Deciphering photoacidity by following electronic charge distribution changes along the photoacid Förster cycle with picosecond nitrogen K-edge x-ray absorption spectroscopy

Photoacids are molecular systems that show a strong increase in acidity in the first electronic excited state. The underlying mechanisms for photoacidity and photobasicity have until now remained unsolved. We use picosecond N K-edge x-ray absorption spectroscopy to determine how the transient electronic-structure changes and hydrogen-bond dynamics determine the acidity of a prototypical photoacid, 8-aminopyrene-1,3,6-trisulfonate (APTS), in aqueous solution. We follow in time the characteristic spectroscopic signatures of N-H σ* and N-lone pair interactions of the proton donating functional amine group as well as aromatic pyrene π* anti-bonding orbitals of APTS along the different stages in the Förster photocycle. With our flatjet system for x-ray absorption spectroscopy in transmission and with the picosecond x-ray pulses at BESSY II (in multibunch mode), we elucidate how UV excitation converts the photoacid into the conjugate photobase form on a time scale of 150 ps, followed by electronic excited state fluorescence decay on nanosecond time scales. With these results we demonstrate that a systematic electronic-structural approach to the ultrafast dynamics of photoacids in aqueous solution can be established.