Abstract
1H-pyrrolo[3,2-h]qinoline (PQ) and 2-(2′-pyridyl)pyrrole (PP) are important systems in the study of proton-transfer reactions. These molecules possess hydrogen bond donor (pyrrole) and acceptor (pyridine) groups, which leads to the formation of cyclic dimers in their crystals. Herein, we present a joint experimental (Raman scattering) and computational (DFT modelling) study on the high-pressure behaviour of PQ and PP molecular crystals. Our results indicate that compression up to 10 GPa (100 kbar) leads to considerable strengthening of the intermolecular hydrogen bond within the cyclic dimers. However, the intramolecular N–H∙∙∙N interaction is either weakly affected by pressure, as witnessed in PQ, or weakened due to compression-induced distortions of the molecule, as was found for PP. Therefore, we propose that the compression of these systems should facilitate double proton transfer within the cyclic dimers of PQ and PP, while intramolecular transfer should either remain unaffected (for PQ) or weakened (for PP).
Highlights
Proton-transfer (PT) reactions are at the heart of many fundamental chemical and biological processes, such as acid–base neutralization [1], electron transfer [2], and enzymatic reactions [3]
When the migration of the proton is confined within one molecule, such a reaction is described as an excited-state intramolecular proton transfer (ESIPT) [4]
The appearance of a hydrogen bond is a prerequisite for PT reactions, both inter- and intramolecular, and the characteristics of this bond strongly influence the fate of the proton transfer
Summary
Proton-transfer (PT) reactions are at the heart of many fundamental chemical and biological processes, such as acid–base neutralization [1], electron transfer [2], and enzymatic reactions [3]. An important class of PT processes includes those where the reaction is initialized by placing the molecule in an excited electronic state—the so-called excited-state proton transfer. The study of systems exhibiting ESIPT receives much attention because of its importance in understanding molecular properties and intermolecular interactions [5,6,7,8], and due to possible technological applications [9]. Among several systems exhibiting such an intramolecular hydrogen bond those containing the pyridine–. High-pressure experiments on molecular crystals yield important information on intermolecular interactions, in particular hydrogen bonds [19,20,21,22,23,24,25,26]. Pressure can be used to tune the electronic and spectroscopic properties of various systems [27]
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