Speciation and Photoluminescent Properties of a 2,6-Bis(pyrrol-2-yl)pyridine in Three Protonation States

2,6-Bis(pyrrol-2-yl)pyridines are important building blocks for supramolecular assemblies and photoluminescent main group and transition metal compounds. Sterically encumbered 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine, (H2PDPPh)-P-Mes, can adopt monomeric and dimeric structures in the solid state and in solution, controlled by competing inter- and intramolecular hydrogen bonds. Deprotonation in the presence of lithium cations provides (Li2PDPPh)-P-Mes, which can be isolated in monomeric and dimeric forms depending on solvent polarity. Protonation of (H2PDPPh)-P-Mes disrupts intramolecular hydrogen bonding and provides the monomeric pyridinium salt [(H3PDPPh)-P-Mes]Cl. Independent of solvent polarity, all three derivatives exhibit intense fluorescence in solution. The absorption and emission spectra are highly sensitive to the level of protonation, which can be rationalized by the effects of (de)protonation on the HOMO and LUMO of the tricyclic pi-system.