Synthetic Regulation of 1,4-Dihydropyridines for the AIE or AIEE Effect: From Rational Design to Mechanistic Views

Article abstract of DOI:10.1002/chem.201705269

Aggregation-induced emission/aggregation-induced emission enhancement (AIE/AIEE) has recently attracted intense research, and a large number of AIE/AIEE luminogens (AIE/AIEEgens) have been constructed for application in diverse scientific fields. The AIE and AIEE effects have similar, but not identical, photophysical behaviors, which are closely related to molecular architectures. However, the current understanding of the inherent differences between AIE and AIEE is still obscure. Herein, a rational design strategy is reported for achieving AIE and AIEE effects by simply incorporating different substituents at the periphery of the same core skeleton. Experimental and theoretical studies on the series of compounds indicated that the restriction of intramolecular twisting motions or/and rotations plays an important role in regard to the corresponding AIE or AIEE behaviors. Moreover, compound 1 a (FW=203, Φ_F=80.9 %) was discovered as the lowest molecular weight AIEEgen with a high quantum yield in the solid state despite having no rotatable units. Compound 2 a also exhibited an AIEE effect with the minimum necessary structure (a single ring).

Full text of DOI:10.1002/chem.201705269

DOI: 10.1002/chem.201705269
Full Paper
&
Aggregation-Induced Emission
Synthetic Regulation of 1,4-Dihydropyridines for the AIE or AIEE
Effect: From Rational Design to Mechanistic Views
Wei Zhang, Na Wang,* Yuan Yu, Yi-Min Shan, Bing Wang, Xue-Mei Pu, and Xiao-Qi Yu*[a]
Abstract: Aggregation-induced emission/aggregation-in-
duced emission enhancement (AIE/AIEE) has recently attract-
ed intense research, and a large number of AIE/AIEE lumino-
gens (AIE/AIEEgens) have been constructed for application
in diverse scientific fields. The AIE and AIEE effects have simi-
lar, but not identical, photophysical behaviors, which are
closely related to molecular architectures. However, the cur-
rent understanding of the inherent differences between AIE
and AIEE is still obscure. Herein, a rational design strategy is
reported for achieving AIE and AIEE effects by simply incor-
porating different substituents at the periphery of the same
core skeleton. Experimental and theoretical studies on the
series of compounds indicated that the restriction of intra-
molecular twisting motions or/and rotations plays an impor-
tant role in regard to the corresponding AIE or AIEE behav-
iors. Moreover, compound 1a (FW=203, FF =80.9%) was
discovered as the lowest molecular weight AIEEgen with a
high quantum yield in the solid state despite having no ro-
tatable units. Compound 2a also exhibited an AIEE effect
with the minimum necessary structure (a single ring).
Introduction
ated with their structures, although their molecular frameworks
may show only subtle differences. To date, the understanding
of the relationship between molecular structure and the corre-
sponding AIE/AIEE effect remains limited. Consequently, the ra-
tional design of luminogens with distinct AIE and AIEE effects
from the same core has never been reported.
Aggregation-induced emission/aggregation-induced emission
enhancement (AIE/AIEE) phenomena have attracted extensive
interest because they are the exact opposite processes to the
notorious aggregation-caused quenching (ACQ) effect and
pave an avenue for creating aggregate-state emitters.[1] The
scaffolds of most AIE luminogens (AIEgens) feature multiple ar-
omatic rotors linked to a stator by single bonds, and the con-
struction of AIEgens generally complies with this principle.[2]
The rotation of the aromatic rings consumes most of the excit-
ed-state energy and results in near nonfluorescence in solu-
tion, whereas restriction of intramolecular rotation (RIR) in ag-
gregates leads to conversion of the energy into photons and
fluorescence turn-on. AIEEgens, which are similar to but not
exactly the same as AIEgens, display gradually enhanced fluo-
rescence with increasing degree of aggregation.[3] These con-
structs generally contain smaller or/and fewer rotatable units
and/or have smaller twisting amplitudes that weaken their ca-
pacity to serve as a relaxation channel for the deactivation of
excitons.[4] AIEE is considered to be a special case of AIE, and
many AIEEgens are also arbitrarily classified as AIEgens from
simple comparisons of the fluorescence efficiencies of the lu-
minogens in the solution and solid states. However, the differ-
ent photophysical behaviors of AIE/AIEEgens are closely associ-
In contrast to their propeller-like counterparts, some newly
emerging AIE/AIEE systems do not contain rotors, and these
systems cannot simply be interpreted by the RIR model.[5]
Thus, the mechanisms of the AIE/AIEE effects are further uni-
fied under the more universal term of restriction of intramolec-
ular motion (RIM).[6] Deciphering the AIE/AIEE processes ena-
bles the design and synthesis of novel AIE/AIEEgens.[7]
Although numerous AIE/AIEE molecules have been construct-
ed, most of them are very complex with high molecular
weights and low quantum efficiencies, and they are often pro-
duced in tedious reaction steps. Such hurdles have greatly im-
peded the advancement of AIE/AIEEgens. Therefore, the devel-
opment of small structures (M<300) with high fluorescence
quantum yields is appealing from both fundamental and prac-
tical viewpoints, yet a significantly challenging task.[8]
1,4-Dihydropyridines (1,4-DHPs) and related heterocyclic
compounds, which have a broad spectrum of biological prop-
erties, are usually obtained by the convenient Hantzsch-type
reaction.[9] Over the past few decades, the photophysics and
photochemical behavior of structurally diverse 1,4-DHPs have
been widely investigated.[10] Recently, we reported a series of
single-ring 1,4-DHPs as the minimal structural requirement for
establishing the AIEE effect.[11] The series of compounds also
exhibited high specificity toward mitochondria with excellent
photostability and storage stability. For these single-ring sys-
tems, gradually restricting rotations of the small ester groups
and distortions of the DHP core in aggregates enables them to
[a] W. Zhang, N. Wang, Y. Yu, Y.-M. Shan, B. Wang, X.-M. Pu, X.-Q. Yu
Key Laboratory of Green Chemistry and Technology, Ministry of Education
College of Chemistry, Sichuan University
Chengdu 610064 (P. R. China)
E-mail: wnchem@scu.edu.cn
xqyu@scu.edu.cn
Supporting information and the ORCID number(s) for the author(s) of this
article can be found under https://doi.org/10.1002/chem.201705269.
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show increased fluorescence in a typical AIEE phenomenon.
Considering the facile modification of 1,4-DHP through a
simple one-pot multicomponent reaction, 1,4-DHP may be an
ideal candidate for constructing AIE/AIEEgens with different
mechanisms. Following this train of thought, we envisioned
that the compounds could also show an AIEE effect through
restriction of the small-amplitude intramolecular motions when
the ester groups are replaced with carbonyl groups. Further at-
tachment of large phenyl rings to the compounds is expected
to create an AIE effect through the strong ability of the phenyl
rotors to consume energy through large-amplitude rotations.
Herein, we report the design and synthesis of several 1,4-
DHP derivatives by a Hantzsch-type reaction. The introduction
of different substituents at the periphery of the DHP core not
only expanded the structural types but also provided insights
into the relationship between the structure and the photo-
physical properties. We explored the fluorescence properties of
these compounds in detail and found that the 1,4-DHPs exhib-
ited distinct AIE/AIEE phenomena: 1) compounds with open
carbonyl groups showed an AIEE effect depending on the re-
striction of intramolecular rotation and twisting motion (RIR-
RITM), 2) compounds with both open carbonyl and phenyl
groups showed an AIE effect depending on the RIR-RITM pro-
cess, and 3) compounds with enclosed carbonyl groups
showed an AIEE effect depending on the restriction of intra-
molecular twisting motion (RITM). To the best of our knowl-
edge, no single luminogen system has yet been developed
whose emission behavior could be regulated between AIE and
AIEE by simply altering the peripheral substituents of a single
core framework.
Figure 1. Molecular design based on 1,4-DHP for the AIE or AIEE effect.
motions. Thus, the two compounds had very similar molecular
structures but completely opposite emission behaviors.
Although CD-5 exhibited an ACQ effect, CD-7 showed typical
AIE characteristics, which originated from its twisting motion
and structural nonplanarity. We proposed that the same strat-
egy could also be applied to the construction of rotor-free AIE/
AIEEgens based on the DHP core to further increase the scope
of the AIE/AIEE family (Figure 1C). Three 1,4-DHP derivatives
that contained two five-, six-, and seven-membered rings on
both sides of the core framework were designed. The rules of
geometry state that the three adjacent vertices of a heptagon
are fixed, which allows this polygon to fold, whereas a penta-
gon is unable to fold. This difference in ring flexibility between
five- and seven-membered rings was responsible for the oppo-
site emission behaviors of the two coumarin derivatives. In the
1,4-DHP system, only two adjacent vertices of each polygon
are fixed, which allows the pentagon to also twist. Further-
more, we deduced that the twisting motions of the flexible ali-
phatic rings would impart weaker nonradiative deactivation
ability than phenyl rotors on aggregation and thus result in an
AIEE process, which is in complete contrast to the ACQ effect
observed for CD-5.
With the molecular design protocols in mind, several 1,4-
DHP derivatives were first synthesized to test the hypothesis.
The derivatives were readily prepared by one-pot Hantzsch re-
actions of acetaldehyde, ammonium acetate, and various b-di-
carbonyl compounds as substrates according to the published
procedure (Scheme 1).[13] All the desired products were purified
by column chromatography with petroleum ether/ethyl ace-
tate or recrystallized from ethanol. The 1,4-DHPs were charac-
terized by 1H and 13C NMR spectroscopy and high-resolution
mass spectrometry (see Supporting Information), which con-
firmed their molecular structures. The 1,4-DHPs were relatively
Results and Discussion
Molecular design
In our previous work, a series of single-ring 1,4-DHPs were pre-
pared, and their progressively enhanced fluorescence intensi-
ties on adding water to their solutions in THF manifested their
AIEE behavior.[11] Restriction of the twisting motions of the core
ring along with the rotations of the two ester groups favors ef-
ficient radiative decay, and thus the fluorescence intensities of
the compounds are enhanced on aggregation. We envisaged
that the attachment of phenyl rotors to the core skeleton
would further improve the ability of the molecule to dissipate
its excited-state energy through intramolecular rotation com-
pared to that containing small ester groups (Figure 1A). On
the basis of this hypothesis, we presumed that the compounds
would be non-emissive in the dissolved or loosely aggregated
state but display bright fluorescence in the closely aggregated
state, as the molecular motions would be almost completely
restricted in the last-named state. Such a photophysical pro-
cess is consistent with a representative AIE phenomenon.
Moreover, Tang et al. recently investigated the optical prop-
erties of two coumarin derivatives, CD-5 and CD-7, containing
five- and seven-membered aliphatic rings, respectively, without
any rotors (Figure 1B).[12] The size of the aliphatic ring played a
crucial role in controlling the molecular rigidity and twisting
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the knr value was substantially lower in the powder samples
than in solution and was close to or even smaller than the kr
value. These results suggest that aggregation greatly sup-
pressed the nonradiative pathway of energy dissipation
through RIM.
To gain insight into the emission processes of the 1,4-DHPs
on gradual aggregation with AIE or AIEE characteristics, we
measured the fluorescence intensity changes of the com-
pounds in THF/water mixtures with varying water fractions fw.
As shown in Figure 2A–C and F, compounds 1a–c exhibited
faint emissions in pure THF at a concentration of 10 mm with
maxima at 395, 421, and 442 nm, respectively. When fw was in-
creased in small increments, the fluorescence intensities of the
compounds also gradually increased due to aggregation, with
a slight redshift in the maximum emission wavelengths. Thus,
this class of compounds clearly exhibited AIEE behavior, as ex-
pected. The redshift in the fluorescence implied narrowing of
the bandgap, possibly because the increased solvent polarity
elevated the energy level of the HOMO.[14] However, the addi-
tion of more water lowered the fluorescence intensity, which
can be attributed to a corresponding decrease in the solubility.
Dynamic light scattering (DLS) measurements demonstrated
that large nanoaggregates started to form once a small
amount of water was added. When fw values were further in-
creased, the increasing trend of the particle sizes became
slower (Figure S4 in the Supporting Information). The average
sizes of the nanoparticle were 300–400 nm at fw =90% (Fig-
ure S5 in the Supporting Information).
Scheme 1. Synthesis of enclosed/open 1,4-DHPs.
insoluble in water but soluble in common organic solvents
such as THF.
AIE/AIEE properties
Three different types of movable parts were incorporated in
the design of the target luminogens, including open carbonyl
groups, enclosed carbonyl groups, and open carbonyl/ester
groups with phenyl groups around the DHP core. On the basis
of the RIM mechanism and our deductions, all of these com-
pounds were expected to exhibit AIE/AIEE characteristics.
Having obtained a series of structurally diverse 1,4-DHP deriva-
tives, we then evaluated their photophysical properties. The
absorption maxima, fluorescence emission maxima, quantum
yields FF, and fluorescence lifetimes t of the derivatives in
both the solution and solid states are summarized in Table 1
and Figures S1–S3 in the Supporting Information. The quan-
tum yields of all these 1,4-DHPs were much higher in the solid
state than in the solution state, and this clearly suggests that
they are indeed AIE/AIEE-active (high FF values up to 0.809).
We also calculated the radiative (kr) and nonradiative (knr) rate
constants from the corresponding FF and t values. In pure
THF solution, the knr value of each compound was larger than
its kr value by 1–3 orders of magnitude, which implies that
nonradiative decay was the dominant decay channel. However,
Unlike in the propeller-shaped AIE/AIEEgens containing ro-
tatable phenyl rings, the common RIR model cannot explain
the AIEE effect observed for these rotor-free fluorogens.
According to a previous report, the fluorescence enhancement
might stem from the restriction of the out-of-plane twisting
motion of the aliphatic ring in the aggregated state.[12] Notably,
the emission behavior of compound 1a was exactly opposite
to that of CD-5. The AIEE effect of 1a indicates that a five-
membered ring backbone can still be distorted after fixation of
two adjacent vertices, which proves the feasibility of our mo-
lecular design strategies developed on the basis of geometry.
We next investigated the fluorescent signatures of the open-
type 1,4-DHPs (2a,b) under the same experimental conditions
(Figure 2D–F). Interestingly, compound 2a showed a very simi-
lar gradual increase in emission intensity with increasing fw to
Table 1. Photophysical properties of 1a-2c in the solution and solid state.
Compound
Solution[a]
labs
Solid state
lex
[c]
[d]
[c]
[d]
lem
t[b]
FF
kr, knr
[107 sꢀ1
lem
t[b]
FF
kr, knr
[107 sꢀ1
]
[nm]
[nm]
[ns]
]
[nm]
[nm]
[ns]
1a
1b
1c
2a
2b
2c
340
357
367
365
380
351
395
421
442
449
–
0.5
3.1
0.2
<0.1
<0.1
<0.1
0.067
0.162
0.002
0.009
0.004
0.009
12.6, 175.0
5.1, 26.6
0.9, 509.3
37.5, 3915.0
3.9, 1077.2
18.4, 1977.6
325
325
365
340
380
345
420
455
475
480
510
480
1.2
3.7
1.4
3.4
4.8
5.0
0.809
0.712
0.037
0.275
0.196
0.204
67.4, 15.9
19.2, 7.8
2.6, 68.8
8.1, 21.3
4.1, 16.8
4.1, 15.9
–
[a] Measured in THF. [b] On excitation at 371 nm. [c] Absolute quantum yield determined with an integrating-sphere system. [d] Rate constants for radiative
(kr) and nonradiative (knr) decay were calculated from the FF and t values according to the formulas kr =FF/t and kn =(1ꢀFF)/t.
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Figure 2. Fluorescence spectra of A) 1a, B) 1b, C) 1c, D) 2a, and E) 2b in THF/water mixtures with different fw values. F) Normalized maximum emission inten-
sity of 1a–2b as a function of fw. G) Photographs of 1a and 2a,b in THF/water mixtures with different fw taken under UV illumination. Concentration: 10 mm;
excitation wavelength: 340 (1a), 357 (1b), 367 (1c), 365 (2a), 380 nm (2b).
that observed for the enclosed-type counterparts. In contrast
to the mechanism of AIEE activity for compound 1, the two
carbonyl groups of 2a act as rotatable moieties and are mainly
responsible for the AIEE activity through an RIR process. Com-
pound 2a has the minimal structure necessary to elicit AIEE
behavior, namely, a single ring. Considering the strong energy-
consumption ability of aromatic rotors, we further explored
the fluorescence spectra of compound 2b in THF/water mix-
tures with increasing fw values. As illustrated in Figure 2E, no
discernible emission peaks were observed as fw was increased
from 0 to 80%; however, an abrupt emission enhancement
was observed at fw ꢁ90%. Compound 2c also showed a similar
phenomenon due to the similarity of its scaffold to that of 2b
(Figure S6 in the Supporting Information). These results are in-
dicative of a typical AIE process, which is in good agreement
with the behavior observed for the propeller-like AIEgens with
multiple aromatic rotors.
these AIE/AIEEgens adopted a nonplanar conformation. There-
fore, we used four atoms in the central ring (C2, N1, C4, and C5)
to define a dihedral angle q for quantitative analysis of the
degree of distortion (Figure 3A).[16] The q value varied from
152.85 to 173.798 depending on the different substituents at
the periphery (Figure 3 and Figure S7 in he Supporting Infor-
mation). For compound 2c, three independent molecules with
different dihedral angles (151.63, 158.46, and 161.828) existed
in the crystalline state. The twisted nature of the 1,4-dihydro-
N-heterocyclic architecture may help to prohibit close p–p
stacking interactions in the aggregated state and thus avoid
the formation of species that are detrimental to emission.
We further scrutinized the packing modes of the 1,4-DHPs.
Compound 1a crystallized in the orthorhombic space group
Cmc21 in the presence of several drops of water. Multiple inter-
molecular hydrogen bonds of the form OꢀH···O and NꢀH···O
(H···O 1.864 and 1.903 ꢁ, respectively) were observed between
the DHP core/carbonyl groups and water molecules, which sta-
bilized the edge-to-face packing. More importantly, the carbon-
yl groups of the five-membered ring on both sides of the cen-
tral ring adopted slightly twisted conformations with torsion
angles of 173.568. The present system was packed in a loose
pattern owing to the nonplanar three-ring skeletons and hy-
drogen-bonding interactions with water, and the conformation
of the molecules was rigidified by the collective forces of the
Crystal structures
To better understand the AIE/AIEE characteristics of the 1,4-
DHPs, we obtained five single-crystal structures to determine
the molecular conformation and packing modes (1a-2a and
2c).[15] Our repeated attempts to grow single crystals of 2b
were unsuccessful. In general, the DHP core frameworks of all
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Figure 3. Chemical structure of the 1,4-DHPs (A). Single-crystal structures and molecular packing modes of 1a (B and C) and 2a (D and E) showing various
key dihedral angles [8] (blue dashed lines) and NꢀH···O and OꢀH···O interactions with distances [ꢁ]. Carbon, oxygen, and nitrogen atoms are shown in gray,
red, and light blue, respectively.
weak interactions. The results differ from the hypothesis that a
planar conformation and tight packing of CD-5 were responsi-
ble for the severe quenching of the emission intensity in the
aggregate state due to the induced p–p interactions. On the
basis of geometric analysis, a fixed number of vertices in a
five-membered ring alters the mobility of the ring.
blades are crucial for the enhanced emission in the aggregated
state.
In our previous study, the distorted DHP core framework
was forced to change continuously from a puckered to spread
geometry along with rotation of the ester groups. Because the
molecules in the current study are from the same family, we ra-
tionalized that the present AIE/AIEEgens would exhibit similar
behavior. In combination with the results described above, we
therefore concluded that all of the collective factors including
twisted annular conformations, rotatable units, intermolecular
interactions, and packing modes, would endow the lumino-
gens with distinct AIE/AIEE properties.[17]
Compounds 1b and 1c crystallized in triclinic space group
ꢀ
P1 and monoclinic space group P2/c, respectively. As shown in
Figure S7 of the Supporting Information, an antiparallel pack-
ing structure with a face-to-face orientation (interplanar distan-
ces of 4.30 and 4.78 ꢁ) is clearly observed in the crystal struc-
ture of 1b, while several different molecular stacking modes
can be identified in 1c. Intermolecular NꢀH···O hydrogen
bonds with a distance of about 2.9 ꢁ were formed between
the carbonyl groups and DHP rings of neighboring molecules.
Furthermore, the six/seven-membered carbonyl rings exhibited
large twisting amplitudes and impeded close p–p stacking.
Therefore, the AIEE effects of compounds 1a-c can be attribut-
ed to their intrinsic twisting motions. When fully dissolved in
solution, the dynamic intramolecular twisted motions of the
isolated molecules promote nonradiative excited-state decay,
and thus result in poor fluorescence. Such motions are imped-
ed in the aggregated state due to physical constraints, which
block the nonradiative relaxation channels and enable strong
fluorescence.
Theoretical calculations and mechanisms
The electronic structures of the AIE/AIEEgens were calculated
by DFT at the B3LYP/6-311+G(d) level of theory by using the
Gaussian program. The optimized structures, molecular orbital
diagrams, and energy levels are plotted in Figure 4. The fron-
tier molecular orbitals of compounds 1a–2a are distributed
across the entire conjugated framework. The similarity in the
orbital distributions implies that the luminogens have similar
photophysical properties. The HOMOs of 2b and 2c are cen-
tered on the DHP backbone, whereas the LUMOs are delocal-
ized on the two phenyl groups, which exhibit partial charge-
transfer character. Moreover, we performed time-dependent
DFT calculations on the 1,4-DHPs and found that the emission
bands can be predominantly ascribed to the electronic transi-
tion from the LUMO to HOMO (Table S1 in the Supporting In-
formation). Such predicted results are consistent with the ex-
perimental spectroscopic data.
Both open carbonyl-substituted molecules (2a and 2c) stack
head-to-tail and participate in hydrogen-bonding interactions
(NH···O 2.943 ꢁ for 2a and 2.991 ꢁ for 2c) in the crystalline
state (Figure 3 and Figure S7 in the Supporting Information).
The face-to-face antiparallel packing structure of 2a is similar
to that of 1b and has an interplanar distance of 4.86 ꢁ (triclinic
ꢀ
space group P1). The introduction of the two phenyl groups
To quantitatively interpret the geometric relaxations of the
AIE/AIEEgen single molecules, we analyzed the structural
changes jD(S0ꢀS1)j on photoexcitation in the gas phase. The
simulated structures of the S0 and S1 states and the corre-
sponding key distortion angles are summarized in Figure S8
and Table S2 of the Supporting Information, respectively. The
into the backbone of DHP in compound 2c resulted in a
zigzag arrangement to avoid steric repulsion and strong p–p
interactions (monoclinic space group P21/c). Overall, the loose
molecular packing structure and RIR of the multiple rotatable
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Figure 4. Optimized structures, frontier molecular orbitals, and energy levels of the HOMOs and LUMOs of 1a–2c.
geometries of the S1 state are obviously different from those
of the S0 state. First, the 1,4-DHPs with open carbonyl groups
(2a,c) have larger twisting amplitudes than those with ring
substituents, because they are not restricted by covalent
bonds. The extent of deformation in the compounds is corre-
lated to the distortion angles jDj in the decreasing order of
2c>2aꢂ1c>1b>1a. Second, the two large seven-mem-
bered rings in 1c enhance the flexibility and greatly facilitate
out-of-plane movements of the molecular skeleton, which are
similar to the free rotations of the carbonyl groups in 2a.
Third, although 1a experiences the least motion among the
studied compounds, it is still very different from CD-5, which
has a more planar conformation because of its intrinsic struc-
tural rigidity. Fourth, the dihedral angles q in the DHP core
rings are highly dependent on the peripheral substituents, and
thus illustrate that the distorted core framework could be
forced to change along with the motion of the peripheral
groups.
We subsequently modeled the effect of aggregation by cal-
culating the geometric structures (S0 and S1 states) of a large
enough cluster of the crystal structures (taking 1a, 2a, and 2c
as representatives of the three types of substituents) through
a combined quantum mechanics/molecular mechanics (QM/
MM) approach (Figures S9–S11 in the Supporting Informa-
tion).[18] The geometric modifications between the S0 and S1
states of these compounds were smaller in the solid phase
than in the gas phase because the intramolecular motions
were largely constrained in the solid state (Figure 5 and
Table 2). For instance, the changes in the torsion angles be-
tween the two phenyl groups and the DHP core in 2c were
smaller (Df3, Df4=12, 118) in the solid phase than in the gas
phase (Df3, Df4=278, 188). Meanwhile, the change in the di-
Figure 5. Optimized S0 and S1 geometric structures of 1a, 2a,and 2c in the gas and solid phases. The rules for defining the relevant distortion angles are
shown in the upper panel. The key changes in the distortion angles from the S0 state to the S1 state are listed in the lower panel.
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Table 2. Distortion angles [8] of 1a, 2a, and 2c in the gas and solid phases and the changes in the distortion angles from the S0 state to the S1 state (jDj
= j(S0ꢀS1)j).
Distortion angle 1a
gas phase
S0 S1
1a
2a
gas phase
jDj S0
2a
2c
gas phase
jDj S0
2c
solid phase
solid phase
solid phase
jDj S0
S1
S1
jDj S0
S1
S1
jDj S0
S1
jDj
q
171 179
8
1
3
9
1
178
ꢀ177
179
179
ꢀ179
180
1
2
1
151 165 14
173 156 17
151
18
164 13
153
170 17
162
3
173 11
f1
f2
f3
f4
ꢀ178 ꢀ179
178 ꢀ179
20
2
3
–
–
ꢀ6
ꢀ13
7
1
2
1
4
ꢀ5 ꢀ2
3
–
–
ꢀ143 ꢀ140
ꢀ167 ꢀ166
ꢀ163 ꢀ167
ꢀ1
8
2
ꢀ0.2
ꢀ0.5 0.3
–
–
–
–
–
–
–
–
ꢀ123 ꢀ150 27
121 139 18
ꢀ127 ꢀ139 12
125 136 11
1
2
6
4
hedral angle Dq in the DHP core ring also decreased to 118.
These results are consistent with the stronger fluorescence ob-
served for the aggregate state relative to the solution state,
which reveals that the excited-state energy was converted into
photons via RIR and RITM.
ly suppressed, and the fluorescence intensity of the compound
remains low. When clustered into larger aggregates, a large
number of molecules become entrapped inside the particles,
and this results in almost complete RIM and fluorescence turn-
on (AIE effect). For the enclosed analogue of 2a, that is, com-
pound 1a, out-of-plane twisting deformation is the major non-
radiative decay channel in solution. The limited twisting mo-
tions of the carbonyl rings are sensitive to and readily inhibited
by the surrounding molecules on aggregation and thus favor
the radiative channel and an AIEE effect.
Considering the influence of the aggregation behavior on
the photophysical phenomena (induced emission or emission
enhancement), we attempted to develop a tentative explana-
tion for the AIE/AIEEgens with different substituents on the
basis of the above experimental and theoretical results as well
as the mechanism proposed by Tang et al. The mechanisms of
1a, 2a, and 2c are shown in detail in Figure 6 as illustrative ex-
Conclusion
We constructed a series of 1,4-DHP derivatives by simple reac-
tions and investigated their AIE and AIEE properties in detail
(1a is the lowest molecular weight AIEEgen with a high abso-
lute quantum yield in the solid state; 2a contains a single ring,
which is the minimum structure necessary for AIEE). The emis-
sion behaviors of these compounds were readily regulated by
altering the substituents at the periphery of the core frame-
work. On the basis of the experimental and theoretical results,
the relationship between the molecular structures and the cor-
responding AIE/AIEE processes was attributed to three factors
(Table 3). First, the DHP core inherently adopted a twisted con-
formation, which hampered strong p–p intermolecular interac-
tions. Second, the types of substituents played critical roles in
creating the AIE or AIEE signature. Finally, the mechanisms
used to explain the main cause of the AIE/AIEE effect included
RIR/RITM. Overall, AIE and AIEE are two similar photophysical
properties but not exactly the same in detail. We believe that
this study will enable the more flexible design of novel AIE/
AIEEgens for high-tech applications in biotechnology and ma-
terials science.
Figure 6. Proposed AIEE mechanisms for 1a and 2a and AIE mechanism for
2c.
amples of the three types of substituents. As an archetype of
the AIE/AIEE family, 2a contains two small rotatable units that
give rise to AIEE characteristics. We inferred that the rotations
of the carbonyl groups are easily restricted by adjacent mole-
cules on aggregation, and this leads to gradual switching of
the energy loss of excitons from the nonradiative pathway to
the radiative channel. Compound 2c has four rotatable units,
including two ester groups and two large phenyl rotors. The
molecules are loosely packed in the crystalline state owing to
steric repulsion imposed by the aromatic rings, which results
in a large intermolecular free volume in small aggregates.
Therefore, the motions of the rotatable blades cannot be total-
Table 3. Factors summarizing the relationship between the molecular
structures and the corresponding AIE/AIEE effect.
Compound Substituent
Type
Motion Mechanism[a] Phenomenon
capacity
2a
2b,c
carbonyl
carbonyl/es- open
open
weak
strong RIR
RIR
AIEE
AIE
ter+phenyl
1a–c
carbonyl
enclosed weak
RITM
AIEE
[a] In addition to RITM of the 1,4-DHP core.
Chem. Eur. J. 2018, 24, 1 – 9
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Full Paper
Song, W. J. Zhang, M. J. Jiang, H. H. Y. Sung, R. T. K. Kwok, H. Nie, I. D.
Williams, B. Liu, B. Z. Tang, Adv. Funct. Mater. 2016, 26, 824–832; d) S.
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Tang, B. Liu, Appl. Phys. Rev. 2017, 4, 021307.
Acknowledgements
This work was financially supported by the National Program
on Key Basic Research Project of China (973 Program,
2012CB720603 and 2013CB328900), the National Science Foun-
dation of China (No. 21232005). We also thank the Comprehen-
sive Training Platform of Specialized Laboratory, College of
Chemistry, Sichuan University.
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Keywords: aggregation
fluorescence · nitrogen heterocycles · polycycles
· aggregation-induced emission ·
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Version of record online: && &&, 0000
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FULL PAPER
Rational design strategy: By incorpo-
rating different substituents at the pe-
riphery of the same core, the 1,4-dihy-
dropyridines show aggregation-induced
emission or aggregation-induced emis-
sion enhancement effects (see figure).
Experimental and theoretical studies in-
dicated that the restriction of intramo-
lecular twisting motions or/and rota-
tions played important roles in the cor-
responding photophysical behaviors.
&
Aggregation-Induced Emission
W. Zhang, N. Wang,* Y. Yu, Y.-M. Shan,
B. Wang, X.-M. Pu, X.-Q. Yu*
&& – &&
Synthetic Regulation of 1,4-
Dihydropyridines for the AIE or AIEE
Effect: From Rational Design to
Mechanistic Views
Chem. Eur. J. 2018, 24, 1 – 9
www.chemeurj.org
9
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ