Photomechanical Luminescence from Through-Space Conjugated AIEgens

Article abstract of DOI:10.1002/anie.201913383

Transforming molecular motions into the macroscopic scale is a topic of great interest to nanoscience. The photomechanical effect is a promising strategy to achieve this goal. Herein, we report an intriguing photomechanical luminescence driven by the photodimerization of 2-phenylbenzo[b]thiophene 1,1-dioxide (P-BTO) in molecular crystals and elucidate the working mechanism and substituent effect through crystallographic analysis and theoretical calculations. Striking splitting, hopping, and bending mechanical behaviors accompanied by a significant blue fluorescence enhancement are observed for P-BTO crystals under UV light, which is attributed to the formation of photodimer 2P-BTO. Although 2P-BTO is poorly π-conjugated because of the central cyclobutane ring, it exhibits prominent through-space conjugation and aggregation-induced emission (AIE), affording strong solid-state blue fluorescence at 415 nm with an excellent quantum yield of up to 96.2 %.

Full text of DOI:10.1002/anie.201913383

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Dancing Brightly Under Light: Intriguing Photomechanical
Luminescence in Constructing Through-Space Conjugated
AIEgens
Authors: Jingjing Guo, Jianzhong Fan, Xinzhi Liu, Zujin Zhao, and Ben
Zhong Tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913383
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10.1002/anie.201913383
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COMMUNICATION
Dancing Brightly Under Light: Intriguing Photomechanical
Luminescence in Constructing Through-Space Conjugated
AIEgens
Jingjing Guo, Jianzhong Fan, Xinzhi Liu, Zujin Zhao,* and Ben Zhong Tang
Abstract: Transforming molecular motions into macroscopic scale is
a topic of great interest to nanoscience. Photomechanical effect is a
promising strategy to realize this purpose but remains challenging to
design molecular materials with superior photomechanical effect that
can be visually monitored under UV light. Herein, we wish to report
intriguing photomechanical luminescence driven by photodimerization
of 2-phenylbenzo[b]thiophene 1,1-dioxide (P-BTO) in molecular
crystals, and elucidate working mechanism and substituent effect via
crystallography analysis and theoretical calculation. Striking splitting,
hopping and bending mechanical behaviors accompanied by
significant blue fluorescence enhancement are observed for P-BTO
crystals under UV light, which is attributed to the formation of
photodimer 2P-BTO. Although 2P-BTO is poorly π−conjugated
because of the central cyclobutane ring, it exhibits prominent through-
space conjugation and aggregation-induced emission (AIE)
characters, affording strong solid-state blue fluorescence at 415 nm
with an excellent quantum yield of up to 96.2%. This work not only
provides novel molecular crystals with photomechanical
luminescence but also explores a new kind of AIE luminogens
(AIEgens) based on a tailored through-space conjugated framework.
alternative, which means the photomechanical movements can
be lit up by a simultaneous fluorescence “turn-on” phenomenon
under UV light irradiation. Therein, the highly efficient
photomechanical luminescence, driven by photochemical
reactions using UV light as the excitation source, plays an
important role in monitoring the process.
In general, the mechanical motions of photoactive crystals
mainly originate from structural transformations during
photochemical
azobenzene,[3a,3b] [2
photocyclization of diarylethenes,[3e,3f] and [4
photodimerization of anthracenes.[3g] Among diverse reported
photochemical reactions, the topochemical [2 2]
reactions,
such
2] cycloaddition of olefins,[3c,3d]
4]
as
isomerization
of
+
+
+
photodimerization has received most attention because of large
structural variation and easy monitoring via NMR spectroscopy.
But the formation of cyclobutane ring breaks the original
π−conjugation of the precursor, causing severe damage to the
luminescence of the product. So, efficient photomechanical
luminescence is rarely achieved from molecular crystals involving
photodimerization. In this work, we report the intriguing
photomechanical luminescence based on photodimerization of 2-
phenylbenzo[b]thiophene 1,1-dioxide (P-BTO) in molecular
crystals. The relevant working mechanism and substituent effect
on the reaction are investigated. Although the generated
photodimer 2P-BTO is poorly π−conjugated because of the
presence of cyclobutane ring, they exhibit prominent aggregation-
induced emission (AIE)[4] property with strong deep blue
fluorescence in solid powder, owing to the synergistic effect of
efficient intramolecular through-space conjugation and restriction
of intramolecular vibration. The formation of new robust AIE
luminogens (AIEgens) is considered to contribute significantly to
intense photomechanical luminescence.
The photodimerization of benzo[b]thiophene 1,1-dioxide
(BTO) can date back to the 1950s,[5] and the related photodimers
structures and reaction mechanisms were subsequently
reported.[6] But all of these reactions were only performed in
solutions. Until very recently, Resendiz et al.[7] reported the
photodimerization of BTO derivatives in solid and elucidated the
impact of reaction conditions on the structures of the photodimers.
In comparison with BTO derivatives reported by Resendiz et al.,
P-BTO has a bulky phenyl substitute. In crystals, two adjacent P-
BTO molecules can align in an antiparallel and head-to-tail
manner (Figure 1), and the intermolecular distance between the
reactive double bonds is 3.629 Å, shorter than 4.2 Å (Figure S8),
well meeting the criteria of Schmidt for photodimerization in solid
state.[8] In the transition state, the two P-BTO molecules approach
each other by maintaining the initial packing mode and the
distance between the reactive carbons is shortened to 3.241 Å,
as revealed by the calculation (Figure S18), which is beneficial for
the formation of anti head-to-tail (anti-ht) conformation of the
product.
Photomechanical-responsive materials, such as photoactive
polymers, elastomers, liquid crystals and molecular crystals, hold
great potentials in light-driven artificial actuators,[1] and have
achieved remarkable progresses in recent years.[2] In particular,
considerable efforts have been devoted to exploring
photomechanical crystals owing to their rapid energy transfer with
less energy dissipation and higher Young’s modulus. They can
effectively amplify microscopic molecular motions into
macroscopic scale, and the ordered molecular structures provide
an opportunity to decipher the related processes. Various
mechanical motions, such as jumping, bending, curling, crawling
and twisting, have been realized in photomechanical crystals.[3]
With the desire to visually monitor and continuously characterize
morphological changes of the crystals especially in dark
background, photomechanical luminescence is a promising
[*]
J. Guo, X. Liu, Prof. Z. Zhao, Prof. B. Z. Tang
State Key Laboratory of Luminescent Materials and Devices,
Guangdong Provincial Key Laboratory of Luminescence from
Molecular Aggregates, South China University of Technology,
Guangzhou 510640, China
E-mail: mszjzhao@scut.edu.cn
Dr. J. Fan
Shandong Province Key Laboratory of Medical Physics and Image
Processing Technology, Institute of Materials and Clean Energy,
School of Physics and Electronics, Shandong Normal University,
Jinan 250014, China
Prof. B. Z. Tang
Department of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201913383
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As expected, upon exposure to UV light (365 nm), the
microcrystalline powder of P-BTO (Figure S2) undergoes
topochemical photodimerization to generate 2P-BTO (Figure 1).
The progress of photodimerization can be well monitored by using
1H NMR spectroscopy (Figure S1). The singlet peak of Ha of BTO
at 8.01 ppm is gradually decreased with the increase of irradiation
time. Meanwhile, a new singlet peak appears at 5.96 ppm and
intensifies progressively along with the irradiation, which is
assigned to Hb in the newly formed cyclobutane ring, validating
the concomitant formation of the photodimerization product. By
calculating the related integral peaks in 1H NMR spectrum, almost
86% photoconversion is achieved within a short exposure time of
2 hours, indicative of a high efficiency of the reaction. Single
crystals of 2P-BTO can be obtained from a solution of P-BTO in
dichloromethane/hexane mixture exposed under sunlight for a
few days. The X-ray crystallography reveals that 2P-BTO only
holds an anti-ht conformation, which is further confirmed by the
generated photodimers. Hence, two new precursors of NP-BTO
and MOP-BTO with more bulky naphthyl and 4-methoxyphenyl,
respectively, are prepared and adopted for the photodimerization.
However, their microcrystalline powders are photochemically inert
under the same conditions. By carefully surveying the crystal
packing, it is found that the distance between the reactive double
bonds of two adjacent NP-BTO molecules is 4.949 Å, which is
larger than 4.2 Å (Figure S9). For MOP-BTO crystals, although
the distance of two reactive double bonds is short enough (3.852
Å), other important geometrical parameter θ1 (the rotational angle
of one double bond with respect to the other) is 54.17o, greatly
deviating from the ideal value of 0o (Table S1, and Figure S10).
Since the molecules are almost immobile in crystalline state, it is
understandable that the photodimerization of these two
precursors can hardly proceed in solid. On the contrary, the
photodimerization can happen with high regioselectivity and high
yields for NP-BTO, MOP-BTO as well as P-BTO in solution state,
because the molecules have large freedom to adjust their relative
positions. The generated photodimers 2NP-BTO and 2MOP-BTO
also adopt anti-ht conformations, as determined by NMR
spectroscopy or single crystal structures (Figure S4). Actually, the
anti-ht conformations are the thermodynamically most stable
conformations for these photodimers, as disclosed by the
calculation (Figure S19). All the photodimers are very stable
under ambient conditions in solid, and possess good thermal
stability with high decomposition temperatures of 358−419 oC at
5% weight loss (Figure S12).
o
single crystal grown by sublimation at 260 C (Figure S3). The
formation of sole anti-ht symmetrical photodimer undoubtedly
demonstrates the excellent region- and stereo-selectivity of the
photodimerization of P-BTO.
O
O
S
Ha
Hb
hv
Hb
Solid
S
O
S
O
O
O
P-BTO
2P-BTO
O
O
S
hv
S
O
Solution
S
O
O
O
NP-BTO
2NP-BTO
O
O
O
S
hv
O
Solution
S
S
O
O
O
O
O
MOP-BTO
2MOP-BTO
d2
d1
d1
d2
Figure 2. Fluorescence microscopy images of P-BTO single crystals during
irradiation with 365 nm UV light for 1 min (A and I: under daylight; B-H: under
UV light). Scale bar: 10 μm.
d1 = 3.629 Å d2 = 4.363 Å
The photodimerization of P-BTO can also occur in its bulky
single crystals, results in significant morphological evolution
(Figure 2, Figure S13 and Video S1-3). It is found that bulky
crystals rapidly split into small pieces, some of which can move
and even jump out of the visual field under UV light irradiation,
within a very short response time of 10 seconds. Moreover, some
crystals can be divided into small belts, and become bent
apparently. The obvious surface cracks of crystals can also be
observed in scanning electron microscope (SEM) images (Figure
Figure 1. Synthetic routes of 2P-BTO, 2NP-BTO and 2MOP-BTO, the distance
between two adjacent antiparallel P-BTO molecules, and crystal structure of 2P-
BTO obtained in P-BTO solution.
Bulky substituents can effectively influence crystal stacking
of the molecules, and thus can be used to tune the
topophotochemical reactions and photophysical properties of the
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S14). Along with the photomechanical effect, an impressive
fluorescence “turn-on” phenomenon is observed. The P-BTO
crystals show weak blue fluorescence (446 nm), but the
fluorescence significantly intensifies after UV light irradiation for 1
To gain in-depth insights into the photomechanical
luminescence, the photophysical property of 2P-BTO is carefully
investigated. In dimethylsulfoxide (DMSO), 2P-BTO shows a
maximum absorption band at ~277 nm, apparently blue-shifted
than that of P-BTO (~341 nm). The fluorescence of 2P-BTO is
also blue-shifted by 31 nm relative to that of P-BTO in solid
powder (Figure S16 and S17). The blue shifts in absorption and
fluorescence spectra are due to the non-conjugated cyclobutane
ring, which greatly undermines the π−conjugation of 2P-BTO.
Meanwhile, a large Stokes shift (12005 cm−1) is observed for 2P-
BTO in solid. And the onsets of absorption and emission are also
apparently separated. These results demonstrate the
conformation change between the ground state and excited state
of 2P-BTO (Figure S17) under photoexcitation.[9] Actually, 2P-
BTO is a very faint emitter in DMSO, as evidenced by a ΦF as low
as 0.8%, which is understandable in view of the poorly
π−conjugated structure and vigorous intramolecular motion in
solution.[10] What surprised us is that its fluorescence becomes
much stronger upon aggregate formation by adding a large
amount of water to DMSO solution (Figure 3C and D). The solid
powder of 2P-BTO can fluoresce strongly at 415 nm with an
excellent ΦF of up to 96.2%, unveiling the prominent AIE property.
2NP-BTO and 2MOP-BTO with different substituents display the
similar AIE behaviors (Table 1).
h
(Figure 3A). The vigorous photoinduced motions and
fluorescence enhancement are also true for P-BTO
microcrystalline powders under the same conditions, with
noticeably increased fluorescence quantum yield (ΦF) from 1.7%
to 92.3% (Figure 3B and Table S2). More importantly, the
photomechanical phenomena can be directly caught by naked
eyes (Video S4-6), thanks to the fluorescence “turn-on”
phenomenon.
To explore the driving force for the above photomechanical
motions in depth, the photoinduced changes in cell parameters
and crystal structures are analyzed by X-ray crystallography.
Upon UV light irradiation for 1 minute, the unit cell of P-BTO
crystal expands by 65.0% along a-axis and 46.4% along b-axis,
but shrinks by 16.9% along c-axis (Table S3). On the other hand,
the bond length of C−C single bond in cyclobutane ring of 2P-BTO
is 1.583 Å, which means P-BTO molecules need to get closer by
~2.046 Å during photodimerization. And the distance between the
outermost hydrogen atoms spreads to 7.562 Å in 2P-BTO
molecule, disclosing obvious photo-triggered structural
transformation (Figure 1C). Eventually, the space group is altered
͞
from P1 for P-BTO crystals to C2/c for 2P-BTO crystals to
Table 1. Photophysical properties of 2P-BTO, 2NP-BTO and 2MOP-BTO.
accommodate these structural changes. Thus, photomechanical
behaviors occur on account of instantaneous release of the
accumulated strain during the photodimerization.
Ф
F [%][c]
λabs
[nm][a]
λem
[nm][b]
[d]
Compound
αAIE
soln
solid
2P-BTO
277
279
276
415
439
451
0.8
0.5
0.7
96.2
50.6
66.4
120.3
101.2
94.9
2NP-BTO
2MOP-BTO
[a]Absorption maximum in DMSO solution (10−5 M). [b]Emission maximum in solid
powder. [c]Fluorescence quantum yields of DMSO solution (soln) and solid
powder, determined by a calibrated integrating sphere (λex = 340 nm). [d]AIE
activity, calculated by ΦF (solid)/ΦF (soln).
The strong fluorescence of 2P-BTO in solid powder obviously
differs from those of P-BTO. To figure out the origin of the strong
fluorescence, the crystal structure of 2P-BTO is studied. There
are two pairs of phenyl rings (ϕ1 and ϕ2) aligning in close proximity
with a shortest interplane distance of 3.147 Å (Figure 1C), which
is short enough to engender noncovalent intramolecular
interactions. To visualize such intramolecular interactions, the
independent gradient model (IGM) method is used on the basis of
single crystal structure.[11] The obvious green sections between
two phenyl rings clearly validate that through-space interaction
exist in 2P-BTO crystal (Figure 4A). Similar interaction is also
found in 2MOP-BTO crystal (Figure S11). Such kind of through-
space interaction is conducive to stabilizing molecular structure
and strengthening intramolecular electronic communication, and
can result in a radiative channel for dissipating excited state
energy.[10b,12]
Figure 3. (A) PL spectra and (B) ΦF values of P-BTO microcrystalline powders
under irradiation with 365 nm UV light for different time. Inset: fluorescent photos
of P-BTO before and after UV light irradiation. (C) PL spectra of 2P-BTO in
DMSO/water mixture with different water fractions (fw) (10−5 M). (D) Plots of
relative PL intensity (I/I0) versus fw (I0 = intensity at fw = 0%). Inset: fluorescent
photo of pure 2P-BTO under UV light irradiation.
Theoretical simulations are also carried out to decipher the
mechanism of the efficient solid-state fluorescence. The large
geometric change occurs between the ground state (S0) and the
lowest energy-excited state (S1). The two aromatic rings employ
an almost parallel and closer stacking manner in the excited state
(Figure S20, Table S4-S6). which is more favorable for interplane
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electronic communication and orbital overlap. Taking 2P-BTO as
an example, the two phenyl rings (ϕ1 and ϕ2) get much closer in
S1 state, and stronger through-space interaction is visualized
between two stacked phenyl rings (Figure 4B). In addition,
apparent electronic delocalization of the lowest unoccupied
molecular orbital (LUMO) is found between the two stacked
phenyl rings, which confirms through-space conjugation character
(Figure 4D).[13] For 2NP-BTO and 2MOP-BTO, the interplane
distances are slightly longer than that of 2P-BTO in S1 state due
to steric hindrance, but short enough for a large overlap degree
of frontier orbitals. The through-space conjugation characters can
be clearly observed in LUMOs as well (Figure S21 and S22).
These results definitely manifest that these photodimers are
governed by eminent through-space conjugation.
Figure 5. (A) PL spectra of 2P-BTO in solid at 77 and 298 K, and (B) the ultrafast
pico-nanosecond fluorescence transients of 2P-BTO in solid monitored at
various wavelengths.
Excited state
Ground state
The calculation by the combined quantum mechanics and
molecular mechanics (QM/MM) method with two-layer ONIOM
approach[16] on the basis of a solid-phase computational model of
2P-BTO crystal structure shows that the radiative decay rate from
S1 to S0 is increased from gas (3.96 × 106 s‒1) to solid (2.00 × 107
s‒1) by about 5 times, partially owing to the reinforced through-
space conjugation in solid. On the other hand, the nonradiative
decay rate in solid (1.09 × 109 s‒1) is decreased by about 3 orders
of magnitude than that in gas (1.28 × 1012 s‒1), indicating the
nonradiative decay has been remarkably blocked upon
aggregation. The Huang-Rhys (HR) factors and reorganization
energies (λ) which are crucial parameters to evaluate the
nonradiative decay processes are determined by DUSHIN
program (Figure 6 and Table S7).[17] They are much smaller in
solid than in gas, especially in low frequency region (<100 cm‒1),
disclosing the vibrational motions of phenyl rings are successfully
restricted in solid relative to in gas, which suppresses nonradiative
decay and contributes to through-space conjugation, rendering
strong fluorescence in solid.
LUMO
HOMO
LUMO
HOMO
Figure 4. Visualization of through-space conjugation in (A) crystal and (B)
lowest energy-excited state of 2P-BTO. The frontier orbital amplitude plots of
2P-BTO in (C) the ground state and (D) the lowest energy-excited state,
calculated based on M06-2X with Grimme’s empirical dispersion correction[14]
and the basis set 6-31G(d,p).
Furthermore, the formation of through-space conjugation in
the excited state is also validated for 2P-BTO in solid. As shown
in Figure 5A, the fluorescence intensity increases as the
temperature rises from 77 to 298 K, which is opposite to the
common observations of decreased fluorescence along with the
increase of temperature due to the nonradiative decay caused by
molecular motion. But in the case of 2P-BTO in solid, certain
molecular motion can induce minor conformational adjustment
and facilitate the formation of an intramolecular cofacial
conformation in the excited state. Consequently, the
intramolecular electronic coupling of 2P-BTO is reinforced,
accounting for the increased fluorescence.[15] The ultrafast pico-
nanosecond fluorescence transients monitored at different
emission wavelengths further reveal the corresponding relaxation
dynamics (Figure 5B). At the blue side of the emission (375 nm),
the fluorescence lifetime is determined to be 5.50 ns. By
monitoring at 415 nm (the fluorescence peak of 2P-BTO in solid),
the lifetime becomes shorter (1.65 ns), which can be attributed to
the enhancement of radiative decay rate. Therefore, the formation
of energy-minimized geometry in S1 state with obvious through-
space conjugation (Figure 4D) can be further confirmed.
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Figure 6. Calculated reorganization energies of 2P-BTO in (A) gas and (C) solid
and HR factors of 2P-BTO in (B) gas and (D) solid versus normal mode
frequencies. Inset: representative vibration modes.
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In summary, the photodimerization reactions of BTO
derivatives with bulky substituents are studied, based on which a
new kind of dynamic crystal photoactuator is explored.
Photoinduced splitting, hopping and bending behaviors, as well
as significant fluorescence enhancement are observed for P-BTO
crystals, owing to the formation of photodimer 2P-BTO under UV
light irradiation. Although 2P-BTO is poorly π−conjugated
because of the central cyclobutane moiety, it exhibits prominent
AIE property with strong blue fluorescence at 415 nm and a
remarkable ΦF of 96.2% in solid powder, owing to efficient
intramolecular through-space conjugation and restriction of
intramolecular vibration. This work provides a new method to
construct AIEgens based on through-space conjugation, with
unequivocal working mechanism. And the presented
photomechanical luminescence is to the benefit of visually
monitoring morphological changes of crystals in high resolution,
and offers great potential for the applications in smart devices and
bionic science.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21788102 and 21673082), and the
Natural Science Foundation of Guangdong Province
(2019B030301003).
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Keywords: photodimerization • photochemical reaction •
photomechanical luminescence • aggregation-induced emission
• through-space conjugation
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10.1002/anie.201913383
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
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COMMUNICATION
Intriguing
photomechanical
Jingjing Guo, Jianzhong Fan, Xinzhi
Liu, Zujin Zhao,* and Ben Zhong Tang
luminescence is observed for P-BTO
crystals, driven by photodimerization
reaction under UV light irradiation.
The generated photodimer 2P-BTO
exhibits prominent through-space
conjugation and aggregation-induced
emission characters with an excellent
fluorescence quantum yield of up to
96.2% in solid.
Page No. – Page No.
Dancing Brightly Under Light:
Intriguing
Luminescence
Photomechanical
in Constructing
Through-Space Conjugated AIEgens
This article is protected by copyright. All rights reserved.