A theoretical study of cis-trans photoisomerization in the bis(glycinato)platinum(II) complex

Abstract

The cis→trans photoisomerization of the bis glycinato complex of Pt II is examined on the basis of a semi-empirical molecular orbital model. It had previously been suggested by Balzani and co-workers that the reaction coordinate for this irreversible, photoisomerization process was a torsional twist of the glycine ligands about an axis that would transform the cis isomer into the trans isomer. The electronic wave functions and energies of Pt(gly) 2 are computed for various geometries generated by this torsional twist operation. Two minima in the total electronic energy of the ground state are found. These are at ϕ (twist angle) = O° (cis) and ϕ=180° (trans), and E (cis) ⩾ E (trans). A single maximum is found at ϕ=90°. The lowest ecxited electronic state appears to have two minima, one between ϕ=80° and 90°, and the other between ρ=90° and 100°. The minimum on the trans side of 90° (i.e., ϕ>90°) is deeper than the one on the cis side. The calculated results indicate that the lowest ecxited electronic state in its ground vibrational level will overlap the upper levels of the vibronic manifold of the trans ground state, whereas overlap with vibronic levels of the cis ground state is zero or nearly so. The implications of these results with respect to the mechanism and quantum yield of the photoisomerization reaction are discussed, and it is concluded that Balzani's original hypothesis is supported by our semi-quantitative theoretical study.