Fast Photochromism of the Imidazole Dimers Bridged by Group 14 Atoms

Article abstract of DOI:10.1021/acs.orglett.0c02072

We developed fast photochromic imidazole dimers bridged by group 14 atoms. These compounds reversibly break the C-N bond to generate the colored open-ring biradical form. The colored form thermally reproduces the initial colorless form in the microsecond

Full text of DOI:10.1021/acs.orglett.0c02072

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Letter
Fast Photochromism of the Imidazole Dimers Bridged by Group 14
Atoms
Hiroki Ito, Sho Tanaka, Katsuya Mutoh, and Jiro Abe*
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ABSTRACT: We developed fast photochromic imidazole dimers bridged by group
14 atoms. These compounds reversibly break the C−N bond to generate the colored
open-ring biradical form. The colored form thermally reproduces the initial colorless
form in the microsecond time scales. Furthermore, the color of the biradical can be
easily controlled by the introduction of two different types of the imidazolyl radicals.
These results give attractive insights for the further development of fast photochromic
imidazole dimers.
Scheme 1. Photochromic Reaction Schemes of (a) [2.2]PC-
Bridged Imidazole Dimer, (b) Binaphthyl-Bridged
Imidazole Dimer, and (c) Group 14 Atom Bridged
Imidazole Dimers
hotoresponsive molecules are one of the attractive
molecules which convert light energy into the changes
P
in photophysical properties. Among the various photo-
responsive molecules, organic photochromic molecules which
reversibly change the color accompanied by the structural
isomerization upon light irradiation have received much
attention as molecular switches.1−6 The artificial photochromic
compounds such as diarylethene, azobenzene, and spiropyran
have been utilized as photoswitches for light-memory,
photomechanical effect, catalysis, fluorescence, and biofunc-
tions.7−19 These attractive applications of photochromic
molecules have been achieved by the efficient utilization of
the thermal bistability of the structural isomers generated upon
light irradiation. However, because these molecules are not
suitable as a switching trigger required for rapid photo-
response, such as real-time control of holographic displays and
dynamic process of molecular assemblies,20,21 the development
of fast switchable photochromic molecules showing the rapid
thermal back reaction within millisecond time scales has been
expected.
We have recently developed fast switchable photochromic
compounds, bridged imidazole dimers (Scheme 1).22−27 The
bridged imidazole dimer is synthesized by the radical
recombination reaction between two imidazolyl radicals to
form the intramolecular C−N bond. Upon UV light
irradiation, the C−N bond homolytically cleaves to produce
the transient colored biradical species. Because the thermal
back reaction rate of the biradical can be controlled from the
microsecond to second time scales, these fast photochromic
compounds have potential applications to ophthalmic lenses,
fluorescence switching,28,29 and real-time holography.30−32 We
also discovered negative photochromic compounds by using a
binaphthyl moiety as a bridging unit for the imidazolyl radicals,
binaphthyl-bridged imidazole dimer (BN-ImD), which shows
Received: June 23, 2020
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© XXXX American Chemical Society
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the photoinduced decoloration upon visible light irradia-
tion.33−35 The most stable colored isomer of BN-ImD has the
unconventional C−N bond between the imidazole ring and the
1-position of the binaphthyl unit, leading to the attractive
negative photochromic properties. These studies show the
potential flexible acceptability of the bridging structure to
combine the imidazolyl radicals. Because the relative stabilities
of the structural isomers possessing different binding modes
between the two imidazole units strongly depend on the
bridging unit, the photochromic properties of bridged
imidazole dimers are drastically changed depending on the
bridging manner. Therefore, the investigation of the effect of
unexplored bridging unit on the fast-photochromic properties
is important for the development of the study on fast-
switchable photoresponsive compounds based on imidazolyl
radicals.
In this study, we present the novel molecular design for the
photochromic imidazole dimers, diphenyl-methane, diphenyl-
dimethyl-silane, and diphenyl-dimethyl-germane bridged imi-
dazole dimers (C-ImD, Si-ImD, and Ge-ImD, respectively).
These imidazole dimers show the fast-thermal recombination
in the microsecond time scale and the straightforward
tunability of the absorption spectra for the colored biradicals
over the visible and near-infrared (NIR) light region.
Compounds C-ImD, Si-ImD, and Ge-ImD were synthe-
sized according to Schemes S1−S3 (Supporting Information).
The molecular structures of C-ImD, Si-ImD, and Ge-ImD
were revealed by X-ray crystallographic analysis (Figure 1).
Figure 2. (a) UV−vis absorption spectra of C-ImD, Si-ImD, and Ge-
ImD in benzene at 298 K. (b) Transient absorption spectra of C-
ImD, Si-ImD, and Ge-ImD (1.0 × 10−3, 1.3 × 10−3, and 1.4 × 10−3
M, respectively) in benzene upon 355 nm laser pulse irradiation
(pulse width = 5 ns, power = 3.0 mJ/pulse).
imidazole dimers show the broad absorption bands in the
whole visible light region because the absorption band at
around 800 nm appears by the through-space interaction
between the generated imidazolyl radicals in addition to the
absorption bands of TPIRs.38 Therefore, the similar absorption
spectra of the biradicals of C-ImD, Si-ImD, and Ge-ImD with
that of TPIR indicate no interaction between the two
imidazolyl radicals despite that the radicals are closely
connected by the bridging unit. It was previously demonstrated
that the through-space interaction between the imidazolyl
radicals can be controlled by changing the relative config-
uration of the two imidazolyl radicals.38 That is, the small
overlap between the wave functions of the imidazolyl radicals
decreases the absorption band at 800 nm. The TDDFT
calculation for the biradicals of C-ImD, Si-ImD, and Ge-ImD
were performed to predict the transient absorption spectra
upon UV light irradiation. The two optimized structures were
predicted for the biradical species by the DFT calculations
(Figure 3). These calculated absorption spectra for C-ImD, Si-
ImD, and Ge-ImD were consistent with the experimental
results. The DFT results suggest that the two imidazolyl
radicals are spatially separated, resulting in the small overlap
integral between the wave functions of the two imidazole
radicals. Therefore, the absorption spectra of the biradicals of
C-ImD, Si-ImD, and Ge-ImD show the similar spectral shapes
with that of the individual TPIR.
Figure 1. ORTEP representations of the molecular structures of C-
ImD, Si-ImD and Ge-ImD (thermal ellipsoids at 50% probability; N,
Si and Ge atoms are shown in blue, orange and green, respectively). H
atoms and solvent molecules are omitted for clarity.
These imidazole dimers have the C−N bond between the two
imidazole rings to form the 1,2′-isomers. The other structural
isomers such as the 1,4′-isomer and the 2,2′-isomer36−38 could
not be obtained by the oxidation reaction of the precursor
lophine with potassium ferricyanide at room temperature. The
lengths of the C−N bonds of C-ImD, Si-ImD and Ge-ImD
were determined to be 1.489, 1.485, and 1.480 Å, respectively,
as long as those of typical imidazole dimers.22−24 Compounds
C-ImD, Si-ImD, and Ge-ImD form the eight-membered ring
structure between the imidazole dimer and the bridging unit in
the closed-ring forms.
Figure 2a shows the absorption spectra of C-ImD, Si-ImD,
and Ge-ImD in benzene. The absorption spectra have the large
absorption band in the UV region. Upon UV light irradiation,
the transient absorption band at 570 nm was observed in
addition to the sharp absorption band below 400 nm (Figure
2b). The transient absorption spectral shapes of C-ImD, Si-
ImD, and Ge-ImD are similar with that of typical triphenyl
imidazolyl radical (TPIR), indicating the generation of the
transient radical species by the C−N bond cleavage.39
Generally, the photogenerated biradical species of the bridged
The time profiles of the transient absorbance at 570 nm for
C-ImD, Si-ImD, and Ge-ImD are shown in Figure 4. The
transient absorption spectra monotonically decay in the whole
spectral region (see the Supporting Information) because the
differences in the thermodynamical stabilities between the two
conformations of the biradicals are negligibly small (approx-
imately <5 kJ/mol, Table S10), indicating that the two
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temperature range from 278 to 313 K to estimate the
activation parameters (the enthalpy and entropy of activation,
ΔH⧧ and ΔS⧧, and the free energy barrier ΔG⧧ = ΔH⧧ −
TΔS⧧) as shown in Table 1. The Eyring plots produced
Table 1. Activation Parameters for the Thermal Back
Reactions of C-ImD, Si-ImD, and Ge-ImD
ΔH⧧ (kJ/mol) ΔS⧧ (J/K mol) ΔG⧧ (kJ/mol) τ1/2 (μs)
C-ImD
Si-ImD
Ge-ImD
35.0
23.8
24.6
−52.1
−51.4
−52.1
50.5
39.1
40.2
79
0.77
1.2
excellent straight lines, and the activation parameters were
estimated from standard least-squares analysis (Supporting
Information). The ΔS⧧ values are same between the thermal
back reactions of C-ImD, Si-ImD, and Ge-ImD, indicating
little difference in the conformational change accompanied
with the thermal recombination reactions. In contrast, the
ΔH⧧ value of C-ImD is much larger than those of Si-ImD and
Ge-ImD. The DFT calculations were performed to obtain the
structures of the transition states and to calculate the activation
energy barriers (Supporting Information). Because the two
states for the optimized structures (BR1 and BR2) of the
biradicals (Figure 3) are almost degenerate, the sum of the
activation barriers from BR1 and BR2 are compared. Although
the calculated values of the activation barriers are not
quantitatively consistent with the experimental data, the results
suggest that the sum of the activation barriers for C-ImD is the
largest in those of C-ImD, Si-ImD, and Ge-ImD. The
structures of the transition states are shown in Figure S69.
The large difference between the transition states of C-ImD,
Si-ImD, and Ge-ImD is the bond distances of the 14 atoms-
bridged unit as similar with the 1,2′-isomers. This difference
causes the change in the orientation of two imidazole rings
which make the C−N bond to produce the 1,2′-isomer. While
these differences may be related to the difference in the
thermal back reaction rates, the detailed explanation of the
remarkable acceleration of the thermal back reactions of Si-
ImD and Ge-ImD have still been under investigation.
Figure 3. Optimized structures of the two conformations of the
biradicals for C-ImD, Si-ImD, and Ge-ImD by DFT (UCAM-
B3LYP/6-31+G(d,p)).
The transient absorption spectra of C-ImD, Si-ImD, and
Ge-ImD can be easily modulated because of the small
interaction between the two imidazolyl radicals. We demon-
strated that the introduction of the two different imidazolyl
radicals gives a superposed absorption spectrum in the visible
light region. Figure 5 shows the transient absorption spectra of
the diphenyl-dimethyl-silane bridged imidazole dimer com-
posed of the diphenyl imidazole and the phenanthro-imidazole
rings (Si-ImPhen) and that composed of the diphenyl
imidazole and pyrenyl-imidazole rings (Si-ImPy) upon UV
light irradiation. The molecular structures of these imidazole
dimers were revealed by X-ray crystallography (Figures S53
and S54). The heteroimidazolyl radical pair was generated by
the photoinduced C−N bond breaking reaction. The photo-
generated radical pairs of Si-ImPhen and Si-ImPy also show
the rapid thermal back reactions in the microsecond time scale
(Figures S65 and S67). The radical pair absorbs the whole
range of visible light. The transient absorption spectra of Si-
ImPhen and Si-ImPy show good overlap with the super-
position of the absorption spectra of the corresponding
individual imidazolyl radicals (Phen• and Si-ImD in Figure
5a or Py• and Si-ImD in Figure 5b), indicating that the
independency of the imidazolyl radicals is maintained even by
Figure 4. (a) Decay profiles of the transient biradicals of C-ImD, Si-
ImD, and Ge-ImD (2.0 × 10−4, 2.0 × 10−4, and 4.7 × 10−4 M,
respectively) in benzene at 298 K monitored at 570 nm upon 355 nm
laser pulse irradiation (pulse width = 5 ns, power = 3.0 mJ/pulse). (b)
The first-order plots for the decay profiles.
conformations are thermally equilibrated at 298 K. The half-
lives of the transient biradicals of C-ImD, Si-ImD, and Ge-
ImD were measured to be 79, 0.77, and 1.2 μs at 298 K,
respectively. The fatigue resistances of C-ImD, Si-ImD, and
Ge-ImD against the repetitive laser irradiation are shown in
Figure S68. The thermal back reactions of C-ImD, Si-ImD,
and Ge-ImD are drastically accelerated into the time scales of
micro- or submicroseconds, compared with that of the
[2.2]PC-bridged imidazole dimer,22 only by bridging the
TPIRs with group 14 atoms. We investigated the temperature
dependence of the thermal back reaction rates in the
C
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AUTHOR INFORMATION
■
Corresponding Author
Jiro Abe − Department of Chemistry, Aoyama Gakuin
University, Sagamihara, Kanagawa 252-5258, Japan;
orcid.org/0000-0002-0237-815X; Email: jiro_abe@
chem.aoyama.ac.jp
Authors
Hiroki Ito − Department of Chemistry, Aoyama Gakuin
University, Sagamihara, Kanagawa 252-5258, Japan
Sho Tanaka − Department of Chemistry, Aoyama Gakuin
University, Sagamihara, Kanagawa 252-5258, Japan
Katsuya Mutoh − Department of Chemistry, Aoyama Gakuin
University, Sagamihara, Kanagawa 252-5258, Japan;
orcid.org/0000-0002-9778-8329
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02072
Figure 5. (a) Transient absorption spectra of Si-ImPhen and Si-ImD
and the absorption spectrum of Phen• in benzene. (b) Transient
absorption spectra of Si-ImPy and Si-ImD and the absorption
spectrum of Py• in benzene.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
the introduction of the π-extended imidazolyl radicals.
Therefore, the various types of the combination of the radical
species will be expected for the further enhancement of the
photochromic properties.
This work was supported partly by JSPS KAKENHI Grant
Number JP18H05263 and JSPS KAKENHI Grant Number
JP17K14475 for K.M.
In conclusion, we developed the novel series of the fast-
photochromic bridged imidazole dimer, group 14 atoms-
bridged imidazole dimers, C-ImD, Si-ImD, and Ge-ImD.
Compounds C-ImD, Si-ImD, and Ge-ImD can reversibly form
the colored biradical by the photoinduced C−N bond
breaking. The thermal back reaction of the biradical accelerates
to the microsecond time scale only by bridging the TPIRs with
group 14 atoms. In addition, the different types of the
imidazolyl radicals can be easily introduced to the bridged
imidazole dimer. Because of the small interaction between the
two imidazolyl radicals, the absorption spectrum of the
biradical reflects the superposition of each absorption spectrum
of the introduced imidazolyl radical, resulting in the on-
demand control of the absorption spectra. These novel
molecular designs bridged by group 14 atoms will open up
the further development of the fast-photochromic imidazole
dimers.
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