Helical chirality of azobenzenes induced by an intramolecular chiral axis and potential as chiroptical switches

Article abstract of DOI:10.1002/chem.201003087

The cis and trans forms of the azobenzene skeleton differ significantly in length and therefore are frequently used as photochromic parts for changing absorption and fluorescence properties, in addition to association constants when they are hosts.Althoug

Full text of DOI:10.1002/chem.201003087

DOI: 10.1002/chem.201003087
Helical Chirality of Azobenzenes Induced by an Intramolecular Chiral Axis
and Potential as Chiroptical Switches
Kazuto Takaishi,*[a] Masuki Kawamoto,[a] Kazunori Tsubaki,[b] Taniyuki Furuyama,[a]
Atsuya Muranaka,[a] and Masanobu Uchiyama[a, c]
The cis and trans forms of the azobenzene skeleton differ
significantly in length and therefore are frequently used as
photochromic parts[1] for changing absorption and fluores-
cence properties,[2] in addition to association constants when
they are hosts.[3] Although optically active compounds with
azobenzene adducts have been reported,[4–6] few studies
have examined the asymmetry of the azobenzene moiety
itself. Haberhauer and Kallweit have investigated the twist-
ing direction of the cis-azobenzene moiety by studying the
helical chirality linked to a chiral cyclic imidazole tetrapep-
tide, possessing four asymmetric centers.[7] The axially chiral
binaphthyl is rigid in the rod direction, but around the axis
has a wide and flexible asymmetric field. Thus, binaphthyl
skeletons have been employed in asymmetric organocataly-
sis,[8] molecular recognition,[9] and chiral doping of liquid
crystals.[10]
chirality of the binaphthyl units in 1–7, and oligonaphthyls
linked to azobenzenes 8 and 9, with an emphasis on changes
in the chiroptical properties due to cis–trans photoisomeriza-
tion. In particular, we have examined the optical rotation
because of its relationship to noise cancellation and nondes-
In addition to studying the half-life of the cis-azobenzene
moieties, we have previously investigated photoswitching of
the optical properties, including absorption, circular dichro-
ism (CD), and optical rotation of axially chiral binaphthyl–
azobenzene cyclic dyads.[11] Although this study included
compounds 1–5, their extensive steric structures were elu-
sive (Scheme 1).[11b] Herein, we investigate the twisting pat-
terns of the cis-azobenzene moieties induced by the axial
[a] Dr. K. Takaishi, Dr. M. Kawamoto, Dr. T. Furuyama,
Dr. A. Muranaka, Prof. Dr. M. Uchiyama
RIKEN (The Institute of Physical and Chemical Research)
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax : (+81)48-467-9389
E-mail: ktakaishi@riken.jp
[b] Prof. Dr. K. Tsubaki
Graduate School of Life and Environmental Sciences
Kyoto Prefectural University, Shimogamo Nakaragi-cho
Sakyo, Kyoto 606-8522 (Japan)
[c] Prof. Dr. M. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 113-0033 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003087.
Scheme 1. Compounds 1–11; Bn=Benzyl.
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COMMUNICATION
tructive photoswitches; both of which have applications in
memory devices.[12]
the integral of the HPLC chromatograms for (R)-6, both
wavelengths gave the same cis–trans isomerization rate of
0.8 (Table 2, entry 1). Figure 1a–b shows the CD spectra of
(R)-6 after photoirradiation. Although the cis and trans
forms varied slightly, the negative split CD at a short wave-
length (around 250 nm) had a typical shape for (R)-1,1’-bi-
In compounds 1–5, the 3,3’-positions of the binaphthyl are
substituted and circularly linked to the azobenzene moiety
at the 2,2’-positions. Compound 6 has dibenzyloxy groups at
the 7,7’-positions, however compound 7 is linked to the azo-
benzene moiety at the 7,7’-positions, but is also substituted
by 2,2’-dimethoxy groups. Compounds 8 and 9 are oligo-
naphthyl–azobenzene dyads with multiple chiral axes. Com-
pounds (R)-10 and 11 are model structures, which were se-
lected for comparison by using computational chemistry.
Compounds 1–9 were synthesized by a tandem Williamson
reaction,[11] with the appropriate axially chiral binol[13] or oli-
gonaphthalene–diol[14] derivative and 2,2’-bis(3-bromopro-
poxy)azobenzene (12, Scheme 2).
1
naphthyls. This shape was largely attributed to the Bb tran-
sition moment.[15] However, further investigation on the
short wavelength side was extremely difficult because the
azobenzene units also exhibited CD signals in this region. In
addition, the negative/flat region, which appeared at the
long wavelengths (400–500 nm), absorbed in the n–p* band
of the azobenzene moiety. Hence, we hypothesized that the
cis-azobenzene moiety of (R)-6, preferentially twisted as
either (P) or (M) along with the chiral axis of a binaphthyl,
displayed a negative Cotton effect in the long-wavelength
region, whereas the trans-azobenzene moiety maintained
planarity without preferential asymmetry.
To confirm the hypothesis that the axial chirality of bi-
naphthyl induces the helical chirality of cis-azobenzene, we
calculated the optimized geometries and corresponding CD
spectra of two diastereomers, the (P) and (M) forms at the
azobenzene moiety.[16] Figure 2 shows the optimized struc-
Scheme 2. Synthesis of 1–9 (Compound 7 is a regioisomer of the above
structure).
The change in absorption near 360 nm indicated a p–p*
transition of the trans form, which confirmed the cis–trans
isomerization of the azobenzene moiety. The absorption re-
gions of the binaphthyl and oligonaphthyl moieties were less
than 350 nm,[13,14] hence most compounds were efficiently
photoisomerized. As an example, Figure 1c–d shows the
changes in the absorption spectra of (R)-6 after irradiation
with 365 and 436 nm light, respectively. Irradiation at
365 nm caused trans!cis isomerization, whereas the reverse
reaction occurs upon irradiation at 436 nm. According to
Figure 2. Optimized structures of (R)-cis-(P)-6 and (R)-cis-(M)-6 ob-
tained by DFT calculations at the B3LYP/6-31G(d) level.
tures of (R)-cis-(P)-6 and (R)-cis-(M)-6 obtained by the
DFT calculations at the B3LYP/6-31G(d) level. By using
these optimized structures, the CD in the “azobenzene
region” (350–600 nm) was predicted (Figure 3). It should be
noted that the CD of (R)-10, the corresponding binaphthyl
skeleton, was silent. Time-dependent (TD)DFT methods at
the B3LYP/6-31G(d) level indicated that (R)-cis-(P)-6 dis-
played a negative Cotton-effect pattern whereas (R)-cis-
(M)-6 exhibited a positive Cotton-effect pattern. The experi-
mental CD indicated that the preferential configuration of
(R)-cis-6 was the (P) form. By the same method, compound
(R)-cis-7 was assigned the (M) configuration (Figure 4).
Figure 1. a) and b) CD spectra of (R)-6. c) and d) absorption spectra of
(R)-6 (horizontal scales are the same) after 365 nm irradiation (solid
line) and after 436 nm irradiation (dashed line). Conditions: 1,4-dioxane
(1.0ꢁ10ꢀ5 m), 208C, light path length=10 mm, irradiation wavelengths=
365 nm (10 mW/cm2, 100 s) and 436 nm (10 mW/cm2, 100 s).
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K. Takaishi et al.
Table 1. Experimental and calculated CDs and estimated helical chirality
of the cis-azobenzene moiety in 1–9 and 11.
Sign of De[a,b]
Entry Compound Experimental Calcd
Calcd
Chirality of cis-
cis-(P) cis-(M) azobenzene[c]
1
2
3
4
5
6
7
8
(R)-1
(R)-2
(R)-3
(R)-4
(R)-5
(R)-6
(R)-7
(S)-3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
+
+
+
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
+
+
+
+
+
+
+
(P)
(P)
(P)
(P)
(P)
(P)
(M)
(M)
(M)
(M)
9
10
11
ACHTUNGTRENNUNG
ꢀ
+
[a] Sign of De at 350–600 nm. [b] Calculated by using the TD-DFT
method with B3LYP/6-31G(d). [c] For entries 1–8, the chirality was deter-
mined by comparing the signs between the experimental and computa-
tional CD. For entries 9 and 10, the chirality observed was due to the ex-
perimental Cotton effect. [d] (S,S,S,S,S,S,S)-9.
Table 2. [a]D of 6–9 after photoirradiation.[a]
[a]D [8]
Compound After 365 nm After 436 nm After 365 nm After 436 nm
cis/trans ratio[b]
irradiation
irradiation
irradiation
irradiation
(R)-6
(R)-7
ACHTUNGTRENNUNG
ꢀ544
+358
+233
ꢀ2
ꢀ7
+282
+141
ꢀ26
80:20
80:20
83:17
83:17
20:80
26:74
41:59
40:60
Figure 4. a) Optimized structures of (R)-cis-(P)-7 and (R)-cis-(M)-7 ob-
tained by DFT calculations at the B3LYP/6-31G(d) level) under C2 sym-
metry. b) CD (calculated by the TD-DFT method at the B3LYP/6-
31G(d) level of (R)-cis-(P)-, (R)-cis-(M)-7 (dashed line), and the experi-
mental CD of (R)-7 after 365 nm irradiation (solid line, 1ꢁ10ꢀ5 m in 1,4-
dioxane at 208C). Gaussian bands with a half-band width of 2500 cmꢀ1
were used to produce the calculated spectra. c) Absorption spectra of
(R)-7 after 365 nm irradiation (1.0ꢁ10ꢀ5 m, 1,4-dioxane at 208C).
[a] Conditions: chloroform, c=0.10 gdLꢀ1
,
208C, light path length=
10 cm, irradiation wavelengths=365 nm (10 mWcmꢀ2, 500 s) and 436 nm
(10 mWcmꢀ2, 500 s). [b] Determined by HPLC (column: Inertsil SIL
100 A (GL science)), eluent: CHCl3/n-hexane 1:1. [c] (S,S,S,S,S,S,S)-9.
The coequal data concerning the asymmetry of the com-
pounds 1–11 (Table 1),[17] illustrates three points regarding
chirality: 1) (R)-2,2’-azobenzene-linked-binaphthyls indu-
cedthe (P) azobenzene conformation (Table 1, entries 1–6),
2) (S)-2,2’-analogues induced the (M) azobenzene conforma-
tion (Table 1, entry 8), and 3) 7,7’-analogues acted as
pseudo-enantiomers of 2,2’-analogues (Table 1, entry 7). The
size of oligonaphthyls 8 and 9 prevented the fine optimiza-
tion of their structures, however the central (S)-axial chirali-
ty induced the cis-(M)-azobenzene conformation (Table 1,
entries 9 and 10). Additionally, the CDs of just the azoben-
zene moieties in cis-(P)-11 and cis-(M)-11 (extracted from
the optimized structures of corresponding (R)-cis-6) were
calculated. Similar to (R)-cis-6, compound cis-(P)-11 exhibit-
ed a negative Cotton effect, with cis-(M)-11 showing a posi-
tive Cotton effect (Table 1, entry 11). Therefore, the CD
patterns described herein are derived from the innate CD of
azobenzene itself and not from binaphthyl-related induced
CD.[18,19] The relationship between the twisting patterns of
the cis-azobenzene moieties and the sign of the Cotton
Figure 3. a) CD (calculated by the TD-DFT method at the B3LYP/6-
31G(d) level) of (R)-cis-(P)-, (R)-cis-(M)-6 (dashed lines), and experi-
mental CD spectra of (R)-6 after 365 nm irradiation (solid line with cir-
cles, 1ꢁ10ꢀ5 m in 1,4-dioxane, 208C) and (R)-10 (solid line with squares,
1ꢁ10ꢀ5 m in 1,4-dioxane, 208C). Gaussian bands with a half-band width
of 2500 cmꢀ1 were used to produce the calculated spectra. b) Absorption
spectra of (R)-6 after 365 nm irradiation (1.0ꢁ10ꢀ5 m, 1,4-dioxane, 208C).
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Chem. Eur. J. 2011, 17, 1778 – 1782

Helical Chirality of Azobenzenes
COMMUNICATION
effect are consistent with those reported by Haberhauer and
Kallweit.[7]
switching of zero-/levo- rotation was achieved with (R)-6.
These conformational and chiroptical results should assist in
the future study of asymmetric azobenzene and chiroptical
switches. Studies to investigate the relationship between
conformation and linker length are currently underway in
our laboratory.
Isomerization was also shown to influence the optical ro-
tation. Table 2 shows the specific optical rotations at the
sodium D-line (589 nm), [a]D, in addition to the cis–trans
ratios of 6–9, after photoirradiation until the values became
constant. Generally, the [a]D value after 365 nm irradiation
reflects the CD intensity and the Cotton-effect pattern at
longer wavelengths. In other words, the [a]D value reflects
the twist of the cis-azobenzene moieties. The [a]D value of
(R)-6 exhibited the largest change (5378), and consequently
(R)-6 is a suitable switch for zero-/levo-rotation. In contrast,
the values of 7–9 remained relatively constant despite effi-
cient isomerization. Hence, 2,2’-azobenzene-substituted bi-
naphthyls are more effective in altering [a]D than the 7,7’-
analogues. However, oligonaphthyls are unsuitable for a-
changes because their structural changes only occur in the
vicinity of the azobenzenes, and subsequent structural
changes do not occur throughout the entire molecule.
Dextro/levo rotation switching has already been realized for
(R)-1, ꢀ3, and ꢀ4.[11b] Hence, selecting the appropriate com-
pound may control the type of sign-changing pattern. More-
over, absorption of the sodium D-line did not occur, and
consequently the target compounds did not degrade during
the measurement of [a]D. Therefore, a switch for a, adapted
from these compounds, should realize the nondestructive
reading of memory devices. Furthermore, at 298 K, the half-
lives for most of these cyclic cis-compounds was longer than
100 h, which is extraordinary for azobenzene derivatives.
These long half-lives, which are due to the cyclic structures,
provide a practical advantage for future applications (see
Table S3 in the Supporting Information).
Experimental Section
Full experimental details, characterization data, and computational re-
sults are given in the Supporting Information.
Synthesis of (R)-6: A suspension of diol (R)-10 (200 mg, 0.401 mmol),
K2CO3 (166 mg, 1.20 mmol, 3.0 equiv), and 2,2’-bis(3-bromopropoxy)azo-
benzene (12, 183 mg, 0.401 mmol, 1.0 equiv) in DMF (30 mL) was stirred
for 22 h at room temperature. Then the reaction mixture was poured into
a mixed solvent of chloroform and water. The organic layer was separat-
ed, and washed successively with 0.1n hydrochloric acid solution, water
(twice), and brine. After drying over sodium sulfate, the solvent was
evaporated under vacuum to give a residue, which was purified by silica
gel column chromatography (with chloroform as the eluent) and gel per-
meation chromatography (GPC) to afford (R)-6 (83 mg, 0.105 mmol,
26%) as a red amorphous solid.
Acknowledgements
This work was supported in part by the Shorai Foundation for Science
and Technology. We are also grateful to members of the molecular char-
acterization team at RIKEN for the spectral measurements.
Keywords: azo compounds
isomerization · photochromism
·
binaphthyl
·
chirality
·
In summary, we have revealed the conformation of sever-
al binaphthyl–azobenzene dyads and oligonaphthyl–azoben-
zene dyads with a focus on the twist pattern of the cis-azo-
benzene moiety. The 2,2’-linked-(R)-binaphthyl induced the
cis-(P) conformation of the azobenzene, whereas symmetri-
cally 7,7’-linked-(R)-binaphthyl induced the cis-(M) confor-
mation of the azobenzene (Scheme 3). A systematic and
simple induction of asymmetrical azobenzenes was realized
by using common binaphthyl skeletons. Moreover, photo-
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Scheme 3. Twisting pattern of cis-azobenzenes induced by axial chirality
of the binaphthyls.
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Received: October 26, 2010
Published online: January 7, 2011
1782
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