Rational design of systematic AIEEgens further modified by substituents from a novel chain structure

Article abstract of DOI:10.1007/s11426-020-9831-y

The expansion of new structures in aggregation-induced emission/aggregation-induced emission enhancement (AIE/AIEE) systems has attracted persistent attention recently, from which more luminescent functional molecules with characteristic skeletons are derived to satisfy specialized applications. In this study, a series of derivatives cored by tetraphenyl enamine with various terminal groups were designed and synthesized based on a novel p-π conjugate chain structure (?C=C–N–). Experimental and theoretical studies reveal that attaching modified groups to enamine core is decisive to achieve successful conversion from non-luminance to AIEE-activity. Moreover, due to different substituent effect on electronic structure, molecular conformation and molecular packing, diverse enamine compounds exhibited prominent substituent tunable emission properties, realizing regulated AIEE effect and multicolor emitting. These results not only offer a new method to design AIEgens/AIEEgens with p-π conjugate chain structures, but also provide in-depth knowledge for functional modifications of more novel AIE/AIEE units and materials. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11426-020-9831-y

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-020-9831-y
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Rational design of systematic AIEEgens further modified by
substituents from a novel chain structure
Hao Lu1, Kun Wang2, Beibei Liu1, Meng Wang1, Mingming Huang1, Yue Zhang1 &
Jiping Yang1*
1Key Laboratory of Aerospace Advanced Materials and Performance, Ministry of Education, School of Materials Science and Engineering,
Beihang University, Beijing 100191, China;
2State Key Laboratory of Advanced Power Transmission Technology, State Grid Global Energy Interconnection Research Institute,
Beijing 102211, China
Received June 2, 2020; accepted July 16, 2020; published online September 15, 2020
The expansion of new structures in aggregation-induced emission/aggregation-induced emission enhancement (AIE/AIEE)
systems has attracted persistent attention recently, from which more luminescent functional molecules with characteristic
skeletons are derived to satisfy specialized applications. In this study, a series of derivatives cored by tetraphenyl enamine with
various terminal groups were designed and synthesized based on a novel p-π conjugate chain structure (–C=C–N–). Experi-
mental and theoretical studies reveal that attaching modified groups to enamine core is decisive to achieve successful conversion
from non-luminance to AIEE-activity. Moreover, due to different substituent effect on electronic structure, molecular con-
formation and molecular packing, diverse enamine compounds exhibited prominent substituent tunable emission properties,
realizing regulated AIEE effect and multicolor emitting. These results not only offer a new method to design AIEgens/AIEEgens
with p-π conjugate chain structures, but also provide in-depth knowledge for functional modifications of more novel AIE/AIEE
units and materials.
aggregation-induced emission enhancement, chain structure, enamine derivatives, substituent tunable emission
Citation:
Lu H, Wang K, Liu B, Wang M, Huang M, Zhang Y, Yang J. Rational design of systematic AIEEgens further modified by substituents from a novel
chain structure. Sci China Chem, 2020, 63, https://doi.org/10.1007/s11426-020-9831-y
1 Introduction
tom molecules and clusterization-triggered luminophores
[10–12]. Among these types of AIEgens/AIEEgens, organic
luminescent molecules with conjugate chain structures are
regarded as a crucial constitution. Generally, most AIE/AIEE
luminogens feature propeller-like configuration with several
rotors attaching to a stator, which are obedient to the most
accepted AIE/AIEE mechanism, namely as the restriction of
intramolecular motion (RIM) [13–15]. From this point of
view, conjugate chains should be a benign candidate of stator
structures, and a series of phenyl-substituted AIE/AIEE
systems owning conjugate chains have been reported, such
as tetraphenylethenes, tetrathienylethenes, distyrylanthra-
cene [16–18].
Aggregation-induced emission/aggregation-induced emis-
sion enhancement luminogens (AIEgens/AIEEgens), in-
itially proposed and investigated by Tang et al. in 2001, are
of great significance due to their wide applications in chemo-
sensors, bio-imaging, optoelectronics [1–9], etc. So far,
hundreds of compounds have been exploited to exhibit AIE/
AIEE properties. Based on the primary characteristic of
molecular structures, they can be divided into the following
categories as pure hydrocarbon aromatics, organic heteroa-
*Corresponding author (email: jyang@buaa.edu.cn)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
chem.scichina.com link.springer.com

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Lu et al. Sci China Chem
On the other hand, conjugate chains are specific structures
of p-π conjugate chain structure fluorophore systems and
luminescent applications.
formed in numerous characteristic reactions. For example,
diene chain (–C=C–C=C–) is prone to be generated in fa-
mous metal coupling reaction [19]. As well, Schiff base re-
action always occurs accompanying with the formation of
characteristic imine chain (–C=N–) [20]. According to these
investigations, conjugate chains may be acted as a bridge to
combine macroscopical design and specific synthetic process
of AIEgens/AIEEgens. In fact, some characteristic chain
structures were already developed in the previous study of
AIE/AIEE molecules. On the basis of 1,3-butadiene struc-
ture, Prof. Dong’s group [21,22] synthesized a series of AIE
molecules, applying in the fields of chemical detection,
biological probe and multicolor switching. Also, Han et al.
[23,24] stated a number of compounds containing Schiff
base chain, which displayed obvious AIE effect and favor-
able biocompatibility. These results showed that conjugate
chains can afford a novel route to design AIE/AIEE systems
with unique photo-physical properties.
However, reported AIEgens/AIEEgens with conjugate
chains are often confined to π-π conjugation [25,26]. By
comparison, facile design and in-depth study about AIE/
AIEE compounds possessing characteristic p-π conjugate
chains have seldom been recorded. Actually, p-π conjugates
are usually related to desirable heteroatom structures, such as
N atom, S atom and B atom. These electron-deficient and
electron-withdrawing parts offer the ability to tune energy
levels, and the attached empty orbitals put up great capacity
to form metal-organic frameworks (MOFs) and covalent
organic frameworks (COFs) [27,28]. Also, the p-π conjugate
chains can be utilized as vital clues to build new AIE/AIEE
systems from the perspective of characteristic reaction [29].
Thus, it is a challenging but appealing task to design novel p-
π conjugate AIEgens/AIEEgens containing heteroatom
chains.
2 Experimental
Totally, five aniline monomer derivatives with different
modified groups in para-position were successfully synthe-
sized, which were elucidated in Scheme 1. According to the
design strategy, we divided these five molecules into three
groups: (1) substituents from weak auxochrome to benign
auxochrome (Br-B1-A1, HO-B1-A1, MeO-B1-A1); (2) dif-
ferent auxochrome substituent number (MeO-B1-A1-OMe);
(3) strong electron-withdrawing substituent (NO2-B1-A1).
These derivatives were all purified by silica gel column
chromatography and recrystallization. As well, their struc-
tures were characterized using mass spectroscopy and
spectroscopic methods. Solubility test indicated that all
compounds were partly soluble in hexane and alcohols and
insoluble in H2O, while most common organic liquids, such
as CH2Cl2, CHCl3, tetrahydrofuran (THF) and N,N-
dimethylformamide (DMF) were their good solvents. Dif-
ferential scanning calorimetry (DSC) test revealed that all
the enamines owned benign thermostability properties with
melting points as 153, 158, 118, 109 and 148 °C for Br-B1-
A1, HO-B1-A1, MeO-B1-A1, MeO-B1-A1-OMe and NO2-
B1-A1, respectively. The specific reaction process, char-
acterization results and thermal analysis results are described
in the supporting information (Sect. S2 and Figures S1–S7,
Supporting Information online).
Stork enamine alkylation is an important organic reaction
with mild reaction condition and approving yield, as well as the
formation of characteristic enamine structure (–C=C–N–),
exerting diverse applications in fields of chemical synthesis,
medicine and biology [30–32], etc. In previous study, our
group has developed a series of oligoaniline-based AIEE-
gens with multiple enamine insertions, yet the substituted
aniline monomer (B1-A1) is not emissive in both solution
and solid state, showing AIE/AIEE-inactive property [33].
So, as a separate research object, the role of p-π conjugate
enamine chain in the design and preparation of novel AIE-
gens/AIEEgens has not been fully recognized. Hence, in this
contribution, we designed a series of aniline monomer de-
rivatives with various substituents (Figure 1). The systema-
tical research reveals the role of p-π conjugate chain structure
in novel AIE/AIEE molecular construction, as well as the
remarkable effect of modified segments to achieve AIE/
AIEE behaviors, providing further guidance to the extension
Figure 1 Reported typical π-π chain AIE/AIEE molecules and the novel
ethylene-N chain AIE/AIEE system (color online).
Scheme 1 Synthesis of enamine derivatives containing different modified
groups (color online).

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Lu et al. Sci China Chem
3 Results and discussion
vatives were recorded in THF (10−4 M), as illustrated in
Figure 2(b). With a view to the transition of local excited
state, all compounds unfolded emission peaks at approxi-
mately 430 nm apart from NO2-B1-A1. At the same time,
some emission shoulders or peaks appeared at about 460 nm
for Br-B1-A1, 500 nm for HO-B1-A1, 510 nm for MeO-B1-A1,
and 540 nm for MeO-B1-A1-OMe. Moreover, it was re-
vealed that corresponding wavelengths of these shoulders
and peaks were dependent on the electronic-donating abil-
ities of diverse modifications. These results mean that the
peaks may be caused by the transition of twisted in-
tramolecular charge transfer (TICT) excited state [35], in-
dicating the possibility to achieve tunable emission by
changing the terminal substituent groups. For NO2-B1-A1,
owing to the existence of conspicuous D-A structure, its PL
emission peak was at around 620 nm with a large Stokes shift
(∆λ>180 nm).
3.1 Photo-physical properties
Firstly, the spectral tests in dilute solution were used to in-
vestigate optical properties of these synthesized enamine
derivatives. Their absorption behaviors and normalized
emission spectra in THF solution (10−4 M) are shown in
Figure 2. The basic spectroscopic parameters are summar-
ized in Table S1 (Supporting Information online).
As shown in Figure 2(a), the compounds with diverse
modifications (bromine, hydroxyl, methoxyl, double meth-
oxyls and nitro) severally showed different absorption bands
with peaks at 319, 304/321, 303/319, 311/336 and 310/410
nm. Also, these enamine derivatives (Br-B1-A1, HO-B1-A1,
MeO-B1-A1, MeO-B1-A1-OMe and NO2-B1-A1) all
showed strong absorption capability with molar extinction
coefficients of 23,626, 27,154, 24,173, 25,014 and
17,900 L mol−1 cm−1, respectively. The absorption at ap-
proximately 310 nm of all substances was typical conjugate
transition of the enamine structure, which corresponded to
the absorption wavelength calculated by the transition en-
ergy of initial B1-A1 structure (Figure S8). Meanwhile, some
shoulder absorptions appeared at the range from 340 to
370 nm for Br-B1-A1, HO-B1-A1, MeO-B1-A1 and MeO-
B1-A1-OMe. Unlike the compounds with moderate terminal
groups, the introduction of strong electron-withdrawing nitro
group greatly changed the absorption feature of corre-
sponding neutral enamine compound, and a sharp long-wa-
velength absorption peak appeared at 410 nm. Valuably, the
bathochromic-shift degree for all enamine derivatives was
consistent with the electron-donating capacities of their di-
verse modifications, implying that the main cause of long-
wavelength absorption should be the intramolecular charge
transfer (ICT) effect from the relatively electron-rich moiety
to the electron-deficient part [34]. Represented by NO2-B1-
A1, the UV absorption spectra in different mixtures of n-
hexane and chloroform demonstrated the long-wavelength
absorptions gradually redshifted accompanying with the in-
creased solvent polarity (Figure S9), further verifying the
ICT feature of long-wavelength absorptions.
3.2 Aggregation-induced emission enhancement char-
acteristics
The AIE/AIEE properties of enamine derivatives were then
explored. The different amounts of water, a poor solvent for
these luminogens, were added to the pure THF solutions by
adjusting the water fractions between 0 and 95%, and the
emission changes were further monitored through PL spec-
tra.
As shown in Figure 3(a), for MeO-B1-A1, when the water
content was less than 70%, slight emission enhancement
occurred. Nevertheless, after the content of water exceeded
70%, the luminous intensity significantly raised. Ultimately,
the luminous intensity of MeO-B1-A1 solution containing
95% water had 90-fold increase in comparison to its pure
THF solution, showing typical AIEE characteristic [36]. In
the inset of Figure 3(a), the fluorescent image also visually
exhibited the tremendous change of MeO-B1-A1 solutions in
THF/water mixtures containing 0 and 95% water. Usually,
the mutation point is closely related to the formation of nano-
aggregates, demonstrating coincident correlation between
aggregating state and fluorescence intensity in the MeO-B1-
A1 system [37]. The PL intensity hardly changed before the
Furthermore, the emission spectra of these enamine deri-
Figure 2 Absorption behaviors (a) and normalized emission (b) spectra of five enamine derivatives in THF (concentration: 10−4 M; excitation wavelengths:
365 nm for Br-B1-A1, HO-B1-A1, MeO-B1-A1 and MeO-B1-A1-OMe, 410 nm for NO2-B1-A1; EX slit: 5 nm; EM slit: 10 nm) (color online).

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Lu et al. Sci China Chem
Figure 3 (a) Emission spectra of MeO-B1-A1 in THF/water mixtures under excitation at 365 nm (10−3 M; EX slit: 5 nm; EM slit: 10 nm). Inset: fluorescent
images of MeO-B1-A1 in THF/water mixtures containing 0 and 95% water. (b) Plots of (I/I0)−1 vs. water content for five enamine derivatives, where I0 is the
luminescence intensity in pure THF (color online).
water content reached 70% as no obvious aggregation ap-
peared. In contrast, evident aggregation occurred while the
content of water was over 70%, resulting in a rapid increase
of emission intensity. Represented by MeO-B1-A1, the ag-
gregation was convincingly demonstrated by the Mie effect
in the UV absorption curves at various proportions of THF
and water (Figure S10) [38]. In the meantime, accompanying
with enhanced water fraction, the long-wavelength emission
gradually disappeared, and a novel emission peak aroused at
475 nm when water content reached to 95%. In comparison
to the dilute solution state, whose emission peak position
located at around 430 nm, the molecular structure of MeO-B1-A1
became more planar due to the crowded space in aggregation
state, contributing to the bathochromic shift emission.
Similar to the PL performance of MeO-B1-A1, other en-
amine derivatives all presented typical AIEE behaviors,
which were displayed in Figure S11. Their turning point
values of fluorescence intensity all focused on 60% or 70%,
which were consistent with their analogous molecular size.
Moreover, there were still some other similarities and dif-
ferences to be elucidated detailly. In the mixed solutions
containing 95 vol% water, the fluorescence wavelength of
Br-B1-A1, HO-B1-A1, MeO-B1-A1-OMe and NO2-B1-A1
was at 414, 471, 508 and 590 nm, respectively. Besides the
fluorescence wavelength, the AIEE behavior of HO-B1-A1
was parallel to MeO-B1-A1 with 104-fold increase of lu-
minous intensity. That means the terminal hydroxyl and
methoxy exhibit similar substituent influence on the emis-
sion performance of original enamine molecule. For Br-B1-
A1, the PL spectra showed multiple peaks or shoulders due
to the weak auxochrome property and heavy atom effect of
bromine substitution [39], and the luminous intensity in-
creased by only seven times at the 95% water volume frac-
tion. With respect to MeO-B1-A1-OMe, although the
introduction of double methoxyls afforded more freedom
space for energy consumption, the enhanced auxochrome
effect was beneficial for stronger emission. Thus, MeO-B1-
A1-OMe also indicated relatively small enhancement
(9-fold) in the 95 vol% water solution. When regarding to
NO2-B1-A1, its PL intensity exhibited a huge 121-fold
promotion from 0% to 95 vol% water solution in AIEE test.
This phenomenon is opposite to common cognizance, as
nitrophenyl group is usually considered as a famous electron
and energy acceptor and acted as a fluorescence quencher.
The reason may be that the single nitro is still not adequate to
overpower the AIE/AIEE effect [40], and the substituent
effect of nitro has greatly changed the aggregation emission
ability of original B1-A1 structure (illustrated in the fol-
lowing analysis). This phenomenon is attracting and further
research of the corresponding cause and application is still
underway.
Generally, the AIEE behaviors of five enamine derivatives
are summarized in Figure 3(b). As we have reported before
[33], the fluorescence quantum efficiency of B1-A1 was just
0.01% and 0.02% in THF solution state and solid powder
state, respectively, exhibiting non-luminous characteristic. In
comparison to the original non-emissive B1-A1 molecule, all
modified enamines brought out AIEE-active properties.
Comparing to their fluorescence intensities in dilute solu-
tions and in 95 vol% water systems, the fluorescence in-
tensities of Br-B1-A1 and MeO-B1-A1-OMe increased by
10 times, and those of HO-B1-A1, MeO-B1-A1 and NO2-
B1-A1 promoted by 100 times. The results of quantum yield
in both solution and solid states of these enamines further
proved these investigations, which are presented in Table S2.
Their quantum yields were remarkably higher in the powder
state contrast to in the solution state. In addition, the in-
creased rations of the quantum yield data are consistent with
these of fluorescence intensity in AIEE experiments for the
derivatives. Combining the aforesaid results, it is concluded
that successful conversion can be achieved from non-lumi-
nance to AIEE-activity through suitable modifications based
on the tetraphenyl enamine structure with p-π conjugate
chain skeleton. Furthermore, the results reveal the notable
substituent effect on the optical properties and AIEE per-
formance of these enamine derivatives, providing valuable
guidance for the modification of undesirable non-emissive
compounds with heteroatom conjugate chains.

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Lu et al. Sci China Chem
3.3 Substituent tunable emission properties
higher ϕF of MeO-B1-A1-OMe is deduced to be stemmed
from its larger kr and smaller knr, and the major cause may be
manifested as the better conjugated molecular structure of
double methoxyl modification [43]. Dissimilarly, in solid
powder state, the kr kept almost constant and the knr was
down by half. These consequences mean that the main cause
of AIEE performance is just dependent by effecting the
nonradiative decay channel in the aggregating condition,
therefore the emission enhancement of MeO-B1-A1-OMe is
relatively small.
As mentioned above, all derivatives are based on the skele-
ton of tetraphenyl enamine and only differ in the type or
number of modified groups. So, to investigate the substituent
influence on optical properties of these enamines deeply,
their pivotal photo-physical parameters and solid emission
behaviors were further researched.
As we all know, the photoluminescent behavior of a mo-
lecule is mainly determined by radiative and nonradiative
decay in excited state. The corresponding rates (kr and knr)
are represented in the following two formulas [41]:
Not only the AIEE behaviors of the compounds, but also
the emission wavelengths were found to be influenced by the
side groups. As mentioned above, various enamines ex-
hibited different emission wavelengths in the 95 vol% water
systems. To gain insight into the distinction of emitting
colors affected by modifications, the emission behaviors of
Br-B1-A1, HO-B1-A1, MeO-B1-A1, MeO-B1-A1-OMe and
NO2-B1-A1 in the solid powder state were also tested, in-
dicating maximum emission wavelengths at 449, 483, 483,
518 and 624 nm, respectively (Figure 4(a)). Comparing to
the solution state, the emission wavelengths of whole en-
amine compounds indicated apparent redshift in the powder
state. Actually, well-organized and flatter conformations
may produce along with the increased degree of aggregation,
consequently leading to the redshift of emission peaks. In
addition, the CIE1931 diagrams and chromaticity co-
ordinates of these powder samples were also illustrated in
Figure 4(b), among which the emission colors of Br-B1-A1,
HO-B1-A1, MeO-B1-A1, MeO-B1-A1-OMe and NO2-B1-
A1 were blue, cyan, cyan, green and red with the chroma-
ticity coordinates represented as (0.16, 0.13), (0.15, 0.32),
(0.16, 0.33), (0.24, 0.41), and (0.61, 0.35), respectively.
Thus, by analyzing the fluorescent performance of enamine
derivatives with different terminal modifications in the
95 vol% water solution, solid powder state and CIE co-
ordinate, it is concluded that different modification strategies
have noteworthy influence on luminescence color, showing
meaningfully substituent tunable characteristics. These re-
sults enlighten that the diverse emitting colors of enamines
can be realized by modification strategies, providing gui-
dance for the design of colorful p-π conjugate chain com-
pounds.
F = kr / (kr + knr)
= 1 / (kr + knr)
(1)
(2)
In the statement, ϕF is the quantum yield and τ refers to the
average fluorescence lifetime. All enamines’ fluorescence
lifetimes were measured in both dilute THF solution and
solid powder states, which were shown in Figure S12 and
further summarized in Table 1. In detail, their PL lifetime
values were gained as 4.04, 2.42, 4.06, 9.13, 0.16 ns in dilute
solutions and 4.30, 4.61, 8.73, 23.55, 0,48 ns in solid powder
states for Br-B1-A1, HO-B1-A1, MeO-B1-A1, MeO-B1-
A1-OMe and NO2-B1-A1, respectively. Assuredly, these
results reveal that terminal modifications have remarkable
effect on their fluorescence lifetimes. The strong electron-
absorbing group (–NO2) attenuates the fluorescence lifetime
drastically, yet the introduction of auxochromes is conducive
to enhance the fluorescence lifetime of these derivatives. It is
worth mentioning that the fluorescence lifetime of MeO-B1-
A1-OMe in powder state was determined to be 23.55 ns,
which is rather long for an organic fluorescent molecule [42].
Furthermore, from dilute THF solution state to solid
powder state, the kr values all increased tremendously for
HO-B1-A1, MeO-B1-A1 and NO2-B1-A1, accompanying
with distinctly decline of their knr values. These results are
beneficial to typical AIEE properties and coincident with
their roughly 100-fold fluorescence enhancement. With re-
spect to Br-B1-A1, its kr improved a lot whereas the knr
decreased rarely from dilute solution to solid powder state,
thus resulting in a relatively smaller increase in emission
intensity. Moreover, in pure THF solution, the obviously
Table 1 Average fluorescence lifetimes and radiative and nonradiative decay rates for five enamine compounds
<τ> (ns)
krc) (×106 s−1)
knr (×108 s−1)
c)
Solna)
4.04
2.42
4.06
9.13
0.16
Solidb)
Soln
0.0495
0.0826
0.148
0.110
3.13
Solid
0.372
2.60
Soln
2.48
4.13
2.46
1.10
62.50
Solid
2.33
2.14
1.13
0.42
18.60
Br-B1-A1
HO-B1-A1
4.30
4.61
MeO-B1-A1
MeO-B1-A1-OMe
NO2-B1-A1
8.73
1.27
23.55
0.48
0.110
227
a) In THF solution (10−4 M). b) Solid powder. c) The radiative decay rate kr=ϕF/τ; the nonradiative decay rate knr=1/τ−kr.

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Lu et al. Sci China Chem
Figure 4 (a) Normalized emission spectra of all five enamine derivatives in solid powder states (excitation wavelengths: 365 nm; EX slit: 2.5 nm; EM slit:
2.5 nm). (b) CIE1931 diagram and the chromaticity coordinates of the solid enamine samples (color online).
To summarize, from elementary tetraphenyl enamine
structure to multiple derivatives with various modifications,
the key role of substituents has been confirmed in the
aforementioned statements. However, the related mechan-
isms are still unclear as following: (1) How is the AIEE
effect of modified enamines realized? (2) What are the rea-
sons of different AIEE behaviors about modified enamines
with different terminal groups? (3) What are the causes of
tunable optical physical properties of diverse enamines?
These issues need further research and discussions.
more planar than that in the ground state. This relaxation
process partly consumes energy to promote non-radiative
transitions, and the planar structures tend to format π-π in-
teractions while aggregating. According to the RIM me-
chanism, these factors are unfavorable to the luminescence
of B1-A1 in aggregation state, thus causing AIE/AIEE in-
active effect.
In the same manner, theoretical calculation was employed
to gain underlying awareness of the relationship between
molecular structure and emission property on these sub-
stituted enamine derivatives. All derivatives exhibited dis-
torted molecular conformations either in the ground state or
the excited state, which contributed to the AIEE feature. In
contrast to initial tetraphenyl enamine, the degree of re-
laxation and planarization from S0 and S1 state in all mod-
ified enamines became smaller (Table S3), revealing the
reduction of non-radiative transition channels and the in-
creased possibilities of the conversions from non-luminance
to AIEE active performance. Moreover, the conformational
change degree is different in various compounds, thus
causing their diverse AIEE behaviors. From these analysis
results, we can infer that the introduced substituents solidify
the conformations of these enamine derivatives, subse-
quently reducing their non-radiative energy consumption in
aggregating state.
In the meantime, to deeply understand substituent tunable
properties of all enamine compounds, their energy levels and
electron cloud distributions of the HOMO and LUMO were
simulated at the B3LYP/6-31G(d) level (Figure 5) [46].
Except NO2-B1-A1, the electronic densities of enamine de-
rivatives in both the HOMO and LUMO levels are equally
distributed, displaying that fully effective conjugations have
been realized in short-length enamine chains and their
structures are much rigid. The calculation also revealed that
the energy levels and gaps could be modestly tuned by
modifications with different electron donor abilities. From
bromine to double methoxyls, the energy levels of the
HOMOs (−4.97 to −4.54 eV) and LUMOs (−0.96 to
−0.67 eV) in these four enamines gradually increased, con-
3.4 Electronic and geometric structures
In order to gain insight into the above questions, theoretical
calculation was conducted to make further analysis. At first,
the nature of nonluminous performance on B1-A1 molecule
should be understood. Theoretical simulation of B1-A1 was
based on the (TD) B3LYP/6-31G(d) level in THF and
modeled by PCM and ONIOM approaches in the crystal state
[44]. Computational results showed that the oscillator
strengths (f) in S1→S0 transition of B1-A1 were gained as
0.04 and 0.20 in the solution and the crystal state, in which
the transition between the LUMO and HOMO (LUMO=
lowest unoccupied molecular orbital, HOMO=highest oc-
cupied molecular orbital) accounted for 99% and 98%, cor-
respondingly (Figure S13). On the basis of the consequences,
B1-A1 should own certain luminescent capability in solid
state theoretically, meaning the actual nonluminous behavior
may be affected by other factors. Actually, the relaxation of
molecular conformation is considered as an important reason
affecting the radiation transition in many organic lumines-
cent molecular systems [45]. Hence, we further compared
and analyzed the optimized structures of B1-A1 in S0 and S1
state. For the sake of illustration, the dihedral angles of ad-
jacent benzene rings in enamine chain were defined as α1 and
α2. As shown in Figure S14, α1 and α2 of B1-A1 were stated
as 67.2° and 70.5° in S0 state, as well as 52.1° and 55.0° in S1
state, respectively. These results indicated the aggregating
conformation of B1-A1 in S1 excited state is prone to be

7
Lu et al. Sci China Chem
amine segments in aggregation. As well, in contrast to the
computational data in ground state (Table S4), their relevant
angle parameters (α1, α2) mostly increase in the crystal state,
which is beneficial to inhibit harmful interactions and pro-
mote AIEE effect. Notably, the degrees of distortion in these
molecules are diverse, further affecting their different optical
performance [48]. The dihedral angles of MeO-B1-A1-OMe
are minimum in all auxochrome substituted enamines, thus
showing higher planarity to produce longer absorption band.
The similar trend also emerged in the solid emission peaks.
Interestingly, the electronic and spatial effects sometimes
work in opposite directions. A flatter conformation of HO-
B1-A1 compound appears than MeO-B1-A1 molecule, while
the auxochrome effect of methoxy is a bit stronger than
hydroxyl. As a result, combining the corresponding elec-
tronic and configurable effects, HO-B1-A1 and MeO-B1-A1
reveal similar emission wavelength in both solution state and
solid state.
The detailed packing structures of Br-B1-A1, MeO-B1-
A1, MeO-B1-A1-OMe and NO2-B1-A1 are given in Figure
6. Unlike common AIE/AIEE substances with parallel-slip-
ped crystal stack, four compounds all exhibit loose molecular
arrangements to hamper planar intermolecular interactions,
which are put down to the high congestion resulting from the
twisted structures. More importantly, multiple inter-
molecular interactions exist in these enamines. On one hand,
these weak forces further assist in restraining molecular
motion, which reduces the non-radiative deactivation of
excitons and contributes to the conversion from non-lumi-
nance to AIEE active performance. On the other hand, the
Figure 5 Energy levels and electron cloud distributions of the HOMO
and LUMO of all enamine derivatives calculated at the B3LYP/6-31G(d)
level by the DFT approach (color online).
forming to the respective electron donating effect of sub-
stituted groups. On account of the neutral feature of tetra-
phenyl enamine segment, the corresponding energy gaps of
enamines are tightly affected by their modifications. Con-
sequentially, as for the energy gaps between the HOMOs and
LUMOs, the values for enamine derivatives lie in the range
from 4.00 to 3.87 eV, with the smallest for MeO-B1-A1-
OMe (3.87 eV) and the largest for Br-B1-A1 (4.00 eV),
showing the agreement with their experimental spectral re-
sults. For NO2-B1-A1, the electron cloud distributions of the
HOMO and LUMO are separated owing to the distinct ICT
effect. The HOMO is distributed on the whole molecule and
mainly centered at the tetraphenyl enamine unit, while the
LUMO mainly localizes on the nitrobenzene part. The strong
electron-withdrawing nitro group tremendously reduces the
energy levels in HOMO and LUMO, and a minor energy gap
value generates to present near infrared luminescence [47].
3.5 Single-crystal structural analysis
To reveal their AIEE mechanism and emission feature, the
single crystal analyses of all enamine derivatives are valu-
able. Analyzable single crystals of these compounds were
able to be obtained through slow evaporation of CH2Cl2-
hexane mixtures under mild conditions. As twin crystal is
contained, it is specially noting that the single crystal
structure of HO-B1-A1 is just used for studying in-
tramolecular structural characteristics. The crystallographic
data can be obtained free of charge from the Cambridge
Crystallographic Data Centre with serial number CCDC
1994588, 2013819, 1994591, 1994592 and 1994595 for Br-
B1-A1, HO-B1-A1, MeO-B1-A1, MeO-B1-A1-OMe and
NO2-B1-A1, respectively.
As shown in Figure S15, these enamine crystals all exhibit
distorted molecular conformations with the dihedral angles
of adjacent benzene ring exceeding 60°, which can effec-
tively restrain the free rotations of phenyl and diphenyl en-
Figure 6 The packing patterns and intermolecular interactions of Br-B1-
A1 (a, b), MeO-B1-A1 (c, d), MeO-B1-A1-OMe (e, f) and NO2-B1-A1 (g,
h), respectively. Hydrogen omitted for clarity (a, c, e, g) (color online).

8
Lu et al. Sci China Chem
intermolecular forces with various distances and strengths
greatly affect the aggregated luminescence behaviors of
these amine derivatives, causing their diverse AIEE beha-
viors. In view of Br-B1-A1 and MeO-B1-A1-OMe, only C–
H···π interactions between neighboring enamine derivative
molecules were observed. Detailly, for Br-B1-A1 molecule,
some C–H···π hydrogen bonds with lengths of 3.4–3.7 Å are
formed between the benzene rings in aniline monomer and
stilbene. As for MeO-B1-A1-OMe, C–H···π interactions
with fixed bond length at 3.046 Å only exist in the adjacent
stilbene moieties. These C–H···π interactions are relatively
limited and weak, thus leading to the smaller enhancement of
fluorescence intensities when aggregating. On the contrary,
the numbers of intermolecular interactions obviously im-
prove in consideration of MeO-B1-A1 and NO2-B1-A1, and
the modified sections are found to participate in their inter-
molecular interactions at the same time. As for MeO-B1-A1,
apart from C–H···π hydrogen bonds in benzene units with
lengths of 3.4–4.1 Å, the C–H···π interactions between
methoxyl and neighboring aniline parts are also discovered
with a length of 3.067 Å. For NO2-B1-A1, the distances of
C–H···π hydrogen bonds range from 3.2 to 3.6 Å, while the
distances of NO2···H interactions are just 2.646 Å. As we
can see, these intermolecular interactions involving sub-
stituent groups are relatively strong to reduce nonradiative
transitions by solidifying the structures of corresponding
molecules, thus leading to higher enhancement ratios of lu-
minous intensities from monomolecular state to aggregating
state [49]. Therefore, AIEE properties of these enamines may
be tuned by selecting specific substituents through regulating
and controlling relevant intermolecular forces, which further
provide a constructive method of building more AIE/AIEE
systems with p-π conjugate chain structures.
to be effectually regulated by diverse substituents. In virtue
of theoretical calculation and single crystal investigation, we
concluded that the introduced side groups readily increased
the degree of conformation distortion and provided multiple
intermolecular interactions to solidify molecular structures,
which promoted radiative decay rate and block nonradiative
channel via the restriction of intramolecular rotation (RIR)
mechanism. In particular, the participation of the modified
sections in intermolecular interactions can dramatically im-
prove AIEE effect of the corresponding enamines. These
results not only contribute to the understanding of the en-
amines’ substituent influence on photo-physical properties,
AIEE properties, electronic structure and molecular packing,
but also offer basic knowledge for further functional mod-
ification of other kinds of AIE/AIEE units to obtain new
AIE/AIEE materials.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21574003).
Conflict of interest The authors declare no conflict of interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
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