Search

In this work, we present three donor–acceptor thermally activated delayed fluorescence (TADF) molecules comprising a 2,3,5,6-tetrafluorobenzonitrile acceptor with various electron donor units: phenoxazine (Phx-BzN), phenothiazine (Pht-BzN), and carbazole (Cz-BzN). These molecules have been studied using steady-state and time-resolved photophysical techniques in solution, film in polymer host and in crystal state. While Cz-BzN displays TADF in solution and polymethyl methacrylate (PMMA) films, Phx-BzN and Pht-BzN are non-emissive in solution and somewhat emissive in polymer films. More interestingly, while Pht-BzN remains virtually non-emissive in all studied solvents, it exhibits strong photoluminescence and TADF in crystal state, attributed to Crystallization Induced Emission (CIE). Interestingly, by applying ultrasound to an amorphous Pht-BzN water suspension we were also able to induce a reorganization of the molecules obtaining a microcrystal suspension. This can represents a new strategy to develop luminescent organic microcrystalline materials. We demonstrate through computational studies that the CIE properties arise due to intermolecular interactions in the crystal structure that result in locking the ground state molecular geometry and blocking relaxation in the excited state. As a result, the oscillator strength in the crystal form is enhanced leading to a highly luminescent behaviour, while in solution it equals nearly zero due to the molecule adopting a perfectly orthogonal D–A orientation in the excited state. [ABSTRACT FROM AUTHOR]

A multi‐tetrazine compound tailored for grafting on graphene sheets has been designed and investigated. It is composed of hydrogen substituted tetrazines involved in inverse Diels‐Alder demand cycloaddition reactions for grafting and a central dialkoxytetrazine to confer redox properties to the final material. The successful incorporation of this compound in reduced graphene oxide was demonstrated by elemental analysis, XPS, Raman spectroscopy, AFM, SEM and XRD. Cyclic voltammetry shows the redox activity of the remaining central tetrazine unit in graphene and allows to assess the specific capacity of the material. A significant increase in the specific capacity was found compared to pristine graphene and another tetrazine model compound bearing only one grafting unit. This result was assigned to an increase in the specific area in relation to bridges created between graphene sheets, as evidenced by AFM and XRD. The performance in energy storage were assessed in symmetrical Swagelok cells confirming the high impact of the functionalization on capacitance values. [ABSTRACT FROM AUTHOR]

A single layer of a dyad made of a cobalt(II) bis‐terpyridine complex functionalized with a Boron‐dipyrromethene (BODIPY) fluorophore on transparent Indium Tin Oxide (ITO) electrode is prepared. A modulation of the photoluminescence intensity of the fluorophore upon the application of an electric stimulus on the ITO electrode that supports 10 picomoles of molecules is demonstrated. The intensity of the fluorescence intensity decreases when the cobalt is at the oxidation state +III, due to CoIII oxidative quenching of BODIPY emission that has a higher driving force than the CoII reductive quenching initially responsible for the low fluorescence intensity of the chemisorbed dyad. [ABSTRACT FROM AUTHOR]

Ferrocene‐BODIPY (Fc‐BDP) conjugates in which one or two ferrocene entities are linked to the β‐positions of the BODIPY core by an ethynyl bridge have been developed. These derivatives were easily and efficiently grafted onto a dual‐clickable fullerene platform using CuAAC reactions, leading to a clickable Fc‐BDP‐C60 triad and a clickable [Fc]2‐BDP‐C60 tetrad which can be used for further derivatization with complex structures. Due to the extended π‐conjugation and the presence of an intramolecular charge transfer band from Fc to BDP, all these conjugates display a broad absorption in the visible region, which is bathochromically shifted when two Fc are appended to the BDP core. Ultrafast multistep electron transfers leading to charge stabilization were demonstrated in the Fc‐BDP‐C60 triad and [Fc]2‐BDP‐C60 tetrad by femtosecond transient absorption studies. [ABSTRACT FROM AUTHOR]

Control of photophysical properties is crucial for the continued development of electroluminescent devices and luminescent materials. Preparation and study of original molecules uncovers design rules towards efficient materials and devices. Here we have prepared 7 new compounds based on the popular donor–acceptor design used in thermally activated delayed fluorescence emitters. We introduce for the first time benzofuro[3,2-e]-1,2,4-triazine and benzothieno[3,2-e]-1,2,4-triazine acceptors which were connected to several common donors: phenoxazine, phenothiazine, carbazole and 3,6-di-tert-butylcarbazole. DFT calculations, and steady-state and time-resolved photophysical studies were conducted in solution and in solid states. While derivatives with azine moieties are non-emissive in any form, the compounds comprising 3,6-di-tert-butylcarbazole display TADF in all cases. More interestingly, the two derivatives substituted with a carbazole donor are TADF active when dispersed in a polymer matrix and phosphorescent at room temperature in neat films (microcrystalline form). [ABSTRACT FROM AUTHOR]

The synthesis, spectroscopic and electrochemical data of donor-acceptor-donor tetrazines containing ferrocene moieties and phenyl unit as a [pi]-bridge are presented. The ultraviolet (UV)-vis spectroscopic and cyclic voltamperometric results have indicated sizable intramolecular charge transfer interactions in the ground state when the ferrocene is directly bound to the tetrazine and have shown reduction of the electron-donor strength of ferrocene moieties when there is a phenyl linkage.

Surface modification by photo grafting constitutes an interesting strategy to prepare functional surfaces. Precision applications, however, demand quantitative methods able to monitor and control the amount and distribution of surface modifications, which is hard to achieve, particularly in operando conditions. In this paper, a label‐free, cost‐effective, all‐optical method based on wavefront sensing which is able to quantitatively track the evolution of grafted layers in real‐time, is presented. By positioning a simple thin diffuser in the close vicinity of a camera, the thickness of grafted patterns is directly evaluated with sub‐nanometric sensitivity and diffraction‐limited lateral resolution. By performing an in‐depth kinetic analysis of the local modification of an inert substrate (glass cover slips) through photografting of arydiazonium salts, different growth regimes are characterized and several parameters are estimated, such as the grafting efficiency, density and the apparent refractive index distribution of the resulting grafted layers. Both focused and widefield‐grafting can be quantitatively monitored in real time, providing valuable guidelines to maximize functionalization efficiency. The association of a well‐characterized versatile photografting reaction with the proposed flexible and sensitive monitoring strategy enables functional surfaces to be prepared, and puts surface micro‐ to submicro‐structuration within the reach of most laboratories. [ABSTRACT FROM AUTHOR]

Broad-frequency dielectric behaviors of multiwalled carbon nanotubes (MWCNTs) embedded in room temperature vulcanization silicone rubber (RT-SR) matrix were studied by analyzing alternating current (ac) impedance spectra, which would make a remarkable contribution for understanding some fundamental electrical properties in the MWCNT/RT-SR nanocomposites. Equivalent circuits of the MWCNT/RT-SR nanocomposites were built, and the law of polarization and mechanism of electric conductance under the ac field were acquired. Two parallel RC circuits in series are the equivalent circuits of the MWCNT/RT-SR composites. At different frequency ranges, dielectric parameters including conductivity, dielectric permittivity, dielectric loss, impedance phase, and magnitude present different behaviors. [ABSTRACT FROM AUTHOR]

Broad-frequency dielectric behaviors of multiwalled carbon nanotubes (MWCNTs) embedded in room temperature vulcanization silicone rubber (RT-SR) matrix are studied by analyzing alternating current (ac) impedance spectra. Dielectric parameters including conductivity, dielectric permittivity, dielectric loss, impedance phase and magnitude have shown different behaviors at different frequency ranges.

Two dyads containing tris- and tetrakis-meso-fluorenyl-substituted porphyrin and ethynylruthenium units, 1 and 2, were investigated by emission spectro-electrochemical (SEC) methods for their potential use as fluorescence switches. The ruthenium group as a potential electron donor and the porphyrin as a potential electron acceptor are connected by a phenylene bridge in 1 and by a fluorenylene bridge in 2. The new fluorenyl-linked dyad 2 was probed by UV-visible, near-infrared (NIR) and infrared (IR) absorption SEC methods, and the properties interpreted with the aid of hybrid-DFT computations, for comparison with reported data for 1. The porphyrin-based fluorescence of 1 decreased in intensity upon oxidation to 1+ and decreased further on oxidising 1+ to 12+. A much weaker change in the fluorescence intensity of 2 was observed upon oxidation to 2+ but the intensity decreased upon subsequent oxidation of 2+ to 22+. These findings contrast with data reported for some other porphyrins appended with redox-active ruthenium or iron units, where fluorescence intensities increase upon oxidation of the peripheral metal centers, but they match data reported more recently for closely related arrays. A rationale for these apparently contrasting observations is proposed. [ABSTRACT FROM AUTHOR]

A novel and unexpected convenient one‐pot synthesis of 1,2,3,4‐thiatriazoles has been discovered while investigating the classical tetrazine "Pinner synthesis". The synthetic route starts from commercially‐available nitrile derivatives and gives good to high yields (51–80 %) with no need to isolate any thioacylating agents. The crucial impact of the solvent on the outcome of the modified "Pinner synthesis" is moreover examined and discussed. Using this new synthetic route, a novel donor‐acceptor thiatriazole derivative has been prepared, which exhibits prominent thermally‐activated delayed fluorescence (TADF) in both solution and film. The photoluminescence quantum yield (PLQY) in methylcyclohexane (MCH) and Zeonex (a cyclo olefin polymer) in oxygen‐free conditions were determined to be 76 and 99 %, respectively. This work provides an efficient and practical synthetic approach to functionalized 1,2,3,4‐thiatriazole derivatives, and will noticeably facilitate the application of 1,2,3,4‐thiatriazole as an electron acceptor in organic electronics. One‐stop shop: A novel and unexpected convenient one‐pot synthesis of 1,2,3,4‐thiatriazoles has been discovered meanwhile investigating on the classical tetrazine "Pinner synthesis". The synthetic route starts from commercially‐available nitrile derivatives and gives good to high yields (51–80 %) with no need to isolate any thioacylating agents. Using this new synthetic route a novel donor‐acceptor thiatriazole derivative has been prepared which exhibits prominent thermally activated delayed fluorescence (TADF). [ABSTRACT FROM AUTHOR]

A new and versatile method to prepare Ag nanoparticles on functionalized graphene sheets is described. In a first step, pyridine‐pyridazine units are introduced on graphene sheets by cycloaddition reaction with a tetrazine derivative. Then the material is coated on an electrode and electrodeposition is carried out to generate silver nanoparticles in a one‐step electroreduction process. The applied potential signal is shown to influence the size and dispersity of the particles in the composite. While larger particles are active in improving sensitivity of Surface‐Enhanced Raman Spectroscopy, smaller ones can be efficiently used in electrocatalysis, like for oxygen reduction. Both effects are demonstrated to highlight the versatility of the applications of these easily prepared nanocomposites. Silver nanoparticles decorated graphene sheets are synthesized in a simple two‐step process : first pyridine‐pyridazine are introduced at the graphene surface by inverse Diels‐Alder reaction. Then electroreduction in presence of silver nitrate leads to nanoparticles. Application to SERS and electrocatalysis according to nanoparticle size are demonstrated. [ABSTRACT FROM AUTHOR]

Because of the limitations of conventional metal‐oxide‐based electrodes, studies on organic redox‐active materials as alternative electrodes for secondary batteries are emerging. However, reported organic electrode materials are still limited to a few kinds of organic redox groups. Therefore, the development of new redox‐active groups for high‐performance electrode materials is indispensable. Here, we evaluate s‐tetrazine derivatives as a new electrode material in Li‐ion batteries and study their charge/discharge mechanisms by ex situ XPS measurements. The porous carbon CMK‐3 was introduced to encapsulate the s‐tetrazines, which allowed 100 % utilization of the theoretical capacity and stable cycle performance of the s‐tetrazines by preventing dissolution of the molecules into the electrolytes. This new class of redox‐active group can pave the way for the next‐generation of energy storage systems. s‐Tetrazines for electrode‐active materials in secondary batteries were developed for the first time. The redox mechanism of the s‐tetrazine electrodes during charge/discharge was studied through ex situ XPS analysis. By fabricating a nanocomposite with a mesoporous carbon CMK‐3 to prevent dissolution of the small s‐tetrazines into the electrolyte, their potential applicability as electrode materials was improved. [ABSTRACT FROM AUTHOR]

Abstract: A facile, efficient and metal‐free synthetic approach to 3‐monosubstituted unsymmetrical 1,2,4,5‐tetrazines is presented. Dichloromethane (DCM) is for the first time recognized as a novel reagent in the synthetic chemistry of tetrazines. Using this novel approach 11 3‐aryl/alkyl 1,2,4,5‐tetrazines were prepared in excellent yields (up to 75 %). The mechanism of this new reaction, including the role of DCM in the tetrazine ring formation, has been investigated by 13C labeling of DCM, and is also presented and discussed as well as the photophysical and electrochemical properties. [ABSTRACT FROM AUTHOR]

In this work, we explore the possibility of tuning the fluorescence intensity of two porphyrin systems through the electrochemical oxidation of an appended ruthenium acetylide bridge. Two electrochemically switchable systems, a dyad (ZnP-Ru, 3) and a triad (ZnP-Ru-P2H, 5), were prepared and investigated. In the ZnP-Ru dyad, the fluorescence of the zinc porphyrin was switched reversibly between the ON and OFF state upon the oxidation of the ruthenium unit, the most probable quenching process involved after oxidation being the electron transfer from the singlet excited state of ZnP to the oxidized ruthenium center. In the ZnP-Ru-P2H triad, we show that both porphyrins’ fluorescence are highly quenched independent of the redox state of the ruthenium bridge owing to the efficient photoinduced charge transfer within the ruthenium complex. [ABSTRACT FROM AUTHOR]

New tetrazine derivatives were prepared and their electrochemical properties as well as their reactivity in the inverse demand Diels−Alder (DAinv) reaction were investigated. DFT calculations were performed to calculate the energy and electronic distribution in the frontier molecular orbitals. In particular, the reactivity of these tetrazine derivatives has been compared with that of the already known dichlorotetrazine. A comparison of the gathered data allows a rough correlation to be established between the reactivity and the electron-withdrawing character of the substituent, but only a closer examination of the involved orbitals can enable an accurate prediction of the DAinv reactivity of the investigated compounds. [ABSTRACT FROM AUTHOR]

A new Ir(iii) cyclometallated complex bearing a fluorenyl 5-substituted-1,10-phenanthroline ligand ([Ir(ppy)2(L1)][PF6], ppy = 2-phenylpyridine) is presented which exhibits enhanced triplet oxygen sensing properties. The efficacy of this complex to act as a photosensitiser for altering the morphology of C6 Glioma cells that represent malignant nervous tumours has been evaluated. The increased heavy metal effect and related spin–orbit coupling parameters on the photophysical properties of this complex are evidenced by comparison with Ru(ii) analogues. The complex [Ir(ppy)2(L1)][PF6] is shown to exhibit relatively high two-photon absorption efficiencies for the lowest energy MLCT electronic transitions with two-photon absorption cross sections that range from 50 to 80 Goeppert–Mayer units between 750 to 800 nm. Quantum yields for the complex were measured up to 23% and the Stern–Volmer quenching constant, KSV was determined to be 40 bar−1 in acetonitrile solution, confirming the high efficiency of the complex as a triplet oxygen sensitiser. Preliminary in vitro experiments with C6 Glioma cells treated with [Ir(ppy)2(L1)][PF6], show that the complex is an efficient sensitizer for triplet oxygen, producing cytotoxic singlet oxygen (1O2) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death. [ABSTRACT FROM AUTHOR]

The synthesis, photophysical and electrochemical properties as well as theoretical calculation studies of a newly designed triphenylamine derivative are described. This original compound displays one neutral form, three oxidized forms, and two protonated forms with distinct photophysical characteristics. The interplay of the emission with the protonation or the redox state (electrofluorochromism) has been explored and an on-off-on-off fluorescence switching was observed in the case of oxidation and an on-on-off fluorescence switching in the case of protonation. [ABSTRACT FROM AUTHOR]

A series of emitting push–pull triarylamine derivatives, models of their widely used homologues in photonics and organic electronics, was investigated by steady-state and time-resolved spectroscopy. Their structural originality stems from the sole change of the electron-withdrawing substituent X (–H: 1, –CN: 2, –NO2: 3, –CHC(CN)2: 4), giving rise to efficient emission tuning from blue to red upon increasing the X electron-withdrawing character. All compounds are highly fluorescent in alkanes. The more polar compounds 2–4 undergo considerable Stokes shift and emission quenching in polar solvents. Femtosecond transient absorption data allowed us to identify the nature of the emissive state which varies as a function of the compound and surrounding polarity. A long-lived ππ* excited state with weak charge transfer character was found for 1. This excited state evolves into a long-lived ICT state with red-shifted emission for 2 in polar solvents. For 3 and 4, the ICT state is directly populated in all solvents. Long-lived and emissive in n-hexane, it relaxes in toluene to a new ICT′ conformation with stronger charge transfer character and enhanced Stokes shift. In more polar THF, ethanol, and nitrile solvents, ICT relaxes to a dark excited state ICT′′ with viscosity-dependent kinetics (＜10 ps). The ICT′′ state lifetime drops with increasing solvent polarity (150 ps for 3 in THF, 8.5 ps in butyronitrile, 1.9 ps in acetonitrile), denoting an efficient radiationless deactivation to the ground state (back charge transfer). This result reveals a very small S0–S1 energy gap at the relaxed ICT′′ geometry, with a possible close-lying S0–S1 conical intersection, which suggests that the ICT → ICT′′ process results from a structural change involving a large-amplitude molecular distortion. This fast structural change can account for the strong fluorescence quenching observed for 3 and 4 in polar solvents. Finally, the magnitude of intersystem crossing between the singlet and triplet excited states largely depends on the electron-deficient X unit and the solvent itself. These observations help one conclude on the prevailing role played by the electron-withdrawing groups and the surrounding polarity in the photophysical performances of triphenylamine derivatives, largely employed in numerous emissive solid-state devices. [ABSTRACT FROM AUTHOR]