Electron transfer in platinum(II) diimine-centered triads: mechanistic insights from photoinduced transient displacement current measurements

The consequences of photoexcitation of a platinum diimine bisacetylide complex and its triads with phenothiazine species (McGarrah, J. E.; Eisenberg, R. Inorg. Chem. 2003, 42, 4355-4365) were investigated by the photoinduced transient displacement current (PTDC) method, with the aim of understanding the role of solvent in defining the nature and extent of intratriad electron transfer. PTDC enables reports on the distance of charge separation in photoexcited states. Photoexcition of the triad, Pt(dbbpy)(CCC(6)H(4)CH(2)(PTZ))(2) (where dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), leads to formation of the (3)MLCT excited state, which in CH(2)Cl(2) is intramolecularly quenched by PTZ to form a charge-separated (CS) state, Pt(dbbpy(*-))(CCC(6)H(4)CH(2)(PTZ) CCC(6)H(4)CH(2)(PTZ(*+)); the CS state features a dipole moment oriented in essentially the opposite direction to that of the ground state. In toluene the last step of charge separation is shut down. In THF these two transient states are equilibrated, approximately as a 1:1 mixture of (3)MLCT and CS states, causing a complex, but instructive, PTDC response. The PTDC response for Pt(dbbpy)(CCC(6)H(5))(2), on the other hand, is similar in all solvents and shows a negative signal corresponding to a long-lived, and comparatively nonpolar, (3)MLCT state. The ground-state dipole moment, mu(g), weakly increases with solvent polarity, from approximately 11.5 D in toluene to approximately 15.5 D in THF and CH(2)Cl(2).