Coordination-driven molecular switch on the base of an ESIPT-capable pyrimidine ligand: Synthesis, fine-tuning of emission by the halide anion and theoretical studies

ESIPT-based materials (ESIPT = Excited State Intramolecular Proton Transfer) find diverse applications in optoelectronics and biomedicine owing to the peculiarities of their luminescence properties. Here, an ESIPT-capable compound 2-(3,5-dimethyl-1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL^4,2,Me) featuring a short O–H⋅⋅⋅N intramolecular hydrogen bond and two N,N-sites for metal binding has been synthesized. HL^4,2,Me is the first reported molecule which can act as an ESIPT molecular switch triggered by metal ion coordination without its deprotonation. Drastic changes in the HL^4,2,Me conformation in the [Zn(HL^4,2,Me)X₂] (X = Cl, Br, I) complexes significantly alter the photoluminescence response compared to the free ligand. In the solid state, HL^4,2,Me exhibits barrierless ESIPT and large Stokes-shifted yellow-orange emission due to the interplay of anti-Kasha S₂ → S₀ fluorescence and Kasha-like T₁ → S₀ phosphorescence radiative channels. The violation of Kasha’s rule for HL^4,2,Me is justified by an extraordinarily large S₂ – S₁ energy gap (ca. 0.9 eV), slowing down the rate of S₂ → S₁ internal conversion. The photoluminescence behavior of the ESIPT–incapable zinc(II) coordination compounds strongly depends on the halide anion: the chlorido complex exhibits only fluorescence, the bromido complex displays a minor phosphorescence channel in addition to a major fluorescence channel, while the iodido complex exhibits predominantly phosphorescence. As a result, the emission color of the [Zn(HL^4,2,Me)X₂] complexes changes gradually from blue for X = Cl to orange for X = I, providing a platform for the fine-tuning of emission by the halide anion.