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 Px and Py 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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry