Structural Monitoring of the Onset of Excited-State Aromaticity in a Liquid Crystal Phase

Article abstract of DOI:10.1021/jacs.7b08021

Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird's rule for triplet aromaticity since 1972 gives the rationale of photoinduced conformational changes and photochemical reactivities of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC) molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state aromaticity can be used as the working mechanism of new photoresponsive materials.

Full text of DOI:10.1021/jacs.7b08021

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Article
Structural monitoring of the onset of excited-
state aromaticity in a liquid crystal phase
Masaki Hada, Shohei Saito, Sei’ichi Tanaka, Ryuma Sato, Masahiko Yoshimura,
Kazuhiro Mouri, Kyohei Matsuo, Shigehiro Yamaguchi, Mitsuo Hara, Yasuhiko Hayashi,
Fynn Roehricht, Rainer Herges, Yasuteru Shigeta, Ken Onda, and R. J. Dwayne Miller
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b08021 • Publication Date (Web): 16 Oct 2017
Downloaded from http://pubs.acs.org on October 16, 2017
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TOC graphic
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Structural monitoring of the onset of excited-state aromaticity in a
liquid crystal phase
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Masaki Hada1*†, Shohei Saito2,3*†, Sei’ichi Tanaka4, Ryuma Sato5,
Masahiko Yoshimura6, Kazuhiro Mouri6, Kyohei Matsuo6, Shigehiro Yamaguchi6,
Mitsuo Hara7, Yasuhiko Hayashi1, Fynn Röhricht8, Rainer Herges8, Yasuteru Shigeta5*,
Ken Onda9*, R. J. Dwayne Miller10,11
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1Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan.
2Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
3JST-PRESTO, Kawaguchi 332-0012, Japan.
4School of Science, Tokyo Institute of Technology, Yokohama 226-8502, Japan.
5Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan.
6Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan.
7Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
8Otto Diels-Institute for Organic Chemistry, Christian-Albrechts University Kiel, Kiel 24119, Germany.
9Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 819-0395, Japan.
10Max Planck Institute for the Structure and Dynamics of Matter, Center for Free Electron Laser Science, Hamburg
Centre for Ultrafast Imaging, University of Hamburg, Hamburg 22761, Germany.
11Departments of Chemistry and Physics, University of Toronto, Toronto M5S 3H6, Canada.
†These authors contributed equally to this work.
ABSTRACT: Aromaticity of photoexcited molecules is an important concept in organic chemistry. Its theory, Baird’s rule for
triplet aromaticity since 1972, gives the rationale of photoinduced conformational changes and photochemical reactivities
of cyclic π-conjugated systems. However, it is still challenging to monitor the dynamic structural change induced by the
excited-state aromaticity, particularly in condensed materials. Here we report direct structural observation of a molecular
motion and a subsequent packing deformation accompanied by the excited-state aromaticity. Photoactive liquid crystal (LC)
molecules featuring a π-expanded cyclooctatetraene core unit are orientationally ordered but loosely packed in a columnar
LC phase, and therefore a photoinduced conformational planarization by the excited-state aromaticity has been successfully
observed by time-resolved electron diffractometry and vibrational spectroscopy. The structural change took place in the
vicinity of excited molecules, producing a twisted stacking structure. A nanoscale torque driven by the excited-state
aromaticity
can
be
used
as
the
working
mechanism
of
new
photoresponsive
materials.
Introduction
changes during aromaticity reversal (Hückel ↔ Baird) in
T1 and S1 have been analyzed by time-resolved absorption
and IR spectroscopies using expanded porphyrins and
their metal complexes,7 and the energetics of Baird
aromaticity has been quantified by circular dichroism
The concept of excited-state aromaticity was first
introduced from a theoretical viewpoint. In 1972, Baird
predicted the photoinduced aromaticity of cyclic 4nπ-
electron systems in their lowest triplet state (T1)1. Baird
aromaticity is in sharp contrast to the commonly observed
Hückel aromaticity of cyclic (4n+2)π-electron systems in
their singlet ground state (S0)2. On the basis of Baird’s rule,
spectroscopy with photoirradiation of
a chiral COT
derivative.8 However, direct structural observation of
Baird aromatic species has been still limited to
spectroscopy. This situation is different from the study of
Möbius and stacked-ring aromaticities, whose structural
analyses have been provided by single-crystal X-ray
diffractometry4c–e,g,5c. The main difficulty derives from the
requirement for the real-time diffraction analysis of short-
lived species in the excited state. In this work, we
overcome this difficulty by the combination of the
synthesis of a 4nπ liquid crystal (LC) molecule and the
ultrafast electron-diffraction analysis. We report a direct
structural evidence of a conformational planarization of π-
photoinduced
conformational
changes
and
photoreactivities of cyclic π-conjugated systems are
rationally explained in a variety of organic compounds3.
Compared to the other anomalous “4nπ aromaticities”,
Möbius aromaticity4 and three-dimensional stacked-ring
aromaticity5, experimental clues of Baird aromaticity have
been found from earlier stage6 mainly because the excited-
state aromaticity is reflected in the results of
photoreactions of common π-conjugated systems. Until
recently, most of the experimental insight for the excited-
state aromaticity has been indirectly obtained on the basis
of their photoreactivities.3b As latest advances, structural
expanded cyclooctatetraene (COT) and
a subsequent
packing deformation in a columnar LC phase, accompanied
by the excited-state aromaticity of its 4nπ electron circuit.
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This work also includes an advancement in the method of
(Figure 1c). The XRD analysis yielded lattice parameters of
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structural characterization of photoresponsive liquid
crystals. In spite of remarkable progress in the time-
resolved diffractometry using ultrabright and ultrashort-
pulsed X-ray9 and electron10 sources, the application has
been limited to the structural characterization of simple
isolated molecules or single crystals in excited state. In this
study, we have characterized the structural dynamics of
photoexcited LC thin films by the combinational use of
time-resolved electron diffractometry and molecular
dynamics (MD) simulations. Since the electron scattering
from organic samples is stronger than the X-ray scattering,
diffraction change can be monitored in real time with
sufficient intensity even for such a thin film that UV
excitation light can penetrate the whole materials.
a = 61.6 Å and b = 41.7 Å. The intermolecular distance in
the stacked column along the c-axis was obscured by a
broad diffraction peak (2θ ≈ 20°; d ≈ 4.4 Å) originating
from the dendritic alkyl chain moiety. This situation is
quite common for typical columnar LC materials14.
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Results and Discussion
Molecular design.
Molecular motion of the 8π COT ring has attracted
attention not only from the viewpoint of aromaticity but
from the viewpoint of materials science. COT has been
utilized for a broad range of applications such as a triplet
quencher of fluorescent conjugated polymers11, an additive
for improving the photostability of organic fluorophores in
imaging techniques12, a molecular viscosity probe13 and a
light-melt adhesive14. The conformational planarization of
the photoexcited COT ring is a key motion in their working
mechanism. Here we synthesized a columnar LC molecule
(see Figures S1–2) that is composed of the π-expanded
COT (π-COT) core unit with typical dendritic alkyl chain
moieties15 (Figure 1a). The saddle-shaped π-COT skeleton
was selected as a photoactive core unit of the LC molecule,
because it has the following unique features: 1) high
conformational flexibility due to the small internal steric
hindrance between the neighboring thiazole rings with the
head-to-tail connection16, 2) stacking ability to form a well-
defined columnar structure despite its nonplanar
molecular frame16, and 3) 8π electron system at the central
COT ring that is expected to undergo a photoinduced
conformational change into a flat form owing to the
excited-state aromaticity.17 In this photoactive columnar
LC system, the orientationally ordered saddle-shaped
molecules afford structural information for the time-
resolved diffraction analysis, and the loosely packed
structure allows the intramolecular conformational
planarization and the following intermolecular packing
deformation in the vicinity of photoexcited molecules.
Characterization of columnar liquid crystal.
The π-COT-based material exhibited a columnar LC phase
between 15 and 84°C (Figure S3), which enabled the time-
resolved structural analysis of the LC phase to be
performed at room temperature. Using polarized optical
microscopy (POM) observations, the LC texture of the
material was confirmed using crossed Nicols (bright
regions in the inset of Figure 1b). Figure 1b shows the XRD
pattern of the LC phase obtained using Cu Kα radiation (see
Figure S4 and Table S1). The LC structure was
characterized as a rectangular columnar form of C2/m, in
which the saddled molecules align on top of one other
Figure 1: Molecular structure and lattice parameters of the
liquid crystal. a, Chemical structure and stereoscopic view of
the π-COT-based LC molecule with
a saddle shaped
conformation in the ground state (S0). b, The static X-ray
diffraction pattern with several peaks indicated by arrows.
Blue arrow particularly indicates the (001) peak. The inset
figure shows POM image of the LC thin film at 80°C under
crossed Nicols. The white scale bar demonstrates 200 μm. c,
The lattice parameters in the LC phase. Rectangular columnar
phase of C2/m was assigned according to the extinction rule.
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Conformational planarization in the excited state.
NICS(1)zz values, magnetic criteria for the aromaticity21b-d
,
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were calculated to be –8.04 and –18.24 ppm at the center
of the COT ring in the planar T1 structure at the GIAO-
UB3LYP/6-311+G*//UB3LYP/6-31+G* level of theory (see
also Figures S24 and S25). By comparison with Baird
aromatic D8h COT in T1 showing NICS(1) = –10.80 and
NICS(1)zz = –32.08 ppm at the same GIAO level (with the
reported C–C bond length of 1.4063 Å17, HOMA = 0.91),
significant Baird aromaticity of π-COT in T1 was indicated.
On the other hand, the saddle-shaped S0 structure of π-COT
calculated at the GIAO-B3LYP/6-311+G*//B3LYP/6-31+G*
level showed NICS(1) = 2.76 and NICS(1)zz = 18.31 ppm. By
comparison with Hückel antiaromatic D4h COT in S0 as a
The photoinduced dynamics of the columnar LC material
consists of a sequence of structural motions at different
scales: first, a conformational change at the molecular level
(Figures 2–4) and, second, a subsequent deformation of
the local packing structure around the photoexcited
molecules in the π-stacked columns (Figures 5–8).
Inspired by the predictions of the triplet aromaticity of
parent COT3a,b,17, the saddle-to-flat conformational change
in the excited state of the π-COT core unit was calculated
using density functional theory (DFT) quantum chemical
calculations. Photochemical formation of the triplet state of
COT requires a multiple process. Upon excitation of the
tub-shaped parent COT with D2d symmetry, the higher
excited singlet states are formed (S0→Sn) and immediate
radiationless decay into the lowest singlet excited state
takes place (Sn→S1).18 Then, a rapid intersystem crossing
to the triplet state (S1→T1) is expected (see Figure S31, in
which the intersystem crossing through S1→T2→T1
scheme has been supported). Although the conformational
planarization of COT in the excited state has been studied
in relation to the triplet energy transfer,19 the
conformational dynamics in S1 has never been observed
experimentally as far as we know, probably because its
lifetime is extremely short. In this context, we performed
(TD-)DFT calculations of the π-COT core unit in the ground
and excited states. While the S0 optimized structure takes a
saddle conformation with the S–C–C–N torsion angle φ of
37° (Figures 2a,c), the optimized structures in S1 and T1
predict flat conformations with φ = 0° (Figures 2b,d for T1
and Figure S21). The CASSCF calculations also indicate the
flat conformation in S1 and T1 (Figure S27). On the
assumption that the intersystem crossing would take place
after the saddle-to-flat conformational change in S1, the
structural optimizations with fixed φ from 0° to 120° in 8°
intervals were performed in S1 (Figure 3a). The resulting
energy diagrams indicated that the S1 energy is
significantly lowered as the molecular shape flattens,
suggesting spontaneous conformational planarization in
the excited state.
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transition state showing NICS(1) = 36.28 and NICS(1)zz
=
109.43 ppm (with the reported C–C bond lengths of 1.3510
and 1.4718 Å17, HOMA = –0.08), nonaromaticity of π-COT
in S0 has been supported.
Excited-state aromaticity.
Importantly, the pronounced excited-state aromaticity of
the T1 structure was confirmed by standard aromaticity
indices, the anisotropy of the induced current density
(ACID)20, the nucleus-independent chemical shift (NICS)21,
and the harmonic oscillator model of aromaticity
(HOMA)22. In these analyses, a model structure of π-COT
bearing biphenyl groups was investigated. The ACID plot
revealed a strong diatropic (aromatic) ring current on the
π-COT core unit (Figure 2e and Figure S28). Distinct
contribution of the 8π electron circuit was confirmed along
the COT ring, while diatropic ring current in the 24π-
electron periphery including the thiazole rings is also
remarkable. The central COT ring remains uncharged
(unlike COT2+ or COT2–) according to a natural bond orbital
(NBO) charge distribution analysis (Figure S23). In
relation to this, it has been recently reported that
localization/delocalization of excited-state aromaticity
depends on each annelated ring system.23 NICS(1) and
Figure 2: Optimized structures and aromaticity indices of π-
COT core unit, in which simplified model (Ar = biphenyl) is
treated. a,b, DFT optimized structures of S0 at the B3LYP/6-
31G* and of T1 at the UB3LYP/6-31+G* level of theory. Full
optimization was performed with the structural symmetry of
C2 and C4, respectively. c,d, Bond lengths, NICS(1), NICS(1)zz,
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and HOMA values on the central COT ring of the optimized
remained in the excited state for over several-hundred-
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structures in S0 (c) and T1 (d). NICS calculations were
performed with the gauge-independent atomic orbital (GIAO)
method at the (U)B3LYP/6-311+G* level of theory. e, Enlarged
ACID plot for the T1 optimized structure. Isosurfaces (yellow)
and current density vectors (green arrows with red heads) are
shown. The current density vectors indicate a diatropic
(clockwise) ring current. The complete ACID plot is shown in
Figure S28.
picoseconds to nanoseconds, which is corresponding to the
third term in the equation (1). This long-lifetime species
was mainly observed in transient electron diffractometry
described in the next Section.
9
The HOMA values, the aromaticity index based on the bond
alternation of a focused ring system, are in agreement with
the results in the NICS calculations. The C–C bond lengths
of the eight-membered COT ring afforded HOMACOT values
of 0.65 (aromatic) in T1 while it was 0.25 (nonaromatic) in
S0. The aromatic indices in the CASSCF-optimized T1 and S0
structures, in which a simplified model structure without
the biphenyl groups was treated, also supported the
above-mentioned features (Figures S26 and S27). Namely,
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strong 8π aromaticity of the COT ring in T1 (HOMACOT
=
0.87, NICS(1) = –11.41 ppm and NICS(1)zz = –28.83 ppm)
and nonaromaticity in S0 (HOMACOT = 0.02, NICS(1) = 0.55
and NICS(1)zz = 13.85 ppm) were indicated in the CASSCF
optimized structure. In S1, significantly small bond
alternation is suggested (HOMACOT = 0.60), but the degree
is not remarkable when compared to T1 (Figure S26 and
S27). These theoretical analyses clearly indicate triplet
aromaticity of the π-COT core unit as well as
nonaromaticity in S0, and it is consistent with the
conformational planarization effectively driven by the
photoinduced excited-state aromaticity.
Spectroscopies on the molecular dynamics.
Figure 3: Transient visible transmission spectroscopy of a
π-COT-based LC thin film. a, Schematic energy diagram in
the conformational change of the π-COT core unit bearing
biphenyl groups from saddle (S0) to flat (S1 and T1) structures.
b, Time-resolved visible transmission spectroscopy on the LC
thin film using pump and probe light with the energies of 4.7
eV (λ = 266 nm) and approximately 2.1 eV (λ = 500–700 nm),
respectively. The rising signal component has a time-constant
of 2 ps (as shown in the inset), and the relaxation time-
constants are 20 and 150 ps, respectively.
We next performed UV-visible absorption spectroscopy
and transient transmission spectroscopy at various pump
and probe energies on thin films of the π-COT-based LC
material in order to confirm the optical excitation and
associated photoinduced dynamics. Although the ultrafast
dynamics of the parent COT molecule have been studied
with ionization/mass spectrometry24, direct structural
evidence of the triplet COT species has never been
obtained. Figure 3b shows the transient visible
transmittance using pump pulse of 4.7 eV (λ = 266 nm) and
probe pulse of approximately 2.1 eV (λ = 500–700 nm),
respectively (Supplementary Section III). The π-COT
molecule that absorbed UV light was immediately excited
to a far-from-equilibrium state (Sn) and transferred to the
excited state within 2 ps. Thus, the photoexcited π-COT
molecule in ~100 ps should be relaxed to the lowest
excited state; therefore, we will mainly discuss on the
photoinduced dynamics of the π-COT molecule on the
relevant timescale. The transient transmittance was fitted
with the following equation:
The conformational planarization in the excited state is
confirmed by the combination method between time-
resolved infrared (IR) vibrational spectroscopy on the LC
film and spectral simulation using DFT calculations on the
corresponding isolated molecules.7e,25 We first compared
the ground state FT-IR spectrum to the calculated spectra
of the saddle conformer of π-COT model structures in S0
with different length of alkoxy chains, and confirmed that
the propoxyl group (-OC3H7) is long enough to reproduce
the observed spectra (Figure S10 and Table S2). Time-
resolved spectra display the evolution of the molecular
vibrational modes of the planar π-COT conformer. Figure
4a shows the differential vibrational spectrum at a delay
time of 100 ps. After the photoexcitation, several peaks
and bleaches in amplitude were observed in the
vibrational spectra. The characteristic peaks e.g., at the
wavenumbers 1183, 1338 and 1489 cm–1, were well
reproduced in the calculated T1–S0 differential vibrational
spectrum (Figure 4b), which was obtained by the
⁄
ꢄ
⁄
ꢈ
ꢄ
⁄
ꢈ
∆ꢀ ꢀ ꢁ ꢂꢃexp ꢅ ꢆ ꢇꢃ ꢉ ꢂꢊexp ꢅ ꢆ ꢇꢊ ꢉ ꢂꢋ
(1)
where the first and second terms indicate exponential
decay with the time-constants of τ1 and τ2. The third term
suggests the decay of even longer timescale. Once excited
in the LC film, most of the molecules returned to S0 in τ1 =
20 ps and τ2 = 150 ps probably through a conical
intersection, but a small proportion of the molecules
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subtraction of the spectrum at the saddle conformation in
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S0 from the spectrum at the flat conformation in T1 (see
Figures S12 for more detail). The calculated T1–S0
spectrum showed better agreement with the experiment
than the calculated S1–S0 spectrum, and therefore we
assigned the excited species as T1, although the optimized
structures in S1 and T1 both take flat conformation and the
possibility of the S1 observation was not ruled out (Figures
S21 and S27). Figure 4c shows the calculated IR spectrum
of the T1 structure and colored bars indicate which part
mainly vibrates in the calculated normal mode vibrations
(Figure S13). On the basis of these vibrational assignments,
the peaks at 1183 and 1489 cm–1 as indicated by red
dashed lines in Figure 4a are related to the breathing
motions of COT and thiazole rings which are non-active in
saddle shape and active in flat structure. The peaks or
shoulders at 1230, 1310, 1338, 1418, 1442, 1578, and
1600 cm–1 as marked black dashed lines in Figure 4a are
related to the stretching modes of the alkoxy or biphenyl
group, which are presumably induced by difference in
charge distribution between the saddle S0 and planer T1
states (Figure S29, S30). The time-dependent evolution of
the peak intensity at 1489 cm–1 is shown in Fig. 4d.
Analysis of the time-dependent evolution and decay at
each characteristic peak indicated the same molecular
dynamics as those observed in the transient visible
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transmission spectroscopy (Figure S6–S9). Namely,
a
saddle-to-flat conformational change of the π-COT core
unit in 2 ps and the relaxation to S0 in τ1 = 10–20, τ2 = 150
ps and longer timescale. Since the 10–20 ps decay was also
observed in solution phase (Figure S10), it was assigned to
the relaxation dynamics of isolated molecules, which are
generally located at the surface, interface or grain
boundaries of the liquid crystals. In addition, the 150 ps
and longer-lifetime decay modes were interpreted as the
relaxation dynamics of photoexcited molecules in the LC
structure. The difference in these decay modes would
originate from the different local environments of excited
molecules, and we assigned the long-lifetime species to the
photoexcited molecule in the stacked column (see next
Section).
Figure 4: Ultrafast time-resolved IR spectroscopy of a π-COT-
based LC thin film. a b, Differential IR vibrational spectrum
measured with a time-delay of 10 ps compared to the
calculated differential vibrational spectrum (T1–S0) The
scaling factor of the calculated spectra is 0.97. c, Vibrational
peak assignment of the T1 spectrum. The peaks are classified
as vibrational modes of COT and thiazole rings, alkoxy group
or biphenyl group. d, Time evolution of the peak intensity at
the representative wavenumber of 1489 cm–1. The fast (20 ps)
and slow (150 ps) decays are identical to the dynamics
observed in isolated molecules and molecules in the LC phase.
Structural observation of the photoexcited liquid
crystal.
The electron diffraction measurements were performed on
a ~100-nm-thick LC thin film in transmission geometry.
The two-dimensional electron diffraction patterns of the
LC thin film showed an ill-defined broad halo ring
originating from the long alkyl chains (Figure 5a), similar
to the wide XRD peak observed at diffraction angle 2θ ≈
20° (Figure 1b). This broad halo, which is typically
observed in columnar LC materials, is composed of a
number of diffraction peaks produced by the long alkyl
chains and several peaks originating from the stacked π-
COT core moieties. Under photoirradiation, structural
deformation is induced around the photoresponsive π-COT
core moieties, and then small peak modulation occurs in
the diffraction pattern. By subtracting the initial diffraction
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pattern from that obtained 500 ps after UV pulse details in Methods and Supplementary Section V). Shape of
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irradiation, we extracted the modulated diffraction pattern
induced by the UV irradiation. The resulting differential
diffraction pattern was well defined with clearly
observable rings (Figure 5b). Radial averages of the
differential diffraction patterns at –50, 100, 300 and 500
ps after the UV pulse irradiation are presented in Figure
6a. The signal-to-noise ratio can be drastically modified by
the subtraction of the diffraction pattern before the
photoexcitation from that after the photoexcitation, in
which negative and positive peaks are obtained. (See
Figure S16 in more detail). In the differential time-
resolved electron diffractometry, molecular dynamics in
periodic structures can be detected sensitively rather than
the dynamics of isolated molecules. Therefore, the
conformational dynamics of excited molecules in the
stacked column can be selectively monitored. The negative
peaks (marked with blue arrows) originate from the initial
structure that disappeared upon photoirradiation, while
the positive peaks (marked with red arrows) indicate the
formation of a new ordered structure. Since the time-
evolution was relatively slow, the packing deformation
should be induced by the long-lifetime species observed in
the spectroscopic measurements.
the simulated diffraction patterns were almost constant
with 2.9, 4.0, and 6.7%ex, although the peak intensity was
dependent on this percentage (Figure 6b and Figure S17).
The MD simulation well reproduced most of the peaks
experimentally observed in the differential electron
diffractometry (Figure 6a).
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Figure 6: Ultrafast time-resolved electron diffraction of a π-
COT-based LC thin film. a, Differential electron diffraction
pattern at –50, 100, 300, and 500 ps. Red and blue arrows
indicate positive and negative peaks. b, Simulated differential
electron diffraction pattern on the basis of the MD calculation
of columnar π-stacked structure containing 2.9% of excited
molecules.
Figure 5: Electron diffraction patterns from a π-COT-based LC
thin film. a, The electron diffraction pattern without
photoexcitation. b, Differential diffraction pattern obtained
with and without photoexcitation.
Combinational analysis of the time-resolved electron
diffractometry and the MD simulation suggested that the
conformational planarization takes place more slowly (τ ~
300 ps) for the excited molecules sandwiched by the
saddle-shaped molecules (Figure 7a), and that the
planarized molecule and neighboring unexcited molecules
starts to rotate (from t ~ 200 ps with τ ~ 125 ps) toward
opposite direction in the stacked column due to the steric
repulsion between them (Figures 7b and 8). In this
columnar structure, intermolecular distance along the c-
To understand the structural dynamics observed by the
time-resolved electron diffraction, we performed MD
simulations. To prepare suitable model for MD
simulations, we considered the number of incident
photons (1.2 mJ/cm2) and molecules per unit area in
advance. Approximately 25% of the π-COT molecules
absorb UV light and undergo the electronic transition
leading to the lowest excited state. Among them, 7–8% of
the photoexcited molecules (at most 2% of all the
molecules of the material) remain in the excited state (Ex)
even after 300–500 ps as observed in the time-resolved
visible and mid-IR spectroscopies in Figures 3b and 4c,
which is corresponding to the third term in the equation
(1). Considering this situation, the differential electron
diffraction patterns were simulated for the stacked π-COT
core moieties containing small ratio of excited species
(%ex), in which this percentage was set to be 20, 6.7, 4.0,
and 2.9%ex (Figure S17). Here, we first considered two
sets of stacked π-COT pentamers arranged with the order
of saddle-saddle-flat-saddle-saddle conformers (S0–S0–Ex–
S0–S0) with 20%ex and all saddle conformers (S0–S0–S0–S0–
S0) with 0%ex, then these pentamers were mixed at
respective ratios in the columnar stacking structure (See
axis was determined as 4.55
Å through fitting of
experimental (Figure 6a) and simulated (Figure 6b)
differential diffraction patterns, then the π-stacking
distance (d
) between the biphenyl moieties was
–
π π
estimated as 3.7 Å. The dynamics of the negative peak
(Figure 7a; Q = 0.245 Å–1) suggested the local destruction
of the π-stacked molecular ordering by the conformational
planarization of excited species, starting just after the
photoexcitation with the time constant of 300 ps. On the
other hand, the dynamics of the broad positive peak
(Figure 7b; Q = 0.37 Å–1) starting to increase after 200 ps
from the photoexcitation suggested the rotation of the
planar excited molecule and the neighboring saddle
molecules. This rotational behavior was induced by an
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indicate the periodicity of the stacked molecules (4.55 Å) as
intermolecular steric repulsion between the rigid biphenyl
moieties (Figure 8). As a result, both the conformational
planarization of the π-COT molecule and the subsequent
packing deformation were directly evidenced by
diffractometry, strongly supporting the exhibition of the
Baird aromaticity in the excited state.
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well as the π-stacking distance between the biphenyl moieties
(3.7 Å). The yellow arrows indicate the displacement of
molecules.
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Methods
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7
UV-Vis transient absorption spectroscopy
8
9
We performed the conventional transient absorption
spectroscopy on the π-COT molecule in LC phase. The
pump pulses are modified its photon energy into 400 or
266 nm by BBO crystal(s). The probe pulse is focused into
a sapphire window to generate white light (500–700 nm).
The two optical pulses are focused onto the sample, and
the transmitted white probe light is dispersed by the
spectrometer and detected with the Si photodiode. The
incident fluence of the pump light was 1 mJ/cm2. The
sample was spread on bulk CaF2 substrate, melt at 100°C
on a hotplate, and cooled gradually to room temperature.
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Time-resolved mid-IR vibrational spectroscopy
UV (266 nm) pump and mid-IR (1050–1700 cm-1) probe
time-resolved spectroscopy were performed in the
transmission mode on π-COT molecule in solution (1 mM
in CH2Cl2 solvent) and in LC phase coated on CaF2
substrate. The experimental details of this optical pump-
probe setup are presented elsewhere25. The incident angle
of the pump and probe light was set nearly parallel to the
surface normal of the sample. The pulse durations of the
UV and mid-IR pulses were 100 fs and <1 ps, respectively.
The repetition rate and the incident fluence of the UV
pump pulse were 500 Hz and 1 mJ/cm2, respectively.
Figure 7: Time evolution of the electron diffraction peaks at
the Q-values of 0.245 (a) and 0.37 Å–1 (b). Here, the Q-value is
defined as the reciprocal number of the lattice distance (d).
Conclusion
Time-resolved electron diffraction
In conclusion, structural monitoring of fast conformational
planarization of π-expanded COT molecules in the excited
state has been realized in a columnar LC phase. The
excited-state aromaticity exerted in the short-lived species
was strongly evidenced experimentally and theoretically.
The molecular motion triggered by the excited-state
aromaticity produces a nanoscale torque for subsequent
structural deformation in a condensed phase, which can be
used as the working mechanism of photoresponsive
materials.14,26 The insights obtained here hold out the
prospect that the excited-state aromaticity will be a key
concept for designing new photofunctional soft materials3a.
The experimental setup of the compact DC-accelerated
electron diffraction is provided elsewhere27. UV (266 nm)
pump was focused to a 210-μm spot size on the ~100 nm
thick film of π-COT molecule. The incident laser fluence
was 1.2 mJ/cm2. From the transmission and reflectivity of
the sample measured to be 40% and 30% respectively,
absorption fluence was determined to be 0.36 mJ/cm2. The
acceleration voltage of the probe electron pulses was 75
keV under a DC electric field. Photoinduced structural
changes inside the material were investigated with
electron pulses containing 2 × 104 electrons (3 fC) confined
to a 100-μm-diameter spot incident on the sample. The UV
optical pulse to generate electrons was stretched to be
>500 fs by a 25-mm-thick fused silica plate on purpose.
The pulse duration of the electron beam was ~1 ps.
Diffracted and directly transmitted electrons were focused
with a magnetic lens onto a 1:2 fiber-coupled charge-
coupled device camera coated with a P43 (Gd2O2S:Tb)
phosphor scintillator. To acquire one electron diffraction
image, 1 × 104 shots of electron pulses were collected at
repetition rates of 500 Hz. The sample solution in
chloroform (10 mg/ml) was spin-coated on SiN thin (30
nm) membrane. The thin sample film was melt on a
hotplate, and cooled gradually to room temperature.
Figure 8: Structural dynamics of the photoexcited columnar
LC. The dynamics of the columnar LC structure were observed
using time-resolved electron diffraction. The white arrows
Synthesis
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Journal of the American Chemical Society
Static characterization: Fig. S3, S4, Table S1
Optical measurements: Fig. S5–S9
Mid-IR vibrational spectroscopy: Fig. S10–S15, Table S2, S3
Time-resolved electron diffraction: Fig. S16–S18, Table S4–
S7
The π-COT-based LC compound was synthesized by the
Suzuki-Miyaura cross coupling reaction of cyclic 2-(4-
chlorophenyl)thiazole tetramer and the 3,4,5-
tridodecyloxyphenyl boronic acid using Pd2(dba)3·CHCl3
and the Buckwald phosphine ligand of XPhos28.
Purification was performed by repeated silica gel column
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Calculations: Fig. S19–S34, Table S8–S25
5
chromatography
chromatography (HPLC). The isolated yield was 78% (see
the Supplementary Section I).
and
high
performance
liquid
6
7
8
AUTHOR INFORMATION
Corresponding Authors
9
hadamasaki@okayama-u.ac.jp, s_saito@kuchem.kyoto-u.ac.jp,
shigeta@ccs.tsukuba.ac.jp, konda@chem.kyushu-univ.jp
Density functional theory (DFT) calculations
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The energy profile of the π-COT core unit was calculated at
B3LYP/6-31+G* level using Gaussian0929, in which the
structural restraint was set by the modredundant keyword
with the structural symmetry of C2. In addition to the DFT
calculations, the full structural optimizations in S0, S1, T1
and T2 of a π-COT were performed in CASSCF(8, 8)/6-31G*
level using Gaussian09. The NICS ((U)B3LYP/6-311+G*)
and HOMA values were calculated using the DFT or
CASSCF optimized structures. The ACID plots are based on
structures and wavefunctions optimized at the UB3LYP/6-
31+G* level of density functional theory. The ACID scalar
fields were calculated using our program at an isosurface
value of 0.03.20,30
Funding Sources
Japan Science Technology Agency (JST), PRESTO, Grant
numbers: JPMJPR13KD, JPMJPR12K5, JPMJPR16P6.
JSPS KAKENHI, Grant Numbers: JP15H02103, JP17K17893,
JP15H05482, JP17H05258, JP26107004. JP17H06375
ACKNOWLEDGMENT
We thank Prof. Takashi Kato at the University of Tokyo, Prof.
Yoshiaki Uchida at Osaka University, Prof. Shin-ya Koshihara
at Tokyo Institute of Technology for valuable discussions, Dr.
Sercan Keskin at the University of Hamburg for the
measurement of static electron diffraction, and Dr. Yumi
Nakaike for the measurement of DSC.
Molecular dynamics (MD) simulation
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ASSOCIATED CONTENT
Supporting information is available free of charge on the ACS
Publishing website at DOI:
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Ottosson, H.; Aida, T.; Itoh, Y. Nat. Commun. 2017, 8, 346.
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