Nanosecond photochromic molecular switching of a biphenyl-bridged imidazole dimer revealed by wide range transient absorption spectroscopy

Article abstract of DOI:10.1039/c4cc06001f

We demonstrate that a biphenyl-bridged imidazole dimer exhibits fast photochromism with a thermal recovery time constant of ～100 ns, which is the fastest thermal back reaction in all reported imidazole dimers. Sub-ps transient absorption spectroscopy reveals that the generation process of the colored species occurs within 1 ps. This journal is

Full text of DOI:10.1039/c4cc06001f

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DOI: 10.1039/C4CC06001F
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Nanosecond Photochromic Molecular Switching
of a Biphenyl-Bridged Imidazole Dimer Revealed
by Wide Range Transient Absorption
Spectroscopy
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Tetsuo Yamaguchi,a Michiel F. Hilbers,b Paul P. Reinders,b Yoichi
Kobayashi,a Albert M. Brouwer,b and Jiro Abe*ac
DOI: 10.1039/x0xx00000x
www.rsc.org/
We demonstrate that a biphenyl-bridged imidazole dimer
exhibits fast photochromism with a thermal recovery time
constant of ~100 ns, which is the fastest thermal back reaction
in all reported imidazole dimers. Sub-ps transient absorption
spectroscopy reveals that the generation process of the
colored species occurs within 1 ps.
Microsecond time scales cover various phenomena such as
electronic relaxation processes of triplets and charge transfer
states, molecular diffusion, enzyme reactions, folding of proteins,
etc. To trigger and precisely control these phenomena by
external stimuli, switches are necessary that respond faster than
these time scales. Photochromic molecules are the best
candidates for such switch molecules because they can
modulate various properties such as color, conductivity,1,2
chirality,3,4 intermolecular interaction,5 etc. As compared to the
structural changes during the electronic relaxation pathways from
the FranckꢀCondon state to the lowest excited state (S1),
photochromic reactions induce drastic structural changes
repeatedly. Therefore, photochromic molecular switches have a
potential to produce mechanical work6–8 and to act as optical
logic gate systems to control electronic relaxation processes
such as charge transfer and coupling.9–13 There are some reports
on photochromic compounds whose switch cycles at ambient
temperature are completed on subꢀꢁs time scales such as
certain oxazine and azo derivatives.14,15 The switching materials
that show fast photochromism on the ns time scale offer novel
applications such as high speed optical actuators16,17 with MHz
repetition frequencies.
scale while other similar derivatives were reported to exhibit the
characteristic negative photochromism. On the other hand,
quantum chemical calculations of bisDMDPIꢀBP suggest a
similarity of bisDMDPIꢀBP to the reported photochromic
imidazole dimers. Quantum chemical calculations of the reported
photochromic imidazole dimers suggest that the C–N bond
between the two imidazole rings has antiꢀbonding character in
the LUMO (Fig. 1b, DFT MPW1PW91/6ꢀ31G(d) level).25 The
calculations of bisDMDPIꢀBP also suggest that the C–C bond
between the two imidazole rings has antiꢀbonding character in
the LUMO. This similarity implicitly suggest that bisDMDPIꢀBP is
very likely to show photoinduced bond cleavage of the C–C bond
between the two imidazole rings as similar processes as the
reported photochromic imidazole dimers.
We have developed a series of bridged imidazole dimers
which act as subꢀsecond to ꢁs fast molecular switches.18–22
However, there are few compounds which act as subꢀꢁs
photochromic switches in bridged imidazole dimers.23 In our
previous study,24 we reported that bisDMDPIꢀBP (Figs. 1a and b)
did not give any photochromic signals later than the ꢁs time
In this study, we apply subꢀps and subꢀꢁs transient
absorption spectroscopy to reveal the photochromism of
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The absorbance of bisDMDPIꢀBP increases rapidly at
wavelengths longer than ~370 nm. Prior to timeꢀresolved
experiments, we examined the durability of bisDMDPIꢀBP by
repetitive exposures to ns laser pulses (See supporting
information for detail). The absorption spectra and HPLC
analyses reveal that bisDMDPIꢀBP almost does not decompose
even after 10000 laser shots under O2 atmosphere (Figs. S2 and
S3 in ESI). It shows that bisDMDPIꢀBP has high durability and
the decomposition during the laser experiments is negligible. We
set the excitation wavelength to 305 nm in ns and 310 nm in fs
transient absorption measurements. The details of the
experimental setup were reported previously29,30 and are briefly
shown in the supporting information.
Fig. 2a shows ns transient absorption spectra of bisDMDPIꢀ
BP at different time delays at room temperature (~20 ºC). We
observed two absorption bands at ~380 and 595 nm
superimposed on a broad background, and the whole spectrum
decays exponentially. The transient spectrum is very similar to
those of the radical species of bridged imidazole dimers reported
previously.21,24 The quantum chemical calculations suggest that
the absorption band at 550ꢀ650 nm can be attributed to the πꢀπ*
like transition (Figs. S13 and S14 in ESI). In addition, the
calculations suggest that the dihedral angle of the biphenyl
moiety changes from 20 to 91° by the photochromic reaction
(Figs. S11 and S12 in ESI). The ns transient absorption
experiment shows that the colored species of bisDMDPIꢀBP has
a weak and broad background at >700 nm, where the absorption
band due to the radicalꢀradical interaction was observed in the
previously studied bridged imidazole dimers.20,21,26 Our previous
work has demonstrated that the absorption band due to the
radicalꢀradical interaction decreases, becomes broad, and shifts
to longer wavelengths with decreasing radicalꢀradical
interactions.21 Since bisDMDPIꢀBP has little transient absorption
at >700 nm as compared to that of reported bridged imidazole
dimers,20,21 it indicates that the radicalꢀradical interaction of the
colored species of bisDMDPIꢀBP is weak because of the large
dihedral angle of the bridging biphenyl. The calculations and
experimental results show that the photochromism of bisDMDPIꢀ
BP induces a large conformational change due to the rotation of
the biphenyl moiety.
bisDMDPIꢀBP. We found that bisDMDPIꢀBP indeed undergoes
the expected ring opening (Fig. 1a) and is one of the fastest
photochromic compounds, with a ring opening within 1 ps and
thermal lifetime of the open form of ~100 ns. The fast and large
conformational change of bisDMDPIꢀBP offers great potential for
a molecular device to produce a large mechanical work and for a
trigger to induce
a
local conformational change to
macromolecular and biological organizations.
Imidazole dimers are categorized into several isomers
depending on the binding manner between two imidazole rings.
BisDMDPIꢀBP is the 2,2’ꢀisomer, in which the C–C bond is
formed between the 2ꢀposition of an imidazole ring and the 2’ꢀ
position of the other imidazole ring,24 while almost all reported
bridged imidazole dimers are the 1,2’ꢀ or the 1,4’ꢀisomers, in
which a C–N bond is formed between the two imidazole
rings.18,20,21,26 While there are several reports on the 2,2’ꢀisomers
of hexaarylbiimidazole (HABI) derivatives,27,28 they are sensitive
to temperature and pressure, and easily converted to the
photochromic 1,2’ꢀisomers via the imidazolyl radical formed by
thermochromism or piezochromism. Therefore, the stable 2,2’ꢀ
isomer of bisDMDPIꢀBP offers a unique opportunity to reveal the
photochromism of a representative 2,2’ꢀisomer.
Fig. 2b shows the decay profile of the colored species of
bisDMDPIꢀBP observed at 400 nm at room temperature. The
decay is fitted with a single exponential function and the lifetime
and the rate constant are 103 ns and 9.7 × 106 s−1 at room
temperature, respectively. This lifetime shows that bisDMDPIꢀBP
exhibits the fastest thermal back reaction in all reported
imidazole dimers. The decays at other wavelengths are quite
similar (see Fig. S4 in ESI), indicating that there is a single
transient species that decays with first order kinetics, as
expected for the biradical.
To reveal the detail of the bond cleavage process of
bisDMDPIꢀBP, we conducted subꢀps transient absorption
spectroscopy in which the sample was excited at 310 nm with a
pulse duration of ~200 fs. Figs. 3a and b show the transient
absorption spectra and dynamics of bisDMDPIꢀBP at different
time delays. The grey dots and solid red curves show the
BisDMDPIꢀBP was synthesized according to our previous
report.24 The steadyꢀstate absorption spectrum of bisDMDPIꢀBP
is shown in Fig. S1 (Electronic supplementary information: ESI).
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experimental data and fitted spectra (and dynamics) obtained by
the global analyses as explained later. We found that the
transient absorption spectra until 1 ps are quite broadened and
the spectra after 3 ps look very similar to those in the ns transient
absorption experiments (Fig. 2a). Interestingly, although the
spectral width earlier than 1 ps is quite broadened, the peak
positions are almost identical, which indicates that the radical
component is already generated on subꢀps time scales. We
apply global analyses to reveal the multiꢀwavelength kinetics of
bisDMDPIꢀBP. These analyses were done by using the TIMP
package with the graphical interface program Glotaran, version
1.0.1 (http://glotaran.org).31 The details of the analyses are
shown in the supporting information. We propose that the
kinetics follows a sequential model because the initial relaxation
pathways of HABI derivatives can be expected as follows (Fig.
S6 in ESI):32,33 After the photoexcitation, the photogenerated
FranckꢀCondon state relaxes to the S1 state. Then, the S1 state
relaxes to the hot state of the colored species. Finally, the
colored species is formed via vibrational cooling. In addition to
the transition from the S1 state to the higher vibrational state of
the colored species, the higher excited state of the colored
species may also contribute to the bond cleavage process
through the coupling with the S1 state of the colorless species.
Therefore, we apply sequential kinetic models for the global
analyses.
colored species (Fig. S9 in ESI). First and second transient
species are therefore best assigned as the transient absorption
of the S1 state and the vibrationally hot electronic ground state of
the colored species formed during the dissociation process,
respectively.
After the photoexcitation, the dynamics from 430 to 750 nm
show a fast decay and a very slow decay. On the other hand, the
transient absorption at 380 nm only shows a fast rise and a very
slow decay. The slow decay is attributed to the colored
photoproduct, which is almost stationary on this time scale.
In conclusion, subꢀps to subꢀꢁs transient absorption
spectroscopy reveals that bisDMDPIꢀBP exhibits extremely fast
photochromism with a large conformational change. The colored
species of bisDMDPIꢀBP forms within 1 ps after the excitation
and the thermal back reaction of the colored species takes only
~100 ns. As compared to other nsꢀscale switching molecules, a
merit for bisDMDPIꢀBP is an easy functionalization with
sustained fast photochromic properties because the rates of the
photochromic properties in bridged imidazole dimers are mostly
dominated by the bridging unit: biphenyl in the present case. This
property of the imidazole dimer supports the potential of
bisDMDPIꢀBP for nsꢀtimeꢀscale photoswitch materials.
Firstly, we applied the twoꢀstate sequential model as follows:
ꢂ ꢃꢄ
ꢀ
ꢂ ꢇ
ꢈ
ꢀ
Sꢁ ꢅꢆ Component 1 → ꢀColored speciesꢂ
(1)
In this model, we assume that the precursor states of the colored
species are not distinguishable and give a single transient signal
(Component 1). This model qualitatively explains the transient
spectra and decays, and the time constant of the generation
process of the colored species (τ1) is 760 fs (Fig. S7 in ESI).
However, it is difficult to reproduce the small amount of another
rise component appearing in the range 520ꢀ600 nm (Indicated as
black arrows in Fig. S8, ESI). The rise component does not look
like noise or an artifact. To give a better explanation to the
transient signals, we introduce the threeꢀstate sequential model
as follows.
This work was supported partly by the Core Research for
Evolutionary Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST) and a GrantꢀinꢀAid
for Scientific Research on Innovative Areas "Photosynergetics"
(No. 26107010) from MEXT, Japan. T. Yamaguchi appreciates
the Research Fellowships of JSPS for Young Scientists.
ꢈ
ꢉ
ꢀ
ꢂ ꢃꢄ
ꢀ
ꢂ ꢇ
ꢀ
ꢂ ꢇ
Sꢁ ꢅꢆ Component 1 → Component 2 → ꢀColored speciesꢂ (2)
The model assumes that the precursor state of the colored
species can be split into two states and they sequentially
generate and decay. The model fits the transient spectra and
decays very well and the time constants τ1 and τ2 are both found
to be 420 fs. The fitted spectra and decay curves are shown as
the red curves in Fig. 3. The fitted spectra of the individual
species and time traces of their population are given in Fig. S9
(ESI). This wellꢀfitted global analysis suggests that the bond
Notes and references
a
Department of Chemistry, School of Science and Engineering,
Aoyama Gakuin University, 5ꢀ10ꢀ1 Fuchinobe, Chuoꢀku,
Sagamihara, Kanagawa 252ꢀ5258, Japan
Eꢀmail: jiro_abe@chem.aoyama.ac.jp; Fax: +81ꢀ42ꢀ759ꢀ6225;
Tel: +81ꢀ42ꢀ759ꢀ6225
cleavage process consists of at least two distinct processes. The b University of Amsterdam, van ’t Hoff Institute for Molecular Sciences,
P. O. Box 94157, 1090 GD Amsterdam, The Netherlands
second component in the spectral evolution looks like a
somewhat shifted and broadened version of the spectrum of the
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DOI: 10.1039/C4CC06001F
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