Exquisite 1D Assemblies Arising from Rationally Designed Asymmetric Donor-Acceptor Architectures Exhibiting Aggregation-Induced Emission as a Function of Auxiliary Acceptor Strength

Article abstract of DOI:10.1002/chem.201503570

One-dimensional nanostructures with aggregation-induced emission (AIE) properties have been fabricated to keep the pace with growing demand from optoelectronics applications. The compounds 2-[4-(4-methylpiperazin-1-yl)benzylidene]malononitrile (PM1), 2-{4

Full text of DOI:10.1002/chem.201503570

DOI: 10.1002/chem.201503570
Full Paper
&
Substituent Effects
Exquisite 1D Assemblies Arising from Rationally Designed
Asymmetric Donor–Acceptor Architectures Exhibiting
Aggregation-Induced Emission as a Function of Auxiliary Acceptor
Strength
Roop Shikha Singh, Sujay Mukhopadhyay, Arnab Biswas, and Daya Shankar Pandey*[a]
Abstract: One-dimensional nanostructures with aggrega-
tion-induced emission (AIE) properties have been fabricated
to keep the pace with growing demand from optoelectron-
ics applications. The compounds 2-[4-(4-methylpiperazin-1-
yl)benzylidene]malononitrile (PM1), 2-{4-[4-(pyridin-2-yl)pi-
perazin-1-yl]-benzylidene}malononitrile (PM2), and 2-{4-[4-
(pyrimidin-2-yl)piperazin-1-yl]benzylidene}malononitrile
a vital role in triggering AIE in these compounds because
the same D–A construct led to aggregation-caused quench-
ing upon replacing A’ with an electron-donating ethyl group
(PM1). Moreover, the effect of restricted intramolecular rota-
tion and twisted intramolecular charge transfer on the
mechanism of AIE has also been investigated. Furthermore,
it has been clearly shown that the optical disparities of
these A’–D–p–A architectures are a direct consequence of
comparative A’ strength. Single-crystal X-ray analyses provid-
ed justification for role of intermolecular interactions in ag-
gregate morphology. Electrochemical and theoretical studies
affirmed the effect of the A’ strength on the overall proper-
ties of the A’–D–p–A system.
(PM3) have been designed and synthesized by melding pi-
perazine and dicyanovinylene to investigate AIE in an asym-
metric donor–acceptor (D–A) construct of A’–D–p–A- topolo-
gy. The synthetic route has been simplified by using phenyl-
piperazine as a weak donor (D), dicyanovinylene as an ac-
ceptor (A), and pyridyl/pyrimidyl groups (PM2/PM3) as auxil-
iary acceptors (A’). It has been established that A’ plays
Introduction
strategic tuning of intermolecular interactions, leading to
nanostructures of desired morphology and tunable solid-state
emission. Through our earlier work, we have tried to devise
a simplified approach toward optical and morphological con-
trol in AIE luminogens.[7] Following the same strategy, we in-
tended to create 1D solid-state fluorescent nanostructures, in
particular, twisted or helical nanofibers because they are en-
dowed with distinct optical and photophysical properties that
broaden their technological applications (see above).
The fabrication of functional soft materials through controlled
self-assembly of molecular building blocks has proven to be
extremely valuable in the construction of various hierarchical
assemblies.[1] Among these, 1D nanostructures have attracted
particular attention owing to their significant role in the con-
struction of high-performance optoelectronic devices, such as
field-effect transistors (FETs), organic light-emitting diodes
(OLEDs), organic light-emitting transistors (OLETs), and solar
cells.[2] However, the realization of their full potential has been
a daunting task due to the aggregation-caused quenching
(ACQ) effect. After the discovery of aggregation-induced emis-
sion (AIE) by Tang and co-workers,[3] the area of AIE lumino-
gens has expanded its horizon from blue- to red-emitting lumi-
nophores.[4,5] The mechanistic interpretation of AIE relies on
several processes, including restricted intramolecular rotation
(RIR), J-aggregate formation, excimer emission, and twisted in-
tramolecular charge transfer (TICT).[6] With the addition of AIE,
it has been possible to control aggregate buildup through the
The AIE luminogens developed so far are blue or green
emitters. Red/yellow emitters are relatively scarce and general-
ly endowed with appropriate p-conjugated donor (D) and ac-
ceptor (A) molecules, which lead to significantly redshifted
emission through intramolecular charge transfer (ICT). In this
context, numerous systems that exhibit fascinating optoelec-
tronic properties have been successfully developed by various
groups.[8] Furthermore, along with conventional symmetrical
D–A systems, current research has also focused on asymmetric
D–A molecules. The presence of more than one D/A unit with
competitive strength unleashes interesting aspects of the
push–pull mechanism, which may enable adjustments in the
molecular structure and desired optoelectronic properties.[9] As
an efficient A, dicyanovinylene (DCV) has found wide applica-
tions in the construction of symmetric and asymmetric D–A ar-
chitectures with resourceful usage in organic photovoltaics
(OPVs).[9b,10] Lately, some reports on AIE-active DCV derivatives
[a] R. S. Singh, S. Mukhopadhyay, A. Biswas, Prof. D. S. Pandey
Department of Chemistry, Faculty of Science
Banaras Hindu University, Varanasi 221 005 U.P. (India)
E-mail: dspbhu@bhu.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503570.
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based on conventional tetraphenylethylene (TPE) donors, in-
volving only modification in the strength of A, have sur-
faced.[11] None of these reports deal with nanostructures of de-
sired morphology with AIE attributes.
Herein, we report on a system with asymmetric D–A archi-
tecture to control AIE as a function of A strength. Our prime
concern was to set up a morphological trademark for achiev-
ing 1D nanostructures with AIE characteristics. Considering the
applicability of flexible spacers in creating helical nanostruc-
tures, piperazine has been chosen to provide conformational
flexibility in the scaffold.[12] Phenylpiperazine has been em-
ployed as D, whereas DCV has been used as the main A and
pyridyl/pyrimidyl as auxiliary acceptors (A’). To ensure that AIE
is unique to this A’–D–p–A system and largely tuned through
competition between the As, an ethyl group has been incorpo-
rated as an electron-donating substituent on the piperazine
core to offer a symmetric D–p–A construct. Herein, we present
three strategically designed piperazine–DCV-based symmetric
(2-[4-(4-methylpiperazin-1-yl)benzylidene]malononitrile (PM1))
and asymmetric (2-{4-[4-(pyridin-2-yl)piperazin-1-yl]benzyl-
idene}malononitrile (PM2) and 2-{4-[4-(pyrimidin-2-yl)piperazin-
1-yl]benzylidene}malononitrile (PM3) D–A architectures, and
a systematic study on the effect of substituents on their prop-
erties. Possessing a simple ethyl donor, symmetric PM1 was
AIE inactive, whereas the A’ pyridyl and pyrimidyl moieties in
PM2 and PM3 satisfied both steric and electronic requirements
and bestowed them with AIE activity.
Scheme 1. Synthetic route to PM1–PM3.
bands for PM1, PM2 and PM3 at almost the same position in-
dicated comparable conjugation lengths in these molecules.
Upon excitation at l=425, 424 and 423 nm, compounds
PM1–PM3 display weak locally excited (LE) emission bands at
lꢀ485 nm with appreciable Stokes shifts (SSs; PM1: l=
481 nm, SS, 56 nm; PM2: l=488 nm, SS, 64 nm; PM3: l=
494 nm, SS, 71 nm).
Possessing A’–D–p–A architecture, these compounds are ex-
pected to show TICT behavior marked by charge separation
between D and A units. In polar solvents, intramolecular rota-
tion brings the luminogen from the LE to TICT state, featuring
quenched and redshifted emission. This prompted us to inves-
tigate the photophysical behavior of PM1–PM3 as a function
of solvent polarity. Whereas there was no significant change in
the absorption behavior of these molecules in different sol-
vents, the fluorescence spectra were quite different, except for
PM1. Both PM2 and PM3 displayed redshifted emissions in
polar solvents (Figure 1). With changes in solvent polarity from
extremely nonpolar to polar, redshifts of 20 and 40 nm were
observed for PM2 and PM3, respectively. It is worth mention-
ing that the emission intensity of PM2 and PM3 diminished in
polar solvents (Figure S9 in the Supporting Information). To
affirm this, we calculated the solvent polarity parameters (Df)
for all solvents under investigation and tried to establish a rela-
tionship between emission intensity and Df for PM2 and PM3
(Figure S9c in the Supporting Information). The plot of emis-
sion intensity versus Df displayed a downward curve, which
also supported the vulnerability of the excited state to decay
through nonradiative processes in polar solvents, leading to
emission quenching.[4b] It has been observed that the variation
in emission intensity with solvent polarity was more consistent
in PM3 than PM2. These results suggested that PM3 showed
better solvatochromic behavior and enhanced ICT than PM2.
Moreover, the excitation spectra of PM2 and PM3, monitored
at l=488 and 494 nm (Figure 2), revealed that the absorption
spectra did not match the excitation spectra at all, which indi-
cated that emission arose from a charge-transfer state.[15]
Considering the polarity-dependent optical properties of
PM1–PM3 and rich lineage of DCV derivatives in photovoltaic
systems, piperazine–DCV conjugates were expected to be en-
dowed with AIE attributes. To understand aggregation behav-
Result and Discussion
Synthesis and characterization of PM1–PM3
The aldehydes 4-(4-ethylpiperazin-1-yl)benzaldehyde (1a), 4-[4-
(pyridin-2-yl)piperazin-1-yl]benzaldehyde (1b), and -[4-(pyrimi-
din-2-yl)piperazin-1-yl]benzaldehyde (1c) were synthesized by
following procedures reported in the literature.[13] These alde-
hydes reacted with malononitrile in the presence of catalytic
amounts of piperidine to afford PM1, PM2,[14] and PM3 in rea-
sonably good yield (70%, PM1; 88%, PM2; 90%, PM3). The
syntheses of PM1–PM3 are depicted in Scheme 1. All com-
pounds were thoroughly characterized by elemental analyses,
1
HRMS, and H and 13C NMR spectroscopy (Figures S1–S5 in the
Supporting Information). The molecular structure of PM3 was
confirmed by X-ray single-crystal analysis. The compounds
under investigation exhibited good solubility in THF, acetoni-
trile, DMF, DMSO, and dichloromethane; moderate solubility in
methanol and ethyl acetate; and poor solubility in nonpolar
solvents.
Photophysical properties
The optical properties of PM1–PM3 have been investigated by
electronic absorption and emission spectral studies in THF (c=
5”10À5 molLÀ1), and the resulting spectra are depicted in Fig-
ure S6 in the Supporting Information. In the absorption spec-
tra, these compounds displayed absorption bands at l=425,
424, and 423 nm, respectively. The occurrence of absorption
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Figure 1. Emission spectra of PM2 (a) and PM3 (b) in solvents of varying polarity (c=5”10À5 molLÀ1).
Figure 2. Excitation spectra of PM2 (a) and PM3 (b) at lem =488 and 494 nm, respectively.
ior, the absorption and emission properties of PM1–PM3 were
examined in mixtures of THF/water with varying compositions;
water is a poor solvent for these compounds and must pro-
mote the aggregation process.
emerged at l=525 nm, which further strengthened at fw =
100%. It is noteworthy to mention that at fw =100%, along
with the major band at l=525 nm, a shoulder appeared at
l=564 nm. On the other hand, compound PM3 manifested an
overall quenching of the emission intensity up to fw =80% for
the band at l=494 nm. At fw =90%, a new band started to
appear at l=580 nm, along with a native emission band at
l=494 nm. A dramatic fluorescence enhancement for the
band at l=580 nm was observed at fw =100%, with complete
loss of the original band at l=488 nm.
Compounds PM2 and PM3 produced analogous absorption
spectra with increasing water fraction (fw), whereas PM1 dis-
played insignificant changes (Figure S7 in the Supporting Infor-
mation). Compounds PM2 and PM3 do not show major
changes up to fw =70%, but as fw reaches 80%, a dramatic de-
crease in the absorption intensity is observed for the band at
lꢀ423 nm. At fw =90%, a leveling-off tail appeared in the visi-
ble region, which might be due to the Mie scattering effect
commonly observed in nanoaggregate suspensions.[16]
The exact THF/water ratio for maximum fluorescence en-
hancement can only be perceived by stepwise monitoring of
the fluorescence intensity as a function of fw. The emission
spectra of PM2 and PM3 (Figure 4) were thus acquired at fw =
92, 94, 96, 98, and 100%. For PM2, as the water content in-
creased to fw =90% a new band appeared at l=525 nm with
slightly enhanced fluorescence. Interestingly, upon increasing
to fw =92%, along with the band at l=525 nm, a shoulder ap-
peared at l=564 nm. Both bands intensified and attained
maxima at fw =96%. Further addition of water (fw =98 and
100%) severely quenched the emission due to apparent pre-
To monitor the excited-state behavior of the compounds
during aggregation, photoluminescence spectra were acquired
by using same solvent composition (Figure 3 and Figure S10 in
the Supporting Information). Compound PM1 displayed prop-
erties similar to an ACQ luminogen (Figure S8 in the Support-
ing Information). The addition of water to PM2 up to fw =80%
produced a continuous decrease in fluorescence intensity for
the band at l=488 nm, and at fw =90% a redshifted band
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Figure 3. Emission spectra of PM2 (a) and PM3 (b) in mixtures of THF/water with different water volume fractions (c=5”10À5 molLÀ1
)
Figure 4. Emission spectra of PM2 (a) and PM3 (b) in mixtures of THF/water from fw =90 to 100% (c=5”10À5 molLÀ1).
cipitation. Compound PM3 displayed a dissimilar emission pro-
file with increasing fw. As the water content exceeded 90%,
the band at l=494 nm diminished and that at l=580 nm
became the major band and attained a maximum at fw =96%.
Upon further increasing the water content, the emission inten-
sity lowered possibly due to decreased solubility and precipita-
tion.[17b] Both PM2 and PM3 showed outstanding fluorescence
enhancement (ꢀ8 and ꢀ2.5 times) upon aggregation, and
thus, can easily be categorized as AIE-active luminogens.
nature of the aggregates at the point of maximum emission
enhancement. These observations advocated that the aggre-
gates of PM2 and PM3 leading to maximum emission en-
hancement at fw =96% possessed a certain degree of crystal-
linity.
Mechanism of AIE and substituent effects
Fascinating optical properties of PM2 and PM3 observed upon
aggregation and variation of the fluorescence with solvent po-
larity prompted us to elucidate the most probable mechanism
of AIE. If RIR is the major route for emission enhancement, it
should be proven experimentally. It is well documented that
AIE-active luminogens relying on RIR exhibit fluorescence en-
hancement upon increasing viscosity of the solution.[17] To
affirm this, the emission spectra of PM2 and PM3 were ac-
quired in a mixture of methanol/glycerol (c=5”10À5 molLÀ1,
0.2 vol% THF) of varying composition (Figure 5a and b). It was
observed that both PM2 and PM3 showed tremendous in-
To evaluate the process of aggregate buildup and its effect
on emission enhancement, we attempted to determine the
nature of the ensuing aggregates. We examined the powder
XRD patterns (Figure S13 in the Supporting Information) for
aggregates of PM2 and PM3 at fw =90 (water fraction for the
onset of aggregation) and 96% (water fraction for maximum
emission enhancement). The powder XRD patterns suggested
the purely amorphous nature of nascent aggregates formed at
fw =90%, whereas the well-defined peaks in the powder XRD
patterns of PM2 and PM3 at fw =96% suggested crystalline
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Figure 5. Emission spectra of PM2 with increasing fractions of glycerol (a) and hexane (c), and emission spectra of PM3 with increasing fractions of glycerol
(b) and hexane (d) (c=5”10À5 molLÀ1).
creases in emission intensity with increasing glycerol content.
Compound PM2 displayed about eight times enhancement in
emission intensity at a glycerol fraction (fg) of 90%, whereas
PM3 manifested about six times enhancement (at fg 80%).
There was no apparent change in the position of emission
maxima at l=488 and 494 nm for PM2 and PM3, respectively.
This eliminated the possibility of fluorescence enhancement
due to aggregation and established that observed changes
were solely due to the viscosity effect. Thus, viscosity-depen-
dent fluorescence enhancement confirmed the involvement of
RIR in the AIE process.
the emission maxima of PM2 and PM3 (Figure 5c and d). As fH
reached 100%, compounds PM2 and PM3 produced total
blueshifts of 30 and 32 nm, respectively. However, the emission
intensity was affected differently. There were increases in the
emission intensities of PM2 and PM3 up to fH =80%; further
addition of hexane (fH =90 and 100%) led to a decrease in
fluorescence intensity. As expected, the subsequent addition of
hexane up to fH =80% led to emission enhancement, which
was previously due to TICT and later due to RIR. At higher fH
(90 and 100%), quenching was induced by precipitation due
to poor solubility of the compounds in hexane.
Compounds PM2 and PM3 exhibited solvatochromism and
quenching of the emission intensity up to fw =90% with red-
shifted emission maxima, which raised the possibility of TICT. It
was reported previously that, if TICT was involved in the AIE
mechanism, the systems would undergo a blueshift and appre-
ciable fluorescence enhancement with increasing hexane
ratio.[18] Hexane is a nonpolar solvent; therefore, upon adding
small portions of hexane, there should be an increase in the
fluorescence intensity. Because hexane is a poor solvent for
PM2 and PM3, larger fractions of hexane should lead to AIE in
these compounds. To further confirm this phenomenon, the
emission spectra of PM2 and PM3 were monitored in a mixture
of THF/hexane by varying the hexane fractions. Upon increas-
ing the hexane fraction, fH, there is an apparent blueshift in
TICT and RIR are competitive processes and affect the AIE at-
tributes differently. The vivid optical response during aggrega-
tion can be explained as follows: As fw increases stepwise, fluo-
rescence quenching, with a redshift in the emission maxima, is
observed up to fw =90%, which is an immediate consequence
of TICT because the addition of water increases the polarity of
the system. At fw >90%, RIR becomes important and weakens
TICT to induce fluorescence enhancement due to aggrega-
tion.[17b,19]
The time-resolved emission decay profiles for PM2 and PM3
have also been examined in a mixture of THF/water with vary-
ing water contents (Figure 6). Different decay dynamics for
PM2 and PM3 with varying water fractions rationalized their
diverse emission behavior. Up to fw =80%, the average lifetime
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The difference in the emission behavior of PM2 and PM3
with increasing water gradient also accounted for the afore-
mentioned mechanism. Compound PM3 possesses a pyrimidyl
group as A’, which is clearly more electron withdrawing than
the pyridyl moiety in PM2. Hence, TICT is more pronounced in
PM3 and results in a more redshifted emission with smaller tav
value (2.59 and 3.29 ns) than that of PM2 (2.66 and 4.85 ns)
upon aggregation. RIR does not remain as effective in PM3 as
that in PM2 due to a more effective TICT, which leads to only
about 2.5 times emission enhancement in PM3 relative to
about 8 times in PM2 upon aggregation. Moreover, more profi-
cient solvatochromism in PM3 than that in PM2 suggested the
same mechanism.
Solid-state emission properties
Figure 6. A logarithmic view of the time-resolved fluorescence of PM2 and
PM3 at varying fw.
The photoluminescence spectra of PM1–PM3 have also been
acquired in the solid state (Figure 7). Compounds PM2 and
PM3 displayed a broad spectral range from l=481 to 700 nm
when excited at lꢀ423 nm. Compound PM1, on the other
hand, did not show any emission. The solid-state emission pro-
files and emission colors of PM2 and PM3 were almost identi-
cal to those of the aggregated state. Visibly, the solid-state
emission color of PM2 was green–yellow and a bit lighter than
yellow–orange for PM3, which could be related to more red-
shifted spectra of aggregates of PM3 (see above).
Because the powder XRD patterns suggested the crystalline
nature of the aggregates, the emission behavior of these mole-
cules in the crystalline state should also be monitored. Al-
though we could not obtain diffraction-quality crystals of the
compounds, gradual evaporation of the solution of aggregate
at fw =100% over a period of 2–3 days led to very fine crystal-
line fibers of PM2 and PM3. The crystalline fibers were fluores-
cent under UV illumination (lex =365 nm); the respective
(tav) was very small and beyond the detection limit of the in-
strument. As the water content rose to fw =90 and 100%, a sig-
nificant increase in tav occurred for PM2 and PM3. The tav
values were 2.66 and 4.85 ns for PM2 at fw =90 and 100%, re-
spectively; the tav values for PM3 were slightly lower: 2.59 and
3.29 ns at fw =90 and 100%, respectively. The increased life-
time at higher water fractions indicated decreased nonradia-
tive decay, which again substantiated competitive TICT and RIR
processes. Increasing water fractions raise the polarity of the
media and support the occurrence of TICT; a “dark state” char-
acterized by fast nonradiative decay and decreased lifetime,
leading to fluorescence quenching. Active intramolecular rota-
tions are restricted by aggregation at higher water fractions,
which effectively prevent formation of the dark state, leading
to enhanced emission with a longer lifetime.
Figure 7. Solid-state emission spectra of PM2 (a) and PM3 (b). Inset: images of crystalline fiber (I) and solid powder (II) of PM2 (a) and PM3 (b) under UV illu-
mination (lex =365 nm).
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images of crystalline fibers and powders under UV illumination
are depicted in Figure 7 (insets). It can be seen that the fluo-
rescence of the crystalline fibers is redshifted relative to those
of the solid powders. For further verification, we acquired
solid-state emission spectra of the crystalline fibers of PM2 and
PM3 (Figure 7). In the solid-state emission spectrum, the crys-
talline fibers of PM2 displayed two bands at l=531 and
583 nm, along with a shoulder at lꢀ631 nm, whereas that of
PM3 displayed a single band at l=600 nm. It is clear that par-
tial crystallization induces a redshift in the emission maxima of
PM2 and PM3. The redshifts in the emission maxima of PM2
and PM3, from solution to crystalline fibers through aggre-
gates, are 95 and 106 nm, respectively. In other words, it can
be concluded that, upon onset of aggregation at fw =90%,
a further increase in water content enables the aggregates to
acquire an ordered structure through increased intermolecular
interactions, which are also accountable for the aforemen-
tioned redshifts.
Figure 8. SEM (a, c) and TEM (b, d) images of the aggregates of PM2 (a, b)
and PM3 (c, d) formed in a mixture of THF/water (fw =90%;
c=5”10À5 molLÀ1).
Aggregate morphology
Because we focused our attention toward the development of
1D nanostructures that exhibited AIE, we verified our assump-
tions in practice by subjecting the aggregates to SEM, TEM,
and fluorescence microscopy. Compounds PM2 and PM3 both
provided 1D nanoaggregates with different morphologies. SEM
data revealed that PM2 consisted of perfect leaf-shaped aggre-
gates, while PM3 formed twisted ribbon-like nanoaggregates
(Figure 8a and Figure S11a in the Supporting Information). A
closer view of the SEM images of aggregates of PM3 showed
a unique morphology, wherein twisted ribbons actually con-
verged at a point to fabricate the flower-shaped edifice (Fig-
ure 8c and Figure S11c in the Supporting Information). Nota-
bly, compound PM3 displayed more than one hierarchical as-
sembly. As shown in Figure 8c, there are some straight ribbons
along with a couple of helical fibers. It is believed that the
twisted ribbons braid together to form helical fibers (Fig-
ure S11 in the Supporting Information).[20]
ized in Figure 8b and d and Figure S11b and d in the Support-
ing Information. For PM3, TEM data revealed the occurrence of
more than one twist in each ribbon-shaped structure. These
twists also support the formation of helical fibers as one of the
hierarchical assemblies. Fluorescence microscopy images of
both PM2 and PM3 (Figure 9 and Figure S12 in the Supporting
Information) display dichromic fluorescence under blue or
green excitation. An overlay of the images produced a yellow-
colored image of the respective aggregates (Figure 9c), and
thus, suggested that dichromic fluorescence originated from
a single entity, namely, a leaf-shaped aggregate in PM2 and
flower-like aggregates in PM3.
Crystal structure
Furthermore, to gain a deeper insight into the aggregation
process and to rationalize the concomitant relationship be-
tween molecular packing and aggregate morphology, we tried
TEM data confirmed the self-assembly of the aggregates.
Branching in the leaf-shaped aggregates of PM2 can be visual-
Figure 9. Fluorescence microscopy images of PM2 and PM3 under red (a) and green (b) excitation, as well as overlaid images (c).
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to obtain single crystals of PM1–PM3 by slow diffusion of
CH2Cl2/CH3OH into a solution of the respective compound. For-
tunately, we obtained crystals of PM3, which crystallizes in the
¯
triclinic system with the P1 space group. Data collection and
refinement parameters for PM3 are gathered in Table S1 in the
Supporting Information, selected geometric parameters can be
found in Table S2 in the Supporting Information, and a perti-
nent view is depicted in Figure 10. It is evident from the crystal
structure that the piperazine unit acquires a chair conforma-
tion. The dihedral angle between piperazine (N4, C6, C5, N3,
C8, C7) and the pyrimidine (C1, N1, C4, C3, C2, N2) core is
17.808 and between piperazine and the phenyl (C9, C10, C11,
C12, C13, C14) core is 9.768. Selection of the piperazine unit as
a spacer was instrumental, since it is now clear from the crystal
structure that the molecule acquires a twisted conformation
due to torsions offered by the piperazine unit.
Figure 10. ORTEP view of PM3; ellipsoids are given at the 30% probability
level (hydrogen atoms are omitted for clarity).
Figure 11. Crystal packing in PM3 through face-to-face CÀH···N interactions
(a). Packing in PM3 through all CÀH···N hydrogen-bonding interactions (b),
displaying a wavy 1D structure and head-to-tail packing in PM3 (c).
Careful examination of the crystal packing in PM3 revealed
that each molecule was surrounded by three neighboring mol-
ecules, mainly through CÀH···NÀH bonding interactions
(Figure 11). Molecular packing through these interactions un-
veiled that DCV units of the adjacent molecules stacked in
a slightly slipped manner through face-to-face CÀH···N interac-
tions (2.715 Š). Of two other CÀH···N interactions, one involves
the piperazine hydrogen and DCV nitrogen atom of one adja-
cent molecule, whereas the other is between the piperazine
hydrogen and DCV nitrogen atom of a neighboring molecule.
These interactions, with bond lengths of 2.722 and 2.603 Š,
enable the molecules to be arranged in an almost head-to-tail
manner, analogous to J aggregates (Figure 11 c).[21] The compa-
ratively short length of these interactions leads to strongly
coupled aggregate structures and also reflects possible elec-
tronic communication between these molecules to promote
the redshift upon aggregation or in the solid state.[22] The
aforementioned interactions facilitate 1D self-assembly in PM3
and make the twisted structure rigid. Thus, these hydrogen-
bonding interactions restrict intramolecular rotations and lead
to enhanced emission in the solid state. DFT studies revealed
almost identical structures for PM1and PM2, which compen-
sated for the unavailability of crystal structures of these com-
pounds (Figure S14 in the Supporting Information)
Cyclic voltammograms of these molecules were acquired in
acetonitrile (c=1”10À4 molLÀ1). In the cyclic voltammogram
(Figure 12), compound PM1 showed a reversible oxidation
wave, whereas PM2 and PM3 displayed irreversible oxidation
waves (weaker in PM3), which may arise due to the formation
of a stable radical cation in the conjugated sequence. The dif-
ference in the oxidation behavior can be rationalized by the
substituent effect. The reversibility of the oxidation curve in
PM1 suggested that the radical cation for this system was
more stable than those of PM2 and PM3. This corresponded
well with the electron-donating ability of the ethyl group in
PM1, whereas the pyridyl (PM2) and pyrimidyl (PM3) moieties
act as electron-withdrawing groups. These compounds (PM1–
PM3) displayed an irreversible reduction wave in their cyclic
voltammograms, which marked the formation of radical anions
at the terminal DCV unit. Experimental HOMO and LUMO ener-
gies were evaluated from the oxidation and reduction poten-
tials, respectively (PM1: E=À5.68 eV (HOMO), E=À3.20 eV
(LUMO); PM2: E=À5.57 eV (HOMO), E=À3.23 eV (LUMO);
PM3: E=À5.59 eV (HOMO), E=À3.24 eV (LUMO); Table 1). The
HOMO is more stabilized in PM1, but the LUMO is more stabi-
lized in PM2 and PM3, which is in good agreement with the
electron-donating ability of the ethyl substituent and -with-
drawing ability of the pyridyl and pyrimidyl substituents. The
HOMO–LUMO gaps (PM1, 2.48 eV; PM2, 2.34 eV; PM3, 2.35 eV;
Table 1) were also calculated and found to be in good agree-
ment with those obtained from absorption studies.
Electrochemical studies
The electronic structures of luminogens play a pivotal role in
determining charge-transfer processes and also provide an ac-
count of the HOMO and LUMO energy levels of the molecules.
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Figure 12. Cyclic voltammograms of PM1 (a), PM2 (b), and PM3 (c) in acetonitrile (c=5”10À5 molLÀ1).
pyrimidyl). It can further be added that charge-transfer transi-
tions are not contributed to by HOMO–LUMO alone, but other
possible transitions involving HOMO-1 and LUMO+1 make
a significant contribution. The HOMO–LUMO transition corre-
sponds to the main absorption band at lꢀ423 nm in both
PM2 and PM3. The broad absorption in these compounds re-
vealed the involvement of other transitions of comparable en-
ergies. These observations account well for the competitive be-
havior of the two acceptor groups. The effect of A’ can be real-
ized particularly on the LUMO level because it was stabilized
to a greater extent in PM3 and PM2 than that in PM1. Theo-
retical results also supported the band gap values for PM1–
PM3 through electrochemical and UV/Vis observations. Time-
dependent (TD) DFT calculations were also performed on
PM1–PM3, and the UV/Vis spectra obtained theoretically were
comparable to those obtained experimentally (Figure S15 in
the Supporting Information).
Table 1. Electronic states (HOMO/LUMO levels) [eV] and energy gaps [eV]
in PM1–PM3.
LUMO
HOMO
Eg
VCV
Vcalcd
VCV
Vcalcd
VCV
Vcalcd
VUV
PM1
PM2
PM3
À3.20
À3.23
À3.24
À2.37
À2.42
À2.42
À5.68
À5.57
À5.59
À5.82
À5.82
À5.85
2.48
2.34
2.35
3.46
3.40
3.43
2.91
2.92
2.93
Theoretical considerations
The electronic structures of PM1–PM3 were further appraised
by means of DFT (B3LYP 6-31G*) and the resulting molecular
orbital distributions of the frontier orbitals are depicted in
Figure 13. Calculated energy levels and HOMO–LUMO gaps are
summarized in Table 1.The computational results indicated
that in PM1–PM3 the HOMO was spread over whole molecule,
Conclusion
Through this work, a highly ambitious approach has been de-
veloped to obtain interesting 1D nanostructures with AIE attri-
butes and solid-state emission. In this context, we used three
rationally designed, and effortlessly synthesized, simple com-
pounds that were composed of piperazine/DCV derivatives to
offer symmetric (PM1) and asymmetric (PM2, PM3) D–A con-
structs. Asymmetry was achieved by decorating the com-
pounds with A’ (pyridyl, pyrimidyl) groups. Surprisingly, the
symmetric D–A molecule was AIE-inactive, whereas asymmetric
D–A molecules were endowed with AIE activity, solvatochrom-
ism, viscochromism, and strong solid-state emissions, which
clearly justified the key role of A’ in triggering AIE in these
compounds. The mechanism for these intriguing properties
was established to be under the competitive control of TICT
and RIR, which also suggested that optical variations in PM2
and PM3 could be directly related to the comparative acceptor
strength of A’. Flexibility provided by the piperazine moiety
and intermolecular hydrogen-bonding interactions led to ex-
quisite 1D nanostructures. The outcomes of this study are en-
couraging for the design of new, improved, AIE-active lumino-
phores by exploiting asymmetry in the structures.
Figure 13. Frontier molecular orbitals of PM1 (left), PM2 (middle), and PM3
(right) obtained from DFT calculations.
whereas the LUMO was mainly attributed to the aryl–DCV
moiety, which was consistent with the strong accepting ten-
dency of cyano groups. Notably, there is no contribution to
the LUMO by A’ groups, that is, pyridyl in PM2 and pyrimidyl
in PM3. It is presumed that the presence of two acceptors cre-
ates competition, wherein DCV, which is the better acceptor,
excels.
To confirm this assumption, we also evaluated the molecular
orbital distribution of HOMO-1 and LUMO+1. The HOMO-1 is
very similar to that of HOMO and spread over the whole mole-
cule, whereas LUMO+1 is only associated with A’ (pyridyl and
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1
Yield: 0.438 g, 88%; H NMR (CDCl3): d=6.66 (t, 1H; pyridyl), 6.84,
(d, J=8.7 Hz, 1H; pyridyl), 7.10 (d, J=9.0 Hz, 2H; phenyl), 7.54 (d,
J=6.9 Hz, 1H; methine), 7.86 (d, J=8.7 Hz, 2H; phenyl), 8.10 ppm
(s, 1H; pyridyl); 13C NMR (CDCl3): d=43.7, 43.4, 70.4, 107.0, 113.0,
115.2, 120.0, 133.6, 137.6, 147.5, 154.1, 158.8 ppm; HRMS: m/z
calcd for C19H17N5 [M+H]+: 316.1552; found: 316.1551; elemental
analysis calcd (%) for C19H17N5: C 72.36, H 5.43, N 22.21; found: C
72.29, H 5.39, N 22.19.
Experimental Section
Reagents
The solvents were dried and distilled prior to use by following
standard procedures.[23] 1-Ethylpiperazine, 1-(2-pyridyl)piperazine,
1-(2-pyridyl)piperazine, and malononitrile were procured from
Sigma Aldrich India and used as received without further purifica-
tions. The aldehydes 1a–c were prepared by following a procedure
reported in the literature.[13]
Synthesis of PM3
General methods
Compound PM3 was prepared by following the same procedure
as that used for PM1 with 1c (0.5 g, 1.87 mmol) in place of 1a.
Yield: 0.451 g, 90%; 1H NMR (CDCl3): d=3.60 (t, 4H; piperazine),
4.01 (t, 4H; piperazine), 6.57 (t, 1H; pyrimidyl), 6.89 (d, J=8.7 Hz,
2H; phenyl), 7.50 (s, 1H; methine), 7.84 (d, J=9.0 Hz, 2H; phenyl),
8.35 ppm (s, 2H; pyrimidyl); 13C NMR (CDCl3): d=25.1, 43.0, 46.2,
92.3, 113.2, 133.7, 139.6, 154.3, 157.8 ppm; HRMS: m/z calcd for
C19H17N5 [M+H]+: 317.1504; found: 317.1509; elemental analysis
calcd (%) for C18H16N6: C 68.34, H 5.10, N 26.56; found: C 68.29, H
5.05, N 26.48.
Elemental analyses for C, H, and N were obtained on an Elementar
Vario EL III Carlo Erba 1108 analyzer from the microanalytical labo-
ratory of the Sophisticated Analytical Instrumentation Facility
(SAIF), Central Drug Research Institute (CDRI), Lucknow, India. Elec-
tronic absorption spectra were acquired on Shimadzu UV-1601
spectrophotometers. H and 13C spectra were acquired on a JEOL
1
AL 300 FT-NMR spectrometer by using Si(CH3)4 as an internal refer-
ence. Fluorescence spectra (95% aqueous methanol) at room tem-
perature were acquired on a PerkinElmer LS 55 fluorescence spec-
trometer. SEM images were acquired on a JEOL JSM 840A scanning
electron microscope. TEM images were obtained on a FEI Technai
20 U twin transmission electron microscope. Fluorescence micro-
scopic images were recorded on an EVOS FL cell imaging system.
High-resolution mass spectra were recorded on a Brucker-Daltonics
micrOTOF-Q II mass spectrometer. Cyclic voltammetry measure-
ments were performed on a CHI 620c electrochemical analyzer at
RT. Experiments were performed in an air-tight single-compartment
cell by using platinum wire as the counter electrode, a glassy
carbon working electrode, and an Ag/Ag+ reference electrode.
Crystal data for PM3 was collected on a Bruker Kappa Apex-II dif-
fractometer at RT with MoKa radiation (l=0.71073 Š). The structure
was solved by direct methods (SHELXS 97) and refined by full-
matrix least-squares on F2 (SHELX 97).[24] Non-hydrogen atoms
were refined with anisotropic thermal parameters. Hydrogen
atoms were geometrically fixed and refined by using a riding
model. The computer program PLATON was used for analyzing in-
teractions and stacking distances.[25]
Theoretical studies
Quantum chemical calculations were performed at the B3LYP DFT
level by using B3LYP/6-31G** for PM1–PM3.[17b] All geometry opti-
mizations and frequency calculations (to verify a genuine mini-
mum-energy structure) were performed by using the Gaussian 09
suite of programs.[26]
Acknowledgement
We acknowledge financial support from the Department of Sci-
ence and Technology (DST), New Delhi, through the scheme
SR/S1/IC-25/2011. R.S.S. acknowledges the University Grants
Commission, New Delhi, India, for a Senior Research Fellowship
(19-12/2010(i) EU-IV).
Keywords: aggregation
fluorescence · nanostructures · self-assembly
·
donor–acceptor
systems
·
CCDC 1418259 contains the supplementary crystallographic data
for this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
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Received: September 7, 2015
Published online on November 30, 2015
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