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.     

Synthesis, computational modeling, and properties of benzo-appended BODIPYs

A series of new functionalized mono- and dibenzo-appended BODIPY dyes were synthesized from a common tetrahydroisoindole precursor following two different synthetic routes. Route?A involved the assembly of the BODIPY core prior to aromatization, while in Route?B the aromatization step was performed first. In general, Route?A gave higher yields of the target dibenzo-BODIPYs, due to the ease of aromatization of the BODIPYs compared with the corresponding dipyrromethenes, probably due to their higher stability under the oxidative conditions (2,3-dichloro-5,6-dicyano-1,4-benzoquinone in refluxing toluene). However, due to the slow oxidation of highly electron-deficient BODIPY 3?c bearing a meso-C(6)F(5) group, dibenzo-BODIPY 4?c was obtained, in 35?% overall from dipyrromethane, only by Route?B. Computational calculations performed at the 6-31G(d,p) level are in agreement with the experimental results, showing similar relative energies for all reaction intermediates in both routes. In addition, BODIPY 3?c had the highest molecular electrostatic potential (MEPN), confirming its high electron deficiency and consequent resistance toward oxidation. X-ray analyses of eight BODIPYs and several intermediates show that benzannulation further enhances the planarity of these systems. The π-extended BODIPYs show strong red-shifted absorptions and emissions, about 50-60?nm per benzoannulated ring, at 589-658 and 596-680?nm, respectively. In particular, db-BODIPY 4?c bearing a meso-C(6)F(5) group showed the longest λ(max) of absorption and emission, along with the lowest fluorescence quantum yield (0.31 in CH(2)Cl(2)); on the other hand monobenzo-BODIPY 8 showed the highest quantum yield (0.99) of this series. Cellular investigations using human carcinoma HEp2 cells revealed high plasma membrane permeability for all dibenzo-BODIPYs, low dark- and photo-cytotoxicities and intracellular localization in the cell endoplasmic reticulum, in addition to other organelles. Our studies indicate that benzo-appended BODIPYs, in particular the highly stable meso-substituted BODIPYs, are promising fluorophores for bioimaging applications.     

Silyl‐ and Disilanyl‐BODIPYs: Synthesis via Catalytic Dehalosilylation and Spectroscopic Properties

We describe herein the first synthesis of silyl‐ and disilanyl‐BODIPYs through transition‐metal‐catalyzed dehalosilylation of iodo‐BODIPYs using a Pd(P(t Bu)₃)₂/Et₃N/toluene system. Various mono‐ and bis‐silyl‐BODIPYs, mono‐ and bis‐disilanyl‐BODIPYs and bis‐BODIPYs linked by silylene and SiOSi groups were synthesized by using this straightforward method. Silyl‐ and disilanyl‐substitution significantly modifies the spectroscopic properties of the BODIPY, in which the fluorescence quantum yields of the silyl‐BODIPYs are remarkably increased, whereas the emission spectra of disilanyl‐BODIPYs are red‐shifted due to effective σ(SiSi)–π(BODIPY) conjugation.     

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.     

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.     

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.     

1,8‐Naphthalimide‐Substituted BODIPY Dyads: Synthesis,Structure, Properties,and Live‐Cell Imaging

A set of 1,8‐naphthalimide (NPI)‐substituted 4,4‐difluoroboradiaza‐s‐indacene (BODIPY) dyads 1 a – 1 c were designed and synthesized by the Pd‐catalyzed Sonogashira cross‐coupling reaction of ethynyl substituted NPI 1 with the meso‐, β‐, and α‐halogenated BODIPYs a , b , and c , respectively. The BODIPY 1 c exhibits redshifted absorption, which suggests better electronic communication with substitution at the α‐position of BODIPY compared with at the meso and β positions, which was further supported by time‐dependent DFT calculations. The optical band gap follows the order 1 a > 1 b > 1 c . The single‐crystal X‐ray structures of dyads 1 a – 1 c are reported, which reflect planar orientations of the BODIPY units with respect to the NPIs. The DFT‐optimized structures show good correlation with the experimental data obtained from the single‐crystal X‐ray structures. The packing diagram of 1 a shows a sheet‐like arrangement, 1 b forms a ladder‐like structural motif, and 1 c forms a complex 3D structural arrangement. The dyads 1 a – 1 c show low cytotoxicity (IC₅₀>100 μm ). The confocal microscopy studies with HeLa and A375 cells (when treated with dyads 1 a – 1 c ) show that all the dyads easily enter the cell membrane and show significant multicolor intracellular fluorescence covering the entire visible range with clear emissions in blue, green, and red channels.     

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.     

Synthesis and studies of covalently linked meso-furyl boron-dipyrromethene-ferrocene conjugates

Four meso-furyl BODIPY-ferrocene conjugates 1–4 in which one or more ferrocene groups were connected directly to BODIPY core or meso-furyl group were synthesized by coupling of appropriate bromo meso-furyl BODIPYs with α-ethynylferrocene under mild Pd(0) coupling conditions. The compounds were characterized by HR-MS mass, NMR, absorption, electrochemistry and fluorescence techniques. The absorption studies of compounds 1–4 showed charge transfer band in addition to BODIPY absorption bands indicating that the BODIPY and ferrocene moieties interact within the conjugates. On the other hand, the charge transfer band is absent in meso-phenyl BODIPY-ferrocene conjugate due to the orthogonal arrangement of ferrocene appended meso-phenyl group with BODIPY core which prevents the interaction between the two moieties. The electrochemical studies showed strong oxidation due to ferrocene moiety and reduction due to meso-furyl BODIPY unit. The compounds 3 and 4 which contain two and three ferrocenyl groups respectively were oxidized at the same potential with two and three electrons involved in the redox process. The compounds 1–4 are weakly fluorescent due to electron transfer from ferrocene unit to BODIPY unit. However, the fluorescence can be restored by oxidizing the ferrocene to ferrocenium ion which prevents the electron transfer between the two moieties. The computational studies support the experimental results.     

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.     

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.     

Large Porphyrin Squares from the Self‐Assembly of meso‐Triazole‐Appended L‐Shaped meso–meso‐Linked ZnII–Triporphyrins: Synthesis and Efficient Energy Transfer

meso‐Triazolyl‐appended Zn^II–porphyrins were readily prepared by Cu^I‐catalyzed 1,3‐dipolar cycloaddition of benzyl azide to meso‐ethynylated Zn^II–porphyrin (click chemistry). In noncoordinating CHCl₃ solvent, spontaneous assembly occurred to form tetrameric array ( 3 )₂ from meso–meso‐linked diporphyrins 3 , and dodecameric porphyrin squares ( 4 )₄ and ( 5 )₄ from the L ‐shaped meso–meso‐linked triporphyrins 4 and 5 . The structures of these assemblies were examined by ¹H NMR spectra, absorption spectra, and their gel permeation chromatography (GPC) retention time. Furthermore, the structures of the dodecameric porphyrin squares ( 4 )₄ and ( 5 )₄ were probed by small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) measurements in solution using a synchrotron source. Excitation‐energy migration processes in these assemblies were also investigated in detail by using both steady‐state and time‐resolved spectroscopic methods, which revealed efficient excited‐energy transfer (EET) between the meso–meso‐linked Zn^II–porphyrin units that occurred with time constants of 1.5 ps^?1 for ( 3 )₂ and 8.8 ps^?1 for ( 5 )₄.     

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 Pd⁰‐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 λ_max 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.     

Systematic Modulation of the Fluorescence Brightness in Boron‐Dipyrromethene (BODIPY)‐Tagged N‐Heterocyclic Carbene (NHC)–Gold–Thiolates

Five different highly fluorescent boron‐dipyrromethene (BODIPY)‐tagged N‐heterocyclic carbene NHC–gold halide complexes were synthesized. The substitution of the halogeno ligand by 4‐substituted aryl thiolates leads to a decrease in the brightness of the complexes. This decrease depends on the electronic nature of the thiols, being most pronounced with highly electron‐rich thiols (4‐R=NMe₂). The brightness of the gold thiolates also depends on the distance between the sulfur atom and the BODIPY moiety. The systematic variation of the electron density of [(NHC–bodipy)Au(SC₆H₄R)] (via different R groups) enables the systematic variation of the fluorescence brightness of an appended BODIPY fluorophore. Based on this and supported by DFT calculations, a photoinduced electron‐transfer quenching appears to be the dominant mechanism controlling the brightness of the appended BODIPY dye.     

Platinum(II) Complexes of P‐Chiral 1, 8‐Bis(diorganophosphino)naphthalenes; Crystal Structures of dmfppn,rac‐[PtCl2(dmfppn)], and rac‐[PtCl2(dtbppn)] (dmfppn = 1, 8‐di(methyl‐pentafluorophenylphosphino)naphthalene and dtbppn = 1, 8‐di(tert‐butylphenylphosphino)naphthalene)

The reaction of 1, 8‐dilithionaphthalene 2 , with 2 equivalents of rac‐Me(C₆F₅)PCl, gave a 6 : 1 mixture of rac‐ and meso‐1, 8‐di(methyl‐pentafluorophenylphosphino)naphthalene (dmfppn, rac‐ 3h and meso‐ 3h ), but no reaction was observed when the sterically crowded rac‐tBu(C₆F₅)PCl was used. In ³¹P NMR experiments, rac‐ 3h and mmeso‐ 3h exhibited characteristic signals (virtual quintets), which indicate that there is significant coupling through space (³J_PF + ⁷ J_PF ≈ 15 Hz). Compound rac‐ 3h was isolated by fractional crystallisation and treated with aqueous H₂O₂ to yield the corresponding bis‐phosphine dioxide, rac‐ 7h . In contrast to rac‐ 3h , there was no sign of through‐space coupling in rac‐ 7h , which again illustrates that the latter operates via the lone pairs at phosphorus. Platinum(II) complexes were prepared from the new, P‐chiral chelate rac‐ 3h , and the related ligand 1, 8‐di(tert‐butylphenylphosphino) naphthalene (rac‐dtbppn, rac‐ 3e ). All isolated new compounds were characterised by multinuclear NMR and IR spectroscopy, mass spectrometry, and elemental analysis. Single‐crystal X‐ray structure determinations were performed for rac‐dmfppn (rac‐ 3h ), rac‐[PtCl₂(dtbppn)] (rac‐ 17e ), and rac‐[PtCl₂(dmfppn)] (rac‐ 17h ). rac‐ 3h displays crystallographic twofold symmetry. In rac‐ 17h , the electron‐withdrawing effect of the C₆F₅ groups causes a shortening of the P_t—P bond to ca. 220 pm (cf. 223 pm in rac‐ 17e ).     

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.     

Straightforward Synthetic Protocol for the Introduction of Stabilized C Nucleophiles in the BODIPY Core for Advanced Sensing and Photonic Applications

A straightforward synthetic protocol to directly incorporate stabilized 1,3‐dicarbonyl C nucleophiles to the meso position of BODIPY (4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene) is reported. Soft nucleophiles generated by deprotonation of 1,3‐dicarbonyl derivatives smoothly displace the 8‐methylthio group from 8‐(methylthio)BODIPY analogues in the presence of Cu^I thiophenecarboxylate in stoichiometric amounts at room temperature. Seven highly fluorescent new derivatives are prepared with varying yields (20–92 %) in short reaction times (5–30 min). The excellent photophysical properties of the new dyes allow focusing on applications never analyzed before for BODIPYs substituted with stabilized C nucleophiles such as pH sensors and lasers in liquid and solid state, highlighting the relevance of the synthetic protocol described in the present work. The attainment of these dyes, with strong UV absorption and highly efficient and stable laser emission in the green spectral region, concerns to one of the greatest challenges in the ongoing development of advanced photonic materials with relevant applications. In fact, organic dyes with emission in the green are the only ones that allow, by frequency‐doubling processes, the generation of tunable ultraviolet (250‐350 nm) radiation, with ultra‐short pulses.     

Metal‐Assisted One‐Pot Synthesis of Isoporphyrin Complexes

Zinc and cadmium complexes of meso‐arylisoporphyrins carrying a pyrrolyl or dipyrrinyl substituent at the sp³ carbon atom were obtained through a simple one‐pot variation of the Alder–Longo porphyrin synthesis. Key to the formation and stabilization of isoporphyrins is the presence of metal acetates during the oxidative macrocyclization step. The characteristic Q‐bands of isoporphyrins are found in the NIR region between 750 nm and 880 nm. All of the isolated pyrrolyl‐ and dipyrrinyl‐appended isoporphyrins are stable under typical laboratory conditions and allow chemical transformations like BF₂ coordination, transmetalation, and ligand exchange.     

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.     

Synthesis and Spectroscopic Properties of Fused‐Ring‐Expanded Aza‐Boradiazaindacenes

A series of fused‐ring‐expanded aza‐boradiazaindacene (aza‐BODIPY) dyes have been synthesized by reacting arylmagnesium bromides with phthalonitriles or naphthalenedicarbonitriles. An analysis of the structure–property relationships has been carried out based on X‐ray crystallography, optical spectroscopy, and theoretical calculations. Benzo and 1,2‐naphtho‐fused 3,5‐diaryl aza‐BODIPY dyes display markedly red shifted absorption and emission bands in the near‐IR region (>700 nm) due to changes in the energies of the frontier MOs relative to those of 1,3,5,7‐tetraaryl aza‐BODIPYs. Only one 1,2‐naphtho‐fused aza‐BODIPY of the three possible isomers is formed due to steric effects, and 2,3‐naphtho‐fused compounds could not be characterized because the final BF₂ complexes are unstable in solution. The incorporation of a  N(CH₃)₂ group at the para‐positions of a benzo‐fused 3,5‐diaryl aza‐BODIPY quenches the fluorescence in polar solvents and results in a ratiometric pH response, which could be used in future practical applications as an NIR “turn‐on” fluorescence sensor.