Quadrupolar Ligand-to-Metal Charge Transfer Excited States in Octahedral Zr(IV) Complexes

Earth-abundant, visible light absorbing zirconium photosensitizers are of great interest for their long-lived excited states. These photosensitizers consist of two electron rich tridentate ligands ligated to an electron deficient d⁰ Zr(IV) metal center, coordination chemistry that gives rise to ligand-to-metal charge transfer (LMCT) excited states. The vectoral nature of an LMCT transition was expected to give rise to an excited state dipole moment that electroabsorption data reported herein show is in fact absent for these photosensitizers. Electroabsorption spectra of three Zr(IV) photosensitizers bearing two tridentate pyridinedipyrrolide ligands, abbreviated Zr(PDP)₂, were measured in a 2-MeTHF glass. Analysis of both traditional and higher-order electroabsorption spectra revealed significant changes in the polarizability between the ground and excited state without a dipole moment change. The spectral data indicates that upon the absorption of a single photon a quadrupolar excited state is formed via a simultaneous and equal charge transfer from both tridentate ligands to the Zr metal center. The data presented herein provides the first direct experimental evidence for such behavior in transition metal complexes with charge transfer excited states. The implications for the design of LMCT photosensitizers and our fundamental understanding of symmetry breaking in transition metal complexes are discussed.