Accurate description of excited state intramolecular proton transfer that involves zwitterionic state using optimally tuned range-separated time-dependent density functional theory

Excited-state intramolecular proton transfer (ESIPT) reaction typically involves the excited-state of charge-transfer (CT) character. Thus, locating the transition state (TS) and providing an accurate description for the canonical and tautomeric forms play important roles and remain as challenges in the investigation of ESIPT reaction. In this study, the ESIPT reaction of a recently developed ESIPT chromophore (compound 1) (N. Suzuki et al., Angew. Chem. Int. Ed. 2014, 53, 8231) that involves the CT and zwitterionic states has been investigated using the time-dependent density functional theory (TDDFT) method with different functionals and the second-order algebraic diagrammatic construction (ADC(2)) method. Theoretical results suggest that the TDDFT method with IP-tuned range-separated functional should be applied to investigate ESIPT reaction when the zwitterionic or CT states are involved. In addition, the TS geometries for the ESIPT reaction of compound 1 in three different solvents have been successfully located, and the Gibbs free energies along the reaction path have been provided. The calculated results can clearly explain the experimentally observed solvent-dependent fluorescence of compound 1.