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 (D-0) state and the excited quartet (Q(1)) state consisting of the excited tripler SiPc and the doubler TEMPO radical. The excited quartet spectrum is simulated using zero field splitting parameters D = 4.3 x 10(-3) and E = 3.3 x 10(-4) cm(-1). The D value observed is almost identical with that calculated under a point charge approximation (= 4.7 x 10-3 cm(-1)). The electron spin polarization of the Q(1) stale is reproduced by selective intersystem crossing (ISC) from the excited doublet states to the \+/-1/2 > sublevels in the Q(1) 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 Q(1) and D-0 states exhibit A and E polarizations, respectively, where the A and E are absorption and emission of microwaves. The E polarization of the D-0 state can be explained by the radical-quarter pair mechanism (RQPM), which originates from an intermolecular interaction between the Q(1) and D-0 SiPc-TEMPO molecules. This RQPM was observed for the first time. The intermolecular interaction between a stable radical and the Q(1) SiPc-TEMPO is clearly demonstrated using a toluene solution containing both SiPc-TEMPO and 2,6-di-tert-butyl-alpha-(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 Q(1) SiPc-TEMPO to Do Galvinoxyl and the RQPM between the Q(1) SiPc-TEMPO and D-0 Galvinoxyl, respectively.