Syntheses, X-ray crystal structures, and emission properties of diprotonated tetrapyridylpyrazine and triprotonated terpyridine

A series of metal-free compounds, that is, planar diprotonated tetraH(2)(PF6)(2) (1), planar monoprotonated bppzHPF(6) (2), nonplanar diprotonated bppzH(2)(PF6)(2) (3), and planar triprotonated terpyH(3)Cl(PF6)(2) (4), were prepared and characterized by electrospray ionization mass spectrometry, Ultraviolet-visible spectroscopy and cyclic voltammetry. Abbreviations used are tetra = tetra-2-pyridylpyrazine, bppz = 2,3-bis(2-pyridyl)pyrazine, and terpy = 2,2':6',2 ''-terpyridine. The X-ray crystal structures of the four compounds 1, 2, 3, and 4 were determined. Both protonated pyridine rings are hydrogen bonded intramolecularly to the adjacent pyridine ring in compounds 1 and 3. The pi-pi* absorption bands in the ultraviolet region for 1, 2, 3, and 4 in acetonitrile were red-shifted relative to those of the corresponding neutral unprotonated compounds. All the cyclic voltammetry for the protonated species, 1, 2, 3, and 4, showed the first reduction waves from -0.37 to -1.18 V, that were more positive than those of the neutral ones. Density functional theory was applied to interpret the planarity in 1. The attachment of two protons to the two terminal tetra nitrogens in 1 leads to the remarkable emission (Phi = 0.031). The attachment of three protons to the three terpy nitrogens in 4 also gives the large quantum yield (Phi = 0.48). Copyright (C) 2015 John Wiley & Sons, Ltd.