Excited-state proton transfer (ESPT): Investigations on the single-molecule level for separating solvation from intrinsic reaction dynamics

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Proton-transfer reactions are amongst the most important chemical transformations. One theoretical and successful approach for its description is based on Marcus-theory which is known for its relevance in electron-transfer reactions. In contrast to this latter reaction type, proton-transfer reactions in the excited-state (ESPT) are known which are associated with bright fluorescence emission. In this special case, the photoacid and the conjugated base exhibit dual emission, i.e. both are fluorescent with different colors. After emission, the proton is shuffled back to the conjugated base and the system is consequently reset in its initial state. These properties enable the observation of an individual reaction multiple times by means of single-molecule fluorescence spectroscopy. We recently synthesized and described a variety of photostable photoacids which undergo ESPT even in a solid matrix. By recording the fluorescence intensities of both the photoacid and the conjugated base as well as the kinetics of ESPT in a temperature-dependent manner, we seek to determine the activation energy as well the free energy of a single elementary reaction. As we expect to meet varying geometries for the photoacid/proton-acceptor pair, these experiments will enable to address the distance dependence of proton-transfer reactions. Preliminary antibunching experiments show that the backreaction in the electronic ground state does not reflect the kinetics of proton diffusion, but allows for studying the kinetics of individual photocycles. Alternatively, varying compositions in the system photoacid/proton-acceptor/surrounding can be resolved from which the solvation contribution to the reaction dynamics can be separated from intrinsic contributions. Our long-term goal is to combine these approaches and to roughly sketch a potential-energy surface of a simple reaction only by experimental means.