Thiophene-functionalized octupolar triindoles: Synthesis and photophysical properties

Article abstract of DOI:10.1016/j.saa.2012.08.008

Two thiophene-functionalized octupolar triindole molecules were synthesized through Suzuki and Sonogashira cross-coupling reactions. Their photophysical and electrochemical properties were explored. The spectroscopic data demonstrate that triindole is an

Full text of DOI:10.1016/j.saa.2012.08.008

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 1020–1024
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Thiophene-functionalized octupolar triindoles: Synthesis and
photophysical properties
a
a
a
a
b
Mao-Sen Yuan a,b, , Tian-Bao Li , Wen-Ji Wang , Zhen-Ting Du , Jun-Ru Wang , Qi Fang
⇑
a College of Science, Northwest A&F University, Yangling 712100, PR China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
Two thiophene-functionalized
octupolar triindole molecules 7 and
8 were synthesized.
"
"
The two triindole compounds exhibit
excellent luminescent properties.
Thiophenylethynyl substituted
triindole 8 forms a p–p stacking in
high concentration.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2012
Received in revised form 28 July 2012
Accepted 7 August 2012
Available online 19 August 2012
Two thiophene-functionalized octupolar triindole molecules were synthesized through Suzuki and Sono-
gashira cross-coupling reactions. Their photophysical and electrochemical properties were explored. The
spectroscopic data demonstrate that triindole is an excellent rich electronic central donor for building
octupolar optoelectronic molecules and possesses excellent luminescent properties. Thiophenylethynyl
substituted triindole compound 3,8,13-tri(2-thiophenylethynyl)-5,10,15-tributyltriindole forms an inter-
molecular aggregation induced by
curves and results of theoretical calculation imply that the two compounds have low ionization potential
and low E1/2 oxd, which may be originated from the high electronic density of both triindole and thiophene
groups.
p–p stacking from the rigid coplanar molecular scaffold. The redox
Keywords:
Triindole
Thiophene
Octupolar framework
Photophysical property
Charge transfer
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
pounds [7–18]. Wherein, the chromophore cores with octupolar
coplanar rigid geometry have especially intrigued organic chemis-
Octupolar organic
p
-conjugated molecules have been attracting
try scientists to achieve versertise optoelectronic materials. Triin-
great interest due to their fascinating structures and unique
properties in organic optoelectronics [1,2]. Recently, considerable
efforts have been devoted to design of new octupolar star-shaped
molecules and investigation their structure–property relationships
[3–6]. For rational design an optoelectronic molecules, the selec-
tion of novel core structure is the most important factor because
of its crucial properties dependence. So far, a few of C3-symmetric
functionalized groups have been used to build star-shaped com-
dole (10,15-dihydro-5H-diindolo[3,2-
a
:30,20-c]carbazole), as an
analogue of truxene, to some extend can be considered as a trimer
in which three carbazole units are sharing an benzene ring. Triin-
dole’s extend
character enables its central donor to accelerate both intermolecu-
lar ( stacking) and intramolecular (donor?acceptor) charge
p-conjugated coplanar geometry and rich electronic
p–p
transfer. Most recently, triindole derivatives have been studied as
a precursor of triazafullerene, potential light-emitting diodes, dis-
cotic liquid-crystalline and two-photon absorption materials
[15,19–26]. In order to endow optoelectronic materials with good
thermal and chemical stability, we availably functioned triindole
unit at its 3,8,13-positions with thiophenyl and thiophenylethynyl
⇑
Corresponding author at: College of Science, Northwest A&F University,
Yangling 712100, PR China. Tel./fax: +86 29 87092226.
E-mail address: yuanms@nwsuaf.edu.cn (M.-S. Yuan).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.08.008

M.-S. Yuan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 1020–1024
1021
respectively to get two octupolar
p-conjugated molecules: 3,8,13-
Synthesis and characterizations of the subject compounds
tri(2-thiophenyl)-5,10,15-tributyltriindole and 3,8,13-tri(2-thio-
phenylethynyl)-5,10,15-tributyltriindole. In this work, we present
a photophysical and redox studies of the two new triindoles in vir-
tue of the results of theoretical calculation.
N-butylindolin-2,3-dione (2), N-butylindolin-2-one (3) and
5,10,15-tributyltriindole (4) were synthesized following Ref. [15]
Synthesis of N-butyl-5-iodo-indolin-2-one (5)
A mixture of compound 3 (2.00 g, 0.011 mol), yellow HgO
(2.40 g, 0.011 mol), I2 (4.30 g, 0.017 mol) and 120 mL solvent
(CH3CH2OH:CCl4 = 2:1) was heated to 50 °C with keeping stirring.
After reacting for 24 h in dark, the mixture were poured into water
and extracted with CHCl3. After washed twice with KI aqueous
solution (10%), the extract liquor was dried over MgSO4. The crude
product was purified by column chromatography on silica gel, elut-
ing with chloroform and compound 5 was obtained (2.49 g, 72%) as
white powder, 1H NMR (CDCl3, 500 MHz, ppm) d 0.84–0.87 (t,
J = 7.1, 3H), 0.98–1.04 (m, 2H), 1.21–1.25 (m, 2H), 3.43 (s, 2H),
3.57–3.62 (m, 2H), 6.87 (s, 1H), 6.99–7.02 (t, J = 7.9, 1H), 7.22–
7.25 (m, 1H). 13C NMR (CDCl3, 125 MHz, ppm) d 179.6, 147.5,
125.9, 125.6, 124.4, 121.1, 93.5, 47.2, 35.3, 33.6, 20.1, 14.1.
ESI: m/z 315.0 (M+, 100%).
Experimental
General
Solvents were dried and distilled for use. Solvents used for
absorption and fluorescence measurements were HPLC grade. 1H
NMR spectra were recorded at 25 °C on a Bruker Avance 400 or
500 MHz spectrometer using CDCl3 as solvent. 13C NMR spectra
were recorded at 25 °C on a Bruker Avance 100 or 125 MHz spec-
trometer using CDCl3 as solvent. Element analyses (C, H, N, S) were
performed using a PE 2400 autoanalyser. Mass spectrometry anal-
yses were performed by a Bruker Biflex III matrix assisted laser
desorption/ionization time of flight (MALDI–TOF) mass spectrome-
ter or a LCQ–FLEET mass spectrometer.
All absorption spectra were recorded on a UV1800 spectrome-
ter. Steady state fluorescence spectra and decay curves were ob-
tained on an Edinburgh FLS920 fluorescence spectrometer.
Reconvolution fit of the decay profiles were made with F900 anal-
ysis software to get the lifetime value.
Cyclic voltammetry (CV) was performed on CHI660D worksta-
tion and measurements were carried out in CH2Cl2 containing
0.1 M n-Bu4NClO4 as the supporting electrolyte, platinum disc as
the working electrodes, platinum wire as the counter electrode
and Ag/AgNO3 as the reference electrode. The scan rate was main-
tained at 100 mV sꢀ1. Under these conditions, the onset oxidation
potential (E1/2 oxd) of ferrocene was 0.36 eV versus Ag/Ag+. The en-
ergy of HOMO level was calculated by HOMO, where HOMO = ꢀE1/
2 oxd ꢀ 4.44 [27].
Synthesis of 3,8,13-triiodo-5,10,15-tributyltriindole (6)
Compound 5 (2.20 g, 0.007 mol) was mixed with POCl3 (20 mL),
and the mixture was heated to reflux under N2. The mixture was
then stirred for 8 h at the temperature; the mixture was poured
into ice water and stirred for 30 min. Then the mixture were neu-
tralized with aqueous NaOH and extracted with CHCl3 and dried
over Na2SO4. The crude product was purified by column chroma-
tography on silica gel, eluting with chloroform-petrol ether (1:3)
and compound 6 was obtained (0.65 g, 31%) as a white powder.
1H NMR (CDCl3, 400 MHz, ppm) d 0.93–0.96 (t, J = 7.5, 9H), 1.32–
1.41 (m, 6H), 1.84–1.95 (m, 6H), 4.50–4.53 (t, J = 7.7, 6H), 7.30–
7.32 (d, J = 8.5, 3H), 7.65–7.67 (d, J = 8.0, 3H), 8.33 (s, 3H). 13C
NMR (CDCl3, 125 MHz, ppm) d 175.9, 146.3, 125.9, 124.6, 123.8,
O
O
THF, KOH
Bu
N
n-C4H9I
N2H4·H2O
POCl3
O
O
O
N
Bu
N
50 oC
N
100 o
C
N
reflux
H
Bu
N
1
Bu
3
I2, HgO
C2H5OH, CCl4
2
I
N
Bu
4
Bu
50 oC,Avoid light
Bu
N
POCl3
I
O
N
Bu
100 o
C
I
N
I
Bu
6
5
S
S
B(OH)2
Pd (PPh3)4, Et3N,
n-Bu4NF, THF
reflux
Pd(PPh3)4, K2CO3, THF
reflux
S
S
Bu
N
Bu
N
N
N
Bu
Bu
N
Bu
S
N
S
Bu
S
S
7
8
Scheme 1. Synthesis of title compounds.

1022
M.-S. Yuan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 1020–1024
a
14.0k
12.0k
10.0k
8.0k
6.0k
4.0k
2.0k
0.0
b
1.2
1.0
0.8
0.6
0.4
0.2
0.0
7
8 in high concentration
7
8
8
4
4
8 in high concentration
300
350
400
450
500
550
350
400
450
500
550
600
Wavelength / nm
Wavelength / nm
Fig. 1. The absorption (a) and fluorescence emission spectra (b) of compounds 4, 7 and 8 with c = 1.0 ꢁ 10ꢀ5 mol/L (solid lines) and c = 1.0 ꢁ 10ꢀ2 mol/L (short dash lines) in
THF.
Table 1
Properties of triindole compounds 4, 7 and 8.
f
absa (nm)
emax (104, Mꢀ1 cmꢀ1
)
kFa (nm)
Ub
sc (ns)
(103 cmꢀ1
d
~
Dv )
E1=2oxd (V)
HOMOg (eV)
LUMOg (eV)
D
fe (103 cmꢀ1
)
~
v=D
Compound
4
7
8
316
337
348
6.3
12.0
10.3
396
424
430,446
0.19
0.37
0.39
6.0
4.2
3.8
6.39
5.32
5.48
–
4.63
4.70
–
0.57
0.56
–
–
ꢀ5.01
ꢀ1.83
ꢀ5.00
ꢀ2.05
a
b
c
d
e
f
Linear absorption and fluorescence maxima with c = 1.0 ꢁ 10ꢀ5 mol/L.
Fluorescence quantum yields determined using coumarin 307 with c = 1.0 ꢁ 10ꢀ5 mol/L as standard.
Fluorescence lifetime.
Stokes-shift.
Â
Ã
The slope of plot of
Dv
versus
D
f in aprotic solvents based on the Lippert-Mataga equation, where Mf ¼ ½ð
e
ꢀ 1Þ=ð2
e
þ 1Þꢂ ꢀ ðn2 ꢀ 1Þ=ð2n2 þ 1Þ .
~
In CH2Cl2 containing 0.1 M n-Bu4NClO4 as a supporting electrolyte; potentials were recorded vs Ag/AgCl.
g
HOMO and LUMO were estimated from E1/2
and absorption (HOMO = ꢀE1/2 oxdꢀ4.44; LUMO = HOMOꢀEg).
oxd
a
8
b
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Hexane
Toluene
THF
8
Hexane
Toluene
THF
CHCl3
CH3CN
0.8
0.6
0.4
0.2
0.0
CHCl3
CH3CN
320
360
400
440
400
450
500
550
Wavelength / nm
Wavelength (nm)
Fig. 2. Normalized absorption (a) and emission (b) spectra of compound 8 in different solvents with c = 1.0 ꢁ 10ꢀ5 mol/L. Those of 7 are very similar to those of 8.
121.7, 92.7, 46.3, 34.8, 20.4, 14.2. MALDI–TOF: m/z 891.0 (M+,
100%), 765.1 (M+ꢀ126, 87%), 639.2 (M+ꢀ252, 35%).
d 0.89–0.92 (t, J = 7.5, 9H), 1.09–1.19 (m, 6H), 1.90–2.02 (m, 6H),
4.80–4.91 (m, 6H), 7.04–7.05 (t, J = 1.7, 3H), 7.30–7.36 (m, 6H),
7.46–7.69 (m, 6H), 8.29–8.31 (d, J = 11.3, 3H). 13C NMR (CDCl3,
125 MHz, ppm) d 141.2, 140.6, 139.2, 127.1, 125.0, 123.4, 121.4,
121.3, 115.6, 113.2, 110.5, 104.8, 46.3, 33.1, 21.3, 14.2. MALDI–
TOF: m/z 760.2 (M+, 100%), 702.9 (M+ꢀ57, 13%). Elemental Anal.
Calcd. for C48H45N3S3: C, 75.85; H, 5.97; N, 5.53; S, 12.66. Found:
C, 75.87; H, 5.69; N, 5.61; S, 12.59.
Synthesis of 3,8,13-tri(2-thiophenyl)-5,10,15-tributyltriindole (7)
A mixture of compound 6 (0.50 g, 0.56 mmol), 2-thiophene-
boronic acid (0.46 g, 3.36 mmol), Pd(PPh3)4 (30 mg, 0.03 mmol),
THF (25 mL) and 2 M aqueous K2CO3 solution (5 mL) was heated
to reflux with stirring under a nitrogen atmosphere for 20 h. The
mixture were cooled to room temperature and poured into water
(100 mL). After extraction with CHCl3, the organic phase was dried
over Na2SO4. The solvent was removed and the residue was
purified by column chromatography on silica gel using DCM–hex-
ane (1:10) as the eluent to get compound 7 (0.34 g, 79%) were ob-
tained as a bright yellow powder. 1H NMR (CDCl3, 500 MHz, ppm):
Synthesis of 3,8,13-tri(2-thiophenylethynyl)-5,10,15-tributyltriindole
(8)
A mixture of compound 6 (0.50 g, 0.56 mmol), 2-ethynylthioph-
ene (0.40 g, 3.36 mmol), Pd(PPh3)4 (20 mg, 0.02 mmol), n-Bu4NF
(50 mg), THF (20 mL) and triethylamine (10 mL) was heated to

M.-S. Yuan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 1020–1024
1023
reflux with stirring after flushed with nitrogen for half an hour.
After reacting for 3 h under nitrogen, the mixture were cooled to
room temperature and poured into water (100 mL). After extrac-
tion with CHCl3 several times, the organic phase was dried over
MgSO4. The solvent was removed and the residue was purified
by column chromatography on silica gel using DCM–hexane (1:5)
as eluent to get compound 8 (0.38 g, 81%) were obtained as a yel-
low powder. 1H NMR (CDCl3, 500 MHz, ppm): 1H NMR (CDCl3,
500 MHz, ppm): d 0.83–0.87 (t, J = 7.4, 9H), 1.21–1.25 (m, 6H),
2.08–2.14 (m, 6H), 4.77–4.81 (m, 6H), 7.05–7.06 (t, J = 1.6, 3H),
7.30–7.32 (t, J = 5.5, 6H), 7.53–7.60 (m, 6H), 8.34–8.36 (d, J = 12.5,
3H). 13C NMR (CDCl3, 125 MHz, ppm) d 140.6, 139.3, 131.3,
127.1, 126.3, 125.0, 123.8, 121.4, 114.1, 113.2, 110.5, 102.8, 94.7,
82.2, 47.1, 32.5, 20.0, 14.1. MALDI–TOF: m/z 831.2 (M+, 100%),
774.5 (M+-57, 11%). Elemental Anal. Calcd. for C54H45N3S3:C,
77.94; H, 5.45; N, 5.05; S, 11.56. Found: C, 77.98; H, 5.67; N,
5.34; S, 11.43.
synthesized. And then, compound 2 was converted to compound
3 by classical Wolff–Kishner reduction. In darkness and with HgO
in the presence of a catalyst, compound 3 was iodinated at 5-posi-
tion to obtain N-butyl-5-iodo-indolin-2-one (5). Compounds 4 and
6 were synthesized by trimerization of 3 and 5 in POCl3 under N2.
After that, the Suzuki and Sonogashira cross-coupling of 5 with 2-
thiophene-boronic acid and 2-ethynylthiophene gave the octupo-
lar molecules 7 and 8 respectively. Details of the synthesis and
characterizations are given in the experimental section.
Photophysical properties measurement
As shown in Fig. 1 and Table 1, the absorption spectra of com-
pounds 7 and 8 in THF with c = 1.0 ꢁ 10ꢀ5 mol/L exhibit a distinct
absorption peak at 316 nm assigned to triindole. And the maxi-
mum absorption peaks of the two title compounds (7,
kmax = 337 nm; 8, kmax = 348 nm) show a 20–30 nm red-shift rela-
tive to that of their precursor 4, which attributed to the extended
p-conjugated character and their rigid planar frameworks. It can
Results and discussion
be regarded that three C(sp)BC(sp) triple bonds are embedded in
three branches of compound 7 to form compound 8. The absorp-
tion spectrum of 8 behaves a slight red-shift in comparison with
Synthesis and characterization
7. In high concentrations (c = 1.0 ꢁ 10ꢀ2 mol/L), compound
8
As shown in Scheme 1, the cheap and innocuous isatin (lindo-
lin-2,3-dione) was used as the starting materials. After N-alkyl-
ation of isatin (1) in THF, N-butylindolin-2,3-dione (2) was
shows an obvious new absorption peak appeared at ꢃ450 nm. Sur-
prisingly, compound 7 does not exhibit similar change. This indi-
cates that an intermolecular aggregation was formed induced by
HOMO-1 (164), -4.9035 eV HOMO(165), -4.9033 eV
LUMO(166), -0.9151 eV LUMO+1 (167), -0.9147 eV
HOMO-1 (182), -4.9721 eV HOMO(183), -4.9720 eV
LUMO(184), -1.2879 eV LUMO+1 (185), -1.2876 eV
Fig. 3. The graphic representations of the frontier molecular orbits of 7 (up) and 8 (down).
3.0u
2.0u
1.0u
0.0
b
a
20.0u
15.0u
10.0u
5.0u
8
7
8
0.0
-1.0u
-5.0u
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Potential (V vs. Ag/Ag+)
Potential (V vs. Ag/Ag+)
Fig. 4. Cyclic voltammogram of compounds 7 and 8 in CH2Cl2; electrolyte 0.1 M n-Bu4NClO4, scan rate 100 mV sꢀ1, scan range 0.2–0.8 V (a), scan range 0.2–1.5 V (b).

1024
M.-S. Yuan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 1020–1024
p–p
stacking which contributes to the rigid coplanar molecular
triindole compounds possess rich electronic character, which
construction.
imply that they can apply as organic electronic materials.
As shown in Fig. 2 the two analogues emit blue fluorescence lo-
cated at 424 and 430 nm in THF and behave moderate emission
Acknowledgments
~
Dm
solvatochromism. The almost same values of
=Df indicate that
they have very similar dipolar change in both ground state and ex-
cited state.
Financial support from the Ph.D. Programs Foundation of Minis-
try of Education of China (No. 20090204120033) and the Funda-
mental Research Funds for the Central Universities (No.
Z109021106) is gratefully acknowledged.
In order to understand the spectral behaviors and charge trans-
fer modes of the compounds 7 and 8, a TD-DFT (time-dependent
density functional theory) calculation at the B3LYP/6-31G⁄ level
was carried out (Fig. 3). To compute feasible, all butyl groups in
the 5, 10, 15 positions of triindole have been replaced by methyl
groups. The calculation results exhibit that the highest occupied
molecular orbital (HOMO) and HOMO-1 of the C3-symmetric octu-
polar compounds 7 and 8 have very close energy, and the lowest
unoccupied molecular orbital (LUMO) and LUMO+1 the same. This
is consistent with our previous work [28,29]. As a result, moderate
multidimensional intramolecular charge transfer take place be-
tween the center triindole and the periphery thiophene.
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The electrochemical behaviors of compounds 7 and 8 were
examined by cyclic voltammetry measurement, and they exhibit
very similar redox properties. As shown in Fig. 4a, both compounds
can be easily oxidized to stable cationic radicals with the onset oxi-
dation potential (E1/2 oxd) at 0.57 for 7 and 0.56 V for 8, respec-
tively. The energy of HOMO levels can be estimated as ꢀ5.01 eV
for 7 and ꢀ5.00 eV for 8 (HOMO = ꢀE1/2 oxdꢀ4.44) [27]. And the en-
ergy of LUMO levels can be roughly estimated from the energy of
HOMO level and energy bandgap (Eg) determined from the longest
wavelength absorption onset (LUMO = HOMOꢀEg) to be ꢀ1.83 eV
for 7 and ꢀ2.05 eV for 8. It should be noted that when the scan
was repeated in the region between 0.2 and 0.8 V, the two com-
pounds showed reversible oxidation waves. However, when the
scan range is enlarged to 0.2–1.5 V, they showed the irreversible
redox properties (Fig. 4b), which is different from the reported tri-
indole derivatives [20,21,24–26]. This may be originated from the
high electronic density of both triindole and thiophene groups.
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Conclusion
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In conclusion, two octupolar coplanar triindole molecules have
been synthesized by incorporating three 2-thiophenyl and 2-thio-
phenylethynyl groups to the 3,8,13-positions of triindole respec-
tively. Their photophysical and electrochemical properties have
been examined. The thiophene-functionalized triindole com-
pounds 7 and 8 emit blue enhanced fluorescence in comparison
with that of the precursor 4, and they would hopefully become
promising luminescent materials. Their electrochemical properties
and results of theoretical calculation demonstrate that the two
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