Binding Modes of Carboxylate- and Acetylacetonate-Linked Chromophores to Homodisperse Polyoxotitanate Nanoclusters

The binding of carboxylate- and acetylacetonate-linked
chromophores
to homodisperse polyoxotitanate nanoclusters with 17 Ti atoms or more
are surveyed and found to be limited to chelate-bidentate and the
bridging modes, the former being dominant for the acetylacetonate-linked
chromophores, the latter for the carboxylate linkers. Chromophores
with acetylacetonate linking groups invariably bind in the chelate
mode, whereas carboxylic acid terminated chromophores more frequently
are observed to have the bridging mode, with the exception of three
cases in which a strong electron-donating substituent is present on
two different sensitizers. The calculations for isonicotinateand nitrophenylacetylacetonate
functionalized Ti17 clusters show the observed binding modes to correspond
to the lower energy functionalized clusters, but do not predict the
difference between the cinnamic acid and dimethylaminocinnamic acid
binding to Ti17, which are bridging and chelate respectively. Both
binding modes were never observed to occur for a single chromophore,
even when synthetic conditions were varied. Density of state calculations
show broadening and splitting of the chromophore LUMO on complexation
due to interaction with the cluster’s conduction band, as well
as frequent penetration of sensitizer orbitals into the bandgap of
the functionalized nanoparticle.