Thermochromic and highly tunable color emitting bis-tolane based liquid crystal materials for temperature sensing devices

Article abstract of DOI:10.1016/j.dyepig.2021.109272

The luminescent compounds based on the D-π-A system consisting of substituted pyrrole as acceptor, π conjugated bis-tolane and -OMe, -N(CH₃)₂, -N(Ph)₂ at termini acting as donors have been synthesized. On heating, all the compounds exhibited distinct mesomorphic behavior depending upon donating groups and substituents at the acceptor unit. These luminescent liquid crystals exhibited emission from blue, green to reddish orange with high quantum efficiency in solid as well in solutions state. We discovered the rarely reported three membered ring interactions between the Br at alpha position of pyrrole and acetylene bond and their impact on luminescence efficiency via single crystal structure of 2Br–CN–N(CH₃)₂ and 2Br–CN–N(Ph)₂. Furthermore, the thermochromic behavior of these LLCs compounds has been explored, especially CN–N(Ph)₂, its reddish orange emission changed to blue at 240 °C–250 °C in pure state, while in dimethyl silicone polymer (PDMS) film, it switched to blue at 200 °C and completely vanished at 240 °C. These simple and highly emissive compounds display outstanding thermochromic properties above 200 °C, thus will be potentiality valuable for high temperature sensing devices.

Full text of DOI:10.1016/j.dyepig.2021.109272

Dyes and Pigments 190 (2021) 109272
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Thermochromic and highly tunable color emitting bis-tolane based liquid
crystal materials for temperature sensing devices
,
,b,*
Majeed Irfana, Idrees Sumraa, Mei Zhanga, Zicun Songa, Tingting Liua **, Zhuo Zenga
a School of Chemistry, South China Normal University, Guangzhou, China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry Chinese Academy of Sciences, Shanghai, 200032, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The luminescent compounds based on the D-π-A system consisting of substituted pyrrole as acceptor, π conju-
Luminescent liquid crystals
Thermochromic
gated bis-tolane and -OMe, -N(CH3)2, -N(Ph)2 at termini acting as donors have been synthesized. On heating, all
the compounds exhibited distinct mesomorphic behavior depending upon donating groups and substituents at
the acceptor unit. These luminescent liquid crystals exhibited emission from blue, green to reddish orange with
high quantum efficiency in solid as well in solutions state. We discovered the rarely reported three membered
ring interactions between the Br at alpha position of pyrrole and acetylene bond and their impact on lumines-
cence efficiency via single crystal structure of 2Br–CN–N(CH3)2 and 2Br–CN–N(Ph)2. Furthermore, the ther-
mochromic behavior of these LLCs compounds has been explored, especially CN–N(Ph)2, its reddish orange
emission changed to blue at 240 ◦C–250 ◦C in pure state, while in dimethyl silicone polymer (PDMS) film, it
switched to blue at 200 ◦C and completely vanished at 240 ◦C. These simple and highly emissive compounds
display outstanding thermochromic properties above 200 ◦C, thus will be potentiality valuable for high tem-
perature sensing devices.
Fluorescence characteristics •
1. Introduction
In 2008, Keto’s group described the change of phase from cubic to
columnar of pyrene derivative LC, along with the change in emitting
Luminescent liquid crystals (LLCs) have numerous potential appli-
cations in optoelectronics [1,2] and biomaterials [3–5] due to their
unique anisotropic liquid crystalline behavior along with the photo-
physical characteristics. The LLCs materials might increase the effi-
ciency of the optoelectronic devices by enhancing their features like
contrast, brightness and viewing angle [6].
color from yellow to blue green at 160 ◦C [13]. They also reported the
couple of liquid crystal phases with emissive color transition from yel-
low to blue of 2,6-diethynylanthracene derivative on cooling the
isotropic phase [14]. In 2012, the Park group reported (2Z,2′Z)-2,2′-(1,
4-phenylene) bis(3-(3,4,5-tris(dodecyloxy)phenyl)acrylonitrile), GDCS,
which has no emission in iso-tropic phase, while, on cooling to LC and
crystalline state, it exhibited evident green and yellow emission
respectively. They also elaborated the role of cyano groups to assist the
formation of hexagonal columnar liquid crystal phase at room temper-
ature, which in turn cause the aggregation-induced enhanced emission
(AIEE) [15].
Generally, luminescent liquid crystals are usually constructed by
connecting the bis-tolane, tolane or some long molecular segment with
the fluorophore. However, the phenomenon of aggregation caused
quenching (ACQ) has remained a serious challenge for their applica-
tions. So, the development of suitable design, adhering to efficient
luminescence became significant field of research. In order to address
these challenges, LC mesogens were incorporated to the periphery of
AIEs [7,8] to construct highly emissive materials in solid state with
liquid crystals characteristics [9–12]. LLCs can be used to develop effi-
cient stimuli responsive materials, as they have tendency to be easily
converted into mesophase, so remarkable changes can take place in
luminescence intensity or emission color with the phase transition.
Considerable progress has been made for LLCs compounds based on
the thermochromism and mechanochromism [16–22]. Most of the LLCs
exhibited blue to yellow emissive color [23–26] and to get the red color
LLC materials has become focused area of research due to their utiliza-
tion in sensor and display technology [27,28]. Ionic liquid crystals
consisted of thienoviologen fluorophore exhibited the luminescence at
640 nm in nematic columnar phase were reported by Veltri et al. [29]
* Corresponding author. School of Chemistry, South China Normal University, Guangzhou, China.
** Corresponding author.
E-mail addresses: liutt5@163.com (T. Liu), zhuoz@scnu.edu.cn (Z. Zeng).
https://doi.org/10.1016/j.dyepig.2021.109272
Received 14 February 2021; Received in revised form 24 February 2021; Accepted 25 February 2021
Available online 9 March 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
Furthermore, electrochromic thienoviologen based ionic LLCs having
variable alkyl chain length exhibiting red emission at 630 and 670 nm in
columnar and smectic phases respectively have been reported [30,31].
Recently, nitro-substituted pyrrolopyrrole derivative (TPPP-NO2) LCs
material having thermochromic and mechano-chromic characteristics
has been published. This material started to display red color emission at
135 ◦C (LC phase), while quenching at 185 ◦C (isotropic phase) under
the turn-on and turn-off mechanism [32].
4-((4-ethynylphenyl)ethynyl)-N,N-diphenylaniline (3c) to generate
corresponding luminescent liquid crystals CN–OMe, CN–N(CH3)2,
CN–N(Ph)2, 2Br–CN–OMe, 2Br–CN–N(CH3)2, 2Br–CN–N(Ph)2,
2F–CN–OMe, 2F–CN–N(CH3)2 and 2F–CN–N(Ph)2 in 90–96% yield.
3. Photophysical properties
3.1. UV and PL spectra
According to best of our knowledge, until now the published work
about thermoresponsive LLC materials indicated the sensing of tem-
perature up to or below to 200 ◦C. We put an effort to use purely organic
luminescent liquid crystals compounds for sensing of high temperature
up to 240 ◦C or more with excellent luminescence efficiency. In this
work, we synthesized simple and efficient bis-tolane based luminescent
liquid crystals compounds, which exhibited emission from blue to
We studied the absorption and emission properties of the LLCs in
DCM (10ꢀ 5 mol/L) as well as in solid state. The spectroscopic data is
given in Fig. 1 and summarized photophysical data is given in Table 1.
The UV–Vis spectra of the LLCs exhibited the two absorption bands in
max
DCM which is due to the D-
π-A system in all the compounds. λ
of
abs
CN–OMe, CN–N(CH3)2, CN–N(Ph)2, 2Br–CN–OMe, 2Br–CN–N(CH3)2,
2Br–CN–N(Ph)2, 2F–CN–OMe, 2F–CN–N(CH3)2 and 2F–CN–N(Ph)2
are 333, 368, 376, 333, 371, 378, 334, 371 and 378 nm respectively as
shown in Table 1. It is evident that there was no significant difference of
λmabasx within the compound series containing either -OMe, -N(CH3)2 or
-N(Ph)2 donating groups, while marked difference of about 35–37 nm
and 43–44 nm was observed, when -OMe group was changed by -N
(CH3)2 and -N(Ph)2 respectively, which is bathochromic shift due to
strong electron donating effect of N-dimethylamino and N-diphenyla-
mino groups.
reddish-orange. Our designed molecules are based on D-π-A system,
where pyrrole (with or without halogens) containing the –CN group act
as withdrawing unit and -OMe, -N(CH3)2 and -N(Ph)2 at terminal are
electron donating groups (Scheme 1).
2. Synthesis
Synthesis of these LLCs is clearly descripted in Scheme 1. Details of
the synthesis of the materials are given in the ESI. Compounds CHO–I
was synthesized through the methods reported in literature in 75% yield
[33]. 2F–CHO–I was synthesized from the reported compound 3,
4-difluoro-1-(4-iodophenyl)-1H-pyrrole using Vilsmeier-Haack reaction
in 90% yield [34]. Compounds CHO–I and 2F–CHO–I were transformed
into the corresponding oxime, further dehydrated with acetic anhydride
to give the 1-(4-iodophenyl)-1H-pyrrole-2-carbonitrile (CN–I) and 3,
4-difluoro-1-(4-iodophenyl)-1H-pyrrole-2-carbonitrile (2F–CN–I) [35].
The 1-(4-iodophenyl)-1H-pyrrole-2-carbonitrile (CN–I) was converted
to 4,5-dibromo-1-(4-iodophenyl)-1H-pyrrole-2-carbonitrile (2Br–CN–I)
using N-bromosuccinimide in DMF at 0 ◦C to room temperature. These
three (CN–I, 2F–CN–I and 2Br–CN–I) intermediates were coupled with
1-ethynyl-4-((4-methoxyphenyl)ethynyl)benzene (1c), 4-((4-ethy-
The λmemax values in solution form of these LLCs as shown in Table 1.
max
There was a slight difference within λ
values of compounds con-
em
taining -OMe and -N(Ph)2 respectively, while in compounds comprising
of -N(CH3)2, it was more pronounced, about 8–11 nm due to variable
positions and electronic cloud of halogens in geometry. Again, the -N
(CH3)2 and -N(Ph)2 groups containing LLCs exhibited remarkable
bathochromic shift of about 73–87 nm from their respective methoxy
group containing compounds. According to the emission spectra in the
solid state, a regular red shift pattern of 6 nm has been observed in -OMe
containing compounds from CN–OMe to 2F–CN–OMe while variable
trend has observed in -N(CH3)2 containing LLCs. The halogen contain-
ing 2Br–CN–N(CH3)2 and 2F–CN–N(CH3)2 exhibited the λbem at 478 nm
and 501 nm while CN–N(CH3)2 exhibited λbem at 511 nm. The -N(Ph)2
nylphenyl)ethynyl)-N,N-diphenylani
-line
(2c)
and
Scheme 1. Synthesis of Bis-tolane based luminescent liquid crystals (i) 1) NH2OH.HCl, 2) (AcO)2O. (ii) NBS, DMF 0 ◦C, rt. (iii) Pd (PPh3)2Cl2, CuI, Et3N, CH3N,
85 ◦C. (iv) Pd (PPh3)2Cl2, CuI, Et3N, THF, rt. (v) 1) TMS, Pd (PPh3)2Cl2, CuI, Et3N, THF, rt. 2) K2CO3, MeOH, rt.
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M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
Fig. 1. a) Normalized absorption, b) Emission spectra in DCM solution (1 × 10ꢀ 5 mol/L). c) Photo-luminescence spectra in solid state, Photographs taken under UV
illumination d) CN–OMe, e) CN–N(CH3)2, f) CN–N(Ph)2, g) 2Br–CN–OMe, h) 2Br–CN–N(CH3)2, i) 2Br–CN–N(Ph)2, j) 2F–CN–OMe, k) 2F–CN–N(CH3)2 and l)
2F–CN–N(Ph)2.
in case of 2Br–CN–N(CH3)2. The CN–N(Ph)2 exhibited the orange-red
Table 1
emission, having 627 nm λbem value, which might be due to difference
Photophysical data of luminescent liquid crystals compounds.
in packing and freely available π-electrons of bis-tolane moiety which
τb(ns)
a
a
a
b
b
Compounds
λabs
(nm)
λem
(nm)
ΦF
(%)
λem
(nm)
ΦF
made an excellent D- π-A system as compared to respective halogenated
(%)
LLCs, which displayed emission in green to yellow range.
These LLCs have exhibited excellent quantum efficiency in solution
as well as in the solid state. The fluorescence quantum yield in the so-
lution ΦaF (%) and solid state ΦF b (%) are shown in Table 1. In solutions,
quantum efficiency of LLCs containing -N(CH3)2 and -N(Ph)2 is higher
due to strong donating effect and relatively strained structure than the
compounds with the -OMe donating group. The highest ΦaF (%) was
exhibited by 2F.
CN–OMe
333
368
376
333
371
407
487
480
411
498
76
89
90
37
80
455
511
627
461
478
32
37
40
6
3.60
5.04
12.7
3.32
3.63
CN–N(CH3)2
CN–N(Ph)2
2Br–CN–OMe
2Br–CN–N
(CH3)2
11
2Br–CN–N(Ph)2
2F–CN–OMe
2F–CN–N
378
334
371
490
409
495
89
68
90
545
467
501
35
38
25
5.60
4.07
3.09
-CN -N(Ph)2 (ΦaF ¼ 92%) and 2Br–CN–OMe exhibited the lowest
(CH3)2
b
(ΦaF ¼ 37%). We measured the absolute quantum yield ΦF in cali-
2F–CN–N(Ph)2
378
488
92
493
36
6.77
brated integrating sphere system in solid state. The non-halogenated
CN–OMe, CN–N(CH3)2 and fluorinated 2F–CN–OMe, 2F–CN–N(CH3)2
exhibited good quantum yield, 25%–38%, while brominated,
2Br–CN–OMe and 2Br–CN–N(CH3)2 displayed lower ΦF b, 6% and 11%
respectively in powder. All the compounds with the terminal -N(Ph)2
group exhibited remarkable luminescence efficiency, 35%–40%, which
might be due to non-planner structure of terminal diphenyl amino
a
b
In DCM (1 × 10ꢀ 5 mol/L).
Emission data and Absolute quantum yield in solid state.
group containing LLCs exhibited blue shift when we move from non-
halogenated to halogenated LLCs. The halogens exerted the with-
drawing effect, but they might have possible interactions with the
π
-electron of acetylene bonds between the molecules due to that reason,
groups to avoid the
π-π stacking or severe halogen interactions with the π
blue shift has been observed in these LLCs and this effect is more evident
electrons of bis-tolane system. It is evident from quantum yield data in
3

M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
solution and solid state that these luminescent compounds exhibited
higher quantum efficiency in solution as compared to solid state due to
intermolecular interactions which lower down the luminescence
summarized in Table 2.
From the heat-weight loss curves, the thermal decomposition tem-
peratures of all compounds are above 300 ◦C, which shows good thermal
stability.
efficiency.
max
Further, we examined the solvatochromic effect on the λmabasx and λ
According to the DSC curves, all the compounds exhibited liquid
crystals phases on heating and the compounds with the -N(CH3)2 ter-
minal group have higher temperature mesophases as compared to
compounds containing -OMe as terminal group. The polarizing optical
microscopy (POM) shows that the CN–OMe displayed the Smectic A
mesophases, while the bromo and fluoro with the -OMe terminal group
exhibited the schlieren nematic texture. The nonhalogenated and
dibromo compounds with -N(CH3)2 exhibited texture of Smectic B,
while their counterpart, difluoro LLC displayed mosaic nematic phase.
Interestingly, Smectic A mesophases was observed in case of CN–N
(Ph)2, while the 2Br–CN–N(Ph)2 and 2F–CN–N(Ph)2 displayed the
em
of each compound. In λmabasx of LLCs compound containing -OMe donating
group, slight blue shift (3–6 nm) has been observed, while the -N(CH3)2
and -N(Ph)2 containing luminescent compounds exhibited slight blue
shift (4–5 nm) and (6–7 nm) respectively, when we move from toluene
to polar acetonitrile solvent.
max
The λ
values measured in non-polar (toluene) to polar (ACN)
em
solvents have shown irregularity in the pattern. All the -OMe and -N
(Ph)2 containing LLCs, first exhibited red shift when we move from
toluene to DCM, then slight blue shift up to THF and then again red shift
has been observed. The overall red shift was not remarkable in case of
-OMe, while compounds containing -N(Ph)2 group displayed red shift of
82–90 nm from toluene to acetonitrile. There was evident bathochromic
effect of about 93–104 nm was observed in case of -N(CH3)2 donating
group containing compounds when we move from toluene up to
acetonitrile and the highest red shit of about 104 nm was observed in
case of 2Br–CN–N(CH3)2. The absorption and emission spectra in
different solvents are shown in Figs. S1 and S2 respectively. (See sup-
porting information).
Table 2
Thermal behavior of the LLCs compounds.
Compound
Transition temperatures/◦Ca
Td/◦Cb
CN–OMe
Cr 196 SmA 204
Cr 239 SmB 244
Cr 241 SmA 250
Cr 204 N 230
Iso
Iso
Iso
Iso
Iso
Iso
Iso
Iso
Iso
349
347
520
356
357
450
340
341
330
CN–N(CH3)2
CN–N(Ph)2
2Br–CN–OMe
2Br–CN–N(CH3)2
2Br–CN–N(Ph)2
2F–CN–OMe
2F–CN–N(CH3)2
2F–CN–N(Ph)2
Cr 272 SmB 284
Cr 172 SmB 180
Cr 187 N 193
3.2. Thermotropic liquid crystalline property
Cr 220 N 226
These bis-tolane based luminescent compounds were able to self-
assemble into LC phase upon thermal treatment. The mesophase prop-
erties of LLCs were studied by differential scanning calorimetry (DSC)
and polarizing optical microscopic (see Fig. 2). The related curves are
listed in Figs. S3–S8, corresponding transition temperatures and thermal
decom -position temperatures (Td) were calculated by TGA analysis
Cr 191 SmB 198
a
Transition temperature were determined by DSC (peak temperature, first
heating scan, 10 K min ꢀ 1) and confirmed by POM. Cr = crystalline solid, N =
nematic, SmA = Smectic A, SmB = Smectic B, iso = isotropic liquid state.
b
Decomposition temperature.
Fig. 2. POM texture images of compound a) CN–OMe, b) CN–N(CH3)2, c) CN–N(Ph)2, d) 2Br–CN–OMe, e) 2Br–CN–N(CH3)2, f) 2Br–CN–N(CH3)2, g) 2F–CN–OMe,
h) 2F–CN–N(CH3)2 and i) 2F–CN–N(Ph)2.
4

M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
Smectic B phase. Furthermore, the temperature ranges of mesophase in
nonhalogenated CN–OMe, CN–N(CH3)2 and fluorinated 2F–CN–OMe,
2F–CN–N(CH3)2 is 6–8 ◦C, while in case of brominated 2Br–CN–OMe
and 2Br–CN–N(CH3)2, it is 26 ◦C and 12 ◦C respectively. In these cases,
addition of bromine significantly increased the transition temperature
ranges and thermal stability. The LLCs containing the -N(Ph)2 terminal
group exhibited the variable behavior and they did not display any
separate mesophase peak in DSC. The fluorinated 2F–CN–N(Ph)2 and
brominated 2Br–CN–N(Ph)2 compounds have lower mesophase tem-
perature, melting point and stability as compared to CN–N(Ph)2.
3.4. Thermoresponsive behavior of CN–N(Ph)2 in PDMS film
As the LC compound CN–N(Ph)2 exhibited the thermochromic
behavior, we prepared temperature sensitive film using the Sylgrad 184
(supporting information) and the PL graphs and images of thermores-
ponsive behavior are given in Fig. 4 and S13 (Supporting information).
The CN–N(Ph)2 in polydimethyl siloxane film exhibited the λmemax value at
622 nm and comparatively different behavior from pure compound.
According to Fig. 4 (a, b), while heating from 30 ◦C to 240 ◦C, there was
no appreciable change in the PL intensity and wavelength up to 90 ◦C.
When we further increased the temperature, the luminescence intensity
decreased rapidly with blue shift from 622 nm to 611 nm in λmemax up to
3.3. Thermoresponsive behavior of compound CN–N(Ph)2
170 ◦C. Upon further heating up to 190 ◦C, the λemmax shifted to the 442
max
As already mentioned, that the LLCs compounds can be excellent
stimulus responsive material and, in this work, we explored the ther-
moresponsive behavior of bis-tolane based LLCs compounds. Generally,
these types of linear compounds display variations in emission intensity
rather than remarkable mechanochromism or thermochomism. Ac-
cording to the PL intensity graphs data in Fig. 3 and S12 (Supporting
information), the LLCs comprising of -N(CH3)2 group exhibited green to
yellow emissions which converted into the blue emission in their LC
phase on heating and same is the case with the 2Br–CN–N(Ph)2 and
2F–CN–N(Ph)2. We selected the CN–N(Ph)2 LLCs compound as it
emitted the orange-red color with broad range of emission and ther-
modynamically quite stable. The PL intensity regularly decrease with
nm with the blue emission. The λ
almost reached to violet region
em
(437 nm) with maximum PL intensity and again started to decrease and
completely vanished at 240 ◦C.
3.5. Crystal structure
As already mentioned that these LLCs exhibited excellent fluores-
cence features in solid as well as in solvents but quantum yield in DCM
solvent is higher as compared to powder form, so to further decipher the
solid-state emission behavior, we scrutinized the packing pattern of
2Br–CN–N(CH3)2 (CCDC 2041597) and 2Br–CN–N(Ph)2 (CCDC
2057432). The suitable crystals for 2Br–CN–N(CH3)2 and 2Br–CN–N
(Ph)2 were obtained by slow diffusion of hexane into chloroform, while
after several attempts unfortunately we were not successful to get
appropriate crystal of CN–N(CH3)2 and CN–N(Ph)2. The 2Br–CN–N
(CH3)2 and 2Br–CN–N(Ph)2 both were crystallized out in triclinic sys-
tem with P-1 space group. In the 2Br–CN–N(CH3)2 dihedral angle be-
tween pyrrole and aryl ring is 67.16◦ (Fig. 5a) while in case of
2Br–CN–N(Ph)2, it was reduced to 59.12◦ (Fig. 6a) and unit cells of both
crystals contain two molecules. (Figs. 5b and 6b). As revealed by the
crystallographic data, the bromine at the alpha position of the pyrrole
and terminal groups has evident effect in the determination of the
packing and luminescence characteristics. There are mainly two types of
interactions has been observed in the relatively close packing 2Br–CN–N
the rise of temperature from 40 ◦C to 200 ◦C with the slight blue shift in
max
the λ
from 627 nm to 605.6 nm due to change in intermolecular
em
interactions. The emission with 509.6 nm became dominant when we
move from 200 ◦C to 210 ◦C, which is the green emission. The compound
max
started to exhibit λ
= 486 nm at 240 ◦C and become completely blue
em
at isotropic phase temperature, 250 ◦C having 478 nm λemmax. There was a
slight change of 4 nm in wavelength while moving from 241 ◦C to
250 ◦C, which was the temperature nearly between the liquid crystal
phase and isotropic phase with respect to DSC data. Further, the emis-
sion was disappeared when we raised the respect temperature up to
251 ◦C.
Fig. 3. Thermoresponsive behavior of CN–N(Ph)2 a) Temperature dependent PL Intensity graph b) Normalized PL graph c) Images of compounds during heating
process from 30 ◦C to 250 ◦C under Uv-light.
5

M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
Fig. 4. Thermoresponsive behavior of CN–N(Ph)2 in PDMS film a) Temperature dependent PL Intensity graph b) Normalized PL graph c) Images of thin film during
heating process from 40 ◦C to 240 ◦C under Uv-light.
Fig. 5. Crystal structures. 2Br–CN–N(CH3)2 (CCDC 2041597) a) Top view (dihedral angle), c) side view, and b) packing in unit cell.
Fig. 6. Crystal structure. 2Br–CN–N(Ph)2 (CCDC 2057432) a) Top view (dihedral angle), c) side view, and b) packing in unit cell.
(CH3)2 I) The Br at the alpha position of every single molecule interact
with acetylene bond of neighboring molecule, so two such interactions
occurred between two molecules. II) The hydrogen on aromatic ring of
one molecule interact with carbon of aromatic ring and again two such
interactions existed between two molecules. The interaction of Br with
acetylene bond is like three membered. The 2Br–CN–N(Ph)2 exhibited
6

M. Irfan et al.
Dyes and Pigments 190 (2021) 109272
several intermolecular interactions (Fig. 6b) including the hydrogen
bonding between the C–H … .Br and C–H… N in the two-dimensional
head to tail packing. In this case Br at the alpha position of one mole-
cule not only formed three-membered ring with the acetylenic bond of
another molecule, it also interacted with the electrons of the aromatic
ring adjacent to the acetylene bond. Unlike to the 2Br–CN–N(CH3)2, It
looked like five membered rings containing three triangles and there
was only one such interaction existed between two molecules. The Br at
beta position of one molecule also interacted with the N of the cyano
group of other molecules in head to head parallel interactions. In the
2Br–CN–N(Ph)2, the terminal non-planar diphenyl group also exhibited
weak intermolecular interactions and played significant role in rela-
tively slipped packing which was minor in case of dimethylamino in
2Br–CN–N(CH3)2. The molecules of 2Br–CN–N(Ph)2 have relatively
more bent geometry than the 2Br–CN–N(CH3)2, rather the linear in
typical head to tail packing. So, the excessive weak intermolecular
forces, the relatively strained and bent structure made the 2Br–CN–N
(Ph)2 highly emissive as compared to 2Br–CN–N(CH3)2. Additionally,
the dislocated two-dimensional head to tail lose packing avoided the
Table 3
HOMO, LUMO, Dipole Moment (D) and ΔE values of LLCs.
Compounds
HOMO (eV)
LUMO (eV)
Dipole Moment (D)
Eg (eV)
CN–OMe
ꢀ 5.65
ꢀ 5.13
ꢀ 5.24
ꢀ 5.73
ꢀ 5.19
ꢀ 5.28
ꢀ 5.76
ꢀ 5.21
ꢀ 5.29
ꢀ 2.08
ꢀ 1.93
ꢀ 2.10
ꢀ 2.19
ꢀ 2.05
ꢀ 2.21
ꢀ 2.28
ꢀ 2.15
ꢀ 2.29
4.641871
7.541696
5.393223
5.576457
8.570052
5.796434
8.771031
11.234129
8.555128
3.57
3.20
3.14
3.54
3.14
3.07
3.48
3.07
3.00
CN–N(CH3)2
CN–N(Ph)2
2Br–CN–OMe
2Br–CN–N(CH3)2
2Br–CN–N(Ph)2
2F–CN–OMe
2F–CN–N(CH3)2
2F–CN–N(Ph)2
2Br–CN–N(NH3)2 and 2Br–CN–N(Ph)2 carring the three membered
type of alpha Br bonding interaction with acetylene electrons exhibiting
different quantum efficiency have been firstly explored. The CN–N(Ph)2
exhibited excellent thermochromic behavior while heating near to LC
phase from 240 ◦C to 250 ◦C in pure state. The thin film of PDMS mixed
with CN–N(Ph)2 displayed evident color switching from orange to
greenish blue at 190 ◦C, which turn to completely blue at 200 ◦C and
turn off phenomenon at 240 ◦C. These compounds display excellent high
temperature luminescence & mesophase behavior and have potential to
be used in temperature sensor devices as these thermochromic changes
are visible under naked eye.
significant
solid state.
π-π stacking ensured the high fluorescence quantum yield in
3.6. Theoretical calculations
Majeed Irfan: Investigation, Resources, Methodology, Data curation,
Writing – original draft, Formal analysis, Sumra Idrees: Resources, Mei
Zhang: Resources, Zicun Song: Resources, Tingting Liu: Project admin-
istration, Funding acquisition, Supervision. Zhuo Zeng: Project admin-
istration, Funding acquisition, Supervision.
The structural optimization and frequency operations were con-
ducted under density functional theory (DFT) with Gaussian 09 software
by using the parameters B3LYP/6-311G in gas phase for better under-
standing of electrochemical properties. As shown in Fig. S14 (supporting
information), the electronic density in HOMOs of all compounds is
mainly localized in the bis-tolane tail, specifically in cases compounds
comprising the -N(CH3)2 and -N(Ph)2 donating group at terminal. The
LUMOs of these LLCs exhibited almost uniform distribution of electron
density over the whole molecules, except the 2Br–CN–OMe and
2Br–CN–N(CH3)2, where the electron density is mainly contained on the
bis-tolane central core. There is appreciable effect on the dipole moment
(D) and Eg (eV) values of these LLCs has been observed by varying the
weak H to strong F electronegative atoms on the pyrrole along with the
CN group as well as from weak -OMe to strong electron donating ter-
minal group -N(CH3)2. As shown in Table 3, dipole moment value of
CN–OMe and CN–N(Ph)2 is 4.641871(D), 5.393223 (D) respectively
while their counterpart CN–N(CH3)2 has dipole moment 7.541696 (D),
so introduction of strong electron-donor at the terminal increase the
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
The authors gratefully acknowledge the support of Science and
Technology
Planning
Project
of
Guangdong
Province
(2017A010103017), National Natural Science Foundation of China
(51703069, 21272080), Special Innovation Projects of Common Uni-
versities in Guangdong Province (20178S0182).
dipole moment value in each case on this D-π-A bis-tolane conjugated
system. Likewise, 2F–CN–N(CH3)2 exhibited 11.234129 (D), highest
value of dipole moment due to most electronegativity fluorine atoms on
pyrrole ring as withdrawing groups. The ΔE (eV) values decreases when
we move from weak to strong electron-donor at terminal in each case as
well as from weak to strong electronegative substituents at pyrrole ring.
The LLCs with -N(Ph)2 donor terminal group exhibited the lowest
HOMO LUMO energy gap difference among their counter parts which
might be one of reason for their better quantum efficiency.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109272.
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