Tunable optical properties of chromophores derived from oligo(p -phenylene vinylene)

Article abstract of DOI:10.1021/ol1007263

A series of 11 symmetric push-pull chromophores consisting of electron-accepting groups connected through a central π-conjugated system derived from oligo(p-phenylene vinylene) (OPV) were designed and synthesized. Electronic and spectroscopic properties were investigated by UV-visible absorption, fluorescence spectroscopy, and cyclic voltammetry. By finely tuning the electron-withdrawing ability of the acceptors as well as the length of the π-conjugated spacer, a wide range of dyes exhibiting strong absorption and emission were obtained.

Full text of DOI:10.1021/ol1007263

ORGANIC
LETTERS
2010
Vol. 12, No. 10
2382-2385
Tunable Optical Properties of
Chromophores Derived from
Oligo(p-phenylene vinylene)
Audrey Guerlin,†,‡ Fre´de´ric Dumur,*,§ Eddy Dumas,† Fabien Miomandre,‡
Guillaume Wantz,| and Ce´dric R. Mayer*,†
Institut LaVoisier de Versailles, UniVersite´ de Versailles Saint Quentin, UMR 8180 CNRS,
45 aVenue des Etats-Unis, 78035 Versailles cedex, France, Laboratoire PPSM,
Ecole Normale Supe´rieure de Cachan, UMR 8531 CNRS, 61 aVenue du Pre´sident
Wilson, 94235 Cachan, France, Laboratoire Chimie ProVence, UMR 6264 CNRS,
UniVersite´s d’Aix-Marseille I, II, III, Faculte´ des Sciences de Saint Je´roˆme,
AVenue Escadrille Normandie-Niemen, Case 542, 13397 Marseille cedex 20, France,
and Laboratoire IMS, UMR 5218 CNRS, Ecole Nationale Supe´rieure de Chimie et de
Physique de Bordeaux, UniVersite´ de Bordeaux, 16 AVenue Pey Berland,
33607 Pessac Cedex, France
frederic.dumur@uniV-proVence.fr; cmayer@chimie.uVsq.fr
Received March 29, 2010
ABSTRACT
A series of 11 symmetric push-pull chromophores consisting of electron-accepting groups connected through a central π-conjugated system
derived from oligo(p-phenylene vinylene) (OPV) were designed and synthesized. Electronic and spectroscopic properties were investigated by
UV-visible absorption, fluorescence spectroscopy, and cyclic voltammetry. By finely tuning the electron-withdrawing ability of the acceptors
as well as the length of the π-conjugated spacer, a wide range of dyes exhibiting strong absorption and emission were obtained.
During the last decades, organic-based optoelectronic materi-
als displaying strong absorption and emission properties have
received great scientific attention. In these fields, linear
π-oligomers1 and notably p-phenylene vinylene (PPV)
derivatives are of particular interest as their high lumines-
cence quantum yields make them excellent candidates for
efficient Organic Light-Emitting Diodes (OLEDs).2 The main
characteristics of OPVs are broad absorption bands, high
luminescence quantum yields in solution and in the solid
state,3 and large Stokes shifts. Moreover, addition of electron-
donating groups such as alkoxy groups onto the phenylene
† Universite´ de Versailles Saint Quentin.
‡ Ecole Normale Supe´rieure de Cachan.
§ Universite´s d’Aix-Marseille I, II, III.
| Universite´ de Bordeaux.
(1) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer
Approach; Wiley-VCH: Weinheim, Germany, 1998.
(2) Mu¨llen, K.; Scherf, U. Organic-Light Emitting DeVices: Synthesis,
Properties and Applications; Wiley-VCH: Weinheim, Germany, 2006.
10.1021/ol1007263  2010 American Chemical Society
Published on Web 04/30/2010

units enables us to tune the absorption and emission positions
as well as the film formation ability.4 When OPVs are
substituted with such electron-donating groups, they can act
as donors in push-pull systems if connected to electron
acceptors. In this configuration, only a few oligomers have
been reported, namely with C60,5 nitro,6 hexylsulfonyl,7
dicyano,8 pyridine,9 and formyl10 groups as acceptors.
Herein, we report the preparation and properties of 11 new
chromophores, SOPV2-5,7 and LOPV2-7, constructed
with two different central π-conjugated cores in conjugation
with six different acceptors. Depending on the nature of the
electron-withdrawing substituents, the newly synthesized
OPVs exhibit either an efficient luminescence or a strong
absorption.
SOPV6 was prepared according to a synthesis reported
in the literature,9 and extended LOPV6 was obtained in two
steps following the procedure shown in Scheme 2. In the
Scheme 2. Synthesis of Extended Oligomer LOPV6
Chromophores SOPV2-7 and LOPV2-5,7 were pre-
pared according to two different strategies. The first one,
used for the synthesis of SOPV2-5 and LOPV2-5,
consisted of a Knoevenagel condensation between com-
mercially available aldehydes SOPV1 or LOPV1 and the
corresponding acceptors 2-5 (Scheme 1). Knoevenagel
first step, 9 was synthesized using a Horner-Wadsworth-
Emmons olefination, starting from 79 and 8,11 and isolated
in 86% yield. Then, the synthesis of LOPV6 relied upon a
symmetrical Heck cross-coupling reaction. Coupling of 9
with 4-vinylpyridine in the presence of Pd(OAc)2, Et3N, and
P(o-tolyl)3 in a benzene/water (1/1) mixture at 120 °C
provided oligomer LOPV6 in 91% yield.
Scheme 1. Routes to Oligomers SOPV2-5 and LOPV2-5
Oligomers SOPV7 and LOPV7 were synthesized by
alkylation of dipyridines SOPV6 and LOPV6 with 4-me-
thylbenzyl bromide 6 (Scheme 3). 6 was used in this study
Scheme 3. Routes to Oligomers SOPV7 and LOPV7
reactions furnished all dyes with yields ranging from 67 to
93%. The same procedure was used for all condensations,
except for SOPV5 and LOPV5 for which highly sterically
hindered acceptor 5 was used (see Supporting Information).
indeed to strengthen the accepting ability of the accepting
fragments of the oligomers but also as a model capable to
mimic 4-mercaptobenzyl bromide that will be subsequently
used to functionalize SOPV6 and LOPV6 for Self-As-
sembled Monolayer (SAM) applications.
Optical properties of SOPV1-7 and LOPV1-7 were
investigated by means of UV-visible and fluorescence
spectroscopies in solution and in the solid state. All
spectroscopic results with fluorescence quantum yields are
summarized in Table 1. All dyes showed a strong intramo-
lecular charge transfer (ICT) absorption band in the visible
region with maximum position ranging from 398 nm for
SOPV6 to 542 nm for SOPV5 in CH2Cl2. Long OPVs,
which possess a stronger electron-releasing group, exhibit
(3) Lincker, F.; Bourgun, P.; Masson, P.; Didier, P.; Guidoni, L.; Bigot,
J.-Y.; Nicoud, J.-F.; Donnio, B.; Guillon, D. Org. Lett. 2005, 7, 1505.
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Escalonilla, M. J.; Rueff, J.-M.; Figueira Duarte, T. M.; Nierengarten, J.-
F.; Palermo, V.; Samori, P.; Rio, Y.; Accorsi, G.; Armaroli, N. Chem.sEur.
J. 2005, 11, 4405.
(6) Chaieb, A.; Knoukh, A.; Brown, R.; Franc¸ois, J.; Dagron-Lartigau,
C. Opt. Mater. 2007, 30, 318.
(7) Tao, Y.; Donat-Bouillud, A.; d’Iorio, M.; Lam, J.; Gorjanc, T. C.;
Py, C.; Wong, M. S. Synth. Met. 2000, 111-112, 417.
(8) (a) Jin, J.-Y.; Jin, Z.-Z.; Xia, Y.; Zhou, Z.-Y.; Wu, X.; Zhu, D.-X.;
Su, Z.-M. Polymer 2007, 48, 4028. (b) Wang, B.-C.; Chang, J.-C.; Pan,
J.-H.; Xue, C.; Luo, F.-T. J. Mol. Struct. 2003, 636, 81.
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15, 2927.
(11) (a) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W.
J. Am. Chem. Soc. 1977, 99, 6077. (b) Kamlet, M. J.; Abboud, J.-L. M.;
Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877.
Org. Lett., Vol. 12, No. 10, 2010
2383

Table 1. Electrochemical and Photophysical Data of 9 and Oligomers SOPV1-7 and LOPV1-7
λabs
λem
Stokes
∆Ered
∆Ered
d
compd [nm]b log ε [nm]c ØF
τ (ns)e shift [nm] ∆E (eV)f λabs [nm]g λem [nm]h E°red (1) [V]i E°red (2) [V]i (1) [V]i (2) [V]i
SOPV1a 403 2.47 467
LOPV1a 449 4.73 521
0.40 <10-3
1.77
64
72
57
87
3.07
2.76
3.02
2.60
2.42
2.26
2.28
3.11
2.53
2.39
2.25
2.03
1.93
2.74
2.32
n.m.
444
416
472
530
n.m.
555
430
n.m.
n.m.
n.m.
n.m.
n.m.
n.o.
n.m.
556
507
567
696
n.m.
n.o.
484
n.m.
n.m.
n.m.
n.m.
n.m.
n.m.
n.m.
n.o.
-1.85
n.o.
n.o.
n.o.
n.o.
-1.31
-1.56
-1.41
-1.70
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
0.21
n.o.
n.o.
n.o.
n.o.
0.21
0.12
0.24
0.33
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
n.o.
1
9
410 4.72 467
477 2.90 564
512 4.30 622
548 4.37 n.o.
542 4.31 n.o.
398 4.50 470
490 4.52 616
518 4.90 624
550 4.95 662
0.36 <10-3
1.67 9.72
1.34 5.40
n.d. n.d.
n.d. n.d.
1.22 <10-3
0.25 1.23
0.25 1.89
0.09 1.22
SOPV2
SOPV3
SOPV4
SOPV5
SOPV6
SOPV7
LOPV2
LOPV3
LOPV4
LOPV5
LOPV6
LOPV7
-1.02
-0.97
-0.85
-0.61
-0.99irr
-1.10
-1.49
-1.40
-1.27irr
-0.96
-1.24irr
-1.30
0.21
0.17
0.23
0.60
n.d.
0.26
0.17
0.09
0.42
0.20
0.40
0.20
110
n.d.
n.d.
72
126
106
112
154
n.d.
118
n.d.
611 4.85 765 <0.01 n.d.
640 4.73 n.o.
452 4.94 570
535 4.22 n.o.
n.d. n.d.
0.07 1.27
n.d. n.d.
n.o.
n.o.
n.o.
n.m.
a Compounds SOPV1 and LOPV1 were purchased from Aldrich and used as supplied commercially. n.d.: not determined. n.m.: not measured. n.o.: not
observed. b Absorption maximum in CH2Cl2. c Emission maximum in CH2Cl2. d Fluorescence quantum yield in CH2Cl2. Compound LOPV1 was used as a
standard (λex ) 495 nm). ØF(LOPV1) ) 0.48 in toluene,15 ØF(LOPV1) ) 0.52 measured in CH2Cl2. e Fluorescence lifetime. f Optical band gap determined
in CH2Cl2. g Solid state absorption maximum. h Solid state emission maximum. i CV in DMF on gold electrode vs Ag/AgCl, with Bu4NPF6 (0.1 M) as
supporting electrolyte. Scan rate: 100 mV/s. E°red is the reduction standard potential, while ∆Ered is the peak to peak potential difference for each redox
signal.
red-shifted ICT bands with maxima ranging from 452 nm
for LOPV6 to 640 nm for LOPV5 in the same solvent
(Figure 1). As expected for each series, a bathochromic shift
The solvatochromic behavior was investigated for all dyes.
The largest shift of the position of the ICT absorption band
was observed for SOPV4 with ∆λ ) 73 nm. A pronounced
solvatochromism was observed for all dyes, except for
SOPV6 (∆λ ) 10 nm) and LOPV7 (∆λ ) 16 nm), either
containing pyridines or pyridiniums as electron acceptors.
Examination of the solvatochromism revealed a good cor-
relation with the solvent parameters π* determined by
Kamlet and Taft.11 A negative solvatochromism was ob-
served for all dyes except for SOPV6 and SOPV7. Surpris-
ingly, the unexpected behavior evidenced for SOPV6 and
SOPV7 was not observed with the corresponding extended
LOPV6 and LOPV7.
Fluorescence spectra of dyes were recorded in solvents
of varying polarity at 495 nm. Similar results were obtained
with degassed and nondegassed solutions. All investigated
dyes exhibited intense fluorescence at room temperature with
maxima ranging from 470 nm for SOPV6 to 765 nm for
LOPV4 in CH2Cl2, except for three dyes (SOPV4,5 and
LOPV5) for which no emission was detected. Fluorescence
properties of SOPV1-3,6,7 and LOPV1-4,6,7 are similar
to that reported for analogue systems. A significant
decrease of the photoluminescence quantum yields is
observed at room temperature when the conjugation length
is increased.5,12 As already mentioned above for absorption
measurements, an increase of the accepting ability of the
acceptor substituents induces a large red shift of the
maximum fluorescence wavelength as illustrated with
LOPV3 (λem ) 662 nm) and LOPV4 (λem ) 765 nm) in
CH2Cl2. Similarly, when the power of the donor fragment
increases, significant bathochromic shifts are observed as
Figure 1. Absorption spectra of LOPV2-7 recorded in CH2Cl2.
of the charge transfer band is observed when the electron-
withdrawing power of the acceptor increases. Replacement
of aldehydes in SOPV1 and LOPV1 by the strong electron
acceptor 5 leads to bathochromic shifts as large as 139 nm
for short OPVs and 191 nm for the long ones. Compared to
results obtained in the literature by substitution of methyl
groups by cyano groups (shift of 60 nm)8b or nitro groups
(shift of 50 nm)6 in OPVs similar to LOPV1, dyes reported
in this letter exhibit much larger shifts. In addition, by using
the electron acceptor power as a tool to modulate the position
of the ICT band, progressive substitution of the aldehyde
functions in SOPV1 and LOPV1 by a series of acceptors
leads to dyes able to partially cover the visible region (from
398 nm for SOPV6 to 640 nm for LOPV5).
(12) Peeters, E.; Ramos, A. M.; Meskers, S. C. J.; Janssen, R. A. J.
J. Chem. Phys. 2000, 112, 9445
.
2384
Org. Lett., Vol. 12, No. 10, 2010

illustrated with SOPV6 and LOPV6 (∆λem ) 100 nm in
CH2Cl2).
repulsion, resulting in a negative shift. The difference
between the two successive reduction potentials is slightly
lower than in the case of a phenyl linker (e.g., 0.58 V for
TCNQ vs 0.29 V15 for SOPV2) and much less than for
similar compounds with an EDOT linker.16 Conversely, in
LOPV derivatives, this interaction becomes much smaller
because the bridge does not favor electronic communication,
probably because of the donor effect of the O-alkyl side
chains and distortion from planarity. Thus, only one redox
wave can be seen on the voltammograms, as in noninteracting
symmetrical systems, occurring unsurprisingly at a more
positive value than for similar OPVs without terminal
accepting groups.17 Nevertheless, the noticeable more nega-
tive potential for this reduction compared to analogous
SOPVs is wondering, as well as the relative peak currents
that do not scale with a two-electron reduction for LOPVs.
We assumed that possible interactions with the electrode
surface could be the reason for this behavior. Actually, albeit
not dependent on the electrode material, the shape of the
cyclovoltammogram is altered upon decreasing the concen-
tration. In that case, the reduction peak splits into two close
but distinguished signals (see Supporting Information),
confirming that the two redox moieties in LOPVs are actually
reduced at nearly the same potential. For most of these
systems, the reversibility is moderate in the conditions of
investigations, and electrochemical kinetics are rather slug-
gish as illustrated by the high value of peak-to-peak potential
differences.
The effect of the solvent polarity on the position of the
maximum fluorescence wavelength was also investigated (see
Supporting Information). An increase of the polarity induced
a red shift of the maxima for all dyes and resulted in larger
Stokes shifts, except for LOPV6 and LOPV7. An increase
of the Stokes shift with the polarity of the solvent is
indicative of significant charge redistribution upon excita-
tion.13 Among all the dyes studied in this letter, LOPV6 is
the chromophore presenting the largest Stokes shifts, ranging
from 118 nm in CH2Cl2 to 183 nm in dioxane. Solid state
photoluminescence investigations revealed a red shift of the
emission maxima compared to the ones obtained in solution,
therefore illustrating intermolecular interactions such as
π-stacking.14
Electrochemical behavior of all dyes was investigated by
cyclic voltammetry (CV) in DMF on a gold electrode. The
resulting redox potentials are gathered in Table 1. As
expected, the reduction potentials shift positively with the
increasing withdrawing power of the terminal substituents
(e.g., from 2 to 5). However, it can be emphasized that short
and long OPV derivatives have very different electrochemical
features (see Figure 2): SOPVs show two distinct and well
To summarize, a series of 11 new chromophores covering
the visible region have been designed and synthesized in high
yields. The absorption and emission properties of these
chromophores were efficiently controlled by tuning both the
acceptor power of the acceptor substituents in the oligomers
and the length of the conjugated electron-releasing core.
Strong solvatochromism was also evidenced for both absorp-
tion and emission. Finally, specificities of their electrochemi-
cal behavior were investigated.
Acknowledgment. We thank the C’NANO IdF, CNRS,
and the ANR agency (ANR-07-JCJC-040, AURUS program)
for financial support.
Figure 2. Reduction waves of LOPV4 (dashed line) and SOPV4
(solid line).
Supporting Information Available: Experimental pro-
cedures and spectroscopic characterizations for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
separated redox waves, while LOPVs display only one wave
at a significantly more negative potential. The SOPV
behavior looks like the one of quinone derivatives, i.e.,
typical of two identical redox moieties in close intramolecular
interactions; thus, addition of the second electron is made
notably more difficult than the first by the electrostatic
OL1007263
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