Magnetosensitive exciplex emission in X-irradiated alkane solutions

The emission of optically generated exciplex is well-known to be magnetosensitive. Magnetic field effects (MFE) are based on the reversibility of transitions between the radical ion pair state and the exciplex in the solvent of corresponding viscosity and polarity.

On the contrary, X-ray irradiation of alkane solution has rarely been used to study exciplex formation and magnetosensitivity of the exciplex emission band. The systems under study were already known from optical excitation, such as excimers of pyrene in non-polar solutions or polymeric systems. The magnetosensitivity of the exciplex emission band in case of X-ray irradiation is caused by irreversible recombination of a radical ion pair to produce an electronically excited state. The presence of the magnetosensitive exciplex emission band in principle allows separation of the bulk and recombination channels of exciplex formation in case of X-ray irradiation of non-polar solutions.

This work reports the spectra of X-ray generated luminescence from several donor-acceptor systems typical for radiation spin chemistry, i.e., solutions of diphenylacetylene/N,N-dimethylaniline (DMA), p-terphenyl/DMA in two different non-polar solvents (n-dodecane, isooctane). Electron acceptor (diphenylacetylene, p-terphenyl) were chosen molecules with varying fluorescence decay time from about 100 ns to about 10 ps, positive charge acceptor (DMA) was held constant. By means of studying the sensitivity of the luminescence spectra to external static magnetic field we have obtained the close to maximally possible under these experimental conditions MFE in exciplex emission band for the system diphenylacetylene/DMA in n-dodecane (19±1%) and isooctane (17±1%). In case of isooctane solutions of diphenylacetylene/DMA system the magnitude of MFE practically equals to the experimentally measured fraction of spin-correlated radical ion pairs.