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

Asymmetric and Symmetric S-zig-zag-Fused BODIPYs: Synthesis and Photophysical and Oxidative Properties

We present a new, straightforward, and versatile approach that utilizes regioselective brominated precursors to synthesize both asymmetric and symmetric S-zig-zag-fused BODIPYs (s-TFB and bis-TFB) in moderate yields (45% and 40%, respectively). X-ray structure analyses reveal that the planar rigidity of the BODIPY skeleton is progressively enhanced with an increasing number of thiopyran rings. The annulation of S-heteroaromatic rings at the zig-zag edge of the BODIPY core results in blue-shifted absorption and emission spectra, with bis-TFB exhibiting maxima at 530 and 539 nm and elevated LUMO energy levels. In contrast, oxidation of s-TFB and bis-TFB with m-CPBA demonstrates significant site selectivity, affording four oxidation products, namely s-s-SFB, s-bis-SFB, bis-s-SFB, and bis-bis-SFB, in yields ranging from 22% to 36%. These oxidated S-zig-zag-fused BODIPY derivatives display large red-shifted absorption and emission spectra (e.g., 648 and 735 nm for s-bis-SFB), along with more stable HOMO and LUMO energy levels and reduced HOMO-LUMO gaps. This S-zig-zag-fused cyclization/oxidation strategy enables precise tuning of the BODIPY optoelectronic properties, opening new avenues in dye design and application.

Advances in Π-Conjugated Benzothiazole and Benzoxazole-Boron Complexes: Exploring Optical and Biomaterial Applications

This mini-review highlights the transformative potential of benzothiazole (BTz)- and benzoxazole (BOz)-based boron-complexed dyes. It represents an innovative evolution of the classic boron-dipyrromethene (BODIPY) structure, which is well established for its superior photophysical properties. Incorporating BTz- or BOz-ligands into the borane (-BR2) component, originates more electron-deficient architecture, enabling novel modes of complexation and addressing limitations such as spectral overlap and self-quenching in traditional BODIPY dyes. The review focuses on the remarkable versatility of boron-benzothiazole (BOBTz)- and boron-benzoxazole (BOBOz)-based complexes, particularly in three rapidly advancing fields: organic light emitting diode (LED) technology, bioimaging, and mechanochromic luminescence (MCL). Over the past 15 years, these complexes have demonstrated exceptional adaptability, showcasing enhanced properties like high fluorescence quantum yields, large molar extinction coefficients, and tunable emissions across visible and near-infrared spectra. The insights described in this review highlight the major role of BOBTz- and BOBOz-complexes in shaping innovative, and sustainable advanced materials while addressing emerging challenges in modern materials science. Besides, the refining of both BOBTz- and BOBOz-complexes offers exciting prospects for technological challenges such as energy-efficient lighting, non-invasive imaging, and creating stimuli-responsive materials for next-generation sensors. Moreover, the environmental sustainability of these materials, including green synthesis approaches and recyclable components represents an important frontier for future exploration.

Recent advances in the synthesis of chalcogenylated heterocycles obtained by chalcogenocyclization

Heterocyclic molecules containing organochalcogens are important scaffolds that have attracted scientific interest due to their remarkable pharmacological properties. As a consequence, in recent years several protocols have been developed for the synthesis of this class of compounds. More specifically, cyclization reactions have become a powerful tool in the synthesis of heterocycles containing two or more chalcogen atoms. This review summarizes the recent advances in the synthesis of heterocycles containing two or more chalcogens (S, Se, and Te), through a wide diversity of cyclization reactions with different substrates, with emphasis on cyclization reactions of chalcogenoalkynes and alkenes in chalcogenocyclization reactions, highlighting their scope, main advantages, synthetic differences, and limitations.

Ring-fused BODlPY derived heavy-atom-free triplet photosensitizers

Triplet photosensitizers are compounds that demonstrate strong absorption of the excitation light, high intersystem crossing (ISC) efficiency for efficient triplet state generation, and long triplet lifetimes to facilitate subsequent photochemical reactions. Among these, heavy-atom-free triplet photosensitizers have attracted particular attention due to their advantages of long-lived triplet states and low dark toxicity in comparison with heavy-atom-containing photosensitizers. Owing to the superior photophysical and chemical characteristics, boron dipyrromethene (BODIPY) dyes have been developed as promising heavy-atom-free triplet photosensitizers through specific molecular design strategies. However, many heavy-atom-free BODIPY-based photosensitizers exhibit relatively short excitation wavelengths in the visible-light region, and their ISC efficiencies dropped significantly with the extension of π-conjugation via the Knoevenagel condensation reaction. Recently, the ring-fused BODIPY skeleton has provided a feasible approach for the design of long-wavelength NIR photosensitizers. This review provides a comprehensive summary of the strategies utilized for the construction of ring-fused BODIPY-based photosensitizers including the installation of a twisted π-conjugation framework, the fusion of thiophene moieties and the formulation of an aggregation-induced ISC process. Meanwhile, some important spectroscopic and photophysical properties of these photosensitizers, along with their related applications, are also described.

Near-Infrared Twisted Polycyclic Arene-Fused BisBODIPYs through a Tandem Inter- and Intramolecular Scholl Reaction

Five novel polyaromatic-ring-fused bisBODIPYs have been synthesized from corresponding readily available α-arylBODIPYs through a FeCl3-mediated tandem inter- and intramolecular oxidative aromatic coupling reaction. These resultant bisBODIPYs show twisted planar conformations, strong absorptions (ε up to 1.8 × 105 M-1 cm-1), and good fluorescence emissions in the near-infrared region (NIR, 660-734 nm) and good intersystem crossing efficiencies (ΦΔ = 18.9% for dimer 2d in toluene), demonstrating their promising potential applications as heavy-atom-free photosensitizers.

Dibenzofuran[a]-Fused BODIPYs: Synthesis, Photophysical Properties, and N2O2-Boron-Chelation Towards NIR Materials for Application in Organic Photodetectors

Highly annulated boron-dipyrromethenes (BODIPYs) were synthesized with the objective to develop a near-infrared (NIR)-absorbing photodetector. Post-functionalization of the dibenzoBODIPY scaffold enabled it to fuse with the dibenzofuran heterocycle at the a-bond of the pyrrole unit to give the related dyes 1 and 2, which absorb far-red light in tetrahydrofuran. Further structural modification by intramolecular B,O-chelation of 2 yielded the benzo[1,3,2]oxazaborinine-containing dye 14 having an intense absorption band with a λmax value of 812 nm (ϵ=1.3×105 M-1 cm-1), as rationalized by time dependent density functional theory (TD-DFT)/DFT calculations. Dye 14 exhibited unique emission properties, wherein irradiation at 375 nm led to a dual emission at 822 nm (Φ=5.1 %) and 470 nm (Φ=7.8 %), which could be attributed to the electronic non-adiabatic coupling due to the large energy difference between the S2 and S1 states, according to the anti-Kasha rule. Using a resistance-heating-type vacuum-deposition method, the rigid π-conjugated structure of 14 enabled its application as an NIR photodetector in a single-component device (indium tin oxide/14/Al). Current-voltage (J-V) measurements under photoirradiation at 870 nm (120 μW cm-2) produced a photocurrent of 6.05×10-7 A cm-2 at a bias potential of 0.1 V.

The Role of Spacers as a Probe in Variation of Photoluminescence Properties of Mono- and Bi-Nuclear Boron Compounds

A series of N,O donor-based mono- and binuclear four-coordinated boron complexes were synthesized. Depending on the substitution and spacer, these complexes exhibit intense blue, green and yellow emission in solution states. Notably, the fluorescence quantum yields (ΦF) and fluorescence decay (lifetime, τ) of mononuclear boron complexes (2 a-2 e) were higher than the binuclear boron complexes (2 f-2 k). The lowest lifetime and quantum yield in binuclear boron complexes were due to intramolecular rotation induced non radiative processes. The disulphide spacer-based boron complexes 2 i-2 k showed aggregation-caused quenching in the THF/H2O mixture whereas no other complexes were ACQ responsive. These complexes show large Stokes shift, one of them i. e. 2 e has the highest Stokes shift of 130 nm. Further, the electrochemical study suggests the presence of two redox incidences. Theoretical studies show close corroboration between the TD-DFT computed and experimentally measured absorption maxima as well as DFT (GIAO) calculated and experimentally measured 11B NMR values. This complements the appropriate selection of the theoretical methods to shed light on the electronic transitions in the mono- and binuclear BF2 complexes.

Chemical feature-based machine learning model for predicting photophysical properties of BODIPY compounds: density functional theory and quantitative structure-property relationship modeling

CONTEXT

Boron-dipyrromethene (BODIPY) compounds have unique photophysical properties and have been applied in fluorescence imaging, sensing, optoelectronics, and beyond. In order to design effective BODIPY compounds, it is crucial to acquire a comprehensive understanding of the relationships between the structures of BODIPY and the corresponding photoproperties. Fifteen molecular descriptors were identified to be strongly correlated with the maximum absorption wavelength. The developed ML/QSPR model exhibited good predictive performance, with coefficients of determination (R2) of 0.945 for the training set and 0.734 for the test set, demonstrating robustness and reliability. A posterior analysis of some of the selected descriptors in the model provided insights into the structural features that influence BODIPY compound properties; meanwhile, it also emphasizes the importance of molecular branching, size, and specific functional groups. This work shows that applied combined cheminformatics and machine learning approach is robust to screen the BODIPY compounds and design novel structures with enhanced performance.

METHODS

In the present study, all the BODIPY models studied were fully optimized, and the corresponding absorption spectrum was obtained at DFT/TDDFT//B3LYP/6-311G(d,p) level. All the above calculations were executed by the Gaussian 16 program. Based upon the theoretical computational results, the machine learning-based quantitative structure-property relationship (ML/QSPR) model was employed for predicting the maximum absorption wavelength (λ) of BODIPY compounds by combining hand-crafted molecular descriptors (MD) and explainable machine learning (EML) techniques using Scikit-learn python library. A dataset of 131 BODIPY compounds with their experimental photophysical properties was used to generate a diverse set of molecular descriptors capturing information about the size, shape, connectivity, and other structural features of these compounds using Chemaxon and Alvadesc software. A genetic algorithm (GA) variable selection together with the multi-linear regression (MLR) method were applied to develop the best predictive model using the Genetic Selection python library.

Diarenofuran[b]-fused BODIPYs: One-Pot SNAr-Suzuki Synthesis and Properties

We present a streamlined methodology that integrates regioselective tetrahalogen-BODIPY and o-hydroxyphenylboronic acid in a one-pot process, leveraging SNAr nucleophilic substitution in conjunction with Suzuki coupling to afford diarenofuran [b]-fused BODIPYs (DBFB1-9) with commendable yields (85-95%) and short reaction times (0.5-1.0 h). X-ray structure analyses of DBFB5,7-9 elucidate that these diarenofuran[b]-fused BODIPYs adopt a "butterfly" conformation, maintaining a highly rigid planarity. A comprehensive examination of the spectroscopic and electrochemical properties of these diarenofuran[b]-fused BODIPY derivatives, incorporating various substituents, reveals intriguing characteristics, including pronounced absorption and emission in the near-infrared region (NIR), elevated fluorescence quantum yields (ΦF = 75-89% in dichloromethane), and tunable HOMO-LUMO levels. Remarkably, compared to DBFB1-8, DBFB9 possesses a large extended π-conjugated system, resulting in significant red shifts in its absorption and emission maxima, reaching 623 and 635 nm, respectively, and a reduced HOMO-LUMO gap. Despite this substantial structural expansion, DBFB9 maintains a surprisingly high fluorescence quantum yield (ΦF = 80%), underscoring its exceptional photophysical performance and strong potential for applications requiring efficient NIR emission and robust fluorescence retention.

Design and Synthesis of BODIPY and Its Application in Inhibiting Intestinal Flora

BODIPY-based photosensitizers were synthesized and tested for antimicrobial photodynamic therapy, revealing structural modifications enhancing the photodynamic therapy (PDT) effects. This research may lead to new PDT strategies for treating bacterial infections, including those resistant to traditional antibiotics, and offers insights into the prevention and treatment of Alzheimer's disease through gut microbiota regulation.

Zig-zag-fused π-extended BODIPYs via gold-catalysed cycloisomerisation

π-Extension of the BODIPY core is an effective strategy to modulate the physical properties of BODIPYs. However, π-extension on the zig-zag edge of BODIPYs remains unexplored. Here, we disclose the synthesis of zig-zag-fused BODIPYs through gold-catalysed cycloisomerisation. The obtained BODIPYs show fascinating photophysical properties including near-infrared absorption and emission.

Thieno[3,2-b]thiophene for the Construction of Far-Red Molecular Rotor Hemicyanines as High-Affinity DNA Aptamer Fluorogenic Reporters

The construction of far-red fluorescent molecular rotors (FMRs) is an imperative task for developing nucleic acid stains that have superior compatibility with cellular systems and complex matrices. A typical strategy relies on the methine extension of asymmetric cyanines, which unfortunately fails to produce sensitive rotor character. To break free from this paradigm, we have synthesized far-red hemicyanines using a dimethylamino thieno[3,2-b]thiophene donor. The resultant probes, designated as ATh2Ind and ATh2Btz, possess excitation maxima (λmax) of >600 nm and have been rigorously characterized by NMR, electrochemistry, and computational methods. The dyes possess alternating charge patterns like indodicarbocyanine (Cy5), but with twisted intramolecular charge transfer (TICT) rotational barriers at 60°, akin to the classical FMR thiazole orange (TO1). ATh2Btz also displays cyanine characteristics, enhancing its response upon binding to nucleic acids and allowing for efficient staining of cellular nuclei. When binding to the DNA aptamer for quinine (MN4), ATh2Btz exhibits a Kd of 17 nM, a 660-fold light-up response, brightness (Φfl x εmax) of ∼37,000 M-1cm-1, and λex/λem of 655/677 nm. The resulting far-red DNA-based MN4-ATh2Btz platform has been termed "pomegranate."

Dithio-Fused Boron Dipyrromethenes: Synthesis and Impact of S-Heteroaromatic Annulation Mode

Three dithio-fused boron dipyrromethenes (BODIPYs), DTFB-1, DTFB-2, and DTFB-3, in which symmetrically S-heteroaromatic ring units fused at [a], zigzag, and [b] bonds of the parent BODIPY core, respectively, were prepared from the facile and efficient post-functionalization of tetra-halogenated BODIPYs through Pd-catalyzed cyclization. Dithio-fusion at various positions of BODIPY effectively tunes their photophysical properties and single-crystal structural packing arrangements. The single-crystalline microribbons of DTFB-2 exhibit commendable hole mobilities in air, reaching up to 0.03 cm2 V-1 s-1.

A Practical and Modular Method for Direct C-H Functionalization of the BODIPY Core via Thianthrenium Salts

Direct structural modification of small-molecule fluorophores represents a straightforward and appealing strategy for accessing new fluorescent dyes with desired functionalities. We report herein a general and efficient visible-light-mediated method for the direct C-H functionalization of BODIPY, an important fluorescent chromophore, using readily accessible and bench-stable aryl and alkenylthianthrenium salts. This practical approach operates at room temperature with extraordinary site-selectivity, providing a step-economical means to construct various valuable aryl- and alkenyl-substituted BODIPY dyes. Remarkably, this protocol encompasses a broad substrate scope and excellent functional-group tolerance, and allows for the modular synthesis of sophisticated symmetrical and asymmetrical disubstituted BODIPYs by simply employing different combinations of thianthrenium salts. Moreover, the late-stage BODIPY modification of complex drug molecules further highlights the potential of this novel methodology in the synthesis of fluorophore-drug conjugates.

BODIPY-based photocages: rational design and their biomedical application

Photocages, also known as photoactivated protective groups (PPGs), have been utilized to achieve controlled release of target molecules in a non-invasive and spatiotemporal manner. In the past decade, BODIPY fluorophores, a well-established class of fluorescent dyes, have emerged as a novel type of photoactivated protective group capable of efficiently releasing cargo species upon irradiation. This is due to their exceptional properties, including high molar absorption coefficients, resistance to photochemical and thermal degradation, multiple modification sites, favorable uncaging quantum yields, and highly adjustable spectral properties. Compared to traditional photocages that mainly absorb UV light, BODIPY-based photocages that absorb visible/near-infrared (Vis/NIR) light offer advantages such as deeper tissue penetration and reduced bio-autofluorescence, making them highly suitable for various biomedical applications. Consequently, different types of photoactivated protective groups based on the BODIPY skeleton have been established. This highlight provides a comprehensive overview of the strategies employed to construct BODIPY photocages by substituting leaving groups at different positions within the BODIPY fluorophore, including the meso-methyl position, boron position, 2,6-position, and 3,5-position. Furthermore, the application of these BODIPY photocages in biomedical fields, such as fluorescence imaging and controlled release of active species, is discussed.

Simultaneous discovery of chiral and achiral dyes: elucidating the optical functions of helical and flag-hinged boron tetradentate complexes

The construction of novel complexes can lead to the manifestation of unexpected structures and properties, thereby making chemical exploration in experiments a potential source for novel discoveries. In this study, by reacting 6,6'-dihydrazineyl-2,2'-bipyridine with acyl chlorides and subsequently coordinating with boron trifluoride, two different boron-tetradentate ligand complexes were simultaneously generated. One of these complexes exhibited a unique structure in which tetra-BF2 moieties coordinated to all four coordination sites of the ligand molecule, forming a flag-hinged structure around the bipyridine part. The second complex featured a helical structure formed by the hybridization of two BF2 and one B-O-B moieties, representing a highly unusual form of the complex. The structures of these two boron complexes were consistently observed when various substituted acyl chlorides were employed. Furthermore, it was found that enhancing electron-donor properties could induce a redshift in emissions. Utilizing the dimethylamino group as the proton receptor promoted a yellow-to-blue fluorescence switch in the tetra-BF2 complex and an OFF/ON fluorescence in the B-O-B bridged complex upon protonation. The helical chirality observed in the latter complex resulted in stable (P)/(M)-enantiomers after optical resolution. This complex exhibited circular dichroism with a |gabs| of up to 1.2 × 10-2 and circularly polarized luminescence with a |glum| on the order of 10-3 in solution and polymer film.

Nucleophilic Aromatic Substitution (SNAr) as an Approach to Challenging Nitrogen-Bridged BODIPY Oligomers

A series of nitrogen-bridged BODIPY oligomers were synthesized via nucleophilic aromatic substitution (SNAr) as a convenient approach. Further transformations achieved novel α,α-aryl BODIPY dimers as well as a BODIPY hexamer efficiently. These BODIPY oligomers showed good photophysical properties, such as apparent absorption and emission both in visible and near-infrared regions. Interestingly, the high air and photothermal stability, strong NIR absorption, and high photothermal conversion rates of hexamer B6 suggest potential applications in photothermal therapy.

Unsymmetrical Benzothieno-Fused BODIPYs as Efficient NIR Heavy-Atom-Free Photosensitizers

Heavy-atom-free photosensitizers are potentially suitable for use in photodynamic therapy (PDT). In this contribution, a new family of unsymmetrical benzothieno-fused BODIPYs with reactive oxygen efficiency up to 50% in air-saturated toluene was reported. Their efficient intersystem crossing (ISC) resulted in the generation of both 1O2 and O2-• under irradiation. More importantly, the PDT efficacy of a respective 4-methoxystyryl-modified benzothieno-fused BODIPY in living cells exhibited an extremely high phototoxicity with an ultralow IC50 value of 2.78 nM. The results revealed that the incorporation of an electron-donating group at the α-position of the unsymmetrical benzothieno-fused BODIPY platform might be an effective approach for developing long-wavelength absorbing heavy-atom-free photosensitizers for precision cancer therapy.

De Novo Access to BODIPY C-Glycosides as Linker-Free Nonsymmetrical BODIPY-Carbohydrate Conjugates

Recent years have witnessed an increasing interest in the synthesis and study of BODIPY-glycoconjugates. Most of the described synthetic methods toward these derivatives involve postfunctional modifications of the BODIPY core followed by the covalent attachment of the fluorophore and the carbohydrate through a "connector". Conversely, few de novo synthetic approaches to linker-free carbohydrate-BODIPY hybrids have been described. We have developed a reliable modular, de novo, synthetic strategy to linker-free BODIPY-sugar derivatives using the condensation of pyrrole C-glycosides with a pyrrole-carbaldehyde derivative mediated by POCl3. This methodology allows labeling of carbohydrate biomolecules with fluorescent-enough BODIPYs within the biological window, stable in aqueous media, and able to display singlet oxygen generation.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Synthesis, Structures and Photophysical Properties of Asymmetric Fulvene-[b]-fused BODIPYs

Herein, we developed a one-pot procedure to synthesize novel fulvene-[b]-fused BODIPYs from α-(2-alkynylphenyl)-pyrrole and acylpyrrole, using 5-exo cyclization as the key transformation. Compared to benzene-[b]-fused BODIPYs, although they have similar chemical compositions, their structures and properties significantly differ from each other, which can be attributed to the less aromaticity of the fulvene linker than benzene. Notably, fulvene-[b]-fused BODIPY 1 b exhibits helical-twisted core skeleton, intensified red-shifted absorption, and peak fluorescence. In addition, the pathway of this one-pot reaction and the mechanism of POCl3 mediated 5-exo cyclization have been proposed by a combining experimental and computational study.

Tunable Yellow to Near-Infrared Fluorescent Boron-Amino-Chelating Complexes with Stokes Shifts >100 nm

A series of diphenylboron-chelating N-substituent 8-aminoquinoline, 5-aminoquinoxaline, and 1-aminophenazine were prepared. They exhibit lowest energy absorption peaks of 444-766 nm, emission peaks of 563-820 nm, and quantum yields of up to 46.5%. Electrochemical and theoretical studies indicate that the N-substituent mainly determines the HOMO and the framework determines the LUMO, thus allowing for a wide-tuning of absorptions/emissions. Intramolecular charge transfer transition leads to large Stokes shifts of up to 166 nm. One selected compound showed satisfactory cytocompatibility and cytoplasm-targeting cell imaging ability.

Biotin and boron-dipyrromethene-tagged platinum(IV) prodrug for cellular imaging and mito-targeted photocytotoxicity in red light

A platinum(IV) prodrug, cis,cis,trans-[Pt(NH3)2Cl2(biotin)(L)] (1), derived from cisplatin, where HL is the PEGylated red-light active boron-dipyrromethene (BODIPY) ligand, was synthesized, characterized and its photocytotoxicity evaluated. The complex showed a near-IR absorption band at 653 nm (ε ∼9.19 × 104 M-1 cm-1) in dimethyl sulfoxide and Dulbecco's phosphate-buffered saline (1 : 1 v/v) at pH 7.2. When excited at 630 nm, it showed an emission band at 677 nm in DMSO with a fluorescence quantum yield of 0.13. The 1,3-diphenylisobenzofuran titration experiment gave a singlet oxygen quantum yield (ΦΔ) of ∼0.32. A mechanistic DNA photocleavage study revealed singlet oxygen as the reactive oxygen species (ROS). The complex with biotin and PEGylated-distyryl-BODIPY showed significantly higher cellular uptake in A549 cancer cells as compared to non-cancerous Beas-2B cells from flow cytometry, indicating selectivity towards cancer cells. A dichlorodihydrofluorescein diacetate assay showed cellular ROS generation. Confocal images revealed predominant internalization in the mitochondria. The prodrug showed remarkable photodynamic therapy (PDT) activity in cancerous A549 and multidrug-resistant MDA-MB-231 cells with a high photocytotoxicity index value (half-maximal inhibitory concentration (IC50): 0.61-1.54 μM in red light), while being non-toxic in the dark. The chemo-PDT activity was significantly less in non-tumorigenic lung epithelial cells (Beas-2B). The prodrug effectively triggered cellular apoptosis, which was confirmed by the Annexin V-FITC/propidium iodide assay, and the alteration of the mitochondrial membrane potential was substantiated by the JC-1 dye assay. The β-tubulin immunofluorescence assay confirmed that incubating the cells with a light-treated complex resulted in the rapture of the cytoskeletal structure and the formation of apoptotic bodies. The results demonstrate that the prodrug triggered apoptosis via DNA damage, a reduction in mitochondrial function and disruption of the cytoskeletal framework.

Tuning the Photophysical Properties of Aza-BODIPYs in the Near-Infrared Region by Introducing Electron-Donating Thiophene Substituents

This study presents the synthesis, the spectroscopic and electrochemical properties of new bis- and tetra-substituted azaboron-dipyrromethene (aza-BODIPY) dyes substituted by different electron donating groups connected to the aza-BODIPY core through a thiophene unit. In line with theoretical calculations, experimental measurements point out the positive impact of the thiophene group that behave as a secondary donor group leading to an enhancement of the intramolecular charge transfer process in comparison to previously reported aza-BODIPY dyes. This heterocycle has also been found to tune the oxidative potential and to stabilize the electro-generated species.

Regioselective Synthesis of Directly Connected BODIPY Dimers through Oxidative Coupling of α-Amino-Substituted BODIPYs

A family of directly β,β-linked BODIPY dimers with amino groups at α-positions were regioselectively prepared by the oxidative coupling reaction of α-amino-substituted BODIPYs. The structure of one representative dimer was elucidated by X-ray diffraction analysis, showing its twisted orientation of two BODIPY units with a dihedral angle of 49°. Comparing with the corresponding monomers, these dimers showed red-shifted absorptions and emissions along with efficient intersystem crossing, giving Φ_Δ of 43% for dimer 4b in toluene, indicating potential use as heavy-atom-free photosensitizers.

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.

Synthesis and Spectroscopic Properties of Selenophene[3, 2-b]-Fused BODIPYs

Fusion selenophene endows the chromophore with more intrinsic and special functions. Herein, nonsymmetric selenophene-fused BODIPYs were designed and synthesized starting from the selenophene unit. The fused ring of selenophene not only maintains the rigid structure of BODIPY but also further modulates its spectral properties. The newly prepared dyes possessed many promising properties including large molar extinction coefficients, low fluorescence quantum yields, and moderate singlet oxygen generation. Quantum calculations affirmed that the smaller singlet-triplet energy gap and larger spin-orbit coupling cause efficient intersystem crossing, thus enhancing the singlet oxygen generation yield. Furthermore, selenophene-fused BODIPY exhibited significant phototoxicity with negligible dark cytotoxicity, based on the fluorescence imaging of the reactive oxygen species detection experiment.

Accurate prediction of 11B NMR chemical shift of BODIPYs via machine learning

In this article, we present the results of developing a model based on an RFR machine learning method using the ISIDA fragment descriptors for predicting the ¹¹B NMR chemical shift of BODIPYs. The model is freely available at https://ochem.eu/article/146458. The model demonstrates the high quality of predicting the ¹¹B NMR chemical shift (RMSE, 5CV (FINALE training set) = 0.40 ppm, RMSE (TEST set) = 0.14 ppm). In addition, we compared the "cost" and the user-friendliness for calculations using the quantum-chemical model with the DFT/GIAO approach. The ¹¹B NMR chemical shift prediction accuracy (RMSE) of the model considered is more than three times higher and tremendously faster than the DFT/GIAO calculations. As a result, we provide a convenient tool and database that we collected for all researchers, that allows them to predict the ¹¹B NMR chemical shift of boron-containing dyes. We believe that the new model will make it easier for researchers to correctly interpret the ¹¹B NMR chemical shifts experimentally determined and to select more optimal conditions to perform an NMR experiment.

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.

Aryl-Boron-Substituted BODIPYs: Direct Access via Aluminum-Chloride-Mediated Arylation from Arylstannanes and Tuning the Optoelectronic Properties

An efficient procedure is presented for functionalization of BODIPYs at boron with arylstannanes as weak nucleophiles in the presence of aluminum chloride, providing new aryl-boron-substituted BODIPY and aza-BODIPY derivatives of singular importance. Most of these aryl-boron-substituted BODIPYs showed bright emission in the aqueous solution with significant aggregation-induced emission enhancement and high solid-state emission as a result of the restricted rotation of the meso-phenyl group and boron-substituted aryl groups as well as the formation of J-type aggregates.

Solvatochromic Sensitivity of BODIPY Probes: A New Tool for Selecting Fluorophores and Polarity Mapping

This research work is devoted to collecting a high-quality dataset of BODIPYs in a series of 10-30 solvents. In total, 115 individual compounds in 71 solvents are represented by 1698 arrays of the spectral and photophysical properties of the fluorophore. Each dye for a series of solvents is characterized by a calculated value of solvatochromic sensitivity according to a semiempirical approach applied to a series of solvents. The whole dataset is classified into 6 and 24 clusters of solvatochromic sensitivity, from high negative to high positive solvatochromism. The results of the analysis are visualized by the polarity mapping plots depicting, in terms of wavenumbers, the absorption versus emission, stokes shift versus - (absorption maxima + emission maxima), and quantum yield versus stokes shift. An analysis of the clusters combining several dyes in an individual series of solvents shows that dyes of a high solvatochromic sensitivity demonstrate regular behaviour of the corresponding plots suitable for polarity and viscosity mapping. The fluorophores collected in this study represent a high quality dataset of pattern dyes for analytical and bioanalytical applications. The developed tools could be applied for the analysis of the applicability domain of the fluorescent sensors.

Symmetry dual functional pyrimidine-BODIPY probes for imaging targeting and activity study

Nondestructive diagnosis of tumor has always been the goal of scientists. Fluorescent dyes have become the rising star in the field of cancer diagnosis because of their excellent characteristics. Therefore, in this work, fluorescence probes d-Y-B and dO-Y-B with anti-tumor activity were constructed by introducing pyrimidine groups with high anti-tumor activity using fluorescence dye BODIPY as parent nucleus. The modified BODIPY group in the structure had the advantage of fluorescent dye, ensuring the strong fluorescence and photosensitivity of the target compound. That ethylenediamine acts as a bridge with two -NH- groups to increase molecular hydrogen bonding, and can bind firmly to multiple proteins. Co-localization of the target compounds d-Y-B and dO-Y-B with the hoechst dye for labeling living cells showed that these compounds had high biocompatibility and photostability for localization to HeLa cells. In vivo imaging in mice can realize specific localization and real-time visualization of tumor cells. The results of cytotoxicity experiments in vitro and computer software simulating molecular docking confirmed the potential of the target compounds as an anticancer agents. The bifunctional probe realized visualization of cancer cells in mice, and can kill cancer cells by anti-proliferation, which may provide a direction for future anticancer drug development.

Sensing lysozyme fibrils by salicylaldimine substituted BODIPY dyes - A correlation with molecular structure

Quick and efficient detection of protein fibrils has enormous impact on the diagnosis and treatment of amyloid related neurological diseases. Among several methods, fluorescence based techniques have garnered most importance in the detection of amyloid fibrils due to its high sensitivity and extreme simplicity. Among other classes of molecular probes, BODIPY derivatives have been employed extensively for the detection of amyloid fibrils. However, there are very few studies on the relationship between the molecular structure of BODIPY dyes and their amyloid sensing activity. Here in a BODIPY based salicylaldimine Schiff base and its corresponding boron complex have been evaluated for their ability to sense amyloid fibrils from hen-egg white lysozyme using steady state and time-resolved spectroscopic techniques. Both dyes show fluorescence enhancement as well as increase in their excited state lifetime upon their binding with lysozyme fibrils. However, the BODIPY derivative which shows more emission enhancement in fibrillar solution has much lower affinity towards amyloid fibrils as compared to other derivative. This contrasting behaviour in the emission enhancement and binding affinity has been explained on the basis of differences in their photophysical properties in water and amyloid fibril originating from the difference in their molecular structure. Such correlation between the amyloid sensitivity and the molecular structure of the probe can open up a new strategy for designing new efficient amyloid probes.

Aryl/Heteroaryl Substituted Boron-Difluoride Complexes Bearing 2-(Isoquinol-1-yl)pyrrole Ligands Exhibiting High Luminescence Efficiency with a Large Stokes Shift

A series of 2-(isoquinol-1-yl)pyrrole-boron complexes possessing (hetero)aryl substituents on the pyrrole and/or isoquinoline moiety were prepared. These compounds exhibited the fluorescence emission character in both solution and solid state. In most cases, the large Stokes shift and high fluorescence quantum yield in the solution were compatible. Furthermore, the structural diversity allowed the precise tuning of emitting colors from light blue to red with strong emission intensity. The present paper describes their comprehensive optical characteristics dependent on the type and position of the substituted aryl groups by the experimental and computational studies.

Stable, Bright, and Long-Fluorescence-Lifetime Dyes for Deep-Near-Infrared Bioimaging

Near-infrared (NIR) fluorophores absorbing maximally in the region beyond 800 nm, i.e., deep-NIR spectral region, are actively sought for biomedical applications. Ideal dyes are bright, nontoxic, photostable, biocompatible, and easily derivatized to introduce functionalities (e.g., for bioconjugation or aqueous solubility). The rational design of such fluorophores remains a major challenge. Silicon-substituted rhodamines have been successful for bioimaging applications in the red spectral region. The longer-wavelength silicon-substituted congeners for the deep-NIR spectral region are unknown to date. We successfully prepared four silicon-substituted bis-benzannulated rhodamine dyes (ESi5a-ESi5d), with an efficient five-step cascade on a gram-scale. Because of the extensive overlapping of their HOMO-LUMO orbitals, ESi5a-ESi5d are highly absorbing (λ_abs ≈ 865 nm and ε > 10⁵ cm^-1 M^-1). By restraining both the rotational freedom via annulation and the vibrational freedom via silicon-imparted strain, the fluorochromic scaffold of ESi5 is highly rigid, resulting in an unusually long fluorescence lifetime (τ > 700 ps in CH₂Cl₂) and a high fluorescence quantum yield (ϕ = 0.14 in CH₂Cl₂). Their half-lives toward photobleaching are 2 orders of magnitude longer than the current standard (ICG in serum). They are stable in the presence of biorelevant concentration of nucleophiles or reactive oxygen species. They are minimally toxic and readily metabolized. Upon tail vein injection of ESi5a (as an example), the vasculature of a nude mouse was imaged with a high signal-to-background ratio. ESi5 dyes have broad potentials for bioimaging in the deep-NIR spectral region.

Engineering BODIPY-based near-infrared nanoparticles with large Stokes shifts and aggregation-induced emission characteristics for organelle specific bioimaging

Lipid droplets (LDs) and lysosomes, as two important subcellular organelles, play specific and indispensable roles in various cellular processes. The development of efficient LD- and lysosome-specific fluorescent bio-probes is of great importance. However, current commercial lipid droplet- (LD) and lysosome-specific fluorescent specific bio-probes often suffer from the aggregation-caused quenching (ACQ) effect, short absorption and emission wavelengths, poor photostability and low specificity. Herein, a typical ACQ luminogen BODIPY was directly conjugated to strong electron donating triarylamine units at its α-positions, giving near-infrared (NIR) fluorescent materials TPAB and 2TPAB with aggregation-induced emission (AIE). Both TPAB and 2TPAB nanoparticles were obtained by self-assembly, and showed NIR emissions, large Stokes shifts, good photostability and two-photon absorption. These nanoparticles presented remarkable bioimaging performances and were shown to specifically localize in LDs or lysosomes, respectively, depending on the number of triarylamine units attached. They have been successfully used to detect endogenous LD overproduction, and monitor abnormal activities of LDs/lysosomes, as well as real-time track the lipophagy process in cells. Their far NIR emission and two-photon excitation further supported their promising bioimaging application for lipid droplet tracking in liver tissue and live zebrafish larvae. Our work here enriches BODIPY based NIR AIE dyes and provides organelle specific bio-probes which are superior to currently used commercial ones.

Highly Fluorescent Bipyrrole-Based Tetra-BF2 Flag-Hinge Chromophores: Achieving Multicolor and Circularly Polarized Luminescence

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF₂ ) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S₁ excited state. As the rotational motion was suppressed around the 2,2'-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF₂ complexes can potentially realize novel photofunctional materials.

Bodipy-carbohydrate systems: synthesis and bio-applications

Luminescent BODIPY-sugar probes have stimulated the attention of researchers for the potential applications of such molecular systems in bio-imaging. The presence of carbohydrate units confers unique structural and biological features, beside enhancement of water solubility and polarity. On the other hand, BODIPY (BOronDiPYrromethene) derivatives represent eclectic and functional luminescent molecules because of their outstanding photophysical properties. This article provides a review on the synthesis and applications of BODIPY-linked glycosyl probes in which the labelling of complex carbohydrates with BODIPY allowed the disclosing of their in vivo behaviour or where the sugar constitutes a recognition element for specific targeting probes, or, finally, in which the stereochemical characteristics of the carbohydrate hydroxyl groups play as structural elements for assembling more than one photoactive subunit, resulting in functional supramolecular molecules with modulable properties. We describe the methods we have used to construct various multiBODIPY molecular systems capable of functioning as artificial antennas exhibiting extremely efficient and fast photo-induced energy transfer. Some of these systems have been designed to allow the modulation of energy transfer efficiency and emission color, and intensity dependent on their position within a biological matrix. Finally, future perspectives for such BODIPY-based functional supramolecular sugar systems are also highlighted.

Silver-mediated, direct phosphorylation of BODIPY dyes at the 3- or 3,5-positions with H-phosphonates

A direct and regioselective C-H/P-H cross-coupling of dialkyl phosphites, and diphenylphosphine oxide to easily available BODIPYs through an Ag-mediated radical addition, resulted in a series of new α-phosphorylated BODIPY fluorophores under mild conditions. Hydrolysis of the phosphonate gave the corresponding BODIPY phosphoric acid, which is soluble and fluorescent in water with a high quantum yield of 0.83.

Simple and efficient visualization of aromaticity: bond currents calculated from NICS values

Aromaticity is a fundamental concept in chemistry, underpinning the properties and reactivity of many organic compounds and materials. The ability to easily and accurately discern aromatic behavior is key to leveraging it as a design element, yet most aromaticity metrics struggle to combine accurate quantitative evaluation, intuitive interpretability, and user-friendliness. We introduce a new method, NICS2BC, which uses simple and inexpensive NICS calculations to generate information-rich and easily-interpreted bond-current graphs. We test the quantitative and qualitative characterizations afforded by NICS2BC for a selection of molecules of varying structural and electronic complexity, to demonstrate its accuracy and ease of analysis. Moreover, we show that NICS2BC successfully identifies ring-current patterns in molecules known to be difficult cases to interpret with NICS and enables deeper understanding of local aromaticity trends, demonstrating that our method adds additional insight.

Red-Shift (2-Hydroxyphenyl)-Benzothiazole Emission by Mimicking the Excited-State Intramolecular Proton Transfer Effect

Novel strategies to optimize the photophysical properties of organic fluorophores are of great significance to the design of imaging probes to interrogate biology. While the 2-(2-hydroxyphenyl)-benzothiazole (HBT) fluorophore has attracted considerable attention in the field of fluorescence imaging, its short emission in the blue region and low quantum yield restrict its wide application. Herein, by mimicking the excited-state intramolecular proton transfer (ESIPT) effect, we designed a series of 2-(2-hydroxyphenyl)-benzothiazole (HBT) derivatives by complexing the heteroatoms therein with a boron atom to enhance the chance of the tautomerized keto-like resonance form. This strategy significantly red-shifted the emission wavelengths of HBT, greatly enhanced its quantum yields, and caused little effect on molecular size. Typically, compounds 12B and 13B were observed to emit in the near-infrared region, making them among the smallest organic structures with emission above 650 nm.

Recent advances in zig-zag-fused BODIPYs

Recent progress in the synthesis of zig-zag-fused BODIPY, structure–property relationships, as well as their applications are summarized.

Synthesis, Reactivity, and Properties of a Class of π-Extended BODIPY Derivatives

A new family of π-extended BODIPY derivatives were obtained through the condensation of aldehyde and pyrrole in aqueous solution in the presence of HCl. The new rigid π-framework extends beyond the dipyrromethene unit, which is significantly different from classical BODIPYs in the electronic configuration. Both π-extended BODIPYs displayed intense absorption and moderate emission with maxima around 565 and 620 nm, respectively, and showed interesting reactivity toward various nucleophiles. Moreover, these π-extended BODIPYs were developed as fluorescent probes for rapid and selective detection of GSH and were successfully applied for live-cell imaging.