Room temperature discotic liquid crystalline triphenylene-pentaalkynylbenzene dyads as an emitter in blue OLEDs and their charge transfer complexes with ambipolar charge transport behaviour

Article abstract of DOI:10.1039/c9tc01178a

Six novel non-symmetric discotic liquid crystalline (DLC) dimers composed of pentaalkynylbenzene (PA) and triphenylene (TP) moieties were synthesized by using the CuI-Et₃N catalyzed click reaction between terminal TP alkyne and PA azide. All the compounds exhibit mesomorphic behaviour at room temperature. Pure compounds 8a-b (n = 6, 8 & m = 1) and 9a-b (n = 6, 8 & m = 3) display a columnar centered rectangular (Col_r) phase while 8c (n = 10, m = 1) and 9c (n = 10, m = 3) show a weak smectic (Sm) phase. Remarkably, the mesophase transforms from Col_r and Sm to columnar hexagonal (Col_h) on doping the pure compounds with electron-acceptor 2,4,7-trinitrofluorenone (TNF) molecules in a 2:1 TNF/compound ratio. The induction of stable Col_h mesomorphism is driven through charge-transfer interactions of electron-rich PA and TP moieties with electron-acceptor TNF molecules. The thermal behaviour of the compounds was studied through polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. The performed grazing incidence small and wide angle X-ray scattering (GISAXS/GIWAXS) study showed good alignment ability of 9a in the Col_r and complex 9a/TNF in the Col_h phase. Additional information about the arrangement of molecules in the Col_r and the Col_h 2D lattice is deduced by the construction of electron density maps (EDM). The pure non-symmetric dimers 8a-c and 9a-c were blue luminescent in the solution state and shifted to green in the solid state because of aggregation. Hence, the fabrication of doped OLED devices by using 8a and 9a as an emitter with the configuration ITO/PEDOT:PSS/host:emitter/TPBi/LiF/Al illustrates the restoration of the electroluminescence (EL) spectrum to narrow and a blue shifted photoluminescence (PL) spectrum, which indicates the suppression of aggregates. Emitters 8a and 9a are doped with three different hosts: (carbazolyl)-1,1′-biphenyl (CBP), 4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) and 2,7-bis(carbazol-9-yl)-9,9-spirobifluorene (Spiro-2CBP), by varying the dopant concentration. The best EL performance is observed for the device fabricated with 8a (5 wt% in the CBP host) and 9a (3 wt% in the CBP host) with a maximum external quantum efficiency (EQE) of 2.1% and CIE coordinates of (0.16, 0.10). One of the studied complexes 8b/TNF exhibited an ambipolar charge carrier mobility of 1.78 × 10^-3 cm² V^-1 s^-1 and 1.25 × 10^-3 cm² V^-1 s^-1 for electrons and holes, respectively, at room temperature. The observed ambipolar mobility of the complex compound reveals its potential application as an organic semiconducting material.

Full text of DOI:10.1039/c9tc01178a

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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C9TC01178A
Journal Name
ARTICLE
Room temperature Discotic Liquid Crystalline Triphenylene-
Pentalkynylbenzene dyads as an emitter in blue OLEDs and their
Charge transfer complexes with ambipolar charge transport
behaviour
Indu Bala,a Wan-Yun Yang,b Santosh Prasad Gupta,c Joydip De,a Rohit Ashok Kumar Yadav,b
Dharmendra Pratap Singh,d Deepak Kumar Dubey,b Jwo-Huei Joub, Redouane Doualid and Santanu
Kumar Pal*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel six non-symmetric discotic liquid crystalline (DLC) dimers composed of pentaalkynylbenzene (PA) and triphenylene
(TP) moieties were synthesized by using the CuI-Et3N catalyzed click reaction between terminal TP alkyne and PA azide. All
the compounds exhibit mesomorphic behaviour at room temperature. Pure compounds 8a-b (n=6, 8 & m=1) and 9a-b (n=6,
8 & m=3) displays columnar centered rectangular (Colr) phase while 8c (n=10, m=1) and 9c (n=10, m=3) shows weak smectic
(Sm) phase. Remarkably, the mesophase transforms from Colr and Sm to columnar hexagonal (Colh) on doping the pure
compounds with electron-acceptor 2,4,7-trinitrofluorenone (TNF) molecule in 2:1 TNF/compound ratio. The induction of
stable Colh mesomorphism is driven through charge-transfer interactions of electron-rich PA and TP moiety with electron-
acceptor TNF molecule. The thermal behaviour of the compounds were studied through polarised optical microscopy (POM),
differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. The performed grazing incidence small and wide
angle X-ray scattering (GISAXS/GIWAXS) study showed the good alignment ability of 9a in Colr and complex 9a/TNF in Colh
phase. Additional information about the arrangement of molecules in the Colr and Colh 2D lattice is deduced by the
construction of electron density maps (EDM). As the pure non-symmetric dimers 8a-c and 9a-c were blue luminescent in
solution state and shifted to green in solid state because of aggregation. Hence, the fabrication of doped OLED devices by
using 8a and 9a as an emitter with the configuration ITO/PEDOT:PSS/Host:Emitter/TPBi/LiF/Al, illustrate the restoration of
electroluminescence (EL) spectrum to narrow and blue shifted photoluminescence (PL) spectrum which indicates the
suppression of aggregates. The emitter 8a and 9a are doped with three different hosts: (carbazolyl)-1,1’-biphenyl (CBP),
4’,4’’-tri(N-carbazolyl)triphenylamine (TCTA) and 2,7-Bis(carbazol-9-yl)-9,9-spirobifluorene (Spiro-2CBP) by varying dopant
concentration. The best EL performance is observed for the device fabricated with 8a (5 wt% on CBP host) and 9a (3 wt% on
CBP host) with maximum external quantum efficiency (EQE) of 2.1% with CIE coordinates of (0.16, 0.10). One of the studied
complex 8b/TNF exhibited ambipolar charge carrier mobility of 1.78 × 10-3 cm2 V-1 s-1 and 1.25 × 10-3 cm2 V-1 s-1, for electron
and hole, respectively at room temperature. The observed ambipolar mobility of complex compound reveals its potential
application as organic semiconducting material.
from its self-organising features.1-6 LCs owing to the presence of
alkyl chains provides the comfort of fabrication by solution
processing or physical vapor deposition technique.6 The
Introduction
Organic light emitting diodes (OLEDs) based on a columnar (col)
liquid crystalline (LC) emitter offers the advantages of large
charge carrier mobility and self-healing from defects, benefitted
development of OLEDs opens a path for the long-term
perspective of biodegradable optoelectronic devices. For OLEDs
device application, three basic color components, i.e red, green
and blue are required for realizing white light emission for full
color display.7 In comparison to red and green emitter, the
advancenment of deep-blue emitting materials still remains a
challenge owing to their intrinsic wide band-gap and stability
issues. Multiynes are known as a blue-light emitter discotic
nematogens,8-10 so their incorporation with the non-emissive
rigid π-conjugated molecules can modify their self-assembly
a. Indian Institute of Science Education and Research (IISER) Mohali, Sector-81, SAS
Nagar, Mohali 140306, India. *E-mail: skpal@iisermohali.ac.in
b. Department of Materials Science and Engineering, National Tsing Hua University,
Hsinchu 30013, Taiwan
c. Department of Physics, Patna University, Patna- 800005.
d. Unité de Dynamique et Structure des Matériaux Moléculaires
(UDSMM), Université du Littoral Côte d’Opale (ULCO), 50 Rue Ferdinand Buisson,
62228, Calais, France.
Electronic Supplementary Information (ESI) available.
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C5H11
C5H11
DOI: 10.1039/C9TC01178A
C6H13
O
OC6H13
OC6H13
+
O
n N3
+
m
C5H11
O
C6H13O
OC6H13
C5H11
C5H11
C5H11
C5H11
C6H13
O
OC6H13
OC6H13
N
N
N
O
C5H11
O
n
m
C6H13O
OC6H13
C5H11
C5H11
Figure 1 Formation of the TP-PA non-symmetric discotic dimer.
behaviour and can induce the emissive properties in the tendency to form charge transfer complexes with multinitro
resulting new material. fluorenone electron acceptors owing to their high electron
In this context, in the literature, non-symmetric LC dimers8-11 richness.47-52 This provides an easy and efficient way to
comprising of two different mesogenic units have attracted manipulate the mesomorphic properties and also gives rise to
their own significance as it can combine the properties of both the good charge transport properties of the resulting
the individual mesogenic units into just one single component. complexes. In literature, triphenylene containing DLCs are
As a result, one can connect two different mesogenic units of known to exhibit good charge carrier conducting properties.53-
57
their own interest to form new LC materials of new interesting
properties. Triphenylene (TP) derivatives are the pillars of Till date, there are only three reports based on
discotic liquid crystals (DLCs) which mainly favours col multialkynylbenzene and TP non-symmetric oligomers which
organisation due to their rigid conjugated core.12 In contrary, were reported by our group.8-10 In one report, we have
multialkynyl benzenes are the most investigated discotic synthesized TP-PA dimer connected via flexible alkyl spacer in
nematogens in the literature.13,10 The induction of discotic between rigid ester group8 showing Colh phase at ambient
nematic (ND) phase in these systems is a result of rotational temperature while in another via simple flexible alkyl spacer
freedom provided by the ethynyl linkers which prevents exhibits ND phase at room temperature.10 In the third report,
effective col stacking of the pentayne disks.13 There are the triphenylene-multiynes-triphenylene triad connected via
numerous reports on TP dimers, oligomers and polymers14 flexible alkyl spacers possess highly ordered col plastic and Colr
while there are a few reports on pentaalkynylbenzene (PA) mesophases.9 As the nature of mesophases is highly dependent
ones.8-10,15 Conventionally, TP-containing DLC oligomers are on the type of linkage unit. So it becomes very important to
largely connected via ether, ester, amide, siloxane, or oligo explore and exploit new discotic dimers with different linkages
methylene functional groups.16-23 A variety of pentayne dimers, from a structure-property relationship point of view.
oligomers and side-chain polymers have been synthesized The formation of TP-PA dyad via triazole linkage was motivated
differing in peripheral substituents and nature of spacers which by three goals. Firstly, the col assembly of TP can be stabilized
involves alkyl, amide, oligo(ethylene oxide), perfluoroalkyl and in a better manner at room temperature via the introduction of
siloxane groups.24,25 Symmetric TP and PA dimers linked via triazole ring along with the PA moiety. Secondly, in addition to
flexible alkyl spacers show glassy structures16 and ND phases, their room temperature col behaviour, the blue light emitting
respectively. In addition, chemoselective click chemistry property can be induced to the resulting mesogen by the
approach has been widely used to produce new LC materials involvement of PA moiety and hence used as a potential emitter
with different molecular structures and physical properties. This in fabricating blue OLED devices. Thirdly, in continuation to
includes molecular propeller DLCs,26,27 gold nanoparticles,28,29 these, additional inter-disc interactions were introduced by
LC dendrimers,30 fullerenes,31 metal complexes,32 aggregation- doping the mesogens with disc-shaped electron acceptors for
induced light emitting oligomers33 and so on. A variety of TP the realization of charge transfer complexes and the ambipolar
containing DLC oligomers have been synthesized via a triazole characteristics of the complex has been revealed from the time
linkage.34-46 Both PA and TP are known to form variety of of flight mobility experiments.
mesophases in their neat as well as in complex form due to their
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DOI: 10.1039/C9TC01178A
C5H11
C5H11
C5H11
C5H11
(iii)
C5H11
C5H11
Br
Br
O
(i)
Br
Br
Br
Br
Br
Br
Br
Br
(ii)
OH
O
O
(CH2)n
Br
(CH2)n
(CH2)n
C5H11
C5H11
C5H11
C5H11
Br
N3
1
2
3
4
OC6H13
OC6H13
OC6H13
C6H13
O
O
C6H13
O
O
C6H13
O
O
OC6H13
OC6H13
(iv)
(v)
OH
m
O
OC6H13
OC6H13
C6H13
C6H13
OC6H13
C6H13
OC6H13
OC6H13
5
6
7
(vi)
C5H11
C5H11
OC6H13
N
8a:
O
n = 6; m = 1
: n = 8; m = 1
: n = 10; m = 1
: n = 6; m = 3
: n = 8; m = 3
: n = 10; m = 3
N
N
OC6H13
OC6H13
n
8b
8c
9a
9b
9c
m
C5H11
O
C6H13
O
OC6H13
8, m=1
9, m=3
C5H11
C5H11
Scheme 1. Synthesis of the target compounds 8a-c and 9a-c, Reagents and conditions: (i) K2CO3, KI, Br(CH2)nBr, acetone, reflux, 12 hrs, Yield-80 % (ii) Pd(PPh3)2Cl2, 4
pentylphenylacetylene, CuI, Et3N, 100 °C, Yield-80 % (iii) NaN3, DMF, RT, 12 hrs, Yield-99% (iv) Cat-B-Br, CH2Cl2, RT, 48 h, Yield-40%; (v) 3-bromo-1-propyne/5-chloro-1-
pentyne, Cs2CO3, KI, butanone, 80 °C, 18 h, Yield-88%; (vi) CuI, NEt3, toluene, RT, overnight, Yield for 8a-60%, 8b-64%, 8c-58 % and 9a-68%, 9b-71%, 9c- 66%.
This paper deals with the first representatives of TP and PA (electron acceptor) stabilises the mesophase and contributes
dimers modified with triazole ring in between the ether chains. towards the ambipolar nature of the resulting complexes.
The synthesis, thermal behaviour, and the mesomorphic
properties of neat compounds and their binary systems Results and discussions
attained after doping with electron acceptor 2,4,7- Synthesis and characterization
trinitrofluorenone (TNF) are discussed in detail. The compounds The synthesis of the target compounds are depicted in Scheme
were doped in different ratios with TNF such as in 1. The synthesis of non-symmetric dimers have been started
TNF:compound ratio of 1:1, 1:2, 2:1. The mesomorphic from the Williamson alkylation of the pentabromophenol as
behaviour were found to be perfectly Colh in case of 2:1 ratio of reported in earlier reports.8-10,58 Then the fivefold sonoghshira
TNF:compound while in another cases the intermediate kind of coupling of compound 2 with 4-pentylphenylacetylene results
behaviour was found which was not clear from X-ray diffraction in the formation of compound 3.
(XRD) techniques. The doped devices have been fabricated by Afterwards, the reaction of compound 3 with sodium azide
using neat compounds 8a and 9a as an emitter with the (NaN3) in DMF leads to the replacement of bromo to azido
configuration ITO/PEDOT:PSS/Host:Emitter/TPBi/LiF/Al which group by the nucleophilic attack of azide anion. Moreover, the
shows maximum EQE of 2.1 %. Additonally, the studied complex hexaalkoxytriphenylene 5, monohydroxytriphenylene 6 and
8b/TNF exhibited both electron and hole charge carrier mobility propargyl bromide and 5-chloropentayne substituted
of value 1.78 × 10-3 cm2 V-1 s-1 and 1.25 × 10-3 cm2 V-1 s-1 which triphenylenes 7, were prepared as reported previously.8, 37 Then
revealed its ambipolar nature. The donor–acceptor molecular the CuI catalysed click reaction between TP substituted terminal
interactions among discogens (electron donor) and TNF alkynes 7 and PA substituted aliphatic azides 4 leads to the
formation of target compounds 8 and 9. All the target
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Thermal Behaviour
View Article Online
DOI: 10.1039/C9TC01178A
a
b
d
The thermal behaviour of all the compounds were examined by
polarising optical microscopy (POM), differential scanning
calorimetry (DSC) and wide-angle X-ray diffraction (WAXD)
studies. All the compounds 8a-c, 9a-c in their pure form were
showing the highly fluidic birefringent textures started
appearing from the room temperature upon heating (Fig. 2)
while on cooling with the rate of 10 °C/min, the weak
birefringence appears (Fig. S16, see ESI) but only at lower
temperatures (below 15 °C, see Table 1). This was the case
examined for all the compounds. The reappearance of textures
at such lower temperatures on cooling can be possibly
explained due to the inability of the mesogens to overcome its
c
Table 1 Thermal behaviour of the pure PA-TP oligomers.
f
e
Compound
Phase transition temperaturesa,b
Heating/Cooling cycle
d
8a
8b
8c
9a
9b
9c
Colr 46.1 (25.15) Iso/ Iso 13.2 (0.04) NC
Colr 62.5 (27.32) Iso/Iso 11.5 (0.27) NC
Sm 51.8 (12.51) Iso/Iso 11.0 (0.01) Smd
Colr 53.0 (33.07) Iso/Iso 12.8 (0.03) NC
Colr 68.8 (36.52) Iso/Iso -3.0c NC
d
Sm 76.5 (23.47) Iso/Iso -14.5 (4.85) Sm
aPhase transition behaviour observed on heating (°C) and enthalpy values in
parentheses in kJ/mol; Abbreviation: Colr = centered Columnar rectangular; Sm=
Disordered smectic; Iso = Isotropic. bPhase transition observed through DSC.
cPhase transition through POM. dcorresponding diffractograms are in ESI.
Figure 2 Polarised optical micrographs (POM) of pure compounds: (a) 8a at 35.2 °C; (b)
8b at 40.2 °C; (c) 8c at 33.4 °C; (d) 9a at 37.1 °C; (e) 9b at 45.3 °C; (f) 9c at 57.4 °C, on
heating with the rate of 5 °C/min (crossed polarizers).
compounds are characterized by NMR, FT-IR and HRMS (Fig. S1-
13, see ESI)
b
a
9c
9a
ha
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
q (Å-1)
q (Å-1)
c
d
9b
b
a
0.5
1.0
1.5
2.0
q (Å-1)
Figure 3 X-Ray diffraction pattern for the pure compounds: (a) Colr phase at 25 °C for 9a; (b) Colr phase at 25 °C for 9b; (c) DLam phase for 9c; (d) Proposed molecular stacking model
for Colr phase in case of 9a and 9b and weak smectic phase in case of 9c.
4 | J. Name., 2012, 00, 1-3
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Table 2 X-ray diffraction data for pure PA-TP oligomers.
TP components are connected through the longest alkyl spacers
weakening the inter-columnar stacking exhibited the
View Article Online
DOI: 10.1039/C9TC01178A
a
b
Compound
Mesophase
Lattice
constants
(Å)
dobs
(Å)
dcal
(Å)
Miller
Indices
(hk)
11
02
20
a
b
9a
Colr at 25 °C
a = 22.27
b = 32.02
18.29 18.28
16.01 16.01
11.07 11.13
8.90
6.12
5.52
4.92
4.55
4.05
3.74
9.14
6.09
5.57
4.81
4.57
4.00
3.85
22
33
40
26
44
08
hc
d
c
9b
Colr at 25 °C
a = 22.38
b = 30.62
18.59 18.58
15.31 15.31
11
02
9.05
5.57
5.00
4.55
4.04
3.76
20.09
4.62
9.29
5.46
5.10
4.64
3.83
3.78
22
42
06
44
08
hc
f
e
9c
Sm at 25 °C
ha
ퟏ
adobs :experimental d-spacing; bdcal : calculated d-spacing by using the relation: 풅풄풂풍ퟐ
풉ퟐ
풂ퟐ
풌ퟐ
풃ퟐ
=
+
;
with h+k = even, h and k are the miller indices of the reflections
[
]
Figure 4 Polarised optical micrographs (POM) of 2:1 TNF: Compound ratio (a) 8a at 57.7
°C; (b) 8b at 93.2 °C; (c) 8c at 62 °C;(d) 9a at 89.9 °C; (e) 9b at 111.1 °C; (f) 9c at 52.2 °C
observed on cooling from isotropic phase with the rate of 5 °C/min (crossed polarizers).
corresponding to the columnar centered rectangular (Colr); a & b are the unit cell
parameters, ha & hc spacing appear due to alkyl chain-chain and core-core
correlation, respectively.
diffractograms similar for the ND phases (Fig. S15c, see ESI and
Fig. 3b). Although, from the microscopic investigation, no
representative textures of ND was found (Fig. 2c & f). Hence,
later on, upon combining the microscopic and XRD information,
the phase was best described to be weak smectic phase (Sm).
For all the hybrid PA-TP dimers 8a-c and 9a-c, the imbalanced
molecular geometrical structure suppressed their tendency to
form long range ordered col structures as indicated by the
characteristic WAXD patterns with no sharp reflection peaks.
The XRD pattern for 9a is shown in the Fig. 3a. It exhibited many
narrow peaks in small as well as in the wide angle region. The
diffraction pattern could be indexed on 2D centered rectangular
lattice (Table 2). The lattice parameter was found to be a =
22.27 Å and b = 32.02 Å. The peak indexed as (46) is also having
contribution from π-π interaction between discs.
flexibility that the molecules have gained into its isotropic state
and hence required lower temperature to reassemble. The
similar behaviour was also supported by DSC study for the pure
series of compounds 8a-c and 9a-c (Fig. S14, see ESI). For all the
pure compounds, the DSC thermogram were recorded in the
temperature range of -50°C to the respective isotropic
temperatures. In the heating cycle in DSC thermogram, the
compounds showed one peak corresponding to its mesophase
to isotropic transition and on cooling cycle weak peak
associated with low enthalpy appears at lower temperatures
(also supported by weak birefringence under POM) and
suggests some weakly ordered mesophase. The weak
birefringence becomes brighter when the sample kept at room
temperature for one day. The transition temperatures of
compounds 8a-c and 9a-c are listed in Table 1.
Table 3 Thermal behaviour of the TNF/Compound in 2:1 ratio.a,b
As from the careful microscopic studies, it was observed that
once the 8a-c and 9a-c compounds are heated to their isotropic
temperatures, it took a time of one day to reassemble properly
into columnar rectangular phase. Hence, the WAXD studies
were carried out for the assembled samples. The X-ray
diffractograms for the compounds 8a-b and 9a-b that differs
from each other in the length of the alkyl spacer connecting the
PA and TP moiety through triazole linkage were found to be
similar (Fig. S15a,b, see ESI and Fig. 3a,c). The diffractograms for
8a-b and 9a-b could be indexed on Colr lattice. However, for the
compound 8c (n=10, m=1) and 9c (n=10, m=3) in which PA and
Compound
8a/TNF
8b/TNF
8c/TNF
9a/TNF
9b/TNF
9c/TNF
Heating (°C)
Colh 70.1 Iso
Colh 132.8 Iso
Colh 83.8 Iso
Colh 108.9 Iso
Colh 135.6 Iso
Colh 73.2 Iso
Cooling (°C)
Iso 63.3 Colh
Iso 106.3 Colh
Iso 69.1 Colh
Iso 100.3 Colh
Iso 129.8 Colh
Iso 68.5 Colh
aPhase transition temperature (°C) and transition enthalpies in kJmol-1 (in
parenthesis). Abbreviation: Colh = Columnar hexagonal; Iso = Isotropic. b Phase
transition observed through POM & XRD.
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DOI: 10.1039/C9TC01178A
a
c
(10)
(10)
103
9a/TNF
9c/TNF
(002)/
hc
103
ha
1.0 ha
0.6 0.8
hc
(11)
102
0.5
1.0
1.5
2.0
0.5
1.0
1.5
2.0
q (Å-1)
q (Å-1)
b
d
104
9b/TNF
(10)
hc
ha
103
hc
0.5
1.0
1.5
2.0
q (Å-1)
Figure 5 X-Ray diffraction pattern for 2:1 ratio of TNF: compound complex: (a) Colh phase at 25 °C for 9a/TNF complex; (b) Colh phase at 25 °C for 9b/TNF complex; (c) Colh phase
at 25°C 9c/TNF complex; (d) Proposed molecular stacking model for Colh phase.
Similarly, compound 9b showed Colr phase with lattice temperatures which corresponds to columnar nematic (NC) for
parameters; a = 22.38 Å and b = 30.62 Å (Fig. 3c, Table 2).
8a-b & 9a-b and smectic for 8c & 9c and well explained in
However, compound 9c exhibited one narrow peak in small supporting information (Fig. S16 see ESI). The same Colr
angle region with spacing 20.09 Å and one broad peak in the mesophase reappears for 8a-b and 9a-b after keeping the
wide angle regime with spacing 4.62 Å, indicating the sample showing NC phase for one day.
occurrence of weak smectic phase (Fig. 3b, Table 3). As for the In conclusion, 8a-b and 9a-b show Colr phase on heating while
long spacer chain length, the disorder effect is getting on cooling back from the isotropic phase, it self-assembles into
pronounced which lead to the destabilization of the Colr phase NC at lower temperatures which turns into Colr on keeping the
and resulting into weak smectic phase. The lattice parameters sample for one day at room temperature. For 8c and 9c smectic
and indexing for 9a-c are listed in Table 2. On careful mesophase was found at heating and cooling as well.
microscopic and DSC investigations of series 8a-c and 9a-c, it On the other hand, when the compounds were doped with the
has been found that all the compounds show Colr mesophase electron acceptor molecule TNF, it shows enantiotropic
on heating while on cooling the mesophase reappears at lower mesomorphic behaviour. After mixing the compounds with TNF
6 | J. Name., 2012, 00, 1-3
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in different TNF/compound molar ratio of 1/2:1 and 1:1, the In the complex form, all the compounds 8a-c/TNF and 9a-c/TNF
View Article Online
intermediate phase behaviour of Colr and Colh was observed but were showing characteristic col texturesDoOnI:h10e.a10ti3n9g/Ca9sTCw0e11ll78aAs
convert to Colh on increasing the TNF ratio to 2. We have on cooling (Fig. 4). The phase was described to be Colh on the
described detailed microscopy (Fig. 4) and XRD studies in case basis of POM with the support of XRD technique. The XRD
of 2:1 molar ratio of TNF:compound (Fig. S17 & 5).
patterns for 2:1 TNF/9a, 9b & 9c are shown in Fig. 5. In case of
9a/TNF, the diffraction pattern showed four peaks in the small
angle region with d-spacing in ratios 1:1/
3
:1/√4:1/√7
a
b
23.0
22.5
22.0
21.5
21.0
20.5
20.0
19.5
32
30
28
26
24
22
20
18
parameter a
parameter b
corresponding to the reflections from (10), (11), (20) and (21)
planes of the Colh phase, respectively. In addition, there are two
peaks in the wide angle regime, namely, ha and hc {(002)} along
with (001) peak of the small angle region. The wide angle ha
peak attribute the correlation mainly due to the molten chains
whereas hc peak is indicative of the π-π interaction of the discs
in the column. The XRD pattern for complex 9b/TNF and
9c/TNF exhibited (10), (11), (20) and (10), (11) peaks,
corresponding to Colh phase, respectively. Similarly, the
complexes 8a-c/TNF also arranges in Colh manner (Fig. S17,
Table S2, see ESI). Variation of lattice parameter for pure
compounds 8a-c, 9a-c and their corresponding 2:1 TNF
complexes are shown in the Fig. S18 (see ESI) and Fig. 6.
In order to have the better understanding about the Colh and
Colr phase, the compounds were additionally investigated by
GIWAXS/GISAXS on aligned films. All the experiments were
carried out on an ITO-coated glass substrate. The thin films
were prepared by shearing the gummy sample along one
direction at room temperature. The morphological observation
of the thin films were observed under POM showing uniformly
oriented films along one direction (Fig. 7b & 7c). The aligned
patterns were obtained by exposing the the X-ray incident
beam perpendicular to the mechanical shearing direction (Fig.
7a).
6
7
8
9
10
6
7
8
9
10
Main spacer chain length, n
Main spacer chain length, n
Figure 6 Variation of lattice parameter for (a) pure compound 9a, 9b and 9c; (b) 2:1
TNF/compound complex: 9a/TNF, 9b/TNF and 9c/TNF.
Table 4 X-ray diffraction data for TNF/Compound in 2:1 ratio.
a
b
Compound Mesophase
Lattice
dobs
dcal
Miller
Indices
(hk)
constants (Å) (Å)
(Å)
9a/TNF
Colh at 25 °C
a = 21.34
c = 7.50
18.48 18.48 10
10.63 10.67 11
9.63
7.65
6.73
4.72
3.75
9.24 20
7.50 001
6.98 21
-
ha
3.75 hc (002)
9b/TNF
9c/TNF
Colh at 25 °C
Colh at 25 °C
a = 21.83
a = 22.70
18.96 19.08 10
11.20 11.02 11
9.54
4.84
3.74
3.32
9.54 20
Firstly, the GIWAXS of blank substrate was recorded to
substract the background peaks (Fig. 7d). The partially aligned
pattern of the compound 9a in Colr phase was obtained (Fig.
7e). In the partly aligned pattern for compound 9a in Colr phase,
the intensity maximum in small angle regime is along the
shearing direction as shown in Fig. 7e. The modelling of the
corresponding mesophases shown by pure compounds as well
as complexes are shown in Fig. 8.
ha
hc
3.18 60
19.64 19.66 10
11.39 11.35 11
4.78
3.77
ha
hc
ퟏ
adobs :experimental d-spacing; bdcal : calculated d-spacing by using the relation: 풅풄풂풍ퟐ
ퟒ
풉ퟐ + 풉풌 + 풌ퟐ
풍ퟐ
=
(
) + 풄ퟐ
;
[
]
h, k, l are the indices of the reflections corresponding to
풂ퟐ
ퟑ
the columnar hexagonal (Colh) phase; a & c are the unit cell parameters, ha & hc
spacing appear due to alkyl chain-chain and core-core correlation, respectively.
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DOI: 10.1039/C9TC01178A
a
d
e
(26)
(44)
(22)
Peak due to substrate
(02)
(11)
Leakage to
main beam
(40)
3500
b
c
3000
2500
2000
1500
1000
500
f
g
0
0
50
100 150
Azimuthal angle (Degree)
Figure 7 (a) Schematic showing the X-ray incident beam was parallel to the film normal direction and mechanical shearing direction; (b & c) POM morphological observation of thin
films used in GIWAXS experiments for 9a and 9a/TNF complex, respectively, sheared at room temperature; (d) GIWAXS of blank ITO-coated glass substrate; (e) Aligned GIWAXS X-
ray diffraction pattern for 9a in Colr phase; (f) GISAXS X-ray diffraction pattern for 9a/TNF complex in Colh phase; (g) azimuthal profiles corresponding to the Colh GIWAXS pattern
shown in “f” recorded along a circle going through the scattering maximum.
Columnar
Hexagonal
(Colh)
Colh
Weak
(Sm)
electron rich
Sm
O
NO2
O2N
O2N
Columnar
Rectangular
(Colr)
electron deficient
Colr
Figure 8 Schematic showing the self-assembly behaviour of the non-symmetric oligomer in pure state and in 2:1 TNF/Compound complex.
11
Significantly, the GIWAXS pattern also shows aligned peaks and corresponding to (10), (01) and ( ) planes of the hexagonal
the pattern is well matching with the un-oriented pattern. On (Colh) lattice (Fig. 7f).
the other hand, the aligned pattern of the complex 9a/TNF in
Colh phase showed strong Bragg peaks represented by three
lattice points in the angular region between 0° to 180°
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peaks at 30°, 90° and 150°, respectively, further confirming the
occurrence of Colh phase.
View Article Online
DOI: 10.1039/C9TC01178A
b
a
Electron Density Map (EDM)
The arrangement of molecules in their respective phases, has
been deduced by reconstructing the electron density map
(EDM) by using the information of their respective peak indexes
and intensities. The EDM for the pure compound 9a in the Colr
phase is shown in Fig. 9a and sketch of the arrangement of the
molecules of compound 9a is displayed in Fig. 9c. Moreover, the
EDM of complex 9a/TNF in the Colh phase and sketch of the
arrangement of the molecules are shown in Fig. 9b and 9d,
respectively.
c
d
Photophysical behaviour
The photophysical studies for 8a-c and 9a-c were done in
solution as well as in thin film state. The UV/Vis absorption and
fluorescence spectroscopic data of newly synthesized TP-PA
dyads in dilute chloroform solution at room temperature are
presented in Table 5. And the normalised UV/Vis and emission
spectra in CHCl3 for compound 8a is shown in Fig. 10a. All the
compounds showed similar absorption and emission behaviour
as they only differ in the length of flexible alkyl spacer which
usually have no effect on the photophysical properties until or
unless it does not affect the assembly of the molecules (Fig. S19
& S20, see ESI). All the compounds were showing blue tinge in
dilute chloroform solution even under daylight and proper blue
Figure 9 Electron density maps of (a) 9a in centered Colr phase; (b) complex 9a/TNF in
Colh phase. Red color represents the high electron density region and deep blue is the
lowest. Rectangle in “Fig. a” represents the unit cell of Colr phase with lattice parameter
a & b and hexagon in “Fig. b” displayed the conventional unit cell of Colh phase with
lattice parameter, a; (c) & (d) Arrangement of molecules in the Colr and Colh phase,
respectively.
The columnar LC phase has planar orientation as indicated by
intense meridional reflections with high in-plane alignment of
the columns (normal to z) but random orientation in the x,y
plane.59 Moreover, the azimuthal plot (Fig. 7g) showed three
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a
c
View Article Online
DOI: 10.1039/C9TC01178A
10000
Prompt/IRF
Experimental decay
Bi-exponential fit
Absorption
Emission
1.0
8000
6000
4000
2000
0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
0
5
10
15
20
25
Decay time (ns)
Wavelength (nm)
d
b
Absorption
Emission
1.0
0.8
0.6
0.4
0.2
0.0
225
300
375
450
525
600
Wavelength (nm)
Figure 10 (a) Absorption (red) and Emission (blue) spectra of compound 8a in HPLC chloroform solution; (b) Absorption (red) and Emission (green) spectra of compound 8a in thin
film; (c) Fluorescence decay curve of compound 8a in chloroform solution fitted by bi-exponential fitting; (d) Photomicrographs showing green (preparing by drop cast) and blue
light emission in thin film and solution (DCM) state, respectively.
light emission under UV light (Fig. 10d). Absorption spectra of solution state. While the number of emission peaks varies for all
compound 8a displayed absorption maxima mainly centred at the derivatives. In addition, in comparison to solution state, the
280 and 337 nm with four shoulder peaks at 261, 270, 381 & absorption bands in solid state are broad and emission peaks
420 nm. The photophysical studies were also performed in neat become broader and red shifted because of aggregation (Fig.
form for all the compounds 8a-c and 9a-c. In solid state 10b and S21, see ESI). In neat film, the compounds were
absorption spectrum, the number and position of the peaks showing green fluorescence (Fig. 10d).
were observed at around the same wavelength as that of in
Table 5 Photophysical properties of the PA-TP oligomers.
a
a,c
c
d
e
Compound
Asolution
Esolution
Asolid
Peaks (nm)
Esolid
Ʈ1(α1)
(ns)
2.40 (0.16)
Ʈ2(α2)
(ns)
4.41 (0.84)
Ʈav
(ns)
φF
Peaks (nm)
261, 270, 280
337,b 382, 419
261, 270, 280
337,b 382, 420
261, 270, 280
337,b 382, 419
261, 270, 280
337,b 382, 419
261, 270, 280
337,b 382, 419
261, 270, 280
337, b 381, 420
Peaks (nm)
Peaks (nm)
454, 513
8a
8b
8c
9a
9b
9c
446, 470
261, 270, 280
335,b 384, 425
262, 271, 279
336,b 384, 423
263, 270, 279
335,b 381, 423
262, 270, 280
338,b 381, 422
262, 270, 280
338,b 381, 422
261, 268, 280
340,b 384, 425
4.10
4.28
4.17
4.23
4.16
4.23
0.46
0.47
0.44
0.35
0.41
0.43
446, 470
446, 470
446, 470
446, 470
446, 471
401, 461,
510
402, 452,
515
2.20 (0.10)
1.02 (0.10)
2.16 (0.10)
2.12 (0.11)
2.19 (0.11)
4.52 (0.90)
4.52 (0.90)
4.45 (0.89)
4.41 (0.89)
4.48 (0.89)
523
455, 515
453, 515
ain micromolar chloroform solution. b λmax c obtained after exciting at their corresponding λmax. Asolution: Absorbance in solution sate, Esolution: Emission in solution state.
.
Asolid: Absorbance in thin film, Esolid: Emission in thin film. dƮav= average lifetime. eφF = Fluorescence quantum yield.
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DOI: 10.1039/C9TC01178A
c
a
d
6.0x10-5
4.0x10-5
2.0x10-5
0.0
ELUMO
-2.0x10-5
-2
-1
0
1
2
3
Potential (V)
b
1.8x10-4
1.2x10-4
6.0x10-5
0.0
EHOMO
-6.0x10-5
-1.2x10-4
-2
-1
0
2
3
Potential (1V)
Figure 11 (a & b) Cyclic voltammograms, (c & d) HOMO and LUMO frontier molecular orbitals for 8a and 9a, respectively.
The absorption and emission peaks for all the TP-PA oligomers Beside this, quantum yield measurements (Fig. S22, see ESI)
in solution and thin film state are listed in Table 5. were done in THF by taking quinine sulfate as a standard as its
To obtain further insight into the fluorescence behaviour of the absorption maxima (λmax) centred at around same wavelength
newly synthesized series, fluorescence decay and lifetime (λmax= 340 nm) as that of our target compounds (λmax= 337nm).
measurement studies were carried out in chloroform solutions The quantum yield values lied in the range of 0.35-0.47 for the
using a time correlated single photon counting technique. The series of compounds. The electronic energy levels of the chosen
excitation wavelength is 337 nm corresponding to the TP-PA oligomers 8a and 9a used to fabricate OLED devices were
absorption maxima of the compounds and a band-pass filter at estimated by using cyclic voltammetry (CV) technique.
447 nm is used to measure the fluorescence decay curves The CV experiments were carried out by using millimolar
shown in Fig. 10c for compound 8a. The decay curves (red solutions of 8a and 9a in oxygen free dichloromethane (Fig. 11a
coloured) were fitted with the Instrument response function & b). 0.1 M solution of TBAH was used as a supporting
(IRF) (black coloured). The fitted (blue) curve is overlapping with electrolyte. The experimental setup conditions for CV
the decay curve (red). In all the cases, the curves were better experiments and electrochemical data for 8a and 9a are listed
fitted by using biexponential function. This implies two radiative in Table 6. Both the derivatives show reversible oxidation and
decay processes occurring from the excited states and giving reduction potential. The HOMO and LUMO energy levels for 8a
two lifetime decay values Ʈ1 and Ʈ2 (Fig. S23, see ESI). The and 9a are found to be -5.36 and -5.43 eV, while the LUMO
average lifetime varied from 4.16-4.28 ns for all the compounds energy levels are -3.68 and -3.52 eV, respectively (Table 6). The
(Table 5).
electrochemical band gap was calculated as, ∆Eg,CV = ELUMO-
EHOMO and found to be 1.69 and 1.90 eV for 8a and 9a,
respectively. The optical band gap (∆EUV) was obtained by using
the red edge absorption wavelength of UV-vis spectrum and is
estimated to be 2.72 eV for both 8a and 9a oligomers. The
electrochemical band gaps (∆Eg,CV) were found to be lesser than
its optical band gap (∆EUV).
Further, the energy minimized structures for chosen compound
8a and 9a were obtained by using Density functional theory
(DFT) calculations using Gaussian 09 software with B3LYP/6-
31G (d,p). The frontier molecular orbitals of 8a and 9a oligomers
are shown in Fig. 11c & d and HOMO of the compound is
primarily located on the electron-rich TP while the LUMO is
distributed on the PA moiety.
Electrochemical properties and DFT calculations
Table 6 Electrochemical dataa
g
C
E1red (V)b
E1oxd (V)c
ELUMO (eV)d EHOMO
(eV)e
∆ECV
(eV)f
1.69
1.90
∆EUV
(eV)
2.72
2.72
8a
8b
-0.61
-0.77
+1.08
+1.14
-3.68
-3.52
-5.36
-5.43
arecorded in micromolar DCM solution. Experimental conditions: Ag/AgNO3 as a
reference electrode, glassy carbon working electrode, platinum wire counter
electrode, and TBAH (0.1m) as a supporting electrolyte. bonset reduction potential.
conset oxidation potential. dEstimated by using the formula: ELUMO = -[4.8-E1/2,Fc/Fc
+
+ E1red] eV. eEstimated by using the formula: EHOMO = -[4.8-E1/2,Fc/Fc+ + E1oxd] eV,
where E1/2,Fc/Fc+ = 0.55 V. fCalculated from the formula: ∆Eg,CV = ELUMO-EHOMO
gCalculated from the formula: ∆EUV = 1240/λonset. Abbreviation: C- Compound.
.
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DOI: 10.1039/C9TC01178A
Device for 8a
Device for 9a
a
c
d
b
(120 nm)
(120 nm)
Al
Al
LiF
LiF
(1 nm)
(30 nm)
(1 nm)
(30 nm)
TPBi
TPBi
(20 nm)
(35 nm)
(20 nm)
(35 nm)
HOST: 8a Emitter (1/3/5)
PEDOT:PSS
ITO
HOST: 9a Emitter (1/3/5)
PEDOT:PSS
ITO
(125 nm)
(125 nm)
Glass substrate
Glass substrate
Figure 12 Schematic diagram of the energy-levels of the solution-processed deep-blue OLED devices composed with three different hosts Spiro-2CBP, TCTA and CBP: (a & b) Device
for 8a, (b & d) Device for 9a.
and CBP. The voltage-luminance, voltage-current density,
luminance-power efficiency, and EL spectra plots are presented
Electroluminescent properties
in Figure 13-14 and the EL characteristics are summarized in
Table 7. For the doped devices, first the emitters are doped in
CBP host at different concentrations (1, 3 and 5 wt%). CBP owes
To fully evaluate the electroluminescence (EL) characteristics of
the newly synthesized deep-blue emitters 8a and 9a initially
non-doped
ITO/PEDOT:PSS/Emitter/TPBi/LiF/Al
devices
with
are
structure
fabricated by
to its favorable alignment of energy levels with those of the
emitters, as this could facilitate host-to-guest energy transfer
and the uniform dispersion of the dopant into the host. Doped
devices illustrate the restoration of EL spectrum to narrow and
blue shifted photoluminescence spectrum which indicates the
suppression of aggregates (Fig. 13d & 14d). The fabricated
devices of compound 8a (1 wt%) and 9a (1 and 3 wt%),
possessed a small hump around 392 nm (Fig. 13d and 14d). It
might be because of the CBP host emission and indicates the
incomplete photoexcitation energy transfer from CBP host to
dopant. This is evident from the decreasing intensity of hump at
employing the dyes as emitting layer Since the non-doped
devices exhibited unsatisfactory EL performance due to the
poor thin film stability and formation of aggregates in solid
state, we fabricated doped devices with the configuration
ITO/PEDOT:PSS/Host:Emitter/TPBi/LiF/Al,
to
mitigate
aggregation caused emission quenching and thereby improve
the EL performances. Figure 12 illustrates the schematic
energy-level diagram of the solution-processed deep-blue OLED
devices composed with three different hosts Spiro-2CBP, TCTA
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103
102
101
View Article Online
Dopant: 8a wt%
Dopant: 8a wt%
(e)
1
3
5
100
1
3
5
100
DOI: 10.1039/C9TC01178A
8a emitter:CBP
100
10
1
(a)
(b)
8
2
3
7
8
9
4 Vo5ltag6e (V)
2
3
4
5
6
7
9
Voltage (V)
101
Dopant: 8a wt%
550
Dopant : 8a wt%
1.0
0.8
0.6
0.4
0.2
0.0
1
3
5
1
3
5
100
100
100
10-1
10-2
10-3
(c)
(d)
720
10
100
1000
400
480
560
640
2)
Wavelength (nm)
Luminance (cd/m
Figure 13 Dopant (Emitter 8a) concentration effect on (a) luminance, (b) current density, (c) power efficiency and (d) electroluminescent (EL) spectra of the solution-processed deep-
blue OLED devices using CBP host with 1, 3, and 5 wt.% dopant concentrations. (e) CIE coordinates obtained for devices at 5 wt.% dye concentration with CBP, Spiro-2CBP and TCTA
host. Inset “e” shows the image of the fabricated device by using 8a emitter doped (5 wt%) with CBP host.
higher dopant concentrations (5 wt%) as the number of electrons from TPBi to emitter. The doped devices exhibited
emitting molecules to confine excitons from CBP host high operating voltage (> 4.7 V at 100 cd m-2) that may arise due
increased. The doped devices exhibited high luminance when to the poor blending of the emitter in CBP host which resulted
compared to non-doped devices of analogous dyes (Fig. 13 & in poor-film stability.
14). It might be due to the effective capture of excitons formed Generally, the doped devices showed superior performance
in the CBP host by dopants via favourable energy transfer.
compared to the non-doped emitting devices. This clearly
A large current density was observed for compound 8a based indicates the role of balanced charge transport at low dopant
devices at low drive voltage when compared to the counter concentration and confinement of recombination well across
device. This may be associated with the imbalance in the charge the CBP layer.60-62 The poor performance of non-doped and
transport and leakage of charges at the terminal electrodes. doped devices with low concentration may be attributed to the
This could arise due to the facile injection of holes from increased crystallinity which could affect the film integrity.
PEDOT:PSS into the emitting layer and poor injection of
Table 7 Effect of doping concentration on the operation voltage (OV), power efficiency (PE), current efficiency (CE), external quantum efficiency (EQE), CIE coordinates, and maximum
luminance of solution-processed deep-blue OLED devices with host CBP, TCTA and Spiro-2CBP.
b
C
Emitter
8a
Host
CBP
Emitter
Concentration
Operation
PEmax/CEmax/EQEmax
(lm W-1/cd A-1/%)
PE100/CE100/EQE100
CIE
Maximum
Luminance
(cd/m2)
648
733
625
10
1289
voltagea (V)
(lm W-1/cd A-1/%)
coordinatesd
(wt %)
1
3
5
100
5
4.7
5.2
5.4
-
4.0
5.1
4.9
1.2/1.6/1.7
0.9/1.4/1.7
1.2/2.0/2.1
- / - / -
1.8/2.1/1.2
1.3/1.7/1.8
0.7/1.0/2.1
1.0/1.4/1.5
0.8/1.3/1.5
0.9/1.6/1.7
- / - / -
1.6/2.0/1.1
0.9/1.4/1.5
0.5/0.7/1.5
(0.17, 0.11)
(0.16, 0.09)
(0.16, 0.10)
-/-
(0.19, 0.23)
(0.16, 0.10)
(0.17, 0.09)
TCTA
Spiro-2CBP
5
1
694
427
9a
CBP
3
5
100
3
5.2
6.1
-
5.2
4.9
1.2/1.8/2.1
0.9/1.6/1.7
- / - / -
0.2/0.7/0.6
1.8/2.4/1.6
0.9/1.4/1.9
0.6/1.2/1.3
- / - / -
0.2/0.7/0.6
0.8/1.3/1.3
(0.16, 0.10)
(0.16, 0.10)
-/-
(0.18, 0.18)
(0.17, 0.13)
481
566
19
2414
750
TCTA
Spiro-2CBP
3
aOperation voltage (the voltage at 100 cd/m2). bPower efficiency (PE), Current efficiency (CE) and EQE at maximum, cPower efficiency (PE), Current efficiency (CE) and
EQE at 100 cd/m2, dCIE coordinates at 100 cd/m2.
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Journal of Materials Chemistry C
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ARTICLE
View Article Online
DOI: 10.1039/C9TC01178A
103
103
102
101
100
(e)
Dopant: 9a wt%
Dopant: 9a wt%
1
3
5
100
9a emitter:CBP
1
3
5
100
102
101
(b)
(a)
3
4
5
6
7
8
9
10
3
4
5
6
7
8
9
10
Voltage (V)
Voltage (V)
101
100
Dopant : 9a wt%
Dopant: 9a wt%
1.0
0.8
0.6
0.4
0.2
0.0
1
3
5
100
1
3
5
100
10-1
10-2
10-3
(d)
(c)
10
100
1000
2)
400
480
560
640
720
Wavelength (nm)
Luminance (cd/m
Figure 14 Dopant (Emitter 9a) concentration effect on (a) luminance, (b) current density, (c) power efficiency and (d) electroluminescent (EL) spectra of the solution-processed deep-
blue OLED devices using CBP host with 1, 3, and 5 wt. % dopant concentration. (e) CIE coordinates obtained for device at 3 wt.% dye concentration with CBP, Spiro-2CBP and TCTA
host. Inset “e” shows the image of the fabricated device by using 9a emitter doped (3 wt %) with CBP host.
The dopant concentration and host material increased as a
result of increasing number of emitting species. Furthermore,
the EL performance of the emitters is optimized by employing
different host materials such as TCTA and Spiro-2CBP at their
best dopant concentration (Table 7). Among the different hosts,
the energy level of CBP host is well aligned with the emitters for
facile charge injection. As a result, the device containing CBP
host showed superior EL performance compared to the other
devices containing different host material.
Charge Carrier Mobility for the complex
The introduced donor-acceptor interactions among discogen
and TNF molecule in the complexes prompted us to measure
their charge transport properties. The charge carrier mobility
measurement for the complex 8b/TNF was carried out by using
the time of flight (ToF) technique. To execute ToF
measurement, the complex was filled in a commercially
purchased homeotropic anchored ITO cell (from AWAT, Poland)
of thickness 5 μm. The sample was excited by a Nd:YAG pulsed
laser having excitation wavelength of 355 nm and 5 ns pulse
width. The measurement was performed in the cooling cycle
o
started from its isotropic temperature with a rate of 2 C/min.
The filling of sample cell was conducted in N2 atmosphere in
order to avoid the recombination of electrons with the space
charges and/or oxygen molecules. To measure the ambipolar
charge mobility, bipolar voltage of ±10V was applied with the
help of Keithley 6487. The induced charge displacement was
observed via transient photocurrent curve which was recorded
by a digital oscilloscope (Agilent, DSO 1012A) connected to a
voltage amplifier. The output transient photocurrent curve was
used to calculate the ambipolar charge carrier mobility.
Figure 15 (a) Ambipolar dispersive transient photocurrent curves of
compound complex, recorded at ±10 V, whereas (b) shows the electron and
hole mobility curves as a function of temperature. Error bars represent the
maximum error of ± 2% in the experimental data.
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Journal of Materials Chemistry C
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The investigated complex 8b/TNF consists of both electron rich
and electron deficient moieties; therefore ambipolar charge
mobility was measured at optimized voltage of ± 10V.
Conflicts of interest
“There are no conflicts to declare”.
View Article Online
DOI: 10.1039/C9TC01178A
We obtained dispersive transient photocurrent curves under
the application of applied bipolar voltage that is attributed to
the positive and negative charge carriers. The transit time (Tr)
of charge carriers was obtained by the photocurrent curves and
the mobility (μ) was calculated using the formula
Acknowledgements
We are thankful to NMR, HRMS and the SAXS/WAXS facility at
IISER Mohali. I. Bala acknowledges CSIR-NET
(09/947(0061/2015-EMR-1) for fellowship. Dr. SK Pal thanks to
CSIR (02(0311)/17/EMR-II) for financial support.
  d 2 /T .V
where Tr is the transient time obtained by
r
photocurrent curves, V is the applied voltage and d is the
thickness of ITO cell. The photocurrent curves and
electron/hole mobility as a function of temperature are shown
in Fig. 15a and 15b, respectively. It is evident from Fig. 15b that
the electron and hole mobilities are almost temperature
invariant which is usually observed in the Colh phase of liquid
crystalline materials. At room temperature (30 oC), DLC
exhibited the average ambipolar charge carrier mobility of 1.78
× 10-3 cm2 V-1 s-1 and 1.25 × 10-3 cm2 V-1 s-1, for the electron and
hole, respectively. We also noticed that the electron mobility is
higher than that of hole mobility. Hence, the additionaly
introduced donor-acceptor interactions among the electron
donor discogens (8a-c and 9a-c) and electron acceptor TNF
molecules stabilises the mesophase and resulting in ambipolar
beahaviour of the complexes.
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16 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C9TC01178A
8a & 9a
CIE (0.16, 0.10)
OLEDs
2.0
1.9
1.8
1.7
TNF
Hole mobility
Electron mobility
Colh (complex)
(Pure comp.) Colr
(26)
(22)
(44)
1.3
1.2
1.1
(02)
(11)
(40)
20
40
60
Temperature ( C)
80
o
100
Novel room temperature discotic liquid crystal dyads for Blue OLEDs
and their Charge transfer complexes exhibiting ambipolar charge
transport behaviour