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

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(+)center dot 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(+)center dot BF4- crystals as the ESIPT active intramolecular O-H center dot center dot center dot N= hydrogen-bonding conformation (Stokes shift approximate to 8500 cm(-1)). In contrast, the H1(+)center dot Cl-, H1(-)center dot Br-, and H1(+)center dot TFA(-) salts exhibit an ESIPT inert structure without any intramolecular O-H center dot center dot center dot N= hydrogen-bonding structure (Stokes shift approximate to 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+center dot center dot center dot X- to N center dot center dot center dot HX, which transforms the cationic H1(+) to neutral 1, and exhibits intense green fluorescence with a large Stokes shift.