Results and Discussion - New Synthetic Routes to Polyoxometalate Containing Ionic Liquids - page 77

Again only a very thin film of the sample 8c could be applied in the measurement due to
shielding from the tungsten atoms, as has already been described in chapter 3.1.4.5. Yet
it was possible to clearly acquire a response. A comparison with the starting material 8
shows a shift of the isomer shift G of +0.52 mms -1 from 0.53 mms -1 to 1.05 mms -1 . As
57 Fe bears a negative sign for ( G R/R) a positive isomer shift implies a decrease of the
electron density at the nucleus, which means that the oxidation state of the iron centre
is decreased in compound 8c (containing Fe in its oxidation state of +III) compared to
compound 8 . Therefore it is proposed that a photochemically controlled reduction
process takes place with the azide(ligand) being oxidized. The strong color change from
orange to dark blue can be explained by formation of a so called "heteropoly blue". It is
known that numerous heteropoly anions can be reduced by one or more electrons,
which are delocalized, according to various time scales, over certain atoms or regions of
the cluster. The reduction products, typically retain the general structures of their
oxidized parents and have a dark blue color. [73, 74] In addition to the higher isomer shift a
strongly increased quadrupole splitting occurs for the photolyzed product 8c . It can be
attributed to a decrease of ligand symmetry surrounding the Fe-centre with azide or a
vacancy at the sixth coordination position for compound 8c compared to the iron being
coordinated to six oxygens in compound 8 . Because of the strong splitting it is assumed
that the additional electron(s) add to an unsymmetrical electron distribution of the
partially filled valance orbitals of the iron center.