Dramatic substituent effects on the photoluminescence of boron complexes of 2-(benzothiazol-2-yl)phenols

Article abstract of DOI:10.1002/chem.201200726

Substituents can induce dramatic changes in the photoluminescence properties of N,O-chelated boron complexes. Specifically, the boron complexes of 2-(benzothiazol-2-yl)phenols become bright deep blue- and orange-red-emitting materials depending on amino substituents at the 5- and 4-positions of 2-(benzothiazol-2-yl)phenol, respectively. Absorption and emission data show that the resulting boron complexes have little or small overlap between the absorption and emission spectra and, furthermore, X-ray crystal structures for both the blue and orange-red complexes indicate the absence of π-π stacking interaction in the crystal-packing structures. These features endow the boron complexes with bright and strong photoluminescence in the solid state, which distinguishes itself from the typical boron complexes of dipyrromethenes (BODIPYs). A preliminary study indicates that the blue complexes have promising electro-optical characteristics as dopant in an organic light-emitting diode (OLED) device and show chromaticity close to an ideal deep blue. The substituent effects on the photoluminescent properties may be used to tune the desired emission wavelength of related boron or other metal complexes. The rainbow connection: Boron complexes of 2-(benzothiazol-2-yl)phenols emit bright deep-blue or orange-red in solution and in solid states, depending on the position of amino substituents (see figure). Analyses reveal that the boron complexes show little absorption-emission spectral overlap and lack π-π stacking interactions in the crystal packing structures. Copyright

Full text of DOI:10.1002/chem.201200726

FULL PAPER
DOI: 10.1002/chem.201200726
Dramatic Substituent Effects on the Photoluminescence of Boron Complexes
of 2-(Benzothiazol-2-yl)phenols
[
a]
[a]
[b]
[b]
Mithun Santra, Hyunsoo Moon, Min-Ho Park, Tae-Woo Lee,*
[
c]
[a]
Young Kook Kim, and Kyo Han Ahn*
Abstract: Substituents can induce dra-
matic changes in the photolumines-
cence properties of N,O-chelated
boron complexes. Specifically, the
boron complexes of 2-(benzothiazol-2-
yl)phenols become bright deep blue-
and orange-red-emitting materials de-
pending on amino substituents at the 5-
and 4-positions of 2-(benzothiazol-2-
yl)phenol, respectively. Absorption and
emission data show that the resulting
boron complexes have little or small
overlap between the absorption and
emission spectra and, furthermore, X-
ray crystal structures for both the blue
and orange-red complexes indicate the
absence of p–p stacking interaction in
the crystal-packing structures. These
features endow the boron complexes
with bright and strong photolumines-
cence in the solid state, which distin-
guishes itself from the typical boron
complexes of dipyrromethenes (BODI-
PYs). A preliminary study indicates
that the blue complexes have promis-
ing electro-optical characteristics as
dopant in an organic light-emitting
diode (OLED) device and show chro-
maticity close to an ideal deep blue.
The substituent effects on the photolu-
minescent properties may be used to
tune the desired emission wavelength
of related boron or other metal com-
plexes.
Keywords: absorption
dyes/pigments luminescence
substituent effects
·
boron
·
·
·
Introduction
plexes of dipyrromethenes, so-called BODIPYs, have been
[2]
extensively explored for various applications. BODIPY
dyes can be considered N,N-chelated boron complexes of
cyanine dyes in which electron delocalization between a-
and w-nitrogen donor/acceptor occurs through an oligo-
methine chain. The boron complexation results in the dis-
tinctive photochemical properties of BODIPY dyes relative
to those of the cyanine dyes, which are also widely used for
biological imaging. Accordingly, other conjugated dye mole-
cules with donor and acceptor ligands that can form boron
complexes would be of interest in the development of novel
photoluminescent compounds. Indeed, N,N- and N,O-chelat-
ed boron complexes based on other types of organic dyes,
Luminescent organic and organometallic compounds have
attracted intense research interest on account of their poten-
tial applications in organic light-emitting diodes (OLEDs),
solar cells, laser dyes, photosensitizers, biomolecular labels,
and molecular probes. Among various types of luminescent
compounds, boron complexes based on N- and O-donor lig-
ands have received particular interest because they show ex-
cellent photophysical properties such as thermal and photo-
chemical stability, high fluorescence quantum yield, and neg-
[1]
ligible triplet-state formation. In particular, boron com-
[3]
which are also simply called BODIPY analogues, have
been explored in search of efficient photoluminescent and
+
+
[
a] M. Santra, H. Moon, Prof. K. H. Ahn
Department of Chemistry
Pohang University of Science and Technology (POSTECH)
San 31, Hyoja-dong, Pohang, 790-784 (Republic of Korea)
Fax : (+82)54-279-5877
[4]
electroluminescent compounds. Most of these boron com-
plexes, however, show inferior photophysical properties to
the conventional BODIPY dyes. Given that photolumines-
cent materials are useful for various applications, it would
be of great value to develop new types of organoboron com-
plexes with promising photophysical properties. Herein, we
wish to disclose the dramatic substituent effects on the pho-
toluminescent properties of N,O-chelated boron complexes,
which resulted in bright blue- and orange-emitting com-
plexes in solution and, furthermore, in the solid state. The
substituent effects observed would also offer us an impor-
tant guideline for the development of related photolumines-
cent and electroluminescent materials.
E-mail: ahn@postech.ac.kr
[
b] M.-H. Park, Prof. T.-W. Lee
Department of Material Science and Engineering
Pohang University of Science and Technology (POSTECH)
San 31, Hyoja-dong, Pohang, 790-784 (Republic of Korea)
Fax : (+82)54-279-2399
E-mail: twlee@postech.ac.kr
[
c] Dr. Y. K. Kim
Next G. EVEN Development Team
Samsung Mobile Display
San 24, Nongseo-dong, Yongin, 446-811 (Republic of Korea)
+
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200726.
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Results and Discussion
In the course of our study on ratiometric fluorescent probes
based on 2-(benzothiazol-2-yl)phenol (BTP) and its deriva-
tives (Figure 1), we noticed that there are dramatic substitu-
[5]
ent effects on their emission properties.
Figure 2. Boron complexes of 2-(benzothiazol-2-yl)phenol derivatives
studied.
Figure 1. 2-(Benzothiazol-2-yl)phenol (BTP, 1c) and its meta- and para-
substituted derivatives.
[8]
as 5a, but it was rather unstable in acetonitrile. Therefore,
we focused on the difluoroboron complexes in the following
studies. Details of the synthesis are described in the Sup-
porting Information.
Specifically, there were significant differences in the emis-
sion wavelengths between the meta- and para-substituted
phenol derivatives 1b and 2b (R=NHCOCH ; NHCOPh).
3
Also, the meta derivatives 1b showed hypsochromic shifts
relative to the parent BTP 1c (55 nm), whereas para ana-
logues 2b showed bathochromic shifts relative to 1c
Substituent effects: The boron complexes of the BTP ana-
[9]
logues, BOBTPs, showed striking emission properties: for
m
(
30 nm). These substituent effects prompted us to explore
example, BOBTP 3a (R’=Me) emitted strong blue lumi-
related BTP derivatives, in particular, amino derivatives 1a
and 2a, and the corresponding N,O-chelated boron com-
nescence (lem =430 nm) when excited at 395 nm in dichloro-
methane, whereas BOBTP 4a (R’=Me) emitted orange lu-
[6]
p
plexes as novel photoluminescent materials.
minescence (lem =597 nm) when excited at 450 nm in di-
chloromethane (Figure 2). The boron complexation thus
leads to a larger shift in the emission wavelength (167 nm)
depending on the substituent position relative to the case of
the noncomplexed BTPs (55–85 nm). Thus, simply by chang-
ing the position of amino substituents, we were able to real-
ize deep blue- and orange-emitting compounds in solution.
Synthesis: The BTP derivatives with amine substituents at
the meta and para positions can be synthesized in four and
three steps starting from commercially available 2-amino-
benzenethiol and 5-amino-2-hydroxybenzoic acid (for the
meta-substituted derivatives) or 4-amino-2-hydroxybenzoic
acid (for the para-substituted derivatives), respectively. A
synthetic route for the meta derivatives is shown in
Scheme 1. The para derivative 2a (R’=Me) was synthesized
m
Both absorption and emission spectra of BOBTP 3a and
its para analogue 4a are given in Figure 3. Both of the com-
plexes emitted strongly when excited at the longest wave-
length of the absorption maxima, 395 and 450 nm, respec-
[6,7]
according to the literature procedure.
Then, we prepared the difluoroboron complexes 3 and 4
m
tively. BOBTP 3a showed narrow absorption and emission
m
p
from the BTP and BTP , respectively (Figure 2), by treat-
ment with boron trifluoride etherate in the presence of tri-
ethylamine. We also prepared diphenylboron analogues such
bands similar to most of the BODIPY dyes, with the full
widths at half-maximum (FWHM) of 40 and 39 nm, respec-
tively, and with a small Stokes shift (17 nm). The para ana-
logue 4a showed an emission band with a larger value of
FWHM (91 nm).
m
Interestingly, BOBTP dyes show small or little spectral
overlap. For example, 3c shows a relatively small absorp-
tion–emission spectral overlap despite a small Stokes shift
(
28 nm), as seen from the normalized spectra (Figure 4a). In
p
the case of the para derivatives, BOBTP 4a, there is a large
Stokes shift (141 nm) with little spectral overlap (Figure 4b).
Such low or small spectral overlap observed for the BOBTP
dyes distinguishes them from the typical BODIPY dyes that
[10]
show significant spectral overlap.
Scheme 1. Synthesis of BTPs 1a. Reagents and conditions: a) polyphos-
p
BOBTP 4a dissolves well in various organic solvents,
phoric acid, 1808C; b) CH
3
I, NaH, DMF, RT; c) CH
N I , NaH, THF, reflux; d) nC12 25SH, NaOH, N-methyl-2-pyrrol-
idone (NMP), 1308C.
3 2 6
CH Br or nC H13Br,
m
+
ꢀ
whereas BOBTP 3a has limited solubility in nonpolar sol-
(
nBu)
4
H
vents. Therefore, we varied the amino substituent (R’) of
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Boron Complexes of 2-(Benzothiazol-2-yl)phenols
FULL PAPER
Figure 3. Electronic absorption (dotted line) and emission (solid line)
m
p
spectra of a) BOBTP 3a (10 mm) and b) BOBTP 4a (100 mm) in di-
chloromethane.
Figure 4. Normalized absorption (dotted line) and emission (solid line)
spectra of a) BOBTP 3c and b) BOBTP 4a in dichloromethane.
m
p
m
BOBTP to longer ones to enhance the solubility in various
m
organic solvents. When the amino substituent in BOBTP
dyes was changed to longer alkyl groups, a slight bathochro-
mic shift in the maximum emission wavelength resulted:
(
R’=Me!Et!n-Hex: l =430!434!448 nm; Figure 5).
em
For a given BOBTP dye, the maximum emission wavelength
showed small bathochromic shifts as the solvent polarity in-
creased (lem =430–454 nm in the case of BOBTP 3c),
m
along with substantial decrease in the emission intensity
(
Figure 6a). Such solvent-dependent behavior was also ob-
p
served in the case of BOBTP 4a (Figure 6b).
m
Of particular note, BOBTP dyes exhibit very bright and
strong luminescence, which suggests that they might find po-
tential applications in OLED materials. The photophysical
properties of BOBTPs in solution are listed in Table 1.
A very high fluorescence quantum yield (F =0.98 in cy-
clohexane) was obtained for BOBTP 3c, as determined
F
m
m
Figure 5. Normalized emission spectra of BOBTP 3a (dotted line),
m
m
BOBTP 3b (dashed line), and BOBTP 3c (solid line) in dichloro-
with 9,10-diphenylanthracene as a reference dye (F =0.90
F
methane.
[
11]
in cyclohexane). With a large molar extinction coefficient
e=73000m cm ), BOBTP 3 dyes thus constitute a prom-
ꢀ1
ꢀ1
m
(
ising class of blue-emitting boron complexes. The corre-
sponding boron complex of the parent BTP ligand 1c that
lacks the 4-amino substituent, BOBTP 6 (Figure 2), is
important role of the substituents on the luminescent prop-
erties of BOBTPs. BOBTP 4a exhibited rather lower fluo-
p
rescence quantum yield (F =0.63 in cyclohexane) and
F
ꢀ
1
ꢀ1
known to have a lower fluorescence quantum yield (F =
molar extinction coefficient (e=5600m cm ) than
F
[12]
m
0.23 in dichloromethane),
which also demonstrates the
BOBTP 3c.
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T.-W. Lee, K. H. Ahn et al.
Table 2. Calculated HOMO–LUMO energy states of BOBTPs 4a, 6, and
[
a]
3
a.
First ex.
E (LUMO)
E (HOMO)
DE
energy [eV]
[eV]
[eV]
[eV]
4
6
3
a
a
2.76
3.58
3.63
ꢀ2.12
ꢀ2.29
ꢀ1.82
ꢀ5.14
ꢀ6.19
ꢀ5.48
3.02
3.90
3.66
[
a] Calculations were done for the singlet ground state and for the first
excited singlet state at the DFT level of theory using the hybrid ex-
change-correlation functional of Beckeꢁs three-parameter and the Lee–
[
13]
[14]
Yang–Parr approximation (B3LYP) implemented in Spartan 08.
Figure 7. Theoretical electron distribution of the HOMO–LUMO energy
states for complexes 4a, 6, and 3a.
There is a clear difference in the electron distribution be-
tween the para complex 4a and the meta complex 3a
(
Figure 7). In the case of 4a, the electron distribution in the
HOMO is confined to the phenol moiety, whereas the elec-
tron distribution in the LUMO is shifted to the benzothia-
zole and the phenol ring. In contrast, the electron distribu-
tion in the HOMO and LUMO of 3a does not
show such a shift but shows a delocalized feature.
The electron distribution shift observed in the case
m
Figure 6. Electronic emission spectra of a) BOBTP 3c (10 mm) and
p
b) BOBTP 4a (100 mm) in dichloromethane (
), cyclohexane (*), aceto-
&
nitrile (~), ethanol (!), and tetrahydrofuran (^).
Table 1. Photophysical data of BOBTPs in solution.
ꢀ
1
ꢀ1
l
abs [nm]
l
em [nm]
Stokes shift e [m cm
]
F
F
F
F
of the para substitution seems to cause a decrease
in the quantum yield.
Experimental HOMO–LUMO energy gaps for
the typical boron complexes were determined by X-
ray photoelectron spectroscopy (XPS) in poly-
in CH
2
Cl
2
in CH Cl
2
2
[nm]
in CH
2
Cl
2
in CH
2
Cl
2
6 12
in cyclo-C H
[
a]
a]
a]
[a]
3
3
3
4
a
b
c
413
417
420
456
430
434
448
597
17
17
28
60000
66000
73000
5600
0.98
0.96
0.91
0.28
0.95
[
[
[a]
0.93
[a]
0.98
[
b]
[b]
a
141
0.63
[
[
a] 9,10-Diphenylanthracene was used as a reference dye (F
b] Fluorescein was used as a reference dye (F =0.79 in 0.1m aqueous NaOH).
F
=0.90 in cyclohexane). (methyl methacrylate) (PMMA) film. We were able
to obtain the HOMO energy levels for the meta de-
F
[15]
rivative 3b (ꢀ5.80 eV) and the para derivative 4a
ꢀ5.70 eV). From these data, the HOMO–LUMO
(
Theoretical calculations: Computational calculations of the
molecular orbitals for the boron complexes of BTPs (4a, 6,
and 3a) suggest that the amino substituents increase both
the HOMO and LUMO energies of the parent boron com-
plex 6, but to different degrees depending on the substitu-
tion position (Table 2).
energy gaps were determined to be 3.63 and 3.02 eV for 3b
and 4a, respectively, based on the LUMO energies estimat-
ed by their maximum absorption wavelengths. These experi-
mental HOMO–LUMO energy gaps match well with the
computational values, as we expect similar values for 3a
(R’=Me) and 3b (R’=Et).
In particular, the para derivative 4a shows a significant in-
crease in the HOMO energy (1 eV) with respect to the
parent complex 6, which is supported by the localized
HOMO electron distribution (Figure 7). In the case of meta
derivative 3a, the perturbations in the HOMO and LUMO
energy levels seem to be comparable in magnitude, and thus
it shows the same bandgap as the parent complex 6 within
computational errors.
Solid-state luminescence: BODIPY dyes typically show
narrow emission spectra, an attractive feature for high color
purity. Most of them, however, exhibit severe spectral over-
lap between the absorption and emission spectra on account
of small Stokes shifts. The spectral overlap leads to self-
quenching of fluorescence at high concentration such as in
the solid state. Furthermore, the p–p stacking in the solid
[10]
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Boron Complexes of 2-(Benzothiazol-2-yl)phenols
FULL PAPER
[
18]
state, which is frequently observed for BODIPYs, leads to
fluorescence quenching. Both undesirable features seem to
limit the application of BODIPY dyes as OLED dopants, as
citonic coupling that leads to quenching.
Many efforts
have been made in the development of organic dye mole-
cules that emit in the solid state, which have produced dif-
ferent types of solid dyes in the J aggregates.
[16]
[19]
indicated by a rare example.
BOBTP
An approach of introducing bulky side substituents to
BODIPY dyes has been explored to enlarge the Stokes
shifts and thus to reduce the undesirable spectral overlap,
and also to reduce the undesirable p–p stacking in the solid
complexes thus constitute a new class of emitting com-
pounds in the solid state.
Quantum yields were determined for the typical boron
m
p
complexes (BOBTP 3a and 3b, and BOBTP 4a) in the
PMMA film by using a spectrometer based on the integrat-
ing sphere method. The results with Commission Internatio-
nale de lꢁEclairage (International Commission on Illumina-
tion, or CIE) values are summarized in Table 3. The meta
[17]
state.
This steric approach has been successful to some
extent; however, the BODIPY derivatives thus developed
by the steric approach still show considerable spectral over-
lap. An inherent problem in this steric approach arises, how-
ever, as larger Stokes shifts (larger structural changes upon
excitation) would lead to more facile nonradiative decay of
excitation energy, thus lowering the fluorescence quantum
yield. Therefore, it is challenging to develop fluorescent
dyes with high quantum yields by reducing the spectral over-
lap and the p–p stacking aggregation while maintaining
narrow emission spectra.
Table 3. Photophysical data of BOBTPs in the solid state.
l
ex
Absorbance
l
em
FWHM
[nm]
F
F
CIE
x
CIE
y
[
nm]
[nm]
3
3
4a
a
b
413
417
456
0.88
0.97
0.72
449
463
598
40
38
99
0.85
0.61
0.43
0.15
0.14
0.27
0.08
0.12
0.15
To our delight, we found that our BOBTP dyes show
bright luminescence even in the solid state. Both the meta
and para complexes 3b and 4a in their crystal states show
bright blue and red-orange luminescence, respectively, upon
irradiation by UV light (Figure 8, top). Photos of the emis-
derivative 3a shows a high quantum yield of 0.85, whereas
the para derivative 4a shows a moderate quantum yield of
0.43. In the film state, the meta derivative 3a shows a chro-
maticity value [CIEx,y (0.15, 0.08)], which is close to an ideal
deep blue [CIEx,y (0.14, 0.08)].
Crystal structures: To gain a further understanding of why
the boron complexes show strong luminescence in the solid
state, we carried out X-ray crystal analysis for the single
m
p
crystals obtained for BOBTP 3b and BOBTP 4a. We
found that, in addition to the small spectral overlap, more
importantly, there were few p–p stacking interactions in the
solid state when we analyzed the crystal-packing patterns of
[20]
m
3
b and 4a, respectively. In the case of BOBTP 3b, of
two parallel crystal planes observed, only one plane at a dis-
tance of 3.9 ꢂ would have p–p interactions, if any (Fig-
ure 9a,c). In this case, however, there is little overlap be-
tween the two interlayered aromatic rings. The benzene
rings of the phenol and the benzothiazole moieties do not
show any overlap between themselves or between each
other. Interestingly, the benzene rings overlap with the
boron-chelated ring moiety that is not a p system. There-
m
fore, BOBTP dyes have low (in the case of 3c) or small (in
the cases of 3a and 3b) spectral overlap, if any, as well as
little p–p stacking interactions in the solid state. These prop-
erties endow them with bright luminescence in the solid
state.
Figure 8. Top: Photos of luminescent crystals of 3b (blue) and 4a
(
3
orange-red) taken by irradiating with UV light in the range of 340–
80 nm and 515–560 nm, respectively. Photos were magnified five times
p
The crystal-packing pattern of BOBTP 4a, which shows
through a microscope. Bottom: Photos of emission colors of 3a (blue)
and 4a (orange) in dichloromethane upon excitation at 395 and 450 nm,
respectively.
two orthogonal parallel layers (Figure 9b), also shows low
p–p stacking interactions between the aromatic rings (Fig-
ure 9d). In this case, the benzene rings (the phenol and the
benzothiazole moieties) also do not overlap with each other,
but overlap with the boron-chelated ring moiety. Thus, 4a
also exhibits bright orange-red luminescence in the solid
state on account of a bathochromic shift in the emission
sion colors of 3a (blue) and 4a (orange) in dichloromethane
are provided for comparison (Figure 8, bottom). Organic
small-molecule dyes have a propensity to lose their lumines-
cence in the solid state, most probably on account of the ex-
[19]
wavelength usually observed in the J-aggregated state.
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T.-W. Lee, K. H. Ahn et al.
dyes have promising optoelectronic properties. A thermo-
m
gravimetric analysis (TGA) for BOBTP 3b indicated that
it is thermally stable up to 3508C (see the Supporting Infor-
m
mation). BOBTP 3b thus was spin-coated as a dopant into
the emitting layer of an OLED with a configuration of
indium oxide (ITO)/gradient hole injection layer
(
GraHIL; 50 nm)/poly
phenylene-[(4-sec-butylphenyl)amino]-1,4-phenylene)]
TFB; 35–40 nm)/(4,4’-N,N’-dicarbazolyl)biphenyl (CBP;
0 nm)/2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzim-
ACHTUNGTRENNUNG[ 2,7-(9,9-di-n-octylfluorene)-alt-(1,4-
(
4
[21]
idazole) (TPBi; 40 nm)/LiF (1 nm)/Al (130 nm). GraHIL,
TFB, CBP, and TPBi were used as the hole-transporting
layer, interlayer, light-emitting layer, and electron-transport-
m
ing layer, respectively. BOBTP 3b was spin-coated with
CBP at a 3 wt% doping level. An energy diagram, photo-
and electroluminescence spectra (Figure 10), a current den-
m
Figure 9. A part of a crystal-packing pattern of a) BOBTP 3b and
p
b) BOBTP 4a that shows no stacking of the benzene rings between adja-
m
cent interlayered crystals; molecular packing of c) BOBTP 3b viewed
p
along the a axis and d) BOBTP 4a viewed down the c axis in single crys-
tals.
Figure 10. a) An energy diagram of the OLED device; b) normalized PL
m
(
dotted line) and EL (solid line) spectra of BOBTP 3b.
The solid structures suggest that such boron complexes
are very promising as optoelectronic materials for OLEDs,
lasers, field-effect transistors, and molecular probes.
sity/voltage characteristic, and a luminance/voltage charac-
[22]
teristic of the device (Figure 11) are provided.
A maxi-
ꢀ
2
Electroluminescent properties: Encouraged by the excellent
mum brightness of 716 cdm was recorded at a driving volt-
age of 8.0 V. The chromaticity of the device at 4.9 V was
CIEx,y (0.15, 0.11), which is close to an ideal deep blue
m
luminescent properties of BOBTP complexes, and BOBTP
derivatives in particular, we evaluated their potential as
OLED materials. A preliminary study on the electrolumi-
ꢀ
1
[CIEx,y (0.14, 0.08)]. A peak current efficiency of 1.7 cdA
was recorded at 3.0 V. These preliminary results are encour-
m
nescence properties of blue dye 3b indicates that BOBTP
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Boron Complexes of 2-(Benzothiazol-2-yl)phenols
FULL PAPER
Experimental Section
Synthesis and characterization: Synthesis of BTP 2a (R’=Me) was car-
[
6]
ried out according to the reported procedure. BTPs 3a–3c were synthe-
sized by a different route, and only characterization data for representa-
tive compounds are described here. Details can be found in the Support-
ing Information.
m
1
BTP 1a (R’=Et): H NMR (300 MHz, CDCl
3
, 208C, TMS): d=12.5 (s,
H), 7.8–7.79 (m, 2H; Ar), 7.48–7.39 (m, 2H; Ar), 7.31–7.25 (m, 1H),
.30–6.27 (m, 2H; Ar), 3.44–3.37 (q, J=7.2 Hz, 4H), 1.23–1.19 ppm (t,
1
6
1
3
J=7.2 Hz, 6H); C NMR (75 MHz, CDCl
3
, 208C, TMS): d=169.7,
1
4
59.9, 152.4, 151.5, 132.1, 130, 126.4, 124.3, 121.3, 121.2, 106, 104.3, 98.1,
4.7, 12.8 ppm.
m
1
BOBTP 3a: H NMR (300 MHz, CD
2
Cl
2
, 208C, TMS): d=8.12–8.09
(
m, 1H; Ar), 7.85–7.81 (m, 1H; Ar), 7.6–7.56 (m, 1H; Ar), 7.48–7.41 (m,
2
H; Ar), 6.46–6.42 (dd, J=11.4, 2.4 Hz, 1H; Ar), 6.27 (d, J=2.4 Hz, 1H;
1
3
Ar), 3.1 ppm (s, 6H); C NMR (125 MHz, CD
2
Cl
2
, 228C, TMS): d=
1
9
68.1, 158, 156.8, 143.6, 128.5, 128.3, 128, 125.7, 122, 118.6, 106.4, 102.8,
1
9
8.7, 40.01 ppm (two carbon atoms); F NMR (300 MHz, CD
2
Cl
2
, 208C,
1
1
TFA): d=ꢀ61.63 ppm (q, J=16.50 Hz, 2F);
B NMR (100 MHz,
2 2 3 2
CD Cl , 208C, BF ·Et O): d=1.29–0.97 ppm (t, J=16 Hz, 1B).
m
1
BOBTP 3b: H NMR (300 MHz, CDCl
3
, 208C, TMS): d=8.19–8.16 (m,
1
2
1
H; Ar), 7.75–7.71 (m, 1H; Ar), 7.54–7.49 (m, 1H; Ar), 7.40–7.35 (m,
H; Ar), 6.35–6.31 (dd, J=2.4, 9 Hz, 1H; Ar), 6.28–6.27 (d, J=2.4 Hz,
H; Ar), 3.45–3.37 (q, J=7.2 Hz, 4H), 1.24–1.19 ppm (t, J=7.2 Hz, 6H);
1
3
C NMR (75 MHz, CDCl
3
, 208C, TMS): d=167.5, 158.2, 154.6 (two
carbon atoms), 134.5, 128.7, 128, 125.5, 121.6, 119, 106.2, 102.5, 98.5, 44.8,
1
9
1
(
1
2.6 ppm; F NMR (300 MHz, CDCl
3
, 208C, TFA): d=ꢀ137.8–138 ppm
q, J=18.6 Hz, 2F); B NMR (100 MHz, CDCl , 208C, BF ·Et O): d=
.36–1.04 ppm (t, J=16 Hz, 1B).
1
1
3
3
2
p
1
Figure 11. a) Current density versus voltage characteristic, and b) lumi-
nance versus voltage characteristic of the OLED device.
3
BOBTP 4a: H NMR (300 MHz, CDCl , 208C, TMS): d=8.37–8.35 (d,
J=8.1 Hz, 1H; Ar), 7.86–7.84 (d, J=8.1 Hz, 1H; Ar), 7.64–7.59 (m, 1H;
Ar), 7.52–7.47 (m, 1H; Ar), 7.16–7.08 (m, 2H; Ar), 6.71–6.70 (d, J=
.4 Hz, 1H; Ar), 2.93 ppm (s, 6H); C NMR (75 MHz, CD
1
3
2
2 2
Cl , 228C,
TMS): d=170, 150, 144.7, 144.3, 130.1, 129.2, 127.6, 125.4, 122.8, 121.2,
aging for further studies on optimal device characteristics
based on boron complexes.
1
9
1
20.6, 113.5, 109.3, 42.2 ppm (two carbon atoms); F NMR (300 MHz,
CDCl
3
,
208C, TFA): d=ꢀ136.19–136.26 ppm (d, J=22.5 Hz, 2F);
1
1
B NMR (100 MHz, CD
2
Cl
2
, 208C, BF
3
·Et
2
O): d=1.50–1.20 ppm (t, J=
1
5 Hz, 1B).
m
p
Conclusion
CCDC-860065 (BOBTP 3b) and 860066 (BOBTP 4a) contain the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
We have demonstrated that substituents can induce dramat-
ic changes in the photoluminescence properties of N,O-che-
lated boron complexes. Specifically, the boron complexes of
OLED device fabrication: Patterned ITO glass was cleaned by sonication
in acetone and then in isopropanol, each for 15 min. The cleaned ITO
glass was treated with UV/ozone for 15 min. The polymeric hole-injection
layer (GraHIL) was blended with equal proportions of poly(3,4-ethylene-
2
-(benzothiazol-2-yl)phenols lead to bright deep blue- and
orange-red-emitting materials depending on amino substitu-
ents at the 5- and 4-positions of 2-(benzothiazol-2-yl)phenol,
respectively. Furthermore, both the boron complexes show
bright luminescence in the solid state on account of low or
small spectral overlap as well as the absence of p–p stacking
interactions in the solid state as analyzed by X-ray crystal-
lography. A preliminary study indicates that the blue com-
plexes have promising optoelectronic characteristics as dop-
ants in an OLED device and show chromaticity close to an
ideal deep blue. The substituent effects on the photolumi-
nescent properties may be used to improve the photo- and
electroluminescent properties of related boron complexes
and also other metal complexes, both of which are under
active investigation.
dioxythiophene)/poly(4-styrenesulfonate) (PEDO-T/PSS) and
a tetra-
fluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulphonic acid co-
polymer (PFI). A 50 nm-thick GraHIL was spin-coated on the ITO
under ambient conditions and baked immediately on a hot plate at
2
008C for 15 min. A 35–40 nm-thick poly AHCTUNGRETNNUN[G 2,7-(9,9-di-n-octylfluorene)-alt-
(1,4-phenylene-[(4-sec-butylphenyl)amino]-1,4-phenylene)] (TFB) was
used as interlayer, which was dissolved in toluene, spin-coated in a nitro-
gen glovebox, and annealed at 1808C for 30 min. The 40 nm-thick emit-
ting layer (EML) was spin-coated on the TFB interlayer and annealed at
08C for 30 min. The EML was composed of (4,4’-N,N’-dicarbazolyl)-
biphenyl (CBP) as a host and BOBTP 3b as a dopant (3 wt%). Other
layers were vacuum-deposited under a base pressure of 5ꢃ10 torr with-
out breaking the vacuum. As an electron-transport layer, 2,2’,2’’-(1,3,5-
benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi) was deposited on
the EML to a 40 nm thickness. Lithium fluoride (1 nm) and aluminum
8
m
ꢀ
7
(
130 nm) were used for cathode layers. Devices were encapsulated by
2
hollow glass and active area of OLED was 6 mm . The fabricated devices
were measured using a Keathley 236 (measurement source) instrument,
and a CS2000 (Konica Minolta) instrument was used for measuring the
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
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These are not the final page numbers!

T.-W. Lee, K. H. Ahn et al.
current/voltage/luminance (I–V–L) characteristics and electrolumines-
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1
m
[
20] Crystal data for BOBTP 3b (C17
H
17BF
2
N
2
OS): M
r
=346.20; triclin-
¯
ic, space group P1 (no. 2); a=8.5043(2), b=8.8157(2), c=
1
7
1
2.0205(3) ꢂ; a=71.629(2), b=76.751(2), g=68.273(2)8; V=
1
503–1513; k) Y. Zhou, J. W. Kim, R. Nandhakumar, M. J. Kim, E.
3
ꢀ1
88.01(3) ꢂ ; Z=2; T=100(2) K; m(l=0.71073 ꢂ)=0.233 mm
;
Cho, Y. S. Kim, Y. H. Jang, C. Lee, S. Han, K. M. Kim, J.-J. Kim, J.
Yoon, Chem. Commun. 2010, 46, 6512–6514; l) Y. Zhou, J. W. Kim,
M. J. Kim, W.-J. Son, S. J. Han, H. N. Kim, S. Han, Y. Kim, C. Lee,
S.-J. Kim, D. H. Kim, J.-J. Kim, J. Yoon, Org. Lett. 2010, 12, 1272–
ꢀ
3
calcd =1.459 gcm
int =0.0793),
;
71806 reflections measured, 12373 unique
(
R
R
1
=0.0548, wR =0.1376 (I>2s(I)), =0.0971,
2
R
1
p
wR
2
=0.1686 (all data); GOF=1.022. Crystal data for BOBTP 4a
OS): M =318.14; monoclinic, space group P2 /c (no.
4); a=8.5412(2), b=11.7529(3), c=13.8346(4) ꢂ; b=102.157(2)8;
(
C
15
H
13BF
2
N
2
r
1
1
275; m) D. Frath, S. Azizi, G. Ulrich, P. Retailleau, R. Ziessel, Org.
1
Lett. 2011, 13, 3414–3417; n) J. F. Araneda, W. E. Piers, B. Heyne,
M. Parvez, R. McDonald, Angew. Chem. 2011, 123, 12422–12425;
Angew. Chem. Int. Ed. 2011, 50, 12214–12217; o) J. Massue, D.
Frath, G. Ulrich, P. Retailleau, R. Ziessel, Org. Lett. 2012, 14, 230–
3
V=1357.63(6) ꢂ ;
Z=4;
T=100(2) K;
m(l=0.71073 ꢂ)=
.263 mm ; 1calcd =1.557 gcm ; 89916 reflections measured, 6997
unique (Rint =0.0830), =0.0570, wR =0.1643 (I>2s(I)),
.1025, wR =0.1902 (all data); GOF=1.108.
ꢀ
1
ꢀ3
0
R
1
2
1
R =
0
2
2
33.
[
21] ITO/GraHIL (50 nm)/TFB (35–40 nm)/CBP (40 nm)/TPBi (40 nm)/
LiF (1 nm)/Al (130 nm). ITO: indium tin oxide. GraHIL: gradient
hole injection layer (T.-W. Lee, Y. Chung, O. Kwon, J.-J. Park, Adv.
Funct. Mater. 2007, 17, 390–396; T.-H. Han, Y. Lee, M.-R. Choi, S.-
H. Woo, S.-H. Bae, B. H. Hong, J.-H. Ahn, T.-W. Lee, Nat. Photonics
[
[
5] M. Santra, B. Roy, K. H. Ahn, Org. Lett. 2011, 13, 3422–3425.
6] Wang and co-workers concurrently pursued similar substituent ef-
fects on the luminescent properties of the BTP ligands themselves:
D. Yao, S. Zhao, J. Guo, Z. Zhang, H. Zhang, Y. Liu, Y. Wang, J.
Mater. Chem. 2011, 21, 3568–3570.
2
012, 6, 105–110). TFB: poly AHCNUTGTERNNUNG[ 2,7-(9,9-di-n-octylfluorene)-alt-(1,4-
[
7] E. Barni, P. Savarino, M. Marzona, M. Piva, J. Heterocycl. Chem.
phenylene-[(4-sec-butylphenyl)amino]-1,4-phenylene)]. CBP: (4,4’-
N,N’-dicarbazolyl)biphenyl (S. H. Kim, J. Jang, J. Y. Lee, Appl. Phys.
Lett. 2007, 90, 223505). TPBi: 2,2’,2’’-(1,3,5-benzinetriyl)-tris(1-
phenyl-1H-benzimidazole) (Y. Byun, Y.-Y. Lyu, R. R. Das, O.
Kwon, T.-W. Lee, Appl. Phys. Lett. 2007, 91, 211106).
1
983, 20, 1517–1521.
[
8] During the publication process of this work, other groups also ob-
served similar substituent effects for related diphenylboron com-
plexes; they focused on the less bright para derivatives: D. Li, H.
Zhang, C. Wang, S. Huang, J. Guo, Y. Wang, J. Mater. Chem. 2012,
[
22] Since the TFB interlayer allows holes to be easily transported to the
emitting layer, the device becomes more hole-dominant. Although
the TFB interlayer lowers the operating voltage by transporting
more holes to the emitting layer, the lower external quantum effi-
ciency of the device with interlayer than that without interlayer may
be caused by reduced balance of hole and electrons in the emitting
layer.
2
2, 4319–4328.
[
9] BOBTP represents the boron complex of 2-(benzothiazol-2-yl)phe-
nol, the nomenclature of which is 6,6-difluoro-6-bora-5-oxa-11-thia-
6a-aza-benzo[a]fluorine.
[
[
[
[
[
[
10] R. Y. Lai, A. J. Bard, J. Phys. Chem. B 2003, 107, 5036–5042.
11] S. Hamai, F. Hirayama, J. Phys. Chem. 1983, 87, 83–89.
12] M. J. Kwak, Y. Kim, Bull. Korean Chem. Soc. 2009, 30, 2865–2866.
13] A. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
14] Spartan’08, Wavefunction, Inc. Irvine, CA.
15] The XPS measurement in the film state was not possible in the case
of 3a due to its low solubility.
Received: March 5, 2012
Revised: May 23, 2012
Published online: && &&, 0000
&
8
&
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Boron Complexes of 2-(Benzothiazol-2-yl)phenols
FULL PAPER
Substituent Effects
M. Santra, H. Moon, M.-H. Park,
T.-W. Lee,* Y. K. Kim,
K. H. Ahn* .................... &&&& — &&&&
The rainbow connection: Boron com-
plexes of 2-(benzothiazol-2-yl)phenols
emit bright deep-blue or orange-red in
solution and in solid states, depending
on the position of amino substituents
boron complexes show little or small
Dramatic Substituent Effects on the
Photoluminescence of Boron
Complexes of 2-(Benzothiazol-2-
yl)phenols
absorption–emission spectral overlap
and the absence of p–p stacking inter-
action in the crystal packing structures,
respectively.
(
see figure). Analyses reveal that the
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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9
&
ÞÞ
These are not the final page numbers!