Synthesis, characterization and nonlinear optical studies of novel blue-light emitting room temperature truxene discotic liquid crystals

Article abstract of DOI:10.1039/c5ra00873e

A new series of discotic liquid crystals based on a truxene core has been synthesized to study the structure-property relationship in view of the self-assembling property and their linear and nonlinear optical properties. All these branched alkyl chain truxene derivatives show a mesogenic property in a columnar hexagonal fashion at room temperature which is studied by a combination of different techniques. The newly synthesized truxene discotic 7a possesses a clearing temperature of 117 °C, one of the lowest clearing temperatures known in truxene discotic liquid crystals. We have discovered that the introduction of branching near to the core even in a small alkyl chain drastically reduces the isotropic temperature. Due to their C₃ symmetry and their large first hyperpolarizability, these truxene derivatives show long-lived emissions in the solution state at room temperature. We also report large effective three-photon absorption in these materials under nanosecond laser pulse excitation at 532 nm, which makes them suitable candidates for optical limiting applications.

Full text of DOI:10.1039/c5ra00873e

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Synthesis, characterization and nonlinear optical studies of novel DbOlIu: 1e0.1-0l3i9g/Ch5tRA00873E
emitting room temperature truxene discotic liquid crystals
Vinayakumara DR, Manish Kumar, P. Sreekanth, Reji Philip and Sandeep Kumar*
Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore - 560 080, India
Phone: +91 80 23610122, Fax: +91 80 23610492, E-mail: skumar@rri.res.in
Abstract
A new series of discotic liquid crystals based on truxene core has been synthesized to study
the structure-property relationship in view of self-assembling property and their linear and
nonlinear optical properties. All these branched alkyl chain truxene derivatives show
mesogenic property in columnar hexagonal fashion at room temperature which is studied by
combination of different techniques. The newly synthesized truxene discotic 7a possess
clearing temperature of 117 ºC, one of the lowest clearing temperatures known in truxene
discotic liquid crystals. We have discovered that the introduction of branching near to the
core even in small alkyl chain drastically reduces isotropic temperature. Due to their C3
symmetry and their large first hyperpolarizability, these truxene derivatives show long-lived
emissions in solution state at room temperature. We also report large effective three-photon
absorption in these materials under nanosecond laser pulse excitation at 532 nm, which
makes them suitable candidates for optical limiting applications.
Keywords: Truxene, discotic liquid crystal, NLO studies, optoelectronics, nanosecond laser
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DOI: 10.1039/C5RA00873E
1. Introduction
Supramolecular structures formed due to non-covalent interactions in functionalized disc-
shaped molecules, commonly referred to as discotic liquid crystals (DLCs), are of enormous
scientific interest because of their extraordinary unidirectional charge and energy migration
properties along the columnar axis as well their optical properties. As these systems are of
great fundamental and technological importance, significant research work is going on
around the globe which has been reviewed in several articles, for example see references.1-19
A majority of discotic liquid crystals are derived from polycyclic aromatic cores such
as, triphenylene, anthraquinone, phthalocyanine, and hexabenzocoronenes etc., which possess
strong π-π interactions favouring columnar stacking of the molecules. In the columnar
mesophase of DLCs, aromatic cores are oriented in columns separated by molten aliphatic
hydrocarbon chains. The intra-columnar (core-core) separation in a columnar mesophase is
usually of the order of 0.35 nm while the intercolumnar (neighbouring columns) separation is
generally in the range of 2-4 nm, depending on the length of flexible chains. Thus, these
materials functions as quasi-one-dimensional semiconductors.1-19
In 1981, Destrade and co-workers first studied the truxene in discotic liquid crystals
by preparing the hexaesters of truxene showing inverted nematic-columnar phase sequence.20
So far limited truxene derivetives are reported as DLCs because of synthetic difficulties.
Truxene based compounds are attractive for many applications such as optoelectronics,
photovoltaic, polymers, dyes, semiconductors, polymeric light emitting devices etc21-30
because of many reasons: it is electron rich system, possess C3 symmetrical structure, its
good florescence capacity, its ability to act as potential semiconductors, its derivatives are
coloured, its extraordinary thermal stability and its electronic properties.
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Liquid crystals possess large and versatile optical nonlinearities suitable for photonic
DOI: 10.1039/C5RA00873E
applications spanning the femtoseconds to milliseconds time scales, and across a wide
spectral window.31 The nonlinear optical properties of liquid crystals (LCs) have been studied
for more than 20 years starting from the paper by Zeldovich et al.32 who reported the first
observation of a Giant Optical Nonlinearity (GON). The presence of extended π electron
conjugation makes discotic liquid crystals highly polarizable, which makes them potential
candidates for nonlinear optical applications. This is due to the particular spatial arrangement
of the molecules in space and their characteristics.33 Large two photon absorption (2PA) and
three photon absorption (3PA) coefficients can be obtained by optimal design and synthesis
of chromophores and dendrimers.33 The molecules with large differences in polarization upon
excitation and extended π-conjugation have received much attention in DLCs ultrafast optical
responses, which depends upon excitation wavelength. The electronic structure of these
materials usually supports strong nonlinear absorption (NLA) in the visible as well as IR
regions. During the last two decades a number of studies like conducting, optical, non-linear
optical and electroluminescent properties34-40 have been reported on different truxene
derivatives.
Though organic materials are well known candidates for non-linear optical
applications, only a few studies have been reported on the preparation and NLO
characterization of truxene derivatives.41, 42 Liquid crystals exhibit high nonlinear optical
responses due to the fact that they are strongly anisotropic and their molecular reorientation is
highly correlated. Their refractive index changes drastically due to collective director
reorientation under an external field. As such, their extraordinarily large nonlinear optical
responses can be observed even under a relatively weak optical field from a low power laser.
The nonlinear optical response of thermotropic liquid crystals has been extensively studied.33,
43, 44 Because of the extreme sensitivity of LCs to the action of external fields, a powerful
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light wave changes the orientation of the molecules. Light-induced structural changes are
DOI: 10.1039/C5RA00873E
reflected in all of the macroscopic properties of LC and are easily observed both in the
mesophase and in the isotropic phase.45-48
To explore the potential applications of these intriguing materials, DLCs having
mesophase at room temperature and mesophase stability over a wide temperature range are
desired. Further, LCs with not very high clearing temperature are required for various
physical studies. Here we wish to report synthesis and characterisation of a series of new
discotic materials based on hexaalkoxytruxene derivatives with branched alkyl chains at
periphery which exhibit mesophase at ambient temperature and also possess low isotropic
temperature compared with reported truxene DLCs. The newly synthesized materials were
characterized from their spectroscopic, optical, thermal, self-assembling behaviour and X-ray
diffraction studies. Our intention is to tune the linear and non-linear optical properties using
Z-scan technique of truxene-based liquid crystals by the introducing alkyl chain at periphery.
These modified truxene derivatives may be used as non-linear optical materials and
applicable in semiconductor industry.
2. Experimental
2.1 Materials
All the chemicals and reagents were purchased locally and used as such without any further
purification. Acme make silica gel of mesh size 200-400 were used to perform the column
chromatographic purification.
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DOI: 10.1039/C5RA00873E
2.2 Characterization
1
The chemical structures were confirmed by Bruker AMX 500 MHz H NMR spectrometer
and Shimadzu FTIR-8400 FTIR spectrometer, respectively. Absorption and fluorescence
spectra were recorded on a Perkin Elmer UV-Vis-lambda 35 double beam-spectrophotometer
and Perkin Elmer Fluormax spectrophotometer, respectively. Elemental analysis was
performed on a Carlo-Erba Flash 1112 analyzer. Optical textures of mesophases were
observed using Olympus BX51 Polarized Optical Microscope (POM) in conjunction with a
Mettler FP82HT hot stage and FP90 central processor. Differential scanning calorimetry
(DSC) of liquid crystals and Composites were taken at the temperature range of 5 °C to 150
°C using Parkin-Elmer Pyris-1 DSC. X-Ray powder diffraction (XRD) patterns were
obtained on DY 1042-Empyrean XRD with Pixel 3D detector at Cu-Kα radiation.
2.3 Synthesis
The synthesis is illustrated in scheme 1.The synthetic work was commenced from acid
treated aldol cylotrimerization of 5, 6-dialkoxy-1-indanone using p-toluenesulfonic acid as
dehydrating agent in mixed solvent toluene and ethylene glycol.49 All required branched
chains alkyl bromides were prepared according to previously reported literature.50, 51
3-(3,4-dimethoxyphenyl) acrylic acid, (2)
A mixture of 3,4-dimethoxybenzaldehyde (10g, 60.0mol), malonoic acid (10.01g, 96.28mol)
and pyridine (11.9g, 150mol) was taken in a 250 mL round bottom flask, the mixture was
heated to 80 ºC with stirring for 16 hours. The crude white solid was then recrystallized using
ethanol to give compound 2 as white crystals (yield 90%).1H-NMR (CDCl3, 500 MHz, ppm):
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δ 7.74 (d, J = 16.0Hz, 1H), 7.14 (d, J = 8Hz, 2H), 7.88 (d, J = 8Hz, 1H), 6.32 (d, J = 1V6ieHw zAr,ticle Online
DOI: 10.1039/C5RA00873E
1H), 3.92 (s, 6H).
3-(3,4-dimethoxyphenyl)propanoic acid, (3)
A mixture of compound 2 (10g) and 10% Pd/C (500mg) was taken in 60 mL of ethylacetate
in a round bottom flask were hydrogenated at 150 atmosphere of H2 gas for 14 hours at 60 ºC.
The reaction mixture was filtered to remove the catalyst then solvent was removed to give
white solid. Solid product was then recrystallized in a mixture of petroleum ether and
1
ethylacetate to yield compound 3 as white powder (yield 95%). H NMR (CDCl3, 500 MHz,
ppm): δ 6.81-6.74 (m, 3H), 3.86 (s, 6H), 2.91 (t, J = 7.5Hz, 2H), 2.67 (t, J = 7.5Hz, 2H).
2,3-dihydro-5,6-dimethoxyinden-1-one, (4)
P2O5 (9g) and 85% H3PO4 (3.5g) were heated to 120 ºC with stirring for about 1 and ½ hours.
To this clear polyphosphoric acid, compound 3 (1g) was added and continued the same
reaction condition for 6 hours. Cooled reaction mixture was added slowly to the crushed ice
and extracted with 3x100ml chloroform; organic layer was concentrated under vacuum. The
crude dark green solid was purified by column chromatography using dichloromethane as
mobile phase to give pure indanone as yellow solid (yield 55%).1H NMR (CDCL3, 500 MHz,
ppm): δ 7.27 (s, 1H), 6.90 (s, 1H), 3.97 (s, 3H), 3.91 (s, 3H), 3.06 (d, J = 5Hz, 2H), 2.68 (d, J
= 5Hz, 2H).52
2,3-dihydro-5,6-dihydroxyinden-1-one, (5)
BBr3 (3.9g, 15.6mol) was added slowly to a stirred solution of compound 4 (1g, 5.2mol) in
dichloromethane (30mL) at -78 ºC. After 4 hours mixture was then stirred at room
temperature for 2 hours. Water was added to the reaction mixture to give red solid which was
1
filtered and dried to afford titled compound 5 as dark red solid (yield, 80%). H NMR
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(CDCl3, 500 MHz, ppm): δ 10.0 (s, 1H), 9.42 (s, 1H), 6.92 (s, 1H), 6.83 (s, 1HD),OI2: 1.08.1903V(9ieb/wCrA5srR,tiAcl0e0O8n7li3nEe
2H), 2.5 (brs, 2H).
2,3-dihydro-5,6-dialkoxyinden-1-one, (6a, 6b and 6c)
A mixture of compound 5 (1g, 6.09mol), K2CO3 (3.4g, 24.39mol), KI as a catalyst and RBr
(18.3mol) in ethanol was refluxed for overnight. After careful observation in TLC, reaction
mass was filtered and mother liquor were concentrated. The crude product was purified in
column chromatogram to yield pure thick yellow liquid (yield, 70%).1H NMR for 6a:
(CDCl3, 500 MHz, ppm): δ 7.08 (s, 1H), 6.78 (s, 1H), 3.8 (d, 4H), 2.98 (brs, 2H), 2.59 (brs,
2H), 1.8-1.4 (m, 30H); 6b: δ 7.27 (s, 1H), 6.87 (s, 1H), 4.0 (m, 4H), 3.0 (brs, 2H), 2.66 (brs,
2H), 1.8-1.4 (m, 38H); 6c: δ 7.27 (s, 1H), 6.89 (s, 1H), 4.1 (m, 4H), 3.0 (brs, 2H), 2.68 (brs,
2H), 1.8-1.4 (m, 58H).
Hexaalkoxytruxene liquid crystals (7a, 7b and 7c).
A well mixture of para-toluenesulfonic acid, ethylene glycol and compound in toluene was
heated to reflux for overnight. Toluene was removed under high vacuum after careful
confirmation of completion of reaction by thin-layer chromatography; resulted red solution
was purified by column chromatography. The crude solid was purified by repeated
recrystallization in diethylether and methanol to give target compound as gummy solid (yield,
1
55%). H NMR for 7a (CDCL3, 500 MHz, ppm): δ 7.49 (s, 3H), 7.27 (s, 3H), 4.23 (s, 6H),
4.0 (d, 12H), 1.89 (m, 6H), 1.4-0.98 (m, 84H), (elemental anal. C75H114O6, calculated C
81.03, H 10.34; found C 81.0, H 10.50%); 7b: δ 7.49 (s, 3H), 7.27 (s, 3H), 4.24 (m, 18H), 2.0
(m, 12H), 1.6-1.2 (m, 102H) (elemental anal. C87H138O6, calculated C 81.63, H 10.87; found
C 81.22, H 10.91%); 7c: δ 7.47 (s, 3H), 7.27 (s, 3H), 4.17 (m, 18H), 1.97 (m, 12H), 1.6-1.1
(m, 162H) (elemental anal. C117H198O6, calculated C 82.62, H 11.73; found C 82.32, H
11.76%).
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DOI: 10.1039/C5RA00873E
Scheme 1. Synthesis of hexaalkoxytruxenes 7a, 7b, 7c; (a) CH2(COOH)2, Pyridine, 80 ºC, 16
h; (b) 10% Pd\C, H2 gas, CH3COOEt, 60 ºC, Overnight; (c) P2O5, 85% H3PO4, 110 ºC, 6h;
(d) BBr3, CH2Cl2, -78 ºC 4h; (e) RBr, K2CO3, KI, EtOH, reflux, 12h; (f) Ts-OH.H2O,
(CH2OH)2, toluene, reflux, overnight.
3. Results and discussion
3.1 Liquid crystalline and structural properties
The thermal behaviour of these newly synthesised compounds was studied by polarising
optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction
studies. In discotic LCs, the use of branched alkyl chains to modify thermal properties has
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53-55
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been well documented.
We have previously used branched alkyl chains to reduce the
DOI: 10.1039/C5RA00873E
melting and isotropic temperatures of discotic nematic and columnar phases.54-55
The polarised optical microscope textures reveals the typical columnar mesophase textures of
discotic liquid crystalline materials. Textures obtained under POM for the compounds 7a, 7b
and 7c are depicted in Fig. 1.
Figure 1. Polarizing optical microscopy (POM) images of truxene discogens (Colh) observed
between two glass slides upon slow cooling (2 °C/min) from isotropic phase at room
temperature: pictures of a, b and c corresponds to 7a, 7b and 7c at 90 °C (crossed polarizers,
magnification X100).
3.2 Differential scanning calorimetry (DSC)
All three different truxene derivatives showed LC properties at room temperature. Upon
heating, the columnar phase of 7a compound transferred to isotropic liquid at about 117 °C.
On slow cooling texture appears at 103 °C it was stable to room temperature. The compound
7b having decyloxy periphery with methyl group branching at 3 and 7 positions cleared to
isotropic liquid phase at 213 °C. Upon slow cooling, typical fan-type of texture appeared at
about 205 °C, reflects the existence of columnar liquid crystalline phase. With the three
methyl branching in dodecyloxy chain at 3, 7 and 11 positions in the periphery, compound 7c
enters to isotropic phase on heating to 126 °C. The well-defined fan-type textures appeared at
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120 °C in POM which confirms the columnar LC phase and it was stabDleOI: t1o0.10r3o9/oCm5RA00873E
temperature. Truxene DLCs consisting of rigid big core unit usually possess high clearing
temperature.27 Interestingly, the clearing temperature of 7a having one ethyl branching at the
β-position of the hexyloxy chain was 117 °C with shorter chain than the 7b containing two
methyl branches in longer decyloxy and 7c having three methyl branching in internal longer
periphery dodecyloxy alkyl chain. Previously it was observed that the branching near the core
destroys mesomorphism due to steric hindrance.54 However, if the π-π interactions between
the cores are large and peripheral space is available, the material can preserve mesomorphism
despite the branching near the core. The DSC traces of compound 7a, 7b and 7c are shown
in Fig. 2 and thermal behaviour of truxene DLCs is presented in Table 1
7a
Heating Scan
117 C
103 C
Cooling scan
20
40
60
80
100
120
140
Temperature(C)
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DOI: 10.1039/C5RA00873E
7b
Heating scan
213 C
cooling scan
205 C
30
60
90
120
150
180
210
240
Temperature(C)
7c
Heating scan
126 C
cooling scan
120 C
40
60
80
100
120
140
Temperature(C)
Fig. 2. Differential scanning calorimetry (DSC) curves of 7a, 7b and 7c recorded at scan rate
of 10 °C min-1 (1st heating/cooling cycle).
Table 1. Phase transition temperatures (peak, °C) observed in DSC of compounds 7a, 7b and
7c. Cr = crystal; Colh = hexagonal columnar mesophase, Iso = Isotropic phase.
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Transition temperature(°C) and enthalpy changesDO(KI: 1J0/.m103Vo9iel/wCiA5nrRtiAcl0e0O8n7li3nEe
parentheses)
Compound
Heating scan
Cooling scan
Colh 117 (2.2) Iso
Colh 213 (7.0) Iso
Colh126 (2.7) Iso
Iso 103 (1.9) Colh
Iso 205 (7.0) Colh
Iso120 (4.6)Colh
7a
7b
7c
3.3 Small angle X-ray scattering (SAXS)
The liquid crystalline phases of the compounds were further studied by SAXS study. The
SAXs pattern of all compounds are recorded at 50 ºC on heating runs and the intensity vs 2θ
diffraction pattern is shown in Fig. 3. All the three compounds 7a, 7b and 7c shows the
diffraction peaks of hexagonal columnar phase in small angle region with the reciprocal ratio
1: 1/√3:1/2:1/√7. As mentioned before, compounds 7a, 7b and 7c were in LC state at room
temperature, so we focused more on results at room temperature compound (7a) with lowest
isotropic temperature as it is more useful for applications. XRD results of 7a at room
temperature shows the diffraction peaks of d spacing d1= 20.26 Å, d2= 11.68 Å, d3= 10.11 Å
and d4= 7.64 Å. In the wide angle region, they showed the relatively broad peak at higher 2θ
value corresponds to stacking of one molecular core on the other in each columns which is
typical characteristic of hexagonal columnar phase and as the chain length increases the core-
core separation is decreased which reveals that the packing is more compact in compound 7c,
theses values are 3.70 Å, 3.59 Å and 3.42 Å, respectively for 7a, 7b and 7c. The above results
confirmed that all these three hexaalkoxytruxene derivatives are self-assembled in the
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columnar hexagonal fashion in their liquid crystal phase. Crystal data and millerDiOnId: 1e0x.10e3Vs9ie/wfCoA5rRrtiAcl0e0O8n7li3nEe
all the compounds is presented in Table 2.
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DOI: 10.1039/C5RA00873E
Figure 3. Small angle X-Ray diffraction pattern of compounds (a) 7a (b) 7b and (c) 7c. (at 50
ºC from heating runs).
Table 2. Crystal data for compounds 7a, 7b and 7c.
Compounds 2θ
d spacing Colh
Miller
Alkyl-chain Core-Core
(degrees) (Å)
Parameter Indexes length (Å)
separation (Å)
4.37
7.56
8.74
11.57
3.78
6.55
7.56
20.26
11.68
10.11
7.64
1
100
110
200
210
100
110
200
4.61
3.70
7a
7b
1/√3
1/2
1/√7
1
23.40
13.50
11.68
4.78
3.59
1/√3
1/2
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DOI: 10.1039/C5RA00873E
10.04
3.25
5.64
6.55
8.57
8.8
1/√7
1
210
27.20
15.68
13.50
10.32
100
110
200
210
4.87
3.42
7c
1/√3
1/2
1/√7
3.4 Linear optical properties
The application of discotic liquid crystals in devices is strongly depend on their
optoelectronic properties. The absorption spectrum and emission spectrum for 7a, 7b and 7c
derivatives were measured. The absorption and photoluminescent (PL) spectrum of these
compounds were recorded in chloroform solution (0.1 mg/mL) as shown in Fig. 4. All these
three compounds show the absorption maximum at 238 nm, 290 nm and 317 nm. The
absorption spectra in chloroform of all compounds is similar to that previously reported
similar compounds .56 Bright strong blue fluorescence peak was observed at 408 nm, 432 nm
and 457 nm when the all three sample were excited at 317 nm (Fig. 4). Both absorption and
Photoluminescence spectrum of 7a, 7b and 7c are almost same and mirror image to each
other, so these results reveals that the difference in branching at peripheral chains in truxene
discotics has no effect on their optical properties of the liquid crystals.
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DOI: 10.1039/C5RA00873E
UV 7a
UV 7b
UV 7c
PL 7a
PL 7b
PL 7c
318
407
430
457
250
300
350
400
450
500
550
600
Wavelength (nm)
Figure 4. Normalized absorption 7a, 7b and 7c and emission spectrum of 7a, 7b and 7c
(obtained upon exited at λmax = 317 nm) (in CH2Cl2 at room temperature).
3.5 Non-linear Optical characterization
Nonlinear optical absorption studies of 7a, 7b and 7c in in the ns excitation time scale was
carried out using the open aperture Z-scan technique (Fig. 5), employing linearly polarized 5
ns Gaussian laser pulses obtained from a Q-switched frequency- doubled Nd:YAG laser,
operating in the single-shot mode. The laser beam was focused using a converging lens of
focal length 10.75 cm. Beam propagation direction is taken as the z axis, and the focal point
is taken as z = 0. The sample were taken in a 1 mm path length cuvette and had a linear
transmission of 69% at the excitation wavelength of 532 nm. The cuvette was mounted on a
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stepper motor controlled linear translational stage and was translated from –z toD+OIz: 10th.10r3o9u/Cg5hRA00873E
the focal point in successive steps, and the transmitted energy was measured at each step
using a pyro-electric energy detector (Laser Probe, RjP-735). The graph plotted between the
sample position and light transmission is known as the Z-scan curve, from which the
coefficient of nonlinear optical absorption can be extracted. All the NLO studies were taken
in molecular magnitude for the sample 7a, 7b and 7c by making samples in chloroform
solution of the concentration of 0.1 mg/mL.
Fig. 5. Representative schematic of the open aperture z-scan setup used for nonlinear optical
measurements. A laser beam is focused using a converging lens. The light energy transmitted
by the sample (taken in a 1 mm cuvette) at different positions is monitored as the sample is
translated along the z-axis, through the focal point.
Nonlinear absorption in a typical Z-scan measurement is indicated by a smooth valley
shaped curve symmetric about z=0 (focal point) position. The Z-scan curve, and nonlinear
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transmission as a function of input intensity calculated from the Z-scan curve, obtained for
DOI: 10.1039/C5RA00873E
compounds 7a, 7b and 7c are shown in Fig. 6 and 7 respectively. These figures indicate a
typical absorptive nonlinearity. To identify the exact nature of the absorptive nonlinearity, the
Z-scan data was fitted to different nonlinear transmission equations. The best fit was obtained
for a three photon absorption (3PA) type process. The net nonlinear absorption coefficient for
a 3PA process can be expressed as
2
( )
훼 푧 = 훼0 + 훾퐼
(1)
Where I is the input intensity, 훼0 is the unsaturated linear absorption coefficient and 훾 is the
3PA coefficient. The transmitted intensity for a given input intensity is calculated by
numerically solving the following propagation equation,
= −훼0I − 훾퐼3
(2)
푑퐼
⁄
푑푧′
Where 푧′ indicates the propagation distance within the sample.57 The calculated nonlinear
optical parameters are presented in Table 2.
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Figure 6. Open aperture Z-scan curves measured in (a) 7a (b) 7b and (c) 7c for D5O3I:21n0.m103,95/Cn5sRA00873E
laser pulse excitation, at laser pulse energy of 50 J. Solid curves are numerical fits to the
data points using equation (2).
Figure 7. Input intensity dependent normalized transmission curves calculated from the open
aperture Z-scan curves measured in (a) 7a (b) 7b and (c) 7c for 532nm, 5ns laser pulse
excitation, at laser pulse energy of 50 J. Solid curves are numerical fits to the data points
using equation.
Table.2 Nonlinear optical absorption coefficient and optical limiting threshold calculated for
samples7a, 7b and 7c.
휸 (x10-22
Sample
name
Input pulse
Optical
limiting
threshold
(J/cm2)
6.75
m3/W2)
energy (J)
532nm, 5ns
excitation
50
50
50
1.3
2.2
2.8
7a
7b
7c
5.4
4.8
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All three truxene discotic liquid crystal compounds (7a, 7b and 7c) exhibits large
DOI: 10.1039/C5RA00873E
three photon absorptive nonlinearity and strong optical limiting property in the ns excitation
domain at 532nm. The optical limiting threshold value (the input fluence at which the
transmission drops to 50% of the linear transmission) values calculated are presented in table
2.
It may be noted that the present intensity range of investigation (obtained by using ns
laser pulses) is not sufficient for the occurrence of a genuine 3PA process (which is a fifth
order nonlinearity) strong enough to result in the high optical limiting observed. In fact open
aperture Z-scans performed with nanosecond laser pulses often lead to dominant excited state
absorption in organic molecules. Thus, even though the obtained nonlinearity fits numerically
to a fifth order process, in reality this needs not be the actual physical process taking place.
For instance, 2PA followed by excited state absorption (ESA), which is a sequential 
process,will give nonlinear transmission curves that fit well numerically to a 3PA ()
process.58 We believe that the 3PA observed in the present sample originates from a similar
process involving excited state absorption. Among the three compounds 7c has the maximum
nonlinear absorption, which can be due to the presence of long chain group associated with
the truxene core compared to 7a and 7b. The nonlinear absorption is increases with increase
in the chain length.59
Conclusion
Three novel hexaalkoxy truxene derivatives with branched peripheral alkyl chains at different
positions have been successfully synthesized, and characterized using different instruments,
to ensure that the compounds are liquid crystalline materials at room temperature. These
complexes are found to be blue-light emissive at room temperature with rich π-electron
conjugation structures. These complexes are also found to exhibit optical limiting property,
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based on nonlinear absorption. Laser induced director axis reorientation, changes in order
DOI: 10.1039/C5RA00873E
parameter and the presence of -electrons in truxene liquid crystals give rise to the observed
large nonlinear optical response.
Acknowledgement
We thank Mrs K. N. Vasudha for her technical support in the characterization of the samples.
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