Chiral Truxene-Based Self-Assembled Cages: Triple Interlocking and Supramolecular Chirogenesis.

Incorporating chiral elements in host-guest systems currently attracts much attention because of the major impact such structures may have in a wide range of applications, from pharmaceuticals to materials science and beyond. Moreover, the development of multi-responsive and -functional systems is highly desirable since they offer numerous benefits. In this context, we describe herein the construction of a metal-driven self-assembled cage that associates a chiral truxene-based ligand and a bis-ruthenium complex. The maximum separation between both facing chiral units in the assembly is fixed by the intermetallic distance within the lateral bis-ruthenium complex (8.4 Å). The resulting chiral cavity was shown to encapsulate polyaromatic guest molecules, but also to afford a chiral triply interlocked [2]catenane structure. The formation of the latter occurs at high concentration, while its disassembly could be achieved by the addition of a planar achiral molecule. Interestingly the planar achiral molecule exhibits induced circular dichroism signature when trapped within the chiral cavity, thus demonstrating the ability of the cage to induce supramolecular chirogenesis.

Polymerization of tetrazine-substituted diacetylenes as aggregates in suspension.

Polydiacetylenes (PDAs) are conjugated polymers that have been widely exploited for their chromogenic and fluorogenic transitions upon exposure to external stimuli and biomolecules of interest. Herein, we propose a comparative study of the polymerization dynamics of two diacetylene derivatives, TzDA1 and TzDA2, in the form of aggregates in suspension prepared by reprecipitation method from organic solvents in water, varying the diacetylene concentration and solvent proportions, and sonication in water, varying the time and temperature. Both derivatives bear a tetrazine fluorophore, which serves both to increase the fluorescence quantum yield of the system and to track the polymerization by fluorescence quenching exclusively by the blue-PDA, and differ by the chain termination. It was shown that adding a butyl ester function in TzDA2 to a simple urethane (TzDA1) influences the polymerizability and kinetics of polymerization of the aggregates in suspension. In addition, we showed that also the preparation method and preparation conditions do have an influence on the polymerization dynamics, suggesting that a careful study of these properties should be carried out before investigating the applications of such objects.

Dynamics of the energy transfer involved in a diarylethene-perylenebisimide dyad: comparison between the molecule and the nanoparticle level.

Photochromic materials are widely used to achieve fluorescence photoswitching. Understanding the energy transfer processes occurring in these systems would be an advantage for their use and better optimization of their properties. In this scope, we studied a diarylethene-perylenebisimide (DAE-PBI) dyad that presents a bright red emission and a large ON-OFF contrast, both in solution and in an aqueous suspension of nanoparticles (NPs). Using ultrafast transient absorption spectroscopy, the excited state dynamics was characterized for this dyad in THF solution and compared to its behavior in NPs state. An efficient energy transfer process between the PBI fluorophore and the DAE photochromic unit in its closed form was demonstrated, occurring in a few hundreds of femtoseconds.

Chloride Binding Modulated by Anion Receptors Bearing Tetrazine and Urea.

Modulation and fine-tuning of the strength of weak interactions to bind anions are described in a series of synthetic receptors. The general design of the receptors includes both a urea motif and a tetrazine motif. The synthetic sequence towards three receptors is detailed. Impacts of H-bond strength and linker length between urea and tetrazine on chloride complexation are studied. Binding properties of the chloride anion are examined in both the ground and excited states using a panel of analytical methods (NMR spectroscopy, mass spectrometry, UV/Visible spectroscopies, and fluorescence). A ranking of the receptors by complexation strength has been determined, allowing a better understanding of the structure-properties relationship on these compounds.

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).

Circularly Polarized Luminescence and Circular Dichroism of Bichromophoric Difluoroboron-ß-diketonates: Inversion and Enhanced Chirality Based on Spatial Arrangements and Self-Assembly.

We synthesized two bichromophoric difluoroboron-ß-diketonates (DFB) connected in para and meta positions by using cyclohexane diamine as a chiral bridge (para and meta (R/S)-CyDFB). TD-DFT calculations revealed that the variation in connectivity of the DFB units leads to different spatial arrangements and a chirality inversion of the bichromophoric DFB. Higher gabs values were obtained in (R/S)-CyDFB connected in para as compared to meta position. Aggregation of para (R/S)-CyDFB in mixture of solvents increase the glum values as compared to its monomeric form. Ultrasonication and heating induced the formation of highly ordered nano-helical wires of para (R/S)-CyDFB that increased the glum values to 0.015. On the other hand, meta (R/S)-CyDFB failed to form highly ordered self-assembled wires due to hindered H-binding sites. These observations indicate that the chiroptical properties of DFB bi-chromophore system can be modulated with self-assembly and spatial arrangement of the chromophores.

Enhanced mechano-responsive fluorescence in polydiacetylene thin films through functionalization with tetrazine dyes: photopolymerization, energy transfer and AFM coupled to fluorescence microscopy studies.

The development of mechano-responsive fluorescent materials is essential for the design and construction of reliable and versatile sensors for mechanical stress. Herein, novel energy transfer-based systems with tetrazine fluorophore and a polydiacetylene (PDA) backbone are synthesized and studied comparatively to a simple polydiacetylene in the form of thin films. Their photopolymerization properties, energy transfer efficiencies and fluorescent response to nanoscale mechanical stimulation are assessed. It is pointed out that the self-assembling group on the PDA chain influences the geometrical arrangement of the chains and the film morphology and, as a consequence, the efficiency and kinetics of polymerization and the energy transfer efficiency. Moreover, we show that the strategy of introducing tetrazine fluorophore provides a new effective route of improving force detectability by fluorescence using polydiacetylenes as mechano-responsive units.

Multi-Directional Mechanofluorochromism of Acetyl Pyrenes and Pyrenyl Ynones.

A series of acetyl pyrenes and pyrenyl ynones with and without tert-butyl groups showed distinct mechanofluorochromism (MFC). Four pairs of polymorphic solids were found out of six compounds and interestingly, each of them showed hypsochromic, bathochromic or off-to-on MFC. The MFC properties were rationalized by categorizing the packing schemes into herringbone, sandwich, beta and gamma motifs depending on the relative contributions of C⋅⋅⋅C (or π-π) against C⋅⋅⋅H contacts. The bulky tert-butyl and trimethylsilyl groups served not only to reduce the number of aggregation patterns but also to prohibit the complete back reactions in solid state. Our results suggest that the simple pyrene derivatives may be promising candidates for a novel group of mechanically-sensitive materials.

Design and property investigation on a five-interaction-based fluorescent anion receptor clip.

A molecular clip combining a doubly substituted fluorescent anion-π donor probe and two flexible arms bearing H-bond motifs constitutes a new generation of anion receptors. Five simultaneous non-covalent interactions are highlighted by theoretical complexation studies with five different anions. A large range of analytical techniques (electrospray-tandem mass spectrometry, NMR, UV-visible, steady-state and time-resolved fluorescence) were deployed to evaluate the stoichiometry and association constants with the selected anions. The photophysical and anion-π donor properties of the tetrazine ring allowed fine characterization of the binding properties of the ligand. Based on previously published results, an anti-cooperativity effect in non-covalent interactions was demonstrated.

Intertwined Detection and Recognition Roles of Tetrazine in Synergistic Anion-π and H-bond Based Anion Receptor.

The intrinsic properties of tetrazine as a π-anion receptor and as an on/off recognition probe merged with H-bond ability of an urea motif into a single architecture constitutes a new generation of well-defined anion receptors. Complexation properties directly benefit from the dual and synergistic contribution of tetrazine and urea. In this study, we report on the synthesis and assessment of binding properties to anions of diverse geometries. Association constants have been predicted by theoretical calculations and evaluated by multiple and complementary experimental techniques including electrospray-mass tandem spectroscopy, NMR, UV-visible, steady state fluorescence spectroscopies and time resolved fluorescence. These results provide the basis for a better understanding of both the complexation and the anion-dependent quenching mechanism.

Mechanofluorochromism of pyrene-derived amidophosphonates.

Aiming to develop a library of organic mechanofluorochromic (MFC) materials, solid-state emission properties of pyrene-derivatives with amidophosphonate groups (diethyl 1-(pyrene-1-carboxamido)alkylphosphonates, alkyl: methyl, isopropyl, phenyl) were investigated. All of these compounds were found to show strong blue fluorescence, and blue-to-green emission shift was identified upon grinding. Monomer and several excimer emissive species were identified by steady-state and time-resolved fluorescence investigations. The characteristic green-to-blue thermal back reaction was observed even at room temperature without additives, which would be related to the flexibility of side groups for molecular rearrangement. We here demonstrated that the pyrene-derived compounds can be attractive platforms to host MFC properties.

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid-BF2 Dimers.

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid-BF2 dyes and the detailed investigation of their solvent-dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly-coupled H-aggregates. The extent of folding and, in turn, of ground-state aggregation is strongly dependent on the nature of the flexible linker. Steady-state and time-resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in-depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two-dimensional 1 H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.

Mechano-responsive fluorescent polydiacetylene-based materials: towards quantification of shearing stress at the nanoscale.

An OFF to ON mechano-responsive fluorescent polydiacetylene derivative has been prepared by photopolymerisation of a diacetylene covalently linked to a tetrazine fluorophore. This system has been studied at the nanoscale and shows a fluorescence emission intensity that increases with the intensity of the mechanical stimulation in the 20-200 nN range.

Mechanofluorochromism of a Difluoroboron-ß-Diketonate Derivative at the Nanoscale.

Mechanofluorochromic nanoparticles have been prepared from a difluoroboron ß-diketonate complex, and their behavior has been investigated at the nanoscale using atomic force microscopy (AFM) coupled with fluorescence spectroscopy. Two types of nanoparticles were observed, associated with green and yellow emission, reflecting the crystalline polymorphism of this material. While the green-emitting nanoparticles are mechanically insensitive under our conditions, the yellow-emitting ones display a marked hypsochromic shift upon shearing with the AFM tip. At the macroscopic level, the grinding of the bulk material is attributed to the amorphization of the crystalline powder. On the contrary, the marked mechanofluorochromism observed at the nanoscale is attributed to a crystal-to-crystal phase transition. This specific behavior at the nanolevel is extremely promising for applications such as nanoprobes of local mechanical stress.

Photoreduction of triplet thioxanthone derivative by azolium tetraphenylborate: a way to photogenerate N-heterocyclic carbenes.

Although N-heterocyclic carbenes (NHCs) have brought profound changes in catalytic organic synthesis, their generation generally requires an inert atmosphere and harsh conditions. To overcome these limitations, an air-stable NHC photogenerator has been developed involving two mild components: 1,3-bis(mesityl)imidazolium tetraphenylborate (IMesH+BPh4-) and electronically excited isopropylthioxanthone (ITX). In this study, the photochemical mechanism is investigated via the accurate identification of the transient species and photoproducts. Electron transfer reaction between the excited triplet state of ITX and BPh4- is demonstrated as being the primary photochemical step. Nanosecond laser spectroscopy shows an efficient quenching and the formation of the expected ITX radical anion. The oxidized borane species is not observed, suggesting that this short-lived species could dissociate very rapidly to give the phenyl radical - successfully identified using electron paramagnetic resonance - and triphenylborane. As regards the final photoproducts, 1H and 13C NMR spectroscopies support the formation of the targeted NHC, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), suggesting the occurrence of a subsequent proton transfer reaction between ITX radical anion and imidazolium cation (IMesH+). Gas chromatography-mass spectrometry reveals three other products: biphenyl, isopropylthioxanthene and ITX. Their formation can be reconciled with a 2-step mechanism of photoinduced electron/proton transfer reactions. 11B NMR spectroscopy demonstrates that the main organoboron photoproduct is diphenylborinic acid formed by oxidation of BPh3. Due to its Lewis acidity, Ph2BOH can react with IMes to yield an NHC-boron adduct.

Impact of Optical Purity on the Light Harvesting Property in Supramolecular Nanofibers.

Supramolecular ordering and orientation of chromophores are tremendously accomplished in photosynthetic light harvesting complexes, which are crucial for long-range transfer of collected solar energy. We herein demonstrate the importance of optical purity on the organization of chromophoric chiral molecules for efficient energy migration. Enantiomeric bichromophoric compounds, which self-assemble into nanofibers capable of chiral recognition, were mixed to form supramolecular coassemblies with variable enantiopurity. The chiral molecules self-assembled into extended fibers regardless of enantiopurity, while their morphology was dependent on the enantiomeric excess. The optical purity of assemblies also had an effect on the emission efficiency; the nanofibers with higher enantiomeric excess afforded a larger emission quantum yield. The presence of an opposite enantiomer is considered to deteriorate the chiral molecular packing suitable for directional growth of the nanofiber, efficient exciton migration, and chiral guest recognition.

Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals.

Mechanofluorochromic molecular materials display a change in fluorescence color through mechanical stress. Complex structure-property relationships in both the crystalline and amorphous phases of these materials govern both the presence and strength of this behavior, which is usually deemed the result of a mechanically induced phase transition. However, the precise nature of the emitting species in each phase is often a matter of speculation, resulting from experimental data that are difficult to interpret, and a lack of an acceptable theoretical model capable of capturing complex environmental effects. With a combined strategy using sophisticated experimental techniques and a new theoretical approach, here the varied mechanofluorochromic behavior of a series of difluoroboron diketonates is shown to be driven by the formation of low-energy exciton traps in the amorphous phase, with a limited number of traps giving rise to the full change in fluorescence color. The results highlight intrinsic structural links between crystalline and amorphous phases, and how these may be exploited for further development of powerful mechanofluorochromic assemblies, in line with modern crystal engineering approaches.

Fluorescent and Electroactive Low-Viscosity Tetrazine-Based Organic Liquids.

New fluorescent molecular liquids with a tetrazine core have been prepared. These compounds remain liquid at least down to -60 °C and display very low viscosities (28 mPa.s for liquid 1, 58 mPa.s for liquid 2). Both compounds remain fluorescent in the condensed phase. For liquid 1, intermolecular quenching is observed to a certain extent, whereas liquid 2 displays similar photophysical properties in dilute solution and in neat film.

Designing NHC-Copper(I) Dipyridylamine Complexes for Blue Light-Emitting Electrochemical Cells.

This study presents the influence of various substituents on the photophysical features of heteroleptic copper(I) complexes bearing both N-heterocyclic carbene (NHC) and dipyridylamine (dpa = dipyridylamine skeleton corresponding to ligand L1) ligands. The luminescent properties have been compared to our recently reported archetypal blue emitting [Cu(IPr)(dpa)][PF6] complex. The choice of the substituents on both ligands has been guided to explore the effect of the electron donor/acceptor and "push-pull" on the emission wavelengths and photoluminescence quantum yields. A selection of the best candidates in terms of their photophysical features were applied for developing the first blue light-emitting electrochemical cells (LECs) based on copper(I) complexes. The device analysis suggests that the main concern is the moderate redox stability of the complexes under high applied driving currents, leading to devices with moderate stabilities pointing to a proof-of-concept for further development. Nevertheless, under low applied driving currents the blue emission is stable, showing performance levels competitive to those reported for blue LECs based on iridium(III) complexes. Overall, this work provides valuable guidelines to tackle the design of enhanced NHC copper complexes for lighting applications in the near future.

Synthesis, regioselective aerobic Pd(ii)-catalyzed C-H bond alkenylation and the photophysical properties of pyrenylphenylpyrazoles.

This paper reports the synthesis, regioselective aerobic Pd(ii)-catalyzed C-H bond alkenylation and the photophysical properties of pyrenylphenylpyrazoles.