Triarylamines with branched multi-pyridine groups: modulation of emission properties by structural variation, solvents, and tris(pentafluorophenyl)borane

Article abstract of DOI:10.1007/s11426-017-9202-5

Six triarylamine derivatives 1–6 with branched multi-pyridine substituents were prepared and characterized. These compounds are distinguished by the substituent on one of the phenyl group with NO₂ for 1, CN for 2, Cl for 3, p-C₆H

Full text of DOI:10.1007/s11426-017-9202-5

SCIENCE CHINA
Chemistry
•ARTICLES• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . https://doi.org/10.1007/s11426-017-9202-5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Triarylamines with branched multi-pyridine groups: modulation of
emission properties by structural variation, solvents,
and tris(pentafluorophenyl)borane
Rui Li1,2, Zhong-Liang Gong1, Jian-Hong Tang1,2, Meng-Jia Sun1,2, Jiang-Yang Shao1,
Yu-Wu Zhong1,2* & Jiannian Yao1,2
1CAS Key Laboratory of Photochemistry, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China;
2University of Chinese Academy of Sciences, Beijing 100049, China
Received December 12, 2017; accepted December 31, 2017; published online January 26, 2018
Six triarylamine derivatives 1–6 with branched multi-pyridine substituents were prepared and characterized. These compounds
are distinguished by the substituent on one of the phenyl group with NO2 for 1, CN for 2, Cl for 3, p-C6H4OMe for 4, OMe for 5,
and NMe2 for 6, respectively. As revealed by single crystal X-ray analysis, these substituents play an important role in
determining the configuration of the triarylamine framework and the crystal packing of 1–6. The emission properties of these
compounds were examined in different solvents (toluene, CH2Cl2, acetone, tetrahydrofuran (THF), and N,N-dimethylformamide
(DMF)) and in solid states. Distinct dual emissions from the localized emissive state and the intramolecular charge transfer state
were observed for compound 5 in CH2Cl2. Compounds 1 and 6 show apparent aggregated enhanced emissions in acetone/H2O.
The emission properties of these compounds were further modulated by the addition of tris(pentafluorophenyl)borane. In
addition, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed on the ground
and singlet excited states to complement the experimental findings.
triarylamines, luminescence, charge-transfer, pyridines, donor-acceptor
Citation:
Li R, Gong ZL, Tang JH, Sun MJ, Shao JY, Zhong YW, Yao J. Triarylamines with branched multi-pyridine groups: modulation of emission properties
by structural variation, solvents, and tris(pentafluorophenyl)borane. Sci China Chem, 2018, 61, https://doi.org/10.1007/s11426-017-9202-5
1 Introduction
gely tune the emission properties are desirable. The frontier
energy band gap and other related properties of conjugated
organic emitting molecules can be considerably varied by
attaching electron donor and/or electron acceptor moieties to
the molecular framework [24–27]. This strategy is often used
to modulate the emissions of organic molecular materials by
balancing the π-π* localized emissive (LE) state and the
intramolecular charge transfer (ICT) state [28–33].
In addition to the synthetic structural modifications, a
complementary postsynthetic modulation strategy has re-
ceived attention in the design of organic emitting molecules
[34,35]. One example is molecules containing nitrogen-
based functional heterocycles, such as pyridine [36–40],
Luminescent molecular materials with controllable and
tunable emissions are appealing for applications in organic
light-emitting devices (OLEDs), sensing, and bioimaging
[1–5]. Modification of chemical structures is a common
synthetic approach to tune the photophysical properties of
organic emitting materials [6–10]. When designing imaging
probes [11–16] and emitting materials for other optoelec-
tronic applications [17–23], molecular frameworks that
would allow us to modify chemical structures and thus lar-
*Corresponding author (email: zhongyuwu@iccas.ac.cn)
© Science China Press and Springer-Verlag GmbH Germany 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
diazine [41–44], quinozoline [45,46], thiazole [47,48],
among others [49,50]. By protonation or coordination with
Lewis acids, the emissions of these materials can be quen-
ched, increased, or distinctly shifted (often to the lower-en-
ergy region). These compounds show interesting multi-
stimuli responsiveness and are potentially useful in bioima-
ging and emitting devices. In this contribution, the synthesis,
single-crystal X-ray analysis, and emission studies of six
triarylamine compounds 1–6 with branched multi-pyridine
substituents are presented (Scheme 1). We considered in the
outset that, in addition to the postsynthetic modulation on the
pyridine groups, the presence of the branched multi-pyridine
substituents around the periphery of triarylamine may restrict
the rotation of pyridine rings when they are closely packed in
solid states. This may lead to solid materials with aggrega-
tion-induced emission (AIE) or aggregation-enhanced
emission (AEE) properties, which have received wide and
intense interest recently [51–59]. We later found that the
crystal packing and emission properties of these compounds
are largely dependent on the substituents on the triarylamine.
The modulation of their emission properties by structural
variation, solvents, and postsynthetic modulation with tris-
(pentafluorophenyl)borane (TPFB) [60–64] is presented
herein. In particular, distinctive phenomena of dual emission
[65–68] and AEE from some of these compounds are dis-
cussed. The experimental results were further rationalized
with the aid of density functional theory (DFT) and time-
dependent DFT (TDDFT) calculations on the ground and
singlet excited states.
xycinnamic acid. Compounds 3,5-(pyrid-2-yl)bromo-
benzene [69] and 4-(p-methoxyphenyl)aniline [70] were
prepared according to reported procedures.
2.1.1 N,N-bis(3,5-di(pyrid-4-yl)phenyl)-p-nitroaniline (1)
A suspension of 3,5-(pyrid-2-yl)bromobenzene (388 mg,
1.25 mmol), 4-nitroaniline (69.0 mg, 0.50 mmol), tris(di-
benzylideneacetone)dipalladium
(Pd2(dba)3,
36.7 mg,
0.040 mmol), 1,1′-bis(diphenylphosphino)ferrocene (dppf,
22.2 mg, 0.040 mmol), and NaOtBu (120 mg, 1.25 mmol) in
20 mL of anhydrous toluene was stirred at 120 °C for 3 d
under N2 atmosphere. After cooling to room temperature, the
mixture was filtered through a pad of celite to afford. The
yellow filtrate was concentrated and purified through column
chromatography on silica gel (eluent: CH2Cl2/MeOH, 10:1)
1
to afford 118.3 mg of 1 as a yellow solid in 40% yield. H
NMR (400 MHz, CDCl3): δ 8.70 (dd, J=4.5, 1.5 Hz, 8H),
8.19 (d, J=9.2 Hz, 2H), 7.72 (s, 2H), 7.54 (d, J=1.5 Hz, 4H),
7.47 (dd, J=4.5, 1.6 Hz, 8H), 7.21 (d, J=9.2 Hz, 2H). 13C
NMR (100 MHz, CDCl3): δ 152.65, 150.63, 147.38, 146.64,
142.05, 141.61, 125.83, 124.75, 123.11, 121.56, 120.35. EI-
HRMS calcd. for [M]+ C38H26N6O2: 598.2117. Found:
598.2126.
2.1.2 N,N-bis(3,5-di(pyrid-4-yl)phenyl)-p-cyanoaniline (2)
A suspension of 3,5-(pyrid-2-yl)bromobenzene (373.4 mg,
1.2 mmol), 4-cyanoaniline (59.1 mg, 0.50 mmol), bis(di-
benzylideneacetone)palladium
(Pd(dba)2,
36.6 mg,
0.040 mmol), Bu3P (8.1 mg, 0.040 mmol), and NaOtBu
(120 mg, 1.25 mmol) in 20 mL of anhydrous toluene was
stirred at 120 °C for 3 d under N2 atmosphere. After cooling
to room temperature, the mixture was filtered through a pad
of celite. The yellow filtrate was concentrated and purified
through column chromatography on silica gel (eluent:
CH2Cl2/MeOH, 12:1) to afford 207 mg of 2 as a white solid
t
2 Experimental
2.1 Synthesis and general experimental
All reactions were performed in N2 atmosphere. Nuclear
magnetic resonance (NMR) spectra were recorded in the
deuterated solvent on a Bruker Avance 400 MHz spectro-
meter (Germany). Spectra are reported in ppm values from
residual protons of deuterated solvent. Mass data were ob-
tained with a Bruker Daltonics Inc. Apex II FT-ICR or Au-
toflex III MALDI-TOF mass spectrometer (USA). The
matrix for matrix-assisted laser desorption/ionization time of
flight (MALDI-TOF) measurements is α-cyano-4-hydro-
1
in 72% yield. H NMR (400 MHz, CDCl3): δ 8.68 (d, J=
3.8 Hz, 8H), 7.68 (s, 2H), 7.58 (d, J=7.9 Hz, 2H), 7.51 (s,
4H), 7.46 (d, J=4.4 Hz, 8H), 7.22 (d, J=7.9 Hz, 2H). 13C
NMR (100 MHz, CDCl3): δ 151.06, 150.83, 147.82, 146.97,
141.69, 134.04, 124.69, 122.90, 121.84, 121.79, 119.10,
105.63. MALDI-HRMS calcd. for [M+H]+ C39H27N6:
579.2292. Found: 579.2292.
Scheme 1 Synthesis of 1–6.

3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.3 N,N-bis(3,5-di(pyrid-4-yl)phenyl)-p-chloroaniline
(3)
Using the similar procedure for the synthesis of 2, compound
(68.1 mg, 0.50 mmol), Pd(dba)2 (23.0 mg, 0.040 mmol),
tBu3P (8.1 mg, 0.040 mmol), and NaOtBu (120 mg,
1.25 mmol) as a light green solid in 35% yield (105 mg of
1
3
was prepared from 3,5-(pyrid-2-yl)bromobenzene
product was isolated). H NMR (400 MHz, CDCl3): δ 8.64
(388 mg, 1.25 mmol), 4-chloroaniline (63.8 mg,
(d, J=6.1 Hz, 8H), 7.44 (d, J=6.3 Hz, 14H), 7.16 (d, J=
8.9 Hz, 2H), 6.75 (d, J=9.0 Hz, 2H), 3.00 (s, 6H). 13C NMR
(100 MHz, CDCl3): δ 150.63, 149.79, 148.71, 147.87,
140.68, 135.55, 128.03, 121.86, 121.57, 119.61, 113.88,
40.86. MALDI-HRMS calcd. for [M]+ C40H32N6: 596.2683.
Found: 596.2683.
0.50 mmol), Pd(dba)2 (23.0 mg, 0.040 mmol), tBu3P
(8.1 mg, 0.040 mmol), and NaOtBu (120 mg, 1.25 mmol) as
a yellow powder in 71% yield (209.2 mg of product was
isolated). 1H NMR (300 MHz, CDCl3): δ 8.67 (d, J=5.7 Hz,
8H), 7.56 (s, 2H), 7.49–7.40 (m, 12H), 7.34 (d, J=8.7 Hz,
2H), 7.20 (d, J=8.8 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ
150.75, 149.00, 147.39, 145.68, 141.20, 130.33, 129.84,
126.18, 123.05, 121.80, 121.19. MALDI-HRMS calcd. for
[M+H]+ C38H27ClN5: 588.1950. Found: 588.1950.
2.2 Spectroscopic measurements
Absorption spectra were recorded at room temperature on a
TU-1810DSPC spectrometer from Beijing Purkinje General
Instrument Co., Ltd. (China). Luminescence spectra were
recorded on an F-380 spectrofluorimeter from Tianjin
Gangdong Sci. & Tech. Development Co., Ltd. (China), with
a red-sensitive photomultiplier tube R928F. All measure-
ments were performed in a quartz cuvette with path length of
1 cm. The absolute quantum yields and emission lifetimes
were measured by using the Edinburgh FLS980 spectro-
fluorometer system (England). Fluorescence microscopy
characterization was carried out using Olympus IX83 In-
verted fluorescence microscope (Japan) equipped with a
spot-enhanced charge couple device (CCD, Diagnostic In-
strument, Inc., USA)
2.1.4 N,N-bis(3,5-di(pyrid-4-yl)phenyl)-p-methoxy(phen-
4-yl)aniline (4)
Using the similar procedure for the synthesis of 2, compound
4
was prepared from 3,5-(pyrid-2-yl)bromobenzene
(357.8 mg, 1.15 mmol), 4-(p-methoxyphenyl)aniline
(99.6 mg, 0.50 mmol), Pd(dba)2 (23.0 mg, 0.040 mmol),
tBu3P (8.1 mg, 0.040 mmol), and NaOtBu (120 mg,
1.25 mmol) as a yellow solid in 62% yield (204 mg of pro-
1
duct was isolated). H NMR (400 MHz, CDCl3): δ 8.66 (d,
J=5.5 Hz, 8H), 7.56 (dd, J=8.2, 4.9 Hz, 6H), 7.51 (d, J=
1.3 Hz, 4H), 7.46 (d, J=6.0 Hz, 8H), 7.30 (d, J=8.5 Hz, 2H),
6.98 (d, J=8.7 Hz, 2H), 3.84 (s, 3H). 13C NMR (100 MHz,
CDCl3): δ 159.35, 150.58, 149.15, 147.42, 145.46, 140.89,
137.01, 132.67, 128.08, 127.89, 125.21, 122.91, 121.72,
120.70, 114.46, 55.47. MALDI-HRMS calcd. for [M]+
C45H33N5O: 659.2680. Found: 659.2675.
2.3 X-ray crystallography
The X-ray diffraction data were collected using a Rigaku
Saturn 724 diffractometer (Japan) on a rotating anode (Mo-K
radiation, 0.71073 Å) at 293 K. The structure was solved by
the direct method using SHELXS-97 and refined with Olex2
[71]. The detailed crystallographic data (CCDC 1577194–
1577199) are given in Tables S1 and S2 (Supporting In-
formation online).
2.1.5 N,N-bis(3,5-di(pyrid-4-yl)phenyl)-p-methoxyaniline
(5)
Using the similar procedure for the synthesis of 2, compound
5
was prepared from 3,5-(pyrid-2-yl)bromobenzene
(388 mg, 1.25 mmol), p-anisidine (61.6 mg, 0.50 mmol),
Pd(dba)2 (23.0 mg, 0.040 mmol), tBu3P (8.1 mg,
0.040 mmol), and NaOtBu (120 mg, 1.25 mmol) as a yellow
2.4 Computational methods
1
solid in 70% yield (203.6 mg of product was isolated). H
Density functional theory (DFT) calculations were carried
out using the B3LYP exchange correlation functional and
implemented in the Gaussian 09 package [72]. The electronic
structures of complexes were determined using the 6-31G*
basis set for all atoms. No symmetry constraints were used in
the optimization (nosymm keyword was used). Frequency
calculations have been performed with the same level of
theory to ensure the optimized geometries to be local mini-
ma. All orbitals have been computed at an isovalue of 0.02
e/bohr3. TDDFT calculations were performed on the DFT-
optimized structures on the same level of theory. The emis-
sion wavelength was calculated based on the TDDFT-opti-
mized S1 excited state geometry [73,74].
NMR (400 MHz, CDCl3): δ 8.65 (d, J=4.7 Hz, 8H), 7.49 (s,
2H), 7.43 (dd, J=5.8, 3.5 Hz, 12H), 7.23 (d, J=7.8 Hz, 2H),
6.95 (d, J=7.9 Hz, 2H), 3.86 (s, 3H). 13C NMR (100 MHz,
CDCl3): δ 157.50, 150.67, 149.60, 147.72, 140.85, 139.64,
127.96, 121.92, 121.82, 120.08, 115.67, 55.76. EI-HRMS
calcd. for [M]+ C39H29N5O: 583.2372. Found: 583.2369.
2.1.6 N,N-dimethyl-N′,N′-bis(3,5-di(pyrid-4-yl)phenyl)-p-
phenylenediamine (6)
Using the similar procedure for the synthesis of 2, compound
6
was prepared from 3,5-(pyrid-2-yl)bromobenzene
(388 mg, 1.25 mmol), N,N-dimethyl-p-phenylenediamine

4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Results and discussion
other two phenyl rings are rather big (53.451° and 76.639°).
This arrangement makes compound 1 take a three-dimen-
sional configuration. In the crystal packing of 1, a dimer-like
structure is observed, where the two nitrobenzene rings are
placed in an antiparallel fashion. The presence of hydrogen
bond interaction between the NO2 oxygen with the pyridine
hydrogen of another molecule is evident (Figure 2(a, b)).
For compounds 5 and 6 with the electron-donating OMe or
NMe2 group, the situation is totally different. The torsion
angle between the central amine plane with the OMe or
NMe2-containing phenyl ring is rather large (57.62° for 5 and
57.348° for 6, respectively) and those with other phenyl rings
are relatively small (21.793°–29.454°). The configurations
of 5 and 6 are relatively flat compared to other four com-
pounds, and they display a columnar stacking along the
central amine core (Figure 2(e, f) and Figure S3). Compound
6 has smaller torsion angles with respect to 5 and it has a
relatively shorter distance between two neighboring amine
planes along the column (4.41 Å for 5 and 4.17 Å for 6,
respectively). There are considerable intermolecular CAr–
H···π interactions (the CAr–H···phenyl centroid distance is
around 2.6 Å) present along the column stacking of 5 and 6.
In contrast, the torsion angle between the central amine plane
with surrounding phenyl rings are rather average for 2–4, in
the range of 33.640°–48.36°. These compounds take a nor-
mal three-wheel propeller configuration as known triar-
ylamine compounds [75,76]. Compared to 1, 5, and 6, the
crystal packing of 2–4 is relatively loose. It should be pointed
3.1 Synthesis and structural studies
As outlined in Scheme 1, triarylamine compounds 1–6 were
prepared in moderate yields by the Pd-catalyzed C–N cou-
pling of 3,5-(pyrid-4-yl)bromobenzene with substituted an-
ilines. The intermediate 3,5-(pyrid-2-yl)bromobenzene was
synthesized via the Suzuki coupling of 1,3,5-tri-
bromobenzene with p-pyridyl boronic acid according to the
known procedure [69]. One of the triarylamine phenyl ring
of 1–6 contains the electron-withdrawing NO2, CN, or Cl or
the electron-donating p-C6H4OMe, OMe, and NMe2 group,
respectively. Each of the other two triarylamine phenyl rings
contains two pyrid-4-yl groups on the meta position of the
phenyl unit.
Single crystals of 1–6 suitable for X-ray analysis were
obtained by slow diffusion of petroleum ether or n-hexane
into the solution of these compounds in CH2Cl2 (Figures 1
and 2, Figures S1–S3, and Tables S1 and S2, Supporting
Information online). The triarylamine core of these com-
pounds has a three-wheel propeller configuration. Interest-
ingly, the torsion angle between the central amine plane (the
triangle plane shown in Figure 1) and the surrounding phenyl
planes of these compounds differ significantly. As a result,
the shapes and crystal packing of these compounds differ
from each other significantly. Specifically, the torsion angle
between the central amine plane of 1 with the NO2-con-
taining phenyl ring is as small as 9.522°, while those with the
Figure 1 Thermal ellipsoid plots at 30% probability of the single-crystal X-ray structure of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6. Hydrogen atoms are
omitted for clarity. Color code: carbon, grey; nitrogen, blue; oxygen, red; chlorine, green (color online).

5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2 Crystal packing in single crystals of (a, b) 1, (c, d) 2, and (e, f) 6 (color online).
out the influence of the electronic properties of substituents
on the above-mentioned torsion angle is not unique to our
compounds. For instance, we analyzed a recently reported
triarylamine derivative [77] containing different substituents
one each aryl groups and found a similar trend. The torsion
angle between the central amine plane and an electron-de-
ficient phenyl group (nitrophenyl, 27.994°) is much smaller
with respect to that with an electron-rich phenyl group
(ethyoxyphenyl, 73.16°) [77]. The small torsion angle be-
tween the central amine plane and the nitro-substituted
benzene ring is caused by the strong electronic donor-
acceptor effect of the amine-benzene-nitro unit. The pre-
sence of an electron rich group tends to reduce the con-
jugation degree between the phenyl group and the central
amine unit, leading to a big torsion angle. However, sys-
tematic studies on the influence of the substituent on the
geometry and crystal packing of triarylamine derivatives
have not been documented, to the best of our knowledge.
CH2Cl2 (Figure 3(a)). In contrast, the other five compounds
(2–6) show rather weak absorptions in this region and an
absorption maxima between 300 and 350 nm is observed.
The shapes of the absorptions of these compounds are es-
sentially independent on the polarity of solvents examined
(Figure S4). Compounds 2–5 are moderately emissive with
the emission maxima (λem,max) ranging from 419 to 488 nm
and a quantum yield (Φ) between 5.8% and 9.8% in dilute
CH2Cl2 (Figure 3(b), Table 1). In comparison, 1 and 6 are
relatively weakly emissive (Φ<2.0%) with a λem,max of 578
and 400 nm, respectively. In the solid state, all compounds
are emissive. The λem,max and emission color of the solid
samples of 2–5 (from blue to cyan) are essentially in ac-
cordance with those of them in dilute CH2Cl2 (Figure 3(c,
d)). The Φ of 2–5 decreased distinctly in the solid state
compared to those of them in dilute CH2Cl2. The emission
properties of 1 and 6 are rather peculiar compared to those of
2–5. Compound 1 emits bright yellow color (λem,max=553 nm)
with an enhanced Φ of 12.5% in the solid state compared to
the weak brown emission in dilute CH2Cl2 (λem,max=578 nm).
In contrast, the observed weak emission of 6 with a λem,max of
400 nm shifted to 502 nm (green emissive) with an enhanced
Φ of 5.4% in the solid state. This suggests that 1 and 6 may
display AEE properties (see further discussions below). The
emission lifetime (τ) of these compounds are normally
shorter than 10 ns, suggesting characters of singlet excited
3.2 Photophysical properties
The variation of the electronic properties of the aniline
moieties leads to distinctly different photophysical properties
of these compounds. The NO2-substituted compound 1 ex-
hibits a broad and strong absorption band between 350 and
450 nm with an absorption maximum (λabs,max) at 395 nm in

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Figure 3 (a) Absorption and (b) normalized emission spectra (excited at the absorption maximum between 300 and 400 nm) of 1–6 in CH2Cl2 (10 μM); (c)
images of solutions in CH2Cl2 (upper) and solid samples (below) of 1–6 under a UV lamp (365 nm); (d) normalized emission spectra of solid samples of 1–6
(Excitation wavelength is 395 nm for 1, 380 nm for 2 and 3, and 400 nm for others. Optimal emission intensities were obtained under these excitation
wavelengths. The emission wavelengths of all solid samples are independent on the excitation wavelength) (color online).
Table 1 Photophysical data of 1–6 in dilute solutions and solid state
a)
a)
Solution in CH2Cl2
Solution in toluene Solution in THF
Solution in acetone
Solution in DMF
Solid
b)
c)
c)
c)
c)
b)
λabs,max λem,max
Φ
λem,max
(nm)
Φ
λem,max
(nm)
Φ
λem,max
(nm)
Φ
λem,max
(nm)
Φ
λem,max
(nm)
Φ
τ (ns)
τ (ns)
(nm)
395
328
318
328
(nm) (%)
(%)
(%)
27.3
10.3
6.4
(%)
(%)
(%)
12.5
3.2
3.2
1
2
3
4
5
6
578
419
442
469
1.9
5.6
6.3
8.9
<1.0
2.6
482
412
423
448
451
532
3.8
525
423
440
467
475
455
580
433
447
482
493
411
0.5
7.0
6.3
3.0
1.7
0.4
576
442
456
500
504
410
0.3
10.2
6.2
3.2
1.6
0.1
553
423
442
449
463
502
2.7
1.8
2.4
3.2
10.2
6.8
10.5
6.8
4.4
7.1
5.5
6.3
3.5
330 398/488 10.4 5.2/10.1
328 400 1.0 5.2
13.9
1.7
8.9
4.4
5.4
0.2
a) THF is tetrahydrofuran, DMF is N,N-dimethylformamide; b) absolute photoluminescence quantum yields determined using a calibrated integrating
sphere system; c) emission quantum yield relative to that of quinine sulfate in 1 N H2SO4 (Φ=55%).
states (Table 1, and Figures S5 and S6).
ferent solvents display an inversed solvatochromic behavior.
It has a lower-energy emission band (λem,max=532 nm) in to-
luene and a higher-energy emission at 410 nm in DMF.
However, the emission quantum yields of 6 in all solvents
examined are rather low. These solvatochromic studies
suggest that the emissions of 1–5 are clearly of ICT char-
acters, while that of 6 is not. One possibility is that com-
pound 6 has two different LE states. In polar solvent such as
acetone, CH2Cl2, and DMF, the emission is of the higher-
energy LE character. In nonpolar solvent (toluene) and solid
state, the emission of 6 is of the lower-energy LE character.
3.3 Solvatochromism
To further understand the luminescent properties of these
compounds, their emissions in solvents of varying polarities
(toluene, THF, CH2Cl2, acetone, and DMF) were examined
and compared (Table 1 and Figure 4). With increasing sol-
vent polarity, the emission maxima of 1–5 showed a distinct
bathochromic shift, characteristics of the ICT excited states.
The emission maxima shift of 1 is most significant. For in-
stance, it has a λem,max of 482 nm in toluene and it shifts to
576 nm in DMF. The emission quantum yield varies in dif-
ferent solvents. In particular, compound 1 was found to
display bright yellowish emission in THF with a Φ of 27.3%
(Figure S7). Interestingly, the emission maxima of 6 in dif-
3.4 Dual emissions of LE and ICT
We note that, in addition to the main emission band, com-
pounds 3–5 exhibit another weaker emission band at the

7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 4 Normalized emission spectra of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, and (f) 6 in toluene, THF, CH2Cl2, acetone, and DMF (10 μM) at room temperature
(color online).
higher-energy side in CH2Cl2, suggesting dual emission
properties of these compounds. The intensity ratio of the two
emission maxima at 398 and 488 nm of 5 changed when the
excitation wavelength was varied (Figure 5(a)). The excita-
tion spectrum of the emission at 488 nm is distinctly red-
shifted with respect to that of the emission at 398 nm (Figure
5(b)). In addition, these dual emissions have distinctly dif-
ferent lifetime (5.2 ns for 398 nm and 10.1 ns for 488 nm,
respectively, Figure 5(c)). The dual emissions of 5 are be-
lieved to originate from a balance between the LE and ICT
excited states. In comparison, the dual emissions of 3 and 4
are less pronounced with respect to those of 5 (Figure S8).
gregate state (similar to the yellow emission of the solid
sample of 1). In comparison, the weak emission of 6 at
411 nm in pure acetone was red-shifted to 515 nm when the
H2O content was increased to 90%, exhibiting obvious green
emission with around 4 times enhancement of the emission
intensity. Similar green emission was observed for the solid
sample of 6. Optical microscopic studies show that fibrous or
diamond plate aggregates are formed for 1 and 6, respec-
tively, in 9:1 H2O/acetone mixed solvent (Figure 6 and
Figure S9). We consider that the dimeric structure of 1, as
revealed in Figure 2, greatly stiffens the twisted molecular
conformation of the molecule and thus restricts the rotation
of aryl groups. This restricted motion is believed responsible
for the observed AEE phenomenon of 1 [55–59]. For the
same reason, the presence of multiple intermolecular inter-
actions confined in the compact column stacking of 6 in the
solid state is believed responsible for its AEE properties. The
AEE properties of 6 are somewhat peculiar with respect to
those of 1 and most reported AIE or AEE examples showing
blue-shifted emissions as result of reduced reorganization
energy [51–59]. The red-shifted emission of 6 upon ag-
gregate may be caused by the excitonic coupling along the
column stacking direction.
3.5 AEE effect in acetone/H2O
As pointed out in the above solid state emission studies,
compounds 1 and 6 demonstrated a possible AEE effect. The
absorption and emission spectra of them were thus further
examined in a mixture of acetone (good solvent) and H2O
(poor solvent) of different volume ratios (Figure 6 and Figure
S9). The absorption spectra of both compounds did not
change distinctly by varying the H2O/acetone ratio from 1:9
to 8:1 (Figure S9). However, when the H2O content was
further increased to 90%, their absorption spectra were sig-
nificantly disrupted, indicating the formation of aggregates.
Accordingly, the emissions of both compounds were sig-
nificantly enhanced in mixed solvents containing 90% of
H2O. Compound 1 emits rather weakly at 574 nm in pure
acetone and the emission band shifts to 552 nm with around
10 times enhancement of the emission intensity in the ag-
3.6 Postsynthetic modulation by TPFB
As an example of postsynthetic modulation, the emission
properties of 1–6 in CH2Cl2 in the presence of TPFB were
examined. TPFB is an electron-deficient Lewis acid and has
been used to modulate the emission properties of a number of

8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 5 (a) Emission spectra of 5 as a function of excitation wavelength in CH2Cl2 (10 μM). (b) Excitation spectra of the emission at 398 and 480 nm of 5.
The absorption spectrum (black curve) is included for comparison. (c) Decay profile of the emission of at 398 and 480 nm of 5 in CH2Cl2 (λexci is 310 nm)
(color online).
Figure 6 (a, d) Emission spectra and (b, e) plot of the emission intensity at 552 and 515 nm, respectively, of (a, b) 1 and (d, e) 6 in acetone/H2O mixtures
with different water fractions. Inset of (b, e) shows the images of 1 and 6 under a UV lamp (365 nm). (c, f) Fluorescence optical microscopic images of the
aggregated samples of (c) 1 and (f) 6 under UV irradiation. The samples were prepared by drop casting of the suspension of 1 or 6 in mixed solvent of H2O
and acetone with a ratio of 9:1 (color online).
organic materials by boron-nitrogen coordination [60–64].
Two different kinds of absorption spectral changes were
observed for these compounds in the presence of TPFB
(Figure S10). These distinct absorption spectral changes
suggest the strong interaction between these molecules and
TPFB in the ground state. For the NO2-containing compound
1, the absorption band between 350 and 450 nm slightly
shifted to the higher-energy side. For the other five com-
pounds, the main absorption band between 300 and 350 nm
decreased, with the emergence of a broad and weak ab-
sorption band between 350 and 500 nm. The appearance of
the latter absorption band is due to the decreased electron
density on the pyridine moieties upon coordination with
TPFB, leading to the stabilization of the lowest unoccupied
molecular orbital (LUMO) levels of these compounds and
thus the decrease of the frontier energy band gap [61]. For
compound 1, the LUMO is believed to be dominated by the
nitrobenzene segment (see DFT results below). The co-
ordination of TPFB to the pyridine rings of 1 will slightly
stabilize the HOMO level and thus increase the band gap.
Accordingly, the emission of 1 at 578 nm was hypsochro-
mically shifted to the slightly higher-energy region (λem,max
around 520 nm) in the presence of TPFB in CH2Cl2 solution
(Figure 7(a)).
Interestingly, compounds 2–6 show completely different
TPFB-induced emission spectral changes with respect to 1
(Figure 7). For compounds 2 and 3, with the addition of
TPFB, the emission band at 417 or 442 nm gradually de-

9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 7 Emission spectral changes of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5 and (f) 6 in CH2Cl2 with different equiv. of TPFB added (color online).
creased and a new emission band around at 520 nm appeared
in both cases. In other words, the emissions of 2 and 3 were
red-shifted in the presence of TPFB, in accordance with their
absorption spectral changes. In contrast, the ICT emission
band at around 480 nm of the dual-emissive compounds 4
and 5 was completely quenched and the weak LE emission
intensity at around 400 nm did not change distinctly in re-
sponse to TPFB. No additional lower-energy emission band
was observed. As for 6, the emission intensity at around
400 nm essentially remains unchanged upon coordination
with TPFB, indicating a similar LE character. The reactions
of these compounds with TPFB normally occur instantly and
quantitatively. Electrospray ionization mass spectrometry
(ESI-MS) of the adduct of 3 with TPFB suggests that it
coordinates with four TPFB molecules (Figure S11). The
emission spectra of 1–6 in the presence of aqueous HCl in
THF solution were also examined, which indicated that the
emission of these compounds was largely quenched upon
protonation (Figure S12).
absorption and emission spectra of 1–5 (Figure 3). Com-
pound 6 was calculated to have a similar small energy gap as
1 due to a destabilized HOMO level. However, the absorp-
tion and emission spectra of 6 in CH2Cl2 solution are located
in the much higher-energy region than that of 1, which seems
inconsistent with the DFT results (see further discussions
below). The difference in the HOMO or LUMO energy level
among these compounds seems to strongly affect their
photophysical properties. For instance, the distinctly stabi-
lized LUMO level of 1 and destabilized HOMO level of 6 is
believed to be an important cause for the peculiar emission
properties of these two compounds.
In addition to the calculations on the ground state (S0), the
geometrical optimizations on the singlet excited states (S1 of
1–6) have been carried out. Table 2 shows the comparison of
the torsion angle between the central amine plane (NC3) and
surrounding aryl groups (C6H4R or C6H3(Py)2) based on the
single-crystal X-ray data and the DFT-calculated ground S0
and excited S1 states. The DFT-optimized ground state
structures of 1–6 show some differences with their single-
crystal X-ray structures. In particular, the calculated torsion
angle ∠NC3–C6H4NO2 of 1 is much larger than that of the X-
ray data (27.06° vs. 9.52°). The observed dimeric structure in
the single-crystal X-ray data of 1, which forces the ni-
trobenzene planes to take an antiparallel packing, is one
possible reason to explain this difference. Such an inter-
molecular dimeric structure was not considered in DFT
calculations. The basic trend of the DFT-optimized structures
is however consistent with the X-ray data. For instance, both
results show that the ∠NC3–C6H4R torsion is the smallest for
1 and relatively large for 5 and 6 among six compounds. In
3.7 Computational studies
DFT calculations show that the highest occupied molecular
orbital (HOMO) orbitals of 1–6 are all dominated by the
triarylamine segment (Figure 8). The LUMO orbitals of 2–6
have major contributions from the two 3,5-dipyridylphenyl
units, while that of 1 is dominated by the nitrobenzene
moiety. Accordingly, the LUMO level of 1 is relatively sta-
bilized with respect to those of 2–6. The HOMO-LUMO
energy gap of 1 was calculated to 3.35 eV, which is smaller
with respect to those of 2–5. This result is consistent with the

10 . . . . . . . . . . . . . . . . . . . . . . . . . . . .Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 8 DFT-calculated energy diagram and LUMO and HOMO orbitals of 1–6 (color online).
Table 2 Comparison of the torsion angle between the central amine plane (NC3) and surrounding aryl groups (C6H4R or C6H3(Py)2) based on the single-
crystal X-ray data and DFT-calculated ground state and singlet excited state of 1–6
X-ray data (°)
Calculated ground state (°)
∠NC3–C6H4R ∠NC3–C6H3(Py)2
Calculated singlet excited state (°)
Comp.
∠NC3–C6H4R ∠NC3–C6H3(Py)2
∠NC3–C6H4R
94.65
∠NC3–C6H3(Py)2
1
2
3
4
5
6
9.52
39.16
43.15
48.36
57.62
57.35
53.45, 76.64
33.64, 42.92
34.03, 42.82
36.79, 37.73
26.29, 29.45
21.79, 23.13
27.06
31.63
42.69
45.95
53.25
56.35
49.41, 50.10
46.87, 47.21
41.22, 41.30
39.30, 39.89
36.42, 37.83
35.46, 35.57
29.37, 31.26
35.98, 49.31
36.96, 52.90
43.59, 60.78
42.02, 61.54
46.10, 65.95
33.26
30.39
22.25
22.97
19.14
addition, both DFT and experimental data show that the
difference between ∠NC3–C6H4R and ∠NC3–C6H3(Py)2 is
relatively small for 2–4; however, the ∠NC3–C6H4R torsion
of 1 is much smaller than its ∠NC3–C6H3(Py)2 torsion and
the ∠NC3–C6H4R torsions of 5 and 6 are much larger with
respect to their ∠NC3–C6H3(Py)2 torsions. Compared to the
ground state structures, big changes occur to these torsions in
the calculated geometries in the S1 state. The torsion angle
∠NC3–C6H4NO2 of 1 becomes 94.65°, compared to 27.06° in
the ground state. This torsion is much bigger than its ∠NC3–
C6H3(Py)2 torsion angles. In contrast, the ∠NC3–C6H4R
torsions of 2–6 are all smaller with respect to their ∠NC3–
C6H3(Py)2 torsions.
are rather small (f<0.04). These results are consistent with
the observed absorption spectra of 1–6 (Figure 3(a)), with an
intense ICT absorption band for 1 and only very weak ab-
sorptions for the others being observed between 350 and
450 nm. The calculated first transitions of the S1 states (S1 →
S0) correspond to the theoretical emission wavelengths of 1–
6. The calculated emission energies decrease (wavelengths
vary from short to long) in an order from 2, 3, 4, 5, to 1,
which are in well agreement with the observed emission
spectra of these compounds. These transitions are all domi-
nated by the electron relaxation from the LUMO to the
HOMO of the excited state (Figure S13). For compounds 2–
5, these emissions are of the ICT character from the triar-
ylamine unit to the dipyridylphenyl segments. For compound
1, the ICT transition from the triarylamine unit to the ni-
trobenzene segment plays the dominant role. These emis-
sions are characteristic of the twisted charge-transfer
emissions [24], where the donor and acceptor segments are
TDDFT calculations were performed on the optimized
ground and excited state geometries (Table 3). The vertical
S0 → S1 excitations of 1–6 are dominated by the ICT tran-
sitions from HOMO to LUMO. The S0 → S1 excitation of 1
has a big oscillator strength (f=0.5498), while those of 2–6

11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Li et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Table 3 TDDFT-calculated vertical electronic transitions based on the ground (GS) and singlet excited state (ES) of 1–6
Comp.
GS-1
GS-2
GS-3
GS-4
GS-5
GS-6
ES-1
ES-2
ES-3
ES-4
ES-5
ES-6
Transition
S0 → S1
S0 → S1
S0 → S1
S0 → S1
S0 → S1
S0 → S1
S1 → S0
S1 → S0
S1 → S0
S1 → S0
S1 → S0
S1 → S0
E (eV)
3.01
3.34
3.19
3.04
3.06
2.83
1.79
2.88
2.68
2.33
2.22
1.77
λ (nm)
411.2
371.6
388.2
407.9
404.8
437.7
692.9
429.9
462.2
531.7
557.5
700.0
f
Dominant transition (percentage contribution)
HOMO → LUMO (99%)
HOMO → LUMO (98%)
HOMO → LUMO (98%)
HOMO → LUMO (97%)
HOMO → LUMO (98%)
HOMO → LUMO (98%)
LUMO → HOMO (99%)
LUMO → HOMO (99%)
LUMO → HOMO (99%)
LUMO → HOMO (99%)
LUMO → HOMO (99%)
LUMO → HOMO (99%)
0.5498
0.0357
0.0312
0.0201
0.0234
0.0126
0.0431
0.0389
0.0265
0.0160
0.0120
0.0086
twisted with a big torsion angle in the excited state as has
been discussed in the above structural analysis.
these compounds. The emission properties of these mole-
cular materials could be largely modulated by substituents,
solvents, and post-coordination with B(C6F5)3. By varying
the solvent and the electronic properties of the substituents
on the aryl group, dual LE and ICT emissions and AEE
properties were observed from some of these compounds.
This work demonstrates the successful modulation of emis-
sion properties of molecular materials by structural variation
and postfunctionalization, which provides useful information
for the molecular design of emissive materials. Work is under
way to further adjust the molecular structures of these
compounds in order to improve their emission quantum
yields for potential optoelectronic applications.
Aside from the structural changes discussed above, the
calculated dipole moments of 1–5 show big difference be-
tween the ground and excited states. The dipole moment of
1–5 in the S0 state was calculated to be 5.24, 4.00, 0.87, 5.05,
and 5.17 D, respectively, which increased to 23.71, 12.64,
16.07, 26.05, 21.37 D, respectively, in the S1 state. This
suggests the increased CT character in the excited state, in
agreement with the positive solvatochromic effects of the
emissions of these compounds (Figure 4).
The previous emission studies and DFT calculations show
that compound 6 is a special example among all compounds
studied. The ICT emission wavelength of 6 was calculated to
be around 700 nm and the dipole moment of 6 was calculated
to be 18.25 D in the S1 state vs. 7.35 D in the ground state.
However, only very weak emission at around 400 nm was
observed for 6 in polar solvents such as CH2Cl2, acetone, and
DMF and the emission shifts to the longer-wavelength region
in THF and toluene. We consider that the ICT emission of 6
is forbidden as supported by the calculated low oscillator
strength (f=0.0086). The observed emission of 6 is likely of
the LE character as has been discussed previously. The above
emission studies suggest that the emission of 6 at around
400 nm in CH2Cl2 essentially remains unchanged in response
to TPFB, which is also consistent with the LE character.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (91622120, 21601194, 21472196, 21501183,
21521062), the Ministry of Science and Technology of China
(2012YQ120060), the Strategic Priority Research Program of the Chinese
Academy of Sciences (XDB 12010400), and Science and Technology
Commission of Shanghai Municipality (16DZ1100300).
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
1
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4 Conclusions
In summary, we present the synthesis, crystal packing, and
emission properties of a series of triarylamine derivatives
containing multi-branched pyridine groups. The substituent
on the aryl group was found to strongly influence the geo-
metry of the triarylamine motif and the crystal packing of
3
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