Benzosiloles with Crystallization-Induced Emission Enhancement of Electrochemiluminescence: Synthesis, Electrochemistry, and Crystallography

Article abstract of DOI:10.1002/chem.202002647

Crystallization-induced emission enhancement (CIEE) was demonstrated for the first time for electrochemilunimescence (ECL) with two new benzosiloles. Compared with their solution, the films of the two benzosiloles gave CIEE of 24 and 16 times. The mechani

Full text of DOI:10.1002/chem.202002647

Accepted Article
Accepted Article
Title: Benzosiloles with Crystallization-induced Emission Enhancement
of Electrochemiluminescence: Synthesis, Electrochemistry, and
Crystallography
Authors: Liuqing Yang, Donghyun Koo, Jackie Wu, Jonathan M. Wong,
Tyler Day, Ruizhong Zhang, Harshana Kolongoda, Kehan Liu,
Jian Wang, Zhifeng Ding, and Brian L. Pagenkopf
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202002647
Link to VoR: https://doi.org/10.1002/chem.202002647
01/2020

10.1002/chem.202002647
Chemistry - A European Journal
RESEARCH ARTICLE
Benzosiloles with Crystallization-induced Emission Enhancement
of Electrochemiluminescence: Synthesis, Electrochemistry, and
Crystallography
Liuqing Yang‖, Donghyun Koo‖, Jackie Wu‖, Jonathan M. Wong, Tyler Day, Ruizhong Zhang, Harshana
Kolongoda, Kehan Liu, Jian Wang, Zhifeng Ding* and Brian L. Pagenkopf*
[a]
L. Yang‖, D. Koo‖, J. Wu‖, J. M. Wong, T. Day, R. Zhang, H. Kolongoda, K. Liu, J. Wang, Prof. Dr. Z. Ding*, Prof. Dr. B. L. Pagenkopf*
Department of Chemistry
The University of Western Ontario
1151 Richmond St., London, N6A 5B7, ON, Canada
E-mail: zfding@uwo.ca (ZD); bpagenko@uwo.ca (BLP)
‖Authors contributed equally
Supporting information for this article is given via a link at the end of the document.
Abstract: Crystallization-induced emission enhancement (CIEE)
was demonstrated for the first time for electrochemilunimescence
(ECL) with two new benzosiloles. Compared with their solution, the
films of the two benzosiloles gave CIEE of 24 times and 16 times.
The mechanism of the CIEE-ECL was examined by spooling ECL
spectroscopy, X-ray crystal structure analysis, photoluminescence,
and DFT calculation. This CIEE-ECL system is a complement to the
well-established aggregation-induced emission enhancement (AIEE)
systems. Unique intermolecular interactions are noted in the
crystalline chromophore. The first heterogeneous ECL system is
established for organic compounds with highly hydrophobic
properties.
decay processes by increasing the steric bulk on both the silicon
atom and the 2,5-substituents to improve photoluminescence
9]
(PL) quantum efficiency.[7a,
Similarly, we demonstrated
enhanced ECL from siloles by decorating them with 2,5-di(2-
thienyl) substituents that are rich in π-electrons and by strategic
steric tuning at the silicon atom.[4c] After these accomplishments,
we chose to investigate 2-thienyl-benzo[b]siloles because (1)
there is a rising interest in fused silole materials and (2)
benzosiloles are synthetically well operable and expected, from
theoretical calculations, to possess electrochemical properties
superior than previously reported siloles including ours.[7a, 10] We
were
hence
intrigued
if
we
could
improve
the
photoelectrochemical performance of benzosioles similarly with
the successful synthetic manipulation of steric and electronic
properties previously applied to enhance siloles.
Introduction
Tang found that some compounds are highly luminescent as
aggregates/clusters but non-emissive or weakly emissive when
molecularly dissolved in solution, and reported the aggregation-
Induced emission (AIE).[11] The restriction of intramolecular
motions such as rotation and vibration gives rise to the AIE
property of these special molecules.[12] In a related content,
crystallization-induced emission enhancement (CIEE) was also
later reported by Tang[13] for hexaphenylsiloles and some other
compounds whose light emission is remarkably stronger when
they exist in the solid crystalline state compared with their
amorphous or solution state. CIEE is undoubtedly desirable for
light-emitting electrochemical cells, organic light-emitting diodes,
etc., since the layer responsible for light emission in these
devices is usually a crystalline film. In fact, some reports
Electrogenerated chemiluminescence (ECL), also known as
electrochemiluminescence, represents an important light-
emitting process induced in the vicinity of working electrodes.[1]
ECL can be produced via either the annihilation route (i.e., the
recombination of radical cations and anions of a luminophore) or
the coreactant route (i.e., an electrochemical reaction between a
luminophore radical and a suitable coreactant).[2] ECL is
increasingly employed in medical research, environmental
monitoring, light-emitting devices, etc.[1-3] because of its
advantages such as ease of control, low detection limit, high
sensitivity, etc. Luminophores applied in an ECL system include
organic
molecules,[4]
inorganic
complexes,[5]
and
nanomaterials.[6] Currently, the most commonly employed ECL
luminophore for different applications is ruthenium tris(2,2’-
bipyridyl), i.e., Ru(bpy)32+. Additional ECL luminophores with
good performance and low cost have always been avidly desired.
adopted
heterogeneous
ECL
to
improve
analytical
performance,[4a, 6c, 14] but such a strategy is not readily feasible
for existing organic ECL chromophores whose solid films are
extreme hydrophobic and minimally wettable in aqueous
systems but dissolve thoroughly in organic solvents.
Highly π-conjugated compounds containing silole rings (1-
silacyclopentadiene)[4a, 7] have been adopted for use in polymeric
solar cells, electrochemiluminescent sensors, and light-emitting
diodes.[8] The unique photophysical properties of siloles arise
from their low-lying LUMO due to the σ*–π* conjugation between
the π* orbital of the butadiene moiety and the σ* orbital of the
two exocyclic σ-bonds on the silicon atom. We previously
prepared 2,5-bis(arylethynyl)siloles and minimized non-emissive
Herein we report two newly developed chromophores, i.e., 2-(2-
thienyl)-benzo[b]silole (4T1) and 2-(2,2'-bithiophen)-5-yl]-
benzo[b]silole (4T2) along with their electrochemistry and ECL in
both annihilation and coreactant routes, with particular emphasis
on the notably enhanced photoluminescence (PL) and ECL of
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10.1002/chem.202002647
Chemistry - A European Journal
RESEARCH ARTICLE
their crystalline films thanks to CIEE.[15] In fact, this is the first
time that CIEE is invoked to describe ECL enhancement, and
our observations and conclusions complement aggregation-
induced electrochemiluminescence (AIECL), a topic recently
reviewed by Xu for several compounds.[16]
compared with those of 4T1. The observed redox potentials
agree well with DFT calculation results (Figure 1A).
Figure 1A also displays the ECL–voltage curve (green curve)
corresponding to the CV described above. Both the first and
second oxidation peaks of 4T2 exhibit ECL, but no ECL was
found in the cathodic scan, and 4T1 has only one cathodic ECL
signal (Figure S1A). For 4T2, the generally proposed
mechanism of the first ECL peak[1-2] involves the reaction
between radical cations and anions, the production of excited
states, and the radiative relaxation of the excited states with light
emission. The second ECL wave (3 nA) is stronger than the first
(0.8 nA) because of the formation of the 4T2 dication, which can
be rationalized as follows: The transfer of one electron in the
HOMO of neutral 4T2 to the LUMO of the 4T2 dication produces
two radical cations to thus increase the intensity of the second
ECL (inset scheme in Figure 1A). The maximum ECL intensity of
Results and Discussion
Among the several available synthetic routes for benzosilole,[17]
we chose to begin our synthesis with a double trimethylsilylation
of the bromobenzonitrile 1 (Scheme 1).[18] Lithium halogen
exchange followed by treatment with trimethylborate gave the
boronic acid 2 (52% yield over three steps). Reaction of 2 with
acetylene 3 (n = 1, 2) by rhodium(I)-catalyzed Chatani alkyne
cycloaddition afforded the thienyl-appended benzosilole adducts
4T1 and 4T2 without any complication from the nitrile.[19]
Mechanistic studies suggest that extremely harsh conditions are
needed to cleave the C(alkyl)–Si bond.[20] In contrast, in the
Chatani reaction, a Rh(I)-mediated activation process gives the
annulative coupling products 4T1 and 4T2 under relatively mild
conditions with 63% and 54% yield, respectively.
4T2 reached
3 nA. When Ru(bpy)3(PF6)2 of the same
concentration was used as the reference, the ECL efficiency of
4T2 and 4T1 was calculated to be 2.3% and 2.1%, respectively.
Scheme 1. Synthesis of benzosiloles 4T1 and 4T2. LDA = lithium diiso-
propylamide, cod = cyclooctadiene, DABCO = 1,4-diazabicyclo[2.2.2]octane.
Figure 1A shows the cyclic voltammograms (CVs, grey curve) of
0.5 mM 4T2 in dichloromethane (DCM) with 0.1
M
tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte. When the potential was scanned positively, 4T2
underwent three oxidation reactions to produce its radical cation
at 1.12 V, dication at 1.90 V, and radical trication at 2.20 V,
respectively. In contrast, only one reduction peak was observed
at −1.99 V in the cathodic scan. DFT calculations visualize the
electron density across the frontier molecular orbitals in the
electronic structure of 4T2 (inset of Figure 1A). Specifically, the
electrons in the HOMO are well delocalized across a conjugated
system that encompasses the silole, the aryl group, and the
bithiophenyl group at the α-position of the silole ring, whereas
the electrons in the LUMO are mainly spread over the
benzosilole core. Note that neither the HOMO nor the LUMO
shows electrons across the β-bithiophenyl. Without doubt, the
oxidation took place in the conjugated system whereas the
reduction occurred mainly within the benzosilole moiety. The
electrochemistry of 4T1 (Figure S1) reveals two oxidation waves
and one reduction wave at 1.18, 1.67, and −1.84 V, respectively.
It is speculated that because 4T2 has one more thiophene unit,
the extended conjugation and coplanarity of the benzosilole and
bithiophenyl moieties in 4T2 facilitate the oxidation reactions
Figure 1. Cyclic voltammograms (CVs, grey) of 0.5 mM 4T2 in DCM at a scan
rate of 0.1 V/s with 0.1 M TPAP as the supporting electrolyte (A) scanning to
both positive and negative potentials in the absence of 5 mM BPO, and (B)
scanning only to negative potential in the presence of 5 mM BPO. Insets in (A)
are the molecular orbitals iso-surface plots for the HOMO and LUMO of 4T2
based on the DFT/B3LYP/6-31G* calculations. The ECL voltage curves are
overlaid at the bottom in green.
We then added benzoyl peroxide (BPO) as an oxidizing
coreactant to improve the observed low ECL signal (Figures 1A
2
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and S1A) from the annihilation route of both 4T2 and 4T1. In the
presence of 5 mM BPO, the ECL intensity of 4T2 was enhanced
significantly to ~550 nA (Figure 1B), but this coreactant ECL
Table 1. PL and ECL peak wavelength and ECL efficiency for both 4T1 and
4T2 in solution and crystal solid state.
PL Peak
ECL Peak
ECL
process gives
a low ECL efficiency of 0.4% when the
Wavelength
Wavelength
Efficiency
Ru(bpy)3(PF6)2/BPO coreactant system was used as the
benchmark. When the voltage was pulsed at 10 Hz between
0.10 V and the cathodic potential limit (−2.40 V), the ECL
intensity of 4T2 reached ~3 μA (Figure S2A). The pulsing
experiment changed the applied potential rapidly enough to
decrease the time for the electrochemically generated silole
radicals to decay, and the ECL intensity thus became much
higher. For 4T1 with 5 mM BPO, the ECL intensity was
increased to 120 nA in potential scanning and to 400 nA in
pulsing, but the 4T1/BPO system has an even lower ECL
efficiency of 0.08% based on Ru(bpy)3(PF6)2/BPO.
4T1 solution
4T1 crystal film
4T2 solution
428 nm
533 nm
475 nm
602 nm
602 nm
528 nm
630 nm
607 nm
0.08%
2.5%
0.4%
6.5%
4T2 crystal film
Two possible mechanisms of excimer generation in the ECL
route are considered. For both, at around −0.80 V, BPO is firstly
reduced to generate a benzoate radical (PhCOO•), which then
oxidizes 4T2 to produce 4T2•+. In the first mechanism, at about
−1.70 V, excess neutral 4T2 is reduced to 4T2•−, which then
reacts with 4T2•+ to generate a dimer as the excimer to emit ECL.
In the second mechanism, after 4T2•+ is generated from the
oxidation by PhCOO•, a typical electrochemical dimerization can
occur[21] at the thiophene units in 4T2 to produce a ground state
dimer, and the ground state dimer then reacts with BPO to
become the excimer via a classical ECL reaction. In the pulsing
experiment, the applied potential is alternated very quickly, and
the ECL would not improve if a slow electrochemical process
such as the generation of ground state dimer[21] is involved.
Hence, the observed ECL enhancement by pulsing effectively
rules out the second mechanism. The solution ECL here has to
have the first mechanism because the emission wavelength in
the spooling spectrum shows only the dimer and no monomer as
the excimer. At −1.70 V, the potential is high enough to produce
both the radical species of the 4T2 dimer and the BPO radical.
As the cathodic scan continues, the increasingly negative
potential generates a greater number of radical species, and the
ECL intensity thus escalates and peaks at −1.98 V. When the
voltage is applied in the reverse direction, the ECL intensity
decreases due to the reverse reaction between the 4T2 dimer
and the BPO radical species before ECL disappears at −1.68 V.
The ECL behaviors from real-time spooling match well with the
ECL–voltage curve in Figure 1B. The ECL peak of 4T1 at 602
nm also has a redshift of 174 nm from the PL at 428 nm (Table 1,
Figure 2B).
Figure 2A displays the spooling ECL spectra of the above
4T2/BPO coreactant system acquired with a potential scanning
cycle between 0.10 V and −2.10 V, in which the two ECL
features are color-labeled in orange (−1.70 V to −1.98 V) and
purple (−1.98 V to −1.68 V), respectively. The wavelength of the
ECL emission peak is at 630 nm throughout the scan, which is
redshifted from the PL at 475 nm (Table 1). This huge difference
is ascribed to the drastically different excited states of the PL
(monomer excited states) and ECL processes (dimer excited
states, excimer).
After recording the low ECL efficiency of 4T2 and 4T1 in solution,
we dropcast them to prepare a film for each on glassy carbon
working electrodes to assess the ECL performance from the
resulting crystal filmed-electrodes. Gratifyingly, upon irradiation
at 254 nm, the crystal film of 4T2 on the electrode gave an
orange emission and the crystal film of 4T1 gave a green
emission (insets in Figure 3A,B). Additional experiments were
carried out to verify the solid films on the electrodes are indeed
crystalline. We firstly recrystallized 4T2 and carefully transferred
the crystals to a mortar. At this point, the crystals gave off an
orange color both in daylight (Figure 4A) and under 254 nm UV
(Figure 4B). The crystals were then ground with a pestle[22] to
give amorphous 4T2, which then showed not an orange color
but a yellow color both in daylight (Figure 4D) and under 254 nm
UV (Figure 4E). Additionally, we dropcast the DCM solution of
4T2 on a piece of quartz glass and also deposited the solution
on a TLC plate (Figure 4C). On the quartz glass, 4T2 remained
in its crystalline state[13c, 23] and gave the same orange color. In
contrast, the 4T2 dispersed in the silica gel matrix became
amorphous,[13c] and the observed color became yellow (Figure
4F). Therefore, the thin film of 4T2 on the glassy carbon
Figure 2. Spooling ECL spectra of 0.5 mM (A) 4T2 and (B) 4T1 in DCM in the
presence of 5 mM BPO at 0.02 V/s scan rate with 2 s accumulation time for
each spectrum.
3
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electrode exhibits crystalline property because the electrode
displays an orange color under UV irradiation (inset of Figure
3A).
Testing the ECL of the filmed-electrode requires a careful
selection of the solvent. The solvent needs to be able to make
intimate conductive contact with the film surface while not
dissolving the film. DCM easily dissolves the films of 4T1 and
4T2 and clearly cannot be used. Although aqueous systems are
6c, 14]
widely reported,[4a,
the films of 4T1 and 4T2 are highly
hydrophobic and show no electrochemical or ECL behavior in
purely aqueous system (Figure S3). After screening a number of
common solvents for electrochemistry and ECL compatibility, we
found that the films have relatively poor solubility in acetonitrile,
although they are not yet insoluble enough in acetonitrile (see
ECL results in Figure S4). Eventually, the filmed-electrode ECL
of 4T1 and 4T2 was assessed in 1:1 v/v water/acetonitrile.
Figure 3. Cyclic voltammogram at 0.1 V/s scan rate of (A) 4T2 films in
annihilation route, (B) 4T1 films in annihilation route, (C) 4T2 films with 5 mM
BPO as coreactant, and (D) 4T1 films with 5 mM BPO as coreactant. The
insets of (A) and (B) are the crystal films of 4T2 and 4T1 modified on glassy
carbon electrodes under UV 254 nm irradiation, respectively. The ECL voltage
curves are overlaid at the bottom in purple (A and C) and blue (B and D).
Figure 3A,B shows the CV and ECL–voltage curves of 4T2 and
4T1 crystal films in 1:1 v/v water/acetonitrile containing 0.1 M
tetramethylammonium perchlorate (TMAP) as the supporting
electrolyte. Although the shapes of the peaks seem somewhat
different, 4T2 crystal films also display three oxidations and one
reduction, and 4T1 films also show two oxidations and one
reduction, both in agreement with the observations of the 4T2
and 4T1 solutions. No ECL was observed in the annihilation
route for the films because they are subject to surface effect.[24]
Specifically, when 4T2 and 4T1 are prepared as crystalline thin
films, the electrochemical and luminescent behaviors become
restricted because their radical species are no longer available
in solution. In contrast, the solution ECL is related to the
concentration change of reactive species in the vicinity of the
working electrode and thus can readily occur via annihilation.[24]
The addition of 5 mM BPO into the solution greatly enhanced
the ECL of the filmed electrodes (Figure 3C,D), with the 4T2
system giving nearly 1.1 μA and the 4T1 system 600 nA. The
ECL intensity became even stronger in the pulsing experiments
(Figure S5), giving 4 μA for 4T2 and 1 μA for 4T1, respectively.
The accumulation ECL spectrum of the 4T2 film shows peak
wavelength at 607 nm (Figure 4G), which is now almost the
same as the PL peak of the 4T2 crystal (602 nm, Figure 4G and
Table 1). Because the film dropcast on the glassy-carbon
electrodes is verified to be crystalline (Figure 4G), the crystals
must have the same excited states in both PL and ECL process.
The PL experiments of the solid films were conducted using the
same spectrometer for ECL along with a 405 nm laser excitation
source. Compared with the solution PL, the crystalline PL
signals of 4T1 and 4T2 are both significantly enhanced and
strongly redshifted (insets of Figure 5). Figure 5 shows spooling
ECL spectra of the filmed electrode with (A) 4T2 and (B) 4T1 in
the presence of 5 mM BPO, which depict the same peak
wavelength for each compound during ECL evolution and
devolution. It is evident by the spooling ECL spectroscopy that
both 4T2 and 4T1 possess only one excited state each.
Figure 4. 4T2 crystals under daylight (A) and under 254 nm irradiation (B). (C)
Amorphous 4T2 on TLC plate (left) and crystalline 4T2 on quartz (right).
Amorphous 4T2 under daylight (D) and under 254 nm irradiation (E). (F)
Amorphous 4T2 in silica gel matrix (left) and crystalline 4T2 on quartz (right).
(G) PL spectra of amorphous (gold) and crystalline (orange) 4T2 along with
ECL spectrum of a crystal film electrode in contact with 5 mM BPO in
water/acetonitrile electrolyte solution.
A single crystal suitable for X-ray analysis was harvested from
the DCM solution of 4T2. In the X-ray structure of 4T2 (Figure
6A), the benzosilole core and the phenyl group resides in the
same plane along with the α-bithiophenyl group. That is, 4T2
has a highly conjugated system consisting of these three parts.
Note that the bithiophenyl group at the β-position of the silole is
twisted away from the conjugated system (see discussion
4
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below). Hence, redox reactions happen in the conjugated
system but not the β-bithiophenyl, as is also seen from the DFT
calculations (inset of Figure 1A).
hydrogen bonding and π-π stacking, key factors for CIEE,[15] are
both lacking in the 4T2 molecule, conformation rigidification is
still noted for 4T2 and established unambiguously from several
intriguing intermolecular interactions (Figure 7). For example,
the hydrogen atom (H18, electron-deficient) at the far end of the
α-bithiophenyl group seems to have an electrostatic interaction
with the β-bithiophenyl group (C3 and C4 in the thiophene ring
containing S1, electron-rich) in the neighboring benzosilole
(Figure 7A, Table 2), and this interaction presumably helps
enhance the properties of 4T2 in comparison with 4T1. Besides,
the nitrogen atom on the 4-cyano substituent of 4T2 is also
affected by the β-bithiophenyl moiety (interaction between N1
and the H6 atom on the thiophene containing S3, Figure 7B,
Table 2). In addition, the methyl groups on the Si atom seem to
sterically restrain the rotation of the thiophene ring in the
neighboring 4T2 molecule through two independent steric
obstructions (Figure 7C,D, Table 2). Lastly, interaction is also
noted between the proximal thiophene (S3) in the α-bithiophenyl
and the distal thiophene (S1) in the β-bithiophenyl (Figure 7E,
Table 2). All these intermolecular interactions contribute to the
conformation rigidification of 4T2 crystal and enhance the PL
emission. The dihedral angle between intersecting benzosilole
planes in 4T2 is 98.91° (Figure S8), which clearly indicates J-
aggregation (Figure 8). The 3-bithiophene moiety intersects the
conjugated benzosilole within the same molecule at 99.60° and
the neighboring benzosilole at 125.50°, respectively (Figure S8).
The 4T2 crystalline film has a relative PL quantum yield of nearly
6%, about three times that of 4T2 in solution (2%). It is also
Figure 5. Spooling ECL spectra of the filmed electrode with (A) 4T2 and (B)
4T1 in the presence of 5 mM BPO at a scan rate of 0.02 V/s and exposure
time of 2 s. Insets on the left in both (A) and (B) are the PL spectra of 4T2 in
solution (grey) and solid state (black). Insets on the right in both (A) and (B)
display the photo of the electrochemical cell used for the ECL experiment
under the UV irradiation (265 nm).
Figure 6. X-ray crystal structures of 4T2: (A) single molecule, (B) within the
packing system. The dichloromethane solvent molecule is removed for clarity.
The conformations of 4T1 and 4T2 in solution are illustrated in
the insets of Figure 1A. The PL emission of 4T1 and 4T2 is less
pronounced in solution because the excited states can be
deactivated nonradiatively by various intramolecular motions
(e.g., rotation of the bonds between the silole core and the
thiophenyl/bithiophenyl group). In contrast,
restrictions on the conformational freedom can be noted for the
crystalline structure of 4T2. Although conventional forces like
a number of
Figure 7. Intermolecular interactions in crystalline 4T2. (A) H-π interaction
between H18 and C2=C3. (B) Interaction between H6 and N1. (C) Interaction
between S1 and S3. (D) Steric hindrance between H20 and C13. (E) Steric
hindrance between H20 and S3.
5
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RESEARCH ARTICLE
recently summarized by Xu[16] has been reported for 1,1'-
disubstituted
2,3,4,5-tetraphenylsiloles,[4a]
carboranyl
carbazoles,[14] tetraphenylethylene nanocrystals,[26] and platinum
complexes,[27] but the performance difference between
amorphous aggregate and crystalline film states has not been
clearly shown prior to this report on benzosiloles. Is CIEE-ECL
just an example of a well-ordered aggregate? We contend that
CIEE-ECL should not be considered equivalent to AIECL, but
rather it should be seen as an essential complement to AIECL.
In the present case, ECL enhancement differs for electrodes
modified with luminophores that exist as crystalline films or
amorphous aggregates, and differentiating the ECL outcome
can be simplified by invoking CIEE-ECL for crystalline films.
Table 2. Short contacts in the crystal structure of 4T2.
Short Contact Atom 1[a] Atom 2[a] Length[b] Length−VdW[b]
H18
H18
C3
C4
2.797
2.897
−0.103
−0.003
#1[c]
#2
H6
N1
2.538
−0.212
H20
H20
C13
S3
2.867
2.913
−0.033
−0.087
#3
#4
S1
S3
3.459
−0.141
[a]
[b]
Atoms numbered as in ORTEP (Figure 6A).
Length given in Å. [c] The
dihedral angle between the S1 thiophene ring and the nearly isosceles triangle
consisting of C3, C4, and H18 is 88.62°. The dihedral angle between the S4
and S1 thiophene rings is 61.21°. The length of the bond between C3 and C4
is 1.374 Å. The altitude from H18 to the base of the triangle (from C3 to C4) is
calculated based on the law of cosines to be 2.756 Å.
Figure 8. Illustration of J-aggregation of 4T2 with the β-bithiophene (a) hidden
and (b) visualized.
reported that the interactions of chromophores can split the
excited states into two energy levels. As for the case of J-
aggregation which was revealed in 4T2 with twisted structure
and bulky groups (Figure 8), the excitation was from the ground
state to the lower energy level thus produces a redshifted PL
emission (~80 nm in the case of 4T2 from solution to crystal).[25]
It is worth commenting that the ECL efficiency of the 4T2 film
reaches 6.5% and is 16 times that of the 4T2 solution. The ECL
efficiency of the 4T1 film also increases by 24 times compared
with that of the 4T1 solution in DCM. That is, when the films of
4T2 and 4T1 are used instead of their solution, both the ECL
intensity and the ECL efficiency improve greatly due to CIEE, a
phenomenon previously reported by Tang for PL but never
illustrated for ECL until the current work (see disambiguation
between CIEE and AIEE below).[13c, 15] The solid crystal films of
4T2 and 4T1 have a rigid molecular packing (Figure 2B) that
Conclusion
In summary, photoluminescence and ECL enhancement of two
benzosiloles have been induced by crystallization, and was
revealed by spooling ECL spectroscopy, X-ray crystallography,
photoluminescence, and DFT calculation. The CIEE-ECL
system is simple to design and convenient to use. Additionally,
this work establishes a heterogeneous ECL performing system
for highly hydrophobic organic compounds, and showcases the
CIEE strategy to greatly enhance ECL intensity. Our
experimental
results
complement
aggregation-induced
electrochemiluminescence (AIECL) and provide a platform for
designing and processing more efficient optoelectronic devices.
prohibits intramolecular rotations, diminishes nonradiative
15]
relaxation, and boosts the PL and ECL.[13c,
In contrast, in
Acknowledgements
solution, the electrochemically generated excited states of 4T2
and 4T1 mainly return to the ground state by nonradiative
relaxation via single bond rotations of the benzosilole–thiophene
and thiophene–thiophene units, which seriously weaken the
emissive process.
We thank the following institutions for the financial support of
this research: the Natural Sciences and Engineering Research
Council of Canada (NSERC, DG RGPIN-2014-04891 (BLP) DG
RGPIN-2013-201697 (ZD) and RGPIN-2018-06556 (ZD)),
Canada Foundation of Innovation/Ontario Innovation Trust
(CFI/OIT, 9040), Premier’s Research Excellence Award (PREA,
2003 (ZD)), Canada Institute of Photonics Innovation (2005
(ZD)), Ontario Photonics Consortium (2002 ZD)), The University
of Western Ontario (WSS NSERC Bridge 47229 (BLP)), and the
Li Xiaoling research fund (BLP). We also thank the Electronic
Shop and ChemBioStores in the Department of Chemistry at
Western for quality service.
For the films ECL is not feasible via the annihilation route, but
their ECL performance in the coreactant system vastly
outperforms the solutions because ECL reactions occur at the
interface between the benzosilole crystalline film and the
water/acetonitrile solution, instead of only within the film as in
the annihilation route. To the best of our knowledge, this is the
first example where the ECL performance is demonstrated to
improve significantly by CIEE. Previously, aggregation-induced
emission enhancement (AIEE) of ECL, also known as AIECL, as
6
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10.1002/chem.202002647
Chemistry - A European Journal
RESEARCH ARTICLE
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Angew. Chem. Int. Ed. 2019, 58, 14173-14178.
Keywords: benzosilole • silole • crystallization-induced emission
enhancement • photoluminescence • electrochemiluminescence
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7
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10.1002/chem.202002647
Chemistry - A European Journal
RESEARCH ARTICLE
Entry for the Table of Contents
The photoluminescence and electrochemiluminescence (ECL) of two benzosiloles are significantly enhanced upon their
crystallization. The developed ECL system with crystallization-induced emission enhancement is clearly differentiated from ECL
systems with aggregation-induced emission enhancement, and it is also anticipated to find applications in ECL sensing and
optoelectronics.
8
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