Kawai Akio, Shibuya Kazuhiko
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ohokayama, Tokyo 152-8551, Japan.
J Phys Chem A. 2007 Jun 14;111(23):4890-901. doi: 10.1021/jp067753d. Epub 2007 May 23.
The exchange interaction, J, producing quartet and doublet energy separation in radical-triplet excited molecule encounter pairs, was investigated in solution by measuring chemically induced dynamic electron polarization (CIDEP) created through the radical-triplet pair mechanism. A time-resolved FT-EPR method was utilized to measure CIDEP of galvinoxyl radical by recording FID signals and an absolute magnitude of CIDEP, P(n), was determined for each radical-triplet system by detailed analysis of the time evolution curves of CIDEP. A transient FT-EPR signal phase remarkably depends on the triplet molecule. The signal phase is related to the sign of J value, which is responsible for the radical-triplet pair interaction. Most of galvinoxyl-triplet systems showed normal negative sign. An unusual positive sign was found in some systems characterized by a small energy gap, DeltaG, between the radical-triplet pair and intermolecular charge transfer (CT) states. A theoretical calculation of J value for radical-triplet encounter pairs was carried out by considering exchange integral and intermolecular CT interaction. According to the calculated J value and the diffusion theory for CIDEP magnitude, experimental Pn values were theoretically reproduced as a function of DeltaG. The present results confirm our previously reported CT model explaining the complicated nature of the sign of J value in the galvinoxyl-triplet encounter pairs. According to the proposed model for CT effect on J value and CIDEP results, nature of J value in radical-triplet pairs is discussed.
通过测量由自由基 - 三重态对机制产生的化学诱导动态电子极化（CIDEP），在溶液中研究了在自由基 - 三重态激发分子遭遇对中产生四重态和二重态能量分离的交换相互作用J。利用时间分辨傅里叶变换电子顺磁共振（FT - EPR）方法，通过记录自由感应衰减（FID）信号来测量加尔文氧基自由基的CIDEP，并通过对CIDEP时间演化曲线的详细分析，确定每个自由基 - 三重态系统的CIDEP绝对大小P(n)。瞬态FT - EPR信号相位显著依赖于三重态分子。该信号相位与J值的符号有关，J值决定了自由基 - 三重态对相互作用。大多数加尔文氧基 - 三重态系统显示出正常的负号。在一些自由基 - 三重态对与分子间电荷转移（CT）态之间能量间隙ΔG较小的系统中发现了不寻常的正号。通过考虑交换积分和分子间CT相互作用，对自由基 - 三重态遭遇对的J值进行了理论计算。根据计算出的J值和CIDEP大小的扩散理论，理论上再现了作为ΔG函数的实验Pn值。目前的结果证实了我们之前报道的CT模型，该模型解释了加尔文氧基 - 三重态遭遇对中J值符号的复杂性质。根据所提出的CT对J值影响的模型和CIDEP结果，讨论了自由基 - 三重态对中J值的性质。
