Theoretical Study on the Photoacidity of Hydroxypyrene Derivatives in DMSO Using ADC(2) and CC2

Abstract

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.

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

“…Regarding the reliability of the selected methods, the results for photoacid B in acetone are very similar to the previous results for photoacid F in DMSO. 62 The absorption energies at the COSMO-CC2//COSMO-CC2 level are in excellent quantitative agreement with the experimental results, whereas the emission energies at the COSMO-CC2//COSMO-ADC(2) level underestimate them by 0.1–0.2 eV. Except for the large deviation for the acid emission energy that is further analyzed later ( vide infra ), these results fall within the typical accuracy of ADC(2) and CC2 that has been observed in benchmarking studies.…”

“…If available, initial structures were taken from the previous calculations in DMSO (see ref. 62; therein available from ref. 96) and re-optimized in the target solvent (mostly acetone).…”

“…62; therein available from ref. 96) and re-optimized in the target solvent (mostly acetone). At the final geometries, vertical electronic excitation energies were then calculated in the larger aug-cc-pVDZ basis set 89,97 with the desired method(s) ADC(2), 98 CC2, 99 COSMO-ADC(2), 94 or COSMO-CC2.…”

“…1) were taken into account based on the conformer sets that were already determined during the previous study in DMSO. 62 Since the geometries used for COSMO-RS are optimized with COSMO for ε r = ∞, i.e. , they are independent of the solvent, no additional changes were needed.…”

“…2) to determine the values of the photoacids A – F in the related solvent DMSO through the ground-state p K a and electronic transition energies. 62 p K a has been calculated using COSMO-RS (COSMO for real solvents), 63–66 while UV/Vis absorption and fluorescence emission energies have been computed using the wavefunction-based methods ADC(2) and CC2 in combination with the conductor-like screening model (COSMO). 66–68 To the best of our knowledge, that study has been the first that applied such higher-level electronic-structure methods to the study of photoacids, in general, and , in particular, apart from some studies by Aquino and co-workers on anthocyanins, also using ADC(2) but not considering .…”