Dual excitation-wavelength-dependent luminescence and proton transfer in ESIPT-active zinc(<scp>ii</scp>) complexes with 2-(1<i>H</i>-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine

Dinuclear ESIPT-active complexes, [Zn2(HL)2X4] (X = Cl, Br, I), based on 2-(1<i>H</i>-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL), have been synthesized. Along with an ESIPT site with a short O-H⋯N hydrogen bond (O⋯N distance: 2.6 Å), the HL ligand molecule features an additional N,N-chelating site as a prerequisite for metal coordination. This dual-site architecture of the ligand molecule has become a key precondition for the design of ESIPT-active metal complexes, as it ensures the coordination of metal ions to the N,N-chelating site while preserving the integrity of the ESIPT site during complex formation. In the solid state, the ESIPT-active zinc(II) complexes exhibit dual excitation-wavelength-dependent emission as a result of the interplay of a phosphorescence band in the orange region and a fluorescence band in the green region. According to TDDFT calculations, the dependence of photoluminescence on excitation energy stems from the coexistence of two minima on the potential energy surface of the S₀ state, corresponding to enol (E) and keto (K) forms. Excitation with high-energy photons (<i>λ</i>_ex = 300-400 nm) facilitates population of the S₁ state in the enol form, <i>i.e.</i>, S₀^E → S₁, followed by intersystem crossing S₁ → T₁ and subsequent phosphorescence T₁^E → S₀. Conversely, low-energy photons (<i>λ</i>_ex > 420 nm) selectively excite molecules in the keto form, S₀^K → S₁, leading to S₁^K → S₀ fluorescence.