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.

ESIPT geometrical isomers with distinct mechanofluorochromism and intra/intermolecular H-bonding controlled tunable fluorescence

“…Most of the organic fluorescent polymorphs undergo structural transformation (from a less stable to a more stable structure) upon applying external stimuli such as heating. 38,40–45 Organic fluorescent polymorphs exhibiting distinct stimuli-induced fluorescence switching without structural transformation are rarely reported. 46–48 Direct heating (110 °C) of 3 -Y / 3 -R also did not show any fluorescence colour change.…”

“…Most of the organic fluorescent polymorphs undergo structural transformation (from a less stable to a more stable structure) upon applying external stimuli such as heating. 38,[40][41][42][43][44][45] Fig. 6 HOMO-LUMO of 1-4.…”

“…37 Acid-sensitive fluorescence tuning and amino acid sensing were achieved with pyridinefunctionalized 2-(2-hydroxypyridyl)benzothiazole. 38,39 Thus, there is a strong interest in designing and synthesizing new ESIPT molecules to establish the structure-property relationship and obtain new functional fluorescence materials.…”

Interplay of Dual-Proton Transfer Relay to Achieve Full-Color Panel Luminescence in Excited-State Intramolecular Proton Transfer (ESIPT) Fluorophores

“…The design and study of organic molecules exhibiting excited-state intramolecular proton transfer (ESIPT) properties have recently gained much momentum, notably because such fluorophores have a wide variety of potential applications. − Typically, ESIPT is a four-level photochemical cycle (E → E* → K* → K) involving photoexcitation of a molecule in the ground enol-state (E) to an excited enol*-state (E*), followed by fast tautomerization ( k ESIPT ≈ 10 12 s –1 ), leading to the excited keto*-state (K*), which then decays radiatively to its ground keto-state (K). − This usually results in large Stokes shifts and dual emission when the radiative decay occurs from both E* and K* states. − In the past few years, important efforts have been made to merge ESIPT properties with useful photophysical phenomena. Hence, diverse molecular designs allowing the combination of ESIPT with thermally activated fluorescence, phosphorescence or long persistent luminescence, aggregation-induced emission, amplified spontaneous emission, as well as multiple stimuli-responsive emissions have been reported, leading to the potential in different fields including sensing, displaying, lasing, imaging, and so forth. − …”

Hexagon and network structured organic geometrical isomers with distinct intramolecular H-bonding and stimuli-induced self-reversible fluorescence switching

“…57 Amino acid sensing and fluorescence tuning are achieved using ESIPT molecules by introducing a pyridine functionality in 2-(2hydroxypyridyl)benzothiazole. 58 The intramolecular H-bonding remains intact and the change of conformation/ intermolecular interaction caused fluorescence modulation. Hence, there is a strong interest in studying the effect of additional functional groups in ESIPT molecules on the structural assembly and fluorescence properties.…”