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.     

Synthesis,Structure, and Optical Studies of Donor–Acceptor‐Type Near‐Infrared (NIR) Aza–Boron‐Dipyrromethene (BODIPY) Dyes

Six donor–acceptor‐type near‐infrared (NIR) aza–boron‐dipyrromethene (BODIPY) dyes and their corresponding aza–dipyrrins were designed and synthesized. The donor moieties at the 1,7‐positions of the aza–BODIPY core were varied from naphthyl to N‐phenylcarbazole to N‐butylcarbazole. The 3,5‐positions were also substituted with phenyl or thienyl groups in the aza–BODIPYs. Photophysical, electrochemical, and computational studies were carried out. The absorption and emission spectra of aza–BODIPYs were significantly redshifted (≈100 nm) relative to the parent tetraphenylaza–BODIPY. Fluorescence studies suggested effective energy transfer (up to 93 %) from donor groups to the aza–BODIPY core in all of the compounds under study. Time‐dependent (TD)‐DFT studies indicated effective electronic interactions between energy donor groups and aza–dipyrrin unit in all the aza–BODIPYs studied. The HOMO–LUMO gap (ΔE) calculated from cyclic voltammetry data was found to be lower for six aza–BODIPYs relative to their corresponding aza–dipyrrins.     

Rational Design of Emissive NIR‐Absorbing Chromophores: RhIII Porphyrin‐Aza‐BODIPY Conjugates with Orthogonal Metal–Carbon Bonds

The facile synthesis of Group 9 Rh^III porphyrin‐aza‐BODIPY conjugates that are linked through an orthogonal Rh?C(aryl) bond is reported. The conjugates combine the advantages of the near‐IR (NIR) absorption and intense fluorescence of aza‐BODIPY dyes with the long‐lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge‐transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh^III conjugates exhibit strong aza‐BODIPY‐centered fluorescence at around 720 nm (Φ_F=17–34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet‐oxygen quantum yield (Φ_Δ=19–27 %, λ_ex=690 nm) have been observed. Nanosecond pulsed time‐resolved absorption spectroscopy confirms that relatively long‐lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.     

Benzo[c,d]indole‐Containing Aza‐BODIPY Dyes: Asymmetrization‐Induced Solid‐State Emission and Aggregation‐Induced Emission Enhancement as New Properties of a Well‐Known Chromophore

A series of symmetric and asymmetric benzo[c,d]indole‐containing aza boron dipyrromethene (aza‐BODIPY) compounds was synthesized by a titanium tetrachloride‐mediated Schiff‐base formation reaction of commercially available benzo[c,d]indole‐2(1H)‐one and heteroaromatic amines. These aza‐BODIPY analogues show different electronic structures from those of regular aza‐BODIPYs, with hypsochromic shifts of the main absorption compared to their BODIPY counterparts. In addition to the intense fluorescence in solution, asymmetric compounds exhibited solid‐state fluorescence due to significant contribution of the vibronic bands to both absorption and fluorescence as well as reduced fluorescence quenching in the aggregates. Finally, aggregation‐induced emission enhancement, which is rare in BODIPY chromophores, was achieved by introducing a nonconjugated moiety into the core structure.     

Azahelicene‐Fused BODIPY Analogues Showing Circularly Polarized Luminescence

Helical carbazole‐based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene‐incorporated BF₂ dyes were elucidated by x‐ray diffraction analysis. DFT calculations revealed that the π‐conjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene‐fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.     

A New Water‐Soluble Phosphorus‐Dipyrromethene and Phosphorus‐Azadipyrromethene Dye: PODIPY/aza‐PODIPY

PODIPY and aza‐PODIPY have been successfully prepared by the treatment of dipyrromethene and azadipyrromethene with POCl₃ in the presence of Et₃N. The new PODIPY and aza‐PODIPY dyes are found to have photophysical properties. PODIPY and aza‐PODIPY are water‐soluble, and aza‐PODIPY is suited for labeling living Hep‐2 cells for imaging assays in the near‐infrared region. Molecular orbital calculations show that the increase in the HOMO–LUMO band gap for the lowest energy absorption bands is observed in the new phosphorus‐containing aza‐PODIPY, and the HOMO and LUMO energies of aza‐PODIPY are found to be higher than those of aza‐BODIPY.     

New 2,6‐Distyryl‐Substituted BODIPY Isomers: Synthesis,Photophysical Properties,and Theoretical Calculations

A 2,6‐distyryl‐substituted boradiazaindacene (BODIPY) dye and a new series of 2,6‐p‐dimethylaminostyrene isomers containing both α‐ and β‐position styryl substituents were synthesized by reacting styrene and p‐dimethylaminostyrene with an electron‐rich diiodo‐BODIPY. The dyes were characterized by X‐ray crystallography and NMR spectroscopy and their photophysical properties were investigated and analyzed by carrying out a series of theoretical calculations. The absorption spectra contain markedly redshifted absorbance bands due to conjugation between the styryl moieties and the main BODIPY fluorophore. Very low fluorescence quantum yields and significant Stokes shifts are observed for 2,6‐distyryl‐substituted BODIPYs, relative to analogous 3,5‐distyryl‐ and 1,7‐distyryl‐substituted BODIPYs. Although the fluorescence of the compound with β‐position styryl substituents on both pyrrole moieties and one with both β‐ and α‐position substituents was completely quenched, the compound with only α‐position substituents exhibits weak emission in polar solvents, but moderately intense emission with a quantum yield of 0.49 in hexane. Protonation studies have demonstrated that these 2,6‐p‐dimethylaminostyrene isomers can be used as sensors for changes in pH. Theoretical calculations provide strong evidence that styryl rotation and the formation of non‐emissive charge‐separated S₁ states play a pivotal role in shaping the fluorescence properties of these dyes. Molecular orbital theory is used as a conceptual framework to describe the electronic structures of the BODIPY core and an analysis of the angular nodal patterns provides a reasonable explanation for why the introduction of substituents at different positions on the BODIPY core has markedly differing effects.     

Chiral Ammonium Betaine‐Catalyzed Highly Stereoselective Aza‐Henry Reaction of α‐Aryl Nitromethanes with Aromatic N‐Boc Imines

A highly stereoselective aza‐Henry reaction of α‐aryl nitromethanes with aromatic N‐Boc imines was established by using C₁‐symmetric chiral ammonium betaine as a bifunctional organic base catalyst. Various substituted aryl groups for both imines and nitromethanes were tolerated in the reaction, and a series of precursors for the synthesis of unsymmetrical anti‐1,2‐diaryl ethylenediamines was provided.     

A Convenient and Efficient Method for the Preparation of Unique Fluorophores of Lariat Naphtho‐Aza‐Crown Ethers

New naphtho‐aza‐crown ethers containing different phenolic side‐arms attached through the ortho‐position of the phenol have been prepared under solvent‐free conditions. The starting macrocyclic naphtho‐aza‐crown ether 2 was obtained by treatment of naphthalene dicarboxylic acid diester 1 with diethylenetriamine in EtOH at room temperature for two days without stirring in 77% yield (Scheme 1). Phenolic ligands ( 3 – 14 ) were synthesized by the Mannich reaction of the secondary macrocyclic amine 2 with the substituted phenols using nontoxic and inexpensive CaCl₂. This procedure was applied successfully for the synthesis of Mannich bases from simple secondary amines. The CaCl₂ powder can be reused up to three times after simple washing with dry acetone.     

N‐Bridged Annulated BODIPYs: Synthesis of Highly Fluorescent Blueshifted Dyes

A series of novel BODIPY dyes has been prepared through the introduction of an N‐bridged annulated meso ‐phenyl ring at one of the β‐positions of the BODIPY core. An unusual blueshift of the main spectral bands is observed, since the fusion of a meso ‐substituent results in a marked relative destabilization of the LUMO. The greater rigidity of the ring‐fused structure leads to very high fluorescence quantum yields. The position of the main spectral bands can be fine‐tuned by introducing electron withdrawing and donating groups onto the meso ‐phenyl ring.     

Asymmetric Donor–π‐Acceptor‐Type Benzo‐Fused Aza‐BODIPYs: Facile Synthesis and Colorimetric Properties

Novel aza‐diisoindolylmethene and their BF₂‐chelating complexes (benzo‐fused aza‐BODIPYs) were synthesized on a large scale and in a facile manner from phthalonitrile in tBuOK‐DMF solution. The unique asymmetric donor–π‐acceptor structure facilitates B? N bond detachment in the presence of trifluoroacetic acid (TFA) in dichloromethane, resulting in sharp color change from red to colorless, with over 250 nm hypsochromic shift in the absorption maximum. This colorimetric process can be reversed by adding a very small amount of proton‐accepting solvents or compounds. A ¹H and ¹¹B NMR spectroscopy study and also density functional theory (DFT) calculations suggest that TFA‐induced B? N bond cleavage may disrupt the whole π‐conjugation of the BODIPY molecule, resulting in significant colorimetric behavior.     

Aza‐BODIPY Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts

A new series of aza‐BODIPY derivatives ( 4 a – 4 c , 5 a , c , and 6 b , c ) were synthesized and their excited‐state properties, such as their triplet excited state and the yield of singlet‐oxygen generation, were tuned by substituting with heavy atoms, such as bromine and iodine. The effect of substitution has been studied in detail by varying the position of halogenation. The core‐substituted dyes showed high yields of the triplet excited state and high efficiencies of singlet‐oxygen generation when compared to the peripheral‐substituted systems. The dye 6 c , which was substituted with six iodine atoms on the core and peripheral phenyl ring, showed the highest quantum yields of the triplet excited state (Φ_T=0.86) and of the efficiency of singlet‐oxygen generation (Φ_Δ=0.80). Interestingly, these dyes were highly efficient as photooxygenation catalysts under artificial light, as well as under normal sunlight conditions. The uniqueness of these aza‐BODIPY systems is that they are stable under irradiation conditions, possess strong red‐light absorption (620–680 nm), exhibit high yields of singlet‐oxygen generation, and act as efficient and sustainable catalysts for photooxygenation reactions.     

Vertically π‐Expanded Imidazo[1,2‐a]pyridine: The Missing Link of the Puzzle

The dehydrogenative coupling of imidazo[1,2‐a]pyridine derivative has been achieved for the first time. In cases in which the most‐electron‐rich position of the electron‐excessive heterocycle was blocked by a naphthalen‐1‐yl substituent, neither oxidative aromatic coupling nor reaction under Scholl conditions enabled the fusion of the rings. The only method that converted the substrate into the corresponding imidazo[5,1,2‐de]naphtho[1,8‐ab]quinolizine was coupling in the presence of potassium in anhydrous toluene. Moreover, we discovered new, excellent conditions for this anion‐radical coupling reaction, which employed dry O₂ from the start in the reaction mixture. This method afforded vertically fused imidazo[1,2‐a]pyridine in 63 % yield. Interestingly, whereas the fluorescence quantum yield (Φ_fl) of compound 3 , despite the freedom of rotation, was close to 50 %, the Φ_fl value of flat naphthalene‐imidazo[1,2‐a]pyridine was only 5 %. Detailed analysis of this compound by using DFT calculations and a low‐temperature Shpol′skii matrix revealed phosphorescence emission, thus indicating that efficient intersystem‐crossing from the lowest‐excited S₁ level to the triplet manifold was the competing process with fluorescence.     

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.     

Luminescent Ionic Liquid Crystals from Self‐Assembled BODIPY Disulfonate and Imidazolium Frameworks

A series of modular mesogenic salts based on the combination of anionic 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (F‐BODIPY) 2,6‐disulfonate dyes and trialkoxybenzyl‐functionalised imidazolium cations has been designed and synthesised. Each salt contains a rigid dianionic BODIPY core associated with two imidazolium cations functionalised by 1,2,3‐trialkoxybenzyl (alkyl=n‐C₈, n‐C₁₂ or n‐C₁₆) units or, in one case, with imidazolium cations functionalised by a trialkylgallate (3,4,5‐trialkoxybenzoate) unit in which the 3,5‐dialkyl groups are terminated with a polymerisable acrylate entity. All these compounds were highly fluorescent in solution with quantum yields ranging from 54 to 62 %. In the solid state, the width of the emission band observed at around 650 nm is a clear signature of aggregation. With the trialkoxybenzylimidazolium cations, polarised optical microscopy (POM) and X‐ray scattering experiments showed that columnar mesophases were formed. Differential scanning calorimetry (DSC) studies confirmed the mesomorphic behaviour from room temperature to about 130 °C for salts with alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of the BODIPY unit was maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The salt containing the gallate‐functionalised imidazolium cations showed no mesomorphism but the acrylate terminal units could be used to engender photoinitiated polymerisation thereby allowing the material to be immobilised on glass plates. The polymerisation process was followed by FTIR spectroscopy and the fixed and patterned films were highly fluorescent with a solid‐state emission close to that of the complex in the solid state.     

A‐D‐A Type Small Molecules Based on Boron Dipyrromethene for Solution‐Processed Organic Solar Cells

The unique properties of boron dipyrromethene (BODIPY) dyes including facile synthesis, high absorption coefficients, and delocalized molecular orbitals as well as excellent photochemical and thermal stability, make them promising as materials for organic solar cells. Accordingly, in this study three A‐D ‐A structural small molecules of BDTT‐BODIPY, FL‐BODIPY, and TT‐BODIPY have been synthesized, in which two BODIPY acceptor units are symmetrically conjugated to 4,8‐bis(5‐(2‐ethylhexyl) thiophen‐2‐yl)benzo[1,2‐b:4,5‐b]dithiophene (BDTT), 9,9‐dioctyl‐9H‐fluorene (FL), and thieno[3,2‐b]thiophene (TT) donor cores, respectively. The manipulation of the structural parameters significantly improves the performances of the BHJ OSCs, which show power conversion efficiencies of 4.75 %, 1.51 %, and 1.67 % based on [6,6]‐phenyl C₇₁‐butyric acid methyl ester (PC₇₁BM) as the acceptor material and BDTT‐BODIPY, FL‐BODIPY, and TT‐BODIPY as the donor materials, respectively.     

Highly near‐infrared photoluminescence from aza‐borondipyrromethene‐based conjugated polymers

Near‐infrared (NIR) emissive conjugated polymers were prepared by palladium‐catalyzed Sonogashira polymerization of diiodobenzene‐functionalized aza‐borondipyrromethene (Aza‐BODIPY) monomers, which were substituted at 3 and 5 or 1 and 7 positions on the Aza‐BODIPY core, with 1,4‐diethynyl‐2,5‐dihexadecyloxybenzene or 3,3′‐didodecyl‐2,2′‐diethynyl‐5,5′‐bithiophene. The structures of the polymers were confirmed by ¹H NMR, ¹³C NMR, ¹¹B NMR, Fourier transform infrared (FT‐IR) spectroscopies, and size exclusion chromatography (SEC). The optical properties were then characterized by UV–vis absorption and photoluminescence (PL) spectroscopies, and theoretical calculation using density‐functional theory (DFT) method. The polymers were fusible and soluble in common organic solvents including tetrahydrofuran (THF), o‐xylene, toluene, CHCl₃, and CH₂Cl₂, etc. The UV–vis absorption and PL spectra of the polymers shifted to long wavelength region in comparison with simple Aza‐BODIPY as the counterpart because of extended π‐conjugation of the polymers. The polymers efficiently emitted NIR light with narrow emission bands at 713～777 nm on excitation at each absorption maximum. Especially, the polymer attached 1,4‐diethynyl‐2,5‐dihexadecyloxybenzene to 3,5‐position on the core revealed intense quantum yields (?_F = 24%) in this NIR region (753 nm). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

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.     

2,3‐Diaryl‐3‐hydroxy­propionic acid inter­mediates in the synthesis of threo forms of 1,2‐diaryl‐1,3‐propane­diols

Racemic threo‐3‐hydroxy‐2,3‐diphenyl­propionic acid, C₁₅H₁₄O₃, (I), crystallizes from ethyl acetate as a conglomerate of separate (+)‐ and (−)‐crystals. The geometries of (I) and its methyl ester are compared. Reduction of (I) gives threo‐1,2‐diphenyl‐1,3‐propane­diol. The synthesis of threo forms of 1,2‐diaryl‐1,3‐propane­diols via 2,3‐diaryl‐3‐hydroxy­propionic acids is discussed.     

Water‐Soluble Red‐Emitting Distyryl‐Borondipyrromethene (BODIPY) Dyes for Biolabeling

A series of water‐soluble red‐emitting distyryl‐borondipyrromethene (BODIPY) dyes were designed and synthesized by using three complementary approaches aimed at introducing water‐solubilizing groups on opposite faces of the fluorescent core to reduce or completely suppress self‐aggregation. An additional carboxylic acid functional group was introduced at the pseudo‐meso position of the BODIPY scaffold for conjugation to amine‐containing biomolecules/biopolymers. The optical properties of these dyes were evaluated under simulated physiological conditions (i.e., phosphate‐buffered saline (PBS), pH 7.5) or in pure water. The emission wavelength (λ_max) of these labels was found in the 640–660 nm range with quantum yields from modest to unprecedentedly high values (4 to 38 %). The bioconjugation of these distyryl‐BODIPY dyes with bovine serum albumin (BSA) and the monoclonal antibody (mAb) 12A5 was successfully performed under mild aqueous conditions.