Enhancing the versatility and functionality of fast photochromic bridged imidazole dimers by flipping imidazole rings

Article abstract of DOI:10.1021/ja501028v

The widely tunable optical properties and the visible sensitivity have been required for fast photochromic molecules whose coloration-decoloration cycle completes in μs to ms time scale not only for practical applications such as full-color holographic di

Full text of DOI:10.1021/ja501028v

Communication
pubs.acs.org/JACS
Enhancing the Versatility and Functionality of Fast Photochromic
Bridged Imidazole Dimers by Flipping Imidazole Rings
Kentaro Shima,† Katsuya Mutoh,† Yoichi 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
‡CREST, Japan Science and Technology Agency (JST), K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan
S
* Supporting Information
Scheme 1. Photochromism of (a) HH, (b) HT, (c) TT, and
(d) Py-TT
ABSTRACT: The widely tunable optical properties and
the visible sensitivity have been required for fast
photochromic molecules whose coloration−decoloration
cycle completes in μs to ms time scale not only for
practical applications such as full-color holographic
displays but also for fundamental researches in biochem-
istry. However, the so far developed [2.2]paracyclophane-
bridged imidazole dimers, which are one of the best
candidates for fast photochromic molecules, have their
weaknesses for these requirements. Herein, we overcome
the issues with sustaining fast photochromism and high
durability by flipping the two imidazole rings (the head-to-
tail and tail-to-tail forms). The alteration in the relative
configuration of the imidazole rings suppresses the broad
absorption band resulting from the radical−radical
interaction. The substitution to the 2-position of the
imidazole ring of the tail-to-tail form gives the drastic
changes in the steady-state and the transient absorption
spectra. The pyrene-substituted tail-to-tail form demon-
strates that the transient absorption spectrum is featured
by the inherent spectrum of the imidazolyl radical. This
molecular framework is easy to functionalize fast photo-
chromic molecules such as sensitizations to the red light,
chirality, and biological tagging, and therefore it is versatile
for various fast photochromic applications.
he μs to ms time scale is one of the crucial time scales for
Thuman daily lives because signals within the time scale
look instantaneous or cannot be detected by human eyes due to
their time resolutions (within a few tens of ms) but can be
easily detected by simple mechanical instruments. Photo-
chromic molecules, which reversely react upon light irradi-
ation,1,2 could act as excellent probes and triggers to reveal and
control phenomena which occur under these time scales.
However, there were few photochromic molecules whose
coloration−decoloration cycle completes in the μs to ms time
scale.3,4 Pseudogem-bisDPI[2.2]PC (HH in Scheme 1a), which
is one of the best candidates for fast photochromic molecules,
has a tunable photochromic reaction in the time range from μs
to hundreds of ms with near unity quantum efficiency and
excellent durability.5,6 Pseudogem-bisDPI[2.2]PC has opened up
various potential applications in industry such as holographic
displays7,8 and security-recognition devices.9 However, their
applications were restricted to the modulation of the total
absorbance or reflection of light with time because of the
following two reasons: (1) the broad transient absorption band
due to the radical−radical interaction restricted the color
selectivity and (2) the effect of substitution was moderate and
therefore the functionalization was restricted.10,11 The spectral
tunability and the visible-light sensitivity would expand the
Received: February 4, 2014
Published: February 24, 2014
© 2014 American Chemical Society
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Communication
potentials of fast photochromic molecules into full-color
holographic displays12 eventually for commercially available
televisions, wavelength-selective high security systems, and
color-selective filters for detectors to avoid instantaneous
saturations of light. In addition to industrial applications, fast
photochromic molecules which respond to visible light can be
used as a trigger pulse to induce unfolding of proteins,13,14
motions of protein motors,15 and isomerization of DNAs,16,17
and they offer a possibility to observe dynamic conformation
changes in real time.
In this study, we overcome these issues with sustaining fast
photochromism and high durability by flipping both imidazole
rings of pseudogem-bisDPI[2.2]PC. The alteration in the
relative configuration of the imidazole rings suppresses the
broad absorption band due to the radical−radical interaction.
When we call pseudogem-bisDPI[2.2]PC as the head-to-head
form (HH), in which the 2-position of both imidazole rings
binds to a [2.2]paracyclophane (PC) moiety, we synthesized
the head-to-tail form of pseudogem-bisDPI[2.2]PC (HT,
Scheme 1b) and the tail-to-tail form of pseudogem-bisDPI[2.2]-
PC (TT, Scheme 1c), in which the 4-position of one of the
imidazole ring or both imidazole rings bind to the [2.2]PC
moiety. The 2-position of the imidazole ring of the tail-to-tail
form is easily substituted by using aldehyde derivatives, and the
substituent can drastically modulate both steady-state and
transient absorption spectra. The aldehyde derivatives are
widely available in various companies and are much easier to
synthesize as compared to diketone derivatives, which are
necessary to substitute the head-to-head form. As a
demonstration, we attached two pyrenes to the 2-position of
both imidazole rings of the tail-to-tail form of the bridged
imidazole dimer (Py-TT, Scheme 1d) and showed how the
spectra drastically changes. Fast photochromic molecules with
the wavelength tunability and the visible sensitivity have
potentials to develop novel practical applications and to
establish new academic fields.
Scheme 1 illustrates the photochromism of HH, HT, TT,
and Py-TT, respectively. The process of their photochromism
is identical irrespective of the configuration of the imidazole
rings. Upon UV irradiation, the radical pair, which is the origin
of the coloration, is generated by the C−N bond cleavage, and
two radicals thermally recombine and recreate the C−N bond.
The steady-state absorption spectra of HT and TT slightly shift
to the red region as compared to that of HH, which gradually
increases from ∼370 nm (Figures S21 and S22), but their
spectral shapes are generally similar. Figure 1 shows transient
absorption spectra of HH, HT, and TT. The vertical lines
indicate the theoretical spectra for the colored species obtained
by the TDDFT calculations (UMPW1PW91/6-31+G(d,p)//
UM052X/6-31G(d)). The insets show ORTEP representations
of their molecular structures obtained by X-ray crystallographic
analysis. As we see from the structures, while the C−N bonds
of HH and HT bind the 1- and 2′-position of the imidazole
rings (1,2′-isomers), TT is the 1,4′-isomer. HT is intrinsically
different from the head-to-tail form of the naphthalene-bridged
imidazole dimer, which is the 1,4′-isomer.18 As shown in Figure
1a, we observe a strong absorption band around 400 nm and a
broad absorption band over the visible region for HH. The
TDDFT calculations indicate that two bands at ∼400 and 500−
600 nm are attributed to the imidazolyl radical, and the band
over 600 nm is attributed to the radical−radical interactions,
which are supported by our previous report.19 The transient
absorption spectrum of HT shows the similar spectrum to that
Figure 1. Transient absorption spectra of (a) HH, (b) HT, and (c)
TT in degassed benzene (excitation wavelength, 355 nm; pulse width,
5 ns; power 4 mJ/pulse; the concentrations of HH, HT and TT are
2.1 × 10−4, 2.2 × 10−4 and 1.1 × 10−4 M, respectively). Vertical lines
indicate the theoretical spectra for the colored species of each
molecule (TDDFT UMPW1PW91/6-31+G(d,p)//UM052X/6-
31G(d)). Insets show ORTEP representations of the molecular
structures obtained by X-ray crystallographic analysis.
of HH, but the absorption band derived from the radical−
radical interaction decreases and shifts to the red region (Figure
1b). The decrease and the shift are more pronounced in the
transient absorption spectrum of TT, while the absorption band
due to the imidazolyl radical is similar to each other (Figure
1c). The transient absorption spectrum of TT has little
absorption around the red region, and only the absorption band
due to the imidazolyl radical is observed because of the reduced
overlap integral of the wave function. The decrease in the
radical−radical interaction is caused by the increase in the
distance and the displacement between two imidazole rings.
The optimized geometries of the colored species of HH, HT,
and TT are shown in Figures S39, S43, and S47, respectively.
The absorption band derived from the radical−radical
interaction is red-shifted in the order corresponding to HH,
HT, and TT. These absorption bands are mainly attributed to
the HOMO−LUMO transition from the orbital localized at an
imidazole ring to the orbital localized at the other imidazole
ring.
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Figure 2. Decay profiles of the colored species generated from HH,
HT, and TT at 298 K. Experimental condition is identical to that
described in Figure 1.
Figure 2 shows the decay profiles of the colored species
generated from HH, HT, and TT observed at 400 nm at 298 K.
We also conducted the same experiments in aerated solutions
and confirmed that the oxygen did not influence the spectra
and the decay profiles. It indicates that HH, HT, and TT do
not form any oxygen adducts as was found in the naphthalene-
bridged imidazole dimer.20 All decays are well fitted with a
single exponential decay function, and the decay of the colored
forms of TT and HT is faster than that of HH (Table 1). The
half-lives (τ1/2) of the colored species of HH, HT, and TT are
33, 6.9, and 3.2 ms at 298 K, respectively. It is intuitively
interpreted that the faster decay is caused by the destabilization
of the colored species due to the repulsion between a phenyl
group of the tail-aligned imidazole ring and the [2.2]PC moiety.
The activation parameters (ΔH⧧, ΔS⧧, and ΔG⧧ at 298 K)
were obtained from the analysis of the temperature dependence
of the decay curves (Table 1). We found that all activation
parameters decreased in the order corresponding to HH, HT,
and TT. These results are consistent with those obtained from
the theoretical calculations.
As a demonstration to show how the tail-to-tail form
effectively modulates its spectrum, we synthesized Py-TT, in
which the 2-position of two imidazole rings is substituted with
two pyrenes. Figure 3a shows the absorption spectra of TT, the
hexaarylbiimidazole derivative whose phenyl group at the 2-
position of the imidazole ring is substituted with a pyrenyl
group (Py-HABI) and Py-TT. As compared to the previous
report of the head-to-head form derivatives whose 4-position of
the imidazole ring is substituted with the pyrenyl group (Py-
HH),11 whose absorption spectrum shifts at most tens of nm,
that of Py-TT largely shifts ∼100 nm to the red. We found that
Py-TT can be excited even at the wavelength longer than 500
nm (the threshold is ∼550 nm, Figures S34−S36). The further
sensitization to the red, which is the best region for biological
probes,17 is easily accomplished by substituting the longer π-
conjugated units. It demonstrates that the substitution to the 2-
position of the imidazole ring of the tail-to-tail form is an
Figure 3. (a) Steady-state absorption spectra of TT, Py-TT, and Py-
HABI in degassed benzene and (b) the transient absorption spectra of
Py-TT and Py-HABI. Vertical lines indicate the theoretical spectrum
for the colored species of Py-TT (TDDFT UMPW1PW91/6-
31+G(d,p)//UM052X/6-31G(d)).
efficient tool to sensitize the visible light. The transient
absorption spectrum of Py-TT is completely different from that
of Py-HH. Interestingly, the transient absorption spectrum of
Py-TT is almost identical to that of Py-HABI (Figure 3b).21 It
shows that the substitution to the 2-position of the tail-to-tail
form drastically modulates the transient absorption spectrum,
and the spectrum is defined only by the imidazolyl radical, i.e.,
the radical−radical interaction is no longer observed. The
imidazolyl radical gives the narrower transient absorption
spectrum than the spectrum due to the radical−radical
interactions, and it is easier to estimate by quantum chemical
calculations. Py-TT is the first bridged imidazole dimer which
exhibits a red coloration upon UV irradiation. Since the tail-to-
tail form allows us to tune the transient colors simply by
changing the substituents from the phenyl to pyrenyl groups
with the similar decoloration time, the tail-to-tail form offers a
great opportunity to develop a real-time RGB full-color
holographic display, which has not been developed until now
because of the lack of these kinds of materials. The half-life of
the colored species of Py-TT is 1.4 ms at 298 K, and the
activation parameters are shown in the Table 1. Moreover, the
absorbance of the colored species of Py-TT decreases only
several percent even after 10 000 shots of the 355 nm laser
pulses (the pulse duration and the power is 5 ns and 4 mJ,
Table 1. Decoloration Reaction Rates, Half-Lives at 298 K, and Activation Parameters of the Colored Species of HH, HT, TT
a
and Py-TT in Degassed Benzene
k [s−1
]
τ1/2 [ms]
ΔH⧧ [kJ/mol]
ΔS⧧ [J/mol·K]
ΔG⧧ [kJ/mol]
HH
HT
21
33
6.9
3.2
1.4
58.8 (70.3)
52.8 (57.8)
48.2 (50.5)
46.5
−22.2 (−10.5)
−29.2 (−32.4)
−38.1 (−62.8)
−37.8
65.5 (72.6)
61.5 (60.0)
59.6 (54.9)
57.7
1.0 × 102
2.2 × 102
4.8 × 102
TT
Py-TT
a
Values in parentheses show calculated ones.
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(19) Mutoh, K.; Nakano, E.; Abe, J. J. Phys. Chem. A 2012, 116, 6792.
(20) Hatano, S.; Abe, J. Phys. Chem. Chem. Phys. 2012, 14, 5855.
(21) Miyasaka, H.; Satoh, Y.; Ishibashi, Y.; Ito, S.; Nagasawa, Y.;
Taniguchi, S.; Chosrowjan, H.; Mataga, N.; Kato, D.; Kikuchi, A.; Abe,
J. J. Am. Chem. Soc. 2009, 131, 7256.
respectively, Figure S37), indicating a high fatigue resistance of
Py-TT.
In conclusion, we designed the head-to-tail and tail-to-tail
forms of the bridged imidazole dimer and systematically
analyzed the effect of the relative configuration of the imidazole
rings on the photochromic properties. Py-TT achieved visible
sensitizations, and no radical−radical interactions were
confirmed in the absorption spectrum of the colored species,
which offers the tunable transient color selectivity from blue to
red. This molecular framework is easy to functionalize fast
photochromic molecules such as sensitizations to the red light,
chirality, and biological tagging, and therefore it is versatile for
various fast photochromic applications.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jiro_abe@chem.aoyama.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was 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 Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan. K. M. appreciates the Research Fellowships of JSPS for
Young Scientists.
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