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

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1 H -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 [Zn 2 L H 2 Cl 2 ] complex and an ionic (H 2 L H ) 4 [ZnCl 4 ] 2 ·3H 2 O solid. Another ionic solid, (H 2 L 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 ( H 2 L 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) T 1 → S 0 phosphorescence of the enol form and (ii) anti-Kasha S 2 → S 0 fluorescence of the keto form, which if feasible due to the large S 2 -S 1 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 [Zn 2 L H 2 Cl 2 ] and (H 2 L H ) 4 [ZnCl 4 ] 2 ·3H 2 O . Despite both (H 2 L H ) 4 [ZnCl 4 ] 2 ·3H 2 O and (H 2 L H )Br have the same ( H 2 L H ) + cation in the structures, their emission properties differ significantly, whereas (H 2 L H )Br shows dual emission associated with two radiative decay channels: (i) S 1 → S 0 fluorescence and (ii) T 1 → S 0 phosphorescence, (H 2 L H ) 4 [ZnCl 4 ] 2 ·3H 2 O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H 2 L H )Br , which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H 2 L H )Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).