Polyanils and Polyboranils: Synthesis, Optical Properties, and Aggregation-Induced Emission

Modulation of fluorescence and phosphorescence of organoboron compounds from ortho-substituted phenolic Schiff bases by structural modification

Light emission from organoboron compounds of Schiff bases is found to depend strongly on their chemical structure. Two of these compounds (OB1 and OB2), which contain a benzene ring between the Schiff base moieties, exhibit weak fluorescence in methanol, with marked viscosity dependence. Fluorescence lifetimes of these compounds are in picosecond timescale, as determined by femtosecond optical gating (FOG). A significant enhancement in fluorescence intensity and lifetime is observed at 77 K, indicating the operation of an activated nonradiative process. Using fluorescence lifetime imaging microscopy (FLIM), OB1 and OB2 are shown to be potential membrane probes. The third (OB3), which is devoid of this benzene ring, exhibits relatively stronger fluorescence with nanosecond lifetimes at room temperature. No viscosity dependence is observed in this case. The emission spectrum at 77 K is markedly more intense and exhibits an additional red shifted structured feature, which persists for a few seconds. Hence, OB3 seems to have greater promise not only as fluorescent probe but also for light harvesting. The marked improvement of the light emission properties of OB3 compared with OB1 and OB2 is likely to serve as a pointer for the design of Schiff base-derived organoboron luminophores with diverse potential applications.

Interplay between Dual-State and Aggregation-Induced Emission with ESIPT Scaffolds Containing Triphenylamine Substituents: Experimental and Theoretical Studies

We detail the synthesis of a series of fluorophores containing triphenylamine derivatives along with their photophysical, electrochemical, and electronic structure properties. These compounds include molecular structures derived from imino-phenol (anil) and hydroxybenzoxazole scaffolds originating from similar salicylaldehyde derivatives and display excited-state intramolecular proton transfer. We show that depending on the nature of the π-conjugated scaffold, different photophysical processes are observed: aggregation-induced emission or dual-state emission, with a modulation of the fluorescence color and redox properties. The photophysical properties are further rationalized with the help of ab initio calculations.

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

Heteroaryl-Substituted Bis-Anils: Aggregation-Induced Emission (AIE) Derivatives with Tunable ESIPT Emission Color and pH Sensitivity

The two-step synthesis, structural, and photophysical properties of a series of heteroaryl-substituted bis-anil derivatives presenting aggregation-induced emission (AIE) coupled with an excited-state intramolecular proton transfer (ESIPT) process is described. The fluorescence color of the aggregates can be fine tuned by changing the electronic nature of the peripheral substitution, leading to a wide range of emission wavelengths (from green to the near infra-red). Moreover, upon introduction of strong electron-withdrawing groups such as cyano (CN), a competition between ESIPT and deprotonation is observed leading to the emission of the anionic species at low water percentage. This observation led to the synthesis of an additional mixed AIE fluorophore, functionalized by methoxy groups on one side and cyano groups on the other side. Upon addition of water, this dye displays first anionic emission, followed by typical AIE/ESIPT red fluorescence upon formation of the aggregates. TD-DFT calculations on selected AIE dyes were performed to rationalize the nature of the emissive transitions in these derivatives.

Boranils: Versatile Multifunctional Organic Fluorophores for Innovative Applications

Multifunctional stimuli-responsive fluorophores showing bright environment-sensitive emissions have fueled intense research due to their innovative applications in the fields of biotechnologies, optoelectronics, and materials. A strong structural diversity is observed among molecular materials, which has been enriched over the years with a growing responsiveness to stimuli. Boron dipyrromethene (BODIPY) dyes have long been the flagship of emissive boron complexes due to their outstanding properties until a decade ago when analogues based on N^O, N^N, or N^C π-conjugated chelates emerged. The finality of developing borate dyes was to compensate for BODIPYs’ lack of solid-state fluorescence and small Stokes shifts while keeping their excellent optical properties in solution. Among them, the borate complexes based on a salicylaldimine ligand, called by the acronym boranils appear as the most promising, owing to their facile synthesis and dual-state emission properties. Boranil dyes have proven to be good alternatives to BODIPY dyes and have been applied in applications such as bioimaging, bioconjugation, and detection of biosubstrates. Meanwhile, ab initio calculations have rationalized experimental results and provided insightful feedback for future designs. This review article aims at providing a concise yet representative overview of the chemistry around the boranil core with the subsequent applications.

Carbazole-modified thiazolo[3,2-c][1,3,5,2]oxadiazaborinines exhibiting aggregation-induced emission and mechanofluorochromism

Two highly emissive carbazole-containing thiazole-fused oxadiazaborinines were designed and synthesized. These N,O-chelated organoboron dyes displayed large Stokes shifts and remarkable solvatofluorochromism in solutions, as well as good thermal stability and comparatively high photoluminescence quantum yields (up to 34%) in the solid state. The presence of a carbazole donor unit, linked with the oxadiazaborinine acceptor via a phenyl linker, restricted intramolecular rotation, leading to enhanced aggregation-induced emission properties of the compounds: in THF/water mixtures with a large water percentage, they demonstrated the formation of emissive nanoaggregates with an average size of 79 and 89 nm for complexes 2 and 3, respectively. The introduction of bulky tert-butyl groups attached to the carbazole moiety induced significant mechanofluorochromic properties of the compounds.

Synthesis and Tunable Optical Properties of C,N-Chelated Borate Luminophores Derived from Potassium Acyltrifluoroborates

A new class of borate luminophores has been synthesized by a simple two-step reaction using potassium acyltrifluoroborates (KATs) as starting materials. The hydrazones obtained from reactions between KATs and 2-hydrazinopyridines followed by a cyclization resulted in the unprecedented formation of C,N-chelated six-membered bora-heterocycles. Under consideration of the results of DFT and TD-DFT calculations, four luminophores based on such bora-heterocycles are designed and synthesized, which exhibit a tunable fluorescence range from blue to red in the solid state. Moreover, one of the luminophores exhibits mechanofluorochromism from blue to yellow/green. As a result of the aforementioned mechanochromism of one of these luminophores, white-color emission was achieved by simply mixing the four luminophores.

Disaggregation-induced ESIPT: a novel approach towards development of sensors for hyperglycemic condition

An oxazolonapthoimidazo[1,2-a]pyridine-based fluorescence probe ONIP1 was designed and synthesized via multicomponent reaction. ONIP1 was able to distinguish human serum albumin (HSA) from and glycated-HSA via modulation of AIEE- and ESIPT-based dual channel emission properties.

Synthesis and fluorescent properties of boroisoquinolines, a new family of fluorophores

First representatives of a new family of isoquinolines, so called boroisoquinolines, were synthesized and characterized. The synthesis was based on the insertion of the difluoroboranyl group into the 1-methylidene-3,4-dihydroisoquinoline core. The optimization of the 2-difluoroboranyl-3,4-dihydroisoquinoline-1(2H)-ylidene core led to efficient fluorescence in a range of 400-600 nm with outstanding (>100 nm) Stokes shifts. The compounds might be suitable for reversible or irreversible labelling of proteins, particularly the cannabinoid receptor CB₂.

The effect of conformational isomerism on the optical properties of bis(8-oxyquinolato) diboron complexes with a 2,2'-biphenyl backbone

A fluorescent bis(8-oxyquinolato) diborinic complex with a central 2,2'-biphenyl backbone 1 and its octafluoro analogue 2 were synthesized to study the optical-structural relationship of sterically encumbered molecules featuring close intramolecular π-stacking interactions involving chromophore units. The crystal structure of 1 revealed a unique π-stacked arrangement of two pendant phenyl groups and two 8-oxyquinolato ligands (Q) located in the inner part of the complex. Unlike 1, the closely related complex 2 features conformational isomerism, and two major forms, namely 2-syn and 2-anti, are observed in solution to a varying extent depending on the solvent polarity. Form 2-syn, a geometrical analogue of 1, is preferable in polar solutions, whereas its rotational isomer 2-anti featuring π-stacking interactions between the terminal phenyl group and Q ligand dominates in benzene and chloromethane solutions. The observed conformational equilibria strongly affect the optical properties of the system, specifically leading to a significant increase of the quantum yield of emission (from 22% in MeCN to 38% in benzene) accompanied by a bathochromic shift (Δλ = 10 nm) of absorption and hypsochromic shifts (Δλ = -8 nm) of emission spectra with decreasing solvent polarity. This effect was ascribed to the variation in frontier orbital distributions.

Four-coordinate triarylborane synthesis via cascade B-Cl/C-B cross-metathesis and C-H bond borylation

To develop a simple and efficient synthetic method for four-coordinate triarylboranes, we herein describe a tandem highly selective B-Cl/C-B cross-metathesis of two of the same or different arylboranes and C-H bond borylation to synthesize four-coordinate triarylboranes with a broad substrate scope. By switching substituent groups of the target molecules, different emission wavelengths can be achieved from 467 nm to 583 nm with aggregation-induced emission (AIE) properties.

Nonconjugated Fluorescent Molecular Cages of Trinuclear Fluoroborate Complexes with Salicylaldehyde-Based Schiff Base Ligands

Fluorescent organic materials are commonly π-conjugated planar molecules. In the present work, however, we report a novel class of nonconjugated fluorescent molecular cages of trinuclear fluoroborate complexes (nine samples) with salicylaldehyde-based Schiff base ligands. Owing to the stress from lone pair electrons of N atom in the triethylamine bridge, these B(III) complexes exhibit unusual enantiomers with a tripodlike side-single-opening structure. They emit blue, green, and red emission with large Stokes shifts (up to 159 nm) and high fluorescence quantum yields in both solution (up to 0.24) and solid state (up to 0.25), which might contribute to their strong intramolecular hydrogen bonds and weak intermolecular and intramolecular π-π interactions. Combining their advantages of nonconjugation and biocompatibility, these flexible complexes have potential applications in living cell imaging and anion hosts. We have examined the inherent relationships between their chemical structures and emission properties and afforded a new stage for the design of nonconjugated fluorescent fluoroborate complexes.