Investigation of the Electronic Excited-State Equilibrium Geometries of Three Molecules Undergoing ESIPT: A RI-CC2 and TDDFT Study.

Energy minima on the potential energy surfaces of the ground and excited states have been characterized for three photoactive molecules that undergo excited-state intramolecular proton transfer: 3-hydroxychromone, N-salicylideneaniline, and 2-(2-hydroxyphenyl)benzothiazole. Both the CC2 method and the TDDFT methodology with different exchange-correlation (XC) functionals differing by the amount of Hartree-Fock (HF) exchange have been employed. Besides the analysis of the structures along the reaction paths, this study has compared the TDDFT and CC2 results to provide guidelines for selecting the best XC functionals. Several geometrical parameters as well as the excitation energies are found to vary monotonically with the amount of HF exchange. Systematically, this study has addressed the ground-state geometries, those of the excited states, and their variations upon excitation, showing that the M06 XC functional provides the closest agreement with the CC2 results. Still, large differences of geometries have been observed between the different levels of approximation, mostly for the excited states: (i) Not all methods locate the same number of minima, (ii) the bond length variations upon excitation might be reversed, and (iii) the H-bond network can be modified from one level to another, changing the keto/enol character. Moreover, TDDFT/M06 and B3LYP-35 vertical excitation energies are in good agreement with the CC2 values. All in all, these results call for being cautious when using these optimized geometries for predicting the spectroscopic signatures of these compounds to understand the processes that take place during photoexcitation.