Excited triplet state zero-field splitting in a ligand-to-ligand charge transfer complex.

Solid-state variable-temperature, variable-field magneto-photoluminescence experiments have been performed on the emissive excited triplet state (T1) of the ligand-to-ligand charge transfer (LL'CT) complex (qdt)Pt(dbbpy) (qdt = quinoxaline-2,3-dithiolate; dbbpy = 4,4'-di-tert-butyl-2-2'-bipyridine). We observe a strong temperature and magnetic field dependence of the PL spectra, and the spin-lattice relaxation (T1) among ms levels within the T1 state appears to be accelerated due to both ZFS in the T1 state and the presence of the applied magnetic field, allowing for an assessment of the axial ZFS parameter (D) for the chromophoric T1 state, which is EPR silent at X-band (∼9 GHz). Analysis of the data indicates a zero-field splitting (ZFS) of the T1 state (-2.9 ± 0.7 cm-1) that derives from the presence of spin-orbit coupling related to the heavy Pt(II) ion. The results have been used to provide an estimate of the ZFS in the quartet excited state of related radical-elaborated (radical-dichalcogenolene)Pt(dbbpy) complexes. This information is necessary to understand how transitions between the 2T1 and 4T1 excited states in these radical-elaborated complexes lead to electron spin polarization within their 2T1 state and how this is transferred to the 2S0 electronic ground state.