G Model
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L. Sun et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
chain arrangement. Meanwhile, upon decreasing the analysis
temperature, as result of the freezing out of torsional modes, these
well-resolved emission peaks in low-temperature PL spectra are
observed for all pristine spin-coated films (Fig. 3b) [13,21], also
confirmed this assumption. Obviously, there is no planar confor-
mation obtained in all three polymer pristine films. It is a common
sense that there is no enough time to allow for adjust chain
rearrangement in the spin-coated processing for the fast volatili-
zation of solvent molecules, which can reasonably explain the
amorphous state for the PODPF pristine spin-coated films
[32,33,40]. As we discussed above, three polymer chains may
rearrange and adjust each other packing model in condensed
structures upon thermal annealing [32]. Therefore, we also
explored the branched effect on aggregation and photophysical
property via thermal annealing pristine films at 220 ꢀC in the air
atmosphere. It is easily to find that the maximum absorption peak
of PEODPF and POYDPF annealed films is about ꢁ395 nm, similar to
the pristine films (Fig. 3c). However, we can also see clearly
absorbance peak at 411 nm and 440 nm for the annealed films of
PODPF, but absent in PEODPF and POYDPF. As presented in Fig. 3d,
the PL spectra of the PEODPF and POYDPF polymers have similar
emission peaks at about 440, 459, and 499 nm, which were
From the Fig. 4, all pristine films had smooth surface, with a
roughness of 0.462 nm for PEODPF, 0.669 nm for POYDPF and
0.480 nm for PODPF, respectively. However, after thermal anneal-
ing, three annealed films had different film surface. As we
expected, similar to pristine films, PEODPF annealed film also
present smooth surface with a slightly higher roughness of
0.819 nm, consistent with the optical and DLS analysis, also
indicated the introduction of isooctyl chain can improve film
forming ability and morphological thermal stability. According to
DLS analysis of POYDPF and PODPF, nano-aggregate (Rh = ꢁ100 nm)
is formed in the precursor solution, which may maintain in the
solution-processed film and dominate their morphology
[12,19,33,36,38,40]. This rough morphology will be enhanced
under thermal treatment at high temperature. Therefore, several
nano- and micro-domain are observed in the both annealed film of
POYDPF and PODPF. Corresponding roughness are estimated about
1.116 nm and 2.523 nm respectively. As reported in our previous
works [19,32,33], the crystalline structure was induced in the film
after thermal annealing. Therefore, PEODPF exhibited much less
aggregates compared to that of POYDPF and PODPF. Besides, X-ray
diffraction is used as a research method to obtain the information
of material morphology. From the XRD spectrum (Fig. S6 in
Supporting information), it can be seen that the three polymers
assigned to 0-0, 0–1, 0–2 p*-p transitions of the typical amorphous
polyfluorene chains [20]. However, the red-shift PL spectra of
PODPF consisted of three peaks at 455, 482, 515 nm, attributed to
the planar β-conformation [32,40]. Therefore, branched PEODPF
and POYDPF appeared a higher luminance efficiency about 36.1%
and 39.5%, enhanced about 20% than those of PODPF (22.6%) but
similar decay time and radiate rate (Fig. S4 and Table S1 in
Supporting information). Besides, Raman spectroscopy is also
introduced to probe these assumption (Fig. S5 in Supporting
information) [32]. The enhancement of the intensity of peaks form
from 1290 cmÀ1 to 1305 cmÀ1 occurred in PODPF annealed films,
also confirmed the formation of β-conformation. Nevertheless, no
obvious shift are observed in Raman spectra of PEODPF and
POYDPF annealed films, also indicated no obvious conformational
transition under thermal annealing. Therefore, it is effectively
concluded that corporation of branched side chain will suppress
the β-conformation forming ability of polydiarylfluorenes. On the
other sides, these branched chains are also inhibited interchain
aggregation to improve morphological and spectral stability.
According to DLS and optical analysis of three polymers above
(Fig. 2), PEODPF, POYDPF and PODPF chain presented various
different aggregation behaviors in the solution and film states.
Therefore, we further explored the detailed morphology of their
pristine film and annealed films by AFM and XRD measurements.
power show multiple peaks. And by the curve analysis, the u angles
were determined to be 6.1ꢀ and 10.1ꢀ, respectively, which the plane
spacing of the lattice of PEODPF were calculated to be 7.3 Å and
4.4 Å, according to the Bragg's Law method. According to this
method, we can get
p-p stacking distance of the three polymers in
the Table S2 (Supporting information). The distance of
p-p
interaction about 7.4 Å, are associated with the alkoxyethyl
arrangement. As displayed in Fig. S5, it was also found that the
peak width of the PEODPF, POYDPF compared with PODPF,
attributed to the weaker interchain interaction. Because of the
presence of β-conformation in PODPF, there is special peak at d
=9.6 Å, attributed to the interphase arrangement between eight of
carbon alkoxane chain and diarylflourene, and the distance
between the main polyfluorene chain formed. Therefore, it is
further confirmed multi-phase formation in PODPF, which is the
result of the synergistic effect of diarylfluorene and octyl, and the
corresponding frequency multiplication value is 3.8 Å. Therefore,
the micro-structure of the prepared samples was characterized by
XRD, consistent with the AFM and optical results.
In the last decades, polyfluorene had excellent luminance
efficiency and robust deep blue emission, which are widely
applications in deep-blue PLEDs and organic lasers [10,17,29,32].
Meanwhile, compared to the solution states, the emission induced
by the interchain excited state are significantly amplified in the
film state under electrical field, which provided an effective
platform to confirm the aggregation behaviour in condensed
Fig. 5. EL spectral stability and device performance of three polymers. (a) Device
configuration of the PLEDs. (b) EL spectra of devices based on three polymer pristine
(Top) and annealed (Bottom) films, together with their (c) performance of device
based on annealed films.
Fig. 4. AFM images of three polymer pristine and annealed films: (a, b) PEODPF
pristine and annealed film, (c, d) POYDPF pristine and annealed film and (e, f) PODPF
pristine and annealed films, respectively.
Please cite this article in press as: L. Sun, et al., Alkyl-chain branched effect on the aggregation and photophysical behavior of
polydiarylfluorenes toward stable deep-blue electroluminescence and efficient amplified spontaneous emission, Chin. Chem. Lett. (2019),