Substituents modification of meso-aryl BODIPYs for tuning photophysical properties

We successfully synthesized eight meso-aryl BODIPYs with 2,6-diethyl- or 1,2,6,7-tetraethyl substituents and characterized their photophysical properties. The steric hindrance resulting from the phenolic group in the meso-aryl moiety and the ethyl groups on the BODIPY core affected the synthesis of dipyrromethanes as an intermediate as well as the UV–Vis absorption and fluorescence emission of the BODIPYs due to the constrained rotation of the aryl ring. The potential use of the meso-hydroxyphenyl BODIPY as a pH sensor was also shown by the pH-dependent fluorescence emissions.     

Electron-Withdrawing Substituents Allow Boosted NIR-II Fluorescence in J-Type Aggregates for Bioimaging and Information Encryption

Developing molecular fluorophores with enhanced fluorescence in aggregate state for the second near-infrared (NIR-II) imaging is highly desirable but remains a tremendous challenge due to the lack of reliable design guidelines. Herein, we report an aromatic substituent strategy to construct highly bright NIR-II J-aggregates. Introduction of electron-withdrawing substituents at 3,5-aryl and meso positions of classic boron dipyrromethene (BODIPY) skeleton can promote slip-stacked J-type arrangement and further boost NIR-II fluorescence of J-aggregates via increased electrostatic repulsion and intermolecular hydrogen bond interaction. Notably, NOBDP-NO₂ with three nitro groups (−NO₂) shows intense NIR-II fluorescence at 1065 nm and high absolute quantum yield of 3.21 % in solid state, which can be successfully applied in bioimaging, high-level encoding encryption, and information storage. Moreover, guided by this electron-withdrawing substituent strategy, other skeletons (thieno-fused BODIPY, aza-BODIPY, and heptamethine cyanine) modified with −NO₂ are converted into J-type aggregates with enhanced NIR-II fluorescence, showing great potential to convert aggregation caused emission quenching (ACQ) dyes into brilliant J-aggregates. This study provides a universal method for construction of strong NIR-II emissive J-aggregates by rationally manipulating molecular packing and establishing relationships among molecular structures, intermolecular interactions, and fluorescence properties.     

Dual Chromophoric Meso-Aryl BODIPYs with Aggregation Induced Red-Emission Characteristics: Optical Properties and their Application in Fe3+ Detection

Herein, we report the design of meso-aryl BODIPYs as a structural motif for aggregation-caused quenching (ACQ) to aggregation-induced emission (AIE) transformation. A series of meso-aryl BODIPY derivatives were synthesized, by systematically increasing the size of the chromophore at the meso-position from phenyl to pyrene. The effect of various factors, such as the aryl ring size, solvents, viscosity, and metal cations, on the photophysical properties was analyzed. The emission properties are well correlated with the flexibility of the aromatic ring for free rotation around the C_aryl−C_BODIPY bond. Accordingly, meso-phenanthrene BODIPY ( PhB ) has the highest emission characteristics. The emission property of less bulky aryl-substituted BODIPYs increases by increasing the solvent viscosity. The interaction of Fe³⁺ ions with aryl-BODIPYs provides a prominent photophysical response based on Lewis-acid supported decomplexation of BF₂ in aryl-BODIPYs. The bichromophoric meso-aryl BODIPYs exhibit notable intramolecular excitation energy transfer from the aromatic ring to the BODIPY core, which is higher in meso-anthracene BODIPY( AB ). Hence, decorating BODIPYs with polycyclic aromatic systems generates a twisted structure, which inhibits the π-π stacking between the planar aromatic molecules. This can be proposed as an effective approach at the molecular level to convert planar aryl luminophores having ACQ to AIEgens. Besides, the meso-pyrene BODIPY derivative shows excellent mechanofluorochromic behaviour.     

Self-assembly of emissive metallocycles with tetraphenylethylene,BODIPY and terpyridine in one system

ABSTRACT

Tetraphenylethylene (TPE) related (supra)molecules have been intensively investigated due to their aggregation-induced emission (AIE) effect based on the restriction of intramolecular rotation (RIR). Meanwhile, boron-dipyrromethene (BODIPY) tends to emit intense fluorescence with high quantum yields. Herein, we combined TPE, BODIPY and terpyridine (TPY) into one system to study the emissive behaviour of organic building block as well as a self-assembled metallo-supramolecule. The TPY and BODIPY substituents with bulky sizes provide strong hindrance to restrict the rotation of the phenyl groups on TPE, leading to enhancement of emissive properties in both solution and aggregation states. Furthermore, the BODIPY-TPE-TPY ligand (L) was assembled with Zn (II) through coordination-driven self-assembly to form a cyclic dimer (D) with typical AIE characteristics.     

Synthesis and Characterization of Far‐Red/NIR‐Fluorescent BODIPY Dyes,Solid‐State Fluorescence,and Application as Fluorescent Tags Attached to Carbon Nano‐onions

A series of π‐extended distyryl‐substituted boron dipyrromethene (BODIPY) derivatives with intense far‐red/near‐infrared (NIR) fluorescence was synthesized and characterized, with a view to enhance the dye’s performance for fluorescence labeling. An enhanced brightness was achieved by the introduction of two methyl substituents in the meso positions on the phenyl group of the BODIPY molecule; these substituents resulted in increased structural rigidity. Solid‐state fluorescence was observed for one of the distyryl‐substituted BODIPY derivatives. The introduction of a terminal bromo substituent allows for the subsequent immobilization of the BODIPY fluorophore on the surface of carbon nano‐onions (CNOs), which leads to potential imaging agents for biological and biomedical applications. The far‐red/NIR‐fluorescent CNO nanoparticles were characterized by absorption, fluorescence, and Raman spectroscopies, as well as by thermogravimetric analysis, dynamic light scattering, high‐resolution transmission electron microscopy, and confocal microscopy.     

Dihydronaphthalene‐Fused Boron–Dipyrromethene (BODIPY) Dyes: Insight into the Electronic and Conformational Tuning Modes of BODIPY Fluorophores

A new series of boron–dipyrromethene (BDP, BODIPY) dyes with dihydronaphthalene units fused to the β‐pyrrole positions ( 1 a – d , 2 ) has been synthesised and spectroscopically investigated. All the dyes, except pH‐responsive 1 d in polar solvents, display intense emission between 550–700 nm. Compounds 1 a and 1 b with a hydrogen atom and a methyl group in the meso position of the BODIPY core show spectroscopic properties that are similar to those of rhodamine 101, thus rendering them potent alternatives to the positively charged rhodamine dyes as stains and labels for less polar environments or for the dyeing of latex beads. Compound 1 d , which carries an electron‐donating 4‐(dimethylamino)phenyl group in the meso position, shows dual fluorescence in solvents more polar than dibutyl ether and can act as a pH‐responsive “light‐up” probe for acidic pH. Correlation of the pK_a data of 1 d and several other meso‐(4‐dimethylanilino)‐substituted BODIPY derivatives allowed us to draw conclusions on the influence of steric crowding at the meso position on the acidity of the aniline nitrogen atom. Preparation and investigation of 2 , which carries a nitrogen instead of a carbon as the meso‐bridgehead atom, suggests that the rules of colour tuning of BODIPYs as established so far have to be reassessed; for all the reported couples of meso‐C‐ and meso‐N‐substituted BODIPYs, the exchange leads to pronounced redshifts of the spectra and reduced fluorescence quantum yields. For 2 , when compared with 1 a , the opposite is found: negligible spectral shifts and enhanced fluorescence. Additional X‐ray crystallographic analysis of 1 a and quantum chemical modelling of the title and related compounds employing density functional theory granted further insight into the features of such sterically crowded chromophores.     

Effect of Alkyl,Aryl, and <Emphasis Type="Italic">meso</Emphasis>-Aza Substitution on the Thermal Stability of BODIPY

The effect of peripheral alkyl, aryl, and meso-aza substitution on the thermal stability of BODIPYs in an argon or oxygen atmosphere has been analyzed using thermogravimetric study results. It has been shown that an increase in the length of 2,6-alkyl substituents to seven carbon atoms is accompanied by the growth of BODIPY thermal stability by 80°C. The greatest increase in the destruction temperature of BODIPY (by 100°C) is attained via the introduction of phenyl groups in the 1,3,5,7-positions of its dipyrromethenmethene framework. meso-Aza substitution does not almost produce any effect on the thermal stability of BODIPY dyes. The BODIPY destruction beginning temperature decreases by 60–90°C in the presence of air oxygen. The thermal stability of BODIPY tends to decrease with reducing degree and symmetry of alkyl substitution in the dipyrromethene framework. A lower thermal stability of BODIPY in comparison with zinc(II) dipyrromethenates is due to the participation of fluorine atoms in intramolecular redox processes.     

meso‐Ester and Carboxylic Acid Substituted BODIPYs with Far‐Red and Near‐Infrared Emission for Bioimaging Applications

A series of meso‐ester‐substituted BODIPY derivatives 1–6 are synthesized and characterized. In particular, dyes functionalized with oligo(ethylene glycol) ether styryl or naphthalene vinylene groups at the α positions of the BODIPY core ( 3 – 6 ) become partially soluble in water, and their absorptions and emissions are located in the far‐red or near‐infrared region. Three synthetic approaches are attempted to access the meso‐carboxylic acid (COOH)‐substituted BODIPYs 7 and 8 from the meso‐ester‐substituted BODIPYs. Two feasible synthetic routes are developed successfully, including one short route with only three steps. The meso‐COOH‐substituted BODIPY 7 is completely soluble in pure water, and its fluorescence maximum reaches around 650 nm with a fluorescence quantum yield of up to 15 %. Time‐dependent density functional theory calculations are conducted to understand the structure–optical properties relationship, and it is revealed that the Stokes shift is dependent mainly on the geometric change from the ground state to the first excited singlet state. Furthermore, cell staining tests demonstrate that the meso‐ester‐substituted BODIPYs ( 1 and 3 – 6 ) and one of the meso‐COOH‐substituted BODIPYs ( 8 ) are very membrane‐permeable. These features make these meso‐ester‐ and meso‐COOH‐substituted BODIPY dyes attractive for bioimaging and biolabeling applications in living cells.     

Self-selection in the self-assembly of isomeric supramolecular squares from unsymmetrical bis(4-pyridyl)acetylene ligands

A new approach wherein steric interactions between substituents of unsymmetrical bis(4-pyridyl)acetylene ligands dictate the self-selection of single isomers of [4 + 4] self-assembled squares is presented. Each [4 + 4] self-assembly is characterized by multinuclear (31)P and (1)H NMR spectroscopies and electrospray ionization mass spectrometry. NMR spectroscopic studies are used to provide a means of evaluating the efficiency of bulky substituents at proximal or remote positions relative to the Pt-N bonding motif to direct self-selection. Molecular modeling using the MMFF force field is utilized to determine the relative energy of different isomers of each assembly, and modeling results reasonably explain the trend in self-selectivity with varying pyridyl substitution.     

Development of Lysosome-Targeted Fluorescent Probes for Cys by Regulating the Boron-dipyrromethene (BODIPY) Molecular Structure

Our previous discovery suggested that substituents on the 1,7 positions delicately modulate the sensing ability of the meso-arylmercapto boron-dipyrromethene (BODIPY) to biothiols. In this work, the impact of delicate modulations on the sensing ability is investigated. Therefore, 1,7-dimethyl, 3,5-diaryl substituted BODIPY is designed and developed and its conformationally restricted species with a meso-arylmercapto moiety ( DM-BDP-SAr and DM-BDP-R-SAr ) as selective fluorescent probes for Cys. Moreover, the lysosome-target probes ( Lyso-S and Lyso-D ) based on DM-BDP-SAr carrying one or two morpholinoethoxy moieties were developed. They were able to detect Cys selectively in vitro with low detection limits. Both Lyso-S and Lyso-D localized nicely in lysosomes in living HeLa cells and exhibited red fluorescence for Cys. Moreover, a novel fluorescence quenching mechanism was proposed from the calculations by density functional theory (DFT). The probes may go through intersystem crossing (from singlet excited state to triplet excited state) to result in fluorescence quenching.     

Aliphatic Amine Discrimination by Pentafluorophenyl Dibromo BODIPY

Two new fluorescent BODIPY dyes have been designed and synthesized. They dyes differ in their meso substituents, which have different electronic properties. Their selective reactivity towards an Ar‐S_N2 reaction has been explored as a potential basis for colorimetric and fluorescent discrimination of primary, secondary and tertiary aliphatic amines. This dual‐mode, instantaneous recognition event is unprecedented.     

Synthesis and Opto-electronic Properties of BODIPY o-OPhos Systems

Herein, we report the versatile synthetic strategy and opto-electronic properties for the phosphorylation of BODIPY derivatives 5aa - 5ak by substituting with an electron-donating/withdrawing group at the ortho position. Nevertheless, this new methodology relatively promotes the tolerance of the aldehyde moiety and the high yield for the synthesis of BODIPY o-OPhos derivatives. The photophysical studies suggest improved optical properties due to the inductive effect of various electron-donating/withdrawing groups. The UV-visible and the emission data suggest that BODIPY o-OPhos derivatives emphasize the property of the excited states with an increase in fluorescence intensity and high quantum yields due to the presence of bulky phospsho-triester at the meso- position which hinders the free rotation around the C-Ar bond and facilitates the development of OLEDs and various organophosphorus warfare agents. Electrochemical studies reveal 5ak depicts the ease of redox activity amongst the 5aa - 5ak derivatives. The density functional theory indicates the highest occupied molecular orbital on the BODIPY moiety whereas the lowest unoccupied molecular orbital delocalized on BODIPY and the phospho-triester moieties. Thus, the unique development of the novel BODIPY derivatives with improved optical and redox properties pave the way for fluorescent probes and bioimaging techniques.     

Topology Switching in [32]Heptaphyrins Controlled by Solvent,Protonation, and meso Substituents

The switching of topology between “figure‐eight”, Möbius, and untwisted conformations in [32]heptaphyrins(1.1.1.1.1.1.1) has been investigated by using density functional theory calculations. Such a change is achieved by variation of one internal dihedral angle and, if properly controlled, can provide access to molecular switches with unique optical and magnetic properties. In this work, we have explored different conformational control methods, such as solvent, protonation and meso substituents. Despite its antiaromatic character, most of the [32]heptaphyrins (R=H, CH₃, CF₃, Ph, C₆F₅) adopt a figure‐eight conformation in the neutral state, owing to their more‐effective hydrogen‐bonding interactions. The aromatic Möbius topology is only preferred with dichlorophenyl groups, which minimize the steric hindrance that arises from the bulky chlorine atoms. The conformational equilibrium is sensitive to the solvent, so polar solvents, such as DMSO, further stabilize the Möbius conformation. Protonation induces a conformational change into the Möbius topology, irrespective of the meso‐aryl groups. In the triprotonated species, the conformational switch is blocked and a non‐twisted conformer becomes much more stable than the figure‐eight conformation. We have shown that the relative energies of the protonated [32]heptaphyrins are dominated by aromaticity. Importantly, this topology switching induces a dramatic change in the magnetic properties and reactivity of the macrocycles, as revealed by several energetic, magnetic, structural, and reactivity indices of aromaticity.     

On the Edge of the Known: Extremely Electron-Rich (Di)Carboranyl Phosphines

The syntheses of the first B9-connected carboranylphosphines (B9-Phos) featuring two carboranyl moieties as well as access to B9-Phos ligands with bulky electron-donating substituents, previously deemed unattainable, is reported. The electrochemical properties of the B9-Phos ligands were investigated, revealing the ability of the mesityl derivatives to form stabilized phosphoniumyl radical cations. The B9-Phos ligands display an extremely electron-releasing character surpassing that of alkyl phosphines and commonly used N-heterocyclic carbenes. This is demonstrated by their very small Tolman electronic parameters (TEPs) as well as extremely low P−Se coupling constants. Cone angles and buried volumes attest to the high steric demand exerted by the (di)carboranyl phosphines. The dicarboranyl phosphine Au^I complexes show superior catalytic performance in the hydroamination of alkynes compared to the monocarboranyl phosphine analogs.     

A meso‐Tetraaryl‐21‐carbaporphyrin: Incorporation of a Cyclopentadiene Unit into a Porphyrin Architecture

Incorporation of a cyclopentadiene moiety into the meso‐tetraarylporphyrin framework, using 1,3‐bis(arylhydroxymethyl)ferrocene as a synthon, resulted in the rational synthesis of a meso‐tetraaryl‐21‐carbaporphyrin. The molecular design preserves all essential virtues of the original tetrapyrrolic architecture of meso‐tetraarylporphyrin, including the perfect match between the ionic radii of an inserted metal and the size of the macrocyclic (CNNN) core, and steric protection provided by thoughtfully chosen meso‐aryl substituents. Its protonation of the inner core reveal an adjustable (trigonal versus tetrahedral) geometry.     

meso‐Ester Corroles

The introduction of ester groups on the 5‐ and 15‐meso positions of corroles stabilizes them against oxidation and induces a redshift of their absorption and emission spectra. These effects are studied through the photophysical and electrochemical characterization of up to 16 different 5,15‐diester corroles, in which the third meso position is free or occupied by an aryl group, a long alkyl chain, or an ester moiety. Single‐crystal X‐ray structure analysis of five 5,15‐diestercorroles and DFT and time‐dependent DFT calculations show that the strong electron‐withdrawing character of the 5,15 ester substituents is reinforced by their π overlap with the macrocyclic aromatic system. The crystal packing of corroles 2 , 4 , 6 , 9 , and 15 features short distances between chromophores that are stacked into columns thanks to the low steric hindrance of meso‐ester groups. This close packing is partially due to intermolecular interactions that involve inner hydrogen and nitrogen atoms, and thereby, stabilize a single, identical corrole tautomeric form.     

An Optically and Thermally Switchable Electronic Structure Based on an Anthracene–BODIPY Conjugate

An optically and thermally responsive boron dipyrromethene (BODIPY) dye, namely, meso‐2‐(9,10‐dihydro‐9,10‐ethanoanthracene‐11,12‐dione) (DK)‐linked, bicyclo[2.2.2]octadiene (BCOD)‐fused BODIPY ( BCOD‐DK ), was synthesized. The weakly luminous structure of BCOD‐DK can be changed quantitatively to that of the strongly fluorescent BODIPY BCOD‐Ant by optical excitation at the DK unit, which induces double decarbonylation of the DK unit to give an anthracene unit. The solvent effect on the fluorescence properties of BCOD‐DK suggests that the dramatic change in fluorescence intensity is controlled by intramolecular electron transfer from the BODIPY moiety to the meso‐DK substituent. BCOD‐DK is converted to meso‐ DK benzene‐fused BODIPY ( Benzo‐DK ) by heating at 220 °C with 64–70 nm redshift of absorption and fluorescence peaks without changing the fluorescence quantum yield of Φ_F=0.08 in dichloromethane. Benzo‐DK can be converted to strongly fluorescent meso ‐ anthracene benzene‐fused BODIPY Benzo‐Ant by optical excitation. Thus, BCOD‐DK can show four different optical performances simply by irradiation and heating, and hence may be applicable for optical data storage and security data encryption.     

Additive-Free Green Light-Induced Ligation Using BODIPY Triggers

Photochemical ligation is important in biomaterials engineering for spatiotemporal control of biochemical processes. Such reactions however generally require activation by high energy UV or short wavelength blue light, which can limit their use as a consequence of the potential of these high energy light sources to damage living cells. Herein, we present an additive-free, biocompatible, chemical ligation triggered by mild visible light. BODIPY dyes with a pendant thioether attached at the meso-position undergo photolysis of the [C−S] bond under green light (λ=530 nm) excitation, producing an ion pair intermediate that can react specifically with a propiolate group. The utility of this photochemical ligation in materials science is demonstrated by the fabrication of hydrogels with specific architectures, photo-immobilization of biomacromolecules, and live cell encapsulation within a hydrogel scaffold.     

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.     

Conformationally Restricted Aza‐Dipyrromethene Boron Difluorides (Aza‐BODIPYs) with High Fluorescent Quantum Yields

A simple approach to the highly fluorescent near‐infrared aza‐BODIPY dyes with higher fluorescence quantum yields (up to 0.81 in toluene) in comparison with their known analogues is presented. Our approach is based on the restricted rotations of the 1,7‐phenyl groups to the mean plane of the aza‐BODIPYs, which is achieved through the installation of bulky substituents on the 1,7‐phenyl groups of aza‐BODIPYs and results in a reduced nonradiative relaxation process in solution. The large torsion angles between the 1,7‐phenyl groups and the aza‐BODIPY core (?₁ and ?₂ in these novel conformationally restricted aza‐BODIPYs) were confirmed by X‐ray diffraction studies.