Solid‐State Fluorescence of Excited‐State Cation–Anion Intermolecular Proton Transfer in 2‐(2‐Hydroxypyridyl)benzothiazole

Abstract A basic pyridyl unit is introduced to the excited‐state intramolecular proton transfer (ESIPT) of 2‐(2‐hydroxyphenyl)benzothiazole fluorescent molecule, forming 2‐(2‐hydroxypyridyl)benzothiazole (1). The formation of the proton‐transferred H1⁺•X⁻ salts is confirmed in X⁻ = Cl⁻, Br⁻, BF₄⁻, and CF₃COO⁻ (trifluoroacetate, TFA⁻) wherein the highly acidic HX species are easily transferred to the pyridyl unit of 1. Normal ESIPT fluorescence is observed in H1⁺•BF₄⁻ crystals as the ESIPT active intramolecular O−H•••N= hydrogen‐bonding conformation (Stokes shift ≈ 8500 cm⁻¹). In contrast, the H1⁺•Cl⁻, H1⁻•Br⁻, and H1⁺•TFA⁻ salts exhibit an ESIPT inert structure without any intramolecular O−H•••N= hydrogen‐bonding structure (Stokes shift ≈ 9500 cm⁻¹), which could not be sufficiently explained by normal ESIPT fluorescence. The excited‐state monoprotonated H1⁺ exhibits an intermolecular cation–anion proton transfer from N−H⁺•••X⁻ to N•••HX, which transforms the cationic H1⁺ to neutral 1, and exhibits intense green fluorescence with a large Stokes shift.