Molecular Design, Synthesis, and Properties of Highly Fluorescent ...

Molecular Design, Synthesis, and Properties of
Highly Fluorescent Fluorinated Polyimides
Shinji ANDO* † , Yuichi URANO † , Hiroyuki SEKINO † , and Yoshiyuki OISHI §
†Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
Ookayama 2-12-1-S1-21, Meguro-ku, Tokyo 152-8552, Japan
e-mail : sando@polymer.titech.ac.jp
§Department of Chemical Engineering, Iwate University,
Morioka-shi, Iwate, 020-8551, Japan
Abstract
Semi-aromatic fluorinated polyimides (PIs) exhibiting enhanced fluorescent emission in
the visible region are newly designed in aid of TD-DFT calculations, and they were successfully
synthesized from the combinations of two kinds of perfluorinated dianhydrides or four kinds of
unfluorinated dianhydrides, and fluorinated /unfluorinated alicyclic diamines. The PI films obtained
are tough, flexible, and transparent, exhibit Tg’s over 200˚C, and the thermal degradation
temperatures are over 410˚C. The intensities of the strongest fluorescence appearing in the
bluish green region for the 10FEDA-derived PIs and those in the violet blue region for the PIs
derived from aromatic dianhydride having low electron affinities are more than 100 times as
strong as that of a conventional PI. Although the fluorescence of P2FDA-derived PIs are 5-8
times stronger than those of the 10FEDA-derived PIs, we developed novel semi-aromatic polyimides
(PIs) that emit enhanced fluorescence of the three primary colors (red, green, and blue).
Introduction
Thermally stable fluorescent polymers have been attracting attention for use in OLED
devices and spatial optical phase modulators. Polyimides, the most popular thermally stable
polymers, are known to show fluorescence in the visible region [1,2]. However, the wavelengths
and intensities are significantly affected by the electronic properties of raw materials and the
states of aggregation formed during
thermal curing because the fluorescence
in PIs is generally associated
with the charge transfer (CT) excitation
and emission mechanism [3]. In
addition, the quantum yields of the
fluorescence observed for conventional
PIs are low due to their strong
CT nature. In the present study, we propose
a novel thermally stable material
of “highly fluorescent fluorinated
polyimdes” and report their thermal,
optical, and fluorescence properties.
O
O O
CH
O
O
3
C C
C
C
N
N
H 2 N
N
NH 2
C C
C
O O
CH 3 O
CH 3
Ref. S.M. Pyo et al., Polymer, 40, 125 (1998). Ref. M. Hasegawa et al., J. Polym. Sci., B36, 827 (1998).
O
C
N
C
O
O
F 3C CF 3
O
C C
N
C
O
CH 2
CH 2
Ref. W. Li et al., J. Phys. Chem., 101, 11068 (1997).
O
C
N
C
O
O
C
N
C
O
O
C
N
C
O O
O
CH 3
C C
O C O
N N
C C
CH 3
O O
O
Ref. H. Chassemi et al., J. Polym. Sci., B33, 1633 (1995).
O
Ref. S.M. Pyo et al., Macromolecules, 31, 4777 (1998).
Scheme 1 Conventional fluorescent polyimides containing
fluorescent-dyemoieties in the main or side chains.
C
C N
O
O

Calculation and Experimental
For evaluating one-electron transitions in PIs,
the time-dependent (TD)-DFT theory with B3LYP
functional was adopted [4], in which 6-311G(d) basis
set was used for geometry optimizations under no constraints,
and 6-311++G(d,p) basis set was used for calculations
of excitation energies and oscillator strengths.
The calculations were performed using Gaussian-98
(Rev.A11) software. 1,4-Bis(3,4-dicarboxytrifluoro
phenoxy)tetra fluorobenzene dianhydride (10FEDA)
and 1,4-difluoropyromellitic dianhydride, which have
been developed for perfluorinated polyimides [5], were
kindly supplied by NTT Corp, and 4,4’-diaminocyclo
hexyl-hexafluoropropane (6FDC) and 2,2’-bis(tri
fluoromethyl)-4,4’-diaminobicyclohexyl (TFDC)
were generously supplied by Central Glass Co.Ltd.
4,4’-Diamino cyclohexyl methane (DCHM) was purchased
from Tokyo Kasei. The methods for preparing
NMP solutions of poly(amic acid silyl ester) (PASE)
and thermal imidization of spin-coated PASE films
have been reported elsewhere [6].
Results and Discussion
Fig.1 shows the molecular orbitals that play important
roles for the CT (HOMO→LUMO) and the
localized excitation (LE: HOMO−n→LUMO) absorptions
for imide model compounds. Comparing
with a standard model-Ph/An, the combination of
polyfluorinated anhydride and alicyclic amine
(model-4FPh/Ch) exhibits a hypsochromic shift and
hypochromic effect for the CT absorption but a slight
bathochromic shift and a significant hyperchromic
effect for the LE
absorption. It is
well-known that
the intra- and intermolecular
CT interactions
of PI
chains markedly
reduce their fluorescence
emission.
Hence, the keypoints
to provide
O
C
O
C
O
O
C
O
C
C
O
C
[Ph/An]
LUMO
e −
P6FDA CF 3 O P2FDAO
F O P3FDA O
C
O
C
O
C
N
C
O
HOMO (363 nm, f=0.003)
O CF 3 O
O F O
O O
Ea = 2.57 eV 2.48 eV 2.37 eV
10FEDA F F
O F
C
O
C
O
F
ODPA
39˚ 39˚
O
1.59 eV
O
F F
O
C
O
C
O
O
F
F F
2.04 eV F
O
C
O
C
O
O
C
O
C
O
HQDEA
Calculated Absorbance (arb.unit)
[Ph4F/Ch]
H
LUMO
e −
O
C
N
C
O
F
HOMO (309 nm, f=0.001)
Fig. 1 Molecular orbitals essential for the
CT (HOMO→LUMO) and LE (HOMOn→LUMO)
absorptions for models. The
calculated wavelengths and oscillator
strengths for the one-electron transitions to
LUMO are shown in parenthesis.
C
O
C
O
C
O
C
O
0.3
0.2
0.1
LE transition
C
O
C
sBPDA
39˚
1.78 eV
O
O O C
O
C
1.41 eV O
0
250 300 350 400 450
O
C
O
C
O
a) Ph/An O b) Ph4F/An O
42° C
46° C
N N
C
C
CF 3
O
C
O
C
O
Wavelength (nm)
O
C
O
C
BISPDA
C
O
PMDA
6FDA
O
CF 3
45˚
C C
O
C
O
O
c) Ph/Ch O d) Ph4F/Ch O
C
H 90°
90° C
N
H N
C
C
O
O
CT transition
Fig. 2 Calculated absorption spectra of four
models using the TD-DFT theroy.
CH 3
C
Fig. 3 Calculated electron affinities of dianhydrides (B3LYP/6-311G(d)).
O
O
O
F
F
F
F
F
F
F
F
O

highly fluorescent PIs are, firstly, the suppression of
CT interactions, secondly, the promotion of LE emission
occurring at the anhydride moiety, and thirdly, the
suppression of the emission-quenching caused by molecular
aggregation. Consequently, 4FPh/Ch is expected
to be more fluorescent than Ph/An. In addition,
the introduction of less polarizable and bulky –CF 3
groups should be effective to strengthen the fluorescence.
Hence, we decided to use a perfluorinated dianhydride,
P2FDA, 10FEDA, and alicyclic diamines,
some of which include –CF 3 .
According to the design concept described above,
the PIs depicted in Scheme 2 were synthesized. In order
to avoid gelation and to improve solubility of precursors,
the in-situ silylesterification method was
adopted [6]. The PI films obtained by thermal curing
at 300˚C for 1.5h under N 2 (ca.10µm-thick) are transparent,
tough, and flexible. The pale yellowish color
of 10FEDA-derived PIs and the deep red color of
P2FDA-derived PIs originate from the characteristic
absorption peaks at 400 and 500 nm, respectively.
These peaks were assigned to the local excitation at
the dianhydride moieties [7]. The Tg’s were observed
at 205–300˚C, and the thermal degradation temperatures
(10%wt-loss) are 410–440˚C. As shown in Fig.
4, the fluorescent emission observed at 470–480 nm
for 10FEDA-derived PI films are >100 times as
strong as that of a conventional fluorescent PI
(BPDA/PDA). Note that the peak at 400 nm in the
excitation spectra (not shown) coincides well with
Absorbance
R :
O
C
N
C
O
O
C
N
C
O
O F O
C C
N
N
C C
O
F
O
P2FDA
F
F F
O O
F F F F
F
10FEDA
H
C
H
DCHM
BPDA
Alicyclic
Structure : R
F O
C
Alicyclic
N
Structure : R
C
F
O
CF 3
C
CF 3
6FDC
CHA
CF 3
O
C
N
TFDC R'
C
R': CF 3 (87%), CH 3 (11%)
O PDA
Scheme 2. Newly designed highly fluorescent
PIs that can be synthized from P2FDA,
10FEDA and alicyclic diamines, and a conventional
aromatic PI (BPDA/PDA). Aromatic
dianhydrides with low Ea can be used.
2
1
0
BPDA/PDA
P2FDA/DCHM
10FEDA/DCHM
HQDEA/DCHM
ODPA/DCHM
300 400 500 600 700 800
Wavelength / nm
Fig. 3 Experimental UV/Vis absorption spectra
of the PIs formed on silica. The PIs derived
from P2FDA and 10FEDA exhibit characteristic
peaks in at 500 and 400nm regardless
of the diamine structures.
Fluorescence Intensity / arb.unit
1000
100
10
10FEDA/TFDC
10FEDA/6FDC
sBPDA/PDA
10FEDA/DCHM
1
400 500 600 700
Wavelength / nm
Fig. 4 Experimental emission (fluorescence) spectra
of PIs derived from 10FEDA and BPDA/PDA.
The excitation peak (not shown) concides with the
characteristic absorption peak in Fig.3.
Fluorescence Intensity / arb.unit
100
10
P2FDA/TFDC
P2FDA/6FDC
P2FDA/DCHM
sBPDA/PDA
1
400 500 600 700 800
Wavelength / nm
Fig. 5 Experimental emission (fluorescence) spectra
of PIs derived from P2FDA and BPDA/PDA.
The excitation peak (not shown) concides with the
characteristic absorption peak in Fig.3.

the characteristic absorption peak as seen in Fig.
3. In addition, the excitation and emission peaks
are located in the same ranges regardless of the
diamine strutures. These facts indicates that the
CT interactions are effectively suppressed, and the
LE excitation/ emission mechanism is predominant
in these PIs. Situation is similar for the
P2FDA-derived PIs, but the fluorescent emission
at 580 and 680–700 nm are much weaker than
those of the 10FEDA-derived PIs. This suggests
that the CT emission mechanism still remains due
to the significant intermolecular interactions between
the P2FDA moieties.
Finally, we tried to use non-fluorinated aromatic dianhydrides having low electron affinities,
ODPA, HQDEA, and BISPDA (Scheme 1). The lowered elecrton-withdrawing properties
of these dianhydrides should reduce the intra- and intermolecular CT interactions, which can
enhance the LE excitation and emission mechanisms. As shown in Fig.6, the PIs synthesized
from such dianhydrides and alicyclic diamines exhibit strong emission at 380-420 nm (violet to
blue). The intensities of the fluorescence is as strong as those of the 10FEDA-derived PIs.
Conclusion
Novel semi-aromatic polyimides
(PIs) that emit enhanced
fluorescence of the three primary
colors (red, green, and blue) were
designed in aid of TD-DFT calculations
and successfully synthesized
using combinations of
perfluorinated dianhydrides or
1
350 400 450 500 550 600 650 700
Wavelength / nm
non-fluorinated dianhydrides having ether-linkages and fluorinated / unfluorinated alicyclic diamines.
The PI films are tough, flexible, and have Tg’s over 200˚C. The wavelengths of the
fluorescence can be predicted from the electron affinities of dianhydries and controled by changing
their structures. The maximum fluorescence intensities in the blue and bluish green regions are
more than 100 times as strong as that of a conventional PI. These PIs are promising as thermally
stable materials for organic light-emitting devices and flat panel displays in near future.
Fluorescence Intensity / arb.unit
1000
100
10
BPDA/PDA
ODPA/DCHM
HQDEA/DCHM
BISPDA/DCHM
BPDA/DCHM
Fig. 6 Experimental emission (fluorescence)
spectra of PIs derived from non-fluorinated
dianhydrides having low electron affinities.
P2FDA/DCHM 10FEDA/DCHM BPDA/DCHM
Fig. 7 Photos of the PIs newly sythesized, and thermally imidized
on silica. The films are illuminated by UV light (350 nm).
References
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