Synthesis and photophysical properties of two strongly fluorescent bis(diquinaldinatoalumino)-9-silafluorenes

Article abstract of DOI:10.1021/om4005803

Two novel highly fluorescent blue light emitting compounds were synthesized: 9,9-bis(diquinaldinatoalumino)-1,3-diphenyl-9-silafluorene (4) and 9,9-bis(diquinaldinatoalumino)-9-silafluorene (7). Combining silafluorene and quinaldinate aluminum moieties re

Full text of DOI:10.1021/om4005803

Communication
pubs.acs.org/Organometallics
Synthesis and Photophysical Properties of Two Strongly Fluorescent
Bis(diquinaldinatoalumino)-9-silafluorenes
Erika Pusztai,*,† Haley Albright,† Yuanjing Cai,† Rongrong Hu,‡ Robert West,† Ben Zhong Tang,‡
and Ilia A. Guzei†
†Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, United States
‡Department of Chemistry and State Key Laboratory of Molecular Neuroscience, Institute of Molecular Functional Materials, The
Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: Two novel highly fluorescent blue light
emitting compounds were synthesized: 9,9-bis-
(diquinaldinatoalumino)-1,3-diphenyl-9-silafluorene (4) and
9,9-bis(diquinaldinatoalumino)-9-silafluorene (7). Combining
silafluorene and quinaldinate aluminum moieties resulted in
molecules with high quantum yield efficiency both in solution
(31−34%) and in the solid state (79−92%). 4 showed intense
electroluminescence, 4090 cd/m2 at 15 V, and the
mechanochromism of 7 was revealed.
ilafluorenes are π-conjugated organosilicon compounds
dinate (iBuAlQ2), can be synthesized according to a known
Swith remarkable optical and electronic properties. Due to method;14 9,9-dichloro-1,3-diphenyl-9-silafluorene (2)4 and
9,9-dichloro-9-silafluorene (5)3 are known as well. The starting
their special electronic structure they can be applied in electron
materials 9,9-dihydroxy-1,3-diphenyl-9-silafluorene (3) and 9,9-
dihydroxy-9-silafluorene (6) were prepared by hydrolysis of 2
and 5, respectively, similarly to some silole derivatives15 (see
the Supporting Information). 9,9-Bis(diquinaldinatoalumino)-
1 , 3 - d i p h e n y l - 9 - s i l a fl u o r e n e ( 4 ) a n d 9 , 9 - b i s -
(diquinaldinatoalumino)-9-silafluorene (7) were formed in
the reactions of iBuAlQ2 (1) with 3 and 6, as depicted in
parts a and b of Scheme 1.
transporters and emitters for fabrication of organic light
emitting diodes (OLEDs) and photovoltaic cells.1,2 Several
kinds of silafluorene derivatives have been synthesized in order
to obtain blue-emitting compounds, oligomers, or polymers.3−5
Some silafluorene complexes have also been reported,6 but the
combination of another fluorescent moiety with silafluorenes in
one molecule seems to be novel. Here we provide the synthesis
and optical characterization of two blue light emitting
silafluorene derivatives attached to diquinaldinato aluminum
groups.
Tris(8-hydroxyquinolinato)aluminum (Alq3) is a favored
material for fabrication of electroluminescent devices as an
electron transporter and emitter.7 It is stable and emits green
light with good quantum efficiency in the solid state. Alq3 can
be transformed into blue- or white-emitting compounds by
doping or introducing different substituents.8,9 Tris(2-methyl-8-
oxyquinolinato)aluminum (AlQ3) showed a significant blue
shift in the emission spectrum (λmax 479 nm) and increased
emission intensity.10,11 AlQ3 is not stable in air; thus, (μ-
oxo)bis(bis(2-methyl-8-quinolinolato)aluminum(III))
(Q2Al(μ-O)AlQ2) was also investigated and found to be very
similar to AlQ3 in its optical properties; it shows the same blue
shift due to methyl substitution.12,13
1
Silafluorene complexes 4 and 7 were identified by H NMR,
13C NMR, 29Si NMR, X-ray crystallography analysis, and mass
spectra. Crystal structures of 4 and 7 are presented in Figures 1
and 2, respectively. We found two stereoisomers of 7, which are
shown in Figure 2, but we could not separate them; thus, all
results in this paper refer to the isomer mixture.
Q2Al(μ-O)AlQ2 exhibits an absorption band at 365 nm and a
fluorescence emission at 485 nm, while Alq3 has its character-
istic absorption band at 395 nm and fluorescence emission at
517 nm. Methyl substitution caused a significant blue shift in
both the absorption and emission spectra.
1,3,9,9-Tetraphenyl-9-silafluorene has an absorption band at
330 nm and an emission maximum at 368 nm; the quantum
yield efficiency in CH2Cl2 solution is 17% while it is 42% in
solid state.4 The absorbance of its unsubstituted analogue 9,9-
diphenyl-9-silafluorene ranges around 310 nm, and the
emission is at 348 nm with 10% quantum yield efficiency in
CH2Cl2 solution and 16% in the solid state.4 It was revealed
that phenyl substitution at positions 1 and 3 on the silafluorene
For high-performance OLED applications a large solid-state
quantum yield is very beneficial. Our objective was to provide
compounds that show high fluorescence quantum yield without
the need for doping, which would simplify the final fabrication
process of electronic devices. This objective was approached by
combining two blue-emitting moieties, diquinaldinatoalumi-
num and silafluorene. Compound 1, isobutylaluminodiquinal-
Received: June 19, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4005803 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
For compounds 4 and 7 the absorbance at about 250−260
nm is the most intense. This range belongs to the absorption of
the phenyl rings. The second most significant peaks range
around 350−360 nm, and they could be used for excitation. In
comparison to Alq3 or to Q2Al(μ-O)AlQ2 their absorbance is
blue-shifted (see the Supporting Information), but in
comparison to the analogous silafluorene molecules, a
significant red shift occurred.
Scheme 1. Synthetic Route for the Synthesis of 9,9-
Bis(diquinaldinatoalumino)-1,3-diphenyl-9-silafluorene (4)
and 9,9-Bis(diquinaldinatoalumino)-9-silafluorene (7)
The fluorescence emission of 4 and 7 is also blue-shifted in
comparison to Alq3 and Q2Al(μ-O)AlQ2 (Figure 3) and red-
shifted in comparison to the analogous silafluorenes.
Figure 3. Fluorescence spectra of 4 and 7 compared to those of Alq3
and Q2Al(μ-O)AlQ2 (10 ppm solutions in THF).
Single peaks on all spectra indicate that in the combined
molecules, 4 and 7, new chromophores were created. They do
not have distinguishable absorption or emission, but the
silafluorene and the aluminoquinaldinate moieties interact and
enhance each other’s emission. According to the excitation
spectra (see the Supporting Information) the aluminoquinaldi-
nate fragment is dominant in the emission in solution, but in
the solid state this dominance might disappear, as is indicated
by the exceptionally high quantum yield efficiencies and the
blue shift of the fluorescence maxima.
Figure 1. Single-crystal structure of 4 determined by X-ray diffraction
(XRD). The displacement ellipsoids are drawn at the 50% probability
level. All H atoms have been omitted for clarity.
When 4 and 7 are compared to previously reported silole−
aluminoquinaldinate compounds,15 the major differences
appear in solution. While the silole complexes are barely
emissive in organic solvents, 4 and 7 are good fluorescing
compounds. On the other hand, 4 and 7 do not show
aggregation-induced emission (AIE) in water−THF solution
like siloles, but they do not show aggregation-caused quenching
(ACQ), either.
Fluorescence quantum yield efficiency (ΦF) was investigated
in THF with quinine sulfate in 0.1 N sulfuric acid (H2SO4) as
the standard.16 Both compounds 4 and 7 exhibit high ΦF values
in solution, and they are even more enhanced in the solid state
(see Table 1). When the silafluorene and the aluminoquinaldi-
nate moieties are blended, enhanced fluorescence efficiency can
be observed also in solution. The outstanding solid-state
fluorescence quantum yield efficiency can be attributed to the
structure of these complexes: none of these compounds are
planar, and therefore they do not have π−π stacking
Figure 2. Single-crystal structures of the two stereoisomers of 7
determined by X-ray diffraction (XRD). The displacement ellipsoids
are drawn at the 50% probability level. All H atoms have been omitted
for clarity.
interactions in solid state, which usually causes ACQ.17
4
displays an extremely high quantum yield of 92%, in
comparison to 35% for Alq3 and 24% for Q2Al(μ-O)AlQ2.
This even more enhanced ΦF in comparison to that for 7 can
be explained by the restricted intramolecular rotation (RIR) of
the phenyl groups. On comparison of 4 and 7, phenyl
ring causes a significant red shift in both absorption and
emission spectra, and also the fluorescence quantum yield
efficiency was improved.4
B
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Communication
Table 1. Photophysical Properties of Silole Complexes 4 and
a
7 in Comparison to Those of Alq3 and Q2Al(μ-O)AlQ2
λem
λem(solid)
compd
λab (nm)
(nm) ΦF(soln)
(nm)
ΦF(solid)
b
4
7
250; 350
476
476
485
0.34
0.31
0.24
470
461
485
0.92
0.79
0.24
b
260; 360
Q2Al(μ-O)
AlQ2
250; 312;
b
365
b
Figure 6. Mechanochromism of 7. Solid-state emission before (A) and
after grinding (B) and then after fuming by acetone vapor (C). The
photographs were taken under 365 nm UV irradiation.
Alq3
260; 395
517
445
0.11
0.54
512
0.35
b
quinine
sulfate
347
a
Abbreviations: λab = absorption maximum in THF solution, λem
=
emission maximum in THF solution, ΦF(soln) = fluorescence
quantum yield in THF solution determined using quinine sulfate in
0.1 N H2SO4 as a standard, λem(solid) = emission maximum in the
solid state, ΦF(solid) = fluorescent quantum yield of solid powder
determined by a calibrated integrating sphere. Excitation wavelength:
b
325 nm. Excitation wavelength in solution.
substitution has a positive effect on ΦF while it does not
influence the emission range and induces a blue shift of the
maximum only in the solid state (Figures 4 and 5).
Figure 7. Photoluminescence spectra of a solid powder of 7 before and
after grinding. Excitation wavelength: 341 nm.
Figure 4. Fluorescing microcrystals (from left to right): Alq3, Q2Al(μ-
O)AlQ2, 4, 7.
4090 cd/m2 (for Alq3: 520 nm and 19500 cd/m2). Its onset
voltage was 4.8 V, the peak external quantum efficiency 1.07%,
and the current efficiency 2.92 cd/A at 39.2 mA/cm2 current
density (for Alq3: 3.0 V, 1.6%, 5.13 cd/A at 8.74 mA/cm2) (see
the Supporting Information.) Though these characteristic
values are less advantageous than those of Alq3, with device
optimization OLEDs made of these novel compounds can be
improved. Due to the exceptionally high solid-state fluores-
cence results other applications will be the subject of further
investigations as well.
In summary, we have prepared two new highly fluorescent
diquinaldinatoalumino−silafluorene complexes. The combina-
tion of strong emitters and electron transporters, silafluorenes
and aluminoquinaldinate, in one molecule has led to an intense
blue emission with a 2.6-fold increase of fluorescence efficiency
in the solid state in comparison to the standard Alq3.
Mechanochromic behavior of 7 was also revealed. An organic
light-emitting device with the structure ITO/NPB/4/TPBi/
LiF/Al was fabricated, which emits greenish blue light at 512
nm with luminance of 4090 cd/m2 at 15 V. Thus, these
silafluorene−aluminoquinaldinate compounds appear to be
suitable candidates for use in organic electronics.
Figure 5. Fluorescence spectra of 4 and 7 compared to Alq3 and
Q2Al(μ-O)AlQ2 in the solid state.
In the preparation of samples for the solid-state fluorescence
measurements, the mechanochromism of 7 was observed and
investigated further. Crystalline 7 had its emission maximum at
461 nm; after grinding it became slightly greenish with λmax 481
nm. When the powder was treated with acetone vapor, the
emission reverted to the original blue color (Figures 6 and 7).
The color change might occur due to very small changes in the
crystal lattice, as the powder X-ray diffraction (XRD) data of 7
before and after grinding do not show significant differences
(see the Supporting Information).
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving detailed experimental
procedures and characterization data for the products and
crystallographic data for 4 and 7. This material is available free
of charge via the Internet at http://pubs.acs.org. The crystal
structures are available free of charge from the Cambridge
Crystallographic Data Center; the CCDC codes are CCDC-
Finally a light-emitting device with the structure ITO/NPB/
4/TPBi/LiF/Al was fabricated. At 15 V this device emitted
greenish blue light at 512 nm with a maximum luminance of
C
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Organometallics
Communication
942623 for compound 4 and CCDC-942624 and CCDC-
966617 for compound 7.
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.P.: pusztaierika@hotmail.com.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Prof. Hoi Sing Kwok and Yibin Jiang for
providing technical support for the EL measurements and the
Rosztoczy Foundation for their support.
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