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.

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.

Dissecting Tetra-N-phenylbenzidine: Biphenyl as the Origin of Room Temperature Phosphorescence.

Amorphous purely organic thin films are able to show efficient phosphorescence under ambient conditions at room temperature. This opens the perspective to a wide range of new applications, which have attracted lots of interest in the field of material science recently. Therefore, an increasing number of different molecules displaying room temperature phosphorescence (RTP) have already been reported. Whereas the efficiency, the lifetime, or the oxygen sensitivity is frequently discussed, the origin of RTP mainly remains vague. Often, material design rules tend to the development of increasingly complex structures. Here, the well-known tetra-N-phenylbenzidine (TPD), an archetypical material showing highly efficient fluorescence and RTP, is broken down to its fragments. As the complexity of the system decreases with the molecule's size, spectroscopic investigation of this molecular family enables a deeper understanding of the appearance of RTP. With spectral and time-resolved measurements, RTP can be detected for all compounds containing a biphenyl core, with lifetimes up to 0.9 s under inert gas conditions. These findings form the basis of a deeper understanding of the appearance of RTP in organic molecules and therefore allow for a more focused investigation of new materials.

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.

Enantioselective Light Harvesting with Perylenediimide Guests on Self-Assembled Chiral Naphthalenediimide Nanofibers.

Self-assembling molecular systems often display amplified chirality compared to the monomeric state, which makes the molecular recognition more sensitive to chiral analytes. Herein, we report the almost absolute enantioselective recognition of a chiral perylenediimide (PDI) molecule by chiral supramolecular nanofibers of a bichromophoric naphthalenediimide (NDI) derivative. The chiral recognition was evaluated through the Förster resonance energy transfer (FRET) from the NDI-based host nanofibers to the guest PDI molecules. The excitation energy was successfully transferred to the guest molecule through efficient energy migration along the host nanofiber, thus demonstrating the light-harvesting capability of these hybrid systems. Furthermore, circularly polarized luminescence (CPL) was enantioselectively sensitized by the guest molecule as the wavelength band and sign of the CPL signal were switched in response to the chiral guest molecule.

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.

Conglomerate, Racemate, and Achiral Crystals of Polymetallic Europium(III) Compounds of Bis- or Tris-ß-diketonate Ligands and Circularly Polarized Luminescence Study.

This work reports (a) conglomerate and racemic crystal structures of [(Δ,Δ,Δ,Δ,Δ,Δ)- or/and (Λ,Λ,Λ,Λ,Λ,Λ)-EuIII 6(TTP)8(OH2)6Na4] n coordination polymers, (b) racemic crystal structures of (Δ,Δ,Δ,Δ)-/(Λ,Λ,Λ,Λ)-EuIII 4(TTP)4(bipy)4(MEK)2(OH2)2 tetrahedral clusters, and (c) the achiral crystal structure of the [EuIII 2(BTP)4(OH2)2Na2] n coordination polymer (where BTP = dianionic bis-ß-diketonate, TTP = trianionic tris-ß-diketonate, and bipy = 2,2'-bipyridine). The screw coordination arrangement of the TTP ligand has led to the formation of homoconfigurational racemic EuIII products. The conglomerate crystallization of [EuIII 6(TTP)8(OH2)6Na4] n appears to be caused by the presence of the sodium, Na+ counterions, and interactions between oxygen atoms and the trifluoromethyl unit of the TTP ligand and Na+ ions. All the EuIII compounds exhibit characteristic red luminescence (5D0 → 7FJ, J = 0-4) in solution or in the solid crystalline state. Circularly polarized luminescence (CPL) was observed in the chiral EuIII 6(TTP)8(OH2)6Na4] n species, displaying a |g lum| value in the range of 0.15 to 0.68 at the 5D0 → 7F1 emission band. Subtle changes of the [EuIII 6(TTP)8(OH2)6Na4] n structure which may be due to selection of twinned crystals or crystals that do not correspond to a perfect spontaneous resolution, are considered to be responsible for the variation in the observed CPL values.

Aromatic Phosphonates: A Novel Group of Emitters Showing Blue Ultralong Room Temperature Phosphorescence.

In recent years, there has been a growing interest in purely organic materials showing ultralong room-temperature phosphorescence with lifetimes in the range of seconds. Still, the longest known phosphorescence lifetimes are only achieved with crystalline systems so far. Here, a rational design of a completely new family of halogen-free organic luminescent derivatives in amorphous matrices, displaying both conventional fluorescence and phosphorescence is reported. Hydrogen bonding between the newly developed emitters and an ethylene-vinyl alcohol copolymer (Exceval) matrix, which efficiently suppresses vibrational dissipation, enables bright long-lived phosphorescence with lifetimes up to 2.6 s at around 480 nm. The importance of the chosen matrix is shown as well as the implementation in an organic programmable luminescent tag.

Blue-Light-Absorbing Thin Films Showing Ultralong Room-Temperature Phosphorescence.

The development of organic materials displaying ultralong room-temperature phosphorescence (URTP) is a material design-rich research field with growing interest recently, as the luminescence characteristics have started to become interesting for applications. However, the development of systems performing under aerated conditions remains a formidable challenge. Furthermore, in the vast majority of molecular examples, the respective absorption bands of the compounds are in the near ultraviolet (UV) range, which makes UV excitation sources necessary. Herein, the synthesis and detailed analysis of new luminescent organic metal-free materials displaying, in addition to conventional fluorescence, phosphorescence with lifetimes up to 700 ms and tailored redshifted absorption bands, allowing for deep blue excitation, are reported. For the most promising targets, their application is demonstrated in the form of organic programmable tags that have been recently developed. These tags make use of reversible activation and deactivation of the URTP by toggling between the presence and absence of molecular oxygen. In this case, the activation can be achieved with visible light excitation, which greatly increases the use case scenarios by making UV sources obsolete.

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.

Mechano-responsive circularly polarized luminescence of organic solid-state chiral emitters.

Two difluoro-boron ß-diketonate complexes bearing chiral amido groups have been synthesized. Their mechano-responsive luminescence and chiroptical properties have been investigated in the solid state. Both compounds display a bright blue-green emission and a significant circularly polarized luminescence (CPL) signal in the crystalline state, with |g lum| values as high as 2.2-2.4 × 10-2. A bathochromic shift in emission, together with a decrease of |g lum| values to c.a. 3 × 10-3, is induced upon application of a shearing stress. For the DFB-Hex-amide compound, interestingly, sign inversion of the anisotropy factor g lum is observed under mechanical stimulation ("mechano-CPL effect"), which can be rationalized by a switching between monomer and excimer emission.

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.