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1. Achieving high circularly polarized luminescence with push–pull helicenic systems: from rationalized design to top-emission CP-OLED applications

   https://doi.org/10.1039/D0SC06895K  

   Dhbaibi, Kais ; Abella, Laura ; Meunier-Della-Gatta, Sylvia ; Roisnel, Thierry ; Vanthuyne, Nicolas ; Jamoussi, Bassem ; Pieters, Grégory ; Racine, Benoît ; Quesnel, Etienne ; Autschbach, Jochen ; et al ( April 2021 , Chemical Science)

   While the development of chiral molecules displaying circularly polarized luminescence (CPL) has received considerable attention, the corresponding CPL intensity, g lum, hardly exceeds 10 −2 at the molecular level owing to the difficulty in optimizing the key parameters governing such a luminescence process. To address this challenge, we report here the synthesis and chiroptical properties of a new family of π-helical push–pull systems based on carbo[6]helicene, where the latter acts as either a chiral electron acceptor or a donor unit. This comprehensive experimental and theoretical investigation shows that the magnitude and relative orientation of the electric ( μe ) and magnetic (μ m ) dipole transition moments can be tuned efficiently with regard to the molecular chiroptical properties, which results in high g lum values, i.e. up to 3–4 × 10 −2 . Our investigations revealed that the optimized mutual orientation of the electric and magnetic dipoles in the excited state is a crucial parameter to achieve intense helicene-mediated exciton coupling, which is a major contributor to the obtained strong CPL. Finally, top-emission CP-OLEDs were fabricated through vapor deposition, which afforded a promising g El of around 8 × 10 −3 . These results bring about further molecular design guidelinesmore »to reach high CPL intensity and offer new insights into the development of innovative CP-OLED architectures.« less
2. Exciton coupling chirality in helicene-porphyrin conjugates

   https://doi.org/10.1039/D1CC03314J  

   Dhbaibi, Kais ; Matozzo, Paola ; Abella, Laura ; Jean, Marion ; Vanthuyne, Nicolas ; Autschbach, Jochen ; Favereau, Ludovic ; Crassous, Jeanne ( October 2021 , Chemical Communications)

   Enantiopure helicene-porphyrin conjugates were prepared. They show strong changes in their circular dichroic response as compared to classical helicene derivatives, with highly intense bisignate Exciton Coupling (EC) signal and Δ ε values up to 680 M −1 cm −1 for the Soret band. They also display circularly polarized fluorescence in the (far-)red region, with dissymmetry factors up to 7 × 10 −4 .
3. Light driven magnetic transitions in transition metal dichalcogenide heterobilayers

   https://doi.org/10.1088/1361-648X/acab49  

   Vogl, Michael ; Chaudhary, Swati ; Fiete, Gregory A ( December 2022 , Journal of Physics: Condensed Matter)

   Abstract Motivated by the recent excitement around the physics of twisted transition metal dichalcogenide (TMD) multilayer systems, we study strongly correlated phases of TMD heterobilayers under the influence of light. We consider both waveguide light and circularly polarized light. The former allows for longitudinally polarized light, which in the high frequency limit can be used to selectively modify interlayer hoppings in a tight-binding model. We argue based on quasi-degenerate perturbation theory that changes to the interlayer hoppings can be captured as a modulation to the strength of the moiré potential in a continuum model. As a consequence, waveguide light can be used to drive transitions between a myriad of different magnetic phases, including a transition from a 120 ∘ Neel phase to a stripe ordered magnetic phase, or from a spin density wave phase to a paramagnetic phase, among others. When the system is subjected to circularly polarized light we find that the effective mass of the active TMD layer is modified by an applied electromagnetic field. By simultaneously applying waveguide light and circularly polarized light to a system, one has a high level of control in moving through the phase diagram in-situ. Lastly, we comment on the experimental feasibilitymore »of Floquet state preparation and argue that it is within reach of available techniques when the system is coupled to a judiciously chosen bath.« less
4. Twisted tetrathiafulvalene crystals

   https://doi.org/10.1039/D2ME00010E  

   Yang, Yongfan ; Zong, Kai ; Whittaker, St. John ; An, Zhihua ; Tan, Melissa ; Zhou, Hengyu ; Shtukenberg, Alexander G. ; Kahr, Bart ; Lee, Stephanie S. ( January 2022 , Molecular Systems Design & Engineering)

   Optically-active optoelectronic materials are of great interest for many applications, including chiral sensing and circularly polarized light emission. Traditionally, such applications have been enabled by synthetic strategies to design chiral organic semiconductors and conductors. Here, centrosymmetric tetrathiafulvalene (TTF) crystals are rendered chiral on the mesoscale by crystal twisting. During crystallization from the melt, helicoidal TTF fibers were observed to grow radially outwards from a nucleation centre as spherulites, twisting in concert about the growth direction. Because molecular crystals exhibit orientation-dependent refractive indices, periodic concentric bands associated with continually rotating crystal orientations were observed within the spherulites when imaged between crossed polarizers. Under certain conditions, concomitant crystal twisting and bending was observed, resulting in anomolous crystal optical behavior. X-ray diffraction measurements collected on different spherulite bands indicated no difference in the molecular packing between straight and twisted TTF crystals, as expected for microscopic twisting pitches between 20–200 μm. Mueller matrix imaging, however, revealed preferential absorption and refraction of left- or right-circularly polarized light in twisted crystals depending on the twist sense, either clockwise or counterclockwise, about the growth direction. Furthermore, hole mobilities of 2.0 ± 0.9 × 10 −6 cm 2 V −1 s −1 and 1.9 ± 0.8 × 10more »−5 cm 2 V −1 s −1 were measured for straight and twisted TTF crystals deposited on organic field-effect transistor platforms, respectively, demonstrating that crystal twisting does not negatively impact charge transport in these systems.« less
5. All-optical nonreciprocity due to valley polarization pumping in transition metal dichalcogenides

   https://doi.org/10.1038/s41467-021-24138-0  

   Guddala, Sriram ; Kawaguchi, Yuma ; Komissarenko, Filipp ; Kiriushechkina, Svetlana ; Vakulenko, Anton ; Chen, Kai ; Alù, Andrea ; M. Menon, Vinod ; Khanikaev, Alexander B. ( June 2021 , Nature Communications)

   Nonreciprocity and nonreciprocal optical devices play a vital role in modern photonic technologies by enforcing one-way propagation of light. Here, we demonstrate an all-optical approach to nonreciprocity based on valley-selective response in transition metal dichalcogenides (TMDs). This approach overcomes the limitations of magnetic materials and it does not require an external magnetic field. We provide experimental evidence of photoinduced nonreciprocity in a monolayer WS₂pumped by circularly polarized (CP) light. Nonreciprocity stems from valley-selective exciton population, giving rise to nonlinear circular dichroism controlled by CP pump fields. Our experimental results reveal a significant effect even at room temperature, despite considerable intervalley-scattering, showing promising potential for practical applications in magnetic-free nonreciprocal platforms. As an example, here we propose a device scheme to realize an optical isolator based on a pass-through silicon nitride (SiN) ring resonator integrating the optically biased TMD monolayer.