Photochemical behavior of Keggin ions and related compounds

The photochemical behavior of Keggin heteropolytungstates is reinvestigated using a useful thin layer electrochemical cell. This new device allows to measure the molar fraction of the polycompound as oxidized and reduced form at the same time. Some previous results can be confirmed but new aspects of the redox behavior of excited HPC have been evidenced. It has been proved that the photoprocess occurs via the charge transfer reaction between terminal oxygen atoms and tungsten atoms. The limiting rate for the primary electron exchange is depending on the free enthalpy of the first reduction step of the HPC in its fundamental state and not depending on the nature of alcohol. The chromophore involved in the excitation process as well as the oscillator strength can be defined and the quantum yields measured. The rate constant appears to be independent of pH and temperature. An unexpected catalytic effect of the reaction products is evidenced and explained by an interconversion transfer between excited HPC and carbonyl compound.