Luminescence tuning of organoboron quinolates through substituent variation at the 5-position of the quinolato moiety

Article abstract of DOI:10.1021/ol0619664

A series of organoboron quinolates with emission colors ranging from blue to red have been prepared. In comparison to the respective AlQ₃ derivatives a distinct blue-shift of the emission is observed. Theoretical calculations serve to provide i

Full text of DOI:10.1021/ol0619664

ORGANIC
LETTERS
2
006
Vol. 8, No. 23
227-5230
Luminescence Tuning of Organoboron
Quinolates through Substituent Variation
at the 5-Position of the Quinolato Moiety
5
Yang Qin, Irene Kiburu, Shimul Shah, and Frieder Ja
1
kle*
Department of Chemistry, Rutgers UniVersity Newark,
3 Warren Street, Newark, New Jersey 07102
7
fjaekle@rutgers.edu
Received August 8, 2006
ABSTRACT
A series of organoboron quinolates with emission colors ranging from blue to red have been prepared. In comparison to the respective AlQ
3
derivatives a distinct blue-shift of the emission is observed. Theoretical calculations serve to provide insight into the nature of the frontier
orbitals and the effect of the substituents in the 5-position of the quinolate ligands on the relative HOMO and LUMO energy levels. An efficient
new blue emitting material with a pinacolborane substituent has been identified.
1
6
Since the discovery by Tang and VanSlyke in 1987 of the
application of tris-8-hydroxyquinoline aluminum (AlQ ) to
as cyclometalated platinum and iridium species are known
3
for their superior performance as phosphorescent emitters.
light emitting devices, intense research in both academia and
industry has been focused on organic light emitting diodes
Modification of the chelating quinolate ligand in AlQ
electron-donating or electron-withdrawing groups has been
3
with
(
OLEDs), which are thought to have the potential to replace
2
traditional liquid crystal displays (LCDs). Inorganic and
organometallic complexes play important roles as active
materials in OLEDs. For instance, main group metal chelate
complexes and organoborane-functionalized conjugated
aromatics have been used as electron conducting and/or
(4) Recent examples: (a) Son, H. J.; Jang, H.; Jung, B.-J.; Kim, D.-H.;
Shin, C. H.; Hwang, K. Y.; Shim, H.-K.; Do, Y. Synth. Met. 2003, 137,
1
001-1002. (b) Middleton, A. J.; Marshall, W. J. P.; Radu, N. S. J. Am.
Chem. Soc. 2003, 125, 880-881. (c) Trieflinger, C.; R o¨ hr, H.; Rurack, K.;
Daub, J. Angew. Chem., Int. Ed. 2005, 44, 6943-6947. (d) Zhang, H.; Huo,
C.; Zhang, J.; Zhang, P.; Tian, W.; Wang, Y. Chem. Commun. 2006, 281-
3
,4
5
2
83. See also refs 7-16.
fluorescent emitters, while transition metal complexes such
(
5) (a) Shirota, Y. J. Mater. Chem. 2005, 15, 75-93. (b) Barbarella, G.;
Melucci, M.; Sotgiu, G. AdV. Mater. 2005, 17, 1581-1593. (c) Jia, W. L.;
Moran, M. J.; Yuan, Y.-Y.; Lu, Z. H.; Wang, S. J. Mater. Chem. 2005, 15,
3326-3333.
(
1) (a) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913-
9
6
15. (b) Tang, C. W.; VanSlyke, S. A.; Chen, C. H. J. Appl. Phys. 1989,
5, 3610-3616.
(6) (a) O’Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R.
Appl. Phys. Lett. 1999, 74, 442-444. (b) Adachi, C.; Baldo, M. A.; Forrest,
S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904-906. (c) Chan,
S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.; Zhu, N.
Chem. Eur. J. 2001, 7, 4180-4190. (d) Lu, W.; Mi, B.-X.; Chan, M. C.
W.; Hui, Z.; Che, C.-M.; Zhu, N.; Lee, S.-T. J. Am. Chem. Soc. 2004, 126,
4958-4971.
(2) (a) Kalinowski, J. Organic Light-emitting Diodes: Principles, Char-
acteristics & Processes; CRC Press: Boca Raton, FL, 2004. (b) M u¨ llen,
K.; Scherf, U. Organic Light Emitting DeVices: Synthesis, Properties and
Applications; Wiley VCH: Weinheim, Germany, 2006.
(
3) Reviews: (a) Chen, C. H.; Shi, J. Coord. Chem. ReV. 1998, 171,
1
61-174. (b) Wang, S. Coord. Chem. ReV. 2001, 215, 79-98.
1
0.1021/ol0619664 CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/10/2006

7
8
established by Anzenbacher and others as a straightforward
method for the tuning of the optical properties of AlQ and
has more recently been exploited by Weck for the color
tuning of related AlQ -modified polymers. Following the
We chose to prepare organoboron quinolate derivatives 3
(Scheme 1) in which Bu groups are attached to the para-
t
3
9
3
initial reports by Wang, organoboron quinolates and related
chelate complexes of boron have also attracted much
Scheme 1. Synthesis of Organoboron Quinolates 3
1
0-15
interest,
especially since they provide efficient lumi-
nescence and in some cases better stability over their
aluminum counterparts. Even bifunctional quinolate ligands
have now been prepared for the assembly of new multi-
15
nuclear boron quinolate materials. However, until recently
very few studies have been performed on the systematic
16
tuning of the luminescence of organoboron quinolates. The
effect of placing methyl groups at different positions of the
2
quinolato moiety in diphenylboron quinolate (Ph BQ) has
been studied experimentally and by means of theoretical
calculations.1 Wang et al. showed in a very recent paper
that attachment of benzothienyl or naphthyl groups in the
1,12
5-position of the quinolate ligand leads to a red-shift in the
emission as a result of extended delocalization of the
1
0c
quinolate π-system into the benzothienyl substituent.
Moreover, substitution with extended π systems in the 5-
and 7-positions of the quinolate ligand in Ph BQ has been
demonstrated by Slugovc to lead to a significant bathochro-
mic shift of the emission wavelength.13 We report here on
the systematic color tuning of organoboron quinolates
through modification of the 5-substituent with electron-
withdrawing and electron-donating groups and compare the
position of the phenyl group for simplified NMR spectro-
scopic analysis and better comparability with related poly-
2
14
meric derivatives prepared in our laboratory. The electronic
structure of the compounds is systematically varied through
placement of different substituents in the 5-position of the
quinolate moiety (Q). The common precursor 2 was readily
prepared with high selectivity through reaction of 1 with
1
-trimethylstannyl-4-tert-butylbenzene (Scheme 1). When the
reaction was conducted at low temperature and stopped after
h, no evidence of side reactions or further conversion to
3
results with those for the respective AlQ derivatives.
4
(
7) (a) Pohl, R.; Anzenbacher, P., Jr. Org. Lett. 2003, 5, 2769-2772.
(
b) Pohl, R.; Montes, V. A.; Shinar, J.; Anzenbacher, P., Jr. J. Org. Chem.
the triarylborane was observed. Subsequent treatment with
a series of different 8-hydroxyquinoline derivatives with
varying substitution pattern as shown in Scheme 1 led to
the new organoboron quinolates 3. For the last reaction step
either the free 8-hydroxy functionalized ligand can be treated
with 2 in the presence of triethylamine as a base or, in a
versatile new method, the methoxy-protected ligand may be
directly reacted with 2 under cleavage of the O-Me bond
with formation of bromomethane. Complexes 3 were ob-
tained as crystalline solids in moderate to high yields.
FAB-MS analysis of compounds 3 showed in all cases
the molecular ion peak and a fragment due to loss of a tert-
butylphenyl group as the most intense peak. The structures
were further confirmed by NMR spectroscopy. Thus, in the
2
004, 69, 1723-1725. (c) Montes, V. A.; Li, G.; Pohl, R.; Shinar, J.;
Anzenbacher, P., Jr. AdV. Mater. 2004, 16, 2001-2003. (d) Montes, V.
A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem. Eur. J. 2006, 12, 4523-
4
535.
(8) See also: (a) Hopkins, T. A.; Meerholz, K.; Shaheen, S.; Anderson,
M. L.; Schmidt, A.; Kippelen, B.; Padias, A. B.; Hall, H. K., Jr.;
Peyghambarian, N.; Armstrong, N. R. Chem. Mater. 1996, 8, 344-351.
(b) Matsumura, M.; Akai, T. Jpn. J. Appl. Phys. 1996, 35, 5357-5360. (c)
Sapochak, L. S.; Padmaperuma, A.; Washton, N.; Endrino, F.; Schmett, G.
T.; Marshall, J.; Fogarty, D.; Burrows, P. E.; Forrest, S. R. J. Am. Chem.
Soc. 2001, 123, 6300-6307. (d) Cheng, J.-A.; Chen, C. H.; Liao, C. H.
Chem. Mater. 2004, 16, 2862-2868.
(
9) (a) Meyers, A.; Weck, M. Macromolecules 2003, 36, 1766-1768.
(
b) Meyers, A.; Weck, M. Chem. Mater. 2004, 16, 1183-1188. (c)
Kimyonok, A.; Wang, X.-Y.; Weck, M. J. Mater. Sci. C 2006, 47-77. (d)
Wang, X.-Y.; Weck, M. Macromolecules 2005, 38, 7219-7224.
(10) (a) Liu, S.-F.; Seward, C.; Aziz, H.; Hu, N.-X.; Popovic, Z.; Wang,
S. Organometallics 2000, 19, 5709-5714. (b) Wu, Q.; Esteghamatian, M.;
Hu, N.-X.; Popovic, Z.; Enright, G.; Tao, Y.; D’Iorio, M.; Wang, S. Chem.
Mater. 2000, 12, 79-83. (c) Cui, Y.; Liu, Q.-D.; Bai, D.-R.; Jia, W.-L.;
Tao, Y.; Wang, S. Inorg. Chem. 2005, 44, 601-609.
1
1
B NMR all compounds showed a relatively narrow signal
at ca. 12 to 13 ppm (w1/2 ca. 400 to 650 Hz) that is strongly
upfield shifted relative to the broad signal of 2 at 65 ppm
(
11) Anderson, S.; Weaver, M. S.; Hudson, A. J. Synth. Met. 2000, 111-
12, 459-463.
12) Teng, Y. L.; Kan, Y. H.; Su, Z. M.; Liao, Y.; Yan, L. K.; Yang, Y.
J.; Wang, R. S. Int. J. Quantum Chem. 2005, 103, 775-780.
13) Kappaun, S.; Rentenberger, S.; Pogantsch, A.; Zojer, E.; Mereiter,
1
1
(
(w1/2 ) 1800 Hz). The H NMR spectra showed the expected
peak patterns and integration confirmed the ratio of tert-
butylphenyl to quinolate groups of 2:1.
DSC analyses reveal that depending on the substituent X
complexes 3 melt between 230 and 330 °C and upon cooling
form glasses with a glass transition in the range of 78 to
(
K.; Trimmel, G.; Saf, R.; M o¨ ller, K. C.; Stelzer, F.; Slugovc, C. Chem.
Mater. 2006, 18, 3539-3547.
(
14) (a) Qin, Y.; Pagba, C.; Piotrowiak, P.; J a¨ kle, F. J. Am. Chem. Soc.
2
004, 126, 7015-7018. (b) Qin, Y.; Shah, S.; Kiburu, I.; J a¨ kle, F. Polym.
Prepr. 2005, 46, 1026-1027.
(
15) Cui, Y.; Wang, S. J. Org. Chem. 2006, 71, 6485-6496.
1
30 °C. In most cases, recrystallization occurs between 137
(16) Related fine-tuning of tetracoordinate B species with chelating
and 226 °C followed by a melt at the same temperature as
in the first heating cycle.
The photophysical data of the new series of organoboron
quinolates are summarized in Table 1. The parent compound
nitrogen ligands: (a) Liu, Q.-D.; Mudadu, M. S.; Thummel, R.; Tao, Y.;
Wang, S. AdV. Funct. Mater. 2005, 15, 143-154. (b) Chen, H.-Y.; Chi,
Y.; Liu, C.-S.; Yu, J.-K.; Cheng, Y.-M.; Chen, K.-S.; Chou, P.-T.; Peng,
S.-M.; Lee, G.-H.; Carty, A. J.; Yeh, S.-J.; Chen, C.-T. AdV. Funct. Mater.
2
005, 15, 567-574.
5228
Org. Lett., Vol. 8, No. 23, 2006

Table 1. Experimental and Computed (Gaussian03)20 Photophysical Data of Compounds 3 in Comparison to the AlQ
experimental data of compounds 3 TD-DFT calculations
λexc (nm) oscillator strength f
Analogues7b,c
3
data of AlQ3 analoguesd
compd
-NO2
λmax a (nm)
ꢀ (L mol-1cm-1
)
λem a,b (nm)
Φ (%)
λmax (nm)
λem (nm)
3
3
3
3
3
3
3
401
389
396
395
410
419
438
14500
4530
4900
3740
4030
4660
4940
456
485
498
506
528
562
625
0.1
39
30
17
8
394c
412
431
428
445
465
504
0.18c
0.10
0.10
0.06
0.07
0.10
0.09
380e
n.a.
388
388
402
410
422
510e
n.a.
516
526
541
564
612
-Bpin
-C6F5
-H
-Cl
-C6H4OMe
-C6H4NMe2
2
0.1
a
Concentrations were ca. 2 × 10 M in THF. b Excited at the absorption maximum. c Transition from HOMO-3/HOMO-2 to LUMO/LUMO+1. d Data
-5
e
in CH2Cl2 unless otherwise noted. Polymeric analogue of AlQ3 in CHCl3; see ref 9b.
3
-H shows an absorption maximum at 395 nm and green
emission at 506 nm, which correlates very well with the
previously reported data for Ph BQ (λmax ) 395 nm; λem )
various 5-substituents of the quinolate moieties is very
interesting, in that blue emitting materials, which continue
to be in strong demand for device applications, should then
be more easily accessible with boron quinolates. Indeed, the
2
04 nm)1 despite the presence of electron-donating tert-
0b
5
butyl groups in the 4-position of the phenyl rings. This
observation is in agreement with reports by Wang et al. that
the nature of the aryl group on boron has only a minor impact
on the photophysical characteristics.1 However, the quan-
tum yield for 3-H of 17% is considerably lower than that
pentafluorophenyl-substituted derivative 3-C
6
F
5
emits at 498
7
b,c
nm (AlQ
3
derivative: 516 nm ) and attachment of a
pinacolborane moiety in 3-Bpin provides for blue emission
at 485 nm. A further blue-shift to 456 nm can be achieved
by attachment of a nitro group on the quinolate in 3-NO2.
0b
1
0b
9d
reported for Ph
2
BQ (30% in CH
2
Cl
2
and 23% in CHCl
3
However, 3-NO shows an unusually small Stoke’s shift of
2
at room temperature). At this point the reasons for this
deviation are not obvious.
Substitution at the 5-position with electron-withdrawing
and electron-donating groups respectively strongly influences
the absorption and emission spectra. The emission bands span
55 nm in THF and displays a very low quantum efficiency
of 0.1%, which could be due to the charge-transfer character
of the lowest energy absorption bands (see vide infra).1
For all other organoboron quinolates significantly higher
quantum efficiencies are observed with electron withdrawing
(larger energy gap) than with donating substituents (Table
1). Most notably, the new pinacolborane-substituted orga-
noboron quinolate 3-Bpin shows a high quantum efficiency
of 39%.
8,19
2
a range from the blue (456 nm; X ) NO ) to the red (625
nm; X ) C NMe ) that covers almost the entire visible
H
6 4
2
spectrum. According to previous observations for AlQ
derivatives
3
7
b,c
and calculations on boron quinolates,12 the
highest occupied π orbital (HOMO) is mainly located on
the phenolate side and the lowest unoccupied π orbital
To further examine the precise electronic structure of
compounds 3 and the nature of the orbitals involved in the
electronic transitions, we optimized their geometries using
DFT calculations (Gaussian03, B3LYP, 6-31G(d)) and
computed the lowest energy excitations using TD-DFT
*
(LUMO) is mainly located on the pyridyl moiety. By
substituting the 5-position on the phenolate ring with
electron-withdrawing groups the HOMO energy is thus
expected to be lowered, leading to an increase in the HOMO-
LUMO energy gap and thus a blue-shift in the absorption
and emission spectra.7 On the other hand, electron-donating
groups and groups capable of extended π conjugation at the
20
calculations (Table 1, Figures 1 and 2). The calculations
reproduce the experimentally observed trends although the
absolute excitation data deviate significantly. According to
the calculations the lowest energy excitation corresponds to
b,c
5-position of the phenolate ring should lead to higher HOMO
levels and smaller HOMO-LUMO gaps, thus resulting in a
red-shift.
The experimental observations for the organoboron quino-
lates 3 seem to confirm the expected trend in that the
(
17) A reviewer suggested that the blue-shift could be due to compounds
R2BQ only containing one quinolato group, while the emission from AlQ3
may involve mixing between the three quinolato ligand states. As proposed
in the literature based on comparison of the structural data and photophyscial
properties of AlQ with those of species AlQ (OR), the relative bond strength
of the M-O (M ) B, Al) bonds may also play an important role; see: La
Deda, M.; Aiello, I.; Grisolia, A.; Ghedini, M.; Lelj, F. Dalton Trans. 2006,
30-339 and references therein.
(18) Charge transfer character has also been proposed for benzothienyl
Bt) substituted derivatives Bt2BQ; see ref 10c.
(
in DMF and DMSO (see the Supporting Information).
(20) DFT calculations were performed with the Gaussian03 program
Revision C.02, Frisch, M. J. et al. Gaussian Inc.: Wallingford CT, 2004;
full listing of authors is in the the Supporting Information). Geometries
and electronic properties were computed by means of the hybrid density
functional B3LYP with the basis set of 6-31G(d). The input files and orbital
representations were generated with Gaussview. Excitation data were
calculated by using TD-DFT (B3LYP) methods.
3
2
emission of the derivatives with NO
in the 5-position is blue-shifted relative to the parent
compound 3-H, while the electron-donating groups C
NMe and C OMe lead to a strongly red-shifted emission.
A comparison of the organoboron compounds with related
2 6 5
, Bpin, and C F groups
3
6
4
H -
(
2
6 4
H
19) The emission spectra showed additional longer wavelength bands
derivatives reported in the literature7 shows that with
b,c
AlQ
the exception of 3-C
quinolate chromophores is blue-shifted by up to 20 nm (Table
3
(
6
H
4
NMe the emission from the boron
2
1
7
10b
1
). Such a blue-shift has been first reported by Wang
2
for Ph BQ, but the observation that this trend holds for
Org. Lett., Vol. 8, No. 23, 2006
5229

2
). In contrast, the bathochromic shift experimentally ob-
served for 3-Cl can be attributed to lowering of the LUMO
level rather than elevation of the HOMO. Different factors
also impact the position of the frontier orbitals of the
complexes, which feature electron-withdrawing substituents
6 5
in the 5-position. While for 3-C F the HOMO-LUMO gap
remains essentially identical with that of 3-H due to lowering
of both the HOMO and LUMO levels, for 3-Bpin a slight
increase in the HOMO-LUMO gap is due to more pro-
nounced elevation of the LUMO than the HOMO. The latter
is possibly a result of overlap between the organic π-system
and the tricoordinate boron center of the Bpin moiety in the
LUMO (Figure 1), a phenomenon that is commonly observed
for triarylborane species (note that the Bpin moiety is a
2
1
2 1
σ-donor/π-acceptor). Excitation of 3-NO to S is very
weak (f ) 0.005) and corresponds to charge transfer from
orbitals that are centered on the B-bound phenyl rings
(
HOMO, HOMO-1) to the quinolato-centered LUMO orbital.
The relatively strong transition to the singlet excited S state
f ) 0.181) occurs from orbitals with contributions of the
B-bound Ph rings and the phenolate moiety (HOMO-3,
HOMO-2) to primarily Q-NO centered orbitals (LUMO,
5
(
2
Figure 1. Computed orbitals for selected compounds 3 (contour
value ) 0.03).
2
2
LUMO+1).
In conclusion, we demonstrate that direct reaction of Me-
protected hydroxyquinoline derivatives with bromoboranes,
a π-π* (HOMO to LUMO) transition for all complexes
2
Ar BBr, can be applied advantageously to the synthesis of a
except for 3-NO
2
.
series of new organoboron quinolate derivatives. Their
emission color has been tuned systematically to range from
blue to red. Particularly interesting is that attachment of a
pinacolborane moiety in 3-Bpin leads to a new efficient blue
emitter with high quantum efficiency of 39%. DFT calcula-
tions provide insight into the effects of 5-substituents on the
HOMO and LUMO levels. Most notably, extended conjuga-
The frontier orbitals are (apart from 3-NO
2
) primarily
situated on the quinolato moiety and do not show a
significant contribution from the boron atom. The HOMO
levels show significant contributions of the pendent substit-
uents at the 5-position, an effect that is especially pronounced
6 4 6 4 2
for 3-C H OMe and 3-C H NMe (Figure 1). The latter is
consistent with recent reports on complexes with 5- and
6 4 6 4 2
tion in the HOMO of 3-C H OMe and 3-C H NMe leads
-substituents with extended delocalization.1
0c,13
Extended
7
to lowering of the energy gap, while overlap of the empty p
orbital of boron with the organic π system in 3-Bpin is likely
responsible for the observed higher energy gap.
conjugation leads to elevation of the HOMO levels and thus
a bathochromic shift of the excitation wavelength (Figure
Acknowledgment is made to the National Science Foun-
dation (CAREER award CHE-0346828 to F.J. and instru-
mentation grant MRI-0116066), the donors of The Petroleum
Research Fund, administered by the American Chemical
Society, and DuPont for support of this research. F.J. is an
Alfred P. Sloan research fellow.
Supporting Information Available: Synthetic procedures
and data for compounds 3 and details of the DFT calcula-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL0619664
Figure 2. Calculated HOMO and LUMO energies of complexes
(
21) (a) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574-
585. (b) J a¨ kle, F. Boron: Organoboranes In Encyclopedia of Inorganic
Chemistry, 2nd ed.; King, R. B., Ed.; Wiley: Chichester, UK, 2005.
3
2
(for 3-NO additional orbital levels involved in the lowest energy
4
excitations are provided).
(22) Nitro-substituted AlQ3 is weakly emissive: see refs 8b and 9b.
5230
Org. Lett., Vol. 8, No. 23, 2006