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.     

Acidic Condensation of BODIPYs with Aldehydes: A Quick and Versatile Route to Alkenyl‐BODIPYs and C(sp3)‐Connected DYEmers

The condensation of aldehydes with BODIPY (boron dipyrrin) luminophores was investigated. Formaldehyde can be used to connect two BODIPYs at each of the three pyrrolic C positions (α‐, β‐, and β′‐positions) in a quick and highly selective manner, yielding new DYEmers (di‐ and oligomeric BODIPY derivatives) with varied photophysical properties. Benzaldehydes form DYEmers only at the β‐ and the β′‐positions. For aliphatic aldehydes the DYEmer formation competes with the elimination of water from a proposed alcohol intermediate, leading to the formation of α‐ and β‐alkenyl‐BODIPYs. 2‐Phenylacetaldehyde and similar precursors exclusively yield elimination products. These acid‐mediated transformations are valuable alternatives to the well‐established, base‐promoted Knoevenagel condensation protocol that is typically employed in the preparation of BODIPYs with near infrared (NIR)‐shifted absorptions.     

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,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‐C₆F₅ 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‐C₆F₅ group showed the longest λ_max of absorption and emission, along with the lowest fluorescence quantum yield (0.31 in CH₂Cl₂); 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.     

Synthesis and Chiroptical Properties of Chiral Carbazole-Based BODIPYs

A series of carbazole-based boron dipyrromethenes (BODIPYs) 2 a – g bearing binaphthyl units have been synthesized by the Et₂AlCl-mediated reaction of the corresponding BODIPY difluorides 1 a – g with 1,1′-binaphthalene-2,2′-diol. Substituents such as halogen, nitrile, and amino groups were tolerated under the reaction conditions, and the reaction of the phenylethynyl-substituted 1 h gave (R,R)- 3 h bearing two binaphthyl units. The chiroptical properties of these dyes with different substituents were investigated by UV/Vis, CD, fluorescence, and circularly polarized luminescence (CPL) spectroscopy. The CD spectra showed Cotton effects in the absorption region of the BODIPY moieties. In addition, they showed CPL both in solution and in the solid state. Interestingly, several dyes recorded higher g_lum values in the solid state, probably due to intermolecular interactions. Because (R,R)- 3 h recorded relatively low g_lum values, the diastereomer (R,S)- 3 h was prepared. The (R,S) diastereomer showed intense CPL, which suggests a synergetic effect of the two binaphthyl groups. Finally, chiral carbazole-based BODIPY dimers have been synthesized for the first time and their chiroptical properties were investigated. They showed redshifted fluorescence and CPL, which reached the near-IR (NIR) region in the solid state.     

Synthesis and Optical and Redox Properties of Symmetric and Asymmetric BODIPYs

Herein, we present the synthetic route and the photophysical, electrochemical as well as laser properties of novel red‐emitting boron‐dipyrromethenes (BODIPYs) bearing arylethyne moieties. Such functionality is added along the main axis of the chromophore leading to single‐ and double‐substituted derivatives. The relationship between the dye structure and the lasing properties is studied in detail with the help of the photophysical and electrochemical properties as well as quantum mechanical simulations. The asymmetric substitution of the parent dye induces inhomogeneities in the charge distribution, which leads to an overall loss of the fluorescence capacity, mainly in polar media. Such non‐radiative deactivation processes can be softened by decreasing the electron‐donor ability of the substituent or even avoided by symmetrical substitution. Thus, grafting of the arylethyne moieties at the longitudinal axis of the indacene core results in an effective strategy to develop red‐edge BODIPYs with highly efficient and photostable laser emission.     

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.     

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.     

Synthesis and Photophysics of BF2‐Rigidified Partially Closed Chain Bromotetrapyrroles: Near Infrared Emitters and Photosensitizers

We report the synthesis, crystallographic, optical, and triplet and singlet oxygen generation properties of a series of BF₂‐rigidified partially closed chain bromotetrapyrroles as near infrared emitters and photosensitizers. These novel dyes were efficiently synthesized from a nucleophilic substitution reaction between pyrroles and the 3,5‐bromo‐substituents on the tetra‐ and hexabromoBODIPYs and absorb in the near‐infrared region (681–754 nm) with high molar extinction coefficients. Their fluorescent emission (708–818 nm) and singlet oxygen generation properties are significantly affected by alkyl substitutions on the two uncoordinated pyrrole units of these dyes and the polarity of solvents. Among them, dyes 4 ca and 4 da show good singlet oxygen generation efficiency and good NIR fluorescence emission (fluorescence quantum yields of 0.14–0.43 in different solvents studied).     

Tuning the Solid‐State Luminescence of BODIPY Derivatives with Bulky Arylsilyl Groups: Synthesis and Spectroscopic Properties

Boron dipyrromethenes (BODIPYs) with bulky triphenylsilylphenyl(ethynyl) and triphenylsilylphenyl substituents on pyrrole sites were prepared via Hagihara–Sonogashira and Suzuki–Miyaura cross‐coupling with ethynyl‐terminated tetraphenylsilane and boronic acid‐terminated tetraphenylsilane. The chromophores are designed to prevent intermolecular π–π stacking interaction and enhance fluorescence in the solid state. Single crystals of 1 a and 2 b for X‐ray structural analysis were obtained, and weak π–π stacking interactions of the neighboring BODIPY molecules were observed. Spectroscopic properties of all of the dyes in various solvents and in films were investigated. Triphenylsilylphenyl‐substituted BODIPYs generally show more pronounced increases in solid‐state emission than triphenylsilylphenyl(ethynyl)‐substituted BODIPYs. Although the simple BODIPYs do not exhibit any fluorescence in the solid state (Φ=0), arylsilyl‐substituted BODIPYs exhibit weak to moderate solid‐state fluorescence with quantum yields of 0.03, 0.07, 0.10, and 0.25. The structure–property relationships were analyzed on the basis of X‐ray crystallography, optical spectroscopy, cyclic voltammetry, and theoretical calculations.     

Tandem Regioselective Substitution and Palladium‐Catalyzed Ring Fusion Reaction for Core‐Expanded Boron Dipyrromethenes with Red‐Shifted Absorption and Intense Fluorescence

A selective method for the core‐extension of boron dipyrromethene (BODIPY) with two annulated indole rings with exclusive syn‐connectivity is reported. The method is based on a regioselective nucleophilic substitution reaction of 2,3,5,6‐tetrabromoBODIPY with aryl amines, followed by palladium‐catalyzed intramolecular C?C coupling ring fusion. The unsymmetrical core‐expanded BODIPY with annulated indole and benzofuran rings was also synthesized by stepwise and regioselective nucleophilic substitution and palladium‐catalyzed intramolecular C?C coupling reaction. The diindole‐annulated BODIPY was unambiguously characterized by single‐crystal X‐ray analysis. The optical properties of the present core‐expanded BODIPYs were studied, revealing clearly red‐shifted absorption and emission bands and enhanced absorption coefficients upon annulation.     

Tuning the Electrical and Optical Properties of Diketopyrrolopyrrole Complexes for Panchromatic Dye‐Sensitized Solar Cells

A series of metal‐free organic dyes that were bridged by a diketopyrrolopyrrole moiety and were composed of indoline and triphenylamine as donor groups and furan and benzene as conjugated spacer groups were designed and synthesized for use in dye‐sensitized solar cells (DSCs). The photophysical properties, electrochemical properties, and performance of the DSCs were related to the structure of their corresponding dyes. Their absorption spectra broadened upon the introduction of the indoline and heterocyclic furan moieties through fine‐tuning of their molecular configuration. The overall conversion efficiencies of DSCs that were based on these dyes ranged from 5.14–6.53 %. Among the four dyes that were tested, indoline‐based ID01 and ID02 showed higher efficiencies (6.35 % and 6.53 %) as a result of their improved light‐harvesting efficiency and larger electron driving force. The ID01 dye, which contained an indoline moiety as an electron donor and a furan group as a π‐conjugated linker, showed an excellent monochromatic incident‐photon‐to‐current‐conversion efficiency (IPCE) spectrum (350–650 nm) with a maximum value of 78 % in the high plateau region and an onset value close to 800 nm. Intensity‐modulated photovoltage spectroscopy (IMVS) and impedance spectroscopy (IS) revealed that dyes that contained benzene conjugation spacers suppressed the charge‐recombination rate more efficiently than dyes that contained furan spacers, thereby resulting in improved photovoltage.     

BODIPY Dyes Bearing Multibranched Fluorinated Chains: Synthesis,Structural, and Spectroscopic Studies

A small series of boron-dipyrromethene (BODIPY) dyes, characterized by the presence of multibranched fluorinated residues, were designed and synthesized. The dyes differ in both the position (para-perfluoroalkoxy-substituted phenyl ring or boron functionalization) and number of magnetically equivalent fluorine atoms (27 or 54 fluorine atoms per molecule). Photophysical and crystallographic characterization of the synthesized BODIPYs was carried out to evaluate the effect of the presence of highly fluorinated moieties on the optical and morphological properties of such compounds.     

Color‐Tunable Solid‐State Fluorescence Emission from Carbazole‐Based BODIPYs

Several carbazole‐based boron dipyrromethene (BODIPY) dyes were synthesized by organometallic approaches. Thiazole, benzothiazole, imidazole, benzimidazole, triazole, and indolone substituents were introduced at the 1‐position of the carbazole moiety, and boron complexation of each dipyrrin generated the corresponding compounds 1 , 2 a , and 3 – 6 . The properties of these products were investigated by UV/Vis and fluorescence spectroscopy, cyclic voltammetry, X‐ray crystallography, and DFT calculations. These compounds exhibited large Stokes shifts, and compounds 1 , 2 a , and 3 – 5 fluoresced both in solution and in the solid state. Complex 2 a showed the highest fluorescence quantum yield (Φ_F) in the solid state, therefore boron complexes of the carbazole–benzothiazole hybrids 2 b – f , which had several different substituents, were prepared and the effects of the substituents on the photophysical properties of the compounds were examined. The fluorescence properties showed good correlation with the results of crystal‐packing analyses, and the dyes exhibited color‐tunable solid‐state fluorescence.     

Synthesis and Molecular Properties of Acceptor‐Substituted Squaraine Dyes

A broad series of more than 20 acceptor‐substituted squaraines was synthesized that feature different acceptor functionalities at the central squaraine four‐membered ring. The influence of these acceptor units on the reactivity of semisquaraine precursors and stability of the respective squaraines were explored. Thereby the dicyanovinyl group was found to be the most versatile acceptor group that enabled various modifications at the donor moiety of the squaraine scaffold, leading to an extended series of dicyanovinyl‐functionalized squaraines. The variation of donor units afforded a set of NIR fluorophores that cover a wavelength region from the visible at about 650 nm far into the NIR up to 920 nm with fluorescence quantum yields between 0.93 and 0.11 and outstanding optical brightness. This excellent optical property is related to a rigid molecular scaffold that is fixed in an all‐cis configuration by the additional dicyanovinyl acceptor unit. The change of the molecular symmetry from C_2h to C_2v upon functionalization of the squaraine core with dicyanovinyl acceptor group has been confirmed in solution by electro‐optical absorption (EOA) spectroscopy, revealing permanent ground‐state dipole moments μ_g in the range between 4.3 and 6.4 D. These dipole moments direct an antiparallel packing of the molecules in the solid state according to single‐crystal X‐ray analyses achieved for four dicyanovinyl‐functionalized squaraines. The structural properties, the EOA results, as well as the band shapes of the optical spectra indicate that these polymethine dyes are cyanine‐type chromophores. It is worth noting that the orientation of the dipole moment vectors is orthogonal to the orientation of the transition dipole moment vectors, which is an uncommon but characteristic feature of this rather novel class of polymethine dyes. With regard to applications of these dyes in organic solar cells, their redox properties were also studied by cyclic voltammetry.     

Synthesis and Optical Properties of Water‐Soluble Polyglycerol‐Dendronized Rylene Bisimide Dyes

Four new water‐soluble polyglycerol‐dendronized perylene, terrylene, and quaterrylene bisimides have been synthesized and characterized with respect to their optical properties in polar organic solvents and water by using UV/Vis and fluorescence spectroscopy. All of these dyes were highly soluble in water, but the size of the chosen polyglycerol dendron was only sufficient to completely suppress dye aggregation for the core‐unsubstituted perylene derivative. Their high solubility in water and their absorption and emission wavelengths up to the NIR region make the core‐unsubstituted perylene and terrylene bisimides ideal candidates for applications in bioimaging, whilst the lack of fluorescence for quaterrylene bisimide in all polar solvents does not warrant further investigation of this chromophore in fluorescence and imaging applications. Likewise, tuning of the emission of rylene bisimides towards longer wavelengths by employing electron‐donating bay substituents is not a promising strategy, owing to the lower fluorescence quantum yields in polar solvents and, in particular, in water.     

A Diradical Approach towards BODIPY‐Based Dyes with Intense Near‐Infrared Absorption around λ=1100 nm

A diradical approach to obtain stable organic dyes with intense absorption around λ=1100 nm is reported. The para‐ and meta‐quinodimethane‐bridged BODIPY dimers BD‐1 and BD‐2 were synthesized and were found to have a small amount of diradical character. These molecules exhibited very intense absorption at λ=1088 nm (?=6.65×10⁵ M ^?1 cm^?1) and 1136 nm (?=6.44×10⁵ M ^?1 cm^?1), respectively, together with large two‐photon‐absorption cross‐sections. Structural isomerization induced little variation in their diradical character but distinctive differences in their physical properties. Moreover, the compounds showed a selective fluorescence turn‐on response in the presence of the hydroxyl radical but not with other reactive oxygen species.     

Benzimidazole Derivatives: Synthesis,Physical Properties,and n‐Type Semiconducting Properties

A series of new benzimidazole derivatives were synthesized by the solid‐state condensation and direct sublimation (SSC‐DS) method and their physical properties were investigated. The reaction yields and product stability were significantly affected by the identity of the diamine and anhydride substituents. On the other hand, the substituents of the benzimidazole ring allowed fine tuning of the emission maxima, fluorescence quantum yields, and redox potentials. The HOMO–LUMO levels were estimated by cyclic voltammetry in film on indium tin oxide (ITO) and compared with values obtained by other methods. The described benzimidazoles showed high crystallinity, which is attributed to a high planarity and interactions between carbon and heteroatoms. These compounds showed n‐type semiconducting behavior in organic field‐effect transistors (OFETs). Optimized devices for fluorinated NTCBI (naphthalene tetracarboxylic bisbenzimidazole) showed respectable electron mobilities of ～10^?2 cm² V^?1 s^?1.