Analytical Applications of Excited-state proton transfer (ESPT) in Pyrene-Based Photoacids

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Proton-transfer as one of the most fundamental reactions is intensively studied for more than 60 years. We recently published a series of photoacids on the basis of the well-known photoacid pyranine (8-Hydroxypyrene-1,3,6-trisulfonate) which exhibit pKA-values in the excited-state down to -4. However, more applications of these photoacids can be foreseen beyond their use for studying reaction dynamics. Protons-on-demand, i.e. by excitation with visible ultrashort laser-pulses, can trigger reaction cascades like keto-enol rearrangements which can then be studied e.g. by time-resolved IR spectroscopy. As the molecules stay in the excited-state for several nanoseconds, dual emission is observed. The lifetime in the excited-state of the photoacid before the proton-transfer, only lasting for picoseconds, allows for distance measurement in the lower nanometer range by Förster-resonance energy transfer with ultrasensitivity. More emissive states can be created when more (photo)chemically active side groups are attached to the pyrene core.The anticipated applications are yet ideas which withstood realization in the past as all three sulfonate side groups were simultaneously transformed to stronger electron-withdrawing moieties. We propose two approaches to overcome the current limitations: on the one hand, pyrene as starting material will be replaced by aza-pyrene from which even stronger photoacids are figured out. These derivatives would be of great interest for two collaboration partners. On the other hand, we introduced helper substitution, verified by x-ray crystallography, which enables us to modify the pyrene core regioselectively. The marrow of the grant proposal is dedicated to the synthesis of these compounds from which we expect maintenance of the beneficial properties of our previous hydroxy-pyrene derivatives, i.e. high fluorescence quantum yields, high photostabilities and electronic transitions in the visible range of the electromagnetic spectrum.