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

Abstract

Abstract2,6‐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, H2MesPDPPh, 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 Li2MesPDPPh, which can be isolated in monomeric and dimeric forms depending on solvent polarity. Protonation of H2MesPDPPh disrupts intramolecular hydrogen bonding and provides the monomeric pyridinium salt [H3MesPDPPh]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 π‐system.

Synthesis and Characterization of Isostructural Th(IV) and U(IV) Pyridine Dipyrrolide Complexes

“…The UV region of each spectrum is dominated by two absorption features that are reminiscent of the absorption maxima of the free ligand, H 2 Mes PDP Ph (364 and 316 nm in THF), and therefore were assigned to originate predominantly from π–π* transitions within the individual ligands. Notably, these absorption bands are slightly shifted from those of the protonated PDP ligand, consistent with successful metalation (Figure S6); for an in-depth discussion of the absorption profile of the ligand, we direct readers to a recent report from Milsmann and co-workers . In the electronic absorption spectrum of 1b , these two intense bands are observed at 318 nm (ε = 14,006 M –1 cm –1– ) and 360 nm (ε = 12,215 M –1 cm –1 ).…”

“…Notably, these absorption bands are slightly shifted from those of the protonated PDP ligand, consistent with successful metalation ( Figure S6 ); for an in-depth discussion of the absorption profile of the ligand, we direct readers to a recent report from Milsmann and co-workers. 74 In the electronic absorption spectrum of 1b , these two intense bands are observed at 318 nm (ε = 14,006 M –1 cm –1– ) and 360 nm (ε = 12,215 M –1 cm –1 ). In addition, complex 1b also contains a lower-energy transition at 458 nm (ε = 6,203 M –1 cm –1– ), distinct from the electronic absorbance spectrum of H 2 Mes PDP Ph .…”