Supramolecular ordering, thermal behavior, and photophysical, electrochemical, and electroluminescent properties of alkoxy-substituted yne-containing poly(phenylene-vinylene)s

Article abstract of DOI:10.1021/ma0488111

The synthesis and the characterization of defect-free alkoxy-substituted yne-containing poly(phenylene-vinylenes) was investigated. It was found that a degree of 7 was obtained for all the polymers enabling a reliable comparison of their properties as a function of the length and geometry of the grafted side chains. A formation of crystalline superstructures of either spherulitic-like or rodlike morphology was identified as following typical nucleation and growth process for polymer having a symmetric substitution pattern. Results show that the high absorption coefficients and high quantum yields in the solid state combined with enhanced oxidation stability made polymers as potential donor materials in the design of organic solar cells.

Full text of DOI:10.1021/ma0488111

Macromolecules 2004, 37, 7451-7463
7451
Supramolecular Ordering, Thermal Behavior, and Photophysical,
Electrochemical, and Electroluminescent Properties of
Alkoxy-Substituted Yne-Containing Poly(phenylene-vinylene)s
Da n iel Ayu k Mbi Egbe,*,† Ben ja m in Ca r bon n ier ,‡ Lim in g Din g,§
Da vid Mu1 h lba ch er , Eck h a r d Bir ck n er ,| Ta d eu sz P a k u la ,*,‡ F r a n k E. Ka r a sz,§ a n d
Ulr ich -Wa lter Gr u m m t|
Institut fu¨r Organische Chemie und Makromolekulare Chemie der Friedrich-Schiller Universita¨t J ena,
Humboldtstrasse 10, D-07743 J ena, Germany, Max-Planck-Institut fu¨r Polymerforschung,
Ackermannweg 10, D-55128 Mainz, Germany, Department of Polymer Science and Engineering,
University of Massachusetts, Amherst, Massachusetts 01003, Konarka Austria Forschungs-u.
Entwicklungs GmbH, Gruberstrasse 40-42, A-4010 Linz, Austria, and Institut fu¨r Physikalische
Chemie der Friedrich-Schiller Universita¨t J ena, Lessingstrasse 10, D-07743 J ena, Germany
Received J une 16, 2004; Revised Manuscript Received J uly 17, 2004
ABSTRACT: Alkoxy-substituted and defect-free yne-containing poly(phenylene-vinylene)s, 15a -d ,
having the general constitutional unit (-Ph-CtC-Ph-CHdCH-Ph-CHdCH-)n synthesized through
the olefination reactions of the fluorophoric dialdehyde 7 [1,2-bis(4-formyl-2,5-dialkoxyphenyl)acetylene]
with various 2,5-dialkoxy-p-xylylenebis(diethylphosphonates), 14a -d , are reported. A new synthetic route
to 1-bromo-2,5-dialkoxybenzaldehyde (starting material for the synthesis of 7) allowing the grafting of
all types of alkoxy side chains is also reported. Bulk state properties of the four polymeric materials
have been analyzed using various experimental methods giving information on the thermal behavior as
well as self-assembling morphologies at different size scales. Crystalline superstructures of either
spherulitic or rodlike morphology, comprised of polymeric backbone layers separated by the side chains
are identified to develop by nucleation and growth process for symmetrically alkoxy substituted 15a -c.
Discrepancy between optical (Egopt) and electrochemical (Egec) band gaps was observed to be dependent
on the bulkiness (15c) and length (15b) of the grafted side chains. Octadecyloxy side chains in 15b not
only lead to well structured photoluminescence and electroluminescence spectra and higher fluorescence
quantum yields but also allow the design of LED-devices of the configuration ITO/PEDOT/polymer/Ca
with improved parameters (ηext ) 2.15%, luminance ) 5760 cd/m2) relative to the octyloxy-substituted
polymer 15a (ηext ) 0.79%, luminance ) 1406 cd/m2). Despite the differences in the conjugation pattern
between polymers 15 and polymers 17 [(-Ph-CtC-Ph-CtC-Ph-CHdCH-Ph-CHdCH-)n], an
identical chromophore system of structure Ph-CtC-Ph-CHdCH-Ph-CHdCH-Ph-CtC-Ph was
found to be responsible for their emissive behavior as confirmed by fluorescence kinetics measurements
(τ, kf, knr) and quantum chemical calculations. The differences observed in their solid-state photophysical
properties are ascribed to the combined effects of the backbone rigidity (as a function of the number of
-CtC- moieties in the repeating unit) and the nature of the side chains. Moreover less -CtC- units
within polymers 15 lead to the more than 100-fold improvement of their electroluminescent parameters
compared to polymers 17.
In tr od u ction
(phenylene-ethynylene)s (PPEs) are among the most
widely studied π-conjugated polymers.2a,7,8 With the
aim to combine the interesting intrinsic properties of
both classes of compounds into one polymeric material,
our group and others designed alkyl- and/or alkoxy-
substituted hybrid phenylene-vinylene/phenylene-
ethynylene polymers (PPV-PPE)s of well-defined re-
peating units backbones (-Ph-CtC-Ph-CHdCH-)n
and (-Ph-CtC-Ph-CtC-Ph-CHdCH-Ph-CHd
CH-)n, 17, corresponding to a 1 to 1 ratio of the -CtC-
and -CHdCH- moieties.9,10
The electronic and optical properties of conjugated
polymers are controlled both by the primary molecular
structure (intramolecular functionality: π-conjugation)
and by the supramolecular organization (intermolecular
interactions: π-stacking) in a similar way to that in
which secondary and ternary structures are fundamen-
tal to the functions of proteins.11,12 Control over the
supramolecular interactions have been mainly imple-
mented and mediated via the structural, chemical, and
electronic properties of side chains.12,13 Thorough in-
vestigations of the influence of the alkoxy side chains
The design and synthesis of soluble conjugated poly-
mers are presently subject of great research interest,
as a result of their wide range of applications in
optoelectronic devices1 such as light-emitting diodes
(LEDs),2 light-emitting electrochemical cells,3 field
effect transistors (FETs),4 lasers,5 and organic solar
cells.6 These materials have the advantage of a facile
color tunability, good film-forming properties, and
adequate mechanical properties in comparison to their
inorganic counterparts.1,7 Poly(phenylene-vinylene)s
(PPVs) and their dehydrogenated congeners poly-
* Corresponding authors. (D.A.M.E.) Telephone: (+49)-3641-
948245. Fax: (+49)-3641-948202. E-mail: c5ayda@uni-jena.de.
(T.P.) Telephone: (+49)-6131-379113. Fax: (+49)-6131-379100.
E-mail: pakula@mpip-mainz.mpg.de.
† Institut fu¨r Organische Chemie und Makromolekulare Chemie
der Friedrich-Schiller Universita¨t J ena.
‡ Max-Planck-Institut fu¨r Polymerforschung.
§ University of Massachusetts.
Konarka Austria Forschungs-u. Entwicklungs GmbH.
| Institut fu¨r Physikalische Chemie der Friedrich-Schiller Uni-
versita¨t J ena.
10.1021/ma0488111 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/08/2004

7452 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
Sch em e 1
on the photophysical properties of polymers 17 have
been previously reported.10b,d In this article, we report
the synthesis and characterization of alkoxy-substituted
yne-containing poly(phenylene-vinylene), 15a-d, whose
repeating units (RU), (-Ph-CtC-Ph-CHdCH-Ph-
CHdCH-)n, incorporate -CtC- and -CHdCH- moi-
eties in a 1 to 2 ratio. Our synthetic strategy consisting
of first applying the Sonogashira cross-coupling reac-
tion8 in combination with chromatographic work up to
obtain pure monomeric dialdehyde 7 before proceeding
with the Horner-Wadsworth-Emmons olefination re-
action in the polycondensation process, using the syn-
thetic and work up protocols as described elsewhere by
us,10 provides highly pure polymers deprived of diyne-
defect structures. Alkoxy side chains of various length
and geometry have been grafted on the phenylene-
vinylene moieties of the polymers. What effect does
macromolecular design (conjugation pattern within the
backbone repeating unit (RU) and nature of side chains)
have on the solid-state properties of the hybrid polymers
15a -d ? An attempt to give an answer to this question
is the subject of this article, in which complementary
analytical methods have been employed to account for
the properties of the compact bulk materials. Thermal
and phase behaviors were determined using thermo-
gravimetric analysis (TGA), differential scanning calo-
rimetry (DSC), and polarized optical microscopy (POM).
The superstructure morphologies were established using
small-angle light scattering (SALS) experiments, while
assignment of the packing structure was documented
by analysis of the 2D diffraction patterns recorded for
uniaxially oriented specimens. Moreover the photo-
physical (absorption and emission in solution as well
as in solid state, fluorescence kinetics in solution,
electroluminescence (EL), and photoconductivity) and
electrochemical properties of the new compounds have
been investigated. The electrochemical data were de-
termined through the combination of cyclic voltammetry
(CV) and electrochemical voltage spectroscopy (EVS).
Comparison of the photophysical properties of hybrid
polymers 15 with those of hybrid polymers 17 revealed
an identical π-conjugated pattern of structure Ph-Ct
C-Ph-CHdCH-Ph-CHdCH-Ph-CtC-Ph to con-
stitute the effective chromophore system responsible for
the emissive behavior of both classes of polymers. This
was confirmed by solution fluorescence kinetics mea-
surements and quantum chemical calculations. How-
ever reducing the number of -CtC- units within the
conjugated backbone from polymers 17 to polymers 15
is the source of more than 100-fold improvement of the
EL parameters.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The prerequisite
for the synthesis of defect-free yne-containing poly-
(phenylene-vinylene)s 15 is the prior synthesis of
fluorophoric dialdehyde 7 [1,2-bis(4-formyl-2,5-dialkoxy-
phenyl)acetylene]. The synthetic pathway to 7 is il-
lustrated in Scheme 1. The most important component
in this synthetic series is 4-bromo-2,5-dialkoxybenzal-
dehyde (4), which can be obtained through two synthetic
routes. According to route A shown in Scheme 1, 4 is
obtained in a three-step reaction from hydroquinone (1)
as described elsewhere.10d The last step of this synthetic
route, the Bouveault formylation14 of 1,4-dibromo-2,5-
dialkoxybenzene (3) using 1 equiv each of butyllithium

Macromolecules, Vol. 37, No. 20, 2004
Yne-Containing Poly(phenylene-vinylene)s 7453
Sch em e 2
Ta ble 1. Da ta fr om GP C (p olystyr en e a s sta n d a r d , THF
a s elu en t)
following text polymer 15a is referred as standard
compound owing to its regular structure (R1 ) R2
)
octyl). Polymers 15b (R1 ) R2 ) octadecyl), 15c (R1 )
R2 ) 2-ethylhexyl) and 15d (R1 ) methyl, R2 ) 2-ethyl-
hexyl) will respectively enable the emphasis of the
effects of side chains extension, side chains geometry
(branched vs linear) and a quasi removal of one side
chain.
code
Mh n
Mh w
Mh z
Mp
Mh w/Mh n
Dh P
15a
15b
15c
15d
7000
11 000
7500
15 000
28 000
23 500
21 000
28 000
61 000
49 000
38 000
8000
15 000
18 000
20 600
2.1
2.9
3.1
2.6
7
8
7
8
8000
and dimethylformamide in diethyl ether while keeping
the temperature as low as possible, is limited to
compounds with side chains not longer than octyl, due
to solubility problems at low temperature. Moreover
many synthetic runs are needed to get a substantial
amount of 4a (4-bromo-2,5-dioctylbenzene) as a result
of the said solubility problems. No side chain limitations
are observed in the case of route B, which is a six-step
reaction also starting from hydroquinone (1), thus
allowing the synthesis for example of 4b (4-bromo-2,5-
dioctadecylbenzaldehyde).
The Sonogashira Pd-catalyzed cross coupling reaction8
of 4a ,b with 1-ethynyl-4-formyl-2,5-dioctyloxybenzene
(6)10d and subsequent chromatographic purification
afforded 7a (R ) octyl) and 7b (R ) octadecyl) as yellow
compounds in 52 and 74% yield, respectively, alongside
a small amount of the diyne derivative 8 [1,4-bis(4-
formyl-2,5-dioctyloxyphenyl)butadiyne-1,3]. The 13C NMR
spectra in deuterated chloroform of 7a and 8 show
identical peaks except the peaks of the acetylenic
carbons, which are detected around 93 ppm in the case
of 7a and around 80 ppm in the case of 8 (See
Supporting Information, Figures S1 and S2).
The Horner-Wadsworth-Emmons olefination reac-
tions15 of 7a with various bisphosphonate esters
14a -d afforded polymers 15a -d as red materials in
yields between 60 and 90% after work up (Scheme 2).
The alkoxy side chains enable good solubility of
the polymers in standard organic solvents, such as
THF, toluene, chlorobenzene, DMSO, dichloromethane,
and chloroform. The 13C NMR spectra of 15c and 15a ,
for example, obtained in deuterated chloroform are
shown in the Supporting Information as Figures S3
and S4, respectively. The chemical structures of the
polymers were further confirmed through 1H NMR,
IR (KBr), and elemental analysis. Weight-average mo-
lecular weights, Mw, between 15 000 and 28 000 g/mol
were obtained, showing polydispersity indices between
Th er m a l An a lysis. TGA carried under air while
heating at a rate of 10 K/min, shows that polymers 15
are thermostable up to temperatures of about 638 to
673 K, at which 5% weight loss was recorded. Thus, the
polymers were handled comfortably without decomposi-
tion up to isotropization temperature. Figure 1 includes
both sets of heating and cooling DSC curves for the four
polymers. DSC thermograms of 15a and 15b have
similar features exhibiting two reversible transitions
with weak and broad peaks extended over 60 K for the
lower temperature transition and over 90 and 130 K
for the higher temperature transition of 15b and 15a ,
respectively. The higher temperature transitions were
detected at slightly lower temperature on cooling. The
lower temperature transition may be assigned to an
order-disorder process of the side chains as previously
reported for similar rigid-rod polymers having aromatic
backbones substituted by alkyl side chains.16 Compound
15c is characterized by the presence of a unique
reversible transition detected at temperatures of
∼500 and ∼470 K during the first and second heating
runs, respectively. Upon cooling, an exothermic peak
occurs in the temperature range between 463 and 363
K, which represents the crystallization of 15c according
to POM observations as will be shown later. Polymer
15d showed two endotherms on first heating at 333 and
515 K while subsequent cooling and heating runs did
not reveal clear transitions. Note that the DSC signals
recorded during cooling of the four polymers were rather
noisy, suggesting a heterogeneous crystallization pro-
cess taking place.
P ola r ized Op tica l Micr oscop y. According to POM
observations, the higher temperature transitions were
detected as the temperatures at which the materials
melt and enter into the isotropic phase. The optical
micrographs for the 15d sample taken during the first
heating run at 10 K/min and observed under crossed
polaroids are shown in Supporting Information Figure
S5. The texture remains almost unchanged below 473
K. At this temperature, the color changes rapidly and
finally the sample goes to an isotropic state when it is
heated to 539 K. Upon cooling, the sample starts to show
birefringence at 508 K, and its color changes and
becomes reddish when it is cooled to a temperature of
473 K. Even at a temperature of 553 K, 15d gives a
highly viscous melt. Since 15d has a molecular weight
2.1 and 3.1 and degree of polymerization (DP) between
7 and 8 (Table 1). The almost identical DP values for
all four polymers are a prerequisite for a reliable
comparison of their various properties.
In vestiga tion s in th e Bu lk . One of the goals of this
study is to evaluate the effects of the structural features
of yne-containing PPVs 15a -d on their thermal behav-
ior, morphology and structure in bulk. All along the

7454 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
tion in space.19 Note that 15d gave an azimuthal angle
independent SALS pattern.
P ow d er Wid e-An gle X-r a y P a tter n . The structure
of yne-containing PPVs was initially characterized by
means of wide-angle X-ray scattering on powder samples.
Figure 3 gives azimuthally averaged X-ray scattered
intensity distributions at room temperature for the four
samples as obtained directly after polymerization (solid
lines) and after being heated to isotropic state and then
cooled to room temperature (dashed lines). Yne-contain-
ing PPVs give a major peak in the small-angle region
reflecting a d-spacing of 1.51-2.2 nm. As shown in lower
left part of Figure 3, diffractograms obtained from as-
polymerized and melt-crystallized 15c samples show a
change of the position of the small-angle peak. This
finding suggests the occurrence of two phases associated
with different molecular packing and that the structure
with the shortest d-spacing (largest S-1 value) is
thermodynamically more stable. Polymers 15a , 15b, and
15c give rise to several well-defined diffraction peaks
in the wide-angle region superimposed to an amorphous
halo suggesting they self-assemble into supramolecular
organizations occurring due to spatial segregation be-
tween aromatic and aliphatic segments, stacking prop-
erties of π-conjugated main chains, and crystallization
behavior of alkoxy side chains. Only an amorphous halo
is seen in the wide-angle region of 15d that originates
from liquidlike intermolecular correlation and repre-
sents a broad distribution of intermolecular distances.
These initial results confirm that both the structural
features of polymer and thermal treatment may affect
the self-organized supramolecular structure of yne-
containing PPVs.
F igu r e 1. DSC traces recorded during heating (solid lines)
and cooling (dashed lines) of 15a and 15b (top) and of 15c and
15d (bottom) at 10 K/min. Both the first and second heating
cycles are shown.
comparable to that of 15a and 15c (Table 1), its high
melt viscosity may be attributed to the fact that 15d
possesses the lowest number of side chain carbon atoms
of the series, which may favor strong interactions
between aromatic backbones.17 Tendency to aggregate
formation for 15d was also evidenced through the
investigation of solid-state photoluminescence and elec-
troluminescence properties as will be shown later on.
Rather similar morphology changes were observed
during first heating of 15a , 15b, and 15c, while in such
cases, first, flowing of polymer was evidenced when
heated to isotropic state and, second, the appearance
and growth of birefringent domains from the isotropic
melt was identified as following nucleation and growth
process. As an example, optical micrographs taken
during cooling of 15a at temperatures of 493 (left) and
303 K (right) are shown in Supporting Information
Figure S6. It is seen that the color changes also upon
cooling and the size of the birefringent domains is on
the order of a few micrometers. POM observations at
the early stage of the crystallization process have given
the impression that 15a leads to the formation of
spherulite-like birefringent domains, while 15b and 15c
give rise to elongated-shape birefringent structures.
Sm a ll-An gle Ligh t Sca tter in g. To further elucidate
the morphology formation during crystallization, SALS
experiments were performed for the samples as obtained
after POM. Hv SALS patterns recorded for 15a , 15b,
and 15c polymers at room temperature exhibit a 4-fold
symmetry but differ in the orientation of the lobes with
respect to the polarization axes (Figure 2).
F iber Wid e-An gle X-r a y P a tter n . The polymers
were processed at elevated temperatures by a micro-
extrusion process allowing the preparation of macro-
scopically oriented specimens for their wide-angle X-ray
structure analysis using a 2-dimensional detector.20 The
four specimens exhibit X-ray patterns showing strong
equatorial reflections resulting from polymer backbones
being extended along the fiber axis coinciding with the
extrusion direction (Figure 4).
The fiber X-ray pattern of 15a shows that regular
macromolecules exhibit a more ordered structure via
π-π stacking of the aromatic rigid main chains and
interactions of flexible alkyloxy side chains. The broad
reflections along the equator at wide-angles give a
distance of ∼0.42 nm characteristic of a side-to-side
distance between aliphatic segments.21 Polymer 15c
gives reflections in the low-angle region with two
spacings d1 ) 2.195 nm and with d2 ) 1.712 nm. In the
wide-angle amorphous halo two reflections at d-spacing
of 0.477 and 0.388 nm are evident. This latter distance
that is larger than the 0.335 nm sheet-to-sheet distance
of graphite is consistent with the π-stacking correlation
usually observed for π-conjugated macromolecular self-
assemblies.22-24 Polymers 15d and 15b, having side
chains of different lengths, give rise to a wide-angle
diffuse scattering halo corresponding to a liquidlike
short-range order in molecular packing.
From the above results one can estimate that a
probable structure adopted by yne-containing PPVs
involves the formation of polymeric backbone layers,
with the flexible alkoxy side chains filling the space
between neighboring layers. The absence of repeating
unit reflections along the meridian rules out correlation
between neighboring polymeric backbones. In such a
The pattern of 15a confirms that spherulitic crystal
morphology develops during cooling.18 The nature of the
scattering patterns obtained for 15b and 15c may be
interpreted in terms of a supramolecular structure
involving a rodlike aggregate of crystals which is
anisotropic and which may not have correlated orienta-

Macromolecules, Vol. 37, No. 20, 2004
Yne-Containing Poly(phenylene-vinylene)s 7455
F igu r e 2. SALS Hv patterns for 15a , 15b, and 15c taken at room temperature after cooling from isotropic melt.
F igu r e 3. Powder wide-angle X-ray diffractograms of 15a , 15b, 15c, and 15d taken at room temperature for as-polymerized
(solid line) and melt-crystallized (dashed lines) samples.
layered model, the space between the layers should be
controlled by the volume fraction of the side chains.25
Thus, the position of the maximum scattered intensity
of the small-angle equatorial reflections is dependent
on the molecular mass of the repeating unit (MRU) and
reflects the correlation between the axes of the back-
bone layers. As illustrated in Figure 5, this correlation
scales with MRU following a regression line whose slope
is unity indicating 1-dimensional dependence of inter-
molecular distances with respect to the side chains
volume fraction.
Extrapolation to the molecular mass of the repeat unit
backbone gives a distance of ∼6 Å indicating monomo-
lecular backbone layers to be formed. For comparison,
values of d-spacing calculated for powder samples are
also included in Figure 5. Typically, shorter layer
spacings were obtained for the sample in powder form,
meaning that the extrusion process leads to conforma-
tionally regular polymer chains with lower packing
density in the lateral direction. The combination of
alkoxy side chains differing in size (from methyloxy to
octadecyloxy) and geometry (linear vs branched) may
result in varied molecular structures so that it is
difficult to ascertain a definite packing model. For
instance, 15c having the same number of carbon atoms
in each side branches as 15a leads to a slightly lower
d-spacing value due to the presence of two branched side
chains (2-ethylhexyloxy) while 15a has solely linear
octyloxy side chains. However, regarding 15a as model
polymer one can try to assign a packing structure.
Assuming a π-stacked structure with end-to-end pack-
ing of fully extended octyloxy side chains, 15a gives an
increment of 1.13 Å per carbon atom (considering the
oxygen as a methylene group), which is lower than the
predicted value of 1.25 Å/CH2.21 This finding suggests
a tilt angle of about 65° of the alkoxy side chains with
respect to the polymer backbone axis.
Tem p er a tu r e-Dep en d en t F iber Wid e-An gle X-
r a y P a tter n . Azimuthally averaged scattering intensity
distributions for 15c filament taken at selected tem-
peratures during heating (H) and subsequent cooling
(C) are presented in Supporting Information Figure S7.
Upon heating, the small-angle reflections corresponding
to the d1-spacing decrease in intensity and disappear
above 353 K together with the 0.388 nm d-spacing
reflections related to the interchain distance within the

7456 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
F igu r e 4. 2D wide-angle X-ray diffraction fiber patterns from
oriented specimens of 15a , 15b, 15c, and 15d taken at room
temperature after extrusion. The arrow indicates the direction
of the fiber axis.
F igu r e 6. 2D wide-angle X-ray diffraction fiber patterns of
an oriented sample of 15a during heating and subsequent
cooling at temperatures of 303, 363, 413, and 473 K.
oriented form did not show any significant changes with
temperature. Typically, higher orders of the small-angle
reflections appear upon cooling suggesting a better
ordering in the direction perpendicular to the polymer
backbone, while a diffuse scattering halo is maintained
in the wide-angle region. Polymer 15a was found to be
more sensitive to thermal treatment as shown by the
2D wide-angle X-ray patterns presented in Figure 6. The
reflections in the wide-angle region narrow and show
increased intensities up to a temperature of 413 K,
above which they almost disappear with the sharp,
intense layer reflections remaining only. Further heat-
ing brings a shift of the amorphous halo to a lower 2θ
value. Upon cooling, beginning from 413 K, redistribu-
tion of the intensities in the wide-angle region is
observed, since azimuthally distributed meridional re-
flections indicating correlation distances of about 0.46
and 0.54 nm clearly show up with increased intensity
upon further cooling. Besides these eight off-meridional
reflections, two more reflections attributed to the short-
range inter-side chains distance show up along the
equator from a temperature of 363 K, which cor-
roborates the results of DSC analysis (Figure 1). These
findings indicate a rearrangement of the aromatic
planes of the PPV structural unit during annealing of
15a .
F igu r e 5. Layer spacing observed by 2D wide-angle X-ray
fiber patterns at room temperature as a function of the
molecular mass of the repeating unit (MRU) for extruded fibers
(0) and powder samples (4).
layers. From a temperature of 383 K, the intensity of
the small-angle reflections corresponding to the d2-
spacing is greatly enhanced. Upon cooling, the reflection
in the wide-angle region is shifted to a smaller 2θ value
and appears much more broader, indicating that finite
disorder is maintained within the layers, while in the
small-angle region the reflections reminiscent of the d1-
spacing do not reappear below 353 K so that the layered
structure with the denser lateral packing is thermody-
namically more stable, in agreement with the X-ray
powder analysis. The two structures reminiscent of the
d1- and d2-spacing developed according to thermal
treatment and owing to extrusion at elevated temper-
ature are both present in the as-extruded specimen.
Diffraction patterns and corresponding scattering in-
tensity distributions obtained for 15d and 15b in
P h ot ocon d u ct ivit y. Photoconductivity is among
the potential functionalities arising from conjugated
compounds.10,26-29 The photoconductivity spectra are
shown in Figure 7. The maximum photocurrents of the
compounds, Iph, (difference between total current in the
presence of light and the dark current) are given in

Macromolecules, Vol. 37, No. 20, 2004
Yne-Containing Poly(phenylene-vinylene)s 7457
F igu r e 7. Photoconductivity spectra of polymers 15a -d , 17b, and 18 (surface type cell, slit width 0.2 mm, threshold voltage 10
(A) or 20 V (B)).
Ta ble 2. Da ta fr om P h otocon d u ctivity Stu d ies
Ta ble 3. Da ta fr om UV-Vis Absor p tion a n d
P h otolu m in escen ce In vestiga tion s in Solu tion a n d in th e
Solid Sta te
a
b
code
voltage (V)
IPh (A)
νmax (cm-1
)
15a
15b
15c
15d
17b
18
20
10
20
20
10
10
5.9 × 10-10
3.0 × 10-10
1.7 × 10-11
3.8 × 10-11
1.1 × 10-9
3.0 × 10-10
18 800
20 000
18 600
19 000
20 000
20 000
c
opt
d
f
λa
ꢀmax
code (nm) (M-1.cm-1
Eg
(eV)
λf
(nm)
Stokes shifte φf
)
(nm (cm-1)) (%)
7a a
410
409
411
423
472
491
473
22 100
30 700
30 990
57 060
60 200
2.79 457
2.69 452
2.72 452
2.61 468
2.36 524
2.17 556, 593
2.34 525
2.18 557, 596
2.34 528
2.21 554, 593
2.34 525
47 (2510)
43 (2330)
41 (2210)
45 (2270)
52 (2100)
65 (2380)
52 (2090)
65 (2370)
53 (2110)
70 (2610)
51 (2050)
109 (3690)
50
49
41
65
70
19
78
28
72
38
64
17
7ba
8a
16a a
15a a
15a b
15ba
a
b
Maximum photocurrent. Wavenumber of irradiating light at
IPh
.
67 560
15bb 492
Table 2. Iph values of order 10-11 and 10-10 A at
wavenumbers between 18 000 and 20 000 cm-1 were
obtained for 15a -d ; however the applied voltage neces-
sary for the detection of photoconductivity is side chain
dependent. A voltage of 20 V was required to detect
photoconductivity in the cases of polymers 15a , 15c, and
15d having short side chains. Comparatively lower
voltage (10 V) was necessary in the case of 15b having
longer octadecyloxy side groups. This is in agreement
with our previous observations for polymers 17b and
18 [poly(1,4-(2,5-dioctadecyloxy)phenylene-vinylene-
1,4-(2,5-dioctyloxy)phenylene-vinylene] (for the chemi-
cal structure see Supporting Information Figure S8)
having same longer side chains.10e The synthesis of
polymers 17a -c is illustrated in Scheme 2. They were
obtained from dialdehydes 16a -c under the same
reaction conditions as described here for the synthesis
of polymers 15a -d .10d
Absor p tion a n d P h otolu m in escen ce In vestiga -
tion s. The photophysical characteristics of the hybrid
polymers 15a -d and 17a -c along with the dialdehydes
7, 8 and 16 were investigated by UV-vis absorption and
photoluminescence measurements in dilute chloroform
solution as well as in solid state. Fluorescence kinetics
measurements in solution and quantum chemical cal-
culations were carried out to substantiate the conclu-
sions derived from the comparison of the properties of
both classes of polymers.
15ca
15cb
475
484
65 500
-
53 040
-
15d a 474
15d b 492
2.19 601
Solution. Solid state. c Per mole of the repeating unit in
a
b
d
the case of the polymers. Italicized data indicate the major peak.
e λf - λa (1/λa - 1/λf). f Fluorescence quantum yield, ( 10%.
Ta ble 4. F lu or escen ce Lifetim e in Dilu te Ch lor ofor m
Solu tion a n d Ra te Con sta n ts of th e Dea ctiva tion
P r ocesses
code τa (ns) kfb (ns-1
)
kf(SB)c (ns-1
)
kf/kf(SB) knrd (ns-1
)
7a
7b
8
1.85
1.90
1.50
1.44
0.77
0.76
0.71
0.74
0.74
0.76
0.76
0.27
0.26
0.27
0.45
0.91
1.03
1.01
0.86
0.99
1.12
0.97
0.21
0.29
0.27
0.51
0.48
0.52
0.52
0.41
0.56
0.68
0.84
1.3
0.9
1.0
0.9
1.9
2.0
2.0
2.1
1.8
1.6
1.2
1.85
1.98
2.16
0.76
0.33
0.21
0.28
0.42
0.27
0.13
0.27
16a
15a
15b
15c
15d
17a
17b
17c
a
b
Fluorescence lifetime, (0.05 ns. Fluorescence rate constant:
kf ) φf/τ; φf values from Table 3 and Supporting Information
Table S9. c Fluorescence rate constant according Strickler and
Berg:30 kf (SB) ) 2.88 × 10-9 × n2 × ∫F(ν)/ν3 dν/∫F(ν) dν ×
∫ꢀ(ν)/ν dν, n ) refractive index of the solvent, F(ν) ) corrected
d
fluorescence spectrum. Rate constant of radiationless deactiva-
tion: knr ) (1 - φf)/τ.
The absorption maxima, λa, the extinction coefficients
at the maximum wavelength, ꢀmax, the optical energy
gaps, Egopt, the emission maxima, λf, the Stokes shifts,
and the fluorescence quantum yields, φf, are presented
in Table 3 and Supporting Information Table S9.10d
Table 4 summarizes data from the kinetics measure-
ments, namely fluorescence lifetime, τ, fluorescence rate
constant, kf, the rate constant of radiationless deactiva-
tion, knr, the fluorescence rate constant according to
Strickler and Berg, kf(SB),30 and the ratio between kf
and kf(SB).
Solution absorption and emission spectra of the
monomeric dialdehydes 7a ,b, 8, and 16a and polymers
15a and 17a can be viewed in Figures 8 and 9,
respectively. Compounds 7 and 8 exhibit similar ab-
sorption maxima at around 410 nm. The differences lie
within the shorter wavelength part of their spectra

7458 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
F igu r e 8. Absorption and emission spectra of dialdehydes
7a ,b, 8, and 16 in dilute chloroform solution.
F igu r e 10. Solid-state absorption and emission spectra of
polymers 15a -d .
practically the same. The difference in the radiative rate
constants kf(SB) is due to the used absorption coef-
ficients which are determined per repeating unit of the
polymer. The RU’s do not correspond to the effective
chromophore segment. From the relation of the rate
constants kf/kf(SB) (Table 4), we conclude an effective
chromophore system extending over approximately two
RU’s in the case of polymers 15 or 1.5 RU’s in the case
of polymers 17 having an absorption coefficient twice
as large as the given value for polymers 15 or 1.5 times
higher than the given value for polymers 17 (Table 3
and Supporting Information Table S9).
We assume the effective fluorophore segment located
around the lower energy phenylene-vinylene units of
the compounds and having the structure Ph-CtC-
Ph-CHdCH-Ph-CHdCH-Ph-CtC-Ph as shown in
Chart 1. This assumption is moreover confirmed by
quantum chemical calculations31 performed with a
series of different segments of the polymer backbone in
order to localize the regions where π-conjugation is
most effectively restricted and also to guide the selection
of the most appropriate model compound (Chart 2).
Alkoxy side groups were replaced by OH groups to
simplify the calculations. The essential outcome is:
If there are two consecutive p-phenylene-ethynylene
moieties linked to two consecutive p-phenylene-vi-
nylene units as described here, then π-conjugation
extends only over one p-phenylene-ethynylene unit; it
drops down rapidly over two units. We are led to this
conclusion from a comparison of the rotational barriers
between aromatic rings linked by ethynylene moieties
which are, for instance 7.5 kJ /mol for DM1 and 1.9 and
3.8 kJ /mol for the outer phenyl ring alone while the
phenylene-ethynylene-phenylene unit is kept coplanar
in DM3, respectively. The same conclusion is drawn
from the inspection of the HOMO’s and LUMO’s which
insignificantly extend over more than one consecutive
phenylene-ethynylene unit. The isodensity contour
maps of the HOMO and LUMO of DM1 and DM3 are
depicted in Supporting Information Figure S10.
F igu r e 9. Absorption and emission spectra of polymers 15a
and 17a in dilute chloroform solution.
between 250 and 350 nm, where the diyne-containing
dialdehyde 8 shows a highly structured band. The larger
chromophore system in 16a compared to 7a explains
the ca. 10 nm bathochromic shift of its absorption (423
nm) and emission (468 nm) and the higher absorption
coefficient. While 7a emits at 457 nm, the emission
maximum of 7b at λf ) 452 nm is 5 nm blue shifted.
λf ) 452 nm is also the emission maximum of dialdehyde
8. The fluorescence quantum yields of 7, 8 and 16a are
high with values of 50, 40, and 65%, respectively. The
radiative rate constants kf of the monomers, determined
with φf and τ and according to Strickler and Berg are
in good accordance, indicating that the emissive state
is the same as the primarily populated one and geo-
metrical changes between S0 and S1 states are negli-
gible.
Despite the differences in the conjugation pattern,
polymers 15 and 17 show almost identical photophysical
behavior in dilute chloroform solution (Figure 9, Table
3, and Supporting Information Table S9). Only a slight
bathochromic shift of the absorption and emission of
polymers 15a -d (λa ) 472-475 nm, λf ) 524-528 nm,
opt
Eg ) 2.34 eV), having one phenylene-ethynylene unit
in the RU less than polymers 17a -c (λa ) 468-470 nm,
The solid-state absorption and emission spectra of
polymers 15 are depicted in Figure 10. For the purpose
of comparison, those of polymers 17 are shown in
Supporting Information Figure S11.10d The differences
in the solid-state emission curves are related to the
various aggregation modes due to the combination of
conjugation pattern and nature of grafted alkoxy side
opt
λf ) 519-521 nm, Eg ) 2.37 eV), is observed. That
gives evidence of an identical chromophore system
responsible for the emission in both types of polymers.
This is moreover confirmed by the kinetic measure-
ments. As well the fluorescence lifetimes τ as the
radiative rate constants kf of the different polymers are

Macromolecules, Vol. 37, No. 20, 2004
Yne-Containing Poly(phenylene-vinylene)s 7459
Ch a r t 1. Blu e-La beled P or tion Con sid er ed a s Ch r om op h or e System Resp on sible for th e Em ission
Ch a r t 2. Rota tion a l En er gy Ba r r ier a s Obta in ed fr om Qu a n tu m Ch em ica l Ca lcu la tion s
Ta ble 5. Com p a r ison of Electr och em ica l Da ta a of 15a -d w ith Th ose of MDMO-P P V31e
opt
code
Eox
(mV) Ered
(mV) Eox
(mV) Ered
(mV) HOMO (eV) LUMO (eV) Egec (eV) Egec - Eg (eV)
peak
peak
onset
onset
15a
15b
15c
15d
∼+1000
-2010
∼-1980
∼-2000
-1930
+565
+665
+740
+645
+510
-1725
-1765
-1695
-1725
-1750
-5.32
-5.42
-5.49
-5.4
-3.03
-2.99
-3.06
-3.03
-3
2.29
2.43
2.43
2.37
2.26
0.12
0.25
0.22
0.18
0.14
+885
+670
MDMO-P P V
-5.26
a
All potential values are shown vs NHE; NHE-level used for HOMO-LUMO calculation was -4.75 eV.33
chains. The enhanced planarisation of the conjugated
backbone in the solid state explains the red shift of the
(15c and 17c) or longer linear octadecyloxy (15b and
17b) side chains is characterized with well structured
emission curves made up of two maxima. The 0-0
transitions are centered around 552-559 nm and 0-1
transitions around 592-596 nm for all the above-
mentioned polymers except polymer 17c, where the
steric hindrance due to bulky 2-ethylhexyloxy side
chains is the source of the ca. 10 nm blue shift of its
emission peaks located at 542 and 582 nm. Stokes shifts
and fwhmf values between 2100 and 2600 cm-1 and
fluorescence quantum yields between 30 and 50% were
obtained for 15a -c and 17b-c, clearly indicating less
aggregates/excimers contribution in the emissive pro-
cess compared to 15d and 17a .
Electr och em ica l In vestiga tion s. The electrochemi-
cal behavior of the polymers was investigated by the
combination of cyclic voltammetry (CV) and electro-
chemical voltage spectroscopy (EVS).32 The correspond-
ing data are given in Table 5.
Figure 11 depicts the cyclic voltammetry curves of
15a -d . The dotted lines indicate the onset potentials
as determined by EVS. The energy diagram presenting
the position of the HOMOs and LUMOs is shown in the
opt
absorption (Eg
≈ 2.20 eV) and emission spectra
relative to the solution. The grafting of methoxy group
in 15d leads to strong π-π interactions (aggregates
formation) and excimers formation, which explains its
broad (full-width at half-maximum, fwhmf ) 3400 cm-1
and structureless emission curve peaked at λf ) 601 nm,
leading to the comparatively large Stokes shift of 109
nm (≈ 3700 cm-1) and the lowest fluorescence quantum
yield of 17% among polymers 15.
)
The effect of the conjugation pattern is clearly ob-
served while comparing polymers 15a and 17a , both
carrying octyloxy side groups in all phenyl rings.
Enhanced rigidity due to the higher number of -Ct
C- units in 17a , compared with 15a , and consequently
enhanced π-π interactions and excimer formation in
the S1 state result in a less structured and broad
emission spectrum (fwhmf ) 3400 cm-1) and a large
Stokes shift of 109 nm (3800 cm-1) and the lowest φf
value of 23% among polymers of type 17. The less rigid
polymer 15a as well as all polymers of type 15 and 17
consisting of either the branched bulky 2-ethylhexyloxy

7460 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
Ta ble 6. Electr olu m in escen t P r op er ties for ITO/P EDOT/
P olym er /Ca Devices
EL
code
λmax (nm)a
ηext (%)
luminance (cd/m2)
15a
15b
15c
15d
17b
557, 587 (6V)
552, 593 (6V)
551, 583 (10V)
584 (13V)
0.79
2.15
1.82
0.22
0.017
1406
5760
1483
595
543, 581 (12V)
27.910e
a
Italicized data indicate the major peak.
contribute to the effect. The EL spectra of 15a , 15b, and
15c LEDs show two bands while 15d LED gives only
one broad emission band. The small -OCH3 group on
the PV units gives more chance for polymer 15d to form
excimers compared with other three samples totally
grafted with long side chains. The excimer emission will
broaden the EL spectrum and also lead to a red shift.
Figure 13 shows the current density-voltage-lumi-
nance characteristics for the LEDs. The turn-on voltages
for LEDs of 15d , 15c, 15a , and 15b are 7, 4, 3.5, and 5
V, respectively. The external quantum efficiencies reach
0.22, 1.82, 0.79, and 2.15% respectively. From 15d to
15c and from 15a to 15b, with increasing side chain
length, the EL efficiency increases remarkably. The
longer side chains will improve polymer solubility, film-
forming capability, and film morphology which affect
device performance significantly. In addition, longer side
chains can form tiny domains to inhibit exciton inter-
chain migration thus improving exciton confinement
and EL efficiency.36 Our previous study36 indicates that
polymers with grafted 2-ethylhexyloxy side chains are
more efficient than those with grafted octyloxy side
chains. In this work, 15c shows higher efficiency than
15a , conforming with previous results very well. 2-Eth-
ylhexyloxy side chains may be more favorable for exciton
confinement. There is more than a 100-fold improve-
ment of the EL parameters from polymer 17b10e to
polymer 15b, which can be ascribed to less -CtC-
units within compound 15b compared to 17b (Table 6).
F igu r e 11. Cyclovoltammograms of 15a -d recorded at 10
mV/s scan speed; the dotted lines indicate the onset potentials
for the polymers according to the color code. The inset displays
the position of the HOMO and LUMO of polymers 15a -d and
MDMO-P P V in the energy scale.
inset of Figure 11. All polymers exhibited partial re-
versibility in both n-doping and p-doping processes.
Bulky substituents seem to disturb the conjugation33 as
the HOMO levels are lowered. Consequently 15c with
two branched 2-ethylhexyloxy substituents shows the
lowest HOMO energy34 of -5.49 eV. This effect is also
strong but in a reduced form in the case of 15b with
longer linear octadecyloxy side groups, where a HOMO
energy value of -5.42 eV was obtained; it reduces when
one 2-ethylhexyloxy group is replaced by a methyloxy
group in the case of 15d (EHOMO ) -5.4 eV). Conse-
quently the electrochemical band gap energies of both
ec
15b and 15c at Eg ) 2.43 eV show a discrepancy of
opt
0.25 and 0.22 eV to their respective Eg values. The
discrepancy of 0.12 eV in the case of polymer 15a lies
within the range of error. For the purpose of comparison
the electrochemical data of poly[2-methoxy-5-(3′,7′-
dimethyloctyloxy)-1,4-phenylene-vinylene] (MDMO-
P P V)32e (for the chemical structure see Supporting
Information Figure S8) are also given in Table 5. The
higher oxidation potential onset values of polymers 15
compared with MDMO-P P V are evidence of enhanced
oxidation stability due to the presence of -CtC- units
in 15.
Electr olu m in escen t P r op er ties. Figure 12 shows
EL spectra of ITO/PEDOT/polymer/Ca LEDs using
polymers 15a -d . The device from 15d gives yellow light
at 584 nm at 13 V. Other three devices emit green light
at 547-557 nm. All the EL spectra display blue shift
with increasing the applied voltage. For 15d LED, when
increasing the voltage from 13 to 21 V, the EL peak
moves from 584 to 565 nm with a 19 nm blue shift. For
15b LED, at 6 V, the EL spectrum shows two peaks at
552 and 593 nm, which correspond to 0-0 and 0-1
vibrational transitions respectively, similar to the PL
spectrum. With voltage increase, the EL spectra show
a blue shift, and the relative intensity of the 0-0 band
continues to increase at the expense of the 0-1 band.
LEDs using 15c and 15a show similar behavior. This
indicates that the vibrational structure of the EL
spectrum is sensitive to the applied electrical field. The
EL blue shift under higher applied voltage is related to
heating in the LEDs, which results in a thermochromic
effect.35 Band gap distributions in the polymers also
Con clu sion s
Defect-free alkoxy-substituted yne-containing poly-
(phenylene-vinylene)s of general structure (-Ph-Ct
C-Ph-CHdCH-Ph-CHdCH-)n, 15a -d , have been
synthesized and characterized. A degree of polymeri-
zation around 7 was obtained for all the polymers
enabling a reliable comparison of their properties as a
function of the length and geometry of the grafted side
chains. Formation of crystalline superstructures of
either spherulitic-like (15a ) or rodlike (15b and 15c)
morphology was identified as following typical nucle-
ation and growth process for polymer having a sym-
metric substitution pattern. A layered structure con-
sisting of π-stacked main-chain layers and the flexible
alkoxy side chains occupying the space between is
proposed for all the studied polymers. The discrepancy
opt
between the electrochemical (Egec) and optical (Eg
)
band gaps was found to be proportional to the bulkiness
of the side chains. The presence of -CtC- units within
the polymers backbone constitutional unit enhances the
oxidation stability of yne-containing PPVs 15 relative
to MDMO-PPV. Octadecyloxy side chains in 15b are
not only required for the easy detection of the photo-
conductivity but also for the design of LED devices of
configuration ITO/PEDOT/polymer/Ca with (compara-
tively to 15a ) improved parameters (ηext ) 2.15%,
luminance ) 5760 cd/m2). These parameters are more

Macromolecules, Vol. 37, No. 20, 2004
Yne-Containing Poly(phenylene-vinylene)s 7461
F igu r e 12. Electroluminescence spectra of 15a -d from ITO/PEDOT/polymer/Ca devices.
F igu r e 13. Current density-voltage (b) and luminance-voltage (O) characteristics for ITO/PEDOT/polymer/Ca devices from
polymers 15a -d .
than a 100-fold better than those of polymer 17b due
to less -CtC- units within 15b . Comparison of the
photophysical properties of polymers 15 and polymers
17 as well as their corresponding monomeric dialde-
hydes 7 and 16 were carried out in dilute chloroform
solution. Despite the differences in the conjugation
pattern, an identical chromophore system, located around
the lower energy portion of the polymers and having
the structure Ph-CtC-Ph-CHdCH-Ph-CHdCH-
Ph-CtC-Ph, is responsible for the emissive behavior
of the both types of compounds. This is moreover
confirmed by fluorescence kinetics measurements (τ, kf,

7462 Egbe et al.
Macromolecules, Vol. 37, No. 20, 2004
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knr) and quantum chemical calculations. The similarities
or differences in solid-state emissive behavior of 15 and
17 result from the combination of backbone rigidity (as
a function of the number of -CtC- moieties in the
repeating unit) and nature of side chains (length,
geometry). The high absorption coefficients and high
fluorescence quantum yields in the solid state combined
with the enhanced oxidation stability (relative to PPV)
make polymers 15a -d potential donor materials in the
design of organic solar cells.37
Ack n ow led gm en t. L.D. and F.E.K. wish to thank
the AFOSR for generous financial support.
Su p p or tin g In for m a tion Ava ila ble: Text describing the
experimental procedures, the synthesis of the materials, and
the characterization of the materials, Figures S1-S4, showing
13C NMR spectra of 7a , 8, 15c, and 15a , Figures S5 and S6,
showing POM observations for thin films of polymers 15c and
15a , Figure S7, showing X-ray diffractograms of 15c, Figure
S8, showing structures of 18 and MDMO-PPV, Table S9
showing UV-vis and photoluminescence data, Figure S10,
showing isodensity contour maps of DM1 and DM3, and
Figure S11, showing solid state absorption and emission
spectra of polymers 17a -c. This material is available free of
charge via the Internet at http://pubs.acs.org.
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