Red to Near-Infrared Isoindole BODIPY Fluorophores: Synthesis, Crystal Structures, and Spectroscopic and Electrochemical Properties

Synthesis and Properties of Bright Red-to-NIR BODIPY Dyes for Targeting Fluorescence Imaging and Near-Infrared Photothermal Conversion

An efficient synthesis of 3-pyrrolylBODIPY dyes has been developed from a rational mixture of various aromatic aldehydes and pyrrole in a straightforward condensation reaction, followed by in situ successively oxidative nucleophilic substitution using a one-pot strategy. These resultant 3-pyrrolylBODIPYs without blocking substituents not only exhibit the finely tunable photophysical properties induced by the flexible meso-aryl substituents but also serve as a valuable synthetic framework for further selective functionalization. As a proof of such potential, one 3-pyrrolylBODIPY dye (581/603 nm) through the installation of the morpholine group is applicable for lysosome-targeting imaging. Furthermore, an ethene-bridged 3,3'-dipyrrolylBODIPY dimer was constructed, which displayed a near-infrared (NIR) emission extended to 1200 nm with a large fluorescence brightness (2840 M-1 cm-1). The corresponding dimer nanoparticles (NPs) afforded a high photothermal conversion efficiency (PCE) value of 72.5%, eventually resulting in favorable photocytotoxicity (IC50 = 9.4 μM) and efficient in vitro eradication of HeLa cells under 808 nm laser irradiation, highlighting their potential application for photothermal therapy in the NIR window.

A new benzo-bodipy based fluorescent probe for the highly sensitive detection of hypochlorous acid and its application in the living cells and zebrafish imaging

Due to the highly significant biological activity of hypochlorous acid, the monitoring of its concentration in vivo has received extensive attention. In this work, a photoinduced electron transfer (PeT) based benzo-bodipy fluorescent probe BBy-T has been developed for the rapid, sensitive, and selective detection of HClO in an aqueous solution. Based on the HClO-specific oxidation reaction, BBy-T exhibited a distinct fluorescence turn-on response to HClO with a remarkable Stokes shift (84 nm), immediate response (less than 20 s), and low detection limit (13.7 nM). In addition, the bioimaging results indicated that the probe BBy-T could be applied to real-time fluorescence imaging of living HeLa cells as well as living zebrafish.

Self-Immolative Photosensitizers for Self-Reported Cancer Phototheranostics

Photosensitizers to precise target and change fluorescence upon light illumination could accurately self-report where and when the photosensitizers work, enabling us to visualize the therapeutic process and precisely regulate treatment outcomes, which is the unremitting pursuit of precision and personalized medicine. Here, we report self-immolative photosensitizers by adopting a strategy of light-manipulated oxidative cleavage of C═C bonds that can generate a burst of reactive oxygen species, to cleave to release self-reported red-emitting products and trigger nonapoptotic cell oncosis. Strong electron-withdrawing groups are found to effectively suppress the C═C bond cleavage and phototoxicity via studying the structure-activity relationship, allowing us to elaborate NG1-NG5 that could temporarily inactivate the photosensitizer and quench the fluorescence by different glutathione (GSH)-responsive groups. Thereinto, NG2 with 2-cyano-4-nitrobenzene-1-sulfonyl group displays excellent GSH responsiveness than the other four. Surprisingly, NG2 shows better reactivity with GSH in weakly acidic condition, which inspires the application in weakly acidic tumor microenvironment where GSH elevates. To this end, we further synthesize NG-cRGD by anchoring integrin α_vβ₃ binding cyclic pentapeptide (cRGD) for tumor targeting. In A549 xenografted tumor mice, NG-cRGD successfully deprotects to restore near-infrared fluorescence because of elevated GSH in tumor site, which is subsequently cleaved upon light irradiation releasing red-emitting products to report photosensitizer working, while effectively ablating tumors via triggered oncosis. The advanced self-immolative organic photosensitizer may accelerate the development of self-reported phototheranostics in future precision oncology.

A benzo BODIPY based fluorescent probe for selective visualization of hypochlorous acid in living cells and zebrafish

Hypochlorous acid (HOCl) is an essential endogenous reactive oxygen species in biological systems, playing a critical role in various physiological processes. Real-time monitoring of HOCl concentration in living organisms is essential for understanding its biological functions and pathological roles. In this study, we developed a novel fluorescent probe based on benzobodipy, BBDP, for rapid and sensitive detection of HOCl in aqueous solutions. The probe exhibited a significant fluorescence turn-on response to HOCl based on its specific oxidation reaction towards diphenylphosphine, with high selectivity, instantaneous response (less than 10 s), and low detection limit (21.6 nM). Furthermore, bioimaging results illustrated that the probe could be applied for real-time fluorescence imaging of HOCl in live cells and zebrafish. The development of BBDP may provide a new tool for exploring the biological functions of HOCl and its pathological roles in diseases.

BODIPY as a Multifunctional Theranostic Reagent in Biomedicine: Self-Assembly, Properties, and Applications

The use of boron dipyrromethene (BODIPY) in biomedicine is reviewed. To open, its synthesis and regulatory strategies are summarized, and inspiring cutting-edge work in post-functionalization strategies is highlighted. A brief overview of assembly model of BODIPY is then provided: BODIPY is introduced as a promising building block for the formation of single- and multicomponent self-assembled systems, including nanostructures suitable for aqueous environments, thereby showing the great development potential of supramolecular assembly in biomedicine applications. The frontier progress of BODIPY in biomedical application is thereafter described, supported by examples of the frontiers of biomedical applications of BODIPY-containing smart materials: it mainly involves the application of materials based on BODIPY building blocks and their assemblies in fluorescence bioimaging, photoacoustic imaging, disease treatment including photodynamic therapy, photothermal therapy, and immunotherapy. Lastly, not only the current status of the BODIPY family in the biomedical field but also the challenges worth considering are summarized. At the same time, insights into the future development prospects of biomedically applicable BODIPY are provided.

Red-to-Near-Infrared Emitting PyrrolylBODIPY Dyes: Synthesis, Photophysical Properties and Bioimaging Application

Near-infrared (NIR) fluorophores with characteristics such as deep tissue penetration, minimal damage to the biological samples, and low background interference, are highly sought-after materials for in vivo and deep-tissue fluorescence imaging. Herein, series of 3-pyrrolylBODIPY derivatives and 3,5-dipyrrolylBODIPY derivatives have been prepared by a facile regioselective nucleophilic aromatic substitution reaction (S_N Ar) on 3,5-halogenated BODIPY derivatives (3,5-dibromo or 2,3,5,6-tetrachloroBODIPYs) with pyrroles. The installation of a pyrrolic unit onto the 3-position of the BODIPY chromophore leads to a dramatic red shift of both the absorption (up to 160 nm) and the emission (up to 260 nm) in these resultant 3-pyrrolylBODIPYs with respect to that of the BODIPY chromophore. Their further 5-positional functionalization provides a facile way to fine tune their photophysical properties, and these resulting dipyrrolylBODIPYs and functionalized pyrrolylBODIPYs show strong absorption in the deep red-to-NIR regions (595-684 nm) and intense NIR fluorescence emission (650-715 nm) in dichloromethane. To demonstrate the applicability of these functionalized pyrrolylBODIPYs as NIR fluorescent probes for cell imaging, pyrrolylBODIPY 6 a containing mitochondrion-targeting butyltriphenylphosphonium cationic species was also prepared. It selectively localized in mitochondria of HeLa cells, with low cytotoxicity and intense deep red fluorescence emission.

Recent Developments in Isoindole Chemistry

Isoindoles are highly reactive aromatic heterocycles that have a variety of important applications in areas such as medicine, analytical detection, and solar energy. Due to their highly reactive nature, isoindoles can be used to access their derivatives, which possess a diverse array of biological activities. However, their reactivity also makes isoindoles unstable and thus, difficult to prepare. Consequently, there has been a need for the development of novel methods that address some of the synthetic challenges and limitations, as well as reactions that utilize isoindoles to access potentially useful compounds. This review will give an overview of the novel reactions reported within the past decade (2012 to 2022) that involve 2H- and 1H-isoindoles and fused isoindoles as reactants, key intermediates, or products. This review is divided into two parts, with the first part focusing on the synthesis of isoindoles and the second part focusing on reactions of isoindoles. The scopes and limitations of the methods described therein will be discussed and the significance of their contributions to the literature will be highlighted. Similar reactions will also be compared.

1 Introduction

2 Synthesis of Isoindoles

2.1 Synthesis of 2H-Isoindoles

2.2 Synthesis of 1H-Isoindoles

3 Reactions of Isoindoles

3.1 Reactions of 2H-Isoindoles

3.2 Reactions of 1H-Isoindoles

4 Conclusions

A Comparison of the Photophysical, Electrochemical and Cytotoxic Properties of meso-(2-, 3- and 4-Pyridyl)-BODIPYs and Their Derivatives

Boron dipyrromethene (BODIPY) dyes bearing a pyridyl moiety have been used as metal ion sensors, pH sensors, fluorescence probes, and as sensitizers for phototherapy. A comparative study of the properties of the three structural isomers of meso-pyridyl-BODIPYs, their 2,6-dichloro derivatives, and their corresponding methylated cationic pyridinium-BODIPYs was conducted using spectroscopic and electrochemical methods, X-ray analyses, and TD-DFT calculations. Among the neutral derivatives, the 3Py and 4Py isomers showed the highest relative fluorescence quantum yields in organic solvents, which were further enhanced 2-4-fold via the introduction of two chlorines at the 2,6-positions. Among the cationic derivatives, the 2catPy showed the highest relative fluorescence quantum yield in organic solvents, which was further enhanced by the use of a bulky counter anion (PF₆^-). In water, the quantum yields were greatly reduced for all three isomers but were shown to be enhanced upon introduction of 2,6-dichloro groups. Our results indicate that 2,6-dichloro-meso-(2- and 3-pyridinium)-BODIPYs are the most promising for sensing applications. Furthermore, all pyridinium BODIPYs are highly water-soluble and display low cytotoxicity towards human HEp2 cells.

Molecular Design of Monochromophore-Based Bifunctional Photosensitizers for Simultaneous Ratiometric Oxygen Reporting and Photodynamic Cancer Therapy

Monitoring the tumor oxygen level when implementing photodynamic therapy (PDT) on malignant cancer has vital significance but remains challenging yet. Herein, by structurally manipulating a 2,4-dimethylpyrrole-engineered asymmetric BODIPY scaffold with different kinds, numbers, and positions of halogen atoms, we rationally designed several monochromophore-based bifunctional photosensitizers, named BDPs (BDP-I, BDP-II, and BDP-III), with self-sensitized photooxidation characteristics for accurate oxygen reporting and photodynamic tumor ablation. We show that different ways of halogen regulation allow available tuning of BDPs' oxygen-dependent ratiometric fluorescence turn-ons upon light irradiation as well as type-II PDT efficiencies before and after self-sensitized photooxidation. Encouragingly, measuring the specific ratiometric signals of the most promising BDP-II enabled the direct observation of initial oxygen concentration in both living 4T1 cells and a tumor-bearing mice model, affording an alternative way for evaluating oxygen supplementation strategies. Meanwhile, the "always on" PDT effect of BDP-II ensured efficient tumor ablation via apoptosis. Our research was thus believed to be of instructive significance for future application of oxygen-related auxiliary strategies and the design of unimolecular multifunctional PDT agents for cancer precision therapy.

Structural and Nanotribological Properties of a BODIPY Self-Assembly

Boron-dipyrromethenes (BODIPY) are promising functional dyes, whose exceptional optical properties are closely related to their supramolecular assembly. Herein, the self-assembly of a BODIPY derivative functionalized with uracil groups is explicitly and thoroughly investigated by using scanning tunneling microscopy (STM). Based on the simulation and calculation by density functional theory (DFT) method, it can be concluded that the construction of ordered self-assembly structure is attributed to the formation of hydrogen bonds between uracil groups. Moreover, the nanotribological property of the self-assembly on HOPG surface is measured by using atomic force microscopy (AFM). The effort on self-assembly of the BODIPY derivative could enhance the understanding of surface assembly mechanism.

Aromatic [b]-fused BODIPY dyes as promising near-infrared dyes

Far-red and near-infrared (NIR) absorbing/emitting dyes have found diverse applications in biomedicine and material science. However, the absorption and emission of classical BODIPY chromophores at short wavelength hamper their applications. Several strategies have been adopted to modify the structure of the BODIPY core to design NIR dyes. Among these, the most efficient approach to expand the π-conjugation of the BODIPY core is via fusion of aromatic rings. So far, many novel BODIPY skeletons fused to aromatic hydrocarbons and heterocycles at the b bond have been reported. This review comprehensively describes the recent advances regarding the development of aromatic [b]-fused BODIPY dyes with the focus on the design and synthesis, the relationships between their photophysical/spectroscopic properties and molecular structures, and the potential applications in bioassays and optoelectronic devices.

Free-radical cyclization approach to polyheterocycles containing pyrrole and pyridine rings

A wide range of derivatives with new pyrido[2,1-a]pyrrolo[3,4-c]isoquinoline skeleton was synthesized by free-radical intramolecular cyclization of o-bromophenyl-substituted pyrrolylpyridinium salts using the (TMS)₃SiH/AIBN system. The cyclization provides generally good yields of pyrido[2,1-a]pyrrolo[3,4-c]isoquinoline hydrobromides having no additional radical-sensitive substituents. The free bases can be obtained from the synthesized hydrobromides in quantitative yield by basification at room temperature. The selectivity control of intramolecular arylation was achieved by replacing the halogen: the use of 1-(2-(ortho-bromophenyl)-4-(ortho-iodophenyl)pyrrol-3-yl)pyridinium bromide makes it possible to obtain a monocyclization product, and the bicyclization product from the dibromo derivative. The procedure is also applicable to obtain 3-arylpyrido[2,1-a]pyrrolo[3,2-c]isoquinoline derivatives including 2-unsubstituted skeletons that are inaccessible via Pd-catalyzed cyclization.

Accessing the Ene-Imine Motif in 1H-Isoindole, Thienopyrrole, and Thienopyridine Building Blocks

A pathway to a range of diverse heterocycles was developed using a nucleophilic cyclization strategy. Lactams and ene-imines are accessed in a few steps from a common precursor, and these moieties are further elaborated to directly provide pyrroles or pyridines with extended conjugation. Reaction conditions are mild, and a broad range of structural types are available within a few steps.

Recent advances in the synthesis of [a]-benzo-fused BODIPY fluorophores

This feature article summarizes the different strategies for the synthesis of [a]-benzo-fused BODIPYs that have been reported in the literature until 2018. These π-extended BODIPYs are promising fluorophores for bio-imaging and organic photovoltaic applications due to both their attractive photophysical properties in the near-infrared area and their higher (photo)chemical stability compared to conventional bis-styryl derivatives. The four main strategies described in this review can be used to access either directly the expected [a]-benzo-fused BODIPYs or through the di-/tetra-hydroisoindole.

Synthesis structural and spectroscopic properties of quinoxaline-bridged bisBODIPYs

Two novel quinoxaline-bridged bisBODIPYs have been synthesized by the condensation of 2,3-bis(5-formylpyrrol-2-yl)quinoxaline with 3-ethyl-2,4-dimethylpyrrole followed by a modification using a Knoevenagel reaction. They were well characterized by X-ray diffraction, NMR, HRMS UV-vis and fluorescence spectroscopy. These two quinoxaline-bridged bisBODIPYs have unusual broad absorption bands, which are different from those of typical BODIPYs They exhibit broad red-shifted emission bands centered at around 610 nm and 730 nm respectively with larger Stokes shifts at the range of 1421–2136 cm[Formula: see text] Both bisBODIPYs show different extent solvent-dependent fluorescence and exhibit fluorescence quenching in polar solvents due to the existence of possible intramolecular charge transfer.

Azine based smart probe for optical recognition and enrichment of Mo(vi )

Single crystal X-ray structure-characterized azine derivative (L) was explored for the selective detection of molybdenum (Mo(vi)) cations through green fluorescence emission. The Mo(vi) cation assisted inhibition of photo-induced electron transfer (PET) resulted in a 37-fold fluorescence enhancement via chelation enhanced fluorescence (CHEF) that allows detection of Mo(vi) with concentration as low as 2 × 10-9 M. The chelation of Mo(vi) cations by L has been confirmed by the single crystal X-ray structure of the resulting complex. The binding constant of L for Mo(vi) is fairly high (1.33 × 106 M-1). Moreover, L is very efficient for enrichment of Mo(vi) from aqueous solution. Density functional theoretical (DFT) studies substantiate the experimental results.

Synthesis of N,O,B-Chelated Dipyrromethenes through an Unexpected Intramolecular Cyclisation: Enhanced Near-Infrared Emission in the Aggregate/Solid State

The first example of the synthesis of mono-N,O-B-chelated dipyrromethene (BODIPY) derivatives through an unexpected intramolecular nucleophilic displacement of the fluorine by alkenols in the presence of boron trifluoride as Lewis acid is reported. The chlorine in the indacene core allowed for further structural modifications through nucleophilic substitutions or palladium-catalysed coupling reactions to afford new fluorophores with tuneable photophysical properties. Their expanded conjugation structure resulted in distinct red-shifted absorption and emission spectra in organic solutions. Furthermore, the twisted steric hindrance of the benzene substitution patterns suppressed aggregation-induced quenching, leading to an enhanced NIR emission in the aggregate/solid state, which was rarely observed for BODIPY dyes. Nanoparticles of the fluorophores formed by the assembly with the polymeric surfactant F127 were successfully used for bioimaging of living cells and for tumour-targeted imaging in a tumour-bearing mouse model.

Tetrafluorobenzo-Fused BODIPY: A Platform for Regioselective Synthesis of BODIPY Dye Derivatives

A novel route for the synthesis of unsymmetrical benzo-fused BODIPYs is reported using 4,5,6,7-tetrafluoroisoindole as a precursor. The reactivity of the 3,5-dibromo tetrafluorobenzo-fused BODIPY was investigated under nucleophilic substitution and Pd(0)-catalyzed cross-coupling reaction conditions. In addition to the 3,5-bromines, one α-fluoro group on the benzo-fused ring can also be functionalized, and an unusual homocoupling with formation of a bisBODIPY was observed. This new class of fluorinated BODIPYs could find various applications in medicine and materials.

Synthesis and characterization of pyrrolyldipyrrin F-BODIPYs

A series of synthetic analogs of the tripyrrolic natural product prodigiosin were complexed with boron trifluoride to generate the corresponding F-BODIPYs.

PyrrolylBODIPYs: Syntheses, Properties, and Application as Environment-Sensitive Fluorescence Probes

Four pyrrole B-ring-functionalized pyrrolylBODIPYs and their B-ring unsubstituted analogues were synthesized from easily accessible starting 5-halo-2-formylpyrroles and were characterized by nuclear magnetic resonance, high-resolution mass spectrometry, X-ray analysis, and optical/electronic properties. In great contrast to the substitution(s) at the other two pyrrolic units, electron-donating substituent(s) at pyrrole B-ring bring significant blue shift of the absorption and emission bands. Cyclic voltammetry and density functional theory calculations indicate that this blue shift may be attributed to the increased highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels and the overall increase in the energy band gaps. These pyrrolylBODIPYs generally show intense absorption (centered at 570-624 nm) and fluorescence emission (582-654 nm) in nonpolar solvents. A gradual decrease in the fluorescence intensity was observed for these dyes with the increase in solvent dipolar moment, which combines with the red to far-red absorption/emission, rendering these pyrrolylBODIPYs potential applications as environment-sensitive fluorescence probes as demonstrated in this work for bovine serum albumin.

Synthesis, Structure, and Properties of β-Vinyl Ketone/Ester Functionalized AzaBODIPYs from FormylazaBODIPYs

Postfunctionalization of azaBODIPY (the BF₂ complex of azadipyrromethene) is highly desirable due to the strong tunable absorption bands at wavelengths above 650 nm and the wide-ranging applications of this class of dyes in biomedicine and materials science. Currently available postfunctionalization methods for this class of dyes have been limited to the Pd-catalyzed coupling reactions on β-halogenated (brominated or iodinated) azaBODIPY platforms. In this work, we report a new strategy for the facile postfunctionalization of the azaBODIPY chromophore with various vinyl ketone and vinyl esters based on a Wittig reaction on our previously developed β-formylazaBODIPYs and our recently developed β-bromo-β'-formylazaBODIPYs. Our strategy uses easily accessible starting materials and mild reaction conditions. It is highly compatible with various common phosphonium ylides (aliphatic, aromatic, and ester substituted ones). These resultant bromo-containing β-vinyl ketone/ester functionalized azaBODIPYs are potential photosensitizers and can be further functionalized via coupling reactions. The ester groups on some of these resultant azaBODIPYs can be further hydrolyzed to achieve the desired water solubility and conjugate with the biomolecule and solid surface.

Th(iv) and Ce(iv) napthylsalophen sandwich complexes: characterization of unusual thorium fluorescence in solution and solid-state

The synthesis, characterization, and surprising electronic spectroscopy of two ML₂ sandwich complexes, where M = Ce(iv) or Th(iv) and L = napthylsalophen²⁻ are described.

Synthesis of 2-aminoBODIPYs by palladium catalysed amination

Palladium catalysed amination of a 2-iodoBODIPY with a range of anilines and a primary alkylamine has been used as the basis for a ‘switch-on’ fluorescence sensor for phosgene.

Regioselective and Stepwise Syntheses of Functionalized BODIPY Dyes through Palladium-Catalyzed Cross-Coupling Reactions and Direct C-H Arylations

Regioselective and stepwise syntheses of a series of functionalized BODIPY dyes through palladium-catalyzed cross-coupling reactions and direct C-H arylations have been developed. In particular, this method allows the straightforward synthesis of 2,6-dibromo-3,5-diarylBODIPYs and 2-bromo-3-arylBODIPYs from polybrominated BODIPYs. The X-ray structure of intermediates 5a-c indicated that the palladium was first inserted into the C-Br bonds at 3,5-positions of brominated BODIPYs. The resulting 2,6-dibromo-substituted BODIPYs are potential long wavelength photosensitizers which are not easily accessible using previous methods.

Solution and solid-state fluorescence of 2-(2′-hydroxyphenyl)-1,5-benzodiazepin-2-one (HBD) borate complexes

A new family of fluorescent 1,5-benzodiazepin-2-one (HBD) borate complexes was prepared in good yields, and fully characterized by means of MS, NMR and IR spectroscopy, as well as X-ray crystal structure analysis for compound 13.