Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch...

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HL), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [ZnLCl] complex and an ionic (HL)[ZnCl]·3HO solid. Another ionic solid, (HL)Br, was obtained from the solution of HL acidified with HBr. In both ionic solids, the H⁺ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HL, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HL molecules and ionic (L)^- and (HL)⁺ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HL on the photoluminescence response and on altering the emission mechanism. The neutral HL compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T₁ → S₀ phosphorescence of the enol form and (ii) anti-Kasha S₂ → S₀ fluorescence of the keto form, which if feasible due to the large S₂-S₁ energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HL is low. Protonation or deprotonation of the HL ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [ZnLCl] and (HL)[ZnCl]·3HO. Despite both (HL)[ZnCl]·3HO and (HL)Br have the same (HL)⁺ cation in the structures, their emission properties differ significantly, whereas (HL)Br shows dual emission associated with two radiative decay channels: (i) S₁ → S₀ fluorescence and (ii) T₁ → S₀ phosphorescence, (HL)[ZnCl]·3HO exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (HL)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (HL)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).