The role of the excited-state geometry in the photochemical isomerization of bis(glycinato) platinum (II)

Abstract The mechanisms of the thermal and photochemical isomerizations of cis -Pt(gly) 2 have been investigated. The thermal isomerization has been found to occur only in the presence of free glycine. On the contrary, the photoisomerization takes place both in the presence and in the absence of free glycine, with constants quantum yield (0.13). When the two isomerization reactions are carried out in the presence of labeled free glycine, the thermal isomerization leads to the formation of labeled trans isomer as well as to an extensive labeling on the starting cis form; whereas the photochemical isomerization occurs without any labeling of the two isomers. These results show that the thermal isomerization takes place by an intermolecular mechanism, whereas the photochemical isomerization must be an intramolecular process. A twisting mechanism is discussed for the photochemical isomerization. This mechanism is related to the recent spectroscopic evidence for tetrahedral stable conformations of some excited states of square planar complexes.