The effect of 2,2-dicyanovinyl groups as electron acceptors in push-pull substituted oligo(1,4-phenylenevinylene)s

Article abstract of DOI:10.1016/S0040-4039(03)00120-5

The newly synthesized oligo(1,4-phenylenevinylene) series 2a-d with bis(2-hexyloctyl)amino groups as electron donors and 2,2-dicyanovinyl groups as electron acceptors represents conjugated oligomers with strong push-pull effects. Due to the decrease of the intramolecular charge transfer with increasing numbers of repeat units (n=1-4), the long-wavelength transition shows a particularly great hypsochromic shift for the extension of the chromophore.

Full text of DOI:10.1016/S0040-4039(03)00120-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1915–1918
The effect of 2,2-dicyanovinyl groups as electron acceptors in
push–pull substituted oligo(1,4-phenylenevinylene)s
Herbert Meier,a,* Ju¨rgen Gerolda and Dominic Jacobb
aInstitute of Organic Chemistry, University of Mainz, Duesbergweg 10–14, 55099 Mainz, Germany
bNirmala College, Muvattupuzha, India
Received 5 October 2002; accepted 7 January 2003
Abstract—The newly synthesized oligo(1,4-phenylenevinylene) series 2a–d with bis(2-hexyloctyl)amino groups as electron donors
and 2,2-dicyanovinyl groups as electron acceptors represents conjugated oligomers with strong push–pull effects. Due to the
decrease of the intramolecular charge transfer with increasing numbers of repeat units (n=1–4), the long-wavelength transition
shows a particularly great hypsochromic shift for the extension of the chromophore. © 2003 Elsevier Science Ltd. All rights
reserved.
Oligo(phenylenevinylene)s (OPV) attract a lot of atten-
tion, because of their excellent properties as photocon-
ductors and electroluminophores; moreover, various
applications in the field of nonlinear optics (NLO) are
known.1–11 Large second order hyperpolarizabilities
have been found for compounds with terminal donor–
acceptor substitution.2,11 Normally, OPVs show a
monotonous bathochromic shift of the absorption and
the fluorescence maxima with increasing numbers n of
repeat units. The convergence for n“ꢀ can be pre-
dicted on the basis of exponential functions.8,10,12 How-
ever, we recently established the unexpected result, that
a monotonous hypsochromic effect can occur with
increasing numbers n, provided that the intramolecular
charge transfer (ICT) is strongly reduced with growing
extension of the chromophore. An example in the OPV
series was found for terminal dialkylamino and nitro
groups.13 This surprising behaviour encouraged us to
look for other OPV systems with strong push–pull
effects.
failed under many reaction conditions. Traces of 4a
could be detected in the process with Pb(OAc)4 in
CH2Cl2. Finally, we succeeded with a radical process
with NBS in the presence of azoisobutyronitrile
(AIBN). The solid target compound 4a14 was obtained
in moderate yields; until now the reaction could not be
applied to higher OPV systems.
The highly branched dialkylamino group in 1a–d, 2a–d,
3a and 4a guarantees a reasonably good solubility in
chlorinated hydrocarbons as CH2Cl2, CHCl3 and CCl4.
In comparison to the purely donor substituted com-
pound 5a (n=1), a significant bathochromic shift is
observed for the (E)-stilbenes with terminal donor–
acceptor substitution (Scheme 2 and Table 1). The
effect increases from the yellow cyano compound 6a
(n=1) via the orange formyl compound 1a, the red
nitro compound 7a (n=1), and the dark red vinyl
system 2a to the deep blue compound 4a.
Starting with the aldehydes 1a–d, we prepared the
2,2-dicyanovinyl derivatives 2a–d by the condensation
reaction with malononitrile in boiling CHCl3 (Scheme
1). The introduction of a third cyano substituent on the
vinyl group proved to be very difficult. The addition of
HCN (2a“3a) works almost quantitatively and with
the regioselecivity expected for a nucleophilic addition;
however, the subsequent oxidation (dehydrogenation)
Obviously, the intramolecular charge transfer (ICT)
plays a dominant role in this series. Therefore one can
expect for the compounds 2a–d a hypsochromic effect
with increasing numbers n. The extension of the conju-
gation should not be able to compensate the decrease of
ICT. Figure 1 illustrates the result of the UV/vis mea-
surement in CHCl3. The series 2a–d exhibits in fact a
strong hypsochromic shift with increasing numbers n of
repeat units. The effect is even more pronounced as in
the nitro series 7a–d. In contrast to the 2,2-dicyanovinyl
group as strong acceptor, the cyano group is a rela-
tively weak acceptor and permits a bathochromic effect
with growing numbers n. Thus Figure 1 comprises the
Keywords: oligomers; conjugation; push–pull effect; UV/vis spectra.
* Corresponding author. Fax: +49(0)6131 3925396; e-mail: hmeier@
mail.uni-mainz.de
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00120-5

1916
H. Meier et al. / Tetrahedron Letters 44 (2003) 1915–1918
Scheme 1. Transformation of the OPV aldehydes 1a–d to the 2,2-dicyanovinyl derivatives 2a–d and of 2a to the tricyanovinyl
compound 4a.
Scheme 2. OPVs with terminal donor–acceptor substitution.
The characterization of the members of the novel OPV
total behaviour of push–pull substituted OPVs, namely
hypsochromic and bathochromic OPV series and the
formyl series, in which the absorption maxima are
almost independent of the extension of the p system
(chromophore).
1
series 2 by H NMR data is summarized in Table 2. A
complete correlation of the 13C NMR signals of 2a was
achieved by heteronuclear shift correlation measure-
ments. The result is shown in Figure 2.

H. Meier et al. / Tetrahedron Letters 44 (2003) 1915–1918
1917
Table 1. Long-wavelength absorption maxima of donor–acceptor substituted (E)-stilbenes. (Measurement in CHCl3)
Compound
5a
6a
1a
7a
2a
4a
umax [nm]
log m
366
4.3
401
4.3
423
4.3
461
4.7
525
4.5
670
4.6
Figure 1. Long–wavelength absorption maxima of the OPV series 2a–d (ꢀ) in comparison to the OPV series 1a–d, 5a–d, 6a–d and
7a–d.15 (Measurement in CHCl3).
Table 2. 1H NMR data of the compounds 2a–d (l values
the C atoms in para position to the donor group are
increased and the l values of the C atoms in para
position to the acceptor group are decreased. More-
over, the high polarization of the olefinic double bond
is strongly reduced; the Dl difference, which amounts
to 12.8 ppm in 2a, is for the two terminal double bonds
in the OPV scaffold reduced with increasing chro-
mophore length to 6–7 ppm. A complete characteriza-
tion of the new compounds by elemental analyses,
NMR and MS data shall be given in a full paper.
in CDCl3 with TMS as internal standard)
CH3 (t, 12 H)
CH2 (m, 40 H)
CH (m, 2 H)
NꢀCH2 (d, 4 H)
CHꢁCH (AB, 2 H)
3J=(16.290.2) Hz
CHꢁC(CN)2 (s, 1 H)
C6H4
0.8790.01
1.2490.01
1.8390.01
3.2290.01
6.85–7.28
7.6690.03
Donor side (AA%BB%, 4 H)
Acceptor side (AA%BB%, 4 H)
Middle (AA%BB%, 4 H)
Middle (s, 4 H)
6.6390.01/7.3490.02
7.5890.04/7.8790.03
7.4590.01/7.4890.02
7.5190.02
Interestingly the absorption of push–pull polyenes with
terminal 4-dibutylaminophenyl and dicyanovinyl
groups shows a bathochromic shift for increasing num-
bers of repeat units.16 Obviously the structure of the
conjugated chain between donor and acceptor groups
has a decisive influence.
The donor–acceptor interaction is diminished, when the
OPV chain is extended. Consequently, the l values of
Figure 2. 13C NMR data of 2a measured in CDCl3.

1918
H. Meier et al. / Tetrahedron Letters 44 (2003) 1915–1918
9. Segura, J. L.; Martin, N. J. Mater. Chem. 2000, 10, 2403.
Acknowledgements
10. Meier, H.; Ickenroth, D. Eur. J. Org. Chem. 2002, 1745.
11. Nalwa, H. S. In Nonlinear Optics of Organic Molecules
and Polymers; Nalwa, H. S.; Miyata, S., Eds.; CRC: New
York, 1997.
12. Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997,
48, 379.
We are grateful to the Deutsche Forschungsgemein-
schaft and the Fonds der Chemischen Industrie for
financial support.
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