Effect of Polymethylene-Chain Dynamics on the Lifetime of a Charge-Separated Biradical Studied by RYDMR Spectroscopy

Abstract

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 B 0 (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.

Optical switching of radical pair conformation enhances magnetic sensitivity

“…For chemical reactions involving bi-ionic radicals in solution, however, the Coulomb attraction leads to a faster re-encounter. Thus several experimental efforts have been undertaken to modify the distribution of the RP re-encounter time, mainly by imposing constraints on the geometry of the system by linking the radicals with a flexible chain [16,17] or restricting the available diffusion volume using micelles [18] . Although these experimental studies confirm a substantial modification of in these systems, they are still interpreted within the phenomenological exponential model with rate k ∼ 0.1–1 μs −1 essentially determined by properties (like solvent viscosity or chain length) that cannot be changed continuously or even independently, and whose effect on the rate k is not easy to predict quantitatively.…”

Electric Field Effects on Fluorescence of Methylene-Linked Compounds of Phenanthrene andN,N-Dimethylaniline in a Poly(methyl methacrylate) Polymer Film

“…6,9 Phenanthrene chromophores act as an electron acceptor (A), and N,N-dimethylaniline chromophores act as a donor (D) in PIET. 16 Samples for the measurements of the electric field effects on absorption and emission spectra were prepared in the same manner as reported elsewhere. [13][14][15]17 A certain amount of benzene solution of PMMA containing linked compounds was poured onto an ITO-coated or semitransparent aluminum (Al)-coated substrate by a spin coating technique.…”

Developments of magnetic-resonance-related spin chemistry in Japan

“…There are several different RYDMR methods such as fluorescence-detected magnetic resonance (FDMR) [30,31], transient-absorption-detected magnetic resonance (ADMR) [33,34] and recently developed PCDMR [37]. The observation of final reaction products of spin-trapped products constitutes a further RYDMR method [32].…”

“…Azumi group [34,120] studied the time-resolved FDMR observation of the ionic biradical formed in the intramolecular electron-transfer reaction in the polymethylene-tinked phenanthrene (Ph) and N,N-dimethylaniline (DMA) system, Ph-(CH~)~0-O-(CH2)2-DMA. This biradical is in the quasiequilibrium with an exciplex that emits fluorescence.…”

“…Another important field of spin effects, namely, the reaction-yielddetected magnetic resonance (RYDMR) [30,31] started in Japan in 1986 with a remarkable work by Okazaki [32] and developed along with the above mentioned work in MFE, TREPR and pulsed EPR. Recent developments in this field enabled us to control the chemical reactions by microwave (MW) field [33][34][35] and to observe quantum beats [36]. A new RYDMR method known as photoconductivity-detected magnetic resonance (PCDMR) was also developed [37].…”