Morphological tuning via structural modulations in AIE luminogens with the minimum number of possible variables and their use in live cell imaging

Article abstract of DOI:10.1039/c5cc02488a

With intent to fine tune the morphological and photophysical properties, three novel AIE luminogens (BQ1-BQ3) based on quinoline-BODIPY have been synthesized. A judicious choice of substituents (-H, -CH₃, -OCH₃) in these systems led to nanoballs in BQ1 and BQ2, while in BQ3 it led to reticulated nanofibers with diverse photophysical behaviours. This journal is

Full text of DOI:10.1039/c5cc02488a

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Morphological tuning via structural modulations
in AIE luminogens with least possible variables
and their use in live cell imaging
Cite this: DOI: 10.1039/x0xx00000x
Roop Shikha Singh,a Rakesh Kumar Gupta,a Rajendra Prasad Paitandi,a
Mrigendra Dubey,a Gunjan Sharma,b Biplob Kochb and Daya Shankar Pandey*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
With intent to fine tune the morphological and photophysical
properties, three novel AIE luminogens (BQ1
BQ3) based on active quinoline derivatives have scantly been reported.9 These facts
quinoline-BODIPY have been synthesized. Judicious choice prompted us to hypothesize that it might be beneficial to use
of substituents ( H, CH3, OCH3) in these systems led to quinoline-BODIPY as a platform to investigate AIE.
D−
A architecture and fluorescent markers in biological systems,8 AIE
−
−
−
−
nanoballs in BQ1 and BQ2 while reticulated nanofibers in
BQ3 along with diverse photophysical behaviour.
The discovery of aggregation induced emission (AIE) by Tang et al, was
much of a surprise, since it challenged our general perception about
luminescence, as organic luminophores witnessed a downfall in their
potential applications due to aggregation caused quenching (ACQ)
effect.1 This revelation followed development of numerous excellent AIE
luminophores based on tetraphenylethene, cyanostilbene, 9,10-
distyrylanthracene, benzoxazoles etc.2 The phenomenon of AIE can be
related to restricted intramolecular rotation (RIR), reduced co-facial
intermolecular
π overlap, specific molecular arrangements such as J-
aggregation, or dimer stacking and intramolecular charge transfer
(ICT).3 Lately, some fluorescent dyes with a variety of functionalities
have been designed on AIE platform to illustrate promising applications
in photonic biosensors and nanomaterials.4 Among these, boron-
dipyrromethenes (BODIPYs) have been fascinating because of their
resourceful applications in light harvesting antenna system, solar cell
sensitizers, molecular photonic wires, fluorescent switches, marker and
sensors in contemporary chemistry and biology.5 Despite being
excellent emitters in solution, the BODIPY dyes hardly fluoresces in
solid state due to severe self-quenching arising from small Stokes shift
and planar π−conjugated structures. To expand their technological
applications, current research has seen a large surge in overcoming
weaknesses of the BODIPY dyes by introducing sterically bulky
substituents to inhibit close packing.2a,6
Fig. 1 Chemical structures of BQ1−BQ3 (a), BQ4 (b) and ORTEP views of BQ2 (c),
BQ3 (d) at 40% ellipsoidal probability.
Although photonics of the AIE luminogens have been investigated
extensively, due attention toward strategies leading to aggregates
and their morphology has been wanting. Developing an approach that
could address aforesaid issues without conventional decorated
luminophores has been challenging. With an intention of tuning the
morphology of aggregates and their optical properties via minimum
possible variations, three novel AIE luminogens have been designed
using quinoline-BODIPY as
SCH3) group has been used as triggering unit and to tune the AIE, a
tuner varying from hydrogen (BQ1), methyl (BQ2) to methoxy group
BQ3) placed diagonal to SCH3 group. Sulfur containing group has
a platform. Strategically, thioether
(
−
(
−
been chosen as sulfur can increase the intra- and intermolecular
electronic motion due to its high polarizability. In addition, to affirm
Apart from being good metal chelators, functionalized quinolines
are highly emissive in solution as well as in solid state with high
quantum yield, good electron transport and excited state intra-
molecular proton transfer (ESIPT).7 Although these serve as good
candidates for fluorescent dyes, molecular sensors, construction of
triggering effect of −SCH3, a model compound having −Cl in its place
has also been synthesized (BQ4). Further, to restrict rotation of the
quinoline unit around BODIPY core triggering unit has been placed in
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DOI: 10.1039J/oCu5CrnCa02l4N8a8Ame
close proximity to the quinoline nitrogen wherein −SCH3 being bulkier and BQ2 two peaks emerged at 571 and 612 nm as the water content
than
offered clear implication of the steric hindrance on RIR as BQ1
are AIE active, while BQ4 is inactive in this regard (Fig. S 16). It is well emission band corresponding to quinoline unit gradually disappeared.
documented that various interactions in the solid state play an However, for BQ3 both the emission bands showed some unusual
−
Cl imposes greater steric hindrance (Fig. 1). This approach exceeded 70%, which intensified with increasing water content and
−
BQ3 attained maximum value at fw 90% (Fig. 2a, S17 18, ESI†), whereas
−
essential role in aggregation and AIE. However, their utility in changes at fw 70% (Fig. 2b), possibly due to progressive aggregation
determining shape of the aggregates needs to be scrutinized as these (Fig. S24, ESI†). Further additions of water quenched the emission at
offer information about distribution of the hydrophilic and 532 nm with emergence of a new peak at 620 nm, which strengthened
hydrophobic moieties. Therefore, hydrogen (BQ1) has been chosen as with greater water content and attained maxima at fw 90% (Fig. 2b). It
tuner for obvious reasons, methyl group (BQ2) as a weak electron is noteworthy to mention that at fw 90% BQ1 displayed emission in
donor that also maintains hydrophobicity in the quinoline core, while aggregated state without any enhancement, whereas 4- and 2-fold
methoxy group (BQ3) as an electron donating group that renders enhancement in fluorescence intensity was observed for BQ2 and
some hydrophilicity to the quinoline moiety.
Dipyrromethanes have been prepared by condensation of the these molecules at fw
aldehydes with an excess of pyrrole in presence of catalytic fluorescence (AIE). Apparently all these compounds displayed red
amounts of trifluoroacetic acid. The BODIPYs BQ1 BQ4 has been shift upon aggregation, a frequently observed feature in AIE
BQ3, respectively. Water being poor solvent promotes aggregation of
5
−
8
>
70% and RIR resulted in an enhanced
1−4
−
4
synthesized by reacting respective dipyrrins, obtained in-situ by systems.2 Multiple emission bands upon aggregation may arise due
−
oxidation of
Elemental analyses, IR, NMR (1H, 13C), HRMS, electronic absorption, Striking variation in the emission pattern and AIE for BQ1
and emission spectroscopic studies well supported their proposed advocated profound substituent effect and if implicit assumptions are
formulations. Structures of BQ1 BQ4 have unambiguously been right, this would affect morphology of the aggregates as well.
5
−
8
using DDQ, with BF3∙Et2O in presence of Et3N (ESI†). to presence of both monomeric and aggregated state in solution.6b
−BQ3
−
authenticated by X-ray single crystal analyses (Fig. S1
ESI†).
−S10, S26,
Optical properties of BODIPYs BQ1−BQ4 have been investigated
in methanol. These displayed typical absorption bands associated
with BODIPY6c,10 at ~ 507 nm due to π−π∗ transitions along with a high
energy band corresponding to n−π∗ at ~ 340 nm (
c, 50 ꢀM) (Fig. S11a,
ESI†). Illumination of BQ1 BQ4 at ~ 340 nm resulted in dual emission
−
due to quinoline and BODIPY moieties at 435 and 530 nm,
respectively. However, upon excitation at 507 nm it gave an emission
band solely due to BODIPY unit (Fig. S11b, 12, ESI†). Excitation
spectra of these compounds have been monitored at both emission
bands which supported the origin of dual emission (Fig. S13, ESI†).
Fig. 2 Fluorescence spectra of BQ2 (a) and BQ3 (b) in methanol/water mixture (c,
50
ꢀ
M) with different volume fractions of water (fW).
Absorption and emission spectra of BQ1−BQ4 have also been
To gain deep insight into aggregation and diverse optical
acquired in solvents with varying polarities. Notably, these
compounds exhibited lower fluorescence intensity in polar solvents
behaviour, water–methanol (90%) mixtures of BQ1−BQ3 were
subjected to scanning electron, transmission electron and atomic
force microscopy (SEM, TEM and AFM). It was observed that these
created different types of nanoaggregates depending upon respective
substituents. As it is clear from the images (Fig. 3, S23, ESI†) BQ1 and
BQ2 gave nano-ball aggregates of uniform size with average diameter
of 220 and 150 nm, BQ3 formed a highly reticulated network of ~300
nm thick fibrous nanoaggregate. Smaller size of the aggregate in BQ2
relative to BQ1 may be due to greater hydrophobicity of BQ2. The
TEM images displayed further assembly of aligned nanoballs in BQ1
and BQ2 (Fig. 3b, 3e). The shape and size of these aggregates are in
good agreement with those obtained from the AFM (Fig. 3c, 3f and
Fig. S23, ESI†). This morphological disparity strengthened and
established the proposition that nanoaggregate morphologies are
tunable by judiciously chosen substituents even using same platform
and analogous conditions.
relative to apolar ones except for BQ4 (Fig. S19−S21, ESI†). On the
other hand, absorption spectra did not show any significant change.
This behaviour reflected some polarization of the emissive state
whereas ground state was unaffected.
Being excellent chromophores, BODIPYs are expected to exhibit
fascinating photophysical properties in methanol/water mixture (
c, 50
ꢀ
displayed almost identical absorption profiles with increasing water
M) of varying compositions. BQ1 BQ3, in their UV-vis spectra,
−
content while BQ4 exhibited insignificant changes. Upon addition of
water upto 90%, BQ1−BQ3 showed a decrease in absorbance along
with a level-off tail in the visible spectral region, a common feature of
the nanoaggregate suspension.11 Remarkably, colour of the solution
also turned red from yellow (Fig. S14-15, S22, ESI†). It may be
attributed to an increase in π−π interactions, thereby, creation of the
nanoaggregates. To substantiate aggregation process photolumi-
The morphological variation for aggregates from nanoballs to
fibers and their discrete photophysical behaviour revealed significant
role of strong directional intermolecular interactions. This prompted
us to look into crystal structure of these compounds. Careful
examination of the crystal structure of BQ1 revealed that it lacks
nescence (PL) of BQ1
system. Unlike absorption, PL spectra displayed significant variations
wherein BQ1 BQ3 showed emission bands at ~435 and ~532 nm.
Intensity of both the bands diminished with increasing water fraction
fw), and solution became almost nonluminescent at fw <70%. In BQ1
−BQ3 has been acquired using aforesaid solvent
−
(
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π−π interactions and quinoline units of one molecule are inter- quinoline unit forms hydrophobic core (Fig. 4). BQ3 displays a self-
connected to the adjacent ones through C–H∙∙∙
in a slightly slipped manner (Fig. S27a, ESI†).
π
interactions (2.691 Å) assembled aggregate of twisted and highly branched fibers. It is
endowed with two hydrophilic groups, methoxy and BF2 units, which
are well separated by hydrophobic quinoline unit. The aforesaid
interactions bring hydrophilic groups on both sides of the fiber
favoring a sideways aggregation via interaction with water (Fig. 4; Fig.
S32, ESI†).
Fig. 3 SEM (a, d); TEM (b, e) and AFM (c, f) images of nanoball in BQ2 (Top) and
ꢀM with fW = 90%. Inset shows the
nanofiber in BQ3 (Bottom) at c, 50
corresponding magnified images.
BQ2 exists in a dimeric form where neighbouring dipyrromethene
units are linked by face to face π−π interaction (3.353 Å; Fig. S28,
ESI†). Rather larger slipped stacking in this molecule weakens the π−π
Fig. 4 Mechanistic interpretation of aggregation from randomly dispersed
molecules to nanoaggregates in BQ2 (top) and BQ3 (bottom) upon increasing the
water content (fw = 90%): A cross sectional insight of nanoball in BQ2 and
interactions and favors
herringbone mode through long-range ordered arrangement and
forms a typical -aggregate in which adjacent molecules are packed in
J-aggregation. Whereas BQ3 stacks in a
nanofiber in BQ3
.
J
To have better understanding of the spectroscopic results,
theoretical calculations have been performed on energy levels of
anti parallel (head to tail) manner. The quinoline moiety of one
molecule interacts with boron-dipyrrin moiety of the nearby molecule
BQ1
the orbitals from quinoline units while LUMO are concentrated on
BODIPY core. The calculated HOMO LUMO gaps for BQ1 BQ2 and
−BQ3 (Fig. S33, ESI†). The HOMO energy levels are dominated by
through π−π (3.397, 3.363 Å) and C–H∙∙∙
(Fig. S30 32, ESI†). A closer look at hydrogen bonding interactions
revealed that BQ1 has one C–H∙∙∙ (2.691 Å) and two face-to-face C–
H∙∙∙F interactions (Fig. S27 b, ESI†). Likewise, BQ2 has one each of
face-to-edge C–H∙∙∙ (2.751 Å) and face-to-edge C–H∙∙∙N (2.679 Å)
interactions (Fig. S29, ESI†). On the other hand BQ3 possesses two C–
H∙∙∙ (vide-supra) and two C–H∙∙∙F (2.614) hydrogen bonding
π (2.872, 2.810 Å) interactions
−
−
,
π
BQ3 are 3.02, 3.02, 2.96 ev respectively, which are in good agreement
with those obtained from UV-vis spectroscopic data.
π
π
interactions all along the direction of the long molecular axis. (Fig.
S31, ESI†). These interactions provide a rigid 3-D network and restrict
intramolecular rotation of quinoline unit around the BODIPY core,
thereby increasing the emission intensity in aggregated state.
Aforesaid interactions also support the reason behind red shifts in
BQ1−BQ3 upon aggregation. BQ3 experiences maximum red shift
(90nm) followed by BQ2 (81nm) and BQ1 (79nm), as BQ3 has
minimum face to face interactions with adjacent units, hence weaker
π
–π
interactions favoring
J-aggregation.
The mechanism of aggregate build-up has been deduced
considering dominance of the solid state interactions in aggregated
state which has also been evidenced by powder XRD patterns (Fig.
S25, ESI†). The anticipated out-turn of intermolecular interactions are
profound as it brings distribution of the hydrophilic and hydrophobic
domains in such a way that polar groups are exposed to aqueous
environment shielding nonpolar groups away from water to form the
hydrophobic core. With increasing water gradient, H-bonding
interactions in BQ1 and BQ2 favor a nanoball wherein hydrophilic BF2
unit comes outwards to maximize the interaction with water and
Fig. 5 (a, b) Fluorescent and (c) bright field images of DL cells stained with
BQ1−BQ3.
The efficacy of these luminogens in biological systems has been
explored by live cell imaging experiments. DL cells were incubated
with BQ1 BQ3 for 2 h and co-stained with Hoechst 33342 to identify
−
nuclear region of the cells. These compounds stained cytoplasm
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excluding the nucleus (Fig. S35, ESI†). All the compounds showed
dichromic (green and red) fluorescence under blue or green excitation
(Fig. 5) due to presence of both monomeric and aggregated species.
The endocytosis is believed to be major route for internalization of
compounds. Noticeably, the green emission was weakest in BQ3
while BQ1 displayed faint red emission owing to their variable extents
of aggregation. Thus these compounds may be potentially useful in
studying the endocytic structures and pathways.
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In conclusion, three novel quinoline-BODIPY based systems (BQ1-
−
−
BQ3) have been designed and synthesized. AIE in these compounds
is triggered under direct influence of steric hindrance on RIR. This
approach relies on simplifying fine tuning of the nanoaggregate
morphology through minimum possible changes in substituents from
−
,
−
−
H, −CH3 to −OCH3. Two types of aggregates viz. spherical (BQ1,
,
,
BQ2) and fibrous (BQ3) were obtained from the same platform. Vital
role of the substituents has also been rationalized from photophysical
properties and crystal packing patterns. The AIE phenomenon has
−
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We acknowledge financial support from the Department of
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