Nanometrology based control: taming radical grafting reactions with attoliter precision.

Precisely controlled micropatterning with organic moieties is a promising route for designing smart surfaces, enabling the development of microsensors and actuators with optimal usage of reactants. Such applications require fine control over the surface modification process, which in turn demands detailed knowledge about the surface modification process. As complex surface kinetics often emerge as a result of even slight modifications of the grafting entity, non-invasive, sensitive and precise closed loop control strategies are highly desirable. In this paper we demonstrate that a nanometrology approach based on quantitative phase imaging (QPI) fulfill all these requirements. We first use the technique to monitor surface photografting kinetics of aryl radicals, comprehensively analyzing the effect of substituents on surface addition reactions. We demonstrate that several aspects of the grafting process are affected in complex ways, rendering open-loop strategies impossible to implement precisely. Then, we show that the operando optical phase signal can be used as a direct feedback, guiding the grafting reaction process. Using relatively simple instrumentation, we demonstrate that general and precise control strategies can be designed and used to control the volume of the grafting material with attoliter precision, in spite of radically different surface modification kinetics spanning several orders of magnitude.

Operando surface optical nanometrology reveals diazonium salts' visible photografting mechanism.

High resolution and quantitative surface modification through photografting is a highly desirable strategy towards the preparation of smart surfaces, enabling chemical functions to be precisely located onto specific regions of inert surfaces. Although promising, the mechanisms leading to direct (without the use of any additive) photoactivation of diazonium salts using visible wavelengths are poorly understood, precluding the generalization of popular diazonium-based electrografting strategies into high resolution photografting ones. In this paper, we employ quantitative phase imaging as a nanometrology tool for evaluating the local grafting rate with diffraction-limited resolution and nanometric precision. By carefully measuring the surface modification kinetics under a range of different conditions, we reveal the reaction mechanism while evaluating the influence of key parameters, such as the power density, the radical precursor concentration and the presence of side reactions.

Novel D-A chromophores with condensed 1,2,4-triazine system simultaneously display thermally activated delayed fluorescence and crystallization-induced phosphorescence.

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).

Novel Easy to Synthesize Benzonitrile Compounds with Mixed Carbazole and Phenoxazine Substituents Exhibiting Dual Emission and TADF Properties.

The photophysical and electrochemical properties of a new class of fluorinated benzonitrile compounds substituted with mixed phenoxazine and carbazole units have been investigated. When absorbing in a large range of the UV-vis spectrum due to both localized and charge-transfer absorptions, these compounds show dual broad emission in solution and intense emission in PMMA films, with photoluminescence quantum yields changing from a few percent in solution to 18% in a more rigid environment. The compounds also exhibit thermally activated delayed fluorescence demonstrated by the role of oxygen in the quenching of delayed fluorescence and by time-resolved luminescence studies, with an efficiency directly related to the number of phenoxazine substituents. Electrochemistry reveals dramatic changes in the reduction mechanisms according to the number of remaining fluorine atoms on the benzonitrile core. All these results demonstrate how it is possible to tune the photophysical and electrochemical properties of easily synthesizable derivatives by controlling the nature and relative number of the substituents on a simple aromatic platform.

Local Surface Chemistry Dynamically Monitored by Quantitative Phase Microscopy.

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.

Thermoresponsive Fluorescence Switches Based on Au@pNIPAM Nanoparticles.

Despite numerous studies emphasizing the plasmonic impact on fluorescence, the design of a dynamic system allowing on-demand fluorescence switching in a single nanostructure remains challenging. The reversibility of fluorescence switching and the versatility of the approach, in particular its compatibility with a wide range of nanoparticles and fluorophores, are among the main experimental difficulties. In this work, we achieve reversible fluorescence switching by coupling metal nanoparticles with fluorophores through stimuli-responsive organic linkers. As a proof of concept, we link gold nanoparticles with fluorescein through thermoresponsive poly(N-isopropylacrylamide) at a tunable grafting density and characterize their size and optical response by dynamic light scattering, absorption, and fluorescence spectroscopies. We show that the fluorescence emission of these hybrid nanostructures can be switched on-demand using the thermoresponsive properties of poly(N-isopropylacrylamide). The described system presents a general strategy for the design of nanointerfaces, exhibiting reversible fluorescence switching via external control of metal nanoparticle/fluorophore distance.

Versatile one-pot synthesis of gold nanoclusters and nanoparticles using 3,6-(dipyridin-2-yl)-(1,2,4,5)-tetrazine.

A one-pot synthesis of gold nano-objects is described by simply mixing a gold salt (HAuCl4), dodecanethiol and 3,6-di-2-pyridyl-1,2,4,5-tetrazine. When a large excess of thiol is used, gold nanoclusters of 2 nm are obtained in a large amount and with a narrow size distribution. The reaction mechanism was investigated by absorption and emission spectroscopies and shows the in situ formation of dihydrotetrazine acting as the reductant of Au(iii) to make Au(0). Au nanoclusters were isolated from the molecular precursors by HPLC. The nature of the ligands stabilizing Au nanoclusters was investigated by various techniques such as mass spectrometry, SEM-EDS, XPS and NMR. Thiol and tetrazine are shown to play both the role of ligand stabilizing the clusters. Finally, when a much smaller amount of thiol is used, a mixture of Au nanoclusters and Au nanoparticles of 10-15 nm, sometimes aggregated into clusters of 50 nm is obtained. The formation of larger nanoobjects is explained by the lower amount of thiol available to block the growth at the early stage as shown by UV-vis absorption monitoring.

Photoinduced Electron Transfer and Energy Transfer Processes in a Flexible BODIPY-C60 Dyad.

A new donor-acceptor dyad composed of a BODIPY (4,4'-difluoro-4-bora-3a,4a-diaza-s-indacene) donor and a fullerene C60 acceptor has been synthesized and characterized. This derivative has been prepared using a clickable fullerene building block that bears an alkyne moiety and a maleimide unit. The post-functionalization of the maleimide group by a BODIPY thiol leads to a BODIPY-C60 dyad, leaving the alkyne moiety for further functional arrangement. On the basis of the combination of semi-empirical and density functional theory (DFT) calculations, spectroelectrochemical experiments, and steady-state and time-resolved spectroscopies, the photophysical properties of this new BODIPY-C60 dyad were thoroughly studied. By using semi-empirical calculations, the equilibrium of three conformations of the BODIPY-C60 dyad has been deduced, and their molecular orbital structures have been analyzed using DFT calculations. Two short fluorescence lifetimes were attributed to two extended conformers displaying variable donor-acceptor distances (17.5 and 20.0 Å). Additionally, the driving force for photoinduced electron transfer from the singlet excited state of BODIPY to the C60 moiety was calculated using redox potentials determined with electrochemical studies. Spectroelectrochemical measurements were also carried out to investigate the absorption profiles of radicals in the BODIPY-C60 dyad in order to assign the transient species in pump-probe experiments. Under selective photoexcitation of the BODIPY moiety, occurrences of both energy and electron transfers were demonstrated for the dyad by femtosecond and nanosecond transient absorption spectroscopies. Photoinduced electron transfer occurs in the folded conformer, while energy transfer is observed in extended conformers.

Donor-Acceptor 1,2,4,5-Tetrazines Prepared by the Buchwald-Hartwig Cross-Coupling Reaction and Their Photoluminescence Turn-On Property by Inverse Electron Demand Diels-Alder Reaction.

A facile efficient synthetic tool, Buchwald-Hartwig cross-coupling reaction, for the functionalization of 1,2,4,5-tetrazines is presented. Important factors affecting the Buchwald-Hartwig cross-coupling reaction have been optimized. Seven new donor-acceptor tetrazine molecules (TA1-TA7) were conveniently prepared in good to high yields (61-72%). They have been subsequently engaged in the inverse electron demand Diels-Alder (iEDDA) reaction with cyclooctyne. The photophysical and electrochemical properties of the new pyridazines have been studied. Some are fluorescent acting as turn-on probes. More importantly, two pyridazines (DA3 and DA6) exhibit room-temperature phosphorescence (RTP) properties.

Self-Assembled Monolayers of Redox-Active 4d-4f Heterobimetallic Complexes.

In this work, we report the preparation of functional interfaces incorporating heterobimetallic systems consisting in the association of an electroactive carbon-rich ruthenium organometallic unit and a luminescent lanthanide ion (Ln = Eu3+ and Yb3+). The organometallic systems are functionalized with a terminal hexylthiol group for subsequent gold surface modification. The formation of self-assembled monolayers (SAMs) with these complex molecular architectures are thoroughly demonstrated by employing a combination of different techniques, including infrared reflection absorption spectroscopy, ellipsometry, contact angle, and cyclic voltammetry measurements. The immobilized heterobimetallic systems show fast electron-transfer kinetics and, hence, are capable of fast electrochemical response. In addition, the characteristic electrochemical signals of the SAMs were found to be sensitive to the presence of lanthanide centers at the bipyridyl terminal units. A positive shift of the potential of the redox signal is readily observed for lanthanide complexes compared to the bare organometallic ligand. This effect is equally observed for preformed complexes and on-surface complexation. Thus, an efficient ligating recruitment of europium and ytterbium cations at gold-modified electrodes is demonstrated, allowing for an easy electrochemical detection of the lanthanide ions along with an alternative preparative method of SAMs incorporating lanthanide cations compared to the immobilization of the preformed complex.

Fluorenylporphyrins functionalized by electrochromic ruthenium units as redox-triggered fluorescence switches.

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.

Convenient One-Pot Synthesis of 1,2,3,4-Thiatriazoles Towards a Novel Electron Acceptor for Highly-Efficient Thermally-Activated Delayed-Fluorescence Emitters.

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.

s-Tetrazines as a New Electrode-Active Material for Secondary Batteries.

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.

Metal-Free Synthetic Approach to 3-Monosubstituted Unsymmetrical 1,2,4,5-Tetrazines Useful for Bioorthogonal Reactions.

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 13 C labeling of DCM, and is also presented and discussed as well as the photophysical and electrochemical properties.

Redox-driven porphyrin based systems for new luminescent molecular switches.

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.

Unprecedented Self-Organized Monolayer of a Ru(II) Complex by Diazonium Electroreduction.

A new heteroleptic polypyridyle Ru(II) complex was synthesized and deposited on surface by the diazonium electroreduction process. It yields to the covalent grafting of a monolayer. The functionalized surface was characterized by XPS, electrochemistry, AFM, and STM. A precise organization of the molecules within the monolayer is observed with parallel linear stripes separated by a distance of 3.8 nm corresponding to the lateral size of the molecule. Such organization suggests a strong cooperative process in the deposition process. This strategy is an original way to obtain well-controlled and stable functionalized surfaces for potential applications related to the photophysical properties of the grafted chromophore. As an exciting result, it is the first example of a self-organized monolayer (SOM) obtained using diazonium electroreduction.

A cyclometallated fluorenyl Ir(iii) complex as a potential sensitiser for two-photon excited photodynamic therapy (2PE-PDT).

A new Ir(iii) cyclometallated complex bearing a fluorenyl 5-substituted-1,10-phenanthroline ligand ([Ir(ppy)2()][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()][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()][PF6], show that the complex is an efficient sensitizer for triplet oxygen, producing cytotoxic singlet oxygen ((1)O2) by two-photon excitation at 740 nm resulting in photodynamic effects that lead to localised cell damage and death.

Redox- and protonation-induced fluorescence switch in a new triphenylamine with six stable active or non-active forms.

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.

Comprehensive investigation of the excited-state dynamics of push-pull triphenylamine dyes as models for photonic applications.

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.

Iridium(III) soft salts from dinuclear cationic and mononuclear anionic complexes for OLED devices.

Two iridium(III) soft salts based on ion-paired dinuclear cationic and mononuclear anionic complexes were designed and investigated as phosphorescent emitters for solution processed OLEDs. New dinuclear cationic complexes were prepared with two different bridging ligands, a carbazole and a phenylene spacer. Best devices were designed with the soft salt bearing a carbazole moiety.