English abstract
Photovoltaic technology is considered as one of the most promising techniques to harvest
solar radiation reaching the Earth. Among the available photovoltaic technologies, dye
sensitized solar cells (DSSCs) provide important features compared to silicone based solar
cells in terms of lower fabrication cost, thin and lighter weight, and their good efficiency
indoors without the need for direct sun light. A major component of a DSSC is the choice
of a dye that absorbs solar radiation, allowing the excited electron to be transferred to the
photoanode semiconductor to generate electricity. The work described in this thesis aims
to synthesize and characterize new dyes that are derivatives of natural chalcones in order
to minimize the environmental impact. The objective is to study the effect of different
substituents in both chalcone benzene rings on the photophysical properties with the aim
to produce absorption and emission in the red and near-infrared region which constitutes
most of the sun rays. To achieve the study objective, six new derivatives of a promising
chalcone molecule (4-dimethylamino-2′-hydroxychalcone, DHC) have been synthesized,
reflecting different substituents in ring A and ring B of the parent molecule DHC. For ring
A, two new derivatives were synthesized by introducing AcHN in the meta-position (AM CLA) and Br in the para-position (Br-CLA) to the OH group, via three- and two-step
reactions, respectively, that involved a final Claisen–Schmidt condensation reaction. For
ring B, four new derivatives were synthesized, each via one-step Claisen–Schmidt
condensation reaction. The four derivatives involved the removal of the N(Me)2 group and
the introduction of three F (CLA-F), two Cl (CLA-Cl), one MeO group (CLA-OMe), and
two MeO groups (Dimoxy). The new derivatives were characterized by 1H-NMR
spectroscopy,
13C-NMR spectroscopy, and DEPT analysis to confirm their structures.
The new derivatives were spectroscopically characterized by measuring their steady-state
absorption and fluorescence spectra, and fluorescence quantum yield in MeOH solvent
and in the solid state. In MeOH, substitution in ring A (AM-CLA and Br-CLA) was found
to reduce or maintain the overall spectroscopic properties of the parent molecule. On the
other hand, replacing the N(Me)2 group in ring B (CLA-Cl, CLA-F, CLA-OMe, and
Dimoxy) dramatically changes the spectral behavior of the parent molecule, where the
absorption and fluorescence peaks are blue-shifted. These observations indicate that the
presence of the N(Me)2 group is essential for maintaining a strong electron donating power
that stabilizes an intramolecular-charge transfer state via the lone pair of electrons on the
N-atom. As solid, AM-CLA shows a high fluorescence quantum yield (27.5%), close to
that of the parent molecule (32%), which indicates the ability of this new derivative to
form J-aggregates in the crystalline form. In all the new derivatives, the smaller molar
absorptivity, compared to that of DHC, was correlated to weaker intramolecular hydrogen
bonding.
The energy of the highest-occupied (HOMO) and lowest-unoccupied (LUMO) molecular
orbitals of the new derivatives were determined from cyclic voltammetry measurements.
In all the molecules, the LUMO orbital was found to be higher than the conduction band
(CB) of the TiO2 semiconductor, making it possible for the excited electron to be injected
to the CB. On the other hand, only AM-CLA, Br-CLA, and CLA-Cl have their HOMO
energy more negative than the redox potential of the electrolyte I-
/I3-that is used in designing the DSSCs. The results point to the potential use of these three derivatives in
solar cell design as they can be regenerated (reduced) by the electrolyte. It is anticipated
that the results obtained in this thesis will help improve the dye component of the DSSCs
to boost solar-to-electrical conversion efficiency. The new derivatives of chalcone
presented here should also be useful as a benchmark for future preparation of more
promising derivatives.
