Y-shaped two-photon absorbing molecules with an imidazole-thiazole core.

Article abstract of DOI:10.1039/b402019g

Two new classes of two-photon absorbing Y-shaped molecules have been developed to possess an imidazole-thiazole core and a stilbene-type conjugation pathway with either nitro or sulfonyl as terminal electron-accepting group.

Full text of DOI:10.1039/b402019g

Y-shaped two-photon absorbing molecules with an imidazole–thiazole
core†
Ke Feng,a Leonardo De Boni,b Lino Misoguti,b C. R. Mendonça,b Michael Meador,c Fu-Lian Hsud and
Xiu R. Bu*a
a Department of Chemistry and NASA Center for High Performance Polymers and Composites, Atlanta,
Georgia 30314, USA. E-mail: xbu@cau.edu
b Departamento de Física e Ciência dos Materiais, Instituto de Física de São Carlos, Caixa Postal 369,
13560-970 São Carlos, SP, Brazil. E-mail: crmendon@.if.sc.usp.br
c
Polymer Branch, NASA Glenn Research Center, Cleveland, OH 44135, USA
d US Army Edgewood Chemical and Biological Center, Aberdeen Proving Ground, MD 21010, USA
Received (in Cambridge, UK) 10th February 2004, Accepted 29th March 2004
First published as an Advance Article on the web 21st April 2004
Two new classes of two-photon absorbing Y-shaped molecules
have been developed to possess an imidazole–thiazole core and
a stilbene-type conjugation pathway with either nitro or
sulfonyl as terminal electron-accepting group.
The emergence of applications of two-photon technology to up-
converted lasing, optical power limiting, photodynamic therapy
and generation of singlet oxygen, three-dimensional micro-
fabrication and data storage, and three-dimensional fluorescence
microscopy,1 has drawn an increasing interest in the design and
development of two-photon absorbing molecules with large
nonlinear absorptivity to generate a high-energy excited state with
relatively low laser energy. The new structure–property correlation
is being sought to establish guidance for molecular design since the
ability of organic molecules to simultaneously absorb two photons
to reach the excited state depends on molecular structures. Several
strategies2,3 based on schemes such as D–p–D, D–p–A–p–D, and
A–p–D–p–A (D: electron donating group, A: electron accepting
group, and p: conjugated pathway) have been found to enhance the
d value, the two-photon absorptivity. The common structural
features in these designs are (1) molecular symmetry and (2) the use
of amine related groups such as diphenylamine and carbazole as D
components. Other molecular architectures for large two-photon
absorption include branched multi-conjugated systems furnished
with D and/or A components,4 dendrimers with terminal D
components5 and fused aromatic systems with planar p-cores such
as fluorene,6 dithienothiophene,7 and perylene.8 We here report two
novel classes of Y-shaped molecules with large two-photon
absorptivities.
Two new series of chromophores 1–3 and 4–6 were synthesized
via intermediates 13–15, which were obtained by a procedure
starting with the condensation9,10 of benzil derivatives with
2-formylthiazole to afford 7–9. The methylation of 7–9 with
iodomethane in the presence of a base gave 10–12, which were
treated with n-butyl lithium, followed by the addition of DMF to
Scheme 1 Structures of two new series of chromophores.
produce 13–15 (Scheme 1). The Wittig reactions of 4-nitrobenzyl
triphenylphosphine bromide with 13–15 led to the formation of 1–3
while the reactions with 4-methylsulfonylbenzyl triphenylphos-
phine bromide yielded 4–6. Chromophores 1–3 absorb with lmax in
the range of 422–433 nm, 4–6 in the range of 398–410 nm.
Nonlinear optical measurements for two-photon absorption
properties of 1–6 were carried out with the Z-scan technique, which
indicates two distinct absorptive nonlinear processes: the two-
photon absorption (2PA), and the saturable absorption (SA). For
wavelengths between the resonant and the onset of nonresonant
regions, these two processes are competing. The SA process at 500
nm is shown in gray and the 2PA process signature at 600 nm is
shown in black (Fig. 1). For wavelengths in the nonresonant region,
the 2PA process is unique.
† Electronic supplementary information (ESI) available: synthesis and
characterization of 1–6, and 2PA measurement. See http://www.rsc.org/
suppdata/cc/b4/b402019g/
Fig. 1 Z-scan signatures at different wavelengths from 4.
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C h e m . C o m m u n . , 2 0 0 4 , 1 1 7 8 – 1 1 8 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

The 2PA spectra profiles in the range of 600–1000 nm for 1–3,
and 550–1000 nm for 4–6 are shown in Fig. 2. The peaks around
700 nm are listed in Table 1 along with additional peaks around
600–640 nm. The increase in the 2PA intensity below 700 nm is due
to resonance enhancement of the optical nonlinearity near the one
photon transition, in agreement with the resonant denominator in
the sum-over states model.11 Table 1 displays the concentration and
the wavelength peak with their correspondent d values (peak)
obtained in the nonresonant region for the six solutions.
The d values obtained for the 2PA cross-sections indicate that all
compounds exhibit a high 2PA process far from the resonance. The
chromophore 6 gives 1700 GM at 760 nm, which is comparable to
some of the leading chromophores:12 for example, the bis donor D–
p–D compound:3 1420 GM; dendrimers with D-end:5 400–800
GM, dithienothiophene core-based compounds:7 130–1900 GM;
perylene core-based compounds: 84–1200 GM;8 and multi-
branched conjugates: 1390–2620 GM.4
Sulfonyl-based chromophores turn out to be more efficient for
2PA, in nonresonant conditions, compared to nitro-based ones. A
close examination of d values from 2PA lmax > 700 nm reveals
that the resultant difference from these two series is significant in
the following: several systems examined so far have demonstrated6
that a stronger acceptor will lead to the d enhancement. That is the
reason why the nitro group is often used.6 Based on the Hammett
constant scheme for para-substitution, the sp is 0.81 for the NO2
group and 0.73 for the SO2Me group,13 respectively, indicating that
the nitro group is a stronger electron-accepting group. However, the
present chromophores show the opposite trend, with sulfonyl
group-based chromophores giving higher d values than nitro group-
based ones do. For example, the value of 6 (R = OMe) is 1.6 times
that of 3 while the value of 5 (R = Me) is 2.2 times that of 2.
It is also interesting that the present chromophores achieve large
2PA without the use of amine-based donor groups such as NMe2
(for R). Methoxy (R = OMe) is the strongest donor used for these
two series.
Fig. 3 The LUMO (top) and HOMO (bottom) of 6.
imidazole.14 The molecular geometry optimized by DF 6-31G*
shows that one phenyl (at the 4-position) is only twisted 26–29°
from the imidazole ring while the other (at the 5-position) is twisted
53–57°, suggesting that one donating group primarily contributes to
the push–pull structure through better conjugation due to less twist
between two adjacent aromatic rings. For the rest of the
chromophores, namely from imidazole to thiazole and all the way
to the accepting group, the geometry is relatively planar. For
instance, the twists of the two phenyls from imidazole in 6 are
28.82° and 53.59°, respectively. Imidazole and thiazole are almost
co-planar, with a deviation of only 1.89°. The vinyl moiety is also
positioned well with two adjacent aromatic rings, thiazole and
phenyl, with deviations of 1.11° and 3.87°, respectively. Accord-
ingly, the HOMO and LUMO mainly reside on the following
pathway: phenyl with accepting group, vinyl, thiazole, imidazole,
and the phenyl at the 4-position (Fig. 3). Both series, regardless of
the type of electron-accepting groups, possess the same conjugation
pathway. This conjugation pathway is responsible for the increase
in permanent dipole moments from H ? OMe for R (nitro group
series: 1: 8.52; 2: 9.22; 3: 11.0 Debye; sulfonyl group series: 4:
7.30; 5: 8.41; and 6: 10.06 Debye). The charge transfer from donor
to acceptor along this pathway leads to an increase in the d values
from H ? OMe (R) in both series of chromophores. The
contribution from the phenyl at the 5-position is minimal due to a
large twist from the imidazole ring. In addition, the higher d values
from the sulfonyl-based chromophores may be ascribed to larger
transition dipole moments Mge and Mee.3
The fact that the large d values from the present chromophores
are highly comparable to those from well-studied D–p–D systems
is remarkably significant because the current structural architecture
is unsymmetric. We have examined the molecular geometry to
determine the conjugation pathway of this unsymmetric system.
Two phenyl rings have different twists from imidazole, and the
conjugation from one phenyl ring is better than the other with
In summary, we have developed two new classes of Y-shaped
molecules that possess large two-photon absorptivity. The imida-
zole–thiazole core provides a planar core geometry and the minimal
deviations between the two rings are due to the fact that thiazole
does not possess any adjacent hydrogen which could potentially
interact with imidazole and cause a twist.15 One series of
chromophores has sulfonyl as an electron-accepting group while
the other has nitro. The chromophores that possess the sulfonyl
group exhibit larger d values than those with the nitro group.
XRB gratefully acknowledges support by the U.S. Army
(DAAD19-01-0746), NASA (NCC3-910 and NCC3-552), NIH
(G12-RR03062) and DOE (DE-FC02-02EW15254). The support
of CRM by FAPESP (Fundação de Amparo à Pesquisa do Estado
de São Paulo), Brazil is gratefully appreciated. We also thank
Professor Dr Seth R. Marder and Professor Dr Sergio Carlos Zilio
for helpful discussions.
Fig. 2 Two-photon absorption.
Table 1 2PA cross-section coefficients
[C]a
2PA lmax (nm)
d (GM)b
1
2
3
4
5
6
2.3
4.8
6.6
5.3
5.9
5.4
610, 690
610, 720
600, 720
590, 740
600, 740
640, 760
2100, 1500
1300, 650
1450, 1050
1100, 460
1500, 1400
2000, 1700
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