Luminescence of a Zinc(II) Complex with a Protonated 1‐Hydroxy‐1H‐imidazole ESIPT Ligand: Direct Excitation of a Tautomeric Form

The complex (HL)2[ZnCl4] (L=2‐(1‐hydroxy‐4,5‐dimethyl‐1H‐imidazol‐2‐yl)pyridine) featuring a short intramolecular hydrogen bond O−H⋅⋅⋅N (O⋅⋅⋅N 2.61 Å) in the HL+ cation is a rare example of protonated ESIPT‐emitters (ESIPT=excited state intramolecular proton transfer). Photochemistry of (HL)2[ZnCl4] is dominated by the ESIPT tautomerization in HL+. In the solid state, (HL)2[ZnCl4] fluoresces in the sky‐blue region (S1tautomeric→S0tautomeric, λmax=500 nm). In MeCN solution, two minima on the S1 potential energy curve lead to dual emission of (HL)2[ZnCl4] (S1normal→S0normal and S1tautomeric→S0tautomeric, λmax=368 and 417 nm). Energy barriers in the ground state allow trapping and the excitation of the tautomeric form of HL+. The complex (HL)2[ZnCl4] (L=2‐(1‐hydroxy‐4,5‐dimethyl‐1H‐imidazol‐2‐yl)pyridine) is a rare example of protonated ESIPT‐emitters. In the solid state, (HL)2[ZnCl4] fluoresces in the sky‐blue region. In MeCN solution, two minima on the S1 potential energy curve lead to dual emission of (HL)2[ZnCl4]. Energy barriers in the ground state allow trapping and the excitation of the tautomeric form of HL+.