Synthesis of Highly Functionalized BOPHY Chromophores Displaying Large Stokes Shifts

Synthesis and Regioselective Functionalization of Tetrafluorobenzo-[α]-Fused BOPYPY Dyes

The synthesis of a new bis-BF2 tetrafluorobenzo-[α]-fused BOPYPY dye from 4,5,6,7-tetrafluoroisoindole and 2-hydrazinopyrazine is reported. The regioselectivity of nucleophilic substitution reactions at the periphery of the tetrafluorinated BOPYPY and its α-bromo derivative were investigated using N-, O-, S-, and C-based nucleophiles. Among the aromatic fluorine atoms, the F2 atom is consistently regioselectively substituted, except when the α-position contains a thiophenol group; in this case, F4 is substituted instead due to stabilizing π-π-stacking between the two aromatic groups. The α-bromo BOPYPY derivative also reacts under Stille cross-coupling reaction conditions to produce the corresponding α-substituted product. The spectroscopic properties of these new fluorinated BOPYPYs were investigated and compared with nonfluorinated analogs.

Aromatic-Ring-Fused BOPPY Fluorophores: Synthesis, Spectral, Redox Properties, and Bioimaging Application

Tetracoordinated organoboron dyes exhibiting strong fluorescence in either solution or the solid state are currently receiving much attraction in view of their photovoltaic, optoelectronic, and biological applications. Herein, a series of aromatic-ring-fused BOPPY dyes have been developed by one-pot condensation of formylated isoindoles or indoles and pyridinylhydrazine followed by subsequent borylation coordination. The facile synthesis provides excellent diversity of these unsymmetrical α-benzo- and β-benzothiophene-fused BOPPY dyes with intriguing photophysical properties owing to their rigid and planar structure and extended π-conjugation while containing a reactive site. They display intense green to orange fluorescence in solution and red-to-near-infrared emission in the solid state, with high fluorescence quantum yields up to 92 and 21%, respectively, relatively large Stokes shifts, and excellent photostability. Furthermore, two representative benzo-fused BOPPY probes with morpholine or benzenesulfonamide groups were developed and used to selectively "light up" the subcellular organelles such as lysosomes and endoplasmic reticulum under ultralow concentration, respectively.

NIR-Active Porphyrin-Decorated Lipid Microbubbles for Enhanced Therapeutic Activity Enabled by Photodynamic Effect and Ultrasound in 3D Tumor Models of Breast Cancer Cell Line and Zebrafish Larvae

Porphyrin is known to enable the photodynamic effect during cancer drug delivery and molecular imaging. However, its hydrophobicity and tendency to aggregate in an aqueous medium create a significant hurdle for its use as an anticancer drug. Loading porphyrin onto biocompatible delivery vehicles can enhance its efficacy. This can be achieved by using gas-filled microbubbles that can be administered intravenously. This study aimed at developing near-infrared (NIR)-active porphyrin-loaded lipid microbubbles with anticancer activity enhanced by sonodynamic and photodynamic effects. The porphyrin-loaded microbubbles were studied for their cell toxicity, cellular uptake of porphyrin, and effect on cellular three-dimensional (3D) invasion of breast cancer cells (MDA-MB-231) in cellulo. Toxicity studies in zebrafish larvae (Danio rerio) in the presence and absence of photodynamic and sonodynamic therapy were also conducted. The results suggest that with a higher concentration of porphyrin loaded on microbubbles, the porphyrin-loaded microbubbles display a higher therapeutic effect facilitated by photodynamic and sonodynamic therapy, which results in enhanced cellular uptake and cellular toxicity. A lower concentration of loaded porphyrin microbubbles exhibits high cellular viability and good fluorescence intensity in the NIR region, which can be exploited for bioimaging applications.

A BOPHY based fluorescent probe for Hg2+via NTe2 chelation

A BOPHY-based organotellurium-containing probe was synthesized and characterized via single crystal XRD for the selective and sensitive detection of Hg²⁺ over other metal ions. The probe detects Hg²⁺ in less than 1 s with a 2.5-fold enhancement in fluorescent intensity. Due to the chalcogenophilicity of mercury, Hg²⁺ was facilely trapped in the NTe₂ chelating cavity of the probe. The probe can detect Hg²⁺ in the nanomolar range (62 nM) and it showed reversibility with S^2- ions. The sensitivity of the probe for the detection of Hg²⁺ was confirmed in living HeLa cells.

Investigations on the Synthesis, Reactivity, and Properties of Perfluoro-α-Benzo-Fused BOPHY Fluorophores

The synthesis and reactivity of 3,8-dibromo-dodecafluoro-benzo-fused BOPHY 2 are reported, via S_N Ar with O-, N- S- and C-nucleophiles, and in Pd(0)-catalyzed cross-coupling reactions (Suzuki and Stille). The resulting perfluoro-BOPHY derivatives were investigated for their reactivity in the presence of various nucleophiles. BOPHY 3 displays reversible color change and fluorescence quenching in the presence of bases (Et₃ N, DBU), whereas BOPHY 7 reacts preferentially at the α-pyrrolic positions, and BOPHY 8 undergoes regioselective fluorine substitution in the presence of thiols. The structural and electronic features of the fluorinated BOPHYs were studied by TD-DFT computations. In addition, their spectroscopic and cellular properties were investigated; BOPHY 10 shows the most red-shifted absorption/emission (λ_max 659/699 nm) and 7 the highest fluorescence (Φ_f =0.95), while all compounds studied showed low cytotoxicity toward human HEp2 cells and were efficiently internalized.

The Stability of Unsubstituted BOPHY in Various Media

In this communication, we report our findings on the stability of unsubstituted BOPHY in different media. Our studies showed that unsubstituted BOPHY is highly unstable in water, methanol, and DMSO. The dye was observed to be stable in aprotic and/or relatively apolar solvents (ACN, acetone, DCM, and toluene) for a routine application. The pH of the medium was shown to affect the stability of the dye dramatically. Herein, we also presented a one-pot procedure for the synthesis of the dye.

A Family of Highly Fluorescent and Membrane-Permeable Bis(BF2) Acyl-Pyridinylhydrazine Dyes with Strong Solid-State Emission and Large Stokes Shifts: The BOAPH Fluorophores

Organic small-molecule fluorescent chromophores have become essential to modern chemical, biological, and materials related investigations. Herein, a straightforward synthesis and subsequent borylation were presented to form a novel family of bisBF₂-anchoring acyl-pyridinylhydrazine, which we named BOAPH. The chromophore enjoys outstanding structural diversities owing to varied acyl chlorides and N-heteroarenylhydrazides. These resultant BOAPH dyes are confirmed by NMR, HRMS, and single-crystal X-ray structure analysis. Their spectroscopic properties were studied, and most of the strong absorbance and bright fluorescence with maximum wavelengths centered in the range of 400 and 650 nm. More importantly, they exhibit promising fluorescence quantum yields up to 0.79 in solution and solid states, good photostability, and large Stokes shifts. Furthermore, a respective BOAPH dye with a para-dimethylaminophenyl group exhibited the interesting ability of ultrafast staining and two-photon imaging, which can specifically label lipid droplets of living cells immediately without the need for incubation.

Crystal structure of 2-phenylethynyl-1,3,6,8-tetramethylBOPHY (BOPHY = bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine), C22H20B2F4N4

C22H20B2F4N4, orthorhombic, Pbca (no. 61), a = 9.2506(2) Å, b = 36.1677(13) Å, c = 12.9475(3) Å, V = 4331.9 Å3, Z = 8, R gt (F) = 0.0729, wR ref (F 2) = 0.1992, T = 294.4 K.

Crystal structure of 2-bromo-1,3,6,8-tetramethylBOPHY (BOPHY = bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine), C14H15B2BrF4N4

C14H15B2BrF4N4, orthorhombic, Pbca (no. 61), a = 11.970(1) Å, b = 10.049(1) Å, c = 27.847(1) Å, V = 3349.6 Å3, Z = 8, R gt(F) = 0.0697, wR ref(F 2) = 0.1585, T = 296 K.

CCDC no.: 2080820

Sterically Protected and Conformation-Restricted BOBHY Dyes with Bright Near-Infrared Fluorescence: N2O-type Expanded BOPHY Dyes Derived from Boronic Acids

A new family of N₂O-type hydrazine-containing bipyrrole boron complexes has been developed via a one-pot condensation of formylisoindole, hydrazine, and various organoboronic acids. Because of the conformation-restricted coplanar structure and the axial-substituted aryl groups, these novel dyes show deep-red absorption, bright near-infrared (NIR) fluorescence in both solution and solid states, and good solubility in organic solvents. The derivative with pyridinium ions also has been synthesized as an NIR mitochondrially targetable fluorescent probe.

Bicyclic 1,3a,6a-Triazapentalene Chromophores: Synthesis, Spectroscopy and Their Use as Fluorescent Sensors and Probes

The 1,3a,6a-triazapentalene (TAP) is an aromatic heterocyclic fluorescent dye with interesting features such as its small size, large Stokes shift, solvatochromism, and emission wavelengths that are spread across the visible spectrum. TAPs have been synthesized via different synthetic strategies involving click−cyclization−aromatization domino reactions, gold-catalyzed cyclization of propargyl triazoles or triazolization of acetophenones. As a result, TAPs with diverse substitution patterns were obtained, showing varying fluorescence properties. Based on these properties, several TAPs have been selected and studied as fluorescent imaging probes in living cells and as sensors. This mini review provides an overview of the research on the bicyclic TAPs and does not comment on the literature about benzo or otherwise fused systems. The synthetic methodologies for the preparation of TAPs, the substituent effects on the fluorescence properties, and the behavior of the TAP core as an element of biological imaging probes and sensors are discussed.

Fundamental studies to emerging applications of pyrrole-BF2 (BOPHY) fluorophores

This review highlights the up-and-coming pyrrole-BF₂ (BOPHY) fluorophores, with a focus on synthetic procedures, photophysical properties – including structure–property analyses – as well as emerging applications.

Synthesis pathways for the preparation of the BODIPY analogues: aza-BODIPYs, BOPHYs and some other pyrrole-based acyclic chromophores

This mini-review summarizes the synthesis strategies for the preparation and post-functionalization of aza-BODIPYs, BOPHYs, “half-Pcs”, biliazines, MB-DIPYs, semihemiporphyrazines, BOIMPYs, BOPPYs, BOPYPYs, BOAHYs, and BOAPYs.

BOPAHY: a doubly chelated highly fluorescent pyrrole-acyl hydrazone -BF2 chromophore

New easily functionalisable and highly fluorescent BOPAHY chromophores are synthesised via a one-pot two-step reaction starting from commercially available pyrrole-2-carbaldehydes and respective acyl hydrazides in the presence of BF₃·OEt₂. Most importantly, all BOPAHY dyes show excellent photophysical properties with quantum yields up to 0.92. Steady-state spectroscopy and quantum chemical calculations provide a first insight into these promising properties.

A Family of BODIPY-like Highly Fluorescent and Unsymmetrical Bis(BF2) Pyrrolyl-Acylhydrazone Chromophores: BOAPY

A new family of pyrrolyl-acylhydrazones anchored with two BF₂ units, named BOAPY, have been developed as BODIPY-like and unsymmetrical bis(BF₂) chromophores via a simple one-pot reaction. The easily accessible scaffold enjoys excellent diversity due to the structural versatilities of 2-formylpyrroles and acylhydrazines. BOAPYs exhibit good molar absorption coefficients, large Stokes shifts, and excellent chemical stability. More importantly, most of them display excellent fluorescence quantum yields both in solution and the solid state (up to 0.88 and 0.64, respectively).

Crystal structure of 2,7-diiodo-1,3,6,8-tetramethyl-bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine, C14H14B2F4I2N4

C14H15B2F4I2N4, triclinic, P1̄ (no. 2), a = 7.5458(7) Å, b = 8.5659(6) Å, c = 8.6089(5) Å, α = 108.189(6)°, β = 110.492(7)°, γ = 98.678(7)°, V = 473.6 Å3, Z = 1, R gt(F) = 0.0334, wR ref(F 2) = 0.0831, T = 296.5 K.

Rigid, yet flexible: formation of unprecedented silver MB-DIPY dimers with orthogonal chromophore geometry

Unprecedented for BODIPY/DIPY and aza-BODIPY/azaDIPY chemistry, MB-DIPY₂Ag₂ dimers with a twisted chromophore geometry were prepared and characterized by spectroscopy, X-ray crystallography, and DFT calculations.

Unveiling the Photophysical Properties of Boron-dipyrromethene Dyes Using a New Accurate Excited State Coupled Cluster Method

Boron-dipyrromethene (BODIPY) molecules form a class of fluorescent dyes known for their exceptional photoluminescence properties. Today, they are used extensively in various applications from fluorescent imaging to optoelectronics. The ease of altering the BODIPY core has allowed scientists to synthesize dozens of analogues by exploring chemical substitutions of various kinds or by increasing the length of conjugated groups. However, predicting the impact of any chemical change accurately is still a challenge, especially as most computational methods fail to describe correctly the photophysical properties of BODIPY derivatives. In this study, the recently developed coupled cluster method called "domain-based local pair natural orbital similarity transformed equation of motion-coupled cluster singles and doubles" (DLPNO-STEOM-CCSD) is employed to compute the lowest vertical excitation energies of more than 50 BODIPY molecules. The method performs remarkably well yielding an accuracy of about 0.06 eV compared to the experimental absorption maxima. We also provide an estimate to the error made by neglecting vibronic effects in the computed spectra. The dyes selected for investigation here span a large range of molecular sizes and chemical functionalities and are embedded in solvents with different polarities. We have also investigated if the method is able to correctly reproduce the impact of a single chemical modification on the absorption energy. To characterize the method in more specific terms, we have studied four large BODIPY analogues used in real-life applications due to their interesting chemical properties. These examples should illustrate the capacity of the DLPNO-STEOM-CCSD procedure to become a method of choice for the study of photophysical properties of medium to large organic compounds.

BOPHYs versus BODIPYs: A comparison of their performance as effective multi-function organic dyes

The computationally-aided photophysical and lasing properties of a selected battery of BOPHYs are described and compared to those of related BODIPY counterparts. The present joined theoretical-experimental study helps to put into context the weaknesses and strengths of both dye families under different irradiation conditions. The chemical versatility of the BOPHY scaffold has been also comparatively explored to modulate key photonic properties towards the development of red-emitting dyes, chiroptical dyes and singlet oxygen photosensitizers. Thus, BOPHY BINOLation by fluorine substitution with enantiopure BINOLs endows the BOPHY chromophore with chiroptical activity, as supporting by the simulated circular dichroism, decreasing deeply its fluorescent response due to the promotion of fluorescence-quenching intramolecular charge transfer (ICT). Interestingly, the sole alkylation of the BOPHY core strongly modulates the promotion of ICT, allowing the generation of highly bright BINOL-based BOPHY dyes. Moreover, 3,3'-dibromoBINOLating BOPHYs can easily achieve singlet-oxygen photogeneration, owing to spin-orbit coupling mediated by heavy-atom effect feasible in view of the theoretically predicted disposition of the bromines surrounding the chromophore. From this background, we have established the master guidelines to design bright fluorophores and laser dyes, photosensitizers for singlet oxygen production and chiroptical dyes based on BOPHYs. The possibility to finely mix and balance such properties in a given molecular scaffold outstands BOPHYs as promising dyes competing with the well-settled BODIPY dyes.

Optoelectronic, Aggregation, and Redox Properties of Double-Rotor Boron Difluoride Hydrazone Dyes

We develop the chemistry of boron difluoride hydrazone dyes (BODIHYs) bearing two aryl substituents and explore their properties. The low-energy absorption bands (λ_max =427-464 nm) of these dyes depend on the nature of the N-aryl groups appended to the BODIHY framework. Electron-donating and extended π-conjugated groups cause a redshift, whereas electron-withdrawing groups result in a blueshift. The title compounds were weakly photoluminescent in solution and strongly photoluminescent as thin films (λ_PL =525-578 nm) with quantum yields of up to 18 % and lifetimes of 1.1-1.7 ns, consistent with the dominant radiative decay through fluorescence. Addition of water to THF solutions of the BODIHYs studied causes molecular aggregation which restricts intramolecular motion and thereby enhances photoluminescence. The observed photoluminescence of BODIHY thin films is likely facilitated by a similar molecular packing effect. Finally, cyclic voltammetry studies confirmed that BODIHY derivatives bearing para-substituted N-aryl groups could be reversibly oxidized (E_ox1 =0.62-1.02 V vs. Fc/Fc⁺ ) to their radical cation forms. Chemical oxidation studies confirmed that para-substituents at the N-aryl groups are required to circumvent radical decomposition pathways. Our findings provide new opportunities and guiding principles for the design of sought-after multifunctional boron difluoride complexes that are photoluminescent in the solid state.

Optical spectroscopic properties recorded for simple BOPHY dyes in condensed media: The mirror-symmetry factor

The BOPHY structural scaffold provides opportunities for the synthesis of innumerable derivatives with linear geometries and well-controlled π-conjugation pathways. The simpler BOPHY chromophores are highly fluorescent but exhibit poor mirror symmetry between absorption and fluorescence spectra at ambient temperature. In particular, the absorption (and excitation) spectra are broad and appear as two overlapping bands of comparable intensity. In constrained media, such as low-temperature rigid glasses or stretched poly(ethylene) films, mirror symmetry is restored. Analysis of the temperature dependence recorded for simple BOPHY derivatives indicates that the vibronic envelope accompanying the electronic transitions can be well described in terms of low- and medium-frequency modes. Whereas the fluorescence spectral profile is only weakly dependent on temperature, the excitation spectrum is far more affected. The magnitude of the low-frequency mode, and the associated electron-phonon coupling, increase substantially with increasing temperature and is responsible for temperature broadening and distortion of the excitation spectrum in solution. This critical low-frequency vibronic mode is associated with out-of-plane torsional bending of the BOPHY unit. Variable temperature NMR studies failed to provide unequivocal evidence for conformational changes of one of the derivatives over the temperature range 193-353 K.

End-to-end communication in a linear supermolecule with a BOPHY centre and N,N-dimethylanilino-based terminals

Long-range electrostatic interactions are sufficient to cause sequential ionization of the terminal groups in a BOPHY-based supermolecule.

A turn-on fluorescent BOPHY probe for Cu2+ ion detection

A novel D–π–A type fluorine–boron compound BOPHY-PTZ was synthesized and employed as a “turn on” fluorescent probe for the sensitive detection of Cu²⁺ ions.

Photocatalysis and self-catalyzed photobleaching with covalently-linked chromophore-quencher conjugates built around BOPHY

Two Chromophore-Quencher Conjugates (CQCs) have been synthesized by covalent attachment of the anti-oxidant dibutylated-hydroxytoluene (BHT) to a pyrrole-BF₂ chromophore (BOPHY) in an effort to protect the latter against photofading.

A Ratiometric Fluorescent Sensor for Cd2+ Based on Internal Charge Transfer

This work reports on a novel fluorescent sensor 1 for Cd²⁺ ion based on the fluorophore of tetramethyl substituted bis(difluoroboron)-1,2-bis[(1H-pyrrol-2-yl)methylene]hydrazine (Me₄BOPHY), which is modified with an electron donor moiety of N,N-bis(pyridin-2-ylmethyl)benzenamine. Sensor 1 has absorption and emission in visible region, at 550 nm and 675 nm, respectively. The long wavelength spectral response makes it easier to fabricate the fluorescence detector. The sensor mechanism is based on the tunable internal charge transfer (ICT) transition of molecule 1. Binding of Cd²⁺ ion quenches the ICT transition, but turns on the π − π transition of the fluorophore, thus enabling ratiometric fluorescence sensing. The limit of detection (LOD) was projected down to 0.77 ppb, which is far below the safety value (3 ppb) set for drinking water by World Health Organization. The sensor also demonstrates a high selectivity towards Cd²⁺ in comparison to other interferent metal ions.

Investigating the optical properties of BOIMPY dyes using ab initio tools

Using a computational approach combining Time-Dependent Density Functional Theory (TD-DFT) and second-order Coupled Cluster (CC2) approaches, we investigate the spectral properties of a large panel of BOIMPY dyes.

A fluorescent dye with large Stokes shift and high stability: synthesis and application to live cell imaging

A new fluorescent dye with large Stokes shift and excellent photo-stability for live cell imaging has been developed.

Reversal aggregation-induced circular dichroism from axial chirality transfer via self-assembled helical nanowires

Two chiral binaphthyl-based enantiomers, (R/S)-7, can produce gradual reversal AICD signals from solution to aggregation, which can be attributed to axial chirality transfer to self-assembled helical nanowires in aggregation state.

Boron difluorides with formazanate ligands: redox-switchable fluorescent dyes with large stokes shifts

The synthesis of a series of (formazanate)boron difluorides and their 1-electron reduction products is described.