Probing the nuclear susceptibility of mesoionic compounds using two-beam coupling with chirp-controlled pulses

Article abstract of DOI:10.1016/j.cplett.2007.10.037

The third-order non-linear optical response of mesoionic compounds (MIC) in dimethylsulfoxide (DMSO) and methanol solutions was investigated by use of collinear pump and probe technique with chirp-controlled femtosecond pulses. The experiments allowed the investigation of non-instantaneous nuclear processes and thermal effects induced by two-photon absorption (TPA). We found that the nuclear non-linearity of MIC in DMSO is ～1/5 the benzene, which was used as a reference material. This result is attributed to the large inertia of MIC to rotation, compared to benzene. The results for MIC in methanol indicate the influence of thermal effects due to TPA.

Full text of DOI:10.1016/j.cplett.2007.10.037

Available online at www.sciencedirect.com
Chemical Physics Letters 449 (2007) 101–106
www.elsevier.com/locate/cplett
Probing the nuclear susceptibility of mesoionic compounds using
two-beam coupling with chirp-controlled pulses
a,
a
a
a
*
Carlos A.C. Bosco , Glauco S. Maciel , Nikifor Rakov , Cid B. de Ara u´ jo ,
a
b
c
c
L u´ cio H. Acioli , Alfredo M. Simas , Petr oˆ nio F. Athayde-Filho , Joseph Miller
a
Departamento de F ı´ sica, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil
Departamento de Qu ı´ mica Fundamental, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil
Departamento de Qu ı´ mica e Laborat o´ rio de Tecnologia Farmac eˆ utica, Universidade Federal da Para ´ı ba, 58051-970 Jo a˜ o Pessoa, Para ı´ ba, Brazil
b
c
Received 20 June 2007; in final form 10 October 2007
Available online 13 October 2007
Abstract
The third-order non-linear optical response of mesoionic compounds (MIC) in dimethylsulfoxide (DMSO) and methanol solutions
was investigated by use of collinear pump and probe technique with chirp-controlled femtosecond pulses. The experiments allowed
the investigation of non-instantaneous nuclear processes and thermal effects induced by two-photon absorption (TPA). We found that
the nuclear non-linearity of MIC in DMSO is ꢀ1/5 the benzene, which was used as a reference material. This result is attributed to the
large inertia of MIC to rotation, compared to benzene. The results for MIC in methanol indicate the influence of thermal effects due to
TPA.
Ó 2007 Elsevier B.V. All rights reserved.
1
. Introduction
scan [5]. These techniques however cannot easily separate
the electronic and nuclear contributions. Recently, two-
beam coupling (TBC) experiments using ultrashort pulses
have been used to discriminate the electronic from the
Time-resolved non-linear (NL) optics has been used to
investigate the dynamics of molecular systems interacting
with strong electric fields [1–3]. When the excitation wave-
length is off-resonance with the electronic transitions, the
NL response of the molecule is practically ‘instantaneous’.
Besides that, there are ‘non-instantaneous’ contributions
associated with molecular rotation and orientation, gener-
ally called nuclear contributions [3].
(3)
nuclear contribution of v [6,7]. TBC is usually employed
in non-resonant (by one-photon transitions) pump–probe
experiments and the energy exchanged between the incident
beams is the quantity of interest. This is the case when a
Raman or Brillouin resonance is exploited. In TBC experi-
ments introducing a ‘chirp’ in the pulses causes temporal
displacements between the frequency components of the
pump and the probe beams and the energy exchange is
determined by the instantaneous frequency difference and
the relative delay between the incident beams. The energy
exchange depends on the fact that the response time of
the non-linearity is finite and comparable to the pulse dura-
tion. When the laser wavelength is far from resonance with
electronic transitions, only the nuclear contribution will
dominate the energy transfer process.
Different techniques have been employed to probe the
(
3)
third-order NL optical susceptibility, v , of liquid solu-
tions containing organic molecules. These materials may
present high NL optical response and as a consequence they
may be considered interesting candidates for all-optical
devices such as ultrafast optical switches. The techniques
most commonly used to analyze the intensity and dynamics
of the non-linearity are optical Kerr gate (OKG) [4] and z-
*
The mesoionic molecule studied in this work, shown in
Fig. 1a, belongs to a class of organic molecules that has
Corresponding author. Fax: +55 81 32710359.
E-mail address: cac_bosco@yahoo.com.br (C.A.C. Bosco).
0
009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2007.10.037

102
C.A.C. Bosco et al. / Chemical Physics Letters 449 (2007) 101–106
a
b
4
2
ωlaser
H3C
3
2
1
0
N
S
S
Cl
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. (a) Molecular structure of the studied mesoionic molecule. (b) Absorption spectra of MIC in two different solvents: DMSO (solid line) and
methanol (dashed line). Concentration: 0.6 mg/ml. Sample thickness: 2 mm.
a
Chirp control
Ti:Sapphire Laser
(
800 nm, 82 MHz)
Probe
L
S
PD
Delay line
λ/2
P1
Pump
Chopper
λ/2
P2
Computer
Lock-in amplifier
b
2
1
1
1
1
1
00
80
60
40
20
00
80
-3
-2
-1
0
C
1
2
3
Fig. 2. (a) Experimental set-up for time-resolved TBC with the frequency chirp-control. (b) Calibration curve for the laser pulse duration as a function of
the chirp parameter.
been recently investigated for photonic applications [8–11].
This molecule has planar heterocyclic dipolar structures in
which the positive and negative charges are separated and
delocalized within the p-electron system resulting in dipole
moments as large as 5D. When the molecule is excited to
the first excited state its dipole is reduced to nearly zero

C.A.C. Bosco et al. / Chemical Physics Letters 449 (2007) 101–106
103
and a large non-linear response is then obtained due to the
low-lying charge transfer excited state and this large varia-
tion of the molecule dipole. The two-photon absorption
cross section of mesoionic compounds (MICs) has been
investigated using infrared and visible nanosecond pulsed
lasers [8,9] and the off-resonance third-order susceptibility
has been investigated using the optical Kerr gate technique
with ultrafast laser source [10] and incoherent light [11].
Here we use the TBC technique to estimate the magni-
tude of the non-instantaneous nuclear contribution to the
third-order susceptibility in a mesoionic compound dis-
solved in two solvents (methanol and dimethylsulfoxide –
DMSO). Benzene was used as a reference material for cal-
ibration of our data.
width: 12.2 nm, repetition rate: 82 MHz) was used as the
excitation source. The chirp was introduced by a pair of
extracavity linear position-controlled diffraction gratings
with a telescope of unity magnification [13]. The laser beam
exiting the grating pair is split into two beams: one with
high intensity (pump beam) and the other beam with low
intensity (probe beam), with an intensity ratio of 1/8. The
polarization of the beams was controlled by a set of polar-
izers and half-wave plates. The pump beam was mechani-
cally chopped and the beams were focused inside the
sample (a quartz cuvette of 2 mm thickness filled with the
organic solution) using a lens (focal length: 10 cm). The
angle between the beams crossing the sample was chosen
in order to prevent NL effects such as self-diffraction [7].
The delay between the pump and probe pulses was con-
trolled with a delay line (0.1 lm resolution). The probe
beam transmitted through the sample was analyzed by a
photodetector connected to a lock-in amplifier synchro-
nized with the chopper frequency. The intensity of the
2
. Experimental section
The synthesis of the mesoionic compound 2-(p-chloro-
phenyl)-3-methyl-4-phenyl-1,3-thiazolium-5-thiolates (MIC)
was performed in three steps as follows [12]: the first step
required the preparation of the amino acid compound,
N-methyl-C-phenylglycine, by a Strecker’s synthesis with
benzaldehyde, methylammonium chloride and potassium
cyanide. In the second step, the amino acid was subjected
to aroylation with p-chlorobenzoyl chloride to furnish the
amide acid compound N-(p-chlorobenzoyl)-N-methyl-C-
phenylglycine. The third step involves two stages in a
one-pot reaction: a cyclodehydration reaction with acetic
anhydride followed by a cycloadition/reversion 1,3-dipolar
reaction with carbon disulphide, as follows: the N-(p-chlo-
robenzoyl)-N-methyl-C-phenylglycine (5.00 g, 16.47 mmol)
was dissolved in acetic anhydride (20 mL), which was then
heated at 55 °C for 15 min. After cooling to room temper-
ature, carbon disulphide (20 mL) was added with shaking
and allowed to stand for 48 h. Methanol/water (1:1 v/v)
was added until the solution became turbid. After 24 h,
the MIC was obtained in the form of orange-colored nee-
dles, and then recrystallized from methanol (57.93% yield
a
C = 1.61
0
0
0
.2
.1
.0
C = 0.94
C = 0.60
C = 0
C = -0.6
C = -0.93
-
-
0.1
0.2
C = -1.60
-
400
-200
0
200
400
Delay (fs)
(
3.03 g); mp 158–159 °C). The successful production pro-
b
C = +0.67
C = -1.20
cess was verified with the characterization of the desired
compound by mass spectrometry, infrared spectroscopy,
1
13
H and C NMR as well as by elemental analysis and
melting point. The MIC solutions used in the TBC experi-
ments were prepared by adding either methanol or DMSO
as solvents. Fig. 1b shows the absorption spectra of MIC
with concentration of 0.6 mg/ml in DMSO and methanol.
The effect of positive solvatochromism is observed when
methanol is replaced by DMSO. Since DMSO has a larger
*
polarity than methanol, a red-shift is observed in p–p elec-
tronic transitions. Using the solution containing MIC in
DMSO we minimize contributions due to electronic reso-
nances excited by two-photon absorption. This guarantees
that the signal observed is essentially due to nuclear
contributions.
The experiments with chirp-control were performed
using the set-up shown in Fig. 2a. A commercial mode-
locked Ti:sapphire laser (center wavelength: 800 nm, band-
-
400
-200
0
200
400
Delay (fs)
Fig. 3. (a) Experimental (solid line) and theoretical fit (dashed line) of the
probe beam transmission changes as a function of delay between pump
and probe beams for benzene solution at different values of C. (b) Probe
beam transmission changes at two different C values and two configura-
tions: parallel (solid line) and perpendicular (dashed line) polarizations of
the pump and probe beams.

104
C.A.C. Bosco et al. / Chemical Physics Letters 449 (2007) 101–106
(3)
signal is proportional to the nuclear contribution to v , if
The nuclear contribution for the probe signal measured
no electronic resonance is present [6,7].
at the photodetector for a delay time s can be written as [3]
Sðs; C; T1=2; T
N
Þ
ꢀ
ꢁ
R
3
. Results and discussion
2
þ1
ꢂ1
2
2
L ꢁ C ꢁ s=T
ꢁ T
N
ꢁ bj ꢁ
jA
1
ðtÞj ꢁ jA
2
ðt ꢂ sÞj ꢁ dt ;
1=2
¼
2
2
First, a calibration curve containing information on the
1 þ ½ðC ꢁ s=T1=2Þ ꢁ T
N
ꢃ
pulse duration change with the chirp was found using the
autocorrelation signal from a 0.1 mm-thick BBO crystal
for different displacements of the grating pair. The results
presented in Fig. 2b shows that the pulse width spreads
for a non-zero chirp as expected. After that, we tested the
accuracy of the system by performing TBC measurements
on benzene that is a standard transparent liquid. Fig. 3a
ð1Þ
with A and A being the electric field amplitude of the
pump and probe beams, respectively, L is the sample
length, T1/2 is the pulse width, T is the molecule response
1
2
N
time corresponding to the nuclear dynamics, and b is the
j
tensor element of the nuclear part of the susceptibility:
bxxxx for parallel polarizations and bxxyy for perpendicular
polarizations. Adjusting the amplitude of Eq. (1) for a gi-
ven value of C we can fit our the experimental data. Choos-
ing an experimental curve, (C = 0.60) for example, allows
fitting all the experimental data, as shown in Fig. 3a
(
solid line) shows the results for parallel polarizations of
the pump and probe beams at different C values. For
C > 0, the probe beam loses energy at negative delay and
gain energy for positive delay. For negative chirp, C < 0,
the reverse is the case. In the case C = 0, the relative delay
of the frequencies inside of the envelope of each pulse is
zero. Therefore the energy exchange vanishes for any delay,
since the two pulses are indistinguishable. The variation of
amplitude of the signal also depends on C.
(
dashed line). Good agreement between simulations and
experimental data is obtained. Fig. 3b shows the ratio of
the TBC signal intensities for parallel (solid curves) and
perpendicular (dashed curves) polarizations of the pump
a
0.015
b
0
0
0
.08
.04
.00
C = 1.6
.54
.1
C =1.6
0
0
.010
.005
0
0
0.000
0.005
0.010
0.015
-
0.2
-
-
-
-
-
0.04
0.08
-
1.3
-
500
-250
0
250
500
-500
-250
0
250
500
Delay (fs)
Delay (fs)
c
0.06
0
0
0
.04
.02
.00
-
-
-
0.02
0.04
0.06
C = 1.60
Benzene
MIC in DMSO
-400
-200
0
200 400
Delay (fs)
Fig. 4. (a) Probe beam transmission changes as function of delay between the pump and the probe beams for MIC in DMSO at different values of C. (b)
Probe beam transmission changes for MIC in DMSO for parallel (solid line) and perpendicular (dashed line) polarizations of the pump and the probe
beams at C = 1.6. (c) Probe beam transmission changes for benzene (solid line) and MIC in DMSO (dashed line) under the same experimental conditions.

C.A.C. Bosco et al. / Chemical Physics Letters 449 (2007) 101–106
105
and probe beams, measured for a positive and a negative C
a
value. Based on Eq. (1), we assumed that the ratio of the
TBC signal intensities for parallel and perpendicular polar-
izations is equal to the ratio between the tensor elements of
the nuclear part of the NL susceptibility and we found that
bxxxx/bxxyy ꢀ 1.3 for both positive and negative C. This is
C = 1.6
0.54
0.04
0.02
0
.1
0
.00
in accordance with the theory which predicts b
bxxyy = 4/3 for nuclear contributions [3,14].
/
xxxx
-
0.2
1.3
-
-
0.02
0.04
Analogous measurements were made to investigate the
MIC compound. Fig. 4a shows the results for the TBC sig-
nal of MIC in DMSO recorded for parallel polarizations of
the pump and probe beams for different C values. These
results are analogous to the results shown in Fig. 3a for
benzene. We also performed measurements on samples of
only DMSO solution and the results showed negligible
contribution to the TBC signal. Fig. 4b shows the ratio
of the TBC signal of MIC in DMSO recorded for parallel
-
-500
-250
0
250
500
Delay (fs)
b
C = -1.3
0.014
0.007
0.000
-0.007
(
solid curves) and perpendicular (dashed curves) polariza-
tions, measured for C = 1.6. Here, we also found that
bxxxx/bxxyy ꢀ 1.3, as expected. In both cases (benzene and
MIC) there are no electronic resonances and therefore
the MIC non-linearity is dominated by nuclear contribu-
tions. Unfortunately, so far there is no report in the litera-
ture of direct measurements of b but it is possible to
estimate its relative value by comparison with standard
materials. Again, we used benzene as a reference material
and compared the transmission change for benzene with
that for MIC using C = 1.6 and 50 mW of laser power,
as shown in Fig. 4c. Assuming that the ratio of the TBC
signal intensities for parallel and perpendicular polariza-
tions is equal to the ratio between the tensor elements of
the nuclear part of the susceptibility for parallel polariza-
tions of the pump and probe beams, using Eq. (1) we
obtain
-
0.014
-
500
-250
0
250
500
Delay (fs)
Fig. 5. Probe beam transmission changes as function of delay between the
pump and the probe beams for MIC in methanol (a) at different values of
C and (b) at two configurations for C = ꢂ1.3: parallel (solid line) and
perpendicular (dashed line) polarizations of the pump and probe beams.
bxxxxðMICÞ
is that when the pump and probe beams have parallel
polarizations a population grating is formed and thermal
effects are present due to strong two-photon absorption.
This conclusion is supported by similar results found when
we did TBC experiments with Coumarin 480, a commercial
organic dye which presents two-photon absorption at
¼
0:19:
ð2Þ
bxxxxðBenzeneÞ
This result shows that the nuclear contributions for the
non-linearity are smaller for MIC compared to that of ben-
zene, under our experimental conditions. One possible
explanation for this result is that MIC is a larger molecule
and therefore it must present a higher resistance to rotation
when compared to the benzene ring. A simple estimate of
the principal axis of the moment of inertia of both MIC
and benzene was made, and we found that the largest axis
of MIC is about one order of magnitude larger than the
one of benzene. This is in accordance with the experimental
results.
ꢀ800 nm [15].
4. Summary
We have performed time-resolved TBC experiments in
mesoionic compounds (MIC). Using two different solvents
we could probe the samples in a transparent regime (MIC
in DMSO) and in a strong two-photon absorption regime
(MIC in methanol). Our results obtained with different fre-
quency chirps show that the nuclear susceptibility of MIC
is smaller than that of benzene. We attribute this result to
the larger structure of MIC, compared to benzene, which
causes a higher inertia to rotate. In the TPA regime, the
TBC measurements show almost independence with chirp.
This indicates that thermal contributions are dominating
the NL response.
We also performed TBC experiments on MIC in meth-
anol solution and the results are shown in Fig. 5a. Note
that differently from the data shown in Fig. 4a, for parallel
polarizations the TBC signal does not change sign with C.
Besides that the ratio between the transmission changes for
parallel and perpendicular polarizations measured at
C = ꢂ1.3 was found to be much larger than the predicted
4
/3, as shown in Fig. 5b (we found the value of the ratio
equal to ꢀ7.7). The most probable reason for this behavior

1
06
C.A.C. Bosco et al. / Chemical Physics Letters 449 (2007) 101–106
[7] S. Smolorz, F. Wise, J. Opt. Soc. Am. B 17 (2000) 1636.
Acknowledgement
[
8] N. Rakov, C.B. de Ara u´ jo, G.B. da Rocha, J. Miller, A.M. Simas,
Chem. Phys. Lett. 332 (2000) 13.
We are grateful to the Brazilian agencies CNPq and FA-
CEPE for financial support of this work.
[
9] V. Pilla, C.B. de Ara u´ jo, B.F. Lira, A.M. Simas, J. Miller, P.F.
Athayde-Filho, Opt. Commun. 264 (2006) 225.
[
[
10] A.G. Bezerra Jr., A.S.L. Gomes, P.F. Athayde-Filho, G.B. da Rocha,
J. Miller, A.M. Simas, Chem. Phys. Lett. 309 (1999) 421.
11] L.deS. Menezes, C.B. de Ara u´ jo, M.A.R.C. Alencar, P.F. Athayde-
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