Synthesis and transformations of 5-chloro-2,2'-dipyrrins and their boron complexes, 8-chloro-BODIPYs

BODIPY derivatives modified with carborane clusters: synthesis, characterization and DFT studies

An efficient approach for the preparation of 3,5-dicarborane-substituted BODIPY conjugates was developed via the functionalization of 3,5-dibromo-8-pentafluorophenyl-BODIPY with neutral and anionic carborane S-nucleophiles. It was found that 3,5-dicarborane-substituted BODIPYs could be easily modified with a third carborane cluster using S_NAr substitution reactions of the para-fluorine atom in the meso-pentafluorophenyl BODIPY substituent with the corresponding carborane S-nucleophile affording boron-enriched BODIPYs in good yields. The influence of bromine atom substitution with carborane moieties on the position of absorption and fluorescence bands and the fluorescence quantum yields of the prepared BODIPYs were analyzed. The crystal structures of BODIPYs 4 and 8 were investigated. Density functional theory methods (DFT wb97xd/6-31G* and wb97xd/lanl2dz) were performed to study the geometrical structures, electronic characteristics, the highest occupied and the lowest unoccupied molecular orbitals (HOMOs and LUMOs) and other chemical descriptors of the synthesized compounds.

BODIPY Photocatalyzed Beckmann Rearrangement and Hydrolysis of Oximes under Visible Light

A novel, efficient, and mild protocol for rearrangement of oximes to amides or hydrolyzing to ketone/aldehyde using a simple BODIPY dye as a photocatalyst and air as an oxidant via propagation reaction under visible-light irradiation is reported. The triplet excited state of BODIPY played a significant role in the catalytic process. It was found that the various substituted ketoximes, both with electron-withdrawing and electron-donating substituents, afforded the corresponding products with moderate to excellent yields, and the catalytic efficiency was up to 0.01 mol %.

Rational Design of Meso-Phosphino-Substituted BODIPY Probes for Imaging Hypochlorite in Living Cells and Mice

Meso-phosphino-substituted BODIPY probes were developed for concise and rapid detection of hypochlorite (ClO^-). Interestingly, the probe BP gave a turn-on fluorescence response by shutting the photoinduced electron transfer (PET) effect and extending the coplanar conjugated π-system. In contrast, the probe TMBP showed a colorimetric response toward ClO^-. The key role of the steric repulsions was revealed to be for altering the electronic distribution of the BODIPY core, resulting in these obviously different responses. Finally, the probe BP, with high selectivity and sensitivity toward ClO^- (LOD = 1.9 nM; response time, <15 s), was further employed in imaging the variations of exogenous and endogenous hypochlorite (ClO^-) in living RAW 264.7 cells and mouse inflammation models. If wisely utilized, this strategy with meso-phosphino BODIPY dyes may serve as a powerful platform for the preparation of novel chemosensors.

Impact of iodine loading and substitution position on intersystem crossing efficiency in a series of ten methylated-meso-phenyl-BODIPY dyes

Four core and six distyryl-extended methylated-meso-phenyl-BODIPY dyes with varying iodine content were synthesized. The influence of iodine loading and substitution position on the photophysical properties of these chromophores was evaluated. Selective iodine insertion at the 2- and 6-positions of the methylated-meso-phenyl-BODIPY core, rather than maximum iodine content, resulted in the highest intersystem crossing efficiency. Iodination of the distyryl-extended BODIPY core afforded intersystem crossing quantum yields comparable to 2,6-diiodo-BODIPY. Inclusion of an iodine at the para-meso-phenyl position generally enhanced non-radiative decay in the BODIPY excited-state, leading to lower fluorescence and intersystem crossing quantum yield values. Iodine substitution at the styryl-positions resulted in negligible changes to the excited-state dynamics. This study highlights: (1) the rate of radiative decay is similar in all ten derivatives (on the order of 1 × 108 s-1), (2) iodination of the 2,6-positions results in the greatest enhancement of intersystem crossing efficiency, (3) care must be taken when modifying the para-meso-phenyl position as it could have detrimental effects on the excited-state dynamics, (4) the excited-state is negligibly affected by iodination of the styryl groups, potentially enabling orthogonal functionalization without modifying the molecular photophysics, (5) distyryl extension of the chromophore core diminishes rates of non-radiative decay and intersystem crossing, resulting in higher fluorescence quantum yields and lower intersystem crossing yields in the π-extended derivatives compared to the core BDP derivatives, and (6) DFT calculations provide insight into the electronic and structural factors regulating intersystem crossing and vibrational relaxation in these molecules.

Torsion-Induced Nonradiative Relaxation of the Singlet Excited State of meso-Thienyl Bodipy and Charge Separation, Charge Recombination-Induced Intersystem Crossing in Its Compact Electron Donor/Acceptor Dyads

We prepared a series of meso-thienyl boron-dipyrromethene (Bodipy) derivatives to investigate the spin-orbit charge transfer intersystem crossing (SOCT-ISC). The photophysical properties of the compounds were studied by steady-state and femtosecond/nanosecond transient absorption spectroscopy, as well as density functional theory (DFT) computations. Different from the meso-phenyl Bodipy analogues, the meso-thienyl Bodipy are weakly fluorescent. Based on femtosecond transient absorption and DFT computations, we propose that the torsion of the thienyl group and the distortion of the Bodipy core (19.7 ps) in the S₁ state lead to a conical intersection on the potential energy surface as an efficient nonradiative decay channel (408 ps), which is responsible for the observed weak fluorescence as compared to the meso-phenyl analogue. The increased fluorescence quantum yield (from 5.5 to 14.5%) in viscous solvents supports this hypothesis. With the electron donor 4'-hydroxylphenyl moiety attached to the meso-thienyl unit, the fast charge separation (CS, 15.3 ps) and charge recombination (CR, 238 ps) processes outcompete the torsion-induced nonradiative decay and induce fast ISC through the SOCT-ISC mechanism. The triplet quantum yield of the electron donor/acceptor dyad is highly dependent on solvent polarity (Φ_T = 1.9-45%), which supports the SOCT-ISC mechanism, and the triplet-state lifetime is up to 247.3 μs. Using the electron donor-acceptor dyad showing SOCT-ISC as a triplet photosensitizer, efficient triplet-triplet annihilation (TTA) upconversion was observed with a quantum yield of up to 6.0%.

Tuning the optical properties of BODIPY dyes by N-rich heterocycle conjugation using a combined synthesis and computational approach

We report a systematic tuning of optical properties of BODIPY dyes by conjugation of nitrogen-rich heterocycles, and underlying nitrogen influence by TDDFT calculations.

Modulating the Optical Properties of BODIPY Dyes by Noncovalent Dimerization within a Flexible Coordination Cage

Aggregation of organic molecules can drastically affect their physicochemical properties. For instance, the optical properties of BODIPY dyes are inherently related to the degree of aggregation and the mutual orientation of BODIPY units within these aggregates. Whereas the noncovalent aggregation of various BODIPY dyes has been studied in diverse media, the ill-defined nature of these aggregates has made it difficult to elucidate the structure-property relationships. Here, we studied the encapsulation of three structurally simple BODIPY derivatives within the hydrophobic cavity of a water-soluble, flexible PdII6L4 coordination cage. The cavity size allowed for the selective encapsulation of two dye molecules, irrespective of the substitution pattern on the BODIPY core. Working with a model, a pentamethyl-substituted derivative, we found that the mutual orientation of two BODIPY units in the cage's cavity was remarkably similar to that in the crystalline state of the free dye, allowing us to isolate and characterize the smallest possible noncovalent H-type BODIPY aggregate, namely, an H-dimer. Interestingly, a CF3-substituted BODIPY, known for forming J-type aggregates, was also encapsulated as an H-dimer. Taking advantage of the dynamic nature of encapsulation, we developed a system in which reversible switching between H- and J-aggregates can be induced for multiple cycles simply by addition and subsequent destruction of the cage. We expect that the ability to rapidly and reversibly manipulate the optical properties of supramolecular inclusion complexes in aqueous media will open up avenues for developing detection systems that operate within biological environments.

Synthesis and investigation of BODIPYs with restricted meso-8-aryl rotation

Three BODIPYs bearing 1,3,5,7-tetramethyl substituents and a meso-8-aryl group were synthesized and investigated, both experimentally and computationally. The presence of the 1,7-methyl groups and of ortho-substituents on the meso-8-aryl ring prevent free rotation of the meso-8-aryl group, resulting in high fluorescence quantum yields. Substitution at the 2,6-positions of these BODIPYs with chlorine atoms causes pronounced red-shifted absorptions and emissions, and in the case of 2,6-dichloro-1,3,5,7-tetramethyl-8-(2,4,6-triphenylphenyl)-BODIPY 2c increases its fluorescence quantum yields to 0.93 in dichloromethane and 0.98 in toluene. The X-ray structure of 1,3,5,7-tetramethyl-8-(2,4,6-triphenylphenyl)-BODIPY shows increased deviation from planarity and smaller dihedral angle of the meso-8-aryl group compared with the meso-8-phenyl- and meso-8-mesityl-BODIPY analogs. The presence of 2,6-chlorine atoms was found to not significantly affect the rotational barriers of the meso-8-aryl-groups.

gem-Dibromovinyl boron dipyrrins: synthesis, spectral properties and crystal structures

A family of new asymmetric and symmetric 1,3,7,9-tetramethyl-4,4-bora difluoro-diaza-s-indacene (BODIPY) derivatives, bearing gem-dibromovinyl substituents, was synthesized by the Corey-Fuchs olefination method. One or two gem-dibromovinyl moieties were attached at either the p-position of 5-phenyl, or the β-position of the pyrrole ring, directly or, through phenyl spacers. The assigned structures were supported by MS, NMR (¹H, ¹³C, ¹⁹F), X-ray diffraction analysis and for some compounds 2D HSQC and ¹¹B NMR as well as optical spectroscopy. Their absorption and fluorescence properties and solvatochromism in different solvents were investigated. The highest absorption and emission maxima were obtained for compounds having two gem-dibromovinyl groups attached directly or through the phenyl spacer. The best correlation (R-coefficient) between the solvent and spectral properties of the BODIPYs were obtained using the refractive index of the solvent. Although these compounds are structurally quite similar, their solid states show remarkable differences in the crystal system, clearly revealing two distinct patterns of gem-dibromovinyl orientation and torsion angles of the 5-phenyl ring and the indacene plane. Hirshfeld surface analysis data were used to visualize various intermolecular interactions.

BODIPY-based hydrazine as a fluorescent probe for sensitive and selective detection of nitric oxide: a new strategy

A novel fluorescent probe 8-HB was developed with a BODIPY as a fluorophore and 8-substituted hydrazine as a reactive site for sensitive and selective detection of nitric oxide (NO), generating major fluorescent dehydrazinated BODIPY and minor non-fluorescent azide BODIPY.

Stability of a Series of BODIPYs in Acidic Conditions: An Experimental and Computational Study into the Role of the Substituents at Boron

Boron-dipyrromethene (BODIPY) dyes have been extensively investigated in recent years for a variety of bioanalytical and bioimaging applications. The success of these applications relies on the stability of BODIPYs, particularly under acidic conditions. In this work, the stability of a series of 4,4'-disubstituted BODIPYs (-F, -CN, -Ph, -Me, -OMe) toward addition of excess trifluoroacetic acid (TFA) was studied systematically and comprehensively through ¹¹B and ¹H NMR, UV-vis, fluorescence, thin layer chromatography, mass spectrometry, and infrared. The results indicate that 4,4'-dicyano-BODIPY 2 is the most stable among this series and remains unchanged even 3 days after addition of excess TFA. On the other hand, 4,4'-dimethyl-BODIPY 3 and 4,4'-dimethoxy-BODIPY 5 are the least stable, toward addition of TFA, and the 4,4'-diphenyl and 4,4'-difluoro-BODIPYs 1 and 4 were found to have intermediate stability. The experimental analysis and comparison with theoretical calculations indicate that the 4,4'-dicyano-BODIPY 2 has the greater aromaticity of the series, as evaluated by the BLA parameter, decreased charge on boron, and upon TFA addition it forms an unusually stable BODIPY 2···TFA complex. On the other hand, all other BODIPYs decompose within hours after TFA addition. Computational modeling demonstrates that 4,4'-dicyano substitution increases aromaticity and stabilizes the B-N bond, resulting in the most stable compound from the series studied.

Further investigation on the nitration of BODIPY with cupric nitrate: crystal structures of 4,4-di-fluoro-1,3,5,7,8-penta-methyl-2-nitro-4-bora-3a,4a-di-aza-s-indacene, 4,4-di-fluoro-3-nitro-8-phenyl-4-bora-3a,4a-di-aza-s-indacene, and 3-chloro-6-ethyl-5,7,8-trimethyl-2-nitro-4,4-diphenyl-4-bora-3a,4a-di-aza-s-indacene

The treatment of non-fully substituted 4,4-di-fluoro-4-bora-3a,4a-di-aza-s-indacene (BODIPY) with cupric nitrate leads to the introduction of a nitro group at different positions of the BODIPY core, depending on the substitution pattern. This methodology complements the treatment of fully substituted BODIPY with cupric nitrate that was previously reported. The crystal structures of 4,4-di-fluoro-1,3,5,7,8-penta-methyl-2-nitro-4-bora-3a,4a-di-aza-s-indacene, C14H16BF2N3O2 (5a) 4,4-di-fluoro-3-nitro-8-phenyl-4-bora-3a,4a-di-aza-s-indacene, C15H10BF2N3O2(5b) and 3-chloro-6-ethyl-5,7,8-trimethyl-2-nitro-4,4-diphenyl-4-bora-3a,4a-di-aza-s-indacene, C26H25BClN3O2 (5d) are presented. In all three structures, the fused ring system is in a very flattened 'V-shape', with dihedral angles between the two outer five membered rings of 8.12 (14), 6.67 (9) and 12.30 (18) Å for 5a, 5b and 5d, respectively. In each case, the central six-membered ring is in a flattened sofa conformation. In the crystal of 5a, mol-ecules are linked by weak C-H⋯O and C-H⋯F hydrogen bonds forming sheets parallel to (10-1). In the crystal of 5b mol-ecules are linked by weak C-H⋯O and C-H⋯F hydrogen bonds and π-π inter-actions forming sheets parallel to (001). In the crystal of 5d, weak C-H⋯O hydrogen bonds link mol-ecules into chains along [001]. In compound 5d, the atoms of the nitro group were refined as disordered over two sets of sites with occupancies 0.618 (12) and 0.382 (12).

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

A series of push-pull BODIPYs bearing multiple electron-donating and electron-acceptor groups were synthesized regioselectively from 2,3,5,6,8-pentachloro-BODIPY, and characterized by NMR spectroscopy, HRMS, and X-ray crystallography. The influence of the push-pull substituents on the spectroscopic and electrochemical properties of BODIPYs was investigated. Bathochromic shifts were observed for both absorbance (up to 37 nm) and emission (up to 60 nm) in different solvents upon introduction of the push-pull moieties. DFT calculations, consistent with the spectroscopic and cyclic voltammetry studies, show decreased HOMO-LUMO energy gaps upon the installation of the push-pull moieties. BODIPY 7 bearing thienyl groups on the 2 and 6 positions showed the largest λmax for both absorption (635-653 nm) and emission (706-707 nm), but also the lowest fluorescence quantum yields. All BODIPYs were nontoxic in the dark (IC50 > 200 μM) and showed low phototoxicity (IC50 > 100 μM, 1.5 J/cm2) toward human HEp2 cells. Despite the relatively low fluorescence quantum yields, the push-pull BODIPYS were effective for cell imaging, readily accumulating within cells and localizing mainly in the ER and Golgi. Our structure-property studies can guide future design of functionalized BODIPYs for various applications, including bioimaging and in dye-sensitized solar cells.

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.

Synthesis and regioselective functionalization of perhalogenated BODIPYs

Three perhalogenated BODIPYs (1b-3b), bearing chloro and bromo groups at all carbon positions, were synthesized and characterized. The reactivity of BODIPY 3b was investigated under Stille cross-coupling reactions, and single crystal X-ray analysis was used to confirm the regioselectivity of the reactions. Further substitution at the boron atom produced nona-functionalized BODIPYs 7a,b, which show 676 and 739 nm emissions with 91 and 100 nm Stokes shifts, respectively.

Synthesis and in Vitro Studies of a Series of Carborane-Containing Boron Dipyrromethenes (BODIPYs)

A series of seven BODIPYs functionalized with ortho-carborane groups at the 8(meso) or 3/5(α) position were synthesized and characterized by NMR, HRMS, HPLC, and in the cases of 2b and 5b, by X-ray analysis. The BODIPYs exhibited low dark toxicity and phototoxicity toward human glioma T98G cells, and their cellular uptake varied significantly, with 5b accumulating the most and 7 the least. All BODIPYs localized mainly within the cell ER. The BODIPYs showed higher permeabilities than lucifer yellow across human hCMEC/D3 brain endothelial cell monolayers as the BBB model. Among this series, 1b showed the highest BBB permeability (Pe = 16.4 × 10(-5) cm/s), probably as a result of its lower MW (366 Da) and favorable hydrophobicity (log P = 1.5). The combination of low cytotoxicity, amphiphilicity, high boron content, high cellular uptake, and moderate BBB permeability renders these compounds promising boron delivery agents for the BNCT of brain tumors.

Synthesis and spectroscopic properties of β,β'-dibenzo-3,5,8-triaryl-BODIPYs

A series of β,β'-bicyclo-3,5-diaryl-BODIPYs were synthesized from the corresponding β,β'-bicyclo-3,5-diiodo-BODIPYs (1a,b) via Pd(0)-mediated Suzuki cross-coupling reactions in 82-92% yields. Subsequent aromatization with DDQ afforded the corresponding β,β'-dibenzo-aryl-BODIPYs, which showed red-shifted absorptions and emissions in the near-IR range. The dibenzo-appended BODIPYs showed characteristic 1H-, 13C-, 11B- and 19F-NMR shifts, and nearly planar conformations by X-ray crystallography.

Panchromatic chromophore–tetrapyrrole light-harvesting arrays constructed from Bodipy, perylene, terrylene, porphyrin, chlorin, and bacteriochlorin building blocks

Five new chromophore–tetrapyrrole arrays bearing an ethynyl linker have been synthesized to explore the effects of chromophore nature and tetrapyrrole attachment site on panchromatic spectral properties.

Synthesis and properties of a series of carboranyl-BODIPYs

A series of four BODIPYs containing one or two ortho- or para-carborane clusters were synthesized using palladium(0)-catalyzed Suzuki cross-coupling or nucleophilic substitution reactions, at the 2,6- or the 8-positions of halogenated boron dipyrromethenes (BODIPYs). The spectroscopic, structural (including one X-ray) and in vitro BBB permeability of the BODIPYs using hCMEC/D3 brain endothelial cells were investigated.

Tailoring the Solid-State Fluorescence Emission of BODIPY Dyes by meso Substitution

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) derivatives bearing varied substituents at the meso position (i.e., CF3 , CH3 , COOR, CHO, CN, Cl, iPr) were synthesized to elucidate the structure-property relationships that give rise to emissive J-aggregates. Several new BODIPY derivatives can be added to the previously reported 1,3,5,7-tetramethyl-8-trifluoromethyl derivative to the list of those forming J-aggregates, in addition to other dyes that are emissive in the solid state without forming J-aggregates.

Stepwise Polychlorination of 8-Chloro-BODIPY and Regioselective Functionalization of 2,3,5,6,8-Pentachloro-BODIPY

An effective, stepwise methodology for polychlorination of BODIPY using trichloroisocyanuric acid (TCCA) in acetic acid was developed. In this way, selectively substituted di-, tri-, tetra-, and pentachloro-BODIPYs 2-5 were prepared. The pentachloro-BODIPY is shown to undergo regioselective Pd(0)-catalyzed Stille and Suzuki coupling reactions, first at the 8-position followed by the 3,5- and then the 2,6-positions; nucleophilic substitution reactions occur first at the 8- followed by the 3,5-positions, while the 2,6 are unreactive.

Synthesis of 3,8-dichloro-6-ethyl-1,2,5,7-tetramethyl-BODIPY from an asymmetric dipyrroketone and reactivity studies at the 3,5,8-positions

The asymmetric BODIPY 1 a (BODIPY=4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), containing two chloro substituents at the 3,8-positions and a reactive 5-methyl group, was synthesized from the asymmetric dipyrroketone 3, which was readily obtained from available pyrrole 2 a. The reactivity of 3,8-dichloro-6-ethyl-1,2,5,7-tetramethyl-BODIPY 1 a was investigated by using four types of reactions. This versatile BODIPY undergoes regioselective Pd(0) -catalyzed Stille coupling reactions and/or regioselective nucleophilic addition/elimination reactions, first at the 8-chloro and then at the 3-chloro group, using a variety of organostannanes and N-, O-, and S-centered nucleophiles. On the other hand, the more reactive 5-methyl group undergoes regioselective Knoevenagel condensation with an aryl aldehyde to produce a monostyryl-BODIPY, and oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) gives the corresponding 5-formyl-BODIPY. Investigation of the reactivity of asymmetric BODIPY 1 a led to the preparation of a variety of functionalized BODIPYs with λmax of absorption and emission in the ranges 487-587 and 521-617 nm, respectively. The longest absorbing/emitting compound was the monostyryl-BODIPY 16, and the largest Stokes shift (49 nm) and fluorescence quantum yield (0.94) were measured for 5-thienyl-8-phenoxy-BODIPY 15. The structural properties (including 16 X-ray structures) of the new series of BODIPYs were investigated.

Meso enyne substituted BODIPYs: synthesis, structure and properties

We report the synthesis of meso enyne substituted BODIPYs by the reaction of 8-chloro BODIPY with terminal alkynes under Sonogashira coupling conditions, and by Pd–Cu catalyzed hydroalkynylation reaction of terminal alkynes, across the –CC– bond of meso alkynylated BODIPYs.

Site-specific conjugation of 8-ethynyl-BODIPY to a protein by [2 + 3] cycloaddition

We report a straightforward synthesis of 8-ethynyl-BODIPY derivatives and their potential as fluorescent labeling compounds using an alkyne–azide click chemistry approach.

Functionalization of 3,5,8-trichlorinated BODIPY dyes

Catalytic hydrogenation of dibenzyl 5-dipyrroketone-2,9-dicarboxylates followed by decarboxylative iodination affords a 2,9-diiododipyrroketone which gives a 2,5,9-trichlorodipyrromethene hydrochloride after nucleophilic addition/elimination, with adventitious chloride to replace the two iodide groups. Treatment with BF3·Et2O gives a 3,5,8-trichloro-BODIPY that readily undergoes regioselective Stille coupling at the 8-position, or homo/mixed couplings at the 3,8- or 3,5- and 8-positions. Stepwise and controlled replacement of the 3,5- and 8-chlorine atoms using Stille reagents results in formation of a completely unsymmetrical trisubstituted BODIPY. Several examples of unsymmetrical BODIPYs were synthesized and characterized using this methodology. Structure features of new BODIPYs are discussed within the context of 14 new X-ray structures, and photophysical parameters of all new BODIPY compounds are reported and discussed.