The novel solvent-viscosity dependence of the time-resolved reaction-yield-detected magnetic resonance (RYDMR) spectra is observed for the polymethylene-linked biradical generated in the photoinduced intramolecular electron-transfer reaction between N,N-dimethylaniline (DMA) and phenanthrene (Ph) in the compound of the type Ph-(CH2)(10)-O-(CH2)(2)-DMA. With decreasing the solvent viscosity,the line width and peak intensity of the RYDMR Bo (external magnetic field) spectrum increases and decreases, respectively. This result is interpreted in terms of the decrease of the Lifetime of intermediate ionic biradicals at lower viscosity. This explicitly shows that the lifetime of the ionic biradical is governed by the reencounter frequency of the two terminal radicals determined by the polymethylene-chain dynamics. The lifetime of the spin states in the ionic biradical is estimated to be less than 2 ns.