Triarylborane-terminalized branched π-conjugative dyes: Synthesis, structure and optoelectronic properties

Article abstract of DOI:10.1016/j.dyepig.2014.03.014

Organic optoelectronic materials with high fluorescence quantum yield, both in solution and in the solid state, have attracted considerable interest in recent years. In this work, we designed and synthesized three triarylborane-terminalized branched π-con

Full text of DOI:10.1016/j.dyepig.2014.03.014

Dyes and Pigments 107 (2014) 60e68
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Triarylborane-terminalized branched
p-conjugative dyes: Synthesis,
structure and optoelectronic properties
,
b
,
,
Mao-Sen Yuan a 1, Dong-En Wang a 1, Tian-Bao Li a, Yongqian Xu a, Wen-Ji Wang a,
Qin Tu a, Yanrong Zhang a, Manlin Li a, Jinyi Wang a
,
*
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
a r t i c l e i n f o
a b s t r a c t
Article history:
Organic optoelectronic materials with high fluorescence quantum yield, both in solution and in the solid
Received 19 January 2014
Received in revised form
9 March 2014
Accepted 10 March 2014
Available online 18 March 2014
state, have attracted considerable interest in recent years. In this work, we designed and synthesized
three triarylborane-terminalized branched
p-conjugative compounds, including two C3-symmetric p-
3A(acceptor) triarylboron dyes: 2,7,12-tri(5-(dimesitylboryl)thiophen-2-yl)-5,50,10,100,15,150-hexaethyl-
truxene and 2,7,12-tri((5-(dimesitylboryl)thiophen-2-yl)ethynyl)-5,50,10,100,15,150-hexaethyltruxene, and
a
2D(donor)-p-A asymmetric dye: 2,7-di(N,N-diphenylamino)-12-(5-(dimesitylboryl)thiophen-2-yl)-
5,50,10,100,15,150-hexaethyltruxene. The three compounds displayed prominent optical properties. The
ethynyl spaced thiophene analogue emitted intense fluorescence both in the THF solution and in the
solid state with excellent quantum yields. Interestingly, their solid powders showed a very different
fluorescence colour from their respective THF solution. The results of theoretical calculations were in
good agreement with the experimental absorption and CV spectra. The high reversibility of the three
boron-containing dyes in their redox process indicates substantial stability of the produced species,
which make them promising light-emitting materials.
Keywords:
Triarylborane
Charge-transfer
Crystal structure
Optoelectronic properties
Solid-state fluorescence
Truxene
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
transporting and emissive materials in organic light-emitting de-
vices (OLEDs) [4], two-photon absorption and emission materials
Three-coordinate organoboron, with a vacant pz orbital, is an
excellent -electronic acceptor when connected to a -conjugated
[5] as well as selective chemosensors for fluoride and cyanide an-
ions [6] and heat-sensitive materials [7]. For practical application,
having enough high solid-state fluorescence efficiency is a funda-
mental issue for luminescent materials. However, most triarylbor-
anes are highly emissive only in dilute solution and tend to show a
decrease of fluorescence efficiency in the solid state due to the
aggregation-caused quenching (ACQ) effect. The successful exam-
ples of stable and highly emissive triarylboranes in both solution
and the solid state with a fluorescence quantum yield close to unity
are still quite limited [8].
p
p
system. In recent years, three-coordinate organoboron compounds
have attracted a great deal of attention because of their excellent
photophysical and electronic properties [1]. Meanwhile, the avail-
ability of the empty pz orbital makes three-coordinate organoboron
compounds highly susceptible to the addition by Lewis bases,
resulting in their relatively poor stability. An effective method of
improving the stability is to utilize the aromatic substituents, which
arise from bulky aryl groups to provide protection for the central
boron atom by blocking the approach of nucleophiles [1a,2]. In the
last decade, stable triarylboron compounds (triarylboranes) have
been exploited extensively in a wide range of applications as op-
toelectronic materials, such as nonlinear optical materials [3],
An effective strategy of molecular design to achieve an intense
emissive organic compound in both solution and the solid-state is
the construction of a multibranched p-conjugated framework with
large building blocks [9]. The branched steric bulkiness will
inherently reduce the intermolecular dipoleedipole interaction
and
cence self-quenching in the aggregation state. In addition, in
comparison to the linear molecules and polymers, branched
conjugated molecules have a number of advantages for applications
in optoelectronic devices, for example, the two- or three-
p$$$p stacking, which will generally result in drastic fluores-
* Corresponding author. Tel./fax: þ86 29 87082520.
p
-
E-mail
addresses:
jywang@nwsuaf.edu.cn,
jinyiwangpharm@gmail.com
(J. Wang).
1
These two authors contributed equally to this work.
http://dx.doi.org/10.1016/j.dyepig.2014.03.014
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
61
dimensional architectures, the well-defined molecular structure
and good film-forming processing [10]. In this context, it is of
particular interest to develop triarylborane-containing branched p-
conjugated organic emissive materials exhibiting high fluorescence
efficiency in both solution and the solid state.
As far as this issue is concerned, truxene (10,15-dihydro-5H-
diindeno[1,2-a;10,20-c]fluorene), a heptacyclic polyarene, was cho-
sen to construct the branched framework. Due to its C3-symmetric
skeleton and three-dimensional topology, the truxene unit has
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 [Mþ], 898.9 [M-29]þ. Elemental Anal. Calcd. for C67H62N2S: C,
86.78; H, 6.74; S, 3.46. Found: C, 86.71; H, 6.84; S, 3.35.
2.1.2. Synthesis of compound S3B3
n-BuLi (2.4 M solution in n-hexane, 0.20 mL, 0.48 mmol) was
added to a stirred solution of compound S3 (100 mg, 0.13 mmol) in
THF (10 mL) under nitrogen at ꢁ78 ꢀC over 5 min, and followed by
warming to room temperature naturally. After reacting for further
4 h, the reactants were cooled to ꢁ78 ꢀC again, and dimesitylboron
fluoride (0.20 g, 0.74 mmol) in THF (5 mL) was injected over 5 min.
The temperature was allowed to naturally rise to room temperature
and the mixture continuously stirred for two days. Then the re-
actants were diluted with ethyl acetate, washed with water and
dried over magnesium sulfate. After removal of the solvents, the
crude product was obtained. After purification by column chro-
matography on silica gel, eluting with dichloromethane-petroleum
ether (1:5), compound S3B3 was obtained (58 mg, 29%). S3B3: a
yellowish-green powder, m.p. 201e203 ꢀC. 1H NMR (CDCl3,
been intensively developed as a
p-conjugated central core to
fabricate star-shaped optoelectronic molecules, such as OLEDs [11],
organic field-effect transistors (OFETs) [12], liquid crystals [13] and
two-photon absorption materials [14]. Considering the good ther-
mal and chemical stability, 2-thienyl and 2-thienyl-ethynyl were
respectively used as the p-bridge to connect the 2, 7,12-positions of
truxene [15]. As for the termini of the branches, to continue our
effort in the development of highly emissive materials utilizing the
boron element, dimesitylboron (mesityl ¼ 2,4,6-trimethylphenyl)
group is a favourable choice for our proposed design principle: this
bulky unit can suppress the p$$$p stacking and the strong electron-
withdrawing ability can facilitate intramolecular/intermolecular
charge transfer. Herein, we report three boron-containing branched
p
-conjugative compounds, including two C3-symmetric
p
-
500 MHz, ppm):
d
0.19e0.22 (t, J ¼ 7.5 Hz, 18H), 1.52e2.06 (m, 42H),
3A(acceptor) triarylboron dyes: 2,7,12-tri(5-(dimesitylboryl)thio-
phen-2-yl)-5,50,10,100,15,150-hexaethyltruxene (S3B3) and 2,7,12-
tri((5-(dimesitylboryl)thiophen-2-yl)ethynyl)-5,50,10,100,15,150-hex
2.32 (s, 18H), 2.94e2.98 (m, 18H), 6.85 (s, 12H), 7.38e7.41 (m, 6H),
7.51e7.53 (d, J ¼ 10 Hz, 3H), 7.58 (s, 3H), 8.29e8.31 (d, J ¼ 10 Hz, 3H).
13C NMR (CDCl3, 125 MHz, ppm):
d 157.4, 153.6, 144.8, 144.3, 141.9,
aethyltruxene (C3B3), and a 2D(donor)-
p
-A asymmetric dye: 2,7-
140.9, 140.0, 138.5, 132.8, 132.4, 128.2, 125.4, 124.7, 124.3, 123.0,
119.5, 56.9, 29.7, 23.5, 21.3, 8.6. MALDI-TOF: m/z 1501.4 [Mþ], 1472.8
[M-29]þ. Elemental Anal. Calcd. for C105H111B3S3: C, 83.98; H, 7.45;
S, 6.41. Found: C, 83.91; H, 7.12; S, 6.42.
di(N,N-diphenylamino)-12-(5-(dimesitylboryl)thiophen-2-yl)-5,50
,10,100,15,150-hexaethyltruxene (N2SB). Their photophysical prop-
erties in both solution and the solid state, theoretical calculations,
electrochemical properties, as well as the X-ray single-crystal
structures of their precursors, have been comprehensively studied.
2.1.3. Synthesis of compound C3B3
A similar synthetic and purification procedure as for C3B3 was
followed for S3B3 using compound C3 as the precursor. Then
compound C3B3 (41.5%) was obtained. C3B3: a light green powder,
2. Experimental section
2.1. Synthesis and characterizations of the subject compounds
m.p. 222e224 ꢀC. 1H NMR (CDCl3, 500 MHz, ppm):
d 0.22e0.25 (t,
J ¼ 7.5 Hz, 18H), 2.10e2.32 (m, 42H), 2.35 (s, 18H), 2.97e3.02 (m,
6H), 6.88 (s, 12H), 7.41e7.43 (d, J ¼ 10 Hz, 6H), 7.54e7.56 (d,
J ¼ 10 Hz, 3H), 7.61 (s, 3H), 8.32e8.34 (d, J ¼ 10 Hz, 3H). 13C NMR
Solvents for reactions and spectral measurements were dried
and distilled before use. The reagents used for reactions were
purchased from J&K Scientific Ltd. 1H NMR spectra were recorded at
25 ꢀC on Bruker Avance 500 MHz spectrometer using CDCl3 as
solvent. 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
assisted laser desorption/ionization time of flight (MALDI-TOF)
mass spectrometer.
(CDCl3,125 MHz, ppm) d 152.9,145.3,141.1,140.9,140.1,138.8,138.5,
135.3, 133.6, 129.8, 128.2, 125.3, 124.6, 120.7, 97.6, 83.6, 57.0, 29.5,
23.5, 21.3, 8.5. MALDI-TOF: m/z 1572.7 [Mþ], 1543.6 [M-29]þ.
Elemental Anal. Calcd. for C111H111B3S3: C, 84.72; H, 7.11; S, 6.11.
Found: C, 84.91; H, 7.02; S, 6.12.
2.1.4. Synthesis of compound N2SB
A similar synthetic and purification procedure as for N2SB was
followed for S3B3 using compound N2S as the precursor. Then
compound N2SB (30.9%) was obtained. N2SB: a light green powder,
Compounds 1e5 were synthesized according to literature
methods [6h] and compounds S3 and C3 were synthesized
following the literature [6i,14c].
m.p. 188e190 ꢀC. 1H NMR (CDCl3, 500 MHz, ppm):
d 0.23e0.27 (m,
18H), 1.89e2.18 (m, 18H), 2.33 (s, 6H), 2.87e2.97 (m, 6H), 6.86 (s,
6H), 7.00e7.05 (m, 6H), 7.18e7.19 (m, 6H), 7.27e7.30 (m, 6H), 7.46e
7.47 (m, 1H), 7.56e7.57 (m, 1H), 7.69e7.71 (d, J ¼ 10 Hz, 6H), 8.08e
8.10 (d, J ¼ 10 Hz, 1H), 8.14e8.16 (d, J ¼ 10 Hz, 1H), 8.24e8.26 (d,
2.1.1. Synthesis of compound N2S
A mixture of compound 5 (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 atmosphere
for 24 h. The mixture were cooled to room temperature and poured
into water (100 mL). After extraction with dichloromethane (DCM),
the organic phase was dried over Na2SO4. The solvent was removed
and the residue was purified by column chromatography on silica
gel using DCM-hexane (1: 20) as the eluent to get compound N2S
(0.16 g, 31.9%). N2S: a yellow powder, m.p. 156e158 ꢀC. 1H NMR
J ¼ 10 Hz, 1H). 13C NMR (CDCl3, 125 MHz, ppm)
d 157.6, 154.2, 153.8,
147.9, 146.5, 143.2, 141.9, 141.4, 140.9, 138.6, 138.5, 135.3, 129.3,
128.2, 125.2, 124.9, 124.3, 122.8, 120.0, 119.7, 117.6, 56.8, 56.7, 29.5,
29.4, 29.2, 23.5, 21.3, 8.7, 8.6. MALDI-TOF: m/z 1173.9 [Mþ], 1144.8
[M-29]þ. Elemental Anal. Calcd. for C85H83BN2S: C, 86.85; H, 7.12; N,
2.38; S, 2.73. Found: C, 86.67; H, 7.02; N, 2.37; S, 2.65.
2.2. Single crystal X-ray diffraction
(CDCl3, 500 MHz, ppm):
d 0.21e0.28 (m, 18H), 1.89e2.15 (m, 6H),
2.84e2.99 (m, 6H), 7.02e7.12 (m, 6H), 7.32e7.33 (m, 3H), 7.46e7.49
(m, 2H), 7.55e7.58 (m, 16H), 7.57e7.67 (m, 2H), 8.09e8.27 (m, 3H).
The single crystals of compounds S3 and N2S were firstly ob-
tained by the slow diffusion of their respective CHCl3: cyclohexane

62
M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
(2:1, v/v) solutions for several days at room temperature. Since the
two crystals are stable under ambient condition, the data collection
was done without any insert gas protection at room temperature on
2.4. Theoretical calculation
In order to understand the spectral behaviour and elucidate the
influence of the different symmetric pattern on the electronic
structures, the orbital energy of compounds S3B3, C3B3 and N2SB
were respectively calculated by using the Gaussian 09 program at
the B3LYP Time-Dependent Density Functional Theory (TD-DFT).
The 6-31G* was used to optimize their single-molecular ground-
state geometries.
a Bruker SMART APEX-II CCD area detector using graphite-
ꢀ
monochromated Mo K
a
radiation (
l
¼ 0.71073 A). Data reduction
and integration, together with global unit cell refinements were
done by the INTEGRATE program of the APEX2 software. Semi-
empirical absorption corrections were applied using the SCALE
program for area detector. The structures were solved by direct
methods and refined by the full matrix least-squares methods on F2
using SHELX. The single-crystal structure of compound C3 has been
reported by our group [14c].
2.5. Electrochemical properties measurement
Cyclic voltammetry (CV) was performed on CHI660D worksta-
tion and measurements were carried out in THF 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 under N2 atmosphere. The scan
2.3. Photophysical properties measurement
UVevis absorption spectra for the solutions and the films were
recorded with a Shimadzu UV-2550 spectrophotometer. Photo-
luminescence (PL) spectra were recorded using a Shimadzu RF-
rate was maintained at 100 mV sꢁ1
.
5301PC spectrofluorimeter. The fluorescence quantum yield (F) in
solution was determined using rhodamine B in ethanol as a refer-
ence according to a previously reported method [16]. Quantum
yields of the films and the solid-state powder were determined
with a PTI C-701 calibrated integrating sphere system [17]. Steady-
state fluorescence spectra and decay curves were obtained using an
Edinburgh FLS920 fluorescence spectrometer equipped with a
time-correlated single photon counting (TCSPC) card. Reconvolu-
tion fits of the decay profiles were performed with F900 analysis
software to obtain the lifetime values. The solid films were pre-
pared through solution process (THF, 1.0 ꢂ 10ꢁ4 mol/L, 1500e
2000 rpm) with 50e100 nm thickness.
3. Results and discussion
3.1. Synthesis
The synthetic approach to the title compounds is outlined in
Scheme 1. Compound 1 was used as the starting material. After
quantitative bromination and/or iodination, compound 1 was
respectively converted to compounds 2, 3 and 4, which has been
reported in our previous work [6h]. Compounds S3 and C3 were
synthesized via conventional Suzuki reaction and Sonogashira cross-
coupling reaction from compounds 2 and 3. The synthesis of
Scheme 1. Synthesis of title compounds. (a) Bromine, CH2Cl2, 25 ꢀC, 12 h; (b) and (j) 2-Thiopheneboronic acid, Pd(PPh3)4, K2CO3, THF, reflux, 8 h; (c), (f) and (k) Dimesitylboron
fluoride, n-BuLi, THF, ꢁ78 ꢀC, 2 d; (d) and (h) HIO3, I2, CH3COOHeH2SO4eH2OeCCl4, 80 ꢀC, 4 h; (e) 2-ethynylthiophene, Pd(PPh3)4, n-Bu4NF, Et3N, THF, reflux, 3 h; (g) Propylene
carbonate, NBS, 60 ꢀC, 2 h; (i) Diphenylamine, K2CO3, Cu (powder), 18-crown-6-ether, 1,2-dichlorobenzene, reflux, 8 h.

M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
63
compound N2S was firstly processed through selective Ullmann
condensation between diphenylamine and compound 4, which
stems from the different reactivities between aryl bromide and aryl
iodide, then a conventional Suzuki reaction. After respective boro-
nation of compounds S3, C3 and N2S using dimesitylboron fluoride,
the titlecompoundsS3B3, C3B3and N2SBwere obtained. All thenew
compounds were characterized with 1H NMR, 13C NMR, elemental
analysis andMALDI/TOF mass spectroscopy. The intermediatesS3, C3
and N2S have been verified by their X-ray single-crystal structures.
respectively. For the S3 single molecule, the distances between the
three thiophene rings and the truxene are respectively 1.482(5),
3
ꢀ
1.466(5) and 1.479(5) A, which are shorter than typical C(sp )e
C(sp3) single bonds and longer than typical C(sp2)¼C(sp2) double
bonds, implying a certain extent of conjugation between truxene
and thiophene. Another structural feature of S3 is that the three
thiophene rings are almost coplanar with the truxene unit. The
average dihedral angle between the thiophene ring and the nearest
benzene is only 11.2(5)ꢀ, which will facilitate the intramolecular
charge transfer. The S3 molecules are packed in the P21/n space
group. Though the molecular skeleton is highly planar, there is no
3.2. X-ray crystallography
p
$$$
almost perpendicular to the grey molecules in Fig. 1(b). In addition,
the intermolecular CeH$$$ interactions between the blue mole-
p stacking found in the crystal packing. The blue molecules are
Determination of X-ray single-crystal structure is the most
effective tool for acquiring the ground-state molecular structure to
reveal the molecular structureeproperty correlations. The greatest
effort has been exerted to get the single crystals of all the in-
termediates and title compounds. But it failed to achieve the single
crystals of the three boron-containing compounds because of their
huge branched fabrications. Fortunately, the single crystals of their
respective precursors S3, C3 and N2S have been obtained by the
slow diffusion of their CHCl3: cyclohexane (2:1, v/v) solution for
several days at room temperature [18]. As shown in Fig.1(a), (d) and
(g), all the truxene moieties of S3, C3, and N2S exhibit highly planar
geometry with the mean deviations from their individual least-
p
cules and the grey molecules possibly play an important role for the
stabilization of the crystal packing.
Unlike S3, the thiophene rings of the C3 molecule are perpen-
dicular to the central truxene. The average dihedral angle between
thiophene rings and truxene is 89.6(9)ꢀ (Fig. 1(d)), which is very
unusual in common aromatic conjugative systems. This special
structure is mainly attributed to two factors. One is the intermo-
lecular CeH$$$
1 space group with layer-by-layer packing character (Fig. 1(e)).
There are notable intermolecular CeH$$$ interactions existing
between the two neighbouring layers (Fig. 1(f)). The 3-position
p interactions. The C3 molecules are packed in the P-
p
ꢀ
squares plane being 0.018(5), 0.016(8) and 0.037(6) A,
Fig. 1. X-ray single-crystal structures of compounds S3 (a, b, c), C3 (d, e, f) and N2S (g, h, i): ORTEP of S3 (a), C3 (d) and N2S (g); (b) side view of S3 packing; (c) illustration of the Ce
H$$$
p
stacking in S3; (e) side view of C3 packing, the molecules in the same packing layer were marked in the same colour and the distance between the green layer and yellow
ꢀ
layer is about 4.50 A; (f) top view of C3 and illustration of the CeH$$$
p
stacking; (h) side view of N2S packing; (i) illustration of the CeH$$$
p
stacking in N2S. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version of this article.)

64
M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
hydrogen atoms of the thiophene units point toward the plane of
the benzene portion of the truxene rings in the neighbouring layer,
which leads to the thiophene plane being perpendicular to the
3.3. Photophysical properties
The UVevis absorption and PL spectra of compounds S3B3,
C3B3 and N2SB in THF solution, films and the solid-state powder
were measured (Figs. 2 and 3). The corresponding data are sum-
marized in Table 1, together with those of their precursors S3, C3
and N2S for comparison.
plane of truxene. The other is the intramolecular
This formed perpendicular configuration will facilitate the overlap
of the parallel p65ebonding orbital of thiophene and the
(pyepy)e
bonding orbital of the bonded acetylene group, and result in good
conjugation between thiophene unit and acetylene group.
pep conjugation.
p
pe
p
As shown in Fig. 2, the UVevis absorptions of the three boron-
containing compounds exhibit obvious spectral features of their
respective precursor. For example, the absorption maxima (labs) of
compound S3B3 peaked at 340 nm is identical with that of com-
pound S3, and the spectral shape of S3B3 is also very similar to that
of S3 (Fig. 2(a)). Likewise, C3B3 exhibits an identical absorption
band (377 nm) with that of C3 (375 nm). The identical absorption
In the single crystal of asymmetric molecule N2S, the dihedral
angle and the distance between thiophene and truxene are 15.3(3)ꢀ
ꢀ
and 1.482(5) A respectively. The three benzene rings around ni-
trogen atom are arranged in a propeller-like fashion, with the
dihedral angles being 83.1(3), 75.5(3) and 64.2(3)ꢀ respectively
(Fig. 1(g)). The N2S molecules are packed in the P21/c space group.
Similar to S3, the blue molecules are perpendicular to the grey
bands indicate the same
pe
p* charge transfer (CT) processes. In
molecules (Fig. 1(h)), and intermolecular CeH$$$
p
interactions link
accordance with expectation, compounds S3B3, C3B3 and N2SB
blue molecules and grey molecules into dimers (Fig. 1(i)).
feature their respective weak shoulder band at the longer
Fig. 2. The normalized UVevis absorption (a, c and e) and photoluminescence (b, d and f) spectra of compounds S3B3, C3B3 and N2SB with their respective precursors S3, C3 and
N2S in THF with c ¼ 1.0 ꢂ 10ꢁ5 mol/L.

M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
65
Fig. 3. The normalized UVevis absorption (a) and photoluminescence (b) spectra of compounds S3B3, C3B3 and N2SB in THF with c ¼ 1.0 ꢂ 10ꢁ5 mol/L (a, b) and in films (c, d). And
their UVevis absorption (e) and photoluminescence (f) in solid powder. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version
of this article.)
wavelength (w406 nm), which is presumably assigned to the
intramolecular epp CT from the highest occupied molecular or-
bitals (HOMOs) to the lowest unoccupied molecular orbitals
(LUMOs). In the PL spectra, the emission peaks of the three boron-
containing compounds respectively show remarkable red shifts in
comparison with those of their precursor in the same solvent (THF).
three compounds showed similar longest-wavelength absorption
at w406 nm. For their emission spectra (Fig. 3(b)), it is surprising
that C3B3 demonstrates the almost same maximum emission peak
as S3B3 at 429 nm irrespective of the difference in the conjugation
length of their branches. Comparing S3B3 with N2SB, we can draw
the conclusion that the substitution of two branches of S3B3 by two
diphenylamino donors results in an 87-nm red-shift of the fluo-
rescence in THF, but a reduced fluorescence quantum yield
(0.75 / 0.22).
For light-emitting materials, most of their applications are
required to be used in the solid state, e.g. thin films, powder or
crystal. In this case, having sufficiently high solid-state fluorescence
efficiency is a fundamental issue. The photophysical properties of
compounds S3B3, C3B3 and N2SB both in films and in the solid
powder were measured (Fig. 3(c)e(f)). As expected, the three
boron-containing compounds exhibit intriguing solid-state
p
The spectral phenomena indicate that the introduction of the
electronic acceptor (three-coordinate organoboron) into the
p
p
-
-
conjugative system can effectively facilitate intramolecular CT and
improve molecular photophysical properties.
When considering the difference of the three boron-containing
compounds, it was discovered that the absorption maxima of
compound C3B3 (377 nm) in THF solution are obviously red-shifted
in comparison with that of compounds S3B3 (340 nm) and N2SB
(363 nm) because of the expanded
p conjugation (Fig. 3(a)).
However, whatever the positions of absorption maxima, all of the

66
M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
Table 1
The photophysical data of the subject compounds.
Solution in THF
Film
Solid powder
b
labs(nm)a εmax(104 Mꢁ1 cmꢁ1
)
εmax(104, Mꢁ1 cmꢁ1
)
lem(nm)a
D
v
Fc
s
(ns)d
labs(nm)
lem(nm)
Fe
lem(nm)
Fe
s(ns)
~
S3
C3
N2S
S3B3
C3B3
N2SB
340
354,375
358
340,352,404
377,405
363,388,407
16.3
10.8,
7.1
10.0, 8.1, 1.4
12.1, 10.2
8.4, 5.7, 4.4
385, 400
390,406
410
383,400,429
429
3.59
3.64
3.62
5.15
3.27
8.14
0.59
0.58
0.68
0.75
0.63
0.22
2.1
1.5
2.3
2.7
2.9
6.3
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
340,409
379,408
365,408
420,440
447
499
0.67
0.60
0.43
455,519
495
462
0.15
0.79
0.57
11.4
15.2
17.6
516
a
With c ¼ 1.0 ꢂ 10ꢁ5 mol/L.
b
c
Stokes-shift.
Fluorescence quantum yield determined using rhodamine B in ethanol as standard.
Fluorescence lifetime.
Fluorescence quantum yield in the solid state is obtained in a calibrated integrating sphere.
d
e
luminescence (Fig. 3(d) and (f), Table 1). Especially compounds
C3B3 and N2SB, their solid-state fluorescence efficiency reached
0.79 (for C3B3) and 0.57 (for N2SB) in powder form, indicating their
potential applications as excellent non-doped luminescent mate-
rials, e.g. OLEDs.
3.4. Theoretical studies
In order to understand the spectral behaviour and intra-
molecular charge transfer modes, we conducted theoretical calcu-
lations of the three boron-containing compounds. The
optimizations of the molecular geometry and total energy calcu-
lations were carried out using time-dependent density-functional
theory (TD-DFT) calculations at the B3LYP/6-31G* level. As shown
in Fig. 5, the calculation results showed that the HOMO and
HOMOꢁ1 (or LUMO and LUMOþ1) of the C3-symmetric com-
pounds S3B3 and C3B3 have very similar energy. Their HOMO and
HOMOꢁ1 mainly delocalized over the truxene-thiophene frame-
work, and the LUMO and LUMOþ1 mainly delocalized over the
However, it is noteworthy that the powdered compounds C3B3
and N2SB show very different fluorescence from their respective
THF solutions. As shown in Figs. 3f and 4, the THF solution of
C3B3 emits blue fluorescence peaked at 429 nm. Unexpectedly, its
solid powder exhibits bright-green fluorescence peaked at
495 nm. In contrast, the THF solution of C3B3 emits green fluo-
rescence, but its solid powder form emits blue. We noted that the
fluorescence lifetime of solid-state powders of the three com-
pounds were significantly longer than those of their THF solution
(Table 1), which may imply that their solid-state fluorescence is
mainly from an aggregation-induced excimer, not the single
molecule [19]. Because of the molecular multibranch configura-
tion and steric bulk of dimesitylboron, it is impossible that these
molecules were packed in the form of H-aggregation (intermo-
triarylboron framework. Notably, the charge transfer of the 2Depe
A compound N2SB was more prominent than those of S3B3 and
C3B3. After being excited, the charge was entirely shifted from
triarylamine (HOMO) to triarylboron (LUMO). Meanwhile, the re-
sults show that the introduction of the electron-donating diphe-
nylamino groups led to a remarkable increase in the HOMO energy
level by 0.65 eV and consequently a decrease of the energy gap
between HOMO and LUMO compared with that of S3B3, which
corresponds to the relatively strong absorption of N2SB in the
right-hand tails (415e440 nm) of the absorption profiles.
lecular
p$$$p stacking interactions) in the solid state. We specu-
late that these molecules organized into the aggregative state in a
staggered staircase-like J-aggregation or infilled arrangements
through weak intermolecular interactions. Moreover, their star-
shaped molecular geometry provides the possibility of the stag-
gered staircase or infilled arrangements, e.g., the terminal of one
branch embeds into the vacancy between the two branches of
another molecule.
3.5. Electrochemical properties
To further evaluate the potential applicability in electronic de-
vices, the electrochemical properties of the three title compounds
were studied by cyclic voltammetry, and their CV diagrams can be
found in Fig. 6. S3B3 and C3B3 show only reversible reduction
waves. Their half-wave reduction potentials (Er1e/2d) are almost
identical regardless of the difference in the conjugation length,
being ꢁ2.35 and ꢁ2.30 eV vs. Ag/Agþ. N2SB shows not only a
reversible reduction wave, but also a reversible oxidation wave due
to the diphenylamino groups. The Er1e/2d of N2SB is higher than those
of S3B3 and C3B3, being ꢁ1.95 eV vs. Ag/Agþ and the half-wave
oxidation potentials (Eo1x/2d) were ꢁ0.51 eV vs. Ag/Agþ. The high
reversibility of all the three compounds in the redox process
demonstrates the substantial stability of the produced species.
Under such experimental conditions, the redox potential of Fc/
Fcþ, which possesses an absolute energy level of ꢁ4.8 eV relative to
the vacuum level for calibration, is located at 0.39 eV [14b]. So, we
can draw the evaluation of the HOMO energy levels by the equa-
oxd
tion: HOMO ¼ ꢁE ꢁ4.41. As a result, the HUMO energy level of
1/2
N2SB is ꢁ4.92 eV, and the LUMO energy level can be calculated by
the equation: LUMO ¼ HOMOꢁEg, where Eg is the energy bandgap
determined from the longest wavelength absorption onset
(430 nm), being ꢁ1.97 eV, which is highly consistent with the re-
sults of the theoretical calculation (Fig. 5).
Fig. 4. Photo of the THF solutions (c ¼ 1.0 ꢂ 10ꢁ5 mol/L) and as-prepared solid
powders of compounds S3B3, C3B3 and N2SB under 365-nm UV light. (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)

M.-S. Yuan et al. / Dyes and Pigments 107 (2014) 60e68
67
Fig. 5. Molecular orbital diagrams of the frontier molecular orbits of S3B3 (left), C3B3 (middle) and N2SB (right). Due to the large molecular size, the ethyl and methyl groups have
been replaced by H atoms.
4. Conclusion
Acknowledgement
In summary, we have designed and synthesized three truxene-
cored branched peconjugative compounds S3B3, C3B3 and N2SB,
in which triarylboron units were used as the termini of the
branches. The X-ray single-crystal structures of their precursors S3,
C3 and N2S were studied. The results show that the intermolecular
This study was supported by the National Natural Science
Foundation of China (21202132, 21175107 and 21375106), the
Natural Science Foundation of Shaanxi (2012JQ2017), the Ministry
of Education of the People’s Republic of China (NCET-08-602 0464),
the Fundamental Research Funds for the Central Universities
(Z109021106 and Z109021303).
CeH$$$
p
interactions play an important role for the stabilization of
$$$ stack-
the three crystal packings. Interestingly there was no
p
p
ing found in any one crystal packing. The photophysical and elec-
trochemical properties of compounds S3B3, C3B3 and N2SB have
been carefully examined. Because of the electron-withdrawing
nature of the three-coordinate organoboron, molecular multi-
branched configuration and dimesitylboron’s steric bulkiness, the
three compounds display prominent optical properties. Especially,
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