Role of synergistic π-π Stacking and X-H...Cl (X = C, N, O) H-bonding interactions in gelation and gel phase crystallization

Article abstract of DOI:10.1039/c5cc00930h

The self-assembly of p-pyridyl-ended oligo-p-phenylenevinylenes (OPVs) in ethanol leads to the formation of either hollow or solid microrods. The corresponding protonated OPVs with n-butyl chains induce transparent gelation and also gel phase crystallization owing to various synergistic noncovalent interactions. The chloride ion-selective gelation, AIEE and stimuli responsiveness of the gel are also observed. This journal is

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COMMUNICATION
Role of Synergistic πꢁπ Stacking and X―H···Cl (X = C, N, O) Hꢁ
Bonding Interactions in Gelation and Gel Phase Crystallization
Subham Bhattacharjee, a and Santanu Bhattacharya*,a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Further the gelation was accompanied with aggregation induced
50 enhancement in emission (AIEE) and the gel also showed multiꢀ
stimuli responsiveness. Moreover, the formation of nanoꢀspheres
from the rigid bolaꢀamphiphilic OPV comprising nꢀoctyl chains
and their transformation into superstructured organogels upon
Selfꢁassembly of pꢁpyridyl ended oligo pꢁphenylenevinylenes
(OPVs) in ethanol leads to the formation of either hollow or
solid microrods. The corresponding protonated OPVs with nꢁ
butyl chains induce transparent gelation and also gel phase
10 crystallization owing to various synergistic noncovalent
interactions. Chloride ionꢁselective gelation, AIEE and stimuli
responsiveness of the gel are also observed.
increasing concentration of the gelator was observed.
A
55 remarkable selectivity towards the Clꢀ counterion on the gelation
of the protonated OPVs is also noteworthy.
Selfꢀassembly of certain low molecularꢀmass organic molecules
and/or macromolecules yields various functional soft materials
15 like micelles,1a vesicles,1b lipoplexes,1c nanoparticles,1d,e gels,2
liquidꢀcrystals,3 etc. Towards this end, the design of oligo (pꢀ
phenylenevinylene) (OPV) based assemblies including gels has
become a fascinating area of research during the last few years
because of their superior optoelctronic properties, liquidꢀ
20 crystallinity, potential candidacy as artificial light harvesting
materials and for numerous other applications.4 Therefore tuning
the morphology of πꢀgels is challenging as well as important in
the field of supramolecular electronics and other areas.5
(a)
(b)
+
H
OR
N
OR
N
H
b
H
a
N
OR
ꢁ
2Cl
N
+
OR
H
1: R = nꢁC4H9
2: R = nꢁC8H17
P1: R = nꢁC4H9
P2: R = nꢁC8H17
20 ꢀm
(c) (d)
(e)
O
N
Simulated pattern
microꢁrod
H
H
N
5
10
15
20
25
30
10 ꢀm
Two Theta (2θ)
On the other hand, weak X―H···Cl (X = C, N, O) Hꢀbonding
25 interactions have been mostly encountered in the solid state
through crystal engineering6a and anionꢀrecognition processes6b in
solution. However, reports comprising their involvement on the
construction of supramolecular gel are rather rare.2q Only
recently, hydrogelation induced by C―H···X (Cl, O) Hꢀbonding
30 among the πꢀπ stacked aromaticꢀunits has been reported.2q One of
the possible ways to probe such interactions inside a gel is to
grow singleꢀcrystals of the gelator within the gel matrix, which is
Figure 1. (a) Chemical structures of the OPV derivatives 1 and 2
60 and their protonated forms P1 and P2. (b,c) SEM images of the
aggregates of 1 (45 mM) and 2 (10 mM) from EtOH respectively.
(d) Part of the unit cell of the crystal of 1 showing C―H···π and
C―H···N Hꢀbonding interactions (viewed along cꢀaxis). (e) A
comparison of the simulated XRD pattern of single crystal with
65 the PXRD pattern of the microꢀrods of 1.
of course
a
challenging task.7 Rare instances include
crystallization of a hydrogelator from its gelling solvents,7c or fast
35 cooling of a twoꢀcomponent system that triggered gelation by
halogenꢀbonding but slow cooling of the same afforded only coꢀ
crystals instead of a gel.7d NMR studies of the gel phase can also
shed light on the existence of such interactions.2q
Herein, we show the important role of the alkyl chain length
40 in the selfꢀorganization of nonꢀprotonated OPV derivatives in
ethanol to fabricate either hollow or solid rods with rectangular
crossꢀsections. Further, we demonstrate that the cooperative πꢀπ
and multiple X―H···Cl (X = C, N, O) Hꢀbonding interactions are
sufficient to trigger supramolecular polymerization leading to the
45 formation of a robust gel from the protonated pꢀpyridyl ended
OPVs. We also report for the first time that the gel matrix of such
protonated OPVꢀbased gelator comprising nꢀbutyl chains may
also serve as a medium for the crystallization of the gelator itself.
A hot solution of 1 (45 mM; Fig. 1a) in EtOH upon
spontaneous cooling to ~25 °C results in the formation of
multiple single crystals that belong to
a centrosymmetric
70 tetragonal space group (I41/a) (Fig. S1e). The C―H···π and
C―H···N Hꢀbonding interactions are mainly responsible for the
selfꢀassembly of 1 into crystals (Fig. 1d and S1bꢀd). In contrast,
when a hot solution of 1 (45 mM) in EtOH was bath sonicated at
2ꢀ5 °C, it afforded a colloidal solution. Investigation of the
75 colloidal aggregates showed the presence of wellꢀdefined 1ꢀD
molecular rods with uniform size and shape (Fig. 1b and S2aꢀe).
Moreover, the rods displayed smooth outside surface and sharp
edges. The lengths of these rods of high aspect ratio vary in the
range of 10ꢀ300 µm. Further, scanning electron microscopic
80 (SEM) images reveal the hollow feature of these rods having an
open rectangular end. However, sonication of a less concentrated
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solution of 1 (10 mM) in EtOH under the same condition yielded
separation within ~2 days (Fig. S8). This suggests that the
molecular rods with rugged rectangular ends (Fig. S2f). The 60 molecular packing in the crystal is presumably maintained in the
etching of the molecules of 1 by EtOH in the central part of the
solid rods during sonication was probably responsible for such
hollow structure formation.8
gel matrix as well. However, the gel of P1 remained stable over
several months without formation of crystals in the gel phase
when the concentration was maintained near the CGC.
5
The simulated XRD pattern from the singleꢀcrystal was then
compared with the powder Xꢀray diffraction (PXRD) pattern
obtained from these hollow rods, which showed remarkable
similarity between the two (Fig. 1e). Thus the molecular
(a)
(b)
(c)
1
P1
HCl
10 arrangements of 1 in the micro rods may be essentially the same
with that of the singleꢀcrystal structure.7
(d)
(e)
(f)
(g)
The compound 2, on the other hand, induced gel formation
albeit at a higher critical gelator concentration (CGC ~46 mM)
when a hot solution of the same in EtOH was sonicated briefly at
15 ~5 °C (Fig. S3 and table S1). The gel of 2 revealed the presence
of solid rectangular rodꢀlike aggregates under SEM (Fig. 1c and
S4aꢀc). The morphology of 2 was further verified by atomic force
microscopy (AFM), polarized optical microscopy (POM) and
fluorescence microscopic investigations (Fig. S4dꢀg).
50 ꢀm
5 ꢀm
5 ꢀm
(j)
Clꢁ ion
N
Clꢁ ion
(h)
(i)
Simulated pattern
Xerogel of P1
N
O
O
0
20
We have further achieved complete protonation of these
pyridyl ended OPVs by either adding HCl (12 M) solution or by
passing HCl gas into a solution in EtOH. The protonation was
10
15
20
25
30
EtOH
Two Theta (2θ)
Figure 2. Photographs of (a,b) solution of 1 and P1 and (c) gel of
also evident from an obvious yellowꢀtoꢀred color change of the 65 P1 in EtOH (6 mM) under normal and UV light (> 365 nm; inset)
solution of 1 (Fig. 2a,b) as well as by large downfield shifts of the
25 pyridyl Ha and Hb protons, from 8.55 to 8.8 and 7.65 to 8.15 ppm
respectively in 1H NMR spectroscopy (Fig. S5).
respectively. (d) Crystal formation inside the gel of P1 within ~48
h. (eꢀg) SEM, AFM and POM images of P1 (0.5 mM) from EtOH
respectively. (h) Unit cell of the crystal of P1 (Hꢀatoms were
removed for clarity). (i) Packing arrangement of P1 in the crystal
Interestingly, the protonated form of 1, i.e, P1 formed upon
addition of HCl (12 M, 30 µL) to a 6 mM solution of 1 in EtOH 70 shows how OPVꢀunits are held together by C―H···Cl Hꢀbonding
(1 mL), displayed excellent thermoreversible gelation when
30 sonicated for ~25 s at ~5 °C (Fig. 2c). The gel obtained initially
was transparent, which on further sonication for ~1 min turned
into a translucent gel and finally yielded an opaque gel when
sonicated for a longer time (~2 min) (Fig. S6). Moreover, P1
could gelate EtOH at a concentration as low as 2.1 mM and the
35 gel formation even did not require a sonication at a higher
concentration (10 mM) of the gelator (Table S1). These gels
showed strong luminescence (inset in Fig. 2c).
interactions (rest of the Hꢀatoms were removed for clarity). (j)
Comparison of a simulated XRD pattern of a singleꢀcrystal with
the PXRD pattern of the xerogel of P1.
75
To address the above issue more quantitatively, the crystal
structure of P1, which was obtained from its parent gel, was
analyzed by single crystal Xꢀray diffraction. The crystal of P1 is
refined in the centrosymmetric triclinic space group (P ꢀ1) with
an asymmetric unit that contains half of the two molecules of P1
In order to check the effect of counterions towards the 80 in the protonated form, two Clꢀ ions and one EtOH molecule (Fig.
gelation of P1, various other concentrated acids such as HBr, HI,
40 HClO4, H2SO4, HNO3 and CH3COOH were used instead of HCl.
In contrast, none of these triggered gelation of 1 and instead only
a gelatinous precipitate was formed in EtOH. Thus the present
system could selectively trap toxic HCl gas via gel formation.2c
In addition, the gel of P1 exhibited multiꢀstimuli responsive
45 property.9 For example, addition of only ~5% water abolished the
gel of P1 (Table S1). Further, addition of an equiv. of NaOH as a
source of OH‾ neutralized P1 to 1, resulting in the formation of a
S9a). One of the associated Clꢀ ion showed positional disorder
(Fig. S9). The OPVꢀunits are stacked via parallelꢀdisplaced πꢀπ
stacking interactions (Fig. 2h and S10a).10 This parallel
arrangement is supported by few N―H···Cl and multiple
85 C―H···Cl Hꢀbonding interactions (Fig. 2i, S10b and table S2).
Multiple C―H···π interactions between the nꢀbutyl chain of one
molecule and the pyridine ring of the other molecules also helps
in inducing further growth of the crystal (Fig. S10c). The EtOH
molecule in the unit cell also participates in C―H···Cl,
sol instantaneously. The representative SEM and AFM of the gel 90 O―H···Cl and C―H···O Hꢀbonding interactions, suggesting the
of P1 showed presence of wellꢀdefined fibers of high aspect
50 ratios (Fig. 2e,f). Moreover, the fibers of P1 exhibited strong
birefringence when viewed under POM, demonstrating an
anisotropic packing of P1 during aggregation (Fig. 2g and S7b).
crucial role of the alcoholic solvents in gelation (Fig. S10d).
These observations clearly indicate that multiple X―H···Cl (X =
C, N, O) interactions among the slipped πꢀπ stacked OPVꢀunits
account for the rapid crystallization and also provide the most
Interestingly, the gel of P1 at moderate concentration (~10 95 acceptable reason for HClꢀselective gelation phenomenon.
mM) on aging for ~2 days at ambient conditions spontaneously
55 furnished few transparent, deep red colored crystals inside the 3ꢀ
D gel matrix (Fig. 2d). When the gelator concentration was
increased to ~65 mM, majority of the gelator molecules slowly
However, a comparison of the simulated XRD pattern from
the singleꢀcrystal with the PXRD pattern obtained from the
xerogel, reveals the presence of identical first order reflection
peak (Fig. 2j). However, significant differences exist between the
transformed into multiple singleꢀcrystals through phase 100 two, indicating the molecular arrangement in the gel is probably
2
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not exactly same with that of the crystals.7c,d
Additional experiments were performed to further prove the 60 fusion at higher concentrations (Fig. 3f).2b Dynamic light
probably originate from the nanoꢀspheres as a result of their
role of C―H···Cl Hꢀbonding in the gelation process.
A
scattering studies also confirmed the presence of aggregates with
an average hydrodynamic diameter of 188 ± 20 nm in the EtOH
solution of P2 (0.9 mM) (Fig. S13).
precipitate of 1.2HClO4 obtained upon addition of HClO4 (11.7
M) to a solution of 1 in EtOH, was washed with excess of EtOH
to remove the excess acid. Interestingly, the DMSO solution of
5
Spectroscopic experiments were performed in order to gain
the dried residue of 1.2HClO4 (25 mM) transformed into an 65 further information about the selfꢀassemblies of both the
opaque gel instantaneously when HCl (12 M, 30 ꢁL) was added
to it (Fig. S11a). Since ClO4ꢀ anions are generally nonꢀinteractive
protonated and nonꢀprotonated OPVs. The appearance of the NꢀH
stretching peak at ~3378 cmꢀ1 for P1 and P2 confirms the
protonation of the pyridyl Nꢀatom (Fig. S14a). The distinct
features in the UVꢀVis absorption spectra include (i) the
10 with the CꢀH protons,11 the above result further indicates that the
C―H···Cl Hꢀbonding plays a crucial role in the gelation process.
Addition of tetrabutylammonium chloride (TBACl), a source of 70 protonated OPVs (P1 and P2) showing wellꢀresolved vibronic
Clꢀ ion, to the above DMSO solution also resulted in aggregation.
Titration of the solution of 1.2HClO4 in DMSOꢀd6 with
15 increasing proportion of TBACl, resulted in progressive upfield
shift of the pyridyl Ha and Hb protons of the OPVꢀbackbone (Fig.
fineꢀstructures for the electronic transition S0→S1 and (ii) the
absorption maxima of 1 and 2 showing large bathochromic shifts
of ~55 nm on protonation (Fig. 4a). This result suggests that the
protonated OPVs remain in the more planarized state in EtOH
S11b), indicating the Clꢀ ion induced aggregation of the 75 solution compared to their nonꢀprotonated forms. Similar features
protonated OPV. However, the evidence of the involvement of
C―H···Cl Hꢀbonding in the aggregation process of the former
20 could not be clearly observed from the NMR studies. This is
because the upfield shift of the protons might be caused due to
the aggregation of the OPV derivatives or due to the combination
of C―H···Cl Hꢀbonding and aggregation in the solution state.
In contrast, P2 formed an opaque gel only at a relatively
25 higher CGC of 13.5 mM (inset in Fig. 3a,c and Table S1). This is
presumably due to the higher solubility of P2 in EtOH.
were also in agreement with the crystal structures of 1 and P1.
(a)
(b)
395 nm
450 nm
1
2
P1
P2
1.2x106
9.0x105
6.0x105
0.4
0.3
0.2
0.1
0.0
15
45
°C
AIEE
°
C
3.0x105
0.0
300
400
500
600
500
550
600
650
700
750
Wavelength (nm)
Wavelength (nm)
(b)
(c)
(a)
Figure 4. (a) UVꢀVis absorption spectra of 1, 2, P1 and P2 (10
80 µM) in EtOH at 25 °C. (b) Temperatureꢀdependent fluorescence
spectra of P1 at 1.75 mM in EtOH.
30
Further, protonation of the pyridyl group caused a significant
quenching of the fluorescence emission with concomitant redꢀ
85 shift of the emission maximum of ~112 nm (Fig. S14b). This
again suggests that protonation promotes the planarization of the
OPVs. Temperatureꢀdependent emission spectra of the gelator P1
were also recorded in EtOH ([P1] = 1.75 mM). The emission
intensity at 586 nm gradually increases as the hot solution at 45
90 °C was cooled gradually to 15 °C, suggesting an AIEE
phenomenon (Fig. 4b). Decreasing temperature causes an
increase in viscosity of the solution through strong πꢀπ
interactions among the OPVꢀbackbone of P1 as observed in the
crystal structure of P1. This, in turn, restricts the intermolecular
100 ꢀm
100 ꢀm
10 ꢀm
(d)
(e)
(f)
35
1 ꢀm
10 ꢀm
40 Figure 3. (aꢀc) AFM, SEM and fluorescence microscopic images
of P2 at high concentration (15 mM) from EtOH respectively.
Insets in (a) and (c) show the photograph of gel of P2 in EtOH
(20 mM) under normal light and UV light (> 365 nm)
respectively. (d,f) AFM (0.2 mM) and SEM (2 mM) images of P2 95 rotation (RIR) and accounts for the AIEE.12 Similar observations
45 from EtOH respectively. (e) height image of a spherical aggregate
in fig. (d).
were also recorded for the gel of P2 (Fig. S14c).
In conclusion, we have demonstrated the fabrication of
microscopic hollowꢀrods with rectangular crossꢀsection from the
Morphological investigation of the gel of P2 also showed the
selfꢀassembly of 1 in EtOH. Interestingly, the molecular
presence of fibers at a concentration larger than the CGC (15 100 arrangements in the microꢀrods were found to be same with that
50 mM). The fibers show high aspect ratios with several hundreds of
micrometers in length and the diameters in the range of ~0.5 to 3
µm (Fig. 3aꢀc and S12a). Whereas, at a concentration much lower
than CGC (< 1 mM), it revealed the formation of wellꢀdefined
in the singleꢀcrystals of 1. In contrast, the gels of 2 induced
formation of solid molecular rods. However, protonation of 1, i.e,
P1 resulted in the formation of a transparent gel in EtOH
comprising fibrous networks. Interestingly, the crystallization of
nanoꢀspheres with diameters in the range of 150ꢀ300 nm and an 105 the gelator in the 3ꢀD gel of P1 occurred within ~2 days of aging.
55 average height of ~20 nm (Fig. 3d,e). Similar spherical
aggregates were also observed under SEM, POM and
fluorescence microscopic studies (Fig. S12cꢀe). Performing SEM
at a moderate concentration reveals that these extended fibers
HCl was the only acid that could induce the gel formation of 1.
The singleꢀcrystal structure of the crystals obtained in the gel
showed the critical role of the Clꢀ ion in the formation of the gel
and also the singleꢀcrystal via multiple, unconventional
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70
75
5
10 the best of our knowledge, this is the first report of an OPVꢀbased
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DOI: 10.1039/C5CC00930H
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Table of contents
Gel phase crystallization in a transparent gel via synergistic nonꢀcovalent interactions has been reported along
with various remarkable features (see figure).
O
O
N
C―H···Cl Hꢁbonding
N
Hollow Rod
HCl
Key Features
C
O
ꢀ Gel Phase
Crystallization
ꢀ Toxic HClTrapping
ꢀ StimuliResponsive
ꢀ AIEE
Clꢁ ion
SingleCrystal
N
Gel Stabilization Mediated by πꢀπand C―H···Cl Hꢁbonding Interactions
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5