Relationship between electron spin polarization, electron exchange interaction, and lifetime: The excited multiplet states of phthalocyaninatosilicon covalently linked to one nitroxide radical

We have studied phthalocyaninatosilicon (SiPc) covalently linked to one nitroxide radical by the combined use of transient absorption and time-resolved electron paramagnetic resonance (TREPR) spectroscopies and succeeded in clarifying the relationship between the electron spin polarization (ESP), excited-state lifetime, and electron exchange interaction between the excited triplet SiPc and doublet nitroxide. It is found that the lifetime of the excited triplet SiPc is dependent on the linking nitroxide radical and correlates well with the magnitude of the electron exchange interaction. The ESPs due to both D₁-Q₁ mixing (the D₁ and Q₁ states consist of the excited triplet SiPc and doublet nitroxide radical) and spin-orbit coupling (SOC) are observed by systematically changing the electron exchange interaction. It is found that the ESP of the radical-triplet pair changes from SOC type to D₁-Q₁ mixing type with increasing degree of electron exchange interaction. These relationships are important for understanding the excited-state dynamics of the radical-triplet pair system.