The design of second-order nonlinear optical chromophores exhibiting blue-shifted absorption and large nonlinearities: The role of the combined conjugation bridge

Article abstract of DOI:10.1039/b007653h

Two new extended chromophoric systems, in which a 4-aminoazobenzene moiety is linked to a cyclo-bridged hexatriene electron withdrawing group, have been synthesized, and show large optical nonlinearities and unexpected blue-shifted absorption in compariso

Full text of DOI:10.1039/b007653h

The design of second-order nonlinear optical chromophores exhibiting
blue-shifted absorption and large nonlinearities: the role of the combined
conjugation bridge†
Jingdong Luo,a Jianli Hua,a Jingui Qin,*a Jiqi Cheng,b Yaochun Shen,b Zuhong Lu,b Peng Wangc and
Cheng Yec
a Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China. E-mail: jgqin@whu.edu.cn;
Tel: +86-27-87684117; Fax: +86-27-87647617
b National Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, P.R. China
c
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P.R. China
Received (in Cambridge, UK) 20th September 2000, Accepted 27th November 2000
First published as an Advance Article on the web 4th January 2001
Two new extended chromophoric systems, in which a
4-aminoazobenzene moiety is linked to a cyclo-bridged
hexatriene electron withdrawing group, have been synthe-
sized, and show large optical nonlinearities and unexpected
blue-shifted absorption in comparison with shorter chain
analogues.
isophorone-protected triene with the terminal acceptors is
linked to a donor-substituted azo benzene bridge.
The first molecular hyperpolarizabilities of chromophores I
and II were measured by Hyper-Rayleigh Scattering (HRS) in
methanol using the fundamental excitation wavelength of 1064
nm; p-nitroaniline (PNA) was used as the external reference.
The b values of I and II were found to be 2890 3 10230 esu and
3490 3 10230 esu, respectively, and by using the two-level
approximation model, the zero-frequency hyperpolarizabilities
b0 of I and II were extrapolated to be 337 3 10230 esu and 198
3 10230 esu, respectively.
Organic and polymeric second-order nonlinear optical (NLO)
materials have been extensively studied for their potential
applications involving optical signal processing and tele-
communications.1 One of the major challenges in this area of
research is to design and synthesize second-order NLO
chromophores simultaneously exhibiting large first molecular
hyperpolarizability (b), high chemical and thermal stability, and
good optical transparency.2,3 Most attempts to design molecules
with large b since the 1980s have been based upon ‘push–pull’
compounds,4 in which a p-conjugated bridge is endcapped with
an electron donor group and an electron acceptor group. From
the two-level model,5 the b value of this type of chromophore is
a strong function of the absorption maximum (lmax), leading to
the so-called nonlinearity–transparency tradeoff.
Among all the attempts to solve this kind of trade-off, it has
been proved that the nature of p-conjugated bridge of
chromophores is one of the crucial factors in determining the
linear and nonlinear optical properties of the chromophores. For
example, Alain et al. recently reported6 that chromophore 1
(Fig. 1), in which a biphenyl moiety is present between the
diphenylpolyene unit, exhibits a 45 nm (2400 cm21) blue-
shifted absorption and similar optical nonlinearity compared
with its all diphenylpolyenic analogue (chromophore 2, Fig. 1),
though the conjugation bridge of 1 is much longer. It seems a
worthwhile job to study further the linear and nonlinear optical
properties of the chromophores by using a combination of
different types of conjugation bridges.
Therefore, we have recently designed two new chromophores
(I and II in Fig. 2)† in which the conjugation bridge of
Fig. 1 Chemical structures and properties of chromophores 1 and 2.
† Electronic supplementary information (ESI) available: synthesis and
structural characterization of several new compounds of this work. See
http://www.rsc.org/suppdata/cc/b0/b007653h/
Fig. 2 Chemical structures of chromophores I, II and their corresponding
triene-only and azo-only analogues (chromophores 5–10).
DOI: 10.1039/b007653h
Chem. Commun., 2001, 171–172
This journal is © The Royal Society of Chemistry 2001
171

Table 1 The linear and nonlinear properties of chromophore I, II and 5–10
Solvatochromism dataa (lmax/nm)
Theoretical investigation7
Experimental results
b
d
CHCl3
AcOEt
DMF
NMP
b0
fc
b0
f2
HRS
I
490
519
—
492
504
—
511
527
—
519
535
534
533
52.6
46.8
—
2.3302
1.5646
—
337
—
60e
55e
1.0
0.72
—
5
6
7
508
505
525
42.5
1.4965
0.86
II
8
9
510
562
568
516
507
—
—
517
—
—
533
—
—
—
—
—
—
—
—
—
—
198
90e
27e,13
—
1.1
—
—
10
524
545
553
0.82
a AcOEt: ethyl acetate, DMF: dimethylformamide, NMP: N-methylpyrrolidone. b AM1/FF results, in units of 10230 esu. c ZINDO/S-CI results. d Dispersion-
corrected b values (in units of 10230 esu) of I and II were calculated by using an approximate two-level model. e b0HRS values are estimated from the data
of b0EFISH previously reported8 according to the following equation: b0
= (6/35)1/2b0
.
HRS
EFISH
Besides the target chromophores I and II, the other six
chromophores (5–10, see Fig. 2), which are the azo-only or
triene-only analogues of I and II, respectively, will also be
discussed below.8,9
As shown in Table 1, the oscillator strengths (f) of I and II,
respectively determined to be 1.0 and 1.1, are significantly
higher than those of the corresponding azo-only or triene-only
analogues. It seems that the larger oscillator strengths of I and
II may be one of the major points for counterbalancing the
effects of blue shifts. Further study is needed for a full
interpretation.
In summary, we have explored two new NLO chromophores
with combined conjugation bridges and found that they possess
blue-shifted absorption and large molecular nonlinearities.
Experimental results indicate that the combined conjugation
bridge tunes the linear and nonlinear properties of the
chromophores in a different style from those of common
homologous chromophores. We expect this methodology to
build up new molecular engineering, thereby providing a new
opportunity for defeating the ‘nonlinearity-transparency trade-
off’. Design and synthesis of further chromophores with
different types of combined conjugation bridge, and a detailed
investigation of the origin of this new effect are currently in
progress.
The linear and the nonlinear properties of these chromo-
phores are summarized in Table 1. It can be seen that I and II,
two chromophores employing the combination of azo benzene
and conjugated triene as their conjugation bridge, display
unexpectedly blue-shifted absorption compared with the corre-
sponding azo-only or triene-only analogues, although the length
of their conjugation bridge is much longer. For instance, the
absorption maxima of I is about 15 nm (500 cm21) blue-shifted
compared with 5, 6, and 7 in different organic solvents. And as
shown in Fig. 3, there is no significant broadening of the main
absorption band for I, if another absorption band around 400 nm
in their UV-Vis spectrum is taken into account, and this is also
the case for II. All these results show that for I and II, the
electron transmission process between donor and acceptor
groups is affected, and the intramolecular charge transfer (ICT)
efficiency is reduced. This reduction may occur through using
different types of bridge with different energy orbitals other
than the most efficiently conjugated bridge possible.
This work was supported by the National Natural Science
Foundation of China and by a grant of the Key Fundamental
Research Programs of China.
Notes and references
1 S. R. Marder, B. Kippelen, A. K-Y. Jen and N. Peyghambarian, Nature,
1997, 388, 845.
2 C. R. Moylan, R. J. Twieg, V. Y. Lee, S. A. Swanson, K. M. Betterton and
R. D. Miller, J. Am. Chem. Soc., 1993, 115, 12599.
3 C. Zhang, A. S. Ren, F. Wang, J. Zhu and L. R. Dalton, Chem. Mater.,
1999, 11, 1966.
4 T. Verbiest, S. Houbrechts, M. Kauranen, K. Clays and A. Persoons,
J. Mater. Chem., 1997, 7, 2175.
5 J. L. Oudar, J. Chem. Phys., 1977, 66, 2664; J. L. Oudar, J. Chem. Phys.,
1977, 67, 446.
6 V. Alain, S. Rédoglia, M. Blandchard-Desce, S. Lebus, K. Lukaszuk, R.
Wortmann, U. Gubler, C. Bosshard and P. Günter, Chem. Phys., 1999,
245, 51.
Fig. 3 Comparison of the absorption spectra of chromophores I, 5, and 7 in
methanol.
7 The computational approach has been described in our previous work: P.
Zhu, P. Wang and C. Ye, Chem. Phys. Lett., 1999, 311, 306.
8 C.-F. Shu, W.-J. Tsai and A. K-Y. Jen, Tetrahedron Lett., 1996, 37, 7055;
S. Ermer, S. M. Lovejoy, D. S. Leung, H. Warren, C. R. Moylan and R. J.
Twieg, Mat. Res. Soc. Symp. Proc., 1998, 488, 243; K. D. Singer, J. E.
Sohn, L. A. King, H. M. Gordon, H. E. Katz and C. W. Dirk, J. Opt. Soc.
Am. B, 1989, 6, 1339; S. R. Marder, L.-T. Cheng, B. G. Tiemann, A. C.
Friedli, M. Blandchard-Desce, J. W. Perry and J. Skindh, Science, 1994,
263, 511.
9 Assuming that the dipole moment of chromophore 9 is equal to that of
chromophore 8, the b value of chromophore 9 is estimated from its mb
product reported in the following reference: L. R. Dalton, W. H. Steier,
B. H. Robinson, C. Zhang, A. Ren, S. Garner, A. Chen, T. Londergan, L.
Irwin, B. Carlson, L. Fifield, G. Phelan, C. Kincaid, J. Amend and A. Jen,
J. Mater. Chem., 1999, 9, 1905.
Although the p-electron delocalization efficiency of I and II
is somewhat reduced, it should be noted that the b0 values of I
and II are quite competitive with those of their corresponding
analogues from the theoretical investigation and the experi-
mental results. This appears to be at variance with the
predictions of the two-level model. However, besides the
nonlinearity–transparency trade-off mentioned above, the two-
level model also adduces that the b value is a strong function of
the oscillator strengths (f) and Dm, the change of dipole moment
upon excitation. Thus it can be phenomenologically deduced
that the reduction of the ICT efficiency of chromophores I and
II does not accompany a decrease in their f and/or Dm values,
and this has been partially verified by the experimental results.
172
Chem. Commun., 2001, 171–172