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

AbstractSemi-aromatic fluorinated polyimides (PIs) exhibiting enhanced fluorescent emission in

the visible region are newly designed in aid of TD-DFT calculations, and they were successfullysynthesized from the combinations of two kinds of perfluorinated dianhydrides or four kinds ofunfluorinated dianhydrides, and fluorinated /unfluorinated alicyclic diamines. The PI films ob-tained are tough, flexible, and transparent, exhibit Tg’s over 200˚C, and the thermal degradationtemperatures are over 410˚C. The intensities of the strongest fluorescence appearing in thebluish green region for the 10FEDA-derived PIs and those in the violet blue region for the PIsderived from aromatic dianhydride having low electron affinities are more than 100 times asstrong as that of a conventional PI. Although the fluorescence of P2FDA-derived PIs are 5-8times stronger than those of the 10FEDA-derived PIs, we developed novel semi-aromatic poly-imides (PIs) that emit enhanced fluorescence of the three primary colors (red, green, and blue).

IntroductionThermally stable fluorescent polymers have been attracting attention for use in OLED

devices and spatial optical phase modulators. Polyimides, the most popular thermally stablepolymers, are known to show fluorescence in the visible region [1,2]. However, the wavelengthsand intensities are significantly affected by the electronic properties of raw materials and thestates of aggregation formed duringthermal curing because the fluores-cence in PIs is generally associatedwith the charge transfer (CT) excita-tion and emission mechanism [3]. Inaddition, the quantum yields of thefluorescence observed for conven-tional PIs are low due to their strongCT nature. In the present study, we pro-pose a novel thermally stable materialof “highly fluorescent fluorinatedpolyimdes” and report their thermal,optical, and fluorescence properties.

N

O

O

N

O

O

CC

CC

O

O

N

O

O

N

O

O

CC

CC

O OCN

C

O

O

N

O

O

C

CH2

F3C CF3

C

C

CH2

N

O

O

C

C

O

C

CH3

NH2

CH3

CH3

H2N

Ref. S.M. Pyo et al., Polymer, 40, 125 (1998).

Ref. S.M. Pyo et al., Macromolecules, 31, 4777 (1998).Ref. W. Li et al., J. Phys. Chem., 101, 11068 (1997).

Ref. M. Hasegawa et al., J. Polym. Sci., B36, 827 (1998).

Ref. H. Chassemi et al., J. Polym. Sci., B33, 1633 (1995).

N

O

O

C

C

O

O

C

CN

CN

C

O

O

C

CN

O

O

CO O

CH3

CH3

Scheme 1 Conventional fluorescent polyimides contain-ing fluorescent-dyemoieties in the main or side chains.

Calculation and ExperimentalFor evaluating one-electron transitions in PIs,

the time-dependent (TD)-DFT theory with B3LYPfunctional was adopted [4], in which 6-311G(d) basisset was used for geometry optimizations under no con-straints, and 6-311++G(d,p) basis set was used for cal-culations of excitation energies and oscillator strengths.The calculations were performed using Gaussian-98(Rev.A11) software. 1,4-Bis(3,4-dicarboxytrifluorophenoxy)tetra fluorobenzene dianhydride (10FEDA)and 1,4-difluoropyromellitic dianhydride, which havebeen developed for perfluorinated polyimides [5], werekindly supplied by NTT Corp, and 4,4’-diaminocyclohexyl-hexafluoropropane (6FDC) and 2,2’-bis(trifluoromethyl)-4,4’-diaminobicyclohexyl (TFDC)were generously supplied by Central Glass Co.Ltd.4,4’-Diamino cyclohexyl methane (DCHM) was pur-chased from Tokyo Kasei. The methods for preparingNMP solutions of poly(amic acid silyl ester) (PASE)and thermal imidization of spin-coated PASE filmshave been reported elsewhere [6].

Results and DiscussionFig.1 shows the molecular orbitals that play im-

portant roles for the CT (HOMO→LUMO) and thelocalized excitation (LE: HOMO−n→LUMO) ab-sorptions for imide model compounds. Comparingwith a standard model-Ph/An, the combination ofpolyfluorinated anhydride and alicyclic amine(model-4FPh/Ch) exhibits a hypsochromic shift andhypochromic effect for the CT absorption but a slightbathochromic shift and a significant hyperchromiceffect for the LEabsorption. It iswell-known thatthe intra- and inter-molecular CT in-teractions of PIchains markedlyreduce their fluo-rescence emission.Hence, the key-points to provide

HOMO (309 nm, f=0.001)HOMO (363 nm, f=0.003)

C

C

O

O

N

F

H

LUMO

C

C

O

O

N

LUMO

[[[[Ph/An]]]] [[[[Ph4F/Ch]]]]

e−−−− e−−−−

Fig. 1 Molecular orbitals essential for theCT (HOMO→LUMO) and LE (HOMO-n→LUMO) absorptions for models. Thecalculated wavelengths and oscillatorstrengths for the one-electron transitions toLUMO are shown in parenthesis.

CO

C CO

C

O O

OO

CF3

CF3

CO

C C

C

O O

OO

F

F

OC

OC C

OC

O O

OO

CF3P6FDA P2FDA P3FDA

CO

PMDA

Ea = 2.57 eV 2.48 eV 2.37 eV

sBPDA

CO

C CO

C

O O

OO

39˚

10FEDA F F

CO

C

O

O

O

O

O

O O

F F

F

F

F

F

F

F

C

C2.04 eV

1.78 eV

6FDA

45˚C

OC

O

O

C

CF3

39˚ 39˚C

OC C

OC

O O

OO

O

1.59 eV

ODPA HQDEA

CO

C

O

O

O

O

O

O O C

C1.41 eV

BISPDA

CO

C

O

O

OO O

C

CH3

0

0.1

0.2

0.3

250 300 350 400 450

Ca

lcu

late

d A

bs

orb

an

ce

(a

rb.u

nit

)

Wavelength (nm)

O

42°N

O

C

C

a) Ph/An b) Ph4F/An

c) Ph/Ch d) Ph4F/Ch

O

FN

O

C

C F

F

F

46°

O

O

NC

CH

90°

O

HF

O

C

C F

F

F

N90°

CT transition

LE transition

Fig. 2 Calculated absorption spectra of fourmodels using the TD-DFT theroy.

Fig. 3 Calculated electron affinities of dianhydrides (B3LYP/6-311G(d)).

highly fluorescent PIs are, firstly, the suppression ofCT interactions, secondly, the promotion of LE emis-sion occurring at the anhydride moiety, and thirdly, thesuppression of the emission-quenching caused by mo-lecular aggregation. Consequently, 4FPh/Ch is ex-pected to be more fluorescent than Ph/An. In addition,the introduction of less polarizable and bulky –CF3

groups should be effective to strengthen the fluores-cence. Hence, we decided to use a perfluorinated di-anhydride, P2FDA, 10FEDA, and alicyclic diamines,some of which include –CF3.

According to the design concept described above,the PIs depicted in Scheme 2 were synthesized. In or-der to avoid gelation and to improve solubility of pre-cursors, the in-situ silylesterification method wasadopted [6]. The PI films obtained by thermal curingat 300˚C for 1.5h under N2 (ca.10µm-thick) are trans-parent, tough, and flexible. The pale yellowish colorof 10FEDA-derived PIs and the deep red color ofP2FDA-derived PIs originate from the characteris-tic absorption peaks at 400 and 500 nm, respectively.These peaks were assigned to the local excitation atthe dianhydride moieties [7]. The Tg’s were observedat 205–300˚C, and the thermal degradation tempera-tures (10%wt-loss) are 410–440˚C. As shown in Fig.4, the fluorescent emission observed at 470–480 nmfor 10FEDA-derived PI films are >100 times asstrong as that of a conventional fluorescent PI(BPDA/PDA). Note that the peak at 400 nm in theexcitation spectra (not shown) coincides well with

CN

C

O

O

N

O

O

O O

F F

F F

FF

F

F

F

F

10FEDA

C

C

CH

H

CCF3

CF3

CF3

R'

AlicyclicStructure : R

R :

R': CF3 (87%), CH3 (11%)

DCHM 6FDC

TFDC

CHA

CN

C

O

O

N

PDABPDA

C

C

O

O

CN

C

O

O

N

O

OF

FC

C

AlicyclicStructure : R

P2FDA

Scheme 2. Newly designed highly fluores-cent PIs that can be synthized from P2FDA,10FEDA and alicyclic diamines, and a con-ventional aromatic PI (BPDA/PDA). Aro-matic dianhydrides with low Ea can be used.

1

10

100

1000

400 500 600 700

Flu

ore

scen

ce I

nte

nsi

ty /

arb

.un

it

Wavelength / nm

10FEDA/TFDC

10FEDA/DCHM

sBPDA/PDA

10FEDA/6FDC

1

10

100

400 500 600 700 800

sBPDA/PDA

P2FDA/DCHM

P2FDA/6FDCP2FDA/TFDC

Flu

ore

scen

ce I

nte

nsi

ty /

arb

.un

it

Wavelength / nm

0

1

2

300 400 500 600 700 800

HQDEA/DCHM

BPDA/PDAP2FDA/DCHM

10FEDA/DCHM

Ab

sorb

ance

Wavelength / nm

ODPA/DCHM

Fig. 3 Experimental UV/Vis absorption spec-tra of the PIs formed on silica. The PIs de-rived from P2FDA and 10FEDA exhibit char-acteristic peaks in at 500 and 400nm regard-less of the diamine structures.

Fig. 4 Experimental emission (fluorescence) spec-tra of PIs derived from 10FEDA and BPDA/PDA.The excitation peak (not shown) concides with thecharacteristic absorption peak in Fig.3.

Fig. 5 Experimental emission (fluorescence) spec-tra of PIs derived from P2FDA and BPDA/PDA.The excitation peak (not shown) concides with thecharacteristic absorption peak in Fig.3.

the characteristic absorption peak as seen in Fig.3. In addition, the excitation and emission peaksare located in the same ranges regardless of thediamine strutures. These facts indicates that theCT interactions are effectively suppressed, and theLE excitation/ emission mechanism is predomi-nant in these PIs. Situation is similar for theP2FDA-derived PIs, but the fluorescent emissionat 580 and 680–700 nm are much weaker thanthose of the 10FEDA-derived PIs. This suggeststhat the CT emission mechanism still remains dueto the significant intermolecular interactions be-tween the P2FDA moieties.

Finally, we tried to use non-fluorinated aromatic dianhydrides having low electron affini-ties, ODPA, HQDEA, and BISPDA (Scheme 1). The lowered elecrton-withdrawing propertiesof these dianhydrides should reduce the intra- and intermolecular CT interactions, which canenhance the LE excitation and emission mechanisms. As shown in Fig.6, the PIs synthesizedfrom such dianhydrides and alicyclic diamines exhibit strong emission at 380-420 nm (violet toblue). The intensities of the fluorescence is as strong as those of the 10FEDA-derived PIs.

ConclusionNovel semi-aromatic poly-

imides (PIs) that emit enhancedfluorescence of the three primarycolors (red, green, and blue) weredesigned in aid of TD-DFT cal-culations and successfully syn-thesized using combinations ofperfluorinated dianhydrides ornon-fluorinated dianhydrides having ether-linkages and fluorinated / unfluorinated alicyclic di-amines. The PI films are tough, flexible, and have Tg’s over 200˚C. The wavelengths of thefluorescence can be predicted from the electron affinities of dianhydries and controled by changingtheir structures. The maximum fluorescence intensities in the blue and bluish green regions aremore than 100 times as strong as that of a conventional PI. These PIs are promising as thermallystable materials for organic light-emitting devices and flat panel displays in near future.

References1. E.D. Wachsman, C.W. Frank, Polymer, 29, 1191 (1988).2. M. Hasegawa et al, Eur. Polym. J., 25, 349 (1989).3. M. Hasegawa and K. Horie, Prog. Polym. Sci., 26, 259 (2001).4. S. Ando, T. Fujigaya, and M. Ueda, Jpn. J. Appl. Phys. 41, L105 (2002).5. S. Ando, T. Matsuura and S. Sasaki, Macromolecules, 25, 5858-60 (1992).6. Y. Oishi, K. Ogasawara, H. Hirayama, and K. Mori, J. Photopolym. Sci. Technol., 14, 37 (2001).7. Y. Urano, M. Yoshida, Y. Sato, and S. Ando, Polym. Prep. Jpn, 51, 843 (2002).

1

10

100

1000

350 400 450 500 550 600 650 700

BPDA/PDA

ODPA/DCHMHQDEA/DCHM

BISPDA/DCHMBPDA/DCHM

Flu

ore

scen

ce I

nte

nsi

ty /

arb

.un

it

Wavelength / nm

P2FDA/DCHM 10FEDA/DCHM BPDA/DCHM

Fig. 6 Experimental emission (fluorescence)spectra of PIs derived from non-fluorinateddianhydrides having low electron affinities.

Fig. 7 Photos of the PIs newly sythesized, and thermally imidizedon silica. The films are illuminated by UV light (350 nm).