Synthesis and characterization of perfluorinated arylenevinylene polymers

Article abstract of DOI:10.1002/pola.23782

Fully fluorinated arylenevinylene polymers have been synthesized via a methodology based on the Stille cross-coupling reaction and characterized by FTIR spectroscopy and MALDI-TOF mass spectrometry. Investigation of thin film properties by cyclic voltammetry and ellipsometry shows that complete substitution of hydrogen atoms with fluorine atoms on the conjugated backbone of the poly(arylenevinylene)s results in a strong increase of the band gap.

Full text of DOI:10.1002/pola.23782

Synthesis and Characterization of Perfluorinated Arylenevinylene
Polymers
1
2
2
2
3
ANTONIO CARDONE, CARMELA MARTINELLI, VITA PINTO, FRANCESCO BABUDRI, MARIA LOSURDO,
3
2
2
2
GIOVANNI BRUNO, PINALYSA COSMA, FRANCESCO NASO, GIANLUCA M. FARINOLA
1
Istituto di Chimica dei Composti OrganoMetallici, CNR-ICCOM, Dipartimento di Chimica, Universit a` degli Studi di Bari,
via Orabona, 4, Bari I-70125, Italy
2
Dipartimento di Chimica, Universit a` degli Studi di Bari, via Orabona, 4, Bari I-70125, Italy
3
Istituto per le Metodologie Inorganiche e dei Plasmi, CNR_IMIP, Dipartimento di Chimica, Universit a` degli Studi di Bari,
via Orabona, 4, Bari I-70125, Italy
Received 22 August 2009; accepted 8 October 2009
DOI: 10.1002/pola.23782
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Fully fluorinated arylenevinylene polymers have
been synthesized via a methodology based on the Stille
cross-coupling reaction and characterized by FTIR spectros-
copy and MALDI-TOF mass spectrometry. Investigation of
thin film properties by cyclic voltammetry and ellipsometry
shows that complete substitution of hydrogen atoms with
fluorine atoms on the conjugated backbone of the poly(arylene-
vinylene)s results in a strong increase of the band gap.
VC 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 48: 285–291, 2010
KEYWORDS: conjugated polymers; fluorinated polymers; poly
(arylenevinylene); Stille cross-coupling reaction; synthesis; thin
films
INTRODUCTION After the first report on organic light emit-
ting diodes (OLEDs) based on poly(p-phenylenevinylene)
air/moisture instability issues affect the resulting devices
unless careful encapsulation is provided. As a different
approach based on material design, functionalization of the
conjugated backbone with appropriate substituents can be
tailored to adjust the HOMO and LUMO energy levels for
matching the metal electrodes work function, thus favouring
well balanced charge injection. According to the theoretical
1
(
PPV) a large number of functionalized PPV polymers and,
more in general, poly(arylenevinylene) (PAV) derivatives
have been used as active materials in several optoelectronic
2
3
devices such as flat panel displays and photovoltaic cells.
Proper modification of the PPV conjugated backbone enables
not only to tune the colour of the light emission all over the
visible spectrum but also to tailor electronic properties in
order to optimise charge injection and transport in organic
light emitting diodes (OLEDs). Most of the PPVs used as elec-
troluminescent materials are poor electron acceptors due to
the high LUMO energy level and behave as p-type semicon-
ducting polymers with better ability to receive and transport
holes rather than electrons. Actually, the imbalance between
positive and negative charge carriers injection and transport
represents a major limitation to the electroluminescent
efficiency.
6
calculation, performed by Bredas and Heeger, introduction
of electron-withdrawing substituents onto either the aryl
ring or the vinyl group of PPVs would lower both the HOMO
and LUMO energy levels of the polymers, thus allowing the
fabrication of efficient devices even with high work function
metals, such as aluminium. Experimental evidence supports
the theoretical predictions: functionalization of vinylene units
7
–10
of PPVs with the cyano group
was in fact proven to
increase the efficiencies in bilayer OLED devices with PPV as
a hole-transporting layer and an aluminium electrode.
Fluorination has been explored as a chemical modification of
several classes of organic semiconductors, including arylene-
Different strategies have been pursued to improve negative
charge injection and mobility properties in PPV-based
OLEDs. Introduction of supplementary charge transporting
1
1
vinylene polymers,
with numerous intriguing features
mainly due to the special properties of the small halogen
atom, such as the high electronegativity (F is the most elec-
tronegative atom) and the strength of the CAF bond (472
kJ/mol). Based on these features, several improvements of
the materials properties can be expected. First, the electron
4
,5
layers between the cathode and the emissive organic thin
film represents a successful approach that, however, adds
complexity to the device architecture. The use of a low work
function metal cathode (e.g. calcium) can also be useful, but
Correspondence to: G. M. Farinola (E-mail: farinola@chimica.uniba.it)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 285–291 (2010) V
2009 Wiley Periodicals, Inc.
C
PERFLUORINATED ARYLENEVINYLENE POLYMERS, CARDONE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
withdrawing character of the fluorine atoms can lower both
the HOMO and LUMO levels of the polyconjugated systems,
thus improving electron transport properties and conferring
to these polymers ambipolar or n-tipe semiconductor charac-
spectra were recorded in CDCl3 on a Bruker AM 500 spec-
trometer at 500 and 125 MHz, respectively. The residual
CHCl3 signal at d ¼ 7.26 ppm and the CDCl at d ¼ 77.0
3
1
13
ppm were used as the standards for H NMR and C NMR
1
2,13
19
ter.
Second, the replacement of the carbon-hydrogen
spectra, respectively.
F NMR spectra were recorded in
bonds with the stronger and less reactive carbon-fluorine
bonds in poly(arylenevinylene) conjugated skeleton should
increase the thermal and chemical stability of the polymers.
At the present, it is clear that one of the major limiting fac-
tors in the wide commercialization of polymer-based LED
devices is represented by their insufficient long-term stabil-
CDCl on a Varian INOVA 400 spectrometer at 376 MHz, by
3
using CFCl3 as the internal standard. MS spectra were
recorded on a Shimadzu GCMS-QP 5000 spectrometer. Ele-
mental analyses were done by a Carlo Erba CHNS-O EA
1108-Elemental Analyzer. Melting points were determined
on a Gallenkamp capillary melting point apparatus. Cyclic
Voltammetry (CV) experiments were carried out under inert
argon atmosphere with an Autolab potentiostat PGSTAT 10
1
4–17
ity, mainly due to the oxidative photodegradation
of or-
ganic semiconductor layers. Fluorination is considered a pos-
sible strategy to improve lifetime of the materials in devices.
in CH CN containing 0.1 M tetrabutylammonium perchlorate
3
(
TBAP) at a scan rate of 50 mV/s. A three-electrode cell was
We have recently contributed synthetic methodologies for sev-
1
1,18,19
used. Pt wire was used as counter electrode, Ag wire was
used as quasi-reference electrode and ITO was used as work-
ing electrode. The energy level of the Ag/Ag quasi-reference
electrode was 4.54 eV below the vacuum level calibrated by
the Ferrocene/Ferrocene redox system. The films of the
polymers were deposited on the ITO working electrode by
thermal evaporation under reduced pressure (about 10
mbar). The optical response of films was investigated by
eral classes of fluorinated compounds
for photonics and
2
0
electronics, including PPV oligomers, and polymers selectively
þ
2
1,22–24
fluorinated on the aromatic rings
or on the vinylene
2
5–28
units. In particular, poly(2,3,5,6-tetrafluorophenyleneviny-
lene) (PTFPV) and random copolymers with dialkoxypheny-
þ
2
1
2
2–24
lene-vinylene and tetrafluorophenylene-vinylene units
ꢀ
5
have been synthesized via organometallic protocols based on
the Stille cross-coupling reaction of 1,4-diiodotetrafluoroben-
zene, and eventually diiododialkoxybenzene, with (E)-1,2-bis
3
2
spectroscopic ellipsometry.
Measurements at multiple
ꢁ
ꢁ
angles of incidence in the range 55 –70 were carried out to
investigate out-of-plane anisotropy in optical properties and
polarization of optical transitions using a phase-modulated
spectroscopic ellipsometer in the energy photon range 0.75–
6.5 eV (UVISEL, Jobin Yvon) with a resolution of 0.01 eV. In
order to check in-plane anisotropy, measurements were also
performed by in-plane rotation of the films. No difference in
the optical spectra was detected by varying the angle of inci-
dence and by in-plane rotation, suggesting that the film is
isotropic due to the deposition methodology. To derive the
spectral dependence of the refractive index, n, and extinction
coefficient, k, films were analyzed using a two-layer optical
model consisting of the isotropic film and the surface rough-
ness (the latter consisting of a mixture of 50% films and
(
tributylstannyl)ethylene, under Pd(0) catalysis. The PTFPV
polymer was used as the emitting layer in an OLED device
obtaining blue-green light emission. More recently, the Stille
reaction polymerization was extended to the preparation of
2
5–28
PPVs containing fluorinated vinylene units,
starting from
diiodoaryl derivatives and (E)-(1,2-difluoro-1,2-ethenediyl)bis
2
5,29
[
tributylstannane].
Interesting marks were obtained regard-
ing the average molecular weights of polymers, which resulted
in higher value compared to those of other typical Stille-based
polymerizations. The new fluorinated polymers turned out to
be deep-blue emitting materials. Theoretical simulation sug-
gested that the observed strong increase of the energy gap is
due not, as could be in principle expected, to the electronic
effect of the F atoms, but rather to steric repulsion between the
fluorine atoms on the vinylene units and the oxygen atoms on
the aromatic rings causing a strong distortion of the conjugated
3
2
5
0% voids). The energy dispersion of the polymer optical
3
3
constants was modelled using two Tauc-Lorentz oscillators.
3
0
backbone. PPVs with fluorinated double bonds were also
Materials
3
1
reported by Jin et al., who demonstrated that fluorine func-
tionalization increases the electroluminescence efficiency of the
materials in a bilayer light emitting diode, compared to the non-
fluorinated counterpart.
All chemical were purchased from Aldrich, Alfa Aesar, and
ABCR GmbH & Co. KG and used without further purification.
THF was distilled from sodium and benzophenone under a
nitrogen atmosphere. CCl4 was distilled from P2O5 under a
nitrogen atmosphere. DMF and iPr NH were distilled from
2
Continuing our investigation on fluorinated arylenevinylene
polymers, we report herein the first examples of fully
fluorinated poly(arylenevinylene)s, namely the poly(1,4-
tetrafluorophenylene-difluorovinylene) 6F-PPV and the
poly(2,5-difluorothienylene-difluorovinylene) 4F-PTV focus-
sing on the synthesis and an evaluation of their basic elec-
trical and optical properties.
4
Å molecular sieves under a nitrogen atmosphere. Chlorotri-
methylsilane was distilled before use. All solvents were
freshly distilled immediately prior to use. Column chroma-
tography was performed using silica gel 60, 40–63 lm from
Merck. Merck silica gel 60 F254 aluminium sheets were used
for analytical TLC.
Preparation of Monomer
EXPERIMENTAL
3,4-Difluoro-2,5-diiodo-thiophene (M3)
Characterization Methods
FT-IR spectra were measured on a Perkin Elmer 1710 spec-
trophotometer using dry KBr pellets. H NMR and C NMR
To a solution of 3,4-difluoro-2,5-bis(trimethylsilyl)thiophene
(1.13 g, 4.27 mmol) in anhydrous CCl4 (30 mL) cooled at
0 C, a solution of ICl (1.53 g, 9.42 mmol) in anhydrous CCl4
1
13
ꢁ
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ARTICLE
SCHEME 1 Synthesis of the 6F-PPV and 4F-PTV polymers.
(
10 mL) was added dropwise. The resulting mixture was
(15 mL) and anhydrous DMF (15 mL) was stirred at room
temperature under a nitrogen atmosphere for 7 days. Then
methanol was added to the reaction mixture and the precipi-
tate transferred into a Soxhlet apparatus and purified in the
same way of polymer 6F-PPV, by extraction with methanol
(24 h), hexane (24 h), and chloroform (12 h). The pure poly-
mer was recovered as an insoluble red powder (0.48 g, yield
allowed to warm to room temperature and stirred overnight.
Than the reaction mixture was poured in 5% aq. Na S O
2 3
(
The organic layers were combined and dried over anhydrous
Na SO . After removing the solvent at reduced pressure, the
crude residue was purified by distillation with a Kugelrohr
2
100 mL) and extracted with dichloromethane (3 ꢂ 40 mL).
2
4
ꢁ
ꢁ
apparatus (P ¼ 100 mbar, T ¼ 60 C), to yield the product
88%) and does not melt up to 300 C.
as a colourless liquid (1.27 g, yield 80%).
RESULTS AND DISCUSSION
1
9
F NMR (376 MHz, CDCl , ppm) d: ꢀ124.28 (s).
3
Synthesis of the Monomers and Polymers
The two polymers 6F-PPV and 4F-PTV were synthesized via
the Stille cross-coupling reaction between the fluorinated
bis(tributylstannyl) alkene M1 and the corresponding bis-
iodo perfluorinated aromatic monomers M2 and M3, respec-
tively, as summarized in Scheme 1.
1
3
C NMR (125 MHz, CDCl
3
, ppm) d: 147.86 (dd, J ¼ 265, 21
Hz), 55.66 (dd, J ¼ 18, 8.6 Hz).
MS (EI, 70 eV): m/z (%) ¼ 372 (M , 100), 245 (9), 118
15).
þ
(
ꢀ
1
FT-IR (KBr, cm ): 1503, 1412, 1174, 1165, 1002.
The effective catalyst for the cross-coupling of (E)-(1,2-
difluoro-1,2-ethenediyl)bis[tributylstannane] M1 with 1,4-
diiodotetrafluorobenzene M2, was generated in situ from
Anal. Calcd. for C F I S: C, 12.92; S, 8.62. Found: C, 12.90; S,
4
2 2
8.63.
Pd (dba)3 and tris(2-furyl)phosphine, and the reaction mix-
2
Preparation of Polymers
Poly[(2,3,5,6-tetrafluoro-1,4-phenylene)-
ture was refluxed for 1 week. The polymer 6F-PPV was puri-
fied by washing the crude product in a Soxhlet apparatus,
subsequently with hexane (24 h), methanol (24 h), and chlo-
roform (12 h), and recovered as an insoluble dark-green
powder in 65% yield.
1
,2-difluorovinylene] (6F-PPV)
A mixture of 1,4-diiodo-2,3,5,6-tetrafluorobenzene (2.5 g,
.22 mmol), (E)-(1,2-difluoro-1,2-ethenediyl)bis[tributylstan-
nane] (4.0 g, 6.22 mmol), catalyst Pd (dba) (0.174 g, 0.19
6
2
3
The polymerization reaction between 3,4-difluoro-2,5-diiodo-
thiophene M3 and (E)-(1,2-difluoro-1,2-ethenediyl)bis[tri-
butylstannane] M1 was carried out using Pd(PPh3)4 as the
catalyst in the presence of CuI in stoichiometric amount, at
room temperature in DMF/THF mixture as solvent, for
mmol) and ligand tri(2-furyl)phosphine (0.088 g, 0.38 mmol)
in anhydrous THF (40 mL) was refluxed under a nitrogen
atmosphere for 7 days. After cooling to room temperature,
the solvent was evaporated at reduced pressure and the
crude residue was purified by extraction in a Soxhlet appara-
tus with hexane (24 h), ethanol (24 h), and chloroform
1
week. The resulting crude polymer 4F-PTV was purified as
in the previous case and recovered as an insoluble red pow-
der in 88% yield.
(
12 h) in order to eliminate catalyst residue, tributyltin
iodide, and low molecular weight fractions. The pure poly-
mer was recovered as an insoluble dark -green powder (0.85
g, yield 65%) and does not melt up to 300 C.
The organometallic monomer M1 was synthesized as
ꢁ
25
reported in literature. Monomer M2 is a commercially
available compound, whereas monomer M3 was synthesized
modifying the literature procedure, as shown in Scheme 2.
Poly[(3,4-difluoro-2,5-thienylene)-
3
4
1
,2-difluorovinylene] (4F-PTV)
A mixture of 3,4-difluoro-2,5-diiodothiophene M3 (1.0 g,
.69 mmol), (E)-(1,2-difluoro-1,2-ethenediyl)bis[tributylstan-
nane] (1.73 g, 2.69 mmol), catalyst Pd(PPh3)4 (0.154 g,
.13 mmol) and CuI (0.51 g, 2.69 mmol) in anhydrous THF
Lithiation of commercial 2,5-dibromothiophene 1 with two
equivalents of LDA followed by quenching with chlorotrime-
thylsilane yielded the intermediate compound 2, that was
fluorinated with (PhSO ) NF and nBuLi to afford the
2
0
2
2
PERFLUORINATED ARYLENEVINYLENE POLYMERS, CARDONE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
preparation of samples for the MALDI-TOF analysis, TOF
mass spectra of the two polymers were recorded using a
sample protocol preparation that we had previously devel-
2
1
oped for the insoluble fluorinated PPV derivative PTFPV.
The TOF mass spectra (Fig. 2) were obtained after matrix
assisted laser desorption-ionization (MALDI), in positive
mode for 4F-PTV and in negative mode for 6F-PPV, using
dithranol and 9-aminoacridine hemihydrate as matrixes,
respectively.
SCHEME 2 Synthesis of the monomer M3.
The masses calculated account for the peaks within the limit
of the accuracy of about 1%, intrinsically due to the particu-
lar sample preparation procedure.
compound 3. The final step was a iodo-desilylation of com-
ꢁ
pound 3, performed with ICl in CCl at 0 C. The monomer
4
M3 was purified by distillation with a Kugelrohr apparatus
and a pure colourless liquid was obtained in 80% yield.
The polymeric chains detected by MALDI-TOF spectrometer
range approximately from 3 to 9 repeating units for 6F-PPV
(
Fig. 3) and from 6 to 12 for 4F-PTV (Fig. 4). Mass discrimi-
Characterization
nation phenomena in favour of low molecular weight compo-
nents, which can occur in MALDI-TOF mass spectrometry of
polydisperse polymers may have prevented detection of
The scarce solubility of both the polymers synthesized not
only in common organic solvents but also in several per-
fluorinated solvents (octafluorotoluene, perfluorooctane, pen-
tafluoropyridine, and pentadecafluorotriethylamine), pre-
vented the structural characterization with classical solution
NMR spectroscopy and molecular weight determination by
GPC technique. Therefore, the materials were characterized
by FT-IR spectroscopy and MALDI-TOF mass spectrometry.
3
5
The FT-IR spectra of polymers 4F-PTV and 6F-PPV are
shown in Figure 1. Three key peaks appear common to the
ꢀ
1
two polymers: the peaks at 1511 cm (4F-PTV) and 1480
ꢀ
1
cm (6F-PPV), that can be assigned to the C ¼¼ C stretching
vibration of the conjugated double bond of the vinylene units
with the carbon atoms bonded to fluorine atoms; the peaks
ꢀ
1
ꢀ1
at 1165 cm (4F-PTV) and 1173 cm (6F-PPV) attributed
to the CAF stretching vibration of the conjugated vinylene
ꢀ
1
ꢀ1
units; the peaks at 1014 cm (4F-PTV) and 989 cm (6F-
PPV) assigned to the CAF stretching vibration of the per-
fluorinated aromatic rings.
Although the polymers are scarcely soluble in the common
organic solvents, including those usually employed for the
FIGURE 2 MALDI-TOF mass spectra of the 6F-PPV (a) and 4F-
PTV (b) polymers. Ions are generated using a 337-nm nitrogen
laser beam. The spectra were recorded with a sum of 1000
shots acquired in linear mode. Baseline corrections were
applied for a better quality of the spectra and the matrix sig-
nals are left outside the mass range reported.
FIGURE 1 FT-IR spectra of the 6F-PPV and 4F-PTV polymers in
KBr disk.
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ARTICLE
longer chains of 6F-PPV and 4F-PTV. On the other hand, in
our case low molecular weights can be expected due to the
precipitation of growing polymer chains caused by their
scarce solubility in the reaction medium.
In principle, the limited extension of the conjugated chains
does not represent a drawback to the application of semi-
conducting polymers in photonics and electronics. In fact,
our previous reports on low molecular weight PPV polymers
typically obtained via the Stille reaction demonstrated that
oligomeric materials can however exhibit optical and electri-
cal properties suitable for applications in devices, with the
FIGURE 4 4F-PTV polymer chains with all the possible
terminations.
Electrochemistry
3
6
benefit of easier processability. This feature is crucial in
the case of insoluble polymers, as only low molecular weight
materials can be expected to be processable by thermal
evaporation. This is in fact the case for the polymers 6F-PPV
and 4F-PTV, that could be thermally evaporated as thin film
The electrochemical properties of the two polymers were
investigated by CV performed in a solution of TBAP in aceto-
nitrile at a scan rate of 50 mV/s at room temperature under
inert atmosphere. An ITO substrate coated with a polymer
ꢀ5
film (thermally evaporated at reduced pressure: 10 mbar)
ꢀ
5
under reduced pressure (about 10 mbar).
þ
was used as working electrode. Pt wire and Ag/Ag elec-
trode were used as counter and reference electrodes, respec-
tively. The CV of the two polymer films onto ITO substrate
are reported in Fig. 5.
The analysis of the MALDI-TOF spectra of the two polymers,
reveals peaks belonging to 6F-PPV and 4F-PTV chains with
some of the possible combinations of terminal groups (Figs.
3
and 4).
The cathodic scan of the 6F-PPV film evidences an irreversi-
ble reduction process at about ꢀ1.40 V, with a weak signal
at ꢀ1.14 V. The anodic scan shows no particular signals
even at very positive potentials. For the 4F-PTV film, two
intense irreversible electrochemical signals at about 1.62 V
and ꢀ1.26 V are recorded. In particular, moving toward neg-
ative potentials, the reduction of 4F-PTV shows a reduction
In the MALDI-TOF spectrum of 6F-PPV, the five series of
peaks labelled as A4, A3, A2, B3, and B2 can be identified
and assigned to five kinds of polymeric chains differing for
the terminal groups. The peaks A4 and B3 correspond to
polymeric chains with only H-atoms as end groups whereas
the peaks A3 and B2 can be assigned to polymeric chains
with I and H atoms as end groups. The peaks designed as
A2 can be attributed to polymeric chains containing the
SnBu3 group attached to the terminal vinylene unit. The
presence of some chains bearing the SnBu3 group is also
confirmed by X-ray photoelectron spectroscopy (XPS) mea-
surements performed on thin film obtained by thermal evap-
oration under high vacuum, and by FT-IR spectrum showing
the presence of asymmetric CAH stretching of the methyl
ꢀ
1,
and methylene groups (2960 and 2920 cm
respectively)
of the SnBu residue.
3
In the MALDI-TOF spectrum of 4F-PTV, three principal series
of peaks (among all the possible reported in Fig. 4) can be
identified, which are labelled as D4, E3, E2. The peaks D4
and E3 can be assigned to polymeric chains with only H-
atoms as end groups and the peaks E2 to polymeric chains
terminating with thiophene rings bearing I and H atoms. No
evidence of SnBu residue was observed in the case of poly-
3
mer 4F-PTV, as also confirmed by XPS analysis on thin film
and by FT-IR spectroscopy.
FIGURE 3 6F-PPV polymer chains with all the possible
FIGURE 5 Cyclic voltammetry of the 6F-PPV and 4F-PTV
terminations.
polymers.
PERFLUORINATED ARYLENEVINYLENE POLYMERS, CARDONE ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
peak at ꢀ1.10 V, that could be attributed to the irreversible
HOMO-LUMO transition, has been found at 2.55 6 0.02 eV
and 4.18 6 0.03 eV.
3
7
loss of fluoride ions, and a more intense reduction peak at
ꢀ
1.26 V due to the irreversible reduction of the p-conjugated
A comparison between the 4F-PTV energy gap value deter-
mined by CV Eg,EC (2.57 eV) and that obtained by optical spec-
troscopy Eg,opt from the onset of absorption (2.07 eV) reveals
system. These potential values are in good agreement with
those reported for a thienylene-vinylene polymer fluorinated
on the vinylene units and prepared via electrochemical poly-
that, as most commonly reported in the literature, E is
g,EC
3
7
merization. The HOMO and LUMO energy levels of the two
polymers were estimated using the equations EHOMO
40–43
higher than Eg,opt
.
This effect has been explained, from a
¼
fundamental point of view, considering that the electrochemi-
cal determination of bandgap involves the formation of charge
carriers, demanding for higher energy than a bound exciton
3
8
ꢀ
e(Eox þ 4.54) (eV) and ELUMO ¼ ꢀe(Ered þ 4.54) (eV),
where E and E are the onset potentials for oxidation and
ox
red
reduction, and the electrochemical band gap Eg,EC was calcu-
lated as the difference between the HOMO and LUMO levels.
Therefore, according to the CV, we estimated the HOMO and
LUMO energy levels for the polymer 4F-PTV at ꢀ6.08 and
40
detectable optically. In addition, higher values of Eg,EC com-
pared to Eg,opt (calculated by absorption onset) can be attrib-
uted to interface energy barriers present between the polymer
film and the electrode surface, which have been demonstrated
to be also related to the insulating (or shielding) ability of the
ꢀ
3.51 eV, respectively, and the Eg,EC ¼ 2.57 eV. For the poly-
mer 6F-PPV, it was not possible to determine the E
and
41
g,EC
substituents bonded to the main chain. Furthermore, it
the HOMO energy level, but only the LUMO energy level at
3.45 eV.
should be also considered that the onset of optical absorption
might be affected by tail band states due to some structural/
morphological disorder of polymeric chains in the film. There-
fore, the onset of absorption assumes a lower value than the
true band gap.
ꢀ
Optical Properties
Optical properties of the polymer thin films obtained by
thermal evaporation were investigated by spectroscopic
ellipsometry. The derived spectra of the refractive index, n,
and extinction coefficient, k, of thin films of the 6F-PPV and
Interestingly, in our case, the maximum of absorption (2.55
eV) approximately coincides with the electrochemical value
4F-PTV polymers are shown in Fig. 6.
Eg,EC ¼ 2.57 eV.
The refractive index of conjugated polymers is an important
parameter for modelling the angular dependence of emission
from polymer LEDs,
waveguiding in potential polymer laser diode structures. We
find the dispersion of the refractive indexes, particularly for
the 6F-PPV, to be significant, with the index increasing to-
ward the absorption band, as one might expect.
The comparison of the above band gap values with those of
the corresponding hydrogenated polymers (poly[2,5-thieny-
3
9
4
4
emission from microcavities, and
lenvinylene] (PTV) band gap ¼ 1.7 Ev and PPV band gap ¼
4
5
2
.4 eV ) clearly shows that the effect of the perfluorination is
a strong blue-shift of the optical bandgap, a shift which is
more pronounced for 6F-PPV than for 4F-PTV. The lower
blue-shift measured for the 4F-PTV can be explained by con-
sidering the electron rich nature of the thiophene ring, which
can act as electron donor moiety able to reduce the band gap.
Interestingly, the 6F-PPV has a band gap even larger than that
Interestingly, an optical Tauc band gap, Eg,opt (defined as the
onset of the absorption, where the extinction coefficient
k=0) of 2.07 6 0.06 eV and 3.34 6 0.05 eV has been deter-
mined for the 4F-PTV and 6F-PPV polymers, respectively.
The maximum of the absorption, which corresponds to the
2
6,27
recently reported for the MEH-PPDFV.
Several structural
factors can be responsible of this strong blue-shift, including
the reduced polymer chains length, the electron-withdrawing
effect of the fluorine atoms and the steric repulsion between
the fluorine atoms ortho to the vinylene units on the aromatic
rings and those on the double bonds.
Theoretical and spectroscopic investigation are warranted to
explain the role of the different structural factors on the opto-
electronic properties of the perfluorinated arylenevinylene poly-
mers, and the synthetic study here reported opens the way to
further research on these intriguing materials.
CONCLUSIONS
We have synthesized two new arylenvinylene polymers fully
fluorinated on the aromatic rings and on the vinylene units.
The polymers were obtained in good yield via the Stille
cross-coupling reaction of (E)-(1,2-difluoro-1,2-ethenediyl)-
bis[tributylstannane] with 3,4-difluoro-2,5-diiodothiophene
or 1,4-diiodo-2,3,5,6-tetrafluorobenzene. The polymers were
insoluble in common organic and in perfluorinated solvents
but were easily processed as thin films by thermal evapora-
tion at reduced pressure and they were characterized by FT-
FIGURE 6 Spectra of the refractive index, n, (left axis) and of
the extinction coefficient, k, (right axis) of thin films of the 6F-
PPV (black lines) and 4F-PTV (grey lines) polymers.
290
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
IR spectroscopy and MALDI-TOF mass spectrometry. Electro-
chemical properties were analyzed by CV and optical proper-
ties by ellipsometry. The results evidence a strong increase
of the band gap caused by complete substitution of hydrogen
atoms with fluorine atoms on the conjugated backbone of
the investigated polymers.
21 Babudri, F.; Cardone, A.; Chiavarone, L.; Ciccarella, G.; Farinola, G.
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