Spectral studies of multi-branched fluorescence dyes based on triphenylpyridine core

Article abstract of DOI:10.1016/j.saa.2013.10.087

A series of novel triphenylpyridine-containing triphenylamine derivatives have been carefully designed and prepared in good yields using the stepwise route reactions. The relationship of photoluminescence property and structure of compounds 9-13 was syste

Full text of DOI:10.1016/j.saa.2013.10.087

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectral studies of multi-branched fluorescence dyes based
on triphenylpyridine core
⇑
⇑
Hai-Ying Wang, Liang-Feng Chen, Xiu-Ling Zhu, Chao Wang, Yu Wan , Hui Wu
School of Chemistry and Chemical Engineering, Jiangsu Normal University, Xuzhou 221116, China
Jiangsu Key Laboratory of Biotechnology on Medical Plant, Xuzhou 221116, China
Jiangsu Key Laboratory of Green Synthesis for Functional Materials, Xuzhou 221116, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
New framework triphenylpyridine derivatives containing triphenylamine groups were synthesized and
the relationship of photoluminescence property and structure were investigated. Quantum chemical cal-
culations were used to obtain optimized ground-state geometry, spatial distributions of the HOMO,
LUMO levels of the compounds.
ꢀ
ꢀ
New fluorescence dyes based on
triphenylpyridine were synthesized.
Compounds exhibited efficient
emission from blue to green with
high quantum yields.
ꢀ
ꢀ
ꢀ
HOMO/LUMO energy levels were
obtained by CV and theoretical
calculation.
The effects of solvents on the
fluorescence characteristics were
investigated.
All of the compounds possess of
suited HOMO ranges (ꢁ5.56 to
ꢁ5.70 eV).
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 August 2013
Received in revised form 29 September 2013
Accepted 15 October 2013
Available online 28 October 2013
A series of novel triphenylpyridine-containing triphenylamine derivatives have been carefully designed
and prepared in good yields using the stepwise route reactions. The relationship of photoluminescence
property and structure of compounds 9–13 was systematically investigated via UV–vis, fluorescence,
thermogravimetric and electrochemical analyzer. The highest occupied molecular orbital and the lowest
unoccupied molecular orbital distributions of compounds 9–13 were calculated by density functional
theory method. The high fluorescence quantum yields, desirable the highest occupied molecular orbital
levels and high thermal stability of compounds 9–13 indicate that the linkage of triphenylpyridine and
triphenylamine is an efficient means to enhance hole-transporting ability and fluorescent quantum yield.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Triphenylpyridine
Triphenylamine
Fluorescent
Cyclic voltammetry
Chemical calculation
Electrochemical
Introduction
Organic fluorescent compounds have been extensively investi-
gated for a myriad of potential applications in the biological labels,
photovoltaic cells, light emitting diodes, and optical sensors etc.
⇑
Corresponding authors. Tel./fax: +86 516 83403163.
E-mail addresses: wanyu@jsnu.edu.cn (Y. Wan), wuhui72@yahoo.com.cn
(H. Wu).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.10.087

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H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
[1–12]. Fluorescent characteristic relies largely on molecular struc-
ture and assembly. Therefore, it is important to clarify the struc-
ture–property relationship of fluorescence because it enable us to
design and employ more useful fluorescent reagents in the fields
of analytical, biological chemistry and OLEDs in the future.
Triphenylamine molecular possesses a propeller-shaped struc-
ture with highly rich electron and it can maintain uninterrupted
conjugation between central nitrogen lone pair electrons and the
arms. In recent years, it has been widely employed in organic light
emitting diodes, organic solar cells or organic field-effect transis-
tors as electron-donating moieties [13–33]. Pyridine has been a
key building block in constructing functional materials in view of
its outstanding mechanical and dielectric properties [34–39].
Therefore, to afford suitable fluorescent materials [40,41] with
higher hole-transporting ability and fluorescent quantum yield
for analytical and biological chemistry, herein we introduced tri-
phenylamine units into the 2,4,6-triphenylpyridine framework.
As expected, the fluorescence emission color of products can be
easily tuned from blue to green by changing the number of tri-
phenylamine moieties and these compounds 9–13 possess higher
fluorescence quantum yields (0.30–0.45) except compound 12.
Particularly, these compounds exhibit good the highest occupied
molecular orbital (HOMO) levels (ꢁ5.56 to ꢁ5.70 eV), which is
lower than that of the widely-used, hole-transporting material,
4,4-bis(1-naphthylphenylamino)biphenyl (NBP) (ꢁ5.50 eV), it
might be beneficial for the hole-transport capacity. As a result,
these compounds lead to promising applications in functional
materials.
Compound 2a: yield 82%, white 1H NMR (400 MHz, CDCl3):
d 9.81 (s, 1H), 7.69 (d, J = 8.8 Hz, 2H), 7.34 (t, J = 8.0 Hz, 4H),
7.17–7.19 (m, 6H), 7.03 (d, J = 8.8 Hz, 2H).
4,40-Diformyl triphenylamine (2b)
Phosphorus oxychloride (9.23 mL, 0.1 mol) was added dropwise
to a stirred (7.74 mL, 0.1 mol) of DMF at 0 °C. The mixture was stir-
red at 0 °C for 1 h and then stirred at room temperature for another
1 h. After the loading of (10.0 g, 0.04 mol) of triphenylamine dis-
solved in chloroform, the mixture was stirred at 100 °C for 48 h.
After cooling, the solution was poured into cold water. The
resulting mixture was neutralized to pH 7 with 5% NaOH aqueous
solution and extracted with dichloromethane. The extract was
washed with plenty of brine and the solvent was removed at
vacuum. The residue was chromatographed on a silica gel column
(silica gel, hexane/dichloromethane, 3/1, v/v) to produce of yellow-
ish solid.
Compound 2b: yield 70%, yellow 1H NMR (400 MHz, CDCl3):
d 9.89 (s, 2H), 7.84 (d, J = 8.4 Hz, 4H), 7.40 (t, J = 7.6 Hz, 2H), 7.26
(t, J = 6.8 Hz, 1H), 7.18–7.20 (m, 6H).
Tris-(4-formyl-phenyl)amine (2c)
Phosphorus oxychloride (9.23 mL, 0.1 mol) was added dropwise
to a stirred (7.74 mL, 1.0 mol) of DMF at 0 °C. The mixture was stir-
red at 0 °C for 1 h and additionally stirred at room temperature for
1 h. After the addition of (6.0 g, 0.02 mol) of 2b in chloroform, the
mixture was stirred at 100 °C for 48 h. After cooling, the solution
was poured into water. The resulting mixture was neutralized to
pH 7 with 5% NaOH aqueous solution and extracted with dichloro-
methane. The extract was washed with plenty of brine and the sol-
vent was removed at vacuum. The residue was chromatographed
on a silica gel column (silica gel, hexane/dichloromethane, 2/1,
v/v) to produce yellowish solid.
Experimental
Chemicals and instruments
All solvents were carefully dried and freshly distilled. All reac-
tants were commercially available and used without further puri-
fication. Melting points were recorded on Electrothermal digital
melting point apparatus and were uncorrected. 1H and spectra
were recorded at 295 K on a Bruker Avance DPX-400 MHz spec-
trometer using CDCl3 or d6-DMSO as solvent and TMS as internal
standard. UV–vis absorption spectra were recorded on a Shimadzu
UV-2501PC spectrometer. Fluorescence spectra were obtained on a
Hitachi FL-4500 spectrofluorometer. HRMS data were measured
using microTOF-Q(ESI) instrument. Thermal properties was per-
formed under nitrogen on a SDT 2960 (heating rate of 20 °C minꢁ1).
Cyclic voltammetry was carried on a Chi 1200 A electrochemical
analyzer with three-electrode cell (Platinum was used as working
electrode and as counter electrode, and SCE (saturated calomel
electrode) as reference electrode) in CH2Cl2 solution in the pres-
ence of TBAHFP (tetrabutylammonium hexafluorophosphate)
(0.10 mol Lꢁ1) as supporting electrolyte.
Compound 2c: yield 20%, yellow 1H NMR (300 MHz, CDCl3):
d 9.95 (s, 3H), 7.86 (d, J = 8.7 Hz, 6H), 7.27 (d, J = 8.1 Hz, 6H).
1,3,5-Trip-tolylpentane-1,5-dione (5)
A mixture of p-tolualdehyde 3 (1.8 g, 15 mmol), 4-methylaceto-
phenone 4 (4.3 g, 32 mmol) and powder NaOH (2.4 g, 60 mmol)
were crashed together with a pestle and mortar for 2 h and then
recrystallized with ethanol to give white needle crystal. Yield:
4.73 g, 85%.
2,6-Diphenyl-4-p-tolyl-pyridine (6d)
1,3,5-trip-tolylpentane-1,5-dione (5d) (3.7 g, 10 mmol) was
added to a stirred solution of ammonium acetate (8 g, excess) in
ethanol (100 mL). The reaction mixture was heated at refluxing
for 10 h. Upon cooling to room temperature; a precipitate was
filtered, washed with water three times and dried to afford the
product. It was purified by flash column chromatography on silica.
Elution with petroleum/ethyl acetate (8:1) gave a white solid 6d.
Yield: 2.8 g, 75%. Mp: 181–183 °C.IR (cmꢁ1): 3027, 2962, 2919,
2858, 1600, 1543, 1389, 1184, 1114, 1018, and 809. 1H NMR
(CDCl3, 400 MHz, ppm) d 8.09 (d, J = 7.9 Hz, 2H), 7.81 (s, 2H),
7.63 (d, J = 7.8 Hz, 2H), 7.30 (d, 6H), 2.42 (s, 9H).
4-(Diphenylamino)benzaldehyde (2a)
Phosphorus oxychloride (1.6 mL, 16.8 mmol) was added
dropwise to DMF (1.3 mL, 19.5 mmol) at 0 °C, and the mixture
was stirred at 0 °C for 1 h. Triphenylamine (3.3 g, 13.3 mmol)
was added and the reaction mixture was stirred at 100 °C for 6 h.
Then, the mixture was cooled to room temperature, poured into
ice water and carefully neutralized to pH 7 with 5% NaOH aqueous
solution. The solution was extracted with dichloromethane
(3 ꢂ 150 mL). Then, the organic phase was washed with water
(2 ꢂ 100 mL) and dried over anhydrous MgSO4. After filtration,
the solvent was removed. The crude product was purified by
column chromatography (silica gel, hexane/dichloromethane,
3/1, v/v) to produce white solid.
Phosphonium salt (8)
Phosphonium salt 8 prepared via free radical bromination of
(6a–c) with NBS (Fig. 1) and was reacted with triethylphosphite
to yield phosphoniumsalt 8a–c (without isolation for further
reaction).

H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
357
Fig. 1. Synthetic routines for compound 2 and 8a. Reagents and conditions: (a) POCl3, DMF, 0 °C, CHCl3, 100 °C; (b) NaOH (c) NH4OAc, EtOH, 80 °C; (d) NBS, CCl4, 80 °C;
(e) PO(OEt)3, 185 °C.
Fig. 2. Synthetic routines of 11. Reagents and conditions: (f) cat. Pd(PPh3)4, 2 mol Lꢁ1 K2CO3, toluene, 90 °C.
4-(2,6-Diphenypyridin-4-yl)-N,N-diphenylaniline (9)
40-(2,6-Diphenypyridin-4-yl)-N,N-diphenylbiphenyl-4-amine (11)
The compound 9 was synthesized through the same procedure
as that of compound 6 described above using 2a and 4 as starting
material.
Under a nitrogen atmosphere, a mixture of compound 6d
(1.0 mmol), Pd(PPh3)4 catalyst (0.04 mmol) and the corresponding
triphenylamine boronic acid 10 was stirred in dry toluene (15 mL).
Then, 2 mol Lꢁ1 K2CO3 (aq) solution (2 mL)was added via syringe.
The reaction mixture was refluxed for 72 h. After cooling, the prod-
uct was extracted with DCM, washed with water, dried over
MgSO4, filtered, concentrated and further purified by column
chromatography (silica gel, hexane/dichloromethane, 10/1, v/v),
to afford pure compounds 11.
Yield 90%, yellow solid, m. p. 165.0–166.1 °C, 1H NMR
(400 MHz, CDCl3): d 8.20 (d, J = 4.0 Hz, 4H), 7.87 (s, 2H), 7.64
(d, J = 4.0 Hz, 2H), 7.54–7.50 (m, 4H), 7.47–7.43 (m, 2H), 7.33–
7.29 (m, 4H), 7.20–7.16 (m, 6H), 7.11–7.05 (m, 2H) ppm. 13C
NMR (100 MHz, CDCl3): d 116.51, 123.18, 123.57, 124.94, 127.16,
127.92, 128.72, 129.01, 129.48, 132.11, 139.75, 147.37, 148.88,
149.56, 157.47 ppm.
Yield 85%, white solid, m. p. 213.1–214.6 oC, 1H NMR (400 MHz,
CDCl3): d 8.22 (d, J = 4.0 Hz, 4H), 7.94 (s, 2H), 7.83 (d, J = 4.0 Hz, 2H),
7.74 (d, J = 4.0 Hz, 2H), 7.56–7.46 (m, 8H), 7.30 (d, J = 4.0 Hz, 4H),
HRMS (ESI) m/z: calc. for C35H26N2, (M + H)+: 475.2174, Found:
475.2131.

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H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
Fig. 3. Synthetic routines of 12 and 13. Reagents and conditions: (g) THF, 60 °C.
7.17 (s, 6H), 7.07 (d, J = 4 Hz, 2H) ppm. 13C NMR (100 MHz, CDCl3):
d 123.16, 123.65, 124.62, 127.20, 127.23, 127.54, 127.73, 128.73,
129.12, 129.34, 133.85, 137.11, 139.42, 141.44, 147.57, 147.68,
149.84, 157.49 ppm.
General procedure for the synthesis of compounds (12 and 13)
Under a nitrogen atmosphere, a mixture of compounds 8a-c and
compound 2a-c (2.0 mmol) in dry THF (5 mL) was added dropwise
and the mixture were refluxed at 60 °C for 2 h. The mixture was
cooled and quenched with EtOH. The solvents were then removed
HRMS (ESI) m/z: calc. for C41H30N2 , (M + H)+: 551.2487, Found:
551.2443.

H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
359
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
550
9
11
500
12a
THF
12b
450
DMF
12c
Toluene
13a
13b
400
350
300
250
200
150
100
50
0
400
500
600
250
300
350
400
450
500
Wavelength / nm
Wavelength / nm
Fig. 6. The emission spectra of compound 13a in different solvents (1 ꢂ 10ꢁ7
-
Fig. 4. The absorption spectra of compounds 9–13 (1 ꢂ 10ꢁ6 mol Lꢁ1 in CH2Cl2).
mol Lꢁ1) as sample.
10000
9000
124.62, 125.10, 126.02, 126.31, 126.95, 127.15, 127.41, 127.51,
128.70, 129.03, 129.23, 129.32, 129.72, 137.53, 138.52, 139.63,
147.48, 147.68, 149.61, 157.55 ppm.
HRMS (ESI) m/z: calc. for C43H32N2 , (M + H)+: 577.2644, Found:
577.2598.
9
11
12a
12b
8000
12c
13a
7000
13b
6000
4-(4-(2,6-Diphenylpyridin-4-yl)styryl)-N-(4-(4-(2,6-diphenylpyridin-
4-yl)styrl)phenyl)-N-phenylaniline (12b)
5000
4000
3000
2000
1000
0
Yield 60%, yellow solid, m. p. 139.0–141.6 °C, 1H NMR
(400 MHz, CDCl3): d 7.74 (d, J = 4.0 Hz, 7H), 7.60–7.58 (m, 4H),
7.40–7.32 (m, 22H), 7.16–7.13 (m, 4H), 5.96 (s, 4H), 5.53 (s, 8H)
ppm. 13C NMR (100 MHz, CDCl3):
d 116.72, 123.66, 123.95,
125.01, 126.38, 127.02, 127.18, 127.45, 127.64, 128.74, 129.07,
129.15, 129.48, 131.67, 137.64, 138.46, 139.66, 147.19, 147.28,
149.59, 157.58 ppm.
HRMS (ESI) m/z: calc. for C68H49N3 , (M + H)+: 908.4005, Found:
908.3998.
400
450
500
550
600
650
Wavelength / nm
Fig. 5. PL emission spectra of compounds 9–13 (1 ꢂ 10ꢁ6 mol Lꢁ1 in THF).
Tris(4-(4-(2,6-diphenylpyridin-4-yl)styryl)phenyl)amine (12c)
Yield 34%, yellow solid, m. p. 158.1–160.5 °C, 1H NMR
(400 MHz, CDCl3): d 8.22 (d, J = 4.0 Hz, 12H), 7.92 (s, 6H), 7.78
(d, J = 4.0 Hz, 5H), 7.67 (d, J = 4.0 Hz, 5H), 7.55–7.45 (m, 26H),
7.23–7.10 (m, 12H) ppm. 13C NMR (100 MHz, CDCl3): d 116.72,
124.36, 126.63, 127.05, 127.17, 127.47, 127.72, 128.74, 129.08,
132.12, 137.72, 138.38, 139.64, 146.88, 149.56, 157.59 ppm.
HRMS (ESI) m/z: calc. for C93H66N4 , (M + H)+: 1240.5399,
Found: 1240.5087.
Table 1
Optical properties of the compounds 9–13.
Compound
Abs. (nm)
CH2Cl2
em (nm)
THF
Ua
Toluene
DMF
9
11
354
353
412
387
403
395
391
415
428
455
463
468
448
452
445
463
491
497
502
474
484
470
501
520
532
540
504
509
0.42
0.45
0.35
0.30
0.27
0.32
0.33
12a
12b
12c
13a
13b
4,40-(1E,10E)-2,20-(4,40-(4-phenylpyridine-2,6-diyl)bis(4,1-
phenylene))bis(ethene-2,1-diyl)bis(N,N-diphenylaniline) (13a)
Yield 68%, yellow solid, m. p. 125.4–126.7 °C, 1H NMR
(400 MHz, CDCl3): d 8.27–8.21 (m, 3H), 7.91–7.85 (m, 2H), 7.78–
7.63 (m, 6H) 7.57–7.42 (m, 6H), 7.36–7.27 (m, 6H), 7.19–6.98 (m,
24H) ppm. 13C NMR (100 MHz, CDCl3): d 116.78, 119.37, 122.66,
123.09, 123.52, 124.17, 124.21, 124.56, 125.10, 126.31, 126.59,
126.64, 127.20, 127.37, 127.47, 127.72, 128.77, 129.11, 129.19,
129.23, 129.30, 129.73, 131.43, 138.35, 138.42, 139.16, 147.51,
147.54, 150.14, 157.01 ppm.
a
The fluorescence quantum yields (U) were measured in THF using quinine
sulfate (U = 0.55) as standard [46].
under vacuum, the solid was dissolved in CH2Cl2, washed in dis-
tilled H2O, dried over MgSO4 and the solvent was removed. The
residue was chromatographed on a silica gel column (silica gel,
hexane/dichloromethane, 4/1, v/v) to produce compound 12
and13.
HRMS (ESI) m/z: calc. for C63H47N3 , (M + H)+: 847.3882, Found:
847.3847.
(E)-4-(4-(2,6-diphenylpyridin-4-yl)styryl)-N,N-diphenylaniline (12a)
Yield 72%, yellow solid, m. p. 182.2–183.5 °C, 1H NMR
(400 MHz, CDCl3): d 8.22 (d, J = 4.0 Hz, 4H), 7.92 (s, 2H), 7.76 (d,
J = 4.0 Hz, 2H), 7.65 (d, J = 4.0 Hz, 2H), 7.55–7.51 (m, 4H), 7.48–
7.42 (m, 4H), 7.29 (d, J = 4.0 Hz, 4H), 7.17–7.03 (m, 10H) ppm.
13C NMR (100 MHz, CDCl3): d 116.72, 119.36, 123.17, 123.39,
4,40,400-(1E,10E,100E)-2,20,200-(4,40,400-(pyridine-2,4,6-triyl)tris(benzene-
4,1-diyl)tris(ethene-2,1-diyl)tris(N,N-diphenylaniline) (13b)
Yield 29%, yellow solid, m. p. 142.1–144.6 °C, 1H NMR
(400 MHz, d6-CDCl3): d 8.22 (d, 4H, J = 4.0 Hz), 7.89 (d, 2H,

360
H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
Table 2
Thermal, and electrochemical properties of the compounds 9–13.
e
Compound Band
gapa
EHOMO
ELUMO
(eV)a
/
Eg
E
oxonsetc
EHOMO
ELUMO
(eV)
/
Tg/Td
(°C)
(eV)b (V)
d
9
3.81
3.59
3.21
3.05
3.05
3.19
3.00
ꢁ5.11/
ꢁ1.30
ꢁ5.03/
ꢁ1.44
ꢁ4.92/
ꢁ1.71
ꢁ4.92/
ꢁ1.87
ꢁ4.78/
ꢁ1.73
ꢁ4.79/
ꢁ1.60
ꢁ4.93/
ꢁ1.93
2.95
3.02
2.61
2.76
2.64
2.70
2.74
1.23
ꢁ5.67/
ꢁ2.72
ꢁ5.66/
ꢁ2.64
ꢁ5.70/
ꢁ3.09
ꢁ5.56/
ꢁ2.80
ꢁ5.68/
ꢁ3.04
ꢁ5.74/
ꢁ3.09
ꢁ5.56/
ꢁ2.82
68/
188
62/
170
95/
208
80/
11
1.26
1.30
1.16
1.28
1.34
1.16
12a
12b
12c
13a
13b
230
89/
226
110/
226
127/
204
Fig. 7. Cyclic voltammogram of compound 13c (1 ꢂ 10ꢁ3 mol Lꢁ1) as sample, in
0.1 mol Lꢁ1 Bu4NPF6–CHCl3, scan rate 100 mV/s.
a
b
c
DFT/B3LYP calculated values.
Optical energy gaps calculated from the edge of the electronic absorption band.
Oxidation potential in CHCl3 (10ꢁ3 mol Lꢁ1) containing 0.1 mol Lꢁ1 (n-C4H9)4-
NPF6 with a scan rate of 100 mV sꢁ1
.
d
EHOMO was calculated by Eox + 4.4 V, and ELUMO = EHOMO–Eg.
e
Measured by TG–DTA analysis under N2 at a heating rate of 20 °C minꢁ1
.
J = 4.0 Hz), 7.76 (d, 2H, J = 4.0 Hz), 7.65 (d, 6H, J = 4.0 Hz), 7.52–7.27
(m, 10H), 7.14–7.01 (m, 38H) ppm. 13C NMR (100 MHz, CDCl3): d
122.82, 123.11, 123.20, 123.44, 123.54, 124.08, 124.23, 124.58,
124.65, 126.67, 126.98, 127.40, 127.51, 127.56, 128.27, 128.77,
129.26, 129.33, 131.45, 138.41, 147.52, 147.56 ppm.
HRMS (ESI) m/z: calc. for C83H62N4 , (M + Na)+: 1137.4872,
Found: 1137.4844.
Fig. 8. Optimized ground-state geometry of compounds 13a with B3LYP/6-31Gꢃ in
gas phase.
Results and discussion
Compounds 9 and 11 have similar fluorescence spectra because
these compounds possess a similar structure. Especially, the simi-
lar shape, position and one emission peak near 450 nm were
observed for these molecules.
The further examination in the different emission behavior of
these compounds found out that these compounds showed a red
shift with increasing solvent polarity (Fig. 6) [43]. These results
indicate that these compounds are more polar in the excited state
than in the ground state [44] and an increase in the polarity of the
solvent will lower the energy level of the charge transfer excited
state [45].
Compounds 2, 6 and 9 were synthesized according to the liter-
ature methods [22,35] (Fig. 1). For products 11, 12 and 13, the tri-
phenylamine was introduced via a Pd(0) catalyzed Suzuki C–C
coupling reaction or Wittig–Horner condensation reactions [41]
(Figs. 2 and 3). New compounds were characterized by MS spec-
trometry, 1H NMR spectroscopy.
Optical properties
The fluorescence quantum yields (U) were measured in the THF
solution using quinine sulfate (U = 0.55) as a standard (Table 1)
[46]. The emission efficiency in dilute solution largely depends
on the molecular structure. The fluorescence quantum yields of
the compounds are in the range of 0.27–0.45. The U value of 0.42
and 0.45 was observed for 9 and 11, which was higher than that
of compounds 12 and 13. They show that the introduction of alke-
nyl bridge and multi-branched structure reduce coplanarity degree
of compounds 12 and 13, which might lead to concentration
quenching effects [47]. Moreover, this difference of the quantum
yields may appear during the process of the exciton migration
[48], or result from the change of the molecular size [49].
The UV–vis absorption and fluorescence properties of com-
pounds 9–13 in CH2Cl2 and THF are shown in Fig. 4 and 5. Their
comprehensive photophysical characteristics are summarized in
Table 1. The absorption spectra of these compounds were compli-
cated due to multiple overlapping broad bands (Fig. 4). The maxi-
mum UV–vis absorptions of the compounds 9–13 are located in the
range of 355–410 nm, which is supposed to be ascribed to the
transition of the conjugated molecular backbone.
The maximum absorption peak of the alkenyl bridged oligomer
12a was red shifted by 65 nm with respect to the Compounds 9
p–
pꢃ
and 11. The
from the UV–vis absorption maximum [42]. It was obvious that the
E value of 9–13 could be reduced by the introduction of alkenyl
p–p energy gap (DE) of these oligomers was calculated
D
spacers at the backbone.
Thermal properties
Fig. 5 shows the fluorescence emission spectra of compounds
9–13 in the excitation of 350 nm. These compounds indicate a blue
to green fluorescence emission with the maximum emission peaks
varying from 445 to 502 nm in THF solutions. With increasing tri-
phenylpyridine or triphenylamine moieties, the fluorescence emis-
sion peaks for 9–13 are gradually red shifted. Compound 12c
exhibits strong green fluorescence maxima at about 502 nm due
to possess maxima conjugation length.
Glass transition temperatures (Tg) and decomposition tempera-
tures (Td) were determined by differential scanning calorimetry
(DSC) and thermal gravimetric analysis (TGA), respectively, using
a heating rate of 20 °C minꢁ1. The glass transition temperatures
of these compounds were observed at ca. 62–127 °C (Table 2).
The experiment results revealed that the thermal stability of these
compounds seemed to be encouraging. Thermal decomposition

H.-Y. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 355–362
361
Fig. 9. Calculated spatial distributions of the HOMO, LUMO levels of compound 13a as sample.
temperatures for 9–13 were observed at 188, 170, 208, 230, 226,
226 and 204 °C, respectively, which is due to the conjugation
length and asymmetric nature of the molecules.
have been prepared by a stepwise route in good yields. The optical
properties clearly indicate that the fluorescent emission properties
of these compounds rely largely on the molecular structure. As
expected, the optical band gaps decrease considerably as the num-
ber of triphenylamine moieties introduced into triphenylpyridine
framework increases. Moreover, compounds exhibited high
fluorescence quantum yields, high thermal stability and excellent
luminescence emission from blue to green. The DFT calculations
establish that they all possess a high HOMO energy level (ꢁ4.92
to ꢁ5.11 eV) due to the presence of the electron-rich amine moie-
ties and increased conjugation lengths, giving rise to more bal-
anced charge-transport characteristics, which have promising
potential for application in OLEDs as a multifunctional material.
Electrochemical properties
The electrochemical properties of compounds 9–13 are
explored by the cyclic voltammetry in the CHCl3 solutions in the
presence
of
tetrabutylammonium
hexafluorophosphate
(0.10 mol Lꢁ1) as the supporting electrolyte (Table 2). All of
compounds have one reversible oxidation peak which is an
indication of a stable cation radical (Fig. 7). According to a refer-
ence to ferrocene (4.8 eV), the HOMO energy of these materials
was calculated to get a range of ꢁ5.56 to ꢁ5.70 eV [50,51]. Since
the HOMO energy level is close to that of the most widely used
hole-transport material 4,40-bis(1-naphthylphenylamino)biphenyl
(NBP) (ꢁ5.20 eV, ꢁ2.4 eV), it might be beneficial for the hole-trans-
port capacity [51]. Similarly, the optical edge was utilized to de-
duce the band gap and the lowest unoccupied molecular orbital
(LUMO) energies. As expected, these compounds are of lower
LUMO (ꢁ2.61 to ꢁ2.74 eV) energies and smaller band gaps com-
pared with other triphenylamine derivatives [52]. Their LUMO lev-
els represent a small barrier for the electron injection from a
commonly used cathode such as barium, which has a work func-
tion of ꢁ2.2 eV [53]. Therefore, these compounds might be very
useful as hole-transporting and electron-transporting materials in
applications for OLEDs [53].
Acknowledgements
We are grateful to Dr. Guo-Lan Dou in China University of
Mining and Technology for great computational assistances.
The work was partially supported by the foundation of the
‘‘Priority Academic Program Development of Jiangsu Higher Edu-
cation Institutions’’, Key Basic Research Project in Jiangsu Univer-
sity (Nos. 10KJA430050, 13KJA430002) and Nature Science of
Jiangsu Province for Higher Education (No. 11KJB150017).
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