Metal Dilution Effects on Entropy and Light-Induced Valence Tautomeric Interconversion in a 1:1 Cobalt-Dioxolene Complex

The entropy and the light-induced valence tautomeric interconversions of solid solutions of the [Co(Me(2)tpa)(diox)](PF6)center dot C6H5CH3 complex [Me(2)tpa = bis(6-methyl-(2-pyridylmethyl))(2-pyridylmethyl)amine and diox = catecholato (DBCat) or semiquinonato (DBSQ) forms of 3,5-ditert-butylquinone] in the electronically innocent [Zn(Me(2)tpa)(DBSQ)](PF6)center dot C6H5CH3 host, in four different molar ratios, have been investigated. It has been found that the entropy driven transition is strongly affected by the Co:Zn molar ratio, whereas the relaxation rates of the optically induced high-spin Co-II-DBSQ metastable state at cryogenic temperatures are nearly independent of the degree of solid dilution. These results are discussed in the framework of the Jortner theory for adiabatic radiationless multiphonon relaxation processes. It is suggested that the optical bistability in these systems is related to the single molecule properties rather than to the cooperative effects active in the lattice.