Asymmetric multibranched conjugated molecules: Synthesis, structure and photophysical properties

Article abstract of DOI:10.1016/j.saa.2014.06.153

The symmetric multibranched π-conjugated compounds with C₃ or C₆ configuration have been intensively studied. The reports on asymmetric multibranched compounds are very limited. In this work, we designed and synthesized two asymmetri

Full text of DOI:10.1016/j.saa.2014.06.153

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 63–68
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Asymmetric multibranched conjugated molecules: Synthesis, structure
and photophysical properties
Li Zhao a,b, Wenji Wang a, Mao-Sen Yuan a,c,
⇑
a College of Science, Northwest A&F University, Yangling 712100, PR China
b Hospital, Northwest A&F University, Yangling 712100, PR China
c State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
Two truxene-cored asymmetric
multibranched -conjugated
molecules were synthesized.
The two compounds exhibit almost
same absorption maxima, but
different emission.
The theoretical results reveal the
HOMO ? LUMO transition is not their
main transition.
p
a r t i c l e i n f o
a b s t r a c t
Article history:
The symmetric multibranched
sively studied. The reports on asymmetric multibranched compounds are very limited. In this work,
we designed and synthesized two asymmetric multibranched -conjugated molecules using truxene as
the central core, diphenylamino and thiophenyl (or thiophenylethynyl) groups as the different branches
respectively: 2,7-di(N,N-diphenylamino)-12-(2-thiophenyl)-5,50,10,100,15,150-hexaethyltruxene and 2,7-
di(N,N-diphenylamino)-12-(2-thiophenylethynyl)-5,50,10,100,15,150-hexaethyltruxene. Their photophysi-
cal properties have been explored combining with their theoretical calculation and X-ray single-crystal
p-conjugated compounds with C3 or C6 configuration have been inten-
Received 29 April 2014
Received in revised form 16 June 2014
Accepted 29 June 2014
p
Available online 8 July 2014
Keywords:
Photophysical properties
Crystal structure
Charge transfer
Truxene
structure of a key intermediate. Though their different
p-conjugation length of branches, the two title
compounds exhibit almost same absorption maxima. However, their emission peaks behave a gradual
red-shift with the increase of the conjugation length. The theoretical calculation results indicate that
the two asymmetric compounds behave a main transition from the HOMO ꢁ 1 to the LUMO or from
the HOMO to the LUMO + 1 upon excited.
Thiophene
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
attracted great interest in this decade [1–7]. In comparison to the
linear molecules and polymers, branched p-conjugated molecules
Multibranched organic
fascinating molecular architecture and unique properties, have
p
-conjugated molecules, due to their
have some advantages for applications in optoelectronic devices,
for example, the three-dimensional architectures, well-defined
molecular structure and good film-forming processing. In recent
years, a large number of multibranched
have been exploited extensively in a wide range of applications
p-conjugated molecules
⇑
Corresponding author at: College of Science, Northwest A&F University,
Yangling 712100, PR China. Tel./fax: +86 29 87092226.
as optoelectronic materials [8–14]. However, more research
E-mail address: yuanms@nwsuaf.edu.cn (M.-S. Yuan).
http://dx.doi.org/10.1016/j.saa.2014.06.153
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

64
L. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 63–68
mainly focused on the symmetric molecular system. In other
words, most of these multibranched molecules exhibit Cn symmet-
ric configurations (mostly C3 and C6), and every branch of the same
molecule behaves the same chemical structure. Studies that focus
on the properties and structure–function relationship investigation
of asymmetric multibranched molecules are very limited [15,16],
which may be on account of the difficulty in synthesis.
Generally, multibranched optoelectronic molecules are con-
structed from an interior functional core with several photo- and
electro-active branching units. Truxene (10,15-dihydro-5H-diinde-
no[1,2-a;10,20-c]fluorene), due to its C3-symmetric skeleton and
assisted laser desorption/ionization time of flight (MALDI-TOF)
mass spectrometer.
Compounds 1–4 and N2 were synthesized according to litera-
ture methods reported by us [16].
Synthesis of compound N2S
A mixture of compound 4 (0.50 g, 0.54 mmol), 2-thiophene-
boronic acid (0.10 g, 0.78 mmol), Pd(PPh3)4 (20 mg, 0.02 mmol),
toluene (30 mL), ethanol (8 mL) and 2 M aqueous K2CO3 solution
(2 mL) was heated and stirred at 80 °C under a nitrogen atmo-
sphere for 24 h. The mixture were cooled to room temperature
and poured into water (100 mL). After extraction with dichloro-
methane (DCM), the organic phase was dried over Na2SO4. The sol-
vent was removed and the residue was purified by column
chromatography on silica gel using DCM-hexane (1:20) as the elu-
ent to get compound N2S (0.16 g, 31.9%). N2S: a yellow powder,
m.p. 156–158 °C. 1H NMR (CDCl3, 500 MHz, ppm): d 0.21–0.28
(m, 18H), 1.89–2.15 (m, 6H), 2.84–2.99 (m, 6H), 7.02–7.12 (m,
6H), 7.32–7.33 (m, 3H), 7.46–7.49 (m, 2H), 7.55–7.58 (m, 16H),
7.57–7.67 (m, 2H), 8.09–8.27 (m, 3H). 13C NMR (CDCl3, 125 MHz,
ppm) d 154.22, 153.67, 152.67, 147.96, 129.24, 129.05, 128.24,
128.09, 126.35, 126.04, 125.31, 124.92, 124.59, 124.21, 122.95,
122.88, 122.69, 122.27, 121.98, 119.47, 117.66, 67.98, 56.65,
29.40, 29.19, 25.63, 21.46, 8.69, 8.65, 8.61. MALDI-TOF: m/z 927.3
[M+], 898.9 [M-29]+. Elemental Anal. Calcd. for C67H62N2S: C,
86.78; H, 6.74; N, 3.02; S, 3.46. Found: C, 86.71; H, 6.84; S, 3.35.
planar
p-conjugated polyarene structure, has been intensively
studied as a
p-conjugated central core to fabricate branched opto-
electronic molecules [17–29]. Herein, we are of particular interest
to develop the asymmetric multibranched compounds using
truxene as central unit. In order to endow the designed molecules
with good optoelectronic properties as well as thermal and chem-
ical stability, we availably functioned truxene at its 2,7,12-posi-
tions with diphenylamino, thiophenyl and thiophenylethynyl
respectively to get two asymmetric
p-conjugated molecules: 2,
7-di(N,N-diphenylamino)-12-(2-thiophenyl)-5,50,10,100,15,150-hexa-
ethyltruxene (N2S) and 2,7-di(N,N-diphenylamino)-12-(2-thiophen-
ylethynyl)-5,50,10,100,15,150-hexaethyltruxene (N2CS). In this work,
we present a photophysical and theoretical calculation studies com-
bining with the X-ray single-crystal structure of an intermediate.
Experimental section
Synthesis of compound N2CS
Synthesis and characterizations of the subject compounds
A mixture of compound 4 (0.50 g, 0.54 mmol), 2-ethynylthioph-
ene (0.10 g, 0.84 mmol), Pd(PPh3)4 (20 mg, 0.02 mmol), n-Bu4NF
(50 mg), THF (20 mL) and triethylamine (10 mL) was heated to
reflux with stirring after flushed with nitrogen for half an hour.
After reacting for 10 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
(1:10) as eluent to get compound N2CS (0.32 g, 62%). N2CS: a
Solvents for reactions and spectral measurements were dried
and distilled before use. The reagents used for reactions were pur-
chased from J&K Scientific Ltd. 1H NMR spectra were recorded at
25 °C on Bruker Avance 500 MHz spectrometer using CDCl3 as sol-
vent. 13C NMR spectra were recorded at 25 °C on Bruker Avance
125 MHz spectrometer using CDCl3 as solvent. Element analyses
(C, H, S) were performed using a PE 2400 autoanalyser. Mass spec-
trometry analyses were performed by a Bruker Biflex III matrix
Scheme 1. Synthesis of title compounds N2S and N2CS. For comparison, compound N2, a known compound reported in our previous study, is also shown. (a) Propylene
carbonate, NBS, 60 °C, 2 h; (b) HIO3, I2, CH3COOH–H2SO4–H2O–CCl4, 80ꢀC, 4 h; (c) Diphenylamine, K2CO3, Cu (powder), 18-crown-6-ether, 1,2-dichlorobenzene, reflux, 8 h; (d)
2-Thiopheneboronic acid, Pd(PPh3)4, K2CO3, Toluene/ethanol/H2O, under N2, 80ꢀC, 24 h; (e) 2-ethynylthiophene, Pd(PPh3)4, n-Bu4NF, Et3N, THF, under N2, reflux, 10 h.

L. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 63–68
65
Fig. 1. X-ray single-crystal structures of compound 3: (a) ORTEP of compound 3; (b) side and (c) top view of compound 3 packing. H-atoms are omitted for clarity.
Table 1
The photophysical data of compounds 1, N2, N2S and N2CS.
a
a
emax (104, Mꢁ1 cmꢁ1
)
kF (nm)
U b
s c (ns)
D
m
d(103, cmꢁ1
)
D
m
fe (103, cmꢁ1
)
~
~
=D
kabs (nm)
1
N2
N2S
N2CS
305
360
357
362
9.0
5.6
7.13
9.57
368
390
410
429
0.07
0.36
0.68
0.65
12.6
1.9
2.0
5.61
2.14
3.62
4.31
–
5.0
7.9
8.3
2.1
a
b
c
Linear absorption and fluorescence maxima with c = 1.0 ꢂ 10ꢁ5 mol/L.
Fluorescence quantum yields determined using coumarin 307 with c = 1.0 ꢂ 10ꢁ5 mol/L as standard.
Fluorescence lifetime.
Stokes-shift.
d
e
The slope of plot of
versus
D
f in aprotic solvents based on the Lippert–Mataga equation, where
D
f = [(
e
ꢁ 1)/(2
e
+ 1)] ꢁ [(n2 ꢁ 1)/(2n2 + 1)].
~
Dm
Fig. 2. Normalized absorption (a) and emission (b) spectra of compounds 1, N2, N2S and N2CS with c = 1.0 ꢂ 10ꢁ5 mol/L in THF.
yellow powder, m.p. 162–164 °C. 1H NMR (CDCl3, 500 MHz, ppm):
d 0.24–0.29 (m, 18H), 1.89–2.11 (m, 6H), 2.84–2.95 (m, 6H), 6.99–
7.31 (m, 9H), 7.42–7.47 (m, 2H), 7.49–7.55 (m, 18H), 8.07–8.25 (m,
3H). 13C NMR (CDCl3, 125 MHz, ppm): d 154.17, 152.99, 147.97,
147.92, 146.52, 146.29, 143.68, 143.40, 142.66, 141.27, 138.64,
135.24, 131.77, 129.59, 129.25, 127.15, 125.31, 125.15, 124.47,
124.25, 124.16, 123.63, 122.74, 122.63, 121.97, 120.42, 117.73,
117.60, 94.21, 82.82, 56.70, 56.65, 56.54, 29.46, 29.28, 29.16,
8.68, 8.63, 8.61. MALDI-TOF: m/z 950.5 [M+], 921.3 [M-29]+. Ele-
mental Anal. Calcd. for C69H62N2S: C, 87.12; H, 6.57; N, 2.94; S,
3.37. Found: C, 87.33; H, 6.41; S, 3.31.
Single crystal X-ray diffraction
The single crystal of compound 3 was firstly obtained by the
slow diffusion of its THF: cyclohexane (2:1, v/v) solution for several
days at room temperature. The data collection was done on a Bru-
ker SMART APEX-II CCD area detector using graphite-monochro-
mated Mo K
a radiation (k = 0.71073 Å) at room temperature.
Semi-empirical absorption corrections were applied using the
SCALE program for area detector. The structure was solved by
direct methods and refined by the full matrix least-squares meth-
ods on F2 using SHELX.

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L. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 63–68
Photophysical properties measurement
Time-Dependent Density Functional Theory (TD-DFT). The 6-
31G⁄ was used to optimize their single-molecular ground-state
geometries. All ethyl substituents were replaced with H atoms
for simplicity.
UV–vis absorption spectra for the solutions were recorded with
a Shimadzu UV-2550 spectrophotometer. Photoluminescence (PL)
spectra were recorded using a Shimadzu RF-5301PC spectrofluo-
rimeter. Fluorescent decay curves were obtained using an Edin-
burgh FLS920 fluorescence spectrometer equipped with a time-
correlated single photon counting (TCSPC) card. Reconvolution fits
of the decay profiles were performed with F900 analysis software
Results and discussion
Synthesis
to obtain the lifetime values. The fluorescence quantum yield (U)
in solution was determined using coumarin 307 in ethanol as a
reference.
The key procedure of synthesizing the asymmetric compounds
N2S and N2CS is the synthesis of the asymmetric halogenide of
truxene, which needs quantitative bromination and iodination of
compound 1. As shown in Scheme 1, we firstly used 1 equivalent
N-bromosuccinimide (NBS) to get the monobromo substitution 2.
Then, compound 2 was fully iodinated by iodine and iodic acid at
the active 7- and 12-positions to obtain 3. The synthesis of com-
pound 4 was processed through a selective Ullmann condensation
Theoretical calculation
The orbital energy of compounds N2S and N2CS were respec-
tively calculated by using the Gaussian 09 program at the B3LYP
Fig. 3. Molecular orbital diagrams of the frontier molecular orbits of N2S (left) and N2CS (right). All of the ethyl substituents were replaced with H atoms for simplicity.

L. Zhao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 135 (2015) 63–68
67
Fig. 4. Photoluminescence spectra of compounds N2S (left) and N2CS (right) in different solvents with c = 1.0 ꢂ 10ꢁ5 mol/L.
between diphenylamine and compound 3, which stems from the
different reactivities between aryl bromide and aryl iodide. Finally,
the conventional Suzuki and Sonogashira cross-coupling of 4 with
2-thiophene-boronic acid and 2-ethynylthiophene gave the target
molecules N2S and N2CS respectively. Details of the synthesis
and characterizations are given in the experimental section.
However, after p-conjugative extension of N2, compounds N2S and
N2CS unexpectedly demonstrate the almost same maximum
absorption peaks as N2.
In order to understand the spectral behavior and charge transfer
mode, we conducted theoretical calculations of compounds N2S
and N2CS. The optimizations of the molecular geometry were car-
ried out using Time-Dependent Density-Functional Theory (TD-
DFT) calculations at the B3LYP/6-31G⁄ level. As shown in Fig. 3,
upon excited, the charge of N2S and N2CS shift from triarylamine
to thiophene-containing branching. The two compounds exhibit
different energy gaps between the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). The results combing with the spectral experiment (their
almost same absorption maxima) indicate that the main transi-
tions of both N2S and N2CS are not the direct transition from the
HOMO to the LUMO. The transition oscillator strength of the two
compounds from the HOMO to the LUMO are very small
(f = 0.0986 for N2S and f = 0.0462 for N2CS), which may correspond
to the weak absorption of N2S and N2CS in the right hand tails
(380–400 nm) of the absorption profiles (Fig. 2a). They carry on
the energy-level transitions from the HOMO ꢁ 1 to the LUMO or
from the HOMO to the LUMO + 1.
Unlike the absorption spectra, their emission spectra demon-
strate different maximum emission peak (Fig. 2b). With the
increase of the conjugation length, their emission behaves a grad-
ual red-shift in THF. Though the strong electron-donating nature of
diphenylamino group, compounds N2S and N2CS exhibit moderate
emission solvatochromism (Fig. 4), which results from the weak
electron-withdrawing property of thiophene. In addition, the high-
~
est value of Dm=Df of compound N2CS reveals its highest excited-
X-ray crystallography
The single crystal of the key intermediate 3 has been obtained
by the slow diffusion of its THF/cyclohexane solution for several
days at room temperature and the X-ray single-crystal structure
ꢁ
has been determined to be with the space group of Pa 3 [30]. As
shown in Fig. 1a, the truxene unit of 3 lies approximately in a plane
and the maximum displacement from the least-square plane by all
the 27 carbon atoms of the truxene framework is 0.119(2) Å. The
average interplanar angle between the three terminal benzene
rings and the central benzene ring is only 3.91(3)°, which indicates
that the planar configuration will facilitate the intramolecular
charge transfer and
Though the molecular skeleton is highly planar, there is no
ꢃ ꢃ ꢃ stacking found in the crystal packing, which probably arises
p-electronic delocalization.
p
p
from the six splaying ethyl groups. As shown in Fig. 1b and c, com-
pound 3 exhibits interesting crystal packing. There are eight mole-
cules in per crystal unit cell and they are marked in different color
for clarity. The truxene plane of the grey molecule is almost paral-
lel with that of the yellow molecule. The interplanar angle between
blue (or green) molecule and grey (or yellow) molecule is
70.53(3)°. From the top view ([111] direction), we can see that
the blue and green molecules exhibit a 63 screw symmetry.
state polar, which indicates compound N2CS carries on more
prominent intramolecular charge transfer than N2 and N2S upon
excited. On the other hand, we also noted that the non-functional-
ized compound 1 exhibits a longer fluorescence lifetime than the
functionalized compounds N2, N2S and N2CS (Table 1), which indi-
cates compound 1 has the higher stability of the photoexcited
state.
Photophysical properties
The UV–vis absorption and PL spectra of compounds N2S
and N2CS in THF solution have been measured. The correspond-
ing photophysical data were summarized in Table 1, together
with those of two known compounds 1 and N2 for comparison
[16].
Conclusion
As shown in Fig. 2a and Table 1, the non-functionalized com-
pound 1 exhibits obvious vibronic feature, which is attributed to
its rigid planar framework. After connecting two diphenylamino
groups to compound 1, the absorption maxima (kabs) of compound
N2 locates at 360 nm, which is red-shifted 55 nm in comparison
with that of compound 1 (305 nm). This strong absorption band
In conclusion, we designed and synthesized two asymmetric
multibranched
p-conjugated molecules using truxene as the cen-
tral core, diphenylamino and thiophenyl (or thiophenylethynyl)
groups as the different branches respectively. Their photophysical
properties have been studied with that of one known compound
which has been reported by us. The three compounds exhibit
is presumably assigned to the intramolecular p–p charge transfer.
p

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