Circularly polarized-thermally activated delayed fluorescent materials based on chiral bicarbazole donors

G. P. thanks the SCBM, the “PTC du CEA” (POLEM) and the ANR (iChiralight, ANR-19-CE07-0040) for funding and David Buisson, Amélie Goudet and Sabrina Lebrequier. J. C. and L. Fa. acknowledge the Ministère de l’Education Nationale, de la Recherche et de la Technologie, the CNRS and the Spectroscopies-CDTP core facility is also acknowledged. The St. Andrews team thanks the China Scholarship Council, 201906250199 to W. S. and 202006250026 to J. W., E. Z.-C. is a Royal Society Leverhulme Trust Senior Research fellow (SRF\R1\201089). We thank the EPSRC (EP/R035164/1) for funding.We describe herein a molecular design to generate circularly polarized thermally activated delayed fluorescence emitters in which chiral bicarbazole donors are connected to acceptor units via a rigid 8-membered cycle and how the nature of the donor and acceptor units affect the photophysical and chiroptical properties.Publisher PDFPeer reviewe

Synthèse d'émetteurs chiraux TADF pour des applications en OLED à électroluminescence circulairement polarisée

The development of organic light emitting diodes (OLEDs) with CP electroluminescence (CP-OLEDs) has recently emerged as an interesting approach to improve displays performances in comparison to currently used unpolarized OLEDs. CP-emitters with thermally activated delayed fluorescence (TADF) have recently attracted significant attention owing to the possibility to simultaneously convert both singlet and triplet excitons to circularly polarized light emission and reach high devices efficiency. This thesis enabled to develop innovative chiral TADF emitters based on C2-symmetrical bicarbazole system along two strategies: i) intramolecular charge transfer (ICT) system; This thesis enabled to develop innovative chiral TADF emitters based on C2-symmetrical bicarbazole system along two strategies: i) intramolecular charge transfer (ICT) system; this strategy leads to efficient inherent chiral TADF emitters with dissymmetry factors glum of up to 2x10-3 and ii) intermolecular excited state charge transfer (Exciplex) with an efficient separation of HOMO (donor) and LUMO (acceptor units) displaying dissymmetry factors glum reaching 1x10-2, inspired by supramolecular arrangement which enables high order of chiral organization. Interestingly, such approach using a covalent model has also afforded an innovating control of CPL sign in solvents with different polarities, originating from different conformers and electronic structures.Les composés chiraux peuvent émettre spontanément une lumière polarisée circulairement (CP), de signe opposé (luminescence CP gauche ou droite, CPL) pour les deux énantiomères. Ces caractéristiques spécifiques ont rendu les émetteurs chiraux particulièrement attrayants pour le développement de diodes électroluminescentes organiques (OLED) à électroluminescence polarisée circulairement (CP-OLED), qui sont récemment apparues comme une approche intéressante pour améliorer les performances des écrans par rapport aux OLED non polarisées actuellement utilisées. Les émetteurs CP à fluorescence retardée activée thermiquement (TADF) ont récemment attiré une attention particulière en raison de la possibilité de convertir simultanément les excitons singulet et triplet en émission de lumière polarisée circulairement et d'atteindre une efficacité élevée des dispositifs. Cette thèse a permis de développer des émetteurs chiraux TADF innovants basés sur le système bicarbazole de symétrie C2 selon les deux stratégies suivantes: i) système à transfert de charge intramoléculaire (ICT) ; cette stratégie conduit à des émetteurs TADF efficaces à chiralité inhérente avec des facteurs de dissymétrie glum atteignant 2x10-3et ii) système à transfert de charge à l'état excité intermoléculaire (Exciplexe) avec une séparation marquée entre la HOMO (donneur) et la LUMO (unités acceptrices), démontrant des glum allant jusqu'à 1x10-2 en s'inspirant du potentiel de la chimie supramoléculaire à obtenir un ordre de grandeur supérieur en terme d’organisation chirale. De manière intéressante, ce type d’approche à l’échelle covalente a également donné lieu à un contrôle innovant du signe de CPL en fonction de la polarité des solvants utilisés, provenant de conformères et de structures électroniques différents

Negative solvatochromism and sign inversion of circularly polarized luminescence in chiral exciplexes as a function of solvent polarity

International audienceThe potential control of circularly polarized luminescence (CPL), especially its sign and switching at the molecular level without any chemical modification, is desirable, but remains a considerable challenge owing to the difficulty to finely control the magnitude and relative orientation of the associated electric and magnetic dipole transition moments. To address this challenge, we report the synthesis and chiroptical properties of innovative non-conjugated chiral donor-acceptor systems displaying CPL sign inversion as a function of solvent polarity. Through the formation of a chiral exciplex, it is possible to achieve control of its (non-) radiative deexcitation pathways using solvents of different polarity, resulting in emission from locally excited (LE) or charge-transfer (CT) states (with positive and negative CPL), and thermally activated delayed fluorescence (TADF). Theoretical calculations offer further evidence of the relationship between the nature of the emitting species and the CPL. These results provide original molecular design guidelines to obtain switchable CPL emitters, and new insights into the combination of CPL and TADF, a feature of crucial importance for the development of efficient technologies based on CPL emitters

Axial and helical thermally activated delayed fluorescence bicarbazole emitters: Opposite modulation of circularly polarized luminescence through intramolecular charge-transfer dynamics

International audienceThe rationalization of the molecular parameters that influence the intensity and sign of circularly polarized luminescence (CPL) for chiral emitters is a challenging task and remains of high interest for future chiral optoelectronic applications. In this report, we explore the design of novel chiral donor-acceptor structures based on C2-symmetric bicarbazole systems and compare the influence of the type of chirality, namely axial versus helical, and the electron withdrawing strength of the acceptor units on the resulting photophysical and CPL properties. By using carbonyl-based acceptors with both axial and helical electron donors, CP-Thermally Activated Delayed Fluoresence (TADF) can be obtained, whose efficiency depends on the dihedral angle between the carbazole moieties, related to the axial and helical chirality of the compounds. The latter also impacts the intensity of the CPL, which shows an opposite trend in function of the polarity of the solvent, with notably a strong increase of the luminescence dissymmetry factor, glum, for the helical donor-acceptor compoundsrelated to a subtle reoarganization of the intramolecular charge-transfer process

Luminescent chiral exciplexes with sky-blue and green circularly polarized-thermally activated delayed fluorescence

International audienceLuminescent exciplexes based on a chiral electron donor and achiral acceptors are reported as a new approach to design circularly polarized (CP) and thermally activated delayed fluorescence (TADF) emitters. This strategy results in rather high CP luminescence (CPL) values with g$_{lum}$ up to 7x10$^{-3}$, one order of magnitude higher in comparison to the CPL signal recorded for the chiral donor alone g$_{lum}$ similar to 7x10$^{-4}$. This increase occurs concomitantly with a CPL sign inversion, as a result of the strong charge-transfer emission character, as experimentally and theoretically rationalized by using a covalent chiral donor-acceptor model. Interestingly, blue, green-yellow and red chiral luminescent exciplexes can be obtained by modifying with the electron accepting character of the achiral unit while keeping the same chiral donor unit. These results bring new (inter)molecular guidelines to obtain simply and efficiently multi-color CP-TADF emitters