Chain Effect and Magnetic Field Effect on the Photoinduced Electron Transfer Reactions of Polymethylene-Linked Donor Acceptor Systems

Abstract

Model compounds of the type A—(CH2)_n—D (with A = pyrene, D = dimethylaniline and n = 4 − 16) have been used to investigate the effect of the polymethylene chain on the intramolecular electron transfer processes, and also the influence of external magnetic fields on the spin dynamics in the radical ion pair ²A⁻—(CH2)_n—²D⁺ from which the molecular triplet state $^3 \mathop \text{A}\limits^ * - (\text{CH}_\text{2} )_\text{n} - \text{D}$ is formed. This was done using the fluorescence lifetime and transient absorption measurements with the aid of nanosecond laser flashes.

The results show that the most frequently occurring (equilibrium) end-to-end distance of the polymethylene chain increases with $\sqrt \text{n}$ and that the chain dynamics are governed by an effective diffusion coefficient, D_eff. Although the latter increases with n², it is several orders of magnitude smaller than the relative diffusion coefficient of the unlinked A and D molecules.

The magnetic field effect can be interpreted quantitatively by assuming that the spin realignment in the radical ion pair leading from the initially produced overall singlet state to the molecular triplet state is governed by the hyperfine interaction in each radical, the exchange interaction of the radical spins in the pair (which increases with decreasing n), and by the Zeeman splitting of the T₊₁, T_O, T₋₁ energy levels of the radical pair triplet state.

The differences in behaviour with respect to molecular triplet formation of the linked compounds with long (n ≥ 12), medium (6 < n < 12) and short (n ≤ 6) polymethylene chains are discussed.