Photochromism of a naphthalene-bridged imidazole dimer constrained to the "anti" conformation

Article abstract of DOI:10.1021/ol401012u

Anti-1,8-bisTPI-naphthalene in which two imidazole rings are constrained to an anti-conformation leading to the first-formed 1,4′-isomer of the bridged imidazole dimer has been synthesized. The color of the radicals is different from that of the previousl

Full text of DOI:10.1021/ol401012u

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Photochromism of a Naphthalene-Bridged
Imidazole Dimer Constrained to the
“Anti” Conformation
Katsuya Mutoh,† Kentaro Shima,† Tetsuo Yamaguchi,† Masayuki Kobayashi,† and
Jiro Abe*,†,‡
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin
University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan, and
CREST, Japan Science and Technology Agency (JST), 7 Gobancho, Chiyoda-ku,
Tokyo 102-0076, Japan
jiro_abe@chem.aoyama.ac.jp
Received April 11, 2013
ABSTRACT
Anti-1,8-bisTPI-naphthalene in which two imidazole rings are constrained to an anti-conformation leading to the first-formed 1,40-isomer of the
bridged imidazole dimer has been synthesized. The color of the radicals is different from that of the previously reported bridged-imidazolyl
radicals because the intramolecular interaction between the radicals becomes weak due to the anti-conformation. This molecular design would be
a profitable strategy to control the color of the radicals of the bridged imidazole dimer for application in ophthalmic lenses.
Photochromic molecules change their structure with the
absorption spectral change by light, and the generated
structural isomer reproduces the initial state photochemi-
cally or thermally. Photochromic materials have great
potential for application in not only industrial uses1 but
also biological research tools.2 Especially, the T-type
photochromic molecules going back to the initial isomer
thermally are expected to be applied to the opthalmic
lenses. However the high optical density in the visible light
region and the rapid thermal bleaching rate are generally
conflicted because the faster decoloration rate of the
colored species is, the lower the population of the colored
speciesinphotostationarystate. Therefore, rational molec-
ular design to solve the dilemma is required. Recently, we
have developed the bridged imidazole dimers which gen-
erate the colored radicals by UV light irradiation with
sufficiently high optical density to recognize the coloration
and the fast back reaction to form the initial imidazole dimer.3
† Aoyama Gakuin University.
‡ Japan Science and Technology Agency (JST).
(3) (a) Hatano, S.; Horino, T.; Tokita, A.; Oshima, T.; Abe, J. J. Am.
Chem. Soc. 2013, 135, 3164. (b) Mutoh, K.; Abe, J. J. Phys. Chem. C
2013, 117, 4808. (c) Ishii, N.; Kato, T.; Abe, J. Sci. Rep. 2012, 2, 819. (d)
Mutoh, K.; Abe, J. Chem. Commun. 2011, 47, 8868. (e) Kishimoto, Y.;
Abe, J. J. Am. Chem. Soc. 2009, 131, 4227. (f) Fujita, K.; Hatano, S.;
Kato, D.; Abe, J. Org. Lett. 2008, 10, 3105. (g) Iwahori, F.; Hatano, S.;
Abe, J. J. Phys. Org. Chem. 2007, 20, 857.
(4) (a) Yamashita, H.; Abe, J. J. Phys. Chem. A 2011, 115, 13332. (b)
Mutoh, K.; Abe, J. J. Phys. Chem. A 2011, 115, 4650. (c) Mutoh, K.;
Hatano, S.; Abe, J. J. Photopolym. Sci. Technol. 2010, 23, 301.
(5) (a) Kawai, S.; Yamaguchi, T.; Kato, T.; Hatano, S.; Abe, J. Dyes
Pigm. 2012, 92, 872. (b) Harada, Y.; Hatano, S.; Kimoto, A.; Abe, J.
J. Phys. Chem. Lett. 2010, 1, 1112.
(1) (a) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and
Thermochromic Compounds; Plenum Press: New York, 1999. (b) D€urr, H.;
Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier:
Amsterdam, The Netherlands, 2003.
(2) (a) Beharry, A. A.; Wong, L.; Tropepe, V.; Woolley, G. A. Angew.
Chem., Int. Ed. 2011, 50, 1325. (b) Yildiz, I.; Impellizzeri, S.; Deniz, E.;
McGaughan, B.; Callan, J. F.; Raymo, F. M. J. Am. Chem. Soc. 2011,
133, 871. (c) Uda, R. M.; Hiraishi, E.; Ohnishi, R.; Nakahara, Y.;
Kimura, K. Langmuir 2010, 26, 5444. (d) Yamada, M. D.; Nakajima, Y.;
Maeda, H.; Maruta, S. J. Biochem. 2007, 142, 691. (e) Fujimoto, K.;
Amano, M.; Horibe, Y.; Inouye, M. Org. Lett. 2006, 8, 285. (f) Woolley,
G. A. Acc. Chem. Res. 2005, 38, 486.
r
10.1021/ol401012u
XXXX American Chemical Society

Scheme 1. Photochromism of 1 and 2
Scheme 2. Synthesis of 1
Moreover, we have succeeded in establishing efficient strate-
gies for molecular design to control photosensitivity4 and the
thermal back reaction rate5 of the bridged imidazole dimers.
However, the strategy for controlling the color of the radicals
has not been developed because of the broad absorption
bands of the radicals ranging from 500 to 1000 nm. The
absorption band from 700 to 1000 nm inevitably appears
because the bridged imidazole dimer generates the radical
pair which can easily interact with each other.
In this study, we have designed a novel naphthalene-
bridged imidazole dimer, anti-1,8-bisTPI-naphthalene (1),
which constrained the two imidazole rings to the anti-
conformation (Scheme 1). All of the bridged imidazole
dimers that we previously reported have the syn-arranged
structure, and the two imidazole rings are connected with
the CꢀNbondbetweenN1andC20 of each of the imidazole
rings. Therefore, this is the first report of the anti-con-
strained bridged imidazole dimer. We revealed that it has a
unique structure and photochromic properties.
Compound 1 was synthesized according to Scheme 2.
Initially, 3 was synthesized by a Suzuki-coupling reaction
of diiodonaphthalene and 4-(phenylethynyl benzene)-
boronic acid, following the oxidation of the ethynyl moiety
with KMnO4 to give 4. The second Suzuki-coupling reac-
tion of 4-(4,5-diphenyl-1H-imidazol-2-yl)-phenylboronic
acid and 5 synthesized by imidazole condensation of 4
enabled us to obtain the desired unsymmetrical bridged
biimidazole 6. The target 1 was synthesized through the
oxidation reaction of 6. Recrystallization of 1 from a
benzene/hexane solution by the liquidꢀliquid diffusion
method gives the yellow plate crystals. The molecular
structure was revealed by X-ray crystallographic analysis.
As shown in Figure 1, the two imidazole rings are con-
nected with the CꢀN bond between N1 and C40 of the two
imidazole rings (1,40-isomer). The distance of the CꢀN
Figure 1. ORTEP representation of the molecular structure of 1
with thermal ellipsoids (50% probability), where nitrogen atoms
are highlighted in blue. The hydrogen atoms, and the solvent
molecule (benzene) are omitted for clarity.
bridged imidazole dimer has never been reported to date.
Hexaarylbiimidazole (HABI) has the possibility to form
six structural isomers (Figure 2).6 Although the main
product is the photochromic isomer, some of the other
isomers with thermochromic or piezochromic properties
have been reported. Especially, the 1,40-isomer of HABI
also shows photochromism upon UV light irradiation.6a
Thus the fast photochromic behavior of 1 was investigated
in detail.
˚
bond is 1.495 A which is identical with that of 1,8-bisTPI-
˚
naphthalene (2) (1.494 A) described in Scheme 1. It should
be emphasized here again that the 1,40-isomer of the
Figure 3 shows the UVꢀvis absorption spectra of 1 and
2 in benzene. The absorption spectrum of 1 is similar to
that of 2 in the fact that both spectra have the absorption
band in the UV region. The cleavage of the CꢀN bond
(6) (a) Goto, T.; Tanino, H.; Kondo, T. Chem. Lett. 1980, 431. (b)
Tanino, H.; Kondo, T.; Okada, K.; Goto, T. Bull. Chem. Soc. Jpn. 1972,
45, 1474.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Activation Parameters of the Themal Back Reaction of
1R and 2R
ΔH‡/
kJ molꢀ1
ΔS‡/
J molꢀ1 Kꢀ1
ΔG‡/
kJ molꢀ1
1R
2R
42.3
44.8
ꢀ105.5
ꢀ94.5
73.7
73.0
Figure 2. Possible structural isomers of HABI.
Figure 3. UVꢀvis absorption spectra of 1 and 2 in benzene.
upon UV light irradiation of 1 produces the radical species
(1R) accompanied by the color change from colorless to
green. In the presence of oxygen, the imidazolyl radicals
react with oxygen, resulting in the formation of the O2
adduct.7 Compound 1 also reacts with oxygen under con-
tinuous UV light irradiation and generates an O2 adduct.
Figure 4a shows the transient visꢀNIR absorption
spectra of 1R in degassed benzene at 25 °C measured by
the laser flash photolysis with 355 nm irradiation. All of the
absorption bands decay monotonically with the same time
constant, indicating the presence of the single component
of the radical. The thermal bleaching process of 1R in degassed
benzene is shown in Figure 4b. Compound 1R goes back to
the initial dimer monoexponentially within 5 s at 25 °C as
rapidly as the thermal bleaching reaction of 2R. The activation
enthalpies and entropies (ΔH‡ and ΔS‡, respectively) for the
Figure 4. (a) Transient visꢀNIR absorption spectra of 1R in
degassed benzene at 25 °C(5.5ꢁ 10ꢀ5 M, light path length: 10 mm).
Each of the spectra was recorded at 400 ms intervals after
excitation with a nanosecond laser pulse (excitation wavelength,
355 nm; pulse width, 5 ns; power 3 mJ/pulse). (b) Decay profiles
of the colored species 1R and 2R monitored at 400 nm in
degassed benzene at 25 °C (1: 5.5 ꢁ 10ꢀ5 M; 2: 1.1 ꢁ 10ꢀ4 M;
light path length: 10 mm).
thermal back reaction calculated from the Eyring plots
(Figure S14) are estimated to be 42.3 kJ molꢀ1 and
ꢀ105.5 J Kꢀ1 molꢀ1, respectively. The free energy barrier
of the thermal back reaction of 1R (ΔG‡ = ΔH‡ ꢀ TΔS‡) is
estimated to be 73.7 kJ molꢀ1 at 25 °C. The negative value of
ΔS‡ suggests that the degrees of freedom are reduced because
of the intramolecular recombination reaction between the
radicals. The influence of ΔS‡ on the thermal recombination
reaction of the bridged imidazole dimers was discussed in
our previous paper.5a The thermal recombination reaction
of the 30,40,50-triphenyl-1,10:20,100-terphenyl-substituted
(7) (a) Edkins, R. M.; Probert, M. R.; Fucke, K.; Robertson, C. M.;
Howard, J. A. K.; Beeby, A. Phys. Chem. Chem. Phys. 2013, 15, 7848. (b)
Hatano, S.; Abe, J. Phys. Chem. Chem. Phys. 2013, 14, 5855.
Org. Lett., Vol. XX, No. XX, XXXX
C

and 2R. The absorption bands around 600 nm in both
absorption spectra of 1R and 2R can be assigned to the
typical absorption of triphenylimidazolyl radicals (TPIRs).
On the other hand, the decrease in the absorption intensity
at 830 nm of 1Risclearly confirmed, comparedwiththatof
2R. The absorption band around 830 nm of the radical
species of the bridged imidazole dimer is attributed to the
radicalꢀradical interaction resulting from the face to face
alignment of a pair of the radicals.10 Therefore, the overlap
between the π orbitals of the two imidazolyl radicals is the
important factor for the color of the radicals. The radical
of the TPIR moiety is delocalized in the phenyl rings
connected to the imidazole ring. By restricting the TPIR
moiety to the anticonformation, the spatial overlap of the
phenyl rings is decreased. Thus, the decrease in the absorp-
tion band at 830 nm suggests that the overlap integral of
the π orbitals between two radicals is reduced due to the
anti-alignment of the TPIRs.
Figure 5. Transient visꢀNIR absorption spectra of 1R and 2R
just after laser excitation in degassed benzene at 25 °C (1: 1.0 ꢁ
10ꢀ4 M; 2: 1.5 ꢁ 10ꢀ4 M; light path length: 10 mm).
In conclusion, we have reported the new bridged imid-
azole dimer 1 in which triphenylimidazolyl parts are con-
nected with naphthalene in an anti-alignment manner. It is
noted that 1 has the CꢀN bond between N1 and C40 of the
two imidazole rings which is the first report of the 1,40-
isomer of the bridged imidazole dimer. The photochromism
of 1 shows the rapid thermal bleaching reaction to be as fast
as that of 2. The colored species of 1 has the only absorption
band which is characteristic of TPIR although the classical
bridged imidazole dimer has the broad absorption bands
which cover the whole visible region. Thus, the control of
the color of the radicals would be facile compared to the
previously reported bridged imidazole dimers. This work
would present a profitable strategy to develop the bridged
imidazole dimer for application in ophthalmic lenses.
[2.2]PC-bridged imidazole dimer is decelerated compared
with that of pseudogem-DPI-PI[2.2]PC due to the large
nuclear configuration change in the transition state, resulting
in the entropically unfavorable large negative ΔS‡ value. By
comparing the ΔS‡ values of 1R and 2R, we cannot confirm
significant differences (Table 1) suggesting that no distin-
guished degrees of freedom are confirmed in the transition
state of 1R compared with that of 2R even though the binding
manner of the products after the thermal back reaction of 1R
and 2R are the 1,40- and 1,20-isomers, respectively. Normally,
the photochromic properties of the film cannot be consistent
with those in solutions because the reaction rate of photo-
chromism is generally influenced by the environment around
the photochrome.8 Therefore the photochromism of 1 would
be also affected by the matrixes. However, we recently found
that the bridged imidazole dimers can keep their photochro-
mic properties in the polymer matrix with a plasticizer.9 Thus,
the application to the ophthalmic lenses should be achievable
with a plasticizer. The fast photochromic behavior of the
anticonformation bridged imidazole dimer in the polymer
matrix will be investigated in future studies.
Acknowledgment. This work was supported partly by
the Core Research for Evolutional Science and Technol-
ogy (CREST) program of the Japan Science and Technol-
ogy Agency (JST) and a Grant-in-Aid for Scientific
Research (A) (22245025) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
1
Supporting Information Available. Synthesis, H, 13C
Figure 5 shows the transient absorption spectra of 1 and
2, measured upon 355 nm excitation in benzene. These
absorption bands can be ascribed to the radical species, 1R
NMR spectra; HPLC chromatogram; X-ray crystallo-
graphic analysis; experimental details for the laser flash
photolysis measurements; kinetics for the thermal back
reaction in benzene. This material is available free of
charge via the Internet at http://pubs.acs.org.
(8) (a) Ercole, F.; Davis, T. P.; Evans, R. A. Macromolecules 2009, 42,
1500. (b) Norikane, Y.; Davis, R.; Tamaoki, N. New J. Chem. 2009, 33,
1327. (c) Ercole, F.; Malic, N.; Davis, T. P.; Evans, R. A. J. Mater.
ꢀ
Chem. 2009, 19, 5612. (d) Sriprom, W.; Neel, M.; Gabbutt, C. D.; Heron,
B. M.; Perrier, S. J. Mater. Chem. 2007, 17, 1885. (e) Such, G. K.; Evans,
R. A.; Davis, T. P. Macromolecules 2006, 39, 1391. (f) Evans, R. A.;
Hanley, T. L.; Skidmore, M. A.; Davis, T. P.; Such, G. K.; Yee, L. H.;
Ball, G. E.; Lewis, D. A. Nat. Mater. 2005, 4, 249.
(10) (a) Hatano, S.; Sakai, K.; Abe, J. Org. Lett. 2010, 12, 4152. (b)
Mutoh, K.; Nakano, E.; Abe, J. J. Phys. Chem. A 2012, 116, 6792.
The authors declare no competing financial interest.
(9) Ishii, N.; Abe, J. Appl. Phys. Lett. 2013, 102, 163301.
D
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