Substituent and solvent effects on the nature of the transitions of pyrenol and pyranine.: Identification of an intermediate in the excited-state proton-transfer reaction

A comparative study of pyrenol and its trisulfonated derivative, pyranine, is undertaken to provide new clues for the understanding of the excited-state proton-transfer reaction (ESPT) of hydroxyarenes (ArOH*). A particular goal is to elucidate the nature of a transient intermediate involved in a three stop mechanism of ESPT, as recently revealed in a dynamical study of excited pyranine in water. The present focus is on the reactant side, before the proton transfer occurs, and particular attention is given to the analysis of the nature of the electronic transitions and to the solute-solvent interactions in the ground and excited states of the ArOHs. Using both quantum chemical calculations and solvatochromism analyses, both (a) the role of electron-withdrawing substituents and H-bond interaction with the solvent in stabilizing the two lowest excited states, L-1(b) and L-1(a), and (b) their relevance to the inversion of these two states are studied. The results allow the identification of the intermediate species in the three stop mechanism of the ESPT of excited pyranine in water as a L-1(a) state acid form, with appreciable charge-trans fer character, as distinct from the L-1(b) acid form reached in absorption. The results, which differ from more standard pictures of ESPT, are discussed within the perspective of a three valence bond form model for the ESPT process.