Dual excitation-wavelength-dependent luminescence and proton transfer in ESIPT-active zinc(II) complexes with 2-(1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine

Abstract

Dinuclear ESIPT-active complexes, [Zn 2 (HL) 2 X 4 ] (X = Cl, Br, I), were synthesized on the base of 2-(1H-pyrazol-1-yl)-4-(2-hydroxyphenyl)pyrimidine (HL). Along with an ESIPT site with a short O-H•••N hydrogen bond (O•••N 2.6 Å), the HL ligand molecule features an additional N,N-chelating site as a prerequisite for metal coordination. Such an architecture of the ligand molecule has served as 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, preserving the ESIPT site intact upon the complex formation. In the solid state, the ESIPT-active zinc(II) complexes demonstrate dual excitation-wavelength-dependent emission as the 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 (λ ex = 300 -400 nm) facilitates population of the S 1 state in the enol form, i.e. S 0 E → S 1 , followed by intersystem crossing S 1 → T 1 and subsequent phosphorescence T 1 E → S 0 . Conversely, low-energy photons (λ ex > 420 nm) selectively excite molecules in the keto form, S 0 K → S 1 , leading to S 1 K → S 0 fluorescence.2