Maria Anita Scandola

Sensitized photochemistry of the nitropentamminecobalt(III) complex ion

Sensitization of the photochemistry of Co(NH3)5NO2+2 in water or water/ethanol solutions has been attempted using a number of triplet energy donors. Acetone, benzaldehyde, and thioxanthone were found to be efficient sensitizers of the redox decomposition reaction of the complex. In the one experimentally accessible case, i.e., with acetone, the nitro-nitrito linkage isomerization reaction was also found to be sensitized, in the same ratio to redox decomposition as observed in the direct photochemistry. Limit sensitization quantum yields for redox decomposition were 0.35 and 0.25 with acetone and benzaldehyde, respectively. Michlers ketone, biacetyl and benzil did not appreciably sensitize the photochemistry of the complex, although the biacetyl phosphorescence was efficiently quenched by Co(NH3)5NO2+2. The correlation of the triplet energy of the donors with their sensitizing ability, and the simultaneous sensitization of redox decomposition and linkage isomerization are taken as evidence of an energy-transfer sensitization mechanism, as opposed to an electron transfer one. The results are discussed in terms of an excited state model which includes ligand-to-metal charge transfer states of various multiplicities corresponding to high-spin and low-spin cobalt(II) as well as ligand field states.