This work applies the thermodynamic Förster cycle to theoretically investigate the pK a *, i.e., excited-state pK a values of pyranine-derived superphotoacids developed by Jung and co-workers. The latter photoacids are strong enough to transfer a proton to the aprotic solvent dimethyl sulfoxide (DMSO). The Förster cycle provides access to pK a * via the ground-state pK a and the electronic excitation energies. We use the conductor-like screening model for real solvents (COSMO-RS) to compute the ground-state pK a and the correlated wavefunction-based methods ADC(2) and CC2 with the continuum solvation model COSMO to calculate the pK a change upon excitation. A comparison of the calculated UV/Vis absorption and fluorescence emission energies to the experimental results leads us to infer that this approach allows for a proper description of the electronic excitations. In particular, implicit solvation by means of the COSMO model appears to be sufficient for the treatment of these photoacids in DMSO. The calculations confirm the presumption that a charge redistribution from the hydroxy group to the aromatic ring and the electron-withdrawing substituents is the origin of photoacidity for these photoacids. Moreover, the calculations with the continuum solvation model predict that the pK a jump upon excitation decreases with increasing solvent polarity, as rationalized based on the Förster cycle.

In this contribution we extent the use of the conductor-like screening model for realistic solvation (COSMO-RS) to the prediction of pK a values in acetone, a commonly used dipolar aprotic solvent. For this, we calculated the Gibbs free energy of dissociation of 120 organic acids (nine acrylic acids, 87 benzoic acids, nine phenols, and 15 benzenesulfonamides) using COSMO-RS at the two levels BP-TZVP and BP-

Super‐photoacids, that is, photoacids with a negative pKnormala value in the electronically excited state, can trigger an excited‐state proton transfer (ESPT) to the solvent. For the neutral pyranine‐derived super‐photoacid studied here, even indications for ESPT in acetoneous solution are reported. The characteristics of ESPT in this environment, that is, which intermediates exist and what the impact of cosolvents is, remain unsettled though. In this work, we study ESPT in acetone‐water mixtures by steady‐state and time‐resolved fluorescence spectroscopy. Various effects are observed: First, the addition of water supports the formation of a hydrogen‐bonded ground‐state complex comprising one water molecule and the photoacid, whose excitation triggers the formation of a hydrogen‐bonded ion pair on a sub‐ns time scale. Second, water has an overall accelerating effect on the fluorescence dynamics of the involved emitting species, whose contributions are disentangled in a global analysis scheme, enabling the identification of emission from the free photoacid, a photoacid‐water complex, a hydrogen‐bonded ion pair, and the deprotonated photoacid. At least two water molecules are necessary for ESPT in the environment. Third, additional acidification thwarts an efficient ground‐state complex formation of the photoacid and water. However, upon excitation, complexation may occur on a timescale faster than the photoacid's excited‐state lifetime, so that emission from a nascent complex emerges.

The phenomenon of photoacidity, i.e., an increase in acidity by several orders of magnitude upon electronic excitation, is frequently encountered in aromatic alcohols capable of transferring a proton to a suitable acceptor. A promising new class of neutral super-photoacids based on pyranine derivatives has been shown to exhibit pronounced solvatochromic effects. To disclose the underlying mechanisms contributing to excited-state proton transfer (ESPT) and the temporal characteristics of solvation and ESPT, we scrutinize the associated ultrafast dynamics of the strongest photoacid of this class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)8-hydroxypyrene-1,3,6-trisulfonate, in acetoneous environment, thereby finding experimental evidence for ESPT even under these adverse conditions for proton transfer. Juxtaposing results from time-correlated single-photon counting and femtosecond transient absorption measurements combined with a complete decomposition of all signal components, i.e., absorption of ground and excited states as well as stimulated emission, we disclose dynamics of solvation, rotational diffusion, and radiative relaxation processes in acetone and identify the relevant steps of ESPT along with the associated time scales. Graphical abstract