Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

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.

Unraveling the Push-Pull Effect in Acenes, Polyenes and Polyynes

Substituent effects (SEs) are fundamental for predicting molecular reactivity, while polyene, polyyne and acene derivatives are precursors to compounds with diverse applications. Computations were performed for Y-R-X systems, where reaction sites Y=NO2 and O- , substituents X=NO2 , CN, Cl, H, OH, NH2 , and spacers R=polyene, polyyne (n=1-5, 10 repeating units) and acene (up to tetracene). The cSAR (charge of the substituent active region) approach allowed to present, for the first time, quantitative relations describing the spacer's electron-donating and withdrawing properties as a function of n and the spacer type. The electronic properties of the X substituents depend on the type of spacer, its length and the Y group, which is an example of the reverse SE. To describe how the SE between Y and X weakens with n, two approaches were compared: cSAR and SESE (SE stabilization energy). The EDDB (electron density of delocalized bonds) characterize changes in electron delocalization in spacers due to the SE. A new approach - EDDB differential maps - allow to extract the effect of X substitution on the electron delocalization. The charges at spacer's C atoms correlate with cSAR; changes in the slopes confirm the charge transfer by resonance.

Octopus-inspired deception and signaling systems from an exceptionally-stable acene variant

Multifunctional platforms that can dynamically modulate their color and appearance have attracted attention for applications as varied as displays, signaling, camouflage, anti-counterfeiting, sensing, biomedical imaging, energy conservation, and robotics. Within this context, the development of camouflage systems with tunable spectroscopic and fluorescent properties that span the ultraviolet, visible, and near-infrared spectral regions has remained exceedingly challenging because of frequently competing materials and device design requirements. Herein, we draw inspiration from the unique blue rings of the Hapalochlaena lunulata octopus for the development of deception and signaling systems that resolve these critical challenges. As the active material, our actuator-type systems incorporate a readily-prepared and easily-processable nonacene-like molecule with an ambient-atmosphere stability that exceeds the state-of-the-art for comparable acenes by orders of magnitude. Devices from this active material feature a powerful and unique combination of advantages, including straightforward benchtop fabrication, competitive baseline performance metrics, robustness during cycling with the capacity for autonomous self-repair, and multiple dynamic multispectral operating modes. When considered together, the described exciting discoveries point to new scientific and technological opportunities in the areas of functional organic materials, reconfigurable soft actuators, and adaptive photonic systems.

Singlet Oxygen Responsive Molecular Receptor to Modulate Atropisomerism and Cation Binding

In switchable molecular recognition, ¹ O₂ stimulus responsive receptors offer a unique structural change that is rarely exploited. The employed [4+2] reaction between ¹ O₂ and anthracene derivatives is quantitative, reversible and easily implemented. To evaluate the full potential of this new stimulus, a non-macrocyclic anthracene-based host was designed for the modular binding of cations. The structural investigation showed that ¹ O₂ controlled the atropisomerism in an on/off fashion within the pair of hosts. The binding studies revealed higher association constants for the endoperoxide receptor compared to the parent anthracene, due to a more favoured preorganization of the recognition site. The fatigue of the ¹ O₂ switchable hosts and their complexes was monitored over five cycles of cycloaddition/cycloreversion.

Near-Infrared-Absorbing B-N Lewis Pair-Functionalized Anthracenes: Electronic Structure Tuning, Conformational Isomerism, and Applications in Photothermal Cancer Therapy

B-N-fused dianthracenylpyrazine derivatives are synthesized to generate new low gap chromophores. Photophysical and electrochemical, crystal packing, and theoretical studies have been performed. Two energetically similar conformers are identified by density functional theory calculations, showing that the core unit adopts a curved saddle-like shape (x-isomer) or a zig-zag conformation (z-isomer). In the solid state, the z-isomer is prevalent according to an X-ray crystal structure of a C₆F₅-substituted derivative (4-Pf), but variable-temperature nuclear magnetic resonance studies suggest a dynamic behavior in solution. B-N fusion results in a large decrease of the HOMO-LUMO gap and dramatically lowers the LUMO energy compared to the all-carbon analogues. 4-Pf in particular shows significant absorbance at greater than 700 nm while being almost transparent throughout the visible region. After encapsulation in the biodegradable polymer DSPE-mPEG₂₀₀₀, 4-Pf nanoparticles (4-Pf-NPs) exhibit good water solubility, high photostability, and an excellent photothermal conversion efficiency of ∼41.8%. 4-Pf-NPs are evaluated both in vitro and in vivo as photothermal therapeutic agents. These results uncover B-N Lewis pair functionalization of PAHs as a promising strategy toward new NIR-absorbing materials for photothermal applications.

Review of the Common Deposition Methods of Thin-Film Pentacene, Its Derivatives, and Their Performance

Pentacene is a well-known conjugated organic molecule with high mobility and a sensitive photo response. It is widely used in electronic devices, such as in organic thin-film transistors (OTFTs), organic light-emitting diodes (OLEDs), photodetectors, and smart sensors. With the development of flexible and wearable electronics, the deposition of good-quality pentacene films in large-scale organic electronics at the industrial level has drawn more research attention. Several methods are used to deposit pentacene thin films. The thermal evaporation technique is the most frequently used method for depositing thin films, as it has low contamination rates and a well-controlled deposition rate. Solution-processable methods such as spin coating, dip coating, and inkjet printing have also been widely studied because they enable large-scale deposition and low-cost fabrication of devices. This review summarizes the deposition principles and control parameters of each deposition method for pentacene and its derivatives. Each method is discussed in terms of experimentation and theory. Based on film quality and device performance, the review also provides a comparison of each method to provide recommendations for specific device applications.

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.

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.

Pillar[5]arene based supramolecular polymer for a singlet oxygen reservoir

A novel type of supramolecular polymer based on pillararene for the storage and control release of singlet oxygen.

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.

Domino Cross-Scholl Reaction of Tetracene with Molecular Benzene: Synthesis, Structure, and Mechanism

A domino-type multiple C-H functionalization of tetracene with molecular benzene is reported. Under the typical conditions of the Scholl reaction, a domino reaction occurs between tetracene and six molecules of benzene in one pot to furnish an aromatic compound with a curved π-system. This reaction sequence involves oxidative cross-dehydrogenative coupling/annulation and Friedel-Crafts-type reactions. Eight C-C bonds are formed via this intermolecular domino reaction without mediation by a metal or the assistance of a specific substituent.

The Pyrazinacenes

Pyrazinacenes are a class of nitrogen-containing heteroacene molecules composed of linearly fused pyrazine units, which might also include dihydropyrazine groups leading to different reduced states of the compounds. While they are structurally similar to hydrocarbon acenes (e.g., pentacene) the presence of increasing numbers of N-heteroatoms introduces several different additional features of the compounds so that they can be considered for investigations beyond those suggested for acenes (i.e., organic field-effect transistors, solar cell components). Pyrazinacenes are in several ways complementary to C-H-only acenes based on the increasing stability of reduced states of the compounds with increasing numbers of fused pyrazine rings, although an acene-like electronic structure persists in the compounds so far studied. However, the introduction of multiple N atoms leads to properties that depart from C-H-only acenes. In particular, the compounds exhibit a delocalization of NH protons in extended reduced compounds and oxidation state switchability in solution and at interfaces. The presence of NH groups also allows an easy introduction of solubilizing groups at the pyrazinacene chromophore. In this Account, we will describe the preparation of extended pyrazinacenes from dipyrazino[2,3-b:2',3'-e]pyrazine (1,4,5,8,9,10-hexaazaanthracene; N6) derivatives up to 1,4,5,6,7,8,9,12,13,14,15,16,17,18-tetradecaazaheptacene (N14) and also assess structures of the relevant compounds based on X-ray crystallographic studies. Emergent properties of the molecules include highly unusual linear tautomeric processes based on a delocalization of protons (and the corresponding formation of orbitals based on multiple adjacent N lone electron pair interactions), which suggest special transport properties based on molecular protonics. Molecules such as decazapentacene (N10) exhibit multistability of oxidation state, and this is predicted to promote the redox catalytic properties of the compounds. The oxidation-state switching of on-surface processes is also described and has been investigated using scanning tunneling microscopy. The longest known pyrazinacene chromophore (N14) exhibits amphiprotism with its state of protonation being strongly coupled to its fluorescence emission properties in the near-infrared region indicating possible uses in pH-coupled bioimaging applications. The synthesis of the pyrazinacenes is flexible and allows the preparation of symmetrically or unsymmetrically substituted derivatives for the development of more complex molecules and for control of the electronic structure of the acene unit. Overall, the pyrazinacenes represent an emerging class of highly nitrogenous heteroacenes with unique properties and excellent potential for development in different applications based on their special supramolecular properties including guest binding or interactions in biological systems.

Detection of Oxidative Stress Induced by Nanomaterials in Cells-The Roles of Reactive Oxygen Species and Glutathione

The potential of nanomaterials use is huge, especially in fields such as medicine or industry. Due to widespread use of nanomaterials, their cytotoxicity and involvement in cellular pathways ought to be evaluated in detail. Nanomaterials can induce the production of a number of substances in cells, including reactive oxygen species (ROS), participating in physiological and pathological cellular processes. These highly reactive substances include: superoxide, singlet oxygen, hydroxyl radical, and hydrogen peroxide. For overall assessment, there are a number of fluorescent probes in particular that are very specific and selective for given ROS. In addition, due to the involvement of ROS in a number of cellular signaling pathways, understanding the principle of ROS production induced by nanomaterials is very important. For defense, the cells have a number of reparative and especially antioxidant mechanisms. One of the most potent antioxidants is a tripeptide glutathione. Thus, the glutathione depletion can be a characteristic manifestation of harmful effects caused by the prooxidative-acting of nanomaterials in cells. For these reasons, here we would like to provide a review on the current knowledge of ROS-mediated cellular nanotoxicity manifesting as glutathione depletion, including an overview of approaches for the detection of ROS levels in cells.

Red-Emitting, Acene-Doped Conjugated Polymer Nanoparticles that Respond Ratiometrically to Photogenerated 1O2

Fluorophores that respond to external stimuli on demand have numerous applications in imaging and chemical or biological sensing. In this paper, we describe conjugated polymer nanoparticles (CPNs) that comprise a donor polymer matrix and a red-fluorescent, singlet oxygen-reactive heteroacene dopant (DE-TMT) that display a ratiometric response upon photo-oxidation. This ratiometric response can be tuned by the level of doping of DE-TMT, the identity of the conjugated polymer matrix used, and the blending of two conjugated polymers together to access red-shifted emission wavelengths. We followed a rational design process that combined (i) fundamental understanding of the influence of the chemical structure on luminescence spectra and efficiencies, energy transfer efficiencies, and reactivity and (ii) systematically determining how blending multiple chromophores in nanoparticles influences energy transfer efficiencies and the speed of optical responses to irradiation. Our approach of refining the compositions of these nanoparticles has yielded materials that combine many desirable characteristics for analytical applications-utility in aqueous environments, high quantum yield, emission of red light, and ratiometric luminescent responses. We anticipate that the type of approach described herein can be of use to others in designing CPNs for luminescence applications.

Hysteretic Three-State Redox Interconversion among Zigzag Bisquinodimethanes with Non-fused Benzene Rings and Twisted Tetra-/Dications with [5]/[3]Acenes Exhibiting Near-Infrared Absorptions

Octaaryl-substituted bisquinodimethanes (BQDs) with a zigzag structure were designed as redox-switchable molecules that undergo four-electron oxidation to produce tetracationic pentacenes with a doubly twisted structure. In contrast to one-stage four-electron oxidation of BQDs, stepwise two-electron reduction of tetracationic pentacenes occurs to give dicationic anthracenes and then the original BQDs, step-by-step. Since both tetracations and dications exhibit near-infrared (NIR) absorptions (-1400 nm) based on an intramolecular charge-transfer interaction, changes in not only their structures but also their UV-vis-NIR spectra can be controlled by redox stimuli. In this Communication, we present an unprecedented one-step π-extension to pentacene from non-fused benzene rings by oxidation, and subsequent two-stage deannulation to benzene rings via anthracene upon reduction. All structures were determined by single-crystal X-ray analyses, and their properties were characterized by spectroscopic and theoretical studies.

Photoinduced versus spontaneous host–guest electron transfer within a MOF and chromic/luminescent response

The MOF shows charge-transfer sensitized Eu(iii) emission and spontaneous/photoinduced guest-to-host electron transfer, which allow chromic and luminescent sensing of NH3 (luminescence turn-off) and O2 (luminescence turn-on).

Transformable fluorescent nanoparticles (TFNs) of amphiphilic block copolymers for visual detection of aromatic amines in water

A kind of novel transformable fluorescent nanoparticle made of block copolymers is constructed for the sensitive detection of aromatic amines in water.