A chiral BINOL-bridged imidazole dimer possessing sub-millisecond fast photochromism

Article abstract of DOI:10.1039/c4cc02710h

We developed a chiral 1,1′-bi-2-naphthol-bridged imidazole dimer possessing 100 μs fast photochromism and high fatigue resistance. It offers great opportunities for the practical applications to fast photoresponsive chiral dopants, invisible security materials and optical trigger molecules to induce the dynamic structural changes in biological matters. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02710h

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A chiral BINOL-bridged imidazole dimer
possessing sub-millisecond fast photochromism†
Takahiro Iwasaki,a Tetsuya Kato,a Yoichi Kobayashia and Jiro Abe*ab
Cite this: Chem. Commun., 2014,
50, 7481
Received 12th April 2014,
Accepted 16th May 2014
DOI: 10.1039/c4cc02710h
www.rsc.org/chemcomm
We developed a chiral 1,10-bi-2-naphthol-bridged imidazole dimer of photochromic bridged imidazole dimer which acts as a 100 ms
possessing 100 ls fast photochromism and high fatigue resistance. fast molecular switch with high durability and sustained chirality.
It offers great opportunities for the practical applications to fast We applied 1,10-bi-2-naphthol (BINOL) as a linker to bind two
photoresponsive chiral dopants, invisible security materials and imidazoles, which gives a chiral property to the bridged imidazole
optical trigger molecules to induce the dynamic structural changes dimer (compound 1 in Scheme 1a). Besides this, we incorporated
in biological matters.
chloro groups in the o-position of phenyl rings at the 2-position of
the imidazole rings to accelerate the photochromic reaction and to
Chirality in chemistry fields is usually used to describe molecular improve fatigue resistance (compound 2 in Scheme 1a). It has been
structures that cannot be superimposed on their mirror images known that o-chlorohexaarylbiimidazole (o-Cl-HABI) shows a faster
and has been widely accepted in asymmetric syntheses,1,2 phase thermal back reaction (tens-of-seconds) and high durability as
transitions of chiral nematic liquid crystals,3–5 and chiroptical compared to HABI possibly because of the distortion of the mole-
sensors.6–8 In recent two decades, light-driven phase transitions cular structure or the heavy atom effect caused by the chloro group.22
of liquid crystals and molecular motors as triggers have been We expect that the effect of the chloro group is applicable to the
extensively studied by using azobenzene,4–5,9–11 diarylethene,12–14 bridged imidazole dimers as well. Since stable fast photochromic
and thioxanthene-based alkene derivatives.15 For example, several molecules with sustained chirality can induce cooperative motions
researchers have demonstrated that the chiral photoswitch molecules and volume changes spontaneously, these chiral photochromic
induce the reversible modulation of circular dichroism (CD) signals molecules will open up novel application fields such as fast photo-
and the pitch of helical structures,14 the inversion of the helicity of responsive chiral dopants for optical gates and filters, security
chiral nematic phases,13 and the rotation of a micrometer-scale glass information systems with chirality and a trigger to probe dynamic
rod through the phase transition of cholesteric liquid crystals.15
However, since the photochromic reactions of azobenzene and
diarylethene derivatives are generally slow except for several azo
and diarylethene derivatives,16–18 the repetition rate to induce photo-
chromism is limited. Recently, we have developed a series of bridged
imidazole dimers, which act as milliseconds to sub-seconds fast
photochromic switches.19–22 The combination of fast photochromic
switches and chiral properties would achieve fast photoswitches
of chiral molecules, which would be promising for the fast
photoresponsive chiral dopants. Here, we developed a new type
a Department of Chemistry, School of Science and Engineering,
Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara,
Kanagawa 252-5258, Japan. E-mail: jiro_abe@chem.aoyama.ac.jp;
Fax: +81-42-759-6225; Tel: +81-42-759-6225
b CREST, Japan Science and Technology Agency, K’s Gobancho, 7 Gobancho,
Chiyoda-ku, Tokyo 102-0076, Japan
Scheme 1 (a) Photochromism of 1 and 2. (b) An ORTEP representation of
the molecular structure of 1 with thermal ellipsoid at 50% probability
obtained by X-ray crystallographic analysis.23
† Electronic supplementary information (ESI) available: Experimental details and
characterization data. CCDC 996722. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cc02710h
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motions of biological matters such as unfolding of proteins and the
isomerization of DNAs.24,25
It is noteworthy that the enantiomers of 1 and 2 can be
synthesized from the enantiomer of BINOL without any optical
resolutions. The chiral synthesis of 2 is shown in Scheme 2. The
incorporation of a bulky group such as a trityl group into BINOL
(the first step) enables retention of chirality of BINOL even at
temperatures above 100 1C. In the case of the previous study of the
naphthalene bridged imidazole dimer,26 the optical resolution
was required for the enantiomer of the product because the
product was obtained as a racemate. The details of the syntheses
of the enantiomers of 1 and 2 are shown in the ESI.†
Scheme 1a illustrates the photochromism of 1 and 2. The
irradiation of 1 and 2 with UV light breaks the C–N bond
between the imidazole rings and radical species 1R and 2R are
generated, which are the origin of the colouration. The radical
species spontaneously recombine to form the initial dimer by
the thermal reaction at room temperature without visible light
irradiation. Scheme 1b shows the ORTEP representation of the
molecular structure of 1 obtained by X-ray crystallographic analysis.
The details of each photochromism are revealed by steady-state
UV-vis absorption and laser flash photolysis measurements.
Fig. 1a shows the steady-state UV-vis absorption spectra and the
transient absorption spectra of 1 and 2. The blue vertical lines indicate
the theoretical absorption spectra of 2 and 2R calculated by the
conventional TDDFT MPW1PW91/6-31+G(d)//M062X/6-31G(d)
and UMPW1PW91/6-31+G(d)//UM062X/6-31G(d) methods, respectively.
It clearly shows that the spectra of 1 and 2 are almost identical
Fig. 1 (a) Steady-state UV-vis absorption spectra and (b) transient absorption
spectra of the racemates 1 (red) and 2 (blue) in degassed benzene (excitation
wavelength, 355 nm; pulse width, 5 ns; power 4 mJ per pulse; the concentra-
tions of 1 and 2 are 2.1 ꢀ 10ꢁ5 and 2.2 ꢀ 10ꢁ4 M, respectively). Blue vertical lines
indicate the theoretical absorption spectra for 2 and 2R.
irrespective of the substitution of the chloro groups. The steady- and a small sharp peak at B330 nm. The transient absorption
state UV-vis absorption spectra of 1 and 2 are very similar to each spectra of the coloured species of 2 are similar to those of 1 as
other, which show an increase in absorbance below B370 nm well as steady-state UV-vis absorption spectra (Fig. 1b). The
transient absorption spectra show a relatively sharp absorption
at 590 nm as compared to those of the [2.2]paracyclophane
bridged imidazole dimers, which show a broad transient absorption
in the near IR region due to the interaction between the two
imidazole radical chromophores.21,22 It suggests that the interaction
between the two imidazole chromophores of 1R and 2R is weak
mostly because of the larger degree of freedom in 1R and 2R as
compared to those of the [2.2]paracyclophane bridged imidazole
dimers. The TDDFT calculations indicate that the peak at 590 nm is
due to the transition from the BINOL moiety to the 4p-imidazole
ring through the phenyl ring at the 2-position of the imidazole ring
(Fig. S35, ESI†). While the transient absorption spectra of the
coloured species of 1 and 2 are very similar to each other, a striking
difference appears in their decay profiles. Fig. 2 shows the decay
profiles of 1R and 2R generated upon a UV light pulse observed at
400 nm at different temperatures (excitation wavelength: 355 nm,
intensity: 4 mJ per pulse). The decays are well fitted with a single
exponential function, and the half-life of 2R at 25 1C is 100 ms, which
is more than 100 times faster than that of 1R (38 ms). We studied the
temperature dependence of the decay profiles of the coloured
species and obtained the activation parameters (Table 1) by using
the Eyring plots (Fig. S29 and S30, ESI†). Compound 2R shows a
smaller DG‡ value that explains the acceleration of the decolouration
rate of 2R, which is well consistent with the relationship between
HABI and o-Cl-HABI.27 Similarly, the origin of the acceleration is
Scheme 2 Synthetic procedure of the enantiomer of 2 with sustained
initial chirality.
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Fig. 3 (a) CD spectra of (R)-2 (solid lines) and (S)-2 (dashed lines) in
acetonitrile (3.3 ꢀ 10ꢁ5 M) at room temperature. The red and black lines
indicate the spectra before laser shots and those after laser shots, respec-
tively. (b) Chiral HPLC chromatograms of (S)-2 before (black) and after
10 000 shots (red) of nanosecond laser pulses (355 nm; pulse width, 5 ns;
power 4 mJ per pulse) at 25 1C. Both optical purities are 99% ee.
Fig. 2 Decay profiles of (a) 1R and (b) 2R monitored at 400 nm in degassed
benzene (2.1 ꢀ 10ꢁ5 and 2.3 ꢀ 10ꢁ5 M for 1 and 2, respectively). The
measurements were performed in the temperature range from 5 to 40 1C.
Table 1 Decolouration half-lives at 298 K, and activation parameters of
1R and 2R in degassed benzene
Compound DH‡/kJ molꢁ1 DS‡/J Kꢁ1 molꢁ1 DG‡/kJ molꢁ1
t
1/2/ms
resistance. We found that the substitution of the chloro groups at the
o-position of the phenyl rings at the 2-position of the imidazole rings
drastically accelerated the photochromic reaction and improved
fatigue resistance. In addition to these interesting molecular
features, the present synthetic procedure for the enantiomer of 2
is simple, has low cost, and retains the initial chirality of the starting
material. It offers great opportunities for the practical applications to
fast photoresponsive chiral dopants, invisible security materials, and
optical-trigger molecules to induce the dynamic structural changes
in biological matters.
This work was financially supported partly by the Core Research
for Evolutional Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST) and a Grant-in-Aid for
Scientific Research (A) (22245025) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
1R
2R
43.5
19.7
ꢁ74.9
65.8
51.1
38.0
ꢁ105.4
0.10
most probably because of the distortion of the molecular structure
and/or the heavy atom effect due to the chloro group.
Fig. 3a shows the CD spectra of (R)- and (S)-2 before 355 nm
laser pulse irradiation and after 10 000 laser shots. We observe the
symmetrical Cotton effects depending on the (R)- and (S)- chirality
below 400 nm, which is due to the BINOL moiety of the imidazole
dimer. Though 1,10-binaphthyl is known to photoracemize
through the rotation of the intraannular C–C bond upon UV light
irradiation,28,29 the chirality of the BINOL moiety of 2 is retained
after many cycles of the photochromic reaction. Interestingly, the
CD spectroscopy and the chiral HPLC analysis (Fig. 3b) revealed
that 2 is robust and is not racemized even after 10 000 shots of ns
laser pulses (5 ns and 4 mJ per pulse). In addition, we examined
fatigue resistance of 2 from the steady-state UV-vis absorption
spectra (Fig. S31, ESI†). While the absorption spectrum of 1
changes after 10 000 shots of ns laser pulses, that of 2 is almost
identical even after 15 000 shots of ns laser pulses. It shows that
the durability of 2 is greatly improved compared with that of 1.
Thus 2 has high fatigue resistance and is suitable for the practical
application to a fast photochromic chiral dopant.
Notes and references
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dimer which acts as a 100 ms fast molecular switch with high fatigue
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