A 1‐Hydroxy‐1H‐imidazole ESIPT Emitter Demonstrating anti‐Kasha Fluorescence and Direct Excitation of a Tautomeric Form

Abstract The ability of 1‐hydroxy‐1 H ‐imidazoles to exist in the form of two prototropic tautomers, the N‐hydroxy and the N‐oxide forms, can be utilized in the design of new types of ESIPT‐fluorophores (ESIPT=excited state intramolecular proton transfer). Here we report the first example of 1‐hydroxy‐1 H ‐imidazole‐based ESIPT‐fluorophores, 1‐hydroxy‐5‐methyl‐2,4‐di(pyridin‐2‐yl)‐1 H ‐imidazole ( HL ), featuring a short intramolecular hydrogen bond O−H⋅⋅⋅N (O⋅⋅⋅N 2.56 Å) as a pre‐requisite for ESIPT. The emission of HL originates from the anti‐Kasha S 2 →S 0 fluorescence in the N‐oxide form as a result of a large S 2 –S 1 energy gap slowing down the S 2 →S 1 internal conversion. Due to an energy barrier between the N‐hydroxy and N‐oxide forms in the ground state, the HL molecules can be trapped and photoexcited in the N‐oxide form leading to the Stokes shift of ca . 60 nm which is the smallest among known ESIPT‐fluorophores.