New bipyridyl ligands bearing azo- and imino-linked chromophores. Synthesis
and nonlinear optical studies of related dipolar zinc complexes†
Adam Hilton,a Thierry Renouard,a Olivier Maury,a Hubert Le Bozec,*a Isabelle Ledouxb and Joseph Zyssb
a
Laboratoire de Chimie de Coordination et Catalyse, UMR 6509 CNRS-Université Rennes 1, Campus de Beaulieu,
35042 Rennes cedex, France. E-mail: lebozec@univ-rennes1.fr
Laboratoire de Physique Quantique Moléculaire, ENS Cachan, 61 avenue du président Wilson, 94235 Cachan,
b
France
Received (in Cambridge, UK) 8th October 1999, Accepted 10th November 1999
The synthesis and optical properties of 4,4A-bis(dialk-
ylaminophenylazo)-2,2A-bipyridine and 4,4A-bis(dialkylami-
nophenylimino)-2,2A-bipyridine ligands are described; the
corresponding dipolar bipyridyl zinc dichloride complexes
have been prepared and their second order nonlinear optical
properties determined by electric field-induced second
harmonic generation (EFISH) at 1.34 mm.
2,2A-bipyridine and diethylaminobenzaldehyde failed, confirm-
ing the already observed deactivation of the amino and carbonyl
functions in the presence of acceptor and donor groups,
respectively.11 Ligands c and d were characterised by 1H NMR
and high resolution mass spectrometry. Their optical spectra
and those of the related aminostyryl derivatives a and b are
displayed in Fig. 1. Most importantly, a large bathochromic
shift of the ICT band is observed on exchanging the CNC bond
for an NNN (Dlmax = 71 nm) or NNC (Dlmax = 36 nm) bond.
This effect has been already described for other chromophores:
Ulman et al.12 reported a bathochromic shift of 40 nm between
4-dimethylamino-4A-dimethylsulfonylazobenzene and the cor-
responding stilbene derivative, while Whitall et al. described a
20 nm shift between an imino-benzene and the corresponding
stilbene ruthenium complex.11a In the latter case the shift was
accompanied by a strong decrease of the molar extinction
coefficient e, an hypsochromic effect which is not observed in
our case.
Tetrahedral zinc complexes 1a–d (Scheme 2) were readily
prepared upon room temperature treatment of the ligands a–d
with ZnCl2 in dichloromethane.5a In their UV-vis spectra (Fig.
1, Table 1), a red-shift of the intense ILCT (intraligand charge-
transfer) band is observed (Dlmax = 45–60 nm), as expected
from the inductive acceptor strength of the Lewis acid.13 As in
the free ligand, lmax is sensitive to the nature of the XNY
transmitter and follows the order NNN > NNC > CNC.
Poled polymers containing second order nonlinear (NLO)
molecules are particularly promising for electrooptic device
applications. Usually, dipolar chromophores have to be aligned
by using electric field poling near the glass transition tem-
perature (Tg) of the polymer.1 Besides this approach, which
requires high temperature treatment, new mild optical poling
techniques have been developed recently. The so-called ‘photo-
assisted electrical’ poling2 and the ‘all optical’ poling3 can be
used at room temperature for the macroscopic noncentrosym-
metric organisation of dipolar and octupolar chromophores,
respectively. These methods require the presence of chromo-
phores featuring a photoisomerisable moiety and use the
‘flexibility’ of the molecule to break the centrosymmetry of the
material.
We have previously reported that metalloorganic complexes
containing p-donor substituted 4,4A-alkenyl-2,2A-bipyridyl li-
gands4 such as a and b (Scheme 1), show large dipolar or
octupolar microscopic nonlinearities.5 As the CNC double bond
is known to be poorly photoisomerisable as compared, for
example, to the NNN double bond,6 we sought to design new
potentially photoisomerisable bipyridines by introducing ni-
trogen atoms into the transmitter. Herein we report the synthesis
and characterisation of new bipyridines bearing azo- as well as
imino-linked donor groups. We also describe the preparation of
the corresponding dipolar zinc(ii) complexes; the influence of
the nature of the p-bridge on the linear and nonlinear properties
of these complexes is also reported.
The synthesis of the azo-containing bipyridyl ligand c was
readily accomplished in two steps from 4,4A-dinitro-2,2A-
bipyridine-1,1A-dioxide 7 [Scheme 1, eqn. (1)]: 4,4A-diamino-
2,2A-bipyridine was first prepared in quantitative yield by using
hydrazine hydrate in the presence of Pd/C. 8 This mild and
efficient method contrasts favourably with the well-known but
poorly yielding (20%) procedure using Fe/AcOH as the
reducing agent.9 Finally, the diazotation of 4,4A-diamino-2,2A-
bipyridine with sodium nitrite and subsequent coupling with
N,N-dibutylaniline afforded derivative c, which was isolated
(yield 25%) as dark red microcrystals after chromatographic
workup. Imino-containing bipyridine d was synthesised in
excellent yield (95%) by a Schiff base condensation reaction
between N,N-diethyl-1,4-phenylenediamine and 4,4A-diformyl-
2,2A-bipyridine10 [Scheme 1, eqn. (2)]. Attempts to isolate the
‘reverse’ imino-bipyridine from the reaction of 4,4A-diamino-
† Electronic supplementary information (ESI) available: experimental
1999/2521/
Scheme 1 (i) Pd/C (10%), N2H4·H2O, EtOH, reflux, 8 h, 100%; (ii) NaNO2,
Bu2NC6H5 in THF, 0–5 °C, 2 h, 25%; (iii) Et2NC6H4NH2, MgSO4, THF,
reflux, 12 h, 95%.
Chem. Commun., 1999, 2521–2522
This journal is © The Royal Society of Chemistry 1999
2521
Hilton, Adam
