The synthesis, photophysical and electrochemical properties of a series of novel 3,8,13-substituted triindole derivatives

Article abstract of DOI:10.1016/j.dyepig.2012.06.011

The synthesis and properties of a series of new 3,8,13-substituted triindole derivatives 1a-1e are reported. The 3,8,13-substituted triindoles were thermally robust with high decomposition temperatures (≥405°C) and high melt transitions (219°C-373°C). Compound 5e was crystallized in the monoclinic system with the space group P2₁/n. These compounds showed UV-Vis absorption (λ_max^Abs) in the range of 311-345 nm in DCM solution and 371-391 nm in solid state, and fluorescence maxima (λ_max^Em) in the range of 394-412 nm in DCM solution and 416-461 nm in solid state. The fluorescence quantum yields ranged from 0.27 to 0.58. The estimated electron affinities (LUMO levels) and estimated ionization potential (HOMO levels) of compounds 1a-1e are 3.54-3.71 eV and 5.12-5.48 eV, respectively. Quantum chemical calculations using DFT B3LYP/6-31G showed nearly identical LUMO (-0.72 to -1.10 eV) and HOMO (-4.65 to -4.84 eV) values. These results demonstrated that the new 3,8,13-substituted triindoles are promising thermally stable host materials for organic light-emitting diodes with reasonable hole mobility.

Full text of DOI:10.1016/j.dyepig.2012.06.011

Dyes and Pigments 95 (2012) 679e688
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis, photophysical and electrochemical properties of a series of novel
3,8,13-substituted triindole derivatives
,
Tianhao Zhu a, Guangke He a, Jin Chang b, Dongdong Zhao a, Xiaolin Zhu a, Hongjun Zhu a
*
a Department of Applied Chemistry, College of Science, Nanjing University of Technology, Nanjing 210009, PR China
b Discipline of Chemistry, Queensland University of Technology, 2 George St., Brisbane, QLD 4000, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and properties of a series of new 3,8,13-substituted triindole derivatives 1ae1e are re-
ported. The 3,8,13-substituted triindoles were thermally robust with high decomposition temperatures
(ꢀ405 ꢁC) and high melt transitions (219 ꢁCe373 ꢁC). Compound 5e was crystallized in the monoclinic
Received 17 April 2012
Received in revised form
16 June 2012
Accepted 18 June 2012
Available online 4 July 2012
system with the space group P21/n. These compounds showed UVeVis absorption (lAmbasx) in the range
Em
of 311e345 nm in DCM solution and 371e391 nm in solid state, and fluorescence maxima (
l
) in the
max
range of 394e412 nm in DCM solution and 416e461 nm in solid state. The fluorescence quantum yields
ranged from 0.27 to 0.58. The estimated electron affinities (LUMO levels) and estimated ionization
potential (HOMO levels) of compounds 1ae1e are 3.54e3.71 eV and 5.12e5.48 eV, respectively.
Quantum chemical calculations using DFT B3LYP/6-31G showed nearly identical LUMO (ꢂ0.72 to
ꢂ1.10 eV) and HOMO (ꢂ4.65 to ꢂ4.84 eV) values. These results demonstrated that the new 3,8,13-
substituted triindoles are promising thermally stable host materials for organic light-emitting diodes
with reasonable hole mobility.
Keywords:
Synthesis
Optical properties
Electrochemical properties
Triindoles
Quantum chemical calculations
Organic light-emitting diodes (OLEDs)
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Carbazolyl groups are well-known hole-transporting units
because of the electron-donating capabilities associated with the
The field of organic electronics developed rapidly in the last few
years. Compared to conventional inorganic crystalline semi-
conductors, organic light-emitting diodes (OLEDs) aroused world-
wide interest in the search for alternative properties for large area,
nitrogen atom. Their good charge-transport properties and
carbazole-related materials are already utilized in a number of
applications [13,14]. Nowadays, researchers are interested in the
electron-rich 10,15-dihydro-5H-diindolo [3,2-a:30,20-c] carbazole
flexible, lightweight, and energy efficient optoelectronics [1,2].
p-
(triindole), which is an extended p-system constituting three
conjugated materials are extensively investigated and explored
for OLEDs because of their potential in the creation of cost-effective,
power-efficient, and eventually, flexible electronic devices [3e5].
Tang and VanSlyke first reported the use of multilayer OLEDs [6].
Among the many factors that determine the performance of OLEDs,
the most important is in relation to the charge balance factor for the
injection and transport of electrons and holes to the recombination
zone in the hole-transport layer [7e9]. Nowadays, aromatic cores
carbazole units that share an aromatic ring. Hence, interesting and
special functional properties are expected from its derivatives,
which have acquired several applications in hole-transporting
materials, discotic liquid crystals, and OLEDs [15e17].
Some triindole derivatives were synthesized using the triindole
core as a precursor [18]. The synthetic approach to obtain the
triindole core is the cyclocondensation of either indolin-2-one [19]
or indole itself [20] through halogenation. However, whether
triindole derivatives can be synthesized directly by trimerization of
aryl derivatives of indolin-2-one remains unknown.
In this context we report a series of novel 3,8,13-substituted
triindoles synthesized by trimerization of derivatives of indolin-
2-one containing various aromatic substituents. Their photo-
physical and electrochemical properties were investigated with the
aim of understanding structuralephysical property relationships in
novel organic light-emitting materials.
with a large p-orbital area attract interest as well. These molecules
have a strong tendency to assemble in highly ordered organizations
caused by stacking, which paves the way for a favorable overlap of
p-orbitals, and consequently, for improved hole-transport proper-
ties [10e12].
* Corresponding author. Tel.: þ86 25 83172358; fax: þ86 25 83587443.
E-mail address: zhuhjnjut@hotmail.com (H. Zhu).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.06.011

680
T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
2. Experimental section
out on a LS-55 spectrofluorometer (PerkineElmer). Cyclic voltam-
metry (CV) was carried out using a CHI 440A electrochemistry
workstation (CHI USA) with platinum electrodes at a scan rate of
40 mV/s against an Ag/Agþ reference electrode. An electrolyte
solution of 0.1 M TBAP in DCM was used in all experiments [26].
2.1. Materials
N-Ethylaniline was purchased from Shanghai Wison Engi-
neering and Technology Co. Ltd. Fluorene and N-bromosuccinimide
were purchased from Beijing Rizhaoshilide Chemical Company. 1-
Bromo-4-tert-butylbenzene and 1-bromo-4-methoxybenzene
were purchased from Qing Zhou Shi Ao Xing Hua Gong Co. Ltd.
Pd(OAc)2 was purchased from Shanghai Yu Rui Chemical Co. Ltd.
POCl3 was purchased from Sandong Chunguang Chemical Science
and Technology. Co. Ltd. Tetrahydrofuran (THF) were refluxed with
sodium and distilled. Other reagents and solvents were purchased
from Sinopharm Chemical Reagent Co. Ltd. and used as received. 5-
Bromo-N-ethyloxindole 3 [21], aryl boronic acid compounds 4ae4c
[22], 4d [23] and 4e [24], indolin-2-one derivatives 5ae5e [25] and
3,8,13-substituted 10,15-dihydro-5H-diindolo[3,2-a:30,20-c]carba-
zole (triindole) derivatives 1ae1e [21] were synthesized according
to the methods reported in literature with some modification. The
synthetic routines are shown in Schemes 1 and 2, respectively.
2.3. Synthesis
2.3.1. N-Ethyloxindole 2
N-Ethylaniline (22 g, 182 mmol) was dissolved in toluene
(250 mL), and chloroacetyl chloride (14.5 mL,193 mmol) was added
dropwise. Then the mixture was refluxed for 2 h. Water (50 mL)
was added after the mixture was cooled to room temperature and
the mixture was stirred for 3 h to decompose the chloroacetyl
chloride. Toluene layer was washed first with 10% K2CO3 in water
solution, then with HCl (2 M) before dried overnight with sodium
sulfate. After toluene was evaporated, AlCl3 (83 g) was added, and
the mixture was stirred for 10 min at 130 ꢁC. Then the above
mixture was stirred at 210 ꢁC for 45 min. The reaction mixture was
poured into quick-stirring ice water slowly, and then condensed
HCl (50 mL) was added. The mixture was extracted with 300 mL
DCM for three times, and then the DCM layer was dried over
sodium sulfate, after DCM was evaporated, the solid was recrys-
tallized with hexane, got N-ethyloxindole 2 (20.3 g) in total yield
76% of the two steps; m.p. 94e95 ꢁC (lit. [21]: 93e95 ꢁC). 1H NMR
2.2. Measurements
1H NMR and 13C NMR spectra were recorded on a Bruker AV-500
spectrometer at 500 MHz, a Bruker AV-400 spectrometer at
400 MHz or a Bruker AV-300 spectrometer at 300 MHz with tet-
ramethylsilane as internal standard. Mass spectra were obtained on
a VG12-250 mass spectrometer. Thermogravimetric analysis (TGA)
of the molecules and differential scanning calorimetry (DSC) were
conducted on a TA Instruments NETZSCH TG 209 and a DSC
Instruments NETZSCH DSC 204, respectively. Optical absorption
spectra were obtained by using a HP-8453 UV/Vis/near-IR Spec-
trophotometer (Agilent). Photoluminescence spectra were carried
(CDCl3, 400 MHz):
d (ppm): 7.24e7.30 (m, 2H), 7.03 (dd, 1H,
J ¼ 7.2 Hz), 6.85 (d, 1H, J ¼ 7.8 Hz), 3.77 (q, 2H, J ¼ 7.2 Hz), 3.53 (s,
2H), 1.27 (t, 3H, J ¼ 7.2 Hz).
2.3.2. 5-Bromo-N-ethyloxindole 3
N-Ethyloxindole (32.4 g, 185 mmol) in acetonitrile (200 mL) was
stirred at ꢂ5 ꢁC, and then NBS (34 g, 191 mmol) in 300 mL aceto-
nitrile was added dropwise. The mixture was stirred at that
1).ClCH2COCl
NH
toluene, reflux
N
NBS
N
O
O
2).AlCl3,130 -210
MeCN 2d
Br
3
2
R
N
4a-4e
R
N
N
POCl3
reflux
0.5 mol% Pd(OAc)2
O
O
N
Br
R
N
K2CO3. 80
H2O-DMF
3
5a-5e
R
1a-1e
1c: R=
OCH3
1a
: R=
1b: R=
1d: R=
N
1e
: R=
Scheme 1. Synthesis of triindole derivatives (1ae1e).

T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
681
Scheme 2. Synthesis of aryl boronic acid compounds (4ae4e).
temperature for 1 h and then stirred for 2 d at ambient tempera-
ture. Then the solution was evaporated and the solid was dissolved
in 1000 mL DCM, washed trice with 1000 mL water. After DCM was
evaporated, the brown solid was recrystallized with 400 mL iso-
propyl ether, got 5-bromo-N-ethyloxindole 3 (35.4 g) in 78.9%
yield; m.p. 107e109 ꢁC (lit. [21]: 108e109 ꢁC). 1H NMR (CDCl3,
Yield was 72.8% as white solid; m.p. 158e160 ꢁC (lit. [22]: 160 ꢁC).
1H NMR (CD3Cl, 300 MHz):
(ppm): 8.17 (d, 2H, J ¼ 6.6 Hz),
7.42e7.54 (m, 2H), 1.38 (s, 9H).
d
2.3.3.3. 4-Methoxyphenylboronic acid 4c. Compound 4c was
synthesized according to the method described for compound 4a.
Yield was 75.4% as white solid; m.p. 158e163 ꢁC (lit. [22]:
400 MHz):
d
(ppm): 7.41 (d, 1H, J ¼ 8.3 Hz), 7.38 (s, 1H), 6.73 (d, 1H,
J ¼ 8.2 Hz), 3.75 (m, 2H), 3.51 (s, 2H), 1.26 (t, 3H, J ¼ 7.2 Hz).
159e164 ꢁC). 1H NMR (CD3Cl, 300 MHz):
d (ppm): 7.84 (d, 2H,
J ¼ 8 Hz), 6.86 (d, 2H, J ¼ 8 Hz), 3.77 (s, 3H).
2.3.3. General procedure for the synthesis of (4ae4e)
2.3.3.1. Benzeneboronic acid 4a. In a 500 mL Schlenk flask, 4-
bromobenzene (450.0 mmol, 70.65 g) in 150 mL THF was added
dropwise to magnesium (495.0 mmol, 12.3 g) turnings. The
resulting Grignard solution was filtered and added slowly dropwise
with stirring to a calculated amount of trimethyl borate (675 mmol,
70.1 g) in 100 mL THF at such a rate that the temperature did not
rise above ꢂ70 ꢁC. Therefore the flask was cooled using liquid
nitrogen. The reaction mixture was poured into quick-stirring ice
water slowly, and then condensed HCl (5 mL) was added. After
stirring for 30 min, the mixture was shaken repeatedly with ether.
The combined organic layers were washed with water, dried over
anhydrous sodium sulfate, filtered, and concentrated to dryness.
The solid washed with hot hexane, got white solid benzeneboronic
acid 4a (45.2 g). Yield was 82.4%; m.p. 214e218 ꢁC (lit. [27]:
2.3.3.4. 9,9-Di(n-butyl)fluorene 6 (lit. [23]). A mixture of fluorene
(33.2 g, 0.2 mol), n-butyliodide (80 g, 0.44 mol), and sodium
hydroxide (24 g, 0.6 mol) in DMSO (240 mL) was stirred overnight
at 90 ꢁC under an inert atmosphere. After cooling to room
temperature, the solution was poured into cold water and the
resulting mixture was extracted with DCM (3 ꢃ 100 mL). The
combined organic layers were washed with water, dried over
anhydrous sodium sulfate, filtered, and concentrated to dryness.
The crude product was then purified by column chromatography on
silica gel using petroleum ether as eluent to afford the desired
product as a colorless solid 6 (43 g, 76.4% yield); m.p. 55e57 ꢁC. 1H
NMR (400 MHz, CDCl3):
d (ppm): 7.69e7.72 (m, 2H), 7.27e7.33 (m,
6H), 1.94e1.98 (m, 4H), 1.03e1.09 (m, 4H), 0.66 (t, 6H, J ¼ 7.4 Hz),
0.54e0.62 (m, 4H).
216e219 ꢁC). 1H NMR (400 MHz, CDCl3):
d (ppm): 8.24 (d, 2H,
J ¼ 6.8 Hz), 7.59 (t, 1H, J ¼ 7.2 Hz), 7.50 (t, 2H, J ¼ 7.2 Hz).
2.3.3.5. 9,9-Di(n-butyl)-2-bromofluorene 7 (lit. [23]). A mixture of
9,9-Di(n-butyl)fluorene (26 g, 92 mmol) and NBS (16.5 g, 92 mmol)
in acetone (200 mL) was stirred under nitrogen at 80 ꢁC for 3 h.
After cooling to room temperature, the solution was poured into
2.3.3.2. 4-tert-Butylbenzeneboronic acid 4b. Compound 4b was
synthesized according to the method described for compound 4a

682
T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
cold water and the resulting mixture was extracted with DCM
(3 ꢃ 200 mL). The combined organic layers were washed with
water, dried over anhydrous sodium sulfate, filtered, and concen-
trated to dryness. The crude product was purified by short-column
chromatography on silica gel using petroleum ether as eluent to
afford the desired product as a colorless solid 7 (29.2 g, 92.7% yield);
for 48 h. Afterward, the reaction solution was extracted four times
with diethyl ether (4 ꢃ 30 mL). The combined organic layers were
washed with water, dried over anhydrous sodium sulfate. The
product was then dried under high vacuum. The further purifica-
tion of the product was achieved by flash chromatography on
a silica gel column. Yield was 80.4% as yellow solid; m.p.110e111 ꢁC.
m.p. 58e61 ꢁC. 1H NMR (400 MHz, CDCl3):
d
(ppm): 7.64e7.66 (m,
1H NMR (CDCl3, 300 MHz):
d (ppm): 7.49e7.56 (m, 4H), 7.40e7.45
1H), 7.50e7.55 (m, 1H), 7.43e7.47 (m, 2H), 7.31e7.34 (m, 3H),
1.88e1.96 (m, 4H), 1.03e1.12 (m, 4H), 0.68 (t, 6H, J ¼ 7.4 Hz),
0.52e0.63 (m, 4H).
(m, 2H), 7.30e7.35 (m, 1H), 6.90 (d, 1H, J ¼ 8.3 Hz), 3.80e3.83 (m,
2H), 3.58 (s, 2H), 1.30 (t, 3H, J ¼ 7.2 Hz). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 174.66, 143.71, 140.92, 135.66, 128.82, 126.95, 126.80,
126.70, 125.35, 123.49, 108.36, 35.98, 34.78, 12.73. Aal. Calcd. (%) for
C16H15N: C, 80.98; H, 6.37; N, 5.90. Found: C, 80.92; H, 6.39; N, 5.88.
2.3.3.6. 9,9-Di(n-butyl)-2-ylboronicfluorene acid 4d (lit. [23]). In
a 250 mL round bottom flask was placed 7 (14.2 g, 40 mmol). The
flask was evacuated under vacuum and flushed with nitrogen three
times. THF (150 mL) was then added. The mixture was cooled
to ꢂ78 ꢁC and n-BuLi (19.2 mL, 48 mmol) was slowly added. After
the mixture was stirred for 5 h at ꢂ78 ꢁC, trimethyl borate (5.6 mL,
48 mmol) was injected. After 2 h, the mixture was slowly warmed
to room temperature. HCl (3 M,150 mL) solution was added and the
mixture was stirred for another 3 h. The mixture was extracted with
DCM three times. The combined organic layers were washed with
water, dried over anhydrous sodium sulfate. The product was then
dried under high vacuum. The crude product was condensed and
purified on a silica-gel column using a mixture of DCM and acetone
(5:1 by volume) as eluent. A white solid 4d (5.83 g, 45.3% yield) was
2.3.4.2. 5-(4-tert-Butylphenyl)-1-ethylindolin-2-one 5b. Compound
5b was synthesized according to the method described for
compound 5a. Yield was 78.6% as yellow solid; m.p. 125e127 ꢁC. 1H
NMR (CDCl3, 300 MHz):
d (ppm): 7.50e7.43 (m, 6H), 6.90 (d, 1H,
J ¼ 8.3 Hz), 3.77e3.79 (m, 2H), 3.58 (s, 2H), 1.39 (s, 9H), 1.31 (t, 3H,
J ¼ 7.2 Hz). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 174.67, 149.96,
143.46, 137.99, 135.51, 126.50, 126.42, 125.76, 125.27, 123.33, 108.33,
35.98, 34.76, 34.50, 31.36, 12.73. Aal. Calcd. (%) for C20H23N: C,
81.87; H, 7.90; N, 4.77. Found: C, 81.85; H, 7.95; N, 4.75.
2.3.4.3. 1-Ethyl-5-(4-methoxyphenyl)indolin-2-one 5c. Compound
5c was synthesized according to the method described for
compound 5a. Yield was 76.4% as yellow solid; m.p. 122e123 ꢁC. 1H
obtained; m.p. 145e148 ꢁC. 1H NMR (300 MHz, d6-DMSO):
d (ppm):
7.98 (s, 2H), 7.83 (s, 1H), 7.74e7.82 (m, 3H), 7.43e7.45 (m, 1H),
7.31e7.34 (m, 2H), 1.95e1.99 (m, 4H), 0.98e1.06 (m, 4H), 0.61 (t, 6H,
J ¼ 7.5 Hz), 0.46e0.50 (m, 4H).
NMR (CDCl3, 300 MHz): d (ppm): 7.49e7.46 (m, 4H), 6.97 (d, 2H,
J ¼ 8.5 Hz), 6.88 (d, 1H, J ¼ 8.8 Hz), 3.85 (s, 3H), 3.75e3.78 (m, 2H),
3.57 (s, 2H), 1.29 (t, 3H, J ¼ 7.2 Hz). 13C NMR (100 MHz, CDCl3):
d
(ppm) ¼ 174.63, 158.92, 143.18, 135.35, 133.52, 127.80, 126.19,
2.3.3.7. 4-Bromo-N,N-diphenylaniline 8. A solution of N-bromo-
succinimide (35.6 g, 200 mmol) in dry DMF (200 mL) was added to
a solution of triphenylamine (49.1 g, 200 mmol) in dry DMF
(100 mL) and stirred at 20 ꢁC. After 24 h, the mixture was poured
into water (500 mL) and extracted with DCM (3 ꢃ 100 mL). The
combined organic layers were washed with water, dried over
anhydrous sodium sulfate. The product was then dried under high
vacuum. After recrystallization from dry ethanol, (48.9 g, 75.3%
yield), of the desired product 8 was obtained as a white powder;
m.p. 102e104 ꢁC (lit. [24] 102e105 ꢁC). 1H NMR (400 MHz, CDCl3):
125.32, 123.15, 114.26, 108.33, 55.35, 35.98, 34.76, 12.73. Aal. Calcd.
(%) for C17H17N: C, 76.38; H, 6.41; N, 5.24. Found: C, 76.35; H, 6.39;
N, 5.25.
2.3.4.4. 5-(9,9-Dibutyl-9H-fluoren-2-yl)-1-ethylindolin-2-one
5d. Compound 5d was synthesized according to the method
described for compound 5a. Yield was 63.7% as light yellow solid;
m.p. 101e103 ꢁC. 1H NMR (CDCl3, 500 MHz):
d (ppm): 7.72 (m, 2H),
7.57 (d, 2H, J ¼ 7.8 Hz), 7.52 (q,1H, J ¼ 8.2 Hz), 7.49 (s, 1H), 7.28e7.36
(m, 4H), 6.93 (d, 1H), 3.80e3.84 (m, 2H), 3.60 (s, 2H), 2.00 (t, 4H,
J ¼ 8.3 Hz), 1.31 (t, 4H, J ¼ 8.2 Hz), 1.24 (t, 3H, J ¼ 7.2 Hz), 1.04e1.10
(m, 6H), 0.66e0.72 (m, 4H). 13C NMR (100 MHz, CDCl3):
d
(ppm): 7.30 (m, 6H), 7.06 (m, 6H), 6.94 (m, 2H).
2.3.3.8. 4-(Diphenylamino)phenylboronic acid 4e. In a 500 mL round
bottom flask was placed 8 (16.2 g, 50 mmol). The flask was evacuated
under vacuum and flushed with nitrogen three times. THF (150 mL)
was then added. The mixture was cooled to ꢂ78 ꢁC and n-BuLi
(24 mL, 60 mmol) was slowly added. After the mixture was stirred
for 3 h at ꢂ78 ꢁC, trimethyl borate (7 mL, 60 mmol) was injected.
After 2 h, the mixture was slowly warmed to room temperature. HCl
(3 M, 200 mL) solution was added and the mixture was stirred for
another 3 h. The mixture was extracted with DCM three times. The
combined organic layers were washed with water, dried over
anhydrous sodium sulfate. The product was then dried under high
vacuum. The crude product was condensed and purified on a silica-
gel column using a mixture of chloroform and acetone (4:1 by
volume) as eluent. Awhite solid 4e (6.6 g, 45.3% yield) was obtained;
m.p. 226e228 ꢁC (lit. [24] 228e229 ꢁC). 1H NMR (300 MHz, d6-
d
(ppm) ¼ 174.70, 151.47, 150.87, 143.56, 140.72, 140.16, 139.74,
136.16, 127.00, 126.81, 126.74, 126.74, 125.58, 125.39, 123.57, 122.89,
121.11, 119.95, 119.67, 108.38, 55.06, 40.25, 36.03, 34.82, 25.99,
23.08, 13.82, 12.74. Aal. Calcd. (%) for C31H35NO: C, 85.08; H, 8.06; N,
3.20. Found: C, 85.10; H, 8.03; N, 3.18.
2.3.4.5. 5-(4-(Diphenylamino)phenyl)-1-ethylindolin-2-one
5e. Compound 5e was synthesized according to the method
described for compound 5a. Yield was 70.3% as light yellow solid;
m.p. 171e173 ꢁC. 1H NMR (CDCl3, 500 MHz):
d (ppm): 7.46 (d, 2H,
J ¼ 7.5 Hz), 7.41 (d, 2H, J ¼ 8.6 Hz), 7.24e7.27 (m, 6H), 7.12 (d, 6H,
J ¼ 7.5 Hz), 7.02 (t, 2H, J ¼ 7.4 Hz), 6.88 (d, 1H, J ¼ 8.6 Hz), 3.77e3.81
(m, 2H), 3.56 (s, 2H), 1.29 (t, 3H, J ¼ 7.0 Hz). 13C NMR (100 MHz,
CDCl3):
d
(ppm) ¼ 174.64, 147.66, 146.89, 143.30, 135.18, 134.89,
129.26, 127.40, 126.17, 125.32, 124.35, 124.03, 123.05, 122.90, 108.35,
35.98, 34.77, 12.73. Aal. Calcd. (%) for C28H24N2O: C, 83.14; H, 5.98;
N, 6.93. Found: C, 83.09; H, 5.99; N, 6.94.
DMSO):
d
(ppm): 7.84 (s, 2H), 7.66 (d, 2H, J ¼ 7.5 Hz), 7.30 (t, 4H,
J ¼ 7.8 Hz), 7.02e7.09 (m, 6H), 6.86e6.88 (m, 2H).
2.3.4. General procedure for the synthesis of (5ae5e) (lit. [25])
2.3.4.1. 1-Ethyl-5-phenylindolin-2-one 5a. A mixture of K2CO3
(2.12 g, 20 mmol), Pd (OAc)2 (10 mg, 0.5 mol %), 5-bromo-N-eth-
yloxindole (2.4 g, 10 mmol), benzeneboronic acid (3 g, 15 mmol),
distilled water (35 mL) and DMF (30 mL) was stirred under nitrogen
2.3.5. General procedure for the synthesis of (1ae1e) (lit. [21])
2.3.5.1. 5,10,15-Triethyl-3,8,13-triphenyl-10,15-dihydro-5H-diindolo
[3,2-a:30,20-c] carbazole 1a. A mixture of 5a (1.19 g, 5 mmol), POCl3
(10 mL) and methylbenzene (20 mL) was stirred and refluxed for
24 h. After cooling, the mixture was poured into ice water, and then

T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
683
extracted with DCM (100 mL) for three times. The combined
organic layers were washed with water, dried over anhydrous
sodium sulfate. The product was then dried under high vacuum,
purified on a silica gel column using DCM/PE ¼ 1/4, as eluent. A
white solid 1a (0.43 g, 39% yield) was obtained; m.p. 254 ꢁC. 1H
extracted from the monobromination of N-ethyloxindole 2, which
was obtained through the synthesis starting from N-ethylaniline
[21]. The aryl boronic acids 4ae4e, as other key compounds, were
obtained by following a similar procedure (Scheme 2). 4ae4c were
easily be synthesized by the treatment of the corresponding aryl
Grignard compounds with trimethyl borate [22]. 4d and 4e were
achieved in a two-step synthesis: lithiation arylhalide with an
excess of n-BuLi was followed by treatment of trimethyl borate,
Suzuki cross-coupling of aryl boronic acids 4ae4e and 5-bromo-N-
ethyloxindole 3, using Pd (OAc)2 as catalyst, afforded 3-substituted
indolin-2-one derivatives 5ae5e in quantitative yields [25]. Tri-
merization of 5ae5e in POCl3 yielded the desired products 1ae1e
in 29.8%e40.5% yields, which are similar with the yields reported
by the unsubstituted triindole core [19]. Triphenylamino
substituted 1e has the least number of yields among these
compounds. The results demonstrate that triindole derivatives can
be synthesized directly by trimerization of aryl derivatives of
indolin-2-one.
NMR (DMSO, 500 MHz)
d
(ppm): 8.57 (s,1H), 7.96 (d,1H, J ¼ 8.5 Hz),
7.84 (d, 2H, J ¼ 7.4 Hz), 7.80 (d, 1H, J ¼ 8.4 Hz), 7.56 (t, 2H, J ¼ 7.5 Hz),
7.40 (t, 1H, J ¼ 7.3 Hz), 5.10e5.14 (m, 2H), 1.62 (t, 3H, J ¼ 6.9 Hz). 13C
NMR (100 MHz, CDCl3):
d
(ppm) ¼ 142.70, 140.36, 139.17, 133.42,
128.89, 127.41, 126.48, 123.95, 122.60, 120.38, 110.48, 103.48, 41.99,
15.82. Aal. Calcd. (%) for C48H39N3: C, 87.64; H, 5.98; N, 6.39. Found:
C, 87.70; H, 5.89; N, 6.37.
2.3.5.2. 3,8,13-tris(4-tert-Butylphenyl)-5,10,15-triethyl-10,15-
dihydro-5H-diindolo [3,2-a:30,20-c]carbazole 1b. Compound 1b was
synthesized according to the method described for compound 1a.
Yield was 40.5% aswhite solid; m.p. 254 ꢁC.1H NMR (CDCl3, 500 MHz):
d
(ppm): 8.57 (s,1H), 7.68e7.71 (m, 4H), 7.52e7.55 (m, 2H), 5.04e5.08
(m, 2H),1.70 (t, 3H, J ¼ 6.9 Hz),1.41 (s, 9H). 13C NMR (100 MHz, CDCl3):
The synthesized compounds 1ae1e were identified by using the
1H NMR spectra, 13C NMR spectra, and element analysis. The data
were found to be in good agreement with the proposed structures.
The compounds 1ae1e were soluble in common organic solvents,
such as DCM, CH3CN, THF, etc., at room temperature.
d
(ppm) ¼ 149.39,140.22,139.77,133.21,126.99,125.85,123.95,122.49,
120.18,110.41,103.50, 41.98, 34.53, 31.46, 26.91,15.86. Aal. Calcd. (%) for
C60H63N3: C, 87.23; H, 7.69; N, 5.09. Found: C, 87.17; H, 7.75; N, 5.01.
2.3.5.3. 5,10,15-Triethyl-3,8,13-tris(4-methoxyphenyl)-10,15-dihydro-
5H-diindolo [3,2-a:30,20-c]carbazole 1c. Compound 1c was synthe-
sized according to the method described for compound 1a. Yield
was 33.7% as light yellow solid; m.p. 367 ꢁC. 1H NMR (CDCl3,
3.2. Thermal properties
Table 1 shows the thermal properties, including melting and
decomposition temperatures, of these molecules. Differential
scanning calorimetry (DSC) was used to investigate the phase
transitions. No glass transition events were observed by the DSC
scans of the molecules. All compounds had melting transitions
ranging from 219 ꢁC to 373 ꢁC. Fluorenyl substituted 1d melting at
the lowest temperature of 219 ꢁC, while triphenylamino substituted
1e melting at the highest temperature, 373 ꢁC. The decomposition
temperatures determined by the thermogravimetric analysis (TGA)
were above 405 ꢁC (Fig. 1). All these results demonstrated that the
series of 3,8,13-substituted triindole derivatives were very robust
molecules.
500 MHz):
d (ppm): 8.52 (s, 1H), 7.65e7.69 (m, 4H), 7.06e7.08 (m,
2H), 5.07e5.09 (m, 2H), 3.90 (s, 3H), 1.70 (t, 3H, J ¼ 6.9 Hz). 13C NMR
(75 MHz, CDCl3):
d
(ppm) ¼ 158.66, 140.08, 139.18, 135.38, 133.14,
128.36, 124.01, 122.30, 119.96, 114.37, 110.43, 103.52, 56.05, 41.97,
15.83. Aal. Calcd. (%) for C51H45N3O3: C, 81.90; H, 6.06; N, 5.62.
Found: C, 81.20; H, 6.11; N, 5.58.
2.3.5.4. 3,8-bis(9,9-Dibutyl-9H-fluoren-2-yl)-13-(9,9-dibutyl-9H-flu-
oren-3-yl)-5,10,15-triethyl-10,15-dihydro-5H-diindolo[3,2-a:30,20-c]
carbazole 1d. Compound 1d was synthesized according to the
method described for compound 1a. Yield was 31.6% as white solid;
m.p. 219 ꢁC.1H NMR (CDCl3, 500 MHz):
d (ppm): 8.68 (s,1H), 7.80e7.85
(m, 2H), 7.75e7.79 (m, 4H), 7.31e7.40 (m, 3H), 5.13e5.15 (m, 2H),
2.06e2.10 (m, 4H),1.85 (t, 3H, J ¼ 7.0 Hz), 1.10e1.18 (m, 4H), 0.71e0.78
3.3. X-ray crystal structures
(m, 10H). 13C NMR (75 MHz, CDCl3):
d
(ppm) ¼ 151.48, 150.88, 141.54,
Crystal 5e was grown by the slow evaporation of its solution in
PE at room temperature. The diffraction data of a crystal were
collected at 298 K. The crystal structure was solved by using direct
methods, and refined by applying full-matrix least-squares tech-
niques implemented in the SHEXTL crystallographic software
[28,29]. The crystal belongs to the monoclinic system, P21/n space
group with formula C28H24N2O; a ¼ 9.2810(19), b ¼ 15.884(3),
140.97, 140.43, 139.71, 139.32, 134.02, 127.36, 126.85, 126.81, 126.20,
124.05, 122.88, 122.73, 121.77, 120.31, 120.00, 119.63, 110.45, 103.51,
55.09, 42.10, 40.43, 26.06, 23.15, 16.05, 13.85. Aal. Calcd. (%) for
C93H99N3: C, 88.73; H, 7.93; N, 3.34. Found: C, 88.61; H, 7.96; N, 3.39.
2.3.5.5. 4,40-(13-(3-(Diphenylamino)phenyl)-5,10,15-triethyl-10,15-
dihydro-5H- diindolo[3,2-a:30,20-c]carbazole-3,8-diyl)bis(N,N-diphe-
nylaniline) 1e. Compound 1e was synthesized according to the
method described for compound 1a. Yield was 28.6% as light yellow
c ¼ 14.752(3) Å;
b
¼ 90.30(3)ꢁ; V ¼ 2174.7(8) Å3; rcal ¼ 1.235 g/cm3,
m
¼ 0.075 mmꢂ1. All H atoms were clearly located in the different
Fourier maps and were refined freely. The refinement converged to
R1 ¼ 0.1933, wR2 ¼ 0.1405, rmax ¼ 0.144 e/Å3, rmin ¼ ꢂ0.176 e/Å3.
The detailed crystallographic data for 5e are presented in Table 2.
The single crystal structures and packing diagrams of 5e are
shown in Figs. 2 and 3, respectively. As shown in Fig. 2, 5e consists
solid. m.p. 372 ꢁC. 1H NMR (CDCl3, 400 MHz):
d (ppm): 8.55 (s, 1H),
7.64e7.68(m, 4H), 7.18e7.31 (m, 9H), 7.02e7.06(m, 3H), 5.05e5.01 (m,
2H),1.70 (t, 3H, J ¼ 7.2 Hz). 13C NMR (75 MHz, CDCl3):
(ppm) ¼ 147.84,
d
146.48, 140.16, 139.14, 136.78, 132.86, 129.26, 129.12, 127.91, 124.36,
124.01,122.77,122.21,119.79,110.48,103.55, 41.99,15.81. Aal. Calcd. (%)
for C84H66N6: C, 87.01; H, 5.74; N, 7.25. Found: C, 87.09; H, 5.61; N, 7.22.
Table 1
Thermal properties of 1ae1e.
3. Results and discussion
Compd.
Tm (ꢁC)
Td (ꢁC)
3.1. Synthesis and characterization
1a
1b
1c
1d
1e
254
254
368
219
373
405
420
448
429
438
The synthesis of 3,8,13-substituted triindole derivatives 1ae1e
was carried out by using a multi-step synthetic route, as shown
in Scheme 1. As a key compound, 5-bromo-N-ethyloxindole 3 was

684
T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
1a
1b
1c
1d
1e
105
100
95
90
85
80
75
70
Fig. 2. The crystal structure of 5e.
65
optical band gaps (Eg), photoluminescence (PL) peak wavelengths,
0
100
200
300
400
500
Em
TemperatureoC
(l
max
), quantum yields in solution, and solid-state.
Based on the electronic absorption spectra of 1ae1e in dilute
DCM solutions (Fig. 4), these triindoles exhibit strong * elec-
pep
Fig. 1. TGA curves of 1ae1e.
tron absorption bands, with two characteristic absorption peaks,
one at 311, 312, 281, 321 and 323 nm, and the other at 332, 334, 311,
344, and 345 nm. Phenyl substituted 1a, alkyl phenyl substituted
1b, and methoxyl substituted 1c have nearly identical absorption
maxima (lAmbasx) at 311, 312, and 311 nm, respectively. The increase in
conjugation length and electron density associated with fluorenyl
groups in 1d, and triphenylamino groups in 1e led to a large
bathochromic shift of absorption maximum in 1d to 344 nm, and in
1e to 345 nm.
of indole and triphenylamine moieties. The crystal structures reveal
that the indole units are relatively planar. The a-linked phenyl
groups are twisted at 32.4ꢁ in 5e from the plane of the indole units.
At the triphenylamine moiety, the central nitrogen and its three
bended carbon atoms are coplanar, which form a quasi-equilateral
trigonal NC3 plane with the sum of the three CeNeC angles
(360.0ꢁ). Around the central nitrogen, three phenyl ring planes are
arranged in a propeller-like fashion.
The solid-state optical absorption spectra of compounds 1ae1e
are shown in Fig. 5. 1ae1e exhibit a strong pep* transition at 371,
371, 371, 386, and 391 nm. Regarding the absorption spectra of
1ae1e in films (Fig. 4), the trends observed with extending the
conjugation upon functionalization in terms of the spectral shapes
and peak positions are nearly the same compared to the spectra we
obtained in dilute solutions.
The photoluminescence (PL) spectra of the derivatives 1ae1e in
dilute DCM solutions (1 ꢃ 10ꢂ7 M) are shown in Fig. 6. 1ae1c
exhibit similar PL spectra with the main emission peak and the
3.4. Optical properties
The absorption and emission spectra of 1ae1e recorded from
dilute DCM solutions (1 ꢃ 10ꢂ6 M) and thin films are shown in
Figs. 4e7. Table 3 shows the absorption peak wavelengths (lAmbasx),
Table 2
shoulder emission peak in solution. The emission peak ranges from
Crystallographic data for 5e.
Em
406 to 407 nm with a Stokes shift of approximately 95 nm. The
l
max
Compound
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
5e
of fluorenyl substituted 1d slightly red shifted (6 nm) compared to
C28H24N2O
Em
1a. However, the
l
of triphenylamino substituted 1e has slightly
404.49
Monoclinic
P21/N
9.2810(19)
15.884(3)
14.752(3)
90.00
90.30(3)
90.00
max
blue shifted (12 nm) compared to 1a. A Stokes shift of 68 and 49 nm
are shown by 1d and 1e, respectively. These experimental data
clearly indicate that the substitutions exerted little considerable
influence on the electronic states in solution. The large Stokes shifts
between 49 and 95 nm observed for 1ae1e are the consequences of
both the nuclear reorganization that takes place after the prior
emission and irradiation, which results from the conformational
changes and electronic redistribution in the phenyl fragments
(planarization) upon excitation [30].
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
V (Å3)
2174.7(8)
1.235
0.075
Dcalc (g cmꢂ3
m
)
(mmꢂ1
)
F (000)
range (ꢁ)
Index range
856
The emission spectra of 1ae1e in film are shown in Fig. 7. All five
q
1.88e25.38
0 ꢄ h ꢄ 11
0 ꢄ k ꢄ 19
ꢂ17 ꢄ l ꢄ 17
4255
3996 (0.085)
Full-matrix least-squares
1.002
0.0782
0.1092
0.1933
0.1405
compounds emit blue light with the emission maximum ranging
Em
from 416 nm, for 1a, to 461 nm for 1d. All the
l
of 1be1e are
max
slightly red shifted compared to 1a. Fluorenyl substituted 1d has
the maximum red shift (44.5 nm). The Stoke shifts are not very
large for all compounds ranging from 45 to 76 nm.
Reflns collected
Unique reflns (Rint
)
Refinement method on F2
GOF on F2
Optical band gaps (Eogpt) determined from the absorption edge of
the solution and the solid-state spectra are shown in Table 3. The
optical band gap varies from 3.07 to 3.17 eV for 1ae1e in solution
and from 2.43 to 2.67 eV for 1ae1e in solid-state.
R1 [I > 2
wR2 [I > 2
s
(I)]
(I)]
s
R1 (all data)
The fluorescence quantum efficiencies (Faf ) of 1ae1e were
measured in dilute DCM solutions using 9,10-diphenylanthracene
wR2 (all data)
Residual (e Åꢂ3
)
0.144 and ꢂ0.176

T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
685
Fig. 3. Crystal packing diagram of 5e.
1a
1b
1c
1d
1e
1a
1b
1c
1d
1e
1.0
0.5
0.0
0.6
0.4
0.2
0.0
400
450
500
Wavelength (nm)
300
400
500
600
700
Fig. 6. PL spectra of 1ae1e in DCM solution (1 ꢃ 10ꢂ7 M).
Wavelength (nm)
Fig. 4. UVeVis absorption spectra of 1ae1e in DCM solution (1 ꢃ 10ꢂ6 M).
efficiencies. However, 1e showed low fluorescence quantum effi-
ciencies and it is likely that triphenylamino effect is little in lumi-
nescence intensity. From among the five compounds synthesized,
(
Faf ¼ 0.91 in ethanol) as a standard [31]. The quantum efficiencies
of 1ae1e ranged from 0.32 to 0.58 in DCM solutions (Table 3). There
was only a small difference in Faf between 1a, 1b, and 1c. Fluorenyl
substituted 1d obviously increased in fluorescence quantum
1d showed the highest fluorescence quantum efficiencies (0.58)
Em
and the largest
l
(412 nm) with high thermal stability
max
0.8
(1a)
1a
1b
1c
1d
1e
(1b)
(1c)
1.0
0.5
(1d)
(1e)
0.6
0.4
0.2
0.0
0.0
200
300
400
500
600
700
800
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 5. UVeVis absorption spectra of 1ae1e in the solid state.
Fig. 7. PL spectra of 1ae1e in the solid state.

686
T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
Table 3
a Pt working electrode. Measurements were performed at room
temperature under nitrogen with a scanning rate of 40 mV/s and
SCE energy level of 4.4 eV vs vacuum [26,32]. Therefore, the HOMO
and lowest unoccupied molecular orbital (LUMO) energy levels as
well as the energy gap of these compounds can be estimated using
the following equations:
UV absorption and PL properties of 1ae1e in DCM solution and solid film.
Abs
Em
Compd.
l
max
(nm)
l
max
(nm)
Stokes shift (nm) Eogpt (eV)
Ffa
Solution/solid Solution/solid Solution/solid
Solution/solid
1a
1b
1c
1d
1e
311/371
312/371
311/371
344/386
345/391
406/416
406/441
407/447
412/461
394/445
95/45
94/70
96/76
68/75
49/49
3.17/2.67
3.14/2.63
3.14/2.64
3.07/2.54
3.10/2.43
0.32
0.36
0.27
0.58
0.37
onset
EAðLUMOÞ ¼ E
IPðHUMOÞ ¼ E
þ 4:4 eV
þ 4:4 eV
(1)
(2)
(3)
red
a
Fluorescence quantum efficiency, relative to 9,10-diphenylanthracene in ethanol
Ff ¼ 0.91).
onset
ox
(
(Tm ¼ 219 ꢁC, Td ¼ 429 ꢁC). This compound is expected to be
onset
ox
onset
red
Egel ¼ E
ꢂ E
applicable as a blue light emitting material.
onset
onset
where E
and E
are the onset potentials for oxidation and
ox
red
reduction relative to the Ag/Agþ reference electrode.
3.5. Electrochemical properties
As seen from Fig. 8, all figures show the first reduction peak
ranging from ꢂ0 V to ꢂ0.34 V because of the impact of DCM. The
molecules of 1a have only one reduction peak, whereas molecules
1b, 1c, 1d, and 1e have two reduction peaks. 1e only has one peak,
The electrochemical behavior of 1ae1e was investigated by
using a cyclic voltammetry (CV) in c ¼ 10ꢂ3 mol Lꢂ1 in CH2Cl2 and
0.1 M tetra-n-butylammonium hexafluorophosphate (TBAP) using
80
1a
1b
80
40
40
0
0
-40
-80
-40
-80
3
2
1
0
-1
-2
-3
3
2
1
0
-1
-2
-3
E(V) vs SCE
E(V) vs SCE
60
30
80
40
0
1c
1d
0
-30
-60
-90
-40
-80
3
2
1
0
-1
-2
-3
3
2
1
0
-1
-2
-3
E(V) vs SCE
E(V) vs SCE
80
1e
40
0
-40
-80
3
2
1
0
-1
-2
-3
E(V) vs SCE
Fig. 8. Cyclic voltammograms of thin films of 1ae1e coated on a Pt wire electrode. Scan rate 40 mV/s.

T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
687
Table 4
Electrochemical properties of 1ae1e.a
EAb (eV)
Eoonxset (V)
IPb (eV)
Eegl (eV)
onset
Compd.
E
(V)
red
1a
1b
1c
1d
1e
ꢂ0.69
ꢂ0.86
ꢂ0.80
ꢂ0.77
ꢂ0.76
3.71
3.54
3.60
3.63
3.66
0.72
1.08
0.78
0.77
0.95
5.12
5.48
5.28
5.17
5.35
1.41
1.94
1.58
1.54
1.71
a
All potentials vs SCE reference.
b
onset
onset
ox
EA (LUMO) ¼ E
þ 4.4 eV; IP (HOMO) ¼ E
þ 4.4 eV; Eegl ¼ IP ꢂ EA.
red
whereas there were two irreversible oxidation peaks in 1a, 1b, 1c,
and 1d.
As shown in Table 4, 1ae1e have electron affinities (LUMO)
between 3.54 and 3.71 eV (below vacuum), which reveal promising
electron-transport properties for OLEDs. The reduction potentials
of all the molecules were relatively unchanged by the substituents
(R groups), which indicate that the electron affinities (LUMO) lied
on the 10,15-dihydro-5H-diindolo [3,2-a:30,20-c] carbazole moiety.
Even the electron-rich fluorene substituted molecule (1d), which
had expectedly the lowest oxidation potential, had essentially the
same LUMO level as the other molecules.
The formal reduction potential (Eorendset) varies from ꢂ0.69 V (vs
SCE), for 1a to ꢂ0.86 V, for 1b (Table 4). As a result, the estimated
ionization potential (HOMO) is reduced from 5.48 eV in 1b to
5.12 eV in 1a, which suggesting a significant lowering of the energy
barrier for hole injection from the indium tin oxide (ITO)/poly(3,4-
ethylenedioxythiophene) doped with poly-(styrenesulfonate)
(PEDOT:PSS) anode (work function ¼ ꢂ5.2 eV) [18]. The electro-
chemically derived band gap (Eegl) of 1ae1e is shown in Table 4. Eegl
varies from 1.41 eV for 1a to 1.94 eV for 1b. From the five
compounds synthesized, 1d shows high HOMO energy level
(5.48 eV), lowest LUMO level (3.54 eV), and wide Eegl value (1.94 eV).
These characteristics suggest that 1ae1e has the potential to be
used as material for hole-transport and injection in OLEDs [33].
Fig. 10. HOMO (bottom) and LUMO (top) for molecules 1d in gas phase.
3.6. Theoretical calculation
in the Gaussian 03. DFT/B3LYP calculation of lowest excitation
energies was performed at the optimized geometries of the ground
states. The optimized structures of all the compounds (1d as an
example, Fig. 9) reveal that the substituted moieties attached to all
ends of the molecule are nearly coplanar with the triindole back-
Quantum chemical calculations were performed to determine
the electronic properties and the geometries of the 3,8,13-
substituted triindole derivatives. In the calculations, the ground
state geometries of 1ae1e were fully optimized by using density
functional theory (DFT) at the B3LYP/6-31G level, as implemented
bone. Therefore,
p-electron delocalization between those units is
unlikely to be negligible. In their HOMO orbitals (Fig. 10), the
p
-
electrons are able to delocalize over the entire triindole backbone
and one end-capped moiety. The LUMO is delocalized through the
triindole backbone and two end-capped moieties, as illustrated in
Fig. 10. Therefore, a significant change in charge distribution would
result in the HOMO-LUMO transition in compounds 1ae1e. The
HOMO-LUMO energy differences (energy band gaps, calculated
Ecgal) at DFT/B3LYP level of theory are presented in Table 5. The
result indicates that Ecgal values decrease as the length of
p-conju-
gated system and the electron-donating ability of end-capped
moieties increase. These predicted Ecgal values are higher than the
estimated figures from the onset of UVeVis absorption (Eogpt) and
Table 5
Energy values theoretically calculated for molecules 1ae1e in gas phase.
Compd.
Energy (eV)
LUMO
HOMO
Ecgal
1a
1b
1c
1d
1e
ꢂ0.90
ꢂ0.83
ꢂ0.72
ꢂ1.10
ꢂ0.91
ꢂ4.84
ꢂ4.77
ꢂ4.68
ꢂ4.78
ꢂ4.65
3.94
3.94
3.96
3.68
3.74
Fig. 9. B3LYP/6-31G optimized geometry of compound 1d.
cal: Energies theoretically calculated for gas phase by means of DFT B3LYP/6-31G.

688
T. Zhu et al. / Dyes and Pigments 95 (2012) 679e688
cyclic voltammograms (Ecgal). Errors may be responsible for this
finding because the orbital energy difference between HOMO and
LUMO is still an approximate estimation to the transition energy.
However, the transition energy contains significant contributions
from two or three electron integrals. The real situation is that an
accurate description of the lowest singlet excited state requires
a linear combination of a number of excited configurations.
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4. Conclusions
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We have synthesized and investigated the thermal stabilities,
electrochemical, and photophysical properties of five novel 3,8,13-
substituted triindoles. These materials showed high thermal
stability with high decomposition temperature (above 405 ꢁC) and
high melt transition (219 ꢁCe373 ꢁC). The molecule structure of the
5e was characterized by using an X-ray analysis. The results showed
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compounds were investigated in the DCM solution and solid film.
Compared to 1a, both the UVeVis absorption maximum peaks
(from 311 to 345 nm in DCM solution and from 371 to 391 nm in
solid film) and PL emission peaks (from 394 to 412 nm in DCM
solution and from 416 to 461 nm in solid film) slightly red shifted in
1be1d. Among the five compounds synthesized, 1d has more
potential to be used as a blue light emitting material. Based on the
high-lying HOMO energy level, these compounds are expected to
be promising hole-transport and injection materials for OLEDs with
a wide band and reasonable electron and hole mobility.
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