Organogels composed of trifluoromethyl anthryl cyanostyrenes: Enhanced emission and self-assembly

Article abstract of DOI:10.1039/c7pp00362e

A series of α-cyanostyrenes bearing anthracene and electron withdrawing trifluoromethyl units were designed and synthesized. The α-cyanostyrene skeleton favors aggregation induced enhanced emission behavior due to the restriction of intramolecular rotatio

Full text of DOI:10.1039/c7pp00362e

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Photochemical & Photobiological Sciences
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DOI: 10.1039/C7PP00362E
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Organogel Composed of Trifluoromethyl Anthryl Cyanostyrenes: Enhanced
Emission and Self-Assemblies
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Jagadish Katla†, Akshay JM Nair†, Abhijeet Ojha‡ and Sriram Kanvah†*
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*†Department of Chemistry, Indian Institute of Technology Gandhinagar, Palaj,
Gandhinagar 382355
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‡Department of Biological Engineering, Indian Institute of Technology Gandhinagar Palaj,
Gandhinagar 382355
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*Eꢀmail: kanvah@gatech.edu
Abstract
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A series of αꢀcyanostyrenes bearing anthracene and electron withdrawing trifluoromethyl
units were designed and synthesized. The αꢀcyanostyrene skeleton favors the aggregation
induced enhanced emission behavior due to the restriction of intramolecular rotations.
Remarkably, the anthryl cyanostyrenes bearing simple trifluoromethyl (CF3) substituents
form stable organogels with enhanced fluorescence emission compared to its solution
state. In water, the CF3 substituted anthrylstyrenes selfꢀassembles to an entangled fibrous
nano/ microstructures through intermolecular Hꢀ bonding, π–π stacking and cyano
substituent interactions. The morphological features of the aggregates and the gels were
substantiated using scanning electron microscopy, TEM, Powder XRD measurements.
The stability of the gels was assessed using rheology investigations.
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Introduction
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πꢀConjugated organic fluorophores have attracted much attention for their use as materials
for optoelectronic and biological applications1ꢀ8. The synthetic feasibility through the
incorporation of suitable pushꢀpull substituents, their desired optical properties such as the
absorption, emission in the solution and the solid state, tunable redox properties,
photochemical interconversions complement their use for several optical and biological
applications9ꢀ11. In particular, conjugated substrates bearing diarylethylene scaffold have
attracted a sustained interest for their use as switches, photochromic materials, and cellular
imaging12ꢀ17. Among materials with this ethylene scaffold, cyanostilbene derivatives, have
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attracted great attention towards the development of novel functional materials particularly due
to their characteristic unique emission, termed Aggregation Induced Enhanced Emission
(AIEE), in aqueous media18ꢀ22. Incorporation of suitable pushꢀpull substituents in the
cyanostilbene architecture allowed construction of various luminogens with characteristic
emission and morphological features21, 23. These fluorophores exhibit remarkable
enhanced emission in an aggregated state in contrast to the weaker emission in the
solution state. This emission behavior is attributed to the restriction of intramolecular
rotation (RIR), the formation of Hꢀ or Jꢀtype aggregates, charge transfer and other excited
state processes24, 25. Apart from their unique emission and morphological behavior, the
lower molecular weight πꢀconjugated organic molecules also selfꢀassemble to micro
structures26. Such selfꢀassembly of functional organic materials bearing suitable
substituents such as steroidal or alkyl chains, triggered by a combination of nonꢀcovalent
interactions like Hꢀbonding, πꢀπ stacking, van der Waals forces, can result in an organogel
formation8, 27ꢀ30. The organogel systems derived of πꢀconjugated molecular systems have
fascinating applications as white light emitting materials, sensing and imaging31ꢀ34. In this
work, we designed and developed πꢀconjugated substrates bearing an anthracene moiety
linked to αꢀcyanostyrene scaffold containing trifluoromethyl (CF3) substituents. Previous
literature reports suggest that introduction of CF3 groups endows excellent gelation (low
molecular weight organogels: LMOG) properties to the molecular systems and also results
in an enhanced emission26, 35ꢀ39. We thought the introduction of fused planar ring systems
such as anthracene could direct more ordered packing in the gels through their πꢀπ
interactions40, 41. Structural modifications to the πꢀ scaffold are also known to influence
the architecture and its electronic properties. Considering these observations, we designed
styrylanthracene fluorophores, anticipating the favorable properties of anthracene and CF3
groups [Figꢀ1], and investigated their absorption, emission, and gelation properties. We
observed the formation of fibrous aggregates with enhanced emission and good gelation
stability. The selfꢀassemblies and the organogels were characterized by scanning electron
microscopy (SEM), transmission electron microscopy (TEM), fluorescent microscopy and
rheology studies. The results are detailed in the following sections.
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Fig. 1 Structures of compounds synthesized
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Experimental
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Materials All the chemicals, reagents required for the synthesis of the anthracene
derivatives were purchased from SigmaꢀAldrich, Alfa Aesar, Acros and S. D. Fine.
Solvents used for absorption and fluorescence investigations were dried and distilled
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before use. All the synthesized samples were characterized using H NMR and C NMR
in CDCl3 with Tetramethylsilane (TMS) as an internal standard using Bruker AvanceIIIꢀ
500 MHz NMR spectrometer. Mass spectral data were obtained using ESIꢀQToF Watersꢀ
Synapt G2S highꢀresolution mass spectrometer. UVꢀVis absorption spectra were recorded
using Analytik Jena specord 210 plus and the fluorescence emission studies were
performed using HoribaꢀJobinꢀYvon Fluolorogꢀ3 spectrofluorometer. Typically the
excitation wavelengths were set at the absorption maxima (λa) of the compounds under
investigation. All the fluorescence spectra were recorded in a 10mm path length quartz
cuvette with a slit width of 1nm. Fluorescence quantum yields of compounds were
estimated using quinine sulfate (Φ = 0.546 in 0.5 M H2SO4) as a reference standard. For
the fluorescence measurements ~10 µM concentrations of chromophores were used.
Scanning electron microscopy (SEM) analysis was carried out using field emission SEM
(JSM 7600F JEOL). For this purpose, one drop of the sample [~10ꢀ6 M solution in water]
was deposited on a Siꢀwafer mounted on an Aluminum stub with the help of a doubleꢀ
sided adhesive carbon tape. The samples were heatꢀdried at 35°C for 12 h and vacuum
dried for 30 min to ensure complete removal of any residual water and coated with
platinum before being analyzed. Fluorescence microscopic images of gels were taken
using Nikon CiꢀS. The rheological properties of gels are analyzed using strain and stress
controlled Anton Paar MCR 302 rheometer. A typical cone and plate geometry with a
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cone diameter 25 mm and truncation angle of 2° with a gap of 0.105 mm at a constant
temperature of 20°C is used, and the small amplitude oscillatory frequency sweep and
large amplitude oscillatory sweep rheological measurements are performed TEM
measurements were carried out using JEOL JEM1011 with an accelerating voltage of 100
kV. Samples were prepared by drop casting of gel (2) on to carbonꢀcoated copper grids at
the required concentrations at ambient conditions. The solvent was removed under
vacuum. Powder XRD patterns were obtained by Xꢀray diffraction (XRD) with Bruker D8
Discover diffractometer using Cu Kα in the Bragg angle range of 5–60°.
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Synthetic Methods
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The desired compounds were synthesized as per the methodology given in Schemeꢀ1. In a
typical procedure, CF3 substituted phenyl acetonitrile (0.01mol) was taken in 25 mL
methanol. To the alcoholic solution, KOH (0.01mol) was added and stirred vigorously for
15min. To this stirred mixture, a solution of aryl aldehyde (0.01mol) in 25mL of methanol
was added slowly and stirred for 10ꢀ15 min at room temperature. Shining yellow crystals
obtained were washed with excess methanol and recrystallized from absolute ethanol.
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Scheme 1. Synthesis of anthraceneꢀbased cyanostyrenes.
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Characterization Data
(Z)-3-(anthracen-9-yl)-2-phenylacrylonitrile (1): UV (dioxane, λabs:388 nm, ε=9005 Mꢀ1
cmꢀ 1) 1H NMR (500Hz, CDCl3): 8.57(s, 1H), 8.47(s,1H), 8.10(d,4H, J=7Hz), 7.93(d, 2H,
J=7.5Hz), 7.54ꢀ7.61(m,7H); 13C NMR(125 Hz): 140.14, 133.34, 131.3, 129.84, 129.30,
129.12, 128.99, 127.85, 126.67, 126.17, 125.53, 125.04, 121.33, 116.50, 96.16, HRMS
(ESIꢀMS) m/z calculated: 305.1204; found 305.1207
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(Z)-3-(anthracen-9-yl)-2-(4-(trifluoromethyl)phenyl)acrylonitrile (2): UV (dioxane,
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λabs:391 nm, ε=6605 Mꢀ1cmꢀ1) H NMR (500 MHz, CDCl3) δ 8.59 (s, 2H), 8.08 (d, J = 8
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Hz, 2H), 8.00ꢀ8.03 (m, 4H), 7.81 (d, J = 8.5 Hz, 2H), 7.52ꢀ7.57 (m, 4H). C NMR (125
MHz, CDCl3) δ 116.02, 120.03, 124.75, 125.63, 126.34, 126.53, 126.96, 127.09, 129.24,
129.37, 129.53, 131.26, 131.62, 131.88, 136.66, 142.53; HRMS (ESIꢀMS) m/z: calcd for
[M]+ 373.1078 found 373.1059.
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(Z)-3-(anthracen-9-yl)-2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile (3): UV (dioxane,
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λabs:404 nm ε=8505 Mꢀ1cmꢀ1 ) H NMR (500 MHz, CDCl3) δ 8.62 (s, 1H), 8.58 (s, 1H),
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8.31 (s, 2H), 8.09 (d, J=8, Hz, 2H), 8.02 (d, J=7.5, 2H), 7.99 (s, 1H), 7.53ꢀ7.59 (m, 4H).
13C NMR (125 MHz, CDCl3) δ 143.93, 135.54, 133.20, 132.93, 131.22, 130.01, 129.33,
127.20, 126.38, 126.13, 125.70, 124.54, 123.31, 118.70, 115.46, 96.13. HRMS (ESIꢀMS)
m/z: calcd for [M]+ 441.0952, found 441.0958.
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Results and Discussions
Absorption and Emission Studies
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Trifluoromethyl substituted phenyl acetonitrile derivatives bearing anthracene were
synthesized and examined for their absorption and emission behavior in the solution and
solid state. Compound (1) has no trifluoromethyl (CF3) group, (2) has one CF3 group
substituted at the paraꢀ position of the phenyl ring and (3) bears two CF3 groups
substituted at the meta- position on the aromatic ring. Through this arrangement, we
sought to examine the effect of trifluoromethyl and the anthracene substitution on the
aggregationꢀinduced emission properties. The absorption shows no significant changes
post introduction of the CF3 groups [Fig 2a] with absorption maxima in the range of 386 ꢀ
411 nm [Fig S1 and Fig 2b] in various solvents. Substitution by strong electron
withdrawing CF3 group and solvent polarity has only a meager effect on the absorption
maxima. However, in water broader bands with a slight redꢀshift of the absorption maxima
are noted for both (2) and (3) but not for (1).
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Fig. 2 Absorption spectra of a) (1-3) in ACN, b) (2) in different organic solvents
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Compound (1) emits at ~430 nm in heptane. With the increase in solvent polarity,
solvatochromic emission shifts (+ 59 nm) were noted. Likewise (2) emits at ~435nm in
heptane and shows similar solvatochromic shifts (+ 56nm) with peaks at 474ꢀ490 nm in
solvents of higher polarity. Compound (3) also shows comparable emission shifts (+
57nm), 440nm in heptane to 471ꢀ497 nm in solvents of higher polarity. Despite its
electron withdrawing strength CF3 has no role play in improving the solvatochromic
properties [Fig 3a]. This is possible as there is no resonance effect due to lack of orbitals
or electron pairs that can overlap with those of the aromatic ring. In water, however, we
observed considerable emission (+16ꢀ23 nm) shifts with emission bands at 505nm for (1),
513nm for (2) and 520nm for (3) [Fig S2, Fig. 3b and 3c]. This unique emission behavior
in aqueous media is attributed to the exquisite phenomenon known as aggregation induced
enhanced emission or in short AIEE and is a result of restriction of intramolecular rotation
that blocks the nonꢀradiative decay pathway.
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Table-1: Absorption and emission of compounds (1-3) in organic solvents and water.
S.No
Solvents
λabs
(nm)
386
388
388
386
389
388
Stokes Shift Quantum yield (ϕf)
(cmꢀ1)
λ
em (nm)
Heptane
Dioxane
THF
Acetonitrile
Methanol
Water
430
461
473
450
489
505
2650
4081
4631
3684
5257
5971
0.00243
0.00365
0.0098
0.00187
0.00345
0.0037
(1)
Heptane
Dioxane
THF
Acetonitrile
Methanol
Water
389
391
391
388
393
402
435
457
474
450
490
513
2718
3693
4478
3550
5037
5382
0.0444
0.0024
0.0027
0.0039
0.0020
0.0046
(2)
(3)
Heptane
Dioxane
THF
Acetonitrile
Methanol
Water
407
404
400
389
400
411
440
455
471
461, 495
497
1842
2754
3768
4014
4879
5100
0.0664
0.0022
0.0041
0.0042
0.0036
0.0035
520
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8
9
10
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Fig. 3 Emission spectra of a) (1-3) in acetonitrile, b) (2), c) (3) in organic solvents and
water.
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Emission in dioxane-water binary mixture
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Compound (1) emits at 461nm in dioxane. As percent water increases in dioxane,
significant enhancement in emission was observed. As the water fraction increases from 0
to 40%, the fluorescence intensity increases ~4.5 folds along with a blue shift (−32 nm). In
water, compound (1) shows incremental emission intensity (1.8 folds) with a red shift of +
90 nm (Fig S4). These initial emission changes are attributed to the formation of dispersed
particles. However, with the further addition of water, the solvatochromism takes over
with the greater formation of aggregates. The general insolubility of the compounds in
water meant that the existing geometry noted in a solution state (dioxane) changes to a
dispersed state in the binary mixture (aqueous dioxane) and aggregated state in water. In
dioxane, (2) with one CF3 group, emits at ~457nm. With added water, the intensity of the
peaks initially increases and later result in solvatochromic emission shifts as a
consequence of polarity increments. At 80% water, the emission maxima show a band at
~502 nm with ~35 fold intensity enhancement [Fig. 4a]. Quenched emission follows this
intensity enhancement at 90% and 100% water, but bathochromic emission shifts (λf = 527
nm) was observed. Similar to (2), compound (3) also shows the highest emission intensity
at 80% water [a ~5ꢀfold increase] with a bathochromic shift at 497 nm, with further redꢀ
shift to 528 nm [Fig. 4b]. These enhanced emissions are due to intra and intermolecular
interactions exerted by aggregation of fluorophores caused by stacking of twisted
cyanostyrenes. The intensity enhanced redꢀshifted emission of these molecular systems is
attributed to the combined effects of aggregation induced planarization and Jꢀaggregate
formation. Compounds (2) & (3) have a similar bathochromic emission shifts (44 nm & 42
nm), but compound (2) shows dramatic intensity enhancement (~35 fold) compared to (3)
(~5 fold) in 80% water. The lower intensity enhancement in (3) could be due to the
presence of two CF3 groups decreasing their packing ability.
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Fig. 4 Emission of compounds (2) and (3) in the dioxaneꢀwater mixture.
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For comparison of the emission with aggregates and solid state, we also recorded
emission in the solid state (Fig. 5a). The designed compounds show emission in the lower
energy regions [535nm (1), 538nm in (2) and 543nm in (3) ] than that observed in
aggregated or dispersed state. These emission shifts are attributed to close packing of the
molecules that restrict the dynamic motion. The bathochromic absorption shift and the
broadening of the absorption spectra indicate the formation of Jꢀ type aggregates (Fig
S3a, b & c).
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Fig. 5 a) Solid state emission of compounds (1-3) and b) emission of (2 & 3) in tꢀBuOH
and after the formation of the gel.
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Morphology of the Aggregates
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The SEM images of (1) were recorded at different water fractions (10%, 40%, 80% and
100%) of samples using the same set of solutions utilized for fluorescence experiments. At
40% water fraction (Fig S5b), images revealed smaller sized aggregates while at 100%
water fraction, larger lumps are observed (Fig S5c) and no aggregates were seen in
dioxane (Fig S5a). The absorption spectra showed in (Fig S3) reveals bathochromic
absorption shifts at 80% or more water fractions indicating the possible formation of
aggregates. The large emission intensity changes observed at 40% water fractions4,5 could
be due to the presence of differently sized aggregates. For (2 and 3), no aggregates are
seen in dioxane, but aggregateꢀlike structures are noted at 80% water. In 100% water rodꢀ
like crystalline structures (Fig. 6 and 7) was observed. The formation of such structures
was aided by the presence of lipophilic CF3 groups and its contribution to the overall
packing.
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Fig.6 SEM images of compound (2) in (a) 100% dioxane (scale: 1ꢁm), (b) 80% water
(scale: 1ꢁm) and in (c) 100% water (scale: 1ꢁm).
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Fig.7 SEM images of compound (3) in (a) 100% dioxane (scale: 100 nm), (b) 80% water
(scale: 1ꢁm) and in (c) 100% water (scale: 1ꢁm).
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Gelation and Rheology studies
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Encouraged by the unique emission in aqueous media and the propensity of CF3
substituted compounds to form organogels, we investigated the formation of low
molecular weight organogelators with compounds (2 & 3) containing trifluoromethyl
substituents. To study the gelation properties, (2) and (3) were dissolved in tꢀbutanol (1.8
wt%) in a glass vial, heated to 70°C to obtain a clear solution. When the hot solutions of (2)
& (3) in tꢀBuOH were kept in the refrigerator for ~5 min and then left at room temperature
for 30min, organic fibers/ribbons have formed gradually in the solution by assembling
through noncovalent interactions (Hꢀbonding and hydrophobic interactions). The hot tꢀ
BuOH solution is followed by sonication and subsequently cooled for five minutes in the
refrigerator. Warming up the solution to the room temperature results in the formation of
a viscous gel [Fig 11]. The gel so obtained has no gravitational flow even after inversion of
the vial (Fig. 8a & b) indicating its overall stability. This gelation was also thermoꢀreversible
with heating (~70 °C) and cooling cycles [Fig. 8]. Examination of its emission reveals a
bathochromic shift in wavelength with a concomitant increase in emission intensity [Fig.
5b] as compared to the solution. The gelation was also successful in toluene and 1, 2ꢀ
dichloroethane but here we report the studies of gels with tꢀbutanol as a solvent.
Compound (1) with no CF3 group shows no gel formation (Fig. S6) due to insufficient
intermolecular interactions.
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Fig. 8 The thermoꢀreversible solꢀgel transitions of the gels of (2 & 3).
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To ascertain the formation of gels and assess the stability and viscoelastic properties of (2)
& (3), rheological properties were characterized by the time, frequency and amplitude
sweep measurements. The samples were equilibrated for 5 minutes before starting the
rheological experiments and further subjected to an oscillatory strain (γ) = 0.1% and
angular frequency (ω) of 100 – 0.1 rad/s. These conditions ensure that the storage
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modulus (G') and loss modulus (G'') are measured in the linear viscoelastic (LVE) regime.
Oscillatory strain sweep measurements probe the nonꢀlinear response of the samples in the
range of γ = 0.01 % ꢀ 100 % at a fixed ω of 6.28 rad/s. Our data indicate that the storage
moduli (G') is quite higher than the loss moduli (G'') indicating low tan δ (tanδ=G''/G')
values obtained for both gel samples (2) and (3) which reveal their gel nature. The G'
values for both gel (2) and (3) were independent of experimental time and frequency
range42, 43 as shown in Figure 9 and 10. The figures further indicate that the G' obtained
for gel (2) is in the order of 103 Pa, while for gel (3) the obtained G' value is in the order of
104 Pa, which suggests the higher strength of gel (3) as compared to gel (2). Even though,
the strong π−π stacking interactions between the rigid anthryl segments provide the
essential driving force for gelation; the relatively higher strength of gel (3) can be
understood by the existence of two CF3 groups in the gel (3). These CF3 moieties play a
key role in the formation of stronger gel networks by reinforcing nonꢀcovalent
interactions26 as opposed to gel (2) that consist of only a single CF3 group. Figure S7
shows that with an increase in strain amplitudes, the gel starts forming liquid by a steep
decrease in the values of both moduli and the reversal of the viscoelastic signal (G'' > G').
The contribution of CF3 groups in the formation of gels by nonꢀcovalent interactions is
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widely reported21, 36, 44
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Fig. 9 a) Time sweep measurement b) Frequency sweep measurement of gel (2) in tꢀ
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Fig. 10 a) Time sweep measurement b) Frequency sweep measurement of gel (3) in tꢀ
BuOH.
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For further insight on aggregation morphology of (2) and (3), a dried organogel sample
was transferred onto a silicon wafer and subjected to the scanning electron microscopy
(SEM) observation. The SEM images in Figure 11 suggest that (2) and (3) form entangled
threeꢀdimensional networks consisting of the bundles of fibrous aggregates, which must
be responsible for the observed gelation. The corresponding fluorescent microscopy
images were shown in Fig S8. The transmission electron microscopy images (TEM) also
confirm this observation and reveal entangled bundles of fibers (2), built up from thin
fibrils (Fig 12). The TEM image also shows intertwined and entangled microfibers. The
image reveal thicker and longer multiple of nanometers in width and micrometers in length)
fibers for gel (2) (Fig. 11a). It should be noted that conjunctions emerged for (2) as shown in
SEM and TEM images, was in accordance with organogel formation.
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Fig. 11 SEM images of the xerogels obtained from tꢀBuOH gelation sols of (a) (2), (b) (3)
(scale: 10 ꢁm). (inset: shows magnified image of scale: 10 ꢁm showing attachment points)
and
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These selfꢀassembled structures are aided by nonꢀcovalent forces such as hydrophobic
forces, πꢀ π interactions of aryl moieties and intermolecular interactions through the CF3
group (CꢀFꢀꢀꢀHꢀAr). The polar cyano group on the vinyl bond helps in regulation of
growth of the molecular selfꢀassembly by strengthening the electrostatic interactions
between two adjacent molecules. For optimum interactions the system becomes
planarized, but the repulsion induced from the relatively bulkier CF3 group lead the
molecules to slide away along the molecular axis. However, the Hꢀbonding arising from
CꢀFꢀꢀꢀHꢀAr and π−π interaction of the neighboring anthryl rings stabilize these repulsive
movements resulting in the construction of 1D or 2D microfibrilꢀlike structures formed
through lengthwise stacking (i.e., headꢀtoꢀtail Jꢀ aggregation).
21
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2
Fig.12 TEM image of organogel formed by (2): (scale: 1 ꢁm) (inset: shows magnified
image of scale: 200 nm)
3
Powder XRD studies
4
5
Powder XRD measurements were performed on the xerogels of (2) & (3), prepared from tꢀ
BuOH, to obtain further information on the molecular arrangement in their gel state
(Fig.S9a and S9b). Wellꢀresolved Xꢀray diffraction signals of (2) are shown in Fig S8a.
The sharper peaks, formation, and disappearance of the new peaks appear in the gel state
indicating structural changes to the morphology, i.e. formation of fibers/microcrystals.
The peak of powder sample at angle 2θ=17.13° (Full Width Half Maximum=0.498) shifts
to 2θ=17.18° (fwhm=0.457) with intense peaks. The low FWHM indicate greater
crystallinity in these xerogels. Similarly, the Xꢀray diffraction (XRD) pattern of gel (3)
also showed new sharper peaks distinct from the powder sample. The data appears that the
xerogels obtained have higher order packing aided by nonꢀcovalent attractions.
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Conclusion
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We have demonstrated the efficient formation of low molecular weight organogels by the
selfꢀassembly of simple anthrylcyanostyrenes bearing CF3 groups into a threeꢀdimensional
network of fibrous structures. The gels exhibit a bathochromic emission wavelength shifts
accompanied by an enhancement in emission intensity compared to that of the solution.
The unique gelation is attributed to the combined intermolecular interactions of the CF3
group and πꢀπ* interactions of the anthryl moiety and the restrictions offered by the
presence of cyano group on the double bond.
22
Acknowledgements
23
24
25
Board of Research in Nuclear Sciences [37(2)/14/05/2016] for the financial grant, NIIST
Thiruvananthapuram for TEM measurements, and IIT Gandhinagar for overall
infrastructural support
26
27
Supporting Information
Fig S1ꢀS12 including NMR characterization spectra are provided
15

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CF3 substituted Anthryl cyanostyrenes were synthesized and examined for their
self-assembly and organogel formation.

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208x81mm (96 x 96 DPI)