Base-Assisted One-Pot Synthesis of N,N′,N″-Triaryltriazatriangulenium Dyes: Enhanced Fluorescence Efficiency by Steric Constraints

5,11-Diazadibenzo[hi,qr]tetracene: Synthesis, Properties, and Reactivity toward Nucleophilic Reagents

5,11-Diazadibenzo[hi,qr]tetracene was synthesized as a new nitrogen-substituted polycyclic heteroaromatic compound by Pd-catalyzed cycloisomerization of an alkyne precursor followed by oxidative cyclization with bis(trifluoroacetoxy)iodobenzene. The substitution of imine-type nitrogen atoms significantly enhanced its electron-accepting character and facilitated the direct nucleophilic addition of arylamines under strongly basic conditions to afford the desired amino-substituted products. The introduction of amino groups induced a remarkable red-shift in their absorption spectra; the tetrasubstituted product exhibited intense near-infrared absorbing property. Furthermore, the π-electronic system, which includes a redox-active 1,4-diazabutadiene moiety, underwent reversible interconversion to its corresponding reduced form upon reduction with NaBH₄ and aerobic oxidation.

Syntheses and Physical Properties of Cationic BN-Embedded Polycyclic Aromatic Hydrocarbons

Cationic BN-embedded polycyclic aromatic hydrocarbons (BN-PAH⁺ s) were synthesized from a nitrogen-containing macrocycle via pyridine-directed tandem C-H borylation. Incorporating BN into PAH⁺ resulted in a remarkable hypsochromic shift due to an increase in the LUMO energy and the symmetry changes of the HOMO and LUMO. Electrophilic substitution or anion exchange of BN-PAH⁺ possessing tetrabromoborate as a counter anion (BN⁺ [BBr₄ ^- ]) afforded air-stable BN-PAH/PAH⁺ s. Of these, BN⁺ [TfO^- ] allowed reversible two-electron reduction and the formation of two-dimensional brickwork-type π-electronic ion pair with 1,2,3,4,5-pentacyanocyclopentadienyl anion, demonstrating the potential application of BN-PAH⁺ as electronic materials.

Synthesis and properties of sulfur-functionalized triarylmethylium, acridinium and triangulenium dyes

Triangulenium dyes functionalized with one, two or three ethylthiol functionalities were synthesized and their optical properties were studied. The sulfur functionalities were introduced by aromatic nucleophilic substitution of methoxy groups in triarylmethylium cations with ethanethiol followed by partial or full ring closure of the ortho positions with nitrogen or oxygen bridges leading to sulfur-functionalized acridinium, xanthenium or triangulenium dyes. For all the dye classes the sulfur functionalities are found to lead to intensely absorbing dyes in the visible range (470 to 515 nm), quite similar to known analogous dye systems with dialkylamino donor groups in place of the ethylthiol substituents. For the triangulenium derivatives significant fluorescence was observed (Φ_f = 0.1 to Φ_f = 0.3).

Fluorometric Recognition of Nucleotides within a Water-Soluble Tetrahedral Capsule

The design of aqueous probes and binders for complex, biologically relevant anions presents a key challenge in supramolecular chemistry. Herein, a tetrahedral assembly with cationic faces and corners is reported that is capable of discriminating between anionic and neutral guests in water. Electrostatic repulsion between subcomponents can be overcome by the addition of an anionic template, or generating a robust covalent framework by incorporating tris(2-aminoethyl)amine (TREN). The resultant TREN-capped, water-soluble, fluorescent cage binds mono- and poly-phosphoric esters, including nucleotides. Its covalent skeleton renders it stable at micromolar concentrations in water, enabling the fluorometric detection of biologically relevant guests in an aqueous environment. Selective supramolecular encapsulants, such as 1, could enable new sensing applications, such as recognition of toxins and drugs, under biological conditions.

Stimuli-Responsive Organic Dyes with Tropylium Chromophore

Tropylium ions possess an interesting combination of structural stability and chemical reactivity due to its Hückel aromaticity and its positively charged polyene nature, respectively. Herein we exploit the chemical versatility and unique structural properties of the tropylium ion to derive a family of novel push-pull organic dyes with strong absorption in the visible range via simple and practical synthetic protocols. These stable organic dyes are highly stimuli-responsive, as demonstrated by their sensitivity towards solvent, pH change, redox reaction, Lewis base and counterion, which marks them as potentially useful compounds for opto-electronic applications.

Influence of electronic and molecular structure on the fragmentation dynamic of even-electron carbocationic triangulenes and helicenes in the gas phase

Stable, long-lived organic cations are directly transferred by electrospray ionization (ESI) from solution into the gas phase where their collision-induced dissociations (CID) are studied by tandem mass spectrometry. Three related types of triphenyl carbenium ions are investigated, in which the meta positions are either substituted by methoxy groups or tertiary nitrogen bridges, including tetramethoxyphenylacridinium (TMPA⁺ ), dimethoxyquinacridinium (DMQA⁺ ), and triazatriangulenium (TATA⁺ ) cations. These ions are triangular in shape with increasing degrees of planarity. Fragmentation occurs at the periphery of the triangular molecule, involving the methoxy groups and the substituent of the nitrogen bridge. Each initial precursor cation is an even electron (EE) system and shows competing dissociations into both even (EE) and odd electron (OE) fragment ions. The latter reaction is a breach of the classic 'even-electron rule' in mass spectrometry. While the EE fragment dissociates similar to the precursor, the OE fragment ion shows a rich radical-induced fragmentation pattern. Two driving forces direct the fragmentation of the EE precursor ion toward OE fragment ions, including the release of stabilized radicals and the extension of the π-system by increasing planarization of the triangulene core. Copyright © 2017 John Wiley & Sons, Ltd.

Stability of Odd- Versus Even-Electron Gas-Phase (Quasi)Molecular Ions Derived from Pyridine-Substituted N-Heterotriangulenes

Electrospray ionisation of N-heterotriangulenes (i.e., dimethylmethylene-bridged triphenylamines) with up to three pyridyl groups at their periphery, produces the true radical cation ([M]^{+ .} ) and the protonated molecule ([M+H]⁺ ) simultaneously. These ions are studied as model systems to illustrate the stability alternation of odd- versus even-electron ions in energy-dependent collision-induced dissociation (CID) experiments. All ions show the same fragmentation pattern, the consecutive loss of three methyl radicals (^. CH₃ ) from the dimethylmethylene bridges of the central triangulene core. [M]^{+ .} ions dissociate at much lower collision energies than their [M+H]⁺ counterparts. The radical cation forms a singlet fragment with an extended aromatic system that is energetically favoured. Ab initio and density functional theory calculations support this interpretation and allow the assignment of the electronic structure of the fragment ions. Consecutive collision-induced dissociations provide a better match with theory when studied with an ion trap, rather than a linear quadrupole. This is attributed to the resonant nature of the excitation of intermediate ions.

Azadioxatriangulenium and Diazaoxatriangulenium: Quantum Yields and Fundamental Photophysical Properties

Over the last decade, we have investigated and exploited the photophysical properties of triangulenium dyes. Azadioxatriangulenium (ADOTA) and diazaoxatriangulenium (DAOTA), in particular, have features that make them useful in various fluorescence-based technologies (e.g., bioimaging). Through our work with ADOTA and DAOTA, we became aware that the reported fluorescence quantum yields (ϕ_fl) for these dyes are lower than their actual values. We thus set out to further investigate the fundamental structure-property relationships in these unique conjugated cationic systems. The nonradiative processes in the systems were explored using transient absorption spectroscopy and time-resolved emission spectroscopy in combination with computational chemistry. The influence of molecular oxygen on the fluorescence properties was explored, and the singlet oxygen sensitization efficiencies of ADOTA and DAOTA were determined. We conclude that, for these dyes, the amount of nonradiative deactivation of the first excited singlet state (S₁) of the azaoxa-triangulenium fluorophores is low, that the rate of such deactivation is slower than what is observed in common cationic dyes, that there are no observable radiative transitions occurring from the first excited triplet state (T₁) of these dyes, and that the efficiency of sensitized singlet oxygen production is low (ϕ_Δ ≤ 10%). These photophysical results provide a solid base upon which technological applications of these fluorescent dyes can be built.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Diazaoxatriangulenium: synthesis of reactive derivatives and conjugation to bovine serum albumin

The azaoxa-triangulenium dyes are characterised by emission in the red and a long fluorescence lifetime (up to 25 ns). These properties have been widely explored for the azadioxatrianguelnium (ADOTA) dye. Here, the syntheses of reactive maleimide and NHS-ester forms of the diazaoxatriangulenium (DAOTA) system are reported. The DAOTA fluorophore was conjugated to bovine serum albumin (BSA) and investigated in comparison to the corresponding ADOTA-BSA conjugate. It was found that the fluorescence of DAOTA experienced a significantly higher degree of solvent quenching if compared to ADOTA as non-conjugated dyes in aqueous solution, while the fluorescence quenching observed upon conjugation to BSA was significantly reduced for DAOTA when compared to ADOTA. The differences in observed quenching for the conjugates can be explained by the different electronic structures of the dyes, which renders DAOTA significantly less prone to reductive photoinduced electron transfer (PET) quenching from e.g. tryptophan. We conclude that DAOTA, with emission in the red and inherent resistance to PET quenching, is an ideal platform for the development of long fluorescence lifetime probes for time-resolved imaging and fluorescence polarisation assay.

Azadioxatriangulenium: exploring the effect of a 20 ns fluorescence lifetime in fluorescence anisotropy measurements

Azaoxatriangulenium (ADOTA) has been shown to be highly emissive despite a moderate molar absorption coefficient of the primary electronic transition. As a result, the fluorescence lifetime is ~20 ns, longer than all commonly used red fluorescent organic probes. The electronic transitions in ADOTA are highly polarised (r ₀  =  0.38), which in combination with the long fluorescence lifetime extents the size-range of biomolecular weights that can be detected in fluorescence polarisation-based experiments. Here, the rotational dynamics of bovine serum albumin (BSA) are monitored with three different ADOTA derivatives, differing only in constitution of the reactive linker. A detailed study of the degree of labelling, the steady-state anisotropy, and the time-resolved anisotropy of the three different ADOTA-BSA conjugates are reported. The fluorescence quantum yields (ϕ _fl) of the free dyes in PBS solution are determined to be ~55%, which is reduced to ~20% in the ADOTA-BSA conjugates. Despite the reduction in ϕ _fl, a ~20 ns intensity averaged lifetime is maintained, allowing for the rotational dynamics of BSA to be monitored for up to 100 ns. Thus, ADOTA can be used in fluorescence polarisation assays to fill the gap between commonly used organic dyes and the long luminescence lifetime transition metal complexes. This allows for efficient steady-state fluorescence polarisation assays for detecting binding of analytes with molecular weights of up to 100 kDa.

Azadioxatriangulenium: Synthesis and Photophysical Properties of Reactive Dyes for Bioconjugation

Azadioxatriangulenium (ADOTA) is a fluorescent triangulenium dye with a long fluorescence lifetime, highly polarized transitions and emission in the red part of the visible spectrum. These properties make the chromophore suited for application in fluorescence polarization/anisotropy assay. To be useful for these applications, reactive forms of the dyes must be available in significant quantities. Here, the synthesis and photophysical properties of amine-reactive NHS esters and a thiol-reactive maleimide derivate of ADOTA are reported. The synthesis involves two steps of nucleophilic bridge forming reactions starting from tris(2,6-dimethoxyphenyl) methylium tetrafluoroborate, which can readily be made on 100 gram scale. In the third and final step the reactive NHS or maleimide groups are formed. The beneficial photophysical properties of the ADOTA chromophore are maintained in these derivatives, and we conclude that these systems are ideal to study protein motion and protein-protein interactions for systems of up towards 1000 kDa.

Triazatriangulenium-based porous organic polymers for carbon dioxide capture

Reversible and preferred adsorption for CO₂ is visible spectroscopically on triazatriangulenium-based cationic porous organic polymers.

Cationic triangulenes and helicenes: synthesis, chemical stability, optical properties and extended applications of these unusual dyes

Cationic triangulenes and helicenes are highly stable carbocations with planar and helical conformations respectively. These moieties are effective dyes with original absorption and emission properties. Over the last decade, they have received greater attention and are considered as valuable tools for the development of innovative applications. In this review, the synthesis of these unique compounds is presented together with their core chemical and physical properties. Representative applications spanning from surface sciences to biology and chemistry are presented.

Azadioxatriangulenium: a long fluorescence lifetime fluorophore for large biomolecule binding assay

Of the many optical bioassays available, sensing by fluorescence anisotropy has great advantages as it provides a sensitive, instrumentally simple, ratiometric method of detection. However, it is hampered by a severe limitation, as the emission lifetime of the label needs to be comparable to the correlation lifetime (tumbling time) of the biomolecule which is labelled. For proteins of moderate size this is on the order of 20-200 ns, which due to practical issues currently limits the choice of labels to the dansyl-type dyes and certain aromatic dyes. These have the significant drawback of UV/blue absorption and emission as well as an often significant solvent sensitivity. Here, we report the synthesis and characterization of a new fluorescent label for high molecular weight biomolecule assay based on the azadioxatriangulenium motif. The NHS ester of the long fluorescence lifetime, red-emitting fluorophore: azadioxatriangulenium (ADOTA-NHS) was conjugated to anti-rabbit Immunoglobulin G (antiIgG). The long fluorescence lifetime was exploited to determine the correlation time of the high molecular weight antibody and its complex with rabbit Immunoglobulin G (IgG) with steady-state fluorescence anisotropy and time-resolved methods: solution phase immuno-assay was performed following either steady-state or time-resolved fluorescence anisotropy. By performing a variable temperature experiment it was determined that the binding of the ligand resulted in an increase in correlation time of more than 75%, and an increase in the steady-state anisotropy of 18%. The results show that the triangulenium class of dyes can be used in anisotropy assay to detect binding events involving biomolecules of far larger size than what is possible with most other red-emitting organic dyes.

Polarization and symmetry of electronic transitions in long fluorescence lifetime triangulenium dyes

To fully exploit the capabilities of fluorescence probes in modern experiments, where advanced instrumentation is used to probe complex environments, other photophysical properties than emission color and emission intensity are monitored. Each dye property can be addressed individually as well as collectively to provide in-depth information unavailable from the standard intensity measurements. Dyes with long emission lifetimes and strongly polarized transitions enable the monitoring of lifetime changes as well as emission polarization (anisotropy). Thus experiments can be designed to follow slow dynamics. The UV and visible electronic transitions of a series of red-emitting dyes based on the triangulenium motif are investigated. We resolve overlapping features in the spectra and assign the orientation of the transition moments to the molecular axes. The result is the complete Jablonski diagram for the UV and visible spectral region. The symmetries of the studied dyes are shown to have a large influence on the optical response, and they are clearly separated into two groups of symmetry by their photophysical properties. The C(2v) symmetric dyes, azadioxatriangulenium (ADOTA(+)) and diazaoxatriangulenium (DAOTA(+)), have high emission anisotropies, fluorescence lifetimes around 20 ns, and fluorescence quantum yields of ∼50%. The trioxatriangulenium (TOTA(+)) and triazatriangulenium (TATA(+)) dyes-nominally of D(3h) symmetry-have fluorescence lifetimes around 10 ns lifetimes and fluorescence quantum yields of 10-15%. However, the D(3h) symmetry is shown to be lowered to a point group, where the axes transform uniquely such that the degeneracy of the E' states is lifted.

Elimination of autofluorescence background from fluorescence tissue images by use of time-gated detection and the AzaDiOxaTriAngulenium (ADOTA) fluorophore

Sample autofluorescence (fluorescence of inherent components of tissue and fixative-induced fluorescence) is a significant problem in direct imaging of molecular processes in biological samples. A large variety of naturally occurring fluorescent components in tissue results in broad emission that overlaps the emission of typical fluorescent dyes used for tissue labeling. In addition, autofluorescence is characterized by complex fluorescence intensity decay composed of multiple components whose lifetimes range from sub-nanoseconds to a few nanoseconds. For these reasons, the real fluorescence signal of the probe is difficult to separate from the unwanted autofluorescence. Here we present a method for reducing the autofluorescence problem by utilizing an azadioxatriangulenium (ADOTA) dye with a fluorescence lifetime of approximately 15 ns, much longer than those of most of the components of autofluorescence. A probe with such a long lifetime enables us to use time-gated intensity imaging to separate the signal of the targeting dye from the autofluorescence. We have shown experimentally that by discarding photons detected within the first 20 ns of the excitation pulse, the signal-to-background ratio is improved fivefold. This time-gating eliminates over 96 % of autofluorescence. Analysis using a variable time-gate may enable quantitative determination of the bound probe without the contributions from the background.