Synthesis and electronic spectra of substituted oligo(phenylenevinylene)s

Article abstract of DOI:10.1002/1099-1395(200010)13:10<587::AID-POC275>3.0.CO;2-I

A series of substituted oligo(p-phenylenevinylene)s (OPVs) with five benzene rings was prepared via PO-activated olefinations and Knoevenagel condensations. The central ring is substituted with two octyloxy groups to ensure good solubility of the OPVs and the lateral styrene units carry further substituents, with either electron-accepting or donating character and also combinations thereof. The spectral features of these OPVs are dominated by the basic chromophore; further auxochrome groups on the lateral rings (meta and para positions) shift the absorption and emission spectra only slightly to longer wavelengths. Significant bathochromic shifts (absorption ca 20 nm, emission ca 40 nm) are observed for OPVs with cyano groups on the terminal vinylene segments. The absorption spectra are independent from the concentration and solvatochromism is very small. The OPVs are photochemically stable to near-UV irradiation (366 nm) in neutral solution, whereas mid-UV irradiation (254 nm) causes decomposition of the chromophore. The presence of traces of acids or amines leads to different photochemical pathways. Copyright

Full text of DOI:10.1002/1099-1395(200010)13:10<587::AID-POC275>3.0.CO;2-I

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 587–590
Review Commentary
Synthesis and electronic spectra of substituted
oligo(phenylenevinylene)s
Heiner Detert* and Erli Sugiono
Institut fuÈ r Organische Chemie, Duesbergweg 10±14, D-55099 Mainz, Germany
Received 27 August 1999; revised 1 March 2000; accepted 17 March 2000
ABSTRACT: A series of substituted oligo(p-phenylenevinylene)s (OPVs) with five benzene rings was prepared via
PO-activated olefinations and Knoevenagel condensations. The central ring is substituted with two octyloxy groups to
ensure good solubility of the OPVs and the lateral styrene units carry further substituents, with either electron-
accepting or donating character and also combinations thereof. The spectral features of these OPVs are dominated by
the basic chromophore; further auxochrome groups on the lateral rings (meta and para positions) shift the absorption
and emission spectra only slightly to longer wavelengths. Significant bathochromic shifts (absorption ca 20 nm,
emission ca 40 nm) are observed for OPVs with cyano groups on the terminal vinylene segments. The absorption
spectra are independent from the concentration and solvatochromism is very small. The OPVs are photochemically
stable to near-UV irradiation (366 nm) in neutral solution, whereas mid-UV irradiation (254 nm) causes
decomposition of the chromophore. The presence of traces of acids or amines leads to different photochemical
pathways. Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: oligo(phenylenevinylene)s; electronic spectra
Since the discovery of the electroluminescence of poly(p-
phenylenevinylene) (PPV) by Burroughes et al.1 a broad
variety of conjugated polymers have been prepared and
investigated for use in light-emitting diodes (LEDs). In
addition to PPV, polythiophenes and poly(p-phenylene)s
have been used extensively;2 solubility has been obtained
by substitution with flexible side-chains3 or sterically
demanding groups.4 Model compounds proved to be very
useful for the elucidation of the photochemical and
photophysical properties of the polymers,5,6 since they
can be prepared with monodisperse chain lengths, in high
purity, and are often much more soluble than the
polymers. Furthermore, oligomeric compounds can be
used as luminescent building blocks in alternating
copolymers7 and as electronic materials in their own
right.8 For the use of conjugated materials such as PPV in
LEDs, low-lying LUMO levels are favourable,9 since
then the injection of electrons from metals with higher
work functions such as aluminium to the organic
semiconductor requires low activation energies and the
performance of the diode is improved. Unfortunately,
most PPV-type materials are predominantly hole con-
ducting;10 a few polymers and oligomers with higher
electron affinity have been described bearing cyano
groups at the vinylene linkage or electron-deficient
heterocycles in the main chain.11–13 We have prepared
a series of oligo(phenylenevinylene)s (OPVs) with five
benzene rings and a variety of substituents of different
character on the lateral rings and studied the influences of
substitution on the absorption and emission spectra and
the photostability of these model compounds.
The fundamental chromophore of the title compounds
is a linear OPV with five benzene rings and two octyloxy
groups in the 2,5-positions of the central ring; further
auxochrome groups are symmetrically located on the
terminal styrene units. The synthetic strategy for these
compounds started with the central distyrylbenzene
segment carrying two carbaldehyde functionalities and
the two octyloxygroups which are essential for the
solubility of the intermediate and the final compounds.
Hydroquinone dioctyl ether was chloromethylated or
bromomethylated and a twofold Michaelis–Arbusov
reaction led to the bisphosphonate (31% via chloro-
methylation, 38% via bromomethylation) (Scheme 1).
The distyrylbenzenedicarbaldehyde was prepared in a
twofold Horner reaction with terephthaldialdehyde
monodiethylacetal followed by acidic cleavage of the
acetals. This synthon was used for the preparation of all
OPVs reported here. The aldehydes were converted to
styrene units via a second twofold Horner reaction with a
variety of substituted benzylphosphonates or to a-
cyanostyrenes with benzyl cyanides in a twofold
*Correspondence to: H. Detert, Institut fu¨r Organische Chemie,
Duesbergweg 10–14, D-55099 Mainz, Germany.
E-mail: detert@mail.uni-mainz.de
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 587–590

588
H. DETERT AND E. SUGIONO
Scheme 1. Synthesis of starting materials
1
Knoevenagel condensation (Scheme 2). The yields in the
final step were in the range 57–96% (except OPV 5,
17%). Synthetic details will be reported elsewhere. All
OPVs were readily soluble with melting-points in the
range 153–203°C; a small mesophase was observed only
for 1 (155–162°C).
10 8 mol l
(fluorescence). For the photochemical
experiments the solvents were purified by additional
filtration through basic alumina and the solutions were
thoroughly degassed. The essential spectroscopic data are
collected in Table 1.
The intensely coloured OPVs can be divided into two
groups: the yellow compounds with substituents on the
benzene rings (1–9, except 5) and the orange to dark-red
OPVs with propenenitrile segments. Upon UV irradia-
The absorption and emission spectra were recorded in
dichloromethane under ambient conditions; the concen-
1
trations were about 10 5 mol l
(absorption) and
Scheme 2. Synthesis and substitution pattern of OPVs 1±12
Copyright 2000 John Wiley & Sons, Ltd. J. Phys. Org. Chem. 2000; 13: 587–590

OLIGO(PHENYLENEVINYLENE)S
589
Table 1. Substituents and spectral data
4
ꢁ Â 10
1
Compound
R1
R2
R3
ꢀmax (nm)
ꢀ0.1 (nm)
(l mol 1 cm ꢀfmax (nm)
)
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
CF3
CN
CH3
OC6H13
Br
CN
NO2
H
424
426
425
434
450
425
425
427
478
478
477
491
514
478
480
480
8.3
9.1
9.1
8.6
7.4
8.5
6.7
9.3
483
500
501
504
499
502
490
489
H
H
9
10
11
12
H
SO2C10H21
OC3H7
426
445
451
450
478
508
519
528
9.8
8.3
7.1
9.3
487
524
531
533
CN
CN
CN
H
CF3
H
H
H
Cl
tion, the solids exhibit intense fluorescence, and again the
red, non-fluorescent nitro-substituted OPV 5 is different.
This division into two groups holds also for the OPVs
in solution. Both types (ring substitution in 1–9 and
vinylene substitution in 10–12) showed very similar UV–
visible spectra with two small maxima at 305 and 360 nm
and an intense maximum at 424–434 nm for the first
group (except 5) and at 445–451 nm for the second group.
Exemplary spectra (OPVs 1, 5, 9 and 11) are depicted in
Fig. 1. It was found that meta substitution of the terminal
rings with electron acceptors (6–8) results in small
changes in the electronic spectra, whereas para-substitu-
tion (4 and 5 and, more pronounced, the vinylene
cyanides 10–12) induces strong bathochromic shifts.
In the emission spectra of 1–4 and 6–9 the maximum
of the fluorescence intensity was found in the range 483–
504 nm, followed by a characteristic shoulder (Dꢀ ꢀ 30
nm) on the red side. This shoulder was lost in the
emission spectra of the a-cyano-substituted OPVs 10–12
and the maxima were shifted to the red (524–533 nm).
Compared with the other investigated OPVs, the
fluorescence intensity of the dinitro compound 5 was
about 0.01%. Two effects are responsible for the reduced
intensity: the quenching of the fluorescence by the nitro
groups and the bathochromic shift of the absorption
spectrum (ꢀmax = 450 nm) leading to strong reabsorption
of the emitted light.
The introduction of electron-withdrawing substituents
into the meta positions of the lateral rings offers the
possibility of tuning the electron affinity of the
chromophore with only slight changes in the electronic
spectra (OPVs 6–9 compared with 1). Birckner et al.
investigated the electronic spectra of substituted 1,4-
distyrylbenzenes14 and observed strong bathochromic
shifts of the absorption (Dꢀ ꢀ 40 nm) and emission
(Dꢀ ꢀ 70 nm) upon introduction of cyano groups to the
vinylene linkages. Additional electron donors increased
the red shift of the absorption maximum, wheras the
fluorescence maximum was preserved. Notable is
compound 9: the bathochromic shift of the fluorescence
(Dꢀ = 17 nm compared with 1) due to the terminal
auxochrome alkoxy groups can be levelled out by the
introduction of electron-withdrawing sulfones. The
emission spectrum is shifted about 13 nm to the blue,
close to OPV 1. Combinations of this type, vicinal
auxochrome and anti-auxochrome groups, offer the
opportunity to fine-tune the emission properties and the
electron affinity of the chromophore. Sulfones and 1,3,4-
oxadiazoles are powerful electron acceptors with an
additional binding site, useful for the introduction of
solubilizing side-chains.
Changing the solvent from cyclohexane to dichlor-
omethane, acetonitrile and ethanol caused shifts of the
absorption maxima of less than 3 nm, but the extinctions
were changed significantly: the extinction coefficients
increased in the sequence ethanol, cyclohexane, aceto-
nitrile for compounds with electron-withdrawing sub-
stituents (7, 9, 11), whereas this order was reversed for
OPV 1.
Figure 1. UV±visible and ¯uorescence spectra of selected
OPVs
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 587–590

590
H. DETERT AND E. SUGIONO
the decrease in the absorption maximum of 11 in the
absence of triethylamine was about 10 times higher than
in the presence of the base. The amine led to a different
photochemistry, substantiated by increasing absorptions
at 365-nm and much more pronounced at 311 nm. The
initial step in this reaction might be a photochemical
electron transfer process, as has been proposed for the
addition of amines to stilbene.16
OPVs with five benzene rings and a variety of different
substituents were prepared from a distyrylbenzene with
two terminal carbaldehyde groups and benzyl phospho-
nates and benzyl cyanides. Whereas vinyl cyanides
caused strong red shifts of the absorption and emission
and are known to reduce the fluorescence quantum yields
strongly, only small changes in the spectral features were
observed when electron-withdrawing substituents were
linked to the aromatic rings. An increase in the electron
affinity of the chromophore by substitution with electron
acceptors is probable and will be quantified by cyclic
voltammetry.
Figure 2. Irradiation of OPV 11 in cyclohexane
For use in electrooptical devices, a high photostability
of the chromophore is required. OPV 11, bearing two
electron-withdrawing groups, was irradiated in different
media with near-UV(NUV) (366 nm) and mid-UV-
(MUV) radiation (254 nm). In cyclohexane, 11 was inert
towards NUV, whereas MUV caused a uniform photo-
reaction with a decrease in the absorption maximum (451
nm), an isosbestic point at 352 nm and the build-up of a
maximum at 320 nm (Fig. 2), indicating a trans–cis
isomerization of one of the four vinylene linkages. Traces
of acids are known to destroy oligo(2,5-dialkoxypheny-
lenevinylene)s in a slow dark reaction and very much
accelerated by irradiation with NUV and even more
pronounced, with MUV.15 A solution of 11 in CH2Cl2
containing 0.5% trifluoroacetic acid was stable in the
dark for prolonged times; irradiation with NUV caused a
slow decomposition of the chromophore, accompanied
by a small increase in the absorption at 301 nm. With
MUV, the rate of the decrease in the absorption
maximum was comparable to the rate observed in
cyclohexane in the absence of acid, but the acid induced
a different photochemical pathway, presumably by
protonation of the excited chomophore and subsequent
addition reactions.
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Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 587–590