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.

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 In imidazole-based ESIPT-fluorophores, the free imidazolic N 3 atom, being incorporated into the ESIPT site, cannot be used for metal binding without the deprotonation of the adjacent 2-hydroxyphenyl group (Scheme 3, top).…”

“…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 In imidazole-based ESIPT-fluorophores, the free imidazolic N 3 atom, being incorporated into the ESIPT site, cannot be used for metal binding without the deprotonation of the adjacent 2-hydroxyphenyl group (Scheme 3, top). However, if we switch the role of the imidazole cycle to that of proton donor by introducing a proton-donating group to the position N 1 and a proton-accepting group to the position C 2 or C 5 , the lone pair of the N 3 atom can be used for binding metal ions (Scheme 3), especially when assisted by other donor groups.…”

Tuning the AIE Properties of Di-tert-Butyl-diphenyldibenzofulvene Derivatives

“…Summarizing, (i) the quantum yield values obtained for the 2,7- dtb -DPBF derivatives in thin films are practically identical to the values obtained for the 3,6- dbt -DPBF derivatives and less emissive than the nonsubstituted DPBF , whereas in (ii) the solid state, powder, the 3,6- dbt -DPBFs are more emissive; in addition, 2,7- dtb -DPBF (monomer) show a blue shift absorption and red shift emission relative to the 3,6- dtb -DPBF.…”

“…proposed the RACI model, suggesting that the suppression of nonradiative decay in crystals is triggered by the limited accessibility of conical intersections . Upon aggregation, constraining intramolecular motion is expected to significantly affect the internal conversion from the first singlet (S 1 ) state to the conical intersection (CI) on the potential energy surface (PES). , In such cases, the more generic term “solid-state luminescence enhancement” could be applicable. , This simple concept opens new possibilities for the design of new luminescent materials in various areas, particularly in the field of organic light-emitting devices (OLED) and optical sensors. − Previous studies indicate that the diphenyldibenzofulvene (DPBF) structure is, in acetonitrile, weakly fluorescent. It is insoluble in water and forms aggregates in acetonitrile/water mixtures, with an increase in the fluorescence quantum yield. ,,, In recent studies, we have demonstrated that the tert -butyl group plays a significant role in shaping the AIE properties of 3,6- dtb -diphenyldibenzofulvene derivatives (3,6- dtb -DPBF’s) .…”

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][103][104][105][106][107] The molecular skeleton of such molecules is shaped by the proton-donating 2-hydroxyphenyl group introduced in the position 2 of the protonaccepting 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.…”