Why Triple Bonds Protect Acenes from Oxidation and Decomposition

How to Stabilize Large Soluble (Hetero-)Acenes

The higher acenes and azaacenes (>(aza)heptacenes) are fascinating, yet elusive materials. Their reactivity and sensitivity increases concomitantly with their size. In recent years, confinement techniques, that is isolation of acenes in matrices and on surfaces, has surpassed solution-based chemistry with respect to accessing the larger (hetero)acenes at the price of the accessibility of no more than a couple thousands of molecules. Isolating acenes in bulk quantities and in processable form is vital for applications in organic electronics as well as from a viewpoint from basic research. In this Perspective, we will discuss after a short historical outline their degradation pathways, and then will selectively highlight recent efforts in stabilizing soluble (aza)acenes.

Naphthalene diimide-Annulated Heterocyclic Acenes: Synthesis, Electrochemical and Semiconductor Properties and their Multifaceted Applications

Acenes and Naphthalene Diimides (NDIs) stand as distinguished classes of organic compounds, each possessing unique and intriguing properties that have garnered significant attention across various scientific disciplines. Acenes, characterized by linearly fused aromatic rings, have captivated researchers due to their diverse electronic structures and promising applications in materials science. On the other hand, NDIs, known for their distinctive electron-accepting properties, exhibit remarkable versatility in fields ranging from organic electronics, supramolecular to spin chemistry. In this review, we navigate through the fascinating realms of both acenes and NDIs before converging our focus on the highly diverse and distinctive subgroup of NDI-annulated heterocyclic acenes. This potentially important subgroup, has emerged as a subject of intense investigation, encapsulating their fascinating synthesis, optical and electrochemical characteristics, and multifaceted applications that span the realms of chemistry, physics, and biology. Through the exploration of their synthetic strategies, unique properties, and diverse applications, this review aims to offer a comprehensive understanding of the pivotal role played by NDI-based heterocyclic acenes in contemporary multidisciplinary research and technological innovation.

Spatiotemporal Release of Singlet Oxygen in Low Molecular Weight Organo-Gels Upon Thermal or Photochemical External Stimuli

The first example of a material capable of spatiotemporal catch and release of singlet oxygen (1O2) in gel phase is presented. Several low molecular weight organogelators based around an oxotriphenylhexanoate (OTHO) core are developed and optimized with regard to; their gelation properties, and ability of releasing 1O2 upon thermal and/or photochemical external stimuli, in both gel phase and solution. Remarkably, reversible phase transitioning between the gel and solution phase are also demonstrated. Taken together two complementary modes of releasing 1O2, one thermally controlled over time, and one rapid release by means of photochemical stimuli is disclosed. These findings represent the first phase reversible system where function and aggregation properties can be controlled independently, and thus pave the way for novel applications in material sciences as well as in life sciences.

Photocatalytic manipulation of Ca2+ signaling for regulating cellular and animal behaviors via MOF-enabled H2O2 generation

The in situ generation of H2O2 in cells in response to external stimulation has exceptional advantages in modulating intracellular Ca2+ dynamics, including high controllability and biological safety, but has been rarely explored. Here, we develop photocatalyst-based metal-organic frameworks (DCSA-MOFs) to modulate Ca2+ responses in cells, multicellular spheroids, and organs. By virtue of the efficient photocatalytic oxygen reduction to H2O2 without sacrificial agents, photoexcited DCSA-MOFs can rapidly trigger Ca2+ outflow from the endoplasmic reticulum with single-cell precision in a repeatable and controllable manner, enabling the propagation of intercellular Ca2+ waves (ICW) over long distances in two-dimensional and three-dimensional cell cultures. After photoexcitation, ICWs induced by DCSA-MOFs can activate neural activities in the optical tectum of tadpoles and thighs of spinal frogs, eliciting the corresponding motor behaviors. Our study offers a versatile optical nongenetic modulation technique that enables remote, repeatable, and controlled manipulation of cellular and animal behaviors.

Stabilization of Acenes: "Geländer"-Pentacenes

We report soluble tetrakis-biphenylyl substituted pentacenes comprised of sp2 carbons and synthesized from pentacene-5,7,12,14-tetraone. Intramolecular Yamamoto coupling of two tetrakis(chlorobiphenylyl)pentacenes yields helical, doubly wrapped pentacenes, in which the quaterphenylene units solubilize the pentacenes and shield their central anthracene units to an unprecedented degree. The criss-cross-bridged pentacenes resist (photo)oxidation, Diels-Alder reactions and are much less reactive than TIPS-ethynylated pentacene. Extension of this concept might provide access to the larger acenes.

Pyrazinacene conjugated polymers: a breakthrough in synthesis and unraveling the conjugation continuum

Pyrazinacenes are next generation N-heteroacenes and represent a novel class of stable n-type materials capable of accepting more than one electron and displaying intriguing features, including prototropism, halochromism, and redox chromism. Astonishingly, despite a century since their discovery, there have been no reports on the conjugated polymers of pyrazinacenes due to unknown substrate scope and lack of pyrazinacene monomers that are conducive to condensation polymerization. Breaking through these challenges, in this work, we report the synthesis of previously undiscovered and highly coveted conjugated polymers of pyrazinacenes. In order to understand the intricacies of conjugation extension within the acene and along the polymer backbone, a series of electronically diverse four pyrazinacene conjugated polymers were synthesized. Polymers synthesis required optimizing a few synthetic steps along the 12-step synthetic pathway. The generated pyrazinacene monomers are not amenable to the popular condensation polymerizations involving Pd or Cu catalysts. Gratifyingly, Pd and Cu free dehydrohalogenation polymerization of the monomer with HgCl2 resulted in high molecular weight organometallic conjugated pyrazinacene polymers within a few minutes at room temperature. The dual role played by the Hg(ii) during the polymerization, combined with the self-coupling of the RHgCl (intermediate), is at the core of successful polymerization. Notably, the self-coupling of intermediates challenges the strict stoichiometric balance typically required for step-growth polymerization and offers a novel synthetic strategy to generate high molecular weight conjugated polymers even with imbalanced monomer stoichiometries. A combination of electrochemical studies and DFT-B3LYP simulations indicated that the presence of the reduced pyrazine ring promotes interacene π-conjugation through the metal center, in contrast to completely oxidized tetrazaazaanthracene. The extension of conjugation results in ca. 2 eV lower reduction potential for polymers compared to the monomer, placing the LUMO energy levels of these polymers on par with some of the best-known n-type polymers. Also, the presence of NH protons in the pyrazinacene polymers show ionochromism and red-shift UV-vis absorption maximum by ca. 100 nm. This work not only shows a way to realize highly desirable and elusive pyrazinacene conjugated polymers but also paves the way for a library of n-type conjugated polymers that can undergo multi-electron reduction.

Enhancing Fractionated Cancer Therapy: A Triple-Anthracene Photosensitizer Unleashes Long-Persistent Photodynamic and Luminous Efficacy

Conventional photodynamic therapy (PDT) is often limited in treating solid tumors due to hypoxic conditions that impede the generation of reactive oxygen species (ROS), which are critical for therapeutic efficacy. To address this issue, a fractionated PDT protocol has been suggested, wherein light irradiation is administered in stages separated by dark intervals to permit oxygen recovery during these breaks. However, the current photosensitizers used in fractionated PDT are incapable of sustaining ROS production during the dark intervals, leading to suboptimal therapeutic outcomes (Table S1). To circumvent this drawback, we have synthesized a novel photosensitizer based on a triple-anthracene derivative that is designed for prolonged ROS generation, even after the cessation of light exposure. Our study reveals a unique photodynamic action of these derivatives, facilitating the direct and effective disruption of biomolecules and significantly improving the efficacy of fractionated PDT (Table S2). Moreover, the existing photosensitizers lack imaging capabilities for monitoring, which constraints the fine-tuning of irradiation parameters (Table S1). Our triple-anthracene derivative also serves as an afterglow imaging agent, emitting sustained luminescence postirradiation. This imaging function allows for the precise optimization of intervals between PDT sessions and aids in determining the timing for subsequent irradiation, thus enabling meticulous control over therapy parameters. Utilizing our novel triple-anthracene photosensitizer, we have formulated a fractionated PDT regimen that effectively eliminates orthotopic pancreatic tumors. This investigation highlights the promise of employing long-persistent photodynamic activity in advanced fractionated PDT approaches to overcome the current limitations of PDT in solid tumor treatment.

Stabilizing Azaheptacenes

The symmetrical 7,16-diaza-6,8,15,17-tetrakis(triisopropylsilylethynyl)heptacene was obtained by a Pd-catalyzed reaction of a 2,3-diamino-1,4-diethynylanthracene with a 2,3-dibromo-1,4-diethynyl anthracene. Positioning the TIPS-ethynyl groups adjacent to the central ring suppresses dimerization via [4+4] cycloadditions and Diels-Alder reactions; the middle pyrazine ring renders this species stable to oxidation. A single crystal structure was obtained, and thin film transistors with μn = 0.042 cm2 V-1 s-1 were produced. Transposition of the alkynyl groups into the 5,8,15,18-positions with a quinoxaline unit in the center of the heptacene decreases the stability, as does the introduction of two more nitrogen atoms into the 5,18-positions. The hydrocarbon 6,8,15,17-tetrakis(triisopropylsilylethynyl)heptacene is reasonably stable with a half-life of 25 h in solution. Four correctly placed TIPS-ethynyl groups protect heptacene cores.

Photosensitizer-singlet oxygen sensor conjugated silica nanoparticles for photodynamic therapy and bioimaging

Intracellular singlet oxygen (1O2) generation and detection help optimize the outcome of photodynamic therapy (PDT). Theranostics programmed for on-demand phototriggered 1O2 release and bioimaging have great potential to transform PDT. We demonstrate an ultrasensitive fluorescence turn-on sensor-sensitizer-RGD peptide-silica nanoarchitecture and its 1O2 generation-releasing-storing-sensing properties at the single-particle level or in living cells. The sensor and sensitizer in the nanoarchitecture are an aminomethyl anthracene (AMA)-coumarin dyad and a porphyrin or CdSe/ZnS quantum dots (QDs), respectively. The AMA in the dyad quantitatively quenches the fluorescence of coumarin by intramolecular electron transfer, the porphyrin or QD moiety generates 1O2, and the RGD peptide facilitates intracellular delivery. The small size, below 200 nm, as verified by scanning electron microscopy and differential light scattering measurements, of the architecture within the 1O2 diffusion length enables fast and efficient intracellular fluorescence switching by the tandem ultraviolet (UV)-visible or visible-near-infrared (NIR) photo-triggering. While the red emission and 1O2 generation by the porphyrin are continually turned on, the blue emission of coumarin is uncaged into 230-fold intensity enhancement by on-demand photo-triggering. The 1O2 production and release by the nanoarchitecture enable spectro-temporally controlled cell imaging and apoptotic cell death; the latter is verified from cytotoxic data under dark and phototriggering conditions. Furthermore, the bioimaging potential of the TCPP-based nanoarchitecture is examined in vivo in B6 mice.

Role of Singlet and Triplet Excited States in the Oxygen-Mediated Photophysics and Photodegradation of Polyacenes

Polyacenes, such as tetracene and pentacene, are common model systems for the study of photophysical phenomena such as singlet fission (SF) and triplet fusion, processes which may lead to increased photovoltaic efficiencies. While they exhibit desirable photophysical properties, these materials are not photostable and convert to unwanted endoperoxides in the presence of oxygen and light, limiting their use in real-world applications. Not only does oxygen degrade polyacenes but also it can affect their photophysics, leading to both the sensitization and quenching of different excited states. In this study, we characterize the effect of oxygen on 5,12-bis(triisopropylsilylethynyl) tetracene (TIPS-Tn) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) using transient absorption spectroscopy, and show that oxygen can significantly influence the population of excited states, in particular enhancing the polyacene triplet population. We additionally combine the time-resolved excited-state dynamics with photodegradation studies to determine the predominant mechanism of photooxidation, which has previously been unclear. We find that both molecules photodegrade predominantly via singlet oxygen; however, for TIPS-Tn, this occurs through the triplet state, whereas for TIPS-Pn, degradation occurs through the excited singlet. The photodegradation of TIPS-Tn is thus enhanced by faster rates of SF, whereas SF in TIPS-Pn increases the molecule's photostability. This work has implications both for the design of new materials for next-generation photovoltaics that can avoid photooxidation and for the study of their photophysics in real-world environments.

Doubly Bridged Anthracenes: Blue Emitters for OLEDs

The photooxidative stability of a series of doubly bridged anthracenes was evaluated after their preparation via twofold macrocyclization of a bis(resorcinyl)anthracene. Lightfastness correlates with the energy levels of the highest occupied molecular orbital (HOMO), resulting in superior stability of the tetraesters compared to the tetraethers. The lengths and steric demand of the linker only plays a minor role for the ester-based compounds, which can be prepared in reasonable yields and thus tested in proof-of-concept organic light-emitting diodes. Double ester-bridging allows deep blue electro-luminescence, highlighting the importance of the choice of the functional groups used for macrocyclization.

Electronic effects of conjugated aryl groups on the properties and reactivities of di(arylethynyl)tetracenes

The photochemical oxidations of acenes can cause challenges with their optoelectronic applications, such as singlet fission and organic transistors. At the same time, these reactions form the basis for many luminescent sensing schemes for 1O2. While diethynyl substitution is arguably the most widely adopted of the various substitution strategies to control oxidation and also improve solubility and processability of long acenes, the extent to which differences between the alkyne groups can influence key properties of long acenes remains largely unknown. This report therefore describes the effects of various arenes and heteroarenes on the electronic structures, optical properites, and reactivity with singlet oxygen for eight 5,12-di(arylethynyl)tetracenes. The fluorescence spectra of these tetracenes span approximately 100 nm, while their observed rate constants for reaction with singlet oxygen correlates strongly with the HOMO level, spanning one order of magnitude. They are also amenable to fluorescent materials that respond ratiometrically to singlet oxygen. Therefore, electronic effects of groups directly conjugated to ethynylacenes offer a useful chemical space for rational acene design.

Hexakis-TIPS-Alkynylated Nonacenes: Persistent and Processible

Four substituted nonacenes were prepared and characterized by UV-vis and EPR spectroscopy and X-ray crystallography. The compounds are the most stable and soluble nonacenes to date - due to six strategically placed triisopropylsilyl(TIPS)-ethynyl groups. They are stable for several weeks in the solid state. In dilute solution their half-life is 5-9 h. Crystal structure analyses of two nonacenes prove their structures. A nonacene derivative was tested in a solution-processed transistor and exhibits ambipolar charge transport (μe =0.007 cm2 /Vs; μh =0.023 cm2 /Vs).

Zwitterionic Bergman cyclization triggered polymerization gives access to metal-graphene nanoribbons using a boron metal couple

With the rapid growth in artificial intelligence, designing high-speed and low-power semiconducting materials is of utmost importance. This investigation provides a theoretical basis to access covalently bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. Systematic optimization of substrates containing remotely placed boryl groups and the transition metals produced the zwitterions via ionic Bergman cyclization (i-BC) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC, the subsequent steps were barrierless, which involved structureless transition regions. Multivariate analysis revealed the strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman (r-BC), ionic Bergman (i-BC), and ionic Schreiner-Pascal (i-SP) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by the three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP.

Imparting Stability to Organic Photovoltaic Components through Molecular Engineering: Mitigating Reactions with Singlet Oxygen

One key challenge in the development of viable organic photovoltaic devices is to design component molecules that do not degrade during combined exposure to oxygen and light. Such molecules should thus remain comparatively unreactive towards singlet molecular oxygen and not act as photosensitizers for the generation of this undesirable species. Here, novel redox-active chromophores that combine these two properties are presented. By functionalizing indenofluorene-extended tetrathiafulvalenes (IF-TTFs) with cyano groups at the indenofluorene core using Pd-catalyzed cyanation reactions, we find that the reactivity of the exocyclic fulvene carbon-carbon double bonds towards singlet oxygen is considerably reduced. The new cyano-functionalized IF-TTFs were tested in non-fullerene acceptor based organic photovoltaic proof-of-principle devices, revealing enhanced device stability.

Photogeneration of Spin Quintet Triplet-Triplet Excitations in DNA-Assembled Pentacene Stacks

Singlet fission (SF), an exciton-doubling process observed in certain molecular semiconductors where two triplet excitons are generated from one singlet exciton, requires correctly tuned intermolecular coupling to allow separation of the two triplets to different molecular units. We explore this using DNA-encoded assembly of SF-capable pentacenes into discrete π-stacked constructs of defined size and geometry. Precise structural control is achieved via a combination of the DNA duplex formation between complementary single-stranded DNA and the local molecular geometry that directs the SF chromophores into a stable and predictable slip-stacked configuration, as confirmed by molecular dynamics (MD) modeling. Transient electron spin resonance spectroscopy revealed that within these DNA-assembled pentacene stacks, SF evolves via a bound triplet pair quintet state, which subsequently converts into free triplets. SF evolution via a long-lived quintet state sets specific requirements on intermolecular coupling, rendering the quintet spectrum and its zero-field-splitting parameters highly sensitive to intermolecular geometry. We have found that the experimental spectra and zero-field-splitting parameters are consistent with a slight systematic strain relative to the MD-optimized geometry. Thus, the transient electron spin resonance analysis is a powerful tool to test and refine the MD-derived structure models. DNA-encoded assembly of coupled semiconductor molecules allows controlled construction of electronically functional structures, but brings with it significant dynamic and polar disorders. Our findings here of efficient SF through quintet states demonstrate that these conditions still allow efficient and controlled semiconductor operation and point toward future opportunities for constructing functional optoelectronic systems.

On-surface synthesis of non-benzenoid conjugated polymers by selective atomic rearrangement of ethynylarenes

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.

Limited Stability of 6,13-Bis(tri(isopropyl)silylethynyl)pentacene upon One-Electron Oxidation: Electrochemically Induced (4 + 2) Cycloaddition between an Alkynyl-Substituted Acene and Its Radical Cation

6,13-Bis(tri(isopropyl)silylethynyl)pentacene, a particularly stable acene derivative important for (opto)electronic materials, turns reactive upon electrochemical one-electron oxidation. One of the typically stabilizing tri(isopropyl)silylethynyl substituents becomes involved in a (4 + 2) cycloaddition after redox umpolung. The electrosynthetic dimerization of the title compound provides easy access under mild conditions to a complex scaffold, which includes an intact pentacene, an anthracene, and a phenylene unit, all electronically separated. The product's electrochemical redox properties are explained by superimposed cyclic voltammetric features of the pentacene and the anthracene moieties. The reaction path is analyzed on the basis of electroanalytical and ESR data, and an oxidation-cycloaddition-reduction sequence is elaborated. The contribution of homogeneous electron transfers (electron transfer chain reaction) is negligible, in accordance with the relative formal redox potentials of the starting compound and the product. Quantum chemical calculations indicate that the central cycloaddition should be described as a two-step process with a distonic radical cation intermediate. We suggest an extended notation to define the contribution of the components with respect to electron count in the two-step cycloaddition, [3 + 1, 1 + 1].

Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.

A Stable Hexaazaoctacene Cruciform σ-Dimer

Buchwald-Hartwig coupling of a triisopropylsilyl (TIPS)-ethynylated dibromo-N,N'-dihydrotetraazapentacene with 1,4-bis(TIPS-ethynyl)-2,3-diaminonaphthalene furnishes a dihydrohexaazaoctacene. Its oxidation with MnO₂ results in a 7,7'-bi(hexaazaoctacenyl). In addition to eight TIPS-ethynyl groups, the bioctacene motif protects the azaoctacene subunits. The biazaoctacenyl displays a τ_1/2 of > 5 d in dilute solution under ambient conditions. In the crystalline state it is persistent for > 10 months.

Aromatic Endoperoxides

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides, and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices. Improvement of the existing methods and development of new methods for the synthesis of endoperoxides is necessary as result of the demand to improve existing and to create new applications for these valuable materials. On the other hand, prevention of endoperoxide formation is crucial to inhibit irreversible oxidative degradation of aromatic hydrocarbons and to extend their lifetimes as useful organic semiconductors.

Porphyrin-Based Multicomponent Metallacage: Host-Guest Complexation toward Photooxidation-Triggered Reversible Encapsulation and Release

The development of supramolecular hosts with effective host-guest properties is crucial for their applications. Herein, we report the preparation of a porphyrin-based metallacage, which serves as a host for a series of polycyclic aromatic hydrocarbons (PAHs). The association constant between the metallacage and coronene reaches 2.37 × 10⁷ M^-1 in acetonitrile/chloroform (ν/ν = 9/1), which is among the highest values in metallacage-based host-guest complexes. Moreover, the metallacage exhibits good singlet oxygen generation capacity, which can be further used to oxidize encapsulated anthracene derivatives into anthracene endoperoxides, leading to the release of guests. By employing 10-phenyl-9-(2-phenylethynyl)anthracene whose endoperoxide can be converted back by heating as the guest, a reversible controlled release system is constructed. This study not only gives a type of porphyrin-based metallacage that shows desired host-guest interactions with PAHs but also offers a photooxidation-responsive host-guest recognition motif, which will guide future design and applications of metallacages for stimuli-responsive materials.

Red to NIR-emissive anthracene-conjugated PMI dyes with dual functions: singlet-oxygen response and lipid-droplet imaging

The rich chemistry of solution-processable red and near-infrared (NIR) organic emitters has emerged as an attractive and progressive research field because of their particular applications in organic optoelectronics and bioimaging. Also, one can see that the research area of perylene monoimide-based red and NIR-emissive fluorophores is underexplored, which prompted us to design and synthesize three anthracene-conjugated PMI dyes exhibiting strong emission in the red and NIR window in solution. Three PMI-based fluorophores were synthesized via conjoining anthracene and donor moieties (-Ph, -N,N-PhNMe₂) with a PMI core via an acetylene linkage at the peri-position, which helped to attain extensive electronic conjugation, which was reflected in red and NIR-emission in solution. The key molecular features to be highlighted here are: all three dyes are strongly emissive in solution, as unveiled by the excellent absolute fluorescence QYs; and they possess tuneable emission properties, guided by the donor strength and a profound Stokes shift (100-200 nm). The three fluorescent dyes demonstrated appreciable singlet-oxygen (¹O₂) sensitivity when photoirradiated with methylene blue (MB) in solution, showing a substantial blue-shift in emission in a ratiometric manner. Further, the treatment of dye-MB solution with α-tocopherol (¹O₂ scavenger) validated the presence of ¹O₂ as the only oxidizing species generated by MB in solution. Computational investigations gave insight into the twisting of donor moieties in their ground-state optimized geometries, the modulation of the FMO energy gap, and the thermodynamic feasibility of the ¹O₂ reaction. Finally, via taking advantage of the red and NIR-emission, we successfully utilized one of the fluorophores as a lipid-droplet marker for bioimaging in HepG2 cells.

Chemically Triggered Release of Singlet Oxygen from Bisphenalenyl Endoperoxides with a Brønsted Acid

Aromatic endoperoxides have emerged as intriguing stimulus-responsive materials for molecular oxygen (O₂) storage and delivery but are currently limited in their application because they require heat to trigger O₂ release. Here we present the first example of acid-triggered singlet oxygen (¹O₂) release that does not require external heating by treating bisphenalenyl endoperoxides (EPOs) with trifluoroacetic acid. Mechanistic studies reveal that diprotonation of EPOs leads to a >10-fold increase in cycloreversion rates by lowering the energy of activation (ΔE_a) by as much as 71.1 kJ mol^-1. Remarkably, acid-catalyzed ¹O₂ release is even demonstrated at room temperature. Chemical trapping experiments indicate that reactive ¹O₂ is present during acid-triggered release, which is promising for the development of these molecular materials for metal-free, on-demand ¹O₂ delivery.

Large Acene Derivatives with B-N Lewis Pair Doping: Synthesis, Characterization, and Application

Here, we report the synthesis of a novel class of B-N Lewis pair (LP_B/N)-doped large acene derivatives (1a-1d) from the well-designed phenanthridine-based precursors. The resultant LP_B/N-doped benzo-tetracene (1a), dibenzo-heptacene (1b), dibenzo-octacene (1c), and V-shaped tribenzo-nonacene (1d) are thoroughly characterized by X-ray crystallography, cyclic voltammetry, UV-vis absorption, and fluorescence spectroscopies together with DFT calculations. As a proof of concept, a 1a-based organic light-emitting diode device is fabricated to demonstrate the promising application in organic optoelectronics.

Reversible [4 + 2] Photooxygenation in Anion-Coordination-Driven-Assembled A2L2-Type Complexes

Two bis-bis(urea) ligands (L¹ and L²) incorporating the photoactive 9,10-diphenylanthracene fragment were designed for the construction of anion-coordination-driven assemblies and subsequent oxygenation of anthracene moieties for singlet oxygen storage. The corresponding A₂L₂-type sulfate complexes [TEA]₄[(SO₄)₂(L¹)₂] (1) and [TEA]₄[(SO₄)₂(L²)₂] (2), where TEA = tetraethylammonium, were achieved by coordinating the ligands L¹ or L² with sulfate anions. Both 1 and 2 were able to undergo [4 + 2] photooxygenation to form endoperoxide photoproducts 1-EPO and 2-EPO, which can be partially converted back to the original anthracene compounds after heating. The structures of 1-EPO and 2-EPO were unambiguously confirmed by X-ray crystallography, NMR and UV-vis spectroscopy, and high-resolution electrospray ionization mass spectrometry.

Self-Sensitized and Reversible O2 Reactivity with Bisphenalenyls for Simple, Tunable, and Multicycle Colorimetric Oxygen-Sensing Films

Monitoring the levels of molecular oxygen (O₂) is critical for numerous applications, but there is still a long-standing challenge to develop robust and cost-effective colorimetric sensors that enable detection by changes in color. Current technologies employ chromophores that require additional additives, which inherently increase the cost and complexity. Here, we report that bisphenalenyls (PQPLs) function as the single active component for colorimetric O₂ sensing through their quantitative conversion into aromatic endoperoxides (EPOs). PQPLs display self-sensitizing reactivity: they are capable of generating singlet oxygen and binding it without the need for external photosensitizers. The rates of PQPL photooxygenation depend on the electron-donating ability of substituents, which highlights a simple strategy for tuning O₂ sensitivity. EPOs are stable under ambient conditions but can be thermally stimulated to convert back to PQPLs and concomitantly release O₂. Polymer-supported (PTMSP) films of PQPLs (2 wt %) reproduce these reactivity trends with a rapid red-to-colorless transition that is visible to the naked eye within 1 h of exposure and show a very low limit of detection (<5 ppm O₂). Films are chemically and thermally robust and maintain up to >99% of their original colorimetric response when reused and subjected to multiple cycles of photooxygenation and O₂ release. The simplicity and solution processability of these materials highlight their potential as "intelligent" inks for printable colorimetric sensors.

π-Topology and Ultrafast Excited-State Dynamics of Remarkably Photochemically Stabilized Pentacene Derivatives with Radical Substituents

Pentacene derivatives with both π-radical- and TIPS-substituents (1m and 1p) were synthesized and their photochemical properties and excited-state dynamics were evaluated. The pentacene-radical-linked systems 1m (1p) showed a remarkable improvement...

Linear Extension of Anthracene via B←N Lewis Pair Formation: Effects on Optoelectronic Properties and Singlet O2 Sensitization

The functionalization of polycyclic aromatic hydrocarbons (PAHs) via B←N Lewis pair formation offers an opportunity to judiciously fine-tune the structural features and optoelectronic properties, to suit the demands of applications in organic electronic devices, bioimaging, and as sensitizers for singlet oxygen generation. We demonstrate that the N-directed electrophilic borylation of 2,6-di(pyrid-2-yl)anthracene offers access to linearly extended acene derivatives Py-BR (R=Et, Ph, C₆ F₅ ). In comparison to indeno-fused 9,10-diphenylanthracene, the formal "BN for CC" replacement in Py-BR selectively lowers the LUMO, resulting in a much reduced HOMO-LUMO gap. An even more extended conjugated system with seven six-membered rings in a row (Qu-BEt) is obtained by borylation of 2,6-di(quinolin-8-yl)anthracene. Fluorinated Py-BPf shows particularly advantageous properties, including relatively lower-lying HOMO and LUMO levels, strong yellow-green fluorescence, and effective singlet oxygen sensitization, while resisting self-sensitized conversion to its endoperoxide.

Substituent Effect in Pyridinium Alkynylanthracenes on their Performance as Photosensitizers for Photodynamic Therapy

Progress in photodynamic therapy (PDT) relies on the design and synthesis of photosensitizers that can efficiently sensitize singlet oxygen using visible light irradiation while displaying limited dark toxicity. Here, we highlight the paper by Linker and coworkers published in this issue of Photochemistry and Photobiology, which evaluates the effect of the regiochemistry of pyridinium rings in three isomeric pyridinium alkynylanthracenes on their performance as photosensitizers for PDT in HeLa cells.

Luminescent lanthanide-macrocycle supramolecular assembly

A series of macrocyclic compounds, including crown ether, cyclodextrin, cucurbituril and pillararene, bound to various specific organic/inorganic/biological guest molecules and ions through various non-covalent interactions, can not only make a single system multifunctional but also endow the system with intelligence, especially for luminescent materials. Due to their excellent luminescence properties, such as long-lived excited states, sharp linear emission bands and large Stokes shift, lanthanides have shown great advantages in luminescence, and have been more and more applied in the design of advanced functional luminescent materials. Based on reported research, we summarize the progress of lanthanide luminescent materials based on different macrocyclic compounds from ion or molecule recognition to functional nano-supramolecular assembly of the lanthanide-macrocycle supramolecular system including photo-reaction mediated switch of lanthanide luminescent molecules, multicolor luminescence, ion detection and cell imaging of rare-earth up-conversion of macrocyclic supramolecular assembly. Finally, we put forward the prospects of future development of lanthanide luminescent macrocyclic supramolecular materials.

Pyridinium Alkynylanthracenes as Sensitizers for Photodynamic Therapy

Photodynamic therapy (PDT) is a mild but effective method to treat certain types of cancer upon irradiation with visible light. Here, three isomeric methylpyridinium alkynylanthracenes 1o─p were evaluated as sensitizers for PDT. Upon irradiation with blue or green light, all three compounds show the ability to initiate strand breaks of plasmid DNA. The mayor species responsible for cleavage is singlet oxygen (¹ O₂ ) as confirmed by scavenging reagents. Only isomers 1m and 1p can be incorporated into HeLa cells, whereas isomer 1o cannot permeate through the membrane. While isomer 1m targets the cell nucleus, isomer 1p assembles in the cellular cytoplasm and impacts the cellular integrity. This is in accordance with a moderate toxicity of 1p in the dark, whereas 1m exhibits no dark toxicity. Both isomers are suitable as PDT reagents, with a CC₅₀ of 3 μm and 75 nm, for 1p and 1m, respectively. Thus, derivative 1m, which can be easily synthesized, becomes an interesting candidate for cancer therapy.

Electron-Deficient Heteroacenes that Contain Two Boron Atoms: Near-Infrared Fluorescence Based on a Push-Pull Effect*

Electron-deficient heteroacenes that contain two tricoordinate boron atoms in their acene skeletons and planarized phenyl ether moieties at their periphery were synthesized via the borylation of silicon-bridged precursors. X-ray crystallographic analysis revealed quinoidal structures, which give rise to two-step reversible redox processes for both the reduction and oxidation. These compounds exhibit intense absorption and sharp fluorescence bands with vibronic structures in the near-infrared (NIR) region. These properties originate from the push-pull effect along the long axis of the molecule derived from the electron-donating ether moieties and the electron-accepting boron moieties. Of particular note is the NIR emission of the thienothiophene-centered heteroacene, which has a maximum at 952 nm with a narrow band width of 309 cm^-1 in cyclohexane. A Franck-Condon analysis revealed the crucial role of the sterically less-hindered thienothiophene spacer in achieving this sharp emission band.

Molecular Plumbing to Bend Self-Assembling Peptide Nanotubes

Light-induced molecular piping of cyclic peptide nanotubes to form bent tubular structures is described. The process is based on the [4+4] photocycloaddition of anthracene moieties, whose structural changes derived from the interdigitated flat disposition of precursors to the corresponding cycloadduct moieties, induced the geometrical modifications in nanotubes packing that provokes their curvature. For this purpose, we designed a new class of cyclic peptide nanotubes formed by β- and α-amino acids. The presence of the former predisposes the peptide to stack in a parallel fashion with the β-residues aligned along the nanotube and the homogeneous distribution of anthracene pendants.

Pyridinium Modified Anthracenes and Their Endoperoxides Provide a Tunable Scaffold with Activity against Gram-Positive and Gram-Negative Bacteria

Due to the emergence of multidrug resistant bacteria, the development of new antibiotics is required. We introduce here asymmetrically modified positively charged bis(methylpyridinium) anthracenes as a novel tunable scaffold, in which the two positive charges can be placed at a defined distance and angle. Our structure-activity relationship reveals that coupling the methylpyridiniums with alkynyl linkers to the central anthracene unit yields antibacterial compounds against a wide range of bacteria, including Escherichia coli, Staphylococcus aureus, and Staphylococcus epidermidis. Also, different mycobacteria, such as Mycobacterium smegmatis and Mycobacterium tuberculosis, are efficiently targeted by these compounds. The antibacterial activity depends on the number of alkynyl linkers and consequently also on the distance of the positive charges in the rigid anthracene scaffold. Additionally, the formation of an anthracene endoperoxide further increases the antibacterial activity, likely due to the release of toxic singlet oxygen that converts the endoperoxide back to the antibacterial anthracene scaffold with half-lives of several hours.

Photothermal Responsive Singlet Oxygen Nanocarriers for Hypoxic Cancer Cell Ablation

Hypoxia in aggressively proliferating tumor cells has been demonstrated to restrict the efficiency of photodynamic therapy owing to its oxygen (O2 )-dependent generation of singlet oxygen (1 O2 ) from photosensitizers under light irradiation. To address this problem, we propose a small-molecule dye-based 1 O2 capturing agent, B1. B1 not only bears a near-infrared absorbing azo-boron dipyrromethene backbone, but also has 1,4-dimethylnaphthalene, which facilitates the capture of 1 O2 to form endoperoxide (B1-SO). B1-SO undergoes a reversible reaction via near-infrared photothermal stimulation, thus allowing 1 O2 release. Based on this mechanism, stable B1-SO containing micelles (B1-SO NPs) were prepared and employed as 1 O2 nanocarriers to ablate cancer cells in vitro. Taking advantage of this O2 -independent 1 O2 releasing ability, B1-SO NPs were demonstrated to have efficient cytotoxicity under near-infrared irradiation, especially in a hypoxic environment. The unique O2 -independent 1 O2 generation process of B1-SO NPs suggests they can be used as novel cancer phototherapy agents.

TIPS-Ethynylated Naphthodiquinoline and Naphthodiacridine: Novel Diazabisacenes

The synthesis of two diazabisacenes is reported. A bisboronated naphthalene was Suzuki‐coupled to substituted ethyl nicotinates, then cyclized by intramolecular Friedel‐Crafts acylation. The resulting diketones were alkynylated and reduced to give the title compounds, bis(TIPS‐ethynyl)‐substituted naphtha[1,8‐gh:5,4‐g′h′]diquinoline and naphtho[1,8‐bc:5,4‐b′c′]diacridine. Nitrogen incorporation stabilizes the bisacenes with respect to oxidation compared to their consanguine nonaza analogs.

DNA-Triggered Enhancement of Singlet Oxygen Production by Pyridinium Alkynylanthracenes

There is an ongoing interest in ¹ O₂ sensitizers, whose activity is selectively controlled by their interaction with DNA. To this end, we synthesized three isomeric pyridinium alkynylanthracenes 2 o-p and a water-soluble trapping reagent for ¹ O₂ . In water and in the absence of DNA, these dyes show a poor efficiency to sensitize the photooxygenation of the trapping reagent as they decompose due to electron transfer processes. In contrast, in the presence of DNA ¹ O₂ is generated from the excited DNA-bound ligand. The interactions of 2 o-p with DNA were investigated by thermal DNA melting studies, UV/vis and fluorescence spectroscopy, and linear and circular dichroism spectroscopy. Our studies revealed an intercalative binding with an orientation of the long pyridyl-alkynyl axis parallel to the main axis of the DNA base pairs. In the presence of poly(dA : dT), all three isomers show an enhanced formation of singlet oxygen, as indicated by the reaction of the latter with the trapping reagent. With green light irradiation of isomer 2 o in poly(dA : dT), the conversion rate of the trapping reagent is enhanced by a factor >10. The formation of ¹ O₂ was confirmed by control experiments under anaerobic conditions, in deuterated solvents, or by addition of ¹ O₂ quenchers. When bound to poly(dG : dC), the opposite effect was observed only for isomers 2 o and 2 m, namely the trapping reagent reacted significantly slower. Overall, we showed that pyridinium alkynylanthracenes are very useful intercalators, that exhibit an enhanced photochemical ¹ O₂ generation in the DNA-bound state.

A Photosensitive Polymeric Carrier with a Renewable Singlet Oxygen Reservoir Regulated by Two NIR Beams for Enhanced Antitumor Phototherapy

Photodynamic therapy (PDT), which utilizes photosensitizer to convert molecular oxygen into singlet oxygen (¹ O₂ ) upon laser irradiation to ablate tumors, will exacerbate the already oxygen shortage of most solid tumors and is thus self-limiting. Herein, a sophisticated photosensitive polymeric material (An-NP) that allows sustained ¹ O₂ generation and sufficient oxygen supply during the entire phototherapy is engineered by alternatively applying PDT and photothermal therapy (PTT) controlled by two NIR laser beams. In addition to a photosensitizer that generates ¹ O₂ , An-NP consists of two other key components: a molecularly designed anthracene derivative capable of trapping/releasing ¹ O₂ with superior reversibility and a dye J-aggregate with superb photothermal performance. Thus, in 655 nm laser-triggered PDT process, An-NP generates abundant ¹ O₂ with extra ¹ O₂ being trapped via the conversion into EPO-NP; while in the subsequent 785 nm laser-driven PTT process, the converted EPO-NP undergoes thermolysis to liberate the captured ¹ O₂ and regenerates An-NP. The intratumoral oxygen level can be replenished during the PTT cycle for the next round of PDT to generate ¹ O₂ . The working principle and phototherapy efficacy are preliminarily demonstrated in living cells and tumor-bearing mice, respectively.

Single-Photon Near-Infrared-Responsiveness from the Molecular to the Supramolecular Level via Platination of Pentacenes

Near-infrared (NIR) responsiveness is important for various applications. Currently, single-photon NIR-responsive systems are rare compared to systems that display two-photon absorption and triplet-triplet annihilation processes. Supramolecular stacking of photo-responsive chromophores results in decreased efficiency due to space-confinement effects. Herein we show that σ-platination of pentacenes is a feasible protocol to build single-photon NIR-responsive systems, with advantages including a low HOMO-LUMO energy gap, high photochemical efficiency, and pathway specificity. The pentacene-to-endoperoxidation transformation is accompanied by color and absorbance changes. The high photo-oxygenation efficiency of σ-platinated pentacenes facilitates NIR responsiveness in one-dimensional supramolecular polymers, resulting in the disappearance of supramolecular chirality signals and disruption of self-assembled nanofibers. Overall, the σ-platination strategy opens up new avenues toward NIR photo-responsive materials at the molecular and supramolecular levels.

An Air-Stable Organic Radical from a Controllable Photoinduced Domino Reaction of a Hexa-aryl Substituted Anthracene

Air-stable organic radicals and radical ions have attracted great attention for their far-reaching application ranging from bioimaging to organic electronics. However, because of the highly reactive nature of organic radicals, the design and synthesis of air-stable organic radicals still remains a challenge. Herein, an air-stable organic radical from a controllable photoinduced domino reaction of a hexa-aryl substituted anthracene is described. The domino reaction involves a photoinduced [4 + 2] cycloaddition reaction, rearrangement, photolysis, and an elimination reaction; ¹H/¹³C NMR spectroscopy, high resolution mass spectrometry, single-crystal X-ray diffraction, and EPR spectroscopy were exploited for characterization. Furthermore, a photoinduced domino reaction mechanism is proposed according to the experimental and theoretical studies. In addition, the effects of employing push and pull electronic groups on the controllable photoinduced domino reaction were investigated. This article not only offers a new blue emitter and novel air-stable organic radical compound for potential application in organic semiconductor applications, but also provides a perspective for understanding the fundamentals of the reaction mechanism on going from anthracene to semiquinone in such anthracene systems.