Electrochromism of fast photochromic radical complexes forming light-unresponsive stable colored radical cation

Article abstract of DOI:10.1039/c9cc00455f

We demonstrated the electrochromism of photochromic radical complexes containing triaryl imidazole: fast photoswitchable pentaarylbiimidazole (PABI) and the phenoxyl-imidazolyl radical complex (PIC). Cyclic voltammetry and spectroelectrochemistry revealed

Full text of DOI:10.1039/c9cc00455f

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Electrochromism of Fast Photochromic Radical Complexes
Forming the Light-Unresponsive Stable Colored Radical Cation
Katsuya Yamamoto, Isshu Gomita, Hajime Okajima, Akira Sakamoto, Katsuya Mutoh and Jiro Abe*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We demonstrated the electrochromism of the photochromic radicals thermally recombine to form the original imidazole
2
0,21
radical complexes containing the triaryl imidazole, fast dimer.
We have developed the fast photochromic bridged-
photoswitchable pentaarylbiimidazole (PABI) and phenoxyl– imidazole dimers showing the fast thermal recombination
imidazolyl radical complexes (PIC). The cyclic voltammetry and reaction by restricting the diffusion of the photogenerated
2
2-32
spectroelectrochemistry revealed that PABI and PIC generate the radicals.
Pentaarylbiimidazole (PABI) and phenoxyl-
highly stable radical cation by one-electron oxidation imidazolyl radical complex (PIC) are recently developed as novel
3
3-36
accompanying with the color change from colorless to green. The types of fast photochromic compounds (Scheme 1).
stability of the radical cation is strongly affected by the dihedral
angle between the imidazole ring and the phenyl ring at the 2-
position of the imidazole ring.
The molecule that can be switched by one or more external
stimuli is an important component for the development of
1
-3
highly elaborate molecular systems. Most of the molecular
switches are the reversible transformations between two
4
different states by an external stimulus. The combination of
several different molecular switches through covalent links has
been adopted as a common strategy to achieve multi-
5
-7
responsive molecular systems. In contrast, the development
of a single molecular switch showing multi-way response to
various external stimuli is also demanded to expand the
versatility of the design of attractive switching materials. A
photochromic molecule is one of the switching molecules that
exhibit the reversible isomerization between the colorless and
Scheme. 1 Photochemical and electrochemical reaction schemes of (a) PABI and
b) PIC.
(
Because they are easy to synthesize and to modify appropriate
substituents, PABI and PIC are expected to become attractive
fast photochromic frameworks. It has also been known that
imidazole dimers show bond cleavage of the C–N bond between
the two imidazole rings by the electrochemical reduction. The
one-electron reduction of the imidazole dimer leads to the
generation of the radical anion which spontaneously breaks the
C–N bond forming the imidazolyl radical and the imidazole
anion. Although it has been reported that the electrochemical
oxidation of the imidazole dimer shows an irreversible oxidation
process, the detail of the oxidation process has not been
reported to date due to the instability of the radical cation of
8
colored states by light irradiation. It has been reported that
some of them respond to the other stimuli such as acid/base
addition and electric stimulation, and therefore, such
photochromic molecules have been accepted as a good
9
-19
candidate for the multi-responsive molecular component.
Photochromic hexaarylbiimidazole (HABI) is readily cleaved
into a pair of the colored 2,4,5-triphenylimidazolyl radicals
(TPIR) upon UV light irradiation and the photogenerated
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
3
7,38
the imidazole dimer.
In this study, we investigated the
†Electronic Supplementary Information (ESI) available: [Synthesis, Raman and time
resolved spectroscopy experiments, experimental details, the other experimental electrochemical oxidation of PABI and PIC derivatives in detail
results.]. See DOI: 10.1039/x0xx00000x
and revealed the excellent stability of the radical cations of PABI
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DOI: 10.1039/C9CC00455F
Fig. 1 Cyclic voltammograms of (a) 5.3 mM PABI and (b) 6.2 mM PIC in acetonitrile
containing 0.1 M TBAPF as a supporting electrolyte (potential scan rate= 100 mV/s).
Spectroelectrochemistry of (c) 1.0 mM PABI and (d) 1.0 mM PIC under the constant
potential at 1.2 V versus Fc/Fc+ in 0.1 M TBAPF
–acetonitrile solution at room
temperature and TDDFT calculation results (UMPW1PW91/6-
1+G(d,p)//UM052X/6-31(d)). The insets show the corresponding color changes of
the solution.
6
6
3
and PIC. To the best of our knowledge, this is the first report of Fig. 2 Resonance Raman spectra of (a) PABI•+ and (b) PIC•+ electrochemically
the stable radical cation of the photochromic radical complexes generated by the controlled potential coulometry at (a) 1.1 V and (b) 1.2 V versus
+
Fc/Fc in the 1.0 mM PABI and PIC solution containing 0.1 M TBAPF
(
6
in acetonitrile
containing the triaryl imidazole and the obvious reversible color
change by electrochromism.
Raman excitation wavelength, 633 nm; power, 6.0 mW; exposure time, (a) 33 min
and (b) 60 min) and the DFT calculation results for the radical cations of the closed-
ring form (UB3LYP/6-31+G(d,p), scale factor is 0.98). The insets show the spin-
Fig 1a and 1b show the cyclic voltammograms (CV) for PABI
and PIC, respectively, in acetonitrile with 0.1 M TBAPF
6
as a density distributions of PABI•+ and PIC•+ (UM052X/6-31G(d), isosurface value =
supporting electrolyte. Although the oxidation waves of 0.003).
conventional imidazole dimers are irreversible, the fully
almost the same value. It has been reported that the C–N bond
cleavage of PABI and PIC was induced by photoirradiation or
reversible oxidation peaks were observed in the case of PABI
and PIC. The half-wave potentials (E1/2) were determined to be
+
+
electrochemical reduction because of the antibonding
0
.88 V (versus Fc/Fc ) for PABI and 0.84 V (versus Fc/Fc ) for PIC.
3
3,35
character of the C–N bond in the LUMO.
On the other hand,
These reversible oxidation reactions indicate the formation of
•+
•+
because the HOMOs are not related to the C–N bond
formation/dissociation (Fig. S23), PABI and PIC would not show
the electrochemical oxidative bond breaking.
the radical cations (PABI and PIC ) by one-electron oxidation
Scheme 1). So as to reveal the electrochemical oxidation
(
products of PABI and PIC, we conducted the UV–vis–NIR
spectroelectrochemistry as shown in Fig 1c and 1d. The neutral
states of PABI and PIC have no absorption band in the visible
The molecular structures of PABI and PIC•+ were also
•+
investigated in detail by resonance Raman
light region.3
3,35
spectroelectrochemistry (RRSE) which is useful to obtain a
beneficial information about molecular structures of
On the other hand, they absorb a wide range of
+
the visible light under a constant potential at 1.2 V versus Fc/Fc
at room temperature. Therefore, the drastic color change from
colorless to bluish green was reversibly observed by the cyclic
3
9
electrochemical products or intermediates. Fig. 2 shows the
resonance Raman spectra of the PABI and PIC solutions (λex.
33 nm) under the constant potential at 1.1 and 1.2 V versus
=
6
voltammetry. The TDDFT calculations for the closed-ring form
of PABI•+ and PIC•+ were performed at the UMPW1PW91/6-
+
Fc/Fc , respectively, at room temperature. The radical cations
never decomposed through the measurement (Fig. S14). The
observed Raman spectra can be well explained with the
theoretical non-resonant Raman spectra for the closed-ring
3
1+G(d,p)//UM052X/6-31G(d) level of the theory. The
experimental absorption spectra are in good agreement with
the calculation results. The absorption band at around 700 nm
can be attributable to the characteristic transition from the
HOMO to the SOMO of the radical cation (Fig. S19 and S20).
•+
•+
form of PABI and PIC calculated at the UB3LYP/6-31+G(d,p)
level of the theory. We also confirmed that the calculated
Raman spectrum of the radical cation of the open-ring form of
PIC is not consistent with the experimentally obtained Raman
•+
•+
Because the SOMOs of PABI and PIC distribute over only the
π-electron triphenyl imidazole ring, the E1/2 values of
6
•+
•+
•+
spectrum for PIC (Fig. S16). Particularly, the Raman bands of
PABI/PABI and PIC/PIC couples can be estimated to be
•+
-1
PIC at 894, 938 and 971 cm are the characteristic bands for
2
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the closed-ring form. The peaks at 894 and 971 cm-1 can be
assigned to the vibrational modes originated from the sp
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DOI: 10.1039/C9CC00455F
3
carbon (Fig. S15). Thus, the RRSE measurements also suggest
that the oxidation of PABI and PIC does not trigger the C–N
bond cleavage reaction. Moreover, we investigated the
photochromic reaction of the electrochemically generated
PABI• by nanosecond laser flash photolysis. As shown in Fig.
S18, the transient absorption signal couldn’t be observed by
exciting with an intense nanosecond lase pulse. (excitation
wavelength, 355 nm; pulse width, 5 ns; power, 6.0 mJ/pulse).
Thus, PABI• was found not to show photochromism as
observed for PABI. It is presumably because the radical cation
does not have distinct transitions to the excited states
possessing anti-bonding character about the C–N bond.
+
+
•
+
We investigated the origin of the excellent stability of PABI
and PIC . The spin-density distributions of PABI and PIC
•+
•+
•+
provide the crucial insight for the stability. The calculated spin-
density distribution of each radical cation species is shown in Fig.
2
2
. In both cases, the electron spin density distributes over the
,4,5-triphenyl imidazole moiety as similar to that of TPIR.
4
0
TPIR has been well-known as one of the stable radicals because
the unpaired spin delocalizes over the three phenyl rings owing
to the high planarity of the molecule. Indeed, it has also been
reported that the planarity of radical cations plays an important
4
1,42
role for the enhancement of the stability.
Thus, we
investigated the relationship between the stability and the
•+
planarity of PIC , especially focusing on the dihedral angle
between the imidazole ring and the phenyl ring at the 2-position
of the imidazole ring of PIC. In contrast to the rigid molecular
framework of PIC, the phenyl ring at the 2-position of the
imidazole ring of the reversed phenoxyl-imidazolyl radical
complex (RPIC) has a degree of freedom of the rotation around
•
+
•+
Fig. 3 (a) Molecular structures and spin-density distributions of PIC2 , RPIC and
Me-RPIC• (UM052X/6-31G*, isosurface value = 0.003). (b) Cyclic voltammograms
and (c) spectroelectrochemistry under the constant potential at 1.2 V versus Fc/Fc+
+
the single bond between the imidazole ring and the phenyl ring
3,44
(
Fig.3a).4
Furthermore, the relevant dihedral angle can be of 1.0 mM PIC2, RPIC and Me-RPIC in 0.1 M TBAPF –acetonitrile solution (potential
6
modulated by introducing bulky substituents at the ortho scan rate= 100 mV/s). (d) The change in the anodic (Ipa.) and cathodic (Ipc.) peak
•
+
•+
currents against the redox cycles of PIC2/PIC2 , RPIC/RPIC , and Me-RPIC/Me-
positions of the phenyl ring. Based on this idea, we newly
synthesized Me-RPIC which has two methyl groups at the ortho
positions of the phenyl ring (Scheme S1 and Fig. 3a). The
imidazole ring and the phenyl ring of Me-RPIC are almost
orthogonal due to the steric hindrance of the methyl groups (Fig.
S8). Thus, we investigated the electrochemical oxidation
processes of PIC2,35 RPIC and Me-RPIC, and the stability of
these radical cations.
RPIC• couples in 0.1 M TBAPF
+
–acetonitrile solution (potential scan rate= 300
6
mV/s) at room temperature.
S9), indicating that Me-RPIC•+ decomposed due to the
instability before the electrochemical reduction will proceed.
Fig. 3c shows the UV–vis–NIR absorption spectra measured by
•+
•+
•+
spectroelectrochemistry for PIC2 , RPIC , and Me-RPIC in 0.1
M TBAPF –acetonitrile solution upon 1.2 V potential application.
6
Fig. 3a shows the spin-density distributions of these PIC
radical cations calculated by DFT (UM052X/6-31G(d) level of the
theory). Although the unpaired spin of PIC2•+ and RPIC
distributes over the triphenyl imidazole moiety, that of Me-
RPIC•+ is relatively localized on only the 4,5-diphenyl imidazole
•+
Unfortunately, the absorption spectrum of Me-RPIC was not
observed because of its instability in this experimental
condition. In the cases of PIC2 and RPIC , the absorption
bands attributable to the formation of the radical cations
appeared in the visible to NIR regions (600 to 850 nm). The
•+
•+
•+
•+
moiety. Therefore, it is expected that Me-RPIC is unstable
compared with PIC2•+ and RPIC . To reveal the reversibility of
the electrochromic reactions, we performed the cyclic
voltammetry for the three compounds as shown in Fig. 3b. The
•+
durability against the repeated redox cycles of PIC2/PIC2 ,
•+
•+
•+
RPIC/RPIC , and Me-RPIC/Me-RPIC couples by sweeping the
potentials between 0.4 and 1.2 V are shown in Fig. 3d and S10.
The anodic (Ipa.) and cathodic (Ipc.) peak currents of Me-RPIC and
RPIC significantly decreased during repetitive 900 cycles,
whereas PIC2 shows acceptable durability against hundreds to
thousands of the redox cycles. From these results, the dihedral
angle between the imidazole ring and the phenyl ring at the 2-
reversible CV was observed, and the E1/2 was determined to be
+
0
.80 V versus Fc/Fc for PIC2, whereas the CVs for RPIC and Me-
RPIC were not fully reversible when the potential scan rate was
00 mV/s. The irreversible CV of Me-RPIC became reversible
1
with increasing the scan rate from 100 mV/s to 1000 mV/s (Fig.
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2
2
2
3 K. Fujita, S. Hatano, D. Kato, J. Abe, Org. Lett., 20V0ie8w,A1rt0ic,le3O1n0lin5e.
position of the imidazole ring was found to strongly affect the
stability of the radical cation.
4 Y. Kishimoto, J. Abe, J. Am. Chem. Soc., 2009, 131, 4227.
DOI: 10.1039/C9CC00455F
5 Y. Harada, S. Hatano, A. Kimoto, J. Abe, J. Phys. Chem. Lett.,
In conclusion, we investigated the electrochemical oxidation
processes of PABI and PIC, and revealed that they generate
2
010, 1, 1112.
2
2
2
2
3
3
3
6 K. Mutoh, K. Shima, T. Yamaguchi, M. Kobayashi, J. Abe, Org.
Lett., 2013, 15, 2938.
•+
•+
highly stable radical cations, PABI and PIC , by one-electron
oxidation with accompanying the color change from colorless to
bluish green. By performing the RRSE measurements, it was
revealed for the first time that the photochromic radical
complexes containing the triaryl imidazole do not show the C–
N bond breaking by electrochemical oxidation. Moreover, by
comparing the electrochemistry for PIC2, RPIC and Me-RPIC, we
found that the coplanarity between the imidazole ring and the
phenyl ring at the 2-position of the imidazole ring is definitely
crucial to stabilize the radical cation of the triphenyl imidazole.
These findings will give not only a fundamental insight of the
electronic structures of the photochromic radical complexes
but also a molecular design for the unique photo- and
electrochromic materials by using the triaryl imidazole.
7 K. Shima, K. Mutoh, Y. Kobayashi, J. Abe, J. Am. Chem. Soc.,
2
014, 136, 3796.
8 T. Iwasaki, T. Kato, Y. Kobayashi, J. Abe, Chem. Commun., 2014,
0, 7481.
5
9 T. Yamaguchi, M. F. Hilbers, P. P. Reinders, Y. Kobayashi, A. M.
Brouwer, J. Abe, Chem. Commun., 2015, 51, 1375.
0 T. Yamaguchi, Y. Kobayashi, J. Abe, J. Am. Chem. Soc., 2016,
1
38, 906.
1 K. Mutoh, Y. Kobayashi, Y. Hirao, T. Kubo, J. Abe, Chem.
Commun., 2016, 52, 6797.
2 Y. Kobayashi, Y. Mishima, K. Mutoh, J. Abe, Chem. Commun.,
2017, 53, 4315.
3 H. Yamashita, J. Abe, Chem. Commun., 2014, 50, 8468.
4 Y. Kobayashi, H. Okajima, H. Sotome, T. Yanai, K. Mutoh, Y.
Yoneda, S. Shigeta, A. Sakamoto, H. Miyasaka, J. Abe, J. Am.
Chem. Soc., 2017, 139, 6382.
3
3
3
5 H. Yamashita, T. Ikezawa, Y. Kobayashi, J. Abe, J. Am. Chem.
Soc., 2015, 137, 4952.
Acknowledgment
This work was supported partly by the JSPS KAKENHI Grant
Numbers JP18H05263, JP26107010, and JSPS KAKENHI Grant
Number JP17K14475 for K.M.
36 T. Ikezawa, K. Mutoh, Y. Kobayashi, J. Abe, Chem. Commun.,
2
016, 52, 2465.
7 E. W. Oliver, D. H. Evans, J. V. Casper, J. Electroanal. Chem.,
996, 403, 153.
3
1
3
3
8 K. Mutoh, E. Nakano, J. Abe, J. Phys. Chem. A, 2012, 116, 6792.
9 D. L. Jeanmaire, M. R. Suchanski, R. P. V. Duyne, J. Am. Chem.
Soc., 1975, 97, 1699.
Notes and references
40 J. Abe, T. Sano, M. Kawano, Y. Ohashi, M. M. Matsushita, T.
Iyoda, Angew. Chem., Int. Ed., 2001, 40, 580.
1 A. Ronlán, J. Coleman, O. Hammerich, V. D. Parker, J. Am.
Chem. Soc., 1974, 96, 845.
1
2
S. Shinkai, Pure Appl. Chem., 1987, 59, 425.
V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, Angew. Chem.,
Int. Ed., 2000, 39, 3348.
G. Ashkenasy, T. M. Hermans, S. Otto, A. F. Taylor, Chem. Soc.
Rev., 2017, 46, 2543.
4
4
4
4
2 A. Nilsson, U. Palmquist, A. Ronlán, V. D. Parker, J. Am. Chem.
Soc., 1975, 97, 3540.
3
3 Y. Kobayashi, K. Shima, K. Mutoh, J. Abe, J. Phys. Chem. Lett.,
4
5
B. L. Feringa, Moleular Switches, Wiley-VCH, Weinheim, 2001.
A. Samat, V. Lokshin, K. Chamontin, D. Levi, G. Pepe, R.
Guglielmetti, Tetrahedron, 2001, 57, 7349.
2
016, 7, 3067.
4 A. Tokunaga, K. Mutoh, T. Hasegawa, J. Abe, J. Phys. Chem.
Lett., 2018, 9, 1833.
6
7
8
D. Gust, J. Andréasson, U. Pischel, T. A. Moore, A. L. Moore,
Chem. Commun., 2012, 48, 1947.
G. Szalóki, G. Sevez, J. Berthet, J-L. Pozzo, S. Delbaere, J. Am.
Chem. Soc., 2014, 136, 13510.
H. Tian, J. Zhang, Eds., Photochromic Materials, Wiley-VCH,
Weinheim, 2016.
9
1
1
1
1
1
Z. F. Liu, K. Hashimoto, A. Fujishima, Nature, 1990, 347, 658.
0 V. I. Minkin, Chem. Rev., 2004, 104, 2751.
1 D. M. White, J. Sonnenberg, J. Am. Chem. Soc., 1966, 88, 3825.
2 A. Peters, N. R. Branda, J. Am. Chem. Soc., 2003, 125, 3404.
3 Y. Tanaka, A. Inagaki, M. Akita, Chem. Commun., 2007, 1169.
4 T. Nakashima, Y. Kajiki, S. Fukumoto, M. Taguchi, S. Nagao, S.
Hirota, T. Kawai, J. Am. Chem. Soc., 2012, 134, 19877.
5 F. Pina, A. Roque, M. J. Melo, M. Maestri, L. Belladelli, V.
Balzani, Chem. Eur. J., 1998, 4, 1184.
1
1
1
6 K. Yumoto, M. Irie, K. Matsuda, Org. Lett., 2008, 10, 2051.
7 S. Chen, Y. Yang, Y. Wu, H. Tian, W. Zhu, J. Mater. Chem., 2012,
2
2, 5486.
1
1
8 S. Chen, Z. Guo, S. Zhu, W. Shi, W. Zhu, ACS Appl. Mater.
Interfaces, 2013, 5, 5623.
9 J. Gurke, M. Quick, N. P. Ernsting, S. Hecht, Chem. Commun.,
2
017, 53, 2150.
2
2
0 T. Hayashi, K. Maeda, Bull. Chem. Soc. Jpn., 1962, 35, 2057.
1 R. Dessauer, Photochemistry, History and Commercial
Applications of Hexaarylbiimidazoles, Elsevier, Amsterdam,
2
006.
2 F. Iwahori, S. Hatano, J. Abe, J. Phys. Org. Chem., 2007, 20,
57.
2
8
4
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