Regulation of aggregation-induced emission color of α-cyanostilbene luminogens through donor engineering of amino derivatives

Article abstract of DOI:10.1016/j.tetlet.2021.152972

In this contribution, α-cyanostilbene-based D-π-A-π-D type compounds consisting of different substituents on the amino donor group (e.g., fused, aryl/alkyl, and dialkyl) were designed, synthesized and their aggregation-induced emission was investigated thoroughly. The incorporation of these substituents furnished AIE features with tunable emission colors ranging from green-to-yellow-to-red. The compounds all showed a weak emission in THF; however, they showed an enhanced emission upon addition of water by forming emissive aggregates, which may arise from the restriction of twisted intramolecular charge transfer and E/Z isomerization. The DFT calculations revealed that the luminogens adopted distinctive conformations including a highly twisted one and a relatively coplanar one with the variation in the amino substituents.

Full text of DOI:10.1016/j.tetlet.2021.152972

Tetrahedron Letters 69 (2021) 152972
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Regulation of aggregation-induced emission color of a-cyanostilbene
luminogens through donor engineering of amino derivatives
⇑
Murat Tonga
Department of Chemistry, University of Massachusetts, Amherst, MA 01003, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
In this contribution,
a-cyanostilbene-based D-p-A-p-D type compounds consisting of different sub-
Received 14 December 2020
Revised 23 February 2021
Accepted 26 February 2021
Available online 2 March 2021
stituents on the amino donor group (e.g., fused, aryl/alkyl, and dialkyl) were designed, synthesized and
their aggregation-induced emission was investigated thoroughly. The incorporation of these substituents
furnished AIE features with tunable emission colors ranging from green-to-yellow-to-red. The com-
pounds all showed a weak emission in THF; however, they showed an enhanced emission upon addition
of water by forming emissive aggregates, which may arise from the restriction of twisted intramolecular
charge transfer and E/Z isomerization. The DFT calculations revealed that the luminogens adopted dis-
tinctive conformations including a highly twisted one and a relatively coplanar one with the variation
in the amino substituents.
Keywords:
Aggregation-induced emission
a-Cyanostilbene
Color-tunability
Substituent effect
Ó 2021 Elsevier Ltd. All rights reserved.
Organic
p
-conjugated compounds have been extensively inves-
luminogens [7]. Among them, a-cyanostilbene has been inten-
tigated for optical and electronic applications [1]. The majority of
these compounds are very emissive in dilute solution but tend to
become weakly emissive or non-emissive at the solid states due
to the formations of unfavorable species such as excimers as a
sively studied as a functional moiety in the AIE compounds due
to its simple structure and high polarizability [8]. Despite the small
size of nitrile in the
a-cyanostilbene structure, it can still induce
steric impacts in the surrounding groups which cause a twisted
result of strong p-p interactions in the aggregate [2]. This is known
conformation. Also, its electron-withdrawing feature enables to
as aggregation-caused quenching (ACQ) which has significantly
restricted their applications since they are typically employed in
aggregated or solid states [3]. However, the luminescent materials
require having a high luminescence efficiency to work primarily at
the solid-state. In 2001, this key challenge has been attempted by
Tang and coworkers with the proposition of a new and an opposite
phenomenon, so-called aggregation-induced emission (AIE) [4]. In
this concept, AIE luminogens demonstrate weak emission in solu-
tion, but a strong emission in the aggregate or solid-state. The
accepted mechanism is the restriction of intramolecular motions
(RIM) which can be obtained by planarization in the configuration,
formation of J-aggregate, E/Z isomerization, twisted intramolecular
charge transfer (TICT), and excited-state intramolecular proton
transfer (ESIPT) [5]. Since then, AIE luminogens have become of
great advanced materials in particular applications including
organic light emitting devices (OLEDs), sensing, and imaging [6].
In general, the AIE compounds can be categorized by their
design donor-acceptor type
desired optical features. Besides, intramolecular charge transfer
(ICT) commonly observed in the donor–acceptor type -conju-
gated systems plays a significant role in the photophysical behav-
iors of AIE luminogens, particularly the emission color [9].
The AIE luminogens with tunable colors (emissions) have found
tremendous attention in optoelectronic devices and biotechnology
[10]. The molecular packing of the luminogens at the solid-state is
significantly affected by the substituents on the luminogens which
p-conjugated compounds showing
p
has no direct electronic contribution to
p-conjugated emissive
core. Particularly, side-chain substituents have demonstrated a
dramatic impact on the emission properties such as color and
quantum yield. For instance, Imoto et al showed the effect of the
chain length, hydroxyl group, and branching structure on the AIE
properties of N-alkyl arylaminomaleimide dyes [11]. Dong et al
reported the effects of side chains on AIE properties of water-sol-
uble TPE derivatives [12]. In another study, Zhang et al investigated
the chain length-dependent behavior of 9,10-bis[(N-alkylphenoth-
iazin-3-yl)vinyl]anthracene compounds [13]. Herein, in the pre-
structural aspects: hydrocarbon, heterocyclic,
a-cyanostilbene,
hydrogen-bonding, polymeric, organometallic, and miscellaneous
sent paper, novel AIE luminogens with D-p-A-p-D configuration
were developed using amino derivatives as a donor unit and cyano
as an acceptor unit. The emission properties of the luminogens can
⇑
Address: Polnox Corporation, 225 Stedman St Suite 23, Lowell, MA 01851,
United States.
https://doi.org/10.1016/j.tetlet.2021.152972
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

M. Tonga
Tetrahedron Letters 69 (2021) 152972
be rationally tuned at the aggregate state emitting from green-to-
yellow-to-red. The introduction of amino derivatives with various
substituents into the
p-conjugated a-cyanostilbene configuration
results in not only a highly twisted conformation but also a planar
one, which generated distinctive AIE properties.
The synthetic routes for AIE luminogens CZ, DPA and DMA are
shown in Scheme 1. Synthetic details are given in the Supporting
Information. The Knoevenagel condensation reaction between a
terminal aldehyde on the donor moieties and an activated methy-
lene group on the acceptor (1,4-phenylenediacetonitrile) (6) gave
all D-p-A-p-D type luminogens in the presence of a base catalyst
potassium-t-butoxide. The synthetic approach for the donor com-
pounds starts with the alkylation of amino donors. The intermedi-
ate compounds 1 and 3 which are the alkylation of carbazole and
diphenylamine, respectively, were prepared according to proce-
dures described previously [14]. Vilsmeier-Haack formylation
readily yielded the intermediates 2 and 4 consisting of a terminal
aldehyde group on the alkylated amino donor compounds. Com-
mercially available donor 4-(dimethylamino)benzaldehyde (5)
was used as-is. The synthesis of DMA has been previously reported
[15]. Notably, all luminogens demonstrated good solubility in com-
mon organic solvents such as dichloromethane, chloroform, THF,
acetone, and acetonitrile. The resulting AIE luminogens were thor-
oughly characterized by H1 NMR and high-resolution mass
spectrometry.
The optical properties (the UV–vis absorption and photolumi-
nescence (PL) spectra) of the AIE luminogens CZ, DMA and DPA
were investigated in THF solution. The UV–vis absorption and PL
spectra of the luminogens are shown in Fig. 1 and the correspond-
ing photophysical data are presented in Table 1. The solution
absorption spectroscopy of the luminogens endows the under-
standing of their excited state behavior, mainly ICT character
which is a common process noted in the donor–acceptor com-
pounds. Generally, the combination of electron-donating groups,
including alkyldiphenylamine or carbazole, and electron-with-
drawing groups such as cyano leads to an ICT process in the
luminogens. The ICT absorptions of the luminogens DPA and
DMA are similar due to nearly identical electron-donating
strengths. As an example, the solvatochromic behavior of the
luminogen DMA was shown in various solvents (Fig. S3). The max-
imum absorption peak of these luminogens is located at ~428 nm,
while it is at 409 nm for the luminogen CZ. A redshift by about
20 nm suggests that the alkyldiphenylamine and dimethylamine
groups are a stronger electron-donor than the carbazole group
Fig. 1. Normalized absorption (a) and normalized photoluminescence spectra (b) of
the luminogens in THF.
[16]. The luminogens absorb strongly through the blue region with
band onsets at 500–525 nm.
The luminogens showed weak photoluminescence at the solu-
tion state. The normalized PL spectra are shown in Fig. 1b. A similar
trend was observed in the PL spectra, as was noted for their absor-
bance spectra. Red-shifting maxima of from 482 to 520 nm were
observed with increasing electron-donating ability of the donor
segments, DPA has the largest shift (91 nm) and CZ has the lowest
shift (73 nm). Related Stokes shifts are also shown in Table 1. The
large Stokes shifts suggest for large dipole moment change at the
excited state.
Molar extinction coefficients of the luminogens are presented in
Table 1. All the luminogens exhibited quite high molar extinction
coefficients ranging from 57,400 MÀ1 cmÀ1 to 77,300 MÀ1 cmÀ1
at the maximum absorption wavelength (kmax) in THF. The lumino-
gen DMA showing the highest extinction coefficient might be due
to its coplanar geometry compared to CZ and DPA.
The luminogens are soluble in THF, but insoluble in water. To
investigate aggregation-induced emission aspects of the lumino-
gens, photoluminescence spectra were evaluated at 5
lM of the
luminogens in THF with different water fractions (fw) changing
from 10% to 90%. With the addition of the high fraction of water,
well-dispersed state in THF was altered to the aggregate state,
which ultimately affects the emission properties. Fig.
2
Scheme 1. Synthetic route to the luminogens CZ, DPA and DMA. (a) 1-bromohexane, 18-crown-6, KOH, toluene, 110 °C, 20 h, 96%. (b) 2-ethylhexylbromide, NaOH, DMSO,
110 °C, 24 h, 80%. (c) POCl3, DMF, 100 °C, 15 h, 34% (2) and 7% (4). (d) t-BuOK, ethanol, reflux, 16 h, 74% (CZ), 67% (DPA), 90% (DMA).
2

M. Tonga
Tetrahedron Letters 69 (2021) 152972
Table 1
Photophysical data of the luminogens.
Absmax
(nm)
PLmax
(nm)
Absonset
(nm)
e
Stokes Shift
Eogpt
(MÀ1cmÀ1
)
(cmÀ1
)
(eV)a
CZ
DMA
DPA
409
428
429
482
513
520
502
505
525
67,600
77,300
57,400
3703
3871
4079
2.47
2.46
2.36
a
Eogpt: optical band gap, estimated from UV–vis absorption edge.
Fig. 2. Photoluminescence spectra of the luminogens in THF-water mixtures: (a) CZ, (b) DPA, and (c) DMA.
demonstrates the PL change upon the addition of water to the THF
solution.
nature of the alkylated diphenylamine moiety also limits inter-
molecular stacking, which generally diminishes the aggregate
p-p
CZ has a D-
p
-A-
p
-D structure and rotator alkyl segments which
emission and therefore rendering the compound DPA to emit
intensely. The change in the PL peak intensity (I/I0 ratio) was plot-
ted in Fig. 3b. When the fw = 90%, the peak intensity rose by about
13-times in comparison with the fw = 50%, which is the most
quenched state and it was roughly enhanced by 1.2-times com-
pared to in the pure THF solution. AIE property occurred with red-
shifts of 5–13 nm for the added fw = 10–70%, it was red-shifted by
24–31 nm when the fw > 70%. These redshifts are lower than that of
the luminogens CZ and DMA, which is possibly due to DPA pos-
sessing many rotator groups [19].
allow demonstrating typical AIE behaviors. The PL intensity of CZ
showed a continuous decrease until the fw reached 70% with some
irregularities at the water fractions of 20% and 50% (Fig. 2a). This
weakened PL intensity can be ascribed to a change in the solvent
polarity. Upon addition of water, the molecules can form weakly
emitting crystalline aggregates, intensely emitting amorphous
aggregates, or both [17]. In the crystalline state, the molecules pack
in a well-ordered structures, but they can easily undergo strong
intermolecular
sion intensity.
p-p interactions that quench or weaken the emis-
The pure THF solution of DMA is less emissive in comparison
with those of CZ and DPA due to the restricted intramolecular
motions of short alkyl chains which deplete excited state energy
by non-radiative decay (Fig. 2c). Similarly, when the fw < 60% in
the mixed solution, the emission is more quenched, which can be
attributed to the formation of the crystalline aggregates upon the
variation in the solvent polarity. The raised polarity of the solvent
progressively stabilizes the TICT process and leads to a decrease in
the emission [20]. The change in the PL intensity (I/I0 ratio) with
the change in fw was also investigated (Fig. 3c). Once the fw reached
70% or more, a drastic improvement was observed in the red emis-
sion which is 10-times higher than that in pure THF. In addition,
the emission intensity was improved up to ~50 times compared
to the most quenched state which was prepared with the added
fw = 50%. It can be explained that the formation of the aggregates
limits the rotational motions of the -NMe2 group, and thereby
enhancing the emission. Hence, DMA exhibits typical AIE behav-
iors. And also, this type of photophysical behavior is very common
in the structures having strong donor–acceptor interaction [21].
The improvement in the PL intensity was also accompanied by
8–20 nm of redshifts in the PL peak for the added fw up to 60%, then
85–95 nm redshifts for the added fw > 60%, which is typical AIE
characteristics. Thus, the aggregates of DMA that were created
with a higher amount of water fraction showed red emission
However, in the amorphous state, the randomly ordered mole-
cules due to the weak intermolecular interactions can activate the
RIM, thereby improving the emission intensity. The emission of CZ
starts to intensify after adding fw = 70% by reaching the intensity of
the pure THF solution. A possible explanation for this behavior
could be the formation of amorphous aggregates which typically
increase the emission.
A similar AIE and twisted intramolecular charge transfer (TICT)
phenomenon were also observed for the luminogen DPA due to the
strong D-A impact and intramolecular rotations in its molecular
structure. To investigate the AIE features of the DPA, different
water fractions were added to the pure THF solutions. Fig. 2b
demonstrates that the PL intensity decreases gradually with the
fw < 50%, and starts to increase gradually at the fw = 50–70%. Then,
it intensifies significantly when the fw > 80%, with emitting yellow
light (kem = 551 nm) under illumination with a 365 nm UV lamp
(Fig. 3b-inset). In the solutions with dilute water fractions, the
intramolecular motions such as phenyl and alkyl rotations can
switch the excited-state energy to non-radiative, causing
a
decrease in the emission by possibly generating crystalline aggre-
gates. At high water fractions, the aggregates could rearrange into a
less ordered fashion to form amorphous aggregates, which usually
enhance the PL intensity [18]. In addition, at higher fw, the twisted
3

M. Tonga
Tetrahedron Letters 69 (2021) 152972
Fig. 3. The changes in the PL intensity and kmax in THF-water mixtures with fw = 10–90%: (a) CZ, (b) DPA and (c) DMA. Inset: fluorescent images of the luminogens in THF–
water mixtures at the fw = 90% and fw = 0% under illumination with a 365 nm UV lamp.
peaked at 608 nm under illumination with a 365 nm UV lamp
(Fig. 3c-inset). Notably, these luminogens demonstrated color-tun-
able properties from green-to-yellow-to-red by simply controlling
the water fractions in the mixed solvent system.
participates
the central phenylene and the phenylene on the electron-donating
amino group which participates -conjugation in the main plane.
p-conjugation in the backbone; c is defined between
p
Table 2 shows the calculated dihedral angles, HOMO, LUMO, and
electronic band gap for all the luminogens. Using the DFT compu-
tational analysis, electronic band gaps were also calculated in the
range of 2.96–3.15 eV, which are consistent with spectroscopic
results. In general, cyano electron-accepting groups are pushed
out of the central backbone due to steric hindrance with neighbor-
ing phenyl rings in these luminogens. In the solution state, the
excited state energy can be deactivated through non-radiative
transitions such as vibrational and rotational motions of the sub-
stituent moieties on the vinylic double bond, thereby resulting in
minimal emission.
The CIE 1931 coordinates of the luminogens are shown in the
chromaticity diagram in Fig. S4. Specifically, three types of emis-
sions were observed with the change in water content in the aggre-
gate state. For instance, the luminogen CZ displayed a cyan
emission at the fw > 40%, a green emission at the fw = 40–60%, a
cyan emission at the fw = 70%, which is anomalous shifting to lower
wavelengths, and a green emission at the fw > 70% (Fig. 4S-a). DPA
showed a yellowish-green emission at the fw = 10–60%, a yellow-
green emission at the fw = 60–80%, and a yellow emission at the
fw = 90% (Fig. 4S-b). DMA showed a quite different color palette
than CZ and DPA: a yellowish-green emission at the fw = 10–50%,
a yellow-green emission at the fw = 60%, and a red emission at
the fw = 70–90% (Fig. 4S-c).
To gain insight into the relationship between the structures and
AIE properties of the luminogens, optimized geometry, and molec-
ular frontier orbitals were studied by density functional theory
(DFT) using the Spartan 04 program at the B3LYP/6-31G** level
(Fig. 4) when the crystal structures are not available. The lumino-
gens all demonstrated similar highest occupied molecular orbital
(HOMO) electron distributions which are mostly delocalized
As illustrated in Fig. 4, CZ and DPA adopt highly twisted pro-
peller-like conformations. Dihedral angles for CZ were found as
a
= 19.9°, b = 10.5° and
were higher for DPA due to rotary N,N-alkyldiphenyl amino
groups: = 21.3°, b = 13.6°, and = 49°. Overall, CZ shows better
c = 43.8°. The calculated dihedral angles
a
c
planarity than DPA due to the extended conjugation. In the aggre-
gate state, such torsional angles due to free rotations are exten-
sively limited in CZ and DPA. Thus, the formation of detrimental
species is suppressed by inducing intense emission. DMA exhibited
a highly planar conformation compared to CZ and DPA. The pres-
throughout the entire
p-conjugated backbone. Cyano, alkyls, and
ence of small dimethylamino groups increased the rigidity of p-
peripheral phenyl groups have no contributions to the HOMO
levels. The lowest unoccupied molecular orbital (LUMO) electron
distributions in all luminogens were similar to one another, were
mainly located in the central region including cyano acceptor
groups, vinylenes, and the central phenylene. Clearly, such an elec-
conjugated backbone by forming a less twisted geometry relative
to the luminogens CZ and DPA. Dihedral angles for DMA were cal-
culated as
a = 19.9°, b = 3.9° and c = 36.2°. Showing AIE activity
with such a structure is an atypical result for DMA. It could be
explained by the photoisomerization of the vinyl double bond via
an E/Z conversion upon excitation. This transformation in the
excited state depletes the energy of the excited state through
non-radiative pathways. However, such a conversion in the excited
state is impeded or weakened in the aggregated state to allow the
compounds to display an intense emission [23]. The E/Z reorgani-
zation is presumably challenging for CZ and DPA due to the steric
hindrance of large alkyl groups which could prevent intermolecu-
lar stacking at the excited state. Additional experiments would be
necessary to explore this further. Moreover, it can be inferred that
a large twisted conformation gave green and yellow emission
while a small one gave a red emission.
tronic distribution reveals intense ICT attributes in the D-p-A-p-D
type luminogens, which may induce quenched or diminished PL in
moderately polar solvents like THF. This behavior is often observed
to decay from the excited state to the ground state via non-radia-
tive channels [22]. Also, these findings are consistent with the
spectroscopic results. In the absence of crystal structures, opti-
mized geometries could be beneficial to understand the structural
features. Their optimized structures are given in Fig. 4. The DFT cal-
culations revealed that the overall planarity of the luminogens
increases in the order of CZ > DPA > DMA.
The dihedral angles between the electron-withdrawing cyano
group, the conjugated phenylene group on the electron-donating
amino group and the central phenylene building block were
This work shows a practical synthetic methodology for modu-
lating emission color by varying the functionalities on the donor
investigated. Particularly, dihedral angle
a
is defined between the
moieties. Connecting amino derivatives to
a-cyanostilbene
electron-withdrawing cyano moiety and the central phenylene; b
is defined between the electron-withdrawing cyano moiety and
the phenylene on the electron-donating amino group which
through -conjugation in the D- -A- -D structures yielded the
AIE luminogens with color-tunable emission ranging from green-
to-yellow-to-red. Specifically, the emission was readily tuned by
p
p
p
4

M. Tonga
Tetrahedron Letters 69 (2021) 152972
Fig. 4. Description of dihedral angles, the optimized geometry, and frontier molecular orbitals of the luminogens.
Table 2
Computational data of the luminogens.
a
(°)
b
(°)
c
(°)
EHOMO
(eV)
ELUMO
(eV)
EDg FT
(eV)
CZ
DMA
DPA
19.9
19.9
21.3
10.5
3.9
13.6
43.8
36.2
49
À5.18
À4.9
À4.7
À2.03
À1.88
À2.01
3.15
3.02
2.96
changing the
p
-skeleton (e.g., fused ring) and the substituents (e.g.,
12-2-007 and W911NF-14-2-0002 from the US Army Natick Sol-
dier Research, Development and Engineering Center (NSRDEC).
Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the authors and do not nec-
essarily reflect the views of the NSRDEC. The author gratefully
thanks Emeritus Prof. P.M. Lahti of University of Massachusetts
for providing resources for this self-guided research project.
N, N-alkyl/diphenyl and N,N-dimethyl) on the amino donor seg-
ments. The employment of the alkyl chains in CZ and DPA resulted
in a twisted conformation that allowed the molecules to show AIE
aspects in the aggregate state. Despite the high planarity observed
in DMA in comparison to CZ and DPA, it displayed an AIE property
that may stem from an E/Z transformation upon excitation. The
DFT results also revealed that the degree of twisting plays a big role
in the emission colors of the aggregate state: a large twisted orien-
tation resulted in green and yellow emission as in the case of CZ
and DPA while a small one in DMA gave a red emission. Therefore,
this study would render luminescent materials to produce tunable
AIE features that could open a versatile strategy for the develop-
ment of optoelectronic applications.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.152972.
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