Twofold terminal post-functionalization of acetylacetone with hole- and electron-transporting fragments

Article abstract of DOI:10.1016/j.tetlet.2010.06.090

A modular synthetic methodology has been developed to prepare β-diketones functionalized with hole- and electron-transporting fragments at their two termini. The optical and electrochemical properties of these new β-diketones are also described in detail.

Full text of DOI:10.1016/j.tetlet.2010.06.090

Tetrahedron Letters 51 (2010) 4612–4616
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Twofold terminal post-functionalization of acetylacetone with hole- and
electron-transporting fragments
a
a
Lingcheng Chen a,b, Junqiao Ding a, Yanxiang Cheng a, Lixiang Wang a, , Xiabin Jing , Fosong Wang
*
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China
b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 March 2010
Revised 8 June 2010
Accepted 18 June 2010
Available online 22 June 2010
A modular synthetic methodology has been developed to prepare b-diketones functionalized with hole-
and electron-transporting fragments at their two termini. The optical and electrochemical properties of
these new b-diketones are also described in detail.
Ó 2010 Elsevier Ltd. All rights reserved.
Acetylacetone (acac) derivatives are extremely attractive for or-
ganic chemists,1 for they play a very important role as building
blocks for construction of a broad diversity of natural products,
including aromatics and heterocycles.2 At the same time, they
are versatile ligands for numerous metallocomplexes.3 Among
them, europium (Eu), terbium (Tb), platinum (Pt) and iridium (Ir)
complexes are extensively used as triplet emitters in organic
light-emitting diodes (OLEDs) nowadays.4 In order to improve their
charge transporting ability, hole- and electron-transporting frag-
ments have been introduced into the acac platform.5 For instance,
Bazan and coworkers reported a simple single-layer OLED with an
external quantum efficiency of 0.08% using the complex tris[1-(N-
ethyl-carbazolyl)(3,5-hexyloxybenzoyl)methane](phenanthroline)-
europium.5a Here, the phenanthroline ligand acted as the
electron-transporting medium, and an attached carbazole was
used for the hole transport. And in 2001, an oxadiazole-function-
lized b-diketonate ligand was synthesized and the Tb complex
was prepared.5b The current density of its OLED increased from
1.3 to 275 mA/cm2 at a driving voltage of 20 V compared with
the unsubstituted terbium tris(acetylacetonate).
In addition, we have developed bifunctional phosphorescent Ir
dendrimers with a ‘self-host’ feature, in which the core acts as
the emissive dopant and the dendron plays the same role as the
host.6 In view of the ease of synthesis in a large scale, the hetero-
leptic green Ir dendrimer with acac as the ancillary ligand was pre-
pared.7 Nonetheless, the nondoped device gave an efficiency of
25.8 cd/A, lower than that of the corresponding homoleptic dendri-
mer (34.7 cd/A). Aiming at high performance of the nondoped de-
vices, it is desirable to link acac ligand with carbazole dendrons to
further prevent the intermolecular interactions between the emis-
sive cores.
usually prepared via the typical Claisen condensation reaction8
from ketones and esters (Fig. 1, Method A),5a,c–e which means
tedious multiple synthetic and purified steps when different func-
tional groups are incorporated. Therefore, the development of a
modular synthetic methodology will be a keystone for acac deriv-
atives in optoelectronic application, for it is believed to allow flex-
ibility in varying the functionality in molecules with relative ease.
In this regard, a series of novel b-diketones, D1-BCzacac–D3-BCza-
cac and BOXDacac, were synthesized in a modular fashion. Their
photophysical and electrochemical properties are also presented
in this study.
As for acac, both
less, -functionalization may affect the basicity and thus the liga-
a- and c
-positions can be modified.1 Neverthe-
a
tion capability to metal ions of acac, and some compounds are
unstable.9 Thus, our basic strategy is based on the direct post-func-
tionalization of acac at its
c-positions (Fig. 1, Method B). As indi-
cated in Figure 1,
c
-alkylation1 is used to prepare the key
intermediate RG-acac-RG with two functionalities at its termini,
which can further react with hole- and electron-transporting frag-
ments. As a result, such functional entities and the acac skeleton
are combined via non-conjugated bond so as to weaken the
electronic coupling between them, and thus ensure their relative
independence, which is a favorable feature for optoelectronic
materials.
It is known that monoalkylations10 of dipotassioacetylacetone
with molecular equivalents of alkyl halides in liquid ammonia
are accompanied by appreciable amounts of dialkylation, whereas
those of disodio- and dilithioacetylacetone are not.11 However, two
successive monoalkylic reactions can provide dialkylic products
through dianions, such as disodio salts.12 This discovery promotes
us to attempt the one-pot dialkylation of acac via trianions.13 To
this end, we first explored the synthesis of 1 by a similar route
(Scheme 1). After treatment of acac with 1 equiv NaH and 2 equiv
n-BuLi, trianion acac3À was formed, and then reacted with 2 equiv
butyl bromide to successfully afford bialkylic product 1 in a mod-
erate yield of 31%. It is worthy noting that, during the reaction,
Because of these motivations, the modification of the acac
framework becomes intriguing. However, these compounds are
* Corresponding author. Tel.: +86 431 85262108; fax: +86 431 85685653.
E-mail address: lixiang@ciac.jl.cn (L. Wang).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.090

L. Chen et al. / Tetrahedron Letters 51 (2010) 4612–4616
4613
Figure 1. The synthetic routes of b-diketones containing hole- and electron-transporting moieties via the typical Claisen condensation (Method A) and direct post-
functionalization (Method B). HT: hole-transporting moiety; ET: electron-transporting moiety.
sponding monosodio salt instead of 4, which was obtained by the
addition of 4 to 1 equiv NaH in THF solution. Based on the standard
nucleophilic substitution conditions,15 as presented in Scheme 2,
we succeeded in performing the reactions of the monosodio salt
of 4 with the first, second and third generation carbazole dendrons
to provide D1-BCzacac (80%), D2-BCzacac (73%) and D3-BCza-
cac(40%), respectively. Likewise, BOXDacac bearing electron-
transporting oxadiazole unit was achieved by Williamson ether
reaction16 in a yield of 70%.
Structures of D1-BCzacac–D3-BCzacac and BOXDacac were
confirmed by 1H NMR, 13C NMR, mass spectrometry, and elemental
analysis. From their 1H NMR spectra, all the synthesized com-
pounds show the characteristic enolic proton absorptions located
at around d 15.42–15.55 ppm. Moreover, the signals appeared at
d 5.34–5.52 and 3.44–3.60 ppm can be ascribed to the
a-position
protons in enol and keto forms, respectively. By comparing the
integration of d 5.34–5.52 with 3.44–3.60 ppm,17 the relative ratios
of the enol tautomer can be estimated to be more than 80%. This
indicates that b-diketones are predominantly enolized in most or-
ganic solvents, which is in accordance with the literature.18
Absorption and emission spectra of D1-BCzacac–D3-BCzacac
with respect to the corresponding carbazole dendrons were re-
corded in dichloromethane (Fig. 2), and the spectral characteristics
are summarized in Table S1 (Supplementary data). As for D1-BCza-
cac, the intensity of the absorption band at 268 nm is significantly
higher than that of D1. This is understandable, for the acac frag-
ment exhibits absorption property with kmax around 270 nm
(Fig. S1, Supplementary data). Furthermore, it is noteworthy that
the lowest-energy absorption band in the range of 310–370 nm
is red-shifted by about 14 nm from D1 to D1-BCzacac. Correspond-
ingly, a red shift of 4 nm for the emission maximum is observed
accompanied by the spectral broadening. These observations illus-
trate that the aggregation exists in D1-BCzacac, which may result
from the intermolecular hydrogen bonds. As discussed above, the
enol tautomer is the predominant form in solutions. Consequently,
intra- and intermolecular hydrogen bonds can be formed, between
which the intermolecular hydrogen bonds are further able to in-
duce the aggregation of the carbazole dendrons at the focal points.
Similar phenomenon has been found previously that hydrogen
bonding in 1,3-diketones has effect on crystal packing processes
by the formation of so-called ‘beta-chains’.19 Interestingly, this
aggregation effect can be completely prohibited when the size of
the attached dendron increases. As evidenced by D3-BCzacac, the
Scheme 1. Dialkylation of acetylacetone with alkyl bromides.
monoalkylic compound was also accompanied, but 1 could be
readily separated by distillation under reduced pressure.
Next, we extended this one-pot reaction to synthesize the bro-
mo-substituent target molecule 4. To avoid the formation of the
by-products, 1,4-dibromobutane was protected by phenol at one
terminus to give the molecule 2 (Scheme 1). Then reactions were
carried out between 2 and the trianion of acac prepared according
to the same procedure as 1 to obtain 3 with a yield of 28%. Notably,
3 is a solid at room temperature, whose melting point is about
63 °C, and can be easily isolated from the monoalkylic compound
by use of recrystallization along with flash chromatography. Quan-
titatively, 3 could be converted to 4 in the presence of BBr3.14
Unfortunately, the followed functionalization directly using 4 as
the electrophilic reagent failed. This was probably due to the reac-
tivity of acac moiety under the basic conditions. Therefore, an
alternative strategy investigated was the adoption of the corre-

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L. Chen et al. / Tetrahedron Letters 51 (2010) 4612–4616
Scheme 2. Synthesis of b-diketones functionalized with hole- and electron-transporting moieties.
absorption and emission spectra are almost identical to those of D3.
This is well correlated to the downfield shift of the enolic proton.
For example, its chemical shift gradually increases from
15.42 ppm of D1-BCzacac to 15.55 ppm of D3-BCzacac, indicative
of the disrupted intermolecular hydrogen bonds with the increas-
ing generation number.17
When going from D1-BCzacac to D2-BCzacac, both the absorp-
tion onsets and the emission maxima display an obvious batho-
chromic shift, owing to the enlargement of the conjugation
length. As listed in Table S1 (Supplementary data), the optical band
gap taken from the absorption onset decreases from 3.42 eV of D1-
BCzacac to 3.14 eV of D2-BCzacac, and the emission maximum is
red-shifted by about 33 nm. In contrast, from D2-BCzacac to D3-
BCzacac this bathochromic effect is almost negligible, and both
the absorption and photoluminescence (PL) spectra of D3-BCzacac
are nearly the same as those of D2-BCzacac. The results demon-
strate that the delocalization of the electronic states is mainly lim-
ited to the second generation carbazole dendrons, and does not
extend obviously to the third generation ones.
Figure 2. The absorption and photoluminescence spectra of D1-BCzacac–D3-
BCzacac compared with the corresponding carbazole dendrons (10À5 M in CH2Cl2
solution).

L. Chen et al. / Tetrahedron Letters 51 (2010) 4612–4616
4615
and D2-BCzacac show reversible oxidation waves with an onset
potential of 0.48 and 0.38 V, respectively. And for D3-BCzacac,
multiple irreversible oxidation peaks together with an onset of
0.39 V are observed. Upon going from D1-BCzacac to D2-BCzacac
and D3-BCzacac, we note that, the onset potential firstly moves to-
wards a negative direction, and then remains unchanged. This var-
iation is corresponding to the extension of the p-conjugation with
the increasing generation number. In comparison to D1-BCzacac,
the HOMO energy levels for D2-BCzacac and D3-BCzacac was en-
hanced by about 0.1 eV, indicating the lower hole-injection barrier.
Simultaneously, the photophysical and electrochemical proper-
ties of BOXDacac were investigated in dichloromethane and aceto-
nitrile. As indicated in Figure 4, BOXDacac shows
a major
absorption band at 298 nm and a maximum emission peak at
363 nm together with a high optical band gap of 3.70 eV. During
anodic and cathodic scans, only irreversible reduction waves were
found (Fig. 4). Based on the onset of the reduction potential
(À2.81 V), its LUMO energy level was calculated to be À1.99 eV.
And combined with the optical band gap, the HOMO energy level
could be estimated to be À5.69 eV.
Figure 3. Cyclic voltammograms of compounds D1-BCzacac–D3-BCzacac. Mea-
sured in dichloromethane (5.0 Â 10À4 M). Scan rate: 100 mV sÀ1
.
The dependence of the conjugation length on the generation
number20 also affects their electrochemical properties. The electro-
chemical properties of these materials in CH2Cl2 (5.0 Â 10À4 M)
were studied in a three-electrode electrochemical cell with Bu4N-
ClO4 (0.1 M) electrolyte, Pt wire counter electrode and Ag/AgCl ref-
erence electrode. Due to the different chemical environment of the
N-heterocycles in the carbazole units, D1-BCzacac show only one
reversible redox wave while D2-BCzacac and D3-BCzacac dis-
played multi-redox waves. As shown in Figure 3, D1-BCzacac
In summary, we report a modular synthesis of novel b-dike-
tones containing hole-transporting carbazole dendrons and elec-
tron-transporting oxadiazole units. In solutions, the enol
tautomer is the predominant form, and the formed intermolecular
hydrogen bonds can cause the aggregation of the carbazole den-
drons at the focal points. With increasing generation number, how-
ever, the aggregation effect can be entirely suppressed. Further
incorporation of these compounds as the ancillary ligands for the
Ir complexes in optoelectronic application is currently under way
in our laboratory.
Acknowledgments
The authors are grateful to the National Natural Science Foun-
dation of China (No. 20923003), Science Fund for Creative Research
Groups (No. 20621401), and 973 Project (2009CB623601) for the
financial support of this research.
Supplementary data
Supplementary data (synthetic details and spectroscopic char-
acterizations for new compounds) associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.2010.06.090.
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