Spiro-Functionalized Diphenylethenes: Suppression of a Reversible Photocyclization Contributes to the Aggregation-Induced Emission Effect

Article abstract of DOI:10.1021/jacs.9b04426

Many aggregation-induced emission (AIE) materials are featured by the diphenylethene (DPE) moiety which exhibits rich photophysical and photochemical activities. The understanding of these activities behind AIE is essential to guide the design of fluorescent materials with improved performance. Herein by fusing a flexible DPE with a rigid spiro scaffold, we report a class of novel deep-blue material with solid-state fluorescent quantum yield (φ_F) up to 99.8%. Along with the AIE phenomenon, we identified a reversible photocyclization (PC) on DPE with visible chromism, which is, on the contrary, popularized in solutions but blocked by aggregation. We studied the steric and electronic effects of structural perturbation and concluded that the PC is a key process behind the RIMs (restriction of intramolecular motions) mechanism for these materials. Mitigation of the PC leads to enhanced fluorescence in solutions and loss of the AIE characteristics.

Full text of DOI:10.1021/jacs.9b04426

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Communication
Spiro-functionalized Diphenylethenes: Suppression of a
Reversible Photocyclization Contributes to the AIE Effect
Zhibiao Zhou, Sheng Xie, Xian Chen, Yujie Tu, Jiannan Xiang,
Jianguo Wang, Zikai He, Zebing Zeng, and Ben Zhong Tang
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2019
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Spiro-functionalized Diphenylethenes: Suppression of a
Reversible Photocyclization Contributes to the AIE Effect
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┴
Zhibiao Zhou,#† Sheng Xie,#‡ Xian Chen,† Yujie Tu,‡ Jiannan Xiang,† Jianguo Wang,‡ Zikai He,§
Zebing Zeng,*† and Ben Zhong Tang*‡||
†State Key Laboratory of Chemo/Biosensing and Chemometrics, Center for Aggregation-Induced Emission, College of
Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China;
‡Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and
Reconstruction, Institute for Advanced Study and HKUST-Shenzhen Research Institute, The Hong Kong University of Science
& Technology, Hong Kong, China;
┴College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China;
§School of Science, Harbin Institute of Technology, Shenzhen, HIT Campus of University Town, 518055 Shenzhen, China.
||Center for Aggregation-Induced Emission, SCUT-HKUST Joint Research Institute, State Key Laboratory of Luminescent
Materials and Devices, South China University of Technology, Guangzhou 510640, China.
Supporting Information Placeholder
Albeit the AIE effect can be generally attributed to suppression
of non-radiative pathways by aggregation, the working mechanism
for specific AIEgens remains to be addressed.7 As for DPE-
featured AIEgens, intramolecular motions such as rotation of Ph,
torsion/twisting of C=C8 and puckering of the central ring,9
formation of through-space aromatic-dimer at excited states,10 and
photochemical E/Z isomerization11 or cyclization12 via conical
intersections, have been suggested to be decisive factors in the
fluorescence light-up process (Fig. 1A).13 Specifically, the PC-
mediated photochemical deactivation was proposed as the
dominant relaxation pathway in the prototypical tetraphenylethene
derivatives (TPEs),14 but lack of experimental evidences due to the
ABSTRACT: Many aggregation-induced emission (AIE) materi-
als are featured by the diphenylethene (DPE) moiety which exhibits
rich photophysical and photochemical activities. The understand-
ing of these activities behind AIE is essential to guide the design of
fluorescent materials with improved performance. Herein by fusing
a flexible DPE with a rigid spiro scaffold, we report a class of novel
deep-blue material with solid-state fluorescent quantum yield (ΦF)
up to 99.8%. Along with the AIE phenomenon, we identified a re-
versible photocyclization (PC) on DPE with visible chromism,
which is, on the contrary, popularized in solutions but blocked by
aggregation. We studied the steric and electronic effects of struc-
tural perturbation and concluded that the PC is a key process behind
the RIMs (restriction of intramolecular motions) mechanism for
these materials. Mitigation of the PC leads to enhanced fluores-
cence in solutions and loss of the AIE characteristics.
Solid-state luminescent materials have attracted attentions
because of their applications in lasing, LED display, chemical
sensing, biological imaging and so on.1 In many cases,
fluorophores that emit strongly in dilute solutions become faint or
even non-emissive in the solid state, where the formation of
molecule aggregates causes self-quenching.2 In a reverse manner,
AIE-active materials fluoresce weakly when molecularly
dissolved, but become much more emissive in the solid state.3
Many AIE luminogens (AIEgens) have flexible rotors, such as aryl
rings featuring a modular pattern of the diarylethene (DAE) moiety
(Fig. 1).4 The structural design has paved the way for solid-state
emitters as well as fluorogenic materials,5 but frequently with
limited tunability on the luminescence color and leaving much
rooms for improving the solid-state ΦF (Fig. S1). One suspected
reason is that some non-radiative pathways popularized in solutions
might be not fully blocked in the solid state. One solution to this
issue is thus to modulate the state-dependent elemental relaxation
processes.6
Figure 1. (A) Photophysical and photochemical processes of the
DPE moiety. (B) Hydrocarbons DPI and SIPs studied in this work.
(C) Synthesis of SIPs.
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reversible nature of PC in an ultrafast process.15 In other aspects,
decolorized within minutes. The colorization-decolorization cycle
could be repeated in degassed solutions (Fig. S13). The color was
less intensive under air, and was not visible in O2-purged solutions.
By refering to other DAE photoswitches, the emerging absorption
at 475 nm was ascribed to the photocyclized -extending SIP-2H,
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DAEs have been known as photochromic materials via the PC
chemistry.16
In the way searching for deep-blue emitters, we found that
hydrocarbon DPI (Fig. 1B) is AIE-active and exhibits deep-blue
emission with 25.5% ΦF in the solid state (Table 1, Fig. S2-4). To
boost the solid-state photoluminescence (PL) further, we envision
that structural modulation by a rigid spiro scaffold to substitute the
dimethyl unit would help restricting the conformation of excited
fluorophores, and thereby modulate the relaxation processes.17
Meanwhile the sp3-hybridized spiro avoids extension of the -
system to a significant degree.18 For the first time, SIPs with
different spiro scaffolds were efficiently synthesized by modifying
the classic synthesis from Clarkson (Fig. 1C).19 We evaluated a
series of SIPs and found an optimal one, SIP-2, showing solid-state
ΦF = 99.8%. Its CIE coordinate is moreover close to the European
Broadcasting Union standard blue (0.15, 0.06).
1
which was tracked and identified by H-NMR (Fig. S14-15). The
isolation of SIP-2Ox as the only product in the air/oxygen-
exposing groups supported the formation of SIP-2H intermediate.
To sum up, SIP-2 readily undergoes the PC to yield SIP-2H in
solutions which is thermally reversible, and the SIP-2H can be
easily oxidized to form the ring-closed SIP-2Ox.
9
On the contrary, the PC-based photochromism is highly
suppressed in the solid state (Fig. 2C). NMR analysis showed trival
formation (<1%) of SIP-2Ox after 8 h irradiation of the powder
(Fig. 2D); while a THF-fumed sample was used, the color change
is almost instant and intense. Meanwhile the irradiated areas
showed diminished fluorescence (Fig. 2C). Similar to the AIE, the
PC-based photochromism is state-dependent: the PC is highly
suppressed in the solid states, but popularized in diluted solutions.
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Table 1 summarizes the photophysical properties of DPI and
SIPs in solution and solid states. Their UV-vis and PL remained
unsubtle to solvent effects (휆ex/휆em = 310/420~5 nm) from non-
polar to polar solvents (Fig. S5-6), indicative of relatively rigid
conformations at the ground and excited states (Fig. S7). Due to the
-conjugation break in the spiro linkage, SIPs retain a deep-blue
emission of 휆em~420 nm, an impressing blue-shifted color to the
prototypical TPE of 휆em~460 nm.
To evaluate the PC sensitivity on the molecule state, we firstly
studied the concentration effect in solutions. As indicated by
molecular dynamic simulations and ultrafast spectroscopyical
results on TPEs,14 the photo-catalyzed forward cyclization, in
general, accomplishes within pico-seconds after photon absorption,
as prohibits a correlation study of the PC dynamics in respond to
its physical state. Anyway, we could monitor the spontaneous ring-
opening process of SIP-2H by following the absorption decay at
475 nm (Fig. 2E). The monoexponential fitting analysis yields a
half-life time of 370 s, 415 s and 523 s for [SIP-2] = 0.2, 5 and 10
mM, respectively (Fig. S16). The positive correlation implies that
the ring-opening step is comparably suppressed in a higher
concentration, a phenomenon similar to concentration-caused
fluorescence quenching. According to the general principles of
microscopic reversibility, the monomolecular cyclization step is
reasonably impacted by the physical state as well.
The SIPs are a new class of AIEgens with deep-blue emission
(Fig. S8-11). Their ΦF values increased significantly in the powder
form, ranging from 30.8% to 99.8%, nearly 4-16 times higher than
the corresponding ones in dilute solutions (Table 1). The ΦF values
are also much higher than that of DPI (ΦF = 25.5%), showing the
intriguing effect exerted by the spiro bridge. The solid-state PL
slightly blue-shifted from the corresponding ones in THF solutions,
and all belong to deep-blue colors. Viewing from the fluorescence
lifetime study, the relaxation pathways of SIPs after
photoexcitation are highly physical-state-dependent (Fig. S12).
These features appear hence to be unique AIE characteristics for
SIPs, as compared to many spiro compounds suffering from the
ACQ.17 Regarding the PL properties, SIPs are comparable to
reported spiro-based materials.20
We further examined the AIE and PC behaviors of SIP-2 in a
series of colloidal solutions made by re-precipitation (Fig. 2F). The
PL and absorption intensity of SIP-2 had negligible change from
0% to 50% water fraction (fw). With a further increase of fw, the
evident Tyndall effect approved the formation of aggregates. In
accordance with the AIE, the PL increased progressively when
aggregation occurred. On the exact contrary, the absorption at 475
nm declined swiftly (after a light irradiation of 3 min), and became
colorless when fw reached 70 vol% (middle). As in Fig. 2G, there
is a negative correlation between the PL and the PC-based
photochromism in these colloids. Similar phenomenon were
observed for other SIPs (Fig. S17-19). It thus can be hypothesized
that the occurrence of PC deactivates the excited states of SIPs,
leading to faint emission in dilute solutions; while the deactivation
of the photochemical PC by molecule aggregation boosts the
emission in the solid state.21
We attributed the AIE properties primarily to the flexible DPE
moiety. Interestingly, in contrast to the similar solution ΦF, the
regioisomeric spiro-scaffolds result in a large difference on the
solid-state ΦF (SIP-2, 99.8% vs SIP-3, 30.8%). The efficient struc-
tural modulation attracts interests on the working mechanism un-
derlying the AIE phenomenon.
Along with the AIE process, we further identified a state-
dependent PC process in DPI and SIPs (Fig. 2A). Upon prolonged
light excitation, the SIP-2 solutions, for example, displayed visible
change from colorless to orange-yellow peaked at 475 nm (Fig.
2B). When the irradiation was removed, the solution restored and
Table 1. Photophysical properties of SIPs at different states
Name
휆ex
THF
280
310
316
310
310
휆em [nm] (ΦF %)
CIE (x, y)
Film
Egopt [eV]b
 [ns] (kr [108 s-1], knr [108 s-1])a
THF
Powder
THF
Powder
DPI
415 (1.2)
425 (7.6)
424 (8.5)
420 (7.1)
424 (7.5)
406 (25.5)
415 (59.8)
416 (99.8)
415 (30.8)
419 (57.2)
2.41 (0.01, 4.13)
2.25 (0.34, 4.10)
1.80 (0.47, 5.08)
1.82 (0.39, 5.10)
1.54 (0.48, 6.01)
1.60 (1.59, 4.66)
3.22 (1.86, 1.24)
2.65 (3.77, 0.01)
2.32 (1.33, 2.09)
2.09 (2.74, 2.04)
(0.152, 0.048)
(0.153, 0.058)
(0.154, 0.056)
(0.154, 0.055)
(0.153, 0.057)
3.58
3.41
3.35
3.43
3.43
SIP-1
SIP-2
SIP-3
SIP-4
akr: radiative decay rate; knr : nonradiative decay rate. bCalculated from UV adsorption.
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In Argon
0.6
0.4
0.2
0.0
In Air
In Oxygen
Ar Air O2
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Figure 2. (A) AIE and PC processes of SIP-2 in the dissolved and aggregated states. (B) UV-vis absorption of SIP-2 in THF (1  10−2 M)
upon irradiation (313 nm, 8 W) under argon, air and oxygen. (C) Photochromism for the dry, and THF-fumed SIP-2 powders. (D) 1H NMR
showing trace SIP-2Ox after 480 min irradiation of the dry sample. (E) Decay of absorption at 475 nm (SIP-2H) after irradiation for 3 min
on samples of different [SIP-2], at 20 oC. (F) Photographs of SIP-2 colloids (showing fractions of water fw in THF) after the light irradiation.
(G) Plots of relative PL intensity (I/I0) and absorption at 475 nm vs the fw; [SIP-2] = 2  10−4 M, 휆ex: 316 nm, I0 = intensity at fw = 0%.
The next question is that to what extent the photochemical PC
process is bound to the AIE effect. The non-radiative decay via the
cyclized intermediate is energetically barrierless in solutions for
crystalline packing too, manifested by the rich and regular C-H
and - interactions between the spiro backbones (Fig. 3, right).
Furthermore, structural mitigation of the PC activity leads to loss
of the AIE characteristics (Fig. 4). The absence of PC is observed
in SIP-5 with bulky naphthalene, and SIP-6 with CN substitution
which deviate the reacting sites away upon photo-activation (Fig.
S7, Table S1). In comparison to SIP-2, both showed enhanced
emission in solutions (SIP-5, 39.1%; SIP-6, 94.5%), which can be
ascribed to the deactivation of the PC-mediated pathways. In
addition likely due to the loss of the PC activity, the ring-closed
SIP-2Ox is also not AIE-active (Fig. S25).
14c
TPEs,14a,
which is also likely the same case here for SIPs.
Therefore, the key is to understand the suppression effects of
aggregation on the photochemical cyclization. In fact, many DAEs
are PC-active in the solid state. There are, however, only empirical
rules for predicting the photochromism activities.22 As appoved by
powder XRD analysis, the SIP powders as prepared are largely in
crystalline states (Fig. S20). Accordingly, we scrutinized the
crystalline structures of SIPs (Fig. 3 and S21). The DPEs of SIPs
all show an antiparallel conformation favorable for the cyclization,
but the distances (d) between the photocyclization reactive carbon
atoms vary largely, depending on the spiro structure (Fig. 3, left).
The large d for SIP-1/3 suggests a less propensity to the PC
pathway in the crystalline,22 as is consistent with the observation
that SIP-1/3 are much more photochromism-resistant than SIP-2/4
(Fig. S22). The case is different for SIP-2/4 having both favorable
orientations and intramolecular distances (d ~ 3.40 Å) for the PC
(comparing to d ~ 3.21 Å in computated S1, Fig. S7). In the
crystalline packing, both DPE rotors are obviously constrained by
the benzofluorene unit from adjacent molecules via multiple C-
H and - interactions (Fig. 3, middle). As indicated by the free-
motion space for the DPE, SIP-2/4 had a low probability for the
PC. From the perspectives of RIMs theory, the conformational
restriction by aggregation likely contributes to the AIE effect via
deactivating the photochemical cyclization.
A
-
3.29
2.60
C-H…
2.73

C-H…

2.60
-
3.49
3.29
SIP-2
Both Phs are restricted.
B
C-H…
2.83

C-H…

-
3.21
2.83
2.83
2.83
Less
restricted
Restricted
SIP-3
Figure 3. Crystalline structures of SIP-2/3 showing parameters for
Notably, it is not right to relate the AIE effect solely to the
suppression of the photochemical processes. Comparably, in all
SIPs with aggregation-caused suppression of the PC, the ultrahigh
PL performance of SIP-2 is likely associated with the rigid
the PC and AIE.
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Figure 4. Mitigation of the PC activity in SIP-2 leading to loss of
the AIE characteristics.
9
In summary, by simply merging a DPE moiety with a spiro-
scaffold, we introduced a new class of AIEgens (DPI and SIPs)
with strong deep-blue emission in the solid state. We disclosed a
dominant role of the photochemical cyclization in the non-radiative
pathways in dilute solutions, and the suppression of the PC is bound
to the boosted emission in the solid state. During the structural
modulation of PL properties, the unique spiro functionalities are
highlighted, by steric, electronic and other effects on the PC
activity, leading to ultrahigh emission efficiency with a good
control on the color shift. The PC-engaged working mechanism for
AIE which has been argued for TPEs, might work for other
unprecedent AIEgens and lead to the design of fluorescent
materials with enhanced performance and applicability.
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ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge on
the ACS Publications website.
Synthesis description, characterization details
AUTHOR INFORMATION
Corresponding Author
*E-mail: tangbenz@ust.hk
*E-mail: zbzeng@hnu.edu.cn
Author Contributions
#These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors thank the National Natural Science Foundation
of China (51573040), Hunan Provincial NSFC
(2018JJ1008).
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