Diyne-containing PPVs: Solid-state properties and comparison of their photophysical and electrochemical properties with those of their yne-containing counterparts

Article abstract of DOI:10.1021/ma0507490

Diyne-containing poly(p-phenylene-vinylene)s, 4a-d, of general chemical structure -(Ph-C≡C-C≡C-Ph-CH=CH-Ph-CH=CH-)_n, obtained through polycondensation reactions of 1,4-bis(4-formyl-2,5-dioctyloxyphenyl)-buta-1,3-diyne (2) with various 2,5-dialk

Full text of DOI:10.1021/ma0507490

Macromolecules 2005, 38, 6269-6275
6269
Diyne-Containing PPVs: Solid-State Properties and Comparison of
Their Photophysical and Electrochemical Properties with Those of Their
Yne-Containing Counterparts
Daniel Ayuk Mbi Egbe,*,† Benjamin Carbonnier,‡ Elisabeth Lekha Paul,§
David Mu1 hlbacher, Thomas Kietzke,§ Eckhard Birckner,# Dieter Neher,§
Ulrich-Walter Grummt,# and Tadeusz Pakula‡
Institut fu¨r Organische Chemie und Makromolekulare Chemie der Friedrich-Schiller Universita¨t Jena,
Humboldtstr. 10, D-07743 Jena, Germany; Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg
10, D-55128 Mainz, Germany; Institute of Physics, University of Potsdam, Am Neuen Palais 10,
D-14469 Potsdam, Germany; Konarka Austria Forschungs-u. EntwicklungsGmbH, Gruberstr. 40-42,
A-4010 Linz, Austria; and Institut fu¨r Physikalische Chemie der Friedrich-Schiller-Universita¨t Jena,
Lessingstr. 10, D-07743 Jena, Germany
Received April 11, 2005; Revised Manuscript Received May 23, 2005
ABSTRACT: Diyne-containing poly(p-phenylene-vinylene)s, 4a-d, of general chemical structure -(Ph-
CtC-CtC-Ph-CHdCH-Ph-CHdCH-)n, obtained through polycondensation reactions of 1,4-bis(4-
formyl-2,5-dioctyloxyphenyl)-buta-1,3-diyne (2) with various 2,5-dialkoxy-p-xylylenebis(diethylphospho-
nates), 3a-d, are the subject of this report. The polymers exhibit great disparity in their degree of
polymerization, n, which might be ascribed to side-chain-related differences in reactivity of the reactive
species during the polycondensation process and which led to n-dependent absorption (solution and solid
state) and emission (solution) behaviors of the polymers. Polarizing optical microscopy and differential
scanning calorimetry are employed to probe their thermal behavior. The structure is investigated by
means of wide-angle X-ray diffraction for both isotropic and macroscopically oriented samples. Comparison
of photophysical (experimental and theoretical) and electrochemical properties of the polymers with those
of their yne-containing counterparts 6a-d [-(Ph-CtC-Ph-CHdCH-Ph-CHdCH-)n] has been carried
out. Similar photophysical behavior was observed for both types of polymers despite the difference in
backbone conjugation pattern. The introduction of a second yne unit in 4 lowers the HOMO and LUMO
levels, thereby enhancing the electron affinity of polymers 4 compared to polymers 6. The “wider opening”
introduced by the second yne unit facilitates moreover the movement of charges during the electrochemical
processes leading to minimal discrepancy, ∆Eg, between the optical and electrochemical band gap energies.
Polymers 6, in contrast, show significant side-chain-dependent ∆Eg values. Low turn-on voltages between
2 and 3 V and maximal luminous efficiencies between 0.32 and 1.25 cd/A were obtained from LED devices
of configuration ITO/PEDOT:PSS/polymer 4/Ca/Al.
Introduction
iftci and Heeger in 1992.11 A great number of PPV
derivatives have been designed to suit various specific
areas of applications.1 Polymers containing diyne group
in the main chain12 have been basically designed for
nonlinear optical investigations, resulting from the fact
that such compounds exhibit very high second- and/or
third-order nonlinear optical susceptibility.13-15 In this
contribution we focus on diyne-containing PPVs of the
following general constitutional unit: -(-Ph-CtC-
CtC-Ph-CHdCH-Ph-CHdCH-)n-, 4a-d. This was
purposely done to compare their properties with those
of their yne-containing congeners: -(Ph-CtC-Ph-
CHdCH-Ph-CHdCH-)n- bearing identical alkoxy
side chains, 6a-d.16 This is in line with our previous
studies on hybrid phenylene-ethynylene-alt-phenylene-
vinylene polymers. Such polymers exhibit enhanced
electron affinity and higher photoluminescence efficien-
cies than PPV.17-20 The question to be answered here
is, what effect does the replacement of yne units through
diyne units have on photophysical (absorption, emission,
fluorescence kinetics) and electrochemical properties of
these PPV derivatives? Theoretical calculations have
been taken into considerations to substantiate the
experimental findings. Solid-state characterization of
the synthesized polymers was performed by means of
various experimental techniques, and the results are
discussed with respect to the molecular characteristics.
Semiconducting polymers have recently attracted
considerable attention from both fundamental and
practical points of view.1 The electronic and optical
properties of such compounds are synthetically tunable
through structural modifications. The grafting of ad-
equate solubilizing agents enable their processability
into transparent films, a precondition for their use as
advanced materials in various domains of applications,
such as light-emitting diodes,2-4 field effect transistors,5-7
and photovoltaic devices.8-10
Among conjugated polymers, poly(phenylene-vi-
nylene)s (PPVs) have been most widely studied, espe-
cially since the first report of electroluminescence by the
group in Cambridge in 1990.2 The focus on PPVs was
furthermore upheaved after the discovery of ultrafast
photoinduced femtosecond charge transfer from alkoxy-
substituted PPV to the Buckminsterfullerene by Saric-
† Institut fu¨r Organische Chemie und Makromolekulare Chemie
der Friedrich-Schiller Universita¨t Jena.
‡ Max-Planck-Institut fu¨r Polymerforschung.
§ University of Potsdam.
Konarka Austria Forschungs-u. EntwicklungsGmbH.
# Institut fu¨r Physikalische Chemie der Friedrich-Schiller-
Universita¨t Jena.
* Corresponding author: Tel (+49)-(0)-3641-948267; Fax (+49)-
(0)-3641-948202; e-mail c5ayda@uni-jena.de.
10.1021/ma0507490 CCC: $30.25 © 2005 American Chemical Society
Published on Web 06/23/2005

6270 Mbi Egbe et al.
Macromolecules, Vol. 38, No. 15, 2005
Scheme 1
The 13C NMR spectrum of 4c in deuterated chloroform
is depicted in Supporting Information Figure S1. The
various signals could be readily assigned to their cor-
responding carbons, expect the signal of the triple bond
carbons, which however was detected in the infrared
spectroscopy around 2140 cm-1 as a weak peak.
Table 1. Molecular and Thermal Characteristics for the
Synthesized Diyne-Containing PPVs
a
a
a
a
code
Mh n
Mh w
Mh z
35900 12900 2.20
24900 4900 2.28
Mp
PDIa na T5%b [K]
4a
4b
4c
4d
8600 19000
5200 11900
8
4
631
614
629
628
24000 70500 130000 70000 2.93 22
4100 8500 15 700 3300 2.07
4
Thermal Behavior. Thermal behavior of the four
diyne-containing PPVs was evaluated by means of TGA,
POM, and DSC. All polymers exhibited remarkable
thermal stability, and the lowest temperature for 5%
weight loss was 614 K for 4b (Table 1). Polymers 4 are
birefringent as observed after polymerization and melt
into isotropic liquid with increasing temperature. The
major differences were noticed upon cooling from the
isotropic states. For 4a, 4b, and 4c birefringent textures
were observed to develop (for an example see Supporting
Information Figure S2), while 4d retained its black
appearance even below room temperature. On the basis
of these microscopic observations, the high-temperature
endotherms seen in the DSC traces are assigned to the
isotropization of the polymers (Figure 1). The complex
thermal behavior of the polymers can be recognized from
the data presented in Figure 1, where especially the
polymers’ endothermic transitions, as observed during
the first and second heating runs, differ considerably.
Despite their rather complex melting, reorganization
ability of 4a and 4c is evidenced by unique exothermic
peak. The transitions observed for 4b in the low-
temperature range are presumed to reflect the order-
disorder transitions of the longer octadecyloxy side
chains. The DSC analysis indicates that polymer 4d is
partially amorphous after fast cooling (10 K min-1) from
isotropic melt confirming the POM observations. No
changes in the microscopic textures were observed for
any of the transition preceding the isotropization. This
finding suggests that the macromolecular arrangements
are nearly the same for all the phases.
a Data from GPC obtained using THF as eluent and polystyrene
as standard. b Temperature at which 5% weight loss was recorded
during the thermogravimetric analysis.
Results and Discussion
Synthesis and Characterization. The synthetic
path to polymers 4a-d is illustrated in Scheme 1. The
compounds were obtained through the Horner-Wad-
sworth-Emmons olefination reaction of various di-
alkoxy-substituted bisphosphonate esters 3 with diyne-
containing dialdehyde 2. Dialdehyde 2 is always obtained
as a byproduct in 2-7% yield during the Sonogashira
Pd-catalyzed cross-coupling reaction21 of 1-ethynyl-4-
formyl-2,5-dioctyloxybenzene (1)17 with aryl halogenide
or aryl dihalogenide, as shown in eq 1.16,18b,19b
0
I
1 + X-aryl-X or X-aryl Pd /Cu , base, Ar8
product + 2 (X ) Br or I) (1)
The direct conversion of 1 into 2 is achieved in yields
around 80% through the jointly Pd- and Cu-catalyzed
oxidative coupling in the presence of iodine and piperi-
dine (Scheme 1). Polymers 4a-d were obtained as
orange-red materials in yields between 72 and 87% after
reaction times between 2 and 3.5 h. They are soluble in
common organic solvents such as chloroform, THF,
dichloromethane, toluene, and chlorobenzene. Data from
GPC (THF and polystyrene as standard) are given in
Table 1. All the polymers are characterized with a
polydispersity index around 2. However, there are
considerable discrepancies between the degrees of po-
lymerization, n, in total contrast to polymers 6a-d,
where n values around 7 were obtained for all the
polymers.16 This might suggest differences in reactivity
depending on the side chains grafted on the bisphos-
phonate esters. An attempt, for example, to increase the
n value of 4b by prolonging the reaction time to 6 h
brought no significant changes.
Structural Ordering. Powder samples were initially
examined both at room temperature and in isotropic
phase (Supporting Information, Figure S3). The data
collected at room temperature indicate correlations over
large distances for the four samples and over short
distances for the octadecyloxy-substituted polymer 4b.
The position of the low-angle diffraction peaks seems
to be correlated with the molecular structure since the
related periodicities increase accordingly with the length
of the side chains. For measurements performed in the
The chemical structure of the polymers was confirmed
1
through H and 13C NMR, IR, and elemental analysis.

Macromolecules, Vol. 38, No. 15, 2005
Diyne-Containing PPVs 6271
Figure 1. DSC thermograms recorded for the bulk diyne-
containing (PPV)s during heating (thick lines) and cooling (thin
lines) with the rate 10 K min-1. Both first (upper traces) and
second heating (lower traces) runs are presented for each
polymer.
isotropic phases, the low-angle intensity maxima reveal
that the long-distance correlations are partially pre-
served and the diffuse wide-angle halos reflect the
disordered molecular arrangement. The structure of
diyne-containing PPVs was further examined for ex-
truded filaments. The mechanical deformation involved
alignment of chains along the fiber axis and in general
has led to an improved order on the molecular scale.
For the uniaxial orientation, long-range correlations
were detected along the equatorial and in some cases
along the meridional directions (Figure 2). The equato-
rial reflections are assigned to the transversal period-
icities between polymer backbone-related structures
such as layers. The meridional scattering indicated
additional ordering in the direction parallel to the
orientation of the macromolecular chains. The periods
extracted from the position of the meridional reflections
for 4a and 4c assumed a value of about 2.2 nm, nearly
matching the chemical repeating unit length in fully
extended conformation (Chem 3D Ultra 7.0). Such
translationally correlated arrangement of the polymer
chains was not detected for polymers 4b and 4d having
the lowest degree of polymerization (n ) 4) of the series.
For 4b, the wide-angle scattering exhibited intensity
maximum along the meridional direction and indicated
a distance of 0.42 nm. These findings suggest that long
octadecyloxy side groups may provide strong cohesive
forces influencing the packing in the solid state. For the
other polymers, we presumed that the wide-angle
intensity maxima detected along the equatorial direction
reflect mainly the interchain distances within self-
ordered assemblies.
Figure 2. (a) 2D X-ray diffraction pattern recorded for
macroscopically oriented filament of 4c. (b) Intensity distribu-
tions measured along the equatorial (horizontal) and meridi-
onal (vertical) directions in the 2D pattern.
nearly the same, but the meridional reflections lost
gradually their intensity and diffuse halos appeared at
wide angles. These scattering effects indicate that
within the high-temperature supramolecular organiza-
tion the chains are translationally uncorrelated. The
changes observed were perfectly reversible, and initial
ordering was recovered after slow cooling to room
temperature.
Photophysical Investigations. The photophysical
properties of the polymers were investigated in dilute
chloroform solution as well as in solid state. The optical
data are summarized in Table 2, namely the absorption
peak maxima, λa, the extinction coefficients at the peak
maxima, ꢀmax, the optical band gap energy, Eopt (calcu-
g
lated from λ10% max, wavelength at which the extinction
has dropped to 10% of the peak value),18b the emission
maxima, λe, the Stokes shift, the full width at half-
maximum of the absorption and emission spectra,
fwhma, fwhme, and the fluorescence quantum yields, Φf.
The emission spectra were obtained after exciting at λa.
Table 3 summarizes data from the fluorescence kinetics
measurements, namely fluorescence lifetime, τ, fluores-
cence rate constant, kf, the rate constant of radiationless
deactivation, knr, the fluorescence rate constant accord-
ing to Strickler and Berg, kf(SB), and the ratio between
kf and kf(SB). Figure 3 depicts the dilute solution
absorption and emission spectra of polymers 4a-d as
well as 6a-d. The photophysical data (λa ∼ 470 nm, λe
∼ 525 nm, Φf ∼ 70%, and τ ∼ 0.7 ns) as well as the
Effect of temperature on the organized structures has
been examined for 4a, 4b, and 4c samples in the
oriented state. In the temperature ranges correlating
well with the transitions detected by means of DSC,
changes were also noticed in the 2D WAXS patterns.
Herein, we will limit our discussion to the changes
observed for 4a and 4c during heating to temperatures
below the isotropic phases. With the temperature in-
crease, the long-range equatorial correlations remained

6272 Mbi Egbe et al.
Macromolecules, Vol. 38, No. 15, 2005
Table 2. Data from Absorption and Emission Spectra in Dilute Chloroform Solution (10-5-10-6 M) and in Solid Thin
Films of the Polymers
code
λa [nm]
ꢀmaxc [M-1 cm-1
]
fwhma [cm-1
]
Eogpt [eV]
λed [nm]
Stokes shifte [nm (cm-1)]
fwhme [cm-1
]
Φf f [%]
4aa
4ab
4ba
4bb
4ca
4cb
4da
4db
6aa
6ab
473
493
465
478
483
520
464
478
468
491
71 700
4100
4800
4700
5500
4000
4400
5000
5700
4500g
5000
2.36
2.17
2.35
2.19
2.33
2.19
2.36
2.21
2.36
2.17
525
553, 592
524
552, 594
529
552, 592
523
551
524
556, 593
52 (2100)
60 (2200)
59 (2400)
74 (2800)
46 (1800)
32 (1100)
59 (2400)
73 (2800)
56 (2300)
65 (2400)
2100
2700
2200
2200
1500
2100
2300
2400
1900
2300
67
21
67
32
68
23
71
44
70
19
53 000
91 000
58 800
60 300
a Solution. b Solid state. c Per mole of the repeating unit in the case of the polymers. d Italic data indicate the major peak. e λe - λa (1/λa
- 1/λf). f Fluorescence quantum yield, (10%. g The fwhma values in solution for 6b, 6c, and 6d are respectively 4300, 4600, and 4300
cm-1
.
Table 3. Fluorescence Lifetimes in CHCl3 Solution and
Rate Constants of the Deactivation Processes
code τ,a ns kf,b ns-1 kf(SB),c ns-1 kf/kf (SB) knr,d ns-1
4a
4b
4c
4d
6a
6b
6c
6d
0.73
0.77
0.66
0.76
0.77
0.76
0.71
0.74
0.92
0.87
1.03
0.93
0.91
1.03
1.01
0.86
0.54
0.43
0.66
0.50
0.48
0.53
0.52
0.41
1.7
2.0
1.6
1.9
1.9
2.0
1.9
2.1
0.45
0.43
0.48
0.38
0.39
0.29
0.39
0.49
a Fluorescence lifetime, (0.05 ns. b Fluorescence rate constant:
kf ) Φf /τ. c Fluorescence rate constant according Strickler and
Berg: kf(SB) ) 2.88 × 10-9 × n2 × ∫F(ν)/ν3 dν/∫F(ν) dν × ∫ꢀ(ν)/ν
dν, where ꢀ(ν) ) absorption coefficient, for the polymers per
repeating unit, n ) refractive index of the solvent, and F(ν) )
corrected fluorescence spectrum. d Rate constant of radiationless
deactivation: knr ) (1 - Φf )/τ.
Figure 4. Normalized thin film absorption and emission
spectra of polymers 4a-d and 6a.
of phenylene units) with increasing polymer chain
length in the S0 state. The more pronounced shoulder
in the fluorescence spectra with increasing chain length
is due to the restricted amplitude of bending motions
of the excited chromophore in longer polymer chains.
The solid-state absorption and emission spectra are
depicted in Figure 4. Similar to the solution, there is a
correlation between the degree of polymerization, n, and
the solid-state absorption maximum, λa, whereby a
gradual red shift of λa is observed with increasing n.
For example, both 4b and 4d with n ) 4 absorb at λa )
478 nm and exhibit almost similar fwhma value around
5500 cm-1 despite the differences in side chains. Whereas
polymers 4a and 6a, both having n ) 8, but different
conjugation patterns, show similar absorption behavior
(λa ∼ 490 nm, fwhma ∼ 5000 cm-1), which moreover
proves that replacing yne through diyne leads to no
changes in the photophysical behavior of the polymers.
Polymer 4c, with n ) 22, has the longest wavelength
maximum at λa ) 520 nm.
The emission spectra of all the polymers except 4d
are well structured and consist of two maxima around
552 and 592 nm. In the case of 4d, the emission
spectrum clearly shows only the peak around 551 nm;
the peak around 592 nm appears as a discrete shoulder.
In general, the shape and the relative intensity of both
emission maxima as well as the fwhme values depend
more on the nature of the side chains attached at R1
and/or R2 than on the degree of polymerization n.
However, polymers 4b and 4d, with lowest n values of
4, show higher solid-state fluorescence quantum yields
of 32 and 44%, respectively, probably due to less
Figure 3. Normalized absorption and emission spectra of
polymers 4a-d and 6a-d in dilute chloroform solution.
spectra show that there are no significant differences
in both types of compounds. The almost twice smaller
fwhme (relative to fwhma) and the more structured
fluorescence spectra are indications of enhanced rigidity
and planarization in the singlet excited state, S1. From
the ratio kf/kf(SB), it can be concluded that the effective
chromophore consists of 1.5-2 repeating units in diyne-
and yne-containing polymers; i.e., the diyne units do not
interrupt the conjugation despite the small rotational
energy barrier between the phenylene units. However,
a closer look at the absorption maxima as well as the
shape of the absorption and emission spectra shows a
small but significant increase of the band position with
increasing degree of polymerization (Figure 3).22
This is ascribed to the higher probability of finding
longer effective conjugated segments (higher coplanarity

Macromolecules, Vol. 38, No. 15, 2005
Diyne-Containing PPVs 6273
Table 4. Comparison of Electrochemical Data of 4a-d and 6a-d with MDMO-PPV25 a
OX
peak
OX
onset
RED
onset
code
E
[mV]
ERpeEaDk [mV]
E
[mV]
E
[mV]
HOMO [eV]
LUMO [eV]
Eegc [eV]
∆Eg [eV]
4a
4b
4c
4d
6a
6b
6c
+1125
+1215
+710
-1570
-1590
-1590
-1560
-1725
-1765
-1695
-1725
-1750
-5.46
-5.51
-5.51
-5.46
-5.32
-5.42
-5.49
-5.4
-3.18
-3.16
-3.16
-3.19
-3.03
-2.99
-3.06
-3.03
-3
2.28
2.35
2.35
2.25
2.29
2.43
2.45
2.37
2.26
0.11
0.16
0.16
0.04
0.12
0.25
0.22
0.18
0.14
+760
+760
+710
+565
+665
+740
+645
+510
+1095
∼+1000
-2010
∼-1980
∼-2000
-1930
6d
PPV
+885
+670
-5.26
a All potential values are shown vs NHE; NHE level used for HOMO-LUMO calculation was -4.75 eV.26,27
radiationless deactivation pathways. Polymers with
higher n values (4a, 4c, and 6a) exhibit Φf values
around 20%.
Theoretical Considerations. Density functional
theory implemented in the Gaussian03 package23 was
used to optimize the geometry of both types of polymers.
The model chemistry used was b3lyp/6-31g(d). Although
alkoxy side chains enhance conjugation due to their
electron-donating property, the calculations were first
done only with unsubstituted polymers in order to
reduce the calculation effort because we frequently
encountered convergence problems.
After successful geometry optimization using periodic
boundary conditions and the default convergence crite-
ria, the difference densities between the S1 and S0 were
calculated for the repetition unit and a series of oligo-
mers comprising up to five repetition units (rhf/6-31g-
Figure 5. CV curves of polymers 4a-d. The vertical lines
(d)). Hydrogen atoms were attached to the chain ter-
minals. The results were visualized with the help of the
GAUSS-VIEW program. It is obvious from Supporting
Information Figures S4 and S5 that the regions of
electron redistribution on the chain remain nearly
unchanged upon increasing the number of repetition
units from 4 to 5. Thorough inspection reveals that the
diyne-containing polymer converges a little faster than
the yne-containing one. However, in addition to the
intrinsic limitation of the conjugation length obtained
for the idealized coplanar chain geometry, there is
another limitation introduced at finite temperature by
the conformational flexibility of the polymer chain. We
have recently calculated the torsional barrier for tolane
as 3.4 kJ/mol.24 With the same model chemistry (b3lyp/
6-31+g(d)), we find 0.85 kJ/mol for 1,4-diphenyldiethy-
nyl. Transition state optimization resulted in a barrier
of 0.92 kJ/mol. The rotational barrier of 2,5,2′,5′-
tetrahydroxytolane was calculated as 6.2 kJ/mol (6.0 kJ/
mol for the transition state). We neither could optimize
a 90° twisted structure nor were able to localize a
transition state due to convergence problems with the
model compound 2,5,2′,5′-diphenylbutadiyne using the
(b3lyp/6-31+g(d) model chemistry.
To get an upper limit of the rotational barrier, we
simply performed single-point calculations of stepwise
twisted structures without geometry optimization. The
largest energy difference calculated was 0.3 kJ/mol
(between 10° and 70° dihedral angles). Contrary to the
expectation and in contrast to tolane, enhancement of
the rotational barrier due to hydroxy substitution was
not detectable with the diyne-containing model com-
pound. However, because of the minuteness of the
barriers for both diyne-containing model compounds, we
should not draw any conclusions other than that both
barriers are well below kT at ordinary temperature.
From a comparison of the calculated rotational barriers,
we ought to expect a somewhat smaller conjugation
show the redox onset values as obtained by EVS.
length for the diyne-linked polymers at ordinary tem-
perature in comparison to the yne-linked polymers. The
optical and photophysical properties are determined by
a very subtle interplay between electronic interactions
within the polymer backbone including donating effects
of substituents, steric restrictions due to the side chains
and packing effects, and π-π interactions in the con-
densed state.
Electrochemical Properties. Electrochemical in-
vestigations were carried out through the combination
of cyclic voltammetry (CV) and electrochemical voltage
spectroscopy (EVS), enabling a more exact determina-
tion of the HOMO and LUMO levels and the concomi-
tant electrochemical energy gap, Eegc.25-27
Figure 5 shows the CV curves of polymers 4a-d
recorded at 10 mV/s scan speed; the lines indicate the
onset potentials for the polymers according to the color
code. All polymers show reversible redox behavior but
are not stable upon many cycles. Comparison of the
electrochemical data of polymers 4a-d and 6a-d
together with MDMO-PPV25 is presented in Table 4.
The insertion of the second yne unit in 4a-d leads
to an increase of the oxidation and reduction onset
potentials and consequently to an enhancement of their
electron affinity compared to the corresponding yne-
containing polymers 6a-d and MDMO-PPV. Their
HOMO and LUMO levels are comparatively lowered as
can be noticed in the Figure 6, where the energy level
diagram of the compounds is presented. The insulating
(i.e., shielding) effects of bulky 2-ethylhexyl or longer
octadecyl side chains, accounting for considerable dis-
crepancy ∆Eg between optical and electrochemical band-
gap energies in in polymers 6,16,28-30 are minimal in
polymers 4 due to the “wider opening” caused by the
introduction of the second yne unit, which not only

6274 Mbi Egbe et al.
Macromolecules, Vol. 38, No. 15, 2005
best EL parameters, however, resulted from the device
made from the 2-ethylhexyl-substituted polymer 4c.
Conclusions
Four diyne-containing poly(p-phenylene-vinylene)
derivatives, 4a-d, have been synthesized, and their
chemical structure as well as solid-state properties have
been thoroughly characterized. The disparity of the
degree of polymerization, n (4 for 4b and 4d, 8 for 4a,
and 22 for 4c), might be explained with the side-chain-
dependent differences in reactivity of the different
reactive species during the polycondensation process.
n-dependent absorption and emission spectra in dilute
chloroform solution as well as n-dependent thin film
absorption spectra were observed. Following conclusions
can be drawn from the comparison of the properties of
polymers 4a-b and their yne-containing counterparts
6a-d: (1) Despite the difference in their conjugation
pattern, both types of compounds show identical pho-
tophysical properties as confirmed from fluorescence
kinetics measurements and theoretical calculations. (2)
The introduction of a second yne unit in 4 lowers their
HOMO and LUMO levels relative to 6 due to enhanced
electron affinity. (3) The “wider opening” in the back-
bone of 4 caused by the diyne unit facilitates the
migration of charges during the n- and p-doping pro-
cesses leading to smaller values of ∆Eg, when bulky
2-ethylhexyloxy or longer octadecyoxy are grafted on
phenylene-vinylene segment of the polymers. This is
in contrast to polymers 6, where the insulating effects
of bulky or longer side chains led to remarkable ∆Eg
values. LED devices of configuration ITO/PEDOT:PSS/
polymer 4/Ca/Al exhibited low turn-on voltage between
2 and 3 V due to enhanced electron affinity.
Figure 6. Position of the HOMO and LUMO of the polymers
4a-d, 6a-d, and MDMO-PPV in the Fermi energy scale.
Supporting Information Available: Experimental de-
scription, synthesis and characterization of the material, 13C
NMR spectrum of polymer 4c, POM morphological observation
for melt-crystallized thin film of polymer 4b, powder X-ray
diffraction intensity distributions of polymers 4a-d, calculated
S1-S0 difference densities for unsubstituted oligomers of 4 and
6, and current density-voltage and luminance-voltage char-
acteristics of LED devices from polymers 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 7. Electroluminescence spectra measured at 5 V of
devices of configuration ITO/PEDOT/polymer 4/Ca/Al.
Table 5. LED Parameters from Devices of Configuration
ITO/PEDOT:PSS/Polymer 4/Ca/Al
References and Notes
λEL
code [nm]
luminance at 100 max efficiency
turn-on
voltage [V]
(1) Hadziioannou, G., van Hutten, P. F., Eds.; Semiconducting
Polymers: Chemistry, Physics and Engineering, 1st ed.;
Wiley-VCH: Weinheim, Germany, 2000.
A/cm-2 [cd/m-2
]
[cd/A]
4a
4b
4c
4d
557
552
552
552
340
400
1200
506
0.32
0.42
1.25
0.54
2.8
3
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facilitates the diffusion of the counterion ClO4- from the
bulk electrolyte to the oxidation site to balance the
charges during the oxidation process but also enables
an easy transport of charges from the polymer to the
electrode (or vice versa).
Electroluminescent Properties. LED devices of
configuration ITO/PEDOT:PSS/polymer 4/Ca/Al, de-
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for 4b-d and at 557 nm for 4a. The EL spectra, depicted
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