Nitrostyrene‐Modified 2‐(2‐Hydroxyphenyl)benzothiazole: Enol‐Emission Solvatochromism by ESICT‐ESIPT and Aggregation‐Induced Emission Enhancement

Abstract

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) are two competitive reactions that occur in the excited states of organic dyes that contain intramolecular hydrogen-bonds and electron acceptors and donors. Determining the mechanisms of these processes is key to understanding their multiple emission features, as the manner in which these processes interact can be modulated by modifying the dye structure. In addition, donor-π-acceptor (D-π-A) molecules often suffer from aggregation-induced quenching. Herein, we report the synthesis of three nitrophenyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives, HBT-s-NO , HBT-d-NO , and HBT-t-NO , which have C-C, C=C, and C≡C bonds between their HBT and nitrophenyl moieties, respectively. Compared with the enol emissions from HBT-s-NO and HBT-t-NO , that from HBT-d-NO exhibits outstanding solvatochromism owing to consecutive ESICT-ESIPT. In addition, X-ray diffraction reveals that despite the highly planar and polar nature of HBT-d-NO , which is strongly H-aggregated, it exhibits highly efficient fluorescence. Hence, this study provides new insight into the design of ESICT/ESIPT-coupled systems and for engendering planar dipolar molecules with excellent emission properties in the solid state.

An ESIPT-based fluorescent switch with AIEE, solvatochromism, mechanochromism and photochromism

“…39 Because of the transient changes involved in the ESIPT process, ESIPT emission is quite easily affected by external stimuli, resulting in the change of fluorescence properties. [40][41][42][43][44] Therefore, making good use of the ESIPT process is an effective way to fabricate smart luminescent materials that are sensitive to external stimuli such as solvent polarity, pH, heat, mechanical force and pressure. [45][46][47][48][49][50] The most studied ESIPT system consists of the derivatives of 2-(2 0 -hydroxyphenyl)benzimidazole (HBI), 2-(2 0 -hydroxyphenyl)benzoxazole (HBO) and 2-(2 0 -hydroxyphenyl)benzothiazole (HBT), where the hydroxyl linked to its conjugated group acts as the proton donor and the imidazole/oxazole/thiazole ring acts as the proton acceptor.…”

ESIPT‐Active Pyrene‐imidazole Fluorophores: GSIPT, Dual Solid‐ and Solution‐State Emission plus Counter‐Intuitive Crystal Packing**

“…All the compounds are obtained in good yields (70-85 %). The compounds have been characterized using 1 H/ 13 C NMR and high-resolution mass spectrometry (HRMS).…”

“…To further support the existence of the keto-form in the solid state, we recorded the Fourier transformation infrared spectroscopy (FT-IR) and solid state (CP-MAS) 13 C NMR of the ESPIT-active compounds. The FT-IR spectra of the solids of 3 and 4 showed stretching frequencies at 1627 cm À 1 and 1671 cm À 1 respectively (Figure S10), corresponding to the carbonyl (À C=O) [41] indicating the existence of keto-isomer.…”

“…Among the large variety of developed molecules, the majority of the compounds were based on imidazoles, thiazoles, oxazoles, Schiff base and azo-units. [4,2,[13][14][15][16][17][18][19][20] Interestingly, for the ESIPT-fluorophores, tautomerism occurs only in the excited state but not in the ground state. This can be attributed to the instability of the keto-isomer as it loses the aromaticity of the aryl unit in the ground state.…”

Progresses in Reactive Fluorescent Probes with Fused Aggregation- Induced Emission (AIE) and Excited State Intramolecular Proton Transfer (ESIPT) Structures

“…性质的分子会产生双荧光发射峰, 即激发态的烯醇式结 构引起的短波长发射(烯醇式发射)和激发态的酮式结构 引起的长波长发射(酮式发射). 近年来, 由于 ESIPT 荧 光团具有较大的斯托克斯位移和双重发射的特点, 将其 用在荧光探针的结构设计上, 很大程度避免了激发光谱 和发射光谱的重叠, 提高了荧光分子的灵敏度, 从而得 到了广泛的应用 [10][11][12][13] . [14] 首次 发现了一系列化合物, 这类化合物的性质与传统的聚集 诱导荧光淬灭效应恰好相反, 在稀溶液中没有荧光或者 荧光微弱, 但随着向溶液中加入不良溶剂后, 体系变浑 浊, 分子处于聚集态或固态时, 产生强烈荧光, 他们把 这 一 现 象 定 义 为 聚 集 诱 导 发 光 (Aggregation-induced emission, AIE) [15][16][17][18] .…”