Metal-Free and Versatile Synthetic Routes to Natural and Synthetic Prodiginines from Boron Dipyrrin

Synthesis and studies of covalently linked (BODIPY)2-3-pyrrolyl BODIPY triads

Two novel covalently linked triads containing one 3-pyrrolyl BODIPY unit and two BODIPY units were synthesized over a sequence of steps using meso-(p-formylphenyl)-3-pyrrolyl BODIPY as a key precursor. Both triads were thoroughly characterized and studied by using HRMS, 1D and 2D NMR spectroscopy, X-ray crystallography, absorption, steady-state and time-resolved fluorescence, cyclic voltammetry, and DFT/TD-DFT techniques. X-ray structural analysis of these triads revealed that the dipyrrin units of both meso-BODIPY and 3-pyrrolyl BODIPY were oriented in the same plane whereas the dipyrrin unit of the α-BODIPY unit was oriented almost perpendicular to that of the 3-pyrrolyl BODIPY unit. Both triads showed two well-separated absorption bands corresponding to BODIPY and 3-pyrrolyl BODIPY units in the 510-675 nm region. In these triads, the BODIPY unit absorbs in the higher energy region and acts as an energy donor whereas the 3-pyrrolyl BODIPY unit absorbs in the lower energy region and acts as an energy acceptor. Steady-state and time-resolved fluorescence studies of these triads indicated the possibility of intramolecular singlet-singlet energy transfer with almost 98% efficiency from the BODIPY units to the 3-pyrrolyl BODIPY unit upon selective excitation of the BODIPY unit in both triads. DFT and TD-DFT investigations corroborated the experimental findings.

An α-benzithiazolyl 3-pyrrolyl BODIPY probe for ratiometric selective sensing of cyanide ions and bioimaging studies

A simple chromo-fluorogenic chemodosimeter probe, α-benzithiazolyl 3-pyrrolyl BODIPY, was synthesized by reacting α-formyl 3-pyrrolyl BODIPY with 2-aminothiophenol in DMF at reflux under basic conditions. The probe was structurally characterized by X-ray, HR-MS, and 1D & 2D NMR techniques. The X-ray structure revealed that the appended pyrrole was almost in the plane with a small deviation of 12.15° from the 12-atom mean plane of the BF2-dipyrrin core and the benzithiazolyl moiety was also deviated by 18.74° from the BF2-dipyrrin core. The α-benzithiazolyl 3-pyrrolyl BODIPY exhibits one intense absorption band at 608 nm and a less intense band at 412 nm corresponding to the 3-pyrrolyl BODIPY and benzithiazolyl moiety, respectively. The strongly fluorescent probe shows one intense emission band at 637 nm with a quantum yield of 0.48. The probe acted as an exclusive colorimetric and chemodosimetric sensor for CN- ions over other anions with high sensitivity (LOD = 13 nM) and quick response time (10 s) in an aqueous CH3CN medium. The CN- ion attacks the imine group of the benzithiazolyl moiety of 3via a nucleophilic addition reaction and converts the sp2 to sp3 carbon which disrupts the conjugation between the 3-pyrrolyl BODIPY and benzithiazolyl moieties, which is reflected in the clear colour change from red fluorescence to blue fluorescence as well as significant changes in the spectral and electrochemical properties. The detection of cyanide with the probe for biological applications was also performed with plant tissue. DFT/TD-DFT studies were in agreement with the experimental observations.

A white light emitting single halochromic hydrazine bridged bis(3-pyrrolyl BODIPY) fluorophore

In search of white light emitting fluorophores, a hydrazine bridged Schiff base compound, bis(3-pyrrolyl BODIPY), was synthesized by condensing readily available α-formyl 3-pyrrolyl BODIPY with hydrazine hydrate in CH3OH under reflux for 5 h followed by recrystallization. Bis(3-pyrrolyl BODIPY) was thoroughly characterized by HR-MS, 1D and 2D NMR, and X-ray crystallography. The X-ray structure revealed that the 3-pyrrolyl BODIPY units in the dyad were arranged trans to each other with respect to the hydrazine moiety. Bis(3-pyrrolyl BODIPY) showed absorption bands in the region of 390-705 nm and exhibited multiple fluorescence bands in the region of 395-720 nm at different excitation wavelengths. The protonated derivative of bis(3-pyrrolyl BODIPY) generated by the addition of TFA to its CH2Cl2 solution showed significant changes in the optical properties and generated white fluorescence under UV light with specific emission bands observed in blue, green, and red regions, indicating that bis(3-pyrrolyl)BODIPY is a single white light emitting halochromic fluorophore under acidic conditions. DFT and TD-DFT studies justify the structural and electronic properties of the protonated derivative of bis(3-pyrrolyl BODIPY) exhibiting white light emission.

Synthesis, Structure, Spectral, Electrochemical, and Theoretical Studies of Ru(II) Complexes of 3-Pyrrolyl BODIPY-Based Ligands

3-Pyrrolyl BODIPY having an appended pyrrolyl group at the 3-position of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) and its functionalized derivatives have been used as ligands to prepare one example of Ru(II) complex of pyrrolyl dipyrrin and three unique examples of bichromophoric BODIPY-Ru(II) complexes in good yields. The complexes were characterized by HR-MS, 1D and 2D NMR, X-ray analysis for two complexes, absorption, cyclic voltammetry, and DFT/TD-DFT techniques. The X-ray structure of the Ru(II) complex of pyrrolyl dipyrrin revealed that the geometry around the Ru(II) ion was pseudo-octahedral with an arene unit occupying three coordination sites in η6-fashion and two nitrogen atoms of the dipyrrin ring and one chloro group adopting the "three legs piano-stool" structure. The X-ray structure of the bichromophoric BODIPY-Ru(II) complex revealed that the BODIPY core was planar and the central B(III) was coordinated with two pyrrole nitrogens of the dipyrrin unit and two axial fluoride ions in a tetrahedral geometry, and Ru(II) was bonded to appended pyrrole "N" and "N" of benzimidazole substituent present at the α-position of appended pyrrole, one chloro group, and one arene ring in a pseudo-octahedral geometry. The spectral studies revealed that the electronic properties of the BODIPY unit in BODIPY-Ru(II) complexes were significantly altered, and electrochemical studies indicated that the BODIPY-Ru(II) complexes exhibit one oxidation corresponding to Ru(II) to Ru(III) and one reduction corresponding to the BODIPY unit. The DFT/TD-DFT studies corroborated the experimental observations.

Anticooperative Supramolecular Oligomerization Mediated by V-Shaped Monomer Design and Unconventional Hydrogen Bonds

After more than three decades of extensive investigations on supramolecular polymers, strategies for self-limiting growth still remain challenging. Herein, we exploit a new V-shaped monomer design to achieve anticooperatively formed oligomers with superior robustness and high luminescence. In toluene, the monomer-oligomer equilibrium is shifted to the monomer side, enabling the elucidation of the molecular packing modes and the resulting (weak) anticooperativity. Steric effects associated with an antiparallel staircase organization of the dyes are proposed to outcompete aromatic and unconventional B-F⋅⋅⋅H-N/C interactions, restricting the growth at the stage of oligomers. In methylcyclohexane (MCH), the packing modes and the anticooperativity are preserved; however, pronounced solvophobic and chain-enwrapping effects lead to thermally ultrastable oligomers. Our results shed light on understanding anticooperative effects and restricted growth in self-assembly.

Substitution at boron in BODIPYs

Recent synthetic achievements have led to 4,4-disubstituted-4-bora-3a,4a-diaza-s-indacenes (BODIPYs) with varying substituents at the meso, pyrrolic and/or boron sites, with each influencing photophysical properties and utility. This Feature article gives an overview of chemistry at the boron atom in BODIPYs, highlighting our contributions that evolved from synthetic curiosities and now offer this dipyrrolic skeleton potential across a wider range of applications. We first summarise preparative routes to BODIPYs through complexation of boron with the dipyrrinato ligand. The role of boron in protecting dipyrrins is then discussed, followed by strategies by which to achieve facile substitution at the boron atom.

Synthesis of pyrrolyl-BODIPY dyes through regioselective SN Ar reactions and application as a fluorescent sensor for fluoride anion

Two pyrrolyl-BODIPY dyes with 3,5-di-[Formula: see text]-butyl-4-hydroxyphenyl group were synthesized through stepwise S[Formula: see text]Ar reactions of 3,5-dibromoBODIPYs, which were used as a fluorescent sensor for basic anions. The intermediate pyrrolyl-BODIPYs 2a–2b were regioselectively synthesized through an efficient S[Formula: see text]Ar reaction between 3,5-dibromoBODIPY 1a and pyrroles. The target pyrrolyl-BODIPYs 3a–3b with a 3,5-di-[Formula: see text]-butyl-4-hydroxyphenyl group at 3-position and a pyrrole substituent at 5-position were obtained through a second S[Formula: see text]Ar reaction between pyrrolyl-BODIPYs 2a–2b and high steric hindrance 2,6-dibutylphenol in 90% and 88% yields, respectively. In contrast, the reaction between pyrrolyl-BODIPYs 2a–2b and phenol gave pyrrolyl-BODIPYs 3c–3d with phenoxy substituent at 3-position. These pyrrolyl-BODIPYs 3a–2d show strong, sharp absorptions (551–604 nm) and emissions (564–634 nm) with high fluorescence quantum yields up to 0.86 in dichloromethane. Importantly, the 3,5-di-[Formula: see text]-butyl-4-hydroxyphenyl group of pyrrolyl-BODIPY 3a showed a turn-off fluorescent response toward fluoride anion.

Facile deprotection of F-BODIPYs using methylboronic acid

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacenes (F-BODIPYs) are deprotected through removal of the -BF₂ moiety upon treatment with methylboronic acid. The tolerance of various substitution patterns about the dipyrrinato core is demonstrated via the deprotection of thirteen F-BODIPYs and an F-aza-BODIPY. Work-up with aq. HBr affords the desired dipyrin HBr salt in quantitative yield without need for purification.

Deuteration and tautomeric reactivity of the 1-methyl functionality of free-base dipyrrins

Regioselective reactivity of the 1-methyl group of free-base dipyrrins is explored, including discussion of tautomerism to provide exocyclic alkenyl reactivity. Deuterium is installed so as to generate dipyrrins substituted with deuterated methyl groups. Furthermore, the 1-methyl group reacts to become involved in C-C bonds involving only sp³-hybridised carbon atoms. The isolation of an elusive framework featuring a dipyrrin substituted with a pyrrole in a non-vinylogous fashion is also reported. The use of asymmetric dipyrrins featuring an electron-withdrawing group on one of the pyrrolic units results in regioselective reaction of the alpha-methyl group distal to the electron-withdrawing group.

Synthesis and characterization of pyrrolyldipyrrin F-BODIPYs

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

Novel Alkoxy-Substituted Dipyrrins and Near-IR BODIPY Dyes-Preparation and Photophysical Properties

Starting from 3-alkoxy-2-aryl-substituted pyrroles and aromatic aldehydes, a collection of new dipyrrins was prepared. Under the standard conditions of Treibs, these were converted into the corresponding boron dipyrrins (BODIPYs). Compounds of this type with alkoxy groups at C-3 position of both pyrrole subunits are new and hence the photophysical properties of this collection of novel dipyrrins and BODIPY dyes were investigated. The dipyrrins show absorption maxima up to 596 nm and emissions of up to 677 nm. For the BODIPY series a remarkable effect of the alkoxy groups was identified, resulting in red shifts for absorptions and emissions. The compound substituted with two 2-thien-2-yl groups and a meso-C₆ F₅ substituent shows an absorption maximum at 725 nm and emits at 754 nm and thus is a new representative of a near-IR BODIPY dye related to certain aza-BODIPYs. Our results demonstrate the influence of the alkoxy groups on the spectroscopic data and reveal the potential of 3-alkoxy-2-aryl-substituted pyrroles for the design of new fluorophores.

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 and anticancer activity of prodigiosenes bearing C-ring esters and amides

Ten novel prodigiosenes with anticancer activity.