Chemistry Reference
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Fig. 20 UV-vis absorption spectrum measured for 20 nominal ML of H
2
THOPP deposited onto
quartz by Langmuir-Blodgett technique. Three overlapping bands are visible. Two of them
(at 2.81 and 3.1 eV) are due to the splitting of the Soret band of the molecule at about 3 eV. A
third weak band is present at 3 eV, being the relic Soret transition of the molecules not aggregated
in the solid phase. In the
inset
: structure of 5,10,15,20-tetrakis-[4-(1-heptyloxy)phenyl] porphyrin
(H
2
THOPP) porphyrin. The dipoles of the molecule are indicated (From Goletti et al. [
53
])
the morphology and structure of the layer would have needed more accurate
calculations to reach a precise attribution of the observed structures.
The same experimental procedure has been used later for LB layers of porphyrin,
in particular a LB film, 20 monolayers thick, of 5,10,15,20-tetrakis-
[4-(1-heptyloxy)phenyl]porphyrin (H
2
THOPP) in a 1/4 M ratio with arachidic
acid [
52
,
53
]. The film was deposited onto rectangle-shaped quartz substrates and
then silver coated at the surface pressure 25 mN/m. The molecular structure is
represented in the inset of Fig.
20
. The molecule possesses two dipoles mutually
perpendicular in the molecular plane. In metalloporphyrins with D
4h
symmetry, the
related transitions are degenerate [
54
]. In free-base porphyrin (as H
2
THOPP), due
to their lower D
2h
symmetry, these transitions are not exactly degenerate [
54
]. How-
ever, there is no difference observed experimentally between them [
54
,
55
], so that
in the discussion, the authors have assumed a D
4h
symmetry for H
2
THOPP.
As it was demonstrated in Goletti et al. [
51
], nominal coverages and effective
coverages are not always coincident in LB layers. Then the authors used the
expression “nominal monolayers” (although the validity of their conclusions is
independent from that). UV-visible absorption spectrum of the layer deposited onto
a quartz substrate is reported in Fig.
20
. As elsewhere reported for similar porphy-
rins [
54
], two absorption bands are visible: one of lower intensity at 3.1 eV and one
of higher intensity at 2.81 eV. Both of them are due to the splitting of the so-called
Soret band at about 3 eV observed in the solution spectrum of H
2
THOPP. Such a
splitting arises from the coupling between neighboring molecules in the layer
[
56
]. The transitions at lower photon energies (known as Q bands) do not suffer
