Time-resolved EPR, fluorescence, and transient absorption studies on phthalocyaninatosilicon covalently linked to one...

The photophysical properties of tetra-tert-butylphthalocyaninatosilicon (SiPc) covalently linked to one or two 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) radicals (R1, R2) have been studied by fluorescence, transient absorption, and time-resolved electron paramagnetic resonance (TREPR) spectroscopies. It is found that the fluorescence quantum yields and lifetimes of R1 and R2 decrease compared with those of (dihydroxy)SiPc ((dihydroxy)SiPc = 6.8 ns, R1 = 4.7 ns and 42 ps, and R2 = 4.7 ns and <30 ps). Transient absorption measurements indicate that the lifetime of the excited triplet SiPc is markedly dependent on the number of linking TEMPO radicals ((dihydroxy)SiPc = 500 micros, R1 = 7.6 micros, and R2 = 3.7 micros). These short lifetimes of R1 and R2 in the excited states are explained as a result of the interaction with TEMPO changing the ISC between the singlet and triplet states to spin-allowed transitions. Quantitative TREPR investigations have been carried out for the radical-quartet pair mechanism of R1 and the photoinduced population transfer of R2. It is determined that the rise and decay times of these electron spin polarizations denote the spin-lattice relaxation time of the ground state and the lifetime of the excited multiplet state, respectively. This study contributes not only to an elucidation of radical-chromophore interactions but also to a novel approach for controlling magnetic properties by photoexcitation.