments of compound
1 with DABCO. In the UV-vis titration of 1 with DABCO,
the absorption spectra of 1 showed a distinct red-shift and
clear isosbestic points at the Soret and Q bands as DABCO
was introduced into the solution, but its absorption intensity
and position showed almost no change when in the presence
of more than 1 equiv of the ligand (Figure 5). Their
solution at 293 K (Figure 2). The diagnostic signals (1-4)
in the meso-phenyl groups gradually disappear in the
nonaromatic field and gradually increase in the aromatic field.
Signals 5 and 6 approach each other and ultimately appear
at the same position (δ 6.78) as the amount of DABCO was
increased. The DABCO methylene unit is characterized by
a sharp signal at δ -4.78.19 Simultaneously, the other
aromatic signals are also largely simplified. These facts
suggest the formation of a sandwich complex in which the
complex adopts a 1:1 host-guest assembly (Figure 3b).
Only the 1:1 inclusion complex is formed, and no other
assembly modes, e.g. the 2:2 sandwich complex formed
between compound 1 and DABCO exist in CHCl3 solution.
In general, ligand exchange in the zinc porphyrin-ligand
systems readily takes place in the presence of the free ligand
in solution because the zinc atom in the center of the
porphyrin is a five-coordinated species,15,19,20 which is
usually embodied by the appearance of an averaged signal
of the ligands. This phenomenon, however, was not observed
in the spectrum of the 1H NMR titration of 1 with DABCO
when 2 equiv of the ligand were added. In this spectrum,
the methylene proton appeared as two isolated peaks: one
(δ 2.30) is attributed to the free ligands and the other (δ
-4.78) is attributed to the complexed ligands (Figure 2e).
This indicates that the sandwich complex 1 ⊃ DABCO
cannot be opened by the excessive DABCO due to the
Figure 5
.
Absorption spectral change in the titration of 1 (2.0 ×
10-6 M) with DABCO in CHCl3 at 293 K. (Inset) ∆A418 vs
[DABCO] plot, the association constant is up to 4.21 × 108 M-1
.
association constants (Kassoc) evaluated by applying a non-
linear curve-fitting method21 are up to 108 M-1.
(19) (a) Ema, T.; Ouchi, N.; Doi, T.; Korenaga, T.; Saka¨ı, T. Org. Lett.
2005, 7, 3985. (b) Zhou, Z. C.; He, L.; Zhu, Y. Z.; Zheng, J. Y. Chin.
J. Chem. 2007, 25, 1632.
The high stability of the inclusion complex (Kassoc 108 M-1)
originates from the distortion of the relative sigma bonds
(Bd; Figure 3) and the cooperative binding of the two nitrogen
(20) Retsek, J. L.; Drain, C. M.; Kirmaier, C.; Nurco, D. J.; Medforth,
C. J.; Smith, K. M.; Sazanovich, I. V.; Chirvony, V. S.; Fajer, J.; Holten,
D. J. Am. Chem. Soc. 2003, 125, 9787
.
Org. Lett., Vol. 11, No. 8, 2009
1783

atoms inside the cavity.15,19,22 It is thought that distortion
of these bonds simultaneously takes place when the 1:1
inclusion complex was formed through rotation of the bonds.
The distortion force of the bonds restricts the movement of
the ligand in the localized microenvironment.
The open tweezer-type molecule 1 can be effectively
closed by virtue of coordination of DABCO, while the close
conformation cannot be completely destroyed by excessive
DABCO. This is a unique property that distinguishes it from
many similar switch systems.2,15,19 Therefore, the complexed
molecule 1 adopts a close tweezer like arrangement, which
means that the molecule is converted to the “OFF” state,
that is, another state of the conformational switch.
spectrum of 1 fully recovered due to its competition for the
central zinc atom of the porphyrin. The DABCO and Cu+
ions readily form stable coordination polymers and expedi-
ently deposit from solution system,23,24 which is a necessary
condition to maintain the repeatable operation of the switch.
Therefore, both of them mutually play the role as capture
reagents in the two processes. The close-1 can be returned
to the open-1 by adding a Cu+ ion, which was also
1
demonstrated by the H NMR titration experiments of 1 ⊃
DABCO with Cu+ ions (see Supporting Information, S-12).
No well-defined switch phenomena of compound 1 were
observed under regulation of DABCO in the polar solvent
DMSO (see Supporting Information, S-13).
The conformation of the switch system is interconvertible
between the “OFF” and “ON” states. The conformation of
molecule 1 can be switched to the “OFF” position with
adding DABCO, and switched to the “ON” position as the
DABCO is removed (Figure 6 top). The reversible UV-vis
In conclusion, we have reported a very efficient molecular
conformational switch using the ordinal regulation of DAB-
CO and a Cu+ ion. The tweezer-type free molecule 1 being
in the “ON” state can be quantitatively switched to the “OFF”
state by coordination of DABCO, and the “OFF” state of
molecule 1 can also be efficiently recovered to its open
conformation by removing the DABCO with Cu+. Further-
more, molecule 1 is a modifiable and potential stereostruc-
ture, which is of major significance since it provides more
avenues for the construction of novel molecular scale devices
and potential applications in the artificial molecular machines.
Acknowledgment. The work was supported by the Nature
Science Foundation of China (20772028) and the Nature
Science Foundation of Hunan province (06JJ2002).
Supporting Information Available: Experimental pro-
cedures, reference titration results, spectroscopic data and
characterization, and HR-MS of compounds (or complexes),
and crystallographic data of 5 in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 6. Conformational interconversions and consequent spectral
shifts of 1 (2 × 10-6 M) in the reversible titration with DABCO
and Cu+ ions in CHCl3 solution. The values beside the colored
lines denote the molar ratio of DABCO (or Cu+ ions) to 1.
OL802976H
(21) Wu, Z. Q.; Shao, X. B.; Li, C.; Hou, J. L.; Wang, K.; Jiang, X. K.;
Li, Z. T. J. Am. Chem. Soc. 2005, 127, 17460.
spectral changes of 1 via two processes result in a clear
evidence for the conformational interconversion of molecule
1, close-1 and open-1 (Figure 6 down). In “ON” process, it
need 4 equiv of the Cu+ ion (from CuClO4•4MeCN) to
remove the complexed DABCO and make the absorption
(22) Zheng, J. Y.; Tashiro, K.; Hirabayashi, Y.; Kinbara, K.; Saigo, K.;
Aida, T.; Sakamoto, S.; Yamaguchi, K. Angew. Chem., Int. Ed. 2001, 40,
1857.
(23) Graham, P. M.; Pike, R. D. Inorg. Chem. 2000, 39, 5152.
(24) (a) Seki, K.; Mori, W. J. Phys. Chem. B 2002, 106, 1380. (b) Seki,
K. Chem. Commun. 2001, 1496.
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