of an electron
in nature. Plastocyanin protein extracted from spinach thylakoid. Electron
carrier properties emerge from the association of an organic skeleton (peptide
chain) and a metal ion centre (Copper). The tertiary structure is the key
function as it maintains the Cu(II) centre in an unfavorable coordination
geometry (tetrahedral). This unstability favours quick electron transfer reactions
based on Cu(I) / Cu(II) redox couple.
strategy we used was inspired from plastocyanin proteins. In our dyes we kept
the metal ion centre (ruthenium), two surrounding bidentate bipyridines and
two monodentate isocyanate ligands. Famous N3 dye contains two 4,4'-dicarboxylic-2,2'-bipyridines.
The idea was to keep one of this anchoring bipyridine and a second one able
to give hydrogen bondings as we find in proteins. Hydrogen bondings derived
from amides bond have for objectives to give:
weak reversible and directed interactions with other surrounding dyes in
order to reduce empty space on semiconductor surface. The monolayer load
should be increased and by the way light harvesting performance.
easy chemical group attaching centre. This should permit to attach any desired
molecule bearing a primary amine. Dye could be designed to be globally more
or less lipophylic or hydrophilic. These two parameters are increasingly
important in protein and proved recently to have an influence on electron
transfers in semiconductors.
active sites with which higher dye molecular ordering on surface could be
reached. This was described already in literature with thiol molecules on a
gold surface. See example given below.
synthesis of diverse ligands including the amide LA6 on which different alkyl
chains length have been attached.
dyes bearing more or less liphophilic groups and amides bonds (C4).
C4 deposition on TiO2 (from an alcoholic solution), IR measurements demonstrated
that addition of 1 or 2 equivalent of urea to the ethanolic dye solution reduces
chains disorder. This macro scale modification has a direct sub nanometric
impact. The antisymmetric vibration mode in the vicinity of 2900 cm-1 is a
diagnostic for the conformational ordering of alkyl chain. Lower values obtained
for C1 in comparison to C4 are an indication of a higher conformational ordering
of alkyl C6H13 chains. Moreover sharper peaks for C4 indicate higher chain
rigidity provided probably by intra-molecular hydrogen bondings. Experiences
realised with urea and C4 revealed that conformational chains ordering can
be improved as data (table 2) revealed a small decrease of antisymmetric stretching
mode frequency from 2930 to 2928 cm-1 and for other C-H frequency from 2961
to 2958 cm-1 and from 2860 to 2858 cm-1.
conclusion, C4 sensitiser yields incident photon-to-current conversion efficiency
values of about 80%. Nevertheless, in addition of good photovoltaic performances
of this novel bis amide complex and the possibility to tune self-organisation
with hydrogen-bonding recognition groups a new area was opened for the development
of supramolecular sciences applied to dye sensitised solar cells.
amide complex C4 [Ru(H2dcbpy)(dhabpy)(NCS)2] presents the best photovoltaïc
results with a photocurrent density of 15.5 mA/cm2 and an open circuit potential
of 706 mV. A better potential Voc=750 mV and Jsc=14.9 mA/cm2 was obtained
with another electrolyte.
overall conversion efficiency reaches 7.4% under standard AM 1.5 sunlight.
An example of the chemical structure of one such amide N3 dye (Ruthenium (dithiocyanato)(4,4'-dicarboxy-2,2'-bipyridine)(4,4'-dihexylamide-2,2'-bipyridine)) is shown below: