A borane-bithiophene-BODIPY triad: intriguing tricolor emission and selective fluorescence response towards fluoride ions

The Advent of Bodipy-based Chemosensors for Sensing Fluoride Ions: A Literature Review

The detection of fluoride ions in water and other sources is crucial because they can harm human health if they exceed the safe limit of 1-1.5 ppm. BODIPY (boron dipyrromethene) dyes are promising fluorophores for chemosensors, and their design and modification have attracted a lot of attention. Their advantages include visible light excitation and emission, high molar absorption coefficients (ε) and fluorescence quantum yields [ϕ (λ)], and flexible scaffold manipulation for various applications. In this article, we review the progress of BODIPY-based sensors for fluoride ions from the early 2000s to the present. We focus on the different scaffold modifications of the sensors and their corresponding responses, as well as the underlying photophysical mechanisms and potential uses of each sensor.

Crystallization induced room-temperature phosphorescence and chiral photoluminescence properties of phosphoramides

We report the design and synthesis of a series of room temperature phosphorescent phosphoramides TPTZPO, TPTZPS, and TPTZPSe with a donor (phenothiazine)-acceptor (P = X, X = O, S, and Se) architecture. All the compounds show structureless fluorescence with a nanosecond lifetime in dilute solutions. However, these compounds show dual fluorescence and room temperature phosphorescence (RTP) in the solid state. Both the intensity and energy of luminescence depend on the heteroatom attached to the phosphorus center. For example, compound TPTZPO with the P[double bond, length as m-dash]O unit exhibits fluorescence at a higher energy region than TPTZPS and TPTZPSe with the P[double bond, length as m-dash]S and P[double bond, length as m-dash]Se groups, respectively. Crystalline samples of TPTZPO, TPTZPS, and TPTZPSe show stronger RTP than the amorphous powder of respective compounds. Detailed steady-state, time-resolved photoluminescence and computational studies established that the 3n-π* state dominated by the phenothiazine moiety is the emissive state of these compounds. Although TPTZPS and TPTZPSe crystallized in the chiral space group, only TPTZPSe showed chiroptical properties in the solid state. The luminescence dissymmetry factor (g lum) value of TPTZPS is small and below the detection limit, and a CPL spectrum could not be observed for this compound.

Synthesis and Characterization of Far-Red Emissive Boron-Based Triads Showing Large Stokes Shifts: Optical, TRANES, and Electrochemical Studies

Herein, we report the design and synthesis of far-red emissive boryl-thiophene-BODIPY triads 1-3. The π-conjugation length and electronic communication between borane and BODIPY moieties are tuned by judiciously varying the size of the oligothiophene spacer in these triads (1, terthiophene; 2, quarterthiophene; and 3, pentathiophene). Conjugates 1-3 showed intriguing triple emissions in the blue to far-red regions. Detailed optical, time-resolved decay kinetics, time-resolved area-normalized emission spectra (TRANES), fluoride binding, and computational studies suggest that the multiple emissions in these triads are due to an inefficient transfer of energy from the boryl-oligothiophene to the BODIPY unit. In addition, all of the conjugates showed a ratiometric fluorescence response to fluoride ions.

Synthesis, Structure, Spectral, and Anion Sensing Studies of an Aromatic meso-Fused Boron(III) Benzitriphyrin(2.1.1) Complex

An aromatic tricoordinated organo B(III) complex of benzitriphyrin(2.1.1) was synthesized by treating the nonaromatic phlorin analogue of meso-fused benzitriphyrin(2.1.1) with BCl₃ in triethylamine/toluene and refluxing for 2 h. The X-ray structure revealed that B(III) was in a trigonal-planar geometry and was coordinated to two nitrogen atoms and one carbon. Insertion of the small B(III) ion into the meso-fused phlorin analogue resulted in transforming the nonaromatic phlorin analogue to an aromatic B(III) benzitriphyrin(2.1.1) complex. Spectral and electrochemical studies supported the aromatic nature of the B(III) complex of benzitriphyrin(2.1.1). Theoretical studies supported the major contribution of the 18π delocalization pathway toward the aromatic nature of the B(III) meso-fused benzitriphyrin(2.1.1) in comparison to the 22π delocalization pathway. The B(III) benzitriphyrin(2.1.1) complex acts as a specific colorimetric and fluorogenic F^-/CN^- ion sensor.

π-Conjugated Materials Derived From Boron-Chalcogenophene Combination. A Brief Description of Synthetic Routes and Optoelectronic Applications

Functional materials composed of Boron-chalcogenophene conjugates have emerged as promising ensemble featuring commendable optoelectronic properties. This review describes the categories, synthetic routes and optoelectronic applications of a range of boron-chalcogenophene conjugates. Conjugation and linking of different types of tri- and tetra-coordinated boron moieties with chalcogenophenes have remained an important strategy for constructing a range of functional materials. Synthetic protocols have been devised to efficiently prepare such chemically robust conjugates, often exhibiting a myriad of photophysical properties, redox capabilities and also solid-state behaviors. Tin-boron and silicon-boron exchange protocols have been efficiently adapted to access these boron-chalcogenophenes. Few other commonly used methods namely, hydroboration of alkynes as well as electrophilic borylations are also mentioned. The chemical and electronic properties of such boron-chalcogenophene conjugates are directly influenced by the strong Lewis acid character of trivalent boranes which can further alter the intra- and inter- molecular Lewis acid-base interactions. Apart from the synthetic protocols, recent advances in the application of these boron-chalcogenophene conjugates towards analyte sensing, organic electronics, molecular switches and several other aspects will be discussed in this review.

Effect of the Molecular Conformation on Excitation Energy Transfer in Conformationally Constrained Boryl-BODIPY Dyads

We studied the dual emission characteristics of a series of boryl-BODIPYs (1-6) comprised of triarylborane (TAB) as an energy donor and BODIPY as an energy acceptor. The molecular conformations of dyads 1-6 were systematically tuned by judiciously changing the spacer that bridged the boryl and BODIPY moieties. Frontier molecular orbitals (FMOs) are localized in 3, 4, and 6 with a twisted molecular conformation. In contrast, FMOs are significantly delocalized in 1, 2, and 5 with the least-twisted molecular conformation. Dyads 1-6 showed dual emission features when they were excited at the TAB-dominated absorption band. However, the ratio between the two emission bands in 1-6 significantly varied depending on the molecular conformations. Systematic photoluminescence (PL) studies (both steady-state and time-resolved PL) together with computational, crystal structure, and anion binding studies established that the frustrated excited-state energy transfer from borane to BODIPY is the cause of the dual emission features in these molecular dyads. These studies also revealed that the energy transfer from borane to BODIPY can be elegantly tuned by modulating the dihedral angle between these two moieties.

Triarylborane-Appended Anils and Boranils: Solid-State Emission, Mechanofluorochromism, and Phosphorescence

Herein, the design, synthesis, optical properties, and mechanofluorochromism characteristics of a series of conjugates having covalently linked triarylborane (TAB) and anil/boranil units (TAB-anil: 1 a-3 a and TAB-boranil: 1-3) are reported. The electronic interactions between TAB and anil/boranil in 1 a-3 a and 1-3 were fine-tuned by changing the boryl moiety's position on the phenyl spacer connecting the BMes₂ (Mes=mesityl) and anil/boranil units. A boryl moiety at the meta position (1 a) of the phenyl spacer stabilizes the enolic form (E-OH), whereas a boryl moiety at the para position (2 a and 3 a) stabilizes the keto form (Z-NH) in the solid state. However, in solution 1 a, 2 a, and 3 a exhibit keto-enol tautomerism in both ground and excited states. Compounds 1 a-3 a and 1-3 show red-shifted absorption compared with 4 a and 4, which are devoid of TAB moieties, which indicate effective participation of an empty p orbital on the boron center in 1 a-3 a and 1-3. Compounds 1 and 2 showed fluorescence variations in response to external stimuli such as mechanical grinding. Long phosphorescence lifetimes of 18-46 ms were observed for compounds 1-3. The observed optical properties of 1 a-3 a and 1-3 are rationalized in the context of quantum mechanical calculations.

Molecular Conformational Effect on Optical Properties and Fluoride Induced Color Changes in Triarylborane-Vinylbithiophene-BODIPY Conjugates

Three new triads [bis(mesityl)boryl (Mes₂B)-vinylbithiophene-BODIPY] bearing zero (1), two (2), and four (3) methyl groups on the BODIPY core are synthesized, and their optical properties are reported. The vinyl linker between the thiophene rings in the spacer moiety improved the electronic communication between the boryl and BODIPY units. It displayed a bathochromic shift in the absorption and emission spectra compared to the Mes₂B-bithiophene-BODIPY triad reported elsewhere. These compounds exhibit intriguing multiple-emission features due to an incomplete energy transfer from donor borane to the acceptor BODIPY unit. These compounds' photoluminescence color can be conveniently fine-tuned by fluoride binding at the coordinatively unsaturated tricoordinate boron center. Triad 3, with a rigid molecular structure, showed a sharp emission band, whereas triads 1 and 2, with flexible molecular structures, displayed broad emission bands with a robust bathochromic shift, ascribed to their excited state structural reorganizations. These triads selectively bind fluoride ions and show colorimetric responses, which can be seen with an unaided eye. DFT computational studies were performed to rationalize the optical signatures of these compounds.

Delayed Fluorescence, Room Temperature Phosphorescence, and Mechanofluorochromic Naphthalimides: Differential Imaging of Normoxia and Hypoxia Live Cancer Cells

We study the effect of molecular conformation on the electronic coupling between the donor amines and acceptor 1,8-naphthalimide (NPI) in a series of D-A systems 1-4 (A = NPI; D = phenothiazine, phenoxazine, carbazole, diphenylamine, respectively, for 1, 2, 3, and 4). Weakly coupled systems show dual emission in the solution state, while strongly coupled systems show single emission bands. The energy of transitions and photoluminescence (PL) quantum yield are sensitive to the molecular conformation and donor strength. These compounds show delayed emission in the solutions and aggregated state and phosphorescence in the solid state. Compounds 3 and 4 with weak donors exhibit intermolecular slipped π···π interactions in the solid state and consequently exhibit dual (intra- and inter-) phosphorescence at low temperature. Steady state and time-resolved PL studies at variable temperature together with computational and crystal structure analysis were used to rationalize the optical properties of these compounds. The delayed emission of these compounds is sensitive to molecular oxygen; accordingly, these molecules are utilized for differential imaging of normoxia and hypoxia cancer cells.

Hydrazone Based Dual - Responsive Colorimetric and Ratiometric Chemosensor for the Detection of Cu2+/F- Ions: DNA Tracking, Practical Performance in Environmental Samples and Tooth Paste

Colorimetric sensors have attracted wide scope of attentions due to its fascinating advantages, like handy, equipment-free and naked eye detections. In this investigation, a new and novel hydrazone based dual-responsive ratiometric/colorimetric chemosensor have been developed for highly selective and sensitive detection of Cu²⁺ and F^- ions in dimethyl sulfoxide (DMSO) solvent. The probe showed highly selective sensing towards Cu²⁺ and F^- ions by exhibiting a color change from pale yellow to yellowish green and pale yellow to yellowish brown respectively., in DMSO without any interference of other ions at same concentration. These experimental results have also substantiated by the NMR, HR-MS, UV-Vis spectroscopic, cyclic voltammetry, differential pulse voltammetry techniques and DFT calculations. The detection limits are found to be 5.8 μM for Cu²⁺ and 0.025 μM for F^- ions which is far below to the values recommended by WHO. The stoichiometric ratios between NAPCBH and Cu2+/ F- ions were confirmed from the Job's plots and ¹H NMR titration experiments which are found to be 2:1 and 1:1 respectively. The tracking ability of the DNA with NAPCBH-Cu²⁺ was studied by UV-Vis titration and Cyclic voltammetry measurements. It shows efficient affinity towards DNA with NAPCBH-Cu²⁺. The probe can also quantitatively determine the Copper and fluoride ions present in environmental samples & toothpaste. The NAPCBH was promptly recovered by utilizing very low concentration of HCl, showing that was found feasible and re-usable sensor for the convenient detection of Cu²⁺ and F^- ions. Graphical Abstract.

Tri-(2-picolyl)amine-modificated triarylborane: Synthesis, photophysical properties and distinguish for cyanide and fluoride anions in aqueous solution

We designed and synthesized a tri-(2-picolyl) amine (TPA) functionalized triarylborane, 1-(6-(4-(dimesitylboryl)phenyl)pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine (PB2). The photophysical properties of PB2 were thoroughly explored. Moreover, PB2 can capture CN^- and F^- in aqueous solution through strong chelation induced by the synergy of a boron atom and metal ion gripped by TPA to display entirely different fluorogenic responses such as fluorescence enhancement for CN^- and fluorescence quenching for F^-. The results of TOF-MS-EI analysis and theoretical calculations indicate that the complexing of PB2 with CN^- formed a 2-to-2 adduct with a stabilized configuration, resulting in strong emission. The complexing of PB2 with F^- formed a 1-to-1 adduct with a loose configuration, resulting in weak emission. In pure water, the detection limit of PB2 for CN^- is 0.79 μM, and in H₂O/THF (1:9 v/v) system, the detection limits of PB2 for CN^- and F^- can reach 0.39 and 2.12 μM, respectively, indicating its potential application for effective detection and discrimination of CN^- and F^-.

Triarylboron Anchored Luminescent Probes: Selective Detection and Imaging of Thiophenols in the Intracellular Environment

The advances in boron incorporated organics have captured overwhelming interest on account of their outstanding properties and promising applications in various fields. Mostly, triarylborane compounds (TAB) are exploited as sensors of F^- and CN^- anions at the expense of the intrinsic Lewis acidic nature of boron. New molecular probes 1 and 2 for detection of toxic thiophenol were designed by conjugating highly fluorescent borylanilines with the luminescent quencher 2,4-dinitrobenzene based sulfonamides (DNBS), wherein the electrophilicity of the DNBS moiety has been modulated by fine-tuning the intrinsic Lewis acidity of boron. The interplay between PET (photoinduced electron transfer) and ICT have been employed for developing the TAB tethered turn-on fluorescent sensor for thiophenol with high selectivity for the first time. The newly developed probes showed very fast response toward thiophenol (within ∼5 min) with limits of detection (LOD) lying in the micromolar range, clearly pointing to their potential. Further, compounds 1 and 2 were explored for detecting thiophenol in the intracellular environment by discriminating biothiols. DFT and TD-DFT calculations were performed to support the sensing mechanism.

Multiple emissive triarylborane-A2H2 and triarylborane-Zn-A2H2 porphyrin conjugates

Triarylborane-A2H2 (1) and triarylborane-Zn-A2H2 porphyrins (2) have been synthesized by acid catalyzed condensation of 4-dimesitylboryl-benzaldehyde and dipyrromethane under ambient conditions. Compounds 1 and 2 showed multiple emission bands upon excitation at the triarylborane dominated absorption region (350 nm). Detailed experimental and computational studies show that the multiple emission features of 1 and 2 arise as a result of a partial energy transfer from the donor (triarylborane) to the acceptor (porphyrin) moieties. Compounds 1 and 2 showed very high selectivities towards fluoride ions compared to other competing anions.

Novel aluminum–BODIPY dyads: intriguing dual-emission via photoinduced energy transfer

Three novel BODIPY-based heterodinuclear complexes were prepared and especially, salen-based aluminum–BODIPY dyads exhibited intramolecularly significant photoinduced energy transfer from aluminum–salen to the BODIPY group.

Merging thiophene with boron: new building blocks for conjugated materials

Recent advances on the use of thienylborane chemistry for the development of new functional materials are highlighted.

White light emissive molecular siblings

White-light emission from boron based molecular siblings.

Benzoindolium–triarylborane conjugates: a ratiometric fluorescent chemodosimeter for the detection of cyanide ions in aqueous medium

A new ratiometric fluorescent chemodosimeter has been synthesized and characterized that exhibits high selectivity and sensitivity toward CN⁻ ions in aqueous medium.

A highly selective ratiometric bifunctional fluorescence probe for Hg2+and F−ions

A dithioacetal-containing triarylborane behaves as a promising ratiometric bifunctional fluorescence probe to detect Hg²⁺and F⁻simultaneously.

Dicyanovinyl substituted triarylboranes: a rational approach to distinguish fluoride and cyanide ions

Two new dicyanovinyl (DCV) functionalized triarylboranes (Mes2B-π-spacer-DCV, for 1: π-spacer = C6H4, for 2: π-spacer = 2,3,5,6-tetramethyl-phenyl) are reported. The molecular structures of 1 and 2 are similar except for the spacer which connects the boryl and DCV units. This small structural perturbation induces drastic changes in the optical properties of 1 and 2. Compound 2 shows weak dual fluorescence emission in nonpolar solvents and a stronger emission in polar solvents. Compound 1 is weakly fluorescent in polar environments but shows an intense single luminescence peak in less polar environments. Compound 1 exhibits a turn-off fluorescence response for both fluoride and cyanide: in contrast, 2 shows a turn on fluorescence response for both anions with different fluorescence signatures. The NMR titration studies reveal that for compound 2, fluoride binds to the boron centre and cyanide binds to the DCV unit. For compound 1, the fluoride ion binds to the boron center, whereas the CN(-) binds to both the Ar3B and DCV units.

Truxene-cored π-expanded triarylborane dyes as single- and two-photon fluorescent probes for fluoride

Two triarylboranes were synthesized and they exhibited high selectivity and sensitivity to fluoride as both single-photon and two-photon fluorescent probes.

A fluorescent probe based on hydroxylnaphthalene 2-cyanoacrylate: fluoride ion detection and its bio-imaging in live cells

A novel ratiometric fluorescent probe based on hydroxylnaphthalene 2-cyanoacrylate shows a high selectivity for fluoride ions and can be used for intracellular imaging.

Organic soluble and uniform film forming oligoethylene glycol substituted BODIPY small molecules with improved hole mobility

A propeller type donor imparts organic solubility to oligoethylene glycol substituted conjugated small molecules. The oligoethylene glycol facilitates better intermolecular contacts and improved organic field effect transistor efficiency.