Synthesis and characterization of highly refractive polyimides derived from thiophene-containing aromatic Diamines and aromatic dianhydrides

Article abstract of DOI:10.1021/ma902013y

Aromatic thiophene-containing diamines, 2,5-bis(4-aminophenylsulfanyl) thiophene (APST) and 2,5-bis{2'-[5'-(4"-aminophenyl)sulfanyl]thienyl} thiophene (APSTT), were synthesized and polymerized with a sulfur-containing dianhydride, 4,4'-[p-thiobis(phenylen

Full text of DOI:10.1021/ma902013y

1836 Macromolecules 2010, 43, 1836–1843
DOI: 10.1021/ma902013y
Synthesis and Characterization of Highly Refractive Polyimides Derived
from Thiophene-Containing Aromatic Diamines and Aromatic
Dianhydrides
Namiko Fukuzaki, Tomoya Higashihara, Shinji Ando, and Mitsuru Ueda*
Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1-H120, O-okayama,
Meguro-ku, Tokyo 152-8552, Japan
Received September 9, 2009; Revised Manuscript Received January 6, 2010
ABSTRACT: Aromatic thiophene-containing diamines, 2,5-bis(4-aminophenylsulfanyl)thiophene (APST)
and 2,5-bis{20-[50-(400-aminophenyl)sulfanyl]thienyl}thiophene (APSTT), were synthesized and polymerized
with a sulfur-containing dianhydride, 4,40-[p-thiobis(phenylenesulfanyl)]diphthalic anhydride (3SDEA) and
4,40-oxydiphthalic anhydride (ODPA), respectively, to afford four poly(amic acid)s (PAAs) with inherent
viscosities of 0.41 to 1.68 dL/g. Flexible and tough polyimide (PI) films obtained from the PAA precursors
showed good thermal, mechanical, and optical properties. The glass-transition temperatures (Tg) of the PIs were
in the range of 157-220 °C,determinedby differential scanning calorimetry (DSC), and 160-216 °C by dynamic
mechanical analysis (DMA), depending on the dianhydride used. The 10% weight loss temperatures were in the
range of 354-456 °C, showing relatively high thermally resistant characteristics of the PI films. The PI films also
showed good optical transparency above 500 nm, which agreed well with the calculated absorption spectra using
the time-dependent density functional theory. The average refractive indices (nav) measured at 633 nm were
1.7228 to 1.7677, and the in-plane/out-of-plane birefringences (Δn) were 0.0067 to 0.0074. In particular, the nav of
PI derived from 3SDEA and APSTT exhibited the highest refractive index, that is, 1.7677 at 633 nm, in the high-
refractive-index polyimides. The high refractive indices originate from the high sulfur content, dense molecular
packing, and the absence of bulky structures. The relatively small birefringence mainly results from the flexible
and bent thioether linkage structures of the diamine.
Introduction
properties and have wide applications in advanced optoelectronic
fields, such as organic field-effect transistors25,26 and electrolumi-
Recently, demand for high-refractive-index (high-n) polymers
with high transparency and low birefringence has increased for
advanced integrated optical applications.1-3 For instance, poly-
mer microlenses for charge-coupled devices (CCDs) and comple-
mentary metal oxide semiconductor (CMOS) image sensors
require a higher refractive index exceeding 1.70, even 1.80, where
the thermal stability of polymers is also becoming an important
consideration in the design of polymer microlens because of the
severe heat environment present during fabrication processes.4
The introduction of substituents with high molar refractions and
low molar volumes can efficiently increase the refractive indices of
polymers according to the Lorentz-Lorenz equation.5 Because a
sulfur atom has a large atomic refraction, many high-n polymers
containing sulfur have been reported.6-13 Aromatic polyimides
(PIs) are well-known for their high thermal stability and often
show inherent high refractive indices because of their high polari-
zable molecular chains and high aromatic contents and are thus
thought to be one of the best candidates for high-n polymers.14-16
In our continuous efforts to develop optical polymers exhibit-
ing high refractive indices combined with low birefringence, high
thermal stability, and optical transparency, a series of PIs was
recently developed on the basis of a diamine containing thioether
linkages in the skeletal structure.17-24 The PIs thus prepared
possess good thermal stability, refractive indices >1.70 at the
wavelength of 633 nm, and birefringences <0.01.
nescent devices.27 Thiophene-containing PIs have also been deve-
loped for microelectronic or photonic applications.28,29 Although
the electrical and optical functions of the thiophene or dibenzothio-
phene moiety have been widely noticed, their contribution to the
high refractive index of polymers as sulfur-containing substituents
has rarely been mentioned.30
In a previous paper, we reported highly refractive PIs with high
sulfur content (Sc) of 20.5 wt % in the repeating unit derived from
2,8-bis(p-aminophenylenesulfanyl)dibenzothiophene and aro-
matic dianhydrides.22 These PIs exhibited good transparency in
the visible region with a transmittance>80% at 500 nm, high
refractive indices ranging from 1.7353 to 1.7578, and low bi-
refringence between 0.0068 and 0.0106. Furthermore, the highly
transparent PI with high Sc of 23.2 wt % obtained from 2,70-
bis(4-aminophenylenesulfanyl)thianthrene (APTT) and 3SDEA
was reported to show a refractive index of 1.7600 at 633 nm with
low birefringence of 0.0084.19 A thiophene unit is effective to
increase Sc while keeping low molar volumes because the sulfur
atom is included in a five-member ring. Therefore, the resulting
polymers containing thiophene units are expected to have higher
refractive indices than corresponding polymers with thioether
linkages.
In this study, we prepared new diamines containing a thiophene
unit combining flexible thioether moieties, which provide polymers
with high refractive indices, high transparency, and low birefrin-
gence. We report the synthesis and properties of optically transpar-
ent PIs derived from 2,5-bis(4-aminophenylsulfanyl)thiophene
(APST), 2,5-bis{20-[50-(400-aminophenyl)sulfanyl]thienyl}thiophene
Thiophene- or dibenzothiophene-containing polymers have been
attracting much attention for their unique electronic or photonic
*Corresponding author. E-mail: Ueda.m.ad@m.titech.ac.jp.
pubs.acs.org/Macromolecules
Published on Web 01/29/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 4, 2010 1837
Figure 1. FTIR spectra of CTST and APSTT.
Scheme 1. Synthesis of APSTT
(APSTT), and aromatic dianhydrides such as 4,40-[p-thiobis-
(phenylsulfanyl)]diphthalic anhydride (3SDEA) and 4,40-oxydi-
phthalic anhydride (ODPA). These PIs exhibited high refractive
indices in the range of 1.7228 to 1.7677 at 633 nm, high thermal
stability (>350 °C), low birefringence in the range of 0.0067 to
0.0074, and good transmittance >80% at 500 nm. In particular, the
nav of PI derived from 3SDEA and APSTT exhibited the highest
refractive index, that is, 1.7677 at 633 nm in the reported highly
refractive PIs.23 The effects of structure on the thermal properties,
optical transparency, and refractive indices of the PIs are discussed
in detail.
analysis (TGA) was recorded on a Seiko TG/DTA 6300 thermal
analysis system at a heating rate of 10 °C/min under nitrogen.
Differential scanning calorimetry (DSC) was performed on a
Seiko DSC 6300 at a heating rate of 10 °C/min. Dynamic
mechanical thermal analysis (DMA) was performed on PI film
specimens (30 mm long, 10 mm wide, and 50-60 μm thick) on a
Seiko DMS 6300 at a heating rate of 2 °C/min with a load
frequency of 1 Hz in air. The glass-transition temperature (Tg)
was determined as the peak temperature of the loss modulus
(E00) plot.
The out-of-plane (nTM) and in-plane (nTE) refractive indices
of PI films were measured with a prism coupler (Metricon,
model PC-2000) equipped with a He-Ne laser light source
(wavelength: 632.8 nm). In plane/out-of-plane birefringence
Experimental Section
(Δn) was calculated as the difference between nTE and nTM
The average refractive index (nav) was calculated according
to eq 1
.
Materials. All chemicals were purchased from TCI (Tokyo,
Japan), if not otherwise specified. 2,5-Dichlorothiophene, 2,5-
dibromothiophene, and 4-aminothiophenol were used as
received. APST30 and 3SDEA18 were prepared according to
our previous work. ODPA was dried at 160 °C overnight
in vacuo prior to use. N-Methyl-2-pyrrolidinone (NMP,
Wako, Japan) and N,N-dimethylformamide (DMF, Wako,
Japan) were purified by vacuum distillation from CaH2 prior
to use.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
nAV
¼
ð2nTE þ nTM Þ=3
ð1Þ
Calculations. The refractive indices of the models for the
aromatic PIs synthesized in this study were calculated on
the basis of the Lorentz-Lorenz theory in the same way as the
previous report.17 One-electron transition energies and the
corresponding oscillator strengths of the PI models were also
calculated using the time-dependent (TD)-DFT theory to pre-
dict the optical absorption in the UV-visible region.31-33 The
6-311G(d) basis set was used for geometry optimizations under
no constraints, and the 6-311þþG(d,p) was used for calcula-
tions of linear polarizabilities, transition energies, and oscillator
strengths. The three-parameter Becke-style hybrid func-
tional (B3LYP) was adopted as a function, and all calculations
were performed using the software package of Gaussian 03
Characterization. The 1H and 13C NMR spectra were re-
corded on a Varian Mercury 300 spectrometer using CDCl3 or
DMSO-d6 as solvent and tetramethylsilane as reference. Inher-
ent viscosity was measured using an Ubbelohde viscometer with
a 0.5 g dL-1 NMP solution at 30 °C. Fourier transform infrared
3
(FTIR) spectra were obtained with a Horiba FT-120 spectro-
photometer. Ultraviolet-visible (UV-vis) optical absorption
spectra were recorded on a Hitachi U-3210 spectrophotometer
at room temperature. PI films were dried at 100 °C for 1 h before
testing to remove the absorbed moisture. Thermogravimetric

1838 Macromolecules, Vol. 43, No. 4, 2010
Fukuzaki et al.
Figure 2. NMR spectra of APSTT (a) 1HNMR and (b) 13CNMR.
(rev. C02 and D01). A typical packing coefficient (Kp) of 0.60
was used for evaluating the intrinsic molecular volumes of
models to predict the refractive indices.14
Synthesis of 2,5-Bis{20-[50-(400-aminophenyl)sulfanyl]thienyl}-
thiophene. In a 200 mL round-bottomed flask equipped
with a magnetic stirrer, a nitrogen inlet, a Dean-Stark trap,
and a condenser was placed p-aminothiophenol (1.38 g, 11.0
mmol), cesium carbonate (2.12 g, 6.50 mmol), DMI (5 mL), and
toluene (5 mL). The mixture was heated with stirring at
140 °C for 4 h under a nitrogen atmosphere. After complete
removal of water, residual toluene was distilled off. Then, the
mixture was cooled to room temperature, and CTST (1.90 g,
5.00 mmol) dissolved in DMI (2.0 mL) was added dropwise to
the mixture. After addition, the reaction mixture was heated to
140 °C and maintained at this temperature for 24 h. The mixture
was poured in water and extracted with acetic acid. The organic
layer was washed with water, dried over anhydrous MgSO4, and
evaporated. The crystalline residue was passed through a silica
gel column chromatography (elution with a 1:1 mixture of acetic
acid and hexane). Evaporation of the solvent gave a solid
substance, which was further purified by HPLC (chloroform)
to give APSTT. Yield: 0.714 g (25%). mp: 95-98 °C. FTIR
(KBr, cm-1): 3421, 3209, 1620, 1597, 1493, 1396, 1281, 1176,
957, 810. 1H NMR (300 MHz, CDCl3/TMS, δ): 7.23-7.26 (d,
4H), 7.00-7.01 (d, 4H), 6.95(s, 2H), 6.90-6.91 (d, 4H),
6.60-6.63 (d, 4H), 3.76 (s, 4H). 13C NMR (300 MHz, CDCl3/
TMS, δ): 147.0, 142.4, 139.0, 136.0, 133.7, 133.6, 132.5, 131.1,
123.2, 115.8. Anal. Calcd for C24H18N2S7: C, 51.58; H, 3.25; N,
5.01. Found: C, 51.41; H, 3.33; N, 4.93.
Monomer Synthesis. Synthesis of 2,5-Bis[(2-thienyl)sulfanyl]-
thiophene (TST). A solution of 2-thiophenethiol (4.81 g, 44.1
mmol) in DMF (15 mL) was added to a stirred mixture of 2,5-
dibromothiophene (2.75 g, 18 mmol), KOH (2.32 g, 41.4 mmol),
and copper oxide (5.67 g, 39.6 mmol) in DMF (30 mL). The
mixture was heated at 135 °C for 24 h, cooled to room temperature,
and poured in 6 M HCl (80 mL), and toluene (300 mL) was added.
The resulting two-phase mixture was stirred for a while and filtered
through a pad of Celite. The organic layer was separated, washed
with water, dried over MgSO4, and evaporated. The residue was
purified by silica gel column chromatography (hexane) to give TST.
Yield: 2.51 g (45%). 1H NMR (300 MHz, CDCl3, δ): 7.10-7.12
(d, 4H), 6.91 (s, 2H), 6.54-6.56 (d, 4H), 5.36 (s, 4H).
Synthesis of 2,5-Bis[20-(50-chlorothienyl)sulfanyl]thiophene
(CTST). NCS (2.14 g, 16 mmol) was added in small portions
to a stirred solution of 2.50 g (8.00 mmol) of TST in acetic acid
(12 mL) and dichloromethane (16 mL) over a period of 1.5 h
at room temperature. The mixture was poured into water
(200 mL), and the organic layer was separated, washed with
saturated aqueous NaHCO3 solution and water, dried over
MgSO4, and evaporated. The obtained solid was recrystallized
from hexane to afford white crystals. Yield: 2.60 g (80%). mp:
58-59 °C. FT-IR (KBr, cm-1): 1515, 1408, 1200, 1014, 1003,
960, 795, 563,536. 1H NMR (300 MHz, CDCl3, δ): 7.02 (d, 2H),
6.99 (s, 2H), 6.79 (d, 2H). 13C NMR (300 MHz, CDCl3/TMS, δ):
139.2, 133.9, 133.7, 132.5, 127.0. Anal. Calcd for C12H6Cl2S5: C,
37.79; H, 1.59. Found: C, 37.66; H, 1.78.
Polyimide Synthesis. A typical polymerization procedure for
the synthesis of poly(amic acid) (PAA) was as follows. APSTT
(0.2794 g, 0.5 mmol) and newly distilled NMP (1.0 mL) were
charged in a 20 mL flask equipped with a magnetic stirrer, a

Article
Macromolecules, Vol. 43, No. 4, 2010 1839
Scheme 2. Synthesis of Polyimides
and 280 °C, respectively, under a nitrogen atmosphere. The PI-1
film was obtained by immersing the silicone wafer in warm
water. The films of PIs 2-4 were prepared by a similar method
from PAA solutions.
Results and Discussion
Monomer Synthesis. A new thiophene-containing aro-
matic diamine APSTT was prepared in three steps, as shown
in Scheme 1. The reaction of thiophene-2-thiol with 2,5-
dibromothiophene in DMF under the Ullman reaction
condition yielded TST, which was treated with N-chloro-
succinimide to yield CTST. CTST was directly reacted with
4-aminothiophenol to give the diamine APSTT, which was
purified by HPLC to remove a small amount of impurity.
Figure 1 compares the FTIR spectra of CTST and APSTT.
The absorption band at 1072 cm-1, assignable to the C-Cl
stretching in CTST, disappears in the spectrum of APSTT.
Alternatively, the characteristic absorptions of the amino
groups at 3436 and 3421 (N-H stretching) and 1597 cm-1
(N-H deformation) are obviously observed. All peaks
1
detected in the H and 13C NMR spectra of APSTT were
consistently assigned (Figure 2). The signal at 3.76 ppm in
Figure 2a was assigned to the amino groups, and the
aromatic proton (Ha) ortho to the amino groups was ob-
served at 6.61 ppm. In the 13C NMR spectrum (Figure 2b),
10 signals were observed, which are consistent with the
expected structure. The structure of the diamine was also
confirmed by elemental analysis.
Polyimide Synthesis. The PIs were prepared by the poly-
condensation of diamines APST and APSTT with aromatic
tetracarboxylic dianhydrides 3SDEA and ODPA via soluble
PAA precursors, followed by thermal imidization at elevated
temperatures (Scheme 2). The PAA solutions have inherent
viscosities of 0.41 to 1.68 dL/g, indicating moderate to high
molecular weights of the polymers (Table 1). Flexible and
tough PI films were cast from the PAA solutions. The FTIR
spectra of the PIs (Figure 3) show the characteristic absorp-
tions of the imide ring at 1774 (asymmetrical stretching of
carbonyl) and 1724 cm-1 (symmetrical stretching of
carbonyl) and the C-N bond at 1373 cm-1. In addition,
absorption peaks at ∼1222 cm-1 attributed to aromatic
thioether (Ar-S-Ar) are observed. The elemental composi-
tions of the PIs were checked by elemental analysis. These
results clearly indicate the formation of the expected PIs.
Thermal Properties. The thermal decomposition and
deformation behavior of the PIs was investigated by TGA,
DSC, and DMA measurements. The results are presented in
Table 2. The PIs derived from APST exhibit good thermal
stability up to 350 °C under nitrogen (Figure 4), whereas it
is inferior to that of the PIs from the phenylene analogue,
3SDEA. The PIs derived from APSTT exhibit somewhat
lower thermal stability with a 10% weight-loss temperature
of 354 °C.
The Tg values of the PIs were determined by DSC
(Figure 5) and DMA (Figure 6) analyses, respectively. Tg is
one of the key parameters of high-n optical polymers when
considering the high-temperature circumstance during fabri-
cation and the long-term heat-releasing environment result-
ing from the miniaturization of optoelectronic devices.34 In
the DMA analysis, the Tg is determined to be the peak
temperature of loss modulus (E00) curves. The Tg values
obtained from these two methods are nearly the same. The
small differences are mainly attributed to the different
frequency responses of the PIs, that is, f = 0 Hz for DSC
and 1 Hz for DMA. The crucial factor that contributed to
the lower Tg of PI-3 is the flexible molecular chains. PI-3
Table 1. Synthesis and Characterization of Polyimides
[η]inh of PAAa
elemental analysis
PI
[dL/g]
1.68
C (%)
H (%)
N (%)
PI-1
calcd
found
calcd
found
calcd
found
calcd
found
63.13
63.26
63.56
63.19
58.62
58.61
58.67
57.49
2.89
3.08
2.67
2.99
2.65
2.76
2.42
2.64
3.35
3.26
4.63
4.50
2.63
2.65
2.63
3.41
PI-2
PI-3
PI-4
0.48
0.61
0.41
a Measured at a concentration of 0.5 g/dL of NMP solution at 30 °C.
nitrogen inlet, and a cold water bath. 3SDEA (0.2713 g, 0.5
mmol) was added, and dehydrated NMP (1.2 mL) was used to
adjust the solid content of the reaction systems to 20 wt %. The
pale-yellow solution was stirred at room temperature for 24 h to
yield a viscous PAA solution. The PAA solution was spin-
coated on a silicon wafer, and the thickness was controlled by
spinning rate. For example, the thickness of a specimen for FT-
IR and UV-vis measurements was controlled to be ∼10 μm,
and the specimen for thermal properties measurement was
adjusted to be 30-50 μm. PI-1 film was obtained by thermally
curing the PAA solution in an oven for 1 h each at 80, 150, 250,

1840 Macromolecules, Vol. 43, No. 4, 2010
Fukuzaki et al.
Figure 3. FTIR spectra of PI films.
Table 2. Thermal Properties of Polyimides
Tg (°C)a
T5%
T10%
b
b
PI
DSC
DMA
(°C)
(°C)
PI-1
PI-2
PI-3
PI-4
185
220
177
157
186
216
177
160
428
396
338
331
456
443
374
354
a Tg: glass-transition temperature. b T5%, T10%: temperatures at 5 and
10% weight loss, respectively.
Figure 5. DSC curves of PI films (under a nitrogen atmosphere,
20 °C/min).
refractive indices (ncalcd). Figure 7 shows the experimental
UV/vis transmission spectra of the PI films and the calcu-
lated absorption spectra of the PIs. The absorptions of
the experimental spectra were normalized at a thickness of
10 μm. The PI films show pale-yellow to yellow colors with
T450 values of 62-86%. The PI-2 obtained from ODPA and
APST exhibited the highest optical transparency with the
smallest value of λcutoff, where the PI-3 from 3SDEA and
APSTT showed the lowest. The introduction of plural
thiophene rings in the main chain slightly deteriorates the
optical transparency of the films. This could be due to the
enhanced delocalization of lone-pair electrons of sulfur
atoms and π-electrons of the thiophene rings in APSTT.
The calculated ionization potential (Ip) of APSTT diamine
(6.85 eV) is smaller than that of APST (6.92 eV), which
indicates that the former possesses a higher electron-donat-
ing property than the latter. In addition, APSTT has a
smaller band gap (4.28 eV) than APST (4.52 eV), which
should be due to the electron delocalization around the
thiophene-S-thiophene moieties. The calculated absorp-
tion spectra agree with the experimental features of the
transmission spectra. The PIs derived from 3SDEA dian-
hydride displayed longer cutoff wavelengths with higher
absorption at 400 nm than those of the PIs from ODPA.
Figure 4. TGA curves of PI films (under a nitrogen atmosphere,
10 °C/min).
possesses flexible thioether linkages in both the dianhydride
and diamine moieties, which should undergo reorientation at
elevated temperatures. According to the thermal decomposi-
tion temperatures and Tg, it can be concluded that the PIs
derived from APST exhibit better thermal stability than
those of APSTT, which can be mainly attributed to the lower
thioether contents.
Optical Properties. Table 3 summarizes the sulfur
content and optical properties of the PI films, including
transmittances at 450 nm (T450), film thicknesses, in-plane
(nTE) and out-of-plane (nTM) refractive indices, average
refractive indices (nav), birefringences (Δn), and calculated

Article
Macromolecules, Vol. 43, No. 4, 2010 1841
In addition, the longest cutoff wavelength of PI-3 and the
lowest absorption (i.e., highest transmittance) of PI-2
at 340-380 nm are also reproduced in the calculations.
These results indicate that the optical transparency of the
PI films prepared in this study is essentially determined
by the inherent electronic structure of PI chains, whereas
Figure 7. Experimental UV/vis transmission spectra of the
PI films (top) and the calculated absorption spectra of the PIs
(bottom).
Figure 6. DMA curves of PIs (1 Hz, 2 °C/min): (a) Storage modulus
E0, (b) loss modulus E00, and (c) tan δ.
Figure 8. Relationship between the experimental and calculated refrac-
tive indices of the PIs.
Table 3. Optical Properties of PIs Films
refractive indices and birefringence at 633 nmc
PI
Sc (wt %)a
T450 (%)b
nTE
nTM
nav
Δn
ncalcd
PI-1
PI-2
PI-3
PI-4
ref-PI
23.0
15.9
30.1
26.0
20.5
86
75
62
64
64
1.7546
1.7251
1.7700
1.7503
1.7505
1.7472
1.7183
1.7629
1.7436
1.7437
1.7521
1.7228
1.7677
1.7481
1.7482
0.0074
0.0068
0.0072
0.0067
0.0068
1.7730
1.7264
1.7934
1.7470
1.7708
a Sulfur content. b Transmittance at 450 nm (10 μm thick). c See measurements.

1842 Macromolecules, Vol. 43, No. 4, 2010
Fukuzaki et al.
Scheme 3. Structure of 3SDEA-3SDA (ref-PI)
the intermolecular interactions do not exert a significant
influence.
Acknowledgment. Thefinancial supportbyJSR corporation,
Japan is gratefully acknowledged.
The in-plane (nTE) and out-of-plane (nTM) refractive indi-
ces of the PI films measured at 633 nm, respectively, ranged
from 1.7251 to 1.7700 and 1.7183 to 1.7629, as listed in
References and Notes
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Table 3. The nTE values are slightly higher than those of nTM
,
which indicates that the PI chains are preferentially oriented
parallel to the film plane.35 All PI films exhibited high nav
values in the range of 1.7228 to 1.7677, and the nav observed
for PI-3 (1.7677) is the highest of any high refractive PI ever
reported.23 Such a high refractive index is primarily due to
the very high sulfur content (30.1 wt %) and the relatively
dense molecular packing originating from the thiophene-
S-thiophene structures. The Abbe number (vD) of PI-3 was
21.6, which is a typical vD for sulfur-containing aromatic
PIs.36 Figure 8 shows the relationship between the experi-
mental (nav) and calculated refractive indices (ncalcd) of the
PIs. The ncalcd values show an approximately linear rela-
tionship with the nav values, though the ncalcd of the PIs
synthesized from 3SDEA (PI-1 and PI-3) was slightly over-
estimated (nav < ncalcd). This indicates that the 3SDEA
dianhydride having two diphenylthioether linkages in its
skeletal structure reduces the molecular packing density of
PI chains, which is supported by the fact the data points
of PI-1 and PI-3 equally deviate from the reference line
of nav = ncalcd. In contrast, the calculations successfully
reproduce the nav values of the PIs from ODPA dianhy-
dride. Furthermore, all PIs show very small birefringence
values (Δn) in the range of 0.0067 to 0.0074. Such low
birefringences should be related to the highly flexible
phenyl-S-thiophene and thiophene-S-thiophene lin-
kages in the main chain. These results indicate that aro-
matic diamines having highly polarizable aromatic-
S-aromatic linkages are effective in increasing the refrac-
tive indices of PIs. In particular, the plural thiophene and
flexible thioether linkages in the APSTT diamine endow
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relatively good optical transparency. The transparency
was comparable to that of our previous high-n PI sample,
such as ref-PI (Scheme 3), indicating 64% transmittance
at 450 nm.18
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