On the Origins of Nonradiative Excited State Relaxation in Aryl Sulfoxides Relevant to Fluorescent Chemosensing

We provide herein a mechanistic analysis
of aryl sulfoxide excited
state processes, inspired by our recent report of aryl sulfoxide based
fluorescent chemosensors. The use of aryl sulfoxides as reporting
elements in chemosensor development is a significant deviation from
previous approaches, and thus warrants closer examination. We demonstrate
that metal ion binding suppresses nonradiative excited state decay
by blocking formation of a previously unrecognized charge transfer
excited state, leading to fluorescence enhancement. This charge transfer
state derives from the initially formed locally excited state followed
by intramolecular charge transfer to form a sulfoxide radical cation/aryl
radical anion pair. With the aid of computational studies, we map
out ground and excited state potential energy surface details for
aryl sulfoxides, and conclude that fluorescence enhancement is almost
entirely the result of excited state effects. This work expands previous
proposals that excited state pyramidal inversion is the major nonradiative
decay pathway for aryl sulfoxides. We show that pyramidal inversion
is indeed relevant, but that an additional and dominant nonradiative
pathway must also exist. These conclusions have implications for the
design of next generation sulfoxide based fluorescent chemosensors.