Synthesis and Spectroscopic Investigation of a Series of Push-Pull Boron Dipyrromethenes (BODIPYs)

Thieno[3,2-b]thiophene-based bridged BODIPY dimers: synthesis, electrochemistry, and one- and two-photon photophysical properties

Four BODIPY dyes (6a-6d) with electron-donating or electron-withdrawing groups at the meso-position were synthesized by the Sonogashira coupling reaction of 2,5-diethynylthieno[3,2-b]thiophene with mono-iodo-BODIPY moieties. All compounds were fully characterized by 1H NMR and MALDI-TOF MS. Their photophysical and electrochemical properties were studied by UV-visible absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, two-photon excitation spectroscopy and cyclic voltammetry. These conjugated dyes exhibit interesting photophysical properties such as a high molar extinction coefficient, large Stokes shift and high two-photon absorption cross section σ2.

Organic BODIPY Based Gels: Optical, Electrochemical and Self-Assembly Properties

Two novel BODIPY dyes, BOC3 and BC12, were synthesized with variable alkyl chains at terminal amide functional units. BC12, featuring a longer alkyl chain (-C12H25), formed a gel compared to BOC3, which has a shorter alkyl chain (< C->CH2OCH3), due to supra molecular self-assembly in film. Both dyes exhibited absorption peaks around 530 nm in the visible region, with a red shift of about 30 nm in the film state, essential for organic electronic applications. Concentration variation studies revealed π-π stacking/aggregates in the solid state causing red shifts in absorption and emission. BC12 exhibited more significant red shifts in film compared to its solution state due to supra molecular self-assembly. Electronic structure analysis using density functional theories (BMK and O3LYP) showed better correlation with absorption using the O3LYP method. Both dyes displayed quasi-irreversible oxidation and reduction couples with suitable HOMO (5.46 eV) and LUMO (3.32 eV) energy levels for organic electronic applications. Transient photoluminescence studies indicated a longer lifetime for BC12 (5.28 ns) than BOC3 (4.50 ns), suggesting π-π aggregation and supra molecular self-assembly. BC12's gelation, attributed to its long alkyl chain and two-dimensional motifs of the BODIPY core, forms spherical-shaped nano networks. These findings underscore the potential of molecularly tuned dyes with alkyl chains for nano-sized self-assembly in organic electronic devices. Red shifts were observed due to combination of aggregation, stacking and columnar meso-phase formation in supramolecular assembly. Absorption spectra of dyes in toluene with various concentrations showed the formation of Aggregation/π-π stacking might be due to head to tailing interactions.

Tracking the Intramolecular Charge Transfer Process of 2,6-Substituted D-A BODIPY Derivatives

Three 2,6-electron donor-substituted boron dipyrromethene (BODIPY) exhibiting an intramolecular charge transfer (ICT) character with large Stokes shift and moderate fluorescence quantum yields were designed and synthesized. Broadband femtosecond transient absorption (fs-TA) spectroscopy measurements were performed to directly detect the CT state in nonpolar or less polar solvents and the charge separation (CS) state in more polar solvents. A solid foundation for the fs-TA assignment can be found in electrolysis experiments. In addition, the ICT character of the newly designed compounds was investigated by density functional theory (DFT) calculations. Meanwhile, the reference compounds without the donor groups were synthesized, and their photophysical behaviors and ultrafast time-resolved spectra confirmed that no ICT process occurred regardless of the nature of the solvent. This work emphasizes the importance of decorating the BODIPY core with electron-donating substituents at 2,6-positions to efficiently adjust its photofunctional behaviors demonstrating the ICT character. Importantly, the photophysical processes could be easily regulated by changing the solvent with different polarities.

Cell Uptake of Steroid-BODIPY Conjugates and Their Internalization Mechanisms: Cancer Theranostic Dyes

Estradiol-BODIPY linked via an 8-carbon spacer chain and 19-nortestosterone- and testosterone-BODIPY linked via an ethynyl spacer group were evaluated for cell uptake in the breast cancer cell lines MCF-7 and MDA-MB-231 and prostate cancer cell lines PC-3 and LNCaP, as well as in normal dermal fibroblasts, using fluorescence microscopy. The highest level of internalization was observed with 11β-OMe-estradiol-BODIPY 2 and 7α-Me-19-nortestosterone-BODIPY 4 towards cells expressing their specific receptors. Blocking experiments showed changes in non-specific cell uptake in the cancer and normal cells, which likely reflect differences in the lipophilicity of the conjugates. The internalization of the conjugates was shown to be an energy-dependent process that is likely mediated by clathrin- and caveolae-endocytosis. Studies using 2D co-cultures of cancer cells and normal fibroblasts showed that the conjugates are more selective towards cancer cells. Cell viability assays showed that the conjugates are non-toxic for cancer and/or normal cells. Visible light irradiation of cells incubated with estradiol-BODIPYs 1 and 2 and 7α-Me-19-nortestosterone-BODIPY 4 induced cell death, suggesting their potential for use as PDT agents.

Mitochondrial Targeting and Imaging with Small Organic Conjugated Fluorophores: A Review

The last decade has seen an increasingly large number of studies reporting on the development of novel small organic conjugated systems for mitochondrial imaging exploiting optical signal transduction pathways. Mitochondria are known to play a critical role in a number of key biological processes, including cellular metabolism. Importantly, irregularities on their working function are nowadays understood to be intimately linked to a range of clinical conditions, highlighting the importance of targeting mitochondria for therapeutic benefits. In this work we carry out an in-depth evaluation on the progress to date in the field to pave the way for the realization of superior alternatives to those currently existing. The manuscript is structured by commonly used chemical scaffolds and comprehensively covers key aspects factored in design strategies such as synthetic approaches as well as photophysical and biological characterization, to foster collaborative work among organic and physical chemists as well as cell biologists.

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.

Effect of the Aza-N-Bridge and Push-Pull Moieties: A Comparative Study between BODIPYs and Aza-BODIPYs

In the field of fluorescent dyes, difluoroboron-dipyrromethenes (BODIPY) have a highly respected position. To predict their photophysical properties prior to synthesis and therefore to successfully design molecules specifically for one's needs, a solid structure-function understanding based on experimental observations is vital. This work delivers a photophysical evaluation of BODIPY and aza-BODIPY derivatives equipped with different electron-withdrawing/-donating substituents. Using combinatorial chemistry, pyrroles substituted with electron-donating/-withdrawing substituents were condensed together in two different manners, thus providing two sets of molecules. The only difference between the two sets is the bridging unit providing a so far lacking comparison between BODIPYs and aza-BODIPYs structural homologues. Replacing the meso-methine bridge with an aza-N bridge results in a red-shifted transition and considerably different, temperature-activated, excited-state relaxation pathways. The effect of electron-donating units on the absorption but not emission for BODIPYs was suppressed compared to aza-BODIPYs. This result could be evident in a substitution pattern-dependent Stokes shift. The outlook of this study is a deeper understanding of the structure-optics relationship of the (aza)-BODIPY-dye class, leading to an improvement in the de novo design of tailor-made molecules for future applications.

Accurate predictions of the electronic excited states of BODIPY based dye sensitizers using spin-component-scaled double-hybrid functionals: a TD-DFT benchmark study

The vertical excitation energies of 13 BODIPY based dye sensitizers are benchmarked by means of TD-DFT, using 36 functionals from different DFT rungs. Most TD-DFT results were found to overestimate the excitation energies, and show mean absolute error (MAE) values in the range 0.2-0.5 eV. The dispersion-corrected, spin-component-scaled, double-hybrid (DSD) functionals DSD-BLYP and DSD-PBEP86 were found to have the smallest MAE values of 0.083 eV and 0.106 eV, respectively, which is close to the range of average errors found in the more expensive coupled-cluster methods. Moreover, DSD-BLYP and DSD-PBEP86 functionals show excellent consistency and quality of results (standard deviation = 0.048 eV and 0.069 eV respectively). However, the range separated hybrid (RSH) and the range separated double hybrid (RSDH) functionals were found to provide the best predictability (linear determination coefficient R 2 > 0.97 eV).

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.

Solvent and Anion Effects on the Electrochemistry of Manganese Dipyrrin-Bisphenols

A series of "N₂O₂-type" manganese dipyrrin-bisphenols (DPP), formulated as (Ar)DPPMn, where Ar = pentafluorophenyl (F₅Ph), phenyl (Ph), or mesityl (Mes), were electrochemically and spectroscopically characterized in nonaqueous media with and without added anions in the form of tetrabutylammonium salts (TBAX where X = ClO₄^-, PF₆^-, BF₄^-, F^-, Cl^-, OH^-, or CN^-). Two major one-electron reductions are observed under most solution conditions, the first of which is assigned as a Mn^III/II process and the second as electron addition to the π-ring system as confirmed by spectroelectrochemistry. Each Mn^III complex also exhibits one or two one-electron oxidations, the exact number depending upon the positive potential limit of the electrochemical solvent. The two oxidations are separated by 580-590 mV in CH₃CN containing 0.1 M TBAPF₆ and are assigned as π-ring-centered electron transfers to stepwise form a (Ar)DPPMn^III π-cation radical and dication under these solution conditions. Comparisons are made between redox properties of (Ar)DPPMn and manganese(III) porphyrins, corroles, and corrolazines each of which contains an innocent trianionic complexing ligand. The redox behavior and spectroscopic properties of [(Ar)DPPMn]ⁿ where n = 0, -1, or +1 are also compared to that of other structurally related [(Ar)DPPM]ⁿ complexes under similar solution conditions where M = Co^II, Cu^II, B^III, or Au^III.

β-AlkenylBODIPY Dyes: Regioselective Synthesis via Oxidative C-H Olefination, Photophysical Properties, and Bioimaging Studies

A series of 2-alkenyl- and 2,6-dialkenylboron dipyrromethene (BODIPY) derivatives were synthesized through Pd(II)-catalyzed regioselective and stereoselective oxidative C-H olefination in one step. The 2-alkenyl BODIPY derivative further reacted with various amines regioselectively at the 5-position through direct oxidative nucleophilic substitution. The photophysical properties of the 2-alkenyl- and 2,6-dialkenyl-substituted BODIPYs were investigated, which showed great potential in fluorescent bioimaging.

A Straightforward Substitution Strategy to Tune BODIPY Dyes Spanning the Near-Infrared Region via Suzuki⁻Miyaura Cross-Coupling

In this study, a series of new red and near-infrared (NIR) dyes derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) were developed by introducing thiophene and its derivatives to the 3- and 5- positions of the dichloroBODIPY core. For the first time, cyclictriol boronates and N-methyliminodiacetic acid (MIDA) boronate were used as organoboron species to couple with 3,5-dichloroBODIPY via the one-step Suzuki–Miyaura cross-coupling. Six kinds of thieno-expended BODIPY dyes were synthesized in acceptable yields ranging from 31% to 79%. All six dyes showed different absorption and emission wavelengths spanning a wide range (c.a. 600–850 nm) in the red and NIR regions with relatively high quantum yields (19–85%). Cellular imaging of 8-(2,6-dimethylphenyl)-re3,5-di(2-thienyl)-BODIPY (dye 1) was conducted using bovine cumulus cells, and the fluorescence microscopy images indicated that the chromophore efficiently accumulated and was exclusively localized in the cytoplasm, suggesting it could be utilized as a subcellular probe. All six dyes were characterized using 1H-NMR and mass spectrometry.

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.

Disilanylene-bridged BODIPY-based D–σ–A architectures: a novel promising series of NLO chromophores

A novel series of disilanylene-bridged BODIPY-based chromophores have been reported.

Design and Evaluation of a Boron Dipyrrin Electrophore for Redox Flow Batteries

A boron dipyrrin (BODIPY) dye was designed as a molecular single-component electrophore for redox flow batteries. All positions of the BODIPY core were assessed on the basis of literature data, in particular cyclic voltammetry and density functional calculations, and a minimum required substitution pattern was designed to provide solubility, aggregation, radical cation and anion stabilities, a large potential window, and synthetic accessibility. In-depth electrochemical and physical studies of this electrophore revealed suitable cathodic behavior and stability of the radical anion but rapid anodic decomposition of the radical cation. The three products that formed under the conditions of controlled oxidative electrolysis were isolated, and their structures were determined by spectroscopy and comparison with a synthetic model compound. From these structures, a benzylic radical reactivity, initiated by one-electron oxidation, was concluded to play the major role in this unexpected decomposition.

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.

Synthesis of BODIPY-Peptide Conjugates for Fluorescence Labeling of EGFR Overexpressing Cells

Regioselective functionalization of 2,3,5,6,8-pentachloro-BODIPY 1 produced unsymmetric BODIPY 5, bearing an isothiocyanate group suitable for conjugation, in only four steps. The X-ray structure of 5 reveals a nearly planar BODIPY core with aryl dihedral angles in the range 47.4-62.9°. Conjugation of 5 to two EGFR-targeting pegylated peptides, 3PEG-LARLLT (6) and 3PEG-GYHWYGYTPQNVI (7), under mild conditions (30 min at room temperature), afforded BODIPY conjugates 8 and 9 in 50-80% isolated yields. These conjugates showed red-shifted absorption and emission spectra compared with 5, in the near-IR region, and were evaluated as potential fluorescence imaging agents for EGFR overexpressing cells. SPR and docking investigations suggested that conjugate 8 bearing the LARLLT sequence binds to EGFR more effectively than 9 bearing the GYHWYGYTPQNVI peptide, in part due to the lower solubility of 9, and its tendency for aggregation at concentrations above 10 μM. Studies in human carcinoma HEp2 cells overexpressing EGFR demonstrated low dark and photo cytotoxicities for BODIPY 5 and the two peptide conjugates, and remarkably high cellular uptake for both conjugates 8 and 9, up to 90-fold compared with BODIPY 5 after 1 h. Fluorescence imaging studies in HEp2 cells revealed subcellular localization of the BODIPY-peptide conjugates mainly in the Golgi apparatus and the cell lysosomes. The low cytotoxicity of the new conjugates and their remarkably high uptake into EGFR overexpressing cells renders them promising imaging agents for cancers overexpressing EGFR.

Enhanced Hypsochromic Shifts, Quantum Yield, and π-π Interactions in a meso,β-Heteroaryl-Fused BODIPY

We report the synthesis and investigation of an unprecedented 8-heteroaryl-fused BODIPY 4. This compound exhibits enhanced π-π stacking in the solid state, unusually large blue-shifts in the absorbance and emission spectra, and higher fluorescence quantum yield than its unfused precursor; DFT calculations suggest a small energy gap for 4 and strong electronic communication between the 8-OPh and the BODIPY core.