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

Abstract

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HL^H), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn₂L^H₂Cl₂] complex and an ionic (H₂L^H)₄[ZnCl₄]₂·3H₂O solid. Another ionic solid, (H₂L^H)Br, was obtained from the solution of HL^H 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^H, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HL^H molecules and ionic (L^H)^- and (H₂L^H)⁺ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HL^H on the photoluminescence response and on altering the emission mechanism. The neutral HL^H 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^H is low. Protonation or deprotonation of the HL^H 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 [Zn₂L^H₂Cl₂] and (H₂L^H)₄[ZnCl₄]₂·3H₂O. Despite both (H₂L^H)₄[ZnCl₄]₂·3H₂O and (H₂L^H)Br have the same (H₂L^H)⁺ cation in the structures, their emission properties differ significantly, whereas (H₂L^H)Br shows dual emission associated with two radiative decay channels: (i) S₁ → S₀ fluorescence and (ii) T₁ → S₀ phosphorescence, (H₂L^H)₄[ZnCl₄]₂·3H₂O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H₂L^H)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H₂L^H)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).