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Y. Wu et al. / Tetrahedron xxx (2015) 1e7
hyperbranched electroluminescent polymers with the three prin-
ciple colors (red,22 green23,24 and blue18) have been synthesized,
and both emission efficiency and thermal stability were effectively
improved with respect to their linear analogies.15,25e27 However,
white-light-emitting hyperbranched polymers have rarely been
reported.
In this paper, hyperbranched fluorescence/phosphorescence
hybrid copolymers with 9,9-dioctylfluorene and bis(1-phenyl-iso-
quinoline)(acetylacetonato)iridium(III) (Ir(piq)2acac) branches and
spiro[3.3]heptane-2,6-dispirofluorene (SDF) core (10 mol %) were
constructed. The three-dimensional-structured SDF exhibits great
morphological stability and intense fluorescence,28 and further-
more, its steric hindrance can prevent rotation of the adjacent aryl
groups, which reduces close packing and intermolecular in-
teractions between the chromophores in the solid-state.29,30 In
order to obtain white-light emission, the orange light-emitting unit
Ir(piq)2acac was introduced with different contents from 0.02 mol
% to 0.05 mol %. Such a highly branched framework may provide
a highly efficient white-light electroluminescence.
absorption peak around 472 nm is mainly belonged to the spin-
inhibited metal-to-ligand charge-transfer (3MLCT) state and
ligand-centered (3LC) state transitions.31 It is obvious that the
emission of copolymer PFSDF and the absorption spectrum of the Ir
complex show good spectra overlap. Therefore, efficient FRET from
the PFSDF to the Ir complex, and white-light emission can be ex-
pected by the combination of blue light-emitting from PFSDF and
orange light-emitting from Ir(piq)2acac through finely adjusting
the content of Ir complex.
The normalized UVevis absorption and PL spectra of the co-
polymers in CHCl3 solution (10ꢂ5 mol/L) and thin film states are
shown in Fig 2. In dilute solution, all of the copolymers exhibit
typical absorption and emission bands of PF with an absorption
maximum around 373e379 nm, and emission peaks at 420, 440,
and a shoulder peak at 474 nm. The presence of vibronic structure
in the emission spectra indicates that the hyperbranched co-
polymers have rigid and well-defined backbone structures. The
absorption and emission bands of Ir(piq)2acac were not detected
due to its comparatively low content (0.02e0.05 mol %) in the co-
polymers. In dilute solution, the energy transfer was exclusively
intrachain.32
2. Results and discussion
In films, the copolymers exhibit UVevis absorption bands at
around 375 nm similar to those in dilute solution. In the PL spectra,
the maximum emission bands of the copolymers are at about 420
and 440 nm, showing no obvious bathochromic shift with respect
to those in dilute solution. This result indicates that the hyper-
branched molecular structure can prevent the aggregation and the
interaction of the copolymer chains efficiently. The emission band
of Ir(piq)2acac centered at 613 nm still can’t be observed due to the
low content of Ir(piq)2acac, as a result of both intra- and interchain
FRET from fluorene unit to Ir(piq)2acac.22,33
2.1. Synthesis and characterization
The synthesis of Ir(Brpiq)2acac even reported used 1-
chloroisoquinoline, 4-bromophenylboronic acid and IrCl3$3H2O as
the orange light-emitting unit. The hyperbranched copolymers
with 9,9-dioctylfluorene and Ir(piq)2acac as the branches and SDF
as the core were prepared by Suzuki polycondensation with yields
ranging from 49% to 59% (Scheme 1). The feed molar ratios of the
branching point SDF were 10% and Ir(piq)2acac were varied from
0.02 % to 0.05 % relative to the fluorenyl units, and the corre-
sponding hyperbranched copolymers were named as PFSDF-Ir2
(Ir(piq)2acac 0.02 mol %), PFSDF-Ir3 (Ir(piq)2acac 0.03 mol %),
PFSDF-Ir4 (Ir(piq)2acac 0.04 mol %) and PFSDF-Ir5 (Ir(piq)2acac
0.05 mol %), respectively. All of the copolymers are readily soluble
at room temperature in common organic solvents such as CHCl3,
THF and toluene. The synthetic and structural results of the co-
polymers were summarized in Table 1. The 1H NMR spectra of the
copolymers were quite similar (Fig. S2, the proton signals of
Ir(piq)2acac were not detected because of its low content), re-
vealing the similar backbone structures of the copolymers. Taking
PFSDF-Ir4 as an example (Fig. S3), the actual content of SDF was
calculated by comparing the peak integral intensities of the proton
2.3. Film forming properties
The morphology of the spin-coated films of the copolymers was
estimated by atomic force microscopy (AFM) at a tapping mode,
and the images are shown in Fig. 3. All the films show smooth
surface with small root-mean-square (RMS) values of 1.295, 1.670,
2.316 and 2.884 nm for PFSDF-Ir2, PFSDF-Ir3, PFSDF-Ir4 and
PFSDF-Ir5, respectively. The results indicate that the hyper-
branched structure with three-dimensional-structured SDF branch
point could provide a homogeneous morphology of spin-coating
films, which could be favorable for the PLED fabrication.
signals of the spiro[3.3]heptane of SDF (
d
3.0e3.5) and the aromatic
2.4. Electroluminescence properties
ring of fluorene ( 7.4e8.0) of the copolymer (8.80:1), which was
d
close to the feed ratio (8.25:1). The number-average molecular
weights (Mns) of the copolymers were all around 13000 with the
polydispersity indexes (PDIs) ranging from 1.56 to 3.35. All of the
copolymers exhibit good thermal stability with the onset de-
composition temperatures (Td, measured at a 5% weight loss) from
407 to 423 ꢀC. DSC data reveals the glass transition temperatures
(Tgs) of copolymers were all around 155 ꢀC (Fig. S4, Supplementary
data).
Using the copolymers as emitting materials, single-layer PLEDs
were fabricated with the configuration of ITO/PEDOT:PSS (40 nm)/
Copolymers (50 nm)/TPBi (35 nm)/LiF (1 nm)/Al (150 nm). The
electroluminescent spectra of the devices are shown in Fig. 4a and
the characteristics were summarized in Table 2.
Generally, all of the copolyemers exhibit much broader EL
spectra than their PL counterparts. At the voltage of 16 V, PFSDF-Ir2
and PFSDF-Ir3 both exhibit blue-light emitting and the broad peaks
locate at the 452 nm and 484 nm, respectively. The spectrum of
PFSDF-Ir4 covers the visible light region with 400e700 nm and the
main peaks locate at 425 and 548 nm, respectively. White-light
emission is obtained with CIE coordinates located at (0.31, 0.35)
(Fig. 4b). The emission peak of PFSDF-Ir5 mainly locates at 533 nm
and yellow-light emission is achieved. For all of the copolymers, the
emission band around 540 nm may be from the excimer of PFSDF
formed under the electric field.5,22,34e36 As the electrons and holes
can be trapped on the Ir complex,22 the formation of the excimer
may be induced by Ir(piq)2acac under electric excitation.35 As
a result, the excimer emission intensity increased with the Ir(pi-
q)2acac contents. In PFSDF-Ir5, the emission from the PFSDF
2.2. Photophysical properties
Fig. 1 shows the UV-visible absorption of Ir(Brpiq)2acac and
photoluminescence (PL) spectra of hyperbranched polyfluorene-
spiro[3.3]heptane-2,6-dispirofluorene (PFSDF, SDF 10 mol %) co-
polymer in CHCl3 solution at a concentration of 1.0ꢁ10ꢂ5 mol/L.
There are two strong absorption peaks seated at 243 nm and
297 nm, which are mainly attributed to the spin-allowed ligand-
centered (1LC) state transitions. The weak absorption peaks seated
at 345 nm are mainly assigned to the spin-allowed metal-to-ligand
charge-transfer (1MLCT) state transitions. The unconspicuous