Photophysical studies of pyrenyl cyanostyrenes: Effect of trifluoromethyl substitution on gelation

Article abstract of DOI:10.1039/c8nj04146f

α-Cyanostyrenes bearing a planar pyrene unit and electron withdrawing trifluoromethyl units were designed and synthesized. The conformational restriction due to the presence of the cyano group on the double bond favors aggregation induced emission in aqueous media. The styrylpyrenes aggregate to form microstructures influenced by π-π stacking, cyano and CF₃ substituent interactions. Importantly confluence of the pyrene ring and simple trifluoromethyl (CF₃) unit allows the formation of a stable organogel with bathochromic shifts in emission. The formation of aggregates and the gel was substantiated using ¹H NMR spectroscopy and scanning electron microscopy. The stability of the gels was assessed using rheology investigations and rationalized by single crystal X-ray data.

Full text of DOI:10.1039/c8nj04146f

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Photophysical studies of pyrenyl cyanostyrenes:
effect of trifluoromethyl substitution on gelation†
Cite this: New J. Chem., 2018,
42, 18297
Jagadish Katla,a Abhijeet Ojha,b Akshay J. M. Nair,a Krishnan Ranganc and
a
Sriram Kanvah
*
a-Cyanostyrenes bearing a planar pyrene unit and electron withdrawing trifluoromethyl units were
designed and synthesized. The conformational restriction due to the presence of the cyano group on
the double bond favors aggregation induced emission in aqueous media. The styrylpyrenes aggregate to
form microstructures influenced by p–p stacking, cyano and CF3 substituent interactions. Importantly
confluence of the pyrene ring and simple trifluoromethyl (CF3) unit allows the formation of a stable
organogel with bathochromic shifts in emission. The formation of aggregates and the gel was substan-
tiated using 1H NMR spectroscopy and scanning electron microscopy. The stability of the gels was
assessed using rheology investigations and rationalized by single crystal X-ray data.
Received 14th August 2018,
Accepted 9th October 2018
DOI: 10.1039/c8nj04146f
rsc.li/njc
classic molecule with excellent absorption and emission properties
that is widely utilized for various optical and biological applica-
Introduction
The desirable chemical, optical, electrical and biological properties tions.9–12 We anticipated that introduction of the flat pyrene
of donor–acceptor (D–A) conjugated organic molecules provide moiety, capable of forming excimers, may aid in the formation
an attractive platform for the design and fabrication of newer of ordered structures. The polar –CN group strengthens the
functional materials.1 Their molecular structures guided by weak electrostatic interactions among the neighboring molecules
intermolecular interactions provide structural tunability enabling and restricts the intramolecular rotations. The CF3 end groups
them to be utilized as functional building blocks for the construc- help in intermolecular interactions due to its associated hydro-
tion of self-assembled molecular micro or nanostructures.2,3 phobic character apart from the interactions associated with
Among many p-conjugated systems, the cyanostilbene scaffold the strong electronegative F group. Furthermore, the incorpora-
with terminal substituents on the phenyl group has been devel- tion of CF3 substituents also helps in the gelation properties
oped for multiple organic electronic or biological applications.4–6 of suitable organic molecular scaffolds.13–15 Previously known
The cyano substitution on the double bond yields fluorophores organogels with pyrene or other molecular structures typically
exhibiting unique emission termed aggregation-induced emission contain long alkyl chain or steroidal substituents16–19 and show
(AIE) driven by their ‘twist elasticity’ characteristics. These fluoro- tremendous promise as functional materials for optical and
phores with good solubility in organic solvents show weak biological applications.20–25 The elongated aryl moieties and CF3
emission but exhibit remarkable emission in aqueous media groups lead to increased intermolecular interactions through
owing to the formation of aggregates7 and self-assemblies.4,8 strong p–p and hydrophobic interactions enhancing the gelation
Tapping into these remarkable properties, we extended our through a self-assembling process.26–28 It is known that the larger
earlier work on the cyanostilbenes by introducing a CF3 group hydrophobic area achieved by longer or bulkier aryl moieties
on the aromatic ring containing the styrylpyrene unit. Pyrene is a also aids in the construction of nanostructures.29,30 On this basis,
we have designed cyanostyrene derivatives with pyrene and tri-
fluoromethyl substitutions (Fig. 1). The results are detailed in the
following sections.
a Department of Chemistry, Indian Institute of Technology Gandhinagar, Palaj,
Gandhinagar 382355, India
b Department of Biological Engineering, Indian Institute of Technology
Gandhinagar, Palaj, Gandhinagar 382355, India
c Department of Chemistry, Birla Institute of Technology and Science,
Materials and methods
Pilani – Hyderabad Campus, Kapra Mandal, Medchal District, Hyderabad,
500 078, India. E-mail: rkrishnan@hyderabad.bits-pilani.ac.in
All the chemicals and reagents required for the synthesis of the
pyrene derivatives and spectroscopic studies were purchased
† Electronic supplementary information (ESI) available: Supporting figures,
single crystal X-ray data tables and synthetic methods. CCDC 1858766 and
from Sigma-Aldrich, Alfa Aesar, Acros Organics and S. D. Fine-
Chem and were used as such. All the synthesized samples were
1858773. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c8nj04146f
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Scheme 1 Synthesis of styrylpyrene derivatives.
Fig. 1 Pyrene derivatives designed and synthesized.
Results and discussion
1
characterized using H NMR and 13C NMR in CDCl3 and with
The synthesized styrylpyrenes are shown in Fig. 1. (Z)-2-Phenyl-
3-(pyren-3-yl)acrylonitrile (1) has no trifluoromethyl substitu-
tion, (Z)-2-(4-(trifluoromethyl)phenyl)-3-(pyren-3-yl)acrylonitrile
(2) has one trifluoromethyl substitution on the aromatic ring and
(Z)-2-(3,5-bis(trifluoromethyl)phenyl)-3-(pyren-3-yl)acrylonitrile (3)
has two trifluoromethyl substituents on the meta positions of the
aromatic ring. All the styrylpyrenes have a cyano group in the
double bond. Molecule (1), devoid of any CF3 groups, shows
absorption bands at B380 nm with a shoulder at B396 nm and
another band at B300 nm (Fig. S1a, ESI†).
tetramethylsilane (TMS) as an internal standard using a Bruker
AvanceIII-500 MHz NMR spectrometer. Mass spectral data were
obtained using an ESI-QToF Waters-Synapt G2S high-resolution
mass spectrometer. UV-vis absorption spectra were recorded
using an Analytik Jena Specord 210 plus and the fluorescence
emission studies were performed using a Horiba-Jobin Yvon
Fluolorog-3 spectrofluorimeter. Typically the excitation wave-
lengths were set at the absorption maxima (labs) of the
compounds under investigation. All the fluorescence spectra
were recorded in a 10 mm path length quartz cuvette with a slit
width of 2 nm. Fluorescence quantum yields of compounds
were estimated using quinine sulphate (F = 0.546 in 0.5 M H2SO4)
as a reference standard. For the fluorescence measurements,
B10 mM concentrations of chromophores were used. Single
crystal X-ray studies were performed using a CrysAlis PRO
on a single crystal Rigaku Oxford XtaLab Pro diffractometer.
Scanning electron microscopy (SEM) analysis was carried
out using a field emission SEM (JSM 7600F JEOL). For
this purpose, one drop of the sample [B10ꢀ6 M solution in
water] was deposited on a Si-wafer mounted on an aluminum
stub with the help of double-sided adhesive carbon tape.
The samples were dried to ensure complete removal of any
residual water and coated with platinum before being analyzed.
The rheological properties of the gels were analyzed using
an Anton Paar MCR 302 rheometer. A typical cone and plate
geometry with a cone diameter of 25 mm and truncation angle of
21 with a gap of 0.105 mm at a constant temperature of 20 1C
was used and the following rheological measurements
were performed: small amplitude oscillatory frequency sweep
and large amplitude oscillatory sweep measurements. Powder
XRD analysis was carried out using a Bruker D8 Discover,
with Cu K-alpha operating at 40 kV and 30 mA. The scan
speed for the analysis was 0.3 s per step with a single step size
of 0.01.
Substituting a CF3 group in (2) results in smaller absorption
maxima shifts (+2 to +3 nm) and increasing the number of
CF3 substituents (3) does not yield any significant absorption
maxima changes (Fig. 2 and Fig. S1b, ESI†). Solvent polarity
The desired compounds were prepared as per the synthetic
methodology given in Scheme 1. In a typical procedure, a
commercially available CF3 substituted phenylacetonitrile
(0.01 mol) derivative was taken in 25 mL methanol. To the
alcoholic solution, KOH (0.01 mol) was added and vigorously
stirred for 15 min. To this, pyrene-1-carboxaldehyde (0.01 mol)
in 25 mL of methanol was added slowly, and the contents were
stirred for 10–15 min at room temperature. The shining yellow
crystals obtained were washed with excess methanol and
recrystallized from absolute ethanol.
Fig. 2 Absorption spectra of (a) (2) in different organic solvents and
(b) (1), (2) and (3) in acetonitrile at a concentration of 10 mM.
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changes exert weaker effects on the absorption maxima. In yields a B2.7 fold increase in the emission intensity. Interestingly
water, however, broad absorption is noted with peaks coincid- a new emission band begins to appear at B560 nm, but it remains
ing with the original absorption at B380 nm and another as a broad shoulder. In contrast to (2), a distinct emission band is
absorption band at B435 nm. This broad and bathochromic noted at B558 nm [Fig. 4b]. The bathochromic emission observed
absorption in water could be due to the formation of aggre- for (1–3) in water is attributed to the emission from the aggregates.
gates. The absorption and emission data are given in Table 1.
However, it is interesting to note that (3) with two CF3
Compound (1) in heptane shows emission maxima at 439 nm groups shows a distinct emission band in comparison to (2)
with a shoulder peak at 463 nm. In dioxane, (1) emits at where only a shoulder is observed. It is possible that the CF3
B460 nm and in acetonitrile, the observed emission is at substitution in (2) may be helping in stabilizing the structure
B477 nm. In water, however, the emission shows a significant through non-covalent forces and restricting the formation of
red-shift with a distinct emission band at B548 nm with a the aggregates. In (3) such stabilization may be hindered due to
shoulder at B476 nm [Fig. S2, ESI†]. Compound (2) shows steric effects of meta-substituted CF3 groups, and in (1), no
moderate solvatochromic emission shifts. In heptane, (2) additional stabilizing forces are present, or the interactions are
shows structured emission with an emission peak at 450 nm purely random readily allowing the formation of the aggregates.
and in water, the emission is noted at B467 nm with a tail
For comparison, the emission was also measured in the
emission in the range of 550 nm to 600 nm. The former peak solid state. Compounds (1) and (3) show comparable emission
indicates the monomer emission and the tail emission could be with emission maxima at B500 nm while compound (2) shows
due to the formation of the aggregates [Fig. 3a]. Compound (3) a significant red-shifted emission band at 580 nm [Fig. 4c]. To
bearing two CF3 groups also shows comparable emission better understand the morphology of the aggregates, we recorded
behavior [Fig. 3b] but with a conspicuous emission band at scanning electron microscopy images at two different water
565 nm. The lower wavelength peak is attributed to the monomer fractions where emission enhancement is maximum (80%) and
emission, and the longer wavelength band is attributed to the at 100%.
aggregates.31 The CF3 groups show a strong –I effect because of the
The SEM images of (2) and (3) show distinct morpho-
presence of electronegative fluorine atoms but exhibit no reso- logical differences (Fig. 5). Compound (2) at 80% water fraction
nance effect with the aromatic ring. The twisted conformation due shows irregular pebble like structures that are close together. At
to the presence of vinyl substitution may hinder the formation of 100% water, irregular short dendritic structures are noted.
excimers. The concentration dependent emission investigations Compound (3) shows different morphologies with sheet like
in 1,4-dioxane indicates concentration quenching with no new structures (80% water) and square/distorted square like struc-
emission bands excluding the possibility of the excimer emission tures (at 100% water) (Fig. 6). SEM dropcast images were
in organic solvents [Fig. S5, ESI†]. The optical measurements were recorded using 10 mM concentration of the compounds.
performed at concentrations of 10 mM.
CF3 substituted small molecules have been shown to form
self-assemblies or organogels due to favorable intermolecular
interactions through weak non-covalent interactions that
provide both strength and enable reversibility.33–36 Fused ring
systems such as pyrene aided by trifluoromethyl units (CF3) can
assist in the formation of stable assemblies at higher concentra-
tions. To study the gel formation, compounds (2) & (3) were
dissolved in t-BuOH (1.8 wt%) in a glass vial, and heated to 70 1C
to obtain a clear solution. The hot t-BuOH solution was sonicated
for 30 minutes at 25–36 1C using a bath sonicator and rapidly cooled
in the refrigerator for 5 minutes and allowed to warm up to room
temperature. The solution (2) subsequently turns to a viscous gel
and shows no gravitational flow even after inversion of the vial
(Fig. 7a) indicating the overall stability. The gel so formed was
also reversible upon heating (B70 1C) and cooling cycles.
Emission in dioxane–water binary mixtures
Increasing the percent water fraction in dioxane results in
B8.5 fold increase (80%) in the emission intensity along with a
B9 nm hypsochromic wavelength shift for (2). Further increase in
water percent decreases the emission intensity, but at 100% water,
a broad emission band at B565 nm appears along with the
470 nm band [Fig. 4a]. Compound (1) without any CF3 group
shows aggregation induced emission at 100% water32 (Fig. S4,
ESI†). The observations for compound (3), containing two CF3
substituents are comparable to that observed for (1).32 The 480 nm
emission noted in dioxane shows a hypsochromic shift (ꢀ20 nm)
with an increase in water percentage (60%) along with enhanced
intensity (2-fold increase). Further increase in water percentage
Table 1 Absorption and emission data of (2) and (3) in organic solvents
(2)
(3)
Solvent
la (nm)
lf (nm)
Stokes shift (cmꢀ1
)
Quantum yield (F) la (nm)
lf (nm) Stokes shift (cmꢀ1
)
Quantum yield (F)
Heptane
Dioxane
THF
DMF
CH3CN
Methanol 382, 409 457
Water
383, 410 450
382, 410 470
383, 410 483
382, 410 498
3887
4991
5337
6097
6395
4296
0.0302
0.0110
0.0112
0.0043
0.0025
0.0037
0.0073
385, 411 458
384, 412 480
385, 412 493
384, 412 508
4139
0.0632
0.0611
0.0051
0.0027
0.0034
0.0035
0.0070
5208
5690
6356
6366
5499
8478
380
502an
385
510
381, 409 482
382, 417 565
419, 425 467, sh 565 2116
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Fig. 3 Emission spectra of (a) (2) and (b) (3) in different organic solvents and (lex: 380 nm) measured at a concentration of 10 mM.
Fig. 4 Emission spectra of compounds (a) (2) and (b) (3) in dioxane–water binary mixture measured at a concentration of 10 mM and (c) solid state
fluorescence spectra of compounds (1–3).
Examination of the gel emission reveals a bathochromic yellow color needles (2) and colorless needles (3). The compound
emission (61 nm) band at 517 nm (Fig. 7b) as compared to (2) crystallized in the monoclinic crystal system and P21/c space
the observed emission in t-BuOH (456 nm). The compounds (1) group, while the compound (3) crystallized in the orthorhombic
and (3) form no gel (Fig. S6, ESI†) under the same experimental system and Pbca space group. The crystal structure details of (2)
conditions with emission at B553 nm.
and (3) are deposited into the Cambridge Crystallography Data
Center, and their CCDC numbers, respectively are 1858766 and
1858773.† The ORTEP diagrams of the compounds (2) and (3) are
Single crystal analysis
The crystal structures of (2) and (3) were determined by single illustrated in Fig. 8. The molecular structure of (2) is composed of
crystal X-ray diffraction. The single crystals were obtained as trans-disposition of the pyrenyl ring group to the p-trifluorotolyl
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Fig. 5 SEM images of compound (2) in (a) 80% water (scale: 100 nm; inset at scale: 1 mm) and in (b) 100% water (scale: 100 nm) measured at a
concentration of 10 mM.
Fig. 6 SEM images of compound (3) in (a) 80% water and in (b) 100% water (scale: 1 mm) measured at a concentration of 10 mM.
group while the pyrenyl group is in the cis-position to the cyano with two hydrogens, and H21 and H17 forming an extended array
group around the olefin double bond. The C17–C18 double bond along the c-crystallographic axis, N1ꢁ ꢁ ꢁH17 = 2.604 Å (with
distance is 1.3566(16) Å while the C19–N1 triple bond distance styrene C–H group) and N1ꢁ ꢁ ꢁH21 = 2.576 Å (with phenyl C–H
is 1.1460(16) Å. There are at least four types of intermolecular group) [Fig. S9, ESI†]. The ORTEP diagram of the compound (2)
non-covalent bond interactions as noted for compound (2): is illustrated in Fig. 8a. The short contact interactions are given
(i) p–p stacking, (ii) F–F interaction,37,38 (iii) F–H interaction, and in Table S1 (ESI†). Similar to (2), the molecular structure of (3)
(iv) N–H interaction. The intermolecular p–p stacking arrange- is composed of trans-disposition of the pyrenyl ring group to
ments are observed in the crystal structure of the mono-CF3 the p-3,5-bis(trifluoromethyl)phenyl group while the pyrenyl
compound [Fig. S7, ESI†]. The intermolecular p–p stacking effect group is in the cis-position to the cyano group around the
can be visualized between the polycyclic pyrene rings as well olefin double bond. The C17–C18 double bond distance is
between the –CF3 substituted phenyl rings. The p–p interaction 1.340(4) Å while the C19–N1 triple bond distance is 1.152(4) Å.
is shown in Fig. S8 (ESI†) with ring centroid–centroid G1–G10, The CRN bond lengths for (2) and (3), respectively are 1.1460(16) Å
G2–G20, G3–G30, G4–G40 and G5–G50 distances of 3.916, 3.873, and 1.152(4) Å. The slight elongation in the cyano group bond
3.854, 3.822 and 3.839 Å, respectively.
length may be due to the involvement in the formation of non-
Along the a-crystallographic axis, the molecules are arranged covalent intermolecular interactions such as N1ꢁꢁꢁC10 = 3.238 Å,
and stabilized through two types of interactions, p–p stacking and C19ꢁ ꢁ ꢁH3 = 2.863 Å and C19ꢁ ꢁ ꢁH7 = 1.861 Å. These inter-
due to the pyrene ring and intermolecular fluoride–fluoride molecular interactions and orientations of the molecules do
interactions.39,40 The F1–F10, F1–F30 and F2–F30 fluoride–fluor- not favor the p–p pyrene ring stacking in the molecular packing
ide non-bonding interaction lengths, respectively are 2.915 Å, arrangements.
2.875 Å, and 2.841 Å. Along the crystallographic b-axis from
Also, the following intermolecular interactions, F3ꢁꢁꢁH7 (2.596 Å),
fluoride to hydrogen of the pyrene ring, the non-bonding F5ꢁꢁꢁC23 (3.095 Å), H13ꢁꢁꢁC7 (2.587 Å) and H13ꢁꢁꢁC4 (2.789 Å)
interaction can be observed with F1–H11 and F30–H12 inter- support the three-dimensional crystal structure formation
action lengths of 2.582 Å and 2.661 Å, respectively [Fig. S8, ESI†]. [Fig. S10, ESI†]. Unlike in (2), in (3), the Fꢁ ꢁ ꢁF and p–p ring
The nitrogen of the cyano group forms a three-centered interaction stacking are not observable. These interactions stabilize the
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phenyl ring plane of compound (2) is oriented with a deviation of
531 with respect to the pyrene ring plane, but in compound (3),
this deviation is only minimal around 31, and the rings
are slightly tilted facing each other. For (2), the dihedral
angles C2–C1–C17–C18 and C17–C18–C20–C21 are ꢀ32.52
and ꢀ18.90 degrees, respectively and for compound (3), these
corresponding dihedral angles are 40.49 and ꢀ37.51 degrees,
respectively. The data for the crystals (1) and (2) are given in
Tables S1–S12 in the ESI.†
Rheology behavior
The stability and viscoelastic properties of the synthesized
organogels were studied by rheological measurements.
The storage modulus (G0) and loss modulus (G00) represent
solid and liquid-like properties of the material. Time and
frequency sweep experiments were conducted at a constant
temperature of 20 1C. Before the rheological measurements, the
sample was equilibrated for 5 minutes. The organogel formed
by (2) in t-BuOH was subjected to an angular frequency (o) of
0.1–100 rad sꢀ1. These experimental conditions ensure that
both the storage modulus (G0) and loss modulus (G00) were
measured in the linear viscoelastic regime. In all cases, the
value of G00 was less than the value of G0 at all frequencies.
Among these different treatment conditions, organogel (2)
showed the highest storage modulus (G0) in the order of 104
Pa as compared to the loss modulus (G00). Low tan d values
(tan d = G00/G0) obtained for gel (2) revealed the gel-like nature
of the sample with possible cross-linked networks. The G0
value for gel (2) was independent of experimental time, and
frequency range as shown in Fig. 9 and also indicates that the
G0 value is in the order of 104 Pa, which shows the higher
mechanical strength of gel (2). The p–p stacking interactions
between pyrenyl segments provide the essential gelation,
and strongly support the view that the aromatic units induce
rigidity in the structure and help in a linear fashion to form an
organogel. The relatively higher strength of the gel can be
understood by the existence of CF3 groups which play a key
role in the formation of stronger gel networks by reinforcing
non-covalent interactions.13,40,41 In the case of (3), the position-
ing of the CF3 groups hinders the formation of stable gels
although different emission bands were observed at higher
concentration (Fig. S12, ESI†).
Fig. 7 (a) Photographs of gel (2) in the solution state (70 1C) and in the gel
state (25 1C) in t-BuOH (1.8 wt%) and (b) emission spectra of the gel formed
by (2) in t-BuOH.
1H NMR studies
The formation of the self-assembled structures and possible
interactions can be gauged by 1H NMR measurements42
[Fig. 10]. The experiments were performed in CDCl3 for (2) at
higher concentrations to probe the driving force for the self-
assembly process. With increase of concentration from 0.4 wt%
1
to 0.8 wt%, the aromatic H NMR signal (7.7–8.7 ppm) shows
Fig. 8 ORTEP diagram of compounds (a) (2) and (b) (3). Thermal dis-
a slight upfield shift [Ha (0.019 ppm), Hb (0.035 ppm), Hc
(0.026 ppm), and Hd (0.0161 ppm)] indicating the possible role
of p–p stacking interactions between the pyrenyl rings. These
placement ellipsoids are drawn at the 50% probability level.
aggregates formed and stronger molecular packing support results support the formation of aggregates through stabilization
better gel formation for (2) than in (3). The orientation of of p–p interactions in the gel phase and support our view that
both the molecules is given in Fig. S11 (ESI†). The substituted the interacting pyrene units assist in the formation of the
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Fig. 9 (a) Time sweep measurement and (b) frequency sweep measurement
of gel (2) in t-BuOH.
1
organogel. We could not conduct the H-NMR measurements
at higher concentrations as the sample tends to lose the
locking ability. The scanning electron microscopy (SEM) of
the dried xerogel of (2) indicates the formation of an entangled
two-dimensional network consisting of bundles of ribbon-
like aggregates [Fig. 11a]. These bundles of self-assembled
aggregates are aided by strong hydrophobic and p–p interactions
of the aromatic pyrene unit and interactions through CF3 groups
Fig. 11 (a) SEM images of the xerogels obtained from t-BuOH gelation
sols of (2) (scale: 10 mm) and (b) powder XRD pattern of the xerogel of (2)
formed in t-BuOH. The powder XRD pattern obtained from single crystals
of (2) is given in the ESI.†
(C–Fꢁ ꢁ ꢁH–Ar). The CF3 group plays a key role in improving
intermolecular interaction much like the long alkyl chain
or steroidal group.
Powder XRD studies
Powder XRD measurements were performed on the xerogels of
(2) prepared from t-BuOH to obtain further information on the
molecular arrangement in their gel state. Well-resolved X-ray
diffraction signals of (2) are shown in Fig. 11b. Sharper
peaks appear in the gel state indicating structural changes to
the morphology, i.e., the formation of micro ribbons/micro
crystals. The peak of the powder sample at angle 2y = 15.211
(full-width half maximum = 0.5657) shifts to 2y = 15.171
(FWHM = 0.3479) with intense peaks. The low FWHM indicates
greater crystallinity in these xerogels. The data show that the
xerogels obtained have higher order packing aided by non-
covalent attractions. The single crystal powder XRD of (2) reveals
sharp and intense signals that correspond to the dominant crystal-
line planes (Fig. S13, ESI†).
Fig. 10 1H NMR spectra of (2) with increasing concentration (0.4–0.8 wt%)
in CDCl3 solution.
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14 C. Dou, L. Han, S. Zhao, H. Zhang and Y. Wang, J. Phys.
Chem. Lett., 2011, 2, 666–670.
15 D. Kim, E. Kwon Ji and Y. Park Soo, Adv. Funct. Mater., 2017,
28, 1706213.
16 K. Wang, Z. Guo, L. Zhang, K. Sun, P. Yu, S. Zhou, W. Wang
and Z. Li, Soft Matter, 2017, 13, 1948–1955.
17 Y.-L. Xu, C.-T. Li, Q.-Y. Cao, B.-Y. Wang and Y. Xie, Dyes
Pigm., 2017, 139, 681–687.
18 K. Lalitha and S. Nagarajan, J. Mater. Chem. B, 2015, 3, 5690–5701.
19 L. Zhang, C. Liu, Q. Jin, X. Zhu and M. Liu, Soft Matter, 2013,
9, 7966–7973.
Conclusions
We have designed pyrene-containing cyanostyrene derivatives
bearing simple trifluoromethyl groups and discussed their
absorption and emission properties in organic solvents and
water. At higher concentrations, the styrylpyrenes efficiently
form organogels despite lacking steroidal or long alkyl chain
substituents. This unique gelation ability is attributed to the
influence of p–p interactions of the planar pyrene moiety
assisted by the interactions from the CF3 units. Such formation
of organogels through the introduction of small molecular
entities such as CF3 groups could boost the development of
luminescent low molecular weight gelators.
20 A. Ajayaghosh and V. K. Praveen, Acc. Chem. Res., 2007, 40,
644–656.
21 J. H. van Esch and B. L. Feringa, Angew. Chem., Int. Ed.,
2000, 39, 2263–2266.
22 J. Seo, J. W. Chung, J. E. Kwon and S. Y. Park, Chem. Sci.,
2014, 5, 4845–4850.
Conflicts of interest
There are no conflicts to declare.
23 B. Xing, C.-W. Yu, K.-H. Chow, P.-L. Ho, D. Fu and B. Xu,
J. Am. Chem. Soc., 2002, 124, 14846–14847.
Acknowledgements
´
24 Y. Ma, M. Cametti, Z. Dˇzolic and S. Jiang, J. Mater. Chem. C,
We thank the Board of Research in Nuclear Sciences [37(2)/14/
05/2016] for the financial grant, IIT Gandhinagar for overall
infrastructural support and Dr Sudhanshu Sharma for general
discussions. Dr R. Krishnan thanks the Birla Institute of
Technology Pilani Hyderabad campus for the infrastructural
support.
2016, 4, 10786–10790.
25 S. Shin, S. H. Gihm, C. R. Park, S. Kim and S. Y. Park, Chem.
Mater., 2013, 25, 3288–3295.
26 M. Ma, Y. Kuang, Y. Gao, Y. Zhang, P. Gao and B. Xu, J. Am.
Chem. Soc., 2010, 132, 2719–2728.
27 S. Ghosh, V. K. Praveen and A. Ajayaghosh, Annu. Rev. Mater.
Res., 2016, 46, 235–262.
´
28 F. Aparicio, S. Cherumukkil, A. Ajayaghosh and L. Sanchez,
Notes and references
Langmuir, 2016, 32, 284–289.
1 S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 29 M. Lee, B.-K. Cho and W.-C. Zin, Chem. Rev., 2001, 101,
2014, 114, 1973–2129. 3869–3892.
2 C. Ren, J. Zhang, M. Chen and Z. Yang, Chem. Soc. Rev., 30 T. Shimizu, Self-Assembled Nanomaterials I: Nanofibers, Springer,
2014, 43, 7257–7266. 2008.
3 Y. Jin, Y. Xia, S. Wang, L. Yan, Y. Zhou, J. Fan and B. Song, 31 J. B. Birks and L. G. Christophorou, Nature, 1963, 197, 1064–1065.
Soft Matter, 2015, 11, 798–805.
4 B.-K. An, J. Gierschner and S. Y. Park, Acc. Chem. Res., 2011,
45, 544–554.
32 J. Katla, H. R. Bhat, P. C. Jha, P. S. Ghalsasi and S. Kanvah,
ChemistrySelect, 2017, 2, 1902–1910.
33 J. Katla, A. J. M. Nair, A. Ojha and S. Kanvah, Photochem.
Photobiol. Sci., 2018, 17, 395–403.
5 L. Zhu and Y. Zhao, J. Mater. Chem. C, 2013, 1, 1059–1065.
6 A. K. Mandal, S. Sreejith, T. He, S. K. Maji, X.-J. Wang, 34 R. M. Ongungal, A. P. Sivadas, N. S. S. Kumar, S. Menon and
S. L. Ong, J. Joseph, H. Sun and Y. Zhao, ACS Nano, 2015, 9,
4796–4805.
7 V. Palakollu and S. Kanvah, New J. Chem., 2014, 38, 5736–5746.
S. Das, J. Mater. Chem. C, 2016, 4, 9588–9597.
35 B.-K. An, J. Gierschner and S. Y. Park, Acc. Chem. Res., 2012,
45, 544–554.
8 M. Paramasivam and S. Kanvah, J. Phys. Chem. C, 2016, 120, 36 C. Dou, D. Chen, J. Iqbal, Y. Yuan, H. Zhang and Y. Wang,
10757–10769.
Langmuir, 2011, 27, 6323–6329.
9 F. M. Winnik, Chem. Rev., 1993, 93, 587–614.
37 S. Maity, P. Das and M. Reches, Sci. Rep., 2015, 5, 16365.
¨
10 J. Huang, Y. Wu, Y. Chen, Z. Zhu, X. Yang, C. J. Yang, K. Wang 38 K. Reichenbacher, H. I. Su¨ss and J. Hulliger, Chem. Soc. Rev.,
and W. Tan, Angew. Chem., Int. Ed., 2011, 50, 401–404. 2005, 34, 22–30.
11 S. S. Lehrer, Proteins: Structure, Function, and Engineering, 39 A. Mukherjee, S. Tothadi and G. R. Desiraju, Acc. Chem. Res.,
Springer, 1995, pp. 115–132. 2014, 47, 2514–2524.
12 B. M. de França, J. S. B. Forero, S. J. Garden, E. S. Ribeiro, 40 R. Berger, G. Resnati, P. Metrangolo, E. Weber and J. Hulliger,
ˆ
R. d. S. Souza, R. S. Teixeira and R. J. Correa, Dyes Pigm.,
Chem. Soc. Rev., 2011, 40, 3496–3508.
¨
41 L. Hu, Y. Zhang and O. Ramstrom, Sci. Rep., 2015, 5, 11065.
2018, 148, 444–451.
13 B.-K. An, D.-S. Lee, J.-S. Lee, Y.-S. Park, H.-S. Song and 42 J. H. Jung, G. John, M. Masuda, K. Yoshida, S. Shinkai and
S. Y. Park, J. Am. Chem. Soc., 2004, 126, 10232–10233. T. Shimizu, Langmuir, 2001, 17, 7229–7232.
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