Synthesis, characterization and third-order non-linear optical properties of novel fluorene monomers and their cross-conjugated polymers

Article abstract of DOI:10.1016/j.polymer.2010.03.046

We designed and synthesized two novel fluorene monomers of D-A-D (donor-acceptor-donor) type (M1 and M2), and their two corresponding polymers (PM1 and PM2) and a copolymer (CPM). These high molecular weight, film-forming polymers were obtained from metal-free, superacid-catalyzed reactions of the monomers with N-phenylisatin. The cubic NLO response (χ⁽³⁾) for these new compounds, in solid thin films, was measured through the use of third-harmonic generation (THG) Maker-Fringes technique at IR wavelengths given values of the order of 10^-12 esu from which, the corresponding second hyperpolarizabilities (γ) were estimated to be of the order of 10^-33 esu for monomers and 10^-31 esu for polymers. Second hyperpolarizabilities have also been estimated theoretically at B3LYP/6-31G(d) level of theory in gas phase and related with the electronic structure of the synthesized molecules.

Full text of DOI:10.1016/j.polymer.2010.03.046

Polymer 51 (2010) 2351e2359
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, characterization and third-order non-linear optical properties of novel
fluorene monomers and their cross-conjugated polymers
,
b
b
,
b
c
G. Ramos-Ortíz a, J.L. Maldonado a , M.C.G. Hernández , M.G. Zolotukhin , S. Fomine , N. Fröhlich ,
*
*
U. Scherf c, F. Galbrecht c, E. Preis c, M. Salmon d, J. Cárdenas d, M.I. Chávez d
a Centro de Investigaciones en Óptica A.C., A.P. 1-948, 37000 León, Gto., Mexico
b Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, CU, Coyoacán 04510, México D.F., Mexico
c Macromolecular Chemistry Group, Wuppertal University, Gauss-Str. 20, D-42097 Wuppertal, Germany
d Instituto de Química, Universidad Nacional Autónoma de México, Apartado Postal 70-360, CU, Coyoacan 04510, México D.F., México
a r t i c l e i n f o
a b s t r a c t
Article history:
We designed and synthesized two novel fluorene monomers of DeAeD (donoreacceptoredonor) type
(M1 and M2), and their two corresponding polymers (PM1 and PM2) and a copolymer (CPM). These high
molecular weight, film-forming polymers were obtained from metal-free, superacid-catalyzed reactions
Received 4 February 2010
Received in revised form
23 March 2010
Accepted 25 March 2010
Available online 31 March 2010
of the monomers with N-phenylisatin. The cubic NLO response (c
(3)) for these new compounds, in solid
thin films, was measured through the use of third-harmonic generation (THG) Maker-Fringes technique
at IR wavelengths given values of the order of 10ꢀ12 esu from which, the corresponding second hyper-
polarizabilities (g
) were estimated to be of the order of 10ꢀ33 esu for monomers and 10ꢀ31 esu for
Keywords:
polymers. Second hyperpolarizabilities have also been estimated theoretically at B3LYP/6-31G(d) level of
theory in gas phase and related with the electronic structure of the synthesized molecules.
Ó 2010 Elsevier Ltd. All rights reserved.
Fluorene polymers
Cubic NLO response
THG Maker-Fringes
1. Introduction
compounds can be readily tuned by varying the nature of the
co-units in the main chain, and the intra-chain charge transfer
There is an enormous interest in developing fluorene deriva-
tives: molecules, oligomers and polymers for different optical
applications, such as organic electroluminescence devices (OLEDs)
and organic photovoltaic cells (OPVs) [1e10]. Other optical uses are
for two photon absorption (TPA) [11e16], and sensors [17]. These
fluorene-based compounds are of great interest since they show
between the electron-deficient and electron-excessive units can
enhance their NLO properties. Additionally, optical phenomena
in fluorene containing polymers can be controlled by the
introduction of cross-conjugated segments into the main chain. In
cross-conjugation three groups are present, two of which are not
conjugated with each other, although each is conjugated with the
third one [20].
a strong
peelectron conjugation, i.e., large electron delocalization
and, additionally, high fluorescent efficiency. Fluorene is a suitable
arylene moiety because it is easy to substitute with solubilizing
groups at its 9-position [18].
Non-linear optical (NLO) properties have been measured for
a few fluorene systems [18,19]. It is believed that the strong
In this work, we have designed and synthesized two novel
fluorene monomers of DonoreAcceptoreDonor type (M1 and M2)
(see chemical structures in Fig. 1), and two corresponding polymers
(PM1 and PM2) and a copolymer (CPM).
Linear (absorption and fluorescence) and NLO properties of
these new fluorene containing monomers and (co)polymers are
peelectron conjugation is crucial factor in attaining high optical
non-linearities and so, it is of great interest to identify or under-
stand the structureeproperty relationship. This knowledge is very
important for a rational design of new second and third-order NLO
materials based on both low molecular weight molecules and
polymers. The electronic transitions of the fluorene-based
reported. The third-order optical susceptibility c(3) (ꢀ3
u,u,u,u),
was estimated through THG Maker-Fringes technique [21].
2. Results and discussion
2.1. Synthesis of monomers and polymers
* Corresponding authors.
E-mail addresses: jlmr@cio.mx (J.L. Maldonado), zolotukhin@iim.unam.mx
(M.G. Zolotukhin).
Monomers M1 and M2 have been prepared using organo metal
catalyzed arylearyl Suzuki cross-coupling (see Experimental
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.03.046

2352
G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
S
O
H3C
CH3
H3C
H3C
CH3
CH3
N
N
H3C
CH3
M2
M1
S
H3C
CH3
H3C
CH3
N
N
C
N
PM1
C
O
n
O
CH3
CH3
H3C
H3C
PM2
C
C
O
N
n
S
H3C
CH3
H3C
CH3
N
N
O
CH3
H3C
CH3
H3C
C
C
O
C
N
O
N
p
m
n
CPM
Fig. 1. Chemical structures of the five analyzed fluorene monomers and polymers.
section). The monomers were obtained in good yields; their
structural integrity and high purity were confirmed.
N-methylpyrrolidon. Transparent, strong, and flexible films could
be cast from the polymer solutions.
Polymers PM1, PM2 and CPM were obtained by superacid-
catalyzed polyhydroxyalkylation of monomers M1 and M2 (and
their mixture) with N-phenylisatin [22]. Thus, the synthesis of
polymer PM1 proceeds as electrophilic aromatic substitution
according to the following Scheme 1.
Since it is often very difficult to achieve high regioselectivity in
electrophilic aromatic polycondensation reactions, each polymer
was subjected to structural studies by NMR spectroscopy and
taking advantage of the good solubility of the polymers obtained in
CDCl3. As an example, the 13C NMR spectrum of polymer PM1 is
presented in Fig. 2a.
The obtained polymers possess a high molecular weight and
a reasonably narrow polydispersity. According to GPC data, the
molecular weights Mw and Mn for PM1, PM2 and CPM were found
to be: 1.22 ꢁ 105 and 8.47 ꢁ 104, 8.55 ꢁ 104 and 6.07 ꢁ 104,
1.55 ꢁ 105 and 1.05 ꢁ 105 g/mol, respectively. In thermo gravimetric
measurements (TGA) the temperature of 2% weight loss in air was
found to be w400 ꢂC. DSC analysis did not reveal any thermal
transitions before decomposition of the polymer starts.
2.2. Electronic structure of a polymer chain
The spectrum is very well resolved and shows all anticipated
resonances, pointing to strictly 2,7-substituion in fluorene frag-
ments. It should be noted that resonances of protons (particularly,
in isatin and N-phenyl nuclei) are somewhat broadened and do not
allow for their correct assignment. Similarly, NMR studies of
polymers PM2 and CPM revealed high regioselectivity of polymer-
forming reactions leading to linear structures with para-substitu-
tion in the main chain. The 1H and 13C NMR spectra and the signal
assignments of polymer PM2 are given in Fig. 2b. Polymers are
soluble in common organic solvents, such as chloroform, methy-
lene chloride, tetrahydrofurane, N,N-dimethylformamide and
Quantum mechanics calculations [23] performed for a model
compound depicted in Fig. 3 has shown the presence of electronic
coupling between fluorene fragments through a sigma bridge. This
effect can be clearly seen in Fig. 4, representing the HOMO energy
level of the model compound optimized at HF/3-21G level of
theory.
As seen from Fig. 4, the HOMO level has contributions from
p-orbitals of both fluorene fragments and sigma orbitals of spiro-
tetrahedral carbon, which connects two -systems. As a matter of
p
fact, this type of connections between different aromatic blocks in
the main chain proceeds through homoconjugation mechanism

G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
2353
S
H3C
CH3
H3C
CH3
O
C
N
N
CH2Cl2, TFSA
-H2O
C O
+
N
S
H3C
CH3
H3C
CH3
N
N
C
C O
N
PM1
n
Scheme 1. Synthesis of polymer PM1.
S
N
a
N
15
15
16
14
15
14
13
14
14
13
15
16
28
28
26
26
24
25 22
21
23
19
23
19
24
25
12
10
7
11
6
9
8
27
18 17
20
18
1
27
22
21
17
20
O
N
2
3
4
3
4
n
5
14
27
25
28
16
22
17
8
9
5
11
13
23
24
18
3
4
15
26
6
2
10
153
150
147
144
141
138
135
132
129
126
123
120
20
f1 (ppm)
21
28.0
27.0
f1 (ppm)
19
1
12
7
180
170
160
150
140
130
120
110
100
90
f1 (ppm)
80
70
60
50
40
30
20
10
0
Fig. 2. a. 13C NMR spectrum of polymer PM1 (solution in CDCl3). b. The 1H and 13C NMR spectra of polymer PM2 (solutions in CDCl3).

2354
G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
O
b
14 15
16
16
19
25
30 31
31 30
15
25 15
24
26
15
14
32
29
29
27 28
13
13
O
16
16
19
24
26
12
N
10
7
17 18
20
28 27
22 23
21
23 22
18 17
11
9
8
21
20
1
2
6
3
3
n
4
4
5
25
15
4
3
9
8
17
27
28
14
16
5
11
31
2
26
29
24
23
30
20 21
13
6
10
140
135
130
125
120
28
27
26
19
Chloroform
22
18
12
32
1
7
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
ppm
3, 23
21
20
15
25, 28
7, 17, 27
5, 14
Chloroform
30
4
8
9
10
8.1
8.0
7.9
7.8
7.7
7.6
7.5
7.4
7.3
7.2
7.1
7.0
1.6
1.5
1.4
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
ppm
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Fig. 2. (continued).

G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
2355
H3C
CH3
CH3
CH3
C O
N
H
Fig. 3. Chemical structure of the model compound.
[20]. Therefore, these polymers should behave similarly to truly
conjugated polymers and show similar properties.
3. Linear and non-lineal optical properties
Fig. 4. HOMO of the model compound calculated at HF/3-21G level of theory.
3.1. Absorption and photoluminescence
Fig. 5 shows the normalized linear absorption and photo-
luminescence (PL) spectra for all compounds. Absorption bands
with main maxima at 316, 348, 324, 354, and 320 nm are found for
M1, M2, PM1, PM2, and CPM, respectively, that are attributed to
pep* transition of the conjugated main chain. M2 also shows
a strong peak at 309 nm. M1, PM1 and CPM show absorption bands
pure electronic NLO effects (important for high bandwidth
photonic applications). For these measurements, solid state films
were prepared by spin cast. Sample thickness was between 85 and
200 nm. As an example of these experiments, Fig. 6 shows the so
called THG Maker-Fringe pattern for a PM1 film. As reference, the
figure also includes the THG pattern measured from the fused silica
substrate alone (thickness: 1 mm). These data were obtained at the
fundamental near infrared wavelength of 1200 nm (THG signal in
400 nm). From these data is estimated that the third-order
non-linear susceptibility of the PM1 film is of the order of
7.5 ꢁ 10ꢀ12 esu at such fundamental wavelength. Table 1 summa-
rizes the cubic measurements for these compounds.
of moderate intensity at 415, 419 and 422 nm, respectively, which
are assigned to the ne
building blocks. M2 and PM2 also show a weak low energy band at
454/455 nm that is associated to the ne * transition of the
p* transition of the benzothiadiazole
p
fluorenone unit [24,25]. These compounds show an intense
photoluminescence (PL) and emit light of green and greenish-
yellow color (See Fig. 5b). Preliminary results give a quantum yield
close to one for some of these compounds, this parameter is being
determined by using an integrating sphere and a fluorescence
standard with known quantum yield (laser dyes). Currently, two
photon absorption (TPA) experiments are conducting. Their PL
maxima are located at 538, 573, 538, 578, and 552 nm for M1, M2,
PM1, PM2, and CPM respectively. Table 1 summarizes the absorp-
tion and PL maxima.
From our measurements, the monomer M2 and the polymer
PM1 showed the largest cubic susceptibilities
c
(3): 1.0 ꢁ 10ꢀ11 esu
and 7.5 ꢁ 10ꢀ12 esu, respectively. It seems that c(3) value of M2
fragments depends strongly on conjugation within the unit. Thus,
conjugation defects frequently occur in a polymer due to globular
conformation of the chain and it may cause a decrease in the cubic
non-linearity for PM2 compared to M2. On the other hand, NLO
properties of M1 are mostly related with strongly polarisable
central fragment, not with the conjugation along the oligomer. This
could be a possible reason in increasing
M1 due to additional contribution of “isatin” unit. It is important to
c
(3) in PM1 compared with
3.2. Cubic optical non-linearities
mention that the absorption coefficients (a) at 400 nm for M1, M2,
The cubic NLO response for these fluorene monomers and
polymers was estimated through the use of third-harmonic
generation (THG) Maker-Fringes technique [21]. The choice of using
PM1, PM2 and CPM are 1.7, 1.5, 6.5, 1.4 and 3.8 ꢁ 104 cmꢀ1
,
respectively, so there is not a significant difference between them
(3) values through three-photon resonance (about a factor of
this technique to measure c
(3) is because it allowed us to measure
in the
c
a
b
1.0
0.8
0.6
0.4
0.2
0.0
M1
1.0
0.8
0.6
0.4
0.2
0.0
M1
M2
M2
PM1
PM2
CPM
PM1
PM2
CPM
450
500
550
600
650
700
750
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 5. a. Normalized optical linear absorption of the monomers M1 and M2 and the polymers PM1, PM2 and the copolymer CPM. b. Normalized photoluminescence of these same
compounds. Samples were dissolved in chloroform. A nitrogen laser (337 nm) was used as the pumping light source.

2356
G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
Table 1
Absorbance and fluorescence maxima; absorption coefficient
a
and c(3) values.
3
b
Sample Absorbancea Fluorescencea
a
ꢁ 104 (cmꢀ1
)
c(3) (ꢁ10ꢀ12 esu)c
PL
lmax (nm)
l
max
(nm)
M1
M2
PM1
PM2
CPM
316
348
324
354
320
538
573
538
578
552
1.7
1.5
6.5
1.4
3.8
4.3
10.0
7.5
5.9
4.6
2
1
0
a
In solution (chloroform).
b
c
Solid state films: At 400 nm: For THG, fundamental wavelength of 1200 nm.
Solid state films:
c
(3) for fused silica ¼ 3.1 ꢁ 10ꢀ14 esu at 1200 nm.
<1.5 times for the largest difference on the cubic susceptibility)
when using the wavelength of 1200 nm (see Eq. (1) in the
Experimental section).
For the well known poly(p-phenylenevinylene) (PPV) polymer,
-40
-20
0
20
40
c
(3) is reported to be of the order of 10ꢀ10e10ꢀ11 esu [26], while for
Angle (degrees)
poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylenevinylene]
(MEH:PPV), a well known conjugated polymer with linear molec-
Fig. 6. THG Maker-fringe pattern for a 101 nm-thin polymer film of compound PM1
(filled circles) and for a 1-mm-thick substrate without a film deposited on it (open
circles). The fundamental wavelength was 1200 nm.
ular structure,
c
(3) is in the range between 1.5 and 3.5 ꢁ 10ꢀ11 esu
(at 1200 nm) [27]. PPV and MEH:PPV polymers are considered as
prototypes of non-linear materials for photonic applications. For
some oligomers of PV (PV-n), c(3) values are reported, measured
through THG Maker-Fringe technique, on the order of
10ꢀ11e10ꢀ13 esu [26,28]. So, the c(3) values for our synthesized
fluorene derivatives, estimated from the THG Maker-Fringe tech-
nique, are in the interval of some PV oligomers and slightly smaller
that for MEH-PPV. In regards to third-order NLO properties, the
standard techniques used to measure non-linearities include
Z-scan, DFWM, Kerr effect and Maker-Fringe (THG). It must be
observed that these techniques determine different tensor values of
The wavelength dependence of the third-order non-linear
susceptibility for compound PM1 was also determined (See Fig. 7a).
To make clear the multi-photon resonance, Fig. 6 includes the linear
absorption spectrum of the film (top and right axes). Note that the
wavelength scales are arranged in such a way that the bottoms
scale is 3 times the top one. Thus, according to the absorption peak
located at 418 nm (see also Fig. 5), it is observed a slight
enhancement (about a factor of 1.5) on cubic non-linearities due to
three-photon resonances. For comparison, this figure (Fig. 7b) also
includes the dispersion measured under the same experimental
conditions on a MEH:PPV film.
c
(3). In addition to these different characterization techniques, the
variety of wavelengths employed, pulse duration of the lasers and
the fact that most of non-linear characterization is performed with
solutions make that the comparison between the optical non-
linearities reported in the literature for similar molecules is not
straightforward. For example, the third-order non-linear optical
properties of fluorene polymers in CHCl3 were measured using
a femtosecond time-resolved optical Kerr effect technique obtain-
3.3. Evaluation of
g
Because in the literature most of molecular characterization is
performed using solutions, then the second molecular hyper-
polarizability
second hyperpolarizabilities were estimated for M1 and M2 theo-
retically. Molecular geometry was optimized at B3LYP/6-31G(d)
ing c
(3) of the order of 10ꢀ14e10ꢀ15 esu [18]. In PPV films, by using
g is more frequently reported than c
(3). Static average
DFWM, c(3) of the order of 10ꢀ10 esu employing fs laser pulses at
1200e1800 nm was estimated [29].
a
THG wavelength (nm)
THG wavelength (nm)
b
400
500
600
400
450
500
550
1.6
1.2
0.8
0.4
0.0
8
7
6
5
7
6
5
4
3
2
1
8
6
4
2
0
1200
1400
1600
1800
1200
1400
1600
Pump wavelength (nm)
Pump wavelength (nm)
Fig. 7. Wavelength dependence of the third-order non-linear susceptibility (bottom and left axes): a. for the polymer PM1 film (Filled circles). b. As comparison, for MEH:PPV
polymer film (Filled squares) [14]. As reference, the absorption for the films is included (top and right axes) (Open circles).

G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
2357
level of theory in gas phase. Larger 6-311þG(d,p) was applied for
the calculation of using B3LYP/6-31G(d) optimized geometry.
CPHF method as implemented in Jaguar 7.6 suite of programs [23]
was used for calculations. The calculated for M1 and M2 were
contribution of “isatin” unit. This conclusion can also be sup-
ported by the theoretical evaluation of , which to some, extent
g
g
correlates with experimentally determined c(3) for oligomers M2
and M1. From our experimental and theoretically results, it can
be induced that compounds possessing high c(3) values, not due
to proper conjugation but due to highly polarisable individual
fragments, seem to be the best candidates for incorporation into
polymer chain in order to obtain a polymer with good third-
order non-linear optical properties.
g
found to be of 1.03 ꢁ 10ꢀ33 and 1.26 ꢁ 10ꢀ33 esu, respectively. It
seems that larger conjugated system that exists in M2 contributes
more than highly polarisable S atom of M1. This hypothesis is also
confirmed by the fact that not only second polarisabilities but
calculated linear polarisabilities and first hyperpolarizabilities are
larger for M2.
The magnitude of
the experimental values. In solid films
g
calculated theoretically was compared with
5. Experimental
g
is given by <g> ¼
c
(3)/L4Ns
where Ns is the number density of molecules in the compound film
and L ¼ (n2 þ 2)/3 is the correction factor due to local field effects
[18,30], n being the refractive index. In our case, the second hyper-
Unless otherwise indicated, all starting materials were obtained
from commercial suppliers and were used without further purifi-
cation. 1H and 13C NMR data were obtained on a Bruker ARX
400-spectrometer. Chemical shifts are given in parts per million
(ppm) using residual solvent protons as internal standards. Split-
ting patterns are designated as “s” (singlet), “d” (doublet), “t”
(triplet), and “m” (multiplet). Low-resolution mass spectra were
obtained on a Varian MAT 311A operating at 70 eV (Electron Impact,
EI) and reported as m/z and percent relative intensity. FD masses
were obtained on a ZAB 2-SE-FDP. Elemental analyses were per-
formed at the University of Wuppertal, Department of Analytical
Chemistry, using a Perkin Elmer 240B. The inherent viscosities of
0.2% polymer solutions in 1-methyl-2-pyrrolidinone (NMP) were
measured at 25 ꢂC using an Ubbelohde viscometer. The chroma-
tography system was equipped with three Waters styragel columns
at 40 ꢂC with THF as the solvent at a flow rate of 1.0 mL/min. The
SEC-MALS measurements were performed at 25 ꢂC using a sepa-
ration system comprising two size-exclusion columns (a Waters
HSPgel HR MB-L and a HR MB-B) with a molecular weight range
from 5 ꢁ102 to 7 ꢁ 105 and 1 ꢁ103 to 4 ꢁ 106, respectively. The light
scattering measurements were carried out on a Dawn Eos multi-
angle light scattering (MALS) instrument (Wyatt Technology, Santa
Barbara, CA).
polarizabilities are estimated to have values of 0.96 ꢁ 10ꢀ33
,
¼
2.35 ꢁ 10ꢀ33, 311 ꢁ 10ꢀ33 and 240 ꢁ 10ꢀ33 esu for M1 (Mw
536.23 g/mol), M2 (Mw ¼ 564.25 g/mol), PM1 (Mw ¼ 122,000 g/mol)
and PM2 (Mw ¼ 85,500 g/mol), respectively. These estimations were
made for a wavelength of 1200 nm. As seen, absolute values for
g
(theoretical values) correlate with experimentally measured c(3)
values. It is noteworthy that M1, having S atom shows lower
(3) and
values compared to M2.
As we pointed out previously, it is no easy to compare cubic
c
g
non-linearities due to the different techniques and experimental
conditions used for different groups. For instance, for some fluo-
rene polymers in CHCl3 where a femtosecond time-resolved optical
Kerr effect technique was used,
g
values of 10ꢀ30e10ꢀ31 esu were
obtained [18]. In Ref. [31] is reported that the molecular second-
order hyperpolarizability for thin films of the thienyleneethynylene
oligomers in poly(methyl methacrylate), measured by THG Marker
fringe technique, is 2.30 ꢁ 10ꢀ33 esu. By using Z-scan technique at
800 nm with 100-fs laser pulses, in Ref. [32] an estimated
g value
up to 2.5 ꢁ 10ꢀ32 esu for oligomers of PV was indicated and in Ref
[28] a
g
of the order of 10ꢀ32 esu by THG Maker-fringe technique
was reported. It is important to point out that effective comparisons
must be carried out with compounds tested under the same tech-
nique, i.e., measurement of non-linearities with same origin such as
5.1. Monomer syntheses
g
(ꢀ3
u
,
u
,
u
,
u
) or
g
(
u
,
u
,ꢀ
u
,
u
). In our case, the use of THG-Maker-
5.1.1. 2-Bromo-9,9-dimethylfluorene
fringes technique at infrared wavelength assured that the
measurement of
c
(3) (ꢀ3
u
,
u
,u
,
u) has pure electronic origin, while
other methods, such as Z-scan (c(3)
(
u,
u,ꢀu,u
)), might include
several competing contributions (for instance: thermal effects) to
the overall non-linear measurements. Nevertheless, it must be also
Br
mentioned that although the relatively large values of
g measured
H3C
CH3
through THG technique only looked at electronic effects, the
observed partial three-photon resonance (see Fig. 7) could imply
time response limitations for high bandwidth photonic
applications.
Aqueous sodium hydroxide solution (40 mL, 50%) and iodo-
methane (31.9 g, 225 mmol) were added to a solution of
2-bromofluorene (25 g, 102 mmol) and tetrabutylammonium
bromide (9.9 g, 31 mmol) in 75 mL of DMSO at 45 ꢂC. The
mixture was stirred at 45 ꢂC for 2 h and then poured into water
(100 mL). The mixture was extracted two times with dieth-
ylether. The combined organic phases were washed with brine,
water and dried over Na2SO4. After evaporation of the solvent
the residue was purified by column chromatography with
hexane as eluent to receive a colorless oil, that solidified under
4. Conclusions
Two new fluorene monomers (M1 and M2) and two corre-
sponding polymers (PM1 and PM2) and a copolymer (CPM) were
synthesized and characterized. Transparent, strong, and flexible
films could be cast from polymer solutions. With respect to their
NLO properties, it seems that c
(3) value of M2 fragments depends
strongly on conjugation within the unit. Thus, conjugation
defects frequently occur in polymers due to globular conforma-
tion of polymer chain decreasing cubic non-linearity for PM2
compared to that one for M2. On the other hand, non-linear
optical properties of M1 are mostly related with strongly polar-
isable central fragment, not with the conjugation along the
oligomer. This could be the reason in increasing c(3) in PM1 in
comparison with cubic susceptibility for M1 with the additional
stirring in vacuum to yield 27.8 g (80%) of 2-bromo-
9,9-dimethylfluorene.
1H NMR (400 MHz, CDCl3):
(s, 6H, CH3) ppm.
d: ppm. 7.3e7.7 (m, 6H, Ar-H), 1.49
13C NMR (100 MHz, CDCl3):
d
¼ 155.7, 153.3, 138.2, 130.1, 127.7,
127.2, 126.1, 122.6, 121.4, 121.0120.0, 47.1, 27.0 ppm.
LR-MS (EI, m/z): 178 (100.0), 272 [Mþ] (93), 274 (93), 273 (45),
274 (43).

2358
G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
5.1.2. 9,9-Dimethyl-2-(4,40,5,50-tetramethyl-1,3,2-dioxaborolano)
5.1.4. 2,7-Bis[20-(90,90-dimethyl)fluorenyl]fluorene-9-one (M2)
fluorene
O
B
O
H3C
CH3
H3C
CH3
O
H3C
CH3
1H NMR (600 MHz, C2D2Cl4):
d
¼ 7.98 (d, 2H), 7.81 (dd, 2H), 7.77
(d, 2H), 7.73 (dd, 2H), 7.68 (d, 2H), 7.62 (d, 2H), 7.60 (dd, 2H), 7.45
(dd, 2H), 7.34 (m, 4H), 1.54 (s, 12H) ppm.
A flame dried 500 mL flask was charged with 2-bromo-9,9-
dimethylfluorene (15 g, 55 mmol) and sealed under argon. Dry
hexane (250 mL) and THF (50 mL) were added and the mixture
cooled to ꢀ78 ꢂC. n-BuLi (55 mmol) was added, the mixture stirred
for 10 min and allowed to warm up to 0 ꢂC. The solution was cooled
again to ꢀ78 ꢂC, and 2-isopropoxy-4,40,5,50-tetramethyl-1,3,2-
dioxaborolane (71 mmol) added at once. The reaction mixture was
stirred at room temperature for 12 h. The mixture was poured into
water and extracted with chloroform. The solution was evaporated
to dryness and the residue purified by column chromatography
using hexane/ethyl acetate (95:5) as eluent. After removing the
solvent the remaining solid was recrystallized from hexane to
afford 12.5 g of 9,9-dimethyl-2-(4,40,5,50-tetramethyl-1,3,2-dioxa-
borolano)fluorene as a white solid in 71% yield.
13C NMR (150 MHz, C2D2Cl4):
d
¼ 194.0,154.5,154.0,143.0,139.2,
138.7, 138.6, 135.2, 133.5, 127.7, 127.2, 125.9, 123.0, 122.8, 121.0,
121.0, 120.5, 120.3, 47.0, 27.3 ppm
5.1.5. Synthesis of polymers
Synthesis of polymer PM1: 4,7-Bis[20-(90,90-dimethyl)fluorenyl]
benzo[1,2,5]thiadiazole (M1) (0.416 g, 0.80 mmol) and N-pheny-
lisatin (0.179 g, 0.80 mmol) were dissolved in dichloromethane
(2.3 mL) and then trifluoromethanesulfonic acid (0.7 mL) was
added and the mixture was stirred at room temperature for 5 h.
The resulting green, viscous solution was then poured slowly into
methanol (200 mL). The precipitated, yellow fibres were filtered
off, extracted with refluxing methanol and acetone, and dried at
100 ꢂC under vacuum. The resulting pure yellow polymer PM1
(0.562 g, 98.2% yield) had an inherent viscosity hinh ¼ 0.90 dL/g
(NMP).
1H NMR (400 MHz, CDCl3):
(m, 3H), 1.52 (s, 6H), 1.39 (s, 12H) ppm.
13C NMR (100 MHz, CDCl3):
d
¼ 7.73e7.90 (m, 4H), 7.32e7.47
d
¼ 154.3, 152.8, 142.2, 139.0, 133.9,
127.8, 127.7, 126.9, 122.6, 120.4, 119.3, 83.7, 46.8, 27.1, 24.9 ppm.
The polymer PM2 was obtained by the following procedure. 2,7-
Bis[20-(90,90-dimethyl)fluorenyl]fluorene-9-one (M2) (0.240 g,
0.42 mmol), N-phenylisatin (0.095 g, 0.42 mmol), dichloromethane
(1.3 mL) and trifluoromethanesulfonic acid (0.3 mL) were stirred at
room temperature for 24 h. The resulting transparent red viscous
solution was poured into methanol, and orange fibres were filtered
off, extracted with refluxing methanol and acetone, and dried at
80 ꢂC in an oven. The resulting polymer PM2 (0.326 g, 99.7% yield)
had an inherent viscosity hinh ¼ 0.51 dL/g (NMP).
LR-MS (EI, m/z): 319.9 [Mþ] (100.0), 304.6 (90.0).
2,7-Dibromofluorene, 2,7-dibromofluorene-9-one, and 4,7-
dibromobenzo[1,2,5]thiadiazole are known compounds:
S
N
N
Br
Br
Br
Br
Br
Br
O
Copolymer CPM was obtained byanalogous procedure. 4,7-Bis[20-
(90,90-dimethyl)fluorenyl]benzo[1,2,5]thiadiazole (M1) (0.208 g,
0.40 mmol), 2,7-Bis[20-(90,90-dimethyl)fluorenyl]fluorene-9-one
(M2) (0.226 g, 0.40 mmol), N-phenylisatin (0.179 g, 0.80 mmol),
dichloromethane (2.3 mL) and trifluoromethanesulfonic acid
(0.7 mL) were stirred at room temperature for 4.5 h. The resulting
transparent brown viscous solution was poured into methanol, and
yellowish fibres were filtered off, extracted with refluxing methanol
and acetone, and dried at80ꢂC inanoven. Theresultingpolymer CPM
(0.59 g, 49.4% yield) had an inherent viscosity hinh ¼ 0.85 dL/g (NMP).
The structure of the polymers synthesized was confirmed by 1H
and 13C NMR analysis.
General procedure for the Suzuki-type coupling: 9,9-Dimethyl-2-
(4,40,5,50-tetramethyl-1,3-2-dioxaborolano)fluorene (10.4 mmol),
the corresponding dibromo compound (3.25 mmol), sodium
carbonate (130 mmol) and aliquat 336 (1.3 mmol) were dissolved in
a mixture of 90 mL of toluene and 60 mL of water under argon. The
Pd(PPh3)4 catalyst (0.162 mmol) was subsequently added and the
reaction mixture was stirred for 48 h at 100 ꢂC. After cooling down
to room temperature the mixture was extracted with dichloro-
methane and washed with aqueous 2 N HCl solution, concentrated
NaHCO3-solution and brine. The organic phase was dried over
Na2SO4 and the solvent removed under vacuum. The solid precip-
itate was recrystallized from heptane/dichlormethane (1:1) to
obtain the trimeric products in 78e82% yield.
5.1.6. Optical measurements
5.1.3. 4,7-Bis[20-(90,90-dimethyl)fluorenyl]benzo[1,2,5]thiadiazole
(M1)
Cubic non-linearities were studied in solid state (solid films),
M1, M2, PM1, PM2 and CPM were dissolved in chloroform. Films
were deposited on fused silica substrates (1 mm-thick) by using the
spin coating technique. The prepared films had thickness between
85 and 200 nm with good optical quality showing negligible light
scattering at visible and NIR wavelengths. Absorption spectra
of spin-coated films were obtained with a spectrophotometer
(PerkineElmer Lambda 900). Sample thickness was measured by
using a Dektak 6M profiler.
S
N
N
H3C
CH3
H3C
CH3
THG Maker-fringes setup is reported elsewhere [33,34]. Briefly,
it consisted of a Nd-YAG laser-pumped optical parametric oscillator
(OPO) that delivered pulses of 8 ns at a repetition rate of 10 Hz. The
output of the OPO system was focused into the films with a 30-cm
focal-length lens to form a spot with a radius of approximately
1H NMR (600 MHz, C2D2Cl4):
d
¼ 8.03 (dd, 4H), 7.88 (dd, 4H),
7.79 (dd, 2H), 7.48 (dd, 2H), 7.36 (m, 4H), 1.57 (s, 12H) ppm.
13C NMR (150 MHz, C2D2Cl4):
d
¼ 154.5, 154.4, 154.3, 139.7, 139.0,
136.7, 133.6, 128.8, 128.4, 127.9, 127.4, 123.9, 123.0, 120.6, 120.4, 47.3,
27.6 ppm.
150 mm. Typical energies in our measurements were set at 1 mJ per

G. Ramos-Ortíz et al. / Polymer 51 (2010) 2351e2359
[5] Ono K, Saito K. Heterocycles 2008;75:2381e413.
2359
pulse at sample position (corresponding to peak intensities of
w0.18 GW/cm2). The third-harmonic beam, as a bulk effect,
emerging from the films was separated from the pump beam by
using a color filter and detected with a PMT and a Lock-in amplifier.
The THG measurements were performed for incident angles in the
range from ꢀ40ꢂ to 40ꢂ with steps of 0.27ꢂ. All the experiment was
computer controlled.
[6] Chen H, Cai XR, Xu ZG, Zhang T, Song BF, Li Y, et al. Polym Bull
2008;60:581e90.
[7] Peng ZK, Tao SL, Zhang XH, Tang JX, Lee CS, Lee ST.
J Phys Chem C
2008;112:2165e9.
[8] Tang WH, Ke L, Tan LW, Lin TT, Kietzke T, Chen ZK. Macromolecules
2007;40:6164e71.
[9] Kong QG, Zhu D, Quan YW, Chen QM, Ding JF, Lu JP, et al. Chem Mater
2007;19:3309e18.
[10] Jaramillo-Isaza F, Turner ML. J Chem Mater 2006;16:83e9.
[11] Guang SH, Loon-Seng T, Qingdong Z, Paras N. Chem Rev 2008;108:1245e330.
[12] Hwan MK, Bong RC. Chem Commun 2009:153e64.
[13] Terenziani F, Katan C, Badaeva E, Tretiak S, Blanchard-Desce M. Adv Mater
2008;20:4641e78.
In the Maker-fringes technique, the third-harmonic peak
u
intensity I3 from the substrate-film structure is compared to one
produced from the substrate alone. Then, the non-linear suscepti-
bility
c
(3) in a film of thickness Lf is determined from [35]:
[14] Castro-Beltran R, Ramos-Ortiz G, Jim CKW, Maldonado JL, Häuber M, Peralta D,
!
!
et al. Appl Phys B-Lasers 2009;97:489e96.
[15] Mongin O, Porres L, Charlot M, Katan C, Blanchard-Desce M. Chem Eur J
2007;13:1481e98.
[16] Belfield KD, Morales AR, Kang BS, Hales JM, Hagan DJ, Van Stryland EW, et al.
Chem Mater 2004;16:4634e41.
[17] Thomas III SW, Joly GD, Swager TM. Chem Rev 2007;107:1339e86.
[18] Zhan XW, Liu YQ, Zhu DB, Huang WT, Gong QH. Chem Mater
2001;13:1540e4.
1=2
u
If3
Is3
2
cðs3Þ Lc;s
p
a
=2
cð3Þ
¼
(1)
ꢀ
ꢁ
u
1 ꢀ exp
a
Lf =2
where cðs3Þ and Lc,s are the non-linear susceptibility and coher-
ence length, respectively, for the substrate at the fundamental
wavelength, and
a is the film absorption coefficient at the
[19] Sanguinet L, Williams JC, Yang ZY, Twieg RJ, Mao GL, Singer KD, et al. Chem
Mater 2006;18:4259e69.
[20] Smith M, March J. March’s advanced organic chemsitry. 6th ed. Willey & Sons;
2007. p. 45.
[21] Kajzar F, Messier J, Rosilio CJ. Appl Phys 1986;60:3040e4.
[22] Hernández MCG, Zolotukhin MG, Maldonado JL, Rehmann N, Meerholz K,
King S, et al. Macromolecules 2009;42:9225e30.
[23] Jaguar, version 7.5, Schrodinger. New York: LLC; 2008.
[24] Zojer E, Pogantsch A, Hennebicq E, Beljonne D, Brédas JL, Scandiucci de
Freitas P, et al. J Chem Phys 2002;117:6794e802.
harmonic wavelength. In our calculation we considered
csð3Þ ¼ 3:1 ꢁ 10ꢀ14 esu for the fused silica substrate and Lc,s
(9
mm at 1200 nm) was calculated from tabulated values of
the refractive index [33,34]. Our samples satisfied the condition
Lf ꢃ Lc;s in which the Eq. (1) is valid.
Acknowledgments
[25] Kulkarni AP, Kong X, Jenekhe SA. J Phys Chem B 2004;108:8689e701.
[26] Mathy A, Ueberhofen K, Schenk R, Gregorius H, Garay R, Müllen K, et al. Phys
Rev B 1996;53:4367e76.
Authors acknowledge the financial support from CONACYT
Mexico (grants 60942 and 55250), and support from DGAPA-UNAM
(PAPIIT IN 111908). Thanks are due to Dr. G. Cedillo for recording the
NMR spectra, S. L. Morales and E. Fregoso-Israel for assistance with
thermal and spectroscopic analysis, and M. Olmos for film prepa-
ration and optical linear characterization.
[27] Bahtiar A, Koynov K, Ahn T, Bubeck C. J Phys Chem B 2008;112:3605e10.
[28] Koynov K, Bahtiar A, Bubeck C, Muhling B, Meier HJ. Phys Chem
2005;109:10184e8.
B
[29] Samoc M, Samoc A, Luther-Davies B. Opt Express 2003;11:1787e92.
[30] Whittall IR, McDonagh AM, Humphrey MG, Samoc M. Adv Organomet Chem
1999;43:349e405.
[31] Geisler T, Petersen JC, Bjørnholm T, Fischer E, Larsen J, Dehu C, et al. J Phys
Chem 1994;98:10102e11.
References
[32] Wong MS, Li ZH, Shek MF, Samoc M, Samoc A, Luther-Davies B. Chem Mater
2002;14:2999e3004.
[33] Ramos-Ortiz G, Maldonado JL, Meneses-Nava MA, Barbosa-García O,
Olmos-López M, Cha M. Opt Mater 2007;29:636e41.
[34] Victorovna-Lijanova I, Reyes-Valderrama MI, Maldonado JL, Ramos-Ortiz G,
Tatiana K, Martínez-García M. Tetrahedron 2008;64:4460e7.
[1] Shirota Y, Kageyama H. Chem Rev 2007;107:953e1010.
[2] Saragi TPI, Spehr T, Siebert A, Fuhrmann-Lieker T, Salbeck J. Chem Rev
2007;107:1011e65.
[3] Park JW, Park SJ, Kim YH, Shin DC, You H, Kwon SK. Polymer 2009;50:102e6.
[4] Jung IH, Jung YK, Lee J, Park JH, Woo HY, Lee JI, et al. J Polym Sci A Polym Chem
2008;46:7148e61.
[35] Wang XH, West DP, McKeown NB, King TA.
J Opt Soc Am B
1998;15:1895e903.