Dual emission of ESIPT-capable 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)pyrimidines: interplay of fluorescence and phosphorescence

Abstract

ESIPT-capable pyrimidine-based compounds featuring short O–H⋯N intramolecular hydrogen bonds, 2-(2-hydroxyphenyl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL¹) and 2-(2-hydroxyphenyl)-4-(3,5-diphenyl-1H-pyrazol-1-yl)-6-methylpyrimidine (HL²), were synthesized by the condensation of 4-hydrazinyl-2-(2-hydroxyphenyl)-6-methylpyrimidine with acetylacetone and dibenzoylmethane. In solution, HL¹ and HL² do not exhibit luminescence due to efficient non-radiative deactivation through S₀/S₁ conical intersection of non-planar nature, whereas in the solid state they show dual emission associated with phosphorescence at 500–700 nm and fluorescence at 400–500 nm. Phosphorescence is not related to ESIPT and occurs in the enol form (E) in accordance with the Kasha rule, T^E₁ → S^E₀. In contrast, fluorescence follows the proton transfer process and corresponds to the anti-Kasha S^K₂ → S^K₀ transition in the tautomeric keto form (K), which is caused by an extraordinarily large S₂–S₁ energy gap (ca. 0.9 eV) and, consequently, low S₂ → S₁ internal conversion rate. The fluorescence/phosphorescence intensity ratio is temperature-dependent for HL¹, resulting in the emission color change from yellow-green at 77 K to blue-green at 300 K, while HL² glows in the orange region in the whole temperature range owing to the predominant contribution of phosphorescence.