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.

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.

B-O-B Bridged BOPPY Derivatives: Synthesis, Structures, and Acid-Catalyzed Cis-Trans Isomeric Interconversion

A new class of BOPPY derivatives has been facilely synthesized by a two-step reaction of coupling 3,5-dimethylpyrrole-2-carbaldehyde with 2,3-dihydrazinoquinoxaline (QDH) followed by coordinating with BF3·OEt2. The reaction mainly produces a...

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.

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.

Similar, Yet Different: Long-Range Metal-Metal Coupling and Electron-Transfer Processes in Metal-Free 5,10,15,20-Tetra(ruthenocenyl)porphyrin

The electronic structure, redox properties, and long-range metal-metal coupling in metal-free 5,10,15,20-tetra(ruthenocenyl)porphyrin (H₂TRcP) were probed by spectroscopic (NMR, UV-vis, magnetic circular dichroism (MCD), and atmospheric pressure chemical ionization (APCI)), electrochemical (cyclic voltammetry, CV, and differential pulse voltammetry, DPV), spectroelectrochemical, and chemical oxidation methods, as well as theoretical (density functional theory, DFT, and time-dependent DFT, TDDFT) approaches. It was demonstrated that the spectroscopic properties of H₂TRcP are significantly different from those in H₂TFcP (metal-free 5,10,15,20-tetra(ferrocenyl)porphyrin). Ruthenocenyl fragments in H₂TRcP have higher oxidation potentials than the ferrocene groups in the H₂TFcP complex. Similar to H₂TFcP, we were able to access and spectroscopically characterize the one- and two-electron oxidized mixed-valence states in the H₂TRcP system. DFT predicts that the porphyrin π-system stabilizes the [H₂TRcP]⁺ mixed-valence cation and prevents its dimerization, which is characteristic for ruthenocenyl systems. However, formation of the mixed-valence [H₂TRcP]²⁺ is significantly less reproducible than the formation of [H₂TRcP]⁺. DFT and TDDFT calculations suggest the ruthenocenyl fragment dominance in the highest occupied molecular orbital (HOMO) energy region and the presence of the low-energy MLCT (Rc → porphyrin (π*)) transitions in the visible region with energies higher than the predominantly porphyrin-centered Q-bands.

Synthesis, Structure and Photophysical Properties of a New Class of Inherently Chiral Boron(III) Chelates-The tert-Leucine Complexes

A new family of boron(III) chelates is introduced whereby molecular chirality, confirmed by circular dichroism, is imported during synthesis such that isolation of the diastereoisomers does not require separation procedures. The photophysical properties of two members of the family have been examined: the N,O,O-salicylaldehyde-based derivative shows pronounced intramolecular charge-transfer character in fluid solution and is weakly fluorescent, with a large Stokes shift. The corresponding 2-methylamino-benzaldehyde-derived N,N,O-chelate absorbs and fluoresces in the visible region with a much smaller Stokes shift. Orange fluorescence is also observed for this compound as a cast film. Temperature-dependence studies show that decay of the fluorescent state is weakly activated but emission is less than quantitative at 77 K. Quite rare for boron(III)-based chelates, this derivative undergoes intersystem crossing to form a meta-stable triplet-excited state. X-ray crystal structures are reported for both compounds, along with simulated ECD spectra.

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.

Probing Electronic Communication and Excited-State Dynamics in the Unprecedented Ferrocene-Containing Zinc MB-DIPY

The electronic communication between two ferrocene groups in the electron-deficient expanded aza-BODIPY analogue of zinc manitoba-dipyrromethene (MB-DIPY) was probed by spectroscopic, electrochemical, spectroelectrochemical, and theoretical methods. The excited-state dynamics involved sub-ps formation of the charge-separated state in the organometallic zinc MB-DIPYs, followed by recovery of the ground state via charge recombination in 12 ps. The excited-state behavior was contrasted with that observed in the parent complex that lacked the ferrocene electron donors and has a much longer excited-state lifetime (670 ps for the singlet state). Much longer decay times observed for the parent complex without ferrocene confirm that the main quenching mechanism in the ferrocene-containing 4 is reflective of the ultrafast ferrocene-to-MB-DIPY core charge transfer (CT).

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).

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.

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.

Highly Luminescent Heavier Main Group Analogues of Boron-Dipyrromethene

The preparation and photophysical properties of two heavier main group element analogues of boron-dipyrromethene (BODIPY) chromophores are described. Specifically, we have prepared dipyrrin complexes of dichlorogallate (GADIPY) or phenylphosphenium (PHODIPY) units. Whereas cationic PHODIPY is labile, decomposing to a phosphine over time, GADIPY is readily prepared in good yield as a crystalline solid having moderate air- and water-stability. Crystallographically characterized GADIPY displays intense green photoluminescence (λ_em = 505 nm, Φ_em = 0.91 in toluene). These inaugural heavier main group element analogues of BODIPY offer a glimpse into the potential for elaboration to a panoply of chromophores with diverse photophysical properties.

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.

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.