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

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Abstract
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 ( 3 MLCT) state to the 3 Pery 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 3 Pery state spectrum with an electron spin polarization (ESP) pattern ( e , a , e , a , e , a ), distinct from the spin‐orbit coupling (SOC)‐generated 3 Pery state ( e , e , e , a , a , a ). Based on the orientation of the two units, the population ratios of the 3 MLCT sublevels were deduced as P x : P y : P z = 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.
Funding Information
  • Fundamental Research Funds for the Central Universities (DUT22LAB610)

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