Tautomerization in Condensed Phases and in Isolated Molecules

Abstract

AbstractWe review our recent investigations of systems revealing single or double proton (hydrogen) transfer in the ground or electronically‐excited states. The results obtained for condensed phases are compared with the data provided by experiments carried out in the regime of molecular isolation, mainly by using supersonic jet spectroscopy techniques.Both intra‐ and intermolecular processes have been studied. Four different bifunctional chromophores are discussed, all sharing a common feature of possessing a proton‐donating NH group and an acceptor nitrogen atom incorporated into a pyridine or pyrrolene ring.Porphycene and derivatives reveal, when isolated, coherent double hydrogen transfer manifested by vibration‐specific tunneling splittings. In condensed phases, tautomerization occurs as a rate process; its dynamics can be probed by techniques using polarized light. Huge dependence of the tautomerization rate on the strength of intramolecular hydrogen bonds has been detected.2‐(2′‐pyridyl)pyrrole provides a rare example of a molecule which undergoes both intramolecular excited‐state single proton transfer and a solvent‐assisted excited‐state double proton transfer reaction. Supersonic jet studies allowed us to identify the vibrational modes that enhance the tautomerization.Photoinduced intramolecular proton transfer in 7‐(2′‐pyridyl)indole is accompanied by twisting of the proton donor and acceptor subunits. The reaction occurs faster in the supersonic beam than in room temperature solutions.1H‐pyrrolo[3,2‐h]quinoline forms 1:1 and 1:2 complexes with alcohols and water. The stoichiometries and vibrational structure could be determined for jet‐isolated complexes. The 1:1 cyclic, doubly hydrogen‐bonded species reveal rapid photoinduced tautomerization both in condensed phases and when isolated in supersonic beams.