Electron spin polarizations of phthalocyaninatosilicon covalently linked to one TEMPO radical in the excited quartet and doublet ground states

Phthalocyaninatosilicon(IV) covalently linked to a 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radical, SiPc-TEMPO, has been studied by time-resolved electron paramagnetic resonance (TREPR). A TREPR spectrum at 20 K is assigned to the doublet ground (D₀) state and the excited quartet (Q₁) state consisting of the excited triplet SiPc and the doublet TEMPO radical. The excited quartet spectrum is simulated using zero field splitting parameters D = 4.3 × 10^-3 and E = 3.3 × 10^-4 cm^-1. The D value observed is almost identical with that calculated under a point charge approximation (= 4.7 × 10^-3 cm^-1). The electron spin polarization of the Q₁ state is reproduced by selective intersystem crossing (ISC) from the excited doublet states to the |±l/2> sublevels in the Q₁ state. This selective ISC is explained by spin-orbit coupling between the P_x and P_y orbitals on oxygen atoms of axial-ligands. In the TREPR spectrum at 293 K, SiPc-TEMPO molecules in the Q₁ and D₀ states exhibit A and E polarizations, respectively, where the A and E are absorption and emission of microwaves. The E polarization of the D₀ state can be explained by the radical-quartet pair mechanism (RQPM), which originates from an intermolecular interaction between the Q₁ and D₀ SiPc-TEMPO molecules. This RQPM was observed for the first time. The intermolecular interaction between a stable radical and the Qi SiPc-TEMPO is clearly demonstrated using a toluene solution containing both SiPc-TEMPO and 2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)- p-tolyloxy (Galvinoxyl). The A and E polarizations are observed for the fast and slow components of Galvinoxyl, respectively, and can be assigned to the electron spin polarization transfer from the Q₁ SiPc-TEMPO to D₀ Galvinoxyl and the RQPM between the Q, SiPc-TEMPO and D₀ Galvinoxyl, respectively.