Photoisomerization locking of azobenzene by formation of a self-assembled macrocycle

Article abstract of DOI:10.1039/c2cc18014f

Reaction of an azobenzene-linked biscatecholate ligand with boron and titanium sources gave ring- and cage-shaped complexes in a self-assembly fashion, respectively. These complexes were inert to photoisomerization though the ligand itself was isomerized upon photoirradiation. The self-assembled macrocyclization caused inhibition of the photoisomerization.

Full text of DOI:10.1039/c2cc18014f

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Showcasing research from Tatsuya Nabeshima’s
Laboratory/Tsukuba Research Center for
Interdisciplinary Materials Science (TIMS),
Graduate School of Pure and Applied Sciences,
University of Tsukuba, Japan.
Photoisomerization locking of azobenzene by formation of
self-assembled macrocycle
Self-assembled ring- and cage-shaped complexes of an
azobenzene-linked biscatecholate ligand were synthesized, and
these complexes were inert to photoisomerization. This fact
indicates that the self-assembly approach is an efficient and new
method for the photoisomerization locking of azobenzenes.
See Tatsuya Nabeshima et al.,
Chem. Commun., 2012, 48, 5724.
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COMMUNICATION
Photoisomerization locking of azobenzene by formation of a
self-assembled macrocyclew
Masaki Yamamura, Yuki Okazaki and Tatsuya Nabeshima*
Received 22nd December 2011, Accepted 27th January 2012
DOI: 10.1039/c2cc18014f
Reaction of an azobenzene-linked biscatecholate ligand with boron
and titanium sources gave ring- and cage-shaped complexes in a
self-assembly fashion, respectively. These complexes were inert to
photoisomerization though the ligand itself was isomerized upon
photoirradiation. The self-assembled macrocyclization caused
inhibition of the photoisomerization.
used photochromic units, features a light-induced trans–cis
isomerization. Locking of the photoswitch on supramolecular
macrocycles having photochromic units has potential for tuning
the isomerization behaviour due to reversible formation of the
macrocyclic structure. Isomerization of macrocyclic azobenzene
derivatives inhibited by a guest or in a condensed state was
reported;8 perfect locking of isomerization by self-assembled
macrocyclization has not been reported as far as we know.
Herein, an azobenzene-linked biscatechol ligand was designed
and synthesized for self-assembled rigid macrocycle formation.
The macrocyclic complexes of azobenzene-biscatechol showed
locking of the photoisomerization behaviour.
Photochromic compounds are fascinating materials because of
their application in optical memory devices, molecular machines,
and switchable molecular receptors.1 On the other hand, locking
of photoreaction has been investigated because the inhibition
plays a critical role in the design of highly stable dyes,2
photorefractive materials,3 and luminescent dyes.4 More
recently, a combination of photo and other external stimuli
was reported to lead to multi-responsive systems.5,6 In these
multi-responsive systems, photoreaction controlled by an
external effector, such as a proton,7 ion,8 Lewis acid,9 or
electron,10 provides molecular switches serving as a dual-input
logic system and nondestructive molecular memory based on
locked and unlocked photoreactions by the addition of the
effector. Here, we focused on a self-assembly approach for the
photoisomerization locking by forming a rigid macrocyclic
azobenzene (Fig. 1). Azobenzene, one of the most commonly
The azobenzene-linked biscatechol ligand trans-LH4 (Scheme 1)
was synthesized as follows: the cross-coupling reaction of
4,40-dibromoazobenzene with 2,3-dimethoxyphenylboronic
acid in the presence of Pd(PPh3)4 gave
a precursor,
4,40-bis(2,3-dimethoxyphenyl)azobenzene (72% yield), which
was demethylated by treatment with BBr3 to give trans-LH4
(75% yield).w The structural analysis of trans-LH4 by X-ray
crystallography revealed the usual bond lengths and angles as seen
in trans-azobenzene.11 In the UV-vis spectrum, absorption maxima
Fig. 1 Control of photoisomerization behaviour of azobenzene by
self-assembly approach.
Tsukuba Research Center for Interdisciplinary Materials Science
(TIMS), Graduate School of Pure and Applied Sciences, University
of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp; Fax: +81 29-853-4507;
Tel: +81 29-853-4507
w Electronic supplementary information (ESI) available: The detailed
synthetic procedure, X-ray crystallographic analysis, 1D NMR spectra,
DOSY, ESI- and MALDI-TOF-MS, and computational study. CCDC
855272. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc18014f
Scheme 1 Self-assembly formation of azobenzene-linked ring
[L4B4]4ꢀ and cage [L3Ti2]4ꢀ
.
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5724 Chem. Commun., 2012, 48, 5724–5726

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of p–p* and n–p* transitions appeared at 360 and 440 nm,
respectively.
Two different types of complexes of trans-LH4 bearing
boron and titanium catecholate, which are known as a joint
part of ring-12 and cage-shaped13 self-assemblies of biscatecholate
ligands, respectively, were synthesized to construct the macrocyclic
structure. First, the borate complex was synthesized by the
reaction of trans-LH4 with triisopropyl borate in the presence of
Et3N to give a red glassy solid in 88% isolated yield (Scheme 1).
1H, 13C and 11B NMR spectra in DMSO-d6 showed highly
symmetric signals assigned to one set of the biscatecholate boron
and symmetric azobenzene moieties with Et3NH+ as a counter
cation (Fig. S2, ESIw). All the protons were characterized by
1H–1H COSY and NOESY. When (nBu4N)OH was used instead
of Et3N, the product showed the same NMR signals in the
aromatic region. No influence of the counter cation on NMR
indicates the separation of an ion pair in solution. The elemental
analysis corresponds to the cyclic oligomer [LnBn](Et3NH)n. In
addition, negative mode ESI-TOF-MS measurement also
confirmed the formation of the cyclic compound by observation
of an ion peak corresponding to [LnBn]nꢀ at m/z of 405.1 as a
main peak and no acyclic oligomer was observed. The isotopic
pattern of the ion peak correlated with the theoretical distribution
of tetramer [L4B4]4ꢀ mixed with trimer [L3B3]3ꢀ is shown in
Fig. S7, ESI.w A diffusion-ordered NMR spectroscopic
(DOSY) experiment strongly supported a single discrete product
by the alignment of a single set of diffusion coefficient peaks for the
product (Fig. S5, ESIw). The diffusion constant was determined to
be 7.06 ꢁ 10ꢀ11 m2 sꢀ1, and the corresponding hydrodynamic
radius, Rh, was a much larger value, 15.6 A, than that of trans-LH4,
4.88 A. A molecular modelling study was performed for the
macrocyclic complexes by DFT calculation.14 There are four
diastereomers in the macrocycle [(trans-L)4B4]4ꢀ due to the
chirality of the boron catecholate moieties (Fig. S15, ESIw).
The modelling study of the four diastereomers suggests that the
most stable isomer is the D4 symmetric one with its four boron
centres having the same chirality. The observed Rh was comparable
to the molecular size of the calculated model structure of tetramer
[(trans-L)4B4]4ꢀ (15.5 A) rather than trimer [L3B3]3ꢀ (12.6 A,
Fig. S14, ESIw). Trimer [L3B3]3ꢀ observed in ESI-MS should exist
as only a minor species with a very small population to be observed
in the NMR spectra. Thus, the ring-shaped self-assembled complex
[(trans-L)4B4]4ꢀ was efficiently formed.
Chart 1 Acyclic complexes [L0 B]ꢀ and [L03Ti]2ꢀ
.
2
and the trans-isomers, [(trans-L)4B4]4ꢀ and [(trans-L)3Ti2]4ꢀ
,
respectively.
The photoisomerization study of trans-LH4, [(trans-L)4B4]4ꢀ
and [(trans-L)3Ti2]4ꢀ was performed using a 400 W high-
pressure mercury lamp. Irradiation of UV light (360 nm) onto
a DMSO solution of trans-LH4 resulted in a decrease in lmax at
360 nm and an increase at 500 nm in the UV-vis spectra with
isosbestic points. This spectral change corresponds to the
typical trans/cis isomerization of azobenzene. The 1H NMR
spectrum showed new peaks assigned to the cis-isomer after
irradiation, and the trans/cis ratio is 56/44 in the photostationary
state at 360 nm. Irradiation of visible light (440 nm) onto the
cis/trans mixture of LH4 resulted in recovery of the absorption
of the trans-form, which represents reversible isomerization. In
the UV-vis spectra of [(trans-L)4B4]4ꢀ in DMSO, a broad
absorption band was observed around 400 nm, which represents
a p–p* transition overlapped with an n–p* band. The lmax was
red-shifted over 30 nm compared to trans-LH4. This shift is due
to the electron-donation of the catecholate complex. The
isomerization experiment of [(trans-L)4B4]4ꢀ was carried out
by irradiation at different wavelengths (360 nm and 440 nm).
However, no spectral change was observed even after prolonged
photoirradiation for 60 min (Fig. 2). The inhibition of photo-
isomerization is based on either the conformational rigidity as
shown in azobenzene-linked rigid cyclophanes17 or the substitution
effect of the catecholate complex unit. The photoisomerization of
the trans-form of acyclic complex [L02B]ꢀ was evaluated under the
same conditions, and isomerization was confirmed by UV-vis and
NMR spectra; the reaction reached the photostationary state after
irradiation at 360 nm for 60 s (Fig. 2). The ratio of isomers in
the photostationary state was determined to be 35 : 65 (trans/cis)
The titanium complex was quantitatively synthesized by the
reaction of trans-LH4 with Ti(O)(acac)2 in the presence of
Na2CO3 (99% yield, Scheme 1). The 1H NMR spectrum
showed only one discrete product (Fig. S3, ESIw). The MALDI-
TOF-MS, DOSY experiment, and the model study by DFT
calculation suggested the formation of the D3 symmetric
trigonal-pyramidal cage-shaped complex [(trans-L)3Ti2]4ꢀ
(Fig. S16, ESIw),15 similarly to reported cage-shaped triscatecholate
Ti complexes.13 To prove the detailed macrocyclic effect,
acyclic boron and titanium complexes [L02B](Et3NH) and
[L03Ti]Na2 were synthesized following a procedure similar to
those for [(trans-L)4B4]4ꢀ and [(trans-L)3Ti2]4ꢀ 16
respectively
,
(Chart 1). Finally, synthesis of the self-assembled complexes of
cis-LH4 was attempted toward photoisomerization locking of
the cis-azobenzene. But the reaction of cis-LH4 with boron and
titanium sources only gave a mixture of unidentified compounds
Fig. 2 UV-vis spectral change of [L0 B](Et3NH) (a dotted line, right
2
axis) and [L4B4](Et3NH)4 (a solid line, left axis) after photoirradiation
with a high-pressure mercury lamp (360 nm) in DMSO.
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Chem. Commun., 2012, 48, 5724–5726 5725

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Notes and references
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Fig. 3 UV-vis spectral change of [L0 Ti]Na2 (a dotted line) and
3
[L3Ti2]Na4 (a solid line) after photoirradiation with a high-pressure
mercury lamp (360 nm) in DMSO.
by the integral ratio in the NMR spectrum. This result is in
contrast with the inert nature of macrocycle [(trans-L)4B4]4ꢀ to
photoirradiation. The locking of photoisomerization of
[(trans-L)4B4]4ꢀ is ascribed to the molecular rigidity of the
macrocyclic structure.
Sonnichsen, F. Tuczek and R. Herges, Science, 2011, 331, 445–448.
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In the UV-vis spectra, the broad shoulder peaks of
[(trans-L)3Ti2]4ꢀ and [(trans-L0)3Ti]4ꢀ, overlapped with the
absorptions of azobenzene, reached the region of over 600 nm
assigned to the MLCT bands (Fig. 3). Cage-shaped titanium
complex [(trans-L)3Ti2]4ꢀ did not isomerize upon photoirradiation.
In contrast, photoirradiation of acyclic [(trans-L0)3Ti]4ꢀ resulted in
a small but detectable spectral change. The spectrum was recovered
by allowing the sample to stand in the dark, indicating the thermal
back-isomerization of the cis-isomer of [L03Ti]2ꢀ. A prolonged
photoirradiation time or heating of [(trans-L0)3Ti]2ꢀ produced
decomposed products. Conversely, cage-shaped [(trans-L)3Ti2]4ꢀ
did not change under the same conditions, revealing the
11 Crystallographic data for LH4ꢂthf2: C32H34N2O6, Mw = 542.61,
%
triclinic, P1, a = 8.9085(12), b = 11.6096(15), c = 14.9458(19) A,
a = 69.838(2), b = 74.1440(10), g = 85.913(2)1, V = 1395.4(3) A3,
Z = 2, Dcalcd = 1.291 g cmꢀ3, l = 0.71073 A, m = 0.089 mmꢀ1
,
T = 120(2) K, 6662 reflections, 4796 unique (Rint = 0.0109),
GOF = 1.015, R1 = 0.0360 (I > 2s(I)), wR2 = 0.1064 (all data),
CCDC 855272.
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S. Yoshigai, I. Azumaya and K. Yamaguchi, J. Am. Chem. Soc.,
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photoisomerization locking and high stability of [(trans-L)3Ti2]4ꢀ
.
13 M. Albrecht and S. Kotila, Angew. Chem., Int. Ed. Engl., 1995, 34,
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In addition, the NMR study also showed no isomerization of
[(trans-L)3Ti2]4ꢀ and isomerization of [(trans-L0)3Ti]4ꢀ. Inhibition
of the isomerization of [(trans-L)3Ti2]4ꢀ should be ascribed to the
macrocyclic structure.
1996, 2, 1264–1268; D. L. Caulder, C. Bruckner, R. E. Powers,
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¨
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In summary, the azobenzene-linked ring [(trans-L)4B4]4ꢀ and
cage [(trans-L)3Ti2]4ꢀ were efficiently synthesized by a coordination-
driven self-assembly of the azobenzene-linked biscatechol ligand
trans-LH4. The acyclic azobenzenes bearing catecholate moieties
trans-LH4, [(trans-L0)2B]ꢀ, and [(trans-L0)3Ti]2ꢀ were isomerized
by photoirradiation though the photoisomerization of the
self-assembled macrocyclic azobenzenes [(trans-L)4B4]4ꢀ and
[(trans-L)3Ti2]4ꢀ did not proceed at all. This fact indicates that
the self-assembly approach is an efficient and new method for
the photoisomerization locking of azobenzenes. Now we are
investigating the application of this strategy for construction
of more sophisticated molecular switches.
R. Frohlich, Chem. Commun., 2005, 157–165.
¨
14 Firstly, equilibrium conformations were determined by Monte-Carlo
method at the PM3 level. Then, each equilibrium conformer was
optimized at the BLYP/6-31G* level.
15 In the negative mode MALDI-TOF-MS of the product, a main ion
peak is observed as an isotopic cluster centred at m/z 1303.1 that
matches with the theoretical isotope distribution for
[L3Ti2+Na+2H]ꢀ. Rh of L3Ti2 in DMSO-d6 was estimated to
be 8.8 A by DOSY experiment. The Rh value of [L3Ti2]4ꢀ is smaller
than that of [L4B4]4ꢀ and well comparable with the calculated
model structure.
16 [L03Ti]2ꢀ was obtained as a mixture of the fac and mer isomers because
of their fast interconversion. See references for the interconversion
of titanium triscatecholate complexes: A. Rosenheim,
B. Raibmann and G. Z. Schendel, Z. Anorg. Allg. Chem., 1931,
196, 160–176; A. V. Davis, T. K. Firman, B. P. Hay and
K. N. Raymond, J. Am. Chem. Soc., 2006, 128, 9484–9496.
17 R. Reuter and H. A. Wegner, Chem. Commun., 2011, 47,
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This research was financially supported by Grants-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
c
5726 Chem. Commun., 2012, 48, 5724–5726
This journal is The Royal Society of Chemistry 2012