COMMUNICATION
Ultra-photostable n-type PPVs
Youngmi Kim and Timothy M. Swager*
Received (in Cambridge, UK) 23rd August 2004, Accepted 11th October 2004
First published as an Advance Article on the web 30th November 2004
DOI: 10.1039/b412948b
Poly(p-phenylenevinylene)s containing trifluoromethyl substi-
tuted aromatic rings (CF3-PPVs) exhibit high photooxidative
stability to give robust materials suitable for demanding
applications.
Luminescent conjugated polymers (CPs), especially poly(p-pheny-
lenevinylene) (PPV) based polymers, have been widely investigated
in a variety of applications including light-emitting diodes,1 solar
cells,2 and optically pumped lasers.3 In spite of their desirable
attributes of high emission efficiency, fine-tunability of their band
gap, and good processibility, dramatic improvements in the
durability and stability of CPs are necessary for many applications.
The design of organic electronic polymers having enhanced
photooxidative stability and charge transport properties is of
particular importance for application in solar cells.4 Efforts to
increase the stability of CPs include encapsulation to protect them
from oxygen and moisture5 and the incorporation of electron-
withdrawing groups such as cyano or trifluoromethyl on the
vinylene units of PPV to decrease photooxidation.6
Scheme 1 (a)NBS,CF3CO2H,H2SO4,60uC;(b)(i)n-BuLi,THF,278uC,
(ii) CO2; (c) BH3–THF, RT; (d) PBr3, THF; (e) KOtBu, THF, RT.
procedure and poly(9,9-dioctylfluorene-2,7-diyl) (PF, Mn 64,000,
PDI 5 2.9) was purchased from H.W. SANDS Corp.9
We also synthesized an alternating copolymer containing CF3-
PV and MEH-PV units, poly-[2-methoxy-5-(29-ethylhexyloxy)-1,
4-phenylenevinylene-alt-2,5-bis(trifluoromethyl)-1,4-phenyle-
nevinylene] (alt-co-PPV) by Wittig condensation of 2,5-
bis(trifluoromethyl)-1,4-benzenedicarboxaldehyde (7) with
2-methoxy-5-(29-ethylhexyloxy)-1,4-xylenebis(triphenylpho-
sphonium bromide) (9) as shown in Scheme 2. The mixture
was quenched with 2% HCl solution and then poured into
methanol. The polymer was isolated by filtration, dried, and
reprecipitated in MeOH to give an orange solid with a yield
of ca. 55%. The alt-co-PPV (Mn 5500, PDI 5 1.35, THF) was
highly soluble in common organic solvents whereas CF3-PPV
(Mn 25,000, PDI 5 1.25, THF) showed poor solubility in
CH2Cl2, or CHCl3 and partial solubility in THF and DMF.{
The latter solvents allowed solution photoluminescence
studies and the spin coating of thin films.
Although PPV derivatives containing electron-withdrawing
substituents at vinylene units have shown improved photooxida-
tive stability, undesirable conformational changes arise from this
substitution as a result of increased steric hindrance.7 The resulting
nonplanarity can disturb the transport properties and result in a
strong reduction of fluorescence yields due to torsional induced
nonradiative deactivation.8
Herein, we report the synthesis of PPVs containing perfluor-
oalkyl groups attached to the phenyl rings and the dramatic
improvement in their stability against photooxidation. The
chemical structures of the perfluorinated CPs and their syntheses
are depicted in Schemes 1 and 2. As shown in Scheme 1, the
synthesis of monomer 5,{ 1,4-bis(trifluoromethyl)-2,5-dibromo-
methylbenzene, involves bromination of 1,4-bis(trifluoromethyl)-
benzene (1) to obtain 2 with 91% yield in the presence of NBS/
CF3COOH and H2SO4. Lithiation of 2 with n-butyllithium in
THF followed by addition of CO2 gives 2,5-bis(trifluoromethyl)-
terephthalic acid (3), and reduction leads to the corresponding diol
(4). Monomer 5 is prepared from bromination of 4 and is
polymerized by treatment with excess of KOtBu in THF at room
temperature for 1 day. The reaction mixture was poured into
methanol, and the resulting precipitate was filtered and washed
with methanol, hexane, and acetone. The solid was dried to give
poly-[2,5-bis(trifluoromethyl)-p-phenylenevinylene] (CF3-PPV) as
a green fluorescent solid with a yield of ca. 65%. In order to
compare its stability with that of other CPs, poly-2-methoxy-5-
(29-ethylhexyloxy)-phenylenevinylene (MEH-PPV), a widely used
molecular electronic material, was prepared by the literature
As shown in Fig. 1, the UV-visible absorption spectrum of the
CF3-PPV in THF has a maximum at 361 nm and the emission
spectrum displays a large Stokes-shift with a peak maximum at
489 nm and a shoulder at 514 nm. The magnitude of the Stokes
shift suggests large differences in the conformation of the
polymer’s ground and excited states and energy migration to
minority segments having greater conjugation lengths.10 As
expected with CF3 substitution the absorption and emission
spectra of alt-co-PPV are blue shifted relative to MEH-PPV. CF3-
PPV showed relatively high fluorescence quantum yields in both
THF solution (0.63, standard: quinine sulfate in 0.1 N H2SO4) and
in thin films (0.25, standard: 0.01 mol% 9,10-diphenylanthracene
in PMMA). In contrast we find MEH-PPV to have lower
fluorescence quantum yields in solution (0.08, standard: fluorescein
in 0.1 N NaOH) and in thin films (0.03, standard: 0.01 mol% 9,10-
diphenylanthracene in PMMA).§
We have investigated photostabilities of CF3-PPV, alt-co-PPV,
MEH-PPV, and PF thin films. The photooxidation studies were
performed by continuous UV irradiation of polymer thin films
using a 450 W steady-state Xe lamp as the irradiation source (slit
width 5 10 nm) under aerobic conditions. Polymer films having
*tswager@mit.edu
372 | Chem. Commun., 2005, 372–374
This journal is ß The Royal Society of Chemistry 2005
Kim, Youngmi
