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

Abstract

AbstractA 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.