Appl. Magn. Reson. 12, 423-430 (1997) AppUed MagneticResonance 9 Springer-Verlag 1997 Printed in Austria Reaction-Yield-Detected Magnetic Resonance in the Intra- and Intermolecular Electron Transfer Reactions K. Enjo 1, K. Maeda 1, H. Murai 1, T. Azumi 1, and Y. Tanimoto 2 1Department of Chemistry, Graduate School of Science, Tohoku University, Sendai, Japan 2Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima, Japan Received January 8, 1997 Abstract. The spin dynamics of the intermediate radical ion pairs formed in the intra- and intermo- lecular electron transfer reactions between N,N-dimethylaniline and phenanthrene is studied by em- ploying the polymethylene-linkedand nonlinked systems. Reaetion-yield-detected magnetic resonance (RYDMR) expe¡ are performed by monitoring the exciplex fluorescente. From the theoretical consideration of the data, we conelude that the lifetime of the radical ion pair in the linked system is shorter than that in the nonlinked system. This tendency is opposite to what is generally observed in linked and nonlinked neutral biradical. 1. Introduction The nature of polymethylene-linked biradicals generated in the photochemical reaction is one of the most attractive themes in the spin chemistry. The lifetime of polymethylene-linked neutral biradicals [1, 2] is thought to be longer than that of nonlinked radical pairs because the polymethylene chain restricts the escape channel of the radical pair. However, the contribution of the polymethylene chain dynamics to the lifetime of ionic biradicals is expected to be different from that of neutral biradicals. In the case of the neutral biradical (initial spin multiplicity is unreactive triplet), it decays only by the recombination reaction of the singlet state converted through a spin realignment in the biradical. On the other hand, the ionic biradical generated from the polymethylene-linked donor-acceptor sys- tem, A-(CH)n-D, [3, 4] recombines from both its singlet (initial spin state) and triplet manifolds. For the last two decades the extemal magnetic field effects upon there combination reaction of the ionic biradical have been investigated exten- sively by Weller, Steark et aL and Tanimoto et al. [3-5]. However, these studies mainly focused their interests on the spin exchange interaction J. The purpose of this paper is to investigate the effect of the polymethylene chain dynamics on the lifetime of the ionic biradical. For this purpose we applied the