population of the excited triplet via a spinforbidden intersystem crossing from singlet to triplet. Moreover, the excited triplet can be quenched quite easily by molecular oxygen or deactivated through nonradiative pathways ( Figure 1A). In the literature, two main approaches have been developed to overcome those issues and achieve persistent RTP in aerated atmosphere. In any case, the phosphors must be contained in a rigid environment, inducing a restriction of molecular motion, be it a crystalline structure [11][12][13] or the use of an external host trapping the luminophore in the amorphous phase. [14,15] In both systems, much progress has been achieved, and efficient materials have been developed reporting long-lasting phosphorescence and high phosphorescence quantum yield. In 2017, Shoji et al. revealed the exceptional phosphorescence properties of a series of simple crystalline arylboronic esters, with lifetimes up to 1.73 s, which is-to the best of our knowledge-one of the longest values ever reported for metal-free organic phosphors. [16] Earlier this year, Su et al. designed an organic molecule that is able to participate in multiple hydrogen bondings. After dispersion into polyvinyl alcohol (PVA) to create drop-coating thin films and long irradiation under strong UV light (65 min, 254 nm), an intense phosphorescence (Φ p up to 11.23%) associated with a long lifetime (up to 0.71 s) could be observed at room temperature in aerated atmosphere. In fine, these films could be used to print and encode information. [17] These examples, as for most of the studies reporting URTP pure organic systems described in the literature, work only with excitation in the UV range to trigger the phosphorescence response. This represents a clear limitation and makes these materials hardly suitable for potential mass commercial purpose. The only exception has been reported by Huang and co-workers in 2017. [18] In this work, the authors describe the rational design of new phosphorescent organic crystalline powders, characterized by very long emission lifetimes (up to 0.84 s), due to the derivatives ability to form H aggregates. For one crystalline target with absorption strength ranging up to 450 nm, phosphorescence could be obtained after excitation with a commercial white light-emitting diode (LED). But the long-term stability of such crystalline structures is unclear and unreliable, and the elaboration of thin films from such materials can be laborious, which makes them impractical for the development of smart tags or security devices.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 (...
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.
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.
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.
Mechano-CPL effect: chiral difluoro-boron β-diketonate complexes show concomitant changes of emission color as well as solid state chiroptical properties upon mechanical stimulation.
Biluminescent organic systems displaying fluorescence and phosphorescence at room temperature under aerated atmosphere still remain uncommon to date. Especially the utilization of the room temperature phosphorescence (RTP) is limited, as it is effectively quenched by oxygen, rendering it incompatible with many applications for use in ambient conditions. So far, only encapsulation to prevent oxygen penetration into the films or the use of special steroid hosts are known concepts to allow conventional biluminescent emitters to show RTP. Here, the design and synthesis of a novel water‐soluble biluminescent emitter is reported. After dispersion in a host matrix, and using host/guest hydrogen‐bonding interactions, easy‐processable highly phosphorescent transparent thin‐films are realized. Their luminescent properties can be activated at will under aerated atmosphere for several weeks without degradation of the system. The polymer (host) mixture can even be readily used for 3D printing, leading to a large range of accessible phosphorescent objects.
Difluoroboron-β-diketones are known to exhibit very promising mechanochromic luminescence but a greater understanding of these materials and their mechanoresponsive properties is still desirable. Here we demonstrate that a combination of spectroscopic and physicochemical techniques is necessary to understand the variation of solid-state fluorescence observed under mechanical stress and thermal annealing. For this study, we decided to focus on a new fluorescent compound showing a polymorphic behavior. Our investigation on the mechanofluorochromic properties is based on a thorough spectroscopic study (steady-state and time-resolved) on powder samples and thin films deposited on paper and coverglass substrates. Three different states were identified: two crystalline states (a stable green-emissive and a metastable yellow-emissive one) and an amorphous phase (yellow orange emission). The detailed photophysical properties highlight the dynamic excimer formation processes, which can take place in the yellow-emissive crystalline and amorphous states, along with the effect of the substrate on the thin films composition. These results were confirmed by studying the thermal annealing process with a combined AFM and fluorescence microscope. The switching between these states under mechanical and thermal treatments underlies the fluorescence color changes.
Mechanofluorochromic nanoparticles have been prepared from a difluoroboron β-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.
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