Tuning the photophysical, thermal properties and electronic energy levels in the triphenylamine derivatives by benzoylhydrazone architecture

Article abstract of DOI:10.1016/j.dyepig.2014.04.021

A novel of triphenylamine derivatives with more extended π-conjugated systems achieved by benzoylhydrazone architecture were synthesized under mild condition with high yields. The dependence of their photophysical, electrochemistry and thermal properties on their chemical structure is discussed. Compared with the N,N-di(4-methylphenyl)aniline, the thermostability of the five triphenylamine-based hydrozones is dramatically promoted. These compounds are stable up to 345 °C according to thermogravimetric analysis. The energy levels of the four compounds were finely tuned by introducing the electron-donating and -withdrawing groups to the triphenylamine core which are supported by the fluorescence spectra and transit fluorescence spectra of the compounds (-H > -CH₃ > -Cl > Pyridine > -NO₂). Their multiple cyclic voltammetry study and density functional study suggest these materials to be promising hole-transport properties.

Full text of DOI:10.1016/j.dyepig.2014.04.021

Accepted Manuscript
Tuning the photophysical, thermal properties and electronic energy levels in the
triphenylamine derivatives by benzoylhydrazone architecture
Lizeng Liu, Xianfang Meng, Wei Li, Xueqin Zhou, Zhengchen Bai, Dongzhi Liu,
Yunrong Lv, Rui-hong Li
PII:
S0143-7208(14)00149-1
10.1016/j.dyepig.2014.04.021
DYPI 4349
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 31 December 2013
Revised Date: 12 April 2014
Accepted Date: 16 April 2014
Please cite this article as: Liu L, Meng X, Li W, Zhou X, Bai Z, Liu D, Lv Y, Li R-h, Tuning the
photophysical, thermal properties and electronic energy levels in the triphenylamine derivatives by
benzoylhydrazone architecture, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.04.021.
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ACCEPTED MANUSCRIPT
Tuning the photophysical, thermal properties and electronic
energy levels in the triphenylamine derivatives by
benzoylhydrazone architecture
1,a,
1,b
2,c
2,d
*
Lizeng Liu , Xianfang Meng , Wei Li , Xueqin Zhou ,
2,f
Zhengchen Bai1,e
Yunrong Lv1,g, Rui-hong Li1,h
, Dongzhi Liu ,
1. College of Science, Tianjin University of Commerce, Tianjin 300134, PR China
2. School of Chemical Engineering, Tianjin University, Tianjin 300072, PR China
a llzeng@tjcu.edu.cn*, b mxfang@tjcu.edu.cn, c jkliwei@yahoo.cn,
f bzhcheng@tjcu.edu.cn,
d zhouxueqin@tju.edu.cn, e dzliu@tju.edu.cn,
g 997516949@qq.com, h 1597178053@qq.com
*Corresponding author: Tel: + 86 22 83692436, Fax: + 86 22 83692436,
e-mail: llzeng@tjcu.edu.cn
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ABSTRACT: A novel of triphenylamine derivatives with more extended
π-conjugated systems achieved by benzoylhydrazone architecture were synthesized under
mild condition with high yields. The dependence of their photophysical, electrochemistry
and thermal properties on their chemical structure is discussed. Compared with the
N,N-di(4-methylphenyl)aniline, the thermostability of the five triphenylamine-based
hydrozones is dramatically promoted. These compounds are stable up to 345 °C
according to thermogravimetric analysis. The energy levels of the four compounds were
finely tuned by introducing the electron-donating and -withdrawing groups to the
triphenylamine core which are supported by the fluorescence spectra and transit
fluorescence spectra of the compounds (-H > -CH3 > -Cl > Pyridine > –NO2). Their
multiple cyclic voltammetry study and density functional study suggest these materials to
be promising hole-transport properties.
Key words: Triphenylamine Crystal structures Photophysical properties
Electrochemistry Thermal stability Quantum chemistry
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1. Introduction
In order to suit the large-scale manufacturing of the opto devices
(electrophotography, organic light emitting diodes, solar cells, field effect transistors etc),
triphenylamine-based molecules of low-molar-mass have attracted much attention [1-14].
Triphenylamine-based hydrazones as hole-transport materials are motivated by their
facile preparation, high solubility, electrode compatibility, good transparency in the
visible region, high LUMO (lowest unoccupied molecular orbital) energy level, and
modest HOMO (highest occupied molecular orbital) level[15,16]. Hydrazones are a class
of molecules that contain the -CH=N- functionality, which could enhance the charge
carrier mobility by enlargement of the molecule of conjugated π-electrons. The study on
structure-property relationship reveals that the absorptivities/ electronic excitation
energies and the HOMO/ LUMO energy levels are affected mainly by the extending
conjugation length, planarity and dimensionality of the π-center architecture [17, 18]. The
other important factor influencing charge transport properties is the origin of the
substituents in hydrazone molecules. On the other hand, the thermal, optical, and charge
transport properties were influenced by the nature of the linking fragment and the
structure of the triphenylamine-based molecular symmetry. Various moieties have been
used in the design and synthesis of triphenylamine-based low-molar-mass hydrazones.
Apparently, the incorporation into the molecules of the rigid phenyl, alkoxyphenyl or
branched alkyl groups, which hinder the packing of molecules, decreases their ability to
form glasses [19-20]. These hydrazones form glasses and their amorphous films could be
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prepared on substrates by casting from solutions. Furthermore, the energy levels of
triphenylamine-based hydrazones can be varied in a relatively wide range by changing
the length of the linking fragment and the substituent of the hydrazide moieties. Density
functional theory (DFT) has been applied to explain electronic structures, electronic
excitation mechanisms, and optical properties of the molecules [21-23]. In order to
develop higher effective triphenylamine-based materials, a serial of new triphenylamine
derivatives with different benzoylhydrazone structure were synthesized, and their
morphology, optical properties, thermal stability, electron structure and electronchemisty
properties were discussed in this paper. In addition, theoretical calculation was applied to
further demonstrate the effect of the crossing-section side groups on the electron structure
of these compounds.
……………………………………Figure 1……………………………………
2. Experiment section
2.1 Materials
All chemicals and solvents were analytical grade and without further purification.
N,N-di(4-methylphenyl)aniline(TPA), 4-[N,N-di(4-tolyl)amino] benzaldehyde(R2-CHO),
4-chlorobenzohydrazide,
4-methylbenzohydrazide,
4-nitrobenzoylhydrazine
4-pyridinecarboxylic hydrazide, benzoylhydrazine were prepared according to the
literature [24-27]. Thin layer chromatography purification was carried out using silica gel
purchased from Qingdao Haiyang Co.LTD, China.
2.2 Instrumentation
Melting points of all compounds were determined on X-6 binocular microscope
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(Beijing Tech. Instrument Co.LTD, Beijing, China), and the thermometer was not
corrected. Infra-red spectra were obtained on Bio-rad Fts3000 spectrometer in the
1
4000-400 cm-1 range as KBr disks. H and 13C NMR spectra were performed using
Bruker AC-P500 Instrument spectrometer at 400 MHz in DMSO-D6 or CDCl3, TMS was
induced as internal standard. High-resolution mass spectrometry (HRMS) data were
recorded on VG ZAB-HS mass spectrometer. X-ray diffraction data of single crystals
were collected by a Bruker Smart 1000 CCD diffractometer. The structure was solved by
direct methods and expanded by difference Fourier techniques with SHELXS-97 program
[28]. Absorption spectra were measured with Helios Gamma UV-vis spectrophotometer.
Steady-state fluorescence spectra were recorded with a Varian Cary Eclipse
fluorospectrophotometer. Fluorescence transients of the samples were recorded using a
time-correlated single photon counting system HORIBA Jobin Yvon IBH instrument, 370
nm Nanoled utilized as pulsed excitation sources. Thermogravimetric (TG) analysis was
investigated by TA Q500 Thermo gravimetric analyzer instrument. Differential scanning
calorimetry (DSC) were carried out in a nitrogen atmosphere with a TA Instruments Q10
calorimeter at a heating rate of 5 °C/min. Cyclic voltammetry (CV) was carried out on a
CHI7600 electrochemical workstation in CH2Cl2, using n-Bu4NPF6 as support electrolyte
(10-1 M); scan rate: 50 mV s-1. The electrochemical cell was equipped with three
electrodes: a working electrode (plate Pt electrode), an auxiliary electrode (platinum wire
electrode), and a reference electrode (SCE, a saturated calomel electrode).
2.3. Theoretical calculations
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In order to investigate the electron distribution of the HOMO and LUMO energy
levels of the four compounds, density functional theory (DFT) calculations were carried
out through the Gaussian 03 program at the B3LYP/6- 31tG(D) level [29].
2.4 Synthetic Procedure for 4-(N,N- diphenylamino)benzaldehyde(R2-CHO)
N,N - dimethylformamide (2.74 g, 37.5 mmol) was added dropwise to the mixture of
TPA (2.27 g, 18.8 mmol) and phosphorous oxychloride (POCl3) (5.65 g, 22.5 mmol) at
25 °C. The reaction mixture was stirred and heated at 130 °C for 6 h. After cooling down
to room temperature, the reaction mixture was poured into 50 mL cold water and
extracted with chloroform (3 × 15 mL). The combined organic layer was dried with
anhydrous MgSO4, filtered. The purified compound R2-CHO was obtained by
recrystallizing from diluted ethanol. Yield: 1.75 g (31.5%). Melting point: 96.3-97.5 °C.
ESI-MS (m/z, [M+H]+)=302.4.
2.5 General Synthetic Procedure for Compound I
Aryl carboxylic esters (I) was synthesized from aromatic acids and ethanol. Thionyl
chloride (32 mmol) was added dropwise to a stirred mixture of aromatic acids (8 mmol)
and ethanol (70 mmol) at room temperature. After the reaction had been refluxed for 6 h,
most of the ethanol was removed. The residue solution was poured into cold water (20
mL) and extracted with ethyl acetate (3 × 15 mL). Then, the organic layer was washed
successively with water (3 × 10 mL). The organic layer was dried over anhydrous sodium
sulfate. After filtration, the solvent was removed under vacuum to give the Compound I
with high yield, which was not further purified.
2.6 General Synthetic Procedure for Compound II
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A 5 mL absolute ethanol solution of hydrazine hydrate (80%) and Compound I (6
mmol) was refluxed for 4-6 h under severely stirring, the product being appeared as
yellowish white solid. For purification of product, it was filtered and washed with 30 ml
water and 30 ml ethanol pre-cooled by ice, and recrystallized from ethanol to yield the
Compound II as white solid.
4-chlorobenzohydrazide (II-1): yield (91.3%). Melting point: 143.6-144.1 °C.
ESI-MS (m/z, [M+H]+)=171.2.
4-methylbenzohydrazide (II-2): yield (92.1%). Melting point: 105.7-106.3 °C.
ESI-MS (m/z, [M+H]+)=151.2.
4-nitrobenzoylhydrazine (II-3): yield (93.4%). Melting point: 189.9-190.2 °C.
ESI-MS (m/z, [M+H]+)=182.2.
4-pyridinecarboxylic hydrazide (II-4): yield (95.1%). Melting point: 170.0-171.0 °C.
ESI-MS (m/z, [M+H]+)=138.2.
Benzoylhydrazine (II-5): yield (91.6%). Melting point: 110.0-112.0 °C. ESI-MS
(m/z, [M+H]+)=137.2.
2.7 General Synthetic Procedure for Compound III
The Compound I (0.79 mmol) was dissolved in 10 mL of absolute ethanol. Then the
mixture of R2-CHO (0.66mmol), 10ꢀL acetic acid and 3 mL absolute ethanol was added
dropwise at 70 °C. The mixture was refluxed for 5 h. The solvent was evaporated. The
crude product was purified by flash chromatography (neutral Al2O3).
4-[N,N-di(4-tolyl)amino] benzaldehyde 4-chlorobenzoyl hydrazone (III-1): yield
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(91.3%). Melting point: 114.2-117.6 °C. IR(KBr, cm-1): 3446 (ν –NH-), 3090, 3027, 2920,
1654, 1598, 1564, 1506, 1273, 817. 1H NMR (CDCl3, 400 MHz, ppm): 7.79(d, 1H, J=6.8
Hz), 7.71-7.69(q, 1H), 7.55-7.52(q, 2H), 7.49(s, 1H, -N=CH-),7.43(d, 2H, J=7.6 Hz),
7.26(s, 1H), 7.10(d, 4H, J=8.0 Hz), 7.01(d, 4H, J=8.0 Hz), 6.94(d, 2H, J=8.4 Hz), 2.33(s,
13
6H, -CH3). C NMR (CDCl3, 100 MHz, ppm): 153.8, 144.3, 139.3 133.8, 130.9, 130.1,
129.1, 128.8, 125.6, 122.2, 120.2, 20.9. HR-ESI-MS (m/z, [M+H]+) calcd. for
C28H25ON3Cl+ 454.1681, found 454.1679.
4-[N,N-di(4-tolyl)amino] benzaldehyde 4-methyl benzoylhydrazone (III-2): yield
(89.4%). Melting point: 111.8-112.5 °C. IR(KBr, cm-1): 3440 (ν –NH-), 3190, 3026, 2918,
1646, 1599, 1557, 1504, 1274, 815. 1H NMR (CDCl3, 400 MHz, ppm): 8.10(s, 1H),
7.45(s, 1H, -N=CH-), 7.19(s, 6H, J=8.4 Hz), 7.03(d, 4H, J=8.4 Hz), 6.95(d, 4H, J=8.0 Hz),
6.89(d, 2H, J=8.8 Hz), 2.60(s, 6H, -CH3), 2.34(s, 3H, -CH3). 13C NMR (CDCl3, 100 MHz,
ppm): 156.7, 143.4, 137.9, 132.7, 129.1, 128,4, 127.8, 126,2, 124.5, 119.3, 20.5, 19.9.
+
HR-ESI-MS (m/z, [M+H]+) calcd. for C29H28ON3 434.2227, found 434.2227.
4-[N,N-di(4-tolyl)amino] benzaldehyde 4-nitro benzoylhydrazone (III-3): yield
(78.3%). Melting point: 152.3-155.3 °C. IR(KBr, cm-1): 3433 (ν –NH-), 3191, 3028, 1660,
1
1597, 1561, 1506, 1291, 818. H NMR (CDCl3, 400 MHz, ppm): 8.31(d, 2H, J=8.0 Hz),
8.14-8.12(m, 1H), 8.04(s, 1H), 7.57-7.53(m, 1H), 7.44(s, 1H, -N=CH-), 7.36(d, 1H, J=8.0
Hz), 7.29(s, 1H), 7.15-7.11(t, 4H), 7.07-7.03(t, 4H), 6.98-6.94(t, 2H), 2.36(s, 3H, -CH3);
2.35(s, 3H, -CH3). 13C NMR (CDCl3, 100 MHz, ppm): 149.5, 145.4, 131.0, 129.7, 129.3,
129.0, 127.6, 126.5, 126.4, 126.3, 124.5, 124.4, 121.1, 55.0, 20.9. HR-ESI-MS (m/z,
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+
[M+H]+) calcd. for C28H25O3N4 465.1921, found 465.1922.
4-[N,N-di(4-tolyl)amino] benzaldehyde 4-pyridine formylhydrazone (III-4): yield
(89.3%). Melting point: 118.6-119.5 °C. IR(KBr, cm-1): 3442 (ν –NH-), 3191, 3026, 2919,
1654, 1597, 1551, 1505, 1288, 816. 1H NMR (CDCl3, 400 MHz, ppm): 8.74(d, 2H, J=4.8
Hz), 8.23(s, 1H), 7.79(t, 1H), 7.72(t, 1H), 7.52(d, 1H, J=8.8 Hz), 7.49(s, 1H, -N=CH-),
7.35(d, 1H, J=8.4 Hz), 7.10(d, 4H, J=8.4 Hz), 7.02(d, 2H, J=8.4 Hz), 7.01(d, 2H, J=8.4
Hz), 6.93(d, 2H, J=8.8 Hz), 2.31(s, 6H, -CH3). 13C NMR (CDCl3, 100 MHz, ppm): 150.5,
149.7, 144.2, 140.7, 134.0, 130.2, 129.0, 128.4, 125.7, 123.8, 121.2, 120.3, 120.0, 20.9.
HR-ESI-MS (m/z, [M+H]+) calcd. for C27H25ON4 421.2023, found 421.2027.
+
4-[N,N-di(4-tolyl)amino] benzaldehyde benzoylhydrazone (III-5): yield (91.5%).
Melting point: 186.5- 189.2°C. IR(KBr, cm-1): 3429 (ν -NH-), 3071, 3029, 2922, 1651,
1599, 1559, 1506, 1286, 816. 1H NMR (CDCl3, 400 MHz, ppm): 9.09(s, 1H), 8.19(s, 1H),
7.86(s, 1H, -N=CH-), 7.83(s, 2H), 7.51(d, 2H, J=8.0 Hz), 7.44(d, 2H, J=8.0 Hz), 7.08(d,
4H, J=8.0 Hz), 7.00(d, 4H, J=8.0 Hz), 6.93(d, 2H, J=8.0 Hz), 2.31(s, 6H, -CH3). 13C
NMR (CDCl3, 100 MHz, ppm): 149.9, 148.1, 144.5, 134.1, 133.8 132.0, 130.7, 128.9,
128.7, 128.0, 127.0, 125.8, 120.4, 20.9. HR-ESI-MS (m/z, [M+H]+) calcd. for
C28H26N3O+ 420.2070, found 420.2068.
3. Results and Discussion
3.1 Synthesis
As to 4-chlorobenzohydrazide, 4-methylbenzohydrazide, 4-nitrobenzoylhydrazine
4-pyridinecarboxylic hydrazide, benzoylhydrazine, the yields were consistently in the
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90-95% range.
In this work, we coupled diarylamines with aryl halides to prepare triarylamines by
the copper-catalyzed Ullmann condensation [30]. 4-(N, N-diphenylamino) benzaldehyde
(R2-CHO) was prepared in high yield via a Vilsmeier formylation reaction of the TPA
with DMF and POCl3 [27]. Hydrazone formation can be catalyzed by acid or base. The
synthetic route to the target molecules (III 1~5) were carried out from the corresponding
aryl hydrazide in ethanol condensation with R2-CHO, which was catalyzed by several
drops of acetic acid.
The method had been found to be more efficient, not only due to higher yields but
also because of the inexpensive reagent, simple reaction conditions, and a cleaner product
(Fig. 1).
3.2 Crystal structures of III-2 and III-3
The single crystal of III-2 and III-3 were grown by slowly evaporating the solvent
from its concentrated methanol solution. The crystal, data collection, and refinement
parameters are listed in Table 1. Selected bond lengths and torsion angles are shown in
Table 2. The molecular structures of III-2 and III-3 are given in Fig. 2, respectively. The
eight phene planes are labeled as P1, P2, P3, P4, P5, P6, P7, P8.
As shown in Fig. 2, all acylhydrazone bonds are in trans-configuration. Between
III-2 and III-3, three phenyl ring planes around the central nitrogen of triphenylamine
moieties are arranged in a propeller-like fashion with angles[31,32]: P1 and P2: 59.3˚, P2
and P3: 110.8˚, P3 and P1: 110.4˚; P5 and P6: 56.2˚, P6 and P7: 125.4˚, P7 and P5:
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117.3˚. The P3 (P7),-C=N-, -C=O-, P4 (P8) plane makes dihedral angles of 15.0˚, 7.4˚,
21.6˚ (0.6˚, 5.5˚, and 8.8˚), respectively, for III-2 (III-3).
It can be seen from Table 2 that all the bond lengths of C-C ( C-N of amide) are
located between the normal C=C double bond (1.34 Å) and C-C single bond (1.54
Å)[33](for C-N: 1.50 Å, C=N: 1.34 Å), which suggest that III-2 or III-3 has a highly
delocalized π-electron system in the molecule.
The plane of the acylhydrazone unit is in the same planar with the amide plane in
III-2: N1/N2/C9/C10= 177.53(9); III-3: N1/N2/C1/C2= 178.82(8), N2/N1/C22/C23= 179.84(7),
N2/N1/C22/O1= 0.52(13), and twisted slightly in III-2: N2/N1/C1/C2= 172.35(9),
N2/N1/C1/O1= 7.14(16). This in turn brings about appropriate changes in the bond lengths
and electron density surrounding the cross section in the III 2 and 3. In conclusion, the
change of bond lengths and the coplanar construction of III 2 and 3 show that there is a
π-conjugated system in the molecule, which is very important for the optical, electronic
and thermal properties of the compounds.
……………………………………Figure 2……………………………………
……………………………………Table 1……………………………………
……………………………………Table 2……………………………………
3.3 Photophysical properties of the III 1~5
The UV-vis absorption and Fluorescence spectra of III 1~5 in dilute solutions of
CH2Cl2 (4 × 10-5M) are shown in Fig. 3, in which the TPA is used as a reference
compound. The corresponding data are summarized in Table 3. In the absorption spectra,
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it is obvious that the absorption spectra of III 1~5 exhibited a large red shift of 71-85 nm
compared with that of the TPA. The III 1~5 dyes exhibit two major prominent bands,
appearing at 275-318 nm, 320-500 nm, respectively. The absorption band of 275-318 nm
can be ascribed to a localized aromatic π-π* transition of typical triarylamine [34, 35] and
the band around 320-431 nm is contributed to the combination of the π-π* transition of
triphenylamine moieties and the π-π* transitions of the substituted aryl groups [36, 37].
The introduction of benzoylhydrazone structure in the triphenylamine moiety increases
the λmax in CH2Cl2, which resulted from the extension of the π-conjugation structures of
the molecule. The molar extinction coefficients of the III 1 and 2 are 5.57×104 M-1 cm-1,
8.10×104 M-1 cm-1 respectively. However, the electron withdrawing group decreases the
molar extinction coefficients of the dyes, such as nitro group and pyridine. In conclusion,
due to the inductive effects of the substituent groups transferred by the double bond
systems in one molecule, the molar extinction coefficient of III 1~5 dramatically varies
depending on the substituent groups (III-2 > III-5 > III-1 > III-3 > III-4).
Upon 380 nm photoexcitation, one band in the 470-550 nm is observed in III-1,
III-2 and III-5 (Fig. 3). The fluorescence of III-2 and III-5 is strong, with a peak at 452
and 491 nm, but III-1 exhibits a weak emission at 456 nm. The presence of electron
withdrawing atom (III-4, X = N) or group (III-3, -NO2) leads to a completely quench
deactivation of the excited state for these triphenylamine derivatives. With introducing
the methyl group to the benzoyl part, the fluorescent intensity dramatically increased.
This should be resulted from the fact that the methyl group act as electron donor. Like the
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molar extinction coefficient in the absorption spectra, for III 1~5, the emissions are
sensitive to the para-substituents on the -CO-Ar group and the emission wavelength is
affected by the electronic properties of R in the order: H > Cl > Me > NO2/Pyridine. The
strong emission band of the molecule with electron-donating moieties is considered to be
the emissions from the higher energy band gap of III-1, III-2 and III-5, while no
emission from the lower energy band gap of III-3 and III-4 is obtained, indicating an
super-fast energy transfer from the excited state to the ground state of the corresponding
molecule.
The fluorescence lifetime measurements were performed on a HORIBA Jobin Yvon
IBH instrument based on time correlated single-photon counting (TCSPC) fluorescence
spectrometer, which the 370 nm Nanoled was operated as an excitation source. The
fitting of the fluorescence decay curves to a model of biexponential decay was performed
using an adapted Datastation software v2.4 package. Compared to those of the TPA, the
slower decay profiles of the III 1~5 as indicates that the molecules with the larger
π-conjugation result in the longer excitation lifetime. The corresponding radiative decay
curves of III 3, 4 are sharper than that of the III 1, 2, which may imply electron donor
group for the longer fluorescence lifetime (Fig. 4). As for extremely fast (8-12 ps) initial
relaxation, the exact fluorescence lifetime could not be obtained.
It should be noted that the absorption intensity of the long wavelength band is much
higher than that of the short wavelength absorption for all these triphenylamine-based
compounds. Furthermore the absorption was observed mainly in the ultraviolet (UV)
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region for the target molecules, suggesting high optical bandgaps with excellent
transparency in the visible region which is important for the hole transfer materials.
……………………………………Figure 3……………………………………
……………………………………Table 3……………………………………
……………………………………Figure 4……………………………………
3.4 Thermal properties of the III 1~5
The thermal properties of compounds III 1~5 are investigated by thermal
gravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Fig. 5, Fig. 6),
and the results are summarized in Table 4. All TGA figures of the compounds (III 1~5)
have the same trend, which showed weight loss at about 330 °C and are decomposed in
one step. The absence of weight loss up to 80 °C indicates that the solids have no water
molecule. As shown in Table 4, compounds III 1~5 possess high decomposition
temperatures (Td) in the range of 320-345 °C in nitrogen, which is significantly higher
than that of the TPA (234 °C). Compounds III 1~5 having a triphenylamine core and
larger π-conjugation extended by benzoylhydrazone struture have improved thermal
stability. With respect to the thermal stability, the glass transition temperature (Tg) is one
of the most important thermophysical parameters for the materials applied in OLED,
DSC, and so on. In this work we have explored the glass transition temperature (Tg). For
compounds III 1~5, the glassy state was confirmed by DSC and glass transition
temperatures for similar amorphous molecular materials is presented in Fig. 6. As shown
in the TPA derivatives studies, Tg can be increased by the incorporation of structurally
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rigid moieties-benzoylhydrazone moiety, because they decrease the flexibility of the
molecule due to strong rigid skeleton.
……………………………………Figure 5……………………………………
……………………………………Table 4……………………………………
……………………………………Figure 6……………………………………
3.5 Electrochemical analysis and theoretical calculations of III 1~5
Cyclic voltammetry was carried out to obtain the information about the
electrochemical stability and the reversibility of the redox process of III 1~5. All the
electrochemical data are summarized in Table 2. In order to demonstrate the good
electrochemical stability of III 1~5, the multiple cyclic voltammograms were carried out
and shown in Fig. 7. The anodic peaks located at 1.004V (III-1), 0.992 V (III-2), 1.014 V
(III-3), 1.056 V (III-3), 1.020 V (III-5). These peaks are due to oxidation processes of
the triphenylamine units by losing electrons. As expected, the multiple cyclic scans of the
corresponding molecular are not so different from its first cycle scan and the curves retain
their shape. The HOMO energy levels of the III 1~5 were determined from their peak
oxidation potentials (EHOMO = - Eoxe - 4.74e). The HOMO energy levels of III 1~5 were
thus calculated to be -5.74, -5.73, -5.75, -5.80, and 5.76 eV, respectively. The LUMO
energy levels of the III 1~5 were calculated from their Eg and HOMO energy levels
based on the relation of ELUMO = EHOMO+ Eg, Eg = 1240/λonset. The LUMO energy levels
were -2.85, -2.82, -3.13, -2.99 eV, and 2.84 eV respectively. The values of HOMO show
these five compounds are good candidates for hole-transporting materials (EHOMO
:
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5.73-5.80 eV) being close to that (5.30 eV) of the NPD materials currently used in
hole-transporting layer [38]. Therefore, these triphenylamines III 1~5 are potential good
materials as hole-transporting materials.
Based on the literature data, a hole-transporting materials with high polarizability
and small dipole moment (P) provides good charge transportation property [39]. DFT
calculations were performed to understand the charge distribution of the III 1~5 at the
B3LYP/6-31tG(D) level (Fig. 8). The calculation results showed that III-3 exhibited the
complete separation of HOMO and LUMO at their respective hole- and
electrontransporting, whereas III 1, 2, 4, 5 exhibits lesser segregation of HOMO and
LUMO. Also, the dipole moment (P) of III 1~5 were calculated to be 6.99, 4.54, 10.36,
7.79 and 4.38 D, respectively. III-5 with the small dipole moment (P) could lead to small
interaction energy workable with a charge of the carrier and small activation energy,
which is required when transporting charges between adjacent organic hole-transporting
materials [39]. The decrease in charge mobility of III 1~5 was III-5 > III-2 > III-1 >
III-4 > III-3.
……………………………………Figure 7……………………………………
4. Conlcusions
In conclusion, we have synthesized and characterized new triphenylamine
derivatives consisting of benzoylhydrazone structure, and studied their photophysical,
electrochemistry and thermal properties. Compared with that of TPA, the TGA results of
III 1-5 show that the model compound has enough resistance against thermal
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decomposition, which is due to the presence of the -CH=N-, C=O groups and the
aromatic ring in the target molecular. The experimental results showed that modification
of the π-conjugated systems with hydrazone structure substituted by the electron-donating
and electron-withdrawing groups on the triphenylamine core can effectively facilitate on
the various properties by comparing compound III-5 with III-1 to 4. It could be
mentioned that the attachment of electron-donating groups does not significantly modify
the optical absorption properties of the molecules (Fig. 3) or their optical gap (Table 3),
but increases the fluorescence lifetimes (Fig. 4). On the other hand, the attachment of
electron-withdrawing groups does not have an effect on the lifetimes, but quenches
completely the low-energy fluorescence band (Fig. 3), and reduces the optical gap (Table
3) and the molar extinction coefficient. None of the groups has a significant effect on the
thermal stability of the molecules. Molecule III-3 (with the NO2 group) is the most
promising one for low recombination rates due to the small spatial overlap of its frontier
orbitals (DFT). The cyclic voltammetry curves of III 1-5 show that the substituted
groups do not have an effect on the oxidation potentials and the multiple electrochemical
oxidation results indicate that the hydrazone series are potential hole-transport molecules
owing to their great stability after oxidation, which are also supported with the dipole
moment values calculated by density functional theory (DFT). Although it is not certain
that the π-conjugated systems represent the best molecular architecture to ensure the
applied properties of triphenylamine derivatives, a possible solution to these problems
could consist in the small π-conjugated molecules and a different substituted group as
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ideal materials. Their possible multifunctionality as organic emitters, hole transfer
materials and ligand precursors for the generation of new metal complexes would render
these materials promising for further investigations.
Acknowledgement
This work was supported by Tianjin University of Commerce (No. 090114), the
Innovation Fund for Small Technology-Based Firms of Tianjin (No. 13zxcxgx01052),
and the National Natural Science Foundation of China (No. 31271935).
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Table and Figure captions:
1 Table 1 Crystal data collection and structure refinement of III-2 and III-3
2 Table 2 Selected bond lengths (Å) and torsion angles of the III-2 and III-3
3 Table 3 Photophysical properties and frontier orbital energy levels of TPA and III 1~5
4 Table 4 Thermal properties of TPA and compounds III 1~5
5 Figure 1 Synthesis of compound III 1~5
6 Figure 2 Crystal structures of the III-2 and III-3
7 Figure 3 UV-visible absorption spectra and Fluorescence spectra of TPA and III 1~5
measured at 4×10-5 M in CH2Cl2
8 Figure 4 Time-resolved fluorescence curves of TPA and III 1~5 measured at 4×10-5 M
in CH2Cl2
9
Figure 5 TGA of compounds III 1~5
10 Figure 6 Differential scanning calorimetry thermograms of III 1~5 with a heating rate
of 5 °C min-1
11 Figure 7 Multiple cyclic voltammograms scans (2-9 cycles) for III 1~5 (10-3 M) in
CH2Cl2. (a) III-1, (b) III 2, (c) III-3, (d) III-4, (e) III-5
12 Figure 8 Frontier orbitals of the dyes III 1~5 optimized at the B3LYP/6-31tG(D) level
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Table 1 Crystal data collection and structure refinement of III-2 and III-3
Compound
III-2
III-3
C29H27N3O
C28H24N3O3
Molecular formula
Formula weight
Crystal system
Space group
433.54
464.52
Triclinic
P-1
Triclinic,
P-1
6.6850(7)
8.8050(10)
20.930(2)
95.04(2)
90.27(2)
96.87(2)
1218.3(2)
2
6.9600(17)
7.2877(18)
25.811(4)
82.645(15)
84.039(17)
64.6560(10)
1171.7(4)
2
a(
b(
c(
Å)
Å)
Å)
α(˚)
β(˚)
γ(˚)
V (Å3)
Z
Dcalcd (g cm-3)
F (000)
1.269
1.078
496
400
1.954-33.918
21241
1.593-36.359
22974
θ range(˚)
Reflections collected
Independent reflections
Rint
9636
10978
0.0571
0.0391
9636
10978
Observed reflections
No. of parameters
R1, wR2[I>2.0σ (I)]
R1, wR2 (all data)
Goodness of fit on F2
328
339
0.0487, 0.1133
0.0744, 0.1218
0.900
0.0476, 0.1206
0.0732, 0.1338
0.993
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Table 2 Selected bond lengths (Å) and torsion angles of the III-2 and III-3
III-2
Bond lengths
C(1)-C(2)
N(1)-C(1)
N(1)-N(2)
N(2)-C(9)
C(9)-C(10)
III-3
Torsion angles
1.4729(15)
1.3549(14)
1.3671(12)
1.2775(14)
1.4417(14)
N(2)-N(1)-C(1)-O(1)
N(2)-N(1)-C(1)-C(2)
N(1)-C(1)-C(2)-C(7)
C(3)-C(4)-C(5)-C(8)
N(1)-N(2)-C(9)-C(10)
7.14(16)
-172.35(9)
22.05(16)
178.75(10)
-177.53(9)
Bond lengths
N(1)-C(22)
N(1)-N(2)
N(2)-C(1)
C(1)-C(2)
C(22)-C(23)
Torsion angles
1.3522(11)
1.3872(11)
1.2834(12)
1.4566(12)
1.4996(12)
N(1)-N(2)-C(1)-C(2)
N(2)-C(1)-C(2)-C(7)
N(2)-N(1)-C(22)-O(1)
N(2)-N(1)-C(22)-C(23)
O(3)-N(4)-C(26)-C(25)
178.82(8)
0.03(14)
-0.52(13)
-179.84(7)
5.68(13)
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Table 3 Optical data, electrochemical data and dipole moment (P) of the TPA and III
1~5
Compound
TPA
III-1
379
5.57
456
77
429
2.89
1.004
-5.74
-2.85
6.99
III-2
377
8.10
452
75
426
2.91
0.992
-5.73
-2.82
4.54
III-3
373
3.42
h
71
473
2.62
1.014
-5.75
-3.13
10.36
III-4
385
3.14
h
III-5
377
6.52
491
75
425
2.92
1.020
-5.76
-2.89
4.38
λabs (nm) a
302
ε (104M-1cm-1) b 4.53
λem (nm) c
ꢀ (nm) d
g
g
83
λonset (nm) e
Eg (eV)f
336
3.69
0.940
-5.68
-1.99
0.45
441
2.81
1.056
-5.80
-2.99
7.79
i
Eox
EHOMO(eV) j
ELUMO(eV) k
P(D) l
a Absorption maximum in CH2Cl2 solution. b The molar extinction coefficient at λmax in
CH2Cl2 solution. c Emission maximum in CH2Cl2 solution. d The red-shift compared
with TPA . e The bond edges in the UV-Vis absorption spectra of the sample. f Obtained
from UV-Vis spectra: Eg = 1240/λonset. g undetermined. h undetected. i Obtained from the
cyclic voltammograms. j Determined from the onset of the oxidation peak: EHOMO =
-
Eoxe - 4.74e. k Calculated by the HOMO values and the energy gaps (Egopt): ELUMO =
EHOMO+ Eg. l. Calculated by the DFT calculations at B3LYP/6-31(d) level.
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Table 4 Thermal properties of TPA and compounds III 1~5
Dye
Td(°C)
234
345
339
321
Tg(°C)
-
Yc(%)
100
TPA
III-1
III-2
III-3
III-4
III-5
94.16
94.16
99.15
69.16
104.47
92.3
90.7
74.0
92.4
82.4
341
320
Td The temperature for which the compound is decomposed.
Tg The glass transition temperature for TPA and III 1~5.
Yc Char yield at 700°C.
28