Chemistry Reference
In-Depth Information
saturation observed from the fourth layer indicates that the separation between the
tetraphenylporphyrin layer and the fourth polyimide layer is too extended to allow
successful electron transfer processes.
In natural pigment assemblies, a quantum efficiency close to 100% has been
observed: protein scaffolds are able to modulate the intermolecular separation that
upgrades electron coupling and energy transfer. The idea of Sakakibara and
coworkers [
43
,
44
] was to covalently link porphyrins to cellulose in order to
avoid two typical problems related to LB films, i.e., their stability and 2D organi-
zation; the improved arrangement is promoted by the rigidity of the cellulose
backbone. Therefore, 2,3-di-
O
-stearoyl-6-
O
-[
p
-(10,15,20-triphenyl-5-porphyrinyl)-
benzoyl]cellulose, already reported in Fig.
11
, was ad hoc synthesized and
immobilized as an LB film. In this way, in the light of the so-called role-sharing
functionalization, the macrocycle derivatization is strategically and regioselectively
inserted at the C-6 position, with the aim of remarkably deactivating self-quenching
of the photoexcited porphyrins; at the same time, the stearoyl moieties at the C-2 and
C-3 positions affect positively the processability, self-organization, and solubility. In
fact, the average distance between two subsequent porphyrin cycles was estimated to
be 1.03 nm, and the synthesized derivative carries out the purposed function of
scaffold.
Analysis of the Langmuir curves of the derivative at the air-water interface
indicates that the dye cycles are normally arranged with respect to the subphase
surface, and the UV-vis absorption spectra of
the LB films evidence a
bathochromic shift suggesting a side-by-side
- (or J-) aggregation.
Photoelectrochemical measurements suggested the porphyrin ring as
photoactive moieties with anodic currents arising from the electron transfer from
the dye to the ITO electrode. The LB monolayer exhibited a quantum efficiency of
1.6% (0 V potential vs. SCE,
π
2.0 mW/cm
2
) that is
significantly improved in comparison with the values reported for other porphyrins
in the literature [
45
]. Moreover, upon utilization of more positive potentials, the
photocurrent quantum yields intensified up to 3.8-4.6% (0.2-0.3 V vs. SCE).
In a subsequent work, as already made by Gervaldo et al. for different substances
[
40
], Sakakibara and Nakatsubo investigated the influence of the central metal in
the dye cycles on the photocurrent performances of such cellulose-based materials
[
46
]. Therefore, in addition to the free base, the derivatives metallated with copper
(II), magnesium(II), palladium(II), and zinc(II) were synthesized and employed.
The solvent used in the spreading solution was toluene, since at low concentrations,
it minimizes the aggregation tendency observed when chloroform was employed.
The most stable and densely packed floating layers on the aqueous subphase surface
were obtained with the free base and the palladium porphyrin derivatives. The films
were deposited onto the solid supports by the horizontal lifting method (Langmuir-
Schaefer technique); the corresponding UV-vis spectra using quartz as the sub-
strate showed bathochromic shifts and peak broadenings less intense than in the
case of solutions in toluene. Investigations carried out using polarized light have
evidenced very similar behavior for different derivatives, and this observation
λ ¼
420 nm, light intensity
¼
