Magnetic field effects subject to specific solvation

Magnetic field effects on yields of chemical reactions are well documented in the scientific literature. Many of these effects can be attributed to the radical pair mechanism, i.e. the coherent interconversion of the overall singlet ant the triplet state in transient, diffusively separated radical ion pairs. It has been postulated, that specific solvation is at the root of the exceptionally large field effects observed in binary solvent mixtures composed of compounds of significantly different polarity. In particular, in analogy to the situation in micelles, the enhanced magnetic field effects have been attributed a prolonged lifetime of the radical ion pair and an increased efficiency of charge separation. However, only indirect evidence has been put forward, and the effect of the micro-heterogeneity on the diffusive trajectories of the pair has never been observed directly. This project focuses on magnetic field effects due to the radical pair mechanism in simple organic model systems in constrained (micro-heterogeneous binary mixtures) as well as homogeneous environments (neat solvents and homogeneous solvent mixtures). The model systems (e.g. pyrene/N,Ndimethylaniline, anthracene/N,N- dimethylaniline, 9,10-dimethylanthracene/1,3-benzenedicarbonitrile, and others) are characterized by the fact that they form emissive exciplexes, which serve as a sensitive probe of the underlying radical pair dynamics. This allows the subtle interplay between diffusion and spin evolution to be studied systematically. While steady-state fluorescence measurements have frequently been applied to these systems, time resolved measurements by the single photon counting or the frequency resolved modulation technique have not yet been applied systematically in the presence of an external magnetic field. We propose to devise a time-resolved magnetic field effect spectrometer, which will allow us to eventually discern the peculiarities of micro-heterogeneous solvent systems in particular and confined media in general. Our experimental efforts are accompanied by model calculations that take the stochastic Liouville-von Neumann equation beyond the low-viscosity approximation as the starting point. The theoretical approach is sufficiently general to also accommodate biomimetic environments as micelles. By comprehensively investigating these systems by both experiment and model calculations, we hope to shed light on the parallels and differences between restricted and freely diffusing systems. Our aim is to find objective criteria when exceptionally large magnetic fields are to be observed for unlinked, charged radical pairs.

The type of reaction mechanism of photo-induced charge transfer reactions is under ongoing discussion since the eighties, not only in respect of solar energy conversion efficiencies. Numerous theoretical and experimental investigations are published on this field. The most probable reaction pathways have been postulated. Loose ion pairs can be formed either directly via distant electron transfer or indirectly via the dissociation of an initially formed electronically excited complex (exciplex) or a contact ion pair. From experimental point of view it is very difficult to distinguish between these possible reaction pathways, making decisions about the consecutive reaction quite uncertain. By applying an of an extern magnetic field in combination with time-resolved fluorescence measurements we could show that the equilibrium between the singlet and triplet radical ion pairs is influenced and that the recombination of the radical ions in solution do not contribute to the observed magnetic field effect. By systematically varying the solvent dielectric constant while leaving the solvent viscosity constant, we could show that the irreversible mechanism where the radical pair is generated first is not able to reproduce our time-resolved experimental observations. Introducing a new model, we demonstrated that it is mandatory to take the dissociation equilibrium of the exciplex into account. We have successfully demonstrated that time-resolved magnetic field effect studies have the potential to provide new insights into reaction dynamics of exciplexes and radical ion pairs that are very difficult to obtain with other methods.