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.     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.     

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.     

Facile Preparation of α‐Cyano‐α,ω‐Diaryloligovinylenes: A New Class of Color‐Tunable Solid Emitters

An efficient Knoevenagel condensation reaction was used to construct a series of α‐cyano‐α,ω‐diaryloligovinylenes, which show prominent fluorescence emission in the solid state. On investigating the effect of conjugation length on fluorescent properties, we found that the diene structure showed superior solid‐state luminescence. Furthermore, the emission color could be adjusted by introducing donor or acceptor functional groups at the terminal aryl groups. Full‐color emission in the visible region can be achieved by adding different functional groups to the α‐cyano‐α,ω‐diaryldivinylene moiety. The structure–property relationships were elucidated and some observations such as the substitution position effects were discussed. These compounds have potential applications as full‐color solid emissive candidates in material science and their simple structures allow them to be easily modified resulting in further interesting properties.     

Blue‐to‐Orange Color‐Tunable Laser Emission from Tailored Boron‐Dipyrromethene Dyes

A series of meso‐substituted boron‐bipyrromethene (BODIPY) dyes are synthesized and their laser and photophysical properties systematically studied. Laser emission covering a wide visible spectral region (from blue to orange) is obtained by just changing the electron donor character of the heteroatom at position 8. The additional presence of methyl groups at positions 3 and 5 results in dyes with a photostability similar to that of the unsubstituted dye but with much improved efficiency. Correlation of the lasing properties of the different dyes to their photophysical properties provides inklings to define synthetic strategies of new BODIPY dyes with enhanced efficiency and modulated wavelength emission over the visible spectral region.     

Self‐Assembly and Aggregate‐Induced Enhanced Emission of Amphiphilic Fluorescence Dyes in Water and in the Solid State

1‐Cyano‐1,2‐bis(biphenyl)ethene (CNBE) derivatives with a hexa(ethylene glycol) group as an amphiphilic side chain were synthesized and the self‐assembling character and fluorescence behavior were investigated. The amphiphilic derivatives showed aggregate‐induced enhanced emission (AIEE) in water and in the solid state. The fluorescence quantum yield increased as the rigidity of the aggregates increased (i.e., in ethyl acetate<in water<in the solid state). As determined from measurements of fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes, a key factor for the enhanced emission is suppression of the nonradiative decay process arising from restricted molecular motion. Additionally, the difference in the emission rate constant is not negligible and can be used to interpret the difference in fluorescence quantum yield in water and in the solid state.     

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.     

N‐Boc‐Indolylbenzothiadiazole Derivatives: Efficient Full‐Color Solid‐State Fluorescence and Self‐Recovering Mechanochromic Luminescence

Herein, the solid‐state emission with good fluorescence quantum yields of N‐Boc‐indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid‐state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3‐methylindolyl derivatives exhibit moreover autonomously self‐recovering mechanochromic luminescence, whereby the original solid‐state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self‐recovering mechanochromic luminescence of 3‐methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.     

Solid‐State Emissive Aroyl‐S,N‐Ketene Acetals with Tunable Aggregation‐Induced Emission Characteristics

N‐Benzyl aroyl‐S,N‐ketene acetals can be readily synthesized by condensation of aroyl chlorides and N‐benzyl 2‐methyl benzothiazolium salts in good to excellent yields, yielding a library of 35 chromophores with bright solid‐state emission and aggregation‐induced emission characteristics. Varying the substituent from electron‐donating to electron‐withdrawing enables the tuning of the solid‐state emission color from deep blue to red.     

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.     

Aggregation‐Induced Emission (AIE): A Historical Perspective

Aggregation‐induced emission (AIE) has attracted considerable interest over the last twenty years. In contrast to the large number of available reviews focusing specifically on AIE, this Essay discusses the AIE phenomenon from a broader perspective, with an emphasis on early observations related to AIE made long before the term was coined. Illustrative examples are highlighted from the 20th century where fluorescence enhancement upon rigidification of dyes in viscous or solid environments or J‐aggregate formation was studied. It is shown that these examples already include typical AIE luminogens such as tetraphenylethylene (TPE) as well as stilbenes and oligo‐ or polyphenylenevinylenes and ‐ethynylenes, which became important fluorescent solid‐state materials in OLED research in the 1990s. Further examples include cyanine dyes such as thiazole orange (TO) or its dimers (TOTOs), which have been widely applied as molecular probes in nucleic acid research. The up to 10 000‐fold fluorescence enhancement of such dyes upon intercalation into double‐stranded DNA, attributable to the restricted intramolecular motion (RIM) concept, afforded commercial products for bioimaging and fluorescence sensing applications already in the early 1990s.     

Aggregation-Induced Circularly Polarized Luminescence from Boron Complexes with a Carbazolyl Schiff Base

A variety of carbazolyl-appended Schiff bases were readily synthesized from 1-formylcarbazoles and aniline derivatives. Boron complexation of the resulting ligands allowed for facile preparation of new carbazole-based BODIPY analogues showing solid-state fluorescence. Furthermore, some dyes were converted into chiral compounds through the Et₂AlCl-mediated incorporation of a binaphthyl unit. The chiral dyes showed aggregation-induced fluorescence and circularly polarized luminescence (CPL) with the Φ_F and g_lum of up to 0.22 and −3.5×10⁻³, respectively, in the solid state. The solid-state fluorescence and CPL were well characterized by the crystal packing analyses and DFT calculations.     

Solid‐State Gas Sensors Developed from Functional Difluoroboradiazaindacene Dyes

This article describes the synthesis and characterization of several new difluoroboradiazaindacene (BODIPY?) dyes functionalized at the central 8‐position by a phenyliodo, phenylheptynoate or phenylheptynoic fragment and at the 3‐ or 3/5‐position(s) by 4‐dimethylaminophenylstyryl residue(s). Single‐crystal structural determinations confirm the planarity of the dyes, while the absorption and fluorescence spectroscopic properties are highly sensitive to the state of protonation (or alkylation) of the terminal anilino donor group(s). Reversible color tuning from green to blue for absorption and from colorless (i.e., near‐IR region) to red for fluorescence is obtained on successive addition of acid and base. The difunctionalized derivative is especially interesting in this respect and shows two well‐resolved pK_a values of 5.10 and 3.04 in acetonitrile. Addition of the first proton causes only small spectral changes and deactivates the molecule towards addition of the second proton. It is this latter step that accommodates the large change in absorption and emission properties, due to the reversible extinction of the intramolecular charge‐transfer character inherent to this type of dye. The main focus of the work is the covalent anchoring of the dyes to inert, porous polyacrylate beads so as to form a solid‐state sensor suitable for analysis of gases or flowing liquids. The final material is highly stable—its performance is undiminished after more than one year—and fully reversible over many cycles. The sensitivity is such that reactions can be followed by the naked eye and the detection limit is about 600 ppb for HCl and about 80 ppb for ammonia. Trace amounts of diphosgene can be detected, as can alkylating agents. The sensing action is indiscriminate and also operates when the beads are dispersed in aqueous media.     

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.     

Solvent‐Driven Conformational Exchange for Amide‐Linked Bichromophoric BODIPY Derivatives

The fluorescence lifetime and quantum yield are seen to depend in an unexpected manner on the nature of the solvent for a pair of tripartite molecules composed of two identical boron dipyrromethene (BODIPY) residues attached to a 1,10‐phenanthroline core. A key feature of these molecular architectures concerns the presence of an amide linkage that connects the BODIPY dye to the heterocyclic platform. The secondary amide derivative is more sensitive to environmental change than is the corresponding tertiary amide. In general, increasing solvent polarity, as measured by the static dielectric constant, above a critical threshold tends to reduce fluorescence but certain hydrogen bond accepting solvents exhibit anomolous behaviour. Fluorescence quenching is believed to arise from light‐induced charge transfer between the two BODIPY dyes, but thermodynamic arguments alone do not explain the experimental findings. Molecular modelling is used to argue that the conformation changes in strongly polar media in such a way as to facilitate improved rates of light‐induced charge transfer. These solvent‐induced changes, however, differ remarkably for the two types of amide.     

Turning on Solid‐State Fluorescence with Light

A bioinspired fluorophore that is analogous to the substrate in the bioluminescence of fireflies was prepared and reacts when exposed to weak blue LED light. Upon excitation, this material is photodecarboxylated with a nearly 81‐fold enhancement of the solid‐state emission, the fluorescence quantum yield of the product in solution is approximately 90 %, and violent disintegrative effects occur as a result of the release of carbon dioxide. Crystallographic and computational results, together with global spectral analysis of the kinetics, confirmed that most of the emission observed in the decay‐associated spectra is intrinsic to the product molecule, with only a minor contribution from an excimer through π–π stacking of the molecules in the crystal.