Chemistry Reference
In-Depth Information
Fig. 23 Amplitude of the
Re(
Δr
/
r
) signal measured at
430 nm (
filled circles
) and
440 nm (
open circles
) for
the sample whose RAS
spectrum is reported in
Fig.
22
versus the azimuthal
angle
˕
(defined in the text).
The fit of experimental data
with the function sin(2
)is
reported (
dashed
and
dotted
lines
). In the configuration
chosen,
˕
˕¼
0 means that
the axes
form angles
equal to, respectively, +45
and -45 with the longer
symmetry axis of the
rectangular sample, in its
turn parallel to the direction
along which the sample was
dipped in the liquid (From
Goletti et al. [
52
])
ʱ
and
ʲ
(similarly to the surface in inorganic crystals) become different from the inner
layers (bulk), mimicking the surface-versus-bulk situation at a traditional crystal-
line solid. Strictly speaking, surface states are not expected to exist in molecular
crystals, since no unsaturated bonds are present at the vacuum-crystal interface.
Moreover, obvious intrinsic limits inherent to LB deposition make this growth
technique incompatible with vacuum: all experiments have been performed in air.
Nevertheless, the authors made the hypothesis that--although small--some effects
of the particular environment experienced by molecules of the outer layer could be
measured, thus paving the way to the application of these porphyrin systems for gas
sensing (see sect 4.3).
In consideration of the symmetry of the single porphyrin molecule used in LB
layers of Goletti et al. [
52
,
53
], the detection of a nonzero RAS signal could be a
surprising result. If we consider a symmetric molecule forming an angle
ʸ
with the
cos
2
(
substrate plane, an anisotropy factor [1
)] can be easily obtained by geo-
metrical considerations. However, this would result in a RAS spectrum propor-
tional
ʸ
.
Consequently, an explanation of the line shape of Figs.
21
and
22
must be related
to a different origin. However, it should be considered that porphyrin molecules in
the LB film are interacting (as evidenced by UV-visible spectrum), and the resulting
aggregation could account for the measured anisotropy. The authors made the
hypothesis that the deep-lying layers are indeed inclined with respect to the
to
ε
(dielectric function of the layer) and not
to the derivative of
ε
