The ground-state minima forms of the 1-HAQ and 1,4-DHAQ were optimized by means of the MP2 method (36) using the cc-pVDZ (37) correlation-consistent atomic basis set. The excited state geometries were optimized with the use of the same basis set while using the CC2 (38, 39) method as implemented in TURBOMOLE software package (40). In the calculation of the vertical excitation energies, ΔEVE, mimicking the absorption spectra, performed on top of the MP2/cc-pVDZ–optimized S0-state geometries, the CC2 (38, 39) method was used to evaluate the response properties.

To elucidate the photophysical mechanism on the molecular level, the important driving coordinates were appropriate for each system so that the ground- and excited-state minimum potential energy (MPE) profiles or surfaces could be constructed to estimate the ground- and excited-state energy barriers determining the photophysics of the both molecules. The 1-HAQ molecule was bound by a single hydrogen bond. In this case, one coordinate, the R1(O1H) distance, is needed to be chosen as a driving coordinate to illustrate the photophysical mechanism of the photo-tautomerization process. The MPE profile for 1-HAQ was constructed in a way that for fixed given value of the R1(O1H) distance, all the remaining nuclear degrees of freedom were optimized; once in the ground state and twice in the two excited states: S1(ππ*) and S1(nπ*).

The 1,4-DHAQ system has an additional intramolecular hydrogen bond binding the molecule, and more tautomeric forms were possible to be formed upon photoexcitation. A convenient method for illustrating the tautomerization process in this molecule was to construct the MPE surface spanning the two driving reaction coordinates describing the two intramolecular hydrogen bonds by the R2(O2H) and R3(O4H) distances. In that case, both the R2 and R3 coordinates were frozen for given values, while the rest of the parameters were optimized in the constructed MPE surface, separately for the ground (S0) and the two excited states: S1(ππ*) and S1(nπ*) with the Cs symmetry constrain.

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