Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non-radiative Decay Pathway

Article abstract of DOI:10.1002/anie.202000943

To broaden the application of aggregation-induced emission (AIE) luminogens (AIEgens), the design of novel small-molecular dyes that exhibit high fluorescence quantum yield (Φ_fl) in the solid state is required. Considering that the mechanism of

Full text of DOI:10.1002/anie.202000943

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Accepted Article
Title: Bridged Stilbenes: AIEgens Designed via a Simple Strategy to
Control the Non-radiative Decay Pathway
Authors: Riki Iwai, Satoshi Suzuki, Shunsuke Sasaki, Amir Sharidan
Sairi, Kazunobu Igawa, Tomoyoshi Suenobu, Keiji Morokuma,
and Gen-ichi Konishi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000943
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10.1002/anie.202000943
Angewandte Chemie International Edition
RESEARCH ARTICLE
Bridged Stilbenes: AIEgens Designed via a Simple Strategy to
Control the Non-radiative Decay Pathway
Riki Iwai,[a] Satoshi Suzuki*[b] Shunsuke Sasaki,[c] Amir Sharidan Sairi,[a] Kazunobu Igawa,[d] Tomoyoshi
Suenobu,[e] Keiji Morokuma**[b] and Gen-ichi Konishi*[a]
Celebration for the 20th anniversary of aggregation-induced emission (AIE)
Abstract: To broaden the application of aggregation-induced
emission (AIE) luminogens (AIEgens), the design of novel small-
molecular dyes that exhibit high fluorescence quantum yield (Φfl) in
the solid state is required. Considering that the mechanism of AIE
can be rationalized based on steric avoidance of non-radiative decay
pathways, a series of bridged stilbenes was designed, and their non-
radiative decay pathways were investigated theoretically. Bridged
stilbenes with short alkyl chains exhibited a strong fluorescence
emission in solution and in the solid state, while bridged stilbenes
with long alkyl chains exhibited AIE. Based on this theoretical
prediction, we developed the bridged stilbenes BPST[7] and DPB[7],
which demonstrate excellent AIE behaviour.
of intramolecular rotation’ (RIR) model, the ‘restriction of
intramolecular motion’ (RIM) model,[7] or by the ‘restricted
access to conical intersection’ (RACI) model.[8]
In our previous studies, we discovered that bis(N,N-
dialkylamino)anthracenes (BDAAs) display AIE.[9,10] We have
reported that the mechanism of AIE for BDAAs can be
explained as follows: 1) the ring-puckered conical intersection
(CI) is lowered in energy via rotation of the amino group, which
leads to internal conversion rather than emission in solution; 2)
fast internal conversion in solution results in a low Φfl in solution;
3) given that ring puckering is prohibited in aggregated states,
internal conversion is inhibited and therefore the Φfl in
aggregated states is enhanced.[10] The term “aggregation-
induced emission” refers to the induction of luminescence when
aggregates form in solution. This definition does not include
explanation of the non-radiative decay in solution. From the
theoretical investigation of the photophysical processes of
BDAA exhibiting AIE behavior in a single molecule,[10] we
concluded that the essence of AIE should be explained by the
“environment-responsive non-radiative decay” model, rather
than an ambiguous phenomenology which involves obtaining
luminescence by decreasing mobility in the aggregate state.
Therefore, the rational design strategy for AIEgens from the
viewpoint of theoretical chemistry is the control of conical
intersection accessibility (CCIA).
Introduction
Over the last two decades, aggregation-induced emission
(AIE)[1] has contributed to great developments in the fields of
materials science,[2] analytical chemistry,[3] and life sciences.[4] In
addition to conventional AIE-active compounds, such as
derivatives of tetraarylsilole[5] and tetraphenylethene,[6] recent
studies have found a wide variety of new AIE-active compounds.
However, to further broaden the potential applications of AIE
luminogens (AIEgens), the design of novel small-molecular dyes
that also exhibit high fluorescence quantum yields (Φfl) in the
solid state is required. The mechanism of AIE has been
thoroughly investigated, which allows us to approach this task
via rational molecular design. The characteristic phenomenon
observed in AIEgens, i.e., quenching in solution and emitting
light in the solid state, can be explained either by the ‘restriction
The rational molecular design of AIEgens based on the RACI
model or CCIA strategy has so far been limited to the design of
dialkylaminoarene derivatives.[10,11] In the present study, we
propose a paradigm shift in AIEgen design. Recent theoretical
investigations have revealed that it is possible to analyze
photophysical processes using a potential energy surface (PES)
in the same manner as a classical thermal chemical reaction.[12]
Controlling access to the CI enables the selective formation of
fluorescent and non-fluorescent molecules, as if controlling
access to the transition states determine reaction rate and
selectivity of products. AIEgens that arise from the CCIA
strategy should satisfy the following three conditions: 1) a CI that
is low in energy whilst in solution in order to ensure that the
AIEgen has a low Φfl in solution; 2) a non-radiative decay
pathway that is inhibited by the surrounding molecules when the
AIEgen is in the solid state or an aggregate. The AIEgen should
also possess a molecular motion of large amplitude that gives
rise to a non-radiative decay pathway; 3) the orbital overlap in
the aggregate form should be low in order to avoid aggregation-
caused quenching (ACQ). Thus, by identifying the non-radiative
decay pathway of the AIEgen, one can understand how an
AIEgen acquires a low Φfl in solution and a high Φfl in the
aggregate form. Therefore, the rational design of AIEgens that
arise from the CCIA strategy can be realized as follows: 1)
search for π-conjugated systems as molecular core with known
[a]
[b]
[c]
Prof. Dr. G. Konishi,* A. S. Sairi, R. Iwai
Department of Chemical Science and Engineering
Tokyo Institute of Technology
2-12-1-H-134 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
E-mail: konishi.g.aa@m.titech.ac.jp
Dr. S. Suzuki,* Prof. Dr. K. Morokuma
Fukui Institute for Fundamental Chemistry
Kyoto University
Takano-Nishibiraki-cho 34-4, Sakyou-ku, Kyoto 606-8103, Japan
E-mail: suzuki.satoshi.8v@kyoto-u.ac.jp
Dr. S. Sasaki
Université de Nantes, CNRS
Institut des Matériaux Jean Rouxel, IMN
F-44000 Nantes, France
E-mail: shun.sasaki213@gmail.com
Prof. Dr. K. Igawa
Institute for Materials Chemistry and Engineering
Kyushu University
Fukuoka 816-8580, Japan
Prof. Dr. T. Suenobu
Department of Advanced Science and Biotechnology
Osaka University
Osaka University, 2-1 Yamada-oka, Suita, Osaka 565, Japan
Prof. Keiji Morokuma was passed away at 27 Nov 2017.
[d]
[e]
[**]
1
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10.1002/anie.202000943
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RESEARCH ARTICLE
non-radiative decay pathways; 2) structurally or electronically
tune the system to lower the CI of the molecule; 3) design the
molecular structure so that its non-radiative decay pathway has
a molecular motion with large amplitude. In contrast, the
molecular design of AIEgens using the RIR or RIM model aims
to suppress the non-radiative decay pathway of the molecule in
the solid state.[7] This is achieved via introduction of a large
to be AIEgens. We will show that the synthesized compounds
show good agreement with the computational prediction.
Furthermore, unlike stilbene, π-extended stilbenes exhibit
fluorescence in the visible region, which would be attractive for
applications in functional materials.
number of biaryl bonds or
a bridged structure between
Results and Discussion
neighboring aryl groups to a molecular core. The design strategy
that we employ here attempts to determine the non-radiative
decay pathway in advance, before we introduce bulky groups at
a specific position so that this pathway is blocked in the solid
state. Using this strategy, we can identify prospective AIEgens
via theoretical calculations and subsequently develop simple
high-performance AIEgens.
Initially, we investigated the photophysical properties of a series
of stilbenes and bridged stilbenes. We calculated the
photophysical properties, synthesized the compounds (Figure 1),
experimentally measured their photophysical properties, and
compared the experimental results with those of the theoretical
calculations.
We chose a π-extended stilbene as the molecular skeleton, as
the non-radiative decay pathway of stilbene[13] and that of
tetraphenylethene (TPE) proceeds via flipping of the double
bond.[14] Accordingly, we focused on this pathway and designed
a molecule that is mechanically constrained around the stilbene
double bond. This approach allowed us to control the energy
level of the CI of the stilbene moiety. As reported previously,[15]
by bridging one end of the C=C bond to one of the two phenyl
rings, it is possible to create a structure similar to that of indene.
The formation of this tightly twisted C=C structure is not
energetically favorable and destabilizes the CI. The
corresponding dye exhibits a high-energy CI, which renders the
non-radiative decay less likely to occur. Consequently, the dye
exhibits strong fluorescence in solution and in the solid state.[15]
In this study, we demonstrate how a series of bridged stilbenes
with different bridge lengths exhibit different photophysical
properties. We show that quantum chemical calculations are
able to predict that a loosely bridged C=C bond should have a
low CI and thus almost no fluorescence in solution. Given that
the twisted structure of the C=C bond should be suppressed in
the solid state, loosely bridged stilbenes would also be expected
Theoretical Investigation of the Non-radiative Decay
pathway for the Stilbenes and the Bridged Stilbenes
In the beginning, we determined the non-radiative decay
pathway of stilbene (ST). It should be noted that a twisted-
pyramidal conical intersection has been reported on the PES of
stilbene.[13]
The minimum energy conical intersections (MECIs) of ST were
calculated using spin-flip time-dependent density functional
theory (TD-DFT) at the Becke-Half-and-Half LYP/6-31G+(d)
level of theory using the branching-plane-updating method.[16]
The Franck-Condon state (FC), the local minimum near the FC
(S1min), and the intermediate near the CI (INT) were located
using the same method. The transition state (TS) connecting
S1min and INT was initially located at the TD-B3LYP/6-31+G(d)
level of theory, but subsequently updated to the SFTD-
BHHLYP/6-31G+(d) level of theory using TD-B3LYP Hessian
matrix calculations. All calculations were performed using
GAMESS (2019 Sep.) and Gaussian 16.C01. [17,18]
2
n-5
2
n-5
DPB[7]
PST
ST
BST[n] (n = 5-7)
BST[7]-tBu
BPST[ ]
n
(n = 5-7)
2
DPST
BDPST[7]
Figure 1. Structures of stilbenes and bridged-stilbenes.
We located the CI of ST and connected the most important
points on the PES. As shown in Figure 2(a), excited ST initially
relaxes to a local minimum (S1min), where fluorescence occurs
with an oscillator strength of ~0.6. After passing through a low
barrier (7.88 kJ/mol), an intermediate structure INT is formed
near to the CI. This structure has a small oscillator strength
(~0.01), and thus this state can be thought of as a “dark state”.
After passing the INT, the S1/S0 MECI was found at 25.16 kJ/mol
higher than the S1min. The structure of the MECI is a typical
twisted-pyramidal structure (Figure 3). Note that this type of CI
can lead E/Z isomerization. Because whether which isomer
generates after passing through CI does not affect to the
2
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10.1002/anie.202000943
Angewandte Chemie International Edition
RESEARCH ARTICLE
fluorescence quantum yield, we do not show isomerization
pathway in the figure.
Subsequently, we calculated the non-radiative decay pathways
for BST[5], BST[6], and BST[7], and Figure 2(b) shows the
energy diagram for the non-radiative decay process for each of
these compounds. The FC for BST[7] is higher in energy than
that of BST[5] and BST[6], which can be rationalized in terms of
the planarity of the π-conjugation observed in these molecules.
In contrast to planar BST[5], BST[6] and BST[7] exhibit a
phenyl group that is shifted out of the plane (Table 1). This shift
causes a decrease in planarity of the molecule, which reduces
the effective conjugation length, especially in BST[7] (Figure 4),
and thus causes a shorter absorption wavelength.
To govern the photophysical properties of an AIEgen, the non-
radiative decay pathway needs to be controlled. In the case of
stilbene-based molecules, the simplest approach to achieve this
is to mechanically restrict the twisting angle of the C=C bond.
With this in mind, we considered a different series of compounds
based around a bridged stilbene core (BST[n]). Here, n denotes
the size of the n-membered ring used to restrict the movement
of the C=C bond. Depending on the length of the alkyl chain,
bridging phenyl groups gradually restrict the twisting angle of the
C=C bond.
Figure 2. Energy diagram of (a) ST and PST, (b) bridged-stilbene BST[n], and (c) bridged-phenylstilbene BPST[n].
Table 1. Geometrical parameters of BST[n].a
Dihedral angle
(degree)
1-2-3-4
2-3-4-5
3-4-5-6
BST[7]
BST[6]
BST[5]
Dihedral
angle
1-2-3-4
2-3-4-5
3-4-5-6
-14.8
0.9
218.4
-14.6
2.7
149.7
0.0
0.0
180.0
aThe atom-numbering scheme is shown in Figure 3.
Figure 3 a) Structures of the CIs of ST and BST[5-7]. Twisted pyramidal
structure of C=C double bonds are highlighted. b) Definition of atom-
numbering for Table 1.
Figure 4 Structure of BST[7] in the FC state.
3
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10.1002/anie.202000943
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RESEARCH ARTICLE
The energy of the MECIs descends in the order BST[5] >
BST[6] > BST[7]. Although all these structures are similar, the
shorter alkyl chain found in BST[5] is more rigid than those
found in BST[6] and BST[7], and thus raises the energy of the
twisted-pyramidal CI (Figure 3). Conversely, the longer alkyl
chains observed in BST[6] and BST[7] keep the energy of the
CI low. A similar trend can be observed in the calculated Φfl in
solution. The low barrier required to form the twisted-pyramidal
MECI in BST[7] (9.59 kJ/mol) would suggest that BST[7] easily
undergoes non-radiative decay, which would amount to a lowΦfl
in solution. Conversely, the PES of BST[6] is different from that
of BST[7], i.e., the MECI for BST[6] resides at an energy level
that is 43.26 kJ/mol higher than that of the S1min, which is still
by 11.11 kJ/mol lower than the FC state. Thus, the non-radiative
decay rate constant for BST[6] would be smaller than BST[7]
and Φfl for BST[6] would be larger than BST[7]. The MECI of
BST[5] is so high that it can be feasible expected that its non-
radiative decay is very difficult to take place. In summary, it can
be expected on the basis of the results of the theoretical
calculations carried out in this study that the low MECI of BST[7]
would render BST[7] a good prospective core structure for
advanced AIEgens. BST[7] can be expected to have an
fluorescence in the visible region is preferable. Therefore, we
decided to examine bridged stilbenes with π-extended structures.
Subsequently, we examined a series of compounds that contain
three benzene rings, 4-phenylstilbene (PST) and BPST[n], and
found that the extended π-conjugation observed in these
molecules greatly affects their absorbance and fluorescence
properties.
The shapes and vibrational structures of the absorption and
fluorescence spectra of BST[n] are similar to previously
[19]
reported spectra of ST
and the π-extended ST, poly(p-
phenylene-vinylene).[20] PST and BPST[n] are related in a
similar fashion. Accordingly, the lowest energy absorptions of
PST and BPST[n] (n = 5-7) can be assigned to the allowed 1Ag
→ 1Bu transition.[22]
Comparing the maximum absorption wavelengths (λabs) of PST
and BPST[n] in solution, only BPST[n] exhibits
a large
hypsochromic shift. In contrast, the maximum fluorescence
wavelengths (λfl) of PST and BPST[n] remain virtually
unchanged. The large Stokes shifts (7200 cm-1) observed for
BPST[7] is in good agreement with the calculated results
(Figure 2b). The FC state of BPST[7] is higher than that of
BPST[5] and BPST[6], albeit that the S1min state of all these
compounds are similar. The structural change between the FC
and S1min states of BPST[7] was confirmed by quantum
calculations (Figure S57).
accessible non-radiative decay pathway due to
a large-
amplitude molecular motion and therefore exhibit a low Φfl in
solution.
As the calculated fluorescence wavelength of the BST[n] series
is too short, we also considered bridged π-extended stilbenes
(BPST[n]) to achieve emission at visible wavelengths. The
calculated fluorescence wavelength of BPST[7] (370 nm) is
within the visible region. The energy diagram for the BPST[n]
series follows the same trend as that of the BST[n] series.
Therefore, BPST[7] would be expected to be a good AIEgen
candidate. A more detailed discussion of the energy diagrams is
given later (vide infra).
Diffuse-reflectance and fluorescence spectra of polycrystalline
PST and BPST[n] are shown in Figure S50. Both the λabs and λfl
values of polycrystalline PST and BPST[n] are bathochromically
shifted relative to those in dilute solution. The same
phenomenon has been reported for the prototypical AIEgen
tetraphenylethene (TPE).[6] In the case of BPST[n],
intermolecular electronic interactions or conformational changes
such as planarization-induced π-extension can be expected,
which we will discuss during the x-ray crystallographic analysis
(vide infra).
Photophysical Properties of the Bridged Stilbenes
The structures and photophysical properties (absorption,
fluorescence in solution and the solid state (polycrystalline
solids), as well as Φfl values) of the synthesized stilbenes and
bridged stilbenes are summarized in Figures 1 and S48 and
Table 2. The synthetic methods used and all characterization
data, including all spectra, are listed in the Supplementary
Information. In some cases, the general synthetic route was not
feasible. In the case of BPST[7], when 4-biphenylboronic acid
was used in the Suzuki-Miyaura cross-coupling reaction,[19] p-
quarterphenyl, a compound that shows strong fluorescence in
solution, was obtained as a byproduct. In order to avoid the
formation of p-quarterphenyl, we performed a stepwise addition
of the benzene rings.
Unlike ST, PST exhibit strong fluorescence in the blue region of
the visible spectrum in solution and the solid state (Фsol = 0.64;
Фsolid = 0.72), i.e., PST is unlike ST not an AIEgen. Similarly,
BPST[5] and BPST[6] emit light in solution and the solid state.
In contrast, BPST[7] shows perfect AIE behavior (Фsol = 0.01;
Фsolid = 0.95) with negligible emission in solution and high
emission in the solid state. DPB[7], which extends the π-
conjugation of the system from a different position to BPST[7],
also shows excellent AIE properties (Фsol = 0.02; Фsolid = 0.75).
However, BPST[8] showed little fluorescence in solution or in
the solid state (Фsol = 0.004; Фsolid = 0.04). In regards to the
absorption spectrum (λmax), compared to PST, BPST[5] has
increased planarity and shows longer π-conjugation length,
BPST[6] has a similar degree of increased planarity, and both
BPST[7] and [8] has decreased planarity and exhibits shorter π-
conjugation length. The effect is reflected in the fluorescence
spectra of these molecules (Table 2), i.e., λabs decreases with
decreasing planarity of the molecule. BPST[8] exhibits not only
the smallest λabs, but also the smallest absorption coefficient (ε),
which is most likely due to a large twist around its C=C double
bond. The shape of the fluorescence spectra of polycrystalline
BPST[8] is similar to those of BPST[n] (n = 5-7), albeit that the
Φfl is very small (< 0.01). The reasons why BPST[8] does not
exhibit fluorescence, neither in solution nor the solid, are that the
We first examined a series of compounds that contains just two
benzene rings, i.e., ST and BST[n]. While ST exhibits typical
AIE behavior (Фsol = 0.01; Фsolid = 0.62), BST[5] shows a strong
fluorescence in the UV region in solution and in the solid state
(Фsol = 0.40; Фsolid = 0.79). BST[6], which contains a more
flexible alkyl chain, exhibits aggregation-induced emission
enhancement (AIEE) behavior (Фsol = 0.06; Фsolid = 0.26). While
BST[7] is unfortunately a liquid at room temperature, BST[7]-
tBu is
a solid and shows AIE behavior. However, the
fluorescence of the BST derivatives is in the UV region and,
especially from an analytical and materials science perspective,
4
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RESEARCH ARTICLE
Figure 5. (a) UV absorption spectra of PST, BPST[5], BPST[6], and BPST[7]
in THF (The concentration of each compound is 1.0
Fluorescence spectra of PST, BPST[5], BPST[6], and BPST[7] in THF (The
concentration of each compound is 1.0 × 10-5 M). Small peaks around 330 nm
were assigned to Raman scattering.
twisted structure is sufficiently large and that the conjugated
system does not spread. In other words, this behavior can be
explained by its twisted structure, which leads relatively smaller
oscillator strength besides high accessibility toward MECI.
Based on the results of the fluorescence spectra, it can be
concluded that the flexible bridge provided by a 7-membered
ring leads to the desired AIE effect in PST.
×
10-5 M). (b)
We have also measured the fluorescence lifetime of PST and
BPST[n] (n = 5-7) in THF and determined the non-radiative (knr)
and radiative transition rates (kr) (Table S2). The obtained knr of
PST and BPST[n] (n = 5-7) are 3.3 × 108, 0.9 × 108, 1.4 × 108,
and 3.7 × 109 [s-1], respectively. The kr of PST and BPST[n] (n =
5-7) are 8.0 × 108, 7.2 × 108, 6.6 × 108, and 4.8 × 107 [s-1],
respectively. Among these compounds, BPST[7] shows kr ≪knr,
while the other dyes exhibit kr ≻ knr. In other words, the reason
that BPST[7] shows very weak fluorescence is due to its high knr.
Comparing PST, BPST[5], and BPST[6] with BPST[7] shows
that the knr value of the latter is large albeit that the ε and kr
values are small. Therefore, the conformation of BPST[7] does
not decrease the planarity and restrict the mode leading to MECI.
An aggregation experiment of BPST[7] was performed, albeit
that the Φfl value was low (0.22). Furthermore, the fluorescence
wavelength (λmax) is shorter than that of the solid due to the
shorter lengths of the aggregates of the conjugated system
(Table S1).
4,4’-Diphenylstilbene (DPST) contains four benzene rings and
shows strong fluorescence in solution, which is quenched by
intermolecular interactions in the solid state (Фsol = 0.67; Фsolid
=
0.16). The fluorescence behavior of DPST is similar to that of
poly(p-phenylene vinylene) (PPV).[23] In contrast, the bridged
form of DPST, BDPST[7], shows AIEE behavior (Фsol = 0.12;
Фsolid = 0.87), which suggests that the 7-membered ring structure
contributes to the AIE effect, but does not compensate the effect
of long conjugated systems such as PPV.
Table 2. Photophysical properties of the stilbenes and bridged stilbenes used in this study in solution and in the solid state.
Number of
benzene rings
ε
λabs
[nm]
λfl (THF)
[nm]
λfl (solid)a
[nm]
Φfl
(THF)
Φfl
(solid)
Entry
[M-1 cm-1]
ST
2
2
2
2
2
3
3
3
3
3
3
4
4
23000
25000
22000
22000
25000
32000
42000
37000
26000
22000
36000
49000
46000
297
315
306
285
288
317
332
324
304
288
309
342
321
350
357
365
308
360
381
388
393
386
341
385
405
408
382
417
399
0.01
0.40
0.06
0.001
0.01
0.71
0.89
0.83
0.01
0.004
0.02
0.67
0.12
0.62
0.79
0.26
BST[5]
BST[6]
BST[7]
b
b
-
-
BST[7]-tBu
PST
380
434
434
445
418
405
413
437
431
0.11
0.72
0.62
0.82
0.95
< 0.01
0.75
0.16
0.87
BPST[5]
BPST[6]
BPST[7]
BPST[8]
DPB[7]
DPST
BDPST[7]
a polycrystalline solids. b BST[7] is a liquid at room temperature.
5
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The single-crystal x-ray structure of BPST[7] is shown in Figure
6. As the adjacent π-planes of molecules of BPST[7] do not
adopt a face-to-face conformation, these do not resemble H-type
aggregates. On the other hand, the excitation spectrum of
BPST[7] in the crystal is red-shifted compared to the absorption
spectrum of BPST[7] in solution (Figure S48). Considering the
maximum wavelengths of absorption and fluorescence in
solution and the polycrystalline solid, the effect of excitonic
coupling or excimer formation could be one possible explanation
for the observed phenomenon. The details of the photophysical
process of polycrystalline BPST[7] will be investigated in the
future.
the solid state. As shown in Figure 3, the twisted-pyramidal CI
accompanies
a large movement of the phenyl groups.
Suppression of this large motion in the aggregate form causes
AIE or AIEE.
Subsequently, we examined the PST and BPST[n] series. The
calculated order of the MECIs, BPST[5] > BPST[6] > BPST[7],
corresponds to the order of the Φfl observed in solution.
However, different from the ST/BST[n] series, PST and
BPST[6] are emissive in solution. To understand the difference
between the ST/BST[n] series and the PST/BPST[n] series, we
compared the energy diagrams shown in Figure 2. Since the
oscillator strengths do not differ much between these two series,
it is the rate constant of the non-radiative decay process that
mainly determines the Φfl. According to Engleman, the
probability of non-radiative transition can be estimated using the
height of the CI (= E(MECI) - E(S1min)) and the molecular
nuclear relaxation energy (= E(FC) - E(S1min)).[25] Here, we
employed a simpler parameter: ΔE = E(MECI) - E(FC). This
value shows how accessible the MECI is. If ΔE is large, it means
that the CI is inaccessible and that the non-radiative decay is
unlikely to occur.
An extension of the π-conjugation of the system lowers the FC
and the S1min. However, as shown in Figure 2, this has a
relatively small effect on the TS and the MECI. At this point, we
have not unambiguously determined the cause of this, but we
currently think that this is partly due to the fact that the local
distortion of the CI is not sensitive to distant substituent groups,
while the orbitals in the FC are delocalized and therefore
sensitive to distant substituent groups. In any case, the π-
extension tends to increase ΔE and therefore raise the Φfl. Thus,
the higher Φfl observed in PST relative to that in ST can be
explained by the ΔE values that are given in Table 3. The high
Φfl of BPST[6] can be explained in the same way as the ΔE for
BPST[6] (+0.84 kJ/mol), which is higher than that of BST[6] (-
11.11 kJ/mol).
Crystal
System
Space
Group
Cell Lengths
(Å)
Monoclinic
Pc
a
b 7.8455(5)
β 93.380(7)
c 6.6878(4)
γ 90
15.1833(14)
α 90
Cell Angles
(°)
Cell Volume
795.27(10)
(Å3)
Figure 6. ORTEP drawing of BPST[7] with thermal ellipsoids set at 60%
probability. The carbons of bridged-alkyl chain are highlighted in red.
The length of the alkyl chain affects both the FC and the MECI.
Shorter alkyl chains keep the FC low and raise the MECI, which
increases the Φfl. In contrast, longer alkyl chains raise the FC
and keep the MECI low, which decreases the Φfl. Calculations
were not performed for BDPST[7] and DPST, but can be
interpreted in the same manner.
Both radiative and non-radiative decay processes can be
observed in BDPST[7]. Picosecond laser-induced transient
absorption measurements were carried out with
a sub-
nanosecond transient-absorption spectroscopy system based on
the randomly interleaved pulse train (RIPT) method.[24] After
photoirradiation, the S1 state (S1-Sn transient absorption: 720
nm) is generated, followed by fluorescence decay and the
generation of an intermediate that follows the non-radiative
decay pathway (420 nm) (Figure S49). The observed
fluorescence lifetime was 0.17 ns. The lifetimes for the decay
(0.21 ns) and the formation of the intermediate (0.24 ns) are
approximately identical (Figure S50). Based on these results, it
can be concluded that a structural change occurred due to
internal conversion, as predicted by our calculations.
Table 3 ΔE values in kJ/mol for selected stilbenes and bridged stilbenes
ST
-14.41
+48.15
-11.11
-82.82
PST
-2.01
Not calculated
+0.84
BST[5]
BST[6]
BST[7]
BPST[5]
BPST[6]
BPST[7]
-57.69
Conclusion
Fluorescence quantum yield of BST[n] and BPST[n]
In this section we will compare the experimental photophysical
properties of the stilbenes and bridged stilbenes with their
calculated values and respective PESs.
We calculated the non-radiative decay pathway for stilbene (ST),
which revealed that ST adopts a twisted-pyramidal structure at
the conical intersection (CI). Based on the idea that control of
conical
intersection
accessibility
enables
control
of
photophysical properties, we investigated a series of bridged
stilbenes (BST[n]) and bridged phenylstilbenes (BPST[n]).
Calculating the non-radiative decay pathway of BST[n], we
qualitatively predicted the trend of the fluorescence quantum
We first examined the ST and BST[n] series. The calculated
order of the MECIs, BST[5] > BST[6] > BST[7], successfully
predicted the fluorescence Φfl observed in solution. Every
member of the series BST[n] shows enhancement of the Φfl in
6
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10.1002/anie.202000943
Angewandte Chemie International Edition
RESEARCH ARTICLE
Konishi, Macromolecules 2017, 50, 3544–3556; g) G. Iasilli, A. Battisti,
F. Tantussi, F. Fuso, M. Allegrini, G. Ruggeri, A. Pucci, Macromol.
Chem. Phys. 2014, 215, 499-506; h) L. Xu, L. Xu, Z. Wang, R. Wang, L.
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Ed. 2020, in press; Angew. Chem. 2020, in press. [DOI:
10.1002/anie.201907678]; i) C. Wang, Q. Qiao, W. Chi, J. Chen, W. Liu,
D. Tan, S. McKechnie, D. Lyu, X. F. Jiang, W. Zhou, N. Xu, Q. Zhang,
Z. Xu, X. Liu, Angew. Chem. Int. Ed. 2020, in press; Angew. Chem.
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Zhang, E. Zhao, L. Zhou, D. Chen, J. Sun, X. Gu, W. Yang, B. Z. Tang,
Angew. Chem. Int. Ed. 2020, in press; Angew. Chem. 2020, in press.
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yields of this series of molecules. The calculated results suggest
that BST[5] has a high fluorescence quantum yield (Φfl) and that
BST[7] has a low Φfl. We then synthesized the BST[n] (5-7) and
the BPST[n] (5-8) and compared the experimentally obtained
results with the calculated predictions. Different luminescent
properties such as aggregation-induced emission (AIE),
aggregation-induced emission enhancement (AIEE), and
aggregation-caused quenching (ACQ) were observed,
depending on the structure of the stilbene. BPST[7] showed
excellent AIE properties (Фsol = 0.02; Фsolid = 0.75). Experimental
Φfl were rationalized qualitatively based on the predicted energy
difference between the Franck-Condon (FC) state and the
minimum energy conical intersection (MECI). We discovered
that the length of the alkyl chain serves two important roles: a)
mechanical control of the CI, and b) controlling the FC via
distortion of the π-conjugation plane. These two effects are
synergistic, and thus shorter alkyl chains increase the Φsol, while
longer alkyl chains decrease the Φsol. The extension of the π-
conjugation of the system lowers the FC and the local minimum
near the FC (S1min). The balance of these three effects
determines the overall photophysical properties of the stilbenes
and the bridged stilbenes. Based on the theoretical predictions,
we succeeded in designing and synthesizing new AIEgens. We
are currently designing new dyes using this strategy and
investigating the use of bridged stilbenes in applications such as
sensors, liquid crystals, and supramolecular chemistry.
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discussion. This work is partially supported by the Grant-in-Aid
for Scientific Research (B) (18H02045), Innovative Areas “π-
System Figuration” (17H05145) and “Soft Crystals” (17H06371),
JSPS Fellows (16J10324), JSPS Overseas Research
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RESEARCH ARTICLE
Entry for the Table of Contents
Bridged-stilbene: Based on theoretical prediction of non-radiative decay, a new family of aggregation-induced emission (AIE)
luminogens is reported. Depending on alkyl chain length, they exhibit different quantum yields. BPST[7] and DPB[7] with excellent
AIE behavior are reported.
9
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