Probe the Electron Spin Polarization of the Otherwise Unobservable 3MLCT Triplet State Through Polarization Transfer Accompanying the Intramolecular Triplet–Triplet Energy Transfer in Pt(II)‐Salen Schiff Base Complexes

The non-Boltzmann population of the triplet sublevels in Pt(II)-Schiff base complexes, and other phosphorescent transition metal complexes, exhibits large zero-field splitting (ZFS, ∼25 cm-1), preventing a time-resolved electron paramagnetic resonance (TREPR) study. To investigate the intersystem crossing (ISC) electron spin selectivity of the Pt(II) complexes, we attached a perylene unit as a triplet energy acceptor. Our method relies on the conservation of spin angular momentum during the ultrafast intramolecular triplet-triplet energy transfer (TTET) from the metal-to-ligand charge transfer (3MLCT) state to the 3Pery state of the appended perylene unit, which has a small ZFS (0.05 cm-1) and can be studied with an X-band TREPR spectrometer. TTET occurs within ∼1 ps. TREPR spectra detected no signal from the native complex, while the modified complex showed the 3Pery state spectrum with an electron spin polarization (ESP) pattern (e, a, e, a, e, a), distinct from the spin-orbit coupling (SOC)-generated 3Pery state (e, e, e, a, a, a). Based on the orientation of the two units, the population ratios of the 3MLCT sublevels were deduced as Px:Py:Pz = 0.58:0.22:0.20 (in acceptor's coordinates). Our approach offers a novel way to deduce the ISC spin selectivity in phosphorescent transition metal complexes.