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.