Time-Resolved EPR, Fluorescence, and Transient Absorption Studies on Phthalocyaninatosilicon Covalently Linked to One or Two TEMPO Radicals

• Published in: Journal of the American Chemical Society Vol. 123; no. 4; pp. 702 - 708
• Main Authors: Ishii, Kazuyuki, Hirose, Yoshiharu, Fujitsuka, Hiroshi, Ito, Osamu, Kobayashi, Nagao
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 31.01.2001
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja002780h

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 μs, R1 = 7.6 μs, and R2 = 3.7 μs). 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.