Computational investigation of explicit solvent effects and specific interactions of hydroxypyrene photoacids in acetone, DMSO, and water.

This work employs the correlated wavefunction-based methods ADC(2) and CC2 in combination with the implicit solvent model COSMO to calculate the UV/Vis absorption and fluorescence emission energies of particularly strong hydroxypyrene photoacids in acetone. According to the Förster cycle, the electronic transition energies are first used to compute , i.e. , the p K _a change upon excitation and then the excited-state p K _a (labeled ) with ground-state p K _a values based on COSMO-RS as additional inputs. Furthermore, for the strongest photoacid of that class, namely tris(1,1,1,3,3,3-hexafluoropropan-2-yl)-8-hydroxypyrene-1,3,6-trisulfonate, the need to go beyond implicit solvation and to account for explicit solvent effects on the electronic transition energies and the resulting Δp K _a is investigated in the solvents acetone, dimethyl sulfoxide (DMSO), and water. For this, a hybrid implicit-explicit approach is followed by comparing micro-solvated structures that are generated based on Kamlet-Taft considerations. While implicit solvent effects are mostly sufficient for the aprotic solvent acetone, one explicit solvent molecule seems relevant for DMSO due to its stronger hydrogen-bond (HB) acceptance and hence larger interaction with the photoacid OH group as a HB donor. For the protic solvent water, the situation is more complicated, involving at least one water molecule at the OH group and up to three water molecules at the O ^- group of the corresponding base. Finally, these results are used to rationalize the experimentally observed spectral evolution of the photoacid absorption band in acetone-water solvent mixtures.