Resonance-Assisted Hydrogen Bonding controlling excited state Proton Transfer

Abstract

Resonance-Assisted Hydrogen Bonds (RAHB) are systems in which a hydrogen bond and a heteroconjugated system (a proton-donor group and a proton-acceptor group) form a cyclic structure known as a quasi-ring. These systems exhibit a significantly stronger bond compared to conventional hydrogen bonds due to resonance effects facilitated by π-electrons. This study investigates RAHB systems with different proton-donor (PD) and proton-acceptor (PA) groups based on oxygen and nitrogen elements. The PD groups include OH and NH2, while the PA groups consist of CHO and CHNH. The systems are further characterized by the addition of different 6-Membered rings (6-MRs), resulting in five distinct topologies: benzene, linear, kinked, L4, and K4. The primary focus of this study is to examine the factors that influence RAHB strength and proton transfer in both singlet and triplet states. Specifically, the influence of PD and PA substituents in the ortho position of a 6-MR within the different topologies is elucidated. The analysis involved 20 different RAHB systems, using the B3LYP/6-311+G(d,p) level of theory. Hydrogen bond distances were collected, along with the activation energies associated with the excited-state intramolecular proton transfer (ESIPT) in both singlet and triplet states. Another significant aspect of this study is to analyze the interplay between aromaticity and HB strength. Aromaticity indices, particularly the para-delocalization index (PDI), are used to quantify electron delocalization within the ring. Emphasis is placed on the ipso-ring as it has shown to be a determining factor in RAHB strength. By examining the various aspects of RAHB systems, including activation energies, hydrogen bond distances, and aromaticity indices, this study aims to provide valuable insights into the behavior and properties of these systems, enhancing our understanding of resonance-assisted hydrogen bonding and its impact on proton transfer mechanisms