Room temperature columnar liquid crystalline self-assembly of acidochromic, luminescent, star-shaped molecules with cyanovinylene chromophores

Article abstract of DOI:10.1039/c7tc05898e

Two new star-shaped tris(N-salicylideneanilines) (TSANs) incorporated with cyanovinylene chromophores were prepared through a multistep synthesis. The position of the cyano group was altered in the target molecules, to understand its impact on the photoph

Full text of DOI:10.1039/c7tc05898e

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Room temperature columnar liquid crystalline self-assembly of acidochromic,
luminescent, star-shaped molecules with cyanovinylene chromophores
Ravindra Kumar Gupta, #a Suraj Kumar Pathak,#a Joydip De,b Santanu Kumar Palb and
Ammathnadu S. Achalkumara*
aDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039,
Assam, India.bDepartment of Chemical Sciences, Indian Institute of Science Education and
Research Mohali, Sector-81, Knowledge City, Manauli 140306, Punjab, India.
# Equal contribution
Abstract Two new star-shaped tris(N-salicylideneanilines) (TSANs) incorporated with
cyanovinylene chromophores were prepared through a multistep synthesis. The position of the
cyano group was altered in the target molecules, to understand its impact on the photophysical
and thermotropic behavior, in comparison to the stilbene derivative without a cyano group. The
presence of the cyano group enhanced the mesophase range in comparison to the non-cyano
stilbene derivative. Further, this enhanced intermolecular interactions led to the stabilization of
columnar rectangular phase in comparison to the columnar hexagonal phase of simple stilbene
derivative. These compounds exhibited the freezing of columnar phase in a glassy state, which is
beneficial from the device fabrication point of view. Thus the introduction of cyano group within
the molecular structure of a star-shaped TSAN enhanced the intermolecular interactions and also
altered the luminescence behavior. Such ordered luminescent molecular assemblies, which
stabilize the columnar order over a long range are promising from the viewpoint of emissive
displays. These compounds can be utilized for the sensing of volatile acids in solution at very
low concentration (in parts per billion level) or in thin film state, either by fluorescence
switching or quenching.
Introduction
Stilbene based molecules are gaining importance in material science due to their unique
photophysical and photochemical properties. The energy levels of the vinylene chromophore in
the molecular structure can be easily tuned by the appropriate molecular design aimed towards
specific applications like organic light emitting diodes, solar cells, photoconductors, nonlinear
optically (NLO) active materials, photoswitchable materials.1-4 The ease of synthesis, structural
tunability and planar molecular structure made them compatible in the design of self-assembled
supramolecular structures like gels and liquid crystals.5-6 The cyanostilbene chromophore is a
widely studied molecular motif due to its interesting photophysical and aggregation behavior.
This has also served as a fluorophore, which suppress the non-radiative decay due to the
restriction of intramolecular rotations (RIR) and also led to the favorable arrangement of the
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molecules in bulk or self-assembled structures that cooperatively led to an enhancement of
emission.7 This enhancement in the emission due to the aggregation in contrast to their solution
state emission, is termed as aggregation induced emission (AIE).8 Development of molecular
materials to overcome the aggregation caused quenching (ACQ) of luminescence will be a
desirable quality from the viewpoint of organic light emitting diodes (OLEDs).
Columnar (Col) liquid crystals (LCs) formed by the one-dimensional (1D) stacking of
shape anisotropic molecules are interesting towards the application in organic electronics. They
can be compared to ‘molecular wires’ due to the 1D stacking of central conjugated cores
surrounded by the peripheral sheath of alkyl chains. They are termed as the next alternatives in
comparison to organic single crystals and amorphous polymers.9 Even though they lack the high
charge carrier mobility in comparison to single crystals, they are far better in comparison to
amorphous polymers, along with a good solution processability. Thus there are several
nonconventional molecular designs that are utilized to stabilize functional Col phases, which are
suitable for applications like OLEDs, solar cells or organic field effect transistors.10 The design
of emissive Col LCs suitable for the application in OLEDs, always comes with a hurdle of ACQ,
due to the increased intermolecular interaction in Col phase.
Recently, the topic of luminescent liquid crystals (LLCs) are attracting increasing
attention,11 as a new class of optical materials to overcome the inherent low brightness and poor
energy efficiency of conventional LCs used in displays.12 Development of such materials is of
paramount importance in the advancement of anisotropic LEDs, polarized lasers, information
storage devices, sensors or one-dimensional conductors.13 However, having highly emissive
molecules in solution does not guarantee the same in the condensed state due to the ACQ
effect.14Recently an interesting phenomenon named as aggregation induced enhanced emission
(AIEE) was reported in silole derivatives15 and α-cyanostilbenic derivatives.16 The solutions of
these molecules exhibited high fluorescence quantum yields in aggregated state in comparison to
the monomers in dilute solution state. Motivated by this fact, several AIEE active molecular
motifs were introduced in the liquid crystal design, to combine the beneficial AIEE property with
the self-organizing behavior of LCs.12a Several liquid crystals based on α-cyanostilbenic
derivatives were synthesized.17-19 Such molecules displayed multistimuli responsive behavior.
Tris(N-salicylideneanilines) (TSANs) are getting acceptance from investigators as
a valuable molecular motif to realize variety of self-assemblies. With a meticulous molecular
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design, one can achieve variety of functional materials, due to its synthetic flexibility and hence
the tunability of the properties. There are numerous reports on the application of TSANs based
molecular structures designed towards the molecular switches,20 chemosensors,21 covalent
organic microporous structures,22 and chiral catalysts.23 We have reported a number of Col LCs
based on TSANs whose photophysical and thermal behavior can be tuned by the appropriate
molecular design.24 Recently we have reported several TSANs containing trans-stilbene
chromophore stabilizing Col phase and bright green luminescence.24f,h This motivated us to study
the corresponding cyanostilbene derivatives.
Columnar rectangular phase
Columnar hexagonal phase
ST
!
α-CNST
β-CNST
"
25
70 115 160 205 250
Temperature (oC)
Figure 1. Molecular structures of the stilbene based TSANs (ST24f,h α-CNST and β-CNST), and
their thermal behavior (based on the cooling scans from DSC)
In the present report we are providing the synthesis, characterization, thermal and
photophysical behavior of star-shaped TSANs comprising cyanostilbene moieties. Here the
position of cyano group with respect to the phenyl group near to the central core was varied.
The molecule with the cyano group in α-position to the phenyl group was termed as α-CNST
and the molecule with the cyano group in the β-position to the phenyl group was termed as β-
CNST (Fig.1). Considering the presence of three basic amine protons in conjugation with
cyanovinylene fluorophore, we envisaged that the photophysical properties of the molecule
can be modulated by protonation. Such molecules can be used as sensors for the detection of
trace amount of acids. It is surprising to notice that such reports are not there with TSANs,
and we decided to explore their acidochromic behavior towards the development of a
chemosensor.
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Synthesis and characterization
The target TSANs were synthesized as briefed in Scheme 1. 1,3,5-Triformyl
phloroglucinol, the key intermediate was synthesized by Duff’s reaction.20,24 Ethyl-3,4,5-tri(n-
Scheme 1. Synthesis of star-shaped TSANs
!-CNST
"-CNST
Reagents and Conditions: (i) LAH, THF, RT, 2 h (95 %); (ii) SOCl2, dry dichloromethane,
room temperature (95%); (iii) NaCN, benzyltriethylammonium chloride, Dry DMF, 65 oC, 72
h (75%); (iv) 4-Nitrobenzaldehyde, dry ethanol+chloroform (1:1), NaOEt, room emperature, 3
h (70 %); (v) Zinc powder, HCOONH4, THF-CH3OH (2:1), room temperature, 1 h (60-70 %);
(vi) (PCC) Pyridinium chlorochromate:silica gel, dichloromethane, room temperature, 1 h (75
%); (vii) 4-nitrophenylacetonitrile, ethanol, piperidine (75%); (viii) 1,3,5-
triformylphloroglucinol, Methanol, reflux, 6 h (67-68%).
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dodecyloxy)benzoate was synthesized as reported earlier.24g This ester was reduced to
corresponding alcohol 2 on treatment with lithium aluminium hydride (LAH). This alcohol was
oxidized to corresponding aldehyde 7 on treatment with pyridinium chlorochromate (PCC). This
aldehyde on treatment with 4-nitrobenzyl cyanide, in presence of piperidine as a base resulted in
the formation of nitro compound 8. The nitro group was selectively reduced with the help of zinc
powder in presence of ammonium formate at room temperature to yield corresponding amine 9
in good yield. This amine was refluxed with 1,3,5-triformyl phloroglucinol to give the tris(N-
salicylideneanilines) denoted as α-CNST. The constitutional isomer of this compound was
prepared in the following way. The benzyl alcohol 2 was converted into its chloride derivative 3
on treatment with thionyl chloride. Heating this chloride with sodium cyanide in presence of
o
benyltriethylammonium chloride in dry DMF at 65 C over three days yielded corresponding
benzyl cyanide 4 in good yield. This was treated with 4-nitrobenzaldehyde in the presence of
base to provide the nitro compound 5. This compound was reduced to corresponding amine 6 as
mentioned earlier. Finally this amine was condensed with 1,3,5-triformyl phloroglucinol to
provide the target TSANs denoted as β-CNST.
Both the target molecules existed as an inseparable mixture of C3h and Cs geometrical
isomers of keto-enamine tautomer, with the Cs isomer in higher amount24f,h (Table 1) as
1
evidenced by H NMR spectra (See SI). In particular, the proton spectra showed multiple peaks
between δ 8.7-8.9 and δ 13.1-13.5 which are arising due to the resonance of vinylic and
secondary amine protons respectively, and their coupling. As given in Fig.S23, the region viz., δ
13.1-13.5 consisted of four doublets arising from the resonance of an amine proton (Ha) of C3h
isomer and three analogous amine protons (Hc, Hg and He) of Cs isomer. The enamine region
viz., δ 8.7-8.9 also consisted of four doublets, arising from the resonance of a proton (Hb) of C3h
isomer and three protons (Hd, Hf and Hh) of Cs isomer. The integration of Ha against Hc, Hg and
He of amine protons or Hb vs Hd, Hf and Hh of enamine protons can be considered to calculate the
ratio of two isomers. In the present work, we have considered amine protons for estimating the
ratio of the two isomers, because of the better separation of peaks (Fig. S23). Molecular
structures were further confirmed by 13C NMR, IR and MALDI-TOF mass spectrometry.
Stilbene based TSANs (ST) without cyano functional group bearing nine n-dodecyloxy chains
was also prepared as reported earlier24f,h for the comparison of the photophysical properties with
respect to α-CNST and β-CNST.
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Thermal behavior
The liquid crystalline phase assignment of the TSANs, phase transitions, associated
enthalpies with these transitions are provided in Table 1 and graphically represented in Figure 1.
The data obtained from X-ray diffraction (XRD) studies are presented in Table 2. The thermal
behavior of compound ST was reported earlier24f,h and to visualize the impact of cyano
substitution (α-CNST and β-CNST), it is included in Table 1 and Figure 1.
Table 1. Phase transition temperatures a (oC) and corresponding enthalpies (kJ/mol) of DLCs
Phase sequence
Entry (C3h: Cs)b
STd (1:2.41)
2nd Heating
1st Cooling
c
Cr 66.5 (1.5) Colh 199.5 (1.6) I
Colr2 187.9 (14.1) Colr1 219.2 (2) I
Colr2 220.4 (25.2) Colr1 237.8 (8.4) I
I 196.5 (1.4) Colh
c
I 212.8 (2.6) Colr1 183.5 (15.2) Colr2
α-CNST (1: 2.9)
β-CNST (1:2.5)
c
I 233 (6.8) Colr1 215 (18.6) Colr2
aPeak temperatures in the DSC thermograms obtained during the second heating and first cooling
o
b
1
cycles at 5 C/min. Ratio obtained from H NMR; cCrystallization was not observed till room
d
temperature. Reported in reference 24f,h. Cr: Crystal phase; Colh: Columnar hexagonal phase; Colr:
Columnar rectangular phase; I: Isotropic phase.
(a)
(b)
(c)
T = 194 oC
T = 177 oC
T = 25 oC
o
o
Figure 2. POM image of compound in Colr1 phase at 194 C (a); in Colr2 phase at 177 C (b);
frozen Colr2 phase at 25 oC (c).
(a)
Colr1
I
Colr2
Colr2
I
Colr1
(b)
Figure 3. DSC scans of compound α-CNST obtained for second heating (red trace) and first
cooling (blue trace) (a); XRD profile depicting the intensity against the 2θ obtained for the Colr1
phase of compound α-CNST at 200 oC (inset provides the image patterns obtained) (b).
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Table 2. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature
(T/oC) of mesophase.
Compoundsa Phase
dobs (Å)
Lattice parameters (Å),
(D/Å)
(T/oC)
Symmetry
Colr1
Lattice area
S
(Å2),
Molecular volume V (Å3)
a = 22.9; b = 41.51;
S = 950.6, V = 4809.9,
Z = 1.2
41.51
22.90
5.06 (ha)
α-CNST
(56.8)
(200)
p2mm
a = 28.7;
b = 39.78;
S = 1141.69,
V = 5697,
Z = 1.4
39.78
28.7
23.01
11.45
4.99 (ha)
Colr2
(140)
p2mm
a = 27.75;
b = 39.68;
S = 1101.12,
V = 5494.6,
Z = 1.3
39.68
27.75
22.55
11.64
4.99 (ha)
Colr2
(25)
p2mm
Colr1
(225)
p2mm
a = 21.02; b = 39.74;
S = 835.33,
39.74
21.02
4.94 (ha)
β-CNST
V = 4126.55,
Z = 1.
a = 22.63; b = 39.72;
S = 898.86,
V = 4314.55,
Z = 1.1.
(56.8)
39.72
22.63
11.63
Colr2
(140)
p2mm
4.8 (ha)
a = 27.17; b = 39.75;
S = 1080.0,
V = 5130.03,
Z = 1.3.
39.75
22.43
11.67
4.75 (ha)
Colr2
(25)
p2mm
aThe diameter (D) of the disk (estimated from Chem 3D Pro 8.0 molecular
model software from Cambridge Soft). dobs: spacing observed; dcal
:
spacing calculated (deduced from the lattice parameters; a for Colh phase).
The spacings marked ha correspond to diffuse reflection in the wide-angle
region arising from correlations between the alkyl chains. a and b are the
lattice parameters obtained for Colr phase; Z indicates the number of
molecules per columnar slice of thickness ha, estimated from the lattice
area S and the volume V.
Stilbene based TSAN with nine n-dodecyloxy chains exhibited a Col phase with hexagonal
o
symmetry over a wide thermal range starting from ambient temperature to ≈ 200 C. TSAN α-
CNST was a waxy solid that was shearable and looked briefringent under polarizing optical
microscope (POM). On heating it with the help of programmable hot stage showed a transition to
more birefringent and fluidic structure in accordance with a transition noticed by differential
o
scanning calorimeter (DSC) at 188 C with an enthalpy change of 14.1 kJ/mol. This was
transformed to an isotropic liquid at ≈ 219 oC (ΔH = 2 kJ/mol) (Fig.3a). On cooling the isotropic
liquid, a transition to a bright mosaic pattern was noticed at 212 oC, which exhibited a reduction
o
in the brightness at around 184 C. This mosaic texture disappeared over further cooling and
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transformed into a dark colored texture with the remnants of high temperature mosaic pattern,
which remained unaltered at room temperature (RT) (Fig.2a-c). No signs of crystallization were
noticed in DSC scans, even though the sample was not shearable at room temperature. This is
probably a transition to the glassy state (Fig.3a). Though the POM texture resembled the Col
phase, the symmetry of the columnar arrangement can be understood only with the help of XRD
studies, and hence the XRD scans were collected at different temperatures. Powder XRD pattern
obtained at 200 oC, on cooling from the isotropic liquid exhibited a sharp peak with a d-spacing
of 41.51 Å along with a peak corresponding to a d- spacing of 22.9 Å (Fig.3b, Table 2). At wide
angle a diffused peak corresponding to a d-spacing of 5.06 Å was observed, which corresponds
the packing of flexible alkyl tails. Peaks at the low angle can be indexed into the (01) (10) Miller
indices of a rectangular lattice with lattice parameters a = 22.9 Å; b = 41.51 Å. The values
calculated for the lattice area and lattice volume were found to be 950.6 Å2 and 4809.9 Å3. The
number of molecules present in the unit rectangular cell (Z) was found to be approximately one.
As noticed in the DSC scans there was a transition at 212 oC (ΔH = 2.6 kJ/mol), which prompted
o
us to conduct the XRD at a temperature lower than this. The XRD pattern obtained at 140 C
showed several peaks at low to mid angle region corresponding to d-spacings of 39.78, 28.7,
23.01 and 11.45 Å that can be indexed in to the Miller indices (01), (10), (11) and (22) of a
rectangular lattice. The low temperature rectangular structure is different from the high
temperature one with a larger lattice area and larger cell volume (Table 2 and Fig.S26). In order
to distinguish these two different Colr phases, we have named the high temperature one as Colr1
b = 39.7 Å
a = 27.8 Å
28.4 Å
Figure 4. Schematic showing the self-assembly of compound α-CNST in Colr phase of p2mm
symmetry (Derived from the XRD pattern obtained at 25 oC, only C3h isomer shown for the sake
of clarity)
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phase, while the low temperature one as Colr2 phase. Similar reflections were observed for the
diffraction pattern obtained at room temperature (Table 2, Fig.S26), which showed the existence
of Colr2 phase with out any crystallization, however in a glassy state. The self-assembly of α-
CNST in Colr phase of p2mm symmetry can be represented as shown in Fig.4.
Constitutional isomer β-CNST was also a waxy solid which showed similar phase
transitions, but with a higher isotropic temperature of 238 oC. The optical texture had rectilinear
defects arising from the homeotropic domains (Fig.5a), which is commonly observed for Col
phase. A transition was noticed at 215 oC (ΔH = 18.6 kJ/mol), with a decreased birefringence in
the optical texture (Fig.5b). This remained as such till room temperature without a sign of
crystallization. It has to be noted that cooling at a slower rate (1oC/min) provided a nice
homeotropically aligned Colr phase with the fern like domains growing at right angles to each
other (Fig.S27). Powder XRD studies were conducted at different temperature intervals. The
o
XRD pattern obtained at 225 C confirmed the Colr phase (Fig.5c and Table 2). The diffraction
o
patterns obtained at 140 C and RT showed the existence of another Colr phase (Fig.S28). The
self-assembly of β-CNST in Colr phase of p2mm symmetry can be represented as shown in
Fig.S29.
Colr1
I
Colr2
Colr2
I
Colr1
(b)
(a)
(c)
o
Figure 5. POM image of compound β-CNST in Colr2 phase at 150 C (a); DSC scans of
compound β-CNST obtained for second heating (heating: black trace) and first cooling (cooling:
red trace) (b); XRD profiles depicting the intensity against the 2θ obtained for the Colr2 phase of
compound β-CNST at 225 oC (inset provides the image pattern obtained) (c).
Similar to the case of α-CNST, we have denoted the high temperature and low temperature Col
phases as Colr1 and Colr2 phases. In comparison to α-CNST, β-CNST exhibited higher clearing
temperature and enhanced mesophase range, which confirms the enhanced intermolecular
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interactions due to this substitution. The introduction of cyano group in the molecular structure
led to the stabilization of Colr phase in comparison to TSANs without a cyano group, i.e. ST,
which again confirmed the enhanced intermolecular interactions in these molecules due to the
polar cyano group.
Photophysical behavior
Photophysical properties of the TSANs, α-CNST and β-CNST were studied in solutions
and in thin films and compared with that of ST (Table 3). The absorption spectra of compounds
in chloroform solutions displayed two bands, which are due to n-π* and π-π* transitions. α-
CNST exhibited blue shifted bands in comparison to ST, while β-CNST exhibited a red shifted
band. Between the two cyanostilbene derivatives, β-CNST exhibited a red shifted absorption
band (Fig.6a). Emission spectra of ST and β-CNST exhibited a single emission maximum
centered at 517-519 nm along with a shoulder band at longer wavelength. In comparison, the
emission spectrum of α-CNST was blue shifted with an emission maximum at 504 nm. The thin
films of the samples were prepared by spin coating of the solutions in chloroform on quartz
plates. The emission spectra of these films showed red shifted bands in comparison to solutions
due to the formation of aggregates.25 The emission spectra of β-CNST were red-shifted in
comparison to that of ST and α-CNST in the thin film state (Fig.6b). Fluorescence lifetime
measurements at micromolar concentrations and thin films revealed the existence of three
different species. The species in the solution state showed lower lifetime in comparison to the
same in thin film state, which points to the formation of aggregates (See SI, Table S2-3 and
Fig.S31). However the aggregates do not show any sort of enhanced emission in comparison to
the solution state (Fig. S32). Both the solutions and thin films showed visually perceivable
emission on exciting with the UV light of long wavelength (Fig.S33). The relative quantum
yields of these compounds were measured in solution as reported earlier (Table 3).25 The
quantum yields were in the range of 0.26-0.3. The optical band gap calculated from the
absorption onset values were found to be in the range of 2.5-2.6 eV.
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(a)
(b)
Figure 6. Normalized absorption spectra of compounds (solid line) and emission spectra (broken
line) in CHCl3 solution (a); normalized absorption spectra of compounds (solid line) and
emission spectra (broken line) obtained for the thin films prepared by spin coating in CHCl3
solution (b);
Table 3. Photophysicala properties of TSANs in solution and thin film state.
Solution
Absorption Emissionb
Thin film
d
Entry
Stokes shift
ɸf
λonset
(nm)
497
487
504
ΔEg, opt
Absorptiong Emissione,g
(nm)
(nm)
517
504
519
(cm-1)
3333
3042
3004
(nm)
376, 436
439
(nm)
560
559
575
ST
α-CNST
β-CNST
357, 441
359, 437
369, 449
0.30
0.28
0.26
2.50
2.55
2.46
456
aMicromolar solutions in CHCl3. bExcited at the respective absorption maxima. cBand gap was determined from
the red edge of the longest wave length in the UV-Vis absorption spectra. dΔEg (eV) = 1242.37/(wavelength in
nm), Excited at the respective absorption maxima in thin film state, fRelative quantum yields are calculated with
e
respect tetrakis(octyl)-1H-phenanthro[1,10,9,8]carbazole-3,4,9,10-tetracarboxylate in THF solution as the
standard, gPrepared by spin coating of the sample.
Acidochromism
Considering the presence of a basic N atom in conjugation with a cyanovinylene
chromophore, we envisioned that these compounds might act as good proton acceptors and the
protonation may change the extent of electron delocalization. This would affect the optical
properties, specially the fluorescence of these compounds, which can be beneficial in the
detection of acids. Due to the high solubility and strong acidity of the trifluroacetic acid (TFA), it
was chosen to protonate the TSANs. As seen from the Fig.7a and c, the emission intensities of
compounds α-CNST and β-CNST were reduced by the addition of trifluoroacetic acid (TFA)
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along with a red shift (26-33 nm). The red shift was more in the case of α-CNST thus the
emission color change from green to yellow was more prominent under UV light of long
wavelength (Fig.S35). However, the absorption spectra of these compounds showed only a small
blue shift (Fig.S34). It is interesting to note that gradual neutralization with the triethylamine
recovered the original green emission, which was confirmed by fluorimetric titration (Fig.7b and
d). Similar experiment was done for the spin-coated thin films too, but with vapors of TFA and
TEA. Due to the low emission intensity, the emission change was not visually detectable, but the
fluorescence spectrum revealed the change in the emission (Fig.8 and Fig.S36). Treating the
films with vapors of TEA showed emission shift to the original position only in the case of α-
CNST, while the original emission maximum was not recovered in the case of β-CNST.
However in both the cases the emission intensity was increased after neutralizing the films with
TEA vapors (Fig.8b,d). From the luminescence quenching in response to TFA addition, we
calculated the limit of detection (LOD) for TFA in solution. The LOD determined for β-CNST
504 nm
504 nm
537 nm
537 nm
(a)
(b)
519 nm
519 nm
545 nm
545 nm
(c)
(d)
Figure 7. Change the emission spectra of compound α-CNST with the gradual addition of TFA
(a); recovery of the emission by neutralization with TEA (b); Change the emission spectra of
compound β-CNST with the gradual addition of TFA (c); recovery of the emission by
neutralization with TEA (d).
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559 nm
559 nm
TFA
TEA
583 nm
583 nm
597 nm
(a)
(b)
575 nm
TEA
TFA
605 nm
605 nm
(c)
(d)
Figure 8. Change the emission spectra of the thin film of compound α-CNST on exposure to
TFA vapors (a); recovery of the emission by neutralization with TEA vapors (b); Change the
emission spectra of compound β-CNST on exposure to TFA vapors (c); recovery of the emission
by neutralization with TEA vapors (d).
was 48.34 parts per billion (ppb), while for α-CNST the LOD was found to be 78.89 ppb (See
SI, Fig.S38 and S39).
In order to make a simple handheld device to detect the acid vapors by naked eye we
have placed the solution used for the preparation of films on a TLC plate with the help of a
dropper (See the word ‘IIT’ in Fig.S37). Under the UV light of long wavelength these letters
have shown bright yellow emission. Placing the droplets of TFA over the word ‘IIT’ quenched
the emission along with a redshift as seen under UV light. After few minutes the droplets of TEA
placed on this word showed the recovery of the yellow emission instantly. Thus these
compounds can be utilized as volatile acid sensors in solution as well as in thin film state either
by switching or by quenching of the emission without the need of any sophisticated
instrumentation.
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Conclusions
In this paper we have prepared two novel star-shaped TSANs through a multistep
synthesis to incorporate a cyanovinylene chromophore between the two unsymmetrically
substituted benzene rings. The position of the cyano group was altered and the target molecules
were compared with the stilbene derivative for their thermal and photophysical behavior. The
presence of the cyano group enhanced the mesophase range in comparison to the non-cyano
stilbene derivative. Further, enhanced intermolecular interactions led to the stabilization of Colr
phase in comparison to the columnar hexagonal phase of simple stilbene derivative. These
compounds exhibited the freezing of Col phase in a glassy state, which is beneficial from the
device fabrication point of view. These compounds exhibited different absorption and emission
behavior in comparison to the stilbene based TSANs reported earlier. The solution state emission
of α-CNST was blue shifted in comparison to stilbene based TSANs (ST), while the β-CNST
exhibited a red shifted emission band in comparison to that of ST. These compounds were
emissive in the thin film state also, which means that they are emissive in Col phase. In the solid
thin films, α-CNST exhibited a similar emission as compound ST, but a red shifted emission
was observed for β-CNST. Thus the stabilization of room temperature Col phase along with the
emissive nature is promising from the viewpoint of emissive displays. These compounds
showed the ability to detect the volatile acids in solution as well as thin film state with an
emission switching or by quenching. Specially, in solution state, they could sense the volatile
acid like trifluoroacetic acid in parts per billion levels by fluorescence switching. It is interesting
to note that they recover their original emission on neutralization with a base.
Acknowledgements. ASA sincerely thanks Science and Engineering Board (SERB), DST, Govt.
of India and BRNS-DAE for funding this work through project No. SB/S1/PC-37/2012 and
No.2012/34/31/BRNS/1039 respectively. We thank the Ministry of Human Resource
Development for Centre of Excellence in FAST (F. No. 5-7/2014-TS-VII). We thank CIF, IIT
Guwahati for instrumentation facility.
Conflicts of interest statement. There are no conflicts of interest to declare
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