New side groups-tuned triphenylamine-based chromophores: Synthesis, morphology, photophysical properties and electronic structures

Article abstract of DOI:10.1016/j.molstruc.2013.07.031

A new series of crossing symmetric triphenylamine-based chromophores with different side groups were synthesized. Their morphological structure, thermal stabilities and photo-physics were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), UV-vis spectrophotometer and fluorescent photometer. The results indicated that these compounds emitted green fluorescence with a quantum yield ranging from 0.34 to 0.80 and a lifetime of 0.89 to 1.34 ns. Meanwhile, these compounds with crystal character or amorphous phase exhibited relatively high decomposition and glass transition temperatures. Their electrochemical behaviors and electronic structures were investigated in detail by cyclic voltammetry (CV), and further confirmed by theoretical calculation. The introduced side groups lowered the electron density of the conjugated system, resulting in a significant decrease of LUMO energy level. In a word, it suggested that the optical properties were regulated by the electronics structures with the different effect of the side groups on the LUMO.

Full text of DOI:10.1016/j.molstruc.2013.07.031

Journal of Molecular Structure 1050 (2013) 151–158
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
New side groups-tuned triphenylamine-based chromophores: Synthesis,
morphology, photophysical properties and electronic structures
⇑
Pei Liu, Peng Zhang , Dongliang Cao, Lihua Gan, Yuanfang Li
College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR China
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
The crossing symmetric triphenylamine diversities were synthesized.
Those compounds showed a relatively high thermal-stability.
Those compounds emitted green fluorescence with a relatively high quantum yield.
The morphology was in the transition from crystalline to amorphous.
The tuning of the LUMO levels was confirmed by CV and theoretical calculation.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2013
Received in revised form 10 July 2013
Accepted 18 July 2013
Available online 25 July 2013
A new series of crossing symmetric triphenylamine-based chromophores with different side groups were
synthesized. Their morphological structure, thermal stabilities and photo-physics were characterized by
X-ray diffraction (XRD), scanning electron microscope (SEM), differential scanning calorimetry (DSC),
thermogravimetric analyzer (TGA), UV–vis spectrophotometer and fluorescent photometer. The results
indicated that these compounds emitted green fluorescence with a quantum yield ranging from 0.34
to 0.80 and a lifetime of 0.89 to 1.34 ns. Meanwhile, these compounds with crystal character or amor-
phous phase exhibited relatively high decomposition and glass transition temperatures. Their electro-
chemical behaviors and electronic structures were investigated in detail by cyclic voltammetry (CV),
and further confirmed by theoretical calculation. The introduced side groups lowered the electron density
of the conjugated system, resulting in a significant decrease of LUMO energy level. In a word, it suggested
that the optical properties were regulated by the electronics structures with the different effect of the
side groups on the LUMO.
Keywords:
Triphenylamine
Electronic structures
Green fluorescence
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
As the core part of OLEDs, the performance of OLEDs was deter-
mined by the EL materials. Materials which provided with two well-
Organic photoluminescent materials for organic light emitting
diodes (OLEDs) are promising optoelectronic materials [1] due to
their merits of high brightness, easy fabrication and tunable wide
range of emission colors [2,4]. Over the past decades, a large effort
has been made to develop and improve the high performance of
OLEDs [5–7]. However, there are still many limits in higher effi-
ciency and longer working life of OLEDs. As is known to all, a excel-
lent OLED usually consists of multilayer structures, an electron
transporting material (ETM) layer, an emitter layer (EL), a hole
transporting material (HTM) layer and positive or negative elec-
trode. Therefore, the balance of carrier transport between layer
and layer is still a main challenge for the efficient OLEDs [4,8,9]
as well as the development in excellent organic photoluminescent
materials.
known properties, thermal stability and high luminescent efficiency
[10–13], have attracted considerable attention because of their
promising application in OLEDs. So far, among the diverse classes
of organic
p-conjugated systems reported [14–16], electron-rich
triphenylamine (TPA) and its derivatives have been widely investi-
gated as active hole transport materials for OLEDs. For example, Hsu
and co-workers reported efficient red OLEDs, in which triphenyl-
amine and 2,7-bis-(phenylsulfonyl)-fluorene units-based com-
pound with good morphological stability, bipolar transporting
was used as active layer [17]. Cias et al. prepared a series of symmet-
rically para-substituted triphenylamine with hole injection and
electron blocking character and investigated their intrinsic relation-
ship between the tunable molecular structure and electron energy
levels (highest occupied molecular orbital (HOMO), and the lowest
unoccupied molecular orbital (LUMO)) [18] by experimental meth-
od and theoretical calculation. Recently, 2,3,3-triphenylacrylonitrile
decorated TPA with donor–acceptor architecture was also reported
⇑
Corresponding author. Tel.: +86 023 68252360.
E-mail address: ybzhang@swu.edu.cn (P. Zhang).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.07.031

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P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
POCl3, DMF
N
OHC
N
40 °C, 1 h
70 °C, 2 h
t
-BuOK,
THF
FTPA
Br2, I2
CH2Cl2
12 °C,
18 h
RT, N2
24 h
Br
Br
Br
NBS, CCl4
reflux, 8 h
PO(EtO)3
reflux, 6 h
(EtO)2OPH2C
CH2PO(OEt)2
BrH2C
Br
CH2Br
Br
Br
XB(OH)2,
DMF,Pd(PPh3)4
Br
X
N
N
80 °C, 24 h
N
N
Br
1
X
3: X =
2: X =
Scheme 1. Synthesis of compounds 1–4.
4: X =
as yellow photoluminescent materials with high quantum yield
[19]. However, the high LUMO level may impede electron injection
from the electron transporting layer, and the mismatch of LUMO en-
ergy level with work function of the negative electrode will cause
serious electron trapping at the doped sites, resulting in the high
operating voltage of the devices [17,20].
Our goal was to regulate the optical properties and electron
structures of the TPA derivatives by crossing symmetric substituent.
Therefore, in this paper, some representative crossing-symmetric
TPA derivatives with different side groups were synthesized and
their morphology, optical properties and electron structure were
discussed in detail. In addition, theoretical calculation was applied
to further demonstrate the effect of the crossing symmetric side
groups on the electron structure of these compounds.
tometer (Purkinje General Instrument Co., Beijing, China). The
morphology structure of these compounds was carried out by an
S-4800 field-emission scanning electron microscope (SEM, Hitachi,
Japan). The thermal behavior of these compounds was performed
with a TA SDTQ600 thermogravimetric analyzer (TGA, USA) and
differential scanning calorimetry (DSC, Model 200F3-Maia,
Germany) under a dry nitrogen gas flow at a heating rate of
10 °C/min, respectively. Ultraviolet–visible absorption (UV–vis)
and emission spectra in solution and in film were recorded on a
UV-3600 spectrometer (Shimadzu, Japan) and F-2500 fluorescence
spectrometer (Hitachi, Japan). The fluorescence lifetimes of these
compounds from dichloromethane (DCM) solutions at room tem-
perature were recorded using FL-TCSPC time resolved fluorescence
spectrometer (France, Horiba Jobin Yvon Inc.). Cyclic voltammetry
were recorded on a CHI-604C electrochemical workstation (Cheng-
hua Co., China) with platinum wire as counter electrode, Pt as
working electrode and Hg as reference electrode, respectively.
Theoretical calculation was performed with time dependent den-
sity functional theory (TD-DFT) on Gaussian 09 quantum chemical
program package.
2. Experimental procedures
2.1. Materials
In this study, all chemical reagents and solvents were used with-
out further purification except that dimethylformamide (DMF),
POCl3 and tetrahydrofuran (THF) were redistilled. Triphenylamine,
Potassiun tert-butoxide (t-BuOK), pheylboronic acid, 4-meth-
ylphenylboronic acid, 1-naphthylboronic acid and Pd(PPh3)4 were
obtained from domestic chemical market. Other chemical reagents
were of analytical grade (Chengdu Kelong Chemical Reagents Fac-
tory, China) and used directly without further purification except
that N-bromosccinimide (NBS) was purified by recrystallization
method.
2.3. Synthesis of the compounds
The synthesis routes of the compounds were shown in Scheme 1.
2,5-Dibromo-p-xylene and 1,4-bis(bromomethyl)-2,5-dibromo-
benzene (DBB) have been synthesized and purified as white solid
according to the literature [21–23]. DBB was refluxed for 6 h in
the presence of triethylphosphite to give 2,5-bis(diethylphosphon-
atomethyl)-1,4-(dibromobenzen) (BDPE-DBB) [24]. 4-Formyltri-
phenylamine (FTPA) was produced by the Vilsmeier formylation
of triphenylamine by POCl3/DMF [25].
2.2. General characterization
1,4-dibromo-2,5-bis(diphenylaminophenyl)benzene (com-
pound 1): A solution of 0.58 g (1.5 mmol) BDPE-DBB and 0.96 g
(3.5 mmol) FTPA in 10 mL THF was stirred under N2 at room tem-
perature (RT) to which was added 0.38 g (3.4 mmol) t-BuOK in
5 mL THF in small portions. Upon the addition of t-BuOK, the color
1H NMR spectra were recorded on a DPX-400 spectrometer
(300 MHz, Brucker). The infrared (IR) spectra were measured as
KBr pellets on
a FTIR-8400 spectrometer (Shimadzu, Japan).
X-ray diffraction (XRD) was measured on an XD-3 powder diffrac-

P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
153
2 (0.12 g, 62%) as light yellow solid (mp 301–303 °C). 1H NMR
(300 MHz, CDCl3): d (ppm): 7.71 (s, 2H), 7.50–7.48 (t, 8H), 7.43–
7.40 (t, 4 H), 7.24–7.23 (m, 10H), 7.13–7.01 (m, 20 H). FT-IR (KBr,
cmꢁ1): 3425 (m), 3028 (w), 2372 (w), 1589 (s), 1489 (s), 1327
(m), 1284 (s), 1176 (w), 1076 (w), 1026 (w), 964 (w), 895 (w),
856 (w), 818 (w), 752 (m), 694 (m), 613 (w), 505 (m).
1,4-dibenzyl-2,5-bis(diphenylaminophenyl)benzene (com-
pound 3): Same as compound 2 but 4-methylphenylboronic acid
(0.2 g, 1.5 mmol) instead of pheylboronic acid was used, resulting
in compound 3 (0.15 g, 75%) as green powder (mp 277–278 °C).
1H NMR (300 MHz, CDCl3): d (ppm): 7.69 (s, 2H), 7.40–7.37 (d,
4H), 7.29–7.21 (m, 20 H), 7.10–7.07 (d, 8H), 7.04–6.97 (m, 8 H),
2.44 (s, 6H). FT-IR (KBr, cmꢁ1): 3448 (m), 3024 (w), 2920 (w),
2866 (w), 2376 (w), 1589 (s), 1493 (s), 1327 (m), 1277 (s), 1176
(w), 1111 (w), 1072 (w), 1018 (w), 960 (w), 899 (w), 856 (w),
818 (w), 752 (m), 694 (m), 617 (w), 505 (m).
Fig. 1. XRD patterns of compounds 1–4.
1,4-dinaphthyl-2,5-bis(diphenylaminophenyl)benzene
(compound 4): Same as compound 2 but 1-naphthylboronic acid
(0.26 g, 1.5 mmol) instead of pheylboronic acid was used, resulting
in compound 3 (0.14 g, 65%) as green powder (mp 277–278 °C). 1H
NMR (300 MHz, CDCl3): d (ppm): 8.06–7.91 (m, 4H), 7.85–7.76 (t,
2H), 7.60–7.48 (m, 6H), 7.43–7.36 (t, 4H), 7.23–7.19 (d, 6H),
7.12–7.09 (d, 6H), 7.03–6.84 (m, 20H). FT-IR (KBr, cmꢁ1): 3425
(m), 3032 (w), 2376 (w), 1589 (s), 1489 (s), 1323 (m), 1277 (s),
1176 (w), 1107 (w), 1076 (w), 1022 (w), 960 (w), 894 (w), 856
(w), 806 (w), 752 (m), 694 (m), 617 (w), 509 (m).
of the reaction mixture turned to yellow. After stirred 24 h at RT,
the reaction mixture was poured into water, extracted with dichlo-
romethane (DCM) and washed with water. The organic layer was
dried over anhydrous magnesium sulfate. Then, the solvent was re-
moved under reduced pressure and the residue was chromato-
graphed on silica gel to yield compound 1 (0.66 g, 57%) as orange
solid (mp 275–277 °C). 1H NMR (300 MHz, CDCl3): d (ppm): 7.86
(s, 2H), 7.43–7.41 (d, 4H), 7.31–7.22 (m, 10H), 7.14–7.12 (d, 8H),
7.07–6.97 (m, 10H). FT-IR (KBr, cmꢁ1): 3425 (m), 3060 (w), 2376
(w), 1585 (s), 1489 (s), 1327 (m), 1284 (s), 1173 (w), 1215 (w),
1053 (m), 960 (w), 891 (w), 856 (w), 818 (w), 752 (m), 698 (m),
621 (w), 509 (m).
3. Results and discussion
1,4-diphenyl-2,5-bis(diphenylaminophenyl)benzene (com-
pound 2): A mixture of 0.19 g (0.25 mmol) compound 1, 0.14 g
(1.5 mmol) pheylboronic acid, 0.16 g (1.5 mmol) Na2CO3 and
0.05 g Pd(PPh3)4 were dissolved in 10 mL DMF under N2 at 80 °C
and stirred 24 h. The reaction mixture was allowed to cool down
to RT and poured into water, extracted with DCM and washed with
water. The organic layer was dried over anhydrous magnesium sul-
fate. Then, the solvent was removed under reduced pressure and
the residue was chromatographed on silica gel to yield compound
3.1. Morphological properties
The powder XRD analysis for compounds 1–4 was shown in
Fig. 1. Compounds 1–3 displayed two or three mainly intensive
and sharp peaks, respectively, demonstrating that these small mol-
ecules were provided with a certain crystalline nature [26]. How-
ever, the XRD patterns of these three powders were different
from each other, suggesting that these results may be attributed
to different crystalline phase or different strong intermolecular
Fig. 2. SEM of compounds 1–4 powders.

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P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
Fig. 3. TGA of compounds 1–4 under nitrogen, inset: 2nd heating DSC curves of
compounds 1–4.
interaction in the solid state. Furthermore, the decreased intensity
and relatively broadened peaks may be attributed to the gradually
decreasing crystalline nature from compounds 1–3. In comparison
with the first three compounds, the compound 4 showed a rela-
tively broad and random scatter peak, demonstrating the amor-
phous characteristics of compound 4 in solid state [27], which
was the reason that the bulky naphthyl side subsitutents led to
twist the main chain and inhibited chain packing and crystallinity
[23]. The morphologic characteristics of compounds 1–4 were val-
idated in advance by SEM measurement. As shown in Fig. 2(a–c),
the SEM images of compounds 1–3 presented irregular crystalline
morphology, respectively, further confirming the different crystal
characteristics of compounds 1–3. On the contrary, compound 4
exhibited a form of cotton-like small grains in Fig. 2(d), suggesting
the amorphous character of compound 4 [28], which was in agree-
ment with the XRD results.
3.2. Thermal behavior
The thermal characteristics of these compounds were measured
by DSC analysis and TGA (Fig. 3). The corresponding thermal
behavior parameters of these compounds were summarized in
Table 1. The DSC curves of compounds 1–3 showed a weak glass
transition endothermic peak (Tg) at 102 °C, 129 °C and 125 °C in
the 2nd heating scan, respectively, which suggested that the
increasing branched chains could hinder the molecular twist,
resulting in such increase in the Tg. In addition, DSC analysis re-
vealed that the Tg for compound 3 was reduced by 4 °C due to
Table 1
The optical characteristics of the compounds 1–4 in DCM solution and films and DSC
and TGA measurements of compounds 1–4.a
a
DSCb
TGAb
em
max
abs
max
Compds.
UF
sF (ns)
k
(nm)
k
(nm)
Fig. 4. (a) Normalized UV–vis absorption spectra (line) and fluorescence emission
spectra (dash) of compounds 1–4 in DCM solution, (b) normalized UV–vis
absorption, and (c) normalized fluorescence emission spectra of compounds 1–4
films on a glass substrate.
Sol
Film
Sol
Film
Tg (°C)
Td5 (°C)
1
2
3
4
425
415
410
419
478
440
454
443
528
495
492
489
522
502
507
512
0.34
0.80
0.74
0.59
0.89
1.29
1.34
1.17
102
129
125
132
334
349
300
306
the introduction of p-methylphenyl, comparing with compound 2
[29]. Compound 4 with amorphous characteristics exhibited a rel-
atively high Tg at 132 °C, which was attributed to an introduction
of larger naphthalene side group. As shown in Fig. 3, the TGA
a
The fluorescence quantum efficiency (U) of each compound in DCM solution
was measured by excitation of the corresponding maximum excitation wavelength,
using fluorescein (UFL = 0.79) [33] in 0.1 M aqueous NaOH solution as a standard.
b
Under nitrogen, heating rate = 10 °C/min.

P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
155
3.3. Optical properties
The absorption and emission spectra of compounds 1–4 in DCM
solution and film were shown in Fig. 4, and the corresponding data
was summarized in Table 1. As shown in Fig. 4(a), two distinctly
broad absorptions of compound 1 were observed at 302 and
425 nm, which could be assigned to a p–p
* transition from styrene
conjugated system and an intramolecular charge-transfer (CT)
transition, respectively [30]. Compounds 2–4 exhibited similar
absorptions with absorption maxima at 415, 410 and 419 nm,
respectively, owing to an introduction of different symmetrical
side groups. Besides, compounds 2–4 exhibited green emissions
in DCM solution and the blue shift in various degrees was ob-
served, due to the effect of side groups on
p-conjugated degree
of main chain, comparing with compound 1 (Table 1). As shown
in Fig. 4(b). The absorption peaks of compounds 1–4 in solid film
were present in the range of 440–480 nm, which exhibited the
marked red shift due to the self-assembled as molecular aggrega-
tion effect [31], in comparison with those in solution.
Fig. 4(c) showed the PL spectra of thin films of compounds 1–4.
Compound 1 exhibited the green emission with the emission max-
imum of 522 nm, and a slightly blue shift comparing with that in
solution which may originate from luminescence rigidochromism
or conformation of H-aggregation [32]. Thin films of compounds
2–4 also exhibited the green emissions, and markedly red shift
than those in solution, respectively, owing to the molecular
Fig. 5. Fluorescence emission spectra of compound
concentration.
1 in DCM with different
curves of all compounds exhibited relatively high thermal decom-
position temperatures (Td) (more than 250 °C). In a word, these
small molecules exhibited excellent thermal stability.
Fig. 6. Normalized fluorescence emission spectra (a–d) of compounds 1–4 in different solution.

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P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
Fig. 7. Cyclic voltammetry of compounds 1–4 in DMF at the scanning rate of 50 mV/s.
aggregation in solid film. In addition, comparing with compound 1,
the blue shift phenomenon in varying degrees for compounds 2–4
was observed due to different side substituents, respectively.
DMF were chosen to evaluate the effect of the solvent polar on
the emission. For example, compound 1 (Fig. 6(a)) exhibited a sig-
nificant red shift in polar solvents than in nonpolar solvents and
the emission maximum was about 53 nm red-shift from TOL to
DMF. Compounds 2–4 (Fig. 6(b–d)) showed analogous solvatochro-
mism to that of compound 1, which might be resulted from the dif-
ference in the dipole moment of these compounds in the excited
state and the ground state [34]. A decrease in the energy of singlet
excited states may occur as a function of an increase in the polarity
of the solvents, indicating intramolecular charge transfer from the
TPA part to the central benzene ring.
The fluorescence quantum yields
(UF) and lifetimes (sF)
(Fig. S1) of compounds 1–4 recorded in DCM were summarized
in Table 1. The low UF value of compound 1 was found to be
0.34, whereas relatively high UF value for compound 2 (0.80)
and 3 (0.74) were measured, and the moderate UF value of 0.59
for compound 4 was given. Fluorescence decays of compounds
1–4 were fitted with mono-exponential functions, indicating
emission from the singlet excited state in each case [34]. All of
these compounds exhibited a relatively short fluorescence lifetime
(about 1 ns).
3.4. Electrochemical properties
The emission spectra of compounds 1–4 were recorded at
different concentrations in DCM (Figs. 5 and S2). From Fig. 5, the
concentration quenching for compound 1 has not been detected
in detection range of 10ꢁ6 M to 10ꢁ4 M, indicating no dimer forma-
tion up to concentration as high as 10ꢁ4 M [35]. Meanwhile, the
critical concentration for compounds 3 and 4, at which the emis-
sion intensity began to drop significantly, was obviously higher
than that of compound 2 but no red shift in their emission spectra.
We hypothesized that the larger side groups played a role in
reducing the effect of concentration quenching.
To assess the electrochemical behavior and electronic structure
of these compounds, the electrochemical experiment of these
compounds in DMF were performed by cyclic voltammetry (CV)
using tetrabutylammonium perchlorate (TBAP) as the supporting
electrolyte. As shown Fig. 7, compounds 1–3 showed continuous
irreversible oxidation processes in the positive scan region, origi-
nating from TPA and styryl units, respectively. But no reduction
reactions were detected in the negative monitoring region. In case
of compound 4, an irreversible oxidation processes in positive
potential range and a reversible reduction processes in negative
potential range were observed. This indicated that the side group
In order to further understand the solvatochromism of these
compounds, toluene (TOL), DMC, THF, ethyl acetate (EA) and

P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
157
Table 2
The electrochemical data and energy levels of the compounds 1-4.
3.5. Theoretical calculation
In order to better understand the electron structure and the
localization of the frontier orbitals, the theoretic investigation of
compounds 1–4 were performed by using the means of TD-DFT
with the B3LYP/6-13G basis sets. The orbital profiles of the HOMO
and LUMO of each compound were presented in Fig. 8, and the cor-
responding data was summarized in Table 2. In case of compound
1, the HOMO was essentially located on the central styrene conju-
gated backbone and the TPA donor units. The LUMO was only
Compds.
Energy (eV)
Exp.a
Energy (eV)
Cal.
b
Eogpt:
Eegl:
EHOMO
ELUMO
EHOMO
ELUMO
Eg
1
2
3
4
ꢁ5.25
ꢁ5.21
ꢁ5.23
ꢁ5.20
ꢁ2.71
–
–
–
3.03
2.54
2.56
2.63
2.58
ꢁ4.88
ꢁ4.73
ꢁ4.71
ꢁ4.73
ꢁ1.68
ꢁ1.45
ꢁ1.40
ꢁ1.48
3.20
3.28
3.31
3.25
ꢁ2.65
ꢁ2.60
ꢁ2.17
a
Electronic structure were determined from the below formula: EHOMO = -
ꢁ(Eox + 4.4) (eV) and ELUMO = ꢁ(Ered + 4.4) (eV), where Eox and Ered were the onset
spread over the whole p-conjugated backbone. On the other hand,
oxidation and onset reduction potentials in Fig. 7. Electrochemical band gap
Eegl: ¼ ELUMO ꢁ EHOMO. Optical band gap Eogpt ¼ h
m=k was from the optical absorption
the main components of the HOMO and LUMO of compounds 2–4
were similar to those of compound 1. Nevertheless, different side
group substituents made a certain participation of the LUMO orbi-
tal, which suggested that the side substituents could perturb the
distribution of the electron density on the LUMO orbital. As a con-
sequence, the optical properties and electrochemical behavior of
these cruciforms were different from each other.
spectra.
b
For compounds 1–3, the HOMO energies values from electrochemical experi-
mental result and the LUMO energies were estimated according to the optical band
gap (ELUMO ¼ Eogpt þ EHOMO). For compound 4, both the HOMO energy and the LUMO
energy were estimated according to the electrochemical experimental result.
On the other hand, the calculated energy levels (HOMO, LUMO
and Eg) were listed in Table 2. The energies of HOMO and LUMO
were ꢁ4.88 and ꢁ1.68 eV for compound 1, respectively. And the
(naphthalene or thiophene) substituents resulted in the increasing
acceptor abilities from the main chain. All the electrochemical data
of these compounds were summarized in Table 2. The HOMO en-
ergy level and LUMO energy level of compound 1 were ꢁ5.25
and ꢁ2.71 eV, respectively. The HOMO energy level of compound
2 (ꢁ5.21 eV) and compound 3 (ꢁ5.23 eV) were very similar to that
of compound 1. Therefore, the phenyl substituent had slightly ef-
fect on the reduction potential of these molecules. Comparing with
compound 1, the higher HOMO energy level of compound 4
(ꢁ5.20 eV) was obtained. In the same time, the LUMO energy level
of compound 4 (ꢁ2.17 eV) was calculated by the reduction poten-
tials. The relatively high HOMO levels of compounds 1–4 were sim-
ilar to that of excellent hole-transporting materials [29,36]. In
addition, the optical band gaps (Eogpt) for compounds 1–4 were ob-
tained through the absorption band edge. The LUMO energy level
for compounds 1–3 could calculate by the combined method of
corresponding band gap (Eg) (3.20 eV) was derived from EHOMO
-
ꢁ ELUMO. Compounds 2–4 exhibited almost no change on the
HOMO but a notable increase on the LUMO in a certain degree.
Meanwhile, the Eg for these compounds increased as: 1 < 4 < 2
< 3, which was basically consistent with the experimental results
except for the individual (Table 2). It should be pointed out that
the calculated results were different from the experiments data
due to the difference in the method.
4. Conclusions
A series of crossing symmetric triphenylamine-based chro-
mophores was synthesized successfully. Their excellent morpho-
logical properties and thermal stability were confirmed by using
the HOMOs and Eogpt
.
Fig. 8. Molecular orbital surfaces of the HOMO and LUMO of compounds 1–4.

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P. Liu et al. / Journal of Molecular Structure 1050 (2013) 151–158
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XRD, SEM, DSC and TGA methods. All compounds exhibited green
emissions with relatively high quantum yields and moderate fluo-
rescence lifetimes. In addition, compounds 1–3 showed irrevers-
ible oxidation behavior, while compound 4 displayed reversible
oxidation reaction and also reduction reaction, indicating efficient
effect of side substituents on electrochemical behavior. On the
other hand, the electronic structures (HOMO and LUMO) of com-
pounds 2–4 were efficiently regulated by the introduction of sym-
metric side groups, which were confirmed in advance by theoretic
calculation. Especially, the LUMO levels for compounds 2–4 were
tuned by the side substituents which were a main reason that im-
proved photophysical and electrochemical properties. In summary,
the results suggested that the triphenylamine-based chromoph-
ores prepared by crossing symmetric substituent might be the
promising organic optoelectronic materials for the application of
photoelectron devices and the further investigation of device was
underway.
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This work was financially supported by the Fundamental Re-
search Funds for the Central Universities (XDJK2010C060) and
the National Science Fundation of China (Grant 21175109B0509).
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