Novel planar and star-shaped molecules: Synthesis, electrochemical and photophysical properties

Article abstract of DOI:10.1016/j.saa.2013.01.047

Three novel planar and star-shaped molecules containing thiophene-functionalized group and acetylenic spacers, 4-((5″-iodo-[2, 2′:5′,2″-terthiophen]-5-yl)ethynyl)aniline (I3TEA), 4,4′-([2,2′:5′,2″-terthiophene]-5,5″- diylbis(ethyne-2,1-diyl))dianiline (3T

Full text of DOI:10.1016/j.saa.2013.01.047

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Novel planar and star-shaped molecules: Synthesis, electrochemical
and photophysical properties
⇑
Qingfen Niu, Yunqiang Lu, Hongjian Sun, Xiaoyan Li
School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, 250100 Jinan, People’s Republic of 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
"
Three novel materials I3TEA, 3TDDA
and TETEB were successfully
synthesized.
"
"
Their optical and electrochemical
properties were investigated.
The compounds were promising
candidates for photovoltaic
materials.
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel planar and star-shaped molecules containing thiophene-functionalized group and acetylenic
spacers, 4-((500-iodo-[2,20:50,200-terthiophen]-5-yl)ethynyl)aniline (I3TEA), 4,40-([2,20:50,200-terthiophene]-
5,500-diylbis(ethyne-2,1-diyl))dianiline (3TDDA), 1,3,5-tris((5-((trimethylsilyl)ethynyl)thiophen-2-yl)-
ethynyl)benzene (TETEB) were synthesized through Sonogashira coupling reaction. Their photophysical
and electrochemical properties were explored by UV–vis, photoluminescence (PL) emission and DFT
calculation. Three compounds possess unusual phenomena of PL, with the concentration of gradually
decreased, the intensity of PL firstly increased and then decreased, this may be due to the aggregate-
enhanced emission (AEE) effect and aggregation-caused quenching (ACQ) effect. Their spectroscopic data
Received 27 October 2012
Received in revised form 4 January 2013
Accepted 17 January 2013
Available online 1 February 2013
Keywords:
Planar molecule
Star-shaped molecule
Acetylenic spacer
Aggregate-enhanced emission effect
Aggregation-caused quenching effect
Photovoltaic material
demonstrate that D–
acceptor units with acetylenic spacers
properties. Dye I3TEA with donor(D)–acceptor(A) structure shows good charge transfer because of amino
functional group and iodine atom. Octupolar organic -conjugated star-shaped molecule TETEB pos-
p
–A–p
–D structured organic dye 3TDDA based on NH2 as donor units and 3T as
p
-conjugated chain between them exhibits good luminescent
p
sesses excellent luminescent properties because of its unique structure applied in the optoelectronic
field. The redox curves and results of DFT calculations suggest that three compounds have low ionization
potential and low Eoxd. All the good properties demonstrate three compounds could be promising
candidates for photovoltaic materials.
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
nature of the C„C group leading to an enhanced electron affinity,
and consequently to a higher oxidation potential and a lower
Acetylenic-based conjugated polymers or oligomers have
drawn much attention of a great many researchers due to their
wide applications in photovoltaic devices [1–10]. Researchers are
interested in an increase of the open circuit voltage, which is de-
fined as the quasi-Fermi level splitting between the donor HOMO
and the acceptor LUMO. However, this specific electronic parame-
ter is involved in solar cells because of the electron-withdrawing
HOMO level [1–3,11–13]. Therefore, the materials of this kind used
as donors may provide a new route to improve photovoltaic device
performances. Recently, a large number of small molecules based
photovoltaic devices have attracted much attention in bulk hetero-
junction solar cells attributed to their reproducible preparation,
facile purification and easy functionalization [14–19]. The concept
of heterojunction has already been exploited in dye–dye, polymer–
dye, polymer–polymer and polymer–fullerene blend photovoltaic
devices on a large scale [20–23]. In parallel, donor–acceptor
jugated polymers or oligomers are explored as active materials
p-con-
⇑
Corresponding author. Tel.: +86 531 88361350; fax: +86 531 88564464.
E-mail address: xli63@sdu.edu.cn (X. Li).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.01.047

378
Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
widely applied in the field of organic solar cells [24–27]. The design
of the class of this material has two main purposes. One is to re-
duce the band gap of the molecule for broadening the range of
absorption and the other is to study the relationship between dif-
ferent donor/acceptor chromophores and their corresponding
properties of photophysics, electrochemistry, optics and electron-
ics [14,28–32]. Ultimately, thiophene-based materials have at-
tracted a great deal of attention for their potential applications in
the fields of organic semiconductors due to their light weight, good
chemical stability, high carrier mobility and the ease of structural
tuning to adjust the optical, electronic and morphological
properties for the time being [33–36]. It has been reported that
oligothiophenes end-capped with strong electron donors as
diphenylaminofluorenyl moieties have become highly efficient
hole-transporting bis-dipolar emitters for organic light emitting
diodes (OLEDs) [37].
properties dependence. Up to now, some of C3-symmetric func-
tionalized groups have been used to build star-shaped molecules
[47–58], but the total number is not much. By reason of what we
said above, the incorporation of arylene ethynylene structural mo-
tifs into p-conjugated compounds is expected to render them more
interesting and desirable properties and hence is of great interest
to us.
In this paper, we synthesized two novel planar molecules 4-
((500-iodo-[2,20:50,200-terthiophen]-5-yl)ethynyl)aniline (I3TEA),
4,40-([2,20:50,200-terthiophene]-5,500-diylbis(ethyne-2,1-diyl))diani-
line (3TDDA) and one octupolar organic p-conjugated star-shaped
molecule 1,3,5-tris((5-((trimethylsilyl)ethynyl)thiophen-2-yl)-
ethynyl)benzene (TETEB) containing thiophene-functionalized
group and acetylenic spacers. Their structures are shown in
Fig. 1. All the objective compounds were successfully characterized
with respect to UV–vis absorption spectroscopy, photolumines-
cence (PL) emission spectroscopy, density functional theory (DFT)
calculation and cyclic voltammetry (CV). Three compounds display
narrow band gaps and a slight aggregation induced emission (AIE)
At the same time, a large number of researchers also have
drawn great interests in octupolar organic
p-conjugated dendri-
mers containing C–C triple bonds facilitated electronic communi-
cation because of their novel structures, unique properties and
unusually narrow band gaps in organic optoelectronic field
[38,39]. More fascinating octupolar star-shaped molecules have
continued to receive much attention due to their potential applica-
effect. D–p–A–p–D structured organic dye 3TDDA based on NH2 as
donor units and 3T as acceptor units with acetylenic spacers
p-
conjugated chain between them possesses good luminescent prop-
erties. Dye I3TEA with donor(D)–acceptor(A) structure shows
weak fluorescence intensity in comparison with dye 3TDDA due
to the presence of iodine. Polymer TETEB exhibits excellent lumi-
nescent properties probably applied in the optoelectronic field.
tion as a kind of novel p-electronic material. Currently, the materi-
als of this kind have been used as electron-transporting and light
emitting layers in the fabrication of highly efficient electrolumi-
nescence (EL) devices. Great efforts have been made to design
and synthesize more fascinating octupolar star-shaped molecules
and to study their structure–property relationships in the recent
years [40–43]. It is obvious that the polymers have been found to
exhibit excellent photoluminescence (PL) and electroluminescence
(EL) properties, so they have been widely used as light-emitting
diodes, photovoltaic cells and even field effect transistors [44–
46]. The selection of novel core structure is the key factor for the
rational design of an optoelectronic molecule due to its crucial
Experimental
Measurements and characterizations
1H and 13C NMR spectra were recorded on a Bruker AVANCE 300
NMR Spectrometer with DMSO-d6 and CDCl3 as the solvent with
tetramethylsilane (TMS) as an internal reference. Infrared mea-
surements with the KBr pellet technique were performed within
the 4000–400 cmꢀ1 region on a Bruker ALPHA FT-IR spectrometer.
LC-MS were obtained from Agilent 6510 Accurate-Mass Q-TOF LC/
MS system. Ultraviolet absorption (UV) spectra of these samples
were recorded using a Hitachi U-4100 spectrometer. Photolumi-
nescence (PL) measurements were recorded on a Hitachi F-4500
fluorescence spectrophotometer with a 150 W Xe lamp. Cyclic vol-
tammetry (CV) measurement was performed on a CHI800 electro-
chemical instrument with a three-electrode cell in a solution of
0.1 M tetrabutylammonium perchlorate (Bu4NClO4) in anhydrous
chloroform at room temperature under nitrogen atmosphere with
a scan rate of 100 mV/s. The working electrode was glass–carbon
electrode, the counter electrode was a platinum wire, and the ref-
erence electrode was Ag/AgCl (Ag in 0.1 M AgNO3 solution of
MeCN) which was separated by a diaphragm. Ferrocene–ferroce-
nium (Fc/Fc+) couple was chosen as internal standard. Thin-layer
chromatography (TLC) was carried out with silica gel GF254 cov-
ered on plastic sheets and visualized by UV light, and flash column
chromatographic separation was performed over silica gel.
S
I
S
S
S
H2N
I3TEA
S
S
NH2
H2N
3TDDA
Me3Si
S
SiMe3
S
Materials
2-Bromothiophene and 2,5-dibromothiophene were purchased
from the China Medicine Shanghai Chemical Reagent Corp, China.
All other chemicals were purchased from Aldrich and Acros and
used as received without further purification. 4-Ethynylaniline
[59], N-iodosuccinimide (NIS) [60], 2,20:50,200-terthiophene (3T)
[61], lithium diisopropylamide (LDA) [62] and trimethyl(thio-
phen-2-ylethynyl)silane [63] (2) were prepared according to the
published procedures. Catalysts Pd(PPh3)2Cl2 [64] and Ni(dppp)Cl2
S
SiMe3
TETEB
.
Fig. 1. The structures of compounds I3TEA, 3TDDA and TETEB.
[65] were prepared from PdCl2 and NiCl2 6H2O according to the

Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
379
published literatures, respectively. Solvents were purified and
dried according to the standard procedures.
triethylamine (40 mL) and dry THF (50 mL) under an atmosphere
of nitrogen. Copper(I) iodide (33.4 mg, 0.2 mmol), triphenylphos-
phine (46.7 mg, 0.2 mmol) and dichlorobis(triphenylphos-
phine)palladium(II) (66.9 mg, 0.07 mmol) were added to the
stirred solution. 4-Ethynylaniline (1.2 g, 10.0 mmol) was dropped
in and the mixture was heated to 50 °C for 6 h. After cooling, the
formed precipitate of triethylamine hydroiodide was filtered off
and washed with diethyl ether. The combined filtrates were evap-
orated under reduced pressure, and the crude product was purified
by silica column chromatography eluting with petroleum ether/
ethyl acetate (V:V = 5:1) to afford compound I3TEA (2.1 g, 42%)
Synthesis
Synthesis of ((5-iodothiophen-2-yl)ethynyl)trimethylsilane (2)
To a solution of diisopropylamine (3.4 g, 33.8 mmol) in diethyl
ether (20 mL) at ꢀ78 °C was added dropwise n-butyllithium
(19.4 mL, 31.0 mmol, 1.60 M in hexane), and then stirred at
ꢀ78 °C for 15 min. The mixture was warmed to 0 °C for 30 min
and then re-cooled to ꢀ78 °C. Trimethyl(thiophen-2-ylethy-
nyl)silane (1) (2.8 g, 15.5 mmol) in diethyl ether (10 mL) at room
temperature was then added dropwise to the LDA solution, and
the solution was warmed from ꢀ78 to 0 °C for 15 min. After re-
cooling to ꢀ78 °C, iodine (8.6 g, 33.8 mmol) in diethyl ether
(60 mL) was added via cannula, and the solution was then warmed
to room temperature and stirred overnight. The mixture was
quenched with water, and the aqueous layer was extracted with
diethyl ether. The combined organic layer was washed with brine
and aqueous sodium thiosulfate, and then dried over magnesium
sulfate. The solvent was removed under vacuum, and the crude
product was purified by silica column chromatography eluting
with petroleum ether to afford compound ((5-iodothiophen-2-
yl)ethynyl)trimethylsilane [65] (2) (4.3 g, 90%) as a yellow liquid.
The product was used instantly for the next step without further
purification.
as an orange powder. m.p. 143–145 °C; IR (KBr, cmꢀ1
) m 3411,
3332, 2923, 2188, 1600, 1514, 1461, 1377, 1282, 1176, 1067,
1015, 944, 829, 790, 533, 473; 1H NMR (300 MHz, DMSO-D6,
ppm) d 5.65 (s, 2H), 6.56 (d, J = 8.7 Hz, 2H), 7.08 (d, J = 3.6 Hz,
1H), 7.20 (d, J = 8.7 Hz, 2H), 7.23 (d, J = 3.9 Hz, 1H), 7.27–7.34 (m,
4H); 13C NMR (75 MHz, DMSO, ppm) d 79.0, 79.4, 107.8, 114.2,
122.9, 125.0, 125.1, 125.5, 126.1, 126.6, 132.8, 133.0, 135.1,
135.4, 136.7, 138.5, 141.9, 150.2.
Synthesis of 4,40-([2,20:50,200-terthiophene]-5,500-diylbis(ethyne-2,1-
diyl))dianiline (3TDDA)
Compound 3TDDA was synthesized from 5,500-diiodo-2,20:50,200-
terthiophene (5) (5.0 g, 10.0 mmol) and 4-ethynylaniline (2.3 g,
20.0 mmol) by the same procedure as compound I3TEA. The crude
product was purified by silica column chromatography eluting
with petroleum ether/ethyl acetate (V:V = 3:1) to afford compound
3TDDA (3.1 g, 64%) as an orange powder; IR (KBr, cmꢀ1
) m 3433,
Synthesis of 1,3,5-triethynylbenzene (4)
The procedure was modified following the Sonogashira cou-
pling protocol. 1,3,5-Tribromobenzene (31.5 g, 100 mmol) was dis-
solved in dry triethylamine (250 mL) under an atmosphere of
nitrogen. Copper(I) iodide (191 mg, 1.0 mmol), dichlorobis(dib-
enzonitrile)palladium(II) (192 mg, 0.5 mmol) were added to the
stirred solution. After addition of 2-methyl-3-butyn-2-ol (12.6 g,
150 mmol) in drops, the reaction mixture was refluxed for 12 h un-
der nitrogen atmosphere. At the room temperature, the formed
precipitate of triethylamine hydroiodide was filtered off and
washed with diethyl ether (3 ꢁ 50 mL). The combined filtrates
were dried over anhydrous Na2SO4 and evaporated to have the
crude product. Re-crystallization of this crude product in 20 mL
of ethanol at ꢀ20 °C delivered white crystals of 2,20,200-(benzene-
1,3,5-triyltris(ethyne-2,1-diyl))tris(propan-2-ol) (3) (30.8 g, 95%).
The hydrolysis of compound 3 was carried out by treatment with
KOH (19.2 g) and toluene 150 mL under stirring at 110 °C for 8 h.
A standard work-up procedure involving evaporation of the organ-
ic solvent, extraction of the residue with diethyl ether, drying over
Na2SO4, and removal of the solvent under reduced pressure yielded
1,3,5-triethynylbenzene [66] (4) (13.0 g, 92%) as colorless needle
crystals. m.p. 95.4–96.0 °C; 1H NMR (300 MHz, CDCl3, ppm) d
3.10 (s, 3H), 7.57 (s, 3H).
3356, 2193, 1897, 1728, 1609, 1516, 1484, 1416, 1271, 1172,
1111, 1051, 960, 866, 830, 778, 621, 521, 475, 396; 1H NMR
(300 MHz, CDCl3, ppm) d 3.68 (s, 1H), 3.85 (s, 2H), 4.10 (s, 1H),
6.47 (dd, J1 = J2 = 2.1 Hz, 2H), 6.64 (dd, J1 = J2 = 3.9 Hz, 2H), 6.83–
6.92 (m, 1H), 6.97–7.05 (m, 2H), 7.06–7.17 (m, 2H), 7.32 (dd,
J1 = 2.7 Hz, J2 = 1.8 Hz, 2H), 7.40 (dt, J1 = J2 = 3.0 Hz, J3 = J4 = 2.1 Hz,
2H), 7.80 (dd, J1 = J2 = 4.8 Hz, 1H); 13C NMR (75 MHz, CDCl3, ppm)
d 79.3, 82.1, 113.7, 114.7, 123.7, 124.5, 124.8, 125.1, 128.1, 130.8,
131.9, 132.8, 137.9.
Synthesis of 1,3,5-tris((5-((trimethylsilyl)ethynyl)thiophen-2-yl)-
ethynyl)benzene (TETEB)
Compound TETEB was synthesized from ((5-iodothiophen-2-
yl)ethynyl)trimethylsilane (2) (3.7 g, 12.0 mmol) and 1,3,5-triethy-
nylbenzene (4) (0.6 g, 3.7 mmol) by the same procedure as com-
pound I3TEA. The crude product was purified by silica column
chromatography eluting with petroleum ether/ethyl acetate
(V:V = 25:1) to afford compound TETEB (1.72 g, 68%) as an pale
yellow powder; m.p. 103–104 °C; IR (KBr, cmꢀ1
) m 2963, 2928,
2858, 2147, 1582, 1463, 1410, 1380, 1251, 1201, 1165, 1095,
1031, 856, 804, 760, 698, 626, 554; 1H NMR (300 MHz, CDCl3,
ppm) d 0.26 (s, 27H), 7.11 (s, 6H), 7.60 (s, 3H); 13C NMR
(75 MHz, CDCl3, ppm) d ꢀ0.03, 78.5, 83.9, 92.1, 96.9, 123.8,
124.0, 125.4, 132.3, 132.7, 134.0; HRMS (ESI) calcd. for C39H36S3Si3
[M+] 685.1320, found 685.1339.
Synthesis of 5,500-diiodo-2,20:50,200-terthiophene (5)
2,20:50,200-Terthiophene (3T) (5.0 g, 20.2 mmol) and NIS (9.9 g,
44.5 mmol) were dissolved in a mixture of dry dichloromethane
(50 mL) and glacial AcOH (50 mL). The mixture was shielded from
light and stirred for 4 h at room temperature. Solvents were re-
moved by evaporation. The residue was dissolved in 20 mL of mix-
ture of dichloromethane and pentane (V:V = 3:1). Crystallization at
ꢀ20 °C gave rise to yellow crystals of 5,500-diiodo-2,20:50,200-terthio-
phene [67] (5) (8.5 g, 84%). The product was used instantly for the
next step without further purification.
Results and discussion
Synthesis and characterization
Schemes 1 and 2 illustrate the synthetic routes for the investi-
gated compounds I3TEA, 3TDDA and TETEB. All of the compounds
were prepared by the Sonogashira coupling reaction. The synthetic
reactions were controlled by TLC. These materials are readily solu-
ble in common solvents such as THF, diethyl ether, dichlorometh-
ane and chloroform, allowing them to be easily purified by column
chromatography. The objective products were characterized by IR,
Synthesis of 4-((500-iodo-[2,20:50,200-terthiophen]-5-yl)ethynyl)anili-ne
(I3TEA)
Following the Sonogashira coupling procedure, 5,500-diiodo-
2,20:50,200-terthiophene (5) (5.0 g, 10.0 mmol) was dissolved in dry

380
Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
S
S
S
LDA/I2
TMSA
Br
SiMe3
I
SiMe3
88%
Pd(PhCN)2Cl2, CuI
Et3N,50°C
1
2
HO
OH
Br
Br
reflux
Pd(PhCN)2Cl2, CuI,
Et3N ,reflux
+
OH
KOH/toulene
Br
OH
3
95%
4 87%
Scheme 1. Synthesis of monomers 2 and 4.
Pd(Ph3P)2Cl2, CuI
Ph3P, Et3N, THF
1 eq H2N
2 eq H2N
I3TEA
42%
S
+
I
I
S
S
Pd(Ph3P)2Cl2, CuI
Ph3P, Et3N, THF
5
3TDDA
64%
Pd(Ph3P)2Cl2, CuI
Ph3P,Et3N, THF
S
TETEB
68%
I
+
SiMe3
2
4
Scheme 2. Synthetic routes to I3TEA, 3TDDA and TETEB.
NMR and MS. The results are consistent with the expected chemi-
cal structures.
Photophysical properties
Absorption spectra
The UV–vis absorption spectra of the three novel compounds
I3TEA, 3TDDA and TETEB were recorded in CHCl3
(C = 1.0 ꢁ 10ꢀ5 M) shown in Fig. 2. In the solution, each compound
shows two distinct absorption bands, one broad absorption band is
around at 300–500 nm, 330–500 nm, 250–450 nm for I3TEA,
3TDDA and TETEB, respectively. The long wavelength band is a di-
pole-allowed,
jugated -electron system of the core of the mesogens. This band
p–
pꢂ electron transition absorption band of the con-
p
corresponds to the transition from the ground state to the lowest
excited singlet state of the molecule. The narrow absorption band
between 230 and 300 nm corresponds to a transition to a higher-
lying singlet state [68,69]. The absorption of the star-shaped mol-
ecule TETEB reveals two absorbance peaks at 339 and 362 nm,
Fig. 2. UV–vis absorption spectra of the dyes I3TEA, 3TDDA and TETEB in
chloroform solution (1.0 ꢁ 10ꢀ5 M).
indicating a weak shoulder similar to
tion maximum of
pꢂ transitions for compound 3TDDA was grad-
ually red-shifted in comparison of I3TEA from 401 to 420 nm with
elongation of the -conjugation length. This can be rationalized by
the increased intermolecular interaction with the expanding
p–
pꢂ transitions. The absorp-
p
–
of the
p-conjugation. The onset absorption wavelengths were
p
466 nm for I3TEA, 493 nm for 3TDDA, 448 nm for TETEB, respec-
tively. However, the onset absorption wavelengths were increased
p–p

Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
381
Fig. 3. UV–vis absorption spectra of compounds I3TEA, 3TDDA and TETEB in different solvents (1.0 ꢁ 10ꢀ5 M).
Table 1
The optical characteristics of three compounds in four different solvents.
Compound
Solvents/the absorption maximum wavelengths kmax (nm)
CHCI3
THF
CH3OH
DMF
I3TEA
3TDDA
TETEB
401
420
339
402
421
340
403
422
341
407
430
343
by the increased conjugation effect [70–72]. Therefore, the HOMO–
LUMO gaps obtained from the end-absorption are 2.66 eV for
I3TEA, 2.52 eV for 3TDDA, and 2.77 eV for TETEB, respectively.
Effect of solvent on the absorption spectra
Typical absorption spectra of compounds I3TEA, 3TDDA and TE-
TEB in different four solvents such as CHCl3, THF, CH3OH and DMF
(C = 1.0 ꢁ 10ꢀ5 M) were shown in Fig. 3 and the corresponding
absorption wavelength maxima of the three compounds were pre-
sented in Table 1. As shown in Table 1, absorption wavelength
maxima for I3TEA in different solvents varied from 401 to
407 nm, 420 to 430 nm for 3TDDA, and 339 to 341 nm for TETEB,
respectively. With the increase of polarity of solvents, the absorp-
tion peak of three compounds was slightly red-shifted. This shift
Fig. 4. Photoluminescence emission (PL) spectra of the three compounds in CHCl3
(1.0 ꢁ 10ꢀ6 M).
However, the other two compounds were highly fluorescent at
room temperature. It should be noted that the spectrum of TETEB
reveals four pronounced peaks centered at 375, 390, 418 and
446 nm, respectively. While, for 3TDDA, one pronounced peak cen-
tered at 496 nm and one weak shoulder centered at 525 nm. This
indicates the existence of molecular conformation deviation be-
tween the ground state and the exited state. It is well worth noting
that TETEB shows a broader band and the fluorescence intensity is
much stronger than the corresponding intensity of the others.
Meanwhile, the result also indicates that their structure–property
suggested that the absorption maximum of
p–
pꢂ transition energy
decreased, which allowed the
p–
pꢂ electron transition. The absorp-
tion spectra of compounds I3TEA, 3TDDA and TETEB in different
solvents exhibit different absorption intensities, however, the
three compounds in CH3OH show the weakest absorption intensi-
ties. This phenomenon may be due to the formation of the intra-
molecular hydrogen bond.
relationships of the novel octupolar organic
p-conjugated star-
shaped molecule on the conjugated backbone by acetylenic spacers
and thiophene-functionalized group could effectively influence the
fluorescence intensities (up to 5364 a.u.) in the emission spectra.
Fluorescence spectra
The chloroform solutions containing I3TEA and 3TDDA exhib-
ited a bright green-yellow fluorescence even under daylight, while
compound TETEB showed an orange-yellow fluorescence. Photolu-
minescence (PL) emission spectra of compounds I3TEA, 3TDDA
and TETEB were measured in chloroform with a concentration of
1.0 ꢁ 10ꢀ6 M (Fig. 4). The corresponding PL emission maximal
wavelengths are located at 482, 496 and 375 nm, respectively. It
can be found that their intensity of fluorescence differs from each
other. The fluorescence intensity of compound I3TEA is extremely
weak compared with the other two compounds. This may be due
to the heavy atom effect. Heavy atom substitution (such as iodine
in the aromatic ring) could result in higher intersystem crossing
efficiency as well as higher triplet yields [73–75]. For compound
I3TEA, the donor–acceptor structure character resulting from the
electron-donating amino functional group and electron-withdraw-
ing iodine atom is in favor of dipole–dipole interactions between
molecules. Therefore, I3TEA exhibited fluorescence quenching.
Therefore, octupolar organic
p-conjugated star-shaped molecule
TETEB exhibits excellent luminescent property probably used as
luminescent materials.
Effect of the concentration on the emission spectra
In order to further study the effect of the concentration on the
fluorescence emission intensity, we observed the fluorescence
emission spectra of compounds I3TEA, 3TDDA and TETEB in chlo-
roform solution with different concentrations shown in Fig. 5. The
emission spectra of I3TEA show two emission bands, one broad
emission band in the range of 430–600 nm, another narrow emis-
sion band in the range of 390–420 nm. The two peaks of the two
bands are located at 407 and 494 nm and the emission maximal
peak centered at 494 nm in the range of the concentration from
5.0 ꢁ 10ꢀ4 to 1.0 ꢁ 10ꢀ6 M. This indicates no existence of molecu-

382
Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
Fig. 5. PL spectra of three compounds in chloroform solution with different concentrations.
Fig. 6. Normalized emission spectra of these compounds in different solvents with the concentration of 1.0 ꢁ 10ꢀ6 M.
Table 2
lar conformation deviation between the ground state and the ex-
ited state. The emission spectra exhibit three emission peaks with
the concentration from 5.0 ꢁ 10ꢀ7 to 1.0 ꢁ 10ꢀ8 M. In the range of
430–600 nm a new weak shoulder peak was found during this
broad emission band. The three peaks are located at 407, 470
and 494 nm and the emission maximal peak centered at 470 nm.
This may be attributed to the existence of molecular conformation
deviation between the ground state and the exited state in this
concentration range. The fluorescence intensities of I3TEA in chlo-
roform decreased significantly (from 392 to 80 a.u.) in this concen-
tration range during the broad emission band, but the fluorescence
intensities increased significantly (from 34 to 416 a.u.) during the
narrow emission band. This indicates that I3TEA does not aggre-
gate in diluted solution. For 3TDDA, the emission spectra exhibit
two emission bands and three peaks, one broad emission band in
the range of 450–600 nm, another narrow emission band in the
range of 400–415 nm. The three peaks of the two bands are located
at 408, 496 and 522 nm and the emission maximal peak centered
at 496 nm. The fluorescence intensities of 3TDDA in chloroform in-
creased significantly (from 510 to 1440 a.u.) with the concentra-
tion from 5.0 ꢁ 10ꢀ4 to 5.0 ꢁ 10ꢀ6 M. However, the fluorescence
intensities decreased significantly (from 1355 to 757 a.u.) with
the concentration from 1.0 ꢁ 10ꢀ6 to 1.0 ꢁ 10ꢀ7 M during the
broad emission band, but the fluorescence intensities have no sig-
nificant change during the narrow emission band. The difference of
the fluorescence behavior between I3TEA and 3TDDA may be
attributed to the heavy atom effect of iodine in I3TEA and different
aggregates in different concentrations. For TETEB, the emission
spectra exhibit four emission peaks, the four peaks are located at
375, 390, 418 and 446 nm, respectively, and the emission maximal
peak centered at 375 nm. The fluorescence intensities of TETEB in
chloroform increased significantly (from 1235 to 6714 a.u.) with
the concentration from 5.0 ꢁ 10ꢀ4 to 5.0 ꢁ 10ꢀ6 M. However,
the fluorescence intensities decreased significantly (from 5364
to 2042 a.u.) with the concentration from 1.0 ꢁ 10ꢀ6 to
The optical characteristics of the dyes in six various solvents.
Compound
Solvents/the emission maximum wavelengths kmax (nm)
C7H8
CHC13
CH3COOC2H5
THF
C2H5OH
CH3CN
I3TEA
3TDDA
TETEB
470
494
376
470
495
376
496
499
375
500
507
421
510
523
369
510
526
368
1.0 ꢁ 10ꢀ7 M. Based on the results, it can be concluded that the
three compounds have the concentration aggregate-enhanced
emission (AEE) effect [76] and aggregation-caused quenching
(ACQ) effect [77].
Effect of solvent on the emission spectra
Normalized fluorescence spectra of the three compounds in var-
ious solvents such as C7H8, CHCl3, CH3COOC2H5, THF, C2H5OH and
CH3CN (C = 1.0 ꢁ 10ꢀ6 M) are recorded in Fig. 6 and the corre-
sponding emission maximum wavelengths are summarized in
Table 2. As depicted in Fig. 6, the emission maxima for I3TEA
and 3TDDA had a red-shift, but the emission maxima for com-
pound TETEB had a bule-shift with the increasing the polarity of
solvents from toluene to acetonitrile. The results might be resulted
from the difference in the dipole moment of compounds I3TEA and
3TDDA in the excited state and the ground state. The relaxed ex-
cited state would be energetically stabilized relative to the ground
state with increasing the polarity of solvents and a significant red-
shift of the fluorescence band was observed. The stronger the inter-
action of solute and solvent is, the lower the energy of the excited
state is, and the larger the red-shift of the emission band is. How-
ever, the TETEB is on the contrary. Furthermore, the fluorescence
intensities of TETEB have a significant change in the different sol-
vents. Fluorescence spectra of the three compounds in various sol-
vents with different polarities had similar fluorescence emission

Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
383
spectra, which suggested that the fluorescence emission could oc-
cur from the first excited state S1 to the ground state S0.
Theoretical calculations
In order to gain insight into geometrical configuration and
photophysical properties and charge transfer modes, the ground-
state optimized geometries and electronic structures of com-
pounds I3TEA, 3TDDA and TETEB were calculated with Gaussian
03 software by making full use of density functional theory (DFT)
based on the Becke’s three parameter gradient-corrected func-
tional (B3LYP) with a polarized 6-31G(d) basis [78]. The geometries
of ground-state optimized structures and the electron-density dis-
tribution of HOMO, HOMOꢀ1 and LUMO, LUMO+1 of optimized
structures are showed in Figs. 7a and 7b, respectively. Fig. 7a
shows the compounds I3TEA and 3TDDA have good planar struc-
Fig. 7a. Optimized geometries of I3TEA, 3TDDA and TETEB. Atom coloring: white,
hydrogen; gray, carbon; yellow, sulfur; purple, iodine; blue, nitrogen; green, silicon.
Isosurface cut-off value: 0.02. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
calculated results show that the introduction of additional thio-
phene units stabilizes the LUMO energy levels through increased
length of conjugation. The amino substitution on the phenyl ring,
tures and TETEB is an octupolar organic p-conjugated star-shaped
molecule. The HOMO, HOMOꢀ1 and LUMO, LUMO+1 energies and
energy band gaps calculated by DFT were listed in Table 3. The DFT
Fig. 7b. The frontier molecular orbitals of I3TEA (top), 3TDDA (middle) and TETEB (bottom) calculated with TD-DFT on B3LYP/6-31Gꢂ.
Table 3
Summaries of physical, electrochemical and DFT calculated measurements of the compounds.
Compound
Optical and electrochemical characteristics
DFT calculation
HOMO (eV)
Eox (V)
HOMOa (eV)
konset (mn)
LUMOc (eV)
LUMO (eV)
Eg (eV)
HOMOꢀ1 (eV)
LUMO+1 (eV)
b
Eogpt (eV)
b
I3TEA
3TDDA
TETEB
0.66
1.20
0.91
ꢀ5.46
ꢀ5.69
ꢀ5.41
466
493
448
2.66
2.52
2.77
ꢀ2.80
ꢀ3.17
ꢀ2.64
ꢀ4.85
ꢀ4.59
ꢀ5.53
ꢀ1.92
ꢀ1.92
ꢀ2.02
2.93
2.67
3.51
ꢀ5.64
ꢀ5.13
ꢀ5.54
ꢀ1.19
ꢀ1.12
ꢀ2.02
a
b
c
HOMO = 4.8 + Eox
.
Obtained from UV spectra Eogpt ¼ 1240=Xonset
.
LUMO ¼ HOMO ꢀ Eogpt
.
Fig. 8. CV curves measured in CHCl3 (C = 1.0 ꢁ 10ꢀ3 M) of I3TEA, 3TDDA and TETEB.

384
Q. Niu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 377–385
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however, destabilized the LUMOs of compounds I3TEA and
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pꢂ band gap energy in the order of
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and TETEB were measured by cyclic voltammetry in chloroform
with a concentration of 1.0 ꢁ 10ꢀ3 M as shown in Fig. 8. All of
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optical data were in good agreement with the DFT calculation.
The good optical and electrochemical properties of these com-
pounds are extremely important characteristics in the conceptions
of materials in photovoltaic application.
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In this work, we have successfully synthesized two novel planar
and one octupolar organic
p-conjugated star-shaped molecules
containing thiophene-functionalized group and acetylenic spacers,
I3TEA, 3TDDA and TETEB. Their photophysical and electrochemi-
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We gratefully acknowledge the support by NSF China No.
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