Chemistry Reference
In-Depth Information
Fig. 42
R
/
R
RAS
spectra measured on an 8-ML-thick Zn-HepOTTP Langmuir-Blodgett film: (
a
)
before (
solid line
,
filled circles
) and after (
dashed line
,
empty circles
) the exposure to hexane
vapors (18%); (
b
) before (
solid line
,
filled circles
) and after (
dashed line
,
empty circles
) the
exposure to ethanol vapors (18%); (
c
) before (
solid line
,
filled circles
) and after (
dashed line
,
empty circles
) the exposure to butylamine (1%). Each exposure stage was 210 s long. The sign of
ΔR
/
R
has been chosen arbitrarily. Zero lines have been reported for each curve (From Bussetti
et al. [
88
]. Reprinted with permission. Copyright 2011. American Chemical Society.)
Δ
in the new experiments, the optical anisotropy modification of an 8-monolayer LB
thin film of a 5,10,15,20-tetrakis(4-heptyloxyphenyl) porphyrin (Zn-HepOTTP) was
measured, in consequence of the exposure to three different VOCs [
88
].
It is interesting to mention that the optical anisotropy of the as-grown LB film
was almost insensitive to analytes: despite the evident variation of the simulta-
neously monitored QMB signal (demonstrating the adsorption of VOC molecules
on the film), RAS measured a nearly constant signal during exposure. It was
necessary to perform a preliminary “activation” of the optical behavior of the
layer: all the reported spectra have been recorded after a preliminary exposure of
the as-grown film to saturated vapors of ethanol diluted with N
2
carrier gas (70%).
After cleaning this activated sample in nitrogen flow, the optical anisotropy (pre-
viously inactive) of the LB film exhibited now a high sensitivity to volatile
compounds introduced in the cell, comparable to the published LS film case
[
86
]. On the contrary, the adsorption isotherms recorded by QMB before and
after activation are identical, proving that further adsorption processes were unaf-
fected by activation. The RAS spectrum for the activated LB film (before exposure
to analytes) is reported in Fig.
42
. It has a different line shape with respect to the
corresponding spectrum of a thicker LS sample of the same porphyrin (see Fig.
40
).
This result is not surprising: the different film thickness (30 ML vs 8 ML) implies a
likely different arrangement of the porphyrin molecules in the sample and, conse-
quently, a different optical anisotropy.
