C3-Symmetric Luminescent Diketone with Amido-Linkage as a Polymorphic Fluorescence Emitter.

The study introduces a novel C3-symmetric ß-diketone compound, BTA-D3, and its monomeric counterpart, D, with a focus on their synthetic procedure, photophysical properties and aggregation behavior. Both compounds exhibit characteristic absorption and weak fluorescence in solution, with BTA-D3 displaying higher absorption coefficients due to its larger number of diketone units. Density Functional Theory (DFT) calculations suggest increased co-planarity of diketone groups in BTA-D3. A significant finding is the Aggregation-Induced Emission (AIE) property of BTA-D3, as its fluorescence intensity increases dramatically when exposed to specific solvent ratios. The AIE behavior is attributed to intermolecular excitonic interaction between BTA-D3 molecules in self-organized aggregates. We also studied fluorescence anisotropy of BTA-D3 and D. Despite its larger size, BTA-D3 showed reduced anisotropy values because of efficient intramolecular energy migration among three diketone units. Furthermore, BTA-D3 demonstrates unique polymorphism, yielding different emission colors and structures depending on the solvent used. A unique approach is presented for promoting the growth of self-organized aggregate structures via solvent evaporation, leading to distinct fluorescence properties. This research contributes to the understanding of C3-symmetric structural molecules and provides insights into strategies for controlling molecular alignment to achieve diverse fluorescence coloration in molecular materials.

In Situ Generation of 1-Acetylpyrene as a Visible-Light Photocatalyst for the Thia-Paternò-Büchi Reaction.

The thia-Paternò-Büchi reaction represents one of the straightforward approaches to build thietane cores. Unfortunately, the significant instability of thiocarbonyls, particularly thioketones and thioaldehydes, has hitherto rendered this photochemical [2+2]-cycloaddition underexploited. To address this limitation, we report here a visible-light photochemical domino reaction including: the in situ generation of thiocarbonyls, though a Norrish type II fragmentation of pyrenacyl sulfides, and the aforementioned thia-Paternò-Büchi reaction with various non-volatile electron-rich alkenes. The highly efficient synthesis of a wide range of unprecedented thietanes from intrinsically highly unstable thiocabonyls, such as thioaldehydes and aliphatic thioketones, was made possible by the multitasking capability of pyrenacyl sulfides, as a source of thiocarbonyl substrates and as precursors of 1-acetylpyrene, which acts as the photocatalyst for the thia-Paternò-Büchi reaction. The photosensitizer properties of the latter have been experimentally established and a triplet-triplet Dexter energy transfer-based mechanism is proposed.

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.

Electroreductive Cross-Coupling of Trifluoromethyl Alkenes and Redox Active Esters for the Synthesis of Gem-Difluoroalkenes.

An electroreductive access to gem-difluoroalkenes has been developed through the decarboxylative/defluorinative coupling of N-hydroxyphtalimides esters and α-trifluoromethyl alkenes. The electrolysis is performed under very simple reaction conditions in an undivided cell using cheap carbon graphite electrodes. This metal-free transformation features broad scope with good to excellent yields. Tertiary, secondary as well as primary alkyl radicals could be easily introduced. α-aminoacids L-aspartic and L-glutamic acid-derived redox active esters were good reactive partners furnishing potentially relevant gem-difluoroalkenes. In addition, it has been demonstrated that our electrosynthetic approach toward the synthesis of gem-difluoroalkenes could use an easily prepared Kratitsky salt as alkyl radical precursor via a deaminative/defluorinative carbofunctionalization of trifluoromethylstyrene.

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.

Mechanofluorochromic Difluoroboron ß-Diketonates Based Polymer Composites: Toward Multi-Stimuli Responsive Mechanical Stress Probes.

Developing mechano-responsive fluorescent polymers that exhibit distinct responses to distinct mechanical stresses requires a careful design of the fluorophore in order to tune its interactions with the polymer. A series of mechanofluorochromic (MFC) polymer composites are prepared by dispersing difluoroboron diketonates complexes with various alkyl side-chain lengths (DFB-alkyl) in linear low-density polyethylene. Observation of the resulting polymer composites under a microscope reveals different aggregate sizes of the three DFB-alkyls, thus confirming the functionalization by alkyl side chains as a powerful approach to control the aggregation process in a polymer. Besides, the three polymer composite samples are shown to be sensitive to both stretching and scratching, thereby consisting in the first reported example of MFC polymer responding to these two distinct mechanical stimuli. To establish a structure-property relationship, the strategy consisted in applying controlled tensile or friction forces while simultaneously monitoring fluorescence changes. Interestingly, the intensity of the MFC response to both stretching and scratching depends on the alkyl chain length and thus on the aggregation properties of the fluorophore. According to a time-resolved fluorescence study, the emission is found to originate from different species following the type of applied stress (tensile or friction force).

First Example of a Heptazine-Porphyrin Dyad; Synthesis and Spectroscopic Properties.

We have prepared the first example of a porphyrin linked to an heptazine photoactive antenna. The two entities, linked with an alkyl spacer, demonstrate the activity of both active moieties. While they behave electrochemically independantly, on the other hand the spectroscopy shows the existence of energy transfer between both partners.

Mechanical Modulation of the Solid-State Luminescence of Tricarbonyl Rhenium(I) Complexes through the Interplay between Two Triplet Excited States.

Mechanoresponsive luminescence (MRL) materials promise smart devices for sensing, optoelectronics and security. We present here the first report on the MRL activity of two ReI complexes, opening up new opportunities for applications in these fields. Both complexes exhibit marked solid-state luminescence enhancement (SLE). Furthermore, the pristine microcrystalline powders emit in the yellow-green region, and grinding led to an amorphous phase with concomitant emission redshift and shrinking of the photoluminescence (PL) quantum yields and lifetimes. Quantum chemical calculations revealed the existence of two low-lying triplet excited states with very similar energy levels, that is, 3 IL and 3 MLCT, having, respectively, almost pure intraligand (IL) and metal-to-ligand charge-transfer (MLCT) character. Transition between these states could be promoted by rotation around the pyridyltriazole-phenylbenzoxazole bond. In the microcrystals, in which rotations are hindered, the 3 IL state induces the prominent PL emission at short wavelengths. Upon grinding, rotation is facilitated and the transition to the 3 MLCT state results in a larger proportion of long-wavelength PL. FTIR and variable-temperature PL spectroscopy showed that the opening of the vibrational modes favours non-radiative deactivation of the triplet states in the amorphous phase. In solution, PL only arises from the 3 MLCT state. The same mechanism accounts for the spectroscopic differences observed when passing from crystals to amorphous powders, and then to solutions, thereby clarifying the link between SLE and MRL for these complexes.

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.

Unexpected Order-Disorder Transition in Diacetylene Alcohol Langmuir Films.

The phase diagram of the Langmuir film of diacetylene alcohol-henicosa-5,7-diyn-1-ol-is investigated by means of surface pressure versus surface area isotherms, Brewster angle microscopy, X-ray reflectivity, and grazing incident X-ray diffraction. Among the usual phases described in the generic phase diagram of small head group molecules, one observes an unexpected reversible transition from an ordered condensed phase to a disordered one upon increasing the surface pressure. We postulate that the origin of this unusual, unprecedented transition results from the competition between the interactions between the diacetylene blocks in the hydrophobic chain and the hydrogen bonds between head groups and water.

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.

Preparation of Chiral Photosensitive Organocatalysts and Their Application for the Enantioselective Synthesis of 1,2-Diamines.

Chiral phosphoric acid based organocatalysis and visible-light photocatalysis have both emerged as promising technologies for the sustainable production of fine chemicals. In this context, we have envisioned the design and the synthesis of a new class of chimeric catalytic entities that would feature both catalytic capabilities. Given their multitask nature, such catalysts would be particularly attractive for the development of new catalytic transformations, tandem processes in particular. Toward this goal, several BINOL-based chiral phosphoric acid backbones presenting one or two visible-light-sensitive thioxanthone moieties have been prepared and studied. The utility of these new photoactive chiral organocatalysts is then demonstrated in the enantioselective tandem three-component electrophilic amination of enecarbamates. Of note, the C1-symmetric organo/photocatalyst has shown a better catalytic activity than those presenting a C2 symmetry.

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.

s-Tetrazine Dyes: A Facile Generation of Photoredox Organocatalysts for Routine Oxidations.

A series of organic dyes derived from s-tetrazine have been synthesized, and their photophysical and electrochemical properties are systematically investigated. Testing these compounds as photoredox catalysts in a model oxidative C-S bond cleavage of thioethers has led us to identify new classes of active s-tetrazines. Moreover, some of them can be formed in situ from commercially available 3,6-dichlorotetrazine, making this photocatalyzed C-S bond functionalization simple and highly practical.

Fluorescent Zr(IV) Metal-Organic Frameworks Based on an Excited-State Intramolecular Proton Transfer-Type Ligand.

We report here the preparation of a series of Zr(IV) metal-organic frameworks (MOFs) of the MIL-140 structure type incorporating a ligand exhibiting an intense excited-state intramolecular proton transfer (ESIPT) fluorescence. These solids were obtained by systematically varying the substitution rate of 4,4'-biphenyldicarboxylate by 2,2'-bipyridine-3,3'-diol-5,5'-dicarboxylate, and they were thoroughly characterized by complementary techniques, including high-resolution powder X-ray diffraction, solid-state NMR spectroscopy, nitrogen sorption experiments, and time-resolved fluorescence. We show that the incorporation of the ESIPT-type ligand induces an increase of the hydrophilicity, leading ultimately to a higher sensitivity toward hydrolysis, a phenomenon rarely observed in this structure type, which is considered as one of the most stable among the Zr carboxylate MOFs. Eventually, optimization of the amount of fluorescent ligand within the structure allowed combining a decent microposity ( SBET > 750 m2·g-1) and a high stability even in boiling water, together with a high fluorescence quantum yield (>30%).

ß-Diketone derivatives: influence of the chelating group on the photophysical and mechanofluorochromic properties.

A diphenyl-boron ß-diketonate complex was synthesized. Its photophysical properties were studied in solution and in the solid-state, and compared to those of its parent diketone and the corresponding difluoro-boron complex. TD-DFT calculations show that the molecular orbitals involved in the first Franck-Condon transition are very different for the three compounds studied. The difluoro-boron complex is strongly fluorescent in solution, and remains fluorescent in the solid-state. The free diketone turns to be very weakly fluorescent in solution and displays significant Aggregation Induced Enhanced Emission (AIEE) in the crystalline state, which can be explained by a rigidification of the molecule, while the diphenyl-boron complex is weakly fluorescent in solution as well as in the solid-state. For the free diketone and the difluoro-boron complex a mechanofluorochromic response is observed upon grinding the crystalline powder in a mortar, while for the diphenyl-boron complex no fluorescence emission change is detected under these conditions. Overall, this study shows that the nature of the chelating group has a crucial influence on the photophysical and mechanofluorochromic properties of ß-diketonate complexes, leading to a wide variety of behaviors within the closely related structures of such derivatives.

A Flexible Fluorescent Zr Carboxylate Metal-Organic Framework for the Detection of Electron-Rich Molecules in Solution.

A novel Zr(IV) dicarboxylate metal organic framework (MOF) built up from an s-tetrazine derived ligand was prepared. This solid, which exhibits a diamond type network, combines a good stability in water, a structural flexibility, and fluorescence properties thanks to the organic ligand. It is noteworthy that this fluorescence is quenched when exposed to electron-rich molecules in solution, such as amines or phenol, this phenomenon being associated with the adsorption of the quencher, as unambiguously proven by X-ray diffraction (XRD) analysis. Finally, the quenching efficiency is shown to be governed not only by electronic and steric factors but also by the relative polarity of the solvent, the MOF, and the quencher. This work thus suggests that it is possible to develop new MOF-based sensors presenting in a given medium (such as water) highly selective responses.

A Robust Infinite Zirconium Phenolate Building Unit to Enhance the Chemical Stability of Zr MOFs.

A novel Zr-chain based MOF, namely MIL-163, was designed and successfully synthesized using a bis-1,2,3-trioxobenzene ligand. Endowed with large square-shaped channels of 12 Šwidth, it shows remarkable water uptake (ca. 0.6 cm(3) g(-1) at saturating vapor pressure) and a remarkable stability in simulated physiological media, where archetypical Zr carboxylate MOFs readily degrade.

Mechanofluorochromism and self-recovery of alkylsilylpyrene-1-carboxamides.

A family of (alkylsilyl)pyrene-1-carboxamides exhibits similar mechanofluorochromic responses upon grinding. However, their spontaneous fluorescence recovery processes are distinct despite their similarity in chemical structures and crystal packings. Fluorescence spectroscopy, crystallography, and nanomechanical tests revealed that the chromic direction is dominated by the packing motif, while the fluorescence recovery is driven by the intermolecular interactions and the reversibility of deformation.

Cascade Fluorescence Modulation in Photochromic Microcapsules.

Certain derivatives of terarylene are able to undergo a highly efficient oxidative cycloreversion cascade effect, a ring opening reaction with quantum yields above unity, resulting in a colored-to-colorless transition in solution. In the presence of chloroform, high-energy UV and X-rays can trigger this phenomenon, potentially acting as a visual detection system for ionizing radiation. However, chloroform is sensitive to different irradiation wavelengths without distinction, making it difficult to adapt to a reusable device. Chlorobenzene was chosen as an alternative halogenated solvent, as it offers wavelength selectivity between photocyclization and cascade effect cycloreversion. Nile Red was also incorporated into the system with the aim of improving the sensitivity of the visual detection via fluorescence photoswitching. Finally, microencapsulation of both terarylene and Nile Red was targeted to obtain both the cascade effect and photoswitching in a single system. In microcapsules made from a Pickering emulsion, this terarylene-Nile Red system showed high fatigue resistance to repeated photocyclization and cycloreversion irradiation, giving access to repeated ON/OFF fluorescence photoswitching. The cascade effect was also successfully demonstrated along with fluorescence recovery, showing the versatility of the two phenomena in different media.