Construction of full-color-tunable and strongly emissive materials by functionalizing a boron-chelate four-ring-fused π-conjugated core

Article abstract of DOI:10.1039/c1jm14606h

2-(2′-Hydroxyphenyl)benzoxazole (HBO) and 2-(2′-hydroxyphenyl) benzothiazole (HBT) reacted with triphenylborane produced two rigid π-conjugated fluorescent cores 1 (BPh₂(BOZ), BOZ = 2-(benzo[d]oxazol-2-yl)phenol) and 2 (BPh₂(BTZ), BTZ = 2-(benzo[d]thiazol-2-yl)phenol). Comparisons of photophysical properties and calculations between para- and meta-diphenylamine-substituted derivatives 5 (BPh₂(para-NPh₂-BTZ)) and 7 (BPh₂(meta-NPh ₂-BTZ)) demonstrated that functionalization at the para-position of the rigid core is effective in tuning the electronic structure and hence the photophysical properties of this type of boron-chelate complex. Simple modification of these frameworks by introducing various amine groups at the para-position allows the synthesis of strongly fluorescent materials 3 (BPh ₂(para-Cz-BTZ), Cz = 9H-carbazol-9-yl), 4 (BPh₂(para- NPh₂-BOZ), NPh₂ = diphenylamino), 5, and 6 (BPh ₂(para-NMe₂-BTZ), NMe₂ = dimethylamino). The emission colors of these newly synthesized complexes together with the parent complexes 1 and 2 covered a wide range from deep blue to saturated red in both solution and the solid state. Crystal structure analysis discloses that two phenyl groups attached to the boron atom effectively keep the luminescent ring-fused π-conjugated skeletons apart, making these fluorophores highly emissive in solid forms (Φ_F = 0.36-0.71). Organic light-emitting diodes employing these boron complexes as emitters not only keep the full-color tunable emission feature but also show high electroluminescent (EL) performance; for instance, the greenish-blue device based on 2 showed the highest efficiency of 7.8 cd A^-1 and the yellow light-emitting device based on 4 exhibited the highest brightness (31220 cd m^-2) among the boron-containing emitters reported so far.

Full text of DOI:10.1039/c1jm14606h

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PAPER
Construction of full-color-tunable and strongly emissive materials by
functionalizing a boron-chelate four-ring-fused p-conjugated core†
*
Di Li, Hongyu Zhang, Chenguang Wang, Shuo Huang, Jianhua Guo and Yue Wang
*
Received 16th September 2011, Accepted 9th November 2011
DOI: 10.1039/c1jm14606h
2-(20-Hydroxyphenyl)benzoxazole (HBO) and 2-(20-hydroxyphenyl)benzothiazole (HBT) reacted with
triphenylborane produced two rigid p-conjugated fluorescent cores 1 (BPh2(BOZ), BOZ ¼ 2-(benzo[d]
oxazol-2-yl)phenol) and 2 (BPh2(BTZ), BTZ ¼ 2-(benzo[d]thiazol-2-yl)phenol). Comparisons of
photophysical properties and calculations between para- and meta-diphenylamine-substituted
derivatives 5 (BPh2(para-NPh2-BTZ)) and 7 (BPh2(meta-NPh2-BTZ)) demonstrated that
functionalization at the para-position of the rigid core is effective in tuning the electronic structure and
hence the photophysical properties of this type of boron-chelate complex. Simple modification of these
frameworks by introducing various amine groups at the para-position allows the synthesis of strongly
fluorescent materials 3 (BPh2(para-Cz-BTZ), Cz ¼ 9H-carbazol-9-yl), 4 (BPh2(para-NPh2-BOZ), NPh2
¼ diphenylamino), 5, and 6 (BPh2(para-NMe2-BTZ), NMe2 ¼ dimethylamino). The emission colors of
these newly synthesized complexes together with the parent complexes 1 and 2 covered a wide range
from deep blue to saturated red in both solution and the solid state. Crystal structure analysis discloses
that two phenyl groups attached to the boron atom effectively keep the luminescent ring-fused p-
conjugated skeletons apart, making these fluorophores highly emissive in solid forms (FF ¼ 0.36–0.71).
Organic light-emitting diodes employing these boron complexes as emitters not only keep the full-color
tunable emission feature but also show high electroluminescent (EL) performance; for instance, the
greenish-blue device based on 2 showed the highest efficiency of 7.8 cd Aꢀ1 and the yellow light-emitting
device based on 4 exhibited the highest brightness (31 220 cd mꢀ2) among the boron-containing emitters
reported so far.
fluorescence efficiency, whereas a lot of fluorophores emit
strongly when molecularly dissolved in solvents, they become
weak fluorophores in the solid state due to either attractive
dipole–dipole interactions or intermolecular p-stacking.3
Considering that the fluorescent materials are used as solid forms
in most applications, how to suppress the quenching process is
thus highly important.
Introduction
The development of highly emissive organic materials in the solid
state is a fundamental and important requirement for various
optoelectronic applications, such as organic light-emitting diodes
(OLEDs), organic lasers, and sensors.1 In the construction of
emitting materials, tunable emission color and high solid-state
quantum efficiency are two important issues in terms of opto-
electronic applications of materials. To realize the effective color
tuning, many strategies including copolymerization have been
empirically pursued, there exists only a limited number of emis-
sive core skeletons with flexibility in their synthetic strategies for
tunable emission wavelengths.1a,2 In this regard, it is urgently
demanded to develop fluorescent cores with flexibility in struc-
tural functionalization for full-color fluorescence. As for the
Extended p-conjugated molecules with ring-fused structures
have attracted considerable attention since their structural
feature of flat and rigid p-conjugated skeletons bring about a set
of desired properties such as intense luminescence, good thermal
stability and high carrier mobility.4 Introducing main group
elements, such as nitrogen, silicon, sulfur, selenium, boron, and
phosphorus into this type of p-plane have proved to be an effi-
cient way to modulate their optoelectronic properties.5 In
particular, incorporation of four-coordinate boron moieties into
the extended p-conjugated framework might be of interest
considering that four-coordinate boron complexes have wide
applications in OLEDs.6,7 Along this line of consideration, we
have recently synthesized several series of brightly fluorescent
solids by incorporation of four-coordinate boron moieties into
the extended p-conjugated skeletons and found that these
State Key Laboratory of Supramolecular Structure and Materials, College
of Chemistry, Jilin University, Changchun, 130012, P. R. China. E-mail:
hongyuzhang@jlu.edu.cn; yuewang@jlu.edu.cn; Tel: +86-431-85168484
† Electronic supplementary information (ESI) available: Theoretical
calculations, electrochemical spectra, DSC and TGA curves. CCDC
reference numbers 837275–837277. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c1jm14606h
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molecules could serve as efficient emitters in OLEDs with an
intrinsic electron-transporting nature.8 Modulating the length of
the core skeletons, such as in five-, six-, and seven-ring-fused
systems, significantly affects the emission color of the produced
molecules, and altering the bridging atoms of frameworks or
substitutes on the boron atoms can further tune their fluores-
cence. While these synthetic strategies allowed the construction
of a family of boron-containing emitting materials with fluores-
cence covering a wide range from pure blue (450 nm) to saturated
red (670 nm), from a viewpoint of simplifying the synthetic
procedure, it is strikingly attractive to develop a rigid boron-
chelate fluorescent core that has flexibility in structural modu-
lation for color tuning.
kind of boron complexes as emitting materials in OLEDs, elec-
troluminescent (EL) devices employing 1–6 as emitters have also
been fabricated and characterized.
Experimental
General information
All starting materials were purchased from Aldrich Chemical Co.
and used without further purification. The solvents for syntheses
were freshly distilled over appropriate drying reagents. All
experiments were performed under a nitrogen atmosphere using
1
standard Schlenk techniques. H NMR spectra were measured
on a Bruker AVANCE 300 or 500 MHz spectrometer with tet-
ramethylsilane as the internal standard. Mass spectra were
recorded on a Shimadzu AXIMA-CFR MALDI-TOF mass
spectrometer. Elemental analyses were performed on a flash EA
1112 spectrometer. UV-vis absorption spectra were recorded
using a PE UV-vis lambdazo spectrometer. The emission spectra
were recorded using a Maya2000 Pro CCD spectrometer. Elec-
trochemical measurements were performed with a BAS 100W
Bioanalytical electrochemical work station. Differential scanning
calorimetric (DSC) measurements were performed on
a NETZSCH DSC204 instrument. Thermogravimetric analyses
(TGA) were performed on a TA Q500 thermogravimeter.
Recently, Kwak and Kim reported two BF2-chelate fluo-
rophores based on 2-(20-hydroxyphenyl)benzoxazole (HBO) and
2-(20-hydroxyphenyl)benzothiazole (HBT) ligands that emit blue
fluorescence with moderate quantum yields (FF ¼ 0.20 and 0.23)
in solution.9 Employing HBO and HBT ligands, we have
synthesized two BPh2-chelate analogs 1 and 2 and found these
boron complexes having four-ring-fused core skeleton together
with bulky side aryl groups emit very bright blue or cyan fluo-
rescence in both solution (FF ¼ 0.55 for 1 and 0.65 for 2) and the
solid state (FF ¼ 0.53 for 1 and 0.60 for 2).8d In addition, our
recent findings disclosed that the electronic structures of the
precursor ligands could be effectively tuned by a para-substitu-
tion with various amine groups.10 In view of these results, it
appeared attractive to attempt para-modification of the fluores-
cent BPh2-chelate four-ring-fused skeleton with an aim to
construct materials with tunable emission colors and high fluo-
rescence efficiencies. Indeed, a simple functionalization of this
type of boron-bridged fluorescent core by introducing amine
groups with varying electron-donating ability (Chart 1) allows us
to synthesize a series of strongly fluorescent materials 3–6 which
together with the parents 1 and 2 exhibit emission colors ranging
from deep blue to saturated red. The substitution position effect
was investigated by a comparison of photophysical properties
between para- and meta-diphenylamine-substituted complexes 5
and 7. Fundamental properties of the newly synthesized
complexes 3–6 together with the parent BPh2-chelate complexes
1 and 2 were fully demonstrated. To test the possibility of this
2-(Benzo[d]thiazol-2-yl)-4-(9H-carbazol-9-yl)phenol
(HL3),
2-(benzo[d]oxazol-2-yl)-4-(diphenylamino)phenol (HL4), 2-
(benzo[d]thiazol-2-yl)-4-(diphenylamino)phenol (HL5), 2-(benzo
[d]thiazol-2-yl)-4-(dimethylamino)phenol (HL6) and 2-(benzo[d]
thiazol-2-yl)-5-(diphenylamino)phenol (HL7) were synthesized
according to the literature procedures.10
General synthetic procedure
To a rapidly stirred solution of ligand in THF, 1.1 mol of BPh3 in
THF was added and the mixture was stirred overnight at room
temperature. The volatiles were removed under vacuum, and the
residual solid was purified by vacuum sublimation to give the
boron complex as a solid.
Complex 3. Yield was 52% as green powder. 1H NMR (CDCl3,
500 MHz, ppm): d 8.15 (d, J ¼ 8.0 Hz, 2 H), 7.87 (d, J ¼ 8.0 Hz, 1
H), 7.72 (d, J ¼ 2.5 Hz, 1 H), 7.62 (dd, J ¼ 8.5 Hz, 2.5 Hz, 1 H),
7.45–7.27 (m, 20 H). MS m/z: 556.6 [M]+ (calcd: 556.2). Anal.
Calcd (%) for C37H25BN2OS: C, 79.86; H, 4.53; N, 5.03; S, 5.76;
Found: C, 79.56; H, 4.42; N, 5.02; S, 5.65.
Complex 4. Yield was 46% as yellow powder. 1H NMR
(CDCl3, 500 MHz, ppm): d 7.58 (d, J ¼ 8.0 Hz, 1 H), 7.54 (d, J ¼
3.0 Hz, 1 H), 7.46–7.44 (m, 4 H), 7.39 (t, J ¼ 7.5 Hz, 1 H), 7.35
(dd, J ¼ 9.0 Hz, 2.5 Hz, 1 H), 7.27–7.24 (m, 11 H), 7.14 (d, J ¼ 9.0
Hz, 1 H), 7.08 (d, J ¼ 8.0 Hz, 4 H), 7.02–6.99 (m, 3 H). MS m/z:
542.2 [M]+ (calcd: 542.2). Anal. Calcd (%) for C37H27BN2O2: C,
81.93; H, 5.02; N, 5.16; Found: C, 81.81; H, 4.93; N, 5.15.
Complex 5. Yield was 59% as orange powder. 1H NMR
(CDCl3, 500 MHz, ppm): d 7.79 (d, J ¼ 8.0 Hz, 1 H), 7.39–7.36
(m, 5 H), 7.30–7.23 (m, 14 H), 7.11–7.06 (m, 5 H), 7.00 (t, J ¼ 7.5
Hz, 2 H). MS m/z: 558.6 [M]+ (calcd: 558.2). Anal. Calcd (%) for
Chart 1 (a) Structural feature of boron-chelate four-ring-fused electron-
system and (b) derivation of the core skeleton at the para- or meta-
position.
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C37H27BN2OS: C, 79.57; H, 4.87; N, 5.02; S, 5.74; Found: C,
79.28; H, 4.70; N, 4.95; S, 5.64.
were prepared in vacuum at a pressure of 5 ꢁ 10ꢀ6 Torr. Organic
ꢀ1
ꢀ
layers were deposited onto the substrate at a rate of 1–2 A s
.
After the organic film deposition, LiF and aluminium were
thermally evaporated onto the organic surface. The thicknesses
of the organic materials and the cathode layers were controlled
using a quartz crystal thickness monitor. The electrical charac-
teristics of the devices were measured with a Keithley 2400 source
meter. The EL spectra and luminance of the devices were
obtained on a PR650 spectrometer. The N,N0-bis(naphthalen-1-
yl)-N,N0-bisphenyl)-benzidine (NPB), 2,20,200-(1,3,5-benzine-
triyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), tris(8-hydroxy-
quinolinato) aluminium (Alq3) and bis(10-hydroxybenzo[h]qui-
nolinato) beryllium (Bebq2) were purchased from Jilin Optical
and Electronic Materials Corp.
1
Complex 6. Yield was 40% as red powder. H NMR (CDCl3,
500 MHz, ppm): d 7.84 (d, J ¼ 8.0 Hz, 1 H), 7.40–7.36 (m, 5 H),
7.24–7.20 (m, 8 H), 7.13 (d, J ¼ 9.0 Hz, 1 H), 7.08 (dd, J ¼ 9.0 Hz,
2.5 Hz, 1 H), 6.71 (d, J ¼ 3.0 Hz, 1 H), 2.88 (s, 6 H). MS m/z:
434.4 [M]+ (calcd: 434.2). Anal. Calcd (%) for C27H23BN2OS: C,
74.66; H, 5.34; N, 6.45; S, 7.38; Found: C, 74,74; H, 5.28; N, 6.50;
S, 7.33.
Complex 7. Yield was 46% as yellow powder. 1H NMR
(DMSO, 300 MHz, ppm): d 8.20 (d, J ¼ 8.4 Hz, 1 H), 7.57 (d, J ¼
8.7 Hz, 1 H), 7.46–7.38 (m, 5 H), 7.29–7.23 (m, 7 H), 7.16 (s, 10
H), 6.98 (d, J ¼ 8.4 Hz, 1 H), 6.29 (dd, J ¼ 9.0 Hz, J ¼ 2.4 Hz, 1
H), 6.11 (d, J ¼ 2.4 Hz, 1 H). MS m/z: 558.4 [M]+ (calcd: 558.2).
Anal. Calcd (%) for C37H27BN2OS: C, 79.57; H, 4.87; N, 5.02; S,
5.74; Found: C, 79.61; H, 4.98; N, 5.16; S, 5.78.
Results and discussion
Synthesis
The molecular structures of BPh2-chelate parent complexes 1 and
2 and synthetic routes of amine-substituted complexes 3–6 are
outlined in Scheme 1. The precursor ligands HL3–HL7 were
synthesized following the reported literatures.10 Simple mixing of
the ligands and triphenylborane in THF at room temperature
and subsequent purification by vacuum sublimation produced
targets 3–6 in moderate yields (40–59%). Boron complex 7 was
synthesized by the same way in 46% yield for the purpose of
defining the substitution position effect. All the newly synthe-
X-Ray diffraction
Diffraction data were collected on a Rigaku RAXIS-PRID
diffractometer using the u-scan mode with graphite-mono-
chromator Mo-Ka radiation. The structures were solved with
direct methods using the SHELXTL programs and refined with
full-matrix least-squares on F2.12 Non-hydrogen atoms were
refined anisotropically. The positions of hydrogen atoms were
calculated and refined isotropically. Crystallographic data
(excluding structure factors) for the structure(s) reported in this
paper have been deposited with the Cambridge Crystallographic
Data Centre: CCDC 837275 for 1, CCDC 837276 for 2, and
CCDC 837277 for 6.†
1
sized boron complexes were fully characterized by H NMR,
mass spectra, and elemental analyses.
Substitution position effect
Crystal data for 1. C25H18BNO2, M ¼ 375.21, monoclinic, a ¼
To evaluate the substitution position (para- or meta-) effect on
the photophysical properties, the absorption and emission
spectra of diphenylamine-substituted boron complexes 5 and 7 in
CH2Cl2 were compared. These two complexes showed
completely different absorption and emission properties merely
due to the subtle difference in substitution position, despite their
3
ꢀ
ꢀ
26.232(5), b ¼ 10.000(2), c ¼ 15.213(3) A, U ¼ 3923.4(13) A , T ¼
293(2) K, space group C2/c, Z ¼ 8, 14832 reflections measured,
3454 unique (Rint ¼ 0.1215) which were used in all calculations.
The final wR(F2) was 0.1540 (all data). Crystal data for 2.
C25H18BNOS, M ¼ 391.27, monoclinic, a ¼ 9.781(2), b ¼ 13.651
3
ꢀ
ꢀ
(3), c ¼ 16.457(5) A, U ¼ 2006.3(9) A , T ¼ 293(2) K, space group
P21/c, Z ¼ 4, 18861 reflections measured, 4559 unique (Rint
¼
0.0678) which were used in all calculations. The final wR(F2) was
0.1502 (all data). Crystal data for 6. C27H23BN2OS, M ¼ 434.34,
ꢀ
monoclinic, a ¼ 13.017(3), b ¼ 12.930(3), c ¼ 18.193(6) A, U ¼
3
ꢀ
2256.9(10) A , T ¼ 293(2) K, space group P21/c, Z ¼ 4, 21232
reflections measured, 5154 unique (Rint ¼ 0.0489) which were
used in all calculations. The final wR(F2) was 0.1137 (all data).
Theoretical calculations
The ground state geometries were fully optimized by the density
functional theory (DFT)13 method with the Becke three-param-
eter hybrid exchange and the Lee–Yang–Parr correlation func-
tional14 (B3LYP) and 6-31G* basis set using the Gaussian 03
software package.15
Fabrication of EL devices
Indium-tin oxide (ITO) coated glass was used as the substrate. It
was cleaned by sonication successively in a detergent solution of
acetone, methanol, and deionized water before use. The devices
Scheme 1 Molecular structures of complexes 1 and 2 and synthetic
routes for complexes 3–7.
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Fig. 1 Comparisons of absorption (solid line) and emission (dashed line)
properties among complexes 2, 5, and 7.
structural similarity. As depicted in Fig. 1, the non-substituted
boron complex 2 is greenish-blue emissive with an absorption
band centred at 389 nm and an emission band which peaked at
485 nm. meta-Diphenlyamine-substituted boron complex 7 is
green fluorescent with the longest absorption maximum of 428
nm and emission band centered at 520 nm, exhibiting 39 and 35
nm redshifts for absorption and emission maxima compared to 2.
Surprisingly, the same group substituted at the para-position
significantly affected the photophysical properties of the
produced boron complex 5 which showed red fluorescence
(labs ¼ 472 nm; lem ¼ 627 nm). Compared with 2, the absorption
and emission bands were red shifted by 83 and 142 nm, respec-
tively. According to density functional theory (DFT) calcula-
tions, the electron-donating group substituted at the para-
position significantly pushed the HOMO level up, hence nar-
rowing the bandgap of the boron complex; whereas that
substituted at the meta-position pushed up both the HOMO and
LUMO levels and thus only weakly influenced the bandgap
(Fig. S1†). In view of these comparisons, it is clear that only the
slight difference in the chemical structure, i.e., whether the amine
group is attached to the para- or meta-position on the hydrox-
yphenyl ring, results in a significant difference in the photo-
physical properties. The calculation and experimental results
demonstrate that derivation at the para-position is more effective
in tuning the luminescent properties of the HBO- and HBT-
chelate boron complexes. Therefore, we next focus the discussion
on the properties of the parent and para-substituted boron
complexes.
Fig. 2 Molecular structures of 1 (a), 2 (c), and 6 (e) with 50% thermal
ellipsoids and packing structures of 1 (b), 2 (d), and 6 (f).
ꢀ
0.14 A for 2, and 0.51 A and 0.11 A for 6 in opposite directions.
ꢀ
ꢀ
ꢀ
ꢀ
The bond lengths of B–O and B–N are 1.51 A and 1.61 A for 1,
ꢀ
ꢀ
ꢀ
ꢀ
1.49 A and 1.64 A for 2, and 1.49 A and 1.65 A for 6, which are
similar to those of the organoboron complexes reported previ-
ously.8 The dihedral angle between the outer benzene ring and
the benzothiazole plane is 13.0ꢂ for 1, 12.4ꢂ for 2, and 10.6ꢂ for 6,
indicating that the four-ring-fused system has certain distortions
from a coplanar framework.
Fig. 2b, d, f shows the molecular packing structures. The
luminescent p-frameworks are effectively separated by two side
phenyl rings attached to boron atoms; as a result, there are no
p–p intermolecular interactions which usually exist in solids of
Crystal structures
Single crystals of complexes 1, 2 and 6 were obtained by vacuum
sublimation and their structures were determined by X-ray
crystallography. All boron atoms adopt typical tetrahedral
geometry to form N,O-chelate six-membered rings which
contribute to constructing the four-ring-fused p-conjugated
skeletons (Fig. 2a, c and e). In the molecular structures, the six-
membered rings constructed by boron chelation take a chair
conformation in which the oxygen and boron atoms deviate from
ꢀ
the central phenyl plane by 0.60 A and 0.04 A for 1, 0.54 A and
ꢀ
ꢀ
Fig. 3 Absorption spectra of complexes 1–6.
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Table 1 Photophysical data of complexes 1–6
lem/nm
lem/nm
(solid)
FF
CH2Cl2
FF
(solid)
a
a
Complex
labs/nm
CH2Cl2
Stokes shift/nm
1
2
3
4
5
6
369
389
416
451
472
469
452
485
543
574
627
642
83
96
127
123
155
173
462
494
519
574
603
655
0.55
0.65
0.72
0.87
0.44
0.31
0.53
0.60
0.71
0.57
0.48
0.36
a
Concentration: 1.0 ꢁ 10ꢀ4 M.
fluorophores with an extended p-conjugated plane and results in
severe fluorescence quenching. Such a molecular packing feature
is important for luminescent fluorophores in maintaining a high
fluorescent intensity in the solid state. This can not only explain
why the BPh2-chelate complexes 1 and 2 exhibit much higher
quantum yields than the BF2-chelate analogs in solution but also
gives a possible explanation for the fact that complexes 1 and 2
keep quantum yields of their solutions and solids at a compa-
rable level.
donor strength, for instance, stronger electron-donating diphe-
nylamine- and dimethylamine-substituted complexes 5 and 6
exhibited a 56 nm and 53 nm redshift, respectively, compared to
carbazole-substituted 3. Another notable feature of the present
system is the bridging atom effect. The longest absorption
maximum of B,O-bridged complex 1 was blue-shifted by about
20 nm compared to that of the B,S-bridged complex 2, demon-
strating that altering the bridging atom also affects the electronic
structure of the four-ring-fused skeleton greatly. This viewpoint
was further verified by the comparison of absorption bands
between B,O-bridged complex 4 and B,S-bridged complex 5
whose molecular structures had the same diphenylamine
substitution.
Photophysical properties
The absorption spectra of complexes 1–6 recorded in CH2Cl2 are
shown in Fig. 3 and their data are summarized in Table 1. All of
the complexes exhibited a broad absorption band (lmax ¼ 369 nm
for 1, 389 nm for 2, 416 nm for 3, 451 nm for 4, 472 nm for 5, and
469 nm for 6) attributable to the electronic transition originating
from the p-molecular orbitals. The amine derivatives 3–6 showed
red-shifted absorption bands when compared to the non-
substituted complexes 1 and 2, reflecting the impact of the amine
groups on the electronic properties. It is worthy of note that the
absorption bands of these amine derivatives are related to the
The fluorescence spectra of complexes 1–6 and their photo-
graph images of samples irradiated by UV lamp (lex ¼ 365 nm) in
both solution and the solid state were recorded and shown in
Fig. 4. All the fluorophores exhibited intense emission in CH2Cl2
solutions (Fig. 4a) when excited at their absorption maxima. The
emission profiles of these complexes in solution, having emission
bands that peaked at 452 nm (1, deep blue), 485 nm (2, greenish
blue), 543 nm (3, green), 574 nm (4, yellow), 627 nm (5, red), and
642 nm (6, deep red), covered a wide range from deep blue to
saturated red, reflecting that the amine derivation at the para-
position was indeed effective in tuning the emission color of this
type of boron-chelate fluorophore. The amine-substituted
complexes 3–6 showed significantly red-shifted emission peaks
compared to the non-substituted complexes 1 and 2, consistent
with the trend observed in the absorption spectra. Similar to the
observation in the absorption profiles, the emission peaks of B,S-
bridged complexes 2 and 5 were red shifted by 33 and 53 nm
compared to their B,O-bridged analogs 1 and 4. The absolute
fluorescence quantum yields (FF) of solutions calculated using an
integrating sphere were estimated to be 0.55 for 1, 0.65 for 2, 0.72
for 3, 0.87 for 4, 0.44 for 5, and 0.31 for 6.
The solid-state fluorescence spectra of these complexes in
powder forms were measured and are shown in Fig. 4b. The
fluorescence of the solid samples with emission peaks at 462 nm
for 1, 494 nm for 2, 519 nm for 3, 574 nm for 4, 603 nm for 5, and
655 nm for 6 also covered a wide range from blue to deep red and
the emission bands of 1, 2, 4, and 6 matched well with those in
solutions. The emission maxima of complexes 3 and 5 were both
blue shifted by about 24 nm compared with those of solution
samples, which reflected crystallochromy effects. All the solid
samples are intensely emissive (FF ¼ 0.53 for 1, 0.60 for 2, 0.71
for 3, 0.57 for 4, 0.48 for 5, and 0.36 for 6, determined using an
integrating sphere) and the fluorescence efficiencies are
Fig. 4 Emission spectra (excited with longest absorption maxima) in
CH2Cl2 (a), and the solid state (b), and photographic images (excited
at 365 nm using a UV lamp) of solution and solid samples for complexes
1–6 (c).
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unoccupied molecular orbital (LUMO) energy levels were
calculated to be in the range of ꢀ2.51 to ꢀ2.82 eV.
Complexes 1 and 2 showed an irreversible anionic redox
process with peak potentials at +1.17 and +1.10 V, respectively,
corresponding to oxidation of the conjugated core. The amine-
substituted derivatives 4–6 displayed two reversible oxidation
1/2
waves and their first oxidation potentials (Eox ¼ +0.40 V for 4,
+0.37 V for 5, +0.20 V for 6) were largely negatively shifted
compared to non-substituted complexes 1 and 2. Carbazole-
substituted complex 3 exhibited the first quasi-reversible anionic
redox process with an oxidation potential at +0.73 V which was
also greatly negatively shifted compared to 2. The significantly
shifted oxidation potentials of the amine-substituted derivatives
are considered to originate from the oxidation of amine groups.
Diphenylamine- or dimethylamine-substituted complexes 4, 5, 6
showed lower oxidation potentials compared to 3, due to their
Fig. 5 Cyclic voltammograms of 1–6 in THF (reduction, 1 mM) and
stronger electron-donating ability. The second oxidation
1/2
CH2Cl2 (oxidation, 1 mM), measured with TBAP (0.1 M) as a supporting
electrolyte at a scan rate of 100 mV sꢀ1
processes of 3–6 had comparable onset potentials (Eox
¼
.
+1.18 V for 3, +1.02 V for 4, +0.99 V for 5, +0.90 V for 6) to
those of 1 and 2. These results mean that the second anionic
redox process of complexes 3–6 corresponds to the oxidation of
the core skeleton. Notably, in comparison with 1 and 2,
complexes 4, 5, and 6 have reversible second oxidation waves
with slightly lower potentials; this is probably due to the presence
of electron communication between the amine segment and the
core skeleton which not only stabilizes the produced dication
species but also enables the oxidation of the p-conjugated core
comparable to those of solutions. It is also worth noting that the
solid-state quantum yields of 5 and 6 are at a very high level for
materials with red or saturated red emissions. This is very
important from the viewpoint of applying these complexes as
emitting materials in OLEDs. The rigid core structure together
with the bulky side phenyl groups attached to the boron atom
play the major roles in maintaining the high fluorescence effi-
ciencies from the solution phases to the solid forms.
Electrochemical properties
The electrochemical properties of these boron complexes 1–6
were characterized by cyclic voltammetry, and their cyclic vol-
tammograms are shown in Fig. 5. The redox potentials are
collected in Table 2.
All the complexes, except for 1, exhibited a reversible cationic
redox process corresponding to the reduction of the core skel-
1/2
eton. The half-wave potentials (Ered ¼ ꢀ2.28 V for 1, ꢀ2.21 V
for 2, ꢀ1.98 V for 3, ꢀ2.29 V for 4, ꢀ2.03 V for 5, ꢀ2.25 V for 6,
vs. Fc/Fc+) are very close to each other, indicating that the
introduced amine segments do not greatly affect the electron
affinity of the conjugated p-system. Complexes 2 and 5 showed
slightly positively shifted reduction waves compared to
complexes 1 and 4, indicative of the certain enhancement of
electron affinity when replacing the bridging oxygen atom by
sulfur. On the basis of the reduction potentials, the lowest
Fig. 6 Calculated molecular orbitals and energy levels of complexes 1–6.
Table 2 Electrochemical and thermal data of complexes 1–6a
HOMO/LUMO/Gap/
eV
Complex
Eox1/2/V
Ered1/2/V
Tm/Tg/Td5/ꢂC
1
2
3
4
5
6
+1.17/n.d.
+1.10/n.d.
+0.73/+1.18
+0.40/+1.02
+0.37/+0.99
+0.20/+0.90
ꢀ2.28
–2.21
ꢀ1.98
ꢀ2.29
ꢀ2.03
ꢀ2.25
ꢀ5.97/ꢀ2.52/3.45
ꢀ5.90/ꢀ2.59/3.31
ꢀ5.53/ꢀ2.82/2.71
ꢀ5.20/ꢀ2.51/2.69
ꢀ5.17/ꢀ2.77/2.40
ꢀ5.00/ꢀ2.55/2.45
160/60/238
211/75/264
303/142/355
n.d./105/324
n.d./115/315
269/129/306
a
n.d. ¼ not observed.
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more easily. On the basis of the first oxidation potentials, the
highest occupied molecular orbital (HOMO) energy levels were
calculated to be in the range of ꢀ5.00 to ꢀ5.97 eV.
Theoretical calculations
To gain a deeper insight into the electronic structures of these
boron complexes, we carried out the density functional theory
(DFT) calculation at the B3LYP/6-31G(d) level. The plots of
HOMOs and LUMOs and energy levels are shown in Fig. 6.
As shown in Fig. 6, the LUMOs of all complexes including the
amine-substituted derivatives are the same and delocalize on the
whole four-ring-fused p-conjugated skeleton. Their LUMO
energy levels are very close, being in the range of ꢀ2.01 eV to
ꢀ2.39 eV. These calculated results demonstrate that the electron-
donating amine groups do not contribute to the LUMOs and can
only slightly tune the energy levels, which is consistent with
observations in the cyclic voltammetry. Replacing the bridging
oxygen atom by a sulfur atom pushes the LUMO energy level
down by certain degrees, which can be seen from comparisons
Fig. 7 Solvatochromic behaviours of complexes 2 (gray line) and 5
(black line).
complexes 1 and 2 keep an identical emission profile when the
solvent polarity is increased from toluene to dichloromethane
and finally to acetonitrile, whereas amine-substituted derivatives
3–6 produce gradually red-shifted emission bands after changing
the solvents in the same order. The normalized emission spectra
of complexes 2 and 5 in solvents of varying polarity are shown in
Fig. 7 as examples and those of other four complexes are
summarized in the supporting information (Fig. S3†). The
emission maxima of all complexes in different solvents are
collected in Table 3. The emission bands of 2 were independent of
the solvent polarity. In contrast, complex 5 showed strong
solvent-polarity dependent emission spectra. When the polarity
of solvents increased from weak (toluene) to strong (acetonitrile),
the emission band underwent remarkable bathochromic shifts
while only a subtle change was observed in the absorption
spectra. The degree of bathochromic shift in the fluorescence of
complex 3 (20 nm) is smaller compared to those of complexes 4
(40 nm), 5 (37 nm), and 6 (40 nm), which is due to the relatively
weak electron-donating ability of the carbazole group in the
series of carbazole, diphenylamine, and dimethylamine.
between complexes 1 (ELUMO ¼ ꢀ2.01 eV) and 2 (ELUMO
ꢀ2.20 eV) as well as 4 (ELUMO ¼ ꢀ2.04 eV) and 5 (ELUMO
ꢀ2.21 eV).
¼
¼
In a sharp contrast, the HOMOs are quite different between
non-substituted complexes (1 and 2) and amine-substituted
derivatives (3–6). The molecular orbitals of 1 and 2 are delo-
calized on the entire ring-fused p-skeleton, whereas those of 3–6
are donated mainly by p-orbitals of the amine groups and the
substituted phenyl rings. As a result, the HOMO levels of
complexes 3–6 (EHOMO ¼ ꢀ5.37 eV for 3, ꢀ4.98 eV for 4, ꢀ4.98
eV for 5, ꢀ5.13 eV for 6) having electron-donating groups are
significantly increased compared to those of complexes 1
(EHOMO ¼ ꢀ5.87 eV) and 2 (EHOMO ¼ ꢀ5.86 eV). It thus can be
expected that the HOMO energy levels are dependent on the
electron-donating strength of the substituted amine groups.
Complex 3 exhibited a lower HOMO level compared to
complexes 4, 5, and 6 due to the weaker electron-donating ability
of the carbazole group. Modulation of the p-frame from the B,
O-bridge to the B,S-bridge failed to tune the HOMO levels
considering the calculated results that 1 and 2 as well as 4 and 5
respectively showed the same HOMO levels though this struc-
tural modification could tune LUMO levels by certain degrees.
Corresponding to the HOMO and LUMO levels, the energy
band gaps of these complexes were determined to be 3.86 eV for
1, 3.66 eV for 2, 2.98 eV for 3, 2.94 eV for 4, 2.77 eV for 5, and
3.02 eV for 6. Amine derivations in 3–6 increase the HOMOs
greatly but affect the LUMOs weakly; hence narrow the band
gap. This result can well explain the red-shifted absorption and
emission bands of 3–6. The HOMO and LUMO distributions of
amine-substituted derivatives 3–6 are donated separately by
amine groups and ring-fused p-conjugated skeletons; therefore,
intramolecular charge transfer (ICT) from the amine group to
the core skeleton is suggested, which could partly explain their
broad absorption and large Stokes shifts.
Thermal properties
The thermal properties of 1–6 were characterized by thermog-
ravimetric analysis (TGA) and differential scanning calorimetry
ꢂ
(DSC) under a nitrogen atmosphere at a heating rate of 10 C
minꢀ1. The resultant data, including melting point (Tm), glass-
transition temperature (Tg) from the re-melt (second heating
Table 3 Emission maxima of complexes 1–6 in different solventsa
lem/nm
(in toluene)
lem/nm
(in CH2Cl2)
lem/nm
in CH3CN
Complex
1
2
3
4
5
6
455
487
533
554
604
630
452
486
543
574
627
642
450
482
553
594
641
670
Solvatochromic behaviours
The solvatochromic behaviours were evaluated to gain insight
a
Concentration: 1.0 ꢁ 10ꢀ4 M.
into the ICT character of complexes 1–6. The non-substituted
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process), and decomposition temperatures (Td5), are shown in
Table 2. Complexes 1 and 2 show relatively low glass-transition
temperatures and melting points with Tg at 60 ꢂC and 75 ꢂC and
Tm at 160 ꢂC and 211 ꢂC, respectively, due to their low molecular
weight. All the amine-substituted derivatives have high glass-
transition temperatures with Tg over 100 ꢂC (142 ꢂC for 3, 105 ꢂC
for 4, 115 ꢂC for 5, 129 ꢂC for 6). The melting points for 3 and 6
are 303 and 269 ꢂC, respectively, which are much higher than that
of 2. Notably, complexes 4 and 5 do not show endothermic peaks
corresponding to their melt, which is indicative of the unique
glassy state of these two complexes. Among the amine-
substituted derivatives, complex 3 with a carbazole group shows
the highest Tg and Tm in the DSC measurement. This is probably
because of the high rotation barrier of the carbazole group in 3
compared with the substituted diphenylamine and dimethyl-
amine groups in 4, 5, and 6. As determined by TGA character-
ization, the decomposition temperatures of amine-substituted
derivatives 3–6 are 355, 324, 315, 306 ꢂC, respectively, which are
significantly higher than those of the non-substituted parent
complexes 1 (238 ꢂC) and 2 (264 ꢂC). High-temperature resis-
tance is a desired feature of OLED materials because of the
inevitable joule heating under the device operating conditions.
The increased Tg, Tm, and Td5 values of 3–6 make these
complexes thermally robust materials.
Fig. 8 Electroluminescent spectra (a), CIE coordinates and photo-
graphic images of devices employing complexes 1–6 as emitters (b),
brightness–voltage (c) and current density–voltage (d) characteristics of
the devices.
Electroluminescent properties
To evaluate the EL properties, OLEDs with the architecture of
[ITO/NPB (35 nm)/boron complexes (3, 4 and 6) (25 nm)/Alq3
(40 nm)/LiF (0.5 nm)/Al] and [ITO/NPB (35 nm)/5 (25 nm)/TPBi
(5 nm)/Bebq2 (35 nm)/LiF (0.5 nm)/Al] were fabricated, where
NPB, boron complex, and Alq3/Bebq2 served as a hole-trans-
porting material, a light emitter, and an electron-transporting
material, respectively, and TPBi served as a hole-blocking layer.
For comparison purposes, the thickness of the hole-transporting
layer, emitting layer, and electron-transporting layer are the
same for all the devices, being 35 nm, 25 nm, and 40 nm,
respectively. As for complex 5, the EL spectrum of the device
comprises the emission profile of Alq3 when this device employed
Alq3 as the electron-transporting layer. Thus, we selected TPBi/
Bebq2 as electron-transporting materials in which TPBi can
effectively block hole injection into the electron-transporting
layer. The devices with the configuration of [ITO/NPB (20 nm)/
BA (15 nm)/1 or 2 (25 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al] based
on complexes 1 and 2 whose EL properties have not reported
previously were also fabricated, in which BA16 served as an
additional hole-transporting buffer layer for the purpose of
hindering the formation of an exciplex between NPB and the
boron complexes. As shown in Fig. 8a, the electroluminescence
(EL) spectra of 1–6 show emission peaks at 465, 500, 544, 560,
612, and 652 nm, respectively, which generally correspond well
with the solid emission spectra in each case; that is, the emission
originates from the boron emitter. The CIE coordinates (X, Y) of
these devices are (0.16, 0.19), (0.19, 0.44), (0.41, 0.55), (0.45,
0.53), (0.61, 0.39), and (0.66, 0.34), respectively, which locate in
the blue, greenish blue, green, yellow, red, and deep red regions,
as depicted in Fig. 8b. Notably, all the EL spectra are voltage-
independent and there is little or no variation in the CIE coor-
dinates with different voltages (Fig. 9). Thus, stable and tunable
Fig. 9 EL spectra of 1 (a), 2 (b), 3 (c), 4 (d), 5 (e), and 6 (f) at different
applied voltages.
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Table 4 EL performance of devices employing boron complexes 1–6 as emitters
c
d
e
Emitter
lELa/nm
Vonb/V
Lmax /cd mꢀ2
hc /cd Aꢀ1
hp /lm Wꢀ1
CIE (X, Y)
1
2
3
4
5
6
465
500
544
560
612
652
5.0
4.8
4.5
3.0
4.5
4.0
2237
3639
9181
31 220
14 010
4662
4.4
7.8
3.1
5.5
4.5
0.59
2.6
4.4
1.8
4.8
3.1
0.47
(0.16, 0.19)
(0.19, 0.44)
(0.41, 0.55)
(0.45, 0.53)
(0.61, 0.39)
(0.66, 0.34)
a
e
Peak electroluminescence. b Turn-on voltage. c Maximum brightness. d Maximum current efficiency. Maximum power efficiency.
EL colors ranging from blue to deep red have been realized by
Acknowledgements
employing the present boron complexes as emitting materials.
This work was supported by the National Natural Science
Foundation of China (50903037, 50733002, and 51173067), the
Major State Basic Research Development Program
(2009CB623600).
The J–V and L–V characteristics of these devices are shown
in Fig. 8c and d and their performance is listed in Table 4.
The blue/greenish-blue emitting devices exhibited a maximum
brightness, a turn-on voltage, and a peak current efficiency of
2237 cd mꢀ2, 5.0 V, and 4.4 cd Aꢀ1 for 1 and 3639 cd mꢀ2, 4.8
V, and 7.8 cd Aꢀ1 for 2. The efficiencies are much higher than
those previously reported boron-contained blue or greenish-
blue light-emitting OLEDs.7d,8a Devices employing 3, 4, 5 as
emitters reached the maximum brightness of 9181, 31 220, and
14 010 cd mꢀ2, turn on voltage of 4.5, 3.0, and 4.5 V, and
peak current efficiency of 3.1, 5.5, 4.5 cd Aꢀ1, respectively.
The brightness of devices fabricated from complex 3, 4, and 5
is significantly improved compared to that of 1 and 2. From
the cyclic voltammetry measurements, the HOMO levels of 3,
4, and 5 are estimated to be at ꢀ5.53 eV, ꢀ5.20 eV, and
ꢀ5.17 eV, respectively, similar to that of the hole-transporting
material NPB (ꢀ5.40 eV). Thus, the presence of amine groups
has a favorable effect in hole-injecting or -transporting, and
thereby improves the luminance. Notably, the efficiency of the
device based on 2 and the brightness of the device based on 4
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