Synthesis and properties of hole-transporting triphenylamine-derived dendritic compounds

Article abstract of DOI:10.1016/j.dyepig.2014.12.013

Materials based on triphenylamino core linked with different alkoxyphenyl-substituents through olefinic spacers were synthesised, and their thermal, photophysical and photoelectrical properties were investigated. The synthesized compounds showed relativel

Full text of DOI:10.1016/j.dyepig.2014.12.013

Dyes and Pigments 115 (2015) 135e142
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of hole-transporting triphenylamine-derived
dendritic compounds
,
Renji R. Reghu a, Jurate Simokaitiene a, Juozas V. Grazulevicius a *, Steponas Raisys b,
Karolis Kazlauskas b, Saulius Jursenas b, Vygintas Jankauskas c, Antonio Reina d
a Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania
b Institute of Applied Research, Vilnius University, Sauletekio 9-III, LT-10222, Vilnius, Lithuania
c Department of Solid State Electronics, Vilnius University, Sauletekio al. 9, LT-10222, Vilnius, Lithuania
d
ꢀ
Departament de Química Analítica i Química Organica, Universitat Rovira i Virgili, Carrer Marcel·lí Domingo s/n, E-43007, Tarragona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Materials based on triphenylamino core linked with different alkoxyphenyl-substituents through olefinic
spacers were synthesised, and their thermal, photophysical and photoelectrical properties were inves-
tigated. The synthesized compounds showed relatively good thermal stability. Derivatives containing
shorter alkyl chains displayed glass-forming ability, while the compounds containing flexible alkyl
termini demonstrated liquid-crystalline behaviour. Flexible alkyl substituents at the periphery of tri-
phenylamine derivatives prevented the close packing of chromophores and hence, they showed rela-
tively similar absorption and fluorescence in thin-films and solutions. Reduction in fluorescence
quantum yield in thin-films upon decreasing the size of peripheral alkyl substituents confirmed
enhanced intermolecular interactions and in turn, suggested migration-induced exciton quenching at
nonradiative decay sites. Ionization potentials of thin solid layers estimated by photoelectron emission
spectroscopy ranged from 5.21 to 5.47 eV. Hole-drift mobility of ethyl substituted triphenylamine
Received 15 October 2014
Received in revised form
7 December 2014
Accepted 12 December 2014
Available online 19 December 2014
Keywords:
Glass-transition
Fluorescence
Photophysics
Ionization potential
Hole-mobility
Triphenylamine
measured using xerographic time-of-flight (XTOF) method was 1.3 ꢀ 10ꢁ3 cm2
V
ꢁ1 sꢁ1 at an electric field
of 106 Vcmꢁ1
.
© 2014 Elsevier Ltd. All rights reserved.
1. Introduction
research interests in recent years because of their high hole-
mobility, stable redox characteristics, electron-donating ability
and/or interesting luminescence characteristics [13e18]. TPA is
highly electron rich and possesses a propeller-like structure, which
enables its derivatives to transport holes and to be solution pro-
cessable. However, exploitation of low-molar-mass TPA com-
pounds in cost-effective device fabrications is often limited because
of their poor film forming property upon solution processing
[19,20]. Hence, these materials must be deposited in optoelectronic
devices by vacuum evaporation method, which is not beneficial for
the commercial applications. Strategic design of electroactive
compounds in the form of dendritic or star-shaped structures
embedding flexible linkages represent a very convenient way to
shift the processing technique in cost-effective way without much
altering the intended decisive properties for application in organic
electronics [21e24]. Though, TPA-based solution processable
compounds possessing superior charge-transporting [23,25e27]
and light harvesting/emitting properties [21,22,24] have been re-
ported, they are still of interest in terms of competent and/or
combined advanced properties. In this article, we report on the
In the last decades, the research and development of sustainable
and renewable energy sources based on organic semiconductors
like organic light emitting diodes (OLEDs), organic photovoltaic
devices (OPVs) or organic field-effect transistors increased in
greater scale due to the quest new technological approaches as well
as new materials [1e5]. Solution processable dendritic materials
are particularly interesting as charge-transporting and/or light-
emitting materials in OLEDs and as light-harvesting antennae in
OPVs [6e9].
Triphenylamine (TPA) based compounds are extensively inves-
tigated as hole-transporting organic semiconductors for organic
electronics since they can form stable aminium radical cations upon
oxidations [10e12]. TPA derived low-molar-mass, dendritic or hy-
per branched and polymeric materials have attracted enormous
* Corresponding author. Tel.: þ37037 300193; fax: þ37037 300152.
E-mail address: juozas.grazulevicius@ktu.lt (J.V. Grazulevicius).
http://dx.doi.org/10.1016/j.dyepig.2014.12.013
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

136
R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
synthesis, thermal, photophysical and photoelectrical properties of
a series of dendritic compounds containing triphenylamino group
as a central core and electron-donating alkoxyphenyl-substituents
as arms linked through olefinic spacers.
H, 11.14; N, 0.68%. MS (MALDI-TOF) m/z
mass ¼ 1956.62).
¼
1956.46 (exact
2.2.3. Tris(4-(3,4,5-triethoxystyryl)phenyl)amine (13)
Yield ¼ 31% (0.18 g); Yellow crystals (FW ¼ 948.22 g/mol); m p.:
2. Experimental section
127e129 ꢂC. 1H NMR (300 MHz, CDCl3),
d
(ppm): 7.41 (d, J ¼ 2.9 Hz,
6H), 7.11 (d, J ¼ 2.9 Hz, 6H), 6.93 (s, 6H), 6.71 (s, 6H), 4.16e4.04 (m,
18H, OCH2), 1.45 (t, J ¼ 4.7 Hz, 18H, CH3), 1.37 (t, J ¼ 4.7 Hz, 9H, CH3).
2.1. Materials
13C NMR (75.5 MHz, CDCl3,
d ppm): 153.1, 146.5, 137.9, 132.8, 132.1,
The starting compounds, i.e., triphenylamine, methyl-3,4,5-
trihydroxybenzoate and diethyl-4-methoxy benzyl phosphonate,
were purchased from Sigma Aldrich and used as received. The re-
agents and the required materials, i.e., phosphoryl chloride,
dimethyl formamide, 18-crown-6, iodoethane, 1-bromododecane,
1-bromodecane, lithium aluminium hydride (2 M solution in THF),
phosphorous tribromide, triphenyl phosphine, potassium hydrox-
ide, potassium carbonate, sodium sulphate, were also purchased
from Sigma Aldrich and used as received.
127.5, 127.2, 127.1, 124.2,105.2, 68.9, 64.7,15.6,15.0. IR (KBr, y
cmꢁ1):
(aliphatic CeH) 2948, 2919, 2856; (Ar C]C) 1596, 1516; (CeN
stretch) 1238; (CeO stretch) 1114. Anal. Calc. for C60H69NO9: C,
76.00; H, 7.33; N, 1.48; O, 15.19%. Found: C, 75.88; H, 7.20; N, 1.52%.
MS (EI) m/z ¼ 949 [M þ H]þ.
2.3. Tris(4-(4-methoxystyryl)phenyl)amine (14)
Diethyl-4-methoxy benzyl phosphonate (0.48 g, 1.88 mmol) and
tris(4-formylphenyl)amine (0.2 g, 0.6 mmol) were dissolved in
30 mL of dry THF and cooled to 0 ꢂC. Potassium ter-butoxide (0.4 g,
3.6 mmol) was added into the solution. It was stirred at 0 ꢂC for 1 h
under nitrogen. Then, it was allowed to warm to room temperature
and stirred for additional 4 h under nitrogen. Water was added to
the reaction mixture followed by ethyl acetate, and the organic
layer was separated. The organic phase was washed with brine and
dried over sodium sulphate before the solvent was evaporated. The
crude product was purified by silica gel column chromatography
using hexane/ethyl acetate (8/2) solvent mixture as an eluent.
Yield ¼ 72% (0.28 g); Yellow crystals (FW ¼ 641.82 g/mol); m p.:
2.2. General procedure for wittig reaction
5-(Bromomethyl)-1,2,3-tris (alkyloxy)benzene, 7,
8 and 9,
(1.88 mmol) was dissolved in 35 mL of DMF and triphenyl phos-
phine (0.49 g, 1.88 mmol) was added into the obtained solution. It
was refluxed for 12 h under nitrogen. DMF was evaporated at the
reduced pressure and the crude product obtained was dissolved in
40 mL of dry THF. Then, the reaction mixture was cooled down to
0
ꢂC. Potassium ter-butoxide (0.4 g, 3.6 mmol) followed by tris(4-
formylphenyl)amine (0.2 g, 0.6 mmol) were added under nitro-
gen. The reaction mixture was stirred at 0 ꢂC for 1hr and then,
allowed to warm to room temperature. It was further stirred for 6 h
at room temperature under nitrogen. Water was added to the re-
action mixture followed by ethyl acetate, and the organic layer was
separated. The organic phase was washed with brine and dried over
sodium sulphate before the solvent was evaporated. The crude
product was purified by silica gel column chromatography using
solvent mixtures of hexane/ethyl acetate (97/3) for 11, (95/5) for 12
and (8/2) for 13 as eluents. Compound 13 was further purified by
recrystallisation from methanol.
199e201 ꢂC. 1H NMR (300 MHz, CDCl3,
d
ppm): 7.45 (d, J ¼ 2.9 Hz,
6H), 7.39 (d, J ¼ 2.9 Hz, 6H), 7.10 (d, J ¼ 2.9 Hz, 6H), 6.96e6.87 (m,
12H), 3.82 (s, 9H, OCH3). 13C NMR (75.5 MHz, CDCl3,
ppm): 159.1,
146.4, 132.4, 130.3, 127.5, 127.1, 126.9, 126.0, 124.2, 114.1, 55.3. IR
(KBr,
cmꢁ1): (arene CeH) 3024; (aliphatic CeH) 2951, 2927, 2851;
d
y
(Ar C]C) 1605, 1511; (CeN stretch) 1249; (CeO stretch) 1173. Anal.
Calc. for C45H39NO3: C, 84.21; H, 6.12; N, 2.18; O, 7.48%. Found: C,
84.64; H, 5.89; N, 2.28%. MS (EI) m/z ¼ 642 [M þ H]þ.
2.4. Instrumentation
2.2.1. Tris(4-(3,4,5-tris(dodecyloxy)styryl)phenyl)amine (11)
Nuclear magnetic resonance spectra were obtained in deuter-
ated chloroform with a Varian Unity Inova spectrometer operating
at 300 MHz and 75.5 MHz for 1H and 13C nuclei respectively. All the
Yield ¼ 35% (0.46 g); Yellow solid (FW ¼ 2210.66 g/mol). 1H
NMR (300 MHz, CDCl3),
d
(ppm): 7.40 (d, J ¼ 2.9 Hz, 6H), 7.10 (d,
J ¼ 2.9 Hz, 6H), 6.92 (s, 6H), 6.69e6.35 (m, 6H), 4.04e3.82 (m, 18H,
OCH2), 1.86e1.70 (m, 18H, CH2), 1.50e1.26 (m, 162H, CH2), 0.87 (t,
data are given as chemical shifts d (ppm) downfield from TMS. IR-
spectroscopy measurements were performed on a Perkin Elmer
Spectrum GX spectrophotometer, using KBr pellets. Mass spectra of
intermediates and low-molar mass target materials were obtained
on a Waters ZQ 2000 spectrometer. The mass spectra of higher-
molecular-weight compounds were determined by MALDI-TOF
using Shimadzu Biotech Axima Performance system and 2, 5-
dihydroxybenzoic acid (DHB) as the matrix under reflector mode
of operation.
J ¼ 4.4 Hz, 27H, CH3). 13C NMR (75.5 MHz, CDCl3,
d ppm): 153.2,
152.7, 146.6, 132.6, 132.2, 127.7, 127.3, 124.2, 107.5, 105.1, 73.4, 69.0,
31.9, 30.3, 29.7, 29.7, 29.7, 29.6, 29.4, 29.4, 29.3, 29.3, 26.1, 26.1, 22.7,
21.7,14.1. IR (KBr, y
cmꢁ1): (aliphatic CeH) 2955, 2919, 2849; (Ar C]
C) 1589, 1511; (CeN stretch) 1240; (CeO stretch) 1122. Anal. Calc.
for C150H249NO9: C 81.50; H, 11.35; N, 0.63; O, 6.51%. Found: C,
81.84; H, 11.79; N, 0.79%. MS (MALDI-TOF) m/z ¼ 2208.38 (exact
mass ¼ 2208.91).
Differential scanning calorimetry (DSC) measurements were
performed on a Perkin Elmer Pyriꢁs1Diamond DSC apparatus at a
heating/cooling rate of 10 ꢂC min under nitrogen atmosphere.
Thermogravimetric analysis (TGA) was executed on TA Instruments
2.2.2. Tris(4-(3,4,5-tris(decyloxy)styryl)phenyl)amine (12)
Yield ¼ 50% (0.58 g); Yellow semi-solid (FW ¼ 1958.17 g/mol). 1H
NMR (300 MHz, CDCl3),
d
(ppm): 7.41e6.33 (m, 6H), 7.03e6.87 (m,
Q100 under nitrogen atmosphere at a heating rate of 20 ꢂC minꢁ1
.
6H), 6.69e6.35 (m, 12H), 4.04e3.89 (m, 18H, OCH2), 1.87e1.69 (m,
18H, CH2), 1.51e1.27 (m, 126H, CH2), 0.88 (t, J ¼ 4.4 Hz, 27H, CH3).
The textures of the mesophases were observed with a polarising
microscope (AXIOLAB Zeiss) equipped with a LINKAM THMS 600
hot stage connected to a TP-92 temperature control unit.
13C NMR (75.5 MHz, CDCl3,
d ppm): 153.2, 152.8, 146.5, 132.8, 132.2,
127.6, 127.2, 124.2, 107.5, 105.0, 73.4, 69.1, 69.0, 68.8, 31.9, 31.9, 30.3,
29.7, 29.6, 29.6, 29.5, 29.4, 29.4, 29.3, 26.1, 26.1, 22.6, 21.7, 14.1. IR
X-ray diffraction (XRD) patterns were recorded on a Siemens
D5000 diffractometer (Bragg-Brentano parafocusing geometry)
with vertical qeq goniometer, and Anton-Paar TTK low temperature
(KBr,
1507; (CeN stretch) 1235; (CeO stretch) 1117. Anal. Calc. for
132H213NO9: C, 80.97; H, 10.96; N, 0.72; O, 7.35%. Found: C, 80.55;
y
cmꢁ1): (aliphatic CeH) 2954, 2924, 2853; (Ar C]C) 1589,
chamber. The goniometer was equipped with a curved graphite
C
diffracted-beam monochromator, receiving (0.03ꢂ), incident and

R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
137
diffracted-beam Soller slits. The detector was
a
scintillation
grade tetrahydrofuran (THF) at 1 ꢀ 10ꢁ5 M concentration. The neat
films of the investigated compounds were prepared from the
1 ꢀ 10ꢁ3 M THF solutions on the glass substrates by drop-casting
technique. Fluorescence quantum yield (FF) of the samples was
estimated by utilizing integrating sphere (Sphere Optics) coupled
to the CCD spectrometer using an optical fibre. Fluorescence tran-
sients were recorded using time-correlated single photon counting
system PicoHarp 300 (PicoQuant GmbH). Pulsed excitation at
1 MHz repetition rate was provided by the picosecond diode laser
with the pulse duration of 70 ps and the emission wavelength of
375 nm.
counter. The angular 2
q
diffraction range was between 1 and 40ꢂ.
Sample was placed on a low background Si(510) plate, which later
was placed on the temperature chamber sample holder. The data
were collected with an angular (0.03ꢂ) step with 3 s per step. The X-
ray diffractometer operated at 40 kV and 30 mA to generate Cu K
radiation.
Absorption spectra of the dilute solutions were recorded by UV-
Vis-NIR spectrophotometer Lambda 950 (Perkin Elmer). Photo-
luminescence (PL) of the solutions and thin films was excited by
365-nm wavelength light emitting diode (Nichia NSHU590-B) and
measured using back-thinned CCD spectrometer (Hamamatsu
PMA-11). For these measurements dilute solutions of the investi-
gated compounds were prepared by dissolving them in spectral
a
The ionization potentials (Ip) were measured by photoelectron
spectroscopy in air as described earlier [28]. The materials were
dissolved in chloroform and coated onto Al plates pre-coated with
O
O
O
O
HO
R-Br / R-I, K2CO3
LiAlH4
18-crown-6 (cat)
60oC, 36 h
OH
0o
THF
C
- RT, 8h
HO
O
O
O
O
R
R
R
R
OH
Acetone
O
R
O
R
1, 2, 3
4, 5, 6
PBr3
RT, 6h
Ether
O
Br
O
O
P
O
+
+
O
O
R
R
O
R
O
O
O
10
7, 8, 9
i) PPh3,DMF, 120 oC, 12h
ii) t-BuOK,THF, RT, 8h
t-BuOK
THF, RT, 6h
O
O
R
O
R
R
O
O
N
14
1, 4, 7, 11 ; R = C12H25
2, 5, 8, 12 ; R = C10H21
3, 6, 9, 13 ; R = C2H5
N
O
R
O
R
R
O
O
11-13
O
R
O
R
O
R
Scheme 1. Synthetic route for the preparation of dendritic compounds 11e14.

138
R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
~0.5
m
m thick methylmethacrylate and methacrylic acid copolymer
curves of compounds 11 and 14 are shown in Fig. 1. The data are
summarized in Table 1.
(MKM) adhesive layer. The function of MKM layer is not only to
improve adhesion, but also to eliminate the electron photoemission
from Al layer. In addition, this layer is conductive enough to avoid
charge accumulation on it during the measurements.
Hole-drift mobility of compound 13 was estimated by xero-
graphic time-of-flight method [29,30]. The sample for the mea-
surement was prepared by drop casting of the THF solution of
compound 13 onto a polyester film with Al layer. After coating the
sample was heated at 70 ꢂC for 1 h. Thus the charge-transporting
layer of the sample was prepared. The thickness of the trans-
During the first heating scan, compound 14 exhibited a sharp
endotherm at 204 ꢂC corresponding to the melting point. However,
when the sample of compound 14 was cooled from the melt, it
showed a glass transition at 91 ꢂC demonstrating the formation of
amorphous phase. During the second heating scan, compound 14
displayed not only the same glass transition observed upon cooling
but also a temptation towards melting. This could be attributed to
the comparatively lower stability of its glassy state [34]. Compound
13 containing ethyl chains demonstrated a glass transition at 107 ꢂC
during the first heating scan. On further heating above glass tran-
sition, compound 13 displayed two endothermic phase transitions
at 130 and 168 ꢂC, respectively (Table 1). The first endotherm at
130 ꢂC suggested the solidesolid phase transition, where one
crystal form apparently was transformed into another crystal form.
The endothermic peak at 168 ꢂC corroborates the melting of the
second crystal structure to give an isotropic phase [34]. However,
the cooling scan and the second heating scan of compound 13 did
not show any thermal transitions which suggested that it can be
transformed into amorphous (glassy) phase when cooled from the
melt [34].
porting layer was ca. 5 mm. Hole drift-mobility (m) was measured by
time-of-flight technique in the xerographic mode. The electric field
was created by positive corona charging. The charge carriers were
generated at the layer surface by illumination with pulses of ni-
trogen laser (pulse duration was 1 ns, wavelength 337 nm). As a
result of pulsed illumination the layer surface potential decreased
down to 2e5% of the initial potential. The capacitance probe con-
nected to the wide frequency band electrometer measured the rate
of the surface potential decrease dU/dt. The transit time tt for the
samples with the transporting material was determined from the
kink of the curve of the dU/dt transient in logelog scale. The hole-
drift mobility was calculated by using the formula
where d is the layer thickness, and U0 the surface potential at the
moment of illumination.
m
¼ d2/U0tt,
Different thermal behaviour was observed for compounds
possessing flexible alkyl chains at the TPA rim. These compounds
did not show any glass transitions. For compound 11 containing
dodecyl alkyl chains, both the first and the second DSC heating
cycles showed multiple endotherms apparently suggesting the
occurrence of a liquid crystalline-like phase transitions. Upon first
3. Results and discussion
3.1. Synthesis and characterisation
Synthesis of triphenylamine-based compounds ramified with
alkoxyphenyl-substituents is shown in Scheme 1. Tris(4-
formylphenyl)amine (10) was synthesized from triphenylamine
by VilsmeiereHaack reaction using the known procedure [31]. For
the preparation of alkoxyphenyl-intermediates (1, 2 and 3), firstly,
methyl-3,4,5-trihydroxybenzoate was O-alkylated in acetone using
potassium carbonate and corresponding bromo or iodo alkanes, i.e.
1-bromododecane for 1, 1-bromodecane for 2, iodoethane for 3,
respectively, in the presence of catalytic amount of 18-crown-6
[32,33]. In the next reaction, the ester functional groups of com-
pounds 1, 2 and 3 were reduced using lithium aluminium hydride
in THF to prepare (3,4,5-tris(alkyloxy)phenyl)methanols 4, 5 and 6,
respectively [32,33]. These derivatives were brominated in the next
step using phosphorous tribromide to synthesize the respective
bromo compounds 7, 8 and 9 [32]. The Wittig reagent (phospho-
nium ylide) was prepared in situ from the bromine-containing
compounds 7, 8 and 9 by refluxing with one equivalent of tri-
phenyl phosphine in DMF. Wittig reactions of compound 10 with
the corresponding alkoxyphenyl derivatives yielded the dendritic
compounds 11e13. Compound 14 was prepared from diethyl-4-
methoxy benzyl phosphonate and compound 10 using the modi-
fied Wittig conditions.
14
cooling
T
g
2nd heating
1st heating
Tm
0
50
100
150
200
Temperature ( o C )
11
cooling
Dendritic compounds 11e13 were found to be soluble in com-
mon organic solvents like dichloromethane, chloroform, toluene or
THF, which was benefited from the alkyl-substituents and the olefin
spacer. Characterization of these materials by mass spectrometry,
elemental analysis, IR spectroscopy and 1H and 13C NMR spec-
troscopy was therefore easily accomplished. The characterization
data were in good agreement with the chemical structures of the
synthesised compounds 11e14.
2
heating
heating
1
-80
-60
-40
-20
0
20
40
60
80
3.2. Thermal properties
Temperature ( o C )
Fig. 1. DSC thermograms of 11 and 14 recorded at a heating/cooling rate of 10 ꢂC minꢁ1
The thermal behaviour of compounds 11e14 under heat was
investigated by DSC and TGA under a nitrogen atmosphere. The DSC
under nitrogen.

R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
139
Table 1
Thermal characteristic of compounds 11e14.
Compound
Tg ꢂCa
Tc ꢂCb
Tm ꢂCc
TID ꢂCd
11
12
13
14
e
2
ꢁ13
e
37
15
130, 168
204
369
320
387
417
e
107
91
e
a
Glass transition temperature.
Crystallization temperature.
Melting temperature.
b
c
d
5% weight loss temperature.
heating, endothermic thermal transitions were observed at 24 ꢂC
and 37 ꢂC with the enthalpy values of
H ¼ 14.5 kJ/mol and
H ¼ 105.3 kJ/mol, respectively. Cooling from the melt displayed
only an exotherm corroborating the crystallization at
ꢂC
H ¼ 115.6 kJ/mol). However, during the second heating cycle, the
endothermic transitions reappeared at 24 ꢂC (
H ¼ 46.3 kJ/mol)
and 35 ꢂC (
H ¼ 32.4 kJ/mol), respectively. Therefore, a liquid-
D
D
2
(D
D
D
crystalline phase was presumably formed on heating and existed
between a short temperature range of 24 ꢂC and 35 ꢂC. The formed
mesophase was found to be monotropic towards heating direction
since, on cooling, the isotropic melt found to be merely solidified
through a unique exotherm to acquire a crystalline phase. This
observation was in concordance with the data of polarized optical
microscopy (POM) measurements. The similar thermal transitions
were also observed for compound 12 containing decyl chains;
however, the mesophase was found to be less stable under heating
and became isotropic liquid below room temperature.
Fig. 2 demonstrates the POM photographs of the mesophase of
compound 11 during the heating of the sample. Upon heating to
24 ꢂC, a viscous anisotropic liquid was formed with a rapid change
in birefringence suggesting the formation of mesophase. At tem-
peratures above 35 ꢂC, the formed mesophase became a low-
viscosity (free-flowing) isotropic fluid. The cooling of this
isotropic melt did not again lead to the mesophase and the melt
remained isotropic up to the room temperature. Nevertheless, the
sample was found to be crystalline after storage at ca. 20 h at the
ambient temperature. Hence, in conclusion, compound 11 could
exist in liquid crystal form (monotropic) over the short range of
temperatures between 24 ꢂC and 35 ꢂC.
Representative X-ray diffraction (XRD) patterns of compound 11
in the crystalline and the liquid crystalline phase are depicted in
Fig. 3. Absorption and fluorescence spectra of the TPA derivatives (a) 11, (b) 12, (c) 13
and (d) 14 in 10ꢁ5 M THF solutions (dashed lines) and neat films (solid lines). Fluo-
rescence quantum yield values are indicated.
Fig. 2. Photographs showing optical textures of compound 11 taken at 32 (a) and 34 ꢂC (b) during heating of the sample. (c) XRD patterns of 11 in the crystalline phase at 15 ꢂC and
ND mesophase at 33 ꢂC.

140
R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
Fig. 2c. The XRD profile of compound 11 recorded at 15 ꢂC showed a
number of sharp diffractions suggesting the occurrence of a crys-
talline phase at this temperature. In the pattern recorded at 33 ꢂC, a
Unlike in dilute solutions the absorption spectra of the TPA
derivatives in the neat films continuously red-shifted and broaden
with decreasing the size of alkoxy chains. For the compounds with
long decyloxy and dodecyloxy chains (compounds 11 and 12) ab-
sorption spectra closely resemble those of dilute solutions. Obvi-
ously, bunches of long alkoxy chains prevent close packing of the
molecules, thus weakening their intermolecular interactions in the
solid state. On the other hand, short ethoxy groups (compound 13)
are unable to suppress this interaction resulting in broadened and
red-shifted (by 5 nm) absorption spectrum with respect to that of
solution. Upon replacement of the long tris(alkoxy) end-moieties
with single methoxy groups at the para positions of the phenyls
(compound 14) intermolecular interactions are enhanced even
more causing much stronger broadening of the band (see Fig. 3).
broad diffraction appeared at 2
q
¼ 20.4ꢂ (4.3 Å). This diffraction
could be attributed to the planar distance between discotic mole-
cules. Moreover, a broad diffraction of low intensity also emerged at
2
q
¼ 2.1ꢂ (42 Å), which apparently indicated the existence of a
short-range columnar order in the mesophase. Therefore, com-
pound 11 could be recognized as monotropic nematic discotic (ND)
mesophase with a short-range of columnar organization between
24 ꢂC and 35 ꢂC.
Compounds 11e14 found to be thermally stable without
showing any decomposition under heating below ca. 320 ꢂC. The
onsets of thermal decompositions (at 5% weight loss) of com-
pounds 11e14 were in the range of 320e417 ꢂC. Such thermal
stability allows their thermal treatment in a wide temperature
range. Compound 13 containing ethyl substituents showed
comparatively higher thermal stability with respect to that of its
closer analogues 11 and 12. Higher thermal stability of 13 could
apparently be attributed to its more rigid structure. Interestingly, a
similar trend was observed for compound 14 containing even
shorter methyl substituents.
3.3. Photophysical properties
Absorption and fluorescence spectra of the dendritic TPA de-
rivatives 11, 12, 13 and 14 are displayed in Fig. 3. The obtained op-
tical properties of the derivatives are summarized in Table 2.
All the studied compounds exhibit similar absorption spectra in
10ꢁ5
M THF solutions, which are typical of phenylethenyl-
substituted TPA derivatives [25,26]. Since the compounds have
identical backbone and different (yet non-conjugated) alkoxy end-
groups, the lowest energy absorption band maxima is located at
similar wavelengths, 392e393 nm. The higher energy absorption
band with half the intensity of the former and peaking at
302e311 nm can be attributed to the local excitation of a single TPA
moiety [25]. Owing to highly twisted and labile molecular structure
of the TPA compounds 11e14, they demonstrate unstructured
fluorescence spectra in THF solutions with the band maxima
peaked at around 455 nm. As it was expected, fluorescence quan-
tum yield (FF) of the compounds in dilute solutions was estimated
to be almost independent on the alkoxy end-groups and ranged
from 49% to 52% (see Table 2).
Table 2
Photophysical properties of the TPA derivatives 11e14.
Solution
Film
a
b
c
a
b
c
f
td
l
l
FF
td
fe
tavg
max
abs
max
FL
max
abs
max
FL
l
l
FF
Compound nm
311
nm
%
ns
nm
nm
%
ns
%
ns
319
0.25 25
11
12
13
392
456
52
50
49
49
1.9 391
453
33
20
8
1.01 51 1.12
2.27 24
0.20 36
0.79 48 0.84
2.45 16
0.20 50
1.12 17 4.46
12.6 33
0.02 74
1.05 14 0.63
3.93 12
311
392
316
1.8 394
456
457
453
456
477
311
393
1.8 398
1.7 396
302
393
460
555
14
4
a
Absorption band maximum.
Fluorescence band maximum.
Fluorescence quantum yield (FF) determined by integrating sphere method.
Fluorescence lifetime measured at.l
Fractional contribution to the total fluorescence intensity.
b
c
d
e
f
Fig. 4. Fluorescence transients of the TPA derivatives (a) 11, (b) 12, (c) 13 and (d) 14 in
10ꢁ5 M THF solutions (grey points) and neat films (black points). Lines mark expo-
nential fits to the experimental data. Instrument response function (IRF) and fluores-
cence lifetimes are indicated.
max
FL
.
P
Average fluorescence lifetime calculated by t
¼
ti ꢃ fi.
avg

R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
141
Similar to the absorption spectra of neat films of compounds
11e14, fluorescence spectra also showed constant red shift as the
size of alkoxy chains is being reduced. Due to the weak intermo-
lecular interaction caused by sufficiently long alkoxy end-groups
fluorescence spectra of the neat films of compounds 11 and 12
demonstrate negligible red shift as compared to those of solutions.
Interestingly, vibronic structure in the spectra of 11 and 12 can be
noticed, which might suggest molecular ordering in the films
caused by the formation of liquid-crystalline phase at room tem-
perature as disclosed by DSC and XRD measurements. The rela-
tively short ethoxy groups (compound 13) already cause noticeable
red shift of the fluorescence spectrum (by 20 nm) as compared to
that in solution. Since the fluorescence spectrum of neat film of 13
contains no traces of vibronic replica and maintains the shape alike
in solutions, it is likely dominated by amorphous character. Fluo-
rescence spectrum of the neat film of compound 14 is found to be
radically different in respect to those of the rest TPA derivatives. The
methoxy groups are too small to impede tight molecule packing
inducing excimer state formation via enhanced intermolecular
forces. Thus, a new significantly red-shifted (by 74 nm) and un-
structured band at 555 nm due to the excimer states is formed.
Interestingly, a small band at the shorter wavelengths (~460 nm)
similar to those observed in the neat films of compounds 11e13 can
be also detected, which likely originates from the areas featuring
weakly interacting molecule species.
Fluorescence transients of the TPA derivatives 11e14 in dilute
solutions demonstrate single exponential decay profiles with
fluorescence lifetime (
t) varying from 1.7 ns to 1.9 ns (Fig. 4). Very
similar of the compounds perfectly correlates with the similar
t
fluorescence quantum yield data. Unlike in solutions, fluorescence
transients in the compound neat films are clearly nonexponential
indicating dispersive exciton hopping through the localized states
in the disordered media [35]. Each transient consists of the very fast
(sub-nanosecond) and slower nanosecond-time-scale components,
which contributes unequally to the overall excited-state decay. The
slower component (
t
ꢄ 1 ns) dominates in compounds 11 and 12
featuring long alkoxy end-groups, whereas the fast one prevails in
compounds 13 and 14 with much shorter end-moieties. Since the
latter two compounds suffer from strong emission quenching in the
neat films (stronger than observed in compounds 11 and 12) due to
exciton migration-facilitated nonradiative decay, it is natural to
associate the initial fast decay in the transients to migration-
induced exciton quenching [25,36,37]. Thus, the transients of the
neat films directly support FF data and unambiguously indicate the
importance of the long alkoxy chains for suppressing exciton
migration to nonradiative decay sites. The long peripheral moieties
are assumed to prevent molecule agglomeration in the solid state
due to steric hindrance effects. Much slower decay components
(t > 4 ns) present only in the transients of compounds 13 and 14
can be explained by long-lived excimer states revealed in the
FF of the neat films of the synthesized compound was found to
be considerably reduced (down to an order of magnitude) as
compared to those of solutions. FF of 33%, 20%, 8% and 4% were
obtained for the compounds 11, 12, 13 and 14, respectively. The
most pronounced reduction of FF occurred for the TPA compounds
with shorter alkoxy end-groups, what corresponded well to the
stronger redshift of absorption and fluorescence spectra in the neat
solid films. This reduction in FF confirmed enhanced intermolec-
ular interactions, and moreover, suggested migration-induced
exciton quenching at nonradiative decay sites to be the main
excited-state deactivation channel [27]. Our previous studies indi-
cated that exciton diffusion can be very efficient in
tris(phenylethenyl)-substituted TPA films [27]. Hence, emission
quenching facilitated by exciton diffusion is indeed probable in
fluorescence spectra of the compound neat films.
3.4. Photoelectrical properties
Ionization potentials (Ip) of thin solid layers of the synthesized
compounds 11e14 were estimated by photoelectron emission
spectroscopy. Photoelectron emission spectra of 11e14 are shown
in Fig. 5. The intersection points of the linear parts of the photo-
electron spectra drawn with the abscissa axis give the ionization
potential values. Ip values of triphenylamine derivatives 11e14 were
comparable and ranged from 5.21 eV to 5.47 eV. It was found that
the extracted photocurrent from the thin films of compounds 11
and 12 was comparatively lower with respect to that extracted from
the solid layers of 13 and 14. This observation can apparently be
attributed to the relatively lower amount of chromophores per unit
area of thin films of compounds 11 and 12. Ip values of the thin
layers of compounds11e14 are rather close to that of commonly
used anode materials in electroluminescent devices, for example,
indiumetin oxide (ITO) [38].
Room temperature hole-drift mobility of a thin layer of com-
pound 13 was measured by xerographic time-of-flight (XTOF)
method. Fig. 6 demonstrates representative dU/dt hole-transients
for the thin film of compound 13. It exhibited a dispersive hole-
transport; which along with the strong electric-field mobility
dependence suggested the trap-dominant charge transport in this
material. Nevertheless, well-defined transit times (tt) established
from the intersection points of two asymptotes of double-
logarithmic plots provided hole-drift mobility at respective
applied fields.
The electric field dependency of hole-drift mobilities of the thin
film of 13 is shown in Fig. 7. Hole-drift mobility of
1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 was observed in the amorphous layer of
compound 13 at an electric field of 106 Vcmꢁ1 at 25 ꢂC. Due to the
formation of crystallites in the thin film after solution casting onto
Al substrate, we were not able to measure the charge carrier
mobility in the layer of compound 14. Compounds 11 and 12 did not
show any significant photosignal of hole-transport in the thin
layers. In the case of compounds 11 and 12, the presence of longer
but flexible alkyl chains increases the distance between electro-
phores. The increased distance between the electrophores restrict
these compounds. Additionally, the significant reduction of FF
,
which is particularly well pronounced in compound 14 and partly
also in compound 13, can be attributed to the appearance of long-
lived excimer states. This is because long-lived states are naturally
more susceptible to trapping by quenching sites such as lattice
distortions, impurity traps and other defects.
8
7
6
5
4
3
2
1
0
11 ; Ip = 5. 47 eV
12 ; Ip = 5. 45 eV
13 ; Ip = 5. 38 eV
14 ; Ip = 5. 21 eV
5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4
hν (eV)
Fig. 5. Photoelectron spectra and ionization potentials of the thin films of 11e14.

142
R.R. Reghu et al. / Dyes and Pigments 115 (2015) 135e142
solid-state molecular packing. Difference in the extracted photo-
current and variations in the ionization potential values of thin
solid layers of synthesized compounds also confirmed their alkyl
substitution influenced molecular packing levels. Hole-drift
mobility of ethyl substituted triphenylamine exceeded
1.3 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 at an electric field of 106 Vcmꢁ1 as measured
by Xerographic time-of-flight (XTOF) method.
Acknowledgement
This research was funded by the European Social Fund under the
Global Grant measure. This research was also supported by FP7
REGPOT-2012e2013-1 ICT project CEOSeR (grant agreement No
316010).
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