Flexible and highly fluorescent aromatic polyimide: design, synthesis, properties, and mechanism

Article abstract of DOI:10.1039/C6TC03889A

To develop high-performance flexible fluorescent aromatic polyimides and to have a deeper insight into the fluorescence mechanism, two diamine monomers PyDA and TzDA with similar chemical structures but totally different electronic effects, were designed and synthesized. PyDA bears an electron-donor pyrrole group while TzDA bears an electron-acceptor 1,2,4-triazole group. The resulting aromatic polyimide TzODPI containing the triazole group showed bright green photoluminescence with a high quantum yield of up to 61% in solution and 13% in the film state. However, fluorescence was totally quenched in the pyrrole-containing polyimide PyODPI. The completely different phenomena were systematically elucidated with the aid of molecular simulations. Theoretical calculations for the molecular orbital distribution, oscillator strength, and the electron transition process between the ground state and excited state of model molecules were applied to clarify the fluorescence mechanism. Highly fluorescent aromatic polyimides can be obtained by appropriate control of the intra-molecular charge-transfer effects between the diamine and dianhydride moieties, and this demonstrates a simple strategy to develop wholly aromatic polyimides with high fluorescence for potential applications in the field of flexible displays.

Full text of DOI:10.1039/C6TC03889A

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Flexible and highly fluorescent aromatic polyimide:
design, synthesis, properties, and mechanism†
Cite this:DOI: 10.1039/c6tc03889a
Zhuxin Zhou, Yi Zhang,* Siwei Liu, Zhenguo Chi, Xudong Chen and Jiarui Xu
To develop high-performance flexible fluorescent aromatic polyimides and to have a deeper insight into
the fluorescence mechanism, two diamine monomers PyDA and TzDA with similar chemical structures
but totally different electronic effects, were designed and synthesized. PyDA bears an electron-donor
pyrrole group while TzDA bears an electron-acceptor 1,2,4-triazole group. The resulting aromatic
polyimide TzODPI containing the triazole group showed bright green photoluminescence with a high
quantum yield of up to 61% in solution and 13% in the film state. However, fluorescence was totally
quenched in the pyrrole-containing polyimide PyODPI. The completely different phenomena were
systematically elucidated with the aid of molecular simulations. Theoretical calculations for the molecular
orbital distribution, oscillator strength, and the electron transition process between the ground state and
excited state of model molecules were applied to clarify the fluorescence mechanism. Highly fluorescent
aromatic polyimides can be obtained by appropriate control of the intra-molecular charge-transfer effects
between the diamine and dianhydride moieties, and this demonstrates a simple strategy to develop wholly
aromatic polyimides with high fluorescence for potential applications in the field of flexible displays.
Received 7th September 2016,
Accepted 14th October 2016
DOI: 10.1039/c6tc03889a
www.rsc.org/MaterialsC
which seriously restrict their applications and the development
of PLEDs. These problems urgently need to be solved to meet
Introduction
Flexible displays bring revolutionary changes to display modes, the advancing fabrication processes in modern electronic and
greatly expand display space forms, and are also the key technical photonic industries, and to maintain a proper lifetime of the
requirements in the fields of wearable devices, smart phones, and devices. Meanwhile, the development of a new generation of
1
other emerging displays. Therefore, critical technologies and new high-performance light-emitting polymeric materials is another
materials in flexible displays have attracted increasing attention efficient way to go.
and have been rapidly developed recently. Among them, interest
Wholly aromatic polyimides (Ar-PIs), the well-established
in polymeric light-emitting materials is growing in this most high-performance engineering polymers, possess excellent thermal
crucial field due to their flexible nature and facile film formation stability, chemical and radiation resistance, mechanical strength
2
3
8
by spin coating or inkjet printing. Conjugated polymers, such as and flexibility. These properties are of significance in modern
4
5
6
poly( p-phenylene vinylene), poly( p-phenylene), poly(thiophene)
device fabrication processes for the requirement of a high glass
7
and polyfluorene, have become important light-emitting materials transition temperature. Though Ar-PIs have great advantages in
in large-area display devices, typically known as Polymer Light- light-emitting device applications over conventional conjugated
Emitting Diodes (PLEDs), showing great potential in advanced polymers, few attempts have been made to use them as light-
flexible displays. However, the solubility, thermal stability, emitting materials because the traditional polyimide is usually of
dimensional stability at high temperature, and photochemical non-luminance or luminance with a pretty low photoluminescence
stability of these conjugated polymers are critical problems, efficiency.
In order to make an improvement to the luminescence efficiency
of Ar-PIs, fluorescent chromophores have been introduced into the
polyimide backbone or used as the pendant groups. However,
PCFM Lab, GD HPPC Lab, Guangdong Engineering Technology Research Centre for
9
High-performance Organic and Polymer Photoelectric Functional Films, State Key
strong p–p interactions occurred between chromophores, and the
strong intermolecular and intramolecular charge transfer between
the occupied and unoccupied molecular orbitals on the diamine and
dianhydride moieties respectively, caused unavoidable fluorescence
quenching. To overcome this problem, aliphatic monomers were
Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China.
E-mail: ceszy@mail.sysu.edu.cn; Fax: +86 20 84112222; Tel: +86 20 84112222
†
Electronic supplementary information (ESI) available: Experimental section,
NMR spectra of the monomers and the polyimides; FTIR spectra, WAXD patterns,
thermal properties, transmittance and absorption behavior of the polyimide
films; cyclic voltammograms of polyimide solutions; chemical structures of
model basic units. See DOI: 10.1039/c6tc03889a
10
utilized. These non-conjugated and low electron-donating moieties
could boost the fluorescence intensity by effectively preventing
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charge-transfer-complex (CTC) formations, though, the thermal the intermolecular interactions as far as possible. In this case,
stability as well as the HOMO level decreased simultaneously, when these diamines were introduced into the polyimide back-
leading to instability of the devices, a mismatch in the work bone by reacting with the same dianhydride ODPA, the intra-
1
1
function of the cathode ITO and dielectric breakdowns. Recent molecular charge-transfer (CT) interaction between the diamine
success has been gained by the introduction of triphenylamine and dianhydride became the main interaction in these two
building blocks to the polyimide backbone. It has been found to aromatic polyimide systems, and the electronic effect of the
be an impactful way to develop wholly aromatic polyimides with diamines played the primary role in the fluorescence properties,
1
2
a high fluorescence quantum yield, which showed attractive which was expected to regulate the charge-transfer strength and
potential applications in up-to-date flexible displays. However, the donor–acceptor interaction intensity along the polymer
the fluorescence mechanism of aromatic polyimide remains backbone. Totally different fluorescence behavior was found
unknown and studies on this would be very important to guide for the resultant polymers. The aromatic polyimide containing
the development of these materials, which may be the next the triazole group, TzODPI, showed bright green photoluminescence
generation of high-performance flexible light-emitting materials. with a high quantum yield (FPL) of up to 61% in solution and
It is well-known that the charge-transfer (CT) character has a 13% in the film state. However, the fluorescence was totally
great impact on the photophysics, photochemistry and optical quenched in the pyrrole-containing polyimide PyODPI. The
properties of polyimides, especially in fully aromatic polyimide completely different phenomena were systematically elucidated
13
systems. Previous reports revealed that this character will quench with the aid of molecular simulations. Theoretical calculations
the fluorescence, hence research rarely focuses on adjusting the for the molecular orbital distribution, oscillator strength, and
CT effect to improve the quantum efficiency of Ar-PIs. However, it the electron transition process between the ground state and
should be interesting to know if one could develop high efficiency excited state of model molecules were applied to clarify the
fluorescent Ar-PIs by appropriate control the electron push–pull fluorescence mechanism. The results indicate that the electron
effect between the diamine moieties and the dianhydride moieties. deficiency in diamine moieties could boost the photoluminescence
In this work, two diamine monomers PyDA and TzDA with efficiency by effectively suppressing the charge-transfer effects
similar chemical structures but totally different electronic that weaken the fluorescence.
effects were designed and synthesized (Scheme 1). PyDA bears
an electron-donor pyrrole group while TzDA bears an electron-
acceptor 1,2,4-triazole group. Both of them contain a large non-
planar and non-polar tetraphenylmethane side group to avoid
Results and discussion
Synthesis and characterization of the diamine monomers and
polyimides
The diamine monomer PyDA was prepared according to our
1
4
previous work. TzDA was synthesized using the multi-step
synthetic route shown in Scheme 1. Mass spectra, elemental
1
13
analysis, IR, H NMR and C NMR spectroscopic techniques
were applied to confirm that the intermediates and the diamino-
compounds fully matched the proposed structures, as shown in
the experimental section and Fig. S1 and S2 (ESI†). The proton
resonance signals at around 5.05 ppm for diamine PyDA and
5
.48 ppm for diamine TzDA, and the infrared absorptions at
ꢀ1
ꢀ1
about 3450 cm and 3340 cm , which are characteristic of
amino groups, indicate the successful preparation of the diamines.
It is worthy to be mentioned that the amino protons of diamine
TzDA undergo resonance at a much higher frequency (5.48 ppm)
1
Scheme 1 Synthesis routes of diamine monomers and their 3D structures.
Fig. 1 H NMR spectra of the diamines (a) TzDA and (b) PyDA.
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than that of the diamine PyDA (5.05 ppm), as detailed in Fig. 1. Table 1 Optical properties of the diamine monomers
Since the spatial structures of the two monomers are almost the
same, the different proton resonance signals could be attributed
to the distinct electron push–pull effect of the molecule core
structures. The 1,2,4-triazole group, which is characterized as an
electron acceptor due to the strong electro-negativity of the two
a
In solution
Solid state
abs
max
ex
max
(nm)
em
max
(nm)
ex
max
em
max
l
l
l
F
(%)
PL
l
l
F
PL
(%)
Diamine
(nm)
(nm)
(nm)
PyDA
TzDA
307
303
317
312
457
404
4.6
29
381
355
413
398
5.5
14
2
sp -hybridized nitrogen atoms, effectively reduces the electron
a
ꢀ5
Measured in THF solution (10 M).
density on the amino groups of diamine TzDA, causing a
resonance shift to a higher frequency field. The absence of
two nitrogen atoms, in contrast, brings the amino proton
signals of diamine PyDA to a lower frequency field. We consider
it an important indication because if such diamine monomers
were polymerized with the same dianhydride respectively, the
difference in the electron push–pull ability for the diamine
moieties would certainly make an impact on the intramolecular
charge-transfer characteristics of the polymers.
The optical properties of the diamines were also character-
ized using the absorption and photoluminescence spectra in
both the solution and solid state. The results are displayed in
Fig. 2 and summarized in Table 1. In tetrahydrofuran (THF)
solution, PyDA and TzDA had absorption peaks at 307 nm and
3
03 nm, respectively. When excited at the wavelength of the
ex
maximum excitation peak (lmax), 312 nm, TzDA showed an
intense emission peak at 404 nm, with a photoluminescence
efficiency of up to 29%. While, PyDA had a weaker and broader
emission peak at 457 nm when excited at 317 nm, with a FPL
value of 4.6%, which was much lower than that of TzDA. As for
the solid state, the emission peak was blue-shifted to 398 nm
and the FPL dropped to 14%, indicating that the aggregation-
Scheme 2 Synthesis of the polyimides PyODPI and TzODPI.
caused fluorescence weakening effect occurred in TzDA. How-
ever, PyDA maintained its FPL of 5.5% in the solid state, and
underwent a blue-shift to 413 nm.
As shown in Scheme 2, the polyimide films were prepared by the
reaction between the as-synthesized diamine and the commercial
4,4-oxydiphthalic anhydride (ODPA), using conventional two-step
polymerization. The mix of the monomers in DMF produced a
viscous precursor poly(amic acid) solution, which was subsequently
cast on a clean dry glass plate followed by programmed thermal
imidization. The resulting flexible and tough PI films were approxi-
mately 35 mm thick. The degree of polymerization was estimated by
inherent viscosity (Zinh) instead of gel permeation chromatography
(GPC), because of the disparity in the structures of polar polyimide
and nonpolar polystyrene, standardly utilized in GPC. It is more
common to evaluate polyimide molecular weights using visco-
metry, as a higher viscosity generally indicates a higher degree
of polymerization. The Zinh values measured using an Ostwald
ꢀ
1
ꢀ1
viscometer were 0.91 dL g and 0.64 dL g for PyODPI and
TzODPI, respectively. As shown in Fig. S3 (ESI†), the infrared
ꢀ
1
absorption bands at 1778 cm (asymmetric CQO stretching) and
ꢀ
1
1
719 cm (symmetric CQO stretching) correspond to the imide
ring, while no amide bands nor carboxyl absorption peaks are
found. Furthermore, the amide and carboxyl proton signals dis-
1
appear in the H NMR spectra (Fig. S4, ESI†), indicating that the
precursor poly(amic acid) was fully imidized. Fig. S5 (ESI†) presents
the wide-angle X-ray diffraction patterns of both the polyimide
films, which suggest the amorphous nature of these two films.
Fig. 2 The optical spectra of the diamine monomers in (a) THF solution
and (b) solid state.
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Table 2 Thermal properties of the polyimides
Table 4 Optical properties of the polyimides
e
a
T
d
(1C)
In solution
Film state
a
b
c
s
d
f
T
g
T
g
T
CTE
Char yield
ꢀ1
ꢀ1
abs
onset
abs
max
em
max
b
c
0
abs
onset
ex
max
em
max
d
b
PI
(1C) (1C) (1C) (mm m 1C
)
5 wt% 10 wt% (wt%)
l
l
l
F
PL
l
l
l
l
t
F
PL
PI
(nm) (nm) (nm) (%) (nm) (nm) (nm) (nm) (ns) (%)
PyODPI 258 282 242 45
TzODPI 285 310 270 42
559
491
584
507
70
47
e
PyODPI 377 329 N.D. N.D. 419 394 N.D. N.D.
—
N.D.
TzODPI 339 288 486 61
372 350 439 536 13 13
a
The transition onset temperature, measured using DSC at a heating rate of
ꢀ1
b
a
2
b
ꢀ2
ꢀ1
1
0 1C min under nitrogen. Glass transition temperature, measured using
Measured in dilute solution of CH Cl2 (ca. 2 ꢁ 10
mg mL ).
ꢀ
1 c
DMA at a heating rate of 5 1C min
using TMA at a heating rate of 10 1C min
.
Softening temperature, measured
Photoluminescence quantum yield determined using a calibrated
ꢀ1
d
c
.
Coefficient of thermal integrating sphere. Cut off wavelength and 80% transmittance wave-
e
d
e
expansion, calculated using TMA curves (50 to 200 1C). Decomposition length. Photoluminescence lifetime, not available for PyODPI. Not
ꢀ
1
temperature, measured using TGA at a heating rate of 20 1C min under detected.
f
nitrogen. Residual weight percentage at 900 1C under nitrogen.
when the flexible diphenyl ether structure in the dianhydride
Thermal properties and solubility of the polyimides
moieties is applied.
Herein, TGA, DSC, DMA and TMA were applied to investigate
the thermal properties of the polyimides. The results are shown
in Table 2 and Fig. S6 (ESI†). The triazole-containing polyimide
TzODPI possessed a much higher glass transition temperature
Optical properties of the polyimides
The optical properties of the polyimides were studied in both the
solid state and the solution state by UV-vis and photoluminescence
spectroscopy, and the results are listed in Table 4.
(T
g
) at 285 1C using the DSC measurement, or 310 1C using
As shown in Fig. 3a and b, and Fig. S7 and S8 (ESI†), both of
the polyimide films are tough and flexible, and exhibit good
DMA, while the T for the pyrrole-containing polyimide PyODPI
g
was lower and was observed at 258 1C using DSC, or 282 1C
using DMA, suggesting a higher rigidity of the TzODPI polymer
backbone or a stronger intermolecular interaction because of
the existence of the triazole groups. Due to this property, the
polyimide TzODPI film also showed relatively higher dimen-
sional stability, with a coefficient of thermal expansion (CTE) of
transparency in the visible region. The cutoff wavelength (l ),
0
abs
max
the maximum absorption wavelength (l ), and the absorption
abs
onset
onset wavelength (l
) of the triazole-containing TzODPI thin
film were 372 nm, 275 nm and 350 nm, respectively, which were
much shorter than that of the pyrrole-containing PyODPI film,
abs
ꢀ
1
ꢀ1
0
which showed 419 nm for l , 332 nm for lmax, and 394 nm for
4
2 mm m 1C in the recorded range of 50–200 1C, than that
abs
abs
abs
ꢀ
1
ꢀ1
l
onset. The lmax and lonset values of the TzODPI film are obviously
of the PyODPI film, which had a CTE of 45 mm m 1C . The
softening temperatures (T ) gauged by TMA, that were 270 1C
blue-shifted by about 57 nm and 44 nm when compared with the
PyODPI film. Also, one can notice a subtle shoulder peak at
around 307 nm in the TzODPI film, which indicates that certain
intermolecular interactions exist because of the strongly polar
s
for TzODPI and 242 1C for PyODPI, proved the stability order as
well. Considering the similarity of the chemical structure for
both polyimide units, the introduction of 1,2,4-triazole groups
results in a more rigid polymer chain and a stronger inter-
molecular interaction thus leading to a higher dimensional and
thermal stability. Between the char yields of the polyimides
obtained from the TGA curves at 900 1C, the PyODPI ranked
higher, which is coincident with its higher carbon content.
Both the as-prepared polyimides showed not only good thermal
stability, but also desirable solubility in common organic solvents.
The solubility test was carried out by adding 10 mg of polyimide
film into 1 mL of solvent. The results are summarized in Table 3.
They could be well dissolved even in the weakly polar solvent
1,2,4-triazole moieties. It is well-known that the coloration of
polyimide originates from the charge-transfer-complex (CTC)
formation, which is greatly influenced by the intra- and inter-
molecular charge-transfer (CT) interactions. The stronger the CT
interactions are, the deeper the color the polyimide has, and vice
versa. Since both of the chemical structures of PyODPI and
TzODPI have bulky triphenyl methyl side groups, the inter-
molecular CT interaction could be alleviated, resulting in a
2 2
CH Cl and in some common polar solvents like NMP at room
temperature, and DMSO, DMF, DMAc and m-cresol at about 90 1C.
The improved solubility mainly results from the bulky triphenyl
methyl side groups as well as the contorted polymer backbone
Table 3 Solubility of the polyimides
PI
2 2
m-Cresol DMSO DMF DMAc NMP THF CH Cl
PyODPI
TzODPI
+
+
+
+
+ꢀ
+ꢀ
+ꢀ
+ꢀ
++
++
+
ꢀꢀ
++
+
‘
‘++’’, soluble at room temperature; ‘‘+’’, partially soluble at room
temperature or soluble when heated at 90 1C; ‘‘+ꢀ’’, partially soluble
when heated at 90 1C; ‘‘ꢀ’’, insoluble when heated at 90 1C. ‘‘ꢀꢀ’’, Fig. 3 Photographs of the flexible polyimide films PyODPI and TzODPI
insoluble at room temperature.
under sunlight (a and b) and 365 nm UV light (c and d).
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Fig. 6 Excitation/emission spectra of the polyimide films (ca. 35 mm).
differences in the emission properties, and the results are
shown in Fig. 3c, d, 5 and 6. For PyODPI, no matter whether
it was in the solution or in the film state, it did not show
fluorescent light. The fluorescence of the diamine PyDA was
totally quenched when it was introduced into the polyimide
backbone by polymerization with dianhydride ODPA (Fig. 3c
and black line in Fig. 6). While for TzODPI, it showed a strong
Fig. 4 The mechanistic charge-transfer intramolecular structure of
TzODPI and PyODPI.
lighter color than conventional polyimides. On the other hand,
when taking into account the intramolecular CT interaction
2 2
emission peak at 486 nm in dilute CH Cl solution (Fig. 5)
(Fig. 4), for TzODPI, the electron acceptor triazole groups may
when exited at 328 nm, with a high absolute fluorescence
quantum yield of 61%, and as far as we know, this is the
highest value ever reported for the luminescent aromatic poly-
imides. In the film state, it exhibited bright green fluorescence
under 365 nm irradiation (Fig. 3d). A strong emission peak at
help to weaken the intramolecular CT interaction, giving
shorter l , lmax and lonset, as well as a lighter color of the film.
0
The alleviated CT interaction in TzODPI could be identified
more clearly by the absorption behavior of the polyimides in
dilute solution (Fig. 5), because the intermolecular interaction
abs
abs
5
36 nm emerged when the film was excited at 439 nm with a
among the macromolecular chains could be ignored in this
abs
short lifetime (t) of 13 ns and a FPL of 13% (red line in Fig. 6),
indicating that the polyimide film could give out fluorescence
when placed under sunlight. The fluorescence quantum yield
of this aromatic polyimide TzODPI is comparatively high,
which has been rarely reported in literature. The lower FPL
in the solid film state than in solution suggests that the
mentioned intermolecular interaction by dense packing exists
in TzODPI and will lower the electron-withdrawing effect of the
case. The l
values of the PyODPI and TzODPI solutions were
max
abs
observed at 329 nm and 288 nm, respectively; and the l
onset
values of them were observed at 377 nm and 339 nm, respec-
tively. The absorption peak of TzODPI is obviously blue-shifted
by about 41 nm when compared with that of PyODPI, which
indicates that the dominant intramolecular CT effect was
greatly weakened due to the strong electron-withdrawing effect
of the 1,2,4-triazole moieties.
1
2
1
,2,4-triazole moieties, thus from another point of view, it will
Inspired by the weakened CT effect in TzODPI, we further
looked into the photoluminescence properties of both poly-
imides to find out whether the mentioned effect would result in
enhance the intramolecular CT effect and subsequently weaken
the fluorescence of the polyimide. Besides, the emission peak
of TzODPI was obviously red-shifted by about 50 nm, from
486 nm to 536 nm, which was a result of the stacking of
macromolecular chains in the solid film state.
The significant differences in the fluorescence behavior of
PyODPI and TzODPI both in solution and the solid state may be
mainly caused by the intensity of the intramolecular CT effect
between the electron donor diamine moieties and the electron
acceptor dianhydride moieties. In PyODPI, the strong intra-
molecular CT interaction between the electron-donor pyrrole-
containing diamine and the dianhydride ODPA, confirmed by
its optical absorption behaviour (Fig. 5), totally quenched the
fluorescence. However, when replacing the pyrrole unit with an
electron-withdrawing unit of 1,2,4-triazole, the electron density
of the diamine moiety is greatly lowered (Fig. 1), thus the
intramolecular CT interactions would be greatly depressed,
and finally this would result in the strong fluorescence emission
of the TzODPI system. However, how these CT effects affect the
Fig. 5 Normalized absorption spectra and emission spectra of the poly-
ꢀ2
ꢀ1
imides in CH
Cl
2 2
solutions (B2 ꢁ 10
mg mL ), and their images
irradiated under 365 nm UV light.
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fluorescence behavior of these aromatic polyimide systems still Table 5 Transition wavelength, oscillator strengths and the assignment of
a
S
0
to S
i
transitions for the model basic units (MBUs) TzM and PyM
needs further study.
To gain a better insight into the photoluminescence pheno-
menon, theoretical calculations of the frontier molecular
Transition
Excited wavelength Oscillator
MBU state (nm) strength Orbitals
Contribution
0.53
orbitals, oscillator strengths and the assignment of S to Si
0
transitions for the model basic units (MBUs) (TzM for TzODPI TzM 1
303
262
0.0021
1.1386
HOMO - LUMO
HOMOꢀ1 - LUMO 0.21
and PyM for PyODPI, Fig. S9, ESI†), were carried out using a
4
HOMO - LUMO+1 0.51
HOMO - LUMO+2 0.28
1
5
Gaussian 09 program package. Time-dependent density func-
tional theory (TD-DFT) with long-range correction method at
the cam-B3LYP/6-31+(d) level was used for dealing with the
charge-transfer properties in the MBUs. The triphenylmethyl
group was excluded since it is not conjugated to the polymer
main chain and does not significantly affect the electronic
properties of the basic units. The dianhydride moiety was
simplified to make a clearer investigation for the influence of
the electron properties in the diamine moiety. Similar to most
fully aromatic polyimides, the highest occupied molecular
orbitals (HOMOs) of both the MBUs are predominantly dis-
tributed on the diamine moieties, while the lowest unoccupied
molecular orbitals (LUMOs) are located on the phthalimide
moieties, as displayed in Fig. 7. However, the introduction of a
triazole group brings the calculated HOMO level of TzDA down
to ꢀ7.50 eV, which is much lower than that of PyDA (ꢀ6.71 eV),
indicating the characteristics of donor and acceptor for the
pyrrole and triazole structures, respectively. The total separa-
tion of these two frontier molecular orbitals would lead to the
PyM 1
323
283
0.0023
0.9756
HOMO - LUMO
0.69
HOMOꢀ2 - LUMO 0.19
HOMO - LUMO+1 0.36
HOMO - LUMO+2 0.53
3
a
Other prohibited transitions are not listed.
and 0.9759 for PyM, Table 5). As LUMO+1 and LUMO+2 delocalize
over the MBUs, the electrons in the excited states are capable of
relaxing easily to the lowest excited state (S ) via rapid internal
1
conversion. These plausible electronic excitation transition
processes match the absorption spectra of the polyimides in
abs
onset
solution. The l
at 339 nm for the TzODPI solution arises
1
from the S formation, consisting of dominant CT transitions
HOMO - LUMO and HOMOꢀ1 - LUMO (calculated transi-
tion wavelength 303 nm). The maximum absorption peak at
2
4
88 nm corresponds to the formation of S , which primarily
comprises locally excited transitions HOMO - LUMO+1 and
HOMO - LUMO+2 (calculated transition wavelength 262 nm).
Analogous results could be found with regard to PyODPI.
After understanding the electron excitation process, we look
into the subsequent transition. According to Kasha’s rule, the
first exited state (S ) with CT effect via a one-electron HOMO to
1
LUMO transition, which is characterized by a small value of
oscillator strength (0.0021 for TzM and 0.0023 for PyM,
Table 5). However, the higher frontier orbitals, LUMO+1 and
LUMO+2, are distributed over the diamine moiety and the
phthalimide moiety. Therefore, electrons in the HOMO are
more initially inclined to hop up to LUMO+1 and LUMO+2
conversion from the upper state S
occurs before the molecule reaches the ground state (S
yield fluorescence emission. Hence, the nature of electron
transition between S and S appears particularly crucial to
i
to the lowest excited state S
1
0
) to
i 4
under sufficient energy to result in an excited state S (S for
1
0
clarifying the significantly different emission properties for
these two similar structures. Herein, the analysis of the electron
transitions was carried out with the aid of a Multiwfn program
TzM and the S for PyM, Table 5), owing to the much larger
oscillator strength values for these transitions (1.1386 for TzM
3
1
6
package after the calculations using Gaussian 09 were finished.
It can explain the electrostatic potential and electron localization
functions of charge-transfer compounds visually by the use of
1
7
wavefunction analysis and quantum chemical studies. The
primary results are summarized in Table 6. Hole and electron
distributions respectively denoting the regions where an electron
leaves and goes to were examined, as displayed in Fig. 8. Con-
cerning the PyM, the hole region (blue) spreads through the
pyrrole moiety and the adjacent phenyl ring, while the electron
region (green) is located at the phthalimide moiety (Fig. 8b).
0 1
Table 6 The analysis of electron transition between S and S by Multiwfn
program package
Integral of
Distance between the
hole–electron centroids of the hole
distribution
MBU overlap (S)
and electron regions (D) Norm of transition
(angstrom)
dipole moment (a.u.)
TzM 0.1947
PyM 0.0795
3.74
5.94
0.076
0.275
Fig. 7 The frontier molecular orbitals of TzM and PyM.
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Table 7 Electrochemical properties of the polyimides
ox
onset (V)
a
b
c
PI
E
E
g
(eV)
HOMO (eV)
LUMO (eV)
TzODPI
PyODPI
0.63
0.50
3.66
3.29
ꢀ5.05
ꢀ4.92
ꢀ1.39
ꢀ1.63
a
abs
b
Energy gap = 1240/lonset of the polyimide solution in CH
2
Cl
2
.
The
HOMO energy levels relative to ferrocene (ꢀ4.80 eV) were calculated
c
from Eonset
g
. LUMO = HOMO + E .
(ESI†) and the results are summarized in Table 7. For TzODPI
and PyODPI, the onset potentials were measured to be 0.63 V
and 0.50 V versus Ag/AgCl, respectively. It is noted that the
Fig. 8 The hole (blue region) and electron (green region) distribution
(a and b) and overlap (c and d) of TzM and PyM.
oxidation potential of external standard ferrocene/ferrocenium
+
(
Fc/Fc ) was measured to be 0.38 V versus Ag/AgCl in NMP.
+
Assuming that the HOMO level for the Fc/Fc standard is ꢀ4.80 eV
with respect to the zero vacuum level, the HOMO level was
determined to be ꢀ5.05 eV for TzODPI and ꢀ4.92 eV PyODPI. Also
note that the band gaps estimated from the absorption edge of the
polymer solutions were 3.66 eV and 3.29 eV or TzODPI and PyODPI,
respectively. The lowest unoccupied molecular orbital (LUMO)
energy levels were calculated to be ꢀ1.39 eV for TzODPI and
Almost no overlap region could be observed except for a tiny overlap
lying on the carbonyl group, and the integral of the overlap of the
hole–electron distribution (S) is tiny with a value of 0.0795. The
calculated results show total separation of the hole and electron
regions in PyM (Fig. 8d), presenting a typical CT transition mode.
While for TzM, the hole region scatters over the 1,2,4-triazole moiety,
the adjacent phenyl ring and the phthalimide moiety, and the
electron region is also located at the phthalimide moiety. The
overlap of the hole and electron regions can be easily noticed in
Fig. 8c. The S value of TzM is much higher with a value of 0.1947,
which is approximately 2.5 times that of PyM. Though the hole and
electron regions are separated indicating that the CT effect still exists
in TzM, this effect is suppressed resulting in the observed larger S
value. Note that a large overlap integral allows effective transitions in
theory. The higher photon emission efficiency of TzM or TzODPI
makes sense. Another charge-transfer feature, the distance between
the centroids of the hole and electron regions (D), is also provided.
The larger the value, the longer distance the charge transfers. Again,
TzM has a shorter D value of 3.74 angstroms, which exhibits a
suppressed CT type characteristic, compared with PyM for which the
D value is 5.94 angstroms. Similarly, the smaller value of transition
dipole moment which TzM has, is in favor of the CT suppression
as well.
ꢀ
1.63 eV PyODPI. The lower HOMO level of TzODPI compared
with that of PyODPI reveals the electron-deficiency of the diamine
moieties in TzODPI. However, the still relatively high HOMO level
value of TzODPI indicates its potential for facile device fabrications.
Conclusions
Two novel wholly aromatic polyimides were prepared from
diamines with totally different electron affinities. They had
facile film forming abilities, high thermal stability and desir-
able solubility. The electron acceptor 1,2,4-triazole containing
polyimide, TzODPI, exhibited bright green fluorescence with a
quantum yield of up to 61% in solution and 13% in the film
state, while PyODPI, bearing electron donor pyrrole moieties,
suffered a fluorescence quenching effect. The entirely opposed
optical properties were expected to originate from the varied CT
interaction intensities, which were elucidated with the aid of
optical spectra and molecular simulations. Highly fluorescent
aromatic polyimides can be obtained by appropriate control of
the intra-molecular charge-transfer effects between the diamine
and dianhydride moieties. This demonstrates a convenient way
for developing novel wholly aromatic polyimides with high
fluorescence efficiency for potential optoelectronic applications.
Consequently, one could conclude that both polyimide structures
perform though certain CT characteristics, with TzODPI having
weaker donor–acceptor interactions and obvious fluorescence
emission. However, the emission of PyODPI was quenched by
the stronger CT effect between the pyrrole-containing diamine
and dianhydride ODPA, due to the deficiency of the transition
1 0
between S and S .
In consideration of the above design concept to develop high
performance polyimides for advanced optical and electronic
applications, it is quite reasonable to question whether the
stronger the electron-withdrawing effect of the diamine moi- Experimental
eties, the higher the emission efficiency of the resulting poly-
imide is. Further investigation has been in progress in this lab
and the results are to be reported in the near future.
The instrumentation and all commercially available starting
compounds are listed in the ESI.† The diamine monomers and
the corresponding polyimides were synthesized by the routes
outlined in Schemes 1 and 2, respectively. The intermediate 4-
Electrochemical properties
0
tritylaniline and the pyrrole containing diamine monomer 4,4 -
Cyclic voltammetry was employed to determine the oxidation (1-(4-tritylphenyl)-pyrrole-2,5-diyl)dianiline (PyDA) were synthe-
1
4
potential of the polymers in NMP solution, as shown in Fig. S10 sized according to a previously reported procedure.
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Synthesis of 1,2-bis(4-nitrophenyl)hydrazine (1)
three-neck flask. The mixture was refluxed with stirring under
argon for 4 h. Then ethanol was removed by rotary evaporation
under vacuum, and the resulting vicious solution was poured into
4
8
-Nitrobenzoyl chloride (9.23 g, 50.0 mmol) was dissolved in
0 mL of anhydrous N-methyl-2-pyrrolidone (NMP). After
4
M NaOH (250 mL). The mixture was stirred vigorously for 2 h,
triethylamine (4.18 mL, 30.0 mmol) was added, the formed
suspension was cooled in an ice-water bath. Then hydrazine
monohydrate (1.50 g, 30.0 mmol) was added dropwise under
vigorous stirring. The mixture was allowed to react at room
temperature for 5 h and then poured into water (200 mL). The
pale yellow precipitation was filtered off and washed with water
extracted with CH Cl and dried under vacuum. Purification of the
2
2
residue by column chromatography and further recrystallization
from ethanol afforded diamine TzDA as a white crystalline solid
1
(
4.69 g, 82%). H NMR (300 MHz, DMSO-d
6
, d, ppm): 7.36–7.31 (m,
6
H), 7.26–7.21 (m, 5H), 7.16–7.07 (m, 8H), 6.97–6.95 (d, 4H), 6.44–
13
2 6
6.41 (d, 4H) and 5.48 (s, 4H, NH ). C NMR (500 MHz, DMSO-d , d,
and ethyl acetate, and dried under vacuum to give a white
1
ppm): 64.79, 113.52, 114.38, 126.84, 128.35, 129.59, 130.97, 132.36,
134.34, 146.26, 148.12, 150.33 and 154.43. Anal. calcd for C H N :
powder (6.70 g, 81%). H NMR (300 MHz, DMSO-d
6
, d, ppm):
1
3
39 31 5
1
(
0.99 (s, 2H), 8.38–8.35 (d, 4H) and 8.15–8.12 (d, 4H). C NMR
500 MHz, DMSO-d , d, ppm): 124.29, 129.50, 138.37, 149.96
and 164.72. MS (EI, m/z): [M] calcd for C14
C, 82.22; H, 5.48; N, 12.29. Found: C, 82.01; H, 5.47; N, 12.21. MS
6
+
+
(EI, m/z): [M] calcd for C H N : 569.26. Found: 569. IR (KBr, n,
3
9 31 5
H
10
N
4
O
6
: 330.06.
Found: 330. IR (KBr, n, cm ): 3222 (N–H, stretching), 1614
CQO, stretching), 1530 (NO , v ) and 1350 (NO , v ).
ꢀ1
ꢀ
1
cm ): 3457, 3339 (N–H, v), 1610 (CQN, stretching) and 1298 (C–N,
stretching).
(
2
as
2
s
Preparation of polyimides
Synthesis of 1,2-bis(4-nitrophenyl)chloromethylene)hydrazine (2)
The synthesis of polyimide TzODPI is used as an example for
the general synthetic route of both polyimides (Scheme 2). To a
solution of diamine TzDA (0.4856 g, 0.8524 mmol) in 5 mL of
anhydrous DMF, was added dianhydride ODPA (0.2644 g,
Compound 1 (9.91 g, 30.0 mmol) and anhydrous toluene (100 mL)
were added to a 250 mL three-neck flask, followed by addition of
PCl (16.66 g, 80.0 mmol) in batches. The suspension was stirred at
5
120 1C for 5 h. Then toluene was removed by rotary evaporation
0.8524 mmol) in one portion. After stirring at room tempera-
under vacuum. Water (100 mL) was added and the mixture was
refluxed for 30 min. After filtration and cooling to room tempera-
ture, a tan crude product was obtained, which was afterwards
purified on a silica-gel column using ethyl dichloromethane/
ture under argon for 8 h, the viscous poly(amic acid) solution
was subsequently coated on a clean glass plate, followed by
thermal imidization in a vacuum oven at 80 1C/1 h, 150 1C/1 h,
230 1C/1 h and 300 1C/1 h. After cooling to room temperature,
n-hexane (v/v = 1/1) as an eluent to give compound 2 as yellow
1
the plate was soaked in warm water and the polyimide film
(approximate 35 mm) peeled off automatically.
needle crystals (8.49 g, 77%). H NMR (300 MHz, DMSO-d ,
6
13
d, ppm): 8.43–8.37 (d, 4H) and 8.33–8.28 (d, 4H). C NMR
500 MHz, DMSO-d , d, ppm): 124.73, 130.16, 138.18, 142.97 and
50.17. MS (EI, m/z): [M] calcd for C14
(
1
3
6
+
H
8
Cl
66. IR (KBr, n, cm ): 1586 (CQN, stretching), 1526 (NO
342 (NO , v ).
2 4 4
N O : 365.99. Found:
Acknowledgements
ꢀ
1
2
, vas) and
1
2
s
The financial supports by the National 973 Program of China
(No. 2014CB643605), the National 863 Program of China (No.
Synthesis of 3,5-bis(4-nitrophenyl)-4-(4-tritylphenyl)-1,2,4-
triazole (3)
2
015AA033408), the National Natural Science Foundation of
China (No. 51373204 and 51233008), the Science and Technology
Project of Guangdong Province (No. 2015B090915003 and
4-Tritylaniline (8.05 g, 24.0 mmol) and 2 (7.34 g, 20.0 mmol) were
2015B090913003), the Doctoral Fund of the Ministry of Education
mixed and dissolved in N,N-dimethylaniline (80 mL). The resulting
solution was stirred at 100 1C under argon for 48 h. After cooling to
room temperature, the mixture was poured into n-hexane (300 mL)
to precipitate the crude product. After filtration, washing
with methanol and purification by column chromatography, a light
of China (No. 20120171130001), and the Fundamental Research
Funds for the Central Universities (No. 161gzd08) are gratefully
acknowledged.
1
yellow crystalline solid was obtained (8.87 g, 70%). H NMR
Notes and references
6
, d, ppm): 8.22–8.19 (d, 4H), 7.65–7.62
(300 MHz, DMSO-d
(
d, 4H), 7.46–7.44 (d, 2H), 7.33–7.29 (m, 6H), 7.24–7.19 (m, 5H)
1
2
3
4
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H
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N
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Synthesis of 4,4 -(4-(4-tritylphenyl)-1,2,4-triazole-3,5-
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Compound 3 (6.30 g, 10.0 mmol), SnCl
1
2
ꢂ2H
2
O (22.56 g,
00.0 mmol) and ethanol (250 mL) were charged into a 500 mL
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1
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