High-Resolution Fluorescence Studies on Excited-State Intra- and Intermolecular Proton Transfer

Abstract

High-resolution fluorescence spectroscopies are useful for studying excited-state intramolecular proton transfer (ESIPT) processes, as well as excited-state double proton transfer (ESDPT) in the case of dimeric species. Recent results are shown for four classes of compounds, i.e., hydroxychromones, aza-indoles, pyrazoloquinolines, and amino-substituted pyridine-N-oxides in liquid solutions, polycrystalline matrices, and/or a supersonic jet. These species can exist as monomers, dimers, complexes with polar solvent molecules, or mixtures of these. Proton/deuteron exchange, which strongly slows down those transfer reactions that take place through tunneling, is an important tool for elucidating the reaction mechanisms. In this chapter, emphasis is on spectral information in the wavelength domain, but time-resolved femtosecond studies (for example, on the ESDPT in dimers of 7-azaindole) are also included. Cryogenic high-resolution (Shpol’skii) spectroscopy – not only in the fluorescence but also in the absorbance mode – receives special attention. It can be used to characterize the various chemical species involved in the photochemistry/physics of the above compounds, but in favorable cases (as demonstrated for hydroxychromones) it can also detailed kinetic information on excited-state proton transfer processes that can be extracted from the homogeneous spectral linewidths.