Synthesis of highly fluorescent diquinaldinatoalumino silole derivatives

Article abstract of DOI:10.1002/chem.201300728

Two-in-one fluorescent compounds: Blending the strong emitter, diquinaldinato aluminum, and the good electron transporter, tetraphenyl silole, resulted in highly fluorescent compounds that are promising candidates for light-emitting devices without the ne

Full text of DOI:10.1002/chem.201300728

DOI: 10.1002/chem.201300728
Synthesis of Highly Fluorescent Diquinaldinatoalumino Silole Derivatives
Erika Pusztai,[a] Seunghyun Jang,[a] Irina S. Toulokhonova,[a] Ilia A. Guzei,[a]
Robert West,*[a] Rongrong Hu,[b] and Ben Zhong Tang[b]
Silicon-containing p-conjugated compounds, especially si-
loles, have recently received significant attention because of
their unusual optical and electronic properties.[1–4] In siloles,
s*–p* conjugation between the s* orbital of the silicon
atom and the p* orbital of the butadiene fragment leads to
a low LUMO energy level and a small HOMO–LUMO gap.
Siloles can be useful as electron-transporting materials in
electronic devices,[5] including light-emitting diodes
(LEDs).[6–8] Many silole derivatives have been obtained re-
cently by simple chemical reactions,[9] but there are few at-
tempts to develop silole derivatives displaying blue lumines-
cence. Herein, we report the synthesis of two new highly flu-
orescent, blue emitting siloles with aluminoquinaldinate
groups attached to the silicon atom.
emission maximum, but this compound displayed no in-
creased emission efficiency.[15]
For high-performance OLEDs, a very large photolumines-
cence quantum yield is required.[16] To achieve this, doping
of a highly fluorescent or phosphorescent molecule (guest)
into an emitting layer (host) is the usual approach. Howev-
er, doping complicates the fabrication process, and it would
be ideal to synthesize a molecule with very high quantum ef-
ficiency and use it for fabrication of OLEDs without doping.
To increase the efficiency of fluorescence, we have de-
signed a system, in which effective electron transport and
emissive moieties are combined in one molecule, by attach-
ing an aluminoquinaldinate complex to a silole group. In the
reactions of ibutylaluminodiquinaldinate, iBuAlQ2 (1), with
1,1-dihydroxy-2,3,4,5-tetraphenylsilole (2), 1,1-bis(alumino-
diquinaldinate)-2,3,4,5-tetraphenylsilole (3) was prepared as
shown in Scheme 1a. In another reaction, iso-butylalumino-
Tris-(8-oxyquinolinato) aluminum (Alq3) is a favored ma-
terial for fabrication of electroluminescent devices as an
electron transporter and emitter.[10–11] It is stable and emits
green light with peak wavelength around lꢀ520 nm with
high quantum efficiency in the solid state. To shift the lumi-
nance of aluminum complex to the blue region, donor
groups at the positions 2 or 4 of the 8-oxyquinoline ligand
or acceptors at position 5 of the ligand have been intro-
duced.[12,13]
Tris-(2-methyl-8-oxyquinolinato)
aluminum
(AlQ3) shows not only a significant blueshift on the UV and
fluorescence spectra, but also increased fluorescence quan-
tum yield efficiency in solution.[12] Because AlQ3 is not
stable in air, its stable derivative, (m-oxo)-bis
quinolinolato)-aluminum
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
vestigated, and it showed very similar optical properties.[14]
Earlier work showed that replacing one 2-methyl-8-oxyqui-
noline in tris(2-methyl-8-oxyquinolinato)aluminum by a tri-
phenylsiloxy substituent resulted in a slight blueshift of the
Scheme 1. Synthetic route for the synthesis of 1,1-bis(diquinaldinatoalu-
mino)silole (3) and 1-methyl-1-diquinaldinatoalumino silole (5).
[a] E. Pusztai, Dr. S. Jang, Dr. I. S. Toulokhonova, Dr. I. A. Guzei,
Prof. R. West
Department of Chemistry
University of Wisconsin
Madison, Wisconsin 53706 (USA)
E-mail: west@chem.wisc.edu
diquinaldinate, iBuAlQ2 (1), with 1-methyl-1-hydroxy-
2,3,4,5-tetraphenylsilole (4), 1-methyl-1-aluminodiquinaldi-
nate-2,3,4,5-tetraphenylsilole (5) was prepared as shown in
Scheme 1b.
[b] Dr. R. Hu, Prof. B. Z. Tang
1
The silole complexes 3 and 5 were identified by H, 13C,
Department of Chemistry and
and 29Si-NMR, X-ray crystallog-
raphy, and mass spectra. The
crystal structure of 3 is present-
ed in Figure 1, whereas that of
5 is shown in Figure 2.
State Key Laboratory of Molecular Neuroscience
Institute of Molecular Functional Materials
The Hong Kong University of Science & Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300728.
8742
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8742 – 8745

COMMUNICATION
Q2Al-mO-AlQ2 exhibited an
absorption band at l=365 nm,
whereas Alq3 has its character-
istic absorption band at 395 nm,
and a similar difference was ob-
served in the fluorescence spec-
tra, because Alq3 emits at l=
517 and Q2Al-mO-AlQ2 at
485 nm. Silole complex 3 exhib-
ited UV absorption maxima at
l=258 and at 365 nm, and
silole complex 5—at 258 and
370 nm, as shown in Figure 3.
For comparison, 1,1-dimethyl-
2,3,4,5-tetraphenylsilole and 1-
Figure 3. UV absorption spectra of 3 (c) and 5 (d) compared to
Alq3 (g) and Q2Al-mO-AlQ2 (a); 10 ppm solutions in THF.
methyl-1,2,3,4,5-pentaphenylsilole exhibited an absorption
band at l=360 and 370 nm, respectively, due to a p–p*
transition of the silole ring.[17,18] On this basis, we assigned
the 365 and 370 nm bands to a similar p–p* transition of the
silole group that overlaps with the absorption band of the 2-
methyl-8-oxyquinaldinate.
The fluorescence spectrum of quinaldinate complexes 3
and 5 are shown in Figure 4 (bottom). The emission band is
a broad band centered at lem =478 nm, strongly blueshifted
compared to 1,1-dihydroxy-2,3,4,5-tetraphenylsilole (2; l=
520 nm) and Alq3 (l=517 nm, Figure 4, top). The blueshift
of fluorescence in some pentacoordinated aluminoquinoli-
Figure 1. X-ray diffraction single-crystal structure of 3. The displacement
ellipsoids were drawn at the 50% probability level. All hydrogen atoms
have been omitted for clarity.
Figure 2. X-ray diffraction single-crystal structure of 5. The displacement
ellipsoids were drawn at the 50% probability level. All hydrogen atoms
have been omitted for clarity.
Figure 4. Fluorescence spectra of 3 (a) and 5 (c) compared to Alq3
(c) and Q2Al-mO-AlQ2 (a); 10 ppm solutions in THF.
Chem. Eur. J. 2013, 19, 8742 – 8745
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8743

R. West et al.
the luminescence efficiency.[26] However, since Tang and co-
workers described the aggregation-induced emission (AIE)
phenomena of siloles,[16] many silole derivatives have been
found to exhibit unusual AIE properties.[27] We have also in-
vestigated the AIE of silole complexes 3 and 5 as shown in
Figure 5. The concentration for 3 and 5 for the fluorescence
À
nates has been attributed to weakening of Al N coordina-
tion bond in these complexes.[19] However, from our struc-
tural data for pentacoordinated aluminoquinolinate com-
plexes,[20] we suggest that the blueshift may arise from a
donor effect at the C-2. La Deda and co-workers calculated
À
that shortening of the Al O bond will increase energy gap
between HOMO and LUMO of the 2-methylquinolin-8-
olate chelants leading to blue emission.[21] Compounds 3 and
5 behave like new chromophores, which show the combined
property of the quinaldinate and the silole moiety, but ac-
cording to the shape and the emission maximum, silole
might take a leading effect in this combined fluorescent be-
havior.[17,18]
Efficiency of emission was investigated in THF with qui-
nine sulfate in H2SO4 (0.1 n) as a standard.[22] Silole com-
plexes 3 and 5 exhibited low emission efficiency <1% in
solution. The solid-state quantum yield was determined with
an integrating sphere according to the method described by
Mello et al.[23] Compound 3 displayed an extremely high
quantum yield of 95%, compared to 35% for Alq3, and
24% of Q2Al-mO-AlQ2, but also 5 exhibited 81% quantum
yield.
Furthermore, light-emitting devices[24] with the structure
ITO/NPB/3/TPBi/Alq3/LiF/Al and ITO/NPB/5/TPBi/LiF/Al
were fabricated (ITO=indium tin oxide; NPB=N,N’-bis
(naphthalen-1-yl)-N,N’-bisACHTNUTRGNE(UNG phenyl)benzidine; TPBi=1,3,5-
tris(N-phenylbenzimidazol) benzene). These devices emitted
greenish-blue light at l=520 and 516 nm with the maximum
luminance of 1770 and 5530 cdmÀ12, respectively, at 15 V.
Compound 5 showed especially good external quantum effi-
ciency 2.75% and outstanding current efficiency 7.89 cdAÀ1
(Table 1). Both devices had the onset voltage at 4.2 V. De-
tails of the solid-state quantum yield and electrolumines-
cence are described in the Supporting Information.
Table 1. Photophysical properties of silole complexes 3 and 5 compared
to Alq3 and Q2Al-mO-AlQ2.[a]
Figure 5. a) Fluorescence spectrum of 3 aggregates in water/THF mix-
tures and b) plot of the relative intensity of 3 aggregates versus vol%
water. c) Fluorescence spectrum of 5 aggregates in water/THF mixtures
and d) plot of the relative intensity of 5 aggregates versus vol% water.
Complex
lab
loge
lem
FFsoln FFsolid
[nm]
[dm3 molÀ1 cmÀ1
]
[nm]
3
5
365
370
365
4.09 (4.08)
3.98
3.66 (3.64)
478
485
485
0.01
0.01
0.24
0.95
0.81
0.24
Q2Al-mO-
AlQ2
Alq3
quinine
sulfate
measurements was 10 mgLÀ1 (10 ppm) in THF. Aggregate
solutions were prepared by rapid injection of THF solutions
containing 3 or 5 to water/THF solutions. Figure 5b and d
show plots for the relative PL intensity of silole complexes
vs% water by volume. Silole-complex aggregates exhibited
a nearly identical emission band, but were more strongly flu-
orescent than the non-aggregated silole complexes. The two
complexes behave somewhat differently; for 3 aggregation
begins at 60% water concentration, whereas for 5 aggrega-
tion begins only at 80% water and then rises exponentially.
Probably, this difference reflects the lower steric hindrance
and greater solubility of 5.
395
347
3.77 (3.66)
3.70
520
445
0.11
0.54
0.35
–
[a] Abbreviations: lab =absorption maximum in THF solution; e=molar
absorption coefficient at lab and at 370 nm; lem =emission maximum in
THF solution, FFsoln =fluorescence quantum yields in THF solution de-
termined by using quinine sulfate in H2SO4 (0.1 n) as a standard. FFsolid
=
fluorescent quantum yield of solid powder determined by a calibrated in-
tegrating sphere.
In general, many conjugated organic emitters are highly
emissive in dilute solution, but become weakly luminescent
when fabricated into thin films due to aggregate formation
in the solid state.[25] The molecules aggregate to form less
emissive species, such as excimers, leading to a reduction in
In summary, two new highly fluorescent diquinaldinatoa-
lumino silole complexes have been prepared. Combination
of strong emitters and electron transporters, tetraphenylsi-
8744
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 8742 – 8745

Synthesis of Diquinaldinatoalumino Silole Derivative
COMMUNICATION
New York, 1998, Chapter 34; b) K. Tamao, S. Yamaguchi, Pure
Appl. Chem. 1996, 68, 139–144.
[10] C. W. Tang, S. A. Vanslyke, Apply. Phys. Lett. 1987, 51, 913–915.
[11] C. W. Tang, S. A. Vanslyke, C. H. Chen, J. Appl. Phys. 1989, 65,
3610–3616.
[12] M. Matsumura, T. Akai, Jpn. J. Appl. Phys. 1996, 35, 5357–5360.
[13] S. A. VanSlyke, P. S. Bryan, F. V. Lovecchio, U.S. Patent 5141671,
1992.
[14] a) T. Iijima, T. Yamamoto, J. Organomet. Chem. 2006, 691, 5016–
5023; b) S. A. VanSlyke, US Patent 5151629, 1992.
lole and aluminoquinaldinate, in one molecule has led to a
sky-blue emission with 2.7 fold increase of fluorescence effi-
ciency in solid state compared to standard Alq3. OLEDs
with the structure of ITO/NPB/3/TPBi/Alq3/LiF/Al and
ITO/NPB/5/TPBi/LiF/Al were fabricated; they emit green-
ish-blue light at l=520 and 516 nm with luminance of 1770
and 5530 cdmÀ12 at 15 V, respectively. Although further opti-
mization will be needed, the silole–(O-AlQ2)2 compounds
appear to be promising candidates for use in organic elec-
tronics.
[15] A. Misra, P. Kumar, S. K. Dhawan, M. N. Kamalasanan, S. Chandra,
Indian J. Pure Appl. Phys. 2005, 43, 522–526.
[16] H. Murata, Z. H. Kafafi, M. Uchida, Appl. Phys. Lett. 2002, 80,
189–191.
[17] J. Luo, Z. Xie, J. W. Lam, L. Cheng, H. Chen, C. Qiu, H. S. Kwok,
X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun. 2001, 1740–
1741.
Experimental Section
[18] B. Z. Tang, X. Zhan, G. Yu, P. P. S. Lee, Y. Liu, D. Zhu, J. Mater.
Chem. 2001, 11, 2974–2978.
[19] X. T. Tao, H. Suzuki, T. Wada, S. Miyata, H. Sasabe, J. Am. Chem.
Soc. 1999, 121, 9447–9448.
[20] I. S. Toulokhonova, I. A. Guzei, M. Kavana, R. West, Main Group
Met. Chem. 2002, 25, 489–495.
For detailed experimental procedures and characterization of products,
see the Supporting Information. CCDC-913897 (2), CCDC-913895 (3),
and CCDC-913896 (5) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
[21] M. La Deda, I. Aiello, A. Grisolia, M. Ghedini, M. Amati, F. Lelj,
Dalton Trans. 2006, 330–339.
[22] G. A. Crosby, J. N. Demas, J. Phys. Chem. 1971, 75, 991–1024.
[23] J. C. de Mello, H. F. Wittmanm, R. H. Friend, Adv. Mater. 1997, 9,
230–232.
Acknowledgements
The authors thank Professor Hoi Sing Kwok and Yibin Jiang for provid-
ing technical support to the EL measurement, Haley Albright for the
help at the syntheses, and the Rosztoczy Foundation for their support.
[24] H. Y. Chen, W. Y. Lam, J. D. Luo, Y. L. Ho, B. Z. Tang, D. B. Zhu,
M. Wong, H. S. Kwok, Appl. Phys. Lett. 2002, 81, 574–576.
[25] R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N.
Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas,
M. Logdlund, W. R. Salaneck, Nature 1999, 397, 121–128.
[26] S. A. Jenekhe, J. A. Osaheni, Science 1994, 265, 765–768.
[27] a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40,
5361–5388; b) W. Z. Wang, P. Lu, S. Chen, J. W. Y. Lam, Y. Liu,
H. S. Kwok, Y. Ma, B. Z. Tang, Adv. Mater. 2010, 22, 2159–2163;
c) M. Wang, G. Zhang, D. Zhang, D. Zhu, B. Z. Tang, J. Mater.
Chem. 2010, 20, 1858–1867; d) J. Liu, Y. Zhong, J. W. Y. Lam, P. Lu,
Y. Hong, Y. Yu, Y. Yue, M. Faisal, H. H. Y. Sung, I. D. Williams,
K. S. Wong, B. Z. Tang, Macromolecules 2010, 43, 4921–4936; e) Z.
Li, Y. Q. Dong, J. W. Y. Lam, J. Sun, A. Qin, M. Haussler, Y. P.
Dong, H. H. Y. Sung, I. D. Williams, H. S. Kwok, B. Z. Tang, Adv.
Funct. Mater. 2009, 19, 905–917; f) Z. Zhao, Z. Wang, P. Lu, C. Y. K.
Chan, D. Liu, J. W. Y. H. Lam, H. Y. Sung, I. D. Williams, Y. Ma,
B. Z. Tang, Angew. Chem. 2009, 121, 7744–7747; Angew. Chem. Int.
Ed. 2009, 48, 7608–7611; g) J. L. Mullin, H. J. Tracy, J. R. Ford, S. R.
Keenan, F. Fridman, J. Inorg. Organomet. Polym. Mater. 2007, 17,
201–213; h) S. Jang, S. G. Kim, D. Jung, H. Kwon, J. Song, S. Cho,
Y. C. Ko, H. Sohn, Bull. Korean Chem. Soc. 2006, 27, 1965–1966;
i) J. Chen, B. Xu, Y. Cao, Synth. Met. 2005, 152, 249–252.
Keywords: aluminoquinaldinates · fluorescence · OLEDs ·
silicon · siloles
[1] R. West, H. Sohn, U. Bankwitz, J. Calabrese, Y. Apeloig, T. Mueller,
J. Am. Chem. Soc. 1995, 117, 11608–11609.
[2] S. Yamaguchi, K. Tamao, J. Chem. Soc. Dalton Trans. 1998, 3693–
3702.
[3] S. Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K. Furukawa, K.
Tamao, Chem. Eur. J. 2000, 6, 1683–1692.
[4] W. W. H. Wong, J. F. Hooper, A. B. Holmes, Austral. J. Chem. 2009,
62, 393.
[5] K. Tamao, M. Uchida, T. Izumizawa, K. Furukawa, S. Yamaguchi, J.
Am. Chem. Soc. 1996, 118, 11974–11975.
[6] H. Sohn, R. R. Huddleston, D. R. Powell, R. West, J. Am. Chem.
Soc. 1999, 121, 2935–2936.
[7] Y. Xu, T. Fujino, H. Naito, T. Dohmaru, K. Oka, H. Sohn, R. West,
Jpn. J. Appl. Phys. 1999, 38, 6915–6918.
[8] S. Yamaguchi, R. Z. Jin, K. Tamao, J. Am. Chem. Soc. 1999, 121,
2937–2938.
[9] a) J. Dubac, C. Guerin, P. Meunier in The Chemistry of Organic Sili-
con Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley,
Received: February 23, 2013
Published online: May 16, 2013
Chem. Eur. J. 2013, 19, 8742 – 8745
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8745