Highly efficient exciplex formation in X-irradiated alkane solutions for luminophores with short fluorescence lifetimes

X-ray generation of electronically excited molecules via the step of radical ion pair recombination offers unique advantages of the possibility of working in the conditions of optically dense sample (typical concentrations of positive charge acceptor are about 10⁻² M or higher) while having an almost homogeneous formation of the excited molecules over the sample, as X-radiation is weakly absorbed by an organic sample. Furthermore, in case of X-ray generation another molecule, corresponding to the second partner of the pair, is always found next to the excited molecule formed upon recombination. This can be either a molecule of the same compound, or a molecule of a different component deliberately introduced into the solution. As a result, not only the expected emission of the excited molecule becomes possible, but also the formation of excited complexes, such as exciplexes. Thus X-ray generation opens the way to exciplexes for systems with a very short fluorescence lifetime of the luminophore molecule, as recombination creates optimal conditions for the high efficiency exciplex formation.

Formation of excited states and exciplexes via the channel of radical ion pair recombination has another very important peculiarity. The multiplicity of the electronically excited state that forms upon recombination, singlet or triplet, corresponds to the collective electronic spin state of the pair, which can be changed by application of external magnetic fields. In magnetic field of several mT the average singlet state population of a recombining pair originally formed in singlet state is higher than the population in zero field due to less spin states available for singlet-triplet mixing. Since in solutions of organic luminophores luminescence and exciplex formation are normally possible only from singlet excited state, the yields of luminescence and exciplex formation via the channel of pair recombination become magnetosensitive.

This work reports the spectra of photo- and radiation-generated luminescence from several donor-acceptor systems typical for radiation spin chemistry, i.e., alkane solutions of naphthalene/N,N-dimethylaniline (DMA), p-terphenyl/DMA, and other. One component of the system (DMA, positive charge acceptor) was held constant, and as the other component (electron acceptor, luminophore) were chosen molecules with widely varying lifetimes of the electronically excited state τ_f, from about 100 ns to about 1 ns and to about 10 ps. In this work we also studied the sensitivity of the obtained emission spectra to external static magnetic field, and obtained the close to maximally possible under these experimental conditions magnetic field effect (up to 20%) in exciplex emission band for the system diphenylacetylene/DMA in n-dodecane.