Influence of protonation on the structural, spectroscopic and electrochemical properties of 1,3-diazaphenalenes.

Abstract

Azaphenalenyl compounds are nitrogen-substituted analogues of the all-carbon phenalenyl. They are capable of undergoing protonation/deprotonation reactions, which can be reversibly controlled by acid-base equilibria. In this study, we successfully isolated different protonation states of 1,3-diazaphenalene (DAP) and its 2,2'-linked dimeric derivative, 2,2'-bi(1,3-diazaphenalene) (BDAP), and subjected them to structural, spectroscopic and electrochemical characterization. Single crystal structures revealed systematic changes in charge distribution, π-conjugation, and intermolecular interactions across the isolated species. Ultraviolet-visible (UV-vis) spectroscopy and voltammetry measurements showed that deprotonation narrowed the HOMO-LUMO gap and dramatically increased the HOMO energies leading to the corresponding red-shift of absorption bands and cathodic shift of oxidation potentials. At the same time, NMR analysis indicated increased negative charge at specific carbon atoms through resonance effects, leading to enhanced π-electron delocalization. Consequently, vibrational mode frequencies of skeletal bonds decreased, and harmonic oscillator model of aromaticity (HOMA) values generally increased. These changes were especially pronounced in the N-heterocyclic ring of DAP-, which achieved 6π-electrons in its aromatic system. BDAP exhibited similar behavior to DAP with respect to protonation/deprotonation processes. One unique feature was the existence of mixed-valent states in BDAP species defined by the splitting of oxidation peaks observed in cyclic voltammograms. These results demonstrated that DAP-based systems offer an additional dimension for modulation of their electronic structures and properties through readily accessible acid-base equilibria that are not available in the parent phenalenyl.