Conformational Effects in Excited State Intramolecular Proton Transfer of Organic Compounds

The results of investigations of excited state intramolecular proton transfer of organic fluorophores with various intramolecular hydrogen bonds are analyzed. The efficiency and mechanism of these reactions and, therefore, spectral luminescent characteristics of these fluorophores are determined to a greater extent by the conformational equilibrium of rotamers in the ground electronic state. The results of femtosecond absorption and time-resolved fluorescence spectroscopy point out that in six-membered complexes with strong intramolecular hydrogen bond, the excited state intramolecular proton transfer is barrierless, has a rate constant of ~1013 s−1 and it is a non-equilibrium process. In the compounds with weaker intramolecular hydrogen bond, the existence of potential barrier results in the decrease in excited state intramolecular proton transfer rate constant by 2–3 orders of magnitude.