Recent Trends in the Design, Synthesis, Spectroscopic Behavior, and Applications of Benzazole-Based Molecules with Solid-State Luminescence Enhancement Properties

Abstract

Molecules that exhibit solid-state luminescence enhancement, i.e. the rare property to be more strongly emissive in the solid state than in solution, find an increasing number of applications in the fields of optoelectronic and nanophotonic devices, sensors, security papers, imaging and theranostics. Benzazole (BZ) heterocycles are of particular value in this context. The simple enlargement of their πelectron system using a -C=C-Ar, or -N=C-Ar moiety is enough for intrinsic SLE properties to appear. Their association with a variety of polyaromatic motifs leads to SLE-active molecules that frequently display attractive electroluminescent properties and are sensitive to mechanical stimuli. The excitedstate intramolecular proton transfer (ESIPT) process that takes place in some hydroxy derivatives reinforces the SLE effect and allows the development of new sensors based on a protection/deprotection strategy. BZ may also be incorporated into frameworks that are prototypical aggregation-induced enhancement (AIE) luminogens, such as the popular tetraphenylethene (TPE), leading to materials with excellent optical and electroluminescent performance. This review encompasses the various ways to use benzazole units in SLE systems. It underlines the significant progresses recently made in the understanding of the photophysical mechanisms involved. A brief overview of the synthesis shows that BZ units are robust building blocks, easily incorporated into a variety of structures. Generally speaking, we try to show how advantage may be taken from these small heterocycles for the design of increasingly efficient luminescent materials.

Aggregation‐Induced Emission Leading to White Light Emission in Diphenylbenzofulvene Derivatives

“…Organic fluorescence molecules that are also solidstate enhanced emitters, i. e., that are more strongly emissive in the solid state than in solution, present a myriad of applications in the fields of optoelectronic and nanophotonic devices, organic solar cells, sensors, security papers, biological imaging and theranostics. [9] Despite successful examples of solid-state emissive materials based on AIE-active cores, it is still challenging to design AIE-active fluorophores with tunable solid-state emission covering the whole visible light spectrum. [10] According to Kasha's rule, photons can only be emitted from the lowest singlet or triplet excited states and since white light emission (WLE) covers 400-800 nm, it is very difficult to obtain WLE from a single molecule.…”

“…A recent work by Zhao and co‐workers [8] provided structure‐property insights on the photophysical properties of several DPBF derivatives, showing that the twist angle around the C=C bond is a key molecular factor controlling the solid‐state fluorescence properties of these luminogens. Organic fluorescence molecules that are also solid‐state enhanced emitters, i. e., that are more strongly emissive in the solid state than in solution, present a myriad of applications in the fields of optoelectronic and nanophotonic devices, organic solar cells, sensors, security papers, biological imaging and theranostics [9] . Despite successful examples of solid‐state emissive materials based on AIE‐active cores, it is still challenging to design AIE‐active fluorophores with tunable solid‐state emission covering the whole visible light spectrum [10] …”

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(ii) complexes with 1-hydroxy-1H-imidazole-based ligands

“…This family includes the well-known ESIPT-platform 2′-(hydroxyphenyl)benzimidazole (HBI) which uses a 2-hydroxyphenyl group as a proton donor and a free N azolic atom as a proton acceptor (Scheme 3). 107,108 Replacing the NH group in the benzimidazole moiety of HBI with other heteroatoms leads to HBI analogues, 2-(2′-hydroxyphenyl)benzothiazoles (HBT) and 2′-(hydroxyphenyl)benzoxazole (HBO). 107,108…”

“…107,108 Replacing the NH group in the benzimidazole moiety of HBI with other heteroatoms leads to HBI analogues, 2-(2′-hydroxyphenyl)benzothiazoles (HBT) and 2′-(hydroxyphenyl)benzoxazole (HBO). 107,108…”

First 1-hydroxy-1H-imidazole-based ESIPT emitter with an O–H⋯O intramolecular hydrogen bond: ESIPT-triggered TICT and speciation in solution

“…Among ESIPT-capable molecules, imidazole-based compounds form an archetypical family of fluorophores. 102–107 The molecular skeleton of such molecules is shaped by the proton-donating 2-hydroxyphenyl group introduced in the position 2 of the proton-accepting imidazole or benzimidazole moiety (Scheme 2). As a result, a short intramolecular hydrogen bond as a pre-requisite for ESIPT is formed between the OH group and the N 3 atom of the imidazole ring.…”