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

• Published in: Advanced optical materials 2022-06, Vol.10 (11), p.n/a
• Main Authors: Takahashi, Keigo, Sambe, Kohei, Kasahara, Yotaro, Takeda, Takashi, Hoshino, Norihisa, Sakai, Ken‐ichi, Akutagawa, Tomoyuki
• Format: Article
• Language: English
• Subjects: Anions; Cations; dielectrics; excited‐state intramolecular proton transfer; Fluorescence; hydrogen‐bonds; Materials science; Optics; Protons; sensing
• ISSN: 2195-1071; 2195-1071
• DOI: 10.1002/adom.202200134

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. A basic pyridyl unit is introduced to the excited‐state intramolecular proton transfer of 2‐(2‐hydroxyphenyl)benzothiazole fluorescent molecule. The excited‐state monoprotonated species exhibit an intermolecular cation–anion proton transfer from N−H+•••X− to N•••HX, which transforms the cationic to neutral states, and exhibit intense green fluorescence with a large Stokes shift.